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J. Phys. Chem. C 2009, 113, 1340–1345
Effect of Bundling on the Tangential Displacement Mode in the Raman Spectra of Semiconducting Single-Walled Carbon Nanotubes during Electrochemical Charging M. Kalbac,*,†,‡ L. Kavan,† and L. Dunsch‡ J. HeyroVsky´ Institute of Physical Chemistry, VVi Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18223 Prague 8, Czech Republic, and Leibniz Institute of Solid State and Materials Research, Group of Electrochemistry and Conducting Polymers, Helmholtzstrasse 20, D-01069 Dresden, Germany ReceiVed: October 16, 2008; ReVised Manuscript ReceiVed: NoVember 24, 2008
A detailed in situ Raman spectroelectrochemical analysis of semiconducting single-walled carbon nanotubes (SWCNTs) in bundles is presented. Special attention has been given to the development of the frequency and intensity of the tangential displacement mode (TG) during electrochemical charging. For negative charging, the frequency of the G+ component of the TG mode increased, decreased, and remained unchanged for the 2.18, 2.54, and 2.41 eV laser photon energies, respectively. For positive charging, only an increase in the frequency of the G+ component has been observed. The maximum frequency upshift (at the applied electrochemical potential of 1.5 V vs Ag-pseudoreference) increased for decreasing photon energies in the series from 2.54 to 2.41 and 2.18 eV. The frequency of the G+ component of the TG mode changes significantly at potentials of approximately 0.8-1.0 and -1.2 V for positive and negative doping, respectively. This is in contrast to the change in intensity of the G+ component of the TG mode of semiconducting SWCNTs, which exhibits distinct changes when the potential of the first Van Hove singularity is achieved (0.5 V). Introduction Raman spectroscopy is a frequently used spectroscopic method to investigate the state of single-walled carbon nanotubes (SWCNTs) due to the resonant enhancement of their Raman signal. The main components of the spectra of SWCNTs are the radial breathing mode (RBM), the tangential displacement mode (TG), which is also known as the G band, the disorderinduced mode (D), and the high-frequency two-phonon mode (G′). The tangential displacement mode is observed in the region of 1450-1630 cm-1. Group theory predicts six Raman-active lines in the TG mode region for chiral nanotubes, 2A1g + 2E1g + 2E2g, which are reduced to three Raman-active bands for zigzag and armchair tubes. However, the E1 and E2 modes have a much weaker Raman intensity than the totally symmetric A1 modes.1 Thus, only two main components of the TG mode are often considered in the analysis. They are usually referred to as the G+ and G- lines, which are found at approximately 1590 and 1560 cm-1, respectively (for tubes with a diameter of ∼1.5 nm). For semiconducting tubes, the G+ line is attributed mainly to the diameter independent A1LO modes, while the G- line is assigned to the diameter dependent A1TO. On the other hand, for metallic tubes, the A1LO mode is broadened and strongly softened to be eventually below the frequency of the A1TO mode because of the Kohn anomaly.2-4 In situ Raman spectroelectrochemistry is a well-established method used to study the electronic structure of carbon nanostructures, such as SWCNTs, double-walled carbon nanotubes (DWCNTs), or fullerene peapods.5-8,12 In contrast to chemical doping, electrochemistry allows precise and wellcontrolled doping of carbon nanostructures to be performed. * Corresponding author. E-mail:
[email protected]. Tel: 420 2 6605 3804. Fax: 420 2 8658 2307. † J. Heyrovsky´ Institute of Physical Chemistry. ‡ Leibniz Institute of Solid State and Materials Research.
Measurements on isolated individual tubes9-11 seem to be easier to evaluate because the data are not disturbed by the overlapping of signals from several tubes or by the effect of bundling. Nevertheless, both individualized and bundled tubes are foreseen for practical applications; hence, the detailed understanding of both individual SWCNTs and SWCNT bundles is necessary. Furthermore, it has been shown recently that the behavior of bundled tubes during charging exhibits some differences from those of individual tubes. For example, in the case of metallic SWCNTs, the fast bleaching of the lowest component of the G- band observed in bundles12 is not found in the case of individual tubes.11 The recent study of the TG mode of individual semiconducting tubes showed that the frequency shift with potential is dependent on the nanotube diameter.13 The wider tubes showed stronger upshift of the TG mode during positive charging because of the simultaneous action of the changed force constant and phonon renormalization effect. For negative charging, these effects have an opposite sign, which leads to the upshift of the TG mode of wider tubes and to the downshift of narrower tubes with increased negative potential.14 In the present work, we inspected the Raman spectra of semiconducting tubes in bundles during electrochemical doping. A precise control of the electrochemical charging enabled us to follow the detailed development of the TG mode with respect to its electrode potential dependence. We compared the results on bundles with the previous ones obtained on individual tubes.13,14 We shall show that the phonon renormalization effect is influencing the behavior of tubes in bundles. However, the interaction of tubes smoothens the effects for tubes in the bundle. In other words, the resulting behavior of the nanotube bundle does not mimic that of each individual tube in the sample, but rather it simulates the behavior of one compact tube with averaged properties.
