Formation mechanism of monodisperse peanut-type α-Fe₂O₃ particles from condensed ferric hydroxide gel

The formation mechanism of monodisperse peanut-type hematite particles obtained in large quantities from highly condensed ferric hydroxide gel in the presence of sulfate ions has been studied. It is disclosed that sulfate ions restrain the growth in all directions normal to the c axis. The anisotropy was elucidated in terms of the specific adsorption of sulfate ions onto the subcrystals of each hematite particle, retarding the surface reaction of ferric complexes such as Fe(OH)⁺₂ on the planes parallel to their c axes. From close observation of the surface structure of a growing anisometric particle with a transmission electron microscope and from X-ray diffraction (XRD) analysis of an oriented particulate monolayer of the anisometric particles, the formation mechanism of the peanut-like shape was explained by a characteristic growth mode of the subcrystals on both ends of an ellipsoidal particle. The fundamental growth process is otherwise the same as that of monodisperse pseudocubic hematite particles grown in the absence of sulfate ions through a two-step phase transformation from Fe(OH)₃ to β-FeOOH and from β-FeOOH to α-Fe₂O₃ via the solution phase.