Plasma Coupled Radio Frequency Furnace: The Synthesis

7626

J. Phys. Chem. C 2010, 114, 7626–7630

Plasma Coupled Radio Frequency Furnace: The Synthesis, Separation, and Elucidation of the Elusive Sc4C82 Fullerene Evangelos Krokos* Rhoenstr. 21, 91056, Erlangen, Germany ReceiVed: NoVember 29, 2009; ReVised Manuscript ReceiVed: March 29, 2010

The influence of cold plasma on the efficiency of production of endohedral fullerenes and the mechanism of endohedral fullerene formation are discussed. For the first time, it was shown experimentally that the nucleation of the endohedral element in the carbon cage takes place in the last stage of the fullerene evolution process (annealing). A plasma-coupled radio frequency induction furnace was examined for the synthesis of scandium endohedral fullerenes. The separation and characterization of the Sc4C2@C80 are reported. Introduction After the discovery of C60 fullerene in 1985 by Kroto et al.,1 these unique molecules have attracted a lot of attention from scientists in different fields such as physics, chemistry, biology and medicine. Almost immediately after the discovery of C60 it was found that it is just a member of a big family of closed carbon cage molecules.2-5 Soon after, fullerenes containing metal atoms inside the cage were discovered,6 and syntheses of several other structurally related archetypes such as heterofullerenes, nanotubes, nanopuds, nano-onions, and nanotori were reported.7 Beside the commonly used arc-discharge approach,8 several alternative methods were developed for the synthesis of fullerenes, such as pyrolysis of polycyclic aromatic hydrocarbons,9,10 synthesis in flame,11-14 laser ablation,15 inductive heating of graphite using radio frequency induction furnace (RF),16 and rational chemical synthesis.17-19 Although the principle of fullerene formation in all known methods (except the last one) seems to be close to each to other, since they give the same distribution of fullerene isomers in the soot (C60 > C70 . C76, C78, C84 . Higher fullerenes), the mechanism of fullerene formation remains widely unexplored. To date, it considers three evolution stages: atomic carbon, cluster growing, and annealing to the fullerene molecule.20 A number of elegant theories of fullerene formation have been proposed, but until now none of them has been universally accepted. The main reason of this is the uncontrollable process of fullerene formation used, which significantly complicates the investigation.20 The RF furnace developed in our group has several advantages over the other methods. It provides more control on many parameters, including, but not limited to, the temperature of carbon and the time and speed of the evaporation of the foreign element. This allowed us to determine the zone of fullerene formation and to measure its thermal parameters. It was also demonstrated that introduction of stable cold plasma in this region essentially increases the formation of Sc endohedral fullerenes. Additionally, as a result of this investigation we report the synthesis, separation, and identification of the Sc4C82 fullerene. * Corresponding author. Tel.: +49 (0)9131/85-27324. Fax: +49 (0)9131/ 85-28307. E-mail: [email protected].

Figure 1. Model of the production chamber in the RF furnace.

Results and Discussion Details of the RF furnace have been described elsewhere.16 Briefly, a cylindrical carbon body is inductively heated while surrounded by pyrolytic boronitride (PBN) which acts as insulation. The foreign element is located at the basis of the carbon cylinder where the temperature is appropriate for its evaporation. During synthesis, He gas flows through the system as presented in Figure 1. The whole arrangement is placed in a water-cooled quartz cylinder. In our experiments with scandium, we noticed that the presence of several Sc3N@C2n compounds, especially Sc3N@C80 and Sc3N@C68, was detected in remarkable amounts, which was unexpected since no external nitrogen source was used during synthesis. The only possible source of N2 in the system is the boronitride shield which can partially decompose at high temperatures releasing nitrogen. Obviously, fullerene formation must at least take place in the area where the metal and nitrogen from the decomposition of the PBN meet in the gas phase. Taking under consideration flow velocity and apparatus geometry, it can be assumed that fullerene formation takes place quite away from the carbon evaporation zone as presented in Figure 1.

