pubs.acs.org/Langmuir © 2011 American Chemical Society
Self-Assembled Graphene/Azo Polyelectrolyte Multilayer Film and Its Application in Electrochemical Energy Storage Device Dongrui Wang and Xiaogong Wang* Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, P. R. China Received November 5, 2010. Revised Manuscript Received December 26, 2010 Graphene/azo polyelectrolyte multilayer films were fabricated through electrostatic layer-by-layer (LbL) selfassembly, and their performance as electrochemical capacitor electrode was investigated. Cationic azo polyelectrolyte (QP4VP-co-PCN) was synthesized through radical polymerization, postpolymerization azo coupling reaction, and quaternization. Negatively charged graphene nanosheets were prepared by a chemically modified method. The LbL films were obtained by alternately dipping a piece of the pretreated substrates in the QP4VP-co-PCN and nanosheet solutions. The processes were repeated until the films with required numbers of bilayers were obtained. The selfassembly and multilayer surface morphology were characterized by UV-vis spectroscopy, AFM, SEM, and TEM. The performance of the LbL films as electrochemical capacitor electrode was estimated using cyclic voltammetry. Results show that the graphene nanosheets are densely packed in the multilayers and form random graphene network. The azo polyelectrolyte cohesively interacts with the nanosheets in the multilayer structure, which prevents agglomeration of graphene nanosheets. The sheet resistance of the LbL films decreases with the increase of the layer numbers and reaches the stationary value of 1.0 106 Ω/0 for the film with 15 bilayers. At a scanning rate of 50 mV/s, the LbL film with 9 bilayers shows a gravimetric specific capacitance of 49 F/g in 1.0 M Na2SO4 solution. The LbL films developed in this work could be a promising type of the electrode materials for electric energy storage devices.
1. Introduction Since it was found by Geim’s group in 2004, graphene has aroused tremendous scientific and industrial interest as a true two-dimensional (2D) macromolecule with many unique properties.1 The properties, such as high Young’s modulus (∼1100 GPa) and fracture strength (125 GPa), high thermal conductivity (5000 W m-1 K-1), and high mobility of charge carriers (200 000 cm2 V-1 s-1), are very attractive for various applications. One potential use of this marvelous material is the application in electrochemical double-layer capacitor (EDLC) owing to its intrinsic conductivity and extremely high specific area of 2600 m2 g-1.2 Electrochemical capacitors (ECs), also known as supercapacitors, can be used as electrical energy storage devices in many fields.3 Compared to traditional energy storage devices, ECs possess advantages such as high-power capability, long lifetime, low weight, and less maintenance costs.4 On the basis of the charge-storage mechanisms, ECs can be classified into pseudocapacitor and electrochemical double-layer capacitor (EDLC). Pseudocapacitor utilizes fast and reversible redox reactions of electroactive materials, such as transition metal oxides and conducting polymers, to store charges.5 On the other hand, *To whom correspondence should be addressed. (1) (a) 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. (b) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451. (c) Park, S.; Ruoff, R. S. Nature Nanotechnol. 2009, 4, 217. (d) Allen, M. J.; Tung, V. C.; Kaner, R. B. Chem. Rev. 2010, 110, 132. (2) 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. (3) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Press: New York, 1999. (4) K€otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (5) (a) Simon, P.; Gogotsi, Y. Nature Mater. 2008, 7, 845. (b) Zhang, Y.; Feng, H.; Wu, X. B.; Wang, L. Z.; Zhang, A. Q.; Xia, T. C.; Dong, H. C.; Li, X. F.; Zhang, L. S. Int. J. Hydrogen Energy 2009, 34, 4889.
