Materials Chemistry and Physics, Vol.163, 88-98, 2015
Controlled phase formation of nanocrystalline iron oxides/hydroxides in solution - An insight on the phase transformation mechanisms
In this work, the effect of pH in the formation of iron oxides/ hydroxides obtained by chemical precipitation, as well as the influence of the presence of Fe(II) in the phases formation process were studied. 1ron(III) chloride nonahydrated and iron(II) chloride tetrahydrated were used as precursors and sodium hydroxide as a base to adjust the pH and to promote the hydrolysis and condensation reactions. Syntheses were performed at three pH values -4, 7 and 12 - and two Fe(II)/Fe(III) molar ratios -0.02 and 0.5. The obtained phases were observed by SEM, showing agglomerates of nanometric crystallites ranging from similar to 1 to similar to 15 nm in size. The iron oxides/hydroxides identification/quantification was performed by the combined use of FTIR, XRD and Miissbauer spectroscopy. It was concluded that at low pH the catalytic effect of Fe(II) in the transformation of ferrihydrite is inhibited independently of the Fe(11) amount. In alkaline medium and at low concentration of Fe(II), goethite was formed. A Fe(I1)/Fe(III) ratio of 0.5, for pH of 7 or 12, led to the simultaneous formation of magnetite and goethite. Thus, the extent of the transformation mechanisms topotactic and reconstructive is strongly influenced by the pH and the Fe(II)/Fe(III) ratio. In alkaline conditions, the predominant mechanism is defined by the ratio of Fe(II)/ Fe(III) - a higher ratio favors the topotatic transformation of ferrihydrite in magnetite, while a low ratio leads to a dominant reconstructive mechanism. (C) 2015 Elsevier B.V. All rights reserved.