Electrical properties of composites with tin-tin oxide core-shell nanostructure and their sensing behaviour

Abstract

Metallic tin having a dendritic structure was grown within a silica-based gel by an electrodeposition technique. The dendrites were shown to consist of nanoparticles with a median diameter of 31 nm. By giving a controlled oxidation treatment to the composite, a tin core-tin oxide shell nanostructure was induced. The shell thicknesses varied from 1.5 to 4.0 nm depending on the duration of oxidation. The electrical conductivity of the composite structure was shown to be controlled by a small polaron hopping conduction. The nanoshell comprised SnO and SnO₂ phases, respectively, and electrical conduction is believed to arise due to the hopping of electrons between Sn²⁺ and Sn⁴⁺ ions at the interfaces of these two regions. The composite films exhibited a three order of magnitude decrease in resistivity as the relative humidity was varied from 35% to 95%. Electrons from hydroxyl groups of adsorbed water molecules are believed to form localized states at the tin ion sites which cause an increase in electrical conductivity. The nanocomposites also exhibit substantial resistivity changes when exposed to CO, C₂H₅OH and NO₂ gases, respectively. The effect is believed to result from reduction or oxidation of the tin ions by the gas molecules concerned.