Large-scale synthesis of high-quality graphene sheets by an improved alternating current arc-discharge method

Abstract

Large-scale synthesis of high-quality graphene sheets is essential for their development from the laboratory to commercial applications. In this work, we report a large-scale production of high-quality graphene sheets through an improved alternating current (AC) arc-discharge method under N₂/H₂ mixed buffer gases, and we successfully obtained ∼2.1 g of graphene sheets in a short arc-discharge period of less than 5 min. The as-prepared graphene sheets exhibit a low-defect structure with no more than 5 layers. Both N₂ and H₂ are thought to be essential to the synthesis of high-quality graphene sheets. Furthermore, the volume ratio of N₂ and H₂ can be a key factor on lowering the defects and reducing the layer numbers of graphene sheets based on the experimental observations. In addition, the as-prepared graphene sheets are applied as an anode material for lithium-ion batteries and show good electrochemical reversibility. In view of the simplicity of the arc-discharge apparatus, the use of environmental friendly and low-cost N₂/H₂ gases, and the outstanding properties of graphene, this AC-powered method may have a great potential on being scaled up to an industrial level.

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Introduction

Graphene, a two-dimensional sheet of sp²-hybridized carbon atoms, has been attracting great interest due to its unique structure and properties since its discovery in 2004.^1–6 Following the burgeoning carbon nanomaterials related research over the past two decades, there have been a plethora of reports on various preparation methods of graphene, including micro mechanical exfoliation of highly ordered pyrolytic graphite,¹ epitaxial growth on SiC⁷ and chemical vapor deposition.^8–11 These methods can produce high-quality graphene but with a low yield, and the requirements for expensive carbon source, high temperature, a sacrificial metal, and trivial transfer processes onto the desired substrates make them hard for low-cost and large-scale production. Other novel approaches also have been employed to overcome these limitations, such as liquid phase exfoliation,¹² electrochemical exfoliation¹³ and unzipping of carbon nanotubes.¹⁴ However, the tedious, multi-step synthesis processes of these methods severely limit the yield of graphene. Besides, bulk-scale synthesis of graphene oxide by chemical exfoliation of graphite is along with massive damage of graphene's structure.¹⁵

In today's scenario, a fast and low-cost method, arc-discharge method using various buffer gases such as H₂, Ar, He, CO₂, and NH₃ or their mixtures^16–25 could produce graphene sheets of integral structure, high thermal stability, and good dispersibility in organic solvents.²⁶ Nevertheless, this strategy provides a possibility for producing carbon nanomaterials in a level of industry. Rao et al. reported for the first time the preparation of graphene flakes with 2–4 layers through arc-discharge method under a mixed atmosphere of H₂ and He.¹⁹ At the same time, Liu et al. prepared heteroatom doped graphene by a catalytic arc-discharge method.²⁷ Recently, a gram-level yield of graphene was achieved by using ZnO as catalyst in direct current (DC) arc-discharge approach.²⁰ However, the atmosphere used in a typical DC arc-discharge process for graphene fabrication usually contains high-cost inert or toxic gases such as NH₃.^21–23 Additionally, the yield and purity of as-obtained graphene sheets often suffer from impurities of spherical carbon aggregates and multi-walled carbon nanotubes in cathode deposit (CD) area. Recently, Gattia and co-workers reported the synthesis of SWCNHs and highly convoluted graphene sheets by alternating current (AC) arc-discharge in air, which could get rid of these harassments.²⁸ A particularly high rate of rod consumption and soot collection was achieved by AC powering two electrodes with the same size and length. However, the graphene sheets obtained by this method were highly convoluted and lack of further research. Therefore, an optimized arc-discharge experimental set-up using non-toxic and low-cost buffer gases for large-scale high-quality synthesis of graphene is still greatly in demand.

In this work, we optimized the production of high-quality graphene sheets by applying a low-cost and green N₂/H₂ atmosphere through an AC arc-discharge method. High-quality graphene sheets were obtained and their yield is twice more than that of DC produced. Through the results from the characterizations, it was found that the properties of graphene sheets, such as layer numbers or defects, are strongly relevant to buffer gases. Based on the experimental results, we proposed a plausible growth mechanism. Furthermore, the electrochemical performance of the as-prepared graphene sheets also has been investigated.

