Stable, Conductive Supramolecular Composite of Graphene Sheets

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Stable, Conductive Supramolecular Composite of Graphene Sheets with Conjugated Polyelectrolyte Huafeng Yang, Qixian Zhang, Changsheng Shan, Fenghua Li, Dongxue Han, and Li Niu* Engineering Laboratory for Modern Analytical Techniques, w/o State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, and Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, PR China Received October 25, 2009 Supramolecularly functionalized graphene-based materials with conjugated poly(2,5-bis (3-sulfonatopropoxy)-1,4ethynylphenylene-alt-1,4-ethynylphenylene) polyelectrolyte were successfully obtained and exhibited high conductivity and stability (even for 8 months without any aid of free polymer in solution). The excellent aqueous solubility and the possibility for self-assembly through electrostatic interactions (i.e., layer-by-layer assembly) will be realized through various applications of graphene. In addition, PPE-SO3- molecules exhibit interesting optoelectronic properties, making the resulting graphene-based materials potentially useful in a variety of optoelectronic device applications.

1. Introduction Sheets of carbon only one atom thick, known as graphene, have attracted numerous investigations because of their unique physical, chemical, and mechanical properties,1-8 which provide potential applications in synthesizing nanocomposites1 and fabricating various microelectrical devices.2,9,10 However, challenges remaining to achieving good, stable dispersion of graphene sheets pose significant obstacles to these goals. Several effective techniques1,3,11-17 have been developed for preparing a good *Corresponding author. Email: [email protected] (L. Niu), Tel: þ86-43185262425, Fax: þ86-431-85262800.

(1) Li, D.; Kaner, R. B. Science 2008, 320, 1170–1171. (2) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652–655. (3) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282– 286. (4) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270–274. (5) Koehler, F. M.; Luechinger, N. A.; Ziegler, D.; Athanassiou, E. K.; Grass, R. N.; Rossi, A.; Hierold, C.; Stemmer, A.; Stark, W. J. Angew. Chem., Int. Ed. 2008, 48, 224–227. (6) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (7) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463–470. (8) Novoselov, K. S.; Morozov, S. V.; Mohinddin, T. M. G.; Ponomarenko, L. A.; Elias1, R. Y. D. C.; Barbolina, I. I.; Blake, T. J. B. P.; Jiang, J. G. D.; Hill, E. W.; Geim, A. K. Phys. Status Solidi B 2007, 244, 4106. (9) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394–3398. (10) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490–493. (11) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856–5857. (12) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155–158. (13) Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535–8539. (14) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon, M. A.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 7720–7721. (15) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105. (16) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (17) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W. F.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 16201–16206.

