Study of ZnO Particles Preparation in Supercritical Water System: Experiments, Molecular Dynamics Simulations, and Photocatalytic Activity Evaluation (Invited)

ZnO nanoparticles have been prepared by hydrothermal synthesis in supercritical water system (SCW). The influences of parameters like reaction temperature, pressure, reactant concentrations and different reactants to the size and morphology of ZnO were studied. Five different morphologies of ZnO nanoparticls were prepared, and molecular dynamics simulations were also performed to investigate the structural and thermodynamic changes in the zinc acetate hydrolysis process in SCW. The results show that Zn(CH₃COO)₂ is prone to aggregate in SCW. On average, one Zn²⁺ ion coordinates with five CH3COO- species and one H₂O molecule, forming an octahedral configuration. However, more water molecules bind Zn^{2 +} at the SCW interface to form Zn(CH₃COO)₂ clusters. The total potential energy of each system decreases after the hydrolysis of Zn(CH₃COO)₂, suggesting that it is a thermally favorable process in SCW. The OH^- reaction product incorporates into the amorphous Zn(CH₃COO)₂ cluster and CH₃COOH is in the SCW phase. Rhodamine B was degradated under UV light with the obtained particles to investigate the photocatalytic activity of ZnO nanoparticles. The impacts of process parameters on the photodegradation rate of Rhodamine B were also studied. It showed that ZnO nanoparticles with the narrowest band gap and the maximum specific surface area and pore volume, could get a photodegradation rate of 99.67% after 6 h and the kinetic constant is 24 times higher than that of direct photolysis.