Nucleation Mechanism of Nano-Iron Oxide in a Supercritical Water Reaction

Despite its ubiquitous application in various branches of engineering and materials science, nucleation with reactions from supercritical water remains elusive. In this work, molecular dynamics with a reactive force field was studied to demonstrate the nucleation process of nano-iron oxide crystals synthesized via the supercritical hydrothermal synthesis method using Fe2(SO4)3 as the precursor. It was shown that during the early stages of nucleation, high local density areas first occurred, making these sites primary candidates for nucleation. Furthermore, the amorphous intermediate reorganizes from the aggregation and coalescence of the prenucleation clusters (PNCs) and precedes the emergence of a crystalline nucleus rather than direct one-step nucleation from classical theory. Through calculation, it was found that the average nucleation rates increase generally with the increase in reaction temperature and system density, and the volumes of nano-iron oxide clusters are maintained at 10–12 nm3 when simulating for 3 ns. It was further confirmed that increasing the reaction temperature and density is conducive to enhancing the crystallinity of iron oxide nanocrystals within a certain range of increments, with larger clusters forming faster at higher temperatures and densities. It was demonstrated that the screw dislocation formation mechanism caused nano-iron oxide lattice defects. Although not all of the nano-iron oxide nuclei generated have lattice defects, all of the perfect nano-iron oxide nuclei are generated by adjusting the configuration of nano-iron oxide nuclei with lattice defects.