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Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2 Thin Films Grown by Atomic Layer Deposition
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C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional Materials

Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2 Thin Films Grown by Atomic Layer Deposition
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2022, 126, 36, 15357–15366
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https://doi.org/10.1021/acs.jpcc.2c04905
Published August 30, 2022

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Abstract

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The physicochemical properties of titanium dioxide (TiO2) depend strongly on the crystal structure. Compared to anatase, rutile TiO2 has a smaller bandgap, a higher dielectric constant, and a higher refractive index, which are desired properties for TiO2 thin films in many photonic applications. Unfortunately, the fabrication of rutile thin films usually requires temperatures that are too high (>400 °C, often even 600–800 °C) for applications involving, e.g., temperature-sensitive substrate materials. Here, we demonstrate atomic layer deposition (ALD)-based fabrication of anatase and rutile TiO2 thin films mediated by precursor traces and oxide defects, which are controlled by the ALD growth temperature when using tetrakis(dimethylamido)titanium(IV) (TDMAT) and water as precursors. Nitrogen traces within amorphous titania grown at 100 °C inhibit the crystal nucleation until 375 °C and stabilize the anatase phase. In contrast, a higher growth temperature (200 °C) leads to a low nitrogen concentration, a high degree of oxide defects, and high mass density facilitating direct amorphous to rutile crystal nucleation at an exceptionally low post deposition annealing (PDA) temperature of 250 °C. The mixed-phase (rutile–brookite) TiO2 thin film with rutile as the primary phase forms upon the PDA at 250–500 °C that allows utilization in broad range of TiO2 thin film applications.

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Introduction

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Titanium dioxide (TiO2) is one of the most widely applied and studied photocatalyst materials, being earth-abundant, non-toxic, and stable in various environments. (1−3) Generally, TiO2 appears in four phases: amorphous, anatase, rutile, and brookite. The first three are the most commonly used having their own advantages, whereas brookite has remained mainly inapplicable due to the challenges in fabrication of its pure form. (4,5) Amorphous titania (am.-TiO2) thin films, typically grown at low temperatures, can provide exceptional optical properties and charge carrier dynamics due to the disordered structure and intrinsic Ti3+ defects. (6−10) However, concerning photocatalytic applications, defect-induced gap states may increase the possibility to detrimental electron–hole recombination, and the chemical instability of am.-TiO2 without additional electrocatalysts limits the operating conditions. (11−16) Crystalline defect-free TiO2, instead, is chemically stable and exhibits reduced charge carrier recombination. (12,17,18)
When comparing the two main crystalline TiO2 phases, anatase and rutile, the metastable anatase TiO2 is often regarded as a better photocatalyst than the thermodynamically stable rutile. (3) In fact, based on recent research, rutile is photocatalytically more active in some reactions, especially, oxidative ones, whereas anatase promotes reduction reactions. (19) Another interesting approach to enhance charge carrier separation and photocatalytic activity is a controllable fabrication of anatase–rutile phase junction structures that are reported to outperform pristine single-phase anatase and rutile, e.g., in photoelectrochemical water splitting applications. (20,21)
Unfortunately, the major challenge with rutile thin films in many applications is the need of high processing temperatures (>400 °C, often even 600–800 °C). (22) Some aqueous-phase processes enable growth of rutile nanocrystals at low temperatures (<200 °C), and atomic layer deposition of the anatase–rutile mixed phase is reported around 300 °C, but particularly, methods for obtaining pure rutile thin films at low temperatures are exceedingly limited. (23−27) Epitaxial ALD of rutile TiO2 thin films on substrates with the rutile-structured SnO2, RuO2, or IrO2 seed layer has been demonstrated around 250 °C. (28) Another potential way to promote rutile formation involves dopant ion-induced oxygen vacancies or oxygen-deficient growth conditions to obtain disordered oxygen sublattice enhancing the rearrangement of ions to form easier the constrained and dense rutile phase. (21,22,29)
One possible approach to modify the defect structure of titania is atomic layer deposition, known for its controllable, uniform, and conformal thin film growth via self-limiting surface reactions. (12,24,30,31) Previously, we have shown that intrinsic precursor traces and oxide defects are highly sensitive to ALD growth temperature when using tetrakis(dimethylamido)titanium(IV) and water as precursors. (10) Interestingly, the growth temperature is also shown to steer the crystallization process toward anatase or rutile TiO2 phases, but understanding of this phenomenon in more detail has remained without comprehensive investigation. (12,18)
This work shows the role of ALD growth temperature-controlled (100–200 °C) intrinsic precursor traces and oxide defects on TiO2 thin film crystallization upon post deposition annealing (PDA, 50 min at 200–500 °C and 500 min at 250 °C). X-ray photoelectron spectroscopy (XPS) is used to investigate the evolution of intrinsic titanium (i.e., Ti3+ and under- and over-coordinated Ti4+) and nitrogen (TDMAT fragments or reaction byproducts) defects within the amorphous titania upon PDA in air. Surface chemical analysis together with grazing incidence X-ray diffraction (GIXRD) measurements, X-ray reflectivity (XRR), and scanning electron microscopy (SEM) offers insights into the defect-mediated crystallization of ALD TiO2 and fabrication of the rutile–brookite TiO2 thin film at an exceptionally low PDA temperature of 250 °C.

Experimental Section

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Substrates

The P-doped (resistivity 1–10 Ω·cm) n-type Si(100) wafers from SIEGERT WAFER GmbH (Germany) cleaved in 10 mm × 10 mm × 0.525 mm pieces were used as substrates in all of the experiments.

Atomic Layer Deposition

The ALD of TiO2 was carried out using a Picosun Sunale ALD R-200 Advanced reactor and tetrakis(dimethylamido)titanium(IV) (Ti(N(CH3)2)4, TDMAT, electronic grade 99.999+%, Sigma-Aldrich) and Milli-Q type 1 ultrapure water as precursors. To reach the proper TDMAT precursor vapor pressure, the bubbler was heated to 76 °C, and to prevent condensation of the precursor gas, the delivery line was heated to 85 °C. The water bubbler was sustained at 18 °C by a Peltier element for stability control. Argon (99.9999%, Oy AGA Ab, Finland) was used as a carrier gas. During the deposition, the continuous Ar flow in the TDMAT and H2O lines was 100 sccm. One ALD cycle consisted of a 1.6 s TDMAT pulse followed by a 0.1 s H2O pulse. The excess precursor was pumped away from the reaction chamber during the 6.0 s purge period between each pulse. TiO2 films were deposited at growth temperatures of 100 and 200 °C. The required numbers of ALD cycles for 30 nm-thick TiO2 at growth temperatures of 100 and 200 °C were 480 and 870, respectively.

Post Deposition Annealing

The post deposition annealing for the samples was performed in atmospheric air by placing the samples into a pre-heated tube furnace for 50 min. After the heat treatment, the samples were removed from the tube furnace and let to cool down freely.

X-ray Photoelectron Spectroscopy

Majority of the X-ray photoelectron spectroscopy measurements were conducted using a NanoESCA spectromicroscope system (Omicron Nanotechnology GmbH) in ultrahigh vacuum (UHV) with a base pressure below 1 × 10–10 mbar. In NanoESCA, focused monochromatized Al Kα (hν = 1486.5 eV) was used as an excitation radiation for XPS. The investigation of evolution of intrinsic Ti and N defects as a function of oxidation temperature was carried out by using a non-monochromatized DAR400 X-ray source (Al Kα) and Argus hemispherical electron spectrometer (Omicron Nanotechnology GmbH). The core level XP spectra were analyzed by the least-squares fitting of Gaussian–Lorentzian lineshapes and using a Shirley-type background. Ti 2p spectra were fitted as in our previous work (10) by using the Ti 2p3/2 reference peak shape measured for crystalline TiO2, i.e., the six-coordinated Ti4+ peak (Ti6c4+), and the amorphous disordered structure was represented by under- and over-coordinated Ti4+ (Ti5/7c4+) and Ti3+ peaks. The binding energy scale of the spectra was calibrated by fixing the O2– peak of TiO2 to 530.20 eV. CasaXPS version 2.3.22 PR1.0 (32) was used as an analysis software and the Scofield photoionization cross-sections as relative sensitivity factors. (33)

Ultraviolet (UV) Light Treatment/Ar+ Ion Bombardment

Ti3+ defects were generated within am.-TiO2 thin films via UV treatment and Ar+ ion bombardment. The treatments were carried out using a NanoESCA system (Omicron Nanotechnology GmbH) equipped with an Hg arc UV source (HBO 103 W/2 type lamp, 4.9 eV, 3 h) and an Ar+ ion gun (30 s with 5 kV acceleration voltage; PAr = 2.5 × 10–5 mbar).

Grazing Incidence X-ray Diffraction and X-ray Reflectivity

Structural properties of TiO2 thin films were analyzed by GIXRD and XRR using two diffractometers (PANalytical Empyrean multipurpose and X’Pert3 MRD diffractometers) equipped with a Cu Kα X-ray source (λ = 1.5406 Å, hν = 8.05 keV). In GIXRD measurement, samples were scanned in the 2θ ranges of 24–34 and 20–52° at the grazing incidence angle of ω = 0.3°. The background was removed from each scan to allow easier comparison of the XRD patterns. In XRR measurement, samples were scanned in the coupled ω-2θ range of 0.5–4°. XRR data was modeled by the GenX program (version 3.5.5.) to extract TiO2 film thickness, mass density, and surface roughness using a single-layer model of TiO2 on a Si substrate. (34)

Scanning Electron Microscopy

The surface morphology of TiO2 thin films was studied by scanning electron microscopy (Zeiss Ultra 55, Carl Zeiss Microscopy GmbH). The SEM images were measured by using in-lens mode with a working distance of 2.3–2.4 mm, electron high tension (EHT) of 1.00 kV, and aperture size of 30.00 μm.

Results and Discussion

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The amount of intrinsic oxide defects and nitrogen traces in am.-TiO2 depends on the ALD growth temperature when using TDMAT and H2O as precursors. (10) The ALD process at 100 °C leaves nitrogen containing TDMAT fragments or reaction byproducts within the am.-TiO2 film, whereas the growth at 200 °C leads to a low nitrogen content but an increased amount of Ti3+. The amount of under- and over-coordinated Ti5/7c4+ ions scales with the amount of Ti3+ defects. Furthermore, we have shown that am.-TiO2 grown at 100 °C prefers crystallization into the anatase structure whereas 200 °C growth temperature induces direct crystallization into rutile at the same oxidative annealing temperature (400–500 °C). (12,18) To understand how the intrinsic defects contribute to the crystallization upon post deposition annealing in air, 30 nm-thick ALD TiO2 films grown at 100 and 200 °C were first investigated by X-ray photoelectron spectroscopy before and after PDA at 500 °C.
Figure 1 shows Ti 2p, O 1s, and N 1s XP spectra of as-deposited amorphous and crystallized (PDA 500 °C) ALD TiO2 grown at 100 and 200 °C (surface concentrations of elements are presented in Table S1). Figure 1a shows that oxide defects, i.e., Ti3+ and Ti5/7c4+ defects, were completely removed from both samples upon PDA at 500 °C despite the difference in the initial amounts. Only six-coordinated Ti4+ ions (Ti6c4+) were present in PDA 500 °C samples supporting the crystallized structure, i.e., Ti–O6 octahedra are the building blocks of crystalline TiO2. (35,36) Few subtle changes were observed in the O 1s transition (Figure 1b) consisting of the O–Ti component (O2–) at 530.2 eV and minor O22–/–OH/O–C and O–N peaks at 532.0 ± 0.2 and 531.1 ± 0.1 eV, respectively. The 532.0 eV component of as-deposited samples corresponds to the interstitial peroxo (O22–) species as discussed in our previous work. (10) The O 1s O–N peak was detected only in the TiO2 (100 °C) PDA 500 °C sample. In addition, a difference was observed in the O 1s linewidths that decreased upon the PDA from 1.16 to 1.14 and 1.07 eV for 100 and 200 °C grown samples, respectively (Figure S1). The narrower O 1s linewidth is attributed to the lower nitrogen concentration and higher degree of crystalline order for the 200 °C grown sample after the PDA. It should be noted that the broader peak and a high binding energy tail of the O 1s spectra could be attributed to hydroxyl groups. (13,37) However, the concentration of possible hydroxyl groups was too low to be reliably differentiated from the O 1s spectra, and no difference was observed between the growth temperatures.

Figure 1

Figure 1. (a) Ti 2p, (b) O 1s, and (c) N 1s XP spectra of 30 nm-thick as-deposited and post deposition-annealed ALD TiO2 grown at 100 and 200 °C.

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The removal of Ti3+ defects upon the PDA was accompanied by the disappearance of the Ti3+ gap state peak at 0.3–0.4 eV from the XPS valence band spectra (Figure S2). Again, a subtle difference was observed between the growth temperatures. Upon the PDA, the valence band edge position shifted 0.20 eV toward lower binding energy for the 200 °C grown sample but no change was observed for the 100 °C grown sample. This may relate to the band gap difference between rutile TiO2 (Eg = 3.0 eV) and anatase TiO2 (Eg = 3.2 eV). (22)
N 1s spectra in Figure 1c reveal strong difference between the samples. Nitrogen containing reaction byproducts and TDMAT fragments within am.-TiO2 were clearly detected for the 100 °C grown sample as represented by the three components at 401.5 ± 0.1, at 400.0 ± 0.1, and at 398.6 ± 0.1 eV. (10,31,38) After the PDA at 500 °C, an additional component at lower binding energy (396.0 ± 0.1 eV) emerged in the ALD TiO2 grown at 100 °C. The component at 396 eV is typically reported to relate to substitutional nitrogen located at O2– sites, i.e., Ti–N-like species within the TiO2 lattice. (39−42) Due to dimethylamine decomposition at >275 °C, (43) unreacted TDMAT ligands (Ti–N(CH3)2) and protonated dimethylamine (H2N(CH3)2+) species are proposed to decompose during the PDA leading to the formation of TiO2 with Ti–O–N and Ti–O–N–O species at 398.4 ± 0.1 and 401.9 ± 0.1 eV, respectively. These species contribute to the O–N component in O 1s (Figure 1b). (44) The formation mechanism of these nitrogen species within the TiO2 lattice is discussed in more detail in Figure 4. The N 1s signal detected at 400.0 ± 0.1 eV was assigned to dimethylamine ALD process byproducts (HN(CH3)2) and their decomposed and oxidized N–O species locating primarily at the surface from where these species may have partially desorbed during the PDA. Indeed, in our previous study, these species (N 1s at 400.0 ± 0.1 eV) showed strong surface enrichment (1.8 at.% vs initial 0.2 at.%) after similar PDA at 350 °C that resulted in partial crystallization. (12) Therefore, it is suggested that the desorption of dimethylamine species is followed by crystallization and the remaining nitrogen species at higher temperatures are oxidized to N–O species that have similar N 1s binding energy with dimethylamine. Surface decomposition of pure dimethylamine takes place at >275 °C, and therefore, it is unlikely to have these species at the surface after PDA at 500 °C. (43) Compared to the am.-TiO2 thin film grown at 100 °C, only a minor amount of nitrogen was observed in ALD TiO2 grown at 200 °C.
Despite the strong chemical changes in Ti and N species upon the PDA, only subtle changes were observed in the elemental surface concentrations (Table S1). The O2–/Ti ratio was close to 2 for all the samples suggesting stoichiometric TiO2, although ionic coordination of titanium differs between as-deposited amorphous and crystallized (PDA 500 °C) TiO2. (10,35,36) The nitrogen concentrations were 0.9 and 0.2 at.% for the samples grown at 100 and 200 °C, respectively. In addition, all the surfaces had 2–5 at.% of carbon. Detailed analysis of carbon species in the film was compromised due to the build-up of adventitious carbon during the sample transfer via air. TiO2 films also contain hydrogen that cannot be directly probed with XPS, but hydrogen bonded to nitrogen species was indirectly analyzed from N 1s spectra. Indeed, Xia et al. recently studied the same TDMAT + H2O ALD process and, by using elastic recoil detection analysis (ERDA), observed elevated hydrogen concentration in ALD TiO2 grown at 100 °C (H/Ti = 0.5) compared to the film grown at 225 °C (H/Ti = 0.1). (37) The difference in the hydrogen content was found to follow the same growth temperature trend with N-bearing TDMAT fragments.
Grazing incidence X-ray diffraction measurements were conducted to study the crystallization of 30 nm-thick ALD TiO2 grown at 100 and 200 °C upon PDA at 300, 400, and 500 °C. Figure 2a reveals that am.-TiO2 grown at 100 °C retains the amorphous phase during the PDA at 300 °C but shows similarly strong anatase peaks at 400 and at 500 °C, suggesting abrupt crystallization. Previously, we have observed that the abrupt am.-TiO2 to anatase TiO2 transition depends on the ALD growth temperature and takes place already at 300 °C for a growth temperature of 150 °C. (18) In contrast, am.-TiO2 grown at 200 °C shows gradual crystallization to mainly rutile TiO2 already at 300 °C. In addition to the rutile phase, some brookite phase characterized by the peaks at 30.8 and 25.8° is also present after the PDA in TiO2 grown at 200 °C. For both growth temperatures, complete crystallization was reached upon the PDA at 500 °C. (12,18) Broader XRD peaks reflect in average smaller grain size for the rutile TiO2 films compared to the anatase TiO2, as supported by the SEM images in the insets of Figure 2.

Figure 2

Figure 2. GIXRD patterns of 30 nm-thick ALD TiO2 grown at (a) 100 °C and (b) 200 °C upon post deposition annealing. The XRD references are from the RRUF database. (45) The insets show the SEM images after PDA at 500 °C.

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As-deposited and PDA 500 °C samples were analyzed by XRR, and TiO2 film thickness, mass density, and roughness were extracted via modeling (Figure S3 and Table 1). The thickness of the sample grown at 100 °C decreased by 3.3% during the PDA, while only a marginal change was observed in the 200 °C grown sample. The densities of TiO2 films grown at 100 and 200 °C were 3.5 and 3.9 g/cm3, respectively. The PDA induced a small decrease in film roughness but had only little if any effect on the film density. The difference in film densities after the PDA can be understood by the higher bulk density of rutile vs anatase TiO2 (4.2 vs 3.9 g/cm3). Quite surprisingly, the apparent mass densities did not change upon crystallization. The decrease in film thickness in the case of the 100 °C grown sample is likely due to the desorption of excess precursor traces and re-structuring of the film.
Table 1. XRR Modeling Results for 30 nm-Thick ALD TiO2 Grown at 100 and 200 °C on Si after Deposition and after Post Deposition Annealing at 500 °C
 TiO2 density (g/cm3)TiO2 thickness (nm)TiO2 roughness (nm)
100 °C, as-deposited3.5230.40.83
100 °C, PDA 500 °C3.5029.40.73
200 °C, as-deposited3.9433.51.06
200 °C, PDA 500 °C3.9333.60.91
These density results are concordant with the values reported by Abendroth et al. for am.-TiO2 grown by using the TDMAT + H2O ALD process at growth temperatures of 120–200 °C. (27) The density increased from 3.65 g/cm3 (120 °C) to 3.95 g/cm3 (200 °C) and reached the highest value of 4.1 g/cm3 at growth temperatures of 320–330 °C, resulting in an anatase–rutile mixed phase with a relatively high N concentration of around 6 at.%. Furthermore, Busani and Devine reported similar densities for anatase (3.62 g/cm3) and mixed anatase–rutile (3.85 g/cm3) TiO2 thin films deposited by PECVD, but before the PDA at 600 °C, the density of their am.-TiO2 was smaller (3.2 g/cm3). (46) Piercy et al. observed ALD TiO2 film density to increase with growth temperature from 3.3 g/cm3 (38 °C) to 3.8 g/cm3 (150 °C) when using TiCl4 and H2O as precursors. (47) Also, the concentration of trace Cl– within the TiO2 films decreased with the growth temperature and was reported to significantly decrease with TiO2 crystallization above 160 °C. Go et al. fabricated ALD TiO2 at 80 °C with TDMAT and O3 using a process where crystallization was induced by the duration of oxygen plasma exposure during the ALD growth cycle. (48) Short (3 s) plasma exposure resulted in less dense (3.73 g/cm3) am.-TiO2, while 30 s plasma exposure was sufficient to grow anatase TiO2 with a density of 4.15 g/cm3. The duration of O3 plasma exposure affected also to the amount of N traces within the TiO2 film and was <2 at.% for the two samples.
Concerning rutile formation, Rafieian et al. reported that tuning of the oxygen concentration during reactive magnetron sputtering can be utilized to fabricate sub-stoichiometric and stoichiometric am.-TiO2, which crystallize into rutile and anatase upon annealing in air at 500 °C, respectively. (29) Furthermore, according to Li et al., oxygen deficiency prefers rutile formation upon rapid thermal annealing (RTA) at 800 °C for 4 min. (21) However, even though the am.-TiO2 grown at 200 °C showed a considerable concentration of Ti3+, the film was not oxygen-deficient as the O2–/Ti ratio was close to 2, and still, the film crystallized as rutile.
The difference in ALD TiO2 crystallization was mediated by the ALD growth temperature that affected essentially the concentration of oxide defects and nitrogen traces. Thus, we tested if the crystallization could be steered toward rutile by the introduction of Ti3+ defects, i.e., oxygen vacancies, to the as-deposited TiO2 using UV light treatment and Ar+ ion bombardment (Figures S4 and S5). The test was performed using ALD TiO2 grown at 150 °C that contains less nitrogen traces compared to TiO2 grown at 100 °C but still favors crystallization to anatase TiO2. (10,18) Neither the UV light treatment nor the Ar+ ion bombardment changed the crystallization from anatase upon the PDA at 500 °C, even though the concentration of Ti3+ defects, within XPS information depth, exceeded that of the 200 °C grown sample. Thus, the result suggests that Ti3+ defects were not the declarative factor determining the crystallization.
Crystallization to anatase TiO2 is more common for PDA temperatures <500 °C. McDowell et al. reported that am.-TiO2 grown from TDMAT and H2O at 150 °C, containing nitrogen impurities but no Ti3+, crystallized into the anatase phase upon 1 h of annealing in air at 500 °C. (49) Furthermore, Pore et al. found that nitrogen and particularly Ti–N bonds prefer formation of pure anatase instead of the anatase–rutile mixed phase. (50) Similar results were also reported by Cheng et al. who considered the anatase phase stabilization to occur due to the compressive stress induced by substitutional nitrogen ions preventing the formation of more dense rutile TiO2. (51) Regarding the anatase to rutile phase transition due to the ionic size effect, nitrogen presumably inhibits the phase transformation, but on the other hand, oxygen vacancies induced by N doping should promote rutile formation. (22) Our results suggest that intrinsic nitrogen defects in am.-TiO2 delay and steer the crystallization toward anatase TiO2, and without nitrogen traces, a more dense am.-TiO2 favors crystallization to also more dense rutile TiO2.
The scanning electron microscopy images in the insets of Figure 2a,b highlight the prominent difference in the morphology and grain size of anatase and rutile TiO2 (PDA 500 °C). The same SEM images in a larger image size are presented in Figure 3. ALD TiO2 grown at 100 °C exhibits exceptionally large anatase grains with a lateral size of >10 μm, which is over 300 times larger than the film thickness. Based on literature, anatase grains with similar magnitude of size have been fabricated by post deposition annealing of ALD grown amorphous Ti–Nb–O or Ti–Ta–O mixed oxide films. (52) Crystallization of undoped ALD grown am.-TiO2 into anatase is reported to result in micron-wide grains, instead. (49,53) The rutile thin film (PDA-treated TiO2 grown at 200 °C), by contrast, consists of much smaller grains (<1 μm).

