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Enhanced Photocatalytic and Antibacterial Ability of Cu-Doped Anatase TiO2 Thin Films: Theory and Experiment

  • Abdullah M. Alotaibi
    Abdullah M. Alotaibi
    Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
    The National Centre for Building and Construction Technology, King Abdulaziz City for Science and Technology (KACST), Riyadh 11442-6086, Saudi Arabia
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  • , 
  • Benjamin A. D. Williamson
    Benjamin A. D. Williamson
    Department of Chemistry, Christopher Ingold Building, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
    Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, U.K.
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    Orcidhttp://orcid.org/0000-0002-6242-1121
  • , 
  • Sanjayan Sathasivam
    Sanjayan Sathasivam
    Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
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    Orcidhttp://orcid.org/0000-0002-5206-9558
  • , 
  • Andreas Kafizas
    Andreas Kafizas
    Grantham Institute, Imperial College London, Exhibition Road, London SW7 2AZ, U.K.
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    Orcidhttp://orcid.org/0000-0002-2282-4639
  • , 
  • Mahdi Alqahtani
    Mahdi Alqahtani
    Electronic & Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.
    Materials Science Research Institute, King Abdulaziz City for Science and Technology (KACST), Riyadh 11442-6086, Saudi Arabia
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    Orcidhttp://orcid.org/0000-0002-9353-1411
  • , 
  • Carlos Sotelo-Vazquez
    Carlos Sotelo-Vazquez
    Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
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  • John Buckeridge
    John Buckeridge
    School of Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, U.K.
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  • , 
  • Jiang Wu
    Jiang Wu
    Electronic & Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.
    University of Electronic Science and Technology of China, North Jianshe Road, Chengdu 610054, China
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    Orcidhttp://orcid.org/0000-0003-0679-6196
  • , 
  • Sean P. Nair
    Sean P. Nair
    Department of Microbial Diseases, UCL Eastman Dental Institute, 256 Gray’s Inn Road, London WC1X 8LD, U.K.
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  • , 
  • David O. Scanlon*
    David O. Scanlon
    Department of Chemistry, Christopher Ingold Building, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
    Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, U.K.
    Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, U.K.
    *E-mail: [email protected]. Fax: (+44) 20-7679-7463 (D.O.S.).
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    Orcidhttp://orcid.org/0000-0001-9174-8601
  • , and 
  • Ivan P. Parkin*
    Ivan P. Parkin
    Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
    *E-mail: [email protected] (I.P.P.).
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Cite this: ACS Appl. Mater. Interfaces 2020, 12, 13, 15348–15361
Publication Date (Web):February 28, 2020
https://doi.org/10.1021/acsami.9b22056

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY.

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Abstract

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Multifunctional thin films which can display both photocatalytic and antibacterial activity are of great interest industrially. Here, for the first time, we have used aerosol-assisted chemical vapor deposition to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates. The films displayed much enhanced photocatalytic activity relative to pure anatase and showed excellent antibacterial (vs Staphylococcus aureus and Escherichia coli) ability. Using a combination of transient absorption spectroscopy, photoluminescence measurements, and hybrid density functional theory calculations, we have gained nanoscopic insights into the improved properties of the Cu-doped TiO2 films. Our analysis has highlighted that the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes observed in the doped samples and the enhanced UV photoactivities observed.

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Introduction

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Anatase TiO2 is the leading chemically and biologically stable, inexpensive semiconductor with photosensitive properties for applications in photocatalysis and antibacterial surfaces. (1−9) For both applications, the general mechanisms for the functional properties are very similar; (3,10) when TiO2 is irradiated with photons with energy greater than or equal to the band gap energy, electrons from the valence band are promoted to the conduction band and concomitant holes are formed in the valence band. These electrons and holes can migrate to the surface of the semiconductor and react with water and oxygen to produce reactive oxygen species (ROS). (10,11) These ROS can oxidize a wide range of organic species, including pollutants, bacteria, cancer cells, and viruses. In the case of bacteria, the ROS produced cause an oxidative stress on bacterial cells that destroy cell walls, DNA, and proteins. (10) This form of antibacterial activity and decontamination of surfaces is innovative as it provides effective relief against bacteria that have grown resistant to many conventional antimicrobial agents, in part due to decades of inappropriate antibiotic use. (12,13) Applying such coatings on high contact touch surfaces such as door handles and keyboards in hospital and health care environments has shown a marked reduction of health care-associated infections. (14)
ROS are destructive not only to bacteria but also to organic pollutants that can be found in air and water. In fact, the primary application of TiO2 as a photocatalyst is as a self-cleaning transparent coating on windows to assist in the breakdown of dirt. (1) The incorporation of dopant quantities of Cu into the TiO2 matrix has dual advantages. Not only can Cu be used as an antibacterial agent by itself, through Fenton-type reactions, but TiO2:Cu can also potentially reduce the recombination of charge carriers which are important for both antibacterial and self-cleaning applications of TiO2. (15−19)
Cu-doped TiO2 has been prepared via solution (17−22) and nonsolution (23)-based routes in nanoparticulate forms mainly for photocatalytic and ferromagnetic applications. Sol–gel techniques (24) and physical vapor deposition methods such as magnetron sputtering (25) have also been used to fabricate thin films.
In this paper, we demonstrate for the first time the synthesis of Cu-doped TiO2 films using an aerosol-assisted chemical vapor deposition (AACVD) method. AACVD is a facile technique that places only the limitation of solubility on the precursors, that is, there are no volatility concerns as with traditional CVD, thus allowing a wider range of precursors to be employed. (26−31) It is also an atmospheric pressure technique that has been used to grow a wide range of films for various applications with the potential for upscale. (32−35)
Both pure and Cu-doped TiO2 films were prepared via AACVD, and their photocatalytic activity to the degradation of a model organic pollutant, stearic acid, and antibacterial activity to bacteria Escherichia coli and Staphylococcus aureus was examined under 365 nm radiation and compared. Cu-doped TiO2 films showed enhanced photocatalytic [formal quantum efficiency (FQE) of 1.1 × 10–4 molecules photon–1 for 5% Cu-doped TiO2] and antimicrobial activities (destruction of E. coli and S. aureus to almost undetectable levels under 4 h of 365 nm irradiation) compared with pure TiO2. Transient absorption spectroscopy (TAS) studies showed that photogenerated charge carriers were longer-lived in Cu-doped TiO2 films. Hybrid functional ab initio density functional theory (DFT) calculations were also carried out in order to determine the mechanism for the enhancement in photocatalytic activities and charge carrier lifetimes that Cu-doped anatase provides.

Experimental Methodology

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All chemicals used were purchased from Sigma-Aldrich Chemical Co. and used as received. Deposition was carried out on an 150 × 45 × 4.5 mm SiO2 (50 nm)-coated float glass (SiO2 acts as a barrier layer preventing the diffusion of ions from within the glass into the deposited film) which has been supplied by Pilkington NSG. Prior to use, the glass substrates were thoroughly cleaned using acetone (99%), isopropanol (99.9%), and distilled water and dried in air.

Deposition Procedure

Deposition was carried out under N2 (BOC Ltd., 99.99% purity) flow using titanium isopropoxide [Ti(OCH(CH3)2)4] (99%), copper nitrate (hydrated), [Cu(NO3)2·3H2O] (99%), and ethyl acetate (99%).
[Ti(OCH(CH3)2)4] (0.5 g, 1.76 mmol) was dissolved in ethyl acetate (30 mL) in a glass bubbler. The resulting solution was atomized using a piezoelectric device (Johnson Matthey Liquifog). For the Cu-doped TiO2 films, [Cu(NO3)2·3H2O] (2, 5, 10, and 20 mol % relative to [Ti(OCH(CH3)2)4]) was added to the [Ti(OCH(CH3)2)4]/ethyl acetate solution. The precursor flow was kept at 1.4 L·min–1. The substrate temperature was 470 °C. After the precursor solution had been transferred, the bubblers were closed, and the substrate was cooled under a flow of N2 to less than 100 °C before it was removed. Coated substrates were handled and stored in air. The coated glass substrate was cut into ca. 1 cm × 1 cm squares for subsequent analysis.

Film Characterization

Powder X-ray diffraction (PXRD) patterns were measured using a modified Bruker-AXS D8 diffractometer with parallel beam optics and a PSD LynxEye silicon strip detector. This instrument uses an unmonochromated Cu Kα source (Kα1, 1.54 Å) operated at 40 kV with a 30 mA emission current. The incident beam angle was set at 1°, and the angular range of the patterns collected was 10° < 2θ < 65° with a step size of 0.05° counted at 1 s/step. Raman spectroscopy was carried out on a Renishaw 1000 spectrometer equipped with a 514.5 nm laser. The Raman system was calibrated using a silicon reference. Scanning electron microscopy (SEM) was performed to determine the surface morphology and film thickness using a JEOL JSM-6301F field emission scanning electron microscope at an accelerating voltage of 5 keV. UV–visible (UV–vis) spectroscopy was performed using a PerkinElmer LAMBDA 950 UV/vis/NIR spectrophotometer over a wavelength range of 300–2500 nm. The spectra were referenced against an air background. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Kα spectrometer with monochromated Al Kα radiation, a dual beam charge compensation system, and a constant pass energy of 50 eV. Survey scans were collected in the range of 0–1200 eV. High-resolution peaks were used for the principal peaks of Ti (2p), O (2p), Cu (2p), C (1s), and Si (2p). The peaks were modeled using sensitivity factors to calculate the film composition. The area underneath these bands is an indication of the element concentration within the region of analysis (spot size 400 μm). Photoluminescence (PL) spectra from 350 to 800 nm were recorded using an Edinburgh spectrofluorometer equipped with a maximum average power of 5 mW. The excitation wavelength used was 380 nm. The PL signals from 900 to 1600 nm were collected through a Newport Oriel monochromator interfaced with a Newport Merlin lock-in amplifier and detected by a TE-cooled Ge photodetector. The samples were excited using a 532 nm diode-pumped solid-state laser. The excitation power was 70 mW. The PL spectra were recorded under air and at room temperature.

Transient Absorption Spectroscopy

TAS was used to determine exciton lifetimes at room temperature (∼22 °C) from the microsecond to second timescale. Samples were excited using pulsed laser excitation (λ = 355 nm, Opolette 355, pulse width = 6 ns, pulse repetition = 0.8 Hz, laser power = 1.55 mJ·cm–2) through a liquid light guide. A quartz halogen lamp (100 W, Bentham, IL 1) with a stabilized power supply (Bentham, 605) was used as the probe light source. To reduce stray light, scattered light, and sample emission, two monochromators and appropriate optical cutoff filters were placed before and after the sample. The probe light passing through the sample was detected using a Si photodiode (Hamamatsu Photonics, S1722-01). This signal was passed through an amplifier (Costronics Electronics) and then measured using a digital oscilloscope. Decays presented are the average between 100 and 200 laser pulses. Samples were measured inside a gas-tight quartz cell under an argon gas atmosphere.

Functional Testing

The photocatalytic activity was measured by monitoring the photocatalytic decomposition of a model organic pollutant, stearic acid (95%, Sigma-Aldrich). To measure the photocatalytic decomposition of a stearic acid coating, samples were attached to an IR sample holder consisting of an aluminum sheet with a circular hole in the middle. The stearic acid coating was applied from a saturated solution of stearic acid in chloroform (0.05 M) through a dip-coating process. Pilkington NSG Activ glass was used as a benchmark and blank float glass as a control. The breakdown of the C–H bonds in stearic acid was measured using Fourier transform infrared (FTIR) spectroscopy between 2800 and 3000 cm–1 using a PerkinElmer Spectrum RX1 FTIR spectrometer. Measurements were taken at 0, 1, 2, 3, 4, 18, and 23 h intervals with the samples irradiated using a 365 nm UVA lamp.
The C–H bonds in stearic acid absorb at 2958 cm–1 (C–H stretch CH3), 2923 cm–1 (symmetric C–H stretch CH2), and 2853 cm–1 (asymmetric C–H stretch CH2). These peaks can be integrated to give an approximate concentration of stearic acid on the surface using a calibration constant (where 1 A·cm–1 in the integrated area between 2800 and 3000 cm–1 corresponds to approximately 9.7 × 1015 molecules cm–2). (36,37) The rate of removal of stearic acid can thus be measured by monitoring the decrease in IR absorbance. The results are typically expressed in terms of FQE, ξ, defined as the number of molecules degraded per incident photon (units, molecule × photon–1). The light source was a black light-bulb lamp, 2 × 8 W (Vilmer-Lourmat). The irradiance of the lamp (I = 3.15 mW cm–2) was measured using a UVX radiometer (UVP).
E. coli (ATCC 25922) and S. aureus (8325-4) strains were maintained by weekly subculture on Brain Heart Infusion (BHI) agar (Oxoid, Basingstoke, UK). Both bacteria were used to inoculate two separate 10 mL aliquots of sterile BHI broths (Oxoid, Basingstoke, UK) and incubated aerobically at 37 °C for 18 h. Bacteria from the overnight culture were harvested by centrifugation at 13,000g for 1 min. The bacteria were then resuspended in phosphate-buffered saline (PBS) (Oxoid, Basingstoke, UK) and again centrifuged at 13,000g for 1 min. Finally, the bacterial pellet was resuspended in PBS before use. The turbidity of the bacterial cell suspension was measured at 600 nm using a spectrophotometer and was adjusted to an optical density, which corresponded to approximately 105 colony-forming units per 25 μL aliquot.
Prior to use, the pure TiO2 and Cu-doped TiO2 slides were cut into 1 × 1 cm sections. A humidity chamber was created to ensure that the suspensions did not dry out. A 25 μL aliquot of the bacterial cell suspension was spread evenly on the surface of each slide and incubated at room temperature (21 ± 2 °C) for the allocated exposure time. For each exposure time (2 and 4 h), triplicate samples were analyzed, and uncoated glass microscope slides were used as a control. The samples were then irradiated for 2 and 4 h using a UVA lamp (Vilber-Lourmat, 2 × 8 W, 365 nm, 0.65 ± 0.23 mW cm–2). A further set of samples (in triplicate) was maintained in the dark for the duration of the irradiation time. Each exposure time was also repeated on two separate occasions.
After incubation, the slides were aseptically transferred to 5 mL of PBS and vortexed for 30 s to release the bacteria into the solution. Serial dilutions of the resulting bacterial suspensions were prepared in PBS, and 100 μL from each dilution was spread onto MacConkey agar (Oxoid, Basingstoke, UK) for E.coli and Mannitol salt agar (MSA, Oxoid Ltd) for S. aureus. Plates were incubated aerobically at 37 °C for 24 h. After incubation, any bacterial colonies were counted, and viable counts of bacteria were calculated.

Theoretical Methodology

All bulk properties, intrinsic defects, and extrinsic dopants together with their respective charge states were simulated using DFT within the plane-wave code VASP. (36,38−40) Defect supercells were created from the geometrically optimized bulk parameters of anatase TiO2 using the hybrid HSE06 (Heyd–Scuseria–Ernzerhof) (41,42) functional. Hybrid functionals are an improvement on standard DFT functionals, which contain a systematic self-interaction error, thus underestimating band gaps. HSE06 has been shown to provide a reasonably accurate description of the electronic and geometric properties of all the polymorphs of TiO2. (2,43−49) In order to describe the interactions between the core and valence electrons, the projector augmented wave (PAW) method (50) was used, which mimics an all-electron method but with greater computational efficiency.
A geometry optimization was carried out on the primitive unit cell of bulk anatase TiO2, allowing volume, lattice parameters, and atoms to relax until the forces on all the atoms were less than 0.01 eV Å–1. An accurate geometry and electronic structure was calculated using a 450 eV plane-wave energy cutoff and a Γ-centered 7 × 7 × 5 k-point grid. From the primitive bulk, a 3 × 3 × 2 supercell containing 108 atoms was created, and thus defective variants were made. Geometric optimizations on the supercells were calculated with only the ions allowed to relax until all the forces were less than 0.01 eV Å–1. Γ-centered k-point meshes of 2 × 2 × 2 combined with plane-wave cutoffs of 450 eV were used, and all defect calculations were spin-polarized. It is known that under Ti-rich conditions, Ti2O3 is a limiting phase as seen in other theoretical calculations; (51) the geometry and total energy of the relaxed unit cell were calculated using a plane-wave cutoff of 450 eV and a Γ-centered k-point mesh of 8 × 8 × 8. For the Cu chemical potential, three limiting phases were considered: CuO, Cu2O, and Cu where, under O-poor conditions, the chemical potential is limited by the formation of Cu and, under O-rich conditions, by the formation of CuO. A geometry optimization of CuO was calculated using a plane-wave cutoff of 450 eV and a Γ-centered k-point mesh of 6 × 6 × 6, and the lowest energy magnetic configuration was found to be antiferromagnetic, consistent with other first-principles calculations and experiment. (52−55) See the Supporting Information for methodology on defect formalism, thermodynamic limits, and optical absorption and emission calculations.

Results and Discussion

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Pure and Cu-doped TiO2 films were synthesized from the AACVD reaction of [Ti(OCH(CH3)2)4] and 2, 5, 10, and 20 mol % of [Cu(NO3)2·3H2O] in ethyl acetate at 470 °C with N2 as the carrier gas. All films were translucent with gray/green coloration, particularly the 20 mol % doped film had excellent coverage and well adhered to the substrate, passing the Scotch tape test. (56)

X-ray Diffraction and Raman Spectroscopy

The films were studied using XRD (Figure 1a) to determine whether the desired solid solution had formed. Only reflections for the anatase phase of TiO2 were visible for all films apart from the 20% doped where, as the possible maximum Cu solubility is reached, additional peaks (43.2 and 50.4°) corresponding to the Cu metal were also observed. No noticeable shifting of the anatase Bragg reflections was seen for the Cu-doped films that would be normally expected upon substitutional or interstitial doping because of the difference in ionic radius among Ti(IV) (0.60 Å), Cu(I) (0.77 Å), and Cu(II) (0.73 Å). This does introduce the possibility that the Cu dopant may not be present in the lattice, that is, as a solid solution but actually as a secondary phase(s). However, the lack of TiO2 lattice distortion is explained by the DFT calculations where substitutional Cu doping of anatase is expected to cause minimal distortion of the lattice (see the Defect Thermodynamics section). Raman spectroscopy (Figure 1b) showed only shifts for the tetragonal anatase phase with four active modes visible for all samples including 20%—this is expected as Cu(0) would be Raman inactive. With increasing Cu concentration, the Raman data also showed a small blue shift in the principal Eg vibrational mode (caused by symmetric O–Ti–O stretching vibrations in TiO2 and indicative of unit cell size) of anatase. This indicates an expansion in the anatase unit cell and is in contrast to XRD results where no change was observed. This discrepancy can be explained by the fact that Raman spectroscopy is more surface-sensitive compared to XRD.

Figure 1

Figure 1. (a) XRD patterns and (b) Raman spectra for the undoped, 2, 5, 10, and 20% Cu-doped TiO2 anatase films grown using AACVD.

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Ultraviolet–Visible Spectroscopy and PL

Figure 2a shows the UV–vis spectra for the pure and Cu-doped films. The spectra are quite similar for pure TiO2 and 2% Cu-doped TiO2, with transmittance in the visible region at 70%. The transmittance in the visible region decreases systematically with increasing Cu concentration reaching 30% for the 20% doped film. The indirect optical band gap, calculated using the Tauc plot, remained at 3.3 eV for the pure, 2, 5, and 10% films. For the 20% Cu-doped TiO2 film however, a small shift was observed at 3.2 eV (see the Supporting Information).

Figure 2

Figure 2. (a) UV–vis spectra encompassing the UV, visible, and near-IR wavelengths and (b) PL spectra for the undoped, 2, 5, 10, and 20% Cu-doped TiO2 anatase films grown using AACVD.

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Figure 2b shows that the emission of Cu-doped TiO2 films in the UV–vis and IR regions was carried out by PL. PL spectral emission can be used to study charge carrier recombination and to investigate the efficiency of charge carrier trapping, migration, separation, and transfer in semiconductors. (57−59) PL emission spectra result from the radiative recombination of photoexcitation, where a higher PL intensity shows a higher degree of radiative recombination. (57,60) There have been examples of materials displaying strong PL and high photocatalytic activity. (61)
PL emission spectra of Cu-doped TiO2 at different Cu-doping concentrations (2, 5, 10, and 20%) in the wavelength range from 350 to 800 and 900 to 1600 nm with excitation wavelengths of 380 and 532 nm (respectively) are presented in Figure 2b. All the samples have a peak at 380–430 nm, which could be due to band gap, defects, or surface trapping of electrons. (60) There are broad peaks from around 500 to 650 nm which intensify with increasing Cu doping, and the peaks located at around 520 nm are thought to be due to a self-trapping of electrons by the TiO6 octahedra. (60) In the NIR region, emission peaks with wavelengths of 1010, 1128, 1230, 1340, and 1525 nm are found to originate as normal glass emissions. In undoped anatase TiO2, the nonglass peaks exist as low-intensity peaks, in particular peaks around 920 and 1050 nm are likely due to defect-centered emissions. In 2, 5, and 10% Cu-doped anatase, the peaks present in anatase (nonglass) are observed here albeit more pronounced. In 2, 5, and 10% TiO2:Cu, additional peaks exist such as the emission at 960, 1250, and 1400 nm. 20% Cu, however, appears more like undoped anatase, with the glass peaks dominating the spectrum. To understand these results, theoretical calculations on the PL of Cu-doped anatase are given below (dopant-centered optical properties).

X-ray Photoelectron Spectroscopy

The oxidation state of the Cu dopant on the surface of the films was determined using XPS (Figure 3). In the 2% Cu-doped film, the Cu 2p3/2 peak is centered at 932.3 eV, matching the transition for Cu(0) and/or Cu(I) [because of heavy peak overlap, it is difficult to distinguish between Cu(0) and Cu(I) from the Cu 2p region alone]. (62,63) The lack of the Cu(II) shake-up satellite peak that is normally found around 945 eV indicates the absence of Cu(II) on this surface. For the higher 5, 10, and 20% doped films, the line shapes of the Cu 2p3/2 and 2p1/2 transitions are asymmetrical and therefore deconvoluted to two different environments. For these films, the principal 2p3/2 peaks are centered at 932.4 eV and matches to Cu(0)/Cu(I). The secondary peaks are centered at 934.5 eV and are assigned to Cu(II). (62,63) The presence of Cu(II) here is further supported by the associated satellite peaks at 945 eV. This supports the DFT results which indicate that both Cu(I) and Cu(II) states are expected in both substitutional and interstitial sites in the anatase crystal lattice (see the Defect Thermodynamics section).

Figure 3

Figure 3. Cu 2p XPS spectra for the (a) 2, (b) 5, (c) 10, and (d) 20% Cu-doped TiO2 anatase films showing the presence of Cu(0), Cu(I), and Cu(II) oxidation states.

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The Cu dopant on the surface of the films was determined using the XPS data. For the 0, 2, 5, 10, and 20 mol % solutions used, the concentration of Cu was 0, 22, 30, and 38 at. %, suggesting surface segregation of Cu.

Transient Absorption Spectroscopy

TAS is a form of laser flash spectroscopy that can monitor the generation, recombination, trapping, charge transfer, and so forth of photogenerated charges in semiconductors. (64,65) The dynamics of photogenerated charges can be studied by tracking transient changes in absorbance at particular wavelengths, (66) where it has been shown that the electrons and holes in anatase TiO2 absorb in the visible region. (67,68) The technique has primarily been used to study charge-transfer processes in solar cells (organic–organic or inorganic–organic) (69−71) and has also been used to study charge transfers in heterojunction photocatalysts (inorganic–inorganic) (72) as well as the kinetics of photocatalytic processes. (73−75)
In this study, the kinetics of electron–hole recombination for the series of Cu-doped anatase TiO2 samples were investigated using TAS. Samples were excited using a UVA laser (355 nm, 1.55 mJ·cm–2·per pulse), and the transient changes in absorption were measured from 450 to 950 nm. Previous studies have shown that the photogenerated electrons and holes in anatase show broad absorptions centered at ∼800 and 450 nm, respectively. (67) An example TAS spectrum of the 5% Cu-doped TiO2 sample is shown in Figure 4a, which shows an extended absorption across the visible typical of anatase TiO2. The decay kinetics did not change with probe wavelength. This showed that charge-transfer processes were not occurring within the material. For instance, it has typically been cited that phase-segregated metal dopants in TiO2, such as Cu, may act as electron sinks. (76) If Cu was indeed acting as an electron sink, our TAS studies would show a difference in decay kinetics in the electron region (∼800 nm) compared with the hole region (∼450 nm). This was not the case for any Cu-doped sample studied herein. As the samples were studied in an argon atmosphere, no photochemistry could occur. This indicated that the transient decay of our absorption signals was due solely to electron–hole recombination. The kinetics of electron–hole recombination were compared across all samples (Figure 4b). These kinetics could be fit to power law decays, typical of the trap/detrapping movement of the charge found in anatase TiO2. (77) The rate of recombination was quantified by finding the time where half the initial absorption was lost (t50%). In the 5% Cu-doped TiO2 sample, recombination was the slowest (t50% ≈ 110 ms), followed by the 2% (t50% ≈ 26 ms), 10% (t50% ≈ 19 ms), 20% (t50% ≈ 3 ms), and undoped sample (t50% ≈ 1 ms). In the undoped sample, the timescale of recombination was typical of anatase TiO2. Moreover, our TAS results showed that Cu doping resulted in an enhancement in charge carrier lifetime. Substantial differences in the initial absorption were also observed (Figure 4b). This indicated that there were critical differences in ultrafast timescale recombination between samples, not observable on the timescale of these measurements. It was clear that the 5% Cu-doped TiO2 sample possessed the greatest number of photogenerated charges from 10 μs (followed by the 2, 10, 20%, and undoped TiO2 sample). This trend was analogous to the lifetime of photogenerated charges, where the 5% Cu-doped TiO2 samples showed both the most charges from 10 μs and the slowest rate of electron–hole recombination.

Figure 4

Figure 4. (a) Transient absorption spectra at select times for the 5% Cu-doped TiO2 thin film. (b) Recombination kinetics at 950 nm for all samples, where the dashed lines represent a fit to a power law decay model [f(t) = a·tb, where t is the time and a and b are variables].

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Photocatalysis Testing

The photocatalytic activities of the as-synthesized films were evaluated against the degradation of a model organic pollutant, octadecanoic (stearic) acid, under ultraviolet (UVA) illumination (I = 3.15 mW cm–2). Stearic acid is highly stable under UV light (in the absence of an underlying effective photocatalyst) and can be easily monitored on transparent materials via infrared spectroscopy. Its photocatalytic degradation can be monitored following the disappearance of characteristic C–H modes at 2958, 2923, and 2853 cm–1 (Figure 5b). The photocatalytic rates were estimated from the linear regression of the initial steps (30–40%) of the curve of integrated area versus illumination time. The corresponding rates were expressed as FQEs, defined as molecules of stearic acid degraded over incident photons (units, molecule × photon–1) (Figure 5a). The variation in FQE values was not attributed to differences in physical properties of the films as all films investigated showed comparable thicknesses and crystallinity based on XRD and SEM analyses (Figure 1a and Supporting Information). In addition, a blank piece of glass (as a control) and Pilkington Activ self-cleaning glass (15 nm TiO2 anatase coating) were also tested for comparison.

Figure 5

Figure 5. (a) FQEs obtained during the degradation of stearic acid under the UVA irradiation of Cu:TiO2 and undoped thin films. Blank glass and Activ samples are included for comparison. (b) IR spectra of stearic acid upon UVA illumination (I = 3.15 mW·cm–2) on a typical Cu-doped TiO2, 5% Cu:TiO2.

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The pure TiO2 sample showed a destruction rate of 7.2 × 10–5 molecules·photon–1; upon doping 2 and 5% of Cu, the rate increases to 9.1 × 10–5 and 1.1 × 10–4 molecules·photon–1 (a higher rate than Activ, 9.4 × 10–5 molecules·photon–1), respectively. At higher concentrations of Cu (10 and 20%), the stearic acid destruction rate decreases to 6.7 × 10–5 and 2.1 × 10–5 molecules·photon–1, respectively. The film photocatalytic activities observed (Figure 5) could be correlated directly with photogenerated charge carrier lifetimes (Figure 4), where high lifetime resulted in high photocatalytic activity. These observations were attributed to the kinetics of the typical processes associated with photocatalysis on TiO2. It is commonly accepted that photocatalysis on TiO2 occurs through the reaction of (i) holes with surface H2O producing highly reactive hydroxyl radicals that subsequently degrade nearby organics and (ii) electrons with O2 forming highly reactive superoxide radicals that also degrade nearby organics. (71) These two processes occur on different timescales, where it has been demonstrated that holes can react with H2O within ∼2 μs and electrons react with O2 from 10 to 900 μs. (72) Our TAS studies showed that the sample with the longest-lived photogenerated charge carrier was 5% Cu-doped TiO2, which showed both the slowest rate of recombination and the highest number of photogenerated charges.

Antibacterial Testing

Because the 5% Cu-doped TiO2 sample showed the best photocatalytic results, this sample was chosen along with pure TiO2 for antimicrobial activity testing against a Gram-negative bacterium, E. coli, and a Gram-positive bacterium, S. aureus, in the dark and under UVA-irradiated conditions. In the case of E. coli, there was no significant reduction in the colony counts for the pure TiO2 sample in the dark from 2 to 4 h (Figure 6a,b). A small reduction was observed under UVA irradiation, which is likely due to the oxidative stress caused by the generation of ROS. This was also observed when the pure TiO2 sample was tested against S. aureus (Figure 6c,d). However, for the Cu-doped sample, there was a small but significant decrease in the colony counts of both E. coli and S. aureus in the dark, which is possibly due to the cytotoxicity of Cu0, Cu+, and Cu2+. Under UVA irradiation of 4 h, there was a rapid decrease in bacterial numbers, to below or almost below detectable levels, for S. aureus and E. coli, respectively (Figure 6b,d). We attribute this to the generation of ROS that attack organic matter when TiO2 is irradiated with energy above its band gap as well as the cytotoxicity of Cu species. The activity was higher in the Cu-doped TiO2 film compared to that in the undoped film, as previously observed from TAS measurements and discussed, because of the Cu states acting as trapping sites that can reduce charge carrier recombination. The lifetimes of the charge carrier are important as they have to travel to the surface of the semiconductor before they can undergo reactions with water and oxygen to produce ROS that then proceed to destroy bacterial cells. The secondary reason for the superior activity of the doped samples over the undoped samples is due to the presence of Cu that has its own antibacterial properties via Fenton-type reactions as discussed in the Introduction. The reasons for the higher killing efficacy for the Gram-positive bacteria S. aureus over the Gram-negative E. coli are not fully understood, but it may be due to their increased permeability toward cytotoxic Cu ions of the Gram-positive bacteria cell envelope that is composed of a cell membrane and a highly permeable cell wall. Although the cell envelope is thinner for Gram-negative bacteria, it contains two membrane layers of which the outer membrane is constructed of mostly tightly packed lipopolysaccharide molecules that are successful permeability barriers. (78)

Figure 6

Figure 6. Antimicrobial test results for pure and 5% doped films in the dark and under UVA conditions for 2 (a,c) and 4 h (b,d) against Gram-negative E. coli (a,b) and Gram-positive S. aureus (c,d) bacteria.

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Defect Thermodynamics

In order to rationalize the increased performance of the Cu-doped TiO2 films over the undoped films, we have carried out a detailed hybrid DFT study of the defect chemistry of Cu-doping anatase TiO2. Figure 7 displays the thermodynamic transition levels for all the intrinsic and extrinsic defects considered in this study under Ti-rich/O-poor and Ti-poor/O-rich conditions.

Figure 7

Figure 7. Transition level diagram under both Ti-rich/O-poor (left) and Ti-poor/O-rich (right) growth regimes. The Fermi energy ranges from the VBM (0 eV) to the CBM (∼3.35 eV).

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In this study, the dominant acceptor and donor defects present in anatase TiO2 are considered. These have been shown in previous theoretical studies to be the oxygen vacancy (VO) and the titanium vacancy (VTi). (46,49,51,79−81) Ti-rich/O-poor growth conditions typically favor the formation of n-type defects, thus VO is lowest in formation energy under these conditions (ΔHf (VO0) = ∼1.19 eV). Under Ti-poor/O-rich conditions, the formation of an oxygen vacancy rises to ∼4.71 eV. VO acts as a resonant two-electron donor with the 2+/0 transition level occurring ∼0.12 eV above the conduction band maximum (CBM). This is in contrast to other wide band gap metal oxides such as ZnO, (82−85) SnO2, (85−88) and BaSnO3 (89,90) where VO acts as a deep donor and is not expected to contribute largely to the intrinsic conductivity of the material. Our results are consistent with previous theoretical studies on anatase. (49,51,89−92)
The titanium vacancy acts as an ultradeep acceptor with the 0/1– transition level occurring ∼0.87 eV above the valence band maximum (VBM). Under typically p-type favorable conditions (Ti-poor/O-rich), the formation energy of the neutral charge state is ∼7.23 eV and almost doubles under Ti-rich/O-poor growth conditions (ΔHf (VTi0) = ∼14.26 eV). All holes are localized on adjacent oxygens around the defect as shown in previous calculations. (93) Under a Ti-poor/O-rich regime, VTi4– begins to compensate VO2+ ∼2.28 eV above the VBM trapping the Fermi energy around this point.
Under Ti-poor/O-rich conditions, CuTi acts as a relatively low formation energy deep acceptor. The formation energy of CuTi0 is ∼2.20 eV, and the 0/1– transition level occurs ∼0.65 eV above the VBM. The 1–/2– transition level occurs ∼1.57 eV above the VBM, and the 3– charge state is not seen over the entirety of the band gap. Copper is therefore incorporated as Cu(II) when in a substitutional configuration, which explains the prevalence of this oxidation state in the experiment. (21,22,94−96)Figure 8a–c shows the hole localization on and around the CuTi defect in the neutral (7c), 1– (7b), and 2– (7c) charge states. In CuTi0, one hole is localized in a d-orbital on Cu and the other two are delocalized on the six joining oxygens. The evolution to the 1– charge state fills a hole with an electron leaving one localized in the Cu d-orbital and the other delocalized as before. In the 2– charge state, (7c), the remaining hole is still mostly localized in the Cu d-orbital but with some density on two equatorial (to the plane of the page) oxygen p-orbitals. Despite being a larger cation than Ti, (97) Cu substitution results in a minimal distortion of the crystal lattice. The Cu atom itself remains in the original Ti position over all charge states, and the adjacent oxygen ions hardly shift from their positions in the neutral, 1–, and 3– charge states. The CuTi2– case displays a small shift of one of the O ions by ∼10%, as seen in Figure 8c. CuTi2– begins to be compensated for by VO2+ ∼1.67 eV above the VBM (∼1.68 eV below the CBM) under Ti-poor/O-rich conditions trapping the Fermi energy at this point. Under n-type favorable conditions, Ti-rich/O-poor, CuTi0 has a very high formation energy of ∼7.76 eV and is thus likely to form in negligible quantities.

