Electrochemically Exfoliated Graphene for Electrode Films: Effect of

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Electrochemically Exfoliated Graphene for Electrode Films: Effect of Graphene Flake Thickness on the Sheet Resistance and Capacitive Properties Jinzhang Liu, Marco Notarianni, Geoffrey David Will, Vincent Tiing Tiong, Hongxia Wang, and Nunzio Motta Langmuir, Just Accepted Manuscript • DOI: 10.1021/la403159n • Publication Date (Web): 03 Oct 2013 Downloaded from http://pubs.acs.org on October 4, 2013

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Electrochemically Exfoliated Graphene for Electrode Films: Effect of Graphene Flake Thickness on the Sheet Resistance and Capacitive Properties Jinzhang Liu*, Marco Notarianni, Geoffrey Will, Vincent Tiing Tiong, Hongxia Wang, Nunzio Motta School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Brisbane, 4001, Australia

ABSTRACT We present an electrochemical exfoliation method to produce controlled thickness graphene flakes by ultrasound assistance. Bi-layer graphene flakes are dominant in the final product by using sonication during the electrochemical exfoliation process, while without sonication the product contains a larger percentage of four-layer graphene flakes. Graphene sheets prepared by using the two procedures are processed into films to measure their respective sheet resistance and optical transmittance. Solid-state electrolyte supercapacitors are made using the two types of graphene films. Our study reveals that films with a higher content of multilayer graphene flakes are more conductive, and their resistance is more easily reduced by thermal annealing, making them suitable as transparent conducting films. The film with

*

Corresponding author. Tel: +61 7 31389125. E-mail: [email protected]

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higher content of bilayer graphene flakes shows instead higher capacitance when used as electrode in a supercapacitor.


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1. INTRODUCTION Graphene is a unique material with exceptional electrical, thermal and mechanical properties. It was initially isolated by mechanical exfoliation of highly ordered pyrolytic graphite by using a piece of adhesive tape. This technique was suitable for obtaining pristine graphene layers for basic scientific research and to prepare proof-of-concept devices1, but provides only small sized graphene flakes, which are not suitable for commercial applications. In recent years, approaches for the preparation of high-quality graphene on a large-scale were intensively explored in order to obtain the amount required for electronic devices,2 transparent electrodes,3 energy storage,4 catalysis,5 drug delivery,6 and sensing.7 Methods for the preparation of large graphene sheets include chemical vapour deposition (CVD),8 ultrasonication-assisted exfoliation of graphite in N-methylpyrrolidone,9 epitaxial growth on electrically insulating surface,10 electrochemical exfoliation of graphite,11-13 and solutionbased chemical reaction of graphene oxide (GO).14,15 The chemical exfoliation of graphite based on the Hummers’ method (oxidation of graphite into graphene oxide sheets) has been widely used.14 However, this method requires a large amount of strong acid and chemicals, and the resulting GO has an extremely high resistivity. Though GO can be reduced either thermally or chemically to improve the electrical conductivity, its quality is still far away from pristine graphene. Also, the reduction to graphene using the chemical method is a complex process. Electrochemical exfoliation is in comparison very simple, low-cost, and non-polluting, as it is based on a mild chemical process without strong oxidzing agent. According to the electrolyte type used there are two different ways to perform electrochemical exfoliation: (1) ionic-liquid-based electrolyte; (2) aqueous solution of others chemicals such as surfactant, acid, and sulphate salt. Recent reports suggest that when using an ionic liquid like [BMIm][BF4] (1-butyl-3-methylimidazolium tetrafluoroborate), BF4- are intercalated into 3 ACS Paragon Plus Environment

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graphite to produce few-layer graphene.12,16 However, large amount of ionic liquid is consumed which makes this method costly. Aqueous solutions of electrolytes are in comparison inexpensive and allow using a wide range of chemicals, like surfactants, such as poly(sodium-4-styrenesulfonate)

and

sodium

dodecyl

sulphate.17,18

Though

many

electrochemical techniques for producing graphene have been reported, including the use of ionic liquid or aqueous electrolyte, much attention was devoted to the material preparation rather that to the practical applications of the resulting graphene. Recently C. Y. Su et al reported a very simple technique, which uses an aqueous solution containing sulfuric acid and potassium ions to rapidly exfoliate highly-oriented pyrolytic graphite, and processed these graphene sheets into transparent conductive films.13 It has been demonstrated that graphene is a good electrode material for applications in transparent conductive films, because of its low absorption coefficient (2.3%) and high conductivity,3 with the perspective to replace indium tin oxide in solar cells and organic lightemitting diodes. So far, CVD-prepared graphene has been exploited to make transparent conductive electrodes, with sheet resistance around 102-103 Ω/sq and visible light transmittance in the range of 60-80%.19,20 However, the preparation of graphene on copper foil by CVD is costly. Reduced GO (rGO) has been proposed as transparent conductive film or as electrode material for supercapacitors. While certainly inferior to the CVD-prepared graphene in transparent electrodes, it might go a good candidate for supercapacitors, which are expected to substitute ionic batteries in the future. Surprisingly, electrochemicallyexfoliated graphene has been rarely used in supercapacitor, though its production process is simpler. In this work, we demonstrate a new electrochemical technique to produce graphene flakes and study their performance as both transparent conduction films and supercapacitor electrodes. It is worth mentioning that in all electrochemical exfoliation techniques, the number of layers in the graphene flakes is variable and difficult to control. Much effort has 4 ACS Paragon Plus Environment

