Synthesis of nano-porous ellipsoid hematite and the crystal growth mechanism

The crystal growth process directly influences crystal morphology. While the growth process of hematite crystals with specific exposed facets is relatively well understood, the growth mechanism of hematite without distinct facets, particularly porous ellipsoidal hematite, remains unclear. This study systematically investigates the hydrothermal synthesis and crystal growth mechanism of nano-porous ellipsoidal hematite. Under elevated temperatures, iron oxyhydroxide (FeOOH) undergoes a sequential transformation involving dissolution–recrystallization, interfacial nucleation, and solid-state phase transition, ultimately forming hematite. Samples collected at specific time intervals (15, 30, 45, 60, 90, and 120 min) were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). The results indicate that lattice rearrangement during the monoclinic-to-hexagonal transition and thermally driven dehydration play critical roles in generating the nano-porous ellipsoidal morphology. This work offers new insights into the controllable hydrothermal synthesis of hematite and its crystallographic transformation pathways.