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EDLC stores the electric energy through charge separation at the electrochemical interface between electrode and electrolyte. Suitable materials for EDLC electrode have been widely searched for years. Carbon-based materials, such as active carbon, carbon nanotube (CNT), and graphite, are currently considered to be promising electrode materials.5 Because of its superior properties as well as low-cost for large-scale production through chemical approaches, graphene could be an ideal candidate for the application as EDLC electrode materials.1c Although graphene nanosheets as the EDLC electrode materials have recently been studied and reported in the literature,6 it is still a challenging task to develop ECs with large capacitance by using the graphene-based electrode materials. One of the main obstacles for graphene as EDLC electrode is the aggregation and formation of irreversible agglomerates, which results in the loss of the surface area and the decrease of capacitance. To minimize this undesirable effect, platinum nanoparticles have been used as physical spacers to separate the exfoliated graphene nanosheets and inhibit the aggregation of graphene.7 Electrostatic layer-by-layer (LbL) self-assembly is a versatile method to construct thin films with controllable architecture and functionality.8 In the process, multilayer films are built up through the alternately dipping a proper substrate into solutions of opposite-charged polyelectrolytes or other materials, which results in the sequential LbL deposition on the substrate. (6) (a) Stoller, M. D.; Park, S. J.; Zhu, Y. W.; An, J. H.; Ruoff, R. S. Nano Lett. 2008, 8, 3498. (b) Vivekchand, S. R. C.; Rout, C. S.; Subrahmanyam, K. S.; Govindaraj, A.; Rao, C. N. R. J. Chem. Sci. 2008, 120, 9. (c) Lv, W.; Tang, D. M.; He, Y. B.; You, C. H.; Shi, Z. Q.; Chen, X. C.; Chen, C. M.; Hou, P. X.; Liu, C.; Yang, Q. H. ACS Nano 2009, 3, 3730. (d) Wang, Y.; Shi, Z. Q.; Huang, Y.; Ma, Y. F.; Wang, C. Y.; Chen, M. M.; Chen, Y. S. J. Phys. Chem. C 2009, 113, 13103. (e) Zhao, X.; Tian, H.; Zhu, M.; Tian, K.; Wang, J. J.; Kang, F.; Outlaw, R. A. J. Power Sources 2009, 194, 1208. (7) Si, Y. C.; Samulski, E. T. Chem. Mater. 2008, 20, 6792. (8) (a) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. (b) Decher, G. Science 1997, 277, 1232.
Published on Web 01/18/2011
DOI: 10.1021/la1044128
2007
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Recently, this method has been used to prepare self-assembled LbL films of graphene with charged polymers, metal nanoparticles, and CNTs to obtain transparent conductive films or biosensors.9,10 Fabrication of graphene-based thin films through the LbL method can be an effective way to produce EC electrodes with high accessible surface area. In the multilayer structure, graphene nanosheets can be separated by alternate layers of polymers or other materials. So far, only graphene/CNT LbL film has been explored for the electrochemical energy-storage application.11 Polymers containing azobenzene and its derivatives (azo polymers for short) are well-known for their interesting photoresponsive properties such as photoinduced chromophore orientation,12 surface-relief-grating (SRG) formation,13 and photomechanical bending of thin films.14 In recent years, azo polyelectrolytes have been prepared and used to fabricate multilayer films through the LbL deposition method.15-17 The multilayer films containing azo polyelectrolytes show interesting properties such as the secondorder nonlinear optical (NLO) effect without the electric-field poling,15 photoinduced film thickness and hydrophilicity changes,16 and electrochromic variation.17 Push-pull type azo compounds, which are the conjugated azobenzene moieties substituted with strong electron donors and acceptors, have large dipole moment and polarizability. It has been showed that dielectric permittivity of polymers is significantly increased by doping with strong push-pull azo compounds.18 Using polyelectrolytes bearing push-pull azo chromophores can be a promising way to improve the perform of the charge storage devices. The LbL films composed of azo polyelectrolyte and graphene as EC electrodes can be prepared to prevent the graphene aggregation and introduce the polymeric material with the high permittivity as buffer layers. However, to our knowledge, a study concerning the graphene/azo polyelectrolyte multilayer construction and its application as EC electrode is still lacking in the literature. In this paper, we report the fabrication of graphene/azo polyelectrolyte multilayer films through LbL self-assembly and the performance of the multilayers as EC electrode. A cationic polyelectrolyte bearing strong push-pull azo chromophores on side-chains (QP4VP-co-PCN) was synthesized. The azo polyelectrolyte and negative-charged graphene nanosheet were used to build the multilayer films through the electrostatic LbL deposition. (9) (a) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nature Nanotechnol. 