Modified solvothermal synthesis of magnetic microspheres with multifunctional surfactant cetyltrimethyl ammonium bromide and directly coated mesoporous shell

The Fe3O4 magnetic microspheres were prepared by a cetyltrimethyl ammonium bromide (CTAB) modified solvothermal process. Then CTAB-modified Fe3O4 microspheres were directly coated by mesoporous SiO2 and γ-AlOOH shell, respectively, and the SiO2@Fe3O4 and γ-AlOOH@Fe3O4 magnetic core–shell composites were obtained. The samples were characterized by FT-IR, XRD, Raman spectroscopy, SEM, TEM, N2 adsorption–desorption technology, and vibrating sample magnetometer (VSM). The results indicate that CTAB molecules play the roles of capping agent, dispersant, and crystal growth oriented agent during the high-temperature solvothermal process. Thus as-prepared Fe3O4 microspheres are assembled by small primary nanocrystals with uniform crystal orientation, and exhibit narrow size distribution, monodispersity, and superparamagnetism with high saturation magnetization (Ms). The formation of Fe3O4 microspheres combines oriented attachment and Ostwald ripening mechanisms. Furthermore, the adsorbed CTAB molecules can serve as nucleation seeds for precipitation of SiO2 and γ-AlOOH, and as templates for growth of mesoporous SiO2. In SiO2@Fe3O4 the mesoporous SiO2 shell presents short-range ordered pores with mean pore size of 2.1 nm. The shell of γ-AlOOH@Fe3O4 is composed of many irregular γ-AlOOH nanosheets with thickness of 3.0–5.0 nm. The BET surface areas of SiO2@Fe3O4 and γ-AlOOH@Fe3O4 reach up to 441 m2/g and 289 m2/g, respectively.