Reverse-Micelle-Induced Exfoliation of Graphite into Graphene

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Reverse-Micelle-Induced Exfoliation of Graphite into Graphene Nanosheets with Assistance of Supercritical CO2 Shanshan Xu, Qun Xu, Nan Wang, Zhimin Chen, Qiuge Tian, Hongxia Yang, and Kaixi Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00092 • Publication Date (Web): 13 Apr 2015 Downloaded from http://pubs.acs.org on April 22, 2015

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Chemistry of Materials

Reverse-Micelle-Induced Exfoliation of Graphite into Graphene Nanosheets with Assistance of Supercritical CO2 Shanshan Xu, Qun Xu*, Nan Wang, Zhimin Chen, Qiuge Tian, Hongxia Yang, Kaixi Wang College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, P.R. China

Abstract: Mass production of graphene with low cost and excellent properties is essential for its practical applications in energy, composites, biotechnology, and electronics. Here for the first time we demonstrate that graphite powder can be efficiently exfoliated into mono- and few-layered nanosheets based on the driving forces originating from the phase inversion, i.e., from micelles to reverse micelles in the emulsion microenvironment built by supercritical carbon dioxide (SC CO2). A series of surfactants have been studied and the experimental results indicate that efficient exfoliation of graphene depends on the suitable surfactant chosen in SC CO2 solution system. In this work, polyvinylpyrrolidone (PVP) is confirmed to be an excellent surfactant to play the critical role on exfoliation of graphite, which leads to a high-yield graphene nanosheets (87.7%, ≤3 layers) with concentration of 1.93 mg/ml, large lateral size (up to 5µm) and low oxidation degree (a C/O ratio of 20.28). And the dispersible graphene can be ink-brushed on A4-size paper to form highly conductive films (2.41Ω sq-1), which confirms that our exfoliation method remains the integrity 1

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of the perfect structure in graphene to the largest extent. Further, the exfoliated graphene was used to prepare electrospun graphene-beaded carbon fibers for supercapacitors. The obtained materials deliver a high specific capacitance of 371.25 F/g, which is 71.6% higher than that of the pristine carbon fibers and exhibit an excellent rate performance. Thus this strategy utilizing reverse-micelle-induced method for exfoliation of graphite to graphene can pave a way for the green solution-processable production of more two-dimensional (2D) nanosheets, which will have great application potential on electronic, biotechnology, energy and information storage, etc.

Keywords: Graphene exfoliation, phase inversion, supercritical carbon dioxide (SC CO2), surfactants, conductive films, supercapacitors

Introduction Owing to its ultrathin, two-dimensional (2D) nature, high surface area, and unprecedented properties, graphene has generated intense interest across multiple scientific disciplines including physics,1,2 material science,3-5 biotechnology6,7 and chemisty.8,9 However, practical use of graphene is limited by its high-quality, solution-processable production on a large scale. To date, several approaches have been explored for graphene preparation. Among them, because of the low throughput yield and high cost of energy, the mechanical cleavage10,11 and CVD-grown graphene approaches12-14do not scale well for applications requiring macroscopic quantities of 2


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graphene. Additionally, chemical oxidation of graphite to graphene oxide followed by reduction is an appealing route to produce reduced graphene oxide at large scale. However, the high content of defects disrupts the graphene lattice and often leads to graphene of poor electronic conductivity.15,16 Therefore, significant efforts have focused on direct exfoliation of pristine graphite powder in the liquid phase to obtain graphene. High-quality dispersions of graphene have been produced by Coleman et al. using such particular organic solvents or solvent blends whose Hansen solubility parameters match well with graphene, so the energy of exfoliation can be minimized.17-19 Meanwhile, some other specialty solvents, such as ionic liquids20,21 and pyrene derivatives solvent22 that can have interactions with the π-electrons on the graphene, have also been reported to successfully render stable graphene dispersions by sonication.23 However, these required particular solvents in the methods above are often not only expensive and high toxicity, but also have a high boiling point and cannot be easily removed, which hinder the further applications of graphene.24 In addition to the methods referred above, surfactant-assisted exfoliation is of particular interest. First, the used solvent is water and so it is benign to environment. Second, the application of surfactants caters for the exfoliation demand that it is necessary to enhance the ratio of surface to mass and to form larger interface. It is well-known that in a micelle, the hydrophobic tail of the surfactant points towards the core while the polar head group forms an outer shell. Similarly, surfactant may also aggregate in non-polar organic solvent, wherein the structure was referred as reverse micelles.25As an excellent and green alternative to conventional organic solvents, 3



