J. Phys. Chem. 1991, 95, 8425-8428 discrepancies described. One such experiment involves the production of higher concentrations of C2HSradicals. This can be done via H + C2H51 HI + C2H5 and in preliminary experiments we have formed lots of butane with C2H5 1OI2 particles/cm3. We hope to continue this work. Rabinovitch et aI.I3J4 have reported results on H + CZH2D2
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(13) Current, J. H.; Rabinovitch, B. S. J . Chem. Phys. 1963, 38, 783. (14) Rabinovitch, B. S.; Dills, D. H.; McLain, W. H.; Current, J. H. J . Chem. Phys. 1960,32,493.
8425
+
and D C2H2D2under conditions nearly comparable to ours (50 mTorr-2 Torr) where they report close to expected yield of butane relative to disproportionation. There are enough complications from wall reactions, concentration gradients of radicals, and secondary reactions to make comparisons with our system extremely tenuous. Acknowledgment. This work has been supported by a grant from the National Science Foundation (CHE-8714666647) and a gift from the Occidental Chemical Corporation, Grand Island, NY.
C60and LlqulbCrystal Mesophase Ian C.Dance: Keith J. Fisher: Gary D. Willett,t and Michael A. Wilson**+*t School of Chemistry, University of New South Wales, PO Box 1, Kensington 2033, NSW, Australia, and CSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde 21 13, NS W,Australia (Received: April 1, 1991; In Final Form: August I , 1991)
Buckminsterfullerene has been detected during the laser ablation of the liquid-crystal carbonization intermediate known as mesophase. Under identical experimental conditions buckminsterfullerene is more readily formed from mesophase than graphite. However, the nature of the collision gas and reaction time affect the amounts of buckminsterfullerene and other carbon clusters formed. Hydrogen gas suppresses the formation of higher C,,n > 70, clusters relative to argon. Benzene and methane were efficient in the production of buckminsterfullerene at longer reaction times (0.1-0.5 s).
The pyrolytic carbonization of many materials proceeds through an intermediate liquid crystal stage formed between 370 and 500 OC and called mesopha~e.I-~Mesophase is detected optically by microscopic examination using polarized light and is believed to consist predominantly of highly aligned polycyclic aromatic^.^ Technological interest in mesophase has stemmed from its utility in forming high tensile strength graphitic fibers ("carbon fibers") and Recently the laser-induced production of buckminsterfullerene and isolation by thermal or electrical arcing techniques has generated great interest.*-I4 Buckminsterfullerene is a c60 soccerball molecule consisting only of carbon and has been shown to form readily from graphite but not in significant amounts from benzene soot.I5 We now report its detection from laser pyrolysis of mesophase. The mesophase was formed using conventional procedures6 as described in the Experimental Section. It was shown to be 95% pure by optical microscopy. Solid-state "C nuclear magnetic resonance by cross polarization and magic angle spinning, taking into account variable spin-lattice relaxation times in the rotating frame showed that the mesophase was 82% aromatic. Elemental analysis showed it contained 92.7% C and 4.8% H. Aliphatic hydrogens (about 20%) were shown to be present by CRAMP!W7 IH N M R employing the BR24 pulse sequence.I8 Laser ablation was carried out using a Spectra Physics DCR-I 1 Nd:YAG laser and detection was by means of a Spectrospin CMS-47 Fourier transform ion cyclotron resonance (FTICR) mass ~pectr0meter.I~The pulse program for these experiments is shown in Figure 1. In some experiments the second delay period D2 was vaned. It is possible that C , may form from less stable carbon species so that during the second delay period C, formation may be enhanced. Typical positive ion FTICR mass spectra are shown in Figure 2. It is clear that both Cboand the expanded soccer ball structure C70with an extra 10 carbons inserted in hexagonal structures University of New South Wales. *CSIRO Division of Coal and Energy Technology. To whom correspondence should be addressed.
