Demonstration of Diameter-Selective Reactivity in the Sidewall

It must be noted that these measurements have been carried out on solid SWNT ropes. Bundling of nanotubes tends to induce changes in the electronic ...
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NANO LETTERS

Demonstration of Diameter-Selective Reactivity in the Sidewall Ozonation of SWNTs by Resonance Raman Spectroscopy

2004 Vol. 4, No. 8 1445-1450

Sarbajit Banerjee† and Stanislaus S. Wong*,†,‡ Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400, and Materials and Chemical Sciences Department, BrookhaVen National Laboratory, Building 480, Upton, New York 11973 Received May 17, 2004; Revised Manuscript Received June 9, 2004

ABSTRACT Strategies for single-walled carbon nanotube (SWNT) separation are critical to developing nanotubes as useful nanoscale building blocks. Herein, we demonstrate that solution-phase ozonolysis of SWNTs fosters diameter selectivity in SWNTs. The main conclusion of our work is that in this sidewall addition reaction, smaller diameter tubes react more extensively than larger diameter tubes, which provides experimental validation for theoretical predictions on the role of pyramidalization and π-orbital misalignment in nanotube reactivity.

Single-walled carbon nanotubes (SWNTs)1 are interesting systems for investigating principles of quantum confinement in one-dimensional systems. Moreover, SWNTs have potential applications in a wide variety of areas, including electronics, structural materials, sensing, field emission, and as components of material composites.2 Several important challenges, however, remain to be overcome before SWNTs can fulfill their potential as technologically relevant nanostructures. Sample homogeneity is one key issue. Most methods associated with the synthesis of SWNTs yield a randomized mixture of different diameters and chiralities. This necessitates post-synthetic treatments, usually chemically based, to separate SWNTs by diameter and electronic structure. In this letter, we limit ourselves to the problem of diameter separation techniques and demonstrate the use of sidewall chemical functionalization, specifically solution-phase ozonolysis, as a means of fostering diameter selectivity in SWNTs. The main conclusion of our work is that, in this sidewall addition reaction, smaller diameter tubes react more extensively than larger diameter tubes, which directly addresses important theoretical concerns raised about the nature of nanotube reactivity. It is predicted that the sidewall addition chemistry of SWNTs differs from that of fullerenes, even though both are curved conjugated carbon systems. While fullerenes can * Corresponding author. E-mail: [email protected]; sswong@ bnl.gov. † SUNY Stony Brook. ‡ Brookhaven National Laboratory. 10.1021/nl049261n CCC: $27.50 Published on Web 07/13/2004

© 2004 American Chemical Society

be thought of as being folded in two dimensions (spherical folding), nanotubes are formed from a graphene sheet, folded in one dimension (cyclic folding). The chemical reactivity in strained carbon systems arises from two factors: (a) pyramidalization at the carbon atom and (b) π-orbital misalignment between adjacent carbon atoms.3,4 In fullerenes, the former is more important and relief of pyramidalization strain energy results in addition reactions to fullerenes being energetically favorable. In SWNTs, the pyramidalization strain is not as acute. Hence, it turns out that in SWNTs, π-orbital misalignment is expected to have a greater influence.5,6 This misalignment, associated with bonds at an angle to the tube circumference (i.e., bonds that are neither parallel to nor perpendicular to the tube axis), is the origin of torsional strain in nanotubes, and the relief of this strain controls the extent to which addition reactions occur with nanotubes. Since π-orbital misalignment as well as pyramidalization scale inversely with tube diameter,4 smaller diameter tubes are expected to be more reactive than larger diameter tubes. Over the years, a number of experiments have been devised to either selectively react with or generate nanotubes in situ of a certain diameter. The preferential solubilization of smaller diameter SWNTs has been reported upon functionalization at the ends and defect sites with poly(ethylene glycol).7 Poly(phenylenevinylene) (PPV) π-stacking interactions with nanotubes were determined to be selective for certain diameters or range of diameters of these SWNTs.8,9 A recent report indicates spectroscopic evidence for diameter selective separation of DNA-wrapped SWNTs upon fractionation by ion-exchange chromatography.10 Smaller-

