Enhanced removal of various dyes from aqueous solutions by UV and simulated solar photocatalysis over TiO2/ZnO/rGO composites

Under UV and simulated solar irradiation, the photo-generated electrons in the conduction band (CB) of ZnO were rapidly transferred to TiO2 and then trapped at the reduced graphene oxide (rGO) surface. By contrast, the photo-generated holes in the valence band (VB) of TiO2 were quickly moved to the VB of ZnO and react with water molecules to generate hydroxyl radical or directly oxidize dye molecules. The separation efficiency of electron-hole pairs for the TiO2/ZnO/rGO (TZR) composites was therefore improved significantly. The formation of reactive species (e.g., h+, OH, O2−, OOH, or H2O2) can attack dye molecules to form the different intermediates and then mineralize them to simple substances (H2O, CO2, NH4+, etc.). [Display omitted] •The TiO2/ZnO/rGO (TZR) composites were prepared by one-step hydrothermal method.•A suitable rGO loading could enhance the photocatalytic activity of TZR composite.•Photocatalytic activity of TZR for various types of dyes (MB, RhB, and MO) was promising.•Photodegradation of dyes occurred by chromophore cleavage, opening-ring, and mineralization. In this study, the nanostructured TiO2/ZnO/reduced graphene oxide (TZR) composites were fabricated by a facile hydrothermal process. The morphology, fine structure, optical and photoelectrochemical properties of the prepared samples were comprehensively examined. The removal of methylene blue, rhodamine B, and methyl orange in water under UV/simulated solar illumination was then studied to evaluate the photocatalytic performance of these composites. The synergism of ternary TZR composites exhibited greater photocatalytic activity than TiO2, ZnO, and TiO2/ZnO. The highest degradation and mineralization reached 99.6% and 59.0% for methylene blue over TZR, respectively, after 120-min UV irradiation. Similarly, these efficiencies also achieved 99.2% and 48.0% for rhodamine B and 99.4% and 44.2% for methyl orange after 180-min UV-C exposure. Under simulated solar irradiation, the degradation of three dyes over TZR5 (TiO2/ZnO/5 wt% GO) was also more effective than that over P25 and TiO2/ZnO. Finally, the mechanisms and pathways for dye degradation by UV photocatalysis over TZR composite were proposed.