9PNQAPQNCLDKLK!EN?MFGPGOGKEA?N@LKOscsharip/IMR2.pdf · Published by Maney Publishing (c) IOM...

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Structure of non-graphitising carbonsP. J. F. Harris

Despite many years of research, the detailedatomic structure of many important carbonmaterials remains poorly understood. In particular,the structure of those carbons which can not betransformed into graphite by high temperatureheat treatment has never been clearly established.These non-graphitising carbons are ofconsiderable commercial importance in a varietyof fields, and a better understanding of theirstructure is clearly needed. Recently, it has beensuggested that non-graphitising carbons may havea microstructure which is related to that offullerenes. In the present paper, the evidence forthis will be considered in detail and theadvantages of the new model over previousmodels of non-graphitising carbons will bediscussed. As well as microporous non-graphitising carbons, other forms of carbonincluding glassy carbon and carbon fibres will beconsidered. IMRj304

© 1997 The Institute of Materials and ASM International.Dr P. J. F. Harris is based in the Department of Chemistry,University of Reading, Whiteknights, Reading RG6 6AD,UK.

IntroductionAlthough graphite is the most stable form of carbonat normal temperatures and pressures, it is a remark-able fact that many carbons can not be transformedinto crystalline graphite even at temperatures of3000°C and above. The so called 'non-graphitising'carbons tend to be hard, low density materials, withisotropic, microporous structures.1-6 In contrast, gra-phitising carbons are soft and non-porous, with densi-ties much closer to that of crystalline graphite. Non-graphitising carbons can develop exceptionally highsurface areas when 'activated' by treatment with amild oxidising agent, and the resulting activated car-bons are widely used as adsorbents and as catalystsupports.4-6 Despite their commercial importance,however, the detailed structure of these carbons atthe atomic level is still poorly understood. The tra-ditional view is that the microstructure consists oftwisted networks of carbon layer planes crosslinkedby bridging groups, explaining both their hardnessand their resistance to graphitisation,2 but the precisenature of such bridging groups has never been prop-erly established. Earlier suggestions that Sp3bondingmay be present in non-graphitising carbons do notappear to stand up to detailed analysis, as discussedin the 'Problems with early models' section later.

The relatively recent discovery of the fullerenes,7-9and subsequently of related structures such as carbonnanotubes10-12 and nanoparticles,13,14 has given usa new perspective on Sp2bonded carbon structures.Most importantly, we now know that carbon struc-tures containing non-six membered rings can be

highly stable. It therefore seems worth consideringthe idea that microporous non-graphitising carbonsmay be fullerene-like in nature. Recent work hasprovided support for this view by showing that hightemperature heat treatments can transform micro-porous carbons into fullerene like nanoparticles.15The purpose of the present paper is to consider furtherthe evidence that non-graphitising carbons containfullerene related elements. In addition to microporouscarbons, a related class of non-graphitising carbonknown as glassy carbon will also be considered, anda brief discussion will be given of the structure ofsoot particles and carbon fibres. First, a brief outlineis given of the types of bonding found in carbonmaterials. A description is then given of the prep-aration and properties of non-graphitising carbons,and conventional models of their structure are criti-cally discussed.

Bonding in carbon materialsA free carbon atom has the electronic structureIs2 2s22p2. In order to form covalent bonds, one ofthe 2s electrons is promoted to 2p, and the orbitalsare then hybridised in one of three possible ways. Ingraphite, one of the 2s electrons hybridises with twoof the 2p electrons to give three Sp2orbitals at 120°to each other in a plane, with the remaining orbitalhaving a pz configuration at 90° to this plane. TheSp2orbitals form the strong (J bonds between carbonatoms in the graphite planes, while pz or n orbitalsprovide the weak van der Waals bonds between theplanes. In naturally occurring or high quality syn-thetic graphite, the stacking sequence of the layers isgenerally ABAB, with an interlayer {0002} spacingof approximately 0·334nm, as shown in Fig. la. In lessperfect graphites, the interplanar spacing is found tobe significantly larger than the value for single crystalgraphite (typically ",0,344 nm), and the layer planesare randomly rotated with respect to each other aboutthe e axis. Such graphites are termed turbostratic.

