Fabrication of graphene from graphite using high-powered ultrasonic vibrators

doi:10.21203/rs.3.rs-3144414/v1

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Fabrication of graphene from graphite using high-powered ultrasonic vibrators

https://doi.org/10.21203/rs.3.rs-3144414/v1

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This paper utilizes an efficient and environmentally friendly method for synthesizing graphene from graphite, namely liquid-phase exfoliation. High-power density ultrasonic vibrators were used to separate graphite layers into graphene in a liquid medium. During layer separation, ultrasonic waves provided mechanical energy to break the Van der Walls bonds and separate graphite layers into graphene. In our study, graphene was synthesized by ultrasonicating graphite in Tween 80 for 1 to 5 hours, followed by magnetic stirring and surfactant removal. The FESEM and Raman measurements demonstrated that high-frequency ultrasound waves were effective at breaking the Van der Waals bonding force between adjacent graphite layers. Average flake sizes (lateral) were reduced with increasing ultrasonication time, reaching a minimum value of 317 nm with 5 hours of ultrasonic treatment. These results show that liquid-phase exfoliation is a cost-effective method to obtain low-defect few-layer graphene.

Graphene

graphite

high-power density

ultrasonication

surfactant

In the recent decade, graphene has attracted the attention of scientists in various fields due to its exceptional physical and chemical properties. With a 2D basic structure, graphene layers are comprised of carbon atoms arranged in a hexagonal shape on a plane, also known as honeycomb structures [1]. Each atom is bonded with its three nearest carbon atoms by stable sigma (σ) covalent bonds, forming the intercalation of sp states, corresponding to the sp² hybridization state. Other allotropes of carbon include three-dimensional (3D) graphite, one-dimensional (1D) carbon nanotube, and zero-dimensional (0D) fullerene. Rolled graphene will form a 0D fullerene, which can be wrapped to form a 1D carbon nanotube or stacked to create 3D graphite [2].

There are many techniques for making graphene materials, such as liquid-phase exfoliation, chemical vapor deposition (CVD), and epitaxial growth [3–5]. Among them, liquid-phase exfoliation (LPE) is a simple method that can produce graphene in large quantities and at low cost [6]. Many variants of LPE exist, such as direct ultrasonic exfoliation, stabilizer-based exfoliation, exfoliation with ionic solvents, mild dissolution exfoliation, sheer exfoliation, electrochemical exfoliation, and functionalization-assisted exfoliation [7]. It should be noted that little is known about how LPE occurs but Li et al. [8] have outlined three stages to the exfoliation process: the rupture of large flakes and the formation of kind band striations on the flake surfaces, followed by cracks forming along said striations and in combination with the intercalation of solvent, resulting in the peeling off of thin graphite strips and finally exfoliation into graphene. Furthermore, it has been shown that numerous factors, such as the dimensions of the container vessel and the height of the fluid in the container, can markedly impact graphene synthesis due to the cavitation effect [9]. Moreover, the lateral size of the flakes is controllable through controlled centrifugation [10].

LPE can be performed by first dissolving graphite into a solvent, followed by sonication, centrifugation, and finally decantation [11, 12]. The choice of solvent is paramount due to the requirement of matching surface energy for graphene exfoliation. However, some of these solvents are expensive, which can hinder the scalability of LPE. Additionally, this method suffers from low graphene concentration [13]. A remedy to this problem involves adding additives or surfactants. As demonstrated by Lotya et al. [13], combining surfactants and ultrasonication can result in high graphene yield and stable exfoliated graphene flakes. Various surfactants, including oligothiophene-terminated poly (ethylene glycol) [14], Nmethyl-pyrrolidone [15], and numerous other ionic and non-ionic surfactants [16], have been used to enhance graphene exfoliation. In general, the use of surfactant in LPE has demonstrated enhancements in the quality of graphene. LPE can also be done solely with solvents such as chloroform [17], 1-propanol [18], and organosilanes [19].

