In Situ Transmission Electron Microscopy of Sequentially Formed FeCoNi Medium-Entropy Nanoparticles Driving Catalytic Graphitization

Metal-catalyzed graphitization offers a low-temperature route to crystalline carbon, yet the formation pathways of active catalysts on polymer-derived carbons remain unclear. We utilized in situ heating scanning transmission electron microscopy to track the conversion of electrospun polyacrylonitrile nanofibers doped with cobalt acetate, nickel acetate, and iron chloride into medium-entropy FeCoNi nanoparticles that catalyze graphitization. Scanning transmission electron microscopy imaging and chemical mapping reveal a sequential reduction and formation wherein Ni nuclei appear at approximately 300°C, alloy with Co between 400°C and 500°C to form CoNi particles, and incorporate Fe at 700°C–800°C to yield medium-entropy FeCoNi nanoparticles. Density functional theory and ab initio molecular dynamics reproduce this order by showing facile ligand pyrolysis and rapid reduction for Ni and Co precursors, whereas FeCl₃ forms refractory oxide intermediates that reduce only at higher temperatures. Ab initio molecular dynamics reveals that smaller FeCoNi clusters exhibit lower thermal stability, becoming mobile at reduced temperatures. Electron diffraction and high-resolution imaging reveal two graphitization pathways that operate in sequence: dissolution–precipitation at lower temperatures, followed by carbide formation and decomposition at higher temperatures, consistent with Fe₃C-like signatures near 700°C. Raman spectroscopy further reveals that FeCoNi nanoparticles exhibit higher structural graphite order and fewer defects than single-metal nanoparticle samples, leading to enhanced catalytic performance.