Multistep Nucleation of High-Entropy Alloy Nanoparticles

High-entropy alloy (HEA) nanoparticles (NPs) represent a promising class of materials with potential applications in catalysis and energy storage. Therefore, a fundamental understanding of their formation is essential for tailoring and optimizing their properties for such uses. Despite their significance, the nanoscale mechanisms underlying the nucleation and growth of HEA NPs remain poorly understood. Here, we employ in situ transmission electron microscopy (TEM) to elucidate the nanoscale steps involved in the thermal-reduction-based synthesis of FeCoNiRhPt HEA NPs directly from the precursor. Our observations reveal that the formation of these NPs proceeds through a sequential alloying process: it begins with the burst nucleation of amorphous RhPt (a-RhPt) clusters, which then crystallize into c-RhPt alloy NPs. Subsequently, these crystalline NPs incorporate Fe, Co, and Ni to transform them into stable HEA NPs. Understanding the intermediate stages in the formation of HEA alloys provides important insights that extend to the broader field of multimetallic material synthesis.