Intermediate pickering emulsion formation as a means for synthesizing hollow mesoporous silica nanoparticles

Hollow mesoporous silica spheres have been successfully synthesized in a one-pot synthesis using an oil-in-water emulsion-based approach under alkaline conditions. Detailed electron microscopy investigations suggest that silica nanoparticle-stabilized emulsion droplets initially form, and the particles grow by continuous attachment of nanoparticles onto this layer. The size of the growing particles is suggested to be limited by electrostatic interactions, paralleling what has been observed for the growth of Stöber particles. Variation of the oil concentration can be used to tune the diameter of the emulsion droplets, and thus the diameter of the hollow core of the final particles. A co-operative emulsion droplet stabilization through free surfactant and surfactant molecules adsorbed on the silica nanoparticles is suggested. The shell is mesoporous with a narrow pore size distribution corresponding to that expected for supramolecular porogens. However, an additional contribution of textural mesopores to the overall shell porosity is observed at higher oil concentrations in the synthesis.