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been dedicated to the production of the thinnest possible flakes, such as single-layer, bi-layer and trilayer, but no much attention has been paid to the effect of the flakes thickness on the film conductivity. We introduced for the first time ultrasound assistance in the electrochemical exfoliation process with the aim of modifying the overall thickness of graphene flakes. We investigated the effect of flake thickness on the film conductivity and capacitive behavior by making transparent conductive films and solid-state electrolyte supercapacitors.

2. EXPERIMENTAL SECTION 2.1. Materials preparation Graphene was produced by electrochemical exfoliation of highly-oriented pyrolytic graphite, (1x1 cm2) used as anode in a 150 ml aqueous solution containing 0.15 M Na2SO4 and 0.01 M sodium dodecyl sulfate. The cathode is a Pt wire. The pH value of the electrolyte was adjusted to ~2.0 by adding few drops of sulfuric acid. The setup was placed into a sonicator (~300 W) during the chemical exfoliation process. The bias was set to 5 V for the first 30 min, and then adjusted to 6 V for the residual time. The bulk graphite expanded quickly and was thoroughly consumed within one hour. After the exfoliation, the dark-color electrolyte was centrifuged at 3000 rpm for 30 min to remove large agglomerates. The remaining dispersion was further diluted with DI water and vacuum filtered onto a porous polymer membrane. The obtained black product was ultrasonically dispersed in DI water and filtered again. This process was repeated three times to ensure the removal of chemical residues. Afterwards, the product was dispersed in dimethylformamide (DMF) containing 1% (v/v) water to get a stable suspension. Two samples, S1 and S2, with and without ultrasound


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assistance in the exfoliation process, respectively, were prepared for comparison. Other electrochemical exfoliation conditions for preparing the two samples are identical. 2.2. Preparation of graphene films To make a graphene film, the graphene sheets in DMF were vacuum filtered onto a porous alumina membrane (Whatmann Co. Pore size 100 nm). After drying, the membrane covered with graphene film was etched away by using a NaOH aqueous solution, leaving the graphene film floating on the surface. After repeatedly removing the solution and refilling DI water, the graphene film was transferred onto a glass substrate and then dried at 70 oC. Thermal annealing (200 oC and 400 oC) in air was performed to reduce the resistance of the films. 2.3. Fabrication of supercapacitor The graphene film made by vacuum filtration was transferred from the alumina membrane to a Au-coated glass substrate. After drying, the electrolyte gel was applied onto the surface. Methods for preparing the electrolyte gel are reported by Kaempgen et al.21 In our case 1 g polyvinyl alcohol is dissolved into 10 ml DI water at 90 oC. After cooling to room temperature, 0.8 g H3PO4 (85 % solution in water) is mixed with the gel with constantly stirring. One capacitor requires two glass substrates coated with graphene films. The electrolyte gel on each substrate was allowed to dry at ambient condition for 5 hours. Then, the two substrates are stacked together face-to-face and the electrolyte gel was further solidified for 10 hours. As a result, the two electrodes are bonded together tightly by the solid-state electrolyte. The gap between two opposite graphene film is about 10 µm. 2.4. Characterization The graphene sheets were drop-cast from DMF suspension onto mica substrates for atomic force microscopy (AFM, BMT Multi-scan 4000). The mica substrate is atomically smooth, allowing a reliable measurement of the graphene sheets thickness by AFM. Room6 ACS Paragon Plus Environment

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temperature Raman spectra of graphene flakes on glass substrates were measured by a Renishaw inVia spectrometer with 532 nm excitation. Conductivity measurements of graphene films were carried out on a four-point probe sheet resistance meter. Fourier transform infrared spectroscopy (FT-IR) measurement was performed using a PerkinElmer Spectrum Two spectrometer. A Varian’s Cary 50 UV-vis spectrophotometer was used to study the UV-visible absorption of the samples. X-ray photoelectron spectroscopy (XPS) analysis of the graphene films were performed using a setup (Omicron Nanotech) with a DAR 400 X-ray source incorporating a 125 mm hemispherical electron energy analyzer. The X-ray source used for the analysis was a Mg Kα (1253.6 eV) at 300 W, incident at 65 degrees to the sample surface. Photoelectrons were collected at a take-off angle of 90 degrees. Base pressure in the analysis chamber was kept at 1.0 x 10-10 mbar and during sample analysis at 4.0 x 10-10 mbar. The transparency of the films was measured by using an ellipsometer (Woollam M2000). An electrochemical station (BioLogic VSP) is used to measure the impedance and capacitor performance of the devices.