Figure 3

Figure 3. SEM images of 30 nm-thick ALD TiO2 grown at (a–d) 100 °C and (e–h) 200 °C: (a, e) as-deposited and after post deposition annealing (50 min) at (b, f) 300 °C, (c, g) 400 °C, and (d, h) 500 °C. All the images were taken with the same magnification.

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As implied by GIXRD results (Figure 2) and sizes of the anatase and rutile grains, the crystallization kinetics of ALD am.-TiO2 grown at 100 and 200 °C are totally different. Figure 3 shows that full crystalline coverage on TiO2 grown at 200 °C is obtained already during PDA at 300 °C (Figure 3f), whereas TiO2 grown at 100 °C shows no crystal nucleation after the same PDA treatment (Figure 3b). However, during PDA at 400 °C (Figure 3c), large grains appear in a 100 °C grown film. To find the nucleation temperature of ALD TiO2 grown at 100 °C, the PDA temperature range of 300–400 °C was studied in more detail. Figure S6 reveals that, upon PDA 350 °C treatment, some tiny crystal nuclei appear probably due to random impurities or defects but PDA at 375 °C leads to partial surface crystallization with large round grains with a diameter over 10 μm.
To understand how the oxide defects and nitrogen precursor traces evolve upon crystallization of ALD TiO2, the Ti and N species at the surface were analyzed as a function of PDA temperature. Figure 4 shows the Ti and N species (cf., Figure 1) for the N-rich ALD TiO2 grown at 100 °C (Figure S8 shows the XPS spectra). The concentration of Ti defects (Ti3+ and Ti5/7c4+ species) was shown to decrease gradually with increasing PDA temperature until they disappeared completely at 400 °C that coincided with the crystallization. In the case of ALD TiO2 grown at 200 °C, the Ti defects disappeared already at 250 °C (Figures S7 and S9) in line with the lower crystallization temperature. The evolution of N species in the case of the 200 °C grown sample was included in our previous work; besides 350 °C PDA temperature where surface enrichment of 1.8 at.% N (N 1s at 400.0 ± 0.1 eV) was detected, the amount of N at the surface was too low (0.2 at.%) for the analysis. (12) In contrast, N species in ALD TiO2 grown at 100 °C showed a clear evolution with PDA temperature while there was only a slight decrease in the total amount of surface N (from 0.86 to 0.78 at.% N and from 2.8 to 1.8 at.% N/Ti). As the temperature increased from 200 to 400 °C, the amount of protonated dimethylamine (H2N(CH3)2+) reaction byproducts (i.e., HN(CH3)2 reacted with the −OH group at 401.5 eV) decreased with a rather concomitant increase in Ti–O–N species (at 398.4 eV). Thus, it is suggested that, during the PDA, N–C bonds break and CH4 desorbs as follows
(1)

Figure 4

Figure 4. Evolution of (a) Ti and (b) N species as a function of the PDA temperature for 30 nm-thick ALD TiO2 grown at 100 °C. The binding energy values of N 1s components in panel (b) are depicted in brackets.

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For the PDA temperature above crystallization, a high binding energy component appears in N 1s at 401.9 eV, slightly above the H2N(CH3)2+ component observed for lower temperatures at 401.5 eV. The binding energy corresponds to strongly oxidized nitrogen species, (54) and it is suggested that, for temperatures >400 °C, Ti–O–N species oxidize to Ti–O–N–O. Substitutional nitrogen (N 1s at 396.0 eV) appeared for temperatures above the crystallization temperature (400 °C). Such substitutional N species induce visible-light absorption that is desirable in photocatalysis. (39) However, in our case, the amount of substitutional nitrogen was too small to induce visible-light absorption (cf., Figure S1c in ref (18)). In addition, at 300 °C, a minor increase in the N 1s peak component at 400.0 eV was observed for the ALD TiO2 grown at 100 °C, which suggests surface segregation and subsequent desorption of HN(CH3)2 that were observed to take place at 350 °C for the ALD TiO2 grown at 200 °C. (12) Furthermore, these dimethylamine reaction byproducts and unreacted TDMAT ligands (at 398.6 eV) will likely react or oxidize to N–O-like species during the annealing.
Nitrogen is known to inhibit the TiO2 crystal nucleation and raise the nucleation temperature. (55) Nitrogen also stabilizes the anatase phase. (22,51) According to Hukari et al., the nucleation onset temperature of stoichiometric am.-TiO2 is in the range of 250–300 °C, whereas an additional nitrogen content can raise the nucleation temperature up to 400 °C. (55) Our results show that the excess of N-bearing TDMAT fragments or reaction byproducts within am.-TiO2 can cause the aforementioned N-mediated effects. Nitrogen defects can also contribute to the exceptionally large anatase crystal size detected for ALD TiO2 grown at 100 °C. Similarly large crystals have been observed to result from explosive crystallization of Ti–Nb–O or Ti–Ta–O mixed oxide films prepared by Pore et al. (52) In the explosive crystallization process, the amorphous material crystallizes rapidly in a self-sustaining manner due to the latent heat released during the crystallization and consequently inducing further amorphous material to convert into crystalline form. (52,56,57) Although the explosive crystallization is an autocatalytic phenomenon, the speed of the process depends still on the temperature and orientation of the crystal growth. (56) However, as shown in Figure S6c and in the work by Pore et al., crystallization fronts proceed until collision with an adjacent grain and therefore inhibited nucleation and low distribution density of crystal nuclei is a desired feature when targeting formation of exceptionally large grains. (52,58) By increasing temperature, further energy barriers for nucleation are overcome and complete crystallization is obtained (Figure S6d). (52)
In contrast to the ALD TiO2 grown at 100 °C, the ALD TiO2 grown at 200 °C containing only little N traces evidences strictly different crystallization properties. As shown in Figure 5a, the ALD TiO2 grown at 200 °C shows partial surface crystallization already at 250 °C. Furthermore, Figure 5 depicts that extending the PDA duration at 250 °C from 50 min (Figure 5a) to 500 min (Figure 5b) resulted in complete surface crystallization with crystal size <1 μm. The GIXRD patterns (Figure 5c) show both rutile and brookite TiO2 peaks indicating the mixed-phase TiO2 thin film with rutile as the primary phase. The rutile to brookite ratio was found to increase with increasing PDA temperature (cf. Figure 2).

Figure 5

Figure 5. SEM images of 30 nm-thick ALD TiO2 grown at 200 °C after PDA at 250 °C (a) for 50 min and (b) for 500 min. (c) GIXRD patterns of the samples. The XRD references are from the RRUF database. (45)

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Two distinct crystal morphologies were observed after crystal nucleation of am.-TiO2: needle- and round-like crystals (Figure 5a). Li et al. stated that the preferential growth of different rutile crystal morphologies in the aqueous-phase process is attributed to the concentration of TiOH3+ ions around the TiO2 nucleus. (25) A low TiOH3+ concentration around the nucleus induces needle-like morphology, whereas a high concentration prefers radial growth of crystalline material. Comparing this crystallization mechanism to our observations, we propose that the different crystal morphologies may be analogously induced by local variations in oxide defect density within the am.-TiO2 structure.
In general, the crystallization of am.-TiO2 can be considered a random process that is affected by many factors. Nucleation, i.e., how TiO6 octahedra join each other, determines whether am.-TiO2 crystallizes as anatase or rutile. (59) Growth of metastable anatase is statistically more likely since there are more possibilities for octahedra to join at right angles compared to joining linearly sharing two edges required for the rutile phase. (22) Apparently, the chemical composition and structure of the am.-TiO2 thin film predetermined the resulting phase structure upon the PDA. We note that the crystallization can be affected by, for example, film thickness, substrate, and PDA process parameters that should be studied separately. Also, it is worth pointing out that crystallization can proceed at even lower temperatures if the duration of the PDA treatment is extended.
The effects of intrinsic defects on crystallization of ALD am.-TiO2 upon PDA are summarized in Table 2. Lower growth temperature (100 °C) prefers trapping of TDMAT-based nitrogen species that leads to delayed crystal nucleation (at 375 °C) and exceptionally large anatase grains (>10 μm). Higher ALD growth temperature (200 °C) results in higher mass density and higher concentration of intrinsic oxide defects but less nitrogen traces. Interestingly, this am.-TiO2 favors direct crystallization into mixed rutile-brookite phase TiO2 with rutile as the primary phase already at 250 °C, which is exceptionally low for TiO2 thin films. ALD of the anatase–rutile mixed phase thin film has been reported at 300 °C. (23,24,27) Fabrication of rutile-rich thin films at relatively low temperature is of interest in optical and photocatalytic applications since rutile exhibits optically anisotropic nature, a high refractive index, and desirable catalytic activity in oxidation reactions. (3,19,60) Lowering the fabrication temperature of photocatalytically active TiO2 thin films enables applications involving temperature-sensitive materials such as polymers. Furthermore, enabling low processing temperatures (250–400 °C) to form rutile-rich TiO2 thin films without an additional seed layer is of interest for high-κ applications, such as dynamic random-access memory (DRAM) capacitors. (61,62) Our work demonstrated that tuning the ALD TiO2 process parameters enabled lowering the fabrication temperature of the rutile-rich TiO2 thin film to 250 °C. Controlling the thin film defects was essential to lowering the crystallization temperature, and we believe that the provided mechanistic understanding paves the way to further optimize the process for even higher rutile purity at even lower temperatures.
Table 2. Summary of Growth and Crystallization of am.-TiO2 Thin Films Grown by ALD Using TDMAT and H2O Precursors at the Growth Temperatures of 100 and 200 °C
a

Nitrogen species originated from dimethylamide ligands of TDMAT molecules. The plus signs represent the surface concentration determined by the XPS measurement.

b

Oxygen vacancies, interstitial peroxo species (O22–), and Ti3+/Ti5/7c4+ ions formed via displacement of oxygen ions within the stoichiometric amorphous TiO2 structure. The plus signs represent the surface concentration determined by the XPS measurement.

c

The results are based on a PDA time of 50 min.

d

The drawings of crystalline TiO2 structures were produced by VESTA software (63) using rutile (64) and anatase (65) crystal structure models provided by the American Mineralogist Crystal Structure Database. (66)

e

Rutile is the main phase. The proportion of brookite decreases at higher PDA temperatures.

Conclusions

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The role of intrinsic defects on crystallization of am.-TiO2 thin films grown by ALD using TDMAT and H2O precursors upon PDA treatment in air was studied as a function of ALD growth temperature (100–200 °C). The am.-TiO2 film grown at 100 °C contained 0.9 at.% of nitrogen-bearing traces of the TDMAT precursor that inhibited crystal nucleation up to 375 °C and led to explosive crystallization of large anatase grains (>10 μm). In contrast, the am.-TiO2 grown at 200 °C contained less precursor traces (0.2 at.% N) and crystallized into the mixed-phase (rutile–brookite) TiO2 thin film with rutile as the primary phase at 250 °C. The concentration of Ti3+ defects increased with ALD growth temperature, and upon crystallization, these oxide defects were completely removed. Increasing the amount of Ti3+ defects within am.-TiO2 either by UV treatment or Ar+-ion sputtering did not affect the crystallization. The mass densities of ALD TiO2 thin films were 3.5 g/cm3 for the film grown at 100 °C and 3.9 g/cm3 for the film grown at 200 °C. The mass densities did not change during the crystallization. Therefore, it is suggested that the growth of denser am.-TiO2 mediates direct crystallization into a more dense and stable rutile phase, while the growth of less dense am.-TiO2 with precursor traces favors crystallization into a metastable anatase phase. Often, am.-TiO2 thin films crystallize first into anatase and subsequently transform from an anatase to rutile phase requiring typically PDA temperatures >500 °C. By controlling the ALD growth temperature, we were able to demonstrate direct amorphous to rutile-rich TiO2 thin film fabrication at 250–500 °C that has potential applications in the fields requiring high-κ thin films such as DRAM capacitors. Moreover, the fabrication of crystalline TiO2 thin films at an exceptionally low temperature of 250 °C enables applications involving temperature-sensitive materials such as polymers.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c04905.

  • Data and analysis from XPS, SEM, XRR, and GIXRD measurements (PDF)

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Author Information

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  • Corresponding Authors
    • Harri Ali-Löytty - Surface Science Group, Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland;  Orcidhttps://orcid.org/0000-0001-8746-7268;  Email: [email protected]
    • Mika Valden - Surface Science Group, Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland;  Email: [email protected]
  • Authors
    • Jesse Saari - Surface Science Group, Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland;  Orcidhttps://orcid.org/0000-0001-6741-0838
    • Kimmo Lahtonen - Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland;  Orcidhttps://orcid.org/0000-0002-8138-7918
    • Markku Hannula - Surface Science Group, Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland;  Orcidhttps://orcid.org/0000-0003-1110-7439
    • Lauri Palmolahti - Surface Science Group, Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland;  Orcidhttps://orcid.org/0000-0001-9992-6628
    • Antti Tukiainen - Faculty of Engineering and Natural Sciences, Tampere University, P.O.B. 692, Tampere FI-33014, Finland
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge Cliona Shakespeare for the XPS measurement series investigating the intrinsic titanium and nitrogen defects as a function of post deposition annealing temperature and Tuomo Nyyssönen for the GIXRD measurements carried out at the Department of Materials Science. This work is part of the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN) (decision number 320165) and was supported by the Academy of Finland (decision numbers 326461 and 326406), by the Jane & Aatos Erkko Foundation (project “Solar Fuels Synthesis”), and by Business Finland (TUTLi project “Liquid Sun”) (decision number 1464/31/2019). J.S. was supported by the Vilho, Yrjö and Kalle Väisälä Foundation of the Finnish Academy of Science and Letters and L.P. by the KAUTE Foundation and Finnish Cultural Foundation.

