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Direct Revealing of the Occupation Sites of Heavy Alkali Metal Atoms in Single-Walled Carbon Nanotube Intercalation Compounds Brigitte Vigolo,*,† Claire He´rold,† Jean-Franc¸ois Mareˆche´,† Patrice Bourson,‡ Samuel Margueron,‡ Jaafar Ghanbaja,† and Edward McRae† Institut Jean Lamour, CNRS-Nancy UniVersite´-UPVMetz, B.P. 70239, 54506 VandoeuVre-le`s-Nancy, France, and Laboratoire Mate´riaux Optiques, Photoniques et Syste`mes, UniVersite´ de Metz et Supe´lec, UMR 7132 CNRS, 2 rue E. Belin, 57070 Metz, France ReceiVed: January 19, 2009; ReVised Manuscript ReceiVed: February 19, 2009
The description and understanding of the intercalation mechanism of electron donors or acceptors in singlewalled carbon nanotube (SWNT) bundles are of critical interest for optimization and improvement of transport properties for future applications of SWNTs in electronics or energy storage. Here, we propose a novel approach based on the analysis of the state of debundling for SWNT bundles, used as a feedback for occupation sites of alkali metal atoms within the SWNT host material. Heterogeneity of the structure might be the origin of the complexity of the intercalation process in SWNTs. 1. Introduction Carbon nanotubes (CNTs) and especially single-walled carbon nanotubes (SWNTs) continue to attract great interest for their intrinsic properties and their potential applications.1 In particular, electronic properties can be modified through intercalation (or doping) of guest species such as alkali metal atoms within their structure.2 These materials motivate research focused on CNT and SWNT intercalation compounds for their applications in nanoelectronics and energy storage.3,4 Nevertheless mechanism of intercalation in SWNT bundles is still poorly understood, and the resulting structural properties of the obtained compounds are undetermined. SWNTs are assembled in bundles formed from few to hundreds of tubes organized in a compact hexagonal lattice. It was proposed that intercalation in CNT structures such as SWNT bundles differs from the well-known behavior observed for other carbon host materials, especially for GIC (graphite intercalation compounds) systems.5 However, the precise location of the alkali metal atoms and the types of sites which are preferentially occupied within a SWNT bundle are difficult to determine.6-9 Intercalation of alkali metals is not a straightforward process because for host materials, it involves some modifications of both their electronic structure upon a reduction reaction and their structural parameters subsequent to the insertion of guest species within the host structures. Two main forces oppose each other throughout the chemical process: (i) favorable forces resulting from spontaneous electron transfer from alkali metals to SWNTs; (ii) nonfavorable mechanical forces acting against an increase of spacing between ordered repeated motifs, i.e., the layers in the case of graphite, the fullerenes in the case of fullerite, and the tubes in the case of SWNT bundles. The obtained structures mainly result from the balance between these two forces. Moreover, in the raw state, samples of SWNTs possess a non negligible number of structural defects and functional groups at their surface, and the hexagonal crystalline structure of the bundles is also defective because of a commonly observed tube diameter distribution. These het* Corresponding author. Tel +33 383684641; fax: +33 383684615, e-mail:
[email protected] (B.V.). † CNRS-Nancy Universite´-UPVMetz. ‡ Universite´ de Metz et Supe´lec.
erogeneities may affect the intercalation process, and they are possibly at the origin of the difficulties in unambiguously determining the favored occupation sites within SWNT bundles obtained from the usual investigation and characterization techniques. Using nonlocal techniques such as X-ray or neutron scattering and extended X-ray absorption fine structure (EXAFS) spectroscopy has led to controversial results; the debate especially concerns occupation by the guest species of the interstitial channels within SWNT bundles.10,11 Here, we present an original approach, utilizing local HRTEM (high resolution transmission electron microscopy) observations of SWNT intercalation compounds that have been dispersed in a polar solvent. 2. Results and Discussion 2.1. The Debundling Process. The electronic transfer between the alkali metal atoms and the SWNTs occurring upon the reduction reaction helps in the dispersion-debundling process as the solvent is added to the reduced powder of SWNTs. The degree of debundling of the SWNTs suspended in the solvent is then a direct consequence of the level of filling of interstitial channels by the guest species. Even if the charges are delocalized over the whole bundle, the dispersion process is certainly favored at the occupation sites of the alkali metal atoms. If we reasonably assume that electron transfer from alkali metals is small,12 the occupation of interstitial sites implies an expansion of the hexagonal lattice of the bundles as has been proposed in the literature.13 Locally, the presence of alkali metal atoms in an interstitial site will favor the debundling process since the increase of the interspacing has already taken place. The dispersion process is afterward expected to lead to separating or debundling of the nanotubes between which alkali metals were inserted. The attractive van der Waals forces between CNTs within a bundle are then counterbalanced through the insertion-dispersion mechanism. If the interstitial channels are not occupied at all, preserved bundles should be observed after dispersion (scheme 1, left side). On the contrary, in the case of a complete occupation of the interstitial sites using HRTEM we should observe, complete debundling of SWNTs leading to their individualization (Scheme 1, right side).
