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2005, 109, 6037-6039 Published on Web 03/10/2005
Dissociation of Electrolytes in a Nano-aqueous System within Single-Wall Carbon Nanotubes M. Zhang,*,† M. Yudasaka,†,‡ and S. Iijima†,‡,§ SORST-JST, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, and Meijo UniVersity, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan ReceiVed: December 10, 2004; In Final Form: February 20, 2005
Research on material incorporation within single-wall carbon nanotubes (SWNTs) through aqueous solutions of various electrolytes is performed for the purpose of providing a foundation for future application of SWNTs to, for example, drug delivery systems. We have determined that the optical spectra of SWNTs were significantly affected when SWNTs that had opened holes or removed caps were treated through immersion in an aqueous solution of electrolytes, followed by drying at room temperature; however, the spectra of SWNTs without opened holes or removed caps were not subjected to such treatment. We infer that when the sucked solutions remained inside the tubes, even after drying (the nano-aqueous system), the electrolyte was dissociated into ions, which was likely to change the electronic states of SWNTs. On the other hand, when the SWNTs were well-dried under vacuum, no remarkable changes in their optical spectra were observed.
Introduction (SWNTs)1
have attracted much Single-wall carbon nanotubes interest, because of their unique structures and properties.2 An important property of SWNTs is their capacity to adsorb molecules. It is well-known that C60 3 can be incorporated inside SWNTs without significantly changing the electronic states of the SWNTs. On the other hand, when incorporated molecules or atoms induce charge transfer, the electrical properties of the SWNTs are changed. This type of incorporation is called intercalation. It has been proven that alkali metals (lithium, sodium, potassium, rubidium, and cesium) and halogens such as iodine,4-8 iron(III) chloride (FeCl3),7,10 and potassium iodide (KI)11 can be intercalated inside SWNTs. To date, the incorporation of molecules or atoms has generally been performed by melting or evaporating the incorporating materials. However, to extend the application of SWNTs in the field of drug delivery systems, incorporation must be performed in the liquid phase at room temperature, because many drugs are thermally unstable. Recently, our group developed a method12 for incorporating C60 into SWNTs in organic solvents. However, we did not know whether aqueous solution of organic or inorganic materials could be incorporated inside SWNTs. In this report, we investigate the changes of the electronic states of SWNTs that have been treated with various aqueous solutions of electrolytes. Raman spectra and optical spectra provide strong evidence that charge transfer occurred between the SWNTs and ions in nano-ionic aqueous systems (i.e., ions and water remaining most likely inside the SWNTs and possibly in interchannels between the SWNT forming bundles). The aqueous solutions used in our * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † SORST-JST. ‡ NEC. § Meijo University.
10.1021/jp044372w CCC: $30.25
experiments are often used in the chemical treatments of SWNTs, such as purification, separation, and chemical reaction, so our results would also be helpful for explaining many phenomena that occur in those processes. Experimental Section SWNTs synthesized using the HiPco13 method were purchased from Carbon Nanotechnologies, Inc. We purified them via heat treatment at 1730 °C under vacuum for 5 h, to reduce the iron content from ∼50% to 2%, and enlarged the SWNT diameters from ∼0.8-1.2 nm to 1.2-1.5 nm.14 To remove caps at the tube tips and open holes at the defect sites of tube walls, the HiPco SWNTs were further treated in an oxygen atmosphere at 550 °C for ∼5 min (we refer to these as HT-HiPco SWNTs).12 This oxygen-gas treatment also changed the SWNTs from hydrophobic to hydrophilic.15 After this treatment, HT-HiPco SWNTs were dispersed into H2O and various aqueous solutions (such as 0.2 N HCl, 0.2 N NaCl, 0.2 N KCl, 0.2 N K2SO4, 0.2 N CH3COONH4, and 30% H2O2). Dispersions of the SWNTs were deposited dropwise onto quartz plates and allowed to form films. They were then dried at room temperature for >24 h in air. Raman spectra (488 nm excitation, Ar+ ion laser) and ultraviolet/visible/near-infrared (UV-Vis-NIR) absorption spectra of the dried films were measured. To determine the effect of adsorbed water on the optical properties, the specimens were further dried at 200 °C under vacuum for 1 h, and their Raman and absorption spectra were again measured and compared with those before the vacuum drying. Results Figure 1 shows results for HT-HiPco SWNTs after treatment with various solutions and dried at room temperature for >24 h. The tangential mode peaks of the Raman spectra of © 2005 American Chemical Society
6038 J. Phys. Chem. B, Vol. 109, No. 13, 2005
Figure 1. Raman spectra (excitation wavelength ) 488 nm) of untreated HT-HiPco SWNTs and those treated with H2O and various aqueous solutions of 0.2 N HCl, H2O2, NaCl, KCl, CH3COONH4, and K2SO4 and then dried at room temperature for >24 h.