In the C60 molecule, shown in Fig. Ib, the carbonatoms are bonded in an icosahedral structure madeup of 20 hexagons and 12 pentagons. Each of thecarbon atoms in C60is joined to three neighbours, sothe bonding is essentially Sp2,although there may bea small amount of Sp3character owing to the curva-ture. Note that all 60 carbon atoms are identical, sothat the strain is evenly distributed over the molecule.Pentagonal rings are also present in carbon nanopart-icles and nanotubes, although these generally havemuch less perfect structures than those of C60 andother fullerenes.

In diamond, each carbon atom is joined to fourneighbours in a tetrahedral structure, as shown inFig. le. The bonding here is Sp3,and results from themixing of one 2s and three 2p orbitals. Diamond isless stable than graphite, and is converted to graphiteat a temperature of 1700°C at normal pressures.

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Harris Structure of non-graphitising carbons 207

a b

a graphite showing unit cell; b eso; c diamond

1 Illustration of bonding in carbon structures

Disordered carbons containing Sp3 bonded atomsare also rapidly transformed into graphitic carbon athigh temperatures.

Non-graphitising carbonsBackgroundIt has been known for about a century that somecarbon materials are more amenable to graphitisationthan others, but the first detailed study of graphitisingand non-graphitising carbons was made by RosalindFranklin in the period before she began her famouswork on DNA. In a paper published in 1951,1Franklin described XRD studies of the effect of hightemperature heat treatments on the structure of avariety of carbons formed by pyrolysis of organicmaterials. She found a clear distinction between car-bons which could be converted into graphite by hightemperature annealing and those which could not.Among the non-graphitising carbons were those pro-duced by the pyrolysis of polyvinylidene chloride(PVDC) and sucrose, while graphitising carbonsincluded those made from polyvinyl chloride (PVC)and petroleum coke. Franklin proposed structuralmodels for the two classes of carbon, and these willbe discussed in the next section.

Since Franklin's time there has been a vast amountof research on the preparation and properties of non-graphitising carbons,2-s and only a very brief outlineis possible here. It is found that non-graphitisingcarbons are invariably highly porous, although someof this porosity is usually inaccessible to gases. Asnoted above, the internal surface area can be enhancedby activation, i.e. mild oxidation with a gas such ascarbon dioxide, steam, or air. A volume distributioncurve for a typical activated carbon is shown inFig. 2.4 It can be seen that most of the internal volumeis in the form of micropores with radii of approxi-mately 1 nm. Larger pores, classified as mesoporesand macropores, are also present, but we are notconcerned with these here. Models of the structure ofmicroporosity in non-graphitising carbons are dis-cussed below.

The effect of heat treatment on graphitising andnon-graphitising carbons has been the subject of alarge number of studies. The structures of the heattreated carbons are usually discussed in terms of theparameters La and Lc, defined as the length andthickness respectively of the graphite lamellae within

the carbons. In carbons prepared at temperaturesbelow ~ 1000°C, La and Lc in both graphitising andnon-graphitising carbons have values around 1 nm,indicating highly disordered structures with relativelylittle graphitisation. The effect of heat treatment onthe two types of carbon differs greatly. This is illus-trated in Fig. 3, taken from the work of Emmerich16which plots La and Lc for graphitising and non-graphitising carbons heat treated at temperatures upto 3000°C (note the La and Lc scales are logarithmic).It can be seen that La for the graphitising carbonreaches a value of 100 nm at 3000°C, while themaximum value for the non-graphitising carbon isonly 10 nm. The Lc value for the graphitising carbonalso approaches 100 nm, while for the non-graphitis-ing carbons the maximum figure is ~4 nm. Extensivegraphite crystallites are not formed in non-graphitis-ing carbons, even at the highest temperatures.

Other physical measurements also demonstratesharp differences between graphitising and non-gra-phitising carbons. Table 1 (Ref. 17) shows the effectof preparation temperature on the surface areas anddensities of a typical graphitising carbon preparedfrom PVC, and a non-graphitising carbon preparedfrom cellulose. It can be seen that the graphitisingcarbon prepared at 700°C has a very low surface

1.0'IE 0.8c

'I0)

ME 0.6CJ

>1 ~ 0.4~.s~(J?v.~

0

parameter on y axis gives measure of volume of gas absorbed bypores with given radius, while x axis is log of radius r in nm

2 Differential volume distribution curve for typicalactivated carbon4

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208 Harris Structure of non-graphitising carbons

100

Ec:

