You can synthesize your composites in an ultrasonic bath tub. That will control the agglomeration.

Have a look to this paper:

http://pubs.acs.org/doi/abs/10.1021/nl0624263

The scalable synthesis of silica coated iron oxide is not difficult. Some detail problems should be noticed. The mixture solution containing ethanol, water, ammonia and nanoparticles should be well-dispersed by ultrasonsic treatment before adding the silica source. You can adopt the equivalently fractionated drop method for adding the TEOS, such as a certain amount of TEOS per half hour.

Finally, good luck for you.

We report herein a novel one pot single step synthesis of magnetite–silica nanocomposites. The concept of co-precipitation of iron(II) and iron(III) salts by a base has been used. The main process differentiator is the use of an alkaline solution of sodium silicate instead of a base. This unique modification helps not only in the formation of iron oxide (due to alkaline conditions) but also introduces silica in the reaction to form a magnetite–silica nanocomposite. Having a silica surface is advantageous, in that it stabilizes the silica particles, makes them biocompatible and gives opportunity to further modify and tag functional groups

via the well established silica surface chemistry. Furthermore, the process also yields porous silica, which increases the overall surface area of the nanocomposite. The nanocomposite samples have been characterized by a host of techniques, such as UV-vis spectroscopy, Fourier transform infrared

spectroscopy (FT-IR), X-ray diffraction (XRD), energy dispersive analysis using X-rays (EDAX), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), zeta potential measurements, dynamic light scattering and magnetic measurements. The utility of these high-surface area nanocomposites for different applications such as tagging (attachment of fluorophores Rhodamine, Rh B), entrapment matrix (zinc loading) and removal of arsenic for water purification has been explored

The details of the synthesis is reported in

Journal of Materials Chemistry A R.SC journal J. Mater. Chem. A, 2013, 1,

2022

You can follow this article given below for controlling size of core-shell (Fe3O4@SiO2) nanostructures