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supercritical CO2 (SC CO2) possesses an important property that it can assist surfactant-water solutions to build the reverse-micelle emulsions microenvironment, and the phase behaviour of emulsions microenvironment can be manipulated by tuning the physical property of the solution.26-28 Specifically, just as a “switch” for the molecular aggregation of surfactants, the tuning of the aggregation behaviours of surfactants by CO2 is reversible, which can be realized by simply pressurization and depressurization.29Although SC CO2, which has low viscosity and high diffusivity, has been utilized to intercalate and delaminate tightly-stacked layered materials such as silicates and graphite,30-32 it has never been studied or explored to induce the reverse emulsions microenvironment for the liquid exfoliation of graphene. In this contribution, we present for the first time a highly facile and versatile method to exfoliate bulk graphite powder by utilizing SC CO2 to induce phase inversion from micelles to reverse micelles in the emulsion microenvironment. We have systematically explored a series of surfactants, such as polyvinylpyrrolidone (PVP), Pluronics F127 and P123, Tween 20, Hexadecyltrimethyl ammonium Bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS), and studied their effect on the exfoliation efficiency of graphite. Experimental results demonstrate that the driving force stemmed from the phase inversion from micelles to reverse micelles is efficient for the exfoliation of graphite, and as well as the ideal surfactant chosen is also a necessary factor. As far as non-ionic surfactant PVP is used, the highly stable suspensions of ultrathin graphene nanosheets can reach 1.93 mg/mL and more than 87% of flakes are 1~3 layers. Large-area conductive graphene films were readily fabricated 4


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on the A4-size paper by simple “painting and drying” process using graphene ink and the supercapacitor exhibited an outstanding rate capability and electrical performance even at the scan rate of 5000 mV/s. Further, flexible graphene/carbon fiber paper with only 5% of graphene was prepared using electrospinning methods. The obtained graphene-beaded carbon fibers displayed a high specific capacitance of 371.25 F/g, 71.6% higher than that of the pristine carbon fibers.

EXPERIMENTAL SECTION Preparation of graphene dispersions. Typical process is as the following. Graphite powder (50 mg) and PVP (200 mg) were added into a group of aqueous solutions (10 mL) with ethanol volume fractions ranging from 0 to 100 % and sonicated for 2 h to form homogeneous solutions. Then the dispersions were quickly transferred into the supercritical CO2 apparatus composed mainly of a stainless steel autoclave (50 mL) with a heating jacket and a temperature controller. The autoclave was heated to 313.2 K, and then CO2 was charged into the autoclave to the desired pressure (16 MPa) under stirring. After a reaction in 3 h, the gas was released. Finally, the dispersion was sonicated in an ice bath for another 2 h, and then the dispersion was centrifuged at 3000 rpm for 45 minutes to remove aggregates. The supernatant (top three quarters of the centrifuged dispersion) was collected by pipette. Preparation of graphene-ink and conductive paper. As-prepared graphene flakes were dispersed in DMF with the concentration of 10 mg/ml followed by sonication of 30 min. The resulting dispersion was applied to commercial paper with a paintbrush. 5



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Then the ‘wet’ paper was dried in the oven for 1 min at 120 °C. This simple ‘painting and drying’ process was repeated for a number of times to increase the graphene loading. The sheet resistance of the conductive paper was measured using a four-point probe system. Preparation of porous graphene/CNFs electrospun nanofibers. Carbon nanofiber and graphene-beaded carbon fibers (G/CNFs) nanofibers were produced by an electrospinning process. Solutions for electrospinning were prepared by dispersing the appropriate amount of graphene and polyacrylonitrile powder (graphene/PAN=1/20) in dimethylformamide (DMF). This solution was fed into a positively charged spinneret attached to an electrospinning apparatus. Finally, the electrospun fiber was stabilized in air and then carbonized in N2 atmosphere at 800 ℃ followed by activated in 400 ℃. Characterization. The morphology and structure of the samples were characterized by field-emission SEM (JEOR JSM-6700F), Tapping-mode AFM (Nanoscope IIIA), HRTEM and TEM (JEOL JEM-2100). UV/Vis spectra (Shimadzu UV-240/PC) were measured to evaluate graphene dispersions concentration. Raman spectra were recorded on a Renishaw Microscope System RM2000 with laser wavelength 532nm. X-ray photoelectron apectroscopy was performed using a Thermo ESCALAB 280 system with Al/K (photon energy =1486.6 eV) anode mono X-ray source. Cyclic voltammetry and the galvanostatic charge–discharge tests of supercapacitors were carried out with the CHI 660D electrochemical work station. RESULTS AND DISCUSSION 6


