Microwave assisted synthesis of high-surface area WO3 particles decorated with mosaic patterns via hydrochloric acid treatment of Bi2W2O9

Abstract

Monoclinic WO₃ particles with mosaic structures on the planes of the particles were synthesized from layered bismuth tungstate (Bi₂W₂O₉) with the alternate stacked structure of Bi₂O₂²⁺ layers and W₂O₇²⁻ layers via a hydrothermal technique using hydrochloric acid at 200 °C under microwave heating. These particles possessed high surface areas, giving a high photocatalytic activity in the degradation of gaseous acetaldehyde. Sequential SEM observations have clarified the dynamic transformations of the structures of Bi₂W₂O₉ under microwave heating in comparison with conventional heating. The WO₃ production through the reaction of Bi₂W₂O₉ with HCl consists of two reaction steps, i.e., H₂W₂O₇ generation via replacement of Bi₂O₂²⁺ with H⁺ (the first step) and conversion of H₂W₂O₇ to WO₃ through dehydration of H₂W₂O₇ (the second step). The first step proceeds even at room temperature, while the second reaction requires temperatures above 180 °C. To investigate the microwave heating effect on the first step (the replacement of Bi₂O₂²⁺), the reaction of Bi₂W₂O₉ and HCl was carried out at 80 °C under both microwave heating and conventional heating. It has been found that the replacement of Bi₂O₂²⁺ with H⁺ is accelerated by the microwave selective heating effect. Interestingly, the WO₃ particles with mosaic patterns were produced only under microwave heating. On the other hand, conventional heating of Bi₂W₂O₉ in the presence of HCl resulted in the formation of plate-like WO₃ particles without mosaic patterns.