Effects Of Co-doping On The Electronic Structures Of Anatase TiO₂ Studied By First Principles Calculations

With the rapid social economy development, the environment pollution tends to be more serious. In order to produce renewable energy, people use semiconductors to convert sunlight into usable electricity and one of the key issues is looking for stable and efficient photocatalytic materials. Titanium dioxide?TiO₂?, due to its low cost and the stable chemical properties, has been regarded as one of the most promising photocatalysts, however, the energy conversion rate of TiO₂ is low because TiO₂ has a wide band gap about 3.2 eV and only could absorb the ultraviolet light?l?387.5 nm?, which accounted for a small part of solar energy. Therefore, in recent years, how to improve the application of TiO₂ as an ideal photocatalyst of effectively utilize sunlight is the most challenging subject for researchers.In this study, we investigate systematically the geometry, the electronic structure and other relative properties of non-metal and metal-doped TiO₂ based on density functional theory. The study is divided into five chapters, and the main contents of this research work are listed as follows:In chapter 1, the crystal structure of TiO₂ and the research background as well as the research progress of TiO₂ photocatalyst are introduced.In chapter 2, we present the density functional theory and introduce the first-principle software package VASP.In chapter 3, the calculated geometric structure, stability, and electronic properties of doped TiO₂ systems are shown and it is found that the?Cu+N? co-doping can prevent the recombination of photo-generated electron-hole pairs and more conductive to improve the photocatalytic activity of the system. By using the first principles method, which based on density functional theory, we calculated the formation energies, the band structure and the electronic density of states of mono-doped TiO₂ system, such as the doping, replacement and adsorption of 3d transition element Cu on the surfaces of?001? and?101?, as well as six possibilities of?Cu+N? co-doping. In this study, the formation energy results show that N atom is preferred to substitute oxygen sites on the horizontal direction when Cu atom is doped at the vacancy of the TiO₂?001? surface. The GGA+U method is also used to analyze the electronic structure of pure TiO₂, Cu doped TiO₂ and?Cu+N? co-doped TiO₂, the results show that the band gap is narrowed and new electronic states appear in the gap, and the occupied and unoccupied states induced by dopants in the band gap can trap the charge carriers. The half-metallic property is found in the?Cu+N? co-doped system. The analysis of density of states and the electronic structure also indicate that Cu-3d orbital interacts with O-2p and N-2p orbital.In chapter 4, we further study the effects of compensated acceptor-donor complexes on co-doped TiO₂. In this section, we describe the geometry, the electronic structure, the band edge position and the charge density distribution of mono-doped Ti O₂, such as substituted metal atoms Zr?Nb and V and substituted nonmetal atoms N?S?P and F. The results show that the impurity hybrid with O-2p orbital or Ti-3d orbital, and the formation of new impurity levels in the band gap would contribute to the separation of electron-hole pairs. Meanwhile, the band-edges of doped system are slightly shifted, of which the F, S, Nb and Zr doped systems could enhance the redox potentials. To prove that the compensated acceptor-donor?P+Nb???S+Zr? and?N+V? complexes which follow the charge compensation principle will be more effective to improve the photocatalytic activity, we also calculated the binding energies and the electronic structures of compensated acceptor-donor co-doped systems and make compasons with the n-type?F+V? co-doped system, it is found that the band gap become narrower and there is red shift of the absorption edges, which could enhance the photoelectrochemical activity. Furthermore, the co-dopant?S+Zr? will be the best candidate to modify the band edge with stronger binding energy.In chapter 5, a summary of this study is given, which reveal the relationship between the electronic structure and the visible light absorption activity of different doping with different methods. Our study provide a powerful theoretical foundation in the research field of Ti O₂ and give support for experiments to seek appropriate dopants.