Langmuir 2009, 25, 1795-1799
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Physico-Chemical Properties of Iodine-Adsorbed Single-Walled Carbon Nanotubes Chiharu Hayakawa, Koki Urita, Tomonori Ohba, Hirofumi Kanoh, and Katsumi Kaneko* Department of Chemistry, Graduate School of Science, Chiba UniVersity, Chiba 263-8522, Japan ReceiVed October 14, 2008. ReVised Manuscript ReceiVed December 1, 2008 I2 was adsorbed on single-walled carbon nanotube from ethanol solution at 303 K. The I2 adsorption isotherm was Langmuirian, giving 35 ((10) mg g-1 of the saturated adsorption amount (coverage 0.06-0.09). The I2-adsorption treatment of SWCNT bundles reduced the N2 adsorption amount at 77 K by only 3%; the adsorption amount of supercritical H2 at 77 K was decreased by 30% because of the I2-adsorption treatment, indicating the blocking of interstitial pores by adsorbed I2. These adsorption results indicated the adsorption of I2 molecules in the narrow interstitial pores. The I2-adsorption treatment increases the Raman intensity coming from metallic SWCNTs, and the dc electrical conductivity increased by 15% because of the I2-adsorption treatment, strongly suggesting the presence of charge-transfer interaction between I2 and SWCNTs irrespective of small coverage by I2.
Introduction Single-walled carbon nanotubes (SWCNTs) have gathered a great deal of attention from fundamental and industrial interests.1-11 SWCNTs are special solids because all carbon atoms are exposed to internal and external surfaces, having different roles for molecules according to the sign of the nanoscale curvature. A recent molecular simulation study showed that physically adsorbed N2 on the internal surface of the negative curvature is more ordered than that on the external surface of the positive curvature.12 Therefore, SWCNTs have a unique potential as an adsorbent for gases. The highly surface-sensitive nature of SWCNTs was found by Eklund et al., who showed that even the exposure of SWCNTs to an inert gas changed the electrical conductivity.13 Ordinary SWCNTs14,15 form the bundle structure, which has three kinds of adsorption sites: groove sites, internal spaces, and interstitial spaces. Because the interstitial spaces have the strongest interaction potential for molecules, increases in the interstitial space volume should be attempted to increase the adsorption capacity. It is well known that I2 molecules can intensively affect the conjugated π-electron system.16 One possible route to increasing the interstitial pore capacity is the * Corresponding author. E-mail:
[email protected]. (1) Iijima, S. Nature 1991, 354, 56. (2) Kauffman, D. R.; Star, A. J. Phys. Chem. C 2008, 112, 4430. (3) Klinke, C.; Hannon, J. B.; Afzali, A.; Avouris, P. Nano Lett. 2006, 6, 906. (4) Lee, S. W.; Jeong, G. H.; Campbell, E. E. B. Nano Lett. 2007, 7, 2590. (5) Ki Kang, K.; Jin Sung, P.; Sung Jin, K.; Hong Zhang, G.; Kay Hyeok, A.; Cheol-Min, Y.; Kentaro, S.; Riichiro, S.; Young Hee, L. Phys. ReV. B 2007, 76, 205426. (6) Zhang, G.; Qi, P.; Wang, X.; Lu, Y.; Li, X.; Tu, R.; Bangsaruntip, S.; Mann, D.; Zhang, L.; Dai, H. Science 2006, 314, 974. (7) Krupke, R.; Hennrich, F.; Kappes, M. M.; v. Lohneysen, H. Nano Lett. 2004, 4, 1395. (8) Li, L.-J.; Nicholas, R. J.; Deacon, R. S.; Shields, P. A. Phys. ReV. Lett. 2004, 93, 156104. (9) Ladislav Kavan, L. D. ChemPhysChem 2003, 4, 944. (10) Hou, S.; Shen, Z.; Zhao, X.; Xue, Z. Chem. Phys. Lett. 2003, 373, 308. (11) Carbon Nanotubes; Jorio, A., Dresselhaus, G., Dresselhaus, M. S., Eds.; Springer: Berlin, 2008; p 1. (12) Ohba, T.; Matsumura, T.; Hata, K.; Yumura, M.; Iijima, S.; Kanoh, H.; Kaneko, K. J. Phys. Chem. C 2007, 111, 15660. (13) 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. (14) Williams, K. A.; Eklund, P. C. Chem. Phys. Lett. 2000, 320, 352. (15) Ohba, T.; Kaneko, K. J. Phys. Chem. B 2002, 106, 7171. (16) Miyajima, N.; Dohi, S.; Akatsu, T.; Yamamoto, T.; Yasuda, E.; Tanabe, Y. Carbon 2002, 40, 1533.
