J.H. Smith, J.E. Millstone
University of Pittsburgh,
United States
Keywords: high entropy, alloy, colloidal, redox reactions, metals
Summary:
Achieving both structural and compositional precision in the synthesis of high-entropy alloy nanoparticles (NPs) requires understanding the interplay between kinetic factors, such as relative precursor reaction rates, and thermodynamic factors, including phase specific properties such as enthalpies of mixing, formation enthalpies, and surface energies. Unlike bulk systems governed largely by equilibrium thermodynamics, metal mixing outcomes in colloidal syntheses are often predefined by the relative reduction kinetics of the metal precursors. At solution accessible temperatures, insufficient thermal energy prevents significant solid-state diffusion, limiting the system’s ability to reach thermodynamic minima unless initially formed during synthesis. Consequently, similar metal ion reaction rates favor simultaneous incorporation into growing NPs, yielding homogeneous alloyed structures, whereas disparities in reaction rates result in phase-segregated, core@shell morphologies. Leveraging this insight, we demonstrate that modulating precursor addition rates alone can dramatically alter metal mixing outcomes between fully phase-segregated and completely alloyed configurations, even when all other synthetic parameters remain constant. We developed a quantitative kinetic model based on two representative bimetallic systems (Au–Pd and Au–Pt) that predicts how relative metal ion reaction rates evolve with varying precursor addition rates. This model then enabled precise synthesis of compositionally controlled nanoparticles in more complex, high-entropy systems comprised of Co, Ni, Cu, Pd, and Pt. Having established the ability to access multiple metal mixing behaviors for a given set of metals, we now further explore how specific metal–metal interactions and intrinsic material properties influence final particle outcomes. By systematically tuning conventional thermodynamic variables alongside nanoscale kinetic parameters, we delineate how bulk alloying rules are modified, or entirely overridden, by length-scale dependent phenomena, revealing fundamental insights into alloy formation at the nanoscale.