2008, 3, 101. (b) Shen, J. F.; Hu, Y. Z.; Li, C.; Qin, C.; Shi, M.; Ye, M. X. Langmuir 2009, 25, 6122. (c) Liu, J. Q.; Tao, L.; Yang, W. R.; Li, D.; Boyer, C.; Wuhrer, R.; Braet, F.; Davis, T. P. Langmuir 2010, 26, 10068. (d) Zeng, G. H.; Xing, Y. B.; Gao, J. A.; Wang, Z. Q.; Zhang, X. Langmuir 2010, 26, 15022. (10) (a) Kong, B. S.; Geng, J. X.; Jung, H. T. Chem. Commun. 2009, 2174. (b) Manga, K. K.; Zhou, Y.; Yan, Y. L.; Loh, K. P. Adv. Funct. Mater. 2009, 19, 3638. (c) Zhu, C. Z.; Guo, S. J.; Zhai, Y. M.; Dong, S. J. Langmuir 2010, 26, 7614. (d) Chen, D.; Wang, X. Y.; Liu, T. X.; Wang, X. D.; Li, J. ACS Appl. Mater. Interfaces 2010, 2, 2005. (e) Hong, T. K.; Lee, D. W.; Choi, H. J.; Shin, H. S.; Kim, B. S. ACS Nano 2010, 4, 3861. (11) Yu, D. S.; Dai, L. M. J. Phys. Chem. Lett. 2010, 1, 467. (12) (a) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403. (b) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100, 1817. (c) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139. (13) (a) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136. (b) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166. (14) (a) Yu, Y. L.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (b) Ikeda, T.; Mamiya, J.; Yu, Y. L. Angew. Chem., Int. Ed. 2007, 46, 506. (15) (a) Lvov, Y.; Yamada, S.; Kunitake, T. Thin Solid Films 1997, 300, 107. (b) Wang, X. G.; Balasubramanian, S.; Li, L.; Jiang, X. L.; Sandman, D. J.; Rubner, M. F.; Kumar, J.; Tripathy, S. K. Macromol. Rapid Commun. 1997, 18, 451. (c) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304. (16) (a) Dante, S.; Advincula, R.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193. (b) Wu, L. F.; Tuo, X. L.; Cheng, H.; Chen, Z.; Wang, X. G. Macromolecules 2001, 34, 8005. (17) Zhang, H. Y.; Yan, X. J.; Wang, Y. W.; Deng, Y. H.; Wang, X. G. Polymer 2008, 49, 5504. (18) (a) Lei, D.; Runt, J.; Safari, A.; Newnham, R. E. Macromolecules 1987, 20, 1797. (b) Luo, D. B.; Deng, L. Appl. Phys. Lett. 2006, 88, 181104.
2008 DOI: 10.1021/la1044128
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High-quality LbL films of the graphene and azo polyelectrolyte (graphene/QP4VP-co-PCN) were obtained through this method. The films show promising properties for application as electrode material in electrochemical energy storage device.
2. Experimental Section 2.1. Materials and Characterization. 4-Vinylpyridine (4VP) and tetrahydrofuran (THF) were dehydrated by CaH2 and distilled before use. N,N-Dimethylformamide (DMF) was azeotropically distilled with benzene three times for dehydration and then distilled under vacuum. 2-(Ethyl(phenyl)amino)ethyl methacrylate (EMA) was prepared according to the method reported previously.19 2,20 -Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol and stored in a freezer before use. Graphite powder (GP, Sinopharm Chemical Reagent Co. Ltd.) with the size of 300-400 mesh was sieved out prior to use. Ultrapure water (resistivity >18.0 MΩ 3 cm) was obtained from a Milli-Q water purification system and used for all experiments. All other reagents and solvents were obtained as analytical grade products and used without further purification. 1 H NMR spectra were recorded on a JEOL JNM-ECA 300 spectrometer (300 MHz for proton). FT-IR spectra were measured using a Nicolet 560-IR spectrometer by incorporating the sample in a KBr disk. Relative molecular weights and molecular weight distributions were measured on a gel permeation chromatography (GPC) apparatus using THF as eluent at a flow rate of 1.0 mL/min at 35 °C. The instrument was equipped with a refractive index detector (Wyatt Optilab rEX) and fitted with a PLgel 5 μm mixed-D column, which was calibrated by using linear polystyrene standards. Thermal phase transitions of the polymers were tested by using TA Instruments DSC 2910 with a heating rate of 10 °C/min in a nitrogen atmosphere. UV-vis absorption spectra were obtained on a Perkin-Elmer Lamda Bio-40 spectrophotometer. The atom force microscope (AFM) images were obtained by using a Nanoscope-IIIa scanning probe microscope in the tapping mode. The scanning electron microscope (SEM) observation was performed on a Hitachi S-4500 microscope with an accelerating voltage of 10 kV. The transmission electron microscope (TEM) observation was performed on a JEOL JSM1200EX microscope with an accelerating voltage of 120 kV. Sheet resistances of the multilayer films deposited on glass slides were measured by using a standard four-point probe configuration (SX1934, China Suzhou Baishen Tech.). For each film, five measurements at different positions were taken and averaged to give the reported value with the standard deviation as the error range. Electrochemical properties of the graphene/QP4VP-coPCN multilayer films as the supercapacitor electrode were measured in a three-electrode cell, which used an Ag/AgCl electrode and Pt wire as the reference and counter electrodes. The graphene/ QP4VP-co-PCN multilayer films absorbed on indium-tin oxide (ITO) coated glasses were used as the working electrode in 1 M Na2SO4 or 1 M H2SO4 solution. Cyclic voltammetry (CV) was measured in the potential range between 0 and 0.8 V versus Ag/AgCl at room temperature at various scan rates from 10 to 1000 mV/s by using a CHI660B bipotentiostat. 2.2. Synthesis of Azo Polyelectrolyte.
