Rules for Selecting Metal Cocatalyst Based on Charge Transfer and Separation Efficiency between ZnO Nanoparticles and Noble Metal Cocatalyst Ag/ Au/ Pt

Loading metal cocatalyst on semiconductor nanoparticles is a general strategy to enhance photocatalytic efficiency, while the consensus of selecting metal cocatalyst matching with semiconductor is still unclear. Herein, we investigated the charge transfer and separation efficiency between ZnO and cocatalysts Ag, Au, and Pt nanoparticles respectively using ultrafast mid-IR transient absorption spectroscopy. We found that Ohmic contact of Ag with ZnO favoring electron transfer and charge separation, while Schottky junction of Pt or Au with ZnO preventing electron transfer from ZnO to metal. Thus Ohmic contact would be better than Schottky contact. Since photocatalytic efficiency is also determined by chemical catalytic efficiency, we proposed a dual metal cocatalyst strategy for improving the overall photocatalytic efficiency, with the inner metal forming Ohmic contact for efficient charge separation and shuttling electrons and the outer-layer metal cocatalyst for optimizing the chemical reactivity.