J. Phys. Chem. B 2008, 112, 12263–12269
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Wrapping of Single-Walled Carbon Nanotubes by a π-Conjugated Polymer: The Role of Polymer Conformation-Controlled Size Selectivity Wenhui Yi,†,| Andrey Malkovskiy,‡ Qinghui Chu,† Alexei P. Sokolov,‡ Marisabel Lebron Colon,§ Michael Meador,§ and Yi Pang*,† Department of Chemistry, UniVersity of Akron, Ohio 44325, Department of Polymer Science, UniVersity of Akron, Ohio 44325, Structures and Materials DiVision, NASA Glenn Research Center, CleVeland, Ohio 44135, and Department of Electronic Science and Technology, School of Information and Electronics Engineering, Xi’an Jiaotong UniVeristy, Xi’an 710049, China ReceiVed: May 8, 2008; ReVised Manuscript ReceiVed: July 31, 2008
Wrapping of a single-walled carbon nanotube (SWNT) was examined by using a poly[(m-phenylenevinylene)alt-(p-phenylenevinylene)] (PmPV) derivative. The polymer’s intrinsic ability in forming a helical conformation was found to play an essential role in the separation of nanotubes. Among about 15 tubes present in the pure SWNT (HiPcoTM) sample, the polymer was found to selectively pick up the tubes (11,6), (11,7) and (12,6), which correspond to tube diameters of 1.19, 1.25 and 1.24 nm, respectively. The SWNTs of smaller diameters were held loosely by the PmPV, and were gradually dropped out under centrifugation. The suspension solution prepared from the SWNT and PmPV was not permanently stable, with precipitation occurring after a few weeks. Irradiation in the UV-vis region exhibited a catalytic effect to shorten the precipitation time to hours. Those tubes, which were held loosely by PmPV, were quickly separated from the suspension during the irradiation process. Introduction Carbon nanotubes are rolled graphite sheets along a certain angle or vector. The electronic structure and properties of the single-walled carbon nanotubes (SWNTs) are defined by their diameters and chiralities (n,m), which separate tubes into metallic or semiconducting types.1 Despite various efforts since Ijima’s discovery in 1991,2 the known methods of carbon nanotubes synthesis produce both metallic or semiconducting tubes simultaneously, with the former content at ∼30%. Studies shows that metallic SWNTs can function as leads in a nanoscale circuit,3 while semiconducting ones can perform as nanoscale Schottky-type field-effect transistors.4-6 To optimize the application potential of SWNT applications in optoelectronic devices and sensors, it is desirable to separate the metallic SWNTs from the semiconducting ones. Isolation of each individual tube from the bulk material will enable the fundamental study of SWNT in general. In the as-prepared sample, the individual tubes are bundled together with others within a mixture of metallic and semiconducting tubes. With the aid of a surfactant and ultrasonic agitation, an aqueous dispersion of raw SWNTs in sodium dodecyl sulfate (SDS)7 can be prepared in which the individual nanotube is broken away from the bundle. By using fluorescence and resonance Raman scattering analysis, the structure of a specific (n,m) nanotube can be recognized and characterized.8,9 In recent years, various attempts have been made to separate metallic and semiconducting SWNTs. Some of the methods are based on the higher chemical reactivity of metallic tubes, which include selective complexation with bromine,10 oxidation with * Corresponding author. † Department of Chemistry, University of Akron. | Department of Electronic Science and Technology, School of Information and Electronics Engineering, Xi’an Jiaotong Univeristy. ‡ Department of Polymer Science, University of Akron. § Structures and Materials Division, NASA Glenn Research Center.
