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.     

Synthesis of novel photocatalytic InVO4-TiO2 thin films with visible light photoactivity

Novel InVO₄-TiO₂ thin films were synthesized by introducing nano InVO₂ into the TiO₂ thin films via a sol-gel method. The photocatalytic efficiency of InVO₄-TiO₂ thin films was investigated by photodegrading of methyl orange in aqueous solution under visible light irradiation. A wide range of characterization techniques, such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-vis absorption spectroscopy (UV-vis), were applied to characterize the obtained thin films. The results indicated that the InVO₄-TiO₂ thin films consist of uniform anatase TiO₂ crystals of 70-80 nm with highly dispersed orthorhombic InVO₄ nanocrystals. The decoloration of methyl orange results illustrated that the InVO₄-TiO₂ thin films with 0.5 wt.% InVO₄ showed the best photoactivities under visible light irradiation, the decoloration rate reached 32.5% after irradiated for 15 h. The InVO₄-TiO₂ thin films exhibited photoactivity under visible light after doped InVO₄ in the TiO₂ films.     

Hydrophilicity of anatase TiO2/Cr-doped TiO2 thin films with different band gaps

Based on the concept that the electron-hole separation effect caused by a different band-gap structure would improve its hydrophilicity, anatase-TiO₂/Cr-doped TiO₂ thin films were synthesized by DC magnetron sputtering. The optical band gaps of TiO₂ thin films decreased from 3.23 to 2.95 eV with increasing Cr-doping content. Multilayer TiO₂ thin films with different band gaps exhibited a superhydrophilicity under UV illumination. In particular, in anatase TiO₂ (3.23 eV)/4.8% Cr-doped TiO₂ (2.95 eV), the hydrophilicity, which indicated a contact angle of less than 20°, lasted for 48 h in the dark after UV illumination was discontinued. This outstanding result has rarely been reported for TiO₂ thin films, which confirmed that the prominent superhydrophilicity of anatase TiO₂/Cr-doped TiO₂/glass could be attributed to the retardation of electron-hole recombination caused by the band-gap difference.     

Oxygen plasma induced hydrophilicity of TiO2 thin films

Anatase nano-TiO₂ thin films were fabricated by reactive magnetron sputtering metal Ti target followed by thermal annealing in air at 450 °C for 2 hrs. The crystalline structure of the sample films were characterized by X-ray diffraction (XRD) and the hydrophilicity was characterized with the diameters of 1 μl water drop. The films were irradiated by oxygen plasmas and the effects of the radio frequency (rf) power, the gas pressure and the irradiation time of the oxygen plasmas on the hydrophilicity of the TiO₂ thin films were investigated. Hydrophilicity can be induced by oxygen plasmas and further more the hydrophilicity shows high stability whenever under the natural light or in dark.     

Hydrophilicity on carbon-doped TiO2 thin films under visible light

Carbon-doped TiO₂ thin films in the anatase phase with dopant concentrations of 1.1, 0.9, and 0.7 mol% were fabricated by a radio-frequency magnetron sputtering method. Dopant carbons were located at the oxygen sites. Carbon substitution caused the absorbance edge and/or the shoulder of TiO₂ to shift to a higher wavelength region. Carbon-doped TiO₂ thin films underwent a hydrophilic conversion when irradiating with visible light (400–530 nm). The hydrophilic property under visible light was inferior to that under ultraviolet light, which is explained by considering that the visible light sensitivity originates from the localized C 2p formed in the band-gap.     

Preparation, characterization and application of TiO2 nanoparticles surface-modified by DDAT

DDAT(S-1-Dodecyl-S′-(α, α′-dimethyl-α″-acetic acid) trithiocarbonate) modified TiO₂ photocatalysts were prepared by hydrothermal treatment before TiO₂ crystallization. The adsorption of DDAT onto the surface of titania nanoparticles led the shifting of the onset wavelength of the optical absorption in the visible range corresponding to ligand-to-metal charge transfer transition within the surface-modified complex. The interaction of TiO₂ nanoparticles with DDAT was investigated by infrared spectra. The XRD indicated that the modification process could not influence the crystallite phase of TiO₂. The photocatalytic studies suggested that the DDAT modified TiO₂ photocatalysts showed enhanced photocatalytic efficiency of photodegradation of 2,4-dichlorophenol compared with the as-prepared TiO₂ under visible-light irradiation.     

