The Journal of
Physical Chemistry
0 Copyright, 1986, by the American Chemical Society
VOLUME 90, NUMBER 4 FEBRUARY 13,1986
LETTERS Reactivity of Large Carbon Clusters: Spheroidal Carbon Shells and Their Possible Relevance to the Formation and Morphology of Soot Q.L. Zhang, S. C. O'Brien: J. R. Heath,+ Y. Liu, R. F. Curl, H. W. Kroto,t and R. E. Smalley* Rice Quantum Institute and Department of Chemistry, Rice University, Houston, Texas 77251 (Received: November 27, 1985)
The reactivity of bare carbon clusters produced by laser vaporization has been studied by a fast-flow gas-phase reactor followed by F2 excimer laser ionization and time-of-flight mass analysis. Although most small carbon clusters (C,, with n < 40) were found to be highly reactive with such small molecules as NO, SO2, and CO, the larger even n clusters (40 < n < 80) were found to be relatively inert. This lack of reactivity was most pronounced for c 6 0 (Buckminsterfullerene). Inertness of the large even C, clusters is evidence that they have closed into edgeless, spheroidal shells composed of 12 pentagonal rings and n/2 - 10 hexagonal rings. The possible role of similar shell structures in the formation and morphology of soot is considered.
Introduction In an initial paper on the vaporization products of graphite we have presented evidence for the formation of a remarkably stable Cm cluster.' We proposed that the stability arises from the ability of a graphitic sheet to close into a'spheroidal shell, thereby eliminating its reactive edges. The structure proposed was that of the soccerball: 12 pentagons and 20 hexagons arrayed on the surface of a sphere, with icosoheral symmetry. Such a structure has all valences satisfied and has been verified by Huckel calculations to be highly aromatic (it has 12 500 Kekule structures2). One remarkable aspect of this object is the fact that it is hollow3 and presents the possibility for binding any of a wide range of atoms inside. In further experiments this aspect was pursued and evidence was obtained for the formation of stable complexes of the formula CWMwhere M = La, Ba, Sr, Ca, ..., et^.^ In both of these papers not only c 6 0 but a range of C, clusters were also observed. Similar but not so prominent stability was evident for Robert A. Welch Predoctoral Fellow. *School of Chemistry and Molecular Sciences, University of Sussex, Brighton, UK BNl 9QJ.
0022-3654/86/2090-0525$01.50/0
such complexes whenever n was an even number greater than 40. As a further indication of the correctness of this structural assignment to the c60 cluster, it seemed reasonable to investigate the relative reactivity of c 6 0 compared with the other carbon clusters produced in this laser-vaporization, supersonic nozzle technique. Remarkably, as detailed below, not only c 6 0 but all of the even clusters in the 40 to 80 carbon atom range were found to be resistant to chemical attack by such reactive small molecules as NO, H2, CO, SO2,02,and NH3, particularly when compared to the smaller clusters in the range below 30 atoms where extensive reactivity was easily observed. (1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. E. Nature (London) 1985,318, 162-163. (2) Klein, D. J.; Schmalz, T. G.; Hite, G. E.; Seitz, W. A. J. Am. Chem. SOC.,submitted for publication. (3) The concept of hollow molecules made from graphite-like sheets was suggested in a remarkably inventive piece by David E. H. Jones in his column "Ariadne" in the New Scientist magazine (Jones, D. E. H. New Scientist, 1966, 3 Nov, 245). (4) Heath, J. R.; OBrien, S . C.; Zhang, Q.;Liu, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7719-1180.
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The relative inertness reported here for the large even clusters is hard to rationalize if all these clusters were simply flat sections of hexagonal graphitic sheets. Such open structures would have too many dangling bonds around the edges to behave as anything but extremely reactive free radicals (at least as reactive as phenyl, or benzyne). Instead, it is argued below that the reactivity of these large even carbon clusters has been reduced by a (apparently) facile rearrangement of the normal hexagonal graphitic structure into a closed shell, much as that postulated previously for Cmalone.
