Single-Walled Carbon Nanotube Films Assembled via Donor

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Polymer/Single-Walled Carbon Nanotube Films Assembled via Donor-Acceptor Interactions and Their Use as Scaffolds for Silica Deposition Jason H. Rouse,*,† Peter T. Lillehei,‡ Joel Sanderson,§ and Emilie J. Siochi‡ National Institute of Aerospace, 144 Research Drive, Hampton, Virginia 23666, Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681, and New Horizons Regional Governors School for Science and Technology, 2032 Butler Farm Road, Hampton, Virginia 23666 Received February 25, 2004. Revised Manuscript Received June 15, 2004

A method of stepwise assembling thin polymer/single-walled carbon nanotube (SWCNT) films using donor-acceptor interactions is demonstrated. When the affinity that amine groups have for nanotubes were utilized, films were formed by the sequential adsorption of polyethylenimine and polyallylamine followed by SWCNTs onto silicon substrates. In an effort to expand this methodology to more thermally and oxidatively stable polymer systems, the ability of the basic nitrogen of the pyridine ring to adsorb SWCNTs was also investigated. These studies demonstrated that the nonsterically hindered, para-substituted pyridine in poly(4-vinylpyridine) (P4VP) also has an affinity for SWCNTs, thus enabling the stepwise formation of P4VP/SWCNT films. Microscopy of the films revealed that they were formed of single tubes and small bundles and that film coverage and thickness were uniform. The ability to use these films as scaffolds for the synthesis of novel hybrid structures is demonstrated by modifying the P4VP films using sol-gel chemistry.

Introduction The development of methods of incorporating or assembling single-walled carbon nanotubes (SWCNT) into structures is an area of intense research given their remarkable properties. SWCNTs are composed of a rolled graphene sheet having a diameter in the nanometer range and lengths that can approach tens of micrometers. Their unique shape and aromaticity have imparted SWCNTs with great strength, stiffness, and high electrical and thermal conductivities.1 Coupled with their low density and high surface area,2 SWCNTs have been promoted as an ideal structural reinforcement, potentially allowing the formation of ultra-light weight, high-strength materials.3 The potential for the use of carbon nanotubes as reinforcements in composites is hindered by the difficulty encountered in uniformly dispersing them throughout the host matrix, a result of both poor solubility4 and strong intertube adhesion.5 With an estimated interaction of 0.5 eV/µm of tube-tube contact,5 SWCNTs are normally found not as single nano* To whom correspondence should be addressed. E-mail: j.h.rouse@ larc.nasa.gov. † National Institute of Aerospace. ‡ NASA Langley Research Center. § New Horizons Regional Governors School for Science and Technology. (1) (a) Ajayan, P. M. Chem. Rev. 1999, 99, 1787-1799. (b) Dresselhaus, M. S., Dresselhaus, G., Avouris, P., Eds. Carbon Nanotubes: Synthesis, Structure, Properties and Applications; Springer: New York, 2001; Vol. 80. (2) Peigney, A.; Laurent, Ch.; Flahaut, E.; Bacsa, R. R.; Rousset, A. Carbon 2001, 39, 507-514. (3) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787-792.

