Interfacial alignment mechanism of forming spherical silica with radially oriented nanopores

When cationic surfactants are added to the Stober process, spherical particles with radially oriented mesopores can be prepared by precipitation of silica from a solution of ethanol, water, and ammonia. Van Tendeloo and co-workers proposed that these particles form by epitaxial growth of cylindrical assemblies from the facets of Ia3d cubic (MCM-48) seeds. [J. Phys. Condens. Matter 2003, 15, S3037.] Here, we reexamine this hypothesis by detailed characterization of intermediate and final mesoporous silica particles formed from ethanol/water/ammonia solutions. We find that the presence of a cubic core is not required to explain the synthesis of spherical particles with radially oriented pores. Instead, we hypothesize that the radial orientation originates at the particle surface because of the preferred alignment of CTAB micelles normal to that interface. Consistent with previous studies of the Stober process, we initially observe small, irregular silica/surfactant clusters. After an induction time, these clusters suddenly form spherical particles larger than 100 nm in diameter because of aggregation or collapse of weakly bound clusters. No ordered micellar structure forms initially, but shortly after the appearance of large spherical particles, cylindrical surfactant micelles appear and align perpendicular to the particle/solution interface. The micelles appear to maintain their alignment normal to the interface even during particle coalescence, supporting the idea that radial orientation originates at the surface rather than the interior of the particles. A study of particles formed with varying amounts of ethanol suggests that ethanol acts as a cosolvent and as a low-dielectric constant solvent to induce cooperative effects on micelle organization and particle morphology, leading to particles with radially oriented pores at a large ethanol concentration.