Influence of polyol structure and molecular weight on the shape and properties of Ni0.5Co0.5Fe2O4 nanoparticles obtained by sol-gel synthesis

This paper presents the formation and decomposition of silica embedded metal carboxylates obtained using Co-, Fe- and Ni- nitrates and polyols with different structure and molecular weight (1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2,3-propanetriol). The thermal behavior of carboxylate precursors (oxalic, malonic, lactic, hydroxymalonic and succinic) is similar for all used polyols, both the formation and decomposition of precursors occurring in two stages. However, the mass loss slightly increases with increasing of the diol chain length and significantly increases with the presence of a supplementary, nonreactive secondary hydroxyl group. X-ray diffraction shows the formation of NiO single phase or NiO and CoFe2O4 mixtures below 750 degrees C, and of mixed Co-Ni ferrite single phase at 1000 degrees C, while Fourier transformed infrared spectroscopy confirms the formation of silica matrix and the formation and decomposition of carboxylate precursors. Transmission electron microscopy shows spherical nanoparticles of different size. The Ni0.5Co0.5Fe2O4 nano crystallites size (4.3-28.1 nm) and relative crystallinity (4-100 a.u.) increase with the annealing temperature, the diol chain length and the distance between hydroxyl groups. The coercive field (0.015-0.6 T) and saturation magnetization (21.5-25 emu/g) of gels annealed at 1000 degrees C also increase for longer chain length diols.