3. RESULTS AND DISCUSSION Figure 1 shows the Raman spectra of two graphene films, made of the S1 (ultrasound assisted exfoliation) and S2 (standard exfoliation) graphene samples, respectively. The films used for Raman measurement were made by vacuum filtration and were about 50 nm in thickness. There are three remarkable peaks in the Raman spectrum which are the D band (defects) around 1350 cm-1, the G band (in-plane vibration of sp2 carbon atoms) around 1574 cm-1, and the 2D band (2nd harmonic of the D band) around 2684 cm-1. Note that the G band peak exhibits a shoulder usually indicated as D’ peak. The G band is associated with the doubly degenerate (iTO and LO) phonon mode (E2g mode) at the Brillouin zone center. In fact, the G-band is the only band coming from a normal first order Raman scattering process in 7 ACS Paragon Plus Environment

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graphene. The D and D’ bands are both related to the presence of structural disorder in graphite/graphene. The ratio of ID/IG, which corresponds to the number of defects, is often used to evaluate the quality of graphene sheets. For the S1sample, produced with ultrasound assistance, ID/IG is ~0.61, much smaller than that of chemically-produced GO and reduced graphene,22 indicating a high structural quality of the as-prepared graphene. For the sample S2 shown in Figure 1b, the ratio ID/IG is ~1.02. In the Raman measurement the laser spot covers plenty of graphene flakes, allowing to assess the overall quality of the sample. Therefore, it can be concluded that the ultrasound-assisted method provides better quality graphene sheets. In the electrochemical process SO ions of the electrolyte are intercalated into graphite to weaken the inter-layer forces. The interaction between ions and graphite layers leads to chemical bonds such as C-OH and C=O and defects due to the dislocation of carbon atoms in the graphene sheets. Ultrasounds provide an extra mechanical force to facilitate the separation of graphene flakes even if the accumulation of ions in the interstice is not sufficient to detach the layers, thereby decreasing the number of defects. Further investigations by AFM on mica show that the ultrasound assistance during the exfoliation process influences the thickness of obtained graphene sheets (Figure 2). The line profiles in Figures 2c and 2d correspond to the red lines in Figures 2a and 2b, respectively. Figure 2c reveals a thickness of 0.8 nm for two graphene flakes and 1.6 nm for the other one. Figure 2d displays graphene sheet thicknesses of 1.2 nm and 1.6 nm. Knowing that the thickness of single-layer graphene is ~0.4 nm, we can conclude that the line profile in Figure 2c includes two bilayer sheets and one four-layer sheet. The statistical analysis for the graphene sheet thickness, obtained by examining the thickness of about one hundred graphene flakes on mica substrates, is shown in Figure 3. Bilayer sheets (0.8 nm-thick) are dominant, about 54 % of the total (Figure 3a) when using the ultrasound assistance, whereas without ultrasound four-layer (1.6 nm-thick) sheets (52% of the total) are the majority (Figure


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3b). This result confirms that the ultrasound assistance has the effect of reducing the thickness of exfoliated sheets. We made two groups of graphene films (F1 and F2) with different thicknesses using respectively the S1 (majority of bilayer graphene) and S2 (majority of four-layer graphene) samples, respectively. Figure 4a shows a typical optical photograph for two transparent graphene films of the S2 sample on glass with different thickness. The scanning electron microscopy (SEM) image in Figure 4b was taken from the thicker film. A side-view SEM image of the graphene film is shown in Figure 4c, revealing the film thickness of ~50 nm. By correlating the volume of graphene suspension for filtration and the film thickness, we can make graphene films with controlled thickness. During the filtration process, graphene sheets evenly deposited onto porous membrane and the sheets are staked to form a film, thus film thickness is uniform. The relationships of sheet resistance versus the optical transmittance of graphene films for F1 and F2 groups are shown in Figures 5a and 5b, respectively. For the F1 group films, the thickness values are 10 nm, 15 nm, 25 nm, and 30 nm, corresponding to transparency of 77.5 %, 66.5 %, 47.5%, and 33.7%, respectively. For the four F2-type films, the thickness are 10 nm, 16 nm, 28 nm, and 52 nm, corresponding to transparency of 77.74%, 61.12%, 43.33%, and 15.7%, respectively. These as-prepared films were firstly annealed at 200 oC in air for 1h before the sheet resistance measurement. After a 400 oC annealing in air for 1h the sheet resistance was measured again. It can be observed that the 400 oC annealing greatly reduces the sheet resistance for both groups of samples. For the F1-group film, at 77.5% transmittance the sheet resistance values corresponding to 200 oC and 400 oC thermal annealing are 5.10 kΩ/sq and 3.21 kΩ/sq, respectively. For the F2-type film, at 75.8 % transmittance the sheet resistance was 4.28 kΩ/sq after annealing at 200 oC, while the value drastically drops to 0.44 kΩ/sq after the 400 oC thermal treatment.


