Light-responsive TiO2 Thin Films Research Articles

Separate-type Pt-free photofuel cell based on a visible light-responsive TiO2 photoanode: Effect of hydrofluoric acid treatment of the photoanode

The present study deals with the effect of a hydrofluoric acid (HF) treatment of visible light-responsive TiO2 (Vis-TiO2) thin film photocatalysts on their photoelectrochemical performances. The Vis-TiO2 thin film photocatalysts have been prepared on Ti metal substrates by a magnetron sputtering deposition method and then treated by an HF solution. The HF treatment of Vis-TiO2 thin film photocatalysts leads to the dramatic enhancement of their photoelectrochemical performances under UV light and visible light irradiation. Moreover, separate-type Pt-free photofuel cells (SPFCs) constructed using the Vis-TiO2 thin films as photoanodes can generate electricity under light irradiation and simultaneously decompose methanol. The SPFC based on the Vis-TiO2 thin film photoanode etched by HF solution shows the enhanced cell performances with two times higher short-circuit current density and higher energy conversion efficiency than that based on the untreated Vis-TiO2 thin film photoanode. In addition to the increase of surface area and surface roughness, the enhanced conductivity by HF treatment, which is confirmed by Mott–Schottky analyses, is found to be responsible for improvement of cell performances of the SPFC.

Effect of the sputtering parameters on the physical properties and photocatalytic reactivity of TiO2 thin films prepared by an RF magnetron sputtering deposition method

Visible light-responsive TiO2 (Vis-TiO2) thin films were successfully developed by applying a radio-frequency magnetron sputtering deposition method by controlling various sputtering parameters such as the substrate temperature, Ar gas pressure, and the target-to-substrate distance. UV–Vis, XRD and SEM investigations revealed that optical property, the crystal structure, and photocatalytic activity of Vis-TiO2 are strongly affected by the sputtering parameters during the deposition step. Vis-TiO2 was found to act as an efficient photocatalyst for the H2 and O2 evolution from water under visible light irradiation (λ ≥ 420 nm). SIMS investigations have revealed that a slight decrease in the O/Ti ratio of the TiO2 thin films plays an important role in the modification of the electronic properties of Vis-TiO2 thin films, enabling them to absorb visible light.

Ion engineering techniques for the preparation of the highly effective TiO2 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.

Photocatalytic oxidation of 2-propanol under visible light irradiation on TiO2 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 (k C 450 nm). The photocata- lytic 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.

Photocatalytic activity of visible light-responsive TiO2 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.

Photocatalytic Decomposition of Water on Double-Layered Visible Light-Responsive TiO2 Thin Films Prepared by a Magnetron Sputtering Deposition Method

A radio-frequency magnetron sputtering (RF-MS) deposition method was applied to prepare a unique double-layered visible light-responsive TiO2 thin film photocatalyst on a Ti foil substrate (DL-TiO2/Ti). The DL-TiO2/Ti consists of a UV light-responsive TiO2 thin film (UV-TiO2) deposited as the inner block layer on a Ti foil substrate and a visible light-responsive TiO2 thin film (Vis-TiO2) deposited as the outer layer. DL-TiO2/Ti exhibited higher photoelectrochemical and photocatalytic performance under both UV and visible light irradiation than a single-layered Vis-TiO2 thin film photocatalyst deposited on a Ti foil substrate. The optimal thickness of the UV-TiO2 film as the inner block layer of DL-TiO2/Ti was found be 100 nm in terms of the photoelectrochemical and photocatalytic performance. Expanding on this work, a novel double-layered TiO2 thin film device, DL-TiO2/Ti/Pt, was also prepared by the RF-MS method where DL-TiO2 was deposited on one side of a Ti foil substrate and Pt was deposited on the other side. The separate evolution of H2 and O2 from H2O was successfully achieved by using an H-type glass cell consisting of two aqueous phases separated by the DL-TiO2/Ti/Pt and a proton-exchange membrane. The separate evolution rate of H2 and O2 was found to be dramatically enhanced on DL-TiO2/Ti/Pt as compared with a single-layered Vis-TiO2 thin film device under white light irradiation from a solar simulator.

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.

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.

