Analysis of electronic structures of 3d transition metal-doped TiO₂ based on band calculations

The electronic structures of titanium dioxide (TiO₂) doped with 3d transition metals (V, Cr, Mn, Fe, Co and Ni) have been analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized-augmented-plane-wave method. When TiO₂ is doped with V, Cr, Mn, Fe, or Co, an electron occupied level occurs and the electrons are localized around each dopant. As the atomic number of the dopant increases the localized level shifts to lower energy. The energy of the localized level due to Co is sufficiently low to lie at the top of the valence band while the other metals produce midgap states. In contrast, the electrons from the Ni dopant are somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. Based on a comparison with the absorption and photoconductivity data previously reported, we show that the t_2g state of the dopant plays a significant role in the photoresponse of TiO₂ under visible light irradiation.