BiVO 4 powders with unique particle architectures have been synthesized using ultrasonic spray pyrolysis (USP). Gases created from the evaporation of solvent and the decomposition of precursor materials shape the morphology of the particles as the solids are formed in the heated aerosol. The BiVO 4 powder was tested as oxygen evolving photocatalysts by monitoring the kinetics of O 2 formation from a AgNO 3 solution irradiated with λ > 400 nm light. USP prepared BiVO 4 was found to have significantly superior photocatalytic activity compared to commercial BiVO 4 and WO 3 , likely due to differences in particle morphology.

Zinc sulfide (ZnS) has found diverse applications as optical phosphors, catalysts, photonic crystals, and light-emitting materials.[1] In addition, ZnS has been examined as a photocatalyst, in part because of its high energy conversion efficiency and the relatively negative redox potential of its conduction band. [1c,d] As a photocatalyst, ZnS has been examined for degradation of water pollutants, reduction of toxic heavy metals, and water-splitting for H 2 evolution. [2] Owing to its large bandgap (3.6 eV), ZnS itself absorbs only in the UV, but its absorbance can be easily tuned by doping with metal ions, including Mn, [1b,1f] Ni, [2c] Cu, [2d] and Pb. [2e] In addition, the optical properties of ZnS strongly depend on particle size and morphology.[3] Our ability, however, to control simply and USP is a robust synthetic method [4][5][6] and has been used to produce a diverse range of nanostructured materials, including metal chalcogenide quantum dots, [5b] metal oxides, [6] carbon, [6] and others. The preparation of ZnS:Ni 2þ hollow microspheres and nanoparticles using USP is illustrated in Figure 1 (cf. Supporting Information (SI) for further description of the USP apparatus). An aerosol of the precursor solution (e.g., 200 mM ZnNO 3 , 1 M thiourea, and 0.2 mM NiNO 3 in water with 3% colloidal silica) was generated by ultrasonic nebulization, and the resulting mist was carried through a heated zone (700 8C) by an inert gas (Ar) flow.As water rapidly evaporates, ZnS starts to precipitate at the droplet surface, solvent evaporates, and decomposition gases (e.g., NO x , NH 3 , HNCS) are evolved. Owing to the relatively low solubility of ZnS and its precursors, the decomposition of the droplet leads to an outer shell enriched in ZnS with a core of primarily silica particles. When the silica template was dissolved using an ethanolic HF solution, nanostructured ZnS:Ni 2þ mesoporous hollow microspheres were obtained ( Fig. 2a and b; see further scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses in SI). When the furnace temperature was increased from 700 8C to 1000 8C, however, the final product after HF treatment consisted of agglomerated nanoparticles (i.e., the microspheres lost structural integrity; Fig. 2c and d; see further SEM and TEM analyses in SI). As a control experiment, when the same precursor solution without the silica colloid template was nebulized and passed through the tube furnace at 1000 8C under Ar flow, only micrometer-sized solid microspheres of ZnS:Ni 2þ were obtained, as shown in Figure 2e and f. Figure 3 shows the diffuse reflectance UV-vis spectra and powder X-ray diffraction (XRD) patterns of ZnS:Ni 2þ hollow microspheres, nanoparticles, and solid microspheres, respectively. The UV-vis spectra of all three morphologies of the ZnS:Ni 2þ show a broad absorbance in the visible region, extending to ca. 550 nm as compared to non-doped ZnS (dashed line), indicating that doped Ni 2þ forms a new energy level in the band structure of ZnS.[2c] Higher-temper...

A novel aerosol synthesis of hollow 300 nm spheres of Al was achieved using an ultrasonic spray technique without the use of templating agents.