Production of Few-Layer Graphene via Enhanced High Pressure

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Production of Few-Layer Graphene via Enhanced High Pressure Shear Exfoliation in Liquid for Supercapacitor Applications Kun Zhang, Jie Tang, Jinshi Yuan, Jing Li, Yige Sun, Yorishige Matsuba, Da-Ming Zhu, and Lu-Chang Qin ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00515 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Nano Materials

Production of Few-Layer Graphene via Enhanced High Pressure Shear Exfoliation in Liquid for Supercapacitor Applications Kun Zhang,† Jie Tang,†, ‡,* Jinshi Yuan,║ Jing Li,†, ‡ Yige Sun,† Yorishige Matsuba,† Da-Ming Zhu,# Lu-Chang Qin,§ †

‡

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan

Doctoral Program in Materials Science and Engineering, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8577, Japan

║

College of Physics, Qingdao University, No. 308 Ningxia Road, Qingdao, 266071, PR China

#

Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City, MO 64110, USA §

Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA

KEYWORDS: few-layer graphene, high pressure fluid, enhanced shear exfoliation of graphite, supercapacitor, rate capability, cycling capability

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ABSTRACT: A novel method for producing graphene via enhanced shear exfoliation in a high pressure fluid has been developed. In the process of exfoliation, graphite flakes, dispersed in a solution of N-Methyl-2-Pyrrolidone (NMP), were pushed through the exfoliation tube by a high pressure pump, which produces high shear stresses on the graphite flakes due to the turbulent flow, causing the peeling-off of graphene sheets from the graphite flake. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images show that produced graphene sheets have thicknesses ranging from monolayer to about twelve layers. The results of X-ray photoelectron spectroscopy (XPS), Raman and infrared (IR)spectroscopy analyses reveal that neither basal-plane defects nor oxides were noticeably introduced to graphene sheets in the process. The graphite solution can be continuously treated in a circular loop. At a pressure of 100 MPa and after two hours of treatment, exfoliated graphene was obtained with a yield of about 15±0.3%. This method is highly efficient and could be facilely scaled up to process large quantities of materials either in batches or continuously. The exfoliated graphene thus produced has been used for preparing the electrodes of supercapacitors which showed a specific capacitance of 135 F/g, along with good cycling and rate capabilities in an aqueous solution of 6M KOH acting as the electrolyte.


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INTRODUCTION Ever since the successful isolation of mono-layer graphene and the demonstration of the feasibility of building electronic devices using such thin carbon layers more than a decade ago, the challenge has been to identify and develop a highly efficient method for producing high quality graphene in a low-cost and large-scale way.1 Over the past decade, various methods, such as the micromechanical cleavage,2 the thermal decomposition,3 the chemical vapor deposition growth,4, 5 and liquid phase exfoliation,6-11 have been proposed and tested. Among the various types of liquid phase exfoliation methods, the one that is based on the Hummers’ method12 which produces graphene oxide (GO) dispersed in solution followed by chemical or thermal reduction to convert GO to graphene, has been most widely adopted.8, 13 However, graphene obtained using this method always suffers from the difficulty in complete removal of undesirable residual functional groups (e.g. epoxides and hydroxyls), which damage the sp2 carbon network in graphene and deteriorate its electronic properties.13 Another liquid phase exfoliation method directly exfoliates graphite through sonication within a stable solvent or a surfactant solution.14-18 Such a method has enjoyed the success in yielding mono- or few-layer graphene sheets of high quality with fewer defects in the basal-planes.14,19 However, the method is limited in relatively small scale as scaling up of the sonication suffered from the requirements that a large amount of energy and power were needed for the process.20,21 The exfoliation of graphite in a liquid under shear was recently reported to be capable of producing graphene.22 It was also shown that a high-speed shear mixing method in liquid phase has the potential to be a scalable alternative for sonication as an exfoliating method for graphite and other layered materials.21,22 Using this method, it is feasible to produce graphene by exfoliation of graphite with volumes up to or even over hundreds of liters, as long as the shear