-J1O 10

100

graphltlsable

non-grophitlsable

graphltlsable

a

Table 1 Effect of temperature on surface areasand densities of carbons prepared fromPVC and cellulose 17

4 Franklin's representations of a graphitising andb non-graphitising carbons 1

depended on the assumption that growth results fromthe movement of whole layers or large fragmentsrather than individual atoms. It follows from this thatthe degree of crystal growth will depend on theorientation of the individual structural units and theamount of crosslinking between them. In graphitisingcarbons, the structural units are approximately paral-lel to each other, as shown in Fig. 4a, and the linksbetween adjacent units are assumed to be relativelyweak. On the other hand, the structural units in anon-graphitising carbon are oriented randomly, asshown in Fig. 4b, and the crosslinks are sufficientlystrong to impede movement of the layers into a moreparallel arrangement. Franklin's models of the struc-ture of graphitising and non-graphitising carbonshave remained popular, and are still reproduced inbooks and review articles.

The advent of high resolution electron microscopy(HREM) in the early 1970s enabled direct images tobe recorded of the structure of non-graphitising car-bons.1s Images of the freshly prepared carbonsshowed a highly disordered structure, but images ofthe carbons after high temperature heat treatmentwere rather more informative. These apparentlyshowed the presence of curved and twisted graphitesheets, typically two or three layer planes thick,enclosing pores of the order of 5-10 nm is size. Theseimages led Ban et al.18 to suggest that heat treatednon-graphitising carbons have a ribbon like structure,as shown in Fig. 5. This structure corresponds to a

bnon-gro phltlsable

1000 2000 3000HEAT TREATMENT TEMPERATURE, °C

For carbons prepared at

o

Specific surface area, m2 g-lPVC 0-58 0-21 0-21 0-71 0-56Cellulose 408 1-60 1-17 2-23 2-25

Helium density, g cm-3

PVC 1-85 2-09 2-14 2·21 2-26Cellulose 1-90 1-47 1-43 1-56 1-70


Startingmaterial

area, which changes little for carbons prepared athigher temperatures, up to 3000°C. The density of thecarbons increases steadily as the prepara-tion temperature is increased, reaching a value of2·26 g cm -3, which is the density of pure graphite, at3000°C. The effect of preparation temperature on thenon-graphitising carbon is very different. A highsurface area is observed for the carbon preparedat 700°C (408 m2 g -1), which falls rapidly as thepreparation temperature is increased. Despite thisreduction in surface area, however, the density of thenon-graphitising carbon is actually lower for highpreparation temperatures than it is at 700°C. Thisindicates that a high proportion of 'closed porosity'is present in the heat treated carbon.

Structure of non-graphitising carbons:early modelsThe first structural models of graphitising and non-graphitising carbons were put forward by Franklin inher classic 1951 paper.1 In these models, the basicunits are small graphitic crystallites containing a fewlayer planes, which are joined together by crosslinks.The precise nature of the crosslinks is not specified.A schematic illustration of Franklin's models is shownin Fig. 4. Her theory of crystallite growth in carbons

3 Variation of La and Lc with heat treatmenttemperature for graphitising and non-graphitising carbons 16

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5 Model of PVDC carbon heat treated at 1950°C, by Ban et al.18

PVDC carbon heat treated at 1950°C. These ribbonlike models are rather similar to an earlier model ofglassy 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 areexclusively Sp2 and are bonded in hexagonal rings.Some authors, notably Ergun and colleagues20,21havesuggested that Sp3bonded atoms may be present inthese carbons, basing their arguments on an analysisof XRD patterns. The presence of diamond likedomains would be consistent with the hardness ofnon-graphitising carbons, and might also explain theirextreme resistance to graphitisation.

Problems with early modelsThe most serious shortcoming of Franklin's modelsfor the structure of graphitising and non-graphitisingcarbons is that the nature of the crosslinks betweenthe graphitic fragments is not described. Suchcrosslinks must be extremely strong, since they aresufficient to prevent graphitisation even at temper-atures of 3000°C and above. The type of crosslinkspresent in polymers, which are usually short linearchains containing a few carbon atoms, would seemto be insufficiently rigid to prevent graphitisation athigh temperatures. The idea that the crosslinks mightcomprise small domains of Sp3bonded carbons alsodoes not appear to stand up to detailed analysis(see below).

