Functionalization of Nanotube Surfaces
Read full article onlineFull Article
Carbon is one of the most prevalent elements in nature, and is critical to a number of important biological and geological structures. Of importance to chemistry, it can form sp-, sp2-, and sp3-hybridized bonds permitting the production of a wide range of geometries and hence, compounds. Graphite, albeit a key component of pencils, has one of the largest in-plane elastic moduli of any material, because of the extended sheets of sp2-hybridized carbon arranged in a honeycomb mesh of hexagons. Diamond is the gem of choice, although it is simply a thermodynamically metastable form of carbon with the largest hardness of any material; it has the highest thermal conductivity and melting point of any solid. Fullerenes, discovered by Kroto et al. in 1985, were produced by using laser vaporization techniques in the gas phase and were initially serendipitously detected using a molecular beam apparatus. They now have been reported to occur naturally in some forms of carbon soot and have been produced by resistive heating of carbon rods in a vacuum, in plasma discharges between carbon electrodes in He, and by oxidative combustion of gasoline/benzene/argon gas mixtures. These molecules, essentially truncated icosahedrons, can be viewed as graphite sheets wrapped into closed carbon cages, consisting of 60 atoms shaped into a soccer ball with pentagons that can then accommodate curvature in the system.
Another potentially more useful class of materials has arisen from the generation of fullerenes, that is essentially a nanometer-scale self-assembled structure of carbon. This time, however, we are not referring to cake and sandwich structures but rather to graphitic carbon, forming seamless cylindrical shells, the carbon nanotubes. Of an intrinsic structural beauty, these tubes consist of shells of sp2-hybridized (trivalent) carbon atoms forming a hexagonal network that is itself arranged helically within the tubular motif. Nanotubes are observed in two distinct structural forms (which can be controlled by growth conditions): multiwall nanotubes (MWNTs), which are made by coaxially nesting successively larger nanotubes separated roughly by the interplanar graphite spacing of ∼ 0.34 Å, and single-wall nanotubes (SWNTs), which consist of a single graphene sheet rolled into a seamless tube.