Page 116 - Carbon Nanotubes

HEMI-TOROIDAL NETWORKS IN PYROLYTIC
CARBON NANOTUBES

A. SARKAR, H. W. KROTO
School of Chemistry and Molecular Science, University of Sussex, Brighton, BN1 9QJ U.K.
and
M. ENDO
Department of Engineering, Shinshu University, Nagano, Japan
(Received 26 May 1994; accepted in revised form 16 August 1994)

Abstract-Evidence for the formation of an archetypal hemi-toroidal link structure between adjacent
concentric walls in pyrolytic carbon nanotubes is presented. The observed and simulated TEM images for
such structures are in excellent agreement. This study suggests that double-walled carbon nanotubes, in
which the inner and outer tubes are linked by such hemi-toroidal seals, may be one viable way of over-
coming the reactivity at the graphene edges of open-ended tubes to engineer stable and useful graphene
nanostructures.
Key Words-Nanotubes, pyrolytic carbon nanotubes, hemi-toroidal nanostructures.

1. INTRODUCTION During the course of a detailed study of the struc-
The discovery by Iijima[ 11 that carbon nanotubes ture and growth of pyrolytic carbon nanotubes (F'CNTs)
form in a Kratschmer-Huffman fullerene generator[2], we have found that complex graphitisation processes
and the subsequent development of techniques by occur during a second stage of heat treatment of these
Ebbeson and Ajayan[3] and others to produce them materials[20,21]. In particular, we here draw attention
in viable quantities, promise to have important impli- to evidence that hemi-toroidal graphite surfaces occur
cations for future nanotechnology. The tubes are es- regularly during nanotube production. In this commu-
sentially elongated fuIlerenes[4] and, thus, description nication we present evidence for the formation of single-
(in particular structural characterisation) in terms of layered hemi-toroid structures, within bulk multi-walled
fullerene chemistry/physics principles is appropriate material during PCNT graphitisation. The structures
and useful. A growth mechanism has been proposed occur in material prepared by pyrolysis of hydrocar-
for the primary tube growth phase[4] which involves bons at ca. 1000°C and subjected to secondary heat
ingestion of carbon fragments (e.g., atoms and chain treatment at 2800°C. The structures have been studied
molecules) at the reactive hemi-fuuerene closed end- experimentally by high resolution transmission electror,
caps of the tubes. An open-ended growth mechanism microscopy (HRTEM)[20] and the resulting images
has also been proposed[5]. A closed-end growth mech- compared with simulations. The simulated structures
anism is consistent with the--observations of Ulmer were generated from hypothetica1 fullerene related
et al. [6] and McElvany et al. [7], that fullerenes can hemi-toroidal networks, which were constructed on
grow by ingestion of carbon fragments into their com- the basis of feasible graphene topologies relaxed by
pletely closed networks. Both growth schemes also are molecular mechanics. During the course of the present
consistent with Iijima's observation that, in general, study of PCNTs, Iijima et al. [5] have obtained TEM
the tubes appear to have their hexagons helically dis- images showing related structures in arc-formed car-
posed in the tube walIs[l]. bon nanotubes (ACNTs).
The discovery that carbon generates closed-cage
fullerenes spontaneousIy[2,8] as well as nanotubes[l,3] 2. OBSERVATIONS
has stimulated the question of whether other topolog-
ically accessible networks are feasible. Predictions on The PCNTs obtained by decomposition of benzene
the possibility of both small fullerenes[9] and giant ful- at ca. 1000-1070°C on a ceramic substrate were gath-
lerenes[lQ,ll] appear to have been vindicated[l2,13]. ered using a toothpick and heat treated at a tempera-
As well as general closed-cage structures, extended ture of 2800°C for 15 minutes[21]. The heat-treated
repeat-pattern networks involving seven- and other- PCNTs were then mounted on a porous amorphous
membered rings have been considered by Terrones and carbon electron microscope grid (so-called carbon
MacKay[l4,l5] and Lenosky et al.1161. Other struc- ultra-microgrid). HRTEM observations were made
tures have been considered and, in particular, Cher- using a LaB6 filament operating at an accelerating
nozatonskii[l.7] and Ihara et a1.[18,19] have discussed voltage of 300 kV. The special aberration coefficient
toroidal structure. C, = 2.0 mm and lambda = 0.0197 A. The under-focus

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