Graphene precursor and manufacturing method of graphene using the same

Abstract

To provide a method for efficiently producing graphene precursor and graphene in a short time.SOLUTION: A graphene precursor is synthesized by applying electric currents to an electrochemical reaction system including a cathode which includes graphite, an anode which may include graphite, and an electrolyte solution which includes a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid as an electrolyte. The obtained graphene precursor is heated to obtain graphene.SELECTED DRAWING: Figure 1

Description

Title of the invention Graphene precursor and method for producing graphene using the same

[0001] Summary of the Invention The present invention relates to the field of inorganic materials chemistry, and in particular to a method for producing graphene. Graphene is expected to have a wide range of applications in fields such as electronics due to its physical properties such as high carrier mobility, thermal conductivity, and transparency. However, conventional methods for producing graphene have faced challenges and limitations.

[0002] Conventional Technology Conventional methods for producing graphene have been proposed, including physical methods using graphite as a raw material and methods using graphene oxide, but these methods have had issues such as inefficiency and safety concerns.

[0003] Physical peeling There have been attempts to exfoliate graphite using physical methods, but the yield is low and the efficiency is poor.

[0004] Reduction of graphene oxide Another method for producing graphene is synthesis by reduction of graphene oxide. Graphene oxide can be mass-produced, but depending on the degree of oxidation, it can be difficult to completely remove oxygen, making it difficult to convert into graphene. This can lead to low-quality graphene with many defects. There are also safety issues during synthesis and wastewater disposal.

[0005] Electrochemical method Electrochemical synthesis is a method for synthesizing graphene oxide, a precursor to graphene. This method uses electricity to exfoliate graphite, and has the advantage of having fewer oxygen and defects than graphene oxide. For the electrochemical method, methods using sulfuric acid or perchloric acid as an electrolyte have been proposed. However, the following issues remain:

[0006] Problems with conventional electrochemical methods Conventional electrochemical methods have used sulfuric acid, sulfates, perchloric acid, perchlorates, tetrafluoroboric acid, tetrafluoroborates, etc. as electrolytes. Although these electrolytes are electrochemically stable, they have disadvantages in the electrochemical method, such as low graphite exfoliation performance despite being inexpensive, low solubility in salts, and explosiveness of perchloric acid and its salts.

[0007] Problem to be solved by the invention An object of the present invention is to solve the conventional problems in the method for producing graphene and to provide an efficient and safe method.

[0008] Summary of the Invention The present invention provides a new method for producing graphene precursors. This method involves immersing graphite in an electrolyte, such as an aqueous solution of trifluoromethanesulfonic acid or methanesulfonic acid, and applying a specific potential to the material, thereby simultaneously oxidizing and exfoliating it. This method allows for the reuse of the electrolyte, produces little waste liquid, and poses a low risk of explosion. Furthermore, the physical properties of the product can be tailored by controlling the electrochemical conditions.

[0009] Specific Examples of the Invention In this invention, we propose a new electrolyte method using distorted tetrahedral ions. In particular, trifluoromethanesulfonic acid and methanesulfonate ions are used. These ions have some sulfate ions substituted with trifluoromethyl or methyl groups. These ions are electrochemically stable and therefore enable the exfoliation of graphene precursors. This method enables the efficient production of graphene precursors.

[0010] Effect of the invention The present invention provides an efficient and safe method for producing graphene precursors, overcoming the challenges in graphene production. Furthermore, the use of distorted tetrahedral ions as a new electrolyte expands the options for electrochemical methods, increasing the possibility of industrialization.

[0011] The specification explains the background of the invention, the prior art, the gist of the invention, specific examples, and the effects of the invention. The contents of the specification are submitted when filing a patent application and are an important document when the patent office examines it. It provides information about the details of the invention and plays a role in establishing the validity of the patent. [Brief explanation of the drawings]

[0012] [Figure 1] A flowchart showing the steps for synthesizing a graphene precursor using an anode containing sheet-like graphite. [Figure 2] Photograph of the appearance of the anode immediately after the reaction in Example 1 [Figure 3] SEM and AFM images and profiles of the product in Example 1 Table 1. Raman spectroscopy I before and after heating of graphene precursors in Examples and Comparative Examples D / I G ratio DETAILED DESCRIPTION OF THE INVENTION

[0013] Various specific embodiments of the present invention will be described in detail below.

[0014] The present invention converts graphite, which is used as an anode, into a graphene precursor through an electrochemical reaction using a specific electrolyte.

[0015] In the present invention, the anode contains layered graphite and can be selected from a wide range of materials without any particular limitation as long as the graphite forms an intercalation compound (intercalates) with the electrolyte of the present invention. Examples include natural graphite, synthetic graphite, graphite obtained by heat-treating a condensation polymerization polymer compound, highly oriented pyrolytic graphite (HOPG), etc.

