Abstract

Tungsten(VI) oxide (WO₃) powders of monoclinic crystal structure with high crystallinity have been synthesized by hydrothermal treatment (HTT), at 523 or 573 K, of aqueous tungstic acid (H₂WO₄) solutions prepared from sodium tungstate by ion-exchange (IE) with a proton-type resin. HTT and/or calcination of home-made and commercial H₂WO₄ solids of tungstite structure also produced WO₃. These WO₃ powders with various physical properties were used as photocatalysts for evolution of oxygen (O₂) from an aqueous silver sulfate solution, and the correlation between their physical properties and photocatalytic activity was examined. WO₃ powders of high crystallinity, e.g., IE-HTT-WO₃ synthesized at 573 K, gave the highest O₂ yield, suggesting that the crystallinity of WO₃ is a key factor in its high activity for this reaction system. Calcination at <1073 K improved the crystallinity of some WO₃ powders, resulting in increases in their activities; however, calcination at a higher temperature reduced the activity, presumably due to formation of oxygen vacancies acting as recombination centers.