10.1021/jp900546n CCC: $40.75 2009 American Chemical Society Published on Web 04/10/2009
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SCHEME 1: Resulting States after the Reduction-Dispersion Process of SWNT Bundles in the Case of (left) Nonoccupation of the Interstitial Channels and (right) Their Complete Occupationa
a
Top: cross sections of bundles for the two chosen examples. Bottom: representation of HRTEM images of the two obtained states of debundling.
Figure 2. Photograph of dispersions in DMSO obtained from pristine SWNTs and SWNTs reduced by potassium.
Figure 1. Raman spectra of pristine SWNTs (top) and SWNTs reduced by potassium (bottom). Measurements were done using an excitation wavelength of 514 nm and in sealed quartz cells under helium to prevent oxidation and to optimize heat dissipation.
2.2. Dispersion of Alkali-Metal-Reduced SWNTs. SWNTs used in this study were synthesized by an arc discharge method and provided by Carbolex Inc. SWNT samples were outgassed under vacuum at 400 °C for at least 4 h. For potassium and rubidium, the reduction reaction was done at about 180 °C in a two-temperature sealed Pyrex glass reactor for 2 weeks. For cesium having a higher saturated vapor pressure, the reaction was conducted at 160 °C.14 SWNT powder was heated at a temperature slightly higher than that of the alkali metal in order to avoid direct condensation of alkali vapor on the SWNT surface. Under these reaction conditions, the compounds should be thermodynamically stable and saturated.15 The reaction with alkali metals leads to an electron enrichment of SWNTs that can be evidenced using Raman spectroscopy. Both modifications of Raman resonance conditions (leading to disappearance of
existing bands and appearance of new bands) and shifts of existing bands of SWNTs are induced by the reaction with alkali metals.16 We have observed some typical signatures for our alkali-reduced SWNT compounds as shown in Figure 1: (i) strong reduction of the Raman intensity; (ii) apparition of bands in the intermediate region; (iii) disappearance of the RBM (radial breathing modes); (iv) downshift of the G band located at 1585 cm-1 in the raw sample, and the high frequency component is found around 1545 cm-1 after reduction reaction. From geometrical consideration, the size of interstitial sites in arc-discharge produced SWNT bundles is estimated at about 300 pm.17 The state of ionization of heavy alkali metals after reaction with SWNTs is not easy to determine, and it has not been investigated in this work. For this reason, we give in Table 1 the diameters of the three used alkali metals at the atomic state and after their first ionization. According to the respective sizes of the sites and the investigated alkali metals, occupation of the interstitial sites of bundles, except for highly ionized potassium and rubidium, is expected to induce an expansion of the SWNT lattice.5 Because of their hydrophobic character and their chemical stability, SWNTs are highly difficult to disperse. Moreover, SWNT bundles are often entangled because of the production process. As a result, their manipulation and processing is a tricky task. Physical adsorption of surfactant molecules, wrapping polymers, or covalent functionalization can be used to increase the affinity between SWNT walls and a chosen medium, but these procedures often lead to a damaged SWNT surface because they require a sonication step. Moreover, they do not allow obtaining highly concentrated dispersions. Stable dispersions of SWNTs can be obtained in a spontaneous way by merely adding alkali metal-reduced SWNTs to an aprotic polar solvent such as NMP (1-methyl-2-pyrrolidone) or DMSO (dimethyl sulfoxide).18 Figure 2 shows a photograph taken one
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Figure 3. Typical spectra obtained for SWNTs dispersed in DMSO. (a) Pristine SWNTs (black curve) and dispersed SWNTs after reduction with potassium (gray curve); excitation wavelength 514 nm. (b) Pristine SWNTs (black curve) and dispersed SWNTs after their reduction with rubidium; excitation wavelength 633 nm. Peaks with an asterisk (*) correspond to the solvent Raman peaks.
