Structure of non-graphitising carbons
Published by Maney Publishing (c) IOM Communications Ltd
P. J. F. Harris
highly stable. It therefore seems worth considering
the idea that microporous non-graphitising carbons
may be fullerene-like in nature. Recent work has
provided support for this view by showing that high
temperature heat treatments can transform microporous carbons into fullerene like nanoparticles.15
The purpose of the present paper is to consider further
the evidence that non-graphitising carbons contain
fullerene related elements. In addition to microporous
carbons, a related class of non-graphitising carbon
known as glassy carbon will also be considered, and
a brief discussion will be given of the structure of
soot particles and carbon fibres. First, a brief outline
is given of the types of bonding found in carbon
materials. A description is then given of the preparation and properties of non-graphitising carbons,
and conventional models of their structure are critically discussed.
Despite many years of research, the detailed
atomic structure of many important carbon
materials remains poorly understood. In particular,
the structure of those carbons which can not be
transformed into graphite by high temperature
heat treatment has never been clearly established.
These non-graphitising carbons are of
considerable commercial importance in a variety
of fields, and a better understanding of their
structure is clearly needed. Recently, it has been
suggested that non-graphitising carbons may have
a microstructure which is related to that of
fullerenes. In the present paper, the evidence for
this will be considered in detail and the
advantages of the new model over previous
models of non-graphitising carbons will be
discussed. As well as microporous nongraphitising carbons, other forms of carbon
including glassy carbon and carbon fibres will be
considered.
IMRj304
Bonding in carbon materials
© 1997 The Institute of Materials and ASM International.
Dr P. J. F. Harris is based in the Department of Chemistry,
University
UK.
of Reading, Whiteknights,
Reading RG6 6AD,
Introduction
Although graphite is the most stable form of carbon
at normal temperatures and pressures, it is a remarkable fact that many carbons can not be transformed
into crystalline graphite even at temperatures of
3000°C and above. The so called 'non-graphitising'
carbons tend to be hard, low density materials, with
isotropic, microporous structures.1-6 In contrast, graphitising carbons are soft and non-porous, with densities much closer to that of crystalline graphite. Nongraphitising carbons can develop exceptionally high
surface areas when 'activated' by treatment with a
mild oxidising agent, and the resulting activated carbons are widely used as adsorbents and as catalyst
supports.4-6 Despite their commercial importance,
however, the detailed structure of these carbons at
the atomic level is still poorly understood. The traditional view is that the microstructure consists of
twisted networks of carbon layer planes crosslinked
by bridging groups, explaining both their hardness
and their resistance to graphitisation,2 but the precise
nature of such bridging groups has never been properly established. Earlier suggestions that Sp3bonding
may be present in non-graphitising carbons do not
appear to stand up to detailed analysis, as discussed
in the 'Problems with early models' section later.
The relatively recent discovery of the fullerenes,7-9
and subsequently of related structures such as carbon
nanotubes10-12 and nanoparticles,13,14 has given us
a new perspective on Sp2 bonded carbon structures.
Most importantly, we now know that carbon structures containing non-six membered rings can be
206
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Materials Reviews
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A free carbon atom has the electronic structure
Is2 2s2 2p2. In order to form covalent bonds, one of
the 2s electrons is promoted to 2p, and the orbitals
are then hybridised in one of three possible ways. In
graphite, one of the 2s electrons hybridises with two
of the 2p electrons to give three Sp2 orbitals at 120°
to each other in a plane, with the remaining orbital
having a pz configuration at 90° to this plane. The
Sp2orbitals form the strong (J bonds between carbon
atoms in the graphite planes, while pz or n orbitals
provide the weak van der Waals bonds between the
planes. In naturally occurring or high quality synthetic graphite, the stacking sequence of the layers is
generally ABAB, with an interlayer {0002} spacing
of approximately 0·334 nm, as shown in Fig. la. In less
perfect graphites, the interplanar spacing is found to
be significantly larger than the value for single crystal
graphite (typically ",0,344 nm), and the layer planes
are randomly rotated with respect to each other about
the e axis. Such graphites are termed turbostratic.
In the C60 molecule, shown in Fig. Ib, the carbon
atoms are bonded in an icosahedral structure made
up of 20 hexagons and 12 pentagons. Each of the
carbon atoms in C60 is joined to three neighbours, so
the bonding is essentially Sp2,although there may be
a small amount of Sp3character owing to the curvature. Note that all 60 carbon atoms are identical, so
that the strain is evenly distributed over the molecule.
