Structure and properties of nitrogen-doped titanium dioxide thin films grown by atmospheric pressure chemical vapor deposition

Using TiCl₄, O₂, and N₂O as precursors, N-doped titanium dioxide thin films with large area and continuous surface were obtained by atmospheric pressure chemical vapor deposition. Measurements of X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscope, transmission electron microscope and ultravoilet-Visible transmission spectra were performed. Using N₂O as N-doped source, anatase-rutile transformation is accelerated through oxygen vacancies formation, and the mean grain size of rutile crystallites decreases with the increase of N₂O flow rate. Compared to the pure TiO₂, N-doped TiO₂ films give a relative narrow optical band-gap, and their visible-light induced photocatalysis is much enhanced. Visible-light-induced hydrophilicity of the TiO₂ thin films enhances with the increase of N₂O flow rate, which might be due to the dentritic islands structure on the surface of the N-doped TiO₂ thin films.     

Conductivity of nanometer TiO2 thin films by magnetron sputtering

The conductivity of nanometer TiO₂ thin films was presented in this paper. The dependence of the conductivity of TiO₂ thin films on the thickness of the film and the substrate material were educed. The TiO₂ films were deposited by reactive magnetron sputtering of a Ti targets in an Ar+O₂ mixture in a conventional sputtering reactor. The thickness of the films deposited on Ti varied in the range from 15 to 225 nm. The resistivity of the films was measured at room temperature in the air. It was found that the conductivity of TiO₂ thin films varies in the range from conductor, semiconductor to nonconductor. This was attributed to electrons transfer at the interface between the TiO₂ and substrates, and the depth of electrons transfer was determined by the difference of work function.     

Chemical vapor deposition and characterization of nitrogen doped TiO2 thin films on glass substrates

Photocatalytically active, N-doped TiO₂ thin films were prepared by low pressure metalorganic chemical vapor deposition (MOCVD) using titanium tetra-iso-propoxide (TTIP) as a precursor and NH₃ as a reactive doping gas. We present the influence of the growth parameters (temperature, reactive gas phase composition) on the microstructural and physico-chemical characteristics of the films, as deduced from X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and ultra-violet and visible (UV/Vis) spectroscopy analysis. The N-doping level was controlled by the partial pressure ratio R = [NH₃]/[TTIP] at the entrance of the reactor and by the substrate temperature. For R = 2200, the N-doped TiO₂ layers are transparent and exhibit significant visible light photocatalytic activity (PA) in a narrow growth temperature range (375-400 °C). The optimum N-doping level is approximately 0.8 at.%. However, the PA activity of these N-doped films, under UV light radiation, is lower than that of undoped TiO₂ films of comparable thickness.     

Nitrogen dioxide gas sensors based on titanium dioxide thin films deposited on langasite

Conductometric sensors, based on pure and Au-doped TiO₂ thin films on langasite (LGS) substrates have been investigated for detecting low concentrations (510 ppb and 1060 ppb) of NO₂ in synthetic air at a temperature range between 220 °C and 320 °C. Thin films of TiO₂ were deposited using the radio frequency (RF) magnetron sputtering method. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were employed for the characterization of thin films. It was observed that the Au-doped TiO₂ sensor has fast response and recovery time of 10 and 29 s respectively towards 510 ppb of NO₂ at 273 °C.     

Low temperature atomic layer deposition of titania thin films

This paper presents a comprehensive study of atomic layer deposition of TiO₂ films on silicon and polycarbonate substrates using TiCl₄ and H₂O as precursors at temperatures in the range 80-120 °C. An in-situ quartz crystal microbalance was used to monitor different processing conditions and the resultant films were characterised ex-situ using a suite of surface analytical tools. In addition, the contact angle and wettability of as-deposited and UV irradiated films were assessed. The latter was found to reduce the contact angle from ≥ 80° to < 10°. Finally, the effect of surface pre-treatment on film toughness and adhesion was investigated and the results show a significant improvement for the pre-treated films.     

Effect of hot-filament annealing in a hydrogen atmosphere on the electrical and structural properties of Nb-doped TiO2 sputtered thin films

