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Combined Structural and Plasmonic Enhancement of Nanometer-Thin Film Photocatalysis for Solar-Driven Wastewater Treatment

  • Desislava Daskalova
    Desislava Daskalova
    Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
    Chair of Electronic Devices, RWTH Aachen University, 52074 Aachen, Germany
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    Orcidhttps://orcid.org/0000-0002-7636-413X
  • , 
  • Gonzalo Aguila Flores
    Gonzalo Aguila Flores
    Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
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  • Ulrich Plachetka
    Ulrich Plachetka
    Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
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  • Michael Möller
    Michael Möller
    Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
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  • Julia Wolters
    Julia Wolters
    Institute of Environmental Engineering, RWTH Aachen University, 52074 Aachen, Germany
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  • Thomas Wintgens
    Thomas Wintgens
    Institute of Environmental Engineering, RWTH Aachen University, 52074 Aachen, Germany
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  • , and 
  • Max C. Lemme*
    Max C. Lemme
    Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
    Chair of Electronic Devices, RWTH Aachen University, 52074 Aachen, Germany
    *Email: [email protected]. Phone: +49 2418867200.
    More by Max C. Lemme
    Orcidhttps://orcid.org/0000-0003-4552-2411
Cite this: ACS Appl. Nano Mater. 2023, 6, 16, 15204–15212
Publication Date (Web):August 16, 2023
https://doi.org/10.1021/acsanm.3c02867

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Titanium dioxide (TiO2) thin films are commonly used as photocatalytic materials. Here, we enhance the photocatalytic activity of devices based on titanium dioxide (TiO2) by combining nanostructured glass substrates with metallic plasmonic nanostructures. We achieve a three-fold increase of the catalyst’s surface area through nanoscale, three-dimensional patterning of periodic, conical grids, which creates a broadband optical absorber. The addition of aluminum and gold activates the structures plasmonically and increases the optical absorption in the TiO2 films to above 70% in the visible and NIR spectral range. We demonstrate the resulting enhancement of the photocatalytic activity with organic dye degradation tests under different light sources. Furthermore, the pharmaceutical drug Carbamazepine, a common water pollutant, is reduced in the aqueous solution by up to 48% in 360 min. Our approach is scalable and potentially enables future solar-driven wastewater treatment.

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Introduction

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The use of titanium dioxide (TiO2) nanoparticles or porous films is well-established in heterogeneous photocatalysis. Under ultraviolet (UV) light irradiation, TiO2 absorbs highly energetic photons, generating electron–hole pairs. These photoexcited charge carriers can then migrate to the surface of the semiconductor to participate in advanced oxidation processes. Owing to their large surface-to-volume ratio, TiO2 nanoparticles in different modifications have been demonstrated to possess excellent photocatalytic performance at short wavelengths. (1) However, certain applications like wastewater treatment require stable TiO2 films instead of nanoparticles because of the inherent difficulties to filter and reuse the nanoparticles. (2) In addition, introducing large amounts of engineered nanomaterials into the biosphere poses toxicological and environmental risks. (3)
As an alternative, the advancement and wide availability of thin film technologies enable the application of thin film photocatalysis on an industrial scale. Nevertheless, thin film photocatalysts lack in efficiency compared to nanoparticles due to their lower surface area and fewer reactive sites. Hence, different strategies have been employed to improve their performance, such as thermally induced nanocrack networks in sputtered TiO2 films to enhance the surface area, (4) or semiconductor heterojunctions to enhance photocatalytic activity under UV (5,6) and extend the absorption to the visible range. (7) Utilizing plasmonic nanostructures to enhance photocatalysis is another promising route toward the commercialization of solar-driven photocatalysis. (8,9) It is based on the collective oscillations of electrons in the metal under illumination (plasmons), which generate intense and strongly localized electric fields in the vicinity of the metallic nanostructures. These plasmonic “hot spots”, when coupled to a semiconducting photocatalyst, can then aid photocatalysis via effects like strong light absorption and scattering, electromagnetic field enhancement, and hot carrier generation. While its fundamentals are well understood, the interplay between different mechanisms of enhancement, like direct electron transfer and plasmonic resonance electron transfer at the metal–semiconductor junction, is still under discussion, (10) in particular when moving away from the nanoparticle picture toward more complex nanostructures or multilayer systems. (11) Designing novel devices and evaluating their performance therefore remain key to progress toward functional and sustainable solutions for advanced wastewater treatment.
Here, we present a combination of a periodically nanostructured broadband light-trapping three-dimensional surface, plasmonic metals like gold and aluminum (Au and Al), and a thin TiO2 layer deposited by atomic layer deposition (ALD). Our approach avoids nanostructuring of noble metals, which is typically achieved by complex and time-consuming wet chemical processing. (12) Furthermore, similar to TiO2 nanoparticles, the impact of such metal nanoparticles on living organisms and the environment remains unclear. (13−15) We circumvent these issues by incorporating the nanostructures in a glass (SiO2) substrate and then creating different material stacks with high surface areas on top of it. We demonstrate the efficiency of our photocatalytic devices through the reduction of methylene blue (MB) dye under different illumination wavelengths. Moreover, we show the successful degradation of the pharmaceutical pollutant carbamazepine under UV-A light using a nanostructured Au surface. Finally, we discuss our results with respect to the state of the art.

Results and Discussion

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We carried out photocatalysis experiments on five different sample types to evaluate the influence of the nanostructures alone versus their combination with the plasmonic metals (Figure 1a, for process details see Methods section). The first reference sample (labeled “SiO2” in figures) was a bare fused silica wafer cut to the standard experiment size, which was assessed to determine the effects of photolysis (if present). The flat surface sample with 25 nm TiO2 was included again for investigating not only photolysis but also standard photocatalysis of the TiO2 itself (labeled “TiO2”). The third reference sample, 25 nm TiO2 on nanostructured SiO2, is expected to utilize the effects of light trapping and surface area increase (labeled “cones + TiO2”). The last two samples differ in the choice of metal (Al and Au) and comprise identically nanostructured SiO2 covered with each metal, followed by the 25 nm TiO2 ALD layer (labeled “cones + Al + TiO2” and “cones + Au + TiO2”). These samples combine the light trapping and surface area increase with the plasmonic effects in the metals and the Schottky barrier/electron transfer effects from the metal–semiconductor junction.

Figure 1

Figure 1. (a) Schematic of the final structures with active layer material combinations under investigation. (b) AFM image of the Al nanodisc hard mask created by nanoimprint lithography. (c) 3D AFM image of the conical nanostructure etched in SiO2. (d) Photograph of the Au/TiO2 photocatalytic panel. The colorful surface effect is observed under ambient light at an angle by the naked eye or with a camera and is due to diffraction and thin film interference effects.

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Optical Absorption and Spectral Overlap Model

The photocatalytic activity of the samples is expected to correlate with their optical absorption. Hence, the optical transmittance and reflectance spectra of the samples were measured by ultraviolet to visible light (UV–vis) spectrophotometry (PerkinElmer Lambda 1050) in the wavelength range from 250 to 800 nm. The samples were laid flat against the ports of a 150 mm integrating sphere housing the photodetector (photomultiplier tube). This ensured the collection of total transmittance (T) and total reflectance (R) for each sample, from which we calculated the total absorbance A = 1 – T – R.
The UV–vis spectra in Figure 2a show that the flat TiO2 reference has an absorption onset at 348 nm (dashed black line). It absorbs 55% of the UV-C radiation at 254 nm. The second reference sample with cones and TiO2 (black line) shows a significant enhancement of UV absorption by 20% and an expansion of the absorption onset to 392 nm, which can be attributed to the light-trapping cones. There is an additional absorption peak of 26% at ∼500 nm, matching the period of the structure. This is because the grid acts as a diffraction grating for light at a wavelength comparable to its period. Here, the subwavelength structure for light with wavelengths larger than the period is turned into a partially diffracting grating for wavelengths smaller than the period. These are known optical effects of gratings, which are less relevant for the photocatalytic application. (16)

Figure 2

Figure 2. (a) Measured optical absorbance of the four fabricated samples superimposed with UV-C and UV-A light source spectra. (b) FDTD simulations of the SiO2–Au–TiO2 nanocone structure: absorption cross-section in comparison to measured absorbance. (c) Simulated electric field cross-section (y-plane) at different illumination wavelengths with plasmonic field enhancement (“hot spot” formation).

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The samples with Al (teal) and Au (orange) layers both show 95–97% absorption in the UV (250–320 nm) and over 70% across the visible and NIR spectrum (400–800 nm). Both types of metal/TiO2 samples have low reflectance over the visible wavelength range, on average 11% for the Al sample and 3% for the Au sample (Figure S3 in the Supporting Information). In the case of gold, absorption reaches a maximum of 97% at 500 nm; again, this effect is attributed to the grating period of the nanostructure. The Au-sample absorption spectrum is reproducible across different positions on the sample surface and different 5 × 5 cm2 panels (Figure S2). Our design was thus successful in creating highly efficient photocatalytic surfaces extended to a broader wavelength range. Figure 2a further shows the source spectra of the UV-C and UV-A lamps we use in the reactor setup, with their peaks at 254 and 365 nm, respectively. This visualizes the spectral overlap between the light source driving the reaction and the absorption capabilities of the photocatalytic surfaces.
The photocatalytic efficiency can be estimated with an optical overlap model for plasmon resonance energy transfer (PRET), (17) which relates and quantifies the photocatalytic reaction with the electric field induced by surface plasmon resonance (SPR). According to the model, the photoactivity of a reaction is directly proportional to the overlap between the illumination source spectrum, the semiconductor absorption spectrum, and the metal SPR spectrum. Typically, nanometer-sized edges, holes, or valleys in a metal surface can give rise to plasmonic “hot spots” or very intense electric fields in the vicinity of the metallic nanostructure. The energy is transferred non-radiatively from the plasmon to the semiconductor photocatalyst via the electric field in the near-field zone, generating electron–hole pairs in the photocatalyst.
We have simulated the optical properties of a periodic array of SiO2–Au–TiO2 nanocones on a SiO2 substrate via finite-difference time domain (FDTD) solver of Maxwell’s equations by Lumerical (for details, see Methods section). From this simulation, we have obtained the absorption cross-section for incident wavelengths between 200 and 800 nm, as well as the e electric field distribution in the xz plane at different incident wavelengths. Figure 2b shows the simulated absorption cross-section together with the measured absorbance and distinguishes between three spectral regimes. Namely, starting from the shortest wavelengths, the TiO2, Al, and Au absorption regions extend to ∼380 nm, where all three materials intensively absorb incoming photons. Then, between ∼380 and ∼520 nm, only Au still has significant interband absorption, while the cone geometry still traps incoming light. Last, above ∼520 nm and up until 800 nm, the dielectric properties of Au (the negative real part of the dielectric function and the imaginary part close to zero, see Figure S7) can enable plasmonic field enhancement.
In addition, Figure 2c shows cross-sectional images of the simulated electric field at illumination wavelengths from this last spectral region above 520 nm. This is where we expect to see the electric field hot spots. The simulations show that they are indeed present, localized on the cone tip for shorter wavelengths of 560 to 590 nm, then at the sidewalls for 662 to 705 nm. As the wavelengths are getting larger than the surface topography dimensions, the simulation shows the electric field building up between the sidewalls of neighboring cones and also in the trenches (753 nm).
Based on the spectral overlap between sample absorption and lamp irradiation, we predict the highest photocatalytic efficiency for the Au sample at UV-C and at UV-A and possibly also for visible wavelengths due to the stronger extinction peak at 520 nm compared to the Al sample. The flat sample is expected to show the lowest efficiency.

