The structure and photocatalytic studies of n-doped TiO2 films prepared by radio frequency reactive magnetron sputtering

N-doped TiO2 films were prepared by a radio frequency reactive magnetron sputtering (RF-MS) deposition method from an undoped TiO2 target in a mixture of Ar/N2 atmosphere on heated quartz glass substrates. The structures and properties of the N-doped were studied by XRD, Raman, XPS, TEM, ultraviolet (UV)-vis and PL spectroscopy. By analyzing the structures and photocatalytic activities of undoped and N-doped TiO2 films under ultraviolet and visible light irradiation, the probable photocatalytic mechanism of N-doped TiO2 films was investigated. Because many oxygen defects are caused in films by nitrogen doping, it is presumed that nitrogen doping and oxygen defect induced the formation of new states closed to the valence band and conduction band, respectively. The cooperation of nitrogen and oxygen defects leads to a significant narrowing of the band gap and greatly improves the absorption in the visible light region. It is found that the degradation efficiencies of N-doped TiO2 films greatly decreased under ultraviolet irradiation, but slowly improved under visible light irradiation, compared with the undoped TiO2 film. It is suggested that the N-doped TiO2 films are formed for the nitrogen to occupy oxygen defect sites directly. The doped nitrogen ions and oxygen defects act as recombination centers that reduce the lifetime of photo-induced electrons and holes, thereby resulting in the decrease of photocatalytic activity under ultraviolet light illumination.