Experimental studies on the formation process of C6o

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CHEMICALPHYSICSLETTERS

Experimental studies on the formation process of Michel Broyer ~, A c h i m G o e r e s b, Michel Pellarin J e a n Louis Vialle" and Ludger W6ste c

2 October 1992

C6o

a Erwin Sedlmayr b,

a Laboratoire de SpectromOtrie lonique et MolOculaire ~, UniversitO Lyon I, Bdttiment 205, 43 Boulevard du 11 Novembre 1918. 69622 Villeurbanne Cedex, France b lnstitutfurAstronomie undAstrophysik, Technische Universit?it Berlin, Hardenber 'strasse 36, W-t000 Berlin 12, German), c Institutj~r Experimentalphysik ( W E t ) der Freien Universitiit Berlin, Arnimallee 14 W-IO00 Berlin 33. Germany

Received 3 March 1992; in final form 14 July 1992

The structure of carbon cluster intermediates along the pathway of C6oformation is investigated. After laser evaporation, the carbon clusters are treated with hydrogen and analyzed by time-of-flightmass spectrometry.The different amount of ligands of the C clusters allows a determination of their structure. The results favour the model of fullerene formation by a naphthalenoctyl polymerization processpublished recently.

1. Introduction

The detection and synthesis of carbon cage molecules (fullerenes) a few years ago [1-3] has had a major impact on chemistry. Many new fields of research have been opened, such as high-temperature organic superconductivity [4], substitution chemistry with fullerenes [ 5,6 ] and endohedral chemistry [ 7,8 ]. In spite of the enormous effort in experimental research there still exists no generally accepted description of the detailed chemical pathway towards C6o. One primary reason for this situation is the extreme complexity of the composition of carbon gas at high temperatures which was the subject of many publications decades ago (e.g. refs. [ 9,10 ] ); another one is the fact that the curvature of the molecules, which is essential in this context, very sensitively depends on the details of the molecular orbitals and, thus, is difficult to calculate. Two different approaches have been published, which aim towards a modelling of the fullerite syn-

Correspondence to: M. Broyer, Laboratoire de Spectromrtrie Ionique et Molrculaire, Universit6 Lyon I, Bhtiment 205, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. I URACNRSNo. 171.

128

thesis occurring in a mixture of vaporized carbon and a helium quenching gas: (1) The hypothesis suggested earlier by Smalley and co-workers [ 5 ], Kroto [ 11 ] and others is based on the conception of irregular growth of two-dimensional clusters by different carbon chain molecules up to a size, where the cluster produces its curvature and icosahedral symmetry by rearrangement processes, driven by the energy gain from reducing the number of dangling bonds. This "rearrangementmodel" is outlined qualitatively and is investigated by numerical experiments like simulated annealing of carbon clusters (e.g. ref. [ 12] ). (2) The other model suggested by Goeres and Sedlmayr [ 13 ] is based on the idea of formation of fullerenes by elementary bricks of clustered carbon compounds Cx with a specific N, which are already two-dimensional structures. Since it is well known that large carbon chains tend to form monocyclic rings [14] and Clo in the carbon gas is the most prominent flat structure [ 15,16 ], in this quantitative model C10 is supposed to be this elementary brick. In the following, we will call it the "monomer", which polymerizes to C6o and higher fullerenes. During addition reactions of C~o rings with the intermediates the monocyclic rings collapse towards a naphthalenoctyl structure, thus by geometry fa-

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vouring the formation of a connecting pentagon which produces the curvature of the intermediates [17]. As in the rearrangement model, rearrangements are necessary to attain icosahedral symmetry, but in this case the process would lead mostly to a conserved curvature. This "Cm monomer model" is able to explain quantitatively the effectiveness of the Kr~itschmer experiment as well as its sensitivity towards the presence of hydrogen [ 17 ]. Since the Cto monomer model has a well defined pathway it gives rise to an experimental approach for an investigation of the fullerite formation. It results in some essential restrictions on the composition of the carbon gas, as far as a highly effective formation of fullerenes occurs, which are different from those the first model is based on: (i) The Cm monomer model predicts the ring-closure reaction of long carbon chains to be the most prominent transition from one- to two-dimensional carbon molecules, whereas the rearrangement model allows for branched chain molecules. (ii) Both models require saucer-shaped intermediates of polyaromatic carbon molecules, including about one third of pentagons among the hexagons of the graphitic structure. In the Cto monomer model the smallest curved intermediate is the C2o molecule, whereas in the rearrangement model curvature is supposed to be introduced by a rearrangement process of flat molecules. However, the critical size for the transition from the flat to the curved structures is unknown in this case. Therefore investigation of the structure and the mass distribution of the intermediates should give rise to some evidence about the C6o formation process. The usual method of such an investigation is radical scavenging which is based on the idea that different structures of the investigated clusters can be classified by different numbers of dangling bonds. Addition of a reactive gas to the clusters leads to ligand complex formation, with the number of ligands being limited by the amount of dangling bonds. This can be analyzed by time-of-flight (TOF) mass spectrometry. If the ligand complexes are chemically inert, the observed amount of ligands allows to determine the initial structural properties of the cluster compound under investigation. Unfortunately in the case of carbon clusters radical scavenging is difficult to perform. On one hand

