Study of the Nitrogen concentration influence on N-Doped TiO2 Anatase from First-Principles Calculations

The nitrogen concentration effects on the formation energies and electronic band structures of N-doped TiO2 anatase have been investigated on the basis of density functional theory (DFT) calculations. At lower doping levels, some localized N 2p states are formed above the valence band in N-doped anatase, leading to the reduction of the photon transition energy. When the doping level rises, the energy gap has little further narrowing compared with that at lower doping levels, and larger formation energies are required during the synthesis of substitutional N- to O-doped anatase. The calculations of optical properties indicate a possible optimum N-doping level in TiO2 with a high photoresponse for visible light. These conclusions are in good agreement with the recent experimental measurements. Two possible mechanisms for the red shift in N-doped TiO2 are tentatively put forward according to the doping levels, which may well explain the recent controversy of the mechanism of the red shift in N-doped TiO2.