Synthesis and characterization of silica-coated magnetic nanoparticles

Abstract

The present work reports on the synthesis and properties of magnetic nanoparticles based on magnetite coated by a silica layer.A series of conditions was tested to allow the synthesis of the nanoparticles, usually performed in an aqueous medium, and could be efficiently coupled with the silica sol-gel process. The resulting particles were characterized by means of X-ray diffraction, transmission electron microscopy associated with electron energy loss spectra and electron spectroscopy imaging, Fourier transform IR, Mo¨ssbauer and magnetization measurements. Initially, magnetite particles with an average crystalline grain size of about 100 Å and a polydispersion of 30%, as revealed by X-ray diffraction and transmission electron microscopy analyses, respectively, were obtained with a mixture of FeCl 2 and FeCl 3 in aqueous and acid solution. The magnetization measurements, at room temperature, show that the particles are in the superparamagnetic regime. Magnetite was also synthesized in acid solutions in an alcoholic environment (25% methanol-to-base ratio), a medium that allows us to proceed with silicacoating by the sol-gel process in a one pot reaction. The resulting particles present size dispersion ranging from around 15 Å to about 200-300 Å as evidenced by electron micrographs. The superparamagnetic behavior is preserved, although its saturation magnetization value decreases from about 92 to about 50 emu g)1 , probably owing to the contribution of the smallest particles as well as to the surface spin disorder induced by addition of methanol to the synthesis medium. For higher values of the alcohol-to-base ratio, the resulting particles are amorphous, becoming crystalline under thermal treatment. When tetraethylorthosilicate is added to a solution containing 25% of methanol to base, iron oxides are SiO 2 -coated at room temperature, as evidenced by electron spectroscopy imaging and Fourier transform IR spectroscopy. The magnetization results are dependent on the Si-to-Fe volume ratio, in such a way that the values decrease as the SiO 2 amount increases, reflecting the nanoparticle coating.

Sulfonic acid-functionalized silica-coated magnetic nanoparticle catalysts

“…The same peaks were observed in the both the bare and silica-coated nanoparticle XRD patterns, indicating retention of the crystalline spinel ferrite core structure during the silica-coating process. The broad peak from 2θ = 20 • to 30 • was consistent with an amorphous silica phase in the shell of the SiMNPs [39].…”

Morphological studies of randomized dispersion magnetite nanoclusters coated with silica

“…We were first examining the different polarities of alcohol that is essential in determining the size distribution and morphology of the nanoclusters. Haddad et al had reported that the volume ratio of solvent-water plays an important role in determining the shape of silica-coated magnetite nanoparticles [26]. They found that low water volume will prevent rapid hydrolysis and condensation of TEOS.…”

Dual-stimuli responsive PNiPAM microgel achieved via layer-by-layer assembly: Magnetic and thermoresponsive

“…in silica since the latter can be readily functionalised to impart the protective shell [13,14]. We recently reported the in situ as well as the post-modification of MNP with polyelectrolytes, without the need for encapsulation in silica [15,16].…”