CHAI Lin, CUI Xiao-jing, QI Yong-qin, TENG Na*,HOU Xiang-lin,*, DENG Tian-sheng,*

（1. Shanxi Engineering Research Center of Biorefinery, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China;2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China;3. CAS key laboratory of carbon materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China;4. Institute of Interface Chemistry and Engineering, Department of Chemistry and Chemical Engineering,Taiyuan Institute of Technology, Taiyuan 030008, China）

Abstract： Ultrasonication is regarded as the most convenient and cleanest approach for graphene preparation from graphite.However, the yields are low in large scale preparation because after ultrasonication the exfoliated graphite is difficult to exfoliate into graphene, which leads to a great deal of waste. A new strategy for the efficient exfoliation of the exfoliated graphite into graphene was investigated by combining ultrasonication and grinding treatments. Results indicated that the exfoliated graphite produced by ultrasonication could be further exfoliated into graphene by combining ultrasonication and grinding. The obtained graphene sheets were all comprised of fewer than 10 layers with a yield of 4.73%. This was attributed to the destruction of the regular stacking of the graphite layers and scrolling and folding their edges to provide entry points for the solvent to overcome the interlayer forces between adjacent layers. This work provides a new strategy for the efficient exfoliation of graphite into few-defect graphene on a large scale.

Key words: Graphene；Exfoliation；Grinding；Ultrasonication；Graphite

1 Introduction

Graphene, an emerging new two-dimensional carbon material with unique hexagonal lattice structure, which endows it talented potentials such as super electrical, thermal, mechanical and electromagnetics properties[1–4]. It was first discovered by Scotch tape method in 2004. Recently, it is widely used in kinds of fields like electronics, light processing, plasmonic, composites materials and energy storage[1]. For decades, graphene has grasped the interests of scientists all over the world, and the preparation methods of graphene were emerging and evolving along with time.

In brief, all the preparation methods were divided into two strategies: top-down methods and bottom-up methods. Top-down methods referred to exfoliation of graphite into graphene by using mechanical exfoliation[5,6], sonication[7–9], solvent thermal exfoliation[8,10], electrical exfoliation[11,12]and oxidation and reduction method[13]. While the bottom-up methods focused on the molecules building blocks that self-assembly building up graphene, such as chemical vapor deposition (CVD) [14,15] and organic synthesis based on precursor molecules[16]. Although high quality of graphene could be fabricated by CVD, the extremely careful fabrication process was considered to be expensive for mass production[5,17]. As mentioned mass production, the oxidation and reduction method should not be ignored[18,19]. Graphite was first oxidized and exfoliated into graphene oxide (GO), which was associated with a disrupted electronic network deficient of novel electronic properties. Meanwhile, GO could hardly be reduced completely to remove all the oxygen atoms[20], and the restoration of the sp2network was incomplete, therefore the properties of the reduced graphene oxide (RGO) were significantly distinguished from those of perfect graphene. However,the serious pollution caused by the large amounts of acid and toxic reducing agent was also a challenge that limited the large-scale production and applications of few-defects graphene. Furthermore, the exfoliation of graphene by physical methods would not introduce defects that caused by oxidation, and could provide sufficient amount of highly pure graphene for certain studies[21].

Among all the preparation methods, ultrasonication was regarded as the most convenient and cleanest approach for few-defects graphene preparation from graphite in organic solvents[22]. In theory, the full exfoliation could be achieved for a small amount of graphite with high power and longtime ultrasonication. High content of graphite in solvent had practical implications in terms of preparation and energy consumption. However, the yield of graphene would be limited, because the fully exfoliation for the large quantities of graphite in solvent was difficult to attain.Thus, it was essential to improve the efficient exfoliation for the large quantities of graphite in solvent[7].Chen[5]and Zhao[23]had reported that before ultrasonication, the graphite was treated mechanically by three-roll mill or ball milling, the yields of graphene increased after ultrasonication. However, for the large quantities of graphite in solvent, the exfoliated graphite after full ultrasonication with regular laboratory equipment still took the majority. Most of the graphite could not be exfoliated into graphene and led to a great deal of waste. It was much desirable to further exfoliate the exfoliated graphite into graphene efficiently. However, few researches were focused on how to take advantage of the exfoliated graphite after full ultrasonication.