References

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    Diversity of TiO2: Controlling the Molecular and Electronic Structure of Atomic-Layer-Deposited Black TiO2
    Ali-Loytty, Harri; Hannula, Markku; Saari, Jesse; Palmolahti, Lauri; Bhuskute, Bela D.; Ulkuniemi, Riina; Nyyssonen, Tuomo; Lahtonen, Kimmo; Valden, Mika
    ACS Applied Materials & Interfaces (2019), 11 (3), 2758-2762CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Visually black, elec. leaky, metallic glasses titania (am-TiO2) thin films were grown by at. layer deposition (ALD) for photocatalytic applications. Broad spectral absorbance in the visible range and exceptional cond. are attributed to trapped Ti3+ in the film. Oxidn. of Ti3+ upon heat treatment leads to a drop in cond., a color change from black to white, and crystn. of am-TiO2. ALD-grown black TiO2, without any heat treatment, is subject to dissoln. in alk. photoelectrochem. conditions. The best photocatalytic activity for solar water splitting is obtained for completely cryst. white TiO2.
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  13. 13
    Hannula, M.; Ali-Löytty, H.; Lahtonen, K.; Sarlin, E.; Saari, J.; Valden, M. Improved Stability of Atomic Layer Deposited Amorphous TiO2 Photoelectrode Coatings by Thermally Induced Oxygen Defects. Chem. Mater. 2018, 30, 1199– 1208,  DOI: 10.1021/acs.chemmater.7b02938
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    13
    Improved Stability of Atomic Layer Deposited Amorphous TiO2 Photoelectrode Coatings by Thermally Induced Oxygen Defects
    Hannula, Markku; Ali-Loytty, Harri; Lahtonen, Kimmo; Sarlin, Essi; Saari, Jesse; Valden, Mika
    Chemistry of Materials (2018), 30 (4), 1199-1208CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Amorphous titania (a-TiO2) combined with an electrocatalyst has shown to be a promising coating for stabilizing traditional semiconductor materials used in artificial photosynthesis for efficient photoelectrochem. solar-to-fuel energy conversion. Here, we report a detailed anal. of 2 methods of modifying an undoped thin film of at. layer deposited (ALD) a-TiO2 without an electrocatalyst to affect its performance in water splitting reaction as a protective photoelectrode coating. The methods are high-temp. annealing in ultrahigh vacuum and at. H exposure. A key feature in both methods is that they preserve the amorphous structure of the film. Special attention is paid to the changes in the mol. and electronic structure of a-TiO2 induced by these treatments. On the basis of the photoelectrochem. results, the a-TiO2 is susceptible to photocorrosion but significant improvement in stability is achieved after heat treatment in vacuum at >500°. On the other hand, the H treatment does not increase the stability despite the ostensibly similar redn. of a-TiO2. The surface anal. allows us to interpret the improved stability to the thermally induced formation of O- species within a-TiO2 that are essentially electronic defects in the anionic framework.
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  14. 14
    Correa, G. C.; Bao, B.; Strandwitz, N. C. Chemical Stability of Titania and Alumina Thin Films Formed by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2015, 7, 14816– 14821,  DOI: 10.1021/acsami.5b03278
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    14
    Chemical Stability of Titania and Alumina Thin Films Formed by Atomic Layer Deposition
    Correa, Gabriela C.; Bao, Bo; Strandwitz, Nicholas C.
    ACS Applied Materials & Interfaces (2015), 7 (27), 14816-14821CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Thin films formed by at. layer deposition (ALD) are being examd. for a variety of chem. protection and diffusion barrier applications, yet their stability in various fluid environments is not well characterized. The chem. stability of titania and alumina thin films in air, 18 MΩ water, 1 M KCl, 1 M HNO3, 1 M H2SO4, 1 M HCl, 1 M KOH, and mercury was studied. Films were deposited at 150 °C using trimethylaluminum-H2O and tetrakis(dimethylamido)titanium-H2O chemistries for alumina and titania, resp. A subset of samples were heated to 450 and 900 °C in inert atm. Films were examd. using spectroscopic ellipsometry, at. force microscopy, optical microscopy, SEM, and X-ray diffraction. Notably, alumina samples were found to be unstable in pure water, acid, and basic environments in the as-synthesized state and after 450 °C thermal treatment. In pure water, a dissoln.-pptn. mechanism is hypothesized to cause surface roughening. The stability of alumina films was greatly enhanced after annealing at 900 °C in acidic and basic solns. Titania films were found to be stable in acid after annealing at or above 450 °C. All films showed a compn.-independent increase in measured thickness when immersed in mercury. These results provide stability-processing relationships that are important for controlled etching and protective barrier layers.
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  15. 15
    Kriegel, H.; Kollmann, J.; Raudsepp, R.; Klassen, T.; Schieda, M. Chemical and Photoelectrochemical Instability of Amorphous TiO2 Layers Quantified by Spectroscopic Ellipsometry. J. Mater. Chem. A 2020, 8, 18173– 18179,  DOI: 10.1039/D0TA04878J
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    Saari, J.; Ali-Löytty, H.; Honkanen, M.; Tukiainen, A.; Lahtonen, K.; Valden, M. Interface Engineering of TiO2 Photoelectrode Coatings Grown by Atomic Layer Deposition on Silicon. ACS Omega 2021, 6, 27501– 27509,  DOI: 10.1021/acsomega.1c04478
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    16
    Interface Engineering of TiO2 Photoelectrode Coatings Grown by Atomic Layer Deposition on Silicon
    Saari, Jesse; Ali-Loytty, Harri; Honkanen, Mari; Tukiainen, Antti; Lahtonen, Kimmo; Valden, Mika
    ACS Omega (2021), 6 (41), 27501-27509CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    Titanium dioxide (TiO2) can protect photoelectrochem. (PEC) devices from corrosion, but the fabrication of high-quality TiO2 coatings providing long-term stability has remained challenging. Here, we compare the influence of Si wafer cleaning and postdeposition annealing temp. on the performance of TiO2/n+-Si photoanodes grown by at. layer deposition (ALD) using tetrakis(dimethylamido)titanium (TDMAT) and H2O as precursors at a growth temp. of 100°C. We show that removal of native Si oxide before ALD does not improve the TiO2 coating performance under alk. PEC water splitting conditions if excessive postdeposition annealing is needed to induce crystn. The as-deposited TiO2 coatings were amorphous and subject to photocorrosion. However, the TiO2 coatings were found to be stable over a time period of 10 h after heat treatment at 400°C that induced crystn. of amorphous TiO2 into anatase TiO2. No interfacial Si oxide formed during the ALD growth, but during the heat treatment, the thickness of interfacial Si oxide increased to 1.8 nm for all of the samples. Increasing the ALD growth temp. to 150°C enabled crystn. at 300°C, which resulted in reduced growth of interfacial Si oxide followed by a 70 mV improvement in the photocurrent onset potential.
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  17. 17
    Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett. 2018, 3, 1795– 1800,  DOI: 10.1021/acsenergylett.8b00920
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    17
    Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency
    Cheng, Wen-Hui; Richter, Matthias H.; May, Matthias M.; Ohlmann, Jens; Lackner, David; Dimroth, Frank; Hannappel, Thomas; Atwater, Harry A.; Lewerenz, Hans-Joachim
    ACS Energy Letters (2018), 3 (8), 1795-1800CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chem. bonds, requires optimization of a photoelectrochem. device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic light absorption and reflection losses. A tailored multifunctional cryst. titania interphase layer acts as a corrosion protection layer, with favorable band alignment between the semiconductor conduction band and the energy level for water redn., facilitating electron transport at the cathode-electrolyte interface. It also provides a favorable substrate for adhesion of high-activity Rh catalyst nanoparticles. Under simulated AM 1.5G irradn., solar-to-H efficiencies of 19.3 and 18.5% are obtained in acidic and neutral electrolytes, resp. The system reaches 0.85 of the theor. limit for photoelectrochem. water splitting for the energy gap combination employed in the tandem-junction photoelectrode structure.
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  18. 18
    Khan, R.; Ali-Löytty, H.; Saari, J.; Valden, M.; Tukiainen, A.; Lahtonen, K.; Tkachenko, N. V. Optimization of Photogenerated Charge Carrier Lifetimes in ALD Grown TiO2 for Photonic Applications. Nanomaterials 2020, 10, 1567,  DOI: 10.3390/nano10081567
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    18
    Optimization of photogenerated charge carrier lifetimes in ALD grown TiO2 for photonic applications
    Khan, Ramsha; Ali-Loytty, Harri; Saari, Jesse; Valden, Mika; Tukiainen, Antti; Lahtonen, Kimmo; Tkachenko, Nikolai V.
    Nanomaterials (2020), 10 (8), 1567CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
    Titanium dioxide (TiO2) thin films are widely employed for photocatalytic and photovoltaic applications where the long lifetime of charge carriers is a paramount requirement for the device efficiency. To ensure the long lifetime, a high temp. treatment is used which restricts the applicability of TiO2in devices incorporating org. or polymer components. In this study, we exploited low temp. (100-150°C) at. layer deposition (ALD) of 30 nm TiO2thin films from tetrakis(dimethylamido)titanium. The deposition was followed by a heat treatment in air to find the min. temp. requirements for the film fabrication without compromising the carrier lifetime. Femto-to nanosecond transient absorption spectroscopy was used to det. the lifetimes, and grazing incidence X-ray diffraction was employed for structural anal. The optimal result was obtained for the TiO2 thin films grown at 150°C and heat-treated at as low as 300°C. The deposited thin films were amorphous and crystd. into anatase phase upon heat treatment at 300-500°C. The av. carrier lifetime for amorphous TiO2 is few picoseconds but increases to >400 ps upon crystn. at 500°C. The samples deposited at 100°C were also crystd. as anatase but the carrier lifetime was <100 ps.
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  19. 19
    Yamakata, A.; Vequizo, J. J. M. Curious Behaviors of Photogenerated Electrons and Holes at the Defects on Anatase, Rutile, and Brookite TiO2 Powders: A Review. J. Photochem. Photobiol., C 2019, 40, 234– 243,  DOI: 10.1016/j.jphotochemrev.2018.12.001
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    19
    Curious behaviors of photogenerated electrons and holes at the defects on anatase, rutile, and brookite TiO2 powders: A review
    Yamakata, Akira; Vequizo, Junie Jhon M.
    Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2019), 40 (), 234-243CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
    A review. Photocatalytic reactions are governed by photogenerated charge carriers upon band gap excitation. Therefore, for better understanding of the mechanism, the dynamics of photocarriers should be studied. One of the attractive materials is TiO2, which has been extensively investigated in the field of photocatalysis. This review article summarizes our recent works of time-resolved visible to mid-IR absorption measurements to elucidate the difference of anatase, rutile, and brookite TiO2 powders. The distinctive photocatalytic activities of these polymorphs are detd. by the electron-trapping processes at the defects on powders. Powders are rich in defects and these defects capture photogenerated electrons. The depth of the trap is crystal phase dependent, and they are estd. to be < 0.1 eV, ∼0.4 eV and ∼0.9 eV for anatase, brookite, and rutile, resp. Electron trapping reduces probability to meet with holes and then elongate the lifetime of holes. Therefore, it works neg. for the reaction of electrons but pos. works for the reaction of holes. In the steady-state reactions, both electrons and holes should be consumed. Hence, the balance between the pos. and neg. effects of defects dets. the distinctive photocatalytic activities of anatase, rutile, and brookite TiO2 powders.
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  20. 20
    Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P. Band Alignment of Rutile and Anatase TiO2. Nat. Mater. 2013, 12, 798– 801,  DOI: 10.1038/nmat3697
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    Band alignment of rutile and anatase TiO2
    Scanlon, David O.; Dunnill, Charles W.; Buckeridge, John; Shevlin, Stephen A.; Logsdail, Andrew J.; Woodley, Scott M.; Catlow, C. Richard A.; Powell, Michael. J.; Palgrave, Robert G.; Parkin, Ivan P.; Watson, Graeme W.; Keal, Thomas W.; Sherwood, Paul; Walsh, Aron; Sokol, Alexey A.
    Nature Materials (2013), 12 (9), 798-801CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO2. The discovery of the photolysis of water on the surface of TiO2 in 1972 launched four decades of intensive research into the underlying chem. and phys. processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO2. We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission expts., that a type-II, staggered, band alignment of ∼ 0.4eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust sepn. of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.
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  21. 21
    Li, A.; Wang, Z.; Yin, H.; Wang, S.; Yan, P.; Huang, B.; Wang, X.; Li, R.; Zong, X.; Han, H.; Li, C. Understanding the Anatase–Rutile Phase Junction in Charge Separation and Transfer in a TiO2 Electrode for Photoelectrochemical Water Splitting. Chem. Sci. 2016, 7, 6076– 6082,  DOI: 10.1039/C6SC01611A
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    21
    Understanding the anatase-rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting
    Li, Ailong; Wang, Zhiliang; Yin, Heng; Wang, Shengyang; Yan, Pengli; Huang, Baokun; Wang, Xiuli; Li, Rengui; Zong, Xu; Han, Hongxian; Li, Can
    Chemical Science (2016), 7 (9), 6076-6082CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    New insight into junction-based designs for efficient charge sepn. is vitally important for current solar energy conversion research. Herein, an anatase-rutile phase junction is elaborately introduced into TiO2 films by rapid thermal annealing treatment and the roles of phase junction on charge sepn. and transfer are studied in detail. A combined study of transient absorption spectroscopy, electrochem. and photoelectrochem. (PEC) measurements reveals that appropriate phase alignment is essential for unidirectional charge transfer, and a junction interface with minimized trap states is crucial to liberate the charge sepn. potential of the phase junction. By tailored control of phase alignment and interface structure, an optimized TiO2 film with an appropriately introduced phase junction shows superior performance in charge sepn. and transfer, hence achieving ca. 3 and 9 times photocurrent d. enhancement compared to pristine anatase and rutile phase TiO2 electrodes, resp. This work demonstrates the great potential of phase junctions for efficient charge sepn. and transfer in solar energy conversion applications.
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  22. 22
    Hanaor, D. A. H.; Sorrell, C. C. Review of the Anatase to Rutile Phase Transformation. J. Mater. Sci. 2011, 46, 855– 874,  DOI: 10.1007/s10853-010-5113-0
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    Review of the anatase to rutile phase transformation
    Hanaor, Dorian A. H.; Sorrell, Charles C.
    Journal of Materials Science (2011), 46 (4), 855-874CODEN: JMTSAS; ISSN:0022-2461. (Springer)
    A review. Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examd., followed by a discussion of the thermodn. of the phase transformation and the factors affecting its observation. A comprehensive anal. of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius vs. the valence characterized by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which exptl. data are unavailable, on the phase transformation are deduced and presented on the basis of this anal.
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    Aarik, J.; Aidla, A.; Kiisler, A.-A.; Uustare, T.; Sammelselg, V. Effect of Crystal Structure on Optical Properties of TiO2 Films Grown by Atomic Layer Deposition. Thin Solid Films 1997, 305, 270– 273,  DOI: 10.1016/S0040-6090(97)00135-1
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    Effect of crystal structure on optical properties of TiO2 films grown by atomic layer deposition
    Aarik, Jaan; Aidla, Aleks; Kiisler, Alma-Asta; Uustare, Teet; Sammelselg, Vaeino
    Thin Solid Films (1997), 305 (1,2), 270-273CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
    The dependence of optical characteristics on the structure of at. layer-deposited TiO2 thin films is studied. Amorphous films grown at 100° have an optical band gap of 3.3 eV, while their refractive index is 2.2-2.3 at 633 nm. The refractive index can be increased up to 2.65 by using growth temps. ∼300°. Compn. and structure studies reveal that the formation of preferentially oriented crystal (anatase) structure contributes to this increase of refractive index most significantly.
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    Reiners, M.; Xu, K.; Aslam, N.; Devi, A.; Waser, R.; Hoffmann-Eifert, S. Growth and Crystallization of TiO2 Thin Films by Atomic Layer Deposition Using a Novel Amido Guanidinate Titanium Source and Tetrakis-Dimethylamido-Titanium. Chem. Mater. 2013, 25, 2934– 2943,  DOI: 10.1021/cm303703r
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    24
    Growth and Crystallization of TiO2 Thin Films by Atomic Layer Deposition Using a Novel Amido Guanidinate Titanium Source and Tetrakis-dimethylamido-titanium
    Reiners, Marcel; Xu, Ke; Aslam, Nabeel; Devi, Anjana; Waser, Rainer; Hoffmann-Eifert, Susanne
    Chemistry of Materials (2013), 25 (15), 2934-2943CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The authors studied the growth of TiO2 by liq. injection at. layer deposition (ALD) using two different amide-based Ti sources, tetrakis(dimethylamido)titanium [(NMe2)4-Ti, TDMAT] and its recently developed deriv., tris(dimethylamido)mono(N,N'-diisopropyldimethylamidoguanidinato)titanium {[(N-iPr)2NMe2]Ti(NMe2)3, TiA3G1}, with H2O vapor as counterreactant. A clear satn. of growth with an increasing precursor supply was found for TDMAT between 150 and 300° and for TiA3G1 between 150 and 330°. Representative growth per cycle (GPC) values at 250° were 0.041 and 0.044 nm/cycle, resp. Compared to that of TDMAT, ALD of TiA3G1 exhibited a significantly higher stability in the GPC values up to 300° coinciding with an improved temp. stability of the precursor. Both processes showed a min. of the growth rate as a function of temp. In all cases, the residual C and N contents of the TiO2 films were <3 atom %. Conformal growth was demonstrated on three-dimensional pinhole structures with an aspect ratio of ∼1:30. Deposition temps. of ≤200° led to quasi-amorphous films. At higher growth temps., the anatase phase developed, accompanied by the brookite and/or the rutile phase depending on process conditions, deposition temp., and film thickness.
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    Li, Y.; Liu, J.; Jia, Z. Morphological Control and Photodegradation Behavior of Rutile TiO2 Prepared by a Low-Temperature Process. Mater. Lett. 2006, 60, 1753– 1757,  DOI: 10.1016/j.matlet.2005.12.012
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    Morphological control and photodegradation behavior of rutile TiO2 prepared by a low-temperature process
    Li, Yuanyuan; Liu, Jinping; Jia, Zhijie
    Materials Letters (2006), 60 (13-14), 1753-1757CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
    Flower-like rutile titania nanocrystals were prepd. via a simple aq.-phase stirring for 24 h at a low temp. of 75°C, employing only TiCl4, HCl as the starting materials. XRD result proved the formation of rutile TiO2. The observations from TEM and SEM showed that the products were large-scale flower-shaped structures composed of radial nanorods. Comparative expts. demonstrated that pinecone-like, needlelike rutile TiO2 could be easily achieved by varying the vol. ratio of TiCl4/H2O. The growth mechanisms of TiO2 nanostructures prepd. under different conditions and their photodegrdn. behavior were also discussed. It was found that the flower-like structures exhibited the highest photocatalytic activity in the photodegrdn. of aq. brilliant red X-3B soln.
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    Yang, K.; Zhu, J.; Zhu, J.; Huang, S.; Zhu, X.; Ma, G. Sonochemical Synthesis and Microstructure Investigation of Rod-like Nanocrystalline Rutile Titania. Mater. Lett. 2003, 57, 4639– 4642,  DOI: 10.1016/S0167-577X(03)00376-8
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    Sonochemical synthesis and microstructure investigation of rod-like nanocrystalline rutile titania
    Yang, Ke; Zhu, Jianmin; Zhu, Junjie; Huang, Shisong; Zhu, Xinhua; Ma, Guobin
    Materials Letters (2003), 57 (30), 4639-4642CODEN: MLETDJ; ISSN:0167-577X. (Elsevier Science B.V.)
    Rod-like and star-like nanocryst. rutile TiO2 were prepd. by a sonochem. method in TiCl4/HCl soln. The product was studied by XRD, TEM, SAED, and HRTEM. The product is purely rutile. The formation of rutile TiO2 may be due to rutile's inherent high-temp. stability. The appearance of star-like structures should be attributed to the conditions of sonochem. method.
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    Abendroth, B.; Moebus, T.; Rentrop, S.; Strohmeyer, R.; Vinnichenko, M.; Weling, T.; Stöcker, H.; Meyer, D. C. Atomic Layer Deposition of TiO2 from Tetrakis(Dimethylamino)Titanium and H2O. Thin Solid Films 2013, 545, 176– 182,  DOI: 10.1016/j.tsf.2013.07.076
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    Atomic layer deposition of TiO2 from tetrakis(dimethylamino)titanium and H2O
    Abendroth, Barbara; Moebus, Theresa; Rentrop, Solveig; Strohmeyer, Ralph; Vinnichenko, Mykola; Weling, Tobias; Stoecker, Hartmut; Meyer, Dirk C.
    Thin Solid Films (2013), 545 (), 176-182CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)
    The at. layer deposition (ALD) of TiO2 from tetrakis(dimethylamino)titanium (TDMAT) and water was studied in the substrate temp. (TS) range of 120 to 330°C. The effect of deposition temps. on the resulting layer microstructure was investigated. Possible interaction mechanisms of TDMAT and H2O precursor mols. and the TiO2 surface at different temps. are discussed. The TiO2 layers were characterized with respect to microstructure, compn., and optical properties by glancing angle x-ray diffraction and reflectometry, x-ray fluorescence anal., photoelectron spectroscopy, and spectroscopic ellipsometry. A const. layer growth with increasing no. of ALD cycles was achieved for all investigated deposition temps. if the inert gas purge time after the H2O pulse was increased from 5 s at temps. below 250°C to 25 s for TS ≥ 320°C. In the investigated temp. range, the growth per cycle varies between 0.33 and 0.67 Å/cycle with a min. at 250°C. The variations of the deposition rate are related to a change from a surface detd. decompn. of TDMAT to a gas phase decompn. route above 250°C. At the same temp., the microstructure of the TiO2 layers changes from amorphous to predominately cryst., where both anatase and rutile are present.
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    Niemelä, J.-P.; Marin, G.; Karppinen, M. Titanium Dioxide Thin Films by Atomic Layer Deposition: A Review. Semicond. Sci. Technol. 2017, 32, 093005  DOI: 10.1088/1361-6641/aa78ce
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    Titanium dioxide thin films by atomic layer deposition: a review
    Niemela, Janne-Petteri; Marin, Giovanni; Karppinen, Maarit
    Semiconductor Science and Technology (2017), 32 (9), 093005/1-093005/43CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
    Within its rich phase diagram titanium dioxide is a truly multifunctional material with a property palette that has been shown to span from dielec. to transparent-conducting characteristics, in addn. to the well-known catalytic properties. At the same time down-scaling of microelectronic devices has led to an explosive growth in research on at. layer deposition (ALD) of a wide variety of frontier thin-film materials, among which TiO2 is one of the most popular ones. In this topical review we summarize the advances in research of ALD of titanium dioxide starting from the chemistries of the over 50 different deposition routes developed for TiO2 and the resultant structural characteristics of the films. We then continue with the doped ALD-TiO2 thin films from the perspective of dielec., transparent-conductor and photocatalytic applications. Moreover, in order to cover the latest trends in the research field, both the variously constructed TiO2 nanostructures enabled by ALD and the Ti-based hybrid inorg.-org. films grown by the emerging ALD/MLD (combined at./mol. layer deposition) technique are discussed.
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    Rafieian, D.; Ogieglo, W.; Savenije, T.; Lammertink, R. G. H. Controlled Formation of Anatase and Rutile TiO2 Thin Films by Reactive Magnetron Sputtering. AIP Adv. 2015, 5, 097168  DOI: 10.1063/1.4931925
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    Controlled formation of anatase and rutile TiO2 thin films by reactive magnetron sputtering
    Rafieian, Damon; Ogieglo, Wojciech; Savenije, Tom; Lammertink, Rob G. H.
    AIP Advances (2015), 5 (9), 097168/1-097168/7CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
    We discuss the formation of TiO2 thin films via DC reactive magnetron sputtering. The oxygen concn. during sputtering proved to be a crucial parameter with respect to the final film structure and properties. The initial deposition provided amorphous films that crystallize upon annealing to anatase or rutile, depending on the initial sputtering conditions. Substoichiometric films (TiOx<2), obtained by sputtering at relatively low oxygen concn., formed rutile upon annealing in air, whereas stoichiometric films formed anatase. This route therefore presents a formation route for rutile films via lower (<500°C) temp. pathways. The dynamics of the annealing process were followed by in situ ellipsometry, showing the optical properties transformation. The final crystal structures were identified by XRD. The anatase film obtained by this deposition method displayed high carriers mobility as measured by time-resolved microwave conductance. This also confirms the high photocatalytic activity of the anatase films. (c) 2015 American Institute of Physics.
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    Dendooven, J.; Detavernier, C. Atomic Layer Deposition in Energy Conversion Applications; Bachmann, J., Ed.; John Wiley & Sons, Ltd., 2017; pp. 1– 40.
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    Head, A. R.; Chaudhary, S.; Olivieri, G.; Bournel, F.; Andersen, J. N.; Rochet, F.; Gallet, J.-J.; Schnadt, J. Near Ambient Pressure X-ray Photoelectron Spectroscopy Study of the Atomic Layer Deposition of TiO2 on RuO2(110). J. Phys. Chem. C 2016, 120, 243– 251,  DOI: 10.1021/acs.jpcc.5b08699
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    Near Ambient Pressure X-ray Photoelectron Spectroscopy Study of the Atomic Layer Deposition of TiO2 on RuO2(110)
    Head, Ashley R.; Chaudhary, Shilpi; Olivieri, Giorgia; Bournel, Fabrice; Andersen, Jesper N.; Rochet, Francois; Gallet, Jean-Jacques; Schnadt, Joachim
    Journal of Physical Chemistry C (2016), 120 (1), 243-251CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The at. layer deposition (ALD) of TiO2 on a RuO2(110) surface from tetrakis(dimethylamido) Ti and H2O at 110° was studied using near ambient pressure XPS (NAP-XPS) at precursor pressures up to 0.1 mbar. In addn. to the expected cyclic surface species, evidence for side reactions was found. Dimethylamine adsorbs on the surface during the TDMAT half-cycle, and a 2nd species, likely Me methylenimine, also forms. The removal of the amide ligand and the formation of an alkylammonium species during the H2O half-cycle are pressure dependent. The O 1s, Ru 3d, and Ti 2p spectra show the formation of the Ru-O-Ti interface, and the binding energies are consistent with formation of TiO2 after 1 full ALD cycle. Dosing TDMAT on the RuO2(110) surface at room temp. promotes a multilayer formation that begins to desorb at 40°. The imine species is not seen until 60°. These insights into the ALD mechanism and precursor pressure dependence on reactivity highlight the utility of NAP-XPS in studying ALD processes and interface formation.
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    Fairley, N.; Fernandez, V.; Richard-Plouet, M.; Guillot-Deudon, C.; Walton, J.; Smith, E.; Flahaut, D.; Greiner, M.; Biesinger, M.; Tougaard, S. Systematic and Collaborative Approach to Problem Solving Using X-ray Photoelectron Spectroscopy. Appl. Surf. Sci. Adv. 2021, 5, 100112  DOI: 10.1016/j.apsadv.2021.100112
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    Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV
    Scofield, J. H.
    Journal of Electron Spectroscopy and Related Phenomena (1976), 8 (2), 129-37CODEN: JESRAW; ISSN:0368-2048.
    The results of calcns. of photoelec. cross-sections for the Kα lines of magnesium at 1254 eV and of aluminum at 1487 eV are presented. All of the subshell cross-sections are given for Z values up to 96. The calcns. were carried out relativistically using the single-potential Hartree-Slater at. model.
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    Björck, M.; Andersson, G. GenX: An Extensible X-ray Reflectivity Refinement Program Utilizing Differential Evolution. J. Appl. Crystallogr. 2007, 40, 1174– 1178,  DOI: 10.1107/S0021889807045086
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    Prasai, B.; Cai, B.; Underwood, M. K.; Lewis, J. P.; Drabold, D. A. Properties of Amorphous and Crystalline Titanium Dioxide from First Principles. J. Mater. Sci. 2012, 47, 7515– 7521,  DOI: 10.1007/s10853-012-6439-6
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    Properties of amorphous and crystalline titanium dioxide from first principles
    Prasai, Binay; Cai, Bin; Underwood, M. Kylee; Lewis, James P.; Drabold, D. A.
    Journal of Materials Science (2012), 47 (21), 7515-7521CODEN: JMTSAS; ISSN:0022-2461. (Springer)
    First-principles methods were used to generate amorphous TiO2 (a-TiO2) models and this simulations lead to chem. ordered amorphous networks. The structural, electronic, and optical properties of the resulting structures were analyzed and compared with cryst. phases. Two peaks found in the Ti-Ti pair correlation correspond to edge-sharing and corner-sharing Ti-Ti pairs. Resulting coordination nos. for Ti (≈6) and O (≈3) and the corresponding angle distributions suggest that local structural features of bulk cryst. TiO2 are retained in a-TiO2. The electronic d. of states and the inverse participation ratio reveal that highly localized tail states at the valence band edge are due to the displacement of O atoms from the plane contg. 3 neighboring Ti atoms; whereas, the tail states at the conduction band edge are localized on over-coordinated Ti atoms. The electronic gap of ≈2.2 eV is comparable to calcd. results for bulk cryst. TiO2 despite the presence of topol. disorder in the amorphous network. The calcd. dielec. functions suggest that the amorphous phase of TiO2 has isotropic optical properties in contrast to those of tetragonal rutile and anatase phases. The av. static dielec. const. and the fundamental absorption edge for a-TiO2 are comparable to those of the cryst. phases.
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    Deskins, N. A.; Du, J.; Rao, P. The Structural and Electronic Properties of Reduced Amorphous Titania. Phys. Chem. Chem. Phys. 2017, 19, 18671– 18684,  DOI: 10.1039/C7CP02940C
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    The structural and electronic properties of reduced amorphous titania