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Graphene preparation in different surfactant emulsions microenvironment The

exfoliation

process

of

graphene

nanosheets

in

the

emulsions

microenvironment built by SC CO2 and different surfactant is schematically illustrated in Figure S1. A series of surfactants including non-ionic surfactant, anionic surfactant and cationic surfactant such as polyvinylpyrrolidone (PVP), Pluronics F127 and P123, Tween 20, Hexadecyltrimethyl ammonium Bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS), have been investigated systematically. The experimental results about the exfoliation of graphite are presented in Figure 1. It can be observed clearly that the dispersed concentration of graphene varied from samples to samples from the photograph pictures shown in Figure 1a. Further by comparing A660/l for the graphene dispersion and their obtained concentration in different surfactants mixture solution, it can be found out that PVP had superlative efficiency for exfoliation of graphene, while CTAB and SDBS had no apparent exfoliation effects. So it can be induced that compared to ionic surfactant, non-ionic surfactant is a good choice to induce exfoliation of graphite in SC CO2. Further we try to answer why PVP is advantageous than Pluronics F127 and P123 and Tween 20. According to their molecular formula shown in Figure S2, it can be suggested that because of the long-chain molecular structure of Pluronics F127, P123 and Tween 20, the adsorption ability of them on graphite are much weaker compared with PVP with the existence of 2D pyrrolidone group. Here it is noteworthy to stress that the adsorption between surfactant and graphite is very important for the exfoliation process, and it will be discussed in detail in the following content of mechanism statement. In this study, 7



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PVP shows the strongest interaction with graphene which is more favourable to adsorb to the graphene surface17,33,34 and then provides requisite forces to exfoliate graphite powder in the process of phase inversion from micelles to reverse micelles. In this regard, surfactant chemistry plays a critical role in the exfoliation of graphite. Since PVP is the excellent surfactant in the SC CO2-assisted process of micelle reversion, our subsequent systematic study about solution effect on the graphene fabrication is carried out in CO2/PVP/water system. Effect of solution component on efficiency of graphite exfoliation The exfoliation of bulk graphite in the emulsions microenvironment built by the CO2/PVP/water system was confirmed by absorption spectra. The UV-vis spectrum of the graphene dispersion is shown in Figure 2a and it is flat and featureless. Besides, PVP shows no absorbance within the wavelength range of 300-900 nm herein (Figure S3), which means that the presence of PVP in graphene suspensions scarcely influences the absorption spectra of graphene nanosheets. Figure 2b shows the measured absorbance per length at the fixed wavelengths of 660nm as a semiquantitative measure of the dispersed graphene concentration (characterized by A/l=αC, where A/l is the absorbance per length, α is the extinction coefficient, and C is the concentration). The experimental result indicates that the concentration of graphene dispersions is strongly dependent on the volume fraction of ethanol in PVP-water solution and reaches maximum value at ethanol volume fraction of 20%, which is consistent with the phenomenon observed in Figure 2c. Figure 2d shows a representative low-magnification TEM image of the graphene flakes on a TEM grid. 8


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A large number of graphene flakes with lateral dimensions in the range of 0.2-1µm and some folded sheets (marked by arrows) are observed. And the measured concentration values of graphene dispersions with ethanol volume fractions ranging from 0 to 100 % are given in Table S1. So it can be concluded that solution composition is decisive on the exfoliation of graphite in micro-emulsion background. Besides to the effect of solution component on exfoliation of graphite, surfactant concentration should be another important factor. So we further investigated the effect of different experimental conditions including the surfactant concentration, initial graphite mass and the centrifugation rate on the graphene concentration in the resultant dispersions. As shown in Figure 3a, with the increase of the surfactant concentration, the graphene concentration initially increased and then decreased. The initial increase of the graphene concentration is due to the more available surfactant molecules that can be absorbed on the graphene surface. However, excess surfactants will be inclined to their self-assemble, leading to a decreased coverage on the graphene. Therefore the phase inversion process for effective exfoliation is suppressed. We subsequently note that increasing concentration of initial graphite at the fixed PVP concentration of 20 mg/mL results in the increased yield of graphene (Figure 3b). And a lower centrifugation speed contributes to a higher graphene concentration because larger graphene flakes are retained in the supernatant.35 Remarkably, the graphene concentration could be readily scaled up depending on the optimal surfactant and graphite concentration as well as the low centrifugation speed. And the yield has been increased to 1.93 mg/ml accordingly. In addition, the graphene dispersion is quite 9