Application of Highly Functional Ti-Oxide-Based Photocatalysts in Clean Technologies

Various Ti-oxide based photocatalysts such as the highly dispersed Ti-oxide species within zeolite frameworks, TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents as well as visible light responsive TiO2 thin films have been successfully prepared. Characterization studies at the molecular level, such as X-ray absorption fine structure (XAFS) and photoluminescence (PL), revealed that the highly dispersed Ti-oxide species within the nano-spaces of zeolites possess a tetrahedral coordination and that they demonstrate unique and high performance for the photocatalytic decomposition of NOx and the photocatalytic reduction of CO2 with H2O. A high photocatalytic reactivity for the TiO2 semiconducting photocatalysts could be achieved by blending them with hydrophobic siliceous zeolites which was equal to the performance of TiO2 deposited with expensive Pt particles. The role of the siliceous zeolites can be described as a so-called “catch and release effect of organic compounds”, i.e., (i) the condensation of the reactants within the hydrophobic cavities of zeolites and; (ii) the efficient diffusion of the reactant onto the TiO2 photocatalytic sites. Furthermore, a novel photocatalytic system which can convert abundant solar energy into renewable H2 energy by the decomposition of H2O into H2 and O2 can also be achieved by using visible light responsive TiO2 thin film photocatalysts prepared by a RF-magnetron sputtering deposition method. The conversion efficiency of solar energy into H2 energy may be estimated at ca. 0.1% from the initial rate of H2 evolution.

Photocatalytic Hydrogen Production from Aqueous Solutions of Alcohol as Model Compounds of Biomass Using Visible Light-Responsive TiO2 Thin Films

Visible light-responsive TiO2 (Vis-TiO2) thin films were prepared by a radio frequency magnetron sputtering deposition method. The photoelectrochemical performance was improved by adding various kinds and concentrations of alcohol, i.e., model compounds of biomass, to water. The separate evolution of pure H2 was observed to proceed efficiently on these Vis-TiO2 thin films under visible light irradiation by using an aqueous solution of methanol.

The Effect of Chemical Etching by HF Solution on the Photocatalytic Activity of Visible Light-responsive TiO2 Thin Films for Solar Water Splitting

The effect of chemical etching by HF solution on the photoelectrochemical performance and photocatalytic activity of visible light-responsive TiO2 (Vis-TiO2) thin films prepared by a radio-frequency magnetron sputtering method has been investigated. It was found that Vis-TiO2 thin films treated with HF solution (HF-Vis-TiO2) exhibit a remarkable enhancement of the photoelectrochemical performance not only under UV but also visible light irradiation as compared to untreated Vis-TiO2. The incident photon to current conversion efficiencies reached 66 and 9.4% under UV (λ = 360 nm) and visible light (λ = 420 nm), respectively. The HF-Vis-TiO2 thin films have a larger surface area and higher donor density than Vis-TiO2, indicating that the remarkable increase in the photocurrent may be due to the short diffusion length of the photoformed holes in reaching the solid–liquid interface as well as to the high conductivity. Moreover, the HF-Vis-TiO2 thin films were found to act as efficient photocatalysts for the decomposition of water with the separate evolution of H2 and O2 from H2O under visible or sunlight irradiation.

Hydrogen Production Using Highly Active Titanium Oxide-based Photocatalysts

Applying the principle of water decomposition over photoelectrochemical cells to heterogeneous photocatalytic systems using powdered semiconductors is now a field of great interest, encouraging new fundamental investigations into chemical reactions which take place at the electrode surfaces in the electrochemical cells. In the present review, we have focused on systems, which can convert solar energy into chemical energy by using TiO2 photocatalysts. The photocatalytic decomposition of water under UV light irradiation has been achieved with systems using various nanoparticle photocatalysts such as TiO2. However, recently, visible light-responsive TiO2 thin films photocatalysts have been successfully prepared by a radio frequency magnetron sputtering (RF-MS) deposition method. These thin film photocatalysts were found to have enough potential for the separate evolution of H2 and O2 from water under solar light. The TiO2 thin films were prepared on metal substrates by RF-MS deposition and mounted on H-type containers filled with water. This unique system enabled the separate evolution of H2 and O2 from water under sunlight irradiation, opening new opportunities for the practical on-site production of pure and clean H2 from water using abundant and clean sunlight in a safe, environmentally harmonious way.

Decomposition of water in the separate evolution of hydrogen and oxygen using visible light-responsive TiO2 thin film photocatalysts: Effect of the work function of the substrates on the yield of the reaction

Abstract The development of visible light-responsive TiO 2 thin films (vis-TiO 2 ) was successfully carried out by applying a radio-frequency magnetron sputtering (RF-MS) deposition method. The Pt-loaded vis-TiO 2 thin films clearly showed high potential for the decomposition of water into H 2 and O 2 by the evolution of H 2 from methanol/water as well as O 2 from a silver nitrate/water system under both UV and visible light irradiation ( λ ≥ 420 nm). These thin films were found to decompose pure water into H 2 and O 2 stoichiometrically under light irradiation of wavelengths longer than 390 nm. Next, a novel TiO 2 thin film photocatalyst was prepared on various metal substrates such as Al, Fe, Pd, Pt, Ti and Zr, while nanoparticles of Pt were deposited on the other side of these substrates. The reaction rate was revealed to increase with a decrease in the work function ( ϕ ) of the substrate. Moreover, the separate evolution of H 2 and O 2 could be successfully achieved under solar light irradiation by applying these thin film photocatalysts in an H-type glass container consisting of two water phases separated by a TiO 2 thin film and proton-exchange membrane.