intercalation or pillaring of molecules or atoms in the bundle. Eklund et al. reported the intercalation of I2 molecules in the SWCNT bundle by immersing SWCNT mats in molten iodine, and they found a great electrical conductivity enhancement. However, Gotovac et al. succeeded in inserting polynuclear aromatic hydrocarbon molecules in the SWCNT bundles using adsorption from solution.17 Adsorption from solution has great merits for the uniform surface modification of SWCNTs, and we can scale up the modification easily.18-20 Because I2 can induce an evident electronic interaction with SWCNTs, as shown by Eklund et al., the SWCNT bundles should electronically interact with I2 molecules even on adsorption of I2 from solution. This article describes the physicochemical properties of SWCNT modified with I2 adsorbed from solution.
Experimental Section We used HiPCO single-walled carbon nanotubes (Carbon Nanotechnologies, Inc.) with an Fe impurity of 9.9 wt % without further purification. SWCNT samples of 2 mg were ultrasonically dispersed in I2-ethanol solutions of 0.05-50 mg L-1 at 298 K, and then the adsorption isotherm of I2 was determined by measurement of the I2 concentration change using UV colorimetry at 440 nm after deconvolution. The calibration curve between the absorbance at 440 nm and the I2 concentration gave a good linear relation. SWCNT samples having different amounts of adsorbed I2 were obtained after drying the adsorption-treated SWCNTs. The nanoporosity change in the I2-adsorbed SWCNTs was examined with N2 and H2 adsorption at 77 K after heating to 423 K and applying 30 mPa for 2 h. The structural change of SWCNTs with I2 adsorption was examined with field-emission scanning electron microscopy (FE-SEM, JEOL JSM6330F), Raman spectroscopy (JASCO NRS-3100), and X-ray photoelectron spectroscopy (JEOL JPS-9010MX). Also, X-ray powder diffraction patterns were measured at room temperature using the synchrotron X-ray with a radiation wavelength of 0.10018 nm at the Super Photon Ring (SPring-8, Hyogo, Japan); the powder sample was set in a Lindemann glass capillary (0.30 mm outside diameter). The electronic interaction of I2 with SWCNTs was studied by dc electrical conductivity measurement over the temperature range (17) Gotovac, S.; Honda, H.; Hattori, Y.; Takahashi, K.; Kanoh, H.; Kaneko, K. Nano Lett. 2007, 7, 583. (18) Zhao, W.; Song, C.; Pehrsson, P. E. J. Am. Chem. Soc. 2002, 124, 12418. (19) An, K. H.; Yang, C.-M.; Seo, K.; Park, K. A.; Lee, Y. H. Curr. Appl. Phys. 2006, 6, e99. (20) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen, R. Nano Lett. 2002, 2, 25.
10.1021/la803395a CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009
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Figure 1. Adsorption isotherm of I2 on SWCNTs at 298 K. (The experimental points (O) are shown with error bars.)
of 20-300 K. The electrical conductivity of SWCNT samples was measured with the four-probe method using rhodium-coated Ni probes for the sheet prepared by suction filtration. The thickness of the SWCNT sheet was 0.042 mm.
