Highly efficient production of graphene by an ultrasound coupled with

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Materials and Interfaces

Highly efficient production of graphene by an ultrasound coupled with a shear mixer in supercritical CO2 Wucong Wang, Yanzhe Gai, Ningning Song, Ding Xiao, Huijun Tan, and Yaping Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04113 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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Industrial & Engineering Chemistry Research

Highly efficient production of graphene by an ultrasound coupled with a shear mixer in supercritical CO2 Wucong Wang, Yanzhe Gai, Ningning Song, Ding Xiao, Huijun Tan, Yaping Zhao * School of chemistry and chemical engineering, Shanghai Jiaotong University, Shanghai, China *Corresponding author. Tel: 0086 021-54743274 E-mail: [email protected] Abstract A novel approach of exfoliation graphite into graphene using an ultrasound coupling with a shear mixer in supercritical CO2 was presented in this paper. The influence of the ultrasound power and the shearing speed on the exfoliation efficiency was systematically studied, and the optimal process parameters were achieved. In comparison with either the ultrasound or the shearing method, the coupling approach obtained higher yield which was up to 82.6%, and the layer number of less than 3 layers was about 60%. The electrical conductivity of the film formed by the obtained graphene was up to 1.18 × 106 S/m. A synergy exfoliation mechanism was proposed: the ultrasound activated the edges of the graphite generating the gap between the graphite layers, and the shear mixer peeled off the graphene layer from the gap simultaneously. The coupling approach of the ultrasound and the shear mixer enable the production of the graphene largely. 1. Introduction Graphene, a two-dimensional honeycomb lattice arranged by sp2-bonded carbon atoms 1-2, has ignited a sheer revolution across the multiple disciplines of science and technology

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due to its fascinating electronic, optical, thermal and mechanical properties 3-6. However, there are still few reports on the integration of graphene into real-life applications, and an efficient approach to produce graphene in large quantities is urgently required. To date, various methods have been developed to prepare graphene, including mechanical cleavage1, chemical vapor deposition7-9, epitaxial growth10-11, reduction of graphite oxide12, and liquid-phase exfoliation13-14. Among these approaches, the liquid-phase exfoliation method is a simple way to obtain high-quality graphene, but the yield is too small 15. The aqueous solution with the surfactant exfoliation

16-20

is a conventional method

because the surfactants can prevent the graphene from agglomerate and restack. However, the surfactant is hard to be wiped out and is limited to the application. Though organic solvent exfoliation

21-22

can produce defect-free few-layer graphene with large quantities

based on the Hansen solubility parameters, most of the organic solvents used are toxic, expensive and have a high boiling point, which brings a severe problem to the application of the graphene. Supercritical solvent extraction has been used as an interactive medium to exfoliate graphite

23

in which the supercritical CO2 show excellent intercalative

medium due to its low critical point 24, small molecule size 25, and without residue. A lot of work has been done on the exfoliating efficiency in supercritical CO2 20, 26-28. One of the key points to the graphene exfoliation in supercritical CO2 was the energy input mode. There were two main energy input modes, ultrasound 27 and shear mixing 26. In previous work, researchers found that improving the input energy would enhance the exfoliation


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efficiency

29.

However, the input energy could not be immortally increased due to the

limitation of the equipment resulting in low exfoliation efficiency. The characteristic of the two energy input modes are different in that the ultrasound provides pitting energy

30,

and the shear mixing offers a region with continuous shear

force. Given the features of the ultrasound and the shear mixer, we propose a strategy to combine the ultrasound and shear mixer to increase the exfoliation efficiency via their synergetic effect for the first time. The influence of the ultrasound power and the shearing speed on the exfoliation efficiency was systematically studied. The coupling of the two modes not only increases the input energy linearly but also creates a distinct synergy. The exfoliation efficiency made by the coupling mode is much higher than the linear superposition of the ultrasound and shear mixer. It might be attributed to the synergy exfoliation: the ultrasound is good at exfoliating the edge of the graphite partly, and the shear mixing is good at peeling of the graphene layer from the partially flaked side. 2. Experimental section 2.1. Materials. Graphite powder (99.99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon dioxide (99.90%) was purchased from Shanghai Hightech Co., Ltd. Absolute ethanol (99.50%) was purchased from Changshu Yangyuan Chemical Co., Ltd. 2.2. Exfoliation of graphene A schematic drawing of the exfoliation system is shown in Figure. 1. The system mainly



