Carbon 41 (2003) 1375–1380
Endohedral metallofullerenes M@C 82 (M5La, Y): synthesis and transport properties I.E. Kareev a,b , V.P. Bubnov a , *, E.E. Laukhina a , V.K. Koltover a , E.B. Yagubskii a a
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka 142432, Moscow Region, Russia b Chemistry Department, Moscow State University, Moscow 119899, Russia Received 13 August 2002; accepted 31 January 2003
Abstract Endohedral metallofullerenes M@C 2n (M5La, Y) were synthesized by the arc-discharge method using optimum electric arc parameters. It was also shown that an organic solvent (N,N-dimethylformamide) is successfully used in selective extraction of M@C 2n . We identified the resulting products by mass spectrometry, electron spectrophotometry, EPR and X-ray fluorescence spectroscopy. The resulting N,N-dimethylformamide extracts of La@C 2n and Y@C 2n are shown to be free of empty fullerenes and appear as a mixture of endometallofullerenes M@C 2n whose main ingredient is M@C 82 (|80 wt%). Transport properties of the extracts of La@C 82 and Y@C 82 were studied using pressed pellets. It was found that oxygen and heat treatment affect conductivity of endometallofullerenes. Heat treatment results in a three-order increase of conductivity from 10 25 upto 10 22 Ohm 21 cm 21 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerene; B. Arc discharge, Heat treatment; C. X-ray diffraction; D. Chemical structure
1. Introduction Endohedral metallofullerenes (EMFs) M@C 2n (M5La, Y and others) are carbon clusters with metal atoms encapsulated inside the fullerene cage. The formation of such compounds is the most typical for C 82 with the group III metals (Sc, Y, and the majority of the lanthanides) [1–3]. The unique structure, physical and chemical properties of EMFs are of great interest as new promising materials with unique magnetic, electrical and optical properties [3,4]. However, the properties of these compounds have not been well studied, due to their unavailability on a large scale. The conducting properties of EMFs have not been investigated at all. The production of EMFs is one of the current problems in studying the chemistry and physics of carbon clusters. It is reported that the EMF content in soot does not exceed 1 wt% [5,6]. We have recently suggested a new procedure for the stepwise extraction of EMFs from endometallofullerene-containing soots [7,8]. The extraction scheme is based on the fact that La@C 82 possesses a large electric dipole moment (P 5 *Corresponding author. Fax: 17-096-515-3588. E-mail address:
[email protected] (V.P. Bubnov).
3.8D) in contrast to molecules of empty fullerenes. Hence, it was concluded that a sample of the EMF-containing soot should be first treated with a nonpolar solvent to extract empty fullerenes, while the rest of the soot should then be treated with a polar solvent to extract M@C 2n [8]. Organic solvents with the values of electric dipole moment close to that of EMFs are preferable for M@C 2n isolation. Here we describe the arc synthesis of EMFs, which together with the selective extraction scheme provides a real possibility to obtain EMFs in larger amounts, and to avoid or simplify the laborious HPLC technique. The transport properties of the N,N-dimethylformamide (DMF) extracts of La@C 82 and Y@C 82 have also been studied.
2. Experimental Endometallofullerene-containing soots were synthesised in an electric reactor by a DC arc-discharge method. The design of the reactor was described in previous papers [8,9]. Analytic grade La– and Y–composite graphite electrodes were used to obtain EMFs La@C 82 and Y@C 82 . A hole 3 mm in diameter was drilled in the centre of a graphite rod (63160 mm). Lanthanum or yttrium
0008-6223 / 03 / $ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00064-2
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metallic filings were mixed with graphite powder and graphite cement (Dylan Industries Inc.) as a binder. The prepared mixture was thoroughly stirred, and then packed into the hole of the graphite electrode. The La / C or Y/ C ratio was varied from 0.5 to 1.5 at%. The M / C ratio ¯1.0 at% was found to be optimum for EMF formation. The modified electrode rods were thermally treated in three stages: (1) treatment in a vacuum furnace at 130 8C for 4–5 h; (2) heating in vacuum (10 23 Torr) at 1100 8C for 4 h; and (3) thermal treatment directly in a reactor in vacuum (10 23 Torr) at 1800–1900 8C for 1 h; the temperature was attained by passing a direct current of 190–200 A through the electrodes. The optimum conditions of arc evaporation of the electrodes were as follows [8]: helium pressure 120 Torr, direct current 90 A, voltage 28–30 V, arc length 5 mm, the distance between arc and a cooled wall of the reactor 50 mm, evaporation rate 1 mm min 21 . EMFs M@C 2n (M5La, Y) were extracted from the endometallofullerene-containing soot at a solvent boiling point under argon according to the two-step scheme: nonpolar–polar organic solvent. Firstly the soot was treated with o-xylene until the next fresh solvent portion was visually colourless. The o-xylene extracts (the yield 1.5–2 wt%) contained C 60 (|80 wt%), C 70 (|20 wt%) and M@C 82 (|0.1 wt%). At the second step of extraction, polar N,N-dimethylformamide was used. The resulting solution was