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ARTICLES Influence of an Extended Fullerene Cage: Study of Chemical and Electrochemical Doping of C70 Peapods by in Situ Raman Spectroelectrochemistry Martin Kalbac,*,†,‡ Ladislav Kavan,‡ Marke´ ta Zukalova´ ,‡ and Lothar Dunsch† Group of Electrochemistry and Conducting Polymers, Leibniz Institute of Solid State and Materials Research, Helmholtzstrasse 20, D-01069 Dresden, Germany, and J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18223 Prague 8, Czech Republic ReceiVed: May 22, 2006; In Final Form: August 22, 2006
C70 peapods (C70@SWCNT) have been studied for the first time under chemical and electrochemical doping by in situ Raman spectroelectrochemistry. The bleaching of Raman features of single-walled carbon nanotubes (SWCNT) and the downshift of the frequency of their tangential mode confirmed the strong chemical doping of C70@SWCNT by potassium vapor. The changes of C70 band intensities and positions upon K-vapor doping indicate the insertion of potassium into the interior of a peapod and the chemical reduction of C70. The subsequent treatment of the K-doped peapod with water caused only a partial dedoping as the potassium outside the peapod is removed while that inside the tube remains intact. Electrochemical charging of the partially dedoped C70 peapods differs from that of C60@SWCNT. The most striking feature was the enhancement of the C70-related Raman bands during anodic charging. The results are interpreted in terms of different electronic structures of C60 and C70 peapods, and their specific feedback to chemical and electrochemical doping.
1. Introduction The empty space inside a single-walled carbon nanotube (SWCNT) offers a possibility for filling with various molecules. At present, fullerenes belong to the most frequently used ones. The filling of SWCNT with C60 fullerene was first reported by Smith et al.1 Supposing a high filling factor, the product, denoted as a peapod, represents an example of a one-dimensional crystal. The changes in the electronic structure of such a carbon nanostructure caused by the doping substantially influence the properties of these materials. This doping can be performed electrochemically or chemically. Electrochemical doping of peapods is preferred because of the precise and easy control of the doping level.2,3 Nonaqueous electrolyte solutions have a sufficiently large potential window from ca. -1.5 to +1.5 V (vs Ag/AgCl reference electrode). However, the electrochemically and chemically doped samples do not reveal the same properties.3-5 The characteristic softening of the C60 fullerene related Ag(2) mode after chemical n-doping was not detectable during electrochemical charging at cathodic potentials. Furthermore, no bands corresponding to a C60 polymer were observed in cathodically n-doped peapods.4 We have demonstrated previously that both electrochemical and chemical doping of C60@SWCNT are governed by different mechanisms.3,5 The electrochemical doping leads to the charging of the SWCNT wall, which slightly affects the intratubular fullerene. On the other hand, the chemical doping with gaseous potassium causes * Corresponding author. Telephone: 420 2 6605 3804. Fax: 420 2 8658 2307. E-mail:
[email protected]. † Leibniz Institute of Solid State and Materials Research. ‡ Academy of Sciences of the Czech Republic.
