Ph.D Thesis


Ph.D StudentAvigail Keller
SubjectConfined Templating of Mesoscopic Metal-Oxides
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professors Frey Gitti
Full Thesis textFull thesis text - English Version


Abstract

Mesostructured materials are periodic materials composed typically from metal or metal oxide networks templated by an organic self-assembled species. These materials are synthesized through two concurrent processes: surfactant self-assembly and metal-oxide polymerization. Under such conditions, the self-organized surfactant liquid-crystalline-phases template the polymerizing amorphous metal-oxide scaffolds. In this dissertation the synthesis of mesostructured silica from a tetrahydrofuran (THF) based sol gel was carried out under cylindrical confinement in anodized alumina membranes (AAM) using the evaporation induced self-assembly (EISA) method. The confinement conditions lead to frustration of the self-assembly process and induce unique channel-imposed mesostructures. Initially the conditions allowing the in-channel hexagonal mesostructure formation were found through controlling the relative humidity during the synthesis process and tuning the silica/surfactant molar ratio within the precursor solution. Two main orientations of the hexagonal phase were identified, in which the hexagonally packed cylinders are oriented either parallel or perpendicular to the membrane surface. This system was then used to study the influence of confinement on the obtained in-channel hybrid mesostructure orientation under various conditions such as channel size and shape, and surfactant type. It was found that while the channel size, in a diameter range of 50-300 nm does not affect the hexagonal orientation, faceted channels promote the perpendicular orientation.

A deeper understanding of the formation mechanism of the in-channel mesostructure allowed the usage of a different tool, channel surface chemistry, to enhance the appearance of the vertical hexagonal in-channel mesostructures. 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 self-assembled monolayer (SAM). 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 is proposed to understand the effect of anions at the channel wall on the competition between mesostructure phase transformation and silica condensation. In addition, replacing the standard ethanol/aqueous precursor solution with a THF based precursor solution allows the incorporation of hydrophobic molecules such as conjugated polymers into the precursor solution and their subsequent incorporation into the in-channel mesostructure.