Synthesis and characterization of Zn1−xNixFe2O4 spinels prepared by a citrate precursor

Nanocrystalline single-phase samples of Zn1−xNixFe2O4 ferrites (image) have been obtained via a soft-chemistry method based on citrate–ethylene glycol precursors, at a relatively low temperature (650 °C). The influence of the nickel and zinc contents as well as that of heat treatments were investigated by means of X-ray powder diffraction, Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. Higher Ni content increases the surface areas, the largest one (∼20 m2/g) being obtained for NiFe2O4 annealed at 650 °C for 15 h. For all compositions, the surface area decreases for prolonged annealing at 650 °C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50 nm) observed in the NiFe2O4 sample (650 °C, 15 h) steadily increase as Ni ions were replaced by Zn, reaching ∼100 nm in the ZnFe2O4 sample (650 °C, 15 h). For all the Zn1−xNixFe2O4 samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650–400 cm−1, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest image-tetrahedral stretching, observed at ∼615 cm−1 in NiFe2O4, shifts towards lower values with increasing Zn, whereas the image-octahedral vibration, observed at 408 cm−1 in NiFe2O4, moves towards higher wavenumbers, reaching 453 cm−1 in ZnFe2O4.