Morphology evolution of α-Fe2O3 controlled via incorporation of alkaline earth metal ions

Abstract

Hematite (α-Fe₂O₃) crystals with different morphologies were successfully fabricated via an environmental friendly hydrothermal route by applying alkaline earth metal ions along the row from Mg²⁺ to Ba²⁺ ions as structure-directing agents without employing any surfactants or templates. It was found that the ionic radii mismatch degree between the alkaline earth metal ions and Fe³⁺ ions played a vital role in determining the crystalline phase and morphology of the iron oxide. Pure α-Fe₂O₃ crystals were created with Mg²⁺, Ca²⁺ and Sr²⁺ ions (30.9%, 81.8% and 114.5% ionic radii mismatch degree with that of Fe³⁺) as additives while goethite iron oxide nanoparticles were obtained with Ba²⁺ ions (145.5% ionic radius mismatch degree with that of Fe³⁺) as additives. Mg²⁺ ion-induced α-Fe₂O₃ crystals had a quasi-cube structure with a rough surface while Ca²⁺-induced α-Fe₂O₃ crystals had a quasi-cube structure with a very smooth surface, and the edge length of which is much smaller than that of the Mg²⁺ ion-controlled α-Fe₂O₃ crystals. However, when Sr²⁺ ions were introduced into the reaction system, the product tended to have plate-like structures. The effects of alkaline earth metal ions on the crystal growth rate and final morphology through anisotropic incorporation were proposed in this paper. Moreover, the effects of incorporation of alkaline earth metal ions on the magnetic properties of α-Fe₂O₃ crystals were investigated.