Ultrasonic Spray Pyrolysis Synthesis of Porous Bi₂WO₆ Microspheres and Their Visible-Light-Induced Photocatalytic Removal of NO

In this study, porous Bi₂WO₆ microsphere photocatalysts were obtained via the ultrasonic spray pyrolysis method using bismuth citrate and tungstic acid as precursors in basic aqueous solution. The characteristics of the resulting samples were investigated in detail by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption/desorption, X-ray photoelectron spectroscopy, and UV−vis diffuse reflectance spectroscopy. The resulting porous Bi₂WO₆ microsphere was of high crystallinity, which means fewer traps and stronger photocatalytic activity. The band-gap energy of Bi₂WO₆ microspheres estimated from the (αhν)² versus photon energy (hν) plots was 2.92 eV. The formation of the porous structure in the as-prepared microspheres can be ascribed to the existence of citrate anions and in situ generated carbon residues that can serve as capping agents and templates, respectively, during the synthesis processes. It was found that the synthesis temperature was an important parameter controlling the morphology of the Bi₂WO₆ microspheres. As compared with the bulk Bi₂WO₆ sample, the resulting porous Bi₂WO₆ microspheres demonstrated superior photocatalytic activities on the removal of NO under either visible light or simulated solar light irradiation. The highest NO removal rates were 110 and 27 ppb/min for the porous Bi₂WO₆ sample under solar light and visible light (λ > 400 nm) irradiation, respectively. On the basis of the analysis of the characterization and experimental observations, a possible mechanism on the formation of porous Bi₂WO₆ microspheres was also proposed.