    Deskins, N. Aaron; Du, Jincheng; Rao, Pratap
    Physical Chemistry Chemical Physics (2017), 19 (28), 18671-18684CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Cryst. titania has been extensively studied using exptl. and theor. tools. Amorphous titania, however, has received less attention in the literature, despite its importance for a no. of applications, such as photocatalysis, batteries, and electronic devices. We modeled amorphous titania using a combination of mol. dynamics and d. functional theory with several stoichiometries (TiOx, 2 ≥ x ≥ 1.75). Our results show that oxygen atom removal from amorphous titania is much easier than from cryst. titania, indicating that reduced amorphous structures are likely common. Ti atoms in amorphous titania exhibit a distribution of coordination nos. (5-7), but the av. coordination no. of oxygen increases upon redn. We also identified that gap states arise in substoichiometric titania due to the formation of Ti3+ centers. Such gap states are highly localized and randomly distributed across different Ti atoms, although we do observe a slight preference for electron localization on 7-coordinated Ti atoms. We observe that band gaps increase with redn. of amorphous titania. We also analyzed a proposed hole hopping mechanism involving oxygen vacancies by calcg. hole hopping distances. We found that such distances are large except in very reduced states, indicating likely slow hole diffusion through an oxygen vacancy mechanism. Our work is the 1st of its kind to thoroughly characterize the structural and electronic properties of amorphous titania in reduced states.
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    Xia, B.; Ganem, J.-J.; Vickridge, I.; Briand, E.; Steydli, S.; Benbalagh, R.; Rochet, F. Water-Rich Conditions during Titania Atomic Layer Deposition in the 100 °C-300 °C Temperature Window Produce Films with TiIV Oxidation State but Large H and O Content Variations. Appl. Surf. Sci. 2022, 601, 154233  DOI: 10.1016/j.apsusc.2022.154233
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    Water-rich conditions during titania atomic layer deposition in the 100 °C-300 °C temperature window produce films with TiIV oxidation state but large H and O content variations
    Xia, Bingbing; Ganem, Jean-Jacques; Vickridge, Ian; Briand, Emrick; Steydli, Sebastien; Benbalagh, Rabah; Rochet, Francois
    Applied Surface Science (2022), 601 (), 154233CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Titania films prepd. by at. layer deposition attract great attention due to the widespread application of the oxide as a photocatalytic material, or more recently, as a promising charge storage material for lithium or proton batteries. We implement here advanced tools (Ion Beam Anal. and XPS, aided by Ellipsometry and X-ray Diffraction) to characterize films grown in the 100 °C-300 °C temp. window, using tetrakis(dimethylamino)titanium (TMDAT) as the metal precursor and water vapor as the oxidant. We examine the outcomes of the ALD process as a function of the deposition temp., applying equal oxidant and precursor half-cycle time lengths, which contrasts with common deposition processes where the water half-cycle is considerably shorter than that of the metal precursor. Under the present ALD scheme, n-type conductive films are obtained where the oxidn. state of titanium is overwhelmingly TiIV at all temps., while the hydrogen content (O/Ti ratio) varies considerably, from ∼ 15 at% (∼1.8) at 100 °C to ∼ 3 at% (∼2) at 300 °C. The ideality of the ALD process is discussed through the identification of nitrogen-contg. mols. detected at the oxide surfaces. By extending the structural and compositional range of ALD titania films, new opportunities of application are expected to appear.
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    Sperling, B. A.; Kimes, W. A.; Maslar, J. E. Reflection Absorption Infrared Spectroscopy during Atomic Layer Deposition of HfO2 Films from Tetrakis(Ethylmethylamido)Hafnium and Water. Appl. Surf. Sci. 2010, 256, 5035– 5041,  DOI: 10.1016/j.apsusc.2010.03.050
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    Reflection absorption infrared spectroscopy during atomic layer deposition of HfO2 films from tetrakis(ethylmethylamido)hafnium and water
    Sperling, Brent A.; Kimes, William A.; Maslar, James E.
    Applied Surface Science (2010), 256 (16), 5035-5041CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Tetrakis(ethylmethylamido)hafnium and water are commonly used precursors for at. layer deposition of HfO2. Using reflection absorption IR spectroscopy with a buried-metal-layer substrate, we probe surface species present during typical deposition conditions. We observe evidence for thermal decompn. of alkylamido ligands at 320 °C. Addnl., we find that complete satn. of the SiO2 substrate occurs in the first cycle at ≈100 °C whereas incomplete coverage is apparent even after many cycles at higher temps. The use of this technique as an in situ diagnostic useful for process optimization is demonstrated.
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    Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269– 271,  DOI: 10.1126/science.1061051
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    Visible-light photocatalysis in nitrogen-doped titanium oxides
    Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y.
    Science (Washington, DC, United States) (2001), 293 (5528), 269-271CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    To use solar irradn. or interior lighting efficiently, the authors sought a photocatalyst with high reactivity under visible tight. Films and powders of TiO2-xNx have revealed an improvement over titanium dioxide (TiO2) under visible light (wavelength < 500 nm) in optical absorption and photocatalytic activity such as photodegrdns. of Methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface. Nitrogen doped into substitutional sites of TiO2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calcns. and x-ray photoemission spectroscopy.
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    Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 Photocatalysis: A Historical Overview and Future Prospects. Jpn. J. Appl. Phys. 2005, 44, 8269,  DOI: 10.1143/JJAP.44.8269
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    TiO2 photocatalysis: A historical overview and future prospects
    Hashimoto, Kazuhito; Irie, Hiroshi; Fujishima, Akira
    Japanese Journal of Applied Physics, Part 1: Regular Papers, Brief Communications & Review Papers (2005), 44 (12), 8269-8285CODEN: JAPNDE ISSN:. (Japan Society of Applied Physics)
    A review. Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has beer used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochem. reaction proceeding on a TiO2 surface when irradiated with UV light. One includes the photoinduced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. The authors review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.
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    Deng, S.; Verbruggen, S. W.; Lenaerts, S.; Martens, J. A.; Van den Berghe, S.; Devloo-Casier, K.; Devulder, W.; Dendooven, J.; Deduytsche, D.; Detavernier, C. Controllable Nitrogen Doping in as Deposited TiO2 Film and Its Effect on Post Deposition Annealing. J. Vac. Sci. Technol., A 2014, 32, 01A123
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    Martínez-Ferrero, E.; Sakatani, Y.; Boissière, C.; Grosso, D.; Fuertes, A.; Fraxedas, J.; Sanchez, C. Nanostructured Titanium Oxynitride Porous Thin Films as Efficient Visible-Active Photocatalysts. Adv. Funct. Mater. 2007, 17, 3348– 3354,  DOI: 10.1002/adfm.200700396
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    Nanostructured titanium oxynitride porous thin films as efficient visible-active photocatalysts
    Martinez-Ferrero, Eugenia; Sakatani, Yoshiaki; Boissiere, Cedric; Grosso, David; Fuertes, Amparo; Fraxedas, Jordi; Sanchez, Clement
    Advanced Functional Materials (2007), 17 (16), 3348-3354CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Nanocryst. mesoporous N-doped titania films were prepd. The introduction of nitrogen into the anatase structure starts at 500°, with N bonding to Ti via O substitution. Increasing the treatment temp. leads to the formation of TiN (TiN1-xOx) and N-doped rutile showing mixed-valence Ti states. Microstructural characterization shows that the ordered mesoporosity is maintained until 700°, where TiN (TiN1-xOx) begins to form. Optical characterization shows that the discrete introduction of N is able to shift the titania absorption edge. The photocatalytic tests give the best results under visible light excitation for the film nitrided at 500°. At this temp. the concn. of nitrogen in the structure is optimal since oxygen vacancies are still not important enough to promote the recombination of the photogenerated electrons and holes.
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    Driessen, J. P. A. M.; Schoonman, J.; Jensen, K. F. Infrared Spectroscopic Study of Decomposition of Ti(N(CH3)2)4. J. Electrochem. Soc. 2001, 148, G178,  DOI: 10.1149/1.1350687
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    Infrared spectroscopic study of decomposition of Ti(N(CH3)2)4
    Driessen, Joost P. A. M.; Schoonman, Joop; Jensen, Klavs F.
    Journal of the Electrochemical Society (2001), 148 (3), G178-G184CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The decompn. of Ti(NMe2)4 (TDMAT) was studied in N2 and H2 environments and surface temps. between 473 and 623 K by using FTIR spectroscopy. The pressure in the system was 5 torr, with a TDMAT partial pressure of 0.3 torr. The evolution of gas-phase species was monitored by characteristic IR absorption. For temps. <478 K, an av. no. of >3 ligands per TDMAT mol. is obsd. Approx. 1 ligand per 2 TDMAT mols. decomps. into methane- and C-contg. species in the coating. This slow decompn. pathway implies intermol. H transfer between multiple TDMAT mols. A decompn. mechanism consistent with the exptl. observations is proposed. For temps. >478 K, the conversion rate of TDMAT into products increases drastically. Less gaseous species are obsd., Auger electron spectroscopy measurements show more C contamination in the coating, and addnl. absorption peaks appear in the IR spectra. NMR spectroscopy indicates that these peaks can be assigned to a mixt. of oligomers of TDMAT reaction fragments that desorb from the hot surface. The increased C content at elevated temps. is attributed to incomplete desorption of ligands or metallacycle formation.
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    Hsu, J.-C.; Lin, Y.-H.; Wang, P. W. X-ray Photoelectron Spectroscopy Analysis of Nitrogen-Doped TiO2 Films Prepared by Reactive-Ion-Beam Sputtering with Various NH3/O2 Gas Mixture Ratios. Coatings 2020, 10, 47,  DOI: 10.3390/coatings10010047
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    X-ray photoelectron spectroscopy analysis of nitrogen-doped TiO2 films prepared by reactive-ion-beam sputtering with various NH3/O2 gas mixture ratios
    Hsu, Jin-Cherng; Lin, Yung-Hsin; Wang, Paul W.
    Coatings (2020), 10 (1), 47CODEN: COATED; ISSN:2079-6412. (MDPI AG)
    Nitrogen-doped TiO2 films were prepd. by reactive ion-beam sputtering deposition (IBSD) in a mixed atm. of NH3 and O2 at a substrate temp. of 400 °C. X-ray photoelectron spectra revealed the presence of six ions, i.e., N3-, N2-, N1-, N+, N2+, and N3+, resp., in the films. The amorphous films had complex, randomly oriented chem. bonds. The Tauc-Lorentz model was employed to det. the bandgap energy of the amorphous films prepd. using different NH3/O2 gas mixing ratios by ellipsometry. In addn., the optical consts. of the films were measured. With the increase in the NH3/O2 gas mixt. ratio to 3.0, the bandgap of N-doped TiO2 narrowed to ∼2.54 eV.
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    Lafuente, B.; Downs, R. T.; Yang, H.; Stone, N. The Power of Databases: The RRUFF Project. In Highlights in Mineralogical Crystallography; Armbruster, T.; Danisi, R. M., Eds.; Walter de Gruyter GmbH & Co KG, 2015; pp. 1– 30 DOI: 10.1515/9783110417104-003 .
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    Busani, T.; Devine, R. A. B. Dielectric and Infrared Properties of TiO2 Films Containing Anatase and Rutile. Semicond. Sci. Technol. 2005, 20, 870– 875,  DOI: 10.1088/0268-1242/20/8/043
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    Dielectric and infrared properties of TiO2 films containing anatase and rutile
    Busani, T.; Devine, R. A. B.
    Semiconductor Science and Technology (2005), 20 (8), 870-875CODEN: SSTEET; ISSN:0268-1242. (Institute of Physics Publishing)
    Elec. and optical properties of low-temp., plasma enhanced chem. vapor deposited films of TiO2 have been studied; the source gases were TiCl4 and O2. The amorphous, as-deposited films had a dielec. const. ∼33 consistent with their measured d. of 3.2 ± 0.2 g cm-3. Films deposited using a -41 V substrate bias contained the anatase phase and some rutile as evidenced from IR spectroscopy and x-ray scattering. Annealing of these films at 600°C resulted in a significant increase in the rutile content of the film.
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    Piercy, B. D.; Leng, C. Z.; Losego, M. D. Variation in the Density, Optical Polarizabilities, and Crystallinity of TiO2 Thin Films Deposited via Atomic Layer Deposition from 38 to 150 °C Using the Titanium Tetrachloride-Water Reaction. J. Vac. Sci. Technol., A 2017, 35, 03E107  DOI: 10.1116/1.4979047
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    Go, D.; Lee, J.; Shin, J. W.; Lee, S.; Kang, W.; Han, J. H.; An, J. Phase-Gradient Atomic Layer Deposition of TiO2 Thin Films by Plasma-Induced Local Crystallization. Ceram. Int. 2021, 47, 28770– 28777,  DOI: 10.1016/j.ceramint.2021.07.037
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    Phase-gradient atomic layer deposition of TiO2 thin films by plasma-induced local crystallization
    Go, Dohyun; Lee, Jaehyeong; Shin, Jeong Woo; Lee, Sungje; Kang, Wangu; Han, Jeong Hwan; An, Jihwan
    Ceramics International (2021), 47 (20), 28770-28777CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
    Atomic layer deposition (ALD) is a thin-film fabrication method that can be used to deposit films with precise thickness controllability and uniformity. The low deposition temp. of ALD, however, often interrupts the facile crystn. of films, resulting in inferior optical and elec. properties. In this study, the extremely localized crystn. of TiO2 thin films was demonstrated by per-cycle plasma treatment during the plasma-enhanced ALD process. By layering cryst. and amorphous films, a phase-gradient TiO2 film with precisely modulated optical and elec. properties was fabricated. Moreover, the ratio between the amorphous and cryst. layer thicknesses for a high dielec. const. and low leakage c.d. was optimized.
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    McDowell, M. T.; Lichterman, M. F.; Carim, A. I.; Liu, R.; Hu, S.; Brunschwig, B. S.; Lewis, N. S. The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 15189– 15199,  DOI: 10.1021/acsami.5b00379
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    The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation
    McDowell, Matthew T.; Lichterman, Michael F.; Carim, Azhar I.; Liu, Rui; Hu, Shu; Brunschwig, Bruce S.; Lewis, Nathan S.
    ACS Applied Materials & Interfaces (2015), 7 (28), 15189-15199CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aq. soln. Amorphous TiO2 deposited by at.-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidn., with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO2 in reducing or oxidizing environments. All of the polycryst. TiO2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO2 layer is neither specific to a particular amorphous or cryst. structure nor detd. wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.
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    Pore, V.; Heikkilä, M.; Ritala, M.; Leskelä, M.; Areva, S. Atomic Layer Deposition of TiO2–xNx Thin Films for Photocatalytic Applications. J. Photochem. Photobiol., A 2006, 177, 68– 75,  DOI: 10.1016/j.jphotochem.2005.05.013
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    Atomic layer deposition of TiO2-xNx thin films for photocatalytic applications
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    Titanium dioxide (TiO2) is recognized as the most efficient photocatalytic material, but due to its large band gap energy it can only be excited by UV irradn. Doping TiO2 with nitrogen is a promising modification method for the utilization of visible light in photocatalysis. In this work, nitrogen-doped TiO2 films were grown by at. layer deposition (ALD) using TiCl4, NH3 and water as precursors. All growth expts. were done at 500° C. The films were characterized by XRD, XPS, SEM and UV-vis spectrometry. The influence of nitrogen doping on the photocatalytic activity of the films in the UV and visible light was evaluated by the degrdn. of a thin layer of stearic acid and by linear sweep voltammetry. Light-induced superhydrophilicity of the films was also studied. It was found that the films could be excited by visible light, but they also suffered from increased recombination.
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    Cheng, H.-E.; Chen, Y.-R.; Wu, W.-T.; Hsu, C.-M. Effect of Nitrogen Doping Concentration on the Properties of TiO2 Films Grown by Atomic Layer Deposition. Mater. Sci. Eng., B 2011, 176, 596– 599,  DOI: 10.1016/j.mseb.2011.02.001
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    Effect of nitrogen doping concentration on the properties of TiO2 films grown by atomic layer deposition
    Cheng, Hsyi-En; Chen, Yu-Ru; Wu, Wen-Tuan; Hsu, Ching-Ming
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    N-doped TiO2 films were grown on n+-silicon substrates by at. layer deposition using titanium chloride and vapor mixt. of ammonia and water as the reactants. The effects of doping concn. on the microstructure and photocurrent response of as-deposited films were investigated. The results show that the doping levels were 0.2, 0.7, 1.2, 1.5, and 4.3 at% for films grown at NH3-to-H2O injection vol. ratios of 350, 380, 440, 520, and 550, resp. The off-plane lattice const. of TiO2 films increased with the increase of doping level, and the transformation of anatase to rutile was inhibited by the doping as the doping concn. reached 1.2 at%. The wavelength-dependent photocurrents suggest an optimal N doping concn. lying between 0.7 and 1.2 at% for the visible light active TiO2 films. Doping with a too-low or a too-high nitrogen level resulted in an inefficient visible light generation or a serious carrier recombination, resp.
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    Pore, V.; Ritala, M.; Leskelä, M.; Saukkonen, T.; Järn, M. Explosive Crystallization in Atomic Layer Deposited Mixed Titanium Oxides. Cryst. Growth Des. 2009, 9, 2974– 2978,  DOI: 10.1021/cg801337f
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    Explosive Crystallization in Atomic Layer Deposited Mixed Titanium Oxides
    Pore, Viljami; Ritala, Mikko; Leskela, Markku; Saukkonen, Tapio; Jarn, Mikael
    Crystal Growth & Design (2009), 9 (7), 2974-2978CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    Anatase crystals with lateral sizes of tens of microns are impossible to prep. as thin films on amorphous substrates by normal thin film deposition techniques. When deposited on amorphous substrates, polycryst. anatase titanium dioxide (TiO2) thin films have typically grain and crystal sizes comparable to the film thickness. We show that when a suitable amt. of Nb2O5 or Ta2O5 is mixed with TiO2 using at. layer deposition (ALD), upon post-deposition annealing large ∼10-100 μm sized anatase monocrystals are formed in films thinner than 100 nm on amorphous substrates. Such an exceptionally extensive lateral grain growth appears to be a result of explosive crystn. when amorphous Ti-Nb-O and Ti-Ta-O thin films are heated, a phenomenon that has not been reported before for TiO2 thin films. The prepd. films will likely have a no. of useful applications where well-ordered anatase surfaces are required, such as photocatalysis, and they also can be used as a transparent conducting oxide.
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    Dufond, M. E.; Chazalviel, J.-N.; Santinacci, L. Electrochemical Stability of n-Si Photoanodes Protected by TiO2 Thin Layers Grown by Atomic Layer Deposition. J. Electrochem. Soc. 2021, 168, 031509  DOI: 10.1149/1945-7111/abeaf3
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    Electrochemical stability of n-Si photoanodes protected by TiO2 thin layers grown by atomic layer deposition
    Dufond, Maxime E.; Chazalviel, Jean-Noel; Santinacci, Lionel
    Journal of the Electrochemical Society (2021), 168 (3), 031509CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
    This work investigates the n-Si photoanodes corrosion protection by Atomic Layer Deposition (ALD) of a TiO2 film. A specific electrochem. exptl. sequence (including successive rest potential measurements and voltammetries under illumination or not) has been established to study the stability of the electrodes in KOH. Depending on the deposition conditions (precursor compn. and temp.), the electrochem. properties of the layers are different. The photoanodes coated using titanium tetraisopropoxide (TTIP) at low temp. exhibit a low photocurrent (jph) that is progressively enhanced during the electrochem. sequence and their stability decreases. When using tetrakis (dimethylamido) titanium (TDMAT), the jph is almost const. and the film prevents from corrosion. The characterizations show that the ALD parameters drive the microstructure of the layer that is found crit. for the electrochem. response. A hydrogen doping occurring during the open circuit potential measurements under illumination is evidenced by IR spectroscopy. It is mainly localized at the grain boundaries and pores of the layers as well as in the n-Si and it modifies the charge transfer at the electrode/soln. junction and the hydrogen diffusion weakens the film causing the Si corrosion. The different charge transfer mechanisms are finally proposed depending on the ALD conditions and the film thickness.
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    Hukari, K.; Dannenberg, R.; Stach, E. A. Nitrogen Effects on Crystallization Kinetics of Amorphous TiOxNy Thin Films. J. Mater. Res. 2002, 17, 550– 555,  DOI: 10.1557/JMR.2002.0077
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    Hukari, Kyle; Dannenberg, Rand; Stach, E. A.
    Journal of Materials Research (2002), 17 (3), 550-555CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
    The crystn. behavior of amorphous TiOxNy (x » y) thin films was investigated by in situ TEM. The Johnson-Mehl-Avrami-Kozolog (JMAK) theory was used to det. the Avrami exponent, activation energy, and the phase velocity pre-exponent. Addn. of nitrogen inhibited diffusion, increasing the nucleation temp., while decreasing the growth activation energy. Kinetic variables extd. from individual crystallites were compared to JMAK anal. of the fraction transformed, and a change of 6% in the activation energy led to agreement between the methods. From diffraction patterns and index of refraction the crystd. phase was found to be predominantly anatase.
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    Molecular Dynamics Study of Explosive Crystallization of SiGe and Boron-Doped SiGe Alloys
    Albenze, Erik J.; Thompson, Michael O.; Clancy, Paulette
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    Explosive crystn. of SiGe alloys and boron-doped SiGe alloys was studied using a special mol. dynamics construction developed by the authors. For the case of Si1-xGex alloys, the simulations indicate that explosive crystn. should occur over the entire compositional range of Si1-xGex alloys at velocities high enough to prevent Ge segregation. The results show qual. agreement with exptl. results for Si-rich systems: Over a range of 0-25% germanium, the simulations predicted a nonlinear decrease in velocity that roughly matches the slope and extent of the rapid drop in interface velocity obsd. exptl. An analogous study of boron-doped Si1-xGex alloys showed that the limit of incorporation of boron during explosive crystn. is strongly dependent on the amt. of germanium present and that explosive crystn. is capable of limiting transient enhanced diffusion throughout the crystn. process.
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    A model for explosive crystn. in a thin amorphous layer on a heat conducting substrate is presented. For the thin layer, the energy equation was used in a 1-dimensional approxn. Heat conduction into the substrate and thermal contact resistance at the interface between layer and substrate are taken into account. Four rate equations were used to describe the kinetics of the homogeneous amorphous-cryst. transition. The whole process was examd. as a plane wave of invariant shape in a moving frame of ref. Heat conduction in the substrate is described by introducing a continuous distribution of moving heat sources at the interface. This gives an integral representation for the temp. in the substrate in terms of the unknown source distribution. The integral term implies that there is a nonlocal influence of the temp. distribution in the layer on the heat loss. A coupled system of an integro-differential equation and four ordinary differential equations was obtained and solved numerically. The propagation velocity of the wave was obtained as an eigenvalue of the system of equations. Varying a nondimensional heat loss parameter, a crit. value is found beyond which no crystn. wave of invariant shape is possible. This can also be interpreted as a certain min. layer thickness. Temp. and crystallinity distributions are shown for some interesting configurations. Predictions of crystn.-wave velocities and min. layer thicknesses are compared with exptl. values for explosive crystn. in Ge. (c) 2015 American Institute of Physics.
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    Interface Engineering for Extremely Large Grains in Explosively Crystallized TiO2 Films Grown by Low-Temperature Atomic Layer Deposition
    Cho, Cheol Jin; Kang, Jun-Yun; Lee, Woo Chul; Baek, Seung-Hyub; Kim, Jin-Sang; Hwang, Cheol Seong; Kim, Seong Keun
    Chemistry of Materials (2017), 29 (5), 2046-2054CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Functionality in thin films is closely linked with both the microstructure of the film as well as the innate nature of the thin film material. The engineering of microstructure, esp. the grain size and its distribution in oxide thin films, is crit. to designing functionalities targeting specific applications. However, the scope for manipulation of the microstructure of an oxide thin film is generally quite limited because of the lack of mobility of atoms at processing temps. that are usually employed for the growth of these films. This work reports the observation of very large grains (∼100 times larger than the film thickness) in TiO2 films grown by at. layer deposition at low temp. (<250°). Large grain sizes were obsd. in the as-grown films even in the absence of postdeposition annealing, wherein substrates with high surface energies were used. The surface energy engineering of substrate offers an efficient method to control the grain size of TiO2 films in low thermal budget processes. The results will provide facile routes to design novel properties of thin films via controlling the grain size even in low temp. processes.
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    Amorphous TiO2 materials, layers and powders, have been produced by different prepn. techniques. The layers have been subjected to electron diffraction and the bulk powders to X-ray diffraction expts. Three-dimensional structure models consistent with the exptl. structure functions have been constructed by reverse Monte Carlo simulations. Partial distribution functions, interat. distances and coordination nos. have been extd. from the model at. configurations. The at. arrangement in all amorphous TiO2 materials investigated has been found to resemble that occurring in brookite, a cryst. modification of TiO2, and to be well described as an assembly of short, staggered chains of Ti-O octahedra. The crystn. behavior of amorphous TiO2-materials has also been discussed.
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    Spectroscopic ellipsometry (SE) measurements were made on thin-film and single-crystal TiO2 anatase using a two-modulator generalized ellipsometer. The TiO2 films were epitaxially stabilized on a LaAlO3 substrate in the anatase crystal structure using reactive sputter deposition. The films were highly cryst., possessing a stepped surface morphol. indicative of at. layer-by-layer growth. The SE results for the anatase film indicate that the material is essentially oriented with the c axis perpendicular to the substrate, but there is some anisotropy near the interface and the surface. Corrugations of the film surface, as obsd. using at. force microscopy, are consistent with a surface structure needed to create cross polarization. Accurate values of the optical functions of cryst. anatase were obtained above and below the band edge using SE. Above the band edge, both the ordinary and extraordinary complex dielec. functions exhibited two crit. points.
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    Rutile-TiO2 thin films have potential for use in high-k applications, such as dynamic random-access memory capacitors; however, they are difficult to realize without using noble-metal-based oxide substrates. Therefore, we proposed a new approach for the prepn. of rutile TiO2 by a small amt. of Sn doping and the insertion of ultra-thin SnO2 to achieve enhanced dielec. performance without using noble-metal-based electrodes. It was confirmed that the crystallinity of rutile TiO2 was remarkably enhanced in Sn-doped TiO2 formed on an ultra-thin SnO2 interlayer. Moreover, 10 nm-thick Sn-doped TiO2 thin film on a 1-nm SnO2 interlayer exhibited a high dielec. const. of about 80.
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    VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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    The lattice consts. and at. parameters of TiO2 (rutile), SnO2, GeO2, and MgF2 were redetd. I(hk0) of TiO2, SnO2, and MgF2 were obtained by careful photometry of Weissenberg photographs (MoKα radiation) and were used for Fourier and (Fo - Fc) projections. The at. parameter of GeO2 was detd. from a 3-dimensional Fourier section using intensities from powder diagrams, because single crystals were not available. Results: TiO2, a = 4.594 ± 0.003, c = 2.959 ±2.002 A., x = 0.306 ± 0.001; SnO2, a = 4.737 ± 0.001, c = 3.185 ± 0.001 A., x = 0.307 ± 0.001; MgF2, a = 4.625 ± 0.002, c = 3.052 ± 0.003 A., x = 0.303 ± 0.001; and GeO2, a = 4.395 ± 0.003, c = 2.860 ± 0.003 A., x = 0.307 ± 0.004. In the TiO6 octahedra of rutile 4 Ti-O distances are shorter than two others by 2%. In the other compds. the 6A-B distances in the AB6 octahedra are equal within the limits of accuracy.
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    A database has been constructed that contains all the crystal structures previously published in the American Mineralogist. The database is called "The American Mineralogist Crystal Structure Database" and is freely accessible from the websites of the Mineralogical Society of America at http://www.minsocam.org/MSA/Crystal_Database.html and the University of Arizona. In addn. to the database, a suite of interactive software is provided that can be used to view and manipulate the crystal structures and compute different properties of a crystal such as geometry, diffraction patterns, and procrystal electron densities. The database is set up so that the data can be easily incorporated into other software packages. Included at the website is an evolving set of guides to instruct the user and help with classroom education.
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Cite this: J. Phys. Chem. C 2022, 126, 36, 15357–15366
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https://doi.org/10.1021/acs.jpcc.2c04905
Published August 30, 2022

