Surface Synthesis of Zinc Sulfide Nanoparticles on Silica Microspheres:  Sonochemical Preparation, Characterization, and Optical Properties

Ultrasonic irradiation of a slurry of amorphous silica microspheres, zinc acetate, and thioacetamide in an aqueous medium for 3 h under ambient air yields zinc sulfide coated on silica. The powder X-ray diffraction of the initial zinc sulfide−silica (ZSS) powder yields diffraction peaks corresponding to the ZnS phase. The TEM image of ZSS shows that the porous ZnS nanoparticles (diameter 1−5 nm) coated the silica (SiO2) surface as thin layers or nanoclusters, depending on the reactant concentration. Infrared spectroscopy illustrates the structural changes that occurred in the siloxane network and surface silanol groups of SiO2 upon the ultrasonic deposition of ZnS. The optical absorption of porous ZnS shows a broad band at around 610 nm, ascribed to unusual surface state transition. The absorption energy of the surface state transition is lower than the band gap of the ZnS particles and probably stems from the dangling surface bonds or defects. On the other hand, the ZSS does not show the surface state transition of ZnS, probably due to the strong surface interaction with SiO2. The classical valence-conduction transition band has been observed in the optical reflectance mode, and it shows an absorption edge at around (290−310 nm), which is markedly blue-shifted compared to that of bulk ZnS (345 nm). The photoluminescence spectrum of the porous ZnS and ZSS shows a band with a maximum centered around 420 nm, which is similar to that of quantum ZnS particles. We propose that the coating process takes place via ultrasonic-cavitation-induced initial grafting of zinc acetate onto the silica surface, followed by the displacement of acetate ion by in situ generated S2- species.