Graphene-Based Nanowire SupercapacitorsZhi Chen,†,∥ Dingshan Yu,‡,∥ Wei Xiong,§ Peipei Liu,† Yong Liu,*,† and Liming Dai*,†,‡

†Institute of Advanced Materials for Nano-bio Applications, School of Ophthalmology and Optometry, Wenzhou Medical University,270 Xueyuan Xi Road, Wenzhou, Zhejiang Province 325027, China‡Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering,Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States§Department of Chemical and Materials Engineering, School of Engineering, University of Dayton, 300 College Park, Dayton, Ohio45469, United States

*S Supporting Information

ABSTRACT: We present a new type of electrochemical super-capacitors based on graphene nanowires. Graphene oxide (GO)/polypyrrole (PPy) nanowires are prepared via electrodepostion ofGO/PPy composite into a micoroporous Al2O3 template, followed bythe removal of template. PPy is electrochemically doped by oxygen-containing functional groups of the GO to enhance the charging/discharging rates of the supercapacitor. A high capacitance 960 F g−1

of the GO/PPy nanowires is obtained due to the large surface area ofthe vertically aligned nanowires and the intimate contact between thenanowires and the substrate electrode. The capacitive performanceremains stable after charging and discharging for 300 cycles. Toimprove the thermal stability and long-term charge storage, GO isfurther electrochemically reduced into graphene and PPy issubsequently thermally carbonized, leading to a high capacitance of 200 F g−1 for the resultant pure reduced grapheneoxide/carbon based nanowire supercapacitor. This value of capacitance (200 F g−1) is higher than that of conventional porouscarbon materials while the reduced graphene oxide/carbon nanowires show a lower Faraday resistance and higher thermalstability than the GO/PPy nanowires.

Electrochemical supercapacitors have been considered asenergy storage devices for a wide range of applications

requiring a high power density, such as consumer devices,electric vehicles, electronic facilities, and energy back-upsystems.1 In a typical supercapacitor, energy is stored in aelectrical double layer at the interface between a high-surface-area electrode and electrolyte solution (electrical double layercapacitors, EDLCs). Pseudocapacitance involving electrontransfers due to Faradic redox reactions could also occur.High-surface-area carbon electrodes, including activated car-bon,2 mesoporous carbon,3 and carbon nanotubes,4−6 havebeen used for EDLCs. However, the capacitance of existingporous carbon materials (normally less than 100 F g−1) is stilllimited by the small effective surface area and low electrolyteaccessibility of the electrodes.7 In order to achieve highcapacitance values, numerous efforts have been tried to improvethe electrochemical active surface area of the electrodes. In thecase of carbon nanotube electrodes, reasonable capacitanceshave been reached (e.g., 102 F g−1 for multiwalled4 and 180 Fg−1 for single-walled8). A new class of supercapacitors with avery high capacitance of 440 F g−1 based on plasma activatedvertically aligned carbon nanotubes and ionic liquid electrolyteshas been reported.9

On the other hand, various electroactive conductingpolymers, such as polypyrrole (PPy), polyaniline (PANi),

polythiophene (PTh), and their derivatives, at differentoxidation states have been considered as an importantsupplementary to improve the capacitive performance ofmicroporous carbon electrodes.10 Of particular interest, PPyhas been attractive as a promising EDLC electrode because ofits unique properties, including high specific capacitance(normally higher than 100 F g−1) and good conductivity inthe doped state.11−13 The reversibility and stability of PPy-based supercapacitors, however, were limited by volumetricshrinkage of the polymer during ejection of doped ions indischarge state, and high ohmic polarization arising from lowconductance when polymer was at the dedoped state. Thecombination of conducting polymers and porous carbonmaterials (e.g., carbon nanotubes) has led to the rapiddevelopment of a new class of electrochemical supercapacitorswith high capacitances up to 500 F g−1.14−16 More recently, themaximum capacitances of 763 F g−1 for the flexible graphene/PANi paper17 and 909 F g−1 for the graphene/PANinanorods18 have been reported.