a strong redox doping of both SWCNT and the intratubular C60 as the potassium vapor penetrates the tube to form exohedral alkali metal fulleride inside a peapod.3,5 This effect was confirmed by Iijima et al. via a direct transmission electron microscopic observation.6 The mechanism of the entering of the alkali metal into SWCNT is not clear. Nevertheless it is suggested that peapods contain a number of “nanowindows”, which are opened at elevated temperature and allow the entering of an alkali metal into the peapod. This is also supported by the theoretical calculation of Tomanek et al.,7 which shows that penetration of C60 into the SWCNT is more effective through the defect in the nanotube wall than through the open ends. The strong interaction between fullerene and alkali metal is suggested to increase the stability of the system. Shinohara et al.8 have shown that exohedral metallofullerene peapod can be formed directly from a fulleride. The exohedral metallofullerenes in peapods possess exceptional stability as they are protected by the nanotube wall from the environment.5,8 Even the treatment of the samples by water did not lead to a complete removal of the alkali metal ions from the doped fullerenes inside the nanotube.5 Consequently, exohedral fullerene peapods were shown to be novel fullerene-based conductors with reduced dimensionality compared to fullerene films.9 Up to now only the chemical doping of C60@SWCNT has been studied in detail.3,5,6,10 However, it was shown that the electrochemical doping of C70@SWCNT differs from that of C60@SWCNT, which pointed at different electronic structures of these two materials.4 Thus it is challenging to investigate the chemical doping of C70@SWCNT. In the present study we show that the results measured on C70@SWCNT are indeed
10.1021/jp063131g CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006
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significantly different from those for C60@SWCNT. Despite the analogous process of K-vapor penetration into the C70 and C60 peapods, there are striking differences between the redox behavior and spectroscopic pattern of C70@SWCNT and C60@SWCNT. 2. Experimental Section The samples of SWCNT and C70@SWCNT peapods (filling ratio 85%) were available from our previous work.4 For the chemical doping, the SWCNT and C70@SWCNT samples were outgassed at 285 °C/10-5 Pa (the residual gas was He) and subsequently exposed at 177 °C to potassium vapor for 25-40 h. The reaction took place in an all-glass ampule interconnected to a Raman optical cell with Pyrex glass window. The rest of the potassium was finally distilled off at 200 °C/10-5 Pa with cooling of the opposite end of the ampule. The water treatment has been done by the following protocol: A part of potassium-doped C70@SWCNT sample (in the form of buckypaper) was treated in water at 90 °C for 2 h in contact with air. Then it was removed from the solution and thoroughly washed with water. Finally the sample was sonicated for 15 min in a large excess of ethanol (containing 5% water). A thin-film electrodes were prepared by evaporation of the resulting slurry on a Pt electrode in air. The film electrode was outgassed overnight at 90 °C in vacuum and then mounted in a spectroelectrochemical cell in a glovebox. The cell was equipped with a Pt counter electrode and Ag-wire pseudoreference electrode. A 0.2 M LiClO4 solution in dry acetonitrile was used as the supporting electrolyte solution. After spectroelectrochemical measurements ferrocene (Fc) was added to the cell and the potential of the Fc/Fc+ couple was measured for reference. The electrode potentials related to the Ag pseudoreference electrode were recalculated and related to the Fc/Fc+ couple. Electrochemical experiments were carried out using PG 300 (HEKA) or EG&G PAR 273A potentiostats. The Raman spectra were measured on a T-64000 spectrometer (Instruments, SA) interfaced to an Olympus BH2 microscope (the laser power impinging on the sample or cell window was between 1 and 5 mW). Spectra were excited by an Ar+ laser at 2.41 and 2.54 eV (Innova 305, Coherent). The Raman spectrometer was calibrated before each set of measurements by use of the F1g line of Si at 520.2 cm-1. 3. Results and Discussion 3.1. K-Vapor Doping: SWCNT-Related Raman Features. The C70@SWCNT samples were strongly doped with potassium vapor and subsequently dedoped by water treatment. In order to estimate the level of doping of C70@SWCNT, we used C60@SWCNT as a probe. Both samples were doped in the same cell to ensure identical conditions for doping. Figure 1 shows the spectrum of potassium-doped C60@SWCNT in the Ag(2) region. The Ag(2) mode of C60 fullerene is known to be redshifted linearly with the number of electrons at the C60 cage. The softening is about 6.5 cm-1 per electron.11 The Ag(2) mode of our heavily doped C60 peapod is at 1428 cm-1, which indicates that six electrons are transferred to the C60 cage. This is in accordance with previous studies.10,12 The observed downshift is slightly larger than expected for C606-, which was explained previously by the formation of a polymer.10,12 However, it could be alternatively explained by the influence of the surrounding tube or the one-dimensional assembly of fullerenes. The Raman spectra of K-doped C70@SWCNT on Figure 2 point at the high level of doping of the peapod sample. The doping causes an almost complete vanishing of the SWCNT
Figure 1. Raman spectra (excited at 2.54 eV) of pristine C60@SWCNT, K-doped C60@SWCNT after treatment with water, and K-doped C60@SWCNT (from top to bottom). Intensities of spectra were normalized using the F1g line of Si at 520.2 cm-1. Spectra are offset for clarity.