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stable, as revealed by an only 7% decrease in its absorbance even after storage in ambient conditions for a month (Figure 3d). Morphological characterizations of exfoliated graphene TEM images were conducted to characterize the morphology of the graphene sheets. As shown in Figure 4, thin graphene sheets with lateral sizes ranging from hundreds of nanometers to several microns are presented. And the corresponding selected area electron diffraction (SAED) patterns of graphene present the typical six-fold symmetry lattices, suggesting a well-crystallized and undistorted graphene structure be retained during the exfoliation process. To further confirm the structure of graphene, the detailed TEM analysis of resultant dispersion is shown in Figure 5. Figure 5a is particularly interesting as the right side of the flake consists of at least two layers, whereas on the left side, a single monolayer protrudes. The selected area electron diffraction (SAED) pattern in Figure 5b exhibits a typical 6-folded symmetric diffraction. Moreover, the stronger diffraction from (0-110) plane than that from the (1-210) plane confirms a monolayer of graphene.36 HRTEM images of graphene shown in Figure 5d-f provide more detailed structural information. HRTEM image of a few-layered graphene nanosheet is supplied in Figure 5d, and inset of it is the Fast Fourier transform of the image. A filtered image of part of the region in the red square is presented in Figure 5e and f. Integrity and uniformity of the lattice structure of the exfoliated graphene confirm again that no defects and deformation were introduced by the exfoliation process. Figure 6a shows a TEM image of a large graphene sheet (3.2 µm), while from the 10


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AFM of graphene, we can also observe a graphene with lateral size up to 5µm. As a powerful method for precisely identifying the layer number by measuring the step height profile of the sample, detailed tapping-mode atomic force microscopy (AFM) images of graphene deposited on a mica substrate were presented in Figure 6b-e. It shows the minimum thickness of 0.75 nm, confirming the monolayer nature, which is comparable with the thickness of pristine graphene on a Si wafer.37And the majority of graphene flakes have a thickness of 0.7-2.1 nm, indicating the few-layered graphene flakes. However, many measured thicknesses of mono- and few-layered nanosheets are larger than the theoretical thickness values, which may be due to the residual surfactants that are absorbed on the graphene surface. The thickness distribution of 100 sheets calculated from the AFM height profile is presented in Figure 6f. Remarkably, more than 87.7% of the graphene nanosheets comprise thin graphene (≤3 layers), where single and bilayer graphenes are the dominant products (together 72.2%).Thus it indicates that this exfoliation method is advantageous. Structural analysis of exfoliated graphene Raman, X-ray photoelectron spectroscopy (XPS) and XRD were conducted to characterize the quality of the exfoliated graphene sheets, and the characterization results are shown in Figure 7. Figure 7a displays the Raman spectra of graphite and the few-layered graphene with excitation at 514 nm. The G band at about 1563 cm-1 is typically assigned to the E2g mode of sp2-hybridized carbon bonds, while D band at about 1334 cm-1 is associated with the breathing mode of k-point phonons of A1g symmetry, which was activated by defects such as edges, functional groups or 11



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structure disorders.38A mean ID/IG ratio of 0.49 , which is much smaller than those of chemically or thermally reduced GO ( 1.1~1.5 )39, may be due to the defects at the edges given the relatively smaller size of the graphene flakes.40 Moreover, a sharp and symmetric 2D band was observed, confirming the formation of monolayer graphene, while the others present characteristic peaks of bi- and few-layered graphene.38 X-ray photoelectron (XPS) was performed to determine the chemical composition of the as-prepared graphene (Figure 7b), which revealed the presence of C (93.47%), O (4.61%) and N (1.92%). The C 1s spectrum is dominated by a feature around 284.6 eV, which corresponds to graphitic carbon. Despite the presence of the weak signals for the C-N band (285.4 eV) and C=O band (287.7 eV) originating from the residual surfactant predominantly, the C/O ratio of 20.28 for graphene was significantly higher than those reported for rGO and other types of exfoliated graphene41-43, indicating a low level of oxidation of graphene. XRD of pristine graphite, exfoliated graphene and rGO are included in Figure 7c and d for comparison. For pristine graphite, it shows a sharp intense diffraction peak at 2θ=26.52° (d-spacing=0.34 nm), which associates with the (002) diffraction characteristic of the graphite structure.44 However, there is no shift in the peak position and d-spacing of (002) of graphene, indicating that the exfoliated graphene remained the original pristine structure. In comparison, the graphene prepared from GO reduction shows an increase in the d-spacing distance and it is due to a large numbers of defects. For our exfoliated graphene with PVP, a new wide diffraction peak can be found at 20.9°corresponding to an interlayer spacing of 0.43 nm, which demonstrates that PVP 12