10.1021/jp8091623 CCC: $40.75 2009 American Chemical Society Published on Web 01/07/2009
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Figure 1. Raman spectra of SWCNTs excited by 2.54, 2.41, and 2.18 eV laser radiation (from top to bottom). The intensity scale is identical for the spectra excited by the same laser energy, except when zooming the intensity scale in the left panel by a factor of 6.
Results and Discussion The RBM and the TG band in the Raman spectra of SWCNTs excited by 2.54, 2.41, and 2.18 eV laser radiation are given in Figure 1. The positions of the most intense RBM bands are at approximately 170 and 181 cm-1 for the 2.54 eV laser excitation energy, 172 and 182 cm-1 for the 2.41 eV laser excitation energy, and 178 cm-1 for the 2.18 eV laser excitation energy. The diameter of SWCNTs can be approximated by the equation
ωRBM ) A ⁄ dt + B -1
(1) -1 15
where A ) 217.8 cm nm and B ) 15.7 cm . Hence, the RBM mode positions of the most intense bands correspond to the tube diameters at approximately 1.40 and 1.32 nm for both the 2.41 and 2.54 eV laser excitation energies and at approximately 1.34 nm for the 2.18 eV laser excitation energy. For the 2.54 and 2.41 eV laser excitation energies, there is a
difference in the relative intensity of the two most intensive RBM bands. In the case of the spectra excited by 2.41 eV laser radiation, the narrower tubes contribute more significantly to the Raman spectra than in the case of the spectra excited by 2.54 eV laser radiation. This is in agreement with the Kataura plot because for one E33 branch the larger diameters are excited by smaller energy. Note that this effect can be modulated by different chiralities of the nanotubes. For example, the tubes within one family exhibit a smaller band gap with decreasing tube diameter in the case of the lower E33 branch. The latter effect is more pronounced for tubes with diameters less than than 1 nm. Here, the diameter of the tubes is about 1.35 nm, and thus the effect is small in our study. In other words, it is reasonable to assume that with an increase in the laser excitation energy the contribution of narrower tubes is more pronounced as the resonance condition is better fulfilled for narrower tubes. Figure 1 also shows the region between 1500 and 1650 cm-1, where the TG mode of SWCNTs appears. The TG mode is not broadened for all three excitation energies, which indicates that almost exclusively semiconducting tubes contribute to the spectra at these particular laser excitation energies. In Situ Raman Spectroelectrochemistry. The detailed in situ Raman spectroelectrochemical data for SWCNT bundles in the TG mode region comprise spectra excited by the 2.54, 2.41, and 2.18 eV laser energies (Figure 2). The most obvious effect is the bleaching of the TG band intensity as a result of electrochemical doping. The attenuation of the Raman intensity can be explained by the filling/depleting of the Van Hove singularities (VHs) and the corresponding bleaching of the E33 optical transition. We have shown previously that an extensive doping of SWCNTs leads to a strong decrease in the Raman
Figure 2. In situ Raman spectroelectrochemical data on SWCNT bundles in the electrode potential range from 1.5 to -1.5 V vs Ag (from top to bottom). The spectra are excited by the 2.18, 2.41, and 2.54 eV laser radiation in the right, central, and left panels, respectively. The bold lines correspond to the spectra at an electrode potential of 0 V vs Ag. The spectra are offset for clarity, but the intensity scale is the same for all spectra in their respective windows. The vertical dotted lines are a guide for the eye and correspond to the position of the various phonon features at 0 V vs Ag pseudoreference electrode. The potential step between the adjacent curves is 0.1 V.