10.1021/jp911321e  2010 American Chemical Society Published on Web 04/14/2010

Plasma Coupled Radio Frequency Furnace

Figure 2. Temperature readings in the fullerene formation area. At approximately 6 cm from the carbon body, a distance at which the foreign element was held, the temperature is 1000 °C, and it decreases to almost 520 °C at a distance of 8.5 cm from the carbon body. The low temperature plasma was contained in that area.

Hence, it can be presumed that the carbon clusters transform to fullerenes (the so-called annealing process) in that region. To additionally verify this, we performed several tests using Ba, as an indicator of endohedral fullerene formation. The Ba source was placed and heated above the carbon body, directly in the assumed fullerene formation zone. Ba was chosen because endohedral fullerenes with Ba readily form in our furnace.21 Taking under consideration that the endohedral element cannot be introduced after the cage has been formed, the formation of Ba endohedral fullerenes with such configuration should unambiguously confirm our hypothesis. Indeed, HPLC analysis of the soot extracts showed that Ba endohedral compounds had formed with rather good yield. Temperature measurements were taken in the area of fullerene formation revealing that the temperature ranges from 1000 to 500 °C as presented in Figure 2. Comparing different fullerene production methods, it is easy to see that the capability of creating endohedral fullerenes seems to be connected with the first ionization potential of the elements intended for incorporation and the temperature achieved in the corresponding method. Although the different methods of fullerene synthesis give approximately the same ratio of isomers of hollow fullerenes, the situation is completely different in the case of synthesis of endohedral ones. Thus, in the case of arc-discharge, only third group elements Sc, Y, and most rare earth elements form endohedral fullerenes with high yields.22-24 From the second group, only Ca gives an acceptable amount of endohedral species.25 It was assumed that the low yield of Sr and Ba endohedral fullerenes was because of the rather large atomic radii of those elements.26 However, this does not seem to be the case since, whereas it is difficult to introduce Sr or Ba in the carbon cage using arc-discharge methods, they are the best elements for endohedral synthesis using RF carbon vaporization.21 Interestingly again Ba and Sr endohedral fullerenes were registered using the PAH pyrolysis approach.27 From the lanthanides, only La was reported to be introduced pyrolytically in the fullerene cage.28 On the other hand, Sc and Y, which tend to form endohedral fullerenes using the arc-discharge method, give only trace amounts of endohedral fullerenes in the RF furnace. To the best of our knowledge, Sc or Y endohedral fullerenes were never obtained in pyrolysis experiments. Summarizing, the low-temperature pyrolysis synthesis (4501000 °C) can only introduce Ba, La, and Sr in the cages which

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7627

Figure 3. HPLC profile of toluene extract of scandium soot showing suppression of higher fullerene formation by a high concentration of Sc in the gas phase (scandium source placed directly above the carbon cylinder). BP column, 4.6 × 250 mm, toluene, 1 mL/min, detection at 300 nm.

have low first ionization energies of 5.2, 5.6, and 5.7 eV accordingly. The RF furnace produces endohedral fullerenes with these elements with good yield but only a small amount of Sc endohedral, which has higher ionization energy at 6.5 eV. When using the high-temperature arc discharge method, Sc, Y, and rare earth elements with ionization energies higher than 5.5 eV form endohedral fullerenes in significant amounts, but elements with ionization energy lower than 5.5 eV do not prefer to form endohedrals (see Supporting Information). This observation led us to the thought that metal in ionized form is essential for creation of endohedral fullerenes. It would appear that the RF furnace does not provide sufficient energy to thermally ionize elements with high ionization energy like Sc. To increase the amount of metal ions in the gas phase, we introduced low temperature plasma in the fullerene formation area for the whole duration of the synthesis (see Supporting Information). Plasma-coupled RF fields used for chemical synthesis have been reported already in the literature.29 Tests with Ba showed to have no remarkable difference in yield of endohedral fullerenes, but plasma coupled synthesis with Sc as a foreign element demonstrated a great difference in the amount of endohedral species obtained. By modifying pressure conditions and Sc amount in the gas phase, it is possible to almost completely suppress the formation of empty higher fullerenes, and mostly endohedral cages form, even if the metal was placed above the carbon body, as can be seen from the chromatogram in Figure 3. Although under these conditions the overall yield of fullerenes decreases, this result demonstrates that almost no annealing process takes place inside the heated carbon cylinder. Effective incorporation of the metal in the cage shows that that it does not have any effect on the early stages of fullerene formation, but it is required before the annealing process. In Figure 4 the difference in yield with and without plasma is being demonstrated. Synthesis and Separation of Sc4C82. The Sc soot extract was analyzed by means of off-line MS analysis. According to LDI-TOF mass spectra, besides empty fullerenes four Sc endohedral fullerene families were observed. Namely, Sc2C2n (n ) 37-50), Sc3N@C2n (n ) 34 and 40), and ScmC82 (m ) 1-4). The majority of these species were reported previously

