Nanoscale observation of morphological transformation during ageing of silica and silica-alumina

Atomic force microscopy (AFM) was used for in-situ observation of nanoscale morphological transformations during the ageing step in sol-gel synthesis. Silica, alumina and silica-alumina samples with different Si/Al ratios were prepared from inorganic salt precursors and geled at low pH. Silica and silica-alumina samples formed branch-like gel network made of nanometer-sized clusters. During ageing at room temperature, the overall structure of the gel network remained unchanged but the clusters underwent phase transformation, coaslesence, coarsening, fragmentation, as well as dissolution resulting in the internal restructuring of the gel material. Morphological transformation associated with crystallization of pseudo-boehmite phase was observed for the alumina samples. These nanometer-scale processes are expected to play a key role in dictating the material properties of the final sol-gel product.