tubes but rather as aligned bundles6 of tubes and larger aggregates that require substantial energy input for exfoliation.7 In addition to standard bulk composite processing techniques (i.e., solution mixing,8,9 melt mixing,10 and in situ polymerization under sonication11) to disperse nanotubes within polymers, efforts to assemble such materials using a “bottom-up” approach have recently been reported. Films containing high (4) (a) Ausman, K. D.; Piner, R.; Lourie, O. Ruoff, R. S.; Korobov, M. J. Phys. Chem. B 2000, 104, 8911-8915. (b) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193-194. (5) Girifalco, L. A.; Hodak, M.; Lee, R. S. Phys. Rev. Lett. 2000, 62, 13104-13110. (6) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483-487. (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.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593-596. (8) (a) Lourie, O.; Wagner, H. D. J. Mater. Res. 1998, 13, 24182422. (b) Stephan, C.; Nguyen, T. P.; de la Chapelle, M. L.; Lefrant, S.; Journet, C.; Bernier, P. Synth. Met. 2000, 108, 139-149. (c) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. Adv. Mater. 2000, 12, 750-753. (d) Mitchell, C. A.; Bahr, J. L.; Arepalli, S.; Tour, J. M.; Krishnamoorti, R. Macromolecules 2002, 35, 8825-8830. (e) Geng, H.; Rosen, R.; Zheng, B.; Shimoda, H.; Fleming, L.; Liu, J.; Zhou, O. Adv. Mater. 2002, 14, 1387-1390. (f) Zhu, J.; Kim, J.; Peng, H.; Margrave, J. L.; Khabashesku, V. N.; Barrera, E. V. Nano Lett. 2003, 3, 11071113. (9) (a) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Catherine, J.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (b) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703. (10) (a) Andrews, R.; Jacques, D.; Rao, A. M.; Rantell, T.; Derbyshire, F.; Chen, Y.; Chen, J.; Haddon, R. C. Appl. Phys. Lett. 1999, 75, 1329-1331. (b) Haggenmueller, R.; Gommans, H. H.; Rinzler, A. G.; Fischer, J. E.; Winey, K. I. Chem. Phys. Lett. 2000, 330, 219-225.

10.1021/cm049708t CCC: $27.50 © 2004 American Chemical Society Published on Web 09/01/2004

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loadings of uniformly dispersed individual nanotubes and small bundles were stepwise assembled onto a substrate via electrostatic interactions12 by sequentially adsorbing a cationic polyelectrolyte followed by oxidized nanotubes.13,14 Initial mechanical characterization of the polyelectrolyte/SWCNT films demonstrated increases in Young’s modulus and tensile strength compared to polyelectrolyte-only films.14 A drawback of the electrostatic adsorption technique is the necessity of introducing oxidized defects into the nanotubes to create sites for adsorption that degrades their properties.15 As researchers have also demonstrated that hydrogen bonding,16,17 charge-transfer interactions,18 and stereocomplex formation19 can be utilized to assemble thin polymer films in a stepwise manner, we have studied the ability of the aminenanotube interaction to promote film assembly using pristine rather than oxidized nanotubes. First reported by Colbert et al.,20 the amine group possesses a high binding affinity for SWCNTs, allowing the selective placement of the nanotubes onto patterned self-assembled monolayers (SAM)20,21 and onto amine-terminated colloidal silica particles.22 Extensive work by Dai and co-workers23 demonstrated that the electron-donating ability of ammonia, alkylamines, and polyethylenimine (PEI) results in the n-doping of SWCNTs (donoracceptor interaction). In the PEI case, doping was even stable in the presence of oxygen.23c With doping most pronounced for semiconducting tubes, the ability to separate metallic from semiconducting nanotubes via (11) (a) Park, C.; Ounaies, Z.; Watson, K. A.; Crooks, R. E.; Smith, J.; Lowther, S. E.; Connell, J. W.; Siochi, E. J.; Harrison, J. S.; St. Clair, T. L. Chem. Phys. Lett. 2002, 364, 303-308. (b) Kumar, S.; Dang, T. D.; Arnold, F. E.; Bhattacharyya, A. R.; Min, B. G.; Zhang, X.; Vaia, R. A.; Park, C.; Adams, W. W.; Hauge, R. H.; Smalley, R. E.; Ramesh, S.; Willis, P. A. Macromolecules 2002, 35, 9039-9043. (12) (a) Decher, G. Science 1997, 277, 1232-1237. (b) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430-442. (13) Rouse, J. H.; Lillehei, P. T. Nano Lett. 2003, 3, 59-62. (14) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nature Mater. 2002, 1, 190-194. (15) (a) Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523526. (b) Yakobson, B. I.; Brabec, C. J.; Bernholc, J. Phys. Rev. Lett. 1996, 76, 2511-2514. (16) (a) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717-2725. (b) Serizawa, T.; Hashiguchi, S. Akashi, M. Langmuir 1999, 15, 5363-5368. (c) Sukhishvili, S.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550-9551. (d) Raposo, M.; Oliveira, O. N., Jr. Langmuir 2000, 16, 2839-2844. (e) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100-2101. (17) (a) Serizawa, T.; Yamamoto, K.; Akashi, M. Langmuir 1999, 15, 4682-4684. (b) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360-1363. (c) Wang, L.; Cui, S.; Wang, Z.; Zhang, X.; Jiang, M.; Chi, L.; Fuchs, H. Langmuir 2000, 16, 10490-10494. (18) (a) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385-1387. (b) Fisher, P.; Laschewsky, A. Macromolecules 2000, 33, 1100-1102. (c) Shimazaki, Y.; Nakamura, R.; Ito, S.; Yamamoto, M. Langmuir 2001, 17, 953-956. (19) (a) Serizawa, T.; Hamada, K.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. J. Am. Chem. Soc. 2000, 122, 1891-1899. (b) Serizawa, T.; Yamashita, H.; Fujiwara, T.; Kimura, Y.; Akashi, M. Macromolecules 2001, 34, 1996-2001. (20) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125-129. (21) (a) Choi, K. H.; Bourgoin, J. P.; Auvray, S.; Esteve, D.; Duesberg, G. S.; Roth, S.; Burghard, M. Surf. Sci. 2000, 462, 195202. (b) Lewenstein, J. C.; Burgin, T. P.; Ribayrol, A.; Nagahara, L. A.; Tsui, R. K. Nano Lett. 2002, 2, 443-446. (22) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Nano Lett. 2002, 2, 531-533. (23) (a) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622-625. (b) Zhou, C.; Kong, J.; Yenilmez, E.; Dai, H. Science 2000, 290, 1552-1555. (c) Shim, M.; Javey, A.; Kam, N. W. S.; Dai, H. J. Am. Chem. Soc. 2001, 123, 1151211513. (d) Kong, J.; Dai, H. J. Phys. Chem. B 2001, 105, 2890-2893.

