17804
J. Phys. Chem. C 2007, 111, 17804-17806
Amplification of Single-Walled Carbon Nanotubes from Designed Seeds: Separation of Nucleation and Growth† Douglas Ogrin,‡ Robin E. Anderson,‡ Ramon Colorado, Jr.,‡ Benji Maruyama,§ Mark J. Pender,§ Valerie C. Moore,‡ Sean T. Pheasant,‡ Laura McJilton,‡ Howard K. Schmidt,‡ Robert H. Hauge,‡ W. Edward Billups,‡ James M. Tour,‡ Richard E. Smalley,‡ and Andrew R. Barron*,‡ Richard E. Smalley Institute for Nanoscale Science and Technology, Carbon Nanotechnology Laboratory, and Department of Chemistry, Rice UniVersity, Houston, Texas 77005, and Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson AFB, Ohio 45433 ReceiVed: February 13, 2007; In Final Form: May 11, 2007
Single-walled carbon nanotubes (SWNTs) may be grown from designed seeds containing an SWNT and the catalyst required for continued growth. Dodecyl side-walled functionalized SWNTs (DD-SWNTs) are endfunctionalized with 4-hydroxypyridine via dicyclohexylcarbodiimide coupling to allow covalent coordination of an inorganic cluster pro-catalyst (FeMoC). DD-SWNT-py-FeMoC on spin-on glass was exposed to H2/ CH4 at 800 °C, resulting in 3-fold growth in the length of 40% of the seed SWNTs. Only ∼1% of the procatalyst alone nucleate SWNTs under the same conditions, suggesting effective separation of the nucleation and growth processes.
It is well understood that the electronic properties of singlewalled carbon nanotubes (SWNTs) are dependent on their helicity, which is closely related to their diameter.1-3 There is much interest, therefore, in the development of methods that allow for the synthesis of SWNTs with particular helicities. In addition, while SWNT production has progressed a long way since their fortuitous discovery in arc soot,4,5 the nucleation process (e.g., cap formation and lift-off6) is rather inefficient and certainly inexact. Even when preformed catalyst particles are used, the yield is often low,7 presumably as a consequence of inefficient initiation of the SWNT growth. For both of these reasons, it would be desirable to use pre-formed SWNTs as seeds for the growth of longer SWNTs, that is, amplification of the original seed. In our approach to the amplification process, it is necessary that pro-catalyst particles be preferentially docked to the end of individual SWNTs (Scheme 1). Heating the SWNT-catalyst conjugate (SWNT-cat) under appropriate conditions would result in the partial consumption of one end of the SWNT while retaining the direct connection, and provide a seed point for subsequent growth upon the introduction of a suitable feedstock. This process would totally eliminate the nucleation step from the growth of SWNTs; furthermore, it is proposed that the diameter and chirality of the grown SWNT section would be controlled by the structure of the original SWNT seed.8 In this regard, we have recently reported the first demonstration that the attachment of an iron catalyst to the end of an individual SWNT allows for the growth of that particular SWNT with retention of the SWNT’s diameter.9 Previous work had demonstrated that continued growth of a bundle of SWNTs does not result in a change in the relative abundance of particular †
Part of the special issue “Richard E. Smalley Memorial Issue”. * Author to whom correspondence should be addressed. E-mail: arb@ rice.edu; URL: www.rice.edu/barron. ‡ Rice University. § Air Force Research Laboratory, Wright Patterson AFB.