7628

J. Phys. Chem. C, Vol. 114, No. 17, 2010

Figure 4. HPLC profile toluene extract of typical experiment with scandium, where the difference in endohedral fullerene yield with (top) and without plasma (bottom) is being demonstrated. Fraction constitution according to MS data. Several empty giant fullerene species are being omitted for clarity.

using the arc-discharge method.23 However, several new species, which form in remarkable amounts, namely, Sc3C2n (n ) 44-49), were detected in the higher fullerene fraction (see Supporting Information). In addition, the distribution of different compounds was found significantly different, in comparison to the ones reported already in the literature. In the ScmC82 (m ) 1-4) family of endohedral fullerenes, especially, the Sc4C82 formed in rather high amounts. The smaller members of this family, ScC82,30 Sc2C82,31,32 and Sc3C8233 have already been characterized. The Sc3C82 member was later demonstrated to enclose a scandium carbide cluster corresponding to a Sc3C2@C80 fullerene.34 Sc4C82 was only mentioned as a byproduct in the synthesis of Sc endohedrals, but no additional data were provided.31 Quantum chemical calculations predict the Sc4C2 cluster inside the Ih symmetric cage C80 (isomer number 7)35 and also show that the valence state Sc4C2@C80 is thermodynamically and kinetically much more favorable than the [email protected] According to these calculations, the Sc4C2@C80 structure attains a stable closed shell electronic configuration with substantial HOMO-LUMO gap (kinetic stability) and has the formal valence state [Sc3+][email protected] Wang et al. have recently reported for the first time the synthesis, isolation, and characterization of this elusive fullerene.36 13C NMR measurements unambiguously confirm the presence of the Ih-C80 cage. The spectral pattern is fully consistent not only with the pattern that all other Ih-C80 endohedral fullerenes exhibit36 but also with the DFT-predicted one. Furthermore, DFT calculations demonstrate that this molecule has a unique Russiandoll-type structure, C2@Sc4@C80, unlike any other endohedral fullerene yet encountered. Since this uncommon molecule had formed in our RF furnace in an appropriate amount, we were able to separate it in pure form. Mass spectrum, isotopic distribution, and UV-vis spectra are presented in Figures 5 and 6. Nonetheless, the amount synthesized was not enough for such detailed investigations, as those from Wang et al. discussed above, thus necessitating a deductive determination of its structure. It is well-known that electronic absorptions of endohedral fullerenes depend heavily on the structure and charge of the cage and do not depend on the nature of the encapsulated part.23 It was found that absorption spectra for different

Krokos

Figure 5. Positive LDI-TOF mass spectrum of the isolated Sc4C2C80 fullerene demonstrating the high purity of the separated sample and isotopic distribution pattern. Measured (bottom) and calculated (top).

