Synthesis, characterisation and processing of zinc sulphide nanoparticles in aerosol-OT reverse micelles

Abstract

Colloidal nanoparticles of the photosemiconductor zinc sulphide, ZnS, were prepared in water-in-oil (w/o) microemulsions of Aerosol-OT/water/isooctane. Characterisation of these nanoparticles was carried out using UV/Visible absorption techniques. The particle radius was calculated using Nedeljkovic's equation, which relates the wavelength of the absorption threshold to particle radius. This study focussed on the various parameters controlling the size of zinc sulphide nanoparticles. The effects of varying water-to-surfactant molar ratio, w, on the stability of zinc sulphide particles were investigated. It was concluded that nanoparticle size is mainly controlled by the micellar size. The changing properties of UV/visible absorption spectra observed when varying the concentration ratio X between the zinc and the sulphide ions were recorded. The results were interpreted in terms of a zinc-ion rich layer. A variation in UV/Visible absorption spectra with nanoparticle concentration was also recorded. Ageing processes associated with nanoparticles were monitored by recording spectra over one week. The time dependence of the spectra revealed a slow growing process of the ZnS nanoparticles attributed to Ostwald ripening. On the other hand, a gradual decrease in absorbance with time was attributed to changes in the photophysical properties of the nanoparticles and explained by a slow oxidation process. The removal of water by addition of Aerosol-OT surfactant to zinc sulphide nanoparticles was studied. Although some growth and photocorrosion did occur, overall stability of the nanoparticles was enhanced. Capping of the particles using thiophenol and pyridine produced a precipitate, which was then readily redispersed in pyridine or toluene. Redispersion using other solvents such as ethers and alkanes was attempted, but pyridine proved to be the better solvent. An increase in particle size was attributed to the presence of a mono layer of thiophenate anions on the cluster surface. Capping was shown to be more effective than removal of water as a method to arrest particle growth and photodegradation. Deposits of the uncapped and capped nanoparticles were obtained by evaporation of the volatile components of the microemulsions. The ZnS nanoparticles thereby obtained were smaller and more stable against photocorrosion than the corresponding microemulsions.