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Posted: Jun 03, 2016

Birth and early history of carbon nanotubes (page 4 of 5)

In 1984, Howard G. Tennent24 of Hyperion Catalysis International, Inc. filed the first patent claiming a process for preparing carbon filaments termed as “carbon fibrils” having nanometer diameter that is substantially free of pyrolytically deposited thermal carbon.
This was achieved by using catalysts on supports such as fumed alumina and the carbon fibrils produced by catalytic vapor phase deposition were composed of multiple continuous layers of ordered carbon atoms with a distinct inner core region. Each of the layers and core are disposed substantially concentrically about the cylindrical axis of the fibril.
Even though, at the time of invention, it was not claimed as multiwalled carbon nanotubes, later studies showed that catalytically grown fibrils have the same morphology as the arc-grown nanotubes25.
During the same time, Harold W. Kroto, a chemist at the University of Sussex, who was interested in long chained carbon molecules found in interstellar space, collaborated with Richard Smalley at Rice University to utilize laser supersonic cluster beam apparatus called AP2 (App-two) available in latter’s lab, which led to the discovery of fullerenes. Using the apparatus, graphite was vaporized by firing a high energy laser beam in a helium-filled vacuum chamber. Analysis of the formed carbon clusters using mass spectroscopy showed remarkably stable cluster consisting of 60 carbon atoms.
After postulating various structural models, they finally proposed a closed cluster having truncated icosahedron structure with carbon atoms placed at each vertex of this structure and all valences being satisfied by two single bonds and one double bond.
The 60 atom cluster of carbon was named buckminsterfullerene after the American architect and engineer of geodesic domes, Richard Buckminster Fuller. The discovery of C60,26 reported in the prestigious journal Nature in 1985, had an enormous impact on carbon research and eventually led to the development of more advanced materials, the most important ones being carbon nanotubes and graphene.
However, it took another five years for Wolfgang Krätschmer of the Max Planck Institute at Heidelberg, and Donald Huffman of the University of Arizona to devise a simpler process to produce large quantities of C6027. They used a simple carbon arc to vaporize graphite in a helium atmosphere and the soot thus obtained contained plate-like crystals of fullerite having 90% C60 and 10% C70. No patents have been filed on fullerene because of their natural existence as reported in terrestrial rocks and interstellar media.
Sumio Iijima, a well known electron microscopy specialist working then at NEC laboratories in Japan, while studying various carbon materials observed long finer hollow fibers in the soot generated on the graphite cathode using the Krätschmer–Huffman method28,29.
Even though the discovery of carbon nanotubes was accidental, the vast accumulated experiences and knowledge of Iijima obtained from electron microscopy studies of various types of carbon materials played a decisive role in identifying carbon nanotube in the graphite cathode soot produced by arc evaporation.
The carbon nanotubes described by Iijima contained more than one graphitic layer having inner diameters of around 4 nm and are closed at both ends. These findings reported in 1991 in the Nature journal unambiguously proved the growth of multiwalled carbon nanotubes using electric arc evaporation of graphite without the need of any metal catalysts.
It is important to note that nano-dimensional carbon tubes were already known to the scientific community at the time of Iijima’s discovery of multiwalled carbon nanotubes, but these studies did not incite wide interest. Many reasons can be attributed to this.
The earlier nanotubes produced using catalytic methods were structurally imperfect and did not have particularly interesting properties. Hence the scientific community was not convinced about their value and potential.
Moreover, the limitations in the investigation tools prevented the elucidation of the structure in nanometer scale.
Another plausible reason for the lack of interest in earlier days was that the studies focused on carbon filaments were not conducted from the point of view of discovering new carbon structures, but rather to identify the growth mechanisms of such structures so that their formation could be prevented.
Riding on the excitement created by the discovery of a new molecular carbon allotrope, fullerene, Iijima’s discovery of carbon nanotubes undoubtedly initiated a flurry of experiments that inspired several researchers to study various aspects of carbon nanotubes.
The discovery of single walled carbon nanotubes30,31 that happened in 1993 was more serendipitous. Continuing experiments with arc-evaporation using modified electrodes at NEC laboratories in Japan, Sumio Iijima and Toshinari Ichihashi observed a different type of soot when the electrodes in the arc-evaporation chamber were impregnated with Fe, and the inside atmosphere changed to a mixture of methane and argon rather than pure helium.
Examination by high-resolution electron microscopy revealed extremely fine single-walled nanotubes running like threads between clusters containing amorphous carbon and metal particles in the soot.
Independent to this finding, Donald Bethune and his colleagues of the IBM Almaden Research Center in San Jose, California, who were interested in developing ferromagnetic transition metal crystallites encapsulated in carbon shells, conducted arc-evaporation experiments using electrodes impregnated with the ferromagnetic transition metals Fe, Co and Ni under an atmosphere of helium. They observed sheets of soot hung like cobwebs from the chamber walls and by examining this strange new material using high-resolution electron microscopy they found ultra-fine nanotubes with single-atomic-layer walls entangled with amorphous soot and of metal or metal carbide particles.
The findings of both groups were published in the same issue of Nature journal in 199330,31.
Continue to page 5 of 5

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