Off-Equilibrium Hydrothermal Synthesis of High-Entropy Alloy Nanoparticles.

High-entropy alloy (HEA) nanoparticles offer unique catalytic properties due to their complex surface coordination and widely tunable electronic structures. Conventional synthesis methods typically involve extreme thermal shock (∼1700 °C) to achieve metal coreduction and mixing. While wet-chemical approaches hold potential for controlling nanoparticle properties, they are hindered by disparities in metal reduction kinetics and a diminished influence of configurational entropy on metal mixing at low temperatures, leading to phase segregation and limited compositional tunability. In this work, we introduce a novel wet-chemical hydrothermal method that enables the synthesis of HEA nanoparticles with enhanced compositional homogeneity and precise property control at low temperatures (∼170 °C). This method utilizes in situ generation of active hydrogen (H) via organic dehydrogenation on nuclei/seed surfaces, creating localized off-equilibrium environments within the near-equilibrium wet-chemical system. These conditions mitigate the thermodynamic and kinetic limitations, enabling synchronized metal reduction, precise compositional tunability over a broad range, and improved alloy uniformity. As a proof of concept, we demonstrate the enhanced electrocatalytic methanol oxidation performance of PtCuNiCoFe HEA nanoparticles through surface composition design. This approach offers a robust platform for synthesizing HEA nanoparticles with tailored properties, expanding their catalytic applications.