Scheme 1 Grinding process for further exfoliation of graphite into graphene.

In order to efficiently exfoliate the graphite into graphene, a strategy of combining ultrasonication and grinding process was proposed in this work. The process was shown in Scheme 1. The exfoliated graphite by full ultrasonication were collected, which could hardly be exfoliated into graphene. Then they were grinded before resetting to ultrasonication. It was illustrated that the exfoliated graphite was re-exfoliated into graphene, even if the yield of graphene was higher than that of exfoliated one from pristine graphite.Although the higher graphene yield was achieved by ball milling treatment, the destruction of graphite caused by ball milling was drastic between the graphite layers and the structure of graphite. The multiscale destruction on the graphite would bring interferences on the understanding of exfoliation mechanism. The destruction caused by manual grinding was relatively mild, which was more conducive to elucidating the relationship between the graphite structure and the graphene yield. Meanwhile, the effects of ultrasonication time and particle size of graphite on exfoliation were investigated. The results indicated that the obtained graphene sheets were all less than 10 layers,and the lateral size was arranging from hundreds of nanometers to several microns. This work provided a new perspective to efficient exfoliation of graphite into graphene, aiming at producing few-defects graphene potentially on a large scale.

2 Experimental

2.1 Materials

All the reagents were analytic purity grade and were used without further purification. Graphite was purchased from Qingdao Tengshengda Carbon Materials Co., Ltd. N-methyl-pyrrolidone (NMP, AR, 99%)was from Sinopharm Chemical Reagent Co., Ltd(Shanghai, China). Ethanol absolute (AR) was bought from Beichen Fangzheng Chemical Reagent Co., Ltd(Tianjin, China).

2.2 Graphene preparation and characterization

A typical procedure for graphite exfoliation was carried out in sonic bath (Kunshan Ultrasonic Instrument Co., LTD, KQ3200DE, 40 KHz, 150 W), with 0.5 g of graphite and 50 mL of NMP for 15, 30, 45,60 min respectively. The resultant dispersion was set for 12 h, and then centrifuged using a HC-2518 (Anhui USTC Zonkiz Scientific Instrument Co., LTD.)for 90 min at 600 r/min. After centrifugation, the decantation was carried out carefully by pipetting off the top half of the dispersion and obtained the graphene dispersion. The sediments were directly collected for further grinding and/or ultrasonication under the same conditions mentioned above. Quartz mortar was employed for grinding. Graphene powder was obtained by pumping and filtration through filter membrane(organic, pore size: 0.22 μm). The membrane with graphene powder was dried for 4 h at 60 °C in a vacuum drying oven. The mass of graphene in graphene dispersion was calculated by weighting the difference of filter membrane before and after filtration. The yield of graphene was then calculated by the ratio of the mass of graphene in graphene dispersion and the mass of graphite by ultrasonication for this time.

The morphology and crystal structure of graphene were investigated by Transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) using Philips Tecnai F30 FEG-TEM operated with an accelerating voltage of 300 kV. Scanning electron microscopy (SEM) was observed by a field emission-scanning electron microscope (S-4800, Hitachi, Japan) at 3-5 kV. The covalent states of C were investigated by X-ray photoelectron spectroscopy (XPS) (an AXIS ULTRA DLD spectrometer with Al-Kα X-ray resource (hv =1 486.6 eV)). The signal of contaminated carbon C1s at 284.8 eV was used as the calibration for binding energy values. The powder X-ray diffraction (XRD) patterns in the 2θrange from 5° to 90° were recorded on a Rigaku Miniflex II diffractometer with CuKɑ radiation at a scanning rate of 4 °/min. Atomic force microscopic (AFM) measurements were performed to characterize the sheet thickness and dimensions using a Nanoscope III MultiMode SPM (Digital Instruments) operated in tapping mode. The samples for AFM measurement were prepared by depositing the corresponding dispersions on 1 cm×1 cm section.