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dispersion of graphene sheets. Among them, the covalent and noncovalent functionalization of graphene have been considered to be important for improving their solubility, self-assembly properties, and further applications in devices. The noncovalent functionalization of graphene sheets with planar aromatic structures, such as pyrene derivatives, has produced chemically modified graphene sheets with small molecules.11 However, the supramolecular functionalization of graphene sheets with conjugated polyelectrolytes (CPEs) has rarely been addressed, although conjugated polymers have recently been found to interact strongly with the CNT surface through π stacking, and the resulting complexes can exhibit high solubility and excellent conductivity properties.18-25 In this work, we prepared a stable, conductive aqueous dispersion of graphene sheets using an anionic water-soluble conjugated polyelectrolyte named poly(2,5-bis(3-sulfonatopropoxy)-1,4-ethynylphenylenealt-1,4-ethynylphenylene) sodium salt (PPE-SO3-), which has a backbone structure that is based on the poly(phenylene ethynylene) (PPE) architecture. In the case of PPE-SO3-, the conjugated polymer backbone should result in behavior of graphene sheets that is similar to that of their nonionic counterparts with the added benefit of imparting excellent aqueous solubility and the possibility for self-assembly through electrostatic interactions (i.e., layer-by-layer assembly). In addition, PPE-SO3- molecules (18) Tang, B. Z.; Xu, H. Y. Macromolecules 1999, 32, 2569–2576. (19) Curran, S. A.; Ajayan, P. M.; Blau, W. J.; Carroll, D. L.; Coleman, J. N.; Dalton, A. B.; Davey, A. P.; Drury, A.; McCarthy, B.; Maier, S.; Strevens, A. Adv. Mater. 1998, 10, 1091–1093. (20) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S. W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721–1725. (21) Chen, J.; Liu, H. Y.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034–9035. (22) Cheng, F.; Adronov, A. Chem.;Eur. J. 2006, 12, 5053–5059. (23) Cheng, F.; Zhang, S.; Adronov, A.; Echegoyen, L.; Diederich, F. Chem.; Eur. J. 2006, 12, 6062–6070. (24) Cheng, F. Y.; Imin, P.; Maunders, C.; Botton, G.; Adronov, A. Macromolecules 2008, 41, 2304–2308. (25) Cheng, F. Y.; Imin, P.; Lazar, S.; Botton, G. A.; de Silveira, G.; Marinov, O.; Deen, J.; Adronov, A. Macromolecules 2008, 41, 9869–9874. (26) Chen, L. H.; McBranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287–12292. (27) Taranekar, P.; Qiao, Q.; Jiang, H.; Ghiviriga, I.; Schanze, K. S.; Reynolds, J. R. J. Am. Chem. Soc. 2007, 129, 8958–8959.

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exhibit interesting optoelectronic properties, making them potentially useful in a variety of device applications.26-28

2. Experimental Section 2.1. Materials. Graphite powders (Spectral pure) were obtained from Shanghai Chemicals, China. All other reagents and solvents were purchased from commercial suppliers and were used as received. All aqueous solutions were prepared with ultrapure water (>18 MΩ) from a Milli-Q Plus system (Millipore). 2.2. Instruments. UV-vis spectra were collected on a Cary 500 UV-vis-NIR spectrophotometer (Varian) using aqueous solutions in water. Fluorescence measurements were carried out on an LS-55 luminescence spectrometer (PerkinElmer). A 1.00 cm path length rectangular quartz cell was used for these measurements. Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer. The 514.5 nm radiation from a 20 mW air-cooled argon ion laser was used as the excitation source. The laser power at the sample position was typically 4 mW with an average spot size of 1 mm diameter. Atomic force microscope (AFM) images were obtained on a Digital Instruments Nanoscope IIIa (Santa Barbara, CA). X-ray photoelectron spectroscopy (XPS) analysis was carried on an ESCALAB MK II X-ray photoelectron spectrometer. The surface resistance was investigated with a 6512 programmable electrometer. Transmission electron microscopy (TEM) pictures were imaged by a JEOL 2000 transmission electron microscope operating at 200 kV.

Figure 1. (A) Photographs of u-CCG and PPE-SO3--G after 24 h

of reduction and (B) photographs of PPE-SO3--G and SDBS-G after removing free polymer. Scheme 1. Chemical Structure of PPE-SO3- and Schematic of the Preparation of PPE-SO3--Modified Graphene Sheets

2.3. Preparation of Graphene Oxide (GO) Nanosheets. Graphene oxide (GO) was prepared by oxidizing natural graphite powder based on a modified Hummers method as originally presented by Kovtyukhova and colleagues.29,30 As-prepared graphene oxide was suspended in ultrapure water to give a brown dispersion, which was subjected to dialysis for 4 days to remove residual salts and acids completely.15 The resulting purified graphene oxide powders were collected by centrifugation and air dried. Graphene oxide powders were dispersed in water to create a 0.05 wt % dispersion. Then the graphene oxide powders were exfoliated through ultrasonication in a water bath (KQ218, 60 W) for 1 h, upon which the bulk graphene oxide powders were transformed into GO nanoplatelets. 2.4. Preparation of PPE-SO3-. PPE-SO3- was prepared according to the literature.31 The as-prepared polymer was dissolved in water/methanol and reprecipitated from methanol/ acetone/ether four more times. The polymer was collected by centrifugation and air dried. FTIR (Vmax 3 cm-1) of PPE-SO3-: 2950, 2876, 1646, 1519, 1472, 1443, 1417, 1281, 1190, 1044, 835, 611, 540. 1H NMR (DMSO-d6; δ from TMS, 100 °C) of PPESO3-) δ 2.141 (t, 4H), 2.781 (t, 4H), 4.215 (t, 4H), 7.147 (S, 2H), 7.581 (broad, 4H).