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The sheet resistance of CVD-prepared graphene film without further doping treatment is normally in the range of 400-700 Ω/sq, with transparency of 85-96 %.23,24 We notice that that our graphene films have much lower resistivity compared to the ones prepared by other solution methods. For example, films of oxide-free graphene flakes, made by sonication of graphite powder in an aqueous solution of sodium cholate, show sheet resistance of 103-106 Ω/sq over the transmittance range of 35-90%.25 Transparent films of graphene flakes, made by ultrasound exfoliation in N-methylpyrrolidone, with 70-90 % transmittance, show sheet resistance of 103-105 Ω/sq.26 Films of reduced graphene oxide show various resistance in the order of 100-104 Ω/sq, depending on the reduction method and film thickness. A representative work shows sheet resistance of 2.2 kΩ/sq at 80 % transmittance for reduced graphene oxide film reported by Jeong et al.27 In Figure 5b, the sample after 400 oC shows sheet resistance of 440 Ω/sq at 76% transmittance, indicating that the electrochemicallyprepared few-layer graphene sheets can be an excellent candidate for inexpensive transparent electrode applications. Techniques like nitrate acid treatment or doping for improving the conductivity and spray or printing for making large-area films may be used to make highperformance transparent electrodes using the electrochemically exfoliated graphene.13,24 FT-IR and UV-vis measurements show no discernable absorption peaks of our graphene films (see Supporting Information). XPS analysis of two 50 nm-thick graphene films, S1 and S2, provides a quantification of the oxygen content before and after 400 oC air-annealing. The oxygen content given by XPS survey is approximately 16.3% for both S1 and S2 samples. This is attributed to the oxidation of graphene, which was unavoidable during the electrochemical process. It also indicates that the ultrasound assistance during the electrochemical exfoliation process has no much influence on the oxidation degree of graphene. After 400 oC annealing in air, the oxygen atomic percentage of both S1 and S2 samples reduce to ~ 13.0%. Hence, the ~3% reduction of oxygen content and the restacking


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of graphene sheets after annealing can be the cause for the large increase of electrical conductivity. Figure 6 shows the C1s peaks of the samples before and after the 400 oC thermal annealing in air. Besides the C-C sp2 bond peak at 284.4 eV and C-C sp3 bond peak at 285.2 eV, other shoulder peaks recognized as C-OH (286.3 eV) and C=O (289.0 eV) are present.28 Deconvoluted XPS spectra reveal that the S1 sample contains 13.1 at.% C-O and 3.2 at.% C=O functional groups. After annealing, the contents change to 9.0 at.% (C-O) and 4.0 at.% (C=O). For S2 sample, thermal annealing leads to the decrease of C-O bonds from 11.6 at.% to 6.3 at.% and increase of C=O from 4.7 at.% to 6.7 at.%, respectively. This indicates that thermal annealing removes part of C-O bonds and induces a slight increase of C=O bonds. This increase might be ascribed to the conversion of C-O to C=O. So the removal of oxygen from graphene surface by thermal annealing could be one of the reasons causing the increase of conductivity. We used S1 and S2 graphene film electrodes to build capacitors, which display different behaviours. The graphene films used in the capacitors are 50 nm thick. Figures 7a and 7b show the cyclic voltammetery (CV) curves of the two devices, S1 and S2, respectively, measured within the voltage range of -0.6 V–+0.6 V and with various scan rates from 20 mV/s to 500 mV/s. These CV curves display nearly rectangular shape, indicating that an efficient electrical double layer is established in both of the graphene-based electrodes. The galvanostatic charge-discharge curves of the two devices, S1 and S2, are shown in Figures 7c and 7d, respectively. The charge curves were nearly symmetric to their corresponding discharge curves in the potential range examined, which indicates a high reversibility between charge and discharge processes. The same counter electrode was used in both CV and charge-discharge measurements. The shapes of charge-discharge curves measured within different voltage ranges of 0-0.8 V and 0-1.2 V are quite similar. As the maximum voltage applied in CV measurement is 0.6 V, stable charge-discharge curves measured in the voltage 11 ACS Paragon Plus Environment