Experimental

Sample preparation

The AC arc welding machine (Tonko BX1-500-2) was purchased from Shanghai GM Technology Co., Ltd. To prepare high-quality graphene sheets, an AC arc-discharge was carried out in a water-cooled stainless steel chamber as described previously.¹⁷ The arc furnace and its schematical diagram are shown in Fig. S1.† A mixture of N₂ and H₂ with different volume ratios were used as buffer gas. Two pure graphite rods with the same diameter of 8 mm and length of 15 and 20 cm respectively were used as electrodes. The electrodes were kept in a distance of ∼1 mm by continuously adjusting one electrode revolving bar. Rated output current of ∼150 A were employed under a fixed frequency of 50 Hz. An initial total pressure of 400 Torr was maintained for various buffer gases including N₂ (400 Torr), H₂ (400 Torr), N₂ (100 Torr)/H₂ (300 Torr), N₂ (133 Torr)/H₂ (267 Torr) and N₂ (200 Torr)/H₂ (200 Torr). And the samples were indicated as AC-N, AC-H, AC-N1/H3, AC-N1/H2, and AC-N1/H1 respectively. Typically, two graphite electrodes were consumed simultaneously in a time range of 4 min to 5 min, which slightly depends on the buffer gases. The erosion rate was calculated to be ∼3.18 g min⁻¹. After the consumption of the electrodes, the generated flue products deposited on the inner and top wall of the chamber were collected under ambient condition. The samples collected were annealed in air from room temperature to 400 °C for 1 h and several grams of products were obtained. For comparison, graphene sheets were also synthesized by DC arc-discharge method under the same conditions. For simplicity, the DC powered samples produced under various buffer gases of N₂ (400 Torr), H₂ (400 Torr), and N₂ (200 Torr)/H₂ (200 Torr) were indicated as DC-N, DC-H, and DC-N1/H2 respectively.

Characterization

The morphology and structure of the samples were examined by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100, 200 kV). For TEM observation, the as-obtained samples were dispersed in ethanol and a droplet of the suspension was deposited on a TEM holey carbon grid. Raman spectra measurements were performed on a Horiba HR800 Raman system with 633 nm excitation. The Raman spectrometer was calibrated before measurements with reference to the F_1g line of silicon at 520.7 cm⁻¹. Atomic force microscopy (AFM) characterization was carried out on a Multimode Picoforce Nanoscope IIIa analyser (Veeco Instruments Co.) in tapping mode under ambient conditions. Samples for AFM images were first dispersed in dimethyl formamide followed by spin-coating it on silicon wafers. Thermogravimetric analysis (TGA) plots were obtained by using a Q600 thermogravimetric analyzer under an air atmosphere at the temperature range of 25–950 °C with a heating rate of 10 min⁻¹. Nitrogen adsorption–desorption isotherm measurements were performed on a Micromeritics ASAP 2010 volumetric adsorption analyser at 77 K. The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2000 using filtered Cu Kα radiation. For the X-ray photoelectronic spectroscopy (XPS) analysis, a Kratos Axis Ultra multitechnique electron spectrometer was used. A CHNOS Elemental Analyzer was used to further determine the elemental content.

Electrochemical measurements were performed by coin type cells (CR2016) which were assembled in a recirculating argon glovebox where both the moisture and oxygen contents were below 1 ppm. The anodes were made as follows: 80 wt% graphene sheets, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride in N-methyl pyrrolidone were mixed; then, the slurry was uniformly applied to a copper foil at room temperature; the coated foil was cut into disk electrodes of 14 mm in diameter and vacuum dried overnight at 120 °C. The counter and reference electrodes were lithium foil (China Energy Lithium Co., Ltd.), and the electrolyte solution was 1 mol L⁻¹ LiPF₆ in a 50 : 50 w/w mixture of ethylene carbonate and dimethyl carbonate. A glass fiber from Whatman was used as separator. A galvanostatic cycling test of the assembled cells was carried out on a LAND-CT2001C system at different rates within a fixed voltage window of 3.0–0.01 V. The rate capability was evaluated by varying the discharge–charge current from 200 to 2000 mA g⁻¹.

Results and discussion

The main text of the article should appear here with headings as appropriate. The AC generated graphene products AC-N1/H2 were collected and the weight of this sample was ∼2.1 g after annealing purification, which was more than twice of DC-N1/H2 (∼0.8 g). Moreover, TGA analysis of AC-N1/H2 in Fig. S2,† which only consisted of a predominated DGA peak at 673 °C, indicating that the product was in high purity of graphene. The consumed anode and cathode by AC arc were shown in Fig. 1c, and CD was hardly found on either electrode. According to previous reports, in case of DC supply, the ionized gas drifts towards cathode and hinders the continuous deposition of carbon ions on cathode. In order to remove the gas ions, the cathode should not be negatively charged,¹⁶ resulting in the formation of CD, which often contained nanotubes and amorphous carbon and did not contribute to the yield of graphene (Fig. S3†). However, in an AC powered apparatus, the two electrodes can be either anode or cathode along with the alternative current changing, leading to the same temperature distribution during equal periods of time and no deposition is observed at either side of the electrodes. Therefore, we can obtain a high quantity of graphene sheets at the expense of these two convertible electrode.