In this paper, we utilized liquid-phase exfoliation to synthesize large quantities of few-layer graphene. Graphene is separated from graphite in the liquid phase under the action of Tween 80. We used the acoustic cavitation effect of high-powered ultrasound to separate the graphite layer into graphene. We postulate that the efficiency of the separation process depends significantly on vibration power. If the ultrasonic power is large, it will create strong pressures that overcome the bonding forces, and thus increase the efficiency of the graphene layering process. The synthesized graphene can be used as highly efficient thermal interface materials (TIMs) [20] or as graphene-based ink for the inkjet printing of electronics [21].

The graphite used in the study is obtained from Vietnam Graphite Group Co., Ltd. Tween 80 solutions were purchased from Sigma-Aldrich. The high-powered ultrasonic vibrator is custom-made and consists of 5 ultrasonic vibrating heads and 1 ultrasonic vibrating probe, each with a frequency of 40 kHz and power of 40W. The vibrating heads were installed on 5 sides of a 5 cm cubic vibration tank and probe is inserted into the cubic vibration tank from the top. The ultrasonic vibrator can achieve high-powered-density up to 2 kW/litre.

The process of synthesizing graphene from graphite is as follows: A solution of 0.25 mg graphite and 2 ml of Tween 80 was magnetically stirred for 2 hours at 600 rpm. The resulting mixture was poured into distilled water and subjected to high-powered ultrasonication for 1 to 5 hours. The resulting mixtures were filtered and washed with distilled water several times.

To survey graphene morphology, we use field emission scanning electron microscopy (FESEM, S-4800; Hitachi) and transmission electron microscopy (TEM, JEM1010; JEOL). Size distribution and zeta potential were determined using a Nano ZS Zetasizer particle analyzer from Malvern. Raman Spectroscopy (LabRAM HR 800, HORIBA Jobin Yvon) was used to study the material's structure.

3.1. Surface morphology

The morphology of the material was surveyed using field emission scanning electron microscopy. The FESEM image of the base graphite is shown in Fig. 1(a), showing an average flake size of approximately 5 µm and demonstrated surface agglomeration. The FESEM results of ultrasonicated graphite with a duration of 1 to 5 hours are shown in Fig. 1(b-f) respectively. The results showed that the surface morphology of graphite was significantly altered after prolonged ultrasonication, with the average flake size becoming smaller and more uniform as ultrasound time increased. Figure 1(f) shows few-layer graphene after 5 hours of ultrasonic treatment, demonstrating the effectiveness of high-powered ultrasonic treatment in breaking the Van der Waals bonding force between adjacent graphite layers. These results are supported by Xin et al., which determined that increasing time or power of sonication correlates with enhanced CNT dispersion in aqueous solutions [22]. For 5 hours of ultrasonication, the thickness, determined through TEM, was approximately 3.27 nm, and given that the distance between graphene layers in bilayer graphene is 0.348 nm [23], the number of layers was estimated to be less than 10, which is considered few-layer graphene. The TEM images of the 5-hour graphene are presented in Fig. 2.

3.2. Graphene structural survey

The structural changes of graphite and graphene were surveyed using Raman Spectroscopy. Figure 3 shows the Raman spectrum of graphite with different ultrasonication times.

The Raman spectral results of graphite initially peak at 1579 cm^-1 (G) and at 2707 cm^-1 (2D), which is indicative of sp² bonding [24, 25]. With an ultrasound time of 1, 2, 3, 4, and 5 hours, peak D can be observed, representing the defect of the material. The intensity ratio of peak D with peak G is presented in Fig. 4.

This suggests that the intensity of peak D increases with the duration of ultrasonic treatment, corresponding to an increase in defect of the graphene structures, possibly due to the generation of sound gaps from high-frequency ultrasonic waves. While the increase in ultrasonication leads to improved dispersion and layer separation of graphene, it also creates additional defects in the material. There likely exists a point of diminishing returns where the detriment of defects outweighs the benefits of enhanced dispersion and layer separation. Further research is needed, particularly in ultrasonication post 5 hours, to determine the optimal ultrasonic treatment time.

