First-principles Study Of Non-metallic Doped TiO₂ Electronic Structures

TiO₂ is a widely used and studied photocatalyst due to its low cost, nontoxicity, high oxidative power and long-term stability against photocorrosion and chemical corrosion. TiO₂ commonly appears as anatase structure. TiO₂?B? is one of the 11 known polymorphs of titanium dioxides such as anatase. TiO₂?B? is being intensively investigated as anode materials for lithium-ion batteries due to their superior safety compared with graphite, Si, Sn or other low voltage anodes. TiO₂?B? meets several of the designed criteria for anode materials:a low cost, a low toxicity, and a pretty high energy density together with a good cycling life. However, a large intrinsic band gap of TiO₂ allows only a small portion of the solar in the ultraviolet ?UV? light region to be absorbed. Therefore, the reduction of TiO₂ band gap is of great importance to improve the optical and photocatalytic properties of this material in solar energy conversion.We mainly focus on the stability and electronic properties of nonmetal-doped anatase TiO₂ and TiO₂?B? in this dissertation. In the first chapter, we presented the research background and progress of TiO₂ in the photocatalytic filed. In the second chapter, we introduced the density functional theory. The electronic properties of nonmetal-doped anatase TiO₂ and TiO₂?B? are investigated by performing PAW potentials with GGA of the PBE as implemented in VASP based on DFT in chapters three and four.Both the external ?cell volume and cell shape? and internal ?atom position? parameters are allowed to relax fully. The doping difficulty is in the order F< N< C< B for nonmetal-doped anatase TiO₂ and TiO₂?B?.After doping nonmetal elements, some band gaps of the doped-TiO₂ become narrow and others become wide, in which impurity states appear in the band gap. The relative positions of the impurity states are much different, mainly caused by the different electronegativities among the nonmetal elements F, O, B, C and N. For non-metal doped TiO₂, the 2p states of the B, C and N atoms are all localized in the band gap mainly because the electronegativity of B, C, N is smaller than that of O. The 2p states of the F atom are localized in the lower energy range of the valence bands. The larger electronegativity should be responsible for it.Through adding H to achieve a charge balance, the impurity states approach to the valence band maximums, by decreasing the electronegativity difference among the nonmetal elements.N-doping and C-doping can cause spin polarization of anatase TiO₂ electronic structure and form 1.0 ?B and 2.0 ?B magnetic moment, respectively. However, N-doping and C-doping can cause spin polarization of TiO₂?B? electronic structure and form 1.0 ?B and 0 ?B magnetic moment, respectively.