Models of the kind illustrated in Fig. 5, which areintended to represent the structure of non-graphitisingcarbons following high temperature heat treatment,also have serious weaknesses. Such models consist ofcurved and twisted graphene sheets enclosing ran-domly shaped pores. However, graphene sheets areknown to be highly flexible, and would therefore beexpected to become ever more closely folded togetherat high temperatures, in order to reduce surfaceenergy. Indeed, tightly folded graphene sheets arequite frequently seen in carbons which have beenexposed to extreme conditions; examples are shownin Fig. 6.22,23Thus, structures like the one shown in

Fig. 5 would be unlikely to be stable at very hightemperatures.

It has also been pointed out by Oberlin24 that theearly models were based on a questionable interpret-ation of the electron micrographs. In most micro-graphs of graphitised carbons, only the {0002}fringesare resolved, and these are only visible when theyare approximately parallel to the electron beam.Therefore, such images tend to have a ribbon likeappearance. However, since only a part of the struc-ture is being imaged, this appearance can be mislead-ing, and the true three-dimensional structure may bemore cage like than ribbon like. This is a veryimportant point, and must always be borne in mindwhen analysing images of graphitic carbons.

As far as models incorporating Sp3bonded carbonatoms are concerned, the main problem is that Sp3bonded carbon is unstable at high temperatures.Diamond is converted to graphite at 1700°C, as notedabove, while tetrahedrally bonded carbon atoms inamorphous films are unstable above about 630°C.25Therefore, the presence of Sp3 bonded atoms in acarbon can not explain the resistance of the carbonto graphitisation at high temperatures. The XRDevidence for Sp3 bonded atoms in non-graphitisingcarbons is also open to question. The main argumentput forward by Ergun et al.20,21 for the presence ofdiamond like domains in non-graphitising carbons isthat the interference functions of such domains arevery similar to those of small graphitic structures.Therefore, Ergun et al.20,21 argue that XRD patternscan not be used to rule out the presence of Sp3bondedatoms. However, a more detailed analysis of XRDpatterns from these carbons by Ruland26 suggeststhat they are indeed inconsistent with a diamond likestructure. In particular, the presence of the graphite{0002}line in patterns from non-graphitising carbonsis difficult to reconcile with a structure containing asignificant proportion of Sp3bonded atoms.

More recent modelsRecently, some models of disordered carbons havebeen put forward in which the carbons are not


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U

4nm--

6 Micrographs showing tightly folded graphite sheets in carbons prepared by a arc evaporation22 andb pyrolysis of acenaphthylene;23 c illustration of folded sheet structure

exclusively bonded in six membered rings. Theseprobably provide a much more realistic basis forunderstanding the structure of non-graphitisingcarbons than the early models based on curved andtwisted graphene sheets. In a book published in 1995,Byrne and Marsh27 discussed the structure of carbonsproduced by the pyrolysis of cellulosic type precur-sors. They suggested that such carbons might be

made up of small structural units such as that illus-trated in Fig. 7a. This structure contains Sp2and Sp3carbons bonded in five, six, and seven memberedrings. A group of such structures, with adsorbatemolecules situated in the gaps between the units, isshown in Fig. 7b.

The discovery of the fullerenes has prompted sev-eral authors to put forward models of all-carbon

b

* An adsorbate molecule

7 a Possible carbonaceous structural unit produced by pyrolysis of cellulosic precursor, according toByrne and Marsh;27 b model of microporous carbon made up of such units

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a

8 a Periodic negatively curved graphitic structure, from the work of Terrones et a/;31 b disorderedschwartzite structure, proposed by Townsend and co-workers32

materials with fullerene related structures. Forexample, several groups have discussed hypothetical'schwartzite' structures, which incorporate negativecurvature owing to the presence of seven memberedrings.28-32 In most cases these theoretical structureshave been ordered, as in the example shown in Fig. 8a,but disordered schwartzites have also been consid-ered. An example of such a structure, taken from thework of Townsend and co-workers,32 is shown inFig. 8b. The fragment shown contains 38 five mem-bered rings, 394 six membered rings, 155 seven mem-bered rings, 12 eight membered rings, and one ninemembered ring. The structure is continuous, withno 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 peratom f1E of the various schwartzite structures relativeto a graphite monolayer. For the random schwartzitethey found a i1E value of 0·23 eV, considerablylower than the value for C60 (0'42 eV), indicatingthat such a structure should have high stability. Theyalso compared the properties of the hypotheticalschwartzite structure with those of evaporated Sp2carbon films, and found good agreement. However,they did not suggest that their model might beappropriate for microporous carbons produced bypyrolysis.

