| Anatase TiO2 has been extensively studied as a promising semiconductor photocatalyst because of its high chemical stability, low cost, non-toxicity and resistance to photocorrosion. Nevertheless, due to its wide intrinsic band gap (3.23eV), TiO2 only shows photocatalyst activity under the ultraviolet (UV) part of sunlight (wavelengths< 380 nm), which crucially restricts its practical application. In order to improve the visible light catalytic absorption of TiO2, a large amount of theoretical and experimental work has been carried out to modify its band gap by introducing impurity atoms into the TiO2 lattice, which have been proved as the most effective method to enhance photocatalytic activity of TiO2 in the visible light region. Therefore, we study the geometry structures, formation energies, density of states, Mulliken charge distribution and optical absorption spectra of non-metal (C, N, B) and metal (V, Cr, Mn, Fe, Co, Ni, Cu, Nb) doped anatase TiO2. We also calculate the properties of metal and non-metal compensated co-doped models.Our results indicate that, non-metal doping has slight influence on the geometric structure of TiO2,the lattice symmetry keeps well after C, N doping, but atoms around B show a serious distortion after B doping; The formation energy of C, N doped TiO2 shows less formation energy, suggesting more energetically favorable; C doping brings the better electronic structure for TiO2, especially at the doping concentration of 4.17 at%, and the band gap can reduce to 2.28eV. At the lower doping concentration, the electronic structures of doped TiO2 are related to the doping position but are not affected by doping concentration. B, N doping can induce impurity states in the mid of the forbidden gap and also reduce the band gap slightly; The optical absorption edge of C, N doped TiO2 obviously moves toward the visible light region, but the absorption edge of B doped TiO2 shifts toward the blue light region.The lattice symmetry of TiO2 is not destroyed after metal doping, but the volumes have different degrees of mild shrink; The formation energy of metal doped TiO2 has less formation energy than non-metal doped TiO2 under Ti-rich conditions; The valence band and conduction band move toward low energy after metal doping, and impurity states appear in the mid of the forbidden gap. Cr doping results in the maximum band gap reduction of 2.28eV; The optical absorption edge of Cr, V doped TiO2 obviously moves toward the visible light region, the absorption edge of Co, Ni and Nb doped TiO2 shifts to about 400nm. But the Mn, Cu and Fe doping have adverse effects on TiO2 optical properties.In the metal and non-metal compensated co-doped models, the doping effect of non-metal is more effective than metal; The formation energy of two elements co-doped TiO2 is higer than mono-doping. But if adding another non-metal in the adjacent position, the formation energy will become much lower; It is also noted that the full charge compensation occurs when the metal cation provides compensation electron, the unoccupied impurity states in the mono-doping conditions may be fully occupied or vanish away. The electronic structures of TiO2 can be improved, the band gap also can be reduced apparently. Among them, Fe2B doping results in the minimum band gap of 2.28eV; The co-doped models of CrC, NiC, Fe2B, Cr2N, NiBN with well electronic structure extend the optical absorption edge to ultraviolet light and visible light region. The optical absorption edge of NbN, VN, CoN, MnN and VNC co-doped TiO2 slightly moves toward the visible light region, but the absorption edge of CuB, NbB, CuNC and NbNC co-doped HO2 shifts toward the blue light region. The absorption coefficient declines in the visible light region, which will not enhance the visible light photocatalytic efficiency of TiO2. |
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