Sonochemical approach to the synthesis of Fe3O4@SiO2 core−shell nanoparticles with tunable properties

In this study, we report a rapid sonochemical synthesis of monodisperse nonaggregated Fe3O4@SiO2 magnetic nanoparticles (NPs). We found that coprecipitation of Fe(II) and Fe(III) in aqueous solutions under the effect of power ultrasound yields smaller Fe3O4 NPs with a narrow size distribution (4−8 nm) compared to the silent reaction. Moreover, the coating of Fe3O4 NPs with silica using an alkaline hydrolysis of tetraethyl orthosilicate in ethanol−water mixture is accelerated many-fold in the presence of a 20 kHz ultrasonic field. The thickness of the silica shell can be easily controlled in the range of several nanometers during sonication. Mössbauer spectra revealed that nonsuperparamagnetic behavior of obtained core−shell NPs is mostly related to the dipole−dipole interactions of magnetic cores and not to the particle size effect. Core−shell Fe3O4@SiO2 NPs prepared with sonochemistry exhibit a higher magnetization value than that for NPs obtained under silent conditions owing to better control of the deposited silica quantities as well as to the high speed of sonochemical coating, which prevents the magnetite from oxidizing