In this work Nb-doped TiO2 thin films were deposited by d.c.-pulsed reactive magnetron sputtering at 500 °C from a composite target with weight fractions of 96% Ti and 4% Nb, using oxygen as reactive gas. In order to enhance the conductive properties, the as-deposited samples were treated in vacuum with atomic hydrogen at a substrate temperature of 500 °C. The atomic hydrogen flow was generated by a hot filament, inside a high-vacuum chemical vapour deposition reactor, at a temperature of 1750 °C. In order to optimise the hydrogen hot-wire treatments, the H₂ pressure was varied between 1.3 and 67 Pa, the treatment time was monitored between 1 and 5 min and the hot-filament current was changed between 12 and 17 A. Dark conductivity was measured as a function of temperature and its value at room temperature was extrapolated and used to assess the effect of the hydrogen annealing on the charge transport properties. A two-order of magnitude increase in dark conductivity was typically observed for optimised hydrogen treatments (10 Pa), when varying the hydrogen pressure, resulting in a minimum resistivity of ~ 3 × 10^− 3 Ω cm at room temperature. The maximum amount of atomic H incorporation in oxygen vacancies was determined to be ~ 5.7 at.%. Carrier mobility and resistivity were also investigated using Hall effect measurements. Correlations between structural and electrical properties and the hydrogen treatment conditions are discussed. The purpose of these films is to provide a transparent and conductive front contact layer for a-Si based photovoltaics, with a refractive index that better matches that of single and tandem solar cell structures. This can be achieved by an appropriate incorporation of a very small amount of cationic doping (Nb^5 +) into the titanium dioxide lattice.     

Photocatalytic degradation of selected dyes by titania thin films with various nanostructures

Titania thin films with various nanostructures of quasi-aligned nanorods, well-aligned nanotubes and nanoparticle aggregates were fabricated and utilized to assist photodegradation of three typical dye derivatives in water, i. e., rhodamine B (RB), methylene blue (MB) and methyl orange (MO). The thin films were characterized by X-ray diffraction, scanning electron microscope and UV-Vis diffuse reflectance spectra. It is found that the bandgap of most lab-fabricated thin films can be estimated more appropriately assuming a direct transition between the valance band and the conduction band of titania. The so-called natural ageing phenomenon was evidence for all the films to assist photodegradation of RB and MB in water; but not discernible for MO, which was much reluctant to photodegradation. MB decomposed much quickly than RB when assisted by titania with predominantly anatase; on the other hand, rutile favored the photodegradation of RB than that of MB. Titania thin films with top morphologies of quasi-aligned nanorods or nanotubes were found to possess advantageous turnover frequency and photonic efficiency over commercial P25 titania and sol-gel derived titania to assist photodegradation of RB and MB in water.     

Enhanced photo-catalytic activity of TiO2 films with doped La prepared by micro-plasma oxidation method

Mesoporous thin titanium dioxide films have been prepared on titanium plates through micro-plasma oxidation. To increase the films' photo-catalytic activity, La ions with different concentrations (0, 0.025, 0.005, 0.075, 0.1g/L) were added into the H(2)SO(4) electrolyte solution. X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy techniques were applied to characterize the modified films. A kind of typical textile industry pollutant (Rhodamine B) was used to evaluate the photo-catalytic activity of the films. The results showed that this activity of the films had been improved by adding La ions into the electrolyte solution. The enhanced photo-catalytic activities might be resulted from the increase of mesopores' number, producing more reactive sites to absorb and oxidize pollutants. Also, the improvement was related to the forming of titanium dioxide lattice distortion, which could accept more photoexcitated holes and produce more strong surface free radicals to oxidate adsorptive molecules.     

Low-temperature deposition of crystallized TiO2 thin films

Crystallized TiO₂ thin films were deposited on a non-heated substrate by two methods: oxygen-ion-assisted reactive evaporation (ORE) and high-rate reactive sputtering (HRS) using two sputtering sources. When the films were deposited on an unheated glass substrate, amorphous films were initially grown on the substrate in case of both deposition methods, although an increase in oxygen-ion energy above 600 eV led to a growth of a crystallized layer on the amorphous films in the case of ORE. When the films were deposited by HRS on a crystallized TiO₂ seed layer, homo-epitaxial growth was observed, and crystallized TiO₂ films with an excellent hydrophilic property were obtained on unheated substrate. In contrast, when the films were deposited by ORE, amorphous films were initially grown on the crystallized TiO₂ seed layer in a similar manner to the deposition of films on a glass substrate, and homo-epitaxial growth was not observed. These results suggest that the large kinetic energy of titanium atoms arriving at the substrate during HRS is a key factor in promoting epitaxial growth of the TiO₂ film at low temperature.     

Effect of crystallographic orientations on electrical properties of sputter-deposited nickel oxide thin films

Nickel oxide thin films of various preferred orientations were deposited by radio-frequency (RF) magnetron sputtering process in different gas ratios of oxygen atmosphere at RF power 200 W on unheated and heated for (673 K) substrates. The relationships among substrate temperature, preferred orientation and electrical properties of the NiO films were investigated. The resulting films were analyzed by grazing-incidence X-ray diffraction, high-resolution transmission electron microscopy (HR-TEM), and ultrahigh resolution scanning electron microscopy (HR-SEM). The electrical properties were measured using four probe and Hall effects measurements. The results show that films deposited at room temperature with the ratio of oxygen varying from 0 to 100% develop a (111) preferred orientation. At temperature of 673 K, while the (111)-orientated film was obtained under a low ratio of oxygen (<50% O₂), a (200) preferred orientation was developed under 100% oxygen. The lowest sheet resistance 0.01 MΩ/□, resistivity 0.83 Ω-cm and higher carrier density 7.35 × 10¹⁸ cm⁻³ could be obtained on (111) preferred orientation samples prepared on unheated substrates in pure oxygen atmosphere. The relationship between preferred orientation and electrical properties was proposed in this paper.     