Fig. 1 (a and b) TEM and HRTEM images of graphene sheets in AC-N1/H2; (c) electrodes after alternative current arc process; (d and e) typical TEM and HRTEM images of DC-N1/H2; (f) AFM image of the AC-N1/H2. The inset of (f) is a magnified picture of rectangle circled area.

The morphologies and microstructures of synthesized samples were shown in Fig. 1a. Few-layer graphene sheets were observed as exclusive species in AC-N1/H2. From the HRTEM images of AC-N1/H2 in Fig. 1b, it could be found that the graphene sheets mainly consisted of 2-layer graphene. This result was further confirmed by our AFM results (Fig. 1f). The height profile of the graphene sheets showed that the thickness of graphene sheets is around 1.8 nm. Considering that the height of single layer graphene sheet is 0.34–1 nm, we could suspect the graphene sheets of AC-N1/H2 are 2–5 layers. Based on the AFM and TEM results, the size of graphene sheets was about 100–300 nm. For comparison, the morphology and structure of DC-N1/H2 were also investigated. Few-layer graphene sheets were observed and the layer number could be clearly distinguished as 3–5 layers (Fig. 1d and e). In addition, the very sharp (002) diffraction peak 2θ and d-spacing value of d₀₀₂ = 3.35 Å existed in both XRD patterns (Fig. S4a†). The number of graphene sheets calculated by Scherrer equation was 6–7, 10–11 for AC-N1/H2 and DC-N1/H2, respectively. Moreover, FT-IR spectra (Fig. S4b†) of these two samples were similar to each other. According to the above results, the AC-N1/H2 and DC-N1/H2 were analogous in the size of graphene sheet and number of graphene layers, implying the structure of graphene sheets in AC and DC arc-discharge process are similar to each other in general.

AC-N, DC-N, AC-H, DC-H, AC-N1/H1, and AC-N1/H3 were also obtained. As shown in Fig. 2a, the TEM image of AC-N was typical ‘dahlia-like’ single-walled carbon nanohorns (SWCNHs), which consisted mostly of tubules ended by a pentagon conical cap. SWCNHs-like species were also found in DC-N (Fig. S5b†), and graphene sheets were hardly found in both samples since N₂ played a key role in the formation of SWCNHs.²⁹ The presence of H₂ was thought to be essential to the synthesis of graphene sheets during arc-discharge process by terminating the dangling carbon bonds and preventing the formation of closed carbon structures.³⁰ However, the products obtained in high amount of H₂ gas (AC-N1/H3, AC-H and DC-H) contained abundant amorphous carbon and sheet-like structures, and most of graphene sheets were much thicker than that of AC-N1/H2 and DC-N1/H2. It seems that a second buffer gas (N₂, He or Ar) is necessary for high-quality synthesis of few-layer graphene sheets.^22–24

Fig. 2 TEM images of (a) AC-N; (b and c) AC-N1/H1; (d and e) AC-N1/H3; (f) AC-H.

Different from AC-N1/H2, not only few-layer graphene sheets but also thick-layer graphene sheets appeared in AC-N1/H3 (Fig. 2d and e). As the amount of H₂ increased, few layer graphene sheets were hardly found in AC-H, implying H₂ being an obstacle for formation of few-layer graphene sheets. However, when N₂ amount increased, spherical carbon aggregates marked by the solid black arrows in Fig. 2b, as well as a three-layer graphene sheet (Fig. 2c) were observed in AC-N1/H1. It seems that N₂ is not only in favour of formation of closed structure like SWCNHs but also plays a key role in tuning thick graphene sheets into thinner ones while H₂ facilitates the formation of sole component of graphene. Therefore, N₂ and H₂ are both necessary for the high-quality synthesis of few-layer graphene sheets, and the volume ratio of N₂ and H₂ is a key factor for the formation of few layer graphene sheets.

