Influence of inorganic and organic additives on the tailored synthesis of iron oxides

Abstract

The influence of the inorganic anions, chloride, phosphate, sulphate and perchlorate and the organic molecules, n-butyl dihydrogen phosphate, 1-naphthyl disodium orthophosphate, methylphosphonic acid, phenylphosphonic acid, methylenediphosphonic acid and 1,2-ethylenediphosphonic acid, on the synthesis of iron oxides by the forced hydrolysis of aqueous Fe^III solutions has been studied by electron microscopy, electron and X-ray diffraction and infrared and X-ray photoelectron spectroscopies. Haematite (α-Fe₂O₃) was formed in the presence of the inorganic additives. The presence of chloride and perchlorate resulted in rhombohedral crystals comprising {1014} or {1012} faces, respectively. Phosphate gave spindle-shaped crystals elongated along the c axis with stabilised faces of {hki0} form. Sulphate behaved in a similar manner but the effect was less pronounced. Computer simulations of relaxed crystal faces predicted the following order of surface energies; {1012} < {1010} < {0001} < {1210} < {1014} < {1011}. Infrared and X-ray photoelectron spectroscopies confirmed the presence of chloride, phosphate or sulphate, but not perchlorate, on the crystal surfaces. The phosphate and sulphate were bound in a bidentate manner. The organic additives, except for the diphosphonates, produced the oxyhydroxide. Iepidocrocite (γ-FeOOH), with no morphological modifications. Prismatic haematite was formed in the presence of methylenediphosphonic acid whereas 1,2-ethylenediphosphonic acid gave akaganeite (β-FeOOH). The results are discussed in terms of stereochemical and geometric interactions between adsorbed additives and selective crystal faces. Predictions of the complexation and hydrolysis behaviour of Fe^III-additive species, using the partial charge model (J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18, 259; N. J. Reeves and S. Mann, to be published) were in agreement with the experimental data.