Laser-generated CuPdAgPtAu High-Entropy Alloy Nanoparticles -- Thermal Segregation Threshold and Elemental Segregation

Abstract: High-entropy alloy nanoparticles synthesized via laser ablation in liquid are promising for catalysis due to their ability to form simple solid solutions despite chemical complexity. In this study, noble metal HEA NPs (CuPdAgPtAu) are produced from equimolar and Cu- or Ag-enriched bulk targets. Advanced electron microscopy, XRD, and atomistic simulations are used for structural and compositional analysis. Equimolar targets and NPs exhibit a single fcc phase. In contrast, Cu- or Ag-enriched targets show phase segregation into two fcc phases, which is not observed in the synthesized NPs. Simulations predict segregation tendencies, including Ag surface enrichment and Pt core enrichment due to surface energy differences. However, experimentally, individual NPs remain compositionally homogeneous. Thermal stability studies reveal that phase segregation can be induced post-synthesis. Upon heating, Cu-Ag segregation occurs, forming a second fcc phase similar to bulk targets. These findings demonstrate that rapid quenching during laser ablation suppresses thermodynamically driven segregation and stabilizes metastable solid solutions under kinetic control. Subsequent slow heating overcomes kinetic barriers, enabling equilibrium phase formation at higher temperatures. The thermal stability of these NPs and their tunable composition, including Cu enrichment beyond equilibrium limits, make them promising for high-temperature catalytic applications while reducing noble metal usage.

• The paper demonstrates that kinetic trapping via laser ablation in liquid stabilizes a single-phase fcc structure in CuPdAgPtAu HEA nanoparticles.
• It reveals that rapid quenching prevents thermodynamic segregation, resulting in a homogeneous alloy even from phase-separated bulk targets.
• The study finds that post-synthesis thermal treatment induces Ag/Cu segregation, informing catalyst design strategies under high-temperature conditions.

Laser-Generated CuPdAgPtAu High-Entropy Alloy Nanoparticles: Kinetic Control, Thermal Stability, and Elemental Segregation

Introduction

This study examines the synthesis, structure, and thermodynamic behavior of compositionally complex CuPdAgPtAu high-entropy alloy nanoparticles (HEA NPs) produced via laser ablation in liquid (LAL). The work demonstrates the capacity of kinetic control during LAL to stabilize metastable, compositionally homogeneous single-phase fcc alloy NPs, even when starting from biphasic or element-enriched alloy targets that exhibit phase segregation under equilibrium. A combination of advanced (S)TEM, XRD, EDS, XPS, and atomistic simulations provides insight into elemental distributions, segregation tendencies, and the role of post-synthetic thermal processing. The implications for tunable noble-metal HEA catalysts, as well as theoretical modeling of NP structure formation, are discussed.

Synthesis and Structural Characterization

Bulk targets of nominally equimolar and element-enriched CuPdAgPtAu alloys were fabricated by arc-melting under inert atmosphere, with subsequent LAL in degassed acetone yielding colloidal HEA NPs. XRD and SAED confirm a single fcc phase for NPs derived from all targets, in marked contrast to the bulk targets themselves: significant Cu or Ag enrichment in the target leads to two-phase fcc segregation at the bulk scale, but the corresponding NPs remain single-phased after LAL.

Morphologically, the NPs are generally spherical, with mean diameters increasing upon Ag and Cu enrichment (NM-Ag: 32 nm; NM-Cu: 22 nm; NM-Eq: 17 nm). EDS and XPS compositional analyses show that NP composition closely reflects the ablated target but with persistent nanoscale inhomogeneity and element-specific deviations driven by surface energy effects and oxygen affinity. Specifically, Ag and Cu are found to be surface-enriched, while Pt and Pd display core enrichment or depletion in the near-surface region.

Kinetic Stabilization and Elemental Segregation

Despite strong thermodynamic driving forces for phase segregation in Ag- and Cu-enriched compositions, only single-phase NPs are produced by LAL. Atom probe tomography and EDS reveal homogenous element distribution at the nanoscale, with only a thin Ag-rich shell observed in particles <10 nm and no extensive phase separation detectable within the resolution of STEM-EDS/SAED. These findings are corroborated by atomistic Monte Carlo and MD simulations. Under equilibrium (MC), and non-equilibrium (MD) with rapid quenching, the simulations predict Ag surface segregation and Pt core clustering, consistent with surface energy and mixing enthalpy considerations. However, the degree of Pt segregation predicted is not directly confirmed experimentally, likely due to rapid quenching and the involvement of solvent-derived species in the actual LAL environment. The kinetic trapping imposed by the extremely high cooling rates ($10^{11}$–$10^{13}$ K/s) during LAL precludes the system from reaching its thermodynamic minimum, stabilizing metastable solid solutions and suppressing phase separation.

Thermal Stability and Post-Synthesis Segregation

In situ TEM heating and ex situ annealing reveal a clear thermal segregation threshold. Below ~400°C–430°C, NPs retain their single-phase structure. Upon heating above this range (e.g., 495°C and 550°C), two fcc phases emerge corresponding to Ag-rich and Cu-rich domains, mimicking the equilibrium phase behavior of the bulk alloys. Post-annealing elemental mapping confirms phase separation, primarily along Ag/Cu lines, with limited Pt segregation into Cu-rich regions for NM-Cu and NM-Ag. Notably, even initially equimolar NPs exhibit segregation into Ag- and Cu-rich phases upon sufficient thermal stimulus, underscoring their metastable, kinetically stabilized nature. The surface-to-volume ratio, size-dependent melting point depression, and accelerated global diffusion kinetics at the nanoscale facilitate this rapid transition relative to bulk alloys.

Implications for Catalysis and Theoretical Modeling

The demonstrated ability to tune HEA NP composition beyond equilibrium solubility limits—particularly to achieve Cu enrichment—while maintaining single-phase fcc metastable structures is of significant importance for high-temperature catalytic applications. The LAL process yields structurally robust, compositionally homogeneous NPs under operating conditions relevant to CO$_2$ reduction and other catalytic cycles; only under prolonged high-temperature exposure does phase segregation become pronounced. The reduction in precious metal content without sacrificing stability or homogeneity offers a sustainable pathway for catalyst design.

From a modeling perspective, the results clearly illustrate the limitations of static or thermodynamic equilibrium-based simulations for predicting NP structure following rapid, non-equilibrium syntheses. Kinetic trapping, surface effects, solvent interactions, and dynamic mixing dictate final NP structures in LAL, mandating the integration of realistic quenching rates, finite-size effects, and chemical environment into future theoretical work.

Conclusions

The laser ablation in liquid synthesis of noble-metal CuPdAgPtAu HEA NPs achieves kinetically stabilized, homogeneous single-phase alloys from even inhomogeneous, phase-separated bulk targets. Ag or Cu enrichment of the starting material can push NP composition beyond the respective bulk equilibrium solubility limits, an effect strictly dependent on rapid quenching and suppressed segregation. Surface enrichment trends for Ag and core retention of Pt are observed in both experiment and simulation, although full agreement on the extent and character of segregation requires further refined atomistic models. Upon thermal annealing, NPs evolve toward the equilibrium two-phase Ag-rich and Cu-rich state, with implications for long-term structural and functional stability under catalytic conditions. Collectively, these results provide both mechanistic insight into NP formation dynamics and practical guidance for designing multimetallic catalysts with tailored thermal and compositional stability.