Figure 8

Figure 8. Partial charge densities for the Cu-related defects. (a–c) Hole charge density for CuTi0, CuTi–, and CuTi2– defects as viewed along the (010) direction. (d,e) Electron charge density for Cui0 and Cui+, respectively, as viewed down the (100) direction. The hole density is shown in blue and the electron density in orange 0–0.015 eV Å–1.

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Our calculations show that the formation energy of CuTi is reduced when adjacent to an oxygen vacancy ([CuTi + VO]). Under O-poor and O-rich conditions, this defect cluster possesses formation energies of ∼2.94 and ∼0.89 eV, respectively. In [CuTi + VO], Cu relaxes slightly away from VO compared to Cu in CuTi2– as expected. These results indicate however that the presence of this defect cluster is unlikely to appear in large quantities under both Ti-poor/O-rich conditions and that Cu doping will not likely induce oxygen vacancies in anatase TiO2 consistent with the work by Mathew et al. (98)
Cui acts as a resonant one-electron donor under both growth regimes and has a quite low formation energy under Ti-rich/O-poor conditions (ΔHf (Cui0) = ∼0.96 eV), which rises to ∼2.41 eV under Ti-poor/O-rich conditions. From our calculations, it is likely that interstitial Cu incorporates as Cu(I) with the 2+/1+ transition level occurring ∼0.37 eV below the CBM. In the literature, Cu is seen as both Cu(II) and Cu(I), and it is likely that both Cui and CuTi coexist in the anatase lattice, as indicated by our calculations. Despite being a resonant donor, the 1+/0 transition level occurs ∼0.05 eV above the CBM, indicating that increasing the copper incorporation into anatase will not lead to high carrier concentrations. When in the neutral charge state (CuTi0), Cu is expected to exist as Cu2+, thus when the Fermi level is very high, that is, with increased Cu incorporation, Cu2+ is expected to be seen (likewise with lower concentrations, Cu+ may be seen). From our calculations, Cui moves from the perfect octahedral interstitial position to a distorted square planar position where it bonds to four oxygens. This can be seen in Figure 8d,e. This behavior is not surprising considering that Cu2+ is well known to be Jahn–Teller active, thus preferring a distorted square planar configuration. Under all charge states, Cu remains in the same position and the Cu–O distances remain at 2.00 Å similar to the average Ti–O bond length of 1.98 Å in anatase. The neighboring four Ti ions move slightly away by an average of 4% from the Cui defect and remain in the same position over all the charge states. In Figure 8d,e, the partial charge density is shown for the neutral (Figure 8d) and 1+ (Figure 8e) charge states. In the neutral charge state, both electrons are delocalized over the Ti 3d orbitals that make up the CBM [indicating Cu(II)], and in the 1+ charge state, there is a localization of the electron density on and around Cu [indicating Cu(I)].

Dopant-Centered Optical Properties

The optical transitions for all defects were evaluated and are shown for the intrinsic defects, VTi and VO, in Figure 9 and substitutional and interstitial Cu in Figure 10.

Figure 9

Figure 9. Configurational coordinate diagrams for VTi (a,b) and VO (c,d). (a,c) Absorption and subsequent emission of an electron to/from the CBM and (b,d) capture of an electron and subsequent emission from/to the VBM.

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Figure 10

Figure 10. One-dimensional configurational coordinate diagram for (a,c) CuTi and (b,d) Cui under electron and hole ionization, respectively. In each example, the absorption and emission energies are independent of the chemical potentials and therefore the growth conditions.

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Absorption

VTi undergoes optical absorption from both the excitation of an electron to the conduction band and the ionization of a hole in the valence band and is shown in Figure 9a,b, respectively. For the excitation of an electron to the conduction band, VTi undergoes four transitions with 414, 401, 371, and 366 nm for VTi4– → VTi3–, VTi3– → VTi2–, VTi2– → VTi–, and VTi– → VTi0, respectively. These absorptions are likely to exist within the absorption edge of anatase and thus will be indistinguishable; however, these may be present in fine structure absorption spectra peaks. (99) VTi can also undergo the capture of an electron (or hole ionization) from the valence band, resulting in four more absorptions with wavelengths of 579, 508, 475, and 496 nm for VTi0 → VTi–, VTi– → VTi2–, VTi2– → VTi3–, and VTi3– → VTi4–, respectively. It is likely that these peaks will be seen with the increasing concentration of VTi and that a greater absorption of visible light will be observed.
Figure 9c,d shows the optical absorption values for VO. Despite VO being thermodynamically resonant within the conduction band, the optical excitation of electrons can still occur. The optical excitation to the conduction band from VO0 → VO+ and VO+ → VO2+ is a two-photon process with a very low absorption energy of 2799 nm whose observation is not likely. A similar two-photon absorption is observed for the capture of an electron from the valence band with much larger absorption energies of around 340 nm.
Figure 10a shows that CuTi undergoes optical absorption from CuTi2– to CuTi– and from CuTi– to CuTi0 in the near-UV (490 nm) and the UV (375 nm) regions, respectively, for the excitation of an electron to the conduction band. These absorptions will occur within the absorption edge of anatase TiO2 and is thus not expected to be distinguishable experimentally. CuTi can also undergo hole ionization from the VBM as shown in Figure 10c where the absorptions occur in the NIR (886 nm; CuTi0 → CuTi–) and the green region (525 nm; CuTi– → CuTi2–) of the visible spectrum. These results can be seen in the transmission data in (Figure 2a) by the dip around the absorption edge and ∼525 nm compared to undoped pure anatase and is shown to be quite pronounced in the 10% Cu samples, signifying larger concentrations of substitutional Cu.
Figure 10b,d displays the electron and hole ionization transitions for interstitial Cu (Cui), respectively. Figure 10b shows that the optical excitation of an electron to the CBM results in the absorption of a photon in the NIR region (969 nm) for Cui+ → Cui2+. From the transmission data in (Figure 2a), broad troughs are seen ∼900–1200 nm for the 10 and 20% Cu samples, suggesting that an increased incorporation of interstitial Cu is present. The broad troughs are indicative of a large relaxation energy (100) (Erel) which for Cui+ to Cui2+ is ∼0.91 eV. A small dip in the transmission occurs for 5% Cu and none at all for 2% Cu, leading to the conclusion that the 2+ → 1+ transition may be suppressed. It is not expected that an optical transition will occur from the 1+ to the 0 charge states as thermal ionization occurs in the conduction band and has thus been omitted from this diagram. The capture of an electron by Cui2+ (from the VBM) facilitates the absorption of a photon in the near-UV with a wavelength of which, as with CuTi, is not expected to be distinguishable experimentally because of its position in the absorption edge of TiO2.

Emission

The intrinsic defect emissions for VTi and VO are shown in Figure 9. Infrared PL also occurs under both electron excitation and capture. Under electron excitation from VTi to the conduction band, it is expected for the transitions: VTi0 → VTi–, VTi– → VTi2–, VTi2– → VTi3–, and VTi3– → VTi4– that emissions of 969, 1261, 1527, and 1344 nm will be seen. Under the emission of an electron to the VBM (Figure 9b), two IR emissions are observed at 2903 and 3768 nm for VTi4– → VTi3– and VTi3– → VTi2–, respectively. The other transitions are likely to be nonradiative as they occur at very small energies (0.033–0.086 eV). Most experimental PL studies are carried out in the UV–vis region of the electromagnetic spectrum and (<900 nm) and are thus not accounted for in the scientific literature. One study on TiO2 nanoribbons, however, shows a PL peak of around 969 nm. (101) The Stokes shifts for each transition (difference between absorption and emission) are fairly large and as such indicate a broad shift between the absorption and emission. The total Stokes shifts for each transition: VTi4– ↔ VTi3–, VTi3– ↔ VTi2–, VTi2– ↔ VTi–, and VTi– ↔ VTi0 are 2.07, 2.28, 2.36, and 2.11 eV, respectively. Within the PL of Cu-doped TiO2 in Figure 2b, the peaks at 969, 1250, and 1340 nm correspond to those by VTi. It is likely that the glass emission around 1531 nm masks the 1527 nm peak from the VTi2– → VTi3– transition.
VO only undergoes PL via the release of an electron to the VBM. This is a two-photon process with a wavelength of 416 nm; thus, the peak intensity is expected to be larger than that of a single photon process. Numerous room-temperature studies observe a peak of around 427–417 nm (where a shift is expected at increasing temperature) which Kernazhitsky et al. identify as an oxygen vacancy-related peak. (99,102−106) In Figure 2b, the peak at 416 nm matches well to that calculated here. The intensity of the peak rises with increasing Cu doping, indicating the formation of oxygen vacancies. The total Stokes shift for the two-electron process is calculated as 1.26 eV, in good agreement with the study of Wang et al., which shows a Stokes shift of 1.36 eV. (102)
The emission energies present for electron (Figure 10a) and hole (Figure 10c) ionization of CuTi range from the mid-IR region to the red region of the visible spectrum. In Figure 10a, the transition of CuTi0 → CuTi– and the subsequent transition of CuTi– to CuTi2– give rise to PL peaks at ∼ 634 and ∼1252 nm, respectively. Upon the release of a hole to the VBM, two PL peaks are expected around 1512 nm (CuTi2– → CuTi–) and ∼31 μm (CuTi– → CuTi0). The latter absorption is expected to be nonradiative because of the very small emission energy and will thus be dissipated through the anatase lattice as phonons. (107) Similar behavior is seen in acceptor defects such as VZn in ZnO (108) as well as VGa in GaN. (107) It is expected that these peaks will be broad because of the relatively large Erel values (∼0.7 eV). Because of the glass emission at 1525 nm in Figure 2b, it is hard to tell whether this peak is present in the AACVD-made Cu-doped TiO2 thin films.
Cui will only photoluminesce after the capture of an electron from the valence band. Figure 10b depicts this with a value of 599 nm and a broad peak (Erel = 0.91 eV). This is seen in the room-temperature PL data in (Figure 2b) from ∼450 to 625 nm. The highest intensity PL occurs in the 20% Cu sample, which, together with the transmission data in (Figure 2a), indicates that the presence of interstitial Cu is more substantial. Figure 10b shows that Cui will not photoluminesce after electron ionization because of the thermodynamic resonance of Cui+ in the conduction band. Instead, it is likely that Cui+ will readily capture an electron with an energy barrier of 0.005 eV to return to the 2+ charge state.
The presence of interstitial Cu in the anatase lattice together with substitutional Cu is likely beneficial to both the photocatalytic activity and the electron-carrier separation lifetimes. The mechanism of this is possibly due to the synergistic effect of deep acceptor/shallow donor pairs whereby an excited electron from CuTi can be readily accepted by an interstitial Cu defect, thereby increasing the separation. Electron capture by substitutional Cu occurs in the visible/NIR region of the spectrum (CuTi0 → CuTi–; Eabs = 886 nm), and electron excitation of Cui also occurs in the visible/NIR region (Cui+ → Cui2+; Eabs = 969 nm) aiding the visible light-enhanced photocatalytic activity. It is possible that competition between these defects may arise with increased Cu incorporation favoring the formation of interstitial Cu over substitutional Cu, thereby losing the beneficial synergistic effect. This is evidenced through the detrimental effect to the carrier separation lifetimes in Figure 4 and to the decrease in photocatalytic activity (Figure 5a) toward higher doping levels where the CuTi:Cui ratio is suggested to be higher (10 and 20%).

Conclusions

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AACVD provides a facile route to highly photoactive and antimicrobial Cu-doped TiO2 thin films. From experimental analysis, the 5% TiO2:Cu film in particular displayed the greatest exciton lifetimes, photocatalytic activity (in the degradation of stearic acid), and antibacterial activity (against E. coli and S. aureus) under 365 nm irradiation. Using hybrid DFT calculations, this was demonstrated to be due to a synergistic effect from interstitial and substitutional Cu within the anatase lattice and confirmed by optical transmission and PL experiments. Effective dopant selection and concentration control are therefore key to providing the maximum efficiency in terms of carrier lifetimes for migration to the surface for the necessary reactions to take place for photocatalysis and antibacterial activity.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.9b22056.

  • Defect formalism, thermodynamic limits, and optical calculations and SEM of the Cu-doped TiO2 films (PDF)

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Author Information

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  • Corresponding Authors
    • David O. Scanlon - Department of Chemistry, Christopher Ingold Building, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, U.K.;  Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, U.K.;  Orcidhttp://orcid.org/0000-0001-9174-8601;  Email: [email protected]
    • Ivan P. Parkin - Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  Email: [email protected]
  • Authors
    • Abdullah M. Alotaibi - Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  The National Centre for Building and Construction Technology, King Abdulaziz City for Science and Technology (KACST), Riyadh 11442-6086, Saudi Arabia
    • Benjamin A. D. Williamson - Department of Chemistry, Christopher Ingold Building, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, U.K.;  Orcidhttp://orcid.org/0000-0002-6242-1121
    • Sanjayan Sathasivam - Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.;  Orcidhttp://orcid.org/0000-0002-5206-9558
    • Andreas Kafizas - Grantham Institute, Imperial College London, Exhibition Road, London SW7 2AZ, U.K.;  Orcidhttp://orcid.org/0000-0002-2282-4639
    • Mahdi Alqahtani - Electronic & Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.;  Materials Science Research Institute, King Abdulaziz City for Science and Technology (KACST), Riyadh 11442-6086, Saudi Arabia;  Orcidhttp://orcid.org/0000-0002-9353-1411
    • Carlos Sotelo-Vazquez - Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
    • John Buckeridge - School of Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, U.K.
    • Jiang Wu - Electronic & Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.;  University of Electronic Science and Technology of China, North Jianshe Road, Chengdu 610054, China;  Orcidhttp://orcid.org/0000-0003-0679-6196
    • Sean P. Nair - Department of Microbial Diseases, UCL Eastman Dental Institute, 256 Gray’s Inn Road, London WC1X 8LD, U.K.
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors would like to thank Professor James R. Durrant for useful discussions and Dr Steve Firth and Dr Tom Gregory for useful discussion on SEM. The authors are grateful to King Abdulaziz City for Science and Technology (KACST), Saudi Arabia, for the provision of a PhD studentship to Abdullah Alotaibi and for financial support from the Saudi Cultural Bureau in London. The authors are also grateful to the UK Materials and Molecular Modelling Hub for computational resources, which are partially funded by EPSRC (EP/P020194/1), and to UCL for the provision of the Legion, Myriad and Grace supercomputers. Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202, EP/R029431), this work was carried out under the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). A.K. thanks Imperial College for a Junior Research Fellowship, the EPSRC for a Capital Award Emphasising Support for Early Career Researchers, and the Royal Society for an Equipment Grant (RSG\R1\180434).

