Photocatalytic oxidation of 2-propanol under visible light irradiation on TiO<Subscript>2</Subscript> thin films prepared by an RF magnetron sputtering deposition method

Visible light-responsive TiO₂ (Vis-TiO₂) thin films able to absorb UV and visible light in wavelength regions of 250–600 nm were successfully developed by applying a radio-frequency magnetron sputtering deposition method. These Vis-TiO₂ thin films exhibited high activity for the photocatalytic oxidation of 2-propanol diluted in water even under visible light irradiation (λ ≥ 450 nm). The photocatalytic activity of Vis-TiO₂ thin films was dramatically enhanced by the deposition of Pt particles on the surface. Secondary ion mass spectrometry measurements revealed that Pt particles are distributed from the top surface to the deep bulk of Vis-TiO₂ thin films with a columnar structure. The unique columnar structure of Vis-TiO₂ thin films plays an important role in the high photocatalytic performance.     

Preparation of the visible light responsive TiO2 thin film photocatalysts by the RF magnetron sputtering deposition method

TiO₂ thin film photocatalysts which could induce photoreactions under visible light irradiation were successfully developed in a single process by applying an ion engineering technique, i.e., the radio frequency (RF) magnetron sputtering deposition method. The TiO₂ thin films prepared at temperatures greater than 773 K showed the efficient absorption of visible light; on the other hand, the TiO₂ thin films prepared at around 573 K were highly transparent. This clearly means that the optical properties of TiO₂ thin films, which absorb not only UV but also visible light, can be controlled by the preparation temperatures of the RF magnetron sputtering deposition method. These visible light responsive TiO₂ thin films were found to exhibit effective photocatalytic reactivity under visible light irradiation (λ > 450 nm) at 275 K for the reductive decomposition of NO into N₂ and N₂O. From various characterizations, the orderly aligned columnar TiO₂ crystals could be observed only for the visible light responsive TiO₂ thin films. This unique structural factor is expected to modify the electronic properties of a TiO₂ semiconductor, enabling the efficient absorption of visible light.     

Development of dye-sensitized solar cells based on visible-light-responsive TiO2 thin films with a unique columnar structure

A visible-light-responsive TiO₂ thin film (Vis-TiO₂), possessing a unique columnar structure oriented perpendicular to the substrate, has been prepared by using a radio-frequency magnetron sputtering deposition method. The unique TiO₂ thin film was used as a key component of N719 dye-sensitized solar cells (DSSCs). The rough morphology caused by the columnar structure leads to enhanced photovoltaic performances of these DSSCs as a consequence of increased amounts of adsorbed dyes and facilitated diffusion of the electrolyte into Vis-TiO₂ film. Additionally, photovoltaic performances of the DSSCs were found to depend strongly on the thickness of the Vis-TiO₂ film, which can be readily controlled by adjusting the sputtering time conducted for deposition. Consequently, a solar-to-electric energy conversion efficiency of 2.6 % under AM 1.5 illumination was observed for an optimally performing DSSC that has a Vis-TiO₂ film thickness of 6.9 μm, prepared by employing a sputtering time of 700 min.     

Photocatalytic activity of visible light-responsive TiO<Subscript>2</Subscript> thin films deposited on various anodized Ti-metal substrates by a RF magnetron sputtering method

Visible light-responsive TiO₂ (Vis-TiO₂) thin film has been deposited on two different kinds of anodized Ti-metal substrates, i.e., plate-type Titanystar (P-Titanystar) or non-woven fabric Titanystar (N-Titanystar) by a radio-frequency magnetron sputtering (RF-MS) method. These Vis-TiO₂ thin film photocatalysts (Vis-TiO₂/P-Titanystar and Vis-TiO₂/N-Titanystar) showed high activity for the oxidation of 2-propanol in water under UV light irradiation while Vis-TiO₂/N-Titanystar showed higher activity than Vis-TiO₂/P-Titanystar. Furthermore, photocatalytic activity of Vis-TiO₂/N-Titanystar was enhanced by the Pt deposition by an RF-MS deposition method. Pt loaded Vis-TiO₂/N-Titanystar (Pt-Vis-TiO₂/N-Titanystar) acted as an efficient photocatalyst for the oxidation of methanol vapor under UV light irradiation even in a flow system. Moreover, it was found that Pt-Vis-TiO₂/N-Titanystar is an effective photocatalyst for the oxidation of various organic compounds in gas phase or ammonia gas even under visible light irradiation (λ > 420 nm) at 293 K.     

