Growth of well-defined cubic hematite single crystals: Oriented aggregation and Ostwald ripening

Single-crystalline alpha-Fe2O3 submicron cubes with a well-defined shape have been successively synthesized via a simple hydrothermal process. X-ray. diffraction patterns, transmission electron microscopy, field emission scanning electron microscopy, and selected area electron diffraction patterns were applied to characterize the as-synthesized samples. The results indicate that the growth of single-crystalline alpha-Fe2O3 cubes can be attributed to cooperation of two principal mechanisms: oriented aggregation and Ostwald ripening. At the beginning, alpha-Fe2O3 nanorods were first fabricated and formed cube-like aggregation following a "one-dimensional (ID) -> three-dimensional (3D)" mode, in which these nanorods orientedly aggregated on the surfaces of some cores by sharing common {012} faces. Then, the resulting polycrystalline aggregates would fuse into a single crystal, in which Ostwald ripening process was expected to carry out because oriented aggregation alone cannot result in well-defined alpha-Fe2O3 cubes. The extended particle would further attract free-standing alpha-Fe2O3 nanorods to orientedly attach on their surface, and the fusion process was repeated until nanorods were completely consumed. This kind of cooperation of two basic mechanisms may give us a new insight into the growth of alpha-Fe2O3 crystals and opens a new way for controllable synthesis of other well-defined crystals in morphology and dimensionality.