Received: January 23, 2014Revised: February 26, 2014Published: March 3, 2014

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In this work, we developed novel supercapacitors based ongraphene and PPy nanowires. As a new class of 2D carbonnanomaterials, graphene and its derivatives have beendemonstrated to be attractive for a wide range of applications,including field effect transmitters,19 biosensors,20 and lithium-ion batteries21 due to its unique high surface area, electricalconductivity, and physical and mechanical properties. Ofparticular interest, graphene oxide (GO) nanosheets are solublein water and many other common polar solvents suitable forthe formation of large-scale uniform films on various substratesby solution processing. The presence of oxygen-containingfunctional groups limits the direct application of GO inelectrically active devices22 but also provides effective ways forcertain practical applications. In the present study, thenegatively charged GO (with carboxyl and hydroxyl groups)was used for electrochemical doping of PPy, imparting the fastcharging and discharging cpability to the resultant composites.Furthermore, the formation of PPy/GO nanowires provides alarge surface area for the electrode. Nanostructures also offerthe possibility for the molecular-level control of the electrode,leading to a systematic variation of different parameters tooptimize the charge storage.23 As a result, an excellentcapacitance value of 960 F g−1 (compared to less than 500 Fg−1 for the most carbon/polymer composites14−16 andmaximum 909 F g−1 for the graphene/PANi nanorods18) wasobtained from the GO/PPy nanowires prepared in this work.To improve the thermal stability and long-term charge storage,GO was further electrochemically reduced into graphene andPPy was subsequently thermally carbonized. The resultantreduced graphene oxide/carbon (rGO/carbon) nanowiresupercapacitor exhibited a high capacitance of 200 F g−1,compared to less than 130 F g−1 for the graphene films24,25 andless than 180 F g−1 for the graphene/carbon nanotubehybrids.26,27

Figure 1a shows schematically the synthetic processes of thepreparation of GO/PPy nanowires and rGO/carbon nanowires.

In a typical experiment, microporous Al2O3 paper (with a holediameter of 220 nm) was sputter-coated with a thin layer ofgold on one side and used as the template for theelectrodeposition of GO/PPy composite. GO/PPy wasdeposited from the GO/pyrrole mixture into the holes of thegold-supported Al2O3 template using cyclic voltammetry withpotential cycling between 0 and 1.2 V for 12 cycles at a scan

rate of 0.1 V s−1 (see the Supporting Information for details). Amuch higher oxidation current was obtained during electro-depositon of PPy/GO compared to the deposition of pure PPy(Figure S1, Supporting Information), suggesting the successfulincorporation of GO into PPy. The successful deposition ofPPy/GO composite was further confirmed using Fouriertransform infrared (FT-IR) spectra (Figure S2, SupportingInformation). As shown in Figure S2 (Supporting Informa-tion), characteristic peaks of GO (CO at 1100 cm−1, COC at 1390 cm−1, CC at 1640 cm−1, CO at 1750 cm−1,and hydroxyl- at 3430 cm−1, blue line) were all observed in thespectrum of the as-synthesized PPy/GO composites (pinkline). In the spectrum of the pristine PPy (green line), peaks at1400 cm−1 and over 500−1000 cm−1 were associated with thealkane (CH) deformation and bend vibration. The peak at1605 cm−1 was attributed to vibration of the NHdeformation. The peak at 1320 cm−1 arose from stretchvibration of CN. All of these peaks were found at thespectrum of the PPy/GO composite, confirming successfulcombination of GO and PPy. TGA results further confirmedthat the thermal stability of both the pristine PPy and GO couldbe enhanced significantly with incorporation of GO with PPy(Figure S3, Supporting Information). Figure 1b shows an AFMimage for the smooth surface of GO nanosheets with anaverage thickness of 1.2 nm, indicating single- or few-layergraphene nanosheets. PPy mixed with GO was electrodepositedinto holes of the Al2O3 (Figure 1a). A highly uniform anddensely packed forest containing GO-doped PPy nanowires wasobserved after removal of the Al2O3 template by soaking thecomposite in 1 M NaOH over 3 h (Figure 1c, d). The diameterof the GO/PPy nanowires was found to be around 220 nmfrom the TEM image, as shown in Figure S4a (SupportingInformation). The GO-doped PPy composite was formed as afree-standing film which was readily used as the electrochemicalelectrode (Figure S5, Supporting Information). To obtain amore stable carbon based graphene nanowire supercapacitor,we electrochemically reduced the GO/PPy/Al2O3 in 0.1 MNaNO3 by cycling potentials between −1 and 0 V over 100cycles, according to a published procedure.28 After the GOreduction, the sample was treated at 700 °C under theprotection of N2 over 1 h to pyrolyze PPy, followed by soakingin 1 M NaOH to remove the template. The free-standing filmstructure remained well after thermal annealing (Figure S2,Supporting Information).SEM images for the resulting rGO/carbon nanowires after

