Synthesis and characterization of WO3 polymorphs: monoclinic, orthorhombic and hexagonal structures

Tungsten trioxide (WO3) is a widely studied semiconductor with a band gap ranging from 2.4 to 3.2 eV with visible light harvesting below 500 nm, which is stable in aqueous solution and has good charge transport properties. An easy methodology is reported for the synthesis of phase-pure WO3 polymorphs crystallized in the orthorhombic, hexagonal and monoclinic crystal systems. The synthesis is based on the preparation of a common precursor (tungstite hydrate) from the alcoholysis of tungsten hexachloride (WCl6), and its subsequent solvothermal treatment in water/ethanol mixtures. In pure ethanol, the precursor transforms into hexagonal WO3, while at higher water content the monoclinic phase prevails. At low water content (10 vol%) the system crystallizes as pure orthorhombic WO3. In the high water content regime, the morphology of the material with the monoclinic structure can be modulated and particles elongated along the [020] direction can be obtained.