Preparation and characterization of ZnO/TiO 2 for the photocatalytic reduction of Cr(VI) in aqueous solution

Abstract

ZnO/TiO 2 photocatalysts coupled with various ZnO dosages (0.5–10 mol%) were prepared by wetness impregnation method and characterized in this study. X-ray photoelectron spectroscopy results indicated that zinc ions did not enter TiO 2 lattice and was more likely to bond with oxygen atoms to form ZnO coupled on the surface of TiO 2. The coupling of ZnO on the surface of TiO 2 retarded the crystal phase transformation from anatase to rutile phase for TiO 2 calcined at temperatures higher than 500 °C. The specific surface area of ZnO/TiO 2 calcined at 700 °C was notably higher than that of pure TiO 2 possibly because the aggregation and agglomeration of particles were inhibited by the indirect contact of TiO 2 particles during the calcination. Photocatalytic reduction of hexavalent chromium, Cr(VI), in aqueous solutions using ZnO/TiO 2 was evaluated under 365 nm UV light illumination. ZnO/TiO 2 containing 2.0 mol% ZnO exhibited the lowest photoluminescence emission intensity and the maximum photocatalytic reduction of Cr(VI) in aqueous solutions due to the enhancement of charge separation by interparticle electron transfer. However, excessive ZnO on the surface of TiO 2 may serve as recombination centers of electron–hole pairs to decrease the photocatalytic activity of ZnO/TiO 2. Moreover, the decreased photocatalytic reduction of Cr(VI) using ZnO/TiO 2 calcined at higher temperatures was assumed to be attributed to the decrease of specific surface areas.