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46427-AC10
Carbon Nanotube Membranes by Templated Growth in Oriented Molecular Sieve Films

Michael Tsapatsis, University of Minnesota

Carbon nanotube membranes have recently attracted attention due to experiments and simulations indicating extremely high fluxes. If selective separations can also be demonstrated, these membranes will be a major breakthrough in gas, liquid, and vapor separations with tremendous implications in energy efficiency.  This research is aiming at the development of a carbon nanotube membrane by growing submicron-thick, densely-packed, sub-nanometer diameter carbon nanotubes in the interior of the pores of oriented aluminophosphate (AlPO₄) molecular sieve films.  The AlPO₄ films alone are of interest because they can serve as highly selective membranes.  Recent progress in our group includes the successful growth of well-intergrown and appropriately-oriented AlPO₄-5 and, for the first time, submicron-thick CoAPO-5 films to be used as catalytic supports for the templated growth of carbon nanotubes.  With these films as hosts for carbon nanotube growth, it is possible that membranes with unprecedented performance will be the ultimate outcome.

To be useful as a catalytic host for the templated growth of carbon nanotubes for separations applications, an aluminophosphate film must have excellent coverage, high orientation, and reliable thickness control during synthesis.  An in situ approach to unmodified silicon substrates produced AlPO₄-5 films with good orientation but poor coverage for low reaction temperatures and short growth times, or good coverage but random orientation for higher temperatures and longer growth times.  To overcome these deficiencies, a seeded growth was introduced which involved substrate functionalization with 3-chloropropyltrimethoxysilane, deposition of AlPO₄-5-related particles on the functionalized substrates to form a seed layer, and secondary (seeded) growth to form a continuous polycrystalline AlPO₄-5 film.  A schematic of the technique is shown in Figure 1.

Figure 1. Schematic representation of the secondary growth technique for AlPO4-5 films (Karanikolos et al. Chem. Matr. 19 (2007) 792-797 and Micro. Meso. Matr. 115 (2008) 11-22).

Since AlPO₄-5 structures consist of one-dimensional straight channels with pore openings 7.3Å in cross-section running along the c- crystallographic axis, it is desirable to obtain c-oriented AlPO₄ films.  For such a film, the reaction must be stopped before intergrowth of misoriented crystals becomes dominant.  To suppress nucleation/growth and make the conditions for film formation more controllable, we introduced an initial heating treatment of the precursor mixture, and subsequently used the pre-treated solution for secondary growth on seeded silicon substrates.  Figure 2 shows the result of such an experiment for a secondary growth time of 1.5 hours.  The crystals have an average size of 1μm and cover the substrate completely, forming a well-intergrown film with an average thickness of 500nm.

Figure 2. Scanning electron microscopy image (SEM, left) and x-ray diffraction pattern (XRD, right) of an AlPO4-5 film indicating excellent coverage and high orientation, respectively (Karanikolos et al. Micro. Meso. Matr. 115 (2008) 11-22).

            To investigate the effect of the structure-directing agent in the formation and morphology of AlPO₄-5 films, experiments were performed using a variety of secondary growth times after substituting for triethylamine (TEA) by tripropylamine (TPA).  The results indicated that an average crystal thickness of less than 50nm was obtained using TEA, while the crystal thickness was increased to approximately 300nm when TPA was used.  The overall thickness of the film was unchanged during the use of TPA.

            Seeded growth experiments were performed to synthesize CoAPO-5 films through the use of a precursor solution containing cobalt acetate tetrahydrate.  Using TEA and TPA as structure-directing agents, the results from Figure 3 indicate that well-intergrown and submicron-thick cobalt-modified films have been achieved which display good coverage and preferred c-orientation by a majority of the crystals.  Again, substituting TEA by TPA yielded films consisting of thicker crystals.  For the first time, submicron-thick CoAPO-5 films were fabricated.  We are currently performing experiments using fabricated AlPO₄-5 and CoAPO-5 films as catalytic supports for the templated growth of carbon nanotubes.

Figure. 3. SEM (left) and corresponding XRD (right) of CoAPO-5 films synthesized using    TEA (top) and TPA (bottom) for secondary growth.  The scale bars correspond to 1μm (Karanikolos et al. Micro. Meso. Matr. 115 (2008) 11-22).

 

The successful fabrications of submicron-thick, well-intergrown, and highly-oriented AlPO₄-5 and CoAPO-5 films have been demonstrated.  Although carbon nanotubes formed in the pores of such hosts will be too small for most separations, the seeded technique developed in our group can be used to synthesize films with larger pore diameter frameworks, such as the 1.2nm diameter aluminophosphate VPI-5.  Introductory research has been performed with VPI-5 and considerable future effort will focus on its application toward oriented films and use as a host for carbon nanotube growth.  Additionally, research focused directly on the templated growth of carbon nanotubes will continue using AlPO₄-5 and CoAPO-5 films.  This work is performed in collaboration with Professor Avelino Corma of the Instituto de Tecnología Química and    Dr. George Karanikolos of the National Center for Scientific Research, Demokritos.

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