Evidence for fullerene like structures innon-graphitising carbonsRecently, the present author and colleagues havecarried out a study of the effect of high temperatureheat treatment on non-graphitising carbons usingHREM.15 It was shown that such heat treatmentscan result in the formation of closed carbon nanopart-icles, which are apparently fullerene like in structure.This suggested that fullerene like elements were pre-sent in the original carbons, and prompted us topropose a new model for non-graphitising carbons.Before describing this model, a brief summary will begiven of the HREM observations.

Two carbons were studied, prepared by the pyrol-ysis of a PVDC polymer and of sucrose, which weretwo of the precursors used by Franklin in her classicwork. Pyrolysis was carried out under nitrogen atabout 700°C, producing carbons which were highlydisordered. Figure 9a shows a micrograph of thefreshly prepared sucrose carbon. The microstructureapparently consists of tightly curled single carbonlayers, although high resolution images of such non-crystalline materials are difficult to interpret directly.Following heat treatment at 2600°C, however, manyregions of the carbon appear to consist largely ofclosed nanoparticles, as shown in Fig. 9b. Highermagnification images in which individual nanopart-icles can be more clearly seen are shown in Fig. 10.15Sometimes, much smaller closed structures with dia-meters of the order of 1 nm or less could be found;examples are shown in Fig. 11. These structures aresimilar in size to small fullerenes like C60 and C70.

However, attempts to extract C60 and other fullerenemolecules from the heat treated carbons have so farbeen unsuccessful.

It should be pointed out that there were manyregions in the heat treated samples in which thetransformation into nanoparticles was not as obviousas it is in Figs. 9b and 10. Many regions were toothick to enable individual nanoparticles to be seen,and the nanoparticles were often partially obscuredby disordered material. This probably explains whyclosed structures such as those shown here have notbeen clearly identified before.

It seems very likely that particles such as thoseshown in Figs. 9b and 10 contain pentagonal carbonrings, as in fullerenes, since it is difficult to envisageany other explanation for their closed structures.Indeed, the nanoparticles shown here are rather simi-lar to particles which can be produced by arc evapor-ation in a fullerene generator,13,14 although in thelatter case the particles usually contain many morelayers. As in the case of fullerenes, 12 pentagons mustbe present in each shell of the nanoparticles in orderto produce closure. Although the shapes of the


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9 a Micrograph of freshly prepared sucrosecarbon; b same carbon following heat treatmentat 2600°C; scale bar 5 nm

nanoparticles shown here are less symmetricalthan in small fullerenes such as C60, this does notargue against a fullerene related structure, since giantfullerenes can have a variety of different shapes, asillustrated in Fig. 12. Features such as the saddlepoints shown in Fig. lOb are evidence for thepresence of seven membered rings as well as penta-gons in the heat treated carbons. The existence oflarge numbers of fullerene like nanoparticles in theheat treated carbons explains the observation of'closed porosity' in such materials, as discussed inprevious sections. The nanoparticles constitute com-pletely sealed capsules which would be impermeableto any gas. Alternative models, such as that proposed


10 a Micrograph showing closed structure inPVDC derived carbon heated at 2600°C;b another micrograph of same sample witharrows showing regions of negative curvature;scale bar 5 nm (Ref. 15)

by Byrne and Marsh27 and illustrated in Fig. 7, donot seem to be capable of explaining the observedclosed porosity.

New model for structure of non-graphitisingcarbonsThe observations described in the previous sectionsuggest that non-graphitising carbons may havefullerene like microstructures. One possible modelfor these materials, therefore, might be the 'randomschwartzite' structure discussed above. However thereare aspects of this model which do not seem appro-priate for non-graphitising carbons. In particular, the