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prohibits the re-aggregation of graphene.45 Exfoliation mechanism in emulsion microenvironment built by SC CO2 The study above indicates that graphene has been exfoliated in the emulsion microenvironment built by SC CO2 and at the same time it also confirms the excellent physical properties of the obtained two-dimensional crystals. Next, in order to supply the exfoliation mechanism of the graphene in the emulsion microenvironment created with assistance of SC CO2, control experiments on the exfoliation of graphite without surfactant or SC CO2 have been performed, respectively. The experimental results of the digital photographs of graphene dispersion are shown in Figure S6. In addition, the calculated concentrations of graphene under different control experimental conditions are also listed in Table 1. It can be judged that the synergic effect of surfactant and SC CO2 played essential roles in exfoliation of graphene. Next a series of high-pressure phase behavior experiment on the CO2/PVP/water system were conducted for the purpose of observing the reaction system of exfoliation process on graphite visually. The experimental results are shown in Figure S7. Figure S7 shows that when CO2 pressure was 6 MPa, there was no apparent phenomenon for graphite powder dispersed in the PVP-water solution. However, a large numbers of bubbles appeared in the aqueous solution when the CO2 pressure reached 8 MPa (Figure S7b), which is the signal of formation of CO2-in-water emulsions.46-48 Subsequently, with continuously increasing CO2 pressure to10, 12, 14 and 16 MPa, a growing number of black droplets floated in the continuous CO2 phase and the size of black droplets became smaller and smaller based on the visual 13



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inspection (FigureS7 c-f), indicating the formation of reverse micelles of water-in-CO2 emulsions.49,50 The phase behavior of emulsions microenvironment can be described according to hydrophilic/CO2-philic balance (HCB) theory (see the Supporting Information).51-53The HCB theory indicates that the inversion and stability of emulsions are strongly dependent on the phase behavior, interfacial tension (γ), and emulsion curvature.54And for supercritical CO2 system, the phase behavior could be effectively manipulated by changing of pressure or the ratio of water and ethanol, which is a convenient and effective route compared with conventional water-in-oil (W/O) emulsions. On the basis of the experimental results above, the possible mechanism is proposed for the exfoliation of graphite in the CO2/PVP/water system. First, an important premise is that because of the strong hydrophobic interaction between surfactant and graphite, they are regarded as a one unit.55,56As far as PVP is concerned, the hydrophilic amide groups of PVP point to the continuous water phase, whereas hydrophobic methylene chains direct towards the inner, and graphite exist in the hydrophobic interior of the micelles as shown in Figure 8a.57,58At the lower CO2 pressures, gaseous CO2 can dissolve into continuous water phase and can enter surfactant interfacial region on account of the interaction between CO2 and the surfactant59,suggesting the formation of CO2-in-water emulsions. When the system reaches the supercritical state, SC CO2 penetrates into the interlayers of graphite with high diffusivity, contributing to the expansion of the distance between adjacent layers and thus the decrease of interaction between them (Figure 8b).31And most importantly, 14


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with further addition of CO2, more CO2 insert into the hydrophobic core and swell the micelles’ core effectively, and simultaneously providing the driving force for the transition from normal micelles to reverse micelles (Figure8 b-d).60,61 So the emulsions became from CO2-in-water emulsions to water-in-CO2 emulsions. In the process of reverse micelles formation, the repulsive forces between hydrophilic amide groups and CO2 promote hydrophilic groups to aggregate together, resulting in curvature increases of the reverse micelles and formation of a water core.62Meanwhile, single- or few-layered graphene nanosheets are exfoliated from the surface of graphite via curvature transition of surfactant caused by the repulsive forces. When CO2 was released, water-in-CO2 emulsions containing reverse micelles transformed into normal micelle solution with water as a continuous phase (Figure 8e). And then a large amount of individual graphene sheets were obtained. Meanwhile, the surfactants adsorb onto the surface of exfoliated graphene flakes, providing steric stabilization against their restacking. Electrical properties of graphene ink-coated thin films. Graphene ink has the great potential for next-generation printable electronic including transistors, solar cells and supercapacitors.63,64To examine the electronic properties of as-prepared graphene flakes, a dispersible graphene “ink” was prepared by dispersing graphene in DMF with a concentration of 10 mg mL-1, and highly conductive graphene films were readily fabricated by brushing the as-prepared graphene ink on commercial paper (Figure 9a).The porous nature of paper provides a strong capillary force for graphene ink, enhancing solvent absorption and contributing to a conformal coating of graphene 15



