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RESEARCH PRODUCT

Supercritical hydrothermal synthesis of ZnO nanopowders beyond the critical point : understanding of nucleation and growth steps

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The supercritical hydrothermal synthesis of nanopowders (especially metal oxide) has been widely studied. To the best of our knowledge, no nanoparticle formation mechanism has been published yet. In this prospect, this study is dedicated to the understanding of metal oxide nanoparticle nucleation and growth mechanisms. For this purpose, zinc oxide is used as a model material. First, the influence of synthesis operating conditions such as pressure, temperature, pH, precursor concentrations and solution flow rates on particle morphological properties (size, particle size distribution or morphologies) has been investigated. Hence, two approaches have simultaneously been carried out. The first approach involves powder characterizations by mean of X-ray diffraction or transmission electron microscopy techniques. The second one consists in the development of a numerical model considering the thermal exchanges, the fluid hydrodynamic behavior and chemical reaction inside the patented reactor by computational fluid dynamics. Results show good agreement between those two approaches. Several ZnO particle formation mechanisms based on powder experimental characterizations are presented in this work depending on operating conditions. In order to enhance the numerical model, a methodology has been set up to evaluate ZnO nanoparticle nucleation and growth rates in supercritical conditions (SCW) by the determination of particle solubility as function of temperature and the pressure.