Ethylene glycol-mediated synthesis of metal oxide nanowires

A simple and convenient method has been demonstrated for large-scale synthesis of metal oxide (including TiO2, SnO2, In2O3, and PbO) nanowires with diameters around 50 nm and lengths up to 30 µm. In a typical procedure, tetraalkoxyltitanium, Ti(OR)4 (with R = –C2H5, –iso-C3H7, or –n-C4H9), was added to ethylene glycol and heated to 170 °C for 2 h under vigorous stirring. The alkoxide was transformed into a chain-like, glycolate complex that subsequently crystallized into uniform nanowires. Similarly, nanowires made of tin glycolate were synthesized by refluxing SnC2O4·2H2O in ethylene glycol at 195 °C for 2 h, and nanowires consisting of indium and lead glycolates were prepared by adding In(OOCC7H15)(OiPr)2 and Pb(CH3COO)2 to ethylene glycol, followed by heating at 170 °C for 2 h. The nanowires could be readily collected as precipitates after the reaction solutions had been cooled down to room temperature. By calcining at elevated temperatures, each glycolate precursor could be transformed into the corresponding metal oxide without changing the wire-like morphology. Electron microscopic and XRD powder diffraction studies were used to characterize the morphology, crystallinity, and structure of these nanowires before and after calcination at various temperatures. A plausible mechanism was also proposed to account for the one-dimensional growth of such nanostructures in a highly isotropic medium. This mechanism was supported by XRD, FT-IR, solid state 13C-NMR, and TGA measurements. As a demonstration of potential applications, the polycrystalline nanowires made of SnO2 were used as functional components to fabricate sensors that could detect combustible gases (CO and H2) with greatly enhanced sensitivity under ambient conditions.