Pentagonal rings are also present in carbon nanoparticles and nanotubes, although these generally have
much less perfect structures than those of C60 and
other fullerenes.
In diamond, each carbon atom is joined to four
neighbours in a tetrahedral structure, as shown in
Fig. le. The bonding here is Sp3,and results from the
mixing of one 2s and three 2p orbitals. Diamond is
less stable than graphite, and is converted to graphite
at a temperature of 1700°C at normal pressures.
Harris
a
Structure of non-graphitising
carbons
207
b
a graphite showing unit cell; b eso; c diamond
1
Illustration of bonding in carbon structures
Published by Maney Publishing (c) IOM Communications Ltd
Disordered carbons containing Sp3 bonded atoms
are also rapidly transformed into graphitic carbon at
high temperatures.
Non-graphitising carbons
Background
It has been known for about a century that some
carbon materials are more amenable to graphitisation
than others, but the first detailed study of graphitising
and non-graphitising
carbons was made by Rosalind
Franklin in the period before she began her famous
work on DNA. In a paper published in 1951,1
Franklin described XRD studies of the effect of high
temperature heat treatments on the structure of a
variety of carbons formed by pyrolysis of organic
materials. She found a clear distinction between carbons which could be converted into graphite by high
temperature annealing and those which could not.
Among the non-graphitising
carbons were those produced by the pyrolysis of polyvinylidene
chloride
(PVDC) and sucrose, while graphitising
carbons
included those made from polyvinyl chloride (PVC)
and petroleum coke. Franklin proposed structural
models for the two classes of carbon, and these will
be discussed in the next section.
Since Franklin's time there has been a vast amount
of research on the preparation and properties of nongraphitising carbons,2-s and only a very brief outline
is possible here. It is found that non-graphitising
carbons are invariably highly porous, although some
of this porosity is usually inaccessible to gases. As
noted above, the internal surface area can be enhanced
by activation, i.e. mild oxidation with a gas such as
carbon dioxide, steam, or air. A volume distribution
curve for a typical activated carbon is shown in
Fig. 2.4 It can be seen that most of the internal volume
is in the form of micropores with radii of approximately 1 nm. Larger pores, classified as mesopores
and macropores, are also present, but we are not
concerned with these here. Models of the structure of
microporosity
in non-graphitising
carbons are discussed below.
The effect of heat treatment on graphitising and
non-graphitising
carbons has been the subject of a
large number of studies. The structures of the heat
treated carbons are usually discussed in terms of the
parameters La and Lc, defined as the length and
thickness respectively of the graphite lamellae within
the carbons. In carbons prepared at temperatures
below ~ 1000°C, La and Lc in both graphitising and
non-graphitising
carbons have values around 1 nm,
indicating highly disordered structures with relatively
little graphitisation. The effect of heat treatment on
the two types of carbon differs greatly. This is illustrated in Fig. 3, taken from the work of Emmerich16
which plots La and Lc for graphitising and nongraphitising carbons heat treated at temperatures up
to 3000°C (note the La and Lc scales are logarithmic).
It can be seen that La for the graphitising carbon
reaches a value of 100 nm at 3000°C, while the
maximum value for the non-graphitising
carbon is
only 10 nm. The Lc value for the graphitising carbon
also approaches 100 nm, while for the non-graphitising carbons the maximum figure is ~4 nm. Extensive
graphite crystallites are not formed in non-graphitising carbons, even at the highest temperatures.
Other physical measurements
also demonstrate
sharp differences between graphitising and non-graphitising carbons. Table 1 (Ref. 17) shows the effect
of preparation temperature on the surface areas and
densities of a typical graphitising carbon prepared
from PVC, and a non-graphitising
carbon prepared
from cellulose. It can be seen that the graphitising
carbon prepared at 700°C has a very low surface
1.0
'I
E
c
0.8
'I
0)
ME
0.6
CJ
>1 ~~ 0.4
~.s
(J?
