Fabrication of graphene from graphite using high-powered ultrasonic vibrators

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 h, 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 h of ultrasonic treatment. These results show that liquid-phase exfoliation is a cost-effective method to obtain low-defect few-layer graphene.


Introduction
In the past decade, graphene has emerged as a captivating subject of study for scientists across diverse fields, drawn by its remarkable physical and chemical properties.Possessing a fundamental 2D structure, graphene layers consist of carbon atoms intricately arranged in a hexagonal pattern on a flat plane, resembling honeycomb structures [1].Each carbon atom forms stable sigma (σ) covalent bonds with its three closest neighbors, creating the intercalation of sp states and giving rise to the sp2 hybridization state.This structure not only imparts remarkable strength and conductivity to graphene but also positions it as a pioneer in the exploration of cuttingedge technologies.The graphene family extends beyond the confines of 2D to embrace various intriguing forms [2].In its three-dimensional manifestation, graphene is stacked to create graphite.It can also transform into one-dimensional carbon nanotubes, resembling microscopic cylinders with exceptional mechanical, electrical, and thermal properties.Moreover, graphene exhibits the ability to roll into zero-dimensional fullerene structures, opening up a realm of possibilities for nanotechnology and material science.Furthermore, the versatility of graphene becomes apparent in its potential applications [3,4].From revolutionizing electronics and energy storage to advancing biomedical technologies, graphene stands as a beacon of innovation.Its impermeability, thermal conductivity, and mechanical strength make it a formidable contender for applications in flexible electronics, supercapacitors, and even medical devices.
There are many techniques for making graphene materials, such as liquid-phase exfoliation, chemical vapor deposition (CVD), and epitaxial growth [5][6][7].Among them, liquid-phase exfoliation (LPE) is a simple method that can produce graphene in large quantities and at low costs [8].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 [9].It should be noted that little is known about how LPE occurs but Li et al [10] 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 [11].Moreover, the lateral size of the flakes is controllable through controlled centrifugation [12].
LPE can be performed by first dissolving graphite into a solvent, followed by sonication, centrifugation, and finally decantation [13,14].By sonicating graphite in a suitable liquid solvent during this process, defect-less graphene can be achieved [15].The choice of solvent is paramount due to the requirement of matching surface energy for graphene exfoliation.However, some of these solvents used in the liquid-phase exfoliation process [16][17][18], such as N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF), are known to be relatively expensive [19,20].This cost factor poses a challenge and may hinder the scalability of LPE for large-scale graphene production.Additionally, this method suffers from low graphene concentration [21].A remedy to this problem involves adding additives or surfactants.As demonstrated by Lotya et al [21], combining surfactants and ultrasonication can result in high graphene yield and stable exfoliated graphene flakes.Various surfactants belonging to different categories, including ionic /non-ionic [22], aromatic/non-aromatic [23], polymeric, etc., have been used to enhance graphene exfoliation [24].In general, the use of surfactant in LPE has demonstrated enhancements in the quality of graphene.
The use of surfactants in the Liquid-Phase Exfoliation (LPE) of graphene is primarily aimed at investigating water as an exfoliating medium and instead of organic solvents.Due to the high surface tension of water, surfactants are commonly used to generate suitable surface energy between graphite and water [25].In this study, we used liquid-phase exfoliation in a water environment with the nonionic surfactant Tween 80 to synthesize substantial quantities of few-layer graphene.Tween 80 prevented agglomeration and restacking by stabilizing the graphene particles formed during graphite delamination in water.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) [26] or as graphene-based ink for the inkjet printing of electronics [27].

Experimental procedure
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 40 W. The vibrating heads were installed on 5 sides of a 5 cm cubic vibration tank and the probe is inserted into the cubic vibration tank from the top.The ultrasonic vibrator can achieve a high power density of 2 k W /l i t re.
The process of synthesizing graphene from graphite is as follows: A solution of 0.25 mg of graphite and 2 ml of Tween 80 was magnetically stirred for 2 h at 600 rpm.The resulting mixture was poured into distilled water and subjected to high-powered ultrasonication for 1 to 5 h.The resulting suspension was filtered and washed with distilled water five times alternately using a vacuum filter to ensure that the final samples no longer contained Tween 80.
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.

