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The morphology of some natural and synthetic graphites
aJoa4D3188uoo+.t3tl &1I988Pergamon Journals Ltd
CarbonVoi 26,No 1,pp.23-32. 1988 Printedut GreaiBntnn.
THE MORPHOLOGY OF SOME NATURAL SYNTHETIC GRAPHITES
AND
A. KAVANAGH Dept. Chemistry, UMIST, Sackville St., Manchester M60 lQD, U.K.
and R. SCHLOGL Fritz Haber Institut der Max-Planck Geseilschaft, Faradayweg 4-6, lOO&Berlin 33, FRG (Received 27 April 1987; accepted 26 June 1987)
Abstract-The morphologies of seven different types of graphite, both natural and synthetic, have been examined using a scanning electron microscope (SEMI. The surfaces are not smooth but were found to exhibit structural detail over the entire range of magnification used. These defects and surface
structures may well affect chemical reactivity and adsorption characteristics of the materials. Key Words-Graphite
morphology, Ticonderoga graphite, pyrolytic graphite, grafoil.
SEM studies. This makes it difficult, on the other hand, to obtain meaningful images with sufficient contrast to recognize the fine structure of the graphite surface under investigation. We consider this study to complement the extensive body of TEM work that has been done, in particular with respect to the elucidation of structural defects[ 1,8]. It further complements recent images obtained with reflection electron microscopy (RlZM)[9] and STM[lO,ll]. In the following, we present a rather large set of micrographs, with several images at various magnifications for each type of graphite in order to give an impression of typical structures.
1. INTRODUCTION
Graphite
has been studied
in many different
physical
and chemical aspects[l], and for many surface-sensitive techniques it has become a reference material with well-known properties[2]. Furthermore graphite finds widespread application in analytical techniques and is an industrially important material. Samples of both natural and synthetic graphites are available in sizes from millimeters up to large plates. However, relatively little is known about the surface morphology of such samples. It is generally accepted that after cleavage essentially featureless and wellordered (atomically smooth) surfaces are obtained[3,11], despite the evidence for cleavage defects obtained from a transmission electron microscope (TEM)[4]. With the advent of the scanning transmission electron microscope @TM) technique, surface studies with atomic resolution have stimulated renewed interest in the morphology of graphite[ll]. As a consequence of the anisotropy of the graphite structure, the chemical reactivity towards oxidation[S] or intercalation[b], for instance, depends strongly on the area ratio of nonreactive basal planes to reactive prismatic faces. This area ratio may deviate considerably from the geometric ratio due to the presence of defects and step structures. Knowledge of this ratio is also required when graphite is used as the substrate in adsorption studies and for the interpretation of some spectral features (e.g., its photoelectron spectrum[7,12]. This article presents a survey of typical morphologies of a variety of natural and synthetic graphites. We employed two scanning electron microscopes in order to obtain images with different levels of resolution. The fact that graphite in bulk samples is a very poor emitter of secondary electrons and is therefore essentially black for SEM is exploited when it is used as a support for low-contrast samples in
2.
EXPERIMENTAL
Seven different types of graphite were examinedtwo batches of highly oriented pyrolytic graphite (HOPG), oriented pyrolytic graphite (OPG), a pyrolytic graphite, two types of naturally occurring graphite (S40 and Ticonderoga single crystal), and a sample of grafoil; a detailed characterization of these samples is given elsewhere[l2]. All graphites were mounted on Al stubs using conductive silver cement and examined in a Camscan S4 SEM operating at typically 15 kV accelerating voltage. Some samples were subsequently examined in a JEOL CX 200 Temscan operating at 200 kV. No attempts were made to decorate the sample surfaces with, for instance, transition metals, to enhance the weak topographic contrast. 3. RESULTS
3.1 Ticonderogu graphite At low magni~~ation, some deposits can be seen on the surface (Fig. 1A); they give different contrast in the absorbed current distri&tion image (AEI) mode and may be mineral impurities not removed 23
24
A. KAVANAGH
and R. SCHL~GL
- _.-__A. 600~
B i3oop
Fig. 1. Surface deposits on Ticonderoga graphite. during purification (a mild process involving dissolution in dilute HCl, washing in distilled water, with no high temperature treatment or use of HF). The disruption visible on the right-hand side of Figs. iB and 2B is probably the result of cleavage of the crystal at a point where it was attached to the mineral deposit. The characteristic defect lines seen in Fig. 1A are typicahy severaf hundred microns in length, intersecting at an angle of 60” to form hexagons. The flake visible in Fig. 1B is probably not made up of a single crystal, but consists of a stack of several individual crystallites. At their boundaries are frequently observed structures such as those in Fig. ZB, where very thin sections protrude from the surface. It can be seen from this micrograph that these agglomerates consist of flakes, plates, and some filamentary structure (see Fig. 2A). We note intersecting defect lines, indicating the mosaic structure, observed previously by optical microscopy[l3]; some folded structures were also occasionally detected. At higher magnification, a surface structure (as in Figs. 3A, B), was revealed, arising from interpenetrating etch pits, covering large areas along defect lines. These etch pits are probably formed under the in-
Fig. 2. Ticonderoga graphite showing the protrusion of thin wafers from the surface.
AL---J
3u
B1OOOnm
Fig. 3. Interpenetrating etch-pits on the surface of Ticonderoga graphite. fluence of alkali-containing minerals, similar to the corrosion effect observed during impregnation of graphite with alkali carbonates[l4]. This is in contrast to the other natural graphite used in this study (S40), which exhibits no pitting either because it is found in seams of graphite rather than as inclusions of single flakes in the surrounding rock or because the etch pits are removed by the oxidative purification method used during manufacture. At still higher magnification, the “smooth” parts of the surface reveal more structure, which could be indicative of vacancy loops (Fig. 4). However, despite this wealth of surface detail, the Ticonderoga graphite was found to have a much less defective surface structure than the other graphites examined. 3.2 S40 Figures 5A and 5B show mutually perpendicular views in the orientation used; the prismatic faces that one would expect in Fig. 5A cannot be seen as clear contours. The flakes are not single crystals but consist of many crystalhtes in random orientation that cover the prismatic faces of the inner stack of crystailites. This has implications for the kinetics of intercalation, since in order to reach the reactive pris-
Fig. 4. Details seen on otherwise smooth areas of the Ticonderoga surface.