3.3. Dispersibility and stability analysis

After ultrasonication of 1 to 5 hours, the graphene mixtures were dispersed into distilled water to evaluate their dispersibility and stability. Figure 5 shows the size distribution of the material with different ultrasonic times (Gr1, Gr2, Gr3, Gr4, and Gr5, corresponding with 1–5 hours respectively).

The size distribution shifts steadily towards smaller graphene flakes, and the full width at half maximum (FWHM) gradually narrows as the ultrasonication time increases. The size distribution of Gr1, Gr2, Gr3, and Gr4 shows the appearance of two size spectra. We postulate that a sonic treatment of 1 hour, 2 hours, 3 hours, and 4 hours exhibit agglomeration of graphene. Gr1, Gr2, and Gr3 possess two size spectra, each with peaks at approximately 245 nm and 4969 nm. Meanwhile, Gr4’s spectra peaks are at about 245 and 1160 nm, respectively. This demonstrates that as ultrasonic vibration time increases, the spectrum peak with large graphene flakes shifts from about 4969 nm to approximately 1160 nm. In addition, the size distribution of Gr5 lies in only one spectrum, demonstrating that Gr5 is well dispersed and does not display large flake agglomeration. The FWHM of Gr5 is also significantly narrower, peaking at about 236nm, and accounts for 20% of the graphene flakes, compared to 8% for Gr4. Figure 6 demonstrates that for 5 hours of ultrasonic treatment, 100% of the graphene flakes are present in the 100–500 nm spectrum. Thus, we’ve determined that further ultrasonic treatment will likely yield minimal size reduction. Furthermore, the reduction in flake size will induce additional defects in the graphene, thus reducing its quality. Additional research is needed, especially on the relationship between ultrasonication time and the number of graphene layers and their defects.

The Zeta potentials of the samples were measured to determine their stability. As demonstrated in Fig. 7, the Zeta potential decreased with increased ultrasonic treatment time, reaching a maximum absolute value of 30 mV for 5 hours of ultrasonication, which is determined to be moderately stable [26]. Furthermore, the 5-hour sample visually showed only slight agglomeration after 5 months.

We have successfully researched and fabricated graphene from graphite using high-powered ultrasonic vibrators. The FESEM results demonstrated an average size reduction and increased uniformity of graphite following prolonged ultrasound treatment. The Raman spectrum showed the intensity ratio I_D/I_G increase with ultrasonication time, corresponding to an increase in graphene defects. Furthermore, the size distribution results indicated an optimal minimum flake size of 317 nm after 5 hours of treatment. Further ultrasonication might further reduce the size of graphene but will introduce additional defects. Additionally, Zeta potential and visual analyses confirmed the moderate stability of graphene after 5 hours of ultrasonic treatment. These results show that high-frequency ultrasound waves are an efficient and economical method to synthesize high-quality few-layer graphene.

Acknowledgements

The study was financially supported by the Ministry of Science and Technology of Vietnam (MOST) under project code NĐT/BY/22/10 and the Vietnam Academy of Science and Technology under project code UDPTCN.04/21-23.

Author contributions

P.M., Y.N, and H.T. conducted experiments and collected data. N.N. wrote the manuscript and conducted analyses. Figures are created by P.M. and N.N. Experiments and methodologies are designed by D.N., T.N., Y.N., and T.P. The project is supervised by P.D. and M.P. T.B. acquired funding and supervised research. All authors reviewed the manuscript.

Conflicts of interest

The authors have no conflicts to declare.

Data availibility statement

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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No competing interests reported.

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Fabrication of graphene from graphite using high-powered ultrasonic vibrators

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Abstract

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1. Introduction

2. Experimental procedure

3. Results and discussion

3.1. Surface morphology

3.2. Graphene structural survey

3.3. Dispersibility and stability analysis

4. Conclusion

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