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consists of a gas cylinder, a chiller, a pump and a reactor with ultrasound and speed-adjustable shear mixer. The gap between the stator and rotor is 100 µm, and the speed is 0-6000 r/min. The volume of the reactor, made from stainless steel 304, is 300 mL. The temperature of the reactor is controlled by an electric heating sleeve. In the specific experiment, 100 mg of graphite was put into the reactor, and carbon dioxide was pumped into the reactor by a pump. When the pressure and the temperature reached preset values, the ultrasound and the shear mixer were turned on.

During the exfoliation,

the pressure and the temperature maintained at 12 MPa and 40°C, respectively (These parameters were selected according to the pre-experiments discussed in Supporting Information.). After the sample was exfoliated in the desired time, the CO2 was exhausted slowly, and the product was collected from the reactor for characterization.

Figure 1. Schematic drawing of the exfoliation apparatus 2.3. Characterization. 10 mg of the exfoliated samples were ultrasonically dispersed in 100 ml ethanol for 20


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min. Then the dispersion liquid was centrifuged for 30 min under the rotate speed of 500 r/min. The supernatant was taken out to calculate the yield. The yield of the graphene was measured by the spectroscopy method according to the Lambert-Beer law, A/l = αC. Herein, A/l (m-1) is the absorbance per cell length measured at 660 nm by a UV–vis spectrophotometer, and α is the absorption coefficient, which is 1639 mL mg-1 m-1 (Method of measuring absorption coefficient can be seen in the Supporting Information). The exact value of the graphene concentration C (mg/mL) was obtained through gravimetric analysis. The yield of the graphene was calculated by the formula: Y (wt%) = (CV/m)×100%, where V is volume and m is initial graphite mass. The morphology of the prepared graphene was recorded by transmission electron microscope (TEM) using JEOL JEM-2100 TEM (Japan) at an acceleration voltage of 120 kV and scanning electrode microscope (SEM) using an FEI Nova NanoSEM 450 (USA). Atomic force microscopy (AFM) images were obtained using Dimension Icon & FastScan Bio AFM (USA). The X-ray diffraction (XRD) recorded on a BRUKER D8 Advance X-Ray Diffractometer (Germany) using a CuKα radiation source (λ = 1.5418Å). The Raman spectroscopy was recorded on a Raman Microscope excitation (USA) with a wavelength of 532 nm. 3. Results and discussion 3.1. Effects of ultrasound, shear mixing, and their combination



Figure 2. Effects of process parameters on the graphene yield at different reaction time. (a) ultrasound, shear mixing, and their combination, (b) ultrasound power, (c) shearing speed. In order to investigate the exfoliation efficiency of the method of coupling an ultrasound and a shear mixing, we designed a series of experiments to examine the influence of the ultrasound power, the shearing speed and the processing time on the yield. Firstly, we compared the exfoliation efficiency made by the coupling method with that by the ultrasound, and the shear mixer applied separately. When the power of the ultrasound and the shearing speed were fixed at 250 W and 2000 r/min, which were selected according to the pre-experimental results as described in the Supporting Information, the influence of the exfoliation time on the yield is shown in Figure 2a. It can be seen that the yield