Experimental Section Carbon clusters were produced in a pulsed supersonic beam apparatus using laser vaporization of a graphite disk, as reported previously. In order to study the reactivity of these carbon clusters, the supersonic nozzle was fitted with a downstream fast-flow reaction tube 1 cm i.d., 10 cm long. As discussed in detail in earlier work from this laboratory on the reactions of transition-metal cluster^,^*^ this reaction device was fitted with a number of pulsed valves which injected reactant gas (diluted in helium) through a set of needles directly into the turbulent flow passing down the reactor tube. The reactions reported here were performed with roughly 100 torr of helium buffer gas flowing through the reaction tube, the reactions occurring at approximately 300 K over a contact time of 150 to 200 ps. The entire experiment was pulsed 9.1 times s-I under computer control, the reactant injectors alternating between shots with a known concentration of the desired reactant species, and control shots where only pure helium was injected. At the end of the reaction tube the cluster-laden reactant gas mixture was allowed to freely expand into a large vacuum chamber, thus producing an intense supersonic expansion and rapidly quenching all further reaction. The supersonic beam skimmed from this free expansion was then passed through the ionization region of a time-of-flight (TOF) mass spectrometer, where the clusters and their reaction products were ionized by an F2excimer laser beam (1 570 A, 7.9 eV). The masses of the resulting ions were measured by T O F mass spectrometry. Results Of the reactants examined in this work, NO, a known efficient free radical scavenger, proved to be the one of the most reactive. Figure 1 displays a broad view of the mass spectrum of carbon clusters obtained when N O was injected in the fast-flow reactor using Fz laser ionization. The top view shows the control experiment. In accord with our previous results, C60, is by far the dominant cluster observed, but here we have purposely adapted the nozzle so as to produce a substantial concentration of other carbon clusters as well. The lower mass spectrum of Figure 1 reveals the changes produced by reaction with the injected NO. Estimates of the various flow parameters indicates the effective NO concentration in the reaction tube was roughly 1 Torr for this expeirment. Note the extensive loss of bare cluster signal for the smaller clusters. In some cases such as the large blackened peaks shown in the figure, reaction products are clearly visible, but most of the product ion intensity has been lost to poorly resolved ion signal on the base line (the broad, substantial base line signal here is real; compare with that of the control experiment on the top panel). The most dramatic observation, however, is not the reactivity of the smaller clusters, but the comparative inertness of c 6 0 and the other even carbon clusters with 40 or more atoms. Also note that this lack of reactivity does not apply to the odd carbon clusters in the same size range. Only the even carbon clusters are sufficiently unreactive to survive these N O conditions. Figure 2 presents similar results with SO2 as the reactant gas. Here again comparison with the control shows there has been extensive reaction with the smaller clusters to produce a variety ( 5 ) Geusic, M. E.; Morse, M. D.; O’Brien, S.C.; Smalley, R. E. Reu. Sci. Instrum. 1985, 56, 2123-2130. ( 6 ) Morse, M. D.; Geusic, M. E.; Heath, J. R.; Smalley, R. E. J . Chem. Phys. 1985, 83, 2293.
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time-of-flight mass spectrum, obtained with roughly 1 Torr of NO in the fast-flow reaction tube, provides a particularly graphic demonstration of the relative chemical stability of the even C, clusters with n > 40 compared to both the smaller carbon clusters and the odd C, clusters in the same mass region. The top spectrum is a control spectrum obtained on alternate shots where only pure helium was injected into the reaction tube. A small (20%) enhancement of the vertical scale was used in plotting the control spectrum to compensate for the fact that the high concentration of NO in the reactant shots produced a small (30 ps) slowing in the cluster beam (see ref 5). of cleanly detected products (note the C z 8 0 and C 3 2 0 peaks blackened in the figure), as well as an extensive unresolved base line signal produced by the reactions. Again, as in the earlier case with NO, the SO, has had little effect on the even carbon cluster signal for Cd0and larger. This is particularly the case for c 6 0 which shows no significant change in intensity as a consequence of addition of SO2. The other large even clusters have been reduced in intensity a small amount, but far less than the odd clusters in the same mass range which have all but disappeared from the spectrum. Results similar to those discussed above have been obtained for O2and NH3. In both cases the most inert species was found to be c60, with the other even clusters in the 40-80 size range showing only slightly more susceptibility to reaction. A second set of experiments was performed in which either Oz or H2was bled into the He carrier so that these gases were present in the hot carbon plasma created by the vaporization laser. With very high reactant concentrations in the main carrier gas (e.g. 120 Torr) substantial reaction was observed with the even, large clusters including c60. But even in these extreme cases, much C60survived unscathed.
