As-prepared single-crystalline hematite rhombohedra and subsequent conversion into monodisperse aggregates of magnetic nanocomposites of iron and magnetite

Monodisperse nanocrystalline rhombohedral composites of Fe and Fe ₃O ₄ magnetic materials have been obtained employing a reduction reaction, in a flowing gas mixture of H ₂ and N ₂, of single-crystalline, submicron-sized α-Fe ₂O ₃ rhombohedral precursors. This synthesis is significant in that we were able to create a nanocomposite with hard and soft magnetic phases juxtaposed within one discrete, anisotropic structure. In turn, the precursor hematite rhombohedra of reproducible shape were successfully prepared using a facile, environmentally friendly, large-scale molten-salt reaction. Rhombohedra represent a high-surface-area, anisotropic formulation of an industrially important material (iron oxide), which is an active component of gas sensors, photocatalysts, and other types of catalytic materials. Moreover, the predictive formation of these materials has been investigated through a systematic variation of experimental parameters. Extensive structural characterization of as-prepared samples has been performed using scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, energy-dispersive X-ray spectroscopy, selected area electron diffraction, X-ray diffraction, and superconducting quantum interference device (SQUID) magnetic measurements.