Control over in-channel mesostructure orientation through AAM surface modification

The synthesis of mesostructured silica from a tetrahydrofuran (THF) based sol-gel was carried out in the channels of an anodic alumina membrane (AAM) using the evaporation induced self-assembly (EISA) method. The effect of channel surface chemistry on the orientation of the in-channel hexagonal mesostructure was studied by treating the channel walls. A variety of channel-surface modifications have been performed, including oxygen plasma treatment, atomic layer deposition (ALD) of pure alumina, and deposition of a hydrophobic monolayer. The in-channel mesostructures were characterized using transmission electron microscopy (TEM) and energy filtered TEM (EFTEM). It was found that these modifications control the concentration of anions at the channel surfaces, and consequently the orientation of the hexagonal mesostructure. Namely, high anion concentration at the channel surface induces the formation of the desired vertically aligned columnar hexagonal phase. A model to understand the effect of anions at the channel wall on the competition between mesostructure phase transformation and silica condensation is proposed. Finally, this study demonstrates that by judiciously modifying the chemistry at the channel walls the formation of desired orientations can be induced.