Fig. 3a showed the N₂ adsorption–desorption isotherms of the graphene sheets of AC-N1/H2 of a type IV adsorption isotherm. The type H3 hysteresis loop and low pressure adsorption indicated the formation of mesopores and a small amount of micropores³¹ The pore size distributions calculated by the Barrett–Joyner–Halenda (BJH) method displays that a main peak centred at ∼2 nm, which originated from the defects induced by N atoms. Brunauer–Emmett–Teller (BET) specific surface area of AC-N1/H2 was 125.39 m² g⁻¹, which is similar to that of DC-N1/H2 (123.73 m² g⁻¹) and higher than the reported 5–7 layers graphene with BET surface area of 77.8 m² g⁻¹.³² Furthermore, with the increasing amount of N₂, the increment of surface area of samples (Table S1†) further demonstrated that the much defects and less intact structure were formed at the present of N₂.

Fig. 3 (a) N₂ adsorption–desorption isotherms of AC-N1/H2 (inset: pore size distribution curves); (b) Raman spectra of AC-N1/H3, AC-N1/H2, AC-N1/H1, and AC-H.

As a powerful tool for charactering structural and electronic properties of carbon-based material, Raman spectroscopy provides useful information on the defects and graphene layers of our prepared graphene sheets. Three dominated peaks at ∼1328, 1575 and 2648 cm⁻¹ in Fig. 3b were corresponding to D, G and 2D band, respectively. The D band at ∼1326 cm⁻¹ is due to the disorder of graphene including structure defects, amorphous carbon or edges that can break the symmetry and selection rule,³³ and the I_D/I_G can reflect the defect quantity of graphene sheets.³⁴ Moreover, the graphene sheets produced in our work by arcing in only H₂ exhibit a relatively high D peak intensity, which was caused by abundant nanosheets edges due to their small size.³⁵ The 2D band originates from a two-phonon double resonance process and it is sensitive to the number of graphene layers. The absence of 2D peak of AC-N (Fig. S2b†) was a characteristic feature of SWCNHs.²⁹ Therefore, the smaller ratio of I_G/I_2D, the fewer the graphene layer number. Table 1 represents the data obtained from the Raman spectra. As shown in Table 1, all the values of I_D/I_G and I_G/I_2D of AC-N1/H1, AC-N1/H2, and AC-N1/H3 were smaller than those of AC-H, indicating more intact structure and fewer layer number of these three samples. Among these three samples, AC-N1/H2 had a minimum value of I_G/I_2D (0.97), showing a sample with fewest layer number. According to previous report that the graphene sheets by arc-discharge are mainly of 4–5 layers with an I_G/I_2D value of ∼1.03,²⁵ it could be concluded that the graphene of AC-N1/H2 is mainly less than five layers, which is in agreement with TEM and AFM results.

Table 1 Raman data of the graphene sheets synthesized under different buffer gases

Sample	Band position (cm^−1)			ID/IG	IG/I2D
	D band	G band	2D band
AC-H	1328	1577	2645	0.92	1.69
AC-N1/H3	1326	1576	2650	0.73	1.16
AC-N1/H2	1326	1573	2648	0.51	0.97
AC-N1/H1	1327	1574	2648	0.55	1.22

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The content of elements of AC-N1/H2 was characterized using an elemental analyser. As seen in Table S2,† the N and O percentages of AC-N1/H2 was 0.64 at% and no more than 2.51 at% O, respectively. This result was also confirmed by XPS result depicted in Fig. 3a. It can be calculated that AC-N1/H2 contains about 0.68 at% N and 2.33 at% O, indicating an intact and low-defect structure. Additionally, we found that with the increment of N₂ in the buffer gas, the content of N element literally increased (Table S3†) but still was below the detectable limits. Moreover, due to the low content of N, all samples produced in a mix gas of N₂ and H₂ did not exhibit the peak for C–N stretch at 1230 cm⁻¹ in the FT-IR spectra (Fig. S6†). The C1s XPS spectrum of AC-N1/H2 (Fig. 4b) showed that it is dominated by a peak due to C=C (284.6 eV) and C–C (285.0 eV). The C–N species at 285.8 eV^36,37 indicated that N atoms successfully doped into the graphene. Due to triple bonds of N₂ is much harder to be broken than N–H bond, the mixed atmosphere of N₂ and H₂ we used as buffer gases instead of NH₃ (ref. 20 and 21) could reduce the chance for enormous formation of C–N bond and generation of SWCNHs. Moreover, the sufficiently low contribution of the heteroatoms demonstrated the formation of high-quality graphene sheets.

Fig. 4 (a) Survey and (b) C1s XPS spectra of AC-N1/H2.