Discussion The c 6 0 cluster is shown in these experiments to be quite inert to chemial attack by small molecules such as NO, SOz, and O2
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The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 527
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Figure 3. One of several plausible structures for the C72cluster which consist of 12 pentagons inserted into and forcing the closure of ( n / 2 10) hexagons where n is the number of carbon atoms in the cluster. Such closed ‘carbon pillow” structures have all valences satisfied for sp2-hy-
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observed high reactivity of these clusters is therefore readily explained. Also consistent with this picture is the fact that the odd carbon clusters in the range larger than C40can only be observed by direct one-photon ionization such as seen in Figures 1 and 2 where the 7.9-eV F2 excimer laser was used. As shown previously,’ the odd clusters are never observed strongly when ionized by an ArF excimer laser where the low (6.4eV) photon energy requires a two-photon process for ionization of carbon clusters in this region. The excess energy involved in this twophoton process apparently fragments the relatively unstable odd clusters to produce even cluster photoions.
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10 20 30 40 50 60 70 CARBON ATOMS PER CLUSTER Figure 2. Reaction of SO2with carbon clusters. The clusters containing less than 40 atoms have almost completely reacted and the predominant peaks in this mass range are product peaks. Note how Cmis essentially unaffected by the reactant whereas the other even large cluster peaks are between 20 and 40% diminished in intensity. As in Figure 1 the large odd clusters have almost completely reacted.
which are known to be extremely active free radical scavengers. This is fully in accord with expectations for a closed, edgeless carbon shell with a highly aromatic electronic structure. The postulated soccerball structure of this C60 molecule, Buckminsterfullerene, is therefore supported by these observations of its low reactivity. Perhaps even more interesting, however, is the result that the other even clusters in the 40-80 size range are quite resistant to attack as well from active free radical scavengers. Like C60, the valence requirements of all their constituent atoms appear to be satisfied. In this regard it is interesting to note that all such even carbon clusters (but not the odd ones!) can form closed shells by using a combination of pentagons and hexagons. As pointed out by Thompson,’ Euler’s relations can be used to deduce that there will always be 12 pentagons in such a structure, together with n / 2 - 10 hexagons, where n is the number of carbon atoms. We have examined such structures for a number of large even-numbered clusters and find there are often excellent candidates available with only moderately strained rings and a high number of resonance forms. The “carbon pillow” structure shown in Figure 4 for the case of C,, is one such example. We suspect that most, if not all, of the even n, n > 40 species which we observe possess such stable structures. The odd carbon clusters in this large size range, on the other hand, cannot close to form an unreactive shell. At least one atom in such a cluster will have a dangling, unsatisfied valence. The
Relevance to the Formation and Morphology of Soot In the paper on Buckminsterfullerene,’ evidence was presented that showed C60 becoming increasingly dominant in the mass spectrum as the helium carrier gas pressure and residence time in the supersonic nozzle was increased. Careful examination of that data shows that Cs0is not increasing much in intensity in the supersonic beam; it is the other clusters that are decreasing. In other words, C60 is a survivor of some nozzle process which is removing the other clusters from the beam. In addition to the relatively large C, clusters that have been the focus this discussion, we also observe quite intense cluster ion signal in the lower mass regions as well. The C3 signal is particularly intense, and an extended series of polyacetyleness are observed as well. Since all these small carbon radicals can only be seen in this apparatus through rather inefficient multiphoton ionization processes, we suspect the density of small clusters in the nozzle is far higher relative to the large clusters than apparent from the mass spectra. In fact, previous studies of laser-vaporized graphite have reported most of the vaporized carbon to be in these very small cluster^.^ The most reasonable candidate for the process which is decreasing the intensity of the large clusters (except for C60) is rapid growth by reaction with the abundant very small carbon clusters to form soot. Even though this soot would be blown through the nozzle and be present in the supersonic beam, it would be too heavy to appear in the mass spectra. As the nozzle conditions are varied to minimize diffusional loss of the very small clusters to the walls, and maximize the time available for reactions, the more reactive of the large clusters will grow to form soot, leaving behind only (7) Thompson, D. W. On Growrh and Form; Cambridge University Press: London, 1942; Chapter IX,p 737. (8) Heath, J. R.; Zhang, Q.;O’Brien, S . C.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. Astrophys. J., submitted for publication. (9) Meyer, R. T.; Lynch, A. W.; Freese, J. M. J . Phys. Chem. 1973, 77, 1083.
