Nitrogen-doped TiO ₂ nanopowders prepared by chemical vapor synthesis: Band structure and photocatalytic activity under visible light

During chemical vapor synthesis of TiO ₂ nanopowders, nitrogen atoms were doped into the crystal lattice of TiO ₂. The nitrogen atoms were predominantly incorporated substitutionally in the crystal lattice of TiO ₂ nanopowders up to the doping level of 1.25 mol% nitrogen, whereas they were in both interstitial and substitutional sites over about 1.43 mol% nitrogen. From the photocatalytic activity of nitrogen-doped TiO ₂ estimated by decomposition of methylene blue under visible light, it was found that the substitutional nitrogen anions appearing at the low level doping was beneficial to its photocatalytic activity, whereas the interstitial ones appearing at the high level doping over 1.25 mol% nitrogen were not. The improved photocatalytic activity due to the substitutionally doped nitrogen was attributed to band gap narrowing which was confirmed by the studies of XPS, near edge X-ray absorption fine structure, and UV-Vis absorption.