Preparation of Aqueous Sols of Tungsten Oxide Dihydrate from Sodium Tungstate by An Ion-Exchange Method

Aqueous sols of tungsten oxide dihydrate (WO₃·2H₂O) were prepared from Na₂WO₄ by an ion-exchange method. An aqueous solution of Na₂WO₄ was let to flow through a glass column packed with protonated cation-exchange resin. The effluent, initially transparent, turned into an opaque viscous fluid (pale yellow) in a few hours, before yellow precipitate deposited to completion in three days. The precipitate was a mixture of a crystalline phase of WO₃·2H₂O and an amorphous phase, and the crystalline part could be separated from another by washing with deionized water and centrifuging. The gel of WO₃·2H₂O thus obtained consisted of platelike crystallite 25 nm thick and 42 nm wide as evaluated from the X-ray diffractometer (XRD) peaks, and could be dispersed well into deionized water to form a stable suspension of colloidal particles with a mean diameter of about 30 nm. The mean particle size as well as the crystallite size tended to increase gradually with the repetition of dispersion in water under ultrasonic wave agitation and gelling by centrifuging. On heating, the gel (WO₃·2H₂O) changed to the monohydrate (WO₃·H₂O) at 100 °C, which in turn changed to the anhydride (WO₃) at 240 °C. Remarkably XRD patterns showed conspicuous preferred orientation of WO₃·2H₂O crystallites in (0 1 0) plane after the sol was centrifuged for a long time (10 h) and, upon dehydration, it was inherited by the dehydrated phases, resulting in the conspicuous orientation of WO₃ crystallites in (0 0 1).


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