Pore shape evolution in mesoporous silica thin films: From circular to elliptical to rectangular

The synthetically tailored morphologies of mesoporous silica thin films are important for applications to sensing, separations, and catalysis. Control of mesopore shapes may be useful for controlling the sizes or orientations of analyte or reactant molecules entering the mesopores. Sol–gel methods using surfactant templates can produce a variety of mesoporous phases, including one composed of long cylindrical micelles in a hexagonal stacking arrangement, aligned parallel to the substrate. This hexagonal phase is particularly useful for analysis of the thin film stresses that produce the circular–elliptical–rectangular shape changes in the micelle cross-sections as the film dries. A simple model is proposed to describe this shape evolution using a minimal set of parameters: mesopore aspect ratios are predicted from the known Poisson ratio of the silica and the measured thickness strain of the film. The model may be of general utility for guiding the structural design of thin film mesoporous materials.