v.~
0
parameter on y axis gives measure of volume of gas absorbed by
pores with given radius, while x axis is log of radius r in nm
2
Differential volume distribution curve for typical
activated carbon4
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Harris
Structure of non-graphitising
carbons
a
100
graphltlsable
E
c:
-J1O
10
non-grophitlsable
100
graphltlsable
non-gro phltlsable
o
1000
2000
Published by Maney Publishing (c) IOM Communications Ltd
HEAT TREATMENT
3
b
3000
TEMPERATURE,
°C
Variation of La and Lc with heat treatment
temperature
for
graphitising
and
nongraphitising carbons 16
area, which changes little for carbons prepared at
higher temperatures, up to 3000°C. The density of the
carbons
increases steadily as the preparation temperature is increased, reaching a value of
2·26 g cm -3, which is the density of pure graphite, at
3000°C. The effect of preparation temperature on the
non-graphitising carbon is very different. A high
surface area is observed for the carbon prepared
at 700°C (408 m2 g -1), which falls rapidly as the
preparation temperature is increased. Despite this
reduction in surface area, however, the density of the
non-graphitising carbon is actually lower for high
preparation temperatures than it is at 700°C. This
indicates that a high proportion of 'closed porosity'
is present in the heat treated carbon.
Structure of non-graphitising
early models
carbons:
The first structural models of graphitising and nongraphitising carbons were put forward by Franklin in
her classic 1951 paper.1 In these models, the basic
units are small graphitic crystallites containing a few
layer planes, which are joined together by crosslinks.
The precise nature of the crosslinks is not specified.
A schematic illustration of Franklin's models is shown
in Fig. 4. Her theory of crystallite growth in carbons
Table 1
Starting
Effect of temperature on surface areas
and densities of carbons prepared from
PVC and cellulose 17
For carbons prepared
at
material
PVC
Cellulose
Specific surface area, m2 g-l
0-21
0-58
1-60
408
PVC
Cellulose
Helium density, g cm-3
1-85
1-90
International
2-09
1-47
Materials Reviews
0-21
1-17
0-71
2-23
0-56
2-25
2-14
2·21
1-56
2-26
1-43
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1-70
NO.5
4
Franklin's representations of a graphitising
b non-graphitising carbons 1
and
depended on the assumption that growth results from
the movement of whole layers or large fragments
rather than individual atoms. It follows from this that
the degree of crystal growth will depend on the
orientation of the individual structural units and the
amount of crosslinking between them. In graphitising
carbons, the structural units are approximately parallel to each other, as shown in Fig. 4a, and the links
between adjacent units are assumed to be relatively
weak. On the other hand, the structural units in a
non-graphitising carbon are oriented randomly, as
shown in Fig. 4b, and the crosslinks are sufficiently
strong to impede movement of the layers into a more
parallel arrangement. Franklin's models of the structure of graphitising and non-graphitising carbons
have remained popular, and are still reproduced in
books and review articles.
The advent of high resolution electron microscopy
(HREM) in the early 1970s enabled direct images to
be recorded of the structure of non-graphitising carbons.1s Images of the freshly prepared carbons
showed a highly disordered structure, but images of
the carbons after high temperature heat treatment
were rather more informative. These apparently
showed the presence of curved and twisted graphite
sheets, typically two or three layer planes thick,
enclosing pores of the order of 5-10 nm is size. These
images led Ban et al.18 to suggest that heat treated
non-graphitising carbons have a ribbon like structure,
as shown in Fig. 5. This structure corresponds to a
Harris
Published by Maney Publishing (c) IOM Communications Ltd
5
Model of PVDC carbon heat treated
Structure of non-graphitising
carbons
209
at 1950°C, by Ban et al.18
PVDC carbon heat treated at 1950°C. These ribbon
like models are rather similar to an earlier model of
glassy carbon proposed by Jenkins and colleagues19
which is discussed further below.
The models of non-graphitising carbons described
so far have assumed that the carbon atoms are
exclusively Sp2 and are bonded in hexagonal rings.
Some authors, notably Ergun and colleagues20,21have
suggested that Sp3 bonded atoms may be present in
these carbons, basing their arguments on an analysis
of XRD patterns. The presence of diamond like
domains would be consistent with the hardness of
non-graphitising carbons, and might also explain their
extreme resistance to graphitisation.
Problems with early models
The most serious shortcoming of Franklin's models
for the structure of graphitising and non-graphitising
carbons is that the nature of the crosslinks between
the graphitic fragments is not described. Such
crosslinks must be extremely strong, since they are
sufficient to prevent graphitisation even at temperatures of 3000°C and above. The type of crosslinks
present in polymers, which are usually short linear
chains containing a few carbon atoms, would seem
to be insufficiently rigid to prevent graphitisation at
high temperatures. The idea that the crosslinks might
comprise small domains of Sp3 bonded carbons also
does not appear to stand up to detailed analysis
(see below).