Methylene Blue Degradation

We have performed dye degradation experiments under different illumination regimes corresponding to different regions of the samples’ optical spectra in order to distinguish between the effects of photolysis, standard semiconductor photocatalysis, and the enhanced photocatalysis of the combined metallic and TiO2 surfaces. The theoretical predictions based on the overlap model were experimentally evaluated using a self-built small-scale reactor in continuous flow mode (Figure 3a,b).

Figure 3

Figure 3. (a) Photocatalytic test reactor with lamp mount. Inset: basin with a 5 × 5 cm2 sample and MB aqueous solution (0.1 L) under illumination. (b) 3D sketch of the reactor, comprising light source over the testing area and water pump with piping to recirculate the liquid. (c) Close-up of the sensor system with red LED and photodiode (PD). The blue arrow indicates the direction of liquid flow through the well-defined volume of the sensor body.

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The sample under study lies in a basin, while an aqueous solution recirculates over it and a lamp shines light on it for the duration of the experiment. We chose three different lamps to study different regimes of the TiO2 photocatalytic reaction: in-bandgap (UV-C) and out-of-bandgap (narrow band UV-A and broadband white light). The experiments with methylene blue (MB) were started with 30 min in the dark to allow for adsorption of MB on the sample surface. The MB degradation was monitored in real time by recording the reduction of its absorption peak at 663 nm with a red LED and a photodiode-based sensor (Figure 3c). Every 10 s, the red LED shines through a well-defined volume of the MB solution and the photodiode measures the transmitted light. Hence, the photodiode signal is a measure of the water transparency, which corresponds to a decrease in MB concentration C/C0, where C is the current concentration and C0 is the initial concentration. The initial concentration of the MB solution was 10–5 mol L–1, and the volume of the solution was 0.1 L. The transparency at this concentration level was measured and normalized to 1 in the figures, while a water transparency of 0 corresponds to clear water according to the detection limit of the sensor. The sensor was calibrated such that the light absorption intensity has a linear dependence on the MB concentration for the measurement range (see Figure S5). The detection range was tuned with the sensor electronics for a large sensitivity between the initial and the final concentration. The water transparency level, related to C/C0, should therefore always lie between one and zero. However, recordings of values larger than one are possible due to water evaporation and do not indicate an increase in concentration beyond the initial C0.
First, we investigated the UV-C regime with a lamp peak at λ = 254 nm. The corresponding photon energies are larger than the bandgap of TiO2, and there is high intrinsic absorption by the semiconductor. The real-time increase of water transparency under UV-C for all samples is shown in Figure 4a. The MB degradation measured on the reference-fused silica substrate reflects photolysis (dashed blue line). After 3 h of runtime, the final water transparency level due to photolysis only was 0.86. Similar results were obtained with the flat TiO2 ALD layer (dashed black line) with water transparency reaching 0.84, indicating that the photocatalytic activity of the as-deposited flat thin film is negligible compared to the pure photolysis under UV-C. In contrast, the performance improved to a relative transparency level of 0.76 when using the sample with TiO2 on the cone surface (solid black line in Figure 4a). The reactor run with this sample combines the effects of photolysis and standard photocatalysis due to TiO2 after the surface area enlargement. The increase in MB degradation is caused by the larger surface that can take part in the exchange of carriers with adsorbed dye molecules. This measurement then is a reference for the metallic samples, since structuring and layer thicknesses remain the same. As soon as one of the two metals are integrated on top of the glass nanostructure and are covered by the same TiO2 layer, other enhancing effects come into play. The data of the two metal samples in Figure 4a confirm the overlap model, as they show enhanced photocatalytic activity in UV-C compared to the TiO2 on cones reference. The water relative transparency reached 0.51 and 0.37 for Al (teal line) and Au (orange line), respectively.

Figure 4

Figure 4. Methylene blue degradation, reactor data recorded in real time. (a) Under UV-C (254 nm) illumination. Dashed blue line: photolysis measured with a blank SiO2 sample. Dashed black line: combined effect of photolysis and standard photocatalysis measured with a flat TiO2 layer. Solid black line: combined effect of photolysis and standard photocatalysis measured with TiO2 on a cone-structured surface. Enhanced photocatalysis is achieved by a stack of Al (teal line) or Au (orange line) and TiO2 on cone-structured surfaces. (b) Under UV-A (365 nm) illumination. No photolysis or standard photocatalysis present, as measured with both a flat TiO2 layer (dashed black line) and TiO2 layer on the cone-structured surface (solid black line). Photocatalysis driven by plasmonics is demonstrated by the Al/TiO2 (teal line) and Au/TiO2 (orange line) layer stacks on cone-structured surfaces. (c) First-order kinetics fit of samples under UV-A illumination. (d) Under white light from high-pressure lamp. No photocatalytic reaction with a flat TiO2 layer (dashed black line) or plasmonic photocatalyst with Al (teal line) present. Lines show an apparent increase in dye concentration because of the lamp’s excessive heating and subsequent water evaporation. The plasmonic photocatalyst with Au (orange line) successfully degrades the dye; the water transparency level increase due to evaporation is compensated for in this data set.

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Second, we examined the UV-A regime with a lamp peak at λ = 365 nm, where there is no discernible photolysis and photon energies are below the bandgap of TiO2. Hence, we expect no photocatalytic dye degradation unless there is a plasmonic enhancement by the metallic cone structures. The water transparency kinetics during a 3 h run are shown in Figure 4b. The reference samples of flat TiO2 (dashed black line) and TiO2 on the cone surface (solid black line) remain at the initial MB concentration, which confirms the lack of photolysis and photocatalysis by TiO2 alone. The substrates with Al (teal line) and Au (orange line), in contrast, increased the water transparency to 0.82 and 0.57, respectively. This is evidence for a plasmonically enhanced photocatalytic reaction.
We have fitted the data for the UV-A regime to first-order kinetics in Figure 4c, using the calibration curve we had previously recorded (Figure S5). The apparent rate constants obtained from the slopes are kAu = 0.02 min–1 for the cones + Au + TiO2 sample and kAl = 0.005 min–1 for the cones + Al + TiO2. For the cones + TiO2 sample (no metal), there is no apparent photocatalytic reaction, the concentration stays very close to the initial value, and ln(C0/C) stays very close to zero. Therefore, the apparent negative slope with kTiO = −0.002 min–1 has no physical meaning. These results were later reproducibly confirmed on different days (example for the cones + Au + TiO2 sample on Figure S3).
Third, MB degradation was investigated under a broadband white light source. In this case, only the Au sample showed photocatalytic activity, reaching a final water transparency level of 0.48 after 3 h (Figure 4d). The measurements taken from the other samples show an apparent increase in dye concentration because the lamp’s excessive heat evaporated some of the water. This was observed despite a built-in Peltier cooling element, which was apparently insufficient. From Figure 4d and the calibration curve (Figure S5), we can estimate the influence of water evaporation to the dye concentration in the experiment with visible light. The final value of water transparency after 3 h of white light illumination for the TiO2 sample (no photocatalysis present) is 1.07, which is equivalent to a 1.8 V sensor signal. Given that at 2 V, the MB concentration is 1 × 10–5 mol L–1 and the linear fit of our calibration curve, we estimate that 1.8 V corresponds to a 1.13 × 10–5 mol L–1 MB concentration. This is an increase in concentration of only one-tenth of the initial value, and it leads to a water transparency decrease of 7% by the end of the measurement. Therefore, we consider the effect of water evaporation on the accuracy of the experiment to be within an acceptable margin, though modifications will be necessary for future experiments.
These three experiments serve to demonstrate the underlying mechanisms. In the UV-C case, the metallic nanostructures led to an improvement of efficiency by up to 51% at the 3 h mark for Au compared to conventional photocatalysis and photolysis. Here, the enhancement can be primarily attributed to the Schottky barrier at the metal–semiconductor interface, which lowers recombination losses of carriers generated by absorption in the TiO2 layer. A linear extrapolation of the measurement data reveals that the metallic samples would clear the water volume completely 2.2 times (Al) to 2.8 times (Au) faster than the TiO2 alone. In the UV-A case, the TiO2 alone barely shows a photocatalytic reaction throughout the measured test interval. Here, the metallic nanostructures are required to induce photocatalytic dye degradation. In the white light case, Au nanostructures are required to induce catalysis.
The superior performance of the Au sample compared to the Al sample at UV-C cannot be conclusively attributed to a difference in optical absorption at this wavelength (compare Figure 2a). This indicates that other enhancement effects, like the Schottky barrier at the metal–semiconductor junction, are driving the improved reaction in the in-gap regime. A higher Schottky barrier would promote better separation of the photoexcited charge carriers. Assuming the same electron affinity of TiO2 in both cases, the Schottky barrier will be higher for the metal with the larger work function, as the barrier is given by the difference between the work function of the metal and the electron affinity of the semiconductor. (17) The work function of Au is typically in the range of 5.31–5.47 eV, while for Al it is 4.06–4.41 eV. (18) Therefore, the Au–TiO2 interface would have the higher potential barrier and would be better at preventing electron–hole recombination.
The Schottky barrier plays an important role for enhancing photocatalytic performance by reducing recombination losses, if the attached semiconductor generates electron–hole pairs due to absorption. The curves measured under white light exposure (Figure 4d) nonetheless show that neither the TiO2 nor the Al–TiO2 produces a decent measurable dye degradation. At wavelengths in the out-of-gap regime, the semiconductor does not absorb light and without generated carriers, a sole reduction of recombination losses clearly cannot lead to any enhancement. Still, the Au–TiO2 sample shows ample dye degradation. The carriers for this reaction presumably result from the interband absorption of light in the Au layer, followed by carrier transfer to the semiconductor. This transfer is most likely caused by a plasmonic enhancement in the nanopatterned metal. Here, the plasmonic field enhancement generates hot carriers in the metal that may overcome the Schottky barrier and be transferred to the semiconductor. The field enhancement is depicted in the simulation of the absorption cross-sections in Figure 2b. It is strongest between 620 and 750 nm where the imaginary part of the dielectric function of Au is lowest (Figure S7). The E-field enhancement is visualized as hot spots in the structure cross-sections in Figure 2c derived from the simulation data.
Benchmarking the performance of our photocatalytic surfaces to the state of the art in literature proves quite difficult because there is great variation in experimental parameters and conditions across reports─from sample composition, to irradiation wavelength and power, to test substance type, volume, and concentration. Pedanekar et al. (19) recently published a table that collects the percentage of degradation efficiency in time as reported by different researchers. We summarize here only entries with MB as the model pollutant (Table 1) and conclude that our best-performing sample (60% in 180 min) matches well the state of the art for thin film photocatalysis. On closer inspection of the references, it becomes apparent that the cited percentages and times mostly refer to smaller testing volumes, for instance: 0.0035–0.005 (20,21) or 0.03, (22) 0.035, (23) 0.05 L, (24) compared to 0.1 L in our case. The initial MB concentration of 10–5 mol L–1 in the literature references is the same as in our experiments. The only other reference with a testing volume of 0.1 L reports 60% degradation in 300 min. (25) Therefore, we conclude that our photocatalytic surfaces indeed constitute the state of the art in efficiency of purifying larger quantities of water.
Table 1. Photocatalytic Performance of Various Metal Oxide and Sulfide Thin Filmsa
thin film photocatalystsynthesis methodirradiationtesting volume [L]b/sample area [cm2]percentage of degradation efficiency in timerefs
N-doped TiO2sol–gelvisible (150 W, Xe > 400 nm)0.005, 4.889% in 150 min (20)
Cu-doped TiO2/reduced graphene oxidespray pyrolysisUV-A, UV-B (300 W, 280–400 nm)0.035, 463% in 180 min (23)
ZnO-doped SiO2sol–gelUV-A (20 W)0.1, 160% in 300 min (25)
ZnSchemical bath depositionUV-C (252 nm, 11 W)0.0035, 292% in 240 min (21)
Bi2VO5.5/Bi2O3chemical solution depositionUV (300 W, Xe)0.03, 1689.97%. in 300 min (22)
CdSSILARUV–visible (500 W, Xe)0.05 powder89% in 120 min (24)
TiO2–Aumagnetron sputtering/ALDUV-A (365 nm, 6 W), visible (125 W)0.1, 2560% in 180 min, 50% in 180 minthis work
a

Adapted with permission from ref (19). Copyright 2020 Elsevier.

b

All entries are for methylene blue with concentration of 10–5 mol/L.

