Microstructural properties of Fe–oxide powders obtained by precipitation from FeCl 3 solutions

Abstract

The precipitation of Fe–oxides was performed by adding NaOH or urotropin to FeCl 3 solutions and aging the precipitation systems at 90°C. Phase composition and microstructural properties of Fe–oxide powders were investigated using Fourier transform infrared spectroscopy, Mössbauer spectroscopy, X-ray diffraction and transmission electron microscopy. β-FeOOH was obtained in all precipitation systems with final pH values up to 1.80. However, when the precipitation was for a long time at pH≤1.80 the appearance of α-Fe 2O 3 phase was also observed. In the presence of urotropin the size of submicrometer β-FeOOH rods increased with time of aging thus forming a fibril structure of near micrometer size. The crystallite size of β-FeOOH varied between 7 and 11 nm and this indicated that β-FeOOH fibrils possessed a substructure. After 30 days of aging, the presence of a α-Fe 2O 3 phase was also detected by XRD. α-Fe 2O 3 was obtained as a single phase by precipitation with NaOH at pH 4.20–4.40 and aging of the precipitation systems up to 3 days. When the pH of precipitation was increased to 6.00–6.50 the formation of α-FeOOH particles, in addition to α-Fe 2O 3, was observed. TEM showed that the size and morphology of particles strongly depended on the precipitation conditions. The mechanism of the formation of Fe–oxide phases is discussed.