On the basis of above results, we proposed a mechanism about the formation of high-quality graphene sheets in this work in Scheme 1. Firstly, the alternating current makes two electrodes consume and evaporate simultaneously, eliminating the formation of cathode deposit which is normally found in direct current arc-discharge process.^19,22–24 Since these two electrodes could behave as anode and cathode alternatively, it results in an extremely high temperature in the arc-discharging region between two electrodes. This means that the diffusion rate of carbon atoms, clusters around the arc-discharge region will increase, and thereby the probability of the collision between all of the carbon species and gas molecules (i.e. N₂ or H₂) reduces. However, the distance from arc-discharge area to the only cooling source (furnace wall) is much larger than that between two electrodes, leading to a gentle temperature gradient. Furthermore, the buffer gas such as H₂ has the highest thermal conductivity and leads to a steep temperature gradient of plasma in the furnace. The cooling rate is fast for hydrogen gas, so carbon clusters are easily formed before atoms deposit to form a crystal structure.³⁸ These clusters do not have enough energy and time to arrange themselves into a long-distance ordered crystalline, and instead form a disordered structure like amorphous carbon or thick graphene sheets. To settle such a deficiency, inert gas like N₂ with low thermal conductivity will be brought in. N₂ are thought to be a key component in synthesis of SWCNHs for producing curved structure by doping N atom into the lattice and altering the temperature gradient.^18,33 Fortunately, the coexistence of H₂ can effectively restrain the formation of dangling bonds and closed structure. Consequently, graphene sheets prepared by a proper ratio of N₂ to H₂ exhibit an intact structure with low heteroatoms content.

Scheme 1 The proposed formation mechanism of graphene sheets at different atmosphere. The blue atoms represent N element; the black atoms represent C element.

The electrochemical performance of the obtained graphene sheets was also studied by applying AC-N1/H2 as an anode material for lithium-ion batteries. According to the TEM results, the graphene sheets of AC-N1/H2 were of small size (100–300 nm). Consequently, the graphene sheets easily re-stacked together and only part of them were electrochemically active at the first several cycles, leading to a low capacity of ∼210 mA h g⁻¹ (Fig. 5a). However, re-stacking graphene sheets tended to separate due to the intercalation and deintercalation of Li ion during the charge/discharge process. As a result, Li⁺ diffusion into the interior space of graphene caused by the mesopores or edge defects in the graphene flats³⁹ that leaded the reversible capacity gradually increased to ∼390 mA h g⁻¹ after a long-term cycling over 300 cycles, indicating a fully active sample with an excellent cyclic performance. In addition, additionally, rate performance of graphene sheets at different current densities (200, 500, 1000, and 2000 mA g⁻¹) was also presented in Fig. 5b. A reversible rate capacity of ∼101 mA h g⁻¹ was delivered at 2000 mA g⁻¹, which was ∼48% of the capacity obtained at 200 mA g⁻¹ in the first 20 cycles, but a discharge capacity of ∼216 mA h g⁻¹ was achieved again during the last 20 cycles at 200 mA g⁻¹. Although the capacity of AC-N1/H2 needs to be improved, it still shows an excellent cyclic retention in 300 cycles.

Fig. 5 (a) Cycle performance of AC-N1/H2 at a current rate of 200 mA g⁻¹; (b) rate capacity test of AC-N1/H2 at current densities of 200, 500, 1000, and 2000 mA g⁻¹.

Conclusions

An AC arc-discharge method using mixed N₂ and H₂ as buffer gases was applied for the large-scale production of high-quality graphene sheets. Consequently, ∼2.1 g of high-quality purified graphene products were obtained after an AC arc process less than 5 minutes and annealing post-treatment. The coexistence of N₂ and H₂ is found to be essential to the synthesis of high-quality graphene sheets with intact structure and few layer numbers. H₂ tends to lead to form the amorphous carbon and thick layer graphene, and N₂ is in favor of the formation of curved graphene by doping N atom into the graphitic lattice. By introducing H₂ which can effectively restrain the formation of dangling bonds and closed structure, the mixed buffer gas yield products of intact structures with low heteroatoms content. Specifically, the graphene sheets produced under N₂ (133 Torr)/H₂ (267 Torr) atmosphere exhibit good crystallinity and few-layer numbers. Moreover, the graphene sheets show an excellent cyclic performance and reversibility when applied as anode material for lithium storage in lithium-ion cells. In view of simplicity of experimental set-up and the use of non-toxic, odorless and low-cost buffer gases, this AC arc-discharge technique can be applied as a potential method for industrial synthesis of graphene.

Acknowledgements

We gratefully thank NSF of China (No. 21171013, No. 21471010) and the Ministry of Science and Technology of China (No. 2013CB933402) for generously supporting this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22273k

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