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range of 0-1.2 V show that the device works well if charged at 1.2 V. The insets in Figures 7c and 7d show typical charge-discharge curves in a continuous operation with 25 µA/cm2 for 24 cycles. Obviously, these curves have the same shape. The specific capacitance can be extracted from either the CV curves or the charge-discharge curves. In the charge-discharge curve the capacitance can be obtained by the slope. However the rising and decreasing curves are not perfectly linear with time. If we treat the charging and discharging as linear process, corresponding to 25 µA/cm2, the capacitance of S1 and S2 devices are 713 µF/cm2 and 483 µF/cm2, respectively. This indicates that the capacitance of S1 is about 1.5 times that of S2 device. Figure 8a shows the Nyquist impedance plots of the two devices, S1, and S2, measured in the frequency range of 0.04 Hz to 1 MHz. The inset gives a magnified view of the plots in the low impedance (high frequency) region, from which the equivalent series resistance of S1 and S2 devices can be extracted as 235 Ω and 206 Ω, respectively. The higher slope at lower frequencies indicates a more capacitive behaviour of electrodes. Figure 8b shows the Ragone plot comparing the supercapacitor performance of S1 and S2 devices. The volumetric energy density and power density are extracted from galvanostatic charge-discharge curves (Supporting Information). For the two devices S1 and S2, the power density is in the order of 1 Wcm-3 and the energy density is in the order of 10-4 Whcm-3. The S1 device is higher than S2 in both power density and energy density and exhibits better supercapacitor performance. Commercial Li-ion film batteries shows low power density in the order of 10-3 Wcm-3, though their energy density is relatively high (10-4–10-3 Whcm-3).29 For Al electrolytic capacitor, the power density was measured to be 100-102 Wcm-3, however the energy density (10-6 Whcm-3) is much lower than in our devices.30 By increasing the power density, S2 device shows larger energy density drop than the S1 device. Hence, the S1 device containing graphene sheets electrochemically exfoliated with ultrasound assistance demonstrates high 12 ACS Paragon Plus Environment

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energy and power performance, which is very promising for practical applications. From the CV curves the capacitance can be calculated by using the current (I) value at V=0 and the scan rate dV/dt, as C=I/(dV/dt). We made capacitor devices using as-prepared graphene films, 200

o

C-annealed films, and 400 oC-annealed films, respectively. The specific

capacitance against scan range (10-500 mV/s) for the two groups of devices, S1, and S2, are shown in Figures 8c and 8d, respectively. These plots show the decrease of the capacitance by increasing the scan rate. This is related with the diffusion of ions in the gelled electrolyte. The lower the scan rate, the more time for the ions to diffuse deeper into the graphene film electrode. Therefore, the capacitance at very low scan rate is close to the full capacitive performance of the device. For devices with unannealed graphene films, in the range of 10500 mV/s, the capacitance of S1 device shows a maximum value of 900 µF/cm2 and the minimum of 567 µF/cm2, while for the S2 device the capacitance values are in the range of 697-382 µF/cm2. Overall, the capacitance of the S1 film with a majority of bi-layer graphene flakes is higher than that of the S2 film with high percentage of four-layer graphene flakes, though the S1 film has lower conductivity than the S2 film. The capacitance of electrolyte supercapacitors using porous reduced graphene oxide as electrode is normally in the range of 200- 400 µF/cm-2.31 Our study shows that the very thin graphene sheets produced by electrochemical exfoliation with ultrasound assistance are suitable for supercapacitor electrode applications. Experimental results in Figures 8c and 8d reveal that thermal annealing of these graphene films decreases their capacitive performance, though their electrical conductivity improves. The conductivity between the 200 oC and 400 oC annealed films show large difference (Figure 5), while their capacitance values are very close. This means that the electrical conductivity of the graphene films has minor influence on the capacitance. As revealed by XPS the surface of graphene flakes has a certain degree of oxidation, and the 13 ACS Paragon Plus Environment