25
Natural and synthetic graphites
High magnification images from the surface of the platelets (Figs. 9A and 9B) reveal a step structure very similar to that seen in HOPG (see below). We have resoived steps of an average height of approx. 250 8, with terraces ranging in width from 250 to 5000 A. The steps have jagged edges, unlike those observed on synthetic graphites or the natural single crystal, and seem to follow c~st~lo~aphic orientations. These edges may again be a result of etching occurring during purification. These defective steps are important for the assessment of chemical reactivity since they provide favourable sites for the commencement of a reaction. Figures 1OA and 1OB show that even at very high magni~cation, the surface is not smooth: the platelets are folded back on themselves (shown by the darker areas in Fig. 10A) and a finer step structure of a few interlayer distances is visible. This fine step structure has been imaged in detail with the STM[ll].
3.3 Grafoil Grafoil is produced from, for example, naturally occurring graphites by intercalation with H$O, or HNO,, after which it is exfoliated by heating it rap-
Fig. 5. View of an edge of S40 graphite in mutually per-
pendicular orientations. matic faces, reactants would have to diffuse past the platelets. Comparing Figs. 1B and 5A, it seems that S40 consists of many platelets with different orien-
tations, whereas the Ticonderoga is composed of a small number of crystals. TEM (Figs. 6A and 6B) has shown that even these platelets are not single crystals. Instead they consist of stacks of randomly oriented single crystals (seen by electron diffraction-not shown). Their surfaces do not seem to be smooth, but exhibit a mottled character. The multiparticulate nature of the surface of S40 is clearly shown in Figs. 7A and 7B. The small surface particles could not be removed by ultrasonic cleaning, indicating that they are integral parts of the graphite structure. The good crystal quality of the naturally grown material is evinced by the sharp edges arising from etching during the purification procedure. (Note the very thin leaves (Fig. 7A) that are penetrated by the 20-kV beam.) The polycrystalline nature of the S40 flakes is shown in Fig. 8, viewing down one of the few accessible prismatic faces. Contrast arises between different packages of layers of individual crystals; the width of these bands shows that this thickness varies from 100 to 250 A.
A 150nm L
_..._1
B t__..... __ j 120nm Fig. 6. TEM images of stacks of platelets in S40.
26
A.
KAVANAGH
and R.
SCHL~~GL
300nm Fig. 8. View down a prismatic face of S40 graphite showing stacked single crystals.
ference in current distribution across the surface arises from resistance across the boundaries between platelets. These features are not due to shadowing effects, which are common at such high magnifications, because they remained when the sample was tilted through several different angles. The electrical resistance of the boundaries of the platelets may well be the result of extensive chemical modification of the outer areas of the platelets during intercalation and exfoliation. In particular, a variety of surfacesoxygen groups attached to the prismatic edges and defects in the crystallites[l2,17]-may cause significant interparticle resistance.
B-
1ocl Fig. 7. Small crystallites attached to the surface of S40.
idly to a high temperature. The high surface area material is then compressed, and sometimes annealed. The method of production of grafoil is assumed to result in a fairly structureless, smooth surface, so that all adsorption sites are of comparable energies and grafoil is considered, therefore, as a suitable substrate for adsorption studies[ 161. The present sample looks disordered at low magnification (Figs. 11A and llB), unlike the natural graphites (see Figs. IA and 5B). The absence of organization in the surface may result from the compression of platelets during manufacture. At higher magnification (Fig. 12A), the surface reveals a rich structure arising from the former individual platelets of the starting graphite. Such a surface should exhibit a broad distribution of adsorption sites, and may hence not be an ideal adsorption substrate. Despite the open, honeycomblike structure of the exfoliated precursor, no pore structure in the macroporous region could be detected. Figure 12B is an AEI (specimen absorbed current) image; the dif-
3.4 Pyrographite Pyrographite is produced by graphitization of carbon, obtained by burning low molecular weight hydrocarbons. At low magnification (Figs. 13A and 13B), it can be seen that the surface in one direction is sectioned (approx. 50 p, wide), with steps perpendicular to this direction with an average height of 1 IL (compare
A-
B32nm
32nm
Fig. 9. High resolution image of the step structures on the surface of S40.
27
Natural and synthetic graphites
A.
8. 50nm
50nm
Fig. 10. High resolution images of the “flat” areas of the S40 surface, showing fine step structure and some surface roughness.
this organ~ation of steps in mutually pe~endicular directions to the complete lack of organization in grafoil (Fig. 12A)). The surface consists of folded sheets of carbon and this becomes more apparent at higher magnifications (Figs. 14A and 14B). The im-
B-
300nm
Fig. 12A. The surface of grafoil showing some structure arising from platelets of the precursor graphite. B. AEI image of grafoil surface.
Fig. 11. The surface of grafoil: note the absence of any organization of the surface structure.
ages show that these sheets that form an approximately rectangular tiling of the surface (Fig. 13) consist of a large number of thin flakes. Inspection of Figs. 15A and 15B shows that there are two levels of steps that differ in height by an order of magnitude. The small steps that can just be detected in Fig. 13 occur as a “staircase” of terminating platelets, many of them bent over at the edge. Note in Fig. 15B how platelets may be folded over a large step with nearly perpendicular orientation (bottom right, Fig. 15B) and how closely a thin platelet may follow the structure of the substrate (step in top right-hand comer of Fig. 15B). It is believed that the richly ordered structure of this graphite results from cleaving the relatively hard material. In contrast to HOPG (see below), this material does not “rearrange” during cleavage because a substantial amount of interlayer defects hinders the gliding of planes. These interlayer defects are responsible for the hardness of this graphite com-
28
A. KAVANAGH
and R. SCHL~GL
AlOOp’
B 3Op-7 Fig. 13. Large steps on the surface of pyrographite.
