Effect of Precipitating Agent and Solvent Polarity on the Size and Magnetic Properties of Magnetite Nanoparticles Prepared by Microwave Assisted Synthesis

We probe the role of precipitating agent and solvent polarity on supersaturation and the size and magnetic properties of precipitated magnetite nanoparticles. Magnetic nanoparticles are synthesized via microwave assisted method using sodium hydroxide, ammonium hydroxide and methylamine as precipitating agents. The solvent dielectric constant is varied between 47-80 by altering the ethanol: water ratio to achieve particles of different sizes. The synthesized nanoparticles are characterized using techniques such as X-ray diffraction, small angle X-ray scattering, transmission electron microscopy, energy dispersive X-ray spectroscopy, vibrating sample magnetometry and Fourier transform infrared spectroscopy. The synthesized nanoparticles are found to be magnetite nanoparticles of average diameter in the range of 2.4-6.9 nm, 4.1-13.6 nm and 4.1-10.8 nm, when sodium hydroxide, ammonium hydroxide and methylamine are used as precipitating agents, respectively. The average particle diameter of nanoparticles obtained using sodium hydroxide is found to be smaller than the nanoparticles obtained using ammonium hydroxide and methylamine. Ammonium hydroxide, as precipitating agent, results in nanoparticles with larger size. A linear correlation between the average particle size and the dielectric constant of solvents is observed. The average particle size increases with an increase in dielectric constant, irrespective of the nature of precipitating agent. Nanoparticles obtained using sodium hydroxide at low solvent dielectric constant yields a mixture of both rod-shaped and spherical particles, whereas, nanoparticles obtained using sodium hydroxide, ammonium hydroxide and methylamine under high solvent dielectric conditions results in spherical particles, as confirmed using TEM analysis. Energy dispersive X-ray spectroscopy confirms the formation of pure magnetite phase and the absence of impurities. FTIR analysis shows the presence of Fe-O tetrahedral and octahedral stretching vibrations in all the prepared samples, further confirming the formation of iron oxide nanoparticles. The saturation magnetization of the synthesized nanoparticles is found to increase with an increase in average particle size.