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oxygen contents of S1 and S2 samples are almost equal. Hence their difference in conductivity is more related with the graphene flake thickness. Overall, the F2-group films have lower sheet resistance because of the higher content of four-layer graphene sheets, showing that the graphene multilayer sheets prepared without sonication are more suitable for transparent electrodes. Graphene flakes with more than two stacked carbon layers, such as trilayer and four-layer, contain outer carbon layers with oxygen bonds, inferior in electrical conduction, which protect the inner layer(s) contributing in large amount to electrical conductivity. Monolayer and bilayer graphene sheets, containing only exposed carbon layers with oxygen groups, are expected to have low conductivity. Therefore, the S1 film with larger percent of bilayer graphene sheets has a higher resistance than that of S2 film containing a higher percentage of four-layer sheets. Moreover, the ultrasound assistance during the graphene preparation process makes the average graphene flake size smaller as revealed by AFM. This in turn boosts the capacitance as charges preferentially reside at edges and corners of graphene sheets.32 Figure 9 shows the plots of specific capacitance against graphene-film electrode thickness for S1 and S2 devices. Within the thickness range of 10-60 nm, both groups of devices show that thicker graphene film provides higher capacitance. The advantage of graphene electrode in supercapacitor is its high specific surface. In the electrode, graphene flakes overlap with each other and charges are stored not only in the outer surface, but also inside the film at the interspace between stacked sheets. In this regard, thicker electrodes containing more graphene sheets can store more charges from the electrolyte. The S1 film with a majority of bilayer graphene sheets would have larger specific surface than the S2 film with a majority of four-layer graphene sheets. Though the S1 film has lower conductivity than S2 film, its larger specific surface gives rise to higher capacitance and makes it surpass the S2 film in supercapacitor performance. Thermal annealing improves the conductivity but 14 ACS Paragon Plus Environment

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results in restacking of graphene sheets to form graphite. Thus, after annealing, the specific surface reduces and the capacitance decreases for both samples.

4. CONCLUSIONS In summary, we have developed a simple electrochemical exfoliation method for fast producing graphene. By introducing ultrasound assistance during the exfoliation process we demonstrate that it can influence the overall thickness of graphene flakes. With ultrasound assistance the final product (S1) contains ~54% bilayer graphene sheets, while without sonication the majority is four-layer graphene sheets (~52%) in the counterpart sample (S2). The two types of graphene films are applied in transparent conducting films and supercapacitors and their performances are compared. Though the ultrasound assistance improves the overall quality of the graphene product as revealed by the Raman analysis, the reduced thickness of graphene flakes leads to a decrease of conductivity in transparent films. We argue that surface oxidation is responsible for the higher sheet resistance of single- and bi-layer graphene sheets. In comparison, graphene flakes with more than three carbon layers have higher conductivity due to the pristine inner layers(s). The films with high percentage of four-layer flakes achieve a sheet resistance as low as 102 Ω/sq, with optical transmittance up to 80%. We made solid-state electrolyte supercapacitors using the S1 and S2 graphene films with thickness of 50 nm, and found that the S1 film, though has higher resistance, is better in supercapacitor performance due to the majority of bilayer graphene flakes. At scan rate of 10 mV/s the specific capacitance of the S1 device is 900 µF/cm2 while that of the S2 device is 689 µF/cm2. Both S1 and S2 devices show power density in the order of 1 Wcm-3 and energy density in the order of 10-4 Whcm-3; the calculated values of S1 being higher than those of S2. We conclude that the multilayer graphene sheets are more suitable for making conductive electrodes but very thin graphene sheets have better performance as supercapacitor electrode due to the increased specific area. Thermal annealing improves the conductivity of graphene 15 ACS Paragon Plus Environment

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films, which is more suitable for transparent electrodes but reduces the capacitance if annealed films are used as electrodes in solid-state electrolyte capacitor.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGEMENTS The authors acknowledge the financial support of QUT through the Vice-Chancellor’s Research Fellowship and of the Australian Research Council through the Discovery Project DP13010212. This research was also partly supported by a Marie Curie International Research Staff Exchange Scheme Fellowship within the 7th European Community Framework Program. This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF) – a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers. We also acknowledge the Central Analytical Research Facility (CARF) at QUT for AFM and SEM characterizations.

REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films, Science 2006, 312, 666-669. (2) Schwierz, F.; Graphene transistors, Nat. Nanotech. 2010, 5, 487-496. (3) Pang, S.; Hernandez, Y.; Feng, X.; Müllen, K.; Graphene as transparent electrode material for organic electronics, Adv. Mater. 2011, 23, 2779-2795. 16 ACS Paragon Plus Environment