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selective solubilization using an alkylamine has also been reported.24 This study reports on the successful application of the amine-nanotube interaction to allow the stepwise formation of polymer/pristine SWCNT films using polyamines. In addition, it will be shown that the more thermally and oxidatively stable pyridyl group also possesses an affinity for nanotubes, allowing the formation of poly(4-vinylpridine) (P4VP)/SWCNT films. Finally, the ability to modify the films formed using P4VP via sol-gel chemistry to produce unique hybrid nanotube/silica/polymer thin film structures is presented. Experimental Section Materials. Polyethylenimine (Mw ) 25000), polyallylamine (Mw ) 65000), poly(propylenimine tetrahexacontaamine) dendrimer generation 4 (DAB-Am-32), poly(4-vinylpyridine) (Mw ) 160000), poly(2-vinylpyridine) (Mw ) 159000), and 3-aminopropyltriethoxysilane (APTES) were purchased from Aldrich and used as received. Dimethylformamide (DMF) and toluene (anhydrous) were purchased from either Aldrich or Fluka and used as-received. Purified single-walled carbon nanotubes (SWCNTs) prepared from the HiPco process were purchased from Carbon Nanotechnologies, Inc. and used as-received.25 The SWCNTs were dispersed in DMF at a concentration of 0.005 g/L of DMF by sonicating the solution for 6 h using a Branson 2310 ultrasonic cleaning bath. After the initial 6 h of sonication, the dispersion was further sonicated for 1 h prior to use each day. Film Formation. Substrates for film formation were either silicon or quartz slides that were first made hydrophilic by treatment with piranha solution (2:1 v/v % concentrated H2SO4 and 30% H2O2) for ∼1 h. Caution: piranha solution reacts violently with organics. After rinsing with deionized water (Milli-Q, 18.2 MΩ) and drying with nitrogen (N2), the substrates were immediately placed into a 0.05 M solution of APTES in toluene. After 4-6 h, the substrates were removed, rinsed with toluene and then ethanol, and dried with N2. Polyethylenimine (PEI)/SWCNT films were stepwise assembled by first dipping the APTES-terminated substrate into a dilute dispersion of HiPco SWCNTs in DMF (0.005 g/L). After 20 min, the substrate was removed, rinsed with DMF, and dried with N2. Rinsing with DMF and drying with N2 were then repeated. The SWCNT-terminated substrate was then placed into a 0.1 M (monomer units) PEI solution in DMF for 10 min. To remove excess PEI, the film was rinsed with DMF and dried with N2 (procedure repeated a second time). The treatment of the substrate with the polymer and then SWCNTs will be referred to as an “adsorption cycle,” and the total number of adsorption cycles noted includes the initial APTES/ SWCNT adsorption step. For poly(4-vinylpyridine) (P4VP) films the concentration of polymer was 0.2 M, and with an absorption time of 20 and 40 min for the P4VP and SWCNT treatments, respectively. Postsynthetic Modification. For the neutral sol-gel condition, a 24-cycle P4VP/SWCNT film was broken into ∼1.0 × 0.5 cm pieces. Three of these pieces were placed horizontally and facedown in glass vials containing aliquots of a solution prepared by mixing 0.60 g of tetraethyl orthosilicate (TEOS, Acros), 3.0 mL of ethanol, and 2.0 mL of deionized water (DI). After various lengths of times1, 3, and 7 dsthe films were removed, rinsed with ethanol, and dried by a stream of nitrogen. A portion of the film treated with TEOS for 7 d was subsequently placed in an oven at 220 °C in air for 7 d. The only modification to this procedure for the acid-catalyzed system was the addition of 0.05 mL of concentrated HCl to (24) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2003, 125, 3370-3375. (25) Chiang, I. W.; Brinson, B. E.; Smalley, R. E.; Margrave, J. L.; Hauge, R. H. J. Phys. Chem. B 2001, 105, 1157-1161.

