Electronic structures and optical properties of Ce-doped anatase TiO<sub>2</sub> with oxygen vacancy

Abstract

<sec> The crystal structures, defect formation energy, electronic structures and optical properties of oxygen vacancy and/or Ce-(co)doped anatase TiO<sub>2</sub> are investigated by using density functional theory plus <i>U</i> calculations. </sec><sec> The calculated results indicate that lattice distortion induces the enhanced octahedral dipole moment in Ce doped TiO<sub>2</sub> crystal when introducing oxygen vacancy into the lattice of the TiO<sub>2</sub> crystal, which is effective for separating the photo-excited electron-hole pairs; meanwhile, compared with the valence band of pure TiO<sub>2</sub> and TiO<sub>2</sub> mono-doped separately with Ce and oxygen vacancy, the valence band of TiO<sub>2</sub> co-doped with Ce and oxygen vacancy broadens drastically, which is mainly contributed from the electronic states of Ce 5d, Ti 4s and O 2p in the valence band shifting toward the lower energy direction. As a result, Ce doped TiO<sub>2</sub> with oxygen vacancy is beneficial to the mobility of photo-generated carriers in TiO<sub>2</sub>. Similarly, the anti-bonding states also move toward the lower band energy direction, which are formed by the mixture of Ce 4f, Ce 5d, Ti 3d, and O 2p orbits in the conduction band. Due to these shifts, the energy gap of Ce and oxygen vacancy codoped TiO<sub>2</sub> is narrowed to 2.67 eV with the emerge of the occupied impurity energy levels near Fermi level. Because of the above-mentioned excellence features, the absorption spectra for doped systems exhibit remarkable red-shift, especially, the intensity of optical absorption of TiO<sub>2</sub> co-doped with Ce and oxygen vacancy in the visible region and the infra-red region are obviously stronger than those of the Ce mono-doped TiO<sub>2</sub>. </sec><sec> When introducing oxygen vacancy into the Ce-doped system, the calculated conduction band energy edge position changes from −0.27 eV to −0.32 eV, which implies that the reducing power of the conduction band edge of TiO<sub>2</sub> is remarkably enhanced. More fascinatingly, the calculated band energy edges for the Ce and oxygen vacancy codoped TiO<sub>2</sub> can satisfy the basic requirement for water splitting under visible light irradiation.</sec><sec> In conclusion, Ce and oxygen vacancy co-doped system can effectively strengthen the photo-catalytic activity of TiO<sub>2</sub> and improve the utilization of the solar light; and our calculated results provide a powerful theoretical basis for the applications of the Ce and oxygen vacancy co-doped anatase TiO<sub>2</sub> in visible-light-driven water splitting in the future research. </sec>