Unraveling Size-Dependent Electronic Structure and Local Environment in FeCoNiCu Multielemental Alloy Nanoparticles

Multielemental alloy nanoparticles (MEA NPs) composed of earth-abundant transition metals have attracted increasing interest due to their compositional complexity and tunable electronic structures. However, precise size control and the structural evolution of non-noble MEA NPs in the ultrasmall regime remain poorly understood. Herein, we report a nonequilibrium colloidal strategy to synthesize FeCoNiCu MEA NPs with tunable sizes ranging from 4.8 to 36.8 nm by employing hot-injection and heat-up routes. While hot injection enables controlled growth down to 10.1 nm through ligand-mediated kinetics, an ultrasmall 4.8 nm FeCoNiCu NP, representing the smallest non-noble MEA reported to date, is exclusively obtained via the heat-up method. Structural characterization reveals a size-dependent transition from a single-phase fcc solid solution to a predominantly amorphous alloy at the ultrasmall limit. X-ray absorption fine structure spectroscopy (XAFS) and hard X-ray photoelectron spectroscopy (HAXPES) collectively demonstrate pronounced electronic heterogeneity with decreasing particle size, characterized by progressive oxidation and electron depletion of Fe, Co, and Ni, while Cu remains predominantly metallic across all sizes. These results establish a clear correlation among synthesis kinetics, size-induced amorphization, and element-specific electronic structure evolution, providing fundamental insights into the design of ultrasmall MEA NPs.