Enhanced visible-light activity of F-N co-doped TiO2 nanocrystals via nonmetal impurity, Ti3+ ions and oxygen vacancies

A visible light active F-N co-doped TiO2 photocatalysts with varying molar ratio of TiO2/NH4F were successfully prepared using an effective yet simple ethanol–water solvothermal method. The photocatalytic activities of the prepared catalysts were tested for the degradation of methylene blue (MB) under visible light irradiation. Superior photocatalytic activity relative to that of undoped TiO2 was observed on the F-N co-doped catalyst with TiO2:NH4F molar ratio of 1.0. Based on the characterization results including FT-IR, XRD, Raman analysis, UV–vis DRS, XPS and ESR, the F and N dopants were effectively incorporated into the anatase TiO2 lattice, and resulted in a decrease in the energy gap, especially for the TONF-2 sample, which shows two optical absorption edge at 2.32 eV and 2.99 eV, respectively. The ESR data showed that the presence of fluorine and nitrogen in the lattice induces the formation of reduced Ti3+ centers and oxygen vacancies. In addition, the surface O2radical dot− radicals were also detected by ESR analysis, which together with radical dotOH radicals are responsible for the MB solution decoloration under visible light. According to the experimental results, it is believed that the enhanced visible light activity arises from the synergistic effect of F-N co-doping, the presence of Ti3+ centers and oxygen vacancies, and thus possible mechanism for both visible light absorption and charge separation were proposed.