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ScienceWeek
CHEMISTRY: ALIGNED CARBON NANOTUBE MEMBRANES
The following points are made by B.J. Hinds et al (Science 2004 303:62):
1) Advances in nanoporous membrane design with improvements in both chemical selectivity and high flux can directly benefit the fields of chemical separations, drug delivery, and wastewater remediation. Matching of pore size to that of the target molecules is critical to further advancement, because it will allow molecular sieving and forced interactions with chemically selective molecules bound to the pore. This is a particularly difficult challenge in the 1- to 10-nm size range.
2) Numerous approaches are being investigated, including functionalized polymer affinity membranes (1), block copolymers (2), and mesoporous macromolecular architectures (3). Nanometer-scale control of pore geometry and demonstration of molecular separations have been achieved through the plating of nanoporous polycarbonate ion track-etch (4) and ordered alumina (5) membranes with initial pore dimensions of 20 to 50 nm. A major challenge to improving the selectivities of pore-plated membranes is minimizing the variations in initial alumina pore diameters, because the resultant diameter is the difference between the plating thickness and the initial pore diameter. Thus, it is beneficial to start with a membrane structure that has an initial pore diameter near that of the target molecule diameter with small dispersion.
3) In principle, the inner cores of carbon nanotubes (CNTs) can enable fine control of pore dimension at the nanometer scale. During the CNT growth process, the nanotube size is set by the diameter of the catalyst particle, offering a practical route for pore diameter control through well-determined catalyst synthesis with nanometer-scale diameter dispersion. Transport through a single CNT with a 100-nm inner-core diameter, embedded across a polymer film, has been successfully demonstrated, but it is a substantial challenge to align large numbers (~10^(11)/cm^(2)) of CNTs with well-controlled nanometer-scale inner diameters across a robust membrane structure. Carbon has also been deposited into porous alumina structures by a template method, making an aligned CNT membrane. However, the inner diameters of these CNTs were 50 nm, limiting their potential usefulness in molecular separation applications.
4) In summary: The authors report that an array of aligned carbon nanotubes (CNTs) was incorporated across a polymer film to form a well-ordered nanoporous membrane structure. This membrane structure was confirmed by electron microscopy, anisotropic electrical conductivity, gas flow, and ionic transport studies. The measured nitrogen permeance was consistent with the flux calculated by Knudsen diffusion through nanometer-scale tubes of the observed microstructure. Data on Ru(NH3)(sub6)(sup3+) transport across the membrane in aqueous solution also indicated transport through aligned CNT cores of the observed microstructure. The lengths of the nanotubes within the polymer film were reduced by selective electrochemical oxidation, allowing for tunable pore lengths. Oxidative trimming processes resulted in carboxylate end groups that were readily functionalized at the entrance to each CNT inner core. Membranes with CNT tips that were functionalized with biotin showed a reduction in Ru(NH3)(sub6)(sup3+) flux by a factor of 15 when bound with streptavidin, thereby demonstrating the ability to gate molecular transport through CNT cores for potential applications in chemical separations and sensing.
References (abridged):
1. E. Klein, J. Membr. Sci. 179, 1 (2000)
2. T. Thurn-Albrecht et al., Adv. Mater. 12, 787 (2000)
3. J. T. Asefa, M. J. MacLachlan, N. Coombs, G. A. Ozin, Nature 402, 867 (1999)
4. K. B. Jirage, J. C. Hulteen, C. R. Martin, Science 278, 655 (1997)
5. E. D. Steinle et al., Anal. Chem. 74, 2416 (2002)
Science http://www.sciencemag.org
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STRUCTURE AND PROPERTIES OF CARBON NANOTUBES
The following points are made by H.F. Bettinger et al (J. Am. Chem. Soc. 2001 123:12849):
1) Multiwall carbon nanotubes, a new tubular form of carbon, were discovered in 1991 by S. Iijima. Carbon nanotubes are comprised of a rolled graphite sheet ("graphene") and closed by fullerene-like caps. Depending on the way the graphene is rolled, different chiralities are possible, and are commonly distinguished by their chiral vector (n,m). The (n,n) tubes are called "armchair" and the (n.0) tubes are called "zigzag" nanotubes. A simple analysis imposing appropriate boundary conditions on the graphene band structure predicts that the armchair tubes are metallic (i.e., the band gap is zero due to band crossing), whereas the zigzag tubes are either semimetals or semiconductors, depending on the value of (n). The computations of the band structures using density functional theory (plane-wave pseudopotential local density approximation) indicate that these simple rules need to be refined, at least for narrow zigzag tubes. It was found by these calculations that the rehybridization of the carbon states due to the strong curvature of small tubes introduces low-lying conduction band states into the band gap of insulating tubes.