References

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  5. 5
    Georgekutty, R.; Seery, M. K.; Pillai, S. C. A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism. J. Phys. Chem. C 2008, 112, 13563– 13570,  DOI: 10.1021/jp802729a
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    5
    A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties, and Mechanism
    Georgekutty, Reenamole; Seery, Michael K.; Pillai, Suresh C.
    Journal of Physical Chemistry C (2008), 112 (35), 13563-13570CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Highly photocatalytically active silver-modified ZnO has been prepd. and the effect of silver modification was studied. The structural and optical properties were characterized by x-ray diffraction, Fourier transform IR, differential scanning calorimetry, BET surface area, Raman, UV-vis, and photoluminescence spectroscopy. The photocatalytic activity of these materials was studied by analyzing the degrdn. of an org. dye, Rhodamine 6G (R6G), and it is found that 3 mol % silver-modified ZnO at 400° C shows approx. four times higher rate of degrdn. than that of unmodified ZnO and a three times higher rate than that of com. TiO2 photocatalyst Degussa P-25. It was also noted that the photocatalytic activity for the modified ZnO sample was five times higher than the unmodified sample using sunlight. The effect of silver in enhancing the photocatalytic activity has been studied by analyzing the emission properties of both ZnO and silver-modified ZnO in the presence (emission increases) and absence (emission decreases) of R6G. The authors attribute these observations to the extent of valence band hole prodn. and the role of silver in trapping the conduction band (CB) electrons in the absence of R6G. In the presence of R6G, the dye preserves the CB electron population in the metal oxide, thus preserving and enhancing emission intensity. The sensitizing property of the dye and electron scavenging ability of silver together constitute to the interfacial charge transfer process in such a way to utilize the photoexcited electrons.
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  6. 6
    Morrison, S. R.; Freund, T. Chemical role of holes and electrons in ZnO photocatalysis. J. Chem. Phys. 1967, 47, 1543– 1551,  DOI: 10.1063/1.1712115
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    Chemical role of holes and electrons in zinc oxide photocatalysis
    Morrison, Stanley Roy; Freund, Thomas
    Journal of Chemical Physics (1967), 47 (4), 1543-52CODEN: JCPSA6; ISSN:0021-9606.
    The role of the electronic properties of a semiconductor in heterogeneous catalysis and electrochemistry was exptl. investigated on single-crystal ZnO. It was shown quant. that the availability of electrons and holes at the surface is dominant in the mechanism of a heterogeneously catalyzed reaction. Chem. rate measurements as well as in situ solid-state measurements were carried out in an aq. medium for the reaction: HCOOH + O2 → H2O2 + CO2, photocatalyzed by ZnO. Two new exptl. electrochem. methods for semiconductor surface reactions were developed: "current doubling" and measurement of "unfilled" electronic surface states by the capacitance. They were devised to characterize reactive sorbed species. The detailed catalytic mechanism was based on studies of individual reaction steps under three solid-state surface conditions: (1) only holes reacting, (2) only electrons reacting, and (3) both holes and electrons reacting. The first two correspond to the ZnO being an electrochem. anode and cathode, resp.; the last corresponds to ordinary catalysis when the hole current is balanced by the electron current so that the net current is zero. The overall catalyzed reaction is not simply the sum of individual oxidn. and redn. reactions which occur with only holes or only electrons, resp., but involves a surface intermediate formed when both holes and electrons are present.
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  7. 7
    Ali, T. T.; Narasimharao, K.; Parkin, I. P.; Carmalt, C. J.; Sathasivam, S.; Basahel, S. N.; Bawaked, S. M.; Al-Thabaiti, S. A. Effect of pretreatment temperature on the photocatalytic activity of microwave irradiated porous nanocrystalline ZnO. New J. Chem. 2015, 39, 321– 332,  DOI: 10.1039/c4nj01465k
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    Effect of pretreatment temperature on the photocatalytic activity of microwave irradiated porous nanocrystalline ZnO
    Ali, Tarek T.; Narasimharao, Katabathini; Parkin, Ivan P.; Carmalt, Claire J.; Sathasivam, Sanjayan; Basahel, Sulaiman N.; Bawaked, Salem M.; Al-Thabaiti, Shaeel A.
    New Journal of Chemistry (2015), 39 (1), 321-332CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
    Porous nanocryst. ZnO photocatalysts were successfully synthesized by microwave irradn. and then thermally treated at different temps. (150 °C, 200 °C, 250 °C and 300 °C). The physico-chem. properties of synthesized samples were detd. by using different characterization techniques. The characterization results indicated that the as-synthesized sample was comprised of both ZnO and Zn(OH)2 phases with a particle size of approx. 50 nm. Thermal treatment of the as-synthesized sample at 150 °C resulted in a pure ZnO phase with a particle size of 40 nm. The results also demonstrated that the surface area, pore diam. and bandgap energy reach a max. value for the ZnO sample treated at 200 °C. The ZnO nanoparticles pretreated at 200 °C showed the highest photocatalytic activity (99% of degrdn.) in a short reaction time (90 min), which can be attributed to the combined effects of several factors including low crystallite size, relatively high surface area, pore diam., pore vol. and bandgap energy. Reusability results show that the catalysts can be readily sepd. from the reaction mixt. by filtration after the photocatalytic reaction and reused at least five times without any loss of activity.
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  8. 8
    Promdet, P.; Quesada-Cabrera, R.; Sathasivam, S.; Li, J.; Jiamprasertboon, A.; Guo, J.; Taylor, A.; Carmalt, C. J.; Parkin, I. P. High Defect Nanoscale ZnO Films with Polar Facets for Enhanced Photocatalytic Performance. ACS Appl. Nano Mater. 2019, 2, 2881– 2889,  DOI: 10.1021/acsanm.9b00326
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    8
    High defect nanoscale ZnO films with polar facets for enhanced photocatalytic performance
    Promdet, Premrudee; Quesada-Cabrera, Raul; Sathasivam, Sanjayan; Li, Jianwei; Jiamprasertboon, Arreerat; Guo, Jian; Taylor, Alaric; Carmalt, Claire J.; Parkin, Ivan P.
    ACS Applied Nano Materials (2019), 2 (5), 2881-2889CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    The fabrication of highly efficient photocatalytic thin films has important consequences for self-cleaning, org. pollutant decompn., and antimicrobial coatings for a variety of applications. Here, we developed a simple synthesis method to produce efficient, high-surface-area zinc oxide (ZnO) photocatalytic films using aerosol-assisted chem. vapor deposition. This approach used mixts. of methanol and acetic acid to promote preferential growth and exposure of polar facets, which favor photocatalytic activity. Interestingly, the initial enhanced efficiency of the films was correlated to structural defects, likely oxygen vacancies, as supported by photoluminescence spectroscopy results. Discussion over the influence of such defects on photocatalytic performance is described, and the need for strategies to develop high-surface-area materials contg. stable defects is highlighted.
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  9. 9
    Hassan, I. A.; Sathasivam, S.; Nair, S. P.; Carmalt, C. J. Antimicrobial properties of copper-doped ZnO coatings under darkness and white light illumination. ACS Omega 2017, 2, 4556– 4562,  DOI: 10.1021/acsomega.7b00759
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    9
    Antimicrobial Properties of Copper-Doped ZnO Coatings under Darkness and White Light Illumination
    Hassan, Iman A.; Sathasivam, Sanjayan; Nair, Sean P.; Carmalt, Claire J.
    ACS Omega (2017), 2 (8), 4556-4562CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    We report the first antimicrobial study of transparent and robust Cu doped ZnO coatings that displayed potent antimicrobial activity that resulted in bacterial (E.coli) redn. below detection limits within 6 h of illumination via a white light source that is found in hospital environments. The same bacterial redn. rate was obsd. even under darkness for 4.0 at.% Cu doped ZnO films. Thus providing an efficient 24-h disinfection. All films were produced via a novel inexpensive and easily scalable route and were also thoroughly analyzed for their material properties.
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  10. 10
    Li, Y.; Zhang, W.; Niu, J.; Chen, Y. Mechanism of photogenerated reactive oxygen species and correlation with the antibacterial properties of engineered metal-oxide nanoparticles. ACS Nano 2012, 6, 5164– 5173,  DOI: 10.1021/nn300934k
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    10
    Mechanism of Photogenerated Reactive Oxygen Species and Correlation with the Antibacterial Properties of Engineered Metal-Oxide Nanoparticles
    Li, Yang; Zhang, Wen; Niu, Junfeng; Chen, Yongsheng
    ACS Nano (2012), 6 (6), 5164-5173CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Oxidative stress induced by reactive oxygen species (ROS) is one of the most important antibacterial mechanisms of engineered nanoparticles (NPs). To elucidate the ROS generation mechanisms, we investigated the ROS prodn. kinetics of seven selected metal-oxide NPs and their bulk counterparts under UV irradn. (365 nm). The results show that different metal oxides had distinct photogenerated ROS kinetics. Particularly, TiO2 nanoparticles and ZnO nanoparticles generated three types of ROS (superoxide radical, hydroxyl radical, and singlet oxygen), whereas other metal oxides generated only one or two types or did not generate any type of ROS. Moreover, NPs yielded more ROS than their bulk counterparts likely due to larger surface areas of NPs providing more absorption sites for UV irradn. The ROS generation mechanism was elucidated by comparing the electronic structures (i.e., band edge energy levels) of the metal oxides with the redox potentials of various ROS generation, which correctly interpreted the ROS generation of most metal oxides. To develop a quant. relationship between oxidative stress and antibacterial activity of NPs, we examd. the viability of E. coli cells in aq. suspensions of NPs under UV irradn., and a linear correlation was found between the av. concn. of total ROS and the bacterial survival rates (R2 = 0.84). Although some NPs (i.e., ZnO and CuO nanoparticles) released toxic ions that partially contributed to their antibacterial activity, this correlation quant. linked ROS prodn. capability of NPs to their antibacterial activity as well as shed light on the applications of metal-oxide NPs as potential antibacterial agents.
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  11. 11
    Chadwick, N. P.; Glover, E. N. K.; Sathasivam, S.; Basahel, S. N.; Althabaiti, S. A.; Alyoubi, A. O.; Parkin, I. P.; Carmalt, C. J. Photo-activity and low resistivity in N/Nb Co-doped TiO2 thin films by combinatorial AACVD. J. Mater. Chem. A 2016, 4, 407– 415,  DOI: 10.1039/c5ta07922e
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    11
    Photo-activity and low resistivity in N/Nb Co-doped TiO2 thin films by combinatorial AACVD
    Chadwick, Nicholas P.; Glover, Emily N. K.; Sathasivam, Sanjayan; Basahel, Sulaiman N.; Althabaiti, Shaeel A.; Alyoubi, Abdulrahman O.; Parkin, Ivan P.; Carmalt, Claire J.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (2), 407-415CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A combinatorial aerosol assisted CVD (cAACVD) cation-anion co-doping study was undertaken for the 1st time, which studies the interplay of nitrogen and niobium co-dopants and the resultant functional properties within TiO2 thin films. This study advantageously creates a single doped TiO2 thin film which incorporates many compns. that transition from nitrogen doped TiO2 to niobium doped TiO2 across the film's width, in a single deposition. The film was split into a grid and the phys. properties of each grid position characterized by x-ray Diffraction (XRD), XPS, SEM and UV-visible transmission spectroscopy (UV/visible). Functional properties such as photocatalytic activity, water contact angles and resistivity were also characterized. The study was successful in creating and identifying the optimum dopant concn. at which these TiO2 films exhibited both a high rate of photo-activity and favorable transparent conducting oxide (TCO) properties. While most co-doping studies report relatively homogeneous film, the inhomogeneity of these films allows both functional properties to exist in conjunction. To the authors knowledge this is the 1st instance cation and anion co-doping was explored in the combinatorial regime.
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  12. 12
    Musat, V.; Teixeira, B.; Fortunato, E.; Monteiro, R. C. C.; Vilarinho, P. Al-doped ZnO thin films by sol–gel method. Surf. Coating. 2004, 180–181, 659– 662,  DOI: 10.1016/j.surfcoat.2003.10.112
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    Al-doped ZnO thin films by sol-gel method
    Musat, V.; Teixeira, B.; Fortunato, E.; Monteiro, R. C. C.; Vilarinho, P.
    Surface and Coatings Technology (2004), 180-181 (), 659-662CODEN: SCTEEJ; ISSN:0257-8972. (Elsevier Science B.V.)
    Transparent and conductive high preferential c-axis oriented ZnO thin films doped with Al have been prepd. by the sol-gel method using zinc acetate and aluminum chloride as cation sources, 2-methoxiethanol as solvent, and monoethanolamine as sol stabilizer. Film deposition was performed by the dip-coating technique at a withdrawal rate of 1.5 cm min-1 on Corning 1737 glass substrate. The effect of dopant concn., heating treatment, and annealing in a reducing atm. on the microstructure as well as on the elec. and optical properties of the thin films is discussed. The optical transmittance spectra of the films showed a very good transmittance, between 85 and 95%, within the visible wavelength region. The min. resistivity of 1.3 × 10-3 Ω-cm was obtained for a film doped with 2 wt.% Al, preheated at 400°, and post-heated at 600°, after annealing under a reduced atm. of forming gas.
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  13. 13
    Matsubara, K.; Fons, P.; Iwata, K.; Yamada, A.; Sakurai, K.; Tampo, H.; Niki, S. ZnO transparent conducting films deposited by pulsed laser deposition for solar cell applications. Thin Solid Films 2003, 431–432, 369– 372,  DOI: 10.1016/s0040-6090(03)00243-8
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    13
    ZnO transparent conducting films deposited by pulsed laser deposition for solar cell applications
    Matsubara, K.; Fons, P.; Iwata, K.; Yamada, A.; Sakurai, K.; Tampo, H.; Niki, S.
    Thin Solid Films (2003), 431-432 (), 369-372CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science B.V.)
    Low resistivity and highly transparent ZnO conducting films for thin film solar cell applications were fabricated at low temp. by pulsed laser deposition. Al-, B- and Ga-doped ZnO films were deposited on Corning 7059 glass substrate at a substrate temp. of 200°. The Al-doped ZnO films were found to have the lowest resistivity of 2.5 × 10-4 Ω·cm and an av. optical transmission of 91% for wavelengths between 400 and 1100 nm. The values of the Ga-doped film were 2.5 × 10-4 Ω·cm and 81%, resp. Owing to the higher optical transmission in the near-IR region, the photovoltaic cell performance of a Cu(In,Ga)Se2 thin-film solar cell with an Al-doped ZnO window outperformed a cell fabricated with a Ga-doped ZnO window.
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  14. 14
    Page, K.; Palgrave, R. G.; Parkin, I. P.; Wilson, M.; Savin, S. L. P.; Chadwick, A. V. Titania and silver–titania composite films on glass—potent antimicrobial coatings. J. Mater. Chem. 2007, 17, 95– 104,  DOI: 10.1039/b611740f
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    14
    Titania and silver-titania composite films on glass-potent antimicrobial coatings
    Page, Kristopher; Palgrave, Robert G.; Parkin, Ivan P.; Wilson, Michael; Savin, Shelley L. P.; Chadwick, Alan V.
    Journal of Materials Chemistry (2007), 17 (1), 95-104CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Titania (anatase) and Ag-doped titania (anatase) coatings were prepd. on glass microscope slides by a sol-gel dip-coating method. The resultant coatings were characterized by x-ray diffraction, x-ray absorption near edge structure (XANES), Raman, SEM, wavelength dispersive x-ray (WDX) anal., XPS and UV-vis techniques and shown to consist of anatase with ∼0.2-1 atom% Ag2O. Photocatalytic activity of the coatings was detd. by photomineralization of stearic acid, monitored by FT-IR spectroscopy. Photocatalytically-active coatings were screened for their antibacterial efficacy against Staphylococcus aureus (NCTC 6571), Escherichia coli (NCTC 10418) and Bacillus cereus (CH70-2). Ag-doped titania coatings were found to be significantly more photocatalytically and antimicrobially active than a titania coating. No antimicrobial activity was obsd. in the dark-indicating that silver ion diffusion was not the mechanism for antimicrobial action. The mode of action was explained in terms of a charge sepn. model. The coatings also demonstrated significantly higher activity against the Gram-pos. organisms than against the Gram-neg. The Ag2O-TiO2 coating is a potentially useful coating for hard surfaces in a hospital environment due to its robustness, stability to cleaning and reuse, and its excellent antimicrobial response.
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  15. 15
    Wu, B.; Huang, R.; Sahu, M.; Feng, X.; Biswas, P.; Tang, Y. J. Bacterial responses to Cu-doped TiO2 nanoparticles. Sci. Total Environ. 2010, 408, 1755– 1758,  DOI: 10.1016/j.scitotenv.2009.11.004
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    15
    Bacterial responses to Cu-doped TiO2 nanoparticles
    Wu, Bing; Huang, Rick; Sahu, Manoranjan; Feng, Xueyang; Biswas, Pratim; Tang, Yinjie J.
    Science of the Total Environment (2010), 408 (7), 1755-1758CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    The toxicity of Cu-doped TiO2 nanoparticles (NPs, 20 nm), synthesized by a flame aerosol reactor, to Mycobacterium smegmatis and Shewanella oneidensis MR-1, is the primary focus of this study. Both doped and non-doped TiO2 NPs (20 nm) tended to agglomerate in the medium soln., and therefore did not penetrate into the cell and damage cellular structures. TiO2 particles (<100 mg/L) did not apparently interfere with the growth of the two species in aq. cultures. Cu-doped TiO2 NPs (20 mg/L) significantly reduced the M. smegmatis growth rate by three fold, but did not affect S. oneidensis MR-1 growth. The toxicity of Cu-doped TiO2 NPs was driven by the release of Cu2+ from the parent NPs. Compared to equiv. amts. of Cu2+, Cu-doped TiO2 NPs exhibited higher levels of toxicity to M. smegmatis (P-value < 0.1). Addn. of EDTA in the culture appeared to significantly decrease the anti-mycobacterium activity of Cu-doped TiO2 NPs. S. oneidensis MR-1 produced a large amt. of extracellular polymeric substances (EPS) under NP stress, esp. extracellular protein. Therefore, S. oneidensis MR-1 was able to tolerate a much higher concn. of Cu2+ or Cu-doped TiO2 NPs. S. oneidensis MR-1 also adsorbed NPs on cell surface and enzymically reduced ionic copper in culture medium with a remediating rate of 61 μg/(liter/OD600/h) during its early exponential growth phase. Since the metal reducing Shewanella species can efficiently "clean" metal-oxide NPs, the activities of such environmentally relevant bacteria may be an important consideration for evaluating the ecol. risk of metal-oxide NPs.
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  16. 16
    Jaimy, K. B.; Safeena, V. P.; Ghosh, S.; Hebalkar, N. Y.; Warrier, K. G. K. Photocatalytic activity enhancement in doped titanium dioxide by crystal defects. Dalton Trans. 2012, 41, 4824– 4832,  DOI: 10.1039/C2DT12018F
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    16
    Photocatalytic activity enhancement in doped titanium dioxide by crystal defects
    Jaimy, Kanakkanmavudi B.; Safeena, V. P.; Ghosh, Swapankumar; Hebalkar, Neha Y.; Warrier, K. G. K.
    Dalton Transactions (2012), 41 (16), 4824-4832CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Visible light sensitive Fe3+ and Ce4+ co-doped nano TiO2 photocatalyst has been prepd. by a modified aq. sol-gel method and the activity has been measured in terms of degrdn. of MB dye. Both dopants shifted the absorption profile of TiO2 to the visible region and improved activity. Fe3+ ions trapped the charge carriers due to the stable electronic configuration and improved their sepn. Ce4+ ions, which were mainly located at the crystallites boundaries, cause dislocations by bending the valence and conduction bands of TiO2 and prevent the recombination of photoexcited electrons and holes. The co-doped TiO2 compns. exhibited higher photocatalytic activity than that of pure titania and com. available Degussa P25 under visible light by utilizing the individual and synergistic contributions of Fe3+ and Ce4+ dopants, resp.
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  17. 17
    Choi, W.; Termin, A.; Hoffmann, M. R. The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. A 1994, 98, 13669– 13679,  DOI: 10.1021/j100102a038
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    17
    The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics
    Choi, Wonyong; Termin, Andreas; Hoffmann, Michael R.
    Journal of Physical Chemistry (1994), 98 (51), 13669-79CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    A systematic study of metal ion doping in quantum (Q)-sized (2-4 nm) TiO2 colloids is performed by measuring their photoreactivities and the transient charge carrier recombination dynamics. The presence of metal ion dopants in the TiO2 cryst. matrix significantly influences photoreactivity, charge carrier recombination rates, and interfacial electron-transfer rates. The photoreactivities of 21 metal ion-doped colloids are quantified in terms of both the conduction band electron redn. of an electron acceptor (CCl4) Fe3+, Mo5+, Ru3+, Os3+, Re5+, V3+, and Rh3+ at 0.1-0.5 at.% significantly increases the photoreactivity for both oxidn. and redn. while Co3+ and Al3+ doping decreases the photoreactivity. The transient absorption signals upon laser flash photolysis (λex = 355 nm) at λ = 600 nm are extended up to 50 ms for Fe3+-, V4+-, Mo5+-, and Ru3+-doped TiO2 while the undoped Q-sized TiO2 shows a complete "blue electron" signal decay within 200 μs. Co3+- and Al3+-doped TiO2 are characterized by rapid signal decays with a complete loss of absorption signals within 5 μs. The quantum yields obtained during CW photolyses are quant. correlated with the measured transient absorption signals of the charge carriers. Photoreactivities are shown to increase with the relative concn. of trapped charge carriers. The photoreactivity of doped TiO2 appears to be a complex function of the dopant concn., the energy level of dopants within the TiO2 lattice, their d electronic configuration, the distribution of dopants, the electron donor concn., and the light intensity.
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  18. 18
    Hassan, I. A.; Parkin, I. P.; Nair, S. P.; Carmalt, C. J. Antimicrobial activity of copper and copper(I) oxide thin films deposited via aerosol-assisted CVD. J. Mater. Chem. B 2014, 2, 2855– 2860,  DOI: 10.1039/C4TB00196F
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    18
    Antimicrobial activity of copper and copper(I) oxide thin films deposited via aerosol-assisted CVD
    Hassan, Iman A.; Parkin, Ivan P.; Nair, Sean P.; Carmalt, Claire J.
    Journal of Materials Chemistry B: Materials for Biology and Medicine (2014), 2 (19), 2855-2860CODEN: JMCBDV; ISSN:2050-7518. (Royal Society of Chemistry)
    Hospital acquired infections are prevalent in many hospitals and contaminated touch surfaces are a route of transmission. To find a soln. for this, copper and copper oxide thin films were deposited via aerosol-assisted chem. vapor deposition using copper nitrate as the precursor, and the films were characterized by a range of techniques, including powder X-ray diffraction and SEM. The antimicrobial activity of the deposited copper and copper oxide films were investigated against Staphylococcus aureus and Escherichia coli. A 5-log10 redn. in the viable counts of E. coli was obsd. on the copper thin films after 30 min while a 2-log10 redn. was obsd. on copper oxide films after 1 h. In the case of S. aureus, both copper and copper oxide films exhibited 4-log10 redn. after 1 h. The high antimicrobial efficacy of the Cu2O films, approaching that of the pure copper films, suggests that oxide formation on copper objects should not significantly impair their antimicrobial activity.
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    Ruparelia, J. P.; Chatterjee, A. K.; Duttagupta, S. P.; Mukherji, S. Strain specificity in antimicrobial activity of silver and copper nanoparticles. Acta Biomater. 2008, 4, 707– 716,  DOI: 10.1016/j.actbio.2007.11.006
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    19
    Strain specificity in antimicrobial activity of silver and copper nanoparticles
    Ruparelia, Jayesh P.; Chatterjee, Arup Kumar; Duttagupta, Siddhartha P.; Mukherji, Suparna
    Acta Biomaterialia (2008), 4 (3), 707-716CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
    The antimicrobial properties of silver and copper nanoparticles were investigated using Escherichia coli (four strains), Bacillus subtilis and Staphylococcus aureus (three strains). The av. sizes of the silver and copper nanoparticles were 3 nm and 9 nm, resp., as detd. through transmission electron microscopy. Energy-dispersive X-ray spectra of silver and copper nanoparticles revealed that while silver was in its pure form, an oxide layer existed on the copper nanoparticles. The bactericidal effect of silver and copper nanoparticles were compared based on diam. of inhibition zone in disk diffusion tests and min. inhibitory concn. (MIC) and min. bactericidal concn. (MBC) of nanoparticles dispersed in batch cultures. Bacterial sensitivity to nanoparticles was found to vary depending on the microbial species. Disk diffusion studies with E. coli and S. aureus revealed greater effectiveness of the silver nanoparticles compared to the copper nanoparticles. B. subtilis depicted the highest sensitivity to nanoparticles compared to the other strains and was more adversely affected by the copper nanoparticles. Good correlation was obsd. between MIC and MBC (r2 = 0.98) measured in liq. cultures. For copper nanoparticles a good neg. correlation was obsd. between the inhibition zone obsd. in disk diffusion test and MIC/MBC detd. based on liq. cultures with the various strains (r2 = -0.75). Although strain-specific variation in MIC/MBC was negligible for S. aureus, some strain-specific variation was obsd. for E. coli.
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  20. 20
    You, M.; Kim, T. G.; Sung, Y.-M. Synthesis of Cu-doped TiO2 nanorods with various aspect ratios and dopant concentrations. Cryst. Growth Des. 2010, 10, 983– 987,  DOI: 10.1021/cg9012944
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    20
    Synthesis of Cu-Doped TiO2 Nanorods with Various Aspect Ratios and Dopant Concentrations
    You, Minkyu; Kim, Tae-Geun; Sung, Yun-Mo
    Crystal Growth & Design (2010), 10 (2), 983-987CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    Colloidal TiO2 nanorods were synthesized with different aspect ratios and successfully doped with copper via a controlled hydrolysis method. Inductively coupled plasma (ICP) at. emission spectroscopy analyses showed that the as-prepd. TiO2 nanorods contained ∼1.7-3.2 at.% Cu. XRD and high-resoln. TEM (HRTEM) analyses revealed that the doped nanorods are in a highly cryst. anatase structure and their crystal growth orientation is preferably [001]. The real doping of Ti lattices with Cu ions was evidenced by the analyses of surface compns. and chem. states of the nanorods using XPS. Through magnetic study using vibration sample magnetometry (VSM), it was verified that the Ti1-xCuxO2 nanorods maintain apparent ferromagnetic ordering at room temp. (300 K). The origin of the ferromagnetic property was explained based on the concn. of oxygen vacancies increased by Cu doping, which was also identified by the XPS analyses. The satn. magnetization showed strong dependency on the aspect ratio of nanorods as well as the Cu content in TiO2 nanorods.
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  21. 21
    Colón, G.; Maicu, M.; Hidalgo, M. C.; Navío, J. A. Cu-doped TiO2 systems with improved photocatalytic activity. Appl. Catal., B 2006, 67, 41– 51,  DOI: 10.1016/j.apcatb.2006.03.019
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    Cu-doped TiO2 systems with improved photocatalytic activity
    Colon, G.; Maicu, M.; Hidalgo, M. C.; Navio, J. A.
    Applied Catalysis, B: Environmental (2006), 67 (1-2), 41-51CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Photocatalytic oxidn. of phenol was performed over copper-doped TiO2 prepd. by a sol-gel method. Different prepn. methods were followed and a comparison with undoped system was also made. Wide structural and surface characterization of catalysts was carried out to establish a correlation between the effect of sulfuric and nitric acids present in the initial soln. and the Cu-TiO2 photocatalytic properties. The presence of sulfuric acid clearly stabilizes Cu-TiO2 and TiO2 structure and surface against sintering, maintaining anatase phase and relatively high surface area values with respect non sulfated Cu-TiO2 or TiO2. Best photocatalytic behavior is found for sulfated TiO2, as previously reported. In addn., incorporation of copper ions into the structure seems to enhance the photoactivity of the system for acidified systems, being the sulfated one the most favorable photocatalyst. The optimum metal loading is found for 0.5 M% of copper ion. A possible explanation of this photocatalytic improvement might be related to the stabilization of Cu2O species in doped TiO2 prepd. in the presence of sulfuric acid. This stabilization could be related to the presence of oxygen vacancies generated in the prepn. procedure using sulfuric acid.
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    Choudhury, B.; Dey, M.; Choudhury, A. Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles. Int. Nano Lett. 2013, 3, 25,  DOI: 10.1186/2228-5326-3-25
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    Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles
    Choudhury, Biswajit; Dey, Munmun; Choudhury, Amarjyoti
    International Nano Letters (2013), 3 (1), 25/1-25/8CODEN: INLNB4; ISSN:2008-9295. (Springer)
    TiO2 doped with Cu2+ initiates the formation of brookite phase along with anatase. Doping of Cu2+ introduces structural defects into TiO2. The direct evidence is the low intense and broad diffraction peaks. Raman peaks of doped TiO2 are also broad and are blueshifted. Pure TiO2 exhibits an absorption in the UV region, the position of which is shifted towards the visible region on incorporation of Cu into it. The visible absorption peaks arise due to the d-d transition of Cu2+ in the cryst. environment of TiO2. Incorporation of Cu2+ distorts the local structure of TiO2, resulting in the loss of octahedral symmetry surrounding Cu2+. The Jahn-Teller distortion splits the 2Egand 2T2g state of Cu2+ into several d states. Interaction of light excites the electron from ground to several of the excited states and gives the visible absorption peaks in the framework of TiO2. These Cu2+ d states and oxygen defects create band states, thereby favoring electronic transition to these levels and resulting in lowering of band gap of TiO2. A direct confirmation is the increase in the magnitude of Urbach energy with the redn. in the band gap of doped TiO2.
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    Park, H. S.; Kim, D. H.; Kim, S. J.; Lee, K. S. The photocatalytic activity of 2.5 wt% Cu-doped TiO2 nano powders synthesized by mechanical alloying. J. Alloys Compd. 2006, 415, 51– 55,  DOI: 10.1016/j.jallcom.2005.07.055
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    23
    The photocatalytic activity of 2.5wt% Cu-doped TiO2 nanopowders synthesized by mechanical alloying
    Park, Ha Sung; Kim, Dong Hyun; Kim, Sun Jae; Lee, Kyung Sub
    Journal of Alloys and Compounds (2006), 415 (1-2), 51-55CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    R to effectively utilize visible light, nanosized Cu-doped TiO2 powders were prepd. by mech. alloying and were characterized by XRD, TEM, DRS, and photoluminescence (PL) spectra. Their photocatalytic activity was measured for 4-chlorophenol photodegrdn. 2.5 Wt% of Cu element entered uniformly into the rutile TiO2 lattice with the av. grain size <15 nm. The Cu-doped rutile TiO2 had new emission peak at 448 nm indicating the decrease of the band gap energy of rutile TiO2. It exhibited an absorption edge at 480-490 nm longer than that for rutile TiO2 powder and led to a 2 times faster degrdn. of 4-chlorophenol.
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    Celik, E.; Gokcen, Z.; Ak Azem, N. F.; Tanoglu, M.; Emrullahoglu, O. F. Processing, characterization and photocatalytic properties of Cu doped TiO2 thin films on glass substrate by sol–gel technique. Mater. Sci. Eng., B 2006, 132, 258– 265,  DOI: 10.1016/j.mseb.2006.03.038
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    Processing, characterization and photocatalytic properties of Cu doped TiO2 thin films on glass substrate by sol-gel technique
    Celik, E.; Gokcen, Z.; Ak Azem, N. F.; Tanoglu, M.; Emrullahoglu, O. F.
    Materials Science & Engineering, B: Solid-State Materials for Advanced Technology (2006), 132 (3), 258-265CODEN: MSBTEK; ISSN:0921-5107. (Elsevier B.V.)
    The present paper describes processing, properties and photocatalytic application of Cu doped TiO2 thin films on glass substrate. Cu doped TiO2 coatings were successfully prepd. on glass slide substrates using sol-gel method. The obtained solns. exhibit acidic characteristics. The phase structure, thermal, microstructure and surface properties of the coatings were characterized by using XRD, DTA/TG, SEM and AFM. Their adhesion properties and spectroscopic anal. were investigated by a scratch tester and UV-vis spectroscopy. Four different solns. were prepd. by changing Cu/Ti ratios. Glass substrates were coated by solns. of Ti-alkoxide, Cu-chloride, glacial acetic acid and isopropanol. The obtained gel films were dried at 300 °C for 10 min and subsequently heat-treated at 500 °C for 5 min in air. The oxide thin films were annealed at 600 °C for 60 min in air. TiO2, CuO, Cu4Ti, Ti3O5 and Cu3TiO4 phases were found in the coating. The org. matters were burned at temps. between 200 and 350 °C and TiO2 crystn. was formed at 450 °C. The wt. loss of the powder during process up to 600 °C is approx. 70%. The microstructural observations demonstrated that CuO content was led an improved surface morphol. while thickness of the film and surface defects were increased in accordance with no. of dipping. According to AFM results, it was found that as the Cu/Ti content increases the surface roughness of the films increases. In addn. structural, thermal and microstructural results, it was found that the films of 0.73 ratio have better adhesion strength to the glass substrate among other coatings. The oxide films were found to be active for photocatalytic decompn. of methylene blue.
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    Zhang, W.; Li, Y.; Zhu, S.; Wang, F. Copper doping in titanium oxide catalyst film prepared by dc reactive magnetron sputtering. Catal. Today 2004, 93–95, 589– 594,  DOI: 10.1016/j.cattod.2004.06.009
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    Copper doping in titanium oxide catalyst film prepared by d.c. reactive magnetron sputtering
    Zhang, Wenjie; Li, Ying; Zhu, Shenglong; Wang, Fuhui
    Catalysis Today (2004), 93-95 (), 589-594CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Copper-doped titanium oxide films were prepd. by d.c. magnetron sputtering using Ti and Cu mixed target. The XPS spectra showed that titanium was in the Ti4+ oxidn. state and oxygen was in the form of O2- in TiO2 and CuO. Apparent high-intensity shake-up satellites of the main Cu 2p peaks indicated the existence of fully oxidized CuO in the Cu-doped TiO2 films. When the copper concn. was low, the Cu-doped TiO2 film had the similar anatase phase as pure TiO2. The samples became amorphous when copper concn. was more than 15.17 at.%. Copper oxides were in the amorphous state in all the films. In the sequence of decreasing copper concn., the surface morphologies changed from flat to nanocryst. with nano-sized pores. The absorption edges of the Cu-doped samples shifted to longer wavelength region and the optical transmittances of these films decreased abruptly with increasing copper concn. The Cu-doped TiO2 films had different photocatalytic behavior in accordance with the amt. of doped copper. The best copper doping concn. was 1.45 at.% in sample I, relating to the most effective photocatalytic activity.
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    Sathasivam, S.; Arnepalli, R. R.; Kumar, B.; Singh, K. K.; Visser, R. J.; Blackman, C. S.; Carmalt, C. J. Solution Processing of GaAs Thin Films for Photovoltaic Applications. Chem. Mater. 2014, 26, 4419– 4424,  DOI: 10.1021/cm501280e
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    Solution Processing of GaAs Thin Films for Photovoltaic Applications
    Sathasivam, Sanjayan; Arnepalli, Ranga Rao; Kumar, Bhaskar; Singh, Kaushal K.; Visser, Robert J.; Blackman, Christopher S.; Carmalt, Claire J.
    Chemistry of Materials (2014), 26 (15), 4419-4424CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The authors present a novel route to high quality GaAs thin films via a soln. processing technique (aerosol assisted CVD) using a novel single source precursor [Me2GaAs(H)tBu]2. The thin films, grown on inexpensive glass substrates, were polycryst. in nature with a Ga to As ratio of 1:1. The morphol. studied via SEM showed the films to be smooth and consisting of compact domes. High-resoln. TEM (HRTEM) revealed the films to have columnar growth and an av. crystallite size of 90 nm. The films also contained low levels of contaminants as detd. via energy dispersive x-ray spectroscopy (EDX) mapping, XPS depth profiling, and secondary ion mass spectrometry (SIMS).
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  27. 27
    Sathasivam, S.; Arnepalli, R. R.; Singh, K. K.; Visser, R. J.; Blackman, C. S.; Carmalt, C. J. A solution based route to GaAs thin films from As (NMe 2) 3 and GaMe 3 for solar cells. RSC Adv. 2015, 5, 11812– 11817,  DOI: 10.1039/c4ra13902j
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    27
    A solution based route to GaAs thin films from As(NMe2)3 and GaMe3 for solar cells
    Sathasivam, Sanjayan; Arnepalli, Ranga R.; Singh, Kaushal K.; Visser, Robert J.; Blackman, Christopher S.; Carmalt, Claire J.
    RSC Advances (2015), 5 (16), 11812-11817CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The novel deposition of GaAs thin films on glass substrates from a soln. based route involving the aerosol assisted chem. vapor deposition (AACVD) of As(NMe2)3 and GaMe3 dissolved in toluene is reported. The gallium arsenide films were analyzed by SEM (SEM), X-ray powder diffraction (XRD), energy dispersive X-ray (EDX) anal., XPS and Raman spectroscopy. Powder XRD showed that cubic polycryst. GaAs had been deposited with films grown at the higher temps. having a Ga to As ratio of 1 : 1. EDX mapping, XPS depth profiling and SIMS showed that the films contained low levels of contaminants. The method described shows the formation of GaAs films with increasing crystallinity and stoichiometry reaching unity with increasing deposition temp.
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    Alotaibi, A. M.; Sathasivam, S.; Nair, S. P.; Parkin, I. P. Antibacterial properties of Cu–ZrO 2 thin films prepared via aerosol assisted chemical vapour deposition. J. Mater. Chem. B 2016, 4, 666– 671,  DOI: 10.1039/c5tb02312b
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    28
    Antibacterial properties of Cu-ZrO2 thin films prepared via aerosol assisted chemical vapour deposition
    Alotaibi, Abdullah M.; Sathasivam, Sanjayan; Nair, Sean P.; Parkin, Ivan P.
    Journal of Materials Chemistry B: Materials for Biology and Medicine (2016), 4 (4), 666-671CODEN: JMCBDV; ISSN:2050-7518. (Royal Society of Chemistry)
    The antibacterial properties of a Cu-ZrO2 film grown via aerosol assisted chem. vapor deposition are presented. The composite film showed high activity against E. coli (Gram-neg.) and S. aureus (Gram-pos.) bacteria with 5 log10 (E. coli) and 4 log10 (S. aureus) decrease in viable bacteria achieved within 20 and 60 min resp. These results were comparable to a pure copper film that was prepd. under the same conditions. The composite film was characterized for material properties using a range of techniques including X-ray photoemission and X-ray diffraction.
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    Bhachu, D. S.; Moniz, S. J. A.; Sathasivam, S.; Scanlon, D. O.; Walsh, A.; Bawaked, S. M.; Mokhtar, M.; Obaid, A. Y.; Parkin, I. P.; Tang, J.; Carmalt, C. J. Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity. Chem. Sci. 2016, 7, 4832– 4841,  DOI: 10.1039/c6sc00389c
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    Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity
    Bhachu, Davinder S.; Moniz, Savio J. A.; Sathasivam, Sanjayan; Scanlon, David O.; Walsh, Aron; Bawaked, Salem M.; Mokhtar, Mohamed; Obaid, Abdullah Y.; Parkin, Ivan P.; Tang, Junwang; Carmalt, Claire J.
    Chemical Science (2016), 7 (8), 4832-4841CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The authors report the synthesis and photoelectrochem. assessment of phase pure tetragonal matlockite structured BiOX (X = Cl, Br, I) films. The materials were deposited using aerosol-assisted CVD. The measured optical bandgaps of the oxyhalides, supported by d. functional theory calcns., showed a red shift with the increasing size of halide following the binding energy of the anion p-orbitals that form the valence band. Stability and photoelectrochem. studies carried out without a sacrificial electron donor showed the n-type BiOBr film to have the highest photocurrent reported for BiOBr in the literature to date (0.3 mA cm-2 at 1.23 V vs. RHE), indicating it is an excellent candidate for solar fuel prodn. with a very low onset potential of 0.2 V vs. RHE. The high performance was attributed to the preferred growth of the film in the [011] direction, as shown by x-ray diffraction, leading to internal elec. fields that minimize charge carrier recombination.
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  30. 30
    Chadwick, N.; Sathasivam, S.; Kafizas, A.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N.; Parkin, I. P.; Carmalt, C. J. Combinatorial aerosol assisted chemical vapour deposition of a photocatalytic mixed SnO2/TiO2 thin film. J. Mater. Chem. A 2014, 2, 5108– 5116,  DOI: 10.1039/C4TA00545G
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    30
    Combinatorial aerosol assisted chemical vapor deposition of a photocatalytic mixed SnO2/TiO2 thin film
    Chadwick, Nicholas; Sathasivam, Sanjayan; Kafizas, Andreas; Bawaked, Salem M.; Obaid, Abdullah Y.; Al-Thabaiti, Shaeel; Basahel, Sulaiman N.; Parkin, Ivan P.; Carmalt, Claire J.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (14), 5108-5116CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Combinatorial Aerosol Assisted Chem. Vapor Deposition (cAACVD) was used to grow a thin film that graduated across its width from tin dioxide to titanium dioxide. This is a relatively new technique that can be used to create a variety of mixed phase and compn. thin films on a single substrate. Here cAACVD was used to deposit a mixed phase TiO2 and SnO2 film and compn. was related to UV photocatalysis, hydrophobicity and microstructure not inherent to anatase TiO2 or cassiterite SnO2. Characterization was achieved using X-ray diffraction (XRD), XPS, SEM (SEM), energy dispersive X-ray spectroscopy (EDX) and UV-Vis spectroscopy. Functional testing to elucidate the differences in functional properties across the film was undertaken by the photo-induced degrdn. of a resazurin 'intelligent' ink, a photo-induced wettability study and two-point resistivity measurements. Functional properties showed enhanced photocatalysis in comparison to Pilkington Activ® with similar formal quantum yield (mols. destroyed per absorbed photon) and formal quantum efficiency (mols. destroyed per incident photon) values.
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  31. 31
    Sathasivam, S.; Kafizas, A.; Ponja, S.; Chadwick, N.; Bhachu, D. S.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N.; Carmalt, C. J.; Parkin, I. P. Combinatorial Atmospheric Pressure CVD of a Composite TiO2/SnO2 Thin Film. Chem. Vap. Depos. 2014, 20, 69– 79,  DOI: 10.1002/cvde.201307081
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    Combinatorial atmospheric pressure CVD of composite TiO2/SnO2 thin film
    Sathasivam, Sanjayan; Kafizas, Andreas; Ponja, Sapna; Chadwick, Nicholas; Bhachu, Davinder S.; Bawaked, Salem M.; Obaid, Abdullah Y.; Al-Thabaiti, Shaeel; Basahel, Sulaiman N.; Carmalt, Claire J.; Parkin, Ivan P.
    Chemical Vapor Deposition (2014), 20 (1-2-3), 69-79CODEN: CVDEFX; ISSN:0948-1907. (Wiley-Blackwell)
    Combinatorial atm. pressure (cAP)CVD is used to deposit a film of graded compn. from mainly TiO2 to TiO2/SnO2 to mainly SnO2. This is the first cAPCVD study of a TiO2/SnO2 system. The thin film is characterized using a range of techniques such as X-ray diffraction (XRD), wavelength dispersive X-ray (WDX) spectroscopy, XPS, SEM (SEM), and ultra violet-visible (UV-vis) spectroscopy. It is found that, at various positions on the film, there are compns. of TiO2 and SnO2. The photocatalytic activity is examd. via the degrdn. of a Resazurin-based 'intelligent ink' under 365 nm wavelength irradn. The change in the concn. of the dye can be monitored by digital imaging alone. The results show how TiO2-rich regions are photocatalytically active, producing a max. formal quantum yield of 3.32 × 10-4 mols. per absorbed photon. The sheet resistance is detd. using a four-point probe via the van der Pauw method. The cond. is highest in the SnO2-rich and thicker regions of the film, however some of the composite regions of TiO2/SnO2 show both cond. and photocatalytic activity.
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    Ponja, S. D.; Sathasivam, S.; Parkin, I. P.; Carmalt, C. J. Transparent conductive aluminium and fluorine co-doped zinc oxide films via aerosol assisted chemical vapour deposition. RSC Adv. 2014, 4, 49723– 49728,  DOI: 10.1039/c4ra09997d
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    32
    Transparent conductive aluminum and fluorine co-doped zinc oxide films via aerosol assisted chemical vapour deposition
    Ponja, Sapna D.; Sathasivam, Sanjayan; Parkin, Ivan P.; Carmalt, Claire J.
    RSC Advances (2014), 4 (91), 49723-49728CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Aerosol assisted chem. vapor deposition (AACVD) was employed to synthesize highly transparent and conductive ZnO, fluorine or aluminum doped and aluminum-fluorine co-doped ZnO thin films on glass substrates at 450 °C. All films were characterized by X-ray diffraction (XRD), wavelength dispersive X-ray spectroscopy (WDX), XPS, SEM (SEM) and UV/Vis/Near IR spectroscopy. The films were 300-350 nm thick, cryst. and displayed high transparency at 550 nm (80-90%). The co-doped film consisted of 1 at.% fluorine and 2 at.% aluminum, exhibiting a charge carrier concn. and a charge carrier mobility of 3.47 × 1020 cm-3 and 9.7 cm2 V-1 s-1, resp. The band gap of the co-doped film was found to be 3.7 eV and the plasma edge crossover was ca. 1800 nm. This film had a highly structured morphol. in comparison to the un-doped and single doped ZnO films for transparent conducting oxide applications.
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    Knapp, C. E.; Carmalt, C. J. Solution based CVD of main group materials. Chem. Soc. Rev. 2016, 45, 1036– 1064,  DOI: 10.1039/c5cs00651a
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    Solution based CVD of main group materials
    Knapp, Caroline E.; Carmalt, Claire J.