diameter SWNTs were found to be more susceptible to oxidation in air than their larger diameter counterparts.11 Taking a different approach, a number of efforts have focused on synthesizing monosized SWNTs in the spatially constrained one-dimensional channels of zeolite single crystals.12,13 Other groups have pursued variations on conventional SWNT growth techniques with a view to obtaining diameter selectivity but with varying levels of success.14,15 In fact, none of the previous work in the literature uses an experimentally viable sidewall addition reaction to the nanotubes toward resolving the diameter selectivity problem. Moreover, to the best of our knowledge, there has been no extensive study of diameter selectivity in existing sidewall functionalization reactions of SWNTs. In fact, the expected diameter dependence was not observed in a previous study of SWNT sidewall functionalization with oxocarbonyl nitrenes.16 Herein we present a systematic multilaser resonance Raman study of diameter selectivity in the solution-phase ozonolysis of SWNTs and correlate our observations with predictions of diameter selectivity, based on the pyramidalization and π-orbital misalignment models. The broad diameter distribution (0.7-1.2 nm) present in HiPco tubes allows us to probe the relative reactivity of different diameters of SWNTs. We recently developed an ozonolysis protocol involving treatment of nanotubes at -78 °C,17 followed by reaction with hydrogen peroxide (H2O2), dimethyl sulfide (DMS), and sodium borohydride (NaBH4) in independent runs, to generate a higher proportion of carboxylic acid/ester, ketone/ aldehyde, and alcohol groups, respectively, on the nanotube surface. This ‘one-pot’ oxidative methodology has three major consequences: first, the purification of as-prepared SWNTs to obtain a high-quality product; second, the chemical functionalization of nanotube sidewalls; and third, a systematic procedure to select for particular distributions of oxygenated functional groups in the resultant purified SWNTs. We used resonance Raman spectroscopy as our primary diagnostic technique to observe changes in the relative reactivity of different diameter distributions in our functionalized, ozonized nanotube samples. Indeed, Raman spectroscopy is a powerful probe of electron-phonon coupling and electronic structure in SWNTs.18 When the incident or scattered photon coincides with an allowed optical transition of a particular nanotube, the Raman spectra for that tube are resonantly enhanced with a very large scattering cross section due to coupling with the optically allowed electronic transitions between van Hove singularities in the electronic density of states of the SWNTs. Thus, different laser excitation wavelengths bring nanotubes of different diameters into resonance.19 Specifically, the Kataura plot, an organized assignment of variously allowed optical transition energies for different diameters of nanotubes, allows for the evaluation of which particular tubes (i.e., specific chiralities and diameters) will be brought into resonance at a given wavelength.20,21 The Raman spectra of SWNTs show three important regions: (a) the radial breathing mode (RBM) mode, which 1446

is dependent on the diameter of the tube, (b) the tangential mode, also known as the G band, with phonons of A1, E1, and E2 symmetries in the 1515-1590 cm-1 region, and (c) the disorder mode, the D band, which is dispersive in the 1280-1320 cm-1 region.18,19 The increased ratio of the disorder band intensity to the tangential mode intensity after chemical treatment is widely accepted as arising from sidewall functionalization, due to the increased numbers of sp3 hybridized carbon atoms in the hexagonal framework of the SWNT sidewalls.22-24 We will focus on the tangential and radial breathing modes in this study. The tangential mode shows distinctive behavior modes for metallic and semiconducting nanotubes. Semiconducting nanotubes have narrow Lorentzians in this region, while metallic nanotubes are characterized by a high-frequency Lorentzian coupled to broad, low-energy Breit-WignerFano (BWF) tails.25 The Fano component in metallic SWNTs essentially arises from the coupling of discrete phonons to an electronic continuum.26 The radial breathing mode (RBM) frequency can be empirically related to the diameter of the tube by ωRBM ) C1/dt + C2

(1)

where dt, the diameter of the tube, can be expressed as dt ) (x3ac(n2 + m2 + mn)0.5)/π