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  • Abstract

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    Figure 1

    Figure 1. (a) Ti 2p, (b) O 1s, and (c) N 1s XP spectra of 30 nm-thick as-deposited and post deposition-annealed ALD TiO2 grown at 100 and 200 °C.

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    Figure 2

    Figure 2. GIXRD patterns of 30 nm-thick ALD TiO2 grown at (a) 100 °C and (b) 200 °C upon post deposition annealing. The XRD references are from the RRUF database. (45) The insets show the SEM images after PDA at 500 °C.

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    Figure 3

    Figure 3. SEM images of 30 nm-thick ALD TiO2 grown at (a–d) 100 °C and (e–h) 200 °C: (a, e) as-deposited and after post deposition annealing (50 min) at (b, f) 300 °C, (c, g) 400 °C, and (d, h) 500 °C. All the images were taken with the same magnification.

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    Figure 4

    Figure 4. Evolution of (a) Ti and (b) N species as a function of the PDA temperature for 30 nm-thick ALD TiO2 grown at 100 °C. The binding energy values of N 1s components in panel (b) are depicted in brackets.

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    Figure 5

    Figure 5. SEM images of 30 nm-thick ALD TiO2 grown at 200 °C after PDA at 250 °C (a) for 50 min and (b) for 500 min. (c) GIXRD patterns of the samples. The XRD references are from the RRUF database. (45)

