Origin of visible-light activity of N-doped TiO2 photocatalyst: Behaviors of N and S atoms in a wet N-doping process

Behaviors of N and S atoms in formation of N-doped TiO2 with a wet N-doping process were analyzed to investigate the origin of photocatalytic activity under visible light. In a titanium oxyhydroxide precursor obtained by mixing titanium oxysulfate and ammonia solutions, two types of NH3 species were formed. One was directly bonded to Ti4+ (coordinated NH3, located at 400.0 eV in X-ray photoelectron spectrum), which was related to photocatalytic activity on NOx removal under visible light (lambda > 405 nm). The other was NH3 in (NH4)(2)SO4 (at 401.8 eV), which was unnecessary for producing the activity. During heat-treatment at 400 degrees C, roughly half amount of the coordinated NH3 was left behind in TiO2 without changing the oxidation state of nitrogen, and the other half was released from the solid phase regardless of partial O-2 pressure. A local unusual structure of TiO2, e.g. crystal defects and distortion, formed by the coordination of NH3 to Ti4+ and/or the release of NH3 seems to be an origin of the visible-light activity of N-doped TiO2. A substitutional doping of nitrogen anions (N3-) into TiO2 was not essential for the activity. The amount of SO42- in N-doped TiO2 was decreased by washing of the precursor or heat-treating the photocatalyst in CO. The activity was significantly improved by decreasing the amount of SO42- in N-TiO2. (C) 2012 Elsevier B.V. All rights reserved.