features (RBM, D, and TG modes) in the spectra of both empty SWCNT and C70@SWCNT. The bleaching of the SWCNT modes upon charging is consistent with the previously published results.13-15 The potassium doping induces a loss of the resonance enhancement due to the quenching of optical transitions between van Hove singularities. The downshift of the TG mode matches the published results for chemically heavily doped SWCNT.13-15 The strong downshift was suggested to be caused by “phase” transformation of crystalline SWCNT ropes.15 However, a similar behavior has been observed recently even for isolated SWCNT;16 thus, it cannot be interpreted as the doping-induced change in the tube-tube interaction. Despite the unclear reason for the downshift, it still represents a qualitative proof of strong doping. The differences between chemically and electrochemically doped C70@SWCNT are obvious. In contrary to the almost negligible downshift of the tangential displacement mode (TG) under electrochemical n-doping of C70@SWCNT,4 the K-vaportreated C70@SWCNT exhibits a strong downshift of the TG mode. Furthermore, the electrochemical n- or p-doping of C70@SWCNT leads to a monotonic intensity attenuation of all C70 features without any changes in their frequencies.4 Figure 2 shows that the chemical K-doping of C70@SWCNT causes strong changes of the frequencies and intensities of the C70 features. Some particular modes vanished completely upon doping; the intensities of others are just slightly changed. A detailed discussion of the spectra of K-vapor-doped C70@SWCNT is given below. 3.2. K-Vapor Doping: C70-Related Raman Features. The potassium-doping of C60@SWCNT leads to a formal transfer of six electrons to C60 cage as indicated by the corresponding downshift of the C60 pentagonal pinch Ag(2) mode.3,5 Since the conditions of both C70@SWCNT and C60@SWCNT doping were identical and the redox behavior of C60 and C70 films seems to be very similar,17 the transfer of six electrons to the C70 cage can be assumed, too. However, as shown below, the correlation between the shift of the C70 fullerene bands and the number of electrons transferred to the C70 fullerene cage is not as straightforward as for the Ag(2) mode in C60@SWCNT. The potassium doping of C70@SWCNT changes the intensities and frequencies of the C70 features, but the tendency upon doping is not uniform for all bands. Some of the C70 bands are strengthened while others are attenuated and their frequencies are reproducibly shifted to red or blue with respect to pristine C70@SWCNT. Despite a rather weak bleaching of the majority of C70 bands as compared to those of SWCNT modes in the
Chemical and Electrochemical Doping of C70 Peapods
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Figure 2. Raman spectra (excited at 2.54 eV) of pristine C70@SWCNT, K-doped C70@SWCNT after treatment with water, K-doped SWCNT, and K-doped C70@SWCNT (from top to bottom). The intensity of the signal was divided by a factor of 20 in the case of pristine C70@SWCNT. The bands corresponding to C70 features are marked by arrows. Intensities of spectra were normalized using the F1g line of Si at 520.2 cm-1. Spectra are offset for clarity, but the intensity scale is identical for all the spectra in the respective windows.
TABLE 1: Summary of Positions and Relative Intensities of C70 Features in Pristine, Potassium-Doped, and Water-Treated, Potassium-Doped C70@SWCNTa,b pristine sample
K-doped
position, cm-1
position, cm-1
220-267 452 568 696 725 1126 1182 1445 1467
270 456 550 689 nd nd nd 1410 1462
K-doped, water treated
intensity
position, cm-1
intensity
0 0 0 -
nd 459 nd 696 nd 1230 1182 1443 1475
0 0 + 0 0 0 +
a nd, the band was not detected. b The changes in intensity are referred to the intensity of this band in a previous step. The sequence of doping treatment was as follows: pristine f K-doped f K-doped and water-treated. -, intensity was decreased; +, intensity was increased; 0, intensity was identical to that of the previous state.
