Preparation of anatase TiO2 thin films by vacuum arc plasma evaporation

Anatase titanium dioxide (TiO₂) thin films with high photocatalytic activity have been prepared with deposition rates as high as 16 nm/min by a newly developed vacuum arc plasma evaporation (VAPE) method using sintered TiO₂ pellets as the source material. Highly transparent TiO₂ thin films prepared at substrate temperatures from room temperature to 400 °C exhibited photocatalytic activity, regardless whether oxygen (O₂) gas was introduced during the VAPE deposition. The highest photocatalytic activity and photo-induced hydrophilicity were obtained in anatase TiO₂ thin films prepared at 300 °C, which correlated to the best crystallinity of the films, as evidenced from X-ray diffraction. In addition, a transparent and conductive anatase TiO₂ thin film with a resistivity of 2.6 × 10^− 1 Ω cm was prepared at a substrate temperature of 400 °C without the introduction of O₂ gas.     

Nitrogen and europium doped TiO2 anodized films with applications in photocatalysis

Micro-arc oxidation method is a useful process for mesoporous titanium dioxide films. In order to improve the photocatalytic activity of the TiO₂ film, N-Eu co-doped titania catalyst was synthesized by micro-arc oxidation in the H₂SO₄/Eu(NO₃)₃ mixture solution.The specific surface area and the roughness of the anodic titania film fabricated in the H₂SO₄/Eu(NO₃)₃ electrolyte, were increased compared to that of the anodic TiO₂ film prepared in H₂SO₄ solution. The absorbance response of N-Eu titania film shows a higher adsorption onset toward visible light region, and the incorporated N and Eu ions during anodization as a dopant in the anodic TiO₂ film significantly enhanced the photocatalytic activity for dye degradation. After dye decomposition test for 3 h, dye removal rates for the anodic TiO₂ film were 60.7% and 90.1% for the N-Eu doped titania film. The improvement of the photocatalytic activity was ascribed to the synergistic effects of the surface enlargement and the new electronic state of the TiO₂ band gap by N and Eu co-doping.     

Nanoporous TiO2 thin film based conductometric H2 sensor

Nanoporous titanium dioxide (TiO₂) based conductometric sensors have been fabricated and their sensitivity to hydrogen (H₂) gas has been investigated. A filtered cathodic vacuum arc (FCVA) system was used to deposit ultra-smooth Ti thin films on a transducer having patterned inter-digital gold electrodes (IDTs). Nanoporous TiO₂ films were obtained by anodization of the titanium (Ti) thin films using a neutral 0.5% (wt) NH₄F in ethylene glycol solution at 5 V for 1 h. After anodization, the films were annealed at 600 °C for 8 h to convert the remaining Ti into TiO₂. The scanning electron microscopy (SEM) images revealed that the average diameters of the nanopores are in the range of 20 to 25 nm. The sensor was exposed to different concentrations of H₂ in synthetic air at operating temperatures between 100 °C and 300 °C. The sensor responded with a highest sensitivity of 1.24 to 1% of H₂ gas at 225 °C.     

Influences of post-annealing on structural and electrical properties of Bi1.5Zn1.0Nb1.5O7 thin films prepared by pulsed laser deposition

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films were prepared on Pt/TiO₂/SiO₂/Si(100) substrates at 650 °C under an oxygen pressure of 10 Pa by using pulsed laser deposition process. The crystallinity, microstructure and electrical properties of BZN thin films were investigated to verify the influences of post-annealing thermal process on them. The X-ray diffractometer (XRD) results indicate that all Bi_1.5Zn_1.0Nb_1.5O₇ thin films without post-annealing process or with post-annealing in situ vacuum chamber and in oxygen ambient exhibit a cubic pyrochlore structure. The improved crystallinity of BZN thin films through post-annealing was confirmed by XRD and scanning electron microscope (SEM) analysis. Dielectric constant and loss tangent of the as-deposited BZN thin films are 160 and 0.002 at 10 kHz, respectively. After annealing, dielectric properties of thin films are significantly improved. Dielectric constant and loss tangent of the in situ annealed films are 181 and 0.0005 at 10 kHz, respectively. But the films post-annealed in O₂ oven show the largest dielectric constant of 202 and the lowest loss tangent of 0.0002, which may attribute to the increase in grain size and the elimination of oxygen vacancies. Compared with the as-deposited BZN thin films, the post-annealed films also show the larger dielectric tunability and the lower leakage current density.     

