Study Of Preparation And Photoelectrochemical Performance Of TiO₂ And Transition Metal Ion Doping TiO₂ Thin Film Electrodes

TiO₂ photocatalyst has been the focus of numerous investigations since 1972, particularly because its application for the chemical translation of light energy, the photocatalytic treatments of organic compounds, the photochemical synthesis, the hydrophile of light inducement of interface and the synthesis of self-cleaning material. Due to the complete mineralization of a wide variety of organic compounds to CO₂, H₂O and inorganic constituents in aqueous/air media, the photocatalytic oxidation of TiO₂ is regarded as an interesting advanced oxidation technique. With regard to photooxidation degradation reactions occurring in aqueous media, there are still many problems to restrict the development of the technique, such as photocatalyst deactivation, slow reaction rate, low quantum efficiency and unpredictable reaction mechanisms. Based on the above consideration, in this paper, the reaction mechanisms of the oxidation of small molecule alcohol and the influence of interface reactions on the photostability of TiO₂ electrodes have been studied; a new method for the preparation of metal ion doping TiO₂ electrodes has been developed, which improved light to electricity conversion efficiency under visible light.This dissertation includes two parts. In the first part, first, nanoporous TiO₂ thin film electrodes was prepared by sol-gel and cathodic electrodeposition techniques. Secondly, in the aqueous media, for the photoelectrooxidation of small molecule alcohol, the processes of ion adsorption and charge transfer at TiO₂/electrolyte interface and the influence of these processes on the photostability of TiO₂ electrodes have been studied. In the second part, the preparation of metal ion doped TiO₂ thin film electrodes by cathodic electrodeposition technique and their photocurrent response under visible light were investigated. The main research shows as follows:1. The preparation of nanoporous TiO₂ thin film electrodes by sol-gel technique.Nanoparticle colloid solution was synthesized by sol-gel method. Stable and even-size nanoparticle colloid solution was obtained by a following heat pressure treatment. Nanoporous TiO₂ thin film electrodes were prepared by soak-sintering method on conductive SnO₂ glass. The SEM (Scanning Electron Microscope), BET(Brunner-Emmett-Teller method), XRD (X-Ray Diffraction) and the measurement of pore size distribution were used to characterize the morphology and size of particle. The crystal lattice of TiO2 thin film sintered at 450 °C is anatase and mean size is about 30nm and mean pore size is about 25-30nm.2. The preparation of nanoporous TiCh thin film electrodes by cathodic electrodeposition technique.Using Ti(O2)SC>4 as precursor, nanoporous TiO2 thin film electrodes were prepared by cathodic electrodeposition technique on Ti slice and ITO glass, respectively. The measurement of photocurrent showed that in 0.5 mol-L"1 Na2SO4+10 mmol-L'1 Ti(O2)SO4+ O.lmol-L"1 KNO3 solution when the deposition potential and time were -1.4V(vs. SCE) and 400s, respectively, the as-prepared film was nanoporous, anatase T1O2 and had the highest response to white light. The influence of two substrates on properties of T1O2 film was also considered.3. The influence of OH" adsorption and charge transfer process on the photoelectrooxidation of small molecule alcohol at TiO2/electrolyte interface.Based on the three-dimensional nanoporous structure of TiO2 thin film, the energy band model of TiO2 thin film electrodes has been advanced in different pH solutions. The onset potential of photocurrent and the saturated photocurrent were determined by the adsorption of OH7H3+O and the charge transfer process at TiO2/electrolyte interface, respectively. The experimental results showed that the adsorption model of OH' on TiO2 surface corresponds to the Langmuir isotherm and under high enough applied bias potential the photooxidation kinetics could be described by Langmuir-Hinshelwood expression. Hydroxide ions were regarded to be the reactant with photogenerated holes and adsorbed water could not be captured by holes and, consequently, the photocurrent was proportional to the density of OH" adsorbed on TiO2 surface under the condition of high enough light intensity, where the rate-limiting step was the formation of photo-driven OH- radical.4. The influence of the interface reactions at different pH values on the photostability of nanoporous T1O2 electrodes.Photostability was evaluated by the change of photocurrent of oxidation methanol at 0V(vs. SCE). In acidic and basic solution, the photocorrosion of different type resulted in the decrease of the photostability of nanoporous TiO2 electrodes. In basic solution, the reaction of photo formed holes to O2"ad producted active O"ad atoms and the resulting O^ad atoms occupied gradually the oxygen vacancies of TiO2 film surface/subsurface, which mean the density of oxygen vacancies who acted as a roleof electron donors under illumination, decreased. This irreversibly decreased the photocatalytic properties of nanoporous TiCh electrodes. The analysis result of ED AX supported the above mechanisms.5. Cathodic electrodeposition preparation of Fe3+-doped TiC>2 thin film electrodes with visible light response.Metal ions(Fe3+> ZrO2+ and Sn2+) doped Tid thin film electrodes were prepared successfully on ITO glass substrates by a novel method of cathodic electrodeposition. The visible light response of electrodes was examined by the measurements of photocurrent under visible light (X>460nm). The morphology, metal ion concentration and existing state were characterized by SEM, EDAX, XRD, UV-Vis and photocurrent measurement.