hydrogen peroxide (H2O2),11 and nitric and sulfuric acids.12 A recent study by Strano et al. shows that the diazonium salts can react with metallic SWNT to the near exclusion of semiconducting SWNTs.13-15 The chemical reactivity of each individual tube is also shown to be dependent on its chirality. It is anticipated that the covalent functionalization methods will have limited uses in separation of valuable metallic SWNTs, because these reactions tend to interrupt the integrated electronic structures of SWNTs. The nondestructive separation of metallic SWNTs has been demonstrated by using an alkyl amine,16-18 porphyrin,19 pyrene,20 and DNA.23,24 In these reports, the added chemical reagent exhibits different affinity toward different types of SWNTs, thereby discriminating them for separation. Recent studies show that the amine-assisted method can effectively enrich the metallic SWNTs to ∼87% purity.17,18 Significant interest exists in using metallic SWNTs in conducting polymer composite.20-22 Different polymers show the ability to discriminate between nanotube species in terms of either diameter or chiral angle.21 As one of soluble π-conjugated polymers, poly[(m-phenylenevinylene)-alt-(p-phenylenevinylene)] (PmPV) represent another interesting system to interact with SWNTs. Several studies of wrapping SWNTs with PmPV derivatives 25-30 have been reported, which suggest selective interaction with semiconducting SWNTs (diameters of 1.28 and 1.35 nm).29,30 Little is known, however, about the origin of this intriguing selectivity. The Mark-Houwink exponent for PmPV in solution is determined to be R≈1.0,31 indicating that the semirigid π-conjugated polymer backbone has certain flexibility to adjust its conformation during wrapping SWNTs. In order to understand the potential role of polymer conformation in the SWNT wrapping, we have performed a molecular modeling study which reveals that the polymer adopts a helical conformation with a cavity size of about 1.3 nm (Figure 1). In addition, the phenylenevi-
10.1021/jp804083n CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
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Yi et al. SCHEME 1: Experimental Procedure for Wrapping of SWNTs by Using PmPV Polymer
Figure 1. Molecular modeling of PmPV oligomer (cyan color) wrapped on a (8,7) SWNT (d ) 0.988 nm, brown color). The nanotube fits the molecular geometry of the bent PmPV conformation (a), and the polymer adopts a helical conformation during the wrapping interaction (b).
nylene (PV) fragments are arranged either perpendicular or parallel to the SWNT surface to facilitate π-π interactions. One of the fundamental issues is how well the tubes can be hold up in the predefined cavity by helical conformation of PmPV, as this will have direct impact on the selectivity of tube wrapping and the stability of the resultant wrapping. An intrinsic property of SWNTs is that they tend to stay in a bundle. Previous studies rely on a single stage of sonication to break up the nanotube bundle, which likely to lead to wrapping of bundled SWNT by PmPV.32 In the current study, the SWNT sample has gone through a sequence of sonication and centrifugation process to optimize the possibility for the individual SWNT wrapping. The wrapping process is monitored in every stage to shed some light on the wrapping of SWNT by PmPV. In this report, we disclose our findings on the wrapping of purified SWNTs by using PmPV. Results and Discussions A typical dispersion procedure is as follows: 0.3 mg of pure SWNTs (HiPcoTM) was added to 20 mL solution of PmPV (Mw)25,000, concentration 0.1 mg/mL) in tetrahydrofuran (THF), and the mixture was sonicated for 3 h in an ice-water bath. The obtained suspension (Sus1) was subject to centrifugation (7000 g, 6 h) to remove the nondispersible SWNTs. The resulting supernatant solution and sediment of SWNTs were designated as Sup-1 and Sed-1, respectively (Scheme 1). Sed-1 was collected and redispersed in PmPV solution. When the sonication-centrifugation process was repeated, the second suspension, supernatant, and sediment were designated as Sus2, Sup-2 and Sed-2, respectively. The sonication-centrifugation process was repeated 7 times to disperse the tubes sufficiently, thereby allowing the tubes to have an optimum interaction with the wrapping polymer chains. The microstructure of a PmPV/SWNTs film, obtained by spin-casting from Sup5, was examined by using an atomic force microscope (AFM) (Figure 2). The phase image showed that the SWNTs were well dispersed in the PmPV/THF solution after repeated sonication-centrifugation processes. The individual nanotube of about 1 nm diameter was coated by PmPV with the polymer coating thickness ranging from 1 to 8 nm. Absorption Spectra. Visible-near-infrared (Vis-NIR) absorption spectra were acquired from the supernatant solutions to monitor the change in population of different tubes. The individual peaks in the Vis-NIR spectrum can be attributed to the valence-to-conduction electronic transitions, which depend
Figure 2. AFM phase image of PmPV/SWNTs film spin-cast from Sup-5 solution.