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      Tunable Ti3+-Mediated Charge Carrier Dynamics of Atomic Layer Deposition-Grown Amorphous TiO2
      Saari, Jesse; Ali-Loytty, Harri; Kauppinen, Minttu Maria; Hannula, Markku; Khan, Ramsha; Lahtonen, Kimmo; Palmolahti, Lauri; Tukiainen, Antti; Gronbeck, Henrik; Tkachenko, Nikolai V.; Valden, Mika
      Journal of Physical Chemistry C (2022), 126 (9), 4542-4554CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Amorphous titania (am.-TiO2) has gained wide interest in the field of photocatalysis, thanks to exceptional disorder-mediated optical and elec. properties compared to cryst. TiO2. Here, we study the effects of intrinsic Ti3+ and nitrogen defects in am.-TiO2 thin films via the at. layer deposition (ALD) chem. of tetrakis(dimethylamido)titanium(IV) (TDMAT) and H2O precursors at growth temps. of 100-200°C. XPS and computational anal. allow us to identify structural disorder-induced penta- and heptacoordinated Ti4+ ions (Ti5/7c4+), which are related to the formation of Ti3+ defects in am.-TiO2. The Ti3+-rich ALD-grown am.-TiO2 has stoichiometric compn., which is explained by the formation of interstitial peroxo species with oxygen vacancies. The occupation of Ti3+ 3d in-gap states increases with the ALD growth temp., inducing both visible-light absorption and elec. cond. via the polaron hopping mechanism. At 200°C, the in-gap states become fully occupied extending the lifetime of photoexcited charge carriers from the picosecond to the nanosecond time domain. Nitrogen traces from the TDMAT precursor had no effect on optical properties and only little on charge transfer properties. These results provide insights into the charge transfer properties of ALD-grown am.-TiO2 that are essential to the performance of protective photoelectrode coatings in photoelectrochem. solar fuel reactors.
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    11. 11
      Ros, C.; Andreu, T.; Morante, J. R. Photoelectrochemical Water Splitting: A Road from Stable Metal Oxides to Protected Thin Film Solar Cells. J. Mater. Chem. A 2020, 8, 10625– 10669,  DOI: 10.1039/D0TA02755C
      11
      Photoelectrochemical water splitting: a road from stable metal oxides to protected thin film solar cells
      Ros, Carles; Andreu, Teresa; Morante, Joan R.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (21), 10625-10669CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A review. Photoelectrochem. (PEC) water splitting has attracted great attention during past decades thanks to the possibility to reduce the prodn. costs of hydrogen or other solar fuels, by doing so in a single step and powered by the largest source of renewable energy: the sun. Despite significant efforts to date, the productivities of stable semiconductor materials in contact with the electrolyte are limited, pushing a growing scientific community towards more complex photoelectrode structures. During the last decade, several groups have focused on the strategy of incorporating state of the art photovoltaic absorber materials (such as silicon, III-V compds. and chalcogenide-based thin films). The stability of these devices in harsh acidic or alk. electrolytes has become a key issue, pushing transparent, conductive and protective layer research. The present review offers a detailed anal. of PEC devices from metal oxide electrodes forming a semiconductor-liq. junction to protected and catalyst-decorated third generation solar cells adapted into photoelectrodes. It consists of a complete overview of PEC systems, from nanoscale design to full device scheme, with a special focus on disruptive advances enhancing efficiency and stability. Fundamental concepts, fabrication techniques and cell schemes are also discussed, and perspectives and challenges for future research are pointed out.
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    12. 12
      Ali-Löytty, H.; Hannula, M.; Saari, J.; Palmolahti, L.; Bhuskute, B. D.; Ulkuniemi, R.; Nyyssönen, T.; Lahtonen, K.; Valden, M. Diversity of TiO2: Controlling the Molecular and Electronic Structure of Atomic-Layer-Deposited Black TiO2. ACS Appl. Mater. Interfaces 2019, 11, 2758– 2762,  DOI: 10.1021/acsami.8b20608
      12
      Diversity of TiO2: Controlling the Molecular and Electronic Structure of Atomic-Layer-Deposited Black TiO2
      Ali-Loytty, Harri; Hannula, Markku; Saari, Jesse; Palmolahti, Lauri; Bhuskute, Bela D.; Ulkuniemi, Riina; Nyyssonen, Tuomo; Lahtonen, Kimmo; Valden, Mika
      ACS Applied Materials & Interfaces (2019), 11 (3), 2758-2762CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Visually black, elec. leaky, metallic glasses titania (am-TiO2) thin films were grown by at. layer deposition (ALD) for photocatalytic applications. Broad spectral absorbance in the visible range and exceptional cond. are attributed to trapped Ti3+ in the film. Oxidn. of Ti3+ upon heat treatment leads to a drop in cond., a color change from black to white, and crystn. of am-TiO2. ALD-grown black TiO2, without any heat treatment, is subject to dissoln. in alk. photoelectrochem. conditions. The best photocatalytic activity for solar water splitting is obtained for completely cryst. white TiO2.
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    13. 13
      Hannula, M.; Ali-Löytty, H.; Lahtonen, K.; Sarlin, E.; Saari, J.; Valden, M. Improved Stability of Atomic Layer Deposited Amorphous TiO2 Photoelectrode Coatings by Thermally Induced Oxygen Defects. Chem. Mater. 2018, 30, 1199– 1208,  DOI: 10.1021/acs.chemmater.7b02938
      13
      Improved Stability of Atomic Layer Deposited Amorphous TiO2 Photoelectrode Coatings by Thermally Induced Oxygen Defects
      Hannula, Markku; Ali-Loytty, Harri; Lahtonen, Kimmo; Sarlin, Essi; Saari, Jesse; Valden, Mika
      Chemistry of Materials (2018), 30 (4), 1199-1208CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Amorphous titania (a-TiO2) combined with an electrocatalyst has shown to be a promising coating for stabilizing traditional semiconductor materials used in artificial photosynthesis for efficient photoelectrochem. solar-to-fuel energy conversion. Here, we report a detailed anal. of 2 methods of modifying an undoped thin film of at. layer deposited (ALD) a-TiO2 without an electrocatalyst to affect its performance in water splitting reaction as a protective photoelectrode coating. The methods are high-temp. annealing in ultrahigh vacuum and at. H exposure. A key feature in both methods is that they preserve the amorphous structure of the film. Special attention is paid to the changes in the mol. and electronic structure of a-TiO2 induced by these treatments. On the basis of the photoelectrochem. results, the a-TiO2 is susceptible to photocorrosion but significant improvement in stability is achieved after heat treatment in vacuum at >500°. On the other hand, the H treatment does not increase the stability despite the ostensibly similar redn. of a-TiO2. The surface anal. allows us to interpret the improved stability to the thermally induced formation of O- species within a-TiO2 that are essentially electronic defects in the anionic framework.
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    14. 14
      Correa, G. C.; Bao, B.; Strandwitz, N. C. Chemical Stability of Titania and Alumina Thin Films Formed by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2015, 7, 14816– 14821,  DOI: 10.1021/acsami.5b03278
      14
      Chemical Stability of Titania and Alumina Thin Films Formed by Atomic Layer Deposition
      Correa, Gabriela C.; Bao, Bo; Strandwitz, Nicholas C.
      ACS Applied Materials & Interfaces (2015), 7 (27), 14816-14821CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Thin films formed by at. layer deposition (ALD) are being examd. for a variety of chem. protection and diffusion barrier applications, yet their stability in various fluid environments is not well characterized. The chem. stability of titania and alumina thin films in air, 18 MΩ water, 1 M KCl, 1 M HNO3, 1 M H2SO4, 1 M HCl, 1 M KOH, and mercury was studied. Films were deposited at 150 °C using trimethylaluminum-H2O and tetrakis(dimethylamido)titanium-H2O chemistries for alumina and titania, resp. A subset of samples were heated to 450 and 900 °C in inert atm. Films were examd. using spectroscopic ellipsometry, at. force microscopy, optical microscopy, SEM, and X-ray diffraction. Notably, alumina samples were found to be unstable in pure water, acid, and basic environments in the as-synthesized state and after 450 °C thermal treatment. In pure water, a dissoln.-pptn. mechanism is hypothesized to cause surface roughening. The stability of alumina films was greatly enhanced after annealing at 900 °C in acidic and basic solns. Titania films were found to be stable in acid after annealing at or above 450 °C. All films showed a compn.-independent increase in measured thickness when immersed in mercury. These results provide stability-processing relationships that are important for controlled etching and protective barrier layers.
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    15. 15
      Kriegel, H.; Kollmann, J.; Raudsepp, R.; Klassen, T.; Schieda, M. Chemical and Photoelectrochemical Instability of Amorphous TiO2 Layers Quantified by Spectroscopic Ellipsometry. J. Mater. Chem. A 2020, 8, 18173– 18179,  DOI: 10.1039/D0TA04878J
      There is no corresponding record for this reference.
    16. 16
      Saari, J.; Ali-Löytty, H.; Honkanen, M.; Tukiainen, A.; Lahtonen, K.; Valden, M. Interface Engineering of TiO2 Photoelectrode Coatings Grown by Atomic Layer Deposition on Silicon. ACS Omega 2021, 6, 27501– 27509,  DOI: 10.1021/acsomega.1c04478
      16
      Interface Engineering of TiO2 Photoelectrode Coatings Grown by Atomic Layer Deposition on Silicon
      Saari, Jesse; Ali-Loytty, Harri; Honkanen, Mari; Tukiainen, Antti; Lahtonen, Kimmo; Valden, Mika
      ACS Omega (2021), 6 (41), 27501-27509CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      Titanium dioxide (TiO2) can protect photoelectrochem. (PEC) devices from corrosion, but the fabrication of high-quality TiO2 coatings providing long-term stability has remained challenging. Here, we compare the influence of Si wafer cleaning and postdeposition annealing temp. on the performance of TiO2/n+-Si photoanodes grown by at. layer deposition (ALD) using tetrakis(dimethylamido)titanium (TDMAT) and H2O as precursors at a growth temp. of 100°C. We show that removal of native Si oxide before ALD does not improve the TiO2 coating performance under alk. PEC water splitting conditions if excessive postdeposition annealing is needed to induce crystn. The as-deposited TiO2 coatings were amorphous and subject to photocorrosion. However, the TiO2 coatings were found to be stable over a time period of 10 h after heat treatment at 400°C that induced crystn. of amorphous TiO2 into anatase TiO2. No interfacial Si oxide formed during the ALD growth, but during the heat treatment, the thickness of interfacial Si oxide increased to 1.8 nm for all of the samples. Increasing the ALD growth temp. to 150°C enabled crystn. at 300°C, which resulted in reduced growth of interfacial Si oxide followed by a 70 mV improvement in the photocurrent onset potential.
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    17. 17
      Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett. 2018, 3, 1795– 1800,  DOI: 10.1021/acsenergylett.8b00920
      17
      Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency
      Cheng, Wen-Hui; Richter, Matthias H.; May, Matthias M.; Ohlmann, Jens; Lackner, David; Dimroth, Frank; Hannappel, Thomas; Atwater, Harry A.; Lewerenz, Hans-Joachim
      ACS Energy Letters (2018), 3 (8), 1795-1800CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chem. bonds, requires optimization of a photoelectrochem. device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic light absorption and reflection losses. A tailored multifunctional cryst. titania interphase layer acts as a corrosion protection layer, with favorable band alignment between the semiconductor conduction band and the energy level for water redn., facilitating electron transport at the cathode-electrolyte interface. It also provides a favorable substrate for adhesion of high-activity Rh catalyst nanoparticles. Under simulated AM 1.5G irradn., solar-to-H efficiencies of 19.3 and 18.5% are obtained in acidic and neutral electrolytes, resp. The system reaches 0.85 of the theor. limit for photoelectrochem. water splitting for the energy gap combination employed in the tandem-junction photoelectrode structure.
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    18. 18
      Khan, R.; Ali-Löytty, H.; Saari, J.; Valden, M.; Tukiainen, A.; Lahtonen, K.; Tkachenko, N. V. Optimization of Photogenerated Charge Carrier Lifetimes in ALD Grown TiO2 for Photonic Applications. Nanomaterials 2020, 10, 1567,  DOI: 10.3390/nano10081567
      18
      Optimization of photogenerated charge carrier lifetimes in ALD grown TiO2 for photonic applications
      Khan, Ramsha; Ali-Loytty, Harri; Saari, Jesse; Valden, Mika; Tukiainen, Antti; Lahtonen, Kimmo; Tkachenko, Nikolai V.
      Nanomaterials (2020), 10 (8), 1567CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
      Titanium dioxide (TiO2) thin films are widely employed for photocatalytic and photovoltaic applications where the long lifetime of charge carriers is a paramount requirement for the device efficiency. To ensure the long lifetime, a high temp. treatment is used which restricts the applicability of TiO2in devices incorporating org. or polymer components. In this study, we exploited low temp. (100-150°C) at. layer deposition (ALD) of 30 nm TiO2thin films from tetrakis(dimethylamido)titanium. The deposition was followed by a heat treatment in air to find the min. temp. requirements for the film fabrication without compromising the carrier lifetime. Femto-to nanosecond transient absorption spectroscopy was used to det. the lifetimes, and grazing incidence X-ray diffraction was employed for structural anal. The optimal result was obtained for the TiO2 thin films grown at 150°C and heat-treated at as low as 300°C. The deposited thin films were amorphous and crystd. into anatase phase upon heat treatment at 300-500°C. The av. carrier lifetime for amorphous TiO2 is few picoseconds but increases to >400 ps upon crystn. at 500°C. The samples deposited at 100°C were also crystd. as anatase but the carrier lifetime was <100 ps.
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    19. 19
      Yamakata, A.; Vequizo, J. J. M. Curious Behaviors of Photogenerated Electrons and Holes at the Defects on Anatase, Rutile, and Brookite TiO2 Powders: A Review. J. Photochem. Photobiol., C 2019, 40, 234– 243,  DOI: 10.1016/j.jphotochemrev.2018.12.001
      19
      Curious behaviors of photogenerated electrons and holes at the defects on anatase, rutile, and brookite TiO2 powders: A review
      Yamakata, Akira; Vequizo, Junie Jhon M.
      Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2019), 40 (), 234-243CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
      A review. Photocatalytic reactions are governed by photogenerated charge carriers upon band gap excitation. Therefore, for better understanding of the mechanism, the dynamics of photocarriers should be studied. One of the attractive materials is TiO2, which has been extensively investigated in the field of photocatalysis. This review article summarizes our recent works of time-resolved visible to mid-IR absorption measurements to elucidate the difference of anatase, rutile, and brookite TiO2 powders. The distinctive photocatalytic activities of these polymorphs are detd. by the electron-trapping processes at the defects on powders. Powders are rich in defects and these defects capture photogenerated electrons. The depth of the trap is crystal phase dependent, and they are estd. to be < 0.1 eV, ∼0.4 eV and ∼0.9 eV for anatase, brookite, and rutile, resp. Electron trapping reduces probability to meet with holes and then elongate the lifetime of holes. Therefore, it works neg. for the reaction of electrons but pos. works for the reaction of holes. In the steady-state reactions, both electrons and holes should be consumed. Hence, the balance between the pos. and neg. effects of defects dets. the distinctive photocatalytic activities of anatase, rutile, and brookite TiO2 powders.
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    20. 20
      Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P. Band Alignment of Rutile and Anatase TiO2. Nat. Mater. 2013, 12, 798– 801,  DOI: 10.1038/nmat3697
      20
      Band alignment of rutile and anatase TiO2
      Scanlon, David O.; Dunnill, Charles W.; Buckeridge, John; Shevlin, Stephen A.; Logsdail, Andrew J.; Woodley, Scott M.; Catlow, C. Richard A.; Powell, Michael. J.; Palgrave, Robert G.; Parkin, Ivan P.; Watson, Graeme W.; Keal, Thomas W.; Sherwood, Paul; Walsh, Aron; Sokol, Alexey A.
      Nature Materials (2013), 12 (9), 798-801CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO2. The discovery of the photolysis of water on the surface of TiO2 in 1972 launched four decades of intensive research into the underlying chem. and phys. processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO2. We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission expts., that a type-II, staggered, band alignment of ∼ 0.4eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust sepn. of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.
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    21. 21
      Li, A.; Wang, Z.; Yin, H.; Wang, S.; Yan, P.; Huang, B.; Wang, X.; Li, R.; Zong, X.; Han, H.; Li, C. Understanding the Anatase–Rutile Phase Junction in Charge Separation and Transfer in a TiO2 Electrode for Photoelectrochemical Water Splitting. Chem. Sci. 2016, 7, 6076– 6082,  DOI: 10.1039/C6SC01611A
      21
      Understanding the anatase-rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting
      Li, Ailong; Wang, Zhiliang; Yin, Heng; Wang, Shengyang; Yan, Pengli; Huang, Baokun; Wang, Xiuli; Li, Rengui; Zong, Xu; Han, Hongxian; Li, Can
      Chemical Science (2016), 7 (9), 6076-6082CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      New insight into junction-based designs for efficient charge sepn. is vitally important for current solar energy conversion research. Herein, an anatase-rutile phase junction is elaborately introduced into TiO2 films by rapid thermal annealing treatment and the roles of phase junction on charge sepn. and transfer are studied in detail. A combined study of transient absorption spectroscopy, electrochem. and photoelectrochem. (PEC) measurements reveals that appropriate phase alignment is essential for unidirectional charge transfer, and a junction interface with minimized trap states is crucial to liberate the charge sepn. potential of the phase junction. By tailored control of phase alignment and interface structure, an optimized TiO2 film with an appropriately introduced phase junction shows superior performance in charge sepn. and transfer, hence achieving ca. 3 and 9 times photocurrent d. enhancement compared to pristine anatase and rutile phase TiO2 electrodes, resp. This work demonstrates the great potential of phase junctions for efficient charge sepn. and transfer in solar energy conversion applications.
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    22. 22
      Hanaor, D. A. H.; Sorrell, C. C. Review of the Anatase to Rutile Phase Transformation. J. Mater. Sci. 2011, 46, 855– 874,  DOI: 10.1007/s10853-010-5113-0
      22
      Review of the anatase to rutile phase transformation
      Hanaor, Dorian A. H.; Sorrell, Charles C.
      Journal of Materials Science (2011), 46 (4), 855-874CODEN: JMTSAS; ISSN:0022-2461. (Springer)
      A review. Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examd., followed by a discussion of the thermodn. of the phase transformation and the factors affecting its observation. A comprehensive anal. of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius vs. the valence characterized by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which exptl. data are unavailable, on the phase transformation are deduced and presented on the basis of this anal.
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    23. 23
      Aarik, J.; Aidla, A.; Kiisler, A.-A.; Uustare, T.; Sammelselg, V. Effect of Crystal Structure on Optical Properties of TiO2 Films Grown by Atomic Layer Deposition. Thin Solid Films 1997, 305, 270– 273,  DOI: 10.1016/S0040-6090(97)00135-1
      23
      Effect of crystal structure on optical properties of TiO2 films grown by atomic layer deposition
      Aarik, Jaan; Aidla, Aleks; Kiisler, Alma-Asta; Uustare, Teet; Sammelselg, Vaeino
      Thin Solid Films (1997), 305 (1,2), 270-273CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
      The dependence of optical characteristics on the structure of at. layer-deposited TiO2 thin films is studied. Amorphous films grown at 100° have an optical band gap of 3.3 eV, while their refractive index is 2.2-2.3 at 633 nm. The refractive index can be increased up to 2.65 by using growth temps. ∼300°. Compn. and structure studies reveal that the formation of preferentially oriented crystal (anatase) structure contributes to this increase of refractive index most significantly.
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    24. 24
      Reiners, M.; Xu, K.; Aslam, N.; Devi, A.; Waser, R.; Hoffmann-Eifert, S. Growth and Crystallization of TiO2 Thin Films by Atomic Layer Deposition Using a Novel Amido Guanidinate Titanium Source and Tetrakis-Dimethylamido-Titanium. Chem. Mater. 2013, 25, 2934– 2943,  DOI: 10.1021/cm303703r
      24
      Growth and Crystallization of TiO2 Thin Films by Atomic Layer Deposition Using a Novel Amido Guanidinate Titanium Source and Tetrakis-dimethylamido-titanium
      Reiners, Marcel; Xu, Ke; Aslam, Nabeel; Devi, Anjana; Waser, Rainer; Hoffmann-Eifert, Susanne
      Chemistry of Materials (2013), 25 (15), 2934-2943CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The authors studied the growth of TiO2 by liq. injection at. layer deposition (ALD) using two different amide-based Ti sources, tetrakis(dimethylamido)titanium [(NMe2)4-Ti, TDMAT] and its recently developed deriv., tris(dimethylamido)mono(N,N'-diisopropyldimethylamidoguanidinato)titanium {[(N-iPr)2NMe2]Ti(NMe2)3, TiA3G1}, with H2O vapor as counterreactant. A clear satn. of growth with an increasing precursor supply was found for TDMAT between 150 and 300° and for TiA3G1 between 150 and 330°. Representative growth per cycle (GPC) values at 250° were 0.041 and 0.044 nm/cycle, resp. Compared to that of TDMAT, ALD of TiA3G1 exhibited a significantly higher stability in the GPC values up to 300° coinciding with an improved temp. stability of the precursor. Both processes showed a min. of the growth rate as a function of temp. In all cases, the residual C and N contents of the TiO2 films were <3 atom %. Conformal growth was demonstrated on three-dimensional pinhole structures with an aspect ratio of ∼1:30. Deposition temps. of ≤200° led to quasi-amorphous films. At higher growth temps., the anatase phase developed, accompanied by the brookite and/or the rutile phase depending on process conditions, deposition temp., and film thickness.
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    25. 25
      Li, Y.; Liu, J.; Jia, Z. Morphological Control and Photodegradation Behavior of Rutile TiO2 Prepared by a Low-Temperature Process. Mater. Lett. 2006, 60, 1753– 1757,  DOI: 10.1016/j.matlet.2005.12.012
      25
      Morphological control and photodegradation behavior of rutile TiO2 prepared by a low-temperature process
      Li, Yuanyuan; Liu, Jinping; Jia, Zhijie
      Materials Letters (2006), 60 (13-14), 1753-1757CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
      Flower-like rutile titania nanocrystals were prepd. via a simple aq.-phase stirring for 24 h at a low temp. of 75°C, employing only TiCl4, HCl as the starting materials. XRD result proved the formation of rutile TiO2. The observations from TEM and SEM showed that the products were large-scale flower-shaped structures composed of radial nanorods. Comparative expts. demonstrated that pinecone-like, needlelike rutile TiO2 could be easily achieved by varying the vol. ratio of TiCl4/H2O. The growth mechanisms of TiO2 nanostructures prepd. under different conditions and their photodegrdn. behavior were also discussed. It was found that the flower-like structures exhibited the highest photocatalytic activity in the photodegrdn. of aq. brilliant red X-3B soln.