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11 Micrographs showing very small closedstructures in sucrose carbon heat treated at2600°C; scale bar 2 nm

random schwartzite structure consists of a singlecontinuous sheet, while non-graphitising carbons arebelieved to be made up of relatively small fragments.24An unbroken sheet such as that illustrated in Fig. 8b,with no edges or dangling bonds, would have a verylow reactivity, unlike most non-graphitising carbonswhich can be quite readily oxidised at moderatetemperatures. The present author and S. C. Tsanghave therefore proposed a modeP5 for the structureof non-graphitising carbons which consists of discretefragments of randomly curved carbon sheets, ratherthan an unbroken sheet, as illustrated in Fig. 13. In


our model we envisage a higher proportion of penta-gons and a smaller proportion of heptagons than inthe random schwartzite structure. This is supportedby the observation that the materials can be convertedby heat treatment to closed carbon nanoparticles,each containing 12 pentagonal rings, suggesting thata large number of such rings may have been presentin the original carbon. The size of the micropores inour model, as well as in the random schwartzitestructure, is of the order 0·5-1 nm, which is similarto the pore sizes observed in typical microporouscarbons, as noted above.

If the model we are proposing for non-graphitisingcarbons is correct, it suggests that these carbons arevery similar in structure to fullerene soot, the lowdensity, disordered material which forms on the wallsof the arc evaporation vessel and from which C60 andother fullerenes may be extracted. Fullerene soot isknown to be microporous, with a surface area, afteractivation with carbon dioxide, of ",700 m2 g-1,33and detailed analysis of high resolution electronmicrographs of fullerene soot has shown that theseare consistent with a random schwartzite type struc-ture.34 It is significant that high temperature heattreatments can transform fullerene soot into nano-particles very similar to those observed in heatedmicroporous carbon.35

Finally in this section, it is worth making a fewcomments on the 'activation' process which is essen-tial for developing a very high surface area in non-graphitising carbons. Activation usually involvestreatment with a mild oxidising agent, such as CO2or water vapour, and it is generally believed that thishas the effect of burning away carbon fragmentsinside micropores, thus enhancing surface area.27

However, if our new model for the structure ofmicroporous carbons is correct, then this activationtreatment may also have a further consequence. It isknown that mild oxidation, for example with CO2 at850°C, can remove the caps from carbon nanotubesby selectively attacking the pentagonal carbon rings.36If microporous carbons have a fullerene like structure,then the effect of such a treatment would be to openclosed pores by selective attack of the pentagons, thusincreasing the surface area significantly.

Glassy carbonThe so called glassy carbons are produced by the slowpyrolysis of certain polymers at temperatures in therange 900-1 aao°c. The resulting carbons are hard,low density materials which can not be graphitised,but unlike most non-graphitising carbons they areimpermeable to gases. Perhaps their most remarkableproperty is their chemical inertness. It has been demon-strated that the rates of oxidation of glassy carbon inoxygen, carbon dioxide, or water vapour are lowerthan those of any other carbon. They are also highlyresistant to attack by acids. Thus, while normal graph-ite is reduced to a powder by a mixture of concentratedsulphuric and nitric acids at room temperature, glassycarbon is unaffected by such treatment, even afterseveral months.37 This property makes glassy carbona useful material for crucibles. It is also used widelyas an electrode material in electrochemistry.

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a C'500 (Ih symmetry); b C600 (D2h symmetry); c C660 (tetrahedral symmetry)

12 Giant fullerene structures with various symmetries, each containing 12 pentagonal rings30

Some of the earliest structural models for glassycarbon assumed that both Sp2and Sp3bonded atomswere present. 38Graphitic domains were envisaged tobe interspersed with tetrahedral domains, perhapslinked by short oxygen containing bridges. Thesemodels were based primarily on an analysis ofXRD measurements and, as mentioned above, suchmeasurements can be open to a number of interpret-ations. It should also be noted that neutron diffractiondata have shown an absence of tetrahedrally bondeddomains in glassy carbon heat treated at 2000°C,39

A different model for the structure of glassy carbonwas put forward by Jenkins and Kawamura19 in 1972.This model, illustrated in Fig. 14, is based on theassumption that the molecular orientation of thepolymeric precursor material is memorised to someextent after carbonisation. Thus, the structure bearssome resemblance to that of a polymer, in which the'fibrils' are very narrow curved and twisted ribbonsof graphitic carbon. The Jenkins-Kawamura modelhas been quite widely accepted, but appears to bedeficient in a number of aspects. For example, astructure such as that shown in Fig. 14, with manyconjoined micropores, would be expected to be per-

13 Schematic illustration of model for structureof non-graphitising carbons based on fullerenelike elements 15


meable to gases, whereas we know that glassy carbonsare highly impermeable. The structure also has a highproportion of edge atoms, which are known to havea relatively high reactivity compared with 'in plane'carbon atoms.

