Study On The Defects Of Ta₃N₅Photocatalyst Based On The First-Principles Calculations

Solar hydrogen production via photocatalytic water splitting is a promising strategy to ease energy shortage and environmental crisis, because it supplies an environmentally approach for splitting water into hydrogen and oxygen gases under irradiation of solar light. In the last decades, metal oxides as photocatalytic semiconductors have been extensively studied. However, the relatively large band gaps of many metal oxides photocatalysts limit their usage under ultraviolet light. Since about43%of the solar energy is constituted by visible light, it is necessary to develop photocatalytic semiconductors which are able to absorb more abundant visible light of solar spectrum. Recently, the tantalum nitride (Ta3N5) as one outstanding representative of non-oxides photocatalysts has attracted a great interest. Due to the smaller optical band gap (about2.1eV) and the appropriate band edge positions, the theoretical maximum solar-to-hydrogen ratio of Ta3N5is as high as15.9%, suggesting that Ta3Ns is one potential semiconductor photocatalyst in future industrial applications.Although Ta3N5is a potential semiconductor photocatalyst, the real photocatalytic performance of Ta3N5has been far below expectations. The valence band maximum and conduction band minimum of Ta3N5straddle the water redox potentials, suggesting that the simultaneous production of H2and O2is theoretically reasonable for Ta3N5. However, real experiments reveal that its photocatalytic ability for H2evolution is much weaker than that for the O2evolution. A proper explanation of this phenomenon comes from the defects in Ta3N5, but what the defects are and how the defects affect the photocatalytic performance of Ta3N5are still less known. With the aid of the first principle calculations, this doctoral dissertation is aiming at revealing the effects of defects on the photocatalytic performance of Ta3N5. Furthermore, based on the in-depth understanding of the true physics of defects in Ta3N5, we propose that the charge compensation elements codoping scheme may improve the H2evolution ability of Ta3N5. The whole doctoral dissertation is composed of six chapters:In the first chapter, we firstly review the research background of this doctoral dissertation and the research progress of Ta3N5in the field of semiconductor photocatalysis. Then, we summarize the research motivation and contents of this dissertation.In the second chapter, we make a brief introduction of the theoretical background of the first principles calculations based on the density functional theory (DFT) and the specific simulation packages employed in this dissertation. In addition, we make a detailed discussion of how to ensure the reliability of the first principles calculations results.In the third chapter, the formation energies, defect transition energy levels and electronic structures of nitrogen vacancy and impurity oxygen in the bulk Ta3N5are studied. The results show that both nitrogen vacancy and impurity oxygen are electron donors and their defect transition energy levels are very shallow. Furthermore, the donated electrons from nitrogen vacancy and impurity oxygen are able to reduce the+5charged Ta. This will induce the downshift of the conduction band of Ta3N5and thus weakening the water reduction ability of Ta3N5. In addition, the effects of oxygen doping on the optical band gaps and band edge positions of the O-doped Ta3N5are systematically investigated. The calculated results are in good agreement with the experimental results.In the fourth chapter, water adsorption and dissociation on the perfect, oxygen containing and nitrogen vacancy containing Ta3N5(100) surfaces are systematically studied. The results show that the perfect Ta3N5(100) surface is very active for water dissociation because of the dangling bonds formed on the perfect Ta3N5(100) surface. Presence of oxygen on surface is able to stabilize the Ta3N5(100) surface but not to facilitate water dissociation, which may be ascribed to the saturation of surface dangling bonds by oxygen. Presence of nitrogen vacancy on surface is able to facilitate water dissociation, but Ta3N5(100) surfaces with nitrogen vacancies are not stable.In the fifth chapter, we propose that the charge compensation elements codoping scheme may be able to improve water splitting ability of Ta3N5. By calculations of formation energies, surface energies and electronic structures of M-O (M=F, Cl, Ti, Zr, Hf, Ge, Sn) codoped Ta3N5, we found that F, Ti, Zr and Hf are promising elements which can be codoped with O to improve the water reduction ability of Ta3N5, providing useful guidance for further experimental study of Ta3N5.In the last chapter, we summarize the main conclusions of this dissertation and propose some possible subjects for the further research of Ta3N5semiconductor photocatalyst.