Visible light active N-doped TiO2 prepared from different precursors: Origin of the visible light absorption and photoactivity

Three kinds of precursors, nanotubular titanic acid (denoted as NTA), raw P25-TiO2 and novel-TiO2, were separately used to prepare visible light active N-doped TiO2 samples by annealing in flowing NH3, aiming to reveal the determinative factors on visible light response. The physicochemical properties of resultant N-doped TiO2 samples were investigated by means of X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectra (DRS), and electron spin resonance (ESR). The visible light photocatalytic activity of the three kinds of N-doped samples was compared by monitoring the photocatalytic oxidation of propylene. It was found that N-doped TiO2 catalyst obtained by using NTA as the precursor possessed the highest photocatalytic activity. The absorption edge observed in the visible spectral region of 2.34-2.53 eV is closely related with single-electron-trapped oxygen vacancy (denoted as SETOV, i.e., F+ color centers), while another absorption edge at 2.95-3.10 is assigned to the intrinsic absorption. The visible light sensitization of N-doped TiO2 was due to the formation of SETOV in NH3-treatment process, and doped-N played a role in preventing photogenerated electrons and holes from recombination, resulting in visible light photocatalytic activity. N-doped TiO2 samples made from different precursors had different concentrations of SETOV and hence different visible light photocatalytic performance. (C) 2011 Elsevier B.V. All rights reserved.