around the middle are present. The narrow-band positive ion mass spectrum of the 718-723 region of the spectrum (resolution peak mass/half-width = 4100, Figure 3) shows three peaks of intensity 1:0.67:0.22 for masses 720, 721, and 722. The expected distribution for a c60 molecule with random I3C substitution is 1 :0.65:0.20. The positive ion mass spectra (Figure 2) also show that a number of other high mass clusters are present. The main envelope centers around m l z 1225, but a further envelope is present at about 1700 daltons. In some experiments a third (1) Brooks, J. D.; Taylor, G. H. Carbon 1965,3, 185. (2) Brooks. J. D.; Taylor, G. H. Nature 1965, 206.697. (3) Brooks, J. D.; Taylor, G. H. Chem. Phys. Carbon 1968, 4, 243. (4) Lewis, I. C. Carbon 1980, 18, 191. (5) Singer, L. S.Carbon 1978, 16, 409. (6) Matsumoto, T. Pure Appl. Chem. 1985, 57. 1553. (7) Smith, G. W.; White, J. L.; Buechler, M. Carbon 1985, 23, 117. (8) Johnson, R. D.; Meijer, G.; Bethune, D. S. J. Am. Chem. Soc. 1990, 112, 8983. (9) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byme, N. E.; Flanagan, S.; Haley, M. M.; OBrien, S. C.; Pan, C.; Xiao, Z.; Billups, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J. Phys. Chem. 1990,94, 8634 and referenccs therein. (10) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kratchmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J . Phys. Chem. 1990,94,8630 and references therein. (1 1) Allermand, P. M.; Koch, A.; Wundl, F.; Rubin, Y.; Diedrich, F.; Alvarez, M. M.; Anz, S. J.; Whetten. R. L. J . Am. Chem. Soc. 1991, 113, 1050. (12) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354 and references therein. (13) Kroto, H. Science 1988, 242, 1139. (14) Taylor, R.; Hare, J. P.; AWul-Sada, A. K.; Kroto, H. W. J . Chem. Soc., Chem. Commun. 1990, 1423. (15) So, H. Y.; Wilkins, C. L. J . Phys. Chem. 1989. 93, 1184. (16) Gerstein, B. C.; Pembleton, R. G.; Wilson, R. C.; Ryan, C. M. J . Chem. Phys. 1977, 66, 361. (17) Ryan, L. M.; Taylor, R. E.; Paff,A. J.; Gerstein, B. C. J. Chem. Phys. 1980. 72, 508. (18) Rhim. W. K.; Burum, D. P. J. Chem. Phys. 1978, 71,944. (19) Allemann, M.; Kellerhals, H. P.; Wanczek, K. P. Int. J. Mass Spectrom., Ion Phys. 1983, 46, 139.
0022-3654191/2095-8425%02.50/0 0 1991 American Chemical Society
Letters
8426 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
j[-"I Quench
Ablation
EXCltC
Delay ion-molecule to allowreaclinns
4
TABLE I: Production of Cm+bv LUKr Ablation of Mcsopb.se re1 ion gauge reaction pressure, Pa time D2.s 5% c60 gas Ar I x 10-4 0 60 100 Ar 1 x 10-4 0.005
Acquire
DE
D,
1x 1x 1x 1x 1x
C6H6 C6H6
19 86
1
IO
3 x 10"
H2 CH, CHI
of ions with molecules to form smaller/larger carbon clusters. envelope at around m f z 2536 could be detected. Figure 2 clearly shows that the total number of ions detected changes when reaction conditions, time, pressure, or gas are varied. In some experiments, e.g., Figure 2a, all species are weak in intensity but Ca predominates. In other experiments, e.g., Figure 2b, a greater concentration of all ions is produced. This variability is probably due to the amount of soot produced.
1 x 10-4 1 x 10-4
Ar Ar Ar
Figure 1. Typical pulse program to obtain FTICR mass spectra. P I = 5 ms, P2 = 1 ms, P, = 3 ps, D , = 6 s, D2 = 0-10 s, DE = 0.5 ms, Acquisition time = 0.022 s. By ion molecule reactions we mean reactions
0.005 0.005 0.005 0.1
10-4 10-4 10-4 10-4 10-4
61 60 61 96 91
0.005
1 I6
0.5
Table I shows relative yields of C , when the reaction time and pyrolysis atmosphere were changed. In this table the data have been normalized against an experiment with argon at 1 X lo4 Pa for 0.005 s so that the formation of species can be compared
0 1000
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~:~ j 40
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1000
1500 mlr
2000
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Figure 2. Effect of changing the reaction time (D2) on relative ion intensities with 1 X IO4 Pa of Ar atmosphere: (a) D2= 0, (b) 0 2 = 0.005 s, (c) D2= 0.1 s, (d) D2 = 1 s, (e) D2= 10 s.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8421
Letters
n
I
'O01 80-
5.aa
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60-
c m
I
40-
B 718
719
720
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722
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mlz
0
Figure 3, High-resolution spectrum of m / r region 718-723 of mesophase
laser pyrolysis product.