    Chemical Society Reviews (2016), 45 (4), 1036-1064CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This crit. review focuses on the soln. based chem. vapor deposition (CVD) of main group materials with particular emphasis on their current and potential applications. Deposition of thin films of main group materials, such as metal oxides, sulfides and arsenides, have been researched owing to the array of applications which utilize them including solar cells, transparent conducting oxides (TCOs) and window coatings. Soln. based CVD processes, such as aerosol-assisted (AA)CVD have been developed due to their scalability and to overcome the requirement of suitably volatile precursors as the technique relies on the soly. rather than volatility of precursors which vastly extends the range of potentially applicable compds. An introduction into the applications and precursor requirements of main group materials will be presented first followed by a detailed discussion of their deposition reviewed according to this application. The challenges and prospects for further enabling research in terms of emerging main group materials will be discussed.
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  34. 34
    Alotaibi, A. M.; Sathasivam, S.; Williamson, B. A. D.; Kafizas, A.; Sotelo-Vazquez, C.; Taylor, A.; Scanlon, D. O.; Parkin, I. P. Chemical vapor deposition of photocatalytically active pure brookite TiO2 thin films. Chem. Mater. 2018, 30, 1353– 1361,  DOI: 10.1021/acs.chemmater.7b04944
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    34
    Chemical Vapor Deposition of Photocatalytically Active Pure Brookite TiO2 Thin Films
    Alotaibi, Abdullah M.; Sathasivam, Sanjayan; Williamson, Benjamin A. D.; Kafizas, Andreas; Sotelo-Vazquez, Carlos; Taylor, Alaric; Scanlon, David O.; Parkin, Ivan P.
    Chemistry of Materials (2018), 30 (4), 1353-1361CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Brookite is the least investigated phase of TiO2 due to the synthetic difficulty of obtaining the pure phase. Here, we present the first ever chem. vapor deposition synthesis of pure brookite TiO2 thin films. The films were highly cryst. and phase pure as detd. by X-ray diffraction and Raman spectroscopy studies. SEM studies showed the films to have a structured morphol. consisting of pyramidal features. The photocatalytic properties of the brookite film, tested using stearic acid under UVA (365 nm) irradn., were superior to both an anatase film grown under similar conditions and NSG Activ glass. Transient absorption spectroscopy showed that the innate electron-hole recombination dynamics are similar in brookite and anatase, akin to previous reports. The superior activity of the brookite film is hence attributed to the higher surface area compared to anatase.
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  35. 35
    Ponja, S. D.; Williamson, B. A. D.; Sathasivam, S.; Scanlon, D. O.; Parkin, I. P.; Carmalt, C. J. Enhanced electrical properties of antimony doped tin oxide thin films deposited via aerosol assisted chemical vapour deposition. J. Mater. Chem. C 2018, 6, 7257– 7266,  DOI: 10.1039/c8tc01929k
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    Enhanced electrical properties of antimony doped tin oxide thin films deposited via aerosol assisted chemical vapor deposition
    Ponja, Sapna D.; Williamson, Benjamin A. D.; Sathasivam, Sanjayan; Scanlon, David O.; Parkin, Ivan P.; Carmalt, Claire J.
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2018), 6 (27), 7257-7266CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    Transparent conducting oxides have widespread application in modern society but there is a need to move away from the current 'industry champion' tin doped indium oxide (In2O3:Sn) due to high costs. Antimony doped tin(IV) oxide (ATO) is an excellent candidate but is limited by its opto-elec. properties. Here, the authors present a novel and scalable synthetic route to ATO thin films that shows excellent elec. properties. Resistivity measurements showed that at 4 at.% doping the lowest value of 4.7 × 10-4 Ω cm was achieved primarily due to a high charge carrier d. of 1.2 × 1021 cm-3. Further doping induced an increase in resistivity due to a decrease in both the carrier d. and mobility. Ab initio hybrid d. functional theory (DFT) calcns. show the thermodn. basis for the tail off of performance beyond a certain doping level, and the appearance of Sb(III) within the doped thin films.
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    Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558,  DOI: 10.1103/physrevb.47.558
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    Ab initio molecular dynamics of liquid metals
    Kresse, G.; Hafner, J.
    Physical Review B: Condensed Matter and Materials Physics (1993), 47 (1), 558-61CODEN: PRBMDO; ISSN:0163-1829.
    The authors present ab initio quantum-mech. mol.-dynamics calcns. based on the calcn. of the electronic ground state and of the Hellmann-Feynman forces in the local-d. approxn. at each mol.-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using sub-space alignment. This approach avoids the instabilities inherent in quantum-mech. mol.-dynamics calcns. for metals based on the use of a factitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows one to perform simulations over several picoseconds.
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    Mills, A.; Wang, J. Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films. J. Photochem. Photobiol., A 2006, 182, 181– 186,  DOI: 10.1016/j.jphotochem.2006.02.010
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    Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films
    Mills, Andrew; Wang, Jishun
    Journal of Photochemistry and Photobiology, A: Chemistry (2006), 182 (2), 181-186CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    The destruction of stearic acid (SA), the SA test, is a popular approach used to evaluate the activities of photocatalytic films. The destruction of SA via semiconductor photocatalysis is monitored simultaneously, using FT-IR spectroscopy, via the disappearance of SA and the appearance of CO2. Sol-gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 1015 mols. of SA cm-2 ≡ 1 cm-1 integrated areas of the peaks in the FT-IR of SA over the range 2700-3000 cm-1, which is three times that reported previously by others. As the SA disappeared the concomitant amt. of CO2 generated was >90% that expected throughout the photomineralization process for the sol-gel titania film. In contrast, the slightly more active, and scattering, P25 titania films generated CO2 levels that dipped as low as 69% during the course of the photoreaction, but eventually recovered to ≃100% that expected based on the amt. of SA present. The importance of these results with respect to SA test and the evaluation of new and existing self-cleaning films are discussed briefly.
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    Kresse, G.; Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal–amorphous-semiconductor transition in germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251,  DOI: 10.1103/physrevb.49.14251
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    Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium
    Kresse, G.; Hafner, J.
    Physical Review B: Condensed Matter and Materials Physics (1994), 49 (20), 14251-69CODEN: PRBMDO; ISSN:0163-1829.
    The authors present ab initio quantum-mech. mol.-dynamics simulations of the liq.-metal-amorphous-semiconductor transition in Ge. The simulations are based on (a) finite-temp. d.-functional theory of the 1-electron states, (b) exact energy minimization and hence calcn. of the exact Hellmann-Feynman forces after each mol.-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nose' dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows the authors to perform simulations over >30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liq. and amorphous Ge in very good agreement with expt.. The simulation allows the authors to study in detail the changes in the structure-property relation through the metal-semiconductor transition. The authors report a detailed anal. of the local structural properties and their changes induced by an annealing process. The geometrical, bounding, and spectral properties of defects in the disordered tetrahedral network are studied and compared with expt.
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    Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15– 50,  DOI: 10.1016/0927-0256(96)00008-0
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    Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
    Kresse, G.; Furthmuller, J.
    Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
    The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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    Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169,  DOI: 10.1103/physrevb.54.11169
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    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Ángyán, J. G. Screened hybrid density functionals applied to solids. J. Chem. Phys. 2006, 124, 154709,  DOI: 10.1063/1.2187006
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    Screened hybrid density functionals applied to solids
    Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Angyan, J. G.
    Journal of Chemical Physics (2006), 124 (15), 154709/1-154709/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Hybrid Fock exchange/d. functional theory functionals have shown to be very successful in describing a wide range of mol. properties. For periodic systems, however, the long-range nature of the Fock exchange interaction and the resultant large computational requirements present a major drawback. This is esp. true for metallic systems, which require a dense Brillouin zone sampling. Recently, a new hybrid functional [HSE03, J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)] that addresses this problem within the context of methods that evaluate the Fock exchange in real space was introduced. We discuss the advantages the HSE03 functional brings to methods that rely on a reciprocal space description of the Fock exchange interaction, e.g., all methods that use plane wave basis sets. Furthermore, we present a detailed comparison of the performance of the HSE03 and PBE0 functionals for a set of archetypical solid state systems by calcg. lattice parameters, bulk moduli, heats of formation, and band gaps. The results indicate that the hybrid functionals indeed often improve the description of these properties, but in several cases the results are not yet on par with std. gradient cor. functionals. This concerns in particular metallic systems for which the bandwidth and exchange splitting are seriously overestimated.
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    Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 2003, 118, 8207– 8215,  DOI: 10.1063/1.1564060
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    Hybrid functionals based on a screened Coulomb potential
    Heyd, Jochen; Scuseria, Gustavo E.; Ernzerhof, Matthias
    Journal of Chemical Physics (2003), 118 (18), 8207-8215CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Hybrid d. functionals are very successful in describing a wide range of mol. properties accurately. In large mols. and solids, however, calcg. the exact (Hartree-Fock) exchange is computationally expensive, esp. for systems with metallic characteristics. In the present work, we develop a new hybrid d. functional based on a screened Coulomb potential for the exchange interaction which circumvents this bottleneck. The results obtained for structural and thermodn. properties of mols. are comparable in quality to the most widely used hybrid functionals. In addn., we present results of periodic boundary condition calcns. for both semiconducting and metallic single wall carbon nanotubes. Using a screened Coulomb potential for Hartree-Fock exchange enables fast and accurate hybrid calcns., even of usually difficult metallic systems. The high accuracy of the new screened Coulomb potential hybrid, combined with its computational advantages, makes it widely applicable to large mols. and periodic systems.
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    Buckeridge, J.; Butler, K. T.; Catlow, C. R. A.; Logsdail, A. J.; Scanlon, D. O.; Shevlin, S. A.; Woodley, S. M.; Sokol, A. A.; Walsh, A. Polymorph engineering of TiO2: demonstrating how absolute reference potentials are determined by local coordination. Chem. Mater. 2015, 27, 3844– 3851,  DOI: 10.1021/acs.chemmater.5b00230
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    Polymorph Engineering of TiO2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination
    Buckeridge, John; Butler, Keith T.; Catlow, C. Richard A.; Logsdail, Andrew J.; Scanlon, David O.; Shevlin, Stephen A.; Woodley, Scott M.; Sokol, Alexey A.; Walsh, Aron
    Chemistry of Materials (2015), 27 (11), 3844-3851CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    We report that the valence and conduction band energies of TiO2 can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an abs. energy ref., using an accurate multiscale quantum-chem. approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO2 as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO2 as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochem. water splitting through polymorph engineering of TiO2.
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    Çelik, V.; Mete, E. Range-separated hybrid exchange-correlation functional analyses of anatase TiO2 doped with W, N, S, W/N, or W/S. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 205112,  DOI: 10.1103/physrevb.86.205112
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    Range-separated hybrid exchange-correlation functional analyses of anatase TiO2 doped with W, N, S, W/N, or W/S
    Celik, Veysel; Mete, Ersen
    Physical Review B: Condensed Matter and Materials Physics (2012), 86 (20), 205112/1-205112/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The electronic properties and at. structures of W, N, S, W/N, and W/S dopings of anatase TiO2 have been systematically investigated using the d. functional theory. The exchange and correlation effects have been treated with the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional. Mixing traditional semilocal and nonlocal screened Hartree-Fock exchange energies, the HSE method corrects the band gap and also improves the description of anion/cation-derived gap states. Enhanced charge-carrier dynamics, obsd. for W/N codoped titania, can partly be explained by mutually passivating modifications of N 2p and W 5d states on its electronic structure. Following this trend, we have found an apparent band gap narrowing of 1.03 eV for W/S codoping. This is due to the large dispersion of S 3p states at the valance band top extending its edge to higher energies and Ti-S-W hybridization appearing at the bottom of the conduction band. W/S-TiO2 might show a strong visible-light response comparable to W/N codoped anatase catalysts.
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    Huy, H. A.; Aradi, B.; Frauenheim, T.; Deák, P. Calculation of carrier-concentration-dependent effective mass in Nb-doped anatase crystals of TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 155201,  DOI: 10.1103/physrevb.83.155201
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    Calculation of carrier-concentration-dependent effective mass in Nb-doped anatase crystals of TiO2
    Huy, Huynh Anh; Aradi, Balint; Frauenheim, Thomas; Deak, Peter
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (15), 155201/1-155201/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The calcd. optical effective masses of electrons, both orthogonal and parallel to the tetragonal axis of the crystal, are reported as a function of carrier concn. in Nb-doped anatase. The calcn. is based on the band structure, obtained by d. functional theory in the generalized gradient approxn. or by using the HSE06 screened hybrid functional. The anisotropy of the crystal and the nonparabolicity of the bands both were taken into account. In the concn. range relevant for transparent conductive oxide applications, the effective mass is detd. by several branches of the conduction band, leading to a complicated dependence on the carrier concn. The form of the function obtained with HSE06 is in line with expt. The strong anisotropy in the effective mass is confirmed, and very large values are predicted along the tetragonal axis for high carrier concns.
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    Janotti, A.; Varley, J.; Rinke, P.; Umezawa, N.; Kresse, G.; Van de Walle, C. Hybrid functional studies of the oxygen vacancy in TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085212,  DOI: 10.1103/physrevb.81.085212
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    Hybrid functional studies of the oxygen vacancy in TiO2
    Janotti, A.; Varley, J. B.; Rinke, P.; Umezawa, N.; Kresse, G.; Van de Walle, C. G.
    Physical Review B: Condensed Matter and Materials Physics (2010), 81 (8), 085212/1-085212/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The electronic and structural properties of the O vacancy (VO) in rutile TiO2 were studied using generalized Kohn-Sham theory with the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional for exchange and correlation. The HSE approach corrects the band gap and allows for a proper description of defects with energy levels close to the conduction band. According to the HSE calcns., VO is a shallow donor for which the +2 charge state is lower in energy than the neutral and +1 charge states for all Fermi-level positions in the band gap. The formation energy of VO2+ is relatively low in n-type TiO2 under O-poor conditions but it rapidly increases with the O chem. potential. This is consistent with exptl. observations where the elec. cond. decreases with O2 partial pressure.
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    Bhachu, D. S.; Sathasivam, S.; Sankar, G.; Scanlon, D. O.; Cibin, G.; Carmalt, C. J.; Parkin, I. P.; Watson, G. W.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N. Solution processing route to multifunctional titania thin films: Highly conductive and photcatalytically active Nb: TiO2. Adv. Funct. Mater. 2014, 24, 5075– 5085,  DOI: 10.1002/adfm.201400338
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    Solution Processing Route to Multifunctional Titania Thin Films: Highly Conductive and Photcatalytically Active Nb:TiO2
    Bhachu, Davinder S.; Sathasivam, Sanjayan; Sankar, Gopinathan; Scanlon, David O.; Cibin, Giannantonio; Carmalt, Claire J.; Parkin, Ivan P.; Watson, Graeme W.; Bawaked, Salem M.; Obaid, Abdullah Y.; Al-Thabaiti, Shaeel; Basahel, Sulaiman N.
    Advanced Functional Materials (2014), 24 (32), 5075-5085CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This paper reports the synthesis of highly conductive niobium-doped titanium dioxide (Nb:TiO2) films from the decompn. of Ti(OEt)4 with dopant quantities of Nb(OEt)5 by aerosol-assisted chem. vapor deposition (AACVD). Doping Nb into the Ti sites results in n-type cond., as detd. by Hall effect measurements. The doped films display significantly improved elec. properties compared to pristine TiO2 films. For 5 at.% Nb in the films, the charge carrier concn. was 2 × 1021 cm-3 with a mobility of 2 cm2.V-1.s-1 . The corresponding sheet resistance was as low as 6.5 Ω.sq-1 making the films suitable candidates for transparent conducting oxide (TCO) materials. This is, to the best of our knowledge, the lowest reported sheet resistance for Nb:TiO2 films synthesized by vapor deposition. The doped films were also blue in color, with the intensity dependent on the Nb concn. in the films. A combination of synchrotron, lab. and theor. techniques confirmed niobium doping into the anatase TiO2 lattice. Computational methods also confirmed exptl. results of both delocalized (Ti4+) and localized polaronic states (Ti3+) states. Addnl., the doped films also functioned as photocatalysts. Thus, Nb:TiO2 combines four functional properties (photocatalysis, elec. cond., optical transparency and blue coloration) within the same layer, making it a promising alternative to conventional TCO materials.
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    Matsubara, M.; Saniz, R.; Partoens, B.; Lamoen, D. Doping anatase TiO 2 with group Vb and VI-b transition metal atoms: a hybrid functional first-principles study. Phys. Chem. Chem. Phys. 2017, 19, 1945– 1952,  DOI: 10.1039/c6cp06882k
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    48
    Doping anatase TiO2 with group V-b and VI-b transition metal atoms: a hybrid functional first-principles study
    Matsubara, Masahiko; Saniz, Rolando; Partoens, Bart; Lamoen, Dirk
    Physical Chemistry Chemical Physics (2017), 19 (3), 1945-1952CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We investigate the role of transition metal atoms of group V-b (V, Nb, and Ta) and VI-b (Cr, Mo, and W) as n- or p-type dopants in anatase TiO2 using thermodn. principles and d. functional theory with the Heyd-Scuseria-Ernzerhof HSE06 hybrid functional. The HSE06 functional provides a realistic value for the band gap, which ensures a correct classification of dopants as shallow or deep donors or acceptors. Defect formation energies and thermodn. transition levels are calcd. taking into account the constraints imposed by the stability of TiO2 and the soly. limit of the impurities. Nb, Ta, W and Mo are identified as shallow donors. Although W provides two electrons, Nb and Ta show a considerably lower formation energy, in particular under O-poor conditions. Mo donates in principle one electron, but under specific conditions can turn into a double donor. V impurities are deep donors and Cr shows up as an amphoteric defect, thereby acting as an electron trapping center in n-type TiO2 esp. under O-rich conditions. A comparison with the available exptl. data yields excellent agreement.
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  49. 49
    Boonchun, A.; Reunchan, P.; Umezawa, N. Energetics of native defects in anatase TiO2: a hybrid density functional study. Phys. Chem. Chem. Phys. 2016, 18, 30040– 30046,  DOI: 10.1039/C6CP05798E
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    Energetics of native defects in anatase TiO2: a hybrid density functional study
    Boonchun, Adisak; Reunchan, Pakpoom; Umezawa, Naoto
    Physical Chemistry Chemical Physics (2016), 18 (43), 30040-30046CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The energetics and electronic structures of native defects in anatase TiO2 are comprehensively studied using hybrid d. functional calcns. We demonstrate that oxygen vacancies (VO) and titanium interstitials (Tii) act as shallow donors, and can form at substantial concns., giving rise to free electrons with carrier densities from 1011 to 1019 cm-3 under oxygen-rich and oxygen-poor conditions, resp. The titanium vacancies (VTi), identified as deep acceptors and induced hole carriers, are incapable of fully compensating for the free electrons originating from the donor-type defects at any oxygen chem. potential. Even under extreme oxygen-rich conditions, the Fermi level, which is detd. from the charge neutrality condition among charge defects, electron and hole carriers, is located 2.34 eV above the valence band max., indicating that p-type cond. can never be realized under any growth conditions without external doping. This is consistent with common observations of intrinsic n-type cond. of TiO2. At a typical annealing temp. and under a typical oxygen partial pressure, the carrier concn. is found to be approx. 5 × 1013 cm-3.
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    There is no expanded citation for this reference.
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    Morgan, B. J.; Watson, G. W. Intrinsic n-type defect formation in TiO2: a comparison of rutile and anatase from GGA+ U calculations. J. Phys. Chem. C 2010, 114, 2321– 2328,  DOI: 10.1021/jp9088047
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    Intrinsic n-type Defect Formation in TiO2: A Comparison of Rutile and Anatase from GGA+U Calculations
    Morgan, Benjamin J.; Watson, Graeme W.
    Journal of Physical Chemistry C (2010), 114 (5), 2321-2328CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The formations of intrinsic n-type defects, i.e., oxygen vacancies and titanium interstitials, in rutile and anatase TiO2 have been compared using GGA+U calcns. In both crystal structures, these defects give rise to states in the band gap, corresponding to electrons localized at Ti3+ centers. O vacancy formation in rutile results in two excess electrons occupying 3d orbitals on Ti atoms neighboring the vacancy. Similarly, for anatase, two Ti 3d orbitals are occupied by the excess electrons, with one of these Ti sites neighboring the vacancy, and the second at a next-nearest Ti position. This localization is accompanied by one oxygen moving toward the vacancy site to give a "split vacancy" geometry. A second fully localized soln. is also found for anatase, with both occupied Ti sites neighboring the vacancy site. This min. is 0.05 eV less stable than the split vacancy and is thus expected to be present in exptl. samples. A partially delocalized soln. corresponding to the split vacancy geometry, with one electron occupying the bottom of the conduction band, is also identified as 0.28 eV less stable. Formation of titanium interstitials donates four electrons to the Ti lattice. In anatase, one of these electrons is located at the interstitial Ti site, and three occupied defect states are hybridized between three nearest neighbor Ti sites. In rutile, these excess electrons are mostly localized at four nearest neighbor Ti sites, with only a small amt. of excess charge found on the interstitial Ti atom. This difference in the charge on the interstitial atom is a consequence of the differing interstitial geometries in the two polymorphs. Calcd. optical absorption spectra for all defects show significant decreases of the optical band gap, with a larger red shift predicted for titanium interstitials in anatase than in rutile. Defect formation energies have been calcd. under oxygen-rich and oxygen-poor conditions for both polymorphs. Under all conditions, O vacancy formation is slightly more favorable in anatase than in rutile, while Ti interstitials form more easily in rutile than anatase. Under O-rich conditions, O vacancies are the favored defect type, but both defect types have high formation energies. Under O-poor conditions, both defect types are stabilized, with Ti interstitials predicted to become the favored defect in rutile samples, particularly at elevated temps.
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    Rocquefelte, X.; Schwarz, K.; Blaha, P. Theoretical investigation of the magnetic exchange interactions in copper (II) oxides under chemical and physical pressures. Sci. Rep. 2012, 2, 759,  DOI: 10.1038/srep00759
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    Theoretical investigation of the magnetic exchange interactions in copper(II) oxides under chemical and physical pressures
    Rocquefelte, Xavier; Schwarz, Karlheinz; Blaha, Peter
    Scientific Reports (2012), 2 (), srep00759, 5 pp.CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    It remains a challenge to understand the unconventional mechanisms that cause high-TC supercond. in cuprate superconductors, high-TC multiferroicity in CuO, or low-dimensional magnetism in the spin-Peierls transition compds. such as CuGeO3. A common feature of all these copper oxide compds. (contg. Cu2+ ions) is the presence of large magnetic superexchange interactions J. It is a general strategy to apply chem. and/or phys. pressure in order to tune these exotic properties. Here we show theor., for the first time, the impact of phys. pressure on J on CuO, for which we predict a strong enhancement of the low-dimensionality of the magnetic interactions and the spin-frustration at high-pressures. Such modifications are expected to strongly influence the multiferroic properties of CuO. We finally demonstrate that PBE0 hybrid DFT calcns. provide reliable J values for a wide range of copper(II) oxides compds., i.e. CuGeO3, BaCu2Si2O7, BaCu2Ge2O7, and La2CuO4.
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    Kuz’menko, A. B.; Van Der Marel, D.; Van Bentum, P. J. M.; Tishchenko, E. A.; Presura, C.; Bush, A. A. Phonon anomalies versus magnetic ordering in CuO. Phys. B 2000, 284–288, 1396– 1397,  DOI: 10.1016/s0921-4526(99)02557-0
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    Phonon anomalies versus magnetic ordering in CuO
    Kuz'menko, A. B.; van der Marel, D.; van Bentum, P. J. M.; Tishchenko, E. A.; Presura, C.; Bush, A. A.
    Physica B: Condensed Matter (Amsterdam) (2000), 284-288 (), 1396-1397CODEN: PHYBE3; ISSN:0921-4526. (Elsevier Science B.V.)
    We report the first measurements of FIR and MIR reflectivity of single-crystal monoclinic antiferromagnetic copper monoxide (CuO) in a wide temp. range (7-300 K) when b-axis and ac-plane polarized modes are completely sepd. and excited in transverse geometry. Previously reported softening of the Au3 mode at the Neel transition is confirmed and found to be up to two times more significant (about 10%). For the first time an emergence of several new IR-active lines at low temps. is obsd. The highest frequency new mode at 690 cm-1 strongly hardens and strengthens with cooling. The data are discussed in terms of strong spin-phonon coupling and possible charge disproportionation at low T.
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    Scanlon, D. O.; Morgan, B. J.; Watson, G. W.; Walsh, A. Acceptor levels in p-type Cu 2 O: rationalizing theory and experiment. Phys. Rev. Lett. 2009, 103, 096405,  DOI: 10.1103/physrevlett.103.096405
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    Acceptor levels in p-type Cu2O. Rationalizing theory and experiment
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    Physical Review Letters (2009), 103 (9), 096405/1-096405/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Understanding conduction in Cu2O is vital to the optimization of Cu-based p-type transparent conducting oxides. Using a screened hybrid-d.-functional approach we have investigated the formation of p-type defects in Cu2O giving rise to single-particle levels that are deep in the band gap, consistent with exptl. obsd. activated, polaronic conduction. Our calcd. transition levels for simple and split Cu vacancies explain the source of the 2 distinct hole states seen in DLTS expts. The necessity of techniques that go beyond the present generalized-gradient- and local-d.-approxn. techniques for accurately describing p-type defects in Cu(I)-based oxides is discussed.
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    Scanlon, D. O.; Morgan, B. J.; Watson, G. W. Modeling the polaronic nature of p-type defects in Cu 2 O: The failure of GGA and GGA+ U. J. Chem. Phys. 2009, 131, 124703,  DOI: 10.1063/1.3231869
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    Modeling the polaronic nature of p-type defects in Cu2O: The failure of GGA and GGA+U
    Scanlon, David O.; Morgan, Benjamin J.; Watson, Graeme W.
    Journal of Chemical Physics (2009), 131 (12), 124703/1-124703/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The exact nature of the hole traps reported deep in the band gap of Cu2O has been a topic of vigorous debate, with copper vacancies and oxygen interstitials both having been proposed as the relevant defects. In this article, the electronic structure of acceptor-forming defects in Cu2O, namely, copper vacancies and oxygen interstitials, is investigated using generalized gradient approxn. (GGA) and GGA cor. for on-site Coulombic interactions (GGA+U). GGA produces notionally semimetallic defect complexes, which is not consistent with the exptl. known polaronic nature of conduction in Cu2O. GGA+U also predicts a semimetallic defect complex for the "simple" copper vacancy but predicts the "split" vacancy and both oxygen interstitials are characterized by localized polarons, with distinct single particle levels found in the band gap. For both methods, however, the positions of calcd. transition levels are inconsistent with exptl. ionization levels. Hence neither GGA nor GGA+U are successful in modeling p-type defects in Cu2O. (c) 2009 American Institute of Physics.
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    Lim, S. P.; Pandikumar, A.; Lim, H. N.; Ramaraj, R.; Huang, N. M. Boosting photovoltaic performance of dye-sensitized solar cells using silver nanoparticle-decorated N, S-Co-doped-TiO2 photoanode. Sci. Rep. 2015, 5, 11922,  DOI: 10.1038/srep11922
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    Boosting Photovoltaic Performance of Dye-Sensitized Solar Cells Using Silver Nanoparticle-Decorated N,S-Co-Doped-TiO2 Photoanode
    Lim Su Pei; Pandikumar Alagarsamy; Huang Nay Ming; Lim Hong Ngee; Ramaraj Ramasamy
    Scientific reports (2015), 5 (), 11922 ISSN:.
    A silver nanoparticle-decorated N,S-co-doped TiO2 nanocomposite was successfully prepared and used as an efficient photoanode in high-performance dye-sensitized solar cells (DSSCs) with N719 dye. The DSSCs assembled with the N,S-TiO2@Ag-modified photoanode demonstrated an enhanced solar-to-electrical energy conversion efficiency of 8.22%, which was better than that of a DSSC photoanode composed of unmodified TiO2 (2.57%) under full sunlight illumination (100 mWcm(-2), AM 1.5 G). This enhanced efficiency was mainly attributed to the reduced band gap energy, improved interfacial charge transfer, and retarded charge recombination process. The influence of the Ag content on the overall efficiency was also investigated, and the optimum Ag content with N,S-TiO2 was found to be 20 wt%. Because of the enhanced solar energy conversion efficiency of the N,S-TiO2@Ag nanocomposite, it should be considered as a potential photoanode for high-performance DSSCs.
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    Patel, S. K. S.; Gajbhiye, N. S. Room temperature magnetic properties of Cu-doped titanate, TiO2 (B) and anatase nanorods synthesized by hydrothermal method. Mater. Chem. Phys. 2012, 132, 175– 179,  DOI: 10.1016/j.matchemphys.2011.11.020
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    Room temperature magnetic properties of Cu-doped titanate, TiO2(B) and anatase nanorods synthesized by hydrothermal method
    Patel, S. K. S.; Gajbhiye, N. S.
    Materials Chemistry and Physics (2012), 132 (1), 175-179CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
    The Cu-doped hydrogen titanate nanorods were synthesized via a hydrothermal reaction and converted into Cu-doped TiO2(B) and anatase phases by calcinations. XRD, FTIR and selected area electron diffraction (SAED) pattern do not show any other phases and thus confirmed the intrinsic ferromagnetic behavior rather than it arising from metallic clusters. The blue shifting in absorption edge and increase in photoluminescence (PL) intensity from Cu-doped hydrogen titanate to anatase phase are strongly correlated with concn. of oxygen vacancies and defect formation. The higher concn. of oxygen vacancies and/or structural defects leads to the coalescence of ferromagnetic domains, which corroborates to the obsd. higher magnetization value of Cu-doped anatase phase compared to other phases.
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    Hasan, M. R.; Suhaimya, S. H. M.; Matb, A. N. C. A sol–gel derived, copper-doped, titanium dioxide–reduced graphene oxide nanocomposite electrode for the photoelectrocatalytic reduction of CO2 to methanol and formic acid. RSC Adv. 2015, 5, 77803,  DOI: 10.1039/c5ra12525a
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    Hasan, Md. Rakibul; Abd Hamid, Sharifah Bee; Basirun, Wan Jeffrey; Meriam Suhaimy, Syazwan Hanani; Che Mat, Ahmad Nazeer
    RSC Advances (2015), 5 (95), 77803-77813CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    A classic Cu-RGO-TiO2 photoelectrocatalyst was fabricated by a facile sol-gel method, deposited on ITO film via electrophoretic deposition and characterized by XRD, FESEM, UV-Vis and FT-IR spectroscopy. A uniformly distributed porous composite film was obsd. on the ITO substrate with an av. particle size of 18 nm. A lower photoluminescence response of the Cu-RGO-TiO2 sample indicates better electron/hole sepn. upon irradn. A max. 1.31 mA cm-2 photocurrent d. was obsd. at -0.61 V bias potential under solar simulator irradn. during CO2 photoelectrocatalysis. Formic acid and methanol were the main products, but longer reaction times led to increased methanol formation. The estd. current efficiency of the prodn. of formic acid and methanol was 32.47%, and the estd. rates of formation of formic acid and methanol were 255 μmol h-1 cm-2 and 189.06 μmol h-1 cm-2, resp.
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    Journal of Colloid and Interface Science (2010), 352 (1), 68-74CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Two percent Cu-doped TiO2 nanoparticles were prepd. by a modified ammonia-evapn.-induced synthetic method, calcined at 450°, and characterized by powder x-ray diffraction, energy dispersive x-ray anal., ESR spectroscopy, SEM, UV-visible diffuse reflectance spectrum, photoluminescence spectroscopy, and electrochem. impedance spectroscopy. Doping shifts the optical absorption edge to the visible region but increases the charge-transfer resistance and decreases the capacitance. Under visible light, the composite nanoparticles very efficiently catalyze the disinfection of E. coli. The prepd. oxide is selective in photocatalysis; under UV light, its photocatalytic activity to degrade sunset yellow, rhodamine B, and methylene blue dyes is less than that of the undoped one.
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    Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn
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    Applied Surface Science (2010), 257 (3), 887-898CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Chem. state x-ray photoelectron spectroscopic anal. of 1st row transition metals and their oxides and hydroxides is challenging due to the complexity of the 2p spectra resulting from peak asymmetries, complex multiplet splitting, shake-up and plasmon loss structure, and uncertain, overlapping binding energies. Current literature shows that all values necessary for reproducible, quant. chem. state anal. are usually not provided. A more consistent, practical and effective approach to curve-fitting the various chem. states in a variety of Sc, Ti, V, Cu and Zn metals, oxides and hydroxides is reported. The curve-fitting procedures proposed are based on a combination of (1) std. spectra from quality ref. samples, (2) a survey of appropriate literature databases and/or a compilation of the literature refs., and (3) specific literature refs. where fitting procedures are available. Binding energies, full-width at half max. (FWHM) values, spin-orbit splitting values, asym. peak-shape fitting parameters, and, for Cu and Zn, Auger parameters values are presented. The quantification procedure for Cu species details the use of the shake-up satellites for Cu(II)-contg. compds. and the exact binding energies of the Cu(0) and Cu(I) peaks. The use of the modified Auger parameter for Cu and Zn species allows for corroborating evidence when there is uncertainty in the binding energy assignment. These procedures can remove uncertainties in anal. of surface states in nano-particles, corrosion, catalysis and surface-engineered materials.
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    Various copper and zinc compds. (Cu, Zn, Cu-Zn alloy, Cu2O, CuO, Cu2S, ZnO, ZnS) have been synthesized and characterized by electron spectroscopy (XPS). Data on the 2p3/2 levels of Cu, Zn, and S, the 1s level of O, the L3M45M45 Auger transitions of Cu and Zn, the valence bands, and the modified Auger parameters are reported. The results obtained with these ref. compds. allow qual. and quant. interpretation of XPS analyses on mixts. of these compds.
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    Pesci, F. M.; Cowan, A. J.; Alexander, B. D.; Durrant, J. R.; Klug, D. R. Charge carrier dynamics on mesoporous WO3 during water splitting. J. Phys. Chem. Lett. 2011, 2, 1900– 1903,  DOI: 10.1021/jz200839n
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    Journal of Physical Chemistry Letters (2011), 2 (15), 1900-1903CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Transient absorption spectroscopy (TAS) has been employed to identify the spectra of photogenerated electrons and holes on WO3. WO3 is a widely studied photoanode for the oxidn. of water, and by measuring the decay of photoholes on the milliseconds to seconds time scale(coating process) in the presence of an electron scavenger, we provide a lower limit for the required lifetime of holes for water oxidn. on WO3. The rate of electron/hole recombination on WO3 has also been examd. at a range of excitation intensities and is found to be nonlinear with excitation intensity.
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    Pendlebury, S. R.; Wang, X.; Le Formal, F.; Cornuz, M.; Kafizas, A.; Tilley, S. D.; Grätzel, M.; Durrant, J. R. Ultrafast charge carrier recombination and trapping in hematite photoanodes under applied bias. J. Am. Chem. Soc. 2014, 136, 9854– 9857,  DOI: 10.1021/ja504473e
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    Ultrafast Charge Carrier Recombination and Trapping in Hematite Photoanodes under Applied Bias
    Pendlebury, Stephanie R.; Wang, Xiuli; Le Formal, Florian; Cornuz, Maurin; Kafizas, Andreas; Tilley, S. David; Gratzel, Michael; Durrant, James R.
    Journal of the American Chemical Society (2014), 136 (28), 9854-9857CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ∼6 ps, ∼103 times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps-μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the assocd. electron trapping are discussed in terms of their implications for efficient water oxidn.
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    Fujishima, Akira; Zhang, Xintong; Tryk, Donald A.
    Surface Science Reports (2008), 63 (12), 515-582CODEN: SSREDI; ISSN:0167-5729. (Elsevier B.V.)
    The field of photocatalysis can be traced back more than 80 years to early observations of the chalking of titania-based paints and to studies of the darkening of metal oxides in contact with org. compds. in sunlight. During the past 20 years, it has become an extremely well researched field due to practical interest in air and water remediation, self-cleaning surfaces, and self-sterilizing surfaces. During the same period, there has also been a strong effort to use photocatalysis for light-assisted prodn. of hydrogen. The fundamental aspects of photocatalysis on the most studied photocatalyst, titania, are still being actively researched and have recently become quite well understood. The mechanisms by which certain types of org. compds. are decompd. completely to carbon dioxide and water, for example, have been delineated. However, certain aspects, such as the photo-induced wetting phenomenon, remain controversial, with some groups maintaining that the effect is a simple one in which org. contaminants are decompd., while other groups maintain that there are addnl. effects in which the intrinsic surface properties are modified by light. During the past several years, powerful tools such as surface spectroscopic techniques and scanning probe techniques performed on single crystals in ultra-high vacuum, and ultrafast pulsed laser spectroscopic techniques have been brought to bear on these problems, and new insights have become possible. Quantum chem. calcns. have also provided new insights. New materials have recently been developed based on titania, and the sensitivity to visible light has improved. The new information available is staggering, but we hope to offer an overview of some of the recent highlights, as well as to review some of the origins and indicate some possible new directions.
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  67. 67
    Wang, X.; Kafizas, A.; Li, X.; Moniz, S. J. A.; Reardon, P. J. T.; Tang, J.; Parkin, I. P.; Durrant, J. R. Transient absorption spectroscopy of anatase and rutile: the impact of morphology and phase on photocatalytic activity. J. Phys. Chem. C 2015, 119, 10439– 10447,  DOI: 10.1021/acs.jpcc.5b01858
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    67
    Transient Absorption Spectroscopy of Anatase and Rutile: The Impact of Morphology and Phase on Photocatalytic Activity
    Wang, Xiuli; Kafizas, Andreas; Li, Xiaoe; Moniz, Savio J. A.; Reardon, Philip J. T.; Tang, Junwang; Parkin, Ivan P.; Durrant, James R.
    Journal of Physical Chemistry C (2015), 119 (19), 10439-10447CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors employed transient absorption spectroscopy (TAS) to study the kinetic dependences of photocatalysis in anatase and rutile TiO2 films of varying morphol. In mesoporous films, anatase was ∼30 times more efficient than rutile in the photocatalytic degrdn. of an intelligent ink model system. Independent of phase, up to 100 lower levels of photocatalysis were found in dense films. Charge carrier lifetimes were probed by TAS on the microsecond to second time scale. For both rutile and anatase, recombination was independent of morphol. Rutile exhibited up to 10 times slower recombination kinetics than anatase. Efficient, irreversible hole scavenging by alcs. was present in mesoporous anatase alone, giving long-lived electrons (τ ≈ 0.7 s) which, upon the addn. of the dye redn. target resazurin, enabled efficient electron transfer (τ ≈ 3 ms). Hole scavenging by alcs. on mesoporous rutile was substantially less efficient and more reversible than anatase, resulting in only a marginal increase in electron lifetime. The lower activity of rutile was not due to differences in recombination but rather to the deficiency of rutile holes to drive efficient and irreversible alc. oxidn.
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  68. 68
    Kafizas, A.; Wang, X.; Pendlebury, S. R.; Barnes, P.; Ling, M.; Sotelo-Vazquez, C.; Quesada-Cabrera, R.; Li, C.; Parkin, I. P.; Durrant, J. R. Where do photogenerated holes go in anatase: rutile TiO2 A transient absorption spectroscopy study of charge transfer and lifetime. J. Phys. Chem. A 2016, 120, 715– 723,  DOI: 10.1021/acs.jpca.5b11567
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    68
    Where Do Photogenerated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime
    Kafizas, Andreas; Wang, Xiuli; Pendlebury, Stephanie R.; Barnes, Piers; Ling, Min; Sotelo-Vazquez, Carlos; Quesada-Cabrera, Raul; Li, Can; Parkin, Ivan P.; Durrant, James R.
    Journal of Physical Chemistry A (2016), 120 (5), 715-723CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood. Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase compn. By exploiting the different signatures in the photoinduced absorption of trapped charges in anatase and rutile, we were able to sep. track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photogenerated holes transfer from rutile to anatase on submicrosecond time scales. This hole transfer can significantly increase the anatase hole yield, with a 20:80 anatase:rutile composite showing a 5-fold increase in anatase holes obsd. from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irresp. of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for detg. the direction of photogenerated charge sepn. across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.
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    Wang, L.; McCleese, C.; Kovalsky, A.; Zhao, Y.; Burda, C. Femtosecond time-resolved transient absorption spectroscopy of CH3NH3PbI3 perovskite films: evidence for passivation effect of PbI2. J. Am. Chem. Soc. 2014, 136, 12205– 12208,  DOI: 10.1021/ja504632z
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    69
    Femtosecond Time-Resolved Transient Absorption Spectroscopy of CH3NH3PbI3 Perovskite Films: Evidence for Passivation Effect of PbI2
    Wang, Lili; McCleese, Christopher; Kovalsky, Anton; Zhao, Yixin; Burda, Clemens
    Journal of the American Chemical Society (2014), 136 (35), 12205-12208CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    MeNH3PbI3 perovskite layered films deposited on substrates with and without a TiO2 support structure were prepd. and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450-800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. For the 1st time the authors could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red shifted at ∼780 nm. An addnl. bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to est. relative amts. of PbI2 in the samples. Kinetic anal. reveals that perovskite films with less PbI2 show faster relaxation rates than those contg. more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples contg. PbI2 are due to a passivation effect, in line with other recently reported work.