filtered off, the solvent was evaporated, and the residue was dried at 90 8C for 1 h in vacuum. The yield of the extracts after the second step reached up to 6 wt% of La@C 2n , or up to 8 wt% of Y@C 2n , relative to the starting soot mass. The mass spectrometry analysis was performed by the laser desorption / ionisation (LDI–TOF) technique using a Finnigan Vision 2000 time-of-flight mass spectrometer. Radiation pulses of 3 ns from a N 2 laser operating at 337 nm were used to desorb the species and the negative ions formed were detected in the linear mode. In addition, a MSBKh mass-spectrometer using ionisation by 2 Cf decay products was applied. Note that for o-xylene extracts the positive ions were detected in the linear mode. Elemental analysis for La and Y was performed using X-ray fluorescence spectroscopy on a VRA-30 analyser. Optical spectra of La@C 82 and Y@C 82 (DMF extracts in o-dichlorobenzene) were recorded in the near IR range (700–1100 nm) with a Hewlett Packard-8453 spectrophotometer. EPR spectra were taken on the X-band Varian E-104A radiospectrometer at room temperature. The samples were prepared as solutions in o-dichlorobenzene in standard quartz tubes with an inner diameter of 3 mm. The samples were thoroughly deoxygenated before the tubes were sealed in vacuum. Electrical conductivity was measured by a conventional four-probe DC method on pressed pellets of EMFs extracts in a helium atmosphere at temperatures ranging from 4.2 up to 300 K. The pellets 7.531.531 mm in size were
pressed at 3800 kg cm 22 . Electrical contacts were made using conducting graphite paste. To exclude errors associated with thermopower, the sample electrical resistivity was measured for two opposite current directions.
3. Results and discussion All the extracts of the M@C 2n -containing soot have been characterised by mass spectrometry. The most intense peaks in the mass spectra of the o-xylene extracts corre1 spond to the molecular masses of C 1 60 (m /z5720) and C 70 (m /z5840) while the weakest peaks correspond to the molecular masses of the highest fullerenes and M@C 82 . The mass spectrum of o-xylene extract of the Y@C 2n containing soot is presented in Fig. 1a. The mass spectra of the o-xylene extracts indicate that empty fullerenes are preferably selected during the first stage of the extraction procedure. Mass spectra of the DMF extracts containing Y@C 2n and La@C 2n are shown in Figs. 1b and 2. All the mass spectra of DMF extracts exhibit molecular masses which correspond to the M@C 2 2n molecular ions. It should be noted that the mass spectra as a result of ionisation by 252 Cf decay products contain a poorer set of the M@C 2n
Fig. 1. Mass spectra of two extracts from the soot sample containing Y@C 2n : o-xylene extract (a) and DMF extract (b); ionisation by 252 Cf decay products.
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Fig. 2. Mass spectra of the DMF extract from the La@C 2n containing soot: laser desorption / ionisation.
molecular masses as compared to that of LDI mass spectra (Figs. 1b and 2). For example, the LDI mass spectrum of the DMF extract of the La@C 2n -containing soot exhibits a great variety of peaks in the range of molecular masses from 1099 to 1243 which correspond to the molecular ions 2 from La@C 2 80 to La@C 92 (Fig. 2). Whatever type of ionisation, the mass spectra of DMF extracts show no molecular masses of empty fullerenes (C 60 , C 70 , and the higher ones); the most intensive peak always corresponds to M@C 82 (see Figs. 1b and 2). The absence of empty fullerenes in the second stage extracts is also confirmed by the optical spectra. Table 1 represents the data on the composition of the extracts obtained by the two-step sequence. These data indicate that the o-xylene extracts are mainly mixtures of empty fullerenes and contain a very small amount of M@C 2n , while the DMF extracts contain EMFs and very few or even no empty fullerenes. The results of the X-ray fluorescence analysis show the content of La or Y in the DMF extracts to be 11.8–12.3 or 7.9–8.1 wt%, respectively. It is seen that the metal content almost coincides with the theoretical lanthanum content in La@C 82 (12.4 wt%) and yttrium content in Y@C 82 (8.3 wt%). According to the elemental analysis data, all the EMF samples dried up to the fixed weight contain about 2–2.4 wt% of N (see Table 1), which corresponds to a DMF content of about 11–13 wt% in the extracts. According to the IR spectra EMFs form complexes with DMF. The IR spectrum of the DMF extract showed absorption bands arising from the stretching vibrations (2925 and 2854 cm 21 ) and from the deformation vibrations (1387 cm 21 ) of the methyl groups of DMF along with a band at 1652 cm 21 arising from the stretching vibrations of the carbonyl group of DMF. The latter was displaced by 20 cm 21 to lower frequency compared to pure DMF (1652 vs. 1672 cm 21 ). This shift may be due to an interaction between DMF and EMFs. To
