Understanding the Growth Mechanism of α‑Fe₂O₃ Nanoparticles through a Controlled Shape Transformation

The growth mechanism of α-Fe₂O₃ nanoparticles in solution has been elucidated from a comprehensive analysis on the shape and morphology of obtained particles. It is found that the hydrothermal synthesis of α-Fe₂O₃ nanoparticles from ferric chloride precursor follows two stages: the initial nucleation of α-Fe₂O₃ nuclei and the subsequent ripening of nuclei into various shapes. The initial nucleation involves the formation of polynuclears from hydrolysis of Fe³⁺ salt precursors, followed by the growth of β-FeOOH nanowires with an akaganeite structure, and then into two-line ferrihydrite nanoparticles through a dissolution–recrystallization process. In the subsequent ripening process, we suggest that the formation of large α-Fe₂O₃ particles follows the dissolution of two-line ferrihydrite and then precipitation and oriented aggregation of α-Fe₂O₃ nuclei rather than the oriented aggregation of ferrihydrite nanoparticles followed by phase transformation. The oriented attachment of {104} facets between α-Fe₂O₃ nuclei results in the formation of oblate spheroid nanocrystals (nanoflower-like particles) either in ethanol or in the beginning stage where the particles first undergo oriented aggregation. With the addition of water, Ostwald ripening process (dissolution–reprecipitation) will play an important role to convert the assembly of nanoflowers into a 3D rhombohedral shape with well-defined edges and surfaces. The proposed mechanism in this article not only allows us to better control the synthesis of iron oxide particles with designed shapes and structures but also provides guidance for theoretical simulations on the oriented attachment process for hematite formation.