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the TEOS/ethanol/DI solution and the use of a 30-cycle P4VP/ SWCNT film. Characterization. Raman spectra were obtained at 532 and 785 nm laser excitations on a Nicolet Almega dispersive spectrometer equipped with a microscope. For silica-treated and untreated P4VP/SWCNT films, spectra were obtained at a laser power of 25%, a 25 µm slit aperture (resolution of 1.52.5 cm-1 over the spectral window), and a 1.0 s acquisition time and with 256 scans averaged. For the bulk sample of HiPco SWCNTs, the spectra were obtained under identical conditions except that only 64 scans were averaged. Spectra were collected at various locations on each sample studied to determine reproducibility. Ultraviolet-visible-near infrared (UV-vis-NIR) spectra were acquired on a Perkin-Elmer Lambda 900. Substrates were quartz slides (ChemGlass) cut to fit diagonally into 1 cm2 matched quartz cuvettes. Spectra were acquired from 1500 to 200 nm at a scan speed of 150 nm/min and a spectra resolution of 1 nm. Atomic force microscopy (AFM) was performed using a Digital Instruments Nanoscope IIIa (Santa Barbara, CA) operated in TappingMode. To minimize tip sample adhesion, the imaging was done under “hard tapping” conditions where the resonance amplitude was set to 3 V and the set-point ratio was operated at or below 70% of the free amplitude. Under these conditions, rapid tip wear was a problem so tip shape was monitored using the Scanning Probe Image Processor (Image Metrology, Denmark). The probes used were NanoSensors TappingMode Etched Silicon Probes (Digital Instruments) with a resonant frequency of ∼360 kHz and a typical tip radius of curvature of ∼10 nm. Scanning electron micrographs (SEM) were obtained on an Hitachi S-5200 high-resolution scanning electron microscope. Samples were mounted on aluminum stubs using graphite paste or mechanically fastened to a cross-sectional film holder. Images were acquired at an accelerating voltage of 0.5-2.0 eV and beam currents below 20 µA.

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Figure 1. Atomic force microscopy (tapping mode) images of a (PEI/SWCNT)4 film prepared on a silicon wafer after each adsorption cycle (a-d, respectively). The images are 5 µm2 and the z-scale in all images is 30 nm.