AlOv
I
B 30~~
Fig. 14. Multiple folding of the surface layers of pyrographite.
B lOOOnmFig. 15A. Fine structure on the surface steps of pyrographite showing bunching of platelets. B. Overlapping, perpendicular platelets. Note the very fine step structure (on the right-hand side) running parallel to the edge of the crystallite. pared to HOPG. Cleavage has exposed many prismatic faces, which will lead to a considerable enhancement of chemical reactivity. The defects also manifest themselves in differences of the photoelectron spectrum of this graphite compared to HOPG[7,17]. The structures in the center of Fig. 15B are thought to mark a location that prevented gliding of differently oriented carbon sheets during cleavage. A cluster of “nongraphitic” carbon atoms as a result of incomplete graphitization may result in such a location acting as an anchor during cleavage. Beam damage occurred after approximately one hour in the microscope. The newly arisen bubblelike structures (Figs. 16A and 16B) imply that some gaseous product has been expelled. The gas (e.g., Hz, CHJ may be concealed at clusters of defects during the pyrolysis step of the low molecular weight precursor. 3.5 Oriented pyrolytic graphite OPG is produced in a similar way as pyrographite, but further graphitizatin under mechanical stress results in improved stacking order. The morphology of freshly cleaved OPG surfaces resembles that of pyrographite and is distincly different from the morphology of HOPG. Our sample exhibited extensive
Natural and synthetic graphites
29
Al 001.1
Al OOOnm
Fig. 16A. Bubblelike structures on the surface of pyrographite caused by beam damage. B. Detail of Fig. 16A showing the “exploded” appearance of one of these “bubbles.”
material contrast, in particular next to steps, as can be seen from Fig. 17 (note the well-aligned occurrence of naturally decorated steps in Fig. 17A). We exclude contamination in the microscope as a source for the overlayer: subsequent examination of this sample with XPS suggested an organic material to be enclosed along defects: in situ cleaving in UHV
Fig. 17A. Steps on the surface of OPG. B. The “dirty” surface of OP materials contrast on the surface of OPG.
B. 1 OOOnm Fig. HA. Overlapping cystallites on the surface of OPG. B. Contour-enhanced AEI image of area shown in A.
results in a contamination of the vacuum and gave UPS spectra of contaminated ~aphite[l7]. No unusual heteroelements except oxygen and nitrogen were detected. Cleavage of this material (which is softer than pyrographite) exposed small crystallites in much more parallel stacking order than in pyrographite (compare the flat surfaces in Figs. 17 and 18 with those in Fig. 14). This probably results from the higher degree of stacking order in the individual graphitic carbon planes. The random orientation between planes can, however, be seen in Figs. 17B and 18A, which show the extensive mosaic spread, wellknown from diffraction experiments. This random orientation of boundaries should be compared with the better ordered surfaces of the natural graphites. The contour-enhanced AEI image of Fig. 18B (taken from a different area than Fig. 18A) demonstrates how several regularly shaped crystallites can overlap
30
A.
KAVANAGH
andR.
SCHL~GL
3op
BU
AU
B-
A-
1OOOnm
200nm
3olJ
Fig. 19A. Step structure of HOPG after cleaving. B. Contour-enhanced AEI image of HOPG.
Fig. 21. Step structure on the otherwise smooth hedges between larger steps on HOPG.
and how they may be covered by lamellae: all this leads again to an unexpected heterogeneity of the absorbed current distribution and points to the heterogeneity of physical properties of such surfaces.
The small number of interlayer defects seems to be second only to the stacking perfection in Ticonderoga flakes. From the contour-enhanced image in Fig. 19B, it can be seen that very thin lamellae follow closely the underlying step structure, although this requires multiple folding. The presence of multiple fractured crystallites on the right-hand side suggests that for this particular surface the cleaving force was not entirely parallel to the basal plane. The first sample exhibits a step structure (Fig. 20A, B, C, D), the distance between steps ranging from 0.2 to several microns. These steps appear to be parallel over large distances, with little disruption of the surface between them. This is probably due to the low number of interlayer defects in this sample. However, at higher magnification (Fig. 21A, B) show that the terraces between steps are not smooth, but exhibit a morphology, with series of parallel steps running over long distances. These steps (height approx. 500 A) are terminating crystallites. To explain this regular step structure, it is suggested that groups of several crystallites may act in concert, and as if they have some elasticity. The individual crystallites in this HOPG sample are of similar thickness but
3.6 Highly oriented pyrolytic graphites HOPG is produced in a similar manner to OPG, but extensive annealing under pressure produces a graphite with a nearly perfect stacking order and varying degrees of rotational disorder (i.e., artificial single crystals). Several samples from two batches of HOPG were examined and they held some features in common: HOPG surfaces are much smoother than those of the OPG and after parallel cleaving low magnification shows a stepped surface arising from a parallel stack of terminating platelets as shown by the examples in Fig. 19. Cleaving of this very soft material did not result in tilting and breaking of large individual crystallites (as in pyrographite), which is due to the high degree of order in HOPG with, in particular, only a small amount of interlayer defects.
B, D,lop 5p
’ ,
Fig. 20. Step structures of varying terrace width on HOPG.
A.
1olJ
B-
1olr
C2oou
Fig. 22. Ridges and multiple folding on the surface of HOPG.
Natural and synthetic graphites
31
24A, B) more than the other HOPG.
