Preparation and characterization of columnar hydrohematite particles from a forced hydrolysis reaction using sodium gluconate

The effects of sodium gluconate (HOCH₂(CHOH)₄COONa; abbreviated as SG) on the morphology of hydrohematite particles produced from a forced hydrolysis reaction were investigated. It is evident from transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FE-SEM) measurements that a fraction of the particles produced in these reactions lost their spherical habit and acquired a columnar shape upon the addition of small amounts of SG. FE-SEM images of the particles reveal that the columnar hydrohematite particles possessed top-face diameters that were the same as their edge lengths (heights). Because of their similar lengths, the columnar particles were imaged as both spheres and squares by TEM observation. The unit cell dimensions of the a-axis of the hydrohematite particles were almost constant at 0.5040 ± 0.0004 nm. The c-axis, however, was slightly larger than was reported in previous studies and became further enlarged upon increase of the SG concentration above 0.3 mol%, at which point the columnar particles were precipitated. This was explained by considering the replacement of O²⁻ ions with OH⁻ to compensate for the charge imbalance resulting from Fe³⁺ vacancies on the basis of the general chemical formula of α-Fe_2−x/3(OH)_xO_3−x. The presence of OH⁻ ions in the hydrohematite particles was identified by in situ Fourier transform infrared (FTIR) spectroscopy and thermogravimetric differential thermal analysis (TG-DTA) measurements. Since the specific surface area of the hydrohematite particles were observed to increase with increasing SG concentration, it was presumed that the hydrohematite particles were produced by the aggregation of clustered particles. Time-resolved TEM observations over a period of 12–18 h after initiation of the reaction for the system with 0.3 mol% SG revealed that columnar particles are produced by the deposition of clustered particles with ca. 10 nm in diameter onto the spherical particles. The edges of columnar particles then become right-angled following this deposition. It can be concluded that the SG molecules behaved as a morphology control agent because no SG molecule remained in the columnar particles.