same spectra, some particular bands of C70 disappear completely (see Table 1). This is the case of the bands at 1182 and 1226 cm-1. The intensities of these bands are similar to those at 1467 cm-1 in the pristine sample. In the doped sample there are no bands corresponding to those at 1182 and 1226 cm-1 of the pristine sample, while the 1467 cm-1 band is still well resolved even after K-doping. The strong decrease in intensity of the two bands at 1182 and 1226 cm-1 was also theoretically predicted for the doping of films of C70 to C706- fulleride.18 The changes in C60 features of C60@SWCNT peapods during K-vapor doping are similar to those in C60 films during chemical
doping.3,5 Hence, a similarity in the behavior of C70 films and C70 in peapods during the doping is expected as well. The radial mode frequencies of K6C70 films are slightly increased compared to the corresponding modes in pristine C70.19 The charge transfer caused by potassium doping induces an increase of the C-C force constant.20,21 However, this effect is compensated by an electrostatic radial restoring force leading finally to a small increase of the radial mode frequencies.19 In this region, the only well-resolved mode of both pristine and doped C70@SWCNT is the band at 452 cm-1 in the pristine material, which upshifts to 456 cm-1 in the doped sample. The band at 270 cm-1 in the doped sample probably corresponds to the quadrupolar modes which are split into several bands between 220 and 267 cm-1 in pristine C70@SWCNT. Thus, the quadrupolar modes are also slightly upshifted upon doping. The small intensity of radial modes is not sufficient for a detailed analysis, but the overall trend in this region confirms the suggested analogy of the doping-induced changes in C70 films and C70@SWCNT. Accordingly, the upshift of the 452 cm-1 and the quadrupolar modes clearly indicates the n-doping of C70 inside a peapod. The values of the upshift both for the 452 cm-1 mode and the quadrupolar modes after doping of C70 to C704- and C706- are very close. Considering the additional influence of the surrounding by the SWCNT wall on the position of the C70 modes, it is difficult to estimate the exact number of electrons transferred to the cage solely from the shift of these modes. Analogously to the doping of C70 films,19 the intermediate and higher frequency tangential modes of C70 in potassiumdoped peapods are downshifted (568 to 550 cm-1, 1467 to 1462 cm-1, and 1445 to 1410 cm-1). As the experimental values for
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Figure 3. In situ Raman spectroelectrochemical patterns of exohedral fullerene C70@SWCNT peapods at different electrode potentials varied by 0.3 V from 1.1 to -1.9 V vs Fc/Fc+. The Raman spectra were excited at 2.41 eV. Intensities of spectra were normalized using the F1g line of Si at 520.2 cm-1. Spectra are offset for clarity, but the intensity scale is identical for all spectra in the respective windows.
both C704- and C706- fulleride films are available, the band at 1445 cm-1 in the pristine sample can be used as an indicator of thechargetransferredtoC70 cageinpotassium-dopedC70@SWCNT peapods. The downshift of about 35 cm-1 is closer to the value of C706- (51 cm-1) than that for C704- (67 cm-1).19 This confirms the charge transfer of six electrons per C70 cage under doping of C70@SWCNT with potassium vapor. The difference indownshiftbetweenthespectraofpotassium-dopedC70@SWCNT and doped C706- films can be attributed to the influence of the surrounding tube. This argument is logical as the charging leads to an increase of the C-C bond length, but the extension is limited by the SWCNT wall in the case of C70@SWCNT. The doping of C70@SWCNT causes a considerable broadening of the C70 features. All fullerene features of the doped sample are broadened by a factor of approximately 3 compared to the starting material. Similar values were observed for the doping of the C70 layers.22 The broadening was reported to be caused by inhomogeneous doping or by electron-phonon interaction.22 However, in the case of our samples no significant differences in the spectra were observed if different spots on the sample were measured. Furthermore the broadening does not directly correlate with the band shift. The broadening of the C70 features can, reportedly, be attributed to the formation of a superconducting phase,22 but this hypothesis still requires confirmation. 3.3. Water Treatment of K-Doped C70@SWCNT. In order to dedope the sample, the potassium-doped C70@SWCNT was treated by water at 90 °C. The water treatment leads to a partial recovery of the SWCNT-related features of C70@SWCNT, which indicates a change in the doping state of the C70@SWCNT sample. The bundling of carbon nanotubes generally complicates the studies of carbon nanostructures. Therefore, we have used a long equilibration time to achieve a homogeneous doping.
