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 Stober 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...