Models of the kind illustrated in Fig. 5, which are
intended to represent the structure of non-graphitising
carbons following high temperature heat treatment,
also have serious weaknesses. Such models consist of
curved and twisted graphene sheets enclosing randomly shaped pores. However, graphene sheets are
known to be highly flexible, and would therefore be
expected to become ever more closely folded together
at high temperatures, in order to reduce surface
energy. Indeed, tightly folded graphene sheets are
quite frequently seen in carbons which have been
exposed to extreme conditions; examples are shown
in Fig. 6.22,23Thus, structures like the one shown in
Fig. 5 would be unlikely to be stable at very high
temperatures.
It has also been pointed out by Oberlin24 that the
early models were based on a questionable interpretation of the electron micrographs. In most micrographs of graphitised carbons, only the {0002} fringes
are resolved, and these are only visible when they
are approximately parallel to the electron beam.
Therefore, such images tend to have a ribbon like
appearance. However, since only a part of the structure is being imaged, this appearance can be misleading, and the true three-dimensional structure may be
more cage like than ribbon like. This is a very
important point, and must always be borne in mind
when analysing images of graphitic carbons.
As far as models incorporating Sp3bonded carbon
atoms are concerned, the main problem is that Sp3
bonded carbon is unstable at high temperatures.
Diamond is converted to graphite at 1700°C, as noted
above, while tetrahedrally bonded carbon atoms in
amorphous films are unstable above about 630°C.25
Therefore, the presence of Sp3 bonded atoms in a
carbon can not explain the resistance of the carbon
to graphitisation at high temperatures. The XRD
evidence for Sp3 bonded atoms in non-graphitising
carbons is also open to question. The main argument
put forward by Ergun et al.20,21 for the presence of
diamond like domains in non-graphitising carbons is
that the interference functions of such domains are
very similar to those of small graphitic structures.
Therefore, Ergun et al.20,21 argue that XRD patterns
can not be used to rule out the presence of Sp3bonded
atoms. However, a more detailed analysis of XRD
patterns from these carbons by Ruland26 suggests
that they are indeed inconsistent with a diamond like
structure. In particular, the presence of the graphite
{0002} line in patterns from non-graphitising carbons
is difficult to reconcile with a structure containing a
significant proportion of Sp3bonded atoms.
More recent models
Recently, some models of disordered carbons have
been put forward in which the carbons are not
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Harris
Structure of non-graphitising
carbons
-U
Published by Maney Publishing (c) IOM Communications Ltd
4nm
6
Micrographs showing tightly folded graphite sheets in carbons prepared by a arc evaporation22 and
b pyrolysis of acenaphthylene;23 c illustration of folded sheet structure
exclusively bonded in six membered rings. These
probably provide a much more realistic basis for
understanding the structure of non-graphitising
carbons than the early models based on curved and
twisted graphene sheets. In a book published in 1995,
Byrne and Marsh27 discussed the structure of carbons
produced by the pyrolysis of cellulosic type precursors. They suggested that such carbons might be
made up of small structural units such as that illustrated in Fig. 7a. This structure contains Sp2and Sp3
carbons bonded in five, six, and seven membered
rings. A group of such structures, with adsorbate
molecules situated in the gaps between the units, is
shown in Fig. 7b.
The discovery of the fullerenes has prompted several authors to put forward models of all-carbon
b
*
7
An adsorbate molecule
a Possible carbonaceous structural unit produced by pyrolysis of cellulosic precursor, according to
Byrne and Marsh;27 b model of microporous carbon made up of such units
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Structure of non-graphitising
carbons
211
a
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8
a Periodic negatively curved graphitic structure, from the work of Terrones et a/;31 b disordered
schwartzite structure, proposed by Townsend and co-workers32
materials
with fullerene related structures.
For
example, several groups have discussed hypothetical
'schwartzite' structures, which incorporate
negative
curvature owing to the presence of seven membered
rings.28-32 In most cases these theoretical structures
have been ordered, as in the example shown in Fig. 8a,
but disordered schwartzites have also been considered. An example of such a structure, taken from the
work of Townsend and co-workers,32 is shown in
Fig. 8b. The fragment shown contains 38 five membered rings, 394 six membered rings, 155 seven membered rings, 12 eight membered rings, and one nine
membered ring. The structure is continuous, with
no edges or unsatisfied valencies, and highly porous,
with typical pore diameters in the range 0·5-1 nm.
