The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), 57Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl3. The XRD analysis of the samples precipitated in the presence of Zn2+ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe2O4 was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe2O3 and ZnFe2O4 were formed for r = 0.1304. In the corresponding FT-IR spectra, the FeOH and FeO stretching bands were sensitive to the Zn2+ substitution, whereas the FeOH bending bands of α-FeOOH at 892 and 796 cm−1 were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in Bhf values in dependence on Zn-doping. A strictly linear decrease in Bhf for α-FeOOH doped with Zn2+ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe2O3 and ZnFe2O4 phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe2O3 with Zn2+ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (∼500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe2O4 particles were around 30–50 nm, and those of α-Fe2O3 particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe2O3 and ZnFe2O4 particles was suggested.