Figure 3. Absorption spectra of supernatant solutions in THF. The spectra are normalized at 1410 nm to compare the relative signal change.
on the size and chirality of the nanotubes. The absorption spectra of supernatant solutions (Sup-1 to Sup-7) (Figure 3a) revealed the details for semiconducting tubes, which exhibit the electronic transitions E11 (900-1600 nm) and E22 (700-900 nm). The absorption peaks at 1410 and 1550 nm were attributed to (11,7) and (11,6) SWNTs with diameters of 1.25 and 1.19 nm, respectively.33 In the original sample, the peak intensity at 1410 nm was weaker than that at ∼1280 nm. However, the relative
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Figure 5. Images of Sup-5 as prepared (a), after setting two weeks (b), exposure to a 40 W lamp for ∼5 s (c) and ∼10 s (d), and then followed by resonication for 3 h (e). Figure 4. Absorption spectra of Sus-5 after subjecting to different centrifugation time.
peak intensity at 1280 nm decreased as the sonicationcentrifugation process was repeated. In the solution of Sup-7, the absorption peak at 1410 nm became predominant, showing that PmPV polymer had the ability to selectively wrap certain nanotubes. To gain further understanding into this process, a freshly prepared suspension solution was subjected to different length of centrifugation times. During the high speed centrifugation, some of the nanotubes precipitated gradually from the suspension. Although the poorly dispersed tubes had a greater tendency to come out, some of the well-dispersed tubes could also be forced out of the polymer solution. It was interesting to notice that the tubes, which gave rise to the 1410 nm absorption, were relatively unaffected by the centrifugation, while those tubes corresponding to 1280 nm absorption gradually decreased with the increasing centrifugation time (Figure 4). This result further confirms the assumption that PmPV polymer had a preference to wrap the nanotubes that gave 1410 and 1550 nm absorption and had a diameter of 1.19 and 1.25 nm respectively. Molecular modeling by using AM1 method also showed that the optimized geometry for PmPV chain could adopt a coil conformation with a cavity size of about 1.3 nm (Figure 1). Clearly, the tubes of comparable diameter would fit into this cavity well, thereby allowing an intimate interaction between the nanotube surfaces and wrapping polymer chains. The tubes being held tightly by the polymers chains would resist the centrifugation force for polymer-nanotube separation. The supernatant solution appeared to be not permanently stable. When the Sup-5 solution was left at room temperature for a week, the absorption peak at 1280 nm was found to decrease similarly as seen in Figure 4. The impact of sample settling for one week was equivalent to about 12 h centrifugation. The freshly prepared Sup-5 was transparent. After being kept for a few weeks, some tuftlike precipitation was formed from the Sup-5 solution (Figure 5). Resonication for 3 h could easily redisperse the SWNTs back into solution. The results clearly indicated that the wrapping process was reversible. It was noted that the precipitation process could occur in a shorter time period (in about 12 h) upon irradiation with UV-visible light. As seen in Figure 6, a large portion of the tubes dropped out of the solution after just a few hours of irradiation. The SWCTs corresponding to absorption peaks at 1410 and 1550 nm were less affected. It was likely that the irradiation altered the conformation of the wrapping polymer, thereby speeding up the unwrapping process and allowing the tubes to slip out more easily. When the nanotube diameters
Figure 6. Absorption spectra of freshly prepared suspension (Sus-5) before (solid line) and after 3 h irradiation (broken line). The spectra are normalized at the peak of 1410 nm for clarity.
closely matched the cavity created by the helical conformation of PmPV, the polymer chains were more tightly wrapped on the surface of the tubes. This local chain rigidity, which is dependent on the wrapped nanotube diameters, hampered the chain movement triggered by irradiation. In contrast, the polymer, which was wrapped on the tubes of smaller diameters, allowed the chain segment motion to occur relatively easily in response to irradiation. In summary, results from both centrifugation and photolytic conditions showed that the stability of nanotubes in PmPV solution was dependent on the tube diameters and chiralities. The observed slower precipitation rate for the tubes, which gave absorption peaks at 1410 and 1550 nm, leads to the conclusion that (11,6) and (11,7) SWNTs had stronger interactions with the PmPV than the others. The tuftlike precipitation formed initially came out as a whole to settle at the bottom (Figure 5b). When an incandescent lamp of 40 W was positioned in close distance (∼15 cm), the entire SWNT mass floated up when the lamp was turned on, and settled down when the lamp was off. This observation indicates that the polymer and nanotubes entangled with each other during the precipitation. Each polymer chain became wrapped around multiple tubes in order to hold them in close proximity. The presence of multiple polymer chains on a single nanotube is also plausible, which is consistent with the formation of a thick PmPV coating on SWNT (Figure 2). Fluorescence. As a complementary measurement, 3D fluorescence spectra provided a comprehensive picture for semiconducting nanotubes. The metallic tubes were nonemissive and thus were not detected by fluorescence. Contour plots showing emission spectra as a function of excitation wavelengths for Sup-5 (Figure 7) reveal the presence of about 20 different semiconducting nanotubes (Table 2). The excitation wavelength (550-830 nm) corresponded to V2 f C2 transition, while the
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Figure 7. Contour plot of fluorescence intensity versus excitation and emission wavelengths for a sample of SWNTs dispersed in PmPV/ THF solution.