(m,n) chirality SWNTs.10 Thus, we believe the concept of SWNT amplification has been demonstrated; however, the catalysts chosen are known to be reasonably efficient at nucleation. We are interested, therefore, in applying the SWNTamplification approach with a pro-catalyst known for its poor nucleation but acceptable growth. The removal of the nucleation step would lower the reaction temperature and thus the energy consumption: an important parameter for large-scale production. The majority of SWNT growth has been shown to occur for iron-based catalysts;11 however, we have shown that the nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98] (“FeMoC”), previously demonstrated to catalyze the growth of SWNTs,12 is actually a poorly initiating catalyst in the absence of added iron.13 Thus, if the use of a seed lowers the activation barrier to SWNT growth, then a significant increase in SWNT yield of FeMoC SWNT-cat growth relative to nucleation and growth from FeMoC alone should be observed. Appropriate reactive groups for functionalization of FeMoC are carboxylates, thiols, and, preferentially, pyridine derivatives.14 In these studies we have developed an end-specific functionalization of the SWNTs to allow attachment of the catalyst precursors via pyridine coordination. Prior to catalyst attachment, the sidewalls of the SWNTs must be protected (from potential coordination of the catalyst precursor) to prevent bundling (to allow for the growth of individual SWNTs to be studied) and allow for solubility in a suitable solvent. As-prepared dodecyl-substituted SWNTs (DD-SWNTs)15 have carboxylate residues at any open end as a consequence of Piranha etching of the SWNTs prior to functionalization. Using the dicyclohexylcarbodiimide (DCC)-catalyzed coupling chemistry of Green and co-workers,16 4-hydroxypyridine was reacted with DD-SWNTs (eq 1, where R ) (CH2)11CH3) to serve as a linkage ligand providing pyridine-functionalized ends for the attachment of metal complexes.17 The subsequent coupling of an appropriate procatalyst to the pyridine-functionalized SWNT is readily accomplished at 55 °C in EtOH/CHCl3. Care was
10.1021/jp0712506 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007
Amplification of SWNTs from Designed Seeds
J. Phys. Chem. C, Vol. 111, No. 48, 2007 17805
SCHEME 1: Schematic Representation of Seeded Growth Process, Showing the Coupling of the Catalyst Precursor to the End-Functionalized SWNT, Followed by the H2 Reduction of the Catalyst Precursor to the Catalyst Particle, and the Subsequent Growth of the New SWNT.
taken to ensure that any unreacted FeMoC is removed prior to characterization or growth. AFM images of DD-SWNT-pyFeMoC show the formation of discrete SWNT-cat complexes between FeMoC and functionalized SWNTs (Figure 1). Upon the basis of the height of the sphere on the end of each SWNT, individual FeMoC molecules are bonded to the end of an SWNT.
Spin coating the FeMoC-based SWNT-cat onto a silicon wafer precoated with spin-on glass (SOG, Honeywell) followed by exposure to H2 at 800 °C results in the majority of the SWNTs being consumed.18 This observation is consistent with the reduction of the catalyst precursor to a Fe/Mo catalyst particle followed by reductive docking of the particle to the SWNT end, and etching of the SWNT by the catalyst and H2 (Scheme 1). In order to examine the growth of larger samples of SWNT-cat, a CHCl3 solution of DD-SWNT-py-FeMoC was spun onto an SOG surface. The Raman spectrum of the sample (Figure 2) clearly showed both the disorder mode (D mode) at
Figure 1. AFM images of DD-SWNT-py-FeMoC showing the presence of the 2 nm FeMoC molecule on the end of the SWNT.
1330 cm-1, consistent with the side-wall functionalization, and the G band at 1590 cm-1 and the G′ band at 2600 cm-1, both associated with the SWNTs.19 Heating the SWNT-cat to 800 °C under a H2/CH4 atmosphere for 15 min results in the loss of the D mode, confirming the loss of all the side-wall groups (Figure 2). The Raman spectrum also shows an increase in the G bands, indicating significant SWNT growth. Moreover, growth was also confirmed by AFM measurements (Figure 3). The lengths of the SWNTs were measured before and after growth. The as-synthesized SWNT-cats have an average length of 115 nm; after growth, the average length of the SWNTs is 450 nm. As may be seen from Figure 4, there is a significant shift in the length distribution. In determining whether the observed growth is “seeded” from SWNT-cat or “new growth” from FeMoC, back-to-back comparisons under identical conditions show that the growth yield from FeMoC is ∼1%,13 while the yield of extended SWNTs is ∼40% (Figure 4). Thus, it appears that the seeded growth from the FeMoC attached to an SWNT seed is much more facile than the nucleation of a new SWNT from the FeMoC catalyst particle. Upon the basis of our previous results comparing growth parallel versus perpendicular to the surface, the magnitude of growth (ca. 3×) is consistent with its being limited by the SWNT-surface interactions.7 Finally, a consideration of Figure 4 suggests that the shorter SWNT-cats are more active than the longer seeds, since ca. 90% of the SWNTs below 200 nm exhibit growth, while longer SWNTs (>1000 nm) show no significant change. We propose that an explanation for this observation is that the shorter
Figure 2. Raman spectra of a DD-SWNT-py-FeMoC “SWNT-cat” on SOG before (black) and after (red) growth.
