Edge-Preferential Graphitization and Nanoscale Structural Heterogeneity in Coal-Derived Carbon Revealed by High-Resolution Transmission Electron Microscopy (HRTEM)

The nanoscale structural heterogeneity of carbons plays a crucial role in regulating the surface activity, catalytic performance, and wettability of substances. Herein, we quantitatively investigate the nanostructural evolution of a coal-derived carbon during high-temperature treatment, using high-resolution transmission electron microscopy (HRTEM). To elucidate the intrinsic nanoscale structural heterogeneity of carbonaceous materials during thermal treatment, a particle-edge preferential graphitization is hypothesized in this study. Based on quantitative HRTEM analysis of coal-derived carbon heat-treated up to 3000 °C, this study demonstrates that particle boundaries act as intrinsic geometric templates and stress concentrators, governing the carbon nanostructural evolution. The length enhancement of the aromatic structural transformation occurs at 300 °C, 1000 °C, and 2550 °C, respectively, with substantially decreased small-size lattice fringes and substantially increased medium- and large-size lattice fringes. The average curvature displays a non-monotonic decrease and a transient increase during the second carbonization and pregraphitization processes, driven by the formation of concentric and twisted nanostructures. During graphitization, lattice fringe curvature becomes geometrically constrained by particle edges, resulting in an unexpected positive correlation with fringe length. Lattice fringes at particle edges develop highly ordered, boundary-parallel orientations, whereas interior regions experience delayed ordering with randomly oriented graphitic domains. Edge-preferential graphitization explains the non-uniform evolution of carbon structures, providing a crucial theoretical basis for the comprehensive utilization of complex amorphous carbon precursors, including coal and biomass-derived fuels.