Article Cite This: ACS Appl. Nano Mater. 2018, 1, 2877−2884
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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⊥ †
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, P. R. China ∥ Department of Physics and Astronomy, University of MissouriKansas City, Kansas City, Missouri 64110, United States ⊥ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3255, United States
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S Supporting Information *
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 Nmethyl-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 12 layers. The results of X-ray photoelectron spectroscopy (XPS) and 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 2 h 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 6 M KOH acting as the electrolyte. KEYWORDS: few-layer graphene, high-pressure fluid, enhanced shear exfoliation of graphite, supercapacitor, rate capability, cycling capability
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INTRODUCTION Ever since the successful isolation of monolayer 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 micromechanical cleavage,2 thermal decomposition,3 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’ method,12 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 of 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 © 2018 American Chemical Society
method directly exfoliates graphite through sonication within a stable solvent or a surfactant solution.14−18 Such a method has enjoyed 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.22,23 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 Received: March 28, 2018 Accepted: June 4, 2018 Published: June 4, 2018 2877
DOI: 10.1021/acsanm.8b00515 ACS Appl. Nano Mater. 2018, 1, 2877−2884
Article
ACS Applied Nano Materials
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 fewer defects. In the exfoliation process, an N-methyl-2pyrrolidone (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 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 the NMP molecule and graphene.19 The 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-atomiclayers 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 material.
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 basalplane defects, 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 is only required that the shear rate exceed a 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 a 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 microfluidizer,29,30 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 largescale graphene production. When this device is used 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 because of 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-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 composed of connected stainless steel tubes forming a closed circulation loop. The exfoliation unit is cooled using a cooling jacket containing cold
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EXPERIMENTAL SECTION
Exfoliation of Graphite. Natural graphite flakes (BF-5A, ∼5 μm, 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 predetermined time, the resulting dispersion was centrifuged for 30 min at 3000 rpm. The supernatant and solid residues were carefully separated and retained for characterization. For an estimation of the yield of exfoliated graphene, 20 mL of obtained supernatant was filtered through a polytetrafluoroethylene (PTFE) membrane filter (Omnipore, 0.2 μm pore size, 47 mm 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 °C for 24 h under a reduced pressure. To rule out the influence of the residual solvent on the mass of exfoliated graphene, a thermal gravimetric analysis (TGA) was conducted by a TG-DTA 2000 (MAC Science Co.) instrument 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. For the preparation of samples for SEM analysis, ca. 20 μL of the dispersion was dropped onto a stainless steel plate and then heated by a hot plate at 120 °C to completely remove NMP. SEM measurements were performed using a JEOL7001F field-emission SEM with an operating voltage of 15 kV. AFM samples were prepared by spin-coating the graphene dispersion
Figure 1. A schematic illustration of the high-pressure system employed for producing graphene by enhanced high-pressure shear exfoliation. 2878
DOI: 10.1021/acsanm.8b00515 ACS Appl. Nano Mater. 2018, 1, 2877−2884
Article
ACS Applied Nano Materials on a Si wafer at 1000 rpm for 20 s and then baking it at 120 °C for 2 h 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 °C in a vacuum oven. TEM images were observed by a JEOL 2100 TEM and a JEOL ARM200F TEM instrument with LaB6 and coldfield 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. For the preparation of 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 °C in a vacuum oven. XPS measurements were performed by a ULVAC-PHI Quantera SXM using a twin anode Al Kα X-ray source. Binding energies were calibrated with the C 1s peak of aliphatic carbon at 284.5 eV. The data were fitted by XPSPEAk 4.1 software using an asymmetric peak shape model for the graphite and graphene C 1s component with a Shirley background. FTIR analyses were performed on a JASCO FT/IR-6100 instrument. Raman measurements were carried out by a RAMAN-11 Raman microscope (Nanophoton) using a laser source with a wavelength of 532 nm. Electrochemical Measurement. For assembly of the twoelectrode 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 6 M 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 multipotentiostat/galvanostat (Biologic) system.
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.
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RESULTS AND DISCUSSION In the exfoliation experiment, graphite flakes were predispersed in 400 mL of NMP with 10 mg/mL concentration and were subsequently transferred to the solution container in the highpressure system. The dispersion was then pushed through a 30 cm long exfoliation tube with a diameter of 0.2 mm. The dispersion was continuously circulated at a pressure of 100 MPa for a predetermined time period. For the prevention of solution temperature from increasing, the exfoliation tube was cooled using water maintained approximately at 5 °C. After centrifugation of the resultant dispersion to separate the unexfoliated graphite, we obtain a dark dispersion of graphene. The homogeneous 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 of storage, the agglomerate can be easily 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 min, 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 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 being broken with the increase of the concentration of graphene and moving 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 to 100 MPa (Figure S4), implying a direct relationship. 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 For an estimation of the flow characteristics of the highpressure 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 γ′ is determined by the energy dissipation rate and can be calculated via γ′ = ερ /μ (where ε represents energy dissipation rate per unit mass). For the fluid with a Re of 1.24 × 104, the γ′ 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 γ′ 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. 2879
DOI: 10.1021/acsanm.8b00515 ACS Appl. Nano Mater. 2018, 1, 2877−2884
Article
ACS Applied Nano Materials
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) corresponding height profile.
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) Histogram of the layer counts of exfoliated graphene sheets.
To analyze the exfoliated graphene produced by the abovedescribed 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