(2)

where ac is the carbon-carbon bond length and n and m are the indices that specify the chirality of the tube. In this work, we shall use values of 223.5 and 12.5 for C1 and C2 based on values derived from studies on individualized HiPco tubes.27,28 Analysis of low wavenumber Raman spectra at different laser excitation wavelengths thus allows us to probe diameter distributions of nanotubes present in the pristine and functionalized nanotube samples. All of the Raman data presented here pertain to pristine HiPco SWNTs, tubes after treatment with O3 for 2 h, and finally, tubes that were further reacted with H2O2 after ozonolysis.17 Raman spectra were obtained on solid samples dispersed in ethanol and placed on a Si wafer. The spectra were obtained on a Renishaw 1000 Raman microspectrometer with excitation from argon ion (514.5 nm), He-Ne (632.8 nm), and diode (780 nm) lasers. A Renishaw setup with a tunable argon ion laser was used to obtain Raman data with 488 nm excitation. Identical power levels of up to 3 mW were used for each set of samples. It has been previously shown that in ozonolysis, the covalent sidewall functionalization process that occurs substantially destroys the electronic band structure of the derivatized SWNTs.17,29 This thus causes those tubes that have been functionalized to no longer be resonance enhanced, and hence there is a decrease in the scattering cross section for functionalized SWNTs. A typical sample of HiPco nanotubes is composed of a mixture of a large distribution of diameters with varied reactivities toward ozone. As nanotubes of a specific diameter react with ozone, the radial Nano Lett., Vol. 4, No. 8, 2004

Figure 1. Raman spectra for pristine (black) and ozonized (blue) SWNTs at 780 nm excitation. (a) Low wavenumber region. The spectra are normalized to the RBM feature at 264 cm-1. (b) Tangential and disorder mode regions. The spectra are normalized to the ωG+ feature.

breathing modes corresponding to those particular reacted tubes will be attenuated, and overall, these will make less of a contribution to the spectral intensity in the RBM region in an ensemble measurement.30 Thus, the rationale for the Raman spectra that we observe for ozonized nanotubes is that the signals actually correspond to the less functionalized or unfunctionalized tubes. As a general comment, normalization at a specific RBM feature allows for evaluation of the relative intensities of different nanotube species present in the pristine and functionalized nanotube samples, respectively. It is of note that there is no net change in the population of nanotubes during the ozonolysis experiment. Figure 1 shows the Raman spectra at 780 nm excitation. At this laser wavelength, the excitation is primarily resonant with the v2 f c2 transitions of semiconducting nanotubes. Figure 1a is normalized to the highest intensity RBM feature at 264 cm-1, which corresponds to a diameter of ∼0.89 nm and which can be assigned to (10,2) or (11,0) nanotubes. It is immediately apparent that for the functionalized, ozonized nanotubes, the lower wavenumber modes have a much higher intensity value with respect to the 264 cm-1 peak than the pristine nanotubes do. Because lower wavenumber peaks correspond to RBMs of larger diameter tubes, after ozonolysis, it is evident that larger diameter tubes make a greater contribution in the RBM region. Since no tubes are lost during the ozonolysis process, this implies that smaller diameter tubes are more heavily functionalized upon ozonolysis and thus show a much greater loss of resonance enhancement. More precisely, for the ozonized tubes, the RBM features at 232 cm-1, corresponding to tubes of a diameter of ∼1.01 Nano Lett., Vol. 4, No. 8, 2004