Results and Discussion Polyamine Films. Initial research into forming polymer/SWCNT films using the amine-nanotube interaction utilized polyethylenimine (PEI), a polymer studied in both electrostatic and hydrogen-bonding film formation methods, and previously shown to adsorb onto single-walled carbon nanotubes (SWCNTs).23c Substrates for film formation were either silicon or quartz slides that were first made hydrophilic and then treated with 3-aminopropyltriethoxysilane (APTES). PEI/SWCNT films were assembled stepwise by first dipping the APTES-terminated substrate into a dilute dispersion of HiPco SWCNTs in dimethylformamide (DMF). After 20 min, the substrate was removed, rinsed with DMF, and dried with nitrogen (N2). Rinsing with DMF and drying with N2 were then repeated. The SWCNT-terminated substrate was then placed into a 0.1 M (monomer units) PEI solution in DMF for 10 min. To remove excess PEI, the film was rinsed with DMF and dried with N2 (procedure repeated a second time). Atomic force microscopy (AFM) images obtained on a (PEI/SWCNT)4 film after each adsorption cycle are shown in Figure 1. Height-profile measurements of the SWCNTs adsorbed onto the APTES-treated surface indicated that approximately 50% was individual nanotubes with the balance being 2-5 nm diameter bundles (Figure 1a). The lengths of the nanotubes and bundles were predominately 1-3 µm. Efforts to increase nanotube surface coverage by increasing the adsorption time up to 24 h did not result in a noticeable increase in surface coverage. However, after treatment of the film

Figure 2. Scanning electron micrograph of a (PEI/SWCNT)4 film adsorbed onto a silicon wafer. The magnification is 1500× and the scale bar is 30 µm.

with PEI and then SWCNTs an increase in nanotube surface coverage was seen (Figure 1b). Repetition of the PEI/SWCNT treatments resulted in a continued increase in the number of nanotubes present, indicating the formation of a multilayered film (Figure 1c,d). Imaging of the film by scanning electron microscopy (SEM) confirmed that after only 4 cycles the adsorption procedure resulted in uniform film coverage (Figure 2). To assess the role of polymer structure on film assembly, two additional polyamines were investigated: (1) a dendrimeric form of PEI (DAB-Am-32) containing 32 primary and 30 secondary amines (formula weight ) 3,514); and (2) a polyallylamine (PAA) containing ∼1500 primary amine units (molecular weight (Mw) ) 65000). The PEI utilized in film formation was a linear polymer with a molecular weight (Mw) of ∼25000 (∼580 secondary amine units). Efforts to form dendrimer/SWCNT films failed, even when high polymer concentrations (1.0 M) and long adsorption times (24 h) were utilized. This result was surprising since

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Figure 3. UV-vis-NIR absorbance measured after each PEI/ SWCNT treatment for a 10-cycle film deposited on a quartz slide. The inset shows the measured absorbance at 1326 nm versus the number of adsorption cycles. The line is a leastsquares fit to the data.

dendrimers of this26 and a closely related structure27 have been used as building blocks in sequential adsorption techniques. The inability to form films using the lower molecular weight dendrimer might be the result of the dendrimer adsorbing onto the nanotube surface to form a tightly bound surface layer, and thus not possessing excess amine groups that could further interact with additional nanotubes. However, films were successfully prepared with PAA and had the same morphology as the PEI/SWCNT films. AFM images of both PEI and PAA films revealed that the nanotubes were covered with a “greasy” layer of polymer (the nodules in Figure 1b-d), thus providing excess amine groups for additional SWCNT adsorption. While AFM proved useful for characterizing initial film growth, or lack thereof, determining whether nanotube adsorption was occurring reproducibly was difficult from AFM imaging alone. Therefore, the growth of a 10-cycle PEI/SWCNT film on a quartz wafer was monitored by ultraviolet-visible-near-infrared spectrophotometry (UV-vis-NIR) (Figure 3). The multiple adsorption peaks present in the spectra are from the van Hove singularities (electronic transitions) of the various metallic and semiconducting nanotubes present in HiPco SWCNTs.1,28 As growth occurs on both sides of the quartz wafer, the linear increase in absorbance at 1326 nm versus number of cycles (inset) represents the reproducible adsorption of the SWCNTs onto the (26) Casson, J. L.; Wang, H.-L.; Roberts, J. B.; Parikh, A. N.; Robinson, J. M.; Johal, M. S. J. Phys. Chem. B 2002, 106, 1697-1702. (27) (a) Watanbe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855-8856. (b) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551-555. (c) Kim, B. Y.; Bruening, M. L. Langmuir 2003, 19, 94-99. (28) While the amine-nanotube interaction is known to be stronger for semiconducting than for metallic nanotubes, comparison of the UVVis-NIR absorbance obtained on the PEI/SWCNT film with that obtained from a bulk nanotube film indicated only a slight decrease in the absorbance intensity at ∼700 nm associated with the van Hove singularities of metallic nanotubes. The presence of metallic nanotubes was further confirmed by Raman spectroscopy. A possible explanation for the lack of enrichment of semiconducting nanotubes in the film is that because both individual nanotubes and small bundles of nanotubes are present in roughly equal amounts within these films, the bundles may contain a sufficient amount of metallic nanotubes to negate the enrichment of semiconducting nanotubes that occurs for the individually adsorbed nanotubes.

