Ultrasonic spray pyrolysis fabrication of solid and hollow PbWO4 spheres with structure-directed photocatalytic activity

Solid and hollow PbWO4 spheres were fabricated by a one-step and template-free ultrasonic spray pyrolysis (USP) using lead nitrate and ammonium metatungstate hydrate as precursors. The characteristics of the resulting samples were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N2 adsorption/desorption, and UV-vis diffuse reflectance spectroscopy in detail. The addition of citric acid (CA) to the precursor solution played a crucial role in producing hollow structured spheres. The hollow PbWO4 spheres were composed of single-crystal nanoparticles. On the basis of the characterization and experimental observations, a possible mechanism on the formation of solid and hollow PbWO4 spheres was proposed. Gaseous products generated due to the decomposition of CA in the pyrolysis process shaped the final hollow morphology. The results also indicated that the CA addition could decrease the crystal size of hollow PbWO4. The specific surface areas and pore volume of hollow PbWO4 spheres were increased simultaneously due to the CA decomposition compared with solid PbWO4 spheres produced without CA addition. The PbWO4 spheres were tested as photocatalyst for NO removal in air. The hollow structured PbWO4 spheres were found to exhibit superior photocatalytic activity to solid spheres due to the differences in microstructure and morphology. The result acquired may shed light on general fabrication strategy for designing hollow structured materials by USP.