P4VP-co-PEMA.
The copolymerization of EMA and 4VP was carried out by using AIBN as radical initiator in anhydrous DMF. For the reaction, EMA (932 mg, 4 mmol) and 4VP (420 mg, 4 mmol) were added into a three-necked round-bottom flask and dissolved by DMF (15 mL). The solution was purged with argon and stirred at room temperature for 30 min. Then, AIBN (27 mg, 2 wt %) was gradually added into the solution. The flask was sealed and immersed in an oil bath at 80 °C. After reaction for 20 h, the polymerization was stopped by pouring the reaction mixture into water. The precipitate was collected by filtration and dried. The (19) Wang, D. R.; Ye, G.; Wang, X. G. Macromol. Rapid Commun. 2007, 28, 2237.
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Figure 1. Synthetic route of the cationic azo polyelectrolyte (QP4VP-co-PCN). crude product was dissolved in THF, precipitated in petroleum ether, collected by filtration, and dried under vacuum at 35 °C for 24 h. Yield: 798 mg (59%). GPC results: Mn = 9800, Mw/Mn = 1.63. 1H NMR (CDCl3), δ (ppm): 8.32 (d, pyridine-H), 7.19 (m, 2H, Ar-H), 6.68 (m, 3H, Ar-H), 6.52(d, pyridine-H), 3.93 (t, 2H, -O-CH2-CH2-N-), 3.23 (m, 4H, -CH2-N-CH2CH3), 1.2-2.0 (br, backbone-H), 1.12 (t, 3H, -N-CH2-CH3), 0.2-1.0 (br, backbone-H). On the basis of the 1H NMR spectral analysis, the molar ratio of P4VP to PEMA was estimated to be 6:4. P4VP-co-PCN. P4VP-co-PCN was prepared through an azocoupling reaction between P4VP-co-PEMA and diazonium salt of 4-aminobenzonitrile. The diazonium salt was prepared by adding an aqueous solution of sodium nitrite (121 mg, 1.75 mmol) dropwise into a solution of 4-aminobenzonitrile (177 mg, 1.5 mmol) in sulfuric acid (0.3 mL) and glacial acetic acid (4.5 mL). After the mixture was stirred at 0 °C for 5 min, the diazonium salt solution was added dropwise into a solution of P4VP-co-PEMA (373 mg, 1 mmol in terms of the aniline group) in DMF (30 mL) at 0 °C. The reaction was carried out with ice-bath cooling for 12 h. Then the reaction mixture was poured into water. The precipitate was collected by filtration and washed with an excessive amount of water. The crude product was purified by dissolving it in THF and precipitating with petroleum ether. The final product was dried at 60 °C under vacuum for 24 h. Yield: 262 mg (52%). GPC results: Mn=12 200, Mw/Mn=1.59. 1H NMR (CDCl3), δ (ppm): 8.34 (d, pyridine-H), 7.86 (m, 4H, Ar-H, azobenzene), 7.71 (d, 2H, Ar-H, azobenzene), 6.68 (m, Ar-H, azobenzene and pyridine), 3.95 (t, 2H, -O-CH2-CH2-N-), 3.29 (m, 4H, -CH2-N-CH2-CH3), 1.2-2.0 (br, backbone-H), 1.12 (t, 3H, -N-CH2-CH3), 0.2-1.0 (br, backbone-H). QP4VP-co-PCN. The azo polyelectrolyte (QP4VP-co-PCN) was prepared by the quaternization of P4VP-co-PCN with bromoethane. P4VP-co-PCN (100 mg) and bromoethane (1 g), which was about 30-fold excess with respect to 4VP units in the copolymer, were dissolved in 20 mL of THF. Then the mixture was refluxed with stirring for 48 h. The quaternized copolymer was obtained by evaporating all the organic solvents and Langmuir 2011, 27(5), 2007–2013
bromoethane under reduced pressure. The crude product was then dissolved in DMF and precipitated in ethanol. The final product was obtained by filtration and drying under vacuum at 60 °C for 24 h. 2.3. Preparation of Graphene Aqueous Suspension. Graphite oxide was prepared using a modified Hummers’s method from natural graphite powders.20 The stable water suspension of graphene with negative charges on surfaces was prepared from the graphite oxide by reduction following the literature method.9a In the process, graphite oxide (2.5 mg) and water (10 mL) were added into the flask immersed in a sonic bath. By sonication for 1 h, graphene oxide (GO) aqueous dispersion was obtained through exfoliation of the graphite oxide. The reduction was carried out by adding hydrazine (5 μL, 35 wt % in water) and ammonia (35 μL, 25 wt % in water) into the dispersion and reacting at 95 °C for 1 h under vigorous stirring. Then the mixture was centrifuged at 5000 rpm for 10 min using a Hitachi CR22G centrifuge with a R20A2 rotor. The suspension was collected and used for the experiments. 