We further benchmarked our results with similar plasmonic photocatalysts and plotted the degradation time to reach maximum transparency (apparent C/C0 = 0) in Figure 5a. We included several reports based on sputtered TiO2 films decorated with metal nanoparticles. (4,26,27) We also added results from metal-doped TiO2 nanoparticle films fabricated by sol–gel methods (28−31) and TiO2 layers without metals. (32) In the interest of fair comparison and to illustrate the complexity of making fair comparisons between vastly different approaches to photocatalysis and diversity in results reporting and presentation, we have also included (Figure 5b) a table with the values of the apparent rate constant k reported in the studies referenced in Table 1 and Figure 5a. Although the shortest times and highest values of k were achieved with TiO2/metallic nanoparticles, there is still the open question of environmental exposure of nanoparticles. In addition, our data is in the same range as these nanoparticle-based methods and provides stable TiO2 thin film surfaces and may therefore be preferable overall. Looking beyond this initial performance evaluation, the possibility of iterative optimization of the nanostructure parameters and the choice of materials, together with the upscaling potential through large-area nanoimprint, makes the proposed photocatalytic surfaces an attractive concept for further studies.

Figure 5

Figure 5. Comparison of typical MB degradation speed for different TiO2-based photocatalysts. (a) In terms of time for C/C0 to reach apparent zero. Literature sources are indicated with references next to the points. Insets: SEM images of (i) gold-covered conical nanostructures as fabricated by us, (ii) a flat TiO2 ALD layer as deposited by us on silicon. (b) In terms of apparent rate constant k for different references.

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Carbamazepine Degradation

As a realistic real-life scenario, we investigated the degradation of the pollutant carbamazepine under UV-A illumination choosing the Au sample for its superior demonstrated performance. The sample surface, the reactor body, and the tubing were thoroughly cleaned with distilled water to remove MB residues from previous experiments. Otherwise, the experimental system and the photocatalytic sample remained unmodified. However, carbamazepine has no absorption peaks in the visible spectrum. This prevented the real-time monitoring in the reactor with the red LED and photodiode combination. Instead, we measured the optical absorption spectra of the initial carbamazepine solution and the solution after 6 h of processing inside the reactor via our UV–vis spectrophotometer with 1 cm path length quartz cuvettes. The total solution volume in the experiment was 0.1 L, and the initial carbamazepine concentration was 1 mg/L. For reference, carbamazepine detected in wastewater samples and in environmental waters is typically in the ng/L to μg/L range. (33) After 6 h of reactor runtime, the characteristic carbamazepine absorption peak at 284 nm falls from 28.4 to 14.7% (Figure 6). The second typical absorption peak close to 200 nm lies at the spectral limit of the measurement tool; therefore, it is much harder to quantify. Nevertheless, the graph shows a clear reduction of the optical absorption─and hence of the concentration─of carbamazepine in the water, demonstrating the viability of our plasmonic nanostructure concept.

Figure 6

Figure 6. Carbamazepine degradation under UV-A illumination. Solution absorbance measured in UV–vis spectrophotometer before and after reactor run time. Black line: absorbance spectrum of the solution with an initial concentration of 1 mg/L. Orange line: absorbance spectrum of the solution after 6 h in the reactor with Au plasmonic photocatalyst. Characteristic peak at 284 nm reduced by 52%.

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Conclusions

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We have demonstrated enhanced photocatalysis efficiency via nanostructured and plasmonically engineered surfaces that act in synergy to circumvent the known limitations of thin film catalysts. Nanostructuring provides a three-fold surface area enlargement to increase the adsorption of reactants. In addition, the subwavelength dimensions of conical structures create a trapping effect for UV and visible light. This red-shifts the TiO2 absorption onset to the violet range of visible light and enhances the UV absorption maximum to 78.4% for a TiO2 thin film of merely 25 nm. Finally, a metal–semiconductor heterostructure formed on top of the textured surfaces (Al or Au and a TiO2 ALD thin layer) further enhances the photocatalytic activity due to both promotion of charge carrier separation and plasmonic resonance. Specifically, the addition of Al or Au accelerates organic dye degradation under UV-C irradiation by a factor of 2.8 compared to bare TiO2 and activates the photocatalytic degradation under UV-A, achieving up to 43% degradation in 180 min for methylene blue and 48% for carbamazepine in 360 min. Furthermore, the large plasmon resonance peak of the Au sample around 520 nm leads to methylene blue degradation of 52% in 180 min under broadband white light, which is relevant for solar-driven applications. We have achieved these results with substrates that were manufactured with scalable nanostructuring and conventional thin-film deposition methods on standard SiO2 substrates. Furthermore, the approach is suitable for addressing other applications of photocatalysis, while being potentially less toxic than commonly employed nanoparticles. Our results hold promise for further up-scaling of thin film-based surfaces for wastewater treatment and other environmentally relevant sectors.

Methods

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We first modeled the devices optical response via finite-difference time domain (FDTD) simulations with Lumerical FDTD photonic simulation software to predict a suitable nanostructure grating periodicity. The critical features for the gratings are their period/distance between structures and their depth of texturing (Figure 1a). One prerequisite is that the grating period needs to be smaller than the wavelength of the incident light. Here, a period of less than or equal to ∼500 nm is required for the enhancement of optical absorption in the UV and visible range. In addition, light-trapping is achieved when the height of the structures is equal to or larger than the wavelength of light, that is, more than 300 nm in the case of UV-solar irradiation.
The chosen geometry closely matched the fabricated structure shape, including the dimensions of period and a height of 480 nm, as well as thin film thicknesses of ∼20 nm. Periodic boundary conditions were applied in the x and y direction, while a perfectly matched layer condition was set in the z direction. The simulation did not take into account surface roughness and effects arising from imperfections in the fabricated arrays. It was also limited to showing the response of the structures when the external electromagnetic field of the light interacts with them, which is not enough to explain the full photocatalytic reaction. Still, the simulation is a good first approximation to guide our understanding of the experimental results.
Common semiconductor technology tools and techniques can successfully produce structures that meet the theoretical requirements. Although this fabrication route may at first seem unusual in the context of TiO2 photocatalysis, it has been a viable choice for other solar-driven devices like nanostructured solar cells. (34) Therefore, employing it for photocatalytic surfaces has the potential to enable innovations within the existing industrial infrastructure, as well as fabricating larger panels to cover the large surface areas required for wastewater treatment plants.
We used nanoimprint lithography (NIL) to fabricate nanostructures on 150 mm SiO2 wafers (fused silica), which provides control of structural parameters like periodicity and lateral dimensions on the order of 10 nm. (35) From a single mold or master wafer, tens of nanostructured stamps can be readily produced and each of them can be used to replicate up to 60 nanostructured wafers, corresponding to an area of ∼1 m2. Here, the master was made with laser interference lithography (LIL) in a setup operating with a laser wavelength of 266 nm. We exposed a 150 mm silicon wafer covered with UV resist with two expanded laser beams that interfered on the substrate (Figure S1 in Supporting Information, process details in ref (36)). This created a periodic 2D grid of light and dark spots in the resist. The substrate was then rotated by 90°, and a second exposure produced a pillar grating with pillar diameters of 240 nm, equal to half of the period defined as the distance from one pillar center to the center of an adjacent pillar (480 nm). Next, the resist grating was transferred into the silicon by reactive ion etching, and the patterned silicon surface was coated with an anti-adhesion layer of octafluorocyclobutane. We then made a reusable soft stamp by casting polydimethylsiloxane (PDMS) onto the imprint master wafer.
The PDMS soft stamp was used to lithographically define the nanostructures of the fused silica wafers. However, the actual etching of the 450 nm deep conical structures in these wafers required an additional Al hard mask, which was deposited by DC magnetron sputtering. Next, AMONIL resist (37) was spin-coated, the soft stamp was pressed onto the resist, the resist was cured under UV illumination, and the stamp was removed. Afterward, reactive ion etching with BCl3-based plasma was used to transfer the pattern from the resist into the Al layer, forming Al nanodiscs as a hard mask on the SiO2 surface (Figure 1b). A second etching step with CHF3-based plasma produced cones in the SiO2 with an average height of 450 nm (Figure 1c). The wafers were diced into 5 × 5 cm2-sized panels for further processing.
Either one of the two metals (Au, Al) was deposited onto the cones via DC magnetron sputtering. The nominal thickness of the metals was 40–50 nm, that is, if they had been deposited on a flat surface. The actual thickness over the conical topography can be roughly estimated by assuming a uniform distribution of the metal over the cones (SEM image in Figure 5a). The geometry of the samples results in a threefold increase of the surface area compared to a flat surface. Thus, the average metal thickness is approximately 15 nm.
Finally, a TiO2 layer with a thickness of tTiO2 = 25 nm was deposited on top of the metals with a conformal ALD plasma process based on a titanium tetrachloride (TiCl4) precursor at 300 °C. Figure 1d is a photograph of the panel surface after nanostructuring and Au/TiO2 thin-film deposition. The structured glass acts as a surface relief diffraction grating for the incoming ambient light. This effect, combined with the thin film interference effect, results in a colorful pattern observable at an angle by the naked eye or with a camera. In addition to the Au/TiO2 and Al/TiO2 panels, a flat TiO2 film on plain SiO2 and a cone sample without metal but with TiO2 were also fabricated as references, both with tTiO2 = 25 nm, in addition to a reference fused silica wafer.
For the MB test in a laboratory-scale continuous flow photocatalytic reactor, our setup design and calibration were guided by the standard ISO 10678:2010 (38) and consulting with the suggestions of Prof. Andrew Mills in his review of the ISO standard. (39)

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsanm.3c02867.

  • Additional experimental details, including: a schematic of the laser interference lithography setup used to fabricate master wafers for nanoimprint lithography; optical absorbance spectra of the nanostructured plasmonic photocatalyst with Au and TiO2 measured on two different panels at three different positions each; and degradation of methylene blue by the nanostructured plasmonic photocatalyst with Au and TiO2 under UV-A with reactor data from measurements on different days (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Desislava Daskalova - Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany;  Chair of Electronic Devices, RWTH Aachen University, 52074 Aachen, Germany;  Orcidhttps://orcid.org/0000-0002-7636-413X
    • Gonzalo Aguila Flores - Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
    • Ulrich Plachetka - Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
    • Michael Möller - Advanced Microelectronic Center Aachen, AMO GmbH, 52074 Aachen, Germany
    • Julia Wolters - Institute of Environmental Engineering, RWTH Aachen University, 52074 Aachen, Germany
    • Thomas Wintgens - Institute of Environmental Engineering, RWTH Aachen University, 52074 Aachen, Germany
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge funding through the German Ministry of Education and Research, BMBF in the project PEPcat (02WCL 1519A) and the European Union’s Horizon Europe research and innovation program under the grant agreement No 101084261 (FreeHydroCells).

