Growth Mechanisms of Iron Oxide Particles of Differing Morphologies from the Forced Hydrolysis of Ferric Chloride Solutions

Since the original work of Matijevic and Scheiner (J. Colloid Interface Sci. 63, 509 (1978)) there has been much interest in the unique morphologies of colloidal particles that can be precipitated from ferric salt solutions. To determine the growth mechanisms responsible for the different morphologies, we used time resolved transmission electron microscopy to follow the growth of iron oxide particles produced by the forced hydrolysis of ferric chloride solutions. The growth of three different hematite particle morphologies were investigated: cubes, spheres, and so-called "double ellipsoids." The morphology of the particles depends on the concentration of FeCl3, the pH, and the temperature of aging. All solutions were seen to first produce rod-like particles of akaganéite (β-FeOOH) which would then transform to hematite (α-Fe2O3), leading under different conditions to spheres, cubes, or double ellipsoids. For all solutions, the initially produced akaganéite rods form by homogeneous nucleation and subsequent growth. The hematite particles are produced by dissolution of the akaganéite rods and reprecipitation as hematite. For the double-ellipsoid-producing solution, the akaganéite rods remain unaggregated in solution. Hematite heterogeneously nucleates on these rods. In addition to growing outward, the hematite particle uses the rod as a template, and a collar forms, which grows along the rod, producing the double-ellipsoid shape. For a sphere-producing solution, the β-FeOOH rods also remain unaggregated in solution but the akaganéite rods which are formed are shorter than those formed for the double-ellipsoids, and the rods dissolve before the growing hematite particles can use the rods as templates. For the cube-producing solution, the initially produced akaganéite rods aggregate into rafts. These rafts, formed from rods of similar length, have a cubic shape that they impart to the hematite which nucleates on the akaganéite raft. The findings indicate that the concentrations of starting compounds not only influence the kinetics of the reaction, but also influence the colloidal behavior. It is this influence on colloidal behavior that allows a similar chemistry, i.e., the chemistry of dissolution and redeposition of aqueous iron species, to produce different morphologies of precipitated particles.