Synthesis of C–N–S co-doped TiO2 mischcrystal with an isobandgap characteristic and its photocatalytic activity under visible light

Abstract

In this paper, a simple thermal decomposition synthesis route for a carbon–nitrogen–sulfur (C–N–S) co-doped titanium dioxide (TiO₂) mischcrystal with an isobandgap property for visible light applications using urea as the C, and N source and titanium(IV) sulfate [Ti(SO₄)₂] as S source is reported. The C–N–S co-doped TiO₂ mischcrystal exhibited a unique isobandgap characteristic and prominent red-shift in the ultraviolet-visible absorption spectrum and a much narrower band gap (2.42 eV and 1.39 eV) because of defects and impurity levels which were confirmed using powder X-ray diffraction, diffuse reflectance spectroscopy (DRS), photoluminescence, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The excellent photocatalytic performance of the C–N–S co-doped TiO₂ mischcrystal samples was demonstrated using degradation of methylene blue and phenol under visible light. The apparent degradation rate of black TiO₂ is 4–7 times higher than that of C–N–S co-doped TiO₂ (anatase or rutile) under visible light irradiation. The valence band XPS and DRS results were carefully examined to understand the band gap reduction of C–N–S co-doped TiO₂. This study confirms that the band gap structure of C–N–S co-doped TiO₂ is determined by the mixed crystalline structure and the formation of impurity levels and defective levels. A new idea is presented that will be useful for the design of band gap structure for photocatalysts with higher photocatalytic activities under visible light irradiation.