References

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    Sani, A.; Cao, C.; Cui, D. Toxicity of Gold Nanoparticles (AuNPs): A Review. Biochem. Biophys. Rep. 2021, 26, 100991,  DOI: 10.1016/j.bbrep.2021.100991
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    Toxicity of gold nanoparticles (AuNPs): A review
    Sani, A.; Cao, C.; Cui, D.
    Biochemistry and Biophysics Reports (2021), 26 (), 100991CODEN: BBRICT; ISSN:2405-5808. (Elsevier B.V.)
    A review. Gold nanoparticles are a kind of nanomaterials that have received great interest in field of biomedicine due to their elec., mech., thermal, chem. and optical properties. With these great potentials came the consequence of their interaction with biol. tissues and mols. which presents the possibility of toxicity. This paper aims to consolidate and bring forward the studies performed that evaluate the toxicol. aspect of AuNPs which were categorized into in vivo and in vitro studies. Both indicate to some extent oxidative damage to tissues and cell lines used in vivo and in vitro resp. with the liver, spleen and kidney most affected. The outcome of these review showed small controversy but however, the primary toxicity and its extent is collectively detd. by the characteristics, prepns. and physicochem. properties of the NPs. Some studies have shown that AuNPs are not toxic, though many other studies contradict this statement. In order to have a holistic inference, more studies are required that will focus on characterization of NPs and changes of phys. properties before and after treatment with biol. media. So also, they should incorporate controlled expt. which includes supernatant control Since most studies dwell on citrate or CTAB-capped AuNPs, there is the need to evaluate the toxicity and pharmacokinetics of functionalized AuNPs with their surface compn. which in turn affects their toxicity. Functionalizing the NPs surface with more peculiar ligands would however help regulate and detoxify the uptake of these NPs.
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    Cheben, P.; Halir, R.; Schmid, J. H.; Atwater, H. A.; Smith, D. R. Subwavelength Integrated Photonics. Nature 2018, 560, 565– 572,  DOI: 10.1038/s41586-018-0421-7
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    Subwavelength integrated photonics
    Cheben, Pavel; Halir, Robert; Schmid, Jens H.; Atwater, Harry A.; Smith, David R.
    Nature (London, United Kingdom) (2018), 560 (7720), 565-572CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometer-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the IR and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technol.
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    Khan, M. R.; Chuan, T. W.; Yousuf, A.; Chowdhury, M. N. K.; Cheng, C. K. Schottky Barrier and Surface Plasmonic Resonance Phenomena towards the Photocatalytic Reaction: Study of Their Mechanisms to Enhance Photocatalytic Activity. Catal. Sci. Technol. 2015, 5, 2522– 2531,  DOI: 10.1039/C4CY01545B
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    Schottky barrier and surface plasmonic resonance phenomena towards the photocatalytic reaction: study of their mechanisms to enhance photocatalytic activity
    Khan, Maksudur R.; Chuan, Tan Wooi; Yousuf, Abu; Chowdhury, M. N. K.; Cheng, Chin Kui
    Catalysis Science & Technology (2015), 5 (5), 2522-2531CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    A review. Metals are doped on semiconductors to enhance the activity of photocatalysts and two possible phenomena can happen at the interfaces of the semiconductors: Schottky barrier formation and Surface Plasmonic Resonance (SPR). Schottky barriers can improve the photoactivity of a reaction by trapping and prolonging the life of the electron. While SPR has the ability to create an electromagnetic field which can improve the photoreaction in three ways: photon scattering, Plasmon Resonance Energy Transfer (PRET) and hot electron excitation. Although both phenomena have been well grounded throughout the field, one crucial ambiguity is still found based on the proposed mechanisms, specifically, what is the direction of electron flow - from metal to semiconductor or vice versa. This feature article reviews the mechanism focusing on how Schottky barrier and SPR phenomena help to improve a photoreaction, as well as the paradox between the Schottky barrier and SPR in the matter of the direction of electron flow in the metal/semiconductor system.
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    Pedanekar, R. S.; Shaikh, S. K.; Rajpure, K. Y. Thin Film Photocatalysis for Environmental Remediation: A Status Review. Curr. Appl. Phys. 2020, 20, 931– 952,  DOI: 10.1016/j.cap.2020.04.006
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    Lee, H.-K.; Fujiwara, T.; Okada, T.; Fukushima, T.; Lee, S.-W. Fabrication of Visible-Light Responsive N-Doped TiO2 Nanothin Films via a Top–down Sol–Gel Deposition Method Using NH4TiOF3 Single Crystals. Chem. Lett. 2018, 47, 628– 631,  DOI: 10.1246/cl.180005
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    Fabrication of visible-light responsive N-doped TiO2 nanothin films via a top-down sol-gel deposition method using NH4TiOF3 single crystals
    Lee, Hack-Keun; Fujiwara, Takumi; Okada, Takuya; Fukushima, Taihei; Lee, Seung-Woo
    Chemistry Letters (2018), 47 (5), 628-631CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
    We report a novel coating method for TiO2 anatase thin-film fabrication via a top-down sol-gel deposition method using NH4TiOF3 single crystals. A cationic nitrogen-rich porphyrin compd. was used to fabricate N-doped TiO2 nanofilms that showed highly efficient photocatalytic activity under visiblelight irradn.
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    Cruz, J. S.; Cruz, D. S.; Arenas-Arrocena, M. C.; De, F.; Flores, M.; Hernández, S. A. M. Green Synthesis of ZnS Thin Films by Chemical Bath Deposition. Chalcogenide Lett. 2015, 12, 277– 285
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    Xie, W.; Zhong, L.; Wang, Z.; Liang, F.; Tang, X.; Zou, C.; Liu, G. Photocatalytic Performance of Bi2VO5.5/Bi2O3 Laminated Composite Films under Simulated Sunlight Irradiation. Solid State Sci. 2019, 94, 1– 7,  DOI: 10.1016/j.solidstatesciences.2019.05.010
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    Photocatalytic performance of Bi2VO5.5/Bi2O3 laminated composite films under simulated sunlight irradiation
    Xie, Wei; Zhong, Lanhua; Wang, Zesong; Liang, Feng; Tang, Xiaoshan; Zou, Changwei; Liu, Guiang
    Solid State Sciences (2019), 94 (), 1-7CODEN: SSSCFJ; ISSN:1293-2558. (Elsevier Masson SAS)
    Bi2VO5.5/Bi2O3 laminated composite thin films with each different layer nos. were prepd. on fused silica substrates by chem. soln. deposition method with a following rapid annealing process. The phase structure, surface morphol., absorption spectrum and photocatalytic degrdn. characteristics of the thin films were characterized. X-ray diffraction patterns show that the laminated thin films are consist of polycryst. Bi2O3 films and c-axis oriented Bi2VO5.5 films. With the increasing of the no. of Bi2VO5.5 layers as well as the decrease of the no. of Bi2O3 layers, the absorption spectrum of the composite film shifted towards the longer wavelength side. Meanwhile, under simulated sunlight, the degrdn. ability of the thin films to Methylene Blue gradually increased and reach a max. at appropriate layer ratio. The optimal laminated composite thin film with the highest degrdn. rate are 6 layers of Bi2VO5.5 films superposed upon 2 layers of Bi2O3 films. The degrdn. rate decreases only by 2% after the 5 cycles of degrdn. The reasons for the enhancement of the photocatalytic properties of the thin film can be attribute to the laminated structure and the matching energy level, which are benefit for the transferring of photo-induced electrons. According to fitting results of the band gap and the XPS valence band spectrum, the mechanism of photocatalytic degrdn. of MB for Bi2VO5.5/Bi2O3 laminated composite films irradn. are analyzed. Under simulated sunlight irradn., the photogenerated electrons on the conduction band of the Bi2O3 can transfer to the conduction band of Bi2VO5.5. Meanwhile, the photogenerated holes on the valence band of Bi2VO5.5 can also be migrated to the valence band of Bi2O3, thus suppress the recombination, prolong the lifetime of photo-generated carriers, and increase the photocatalytic degrdn. efficiency.
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    Pham, T.-T.; Nguyen-Huy, C.; Lee, H.-J.; Nguyen-Phan, T.-D.; Son, T. H.; Kim, C.-K.; Shin, E. W. Cu-Doped TiO2/Reduced Graphene Oxide Thin-Film Photocatalysts: Effect of Cu Content upon Methylene Blue Removal in Water. Ceram. Int. 2015, 41, 11184– 11193,  DOI: 10.1016/j.ceramint.2015.05.068
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    Cu-doped TiO2/reduced graphene oxide thin-film photocatalysts: Effect of Cu content upon methylene blue removal in water
    Pham, Thanh-Truc; Nguyen-Huy, Chinh; Lee, Hyun-Jun; Nguyen-Phan, Thuy-Duong; Son, Tae Hwan; Kim, Chang-Koo; Shin, Eun Woo
    Ceramics International (2015), 41 (9_Part_A), 11184-11193CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
    Photocatalytic thin films on quartz substrates were simply prepd. by spraying a sol of copper metal (Cu)-doped titanium dioxide (TiO2) combined with reduced graphene oxide (RGO). Cu-doped TiO2/RGO powders contg. different amts. of Cu were synthesized to investigate the effect of Cu content upon the photodegrdn. of methylene blue in water, and were used in the prepn. of sols of Cu-doped TiO2/RGO. The Cu-doped TiO2/RGO film photocatalysts showed better performance in the photodegrdn. of methylene blue than an undoped TiO2/RGO film. After 180 min of UV irradn., 7.5-CTG-5 (7.5 wt% of Cu and 5 wt% of RGO) photocatalyst removed 63% of methylene blue from aq. soln. with an initial concn. of 10 ppm, whereas 0-CTG-5 (0 wt% of Cu and 5 wt% of RGO) decompd. only 28% of the methylene blue, indicating that the presence of Cu enhanced the photocatalytic activity. Doped Cu could narrow the bandgap of TiO2, thereby influencing the redn. of graphene oxide and enhancing the hydrophilicity of the materials, and thus producing the obsd. increase in photocatalytic activity. The relationships between hydrophilicity and photocatalytic activity are discussed herein.
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    Ravichandran, K.; Porkodi, S. Addressing the Issue of Under-Utilization of Precursor Material in SILAR Process: Simultaneous Preparation of CdS in Two Different Forms–Thin Film and Powder. Mater. Sci. Semicond. Process. 2018, 81, 30– 37,  DOI: 10.1016/j.mssp.2018.02.037
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    Addressing the issue of under-utilization of precursor material in SILAR process: Simultaneous preparation of CdS in two different forms - Thin film and powder
    Ravichandran, K.; Porkodi, S.
    Materials Science in Semiconductor Processing (2018), 81 (), 30-37CODEN: MSSPFQ; ISSN:1369-8001. (Elsevier Ltd.)
