US Patent 7591984 Preparation of tungsten oxide

Tungsten trioxide hydrate (WO3.H2O) was prepared from a precursor solution of ammonium paratungstate in concentrated aqueous hydrochloric acid. The precursor solution was rapidly added to water, resulting in the crash precipitation of a yellow white powder identified as WO3.H2O nanosized platelets by x-ray diffraction and scanning electron microscopy. Annealing of the powder at 200° C. provided cubic phase WO3 nanopowder, and at 400° C. provided WO3 nanopowder as a mixture of monoclinic and orthorhombic phases.