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    26. 26
      Yang, K.; Zhu, J.; Zhu, J.; Huang, S.; Zhu, X.; Ma, G. Sonochemical Synthesis and Microstructure Investigation of Rod-like Nanocrystalline Rutile Titania. Mater. Lett. 2003, 57, 4639– 4642,  DOI: 10.1016/S0167-577X(03)00376-8
      26
      Sonochemical synthesis and microstructure investigation of rod-like nanocrystalline rutile titania
      Yang, Ke; Zhu, Jianmin; Zhu, Junjie; Huang, Shisong; Zhu, Xinhua; Ma, Guobin
      Materials Letters (2003), 57 (30), 4639-4642CODEN: MLETDJ; ISSN:0167-577X. (Elsevier Science B.V.)
      Rod-like and star-like nanocryst. rutile TiO2 were prepd. by a sonochem. method in TiCl4/HCl soln. The product was studied by XRD, TEM, SAED, and HRTEM. The product is purely rutile. The formation of rutile TiO2 may be due to rutile's inherent high-temp. stability. The appearance of star-like structures should be attributed to the conditions of sonochem. method.
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    27. 27
      Abendroth, B.; Moebus, T.; Rentrop, S.; Strohmeyer, R.; Vinnichenko, M.; Weling, T.; Stöcker, H.; Meyer, D. C. Atomic Layer Deposition of TiO2 from Tetrakis(Dimethylamino)Titanium and H2O. Thin Solid Films 2013, 545, 176– 182,  DOI: 10.1016/j.tsf.2013.07.076
      27
      Atomic layer deposition of TiO2 from tetrakis(dimethylamino)titanium and H2O
      Abendroth, Barbara; Moebus, Theresa; Rentrop, Solveig; Strohmeyer, Ralph; Vinnichenko, Mykola; Weling, Tobias; Stoecker, Hartmut; Meyer, Dirk C.
      Thin Solid Films (2013), 545 (), 176-182CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)
      The at. layer deposition (ALD) of TiO2 from tetrakis(dimethylamino)titanium (TDMAT) and water was studied in the substrate temp. (TS) range of 120 to 330°C. The effect of deposition temps. on the resulting layer microstructure was investigated. Possible interaction mechanisms of TDMAT and H2O precursor mols. and the TiO2 surface at different temps. are discussed. The TiO2 layers were characterized with respect to microstructure, compn., and optical properties by glancing angle x-ray diffraction and reflectometry, x-ray fluorescence anal., photoelectron spectroscopy, and spectroscopic ellipsometry. A const. layer growth with increasing no. of ALD cycles was achieved for all investigated deposition temps. if the inert gas purge time after the H2O pulse was increased from 5 s at temps. below 250°C to 25 s for TS ≥ 320°C. In the investigated temp. range, the growth per cycle varies between 0.33 and 0.67 Å/cycle with a min. at 250°C. The variations of the deposition rate are related to a change from a surface detd. decompn. of TDMAT to a gas phase decompn. route above 250°C. At the same temp., the microstructure of the TiO2 layers changes from amorphous to predominately cryst., where both anatase and rutile are present.
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    28. 28
      Niemelä, J.-P.; Marin, G.; Karppinen, M. Titanium Dioxide Thin Films by Atomic Layer Deposition: A Review. Semicond. Sci. Technol. 2017, 32, 093005  DOI: 10.1088/1361-6641/aa78ce
      28
      Titanium dioxide thin films by atomic layer deposition: a review
      Niemela, Janne-Petteri; Marin, Giovanni; Karppinen, Maarit
      Semiconductor Science and Technology (2017), 32 (9), 093005/1-093005/43CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
      Within its rich phase diagram titanium dioxide is a truly multifunctional material with a property palette that has been shown to span from dielec. to transparent-conducting characteristics, in addn. to the well-known catalytic properties. At the same time down-scaling of microelectronic devices has led to an explosive growth in research on at. layer deposition (ALD) of a wide variety of frontier thin-film materials, among which TiO2 is one of the most popular ones. In this topical review we summarize the advances in research of ALD of titanium dioxide starting from the chemistries of the over 50 different deposition routes developed for TiO2 and the resultant structural characteristics of the films. We then continue with the doped ALD-TiO2 thin films from the perspective of dielec., transparent-conductor and photocatalytic applications. Moreover, in order to cover the latest trends in the research field, both the variously constructed TiO2 nanostructures enabled by ALD and the Ti-based hybrid inorg.-org. films grown by the emerging ALD/MLD (combined at./mol. layer deposition) technique are discussed.
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    29. 29
      Rafieian, D.; Ogieglo, W.; Savenije, T.; Lammertink, R. G. H. Controlled Formation of Anatase and Rutile TiO2 Thin Films by Reactive Magnetron Sputtering. AIP Adv. 2015, 5, 097168  DOI: 10.1063/1.4931925
      29
      Controlled formation of anatase and rutile TiO2 thin films by reactive magnetron sputtering
      Rafieian, Damon; Ogieglo, Wojciech; Savenije, Tom; Lammertink, Rob G. H.
      AIP Advances (2015), 5 (9), 097168/1-097168/7CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
      We discuss the formation of TiO2 thin films via DC reactive magnetron sputtering. The oxygen concn. during sputtering proved to be a crucial parameter with respect to the final film structure and properties. The initial deposition provided amorphous films that crystallize upon annealing to anatase or rutile, depending on the initial sputtering conditions. Substoichiometric films (TiOx<2), obtained by sputtering at relatively low oxygen concn., formed rutile upon annealing in air, whereas stoichiometric films formed anatase. This route therefore presents a formation route for rutile films via lower (<500°C) temp. pathways. The dynamics of the annealing process were followed by in situ ellipsometry, showing the optical properties transformation. The final crystal structures were identified by XRD. The anatase film obtained by this deposition method displayed high carriers mobility as measured by time-resolved microwave conductance. This also confirms the high photocatalytic activity of the anatase films. (c) 2015 American Institute of Physics.
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    30. 30
      Dendooven, J.; Detavernier, C. Atomic Layer Deposition in Energy Conversion Applications; Bachmann, J., Ed.; John Wiley & Sons, Ltd., 2017; pp. 1– 40.
      There is no corresponding record for this reference.
    31. 31
      Head, A. R.; Chaudhary, S.; Olivieri, G.; Bournel, F.; Andersen, J. N.; Rochet, F.; Gallet, J.-J.; Schnadt, J. Near Ambient Pressure X-ray Photoelectron Spectroscopy Study of the Atomic Layer Deposition of TiO2 on RuO2(110). J. Phys. Chem. C 2016, 120, 243– 251,  DOI: 10.1021/acs.jpcc.5b08699
      31
      Near Ambient Pressure X-ray Photoelectron Spectroscopy Study of the Atomic Layer Deposition of TiO2 on RuO2(110)
      Head, Ashley R.; Chaudhary, Shilpi; Olivieri, Giorgia; Bournel, Fabrice; Andersen, Jesper N.; Rochet, Francois; Gallet, Jean-Jacques; Schnadt, Joachim
      Journal of Physical Chemistry C (2016), 120 (1), 243-251CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The at. layer deposition (ALD) of TiO2 on a RuO2(110) surface from tetrakis(dimethylamido) Ti and H2O at 110° was studied using near ambient pressure XPS (NAP-XPS) at precursor pressures up to 0.1 mbar. In addn. to the expected cyclic surface species, evidence for side reactions was found. Dimethylamine adsorbs on the surface during the TDMAT half-cycle, and a 2nd species, likely Me methylenimine, also forms. The removal of the amide ligand and the formation of an alkylammonium species during the H2O half-cycle are pressure dependent. The O 1s, Ru 3d, and Ti 2p spectra show the formation of the Ru-O-Ti interface, and the binding energies are consistent with formation of TiO2 after 1 full ALD cycle. Dosing TDMAT on the RuO2(110) surface at room temp. promotes a multilayer formation that begins to desorb at 40°. The imine species is not seen until 60°. These insights into the ALD mechanism and precursor pressure dependence on reactivity highlight the utility of NAP-XPS in studying ALD processes and interface formation.
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    32. 32
      Fairley, N.; Fernandez, V.; Richard-Plouet, M.; Guillot-Deudon, C.; Walton, J.; Smith, E.; Flahaut, D.; Greiner, M.; Biesinger, M.; Tougaard, S. Systematic and Collaborative Approach to Problem Solving Using X-ray Photoelectron Spectroscopy. Appl. Surf. Sci. Adv. 2021, 5, 100112  DOI: 10.1016/j.apsadv.2021.100112
      There is no corresponding record for this reference.
    33. 33
      Scofield, J. H. Hartree-Slater Subshell Photoionization Cross-Sections at 1254 and 1487 eV. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129– 137,  DOI: 10.1016/0368-2048(76)80015-1
      33
      Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV
      Scofield, J. H.
      Journal of Electron Spectroscopy and Related Phenomena (1976), 8 (2), 129-37CODEN: JESRAW; ISSN:0368-2048.
      The results of calcns. of photoelec. cross-sections for the Kα lines of magnesium at 1254 eV and of aluminum at 1487 eV are presented. All of the subshell cross-sections are given for Z values up to 96. The calcns. were carried out relativistically using the single-potential Hartree-Slater at. model.
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    34. 34
      Björck, M.; Andersson, G. GenX: An Extensible X-ray Reflectivity Refinement Program Utilizing Differential Evolution. J. Appl. Crystallogr. 2007, 40, 1174– 1178,  DOI: 10.1107/S0021889807045086
      There is no corresponding record for this reference.
    35. 35
      Prasai, B.; Cai, B.; Underwood, M. K.; Lewis, J. P.; Drabold, D. A. Properties of Amorphous and Crystalline Titanium Dioxide from First Principles. J. Mater. Sci. 2012, 47, 7515– 7521,  DOI: 10.1007/s10853-012-6439-6
      35
      Properties of amorphous and crystalline titanium dioxide from first principles
      Prasai, Binay; Cai, Bin; Underwood, M. Kylee; Lewis, James P.; Drabold, D. A.
      Journal of Materials Science (2012), 47 (21), 7515-7521CODEN: JMTSAS; ISSN:0022-2461. (Springer)
      First-principles methods were used to generate amorphous TiO2 (a-TiO2) models and this simulations lead to chem. ordered amorphous networks. The structural, electronic, and optical properties of the resulting structures were analyzed and compared with cryst. phases. Two peaks found in the Ti-Ti pair correlation correspond to edge-sharing and corner-sharing Ti-Ti pairs. Resulting coordination nos. for Ti (≈6) and O (≈3) and the corresponding angle distributions suggest that local structural features of bulk cryst. TiO2 are retained in a-TiO2. The electronic d. of states and the inverse participation ratio reveal that highly localized tail states at the valence band edge are due to the displacement of O atoms from the plane contg. 3 neighboring Ti atoms; whereas, the tail states at the conduction band edge are localized on over-coordinated Ti atoms. The electronic gap of ≈2.2 eV is comparable to calcd. results for bulk cryst. TiO2 despite the presence of topol. disorder in the amorphous network. The calcd. dielec. functions suggest that the amorphous phase of TiO2 has isotropic optical properties in contrast to those of tetragonal rutile and anatase phases. The av. static dielec. const. and the fundamental absorption edge for a-TiO2 are comparable to those of the cryst. phases.
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    36. 36
      Deskins, N. A.; Du, J.; Rao, P. The Structural and Electronic Properties of Reduced Amorphous Titania. Phys. Chem. Chem. Phys. 2017, 19, 18671– 18684,  DOI: 10.1039/C7CP02940C
      36
      The structural and electronic properties of reduced amorphous titania
      Deskins, N. Aaron; Du, Jincheng; Rao, Pratap
      Physical Chemistry Chemical Physics (2017), 19 (28), 18671-18684CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Cryst. titania has been extensively studied using exptl. and theor. tools. Amorphous titania, however, has received less attention in the literature, despite its importance for a no. of applications, such as photocatalysis, batteries, and electronic devices. We modeled amorphous titania using a combination of mol. dynamics and d. functional theory with several stoichiometries (TiOx, 2 ≥ x ≥ 1.75). Our results show that oxygen atom removal from amorphous titania is much easier than from cryst. titania, indicating that reduced amorphous structures are likely common. Ti atoms in amorphous titania exhibit a distribution of coordination nos. (5-7), but the av. coordination no. of oxygen increases upon redn. We also identified that gap states arise in substoichiometric titania due to the formation of Ti3+ centers. Such gap states are highly localized and randomly distributed across different Ti atoms, although we do observe a slight preference for electron localization on 7-coordinated Ti atoms. We observe that band gaps increase with redn. of amorphous titania. We also analyzed a proposed hole hopping mechanism involving oxygen vacancies by calcg. hole hopping distances. We found that such distances are large except in very reduced states, indicating likely slow hole diffusion through an oxygen vacancy mechanism. Our work is the 1st of its kind to thoroughly characterize the structural and electronic properties of amorphous titania in reduced states.
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    37. 37
      Xia, B.; Ganem, J.-J.; Vickridge, I.; Briand, E.; Steydli, S.; Benbalagh, R.; Rochet, F. Water-Rich Conditions during Titania Atomic Layer Deposition in the 100 °C-300 °C Temperature Window Produce Films with TiIV Oxidation State but Large H and O Content Variations. Appl. Surf. Sci. 2022, 601, 154233  DOI: 10.1016/j.apsusc.2022.154233
      37
      Water-rich conditions during titania atomic layer deposition in the 100 °C-300 °C temperature window produce films with TiIV oxidation state but large H and O content variations
      Xia, Bingbing; Ganem, Jean-Jacques; Vickridge, Ian; Briand, Emrick; Steydli, Sebastien; Benbalagh, Rabah; Rochet, Francois
      Applied Surface Science (2022), 601 (), 154233CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Titania films prepd. by at. layer deposition attract great attention due to the widespread application of the oxide as a photocatalytic material, or more recently, as a promising charge storage material for lithium or proton batteries. We implement here advanced tools (Ion Beam Anal. and XPS, aided by Ellipsometry and X-ray Diffraction) to characterize films grown in the 100 °C-300 °C temp. window, using tetrakis(dimethylamino)titanium (TMDAT) as the metal precursor and water vapor as the oxidant. We examine the outcomes of the ALD process as a function of the deposition temp., applying equal oxidant and precursor half-cycle time lengths, which contrasts with common deposition processes where the water half-cycle is considerably shorter than that of the metal precursor. Under the present ALD scheme, n-type conductive films are obtained where the oxidn. state of titanium is overwhelmingly TiIV at all temps., while the hydrogen content (O/Ti ratio) varies considerably, from ∼ 15 at% (∼1.8) at 100 °C to ∼ 3 at% (∼2) at 300 °C. The ideality of the ALD process is discussed through the identification of nitrogen-contg. mols. detected at the oxide surfaces. By extending the structural and compositional range of ALD titania films, new opportunities of application are expected to appear.
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    38. 38
      Sperling, B. A.; Kimes, W. A.; Maslar, J. E. Reflection Absorption Infrared Spectroscopy during Atomic Layer Deposition of HfO2 Films from Tetrakis(Ethylmethylamido)Hafnium and Water. Appl. Surf. Sci. 2010, 256, 5035– 5041,  DOI: 10.1016/j.apsusc.2010.03.050
      38
      Reflection absorption infrared spectroscopy during atomic layer deposition of HfO2 films from tetrakis(ethylmethylamido)hafnium and water
      Sperling, Brent A.; Kimes, William A.; Maslar, James E.
      Applied Surface Science (2010), 256 (16), 5035-5041CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Tetrakis(ethylmethylamido)hafnium and water are commonly used precursors for at. layer deposition of HfO2. Using reflection absorption IR spectroscopy with a buried-metal-layer substrate, we probe surface species present during typical deposition conditions. We observe evidence for thermal decompn. of alkylamido ligands at 320 °C. Addnl., we find that complete satn. of the SiO2 substrate occurs in the first cycle at ≈100 °C whereas incomplete coverage is apparent even after many cycles at higher temps. The use of this technique as an in situ diagnostic useful for process optimization is demonstrated.
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    39. 39
      Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269– 271,  DOI: 10.1126/science.1061051
      39
      Visible-light photocatalysis in nitrogen-doped titanium oxides
      Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y.
      Science (Washington, DC, United States) (2001), 293 (5528), 269-271CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      To use solar irradn. or interior lighting efficiently, the authors sought a photocatalyst with high reactivity under visible tight. Films and powders of TiO2-xNx have revealed an improvement over titanium dioxide (TiO2) under visible light (wavelength < 500 nm) in optical absorption and photocatalytic activity such as photodegrdns. of Methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface. Nitrogen doped into substitutional sites of TiO2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calcns. and x-ray photoemission spectroscopy.
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    40. 40
      Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 Photocatalysis: A Historical Overview and Future Prospects. Jpn. J. Appl. Phys. 2005, 44, 8269,  DOI: 10.1143/JJAP.44.8269
      40
      TiO2 photocatalysis: A historical overview and future prospects
      Hashimoto, Kazuhito; Irie, Hiroshi; Fujishima, Akira
      Japanese Journal of Applied Physics, Part 1: Regular Papers, Brief Communications & Review Papers (2005), 44 (12), 8269-8285CODEN: JAPNDE ISSN:. (Japan Society of Applied Physics)
      A review. Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has beer used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochem. reaction proceeding on a TiO2 surface when irradiated with UV light. One includes the photoinduced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. The authors review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.
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    41. 41
      Deng, S.; Verbruggen, S. W.; Lenaerts, S.; Martens, J. A.; Van den Berghe, S.; Devloo-Casier, K.; Devulder, W.; Dendooven, J.; Deduytsche, D.; Detavernier, C. Controllable Nitrogen Doping in as Deposited TiO2 Film and Its Effect on Post Deposition Annealing. J. Vac. Sci. Technol., A 2014, 32, 01A123
      There is no corresponding record for this reference.
    42. 42
      Martínez-Ferrero, E.; Sakatani, Y.; Boissière, C.; Grosso, D.; Fuertes, A.; Fraxedas, J.; Sanchez, C. Nanostructured Titanium Oxynitride Porous Thin Films as Efficient Visible-Active Photocatalysts. Adv. Funct. Mater. 2007, 17, 3348– 3354,  DOI: 10.1002/adfm.200700396
      42
      Nanostructured titanium oxynitride porous thin films as efficient visible-active photocatalysts
      Martinez-Ferrero, Eugenia; Sakatani, Yoshiaki; Boissiere, Cedric; Grosso, David; Fuertes, Amparo; Fraxedas, Jordi; Sanchez, Clement
      Advanced Functional Materials (2007), 17 (16), 3348-3354CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Nanocryst. mesoporous N-doped titania films were prepd. The introduction of nitrogen into the anatase structure starts at 500°, with N bonding to Ti via O substitution. Increasing the treatment temp. leads to the formation of TiN (TiN1-xOx) and N-doped rutile showing mixed-valence Ti states. Microstructural characterization shows that the ordered mesoporosity is maintained until 700°, where TiN (TiN1-xOx) begins to form. Optical characterization shows that the discrete introduction of N is able to shift the titania absorption edge. The photocatalytic tests give the best results under visible light excitation for the film nitrided at 500°. At this temp. the concn. of nitrogen in the structure is optimal since oxygen vacancies are still not important enough to promote the recombination of the photogenerated electrons and holes.
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    43. 43
      Driessen, J. P. A. M.; Schoonman, J.; Jensen, K. F. Infrared Spectroscopic Study of Decomposition of Ti(N(CH3)2)4. J. Electrochem. Soc. 2001, 148, G178,  DOI: 10.1149/1.1350687
      43
      Infrared spectroscopic study of decomposition of Ti(N(CH3)2)4
      Driessen, Joost P. A. M.; Schoonman, Joop; Jensen, Klavs F.
      Journal of the Electrochemical Society (2001), 148 (3), G178-G184CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The decompn. of Ti(NMe2)4 (TDMAT) was studied in N2 and H2 environments and surface temps. between 473 and 623 K by using FTIR spectroscopy. The pressure in the system was 5 torr, with a TDMAT partial pressure of 0.3 torr. The evolution of gas-phase species was monitored by characteristic IR absorption. For temps. <478 K, an av. no. of >3 ligands per TDMAT mol. is obsd. Approx. 1 ligand per 2 TDMAT mols. decomps. into methane- and C-contg. species in the coating. This slow decompn. pathway implies intermol. H transfer between multiple TDMAT mols. A decompn. mechanism consistent with the exptl. observations is proposed. For temps. >478 K, the conversion rate of TDMAT into products increases drastically. Less gaseous species are obsd., Auger electron spectroscopy measurements show more C contamination in the coating, and addnl. absorption peaks appear in the IR spectra. NMR spectroscopy indicates that these peaks can be assigned to a mixt. of oligomers of TDMAT reaction fragments that desorb from the hot surface. The increased C content at elevated temps. is attributed to incomplete desorption of ligands or metallacycle formation.
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    44. 44
      Hsu, J.-C.; Lin, Y.-H.; Wang, P. W. X-ray Photoelectron Spectroscopy Analysis of Nitrogen-Doped TiO2 Films Prepared by Reactive-Ion-Beam Sputtering with Various NH3/O2 Gas Mixture Ratios. Coatings 2020, 10, 47,  DOI: 10.3390/coatings10010047
      44
      X-ray photoelectron spectroscopy analysis of nitrogen-doped TiO2 films prepared by reactive-ion-beam sputtering with various NH3/O2 gas mixture ratios
      Hsu, Jin-Cherng; Lin, Yung-Hsin; Wang, Paul W.
      Coatings (2020), 10 (1), 47CODEN: COATED; ISSN:2079-6412. (MDPI AG)
      Nitrogen-doped TiO2 films were prepd. by reactive ion-beam sputtering deposition (IBSD) in a mixed atm. of NH3 and O2 at a substrate temp. of 400 °C. X-ray photoelectron spectra revealed the presence of six ions, i.e., N3-, N2-, N1-, N+, N2+, and N3+, resp., in the films. The amorphous films had complex, randomly oriented chem. bonds. The Tauc-Lorentz model was employed to det. the bandgap energy of the amorphous films prepd. using different NH3/O2 gas mixing ratios by ellipsometry. In addn., the optical consts. of the films were measured. With the increase in the NH3/O2 gas mixt. ratio to 3.0, the bandgap of N-doped TiO2 narrowed to ∼2.54 eV.
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    45. 45
      Lafuente, B.; Downs, R. T.; Yang, H.; Stone, N. The Power of Databases: The RRUFF Project. In Highlights in Mineralogical Crystallography; Armbruster, T.; Danisi, R. M., Eds.; Walter de Gruyter GmbH & Co KG, 2015; pp. 1– 30 DOI: 10.1515/9783110417104-003 .
      There is no corresponding record for this reference.
    46. 46
      Busani, T.; Devine, R. A. B. Dielectric and Infrared Properties of TiO2 Films Containing Anatase and Rutile. Semicond. Sci. Technol. 2005, 20, 870– 875,  DOI: 10.1088/0268-1242/20/8/043
      46
      Dielectric and infrared properties of TiO2 films containing anatase and rutile
      Busani, T.; Devine, R. A. B.
      Semiconductor Science and Technology (2005), 20 (8), 870-875CODEN: SSTEET; ISSN:0268-1242. (Institute of Physics Publishing)
