Page 250 - Adsorbents fundamentals and applications

CARBON NANOTUBES 235

propane. Thus, the appropriate reaction conditions depend on the gas as well as
the activity of the catalyst. Nanotubes formed by a myriad of combinations of
catalyst/support/gas/reaction conditions have appeared in the literature. Reviews
for some of the reports are available (Ding et al., 2001; Dai, 2001). Ding et al.
(2001) have given a list of detailed reaction conditions for a large number of
them. A summary of these reports is given in Table 9.1.
The following mechanistic steps for filament growth have been generally
accepted since the 1970’s (Baker and Harris, 1978; Tibbetts, 1987). The hydrocar-
bon or CO first decomposes on the front-exposed surfaces of the metal particle to
release hydrogen and carbon, which dissolves in the metal particle. The dissolved
carbon diffuses through the particle and is precipitated on the rear faces to form
the body of the filament. These steps are shown schematically in Figure 9.4. Bur-
ton (1974) has shown that the structure and melting point of a small metal particle
differs significantly from that of the bulk metal, and that the melting point can
be substantially lower than that of the bulk. Hence the supported metal particle
acquires a liquid-like behavior. From in situ TEM studies, Baker has concluded
that the metal particles (of sizes below a fraction of a micrometer) are generally
shaped like a pear, with a truncated rear end, as illustrated in Figure 9.4. Thus,
carbon solubility and diffusivity in the metal catalyst are prerequisites for fila-
◦
ment growth. The solubilities of carbon in Ni are 0.29 at % (700 C) and0.37at
2
◦
◦
% (750 C), and the diffusivity at 700 Cis4.0 × 10 −9 cm /s (Yang et al., 1990).
The carbon dissolution/diffusion/precipitation mechanism for filament growth
has been studied in detail for hollow graphitic filaments formed on Ni, Co, and α-
Fe from methane decomposition, by Yang and Chen (1989). The crystallographic
orientations of the graphite/metal interface were examined with TEM/selected
area electron diffraction. Epitaxial matching of the graphite lattice with different
faces of the metals was identified. The structures of four different faces of Ni are
shown in Figure 9.5. Their possible epitaxial matchings with the graphite lattice

Table 9.1. Catalytic formation of carbon nanotubes from reports after 1996
Gas Reaction Catalyst Support
Conditions
◦
CO Typically 700 C, Fe, Co, Ni, Co-Mo, SiO 2 (most used),
1–5 atm Fe-Mo, Co-Fe, Al 2 O 3 ,MgO,
◦
Unsaturated 600–900 C Carried Co-V, Fe-Ru, Al 2 O 3 -SiO 2 , zeolite,
hydrocarbons in N 2 or another Ni-Cu, Ni-MgO, clay
(C 2 H 2 ,C 2 H 4 , inert atmosphere at Fe-MgO, Ferrocene
C 3 H 6 ,C 4 H 6 , 1atm vapor (Fe(C 5 H 5 ) 2 ),
C 6 H 6 , acetone) Fe(CO) 5 vapor
Saturated 700–1000 C
◦
◦
hydrocarbons, typically 900 C,
mainly CH 4 1atm
Both SWNT and MWNT have been reported, depending on the particle sizes, see Figure 9.4.