rate in the liquid surpasses 104 s-1. This method has been extensively investigated using a simple experimental configuration in which the shear stress was produced by a high speed mixer, of which the rotor/stator combination generates a Couette flow in a graphite dispersed solution. The graphene produced by this method has been found to be devoid of oxidation and basal-plane defect, and is almost indistinguishable from that produced by the sonication method.14,15,24-26 The results were quite astonishing as for exfoliation of graphite to occur, it requires only that the shear rate exceed certain threshold, and high speed shear mixing typically delivers the solution of the exfoliated material a power density which is only a small fraction of that delivered in sonication.21 However, one problem with this method is that exfoliation of graphite takes place only in the vicinity of the rotor/stator, limiting the production rate. To overcome this problem, attempts have been made to produce high shear rate throughout the entire liquid with approaches such as exfoliation of graphite using a kitchen blender with high speed rotating blades,27,28 a microfluidizer29,30 and a high pressure homogenizer.7,31-33 As a widely-used instrument in the food industry,34 the high pressure homogenizer may potentially be used for low cost and large scale graphene production. When using this device for exfoliation, the graphite dispersion is forced through a valve under high pressure. This process exerts high shear stress, cavitation and impact on graphite flakes.31,32 As reported by Arao et. al., graphene is mainly produced by the high shear stress generated as the graphite dispersion passes through the narrow valve gap on the scale of several millimeters.31 Meanwhile, cavitation occurs due to the pressure drop caused by sudden expansion of the flow when exiting the valve. Unfortunately, cavitation also tends to fragment rather than exfoliate graphite flakes, resulting in the production of graphene with small lateral size31,32 and pore defects in its 2D crystal structure.7,33 In this work, we explore an alternative yet facile method of enhanced shear exfoliation in a high


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pressure fluid to produce mono-/few-layer graphene. In Figure 1, a schematic illustration of the high pressure system is presented. The system consists of a solution container, a high pressure pump, and an exfoliation unit which is comprised of connected stainless steel tubes forming a closed circulation loop. The exfoliation unit is cooled using a cooling jacket containing cold water to prevent sample temperature from increasing. Exfoliation of graphite takes place in a 30 cm length exfoliation tube with a diameter of 0.2 mm. Compared to the very short flow path in the valves of high pressure homogenizers, the long exfoliation tube allows graphite flakes to be exposed to the shear stress for a longer time, which enhances efficient exfoliation. Without the use of valves, the flow passes through the exfoliation tube with constant pressure, restraining the cavitation. This helps produce graphene with larger lateral sizes and less defects. In the exfoliation process, an N-Methyl-2-Pyrrolidone (NMP) was used to disperse the pristine graphite and graphene since there is a good match between the surface tension of NMP and the Hansen solubility parameters of graphene.19,35 Although the graphite also can be exfoliated in surfactant or polymer aqueous solutions, the residual surfactant or polymer on the obtained graphene deteriorates its electronic properties and limits its applications.19,25,29 When the NMP solution with dispersed graphite flakes is pushed through the exfoliation tube by a high pressure pump which produces a high shear stress on the graphite flakes, leading to the peeling-off of graphene sheets from the graphite. The produced graphene sheets are stabilized by the interactions between NMP molecule and graphene.19 Graphite solution can be continuously treated in the circular loop. Under the conditions that we have tested so far, exfoliated graphene was obtained with a yield of about 15±0.3% which corresponds to a concentration of 1.5 ± 0.3 mg/mL in the solution. Morphologies and structures of the obtained graphene were examined by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy



(TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy analyses. Results confirmed that the graphene obtained is mono-/few-atomic layer thick without introducing noticeable basal-plane defects. Furthermore, electrochemical properties of the exfoliated graphene were also characterized when it is applied as the supercapacitor electrode materials.

Figure 1. A schematic illustration of the high pressure system employed for producing graphene by enhanced high pressure shear exfoliation.

