It was recalled that, although numerous results were available concerning the origin of the visible-light photocatalytic activity of N-doped TiO2, it remained unclear how nitrogen-doping affected the visible light photocatalytic activity of TiO2. Thus N-doped TiO2 was prepared by heat treatment of commercial P25-TiO2 in flowing NH3, aiming at revealing the origin of visible light sensitization of N-doped TiO2. The resulting N-doped TiO2 was characterized by means of electron spin resonance, X-ray photo-electron spectroscopy, diffusion reflectance spectrometry and X-ray diffraction. Results indicated that N-doped TiO2 showed triplet g value electron spin resonance signals (g = 1.987, 2.004 and 2.024), which was attributed to a single-electron-trapped oxygen vacancy in a certain chemical environment. Its visible light photocatalytic activity was proportional to the intensity of the triplet g value signals, which implied that the visible light photocatalytic activity of N-doped TiO2 was closely correlated to the formation of a single-electron-trapped oxygen vacancy during heat treatment in flowing NH3. Besides, N-doped TiO2 catalyst calcinated at 600C possesses the highest photocatalytic activity, but that calcined at 700C had drastically decreased photocatalytic activity and showed no X-ray photo-electron spectroscopy signal for nitrogen. Moreover, N-doped TiO2 showed visible light absorption in a wavelength range of 400–520nm, which was attributed to the formation of a single-electron-trapped oxygen vacancy and a phase transformation from anatase to rutile. It was suggested that the visible light photocatalytic activity of N-doped TiO2 was co-determined by the formation of a single-electron-trapped oxygen vacancy in TiO2 matrix and existence of doped-N on the surface. In other words, in the absence of either a single-electron-trapped oxygen vacancy in TiO2 matrix or doped-nitrogen on the surface, N-doped TiO2 will not show visible light photocatalytic activity; and the higher the single-electron-trapped oxygen vacancy concentration, the better the visible light photocatalytic activity will be.

Enhanced Visible Light Photocatalytic Activity of N-Doped TiO2 in Relation to Single-Electron-Trapped Oxygen Vacancy and Doped-Nitrogen. Y.Wang, C.Feng, M.Zhang, J.Yang, Z.Zhang: Applied Catalysis B, 2010, 100[1-2], 84-90