Results and Discussion Adsorption of I2 on SWCNTs from Solution. Figure 1 shows the adsorption isotherm of I2 on SWCNTs at 298 K. The I2 adsorption isotherm is Langmuirian. I2 adsorption is complete
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below 5 mg L-1 of the low I2 concentration, indicating the presence of an intensive interaction between I2 and SWCNTs. The saturated adsorption amount is estimated to be 35 (( 10) mg g-1. The surface coverage of SWCNTs with I2 was evaluated under the assumption of a single I2 molecule and the 1:1 I2-ethanol molecular complex as the surface-adsorbed species. The surface coverages were 0.06 and 0.09, corresponding to the single molecular adsorption of I2 and the adsorption with the 1:1 complex form, respectively. Thus, the adsorption of I2 on SWCNTs is very limited. However, this I2-adsorption treatment affects various properties of SWCNTs, as described below. Structural Characterization of I2-Adsorbed SWCNTs. Figure 2 shows FE-SEM images of SWCNTs and I2-adsorbed SWCNTs. Although we can see clearly the entangled structure of SWCNT bundles before I2 adsorption, the I2-adsorbed SWCNTs have more continuous and less differntiated bundle structures, and thereby the explicit entangled structure cannot be observed. Then, the adsorbed I2 should link the SWCNT bundles to each other because of the strong electronic interaction, which will be described later. Figure 3 shows the adsorption isotherms of N2 on the SWCNTs and the I2-adsorbed SWCNT samples at 77 K. Both of the N2 adsorption isotherms are similar to each other. The I2 adsorption
Figure 2. SEM images of SWCNT bundles before (a, b) and after (c, d) I2 adsorption.
Properties of Iodine-Adsorbed SWCNTs
Figure 3. Adsorption isotherms of N2 on a SWCNT (b) and an I2adsorbed SWCNT (4) at 77 K.
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Figure 5. Adsorption isotherms of H2 on a SWCNT (b) and an I2adsorbed SWCNT (4) at 77 K.
be associated with bundle growth, as shown by FE-SEM. The blocking of the narrow interstitial nanopores of the SWCNT bundles is evidenced by adsorption of supercritical H2 at 77 K. Because supercritical H2 is preferentially adsorbed in narrower nanopores that are not accessible to N2 molecules, the change in the narrow pore volume can be sensitively detected by H2 adsorption, as shown in Figure 5. Figure 5 shows a remarkable difference in the H2 adsorptivity of I2-adsorbed and nontreated SWCNTs; the I2-adsorption treatment markedly reduces the H2 adsorption amount by about 30%. If we assume that the density of H2 adsorbed in nanopores is equal to the bulk liquid density (0.078 g mL-1) at 20 K, then the adsorption amount of 7 mg g-1 at 0.10 MPa for SWCNT corresponds to 0.10 mL g-1, being about half of the nanopore volume determined by N2 adsorption. Hence, these narrower nanopores are preferentially blocked by I2. This is also supported by the isosteric heat of N2 adsorption from the Dubinin-Radushkevich (DR) plot. The DR equation is shown in eq 1.
W/W0)exp[-(A/E)2], A ) RT ln(P0/P), E ) βE0 Figure 4. Comparison plot of N2 adsorption isotherms of I2-adsorbed SWCNTs and SWCNTs. The broken line indicates adsorption on mutually equivalent surfaces.