electrochemical reduction and annealing treatment of GO/PPynanowires at 700 °C under N2 are given in parts a and b ofFigure 2, respectively, which shows that the resulting nanowirestructure remains aligned well after the reduction and pyrolysistreatment. The TEM image further confirmed that the diameterof the rGO/carbon nanowires was reduced to about 160 nm(Figure S4b, Supporting Information) which could beattributed to the decomposition of PPy during the annealingprocess. As can be seen, the nanowires exhibit a uniformdiameter and length. The presence of PPy and GO in theresulting GO/PPy nanowires before annealing is confirmed byRaman spectra given in Figure 2c. Two peaks at 1337 and 1573cm−1 (pink line, Figure 2c) can be attributed to the D band(associated with induced disorder) and G band (from thesymmetric graphitic structure) of graphene, respectively. Thepeak intensity ratio of the D band to the G band (ID/G) isaround 0.5, higher than that of a high crystalline qualitygraphene sheet (normally 0.06−0.25).29 The relatively high D

Figure 1. (a) Schematic synthesis of graphene-containing nanowires,(b) a typical AFM image of graphene oxide, (c) side view for SEMimages of polypyrrole/graphene oxide nanowires after the removal ofthe Al2O3 template, and (d) top view for SEM images of thepolypyrrole/graphene oxide nanowires after the removal of the Al2O3template.

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band for the GO/PPy nanowire arises, most probably, from thepresence of defects associated with the oxygen-containingfunctional groups in the GO nanosheets. The presence of PPyis evidenced by the bands around 1000 cm−1, as observedelsewhere.30 The absence of PPy-related bands over 1000 cm−1

after electrochemical reduction and annealing treatment (blueline, Figure 2c), together with the increase in the ID/G ratiofrom 0.5 for the GO/PPy nanowires to 0.89, suggests thatsuccessful reduction of GO to graphene and the pyrolysis ofPPy could have led to the formation of amorphous carbon(AC) on the graphene nanowires. Decomposition of PPy wasfurther confirmed using FT-IR spectra (Figure S2, SupportingInformation). Both characteristic peaks at 1605 cm−1 (N−H)and 1320 cm−1 (C−N) related to the presence of PPy wereascent at the spectrum of the rGO/carbon after annealing at700 °C (black line), confirming decomposition of PPy at thehigh temperature.To measure the supercapacitor performance for both the

GO/PPy nanowire and rGO/carbon nanowire electrodes, weused a standard three-electrode electrochemical cell and 1 MNaCl as the electrolyte. Figure 3 shows cyclic voltammograms(CVs) at the PPy nanowires with various contents of GO at ascan rate of 0.1 V s−1. The CVs of the GO/PPy nanowires arerectangle-shaped with anodic and cathodic charging currentssymmetrical around zero when the relative content of GO inthe GO/PPy composite is less than 30% (w/w), suggesting anideal capacitive behavior for the nanowire electrode. Morespecifically, the capacitive current is found to increase by 1.5times with introduction of 10% (w/w) GO (black line, Figure3a) into the PPy nanostructure compared to the pristine PPynanowires (pink line, Figure 3a). The capacitive current of thePPy/GO composite can be further increased up to 10 times(blue line, Figure 3a) that of the pure PPy nanoelectrode whencomposite nanowires consist of 30% (w/w) GO. The increasedcapacitance with the addition of GO is attributable to thefollowing: (1) The large surface area of GO, combined with themicroporous nanowire structure, provides a sufficient largeelectrochemical active surface area for charging and discharging,and (2) the presence of negative charge functional groups (e.g.,carboxyl) in GO enables the electrochemical doping of PPy,enhancing the conductivity for charge transport in thesupercapacitor. Furthermore, the combination of the dopedconductive GO/PPy nanowires and their vertical alignmentwith a large surface area allows a fast interfacial electron transfer