2 October 1992

the low mass of the carbon atoms requires low-mass ligands like hydrogen to derive mass spectra which are not too complicated, on the other hand the products of saturation reactions in most cases are chemically reactive, leading to a series of secondary reactions. As a consequence, the mass spectra do not simply show the primary composition of the pure carbon gas. Hence, some authors investigated the carbon gas by adding hydrogen to the quenching gas and analyzing the resulting secondary composition, where changing the reaction conditions allows secondary reactions of the carbon hydrogen mixture to be studied [18]. Nevertheless, we think that some of the primary properties of the carbon gas remain unaffected by the presence of the hydrogen. Therefore we likewise analyzed the reaction products of a carbon gas in a helium-hydrogen mixture, looking for such conserved primary properties which are important with respect to the fullerene formation process. The probability of secondary reactions then has to be discussed concerning the particular structure under investigation. The details of the applied method of mass spectra analysis are outlined in previous papers [19,20 ].

2. Experiments A pulsed frequency-doubled Nd:YAG (BMI) laser, focused on a rod of pure graphite, produces a cloud of superheated largely ionized carbon gas with carbon cluster formation. Before the laser shots take place, a synchronized pulsed gas valve generates a carrier gas pulse (duration tl ) of a helium-hydrogen mixture, leading to partial neutralization and saturation of the clusters inside the high-pressure pre-expansion zone. Since the laser pulse is released with a delay time t2 in the range between about 300 and 700 ~ts, carbon vaporization occurs during the arrival of the carrier gas front. Subsequent free-jet expansion quenches the nucleation and cools the clusters and the PAH molecules. Then the cluster expansion is skimmed (a few cm downstream of the nozzle) to form a supersonic molecular beam which passes through the accelerating plates of the TOF mass spectrometer. A high voltage (HV) pulse with a delay time t3 in the range of 150 to 300 ~ts deflects 129

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the positive ions of the molecular beam towards an electron multiplier, where the mass-separated cluster ions are detected. The TOF spectra are recorded in a microcomputer connected with a fast digital oscilloscope (Lecroy 9400). The three different pulse and delay times t~, tz and t3 can be adjusted within a small range of some hundred ~ts (fig. 1 ). The duration tl of the gas pulse has been chosen as t~ = 540 ~ts throughout' the experiment, which results in a maximum possible saturation of the carbon clusters with hydrogen. In order to increase the signal-to-noise ratio, a series of subsequent TOF spectra is averaged for each fixed set of TOF transmission parameters. Although most of the mass spectra of a single laser shot exhibit the qualitative pattern, typically a few hundred spec: tra are integrated to gain highly reproducible spectra. Instead of the pure helium quenching gas customary in fullerene condensation experiments, a mixture of 10% hydrogen and 90% helium is used to treat the carbon clusters immediately after formation. Further increasing this percentage does not lead to a higher degree of saturation, but merely reduces the probability of formation of the larger carbon clusters.

t2 GAS PULSE GRAPHITE ROD

Li4Z3 I

R

"fAG HV

. ACCELERATING PLATES

SKIMMER

. . . . . . . . . . . . . .

°++++ II ELECTRON MULTIPLIER

~

Fig. 1. Schematic experimental apparatus. Upper margin: Diagram of the electricalpulses and definitionof the three adjustable delay times. 130