Enhancement of visible light-induced hydrophilicity on nitrogen and sulfur-codoped TiO2 thin films

We fabricated N, S-codoped, N-doped, S-doped TiO₂ anatase thin films by a radio-frequency (RF) sputtering method and evaluated the photoelectrochemical and photoinduced hydrophilic activities. The N, S-codoped TiO₂ thin film showed obviously higher activities than either the N-doped or S-doped TiO₂ under visible light irradiation. The photoinduced hydrophilic activity of the N, S-codoped TiO₂ was also greater than that of the undoped TiO₂ even under fluorescent light bulb, which contained both visible and UV lights. The high activities of the N, S-codoped TiO₂ could be attributed to the hybridization of the introduced N 2p and S 3p, which was supported by the results of ab initio calculations.     

Investigation of phase transition behaviors of the nitrogen-doped Sb-rich Si-Sb-Te films for phase-change memory

The phase transformation properties of the nitrogen-doped Sb-rich Si-Sb-Te films were investigated in detail. It was found that the addition of N atoms into the Si-Sb-Te films increases the temperature for phase transition from the amorphous phase to a stable hexagonal structure and enhances the sheet resistance of the films following grain refinement. The surface topography of the crystalline films was improved by doping nitrogen atoms. The activation energy for crystallization of the films was increased from 1.84 to 2.89 eV with the increased nitrogen content from 0 to 21 at.%, which promises an improved thermal stability. A prolonged data lifetime up to 10 years at 149.4 °C was realized. From the device performance point of view, the N-doped Si-Sb-Te film with a moderate nitrogen content was preferable for the phase-change memory applications due to its advantage of higher reliability.     

Synthesis of mixed-phase titania films by low-temperature ultrasonic spray pyrolysis

Fully dense TiO₂ films with (1) mixed-phase rutile and anatase and (2) anatase (sole phase) were deposited on fused quartz substrates by ultrasonic spray pyrolysis at nominally 400 °C. The presence and absence of insulation around the entrainment pathway traversing 20 cm above the substrate/hot plate were investigated (174 °C vs 122 °C). The thick films were assessed in terms of mineralogies (qualitative and quantitative), microstructures (topography, thickness and grain size), and visible light transmission (optical and microstructural considerations). With insulation, opaque mixed anatase (∼ 30 vol.%; < 50 nm) and rutile (∼ 70 vol.%; ∼ 1 μm) were observed; without insulation, only transparent anatase (< 50 nm) was observed.     

Photocatalytic activities of Ion doped TiO2 thin films when prepared on different substrates

Pure and ion doped TiO₂ thin films were prepared by sol-gel dip coating process on metallic and non-metallic substrates. Test metal ion concentration ranged from 0.000002 to 0.4 at.%. The resulting films were annealed in air and characterized by optical spectroscopy and X-ray diffraction. The photodegradation of methyl orange under UV irradiation by pristine and ion-doped TiO₂ films was quantified in a photocatalytic reactor developed in this study. In general, both doped and undoped TiO₂ crystals appeared in anatase phase and the photocatalytic activities of the TiO₂ thin films varied with substrates, calcination temperature, doping ions and their concentrations. The best calcination temperature for different substrates ranged from 450 to 580 °C. Films prepared on the metallic substrates resulted in higher photocatalytic activities, while ion doping lowered their efficiencies. On the contrary, for non-metallic substrates except ceramic the photocatalytic efficiencies of undoped films were much lower (< 30%), while ion doping was shown to increase the photocatalytic efficiencies remarkably in some cases, e.g., Cr³⁺ with the tile substrate. Overall, ion doping affected the photocatalytic efficiency of TiO₂ films, and an optimal doping concentration of between 0.0002 and 0.002 at.%, close to an estimate by the Debye length equation, resulted in the highest efficiency for most substrates.     

