Preparation and characterization of ZnO/TiO2 for the photocatalytic reduction of Cr(VI) in aqueous solution

ZnO/TiO2 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 TiO2 lattice and was more likely to bond with oxygen atoms to form ZnO coupled on the surface of TiO2. The coupling of ZnO on the surface of TiO2 retarded the crystal phase transformation from anatase to rutile phase for TiO2 calcined at temperatures higher than 500 °C. The specific surface area of ZnO/TiO2 calcined at 700 °C was notably higher than that of pure TiO2 possibly because the aggregation and agglomeration of particles were inhibited by the indirect contact of TiO2 particles during the calcination. Photocatalytic reduction of hexavalent chromium, Cr(VI), in aqueous solutions using ZnO/TiO2 was evaluated under 365 nm UV light illumination. ZnO/TiO2 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 TiO2 may serve as recombination centers of electron–hole pairs to decrease the photocatalytic activity of ZnO/TiO2. Moreover, the decreased photocatalytic reduction of Cr(VI) using ZnO/TiO2 calcined at higher temperatures was assumed to be attributed to the decrease of specific surface areas.

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► ZnO/TiO2 is prepared for the photocatalytic reduction of hexavalent chromium.
► The coupled ZnO retards the TiO2 crystal phase transformation from anatase to rutile.
► The aggregation of ZnO/TiO2 particles is inhibited during calcinations.
► ZnO/TiO2 shows the higher activity for Cr(VI) reduction as compared to pure TiO2.
► 2.0 mol% ZnO/TiO2 exhibits the maximum photocatalytic reduction of Cr(VI).