It typically exhibits a surface comprised of overlapping platelets (compare Fig. 5). However, we also observed the layers of folds and series of steps typical of the first sample. On otherwise smooth areas of terraces, we observed complex aggregates (approx 2 )Ldiameter) of small platelets. This highly defective, and thus “hard,” material may be left behind by the cleavage procedure (see Fig. 24B). 4. DISCUSSION AND CONCLUSIONS
Fig. 23. Multiple folding and bending of surface layers on HOPG.
considerably larger than those of the natural S40 graphite (Fig. 9A, B). We note that it was only possible to obtain such images at accelerating voltages below 15 kV with the sample tilted 15”. At 200 kV, accelerating voltage no contrast similar to that of Fig. 9 could be obtained in any o~entation of the specimen, Figure 22A, B, and C (C is an overview of the regions in A and B) show that these crystallites would cover up underlying macroscopic defects, such as grain boundaries. Figure 22B shows how multiple folding may arise when one part of a layer is “pinned down” by point defects or an emergent screw dislocation, which prevents the material from cteaving smoothly. The graphite layer along which cleavage occurred was fixed to the bulk at a few points, one of which forms the center of Fig. 22B and so as the mobile part of the layer glided during cleavage, it buckled (Fig. 22C). Figure 23A shows a very fine platelet, multiply folded and loosely attached to the surface; bending such a package of layers may cause them to “open out” as seen in Fig. 23B. Similar effects of cleavage, although on a coarser scale, have been seen previously by optical microscopy: Thomas[l3] presented evidence for extensive rearrangement of Ticonderoga graphite that he attributed to plastic flow during cleavage. The second sample of HOPG morphologically resembles the poiycrystalline natural graphite, (Fig.
Harris et al. [15] examined the surface of annealed pyrolytic graphite in conjunction with adsorptiondesorption studies. They used a Cr-shadowing technique, which showed that some areas of the surface were rougher than others, and some slightly raised. However, the grain structure of the nucleation pattern of Cr prevented them from seeing any finer details (e.g., closely spaced steps). Nevertheless, they noticed two different types of surface morphologies, similar to our observations. We have tried to give a description of the variety of typical surface features found on several different graphites. We used direct imaging and did not require decoration techniques, which might have affected the nature of the structures seen. What has been shown clearly in this study is that there exists a wide variety of defects that manifest themselves after, or are induced by, cleavage.
Fig. 24. Surface of HOPG, showing overlapping platelets.
32
A. KAVANAGH
(Thomas has already described how steps may be formed when a cleavage crack intersects an array of dislocations.) These defects can be seen directly, and are visible in every magnification range used (lOOO150,000x). It has been shown that the graphites examined were not, in general, atomically smooth: several chemically different carbon species are present (e.g., those adjacent to loops, steps, and corners) in appreciable amounts, even on the basal planes. Because of the presence of the surface defects imaged, transport properties on a microscopic scale are likely to be nonuniform in the surface and surface-near region of these graphites; these defects will also affect the local chemical properties of the graphites. In several samples of nominally the same material (OPGs), distinctly different surface features, and hence distinct differences in the quantity and quality of defects, have been found. These differences would not easily be detected by integral methods but may affect several applications, such as use as a host material for intercalation. In particular, grafoil is used in adsorption studies, since it is thought to provide an homogeneous substrate; however, as has been shown, the surface is structurally heterogeneous, implying that adsorption may well occur selectively on certain sites, only averaging out when the size of the area under study is large compared to the areas over which heterogeneities occur. We have also seen evidence of a certain amount of elasticity in HOPG, as the planes glide during cleavage. This study has shown a variety of structural detail in the size range below optical detection and above the level of atomic resolution. Step structures, different patterns of surface organisation and terrace sites have been directly imaged, and these are likely to determine the local properties of a particular sample of graphite used in a wide range of experiments, from adsorption to intercalation and studies of chemical kinetics. Crit-
and R. SCHL~GL
ical evaluation of these experiments is hoped to benefit from this work. Acknowledgments--The
authors acknowledge Dr. T. Page and Mr. M. Stocker, Dept. Material Science, and Prof. J. M. Thomas, Dept. Physical Chemistry, Cambridge, for allowing us to use their electron microscopes. The samples of HOPG were kindly donated by Dr. A. W. Moore, Union Carbide, U.S.A.
REFERENCES 1. P. L. Walker and P. A. Thrower (eds.), ChemBtry and Physics of Carbon, ~01s. l-20, Marcel-Dekker, New
York (1965-1987). 2. B. T. Kelly, ADDI. Sci. XX, X (1981). and Physics of Carbon, vol. 3. G. R. He&g,-Chemistry 2. D. 7. Marcel Dekker. New York (19xX). 4. S: ‘Amelinckx, P. Delavignette and M. Heerschap, Chembtry and Physics of Carbon, vol. 1, p. 1, Marcel Dekker, New York (1965). 5. J. M. Thomas, Chemhtry and Physics of Carbon, vol. 1, p. 121, Marcel Dekker, New York (1965). 6. A.*Herold, Mat. Sci. Eng. 31, 1 (1979). 7. T. Takahashi, H. Tokaihn and T. Sagawa, Phys. Rev. B 32, 8317 (1985).
8. P. A. Thrower, Chemistry and Physics of Carbon, vol. 5. D. 217. Marcel Dekker. New York (1969). 9. J: ‘M. Cdwley Mater, Rei. Sot. Sym.‘ Pro;. 31, 177 (1984). 10. G. Binnig, H. Fuchs, C. H. Gerber, H. Rohrer, E. Stoll and E. Tosatti, Europhysics Lett. 1, 31 (1986). 11. R. Wiesendanger, R. Schliigl and H. J. Guntherodt, Extended
Abstracts,
Carbon
‘86 Int. Carbon
Conf.,
Baden-Baden, p. 79 (1986). 12. R. Schliigl and H. P. Boehm, Carbon 21, 145 (1983). 13. J. M. Thomas and C. Roscoe, Chemistry and Physics of Carbon, vol. 3, p. 1, Marcel Dekker, New York (1968). 14. E. Ferguson, R. Schlijgl and W. Jones, Fuel 63, 1048 (1984).
15. L. B. Harris, J. B. Hudson and J. Ross, J. Phys. Chem. 71, 377 (1967). 16. P. Vora, S. K. Sinha and R. K. Crawford, Phys. Rev. Lett. 41, 704 (1979).