2.4. Multilayer Preparation. The multilayer films composed of graphene and QP4VP-co-PCN were prepared by electrostatic LbL adsorption on different substrates. Multilayer films prepared on quartz slides (50 15 1 mm) and ITO-coated glass slides (50 10 1 mm) were used to perform optical and electrochemical measurements, respectively. The films adsorbed on silicon wafers were used for AFM and SEM characterization. The quartz slides and silicon wafers were pretreated by sonication in a mixture of 98% H2SO4 and 30% H2O2 (7/3, v/v) and then treated with a mixture of H2O/H2O2/NH3 (5:1:1) at 80 °C for 1 h. The substrates were all thoroughly washed with deionized water prior to use. For a typical dipping process, a piece of the pretreated substrates was first dipped in a DMF solution of QP4VP-co-PCN (0.2 mg/mL) for 10 min, rinsed with excess water, and blown dry with a gentle air stream. Then, it was dipped in the graphene suspension for 60 min, rinsed with excess water, and blown dry through the same way. The procedures were (20) (a) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (b) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856.
DOI: 10.1021/la1044128
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Figure 3. FT-IR spectra of P4VP-co-PCN and QP4VP-co-PCN.
Figure 2. 1H NMR spectra of (a) P4VP-co-PEMA and (b) P4VPco-PCN in CDCl3. repeated until the multilayer films with the required numbers of bilayers were obtained. After each adsorption step, the UV-vis absorption spectra were recorded to monitor the multilayer growth on the substrates. To measure the thickness of the LbL films, the films were scratched by a needle, and then the heights of grooves were obtained by using AFM. For each film, thicknesses at three different positions were measured and averaged to give the reported values.
3. Results and Discussion 3.1. Synthesis and Characterization. Azo polycation (QP4VP-co-PCN) was synthesized through the synthetic route given in Figure 1. The random copolymer of 4-vinylpyridine (4VP) and 2-(ethyl(phenyl)amino)ethyl methacrylate (EMA) was prepared through radical copolymerization. The composition and structure of P4VP-co-PEMA were analyzed by spectroscopic methods. Figure 2a gives the 1H NMR spectrum of the polymer. The molar ratio of P4VP to PEMA was estimated to be 6:4 from the integration ratios of resonance signals at 8.32 and 7.19 ppm. P4VP-co-PCN was then synthesized by the azo-coupling reaction between P4VP-co-PEMA and the diazonium salt of 4-aminobenzonitrile. Comparing with the 1H NMR spectrum of P4VP-coPEMA, a downfield chemical shift (from 7.19 to 7.71 ppm) was observed for the protons at the meta-position of the amino groups (Figure 2b). It indicates that the azo-coupling reaction occurs at the para-position of the amino groups of PEMA with high yield.21 The new signal appearing at 7.86 ppm, corresponding to the resonance of the introduced benzoid protons, can further confirm this point. The complete disappearance of resonance signal at 7.19 ppm indicates the conversion to be near 100%. QP4VP-co-PCN was obtained by quaternization of P4VP-co-PCN with bromoethane. Compared to the infrared spectrum of P4VP-coPCN, QP4VP-co-PCN shows new absorption bands at 1639 and 1470 cm-1 and the concomitant disappearance and decrease (21) Wang, D. R.; Ren, H. F.; Wang, X. Q.; Wang, X. G. Macromolecules 2008, 41, 9382.
2010 DOI: 10.1021/la1044128
Figure 4. UV-vis absorption spectrum of QP4VP-co-PCN spincoated film on quartz slide.
