Carbon-doped TiO₂ photocatalyst synthesized without using an external carbon precursor and the visible light activity

Carbon-doped TiO₂ (C-TiO₂) was successfully prepared from a conventional sol-gel synthesis without using external carbon precursors whereas all the previous reports on the synthesis of C-TiO₂ utilized them. The carbons contained in titanium alkoxide precursor could be incorporated into the lattice of TiO₂ with creating mid-bandgap electronic states through the controlled calcination. The level of carbon doping was changed sensitively depending on the calcination temperature, which was verified by UV-visible diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The charge pair generation/recombination in C-TiO₂ was compared under visible and UV light through a time-resolved diffuse reflectance spectroscopy study, which suggested the presence of midgap energy levels induced by the incorporation of carbon dopants. The carbon doping was maximal when the calcination temperature was around 200-250 °C and hindered at higher temperatures. The visible light activities of the prepared TiO₂ samples for the conversion of 4-chlorophenol and iodide were also strongly dependent on the calcination temperature and maximized at around 250 °C. This study implies that the carbon doping in TiO₂ can be obtained even unintentionally in the conventional sol-gel synthesis.