3 Results and discussion

In this work, NMP was chosen as the solvent due to its effectiveness in graphite exfoliation by sonication[24]. It was suggested that the graphite was exfoliated into graphene successfully according to the TEM images in Fig. 1a-e, and the obtained graphene sheets were less than 10 layers. The lateral sizes of graphene sheets were irregularity, arranging from hundreds of nanometers to several microns. Base on the AFM results (Fig. 1f-g), the thickness of obtained graphene sheets was less than 8 nm, which were in agreement with the TEM results.

3.1 Effect of ultrasonication time on the exfoliation of graphite

Ultrasonication time was an important factor affecting the exfoliation. Thus, the ultrasonication time was arranged from 15 to 60 min with 15 min as a gradient. The yields of graphene were calculated by filtration and weighing, and the ultrasonication results were showed in Fig. 2. It was clear that the yields of graphene were increased with the ultrasonication time prolonging. When the ultrasonication time reached 60 min, the yield of graphene was three times higher than that of 15 min. In Fig. 2b, the colors of graphene dispersion under different ultrasonication time were also an obvious proof for the exfoliation results. It was found that the colors of graphene were gray and dark under ultrasonication for 15 min and 60 min respectively. Previous researchers had reported that the relationship between concentration of graphene (CG) and the sonication time followed theCG∝[9]. The trendline of graphene yields displayed in Fig. 2a also showed the similar tendency. Although the yields of graphene were actually promoted with the ultrasonication time increasing, the quality of graphene were impacted because of more defects caused by the prolonged ultrasonication time. The excessive sonication could lead to destruction of the graphene[7,25].

Fig. 1 TEM and AFM images of graphene exfoliated from graphite by direct ultrasonication. (a-e) TEM images of graphene flakes, (f) AFM images of graphene flakes, (g) thickness of graphene flakes. Experimental conditions: graphite, 0.5 g; NMP, 50 mL; ultrasonication time, 30 min.

Fig. 2 (a) Influence of sonication time on graphene yields. (b) Corresponding images of graphene with different ultrasonication times. Experimental conditions: graphite, 0.5 g; NMP, 50 mL; ultrasonication time was 15, 30, 45, 60 min, respectively.

3.2 Exfoliation of graphite into graphene combining ultrasonication and grinding

The TEM images of obtained graphene by combining ultrasonication and grinding treatment in Fig.S1 suggested that the lateral size and the thickness of graphene were similar with the graphene produced without grinding treatment. The effect of ultrasonication times on the yield of graphene was showed in Fig. S3 and Table 1. After each time of ultrasonication treatment, the exfoliated graphite was directly collected to reset to the grinding and/or the next time of ultrasonication treatment. From Fig. S3 (a), it was observed that yields of graphene were decreased with the increase of the ultrasonication times. The graphene yields for both first and second time of ultrasonication were 1.5%–1.6% (weight percentage) (entries 1-2, Table 1). However, the yields for the next three times were 0.5%–0.6% (entries 3-5, Table 1). It could be seen from the pictures in Table 1 that the color of graphene dispersion for ultrasonication several timeswas much lighter than that of ultrasonication for the first time. The facilitation of exfoliation was impaired with the increasing of ultrasonication time, indicating that the exfoliation capacity of ultrasonication was confined. Besides, the total yield of graphene was increased with the increasing of ultrasonication time(Fig. S3 (b)). In addition, the exfoliated graphite after the 5thultrasonication was considered as the graphite which could hardly be exfoliated.

