Preparation of tungsten oxide nanowires and their application to NO2 sensing

Abstract

Tungsten oxide nanowires were prepared by a vapor transport method using WO3 powder as a raw material. The crystal structure and morphology of WO3 nanowires were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The obtained nanowires were hexagonal WO3. The major factors that influenced the morphology were the furnace temperature and the substrate position. The diameter of the nanowires decreased as the distance of the substrate from the raw material increased. Sensors were fabricated by pouring a few drops of nanowire-suspended ethanol onto oxidized Silicon substrates equipped with a pair of interdigitated Pt electrodes. The sensor made of the nanowires as thin as 50nm showed the highest response to NO2 at a low operating temperature of 100°C. The temperature dependence of the response was discussed in relation to the formation of NO2− and NO3− ions on the surface of WO3. The response slightly increased with decreasing diameter if the nanowires are regional depleted in NO2, while it largely increased if the nanowires are in volume depletion. A theoretical calculations based on assumptions were proposed in order to clarify the correlation between the nanowire response and their diameter.