An alternative model for glassy carbon would beone in which the basic structural units are fullerene-like closed particles, as shown in Fig. 15. Such astructure would be impermeable and have a muchlower reactivity than the Jenkins-Kawamura struc-ture. In the schematic illustration shown in Fig. 15,the individual particles are of the order of 1 nm insize. This is consistent with high resolution electronmicrographs of glassy carbon, which show little evi-dence of any structure, suggesting the basic structuralunits are extremely small.

A model of heat treated glassy carbon which alsoinvolves cage like components was proposed byJapanese workers in 1984.40,41This is illustrated inFig. 16. Here, the particles are multilayered and haveinner cavities ~ 50 nm dia. This model was put for-ward before the discovery of C60, and the possibilitythat the closed particles might contain non-six mem-bered rings was not considered.

Carbon fibresMost commercial carbon fibres are produced eitherfrom polyacrylonitrile (PAN) or from mesophasepitch.42Fibres derived from pitch are highly graphitic,and have high elastic moduli, while those derivedfrom PAN have a much more imperfect, lower densitystructure. The lack of extended structure in PANderived fibres makes them relatively insensitive toflaws, giving them higher strength but lower modulusthan pitch derived fibres. The properties of PANderived fibres result from the fact that PAN is non-graphitising, and a brief discussion of their structureis therefore appropriate here.

XRD of PAN derived fibres produces La values ofapproximately 4-10 nm depending on the annealingtemperature.43 HREM shows that the fibres have animperfect structure, containing many elongated voids.Several models have been put forward for the struc-ture of PAN derived carbon fibres, all based on theassumption that the basic structural units are graphite

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!Lc

1

14 Model by Jenkins and Kawamura for structure of glassy carbon 19

sheets or ribbons. A model suggested by Crawfordand Johnson44 is shown in Fig. 17. Here, the structureconsists of a random arrangement of flat or crumpledgraphite sheets, with all the a-b planes running paral-lel to the fibre axis. However, given the flexibility ofgraphite sheets, illustrated in Fig. 6, it is difficult to seehow the voids in such a structure could survive hightemperature heat treatment. Therefore, the possibilitythat the voids in fact result from the presence offullerene like elements is worthy of consideration. Theelongated shapes of the voids suggests that they mayhave structures related to those of carbon nanotubes.

Soot and carbon blackSoot is another carbon material whose structure ispoorly understood. It usually consists of quasi spheri-cal particles ranging from about 10 to about 500 nmin size, which are often joined together in clusters or'necklace' chains. Carbon black is essentially a verypure form of soot, which is of great commercialimportance as a pigment and as a filler in rubber

15 Model for structure of glassy carboncontaining closed, fullerene like particles

products.45 The possibility that soot particles mightbe fullerene like was first suggested by Smalley andco-workers in 1986,46 and discussed further by Krotoand McKay in 1988.47 In these papers it was proposedthat the soot particles grew by a mechanism relatedto the so called 'pentagon road' model, which involvesthe incorporation of pentagonal rings into a growingcarbon network, driven by the need to eliminatedangling 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 andthe growing shell will then tend to curl around onitself like a nautilus shell. However, this model wasnot greeted favourably by soot experts,48 who argued

15 nm I

16 Model for structure of glassy carbon derivedfrom phenol resin following heat treatment at2800°C (Refs. 40, 41)


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17 Model suggested by Crawford and Johnson for structure of PAN derived carbon fibres44

that it was inconsistent with both the kinetics of sootformation and the structural characteristics of theparticles. Concerning kinetics, it was pointed out thatthe growth of curved shell structures would be muchslower than those of planar fragments. Soot formationis known to be extremely rapid, so it seemed unlikelythat the growth of fullerene like shells could beinvolved. Their structural arguments were based onXRD and 13C NMR patterns of combustion soot,which they suggested were inconsistent with the ico-spiral model. They pointed out that d spacings forcontinuously curving icospirals would be lower thanthose observed experimentally in XRD studies ofsoot, while 13CNMR spectra of soot resemble thoseof aromatic molecules much more closely than thoseof fullerenes. However, evidence from XRD and 13eNMR of soot particles is difficult to interpret whenone is dealing with disordered materials such ascarbon blacks, and can not be said to provide defini-tive proof of the structure.

