First-principles study on electronic structure and optical properties of anatase TiO2 codoped with nitrogen and iron

The crystal structure, electronic structure and optical properties of nitrogen and iron codoped anatase TiO2 were studied by using the plane-wave ultrasoft pesudopotentials method based on density functional theory. The calculated results show that the octahedral dipole moments in nitrogen and iron codoped TiO2 increase due to the changes in lattice parameters, bond length and charge of atoms, which is very effective for the separation of photoexcited electron-hole pairs and the improvement of the photocatalytic activity of TiO2. Some impurity energy levels of codoped TiO2 are below the conduction band minimum, and others are above the valence band maximum. The distance between them is narrowed, which results in the redshift of the optical absorption edges to visible-light region. These impurity energy levels can reduce the recombination rate of photoexcited carriers and improve the photocatalytic efficiency of TiO2. Compared with that of Fe doped TiO2, for the codoped TiO2, the density of states peak of impurity energy levels above the valence band maximum increase apparently, which increases the electronic transition probability from the impurity energy levels to the conduction band, and improves the solar energy utilization. If the impurity level is not taken into account, compared with that of pure TiO2, the CB edge position and the VB edge position of codoped TiO2 is only slightly changed, it means that the strong redox capacity of codoping photocatalysts is still excellent.