N-doped Titanium Dioxide, Visible Light Activity And Mechanistic Studies

TiO₂-based semiconductor photocatalysis technology has been shown to be potentially advantageous for environmental remediation as it may lead to complete mineralization of pollutants at ambient conditions with the use of ultraviolet light as the energy source. However, TiO₂ is actived only by irradiating with ultraviolet light due to its large band gap of 3.2 eV, corresponding to wavelengths shorter than 388 nm. This means that TiO₂ could make use of only a small fraction (⁵%) of the sun's energy compared to the visible region (45%). So it is important to look for a method that can shift the optical response of TiO₂ and make it be able to photocatalyze oxidation of organic in low energy light such as visible light.An effective approach to shifting the optical response of TiO₂ from the ultraviolet to the visible spectral range is the doping of TiO₂ with nonmetal nitrogen. In this paper,N-doped TiO₂ photocatalysts, which can respond to visible light, were prepared by a sol–gel procedure using ammonia as nitrogen source. The structural and photoelectric properties of such photocatalyst were characterized by XRD, DSC, IR, UV-VIS and XPS, and the mechanism of N-doped TiO₂ response to visible light was also discussed.The results showed: (1) Compared with TiO₂, The N-doped TiO₂ had obviously visible-light-activity. When the calcination temperature was 400℃, mixing velocity was quick and the value of pH was 4.6, the N-doped TiO₂ could decompose 4-chlorophenol best under visible light. (2) Calcining temperature had important effect on the crystal phase and particle size of N-doped TiO₂, the doped samples exhibited anatase structure at 400℃, and had minimum size, when temperature rised, the crystal sizes of the samples increased, and rutile phase started to appear and its relative abundance increased. (3) The doping of N made the response light energy decrease to visible light, and when the calcination temperature was 400℃, the expansion of absorbed light strength in visible area was most obvious.(4) The NH4+ species, which would be formed by the protonation of NH3, were absorbed on the surface of TiO₂ during the preparation, and were ultimately incorporated into the TiO₂ lattice after calcination. The form of nitrogen was interstitial and is probably bound to hydrogen, and it give rise to a midgap (N-2p) level slightly above the top of the (O-2p) valence band. Under visible-light illumination, the electron transfered from the the midgap band to conduction band and the photocatalytic reaction occured. (5) During N donping, oxygen vacancies maybe be brought into TiO₂ lattice, it maybe come from the reduction by H2 which was decomposed from NH3.