of absorptions at 1770 and 1557 cm-1 (Figure 3), which indicates that the pyridine groups are quaternized. The degree of quaternization is near 90% estimated from the decrease of the 1557 cm-1 band with respect to bands unaffected by the reaction. Figure 4 shows the UV-vis absorption spectrum of spin-coated QP4VPco-PCN film on quartz slide. The absorption at 450 nm is the π-π* transition band of the pseudostilbene-type azo chromophores.19,21 The absorption with λmax = 270 nm corresponds to the j-j* transition of the aromatic cores.22,23 Differential scanning calorimetry (DSC) analysis shows that there is no observable endothermic peaks between 0 and 200 °C (N2, heating rate = 10 °C/min), which is similar to other cationic azo polyelectrolytes.23 The QP4VP-co-PCN can be well dissolved in DMF and DMSO but not in water. Negatively charged graphene nanosheets were prepared by reducing exfoliated graphite oxide with hydrazine.9a The negatively charged graphene nanosheets can be well dispersed in aqueous suspension because of the electrostatic repulsion between the residual carboxylic acid groups on the graphene surfaces. Ammonia was added into the reaction system to adjust pH and improve the stability of the suspension. Figure 5 shows a typical photograph of the graphene suspension and the AFM image of the single-layered graphene nanosheets spin-coated on the silicon wafer. The suspension can remain stable and homogeneous for at least 3 days under room temperature. The graphene nanosheets possess irregular shapes with the plane sizes in the range from tens to thousands of nanometers and the thickness about 0.9 nm. Figure 6 gives the UV-vis absorption spectrum of the diluted graphene aqueous suspension. The suspension shows absorption in the UV and visible region (200-700 nm) with the maximum absorption appearing at 270 nm (λmax). The spectroscopic feature (22) Jung, B. D.; Stumpe, J.; Hong, J. D. Thin Solid Films 2003, 441, 261. (23) (a) Koetse, M.; Laschewsky, A.; Mayer, B.; Rolland, O.; Wischerhoff, E. Macromolecules 1998, 31, 9316. (b) Kumar, S. K.; Hong, J. D.; Lim, C. K.; Park, S. Y. Macromolecules 2006, 39, 3217. (24) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228.
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Figure 5. AFM tapping-mode image of graphene nanosheets spin-coated on silicon wafer together with the section analysis. Inset: photograph of the graphene suspension.
Figure 7. (a) UV-vis absorption spectra of graphene/QP4VP-coFigure 6. UV-vis absorption spectrum of the diluted graphene water suspension.
is consistent with the numerous reports on chemical converted graphene,1c,d,24 which confirms that the suspension contains welldispersed graphene nanosheets. 3.2. Multilayer Fabrication. Multilayer films of graphene and QP4VP-co-PCN were fabricated by the electrostatic LbL selfassembly. The fabrication was accomplished by alternately dipping a piece of the substrates in the QP4VP-co-PCN solution and graphene suspension. It has been reported by us that high-quality LbL films can be obtained by dissolving azo polyelectrolytes in polar organic solvent.25 In this study, DMF was used as solvent to dissolve the cationic QP4VP-co-PCN due to its poor solubility in water. The LbL films of graphene and QP4VP-co-PCN were successfully fabricated on quartz substrate, silicon wafer, and ITO glass. The multilayer growth processes on quartz slides and ITO glass were monitored by UV-vis absorption spectroscopy. In the following discussions, the multilayer films are denoted as (graphene/QP4VP-co-PCN)n, where n means the number of bilayers. Figure 7a shows the UV-vis spectra of the LbL films with different numbers of bilayers (n = 3, 6, 9, 12, 15) deposited on a quartz substrate. The spectra show two absorption bands at 270 and 455 nm. The strong absorption band at 270 nm is attributed to the overlapped graphene π-π* transition and j-j* absorbance of the azobenzene moieties. The absorption band at 455 nm is assigned to the π-π* transition of the azo chromophores. Figure 7b shows the relationship between the absorption intensities at 270, 455, and 650 nm and the number of bilayers. (25) Wang, H. P.; He, Y. N.; Tuo, X. L.; Wang, X. G. Macromolecules 2004, 37, 135.
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PCN LbL films with the different numbers of bilayers. (b) Relationship between the absorbance at 270, 455, and 650 nm and the number of bilayers. (c) Plot of the film thickness measured by AFM vs the number of bilayers.
It can be seen that the intensities almost linearly increase with the multilayer growth. In the process, the increment of absorption intensity corresponding to each bilayer slightly decreases after the deposition for about 10 bilayers, which indicates the slight decrease of the amount of material adsorbed on the substrate. Considering their large sizes, the diffusion of the graphene nanosheets needs much longer time than that for molecules. The dipping time (60 min) could be insufficient for the graphene nanosheets to diffuse close to and adsorb on the surfaces, which would result in the slight decrease of the amount of the adsorbed nanosheets. It has been reported that increasing the dipping time in graphene suspension can result in the linear growth of the multilayer films.9b The thickness of the LbL films was measured by the AFM scratch method. The result shows that the film thickness almost linearly increases with the increase of the bilayer numbers (Figure 7c). As a typical example, the (graphene/ QP4VP-co-PCN)15 film has the thickness of 38.3 nm. The surface morphology of the LbL films was characterized by AFM and SEM. Figure 8a,b gives the typical AFM and SEM images of the films with 15 bilayers (graphene/QP4VP-co-PCN)15 deposited on silicon wafer. The AFM image shows that the multilayer film possesses the root-mean-squared (rms) roughness of 4 nm. The graphene nanosheets can be distinguished by their irregular edges. The graphene nanosheets are densely packed with most of their basal planes parallel to the surface and form randomly organized graphene networks. SEM reveals the smooth surface of the LbL film with tiny wrinkles, which are caused by the compositional and topographic contrast of the nanosheets. The DOI: 10.1021/la1044128
2011
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Figure 9. Plot of the sheet resistance of the graphene/QP4VP-coPCN multilayer films vs the number of bilayers.
