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CARBON NANOTUBES 243

energy of curvature of the graphite sheet and the dangling-bond energy of the
open edge.
In all the studies since Bacon’s work, “graphite rod” or “graphite disk” was
used as the source of carbon for vaporization. No impurity data were given. The
electrode graphite with the highest purity is used as anodes for aluminum smelt-
ing, with a total “ash” content of >0.1% (wt) (Yang, 1979). The commercially
available electrode graphites have “ash” contents significantly higher than this
level. The main impurity in electrode graphite is Fe. The question concerning
the role of the Fe impurity in the formation of MWNTs (and possibly fullerenes)
has not been addressed. With these impurities, SWNTs were likely to have been
formed; however, they have not yet been found probably because of their scarcity.

9.1.3. Adsorption Properties of Carbon Nanotubes
The most unexpected and potentially most important adsorption property of car-
bon nanotubes is hydrogen storage. While controversy remains, intensive research
efforts on this subject are on-going worldwide. This subject will be discussed
separately in Chapter 10.
Carbon nanotubes have cylindrical pores. An adsorbate molecule interacts
with the carbon atoms on the surrounding walls. As discussed in Chapter 2, the
resulting potential in the cylindrical pore can be substantially higher than that
in a slit-shaped pore with the same dimension. In addition, carbon nanotubes
are highly graphitic (much more so than activated carbon). The surface of the
nanotubes is highly aromatic and contains a high density of π electrons. With
these two factors, it is expected that the carbon nanotubes can adsorb molecules
much more strongly than activated carbon (which has slit-shaped or wedge-
shaped pores). This expectation has indeed been shown by a number of simulation
studies of adsorption for He, Xe, CH 4 ,and N 2 . The general results showed that
the interactions are of the order of two when compared with that on planar
graphite, as to be discussed shortly.
Because SWNTs are grown in the form of bundles and ropes from both cat-
alytic route (e.g., Colomer et al., 2000) and graphite vaporization (e.g., Thess
et al., 1996), the inter-tube spaces (bounded by the outer surfaces of the tubes)
are also important for adsorption. The SWNT bundles are arranged in a triangular
lattice structure, held together by van der Waals forces. Hexagonal close-packed
configuration without tube-tube contact has been used in most simulations. In
such cases, the dimensions of the inter-tube pores are often smaller than that
inside the tubes, thus, adsorption in the inter-tube spaces can be stronger. Adsorp-
tion in the inter-tube spaces has been assumed in many theoretical calculations,
particularly in earlier work, when synthesis of small-diameter SWNTs were not
reported. In such calculations, inter-tube spacing as small as 2.6 ˚ Awas used.
Since the first study on adsorption in carbon nanotubes by Pederson and
Broughton (1992), most have been simulations. Few experimental studies have
appeared, however. The observations from both simulations and experiments are
discussed below.