Synthesis and characterization of magnetic nanoparticles coated with silica through a sol-gel approach

This paper investigates the influence of reaction medium pH on silica-coating of magnetite nanoparticles. Magnetite nanoparticles were prepared by means of a reduction-precipitation method using ferric chloride as a starting material, which was partially reduced to ferrous salts by Na2SO3 before alkalinizing with ammonia. The particles were coated by sol-gel method with either ammonia or HCl aqueous solutions for either base- or acid-catalyzed hydrolysis, respectively. Powder X-ray diffraction, Fourier-transform infrared, and Zeta Potential were used for the characterization of oxides and of the coated magnetic nanoparticles. The observed difference of pH IEP in KCl solution for pure silica (2.0), magnetite (5.0), and silica-coated magnetite (2.3) samples confirms that the coating process was effective since the charge surface properties of coated magnetic nanoparticles are close to that of pure silica, even though the Fourier-transform infrared spectra did not evidence the formation of Fe-O-Si bonds.