Photocatalytic performance of TiO2 films produced with combination of oxygen-plasma and rapid thermal annealing

In this work, a combination of oxygen plasma and rapid thermal annealing was suggested in order to oxidize the surface of titanium into TiO₂. A plasma was formed by employing pure oxygen at 150 W, 300 W, and 400 W under a pressure of 7.5 to 8.5 Pa for 5 to 10 min. The TiO₂ was then subjected to rapid thermal annealing (RTA) at a temperature of 400 to 500 °C for 1 min. From the attained results, an RF power of 300 W for 5 min was observed to be sufficient to produce an optimal photocatalytic TiO₂ film. Optimal conditions were confirmed by additional experiments involving humic acid (HA) degradation of the TiO₂ films. When compared to a traditional TiO₂ film, a TiO₂ film prepared with an oxygen-plasma treatment and RTA system exhibited improved photocatalytic capability for HA photodegradation in an aqueous solution. Therefore, this process proposed in this work can be an excellent alternative to the traditional method for fabricating photocatalytic TiO₂ films.     

Microstructures and Photocatalytic Properties of S Doped Nanocrystalline TiO2 Films

The nanocrystalline S doped titanium dioxide films were successfully prepared by the improved sol-gel process. Here TiO(C₄H₉O)₄ and CS(NH₂)₂ were used as precursors of titania and sulfur, respectively. The as-prepared specimens were characterized using x-ray diffraction (XRD), x-ray energy dispersive spectroscopy (EDS), high-resolution field emission scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller (BET) surface area, and ultraviolet-visible diffuse reflectance spectroscopy. The photocatalytic activities of the films were evaluated by degradation of organic dyes in aqueous solution. The results of XRD, FE-SEM, and BET analyses indicated that the TiO₂ films were composed of nanoparticles. S doping could obviously not only suppress the formation of brookite phase but also inhibit the transformation of anatase to rutile at high temperature. Compared with pure TiO₂ film, S doped TiO₂ film exhibited excellent photocatalytic activity. It is believed that the surface microstructure of the modified films is responsible for improving the photocatalytic activity.     

Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications

Different amounts of Co-doped TiO₂ powders and thin films were prepared by following a conventional co-precipitation and sol–gel dip coating technique, respectively. The synthesized powders and thin films were subjected to thermal treatments from 400 to 800 °C and were thoroughly investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive analysis with X-rays, FT-infrared, FT-Raman, diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, BET surface area, zeta potential, flat band potential measurements, band-gap energy, etc. The photocatalytic ability of the powders was evaluated by methylene blue (MB) degradation studies. The thin films were characterized by photocurrent and ultraviolet–visible (UV–Vis) spectroscopy techniques. The characterization results suggest that the Co-doped TiO₂ powders synthesized in this study consist mainly anatase phase, and possess reasonably high specific surface area, low band gap energy and flat band potentials amenable to water oxidation in photoelectrochemical (PEC) cells. The photocatalytic degradation of MB over Co-doped TiO₂ powders followed the Langmuir–Hinshelwood first order reaction rate relationship. The 0.1 wt.% Co-doped TiO₂ composition provided the higher photocurrent, n-type semi-conducting behavior and higher photocatalytic activity among various Co-doped TiO₂ compositions and pure TiO₂ investigated.     

Structural stability of ZnFe2O4 nanoparticles under different annealing conditions

We study the structural stability of surfactant coated ZnFe₂O₄ (ZF) nanoparticles of average particle size 10 nm annealed under different environments. The X-ray diffraction studies in oleic acid coated ZF (OC-ZF) show distinctly different phase transitions under different annealing conditions. The OC-ZF is reduced to α-Fe/ZnO phase under vacuum while it forms FeO/ZnO under argon whereas the ZnFe₂O₄ phase remains stable under air annealing. The simultaneous thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC) coupled mass spectra (MS) in OC-ZF under argon atmosphere suggests that the residual carbon removes oxygen from the lattice to reduce the ZnFe₂O₄ phase into FeO/ZnO during argon annealing. Apart from CO and CO₂ gas evolution at high temperature under argon annealing, creation of oxygen vacancies due to the random removal of oxygen under vacuum annealing, leads to direct interaction between Fe–Fe and the formation of metal Fe. It appears that the residual carbon aids the reduction of ZF and the formation of α-Fe/ZnO during vacuum annealing. After annealing at 1000 °C in vacuum, the magnetization is increased abruptly from 13.8 to 106.5 emu g⁻¹. In sharp contrast, the air and argon annealed samples show a diminished magnetization of 1 emu g⁻¹. The field cooled (FC) and zero FC magnetization of vacuum and argon annealed samples exhibit superparamagnetic and spin-glass type behavior respectively. Our results offer possibilities to switch a magnetically inactive material to an active one.     

Room-temperature ferromagnetism in Cu-doped TiO2 thin films

We report here the observation of significant room-temperature ferromagnetism in a semiconductor doped with nonmagnetic impurities, Cu-doped TiO₂ thin films grown by reactive magnetron sputtering. Films annealed in air were not magnetic while those annealed in vacuum were ferromagnetic with a Curie temperature about 350 K. The magnetic moment per copper atom decreased as the copper concentration increased. These results show that both the oxygen vacancies and the distance between nearest-neighbor copper atoms play a crucial role for the appearance of magnetism.     