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3. Results In a first attempt, the experiment was performed with helium quenching gas only in order to confirm the presence of the fullerene peaks in the mass spectrum. The addition of hydrogen then suppresses the formation of carbon cages [ 17 ]. In this case, for different sets of parameters, the experiments essentially show two different situations with clearly distinct structures of the carbon clusters. Since the time of the cluster treatment by hydrogen is not sufficient to obtain a total saturation, also with maximum t2 and minimum t3 the spectra show partially saturated carbon clusters with an essentially statistical distribution of ligand numbers. Inside of each group of mass peaks for the carbon clusters of the same size, an overall pattern of alternating abundances between dominating odd C,H~-m+~ and less abundant even CnH~-m exists, independently from cluster size n (fig. 2A). Since the clusters in this case allow for addition of more than 12 ligands, the highly saturated CnH + clusters reach into the mass peaks of larger + Cn+~Hm-12 ones with a small number of ligands. Therefore the different groups of clusters with subsequent sizes n and n + 1 overlap for higher masses. Hence the gaps between the groups of the fine-structure of hydrogenation reduce with larger cluster sizes. With a minimum delay time t2 between gas pulse and laser pulse and a maximum HV-laser delay t3, for smaller cluster sizes the pattern of the fine-structure of hydrogenation is more complicated than in the above case (fig. 2B). The alternating odd-even scheme is only shown up to C ¢ , C~" nearly shows the inverse scheme, and the C 6 H ~ - peak is surprisingly dominant. With a cluster size larger than n = 8 the odd-even pattern rapidly vanishes in favour of a smooth distribution which seems to be truncated between m = 7 and m = 9. In contrast to the case of fig. 2A the mass spectra of the larger clusters show well separated groups of fine-structure for different cluster sizes n. The width of the groups of distinct cluster sizes n > 16, where the fine-structure of hydrogenation cannot be resolved, is remarkably constant. From n = 20 to n = 30 the half-width of the groups (in mass units) only increases by about 8% (fig. 2C).

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n m=

A

108

120

n=4 m=

14-4

132

156

S

S

3

168

6 4

5

180

7

7

192

204

8

357

216

228

9

24- 7

23

240

10 7

236

252 a m u

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I I

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+8

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8'4

20 1

2

22

188

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26

12'o 28

1~2 30

~i+ ~ 32

1

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ainu

300

400

Fig. 2. Mass spectra of partially hydrogenized carbon clusters. (A) t2= 700 ~ts, 13 150 laS,minimum cluster formation time. (B) and (C): t2= 13 300 ItS, maximum cluster formation time, two different size regimes. The abscissa is scaled by atomic mass units, n is the carbon cluster size and m the number of hydrogen ligands corresponding to the formula C~H + . =

=

4. D i s c u s s i o n In fig. 2A, the mass spectra show that the majority o f carbon clusters in this case attach an o d d n u m b e r o f ligands. The ratio o f the odd-to-even abundances is about a factor 10, since the peaks for even numbers o f ligands are still increased by the o d d peaks o f the C ,~2 _ ~C~3 cluster c o m p o u n d . This o d d - e v e n pattern o f ligands can be explained by linear, unb r a n c h e d carbon chains. C a r b o n chain ions irrespective o f n show four terminal radical sites. The charge appears to be delocalized by resonance, but with a m a x i m u m charge density residing on the terminal a t o m s [21 ]. As a consequence, the approximately three t e r m i n a l binding electrons easily allow

for saturation by three hydrogen a t o m s forming pol y y n i u m (C2+2nH + ) or polyenylidenium ions (C1+2,HJ-) [22], respectively, which represent the first d o m i n a n t peaks a t m = 3. Each step of further saturation requires opening o f one additional n bond, thus favouring a d d i t i o n o f t w o hydrogen atoms. F o r this reason, chains with o d d numbers o f hydrogen a t o m s are much m o r e abundant. Moreover, the large difference between the intensities o f the peaks with o d d a n d even m, respectively, excludes significant a m o u n t s o f b r a n c h e d chain molecules: Since each branching o f the chains would allow for several structural isomers, the pattern would be blurred i m m e d i a t e l y by the statistical distribution o f the additional radical sites. 131

