| Among various semiconductors, TiO2 has proven to be the most suitable for environmental applications and has been widely employed to promote photocatalytic degradation of harmful organic compounds. However, due to the wide band-gap, pure anatase TiO2 photocatalyst mainly absorbs ultraviolet photons, which amounts to only 5% of the incoming solar energy on the earth's surface. Therefore, these decades, efforts have been devoted to extending the spectral response of pure TiO2 material to visible light.Using density-functional theory (DFT) calculations within the generalized gradient corrected approximation, we investigate various models of nonmetallic impurities X (=C, N, S, F) substitute for O or Ti sites in anatase TiO2. The model structures of X-doped TiO2 are constructed by using the 96-atom 2×2×2 supercell of anatase TiO2. The unit cell parameters are optimized by first-principles DFT calculations. Our work shows the doping at an O site is much more favorable under the Ti-rich condition than under the O-rich condition, while under O-rich condition it is more favorable to subsititute S for a Ti site. For C, N and S anion-doped TiO2, the dopant level is localized and lying above the top of the valence band. For the cation-doped TiO2, the mixing states in the midgap consist of dopant and parts of O 2p states. However for F-doped TiO2, F 2p states are filled and contribution to the VBs in both anion and cation doping. Compare the oxygen vacancy formation energy of all these diverse species with pure TiO2, associate to the electronic structures we confirm the presence of C4+, N3+, S6+ ions at Ti sites.
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