Synthesis and Applications of Thin Ceramic Films with Oriented Nanopores Formed by Evaporation-Driven Self-Assembly

Venkat R. Koganti, University of Kentucky, 177 F Paul Anderson tower, Lexington, KY 40506-0046 and Stephen E. Rankin, Chemical & Materials Engineering, University of Kentucky, 177 F Paul Anderson tower, Lexington, KY 40506-0046.

When sol-gel ceramic precursors, surfactant, water, acid and ethanol are combined and dip coated, the ingredients co-assemble into an ordered mesophase. Hydrolysis and condensation of the precursors within the mesophase results in a stable inorganic frame work, and removal of the templates (surfactants) creates pores which are a replica of the surfactant mesophase previously present. Ordered nanopores of controlled size and morphology are formed by using surfactants as templating agents. Many research groups have been working in the direction of preparing new materials using this approach and developing applications for these materials. Control of orientation of the pores in thin film was until recently an unanswered challenge that would increase the applications of these films in membrane based separation separations, model surfaces for heterogeneous catalysis, semiconductor applications, etc.

Of the different available pore structures, 2D close packed hexagonal (HCP) cylindrical pores are of interest because the pores themselves are 1-dimensional and do not provide alternate paths for diffusing species. However, the cylindrical pores in HCP films usually align parallel to the substrate because of the preferential interactions between the surfactant molecules and the substrate surface. Taking inspiration from molecular simulations, we have shown that it is possible to prepare films with HCP cylindrical pores normal to the film by neutralizing these interactions (functionalizing the substrate to provide equal interactions with the surfactant head and tail). X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) evidence will presented to prove the orientation of these mesochannels in silica films on porous and non-porous substrates.

Forming these oriented films sometimes involves sandwiching the films between two chemically neutral surfaces after coating, which we hypothesize requires reorientation of the micelles in the film. To understand when and how this is possible, we investigate the kinetics of silica condensation in self-assembled films after coating. Using Fourier Transformed Infrared Spectroscopy (FT-IR) as our major characterization tool we study the effect of the type of surfactant mesophase, which create a polar environment for the condensing precursors. Effect of coating parameters like the humidity of the coating environment on the silica condensation will also be investigated, and it will be shown that there is indeed a period of slow condensation during which the micelle structure can be modulated, and that it can be influenced by the surfactant and process parameters.

When films with orthogonal HCP pores are deposited on porous membranes, we can use these membranes with smaller pores (compared to the commercially available ones) to perform size selective separations. The pore size can be easily controlled by choosing from a variety of surfactants, which can allow these films to be used for size exclusion separations. Also these mesopores can be readily functionalized with many functional groups which would find applications for these films in the area of site selective separations. We will liquid permeation and nanoparticle separation results for orthogonal HCP films on anodized alumina substrates that show that this application can be realized.

Finally, we demonstrate the extension of the synthesis approach to a transition metal oxide: titania. The amorphous walls and electrically insulating nature of surfactant templated mesoporous silica films limits their applications to fields such as separations and low-k dielectrics. Transition metal oxides like titania which have crystalline walls are excellent alternatives for photovoltaic, catalytic and electronic applications. We extended the idea of neutral surfaces to create crystalline titania films with orthogonally oriented HCP cylindrical mesopores.