Development and Sensitization of N- or S-Doped TiO₂ Photocatalysts

Abstract

Titanium dioxide photocatalysts are promising substrates for photodegradation of pollutants in water and air, but show photocatalytic activities only under UV light. To utilize a wider range of incident wavelengths such as solar light, development of photocatalysts active under visible light is very important. Chemically modified titanium dioxide photocatalysts containing anatase phase with S (S⁴⁺) substituted for some lattice Ti atoms or N substituted for some lattice O atoms were prepared. In addition, S, C-co-doped TiO₂ having rutile phase were also prepared. These catalysts showed strong absorption of visible light and high activities for degradation of 2-propanol in aqueous solution, partial oxidation of adamantine, and 2-methylpyridine under irradiation at wavelengths longer than 440 nm. The oxidation states of the S, C, and N atoms incorporated into the TiO₂ particles were determined to be mainly S⁴⁺, C⁴⁺, and N³⁻ from XPS spectra, respectively. The photocatalytic activities of S- or N-doped TiO₂ photocatalysts with adsorbed Fe³⁺ ions were markedly improved for oxidation of 2-propanol compared to those of S- or N-doped TiO₂ without Fe³⁺ ions under a wide range of incident wavelengths, including UV light and visible light. The photocatalytic activity reached maximum with 0.90 wt% Fe³⁺ ions adsorbed on S-doped TiO₂, and 0.36 wt% Fe³⁺ ions on N-doped TiO₂. Furthermore, redox treatment of S- or N-doped TiO₂ photocatalysts with adsorbed Fe³⁺ ions by reduction with NaBH₄ followed by air oxidation resulted in further improvements in photocatalytic activities. In this case, the optimum amounts of Fe³⁺ were 2.81 and 0.88 wt% on the surfaces of S- and N-doped TiO₂ photocatalysts, respectively.