In Situ X-ray Scattering Observation and Collective Analyses of the Complete Formation Process of Mesoporous SBA-15

The dynamic formation process of mesoporous silica (SBA-15) using hexane as a swelling agent is fully elucidated through the in situ small-angle X-ray scattering (SAXS) technique in a custom-made reactor. The micelles rapidly transform from spherical to cylindrical shapes following the addition of tetraethyl orthosilicate (TEOS) and subsequently self-assemble into a two-dimensional hexagonal structure. Principal component analysis (PCA) shows that the growth of SBA-15 involves two distinct processes related to the formation of micelles and superlattices. The time evolutions of the size and electron density distribution of micelles are obtained by fitting the SAXS data with a three-layer cylindrical model and analyzing the reaction kinetics. Pair distribution function (PDF) analysis shows that the sizes of the core and corona of micelles decrease over time. The micelles initially form highly ordered hexagonal superlattices, but as the synthesis proceeds, the lattice constant and ordering decrease, as evidenced by the shift and broadening of the Bragg peaks. The electron density distribution inside superlattices is obtained by simulating the Bragg peak intensities, considering contributions from both form and structure factors and directly comparing with the experimental data. The formation of large superlattice aggregates and the partial disassembly of superlattices in the later stage of the reaction were also observed. The methodology developed here holds promise for widespread applications in the in situ X-ray scattering observation of various chemical reaction processes, paving the way for the design and synthesis of complex functional materials.