We also tested different times and conditions for water treatment to achieve a homogeneous sample. The Raman spectra of our samples do not change significantly with moving the examined spot, which points to their homogeneity. Nevertheless, it is practically impossible to avoid the formation of local inhomogeneities, especially in the voids in between the peapods. This is partly the reason for somewhat broader bands in the Raman spectra of these samples. The TG mode shifts to its original position, which also indicates the recovery of original state of peapods. However, the intensity of the SWCNT modes still remains approximately 20 times lower than that of the pristine sample. (Note that the intensity of the lines of pristine C70@SWCNT is divided by a factor of 20 in Figure 2.) Obviously, potassium was removed only from the interstitial voids between the tubes in a rope, but it still remains in the interior of the peapods. This is in agreement with our results reported previously on potassium doping of
[email protected],5 The treatment of doped samples with water affects the C70 modes. The intensities of most bands decrease compared to those in doped sample, and some of the bands disappear completely (270 cm-1, 550 cm-1) despite being visible in the potassiumdoped C70@SWCNT sample. On the other hand, the two modes at 1182 and 1226 cm-1 (position in pristine sample) that disappeared completely in doped sample reappeared again after dedoping by water treatment. Nevertheless, the overall intensity of the C70 modes in the dedoped sample is much lower than that of the pristine one. The results indicate a change of the doping state of the C70 in peapods. As the bands did not reach their original intensity, the complete recovery of the pristine state of C70 was not achieved. Therefore, potassium is confirmed to be present inside C70@SWCNT similarly to the potassium-
Chemical and Electrochemical Doping of C70 Peapods doped C60@SWCNT5 and the exohedral fullerene (K-fulleride) in C70@SWCNT peapods has been formed. The potassium-doped C60@SWCNT sample, which was treated in the same way with water as the doped C70@SWCNT, exhibits a charge of four electrons remaining on the fullerene cage as indicated by the shift of the Ag(2) mode (Figure 1), and therefore confirms our earlier results.3,5 Thus, four electrons on the C70 cage are also expected for the water-treated potassium-doped C70@SWCNT and the C70 modes should appear between their positions in the doped and pristine samples. This is, indeed, the case for the broad band around 700 cm-1 or the tangential C70 mode at 1145 cm-1. The reappeared band at 1182 cm-1 has almost the same position as that in a pristine C70@SWCNT sample. On the other hand, some shifts of the C70 bands do not follow the above-mentioned assumption. For example, the mode at 456 cm-1 in the doped sample is further upshifted to 459 cm-1 instead of downshifted toward the position in the pristine C70@SWCNT sample (452 cm-1). Finally, the band at 1467 cm-1 in the pristine sample, which was downshifted in the doped sample, exhibits an upshift to 1475 cm-1 in the water-treated sample. The reason for such an inverse direction of shifts has not been understood yet. Nevertheless, our experimental results indicate that there is no simple correlation between the shift of C70 features and the number of electrons transferred to the C70 cage as it was found in the case of
[email protected],5 This makes the evaluation of the charged state after treatment with water by Raman spectroscopy more complicated. 3.4. In Situ Spectroelectrochemistry. To get deeper insight into the properties of doped C70@SWCNT peapods, an in situ spectroelectrochemical study was carried out on the exohedral fullerene in C70@SWCNT peapods. Figure 3 shows the Raman spectra obtained using 2.41 eV laser excitation energy. The electrochemical charging of C70@SWCNT peapods causes similar changes of the SWCNT features as found for empty SWCNT. There is a gradual bleaching of the TG mode of SWCNT as the doping level is increased. This is because the change of electrochemical potential leads to a shift of the Fermi level. As the energy of van Hove singularities is achieved, the corresponding interband transition is bleached and the Raman resonance quenched. The second remarkable effect observed during electrochemical anodic doping of empty SWCNT is a hardening of the TG mode, which is explained by stiffening of the graphene sheet by the introduction of holes.13,23 Thus, as the behavior of SWCNT features in Raman spectra is similar for both empty SWCNT and C70@SWCNT peapods, no significant influence of the encapsulated fullerene structure on the electronic structure of the SWCNT is expected. The situation becomes more complicated after the doping of C70@SWCNT peapods with potassium. The detailed analysis of spectroelectrochemical data for exohedral fullerene in C70@SWCNT peapods showed that the TG mode is not upshifted during either cathodic or anodic electrochemical doping. This remarkable behavior has been observed already for potassiumdoped C60@SWCNT peapods,5 and it further confirms the presence of the potassium in the interior of C70@SWCNT peapods. The stability of an exohedral fullerene in C70@SWCNT peapods is similar to that of exohedral fullerene in C60@SWCNT peapods. However, the behavior of fullerene modes during charging differs. During cathodic doping the C70 features are not changed or are only slightly bleached. On the other hand, most of the fullerene bands become more apparent after anodic doping. Furthermore, the behavior of individual bands is