Figure 6. UV-vis spectra of purified Sc4C2@C80 in toluene.

endohedral are very similar irrespective of the kind of enclosed moiety when the cage isomer and the charge state are the same.23,37 However, it was demonstrated that large clusters could influence the frontier orbitals of the cage and cause a shift of absorption spectra.38,39 In addition, with the size of the moiety, when examining endohedral fullerenes with the same cage, a minor difference in the amount of the electron exchange between the encapsulated cluster and the fullerene cage is considered to also cause shifting of absorption bands.39 The effect the cage charge has on absorption spectra was recently examined by Zhang et al.40 Specifically, two endohedral fullerenes were modeled in their calculations, namely, Sc2C2@C68 and Sc3N@C68, together with their corresponding absorption spectra. Their investigations demonstrated that an increase of π-charges on the cage resulted in a larger absorption cross section, i.e., red shifting. Regarding the family of Ih-C80 endohedral fullerenes, in all known Me3N@C80 species the Me3N cluster donates six electrons to the fullerene cage, thus attaining the same electronic structure. Nevertheless, significant red shifting of the absorption spectra of Ih-C80 endohedral fullerenes has been

Plasma Coupled Radio Frequency Furnace

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7629

observed when the size of the encapsulated cluster increases.41-43 Therefore, it is not unreasonable to assume that a Sc4C2@C80 fullerene would have a similar spectrum but even more redshifted, due to the size of the cluster encapsulated, influencing even more its host. Indeed, the spectrum of separated Sc4C2@C80 shows remarkable similarities with other Ih-C806- cages, with their difference being located in the shifting of its absorption toward greater wavelengths (see Supporting Information). Shifting of absorption spectra can additionally be attributed to slight disparities in the extent of electron transfer from different clusters to the fullerene cage.39,40 Nevertheless, these investigations are beyond the scope of this document and were not undertaken. Another point that supports the Sc4C2@C80 structure is its retention time which is shorter than that of Sc3N@C80 (Figure 3). This is in good agreement with the correlation found between the size of the moiety and the retention time for the Y2Cn@C82 (n ) 0, 2) and the ScxYyN3@C80 families where the greater size of the cluster results in shorter retention time.39,43

Supporting Information Available: Image of the RF furnace in operation under conditions with plasma, comparison of UV-vis spectra of different Ih-C806- endohedral fullerenes, chromatogram of recycling and isolation of Sc4C2@C80 species, chromatogram of toluene extract of a typical experiment with Sc concentrating in the higher fullerene zone plus mass spectrometer analysis of it, and diagram demonstrating the correlation between the first ionization energy of different metals and the ability to form endohedral fullerenes with the temperatures reached in different production techniques. This material is available free of charge via the Internet at http://pubs.acs.org.

Experimental Section

References and Notes

For experiments without the use of plasma, the standard setup has been described in detail previously.16 For scandium endohedral production, the RF furnace was coupled with lowtemperature plasma.29 A mixture of Sc2O3 (500 mg) with graphite (100 mg) or pure scandium (250 mg) was used as a source of scandium in the gas phase. The optimal conditions for Sc endohedral production were under 400 mbar pressure and 5 mL/s He flow. The obtained soot after 18 runs was Soxhlet extracted with CS2, and the solvent was afterward gradually changed to toluene. For MS off-line analysis, fractions were collected every 1 min using an analytical buckyprep column with toluene as the mobile phase. The isolation of Sc4C2@C80 was performed in three-step HPLC separation. First the endohedral containing fraction was separated from C60, C70, and higher fullerenes using the preparative Buckyprep - M column (toluene 20 mL/min). Afterward, the fraction containing Sc4C2@C80 together with C84 and Sc2@C76 was collected using the analytical Buckyprep column (toluene 1 mL/min). Pure Sc4C2@C80 was obtained with recycling 9 times using Buckyprep column with a mixture of toluene-hexane (3:1) as a mobile phase. The comparatively low amount of Sc2@C76 is a key point to the successful isolation of the Sc4C2@C80 compound.