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remove the co-crystallised solvent, the extracts were heated under vacuum for several hours at temperatures higher than 100 8C. EPR was used as the principal method in identification of paramagnetic EMFs [3]. Fig. 3 shows the EPR spectra of the DMF extracts in o-dichlorobenzene for two different soot samples containing La@C 2n or Y@C 2n . Both spectra are identical to well-known spectra of La@C 82 and Y@C 82 in chlorobenzene, toluene, o-xylene, and CS 2 [3,10,11]. The explanation of the second octet (Fig. 3a) or doublet (Fig. 3b) is given by the existence of two different isomers of the La@C 82 or Y@C 82 molecule. It should be noted that in addition to the major signals of La@C 82 or Y@C 82 some weak EPR lines are observed in the spectra. These weak lines obviously arise from minor amounts of other La@C 2n or Y@C 2n , respectively, and the lines of the hyperfine structure on 13 C also contribute to the total intensity. The data obtained by mass-spectrometry, X-ray fluorescence spectroscopy, optical spectrophotometry and EPR spectroscopy show the main components of the DMF extracts to be La@C 82 and Y@C 82 . The EPR spectra for the extracts in o-dichlorobenzene solution gave signals with a hyperfine structure corresponding to two isomers of La@C 82 or Y@C 82 . Other EMFs M@C 2n (n540, 42–46) were found in small amounts in the extracts, while empty fullerenes were almost absent. Thus optimisation of the conditions of electrode evaporation in the arc and an efficient method for selective extraction allow a record yield of EMFs with lanthanum and yttrium of up to 6–8 wt% of the weight of the soot. Conductivity measurements showed low conductivity (s295 |10 25 Ohm 21 cm 21 ) of the EMFs extracts. Heat treatment of the extracts in vacuum resulted in higher conductivity due to partial solvent loss. Fig. 4 shows the dependency of room temperature conductivity of Y@C 82 EMFs vs. solvent content. It is seen that conductivity of pellets increases by several orders of magnitude with a reduction of the amount of solvent. The solvent was removed from the DMF extract of Y@C 82 EMFs by heating in vacuum at 220, 300, 350, and 400 8C for 6 h. The solvent content was monitored by elemental analysis for N. It was found that exposing the La@C 82 pressed pellet which had been heat treated in vacuum at 220 8C to air results in a drop of conductivity. The resistance of the pellet increased from 340 up to 1100 Ohm during |3 days. However, heat treatment in vacuum at 120 8C for 2 h results in full recovery of the initial value of conductivity of the pellet. These experiments were reproduced and the initial low-resistant state of the La@C 82 sample was recovered in every experiment (Fig. 5). The exposure of the pellet to air seems to be accompanied by the interaction of endometallofullerenes with oxygen thus resulting in the
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Solvent
P, D
bp (8C)
Yield (%) of extract from M@C 2n containing soot
Extract composition Content of fullerenes (%)a
Content of nitrogen (N) and metal (M)b
M
%
C 60
C 70 1C 2n
M@C 2n solvated
o-Xylene
0.62
143
La Y
2.5 2–3
80 80
¯20 ¯20
.0.1 .0.1
DMF
3.86
152
La Y
3–6 4–8
–
–
¯100 ¯100
a b
The content of empty fullerenes was determined by the optical spectroscopy, the presence of M@C 2n in o-xylene extracts was tested by the EPR method. The content of nitrogen was determined by conventional elemental analysis; the amount of metal was found from the X-ray fluorescence spectroscopy.
N (wt%)
M (wt%)
– – 2.38 2.23
12.360.2 8.160.2
I.E. Kareev et al. / Carbon 41 (2003) 1375–1380
Table 1 The yield and the composition of different extracts obtained from the M@C 2n containing soot
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Fig. 5. Dependency of resistance of La@C 82 DMF extract pellet vs. time of exposure to air and heat treatment in vacuum.
o-dichlorobenzene [12]. The EPR spectra of these samples are characterised by high sensitivity of the hfs lines to the presence of oxygen in solution. The effect of oxygen on a spectral linewidth was found to reversible. However, long duration (2–3-week) exposure of the solutions to air resulted in irreversible changes in the EPR spectra.
Fig. 3. EPR spectra of the DMF extracts obtained from the M@C 2n -containing soots in o-dichlorobenzene: M5La (a) and M5Y (b).
4. Conclusion The use of optimal parameters relating to the electric arc and the efficient extraction method may bring the yield of M@C 2n (M5La, Y) to the level of 6–8% of the primary soot. Conducting properties of EMFs were studied for the fist time. The conductivity of solvent-free EMFs is 10 22 Ohm 21 cm 21 . This value is more than five orders of magnitude higher than that for empty fullerenes. Oxygen affects the conducting properties of EMFs. The interaction of EMFs with oxygen was found to be reversible.
Acknowledgements
Fig. 4. Dependency of room temperature conductivity of Y@C 82 DMF extract vs. N,N-dimethylformamide content.
increased resistance. Heat treatment of the pellet in vacuum enables the removal of oxygen and the recovery of the initial value of conductivity. Therefore, the interaction of EMFs with oxygen is reversible. A similar effect was observed in solutions of the EMFs La@C 82 and Y@C 82 in
This work was supported by the Russian Foundation for Basic Research (Project No. 02-03-33352 and No. 00-0306364), Russian State Research Programme (project ‘Guided Synthesis Fullerenes and Other Atomic Clusters’) and the Program of Russian Academy of Sciences (project ‘Nanocarbon Materials’).
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