Photocatalytic Oxidation of Organic Compounds and Reduction of Dioxygen with Quantum-Sized Semiconductors

Abstract

Aqueous, oxygenated suspensions of quantum-sized ZnO particles with added hole scavengers are shown to produce steady-state hydrogen peroxide concentrations as high as 2 mM upon bandgap irradiation. Quantum yields for H_2O_2 as high as 30% are observed at the lowest illumination intensities. The quantum yield is shown to follow an inverse square-root dependency on absorbed light intensity. The initial rate of H_2O_2 production is 100 to 1000 times faster with Q-sized particles (4-5 run) than with bulk ZnO particles (10-1000 μm). Maximum hydrogen peroxide concentrations are observed only with added hole scavengers. The order of efficiency of hole scavengers is: formate > oxalate > acetate > water > citrate. H_2O_2 formation results from the reaction of conduction band electrons with adsorbed oxygen. Observed reactions products other than H_2O_2 are consistent with oxidation of the hole scavenger by hydroxyl radicals which are produced by reaction of the bole with surface hydroxyl species (□-ZnOH; i.e., a trapped hole) or with water.

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