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Figure 4. One possible model of a growing soot particle as it nears completion of the second spheroidal shell and begins forming the third. This model assumes the large polycyclic aromatic molecules known to be present in sooting flames will tend to maximize carbon-carbon bonding by incorporating pentagons into their aromatic bonding network. These pentagons generate a curvature which brings the net back on itself to form a spheroidal shell. Under fast growth conditions each new shell begins to form before the current shell is finished.
the relatively inert even C, clusters in the n = 40 to 80 region, particularly c 6 0 and C70. One of the most striking features known about soot is that it is almost always found in the form of small spheres, which grow very rapidly to a diameter between 100 and 500 A before they coalesce into strands of beads or more complicated structure^.'^'^ Although there are several excellent kinetic models for soot growth,IWl4the initial particle nucleation process is not well understood. A variety of explanations have been offered over the years for this spherical morphology of the initial growth particle, ranging from a symmetrical coagulation of tiny graphitic microcrystals, to a postulate that the early soot nucleus i's actually (10)Haynes, B. S.; Wagner, H. 2.Phys. Chem. (Frankfurt an Main)
Letters a liquid ball15 of high boiling hydrocarbons. If our postulate of the stability and facile formation of these spheroidal carbon shells is correct, there is obviously another explanation for the sphericity of soot particles: carbon actually prefers to form spherical shells when faced with the necessity of satisfying its valence requirements without the aid of other atoms. In the soot nucleation region of a typical flame, hydrogen is quite abundant, but at flame tempratures (1400-1700 K) dehydrogenation are known to be favored. The polycyclic aromatic molecules known to be present in high concentrations in sooting flames13J5 may therefore adopt pentagonal rings as they grow, so as to generate spheroidal structures which maximize the number of C-C linkages. It is unlikely that many of these aromatic carbon nets will succeed in closing perfectly to form one of the even C, spheroids. With hydrogen so abundant in the flame, some of the dangling bonds will be at least temporarily satisfied by hydrogen. Thus the rapidly growing carbon net is likely to close imperfectly, and begin a second shell before the first is finished. Since successive shells must be spaced by roughly the 3.3 A intersheet distance in graphite, it appears that bonds must break between the imperfect inner shell and the growing outer net before this net can wrap around the inner shell and continue growing to form the next shell. The result of such a process would be a soot nucleus consisting of concentric, but slightly imperfect spheres16-a structure which continues to present a very active growth front at the edges of the outermost shell. Figure 4 displays a molecular model of such a spheroidal growing soot particle. Although we are by no means certain that this is the only possible nucleation route for soot, it does appear worthy of detailed further consideration.
Acknowledgment. We thank Anthony Haymet and T. G. Schmalz for valuable discussions and communication of their results on carbon shell calculations prior to publication. We thank Martin Poliakoff for bringing the article by David Jones to our attention. This research was supported by the National Science Foundation and the Robert A. Welch Foundation, and used a laser and molecular beam apparatus supported by the U.S.Army Research Office and the US.Department of Energy. H.W.K thanks SERC for travel support.
1982, 133, 201.
(11) Harris, S. J.; Weiner, A. Combust. Sci. Technol. 1983, 31, 155. (12) Harris, S.J.; Weiner, A. Combust. Sci. Technol. 1983, 32, 267. (13) Harris, S. J.; Weiner, A. Annu. Reu. Phys. Chem. 1985, 36, 31. (14) Dasch, C.J. Combust. Flame 1985, 61, 219. (15) Lahaye, J.; Prado, G. Particulate Carbon; Plenum: New York,1981; pp 31-56, 143-176.
(16) For very small particles where a substantial fraction of the atoms are on the surface, such onion layer carbon shells are likely to be more stable than any of the alternative graphite or diamond structures. The sphere has the smallest possible surface to volumn ratio and can be constructed from carbon with the fewest possible dangling bonds.
