J. Phys. Chem. C 2007, 111, 10181-10184
10181
Spectroscopic Evidence on Weak Electron Transfer from Intercalated Iodine Molecules to Single-Walled Carbon Nanotubes Yeongri Jung, Seong-Ju Hwang,* and Sung-Jin Kim* Center of Intelligent Nano-Bio Materials (CINBM), DiVision of Nano Sciences and Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea ReceiVed: December 15, 2006; In Final Form: April 27, 2007
The chemical bonding character of iodine-intercalated single-walled carbon nanotube (I-SWNT) has been systematically investigated with combinative X-ray spectroscopies. According to I LI-edge X-ray absorption near-edge structure (XANES) analysis, a white line peak corresponding to 2s f 5p transition shows nearly identical but slightly stronger intensity for the I-SWNT than for neutral iodine, indicative of a weak electron transfer from intercalated iodine to SWNT. In contrast, there are remarkable spectral differences in the I LI-edge XANES spectra of I-SWNT and iodine-intercalated Bi2Sr2CaCu2Oy. This allows us to rule out the possibility of negatively charged polyiodide ions in the present I-SWNT sample prepared by vapor-phase reaction. Also, we have found that a contact between iodine and SWNT during the synthesis has negligible influence on the chemical bonding nature of intercalated iodine species. The electron transfer from iodine to SWNT is further supported by C 1s X-ray photoelectron spectroscopy revealing the reduction of carbon in SWNT upon iodine intercalation.
Introduction Over the past decade, carbon nanotube has attracted intense research interests because of its various potential applications such as electronic and optoelectronic devices, chemical sensors, and biochemical sensors.1-3 Along with its high chemical stability and high mechanical strength, the carbon nanotube has a unique geometric characteristic that is very advantageous for the use of field electron emitters.4 For a practical application of this material, a fine-tuning of its electronic structure is very crucial to optimize its optical and electrical properties. In this context, a doping of foreign species into the carbon nanotube has received special attention as an effective tool for tuning its electronic structure and its physical properties. In one instance, the intercalation of iodine reduces significantly the electrical resistance and thermoelectric power of single-walled carbon nanotube (SWNT) mats over a wide temperature range.5 Like graphite, SWNT synthesized by the arc discharge method has been known to exhibit an amphoteric behavior. Hence it can be doped with either electron donors (K, Rb) or acceptors (Br2).6 In this regard, iodine-doped SWNT (I-SWNT) could be easily synthesized by reacting SWNT mats with molten iodine.7-9 Grigorian et al. interpreted the characteristic low frequency Raman peaks of I-SWNT prepared by the molten salt method as the vibration modes of negatively charged polyiodide I3- or I5- ion.5 Alternatively, iodine can be intercalated through a vapor-phase reaction.10 For this sample, a micro-Raman spectroscopic study has also been carried out to investigate the variation of lattice vibrations depending on excitation wavelength.11 In order to quantitatively estimate the degree of electron transfer between iodine and the SWNT, we have adopted I LIedge X-ray absorption near-edge structure (XANES) analysis, * To whom correspondence should be addressed: Telephone: +82-23277-2350 (S.-J.K.); +82-2-3277-4370 (S.-J.H.). Fax: +82-2-3277-3419 (S.-J.H.). E-mail:
[email protected] (S.-J.K.); hwangsju@ ewha.ac.kr (S.-J.H.).
because the intensity of a characteristic resonance peak in this spectrum can provide a straightforward and quantitative measure for the oxidation state of iodine. In this work, we have carried out combinative X-ray absorption and X-ray photoelectron spectroscopic studies on I-SWNT to elucidate the chemical bonding character of iodine intercalated in SWNT. Experimental Section Iodine intercalation into SWNT was achieved by heating an evacuated quartz tube containing SWNT (ILJIN Nanotech Co. Korea) and an excess of iodine at 250 °C for 3 days. To avoid direct contact between the reactants, the SWNT bundles are separated from excess iodine by a small quartz container (hereafter this method is denoted as an indirect method). Alternatively, the I-SWNT sample was prepared without the container; hence the intercalation reaction proceeds with a direct contact between the SWNT and iodine melt (hereafter this method is denoted as a direct method). However, it is worthwhile to note that, even in the direct method, the host SWNT is not completely soaked in the liquid of molten iodine. After the completion of the reaction, residual iodine on the sample surface was removed by heating the product at 120 °C for 2-4 h to submerge on the other end of the tube cooled by liquid nitrogen. The complete removal of surface-adsorbed iodine species was verified by the thermogravimetric analysis (TGA) results (see Supporting Information), in which an abrupt weight loss at 50-110 °C corresponding to the evaporation of surfaceadsorbed iodine species disappears after the heat treatment. The resulting I-SWNT samples were found to be stable under ambient conditions, permitting measurements in air. To check the reversibility of the iodine intercalation, I-SWNT samples were heated to 700 °C for 6 h under flowing N2 gas. Energy dispersive spectrometric (EDS) analysis clearly demonstrated that the heat treatment gives rise to a remarkable decrease of
10.1021/jp068629x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
10182 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Figure 1. PXRD data of (a) pristine SWNT and I-SWNT compounds prepared by (b) direct and (d) indirect methods, and (c,e) their disintercalated products.