Figure 8. Surface morphology of LbL thin film (graphene/ QP4VP-co-PCN)15 deposited on Si wafer and photograph of the LbL films on ITO glass slides with the different numbers of bilayers: (a) AFM image, (b) SEM image, (c) photographic image.
above results indicate that the graphene nanosheets are submerged in the azo polyelectrolyte layers, and the aggregation of the nanosheets is avoided by the cohesive interaction with azo polyelectrolyte. TEM observation shows that the graphene is uniformly dispersed in the multilayer films (Figure S1, in the Supporting Information). Figure 8c gives the optical photograph of the self-assembled multilayer films on ITO glass slides. The films show good optical transparency and the stepwise darkening with the increasing numbers of the deposition layers. The gradual darkening is caused by the light absorption of the azo chromophores and graphene. 3.3. Electrochemical Capacitor Application. The sheet resistance of the LbL multilayer films assembled on quartz substrates was measured by using a standard four-point probe method. Figure 9 gives the relationship between the resistance and the number of bilayers at room temperature. With the increase of the number of bilayer from 3 to 15, the sheet resistance decreases from 1.1 107 to 1.0 106 Ω/0; i.e., the conductivity of the films increases with the growth of the multilayer. For spin-coating film of QP4VP-co-PCN, the sheet resistance is significantly larger (>109 Ω/0). The lower resistance of LbL films can be attributed to the presence of graphene in the LbL structures. As the bilayer number increases, more continuous intercalated pathways for charge transport can be developed in the films. The saturated value of the conductance, which is close to the inherent property of the material, is comparable to those of graphene films 2012 DOI: 10.1021/la1044128
Figure 10. Electrochemical properties of the graphene/QP4VPco-PCN LbL films. (a) Cyclic voltammograms of the LbL multilayer films with different bilayer numbers at the scanning rate of 10 mV/s in the 1.0 M Na2SO4 solution. (b) Cyclic voltammograms of (graphene/QP4VP-co-PCN)9 in the 1.0 M Na2SO4 solution at different scanning rates. (c) Cyclic voltammograms of (graphene/ QP4VP-co-PCN)9 in two different electrolyte solutions (1.0 M H 2 SO 4 and 1.0 M Na2SO 4 ) under the same scanning rate of 50 mV/s.
fabricated by various methods including spray-coating, 9a vacuum filtration,26 and Langmuir-Blodgett (LB) assembly.27 The performance of the LbL multilayer films as the EDLC electrode was evaluated by using cyclic voltammetry (CV). Figure 10a (26) Eda, G.; Fanchini, G.; Chhowalla, M. Nature Nanotechnol. 2008, 3, 270. (27) (a) Cote, L. J.; Kim, F.; Huang, J. X. J. Am. Chem. Soc. 2009, 131, 1043. (b) Li, X. L.; Zhang, G. Y.; Bai, X. D.; Sun, X. M.; Wang, X. R.; Wang, E.; Dai, H. J. Nature Nanotechnol. 2008, 3, 538.
