J . Phys. Chem. 1992,96,4935-4937
4935
Production of Endohedrai Yttrium-Fuilerene Cations by Direct Laser Vaporization Stephen W. McElvany Code 61 131Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5000 (Received: January 28, 1992)
Direct laser vaporization of samples containing graphite, yttrium oxide, and fullerenes is used to generate gas-phase yttrium-fullerene adduct ions, Y,C,+ (x = 1 , 2 with even-numbered n = 60-100), in a Fourier transform ion cyclotron resonance mass spectrometer. Colliiion-induceddissociation and oxidation reactions are used to compare these species with extemally-bound adducts, Y(C,)+,generated by gas-phase association reactions. The stability toward fragmentation and the remarkable oxidative stability suggest that the laser-generated species have endohedral structures, Y,@C,+. Possible mechanisms are considered for the production of these endohedral metal-containing fullerene cations.
Introduction The production of macroscopic quantities of fullerenes has resulted in both extensive physical characterization of these species and chemical modification to produce new materials. The majority of the derivatives have resulted from modification of the outside of the fullerene cage. However, the cage structures offer the unique opportunity to encapsulate atoms or molecules. This was demonstrated in early gas-phase experiments by Smalley and co-workersl in which lanthanum was incorporated into the carbon clusters. Subsequent photodissociation experiments, wherein the incorporated metal was varied, provided convincing evidence that these carbon cluster ions were spherical in nature.2 In these experiments, the metal was presumed to be trapped inside the fullerenes during the cluster growth process in the laser vaporization/molecular beam source. The inevitable production of macroscopic quantities of such species has recently been reported by Smalley and co-workers for La@Cs2that was produced by both laser vaporization/condensation and carbon arc methods3 (The nomenclature instituted by Smalley3 will be used throughout this paper in which M@C, represents an endohedral complex with M inside the fullerene cage, M(C,) is an externally bound Mfullerene complex, and MC, carries no structural significance). These results have recently been confirmed by groups a t IBM4 and UCLA.s However, Whetten and co-workers5 observed La2@Csoin addition to La@Cs2in soot generated by resistively heating La-doped graphite rods. Gas-phase endohedral fullerene cations containing inert gas atoms (e.g., (He@C60)+)have also been observed resulting from high-energy C,+/He collisions through penetration of the cage by the neutral target.&* In this study, endohedral yttrium-fullerene cations, (Y,@ C,)+ (x = 1, 2), are formed by direct laser vaporization of samples containing yttrium oxide and bulk fullerenes. The location of the metal in these species is unambiguously determined by comparison of their fragmentation and oxidation reactions with externallybound Y(C,)+, which are generated by gas-phase ion/molecule association reactions. The results suggest the formation of Y,@C, species in the bulk through coalescence reactions which is consistent with the recent study on the production of La@C, by laser vaporization of Laz03/graphite in a molecular beam ~ o u r c e . ~ Experimental Section
All experiments were performed on a Fourier transform ion cyclotron resonance (ICR) mass spectrometer (FTMS) consisting of a Nicolet FTMS-1000 data system and a 3-T superconducting magnet9 coupled with a custom vacuum system and ICR cell. This apparatus has been described in detail.1° Pressed pellets containing various combinations of yttrium oxide (Y2O3), graphite, and bulk fullerenes (- 85% C, and 15% C7,,) were placed on the solids probe located flush with one of the trapping plates (90% transparent nickel mesh). The frequency-doubled output (532 nm) of a Quanta-Ray DCR-2 Nd:YAG laser was used for direct laser vaporization (DLV). After passing through an iris to adjust the diameter and pulse energy, the beam was focused by a 1 m
focal length lens located outside the vacuum chamber and then traversed the cell to vaporize/ionize the sample. Ions were mass-selected for collision-induced dissociation (CID) and reactivity studies using a combination of swept frequency and SWIFT" ion ejection events. A static pressure of xenon (1 X lO-'-l X 10" Torr) was used as the target gas in the CID experiments. Oxidation reactions were examined using either background water present in the vacuum system or nitrous oxide (N20), which was pulsed into the system via a General Valve Series 9 pulsed valve and controller. Gas-phase neutral fullerenes were generated by sublimation of bulk samples from the solids insertion probe heated to approximately 400 OC. All chemicals were obtained from commercial sources and used as supplied with the exception of the bulk fullerenes which were extracted with toluene from fullerenerich soot (Texas Fullerenes, Houston, TX) and evaporated to dryness.
