Synthesis of tungsten oxide thin film by liquid phase deposition

High purity and well crystallized tungsten acid hydrates (H2WO4·H2O) thin films were prepared from H2WO4–HF(aq.) and H3BO3 as precursors by the liquid phase deposition method. The crystal structure was indexed as monoclinic with unit cell lattice constants a = 7.517 Å, b = 6.907 Å, c = 3.694 Å and β = 89.58°. The monoclinic phase was transformed into orthorhombic WO3·H2O after heating at 100 °C. Further heating from 300 to 500 °C resulted in an anhydrous monoclinic WO3 films. The effects of the composition and the reaction time on the deposition and the microstructures of the deposited films were studied by the means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The film formation showed strong dependence upon the composition, whereas the amount of deposition, the shape and the films thickness could be controlled by the reaction time. Cross-sectional TEM image of WO3 film deposited on Au wire indicated that the epitaxial growth of the film was maintained after calcination at 500 °C. XPS analysis also revealed the existence of W6+ ions in both the deposited and calcined films.