17. R. Schliigl, Surf. Sci., in press.
CarbonVoi 26,No 1,pp.23-32. 1988 Printedut GreaiBntnn.
THE MORPHOLOGY OF SOME NATURAL SYNTHETIC GRAPHITES
AND
A. KAVANAGH Dept. Chemistry, UMIST, Sackville St., Manchester M60 lQD, U.K.
and R. SCHLOGL Fritz Haber Institut der Max-Planck Geseilschaft, Faradayweg 4-6, lOO&Berlin 33, FRG (Received 27 April 1987; accepted 26 June 1987)
Abstract-The morphologies of seven different types of graphite, both natural and synthetic, have been examined using a scanning electron microscope (SEMI. The surfaces are not smooth but were found to exhibit structural detail over the entire range of magnification used. These defects and surface
structures may well affect chemical reactivity and adsorption characteristics of the materials. Key Words-Graphite
morphology, Ticonderoga graphite, pyrolytic graphite, grafoil.
SEM studies. This makes it difficult, on the other hand, to obtain meaningful images with sufficient contrast to recognize the fine structure of the graphite surface under investigation. We consider this study to complement the extensive body of TEM work that has been done, in particular with respect to the elucidation of structural defects[ 1,8]. It further complements recent images obtained with reflection electron microscopy (RlZM)[9] and STM[lO,ll]. In the following, we present a rather large set of micrographs, with several images at various magnifications for each type of graphite in order to give an impression of typical structures.
1. INTRODUCTION
Graphite
has been studied
in many different
physical
and chemical aspects[l], and for many surface-sensitive techniques it has become a reference material with well-known properties[2]. Furthermore graphite finds widespread application in analytical techniques and is an industrially important material. Samples of both natural and synthetic graphites are available in sizes from millimeters up to large plates. However, relatively little is known about the surface morphology of such samples. It is generally accepted that after cleavage essentially featureless and wellordered (atomically smooth) surfaces are obtained[3,11], despite the evidence for cleavage defects obtained from a transmission electron microscope (TEM)[4]. With the advent of the scanning transmission electron microscope @TM) technique, surface studies with atomic resolution have stimulated renewed interest in the morphology of graphite[ll]. As a consequence of the anisotropy of the graphite structure, the chemical reactivity towards oxidation[S] or intercalation[b], for instance, depends strongly on the area ratio of nonreactive basal planes to reactive prismatic faces. This area ratio may deviate considerably from the geometric ratio due to the presence of defects and step structures. Knowledge of this ratio is also required when graphite is used as the substrate in adsorption studies and for the interpretation of some spectral features (e.g., its photoelectron spectrum[7,12]. This article presents a survey of typical morphologies of a variety of natural and synthetic graphites. We employed two scanning electron microscopes in order to obtain images with different levels of resolution. The fact that graphite in bulk samples is a very poor emitter of secondary electrons and is therefore essentially black for SEM is exploited when it is used as a support for low-contrast samples in
2.
EXPERIMENTAL
Seven different types of graphite were examinedtwo batches of highly oriented pyrolytic graphite (HOPG), oriented pyrolytic graphite (OPG), a pyrolytic graphite, two types of naturally occurring graphite (S40 and Ticonderoga single crystal), and a sample of grafoil; a detailed characterization of these samples is given elsewhere[l2]. All graphites were mounted on Al stubs using conductive silver cement and examined in a Camscan S4 SEM operating at typically 15 kV accelerating voltage. Some samples were subsequently examined in a JEOL CX 200 Temscan operating at 200 kV. No attempts were made to decorate the sample surfaces with, for instance, transition metals, to enhance the weak topographic contrast. 3. RESULTS
3.1 Ticonderogu graphite At low magni~~ation, some deposits can be seen on the surface (Fig. 1A); they give different contrast in the absorbed current distri&tion image (AEI) mode and may be mineral impurities not removed 23
24
A. KAVANAGH
and R. SCHL~GL
- _.-__A. 600~
B i3oop
Fig. 1. Surface deposits on Ticonderoga graphite. during purification (a mild process involving dissolution in dilute HCl, washing in distilled water, with no high temperature treatment or use of HF). The disruption visible on the right-hand side of Figs. iB and 2B is probably the result of cleavage of the crystal at a point where it was attached to the mineral deposit. The characteristic defect lines seen in Fig. 1A are typicahy severaf hundred microns in length, intersecting at an angle of 60” to form hexagons. The flake visible in Fig. 1B is probably not made up of a single crystal, but consists of a stack of several individual crystallites. At their boundaries are frequently observed structures such as those in Fig. ZB, where very thin sections protrude from the surface. It can be seen from this micrograph that these agglomerates consist of flakes, plates, and some filamentary structure (see Fig. 2A). We note intersecting defect lines, indicating the mosaic structure, observed previously by optical microscopy[l3]; some folded structures were also occasionally detected. At higher magnification, a surface structure (as in Figs. 3A, B), was revealed, arising from interpenetrating etch pits, covering large areas along defect lines. These etch pits are probably formed under the in-
Fig. 2. Ticonderoga graphite showing the protrusion of thin wafers from the surface.
AL---J
3u
B1OOOnm
Fig. 3. Interpenetrating etch-pits on the surface of Ticonderoga graphite. fluence of alkali-containing minerals, similar to the corrosion effect observed during impregnation of graphite with alkali carbonates[l4]. This is in contrast to the other natural graphite used in this study (S40), which exhibits no pitting either because it is found in seams of graphite rather than as inclusions of single flakes in the surrounding rock or because the etch pits are removed by the oxidative purification method used during manufacture. At still higher magnification, the “smooth” parts of the surface reveal more structure, which could be indicative of vacancy loops (Fig. 4). However, despite this wealth of surface detail, the Ticonderoga graphite was found to have a much less defective surface structure than the other graphites examined. 3.2 S40 Figures 5A and 5B show mutually perpendicular views in the orientation used; the prismatic faces that one would expect in Fig. 5A cannot be seen as clear contours. The flakes are not single crystals but consist of many crystalhtes in random orientation that cover the prismatic faces of the inner stack of crystailites. This has implications for the kinetics of intercalation, since in order to reach the reactive pris-
Fig. 4. Details seen on otherwise smooth areas of the Ticonderoga surface.