Results and Discussion Production of YC,+. Initial experiments to generate YC,+ by direct laser vaporization (DLV) used a sample consisting of graphite and Y203(-9:1 wt %), analogous to the previous studies which used lanthana-doped graphite. Ion/molecule reactions are believed to be important in the formation of large (n > 32)I2-l4 carbon cluster ions by DLV of graphite. Thus, DLV of a graphite/Y203 sample may be expected to result in (Y@C,)+ formation by encapsulation of Y (or Y') during the growth of fullerenes in the laser-induced plasma. However, DLV of this sample produced only Y+, YO+, YO2+ and minor amounts of YC2+and YC4+ with no detectable signal from C,+ or YC,+ (n > 4 ) . The absence of small C,+ (n < 32) is somewhat surprising because DLV of tantalum/graphite mixtures yields abundant C,+ (n < 32) and Ta,C,+ (n < 2O).I5 The lack of carbon cluster ion formation from graphite/YZO3may be due to the relatively low ionization potentials of Y (6.2 eV)I6 and YO (5.85 eV)16 which dominate the positive ion distribution in the laser-generated plasma. In another attempt to generate YC,+ by DLV, a sample doped with bulk fullerenes (C60/C70mixture) was used which consisted of graphite/Y203/fullerenes( - 5 4 : 1 wt 56). Laser vaporization of this sample yielded the mass spectrum shown in Figure 1. In addition to the presence of fullerene cations (c64+4&+), there are peaks corresponding to YC,+ (n = 56-96) with an enhanced abundance of YC,+ ( m / z 809). These YC,+ species are relatively abundant (e&, Yc60+:c70+ -0.5:l .O), and a direct correlation was observed between their relative abundance (compared to C,+) and the relative fraction of Y2O3 in the bulk sample. At higher m / z (see inset of Figure l ) , additional peaks are observed that correspond to Y2C,+ and are maximum for n = 68-90. The Y2Cn+ species become more abundant than the corresponding YC,+ species for n > 82. The origin and structure of the YxCn+species generated by DLV are unclear. The production of larger carbon cluster ions, C,+ (up to n = 106 in Figure l), which are not present in the bulk
This article not subject to U S . Copyright. Published 1992 by the American Chemical Society
4936 The Journal of Physical Chemistry, Vol. 96, No. 12, I992
McElvany
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a) Isolate Y(c,)+ No Reaction
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mh Figure 1. Direct laser vaporization (focused) mass spectrum of a sample containing graphite, yttrium oxide, and fullerenes. See text for details.
I' I
200
300
400
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600
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'1
b)
lxlO" Torr N,O 0.2 sec reaction
VIP
I+
YO+
(c, I
7 lodo
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1200
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m/z Figure 2. Direct laser vaporization (unfocused) mass spectrum of a sample containing graphite, yttrium oxide, and fullerenes. See text for details.
fullerene mixture, suggests that reactions (bulk or gas phase) are occurring between the fullerenes and small carbon species (probably from the laser vaporization of graphite). Because fullerenes are present in the bulk sample, gas-phase recombination of Y+/O and C,O/+might be expected to result in an externallybound adduct, e.g., Y(C,)+. The YC,+ signal generated by DLV was dependent upon the focused laser irradiation conditions. After the YC,+ signal was maximized, it slowly diminished with increasing time of laser irradiation (-50-100 laser pulses; 0.1-1.0 mJ/pulse) but could be regenerated by several (10-20) higher energy pulses (- 50 mJ/pulse). This indicates that the YC,+ may be formed in the bulk through coalescence reactions induced by the higher power laser irradiation, which is followed by desorption/ionization of intact Y,C, species by the low-energy laser pulses. Evidence for this formation mechanism of M@C, has also been observed by Smalley and co-workers3in laser vaporization studies of La203/graphite using a molecular beam cluster source. Thus, it is quite possible that the YC,+ species formed by DLV (Figure 1) have endohedral (Y @C,) structures resulting from coalescence reactions in the bulk. The production of yttriumfullerene cations was found to occur over much longer irradiation times and without the need to periodically alter the laser conditions when the unfocused laser output was used. The mass spectrum in Figure 2 was obtained without focusing the laser output ( 100-200 mJ/pulse). The overall similarity of the mass spectra in Figures 1 and 2 is expected as similar laser power densities (- lo7 W/cm2) were used in both cases. However, the more abundant production of Y,C,+ (relative to C,+) and the long-term stability of the signal using unfocused laser conditions are not understood at this time. The above attributes of the unfocused vaporization conditions suggest its possible use as an alternate method for the production of macroscopic amounts of these new materials.