nm (possibly (11,3) nanotubes), show a much higher intensity with respect to the peak at 264 cm-1, relative to the signals associated with their pristine, raw nanotube counterparts. Thus, even a 1.2 Å difference in diameter has significant implications in reactivity toward ozone. It must be noted that these measurements have been carried out on solid SWNT ropes. Bundling of nanotubes tends to induce changes in the electronic dispersion along the tube axis. For instance, a decrease in the energy gap is observed for semiconducting tubes, and in armchair tubes a pseudogap is opened, thereby perturbing the pristine nanotube band structure.31 This can lead to deviations from predicted RBM positions as changes in bundling characteristics of the ropes may cause small shifts in the RBM features, thereby making it difficult to unequivocally assign chiralities to observed RBM features. Experimentally, shifts in RBM features have previously been ascribed to changes in the stacking behavior of nanotubes on functionalization.32 Thus, the transition energies for covalently functionalized vs pristine nanotubes (and even vs noncovalently functionalized tubes) are not identical, and as such it would not be appropriate to describe our results in terms of ‘enrichment factors’ of very specific diameters of tubes.10 Figure 1b shows an increase in intensity of the D band on sidewall functionalization, which results from the disorder induced by sp3 hybridization of sidewall carbons upon ozonolysis.29 The chemical derivatization process effectively perturbs excitations between π-bands of the bare sp2hybridized HiPco tubes. Sidewall functionalization breaks the translational symmetry along the nanotube axis, causing these phonons to become Raman active.33 BWF broadening is absent, since primarily semiconducting nanotubes are in resonance at this wavelength. Excitation at 632.8 nm brings into resonance both metallic and semiconducting tubes. Smaller diameter semiconducting tubes with resonant v2 f c2 transitions and larger diameter metallic nanotubes with resonant v1 f c1 transitions are seen in Figure 2.21,30 In Figure 2a, the higher wavenumber (i.e., smaller nanotubes) RBM features above 240 cm-1 arise from semiconducting tubes with diameters ranging from 0.78 to 0.93 nm. These features have been assigned to (10,3), (7,6), and (8,3) nanotubes.10 RBM features below 240 cm-1 have been assigned to metallic nanotubes including (12,3), (13,4), and (9,9) nanotubes with diameters ranging from 1.09 to 1.24 nm. The larger diameter features are greatly enhanced in intensity in the ozonized nanotubes. Figure 2b shows an increase in the intensity of the D band on sidewall functionalization as well as interesting changes in the profile of the G band. A small softening of the Lorentzian mode seen here as well as at 780 nm excitation is consistent with a preponderance of large-diameter tubes.34 It is noteworthy that in ensemble measurements of nanotube ropes, the G band phonons are weakly or nondiameter dispersive. Thus, we do not expect to see substantial dependence of this feature on the diameter distribution in the sample.35 In metallic tubes, the tangential mode splits into axial and circumferential components with a phonon of A1 symmetry coupled to the electronic continuum to give a broadened low 1447

Figure 2. Raman spectra for pristine (black) and ozonized (blue) SWNTs at 632.8 nm excitation. (a) Low wavenumber region. The spectra are normalized to the RBM feature at 256 cm-1. (b) Tangential and disorder mode regions. The spectra are normalized to the ωG+ feature.

energy BWF line shape.36 In Figure 2b, for the functionalized nanotubes, the G band appears to be broadened and has a

higher intensity at the low energy end, indicating a substantially greater contribution from the Fano shape. This is consistent with what would be expected from the RBM picture. Since the smaller diameter tubes that get extensively ozonized are semiconducting nanotubes, the remaining larger diameter tubes have an increased metallic component, which contributes to the increased spectral weight of the Fano line. Figures 3 a and b show Raman spectra acquired at 514.5 nm excitation, which is resonant with v1 f c1 transitions in relatively small diameter metallic tubes and v3 f c3 transitions in larger diameter semiconducting tubes.21,37,38 RBM features above 200 cm-1 arise from metallic nanotubes with diameters ranging from 0.86 to 1.20 nm. The RBM features at ∼187 cm-1 arise from semiconducting nanotubes with diameters of about 1.28 nm. From Figure 3a, it is evident that the longer wavenumber features, corresponding to smaller diameter tubes, are much reduced in intensity in the ozonized SWNT sample. The most prominent features for the pristine nanotubes at 262 and 272 cm-1, which have been assigned to (9,3) and (8,5) nanotubes with diameters of 0.86 and 0.90 nm respectively, are greatly suppressed in the spectrum associated with functionalized tubes, while the initial features at 187 cm-1 arising from 1.28 nm tubes, which have v3 f c3 transitions in resonance with the laser, are enhanced for the functionalized tubes. These data are in line with the view that smaller diameter nanotubes preferentially react with ozone. A possible candidate for the feature at 247 cm-1 is a (12,0) semimetallic zigzag nanotube (dt ) 0.95 nm), while the 187

