A Programmable Nanoparticle Conversion Pathway to Monodisperse Polyelemental High Entropy Alloy, Intermetallic, and Multiphase Nanoparticles.

Polyelemental nanoparticles (PE NPs), those consisting of four or more elements, exhibit unique properties from synergistic compositional effects. Examples include high entropy alloys, high entropy intermetallics, and multiphase types, including Janus and core-shell architectures. Although colloidal syntheses offer excellent structural control for mono- and bi-elemental compositions, achieving the same control for PE NPs remains challenging. Here, this challenge is addressed with a NP conversion strategy wherein different types of PE NPs - including high entropy alloy, high entropy intermetallic, and multiphase Janus nanoparticles - are achieved through thermal transformation of readily synthesized colloidal core-shell NPs. Through systematic variations in stoichiometry and metal identity to the core-shell precursor NPs, along with atomistic simulations that probe phase stabilities, we deduce that the final mixing states of the various NPs are governed by the balance between the enthalpy and entropy of mixing. Moreover, our annealing method allows us to trap NPs at intermediate states of mixing, creating distinct surface ensembles that were evaluated as catalysts for the hydrogen evolution reaction. This study is the first, to our knowledge, to report colloidally derived precursor NPs enabling the synthesis of all types of PE NPs in a single process. This NP conversion strategy offers a general route to diverse PE NPs.