Water as reactive gas to prepare titanium oxynitride thin films by reactive sputtering

Thin films of titanium oxynitride were successfully prepared by dc reactive magnetron sputtering using a titanium metallic target, argon, nitrogen and water vapour as reactive gases. The nitrogen partial pressure was kept constant during every deposition whereas that of the water vapour was systematically changed from 0 to 0.1 Pa. The evolution of the deposition rate with an increasing amount of water vapour injected into the process was correlated with the target poisoning phenomenon estimated from the target potential. Structure and morphology of the films were analysed by X-ray diffraction and scanning electron microscopy. Films were poorly crystallised or amorphous with a typical columnar microstructure. Nitrogen, oxygen and titanium concentrations were determined by Rutherford backscattering spectroscopy and nuclear reaction analysis, and the amount of hydrogen in the films was also quantified. Optical transmittance in the visible region and electrical conductivity measured against temperature were gradually modified from metallic to semiconducting behaviour with an increasing supply of the water vapour partial pressure. Moreover, an interesting maximum of the electrical conductivity was observed in this transition, for a small amount of water vapour.     

Crystallinity and photocatalytic activity of TiO2 films deposited by reactive sputtering with radio frequency substrate bias

TiO₂ films with thicknesses of 400-460 nm were deposited on the unheated non-alkali glass by radio frequency (rf) reactive magnetron sputtering using a Ti metal target. Depositions were carried out using a 3-in. 1000 G magnetron cathode with various rf substrate bias voltages (V_sb, dc component of self bias) of 10-80 V under total gas pressure of 1.0 or 3.0 Pa. The oxygen flow ratio [O₂/(O₂ + Ar)] and rf sputtering power were kept constant at 60% and 200 W, respectively. Photocatalytic activity on photoinduced oxidative decomposition of acetaldehyde (CH₃CHO) of the TiO₂ films showed a clear tendency to decrease with the increase in the V_sb during the deposition. Most of the films consisted of the mixture of anatase and rutile polycrystalline portions. It was confirmed that the rutile phase content increased and anatase phase content decreased markedly with increasing V_sb, where the crystallinity of anatase phase was much higher than that of rutile phase.     

Photoinduced hydrophilicity of titanium dioxide thin films prepared by cathodic electrodeposition

Crystal structure and microstructural properties of titanium dioxide thin films prepared by cathodic electrodeposition on indium-tin-oxide coated glass substrates from aqueous peroxo-titanium complex solutions have been investigated as a function of sintering temperature (25-500 °C) for the first time. We have noticed pronounced photoinduced hydrophilicity for such thin films on exposure to ultraviolet (UV) light illumination. It was observed that all the films, irrespective of their crystalline nature (amorphous and crystalline), display transformation from hydrophobic to super-hydrophilic behavior upon UV illumination. This observation can be correlated with typical nanoporous morphology of electrodeposited TiO₂ films.     

Anatase-rutile transformation through high-temperature annealing of titania films produced by ultrasonic spray pyrolysis

Titanium dioxide films were prepared by ultrasonic spray pyrolysis on (0001) α-quartz substrates at 400 °C and/or annealed in air at 600°, 800°, or 1000 °C. The as-deposited films at 400 °C (50 nm grains) and annealed at 600 °C (100 nm grains) showed single-phase anatase of high transparency in the visible region. Films annealed at 800 °C and 1000 °C showed mixed-phase anatase (100 nm grains) plus rutile (700 nm agglomerates) and pure rutile, (700 nm grains), respectively. Raman and X-ray data indicate the presence of residual stress in the films, which may arise from contamination, silicon diffusion into the films, and/or oxygen deficiency.     

Bismuth doping effect on the phase-change characteristics of nitrogen-doped GeTe films

The microstructures and electrical properties of 8.4% nitrogen-doped GeTe and GeBi(6 at.%)Te films thermally annealed in N₂ atmosphere were investigated. With the addition of Bi to N-doped GeTe films, the initial crystallization temperature was reduced and crystallization speed slowed. The N-doped GeBiTe films showed a rapid increase in crystallite size compared to the N-doped GeTe films. The formation energy of the nucleus may be lower due to the Bi atoms and the growth speed may be slower.     

Nitrogen-doped titanium dioxide photocatalysts for visible response prepared by using organic compounds

In order to utilize visible light in photocatalytic reactions, nitrogen atoms were doped in commercially available photocatalytic TiO₂ powders by using an organic compound such as urea and guanidine. Analysis by X-ray photoelectron spectroscopy (XPS) indicated that N atoms were incorporated into two different sites of the bulk phase of TiO₂. A significant shift of the absorption edge to a lower energy and a higher absorption in the visible light region were observed. These N-doped TiO₂ powders exhibited photocatalytic activity for the decomposition of 2-propanol in aqueous solution under visible light irradiation. The photocatalytic activity increased with the decrease of doped N atoms in O site, while decreased with decrease of the other sites. Degradation of photocatalytic activity based on the release of nitrogen atoms was observed for the reaction in the aqueous suspension system.