Theory Study Of Charge Compensation Mechanism Reflected In Doping Modification Of Semiconductor Photocatalytic Materials

Recently, with the development of theoretical methods and computer technique, first principle method has been used as a routine research mean in condensed matter physics, quantum chemistry and material science. Base on charge compensation effect, this thesis has developed the doping method to control the band structure to improve the photoresponse range and photocurrent density of semiconductor photocatalysts. The effect of different doping forms on electronic properties has been studied to reveal charge compensation mechanism induced by doping and further improve photocatalytic activity. This work can supply a theoretical sight to understand photocatalysis and conduct the experiments to synthesize semiconductor photocatalysts with high activity. The main research contents are listed as follow:In the first chapter, we briefly introduce the research significance and progresses of semiconductor photocatalysts, and summarize the research status of charge compensation effect. On this basis, the research contents of this dissertation have been briefly introduced.In chapter2, we briefly introduce the DFT framework and its recent progress. From the initial LDA, GGA to hybridization functional, computational results based on DFT are more and more accurate. On the basis, the computational softwares in this thesis are summarized in brief.Starting from chapter3, this work introduces different doping forms, including nonmetal-nonmetal codoping and nonmetal-metal codoping. We reveal the charge compensation mechanism of different doping forms by analyzing geometry structure and band structure. In chapter3, we study the origin of improving visible light activity of N-doped CsTaWO6, and mainly discuss three kinds of doping forms (NHS,2N5and2N+Vo). The results indicate that three kinds of charge compensation forms are formed. For NHs-doping, N and H atoms form strong covalent bond, H atom as a donor supplys an electron for N atoms and H-N forms donor-acceptor pair charge compensation form. In2Ns-doped configuration, the two nitrogen atoms tend to form an N-N cluster structure, in which the single electron induced by one nitrogen atom is paired through the charge compensation effect between the two nitrogen impurities. And thus an N-N π*bonding orbital is formed in the band structure, which acts as the highest occupied orbitals. For2Ns+Vo-doping, Some electrons are transferred from oxygen vacancy to the N impurity ions inducing a N2-(doublet) change to a N3-(singlet) valence state, and N-Vo-N forms an acceptor-donor-acceptor pair. Above the formation of three kinds of charge compensation effect is responsible for improving the visible light photocatalytic activity of N-doped CsTaWO6.In chapter4, we focus on the microscopic mechanisms of nonmetal-metal ions codoped TiO2different surfaces on photocatalytic activity. The main contents are W/2N codoped TiO2anatase surface (101) and rutile surface (110), and La/F codoped TiO2anatase (101) surface. The results indicate as follow,(1)In doping the TiO2anatase (101) and rutile (110) surfaces, N2p impurity states induced by N-doping locate in the gap, thereby lack of an electron N acts as an acceptor and requires a donor to provide an electron to keep the charge of system balance. For W-doping, W5d states locate at shallow levels of CBM and acts as donor level. For W/N-codoping, an N2p and W5d hybridized state is formed and the hybridized state locates mainly at the edge of the valence band, whereas other W5d states are at the edge of the conduction band. The W5d states contribute to the lowering of the energy levels of the N2p states, bringing the N states closer to the valence band, and therefore enhancing mixing of N2p and O2p states and eliminating the gap states induced by single N doping. The introduction of W compensates for single electron of N atom, which reflects primary charge compensation effect. In W/2N codoping, two N atoms obtain an electron from W atom, respectively, and form N-W-N acceptor-donor-acceptor charge compensation form. Compared to above all doping forms, W/2N codoping shows the highest photocatalytic activity.(2)In doping TiO2anatase (101) surface, F-doping induces VBM moving to the lower energy level, the Fermi level moving to CBM, and a donor appearing at the edge of CBM. For La-doping, the introduction of La causes an acceptor appearing at the edge of VBM. In La/F codoping, the electron transfer between La atom and F atom induces La-F forming acceptor-donor charge compensation form, which makes the charges of system keep balance. In chapter5, we summarize the conclusions and innovative points of this dissertation, and preview the further studies.