Page 17 of 29

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(4) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, S.; Graphene-based ultracapacitors, Nano Lett. 2008, 8, 3498-3502. (5) Machado, B. F.; Serp, P.; Graphene-based materials for catalysis, Catal. Sci. Tech. 2012, 2, 54-75. (6) Sun, X.; Liu Z.; Welsher, K.; Robinson, J. T.; Goodwin, A.; Zaric, S.; Dai, H.; Nanographene oxide for cellular imaging and drug delivery, Nano Res. 2008,1, 203-212. (7) Hill, E. W.; Vijayaragahvan, A.; Novoselov, K.; Graphene sensors, IEEE Sens. J. 2011, 11, 3161-3170. (8) Mattevi, C.; Kim, H.; Chhowalla, M.; A review of chemical vapour deposition of graphene on copper, J. Mater. Chem. 2011, 21, 3324-3334. (9) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N.; High-yield production of graphene by liquid-phase exfoliation of graphite, Nat. Nanotech. 2008, 3, 563-568. (10) Strupinski, W.; Grodecki, K.; Wysmolek, A.; Stepniewski, R.; Szkopek, T.; Gaskell, P. E.; Gruneis, A.; Haberer, D.; Bozek, R.; Krupka, J.; Baranowski, J. M.; Graphene epitaxy by chemical vapor deposition on SiC, Nano Lett. 2011, 11, 1786-1791. (11) Bourelle, E.; Glaude-Montigny, B.; Metrot, A.; Electrochemical exfoliation of HOPG in formic-sulfuric acid mixtures, Mol. Cryst. Liq. Cryst., 1998, 310, 321-326. (12) Liu, N.; Luo, F.; Wu, H.; Liu, Y.; Zhang, C.; Chen, J.; One-step ionic-liquid-assisted electrochemical synthesis of ionic liquid-functionalized graphene sheets directly from graphite, Adv. Funct. Mater. 2008, 18, 1518-1525. (13) Su, C. Y.; Lu, A. Y.; Xu, Y.; Chen, F. R.; Khlobystov, A. N.; Li, L. J.; High-quality thin graphene films from fast electrochemical exfoliation, ACS Nano 2011, 5, 2332-2339.


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Page 18 of 29

(14) Hummers, W. S.; Offeman, R. E.; Preparation of graphitic oxide, J. Ame. Chem. Soc. 1958, 80, 1339-1339. (15) Park, S.; Ruoff, R. S.; Chemical methods for production of graphenes, Nat. Nanotech. 2009, 4, 217-224. (16) Lu, J.; Yang, J. X.; Wang, J.; Lim, A.; Wang, S.; Loh, K. P.; One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids, ACS Nano 2009, 3, 2367-2375. (17) Wang, G.; Wang, B.; Park, J.; Wang, Y.; Sun, B.; Yao, J.; Highly efficient and largescale synthesis of graphene by electrolytic exfoliation, Carbon 2009, 47,3242-3246. (18) Alanyalioglu, M.; Segura, J. J.; Casaň-Pasor, N.; The synthesis of graphene sheets with controlled thickness and order using surfactant-assisted electrochemical process, Carbon 2012, 50, 142-152. (19) De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Dyu, K.; Thompson, M. E.; Zhou, C.; Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics, ACS Nano 2010, 4, 2865-2873. (20) Kim, J.; Ishihara, M.; Koga, Y.; Tsugawa, K.; Hasegawa, M.; Lijima, S.; Lowtemperature synthesis of large-area graphene-based transparent conducive films using surface wave plasma chemical vapor deposition, Appl. Phys. Lett. 2011, 98, 091502. (21) Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G.; Printable thin film supercapacitors using single-walled carbon nanotubes, Nano Lett. 2009, 9, 1872-1876. (22) Chen, D.; Li, L.; Guo, L.; An environment-friendly preparation of reduced graphene oxide nanosheets via amino acid, Nanotechnology 2011, 22, 325601. (23) Jo, G.; Choe, M.; Lee, S.; Park, W.; Kahng, Y. H.; Lee, T.; The application of graphene as electrodes in electrical and optical devices, Nanotechnology 2012, 23, 112001.


Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(24) Kobayashi, T.; Bando, M.; Kimura, N.; Shimizu, K.; Kadono, K.; Umezu, N.; Miyahara, K.; Hayazaki, S.; Nagai, S.; Mizuguchi, Y.; Murakami, Y.; Hobara, D.; Production of a 100m-long high-quality graphene transparent conductive film by roll-to-roll chemical vapour deposition and transfer process, Appl. Phys. Lett. 2013, 102, 023112. (25) De, S.; King, P. J.; Lotya, M.; O’Neill, A.; Doherty, E. M.; Hernandez, Y.; Duesberg, G. S.; Coleman, J.; Flexible, Transparent, Conducting Films of Randomly Stacked Graphene from Surfactant-Stabilized, Oxide-Free Graphene Dispersions, Small 2010, 6, 458-464. (26) Oh, S. Y.; Kim, S. H.; Chi, Y. S.; Kang, T. J.; Fabrication of oxide-free graphene suspension and transparent thin films using amide solvent and thermal treatment, Appl. Surf. Sci. 2012, 258, 8837-8844. (27) Jeong, S. Y.; Kim, S. H.; Han, J. T.; Jeong, H. J.; Yang, S.; Lee, G. W. Highperformance transparent conductive films using rheologically derived reduced graphene oxide. ACS Nano 2011, 5, 870-878. (28) http://cnx.org/content/m34549/latest/ (29) Pech, D.; Brunet, M.; Durou, H.; Huang, P.; Mochalin, V.; Gogotsi, Y.; Taberna, P. L.; Simon, P.; Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon, Nat. Nanotech. 2010, 5, 651-654. (30) Ei-Kady, M. F.; Kaner, R. B. Scalable fabrication of high-power graphene microsupercapacitors for flexible and on-chip energy storage, Nat. Commun. 2013,4, 1475. (31) Sheng, K.; Sun, Y.; Li, C.; Yuan, W.; Shi, G. Ultrahigh-rate supercapacitors based on electrochemically reduced graphene oxide for ac line-filtering, Sci. Rep. 2012, 2, 247. (32) Wang, Z.; Scharstein, R. W. Electrostatics of graphene: Charge distribution and capacitance. Chem. Phys. Lett. 2010, 489, 229-236.