Jung et al.
Figure 2. I LI-edge XANES spectra for I-SWNT compounds prepared by direct (solid lines) and indirect (dashed lines) methods, in comparison with those for the reference iodine (circles), I-Bi2Sr2CaCu2Oy (dotteddashed lines), and LiI (triangles).
Results and Discussion iodine/carbon ratio from 19.4/80.6 to 0.1/99.9, confirming the complete disintercalation of iodine. The formation of I-SWNT was confirmed by powder X-ray diffraction (PXRD) analysis with a monochromatized beam of Cu KR radiation (λ ) 1.5418 Å). As shown in Figure 1, the iodine intercalation through direct and indirect methods commonly suppresses diffraction peaks corresponding to the hexagonal packing lattice of the pristine SWNT bundles, indicating that iodine acts as a chemical wedge entering the interstitial channels between the nanotubes.5 After the heat treatment at 700 °C, the (100) peak of the pristine SWNT is restored near its original position with weaker intensity,5 strongly suggesting that the long-range ordering of the SWNT is recovered by the disintercalation of iodine. In spite of the complete disintercalation of iodine, the heat-treated I-SWNT samples show a much weaker (100) peak compared to the pristine SWNT. This can be interpreted as evidence of the incomplete restoration of hexagonal ordering of the SWNT fibers. X-ray photoelectron spectroscopy (XPS) data were recorded by using a VG Scientifics ESCALAB 250 XPS spectrometer. The binding energy (BE) of each spectrum was referenced to the Ag 3d peak at 368.26 eV. X-ray absorption spectroscopic (XAS) experiments were performed at the I LI-edge using an extended X-ray absorption fine structure (EXAFS) facility installed at beam line 7C of the Pohang Accelerator Laboratory (PAL) in Pohang, Korea. The XAS measurements were carried out at room temperature in a transmission mode using gasionization detectors. The silicon (111) double crystal monochromator, detuned to 60% of the maximum intensity to minimize the higher harmonics, was utilized for the I LI-edge measurement. All the present spectra were calibrated by measuring the spectrum of iodine element. Energy resolution for the present I LI-edge XANES spectra is about 0.5 eV. The data analysis for the experimental spectra was performed by the standard procedure reported previously.12 The inherent background in the data was removed by fitting a polynomial to the pre-edge region and extrapolated through the entire spectrum, from which it was subtracted. The resulting spectra were normalized to an edge jump of unity for comparing the XANES features directly with one another. For the quantitative analysis of spectral features in the XANES region, the normalized spectra are carefully convoluted with Lorentzian and sigmoid step functions.