Optical and hydrophilic properties of nanostructure Cu loaded brookite TiO2 thin film

Copper loaded TiO₂ brookite thin films were deposited on glass substrates using the dip-coating method. The crystalline structure of the films was characterized by X-ray diffraction analysis. X-ray photoelectron spectroscopy was used to evaluate the properties of the film surfaces. The transmittance spectra of the films were obtained by the Shimadzu multi-purpose spectrophotometer. The water contact angle on the film surfaces during irradiation and storage in a dark place was measured by a contact angle analyzer. The results indicate that Cu loading did not affect the transmittance spectra, whereas it had a significant effect on the hydrophilicity of the TiO₂ film surface.     

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.     

Photoactive and self-cleaning TiO2-SiO2 thin films on 316L stainless steel

In this study, TiO₂-SiO₂ nanocomposite films with different amounts of SiO₂ were prepared by sol-gel process and were coated onto stainless steel 316L. The effect of addition of various amount of SiO₂ in the precursor solution on the photocatalysis, photo-generated hydrophilicity and self-cleaning property of TiO₂ thin films was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, water contact angle measurements and UV spectroscopy. In the tested ranges of SiO₂ content and sintering temperature, the highest photocatalytic activity and self-cleaning property were observed in the 15 mol% SiO₂ sample sintered at 750 °C. Addition of less than 30 mol% SiO₂ had a suppressive effect on the transformation of anatase to rutile and on the crystal growth of anatase in the sintering heat treatment. Phase separation occurred in the composite films for SiO₂ content of 30 mol% or more.     

Substrate and Fe-doping effects on the hydrophilic properties of TiO2 thin films

Titanium dioxide films are known for their hydrophilic and photocatalytic characteristics. Increasing specific surface area and doping can enhance their photocatalytic activity and hydrophilicity. We report here results regarding the enhancement of the photocatalytic properties of titania by both controlling surface morphology and the anatase/rutile ratio. The samples were deposited on glass, indium tin oxide covered glass, and SrTiO₃ by sputtering and laser ablation techniques. Film structure and surface morphology were investigated by X-ray diffraction and atomic force microscopy. Film hydrophilicity was assessed from contact angle measurements during- and post-irradiation with UV light. The contact angle data are discussed in terms of the synergic effects of surface morphology, structure and composition of the films.     

Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 μm thick nanocrystalline TiO₂ film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 μm thick nanocrystalline WO₃ film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm²) at 1.0 V vs. NHE while for the second, complete reduction at −1.3 V (74 mC/cm²) leads to A=2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li⁺ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation.     

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.     

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 film thickness and agglomerate size on the superwetting and fog-free characteristics of TiO2 films

The effect of TiO₂ film thickness and agglomerate size on the non-UV activated superhydrophilic wetting and antifogging characteristics of TiO₂ films was investigated. Evidence from Atomic Force Microscopy analysis showed that surface roughness is the key parameter requiring control so as to retain the superhydrophilic wetting and antifogging behaviour of the synthesised films. Surface roughness can be tuned by simple manipulation of the multilayer assembly of TiO₂ nanoparticles through varying the film thickness and agglomerate size. A film thickness of ~ 140 nm yielded the optimum roughness (root mean square = 23 nm) to give the best superhydrophilic wetting behaviour. Thicker films reduced the film roughness and were detrimental to their superhydrophilic wetting properties. Smaller agglomerate size was also found to be important in retaining film roughness.     

Photocatalysis and characterization of the gel-derived TiO2 and P-TiO2 transparent thin films

The gel-derived TiO₂ and P-TiO₂ transparent films coated on fused-SiO₂ substrates were prepared using a spin-coating technique. Effects of phosphorus dopants and calcination temperature on crystal structure, crystallite size, microstructure, light transmittance and photocatalytic activity of the films were investigated. By introducing P atoms to Ti-O framework, the growth of anatase crystallites was hindered and the crystal structure of anatase-TiO₂ could withstand temperature up to 900 °C. The photocatalytic activities of the prepared films were characterized using the characteristic time constant (τ) for the photocatalytic reaction. The titania film with a smaller τ value possesses a higher photocatalytic ability. After exposing to 365-nm UV light for 12 h, the P-TiO₂ films calcined between 600 °C and 900 °C can photocatalytically decomposed ≥ 84 mol% of the methylene blue in water with corresponding τ ≤ 7.1 h, which were better than the pure TiO₂ films prepared at the same calcination temperature.     