Townsend and co-workers determined the energy per
atom f1E of the various schwartzite structures relative
to a graphite monolayer. For the random schwartzite
they found a i1E value of 0·23 eV, considerably
lower than the value for C60 (0'42 eV), indicating
that such a structure should have high stability. They
also compared the properties of the hypothetical
schwartzite structure with those of evaporated Sp2
carbon films, and found good agreement. However,
they did not suggest that their model might be
appropriate
for microporous
carbons produced by
pyrolysis.
Evidence for fullerene like structures in
non-graphitising carbons
Recently, the present author and colleagues have
carried out a study of the effect of high temperature
heat treatment on non-graphitising
carbons using
HREM.15 It was shown that such heat treatments
can result in the formation of closed carbon nanoparticles, which are apparently fullerene like in structure.
This suggested that fullerene like elements were present in the original carbons, and prompted us to
propose a new model for non-graphitising
carbons.
Before describing this model, a brief summary will be
given of the HREM observations.
Two carbons were studied, prepared by the pyrolysis of a PVDC polymer and of sucrose, which were
two of the precursors used by Franklin in her classic
work. Pyrolysis was carried out under nitrogen at
about 700°C, producing carbons which were highly
disordered. Figure 9a shows a micrograph
of the
freshly prepared sucrose carbon. The microstructure
apparently consists of tightly curled single carbon
layers, although high resolution images of such noncrystalline materials are difficult to interpret directly.
Following heat treatment at 2600°C, however, many
regions of the carbon appear to consist largely of
closed nanoparticles,
as shown in Fig. 9b. Higher
magnification images in which individual nanoparticles can be more clearly seen are shown in Fig. 10.15
Sometimes, much smaller closed structures with diameters of the order of 1 nm or less could be found;
examples are shown in Fig. 11. These structures are
similar in size to small fullerenes like C60 and C70.
However, attempts to extract C60 and other fullerene
molecules from the heat treated carbons have so far
been unsuccessful.
It should be pointed out that there were many
regions in the heat treated samples in which the
transformation into nanoparticles was not as obvious
as it is in Figs. 9b and 10. Many regions were too
thick to enable individual nanoparticles to be seen,
and the nanoparticles were often partially obscured
by disordered material. This probably explains why
closed structures such as those shown here have not
been clearly identified before.
It seems very likely that particles such as those
shown in Figs. 9b and 10 contain pentagonal carbon
rings, as in fullerenes, since it is difficult to envisage
any other explanation
for their closed structures.
Indeed, the nanoparticles shown here are rather similar to particles which can be produced by arc evaporation in a fullerene generator,13,14 although in the
latter case the particles usually contain many more
layers. As in the case of fullerenes, 12 pentagons must
be present in each shell of the nanoparticles in order
to produce closure. Although the shapes of the
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Materials Reviews
1997
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Harris
Structure of non-graphitising
carbons
Published by Maney Publishing (c) IOM Communications Ltd
212
10
9
a Micrograph
of freshly prepared sucrose
carbon; b same carbon following heat treatment
at 2600°C; scale bar 5 nm
nanoparticles shown here are less symmetrical
than in small fullerenes such as C60, this does not
argue against a fullerene related structure, since giant
fullerenes can have a variety of different shapes, as
illustrated in Fig. 12. Features such as the saddle
points shown in Fig. lOb are evidence for the
presence of seven membered rings as well as pentagons in the heat treated carbons. The existence of
large numbers of fullerene like nanoparticles in the
heat treated carbons explains the observation of
'closed porosity' in such materials, as discussed in
previous sections. The nanoparticles constitute completely sealed capsules which would be impermeable
to any gas. Alternative models, such as that proposed
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Materials Reviews
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a Micrograph
showing closed structure in
PVDC derived
carbon
heated at 2600°C;
b another micrograph of same sample with
arrows showing regions of negative curvature;
scale bar 5 nm (Ref. 15)
by Byrne and Marsh27 and illustrated in Fig. 7, do
not seem to be capable of explaining the observed
closed porosity.
New model for structure of non-graphitising
carbons
The observations described in the previous section
suggest that non-graphitising carbons may have
fullerene like microstructures. One possible model
for these materials, therefore, might be the 'random
schwartzite' structure discussed above. However there
are aspects of this model which do not seem appropriate for non-graphitising carbons. In particular, the
Harris
Structure of non-graphitising
carbons
213
Published by Maney Publishing (c) IOM Communications Ltd
our model we envisage a higher proportion of pentagons and a smaller proportion of heptagons than in
the random schwartzite structure. This is supported
by the observation that the materials can be converted
by heat treatment to closed carbon nanoparticles,
each containing 12 pentagonal rings, suggesting that
a large number of such rings may have been present
in the original carbon. The size of the micropores in
our model, as well as in the random schwartzite
structure, is of the order 0·5-1 nm, which is similar
to the pore sizes observed in typical microporous
carbons, as noted above.