EXPERIMENTAL SECTION Exfoliation of Graphite. Natural graphite flakes (BF-5A, ~5µm, purity: 99%) were used as received from Chuetsu Graphite Works Co., Ltd.. The 10 mg/mL graphite dispersion in NMP was passed through the exfoliation unit under a pressure of 100 MPa with a constant volume flow rate of 3.15×10-3 L/s. After a pre-determined time, the resulting dispersion was centrifuged for 30 min at 3000 rpm. The supernatant and solid residues were carefully separated and retained for characterization. To estimate the yield of exfoliated graphene, 20 mL of obtained supernatant 6 Environment ACS Paragon Plus

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was filtered through a polytetrafluoroethylene (PTFE) membrane filter (Omnipore, 0.2 µm pore size, 47mm diameter) using vacuum filtration. The exfoliated graphene on the filter was subsequently washed 3 times with 50 mL to remove the residual NMP. The mass of exfoliated graphene was measured after drying at 120 ℃ for 24 hours under a reduced pressure. To rule out the influence of the residual solvent on the mass of exfoliated graphene, thermal gravimetric analysis (TGA) was conducted by a TG-DTA 2000 (MAC Science Co.) under Ar flow.36 As shown in Figure S1, no appreciable weight loss was observed from room temperature to 800 °C, indicating successful removal of solvents during the drying process. Characterization of exfoliated graphene. To prepare samples for SEM analysis, ca. 20 µL of the dispersion was dropped onto a stainless steel plate and then heated by a hotplate at 120 ℃ to completely remove NMP. SEM measurements were performed using a JEOL7001F fieldemission SEM with an operating voltage of 15 kV. AFM samples were prepared by spin-coating the graphene dispersion on a Si wafer at 1000 rpm for 20 seconds and then baking it at 120 ℃ for 2 hours to remove any residual NMP. AFM measurements were conducted by a scanning probe microscope (JEOL JSPM–5200) using a tapping mode. TEM samples were prepared by dropping ca. 20 µL of the graphene dispersion onto a holey carbon mesh grid and drying under 120 ℃ in a vacuum oven. TEM images were observed by a JEOL 2100 TEM and a JEOL ARM200F TEM with LaB6 and cold-field emitters, respectively. High-resolution TEM analysis using the ARM200F TEM, was conducted at 80 kV with a CEOS aberration corrector. Absorption measurements were taken using a Shimadzu UV/Vis 3600 spectrophotometer. To prepare samples for XPS, FTIR and Raman spectroscopy analyses, the graphene dispersion was filtered through a PTFE membrane filter by vacuum filtration and dried under 80 ℃ in a vacuum oven. XPS measurements were performed by a ULVAC-PHI Quantera SXM using a twin anode Al Kɑ



x-ray source. Binding energies were calibrated with the C1s peak of aliphatic carbon at 284.5 eV. The data were fit by the XPSPEAk 4.1 software using an asymmetric peak shape model for the graphite and graphene C1s component with a Shirley background. FTIR analysis were performed on a JASCO FT/IR-6100. Raman measurements were carried out by a RAMAN-11 Raman microscope (Nanophoton) using a laser source with a wavelength of 532 nm. Electrochemical Measurement. To assemble the two-electrode test cell, the filtered graphene films on PTFE filter were cut into 15 mm circular disks, as we previously reported.37 The weight of each electrode was 2 mg. For cell assembly, one piece of PTFE filter paper, two Pt foils and 6M KOH were used as the separator, current collectors and electrolyte, respectively. Galvanostatic charge/discharge (GC) and cyclic voltammetry (CV) measurements were recorded with a VMP3 multi-potentiostat/galvanostat (Biologic) system.

RESULTS AND DISCUSSION In the exfoliation experiment, graphite flakes were pre-dispersed in 400 ml NMP with 10 mg/mL concentration and were subsequently transferred to the solution container in the high pressure system. The dispersion was then pushed through a 30 cm long exfoliation tube with a dimeter of 0.2 mm. The dispersion was continuously circulated at a pressure of 100 MPa for a pre-determined time period. To prevent solution temperature from increasing, the exfoliation tube was cooled using water maintained approximately at 5 ℃. After centrifugation of the resultant dispersion to separate the unexfoliated graphite, we obtain a dark dispersion of graphene. The homogenous dispersion of graphene was confirmed by the observation of the Tyndall effect (inset of Figure 2a) with a laser passing through the diluted dispersion. Although ~10% graphene was agglomerated after 2 weeks storage, the agglomerate can be easily