between the electrode and the electrolyte solution. However,the CV curve for the GO/PPy nanowires with a GO masspercentage higher than 30% (e.g., 50%, orange line, Figure 3a)is no longer a rectangle shape, which indicates a resistance-likeelectrochemical behavior. Therefore, the addition of anexcessive amount of GO might have led to the formation ofaggregates with a high electrical resistance, and hence thedecrease in capacitance. The 30% GO/PPy composite wasfurther measured by cycling potentials between −0.1 and 0.5 Vfor 100 cycles to identify the long-term cycling stability. Asshown in Figure S6a (Supporting Information), the differencebetween the first cycle and the 100th cycle was ignorable,suggesting good long-term cycling stability of the nanowireelectrode. The specific capacitances of both the GO/PPynanowire and the PPy nanowire were calculated from theirrespective CV curves (Figure S7, see the SupportingInformation for calculation details). Our results indicate thatthe highest specific capacitance of the GO/PPy nanowires isaround 960 F g−1, 6 times that of the pure PPy nanowires(∼160 F g−1). These values of capacitance are stable over awide range of potential scan rates from 0.01 to 0.1 V s−1

(Figure 3b), indicating a fast charging/discharging process inthe capacitor.31

We further carried out constant current charging anddischarging measurements on GO/PPy nanowires (Figure3c). During the charge and discharge steps, a slight, butnoticeable, slope variation of the potential vs time is evident,indicating somewhat pseudocapacitance behavior originatedfrom electrochemical redox reaction at the electrode/electrolyteinterface.32 The apparent symmetry of the charge and dischargecurves suggests the more or less ideal capacitive nature andsuperb reversible redox reaction of the GO/PPy nanowires.The specific capacitance can be calculated from the dischargingcurve: C = Ip × Δt/ΔV, where Ip is the constant dischargingcurrent density, Ip = 1 mAcm−2 (around 14 A g−1), Δt is thedischarge time, and ΔV represents the potential drop duringdischarge. Thus, the specific capacitance is 933 F g−1 at a

Figure 2. (a, b) SEM images of the reduced graphene oxide/carbonnanowires after annealing and (c) Raman spectra of the GO/PPynanowires (pink line) and reduced graphene oxide/carbon nanowires(blue line).

Figure 3. (a) Cyclic voltammogram at GO/PPy nanowires withvarious GO contents in 1 M NaCl(aq), scan rate = 0.1 V s−1. (b) Scanrate dependent specific capacitances of the GO/PPy nanowirescompared to the pure PPy nanowires. (c) The first 10 (blue line) andlast 10 (pink line) cycles of the charge−discharge curve during 300cycles at a constant current density of 1 mA cm−2 in 1 M NaClbetween −0.1 and +0.5 V. (d) EIS Nyquist plots of the GO/PPynanowires (blue line) and the PPy nanowires (pink line) at ac voltageamplitude of 5 mV in the 500 kHz to 0.01 Hz frequency range with abias voltage of 0.4 V.

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constant current of 1 mA cm−2 in the potential range of −0.1 to0.5 V. This value is similar to the capacitance obtained fromCV. The excellent electrochemical stability of the GO/PPynanowire can also be seen from Figure 3c. The charging anddischarging curves remain unchanged after 300 cycles,suggesting a good long-term stability of the GO/PPy nanowiresupercapacitor.Figure 3d shows the Nyquist plots produced by electro-

chemical impedance spectroscopy (EIS) using a sine-waveamplitude of 5 mV, a frequency range of 500 000 to 0.01 Hz,and a bias voltage of 0.4 V. Both plots can be divided into ahigh-frequency component with well-defined semicircles and alow frequency range with straight sloped lines. The interfacialcharge-transfer resistance (Rct), also called the Faradayresistance, can be determined by the diameter of the semicircle.The value of Rct would determine the power density of apseudocapacitor.33 Rct of the GO/PPy nanowire is around 20Ω, decreased from 200 Ω for the PPy nanowire electrode. Thelower Rct could significantly improve the power density of thecapacitor.In addition to the above electrochemical characterization for