    To overcome the major limitation of the successive ionic layer adsorption and reaction (SILAR) technique - the under-utilization of the precursor material - in the present work, simultaneous fabrication of CdS thin films and CdS powder is attempted. CdS films were prepd. by employing SILAR technique using cadmium acetate dihydrate and thiourea as cationic and anionic precursor materials for Cd and S, resp. The pptd. powder materials in the cationic and anionic baths which are usually discarded as wastage are collected and processed to obtain CdS powders. In another trial, the cationic and anionic bath solns. are mixed and the resultant ppt. is collected for characterization. The obtained CdS in thin film and powder forms are analyzed for structural, surface morphol., optical, compositional and photocatalytic properties. The results clearly showed that the obtained CdS films are good candidates for considering them for solar cell applications. Similarly, CdS powders obtained from the mixt. of cationic and anionic bath solns. are found to have good photocatalytic efficacy. Thus the demerits of under-utilization of precursor materials in SILAR process for the deposition of CdS films is overcome to certain extent as 63% of the wastage is recovered as useful product.
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    Ali, A. M.; Ismail, A. A.; Najmy, R.; Al-Hajry, A. Preparation and Characterization of ZnO–SiO2 Thin Films as Highly Efficient Photocatalyst. J. Photochem. Photobiol., A 2014, 275, 37– 46,  DOI: 10.1016/j.jphotochem.2013.11.002
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    Preparation and characterization of ZnO-SiO2 thin films as highly efficient photocatalyst
    Ali, Atif Mossad; Ismail, Adel A.; Najmy, Rasha; Al-Hajry, Ali
    Journal of Photochemistry and Photobiology, A: Chemistry (2014), 275 (), 37-46CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    ZnO doped SiO2 thin films were prepd. by the sol-gel method and annealed at different temps. from 200 to 1100 °C. SiO2 matrix is selected as support due to their high flexibility, thermal stability and high porosity and surface areas. The XRD patterns showed that the hexagonal wurtzite structure of the ZnO film was formed for all the prepd. samples. TEM images of ZnO synthesized nanoparticles are almost spherical with a relatively narrow size distribution in the range of 5-20 nm according to the annealing temp. These images clearly show the (0 0 0 1) at. planes (interplanner distance is 0.52 nm) perpendicular to the c-axis, thus indicating that (0 0 0 1) is the preferred growth direction of these wurtzite-type ZnO. The newly prepd. photocatalysts ZnO-SiO2 films have been evaluated by the detn. of their photonic efficiencies for degrdn. of methylene blue. The photonic efficiencies of 10 wt% ZnO-SiO2 increase from 0.9 to 2.3% with increasing annealing temp. from 200 to 600 °C and then gradually decrease to 1.56% at 1100 °C. The results indicate that 10 wt% ZnO-SiO2 annealed at 600 °C showed the highest photocatalytic activity for the MB photodegrdn. Our work demonstrates the ability to reduce the metalworking temp. as well as to increase the response of ZnO thin film as a highly efficient photocatalyst, which would be of great merit for commercialized applications.
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    Ghori, M. Z.; Veziroglu, S.; Hinz, A.; Shurtleff, B. B.; Polonskyi, O.; Strunskus, T.; Adam, J.; Faupel, F.; Aktas, O. C. Role of UV Plasmonics in the Photocatalytic Performance of TiO2 Decorated with Aluminum Nanoparticles. ACS Appl. Nano Mater. 2018, 1, 3760– 3764,  DOI: 10.1021/acsanm.8b00853
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    26
    Role of UV Plasmonics in the Photocatalytic Performance of TiO2 Decorated with Aluminum Nanoparticles
    Ghori, Muhammad Zubair; Veziroglu, Salih; Hinz, Alexander; Shurtleff, Bill Brook; Polonskyi, Oleksandr; Strunskus, Thomas; Adam, Jost; Faupel, Franz; Aktas, Oral Cenk
    ACS Applied Nano Materials (2018), 1 (8), 3760-3764CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    We present a facile method, combining sputtering and gas aggregation techniques, to prep. a photocatalytic TiO2 thin film decorated with stable aluminum plasmonic nanoparticles (Al NPs) to reveal the localized surface plasmon resonance (LSPR) effect on TiO2 photocatalysis under UV irradn. We demonstrate for the first time the neg. and pos. influences of LSPR on UV photocatalysis by irradiating Al NPs/TiO2 hybrid structures at two different UV wavelengths: both at and above the plasmonic absorption of Al NPs. These findings open the door to design low-cost Al/TiO2 photocatalytic hybrid surfaces that function in a broad spectral range from deep-UV to visible wavelengths.
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    Veziroglu, S.; Ghori, M. Z.; Obermann, A.-L.; Röder, K.; Polonskyi, O.; Strunskus, T.; Faupel, F.; Aktas, O. C. Ag Nanoparticles Decorated TiO2 Thin Films with Enhanced Photocatalytic Activity. Phys. Status Solidi A 2019, 216, 1800898,  DOI: 10.1002/pssa.201800898
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    Kaleji, B. K.; Sarraf-Mamoory, R.; Fujishima, A. Influence of Nb Dopant on the Structural and Optical Properties of Nanocrystalline TiO2 Thin Films. Mater. Chem. Phys. 2012, 132, 210– 215,  DOI: 10.1016/j.matchemphys.2011.11.034
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    Influence of Nb dopant on the structural and optical properties of nanocrystalline TiO2 thin films
    Kaleji, Behzad Koozegar; Sarraf-Mamoory, Rasoul; Fujishima, Akira
    Materials Chemistry and Physics (2012), 132 (1), 210-215CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
    In this study, prepn. of Nb-doped (0-20 mol% Nb) TiO2 dip-coated thin films on glazed porcelain substrates via sol-gel process has been investigated. The effects of Nb on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and SEM. Surface topog. and surface chem. state of thin films was examd. by at. force microscope and XPS. XRD and Raman study showed that the Nb doping inhibited the grain growth. The photocatalytic activity of the film was tested on degrdn. of methylene blue. Best photocatalytic activity of Nb-doped TiO2 thin films were measured in the TiO2-1 mol% Nb sample. The av. optical transmittance of about 47% in the visible range and the band gap of films became wider with increasing Nb doping concn. The Nb5+ dopant presented substitutional Ti4+ into TiO2 lattice.
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    Wada, N.; Yokomizo, Y.; Yogi, C.; Katayama, M.; Tanaka, A.; Kojima, K.; Inada, Y.; Ozutsumi, K. Effect of Adding Au Nanoparticles to TiO2 Films on Crystallization, Phase Transformation, and Photocatalysis. J. Mater. Res. 2018, 33, 467– 481,  DOI: 10.1557/jmr.2018.16
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    Effect of adding Au nanoparticles to TiO2 films on crystallization, phase transformation, and photocatalysis
    Wada, Noriyuki; Yokomizo, Yuji; Yogi, Chihiro; Katayama, Misaki; Tanaka, Atsuhiro; Kojima, Kazuo; Inada, Yasuhiro; Ozutsumi, Kazuhiko
    Journal of Materials Research (2018), 33 (4), 467-481CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
    To investigate the effects of adding Au nanoparticles (AuNPs) to TiO2 films on the crystn., phase transformation, and photocatalysis, films of both TiO2 and TiO2 embedded with AuNPs (Au-TiO2) with various characteristics were prepd. by using the dip-coating method with preheating and post-heating treatments. The AuNPs acted as anatase nucleation agents and crystd. a lot of small anatase crystals with sizes of tens of nanometers, which suppressed the growth of anatase crystals that are large enough for them to transform into rutile crystals, resulting in repression of the transformation from anatase into rutile. The AuNPs affected the progress of the photocatalytic and adsorption reactions, resulting in improved photocatalytic activity. Of all the films we tested, the Au-TiO2 film preheated at 400°C and post-heated at 400°C (AT400-400), which consisted of small anatase crystals with high covalent character and high crystallinity, contained dispersed AuNPs with the smallest av. crystallite size and showed the highest photocatalytic activity. This high activity resulted from the high reaction rate consts. for adsorption and photocatalysis.
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    Sonawane, R. S.; Kale, B. B.; Dongare, M. K. Preparation and Photo-Catalytic Activity of Fe:TiO2 Thin Films Prepared by Sol–Gel Dip Coating. Mater. Chem. Phys. 2004, 85, 52– 57,  DOI: 10.1016/j.matchemphys.2003.12.007
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    Preparation and photocatalytic activity of Fe-TiO2 thin films prepared by sol-gel dip coating
    Sonawane, R. S.; Kale, B. B.; Dongare, M. K.
    Materials Chemistry and Physics (2004), 85 (1), 52-57CODEN: MCHPDR; ISSN:0254-0584. (Elsevier Science B.V.)
    Thin films of iron (Fe) doped titanium dioxide (Fe-TiO2) were prepd. on a variety of substrates by using Ti-peroxy sol-gel dip coating method. The surface structure of the film was modified by adding different concns. of polyethylene glycol (PEG) into the TiO2 sol. Most of the metal ion doped entered TiO2 lattice resulting the shift in optical absorption edge towards visible side. Addn. of PEG alters the surface morphol. and structure of the films. The increase in concn. of PEG increases the no. and size of the pores on surface of the film by decompn. of PEG when the films are subjected to heat treatment. The adsorbed hydroxyl content of such porous films is found to increase with amt. of PEG added. Photocatalytic properties of the surface modified and Fe ion doped TiO2 catalyst was investigated by degrdn. of Methyl orange in sunlight. The photocatalytic activity of the PEG added Fe-TiO2 catalyst was enhanced by 2-2.5 times than undoped TiO2.
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    Khairy, M.; Zakaria, W. Effect of Metal-Doping of TiO2 Nanoparticles on Their Photocatalytic Activities toward Removal of Organic Dyes. Egypt. J. Pet. 2014, 23, 419– 426,  DOI: 10.1016/j.ejpe.2014.09.010
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    Wardhani, S.; Purwonugroho, D.; Fitri, C. W.; Prananto, Y. P. Effect of PH and Irradiation Time on TiO2-Chitosan Activity for Phenol Photo-Degradation. AIP Conf. Proc. 2018, 2021, 050009,  DOI: 10.1063/1.5062759
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    Effect of pH and irradiation time on TiO2-chitosan activity for phenol photo-degradation
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic of the final structures with active layer material combinations under investigation. (b) AFM image of the Al nanodisc hard mask created by nanoimprint lithography. (c) 3D AFM image of the conical nanostructure etched in SiO2. (d) Photograph of the Au/TiO2 photocatalytic panel. The colorful surface effect is observed under ambient light at an angle by the naked eye or with a camera and is due to diffraction and thin film interference effects.

