Study On Modification, Characterization And Photoelectrocatalytic Activity Of TiO₂ Nanotube Array Film

TiO₂ nanotube array films, as a novel nano-material, are of considerable interest due to unique highly ordered array structure, good mechanical and chemical stability, as well as excellent corrosion resistance, and have exhibited the attractive applications for the photocatalytic degradation of pollutants, solar cells, gas sensor materials, photolysis water and so on. However, TiO₂ is a high band gap semiconductor (Eg = 3.2 eV, anatase), electron-hole pairs generation can only be activated by UV-light irradiation (λ< 387 nm), and only 3-4 % solar energy can be used. On the other hand, the efficiency of photocatalytic reaction is limited by the high recombination of photo-induced electron-hole pairs. Therefore, (1) how to increase the absorbance of TiO₂ to visible light and (2) how to enhance the photocatalytic efficiency of TiO₂ are two main technical problems.The main goal and motivation of the present work is to extend the photoresponse range of TiO₂ into the visible range, restrain electron-hole recombination and increase the photocatalytic efficiency. On the one hand, TiO₂ nanotube array film was modified through a simple and feasible method. On the other hand, improvement of the separation rate of photo-generated electron-hole pairs could be achieved by employing electrochemically-assisted method. The SEM, XPS, XRD, Raman spectroscopy, UV-vis absorption spectroscopy and photocurrent spectroscopy were performed to characterize the morphology, composition, crystalline phase photo-absorption ability and phtoelectrochemical property of the modified TiO₂ nanotube array films. The photocatalytic activity of the modified TiO₂ nanotube array films was evaluated by the degradation of methylene blue (MB) in aqueous solution. The effect of bias potential on photoelectrochemical activity was investigated by photocurrent spectroscopy and EIS. The photoelectrocatalytic property of TiO₂ nanotube array film was examined by the degradation of phenol aqueous solution. The main progresses and results of this work are outlined as following:1. Fe³⁺-TiO₂ nanotube array film has been successfully fabricated in Fe (NO₃)₃-HF mixing solution directly by electrochemical anodic oxidation method. Fe³⁺-TiO₂ nanotube array film can remain highly ordered array structure as the concentration of Fe(NO₃)₃ less than 0.2 mol/L. Compared with the undoped TiO₂ nanotube array film, Fe³⁺-TiO₂ nanotube array film has a greater photoelectrochemical activity and its photo-absorption edge appears obvious red shifting. The results of photocatalytic degradation of MB indicate that compared with pure TiO₂ nanotube array film, Fe³⁺-TiO₂ nanotube array film behaves a higher photocatalytic activity irradiated either under UV light or under visible light.2. Ag-loaded TiO₂ nanotube array photocatalyst is prepared by the combination of ultrasonic- and photochemical routes. Ag nanoparticles can be highly dispersed on the surface of TiO₂ nanotube array and Ag content can be controlled. Compared with TiO₂ nanotube array film, Ag-loaded TiO₂ nanotube array film promotes the separation rate of photo-generated electron-hole pairs, which confirmed by photocurrent spectroscopy. The photocatalytic avtivity of Ag-loaded TiO₂ nanotube array film is significantly higher than that of pure TiO₂ nanotube array film under UV illumination.3. The photoelectrochemical properties are investigated by photoelectrochemical associate system. The results indicate that the crystalline structure of TiO₂ affects the photoelectrochemical activity, and anatase TiO₂ is the most favorable structure for photoelectrochemical activity. The EIS spectroscopy reveals the mechanism of separation and transportation of photo-generated electron-hole pairs. Furthermore, it is found that extra bias potential can decrease the values of the electron-transfer resistance and increase the separation efficiency of photo-generated electron-hole pairs.4. The photoelectrocatalytic properties of TiO₂ nanotube array film are evaluated by the degradation of phenol aqueous solution and are compared with photocatalysis. The results demonstrate that the surface reaction step was the only rate dominated step either for the photocatalytic process or for the photoelectrocatalytic process. The photoelectrocatalytic degradation rate of phenol solution is greater than photocatalytic degradation rate for TiO₂ nanotube array film. The anatase TiO₂ nanotube array film with fully crystallization has a higher activity for degradation phenol solution when the bias potential of 0.6 V is applied.