the GO/PPy and PPy electrodes, we have also investigated therGO/carbon nanowires after pyrolysis of PPy in the GO/PPynanowires using CV, EIS, and chronopotentiometry. As shownin Figure 4a, the GO/carbon nanowire electrode in 1 M NaCl

also exhibited a rectangularly shaped CV, suggesting a goodcapacitive behavior for the graphene based nanowires.Compared to the GO/PPy nanowires, the rGO/carbonnanowires showed a 5 times decrease in the charging currents,suggesting the decreased capacitance without the redoxreactions of PPy. The rGO/carbon nanowire electrodeexhibited a well long-term cycling stability when the potentialwas cycled between −0.1 and 0.5 V for 100 cycles (Figure S6b,Supporting Information). The scan rate dependent specificcapacitance given in Figure 4b shows a steady value of 200 Fg−1 over scan rates from 0.01 to 0.4 V s−1 (Figure S8, see theSupporting Information for calculation details). The capaci-

tance of the rGO/carbon nanowires is about 5 times lower thanthat of the GO/PPy nanowires (960 F g−1), while thecapacitance of the rGO/carbon nanowire electrode remainsunchanged even at a scan rate up to 0.4 V s−1, indicating a veryfast electron transfer and low resistance of the electrode. This isalso evidenced by the EIS (Nyquist) plots given in Figure 4d,which shows a lower resistance (5 Ω) for the rGO/carbonnanowire than that of the GO/PPy nanowire (20 Ω). Figure 4cshows the charging and discharging curves for the rGO/carbonnanowire supercapacitor over 300 cycles, indicating an excellentelectrochemical stability. Since the rGO/carbon nanowireswere obtained after annealing GO/PPy nanowires at 700 °Cover 60 min, the excellent thermal stability of the rGO/carbonsupercapacitor is self-evident. The specific capacitance of therGO/carbon nanowire capacitor was calculated to be 190 F g−1

from the discharging curve, which is also consistent with thecorresponding value obtained from CVs. The specificcapacitance of the rGO/carbon nanowire is also higher thanthose of conventional porous carbon materials and evengraphene/carbon hybrids (normally, less than 180 F g−1) dueto the aligned structure of the as-synthesized nanowires.24−27

We have developed a new type of electrochemical super-capacitor based on vertically aligned GO/PPy hybrid nanowireswith the PPy electrochemically doped by GO. A rather highspecific capacitance (960 F g−1) was obtained from the GO/PPy nanowires, which is about 6 times higher than that of thepristine PPy nanowires, due to the large surface area of thevertically aligned nanowires and the intimate contact betweenthe nanowires and the substrate electrode. The GO/PPynanowire supercapacitor also shows a good long-term stabilitywith a high power density, suggesting potential applications inenergy storage devices. To improve the thermal stability andlong-term charge storage, GO was further electrochemicallyreduced into graphene and PPy was subsequently thermallycarbonized, leading to a high capacitance of 200 F g−1 for theresultant pure rGO/carbon based nanowire supercapacitor.This value of capacitance (200 F g−1) is higher than that ofconventional porous carbon materials. The rGO/carbonsupercapacitor also shows a superb electrochemical and thermalstability.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental details, cyclic voltammograms, FT-IR, spectra,TGA, TEM, electrode preparation, long-term CVs, and specificcapacitance calculation. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: yongliu1980@hotmail.com.*E-mail: liming.dai@case.edu.Author Contributions∥These authors contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support for this work from the Chinese NationalNature Science Foundation (21374081, 81301320), theMinistry of Education of China (IRT1077, SRF for ROCS),the National “Thousand Talents Program”, the Zhejiang

Figure 4. (a) Cyclic voltammogram at the rGO/carbon nanowires in 1M NaCl(aq), scan rate = 0.1 V s−1, (b) scan rate dependent specificcapacitances of the graphene/carbon nanowires, (c) EIS Nyquist plotsof the GO/PPy nanowires (blue line) and the rGO/carbon nanowires(green line) at an ac voltage with an amplitude of 5 mV in the 500 kHzto 0.01 Hz frequency range with a bias voltage of 0.4 V, and (d) thefirst 10 (green line) and last 10 (red line) cycles of the charge−discharge curve during 300 cycles at a constant current density of 1mA cm−2 in 1 M NaCl between −0.1 and 0.5 V.

