Influence of electronic structure on visible light photocatalytic activity of nitrogen-doped TiO2

Nitrogen (N)-doped TiO2 thin films were synthesized by sol–gel methods from precursor solutions with or without urea and post calcination in NH3 gas. Their structural and electronic properties were characterized by X-ray diffraction and X-ray photoelectron spectroscopy utilizing synchrotron radiation. N-doped TiO2 powders were synthesized for the estimation of visible light photocatalytic activities. N-doped TiO2 thin films revealed polycrystalline anatase phases. N was chiefly doped into substitutional sites. The densities of N and defects (oxygen vacancies and reduced Ti species) increased with elevating calcination temperature. Localized states associated with doped N were successfully found from shoulder structures of valence band spectra, which were located at 0.24 eV to 0.34 eV above the valence band maximum in the band gap. Incorporated N enhanced the photocatalytic activity, whereas defects reduced it. The highest photocatalytic activity was obtained by synthesizing N-doped TiO2 powders from a precursor solution with urea and subsequent calcination at 550 °C due to high-density N and low-density defects. Consequently, the optimum N/O atomic ratio was shown to be approximately 0.06. It became possible to achieve heavier N doping and better photocatalytic activity under vis light irradiation using urea than any other study only using NH3 gas for nitridation.