If the model we are proposing for non-graphitising
carbons is correct, it suggests that these carbons are
very similar in structure to fullerene soot, the low
density, disordered material which forms on the walls
of the arc evaporation vessel and from which C60 and
other fullerenes may be extracted. Fullerene soot is
known to be microporous, with a surface area, after
activation with carbon dioxide, of ",700 m2 g-1,33
and detailed analysis of high resolution electron
micrographs of fullerene soot has shown that these
are consistent with a random schwartzite type structure.34 It is significant that high temperature heat
treatments can transform fullerene soot into nanoparticles very similar to those observed in heated
microporous carbon.35
Finally in this section, it is worth making a few
comments on the 'activation' process which is essential for developing a very high surface area in nongraphitising
carbons. Activation
usually involves
treatment with a mild oxidising agent, such as CO2
or water vapour, and it is generally believed that this
has the effect of burning away carbon fragments
inside micropores,
thus enhancing surface area.27
However, if our new model for the structure of
microporous carbons is correct, then this activation
treatment may also have a further consequence. It is
known that mild oxidation, for example with CO2 at
850°C, can remove the caps from carbon nanotubes
by selectively attacking the pentagonal carbon rings.36
If microporous carbons have a fullerene like structure,
then the effect of such a treatment would be to open
closed pores by selective attack of the pentagons, thus
increasing the surface area significantly.
Glassy carbon
11
Micrographs
showing
very small
closed
structures in sucrose carbon heat treated at
2600°C; scale bar 2 nm
random schwartzite structure consists of a single
continuous sheet, while non-graphitising
carbons are
believed to be made up of relatively small fragments.24
An unbroken sheet such as that illustrated in Fig. 8b,
with no edges or dangling bonds, would have a very
low reactivity, unlike most non-graphitising
carbons
which can be quite readily oxidised at moderate
temperatures. The present author and S. C. Tsang
have therefore proposed a modeP5 for the structure
of non-graphitising carbons which consists of discrete
fragments of randomly curved carbon sheets, rather
than an unbroken sheet, as illustrated in Fig. 13. In
The so called glassy carbons are produced by the slow
pyrolysis of certain polymers at temperatures in the
range 900-1 aao°c. The resulting carbons are hard,
low density materials which can not be graphitised,
but unlike most non-graphitising
carbons they are
impermeable to gases. Perhaps their most remarkable
property is their chemical inertness. It has been demonstrated that the rates of oxidation of glassy carbon in
oxygen, carbon dioxide, or water vapour are lower
than those of any other carbon. They are also highly
resistant to attack by acids. Thus, while normal graphite is reduced to a powder by a mixture of concentrated
sulphuric and nitric acids at room temperature, glassy
carbon is unaffected by such treatment, even after
several months.37 This property makes glassy carbon
a useful material for crucibles. It is also used widely
as an electrode material in electrochemistry.
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Harris
a
C'500
(Ih
Structure of non-graphitising
symmetry); b
C600
(D2h symmetry); c
carbons
C660
(tetrahedral symmetry)
Published by Maney Publishing (c) IOM Communications Ltd
12 Giant fullerene structures with various symmetries, each containing
Some of the earliest structural models for glassy
carbon assumed that both Sp2and Sp3bonded atoms
were present. 38Graphitic domains were envisaged to
be interspersed with tetrahedral domains, perhaps
linked by short oxygen containing bridges. These
models were based primarily on an analysis of
XRD measurements and, as mentioned above, such
measurements can be open to a number of interpretations. It should also be noted that neutron diffraction
data have shown an absence of tetrahedrally bonded
domains in glassy carbon heat treated at 2000°C,39
A different model for the structure of glassy carbon
was put forward by Jenkins and Kawamura19 in 1972.
This model, illustrated in Fig. 14, is based on the
assumption that the molecular orientation of the
polymeric precursor material is memorised to some
extent after carbonisation. Thus, the structure bears
some resemblance to that of a polymer, in which the
'fibrils' are very narrow curved and twisted ribbons
of graphitic carbon. The Jenkins-Kawamura model
has been quite widely accepted, but appears to be
deficient in a number of aspects. For example, a
structure such as that shown in Fig. 14, with many
conjoined micropores, would be expected to be per-
12 pentagonal rings30
meable to gases, whereas we know that glassy carbons
are highly impermeable. The structure also has a high
proportion of edge atoms, which are known to have
a relatively high reactivity compared with 'in plane'
carbon atoms.
