Kinetic control preserves the single-phase fcc structures of CuPdAgPtxAu high-entropy alloy nanoparticles.

Abstract

High-entropy alloy nanoparticles (HEA NPs) constitute a promising class of materials for heterogeneous electro- and thermal catalysis due to their exceptional compositional tunability. However, precise structural control remains a challenge-particularly for quinary systems, where composition and structure are often intertwined. A structure-robust synthesis could disentangle these effects, enabling clearer correlations between composition and functionality in application development. Laser ablation in liquids (LAL) yields colloidally stable, surfactant-free HEA NPs under kinetic control. Although a few studies have emerged in the last few years, the structural development under compositional variation has not yet been thoroughly studied. In this work, we explore the phase structure of the quinary CuPdAgPtxAu high-entropy alloy nanoparticle model system, with particular focus on the influence of platinum content x. As the element with the highest melting point and surface energy in this system, platinum is expected to affect phase formation, as shown earlier for LAL-generated binary alloy nanoparticles. Using high-resolution transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (STEM-EDS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), we investigated changes in the composition and phase structure of the ∼10 nm nanoparticles with platinum contents of nominal 20-80 at%. We find that the increase in the platinum content does not affect the formation of fcc phase structures within the produced HEA NPs. Furthermore, correlated EDS/XPS analysis hints toward a slight platinum confinement within the nanoparticle core, an observation we attribute to element-specific properties rather than the enrichment of Pt. Molecular dynamics simulations were performed for 4033 atoms (about 5 nm rounded nanoparticle diameter), in which the particles were formed by cooling of a hot metal vapor within nanoseconds, mimicking the typical cooling rate of 1011 K s-1 of NP droplets in a LAL-generated plume. From the derived next-neighbor (NN) distributions and radial distributions (RD), the simulations shed light on the effect of kinetics on the final structure, next-neighbor atom statistics, and final atom arrangement. Here, deviations of the particles' structures relative to those of ideal solid solutions are found with platinum and silver atoms showing tendencies towards core and surface-near segregations, respectively, in agreement with the experiments.