Synthesis of nickel ferrite nanoparticles by sol-gel method

NiFe₂O₄ nanoparticles were prepared by the sol-gel method using polyacrylic acid (PAA) as a chelating agent. It was shown that pure spinel NiFe₂O₄ nanoparticles with a diameter of 5∼30 nm and a specific surface area of 20.0∼55.2 m²/g could be produced by calcining the gel precursors with various molar ratios of PAA to total metal ions (0.5∼2.0) in air at 300°C for 2h. As the molar ratio of PAA to total metal ions increased particle sizes decreased but the crystallinity increased revealing PAA not only worked as a chelating agent to disperse the cations homogeneously but also contributed its combustion heat to increase the crystallinity of NiFe₂O₄ nanoparticles. Moreover the magnetic analysis showed the resultant NiFe₂O₄ nanoparticles might be superparamagnetic. Their saturation magnetization (0.19 emu/mol at 298 K) was slightly lower than that for bulk materials due to small size and increased slightly with decreasing the temperature below 360 K.