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    Clarke, T.; Ballantyne, A.; Jamieson, F.; Brabec, C.; Nelson, J.; Durrant, J. Transient absorption spectroscopy of charge photogeneration yields and lifetimes in a low bandgap polymer/fullerenefilm. Chem. Commun. 2008, 89,  DOI: 10.1039/B813815J
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    Anderson, L. J. E.; Mayer, K. M.; Fraleigh, R. D.; Yang, Y.; Lee, S.; Hafner, J. H. Quantitative measurements of individual gold nanoparticle scattering cross sections. J. Phys. Chem. C 2010, 114, 11127– 11132,  DOI: 10.1021/jp1040663
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    Quantitative Measurements of Individual Gold Nanoparticle Scattering Cross Sections
    Anderson, Lindsey J. E.; Mayer, Kathryn M.; Fraleigh, Robert D.; Yang, Yi; Lee, Seunghyun; Hafner, Jason H.
    Journal of Physical Chemistry C (2010), 114 (25), 11127-11132CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Dark field microspectroscopy is a powerful tool for studying plasmon resonances of noble metal nanoparticles and for developing their applications in sensing and imaging. Here we calibrate a dark field microspectrometer with measurements on gold nanospheres in a uniform dielec. medium to yield quant. spectral scattering cross sections for elongated nanoparticle shapes. Gold bipyramids, 135 nm in length, were found to have a peak differential cross section of 1.2 × 10-16 m2. Measurements of a small ensemble of gold nanorods, 13 nm in diam. and 45 nm in length on av., were found to have a peak differential cross section of only 1 × 10-18 m2. For the smaller gold nanorods, approx. expressions for the total scattering cross section may be used to indicate their scattering signal in microscopy applications.
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    Wang, L.; Wang, H.-Y.; Gao, B.-R.; Pan, L.-Y.; Jiang, Y.; Chen, Q.-D.; Han, W.; Sun, H.-B. Transient absorption spectroscopic study on band-structure-type change in CdTe/CdS core-shell quantum dots. IEEE J. Quantum Electron. 2011, 47, 1177– 1184,  DOI: 10.1109/jqe.2011.2159853
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    Transient absorption spectroscopic study on band-structure-type change in CdTe/CdS core-shell quantum dots
    Wang, Lei; Wang, Hai-Yu; Gao, Bing-Rong; Pan, Ling-Yun; Jiang, Ying; Chen, Qi-Dai; Han, Wei; Sun, Hong-Bo
    IEEE Journal of Quantum Electronics (2011), 47 (9), 1177-1184CODEN: IEJQA7; ISSN:0018-9197. (IEEE Photonics Society)
    We report on the band edge alignment in zinc-blended-type CdTe/CdS core-shell quantum dots, using femtosecond spectroscopy. Time-resolved transient absorption spectroscopy directly shows that the system gradually developed from quasi-type-I to type-II as the shell thickness increased. Following photoexcitation of CdS, the injection of a hole from CdS valence band into CdTe valence band is obsd. The hole transfer occurs on the 1000 fs time scale in these heteromaterials. Furthermore, a strain ca. 150 meV is obtained by comparing the transient with steady-state spectra.
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    Peiró, A. M.; Colombo, C.; Doyle, G.; Nelson, J.; Mills, A.; Durrant, J. R. Photochemical reduction of oxygen adsorbed to nanocrystalline TiO2 films: A transient absorption and oxygen scavenging study of different TiO2 preparations. J. Phys. Chem. B 2006, 110, 23255– 23263,  DOI: 10.1021/jp064591c
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    73
    Photochemical Reduction of Oxygen Adsorbed to Nanocrystalline TiO2 Films: A Transient Absorption and Oxygen Scavenging Study of Different TiO2 Preparations
    Peiro, Ana M.; Colombo, Claudia; Doyle, Gerry; Nelson, Jenny; Mills, Andrew; Durrant, James R.
    Journal of Physical Chemistry B (2006), 110 (46), 23255-23263CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Transient absorption spectroscopy (TAS) has been used to study the interfacial electron-transfer reaction between photogenerated electrons in nanocryst. titanium dioxide (TiO2) films and mol. oxygen. TiO2 films from three different starting materials (TiO2 anatase colloidal paste and com. anatase/rutile powders Degussa TiO2 P25 and VP TiO2 P90) have been investigated in the presence of ethanol as a hole scavenger. Sep. investigations on the photocatalytic oxygen consumption by the films have also been performed with an oxygen membrane polarog. detector. Results show that a correlation exists between the electron dynamics of oxygen consumption obsd. by TAS and the rate of oxygen consumption through the photocatalytic process. The highest activity and the fastest oxygen redn. dynamics were obsd. with films fabricated from anatase TiO2 colloidal paste. The use of TAS as a tool for the prediction of the photocatalytic activities of the materials is discussed. TAS studies indicate that the rate of redn. of mol. oxygen is limited by interfacial electron-transfer kinetics rather than by the electron trapping/detrapping dynamics within the TiO2 particles.
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    Devahasdin, S.; Fan, C., Jr.; Li, K.; Chen, D. H. TiO2 photocatalytic oxidation of nitric oxide: transient behavior and reaction kinetics. J. Photochem. Photobiol., A 2003, 156, 161– 170,  DOI: 10.1016/s1010-6030(03)00005-4
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    TiO2 photocatalytic oxidation of nitric oxide: transient behavior and reaction kinetics
    Devahasdin, Sid; Fan, Chiun; Li, Kuyen; Chen, Daniel H.
    Journal of Photochemistry and Photobiology, A: Chemistry (2003), 156 (1-3), 161-170CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science B.V.)
    Photocatalytic oxidn. (PCO) of nitric oxide (NO) over TiO2 catalyst was studied at source levels (5-60 ppm). The PCO process involves a series of oxidn. steps by the OH√ radical: NO HNO2, NO2, HNO3. The product NO2 can be collected in an adsorbent bed and either recycled back to the combustion chamber or recovered as nitric acid. The ratio of NO2- to NO3- from spent catalyst liquor drops with irradn. time until a steady state is reached. The reactions are limited by thermodn. equil. after ∼12 s space time. The steady-state exptl. data from space time and inlet concn. effects can be described with the Langmuir-Hinshelwood (L-H) kinetic model with R2=0.9208.
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    Tang, J.; Durrant, J. R.; Klug, D. R. Mechanism of photocatalytic water splitting in TiO2. Reaction of water with photoholes, importance of charge carrier dynamics, and evidence for four-hole chemistry. J. Am. Chem. Soc. 2008, 130, 13885– 13891,  DOI: 10.1021/ja8034637
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    75
    Mechanism of Photocatalytic Water Splitting in TiO2. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for Four-Hole Chemistry
    Tang, Junwang; Durrant, James R.; Klug, David R.
    Journal of the American Chemical Society (2008), 130 (42), 13885-13891CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors show for the first time that the photogenerated hole lifetime in TiO2 is a strong determinant of the ability of TiO2 to split water. Hole lifetimes were measured using transient absorption spectroscopy over a range of excitation intensities. The lifetimes of the holes were modulated by the use of exogenous scavengers and were also found to vary systematically with the excitation intensity. In all cases the quantum yield of oxygen prodn. is found to be linked to the light intensity used, ranging from below 1 sun equiv. to nearly 1 sun equiv. The authors also provide evidence that oxygen prodn. requires four photons for each mol. of oxygen, which is reminiscent of the natural photosynthetic water-splitting mechanism. This in turn suggests a mechanism for oxygen prodn. which requires four-hole chem., presumably via three, as yet unidentified intermediates. It is also shown that at excitation densities on the order of 1 sun, nongeminate electron-hole recombination limits the quantum yield significantly.
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    Kundu, S.; Kafizas, A.; Hyett, G.; Mills, A.; Darr, J. A.; Parkin, I. P. An investigation into the effect of thickness of titanium dioxide and gold–silver nanoparticle titanium dioxide composite thin-films on photocatalytic activity and photo-induced oxygen production in a sacrificial system. J. Mater. Chem. 2011, 21, 6854– 6863,  DOI: 10.1039/c0jm03492d
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    76
    An investigation into the effect of thickness of titanium dioxide and gold-silver nanoparticle titanium dioxide composite thin-films on photocatalytic activity and photoinduced oxygen production in a sacrificial system
    Kundu, Sujata; Kafizas, Andreas; Hyett, Geoffrey; Mills, Andrew; Darr, Jawwad A.; Parkin, Ivan P.
    Journal of Materials Chemistry (2011), 21 (19), 6854-6863CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Thin films of TiO2 and TiO2 with incorporated Au and Ag nanoparticles were deposited onto glass microscope slides, steel and Ti foil coupons by two sol-gel dip-coating methods. The film's photocatalytic activity and ability to evolve oxygen in a sacrificial soln. were assessed. It was found that photocatalytic activity increased with film thickness (from 50 to 500 nm thick samples) for the photocatalytic degrdn. of Methylene blue in soln. and Resazurin redox dye in an intelligent ink dye deposited on the surface. Contrastingly, an optimum film thickness of ∼200 nm for both composite and pure films of TiO2 was found for water oxidn., using persulfate (S2O82-) as a sacrificial electron acceptor. The nanoparticle composite films showed significantly higher activity in oxygen evolution studies compared with plain TiO2 films.
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    Cowan, A. J.; Tang, J.; Leng, W.; Durrant, J. R.; Klug, D. R. Water splitting by nanocrystalline TiO2 in a complete photoelectrochemical cell exhibits efficiencies limited by charge recombination. J. Phys. Chem. C 2010, 114, 4208– 4214,  DOI: 10.1021/jp909993w
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    77
    Water Splitting by Nanocrystalline TiO2 in a Complete Photoelectrochemical Cell Exhibits Efficiencies Limited by Charge Recombination
    Cowan, Alexander J.; Tang, Junwang; Leng, Wenhua; Durrant, James R.; Klug, David R.
    Journal of Physical Chemistry C (2010), 114 (9), 4208-4214CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The mechanism of H2O splitting by TiO2, in the absence of chem. scavengers, in a fully functional photoelectrochem. (PEC) cell, was studied. The application of a pos. potential to a nanocryst.-TiO2 film is shown to give long-lived holes which oxidize H2O in milliseconds. These time-resolved studies of a nanocryst.-TiO2 (nc-TiO2) film in a complete PEC cell also showed that all of the long-lived photo-holes generate O2, and that there are no major branching inefficiencies in the catalysis itself, which appears to be operating at efficiencies near 100%. The overall quantum yield of O prodn. under pulsed illumination (355 nm) is ∼8% at excitation densities of 4.4 photons per particle. Under all conditions examd., electron-hole recombination is the dominant loss pathway.
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    Raffi, M.; Mehrwan, S.; Bhatti, T. M.; Akhter, J. I.; Hameed, A.; Yawar, W.; ul Hasan, M. M. Investigations into the antibacterial behavior of copper nanoparticles against Escherichia coli. Ann. Microbiol. 2010, 60, 75– 80,  DOI: 10.1007/s13213-010-0015-6
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    Investigations into the antibacterial behavior of copper nanoparticles against Escherichia coli
    Raffi, Muhammad; Mehrwan, Saba; Bhatti, Tariq Mahmood; Akhter, Javed Iqbal; Hameed, Abdul; Yawar, Wasim; ul Hasan, M. Masood
    Annals of Microbiology (Heidelberg, Germany) (2010), 60 (1), 75-80CODEN: AMNIC7; ISSN:1590-4261. (University of Milan, Dep of Food Science and Microbiology)
    Zerovalent copper nanoparticles (Cu0) of 12 nm size were synthesized using an inert gas condensation method in which bulk copper metal was evapd. into an inert environment of argon with subsequent cooling for nucleation and growth of nanoparticles. Cryst. structure, morphol. and estn. of size of nanoparticles were carried out by X-ray diffraction and transmission electron microscopy. The antibacterial activity of these nanoparticles against the Gram-neg. bacterium Escherichia coli was assessed in liq. as well as solid growth media. It was obsd. from scanning electron microscopic anal. that the interaction of copper nanoparticles with E. coli resulted in the formation of cavities/pits in the bacterial cell wall. The antibacterial property of copper nanoparticles was attributed mainly to adhesion with bacteria because of their opposite elec. charges, resulting in a redn. reaction at the bacterial cell wall. Nanoparticles with a larger surface-to-vol. ratio provide more efficient means for antibacterial activity.
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    Mattioli, G.; Alippi, P.; Filippone, F.; Caminiti, R.; Amore Bonapasta, A. Deep versus Shallow Behavior of Intrinsic Defects in Rutile and Anatase TiO2 Polymorphs. J. Phys. Chem. C 2010, 114, 21694– 21704,  DOI: 10.1021/jp1041316
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    79
    Deep versus Shallow Behavior of Intrinsic Defects in Rutile and Anatase TiO2 Polymorphs
    Mattioli, Giuseppe; Alippi, Paola; Filippone, Francesco; Caminiti, Ruggero; Bonapasta, Aldo Amore
    Journal of Physical Chemistry C (2010), 114 (49), 21694-21704CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The structural and electronic properties of oxygen vacancies (VOx) and titanium interstitials (Ti(i)) in the bulk of the rutile and anatase forms of TiO2 have been investigated with LSD-GGA + U ab initio simulations. In particular, formation energies of the charged and neutral forms of the VOx and Ti(i) defects as well as the corresponding vertical and thermodn. transition levels have been estd. The achieved results can reconcile the apparent inconsistency of exptl. obsd. deep donor levels with the n-type cond. obsd. in reduced TiO2. They show indeed that both defects give rise to vertical transition levels about 1 eV below the conduction band (CB), in agreement with exptl. measures, and to thermodn. transition levels close to the CB. That is, these defects behave as deep donors, when looking at vertical transitions, and as shallow donors, when the effects of the structural relaxations are taken into account. A major part of the explanation of this behavior is played by the polaron-like character of the defect states, which was already noted, but not deepened, in literature. Finally, it is shown that the application of the U correction to both Ti and O species gives qual. similar results, but with a better agreement to exptl. findings, with respect to the application to Ti only. The former approach gives pretty similar results, for both rutile and anatase bulk properties, to those coming from HSE hybrid functional calcns.
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    Deák, P.; Aradi, B.; Frauenheim, T. Polaronic effects in TiO2 calculated by the HSE06 hybrid functional: Dopant passivation by carrier self-trapping. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 155207,  DOI: 10.1103/PhysRevB.83.155207
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    80
    Polaronic effects in TiO2 calculated by the HSE06 hybrid functional: Dopant passivation by carrier self-trapping
    Deak, Peter; Aradi, Balint; Frauenheim, Thomas
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (15), 155207/1-155207/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Metal oxides are extremely challenging for defect calcns. within d.-functional theory because of the underestimation of the band gap and of the polaronic effects due to spurious electron self-interaction in the std. (semi)local implementations. We show here that-similarly to Group-IV semiconductors-the HSE06 screened hybrid functional provides not only a good description of the ground-state properties and an accurate gap of TiO2 modifications, but fulfills the generalized Koopmans' theorem for host-related defect states-independent of localization. This means that the total energy has the correct dependence on the fractional occupation no., lending credibility to the calcd. optical and thermodn. charge transition levels for acceptors (Al, Ga, In, Sc, and Y on Ti site) and donors (Nb on Ti site, and interstitial H). We find deep, localized hole polarons in anatase and electron polarons in rutile. Therefore, p-type doping in anatase and n-type doping in rutile are counteracted by carrier self-trapping. Donors in anatase and the acceptors in rutile give rise to effective mass-like states, but the latter are deeper. In fact, the polaron bound at In in anatase is about as deep as the effective mass-like state induced by In in rutile. The shallow state in anatase and the deep one in rutile for the Nb donor, as well as the deep state for Al in anatase, agree well with exptl. observations.
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    Morgan, B. J.; Watson, G. W. Polaronic trapping of electrons and holes by native defects in anatase TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 233102,  DOI: 10.1103/PhysRevB.80.233102
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    81
    Polaronic trapping of electrons and holes by native defects in anatase TiO2
    Morgan, Benjamin J.; Watson, Graeme W.
    Physical Review B: Condensed Matter and Materials Physics (2009), 80 (23), 233102/1-233102/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The authors have studied the formation of native defects in anatase TiO2 using d. functional theory (DFT) modified with on-site Coulomb terms (DFT+U) applied to both Ti d and O p states. O vacancies and Ti interstitials are deep donors that trap two and four electrons, with transition levels that explain the two features seen in deep level transient spectroscopy expts. Ti vacancies are deep acceptors accommodating four holes. Self-trapping of both electrons and holes is also predicted. In all cases both donor and acceptor trap states correspond to strongly localized small polarons, in agreement with exptl. EPR data. Variation in defect formation energies with stoichiometry explains the poor hole-trapping of reduced TiO2.
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    Oba, F.; Nishitani, S. R.; Isotani, S.; Adachi, H.; Tanaka, I. Energetics of native defects in ZnO. J. Appl. Physiol. 2001, 90, 824– 828,  DOI: 10.1063/1.1380994
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    Energetics of native defects in ZnO
    Oba, Fumiyasu; Nishitani, Shigeto R.; Isotani, Seiji; Adachi, Hirohiko; Tanaka, Isao
    Journal of Applied Physics (2001), 90 (2), 824-828CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The authors have studied the formation energies and electronic structure of native defects in ZnO by a 1st-principles plane-wave pseudopotential method. When p-type conditions are assumed, the formation energies of donor-type defects can be quite low. The effect of self-compensation by the donor-type defects should be significant in p-type doping. Under n-type conditions, the oxygen vacancy exhibits the lowest formation energy among the donor-type defects. The electronic structure, however, implies that only the zinc interstitial or the zinc antisite can explain the n-type cond. of undoped ZnO.
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    Oba, F.; Togo, A.; Tanaka, I.; Paier, J.; Kresse, G. Defect energetics in ZnO: A hybrid Hartree-Fock density functional study. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 245202,  DOI: 10.1103/PhysRevB.77.245202
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    83
    Defect energetics in ZnO: A hybrid Hartree-Fock density functional study
    Oba, Fumiyasu; Togo, Atsushi; Tanaka, Isao; Paier, Joachim; Kresse, Georg
    Physical Review B: Condensed Matter and Materials Physics (2008), 77 (24), 245202/1-245202/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    First-principles calcns. based on hybrid Hartree-Fock d. functionals provide a clear picture of the defect energetics and electronic structure in ZnO. Among the donorlike defects, the oxygen vacancy and hydrogen impurity, which are deep and shallow donors, resp., are likely to form with a substantial concn. in n-type ZnO. The zinc interstitial and zinc antisite, which are both shallow donors, are energetically much less favorable. A strong preference for the oxygen vacancy and hydrogen impurity over the acceptorlike zinc vacancy is found under oxygen-poor conditions, suggesting that the oxygen vacancy contributes to nonstoichiometry and that hydrogen acts as a donor, both of which are without significant compensation by the zinc vacancy. The present results show consistency with the relevant exptl. observations.
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    Janotti, A.; Van de Walle, C. G. Native point defects in ZnO. Phys. Rev. B 2007, 76, 165202,  DOI: 10.1103/PhysRevB.76.165202
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    Native point defects in ZnO
    Janotti, Anderson; Van de Walle, Chris G.
    Physical Review B: Condensed Matter and Materials Physics (2007), 76 (16), 165202/1-165202/22CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The authors have performed a comprehensive 1st-principles study of native point defects in ZnO based on d. functional theory within the local d. approxn. (LDA) as well as the LDA + U approach for overcoming the band-gap problem. O deficiency, manifested in the form of O vacancies and Zn interstitials, has long been invoked as the source of the commonly obsd. unintentional n-type cond. in ZnO. However, contrary to the conventional wisdom, the authors find that native point defects are very unlikely to be the cause of unintentional n-type cond. O vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in n-type ZnO (and are therefore unlikely to form). Zn interstitials are shallow donors, but they also have high formation energies in n-type ZnO and are fast diffusers with migration barriers as low as 0.57 eV; they are therefore unlikely to be stable. Zn antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equil. conditions. We have, however, identified a different low-energy at. configuration for Zn antisites that may play a role under nonequil. conditions such as irradn. Zn vacancies are deep acceptors and probably related to the frequently obsd. green luminescence; they act as compensating centers in n-type ZnO. O interstitials have high formation energies; they can occur as elec. neutral split interstitials in semi-insulating and p-type materials or as deep acceptors at octahedral interstitial sites in n-type ZnO. O antisites have very high formation energies and are unlikely to exist in measurable concns. under equil. conditions. Based on the results for migration energy barriers, the authors calc. activation energies for self-diffusion and est. defect-annealing temps. The results provide a guide to more refined exptl. studies of point defects in ZnO and their influence on the control of p-type doping.
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    Ágoston, P.; Albe, K.; Nieminen, R. M.; Puska, M. J. Intrinsic n-Type Behavior in Transparent Conducting Oxides: A Comparative Hybrid-Functional Study of In2O3, SnO2 and ZnO. Phys. Rev. Lett. 2009, 103, 245501,  DOI: 10.1103/PhysRevLett.103.245501
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    Intrinsic n-Type Behavior in Transparent Conducting Oxides: A Comparative Hybrid-Functional Study of In2O3, SnO2, and ZnO
    Agoston, Peter; Albe, Karsten; Nieminen, Risto M.; Puska, Martti J.
    Physical Review Letters (2009), 103 (24), 245501/1-245501/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We present a comparative study of oxygen vacancies in In2O3, SnO2, and ZnO based on the hybrid-functional method within the d.-functional theory (DFT). For In2O3 and SnO2, our results provide strong evidence of shallow donor states at oxygen vacancies. In comparison with the (semi)local exchange-correlation approxns. in DFT, the hybrid-functional method strongly lowers the formation energy of the pos. charge state and keeps that of the neutral state nearly intact. The trend is analyzed in terms of changes in lattice relaxation energies and in electron energy levels near the band gap. The existence of shallow donor states at oxygen vacancies and the consequent n-type cond. are in line with exptl. findings. The results invalidate some former theor. interpretations based on std. DFT calcns.
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    Scanlon, D. O.; Watson, G. W. On the possibility of p-type SnO2. J. Mater. Chem. 2012, 22, 25236– 25245,  DOI: 10.1039/C2JM34352E
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    On the possibility of p-type SnO2
    Scanlon, David O.; Watson, Graeme W.
    Journal of Materials Chemistry (2012), 22 (48), 25236-25245CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    SnO2 is an abundant, low cost, natively n-type, wide band gap oxide, which can achieve high conductivities due to facile donor doping. Realization of a p-type SnO2 would, however, open up many new avenues in device applications, and has become a major research goal. Previous exptl. and theor. studies have proved inconclusive, with the p-type ability of SnO2 being both supported and questioned in equal measure. We use state of the art hybrid d. functional theory to investigate the nature of intrinsic and extrinsic p-type defects in SnO2. All the p-type defects considered in SnO2 produce localized hole polarons centered on anion sites. We calc. the thermodn. ionization energies of these defects, and demonstrate that an efficient p-type SnO2 is not achievable.
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    Kılıç, Ç.; Zunger, A. Origins of Coexistence of Conductivity and Transparency in SnO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 88, 095501,  DOI: 10.1103/PhysRevLett.88.095501
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    Origins of coexistence of conductivity and transparency in SnO2
    Kilic, Cetin; Zunger, Alex
    Physical Review Letters (2002), 88 (9), 095501/1-095501/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    SnO2 is a prototype transparent conductor, exhibiting the contradictory properties of high metallic cond. due to massive structural nonstoichiometry with nearly complete, insulator-like transparency in the visible range. The authors found, via 1st-principles calcns., that the Sn interstitial and O vacancy have surprisingly low formation energies and strong mutual attraction, explaining the natural nonstoichiometry of this system. The stability of these intrinsic defects is traced back to the multivalence of Sn. These defects donate electrons to the conduction band without increasing optical interband absorption, explaining coexistence of cond. with transparency.
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    Singh, A. K.; Janotti, A.; Scheffler, M.; Van de Walle, C. G. Sources of Electrical Conductivity in SnO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 101, 055502,  DOI: 10.1103/PhysRevLett.101.055502
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    Sources of electrical conductivity in SnO2
    Singh, Abhishek Kumar; Janotti, Anderson; Scheffler, Matthias; Van de Walle, Chris G.
    Physical Review Letters (2008), 101 (5), 055502/1-055502/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    SnO2 is widely used as a transparent conductor and sensor material. Better understanding and control of its cond. would enhance its performance in existing applications and enable new ones, such as in light emitters. Using d. functional theory, we show that the conventional attribution of n-type cond. to intrinsic point defects is incorrect. Unintentional incorporation of hydrogen provides a consistent explanation of exptl. observations. Most importantly, we find that SnO2 offers excellent prospects for p-type doping by incorporation of acceptors on the Sn site. Specific strategies for optimizing acceptor incorporation are presented.
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    Scanlon, D. O. Defect engineering of BaSnO3 for high-performance transparent conducting oxide applications. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 161201,  DOI: 10.1103/PhysRevB.87.161201
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    Defect engineering of BaSnO3 for high-performance transparent conducting oxide applications
    Scanlon, David O.
    Physical Review B: Condensed Matter and Materials Physics (2013), 87 (16), 161201/1-161201/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The high cost and low abundance of indium has driven research to find In-free n-type transparent conducting oxides (TCOs). La-doped cubic perovskite BaSnO3 has been reported to possess electron mobilities as high as 320 cm2 V-1 s-1 for carrier concns. of 8 × 1019 cm-3, comparable to the very best TCOs. To date, however, the origins of cond. in this material have remained unclear. Here we study the defect chem. of BaSnO3 using a hybrid d. functional theory approach, in order to understand how to control the n-type cond. We show that in undoped samples, native defects cannot cause high levels of n-type cond.; however, adventitious H present in samples can act as shallow donors. By studying the effect of a range of donor dopants, we pinpoint ideal growth conditions and donor dopants for high-performance n-type BaSnO3 samples.
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    Sallis, S.; Scanlon, D. O.; Chae, S. C.; Quackenbush, N. F.; Fischer, D. A.; Woicik, J. C.; Guo, J.-H.; Cheong, S. W.; Piper, L. F. J. La-doped BaSnO3—Degenerate perovskite transparent conducting oxide: Evidence from synchrotron x-ray spectroscopy. Appl. Phys. Lett. 2013, 103, 042105,  DOI: 10.1063/1.4816511
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    La-doped BaSnO3-Degenerate perovskite transparent conducting oxide: Evidence from synchrotron x-ray spectroscopy
    Sallis, S.; Scanlon, D. O.; Chae, S. C.; Quackenbush, N. F.; Fischer, D. A.; Woicik, J. C.; Guo, J.-H.; Cheong, S. W.; Piper, L. F. J.
    Applied Physics Letters (2013), 103 (4), 042105/1-042105/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We report direct evidence of conduction band filling in 3% La-doped BaSnO3 using hard XPS. Direct comparisons with hybrid d. functional theory calcns. support a 3.2 eV indirect band gap. The use of hybrid DFT is verified by excellent agreement between our photoelectron spectra and O K-edge x-ray emission and absorption spectra. Our exptl. and computational results demonstrate that the conduction band is primarily of Sn 5s orbital character with little O 2p contribution, which is a prerequisite for designing a perovskite-based transparent conducting oxide. (c) 2013 American Institute of Physics.
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    Deák, P.; Aradi, B.; Frauenheim, T. Quantitative theory of the oxygen vacancy and carrier self-trapping in bulk TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 195206,  DOI: 10.1103/PhysRevB.86.195206
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    Quantitative theory of the oxygen vacancy and carrier self-trapping in bulk TiO2
    Deak, Peter; Aradi, Balint; Frauenheim, Thomas
    Physical Review B: Condensed Matter and Materials Physics (2012), 86 (19), 195206/1-195206/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Std. approxns. of d. functional theory often fail for defects in transition metal oxides. Despite its significance in applications and decades of research, the oxygen vacancy in TiO2 is also poorly understood from the theory point of view. The Heyd-Scuseria-Ernzerhof functional (HSE06) provides a total energy that is a linear function of the occupation no. (as it should be for the exact functional) also in TiO2. This allows reprodn. of the measured IR absorption, photoluminescence, and thermal ionization data within ∼0.1 eV. From our calcns., a consistent and quant. interpretation of the conflicting expts. emerges. Electron self-trapping in rutile makes the properties of the vacancy concn. dependent. In oxidized samples the vacancies are passivated by native Ti3+/Ti4+ traps. This explains why electrons localized to the vacancy could only be obsd. after illumination at very low temp. in magnetic resonance expts. In strongly reduced samples, electrons may stay localized in the vacancy and even the neutral state gives rise to two vertical electronic transitions (at 0.8 and 1.2 eV). The situation is much simpler in anatase, where only holes are self-trapped by O2-/O1- transitions. The oxygen vacancy is a shallower donor in anatase than in rutile.
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    Deák, P.; Aradi, B.; Frauenheim, T. Oxygen deficiency in TiO2: Similarities and differences between the Ti self-interstitial and the O vacancy in bulk rutile and anatase. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 045204,  DOI: 10.1103/PhysRevB.92.045204
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    Oxygen deficiency in TiO2: similarities and differences between the Ti self-interstitial and the O vacancy in bulk rutile and anatase
    Deak, Peter; Aradi, Balint; Frauenheim, Thomas
    Physical Review B: Condensed Matter and Materials Physics (2015), 92 (4), 045204/1-045204/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    TiO2 is an oxygen-deficient, intrinsically n-type material, but it is often debated whether the electrons are donated by oxygen vacancies (VO) or titanium interstitials (Tii). Investigating this issue is complicated by the fact that rutile can self-trap electrons in intrinsic small polaron states, while bulk anatase cannot. The screened hybrid functional HSE06 was proven to account for this phenomenon and has provided quant. correct results for VO in our earlier study. Here, we use it for Tii in both rutile and anatase, allowing full spin and symmetry freedom, to shed light on the similarities and differences to VO. We find that these two defects give rise to very similar fingerprints in ESR, IR absorption, or photoelectron spectra. In weakly reduced rutile, the ground state of both defects is (2+), with two electrons in polaronic traps, bound loosely to the defect. Most of the time, only these latter states (crudely resembling a hydrogenic series, with increasing distance from the defect) are likely to be detected. In anatase, both VO and Tii can be expected to be ionized at room temp. (singly and doubly, resp.), and the next vertical ionization energy is similar in the two defects-and very close to the ionization energy of the bound polarons in rutile. Most signals in paramagnetic resonance expts. on rutile must also be related to the polaron states, and, in general, very special conditions have to be fulfilled to detect electrons localized to VO or Tii itself. We show that, in thermal equil., the dominant defect in intrinsic samples is VO, and Tii can be the majority defect only in strongly reduced anatase, or in case of p-type doping.
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    Quesada-Gonzalez, M.; Williamson, B. A. D.; Sotelo-Vazquez, C.; Kafizas, A.; Boscher, N. D.; Quesada-Cabrera, R.; Scanlon, D. O.; Carmalt, C. J.; Parkin, I. P. Deeper Understanding of Interstitial Boron-Doped Anatase Thin Films as A Multifunctional Layer Through Theory and Experiment. J. Phys. Chem. C 2018, 122, 714– 726,  DOI: 10.1021/acs.jpcc.7b11142
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    93
    Deeper Understanding of Interstitial Boron-Doped Anatase Thin Films as A Multifunctional Layer Through Theory and Experiment
    Quesada-Gonzalez, Miguel; Williamson, Benjamin A. D.; Sotelo-Vazquez, Carlos; Kafizas, Andreas; Boscher, Nicolas D.; Quesada-Cabrera, Raul; Scanlon, David O.; Carmalt, Claire J.; Parkin, Ivan P.
    Journal of Physical Chemistry C (2018), 122 (1), 714-726CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Thin films of interstitial boron-doped anatase TiO2, with varying B concns., were deposited via one-step atm. pressure chem. vapor deposition (APCVD) on float glass substrates. The doped films showed a remarkable morphol. and enhanced photoactivity when compared to their undoped analogs. The TiO2:B films also presented enhanced cond. and electron mobility as measured by a Hall effect probe as well as a high adherence to the substrate, stability and extended lifetime. The structure and compn. of the different samples of TiO2:B films were studied by X-ray diffraction (XRD), Raman spectroscopy, SEM, and dynamic secondary ion mass spectrometry (D-SIMS). Hybrid d. functional theory was used to explore the defect chem. of B-doped anatase and to understand the exptl. results.
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    Navas, J.; Sánchez-Coronilla, A.; Aguilar, T.; Hernández, N. C.; de los Santos, D. M.; Sánchez-Márquez, J.; Zorrilla, D.; Fernández-Lorenzo, C.; Alcántara, R.; Martín-Calleja, J. Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO2. Phys. Chem. Chem. Phys. 2014, 16, 3835– 3845,  DOI: 10.1039/c3cp54273d
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    Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO2
    Navas, Javier; Sanchez-Coronilla, Antonio; Aguilar, Teresa; Hernandez, Norge C.; de los Santos, Desiree M.; Sanchez-Marquez, Jesus; Zorrilla, David; Fernandez-Lorenzo, Concha; Alcantara, Rodrigo; Martin-Calleja, Joaquin
    Physical Chemistry Chemical Physics (2014), 16 (8), 3835-3845CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A good correlation was obtained between the electronic properties of Cu-doped anatase TiO2 by virtue of both phys. chem. characterization and theor. calcns. Pure and Cu-doped TiO2 were synthesized. The compn., structural and electronic properties, and the band gap energy were obtained using several techniques. The method of synthesis used produces Cu-doped anatase TiO2, and XRD, XPS and Raman spectroscopy indicate that Cu atoms are incorporated in the structure by substitution of Ti atoms, generating a distortion of the structure and O vacancies. In turn, the band gap energy of the synthesized samples decrease drastically with the Cu doping. Moreover, periodic d. functional theory (DFT-periodic) calcns. were carried out both to model the exptl. obsd. doped structures and to understand theor. the exptl. structures obtained, the formation of oxygen vacancies and the values of the band gap energy. From the anal. of d. of states (DOS), projected d. of states (PDOS) and the electron localization function (ELF) a decrease in the band gap is predicted upon increasing the Cu doping. Thus, the inclusion of Cu in the anatase structure implies a covalent character in the Cu-O interaction, which involves the appearance of new states in the valence band max. with a narrowing in the band gap.
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    Pongwan, P.; Wetchakun, K.; Phanichphant, S.; Wetchakun, N. Enhancement of visible-light photocatalytic activity of Cu-doped TiO2 nanoparticles. Res. Chem. Intermed. 2016, 42, 2815– 2830,  DOI: 10.1007/s11164-015-2179-y
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    Enhancement of visible-light photocatalytic activity of Cu-doped TiO2 nanoparticles
    Pongwan, Paruchai; Wetchakun, Khatcharin; Phanichphant, Sukon; Wetchakun, Natda
    Research on Chemical Intermediates (2016), 42 (4), 2815-2830CODEN: RCINEE; ISSN:0922-6168. (Springer)
    Bare TiO2 and Cu-doped TiO2 nanoparticles with different nominal doping amts. of Cu ranging from of 0.5 to 5.0 mol% were synthesized using the modified sol-gel method. The samples were phys. characterized by x-ray diffraction, SEM, transmission electron microscopy, energy-dispersive x-ray spectroscopy, Brunauer-Emmett-Teller-sp. surface area, UV-visible diffuse reflectance spectroscopy, zeta potential, XPS, inductively coupled plasma, and photoluminescence techniques. The Cu-doped TiO2 exhibited good photocatalytic activity in mineralization of oxalic acid and formic acid under visible light irradn. Photomineralization of oxalic and formic acids under visible light irradn. revealed greatly enhanced photoactivity exhibited by the 2.0 mol% Cu-doped TiO2 photocatalyst compared to bare TiO2 . The enhanced photocatalytic performance arises from copper ion doping in the TiO2 structure, leading to an extended photoresponsive range, enhanced photogenerated charge sepn., and transportation efficiency.
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    Yoong, L. S.; Chong, F. K.; Dutta, B. K. Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light. Energy 2009, 34, 1652– 1661,  DOI: 10.1016/j.energy.2009.07.024
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    Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light
    Yoong, L. S.; Chong, F. K.; Dutta, Binay K.
    Energy (Oxford, United Kingdom) (2009), 34 (10), 1652-1661CODEN: ENEYDS; ISSN:0360-5442. (Elsevier Ltd.)
    The advantage of copper doping onto TiO2 semiconductor photocatalyst for enhanced hydrogen generation under irradn. at the visible range of the electromagnetic spectrum has been investigated. Two methods of prepn. for the copper-doped catalyst were selected, complex pptn. and wet impregnation methods, using copper nitrate trihydrate as the starting material. The dopant loading varied from 2% to 15%. Characterization of the photocatalysts was done by thermogravimetric anal. (TGA), temp. programmed redn. (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), SEM, Fourier transform IR (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temps. were selected, 300°, 400°, and 500°. It was found that 10 wt.% Cu/TiO2 calcined at 300° for 30 min yielded the max. quantity of hydrogen. The redn. of band gap as a result of doping was estd. and the influence of the process parameters on catalytic activity is explained.
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    Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Found. Crystallogr. 1976, 32, 751– 767,  DOI: 10.1107/s0567739476001551
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    Applied Sciences (2018), 8 (11), 2067CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
    Surface contamination by microbes is a major public health concern. A damp environment is one of potential sources for microbe proliferation. Smart photocatalytic coatings on building surfaces using semiconductors like titania (TiO2) can effectively curb this growing threat. Metal-doped titania in anatase phase has been proven as a promising candidate for energy and environmental applications. In this present work, the antimicrobial efficacy of copper (Cu)-doped TiO2 (Cu-TiO2) was evaluated against Escherichia coli (Gram-neg.) and Staphylococcus aureus (Gram-pos.) under visible light irradn. Doping of a minute fraction of Cu (0.5 mol %) in TiO2 was carried out via sol-gel technique. Cu-TiO2 further calcined at various temps. (in the range of 500-700 °C) to evaluate the thermal stability of TiO2 anatase phase. The physico-chem. properties of the samples were characterized through X-ray diffraction (XRD), Raman spectroscopy, X-ray photo-electron spectroscopy (XPS) and UV-visible spectroscopy techniques. XRD results revealed that the anatase phase of TiO2 was maintained well, up to 650 °C, by the Cu dopant. UV-vis results suggested that the visible light absorption property of Cu-TiO2 was enhanced and the band gap is reduced to 2.8 eV. D. functional theory (DFT) studies emphasize the introduction of Cu+ and Cu2+ ions by replacing Ti4+ ions in the TiO2 lattice, creating oxygen vacancies. These further promoted the photocatalytic efficiency. A significantly high bacterial inactivation (99.9999%) was attained in 30 min of visible light irradn. by Cu-TiO2.
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    Despite decades of research on the role of intrinsic defects in enhancing the performance of reduced TiO2 based materials, unambiguous identification of defects responsible for visible light absorption, and near-IR (NIR) photoluminescence from undoped TiO2 has remained challenging. Herein, through in situ photoluminescence (PL) studies under a controlled environment, the authors studied the origin of an extended visible absorption, visible and NIR PL emission from undoped TiO2 nanoribbons grown by a solvothermal route. Studies reveal that O vacancies, Ti3+, and F+ center in TiO2 are responsible for absorption in the violet and blue-green region and the PL emission in the visible region. However, absorption in the yellow-red to NIR region and PL emission in the NIR region at 1.47 and ∼1.30 eV are due to Ti4+ and Ti3+ interstitials, resp., near the surface identified for the 1st time. The above conclusions are supported by ESR and XPS analyses. The development of such nanoporous undoped TiO2 nanoribbons with strong visible absorption and optical identification of Ti interstitial induced band gap states serves as an important milestone toward realizing improved visible light photocatalytic and photovoltaic applications of this novel material.
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    Nonaq. reactions between Ti(IV) chloride and alcs. (benzyl alc. or n-BuOH) were used for the synthesis of anatase TiO2 particles, while rutile TiO2 particles were synthesized in aq. media by acidic hydrolysis of Ti(IV) chloride. The x-ray diffraction measurements proved the exclusive presence of either the anatase or the rutile phase in prepd. samples. The photoluminescence of both kinds of particles (anatase and rutile) with several well-resolved peaks extending in the visible spectral region was obsd., and the quantum yield at room temp. is 0.25%. Photon energy up-conversion from colloidal anatase and rutile TiO2 particles was obsd. at low excitation intensities. The energy of up-converted photoluminescence spans the range of emission of normal photoluminescence. The explanation of photon energy up-conversion involves mid-gap energy levels originating from O vacancies.
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    First-principles theory of acceptors in nitride semiconductors
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    Physica Status Solidi B: Basic Solid State Physics (2015), 252 (5), 900-908CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Acceptor defects and impurities play a crit. role in the performance of GaN-based devices. Mg is the only acceptor impurity that gives rise to p-type cond., while other acceptors (such as CN impurities and VGa defects) act as sources of compensation and trapping. From the point of view of theory, understanding the physics of acceptor species in GaN has long been a challenge. In the past, limitations of computational techniques made it difficult to quant. predict crucial quantities such as thermodn. and optical transition levels. However, advances in first-principles calcns., including the use of hybrid functionals in d. functional theory, have led to a resurgence in efforts to understand properties of acceptors in nitrides. After briefly discussing advances in theor. techniques, we review recent computational work on acceptor impurities in GaN and compare theor. results with the available exptl. data. We also present new hybrid d. functional calcns. on the transition levels of VGa and its complexes with O and H impurities. The results indicate that donor impurities significantly lower VGa transition levels, and that VGa3H and VGaON2H complexes give rise to yellow luminescence. We also discuss the properties of acceptor impurities in AlN and InN.
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    This work explores the Zn vacancy in ZnO using hybrid d. functional theory calcns. The Zn vacancy is predicted to be an exceedingly deep polaronic acceptor that can bind a localized hole on each of the four nearest-neighbor O ions. The hole localization is accompanied by a distinct outward relaxation of the O ions, which leads to lower symmetry and reduced formation energy. Notably, we find that initial symmetry breaking is required to capture this effect. We present a simple model to rationalize our findings with regard to the approx. equidistant thermodn. charge-state transition levels. Furthermore, by employing a one-dimensional configuration coordinate model with parameters obtained from the hybrid d. functional theory calcns., luminescence line shapes were calcd. The results show that the isolated Zn vacancy is unlikely to be the origin of the commonly obsd. luminescence in the visible part of the emission spectrum from n-type material, but rather the luminescence in the IR region.
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  2. Saleh Alofi, Christopher O’Rourke, Andrew Mills. Study and Modeling of the Kinetics of the Photocatalytic Destruction of Stearic Acid Islands on TiO2 Films. The Journal of Physical Chemistry C 2023, 127 (25) , 12194-12205. https://doi.org/10.1021/acs.jpcc.3c00952
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  • Abstract

