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

The growth mechanism of alpha-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 alpha-Fe2O3 nanoparticles from ferric chloride precursor follows two stages: the initial nucleation of alpha-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 beta-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 alpha-Fe2O3 particles follows the dissolution of two-line ferrihydrite and then precipitation and oriented aggregation of alpha-Fe2O3 nuclei rather than the oriented aggregation of ferrihydrite nanoparticles followed by phase transformation. The oriented attachment of {104} facets between alpha-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.