Sonochemically produced ZnS-coated polystyrene core-shell particles for use in photonic crystals

Zinc sulfide films were grown on carboxyl-modified polystyrene microspheres (PS-CO2) through sonochemical deposition in an aqueous bath containing zinc acetate and sulfide, released through the hydrolysis of thioacetamide. The resulting particles were "optically hollow", due to a large refractive index contrast between the core and shell materials. Continuous, uniform films were obtained after 3-4 h and reached a maximum thickness of 70-80 nm after 13 h of growth, as characterized by transmission electron microscopy (TEM). Aggregation was minimized by subsequent modification of the core-shell particles with mercaptoacetic acid to increase their surface charge and produce good colloidal suspensions. Oscillations in the optical spectra of dilute suspensions of the particles were indicative of interference patterns as expected from Mie light scattering calculations. X-ray diffraction (XRD) patterns match the zinc blende structure of ZnS and indicate a compression in the crystal lattice (alpha = 5.305 +/- 0.037 Angstrom), as compared to the bulk material (alpha = 5.406 Angstrom). Hollow ZnS shells were formed by annealing the core-shell particles in a thermal gravimetric analysis (TGA) oven, at 400 degreesC. A 34% weight loss was observed upon heating, a value approximately equal to that of the polystyrene core. The hollow shells remained intact and readily resuspended in water. Both core-shell and hollow ZnS particles self-assemble to form well-ordered, hexagonal close-packed layers.