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    Figure 2

    Figure 2. (a) Measured optical absorbance of the four fabricated samples superimposed with UV-C and UV-A light source spectra. (b) FDTD simulations of the SiO2–Au–TiO2 nanocone structure: absorption cross-section in comparison to measured absorbance. (c) Simulated electric field cross-section (y-plane) at different illumination wavelengths with plasmonic field enhancement (“hot spot” formation).

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    Figure 3

    Figure 3. (a) Photocatalytic test reactor with lamp mount. Inset: basin with a 5 × 5 cm2 sample and MB aqueous solution (0.1 L) under illumination. (b) 3D sketch of the reactor, comprising light source over the testing area and water pump with piping to recirculate the liquid. (c) Close-up of the sensor system with red LED and photodiode (PD). The blue arrow indicates the direction of liquid flow through the well-defined volume of the sensor body.

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    Figure 4

    Figure 4. Methylene blue degradation, reactor data recorded in real time. (a) Under UV-C (254 nm) illumination. Dashed blue line: photolysis measured with a blank SiO2 sample. Dashed black line: combined effect of photolysis and standard photocatalysis measured with a flat TiO2 layer. Solid black line: combined effect of photolysis and standard photocatalysis measured with TiO2 on a cone-structured surface. Enhanced photocatalysis is achieved by a stack of Al (teal line) or Au (orange line) and TiO2 on cone-structured surfaces. (b) Under UV-A (365 nm) illumination. No photolysis or standard photocatalysis present, as measured with both a flat TiO2 layer (dashed black line) and TiO2 layer on the cone-structured surface (solid black line). Photocatalysis driven by plasmonics is demonstrated by the Al/TiO2 (teal line) and Au/TiO2 (orange line) layer stacks on cone-structured surfaces. (c) First-order kinetics fit of samples under UV-A illumination. (d) Under white light from high-pressure lamp. No photocatalytic reaction with a flat TiO2 layer (dashed black line) or plasmonic photocatalyst with Al (teal line) present. Lines show an apparent increase in dye concentration because of the lamp’s excessive heating and subsequent water evaporation. The plasmonic photocatalyst with Au (orange line) successfully degrades the dye; the water transparency level increase due to evaporation is compensated for in this data set.

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    Figure 5

    Figure 5. Comparison of typical MB degradation speed for different TiO2-based photocatalysts. (a) In terms of time for C/C0 to reach apparent zero. Literature sources are indicated with references next to the points. Insets: SEM images of (i) gold-covered conical nanostructures as fabricated by us, (ii) a flat TiO2 ALD layer as deposited by us on silicon. (b) In terms of apparent rate constant k for different references.

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    Figure 6

    Figure 6. Carbamazepine degradation under UV-A illumination. Solution absorbance measured in UV–vis spectrophotometer before and after reactor run time. Black line: absorbance spectrum of the solution with an initial concentration of 1 mg/L. Orange line: absorbance spectrum of the solution after 6 h in the reactor with Au plasmonic photocatalyst. Characteristic peak at 284 nm reduced by 52%.