Figure 3. Raman spectra for pristine (black) and ozonized (blue) SWNTs. (a) Low wavenumber region at 514.5 nm excitation. The spectra are normalized to the RBM feature at 205 cm-1. (b) Tangential and disorder mode regions at 514.5 nm excitation. The spectra are normalized to the ωG+ feature. (c) Low wavenumber region at 488 nm excitation. The spectra are normalized to the RBM feature at 201.5 cm-1. (d) Tangential and disorder mode regions at 488 nm excitation. The spectra are normalized to the ωG+ feature. 1448

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cm-1 feature could be assigned to a (16,0) semiconducting zigzag nanotube (dt ) 1.28 nm).37 In the spectrum of functionalized nanotubes, the 247 cm-1 RBM feature (corresponding to 0.95 nm tubes) is greatly reduced in intensity, while the low energy 187 cm-1 RBM feature (corresponding to 1.28 nm tubes) is increased in intensity, indicating that a larger quantity of the smaller diameter tubes were extensively functionalized. It is instructive to compare the reactivity of these two zigzag tubes, which is different, because the pyramidalization angles and π-orbital misalignment angles of SWNTs scale inversely with tube diameter. The pyramidalization angles for (12,0) and (16,0) SWNTs are 4.30 and 3.24 degrees respectively, while the π-orbital misalignment angles between adjacent carbons are 0° and 15.3° for the (12,0) nanotube and 0° and 11.4° for the (16,0) nanotube.5,6 The (12,0) nanotube thus has a larger π-orbital misalignment and pyramidalization strain. Hence, this higher strain energy likely drives the sidewall ozonolysis process and renders the (12,0) nanotube more reactive than the larger diameter tubes. The G band profile in Figure 3b shows the opposite trend from that seen at 632.8 nm in Figure 2b. At 514.5 nm, the pristine tubes have a large Fano component since mostly metallic nanotubes are brought into resonance at this wavelength. From the RBM data, it appears that the smaller diameter nanotubes that are heavily functionalized are metallic ones. Thus, the loss of Raman enhancement for these tubes is reflected in a decreased intensity of the Fano component in the G band profile of the functionalized nanotubes as compared with that of the pristine tubes. Hence, from the Raman spectra at 514.5 and 632.8 nm respectively, it is apparent that, as opposed to chiral selectivity observed in other sidewall addition reactions,21,30 in this specific ozonolysis reaction there is a clear case for diameter selectivity, as would be expected based on curvature considerations. Some changes in Fano features may originate from changes in the state of aggregation upon functionalization. However, opposite behaviors at 514.5 and 632.8 nm excitation wavelengths, coupled with lack of noticeable upshifts in the RBM modes characteristic of aggregation,32 indicate that changes in the Fano contribution likely arise from chemical functionalization of smaller diameter tubes. Figures 3c and 3d show Raman spectra acquired at 488 nm excitation, which probes primarily metallic tubes. The data are again consistent with the increased reactivity of smaller diameter nanotubes, as the low energy RBM features, corresponding to larger diameter tubes, are substantially conserved. Furthermore, as at 514.5 nm, the observed decrease in the intensity of the Fano component is due to the fact that most of the smaller diameter tubes are metallic. It is worth noting that the G band profiles at all laser excitations show reduced splitting, which is likely because of the smaller subset of tubes in the sample that are resonance enhanced, consistent with the increased proportion of large diameter tubes that remain upon functionalization. Furthermore, the overall intensities of the RBM features are reduced with respect to the G band at all laser excitation wavelengths, further corroborating an increased defect density in the lattices of functionalized nanotubes.16 Broadening of the line Nano Lett., Vol. 4, No. 8, 2004

Figure 4. Low wavenumber Raman spectra for pristine (black), ozonized (blue), and ozonized/H2O2-cleaved (red) SWNTs. (a) Spectra acquired at 632.8 nm excitation. The spectra are normalized to the RBM feature at 256 cm-1. (b) Spectra acquired at 780 nm excitation. The spectra are normalized to the RBM feature at 264 cm-1.

