EPITAXIAL OVERGROWTH OF GOETHITE ON HEMATITE SYNTHESIZED IN PHOSPHATEMEDIA - A SCANNING FORCE AND TRANSMISSION ELECTRON-MICROSCOPY STUDY

We used X-ray diffraction (XRD), scanning force microscopy (SFM), tran smission electron microscopy (TEM), and color to investigate the effec t of phosphate on the crystallization rate, nature, and morphology of iron oxides prepared from ferrihydrite in the laboratory. Synthesis wa s performed at two temperatures (323 and 373 K) and two pH values (9 a nd 12) from ferric nitrate, for P/Fe atomic ratios ranging from 0 to 2 .5%. The presence of phosphate retarded crystallization, tended to fav or hematite over goethite, and markedly influenced the morphology of t he goethite crystals formed at high pH. Application of SFM in the defl ection mode was useful to investigate the morphology of the small goet hite crystals, with careful attention paid to operating conditions; in particular, sharp silicon probes were found to produce fewer artifact s than coarser silicon nitride ones. At low P/Fe ratios (<0.2%), the g oethite crystals were thin, elongated, multidomain laths; at high P/Fe ratios (>1.5%), star-shaped, twinned crystals were produced. All the theoretical shapes, derived from the assumption that star-shaped cryst als result from the epitaxial growth of goethite on a hematite core, w ere observed by SFM and TEM. The presence of such hematite nuclei was supported by XRD, selected-area electron diffraction, color, and prefe rential dissolution of the samples in HCl, because it is known that he matite dissolves faster than goethite in acid. With increasing P/Fe ra tio, the arms of the star-shaped crystals became shorter. This was lik ely due to the higher density of P-adsorbing pairs of singly coodinate d OH groups on terminal {021} faces relative to prismatic {110} arm fa ces.