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.

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