Langmuir 2011, 27(5), 2007–2013
Wang and Wang
gives the CV curves of the LbL films, deposited on ITO glasses with the area of 15 10 mm, which have the different numbers of bilayers. The measurements were carried out at room temperature in 1.0 M Na2SO4 solution within the potential range of 0-0.8 V at the scanning rate of 10 mV/s. The obtained voltammograms show approximate rectangular shapes, indicating the capacitive characteristics. The capacitance of the film increases with the increase of the number of bilayers. For the films with nine bilayers (graphene/QP4VP-co-PCN)9, the CV curves under different scan rates are given in Figure 10b. The curve remains rectangular in shape with only small distortion even at exceedingly high scanning rate of 1 V/s, which suggests that the film can have good performance in rapid charging/discharging applications.6d The gravimetric specific capacitance (Cg) is calculated by the following equation28 ZV Final 1 I dV ð1Þ Cg ¼ 2mΔV VInitial dV=dt
Article
QP4VP-co-PCN. To use the multilayer in acidic condition for the practical ECs application, adhesion of the self-assembled films to the substrate could be improved by substrate pretreatments such as coating with silane coupling agent. The above results demonstrate that the graphene/azo polyelectrolyte multilayer films are promising for application as EDLC electrode materials. Electrochemical capacitors store the electric energy in the electrochemical double layer (Helmholtz Layer) formed at the solid/electrolyte interface.3,4 Increasing the electrode surface area is a critical requirement to achieve a higher capacitance. Through the LbL assembling approach, the aggregation of the graphene sheets can be minimized or even prevented. Therefore, the graphene nanosheets embedded in the azo polyelectrolyte will possess ultrahigh surface area. The azo polyelectrolyte containing push-pull type azo chromophores can also function as charge storage layer through the charge separation and dipolar polarization. However, due to the complicated structure of the layered structures, further investigation will be required to understand the exact mechanism of the charge storage in the LbL films. Future improvement of the capacitance performance of this type LbL film can be expected through optimizing preparation conditions and the film structure such as altering the assembly pH, controlling dipole orientation direction of the azobenzene groups, and enhancing the adhesion of the LbL film to substrate.
where m is the mass of the electrode material, ΔV is the potential window, VInitial and VFinal are the starting and end potential in one cycle, I is the instantaneous current at a given potential, and dV/dt is the potential scanning rate. For the films with nine bilayers (graphene/QP4VP-co-PCN)9, the mass of graphene and azo polyelectrolyte deposited on the substrate was estimated to be 4.1 and 2.1 μg, respectively. Under a relatively low sweeping rate of 10 mV/s, the Cg in Na2SO4 solution was calculated to be 71 F/g. The Cg of the film decreases with the increase of scanning rate. The Cg decreases to 49 F/g at the scanning rate of 50 mV/s and further decreases to 27 F/g for the sweeping rate of 1 V/s. The tendency is consistent with the previous observations.6b,28,29 The LbL multilayer films show much higher capacitance in acidic electrolyte solution than in the neutral electrolyte solution. Figure 10c gives the comparison of CV curves of the film with nine bilayers (graphene/QP4VP-co-PCN)9 obtained in two different electrolyte solutions under the same sweeping rate of 50 mV/s. In 1.0 M H2SO4, the CV curve remains a nearly rectangular shape, and the integration area is remarkably increased. The Cg is calculated to be 173 F/g for the LbL film, which is about 3.5 times higher than that in Na2SO4 solution (49 F/g). However, on the other hand, the self-assembled multilayer films showed poor stability when immersed in acidic electrolyte solution. The films began to peel away from the ITO glass surface after immersion in the H2SO4 solution for few minutes. It is in contrast to the high stability of the multilayer films in the neutral Na2SO4 solution. The poor stability of the multilayer films in the H2SO4 solution could be attributed to the strong interaction between H2SO4 and
Acknowledgment. The financial support from the NSFC (50873054, 20774055) and the China Postdoctoral Science Foundation (20100470336) is gratefully acknowledged.
(28) Du, Q.; Zheng, M.; Zhang, L.; Wang, Y.; Chen, J.; Xue, L.; Dai, W.; Ji, G.; Cao, J. Electrochim. Acta 2010, 55, 3897. (29) Li, F. H.; Song, J. F.; Yang, H. F.; Gan, S. Y.; Zhang, Q. X.; Han, D. X.; Ivaska, A.; Niu, L. Nanotechnology 2009, 20, 455602. (30) Zhu, Y. W.; Murali, S.; Stoller, M. D.; Velamakanni, A.; Piner, R. D.; Ruoff, R. S. Carbon 2010, 48, 2118.
Supporting Information Available: TEM images of the graphene/azo polyelectrolyte LbL multilayer films. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2011, 27(5), 2007–2013
4. Conclusions Self-assembled multilayer films composed of graphene and azo polyelectrolyte were fabricated through the electrostatic layer-bylayer adsorption. In the multilayer films, the graphene nanosheets are densely packed to form a random graphene networks, and the azo polyelectrolyte fills in the space between the nanosheets. The multilayer structure shows advantage to prevent agglomeration of graphene nanosheets and increase the accessible surface area. The film with 15 bilayers shows the sheet resistance of 1.0 106 Ω/0. The low resistance and large accessible surface area are promising as ideal electrode for electrochemical capacitor devices. Cyclic voltammetric measurements show that the capacitance of the LbL film is proportional to the number of the bilayers and inversely proportional to the potential scanning rate. The gravimetric specific capacitance of the film with nine bilayers is 49 F/g in 1.0 M Na2SO4 solution under the scanning rate of 50 mV/s.
DOI: 10.1021/la1044128
2013
