Theoretical Study For Codoping Of Sulfur And Different Ions For Titanium Dioxide

At present, the energy crisis and water pollution problems are increasingly outstanding, and have become one of the important factors that restrict the survival and development of human beings. As an ideal semiconductor photocatalyst, TiO2has great potential application in the fields of air purification, waste-water treatment, and renewable energy conversion etc. And it has the advantages of strong photocatalytic activity, chemical stability, non-toxic, inexpensive, no secondary pollution, resource rich, and so on. However, as a wide band gap oxide semiconductor, anatase TiO2(its band gap is about3.2eV) exhibits photocatalytic activity only under UV light irradiation, resulting in solar energy utilization rate is very low. In addition, due to the electron-hole pair excitation light produced by easily composite, its quantum efficiency is very low. Therefore, narrowing the band gap of TiO2, in order to absorb visible light, and reducing the photogenerated electron-hole recombination rate, is the key factors to improve the photocatalytic performance of TiO2and application in industrialization. Ions doping could efficient achieve this purpose. However, due to different preparation technology or evaluation standards by different research groups, there are inconsistent understands for the effects of ions doping. While the materials simulations and calculations based on the density functional theory is an effective method to study the microstructure and properties of ions doping on the electronic and atomic scales. Using this method, one not only can deeply understand the micro geometric structure and electronic structure of the doped materials, which can verify experiment observations and predict the unexpected performance, but also can save the research cost.In this thesis, the method of GGA+U has been systematically to investigate S-doped TiO2photocatalyst, which include S replaces O or Ti and S occupies vacancy. Based on the study, Ta and nonmetal co-doping with S could used to further improve the photocatalytic performance of S mono-doped TiO2. The main subject of this thesis is to study the crystal structure, impurity formation energy, and electronic structure of S-doped TiO2photocatalyst. The main calculation results are listed as following:(1) In rutile and anatase phase, three forms of S (substituted oxygen, substituted titanium, and interstitial doping) has similar doping effects:substituted oxygen doping can form impurity levels at the top of valence band, but still with a certain distance; substituted Ti doping can form impurity levels in the middle of band gap, which is disadvantageous for photocatalysis; interstitial doping can form impurity levels at the top of valence band and the bottom of conduction band, which are fully overlapping with the top of valence band and the bottom of conduction band. Comparing with the doping effects on the crystal structure, electronic structure, and optical properties, S interstitial doping for anatase TiO2has the best modification effect.(2) The introduction of Ta co-doping is based on S mono-doped TiO2, in which S replace O. By Ta co-doping, the distance between the S-related impurity levels and the top of valance band is further decreased. At the same time, the introduction of Ta co-doping not only has the similar UV absorption characteristics with that of S mono-doping, and has higher visible-light absorption characteristics than that of S mono-doping.(3) Through analysis and comparison different nonmetal ions codoping with S, we found that when B, C, O occupy the lattice vacancies and S substituted titanium is expected to enhance the photocatalytic performance of TiO2:the impurity levels in the middle of band gap induced by S mono-doping is eliminated, and built-in electric of octahedron is also increased, resulting in further improve the photocatalytic performance.The innovation of this thesis mainly manifests in a synergistic effect on the crystal structure, electronic structure and optical properties through the analysis and comparison of different co-doping system, and then preliminary put forward the basic principles of co-doping modification method:co-doping can be optimized by stepwise synthesis process and more easily implemented; if two kinds of dopants have compensation effect and larger electronegativity, ions co-doping maybe has high efficient and better synergistic effects; non-metallic ions with variable valence states are easier to implement effective synergistic effect of co-doping. At the same time, as a sample of S doping, the results of theoretical study in this thesis reveals the photocatalytic performance of TiO2could be effectively improved under visible-light irradiation by the synergistic effects of proper ions pairs doping, which provid theoretical basis for further development of novel photocatalysts.