Photocatalytic Production of H_2O_2 and Organic Peroxides on Quantum-Sized Semiconductor Colloids

Abstract

Illuminated (320 ≤ X ≤ 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 H_2O_2 as high as 2 mM. Maximum H_2O_2 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 ^(18)O_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 H_2O_2 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 [Φ ∝ ((I_(abs))^(-1)½)], with the wavelength of excitation, and with the diameter of the Q-sized colloids. The initial rate of H_2O_2 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 µm).