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1083 different. The fullerene feature at 452 cm-1 is not changed by either cathodic or anodic doping. The band at 620 cm-1 is not resolved in the K-doped C70@SWCNT after water treatment. However, it appears in the spectra at a potential of 0.5 V (vs Fc/Fc+). It reaches its maximum intensity at 0.8 V (vs Fc/Fc+) and bleaches at 1.1 V (vs Fc/Fc+). During cathodic doping this band does not arise. The spectral region between 850 and 1000 cm-1 (not shown) is complicated by intermediate frequency modes of the SWCNT. Therefore, the fullerene bands in this region were not analyzed. The band at 1182 cm-1 increases in intensity during the anodic doping. The maximum intensity is reached at the highest anodic potential applied, i.e., +1.1 V (vs Fc/Fc+). On the other hand, the band bleaches during cathodic doping and vanishes completely at the limiting negative potential applied -1.9 V (vs Fc/Fc+). The neighboring band at 1226 cm-1 does not change significantly during doping, except for slight bleaching during cathodic doping. The C70 fullerene band around 1445 cm-1 remains almost unchanged during the anodic doping, but it bleaches at cathodic potentials. Finally, the band at 1480 cm-1 remains intact during electrochemical doping. Figure 4 shows the Raman spectroelectrochemical data for K-doped and water-treated C70@SWCNT at 2.54 eV laser excitation. In general, the changes in SWCNT Raman modes going from 2.41 to 2.54 eV excitation should be expected as the peapods with slightly different diameters fulfill the resonance condition. Nevertheless, the experimental spectra for the excitation at 2.54 eV and those at the 2.41 eV are very similar in the region of SWCNT features. This is partly because of a very narrow diameter distribution of peapods. On the other hand, the fullerene features exhibit different intensities in the 2.41 and 2.54 eV spectra, because of the resonance enhancement of C70 spectra. This is obviously the reason why the band at 568 cm-1 does not appear during anodic doping at 2.54 eV excitation. On the other hand, the band at 1480 cm-1 is better resolved and therefore slightly bleached at high cathodic potentials. The electrochemical charging of the exohedral fullerene in C70@SWCNT peapods causes no shift of the fullerene bands for both the 2.54 and 2.41 eV laser excitations (see Figures 3 and 4). This behavior resembles the electrochemical doping of pristine peapods. Therefore, only the chemical redox reactants that are in contact with the fullerene are able to change the band position. On the other hand, the intensity of fullerene bands can be changed by both direct chemical doping and indirect doping by charging of the nanotube wall. The slight bleaching of C70 modes in exohedral fullerene C70@SWCNT peapods at high cathodic potentials is similar to the behavior of C60 features in exohedral fullerene C60@SWCNT peapods.5 This effect has been explained by a shadowing of the fullerene with higher electron density on the peapod wall, which is caused by the filling of van Hove singularities with electrons by cathodic doping. The anodic doping of exohedral fullerene C60@SWCNT peapods leads to the vanishing of the C60 mode at 1440 cm-1.5 The fullerene modes at 283, 493, and 498 cm-1, which are also resolved in water-treated K-doped C60@SWCNT, are not enhanced in intensity during anodic doping. On the other hand, the band around 1470 cm-1 is increased during anodic doping.5 However, this band is known to be strongly enhanced by anodic electrochemical doping of pristine (potassium-free)
[email protected] This effect was not observed for electrochemical doping of pristine
[email protected] Thus, the uniform bleaching of C70 features could be expected during electrochemical anodic doping of exohedral fullerene
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Figure 4. In situ Raman spectroelectrochemical data on exohedral fullerene C70@SWCNT peapods at different electrode potentials varied by 0.3 V from 1.1 to -1.9 V vs Fc/Fc+. The Raman spectra were excited at 2.54 eV. Intensities of spectra were normalized using the F1g line of Si at 520.2 cm-1. Spectra are offset for clarity, but the intensity scale is identical for all spectra in the respective windows.