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    Figure 1

    Figure 1. (a) XRD patterns and (b) Raman spectra for the undoped, 2, 5, 10, and 20% Cu-doped TiO2 anatase films grown using AACVD.

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    Figure 2

    Figure 2. (a) UV–vis spectra encompassing the UV, visible, and near-IR wavelengths and (b) PL spectra for the undoped, 2, 5, 10, and 20% Cu-doped TiO2 anatase films grown using AACVD.

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    Figure 3

    Figure 3. Cu 2p XPS spectra for the (a) 2, (b) 5, (c) 10, and (d) 20% Cu-doped TiO2 anatase films showing the presence of Cu(0), Cu(I), and Cu(II) oxidation states.

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    Figure 4

    Figure 4. (a) Transient absorption spectra at select times for the 5% Cu-doped TiO2 thin film. (b) Recombination kinetics at 950 nm for all samples, where the dashed lines represent a fit to a power law decay model [f(t) = a·tb, where t is the time and a and b are variables].

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    Figure 5

    Figure 5. (a) FQEs obtained during the degradation of stearic acid under the UVA irradiation of Cu:TiO2 and undoped thin films. Blank glass and Activ samples are included for comparison. (b) IR spectra of stearic acid upon UVA illumination (I = 3.15 mW·cm–2) on a typical Cu-doped TiO2, 5% Cu:TiO2.

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    Figure 6

    Figure 6. Antimicrobial test results for pure and 5% doped films in the dark and under UVA conditions for 2 (a,c) and 4 h (b,d) against Gram-negative E. coli (a,b) and Gram-positive S. aureus (c,d) bacteria.

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    Figure 7

    Figure 7. Transition level diagram under both Ti-rich/O-poor (left) and Ti-poor/O-rich (right) growth regimes. The Fermi energy ranges from the VBM (0 eV) to the CBM (∼3.35 eV).

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    Figure 8

    Figure 8. Partial charge densities for the Cu-related defects. (a–c) Hole charge density for CuTi0, CuTi–, and CuTi2– defects as viewed along the (010) direction. (d,e) Electron charge density for Cui0 and Cui+, respectively, as viewed down the (100) direction. The hole density is shown in blue and the electron density in orange 0–0.015 eV Å–1.

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    Figure 9

    Figure 9. Configurational coordinate diagrams for VTi (a,b) and VO (c,d). (a,c) Absorption and subsequent emission of an electron to/from the CBM and (b,d) capture of an electron and subsequent emission from/to the VBM.

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    Figure 10

    Figure 10. One-dimensional configurational coordinate diagram for (a,c) CuTi and (b,d) Cui under electron and hole ionization, respectively. In each example, the absorption and emission energies are independent of the chemical potentials and therefore the growth conditions.