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on paper(Figure S9c and d). Figure 9b presents the relationship between the paper resistance and the graphene loading amount. It can be observed that the resistance dramatically decreases with the loading of graphene. And a sheet resistance of 2.41Ω sq-1 was obtained with a graphene loading of 0.682 mg cm-2, which is much better than that of rGO films (43 KΩ sq-1).65 Besides this, it’s fascinating to find out that the value of sheet resistance is also lower than those of other types of exfoliated graphene when they were fabricated to films. For example, the sheet resistance of graphene exfoliated by microwave and the nitronium ion was 1000 Ω sq-1.43 And graphene-based paper exfoliated by electrochemical methods displayed a sheet resistance of 11Ω sq-1 with a graphene loading of 0.74 mg cm-2.37,66 Therefore, it confirms that our exfoliation method remains the integrity of the perfect structure in graphene to the largest extent, thereby producing graphene’s remarkable electrical properties. The electrochemical performance of the electrode materials was further investigated by cyclic voltammetry. This binder- and additive-free graphene paper-based supercapacitor presents typical double-layer capacitive behaviors at different scanning rates (Figure 9c and d). The area capacitance of the flexible supercapacitor was 19.52 mF cm-2 at the scanning rate of 5 mV s-1, which is much higher than that of thin-film rGO (0.394 mFcm-2).67 Remarkably, the electrode material was found to exhibit a perfect rectangular mirror image even at the scanning rate of 5000 mV s-1, indicating an outstanding rate capability and electrical performance. Graphene-beaded carbon nanofibers for supercapacitors. As electrode material, 16


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carbon fiber may have the low charge-transfer efficiency because of the insufficient fiber-fiber connection and relatively low specific surface area. Graphene, owing to its excellent electrical conductivity and large specific surface area, is expected to combine with CNFs to improve the conductivity and the capacitance. Adding graphene oxide to carbon fibers has been reported, but no one has ever attempted to use the liquid-exfoliated graphene. In this work, the flexible graphene/carbon fiber paper with only 5% of graphene was successfully prepared using electrospinning methods. It suggested that the presence of PVP can enhance the dispersion properties of exfoliated graphene flakes in both aqueous and organic solvents and facilitates incorporation of graphene into homogeneous nanocomposites with various polymers.68 The obtained graphene/CNFs displayed a necklace-like structure which is in favor of increasing the surface area and thereby enhancing the capacitive performance (Figure 10a and b). Figure10c shows the CV curves of the samples at the scan rate of 5 mV s-1. Both the pristine carbon fibers and Graphene/CNFs sample exhibit rectangular shapes, which is typical of the double-layer capacitive behavior. The specific capacitance of graphene/CNFs at the scanning rate of 5 mV s-1 was calculated to be 725.75 F/g, while the pure carbon fibers was only 429.25 F/g. Besides, the CVs of graphene/CNFs electrodes were found to be roughly rectangular shape at scanning rate ranging from 10 to 200 mV s-1, demonstrating a highly stable rate capability (Figure 10d).The specific capacitance of the graphene/CNFs electrodes based on the charge/discharge curve was calculated as 371.25 F/g, 71.6% higher than that of the pure carbon fibers (216.3 F/g). Further, with current density increasing 17



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from 0.5 A/g to 5 A/g, the specific capacitance of graphene/CNFs electrodes decreased slightly and still remained a high value of 325 F/g even at a high discharge current density up to 5 A/g, while the pure carbon fiber electrode was only 187.5 F/g (Figure 10f). The detailed comparison of the capacitance with those reported is presented in Supplementary Table S3. The excellent performance of graphene-beaded carbon fiber achieved in this work is superior to those reported for GO/CNFs on account of the relatively low electrical conductivity of GO.69 Further, compared to CNT with high electrical conductivity, our exfoliated graphene-beaded CNFs also display an apparent higher specific capacitance.70

Conclusion In summary, we have established a versatile and efficient method for producing few-layered graphene nanosheets in the emulsion microenvironment built by SC CO2. Our experimental results confirm that the driving forces originating from the phase inversion in the emulsion microenvironment efficiently facilitate the exfoliation of graphene. Compared with ionic surfactant, non-ionic surfactant such as PVP is a superior choice to induce exfoliation of graphite in SC CO2. This reported method effectively reduces the oxidation degree of graphene (C/O=20.28) and thereby significantly improves the chemical and electronic properties of graphene. Subsequently the conductive films fabricated by this graphene ink exhibited the resistance of 2.41Ω sq-1, which is the lowest value reported up to now for the graphene ink-coated film, indicating the perfect structure remained of the 2D 18


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nanosheets of graphene. Therefore it can be anticipated that this low-cost and environmental-friendly production of graphene will supply a new platform to scale-up fabrication of more two-dimension layered nanomaterials in the near future.

