Nonequilibrium Process for Doping Under Continuous-Flow Hydrothermal Synthesis of Cerium Oxide-Based Nanoparticles

The nonequilibrium composition and its formation process are critical aspects of nanoparticle production technology. Understanding the dynamics of nanoparticle formation under nonequilibrium conditions is essential. In this study, Cr-doped CeO₂ nanoparticles are synthesized via continuous-flow hydrothermal synthesis at various temperatures (300, 350, 400 °C) with reaction times precisely controlled on the order of seconds. At the initial stage of particle formation, Cr-rich CeO₂ particles form due to a low surface energy. Over time, the Cr content decreases as the particles relax toward the equilibrium structure. This process yields an unusual nonequilibrium composition through rapid heating and short residence times. Similar nonequilibrium compositions are also observed for other dopants, such as Fe and Eu. Continuous-flow hydrothermal synthesis thus presents an efficient method for fabricating nanomaterials with unique compositions that are unattainable using conventional batch methods.