Photocatalytic Water Splitting on Visible Light-responsive TiO2 Thin Films Prepared by a RF Magnetron Sputtering Deposition Method

The development of visible light-responsive TiO2 (Vis-TiO2) thin films has been achieved by applying a radio-frequency magnetron sputtering deposition (RF-MS) method. Pt-loaded Vis-TiO2 thin films act as photocatalysts to decompose water involving sacrificial reagent such as methanol or silver nitrate even under visible light (λ ≧ 420 nm) irradiation. It was also found that Pt-loaded Vis-TiO2 thin films decompose pure water into H2 and O2 stoichiometrically under light irradiation of wavelengths longer than 390 nm. Vis-TiO2 thin films exhibit columnar structures perpendicular to the substrate and a declined composition of the O/Ti ratio from the surface (O/Ti = 2.00) to bottom (O/Ti = 1.93). This unique structure (anisotropic structure) of Vis-TiO2 can be considered an important factor in the modification of the electronic properties of Vis-TiO2 thin films, enabling the absorption of visible light. Furthermore, the effect of the Pt loadings on the photocatalytic activity of the TiO2 thin films was investigated and the optimum Pt loading was determined to be 21 μ g/cm2 as Pt metal

Preparation and Characterization of the Visible Light Responsive TiO<sub>2</sub> Thin Film Photocatalysts Prepared by Magnetron Sputtering Method and Their Photocatalytic Activities for the Water Splitting Reactions

Visible light responsive TiO2 (Vis-TiO2) thin films were successfully developed by the radio-frequency magnetron sputtering method, and their photocatalytic activities for the water splitting reactions were investigated. Pt-loaded Vis-TiO2 thin films acted as photocatalysts to decompose water involving sacrificial reagent such as methanol or silver nitrate under visible light irradiation (λ³ 420 nm). Furthermore, the separate evolution of H2 and O2 was successfully achieved under solar light irradiation by applying these photocatalysts in a H-type glass container, which consists of two water phases separated by a TiO2 thin film and proton-exchange membrane.

Preparation of Visible Light-responsive TiO2 Thin Film Photocatalysts by an RF Magnetron Sputtering Deposition Method and Their Photocatalytic Reactivity

Abstract Visible light-responsive TiO2 (Vis-TiO2) thin films were successfully developed by a radio-frequency (RF) magnetron sputtering deposition method. These Vis-TiO2 thin films exhibited a declined composition of the O/Ti ratio from the surface (O/Ti = 2.00) to bottom (O/Ti = 1.93), enabling the absorption of visible light. Pt-loaded Vis-TiO2 was, thus, found to decompose water involving methanol or a 0.05 M silver nitrate solution under visible light irradiation.

Preparation, Characterization, and Reactivities of Highly Functional Titanium Oxide-Based Photocatalysts Able to Operate under UV–Visible Light Irradiation: Approaches in Realizing High Efficiency in the Use of Visible Light

Abstract A review of the research advances made in the design and development of highly reactive and functional titanium oxide photocatalysts, which can utilize not only UV but also visible or solar light, and a clarification of the active sites as well as the detection of the reaction intermediates at the molecular level have been presented here. The potential for the effective utilization and conversion of solar energy into useful and safe chemical energy by the modification of the electronic properties of such TiO2 photocatalysts is great when one considers their myriad applications as well as their non-polluting qualities. Such a modification process by methods such as ion-implantation can be applied not only for semiconducting bulk TiO2 photocatalysts but also for TiO2 thin film photocatalysts and titanium oxide photocatalysts highly dispersed within zeolite frameworks. Moreover, the photocatalytic reactivity of semiconducting TiO2 nano-powders was found to be dramatically enhanced by the loading of small amounts of Pt. This worked to enhance the reduction reaction, resulting in the charge separation of the electrons and holes generated by light irradiation. In addition, highly dispersed titanium oxide species prepared within zeolite frameworks as well as SiO2 or Al2O3 matrices showed much higher and unique photocatalytic performances as compared to semiconducting bulk TiO2 photocatalysts. Significantly, a new alternative method to directly prepare such visible light-responsive TiO2 thin film photocatalysts on various substrates has been successfully developed by applying a RF magnetron sputtering deposition method.