A Novel Approach to Enhance High Temperature Therma Stability of Superparamagnetic Fe3O4 Nanoparticles

We report a novel approach to enhance thermal stability of magnetite nanoparticles by entrapping them in silver matrix. The oleic acid capped nanoparticles of average crystallite size similar to 10 nm were synthesized through co-precipitation, followed by hydrothermal microwave assisted capping of magnetite nanoparticles with Ag matrix in ethylene glycol solvent. At similar to 160 degrees C the oleic acid is desorbed from the magnetite nanoparticles, and silver is deposited on negatively charged magnetite seeds through electrostatic attraction. The prepared particles are characterized by X-ray diffraction, high resolution transmission electron microscopy, dynamic light scattering and UV-vis spectroscopy. The surface plasmon resonance band at 479 nm and large average hydrodynamic diameter of similar to 432 nm confirm the Ag matrix around the magnetite particles. Thermal stability of prepared particles was assessed using in situ high temperature XRD, thermo gravimetric analysis and differential scanning calorimetry and the results are compared with that of oleic acid capped particles. Magnetite nanoparticles in silver matrix showed no phase conversion up to 900 degrees C, while oleic acid capped and bare nanoparticles are phase converted to nonmagnetic alpha-Fe2O3 phase between 400-450 degrees C, under vacuum annealing. A rapid growth of Fe3O4 nanoparticles at higher temperature was observed due to faster coalescence through diffusion and higher surface free energy of magnetite as compared to silver. The saturation magnetization of oleic acid capped magnetite particles in silver matrix before and after vacuum annealing at 900 degrees C were 25.4 and 46.7 emu/g, respectively. The retention of superparamagnetic behavior in silver matrix up to 900 degrees C is interesting for high temperature applications.