A facile preparation method for single-hole hollow Fe3O4@SiO2 microspheres

Abstract

Hollow Fe3O4@SiO2 microspheres with single holes in their shells and Fe3O4 nanoparticles encapsulated in silica shells were prepared using a precipitation-phase separation method. The synthesis was performed by mixing a tetrahydrofuran (THF)–acetonitrile (MeCN) solution containing polymer, tetraethyl orthosilicate (TEOS) and n-dodecyltrimethoxysilane (DTMS)-modified Fe3O4 nanoparticles with an aqueous solution containing cetyltrimethyl ammonium bromide (CTAB) before adding ammonia to catalyze the hydrolysis of TEOS and finally removing the polymer by dissolving it in THF. The polymer used was poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) or a mixture of polystyrene (PS) and polymethyl methacrylate (PMMA). The effects of modifying the Fe3O4 NPs, tailoring the particle size and the hole size and controlling the Fe3O4 content in the Fe3O4@SiO2 hollow microspheres were investigated. The prepared Fe3O4@SiO2 single-hole hollow microspheres were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). We observed that modifying Fe3O4 NPs with DTMS is essential for encapsulating the Fe3O4 NPs in a silica matrix. The hole size of the microspheres increased with the PMMA-to-PS mass ratio, and the hole size could be adjusted from 16nm to 135nm. The size of the microspheres could be tailored from 240 to 1270nm by modulating the process parameters. The Fe3O4@SiO2 microspheres exhibited superparamagnetic properties, and the corresponding saturation magnetization (Ms) values varied from 10 to 46emu/g when controlling the modified Fe3O4 NP content in the microspheres. Based on our results, we propose a possible formation mechanism for the Fe3O4@SiO2 hollow microspheres.