to the argon experiment. The concentration of the 720 (C,+) molecular ion decreased relative to the other ions detected when the pressure of argon was decreased from 1 X lo-" Pa to 3 X IW5 Pa, and increased when the reaction time D, was optimized to 0.005 s. At lower and longer reaction times the intensity of the C , + ion decreased. This suggests C, precursor formation is dependent on the destruction of less stable mass species and inhibition of more stable species. The precursor may not be a , species. Qualitative soccerball but could be an unclosed C differences were also obtained in the production of C, ions where n > 70 (Figure 2). Hydrogen gas suppressed the formation of , (Table I) and the higher mass ions all the species including C relative to Argon (Figure 4a,b), although no hydrogenated species have so far been observed in our studies. Benzene and methane were efficient in the production of C, species at longer reaction times (Table I and Figure 4c). We have also been able to reproduce earlier reported laser albation experiments on graphitew2, in which both C, and C,,, are produced. However, we have now been able to show that Cso forms much more readily from mesophase than graphite. Under identical conditions (0.1 MW/cm2 to the mesophase experiments ,+ was not formed from graphite nor were any reported here C carbon species detected above mass 240. To generate C , from graphite in this experiment a laser power of at least 6.5 times that for mesophase was required. Our new results have some significant implications. Cso and related molecules may have a wide range of potential new uses as batteries, catalysts, lubricants, or absorbents. Mesophase can be formed from a wide range of cheap organic materials such as petroleum pitches, residues, low volatility bituminous c081s,~-~3~Jl and even sucrose.25 Thus sources of Cbomay be found from materials other than graphite. It is also significant that Cbois formed more readily than from graphite. In graphite, units of the covalent solid have to be disrupted. Maophase is a molecular solid and the discrete aromatic units will not have to undergo the extensive degradation of graphite. Indeed it may be possible to modify mesophase structure to greatly enhance C , formation.
Experimental Section Preparation of Mesophase. Pitch derived from continuous hydrogenation of Yalloum brown coal" was refined by maturation at 425 OC with 10%nickel/molybdenum (Cyanamid HDS 3A) catalyst in an autoclave (Parr) for 2.5 h under hydrogen at 10.4 MPa cold pressure. The product was then heated at 400 OC under (20) McElvany, S.W.; Nelson, H.H.;Baronavski, A. P.; Watson, C. H.; Eyler, J. R. Chem. Phys. Len. 1987, 131, 214. (21) Knight, R. D.; Walch. R.A.; Foster, S.C.; Miller, T. A.; Mullen, S. L.; Marshall, A. G. Chem. Phys. Len. 1986, 129, 331. (22) Greenwood, P. F.; Strachan. M. G.; Willett, G. D.; Wilson, M. A. Org. Mass.Specrrom. 1990, 25, 353-362. (23) Barron, P. F.; Collin, P.J.; Russell, N. J.; Wilson, M. A. Fuel Proc. Technol. 1982, 6, 147. (24) Wilson, M. A.; Heng, S.;Frcdericks, P. M.;Collin, P. J.; Vassallo, A. M. Fuel Proc. Technol. 1986. 13. 243. (25) Vassallo. A. M.; Cdd, R. Carbon 1988, 26, 553. (26) Bien. C. N.;Luttin, K.P.;Smith, B. E.;White, N. Energy Fuels 1988. 2, 807.