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redispersed by sonication for 1 min (Figure S2). In Figure 2a, the yield of exfoliated graphene is plotted against treatment time. The yield increases almost linearly with increasing treatment time and gradually levels off after about 100 minutes, approaching a yield of about 15±0.3% which corresponds to a concentration of 1.5±0.3 mg/mL for exfoliated graphene. This concentration is much higher than those reported in shear exfoliation of graphene using the rotor/stator mixer (~0.1 mg/mL)21 and the kitchen blender (~1 mg/mL).27 It should be mentioned that the yield of exfoliated graphene is almost constant with the increase of the initial graphite concentration from 5 mg/mL to 30 mg/mL (Figure S3).This result is in good agreement with those observed in other shear exfoliation methods.21,27 However, the yield decreased with further increases of the initial concentration of graphite. This may be explained as the equilibrium between dispersion and restacking of exfoliated graphene is broken with the increase of the concertation of graphene and move toward the direction where the exfoliated graphene is more likely to restack. It was also found that the percent yield of exfoliated graphene increased from 0.9±0.35 % to 15±0.3% when the applied pressure increased from 25 MPa to 100 MPa (Figure S4), implying a direct relationship.



Figure 2. (a) Yield of exfoliated graphene over treatment time with a pressure of 100 MPa. The error bars show the standard deviation of five independent experimental runs. (b) Schematic illustration of production of graphene by enhanced high pressure shear exfoliation.

The exfoliation mechanism, illustrated in Figure 2b, provides an explanation for the high efficiency of this method. Paton et al. demonstrated that graphene is producible in both laminar and turbulent flow as long as the local shear rate reaches 104 s-1.21 To estimate the flow characteristics of the high pressure fluid flowing through the exfoliation tube, the Reynolds number, Re, was calculated by Re =ρuD/µ ( ρ represents the fluid density, u represents the flow velocity, D represents the tube diameter, and µ represents the dynamic viscosity of the fluid).38 With a pressure of 100 MPa, it gives an Re of 1.24 × 104, indicating a fully developed turbulent flow inside the tube (see the supporting information). The turbulent shear rate γ̇ is determined by


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the energy dissipation rate and can be calculated via γ̇= ඥߝߩ/ߤ (where ε represents energy dissipation rate per unit mass). For the fluid with a Re of 1.24 × 104, the γ̇ is about 1.6 × 107 s-1, (Supporting Information) which is about 103 times higher than the critical value required for exfoliation of graphite.21 Furthermore, the high shear force is uniformly applied to all graphite flakes, leading to exfoliation of graphite throughout the exfoliation tube. In contrast, in a high shear rotor/stator mixer or a rotating blade mixer, a lower γ̇ ranging from∼ 2 × 104 to 1 × 106 s-1 is only obtainable near the rotor/stator probe18,29 or the rotating blade.24 As a result, exfoliation takes place only in a localized area, indicating a limited efficiency in exfoliation of graphite. Thus, our method can lead to a high efficiency in producing few layered graphene sheets and is superior to other shear exfoliation methods using rotor/stator mixers or rotating blade mixers. In order to analyze the exfoliated graphene produced by the above described method, we carried out SEM measurements. Figure 3a and Figure S5a,b are typical SEM images of the pristine graphite flakes with lateral dimension of ca. 5 µm and thickness < 300 nm. In comparison, the SEM images of the exfoliated graphene (Figure 3b and Figure S5c), obtained after treatment of the pristine graphite for 120 minutes under a pressure of 100 MPa, show that graphene sheets have lateral dimensions varying between several hundred nanometers and 1~2 µm. The thickness of exfoliated graphene sheets was characterized using AFM. Figure 3c is a typical AFM image of the exfoliated graphene, showing flakes of submicrometer size similar to those observed by SEM. Figure 3d is a line profile of the graphene sheets along that indicated in Figure 3c. The thickness of the graphene sheets is about 0.8 nm, consistent with the thickness of monolayer graphene on a Si wafer.2,16,39 As displayed in Figure S6, graphene sheets consisting of two layers, three layers and multilayers are also observed in solution after the exfoliation process.