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Figure 3. Development of the G+ mode intensity in dependence on electrode potential. The circles, triangles, and squares correspond to the spectra excited by the 2.18, 2.41, and 2.54 eV laser radiation, respectively. The intensity of the G+ mode is normalized to the intensity at the potential of 0 V vs Ag for each particular laser excitation. The dashed lines correspond to fits of the parts of the dependence of the G+ mode intensity on potential in the following regions: (1) from -1.5 to 0.6 V, (2) from -0.5 to 0 V, (3) from 0 to 0.5 V, and (4) from 0.6 to 1.0 V (for the 2.18 eV laser radiation).
intensity.16 It is generally assumed that the change in Raman intensity is connected to the change of resonance condition. The intensity of the Stokes resonant Raman scattering (I) is given by
I)
c |(EL - Eii - iγ)(EL - Eph - Eii - iγ)|2
(2)
where EL is laser photon energy, Eii is the optical transition energy, Eph is the phonon energy, and γ ≈ 0.05 eV is the damping constant. The c term includes the electron-phonon matrix elements. As was mentioned previously, the electrochemical charging of SWCNTs leads to a shift in the Fermi level. As soon as the Fermi level surpasses the energy of the Van Hove singularity, it suppresses the electronic transitions from or to this particular singularity. If the Raman signal is in resonance, a strong bleaching of the Raman intensity for that feature is expected with such a singularity. However, recent studies of the RBM mode during charging have shown that the spectra significantly change when the charge is injected or depleted to or from the first electronic states (E11 transition).17 The change in the RBM band intensity occurs even if these particular states are not involved in the resonance Raman effect.17 Figure 3 shows a plot of the normalized intensity of the G+ mode in dependence on the applied potential for three laser excitation energies (2.18, 2.41, and 2.54 eV). For all of the probed laser excitation energies, the intensity of the G+ mode begins to be significantly attenuated at the potential of approximately (0.5 V vs Ag pseudoreference electrode. The intensity attenuation is similar for all three laser excitation energies. However, detailed analysis of the data shows that the width of the intensity/potential profile of the G+ mode slightly increases with decreasing excitation laser energy (from 2.54 to 2.18 eV). This means that for the bundled sample the electronic
structure of wider tubes is less sensitive to electrochemical charging than that of narrower tubes. The G+ mode intensity/potential dependence is analogous to the development of the RBM intensity during electrochemical charging.17 Thus, the electronic states are changed already when the first Van Hove singularity is filled or depleted. In fact, the energy level of the first Van Hove singularity of semiconducting tubes under study is about 0.35 eV vs the Fermi level. For an ideal metal electrode, this energy level should be reached by applying the potential of about (0.35 V. Nevertheless, it has been shown recently that for SWCNTs the Fermi level shift is about 50-70% of the applied potential,18 e.g., the potential of 0.5 V corresponds roughly to the Fermi level shift of about 0.35 eV. Note that the discontinuity at 0.5 V in the behavior of the intensity with the applied potential is not very sharp, as might be expected. This could be explained by the presence of nanotube bundles and/or by the changes in the nanotube environment (electrolyte ion concentration) during the charging. It is interesting to note that the intensity attenuation begins to be traceable at a similar potential ((0.5 V) for both the RBM and the G+ mode despite the resonance window for the G+ mode being considerably broader than that for the RBM band.19,20 Hence, it seems that for nanotube bundles the resonance window of the particular phonon mode does not affect the bleaching behavior with applied potential. This might be explained by the presence of many different tubes, which contribute to the spectra, and thus all effects are “smoothened”. The additional broadening due to the broad resonance window might be negligible. The electrochemical charging of SWCNT bundles also leads to a change in the frequency of the G+ band. The change in the G- frequency with applied potential is difficult to analyze because the intensity of the G- component is much weaker than that of the G+ component, and it drops below the traceable level at mild potentials. Nevertheless, the G- mode frequency is not
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Figure 4. Development of the G+ mode frequency in dependence on the electrode potential. The circles, triangles, and squares correspond to the spectra excited by the 2.18, 2.41, and 2.54 eV laser radiation, respectively.