ASSOCIATED CONTENT Supporting Information Supplemented experimental details, UV/Vis absorption spectra of PVP, additional morphology characterization of the starting graphite powder and graphene flakes; the phase behavior of emulsions; Hydrophilic/CO2-philic balance (HCB) theory; SEM image of graphene-coated paper; summary of comparison of the specific capacitances of carbon fibers. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author: Qun Xu, Mailing Address: Da Xue road 75, Zhengzhou University; Zhengzhou, 450052, China; Fax: +86 371 67767827; Tel.: +86 371 67767827. E-mail: [email protected]

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the National Natural Science Foundation of China (Nos. 51173170, 21101141), the financial support from the Innovation Talents Award of Henan Province (114200510019) and the Key program of science and technology (121PZDGG213) from Zhengzhou Bureau of science and technology.

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Figure captions

Table 1. Concentration of the graphene samples of control experiments (CG,i=5 mg mL-1, CPVP=20 mg mL-1).

Figure 1. (a) Digital photographs of graphene suspensions exfoliated with various surfactants. (b) The left axis is the absorbance of the graphene suspensions corresponding to the surfactants. The right axis shows the dispersed graphene concentration calculated using α=1390 mL mg-1 m-1. (c) The structures of PVP surfactant.

Figure 2. (a) The typical absorption spectrum for the graphene. (b) The absorbance of graphene suspensions with different volume fractions of ethanol at the set wavelengths of 660 nm are shown as dots. (c) Photographs of graphene suspensions with ethanol volume fractions varied from 0 to 90 %, which have been stored under ambient conditions for a month. (d) A representative low-magnification TEM image of the graphene flakes.

Figure 3. Dependence of graphene concentration after centrifuging at 3000 rpm (a) on the surfactant concentration (CG,i=15 mg/mL). (b) on the initial graphite concentration. (c) Dependence of graphene concentration on centrifugation speed for the supernatant (CG,i=15 mg/mL, CPVP=20 mg/mL). (d) Absorbance as a function of time for a sample.

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(CG,i=3 mg/mL, CPVP=20 mg/mL).

Figure 4. TEM images of graphene nanosheets dispersed in PVP-water solutions with different volume fractions of ethanol (a) 0 % (b) 10 % (c) 20 % (d) 30 % (e) 40 %(f) 60 % (g) 80 % (h) 100%. The insets are the selected area electron diffraction (SAED) patterns of the corresponding graphene nanosheets.

Figure 5. (a) TEM image of a bi-layered graphene flake. (b-c) SAED pattern of a monolayer graphene flake. (d) HRTEM images of a few-layered graphene nanosheet. Inset: Fast Fourier transform of the image. (e, f) A filtered image of part of the region in the red square. (g) Schematic drawing of the atomic structure of graphene.

Figure 6. (a) TEM image of a relatively large flake of graphene. (b-e) Typical AFM images of exfoliated graphene nanosheets on mica surface. (f) Statistical thickness analysis of graphene by AFM.

Figure 7. (a) Raman spectra of graphite powder and individual graphene sheets. (b) XPS C1s spectra of graphene. (c, d) XRD patterns of graphite powder, rGO, graphene with PVP washed away and graphene with PVP.

Figure 8. A possible mechanism for exfoliation process of gaphene in the emulsion microenvironment. (a) CO2 dissolving in continuous water phase. (b-d) Expansion of 28


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micelles by CO2 entering graphite interlayers, and phase inversion of emulsions result in curvature transition of surfactant and formation of a water core. (e) Reverse micelles transform into normal micelles after CO2 is released.

Figure 9. (a) Photograph of graphene in DMF (10mg mL-1) used as ink and a piece of graphene-coated conductive paper. (d) Relationship between the resistances of paper with graphene loading. (e and f) Cyclic voltammetry of graphene paper-based supercapacitor (loading 0.682 mg cm-2) at scan rates from 5 to 100 mV s-1 and 200 to 5000 mV s-1, respectively.