2.5. Preparation of Unfunctionalized Chemically Converted Graphene (u-CCG) Nanosheets. Unfunctionalized, chemically converted graphene (u-CCG) was synthesized from as-purified graphene oxide nanosheets.15 Briefly, 20 mL of graphene oxide sheets (in water, 0.05 wt %) was mixed with 20 mL of water and 0.012 mL of hydrazine solution (50% in water) in a 100 mL glass vial. After being vigorously shaken or stirred for a few minutes, the GO nanosheets were reduced to graphene nanoplatelets by putting the mixture in an oil bath (∼80 °C) for 24 h. After reduction, a dispersion of u-CCG with visible black floccules was obtained.

2.6. Preparation of Poly(2,5-bis(3-sulfonatopropoxy)1,4-ethynylphenylene-alt-1,4-ethynylphenylene) Sodium Salt-Modified Graphene (PPE-SO3--G), Sodium Dodecyl (28) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Adv. Mater. 1998, 10, 1452–1455. (29) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (30) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771–778. (31) Tan, C. Y.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 5, 446–447.

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Benzene Sulfonate-Modified Graphene (SDBS-G), Poly(sodium 4-styrenesulfonate)-Modified Graphene Sheets (PSS-G), and Poly(vinyl pyrrolidone)-Modified Graphene Sheets (PVP-G). Twenty milligrams of as-prepared GO was distributed in 40 mL of ultrapure water to obtain a homogeneous, stable dispersion of GO with the aid of ultrasonication in a water bath (KQ218, 60 W) for 15 min, and then 60 mg of PPE-SO3(SDBS or PSS or PVP) was added to the GO dispersion. After the mixture was subjected to ultrasonication in a water bath (KQ218, 60 W) for another 15 min, the mixture was reduced with hydrazine monohydrate (1.950 mL, 50%) at 80 °C for 24 h. After reduction, a homogeneous black dispersion was obtained. The resulting solution was then filtered through a polycarbonate membrane (0.22 μm pore size) and was repeatedly washed with water to remove the excess free PPE-SO3- (SDBS or PSS or PVP). The collected PPE-SO3--modified graphene (PPE-SO3--G, black powder) was redistributed in water by ultrasonication in a water bath (KQ218, 60 W) for 15 min and centrifugation at 5000 rpm for 20 min. A dark, homogeneous dispersion was obtained after removing a little of the sedimentation.

3. Results and Discussion 3.1. Formation Mechanism and Stability. Scheme 1 shows the chemical structure of PPE-SO3- and illustrates the preparation of the PPE-SO3--modified graphene sheets. A dark, homogeneous supernatant solution was obtained after removing a little of the sedimentation, and the resulting solution was found to remain stable with no visible precipitate of graphene sheets for more than 8 months. In the control experiment, unfunctionalized chemically converted graphene (u-CCG) was prepared in a similar way but without PPE-SO3- addition. As shown in Figure 1A, a dispersion of u-CCG with visible black floccules DOI: 10.1021/la100365z

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Figure 3. (A) UV-vis absorption spectra. (B) Fluorescence spectra with 436 nm excitation.

