(556d) Directed Growth of Surfactant-Silica Nanostructured Hybrid Materials through Electroosmotic Flow

Nanostructured surfactant-silica hybrid materials (particles in the bulk and thin films at interfaces) spontaneously grow when a small amount of tetraethoxysilane (TEOS) is mixed into a dilute, acidified (pH ~ 0.5-1) cetyltrimethylammonium chloride (CTAC) solution. The CTAC reorganizes into hexagonally packed tubules incased in a silicate matrix [1] and can be removed to leave behind a nanoporous silica matrix suitable for functionalization for catalysis, filtration, sensors, and high aspect-ratio nanowire deposition. However despite a well-defined nanometer-level structure, the micrometer-level structure in the thin films meanders in 2-D parallel to the film plane, reducing pore accessibility, and particles are not suitable for all applications. To increase the usefulness of the final nanoporous silica, a method for guiding the structure across multiple length scales and over large areas as the hybrid material forms is needed. Here we demonstrate that weak electric fields can be used to guide the structure in continuous air-water interface thin films over large areas and over both the nanometer and micrometer length scales [2]. Scanning electron microscopy (SEM), polarized optical microscopy (POM), and transmission electron microscopy (TEM) results indicate that applying an electric field (3 V/m) during film growth results in a large area film with micrometer and nanometer structures oriented parallel to the field lines. Experiments indicate that the electrodes must pierce the interface/film in order to result in an oriented structure, supporting the hypothesis that electroosmotic flow through the hybrid guides the growth [2,3]. An electrokinetic model predicts stresses on the silicate matrix and CTAC tubules that agree with experiment and exceed the yield stresses of the materials.

[1] N. Yao, A.Y. Ku, N. Nakagawa, T. Lee, D.A. Saville, I.A. Aksay, Chem. Mater. 12, 1536 (2000).

[2] K.J. Tatum, J. Chun, J. Liu, D.A. Saville, I.A. Aksay, in preparation to be submitted to Langmuir (2006).

[3] A.Y. Ku, D.A. Saville, I.A. Aksay, in preparation to be submitted to Langmuir (2006).