Thermal Evolution of Leaf-Shaped ZIF Nanostructures into ORR-Active N-Doped Carbons

Metal-organic-frameworks (MOFs) are versatile precursors for electrically conductive heteroatom-doped-carbons (e.g., N-doped carbon) used as electrocatalysts. Spatiotemporally resolved mechanisms governing phase evolution during pyrolysis are essential for designing robust catalysts, yet real-time investigation of temperature-dependent crystalline-to-amorphous transitions, diffusion within amorphous intermediates, and subsequent nucleation-growth remains challenging. ZIF-L, a two-dimensional, leaf-shaped polymorph of zeolitic imidazolate frameworks (ZIFs), differs markedly from cubic ZIF-8 in structure, coordination, and thermal behavior, and shows strong promise for oxygen reduction reaction (ORR) catalysis. However, the atomic-scale pathway governing the evolution of N-doped carbon during ZIF-L pyrolysis remains unclear, motivating this study. Here, we combine in-situ transmission electron microscopy (TEM) and X-ray diffraction (XRD) to directly visualize the thermal decomposition of ZIF-L, revealing the sequential collapse of its layered framework and associated morphological changes under controlled heating to 650 °C, decoupled from electron-beam-induced effects. Ex-situ pyrolysis (700-1100 °C), together with high-resolution TEM, synchrotron X-ray pair-distribution-function (X-PDF) analysis, and electron-energy-loss-spectroscopy (EELS), elucidates the diffusion processes and compositional transformations within the amorphous matrix. We identify progressive, temperature-dependent ligand breakdown, Zn-Nₓ species migration, and Zn-loss, ultimately yielding porous N-doped carbon at 1000 °C, which exhibits the highest ORR activity in our temperature series. These results establish temperature-structure-activity correlations for ZIF-L-derived catalysts and provide mechanistic guidance for the thermal engineering of MOF-derived electrocatalysts.

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