Site Structure and Photocatalytic Role of Sulfur or Nitrogen-Doped Titanium Oxide with Uniform Mesopores under Visible Light

Anion doping to titanium oxide (TiO2) has been extensively studied to explore visible light-responsive photocatalysts. Among them, sulfur doping was most effective; however, the site structure and the implication to promoted photocatalysis have not been reported yet, except for valence state information based on X-ray photoelectron spectroscopy. Sulfur and/or nitrogen was doped to TiO2 with a narrow pore size distribution, at 3 nm, using thiourea or urea. They were compared to the uniform mesoporous TiO2 modified via the chemical vapor deposition (CVD) of hydrogen sulfide and the titanate/TiO2 nanotube synthesized hydrothermally using TiS2. In ethanol and O-2 under visible light (wavelengths of >370, >420, >480, >520, or >580 nm), water formation was promoted by a factor of 12, in clear contrast to the only 2.3x enhancement for acetaldehyde formation by doping sulfur. These anion-doped mesoporous TiO2 catalysts were characterized using Ti and S K-edge extended X-ray absorption fine structure for the first time. The Ti-S bonds were detected at 2.283-2.44 angstrom for sulfur-doped mesoporous TiO2 using thiourea and sulfur-doped mesoporous TiO2 via CVD, demonstrating the presence of substitutional anionic S on the O sites. The doped anionic sulfur created impurity level(s) at 0.8-2.2 eV below the conduction band (CB) minimum of mesoporous TiO2. It enabled the electron excitation to the CB and then to O-2 to convert to water under visible light. In contrast, the impurity level(s) at 0.8-2.2 eV below the CB minimum was/were not effective to (directly or indirectly) receive electrons from the substrate (ethanol/ethoxyl). Preferential water formation under visible light was also observed for TiO2 nanotubes that were synthesized hydrothermally from TiS2.