Tungsten oxide nanowires synthesized by a catalyst-free method at low temperature

We report the synthesis of tungsten oxide W18O49 nanowires by thermal evaporation at low temperature without using catalysts. By placing tungsten powder in the high temperature zone at 650 degrees C and tungsten substrate in the low temperature zone at 400 degrees C of a tube furnace, W18O49 nanowires with diameters of 10-50 nm are produced with high yield. The nanowires extend to 500-1500 nm length, and the cross-sectional shapes are circular or polygonal. The roles of the source material, the pre-oxide layer on the substrate and the temperature of the reactor are also investigated. It is shown that the presence of oxygen on the W surface is essential for tungsten oxide nanowire growth.