Ce和O空位共掺杂TiO_2的电子结构与光学性质

采用基于密度泛函理论加U的计算方法,研究了Ce和O空位单(共)掺杂锐钛矿相TiO_2的电子结构和光吸收性质.计算结果表明,Ce和O空位共掺杂TiO_2的带隙中出现了杂质能级,且带隙窄化为2.67 eV,明显比纯TiO_2和Ce,O空位单掺杂TiO_2的要小,因而可提高TiO_2对可见光的响应能力,使TiO_2的光吸收范围增加.光吸收谱显示,掺杂后TiO_2的光吸收边发生了显著红移;在400.0—677.1 nm的可见光区,共掺杂体系的光吸收强度显著高于纯TiO_2和Ce单掺杂TiO_2,而略低于O空位单掺杂TiO_2.此外,Ce掺杂TiO_2中引入O空位后,TiO_2的导带边从-0.27 eV变化为-0.32 eV,这表明TiO_2的导带边的还原能力得到了加强.计算结果为Ce和O空位共掺杂TiO_2在可见光光解水方面的进一步研究提供了有力的理论依据.

Electronic structures and optical properties of Ce-doped anatase TiO2 with oxygen vacancy

The crystal structures, defect formation energy, electronic structures and optical properties of oxygen vacancy and/or Ce-(co)doped anatase TiO₂ are investigated by using density functional theory plus U calculations. The calculated results indicate that lattice distortion induces the enhanced octahedral dipole moment in Ce doped TiO₂ crystal when introducing oxygen vacancy into the lattice of the TiO₂ crystal, which is effective for separating the photo-excited electron-hole pairs; meanwhile, compared with the valence band of pure TiO₂ and TiO₂ mono-doped separately with Ce and oxygen vacancy, the valence band of TiO₂ 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 TiO₂ with oxygen vacancy is beneficial to the mobility of photo-generated carriers in TiO₂. 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 TiO₂ 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 TiO₂ 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 TiO₂. 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 TiO₂ is remarkably enhanced. More fascinatingly, the calculated band energy edges for the Ce and oxygen vacancy codoped TiO₂ can satisfy the basic requirement for water splitting under visible light irradiation. In conclusion, Ce and oxygen vacancy co-doped system can effectively strengthen the photo-catalytic activity of TiO₂ 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 TiO₂ in visible-light-driven water splitting in the future research.