Influence of Metal Ions on the Transformation of γ-FeOOH into α-FeOOH

The transformation of γ-FeOOH into α-FeOOH in solutions at 50°C was investigated in two different ways by dissolving Ti(IV), Cr(III), Cu(II), Ni(II), and Mn(II) in the solutions at atomic ratios of metal/Fe of 0-0.1 and adding these metal ions to the γ-FeOOH particles. The transition into α-FeOOH was examined by Fourier transform infrared spectroscopy and X-ray diffraction. Ti(IV), Cr(III), and Cu(II) dissolved in the solutions markedly interfered with the transition into α-FeOOH, whereas Ni(II) and Mn(II) showed no influence. The inhibitory effect of the former three metal ions is ascribed to the protection of γ-FeOOH particles against dissolution in solution by surface coating with the hydrolysis species of metal ions, because transmission electron microscopy and inductively coupled plasma-Auger electron spectroscopy demonstrated that the conversion proceeds by dissolution and recrystallization. The metal ions added in γ-FeOOH particles promoted the conversion into α-FeOOH, which was interpreted by the enhancement of dissolution of γ-FeOOH particles due to the decrease in particle size by the addition of metal ions. © 2004 The Electrochemical Society. All rights reserved.