| Nowadays, the threats from energy shortage and environment pollution have seriously impeded the development of society and economy. Thus, it should not hesitate to explore the new energy and solve the environment problem. As the most promising photocatalyst, TiO2has attracted much attention because of the excellent properties such as long-term stability against chemical corrosion, nontoxicity and low cost. However, the wide band gap of TiO2about3.2eV limits its application. Pure TiO2can only absorb ultraviolet light, which accounts for just about5%of solar energy. However, the visible light about43%of the energy could not be absorbed by TiO2. Thus, modification of the electronic structure of TiO2to enable the visible light absorption is of great importance.Recent years, a lot of scientific workers have been working in promoting the photocatalytic properties of TiO2. It is found that doping is an efficient way to reach this aim. The dopants in TiO2lattice could change the local morphology and thus the band structure. The band structure change will lead to visible light absorption and higher photocatalytic ability. Nonmetal elements such as C, N, P, S, F and metal elements such as Fe, Mn, Cr, and Co could shift the band structure and lead to visible light absorption. However, the gap states introduced by single element doping always act as combination centers of photogenerated carrier which would reduce the catalytic rate. Two kinds of dopants may overcome the weakness and, so, doping TiO2with metal and nonmetal simultaneously may be a better method. The nonmetal element doping makes the light response to the visible light and the metal ion will depress the carrier recombination. The synthetic effects of the two kind elements will promote the photocatalytic activity further. For example, N-La codoped, and N-Pt codoped TiO2are found to exhibit better photocatalytic activity than single element doped TiO2. Recently, it is separately reported that Fe-F and Fe-S codoped TiO2have higher photocatalytic abilities under visible light than single doped TiO2. So, it is deserved to research what roles the dopants play and why they are more efficient than single doped TiO2and what the differences between the two codoped systems are. In this thesis, we study the crystal structures, thermodynamic stability, electronic structures and the roles on the photocatalytic ability of Fe, S single doped and codoped anatase TiO2.The frame structures of this thesis are as follows:In the first chapter, we introduce the basic character of the three types of TiO2including the crystal structures and the properties. We also have a brief summary of the applications, photocatalytic mechanism, preparation methods and the modification of TiO2.In the second chapter, we focus on the theoretical basis of density functional theory and the common used exchange-correlation functional approaches.In the third chapter, we study the stability of Fe and F codoped TiO2with different dopants distances and the electronic structures. It is found that when the two dopants are at the fist nearest neighboring distance, the configuration has the best stability. The analysis from the electronic structures shows that Fe will exist as Fe+3, and it will be an electron trap and separate photogenerated carriers efficiently. Besides, Fe doping leads to the localized gap states in the band gap and reduces the electron excitation energy and thus leads to the visible light absorption.In the forth chapter, we have examined the crystal structures, electronic and optical properties of Fe-F and Fe-S co-doped anatase TiO2based on DFT calculations. For comparison, Fe, S, F mono-doped Fe-F codoped TiO2are also studied. Formation energy of the co-doped system is much lower than that of the mono-element doping indicating the synergic effects of co-dopants on the stability of the doped structure. The calculated results indicate that the co-doped atoms introduce impurity energy levels in the band gap mainly composed of Fe3d states. Due to the less energy needed for an electron transition from the impurity energy levels to the conduction band bottom, co-doped anatase TiO2may show higher photocatalytic activity than the mono-doped one under visible light, which may account for the experimentally observed phenomenon. However, localized gap states introduced by Fe-F co-doping may result in visible light adsorption but decline the photocatalytic activity. Compared with Fe-F codoping, Fe-S co-doped TiO2produces gap states near the band edge are extended and may greatly enhance the visible light absorption and reduce the carrier recombination. Consequently, the photocatalytic performance under visible light of Fe-S co-doped TiO2is better than that of Fe-F co-doped one and Fe-S should be a better co-doping pair. |
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