      Elec. and optical properties of low-temp., plasma enhanced chem. vapor deposited films of TiO2 have been studied; the source gases were TiCl4 and O2. The amorphous, as-deposited films had a dielec. const. ∼33 consistent with their measured d. of 3.2 ± 0.2 g cm-3. Films deposited using a -41 V substrate bias contained the anatase phase and some rutile as evidenced from IR spectroscopy and x-ray scattering. Annealing of these films at 600°C resulted in a significant increase in the rutile content of the film.
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    47. 47
      Piercy, B. D.; Leng, C. Z.; Losego, M. D. Variation in the Density, Optical Polarizabilities, and Crystallinity of TiO2 Thin Films Deposited via Atomic Layer Deposition from 38 to 150 °C Using the Titanium Tetrachloride-Water Reaction. J. Vac. Sci. Technol., A 2017, 35, 03E107  DOI: 10.1116/1.4979047
      There is no corresponding record for this reference.
    48. 48
      Go, D.; Lee, J.; Shin, J. W.; Lee, S.; Kang, W.; Han, J. H.; An, J. Phase-Gradient Atomic Layer Deposition of TiO2 Thin Films by Plasma-Induced Local Crystallization. Ceram. Int. 2021, 47, 28770– 28777,  DOI: 10.1016/j.ceramint.2021.07.037
      48
      Phase-gradient atomic layer deposition of TiO2 thin films by plasma-induced local crystallization
      Go, Dohyun; Lee, Jaehyeong; Shin, Jeong Woo; Lee, Sungje; Kang, Wangu; Han, Jeong Hwan; An, Jihwan
      Ceramics International (2021), 47 (20), 28770-28777CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
      Atomic layer deposition (ALD) is a thin-film fabrication method that can be used to deposit films with precise thickness controllability and uniformity. The low deposition temp. of ALD, however, often interrupts the facile crystn. of films, resulting in inferior optical and elec. properties. In this study, the extremely localized crystn. of TiO2 thin films was demonstrated by per-cycle plasma treatment during the plasma-enhanced ALD process. By layering cryst. and amorphous films, a phase-gradient TiO2 film with precisely modulated optical and elec. properties was fabricated. Moreover, the ratio between the amorphous and cryst. layer thicknesses for a high dielec. const. and low leakage c.d. was optimized.
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    49. 49
      McDowell, M. T.; Lichterman, M. F.; Carim, A. I.; Liu, R.; Hu, S.; Brunschwig, B. S.; Lewis, N. S. The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 15189– 15199,  DOI: 10.1021/acsami.5b00379
      49
      The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation
      McDowell, Matthew T.; Lichterman, Michael F.; Carim, Azhar I.; Liu, Rui; Hu, Shu; Brunschwig, Bruce S.; Lewis, Nathan S.
      ACS Applied Materials & Interfaces (2015), 7 (28), 15189-15199CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aq. soln. Amorphous TiO2 deposited by at.-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidn., with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO2 in reducing or oxidizing environments. All of the polycryst. TiO2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO2 layer is neither specific to a particular amorphous or cryst. structure nor detd. wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.
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    50. 50
      Pore, V.; Heikkilä, M.; Ritala, M.; Leskelä, M.; Areva, S. Atomic Layer Deposition of TiO2–xNx Thin Films for Photocatalytic Applications. J. Photochem. Photobiol., A 2006, 177, 68– 75,  DOI: 10.1016/j.jphotochem.2005.05.013
      50
      Atomic layer deposition of TiO2-xNx thin films for photocatalytic applications
      Pore, Viljami; Heikkilae, Mikko; Ritala, Mikko; Leskelae, Markku; Areva, Sami
      Journal of Photochemistry and Photobiology, A: Chemistry (2006), 177 (1), 68-75CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      Titanium dioxide (TiO2) is recognized as the most efficient photocatalytic material, but due to its large band gap energy it can only be excited by UV irradn. Doping TiO2 with nitrogen is a promising modification method for the utilization of visible light in photocatalysis. In this work, nitrogen-doped TiO2 films were grown by at. layer deposition (ALD) using TiCl4, NH3 and water as precursors. All growth expts. were done at 500° C. The films were characterized by XRD, XPS, SEM and UV-vis spectrometry. The influence of nitrogen doping on the photocatalytic activity of the films in the UV and visible light was evaluated by the degrdn. of a thin layer of stearic acid and by linear sweep voltammetry. Light-induced superhydrophilicity of the films was also studied. It was found that the films could be excited by visible light, but they also suffered from increased recombination.
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    51. 51
      Cheng, H.-E.; Chen, Y.-R.; Wu, W.-T.; Hsu, C.-M. Effect of Nitrogen Doping Concentration on the Properties of TiO2 Films Grown by Atomic Layer Deposition. Mater. Sci. Eng., B 2011, 176, 596– 599,  DOI: 10.1016/j.mseb.2011.02.001
      51
      Effect of nitrogen doping concentration on the properties of TiO2 films grown by atomic layer deposition
      Cheng, Hsyi-En; Chen, Yu-Ru; Wu, Wen-Tuan; Hsu, Ching-Ming
      Materials Science & Engineering, B: Advanced Functional Solid-State Materials (2011), 176 (7), 596-599CODEN: MSBTEK; ISSN:0921-5107. (Elsevier B.V.)
      N-doped TiO2 films were grown on n+-silicon substrates by at. layer deposition using titanium chloride and vapor mixt. of ammonia and water as the reactants. The effects of doping concn. on the microstructure and photocurrent response of as-deposited films were investigated. The results show that the doping levels were 0.2, 0.7, 1.2, 1.5, and 4.3 at% for films grown at NH3-to-H2O injection vol. ratios of 350, 380, 440, 520, and 550, resp. The off-plane lattice const. of TiO2 films increased with the increase of doping level, and the transformation of anatase to rutile was inhibited by the doping as the doping concn. reached 1.2 at%. The wavelength-dependent photocurrents suggest an optimal N doping concn. lying between 0.7 and 1.2 at% for the visible light active TiO2 films. Doping with a too-low or a too-high nitrogen level resulted in an inefficient visible light generation or a serious carrier recombination, resp.
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    52. 52
      Pore, V.; Ritala, M.; Leskelä, M.; Saukkonen, T.; Järn, M. Explosive Crystallization in Atomic Layer Deposited Mixed Titanium Oxides. Cryst. Growth Des. 2009, 9, 2974– 2978,  DOI: 10.1021/cg801337f
      52
      Explosive Crystallization in Atomic Layer Deposited Mixed Titanium Oxides
      Pore, Viljami; Ritala, Mikko; Leskela, Markku; Saukkonen, Tapio; Jarn, Mikael
      Crystal Growth & Design (2009), 9 (7), 2974-2978CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
      Anatase crystals with lateral sizes of tens of microns are impossible to prep. as thin films on amorphous substrates by normal thin film deposition techniques. When deposited on amorphous substrates, polycryst. anatase titanium dioxide (TiO2) thin films have typically grain and crystal sizes comparable to the film thickness. We show that when a suitable amt. of Nb2O5 or Ta2O5 is mixed with TiO2 using at. layer deposition (ALD), upon post-deposition annealing large ∼10-100 μm sized anatase monocrystals are formed in films thinner than 100 nm on amorphous substrates. Such an exceptionally extensive lateral grain growth appears to be a result of explosive crystn. when amorphous Ti-Nb-O and Ti-Ta-O thin films are heated, a phenomenon that has not been reported before for TiO2 thin films. The prepd. films will likely have a no. of useful applications where well-ordered anatase surfaces are required, such as photocatalysis, and they also can be used as a transparent conducting oxide.
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    53. 53
      Dufond, M. E.; Chazalviel, J.-N.; Santinacci, L. Electrochemical Stability of n-Si Photoanodes Protected by TiO2 Thin Layers Grown by Atomic Layer Deposition. J. Electrochem. Soc. 2021, 168, 031509  DOI: 10.1149/1945-7111/abeaf3
      53
      Electrochemical stability of n-Si photoanodes protected by TiO2 thin layers grown by atomic layer deposition
      Dufond, Maxime E.; Chazalviel, Jean-Noel; Santinacci, Lionel
      Journal of the Electrochemical Society (2021), 168 (3), 031509CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
      This work investigates the n-Si photoanodes corrosion protection by Atomic Layer Deposition (ALD) of a TiO2 film. A specific electrochem. exptl. sequence (including successive rest potential measurements and voltammetries under illumination or not) has been established to study the stability of the electrodes in KOH. Depending on the deposition conditions (precursor compn. and temp.), the electrochem. properties of the layers are different. The photoanodes coated using titanium tetraisopropoxide (TTIP) at low temp. exhibit a low photocurrent (jph) that is progressively enhanced during the electrochem. sequence and their stability decreases. When using tetrakis (dimethylamido) titanium (TDMAT), the jph is almost const. and the film prevents from corrosion. The characterizations show that the ALD parameters drive the microstructure of the layer that is found crit. for the electrochem. response. A hydrogen doping occurring during the open circuit potential measurements under illumination is evidenced by IR spectroscopy. It is mainly localized at the grain boundaries and pores of the layers as well as in the n-Si and it modifies the charge transfer at the electrode/soln. junction and the hydrogen diffusion weakens the film causing the Si corrosion. The different charge transfer mechanisms are finally proposed depending on the ALD conditions and the film thickness.
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    54. 54
      Viswanathan, B.; Krishanmurthy, K. R. Nitrogen Incorporation in TiO2: Does It Make a Visible Light Photo-Active Material?. Int. J. Photoenergy 2012, 2012, 1– 10,  DOI: 10.1155/2012/269654
      There is no corresponding record for this reference.
    55. 55
      Hukari, K.; Dannenberg, R.; Stach, E. A. Nitrogen Effects on Crystallization Kinetics of Amorphous TiOxNy Thin Films. J. Mater. Res. 2002, 17, 550– 555,  DOI: 10.1557/JMR.2002.0077
      55
      Nitrogen effects on crystallization kinetics of amorphous TiOxNy thin films
      Hukari, Kyle; Dannenberg, Rand; Stach, E. A.
      Journal of Materials Research (2002), 17 (3), 550-555CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
      The crystn. behavior of amorphous TiOxNy (x » y) thin films was investigated by in situ TEM. The Johnson-Mehl-Avrami-Kozolog (JMAK) theory was used to det. the Avrami exponent, activation energy, and the phase velocity pre-exponent. Addn. of nitrogen inhibited diffusion, increasing the nucleation temp., while decreasing the growth activation energy. Kinetic variables extd. from individual crystallites were compared to JMAK anal. of the fraction transformed, and a change of 6% in the activation energy led to agreement between the methods. From diffraction patterns and index of refraction the crystd. phase was found to be predominantly anatase.
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    56. 56
      Albenze, E. J.; Thompson, M. O.; Clancy, P. Molecular Dynamics Study of Explosive Crystallization of SiGe and Boron-Doped SiGe Alloys. Ind. Eng. Chem. Res. 2006, 45, 5628– 5639,  DOI: 10.1021/ie051361w
      56
      Molecular Dynamics Study of Explosive Crystallization of SiGe and Boron-Doped SiGe Alloys
      Albenze, Erik J.; Thompson, Michael O.; Clancy, Paulette
      Industrial & Engineering Chemistry Research (2006), 45 (16), 5628-5639CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Explosive crystn. of SiGe alloys and boron-doped SiGe alloys was studied using a special mol. dynamics construction developed by the authors. For the case of Si1-xGex alloys, the simulations indicate that explosive crystn. should occur over the entire compositional range of Si1-xGex alloys at velocities high enough to prevent Ge segregation. The results show qual. agreement with exptl. results for Si-rich systems: Over a range of 0-25% germanium, the simulations predicted a nonlinear decrease in velocity that roughly matches the slope and extent of the rapid drop in interface velocity obsd. exptl. An analogous study of boron-doped Si1-xGex alloys showed that the limit of incorporation of boron during explosive crystn. is strongly dependent on the amt. of germanium present and that explosive crystn. is capable of limiting transient enhanced diffusion throughout the crystn. process.
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    57. 57
      Buchner, C.; Schneider, W. Explosive Crystallization in Thin Amorphous Layers on Heat Conducting Substrates. J. Appl. Phys. 2015, 117, 245301,  DOI: 10.1063/1.4922787
      57
      Explosive crystallization in thin amorphous layers on heat conducting substrates
      Buchner, Christoph; Schneider, Wilhelm
      Journal of Applied Physics (Melville, NY, United States) (2015), 117 (24), 245301/1-245301/12CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      A model for explosive crystn. in a thin amorphous layer on a heat conducting substrate is presented. For the thin layer, the energy equation was used in a 1-dimensional approxn. Heat conduction into the substrate and thermal contact resistance at the interface between layer and substrate are taken into account. Four rate equations were used to describe the kinetics of the homogeneous amorphous-cryst. transition. The whole process was examd. as a plane wave of invariant shape in a moving frame of ref. Heat conduction in the substrate is described by introducing a continuous distribution of moving heat sources at the interface. This gives an integral representation for the temp. in the substrate in terms of the unknown source distribution. The integral term implies that there is a nonlocal influence of the temp. distribution in the layer on the heat loss. A coupled system of an integro-differential equation and four ordinary differential equations was obtained and solved numerically. The propagation velocity of the wave was obtained as an eigenvalue of the system of equations. Varying a nondimensional heat loss parameter, a crit. value is found beyond which no crystn. wave of invariant shape is possible. This can also be interpreted as a certain min. layer thickness. Temp. and crystallinity distributions are shown for some interesting configurations. Predictions of crystn.-wave velocities and min. layer thicknesses are compared with exptl. values for explosive crystn. in Ge. (c) 2015 American Institute of Physics.
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    58. 58
      Cho, C. J.; Kang, J.-Y.; Lee, W. C.; Baek, S.-H.; Kim, J.-S.; Hwang, C. S.; Kim, S. K. Interface Engineering for Extremely Large Grains in Explosively Crystallized TiO2 Films Grown by Low-Temperature Atomic Layer Deposition. Chem. Mater. 2017, 29, 2046– 2054,  DOI: 10.1021/acs.chemmater.6b04090
      58
      Interface Engineering for Extremely Large Grains in Explosively Crystallized TiO2 Films Grown by Low-Temperature Atomic Layer Deposition
      Cho, Cheol Jin; Kang, Jun-Yun; Lee, Woo Chul; Baek, Seung-Hyub; Kim, Jin-Sang; Hwang, Cheol Seong; Kim, Seong Keun
      Chemistry of Materials (2017), 29 (5), 2046-2054CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Functionality in thin films is closely linked with both the microstructure of the film as well as the innate nature of the thin film material. The engineering of microstructure, esp. the grain size and its distribution in oxide thin films, is crit. to designing functionalities targeting specific applications. However, the scope for manipulation of the microstructure of an oxide thin film is generally quite limited because of the lack of mobility of atoms at processing temps. that are usually employed for the growth of these films. This work reports the observation of very large grains (∼100 times larger than the film thickness) in TiO2 films grown by at. layer deposition at low temp. (<250°). Large grain sizes were obsd. in the as-grown films even in the absence of postdeposition annealing, wherein substrates with high surface energies were used. The surface energy engineering of substrate offers an efficient method to control the grain size of TiO2 films in low thermal budget processes. The results will provide facile routes to design novel properties of thin films via controlling the grain size even in low temp. processes.
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    59. 59
      Petkov, V.; Holzhu, G. Atomic-Scale Structure of Amorphous TiO2 by Electron, X-ray Diffraction and Reverse Monte Carlo Simulations. J. Non-Cryst. Solids 1998, 231, 17– 30,  DOI: 10.1016/S0022-3093(98)00418-9
      59
      Atomic-scale structure of amorphous TiO2 by electron, x-ray diffraction and reverse Monte Carlo simulations
      Petkov, V.; Holzhuter, G.; Troge, U.; Gerber, Th.; Himmel, B.
      Journal of Non-Crystalline Solids (1998), 231 (1,2), 17-30CODEN: JNCSBJ; ISSN:0022-3093. (Elsevier Science B.V.)
      Amorphous TiO2 materials, layers and powders, have been produced by different prepn. techniques. The layers have been subjected to electron diffraction and the bulk powders to X-ray diffraction expts. Three-dimensional structure models consistent with the exptl. structure functions have been constructed by reverse Monte Carlo simulations. Partial distribution functions, interat. distances and coordination nos. have been extd. from the model at. configurations. The at. arrangement in all amorphous TiO2 materials investigated has been found to resemble that occurring in brookite, a cryst. modification of TiO2, and to be well described as an assembly of short, staggered chains of Ti-O octahedra. The crystn. behavior of amorphous TiO2-materials has also been discussed.
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    60. 60
      Jellison, G. E., Jr.; Boatner, L. A.; Budai, J. D.; Jeong, B.-S.; Norton, D. P. Spectroscopic Ellipsometry of Thin Film and Bulk Anatase (TiO2). J. Appl. Phys. 2003, 93, 9537– 9541,  DOI: 10.1063/1.1573737
      60
      Spectroscopic ellipsometry of thin film and bulk anatase (TiO2)
      Jellison, G. E., Jr.; Boatner, L. A.; Budai, J. D.; Jeong, B.-S.; Norton, D. P.
      Journal of Applied Physics (2003), 93 (12), 9537-9541CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Spectroscopic ellipsometry (SE) measurements were made on thin-film and single-crystal TiO2 anatase using a two-modulator generalized ellipsometer. The TiO2 films were epitaxially stabilized on a LaAlO3 substrate in the anatase crystal structure using reactive sputter deposition. The films were highly cryst., possessing a stepped surface morphol. indicative of at. layer-by-layer growth. The SE results for the anatase film indicate that the material is essentially oriented with the c axis perpendicular to the substrate, but there is some anisotropy near the interface and the surface. Corrugations of the film surface, as obsd. using at. force microscopy, are consistent with a surface structure needed to create cross polarization. Accurate values of the optical functions of cryst. anatase were obtained above and below the band edge using SE. Above the band edge, both the ordinary and extraordinary complex dielec. functions exhibited two crit. points.
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    61. 61
      Lee, D.-K.; Kwon, S.-H.; Ahn, J.-H. Growth of Rutile-TiO2 Thin Films via Sn Doping and Insertion of Ultra-Thin SnO2 Interlayer by Atomic Layer Deposition. Mater. Lett. 2019, 246, 1– 4,  DOI: 10.1016/j.matlet.2019.03.018
      61
      Growth of rutile-TiO2 thin films via Sn doping and insertion of ultra-thin SnO2 interlayer by atomic layer deposition
      Lee, Dong-Kwon; Kwon, Se-Hun; Ahn, Ji-Hoon
      Materials Letters (2019), 246 (), 1-4CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
      Rutile-TiO2 thin films have potential for use in high-k applications, such as dynamic random-access memory capacitors; however, they are difficult to realize without using noble-metal-based oxide substrates. Therefore, we proposed a new approach for the prepn. of rutile TiO2 by a small amt. of Sn doping and the insertion of ultra-thin SnO2 to achieve enhanced dielec. performance without using noble-metal-based electrodes. It was confirmed that the crystallinity of rutile TiO2 was remarkably enhanced in Sn-doped TiO2 formed on an ultra-thin SnO2 interlayer. Moreover, 10 nm-thick Sn-doped TiO2 thin film on a 1-nm SnO2 interlayer exhibited a high dielec. const. of about 80.
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    62. 62
      Kim, B.; Choi, Y.; Lee, D.; Cheon, S.; Byun, Y.; Jeon, H. Atomic Layer Deposition for Rutile Structure TiO2 Thin Films Using a SnO2 Seed Layer and Low Temperature Heat Treatment. Nanotechnology 2021, 33, 115701
      There is no corresponding record for this reference.
    63. 63
      Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272– 1276,  DOI: 10.1107/S0021889811038970
      63
      VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data
      Momma, Koichi; Izumi, Fujio
      Journal of Applied Crystallography (2011), 44 (6), 1272-1276CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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    64. 64
      Baur, W. H. Über die Verfeinerung der Kristallstrukturbestimmung einiger Vertreter des Rutiltyps: TiO2, SnO2, GeO2 und MgF2. Acta Cryst. 1956, 9, 515– 520,  DOI: 10.1107/S0365110X56001388
      64
      The refinement of the crystal-structure determination of some representatives of the rutile type: TiO2, SnO2, GeO2, and MgF2
      Baur, Werner H.
      Acta Crystallographica (1956), 9 (), 515-20CODEN: ACCRA9; ISSN:0365-110X.
      The lattice consts. and at. parameters of TiO2 (rutile), SnO2, GeO2, and MgF2 were redetd. I(hk0) of TiO2, SnO2, and MgF2 were obtained by careful photometry of Weissenberg photographs (MoKα radiation) and were used for Fourier and (Fo - Fc) projections. The at. parameter of GeO2 was detd. from a 3-dimensional Fourier section using intensities from powder diagrams, because single crystals were not available. Results: TiO2, a = 4.594 ± 0.003, c = 2.959 ±2.002 A., x = 0.306 ± 0.001; SnO2, a = 4.737 ± 0.001, c = 3.185 ± 0.001 A., x = 0.307 ± 0.001; MgF2, a = 4.625 ± 0.002, c = 3.052 ± 0.003 A., x = 0.303 ± 0.001; and GeO2, a = 4.395 ± 0.003, c = 2.860 ± 0.003 A., x = 0.307 ± 0.004. In the TiO6 octahedra of rutile 4 Ti-O distances are shorter than two others by 2%. In the other compds. the 6A-B distances in the AB6 octahedra are equal within the limits of accuracy.
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    65. 65
      Howard, C. J.; Sabine, T. M.; Dickson, F. Structural and Thermal Parameters for Rutile and Anatase. Acta Cryst. B 1991, 47, 462– 468,  DOI: 10.1107/S010876819100335X
      There is no corresponding record for this reference.
    66. 66
      Downs, R. T.; Hall-Wallace, M. The American Mineralogist Crystal Structure Database. Am. Mineral. 2003, 88, 247– 250
      66
      The American Mineralogist Crystal Structure Database
      Downs, Robert T.; Hall-Wallace, Michelle
      American Mineralogist (2003), 88 (1), 247-250CODEN: AMMIAY; ISSN:0003-004X. (Mineralogical Society of America)
      A database has been constructed that contains all the crystal structures previously published in the American Mineralogist. The database is called "The American Mineralogist Crystal Structure Database" and is freely accessible from the websites of the Mineralogical Society of America at http://www.minsocam.org/MSA/Crystal_Database.html and the University of Arizona. In addn. to the database, a suite of interactive software is provided that can be used to view and manipulate the crystal structures and compute different properties of a crystal such as geometry, diffraction patterns, and procrystal electron densities. The database is set up so that the data can be easily incorporated into other software packages. Included at the website is an evolving set of guides to instruct the user and help with classroom education.
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  • Supporting Information

    Supporting Information

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