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

Nanocrystalline single-phase samples of Zn1-xNixFe2O4 ferrites (0<x<1) have been obtained via a soft-chemistry method based on citrate-ethylene glycol precursors, at a relatively low temperature (650 deg. 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 (∼20m2/g) being obtained for NiFe2O4 annealed at 650 deg. C for 15h. For all compositions, the surface area decreases for prolonged annealing at 650 deg. C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50nm) observed in the NiFe2O4 sample (650 deg. C, 15h) steadily increase as Ni ions were replaced by Zn, reaching ∼100nm in the ZnFe2O4 sample (650 deg. C, 15h). For all the Zn1-xNixFe2O4 samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650-400cm-1, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest ν1-tetrahedral stretching, observed at ∼615cm-1 in NiFe2O4, shifts towards lower values with increasing Zn, whereas the ν2-octahedral vibration, observed at 408cm-1 in NiFe2O4, moves towards higher wavenumbers, reaching 453cm-1 in ZnFe2O4. in ZnFe2O4