Sol–gel auto-combustion synthesis of samarium-doped TiO2 nanoparticles and their photocatalytic activity under visible light irradiation

High photocatalytic activity of Sm3+-doped TiO2 nanocrystalline under visible light has been successfully prepared by sol–gel auto-combustion technique. The samples were characterized by X-ray diffraction, UV–vis diffuse reflectance spectroscopy and photoluminescence (PL) spectra. UV–vis diffuse reflectance spectra showed a slight shift to longer wavelengths and an extension of the absorption in the visible region for almost all the samarium-doped samples, compared to the non-doped sample. The results of photocatalytic decomposition of methylene blue (MB) over 0.5 mol% Sm3+-TiO2 prepared at various calcinations temperatures show that the sample calcinated at 600 °C consists of mixed phases with 51.61% rutile shows the highest photocatalytic activity, which suggests the existence of a synergistic effect between anatase and rutile powders under visible light. Doping with the samarium ions significantly enhanced the overall photocatalytic activity for MB degradation under visible light irradiation because the larger specific surface area and the higher separation efficiency of electron–hole pairs were obtained simultaneously for Sm3+-doped TiO2 nanocrystalline. It was found that there were certain relationships between PL spectra and photocatalytic activity, namely, the stronger the PL intensity, the larger the content of oxygen vacancies and defects, the higher the photocatalytic activity. Therefore, in this study, 0.5 mol% may be the most suitable content of Sm3+ in the titania, at which the recombination of photoinduced electrons and holes could be effectively inhibited and thereby the highest photocatalytic activity is formed.