Systematic control of size, shape and internal structure of monodisperse α-Fe₂O₃ particles

The size, shape, and internal structure of monodisperse hematite (α- Fe₂O₃) particles have been controlled systematically on the basis of the gel-sol method recently developed. The size could be controlled in the range from ca. 2 μm to ca. 0.3 μm without degrading the monodispersity by changing the temperature during the preparation of Fe(OH)₃ gel, or to ca. 30 nm by adding different amounts of ultrafine α-Fe₂O₃ seeds. On the other hand, anions such as Cl^-, OH^-, SO₄/^2-, and PO₄/^3- have remarkably different effects on the shape and internal structure. In this study in particular, the aspect ratio of the ellipsoidal particles was controlled from 1.1 to 5.2 by shape controllers such as sulfate and phosphate ions. It was found at the same time that sulfate ions yielded polycrystalline particles consisting of much smaller subcrystals of a definite orientation, while phosphate produced nearly monocrystalline particles at a reduced concentration of chloride ions. Also, the decrease in concentration of chloride ions and elevation of aging temperature continuously increased the size of the subcrystals of each particle. Finally, it has been shown that fairly uniform nanosized hematite particles can be prepared in large quantities as an application of a new technique for the preparation of the ultrafine α-Fe₂O₃ seeds. As a result, it has become possible to control the mean size, shape, and internal structure of monodispersed hematite particles independently over a wide range.