Separate evolution of H2 and O2 from H2O on visible light-responsive TiO2 thin film photocatalysts prepared by an RF magnetron sputtering method

Visible light-responsive TiO₂ thin film photocatalysts (Vis-TiO₂) have been prepared on Ti metal foil (Vis-TiO₂/Ti) or ITO glass (Vis-TiO₂/ITO) substrates by a radio-frequency magnetron sputtering (RF-MS) method. The UV–Vis spectra as well as photoelectrochemical performance of Vis-TiO₂ were affected by various calcination treatments such as calcination in air or NH₃. Calcination treatment in NH₃ (1.0 × 10⁴ Pa, 673 K) was particularly effective in increasing the visible light absorption of Vis-TiO₂ as well as in enhancing its photoelectrochemical performance and photocatalytic activity. A novel Vis-TiO₂ thin film photocatalyst (Vis-TiO₂/Ti/Pt) was prepared by an RF-MS method where Vis-TiO₂ was deposited on one side of a Ti metal foil substrate and nanoparticles of Pt were deposited on the other side. The separate evolution of H₂ and O₂ from H₂O could be successfully achieved by using an H-type glass cell consisting of two aqueous phases separated by Vis-TiO₂/Ti/Pt and a proton-exchange membrane. It was found that the rate of the separate evolution of H₂ and O₂ was also dramatically enhanced by calcination treatment of Vis-TiO₂ in NH₃.     

Ion engineering techniques for the preparation of the highly effective TiO<Subscript>2</Subscript> photocatalysts operating under visible light irradiation

The successful application of ion engineering techniques for the development of TiO₂ photocatalysts operating under visible and/or solar light irradiations has been summarized in this review article. First, we have physically doped various transition metal ions within a TiO₂ lattice on an atomic level by using an advanced metal ion implantation method. The metal ion implanted TiO₂ could efficiently work as a photocatalyst under visible light irradiation. Some field tests under solar light irradiation clearly revealed that the Cr or V ions implanted TiO₂ samples showed 2–3 times higher photocatalytic reactivity than the un-implanted TiO₂. Second, we have developed the visible light responsive TiO₂ thin film photocatalyst by a single process using an RF-magnetron sputtering (RF-MS) deposition method. The vis-type TiO₂ thin films showed high photocatalytic reactivity for various reactions such as reduction of NOx, degradation of organic compounds, and splitting of H₂O under visible and/or solar light irradiations.     

Preparation of tantalum oxynitride thin film photocatalysts by reactive magnetron sputtering deposition under high substrate temperature

This work spotlights the formation behavior of visible light-responsive tantalum oxynitride (TaON) thin film photocatalysts under high substrate temperature in radiofrequency reactive magnetron sputtering deposition. The results emanating from the optimization of the sputtering conditions demonstrated that sputtered N atoms with high kinetic energy generated by controlling target–substrate distances and total pressures in the sputtering chamber were necessary to grow TaON phase even under N₂-rich atmosphere. Based on these findings, TaON thin film photocatalysts were successfully synthesized by single-step sputtering under a high substrate temperature of 1073 K before heat treatment. The optimal thickness of TaON thin film photocatalysts was extrapolated to be 450 nm by photoelectrochemical measurements under visible light irradiation (λ > 450 nm), in which distinct photocurrents corresponding to water oxidation were observed. Moreover, the photoelectrochemical activity was able to be improved by postsynthetic heat treatment in gaseous NH₃ and loading with IrO₂ nanocolloids as cocatalysts. This finding would be because the thin film photocatalyst after heat treatment in NH₃ under appropriate conditions possessed better crystallinity and moderate donor density. The optimized TaON thin film photocatalysts with IrO₂ nanocolloids also exhibited photocatalytic activity for H₂ evolution from aqueous medium containing methanol as a sacrificial electron donor under visible light irradiation (λ > 450 nm).     

Effect of Pt loading on the photocatalytic reactivity of titanium oxide thin films prepared by ion engineering techniques

Platinum-loaded titanium oxide thin-film photocatalysts were prepared by using an ionized cluster beam (ICB) deposition method and a RF magnetron sputtering (RF-MS) deposition method as dry processes. From the results of the photocatalytic oxidation of acetaldehyde with O₂ under UV light irradiation, small amounts of Pt loading (less than 10 nm film thickness) were found to dramatically enhance the photocatalytic reactivity. However, when TiO₂ thin films were loaded with relatively larger amounts of Pt (more than 30 nm as the film thickness), the photocatalytic reactivity became lower than for the pure TiO₂ thin films. Moreover, investigations of the ratio of Pt loaded onto the surface of the thin film catalysts by XPS measurements revealed that the small amounts of Pt loaded exist as very small clusters working to efficiently enhance the charge separation, whereas, large amounts of Pt covers the entire surface of the TiO₂ thin films, resulting in a decrease of the photocatalytic reactivity.     