TABLE 1: Raman Spectral Frequency (ωRBM), SWNT Chirality, and Diameter of Purified SWNTs, Sed-10, and Sus-7 ωRBMa purified SWNTs
Sed-10b
Sus-7
192.43 217.11 249.86 257.42 280.71 295.76
195.47 218.365 251.675 263.045 282.685 298.050
198.7233 (very weak) 255.2867 266.5289 286.1189 302.4422
SWNT SWNT assignment (n,m) diameter (nm) 12,6 8,8 10,3 7,6 7,5 8,3
1.24215 1.092322 0.941608 0.912543 0.833302 0.789028
a ωRBM represents the Raman frequency in wavenumber in radial breathing mode. b The sample of Sed-10 was treated by stirring in massive THF, followed by centrifugation for 1 h. The process was repeated three times before Raman measurement.
TABLE 2: Comparison of Fluorescence Spectra Data of PmPV/SWNTs Dispersion and SWNTs in Aqueous Surfactant Suspensions λex-Sa (nm)
λex-Rb (nm)
∆λex (nm)
λem-Sa (nm)
λem-Rb (nm)
∆λem (nm)
assignment
diameter (nm)
679 656 747 598 657 733 733 644 687 741 798 805 805 833 774 834 774 800 825
663 644 734 587 647 720 716 633 671 728 786 792 790 859 756 858 760 795 836
16 12 13 12 10 13 17 11 16 13 12 13 15 26 18 24 14 15 11
978 1062 1086 1140 1141 1136 1211 1282 1278 1286 1282 1233 1355 1350 1416 1437 1542 1542 1542
952 1023 1053 1113 1122 1101 1172 1250 1244 1267 1250 1197 1323 1307 1380 1397 1498 1499 1516
26 39 33 27 19 35 39 32 34 19 32 37 32 43 36 40 44 43 36
(8,3) (7,5) (10,2) (8,4) (7,6) (9,4) (8,6) (10,3) (9,5) (8,7) (10,5) (11,3) (9,7) (13,2) (10,6) (11,6) (13,3) (12,5) (11,7)
0.782 0.829 0.794 0.840 0.895 0.916 0.966 0.936 0.976 1.032 1.050 1.014 1.103 1.120 1.111 1.186 1.170 1.201 1.248
a λex-S and λem-S refer to excitation and emission wavelength of PmPV/SWNTs dispersion samples, respectively. b λex-R and λem-R refers to excitation and emission wavelength of SWNTs in aqueous surfactant suspensions cited from reference [8,30] (Science 298, 2361, 2002, and Nano. Lett. 3, 1235, 2003).
emission (900-1400 nm) matched the C1 f V1 transition of SWNTs. Each emission peak in this region is assigned to a certain semiconducting tube. Emission maxima from the SWNTPmPV solution in THF are listed in Table 2, in comparison with those values reported in previous studies.8,33 It is interesting
Figure 8. Raman spectra of purified SWNTs and Sed-10 in radial breathing mode (RBM). The nanotube parameters are assigned in parentheses.
