Theoretical Study On S-Transition Metal Codoping Modification Of Titania Photocatalysts

In recent years, as one of the constraints for human survival and development, the energy crisis and environmental problems such as water pollution is becoming day by day prominent. Titania (T1O2), as an important metal oxide, is widely used in environmental protection due to its low cost, nontoxic, long-term stability and high oxidative power. But, as a wide band gap oxide semiconductor, its solar energy utilization is very low, because its large intrinsic band gap can only absorb the ultra-violet portion of the solar spectrum. Furthermore, its photo quantum yield is also very low due to its photoexcited electron-hole pairs easily recombining. Therefore, narrowing the band gap and reducing the recombination rate of photoexcited electron-hole pairs are the effective approach to improve the photocatalytic activity of TiO2.Currently, first principles calculation based on density functional theory is an effective method to study the microstructure and properties of dopant. Using this theoretical approach to doping, which plays an important role in the scientific research and development of new materials, can give us an insight into the geometric and electronic structure of the materials, verify some experimental results and save research costs.In this thesis, the crystal structure, impurity formation energy, electronic structure, optical properties and band edge position of sulfur and transition metal co-doped anatase TiO2were studied by using the plane-wave ultrasoft pesudopotentials method based on density functional theory. According to the calculated results, the main conclusions gained in this thesis are as follows.(1) Compared with pure TiO2, the dipole moments of octahedral in the new crystal increase especially in sulfur and transition metal co-doped anatase TiO2, which is very effective for the separation of photogenerated electron-hole pairs and the improvement of the photocatalytic activity of TiO2. (2) As for sulfur and transition metal co-doped TiO2, an isolated S3p state locates above the top of valence band and mixes with O2p states, and the3d metal doping creates an occupied level in the band gap due to the3d state of the dopant. As the atomic number of the dopant increases, the localized level shifted to a lower energy. The impurity energy levels locates below the conduction band or/and above the valence band could act as donor levels and acceptor levels, which can reduce the recombination rate of charge carriers and thus improve the photocatalytic activity of TiO2. The other impurity energy levels locates near the middle of band gap can be transitional levels.(3) Compared with pure TiO2, sulfur and transition metal co-doped codopings do not obviously change the band edge position of TiO2. It indicates that codoped TiO2photocatalysts maintain the strong redox potentials and is helpful to improve the response to visible-light.These calculated results illustrate the intrinsic nature of improving visible light activities of TiO2through choosing different ions, doping concentration and styles, which can provide a theoretical basis for experimental research.