Degradation of thin films of YBa2Cu3O7 by annealing in air and in vacuum

The degradation of epitaxial thin films of YBa₂Cu₃O₇ has been studied as a function of annealing temperature in air and in vacuum; some samples had an evaporated overlayer of CaF₂. Degradation was monitored by the measurement of electrical properties after consecutive 30-min annealing treatments. The room-temperature resistance registered significant increases for all samples after annealing at temperatures above about 200°C; the critical current density at 77 K was degraded for annealing temperatures 400°C in air, and 200–250°C in vacuum. By annealing in oxygen at 550°C, electrical properties were restored in degraded bare YBCO samples annealed in vacuum, but not for those annealed in air.     

The effect of the H2 flow rate on the structure and optical properties of TiO2 films deposited by inductively coupled plasma assisted chemical vapor deposition

The optical and photocatalytic properties of TiO₂ are closely related to crystalline structures, such as rutile and anatase. In this paper, TiO₂ films were produced by inductively coupled plasma (ICP) assisted chemical vapor deposition (CVD) without extra heating of the substrate, and the effect of H₂ addition on the structure and optical properties of the films was investigated. After increasing the partial pressure of H₂, the structure of the TiO₂ films changed from anatase to rutile, which usually appears at high temperatures (> 600 °C). The light transmittance decreased with increasing the H₂ flow rate due to the increased surface roughness. The photocatalytic activity of the anatase TiO₂ film was better than that of the rutile TiO₂ film.     

The effect of sputtering gas pressure on structure and photocatalytic properties of nanostructured titanium oxide self-cleaning thin film

Titanium oxide thin films were deposited by DC reactive magnetron sputtering on ZnO (80 nm thickness)/soda-lime glass and SiO₂ substrates at different gas pressures. The post annealing on the deposited films was performed at 400 °C in air atmosphere. The results of X-ray diffraction (XRD) showed that the films had anatase phase after annealing at 400 °C. The structure and morphology of deposited layers were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface grain size and roughness of TiO₂ thin films after annealing were around 10-15 nm and 2-8 nm, respectively. The optical transmittance of the films was measured using ultraviolet-visible light (UV-vis) spectrophotometer and photocatalytic activities of the samples were evaluated by the degradation of Methylene Blue (MB) dye. Using ZnO thin film as buffer layer, the photocatalytic properties of TiO₂ films were improved.     

Photocatalytic properties of Cr-doped TiO2 films prepared by oxygen cluster ion beam assisted deposition

We prepared Cr-doped titanium dioxide (TiO₂) films by oxygen (O₂) cluster ion beam assisted deposition method, and investigated photocatalytic properties of the films as well as crystallographic property, optical property and surface morphology. The films prepared at a substrate temperature below 200 °C were found to be amorphous from the X-ray diffraction measurement. For the substrate temperatures such as 300 °C and 400 °C, the films exhibited rutile and/or anatase structures. The film surface measured by the atomic force microscope (AFM) was smooth at an atomic level. Furthermore, the optical band gap decreased with increase of Cr-composition, and it was approximately 3.3 eV for the non-doped films, 3.2 eV for the 1% Cr-doped films and 3.1 eV for the 10% Cr-doped films, respectively. With regard to the photocatalytic properties of the Cr-doped TiO₂ films, we measured the change of contact angle as well as the photocatalytic degradation of methylene blue by the UV light irradiation. Compared with the non-doped films, the 1% Cr-doped films prepared at a substrate temperature of 400 °C showed high degradation efficiency. In addition, the contact angle of the 1% Cr-doped films with an initial value of 60° decreased to 10° by the UV light irradiation for 20 min, and the films exhibited the predominant properties of photocatalytic hydrophilicity even for the UV light irradiation with longer wavelengths.     

Enhanced photocatalytic activity of TiO2 by polydimethylsiloxane deposition and subsequent thermal treatment at 800 °C

A thin film of polydimethylsiloxane (PDMS) was coated on TiO₂ nanoparticles (P-25, Dagussa), and surface structures of the thin films were analyzed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Furthermore, photocatalytic activity of PDMS-coated TiO₂ samples with various annealing temperatures was determined using UV irradiation. We show that the 800 °C-annealed sample of PDMS-coated TiO₂ showed a two-fold higher photocatalytic activity with respect to the bare TiO₂. The enhanced photocatalytic activity was attributed to the greater hydrophilicity of the annealed PDMS coating.     

