Thermal-direction-controlled architecture in magnetic polymer/carbon microspheres

The precise control over the architecture of magnetic core-shell microspheres is crucial for advanced applications. While synthetic methods are well-established, the influence of heat-transfer direction during polymer curing on the final carbon structure remains unexplored. Herein, monodisperse Fe3O4@resorcinol-formaldehyde core-shell microspheres were synthesized as a model platform to investigate this principle. We consistently show that the thermal pathway governs the morphological evolution during carbonization, as supported by time-resolved electron microscopy: conventional external heating (outside-in) yields yolk-shell structures, whereas magnetically inductive heating (inside-out) produces solid core-shell configurations. Time-resolved electron microscopy reveals that this distinct structural divergence stems from gradient-induced shell shrinkage. This thermal-direction control was extended to other substrates, highlighting its generality. Furthermore, the resulting nitrogen-doped Fe3O4@C-N microspheres serve as efficient magnetically recoverable solid-base catalysts, achieving near-quantitative conversion (99.78%) and yield (98.57%) in biodiesel production from corn oil. This work establishes heat-flow manipulation as a versatile strategy for tailoring carbon architectures and provides insights for designing functional catalytic materials.