First principle calculations of the electronic and optical properties of pure and (Mo, N) co-doped anatase TiO2

Density functional theory calculations were performed in order to investigate the effect of (Mo, N) co-doping on the electronic and optical properties of anatase TiO2. Comparative theoretical study is organized using different doping models including single Mo doping, single N doping in anatase TiO2 and three different models of (Mo, N) co-doped TiO2 regarding the position of the dopants with respect to each other. Mo doping in anatase TiO2 reduced the band gap of pure TiO2 from 2.12 eV to 1.90 eV by introducing Mo 4d state below the conduction band and shifted the Fermi level from the top of the valence band to the bottom of the conduction band which verifies the n-type doping nature of Mo in TiO2. Isolated N 2p state was created above the top of the valence band due to the N doping and the band gap of N-TiO2 was effectively reduced to about 0.78 eV; however the unoccupied N 2p states annihilate the electron-hole pairs which will limit the efficiency of N-TiO2 in visible light photocatalytic activity. (Mo, N) co-doped TiO2 has narrowed band gap of about 1.50 eV and simultaneous impurity states, one is above the top of the valence band (N 2p) and the other is just below the conduction band (Mo 4d). The introduction of Mo 4d state changes the character of N 2p states from unoccupied to occupied states which will lead to removal of the electron-hole recombination center and enhance the visible light photocatalytic activity. Furthermore, optical absorption coefficient spectra results show that (Mo, N) co-doping in anatase TiO2 can have enhanced optical absorption in the visible region compared with that of N-TiO2 and Mo-TiO2, which is attributed to the reduced recombination centers. It is argued that (Mo, N) co-doped TiO2 shows enhanced visible light photocatalytic activity due to the effective utilization of electron-hole pairs in the oxidation/reduction process. (C) 2011 Elsevier B.V. All rights reserved.