Production of metal oxide nanoparticles by supercritical emulsion reaction

A technique derived from SAS is proposed for the production of silicon, tungsten and molybdenum oxides nanoparticles in which supercritical CO2 plays a double role, acting as antisolvent and as reactant. A liquid mixture, that will be used as the continuous phase, containing i-octane, n-heptane, cyclohexane or ethyl acetate and sodium bis (2-ethylhexyl) sulfosuccinate (AOT) or Tween80 as surfactant is first prepared. Then, a solution of sodium metal hydrate in water is added to the above mixture, to form a reverse micellar solution. Oxide nanoparticles of various materials are synthesized in the reverse micelles by reaction with CO2; in the meanwhile, the organic solvent is solubilized in CO2. At the end of the process, the residual components are eliminated by continuous purging with CO2 in a washing step. Semi-continuous SAS-like process has been adopted and process parameters affecting nanoparticles production have been studied such as emulsioning conditions, pressure, water and surfactant concentration. Various recovery methods of the precipitated nanoparticles have been explored such us ultrafiltration (UF) or separation by ultrasonic probe. Nanoparticles have been characterized by Field Emission Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Analysis (EDX). The results show that particles dimensions can be easily controlled by tuning the process parameters. Nanoparticles can be obtained with a narrow particle size distribution and with a mean diameter as small as 29 nm.