Coating with silica works most of the time, with problems attached. The problem is indeed how to stop the residual reaction of silica precursor during the separation of the coated particles. If you can somehow minimise the condensation rate of silica precursor during the separation process this may reduce significantly the aggregation observed.

Looking at a different angle this is not surprising at all. Unless SiO2 has a very strong interaction with Fe3O4 that can overcome the SiO2-SiO2 interactions, they will prefer to aggregate under thermal conditions.

Did you do phase transfer of hydrophobic Fe3O4 from organic phase to aqueous phase via CTAB assisted emulsification and solvent evaporation? Do you want to get a mesoporous silica shell or a nonporous silica shell? When did the aggregation take place, during phase transfer or after silica coating?

I think you should establish a careful control over the stirring speed and the surfactant quantities, before the adding of silicon source

Thank all so much! I did the experiments with the phase transfer via CTAB. then coated silica . I was aggregated. I need the particles that contain some Iron Oxide inside each silica shell, in small size. With the phase transfer. I did not see any precipitation , it is dispersed solution. I am now preparing experiments for silica coating steps (NH3-TEOS-EthOH-H2O- ...) If anyone has hints, pls help me !!

If you want to obtain a mesoporous silica coating after CTAB induced phase transfer, you can check your experimental parameters by comparing with the literatures (http://onlinelibrary.wiley.com/doi/10.1002/anie.200802469/abstract;jsessionid=296589901D707AB9A5EA04F544F9900A.f03t04; http://pubs.acs.org/doi/abs/10.1021/cm901259n). If you want to achieve a nonporous silica coating, excess amount of CTAB from phase transfer needs to be removed before you add NH3/EtOH/H2O/TEOS.

Right !!! Thank you so much!!!! :)

10.1021/jp1075498 -- J. Phys. Chem. C 2011, 115, 1436–1443

Let me see :) Thanks