Charge Transfer Modulation (e−/h+) Between TiO2, ZnO, and Ag for a Superior Photocatalytic Performance

The heterostructure composites ZnO/Ag/TiO2, TiO2/ZnO/Ag, and TiO2/ZnO were synthesized in three steps using the sol–gel method. The photo-oxidation rate of formic acid (FA) was in the following order ZnO/Ag/TiO2 > TiO2/ZnO/Ag > TiO2/ZnO under ultraviolet irradiation. Rearranging the introduction of TiO2, ZnO, and Ag throughout the synthesis had important quantitative consequences particularly on the electron displacement from ZnO/Ag (electron donors) to TiO2 (electron acceptor) in order for the electron density in the TiO2 conduction band and the holes in the valence band of ZnO to increase. This was confirmed by the experimental results which showed that the ZnO/Ag/TiO2 photocatalyst had a catalytic activity twice as high as that of TiO2-P25 and higher than that of TiO2/ZnO/Ag. Two mechanisms explaining the degradation of FA on the ZnO/Ag/TiO2 and TiO2/ZnO/Ag surfaces have been proposed, involving either direct oxidation of FA with electron–holes or oxidation with hydroxyl radicals. Besides, the ZnO/Ag/TiO2 heterostructure developed in our work could provide a new platform for designing highly efficient photocatalysts.