Synthesis of Hollow and Porous Semiconductor Nano-crystals Using a Spray Pyrolysis Reactor
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Nanostructured zinc sulfide (II-VI wide band gap semiconductor) can provide unique photonic, electronic, and catalytic properties that are of interest for applications ranging from bioimaging to photocatalysis. Here we report an easily-controllable, reproducible, and continuous method to produce porous and hollow ZnS nano and micro-spheres. We used poly (ethylene glycol) methyl ether (PEG), polyvinylpyrrolidone (PVP), ethylene oxide/propylene oxide block copolymer (Pluronic F-38) and cetyl trimethylammonium bromide (C-TAB) as templates to synthesize ZnS nano and micro-spheres with controlled internal morphology in a spray pyrolysis process. Spherical particles were produced by droplet-to-particle conversion of spherical droplets. Zinc acetate and thiourea, used here as precursors for ZnS, react in solution to form bis-thiourea zinc acetate (BTZA) that precipitates with the evaporation of solvent. Upon further heating, BTZA decomposes to yield ZnS. During solvent evaporation, PEG and Pluronic precipitate after BTZA, driving formation of a shell of ZnS and a hollow core. In contrast, PVP and C-TAB interact strongly with BTZA and ZnS, such that the PVP and ZnS remain intermixed. After evaporation of solvent, the templating agents can be pyrolyzed at high temperature to leave behind porous or hollow ZnS microspheres composed of many much smaller nanocrystals. Core-satellite micro- and nano-assemblies with silver nanoparticles surrounding zinc sulfide micro- and nanospheres were synthesized by coupling silver nanoparticles prepared in solution to ZnS prepared by spray pyrolysis. Here, we report a two-step synthesis of ZnS-Ag hybrid structures stabilized by a cationic surfactant C-TAB in water. Coupling ZnS with silver nanoparticles in solution could be useful for enhancing the photo-degradation capability of ZnS which is a widely used photocatalyst. As silver has antibacterial properties, we expect it to be a potential photocatalyst in biological systems for germicidal action. Optical measurements of UV-visible (UV-vis) absorbance and photoluminescence spectra (PL) show quenching of photoluminescence of ZnS particles by silver through a non-radiative recombination of electron- hole pairs. ZnS-Ag hybrid particles with diameters ranging from 50 nm to 500 nm were produced. This finding demonstrates a potential way for synthesizing core-satellite structures by first synthesizing core material using spray pyrolysis reactor and later coupling with satellite material nanoparticles through a solution route.