100-
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Figure 4. Effect of changing gas at 1 X 10-4 Pa pressure, DI= 0.005 s: (a) Ar, (b) HI,(c) CH,.
vacuum (9.5 @a) to remove volatile products in a 100 cm3stirred autoclave (Autoclave Engineers). At regular intervals samples were withdrawn for optical analysis. Maximum mesophase generation had occurred after 6 h. The product pitch had a softening range of 280-295 OC, contained 95% mesophase, and had an aromaticity 0.82 and an elemental composition of C 92.7%. H 4.8%, N 1.0%, 0 1.5%. Laser-Ablation Mass Spectrometry. The method has been described in detail elsewhere.22 In brief 5 mg of sample was powdered and compressed into a 2-mm cylindrical stainless-steel satellite probe tip. The assembly was subsequently inserted into the FTICR cell, so that it was flush and in contact with the bottom trapping plate, using a Spectrospin direct insertion probe. A typical pulse sequence for the experiments was as follows: After the removal of unwanted ions from the cell by a quench pulse, the 1064-nm fundamental beam of the Nd:YAG laser was focused to a small area (-0.1 mm2) at the surface of the sample, to induce ionization. It is necessary to allow the ICR cell chamber to reapproach the preionization vacuum conditions prior to ion detection. This requires a short delay time following the ionization pulse. Such a time interval allows neutral species, initially present in the laser-desorbed plasma, to be evacuated, and also a reaction time for collision reactions.
8428
J. Phys. Chem. 1991, 95, 8428-8430
Apart from the ionization pulse width of the Nd:YAG laser, all parameters were under computer control. Two laser irradiance times were used in these experiments, one corresponding to a long-pulse mode (230 rs) and the other to a Q-switched mode (8 ns). Irradiances in the range 0.005-1000 MW cm-' were also used, and neutral density filters were used to obtain reproducible irradiance variations. Although sufficient ions were generally
generated by a single laser pulse to acquire a complete mass spectrum, the spectra reported in this paper were obtained by time averaging of five cycles. No problems were observed with shot-to-shot reproducibility. Acknowledgment. We thank A. Palmisano for technical assistance in preparing the mesophase pitches.
Laser- Induced Polymerization of Submonolayer Formaldehyde on Ag( 111) L.E. Fleck, W. F. Feehery, E. W. Plummer, Z. C. Ying, and H. L. Dai* Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6202 (Received: July 5, 1991; In Final Form: August 21, 1991)
Formaldehyde (H2CO)physisorbed at submonolayer coverages on Ag(ll1) at 80 K is observed to polymerize upon irradiation with 355-nm laser pulses in a nonthermal process. Polymerization occurs over a wide coverage range-from greater than 1 monolayer to as low as 0.1 monolayer. Polymerization is most likely initiated by radicals generated from H2C0dissociation. In contrast to the monomer which desorbs at 110 K, the polymer remains stable on the surface up to 210 K.
Photochemical processes of molecules adsorbed on singlecrystal surfaces have recently been investigated under ultrahigh-vacuum (UHV) conditions using modern surface science techniques. Photoinduced di~sociation,'-~d e s o r p t i ~ nsite , ~ ~rearrangement?.* and bimolecular reactionsg-I0on surfaces have been studied. In this paper, we report the first observation of photoinduced polymerization of a surface adsorbate in UHV. Formaldehyde (H2CO) physisorbed on Ag(ll1) at coverages from 0.1 to >1 monolayer (ML) is observed to polymerize upon irradiation with nanosecond pulses of 355-nm radiation. The polymerization mechanism is determined to be nonthermal. Thermally induced polymerization of H 2 C 0 has been observed previously on several different metal surfaces."