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increased with increasing time. When only the ultrasound was applied, the yield was 2.1% at 30 min and rose slightly to 3.5% at 120 min. It suggests that though the ultrasound can exfoliate the graphite to the graphene, the efficiency was low under this condition. When only the shear mixer was used, the yield was 4.9% at 30min and increased to 12.9% at 120 min. The shear mixer is more efficient than the ultrasound. However, the yield made by the coupling method was 29.7% at 120 min, which is near twice the sum of the yield produced by the ultrasound and the shear mixer. Moreover, not only at 120 min, the yield made by the coupling method at each reaction time was much more than the linear sum produced by the ultrasound and the shear mixer. The yield made by the coupling method reached 10.5% at 30 min and increased by about 6.5% in the following each 30 min. This result indicates that there was a distinct synergy during the exfoliating when the ultrasound and the shear mixer worked together. The ultrasound and the shear mixer may play different roles during the exfoliation, and their contribution mutually promoted the exfoliation when being used at the same time. The mechanism will be discussed in Section 3.4. Then, we further investigated the influence of the ultrasound power, the shearing speed and the reaction time on the yield made by the coupling method. Figure 2b exhibits that the impact of the reaction time on the yield at the different ultrasound powers of 125 W, 250 W, and 500 W, respectively, when the shearing speed was fixed at 2000 r/min. The yield for the three powers was about 10% at 30min. In the next 90min, the yield for the 250 W and 500 W, respectively, increased nearly the same and reached approximately 30%



at 120min. For the 125 W, the yield increased by about 4% in the following each 30min and reached 20.7% at 120min. This result indicates that the influence of the ultrasound power on the yield was not very obvious, and the 500 W was superfluous for this condition. The reason for this observation will be discussed in Section 3.4. Thus, in the following work, the ultrasound power was fixed at 250 W. Figure 2c exhibits that the influence of the reaction time on the yield at the different shearing speed of 2000 r/min, 3000 r/min, and 4000 r/min, respectively, and at the fixed 250 W. The yield was 10.5% at 30min and increased about 6% in the following each 30 min when the shearing speed was 2000 r/min. When the shearing speed increased, the yield increased obviously. The yield for the 3000 r/min and 4000 r/min was 20% and 26%, respectively, at 30min and rose about 9% and 13% respectively in the following each 30 min. The results show that higher shearing speed generated higher exfoliating efficiency. We did not try a higher rate than 4000 r/min because of the limitation of the equipment. Extending the reaction time can increase the yield which reached around 80% at 150 min under the shearing speed of 4000 r/min. However, the yield could not increase obviously in more extended reaction time. We conjecture that 80% might be the maximum value under these conditions. The reason was discussed in Supplementary Information. 3.2 Characterization of graphene sheets Figure 3 exhibits the SEM images of the raw graphite and the exfoliated graphene. The exfoliated graphene was obtained at the optimal conditions of 250 W, 4000 r/min, and


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150 min. It can be seen that the raw graphite appeared regular shape (Figure 3a), while the exfoliated graphene became shattering (Figure 3b). The high magnification image (Figure 3c) indicates that the exfoliated graphene sheets were contorted and shrank. This structure prevents their irreversible accumulation effectively.

Figure 3. SEM images of (a) raw graphite, (b) and (c) exfoliated graphene. To further characterize the morphology and structure of the exfoliated graphene sheets, we applied TEM, AFM, Raman spectra, and XRD to characterize the samples obtained at the optimal conditions of 250 W, 4000 r/min, and 150 min.



Figure 4. (a)TEM image of graphene, and high-resolution image and electron diffraction pattern (Inset), (b) AFM image of graphene sheets, (c) height profile along the lines shown in panel (b), (d) distribution of the layer number of the graphene. Figure 4a shows TEM images and the selected area electron diffraction (SAED) of the selected area of the graphene sheets. It can be observed that the graphene sheets were thin and transparent, and some part of the sheets were folded and wrinkled. The SAED exhibited a typical 6-fold symmetric diffraction and indicated the excellent crystallinity of the graphene

31.

It confirms that the crystallinity of the graphene structure was not

damaged during the exfoliation process. Moreover, the stronger diffraction from the


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(0-110) plane than that from the (1-210) plane confirmed a monolayer of graphene 32. The thickness and the size of the graphene sheets were analyzed by AFM. The mean thickness was less than 1 nm suggesting that the layer number of the graphene sheets were less than 3 layers as shown in Figure 4b and 4c. The lateral size was 1-3 µm. We counted statistically 218 graphene sheets from AFM images (shown in Figure S2a-2f) to obtain the layer number distribution of the graphene. As shown in Figure 4d, over 90% of the graphene sheets were less than 5 layers, among which about 60% were 1-2 layers.