to noted that the emission peaks of SWNTs-PmPV in THF were consistently red-shifted, by about 30-40 nm, from the corresponding peaks of a SWNT suspension in H2O. In addition, the excitation maximum (λex) of SWNT-PmPV in THF was also red-shifted by about 15 nm, revealing that there is a strong π-π electronic interaction between the nanotubes and PmPV chains tightly wrapped on the nanotube surfaces. Raman Characterization. Raman spectra provide important information in terms of the tube chirality, diameter and electronic properties of SWNTs.8,34-37 Recently, the chiral index (n,m) of about 50 nanotubes, including both metallic and semiconducting nanotubes have been assigned on the basis of the 3D Raman with the excitation energy ranging from 1.52 to 2.71 eV.35-37 To determine the SWNT diameters in our samples, Raman spectra were acquired in the region assigned to SWNT the radial breathing mode (RBM) frequency (100-400 cm-1) when the samples were excited at 647.1 nm (Figure 8). The tube diameters in the sample were calculated and listed in Table 1, by using the relation ωRBM ) C1/d + C2 where C1 and C2 are empirical constants.8,35,36 The results were complementary to the fluorescence study, as the later only detected individual semiconducting tubes. The original SWNT sample was compared with the Sed-10, which had gone through repeated extraction with PmPV solution, sonication, and centrifugation processes. The Raman absorption bands in the RBM region (Figure 8) were assigned according to previous studies.34-37,12 The peaks at 192 cm-1 and 217 cm-1 were attributed to two metallic tubes (12,6) and (8,8), while the peaks at 249, 257, 280, and 295 cm-1 to the semiconducting (10,3), (7,6), (7,5), and (8,3) tubes, respectively. Clearly, the content of metallic tubes in the Sed-10 was reduced while that of semiconducting ones was enhanced. The depletion of metallic tubes in the sediment indicated that the metallic tubes were enriched in the supernatant. The characteristic Raman G-band (Figure 9) further confirmed the reduction in metallic tube content in the sediment sample. The original SWNT sample displayed a relatively broad G-band, which consists of two superimposed peaks at ∼1550 and 1587 cm-1. The broadened G-band feature, which is characteristic of metallic tubes, indicated a higher content of metallic tubes in the original SWNT sample. The narrower and weaker Gband observed from the Sed-10 sample indicates a significantly lower metallic tube content as they were selectively retained in the PmPV supernatant.
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Figure 9. Raman G-bands of SWNTs and Sed-10 powders.
The Raman D-bands at 1350 cm-1, which are defect sensitive for carbon nanotubes, were nearly identical for both the original and Sed-10 samples (Figure 9). Therefore, no detectable defects were introduced into the nanotubes during the repeated separation procedures. Examination of a series of suspension samples revealed that the selectivity in wrapping SWNTs by PmPV was dependent not only on the type of tubes but also on the individual tube chirality indices (n,m). The Raman spectra for the series (Susn) were normalized against the solvent peak for comparison, since the concentration of PmPV solution remained to be constant throughout the study. As shown in Figure 10a, the relative intensity of the first RBM peak at around 198 cm-1, which corresponds to the population of metallic (12,6) nanotube, decreased significantly (24.1%) from Sus-1 to Sus-7. The quantitative analysis of selective efficiency of PmPV in each sonication-centrifugation step is shown in Table 3. A plot of the RBM peak area over the entire Sus-series (1 through 7) revealed a linear correlation (Figure 10b). In contrast, the relative intensities of other RBM peaks, which are attributed to semiconducting nanotubes, increased slightly but linearly over the Sus-series. As illustrated in Scheme 1, the nanotubes present in the suspension sample Sus-n should be equal to that in the preceding sediment sample Sed-(n-1), since all the tubes in the sediment were well dispersed into the suspension. In other words, the observed enrichment of semiconducting tubes in the suspension was equivalent to the enrichment in the sediment, which agreed well with the study of powder samples (Figures 7 and 8). The results from the suspension study, therefore, further confirmed the conclusion that metallic tubes were enriched in the supernatant, while semiconducting ones were enriched in the sediment. The assumption of good dispersion was confirmed by examining the Raman G band (1500-1600 cm-1). The height of the Raman G- peak (1520-1560 cm-1), which corresponds to bundled metallic nanotubes,38 was indistinguishable for all the samples (Figure 10a), indicating the absence of the nanotubes bundling. All the samples exhibited the same feature that the very weak G- peak was nearly buried in the baseline of G+ peak (at ∼1587 cm-1) of the semiconducting nanotubes. The Raman RBM bands of SWNT sediment and suspension samples were examined to gain further understanding on the impact of polymer wrapping. In comparison with the original SWNTs, the RBM frequencies of Sus-7 were red-shifted by about 6-7 cm-1 (Table 1), which was comparable to that observed from the polycyclic aromatic hydrocarbon-SWNT
Figure 10. (a) Raman spectra of Sus-1 and Sus-7 in RBM frequency (top). (b) The variation of normalized area of each Raman peak in consecutive suspension samples (bottom).