These results suggest that the exfoliated graphene sheets obtained in this work have typical thicknesses of several atomic layers.

Figure 3. SEM image of (a) pristine graphite flakes and (b) exfoliated graphene sheets after centrifugation. (c)AFM image of exfoliated graphene spin-coated onto a Si wafer taken in tapping mode and (d) the corresponding height profile. To further examine the structure of exfoliated graphene sheets obtained using our method, TEM imaging was performed on the graphene samples. Figure 4a shows a representative TEM image of many exfoliated graphene sheets, revealing that the graphene sheets have irregular shapes with features similar to graphene produced by the sonication exfoliation in solvents.14,15,24-26 Figure 4b shows a mono-layer graphene sheet overlapping with several other graphene sheets. The partial stacking is resulted from the evaporation of solvent in the sample preparation process for TEM imaging. To determine whether or not the graphene sheet is single atomic layer, its electron diffraction pattern can be analyzed. As displayed by the inset of Figure 4b, the electron diffraction pattern acquired from a selected area on the graphene sheet denoted 12 Environment ACS Paragon Plus

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by the circle shows a typical six-fold symmetry. The observation of a stronger intensity in the inner set {1100} of spots versus the secondary set {2110} (as shown in Figure S7, the ratios of the intensity of {1100} and {2110} diffraction peaks are between 2.5 to 5.5), confirms that the graphene sheet consists of single atomic layer of carbon.14 Using the aberration-corrected highresolution TEM (HRTEM), a direct determination of the atomic structure of the exfoliated graphene was also performed. Figure 4c is an unprocessed HRTEM image taken from the edge of monolayer graphene in Figure 4b. The power spectrum of the HRTEM image which shows again the fingerprint of monolayer graphene with strong {1100} spots and weak {2110} spots25,40 is also shown in the inset of Figure 4c. Defect-free hexagonal arrangement of carbon is observed in the atomic resolution lattice image of exfoliated graphene, indicating that no defect is introduced into the 2D plane of the graphene during this exfoliation process. To find the thickness distribution of the exfoliated graphene sheets, we randomly picked 120 graphene sheets to perform more extensive characterization and analysis through TEM imaging. The number of graphene layers can be counted from the edge of each sheet as shown in Figure 4d-g. Figure 4h is the histogram of the distribution which reveals that the sheet thickness varies from 1 to 12 layers with an averaged thickness to be 5 atomic layers. Among them, the number fraction of mono-layer graphene is around 5%. These results prove the enhanced shear exfoliation in a high pressure fluid as an effective method to produce graphene sheets with mono- or few-atomic layers.



Figure 4. (a) TEM image of the obtained few-layer exfoliated graphene sheets. (b) TEM image of a monolayer graphene sheet. Inset: electron diffraction pattern of graphene sheet acquired from a selected area marked by the circle. (c) HRTEM image of a monolayer graphene sheet. Inset: power spectrum of the image. (d-g) HRTEM images of exfoliated graphene sheets with two, three, five and seven atomic layers. (h) A histogram of the layer counts of exfoliated graphene sheets. The chemical composition analyses of the exfoliated graphene were also performed using XPS measurements. The spectrum of the exfoliated graphene (Figure 5a) shows a strong C1s peak (~284.5 eV) and a weak O1s peak (~532 eV) with a C/O ratio of 96.1/3.9. Fitting the high resolution spectrum of C1s (Figure 5b and Table S1,) shows a major peak at 284.6 eV, corresponding to the C-C bonds, and two minor peaks at 285.5 and 286.5 eV, corresponding to the C-O-R and C=O groups, respectively.41,42 This is similar to the XPS spectrum of pristine graphite which shows a C/O of 96.4/3.6, indicating that oxidation of graphene sheets does not


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take place during the high pressure exfoliation process. This result is also confirmed by the Fourier transform infrared (FTIR) spectrum (Figure S8) of the obtained graphene which is nearly identical to that of the pristine graphite.

Figure 5. (a) XPS survey spectra and (b) deconvoluted high-resolution C1s spectra of exfoliated graphene sheets (bottom) and pristine graphite (top).