Figure 5. (a) Raman spectra of graphite and graphene, (b) XRD patterns of graphite and graphene. Raman spectrum was employed to verify the quality and layer number of the graphene further. Figure 5a exhibits Raman spectrum of the graphite and the graphene. The D band is associated with the structure imperfections or hexagonal symmetry breaking of the disordered graphite. The D band of the graphene became stronger because the graphene was thinner and smaller in size. The G peak was caused by the doubly degenerate zone center E2g mode, which was associated with phonon vibrations in sp2 carbon atoms. The ID/IG was 0.064 of raw graphite, and 0.12 of the exfoliated graphene. The 2D peak was



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derived from the inelastic scattering of the second-order zone boundary phonons. From the significant change in shape, intensity, position and full width at half maximum of the 2D peak, we can judge that the exfoliated sample was a mixture of graphene sheets with the layer number 1-4 33. Figure 5b displays XRD patterns of the graphite and the graphene sheets. The (002) peak of the graphene decreased in intensity with no position shift indicating that the layer number

was

reduced

and

the

original

pristine

structure

retained

without

oxygen-containing groups28. We measured the electrical conductivity of the obtained graphene to examine the electrical properties of the exfoliated graphene. The thickness of the graphene film was 3.71µm, and the sheet resistance was 0.228Ω/. The calculated conductivity of the graphene film was 1.18 × 106 S/m, much higher than that of the reduced graphene oxide (up to 104 S/m) 34. The morphology and structure of the graphene film and its electrical conductivity can be seen in Supplementary Information. 3.3 Comparison with other published results We compared the result of this work with four published results studied on the graphene exfoliation in supercritical CO2. Table 1 lists the best reaction time, maximum yield and minimum thickness of the obtained graphene. It can be seen that the best reaction time and the maximum yield of this work were shorter and higher than corresponding results, respectively, published on other studies. At the same time, the minimum thickness of the graphene obtained by this method was less than that made by most other studies,


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indicating that the quality of the graphene obtained by this method is also better. Table 1. Comparison of graphene exfoliation in supercritical CO2

Samples

Assistant

Maximum

Minimum

Yield

Thickness

150 min

~80%

~0.9 nm

240 min

/

~4.2 nm

150 min

~60%

~1 nm

240 min

~30%

~1 nm

120 min

~20%

~0.6 nm

Reaction Time

Ultrasound and 1(This work) Shear mixing Ball-Milling 2(reference 20) and PVP Ultrasound, 3(reference 35) H2O, and PVP Shear mixing 4(reference 26) and SDBS 5(reference 36)

Ultrasound

3.4 Exfoliation mechanism An exfoliation mechanism was proposed based on the results of the experiment. We supposed that the graphene sheets were exfoliated from the surface of the graphite piece, as shown in Figure 6a-6g.



Figure 6. Schematic exfoliating illustration The exfoliation of the graphene is caused by the external forces which are divided into two kinds, normal force (𝑓𝑛) and lateral force(𝑓𝑙), both of which are prerequisites for the exfoliation of graphene29. The 𝑓𝑙 causes relative motion between the surface graphene layer and the others, as shown in Figure 6a and 6b. The 𝑓𝑛 can overcome the van der Waals attraction to peel the graphene layers apart, as shown in Fig. 6d and 6e. From the point of the probability, we think that most lateral force or normal force cannot offer enough energy to exfoliate the graphene layer completely, but transform them to a new state, as shown in Figure 6c and 6g. In this state, the surface layers are partly apart from the other layers. We call these parts active edges. As the active edges are apart from other layers, they are more likely to be effected by the external force than the bulk piece. The generated areas as shown in the red part in Figure 6c and 6g become weak places, in which the exfoliation easily occurs, like peeling off adhesive tape from a smooth flat


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surface. Therefore, the exfoliation includes two main steps during the exfoliation: generating active edges and peeling off the layers. An ultrasound and a shear mixer play different roles in exfoliation process. The ultrasound provides partial high energy in a fluid, such as local high pressure, local high temperature, or cavitation14, 37. However, it is not enough to exfoliate the graphite with the large area though it may exfoliate small piece of graphite27, 35. It may only affect the edge of the graphite. In other words, the ultrasound may only generate the active edges rather than exfoliate the graphite out in this case.