Characterization of TiO2 nanotube arrays prepared via anodization of titanium films deposited by DC magnetron sputtering

Highly ordered TiO₂ nanotube arrays were fabricated on a conducting glass substrate in NH₄HF₂/glycol electrolyte via anodization of titanium film which was deposited by direct current magnetron sputtering (DCMS) at different temperatures. The results showed that Ti films with good homogeneity and high denseness could be formed under the conditions Ar pressure 0.35 Pa, direct current 3.5 A, and 2 h at 300 °C. Characterization of the TiO₂ nanotube arrays was investigated comparatively, by altering anodization time. The surface morphology of the samples changed as the anodization time was prolonged from 10 to 150 min at 30 V. The TiO₂ thin film was amorphous and could be transformed into anatase by annealing at 450 °C. On the basis of UV?Cvisible transmission spectra the bandgap of the thin film was calculated to be 3.12 eV, and its tail extended to 2.6 eV.     

Dye degradation studies of Mo‐doped TiO2 thin films developed by reactive sputtering

TiO₂ thin films with various Mo concentrations have been deposited on glass and n‐type silicon (100) substrates by this radio‐frequency (RF) reactive magnetron sputtering at 400°C substrate temperature. The crystal structure, surface morphology, composition, and elemental oxidation states of the films have been analyzed by using X‐ray diffraction, field emission scanning electron microscopy, atomic force microscopy, and X‐ray photoelectron spectroscopy, respectively. Ultraviolet‐visible spectroscopy has been used to investigate the degradation, transmittance, and absorption properties of doped and undoped TiO₂ films. The photocatalytic degradation activity of the films was evaluated by using methylene blue under a light intensity of 100 mW cm⁻². The X‐ray diffraction patterns show the presence of anatase phase of TiO₂ in the developed films. X‐ray photoelectron spectroscopy studies have confirmed that Mo is present only as Mo⁶⁺ ions in all films. The Mo/TiO₂ band gap decreases from ~3.3 to 3.1 eV with increasing Mo dopant concentrations. Dye degradation of ~60% is observed in Mo/TiO₂ samples, which is much higher than that of pure TiO₂.     

Recent advances in visible light-responsive titanium oxide-based photocatalysts

This paper presents an overview of the recent advances achieved on visible light-responsive TiO₂-based photocatalysts. In order to provide a focal point, we will highlight the work on transition metal ion-doped TiO₂ and TiO₂ thin films prepared by an RF magnetron sputtering deposition method, as well as their corresponding photoactivities on such significant reactions as organic decomposition and water splitting. Some of their proposed mechanisms are presented together with experimental physicochemical evidence to support the conclusions.     

Nitrogen-doped TiO2 from TiN and its visible light photoelectrochemical properties

Homogeneous titanium nitride (TiN) thin film was produced by simple electrophoreic deposition process on Ti substrate in an aqueous suspension of nanosized TiN powder. Nitrogen-doped titanium dioxide (TiO_2−xN_x) was synthesized by oxidative annealing the resulted TiN thin film in air. Photoelectrochemical measurements demonstrated visible light photoresponse for the electrode of annealed electrophoreic deposited TiN samples. It is found that the synthesized TiO_2−xN_x electrode showed higher photo potential under white and visible light illumination than the pure TiO₂ electrode. The photocurrent under visible light illumination was also observed, which increased with the increase of deposition time of TiN thin films.     

Atomic layer deposition of TiO2−xNx thin films for photocatalytic applications

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 irradiation. 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 atomic layer deposition (ALD) using TiCl₄, NH₃ and water as precursors. All growth experiments 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 degradation 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.     

Preparation of nitrogen-substituted TiO2 thin film photocatalysts by the radio frequency magnetron sputtering deposition method and their photocatalytic reactivity under visible light irradiation

Nitrogen-substituted TiO2 (N-TiO2) thin film photocatalysts have been prepared by a radio frequency magnetron sputtering (RF-MS) deposition method using a N2/Ar mixture sputtering gas. The effect of the concentration of substituted nitrogen on the characteristics of the N-TiO2 thin films was investigated by UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The absorption band of the N-TiO2 thin film was found to shift smoothly to visible light regions up to 550 nm, its extent depending on the concentration of nitrogen substituted within the TiO2 lattice in a range of 2.0-16.5%. The N-TiO2 thin film photocatalyst with a nitrogen concentration of 6.0% exhibited the highest reactivity for the photocatalytic oxidation of 2-propanol diluted in water even under visible (lambda > or = 450 nm) or solar light irradiation. Moreover, N-TiO2 thin film photocatalysts prepared on conducting glass electrodes showed anodic photocurrents attributed to the photooxidation of water under visible light, its extent depending on wavelengths up to 550 nm. The absorbed photon to current conversion efficiencies reached 25.2% and 22.4% under UV (lambda = 360 nm) and visible light (lambda = 420 nm), respectively. UV-vis and photoelectrochemical investigations also confirmed that these thin films remain thermodynamically and mechanically stable even under heat treatment at 673 K. In addition, XPS and XRD studies revealed that a significantly high substitution of the lattice O atoms of the TiO2 with the N atoms plays a crucial role in the band gap narrowing of the TiO2 thin films, enabling them to absorb and operate under visible light irradiation as a highly reactive, effective photocatalyst.     

