Formation of mesoporous SBA-15 from a colloidal perspective

The formation of the hexagonal ordered mesoporous SBA-15 material has been invetsigated. The evolution from free spherical micelles into an ordered material was studied with time-resolved in-situ SAXS, USAXS and SANS measurements. Both the mesoscopic region and the colloidal region were monitored. Temperature, precursor and salt additions have been varied. From the time-resolved measurements the events taking place in the formation of SBA-15 was identified and put on a time line. An additional step was found in the formation of plate-like particles, 7 smaller primary particles build up a larger secondary particle. The aggregation of primary particles was highly specific, an oriented aggregation. It was possible to hinder the oriented aggregation by diluting the synthesis reaction at a specific time. Moreover, a salt addition at this specific time give rise to particles with a very large diameter while the thickness was constant. A model to explain the morphology of the mesoporous particles was proposed. The model is based on colloid and surface chemistry arguments and stresses the importance of the surface free energy of the respective interfaces.