C70@SWCNT peapods. In contrast to that, a clear increase in intensity of several C70 features during electrochemical anodic doping was observed. This effect is furthermore in contrast to the behavior of pristine C70 peapods, where the C70 feature bleaches during the anodic doping.4 This behavior resembles that of pristine C60@SWCNT where the so-called anodic Raman enhancement was observed.4 The different responses of Raman signals of C60 and C70 during anodic doping of pristine C60@SWCNT and C70@SWCNT peapods result from different electronic structures of these two materials as discussed previously.4 The small difference of the Fermi level of the SWCNT and the lowest unoccupied molecular orbital (LUMO) of the C60 fullerene was proposed to allow the electronic interaction between SWCNT and C60 fullerene. In the case of standing C70 in C70@SWCNT peapod, the LUMO of the fullerene is located well above the edge of the conduction band of SWCNT and the interaction of both is suppressed. 4 The presence of potassium inside the peapods can change the situation dramatically. A recent calculation using the local density approximation in the density-functional theory showed that intercalation results in the charge transfer from the 4s electron of potassium atoms to the peapods so that both the t1u states of C60 and the π* bands of the nanotubes cross the Fermi energy. The gradual doping of peapods by potassium vapor leads to an upshift of the energy of the t1u and hu states. For metallic tubes the latter states are not close to the crossing point of the two linear dispersion bands of this tubes any more.9 Accordingly, the spectroelectrochemical results obtained on K-doped C60 peapods are changed.5 We suggest that a similar change in electronic structure is responsible for the enhancement of C70 features by anodic doping of exohedral fullerene C70@SWCNT peapods.
4. Conclusion The state of the C70@SWCNT peapods during chemical and electrochemical doping was studied in detail by Raman spectroscopy. The chemical doping by potassium vapor led to significant changes in the vibration patterns of both SWCNT and C70 in peapods. The change in the positions of C70 Raman bands in chemically K-vapor-doped C70@SWCNT supports our finding that six electrons are transferred to the C70 cage. The water treatment caused a partial recovery of both SWCNT and C70 features. However, the original state of the peapod has not been achieved. This indicates that potassium is still located inside the peapod after doping and leaching with water, similar to the potassium-doped C60 peapods. The formation of exohedral fullerene in K-doped C70@SWCNT peapods was confirmed. Both SWCNT and C70 modes change upon electrochemical charging of the C70 peapod. The SWCNT modes exhibit two anomalies during the electrochemical doping of K-doped C70@SWCNT: (1) the TG mode is not shifted during the doping and (2) the bleaching of TG mode is not monotonic during anodic doping. Both effects seem to be associated with residual “inner” doping of peapods as there is no indication of such a behavior in the case of empty SWCNT or pristine peapods, respectively. The C70 features exhibit complex changes during electrochemical doping. However, most of the bands bleach during cathodic doping and increase in intensity during anodic doping. The latter effect contrasts the decrease of intensity of all C70 features during anodic doping of pristine (potassium free) C70 peapods and indicates the change in electronic structure induced by potassium inside the peapod. Acknowledgment. This work was supported by the Academy of Sciences of the Czech Republic (Contract Nos.
Chemical and Electrochemical Doping of C70 Peapods A4040306 and KJB400400601) and by the Czech Ministry of Education, Youth and Sports (Contract No. LC-510). M.K. acknowledges a grant from the Alexander von Humboldt Foundation. References and Notes (1) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323. (2) Kavan, L.; Dunsch, L.; Kataura, H. Chem. Phys. Lett. 2002, 361, 79. (3) Kavan, L.; Kalbac, M.; Zukalova, M.; Dunsch, L. Carbon 2006, 44, 99. (4) Kavan, L.; Dunsch, L.; Kataura, H.; Oshiyama, A.; Otani, M.; Okada, S. J. Phys. Chem. B 2003, 107, 7666. (5) Kalbac, M.; Kavan, L.; Zukalova, M.; Dunsch, L. J. Phys. Chem. B 2004, 108, 6275. (6) Guan, L. H.; Suenaga, K.; Shi, Z. J.; Gu, Z. N.; Iijima, S. Phys. ReV. Lett. 2005, 94, 045502. (7) Berber, S.; Kwon, Y. K.; Tomanek, D. Phys. ReV. Lett. 2002, 88, 185502. (8) Sun, B. Y.; Sato, Y. Suenaga, K.; Okazaki, T.; Kishi, N.; Sugai, T.; Bandow, S.; Iijima, S.; Shinohara, H. J. Am. Chem. Soc. 2005, 127, 17972. (9) Okada, S. Phys. ReV. B 2005, 72, 153409. (10) Pichler, T.; Kuzmany, H.; Kataura, H.; Achiba, Y. Phys. ReV. Lett. 2001, 87, 267401.
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