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Department of Education (T200917), AFOSR (FA9550-12-1-0069), and Zhejiang National Nature Science Foundation(Y13H180013) is acknowledged.

■ REFERENCES(1) Conway, B. E. Electrochemical supercapacitors: scientific funda-mentals and technological applications; Kluwer Academic/PlenumPublishers: New York, 1999.(2) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937.(3) Yoon, S.; Lee, J.; Hyeon, T.; Oh, S. M. J. Electrochem. Soc. 2000,147, 2507.(4) Niu, C. M.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl.Phys. Lett. 1997, 70, 1480.(5) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung,D.; Bae, D. J.; Lim, S. C.; Lee, Y. Adv. Mater. 2001, 13, 497.(6) Du, C.; Yeh, J.; Pan, N. Nanotechnololgy 2005, 16, 350.(7) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. J. Electrochem. Soc.2000, 147, 4580.(8) Baughman, R. H.; Zakhidov, A.; de Heer, W. Science 2002, 297,787.(9) Lu, W.; Qu, L.; Henry, K.; Dai, L. J. Power Sources 2009, 189,1270−1277.(10) Oh, J.; Kozlov, M. E.; Kim, B. G.; Kim, H.; Baughman, R. H.;Hwang, Y. H. Synth. Met. 2008, 158, 638.(11) Otero, T. F.; Cantero, I. J. Electrochem. Soc. 1999, 146, 4118.(12) Ingram, M. D.; Staesche, H.; Ryder, K. S. Solid State Ionics 2004,169, 51.(13) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds,J. R. Adv. Mater. 2000, 12, 481.(14) Khomenko, V.; Frackowiak, E.; Beguin, F. Electrochim. Acta2005, 50, 2499.(15) Wu, Y.; Xu, Q.; Yao, Z.; Liu, A.; Shi, G. ACS Nano 2010, 4,1963.(16) Zhang, K.; Zhang, L.; Zhao, X.; Wu, J. Chem. Mater. 2010, 22,1392.(17) Cong, H.; Ren, X.; Wang, P.; Yu, S. Energy Environ. Sci. 2013, 6,1185.(18) Xiong, S.; Shi, Y.; Chu, J.; Gong, M.; Wu, B.; Wang, X.Electrochim. Acta 2014, DOI: 10.1016/j.electracta.2014.01.163.(19) Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.;Guo, J.; Dai, H. Science 2009, 324, 768.(20) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Nano Lett.2008, 8, 3498.(21) Lu, C.; Yang, H.; Zhu, C.; Chen, X.; Chen, G. Angew. Chem., Int.Ed. 2009, 48, 4785.(22) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.;Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano Lett. 2009, 9,1058.(23) Cassagneau, T.; Fendler, J. H. Adv. Mater. 1998, 10, 877.(24) Vivekchand, S. R. C.; Rout, C. S.; Subrahmanyam, K. S.;Govindaraj, A.; Rao, C. N. R. J. Chem. Sci. 2008, 120, 9.(25) Le, L.; Ervin, M.; Qiu, H.; Fuchs, B. E.; Lee, W. Y. Electrochem.Commun. 2011, 13, 355.(26) Das, T. K.; Prusty, S. Polym.-Plast. Technol. Eng. 2013, 52, 319.(27) Kim, T.; Jung, G.; Yoo, S.; Suh, K.; Ruoff, R. ACS Nano 2013, 7,6899.(28) Kristen, N.; Simulescu, V.; Vullings, A.; Laschewsky, A.; Miller,R.; Klitzing, R. J. Phys. Chem. B 2009, 113, 7986.(29) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.;Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30.(30) Liu, Y.; Liu, X.; Chen, J.; Gilmore, K. J.; Wallace, G. G. Chem.Commun. 2008, 3729.(31) Ghosh, S.; Inganas, O. Adv. Mater. 1999, 11, 1214.(32) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y.Angew. Chem., Int. Ed. 2003, 42, 4092.(33) Bard, A. J., Faulkner, L. R. Electrochemical methods: fundamentalsand applications, 2nd ed.; Wiley: New York, 2001.

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