Structure and magnetic properties of silica-coated magnetite-nanoparticle composites

Magnetite (Fe₃O₄) nanoparticles and amorphous-silica-coated magnetite-nanoparticle (Fe₃O₄/a-SiO₂) composites have successfully been synthesised using co-precipitation and modified Stöber methods, respectively. Instead of using a commercial product, the Fe₃O₄ powder was synthesised via co-precipitation method from local ironstone in Indonesia, while the a-SiO₂ was obtained from a tetraethyl orthosilicate (TEOS) precursor. The modified Stöber method was utilised to form the Fe₃O₄/a-SiO₂ core/shells. The samples were characterised using x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), and vibrating sample magnetometer (VSM) techniques. XRD and FTIR data confirmed the formation of single phase Fe₃O₄ nanoparticles and the Fe₃O₄/a-SiO₂ composites. HRTEM images showed that silica coated the Fe₃O₄ surface with an increased thickness with the addition of the TEOS. Magnetite-silica multi-core/shells were observed in the HRTEM data examination. EXAFS spectra analysis confirmed that the presence of a-SiO₂ shells does not change the crystal and local structures of the magnetite nanoparticles. Room-temperature VSM measurements showed a ferrimagnetic behaviour in both the Fe₃O₄ nanoparticle and the Fe₃O₄/a-SiO₂ core-shell, with decreased magnetisation appearing with an increased thickness of the a-SiO₂ coating.