Illuminated (320 < < 370 nm), aqueous suspensions of transparent quantum-sized (Q-sized) ZnO semiconductor colloids in the presence of carboxylic acids and oxygen are shown to produce steady-state concentrations of H2O2 as high as 2 mM. Maximum H2O2 concentrations are observed only with added electron donors (i.e., hole scavengers). The order of efficiency of hole scavengers is as follows: formate > oxalate > acetate > citrate. Isotopic labeling experiments with 18C>2 are consistent with the hypothesis that hydrogen peroxide is produced directly by the reduction of adsorbed oxygen by conduction band electrons. Quantum yields for H2O2 production are near 30% at low photon fluxes. However, the quantum yield is shown to vary with the inverse square root of absorbed light intensity [ < ((/abs)"1)1/2], with the wavelength of excitation, and with the diameter of the Q-sized colloids. The initial rate of H2O2 production is 100-1000 times faster with Q-sized ZnO particles (Dp = 4-5 nm) than with bulksized ZnO particles (Dp = 0.1 µ ).

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