Study On Modification Of TiO₂ Nanotube Arrays And Its Photoelectrocatalytic Activity

nanotube arrays, as a novel multi-functional semiconductor material, have shown great value in applications for solving the energy shortage and environmental pollution due to its attractive properties such as light corrosion resistance, acid and alkali resistance, chemical stability, non-toxic. TiO2 nanotube array film prepared by anodic oxidation directly grows on Ti substrates and has a firm combination with the Ti metal substrate. Compared with TiO2 nanoparticles, TiO2 nanotube arrays are well distributed, highly ordered and have large surface area, high quantum efficiency. By changing the experimental parameters of anodic oxidation, the length, diameter, wall thickness and morphology of TiO2 nanotube arrays can be precisely controlled. In the practical application, by using TiO2 nanotube arrays as the photocatalyst, not only could the fixation and reutilization of the catalysts in a suspension system be solved, but higher photoelectric conversion efficiency could be achieved since the highly ordered structure can offer an excellent channel for photogenerated charged carriers transfer. However, TiO2 nanotube arrays have disadvantages including low utility of solar spectrum, high recombination rate of electron-hole and low quantum efficiency, which limit its utility in industrialization.This work is aimed at expanding the photoresponse range of TiO2 nanotube arrays, reducing the photogenerated electron-hole recombination rate, consequently improving the efficiency in photoelctrocatalytic degradation of organic pollutants and hydrogen production. Some methods including metal ion doping, semiconductor combination with narrow-band-gap semiconductor and quantum dots decoration were used to modify TiO2 nanotube arrays. Furthermore, an external voltage was applied to further enhance the photogenerated electrons-hole separation. The morphology, composition, crystalline phase and structure of the modified TiO2 nanotube arrays were characterized by SEM, TEM, XRD and XPS. The optical activity, photoelectrochemical activity were examined by UV-visible diffuse reflectance spectroscopy, photocurrent spectroscopy and electrochemical impedance spectroscopy (EIS). The photoelectrocatalytic activity was evaluated by photoelectrocatalytic degradation of methylene blue (MB) and photoelectrocatalytic hydrogen production. The main progress and results are outlined as following:(1) Fe3+ doped TiO2 nanotube arrays were prepared by direct anodization of TiFe alloys containing different amounts of Fe. Fe elements incorporated in the lattice of TiO2 existed in+3 valence state. Doped TiO2 nanotube arrays showed enhanced absorption in the visible region. With the increasing doping level of Fe the photocurrent of doped TiO2 nanotube arrays first increased and then decreased. TiO2 nanotube arrays based on TiFe alloy containing 0.8wt% Fe had a maximum photocurrent. EIS measurement showed that electron-hole separation was enhanced after Fe3+ doping. Photoelectrocatalytic degradation experiment of MB showed that Fe3+ doping improved the photoelctrocatalytic efficiency of TiO2 nanotube arrays.(2) CdS/TiO2NRA/TiO2NTAs composite film was prepared by combining anodic oxidation, hydrothermal treatment and chemical bath deposition. The absorption edge of TiO2 nanotube arrays was obviously red-shifted after deposition of CdS, and the layer made of flower-like rutile nanorods further increased its ability to absorb visible light. Under visible light irradiation and with an external bias of -0.5 V applied, the composite film electrode showed a 17.94-fold enhancement in photocurrent compared with pure TiO2 nanotube arrays. CdS/TiO2NRA/TiO2NTAs composite film hydrothermal treated for 4 h and deposited for 20 cycles exhibited the highest photocurrent.(3) CeO2 quantum dots decorated TiO2 nanotube arrays were synthesized through combining anodic oxidation and solvothermal treatment. The results indicated that the crystalline phase of CeO2 was cubic fluorite, and Ce existed in the state of both Ce3+ and Ce4+. UV-Vis diffuse reflectance spectrum showed that the absorption edge of TiO2 nanotube arrays was red-shifted after deposition of CeO2.