25
Natural and synthetic graphites
High magnification images from the surface of the platelets (Figs. 9A and 9B) reveal a step structure very similar to that seen in HOPG (see below). We have resoived steps of an average height of approx. 250 8, with terraces ranging in width from 250 to 5000 A. The steps have jagged edges, unlike those observed on synthetic graphites or the natural single crystal, and seem to follow c~st~lo~aphic orientations. These edges may again be a result of etching occurring during purification. These defective steps are important for the assessment of chemical reactivity since they provide favourable sites for the commencement of a reaction. Figures 1OA and 1OB show that even at very high magni~cation, the surface is not smooth: the platelets are folded back on themselves (shown by the darker areas in Fig. 10A) and a finer step structure of a few interlayer distances is visible. This fine step structure has been imaged in detail with the STM[ll].
3.3 Grafoil Grafoil is produced from, for example, naturally occurring graphites by intercalation with H$O, or HNO,, after which it is exfoliated by heating it rap-
Fig. 5. View of an edge of S40 graphite in mutually per-
pendicular orientations. matic faces, reactants would have to diffuse past the platelets. Comparing Figs. 1B and 5A, it seems that S40 consists of many platelets with different orien-
tations, whereas the Ticonderoga is composed of a small number of crystals. TEM (Figs. 6A and 6B) has shown that even these platelets are not single crystals. Instead they consist of stacks of randomly oriented single crystals (seen by electron diffraction-not shown). Their surfaces do not seem to be smooth, but exhibit a mottled character. The multiparticulate nature of the surface of S40 is clearly shown in Figs. 7A and 7B. The small surface particles could not be removed by ultrasonic cleaning, indicating that they are integral parts of the graphite structure. The good crystal quality of the naturally grown material is evinced by the sharp edges arising from etching during the purification procedure. (Note the very thin leaves (Fig. 7A) that are penetrated by the 20-kV beam.) The polycrystalline nature of the S40 flakes is shown in Fig. 8, viewing down one of the few accessible prismatic faces. Contrast arises between different packages of layers of individual crystals; the width of these bands shows that this thickness varies from 100 to 250 A.
A 150nm L
_..._1
B t__..... __ j 120nm Fig. 6. TEM images of stacks of platelets in S40.
26
A.
KAVANAGH
and R.
SCHL~~GL
300nm Fig. 8. View down a prismatic face of S40 graphite showing stacked single crystals.
ference in current distribution across the surface arises from resistance across the boundaries between platelets. These features are not due to shadowing effects, which are common at such high magnifications, because they remained when the sample was tilted through several different angles. The electrical resistance of the boundaries of the platelets may well be the result of extensive chemical modification of the outer areas of the platelets during intercalation and exfoliation. In particular, a variety of surfacesoxygen groups attached to the prismatic edges and defects in the crystallites[l2,17]-may cause significant interparticle resistance.
B-
1ocl Fig. 7. Small crystallites attached to the surface of S40.
idly to a high temperature. The high surface area material is then compressed, and sometimes annealed. The method of production of grafoil is assumed to result in a fairly structureless, smooth surface, so that all adsorption sites are of comparable energies and grafoil is considered, therefore, as a suitable substrate for adsorption studies[ 161. The present sample looks disordered at low magnification (Figs. 11A and llB), unlike the natural graphites (see Figs. IA and 5B). The absence of organization in the surface may result from the compression of platelets during manufacture. At higher magnification (Fig. 12A), the surface reveals a rich structure arising from the former individual platelets of the starting graphite. Such a surface should exhibit a broad distribution of adsorption sites, and may hence not be an ideal adsorption substrate. Despite the open, honeycomblike structure of the exfoliated precursor, no pore structure in the macroporous region could be detected. Figure 12B is an AEI (specimen absorbed current) image; the dif-
3.4 Pyrographite Pyrographite is produced by graphitization of carbon, obtained by burning low molecular weight hydrocarbons. At low magnification (Figs. 13A and 13B), it can be seen that the surface in one direction is sectioned (approx. 50 p, wide), with steps perpendicular to this direction with an average height of 1 IL (compare
A-
B32nm
32nm
Fig. 9. High resolution image of the step structures on the surface of S40.
27
Natural and synthetic graphites
A.
8. 50nm
50nm
Fig. 10. High resolution images of the “flat” areas of the S40 surface, showing fine step structure and some surface roughness.
this organ~ation of steps in mutually pe~endicular directions to the complete lack of organization in grafoil (Fig. 12A)). The surface consists of folded sheets of carbon and this becomes more apparent at higher magnifications (Figs. 14A and 14B). The im-
B-
300nm
Fig. 12A. The surface of grafoil showing some structure arising from platelets of the precursor graphite. B. AEI image of grafoil surface.
Fig. 11. The surface of grafoil: note the absence of any organization of the surface structure.
ages show that these sheets that form an approximately rectangular tiling of the surface (Fig. 13) consist of a large number of thin flakes. Inspection of Figs. 15A and 15B shows that there are two levels of steps that differ in height by an order of magnitude. The small steps that can just be detected in Fig. 13 occur as a “staircase” of terminating platelets, many of them bent over at the edge. Note in Fig. 15B how platelets may be folded over a large step with nearly perpendicular orientation (bottom right, Fig. 15B) and how closely a thin platelet may follow the structure of the substrate (step in top right-hand comer of Fig. 15B). It is believed that the richly ordered structure of this graphite results from cleaving the relatively hard material. In contrast to HOPG (see below), this material does not “rearrange” during cleavage because a substantial amount of interlayer defects hinders the gliding of planes. These interlayer defects are responsible for the hardness of this graphite com-
28
A. KAVANAGH
and R. SCHL~GL
AlOOp’
B 3Op-7 Fig. 13. Large steps on the surface of pyrographite.