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    10. 10
      Abed, J.; Rajput, N. S.; Moutaouakil, A. E.; Jouiad, M. Recent Advances in the Design of Plasmonic Au/TiO2 Nanostructures for Enhanced Photocatalytic Water Splitting. Nanomaterials 2020, 10, 2260,  DOI: 10.3390/nano10112260
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      Recent advances in the design of plasmonic Au/TiO2 nanostructures for enhanced photocatalytic water splitting
      Abed, Jehad; Rajput, Nitul S.; El Moutaouakil, Amine; Jouiad, Mustapha
      Nanomaterials (2020), 10 (11), 2260CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
      A review. Plasmonic nanostructures have played a key role in extending the activity of photocatalysts to the visible light spectrum, preventing the electron-hole combination and providing with hot electrons to the photocatalysts, a crucial step towards efficient broadband photocatalysis. One plasmonic photocatalyst, Au/TiO2, is of a particular interest because it combines chem. stability, suitable electronic structure, and photoactivity for a wide range of catalytic reactions such as water splitting. In this review, we describe key mechanisms involving plasmonics to enhance photocatalytic properties leading to efficient water splitting such as prodn. and transport of hot electrons through advanced anal. techniques used to probe the photoactivity of plasmonics in engineered Au/TiO2 devices. This work also discusses the emerging strategies to better design plasmonic photocatalysts and understand the underlying mechanisms behind the enhanced photoactivity of plasmon-assisted catalysts.
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      Zhang, P.; Wang, T.; Gong, J. Mechanistic Understanding of the Plasmonic Enhancement for Solar Water Splitting. Adv. Mater. 2015, 27, 5328– 5342,  DOI: 10.1002/adma.201500888
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      Mechanistic Understanding of the Plasmonic Enhancement for Solar Water Splitting
      Zhang, Peng; Wang, Tuo; Gong, Jinlong
      Advanced Materials (Weinheim, Germany) (2015), 27 (36), 5328-5342CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. H2 generation by solar water splitting is one of the most promising solns. to meet the increasing energy demands of the fast developing society. However, the efficiency of solar-water-splitting systems is still too low for practical applications, which requires further enhancement via different strategies such as doping, construction of heterojunctions, morphol. control, and optimization of the crystal structure. Recently, integration of plasmonic metals to semiconductor photocatalysts has been proved to be an effective way to improve their photocatalytic activities. Thus, in-depth understanding of the enhancement mechanisms is of great importance for better utilization of the plasmonic effect. This review describes the relevant mechanisms from three aspects, including: (i) light absorption and scattering; (ii) hot-electron injection and (iii) plasmon-induced resonance energy transfer (PIRET). Perspectives are also proposed to trigger further innovative thinking on plasmonic-enhanced solar water splitting.
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      Dimcheva, N. Nanostructures of Noble Metals as Functional Materials in Biosensors. Curr. Opin. Electrochem. 2020, 19, 35– 41,  DOI: 10.1016/j.coelec.2019.09.008
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      Nanostructures of noble metals as functional materials in biosensors
      Dimcheva, Nina
      Current Opinion in Electrochemistry (2020), 19 (), 35-41CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
      A review. Recent advancement in nanoscience and nanotechnologies inspired a wide spectrum of uses of nanodimensional materials ranging from industrial sector to biomedical applications. Inorg. nanomaterials made of noble metals, which are corrosion-resistant, are often included as electrode modifiers in designing electrochem. chemosensors and biosensors because of their unique catalytic, elec., and surface-related properties. This review summarizes the developments in electrochem. biosensors with integrated in their architecture metal nanostructures reported mainly during the last two years with a summary on some of the commonly used methods for the synthesis of metallic nanostructures. Nanodimensional noble metal structures might be considered as multipurpose electrode modifiers because of their abilities to act at the same time as electrocatalysts, signal amplifiers, and tools for immobilization and spatial orientation of redox proteins/enzymes or other type of bioreceptors.
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      Siegel, J.; Staszek, M.; Polívková, M.; Valová, M.; Šuláková, P.; Švorčík, V. Structure-Dependent Biological Response of Noble Metals: From Nanoparticles, through Nanowires to Nanolayers; IntechOpen, 2017.
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      Carnovale, C.; Bryant, G.; Shukla, R.; Bansal, V. Identifying Trends in Gold Nanoparticle Toxicity and Uptake: Size, Shape, Capping Ligand, and Biological Corona. ACS Omega 2019, 4, 242– 256,  DOI: 10.1021/acsomega.8b03227
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      Identifying Trends in Gold Nanoparticle Toxicity and Uptake: Size, Shape, Capping Ligand, and Biological Corona
      Carnovale, Catherine; Bryant, Gary; Shukla, Ravi; Bansal, Vipul
      ACS Omega (2019), 4 (1), 242-256CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      The drive behind the growing interest in understanding gold nanoparticle (AuNP) cytotoxicity originates from the promise of AuNPs for diverse biol. applications across the fields of drug delivery, biosensing, biol. imaging, gene therapy and photothermal therapy. While we continue to investigate the novel biomedical applications of AuNPs, progress is currently stalled at the periphery of understanding the forces that govern crit. nano-bio interactions. In this work, we systematically probe the size, shape and surface capping effects of nano-gold by designing a set of eight unique AuNPs. This allowed us to undertake a systematic study involving each of these parameters in the context of their influence on the cytotoxicity and cellular uptake by human prostate cancer cells (PC3) as a model biol. system. While studying the influence of these parameters, our study also investigated the influence of serum proteins in forming different levels of biol. corona on AuNPs, thereby further influencing the nano-bio interface. As such, increased cellular uptake (by nanoparticle no.) was obsd. with decreasing the AuNP size and increased uptake levels were obsd. for AuNS (of the same size) stabilized with amino acids compared to citrate or CTAB. Spherical particles were found to be taken up in greater nos. compared to the shapes with broad flat faces. When measuring cytotoxicity, cetyltrimethylammonium bromide (CTAB)-stabilized rod- and cube-shaped particles were well tolerated by the cells, while toxicity was obsd. in the case of CTAB-stabilized spherical and prismatic particles. These effects, however, are underpinned by different mechanisms. Further, it is demonstrated that it is possible for different chem. stabilizers to elicit varied cytotoxic effects. While we find the limited role of serum proteins in mediating toxicity, they do play a crit. role in influencing the cellular uptake of AuNPs, with lower levels of uptake generally obsd. in the presence of serum. Our findings offer a useful step in the direction of predicting the biol. interactions of AuNPs based on specific parameters of AuNP design.
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      Sani, A.; Cao, C.; Cui, D. Toxicity of Gold Nanoparticles (AuNPs): A Review. Biochem. Biophys. Rep. 2021, 26, 100991,  DOI: 10.1016/j.bbrep.2021.100991
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      Toxicity of gold nanoparticles (AuNPs): A review
      Sani, A.; Cao, C.; Cui, D.
      Biochemistry and Biophysics Reports (2021), 26 (), 100991CODEN: BBRICT; ISSN:2405-5808. (Elsevier B.V.)
      A review. Gold nanoparticles are a kind of nanomaterials that have received great interest in field of biomedicine due to their elec., mech., thermal, chem. and optical properties. With these great potentials came the consequence of their interaction with biol. tissues and mols. which presents the possibility of toxicity. This paper aims to consolidate and bring forward the studies performed that evaluate the toxicol. aspect of AuNPs which were categorized into in vivo and in vitro studies. Both indicate to some extent oxidative damage to tissues and cell lines used in vivo and in vitro resp. with the liver, spleen and kidney most affected. The outcome of these review showed small controversy but however, the primary toxicity and its extent is collectively detd. by the characteristics, prepns. and physicochem. properties of the NPs. Some studies have shown that AuNPs are not toxic, though many other studies contradict this statement. In order to have a holistic inference, more studies are required that will focus on characterization of NPs and changes of phys. properties before and after treatment with biol. media. So also, they should incorporate controlled expt. which includes supernatant control Since most studies dwell on citrate or CTAB-capped AuNPs, there is the need to evaluate the toxicity and pharmacokinetics of functionalized AuNPs with their surface compn. which in turn affects their toxicity. Functionalizing the NPs surface with more peculiar ligands would however help regulate and detoxify the uptake of these NPs.
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      Cheben, P.; Halir, R.; Schmid, J. H.; Atwater, H. A.; Smith, D. R. Subwavelength Integrated Photonics. Nature 2018, 560, 565– 572,  DOI: 10.1038/s41586-018-0421-7
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      Subwavelength integrated photonics
      Cheben, Pavel; Halir, Robert; Schmid, Jens H.; Atwater, Harry A.; Smith, David R.
      Nature (London, United Kingdom) (2018), 560 (7720), 565-572CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometer-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the IR and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technol.
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      Khan, M. R.; Chuan, T. W.; Yousuf, A.; Chowdhury, M. N. K.; Cheng, C. K. Schottky Barrier and Surface Plasmonic Resonance Phenomena towards the Photocatalytic Reaction: Study of Their Mechanisms to Enhance Photocatalytic Activity. Catal. Sci. Technol. 2015, 5, 2522– 2531,  DOI: 10.1039/C4CY01545B
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      Schottky barrier and surface plasmonic resonance phenomena towards the photocatalytic reaction: study of their mechanisms to enhance photocatalytic activity
      Khan, Maksudur R.; Chuan, Tan Wooi; Yousuf, Abu; Chowdhury, M. N. K.; Cheng, Chin Kui
      Catalysis Science & Technology (2015), 5 (5), 2522-2531CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      A review. Metals are doped on semiconductors to enhance the activity of photocatalysts and two possible phenomena can happen at the interfaces of the semiconductors: Schottky barrier formation and Surface Plasmonic Resonance (SPR). Schottky barriers can improve the photoactivity of a reaction by trapping and prolonging the life of the electron. While SPR has the ability to create an electromagnetic field which can improve the photoreaction in three ways: photon scattering, Plasmon Resonance Energy Transfer (PRET) and hot electron excitation. Although both phenomena have been well grounded throughout the field, one crucial ambiguity is still found based on the proposed mechanisms, specifically, what is the direction of electron flow - from metal to semiconductor or vice versa. This feature article reviews the mechanism focusing on how Schottky barrier and SPR phenomena help to improve a photoreaction, as well as the paradox between the Schottky barrier and SPR in the matter of the direction of electron flow in the metal/semiconductor system.
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      Hölzl, J.; Schulte, F. K. Work Function of Metals. In Solid Surface Physics; Hölzl, J., Schulte, F. K., Wagner, H., Eds.; Springer Tracts in Modern Physics; Springer: Berlin, Heidelberg, 1979; pp 1– 150.
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      Pedanekar, R. S.; Shaikh, S. K.; Rajpure, K. Y. Thin Film Photocatalysis for Environmental Remediation: A Status Review. Curr. Appl. Phys. 2020, 20, 931– 952,  DOI: 10.1016/j.cap.2020.04.006
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      Lee, H.-K.; Fujiwara, T.; Okada, T.; Fukushima, T.; Lee, S.-W. Fabrication of Visible-Light Responsive N-Doped TiO2 Nanothin Films via a Top–down Sol–Gel Deposition Method Using NH4TiOF3 Single Crystals. Chem. Lett. 2018, 47, 628– 631,  DOI: 10.1246/cl.180005
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      Fabrication of visible-light responsive N-doped TiO2 nanothin films via a top-down sol-gel deposition method using NH4TiOF3 single crystals
      Lee, Hack-Keun; Fujiwara, Takumi; Okada, Takuya; Fukushima, Taihei; Lee, Seung-Woo
      Chemistry Letters (2018), 47 (5), 628-631CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
      We report a novel coating method for TiO2 anatase thin-film fabrication via a top-down sol-gel deposition method using NH4TiOF3 single crystals. A cationic nitrogen-rich porphyrin compd. was used to fabricate N-doped TiO2 nanofilms that showed highly efficient photocatalytic activity under visiblelight irradn.
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      Cruz, J. S.; Cruz, D. S.; Arenas-Arrocena, M. C.; De, F.; Flores, M.; Hernández, S. A. M. Green Synthesis of ZnS Thin Films by Chemical Bath Deposition. Chalcogenide Lett. 2015, 12, 277– 285
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      Xie, W.; Zhong, L.; Wang, Z.; Liang, F.; Tang, X.; Zou, C.; Liu, G. Photocatalytic Performance of Bi2VO5.5/Bi2O3 Laminated Composite Films under Simulated Sunlight Irradiation. Solid State Sci. 2019, 94, 1– 7,  DOI: 10.1016/j.solidstatesciences.2019.05.010
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      Photocatalytic performance of Bi2VO5.5/Bi2O3 laminated composite films under simulated sunlight irradiation
      Xie, Wei; Zhong, Lanhua; Wang, Zesong; Liang, Feng; Tang, Xiaoshan; Zou, Changwei; Liu, Guiang
      Solid State Sciences (2019), 94 (), 1-7CODEN: SSSCFJ; ISSN:1293-2558. (Elsevier Masson SAS)
      Bi2VO5.5/Bi2O3 laminated composite thin films with each different layer nos. were prepd. on fused silica substrates by chem. soln. deposition method with a following rapid annealing process. The phase structure, surface morphol., absorption spectrum and photocatalytic degrdn. characteristics of the thin films were characterized. X-ray diffraction patterns show that the laminated thin films are consist of polycryst. Bi2O3 films and c-axis oriented Bi2VO5.5 films. With the increasing of the no. of Bi2VO5.5 layers as well as the decrease of the no. of Bi2O3 layers, the absorption spectrum of the composite film shifted towards the longer wavelength side. Meanwhile, under simulated sunlight, the degrdn. ability of the thin films to Methylene Blue gradually increased and reach a max. at appropriate layer ratio. The optimal laminated composite thin film with the highest degrdn. rate are 6 layers of Bi2VO5.5 films superposed upon 2 layers of Bi2O3 films. The degrdn. rate decreases only by 2% after the 5 cycles of degrdn. The reasons for the enhancement of the photocatalytic properties of the thin film can be attribute to the laminated structure and the matching energy level, which are benefit for the transferring of photo-induced electrons. According to fitting results of the band gap and the XPS valence band spectrum, the mechanism of photocatalytic degrdn. of MB for Bi2VO5.5/Bi2O3 laminated composite films irradn. are analyzed. Under simulated sunlight irradn., the photogenerated electrons on the conduction band of the Bi2O3 can transfer to the conduction band of Bi2VO5.5. Meanwhile, the photogenerated holes on the valence band of Bi2VO5.5 can also be migrated to the valence band of Bi2O3, thus suppress the recombination, prolong the lifetime of photo-generated carriers, and increase the photocatalytic degrdn. efficiency.
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    23. 23
      Pham, T.-T.; Nguyen-Huy, C.; Lee, H.-J.; Nguyen-Phan, T.-D.; Son, T. H.; Kim, C.-K.; Shin, E. W. Cu-Doped TiO2/Reduced Graphene Oxide Thin-Film Photocatalysts: Effect of Cu Content upon Methylene Blue Removal in Water. Ceram. Int. 2015, 41, 11184– 11193,  DOI: 10.1016/j.ceramint.2015.05.068
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      Cu-doped TiO2/reduced graphene oxide thin-film photocatalysts: Effect of Cu content upon methylene blue removal in water
      Pham, Thanh-Truc; Nguyen-Huy, Chinh; Lee, Hyun-Jun; Nguyen-Phan, Thuy-Duong; Son, Tae Hwan; Kim, Chang-Koo; Shin, Eun Woo
      Ceramics International (2015), 41 (9_Part_A), 11184-11193CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
