Facile highly scalable method for templating hollow silica spheres using a two step synthesis

In this work we have developed a facile highly scalable two step method for templating hollow silica nanospheres using catanionic vesicles as templates. The template consisted of equilibrium unilamelar catanionic vesicles formed from mixtures of cationic and anionic surfactants. A thorough investigation of the template's behavior was necessary in order to proceed in templating synthesis. The template's (catanionic vesicles) structure, phase and stability were characterized by independent techniques such as cryo-Transmission Electron Microscopy, Small Angle Neutron Scattering and Dynamic Light Scattering. Hollow silica spheres were synthesized using catanionic vesicles as a template and tetramethoxysilane (TMOS) as the silica precursor under acidic conditions. Transmission Electron Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy and Dynamic Light Scattering were the techniques that helped us to characterize the hollow silica nanospheres, further understand and face the challenges of the particular templating sol-gel process. The competition between the adsorption of the hydrolyzed silica precursor onto the vesicular surface and the homogeneous nucleation in the bulk (resulting in gel formation and particle trapping-caging into the gel) was never faced as an optimization challenge. We find that by introducing a second step, under conditions close to the Stober synthesis, non adsorbed silicate species condense resulting in solid particle formation and produce two distinctly different particle size populations of hollow and solid silica beads, which can easily be separated due to their significant density difference. Thus, the second step maximizes the yield of the templated synthesis and optimizes the sol-gel batch process.