Raman spectroscopy of the exfoliated graphene sheets was also conducted. The spectrum of pristine graphite, as shown in Figure 6 (top), has two main bands at 1580 cm-1 (G peak) and 2714 cm-1 (2D peak), and a weak band at 1355 cm-1 (D peak).43 The D peak indicates the presence of basal-plane defects and/or edges. The weak D peak in the pristine graphite is attributed to the edges of graphite.43 Figure 6 (bottom) shows the spectrum of a graphene film prepared by vacuum filtration of the obtained exfoliated graphene sheet dispersion. The spectrum of the exfoliated graphene is similar to that of graphite but with a significant increase in the D peak intensity, indicating additional defects were introduced during the exfoliation process. To quantitatively assess these defects, we calculated the intensity ratio of the D peak and the G peak (ID/IG) of exfoliated graphene and find it to be 0.54 ± 0.03 (Figure S9). This value is similar to that of a typical graphene sheet with a lateral size of several hundred nanometer.14 This result suggests the increase of D peak is due to generation of additional edges, which suggests that exfoliation processes break the graphene sheets to pieces of smaller size as compared to those obtained in mechanical sonication of graphite in solvents.14,15,24-26 In order to estimate the formation of basal-plane defects, the intensity ratio of the D peak and the D’ peak (appearing as a shoulder on the right of the G peak at 1620 cm-1) is calculated in Figure S10. According to the findings of Eckmann,44 the ratio of ID/ID’ provides indications the type of defects present in graphene sheets. The edge defects, vacancy-like defects, and sp3 defects have the values of ID/ID’ of 3.5, 7, and 13, respectively. As shown in Figure S9, the exfoliated graphene gives a ID/ID’ of 3.7 which is very close to that reported by Eckmann44 (3.5), and by Coleman14 (~4). These results highly imply that the high pressure exfoliation does not add extra basal-plane defects into the produced graphene.


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Another important feature in the Raman spectra is the change of the 2D peak of exfoliated graphene. As shown in figure 6(top), the 2D peak of pristine graphite shows a peak at 2714 cm-1 and a shoulder at 2693 cm-1, which is characteristic of a stacked structure. However, the 2D peak in the exfoliated graphene show a symmetrical shape with a slight shift to low frequency (2697 cm-1). This feature strongly suggests that majority of the obtained graphene sheets are mono/few-layer in thickness.45,46 Being consistent with the AFM and TEM results, the results of Raman spectroscopy measurements again confirm that the graphene obtained from high pressure exfoliation is mono-/few-atomic layer thick without introduction of basal-plane defects.

Figure 6. Raman spectra of exfoliated graphene sheets (bottom) and pristine graphite (top). To explore the application of exfoliated graphene as supercapacitor electrode materials, twoelectrode symmetrical supercapacitors were assembled using filtered graphene films. Figure 7a show the cyclic voltagrams (CVs) of the graphene supercapacitor using a 6M KOH solution as the electrolyte at different scan rates. The rectangular shape of the CVs indicates the typical double-layer capacitive behavior of this graphene supercapacitor even at a scan rate of



3000 mV/s. The electrical conductivity of the graphene film is determined to be 10570 S/m, which is comparable to that of exfoliated graphene produced by other methods.28,29,47-49 From the GC curves, the specific capacitance of the graphene electrode was calculated to be 135 F/g with the constant current density of 0.1 A/g (Figure 7b). This value is greater than previously reported values (Table S2) of the exfoliated graphene based supercapacitors.50-55 The specific capacitance of the graphene at various charge/discharge current densities is plotted in Figure 7c. When the current density is increased to 50 A/g, the specific capacitance remains as high as 95 F/g, corresponding to 70% of the specific capacitance at 0.1 A/g, which suggests a good rate performance. Figure 7d displays the cycling performance of the exfoliated graphene electrode within an aqueous 6M KOH electrolyte at a current density of 1 A/g. As can be observed, almost 100% of its initial specific capacitance was preserved after 10000 cycles. The superior performance of the cycling stability is ascribed to the structural perfection (less defects and no additional oxidation) of the exfoliated graphene sheets. These results illustrate that the exfoliated graphene, prepared by the high pressure exfoliation process, is a promising material for the supercapacitors.