changed markedly up to the potential of approximately (1 V, where the band has disappeared for all tested laser excitation energies. In contrast to the G- mode, the frequency of the G+ component is significantly changed if the electrode potential is applied. For example, at the 2.54 eV laser excitation the frequency of the G+ mode is increased from 1592 cm-1 at 0 V vs Ag to 1606 cm-1 at 1.5 V vs Ag. Decreasing the potential to -1.5 V results in the final line downshift of 7 cm-1 with respect to the position of the G+ component at 0 V (Figures 2 and 4). This effect is qualitatively consistent with the previous results on graphite,21 and thus it can be explained in the same way: The n-doping (at negative potentials) leads to the filling of antibonding orbitals of the C-C bond. Thus, the bond is softened, and consequently, the appropriate vibration modes shift to lower frequencies. However, a more detailed analysis of the change in the frequency of the G+ mode with potential shows that the shift is dependent on the excitation laser energies. The maximum upshift of 21 cm-1 (at 1.5 V) is at the 2.41 eV laser excitation, and there is no downshift at negative potentials. For the 2.18 eV laser excitation energies, the upshift is 20 cm-1 (at 1.5 V), and surprisingly, there is also an upshift of 2 cm-1 for negative potentials (at -1.5 V). The detailed analysis also shows that the frequency of the G+ mode starts to change at approximately +0.8-1 V for positive doping and at approximately -1.2 V for negative doping. Hence, contrary to the change in intensity which occurs at (0.5 V, the frequency starts to change at significantly higher potentials. There is a slight dependence on laser excitation energies. For example, in the case of positive doping for the 2.18 eV laser radiation the shift of the TG mode begins to be apparent at 0.8 V. For the 2.41 and 2.54 eV laser excitation energies, the significant changes in frequency begin to occur at 1.0 V. Note that for individual SWCNTs the magnitude of the electrode potential, where the change in the frequency of the G+ mode occurs, is dependent on the diameter of the nanotubes.14 This diameter dependence is not significant for SWCNT bundles here. The actual electrode potential at which the frequency change begins is different for positive and negative doping. The reason
for this unexpected asymmetry is not clear. Nevertheless, the absolute values of the G+ mode frequency shift are much smaller for negative potentials, and thus it is difficult to evaluate precisely the electrode potential where the first change in frequency occurs. In other words, the effect might be too small for negative doping, and thus it is recognized much later than in the case of positive doping. It has been shown recently for individual SWCNTs that the change of the G+ mode during charging depends on the SWCNT diameter.13,14 There are two important effects that influence the frequency of the G+ mode of semiconducting tubes: (a) the phonon renormalization effect and (b) a change in the spring force constant of C-C bonds. While the phonon renormalization effect causes a frequency upshift for both positive and negative doping, the change of the spring force constant causes a downshift for negative doping and an upshift for positive doping. Hence, for positive charging only upshift of the G+ mode is expected. However, for negative doping both effects are competing with each other; thus, the resulting behavior of the G+ mode is dependent on the contribution of each effect. The phonon renormalization is diameter dependent; it is strong for wider tubes, but it is weak for narrower tubes. Therefore, because of the dependence on the SWCNT diameter, a downshift or upshift is expected for the G+ frequency during negative charging. Indeed, for isolated tubes both upshift and downshift have been observed. The situation is more complex for nanotube bundles, where several tubes are in resonance at the same time. These tubes can have different diameters. Hence, the resulting behavior of the G+ mode during charging should reflect all of the different behaviors of tubes with different diameters. Furthermore, an interaction between the tubes may also play a role. The detailed development of the G+ mode at high anodic potentials in the Raman spectra excited by 2.41 eV laser radiation (Figure 5) points to a turn at a potential of 1.0 V, where the shape of the G+ mode starts to change, resulting in an asymmetry with the tail toward higher frequencies. As the potential is increased toward 1.5 V, the high-frequency portion of the G+ mode dominates the spectrum, and it is also upshifted to even higher frequencies. Similar effects have been observed also for the 2.54 and 2.18 eV laser excitation energies. This
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Kalbac et al. renormalization effect and the spring force constant are similar, and δωG+/δVe is approximately 0. Hence, the diameter dependence of δωG+/δVe observed for individual tubes is now reproduced in bundles. But it is important to note that the G+ mode in our spectra is composed of the G+ modes of many SWCNTs, which are not separated at a neutral state. These bands, however, correspond to tubes with different diameters, and thus at higher positive or negative electrode potentials the frequencies of these particular modes are changed in a different way. This results in the broadening and the change in shape of the G+ mode of the SWCNT bundle in the Raman spectra. Furthermore, there are interactions between the tubes in the bundle. These interactions seem to be responsible for the smooth change in the shape of the G+ mode. Hence, the G+ mode is not dissipating into several bands, each corresponding to a particular tube diameter, but rather acts like one compact tube with averaged properties. Conclusions
Figure 5. Details of the Raman spectra of the TG mode at the potentials of 1.1, 1.2, 1.3, 1.4, and 1.5 V vs Ag. The spectra are excited by 2.41 eV laser radiation. The spectra are offset for clarity, but the intensity scale is the same for all spectra.
