Optical absorption in composites containing copper core–copper-oxide shell nanostructure in a silica gel

Abstract

Copper–silica nanocomposites were synthesized by a sol–gel technique followed by a reduction treatment at 923 K for ½ h. The copper particle diameter was 6.7 nm. By subjecting these powders to an oxidation treatment at temperatures varying from 723 to 1123 K copper oxide nanoshells of thickness in the range 0.5–1.8 nm were grown on the metal particles. Optical absorption measurements were carried out on the composite particles. Absorption peaks in the range 350–500 nm were recorded. The data were analysed by the theory of scattering by a core–shell nanostructure. A particle-size dependent dielectric permittivity expression was incorporated in this analysis. The theoretical equations were fitted to experimental data using several parameters, including the electrical conductivity of the metallic core. From the analyses it was found that copper particles having diameters of ∼3 nm and less exhibited a metal to insulator transition.