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Figure 7. Performance of graphene supercapacitor with an aqueous solution of 6M KOH as the electrolyte. (a) CV curves at scan rates between 10 and 3000 mV/s. (b) GC curves at current densities varying from 0.1 to 50 A/ g. (c) Specific capacitance at various discharge current densities. (d) Cycling performance of the supercapacitor.

CONCLUSIONS We have developed a novel method which uses high pressure shear exfoliation in fluid to produce mono-/few-layer graphene. We have characterized the graphene produced using techniques including SEM, TEM, AFM, XPS, IR, and Raman spectroscopy. Our results reveal that the thickness of graphene sheets produced is in the range from monolayer to twelve atomic



layers, with the averaged thickness being five atomic layers. The high pressure exfoliation process does not introduce noticeable defects or additional oxidation in the graphene sheets. The main advantages of this method are its high efficiency and the controlled quality of exfoliated graphene sheets obtained. Furthermore, the method could be smoothly scaled up for processing large amounts of graphite dispersed in fluids either in batch or continuously, and thus has the potential for being implemented for future large scale industrial production. When exfoliated graphene is applied as the supercapacitor electrode material, it shows a specific capacitance of 135 F/g with good cycling and rate capabilities in a 6M KOH aqueous solution as the electrolyte.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Dispersion stability of exfoliated graphene in NMP; Estimation of the flow characteristics of the high pressure fluid; SEM measurements of pristine graphite and exfoliated graphene; AFM measurements of exfoliated graphene; HRTEM measurements of exfoliated graphene; IR spectroscopy measurements of pristine graphite and exfoliated graphene; XPS spectroscopy analysis of exfoliated graphene

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This study was financially supported by JSPS Grants-in-aid for Scientific Research (No. 22310074), JST ALCA program, and JSPS KAKENHI Grant (No. JP17K14089). A part of this study was supported by the NIMS Microstructural Characterization Platform as a program of the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Da-Ming Zhu is supported by a Fellowship from JSPS.

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TOC



Figure 1. A schematic illustration of the high pressure system employed for producing graphene by enhanced high pressure shear exfoliation. 66x52mm (300 x 300 DPI)

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Figure 2. (a) Yield of exfoliated graphene over treatment time with a pressure of 100 MPa. The error bars show the standard deviation of five independent experimental runs. (b) Schematic illustration of production of graphene by enhanced high pressure shear exfoliation. 100x118mm (300 x 300 DPI)



Figure 3. SEM image of (a) pristine graphite flakes and (b) exfoliated graphene sheets after centrifugation. (c)AFM image of exfoliated graphene spin-coated onto a Si wafer taken in tapping mode and (d) the corresponding height profile. 110x81mm (300 x 300 DPI)


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Figure 4. (a) TEM image of the obtained few-layer exfoliated graphene sheets. (b) TEM image of a monolayer graphene sheet. Inset: electron diffraction pattern of graphene sheet acquired from a selected area marked by the circle. (c) HRTEM image of a monolayer graphene sheet. Inset: power spectrum of the image. (d-g) HRTEM images of exfoliated graphene sheets with two, three, five and seven atomic layers. (h) A histogram of the layer counts of exfoliated graphene sheets. 84x47mm (300 x 300 DPI)



Figure 5. (a) XPS survey spectra and (b) deconvoluted high-resolution C1s spectra of exfoliated graphene sheets (bottom) and pristine graphite (top). 166x324mm (300 x 300 DPI)


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Figure 6. Raman spectra of exfoliated graphene sheets (bottom) and pristine graphite (top). 79x79mm (300 x 300 DPI)



Figure 7. Performance of graphene supercapacitor with an aqueous solution of 6M KOH as the electrolyte. (a) CV curves at scan rates between 10 and 3000 mV/s. (b) GC curves at current densities varying from 0.1 to 50 A/ g. (c) Specific capacitance at various discharge current densities. (d) Cycling performance of the supercapacitor. 112x83mm (300 x 300 DPI)


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Production of Few-Layer Graphene via Enhanced High Pressure

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