Figure 4. Atomic force microscopy (tapping mode) images of a (P4VP/SWCNT)4 film prepared on a silicon wafer after each adsorption cycle (a-d, respectively). The images are 5 µm2 and the z-scale in all images is 25 nm.

film over a total of 20 cycles.29 Additionally, strong adsorption in the visible region from the nanotubes resulted in a noticeable darkening of the quartz slide after only 20 cycles. Polyvinylpyridine Films. While the results obtained using PEI and PAA successfully demonstrated the utility of using the amine-nanotube interaction to foster polymer/SWCNT film formation, the poor thermal and oxidative stability of amines limits the possible applications of these systems. Given that the pyridyl group contains both a basic nitrogen atom and improved stability due to its aromaticity, the ability to form films using polyvinylpyridines was investigated. Attempts to form films using poly(4-vinylpyridine) (P4VP) under the identical adsorption conditions used for PEI resulted in little sequential growth. However, increasing the P4VP concentration and adsorption time to 0.2 M and 20 min, respectively, and the nanotube adsorption step to 40 min resulted in reproducible film growth. AFM images obtained after each adsorption cycle of a (P4VP/ SWCNT)4 film (Figure 4) revealed a structure similar to that seen in the PEI system. Rather than determining the reproducibility of film growth by UV-vis-NIR, the thicknesses of films of a varying number of cycless14, 20, 24, 30, 34, and 40swere determined by crosssectional SEM. The thickness of the films appeared very uniform over the ∼5-7 mm of length imaged of each sample, in agreement with the uniform color displayed by films greater than 20 cycles.30 High-magnification images of the film clearly demonstrated that the films (29) The discontinuity in the baseline at approximately 869 nm, caused from changing the NIR source to the UV-vis source, necessitated plotting absorbance at 1326 nm, an electronic transition for semiconducting nanotubes. (30) The presence of an interference color is the result of the interference of light reflected from the air-film interface with that reflected from the film-substrate interface, see: Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007. Pliskin, W. A.; Conrad, E. E. IBM J. Res. Dev. 1964, 8, 43.

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Figure 5. Cross-sectional scanning electron micrographs of 14-cycle (a and b) and 40-cycle (c and d) P4VP/SWCNT films on a silicon wafer. The scale bars in (a) and (c) are 1 µm (50000×) and in (b) and (d) are 200 nm (250000×).

Figure 7. Scanning electron micrographs of the surfaces of a (P4VP/SWCNT)24 film (a); the (P4VP/SWCNT)24 film after sol-gel treatment under neutral conditions for 1 d (b) and 7 d (c); and a (P4VP/SWCNT)30 film after sol-gel treatment under acidic conditions for 1 d (d) and 7 d (e). The magnification in all images is 50000× and the scale bars are 1 µm.

Figure 6. A plot of (P4VP/SWCNT)x film thickness versus the number of adsorption cycles (x). The line is a linear leastsquares fit to the data and error in film thickness was estimated as +5 nm.

were comprised of layers of nanotubes. Representative images obtained from the 14- and 40-cycle films are shown in Figure 5. A plot of film thickness versus number of adsorption cycles resulted in linear growth of ∼2-nm per cycle (Figure 6), consistent with films assembled from predominately individual nanotubes and a thin layer of polymer. From the AFM and SEM images we estimate that the films are ∼80% porous with roughly equal amounts of P4VP and SWCNT present (∼10 vol % nanotubes). To determine if the mechanism for nanotube adsorption onto P4VP was in fact due to direct nitrogennanotube interaction and not simply a π-π interaction between the aromatic ring and the nanotube surface, the ability to form films with poly(2-vinylpyridine) (P2VP) and polystyrene were investigated. Efforts to form films using either polymer were unsuccessful, even using high polymer concentrations (1.0 M) and long adsorption times (24 h). These results clearly indicate that the lone pair of the pyridyl nitrogen, and not the aromatic ring, directly interacts with the π-system of the nanotube. Postsynthetic Modification. The porous structure of the PEI and P4VP/SWCNT films offered the possibil-