Table 1 Effect of ultrasonication time and the grinding treatment on the yield of graphene＊

When the pristine graphite was grinded for 1 h,the yield of graphene achieved 3.42% after ultrasonication (entry 6, Table 1). When the exfoliated graphite after the ultrasonication for one time was treated as the ingredient to be grinded for 1 h and then treated with ultrasonication, the yield of graphene reached 3.58% (entry 7, Table 1). The exfoliated graphite after the 5thultrasonication was taken for the starting materials for grinding and ultrasonication. The yield of was 4.73% (entry 8, Table 1). When the total time of ultrasonication were the same, the yields of graphene obtained by ultrasonication were lower than that of the samples obtained by ultrasonication and grinding for onc time (Fig. S4). All these results by grinding confirmed that grinding was benefited for the exfoliation of graphite into graphene.

Moreover, the yield of 4.73% was more than that of the sample by direct ultrasonication with the yield of 1.68%. The yield of graphene by direct ultrasonication was higher than that of the samples reported and summarized in previous researches as less than 1%[7,11]. There were no doubts that grinding was benefited for exfoliation of graphite into graphene, especially for the exfoliated graphite after ultrasonication.It could be speculated that if the exfoliated graphite was treated by combining ultrasonication and grinding for several times, all the graphite should be exfoliated into graphene in theory, which would promote the graphene yield for high content graphite in solvent to the greatest extent.

3.3 Mechaism of graphite exfoliation by combining ultrasonication and grinding

It had indicated that the ball milling or grinding could facilitate the exfoliation of graphite[5,23].However, the effect of ultrasonication and grinding treatment on the efficient exfoliation of graphite was not mentioned above.

In order to explore the combining of ultrasonication and grinding could restart the exfoliation of exfoliated graphite after ultrasonication into graphene, the morphology of graphite after different treatment was detected by SEM (Fig. 3). It can be seen that the interlayers were densely arranged due to the π-π interaction in the pristine graphite. A certain number of edges in graphite sheets were curled up, which were pointed out by circle of dot line (Fig. 3a and b). The arrangement of graphite layers was regular, and the edges of the exfoliated graphite sheets after ultrasonication turned to be straight and intact with few curled-up edges (Fig. 3c and d). When the exfoliated graphite was treated by grinding, the regular stacking of graphite layers was destroyed to be stagger arrangement and the edges tended to scroll and fold slightly and the curled-up edges returned (Fig. 3e and f). The stagger arrangement of graphite layers and formation of curled-up edges provided approaches for certain solvents to wedge into the interlayer of graphite,which might be the reason why grinding facilitated theexfoliation of graphite into graphene. Otherwise, the curled-up edges would fall from graphite sheets assisted by ultrasonication and dispersed in NMP, leading to a high yield of graphene. These results were consistent with previous research[24]. In addition, it could be revealed that the lateral size of graphite was reducing after ultrasonication and/or grinding treatment. Furthermore, this collision among pestle, graphite and mortar was similar to the shear in liquid environment, which was reported as another approach to achieve exfoliation of graphite[26]. In short, it was suggested that the irregular curled-up edges were the necessary factor for exfoliation of graphene induced by ultrasonication.

The XRD patterns of obtained graphene and graphite with different treatment were showed in Fig. 4. Based on the Scherrer formula, the XRD patterns of the samples were analyzed[27]. Therefore, the typical diffraction angle (2θ), d-spacing (d), full width at a half maximum (FWHM) and the vertical distance along the plane of (002) for each sample (D) were calculated and collected in Table 2. It was suggested that no matter what treatment was put on, the shapes of(002) peaks remained sharp and the intensities were relatively higher than that of graphene. Compared with the pristine graphite, thed-spacing of other samples increased. What’s more, the graphite particle lateral size in the (002) plane direction reduced drastically for grinded graphite after ultrasonication,while it increased after ultrasonication. By the way,thed-spacing and the FWHM of graphene powder were the same with graphite, while the intensity declined sharply, indicating that the graphene sheets reaggregated and re-stacking into graphite to some extent during the solvent evaporation[24,28,29]. Furthermore, the XPS results in Fig. S2 showed that the graphite did not be oxidized during the grinding process, which indicated that few defects would be induced into graphene.