Since structural measurements on the soot particlesthemselves is difficult, it is worth considering whetherany insights into their structure can be gained fromthe way in which the particles are transformed byhigh temperature heat treatment. It is well establishedthat such treatments transform carbon black particlesinto faceted particles which sometimes appear to haveclosed shell like structures. The precise structure ofthe graphitised particles depends on the nature of theoriginal 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 Othergraphitised carbon blacks have a less well definedstructure as in Fig. 18b, from the work of Marsh andco-workers, 50 with many bent and faceted layer planesand some apparently closed shell structures. Thepresence of sharply bent planes and closed particles


is indicative of the presence of pentagonal rings, andsuggests that fullerene like elements may have beenpresent in the original carbon black and soot particles.Further detailed work on the graphitisation of carbonblacks might help to confirm this.

DiscussionThe discovery of the fullerenes has prompted anumber of workers to speculate that fullerene likeelements, i.e. curved structures containing non-sixmembered rings, may be present in well known formsof carbon.46,47,51To date, most of this speculationhas centred on spheroidal structures such as soot andcarbon black particles. However, as discussed in thepresent paper, there are good reasons to believe thatfullerene related structures may be present in otherwell known carbon materials such as microporous,non-graphitising carbon, glassy carbon, and carbonfibres. If correct, this idea would help in explainingthe properties of these carbons. Most importantly,the presence of fullerene like elements could explainwhy certain carbons can not be transformed intographite by high temperature heat treatment, a prob-lem which has not been fully understood since the1951 work of Rosalind Franklin.

One of the main reasons for believing that non-graphitising carbons may be fullerene like, is thatthey can be transformed by high temperature heattreatment into structures containing many closedcarbon cages. Of course, it could be argued that thepentagonal rings form during the high temperatureheat treatment, and are not present in the originalcarbon. However, this raises the question of whypentagons are not formed during the heat treatmentof graphitising carbons such as PVC derived carbonand petroleum coke. The presence of non-six

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18 a Faceted particles in graphitised carbon blacksample,49 scale bar 100 nm; b graphitisedcarbon black containing bent and faceted layerplanes, and some closed particles,5o scale bar10nm

membered rings in non-graphitising carbons alsoexplains many of their physical properties such asmicroporosity and hardness.


The nature of the transformation from microporousnon-graphitising carbon to a structure containingclosed carbon cages is not well understood, andfurther work on this problem would be welcome. Themechanism may involve ring migration mechanismssuch as the Stone-Wales rearrangement, 52 which hasbeen invoked to explain fullerene isomerisation. It isnotable that the cage structures observed in heattreated non-graphitising carbons fall into quite anarrow size range, typically 5-15 nm, suggesting thatstructures in this size range have a special stability.Recent work has demonstrated that crystalline C60 isalso transformed into nanoparticles in this size rangeby high temperature heat treatments. 53

There are other aspects of non-graphitising carbonswhich remain inadequately understood. In particular,there is the very basic question of why some organicmaterials produce graphitising carbons and othersyield non-graphitising carbons. If the ideas put for-ward in this review are correct, this question becomes:why do some precursors form five membered ringswhen carbonised, and others only hexagonal? Thereis unlikely to be a simple answer to this question,since the carbonisation of organic materials is animmensely complex process, and there is rarely asimple relationship between the structure of the orig-inal precursor and the nature of the carbon producedby pyrolysis. For example, structures which containfive membered rings can produce either graphitisingor non-graphitising carbon.54 In fact, it is generallybelieved that the physical properties of the precursors,and the conditions under which pyrolysis is carriedout, are more important than chemical structure indetermining whether the final carbon is graphitisingor non-graphitising. Thus, graphitising carbons usu-ally form a liquid on heating to temperatures around400-500°C, while non-graphitising carbons generallyform solid chars without melting. The liquid phaseproduced on heating graphitising carbons is believedto provide the mobility necessary to form orientedregions. However, this may not be a complete expla-nation, since some precursors, such as sucrose, formnon-graphitising carbons despite passing through aliquid phase. There is clearly a need for more workin this area, and on many other aspects of non-graphitising carbon, in the light of the new knowledgewe have gained about carbon since the discovery ofthe fullerenes.

AcknowledgementThe author thanks Dr S. C. Tsang for discussions.

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