| Photocatalytic oxidation using TiO2 semiconductor as catalyst is a new technology having the broad application prospect for environmental remediation as it may lead to complete mineralization of pollutants at ambient condition using the ultraviolet light as the energy source. However, it is activated only under ultraviolet light irradiation (about 3% of the solar spectrum) because of its large band gap. 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 to be used.In this thesis, the electronic structure and optical properties of anatase TiO2, metallic ions(Co2+, Fe3+, Zr4+, V5+, W6+) substituted Ti4+, metallic ions interstitial in the lattice cell, S doped TiO2 and Fe3+-S2-co-doped TiO2 were calculated by Materials Studio module using the first-principle plane-wave ultrasoft pseudopotential methods which was based on the density functional theory. The main contents of the thesis were discussed as follows:(1) The conduction band and valence band of anatase TiO2 distribute were mainly attributed to Ti 3d and O 2p orbital respectively. There was strong absorption peak at 220nm in the UV region, and the absorption edge had a red shift nearly 400nm.(2) The different positions of impurity levels had a noticeable effect on the band structure of doped TiO2, which were determined by choosing metallic ions and doping styles. The formation of impurity level was mainly contributed by mixing with Co 3d, Fe 3d, Zr 4p, Zr 4d, V 3p, V 3d, W5p, W 5d orbital of the metal. Doped with metallic ions should be responsible for red shift of TiO2 absorption wavelength or the appearance of new absorption peak in the visible light region.The light response threshold of substitution doped TiO2 from small to large order was:Fe3+, Co2+, V5+, Zr4+, W6+ doped TiO2. The doping of Co2+, Fe3+, Zr4+, V5+ brought the red shift of TiO2 absorption wavelength obviously and W-doping occurred a strong absorption peak in the visible light region. There was greater absorption at 400-600nm range in the visible light region when interstitial metallic ions in anatase TiO2.(3) The result of S doped TiO2 showed that the presence of the impurity state of S 3p on the upper edge of valence band was responsible for the engineering of the band gap of TiO2. The excitations from the extended valence band to the conduction band may be responsible for the red shift of absorption edge in S-doped TiO2. The position of the impurity state of S2- substituting for O2- was lower than S6+ substituting for Ti4+, so S2-doped TiO2 showed enhanced photocatalytic activity and the absorption edge had a red shift nearly 600nm. Especially, interstitial S in anatase TiO2 creating one more transition impurity level in band structure than the other doping styles, which exhibited high photocatalytic activity under 600-700nm visible light irradiation.(4) The band structure of Fe3+-S2- co-doped TiO2 was similar to the superposition of single-ion doping. The presence of complex valence band top and the impurity state of Fe 3d on the under edge of valence band was responsible for the engineering of the band gap of TiO2. The formation of impurity states crossing the Fermi level was mainly contributed by mixing with Fe 3d and S 3p orbital, which changed the process of electron transition and reduced the energy required for electronic transition indirectly. Co-doped TiO2 exhibited higher photocatalytic activity than the single-ion modified, which broaden the light adsorption spectrum into the visible region (600-700nm).These computational results illustrated the intrinsic nature of improving visible light activities of TiO2 through choosing different ions, doping concentration and styles, which can provide basic theory for experimental research. |
PDF Full Text Request