Abstract

Mesoporous silica particles SBA -15 (p6mm) have been studied with TEM and the growth has been followed by in-situ small x-ray angle scattering (SAXS) and ultrasmall angle x-ray scattering (USAXS). A difference in synthesis temperature of only 5 degrees dramatically changes the morphology of the mesoporous silica particles. The shape of the particles suggests a difference in growth mechanism between these two temperatures. In our previous work, we concluded that the embryonic particle is a floc consisting of micelles of the structure-directing amphiphilic polymer in a matrix of silica. With time, as silica condenses, the micelles rearrange and eventually the ordered, in this case p6mm, structure is generated. Based on the particulate shape at the lower synthesis temperature, a model is proposed to explain the growth behaviour. Once the flocs have started to organise into the ordered phase they will go through a second aggregation step with seven smaller particles/flocs joining into one larger. This appears to be an oriented aggregation with crystallographic equivalent faces joining. Due to the liquid nature of the material at this point of the synthesis the joining interface will almost “heal” forming a more or less perfect structure. However there is some reminiscence of the primary particles. The electron micrographs show particles with holes within and low contrast at the interface where the particles have joined. If the synthesis is performed at the higher temperature this oriented aggregation step does not occur and the particles have a very different morphology. The combined SAXS and USAXS measurements provide information regarding micellar evolution, formation of the ordered structure and formation of larger particles.