Electrochromic properties of sputtered TiO2 thin films

TiO₂ thin films were deposited on ITO/Glass substrates by the rf magnetron sputtering in this study. The electrochromic properties of TiO₂ films were investigated using cyclic voltammograms (CV), which were carried out on TiO₂ films immersed in an electrolyte of 1 M LiClO₄ in propylene carbonate (PC). As- deposited TiO₂ thin film was amorphous, while the films post-annealed at 300~600°C contained crystallized anatase and rutile. With the increase of the annealing temperature, the surface roughness of film increased from 1.232 nm to 1.950 nm. Experimental results reveal that the processing parameters of TiO₂ thin films will influence the electrochromic properties such as transmittance, ion-storage capacity, inserted charge, optical density change, coloration efficiency and insertion coefficient.     

氧化钼-二氧化钛复合膜的可见光致变色性能研究

采用溶胶凝胶法制备了一系列氧化钼-二氧化钛纳米复合物, 用柠檬酸作分散剂, 将得到的纳米复合物粉末分散成溶胶, 涂膜制备了具有可见光变色性能的均匀透明的光致变色(λ≥420 nm)复合膜. 通过红外、XPS、ESR测定表明, 在氧化钼、二氧化钛复合界面上生成了Mo—O—Ti键, 此键的存在是该复合膜具有可见光致变色响应的内在原因. 在可见光的照射下, 电子从二氧化钛的价带经由Mo—O—Ti键, 被激发到氧化钼的导带上. 研究了光致变色过程的动力学, 发现光致变色反应的速率取决于膜中复合物的摩尔比([MoO₃]/[TiO₂])以及电荷转移能量(CTE).     

The effect of heat treatment in the reducing atmosphere on the physical properties of TiO2 thin films prepared by sol–gel method

Multilayered nanostructured TiO₂ thin films were prepared by sol–gel and dipping deposition on quartz substrate followed by thermal treatment under reducing atmosphere (20 %H₂–80 %Ar). Heat treatment at progressively higher temperatures caused structural, morphological, and optical changes, which were investigated by X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, and UV–Vis spectroscopy. The conductivities of the thin films were also measured by 4-point probe method. The XRD results showed that the calcined TiO₂ thin films consist of single anatase phase which was completely transformed into rutile phase after heat treatment at 1,000 °C. The grains of films grew by intra-agglomerate densification after heat treatment at higher temperatures. The root mean square roughness of the samples was found to be in the range of 0.58–3.36 nm. The partially reduced TiO₂ samples have red-shifted transmittance bands due to new energy band formed by oxygen vacancies. The electrical conductivity of the films was also enhanced after heat treatment in reducing atmosphere.     

Photovoltaic Behaviour of Titanyl Phthalocyanine Thin Films and Titania Bilayer Films

Summary: Titanyl phthalocyanine (TiOPc) thin films were prepared using evaporation and surface polymerization by ion-assisted deposition (SPIAD) in a vacuum deposition system. These films were characterized by means of ultraviolet and X-ray photoelectron spectroscopy as well as UV/Vis absorption spectroscopy. Valence band and elemental content indicated that phthalocyanine electronic and chemical structures were largely preserved during SPIAD. Further, bilayer thin films of titania (TiO₂) and SPIAD TiOPc were prepared. TiO₂ film was deposited by reactive magnetron sputtering of TiO₂ target. Study of the structured samples was focused on the optical and electrical properties of the composite films. The films were characterized by non-contact photovoltage measurements and UV-Vis spectroscopy. These results suggest there is a possibility to use these bilayer thin films in photovoltaic solar cells, however further experiments to improve conductivity of the films will be required.     

Synthesis and characterization of novel InVO<Subscript>4</Subscript>–TiO<Subscript>2</Subscript> thin films using the low temperature sol

The InVO₄ sol was obtained by a mild hydrothermal treatment (the precursor precipitation solution at 423 K, for 4 h). Novel visible-light activated photocatalytic InVO₄–TiO₂ thin films were synthesized through a sol–gel dipping method from the composite sol, which was obtained by mixing the low temperature InVO₄ sol and TiO₂ sol. The photocatalytic activities of the new InVO₄–TiO₂ thin films under visible light irradiation were investigated by the photocatalytic discoloration of methyl orange aqueous solution. The thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and UV–Vis absorption spectroscopy (UV–Vis). The results revealed that the InVO₄ doped thin films enhanced the methyl orange degradation rate under visible light irradiation, 3.0 wt% InVO₄–TiO₂ thin films reaching 80.1% after irradiated for 15 h.