Origin of the visible-light photoactivity of NH_3-treated TiO_2: Effect of nitrogen doping and oxygen vacancies

摘要

N-doped and oxygen-deficient TiO_2 photocatalysts were obtained by heating commercial TiO_2 in NH_3 atmosphere, followed by a postcalcination process. Catalysts were characterized by X-ray diffraction (XRD), N_2-sorption BET surface area, X-ray photoelectron spectroscopy (XPS), Elemental analysis (EA), UV/vis diffuse reflectance spectroscopy (DRS), Electron spin resonance (ESR) and Photoluminescence (PL). It shows that the NH_3-heat-treatment of TiO_2 resulted in not only nitrogen doping but also creation of oxygen vacancies with optical absorption in visible-light region. The postcalcination achieved several beneficial effects including dramatic removal of surface amino species, a rapid decrease in surface Ti~(3+) species, and a low recombination rate of photogenerated carriers on the co-doped TiO_2. The photocatalytic measurement was carried out by the degradation of gas-phase benzene under visible light irradiation. At steady state, the photocatalytic conversion rate of benzene over the postannealed catalyst was 35.8%, accompanied by the yield of 115 ppmv CO_2, which was much higher than that on the NH_3 -treated TiO_2 before postcalcination or the H_2-treated TiO_2 catalysts. Results show that the visible-light activity of the NH3-treated TiO_2 is attributed to a synergistic effect of substitutional nitrogen species and oxygen vacancies in TiO_2.