decreases the N2 adsorption amount over the whole pressure range, indicating the partial blocking of nanopores with adsorbed I2. The surface areas of SWCNT and I2-adsorbed SWCNT from the subtracting pore effect method21 are 585 and 560 m2 g-1, respectively; the nanopore volumes of SWCNT and I2 adsorbed SWCNT from the subtracting pore effect method are 0.220 and 0.190 mL g-1, respectively. A more reliable porosity change due to I2 adsorption can be clearly understood through the comparison plot shown in Figure 4. Here, the ordinate and abscissa are expressed in terms of the N2 adsorption amounts of the I2-adsorbed and -nonadsorbed SWCNT samples at the same relative pressure, respectively. The upward and downward deviations indicate the enhancement and depression of adsorption in the definite pressure range, respectively. A downward deviation in the initial region is observed, accompanying by an almost linear plot in the higher adsorption amount region. The N2 adsorption depression of 20 mg g-1 in the low-pressure region corresponds to 0.025 mL g-1 of the liquid N2 volume, whereas the exclusion volume occupied by molecular I2 of 35 mg g-1 (the saturated adsorption amount) is estimated to be 0.004 mL g-1. Hence, adsorbed I2 blocks the narrow interstitial pores of SWCNT bundles; the smallest diameter of an I2 molecule is 0.40 nm; therefore, part of the narrow nanopores should be occupied by I2 molecules. A downward deviation in the higher adsorption range is observed, indicating the decrease in the external surface area. This reduction should
(1)
Here, W is the amount of adsorption at P/P0, W0 is the limiting amount of adsorption, which often corresponds to the nanopore volume, E0 is the characteristic adsorption energy, and β is the affinity coefficient. βE0 is associated with the isosteric heat of adsorption, qst,1/e, at the fractional filling φ of e-1 using the enthalpy of vaporization ∆Hv at the boiling point through eq 2.
qst,1⁄e ) ∆Hv + βE0
(2)
Because both of the DR plots were linear in the low-pressure region, we obtained the qst,1/e values of 11.1 kJ mol-1 for a SWCNT and 10.4 kJ mol-1 for an I2-adsorbed SWCNTs. The smaller qst,1/e value of the I2-adsorbed SWCNT suggests the relative increment of the wider pores due to the blocking of the narrow nanopores by preadsorbed I2. The X-ray diffraction patterns of SWCNTs and I2-adsorbed SWCNTs were very broad irrespective of using synchrotron X-rays (Supporting Information). Neither of the SWCNT samples gives a clear peak stemming from the ordered bundle structure; the intensity of the peak due to the intertubular spacing is intensified by the I2-adsorption treatment. This must be associated with the bridging effect of adsorbed I2, which induces a partial ordering in the less differentiated bundles. Figure 6 shows the Raman spectral change as a function of the I2-adsorption amount in the wavenumber region of 1200 to 1700 cm-1. Also, the difference spectral change is shown in Figure 6B. Here, the difference spectrum of an I2-adsorbed SWCNT is obtained from the subtraction of the Raman spectrum of the original SWCNT. Although we cannot observe an explicit change in the original spectral change, the difference spectral
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Figure 8. Temperature dependence of the electrical conductivity for a SWCNT (b) and an I2-adsorbed SWCNT (4). Figure 6. (A) Raman spectra and (B) differential spectra as a function of I2-adsorption amount (mg g-1): (a) 36, (b) 30, (c) 26, (d) 14, (e) 0.4, and (f) none. In B, a straight vertical line represents 1544 cm-1, and the broken line (f) indicates the original spectrum.
Figure 9. Mott plots of the logarithm of the electrical resistivity vs T -(n+1) for a SWCNT (b) and an I2-adsorbed SWCNT (4): n ) 1 (a), n ) 2 (b), and n ) 3 (c).
Figure 7. C 1s (left) and I 3d (right) XPS spectra of I2-adsorbed SWCNTs (a) and SWCNTs (b).