widths of RBMs, corresponding to functionalized tubes, may also be explained by the presence of defects and weaker resonance enhancement conditions.39 Increased oxidative sidewall functionalization,40 such as solution-phase ozonolysis followed by H2O2 cleavage,17 causes an even further increase in the relative intensity of the normalized RBM features corresponding to larger diameter tubes (Figure 4a and 4b), suggestive of the preferential reactivity of smaller diameter tubes in this addition reaction. Moreover, with increasing levels of functionalization, the relative intensity of the disorder band also increases (Supporting Information). The D band is dispersive and varies with the laser excitation wavelength from 1290 cm-1 at 780 nm excitation to 1339 cm-1 at 488 nm excitation.33 In conclusion, we have established that chemical reactivity of nanotubes in this sidewall addition reaction, i.e., solutionphase ozonolysis, is dependent on diameter. Smaller diameter nanotubes have greater strain energy per carbon atom due to increased curvature strain and greater rehybridization.5,6 The radial breathing modes in the low wavenumber region of nanotube Raman spectra indicate that, after functionalization, features corresponding to small diameter tubes are relatively diminished in intensity as compared with the profile of larger diameter tubes. Unlike other sidewall addition reactions,30,41 we find no chiral selectivity in the ozonolysis of nanotubes and determine that, in this case, the reaction pathway can be explained 1449

by a simple diameter dependence argument. While the pyramidalization and π-orbital misalignment formalism has been predicted theoretically,4-6 to the best of our knowledge the current work represents the first detailed validation of this dependence in addition reactions on SWNT surfaces. Indeed, the higher curvature of the outer convex surface of the nanotube, especially for smaller diameter tubes, causes the hybrid-π orbitals on the tube exterior to become exposed, thereby favoring overlap with incoming addends.4 This fact, combined with assistance from the σ system toward essentially strain-free bond formation in the region of the adduct, leads to smaller diameter tubes being favored in the sidewall ozonolysis of carbon nanotubes. In fact, the chemically induced diameter selectivity observed here could be the basis for size separation techniques of nanotube species. Acknowledgment. We acknowledge support of this work through startup funds provided by the State University of New York at Stony Brook as well as Brookhaven National Laboratory. Acknowledgment is also made to the National Science Foundation for a CAREER award (DMR-0348239) and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. S.S.W. thanks 3M for a nontenured faculty award. We thank Tim Kelly (Materials Science, Drexel University) for assistance with the Raman measurements. Supporting Information Available: Figure S1: Tangential and disorder mode region of Raman spectra for pristine (black), ozonized (blue), and ozonized and H2O2cleaved (red) SWNTs. (a) Spectra acquired at 632.8 nm excitation. The spectra are normalized to the ωG+ feature. (b) Spectra acquired at 780 nm excitation. The spectra are normalized at the ωG+ feature. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; SpringerVerlag: Berlin, 2001. (2) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (3) Haddon, R. C. Acc. Chem. Res. 1988, 21, 243. (4) Chen, Z.; Thiel, W.; Hirsch, A. ChemPhysChem. 2003, 4, 93. (5) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (6) Hamon, M. A.; Itkis, M. E.; Niyogi, S.; Alvaraez, T.; Kuper, C.; Menon, M.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 11292. (7) Huang, W.; Fernando, S.; Lin, Y.; Zhou, B.; Allard, L. F.; Sun, Y.P. Langmuir 2003, 19, 7084. (8) Dalton, A. B.; Stephan, C.; Coleman, J. N.; McCarthy, B.; Ajayan, P. M.; Lefrant, S.; Bernier, P.; Blau, W. J.; Byrne, H. J. J. Phys. Chem. B 2000, 104, 10012. (9) Keogh, S. M.; Hedderman, T. G.; Gregan, E.; Farrell, G.; Chambers, G.; Byrne, H. J. J. Phys. Chem. B 2004, 108, 6233. (10) Strano, M. S.; Zheng, M.; Jagota, A.; Onoa, G. B.; Heller, D. A.; Barone, P. W.; Usrey, M. L. Nano Lett. 2004, 4, 543.

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