Thermal optical properties of TiO2 films

A fibre optic experimental arrangement was used to determine the thermo-optic coefficient (dn/dT) of electron beam deposited titanium dioxide coatings on the cleaved end faces of multimode optical fibres for a wavelength range between 600 and 1050 nm. The temperature-induced change in the index of refraction (n) and extinction coefficient (k) were successfully determined from reflection spectra. Measurements of n and k at various wavelengths for different temperatures enabled the determination of dn/dT and dk/dT. It was found that dn/dT takes different values at different temperature ranges. For example, at 800 nm, dn/dT was (−1.77±0.7)×10⁻⁴ K⁻¹, between 18°C and 120°C, and took a value of (−3.04±0.7)×10⁻⁴ K⁻¹ between 220°C and 325°C.     

Study of Nd-doping effect and mechanical cracking on photoreactivity of TiO2 thin films

Pure and Nd-doped TiO₂ thin films were fabricated by reactive d.c. magnetron sputtering at low-temperature from a pure Ti target. The structure of films was analyzed by X-ray diffraction (XRD). In order to study the Nd-doping effect on TiO₂ photocatalytic activity, some films were deposited on microscope glass substrates under a constant total sputtering pressure and using different Nd-doping concentrations. The effect of different Nd-dopant concentrations and its influence on the photocatalytic efficiency has been explored by measuring the photodegradation of rhodamine-B (RhB) aqueous solution under radiation of UV light. It is principally that for comparison between heat treatment and doping process on TiO₂ photocatalytic efficiency (important under the point of view of energy costs related with industrial sputtering techniques), crystalline TiO₂ films were also produced by thermal annealing. It was found that comparing with annealed pure TiO₂ films, Nd-doping do not improve the photocatalytic activity. At the same time, it was observed that there seems to exist a dopant concentration band for optimal photoreactivity. In order to study the effect of the film mechanical strength on photocatalytic activity, fragmentation tensile tests were also done on TiO₂/PET (polyethylene terephthalate) substrates at increasing applied strains. It was found that increasing the magnitude of the applied tensile strain, pure TiO₂ becomes more photocatalytic efficient.     

Study on TiO2 thin films grown by advanced pulsed laser deposition on ITO

TiO₂ films were grown by an advanced pulsed laser deposition method (PLD) on ITO substrates to be used as functional electrodes in the manufacturing of solar cells. A pure titanium target (99.99%) was irradiated by a Nd:YAG laser (355 and 532 nm, 5 ns, 35 mJ, 3 J/cm²) in an oxygen atmosphere at different pressures (20-160 mTorr) and at room temperature. After deposition, the films were subjected to an annealing process at 350 °C. The film structure, surface morphology, thickness, roughness, and optical transmission were investigated. Regardless of the wavelength used, the films deposited at room temperature presented only Ti₂O and TiO peaks. After thermal treatment, the TiO₂ films became strongly crystalline, with a tetragonal structure and in the anatase phase; the threshold temperature value was 250 °C. The deposition rate was in the range of 0.035-0.250 nm/pulse, and the roughness was 135-305 nm. Optical transmission of the films in the visible range was between 40% and 60%.     

Microstructure and optical properties of photoactive TiO2:N thin films

In this work, we have chosen oxidation of TiN thin films as a feasible method for preparation of nitrogen-doped titanium dioxide thin films, TiO₂:N, for photocatalytic applications. DC reactive magnetron sputtering with the plasma emission control was used for deposition of stoichiometric TiN thin films. The microstructure and chemical composition of films before and after oxidation were investigated by means of RBS, X-ray diffraction (XRD) in grazing incidence diffraction (GID) configuration, AFM and XPS techniques. The electrical conductivity was measured by the van der Pauw method as a function of the oxidation temperature. The optical transmittance and reflectance spectra of the films were measured over the visible and UV ranges of the light spectrum. GID diffraction patterns of as-sputtered TiN thin films and those after oxidation indicate that TiO₂ rutile is formed at around 300 °C. Nitrogen is still present as indicated by XPS studies even when XRD detects the rutile only. Optical absorption of thin films oxidized at 450 °C is shifted towards the visible range of the light spectrum.     

Effect of crystallization on humidity sensing properties of sol-gel derived nanocrystalline TiO2 thin films

In the present work, we describe the effect of crystallization on humidity sensing properties of nanocrystalline TiO₂ thin films prepared by sol-gel techniques. Here, we report an enhancement in the relative humidity (RH) sensitivity just after the crystallization at 375 °C, which is attributed to increased surface activity near crystallization and lower crystallite size. After crystallization, the RH sensitivity was found to decrease with increasing grain size. The complex impedance of the sensor, measured using impedance spectroscopy, fits well with an equivalent circuit consisting of inter-granular resistance and capacitance in parallel. It was found that with the change in RH, only resistance changes significantly, when compared with the capacitance.