change indicates a gradual increase of the intensity due to the metallic SWCNT at 1544 cm-1. Consequently, adsorbed I2 molecules interact strongly with the SWCNT to inject the carrier electrons even though the surface coverage is less than 0.1. However, the RBM band was invariant on adsorption of I2. Electronic Interaction of I2 with SWCNTs. The Raman spectral change with I2 adsorption suggests an increase in the Raman intensity of metallic SWCNTs. The electronic structural change with I2 adsorption was studied with X-ray photoelectron spectroscopy (XPS) and electrical conductivity. Figure 7 shows XPS C 1s and I 3d spectra of I2-adsorbed and -nonadsorbed SWCNTs. The C 1s XPS spectra of both samples are very similar to each other. The C 1s peak at 284.5 eV is assigned to the sp2 carbon, and a weak subpeak at around 290 eV indicates the presence of a small number of surface functional groups such as COO.22 A nontreated SWCNT has no I 3d peak; only an I2-adsorbed SWCNT has the I 3d peak. Accordingly, XPS examination reveals that I2 is adsorbed on SWCNTs. I2 should be adsorbed on SWCNTs with weak charge-transfer interaction with the valence electrons of SWCNT, agreeing with the Raman results. The XPS O 1s band of I2-adsorbed SWCNTs was very similar to that of nontreated SWCNT samples. Accordingly, I2 is not adsorbed specifically on the surface oxygen groups but on (21) Kaneko, K.; Ishii, C. Colloids Surf. 1992, 67, 203. (22) Utsumi, S.; Honda, H.; Hattori, Y.; Kanoh, H.; Takahashi, K.; Sakai, H.; Abe, M.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. C 2007, 111, 5572.
the conjugated π-electron system through the charge-transfer interaction. Because the core electron levels of carbon atoms are insensitive to the I2-adsorption treatment, C 1s does not change on adsorption of I2. The I2-adsorption effect is observed via electrical conductivity, as shown in Figure 8. Figure 8 shows the logarithm of electrical conductivity σ against reciprocal temperature for I2-adsorbed and -nonadsorbed SWCNTs. The ln σ versus 1/T plots are not linear. I2 adsorption increases the electrical conductivity by 15%. This electrical conductivity increase should be caused by weak charge transfer between I2 and SWCNTs, agreeing with the Raman results. The activation energies of I2-adsorbed and nontreated SWCNT samples for conduction from the linear parts in the high-temperature region are 5.2 and 5.7 meV, respectively. The smaller activation energy of I2-adsorbed SWCNT should be associated with the charge-transfer interaction between I2 and SWCNT. Because the ln σ versus 1/T plot is not linear, in particular, in the lower-temperature region, Mott plots are obtained. Figure 9 shows Mott plots23-25 of the logarithm of the electrical resistivity against the reverse of T n+1. Here n ) 1, 2, and 3. Although n ) 1 gives a narrow linear region, both n ) 2 and 3 lead to a wider linear region. Consequently, electronic conduction should be 2D or 3D. Although we cannot determine the dimensionality of the electronic conduction absolutely, it is noteworthy that the electrical (23) Mott, N. F.; Davis, E. A. Electronic Processes in Non-Crystalline Materials; Oxford University Press: London, 1971, p 39. (24) Bandow, S.; Numao, S.; Iijima, S. J. Phys. Chem. C 2007, 111, 11763. (25) Tang, J.; Qin, L. C.; Sasaki, T.; Yudasaka, M.; Matsushita, A.; Iijima, S. Synth. Met. 2001, 121, 1245.
Properties of Iodine-Adsorbed SWCNTs
conductivity increases because of I2 adsorption of less than 0.1 coverage.
Conclusions A SWCNT is efficiently modified with I2 molecules by liquidphase adsorption. The adsorption of I2 on a SWCNT at only 0.06-0.09 coverage increases the conductivity of metallic SWCNT from Raman spectroscopic examination, inducing an enhancement in the dc electrical conductivity by 15%. The I2 molecules should be adsorbed near the interstitial pore entrances of the SWCNT bundles with the charge-transfer interaction, which is evidenced by remarkable blocking of adsorption of supercritical
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H2 at 77 K. Because the modification of SWCNT with adsorption from solution is scalable and it is hoped that it will be useful in technological applications, we need to search the appropriate adsorbate molecules for property control of SWCNTs in future studies. Acknowledgment. This work was supported by a grant-inaid for scientific research (S). Supporting Information Available: Synchrotron X-ray diffraction patterns of SWCNTs and I2-adsorbed SWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org. LA803395A
