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Abstract 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. 

Galvanic replacement (GR) of monometallic nanoparticles (NPs) provides a versatile route to interesting bimetallic nanostructures, with examples such as nanoboxes, nanocages, nanoshells, nanorings, and heterodimers reported. The replacement of bimetallic templates by a more noble metal can generate trimetallic nanostructures with different architectures, where the specific structure has been shown to depend on the relative reduction potentials of the participating metals and lattice mismatch between the depositing and template metal phases. Now, the role of reaction stoichiometry is shown to direct the overall architecture of multimetallic nanostructures produced by GR with bimetallic templates. Specifically, the number of initial metal islands deposited on a NP template depends on the reaction stoichiometry. This outcome was established by studying the GR process between intermetallic PdCu (i-PdCu) NPs and either AuCl 2 − (Au 1+ ) or AuCl 4 − (Au 3+ ), producing i-PdCu–Au heterostructures. Significantly, multiple Au domains form in the case of GR with AuCl 2 − while only single Au domains form in the case of AuCl 4 − . These different NP architectures and their connection to reaction stoichiometry are consistent with Stranski–Krastanov (SK) growth, providing general guidelines on how the conditions of GR processes can be used to achieve multimetallic nanostructures with different defined architectures.