Figure 2. C 1s XPS spectra of (A) GO, (B) u-CCG, and (C) PPE-

SO3--G.

was obtained after reduction for 24 h compared with the dispersion of PPE-SO3--G, indicating that PPE-SO3- in such chemically converted graphene sheets helped to stabilize the dispersion of graphene sheets. Moreover, the solution stability owing to the anionic conjugated polyelectrolyte could be comparable with those anionic small molecules (such as sodium dodecyl benzene sulfonate, (SDBS)-modified-graphene sheets). The SDBSmodified graphene sheets precipitated in 24 h after the free adsorbate was removed from the solution (as shown in Figure 1B), indicating that the π stacking of PPE-SO3- greatly increased the strength of the interaction between the conjugated polymer and graphene sheets. The stability of the solution of PPESO3--modified graphene should originate from the aqueous solubility imparted by the sulfonate functionality of PPE-SO3as well as the prevention of aggregation due to the intermolecular electrostatic repulsion of these functional groups. 3.2. X-ray Photoelectron Spectroscopy Analysis. GO, u-CCG, and PPE-SO3--G obtained in this work have been further characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figure 2A, the C 1s XPS spectra of GO clearly indicate a considerable degree of oxidation with four components corresponding to carbon atoms in different functional groups:7,12 the C in graphite (BE, 284.59 eV), the C in C-OH (BE, 285.64 eV), the C in C-O epoxy/ether groups (BE, 286.65 eV), and the carbonyl C (BE, 288.49 eV). Although the C 1s XPS spectra of u-CCG (Figure 2B) and PPE-SO3--G (Figure 2C) also exhibit the same species, the peak intensities of oxide species are much weaker than in the spectrum of GO, suggesting considerable deoxygenation by the chemical reduction process. 3.3. UV-Vis Spectroscopic and Fluorescence Analysis. The UV-vis absorption and fluorescence spectra of PPE-SO3and PPE-SO3--G are depicted in Figure 3. In Figure 3A, the 6710 DOI: 10.1021/la100365z

Figure 4. Raman spectra of u-CCG, PPE-SO3--G, and PPESO3- (blue line, PPE-SO3- (radiation at 785 nm); black line, uCCG (radiation at 514.5 nm); and red line, PPE-SO3--G (radiation at 514.5 nm)).

spectrum of PPE-SO3--G exhibits feature of the original PPE-SO3- with a characteristic shoulder peak at ∼430 nm.25 The PPE-SO3--G spectrum is broadened when compared to the spectrum of free PPE-SO3- in solution. This broadening is indicative of the stacking interaction that occurs between the π system of PPE-SO3- and the graphene sheets as with the interaction of CPEs and CNTs.25 Moreover, the absorption peak of the GO dispersion at 230 nm gradually red shifts to 275 nm (as shown in curves PPE-SO3--G and u-CCG), suggesting that the electronic conjugation within graphene sheets is restored upon hydrazine reduction.15 The fluorescence spectrum of PPE-SO3show a clear and dramatic change upon polymer adsorption to the graphene sheet surface (Figure 3B). As shown in the spectrum, significant quenching of the polymer emission occurs upon PPESO3--G complex formation, which is likely a result of effective electron or energy transfer between these two components.11 3.4. Raman Spectroscopy. To explore the interaction between PPE-SO3- and graphene further, the resulting PPE-SO3--G and PPE-SO3- were characterized by Raman spectroscopy Langmuir 2010, 26(9), 6708–6712

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Figure 5. AFM images of (A) a GO dispersion in water (0.25 mg/mL) and (B) a PPE-SO3--G dispersion in water (0.25 mg/mL) on freshly cleaved mica surfaces through drop casting.

