Au nanoparticle (NP)-enhanced activity of a semiconductor in ultraviolet (UV) photocatalysis is generally observed. However, the photoinduced charge transfer behavior and the beneficial role of Au NPs in promoting photocatalytic reactions remain controversial. In the present work, the surface potentials (SPs) of Au NP-nonpolar ZnO composites in the dark and under UV irradiation were measured using Kelvin probe force microscopy (KPFM). On the basis of the KPFM results, the surface photovoltages (SPVs) of Au NP-ZnO composites were obtained by calculating the difference between the SP values acquired under UV irradiation and in the dark. Three-dimensional band diagrams of the Au NP-ZnO photocatalysts after equilibrium in the dark and under UV irradiation were thus constructed. Accordingly, charge transfer between three fundamental interfaces, namely Au NP/ZnO, ZnO/solution, and Au NP/solution, in the tested photocatalytic system is clearly described. With the positive SPV values of photocatalysts, the excess holes in the photocatalyst under steady-state UV irradiation are likely the major contributor in the present work. Furthermore, the SPV values of the Au NP-ZnO photocatalysts, as an indication of average excess carrier concentration, show a systematic correlation with photocatalytic activity. This result suggests that the SPV value of a photocatalyst could be a reasonable index for the evaluation of photocatalytic activity.
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