[0016] The anode may be one obtained by immersing graphite in a strong acid such as concentrated sulfuric acid or nitric acid, followed by a heat treatment step in an expansion furnace, and then molding the resulting expanded graphite using a high-pressure press. By using this as the anode, a graphene precursor can also be produced efficiently.

[0017] The graphite-containing anode is preferably in the form of a sheet.

[0018] The present invention uses an electrolyte solution containing a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid as the electrolyte. Here, the salt of trifluoromethanesulfonic acid or the salt of methanesulfonic acid as the electrolyte can be selected from compounds of salts with an anion of trifluoromethanesulfonic acid or a cation capable of forming a salt with the anion of methanesulfonic acid, but alkali metal salts, alkaline earth metal salts, and ammonium salts of trifluoromethanesulfonic acid or methanesulfonic acid are preferred. Of these, sodium salts or ammonium salts are particularly preferred.

[0019] The cathode used in the method for producing a graphene precursor of the present invention is an electrode that forms a pair with the anode. However, the cathode is not particularly limited and can be appropriately selected from a wide range of materials as long as it has the function of donating electrons to cation species generated as a result of the anodic reaction and can form an electrochemically stable system. For example, it can be selected from metals or carbonaceous materials such as platinum, graphite, titanium, stainless steel, copper, zinc, and lead. The shape of the cathode can be appropriately selected from rod-like, cylindrical, wire-like, plate-like, mesh-like, and the like.

[0020] In the production method of the present invention, an ion exchange membrane or a spacer may be placed between the anode and cathode to prevent undesired reactions from occurring at the anode or cathode or to prevent short-circuiting between the anode and cathode.

[0021] The electrode system in the manufacturing method of the present invention can achieve its purpose with the simplest configuration consisting of only the anode and cathode described above.

[0022] The electrolyte solution is a solution of the electrolyte in a solvent. Usable solvents can be appropriately selected from among solvents that are miscible with a salt of trifluoromethanesulfonic acid, a salt of methanesulfonic acid, or an aqueous solution thereof, and that are electrochemically stable when producing a graphene precursor.

[0023] Preferred solvents are protic polar solvents such as water, lower alcohols such as methanol, ethanol, and propanol, and aprotic polar solvents such as acetonitrile, dimethoxyethane, dimethyl carbonate, and propylene carbonate, etc. One of these may be used alone, or two or more may be used in combination.

[0024] Among these, water is a particularly preferred solvent in terms of electrolyte solubility, cost, and safety.

[0025] The concentration of the electrolyte in the electrolyte solution may be any concentration as long as the electrical resistance of the electrochemical reaction system is not too high and the anions of the electrolyte are quickly supplied to the graphite of the anode to obtain a graphene precursor.

[0026] The present invention uses trifluoromethanesulfonic acid, methanesulfonic acid, or a salt thereof as the electrolyte. These may also be used in combination. These electrolytes are available in pure form, and can be used after dilution and pH adjustment by adding the appropriate solvent as described above, as needed. The concentration of the electrolyte in the electrolyte solution used in this electrochemical reaction system is preferably 5.0 to 50% by mass, and more preferably 15 to 40% by mass.

[0027] In the present invention, a direct current is applied to an electrochemical reaction system comprising the above-described anode, cathode, and electrolyte solution. The voltage generated at this time should be at least sufficient to ensure the potential required for the anions of the electrolyte to be inserted between the graphite layers of the anode, but an overvoltage may be applied to rapidly obtain a graphene precursor. A practical applied voltage may be set to overcome the electrical resistance of the electrolysis system, which is governed by factors such as the electrolyte concentration, the solvent composition of the electrolyte solution, the distance between the anode and cathode, and the electrolysis temperature. The applied voltage is preferably in the range of 1.5 to 10 V, and more preferably 1.5 to 6 V.

[0028] The density of the current supplied to the anode is controlled by the applied voltage and the area of the electrode. When a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid is used as the electrolyte according to the present invention, the anions thereof intercalate extremely quickly between the graphite layers, making it possible to uniformly expand the spacing between the graphene layers. Therefore, the current density can be set over a wide range, from very small to high. For example, the current density in a relatively smooth anode such as a sheet or plate is preferably 10 to 500 mA / cm. 2 and more preferably 50 to 200 mA / cm 2 is.

[0029] In the present invention, it is also preferable to set the current supplied to the anode to a constant value. In this case, the preferable set current value is set so as to fall within the above-mentioned preferable current density range.

[0030] The temperature of the electrolyte solution when passing a current through the electrochemical reaction system may vary depending on the type of solvent in which the electrolyte is dissolved and the concentration of the electrolyte solution, but in practice, the lower limit is a temperature at which the electrolyte solution does not freeze, and the upper limit is the boiling point of the electrolyte solution. The temperature range is preferably 0 to 100°C. More preferably, the temperature range is 20 to 60°C.