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mh Figure 3. (a) Mass spectrum of the mass-selected externally-bound adduct Y(C,)+ formed by gas-phase association (reaction 1). (b) Mass spectrum obtained after a 0.24 reaction with N20(1 X lo-' Torr) illustrating the reactive nature of the externally-bound species.
Characterizationof YC,+. Collision-induced dissociation and ion/molecule reactions were used for detailed characterization of both the YC,+ generated by direct laser vaporization and the externally-bound Y(C,)+ species formed by gas-phase association of c 6 0 with Y+:
Y+ + (260
-
Y(C60)'
(1)
In the latter experiments, mass-selected Y+ (generated by DLV of Y203) was allowed to react with gas-phase neutral fullerenes that are continuously sublimed from a bulk sample outside the ICR cell. Collision-induced dissociation (CID) of M(C,)+ containing later transition metals (e.g., M = Fe, Co, Ni, etc.) generated by assoCiation (reaction 1) results in facile fragmentation to yield M+ or C,+ depending on their relative ionization potentials which is consistent with an externally-bound adduct.17 However, under similar CID conditions (40-eVh,, collisions with Xe) Y(C,)+ formed by association (reaction 1) is apparently much more stable than the later transition metal/&+ species because relatively inefficient fragmentation (-5%) was observed under these conditions to yield Y+ (IP = 6.2 eVI6) and C, (IP = 7.6 eVI8). In contrast, no fragmentation was observed for the DLV-generated YC,+. The relatively high stability toward fragmentation of both YC,+ species does not provide conclusive evidence for the internal or external nature of yttrium in the DLV-generated cations. Oxidation of early transition metal ions is very exothermic as shown in reaction 2 for Y+ with NzO. Thus, the oxidative stability Y+ + N 2 0 YO+ + N2 AH = -139.6 kcal/mol (2)
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of YC,+ should indicate whether the metal is internal or external to the carbon cage. Figure 3a shows the mass spectrum of mass-selectedY(C,)+ following formation of the extemally-bound species by gas-phase association (reaction 1). The mass spectrum in Figure 3b results after reaction with N 2 0 (1 X lo-' Torr; 0.2 s), illustrating the reactive nature of the externally bound metal: Y(C60)' + N20 * YO' N2 C6o (3) The loss of the Cm ligand in reaction 4 and the overall reaction
+ +
The Journal of Physical Chemistry, Vol. 96, No. 12, I992 4931
Endohedral Yttrium-Fullerene Cations
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Conclwions Direct laser vaporization of graphite/Y20,/fullerene samples has been shown to generate relatively abundant YC,' and Y 2 C 2 (n L 60) species. The oxidative stability of these species, when compared to externallybound Y (C,& generated by gas-phase association, suggests endohedral structures with the metal atom(s) within the fullerene cage, (Y,@C,)+. The formation of these species is considered to occur through coalescence reactions in the bulk induced by the laser irradiation. These Y,@C, species are then desorbed/ionized by subsequent laser pulses. Further characterization of these endohedral species by their gas-phase reactions are currently under way, including determination of the ionization potentials of M@C, species by charge-transfer bracketing in the FTMS which can be directly compared to the corresponding fullerene IPSpreviously determined by this technique. Acknowledgment. I thank Mark Ross for helpful discussions and acknowledge the Office of Naval Research for support of this research.
a) DLVgenerated YC:
]
b) 1x10' Torr NO ,
References and Notes 9 ) .
.-c> m
.
.
9 ) .