result is in agreement with the diameter dependence of the change in the G+ mode frequency with applied potential (δωG+/ δVe) observed for individual semiconducting tubes. In the spectra excited by 2.41 eV laser radiation (Figure 1), the RBM bands appear between 160 and 200 cm-1, corresponding to the range of diameters from 1.2 to 1.5 nm (eq 1). The TG mode is known to have a broader resonance window; hence the range of diameters of tubes that are in resonance is more extended for this particular mode. Because each tube is likely to exhibit slightly different δωG+/δVe dependence, broadening of the G+ mode is expected as the applied potential is increased. This is in agreement with the observed spectra (Figure 5). The broadening of the band causes difficulties in evaluating the G+ mode position at higher anodic potentials. Therefore, the values must be used with caution. Nevertheless, the different frequency shift of the G+ mode with electrochemical charging for different laser excitation energies (Figures 2 and 4) is consistent with the diameter dependence of δωG+/δVe discussed above. In general, a higher laser excitation energy leads to a resonance of narrower diameter tubes (for a given Eii, neglecting nanotube family effects). In this study, the diameter of the tubes in resonance increases in the series of 2.54, 2.41, and 2.18 eV. Because the phonon renormalization effect increases with tube diameter, the change in the G+ mode frequency due to this effect should be the strongest in the spectra excited by 2.18 eV, weaker in the spectra excited by 2.41 eV, and the weakest in the spectra excited by 2.54 eV. For negative doping, the contribution of the phonon renormalization effect to δωG+/δVe is positive, while the contribution of the spring constant is negative. The resulting δωG+/δVe should be dependent on the diameter. Hence, the different laser excitations should lead to different δωG+/δVe. The strong phonon renormalization effect for tubes in the spectra excited by 2.18 eV laser energy dominates, and the resulting δωG+/δVe is positive. On the other hand, the weakest phonon renormalization effect for tubes in the spectra excited by 2.54 eV laser energy is dominated by the contribution of the spring force constant, and the resulting δωG+/δVe is negative. Finally, for a 2.41 eV laser excitation the contribution of phonon
A detailed study is presented on the change in the G+ mode of semiconducting tubes with the applied electrochemical potential, using three different laser energies to excite the Raman spectra. We demonstrated that the intensity of the G+ mode is attenuated as soon as electrons are injected into or removed from the first VHs even if these energy levels are not involved in the resonance enhancement of Raman spectra. On the other hand, the frequency of the G+ mode is changed at much higher electrode potentials (about -1.2 V for negative charging and 0.8-1.0 V for positive charging). This is in contrast to the individual tubes, where the frequency change can be observed even at low electrode potentials (for wide diameter tubes). The δωG+/δVe is found to be dependent on the laser excitation energies as a result of the different diameters of SWCNTs in the bundle, which dominate the spectra excited by a particular laser excitation energy. This effect is most obvious in the case of negative doping, where δωG+/δVe was found to be positive, negative, or ≈ 0 for 2.18, 2.54, and 2.41 eV laser radiation, respectively. This effect is similar to that observed in individual tubes. However, in bundled tubes there is a different dependence of the G+ mode frequency on the electrode potential (δωG+/ δVe) for different diameters, and separation of the G+ mode into several individual bands is not observed. It only manifests itself at high anodic and high cathodic potentials, where the G+ mode is broadened and changes shape. Furthermore, the change in the shape of the G+ mode is smooth, probably because of interactions of the tubes in the bundle. Experimental Section Single-walled carbon nanotubes were available from SINEUROP and from our earlier work.22 Briefly, the SWCNTs were prepared by laser ablation,23 using a Ni/Co catalyst and purified by reflux in 15% H2O2, which was followed by washing with HCl to remove residual catalysts.24 The electrodes for in situ spectroelectrochemical studies were fabricated by the evaporation of a sonicated ethanolic slurry of SWCNTs on Pt electrodes. The film was outgassed at 80 °C in vacuum, and then the electrode was mounted in the Raman spectroelectrochemical cell. The spectroelectrochemical cell was airtight, had a single compartment, and was equipped with a glass optical window for spectroscopic measurements. The cell was assembled in a glovebox (MBRAUN), and the glovebox atmosphere was N2 containing