2) Whereas Iijima's multiwall carbon nanotubes consist of at least 2 concentric tubes, single wall carbon nanotubes are produced by laser vaporization of a metal-graphite (Co, Ni) target. These single-wall nanotubes are comprised of a single rolled graphene sheet, but have a tendency to form "ropes", i.e., bundles of single-wall nanotubes. The tubes produced by the laser-oven technique have very uniform diameters of approximately 1.38 +- 0.02 nanometers, according to x-ray diffraction, and an intertube distance of 0.315 nanometers in the ropes, similar to that in crystalline C(sub60). It has been concluded that the ropes are made up of (10,10) armchair single-wall nanotubes.
J. Am. Chem. Soc. http://pubs.acs.org/JACS
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CARBON NANOTUBES -- THE ROUTE TOWARD APPLICATIONS
The following points are made by R.H. Baughman et al (Science 2002 297:787):
1) There are two main types of carbon nanotubes that can have high structural perfection. Single-walled nanotubes (SWNTs) consist of a single graphite sheet seamlessly wrapped into a cylindrical tube. Multiwalled nanotubes (MWNTs) comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk.
2) Despite structural similarity to a single sheet of graphite, which is a semiconductor with zero band gap, SWNTs may be either metallic or semiconducting, depending on the sheet direction about which the graphite sheet is rolled to form a nanotube cylinder. This direction in the graphite sheet plane and the nanotube diameter are obtainable from a pair of integers (n,m) that denote the nanotube type (1). Depending on the appearance of a belt of carbon bonds around the nanotube diameter, the nanotube is either of the armchair (n = m), zigzag (n = 0 or m = 0), or chiral (any other n and m) variety. All armchair SWNTs are metals; those with n - m = 3k, where k is a nonzero integer, are semiconductors with a tiny band gap; and all others are semiconductors with a band gap that inversely depends on the nanotube diameter (1).
3) The electronic properties of perfect MWNTs are rather similar to those of perfect SWNTs, because the coupling between the cylinders is weak in MWNTs. Because of the nearly one-dimensional electronic structure, electronic transport in metallic SWNTs and MWNTs occurs ballistically (i.e., without scattering) over long nanotube lengths, enabling them to carry high currents with essentially no heating (2,3). Phonons also propagate easily along the nanotube: The measured room temperature thermal conductivity for an individual MWNT (>3000 W/m.K) is greater than that of natural diamond and the basal plane of graphite (both 2000 W/m.K) (4). Superconductivity has also been observed, but only at low temperatures, with transition temperatures of about 0.55 K for 1.4-nm-diameter SWNTs (5) and ~5 K for 0.5-nm-diameter SWNTs grown in zeolites.
4) In summary: Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.
References (abridged):
1. S. G. Louie, Top. Appl. Phys. 80, 113 (2001)
2. W. Liang, et al., Nature 411, 665 (2001)
3. S. P. Frank et al., Science 280, 1744 (1998)
4. P. Kim et al, Phys. Rev. Lett. 87, 215502 (2001)
5. M. Kociak, et al., Phys. Rev. Lett. 86, 2416 (2001)
ScienceWeek http://scienceweek.com
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