(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Taylor, R.; Hare, J. P.; Abdulsada, A. K.; Kroto, H. W. J. Chem. Soc., Chem. Commun. 1990, 20, 1423–1424. (3) Ettl, R.; Chao, I.; Diederich, F.; Whetten, R. L. Nature 1991, 353, 149–153. (4) Diederich, F.; Whetten, R. L.; Thilgen, C.; Ettl, R.; Chao, I.; Alvarez, M. M. Science 1991, 254, 1768–1770. (5) Kikuchi, K.; Nakahara, N.; Wakabayashi, T.; Suzuki, S.; Shiromaru, H.; Miyake, Y.; Saito, K.; Ikemoto, I.; Kainosho, M.; Achiba, Y. Nature 1992, 357, 142–145. (6) Johnson, R. D.; DeVries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355, 239–240. (7) Delgado, J. L.; Herranz, Ma. A.; Martin, N. J. Mater. Chem. 2008, 18, 1415–1426. (8) Kraetschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354–358. (9) Taylor, R.; Langley, G. L.; Kroto, H. W.; Walton, D. R. M. Nature 1993, 366, 728–731. (10) Taylor, R.; Langley, G. L. J. Proc. Electrochem. Soc. 1994, p. 68– 79. (11) Howard, J. B.; McKinnon, J. T.; Makarovsky, Y.; Lafleur, A. L.; Johnson, M. E. Nature 1991, 352, 139–141. (12) Howard, J. B.; McKinnon, J. T.; Johnson, M. E.; Makarovsky, Y.; Lafleur, A. L. J. Phys. Chem. 1992, 96, 6657–6662. (13) Howard, J. B.; Lafleur, A. L.; Makarovsky, Y.; Mitra, Y.; Pope, C. J.; Yadav, T. K. Carbon 1992, 30, 1183–1201. (14) Pope, C. J.; Marr, J. A.; Howard, J. B. J. Phys. Chem. 1993, 97, 11001–11013. (15) Okeefe, A.; Ross, M. M.; Baronavski, A. P. Chem. Phys. Lett. 1986, 130, 17–19. (16) Peters, G.; Jansen, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 223– 224. (17) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500–1503. (18) Amsharov, K. Yu.; Jansen, M. Chem. Commun. 2009, 19, 2691– 2693. (19) Amsharov, K. Yu.; Jansen, M. J. Org. Chem. 2008, 73, 2931– 2934. (20) Hirsch, A.; H.; Brettreich, M. Fullerenes chemistry and reactions; LinkWiley-VCH: Weinheim, 2005. (21) Flot, D.; Friese, K.; Haufe, O.; Modrow, H.; Panthofer, M.; Reich, A.; Rieger, M.; Wu, G.; Jansen, M. Mol. Nanostruct.: XVII International Winterschool Euroconference on Electronic Properties of NoVel Materials, AIP Conf. Proc. 2003, 685, 37-40. (22) Shinohara, H. Fullerene: Chem. Phys. Technol.; Kadish, K. M., Ruoff, R. S., Eds.; Wiley: New York, 2000; p 357. (23) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843–892. (24) Nagase, S.; Kobayashi, K.; Akasaka, T.; Wakahara, T. Fullerene: Chem. Phys. Technol. 2000, 395. (25) Kubozono, Y.; Ohta, T.; Hayashibara, T.; Maeda, H.; Ishida, H.; Kashino, S.; Oshima, K.; Yamazaki, H.; Ukita, S.; Sogabe, T. Chem. Lett. 1995, 6, 457–458. (26) John, T.; Dennis, S.; Shinohara, H. Appl. Phys. A: Mater. Sci. Process. 1998, 66, 243–247.

Conclusion In the present work, we showed for the first time that the endohedral element is encapsulated in the carbon cage at the last stage of the fullerene evolution. It was found that metals can be used as an effective marker to locate the area of annealing process. The introduction of low-temperature plasma in this zone has a significant impact on the endohedral fullerene formation process. The increased yield seems to be caused by the presence of metal cations in the gas phase, which catch carbon clusters before the annealing process takes place. Moreover, the data obtained show that the RF furnace has a high potential for the investigation of the fullerene formation mechanism in general. Namely, the intermediate products can be quenched by direct introduction of reactive agents in the formation zone thus avoiding the main problem of mechanism investigation, the influence of reactive agents on the first stages of fullerene evolution.20 Additionally, the Sc4C2@C80 fullerene has been synthesized, isolated, and spectroscopically characterized. Although the

amount produced was not enough for additional measurements, the data obtained confirm the Sc4C2@C80-Ih structure. Acknowledgment. The author thanks Dr. Konstantin Yu. Amsharov for his invaluable help during the experiments and the evaluation of the results.

