Hollow-Carbon Confinement Annealing: A New Synthetic Approach to Make High-Entropy Solid-Solution and Intermetallic Nanoparticles

High-entropy alloy (HEA) nanoparticles (NPs) have been emerging with superior compositional tunability and multielemental synergy, presenting a unique platform for material discovery and performance optimization. Here we report a synthetic approach utilizing hollow-carbon confinement in the ordinary furnace annealing to achieve the nonequilibrium HEA-NPs such as Pt_0.45Fe_0.18Co_0.12Ni_0.15Mn_0.10 with uniform size ∼5.9 nm. The facile temperature control allows us not only to reveal the detailed reaction pathway through ex situ characterization but also to tailor the HEA-NP structure from the crystalline solid solution to intermetallic. The preconfinement of metal precursors is the key to ensure the uniform distribution of metal nanoparticles with confined volume, which is essential to prevent the thermodynamically favored phase separation even during the ordinary furnace annealing. Besides, the synthesized HEA-NPs exhibit remarkable activity and stability in oxygen reduction catalysis. The demonstrated synthetic approach may significantly expand the scope of HEA-NPs with uncharted composition and performance.