HT-HiPco SWNTs appeared at ∼1591 and 1567 cm-1. The radial breathing mode (RBM) peaks appeared at 164, 185, and 202 cm-1, which corresponded to SWNT diameters of 1.49, 1.35, and 1.2 nm, respectively.16 When the SWNTs were treated with H2O, the intensity of the RBM peak at 1.49 nm decreased, whereas the other RBM peaks remained almost unchanged. When the HT-HiPco SWNTs were treated with a solution of CH3COONH4 or HCl, all peaks greatly decreased. When an ionic solution of NaCl or KCl was used, the RBM peaks at 164 and 185 cm-1 vanished completely and the peak at 202 cm-1 and the tangential peaks were greatly reduced. Treatment of the SWNTs with H2O2 caused all RBM peaks to disappear. On the other hand, the Raman spectra of SWNTs treated with a K2SO4 solution showed that their RBM peaks did not decrease but did seem to increase slightly. All of these results indicate that the treatment with aqueous solutions of electrolytes changed the electronic states of the SWNTs. Thermogravimetric and mass spectroscopic (TG-MS) measurements revealed that ∼5-8 wt % H2O remained in the HT-HiPco SWNTs. To remove the remaining water, we further dried the SWNT samples under vacuum at 200 °C for 1 h. The Raman spectra from these samples are shown in Figure 2. For SWNTs treated with H2O, HCl, H2O2, CH3COONH4, or K2SO4, the peak distribution of the Raman spectra was almost the same as that for the HT-HiPco SWNTs. For the SWNTs treated with the solutions of NaCl and KCl, the intensities of the RBM peaks corresponding to 1.33- and 1.49-nm SWNTs were slightly less than those observed before treatment; however, the intensities of peaks corresponding to the 1.2-nm SWNTs and of G-band peaks were similar to those of the HT-HiPco SWNTs. The UV-Vis-NIR spectra of HT-HiPco SWNTs treated with H2O (Figure 3a) showed a broad peak between 1100 nm and 1800 nm, and this peak corresponds to the first interband transition (S11) of the semiconductor-type SWNTs forming bundles.17 In contrast, there was no peak in this range when the HT-HiPco SWNTs were treated with an aqueous solution such as the KCl or K2SO4 solution (see Figure 3b and 3c). When we further dried the samples under vacuum at 200 °C, the UVVis-NIR absorption spectra again showed the S11 peak for SWNTs treated with H2O, and a S11 peak also was restored
Letters
Figure 2. Raman spectra (excitation wavelength ) 488 nm) of untreated HT-HiPco SWNTs and those treated with H2O and various aqueous solutions of HCl, H2O2, NaCl, KCl, CH3COONH4, and K2SO4 and then dried at 200 °C under vacuum for 1 h.
Figure 3. UV-Vis-NIR absorption spectra of HT-HiPco SWNTs treated with (a) H2O, (b) 0.2 N KCl, and (c) 0.2 N K2SO4; the broken lines show the spectra of HT-HiPco SWNTs treated with H2O and various solutions, dried at room temperature, and the solid lines show the absorption spectra of the SWNTs after they were further dried at 200 °C under vacuum for 1 h.
for SWNTs treated with KCl or K2SO4. These results were consistent with those observed in the Raman spectra. Discussion Research on the intercalation of SWNTs has revealed that charge transfer between SWNTs and ions might induce the Fermi level of SWNTs to shift and then influence the resonance condition of the Raman scattering of SWNTs.4,7,10,18-21 The significant changes in the Raman spectra and the disappearance
Letters TABLE 1: Radii of (a) Intertube Channels within a Triangular Lattice of Bundles and (b) Some Ions (a) Radii of Intertube Channels SWNT diameter (nm) 1.0 1.1 1.2 1.3 1.4 1.5 Rinter (nm) 0.104 0.111 0.119 0.127 0.135 0.142 (b) Radii of Some Ions ion OH- ClCH3COO- SO42- K+ Na+ NH4+ Rion (nm) 0.132 0.167 0.18 0.23 0.152 0.113 0.127
of the peak in the S11 region in the UV-Vis-NIR spectra of SWNTs after treatment with various solutions suggest that charge transfer occurred between SWNTs and ions or an aqueous solution of electrolytes.22 When SWNTs treated with aqueous solutions of salts were further dried under vacuum at 200 °C, the anions and cations in the SWNTs should have crystallized as salts, because of the evaporation of water. Transmission electron microscopy (TEM) observation (not shown) and energy-dispersive X-ray (EDX) analysis (not shown) indicated that there were particles of, perhaps, the salt crystal inside the tubes and at the surface of the bundles of SWNTs when the HT-SWNTs were treated with solutions of KCl, NaCl, and K2SO4. Therefore, the Raman spectra of the SWNTs were restored to their original spectra, and the S11 peak in the UVVis-NIR spectra reappeared. The question is: where were the ions and water? Raman spectra of the HiPco SWNTs that did not undergo the cap removal and hole opening did not show any remarkable changes by treatment using an aqueous solution of NaCl, etc.; therefore, we infer that the changes in electronic state were mainly caused by aqueous solution entrapped inside the SWNTs. On the other hand, the SWNT diameters (>1 nm) were much larger than the diameter of the water