K
,
m/z
F v 4. (a) Mass spectrum of mass-selected YCat(and Ca+)formed by direct laser vaporization. (b) Mass spectrum obtained after a 2.0-s reaction with N20(1 X Torr) illustrating the unreactive nature of the DLV-generated species which would be consistent with an endohedral structure, Y@Cat. (The small peak a t m / z 105 does not increase with time or pressure and probably results from oxidation of unejected Yt. The other peaks, e.g. m / z 404.5, are harmonics of the more abundant ions.)
exothermicity (reaction 3, places an upper limit On D(y+-c60) < 6.05 eV. The mass spectra in Figure 4 correspond to an analogous experiment with DLV-generated YCm+. In this case, however, the YCW+ is completely unreactive even when the pressure/time exposure with NzO (1 X lo-' Torr; 2.0 s) is lo3 greater than for Y(C,)+ in Figure 3. The difference in reactivity clearly demonstrates that the DLV-generated YCa+ is dramatically different than the externallybound adduct, Y(C&)+, generated by gas-phase association. The reactivity data are most consistent with the DLV-generated YCW+being an endohedral isomer, Y@Ca+. In addition, all of the other YC,+ and Y2C,+ ions formed by DLV ( ~ 1 and i2) are ~also stable ~ toward ~ oxidation, indicating that the metal atom(s) is (are) inside the fullerene for these species as well.
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(1) 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, 7779. (2) Weiss, F. D.; Elkind, J. L.; OBrien, S.C.; Curl, R. F.;Smalley, R. E. J . Am. Chem. Soc. 1988, 110,4464. (3) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. (4) Johnson, R. D.; de Vries, M. S.;Salem, J.; Bethune, D. S.;Yannoni, C. S.Narure, submitted for publication. (5) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K.S.; Whetten, R. L. J. Phys. Chem. 1991, 95, 10561. (6) Weiske, T.; BBhme, D. K.; HrusHk, J.; Krltschmer, W.; Schwarz, H. Angew. Chem., In?. Ed. Engl. 1991, 30,884. Weiske, T.; Hrudk, J.; Ehme, D. K.; Schwarz, H. J. Phys. Chem. 1991,95,8451. Weiske, T.; HrusBk, J.; Bohme, D. K.; Schwarz, H. Chem. Phys. Let?. 1991,186,459. Weiske, T.; Hrusik, J.; BBhme, D. K.; Schwarz, H. Helu. Chim. Acta, in press. (7) Ross, M. M.; Callahan, J. H. J . Phys. Chem. 1991,95,5720. Mowrey, R. C.; Ross, M. M.; Callahan, J. H. J. Phys. Chem., in press. (8) Caldwell, K. A.; Giblin, D. E.; Hsu,C. S.;Cox, D.; Gross, M. L. J . Am. Chem. Soc. 1991. 113. 8519. (9) Presently sold by Extrel, P.O. Box 4508, Madison, WI 53711. (IO) Parent, D. C.; McElvany, S.W. J. Am. Chem.Soc. 1989,111,2393. (11) T. L. Ricca Associates, 1413 Wyandotte Rd., Columbus, OH 43212. Chen, L.; Wang, T. C.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1987,59,
449. (12) O'Keefe, A.; Ross, M. M.; Baronavski, A. P. Chem. Phys. Le??.1986, 130i 17. (13) McElvany, S.W.; Nelson, H. H.; Baronavski, A. P.; Watson, C. H.; ~ ~J . R. l(-hem.~ phys. ~ Lett. , 1987,134,214. (14) Creasy, W. R.; Brenna, J. T. Chem. Phys. 1988,126,453. Creasy, W. R.; Brenna, J. T. J. Chem. Phys. 1990,92,2269. Crtasy, W. R. J . Chem. Phys. 1990, 92, 7223. (15) McElvany, S . W.; Cassady, C. J. J. Phys. Chem. 1990, 94, 2057. (16) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref Data 1988, Suppl. NO.1. (17) Roth, L. M.; Huang, Y.; Schwedler, J. T.; Cassady, C. J.; Ben-Amotz, Kahr, B.; Freiser, B. S.J. Am. Chem. Soc. 1991,113,6298. Huang, H.; ~ D.; Freiser,~B. S . J. Am. Chem. Soc. 1991, 113, 8186. (18) Zimmerman, J. A.; Eyler, J. R.; Bach, S.B. H.; McElvany, S . W. J. Chem. Phys. 1991, 94, 3556.