TABLE 3: Selective Efficiency of PmPV in Each Sonication-Centrifugation Step relative peak change,a %
PmPV/SWNTs
metallic tube
semiconducting tube
separation step
peak 1
peak 2
peak 3
peak 4
peak 5
1 2 3 4 6 over all 1-6
-2.52 -3.02 -8.58 -5.4 -4.58 -24.10
-6.09 3.95 6.75 11.88 1.76 18.25
8.98 1.69 13.88 0.92 4.65 30.12
9.06 1.86 10.5 6.96 5.14 33.52
28.17 36.79 32.31 14.84 97.64
a The peak area changes are based on Raman spectral analysis of various samples.
interactions (∼8 cm-1 red-shift).39 Clearly, the surrounding polymers on the nanotube surface affected the electronic and vibrational modes of SWNT, possibly through π-π interactions. In another experiment, the Sed-10 sample was soaked and stirred in massive amounts of THF, and then centrifuged for 1 h to separate loose PmPV from SWNT. This process was repeated 3 times prior to Raman measurement. The RBM of the resulting Sed-10 exhibited a smaller red-shift by about 2-3 cm-1 (Table 1), due to less PmPV chains on the nanotube surface. This result also suggests that some PmPV chains were more tightly wrapped on the SWNT, and could not be easily separated from the tubes.
12268 J. Phys. Chem. B, Vol. 112, No. 39, 2008 Experimental Data. Optical absorption spectra were recorded with a PerkinElmer Lambda 950 spectrophotometer at room temperature. Fluorescence spectra were measured with a HoribaJobin Yvon Nanolog fluorometer, equipped with double-grating monochromator in excitation and emission, and a liquid-N2cooled solid state InGaAs detector (model 1427B). Measurements of Raman scattering signal were performed, either in solid (initial SWNT and Sed-10 Powder) or liquid form (Suspension series), by using the excitation wavelength of 647.1 nm from a Krypton Lexel 95 Laser. These measurements were done in the backscattering geometry, using a Horiba-Jobin Yvon Labram HR800 monochromator equipped with a nitrogen-cooled CCD camera. The elastic line was suppressed by a 647.1 nm notch filter. Purification of HiPCO SWNTs. Single wall carbon nanotubes prepared by the HiPCO process were furnished by Rice University and purified using the following procedure. A solution of crude HiPCO SWNTs (0.5 - 1.0 g) was refluxed in aq. HNO3 (2.6 M, 300 mL) for a maximum of 48 h. The resulting mixture was filtered using a 0.45 µm pore membrane from Millipore, Inc. The solid was added to DMF and then sonicated for 0.25 h using a water bath sonicator (S-3000 Sonicator from Misonix, Inc.). After filtration, the HiPCO SWNTs were dried in a vacuum oven at 150 °C for 3 h. The dried nanotubes were then heated at 225 °C in a tube furnace (Lindbergh Blue) for 18 h under wet air. The cooled SWNTs were then transferred to a beaker containing 50 mL conc. HCl, sonicated for 0.5 h, and filtered. The SWNTs were washed several times with distilled H2O and MeOH and allowed to airdry. The SWNTs were placed in a Soxhlet extraction thimble and extracted for 24 h with 6.5 M aq. HCl. The resulting solid was washed several times with distilled H2O and MeOH, and dried in a vacuum oven at 200 °C for 3 h. The dried nanotubes were heated in a tube furnace under moist air at 325 °C for 2 h, followed by 1 h at 425 °C for 1 h under wet argon. The usual yield of a purified HiPCO SWNTs by this method ranged from 30 to 35%. Analysis of the HiPCO SWNTs using inductively coupled plasma spectroscopy (ICP) revealed that the iron content of the nanotubes was reduced from 22.7 wt % to less than 0.05 wt %. Conclusion Absorption, fluorescence, and Raman spectroscopies were used to monitor the SWNT population changes among different generations of supernatant, suspension, and sediment samples. On the basis of Raman RBM bands, the PmPV was found to selectively wrap (12,6) and (8,8) metallic SWNTs, whose diameters are 1.24 and 1.09 nm, respectively. Complimentary to Raman results, the vis-NIR absorption spectra showed that PmPV also selectively wrapped two semiconducting tubes (11,7) and (11,6), which exhibit absorption λmax at 1550 and 1410 nm and have diameter of 1.25 and 1.19 nm, respectively. The experimental results show that PmPV tends to have stronger interaction with those tubes whose diameter is between 1.1-1.3 nm. The diameters of these tubes closely match the size of the cavity created by the helical conformation of PmPV during the wrapping. It should be noted that the conformational cavity size of 1.3 nm in Figure 1 is estimated by considering the approximate atom-to-atom distance. By considering the Van der Walls radius, which increases the effective nanotube size and decreases the cavity of the PmPV, a SWNTwith a diameter of 1.1 nm diameter would fit nearly perfectly within the cavity formed by the PmPV helix. The origin of this selective wrapping of SWNTs, therefore, could be attributed to the SWNT diameter
