Non-metallic Element Doped Tio <sub> 2 </ Sub> Based Photocatalysts Theory

Titanium dioxide (TiO₂) is a well-known photocatalyst because of its cheap, nontoxic peculiarities and stable, efficient performances in the depuration of air and water. However, it is activated only under UV light irradiation (about 3% of the solar spectrum) because of its large band gap (3.2eV for anatase). So, it is in urgent need to develop efficient visible-light-driven photocatalysts by modification of TiO₂ which allow the main part of the solar spectrum (45% or so) to be used. Doping with transition metals is one of the methods to tailor the band gap of TiO₂, but this kind of doped TiO₂ generally has lower photocatalytic activity because of thermal instability and higher carrier recombination rate. Recently, nonmetal-doped TiO₂ received a lot of attention as the doping of the nonmetals (S, N, C, B, P, F, etc.) could efficiently extend the photoresponse of TiO₂ to low energy (visible) region.In this dissertation, we provide a systematical study on the S, C and N-doped anatase TiO₂ system by theoretical calculation based on the state-of-the-art plane-wave based pseudopotential density functional theory (DFT).Main contents of this dissertation included:1. S-doped TiO₂, the substitutional doping on lattice O and lattice Ti sites in anatase TiO₂ crystal, were investigated detailedly. At the same time, the interstitial doping and multi-doping of S in anatase TiO₂ were also involved.(a) The doping effect of S substituting for O in anatase TiO₂.Through band structure calculation, a direct-band-gap is predicted in TiO_2-xS_x. Electronic structure analysis shows that the doping S could substantially lower the band gap of TiO₂ by the presence of an impurity state of S3p on the upper edge of valence band. Excitations from the impurity state of S3p to conduction band may be responsible for the red shift of absorption edge observed in the S-doped TiO₂. The band gap lowering and red shift of absorption edge are found to increase as sulfur concentration increased.(b) The doping effect of S substituting for Ti in anatase TiO₂Electronic structure analysis indicated that, just as in the anionic doping case (S...