Guided self-assembly and directed restructuring of mesoscopic silica using electric fields.

摘要

Surfactant-templated synthesis of ceramics has received considerable attention due to the ease with which these methods produce materials with well-controlled nanometer-sized porosity. A key limitation with this approach is the pores produced by the surfactant template do not always adopt the desired long-range order. Specifically, the ability to straighten the pores, orient them in desired directions, and eliminate domain boundaries must be improved in order for these materials to be useful as membranes for filtration, sensing or catalysis.; One solution to this problem is to guide the self-assembly using applied fields. The idea is to produce short-range order using self-assembly while generating long-range order using the field. Electric fields are an intriguing option because they are easily controlled and can produce orientation in different directions. Recent work by Trau et al. found that mesoscopic silica prepared under the combined influence of high strength electric fields, surface registry and confinement effects possessed oriented nanochannels. The goal of this project was to study the effects of electric fields on surfactant-templated mesoscopic silica.; It was found that low strength (E ∼ 200 V/m) electric fields can both guide the self-assembly of the structure as it forms and alter the organization of material that has already ordered. This is the first demonstration of the latter effect in a templated ceramic system. Experimentally, micron-sized particles transformed into continuous fibers under the influence of the field. This macroscopic change in morphology was accompanied by alignment of the surfactant-templated pores in the direction of the field. Quantitative analysis of the response indicates these structural changes occur through an electrokinetic mechanism rather than dielectric polarization. The distinction is important as electrokinetic phenomena arise from the action of a field on free charge while polarization depends on the dielectric contrast at an interface. This suggests it is possible to orient the structure using relatively weak fields when charged surfactants are used as templates.; The low strength electric field effect described in this dissertation is a simple, but powerful, addition to the existing collection of techniques for controlling the structure of mesoscopic silica.