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 figure 1(a), indicating 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 h are shown in figures 1(b)-(g) 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(g) shows few-layer graphene after 5 h of ultrasonic treatment, demonstrating the effectiveness of high-powered ultrasonic treatment in breaking the Van der Waals bonding force between adjacent graphite layers.In a liquid environment, ultrasound waves act as both a compressive and expansive force, rapidly generating ultra-small vacuum bubbles.With their minuscule size, these bubbles can penetrate between graphite layers.The thermal energy generated throughout this process, coupled with the pressure and cavitation effects, causes the resonating gas bubbles to rupture, resulting in an increased pressure that facilitates the delamination of 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 [28].

Graphene structural survey
The structural changes of graphite and graphene were surveyed using Raman Spectroscopy.The Raman spectra revealed the characteristic peaks of the graphene structure using a 532 nm laser, including the peak at 1347 cm −1 (D), 1579 cm −1 (G), and 2707 cm −1 (2D) [29,30].The G peak serves as a hallmark for all sp 2 carbon systems, indicating in-plane vibrations.In contrast, the 2D peak is linked to the second-order scattering process and stands out for its pronounced intensity, narrowness, and symmetry, exclusively observed in materials resembling graphene [31].Figure 2 illustrates that the 2D peak at 2707 cm −1 undergoes an asymmetry-tosymmetry transition during short-duration ultrasonication, demonstrating the effectiveness of the graphite exfoliation process in the liquid phase using high-power ultrasonic equipment.The ratio I 2D /I G and the FWHM of the 2D peak enable estimating the number of layers [32].In their review of 50 references, Yannick Bleu et al compiled correlations between the I 2D /I G ratio and FWHM(2D) for various graphene layers [33], and based on the I 2D /I G ratio (ultrasonication 5 h) of 0.46 in this study, it can be estimated that the graphene sample comprises more than five layers.However, the FWHM of the 2D peak enables estimating the number of layers (within 1-5) in few-layer graphene with a reasonably high degree of accuracy.For more than four and five layers, the FWHM(2D) cannot be used to quantify the number of graphene layers [33].
Furthermore, Raman spectroscopy reveals changes in graphite structure after vibration, and the intensity of peak D increases with the duration of ultrasonic treatment.In the case of the original graphite, the D peak intensity is low, and the I D /I G ratio is 0.1063.With varying ultrasonic exposure time ranging from 1 to 5 h, the corresponding I D /I G ratios are 0.2738, 0.4127, 0.6467, 0.7186, and 0.8729, respectively.An approximate determination of crystallite size in the graphene material ( a L ), along with defect distance (L D ) and defect density (n D , cm −2 ), can be achieved using the equations [34][35][36][37].
λ represents the laser wavelength (532 nm), and the I D /I G ratio after 5 h of ultrasonication is 0.8729.When applying this formula to our graphene, which features crystallites with crystallite size (L α ) ranging from 12 to 22 nm and defect distances (L D ) in the range of 11 to 14.8 nm, along with a defect density (n D ) within the range of 1.4 to 2.6 × 10 11 cm −2 , indicating a low defect density (L D 10 nm) [37,38].With a low defect density, we believe that the synthesized material maintains a well-structured graphene configuration by avoiding any chemical surface modification.The presence of the D band in our samples is primarily associated with enhanced disorder caused by the formation of voids induced by high-frequency ultrasonic waves.Although increased ultrasonication enhances the dispersion and layer separation of graphene, it concurrently introduces additional defects to the material.Furthermore, the mechanical properties of graphene are influenced by defect density.The defect density (n D ) was estimated to be within the range of 1.4 to 2.6 × 10 11 cm −2 in this study, below the threshold of 10 12 cm −2 , therefore the stability of graphene stiffness is maintained [39].This further emphasizes that graphene made by our proposed process has low defect density.