ity of their postsynthetic modification to create unique hybrid films. Since poly(4-vinylpyridine) is known to catalyze the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) to form silica,31 the ability of P4VP/ SWCNT films to act as a scaffold for silica formation was investigated.32 Due to known effects of catalysis conditions on silica morphologies (i.e., basic, acidic, neutral, and catalysis identity),33 both neutral and acidic conditions on film structure were investigated. For the neutral sol-gel condition, a 24-cycle P4VP/ SWCNT film was broken into ∼1.0 × 0.5 cm pieces. Three of these pieces were placed horizontally and facedown in glass vials containing aliquots of a solution prepared by mixing TEOS, ethanol, and deionized water. After various lengths of times1, 3, and 7 dsthe films were removed, rinsed with ethanol, and dried by a stream of N2. A portion of the film treated with TEOS for 7 d was subsequently placed in an oven at 220 °C in air for 7 d to determine the effect of heating on film structure. The only modification to this procedure for the acid-catalyzed sol-gel system was the addition of a small amount of HCl to the TEOS/ethanol/DI solution and the use of a 30-cycle P4VP/SWCNT film as a scaffold. Scanning electron micrographs of the upper surfaces of the films treated for 1 d under both neutral and acidic (31) Cho, G.; Jang, J.; Jung, S.; Moon, I.-S.; Lee, J.-S.; Cho, Y.-S.; Fung, B. M.; Yuan, W.-L.; O’Rear, E. A. Langmuir 2002, 18, 34303433. (32) (a) Seegar, T.; Redlich, Ph.; Grobert, N.; Terrones, M.; Walton, D. R. M.; Kroto, H. W.; Ruhle, M. Chem. Phys. Lett. 2001, 339, 41-46. (b) Fu, Q.; Lu, C.; Liu, J. Nano Lett. 2002, 2, 329-332. (c) Whitsitt, E. A.; Barron, A. R. Nano Lett. 2003, 3, 775-778. (33) Brinker, C. J.; Schere, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990.

Polymer/Single-Walled Carbon Nanotube Films

Figure 8. Scanning electron micrographs of cross sections of the (P4VP/SWCNT)24 film after sol-gel treatment under neutral conditions for 1 d (a) and 7 d (b); and a (P4VP/ SWCNT)30 film after sol-gel treatment under acidic conditions for 1 d (c) and 7 d (d). The magnification in all images is 250000× and the scale bars are 200 nm.

sol-gel conditions are shown in Figure 7b and d, respectively. The effect of the addition of the HCl to the TEOS solution on the resulting film structure was pronounced. Under neutral conditions, silica growth appears to be mediated by the P4VP covering the nanotubes because the previously smooth polymercovered surfaces (Figure 7a) had a nodular appearance and film porosity was mostly maintained. In contrast, the acid-catalyzed system resulted in a dramatic reduction in the porosity of the P4VP/SWCNT film with silica growth not appearing to be concentrated at the nanotube surface. Images obtained after 7 d of neutral solgel treatment further demonstrated the surface-initiated growth of silica from the P4VP-treated nanotube surfaces (Figure 7c); nanotubes within the film were uniformly covered with ∼10-nm silica particles. In contrast, the film treated under acid-catalyzed conditions for 7 d had a surface devoid of linear structures, indicating the presence of the nanotubes and film porosity was nearly eliminated (Figure 7e). To gain further insight into the structure of these hybrid polymer/nanotube/silica films, cross-sectional scanning electron micrographs were obtained of each series. These images not only confirmed that silica growth occurred homogeneously throughout the entire film thickness but also revealed that the sol-gel-treated films were substantially thicker than the initial film scaffold (Figure 8). For the 24-cycle P4VP/SWCNT film, treated under neutral conditions, the thicknesses of the initial film and after 1 d (see Figure 8a), after 3 d, and after 7 d (see Figure 8b) of sol-gel treatment were 50 ( 5 nm, 120 ( 10 nm, 135 ( 10 nm, and 150 ( 10 nm, respectively. The corresponding values obtained for the 30-cycle film, treated under acidic conditions, were 62 ( 5 nm, 120 ( 10 nm (Figure 8c), 165 ( 10 nm, and 190 ( 10 nm (Figure 8d), respectively. These values after 7 d of treatment represent an increase in film thickness of ∼300% compared to the initial film thicknesses in both cases. It is estimated that this increase in thickness has reduced the nanotube volume percent