TABLE 1: Atomic and Ionic Diameters for Potassium, Rubidium, and Cesium diameter (pm) potassium rubidium cesium
atomic
ionic (first ionization)
470 496 534
266 296 338
month after preparation of the dispersions in DMSO conserved in an airtight bottle. For reduced SWNTs, the solution is instantaneously darkened as solvent is added to the powder and remains stable for months. On the contrary, the pristine sample remains aggregated and rapidly precipitates. Dispersed SWNTs were characterized using Raman spectroscopy (Figure 3 and Table 2). The Raman response of DMSO in the dispersions of the alkali-reduced SWNTs over a wide range of wavenumber (100-2000 cm-1) does not show any modification compared to the two checked references, i.e., pure DMSO and DMSO mixed with pristine SWNTs. DMSO seems not to react with the reduced SWNTs but mainly plays a role of solvation. For this reason, we think that the solvation phenomenon from DMSO is good for these reduced SWNTs, leading to an efficient dispersion which will locally occur on the occupied sites by the alkali metal atoms. Surprisingly and contrary to the observed trends for the Raman response of the reduced powder (see Figure 1), the G band of the dispersed SWNTs shows a significant upshift of several cm-1 at 514 nm and which is even more pronounced (from 20 to 28 cm-1) at 633 nm. The mean diameter of the used SWNTs is 1.4 nm ( 0.2 nm. Considering the Kataura selection rules, only semiconducting tubes contribute to the signal at 514 nm, and at 633 nm only signals from tubes that are metallic are recorded (giving the well-known Breit-Wigner-Fano asymmetric line shape visible in Figure 3 left, for pristine SWNTs). The greater upshift observed at 633 nm seems to be a sign of higher reactivity for metallic tubes toward the electron transfer occurring during the reduction reaction. Intuitively, we may think that the electron enrichment of carbon-carbon bonds which leads to a bond expansion should induce a diminution of the G band vibration frequency as was observed for reduced powders (see Figure 1). The anomalous hardening observed here for the dispersed SWNTs has already been observed and reported for SWNTs in the low-electronic transfer stage.13 It is however difficult to conclude on the exact origin of the unexpected upshift of the G-band for the dispersed SWNTs in our system. Even if this
TABLE 2: Observed Upshifts for the G Band in Dispersions of Reduced SWNT in DMSO G band upshift (cm-1)
excitation wavelength 514 nm
excitation wavelength 633 nm
K@SWNT Rb@SWNT Cs@SWNT
10 3 5
28 20 28
phenomenon is still poorly understood, it certainly depends on a charge effect at the SWNT surface in the solutions. The electron transfer between alkali metal atoms and SWNTs upon the reduction reaction first favors the dispersion of SWNTs in a spontaneous process. After dispersion, a residual charge on the SWNT surface may prevent natural reaggregation commonly encountered in nonstabilized Brownian colloidal dispersions, electrostatic repulsion forces being at the origin of dispersion stability. 2.3. Localization of Alkali Metal Atoms within SWNT Bundles. For HRTEM observations, a droplet of SWNT dispersion is deposited on holey or lacey carbon copper grids. HRTEM analysis remains local and qualitative; however, we used a statistical approach multiplying the number of observed zones on each sample. Figure 4 shows typical images for the three intercalation compounds after reaction with potassium (a-c), rubidium (d-f), and cesium (g-i) and dispersion in DMSO. The images first demonstrate no significant differences in behavior between the three different samples. We have systematically observed a partial debundling of SWNTs with a qualitatively similar advancement in the debundling process. SWNT bundles have a high tendency to split into smaller bundles which are only partially separated from the larger initial bundles; such a phenomenon is visible for potassium-reduced compounds in Figures 4a and 4b, for rubidium in Figures 4e and 4f, and for cesium intercalated compounds in Figure 4h. Moreover, we have repeatedly observed SWNTs that have been completely or partially isolated from the bundles as shown in Figures 4c, 4d, 4g, and 4i (shown with arrows). The high dispersion quality (see Figure 2) indicates that alkali metal atoms certainly occupy the external sites of the bundles including the groove sites. Regarding the filling of the interstitial sites, their accessibility to alkali metal vapor is certainly reduced since the debundling mechanism is not complete and it is only favored (i) at the bundle extremities leading to partial breakage
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Figure 4. Typical HRTEM images of SWNTs dispersed in DMSO after reduction by potassium (a-c), rubidium (d-f), and cesium (g-i).
