Solvent effects on morphologies of mesoporous silica spheres prepared by pseudomorphic transformations

In surfactant-induced, pseudomorphic transformations of submicrometer-sized nonporous spheres to mesoporous silica spheres, the surface morphologies of the products depend on the solvent used during the initial Stöber synthesis. After hydrothermal transformations employing cetyltrimethylammonium bromide (CTAB) as a surfactant, pseudomorphic products of parent silica spheres synthesized in ethanol (HT-SiO₂-EtOH) are mesoporous throughout and have smooth surfaces. In contrast, products from spheres synthesized in isopropanol (HT-SiO₂-iPrOH) or butanol (HT-SiO₂-BuOH) possess highly corrugated shells surrounding a nonporous core. On the basis of ²⁹Si solid-state magic angle spinning (MAS) NMR spectra, this significant change in surface morphology after the hydrothermal transformation is related to small differences in the degree of condensation of the parent silica spheres. In the case of HT-SiO₂-EtOH, the higher degree of condensation of the parent spheres limits sphere dissolution, and the transformation is mostly pseudomorphic. For the other two systems, parent spheres are more reactive and release more silica into solution. Porous shells are therefore formed on the surface of the remaining spheres. Morphological changes were investigated by scanning and transmission electron microscopy. The porosity of the mesoporous silica spheres produced by these reactions was determined by nitrogen sorption measurements and small-angle X-ray scattering. The diffusion depth of CTAB was revealed by nanocasting carbon into the mesoporous silica spheres via phenol-paraformaldehyde gas-phase polymerization and subsequently removing the silica structure. As a result of limited surfactant penetration into the cores of HT-SiO₂-iPrOH spheres, replicated mesoporous carbon spheres possess a corrugated mesoporous shell and a hollow core.