Stable visible-light photocatalytic degradation of organic pollutant by silver salt of Ti-substituted Keggin-type polyoxotungstate

Abstract Nano-sized heterogeneous polyoxometalate-based photocatalyst, Ag 4 H 3 PTi 2 W 10 O 40 , was synthesized in aqueous solution at room temperature. As-prepared samples showed high photocatalytic activity in the degradation of methyl orange (MO) dye aqueous solution under visible-light irradiation ( λ  > 400 nm), about 2.5 and 3.8 times faster than Ag 3 PW 12 O 40 and N-doped TiO 2 , respectively. Remarkably, it exhibited extraordinary stability in repeated photocatalytic process, which is unusual for Ag salts because they are usually unstable under light irradiation. We attributed this mainly to the Ti 4+ ion substitution. In brief, the incorporation of Ti 4+ ions produces a new energy level (composed of Ti 3 d state) in the oxygen-to-tungsten charge transfer energy gap, acting as electron trap and active center. In this way, photo-excited electrons are captured by Ti 4+ ions and then immediately react with H 2 O 2 , not only decreasing the recombination of photo-generated electrons and holes, but also inhibiting the reduction of Ag + into Ag atom and thus enhancing the photocatalytic activity and stability.