Yi et al. matching with the helical conformational cavity of PmPV. The electronic nature of the SWNTs, however, may also play an important role in the observed selectivity, as the Raman RBM bands show strong preference for wrapping metallic tubes over the semiconducting ones. We believe that both factors of electronic and polymer conformational preference are responsible for the observed great selectivity (Figure 10b). In the well dispersed sample, each polymer chain can be viewed as wrapping either a single or several SWNTs, which can have a direct impact on the resulting suspension. In addition, each nanotube can be wrapped by more than one polymer chains. Precipitation of SWNTs from the suspension is attributed to a suitable combination of two factors. In addition to the simple SWNT dropping out of the wrapped polymer chain, the crosstube entanglement can also lead to a tuftlike precipitate, which is likely to be associated with the local chain wrapping and dewrapping process. Irradiation with UV-visible light is assumed to speed up the dewrapping process, possibly by a photochemically induced change in the polymer conformational change. The dramatic depletion of other tubes, except (11,7) and (11,6), raises the prospect that irradiation could be useful in the separation of SWNT. Further study is under way to tune the polymer conformational cavity and the nature of π-π interactions for improved selectivity. Acknowledgment. Y.P. is grateful for the financial supports from The University of Akron and NASA (Grant NNC3-1044). A.P.S. acknowledges the financial support from the Air Force through the Cooperative Center in Polymer Photonics. References and Notes (1) Reich, S.; Thomsen, C.; Maultzsch, J. Carbon nanotubes; Wiley: Weinheim, Germany, 2004. (2) Iijima, S. Nature (London) 1991, 354, 56. (3) Yao, Z.; Kane, C. L.; Dekker, C. Phys. ReV. Lett. 2001, 84, 2941. (4) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (5) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 2773. (6) Wind, S. J.; Appenzeller, J.; Marte, R.; Derycke, V.; Avouris, P. Appl. Phys. Lett. 2002, 80, 3817. (7) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (8) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Weisman, R. B. Science 2002, 298, 2361. (9) Bachilo, S. M.; Balzano, L.; Herrera, J. E.; Pompeo, F.; Resasco, D. E.; Weisman, R. B. J. Am. Chem. Soc. 2003, 125, 11186. (10) Chen, Z. H.; Du, X.; Du, M. H.; Rancken, C. D.; Cheng, H. P.; Rinzler, A. G. Nano Lett. 2003, 3, 1245. (11) Miyata, Y.; Maniwa, Y.; Kataura, H. J. Phys. Chem. B 2006, 110, 25. (12) Yang, C. M.; Park, J. S.; An, K. H.; Lim, S. H.; Seo, K. Y.; Kim, B.; Park, A. K.; Han, S. W.; Park, C. Y.; Lee, Y. H. J. Phys. Chem. B 2005, 109, 19242. (13) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H. W.; Kittrel, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519. (14) Strano, M. S. J. Am. Chem. Soc. 2003, 125, 16148. (15) Nair, N.; Kim, W. J.; Usrey, M. L.; Strano, M. S. J. Am. Chem. Soc. 2007, 129, 3946. (16) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. Am. Chem. Soc. 2003, 125, 3370. (17) Maeda, Y.; Kimura, S.; Kanda, M.; Hirashima, Y.; Hasegawa, T.; Wakahara, T.; Lian, Y.; Nakahodo, T.; Tsuchiya, T.; Akasaka, T.; Lu, J.; Zhang, X.; Gao, Z.; Yu, Y.; Nagase, S.; Kazaoui, S.; Minami, N.; Shimizu, T.; Tokumoto, H.; Saito, R. J. Am. Chem. Soc. 2005, 127, 10287. (18) Maeda, Y.; Kanda, M.; Hashimoto, M.; Hasegawa, T.; Kimura, S.; Lian, Y.; Wakahara, T.; Akasaka, T.; Kazaoui, S.; Minami, N.; Okazaki, T.; Hayamizu, Y.; Hata, K.; Lu, J.; Nagase, S. J. Am. Chem. Soc. 2006, 128, 12239.
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