First, a charge transfer between host and guest has been investigated using I 3d XPS analysis (see Supporting Information). A set of spin doublets is observed for the I-SWNT, like the references KI and KIO4. The BE of I 3d5/2 peak for the I-SWNT (619.6-619.7 eV) appears to be slightly higher than that for KI (619.2 eV). However, it has been well-known that a minute change of iodine oxidation state from neutral molecular state gives only a slight shift of BE (I-, 618.0-619.5 eV; I0, 619.9 eV), which is comparable to the BE variation of the same valent iodine species upon change in the chemical environment such as the type of countercation.13 Hence, it is very difficult to conclusively assign the iodine oxidation state on the basis of the XPS results. In this regard, we have applied I LI-edge XANES spectroscopy for determining the oxidation state of iodine, since the intensity of an XANES feature in the I LIedge can provide a quantitative measure for the density of unoccupied I 5p state. The normalized I LI-edge XANES spectra of I-SWNT compounds prepared by direct and indirect methods are presented in Figure 2, in comparison with those of several references. In addition to I2 and LiI, iodine-intercalated Bi2Sr2CaCu2O superconductor has been adopted as a reference, since the iodine species intercalated in this compound has been wellknown to exist as triiodide (I3-) ions.12 As shown in Figure 2, a sharp absorption resonance called white line (WL) corresponding to the 2s f 5p transition is discernible at ∼5186 eV for all the present compounds except LiI.13,14 Since the WL intensity is associated with the density of unoccupied final states,14 it can be used to estimate the number of unoccupied p-orbitals. In the spectrum of neutral iodine molecule with an electronic configuration of 5p5, the WL peak appears rather intense due to the presence of an unoccupied hole in the final 5p state. However, this peak is not detectable for LiI because the iodine in this compound has fully occupied 5p6 states. Instead, another peak C corresponding to the transition to continuum state is observed at a higher energy of ∼5193 eV. On the other hand, the WL intensity for I-Bi2Sr2CaCu2Oy is somewhat depressed compared to that for neutral iodine, which indicates that the unoccupied I 5p orbital is partially filled with the electron transferred from the host lattice.13 Unlike I-Bi2Sr2CaCu2Oy, both I-SWNT compounds prepared by the direct and indirect methods show a slightly enhanced WL feature compared to neutral iodine, suggestive of a little electron transfer from iodine to SWNT. Of special note is that a contact between
Chemical Bonding Character in I-SWNT
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10183
Figure 3. Experimental I LI-edge XANES data (open circles) and the fit to the sum (solid lines) of the Lorentzian (dashed lines) and sigmoidal functions (dotted-dashed lines) for (a) solid iodine and I-SWNT compounds prepared by (b) direct and (c) indirect methods.
SWNT and iodine during the synthesis has only a negligible influence on the bonding nature of the intercalated iodine species. In order to quantitatively determine the degree of electron transfer between iodine and SWNT, we have evaluated the area of the WL feature by convoluting the spline spectra with Lorentzian and sigmoidal functions, as plotted in Figure 3. The peak areas of both I-SWNTs were determined to be somewhat larger than that of the neutral iodine molecule (Table 1), indicating that the iodine species in the I-SWNTs are weakly positive-charged with the average oxidation state of I+0.08-0.1. In fact, such a small positive charge in the intercalated iodine has been reported for iodine species intercalated fullerene C60.14 However, it should be taken into consideration that there is a report claiming the existence of negatively charged polyiodide ions in I-SWNT synthesized with molten iodine.5 In that study, the authors claimed that their preliminary I 3d XPS results support the presence of polyiodide chains in I-SWNT. However, as described above, I 3d XPS is not a suitable tool for probing the existence of partially charged polyiodide (I-1/n)n species, since the variation of BE is not sensitive enough to distinguish this partially charged state from neutral I0 and negative I-.13 Moreover, recent XPS study on I-SWNT reported