Effect of temperature on the growth of TiO2 thin films synthesized by spray pyrolysis: Structural, compositional and optical properties

Large surface area coatings of oxygen deficient nanocrystalline TiO₂ are of immense use in antifogging mirrors and self cleaning windows. Spray pyrolysis is a simple versatile technique to coat relatively large surface area. A clear understanding of effect of substrate temperature on the coating morphology, structure, composition and optical properties is essential to produce coatings of desired properties. Oxygen deficient nanocrystalline anatase–TiO₂ thin films were synthesized on Si(1 0 0), quartz and glass substrates at 300–550 °C. Well defined platelets like nanograins standing on their edge were obtained at 500 °C. The crystallites were found to be of ∼12 nm thickness and ∼30 nm major diameter. The secondary ion mass spectrometric studies of the films revealed uniform distribution of titanium and oxygen across the thickness of the film up to the film–substrate interface. Presence of lower valent Ti ions and oxygen vacancies were confirmed from XPS studies. The indirect and direct band gap values evaluated from the Tauc plot for films synthesized at 500 °C are 3.3 and 3.62 eV respectively.     

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.     

Hydrophilic properties of nano-TiO2 thin films deposited by RF magnetron sputtering

Microstructure and hydrophilicity of nano-titanium dioxide (TiO₂) thin films, deposited by radio frequency magnetron sputtering, annealed at different temperatures, were studied by field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and water contact angle methods. It is found that the crystal phase transforms from amorphous to rutile structure with increase of annealing temperature from room temperature to 800 °C. It is also indicated that the organic contaminants on the surface of the films can be removed and the oxygen vacancies can be reduced by the annealing treatment. Annealed at the temperature below 300 °C, amorphous TiO₂ thin films show rather poor hydrophilicity, and annealed at the temperature range from 400 to 650 °C, the super hydrophilicity anatase of TiO₂ thin films can be observed. However, when the annealing temperature reaches 800 °C, the hydrophilicity of the films declines mainly derived from the appearance of rutile.     

Fabrication of porous TiO2 film via hydrothermal method and its photocatalytic performances

Porous anatase (TiO₂) films were fabricated onto stainless steel substrates with Ti(OC₄H₉)₄ as a precursor via hydrothermal process. The crystallization and porous structure of TiO₂ film were dependent on the time and temperature of the hydrothermal reaction. A TiO₂ film with orderly porous structure and high crystallization was obtained upon treatment at 150 °C for 2 h. The grain size of TiO₂ is ca. 6 nm, and pore diameter is ca. 10 nm. Diffusion of Fe into the porous TiO₂ film occurred; Fe also diffused onto the surface of the film with the extension of hydrothermal reaction time or increase of the reaction temperature. The diffusion reaction has a large effect on the formation of porous TiO₂ film as well as its interface texture. However, it does not change the crystal phase of the TiO₂. The resultant TiO₂ film showed high photocatalytic activity towards degradation of gaseous formaldehyde.     

Defects in TiO2 films on p+-Si studied by positron annihilation spectroscopy

Variable-energy positron annihilation spectroscopy has been applied to the study of defects in TiO₂/p⁺-Si structures, in the as-grown state and after annealing in vacuum and in hydrogen, to investigate whether annealing (and film thickness) resulted in an increase of vacancy-type defects in the oxide films. It was found that the concentration of such defects remained unchanged after vacuum annealing for all films studied, but after H₂ annealing more than doubled for 150 nm-thick films, and increased by an order of magnitude for 100 nm-thick films. The nature of the vacancies was examined further by measuring high-precision annihilation lines and comparing them with a reference Si spectrum. The changes observed in the ratio spectra associated with oxygen electrons suggest that the defects are oxygen vacancies, which have been shown to enhance electroluminescence from TiO₂/p⁺-Si heterostructure-based devices.     

Studies on nitrogen modified TiO2 photocatalyst prepared in different conditions

Nitrogen modified titania photocatalysts (TiO₂/N) were characterized using high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), dynamic light scattering (DLS), Raman spectroscopy and BET surface area method. The presence of nitrogen in modified photocatalysts has been studied using FT-IR and XPS analyses. The influence of the calcination temperature in the range of 100-350 °C on nanocrystallite as well as particle size of the samples and their photocatalytic activity was investigated. The calcination of TiO₂/N samples caused a growth of the particle size and an increase of their crystallinity. TEM studies present changes of the diameter and shape of TiO₂ particles and nanocrystallites. The XRD and the Raman response of the samples confirmed an increase of the crystallinity of the samples when annealed at higher temperatures.The photocatalytic activity of the modified photocatalysts was determined using the reaction of phenol decomposition. It was shown that phenol decomposition rate was greatly influenced by pH of the solution. The highest phenol degradation using all the modified samples was observed for pH 7.1 which is close to the PZC point established for pristine TiO₂ at pH 6.8.