Flower-shaped tungsten oxide with inorganic fullerene-like structure : Synthesis and characterization

Relatively thick (1.2 μm), novel, flower-like nanostructured tungsten oxide thin films are obtained by electrochemically anodizing tungsten foil in a fluoride containing acidified electrolyte solution. X-ray diffraction analysis reveals the presence of monoclinic hydrated tungstite (WO₃· 2H₂O) in the as-prepared samples, while films annealed at 400 °C for 4 h contain predominantly orthorhombic WO₃ phase. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and UV-vis spectroscopy are utilized to determine the surface morphology, crystal structure, and optical properties of these WO₃ films. An inorganic fullerene-like WO₃·2H₂O structure is observed, with the water molecules acting as the coordination solvent and allowing crystallographically specific growth of crystallized WO₃·2H₂O through oriented attachment. We propose here that the formation of the flower-like structured hydrated tungstite film occurs through an anodization/precipitation-recrystallization process. The photocurrent measurements under visible light illumination show that the flower-like structure with enhanced surface area exhibits a higher photocurrent density of 0.28 mA cm^-2, compared to a photocurrent density of 0.16 mA cm^-2 obtained for the mesoporous structure. The possibility of controlling and adjusting the morphology by tuning the anodization conditions makes the technique an interesting candidate for fabricating photodevices over a large substrate area. © 2010 American Chemical Society.