Tuning metal segregation and surface composition of Pt-Pd nanoparticles in microemulsions: the role of nucleation kinetics.

A kinetic simulation model was used to investigate the formation of Pt-Pd nanoparticles synthesized in microemulsions by the one-pot method and to identify the mechanisms governing metal segregation and surface composition. Metal segregation is determined by the interplay between reduction, nucleation, reactant concentration, and intermicellar exchange. High reactant concentrations increase the availability of metal atoms, allowing the nucleation requirements to be satisfied earlier. This promotes Pt incorporation during the initial stages of nanoparticle formation, favouring Pt-core/Pd-shell structures. In contrast, low reactant concentrations produce alloyed or partially segregated nanoparticles. Surfactant film flexibility mainly affects the surface composition by modulating the contribution of Ostwald ripening to particle growth, with higher flexibility promoting Pt surface enrichment. The critical nucleus size for heteroatomic nucleation is identified as the dominant nucleation parameter governing the degree of metal segregation. The model successfully rationalizes experimental results exhibiting markedly different atomic arrangements and provides mechanistic insight into Pt-Pd nanoparticle formation in microemulsions. These findings provide practical guidelines for tailoring the internal architecture and surface composition of Pt-Pd nanoparticles through the appropriate selection of synthesis conditions.