-I6 H 2 C 0 polymerization was observed on oxygen-covered Ag( l lo), but not on clean Ag(1 lo)." It was speculated that an oxygen atom reacts with H 2 C 0 to form the radical species H2C02,which initiates the polymerization." On Cu(1 10)12and Zn(Wl)," in contrast, polymerization of H 2 C 0 was observed on both the clean and the oxygen-covered surfaces. Polymerization of HzCO has also been observed on the clean Ni(1 10),14 Pt(l1 l),I5 and Rh(l1 1)16surfaces. The mechanism for initiation of polymerization on these clean surfaces is not clear. The gas-phase photochemistry of HzCO is well established." One of the available reaction channels is the dissociation into H ( I ) Hasselbrink, E.; Jakubith, S.;Nettesheim, S.;Wolf; M.; Cassuto, A.; Ertl, G. J . Chem. Phys. 1990, 92, 3154. (2) Marsh, E. P.; Tabares, F. L.; Schneider, M.R. Gilton, T. L.; Meier, W.; Cowin, J. P. J . Chem. Phys. 1990, 92, 2004. White, J. M. J . Chem. Phys. 1990, 92, 1504. (3) Zhou, X.-L.; (4) Ying, 2.C.; Ho, W. Phys. Rev. Len. 1990, 65, 741. (5) Hanky, L.; Guo, X.; Yates. J. T., Jr. J . Chcm. Phys. 1989, 91, 7220. (6) Domen, K.;Chuang, T. J. Phys. Rev. Lcrr. 1987,59, 1484. (7) Richter, L. J.; Buntin, S.A.; King, D. S.;Cavanagh, R. R. Phys. R w . krr. 1990, 65, 1957. (8) Wolf, M.; Hasselbrink, E.; White, J. M.; Ertl, 0.J . Chem. Phys. 1990, 93. 5327. (9) Cho, C.-C.; Polanyi, J. C.; Stannen, C. D. J . Chem. Phys. 1989, 90, 598. (IO) Mieher, W. D.; Ho, W. J . Chem. Phys. 1989.91, 2755. (1 1) Stuve, E. M.; Madix. R. J.; Sexton, B. A. Surf. Sci. 1982, 119, 279. (12) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surfl Sci. 1985,155,366. (13) Sen. P.; Rao, C. N. R. Surf. Scl. 1986, 172, 269. (14) Richter, L. J.; Ho. W. J . Chem. Phys. 1985, 83, 2165. (15) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf Sci. 1987,188, 206. (16) Houtman, C.; Barteau, M. A. Surf. Scf. 1991, 248, 57. (17) Moore, C. 9.;Weisshaar, J. C. Annu. Rev. Phys. Chem. 1983, 34, 525. and references therein.
0022-365419112095-8428$02.50/0
and HCO radicals. In the presence of other HzCO molecules, these radicals can induce polymerization even in the gas phase. If H 2 C 0 is physisorbed on a surface, it may retain its gas-phase photochemical behavior. On a surface, radicals generated from photodissociation may interact with neighboring molecules and induce polymerization chain reactions. Light-induced polymerization of monolayer adsorbates not only involves many interesting fundamental questions in surface reaction and energy-transfer dynamics, it also has many potentially important applications. For example, it is conceivable that lightinduced polymerization could be used to construct a chemically inert protective coating on selected areas of a surface. It could also be used to produce conductive polymers in specific one- or two-dimensional patterns on surfaces. The experiments were performed in a UHV chamber (base pressure 1 X 1O-Io Torr) equipped with an electron energy loss (EEL)spectrometer (McAllister Technical Services) for vibrational spectroscopy and a mass spectrometer (UTI 1OOC) for temperatureprogrammed desorption (TPD) and gas analysis. The Ag( 1 11) substrate (15 mm diameter) was cleaned before each experiment by two cycles of sputtering and annealing. The crystal was held by tantalum clamps on a resistive heating unit (Spectra-Mat) in contact with a liquid-nitrogen reservoir. The temperature was measured by a chromel-alumel thermocouple spotwelded to one of the clamps. Formaldehyde gas was prepared from paraformaldehyde by pyrolysis and fractional distillation. The sample was dosed by back-filling the chamber through a variable leak valve. During TPD experiments, the sample was biased -67 V to repel electrons emitted from the mass spectrometer filaments. (Without the negative bias, electron-induced chemistry in the H 2 C 0 layer is observed.) The photon source was a 20-Hz pulsed Nd-YAG laser (Continuum YG661). The laser was operated at below 50% of its optimum output power. The laser pulses were measured to be 14 ns long (fwhm) under these conditions. The linearly polarized beam was expanded to a diameter of 15 mm and was incident on the surface at a 45O angle. p-polarized light was used in all of the experiments described here. The middle panel of Figure 1 shows an EEL spectrum of a 2 langmuir (1 langmuir = lo4 Toms) exposure of H2CO on Ag (1 1 1) at 80 K. The spectrum is assigned to molecularly adsorbed H2C0. The peak assignment is as follows: 352 meV, the CHI symmetric and asymmetric stretches (unresolved); 2 10 meV, the C=O stretch; 183 meV, the CH2 scissor; 151 meV, the CH2 in-plane and out-of-plane bends (unresolved). The positions of these vibrational peaks differ from the corresponding gas-phase 0 1991 American Chemical Society