      Photocatalytic thin films on quartz substrates were simply prepd. by spraying a sol of copper metal (Cu)-doped titanium dioxide (TiO2) combined with reduced graphene oxide (RGO). Cu-doped TiO2/RGO powders contg. different amts. of Cu were synthesized to investigate the effect of Cu content upon the photodegrdn. of methylene blue in water, and were used in the prepn. of sols of Cu-doped TiO2/RGO. The Cu-doped TiO2/RGO film photocatalysts showed better performance in the photodegrdn. of methylene blue than an undoped TiO2/RGO film. After 180 min of UV irradn., 7.5-CTG-5 (7.5 wt% of Cu and 5 wt% of RGO) photocatalyst removed 63% of methylene blue from aq. soln. with an initial concn. of 10 ppm, whereas 0-CTG-5 (0 wt% of Cu and 5 wt% of RGO) decompd. only 28% of the methylene blue, indicating that the presence of Cu enhanced the photocatalytic activity. Doped Cu could narrow the bandgap of TiO2, thereby influencing the redn. of graphene oxide and enhancing the hydrophilicity of the materials, and thus producing the obsd. increase in photocatalytic activity. The relationships between hydrophilicity and photocatalytic activity are discussed herein.
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      Ravichandran, K.; Porkodi, S. Addressing the Issue of Under-Utilization of Precursor Material in SILAR Process: Simultaneous Preparation of CdS in Two Different Forms–Thin Film and Powder. Mater. Sci. Semicond. Process. 2018, 81, 30– 37,  DOI: 10.1016/j.mssp.2018.02.037
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      Addressing the issue of under-utilization of precursor material in SILAR process: Simultaneous preparation of CdS in two different forms - Thin film and powder
      Ravichandran, K.; Porkodi, S.
      Materials Science in Semiconductor Processing (2018), 81 (), 30-37CODEN: MSSPFQ; ISSN:1369-8001. (Elsevier Ltd.)
      To overcome the major limitation of the successive ionic layer adsorption and reaction (SILAR) technique - the under-utilization of the precursor material - in the present work, simultaneous fabrication of CdS thin films and CdS powder is attempted. CdS films were prepd. by employing SILAR technique using cadmium acetate dihydrate and thiourea as cationic and anionic precursor materials for Cd and S, resp. The pptd. powder materials in the cationic and anionic baths which are usually discarded as wastage are collected and processed to obtain CdS powders. In another trial, the cationic and anionic bath solns. are mixed and the resultant ppt. is collected for characterization. The obtained CdS in thin film and powder forms are analyzed for structural, surface morphol., optical, compositional and photocatalytic properties. The results clearly showed that the obtained CdS films are good candidates for considering them for solar cell applications. Similarly, CdS powders obtained from the mixt. of cationic and anionic bath solns. are found to have good photocatalytic efficacy. Thus the demerits of under-utilization of precursor materials in SILAR process for the deposition of CdS films is overcome to certain extent as 63% of the wastage is recovered as useful product.
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      Ali, A. M.; Ismail, A. A.; Najmy, R.; Al-Hajry, A. Preparation and Characterization of ZnO–SiO2 Thin Films as Highly Efficient Photocatalyst. J. Photochem. Photobiol., A 2014, 275, 37– 46,  DOI: 10.1016/j.jphotochem.2013.11.002
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      Preparation and characterization of ZnO-SiO2 thin films as highly efficient photocatalyst
      Ali, Atif Mossad; Ismail, Adel A.; Najmy, Rasha; Al-Hajry, Ali
      Journal of Photochemistry and Photobiology, A: Chemistry (2014), 275 (), 37-46CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      ZnO doped SiO2 thin films were prepd. by the sol-gel method and annealed at different temps. from 200 to 1100 °C. SiO2 matrix is selected as support due to their high flexibility, thermal stability and high porosity and surface areas. The XRD patterns showed that the hexagonal wurtzite structure of the ZnO film was formed for all the prepd. samples. TEM images of ZnO synthesized nanoparticles are almost spherical with a relatively narrow size distribution in the range of 5-20 nm according to the annealing temp. These images clearly show the (0 0 0 1) at. planes (interplanner distance is 0.52 nm) perpendicular to the c-axis, thus indicating that (0 0 0 1) is the preferred growth direction of these wurtzite-type ZnO. The newly prepd. photocatalysts ZnO-SiO2 films have been evaluated by the detn. of their photonic efficiencies for degrdn. of methylene blue. The photonic efficiencies of 10 wt% ZnO-SiO2 increase from 0.9 to 2.3% with increasing annealing temp. from 200 to 600 °C and then gradually decrease to 1.56% at 1100 °C. The results indicate that 10 wt% ZnO-SiO2 annealed at 600 °C showed the highest photocatalytic activity for the MB photodegrdn. Our work demonstrates the ability to reduce the metalworking temp. as well as to increase the response of ZnO thin film as a highly efficient photocatalyst, which would be of great merit for commercialized applications.
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      Ghori, M. Z.; Veziroglu, S.; Hinz, A.; Shurtleff, B. B.; Polonskyi, O.; Strunskus, T.; Adam, J.; Faupel, F.; Aktas, O. C. Role of UV Plasmonics in the Photocatalytic Performance of TiO2 Decorated with Aluminum Nanoparticles. ACS Appl. Nano Mater. 2018, 1, 3760– 3764,  DOI: 10.1021/acsanm.8b00853
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      26
      Role of UV Plasmonics in the Photocatalytic Performance of TiO2 Decorated with Aluminum Nanoparticles
      Ghori, Muhammad Zubair; Veziroglu, Salih; Hinz, Alexander; Shurtleff, Bill Brook; Polonskyi, Oleksandr; Strunskus, Thomas; Adam, Jost; Faupel, Franz; Aktas, Oral Cenk
      ACS Applied Nano Materials (2018), 1 (8), 3760-3764CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      We present a facile method, combining sputtering and gas aggregation techniques, to prep. a photocatalytic TiO2 thin film decorated with stable aluminum plasmonic nanoparticles (Al NPs) to reveal the localized surface plasmon resonance (LSPR) effect on TiO2 photocatalysis under UV irradn. We demonstrate for the first time the neg. and pos. influences of LSPR on UV photocatalysis by irradiating Al NPs/TiO2 hybrid structures at two different UV wavelengths: both at and above the plasmonic absorption of Al NPs. These findings open the door to design low-cost Al/TiO2 photocatalytic hybrid surfaces that function in a broad spectral range from deep-UV to visible wavelengths.
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      Veziroglu, S.; Ghori, M. Z.; Obermann, A.-L.; Röder, K.; Polonskyi, O.; Strunskus, T.; Faupel, F.; Aktas, O. C. Ag Nanoparticles Decorated TiO2 Thin Films with Enhanced Photocatalytic Activity. Phys. Status Solidi A 2019, 216, 1800898,  DOI: 10.1002/pssa.201800898
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      Kaleji, B. K.; Sarraf-Mamoory, R.; Fujishima, A. Influence of Nb Dopant on the Structural and Optical Properties of Nanocrystalline TiO2 Thin Films. Mater. Chem. Phys. 2012, 132, 210– 215,  DOI: 10.1016/j.matchemphys.2011.11.034
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      Influence of Nb dopant on the structural and optical properties of nanocrystalline TiO2 thin films
      Kaleji, Behzad Koozegar; Sarraf-Mamoory, Rasoul; Fujishima, Akira
      Materials Chemistry and Physics (2012), 132 (1), 210-215CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
      In this study, prepn. of Nb-doped (0-20 mol% Nb) TiO2 dip-coated thin films on glazed porcelain substrates via sol-gel process has been investigated. The effects of Nb on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and SEM. Surface topog. and surface chem. state of thin films was examd. by at. force microscope and XPS. XRD and Raman study showed that the Nb doping inhibited the grain growth. The photocatalytic activity of the film was tested on degrdn. of methylene blue. Best photocatalytic activity of Nb-doped TiO2 thin films were measured in the TiO2-1 mol% Nb sample. The av. optical transmittance of about 47% in the visible range and the band gap of films became wider with increasing Nb doping concn. The Nb5+ dopant presented substitutional Ti4+ into TiO2 lattice.
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      Wada, N.; Yokomizo, Y.; Yogi, C.; Katayama, M.; Tanaka, A.; Kojima, K.; Inada, Y.; Ozutsumi, K. Effect of Adding Au Nanoparticles to TiO2 Films on Crystallization, Phase Transformation, and Photocatalysis. J. Mater. Res. 2018, 33, 467– 481,  DOI: 10.1557/jmr.2018.16
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      Effect of adding Au nanoparticles to TiO2 films on crystallization, phase transformation, and photocatalysis
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      Journal of Materials Research (2018), 33 (4), 467-481CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
      To investigate the effects of adding Au nanoparticles (AuNPs) to TiO2 films on the crystn., phase transformation, and photocatalysis, films of both TiO2 and TiO2 embedded with AuNPs (Au-TiO2) with various characteristics were prepd. by using the dip-coating method with preheating and post-heating treatments. The AuNPs acted as anatase nucleation agents and crystd. a lot of small anatase crystals with sizes of tens of nanometers, which suppressed the growth of anatase crystals that are large enough for them to transform into rutile crystals, resulting in repression of the transformation from anatase into rutile. The AuNPs affected the progress of the photocatalytic and adsorption reactions, resulting in improved photocatalytic activity. Of all the films we tested, the Au-TiO2 film preheated at 400°C and post-heated at 400°C (AT400-400), which consisted of small anatase crystals with high covalent character and high crystallinity, contained dispersed AuNPs with the smallest av. crystallite size and showed the highest photocatalytic activity. This high activity resulted from the high reaction rate consts. for adsorption and photocatalysis.
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      Thin films of iron (Fe) doped titanium dioxide (Fe-TiO2) were prepd. on a variety of substrates by using Ti-peroxy sol-gel dip coating method. The surface structure of the film was modified by adding different concns. of polyethylene glycol (PEG) into the TiO2 sol. Most of the metal ion doped entered TiO2 lattice resulting the shift in optical absorption edge towards visible side. Addn. of PEG alters the surface morphol. and structure of the films. The increase in concn. of PEG increases the no. and size of the pores on surface of the film by decompn. of PEG when the films are subjected to heat treatment. The adsorbed hydroxyl content of such porous films is found to increase with amt. of PEG added. Photocatalytic properties of the surface modified and Fe ion doped TiO2 catalyst was investigated by degrdn. of Methyl orange in sunlight. The photocatalytic activity of the PEG added Fe-TiO2 catalyst was enhanced by 2-2.5 times than undoped TiO2.
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      Untreated phenol waste causes environmental pollution. The phenol waste can be treated using the photocatalyst method. This study aims to examine the effect of pH and irradn. time on the degrdn. of phenol compds. using TiO2-chitosan thin film photocatalysts. The synthesis of a thin layer photocatalyst was prepd. by a dip-coating method or dye method on a glass media. The TiO2 was characterized by powder-XRD. TiO2-chitosan thin photocatalyst activity was tested using 100 mg/L phenol soln. with pH variations of 4, 6, 8, 10, and 12, with and without UV irradn. for 5 h. The irradn. times used were 1, 2, 3, 4, and 5 h and only applied at the optimum pH. The results were statistically tested using BNT. The concns. of phenol before and after treatments were measured by a UV-Vis spectrophotometer at 269.7 nm. The XRD characterization results show that the TiO2 that is used has an anatase structure. Both pH and irradn. time influences the phenol degrdn. percentage. The optimum phenol pH was 8 for both with and without UV light, whereas the optimum irradn. time was 5 h with phenol degrdn. of 33.85% (using UV light) and 4.42% (without UV light). (c) 2018 American Institute of Physics.
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      A review. The presence of pharmaceutical compds. in the environment is a reality that calls for more efficient water treatment technologies. Photocatalysis is a powerful technol. available but the high energy costs assocd. with the use of UV irradn. hinder its large scale implementation. More sustainable and cheaper photocatalytic processes can be achieved by improving the sunlight harvesting and the synthesis of semiconductor/carbon composites has proved to be a promising strategy. Carbamazepine, diclofenac, and sulfamethoxazole were selected as target pharmaceuticals due to their recalcitrant behavior during conventional wastewater treatment and persistence in the environment, as properly reviewed. The literature data on the photocatalytic removal of carbamazepine, diclofenac, and sulfamethoxazole by semiconductor/carbon materials was critically revised to highlight the role of the carbon in the enhanced semiconductor performance under solar irradn. Generally it was demonstrated that carbon materials induce red-shift absorption and they contribute to more effective charge sepn., thus improving the composite photoactivity. Carbon was added as a dopant (C-doping) or as support or doping materials (i.e., nanoporous carbons, carbon nanotubes (CNTs), graphene, and derived materials, carbon quantum dots (CQDs), and biochars) and in the large majority of the cases, TiO2 was the semiconductor tested. The specific role of carbon materials is dependent on their properties but even the more amorphous forms, like nanoporous carbons or biochars, allow to prep. composites with improved properties compared to the bare semiconductor. The self-photocatalytic activity of the carbon materials was also reported and should be further explored. The removal and mineralization rates, as well as degrdn. pathways and toxicity of the treated solns. were also critically analyzed.
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      A review of nanostructured solar cell technol.
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      Koo, N.; Plachetka, U.; Otto, M.; Bolten, J.; Jeong, J.; Lee, E.; Kurz, H. The Fabrication of a Flexible Mold for High Resolution Soft Ultraviolet Nanoimprint Lithography. Nanotechnology 2008, 19, 225304,  DOI: 10.1088/0957-4484/19/22/225304
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      Nanotechnology (2008), 19 (22), 225304/1-225304/4CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
      One key issue for all nanoimprint techniques is an appropriate method for the fabrication of desirable molds. We report on a novel flexible mold fabrication process-pressure-assisted molding (PAM)-for high resoln. soft UV nanoimprint lithog. (soft UV-NIL). In PAM, enhanced master filling is achieved by applying an external pressure during the mold fabrication process. Flexible molds, fabricated with PAM using different pressures in the range of 10-90 kPa, are compared to det. the role of pressures applied in the imprint performance.
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      In this work, n-type triple-gate metal-oxide-semiconductor field effect transistors (MOSFETs) are presented, where laser interference lithog. (LIL) is integrated into a silicon-on-insulator (SOI) CMOS process to provide for the crit. definition of the transistor channels. A mix and match process of optical contact lithog. and LIL is developed to achieve device relevant structures. The triple-gate MOSFETs are elec. characterized to demonstrate the feasibility of this low cost fabrication process.
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      An overview of the methylene blue ISO test for assessing the activities of photocatalytic films
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      The main features are described of the ISO published std.: 10678:2010, namely the Detn. of photocatalytic activity of surfaces in an aq. medium by degrdn. of methylene blue'. The main underlying assumptions of the std. are considered, namely: (i) dye purity, (ii) adsorption and pH, (iii) light source, (iv) stirring and diffusion, (v) reaction mechanism and (vi) kinetics. Possible sources of errors arising form these assumptions are identified and changes to the std.'s protocol suggested. The major suggested changes are: (i) a source of MB+ of known and proven on site high purity (>90%) should be used to make up the std. test soln., which should have a referenced absorbance, 0.74, at 665 nm; (ii) the conditioning soln. should be the same concn. as the std. test soln., (iii) the initial pH of the reaction soln. should lie in the range: 5.5-6.0; (iv) a BLB UVA light with a europium-doped strontium fluoroborate or borate phosphor should be used; (v) the reaction soln. should be vigorously and continuously stirred if possible; (vi) the soln. height should be the min. recommended value of 2 cm, and the photoreactor cylinder i.d., 4.7 cm; (vii) after the 3 h irradn. the reaction soln. should be left stirring overnight to check that no dye photoreductive bleaching has occurred. Application of most of these suggestions should improve considerably the std.'s reported current poor percentage errors for repeatability within a lab (9.2%) and reproducibility between labs (30.6%).
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    • Additional experimental details, including: a schematic of the laser interference lithography setup used to fabricate master wafers for nanoimprint lithography; optical absorbance spectra of the nanostructured plasmonic photocatalyst with Au and TiO2 measured on two different panels at three different positions each; and degradation of methylene blue by the nanostructured plasmonic photocatalyst with Au and TiO2 under UV-A with reactor data from measurements on different days (PDF)


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