Direct imaging of the layer-by-layer growth and rod-unit repairing defects of mesoporous silica SBA-15 by cryo-SEM

The formation process of highly ordered mesoporous silica SBA-15 in the final growth stage is investigated by combined techniques of cryogenic SEM, TEM, SAXS and nitrogen sorption analysis. SBA-15 matrices at different reaction times after the addition of silica source are captured using liquid nitrogen quenching. The effects of the hydrothermal treatments, "Pluronic P123/silica" rod-units for repairing and "hole-like'' surface defects are studied in detail. Well-dispersed flocs with spheroidal-like and elongated-rod-like morphology made of "Pluronic P123/silica'' composites are progressively formed on the surface of SBA-15 rods to repair the structural defects after the phase separation, accompanied by a process of both further cross-linking of silica species and mesostructure reorganization. Direct evidence from cryogenic SEM and TEM images confirms that the sizes of the repairing units evolve from several nanometres to several tens nanometres. A hydrothermal treatment at 130 degrees C further confirms that the "Pluronic P123/silica'' repairing units is supplied from the original solution, suggesting that it plays a critical role in the reorganization of mesostructures. Moreover, closed and concaved terminals of the rod-like SBA-15 particles induced by inhomogeneous growth speeds are observed. A mechanism of rod-unit repairing defects and "layer-by-layer" growth is proposed. Our findings might give insights into the understanding of both the effects of the hydrothermal treatment on the structural reorganization of SBA-15 and of the growth process of nano-structures.