284001 Polymer and Surfactant-Templated Synthesis of Nanostructured ZnS Nano- and Micro-Spheres in a Spray Pyrolysis Reactor
ZnS is a direct (II-VI) wide band gap semiconductor. It is used in as a phosphor in luminescent devices, as a photocatalyst for water splitting, in solar cells for energy conversion and in anti-reflection coatings. We present here a single step approach to synthesize nano- and micro-spheres of ZnS nanocrystals using spray pyrolysis. In this method, an atomizer produces a polydisperse fine mist of aerosol droplets which enters in a tubular reactor maintained at a constant temperature (500 to 700 °C). Inside the reactor, solvent evaporates and both inorganic solutes and polymers or surfactants precipitate or self-assemble. The interactions between the semiconductor nanocrystals and the polymer or surfactant template determine the internal morphology of the ZnS nanospheres, while the overall spherical shape is templated by the droplets from which they form. The spheres were characterized via X-ray diffraction, scanning electron microscopy, transmission electron microscopy with selected area electron diffraction, and energy dispersive spectroscopy. Thermogravimetric analysis was used to determine the amount of residual templating agent remaining in the particles. Templating with poly(ethylene glycol) results in hollow nanospheres, while templating with poly(vinylpyridine) produces porous nanospheres. These hollow and porous nanospheres can be of potential use in photocatalysis due to their large specific surface area and large pore volume. They may also find use as contrast agents and carriers for bioimaging and drug delivery applications.
In addition to polymeric templates, we are currently using surfactants such as cetyl trimethylammonium bromide (C-TAB) to template the internal structure of the ZnS nanospheres. In this case, surfactant templated assembly is expected to proceed via micelle formation with polar head groups pointing outwards into the aqueous solution. Like the polymer template, these surfactants decompose within the reactor, after ZnS formation. Surfactant templates have the potential to produce a uniform pore size distribution, in contrast to the homopolymer templates. In this presentation, we will discuss the spray pyrolysis reactor and reaction scheme in brief followed by polymer and surfactant templated synthesis of ZnS nanocrystals.