Solvothermal synthesis and characterization of tungsten oxides with controllable morphology and crystal phase

Tungsten oxides with various morphologies and crystal phases were synthesized by solvothermal reactions at 200 °C for 7-12 h using different solvents. The morphology and crystal phase of tungsten oxides changed depending on the solvents, i.e., spherical particles of ca. 1 μm in diameter consisting of nanowires, spindle shaped bundles of ca. 1 μm in length consisting of nanowires and accumulations consisting of micrometer sized plates and/or rods of tungsten oxides were obtained using ethanol, 1-propanol and water-ethanol mixed solution, respectively. When water-ethanol mixed solution was used, the crystallinity of tungsten oxide increased but the specific surface area greatly decreased. Crystallinity of tungsten oxides had more important effects on the NO degradation under light irradiation. The product using 42.9 vol.% water-ethanol mixed solvent consisted of the mixture of anhydrous tungsten oxide and hydrous tungsten oxide with preferential orientation of (0 0 2) plane and small band gap energy (2.43 eV), and showed higher photocatalytic degradation of NO even though it had a much smaller specific surface area than those prepared using ethanol and 1-propanol.