Characterization of hematite particles of different shapes

Characterization has been performed on colloidal hematite particles of different shapes: viz., pseudocube, platelet, and spindle. The pseudocubic and platelet-type particles were prepared by a method provisionally referred to as the gel-sol method through aging highly condensed Fe(OH)₃, while the spindle-type particles were obtained in a diluted homogeneous solution of ferric chloride in the presence of phosphate ions after Ozaki et al. (J. Colloid Interface Sci. 102, 146 (1984)). Close observation of their surface structure with SEM and XRD analysis on the powders suggested that a pseudocube (1.65 μm in edge length) is a polycrystal consisting of subcrystals of about 250 Å, a platelet (1.90 μm in diameter and 0.20 μm in thickness) is a single or multilayered crystal, and a spindle (0.43 μm in length and 0.13 μm in width) is a single crystal. The polycrystallinity of the pseudocubic particles was confirmed by clear evidence of remarkable growth of the subcrystals without changing each particle volume on annealing. The crystallographic orientation of these particles was determined by XRD of an oriented particulate monolayer set on a glass plate with gelatin (OPML-XRD): viz., the face index of surface planes of a pseudocube is (012), that of the basal planes of a platelet is mostly (001) and maybe partly (104) and the revolution axis of a spindle is [001]. Moreover, pseudocubic hematite particles prepared by aging a diluted ferric chloride solution with a mixed solvent of ethanol/water (1:1) after Hamada and Matijevieć (J. Colloid Interface Sci. 84, 274 (1981)) were found to be polycrystals consisting of subcrystals of the order of 1000 Å. The growth mechanisms of these hematite particles of different shapes were also discussed.