Bundled tungsten oxide nanowires under thermal processing

Ultra-thin W18O49 nanowires were initially obtained by a simple solvothermal method using tungsten chloride and cyclohexanol as precursors. Thermal processing of the resulting bundled nanowires has been carried out in air in a tube furnace. The morphology and phase transformation behavior of the as-synthesized nanowires as a function of annealing temperature have been characterized by x-ray diffraction and electron microscopy. The nanostructured bundles underwent a series of morphological evolution with increased annealing temperature, becoming straighter, larger in diameter, and smaller in aspect ratio, eventually becoming irregular particles with size up to 5 μm. At 500 deg. C, the monoclinic W18O49 was completely transformed to monoclinic WO3 phase, which remains stable at high processing temperature. After thermal processing at 400 deg. C and 450 deg. C, the specific surface areas of the resulting nanowires dropped to 110 m2 g-1 and 66 m2 g-1 respectively, compared with that of 151 m2 g-1 for the as-prepared sample. This study may shed light on the understanding of the geometrical and structural evolution occurring in nanowires whose working environment may involve severe temperature variations