17806 J. Phys. Chem. C, Vol. 111, No. 48, 2007
Ogrin et al. the performance of a particular catalyst composition with regard to growth as opposed to nucleation. This might also allow for greater flexibility in the selection of catalyst materials. Acknowledgment. This paper is dedicated to our late colleague Rick Smalley for his leadership in the creation of the first center for nanotechnology in the world, which now proudly bears his name. Financial support for this work is provided by the Defense Advanced Research Project Agency and the Robert A. Welch Foundation. R.C. gratefully thanks the National Research Council and the Ford Foundation for a Postdoctoral Fellowship. Supporting Information Available: Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 3. AFM images of SWNTs after being grown from DD-SWNTpy-FeMoC.
Figure 4. Plot of SWNT length after growth (gray bars) as compared the DD-SWNT-py-FeMoC “SWNT-cat” seeds (black bars).
SWNTs were those on which the ends were “activated” during Piranha purification (i.e., oxidation and cutting), creating sites for the coupling chemistry to occur. In the absence of carboxylate (or hydroxyl) groups (i.e., SWNTs with closed ends), there will be no DCC-mediated coupling with the amino pyridine, and hence no ligand suitable for the attachment of the catalyst precursors FeMoC or similar catalyst. We have reported that the growth yield from an SWNT-cat conjugate is significantly higher than that from the catalyst alone, demonstrating the use of a preformed SWNT seed to decouple the nucleation and growth reactions. We propose that the use of a seeded growth for SWNTs will allow for studies to decouple
(1) Tans, S. J.; Devoret, M. H.; Dai, H.; Thess, A.; Smalley, R. E.; Geerligs, L. J.; Dekker, C. Nature 1997, 386, 474. (2) Choi, H.-J.; Ihm, J. H. Phys. ReV. B 1999, 59, 2267. (3) Bachilo, M.; Strano, S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298, 2361. (4) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (5) Bethune, D. S.; Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (6) Gavillet, J.; Loiseau, A.; Journet, C.; Willaime, F.; Ducastelle, F.; Charlier, J. C. Phys. ReV. Lett. 2001, 87, 275504. (7) Ogrin, D.; Colorado, R., Jr.; Maruyama, B.; Pender, M. J.; Smalley, R. E.; Barron, A. R. Dalton Trans. 2006, 229. (8) Such a process is likened to the polymerase chain reaction used for producing relatively large numbers of copies of DNA molecules from minute quantities of source DNA material. (9) Smalley, R. E.; Li, Y.; Moore, V. C.; Price, K.; Colorado, R., Jr.; Schmidt, H.; Hauge, R. H.; Barron, A. R.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 15824. (10) Wang, Y. H.; Kim, M. J.; Shan, H. W.; Kittrell, C.; Fan, H.; Ericson, L.; Hwang, W. F.; Arepalli, S.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2005, 5, 997. (11) Liu, J.; Fan, S. S.; Dai, H. J. MRS Bull. 2004, 29, 244. (12) An, L.; Owens, J. M.; McNeil, L. E.; Liu, J. J. Am. Chem. Soc. 2002, 124, 13688. (13) Anderson, R. E.; Colorado, R., Jr.; Crouse, C.; Ogrin, D.; Maruyama, B.; Pender, M. J.; Edwards, C. L.; Whitsitt, E.; Moore, V. C.; Koveal, D.; Lupu, C.; Stewart, M.; Tour, J. M.; Smalley, R. E.; Barron, A. R. Dalton Trans. 2006, 3097. (14) Ogrin, D.; Barron, A. R. J. Cluster Sci., 2007, 18, 113. (15) Liang, F.; Sadana, A. K.; Peera, A.; Chattopadhyay, J.; Gu, Z.; Hauge, R. H.; Billups, W. E. Nano Lett. 2004, 4, 1257. (16) Azamian, B. R.; Coleman, K. S.; Davis, J. J.; Hanson, N.; Green, M. L. H. J. Chem. Soc., Chem. Commun. 2002, 366. (17) To determine whether the coupling reaction was successful, samples of the functionalized tubes were placed in a UV-vis cuvette with a 0.5 mM EtOH solution of FeCl3. The DD-SWNTs-py are insoluble in EtOH, and therefore any reduction in the intensity of the bands due to the Fe3+ is due to complexation of the metal to the pyridine substituents. Ogrin, D.; Hamilton, C.; Barron, A. R. Dalton Trans., submitted for publication, 2007. (18) SOG was used as a substrate instead of silicon or HOPG to allow direct comparison with our prior studies of FeMoC (see ref 13). (19) Jorio, A.; Pimenta, M. A.; Souza-Filho, A. G.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. New J. Phys. 2003, 5, 139.
