First-principle calculations on optical properties of C-N-doped and C-N-codoped anatase TiO2

The electronic structures, dipole moment and optical properties of C-N-doped and C-N-codoped anatase titanium dioxide (TiO₂) are studied using the plane-wave ultrasoft pseudopotential method of density functional theory (DFT). The results revealed that the absorption coefficients of pure TiO₂ and N-doped TiO₂ are consistent with experimental values in the visible-light region. The bands originating from C/N-2p states lie in the band gap of doped TiO₂. A visible-light absorption edge red-shift can be observed. The atomic charges have changed, resulting in devation of the center of gravity of the negative electric charge from the positive electric charge in the super-cell, and their dipole moment would not be zero. The dipole moment has large influence on the optical responses in the visible region of TiO₂. Because of the small distance (0.531 nm) between C and N atoms, the covalent bond component was easily enhanced between C atom and adjacent O atom, the covalent bonds making it more difficult for the carrier transfer. Moreover, its optical absorption coefficient is going to reduce in the visible-light region. Under the condition of the larger distance (0.691 nm) between C and N atoms, their interaction can be reduced, which is beneficial to electrons transition; as a result, a significant improvement of the photocatalytic activity of TiO₂ has been found under the visible-light irradiation.