that the BE value of the I 3d5/2 peak is within those accepted for neutral I2.15 While Grigorian et al. interpreted Raman peaks of I-SWNT at 175 and 109 cm-1 as the vibration modes of negatively charged polyiodide ions,5 the present I-SWNT sample prepared by vapor-phase reaction does not exhibit any resonance of the Raman peak at 174 cm-1 with a 647.1 nm laser.11 The photoluminescence spectrum of I-SWNT indicates that the phonon mode of polyiodide should be resonant with this excitation source.16 In this regard, this Raman peak was assigned as a radial breathing mode of the pristine SWNT, not as a stretching vibration of I3- or I5- molecule.17 Hence, it becomes certain that the iodine species in the present I-SWNT sample are not stabilized as polyiodide chain molecules. This conclusion is further supported by the fact that there are remarkable differences between the WL peak areas of I-SWNT and I-Bi2Sr2CaCu2Oy (Figure 2), confirming the absence of negatively charged polyiodide species in the present I-SWNT sample prepared by vapor-phase reaction. Based on the present experimental findings, we are able to conclude that small amount of electrons (0.08-0.1e- per iodine) is transferred from iodine to SWNT. It is noteworthy that the present result is surely different from the previous report regarding the negatively charged state of bromine species intercalated in SWNT.18 Such a difference between Br-intercalated SWNT (i.e., Br-SWNT) and I-SWNT can be explained from the viewpoint of their electronic structures. According to the band calculation on the BrSWNT,19 the LUMO of bromine has energy similar to the Fermi level of metallic SWNT or the valence band of semiconducting SWNT. This makes possible electron transfer from the SWNT to the bromine. Taking into account the fact that the energy level of an element is inversely proportional to its electronegativity,20 the lower electronegativity of the iodine lifts its LUMO energy above the Fermi energy or valence band position of the SWNT. This prevents the SWNT from donating electrons to the iodine. Instead, the increase of the HOMO energy of iodine is supposed to induce electron transfer from iodine to the SWNT, which is supported by the present experimental findings. In addition, the theoretical calculation studies clearly demonstrated that the intercalated halogen becomes mostly stabilized on the top of carbon in the SWNT.18,19 Since the bond distance of Br-Br is slightly shorter than that of C-C,21 the elongation of the former bond enhances the matching with the latter bond. Since the reduction of bromine increases electron density in antibonding σ*(Br 4p) orbitals, it gives rise to the elongation and stabilization of Br-Br bonds. Therefore, the electron transfer from the SWNT to bromine is also favorable from the viewpoint of crystal structure. In contrast to the Br-
TABLE 1: Lorentzian and Sigmoid Line Fitting Results of I LI-Edge XANES Spectra for I2 and I-SWNT Compounds Prepared by Direct and Indirect Methods Lorentzian linea c
a0 I2 I-SWNT-direct I-SWNT-indirect
1.22 1.29 1.27
wc
(eV)
3.43 3.60 3.56
sigmoid lineb
E0 (eV)
b0
b1 (eV)
E1 (eV)
WL areac,d (eV)
hole
iodine oxidation state
5186.30 5186.23 5186.25
0.96 0.98 0.98
0.88 1.08 1.09
5191.27 5190.82 5190.63
6.61 7.28 7.11
1.00 1.10e 1.08e
0 +0.10 +0.08
a The symbols a0, w, and E0 represent the maximum amplitude, the full width at the half-maximum (fwhm), and the energy at a0 of the Lorentzian line, respectively, determined by fitting the following Lorentzian equation to the normalized XANES data: f(E) ) a0[(w/2)2/((w/2)2 + (E - E0)2)], where a0 is c/[π(w/2)] with amplitude constant (c). b The symbols b0, b1, and E2 represent the step, the fwhm, and the inflection position of the sigmoid line in energy, respectively, determined by fitting the following sigmoid step function to the normalized XANES data: f(E) ) b0/[1 + exp{-(E - E1)}/b1]. c The uncertainty of a0 estimation is (0.01 for all the samples. The uncertainty of w estimation is (0.04 for I2 and (0.03 for both I-SWNTs. The uncertainty of area estimation is (0.13 for I2, (0.10 for I-SWNT-direct, and (0.11 for I-SWNT-indirect. d The WL area was calculated by integrating the Lorentzian line. e The amount of holes was estimated by imposing spectral weight to the area of each WL peak.
10184 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Jung et al. a contact between reactants does not significantly affect the oxidation state of intercalated iodine species. In agreement with the I LI-edge XANES results presented here, C 1s XPS analysis reveals a slight reduction of carbon in SWNT upon iodine intercalation. From the present experimental findings, it becomes certain that iodine can act as a weak electron donor for SWNT. Acknowledgment. This work was supported by the Seoul Research and Business Development Program (10816) and by the SRC/ERC program of the MOST/KOSEF (Grant No. R112005-008-00000-0). The experiments at PAL were supported in part by MOST and POSTECH.
Figure 4. C 1s XPS data for (a) pristine SWNT (solid lines) and (b) I-SWNT (dashed lines).