An alternative model for glassy carbon would be
one in which the basic structural units are fullerenelike closed particles, as shown in Fig. 15. Such a
structure would be impermeable and have a much
lower reactivity than the Jenkins-Kawamura structure. In the schematic illustration shown in Fig. 15,
the individual particles are of the order of 1 nm in
size. This is consistent with high resolution electron
micrographs of glassy carbon, which show little evidence of any structure, suggesting the basic structural
units are extremely small.
A model of heat treated glassy carbon which also
involves cage like components was proposed by
Japanese workers in 1984.40,41This is illustrated in
Fig. 16. Here, the particles are multilayered and have
inner cavities ~ 50 nm dia. This model was put forward before the discovery of C60, and the possibility
that the closed particles might contain non-six membered rings was not considered.
Carbon fibres
13
Schematic illustration of model for structure
of non-graphitising carbons based on fullerene
like elements 15
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Most commercial carbon fibres are produced either
from polyacrylonitrile (PAN) or from mesophase
pitch.42 Fibres derived from pitch are highly graphitic,
and have high elastic moduli, while those derived
from PAN have a much more imperfect, lower density
structure. The lack of extended structure in PAN
derived fibres makes them relatively insensitive to
flaws, giving them higher strength but lower modulus
than pitch derived fibres. The properties of PAN
derived fibres result from the fact that PAN is nongraphitising, and a brief discussion of their structure
is therefore appropriate here.
XRD of PAN derived fibres produces La values of
approximately 4-10 nm depending on the annealing
temperature.43 HREM shows that the fibres have an
imperfect structure, containing many elongated voids.
Several models have been put forward for the structure of PAN derived carbon fibres, all based on the
assumption that the basic structural units are graphite
Harris
Structure of non-graphitising
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215
!
Lc
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1
14 Model by Jenkins and Kawamura for structure of glassy carbon 19
sheets or ribbons. A model suggested by Crawford
and Johnson44 is shown in Fig. 17. Here, the structure
consists of a random arrangement of flat or crumpled
graphite sheets, with all the a-b planes running parallel to the fibre axis. However, given the flexibility of
graphite sheets, illustrated in Fig. 6, it is difficult to see
how the voids in such a structure could survive high
temperature heat treatment. Therefore, the possibility
that the voids in fact result from the presence of
fullerene like elements is worthy of consideration. The
elongated shapes of the voids suggests that they may
have structures related to those of carbon nanotubes.
Soot and carbon black
products.45 The possibility that soot particles might
be fullerene like was first suggested by Smalley and
co-workers in 1986,46 and discussed further by Kroto
and McKay in 1988.47 In these papers it was proposed
that the soot particles grew by a mechanism related
to the so called 'pentagon road' model, which involves
the incorporation of pentagonal rings into a growing
carbon network, driven by the need to eliminate
dangling bonds. If the pentagons occur in the 'correct'
positions then C60 and other fullerenes will result but,
in general, closed structures will not be formed and
the growing shell will then tend to curl around on
itself like a nautilus shell. However, this model was
not greeted favourably by soot experts,48 who argued
Soot is another carbon material whose structure is
poorly understood. It usually consists of quasi spherical particles ranging from about 10 to about 500 nm
in size, which are often joined together in clusters or
'necklace' chains. Carbon black is essentially a very
pure form of soot, which is of great commercial
importance as a pigment and as a filler in rubber
15 nm I
15
Model
for
structure
of
glassy
carbon
containing closed, fullerene like particles
16 Model for structure of glassy carbon derived
from phenol resin following heat treatment at
2800°C (Refs. 40, 41)
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216
17
Harris
Structure of non-graphitising
carbons
Model suggested by Crawford and Johnson for structure of PAN derived carbon fibres44
that it was inconsistent with both the kinetics of soot
formation and the structural characteristics of the
particles. Concerning kinetics, it was pointed out that
the growth of curved shell structures would be much
slower than those of planar fragments. Soot formation
is known to be extremely rapid, so it seemed unlikely
that the growth of fullerene like shells could be
involved. Their structural arguments were based on
XRD and 13C NMR patterns of combustion soot,
which they suggested were inconsistent with the icospiral model. They pointed out that d spacings for
continuously curving icospirals would be lower than
those observed experimentally in XRD studies of
soot, while 13CNMR spectra of soot resemble those
of aromatic molecules much more closely than those
of fullerenes. However, evidence from XRD and 13e
NMR of soot particles is difficult to interpret when
one is dealing with disordered materials such as
carbon blacks, and can not be said to provide definitive proof of the structure.
