Influence of Ni-dopant on the properties of synthetic goethite

The influence of Ni-dopant on the properties of α-FeOOH was investigated by XRD, FT-IR, 57Fe Mössbauer spectroscopy and transmission electron microscopy. α-FeOOH was synthesized at a highly alkaline pH by precipitation from the FeCl3 solution with the addition of tetramethylammonium hydroxide and autoclaving at 160 °C. The samples doped with Ni2+ ions were precipitated in the same way, but in the presence of varying concentrations of NiCl2. Solid solutions, having the structure of α-FeOOH, were observed in samples with the Ni/Fe ratio up to 0.05. Upon increasing the amount of Ni-dopant the XRD lines were gradually broadened. The sample with the ratio Ni/Fe = 0.10 showed NiFe2O4, besides the dominant phase having the structure type of α-FeOOH. Shifts of IR bands at 892 and 796 cm−1 were not observed in all samples doped with Ni. For the ratio Ni/Fe = 0.10, the IR bands centered at 631 and 404 cm−1 were significantly broadened. RT Mössbauer spectrum of undoped α-FeOOH and Ni-doped α-FeOOH showed distributions of hyperfine magnetic fields. Bhf decreased from 35.1 T for an undoped α-FeOOH to 32.1 T for α-FeOOH containing Ni2+ ions (Ni/Fe = 0.05). The saturation of the α-FeOOH structure with Ni2+ ions in amounts higher than ∼5 mol% was also observed by Mössbauer spectroscopy. The particle size (length) of acicular α-FeOOH particles with a maximum in the interval 180-220 nm was slightly decreased with Ni-doping, but the distribution of the length/width ratio showed no change, having a maximum at 4-5. TEM photographs additionally showed small populations of cubic-shaped or pseudocubic particles of ∼10 nm in size for the ratio Ni/Fe = 0.05 and about 10-20 nm in size for the ratio Ni/Fe = 0.10. These particles were assigned to NiFe2O4.