Miniemulsions as chemical nanoreactors for the room temperature synthesis of inorganic crystalline nanostructures: ZnO colloids

We successfully explored, for the first time, the use of the w/o inverse miniemulsion route to prepare surfactant-functionalised nanocrystalline ZnO colloids. The adopted route exploits the micelles as nanoreactors for the precipitation of the desired oxide in a confined space. Triton X-100 (TritX-), sodium dodecyl sulfate (SDS-) and polyvinylpyrrolidone (PVP-) coated ZnO crystalline nanoparticles (NPs) have been obtained at room temperature (RT) with no need for post-treatment, by precipitation of zinc chloride with ammonium or sodium hydroxide into w/o inverse micelles. Their hydrodynamic diameter, evaluated by Dynamic Light Scattering (DLS), is about 35 nm. X-Ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), Fourier Transform Infrared (FT-IR) spectroscopy and Thermogravimetric Analysis (TGA) have been used to characterize powders separated by miniemulsions. The NP inorganic core is constituted of wurtzite ZnO, with a high degree of crystallinity, as determined by XRD. XRD data and TEM images revealed the formation, in the case of ZnOTritX, of anisotropic plate-like crystallites, with an average diameter of 72 nm and a thickness of 15-20 nm. The RT photo-luminescent (PL) spectrum of ZnOPVP NPs shows a strong UV emission band, attributed to the free exciton recombination, with a relevant tail in the Vis region due to the presence of structural defects. The morphology of these systems, investigated by SEM, corresponds to a homogeneous dispersion of globular sponge structures in a compact and fibrous matrix.