molecule. Therefore, water and aqueous solutions of electrolytes could easily enter the SWNTs. Rossi et al. have shown23 that water can enter nanotubes, and they were able to observe the liquid flow, condensation, and evaporation inside carbon nanotubes. Thus, we are confident that a nano-aqueous solution can exist in the SWNTs. Calculated results for intertube channels within a triangular lattice of bundles10 indicate that water insertion into the intertube channels of an SWNT bundle is unlikely to occur. The interchannel radii shown in Table 1 were calculated using the equation Rinter ) (RNT + Rvdw)(2/31/2 - 1), where Rinter is the radius of the intertube channels within a triangular lattice of bundles, RNT is the nanotube radius, and Rvdw is the van der Waals radius of carbon (Rvdw ) 0.17 nm). The calculated intertube channel diameters are much narrower than the diameter of the water molecule (0.3 nm). It is also notable that an intercalated aqueous solution of organic or inorganic electrolyte in SWNTs could be removed by washing with water several times. Therefore, we think that the solution was not so strongly adsorbed inside the SWNTs. This would be meaningful for future applications such as drug delivery systems, in terms of the release of drugs in living bodies. Summary We have found that the Raman spectra and optical absorption spectra of single-wall carbon nanotubes (SWNTs) were greatly changed after the SWNTs were immersed in aqueous solutions of organic and inorganic electrolytes and then dried at room
J. Phys. Chem. B, Vol. 109, No. 13, 2005 6039 temperature; however, these changes could be reversed by further treatment at 200 °C under vacuum. We infer, from these results, that charge transfer occurred between the SWNTs and ions or the aqueous solution of ions. This could be a result of dissociation of the electrolytes with water inside the SWNTs. Because the aqueous solutions inside the SWNTs caused enormous changes to the electronic states of the SWNTs, we refer them as “nano-aqueous systems”, to indicate their strong effects. In this letter, we do not discuss the details of the charge transfer and its relation to the changes in electronic structure, because we could not clarify the details of the nano-aqueous system (i.e., the concentrations of ions/electrolytes, the dissociation constants of the electrolytes, or the adsorption sites of ions/electrolytes inside the tubes). To clarify these, we plan to conduct experiments to study the release of the incorporated salts in water, and a detailed discussion will be reported in the future. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph. Carbon Nanotube Synthesis, Structure, Properties, and Applications; Springer-Verlag: Berlin, Heidelberg, New York, 2001. (3) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 369, 323. (4) Rao, R. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 257. (5) Lee, R. S.; Kim, H. J.; Fischer, J. E.; Thess, A.; Smalley, R. E. Nature 1997, 388, 255. (6) Pichler, T.; Sing, M.; Knupfer, M.; Golden, M. S.; Fink, J. Solid State Commun. 1999, 109, 721. (7) Pichler, T.; Kukovecz, A.; Kuzmany, H.; Kataura, H. Synth. Met. 2003, 135, 717. (8) Jhi, S. H.; Louie, S. G.; Cohen, M. L. Solid State Commun. 2002, 123, 495. (9) Pichler, T.; Liu, X.; Knupfer, M.; Fink, J. New J. Phys. 2003, 5, 156. (10) Kukovecz, A.; Pichler, T.; Pfeiffer, R.; Kramberger, C.; Kuzmany, H. Phys. Chem. Chem. Phys. 2003, 5, 582. (11) Sloan, J.; Novotny, M. C.; Bailey, S. R.; Brown, G.; Xu, C.; Williams, V. C.; Friedrichs, S.; Flahaut, E.; Callender, R. L.; York, A. P. E.; Coleman, K. S.; Green, M. L. H.; Dunin-Borkowski, R. E.; Hutchison, J. L. Chem. Phys. Lett. 2000, 329, 61. (12) Yudasaka, M.; Ajima, K.; Suenaga, K.; Ichihashi, T.; Hashimoto, A.; Iijima, S. Chem. Phys. Lett. 2003, 380, 42. (13) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (14) Yudasaka, M.; Kataura, H.; Ichihashi, T.; Qin, L. C.; Kar, S.; Iijima, S. Nano Lett. 2001, 1, 487. (15) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.; Hiura, H. Nature 1993, 362, 522. (16) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (17) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555. (18) Claye, A.; Rahman, S.; Fischer, J. E.; Sirenko, A.; Sumanasekera, G. U.; Eklund, P. C. Chem. Phys. Lett. 2001, 333, 16. (19) Kavan, L.; Rapta, P.; Dunsch, L.; Bronikowski, M. J.; Willis, P.; Smalley, R. E. J. Phys. Chem. B 2001, 105, 10764. (20) Stoll, M.; Rafailov, P. M.; Frenzel, W.; Thomsen, C. Chem. Phys. Lett. 2003, 375, 625. (21) Corio, P.; Jorio, A.; Demir, N.; Dresselhaus, M. S. Chem. Phys. Lett. 2004, 392, 39. (22) The salts on the surface of SWNT bundles should be in the solid state, because the water on the surface vaporized easily at room temperature. Their effect on the Raman and absorption spectra was considered to be the same as that after further drying under vacuum. (23) Rossi, M. P.; Ye, H.; Gogotsi, Y.; Babu, S.; Ndungu, P.; Bradley, J. C. Nano Lett. 2004, 4, 989.