7630

J. Phys. Chem. C, Vol. 114, No. 17, 2010

(27) Rose, H. R.; Dance, I. G.; Fisher, K. J.; Smith, D. R.; Willett, G. D.; Wilson, M. A. J. Chem. Soc., Chem. Commun. 1993, 17, 1361– 1362. (28) Crowley, C.; Taylor, R.; Kroto, H. W.; Walton, D. R. M.; Cheng, P. C.; Scott, L. T. Synth. Met. 1996, 77, 17–22. (29) Yoshie, K.; Kasuya, S.; Eguchi, K.; Yoshida, T. Appl. Phys. Lett. 1992, 61, 2782–2783. (30) Ito, Y.; Fujita, W.; Okazaki, T.; Sugai, T.; Awaga, K.; Nishibori, E.; Takata, M.; Sakata, M.; Shinohara, H. ChemPhysChem 2007, 8, 1019– 1024. (31) Wang, C. R.; Inakuma, M.; Shinohara, H. Chem. Phys. Lett. 1999, 300, 379–384. (32) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Saito, Y.; Wang, X. D.; Hashizume, T.; Sakurai, T. J. Phys. Chem. 1993, 97, 4259– 4261. (33) Takata, M.; Nishibori, E.; Sakata, M.; Shinohara, H. Fullerene Based Materials: Structures and Properties (Structure and Bonding); Springer: New York, 2004; Vol. 109, pp 59-84. (34) Liduka, Y.; Wakahara, T.; Nakahodo, T.; Tsuchiya, T.; Sakuraba, A.; Maeda, Y.; Akasaka, T.; Yoza, K.; Horn, E.; Kato, T.; Liu, M. T. H.; Mizorogi, N.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 2005, 127, 12500–12501.

Krokos (35) Tan, K.; Lu, X.; Wang, C. R. J. Phys. Chem. B 2006, 110, 11098– 11102. (36) Wang, T. S.; Chen, N.; Xiang, J. F.; Li, B.; Wu, J. Y.; Xu, W.; Jiang, L.; Tan, K.; Shu, C. Y.; Lu, X.; Wang, C. R. J. Am. Chem. Soc. 2009, 131, 16646–16647. (37) Bucher, K.; Epple, L.; Mende, J.; Mehring, M.; Jansen, M. Phys. Status Solidi B 2006, 243, 3025–3027. (38) Akiyama, K.; Sueki, K.; Kodama, T.; Kikuchi, K.; Ikemoto, I.; Katada, M.; Nakahara, H. J. Phys. Chem. A 2000, 104, 7224–7226. (39) Inoue, T.; Tomiyama, T.; Sugai, T.; Okazaki, T.; Suematsu, T.; Fujii, N.; Utsumi, H.; Nojima, K.; Shinohara, H. J. Phys. Chem. B 2004, 108, 7573–7579. (40) Cheng, W. D.; Hu, H.; Wu, D. S.; Wang, J. Y.; Huang, S. P.; Xe, Z.; Zhang, H. J. Phys. Chem. A 2009, 113, 5966–5971. (41) Iezzi, E. B.; Duchamp, J. C.; Fletcher, K. R.; Glass, T. E.; Dorn, H. C. Nano Lett. 2002, 2, 1187–1190. (42) Chen, N.; Fan, L. Z.; Tan, K.; Wu, Y. Q.; Shu, C. Y.; Lu, X.; Wang, C. R. J. Phys. Chem. C 2007, 111, 11823–11828. (43) Yang, S.; Kalbac, M.; Popov, A.; Dunsch, L. ChemPhysChem 2006, 7, 1990–1995.

JP911321E