AlOv
I
B 30~~
Fig. 14. Multiple folding of the surface layers of pyrographite.
B lOOOnmFig. 15A. Fine structure on the surface steps of pyrographite showing bunching of platelets. B. Overlapping, perpendicular platelets. Note the very fine step structure (on the right-hand side) running parallel to the edge of the crystallite. pared to HOPG. Cleavage has exposed many prismatic faces, which will lead to a considerable enhancement of chemical reactivity. The defects also manifest themselves in differences of the photoelectron spectrum of this graphite compared to HOPG[7,17]. The structures in the center of Fig. 15B are thought to mark a location that prevented gliding of differently oriented carbon sheets during cleavage. A cluster of “nongraphitic” carbon atoms as a result of incomplete graphitization may result in such a location acting as an anchor during cleavage. Beam damage occurred after approximately one hour in the microscope. The newly arisen bubblelike structures (Figs. 16A and 16B) imply that some gaseous product has been expelled. The gas (e.g., Hz, CHJ may be concealed at clusters of defects during the pyrolysis step of the low molecular weight precursor. 3.5 Oriented pyrolytic graphite OPG is produced in a similar way as pyrographite, but further graphitizatin under mechanical stress results in improved stacking order. The morphology of freshly cleaved OPG surfaces resembles that of pyrographite and is distincly different from the morphology of HOPG. Our sample exhibited extensive
Natural and synthetic graphites
29
Al 001.1
Al OOOnm
Fig. 16A. Bubblelike structures on the surface of pyrographite caused by beam damage. B. Detail of Fig. 16A showing the “exploded” appearance of one of these “bubbles.”
material contrast, in particular next to steps, as can be seen from Fig. 17 (note the well-aligned occurrence of naturally decorated steps in Fig. 17A). We exclude contamination in the microscope as a source for the overlayer: subsequent examination of this sample with XPS suggested an organic material to be enclosed along defects: in situ cleaving in UHV
Fig. 17A. Steps on the surface of OPG. B. The “dirty” surface of OP materials contrast on the surface of OPG.
B. 1 OOOnm Fig. HA. Overlapping cystallites on the surface of OPG. B. Contour-enhanced AEI image of area shown in A.
results in a contamination of the vacuum and gave UPS spectra of contaminated ~aphite[l7]. No unusual heteroelements except oxygen and nitrogen were detected. Cleavage of this material (which is softer than pyrographite) exposed small crystallites in much more parallel stacking order than in pyrographite (compare the flat surfaces in Figs. 17 and 18 with those in Fig. 14). This probably results from the higher degree of stacking order in the individual graphitic carbon planes. The random orientation between planes can, however, be seen in Figs. 17B and 18A, which show the extensive mosaic spread, wellknown from diffraction experiments. This random orientation of boundaries should be compared with the better ordered surfaces of the natural graphites. The contour-enhanced AEI image of Fig. 18B (taken from a different area than Fig. 18A) demonstrates how several regularly shaped crystallites can overlap
30
A.
KAVANAGH
andR.
SCHL~GL
3op
BU
AU
B-
A-
1OOOnm
200nm
3olJ
Fig. 19A. Step structure of HOPG after cleaving. B. Contour-enhanced AEI image of HOPG.
Fig. 21. Step structure on the otherwise smooth hedges between larger steps on HOPG.
and how they may be covered by lamellae: all this leads again to an unexpected heterogeneity of the absorbed current distribution and points to the heterogeneity of physical properties of such surfaces.
The small number of interlayer defects seems to be second only to the stacking perfection in Ticonderoga flakes. From the contour-enhanced image in Fig. 19B, it can be seen that very thin lamellae follow closely the underlying step structure, although this requires multiple folding. The presence of multiple fractured crystallites on the right-hand side suggests that for this particular surface the cleaving force was not entirely parallel to the basal plane. The first sample exhibits a step structure (Fig. 20A, B, C, D), the distance between steps ranging from 0.2 to several microns. These steps appear to be parallel over large distances, with little disruption of the surface between them. This is probably due to the low number of interlayer defects in this sample. However, at higher magnification (Fig. 21A, B) show that the terraces between steps are not smooth, but exhibit a morphology, with series of parallel steps running over long distances. These steps (height approx. 500 A) are terminating crystallites. To explain this regular step structure, it is suggested that groups of several crystallites may act in concert, and as if they have some elasticity. The individual crystallites in this HOPG sample are of similar thickness but
3.6 Highly oriented pyrolytic graphites HOPG is produced in a similar manner to OPG, but extensive annealing under pressure produces a graphite with a nearly perfect stacking order and varying degrees of rotational disorder (i.e., artificial single crystals). Several samples from two batches of HOPG were examined and they held some features in common: HOPG surfaces are much smoother than those of the OPG and after parallel cleaving low magnification shows a stepped surface arising from a parallel stack of terminating platelets as shown by the examples in Fig. 19. Cleaving of this very soft material did not result in tilting and breaking of large individual crystallites (as in pyrographite), which is due to the high degree of order in HOPG with, in particular, only a small amount of interlayer defects.
B, D,lop 5p
’ ,
Fig. 20. Step structures of varying terrace width on HOPG.
A.
1olJ
B-
1olr
C2oou
Fig. 22. Ridges and multiple folding on the surface of HOPG.
Natural and synthetic graphites
31
24A, B) more than the other HOPG.