Since structural measurements on the soot particles
themselves is difficult, it is worth considering whether
any insights into their structure can be gained from
the way in which the particles are transformed by
high temperature heat treatment. It is well established
that such treatments transform carbon black particles
into faceted particles which sometimes appear to have
closed shell like structures. The precise structure of
the graphitised particles depends on the nature of the
original carbon black. In some cases, relatively large,
discrete particles are formed, as shown in Fig. 18a,
taken from the work of Graham and Kay.49 Other
graphitised carbon blacks have a less well defined
structure as in Fig. 18b, from the work of Marsh and
co-workers, 50 with many bent and faceted layer planes
and some apparently closed shell structures. The
presence of sharply bent planes and closed particles
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is indicative of the presence of pentagonal rings, and
suggests that fullerene like elements may have been
present in the original carbon black and soot particles.
Further detailed work on the graphitisation of carbon
blacks might help to confirm this.
Discussion
The discovery of the fullerenes has prompted a
number of workers to speculate that fullerene like
elements, i.e. curved structures containing non-six
membered rings, may be present in well known forms
of carbon.46,47,51To date, most of this speculation
has centred on spheroidal structures such as soot and
carbon black particles. However, as discussed in the
present paper, there are good reasons to believe that
fullerene related structures may be present in other
well known carbon materials such as microporous,
non-graphitising carbon, glassy carbon, and carbon
fibres. If correct, this idea would help in explaining
the properties of these carbons. Most importantly,
the presence of fullerene like elements could explain
why certain carbons can not be transformed into
graphite by high temperature heat treatment, a problem which has not been fully understood since the
1951 work of Rosalind Franklin.
One of the main reasons for believing that nongraphitising carbons may be fullerene like, is that
they can be transformed by high temperature heat
treatment into structures containing many closed
carbon cages. Of course, it could be argued that the
pentagonal rings form during the high temperature
heat treatment, and are not present in the original
carbon. However, this raises the question of why
pentagons are not formed during the heat treatment
of graphitising carbons such as PVC derived carbon
and petroleum coke. The presence of non-six
Harris
Structure of non-graphitising
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The nature of the transformation from microporous
non-graphitising
carbon to a structure containing
closed carbon cages is not well understood,
and
further work on this problem would be welcome. The
mechanism may involve ring migration mechanisms
such as the Stone-Wales rearrangement, 52 which has
been invoked to explain fullerene isomerisation. It is
notable that the cage structures observed in heat
treated non-graphitising
carbons fall into quite a
narrow size range, typically 5-15 nm, suggesting that
structures in this size range have a special stability.
Recent work has demonstrated that crystalline C60 is
also transformed into nanoparticles in this size range
by high temperature heat treatments. 53
There are other aspects of non-graphitising carbons
which remain inadequately understood. In particular,
there is the very basic question of why some organic
materials produce graphitising carbons and others
yield non-graphitising
carbons. If the ideas put forward in this review are correct, this question becomes:
why do some precursors form five membered rings
when carbonised, and others only hexagonal? There
is unlikely to be a simple answer to this question,
since the carbonisation
of organic materials is an
immensely complex process, and there is rarely a
simple relationship between the structure of the original precursor and the nature of the carbon produced
by pyrolysis. For example, structures which contain
five membered rings can produce either graphitising
or non-graphitising
carbon.54 In fact, it is generally
believed that the physical properties of the precursors,
and the conditions under which pyrolysis is carried
out, are more important than chemical structure in
determining whether the final carbon is graphitising
or non-graphitising.
Thus, graphitising carbons usually form a liquid on heating to temperatures around
400-500°C, while non-graphitising
carbons generally
form solid chars without melting. The liquid phase
produced on heating graphitising carbons is believed
to provide the mobility necessary to form oriented
regions. However, this may not be a complete explanation, since some precursors, such as sucrose, form
non-graphitising
carbons despite passing through a
liquid phase. There is clearly a need for more work
in this area, and on many other aspects of nongraphitising carbon, in the light of the new knowledge
we have gained about carbon since the discovery of
the fullerenes.
Acknowledgement
The author thanks Dr S. C. Tsang for discussions.
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