Carbon nanotubes have intrinsic properties, which include, high surface area, unique physical properties and morphology, high electric conductivity and their inherent size and hollow geometry can make them extremely attractive as supports for heterogeneous catalysts. The applications and scope of carbon nanotubes (CNTs) have dramatically increased and continue to expand since their rediscovery in 1991. Oberlin and Endo reported in 1976 that carbon fibers had been prepared with various external shapes which contained a hollow tube with diameters ranging from 20 to more than 500 Å along the fiber axis. They observed stacks of carbon layers, parallel to the fiber axis, which where arranged in concentric sheets. They also noted that very small cementite crystals, typically about 100 Å in a diameter, where formed at the tip of the central tube of each fiber. In 1978 Wiles and Abrahamson first mentioned carbon fibres down to 4 nm in diameter (viz. carbon nanotubes although the term "nanotubes" originates from the 1990's) found on a graphite electrode. In their 1978 publication in Carbon, Wiles and Abrahamson described a thick mat of fine fibers and crystallites which they found on graphite and carbon anodes following low current arc operation in nitrogen at atmospheric pressure. They observed fibers ranging in diameter from ca. 4 nm up to 100 nm with lengths up to 15 micrometers which also held many small crystalline particles. Further details of the structure of the carbon fibres was presented at the 14th conference on carbon in 1979 (note that this work has been republished in carbon ) From the electron diffraction study Abrahamson et al reported that the fibers consisted of wrapped graphitic basal layers with a hollow core. Also they noted that the basal layer spacing was distorted from the normal graphitic spacing, and larger. Carbon nanotubes were reported by Iijima in 1991 and the number of publications each year utilizing carbon nanotubes escalates at a hugely increasing rate.
Carbon electrodes, and in particular CNTs, are excellent electrode materials due to their good electrical conductivity and mechanical strength, as well as being relatively chemically inert in most electrolyte solutions, yet retaining a high surface activity and a wide operational potential window. Structurally CNTs can be approximated as ‘‘rolled-up’’ sheets of graphite. Conceptually, the way in which the graphite sheet is rolled up affects the electronic properties of the CNTs. In general any lattice point in the graphite sheet can be described as a vector position (n, m) relative to any given origin.
The graphite sheet can then be rolled into a tube such that the chosen lattice point is coincident with the origin. It has been shown that when |n–m| = 3q where q is an integer, then the CNT is metallic or semi-metallic, and the remaining CNTs are semi-conducting.
CNTs are formed in two principal types: single-walled carbon nanotubes (SWCNTs) which consist of a single tube of graphite, and multi-walled carbon nanotubes (MWCNTs) which consist of several concentric tubes of graphite fitted one inside the other. The diameters of CNTs can range from just a few nanometres in the case of SWCNTs to several tens of nanometres for MWCNTs. The lengths of the tubes are usually in the micrometer range. Shown below is a HR-TEM image of 'bamboo' MWCNTs randomly immobilized onto a surface. Note the 4 layers of graphite separating the nanotube into compartments akin to that observed with bamboo.
It is the small size of CNTs which often accounts for their unique properties and imparts important advantageous properties for electro-analysis such as their large surface area (typically 200–300 m2 per gram) and the ability to construct arrays of nanoelectrodes.
A. Oberlin, M. Endo, J. Crystal Growth 1976, 32, 335
P. G. Wiles, J. Abrahamson, Carbon 1978, 16, 341
J. Abrahamson, P. G. Wiles, B. L. Rhoades, 1979, 14th Biennial
J. Abrahamson, P. G. Wiles, B. L. Rhoades, Carbon 1999, 37, 1873