It typically exhibits a surface comprised of overlapping platelets (compare Fig. 5). However, we also observed the layers of folds and series of steps typical of the first sample. On otherwise smooth areas of terraces, we observed complex aggregates (approx 2 )Ldiameter) of small platelets. This highly defective, and thus “hard,” material may be left behind by the cleavage procedure (see Fig. 24B). 4. DISCUSSION AND CONCLUSIONS
Fig. 23. Multiple folding and bending of surface layers on HOPG.
considerably larger than those of the natural S40 graphite (Fig. 9A, B). We note that it was only possible to obtain such images at accelerating voltages below 15 kV with the sample tilted 15”. At 200 kV, accelerating voltage no contrast similar to that of Fig. 9 could be obtained in any o~entation of the specimen, Figure 22A, B, and C (C is an overview of the regions in A and B) show that these crystallites would cover up underlying macroscopic defects, such as grain boundaries. Figure 22B shows how multiple folding may arise when one part of a layer is “pinned down” by point defects or an emergent screw dislocation, which prevents the material from cteaving smoothly. The graphite layer along which cleavage occurred was fixed to the bulk at a few points, one of which forms the center of Fig. 22B and so as the mobile part of the layer glided during cleavage, it buckled (Fig. 22C). Figure 23A shows a very fine platelet, multiply folded and loosely attached to the surface; bending such a package of layers may cause them to “open out” as seen in Fig. 23B. Similar effects of cleavage, although on a coarser scale, have been seen previously by optical microscopy: Thomas[l3] presented evidence for extensive rearrangement of Ticonderoga graphite that he attributed to plastic flow during cleavage. The second sample of HOPG morphologically resembles the poiycrystalline natural graphite, (Fig.
Harris et al. [15] examined the surface of annealed pyrolytic graphite in conjunction with adsorptiondesorption studies. They used a Cr-shadowing technique, which showed that some areas of the surface were rougher than others, and some slightly raised. However, the grain structure of the nucleation pattern of Cr prevented them from seeing any finer details (e.g., closely spaced steps). Nevertheless, they noticed two different types of surface morphologies, similar to our observations. We have tried to give a description of the variety of typical surface features found on several different graphites. We used direct imaging and did not require decoration techniques, which might have affected the nature of the structures seen. What has been shown clearly in this study is that there exists a wide variety of defects that manifest themselves after, or are induced by, cleavage.
Fig. 24. Surface of HOPG, showing overlapping platelets.
32
A. KAVANAGH
(Thomas has already described how steps may be formed when a cleavage crack intersects an array of dislocations.) These defects can be seen directly, and are visible in every magnification range used (lOOO150,000x). It has been shown that the graphites examined were not, in general, atomically smooth: several chemically different carbon species are present (e.g., those adjacent to loops, steps, and corners) in appreciable amounts, even on the basal planes. Because of the presence of the surface defects imaged, transport properties on a microscopic scale are likely to be nonuniform in the surface and surface-near region of these graphites; these defects will also affect the local chemical properties of the graphites. In several samples of nominally the same material (OPGs), distinctly different surface features, and hence distinct differences in the quantity and quality of defects, have been found. These differences would not easily be detected by integral methods but may affect several applications, such as use as a host material for intercalation. In particular, grafoil is used in adsorption studies, since it is thought to provide an homogeneous substrate; however, as has been shown, the surface is structurally heterogeneous, implying that adsorption may well occur selectively on certain sites, only averaging out when the size of the area under study is large compared to the areas over which heterogeneities occur. We have also seen evidence of a certain amount of elasticity in HOPG, as the planes glide during cleavage. This study has shown a variety of structural detail in the size range below optical detection and above the level of atomic resolution. Step structures, different patterns of surface organisation and terrace sites have been directly imaged, and these are likely to determine the local properties of a particular sample of graphite used in a wide range of experiments, from adsorption to intercalation and studies of chemical kinetics. Crit-
and R. SCHL~GL
ical evaluation of these experiments is hoped to benefit from this work. Acknowledgments--The
authors acknowledge Dr. T. Page and Mr. M. Stocker, Dept. Material Science, and Prof. J. M. Thomas, Dept. Physical Chemistry, Cambridge, for allowing us to use their electron microscopes. The samples of HOPG were kindly donated by Dr. A. W. Moore, Union Carbide, U.S.A.
REFERENCES 1. P. L. Walker and P. A. Thrower (eds.), ChemBtry and Physics of Carbon, ~01s. l-20, Marcel-Dekker, New
York (1965-1987). 2. B. T. Kelly, ADDI. Sci. XX, X (1981). and Physics of Carbon, vol. 3. G. R. He&g,-Chemistry 2. D. 7. Marcel Dekker. New York (19xX). 4. S: ‘Amelinckx, P. Delavignette and M. Heerschap, Chembtry and Physics of Carbon, vol. 1, p. 1, Marcel Dekker, New York (1965). 5. J. M. Thomas, Chemhtry and Physics of Carbon, vol. 1, p. 121, Marcel Dekker, New York (1965). 6. A.*Herold, Mat. Sci. Eng. 31, 1 (1979). 7. T. Takahashi, H. Tokaihn and T. Sagawa, Phys. Rev. B 32, 8317 (1985).
8. P. A. Thrower, Chemistry and Physics of Carbon, vol. 5. D. 217. Marcel Dekker. New York (1969). 9. J: ‘M. Cdwley Mater, Rei. Sot. Sym.‘ Pro;. 31, 177 (1984). 10. G. Binnig, H. Fuchs, C. H. Gerber, H. Rohrer, E. Stoll and E. Tosatti, Europhysics Lett. 1, 31 (1986). 11. R. Wiesendanger, R. Schliigl and H. J. Guntherodt, Extended
Abstracts,
Carbon
‘86 Int. Carbon
Conf.,
Baden-Baden, p. 79 (1986). 12. R. Schliigl and H. P. Boehm, Carbon 21, 145 (1983). 13. J. M. Thomas and C. Roscoe, Chemistry and Physics of Carbon, vol. 3, p. 1, Marcel Dekker, New York (1968). 14. E. Ferguson, R. Schlijgl and W. Jones, Fuel 63, 1048 (1984).
15. L. B. Harris, J. B. Hudson and J. Ross, J. Phys. Chem. 71, 377 (1967). 16. P. Vora, S. K. Sinha and R. K. Crawford, Phys. Rev. Lett. 41, 704 (1979).
17. R. Schliigl, Surf. Sci., in press.