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.