Figure 10. (a) SEM micrograph of CNFs. (b) SEM micrographs of graphene/CNFs. Inset: the morphology of ‘graphene-bead’ in necklace-like graphene/CNFs. (c) CV of carbon fibers and Graphene/CNFs in 1 M H2SO4 solution at 5 mV s-1. (d) CV curves of Graphene/CNFs at different scan rates. (e) Galvanostatic charge–discharge curves of CNFs and Graphene/CNFs at current density of 0.5 A/g. (f) Specific capacitance values at different current density.

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Table 1. Concentration of the graphene samples of control experiments (CG,i=5 mg mL-1, CPVP=20 mg mL-1). Control experiments

Absorbance (A660nm)/cm-1

Concentration / mg mL-1

Experiment without SC CO2

0.314

0.023

Experiment without PVP

0.067

0.005

This work

1.908

0.137

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Figure 1. (a) Digital photographs of graphene suspensions exfoliated with various surfactants. (b) The left axis is the absorbance of the graphene suspensions corresponding to the surfactants. The right axis shows the dispersed graphene concentration calculated using α=1390 mL mg-1 m-1. (c) The structures of PVP surfactant.

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Figure 2. (a) The typical absorption spectrum for the graphene. (b) The absorbance of graphene suspensions with different volume fractions of ethanol at the set wavelengths of 660 nm are shown as dots. (c) Photographs of graphene suspensions with ethanol volume fractions varied from 0 to 90 %, which have been stored under ambient conditions for a month. (d) A representative low-magnification TEM image of the graphene flakes.

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Figure 3. Dependence of graphene concentration after centrifuging at 3000 rpm (a) on the surfactant concentration (CG,i=15 mg/mL). (b) on the initial graphite concentration. (c) Dependence of graphene concentration on centrifugation speed for the supernatant (CG,i=15 mg/mL, CPVP=20 mg/mL). (d) Absorbance as a function of time for a sample. (CG,i=3 mg/mL, CPVP=20 mg/mL).

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Figure 4.TEM images of graphene nanosheets dispersed in PVP-water solutions with different volume fractions of ethanol (a) 0 % (b) 10 % (c) 20 % (d) 30 % (e) 40 %(f) 60 % (g) 80 % (h) 100%. The insets are the selected area electron diffraction (SAED) patterns of the corresponding graphene nanosheets.

Figure 5. (a) TEM image of a bi-layered graphene flake. (b-c) SAED pattern of a monolayer graphene flake. (d) HRTEM images of a few-layered graphene nanosheet. Inset: Fast Fourier transform of the image. (e, f) A filtered image of part of the region in the red square. (g) Schematic drawing of the atomic structure of graphene.

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Figure 6. (a) TEM image of a relatively large flake of graphene. (b-e) Typical AFM images of exfoliated graphene nanosheets on mica surface. (f) Statistical thickness analysis of graphene by AFM.

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Figure 7. (a) Raman spectra of graphite powder and individual graphene sheets. (b) XPS C1s spectra of graphene. (c, d) XRD patterns of graphite powder, rGO, graphene with PVP washed away and graphene with PVP.

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Figure 8. A possible mechanism for exfoliation process of gaphene in the emulsion microenvironment. (a) CO2 dissolving in continuous water phase. (b-d) Expansion of micelles by CO2 entering graphite interlayers, and phase inversion of emulsions result in curvature transition of surfactant and formation of a water core. (e) Reverse micelles transform into normal micelles after CO2 is released.

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Figure 9. (a) Photograph of graphene in DMF (10mg mL-1) used as ink and a piece of graphene-coated conductive paper. (d) Relationship between the resistances of paper with graphene loading. (e and f) Cyclic voltammetry of graphene paper-based supercapacitor (loading 0.682 mg cm-2) at scan rates from 5 to 100 mV s-1 and 200 to 5000 mV s-1, respectively.

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Figure 10. (a) SEM micrograph of CNFs. (b) SEM micrographs of graphene/CNFs. Inset: the morphology of ‘graphene-bead’ in necklace-like graphene/CNFs. (c) CV of carbon fibers and Graphene/CNFs in 1 M H2SO4 solution at 5 mV s-1. (d) CV curves of Graphene/CNFs at different scan rates. (e) Galvanostatic charge–discharge curves of CNFs and Graphene/CNFs at current density of 0.5 A/g. (f) Specific capacitance values at different current density. 40


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TOC

Reverse-Micelle-Induced Exfoliation of Graphite into Graphene Nanosheets with Assistance of Supercritical CO2 Shanshan Xu, Qun Xu*, Nan Wang, Zhimin Chen, Qiuge Tian, Hongxia Yang, Kaixi Wang College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, P.R. China

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Reverse-Micelle-Induced Exfoliation of Graphite into Graphene

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