Figure 7. The photograph of the shear mixer (A) and the illustration of velocity distribution between the stator and rotor (B and C). The shear mixer used consists of a stator and a rotor, as shown in Figure 7A. When the rotor runs at a designed speed, the linear velocity of the fluid at the surface of the rotor is nearly the same as that of the rotor. At the same time, the linear velocity of the fluid at the surface of the stator is almost the same as that of the stator equaling zero. Thus there is a high-velocity gradient between the stator and rotor, as shown in Figure 7B and C. A continuous high shear force is created by the velocity gradient in this region. This



continuous shear force produced by the mixer was quite good at peeling off a graphene sheet with active edges though its energy was not high enough to produce active edges in the shear mixing system. Thus, the combination of the ultrasound (generating active edges) and the shear mixer (peeling off the graphite with active edges) can effectively exfoliate the graphite. The yield made by the combined method should be more than the linear sum made by the ultrasound and the shear mixer, respectively. We characterized the samples obtained at the power of 250 W, and the speed of 2000 r/min by SEM and TEM to prove the mechanism. The active edges can be seen on many partly exfoliated graphite as shown in Figure 8a and 8b. It indicates that the active edges were excess under this condition. According to the mechanism that the ultrasound generates active edges and the shear mixing facilitates to peel off the graphene sheets with active edges, we can expect that the yield of the graphene increases with increasing the shearing speed when the active edges are excess, but increase little with increasing the ultrasound power. This mechanism has been supported by the experimental results (Figure 2b and 2c). When the power of the ultrasound and the shearing speed were 250 W and 2000 r/min, respectively, the active edges were excess. It means that the power provided by the shear mixer (2000 r/min) was not enough to peel off too many active edges. Thus, even though the power of the ultrasound increased to 500 W, the exfoliation yield increased little. However, the yield increased obviously when the shearing speed increased to 3000 r/min


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or 4000 r/min because the shear mixer could provide more energy to peel off the graphite with active edges.

Figure 8. (a) SEM and (b) TEM images of partly exfoliated graphite with active edges 4. Conclusion A new method combined ultrasound and shear mixer in supercritical CO2 is developed for the exfoliation of graphene. The yield of graphene achieved 82.6% under the shearing speed of 4000 r/min, ultrasound power 250 W, and reaction time 180min. About 60% of the obtained graphene sheets were 1-2 layers without oxygen-containing groups. The conductivity of the film formed by the obtained graphene was 1.18 × 106 S/m. A two-steps exfoliation mechanism was proposed: the ultrasound generated the active edges on the graphite and the graphite was followed to be peeled off from the active edge by the shear mixer. The ultrasound mainly produces the active sides, and the shear mixing primarily peels off the graphene layer with active edges. The ultrasound coupled with a shear mixer in supercritical CO2 approach is potential to be applied to produce graphene



largely because it can make defect-free graphene with the existence of the solid powder or dispersion solution and also a green and simple process Acknowledgment We are thankful for the financial support of the National Natural Science Foundation of China (21576165), and Ms. Huiqin Li of Instrumental Analysis Center of SJTU for analysis. Conflict of Interest The authors declare no conflict of interest. Supporting Information The residual graphite. The formation of graphene film and its electrical conductivity References (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306 (5696), 666. (2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102 (30), 10451. (3) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior thermal conductivity of single-layer graphene. Nano Lett. 2008, 8 (3), 902. (4) Geim, A. K. Graphene: status and prospects. Science 2009, 324 (5934), 1530-4. (5) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and


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Highly efficient production of graphene by an ultrasound coupled with

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