Synthesis and characterization of NiFe2O4/SiO2 superparamagnetic hollow spheres

We have synthesized a series of hollow NiFe2O4/SiO2 nanospheres to study the NiFe2O4 particle size effect on their magnetic properties. In this work, we used the functional polymer poly(MMA-co-MAA) latex (500 nm) as a core template to prepare hollow spheres. The NiFe2O4 nanoparticles were firstly deposited on the surface of the polymer sphere using the co-precipitation method. Then, the silica layer was coated on the PMMA/NiFe2O4 core-shell spheres by the sol-gel method. To create silica-coated nickel ferrite hollow spherical structures, these hybrid PMMA/NiFe2O4/SiO2 core-shell spheres were subsequently calcined in the temperature range from 450 to 900°C for 4h. X-ray diffraction pattern shows that the coated phase has a cubic spinel ferrite structure. The average crystallite sizes of the coated NiFe2O4 nanoparticles, depending on the calcined temperature, are in the range from 2.1 to 8.9 nm. The scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs show that the hollow spheres are uniform and the thickness of the composite shell of NiFe2O4/SiO2 sphere is about 50 nm. Based on the thermogravimetric analysis (TGA), we find that the content of NiFe2O4 is 83.1 wt% in the NiFe2O4/SiO2 composite shell. We have measured the magnetization σ (H) isotherms for all samples at room temperature. All hollow spheres exhibit the superparamagnetic behaviour. The spontaneous magnetization σs (300 K) clearly decreased as the particle size decreased. This phenomenon can be interpreted as the effect of surface spin canting when the particle size is reduced. It is supposed that the complete silica network formed through sol-gel method based on hydrolysis and condensation, which in turn hinders the growth of NiFe2O4 nanoparticles and results in an effective refinement of NiFe2O4 particles.