Interfacial Alignment Mechanism of Forming Spherical Silica with Radially Oriented Nanopores

Abstract

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 c...