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236 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

C x H y

SWNT
Fe
C x H y
Fe MWNT

Fe

Catalyst support Catalyst support
Figure 9.4. Growth modes of catalytically formed carbon nanotubes. Fe, Co, and Ni are the
catalysts and hydrocarbons, and CO are the carbon precursors. SWNTs are grown on particles
<∼40–50 ˚ A, whereas MWNTs are grown on larger particles. The outside tube diameter is the
same as the particle diameter.

2.492 Å

2.492 Å

2.492 Å 3.523 Å
(100) (110)

2nd layer

2.492 Å
2.492Å

4.131Å
(111) (311)

Figure 9.5. Structures and lattice distances of four Ni faces.

are illustrated by Figure 9.6. The distance between two parallel C−C bonds of the
hexagonal ring of graphite is 2.46 ˚ A. This distance is nearly perfectly matched
with the lattice distance of 2.49 ˚ A of Ni. Similarly, the lattice constant of Co
(hcp)is a = 2.51 ˚ A and that of α-Fe (bcc)is a = 2.86. These three metals
are among those with lattice constants that are closest to 2.46 ˚ A (Kittel, 1953).