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According to the experimental conditions, the situation shown in fig. 2A corresponds to vaporization of carbon during maximum pressure of the hydrogen rich quenching gas in the evaporation chamber (large t~). In this case, the carbon molecules initially dissociated from the carbon surface become saturated before the carbon gas relaxes towards chemical equilibrium. Since the subsequent reactions of saturated carbon valencie~s with dangling bonds are endothermic and much slower than the radical-radical reactions which form the clusters, we expect the reactions of saturated clusters with carbon not to essentially affect the general structural properties of the carbon clusters. On the other hand, the reaction of carbon chain radicals with molecular hydrogen is slightly exothermic (typically 20 kJ tool- ~). Thus we cannot exclude dissociation or unimolecular reactions of the primary clusters after saturation of the clusters with hydrogen. Nevertheless, for three reasons we conclude that laser-heated carbon evaporates by fragmentation into short chains which allow for the formation of longer linear chains by very fast addition reactions between the terminal radical sites: ( 1 ) The absence of rings can be explained by the fast saturation of the terminal radical sites of the chains, which hinders subsequent ring closure reactions. (2) The destruction of preexisting rings by secondary reactions is unlikely, because the spectra of fig. 2B (see below) prove the stability of the rings towards hydrogenation. (3) The absence of branched chains can be explained by the thermal instability of the C - C single bonds at the temperatures of the carbon gas which has been predicted by estimates using R R K theory [131. To derive the spectra in figs. 2B and 2C, the YAG/ gas-pulse delay has been decreased to the minimum possible value, whereas the HV delay time has been increased to its maximum. Thus, the carbon gas has more time to relax, but still sufficient time for saturation reactions. In fig. 2B the C J- to C ~-2 groups are clearly separated by gaps without any signal. This allows for comparison of the various peaks of pure carbon clusters without a contamination by overlapping of the 132

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groups. The first strong peak of a pure carbon cluster ion is the C i~ signal, whereas the C ~o and C ~-2 peaks are significantly lower. These signals correspond to monocyclic rings: Since such rings have no terminal radical sites, the addition of the first hydrogen atom requires a (small) activation energy to open one of the cumulative n bonds. Thus, the observed pure carbon peaks are caused by ring molecules which did not react with the hydrogen. As a consequence, they have to be a primary component of the pure carbon gas. In the case of neutral clusters, according to theoretical calculations C~o is the most abundant monocyclic ring molecule, because it has a closed shell electronic structure [15]. In contrast, its positive ion C~-0 has an unfilled orbital and, thus, the C~-~ ring is more abundant. With the appearance of the ring molecules the odd-even pattern vanishes, because the activation energy of the stressed rt bonds of the rings is much lower than in the linear chain molecules. The lack of a C~ H+o peak, which should be the most saturated monocyclic C~ ring, indicates the collapse of the cluster towards a bicyclic structure [ 17 ]. The unexpected deviations of the C6~ and C~groups from the overall pattern of the chains can be either a consequence of secondary reactions or a superposition of the chain pattern with coexisting monocyclic rings, a question which cannot be answered from our experiments. The partial disagreement between our results and previous work with a resembling experimental apparatus [ 23 ] stresses the essential difference between hydrogenation of positive cluster ions (as in our case) and ionization of hydrogenated carbon clusters which may easily lead to fragmentation processes. The gaps in the spectrum of fig. 2C prove the curved polyaromatic structure of the larger hydrogenated carbon clusters: The maximum number of hydrogen atoms, which can be added to long chains as well as monocyclic rings, is approximately proportional to the number n of C atoms, so that the width W(n ) of the groups of such structures (in mass units) also has to be proportional to n. Since the spectrum shows a ratio of W(30)/W(20)~l.08 <30/20, chains and monocyclic rings should not have any significant abundances. In the case of flat polyaromatic hydrocarbons (PAHs) the dependence of the group width on molecule size can be estimated

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by comparing the number of hydrogens, for example ofbenzopyrene(C2oHI2)and pyranthrene (C3oH16), and we derive W ( 3 0 ) / W ( 2 0 ) = 16/12, which still is an increase of the group width four times larger than in our experiments. Thus we can conclude, that the hydrogenated carbon clusters with n >/20 cannot be flat, but should be saucer shaped, because curvature reduces the increase of the number of dangling bonds with cluster size. (For example, a cluster including six pentagons can grow like a tube with a constant number of 12 dangling bonds. ) For three reasons it is unlikely that the curvature of the clusters is a consequence of secondary reactions: (1) Reactions of molecular hydrogen with polyaromatic radicals are slightly endothermic (e.g. phenyl+hydrogen, AG(3000 K) ~ - 2 0 kJ mol-~ ). Therefore hydrogenation does not induce rearrangement processes towards curved structures. (2) Hydrogenation of the reactive edges of flat carbon clusters reduces the probability of reduction of the number of dangling bonds by rearrangement processes [ 17 ]. (3) PAH-formation processes (e.g. in flames [22] ) favour flat structures at least up to n=30. Therefore we conclude the curvature of the carbon clusters also to be a property of the primary carbon gas composition. Although the experiments were restricted on analyzing positive carbon cluster ions, the results can be extended to neutral clusters, because there is little difference between the composition of the ionized and the neutral component [9 ]. Thus, in spite of the influence of secondary reactions on the mass spectra, the experiments indicate some conclusions on the structure of carbon clusters on the pathway to C6o, summarized below: (i) Linear, unbranched chains seem to be the dominating component of the pure carbon gas below n=10. (ii) If the carbon gas is being relaxed by collisions sufficiently, the long chains (n 1> 10) undergo ring closure reactions towards monocyclic rings. (iii) The pattern, especially of the Ci~ group, points to a collapse of the small monocyclic rings towards bicyclic structures after reacting with hydrogen (in agreement with ref. [ 17 ] ). However, the de-

