Formation process of nanometer-sized cubic ferric oxide single crystals

Cubic ferric oxide (alpha-Fe2O3) single crystals (several tens of nanometers) of narrow size distribution were prepared by forced hydrolysis of acidified FeCl3 aqueous solutions in open vessels. The X-ray diffraction (XRD) and electron diffraction patterns of the single crystals were produced. The formation of the single crystals was studied using time resolved TEM and XRD. Two stages were considered to exist in the formation process. During the nucleation stage, only amorphous iron oxide particles of 3-5 mm can be found in the solution; in the growth stage, phase transformations occurred from amorphous iron oxide to beta-FeOOH and from beta-FeOOH to alpha-Fe2O3. The three phases coexist in the solution during the formation process. The middle phase, beta-FeOOH, possesses two changing tendencies: to aggregate or to break up. The former tendency leads to the formation of the templates on which cubes form; the latter results in the transformation from beta-FeOOH to amorphous iron oxide. A fast nucleation method was introduced in this work, through which the nucleation process takes only several seconds, and the cubes so prepared are slightly larger than those prepared by the ordinarily used method (nucleation lasting several minutes). Hydrolysis at lower temperature produced quasi-cubes which are not single crystals. (C) 1996 Academic Press, Inc.