Photochemical synthesis and optical properties of binary and ternary metal-semiconductor composites based on zinc oxide nanoparticles

Photochemical properties of zinc oxide nanoparticles and their photocatalytic activity in the processes of the reduction of Cu2+ and Pb2+ ions as well as mixtures of Cu2+-Ag+ and Cd2+-Ag+ ions have been investigated. Optical properties of ZnO/Cu, ZnO/Pb, ZnO/Ag/Cu, ZnO/Ag/Cd and ZnO/Ag/Zn nanocomposites have been examined. It has been found that a decrease in the size of ZnO nanoparticles from 6.0 nm to 4.4 nm results in the strengthening of quantum confinement of photogenerated charge carriers, growth of the band gap and the potentials of the conduction band and the valence band of the semiconductor. It has been found that ZnO nanoparticles smaller than 2R = 6.0 nm photocorrode with the formation of metallic zinc when the colloidal solution is irradiated with the mild UV light. An increase in the potential of the conduction band electrons and the ability of ZnO nanoparticles to accumulate excessive negative charge have been supposed to be the principle reasons of advanced photochemical activity of ZnO nanoparticles with 2R < 6.0 nm. Mie theory has been applied to analyze the parameters of surface plasmon resonance bands of metallic copper in ZnO/Cu nanocomposites as well as to estimate an average size of Cu-0 nanoparticles formed on the surface of the semiconductor. It has been found that Cu(II) photoreduction rate increases substantially when Ag+ ions are present in the irradiated systems. Photocatalytic reactions in these systems result in the formation of ternary bimetallic ZnO/Ag/Cu nanocomposites. It has been shown that Cd(II) ions, which are stable towards photoreduction on the surface of ZnO nanoparticles, can be co-reduced with Ag+ cations giving ternary ZnO/Ag/Cd nanocomposites with peculiar optical properties. Correlations between the conditions of co-reduction of metal cations and the parameters of partially disturbed silver plasmon bands in the optical spectra of colloidal ZnO/Ag/Cd and ZnO/Ag/Zn have been established. (c) 2005 Elsevier B.V. All rights reserved.