Single-Crystalline Dodecahedral and Octodecahedral α-Fe2O3 Particles Synthesized by a Fluoride Anion-Assisted Hydrothermal Method

Despite significant advances in iron oxide nanoparticles, it is still a challenge to synthesize regular polyhedral single-crystalline alpha-Fe2O3 particles because the surface energies of several low-index planes are fairly similar. In the work presented here, well-dispersed and single-crystalline dodecahedral and octodecahedral alpha-Fe2O3 particles are synthesized by a facile hydrothermal method with the aid of F- anions. The crystalline structure of the polyhedral particles is disclosed by various characterization techniques. The dodecahedral particles are of hexagonal bipyramidal shape and enclosed by twelve equivalent (101) planes. The octodecahedral particles are formed by adding six equivalent (111) planes on the two tips of a dodecahedral particle, that is, they are enclosed by twelve (101) planes and six (111) planes. The existence of F- anions plays a crucial role in the control of polyhedral particle shape. The function of F- anions in the shape formation of the polyhedral particles is proposed as follows: 1) A high concentration of exposed Fe3+ cations induces preferential adsorption of F- anions on the (100) plane and leads to the slowest growth along the [100] direction. When the concentration of F- anions is higher than 24 mm, a stable speed ratio of growth along the [001] and [100] directions results in the exposure of (101) planes. 2) With a lower concentration of F- anions, six symmetrical (111) planes with low concentration of exposed Fe3+ cations are present at the tops of a dodecahedral particle to form an octodecahedron. Furthermore, the dodecahedral and octodecahedral alpha-Fe2O3 particles show much stronger magnetism than the previously reported alpha-Fe2O3 nanostructures, having coercivities of 4986 Oe and 6512 Oe, respectively. Such high coercivities are attributed to a large local magnetic anisotropy, which might be induced by the polyhedron with equivalent crystallographic planes and/or the presence of F- anions.