Effects of samarium dopant on photocatalytic activity of TiO₂ nanocrystallite for methylene blue degradation

Sm³⁺-doped TiO₂ nanocrystalline was synthesized by a sol–gel auto-combustion method and characterized by X-ray diffraction, Brunauer-Emmett-Teller method (BET), UV–vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Sm³⁺–TiO₂ catalyst was evaluated by measuring degradation rates of methylene blue (MB) under either UV or visible light. The results showed that doping with the samarium ions significantly enhanced the photocatalytic activity for MB degradation under UV or visible light irradiation. This was ascribed to the fact that a small amount of samarium dopant simultaneously increased MB adsorption capacity and separation efficiency of electron-hole pairs. The results of DRS showed that Sm³⁺-doped TiO₂ had significant absorption between 400 nm and 500 nm, which increased with the increase of samarium ion content. The adsorption experimental demonstrated that Sm³⁺–TiO₂ had a higher MB adsorption capacity than undoped TiO₂ and adsorption capacity of MB increased with the increase of samarium ion content. It is found that the stronger the PL intensity, the higher the photocatalytic activity. This could be explained by the points that PL spectra mainly resulted from surface oxygen vacancies and defects during the process of PL, while surface oxygen vacancies and defects could be favorable in capturing the photoinduced electrons during the process of photocatalytic reactions, so that the recombination of photoinduced electrons and holes could be effectively inhibited.