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Figure Captions

Figure 1. Raman spectra of graphene films. The graphene samples are produced through the electrochemical exfoliation process (a) with and (b) without ultrasound assistance. Figure 2. AFM images of graphene flakes for S1 (a) and and S2 (b) samples. (c) and (d) Line profiles corresponding to the lines in (a) and (b), respectively. (e) and (f) Statistical thickness analysis for the graphene sheet ensembles (selected more than 100 sheets in AFM measurement results) of two samples S1 and S2, respectively. Figure 3. (a) and (b) Statistical thickness analysis for the graphene sheet ensembles (selected more than 100 sheets in AFM measurement results) of two samples S1 and S2. Figure 4. (a) Optical photograph of two graphene films. (b) and (c) Plane-view and crosssectional view SEM images of a graphene film on Si. Figure 5. (a and b) Relationship of sheet resistance versus transmittance for two groups of graphene films. Figure 6. XPS characterizations (C1s binding energy) for electrochemically exfoliated graphene samples. (a) and (b) Graphene films prepared with and without ultrasound assistance, respectively. (c) and (d) The two samples S1 and S2 after 400 oC annealing in air. Figure 7. (a) and (b) CV curves for S1 and S2 graphene films electrodes at different scan rates. (c) and (d) Galvanostatic charge-discharge measurement of two devices using S1 and S2 films as electrodes, with different voltage ranges of 0-0.8 V and 0-1.2 V. The insets are cyclic charge-discharge curves measured with current of 25 µA/cm2. Figure 8. (a) Complex impedance spectrum of S1 and S2 devices with electrode thickness of 50 nm. (b) Energy and power densities of S1 and S2 devices. (c) and (d) Capacitance S1 and S2 devices against different scan rates, obtained from the cyclic voltammetry curves. Capacitance of devices using as-prepared, 200 oC-annealed and 400 oC-annealed graphene films are compared. Figure 9. Relationships of specific capacitance and graphene electrode thickness for S1 and S2 devices.


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Figures & Captions

Figure1. Raman spectra of graphene films. The graphene flakes are produced through the electrochemical exfoliation process (a) with and (b) without ultrasound assistance.


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Figure 2. AFM images of graphene flakes for S1 (a) and and S2 (b) samples. (c) and (d) Line profiles corresponding to the lines in (a) and (b), respectively.

Figure 3. (a) and (b) Statistical thickness analysis for the graphene sheet ensembles (selected more than 100 sheets in AFM measurement results) of two samples S1 and S2. 22 ACS Paragon Plus Environment

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Figure 4. (a) Optical photograph of two graphene films. (b) and (c) Plane-view and crosssectional view SEM images of a graphene film on Si.


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Figure 5. (a and b) Relationship of sheet resistance versus transmittance for two groups of graphene films.


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Figure 6. XPS characterizations (C1s binding energy) for electrochemically exfoliated graphene samples. (a) and (b) Graphene films prepared with and without ultrasound assistance, respectively. (c) and (d) The two samples S1 and S2 after 400 oC annealing in air.


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Figure 7. (a) and (b) CV curves for S1 and S2 graphene films electrodes at different scan rates. (c) and (d) Galvanostatic charge-discharge measurement of two devices using S1 and S2 films as electrodes, with different voltage ranges of 0-0.8 V and 0-1.2 V. The insets are cyclic charge-discharge curves measured with current of 25 µA/cm2.


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Figure 8. (a) Complex impedance spectrum of S1 and S2 devices with electrode thickness of 50 nm. (b) Energy and power densities of S1 and S2 devices. (c) and (d) Capacitance S1 and S2 devices against different scan rates, obtained from the cyclic voltammetry curves. Capacitance of devices using as-prepared, 200 oC-annealed and 400 oC-annealed graphene films are compared.


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Figure 9. Relationships of specific capacitance and graphene electrode thickness for S1 and S2 devices.


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Electrochemically Exfoliated Graphene for Electrode Films: Effect of

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