Effect of Different Anions Upon the WO₃ Morphology and Structure.

In this study the effects of various anions (SO₄, ClO₄ and PO₄) were investigated on the hydrothermal treatment of WO₃ from Na₂WO₄ and HCl at 180 and 200 °C. The products were analyzed by XRD and SEM. With the usage of SO₄ the obtained product was hexagonal (h-) WO₃ in the form of nanorods at both temperatures. Applying ClO₄ resulted in a mixture of WO₃·0.33H₂O and small amount of m-WO₃ at 180 °C and pure WO₃·0.33H₂O at 200 °C. The morphology was consisted of cuboid shapes arranged into spherical structures at 180 °C and longitudinal ones at 200 °C. By the application of PO₄ no product formed at either temperature. Using the combination of SO₄, and ClO₄ the product was h-WO₃ at both 180 and 200 °C with rod-like crystals; thus, the effect of ClO₄ was overdominated by the SO₄ions. Utilization of PO₄ together with SO₄, and/or ClO₄ resulted again in no product, meaning that adding PO₄ to the reaction mixture completely blocks the hydrothermal formation of solid products by forming water soluble phosphotungstic acids.