Search

The control of the structure of carbon nanotubes remains a central issue in rapidly developing science and technology. Of the various structural parameters of nanotubes, namely, diameters, number of walls, length, and chirality, it is the control of chirality which presents significant challenges as well as opportunities. It is known from our earlier work that the addition of nitrogen as a heterocyclic compound in the hydrocarbon feedstock leads to the growth of carpets of multiwall nanotubes in which all the walls are either of the armchair or zigzag structure. The research here shows the role of nitrogen in stabilizing iron carbide catalyst particles, the belt nucleation of the nanotubes, the orientational relationship between particle and nanotube structure, and the reconstruction of the surface layers of the carbide in contact with the growing ends of the tubes. By combining the various pieces of new experimental evidence it has proved possible to suggest why such unique types of nanotubes should form...

The article examines the in situ observation of the effect of nitrogen on carbon nanotube synthesis. Environmental TEM (ETEM) is a relatively new technique ideal for in situ measurements. Researchers apply ETEM to study the effect of nitrogen on CNT growth, elucidating the mechanisms by which nitrogen acts, to guide efforts to optimize synthesis. To the best of our knowledge, this is the first in situ TEM study of the effect of nitrogen on CNT growth. Therefore, in the absence of ammonia, carbon should precipitate first, resulting in an interface with pure iron. Pure iron has a higher surface tension with the nanotube, which explains the separation of the CNT and catalyst immediately following carbon precipitation. A carbide particle will have a lower surface tension with the nanotube due to its higher carbon chemical potential and thus maintain its contact. The pure iron catalyst may become carbide following cap separation, but this will not affect CNT nucleation.