Mechanism of higher photocatalytic activity of anatase TiO2 doped with nitrogen under visible-light irradiation from density functional theory calculation

We have systematically investigated the modification mechanism of anatase TiO2 doped with nitrogen using the plane-wave ultrasoft pseudopotentials method based on density functional theory. The crystal structure, impurity formation energy, electronic structure and optical properties of TiO2 supercells containing subsitutional nitrogen, interstitial nitrogen or oxygen vacancies were calculated. TiO2 doped with substitutional nitrogen has shallow acceptor states above the valence band. In contrast, TiO2 doped with interstitial nitrogen has isolated impurity states in the middle of the band gap. These impurity energy levels are mainly hybridized by N 2p states with O 2p states and Ti 3d states. TiO2 containing oxygen vacancy has shallow donor states below the conduction band, which consists of Ti 3d states. Except for containing oxygen vacancies, the band gaps of N-doped TiO2 are narrowed in the range 0.03-0.23 eV, and thus their fundamental absorption edges extend to the visible-light region. By nitrogen doping, the octahedral dipole moments in TiO2 increase due to the changes in lattice parameters, bond length and charges on atoms, which is very effective for the separation of photoexcited electron-hole pairs. The calculated results are consistent with the existing experimental data. Based on these results, the effects on the electronic structure and photocatalytic activity of anatase TiO2 doped with nitrogen were analysed and compared in detail, and the mechanism by which anatase TiO2 doped with nitrogen has higher photocatalytic activity than that of pure TiO2 was explained. The conclusions have important significance for understanding and further developing TiO2 photocatalysts that are active under visible-light irradiation.