of the bundles and (ii) on an external part of the bundles responsible for the detachment of some individualized nanotubes. By using the usual experimental conditions for preparation of heavy alkali metal-SWNT compounds, a partial occupation of interstitial sites in SWNT bundles is observed. Even if a comprehensive scenario is still missing for the intercalation process, we point out that the inhomogeneous structures we reveal here for heavy alkali-metal-reduced SWNT bundles is certainly at the origin of the disagreements between previous works. Interestingly, compounds obtained from each of the three investigated alkali metals behave similarly, which means that intercalation compounds show equivalent fractions of occupied sites especially regarding the interstitial sites whatever the metal. This observation raises a critical question relative to the thermodynamic equilibrium of these compounds. If we consider that these SWNT intercalation compounds are thermodynamically stable, the energy associated with the observed occupied interstitial sites (i.e., the external interstitial sites close to the surface of the bundles and the interstitial sites at the bundle extremities) has to be considered different from that of inner interstitial sites. Furthermore, even under these conditions, because the intensity of unfavorable mechanical forces and that of favorable affinity between SWNTs and heavy alkali metals are simultaneously increased in going from potassium to cesium, it is difficult to imagine that these opposing forces accurately balance for all three compounds. Obviously, an additional force must act against the positioning of metal atoms between the tubes within the bundles. As it can be observed in the case of GICs,19 the presence of structural defects disturbing the intertube space such as tube-tube covalent bonds may be responsible
for this limitation in accessibility of alkali vapor to SWNT interstitial channels. Using stronger experimental conditions for the reaction, for example increasing of the reaction temperature, is certainly necessary to assist the breaking of these bonds and to obtain homogeneous structures for the heavy alkali metal intercalation compounds. 3. Conclusion A direct observation of the occupation sites upon intercalation of heavy alkali metal within SWNT bundles has been investigated using a local technique. Electron transfer occurring during the reduction reaction facilitates the dispersion of SWNT bundles in a polar solvent. From the state of preservation of the bundles observed by TEM within these dispersions, the intercalation process itself has been investigated especially regarding the occupation of the interstitial sites. Heterogeneities can be at the origin of the difficulties currently encountered for a complete understanding of the intercalation process in such materials. Acknowledgment. The authors thank J.-M. Chassot for his technical help for Raman experiments. References and Notes (1) Carbon Nanotubes: Synthesis, Structure Properties and Applications; Dresselhaus, M.; Dresselhaus, G.; Avouris, P.; Eds.: Springer-Verlag: Berlin, 2001. (2) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature (London) 1997, 331, 257. (3) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, P. Nano Lett. 2001, 1, 453. (4) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91.
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(5) Duclaux, L. Carbon 2002, 40, 1751. (6) Iwasa, Y.; Fudo, H.; Yatsu, Y.; Mitani, T. Synth. Met. 2001, 121, 1203. (7) Rauf, H.; Pichler, T.; Pfeiffer, R.; Simon, F.; Kuzmany, H.; Popov, V. N. Phys. ReV. B 2006, 74, 235419. (8) Claye, A.; Rahman, S.; Fischer, J. E.; Sirenko, A.; Sumanasekera, G. U.; Eklund, P. C. Chem. Phys. Lett. 2001, 333, 16. (9) Duclaux, L.; Salvetat, J.-P.; Lauginie, P.; Cacciaguera, T.; Fauge`re, A. M.; Goze-Bac, C.; Bernier, P. J. Phys. Chem. Sol. 2003, 64, 571. (10) Bantignies, J.-L.; Alvarez, L.; Aznar, R.; Almairac, R.; Sauvajol, J.-L.; Duclaux, L.; Villain, F Phys. Chem. B 2005, 71, 195419–1/6. (11) Challet, S.; Azaı¨s, P.; Pellenq, R. J.-M.; Duclaux, L. Chem. Phys. Lett. 2003, 377, 544. (12) Graphite Intercalation Compounds and Applications; Enoki, T.; Suzuki, M.; Endo, M., Eds.; Oxford University Press: New York, 2003.
Vigolo et al. (13) Duclaux, L.; Salvetat, J. P.; Lauginie, P.; Cacciaguera, T.; Fauge`re, A. M.; Goze-Bac, C.; Bernier, P. J. Phys. Chem. Solids 2003, 64, 571– 581. (14) He´rold, C.; Lagrange, P. C. R. Chimie 2003, 6, 457. (15) Bendiab, N.; Spina, L.; Zahab, A.; Poncharal, P.; Marlie`re, C; Bantignies, J.-L.; Anglaret, E.; Sauvajol, J.-L. Phys. ReV. B 2001, 63, 153407. (16) Sauvajol, J.-L.; Bendiab, N.; Anglaret, E.; Petit, P. C. R. Physique 2003, 4, 1035. (17) Arab, M.; Picaud, F.; Ramseyer, C.; Babaa, R.; Valsaque, F.; McRae, E. J. Phys. Chem. 2007, 126, 054709-1/10. (18) Pe´nicaud, A.; Poulin, P.; Derre´, A.; Anglaret, E.; Petit, P. J. Am. Chem. Soc. 2005, 127, 8. (19) Synthesis of Graphite Intercalation Compounds; Legrand, A. P.; Frandrois, S.; Eds.; NATO ASI Series, Series B: Physics, 1987, 172, pp 1-43.
JP900546N
