Particle morphology and chemical microstructure of colloidal silica spheres made from alkoxysilanes

Abstract

Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.