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      Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results
      Linsebigler, Amy L.; Lu, Guangquan; Yates, John T. , Jr.
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      A review with 160 refs. Interfacial processes and operating principles of heterogeneous photocatalysis are discussed. Electronic excitation processes in a mol. and in a semiconductor substrate are described. The electronic interactions between the adsorbate mol. and the catalyst substrate are described in terms of the catalyzed or sensitized photoreactions. Thermal and photocatalytic studies on TiO2 are summarized.
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    2. 2
      Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P.; Watson, G. W.; Keal, T. W.; Sherwood, P.; Walsh, A.; Sokol, A. A. Band alignment of rutile and anatase TiO2. Nat. Mater. 2013, 12, 798,  DOI: 10.1038/nmat3697
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      Band alignment of rutile and anatase TiO2
      Scanlon, David O.; Dunnill, Charles W.; Buckeridge, John; Shevlin, Stephen A.; Logsdail, Andrew J.; Woodley, Scott M.; Catlow, C. Richard A.; Powell, Michael. J.; Palgrave, Robert G.; Parkin, Ivan P.; Watson, Graeme W.; Keal, Thomas W.; Sherwood, Paul; Walsh, Aron; Sokol, Alexey A.
      Nature Materials (2013), 12 (9), 798-801CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO2. The discovery of the photolysis of water on the surface of TiO2 in 1972 launched four decades of intensive research into the underlying chem. and phys. processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO2. We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission expts., that a type-II, staggered, band alignment of ∼ 0.4eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust sepn. of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.
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    3. 3
      McCullagh, C.; Robertson, J. M. C.; Bahnemann, D. W.; Robertson, P. K. J. The application of TiO2 photocatalysis for disinfection of water contaminated with pathogenic micro-organisms: a review. Res. Chem. Intermed. 2007, 33, 359– 375,  DOI: 10.1163/156856707779238775
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      3
      The application of TiO2 photocatalysis for disinfection of water contaminated with pathogenic micro-organisms: a review
      McCullagh, Cathy; Robertson, Jeanette M. C.; Bahnemann, Detlef W.; Robertson, Peter K. J.
      Research on Chemical Intermediates (2007), 33 (3-5), 359-375CODEN: RCINEE; ISSN:0922-6168. (VSP)
      A review. The use of semiconductor photocatalysis for treatment of water and air was the topic of intense research activity over the past 20 years. This powerful process was also extended to the disinfection of environments contaminated with pathogenic microorganisms. This review summarizes recent developments concerned with the photocatalytic treatment of water contaminated with pathogenic microorganisms presenting a potential hazard to animals and human beings.
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    4. 4
      Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 photocatalysis: a historical overview and future prospects. Jpn. J. Appl. Phys. 2005, 44, 8269,  DOI: 10.1143/jjap.44.8269
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      4
      TiO2 photocatalysis: A historical overview and future prospects
      Hashimoto, Kazuhito; Irie, Hiroshi; Fujishima, Akira
      Japanese Journal of Applied Physics, Part 1: Regular Papers, Brief Communications & Review Papers (2005), 44 (12), 8269-8285CODEN: JAPNDE ISSN:. (Japan Society of Applied Physics)
      A review. Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has beer used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochem. reaction proceeding on a TiO2 surface when irradiated with UV light. One includes the photoinduced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. The authors review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.
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    5. 5
      Georgekutty, R.; Seery, M. K.; Pillai, S. C. A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism. J. Phys. Chem. C 2008, 112, 13563– 13570,  DOI: 10.1021/jp802729a
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      5
      A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties, and Mechanism
      Georgekutty, Reenamole; Seery, Michael K.; Pillai, Suresh C.
      Journal of Physical Chemistry C (2008), 112 (35), 13563-13570CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Highly photocatalytically active silver-modified ZnO has been prepd. and the effect of silver modification was studied. The structural and optical properties were characterized by x-ray diffraction, Fourier transform IR, differential scanning calorimetry, BET surface area, Raman, UV-vis, and photoluminescence spectroscopy. The photocatalytic activity of these materials was studied by analyzing the degrdn. of an org. dye, Rhodamine 6G (R6G), and it is found that 3 mol % silver-modified ZnO at 400° C shows approx. four times higher rate of degrdn. than that of unmodified ZnO and a three times higher rate than that of com. TiO2 photocatalyst Degussa P-25. It was also noted that the photocatalytic activity for the modified ZnO sample was five times higher than the unmodified sample using sunlight. The effect of silver in enhancing the photocatalytic activity has been studied by analyzing the emission properties of both ZnO and silver-modified ZnO in the presence (emission increases) and absence (emission decreases) of R6G. The authors attribute these observations to the extent of valence band hole prodn. and the role of silver in trapping the conduction band (CB) electrons in the absence of R6G. In the presence of R6G, the dye preserves the CB electron population in the metal oxide, thus preserving and enhancing emission intensity. The sensitizing property of the dye and electron scavenging ability of silver together constitute to the interfacial charge transfer process in such a way to utilize the photoexcited electrons.
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    6. 6
      Morrison, S. R.; Freund, T. Chemical role of holes and electrons in ZnO photocatalysis. J. Chem. Phys. 1967, 47, 1543– 1551,  DOI: 10.1063/1.1712115
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      6
      Chemical role of holes and electrons in zinc oxide photocatalysis
      Morrison, Stanley Roy; Freund, Thomas
      Journal of Chemical Physics (1967), 47 (4), 1543-52CODEN: JCPSA6; ISSN:0021-9606.
      The role of the electronic properties of a semiconductor in heterogeneous catalysis and electrochemistry was exptl. investigated on single-crystal ZnO. It was shown quant. that the availability of electrons and holes at the surface is dominant in the mechanism of a heterogeneously catalyzed reaction. Chem. rate measurements as well as in situ solid-state measurements were carried out in an aq. medium for the reaction: HCOOH + O2 → H2O2 + CO2, photocatalyzed by ZnO. Two new exptl. electrochem. methods for semiconductor surface reactions were developed: "current doubling" and measurement of "unfilled" electronic surface states by the capacitance. They were devised to characterize reactive sorbed species. The detailed catalytic mechanism was based on studies of individual reaction steps under three solid-state surface conditions: (1) only holes reacting, (2) only electrons reacting, and (3) both holes and electrons reacting. The first two correspond to the ZnO being an electrochem. anode and cathode, resp.; the last corresponds to ordinary catalysis when the hole current is balanced by the electron current so that the net current is zero. The overall catalyzed reaction is not simply the sum of individual oxidn. and redn. reactions which occur with only holes or only electrons, resp., but involves a surface intermediate formed when both holes and electrons are present.
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    7. 7
      Ali, T. T.; Narasimharao, K.; Parkin, I. P.; Carmalt, C. J.; Sathasivam, S.; Basahel, S. N.; Bawaked, S. M.; Al-Thabaiti, S. A. Effect of pretreatment temperature on the photocatalytic activity of microwave irradiated porous nanocrystalline ZnO. New J. Chem. 2015, 39, 321– 332,  DOI: 10.1039/c4nj01465k
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      7
      Effect of pretreatment temperature on the photocatalytic activity of microwave irradiated porous nanocrystalline ZnO
      Ali, Tarek T.; Narasimharao, Katabathini; Parkin, Ivan P.; Carmalt, Claire J.; Sathasivam, Sanjayan; Basahel, Sulaiman N.; Bawaked, Salem M.; Al-Thabaiti, Shaeel A.
      New Journal of Chemistry (2015), 39 (1), 321-332CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
      Porous nanocryst. ZnO photocatalysts were successfully synthesized by microwave irradn. and then thermally treated at different temps. (150 °C, 200 °C, 250 °C and 300 °C). The physico-chem. properties of synthesized samples were detd. by using different characterization techniques. The characterization results indicated that the as-synthesized sample was comprised of both ZnO and Zn(OH)2 phases with a particle size of approx. 50 nm. Thermal treatment of the as-synthesized sample at 150 °C resulted in a pure ZnO phase with a particle size of 40 nm. The results also demonstrated that the surface area, pore diam. and bandgap energy reach a max. value for the ZnO sample treated at 200 °C. The ZnO nanoparticles pretreated at 200 °C showed the highest photocatalytic activity (99% of degrdn.) in a short reaction time (90 min), which can be attributed to the combined effects of several factors including low crystallite size, relatively high surface area, pore diam., pore vol. and bandgap energy. Reusability results show that the catalysts can be readily sepd. from the reaction mixt. by filtration after the photocatalytic reaction and reused at least five times without any loss of activity.
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    8. 8
      Promdet, P.; Quesada-Cabrera, R.; Sathasivam, S.; Li, J.; Jiamprasertboon, A.; Guo, J.; Taylor, A.; Carmalt, C. J.; Parkin, I. P. High Defect Nanoscale ZnO Films with Polar Facets for Enhanced Photocatalytic Performance. ACS Appl. Nano Mater. 2019, 2, 2881– 2889,  DOI: 10.1021/acsanm.9b00326
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      8
      High defect nanoscale ZnO films with polar facets for enhanced photocatalytic performance
      Promdet, Premrudee; Quesada-Cabrera, Raul; Sathasivam, Sanjayan; Li, Jianwei; Jiamprasertboon, Arreerat; Guo, Jian; Taylor, Alaric; Carmalt, Claire J.; Parkin, Ivan P.
      ACS Applied Nano Materials (2019), 2 (5), 2881-2889CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      The fabrication of highly efficient photocatalytic thin films has important consequences for self-cleaning, org. pollutant decompn., and antimicrobial coatings for a variety of applications. Here, we developed a simple synthesis method to produce efficient, high-surface-area zinc oxide (ZnO) photocatalytic films using aerosol-assisted chem. vapor deposition. This approach used mixts. of methanol and acetic acid to promote preferential growth and exposure of polar facets, which favor photocatalytic activity. Interestingly, the initial enhanced efficiency of the films was correlated to structural defects, likely oxygen vacancies, as supported by photoluminescence spectroscopy results. Discussion over the influence of such defects on photocatalytic performance is described, and the need for strategies to develop high-surface-area materials contg. stable defects is highlighted.
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    9. 9
      Hassan, I. A.; Sathasivam, S.; Nair, S. P.; Carmalt, C. J. Antimicrobial properties of copper-doped ZnO coatings under darkness and white light illumination. ACS Omega 2017, 2, 4556– 4562,  DOI: 10.1021/acsomega.7b00759
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      9
      Antimicrobial Properties of Copper-Doped ZnO Coatings under Darkness and White Light Illumination
      Hassan, Iman A.; Sathasivam, Sanjayan; Nair, Sean P.; Carmalt, Claire J.
      ACS Omega (2017), 2 (8), 4556-4562CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      We report the first antimicrobial study of transparent and robust Cu doped ZnO coatings that displayed potent antimicrobial activity that resulted in bacterial (E.coli) redn. below detection limits within 6 h of illumination via a white light source that is found in hospital environments. The same bacterial redn. rate was obsd. even under darkness for 4.0 at.% Cu doped ZnO films. Thus providing an efficient 24-h disinfection. All films were produced via a novel inexpensive and easily scalable route and were also thoroughly analyzed for their material properties.
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    10. 10
      Li, Y.; Zhang, W.; Niu, J.; Chen, Y. Mechanism of photogenerated reactive oxygen species and correlation with the antibacterial properties of engineered metal-oxide nanoparticles. ACS Nano 2012, 6, 5164– 5173,  DOI: 10.1021/nn300934k
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      10
      Mechanism of Photogenerated Reactive Oxygen Species and Correlation with the Antibacterial Properties of Engineered Metal-Oxide Nanoparticles
      Li, Yang; Zhang, Wen; Niu, Junfeng; Chen, Yongsheng
      ACS Nano (2012), 6 (6), 5164-5173CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Oxidative stress induced by reactive oxygen species (ROS) is one of the most important antibacterial mechanisms of engineered nanoparticles (NPs). To elucidate the ROS generation mechanisms, we investigated the ROS prodn. kinetics of seven selected metal-oxide NPs and their bulk counterparts under UV irradn. (365 nm). The results show that different metal oxides had distinct photogenerated ROS kinetics. Particularly, TiO2 nanoparticles and ZnO nanoparticles generated three types of ROS (superoxide radical, hydroxyl radical, and singlet oxygen), whereas other metal oxides generated only one or two types or did not generate any type of ROS. Moreover, NPs yielded more ROS than their bulk counterparts likely due to larger surface areas of NPs providing more absorption sites for UV irradn. The ROS generation mechanism was elucidated by comparing the electronic structures (i.e., band edge energy levels) of the metal oxides with the redox potentials of various ROS generation, which correctly interpreted the ROS generation of most metal oxides. To develop a quant. relationship between oxidative stress and antibacterial activity of NPs, we examd. the viability of E. coli cells in aq. suspensions of NPs under UV irradn., and a linear correlation was found between the av. concn. of total ROS and the bacterial survival rates (R2 = 0.84). Although some NPs (i.e., ZnO and CuO nanoparticles) released toxic ions that partially contributed to their antibacterial activity, this correlation quant. linked ROS prodn. capability of NPs to their antibacterial activity as well as shed light on the applications of metal-oxide NPs as potential antibacterial agents.
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    11. 11
      Chadwick, N. P.; Glover, E. N. K.; Sathasivam, S.; Basahel, S. N.; Althabaiti, S. A.; Alyoubi, A. O.; Parkin, I. P.; Carmalt, C. J. Photo-activity and low resistivity in N/Nb Co-doped TiO2 thin films by combinatorial AACVD. J. Mater. Chem. A 2016, 4, 407– 415,  DOI: 10.1039/c5ta07922e
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      11
      Photo-activity and low resistivity in N/Nb Co-doped TiO2 thin films by combinatorial AACVD
      Chadwick, Nicholas P.; Glover, Emily N. K.; Sathasivam, Sanjayan; Basahel, Sulaiman N.; Althabaiti, Shaeel A.; Alyoubi, Abdulrahman O.; Parkin, Ivan P.; Carmalt, Claire J.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (2), 407-415CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A combinatorial aerosol assisted CVD (cAACVD) cation-anion co-doping study was undertaken for the 1st time, which studies the interplay of nitrogen and niobium co-dopants and the resultant functional properties within TiO2 thin films. This study advantageously creates a single doped TiO2 thin film which incorporates many compns. that transition from nitrogen doped TiO2 to niobium doped TiO2 across the film's width, in a single deposition. The film was split into a grid and the phys. properties of each grid position characterized by x-ray Diffraction (XRD), XPS, SEM and UV-visible transmission spectroscopy (UV/visible). Functional properties such as photocatalytic activity, water contact angles and resistivity were also characterized. The study was successful in creating and identifying the optimum dopant concn. at which these TiO2 films exhibited both a high rate of photo-activity and favorable transparent conducting oxide (TCO) properties. While most co-doping studies report relatively homogeneous film, the inhomogeneity of these films allows both functional properties to exist in conjunction. To the authors knowledge this is the 1st instance cation and anion co-doping was explored in the combinatorial regime.
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    12. 12
      Musat, V.; Teixeira, B.; Fortunato, E.; Monteiro, R. C. C.; Vilarinho, P. Al-doped ZnO thin films by sol–gel method. Surf. Coating. 2004, 180–181, 659– 662,  DOI: 10.1016/j.surfcoat.2003.10.112
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      12
      Al-doped ZnO thin films by sol-gel method
      Musat, V.; Teixeira, B.; Fortunato, E.; Monteiro, R. C. C.; Vilarinho, P.
      Surface and Coatings Technology (2004), 180-181 (), 659-662CODEN: SCTEEJ; ISSN:0257-8972. (Elsevier Science B.V.)
      Transparent and conductive high preferential c-axis oriented ZnO thin films doped with Al have been prepd. by the sol-gel method using zinc acetate and aluminum chloride as cation sources, 2-methoxiethanol as solvent, and monoethanolamine as sol stabilizer. Film deposition was performed by the dip-coating technique at a withdrawal rate of 1.5 cm min-1 on Corning 1737 glass substrate. The effect of dopant concn., heating treatment, and annealing in a reducing atm. on the microstructure as well as on the elec. and optical properties of the thin films is discussed. The optical transmittance spectra of the films showed a very good transmittance, between 85 and 95%, within the visible wavelength region. The min. resistivity of 1.3 × 10-3 Ω-cm was obtained for a film doped with 2 wt.% Al, preheated at 400°, and post-heated at 600°, after annealing under a reduced atm. of forming gas.
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    13. 13
      Matsubara, K.; Fons, P.; Iwata, K.; Yamada, A.; Sakurai, K.; Tampo, H.; Niki, S. ZnO transparent conducting films deposited by pulsed laser deposition for solar cell applications. Thin Solid Films 2003, 431–432, 369– 372,  DOI: 10.1016/s0040-6090(03)00243-8
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      13
      ZnO transparent conducting films deposited by pulsed laser deposition for solar cell applications
      Matsubara, K.; Fons, P.; Iwata, K.; Yamada, A.; Sakurai, K.; Tampo, H.; Niki, S.
      Thin Solid Films (2003), 431-432 (), 369-372CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science B.V.)
      Low resistivity and highly transparent ZnO conducting films for thin film solar cell applications were fabricated at low temp. by pulsed laser deposition. Al-, B- and Ga-doped ZnO films were deposited on Corning 7059 glass substrate at a substrate temp. of 200°. The Al-doped ZnO films were found to have the lowest resistivity of 2.5 × 10-4 Ω·cm and an av. optical transmission of 91% for wavelengths between 400 and 1100 nm. The values of the Ga-doped film were 2.5 × 10-4 Ω·cm and 81%, resp. Owing to the higher optical transmission in the near-IR region, the photovoltaic cell performance of a Cu(In,Ga)Se2 thin-film solar cell with an Al-doped ZnO window outperformed a cell fabricated with a Ga-doped ZnO window.
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    14. 14
      Page, K.; Palgrave, R. G.; Parkin, I. P.; Wilson, M.; Savin, S. L. P.; Chadwick, A. V. Titania and silver–titania composite films on glass—potent antimicrobial coatings. J. Mater. Chem. 2007, 17, 95– 104,  DOI: 10.1039/b611740f
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      14
      Titania and silver-titania composite films on glass-potent antimicrobial coatings
      Page, Kristopher; Palgrave, Robert G.; Parkin, Ivan P.; Wilson, Michael; Savin, Shelley L. P.; Chadwick, Alan V.
      Journal of Materials Chemistry (2007), 17 (1), 95-104CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      Titania (anatase) and Ag-doped titania (anatase) coatings were prepd. on glass microscope slides by a sol-gel dip-coating method. The resultant coatings were characterized by x-ray diffraction, x-ray absorption near edge structure (XANES), Raman, SEM, wavelength dispersive x-ray (WDX) anal., XPS and UV-vis techniques and shown to consist of anatase with ∼0.2-1 atom% Ag2O. Photocatalytic activity of the coatings was detd. by photomineralization of stearic acid, monitored by FT-IR spectroscopy. Photocatalytically-active coatings were screened for their antibacterial efficacy against Staphylococcus aureus (NCTC 6571), Escherichia coli (NCTC 10418) and Bacillus cereus (CH70-2). Ag-doped titania coatings were found to be significantly more photocatalytically and antimicrobially active than a titania coating. No antimicrobial activity was obsd. in the dark-indicating that silver ion diffusion was not the mechanism for antimicrobial action. The mode of action was explained in terms of a charge sepn. model. The coatings also demonstrated significantly higher activity against the Gram-pos. organisms than against the Gram-neg. The Ag2O-TiO2 coating is a potentially useful coating for hard surfaces in a hospital environment due to its robustness, stability to cleaning and reuse, and its excellent antimicrobial response.
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    15. 15
      Wu, B.; Huang, R.; Sahu, M.; Feng, X.; Biswas, P.; Tang, Y. J. Bacterial responses to Cu-doped TiO2 nanoparticles. Sci. Total Environ. 2010, 408, 1755– 1758,  DOI: 10.1016/j.scitotenv.2009.11.004
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      15
      Bacterial responses to Cu-doped TiO2 nanoparticles
      Wu, Bing; Huang, Rick; Sahu, Manoranjan; Feng, Xueyang; Biswas, Pratim; Tang, Yinjie J.
      Science of the Total Environment (2010), 408 (7), 1755-1758CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      The toxicity of Cu-doped TiO2 nanoparticles (NPs, 20 nm), synthesized by a flame aerosol reactor, to Mycobacterium smegmatis and Shewanella oneidensis MR-1, is the primary focus of this study. Both doped and non-doped TiO2 NPs (20 nm) tended to agglomerate in the medium soln., and therefore did not penetrate into the cell and damage cellular structures. TiO2 particles (<100 mg/L) did not apparently interfere with the growth of the two species in aq. cultures. Cu-doped TiO2 NPs (20 mg/L) significantly reduced the M. smegmatis growth rate by three fold, but did not affect S. oneidensis MR-1 growth. The toxicity of Cu-doped TiO2 NPs was driven by the release of Cu2+ from the parent NPs. Compared to equiv. amts. of Cu2+, Cu-doped TiO2 NPs exhibited higher levels of toxicity to M. smegmatis (P-value < 0.1). Addn. of EDTA in the culture appeared to significantly decrease the anti-mycobacterium activity of Cu-doped TiO2 NPs. S. oneidensis MR-1 produced a large amt. of extracellular polymeric substances (EPS) under NP stress, esp. extracellular protein. Therefore, S. oneidensis MR-1 was able to tolerate a much higher concn. of Cu2+ or Cu-doped TiO2 NPs. S. oneidensis MR-1 also adsorbed NPs on cell surface and enzymically reduced ionic copper in culture medium with a remediating rate of 61 μg/(liter/OD600/h) during its early exponential growth phase. Since the metal reducing Shewanella species can efficiently "clean" metal-oxide NPs, the activities of such environmentally relevant bacteria may be an important consideration for evaluating the ecol. risk of metal-oxide NPs.
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    16. 16
      Jaimy, K. B.; Safeena, V. P.; Ghosh, S.; Hebalkar, N. Y.; Warrier, K. G. K. Photocatalytic activity enhancement in doped titanium dioxide by crystal defects. Dalton Trans. 2012, 41, 4824– 4832,  DOI: 10.1039/C2DT12018F
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      16
      Photocatalytic activity enhancement in doped titanium dioxide by crystal defects
      Jaimy, Kanakkanmavudi B.; Safeena, V. P.; Ghosh, Swapankumar; Hebalkar, Neha Y.; Warrier, K. G. K.
      Dalton Transactions (2012), 41 (16), 4824-4832CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Visible light sensitive Fe3+ and Ce4+ co-doped nano TiO2 photocatalyst has been prepd. by a modified aq. sol-gel method and the activity has been measured in terms of degrdn. of MB dye. Both dopants shifted the absorption profile of TiO2 to the visible region and improved activity. Fe3+ ions trapped the charge carriers due to the stable electronic configuration and improved their sepn. Ce4+ ions, which were mainly located at the crystallites boundaries, cause dislocations by bending the valence and conduction bands of TiO2 and prevent the recombination of photoexcited electrons and holes. The co-doped TiO2 compns. exhibited higher photocatalytic activity than that of pure titania and com. available Degussa P25 under visible light by utilizing the individual and synergistic contributions of Fe3+ and Ce4+ dopants, resp.
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    17. 17
      Choi, W.; Termin, A.; Hoffmann, M. R. The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. A 1994, 98, 13669– 13679,  DOI: 10.1021/j100102a038
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      17
      The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics
      Choi, Wonyong; Termin, Andreas; Hoffmann, Michael R.
      Journal of Physical Chemistry (1994), 98 (51), 13669-79CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      A systematic study of metal ion doping in quantum (Q)-sized (2-4 nm) TiO2 colloids is performed by measuring their photoreactivities and the transient charge carrier recombination dynamics. The presence of metal ion dopants in the TiO2 cryst. matrix significantly influences photoreactivity, charge carrier recombination rates, and interfacial electron-transfer rates. The photoreactivities of 21 metal ion-doped colloids are quantified in terms of both the conduction band electron redn. of an electron acceptor (CCl4) Fe3+, Mo5+, Ru3+, Os3+, Re5+, V3+, and Rh3+ at 0.1-0.5 at.% significantly increases the photoreactivity for both oxidn. and redn. while Co3+ and Al3+ doping decreases the photoreactivity. The transient absorption signals upon laser flash photolysis (λex = 355 nm) at λ = 600 nm are extended up to 50 ms for Fe3+-, V4+-, Mo5+-, and Ru3+-doped TiO2 while the undoped Q-sized TiO2 shows a complete "blue electron" signal decay within 200 μs. Co3+- and Al3+-doped TiO2 are characterized by rapid signal decays with a complete loss of absorption signals within 5 μs. The quantum yields obtained during CW photolyses are quant. correlated with the measured transient absorption signals of the charge carriers. Photoreactivities are shown to increase with the relative concn. of trapped charge carriers. The photoreactivity of doped TiO2 appears to be a complex function of the dopant concn., the energy level of dopants within the TiO2 lattice, their d electronic configuration, the distribution of dopants, the electron donor concn., and the light intensity.
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    18. 18
      Hassan, I. A.; Parkin, I. P.; Nair, S. P.; Carmalt, C. J. Antimicrobial activity of copper and copper(I) oxide thin films deposited via aerosol-assisted CVD. J. Mater. Chem. B 2014, 2, 2855– 2860,  DOI: 10.1039/C4TB00196F
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      Antimicrobial activity of copper and copper(I) oxide thin films deposited via aerosol-assisted CVD
      Hassan, Iman A.; Parkin, Ivan P.; Nair, Sean P.; Carmalt, Claire J.
      Journal of Materials Chemistry B: Materials for Biology and Medicine (2014), 2 (19), 2855-2860CODEN: JMCBDV; ISSN:2050-7518. (Royal Society of Chemistry)
      Hospital acquired infections are prevalent in many hospitals and contaminated touch surfaces are a route of transmission. To find a soln. for this, copper and copper oxide thin films were deposited via aerosol-assisted chem. vapor deposition using copper nitrate as the precursor, and the films were characterized by a range of techniques, including powder X-ray diffraction and SEM. The antimicrobial activity of the deposited copper and copper oxide films were investigated against Staphylococcus aureus and Escherichia coli. A 5-log10 redn. in the viable counts of E. coli was obsd. on the copper thin films after 30 min while a 2-log10 redn. was obsd. on copper oxide films after 1 h. In the case of S. aureus, both copper and copper oxide films exhibited 4-log10 redn. after 1 h. The high antimicrobial efficacy of the Cu2O films, approaching that of the pure copper films, suggests that oxide formation on copper objects should not significantly impair their antimicrobial activity.
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      Ruparelia, J. P.; Chatterjee, A. K.; Duttagupta, S. P.; Mukherji, S. Strain specificity in antimicrobial activity of silver and copper nanoparticles. Acta Biomater. 2008, 4, 707– 716,  DOI: 10.1016/j.actbio.2007.11.006
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      19
      Strain specificity in antimicrobial activity of silver and copper nanoparticles
      Ruparelia, Jayesh P.; Chatterjee, Arup Kumar; Duttagupta, Siddhartha P.; Mukherji, Suparna
      Acta Biomaterialia (2008), 4 (3), 707-716CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
      The antimicrobial properties of silver and copper nanoparticles were investigated using Escherichia coli (four strains), Bacillus subtilis and Staphylococcus aureus (three strains). The av. sizes of the silver and copper nanoparticles were 3 nm and 9 nm, resp., as detd. through transmission electron microscopy. Energy-dispersive X-ray spectra of silver and copper nanoparticles revealed that while silver was in its pure form, an oxide layer existed on the copper nanoparticles. The bactericidal effect of silver and copper nanoparticles were compared based on diam. of inhibition zone in disk diffusion tests and min. inhibitory concn. (MIC) and min. bactericidal concn. (MBC) of nanoparticles dispersed in batch cultures. Bacterial sensitivity to nanoparticles was found to vary depending on the microbial species. Disk diffusion studies with E. coli and S. aureus revealed greater effectiveness of the silver nanoparticles compared to the copper nanoparticles. B. subtilis depicted the highest sensitivity to nanoparticles compared to the other strains and was more adversely affected by the copper nanoparticles. Good correlation was obsd. between MIC and MBC (r2 = 0.98) measured in liq. cultures. For copper nanoparticles a good neg. correlation was obsd. between the inhibition zone obsd. in disk diffusion test and MIC/MBC detd. based on liq. cultures with the various strains (r2 = -0.75). Although strain-specific variation in MIC/MBC was negligible for S. aureus, some strain-specific variation was obsd. for E. coli.
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    20. 20
      You, M.; Kim, T. G.; Sung, Y.-M. Synthesis of Cu-doped TiO2 nanorods with various aspect ratios and dopant concentrations. Cryst. Growth Des. 2010, 10, 983– 987,  DOI: 10.1021/cg9012944
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      20
      Synthesis of Cu-Doped TiO2 Nanorods with Various Aspect Ratios and Dopant Concentrations
      You, Minkyu; Kim, Tae-Geun; Sung, Yun-Mo
      Crystal Growth & Design (2010), 10 (2), 983-987CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
      Colloidal TiO2 nanorods were synthesized with different aspect ratios and successfully doped with copper via a controlled hydrolysis method. Inductively coupled plasma (ICP) at. emission spectroscopy analyses showed that the as-prepd. TiO2 nanorods contained ∼1.7-3.2 at.% Cu. XRD and high-resoln. TEM (HRTEM) analyses revealed that the doped nanorods are in a highly cryst. anatase structure and their crystal growth orientation is preferably [001]. The real doping of Ti lattices with Cu ions was evidenced by the analyses of surface compns. and chem. states of the nanorods using XPS. Through magnetic study using vibration sample magnetometry (VSM), it was verified that the Ti1-xCuxO2 nanorods maintain apparent ferromagnetic ordering at room temp. (300 K). The origin of the ferromagnetic property was explained based on the concn. of oxygen vacancies increased by Cu doping, which was also identified by the XPS analyses. The satn. magnetization showed strong dependency on the aspect ratio of nanorods as well as the Cu content in TiO2 nanorods.
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    21. 21
      Colón, G.; Maicu, M.; Hidalgo, M. C.; Navío, J. A. Cu-doped TiO2 systems with improved photocatalytic activity. Appl. Catal., B 2006, 67, 41– 51,  DOI: 10.1016/j.apcatb.2006.03.019
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      Cu-doped TiO2 systems with improved photocatalytic activity
      Colon, G.; Maicu, M.; Hidalgo, M. C.; Navio, J. A.
      Applied Catalysis, B: Environmental (2006), 67 (1-2), 41-51CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Photocatalytic oxidn. of phenol was performed over copper-doped TiO2 prepd. by a sol-gel method. Different prepn. methods were followed and a comparison with undoped system was also made. Wide structural and surface characterization of catalysts was carried out to establish a correlation between the effect of sulfuric and nitric acids present in the initial soln. and the Cu-TiO2 photocatalytic properties. The presence of sulfuric acid clearly stabilizes Cu-TiO2 and TiO2 structure and surface against sintering, maintaining anatase phase and relatively high surface area values with respect non sulfated Cu-TiO2 or TiO2. Best photocatalytic behavior is found for sulfated TiO2, as previously reported. In addn., incorporation of copper ions into the structure seems to enhance the photoactivity of the system for acidified systems, being the sulfated one the most favorable photocatalyst. The optimum metal loading is found for 0.5 M% of copper ion. A possible explanation of this photocatalytic improvement might be related to the stabilization of Cu2O species in doped TiO2 prepd. in the presence of sulfuric acid. This stabilization could be related to the presence of oxygen vacancies generated in the prepn. procedure using sulfuric acid.
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      Choudhury, B.; Dey, M.; Choudhury, A. Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles. Int. Nano Lett. 2013, 3, 25,  DOI: 10.1186/2228-5326-3-25
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      Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles
      Choudhury, Biswajit; Dey, Munmun; Choudhury, Amarjyoti
      International Nano Letters (2013), 3 (1), 25/1-25/8CODEN: INLNB4; ISSN:2008-9295. (Springer)
      TiO2 doped with Cu2+ initiates the formation of brookite phase along with anatase. Doping of Cu2+ introduces structural defects into TiO2. The direct evidence is the low intense and broad diffraction peaks. Raman peaks of doped TiO2 are also broad and are blueshifted. Pure TiO2 exhibits an absorption in the UV region, the position of which is shifted towards the visible region on incorporation of Cu into it. The visible absorption peaks arise due to the d-d transition of Cu2+ in the cryst. environment of TiO2. Incorporation of Cu2+ distorts the local structure of TiO2, resulting in the loss of octahedral symmetry surrounding Cu2+. The Jahn-Teller distortion splits the 2Egand 2T2g state of Cu2+ into several d states. Interaction of light excites the electron from ground to several of the excited states and gives the visible absorption peaks in the framework of TiO2. These Cu2+ d states and oxygen defects create band states, thereby favoring electronic transition to these levels and resulting in lowering of band gap of TiO2. A direct confirmation is the increase in the magnitude of Urbach energy with the redn. in the band gap of doped TiO2.
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      Park, H. S.; Kim, D. H.; Kim, S. J.; Lee, K. S. The photocatalytic activity of 2.5 wt% Cu-doped TiO2 nano powders synthesized by mechanical alloying. J. Alloys Compd. 2006, 415, 51– 55,  DOI: 10.1016/j.jallcom.2005.07.055
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      23
      The photocatalytic activity of 2.5wt% Cu-doped TiO2 nanopowders synthesized by mechanical alloying
      Park, Ha Sung; Kim, Dong Hyun; Kim, Sun Jae; Lee, Kyung Sub
      Journal of Alloys and Compounds (2006), 415 (1-2), 51-55CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      R to effectively utilize visible light, nanosized Cu-doped TiO2 powders were prepd. by mech. alloying and were characterized by XRD, TEM, DRS, and photoluminescence (PL) spectra. Their photocatalytic activity was measured for 4-chlorophenol photodegrdn. 2.5 Wt% of Cu element entered uniformly into the rutile TiO2 lattice with the av. grain size <15 nm. The Cu-doped rutile TiO2 had new emission peak at 448 nm indicating the decrease of the band gap energy of rutile TiO2. It exhibited an absorption edge at 480-490 nm longer than that for rutile TiO2 powder and led to a 2 times faster degrdn. of 4-chlorophenol.
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    24. 24
      Celik, E.; Gokcen, Z.; Ak Azem, N. F.; Tanoglu, M.; Emrullahoglu, O. F. Processing, characterization and photocatalytic properties of Cu doped TiO2 thin films on glass substrate by sol–gel technique. Mater. Sci. Eng., B 2006, 132, 258– 265,  DOI: 10.1016/j.mseb.2006.03.038
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      24
      Processing, characterization and photocatalytic properties of Cu doped TiO2 thin films on glass substrate by sol-gel technique
      Celik, E.; Gokcen, Z.; Ak Azem, N. F.; Tanoglu, M.; Emrullahoglu, O. F.
      Materials Science & Engineering, B: Solid-State Materials for Advanced Technology (2006), 132 (3), 258-265CODEN: MSBTEK; ISSN:0921-5107. (Elsevier B.V.)
      The present paper describes processing, properties and photocatalytic application of Cu doped TiO2 thin films on glass substrate. Cu doped TiO2 coatings were successfully prepd. on glass slide substrates using sol-gel method. The obtained solns. exhibit acidic characteristics. The phase structure, thermal, microstructure and surface properties of the coatings were characterized by using XRD, DTA/TG, SEM and AFM. Their adhesion properties and spectroscopic anal. were investigated by a scratch tester and UV-vis spectroscopy. Four different solns. were prepd. by changing Cu/Ti ratios. Glass substrates were coated by solns. of Ti-alkoxide, Cu-chloride, glacial acetic acid and isopropanol. The obtained gel films were dried at 300 °C for 10 min and subsequently heat-treated at 500 °C for 5 min in air. The oxide thin films were annealed at 600 °C for 60 min in air. TiO2, CuO, Cu4Ti, Ti3O5 and Cu3TiO4 phases were found in the coating. The org. matters were burned at temps. between 200 and 350 °C and TiO2 crystn. was formed at 450 °C. The wt. loss of the powder during process up to 600 °C is approx. 70%. The microstructural observations demonstrated that CuO content was led an improved surface morphol. while thickness of the film and surface defects were increased in accordance with no. of dipping. According to AFM results, it was found that as the Cu/Ti content increases the surface roughness of the films increases. In addn. structural, thermal and microstructural results, it was found that the films of 0.73 ratio have better adhesion strength to the glass substrate among other coatings. The oxide films were found to be active for photocatalytic decompn. of methylene blue.
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    25. 25
      Zhang, W.; Li, Y.; Zhu, S.; Wang, F. Copper doping in titanium oxide catalyst film prepared by dc reactive magnetron sputtering. Catal. Today 2004, 93–95, 589– 594,  DOI: 10.1016/j.cattod.2004.06.009
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      25
      Copper doping in titanium oxide catalyst film prepared by d.c. reactive magnetron sputtering
      Zhang, Wenjie; Li, Ying; Zhu, Shenglong; Wang, Fuhui
      Catalysis Today (2004), 93-95 (), 589-594CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Copper-doped titanium oxide films were prepd. by d.c. magnetron sputtering using Ti and Cu mixed target. The XPS spectra showed that titanium was in the Ti4+ oxidn. state and oxygen was in the form of O2- in TiO2 and CuO. Apparent high-intensity shake-up satellites of the main Cu 2p peaks indicated the existence of fully oxidized CuO in the Cu-doped TiO2 films. When the copper concn. was low, the Cu-doped TiO2 film had the similar anatase phase as pure TiO2. The samples became amorphous when copper concn. was more than 15.17 at.%. Copper oxides were in the amorphous state in all the films. In the sequence of decreasing copper concn., the surface morphologies changed from flat to nanocryst. with nano-sized pores. The absorption edges of the Cu-doped samples shifted to longer wavelength region and the optical transmittances of these films decreased abruptly with increasing copper concn. The Cu-doped TiO2 films had different photocatalytic behavior in accordance with the amt. of doped copper. The best copper doping concn. was 1.45 at.% in sample I, relating to the most effective photocatalytic activity.
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    26. 26
      Sathasivam, S.; Arnepalli, R. R.; Kumar, B.; Singh, K. K.; Visser, R. J.; Blackman, C. S.; Carmalt, C. J. Solution Processing of GaAs Thin Films for Photovoltaic Applications. Chem. Mater. 2014, 26, 4419– 4424,  DOI: 10.1021/cm501280e
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      26
      Solution Processing of GaAs Thin Films for Photovoltaic Applications
      Sathasivam, Sanjayan; Arnepalli, Ranga Rao; Kumar, Bhaskar; Singh, Kaushal K.; Visser, Robert J.; Blackman, Christopher S.; Carmalt, Claire J.
      Chemistry of Materials (2014), 26 (15), 4419-4424CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The authors present a novel route to high quality GaAs thin films via a soln. processing technique (aerosol assisted CVD) using a novel single source precursor [Me2GaAs(H)tBu]2. The thin films, grown on inexpensive glass substrates, were polycryst. in nature with a Ga to As ratio of 1:1. The morphol. studied via SEM showed the films to be smooth and consisting of compact domes. High-resoln. TEM (HRTEM) revealed the films to have columnar growth and an av. crystallite size of 90 nm. The films also contained low levels of contaminants as detd. via energy dispersive x-ray spectroscopy (EDX) mapping, XPS depth profiling, and secondary ion mass spectrometry (SIMS).
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    27. 27
      Sathasivam, S.; Arnepalli, R. R.; Singh, K. K.; Visser, R. J.; Blackman, C. S.; Carmalt, C. J. A solution based route to GaAs thin films from As (NMe 2) 3 and GaMe 3 for solar cells. RSC Adv. 2015, 5, 11812– 11817,  DOI: 10.1039/c4ra13902j
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      27
      A solution based route to GaAs thin films from As(NMe2)3 and GaMe3 for solar cells
      Sathasivam, Sanjayan; Arnepalli, Ranga R.; Singh, Kaushal K.; Visser, Robert J.; Blackman, Christopher S.; Carmalt, Claire J.
      RSC Advances (2015), 5 (16), 11812-11817CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The novel deposition of GaAs thin films on glass substrates from a soln. based route involving the aerosol assisted chem. vapor deposition (AACVD) of As(NMe2)3 and GaMe3 dissolved in toluene is reported. The gallium arsenide films were analyzed by SEM (SEM), X-ray powder diffraction (XRD), energy dispersive X-ray (EDX) anal., XPS and Raman spectroscopy. Powder XRD showed that cubic polycryst. GaAs had been deposited with films grown at the higher temps. having a Ga to As ratio of 1 : 1. EDX mapping, XPS depth profiling and SIMS showed that the films contained low levels of contaminants. The method described shows the formation of GaAs films with increasing crystallinity and stoichiometry reaching unity with increasing deposition temp.
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    28. 28
      Alotaibi, A. M.; Sathasivam, S.; Nair, S. P.; Parkin, I. P. Antibacterial properties of Cu–ZrO 2 thin films prepared via aerosol assisted chemical vapour deposition. J. Mater. Chem. B 2016, 4, 666– 671,  DOI: 10.1039/c5tb02312b
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      28
      Antibacterial properties of Cu-ZrO2 thin films prepared via aerosol assisted chemical vapour deposition
      Alotaibi, Abdullah M.; Sathasivam, Sanjayan; Nair, Sean P.; Parkin, Ivan P.
      Journal of Materials Chemistry B: Materials for Biology and Medicine (2016), 4 (4), 666-671CODEN: JMCBDV; ISSN:2050-7518. (Royal Society of Chemistry)
      The antibacterial properties of a Cu-ZrO2 film grown via aerosol assisted chem. vapor deposition are presented. The composite film showed high activity against E. coli (Gram-neg.) and S. aureus (Gram-pos.) bacteria with 5 log10 (E. coli) and 4 log10 (S. aureus) decrease in viable bacteria achieved within 20 and 60 min resp. These results were comparable to a pure copper film that was prepd. under the same conditions. The composite film was characterized for material properties using a range of techniques including X-ray photoemission and X-ray diffraction.
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    29. 29
      Bhachu, D. S.; Moniz, S. J. A.; Sathasivam, S.; Scanlon, D. O.; Walsh, A.; Bawaked, S. M.; Mokhtar, M.; Obaid, A. Y.; Parkin, I. P.; Tang, J.; Carmalt, C. J. Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity. Chem. Sci. 2016, 7, 4832– 4841,  DOI: 10.1039/c6sc00389c
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      29
      Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity
      Bhachu, Davinder S.; Moniz, Savio J. A.; Sathasivam, Sanjayan; Scanlon, David O.; Walsh, Aron; Bawaked, Salem M.; Mokhtar, Mohamed; Obaid, Abdullah Y.; Parkin, Ivan P.; Tang, Junwang; Carmalt, Claire J.
      Chemical Science (2016), 7 (8), 4832-4841CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The authors report the synthesis and photoelectrochem. assessment of phase pure tetragonal matlockite structured BiOX (X = Cl, Br, I) films. The materials were deposited using aerosol-assisted CVD. The measured optical bandgaps of the oxyhalides, supported by d. functional theory calcns., showed a red shift with the increasing size of halide following the binding energy of the anion p-orbitals that form the valence band. Stability and photoelectrochem. studies carried out without a sacrificial electron donor showed the n-type BiOBr film to have the highest photocurrent reported for BiOBr in the literature to date (0.3 mA cm-2 at 1.23 V vs. RHE), indicating it is an excellent candidate for solar fuel prodn. with a very low onset potential of 0.2 V vs. RHE. The high performance was attributed to the preferred growth of the film in the [011] direction, as shown by x-ray diffraction, leading to internal elec. fields that minimize charge carrier recombination.
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    30. 30
      Chadwick, N.; Sathasivam, S.; Kafizas, A.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N.; Parkin, I. P.; Carmalt, C. J. Combinatorial aerosol assisted chemical vapour deposition of a photocatalytic mixed SnO2/TiO2 thin film. J. Mater. Chem. A 2014, 2, 5108– 5116,  DOI: 10.1039/C4TA00545G
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      30
      Combinatorial aerosol assisted chemical vapor deposition of a photocatalytic mixed SnO2/TiO2 thin film
      Chadwick, Nicholas; Sathasivam, Sanjayan; Kafizas, Andreas; Bawaked, Salem M.; Obaid, Abdullah Y.; Al-Thabaiti, Shaeel; Basahel, Sulaiman N.; Parkin, Ivan P.; Carmalt, Claire J.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (14), 5108-5116CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Combinatorial Aerosol Assisted Chem. Vapor Deposition (cAACVD) was used to grow a thin film that graduated across its width from tin dioxide to titanium dioxide. This is a relatively new technique that can be used to create a variety of mixed phase and compn. thin films on a single substrate. Here cAACVD was used to deposit a mixed phase TiO2 and SnO2 film and compn. was related to UV photocatalysis, hydrophobicity and microstructure not inherent to anatase TiO2 or cassiterite SnO2. Characterization was achieved using X-ray diffraction (XRD), XPS, SEM (SEM), energy dispersive X-ray spectroscopy (EDX) and UV-Vis spectroscopy. Functional testing to elucidate the differences in functional properties across the film was undertaken by the photo-induced degrdn. of a resazurin 'intelligent' ink, a photo-induced wettability study and two-point resistivity measurements. Functional properties showed enhanced photocatalysis in comparison to Pilkington Activ® with similar formal quantum yield (mols. destroyed per absorbed photon) and formal quantum efficiency (mols. destroyed per incident photon) values.
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    31. 31
      Sathasivam, S.; Kafizas, A.; Ponja, S.; Chadwick, N.; Bhachu, D. S.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N.; Carmalt, C. J.; Parkin, I. P. Combinatorial Atmospheric Pressure CVD of a Composite TiO2/SnO2 Thin Film. Chem. Vap. Depos. 2014, 20, 69– 79,  DOI: 10.1002/cvde.201307081
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      Combinatorial atmospheric pressure CVD of composite TiO2/SnO2 thin film
      Sathasivam, Sanjayan; Kafizas, Andreas; Ponja, Sapna; Chadwick, Nicholas; Bhachu, Davinder S.; Bawaked, Salem M.; Obaid, Abdullah Y.; Al-Thabaiti, Shaeel; Basahel, Sulaiman N.; Carmalt, Claire J.; Parkin, Ivan P.
      Chemical Vapor Deposition (2014), 20 (1-2-3), 69-79CODEN: CVDEFX; ISSN:0948-1907. (Wiley-Blackwell)
      Combinatorial atm. pressure (cAP)CVD is used to deposit a film of graded compn. from mainly TiO2 to TiO2/SnO2 to mainly SnO2. This is the first cAPCVD study of a TiO2/SnO2 system. The thin film is characterized using a range of techniques such as X-ray diffraction (XRD), wavelength dispersive X-ray (WDX) spectroscopy, XPS, SEM (SEM), and ultra violet-visible (UV-vis) spectroscopy. It is found that, at various positions on the film, there are compns. of TiO2 and SnO2. The photocatalytic activity is examd. via the degrdn. of a Resazurin-based 'intelligent ink' under 365 nm wavelength irradn. The change in the concn. of the dye can be monitored by digital imaging alone. The results show how TiO2-rich regions are photocatalytically active, producing a max. formal quantum yield of 3.32 × 10-4 mols. per absorbed photon. The sheet resistance is detd. using a four-point probe via the van der Pauw method. The cond. is highest in the SnO2-rich and thicker regions of the film, however some of the composite regions of TiO2/SnO2 show both cond. and photocatalytic activity.
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    32. 32
      Ponja, S. D.; Sathasivam, S.; Parkin, I. P.; Carmalt, C. J. Transparent conductive aluminium and fluorine co-doped zinc oxide films via aerosol assisted chemical vapour deposition. RSC Adv. 2014, 4, 49723– 49728,  DOI: 10.1039/c4ra09997d
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      32
      Transparent conductive aluminum and fluorine co-doped zinc oxide films via aerosol assisted chemical vapour deposition
      Ponja, Sapna D.; Sathasivam, Sanjayan; Parkin, Ivan P.; Carmalt, Claire J.
      RSC Advances (2014), 4 (91), 49723-49728CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Aerosol assisted chem. vapor deposition (AACVD) was employed to synthesize highly transparent and conductive ZnO, fluorine or aluminum doped and aluminum-fluorine co-doped ZnO thin films on glass substrates at 450 °C. All films were characterized by X-ray diffraction (XRD), wavelength dispersive X-ray spectroscopy (WDX), XPS, SEM (SEM) and UV/Vis/Near IR spectroscopy. The films were 300-350 nm thick, cryst. and displayed high transparency at 550 nm (80-90%). The co-doped film consisted of 1 at.% fluorine and 2 at.% aluminum, exhibiting a charge carrier concn. and a charge carrier mobility of 3.47 × 1020 cm-3 and 9.7 cm2 V-1 s-1, resp. The band gap of the co-doped film was found to be 3.7 eV and the plasma edge crossover was ca. 1800 nm. This film had a highly structured morphol. in comparison to the un-doped and single doped ZnO films for transparent conducting oxide applications.
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      Knapp, C. E.; Carmalt, C. J. Solution based CVD of main group materials. Chem. Soc. Rev. 2016, 45, 1036– 1064,  DOI: 10.1039/c5cs00651a
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      Solution based CVD of main group materials
      Knapp, Caroline E.; Carmalt, Claire J.
