Page 11 - Carbon Nanotubes

PYROLYTIC CARBON NANOTUBES FROM VAPOR-GROWN
CARBON FIBERS

MORINOBU ENDO,' Kmn TAKEUCHI,' KIYOHARU KOBORI,' KATSUSHI TAKAHASHI,
I
HAROLD W. KROTO,~ and A. SARKAR'
'Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan
'School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNl SQJ, U.K.
(Received 21 November 1994; accepted 10 February 1995)

Abstract-The structure of as-grown and heat-treated pyrolytic carbon nanotubes (PCNTs) produced by
hydrocarbon pyrolysis are discussed on the basis of a possible growth process. The structures are com-
pared with those of nanotubes obtained by the arc method (ACNT, arc-formed carbon nanotubes). PCNTs,
with and without secondary pyrolytic deposition (which results in diameter increase) are found to form
during pyrolysis of benzene at temperatures ca. 1060°C under hydrogen. PCNTs after heat treatment at
above 2800°C under argon exhibit have improved stability and can be studied by high-resolution trans-
mission electron microscopy (HRTEM). The microstructures of PCNTs closely resemble those of vapor-
grown carbon fibers (VGCFs). Some VGCFs that have micro-sized diameters appear to have nanotube
inner cross-sections that have different mechanical properties from those of the outer pyrolytic sections.
PCNTs initially appear to grow as ultra-thin graphene tubes with central hollow cores (diameter ca. 2 nm
or more) and catalytic particles are not observed at the tip of these tubes. The secondary pyrolytic depo-
sition, which results in characteristic thickening by addition of extra cylindrical carbon layers, appears to
occur simultaneously with nanotube lengthening growth. After heat treatment, HRTEM studies indicate
clearly that the hollow cores are closed at the ends of polygonized hemi-spherical carbon caps. The most
commonly observed cone angle at the tip is generally ca. 20", which implies the presence of five pentago-
nal disclinations clustered near the tip of the hexagonal network. A structural model is proposed for PCNTs
observed to have spindle-like shape and conical caps at both ends. Evidence is presented for the forma-
tion, during heat treatment, of hemi-toroidal rims linking adjacent concentric walls in PCNTs. A possi-
ble growth mechanism for PCNTs, in which the tip of the tube is the active reaction site, is proposed.
Key Words-Carbon nanotubes, vapor-grown carbon fibers, high-resolution transmission electron micro-
scope, graphite structure, nanotube growth mechanism, toroidal network.

1. INTRODUCTION been proposed involving both open-ended1131 and
closed-cap[l 1,121 mechanisms for the primary tubules.
Since Iijima's original report[l], carbon nanotubes
have been recognized as fascinating materials with Whether either of these mechanisms or some other oc-
nanometer dimensions promising exciting new areas curs remains to be determined.
of carbon chemistry and physics. From the viewpoint It is interesting to compare the formation process
of fullerene science they also are interesting because of fibrous forms of carbon with larger micron diam-
they are forms of giant fuIlerenes[2]. The nanotubes eters and carbon nanotubes with nanometer diameters
from the viewpoint of "one-dimensional)) carbon struc-
prepared in a dc arc discharge using graphite elec- tures as shown in Fig. 1. The first class consists of
trodes at temperatures greater than 3000°C under
helium were first reported by Iijima[l] and later by graphite whiskers and ACNTs produced by arc meth-
Ebbesen and Ajyayan[3]. Similar tubes, which we call ods, whereas the second encompasses vapor-grown car-
bon fibers and PCNTs produced by pyrolytic processes.
pyrolytic carbon nanotubes (PCNTs), are produced A third possibIe class would be polymer-based nano-
by pyrolyzing hydrocarbons (e.g., benzene at ca. tubes and fibers such as PAN-based carbon fibers,
1 10OoC)[4-9]. PCNTs can also be prepared using the
same equipment as that used for the production of which have yet to be formed with nanometer dimen-
so called vapor-grown carbon fibers (VGCFs)[lOJ. The sions. In the present paper we compare and discuss the
VGCFs are micron diameter fibers with circular cross- structures of PCNTs and VGCFs.
sections and central hollow cores with diameters ca.
a few tens of nanometers. The graphitic networks are 2. VAPOR-GROWN CARBON FIBERS AND
arranged in concentric cylinders. The intrinsic struc- PYROLYTIC CARBON NANOTUBES
tures are rather like that of the annual growth of trees.
The structure of VGCFs, especially those with hollow Vapor-grown carbon fibers have been prepared by
cores, are very similar to the structure of arc-formed catalyzed carbonization of aromatic carbon species
carbon nanotubes (ACNTs). Both types of nanotubes, using ultra-fine metal particles, such as iron. The par-
the ACNTs and the present PCNTs, appear to be ticles, with diameters less than 10 nm may be dispersed
essentially Russian Doll-like sets of elongated giant on a substrate (substrate method), or allowed to float
ful,lerenes[ll,12]. Possible growth processes have in the reaction chamber (fluidized method). Both
1