(Figure 4). The Raman data reveal the dramatic changes in the signals of the graphene sheets upon supramolecular functionalization (red line, PPE-SO3--G; black line, u-CCG). G (1596 cm-1) is clearly present, and a symmetrical G0 (or 2D) band could be seen in the 2500-2900 cm-1 region, as expected for wellexfoliated graphene sheets.32 Compared with u-CCG, the intensity ratio of the D/G of PPE-SO3--G dramatically decrease, indicating that the functionalization of graphene with PPE-SO3greatly enhances the intensity of the G band of graphene sheets because of its conjugated structure. Compared with the Raman data of the free PPE-SO3- molecule (blue line), the new features at 1084, 1173, and 2184 cm-1 that appeared in the Raman spectra of PPE-SO3--G could be assigned to PPE-SO3- absorbed onto the graphene sheets. 3.5. Atomic Force Microscopy Analysis and Morphology. Figure 5 shows the atomic force microscopy (AFM) image of well-exfoliated GO nanosheets and monolayer and bilayer PPE-SO3--G sheets. The samples were prepared through drop casting onto freshly cleaved mica surfaces. The micas were dried under ambient conditions for 24 h. As shown in Figure 5B, the mean thicknesses of monolayer and bilayer PPE-SO3--G sheets were determined to be ca. 1.66 nm and ca. 3.31 nm, respectively. The height of the monolayer GO sheets is ca. 0.96 nm (Figure 5A). The distance between PPE-SO3--G sheets is greater than that between GO sheets, as would be expected. This is due to the presence of PPE-SO3- molecules absorbed onto both sides of the graphene sheet. In Figure 6, GO and PPE-SO3--G were analyzed (32) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 187401–187404.

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Figure 6. TEM images of (A) GO and (B) PPE- SO3--G.

by TEM and the images show the crumpled silk veil waves of these sheets. All results indicate that the complex of PPE-SO3- and graphene successfully formed in this work. 3.6. Conductivity. Finally, to quantify the conductivity of CPEs-modified graphene sheets and other polymer-modified graphene sheets, poly(sodium 4-styrenesulfonate)-modified graphene sheets (PSS-G) and poly(vinyl pyrrolidone)-modified graphene sheets (PVP-G) were synthesized according to previous reports.12,33 The photographs of three stable, homogeneous supernatant solutions are shown in the inset of Figure 7. The conductivity was investigated with the aid of one two-band electrode. The two-band electrode is 20 μm wide and 3 mm long. The samples were prepared as follows: 200 μL of the supernatant solution was dripped onto the surface of an as-purified two-band electrode and dried in air for 6 h. Then the sample was directly investigated by a programmable electrometer at room temperature. The results are shown in Figure 7. It is clear that PPE-SO3-G has the lowest surface resistance (30 KΩ), owing to the better (33) Shan, C.; Yang, H.; Song, J.; Han, D.; Ivaska, A.; Niu, L. Anal. Chem. 2009, 81, 2378–2382.

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time. The resulting graphene sheets may be very facile for further applications in electronic devices because of their unique properties, such as electrostatic self-assembly based on their negatively charged side chains or inkjet printing fabrication because of their good dispersibility and stability.

Figure 7. Surface resistance of PPE-SO3--G, PSS-G, and PVP-G at room temperature. (Inset) The left, middle, and right vials contain PPE-SO3--G, PSS-G, and PVP-G, respectively.

conductivity of conjugated polyelectrolyte (PPE-SO3-) compared to that of PSS and PVP. Although the chemical reduction of the unfunctionalized graphene oxide with hydrazine has produced conductive graphene sheets (u-CCG),15 the PPE-SO3--G produced in this work is very conductive like u-CCG (10 KΩ) but, importantly, water-dispersible and stable for a long period of

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4. Conclusions By taking advantage of the solubility in water of PPE-SO3- and the π stacking interaction between PPE-SO3- and graphene sheets, this work has reported a facile method of processing graphene through noncovalent functionalization to obtain highly conductive graphene-based materials that are stable for a long period of time. The successful attachment of PPE-SO3- onto graphene not only stabilizes the graphene dispersion in water but also endows the resulting graphene with negative charges, which makes the further functionalization of graphene feasible. Moreover, PPE-SO3- molecules exhibit interesting optoelectronic properties, and the resulting graphene sheets attached with PPE-SO3- will realize a variety of optoelectronic device applications of graphene. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (nos. 20673109 and 20827004) and the Chinese Academy of Sciences (nos. KGCX2-YW-231 and YZ200906).

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