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gree of saturation of these clusters is not high enough to finally decide this question. (iv) The data strongly support the conception of saucer-shaped polyaromatic carbon clusters consisting of pentagons and hexagons. The size for the onset of curvature is approximately in the regime of 15~
Acknowledgement The authors acknowledge the financial support from the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 337. AG and ES acknowledge the support of the astrochemistry Project Se 430/33 by the Deutsche Forschungsgemeinschaft.

References [ 1] H.W. Kroto, J.R. Heath, S.C. O'Brian, R.F. Curl and R.E. Smalley, Nature 318 ( 1985) 162. [2 ] W. Kr~itschmer,K. Fostiropoulosand D.R. Huffman,Chem. Phys. Letters 170 (1990) 167. [3] R. Taylor, J.P. Hare, A.K. Abdul-Sada and H. Kroto, J. Chem. Soc. Chem. Commun. (1990) 1423. [4] Z. Iqbal, R.H. Boughman,B.L. Ramakrishna, S. Khare, N.S. Murthy, J.S. Bornemann and D.E. Morris, Science 254 (199l) 826. [ 5 ] T. Guo, C. Jin and R.E. Smalley,J. Phys. Chem. 95 ( 1991 ) 4949. [6] R.E. Haufler, J. Conceicao,UP.F. Chibante, Y. Chai, N.E. Byrne, S. Flanagan, M.M. Haley, S.C. O'Brien, C. Pan, Z. Miao, W.E. Billups, M.A. Cuifolini, R.H. Hauge, J.L. 133

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Margrave, L.J. Wilson, R.F. Curl and R.E. Smalley, J. Phys. Chem. 94 (1990) 8634. [7] F.D. Weiss, J.L. Elkind, S.C. O'Brien, R.F. Curl and R.E. Smalley, J. Am. Chem. Soc. 110 ( 1988 ) 4464. [8] T. Weiske, J. Hrusak, D.K. B/Shme and H. Schwarz, Chem. Phys. Letters 186 ( 1991 ) 459. [ 9 ] H.P. Palmer and M. Shelef, Chem. Phys. Carbon 4 ( 1968 ) 85. [ 10] J. Abrahamson, Carbon 12 (1974) 111. [ 11 ] H. Kroto, Science 242 (1988) 1139. [ 12] P. Ballone and P. Milani, Phys. Rev. B 42 (1990) 3201. [ 13 ] A. Goeres and E. Sedlmayr, Chem. Phys. Letters 184 ( 1991 ) 310. [14] E.H. Pietsch and R.J. Meyer, Gmelins Handbuch der anorganischen Chemie Bd. 14, ed. Gmelin Institut (Verlag Chemie, Weinheim, 1967) p. 162. [ 15 ] K. Raghavachari and J.S. Binkley, J. Chem. Phys. 87 ( 1987) 2191.

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[16] S. Yang, K.J. Taylor, M.J. Craycraft, J. Conceiao, C.L. Pelliette, O. Cheshnovsky and R.E. Smalley, Chem. Phys. Letters 144 (1988) 431. [ 17] A. Goeres and E. Sedlmayr, Hydrogen-blocking in C6oformation theories, to be published. [ 18] E.A. Rohlfing, J. Chem. Phys. 93 (1990) 7851. [ 19] P. Fayet, M.J. McGlinchey and L.H. WtJste, J. Am. Chem. Soc. 109 (1987) 1733. [20] G. Delacr6taz, P. Fayet, J.P. Wolf and L.H. W6ste, in: Structure and reactivity of surfaces, eds. C. Morterra, A. Zeccina and D. Kosta (Elsevier, Amsterdam, 1989 ) p. 359. [21 ] S.W. McElvany, B.I. Dunlap and A. O'Keefe, J. Chem. Phys. 86 (1987) 715. [22] Ph. Gerhardt and K.H. Homann, J. Phys. Chem. 94 (1990) 5381. [23] M. Doverstfil, B. Lindgren, U. Sassenberg and H. Yu, Z. Physik D 19 (1991) 447.