3.4 Effect of particle size on the exfoliation of graphite

Fig. 4 XRD patterns of graphene and graphite with different treatment.(a) pristine graphite, (b) sample treated with ultrasonication and exfoliation by pristine graphite, (c) sample treated with ultrasonication, exfoliation and grind by pristine, (d) sample treated with ultrasonication, exfoliation, grind,ultrasonication, and exfoliation by pristine graphite, (e) sample treated with ultrasonication and extraction filtration by pristine graphite, (f) sample treated with ultrasonication, exfoliation, grind, ultrasonication and extraction filtration by pristine graphite. Experimental conditions: graphite, 0.5 g;NMP for ultrasonication, 50 mL; NMP for grinding, 5 mL; ultrasonication time, 30 min; grinding, 1 h; centrifugation at 600 r/min for 90 min.

Table 2 Characteristics of graphene and graphite with different treatments＊

Fig. 5 Yields of graphene with different sizes of graphite as starting materials by ultrasonication treatment. Experimental conditions: graphite,0.5 g; NMP for ultrasonication, 50 mL; ultrasonication time, 30 min; centrifugation at 600 r/min for 90 min.

The particle size of graphite was also another factor impacting the graphene yield. All the experiments mentioned above were conducted with graphite of 325 mesh. The yields of graphene with other sizes of graphite as starting materials were collected in Fig. 5. It could be seen that the yields of graphene increased with the decreasing size of graphite, and the color of graphene dispersion became darker with the particle size decreasing. It was relatively easy to overcome the interlayer forces between neighboring layers of small graphite particles. Compared with the SEM images of graphite of 325 mesh and 3 000 mesh before and after ultrasonication (Fig. S5), although the shape of graphite sheets hardly changed, more fragments generated after ultrasonication. For graphite of 325 mesh, there were many little fragments after ultrasonication, and the size distribution of graphite of 325 mesh was in disorder. The corresponding XRD results were collected in Fig. S6 and Table S1. It could be found that the shapes of (002) peaks for graphite of 325 mesh and 3 000 mesh were similar, and the intensity of (002) peaks decreased acutely with the increasing of graphite particle size. For graphene powder, no matter which size of graphite it came from, the intensity of (002) peaks were much lower than that of graphite. The d-spacing of graphite in different meshes was not varied, and the vertical distance along the plane of (002) peaks reduced after ultrasonication from 1.72 nm to 1.38 nm for graphite of 3 000 meshes, while the vertical distance along the plane of (002) peaks of graphene came from graphite of 325 and 3 000 meshes was 1.38 nm and 0.86 nm,respectively, which attributed to the re-aggregation of graphene.

4 Conclusion

In summary, a new strategy for efficient exfoliation of graphite into graphene by combining ultrasonication and grinding was investigated to improve the exfoliation efficiency of the high content graphite in solvent. The results showed that the exfoliated graphite after ultrasonication could be exfoliated again with the yield increased to 4.5%. In addition, the obtained graphene sheets were all less than 10 layers.The mechanism of graphite exfoliation by combining ultrasonication and grinding might be attributed to the the destruction of the regular stacking of graphite layers and the scrolled and folded edges of graphite sheets. In theory, complete exfoliation of graphite into graphene would be achieved with repeat grinding and ultrasonication treatment for several times. This work provided a new strategy for the efficient exfoliation of graphite into graphene on a large scale.

Acknowledgement

This work was supported by the NSFC-Shanxi Coal-based Low-carbon Coalition Fund ‘Structural analysis of high-performance denitrification porous carbon materials based on coal’ (U1710252) and Applied Basic Research Programs of Science and Technology Commission Foundation of Shanxi Province(201901D111006/ZD).