      Chemical Society Reviews (2016), 45 (4), 1036-1064CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This crit. review focuses on the soln. based chem. vapor deposition (CVD) of main group materials with particular emphasis on their current and potential applications. Deposition of thin films of main group materials, such as metal oxides, sulfides and arsenides, have been researched owing to the array of applications which utilize them including solar cells, transparent conducting oxides (TCOs) and window coatings. Soln. based CVD processes, such as aerosol-assisted (AA)CVD have been developed due to their scalability and to overcome the requirement of suitably volatile precursors as the technique relies on the soly. rather than volatility of precursors which vastly extends the range of potentially applicable compds. An introduction into the applications and precursor requirements of main group materials will be presented first followed by a detailed discussion of their deposition reviewed according to this application. The challenges and prospects for further enabling research in terms of emerging main group materials will be discussed.
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    34. 34
      Alotaibi, A. M.; Sathasivam, S.; Williamson, B. A. D.; Kafizas, A.; Sotelo-Vazquez, C.; Taylor, A.; Scanlon, D. O.; Parkin, I. P. Chemical vapor deposition of photocatalytically active pure brookite TiO2 thin films. Chem. Mater. 2018, 30, 1353– 1361,  DOI: 10.1021/acs.chemmater.7b04944
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      34
      Chemical Vapor Deposition of Photocatalytically Active Pure Brookite TiO2 Thin Films
      Alotaibi, Abdullah M.; Sathasivam, Sanjayan; Williamson, Benjamin A. D.; Kafizas, Andreas; Sotelo-Vazquez, Carlos; Taylor, Alaric; Scanlon, David O.; Parkin, Ivan P.
      Chemistry of Materials (2018), 30 (4), 1353-1361CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Brookite is the least investigated phase of TiO2 due to the synthetic difficulty of obtaining the pure phase. Here, we present the first ever chem. vapor deposition synthesis of pure brookite TiO2 thin films. The films were highly cryst. and phase pure as detd. by X-ray diffraction and Raman spectroscopy studies. SEM studies showed the films to have a structured morphol. consisting of pyramidal features. The photocatalytic properties of the brookite film, tested using stearic acid under UVA (365 nm) irradn., were superior to both an anatase film grown under similar conditions and NSG Activ glass. Transient absorption spectroscopy showed that the innate electron-hole recombination dynamics are similar in brookite and anatase, akin to previous reports. The superior activity of the brookite film is hence attributed to the higher surface area compared to anatase.
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    35. 35
      Ponja, S. D.; Williamson, B. A. D.; Sathasivam, S.; Scanlon, D. O.; Parkin, I. P.; Carmalt, C. J. Enhanced electrical properties of antimony doped tin oxide thin films deposited via aerosol assisted chemical vapour deposition. J. Mater. Chem. C 2018, 6, 7257– 7266,  DOI: 10.1039/c8tc01929k
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      Enhanced electrical properties of antimony doped tin oxide thin films deposited via aerosol assisted chemical vapor deposition
      Ponja, Sapna D.; Williamson, Benjamin A. D.; Sathasivam, Sanjayan; Scanlon, David O.; Parkin, Ivan P.; Carmalt, Claire J.
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2018), 6 (27), 7257-7266CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Transparent conducting oxides have widespread application in modern society but there is a need to move away from the current 'industry champion' tin doped indium oxide (In2O3:Sn) due to high costs. Antimony doped tin(IV) oxide (ATO) is an excellent candidate but is limited by its opto-elec. properties. Here, the authors present a novel and scalable synthetic route to ATO thin films that shows excellent elec. properties. Resistivity measurements showed that at 4 at.% doping the lowest value of 4.7 × 10-4 Ω cm was achieved primarily due to a high charge carrier d. of 1.2 × 1021 cm-3. Further doping induced an increase in resistivity due to a decrease in both the carrier d. and mobility. Ab initio hybrid d. functional theory (DFT) calcns. show the thermodn. basis for the tail off of performance beyond a certain doping level, and the appearance of Sb(III) within the doped thin films.
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      Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558,  DOI: 10.1103/physrevb.47.558
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      Ab initio molecular dynamics of liquid metals
      Kresse, G.; Hafner, J.
      Physical Review B: Condensed Matter and Materials Physics (1993), 47 (1), 558-61CODEN: PRBMDO; ISSN:0163-1829.
      The authors present ab initio quantum-mech. mol.-dynamics calcns. based on the calcn. of the electronic ground state and of the Hellmann-Feynman forces in the local-d. approxn. at each mol.-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using sub-space alignment. This approach avoids the instabilities inherent in quantum-mech. mol.-dynamics calcns. for metals based on the use of a factitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows one to perform simulations over several picoseconds.
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      Mills, A.; Wang, J. Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films. J. Photochem. Photobiol., A 2006, 182, 181– 186,  DOI: 10.1016/j.jphotochem.2006.02.010
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      Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films
      Mills, Andrew; Wang, Jishun
      Journal of Photochemistry and Photobiology, A: Chemistry (2006), 182 (2), 181-186CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      The destruction of stearic acid (SA), the SA test, is a popular approach used to evaluate the activities of photocatalytic films. The destruction of SA via semiconductor photocatalysis is monitored simultaneously, using FT-IR spectroscopy, via the disappearance of SA and the appearance of CO2. Sol-gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 1015 mols. of SA cm-2 ≡ 1 cm-1 integrated areas of the peaks in the FT-IR of SA over the range 2700-3000 cm-1, which is three times that reported previously by others. As the SA disappeared the concomitant amt. of CO2 generated was >90% that expected throughout the photomineralization process for the sol-gel titania film. In contrast, the slightly more active, and scattering, P25 titania films generated CO2 levels that dipped as low as 69% during the course of the photoreaction, but eventually recovered to ≃100% that expected based on the amt. of SA present. The importance of these results with respect to SA test and the evaluation of new and existing self-cleaning films are discussed briefly.
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      Kresse, G.; Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal–amorphous-semiconductor transition in germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251,  DOI: 10.1103/physrevb.49.14251
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      Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium
      Kresse, G.; Hafner, J.
      Physical Review B: Condensed Matter and Materials Physics (1994), 49 (20), 14251-69CODEN: PRBMDO; ISSN:0163-1829.
      The authors present ab initio quantum-mech. mol.-dynamics simulations of the liq.-metal-amorphous-semiconductor transition in Ge. The simulations are based on (a) finite-temp. d.-functional theory of the 1-electron states, (b) exact energy minimization and hence calcn. of the exact Hellmann-Feynman forces after each mol.-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nose' dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows the authors to perform simulations over >30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liq. and amorphous Ge in very good agreement with expt.. The simulation allows the authors to study in detail the changes in the structure-property relation through the metal-semiconductor transition. The authors report a detailed anal. of the local structural properties and their changes induced by an annealing process. The geometrical, bounding, and spectral properties of defects in the disordered tetrahedral network are studied and compared with expt.
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      Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15– 50,  DOI: 10.1016/0927-0256(96)00008-0
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      Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
      Kresse, G.; Furthmuller, J.
      Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
      The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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      Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169,  DOI: 10.1103/physrevb.54.11169
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      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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      Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Ángyán, J. G. Screened hybrid density functionals applied to solids. J. Chem. Phys. 2006, 124, 154709,  DOI: 10.1063/1.2187006
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      Screened hybrid density functionals applied to solids
      Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Angyan, J. G.
      Journal of Chemical Physics (2006), 124 (15), 154709/1-154709/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Hybrid Fock exchange/d. functional theory functionals have shown to be very successful in describing a wide range of mol. properties. For periodic systems, however, the long-range nature of the Fock exchange interaction and the resultant large computational requirements present a major drawback. This is esp. true for metallic systems, which require a dense Brillouin zone sampling. Recently, a new hybrid functional [HSE03, J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)] that addresses this problem within the context of methods that evaluate the Fock exchange in real space was introduced. We discuss the advantages the HSE03 functional brings to methods that rely on a reciprocal space description of the Fock exchange interaction, e.g., all methods that use plane wave basis sets. Furthermore, we present a detailed comparison of the performance of the HSE03 and PBE0 functionals for a set of archetypical solid state systems by calcg. lattice parameters, bulk moduli, heats of formation, and band gaps. The results indicate that the hybrid functionals indeed often improve the description of these properties, but in several cases the results are not yet on par with std. gradient cor. functionals. This concerns in particular metallic systems for which the bandwidth and exchange splitting are seriously overestimated.
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      Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 2003, 118, 8207– 8215,  DOI: 10.1063/1.1564060
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      Hybrid functionals based on a screened Coulomb potential
      Heyd, Jochen; Scuseria, Gustavo E.; Ernzerhof, Matthias
      Journal of Chemical Physics (2003), 118 (18), 8207-8215CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Hybrid d. functionals are very successful in describing a wide range of mol. properties accurately. In large mols. and solids, however, calcg. the exact (Hartree-Fock) exchange is computationally expensive, esp. for systems with metallic characteristics. In the present work, we develop a new hybrid d. functional based on a screened Coulomb potential for the exchange interaction which circumvents this bottleneck. The results obtained for structural and thermodn. properties of mols. are comparable in quality to the most widely used hybrid functionals. In addn., we present results of periodic boundary condition calcns. for both semiconducting and metallic single wall carbon nanotubes. Using a screened Coulomb potential for Hartree-Fock exchange enables fast and accurate hybrid calcns., even of usually difficult metallic systems. The high accuracy of the new screened Coulomb potential hybrid, combined with its computational advantages, makes it widely applicable to large mols. and periodic systems.
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      Buckeridge, J.; Butler, K. T.; Catlow, C. R. A.; Logsdail, A. J.; Scanlon, D. O.; Shevlin, S. A.; Woodley, S. M.; Sokol, A. A.; Walsh, A. Polymorph engineering of TiO2: demonstrating how absolute reference potentials are determined by local coordination. Chem. Mater. 2015, 27, 3844– 3851,  DOI: 10.1021/acs.chemmater.5b00230
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      43
      Polymorph Engineering of TiO2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination
      Buckeridge, John; Butler, Keith T.; Catlow, C. Richard A.; Logsdail, Andrew J.; Scanlon, David O.; Shevlin, Stephen A.; Woodley, Scott M.; Sokol, Alexey A.; Walsh, Aron
      Chemistry of Materials (2015), 27 (11), 3844-3851CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      We report that the valence and conduction band energies of TiO2 can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an abs. energy ref., using an accurate multiscale quantum-chem. approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO2 as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO2 as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochem. water splitting through polymorph engineering of TiO2.
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      Çelik, V.; Mete, E. Range-separated hybrid exchange-correlation functional analyses of anatase TiO2 doped with W, N, S, W/N, or W/S. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 205112,  DOI: 10.1103/physrevb.86.205112
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      Range-separated hybrid exchange-correlation functional analyses of anatase TiO2 doped with W, N, S, W/N, or W/S
      Celik, Veysel; Mete, Ersen
      Physical Review B: Condensed Matter and Materials Physics (2012), 86 (20), 205112/1-205112/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The electronic properties and at. structures of W, N, S, W/N, and W/S dopings of anatase TiO2 have been systematically investigated using the d. functional theory. The exchange and correlation effects have been treated with the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional. Mixing traditional semilocal and nonlocal screened Hartree-Fock exchange energies, the HSE method corrects the band gap and also improves the description of anion/cation-derived gap states. Enhanced charge-carrier dynamics, obsd. for W/N codoped titania, can partly be explained by mutually passivating modifications of N 2p and W 5d states on its electronic structure. Following this trend, we have found an apparent band gap narrowing of 1.03 eV for W/S codoping. This is due to the large dispersion of S 3p states at the valance band top extending its edge to higher energies and Ti-S-W hybridization appearing at the bottom of the conduction band. W/S-TiO2 might show a strong visible-light response comparable to W/N codoped anatase catalysts.
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      Huy, H. A.; Aradi, B.; Frauenheim, T.; Deák, P. Calculation of carrier-concentration-dependent effective mass in Nb-doped anatase crystals of TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 155201,  DOI: 10.1103/physrevb.83.155201
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      Calculation of carrier-concentration-dependent effective mass in Nb-doped anatase crystals of TiO2
      Huy, Huynh Anh; Aradi, Balint; Frauenheim, Thomas; Deak, Peter
      Physical Review B: Condensed Matter and Materials Physics (2011), 83 (15), 155201/1-155201/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The calcd. optical effective masses of electrons, both orthogonal and parallel to the tetragonal axis of the crystal, are reported as a function of carrier concn. in Nb-doped anatase. The calcn. is based on the band structure, obtained by d. functional theory in the generalized gradient approxn. or by using the HSE06 screened hybrid functional. The anisotropy of the crystal and the nonparabolicity of the bands both were taken into account. In the concn. range relevant for transparent conductive oxide applications, the effective mass is detd. by several branches of the conduction band, leading to a complicated dependence on the carrier concn. The form of the function obtained with HSE06 is in line with expt. The strong anisotropy in the effective mass is confirmed, and very large values are predicted along the tetragonal axis for high carrier concns.
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      Janotti, A.; Varley, J.; Rinke, P.; Umezawa, N.; Kresse, G.; Van de Walle, C. Hybrid functional studies of the oxygen vacancy in TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085212,  DOI: 10.1103/physrevb.81.085212
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      Hybrid functional studies of the oxygen vacancy in TiO2
      Janotti, A.; Varley, J. B.; Rinke, P.; Umezawa, N.; Kresse, G.; Van de Walle, C. G.
      Physical Review B: Condensed Matter and Materials Physics (2010), 81 (8), 085212/1-085212/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The electronic and structural properties of the O vacancy (VO) in rutile TiO2 were studied using generalized Kohn-Sham theory with the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional for exchange and correlation. The HSE approach corrects the band gap and allows for a proper description of defects with energy levels close to the conduction band. According to the HSE calcns., VO is a shallow donor for which the +2 charge state is lower in energy than the neutral and +1 charge states for all Fermi-level positions in the band gap. The formation energy of VO2+ is relatively low in n-type TiO2 under O-poor conditions but it rapidly increases with the O chem. potential. This is consistent with exptl. observations where the elec. cond. decreases with O2 partial pressure.
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    47. 47
      Bhachu, D. S.; Sathasivam, S.; Sankar, G.; Scanlon, D. O.; Cibin, G.; Carmalt, C. J.; Parkin, I. P.; Watson, G. W.; Bawaked, S. M.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N. Solution processing route to multifunctional titania thin films: Highly conductive and photcatalytically active Nb: TiO2. Adv. Funct. Mater. 2014, 24, 5075– 5085,  DOI: 10.1002/adfm.201400338
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      Solution Processing Route to Multifunctional Titania Thin Films: Highly Conductive and Photcatalytically Active Nb:TiO2
      Bhachu, Davinder S.; Sathasivam, Sanjayan; Sankar, Gopinathan; Scanlon, David O.; Cibin, Giannantonio; Carmalt, Claire J.; Parkin, Ivan P.; Watson, Graeme W.; Bawaked, Salem M.; Obaid, Abdullah Y.; Al-Thabaiti, Shaeel; Basahel, Sulaiman N.
      Advanced Functional Materials (2014), 24 (32), 5075-5085CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This paper reports the synthesis of highly conductive niobium-doped titanium dioxide (Nb:TiO2) films from the decompn. of Ti(OEt)4 with dopant quantities of Nb(OEt)5 by aerosol-assisted chem. vapor deposition (AACVD). Doping Nb into the Ti sites results in n-type cond., as detd. by Hall effect measurements. The doped films display significantly improved elec. properties compared to pristine TiO2 films. For 5 at.% Nb in the films, the charge carrier concn. was 2 × 1021 cm-3 with a mobility of 2 cm2.V-1.s-1 . The corresponding sheet resistance was as low as 6.5 Ω.sq-1 making the films suitable candidates for transparent conducting oxide (TCO) materials. This is, to the best of our knowledge, the lowest reported sheet resistance for Nb:TiO2 films synthesized by vapor deposition. The doped films were also blue in color, with the intensity dependent on the Nb concn. in the films. A combination of synchrotron, lab. and theor. techniques confirmed niobium doping into the anatase TiO2 lattice. Computational methods also confirmed exptl. results of both delocalized (Ti4+) and localized polaronic states (Ti3+) states. Addnl., the doped films also functioned as photocatalysts. Thus, Nb:TiO2 combines four functional properties (photocatalysis, elec. cond., optical transparency and blue coloration) within the same layer, making it a promising alternative to conventional TCO materials.
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    48. 48
      Matsubara, M.; Saniz, R.; Partoens, B.; Lamoen, D. Doping anatase TiO 2 with group Vb and VI-b transition metal atoms: a hybrid functional first-principles study. Phys. Chem. Chem. Phys. 2017, 19, 1945– 1952,  DOI: 10.1039/c6cp06882k
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      Doping anatase TiO2 with group V-b and VI-b transition metal atoms: a hybrid functional first-principles study
      Matsubara, Masahiko; Saniz, Rolando; Partoens, Bart; Lamoen, Dirk
      Physical Chemistry Chemical Physics (2017), 19 (3), 1945-1952CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We investigate the role of transition metal atoms of group V-b (V, Nb, and Ta) and VI-b (Cr, Mo, and W) as n- or p-type dopants in anatase TiO2 using thermodn. principles and d. functional theory with the Heyd-Scuseria-Ernzerhof HSE06 hybrid functional. The HSE06 functional provides a realistic value for the band gap, which ensures a correct classification of dopants as shallow or deep donors or acceptors. Defect formation energies and thermodn. transition levels are calcd. taking into account the constraints imposed by the stability of TiO2 and the soly. limit of the impurities. Nb, Ta, W and Mo are identified as shallow donors. Although W provides two electrons, Nb and Ta show a considerably lower formation energy, in particular under O-poor conditions. Mo donates in principle one electron, but under specific conditions can turn into a double donor. V impurities are deep donors and Cr shows up as an amphoteric defect, thereby acting as an electron trapping center in n-type TiO2 esp. under O-rich conditions. A comparison with the available exptl. data yields excellent agreement.
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      Boonchun, A.; Reunchan, P.; Umezawa, N. Energetics of native defects in anatase TiO2: a hybrid density functional study. Phys. Chem. Chem. Phys. 2016, 18, 30040– 30046,  DOI: 10.1039/C6CP05798E
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      Energetics of native defects in anatase TiO2: a hybrid density functional study
      Boonchun, Adisak; Reunchan, Pakpoom; Umezawa, Naoto
      Physical Chemistry Chemical Physics (2016), 18 (43), 30040-30046CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The energetics and electronic structures of native defects in anatase TiO2 are comprehensively studied using hybrid d. functional calcns. We demonstrate that oxygen vacancies (VO) and titanium interstitials (Tii) act as shallow donors, and can form at substantial concns., giving rise to free electrons with carrier densities from 1011 to 1019 cm-3 under oxygen-rich and oxygen-poor conditions, resp. The titanium vacancies (VTi), identified as deep acceptors and induced hole carriers, are incapable of fully compensating for the free electrons originating from the donor-type defects at any oxygen chem. potential. Even under extreme oxygen-rich conditions, the Fermi level, which is detd. from the charge neutrality condition among charge defects, electron and hole carriers, is located 2.34 eV above the valence band max., indicating that p-type cond. can never be realized under any growth conditions without external doping. This is consistent with common observations of intrinsic n-type cond. of TiO2. At a typical annealing temp. and under a typical oxygen partial pressure, the carrier concn. is found to be approx. 5 × 1013 cm-3.
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      Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953,  DOI: 10.1103/physrevb.50.17953
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      Projector augmented-wave method
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      There is no expanded citation for this reference.
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      Morgan, B. J.; Watson, G. W. Intrinsic n-type defect formation in TiO2: a comparison of rutile and anatase from GGA+ U calculations. J. Phys. Chem. C 2010, 114, 2321– 2328,  DOI: 10.1021/jp9088047
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      Intrinsic n-type Defect Formation in TiO2: A Comparison of Rutile and Anatase from GGA+U Calculations
      Morgan, Benjamin J.; Watson, Graeme W.
      Journal of Physical Chemistry C (2010), 114 (5), 2321-2328CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The formations of intrinsic n-type defects, i.e., oxygen vacancies and titanium interstitials, in rutile and anatase TiO2 have been compared using GGA+U calcns. In both crystal structures, these defects give rise to states in the band gap, corresponding to electrons localized at Ti3+ centers. O vacancy formation in rutile results in two excess electrons occupying 3d orbitals on Ti atoms neighboring the vacancy. Similarly, for anatase, two Ti 3d orbitals are occupied by the excess electrons, with one of these Ti sites neighboring the vacancy, and the second at a next-nearest Ti position. This localization is accompanied by one oxygen moving toward the vacancy site to give a "split vacancy" geometry. A second fully localized soln. is also found for anatase, with both occupied Ti sites neighboring the vacancy site. This min. is 0.05 eV less stable than the split vacancy and is thus expected to be present in exptl. samples. A partially delocalized soln. corresponding to the split vacancy geometry, with one electron occupying the bottom of the conduction band, is also identified as 0.28 eV less stable. Formation of titanium interstitials donates four electrons to the Ti lattice. In anatase, one of these electrons is located at the interstitial Ti site, and three occupied defect states are hybridized between three nearest neighbor Ti sites. In rutile, these excess electrons are mostly localized at four nearest neighbor Ti sites, with only a small amt. of excess charge found on the interstitial Ti atom. This difference in the charge on the interstitial atom is a consequence of the differing interstitial geometries in the two polymorphs. Calcd. optical absorption spectra for all defects show significant decreases of the optical band gap, with a larger red shift predicted for titanium interstitials in anatase than in rutile. Defect formation energies have been calcd. under oxygen-rich and oxygen-poor conditions for both polymorphs. Under all conditions, O vacancy formation is slightly more favorable in anatase than in rutile, while Ti interstitials form more easily in rutile than anatase. Under O-rich conditions, O vacancies are the favored defect type, but both defect types have high formation energies. Under O-poor conditions, both defect types are stabilized, with Ti interstitials predicted to become the favored defect in rutile samples, particularly at elevated temps.
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      Rocquefelte, X.; Schwarz, K.; Blaha, P. Theoretical investigation of the magnetic exchange interactions in copper (II) oxides under chemical and physical pressures. Sci. Rep. 2012, 2, 759,  DOI: 10.1038/srep00759
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      Theoretical investigation of the magnetic exchange interactions in copper(II) oxides under chemical and physical pressures
      Rocquefelte, Xavier; Schwarz, Karlheinz; Blaha, Peter
      Scientific Reports (2012), 2 (), srep00759, 5 pp.CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      It remains a challenge to understand the unconventional mechanisms that cause high-TC supercond. in cuprate superconductors, high-TC multiferroicity in CuO, or low-dimensional magnetism in the spin-Peierls transition compds. such as CuGeO3. A common feature of all these copper oxide compds. (contg. Cu2+ ions) is the presence of large magnetic superexchange interactions J. It is a general strategy to apply chem. and/or phys. pressure in order to tune these exotic properties. Here we show theor., for the first time, the impact of phys. pressure on J on CuO, for which we predict a strong enhancement of the low-dimensionality of the magnetic interactions and the spin-frustration at high-pressures. Such modifications are expected to strongly influence the multiferroic properties of CuO. We finally demonstrate that PBE0 hybrid DFT calcns. provide reliable J values for a wide range of copper(II) oxides compds., i.e. CuGeO3, BaCu2Si2O7, BaCu2Ge2O7, and La2CuO4.
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      Kuz’menko, A. B.; Van Der Marel, D.; Van Bentum, P. J. M.; Tishchenko, E. A.; Presura, C.; Bush, A. A. Phonon anomalies versus magnetic ordering in CuO. Phys. B 2000, 284–288, 1396– 1397,  DOI: 10.1016/s0921-4526(99)02557-0
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      Phonon anomalies versus magnetic ordering in CuO
      Kuz'menko, A. B.; van der Marel, D.; van Bentum, P. J. M.; Tishchenko, E. A.; Presura, C.; Bush, A. A.
      Physica B: Condensed Matter (Amsterdam) (2000), 284-288 (), 1396-1397CODEN: PHYBE3; ISSN:0921-4526. (Elsevier Science B.V.)
      We report the first measurements of FIR and MIR reflectivity of single-crystal monoclinic antiferromagnetic copper monoxide (CuO) in a wide temp. range (7-300 K) when b-axis and ac-plane polarized modes are completely sepd. and excited in transverse geometry. Previously reported softening of the Au3 mode at the Neel transition is confirmed and found to be up to two times more significant (about 10%). For the first time an emergence of several new IR-active lines at low temps. is obsd. The highest frequency new mode at 690 cm-1 strongly hardens and strengthens with cooling. The data are discussed in terms of strong spin-phonon coupling and possible charge disproportionation at low T.
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      Scanlon, D. O.; Morgan, B. J.; Watson, G. W.; Walsh, A. Acceptor levels in p-type Cu 2 O: rationalizing theory and experiment. Phys. Rev. Lett. 2009, 103, 096405,  DOI: 10.1103/physrevlett.103.096405
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      Acceptor levels in p-type Cu2O. Rationalizing theory and experiment
      Scanlon, David O.; Morgan, Benjamin J.; Watson, Graeme W.; Walsh, Aron
      Physical Review Letters (2009), 103 (9), 096405/1-096405/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Understanding conduction in Cu2O is vital to the optimization of Cu-based p-type transparent conducting oxides. Using a screened hybrid-d.-functional approach we have investigated the formation of p-type defects in Cu2O giving rise to single-particle levels that are deep in the band gap, consistent with exptl. obsd. activated, polaronic conduction. Our calcd. transition levels for simple and split Cu vacancies explain the source of the 2 distinct hole states seen in DLTS expts. The necessity of techniques that go beyond the present generalized-gradient- and local-d.-approxn. techniques for accurately describing p-type defects in Cu(I)-based oxides is discussed.
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      Scanlon, D. O.; Morgan, B. J.; Watson, G. W. Modeling the polaronic nature of p-type defects in Cu 2 O: The failure of GGA and GGA+ U. J. Chem. Phys. 2009, 131, 124703,  DOI: 10.1063/1.3231869
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      Modeling the polaronic nature of p-type defects in Cu2O: The failure of GGA and GGA+U
      Scanlon, David O.; Morgan, Benjamin J.; Watson, Graeme W.
      Journal of Chemical Physics (2009), 131 (12), 124703/1-124703/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The exact nature of the hole traps reported deep in the band gap of Cu2O has been a topic of vigorous debate, with copper vacancies and oxygen interstitials both having been proposed as the relevant defects. In this article, the electronic structure of acceptor-forming defects in Cu2O, namely, copper vacancies and oxygen interstitials, is investigated using generalized gradient approxn. (GGA) and GGA cor. for on-site Coulombic interactions (GGA+U). GGA produces notionally semimetallic defect complexes, which is not consistent with the exptl. known polaronic nature of conduction in Cu2O. GGA+U also predicts a semimetallic defect complex for the "simple" copper vacancy but predicts the "split" vacancy and both oxygen interstitials are characterized by localized polarons, with distinct single particle levels found in the band gap. For both methods, however, the positions of calcd. transition levels are inconsistent with exptl. ionization levels. Hence neither GGA nor GGA+U are successful in modeling p-type defects in Cu2O. (c) 2009 American Institute of Physics.
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      Mittal, K. L. Adhesion measurement of thin films. Act. Passive Electron. Components 1976, 3, 21– 42,  DOI: 10.1155/apec.3.21
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      Lim, S. P.; Pandikumar, A.; Lim, H. N.; Ramaraj, R.; Huang, N. M. Boosting photovoltaic performance of dye-sensitized solar cells using silver nanoparticle-decorated N, S-Co-doped-TiO2 photoanode. Sci. Rep. 2015, 5, 11922,  DOI: 10.1038/srep11922
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      Boosting Photovoltaic Performance of Dye-Sensitized Solar Cells Using Silver Nanoparticle-Decorated N,S-Co-Doped-TiO2 Photoanode
      Lim Su Pei; Pandikumar Alagarsamy; Huang Nay Ming; Lim Hong Ngee; Ramaraj Ramasamy
      Scientific reports (2015), 5 (), 11922 ISSN:.
      A silver nanoparticle-decorated N,S-co-doped TiO2 nanocomposite was successfully prepared and used as an efficient photoanode in high-performance dye-sensitized solar cells (DSSCs) with N719 dye. The DSSCs assembled with the N,S-TiO2@Ag-modified photoanode demonstrated an enhanced solar-to-electrical energy conversion efficiency of 8.22%, which was better than that of a DSSC photoanode composed of unmodified TiO2 (2.57%) under full sunlight illumination (100 mWcm(-2), AM 1.5 G). This enhanced efficiency was mainly attributed to the reduced band gap energy, improved interfacial charge transfer, and retarded charge recombination process. The influence of the Ag content on the overall efficiency was also investigated, and the optimum Ag content with N,S-TiO2 was found to be 20 wt%. Because of the enhanced solar energy conversion efficiency of the N,S-TiO2@Ag nanocomposite, it should be considered as a potential photoanode for high-performance DSSCs.
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      Patel, S. K. S.; Gajbhiye, N. S. Room temperature magnetic properties of Cu-doped titanate, TiO2 (B) and anatase nanorods synthesized by hydrothermal method. Mater. Chem. Phys. 2012, 132, 175– 179,  DOI: 10.1016/j.matchemphys.2011.11.020
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      Room temperature magnetic properties of Cu-doped titanate, TiO2(B) and anatase nanorods synthesized by hydrothermal method
      Patel, S. K. S.; Gajbhiye, N. S.
      Materials Chemistry and Physics (2012), 132 (1), 175-179CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
      The Cu-doped hydrogen titanate nanorods were synthesized via a hydrothermal reaction and converted into Cu-doped TiO2(B) and anatase phases by calcinations. XRD, FTIR and selected area electron diffraction (SAED) pattern do not show any other phases and thus confirmed the intrinsic ferromagnetic behavior rather than it arising from metallic clusters. The blue shifting in absorption edge and increase in photoluminescence (PL) intensity from Cu-doped hydrogen titanate to anatase phase are strongly correlated with concn. of oxygen vacancies and defect formation. The higher concn. of oxygen vacancies and/or structural defects leads to the coalescence of ferromagnetic domains, which corroborates to the obsd. higher magnetization value of Cu-doped anatase phase compared to other phases.
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      Hasan, M. R.; Suhaimya, S. H. M.; Matb, A. N. C. A sol–gel derived, copper-doped, titanium dioxide–reduced graphene oxide nanocomposite electrode for the photoelectrocatalytic reduction of CO2 to methanol and formic acid. RSC Adv. 2015, 5, 77803,  DOI: 10.1039/c5ra12525a
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      A sol-gel derived, copper-doped, titanium dioxide-reduced graphene oxide nanocomposite electrode for the photoelectrocatalytic reduction of CO2 to methanol and formic acid
      Hasan, Md. Rakibul; Abd Hamid, Sharifah Bee; Basirun, Wan Jeffrey; Meriam Suhaimy, Syazwan Hanani; Che Mat, Ahmad Nazeer
      RSC Advances (2015), 5 (95), 77803-77813CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      A classic Cu-RGO-TiO2 photoelectrocatalyst was fabricated by a facile sol-gel method, deposited on ITO film via electrophoretic deposition and characterized by XRD, FESEM, UV-Vis and FT-IR spectroscopy. A uniformly distributed porous composite film was obsd. on the ITO substrate with an av. particle size of 18 nm. A lower photoluminescence response of the Cu-RGO-TiO2 sample indicates better electron/hole sepn. upon irradn. A max. 1.31 mA cm-2 photocurrent d. was obsd. at -0.61 V bias potential under solar simulator irradn. during CO2 photoelectrocatalysis. Formic acid and methanol were the main products, but longer reaction times led to increased methanol formation. The estd. current efficiency of the prodn. of formic acid and methanol was 32.47%, and the estd. rates of formation of formic acid and methanol were 255 μmol h-1 cm-2 and 189.06 μmol h-1 cm-2, resp.
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      Karunakaran, C.; Abiramasundari, G.; Gomathisankar, P.; Manikandan, G.; Anandi, V. Cu-doped TiO2 nanoparticles for photocatalytic disinfection of bacteria under visible light. J. Colloid Interface Sci. 2010, 352, 68– 74,  DOI: 10.1016/j.jcis.2010.08.012
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      Cu-doped TiO2 nanoparticles for photocatalytic disinfection of bacteria under visible light
      Karunakaran, C.; Abiramasundari, G.; Gomathisankar, P.; Manikandan, G.; Anandi, V.
      Journal of Colloid and Interface Science (2010), 352 (1), 68-74CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Two percent Cu-doped TiO2 nanoparticles were prepd. by a modified ammonia-evapn.-induced synthetic method, calcined at 450°, and characterized by powder x-ray diffraction, energy dispersive x-ray anal., ESR spectroscopy, SEM, UV-visible diffuse reflectance spectrum, photoluminescence spectroscopy, and electrochem. impedance spectroscopy. Doping shifts the optical absorption edge to the visible region but increases the charge-transfer resistance and decreases the capacitance. Under visible light, the composite nanoparticles very efficiently catalyze the disinfection of E. coli. The prepd. oxide is selective in photocatalysis; under UV light, its photocatalytic activity to degrade sunset yellow, rhodamine B, and methylene blue dyes is less than that of the undoped one.
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      Liqiang, J.; Xiaojun, S.; Baifu, X.; Baiqi, W.; Weimin, C.; Honggang, F. The preparation and characterization of La doped TiO2 nanoparticles and their photocatalytic activity. J. Solid State Chem. 2004, 177, 3375– 3382,  DOI: 10.1016/j.jssc.2004.05.064
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      Biesinger, M. C.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887– 898,  DOI: 10.1016/j.apsusc.2010.07.086
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      Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn
      Biesinger, Mark C.; Lau, Leo W. M.; Gerson, Andrea R.; Smart, Roger St. C.
      Applied Surface Science (2010), 257 (3), 887-898CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Chem. state x-ray photoelectron spectroscopic anal. of 1st row transition metals and their oxides and hydroxides is challenging due to the complexity of the 2p spectra resulting from peak asymmetries, complex multiplet splitting, shake-up and plasmon loss structure, and uncertain, overlapping binding energies. Current literature shows that all values necessary for reproducible, quant. chem. state anal. are usually not provided. A more consistent, practical and effective approach to curve-fitting the various chem. states in a variety of Sc, Ti, V, Cu and Zn metals, oxides and hydroxides is reported. The curve-fitting procedures proposed are based on a combination of (1) std. spectra from quality ref. samples, (2) a survey of appropriate literature databases and/or a compilation of the literature refs., and (3) specific literature refs. where fitting procedures are available. Binding energies, full-width at half max. (FWHM) values, spin-orbit splitting values, asym. peak-shape fitting parameters, and, for Cu and Zn, Auger parameters values are presented. The quantification procedure for Cu species details the use of the shake-up satellites for Cu(II)-contg. compds. and the exact binding energies of the Cu(0) and Cu(I) peaks. The use of the modified Auger parameter for Cu and Zn species allows for corroborating evidence when there is uncertainty in the binding energy assignment. These procedures can remove uncertainties in anal. of surface states in nano-particles, corrosion, catalysis and surface-engineered materials.
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      Deroubaix, G.; Marcus, P. X-ray photoelectron spectroscopy analysis of copper and zinc oxides and sulphides. Surf. Interface Anal. 1992, 18, 39– 46,  DOI: 10.1002/sia.740180107
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      X-ray photoelectron spectroscopy analysis of copper and zinc oxides and sulfides
      Deroubaix, G.; Marcus, P.
      Surface and Interface Analysis (1992), 18 (1), 39-46CODEN: SIANDQ; ISSN:0142-2421.
      Various copper and zinc compds. (Cu, Zn, Cu-Zn alloy, Cu2O, CuO, Cu2S, ZnO, ZnS) have been synthesized and characterized by electron spectroscopy (XPS). Data on the 2p3/2 levels of Cu, Zn, and S, the 1s level of O, the L3M45M45 Auger transitions of Cu and Zn, the valence bands, and the modified Auger parameters are reported. The results obtained with these ref. compds. allow qual. and quant. interpretation of XPS analyses on mixts. of these compds.
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    64. 64
      Pesci, F. M.; Cowan, A. J.; Alexander, B. D.; Durrant, J. R.; Klug, D. R. Charge carrier dynamics on mesoporous WO3 during water splitting. J. Phys. Chem. Lett. 2011, 2, 1900– 1903,  DOI: 10.1021/jz200839n
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      64
      Charge Carrier Dynamics on Mesoporous WO3 during Water Splitting
      Pesci, Federico M.; Cowan, Alexander J.; Alexander, Bruce D.; Durrant, James R.; Klug, David R.
      Journal of Physical Chemistry Letters (2011), 2 (15), 1900-1903CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Transient absorption spectroscopy (TAS) has been employed to identify the spectra of photogenerated electrons and holes on WO3. WO3 is a widely studied photoanode for the oxidn. of water, and by measuring the decay of photoholes on the milliseconds to seconds time scale(coating process) in the presence of an electron scavenger, we provide a lower limit for the required lifetime of holes for water oxidn. on WO3. The rate of electron/hole recombination on WO3 has also been examd. at a range of excitation intensities and is found to be nonlinear with excitation intensity.
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    65. 65
      Pendlebury, S. R.; Wang, X.; Le Formal, F.; Cornuz, M.; Kafizas, A.; Tilley, S. D.; Grätzel, M.; Durrant, J. R. Ultrafast charge carrier recombination and trapping in hematite photoanodes under applied bias. J. Am. Chem. Soc. 2014, 136, 9854– 9857,  DOI: 10.1021/ja504473e
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      65
      Ultrafast Charge Carrier Recombination and Trapping in Hematite Photoanodes under Applied Bias
      Pendlebury, Stephanie R.; Wang, Xiuli; Le Formal, Florian; Cornuz, Maurin; Kafizas, Andreas; Tilley, S. David; Gratzel, Michael; Durrant, James R.
      Journal of the American Chemical Society (2014), 136 (28), 9854-9857CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ∼6 ps, ∼103 times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps-μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the assocd. electron trapping are discussed in terms of their implications for efficient water oxidn.
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    66. 66
      Fujishima, A.; Zhang, X.; Tryk, D. TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 2008, 63, 515– 582,  DOI: 10.1016/j.surfrep.2008.10.001
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      TiO2 photocatalysis and related surface phenomena
      Fujishima, Akira; Zhang, Xintong; Tryk, Donald A.
      Surface Science Reports (2008), 63 (12), 515-582CODEN: SSREDI; ISSN:0167-5729. (Elsevier B.V.)
      The field of photocatalysis can be traced back more than 80 years to early observations of the chalking of titania-based paints and to studies of the darkening of metal oxides in contact with org. compds. in sunlight. During the past 20 years, it has become an extremely well researched field due to practical interest in air and water remediation, self-cleaning surfaces, and self-sterilizing surfaces. During the same period, there has also been a strong effort to use photocatalysis for light-assisted prodn. of hydrogen. The fundamental aspects of photocatalysis on the most studied photocatalyst, titania, are still being actively researched and have recently become quite well understood. The mechanisms by which certain types of org. compds. are decompd. completely to carbon dioxide and water, for example, have been delineated. However, certain aspects, such as the photo-induced wetting phenomenon, remain controversial, with some groups maintaining that the effect is a simple one in which org. contaminants are decompd., while other groups maintain that there are addnl. effects in which the intrinsic surface properties are modified by light. During the past several years, powerful tools such as surface spectroscopic techniques and scanning probe techniques performed on single crystals in ultra-high vacuum, and ultrafast pulsed laser spectroscopic techniques have been brought to bear on these problems, and new insights have become possible. Quantum chem. calcns. have also provided new insights. New materials have recently been developed based on titania, and the sensitivity to visible light has improved. The new information available is staggering, but we hope to offer an overview of some of the recent highlights, as well as to review some of the origins and indicate some possible new directions.
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    67. 67
      Wang, X.; Kafizas, A.; Li, X.; Moniz, S. J. A.; Reardon, P. J. T.; Tang, J.; Parkin, I. P.; Durrant, J. R. Transient absorption spectroscopy of anatase and rutile: the impact of morphology and phase on photocatalytic activity. J. Phys. Chem. C 2015, 119, 10439– 10447,  DOI: 10.1021/acs.jpcc.5b01858
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      67
      Transient Absorption Spectroscopy of Anatase and Rutile: The Impact of Morphology and Phase on Photocatalytic Activity
      Wang, Xiuli; Kafizas, Andreas; Li, Xiaoe; Moniz, Savio J. A.; Reardon, Philip J. T.; Tang, Junwang; Parkin, Ivan P.; Durrant, James R.
      Journal of Physical Chemistry C (2015), 119 (19), 10439-10447CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors employed transient absorption spectroscopy (TAS) to study the kinetic dependences of photocatalysis in anatase and rutile TiO2 films of varying morphol. In mesoporous films, anatase was ∼30 times more efficient than rutile in the photocatalytic degrdn. of an intelligent ink model system. Independent of phase, up to 100 lower levels of photocatalysis were found in dense films. Charge carrier lifetimes were probed by TAS on the microsecond to second time scale. For both rutile and anatase, recombination was independent of morphol. Rutile exhibited up to 10 times slower recombination kinetics than anatase. Efficient, irreversible hole scavenging by alcs. was present in mesoporous anatase alone, giving long-lived electrons (τ ≈ 0.7 s) which, upon the addn. of the dye redn. target resazurin, enabled efficient electron transfer (τ ≈ 3 ms). Hole scavenging by alcs. on mesoporous rutile was substantially less efficient and more reversible than anatase, resulting in only a marginal increase in electron lifetime. The lower activity of rutile was not due to differences in recombination but rather to the deficiency of rutile holes to drive efficient and irreversible alc. oxidn.
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    68. 68
      Kafizas, A.; Wang, X.; Pendlebury, S. R.; Barnes, P.; Ling, M.; Sotelo-Vazquez, C.; Quesada-Cabrera, R.; Li, C.; Parkin, I. P.; Durrant, J. R. Where do photogenerated holes go in anatase: rutile TiO2 A transient absorption spectroscopy study of charge transfer and lifetime. J. Phys. Chem. A 2016, 120, 715– 723,  DOI: 10.1021/acs.jpca.5b11567
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      68
      Where Do Photogenerated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime
      Kafizas, Andreas; Wang, Xiuli; Pendlebury, Stephanie R.; Barnes, Piers; Ling, Min; Sotelo-Vazquez, Carlos; Quesada-Cabrera, Raul; Li, Can; Parkin, Ivan P.; Durrant, James R.
      Journal of Physical Chemistry A (2016), 120 (5), 715-723CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood. Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase compn. By exploiting the different signatures in the photoinduced absorption of trapped charges in anatase and rutile, we were able to sep. track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photogenerated holes transfer from rutile to anatase on submicrosecond time scales. This hole transfer can significantly increase the anatase hole yield, with a 20:80 anatase:rutile composite showing a 5-fold increase in anatase holes obsd. from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irresp. of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for detg. the direction of photogenerated charge sepn. across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.
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    69. 69
      Wang, L.; McCleese, C.; Kovalsky, A.; Zhao, Y.; Burda, C. Femtosecond time-resolved transient absorption spectroscopy of CH3NH3PbI3 perovskite films: evidence for passivation effect of PbI2. J. Am. Chem. Soc. 2014, 136, 12205– 12208,  DOI: 10.1021/ja504632z
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      69
      Femtosecond Time-Resolved Transient Absorption Spectroscopy of CH3NH3PbI3 Perovskite Films: Evidence for Passivation Effect of PbI2
      Wang, Lili; McCleese, Christopher; Kovalsky, Anton; Zhao, Yixin; Burda, Clemens
      Journal of the American Chemical Society (2014), 136 (35), 12205-12208CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      MeNH3PbI3 perovskite layered films deposited on substrates with and without a TiO2 support structure were prepd. and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450-800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. For the 1st time the authors could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red shifted at ∼780 nm. An addnl. bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to est. relative amts. of PbI2 in the samples. Kinetic anal. reveals that perovskite films with less PbI2 show faster relaxation rates than those contg. more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples contg. PbI2 are due to a passivation effect, in line with other recently reported work.
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    70. 70
      Clarke, T.; Ballantyne, A.; Jamieson, F.; Brabec, C.; Nelson, J.; Durrant, J. Transient absorption spectroscopy of charge photogeneration yields and lifetimes in a low bandgap polymer/fullerenefilm. Chem. Commun. 2008, 89,  DOI: 10.1039/B813815J
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      Anderson, L. J. E.; Mayer, K. M.; Fraleigh, R. D.; Yang, Y.; Lee, S.; Hafner, J. H. Quantitative measurements of individual gold nanoparticle scattering cross sections. J. Phys. Chem. C 2010, 114, 11127– 11132,  DOI: 10.1021/jp1040663
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      71
      Quantitative Measurements of Individual Gold Nanoparticle Scattering Cross Sections
      Anderson, Lindsey J. E.; Mayer, Kathryn M.; Fraleigh, Robert D.; Yang, Yi; Lee, Seunghyun; Hafner, Jason H.
      Journal of Physical Chemistry C (2010), 114 (25), 11127-11132CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Dark field microspectroscopy is a powerful tool for studying plasmon resonances of noble metal nanoparticles and for developing their applications in sensing and imaging. Here we calibrate a dark field microspectrometer with measurements on gold nanospheres in a uniform dielec. medium to yield quant. spectral scattering cross sections for elongated nanoparticle shapes. Gold bipyramids, 135 nm in length, were found to have a peak differential cross section of 1.2 × 10-16 m2. Measurements of a small ensemble of gold nanorods, 13 nm in diam. and 45 nm in length on av., were found to have a peak differential cross section of only 1 × 10-18 m2. For the smaller gold nanorods, approx. expressions for the total scattering cross section may be used to indicate their scattering signal in microscopy applications.
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    72. 72
      Wang, L.; Wang, H.-Y.; Gao, B.-R.; Pan, L.-Y.; Jiang, Y.; Chen, Q.-D.; Han, W.; Sun, H.-B. Transient absorption spectroscopic study on band-structure-type change in CdTe/CdS core-shell quantum dots. IEEE J. Quantum Electron. 2011, 47, 1177– 1184,  DOI: 10.1109/jqe.2011.2159853
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      Transient absorption spectroscopic study on band-structure-type change in CdTe/CdS core-shell quantum dots
      Wang, Lei; Wang, Hai-Yu; Gao, Bing-Rong; Pan, Ling-Yun; Jiang, Ying; Chen, Qi-Dai; Han, Wei; Sun, Hong-Bo