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Figure 9. Low-magnification cross-sectional scanning electron micrograph (P4VP/SWCNT)30 film after sol-gel treatment under acidic conditions for 7 d and subsequent heating at 220 °C for 7 d. The magnification is 5000× and the scale bar is 10 µm.

Figure 10. Raman spectra of the radial breathing mode (top) and tangential mode (bottom) regions of various films: (solid line) bulk HiPco nanotubes; (dotted line) (P4VP/SWCNT)30 film; (dashed line) (P4VP/SWCNT)30 film after sol-gel treatment under acid-catalyzed conditions for 7 d; and (dot-dashed) (P4VP/SWCNT)30 film after sol-gel treatment under acidcatalyzed conditions for 7 d and subsequent heating at 220 °C for 7 d. The laser excitation energy was 785 nm, which probes predominately semiconducting nanotubes.

to ∼3%, assuming the films are dense. Because the films treated under neutral sol-gel conditions are at least 50% porous, the nanotube loading is higher based on solid content only. Heating the 7 d sol-gel-treated films at 220 °C for 7 d in air caused a slight reduction in film thickness (∼10%) and neither sol-gel treatment nor heating had any effect on the uniformity of the thicknesses of these films (Figure 9). While cross-sectional imaging demonstrated the presence of nanotubes within the films, the near absence of linear structures in the top-down images of the films treated under acid-catalyzed sol-gel conditions necessitated further characterization to ensure that the nanotubes were not rebundled during treatment. Since the positions of certain Raman active modes are sensitive to the size of the nanotube bundle,34 spectra of the tangential and radial breathing modes of the (P4VP/

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SWCNT)30 film treated under various conditions were compared to that of a bulk film of the HiPco nanotubes. In the radial breathing mode region, each peak was shifted 3-5 cm-1 to higher frequency in the as-assembled (P4VP/SWCNT)30 film compared to the bulk HiPco film (Figure 10 top). A corresponding increase in the positions of the tangential mode peaks for the (P4VP/SWCNT)30 film were also seen and a noticeable decrease in the width of the ∼1590 cm-1 peak was evident (Figure 10 bottom). These changes in Raman spectra are in agreement with the individual nanotubes and small bundles present in the AFM images of these films. The spectra obtained of both the film after solgel treatment for 7 d and the film after further heating at 220 °C were nearly identical to the spectra of the asassembled film, indicating that sol-gel treatment did not cause the nanotubes to rebundle during silica growth under acid-catalyzed conditions. (34) (a) Henrad, L.; Hernandez, E.; Bernier, P.; Rubio, A. Phys. Rev. B 1999, 60, R8521-B8524. (b) Jorio, A.; Saito, R.; Hafner, J. H.; Lieber, C. M.; Hunter, M.; McClure, T.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. Lett. 2001, 86, 1118-1121.

Rouse et al.

Conclusions When the affinity that amines have for single-walled carbon nanotubes has been utilized, a method of assembling polymer/SWCNT films has been developed. Individual nanotubes and small bundles were incorporated within multilayered films via the sequential adsorption of polyethylenimine and polyallylamine followed by SWCNTs onto substrates. The basic nitrogen of the pyridine ring was also found to possess an affinity for SWCNTs, allowing this sequential adsorption technique to be expanded to more thermally and oxidatively stable polymer systems. Films prepared using poly(4vinylpyridine) (P4VP) possessed the same morphology as the polyamine films and had very uniform thicknesses as evidenced by scanning electron microscopy. The ability to use these porous films as scaffolds for the synthesis of novel hybrid structures was demonstrated by modifying the P4VP films using sol-gel chemistry. Depending upon the sol-gel conditions, novel polymer/ nanotube/silica films with either dense or nanotube coated morphologies were formed. CM049708T