Dispersibility and stability analysis
After ultrasonication of 1 to 5 h, the graphene mixtures were dispersed into distilled water to evaluate their dispersibility and stability.Figure 3 shows the size distribution of the material with different ultrasonic time (Gr1, Gr2, Gr3, Gr4, and Gr5, corresponding with 1-5 h 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 h, 2 h, 3 h, and 4 h 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 236 nm, and accounts for 20% of the graphene flakes, compared to 8% for Gr4.
Figure 4 demonstrates that for 5 h 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 potential values of the samples were measured to determine their stability.As demonstrated in figure 5(a), the Zeta potential values increase with increased ultrasonic treatment time, reaching a maximum absolute value of 30 mV for 5 h of ultrasonication, which is determined to be moderately stable [39].Despite the Zeta potential values suggesting comparable stability across the samples Gr3-5, the variations in size distribution may indicate differences in the degree of exfoliation, leading to variations in the physical dimensions of the graphene particles.In our investigation, the surface of liquid-phase exfoliated graphene remained unaltered.Consequently, this sample maintained consistent electrical and chemical properties on its surface, affirming its Zeta potential stability.This indicates that ultrasonic treatment primarily impacts the physical dispersion state of graphene sheets, exerting minimal influence on substantial chemical alterations to the surface.Moreover, the 5-hour sample visually showed only slight agglomeration after 5 months, as depicted in figure 5(b).This emphasizes the long-lasting stability achieved through the 5 h ultrasonication process.
The aforementioned results indicate that a 5 h ultrasonic treatment yielded the optimal outcome in terms of size, dispersion, and stability.Therefore, after 5 h of ultrasonication, the thickness was determined through TEM analysis.The majority of graphene sheets measured between 200 nm and 400 nm in dimension and each lateral size of a single sheet was measured from the maximum edge-edge length (figure 6   between two graphene layers of about 0.348 nm [40], the number of layers was estimated to be less than 10, which is considered a few-layer graphene.

The yield of graphene preparation
We meticulously conducted the graphene exfoliation by iterating the process five times, as outlined in table 1.The results revealed a significant and consistent average yield of 81.5%.This noteworthy outcome not only underscores the reliability and reproducibility of our method but also positions it as a robust and practical approach for graphene synthesis.

Conclusion
We successfully synthesized graphene from graphite using high-powered ultrasonic vibrators.Our approach involves the use of water instead of a solvent during the exfoliation process, presenting a cost-effective means to obtain few-layer graphene.Prolonged ultrasound treatment resulted in a consistent reduction in graphite size and improved uniformity, as revealed by FESEM.The Raman spectrum indicated increased defects and enhanced layer exfoliation efficiency, while low defect density confirmed graphene quality.Size distribution analysis showed an optimal minimum flake size of 317 nm after 5 h.Our emphasis is on the predominant influence of ultrasonic treatment on graphene's physical dispersion, with minimal impact on significant chemical changes.Zeta potential and visual analyses confirmed moderate stability after 5 h.TEM analysis indicated a graphene thickness of approximately 3.27 nm, corresponding to fewer than 10 layers.The statistical table demonstrated a high efficiency of over 80%, affirming the practicality of our method.These results show that high-frequency ultrasound waves are an efficient and economical method to synthesize high-quality fewlayer graphene.

Figure 2 .
Figure 2. Raman spectrum of graphite with different ultrasonication time.

Figure 3 .
Figure 3. Size distribution of graphene at different ultrasonication time.
(a)).The wrinkleless and sharp edges of the graphene sheets indicated a low level of defects in the basal plane structure.At another magnification of TEM (figure 6(b)), the results revealed an approximate graphene thickness of 3.27 nm.Given the distance

Figure 6 .
Figure 6.TEM results of graphene after 5 h of ultrasonication at resolutions of 200 nm (left) and 20 nm(right).