Br bond, the I-I bond has a longer distance than the C-C bond.21 Hence, in this case, the oxidation of iodine enhances the matching between both bonds through the decrease of electron density in antibonding π*(I 5p) orbitals and the shortening of the I-I bonds, leading to the stabilization of the I-I bonds. The effect of iodine intercalation on the chemical bonding nature of carbon in SWNT was probed with C 1s XPS tool to confirm the direction of electron transfer between iodine and SWNT. Figure 4 represents C 1s XPS spectra of the pristine SWNT and the iodine-intercalated I-SWNT. Both present compounds commonly display a single asymmetric peak in the region of 283-286 eV. This feature shows a slight but distinct shift by ∼0.2 eV toward the low-energy side upon iodine intercalation, evidencing the reduction of carbon in SWNT. This is in good agreement with the I LI-edge XANES result showing the slight oxidation of iodine species. Similarly, a distinct C 1s peak toward the lower energy side is induced by the intercalation of a large amount of alkali metal ions into the fullerene, resulting in the formation of KxC60 (x ∼ 2.8) compound.22,23 It is very reasonable that the provision of electron density for the SWNT results in the low-energy shift of the C 1s XPS peak, since the increase of electron density in the carbon of the SWNT can lead to more effective shielding of nuclear charge. In this regard, the observed downshift of the C 1s XPS peak upon iodine intercalation can be interpreted as evidence of the increase of electron density in the SWNT by intercalated iodine. In addition, such a weak electron transfer between iodine and SWNT would reflect their amphoteric natures. Conclusion In the present study, we have investigated electron transfer between iodine and SWNT using combinative XAS and XPS analysis. The I LI-edge XANES analyses clearly demonstrate a slight increase of the white line peak upon iodine intercalation, indicative of a partially positive-charged state of I+0.08-0.1. Remarkable spectral differences are observed for the XANES spectra of I-SWNT and iodine-intercalated Bi2Sr2CaCu2Oy, underscoring the absence of polyiodide ions in the I-SWNT sample prepared by vapor-phase reaction. It is also found that
Supporting Information Available: TGA curves of the asprepared I-SWNT and its derivative heat-treated at 120 °C, and the I 3d XPS spectra of I-SWNT, KIO4, and KI. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (2) Rueekes, T.; Kim, K.; Jeselvieh, E.; Tseng, G.; Cheung, G. L.; Lieber, C. M. Science 2000, 289, 94. (3) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (4) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (5) Grigorian, L.; Williams, K. A.; Fang, S.; Sumanasekera, G. U.; Loper, A. L.; Dickey, E. C.; Pennycook, S. J.; Eklund, P. C. Phys. ReV. Lett. 1998, 80, 5560. (6) Lee, R. S.; Kim, H. J.; Fisher, J. E.; Thess, A.; Smally, R. E. Nature 1997, 388, 255. (7) Michel, T.; Alvarez, L.; Sauvajol, J.-L.; Almairac, R.; Aznar, R.; Bantignies, J.-L. Phys. ReV. B 2006, 73, 195419. (8) Fan, X.; Dickey, E. C.; Eklund, P. C.; Williams, K. A.; Grigorian, L.; Buczko, R.; Pantelides, S. T.; Pennycook, S. J. Phys. ReV. B 2000, 84, 4621. (9) Venkateswaren, U. D.; Brandsen, E. A.; Katakowski, M. E. Phys. ReV. B 2002, 65, 054102. (10) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 257. (11) Nguyen, V. M.; Yang, I. S.; Jung, Y.; Kim, S. J.; Oh, J.; Yi, W. IEEE Nanotechnol. 2007, 6, 126. (12) Park, D. H.; Hur, S. G.; Jun, J. H.; Hwang, S.-J. J. Phys. Chem. B 2004, 108, 18455. (13) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. H., Muilenberg, G. E., Eds.; PerkinElmer Corp.: Eden Prairie, MN, 1979. (14) Park, N. G.; Choy, S. W.; Kim, S.-J.; Choy, J.-H. Chem. Mater. 1996, 8, 324. (15) Kissell, K. R.; Hartman, K. B.; Van der Heide, P. A. W.; Wilson, L. J. J. Phys. Chem. B 2006, 110, 17425. (16) Baskin, L. M.; Lvov, O. I.; Fursey, G. N. Phys. Status Solidi B 1971, 47, 49. (17) Huong, P. V.; Verma, A. L. Phys. ReV. B 1993, 48, 9869. (18) Jhi, S.-H.; Louie, S.; Cohen, M. L. Solid State Commun. 2002, 123, 495. (19) Park, N.; Miyamoto, Y.; Lee, K.; Choi, W. I.; Ihm, J.; Yu, J.; Han, S. Chem. Phys. Lett. 2005, 403, 135. (20) Huheey, J. H.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and ReactiVity; HarperCollins: New York, 1993; p 171. (21) Wells, A.-F. Structural Inorganic Chemistry; Clarendon Press: Oxford, 1984. (22) Poirier, D. M.; Ohno, T. R.; Kroll, G. H.; Benning, P. J.; Stepniak, F.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Phys. ReV. B 1993, 47, 9870. (23) Pichler, T.; Rauf, H.; Knupfer, M.; Fink, J.; Kataura, H. Synth. Met. 2005, 153, 333.
