Carbon Nanotube-Based Permeable Membranes
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Carbon Nanotube-Based Permeable Membranes Jason K. Holt, Hyung Gyu Park, Olgica Bakajin, Aleksandr Noy, Thomas Huser, and David Eaglesham Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory Livermore, CA 94551, USA ABSTRACT A membrane of multiwall carbon nanotubes embedded in a silicon nitride matrix was fabricated for use in studying fluid mechanics on the nanometer scale. Characterization by fluorescent tracer diffusion and scanning electron microscopy suggests that the membrane is void-free near the silicon substrate on which it rests, implying that the hollow core of the nanotube is the only conduction path for molecular transport. Assuming Knudsen diffusion through this nanotube membrane, a maximum helium transport rate (for a pressure drop of 1 atm) of 0.25 cc/sec is predicted. Helium flow measurements of a nanoporous silicon nitride membrane, fabricated by sacrificial removal of carbon, give a flow rate greater than 1x10-6 cc/sec. For viscous, laminar flow conditions, water is estimated to flow across the nanotube membrane (under a 1 atm pressure drop) at up to 2.8x10-5 cc/sec (1.7 µL/min). INTRODUCTION Carbon nanotubes, with their unique mechanical, electrical, and chemical properties, have been used for a variety of applications, ranging from reinforced polymer composites [1] to field-emission devices [2] and DNA sensors [3]. Consideration of their hollow cores, ranging from ~1 nm in the case of single wall nanotubes to ~10 nm in the case of multiwall nanotubes, suggests they may also find applications in the area of molecular separations. This size scale is comparable to that of typical biological macromolecules, making these materials candidates for biosensing. On this size scale, however, little is known about the behavior of fluids; deviations from continuum transport are to be expected as the channel size becomes comparable to molecular diameters. Furthermore, questions arise as to the wetting properties of polar fluids like water in the hydrophobic channel of a carbon nanotube. In recent years, a number of molecular dynamics simulations have attempted to model this system. Many exotic predictions have been made, from the formation of novel phases of ice [4] and pulsed one-dimensional water transport [5], to the spontaneous insertion of ss-DNA into a single wall carbon nanotube [6]. What has been lacking, however, is a platform for experimentally verifying some of these predictions. To date, there has been just one experimental study [7] claiming to demonstrate gas and ionic transport through a polystyrene/nanotube membrane. Here we aim to reproducibly produce a robust, nanoporous membrane in which the hollow cores of multiwall carbon nanotubes serve as the only conduit for molecular transport. Alternatively, these nanotubes would serve as a template for the production of nanoporous silicon nitride and silicon dioxide, materials that would allow for relatively easy surface functionalization to permit chemically specific as well as size-based separations.
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