Partial Silica Encapsulation of Fe3O4 Nanoparticles in Reverse Emulsion by Internal Energy Modulation

While the reverse emulsion methods traditionally only give core–shell nanostructures, we show that Janus nanostructures could be readily obtained by introducing salt solutions. Given the isolated pockets of reverse emulsion, the salt solutions could reach extreme concentrations, up to 700 times higher than what is possible via the Stöber method. Both the increasing amount of the doped cations in the silica and the decreasing cationic radius would enhance the interactions with the negatively charged silica precursor, resulting in poorer "wetting" of the silica domain on the Fe3O4 nanoparticles. In other words, with increasing strength of the internal interactions, the growth domain would have a higher tendency to curl up and reduce its unfavorable interfaces, leading to poorer wetting. The tuning of the solid–solid wetting is of great significance for the design and synthesis of Janus nanoparticles, in particular their precise structural modulation. We believe that our new means of structural modulation beyond the interface itself would open new synthetic routes at the nanoscale.