Time-resolved in situ raman and small-angle X-ray diffraction experiments: From silica-precursor hydrolysis to development of mesoscopic order in SBA-3 surfactant-templated silica

Time-resolved in situ Raman spectroscopy and small-angle X-ray diffraction have been combined in order to characterize the formation of hexagonal mesostructured SB A-3 silica. Experiments were performed under identical synthesis conditions in order to correlate the molecular and mesoscopic scales. Cetyltrimethylammonium bromide (CTAB) was used as the structuring agent with tetraethoxysilane (TEOS) as the silica precursor at a constant CTAB/TEOS molar ratio under highly acidic conditions and for various dilutions. Raman spectra allowed quantification of the release of ethanol that was produced solely by hydrolysis of TEOS under the experimental conditions used. The hydrolysis kinetics of TEOS was greatly enhanced in the presence of the surfactant as a result of micellar solubilization and surfactant stabilization of emulsion droplets. At the time of precipitation, the degree of TEOS hydrolysis depended strongly on the TEOS/CTAB concentration, decreasing with increasing concentration. The time evolution of the d-spacing and the full width at half-maximum of the (10) main reflection in the XRD pattern showed two distinct stages that were also strongly dependent on the sol composition. There seemed to be a coexistence of an ordered phase and a disordered one for a longer time in SBA-3 compared with MCM-41, clearly indicating that the ordering process is slower for SBA-3. © 2008 American Chemical Society.