PHOTOCATALYTIC PRODUCTION OF H2O2 AND ORGANIC PEROXIDES ON QUANTUM-SIZED SEMICONDUCTOR COLLOIDS

Illuminated (320 less-than-or-equal-to lambda less-than-or-equal-to 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 O-18(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 [PHI is-proportional-to ((I(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 (D(p) = 4-5 nm) than with bulk-sized ZnO particles (D(p) = 0.1 mum).