[0031] In the present invention, the electrolyte solution used for producing a graphene precursor can be repeatedly reused. In this case, the amount of electrolyte lost due to adhesion to the crude graphene precursor product extracted from the electrolyte solution may be replenished to the reaction system as needed.

[0032] In the above-described electrolyte solution recovery step, recovery of an electrolyte solution containing a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid does not require highly acid-resistant equipment or an operating environment, as compared with, for example, using free acids such as trifluoromethanesulfonic acid or methanesulfonic acid as the electrolyte. Therefore, recovery of an electrolyte solution containing a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid can be performed in a mild environment using a wide selection of general-purpose devices, making it more advantageous in large-scale production of graphene precursors.

[0033] The crude graphene precursor product extracted from the electrolyte solution can be washed with excess deionized water until the washing liquid becomes neutral, thereby removing the electrolyte solution components from the structure.

[0034] The crude product obtained by the above steps can be supplied in a wet state to the subsequent step of producing a graphene precursor, or may be subjected to a drying step as necessary before being supplied to the subsequent step of producing a graphene precursor. The specific drying method is not particularly limited, but may be, for example, drying at a temperature of 80°C or less in a constant temperature dryer or a vacuum dryer.

[0035] As described above, in the present invention, by using an anode containing graphite and passing a current through an electrochemical reaction system using an electrolyte solution containing a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid as the electrolyte, anions of the electrolyte are quickly and uniformly intercalated between the graphite layers without bias, thereby obtaining a graphene precursor in which the interlayer distance between each graphene constituting the graphite is uniformly enlarged.

[0036] By subjecting the graphene precursor obtained by the present invention to an exfoliation operation, it is possible to obtain a graphene precursor preferably having a thickness of 100 nm or less.

[0037] The exfoliation operation is not particularly limited and a wide variety of general-purpose operations can be applied. Examples include an exfoliation operation by ultrasonic irradiation, an exfoliation operation by applying a mechanical exfoliation force, and an exfoliation operation by heating. More specifically, examples include a method in which the graphene precursor is dispersed in an appropriate amount of deionized water and then subjected to an ultrasonic irradiation device, or a method in which the graphene precursor is treated with a mixer or a device capable of applying shear force. After the exfoliation operation, the treated product may be freeze-dried, or the cake obtained by filtering or centrifuging may be subjected to a drying treatment similar to the drying treatment for the graphene precursor described above.

[0041] As a result, a graphene precursor having a thickness of 100 nm or less can be advantageously obtained. The average thickness of the graphene precursor is more preferably 50 nm or less, and even more preferably 3 nm or less. A graphene precursor having a thickness of 1 nm or less is particularly preferred.

[0038] The graphene precursor suitably produced by the present invention reflects the features of the production method of the present invention, and is characterized by a small average particle thickness, high purity, and a low content of impurities, as well as a low content of heavy metal components and sulfur components. [Example]

[0039] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0040] <Method for measuring the minimum thickness of graphene precursor> The minimum thickness of the graphene precursor was measured using an atomic force microscope (AFM). Specifically, a dilute dispersion of the graphene precursor was applied to a mica substrate, and measurements were performed using a Shimadzu SPM-9700HT in tapping mode. [Example]

[0041] 100 mL of a 2 M aqueous solution of trifluoromethanesulfonic acid was added to a glass reactor as an electrolyte. A graphite foil (a foil made by heat-treating an aromatic polyimide, a condensation polymerization polymer compound, to make graphite) was placed in the reactor, covering 15 cm of its area, as an anode. 2 The graphite (120 mg) was fixed so that it was immersed in the electrolyte, and a platinum wire was set as the cathode. The outline of the embodiment is shown in Figure 1. This was connected to a DC power source and 100 mA / cm was applied at room temperature. 2 Electrolysis was performed for 15 minutes at a constant current of 0.05V. During this process, a smooth increase in the thickness of the graphite foil and browning of the surface were observed at the working electrode in the area immersed in the electrolyte. Figure 2 shows a photograph of the anode immediately after the reaction. After the reaction, the anode maintained its intact sheet shape with almost no peeling or shedding in the electrolyte. However, the thickness of the immersed area was clearly increased compared to before the reaction. The thickened sheet could be easily penetrated with a plastic spatula, indicating that the electrolyte intercalated uniformly and quickly to the deepest part of the graphite foil, even during the extremely short electrolysis period, resulting in the formation of a graphene precursor.

[0042] After the reaction, the crude product sheet was removed from the electrolyte and washed with deionized water until the washings were neutral, yielding a dark brown crude graphene precursor. A small amount of deionized water was added, followed by ultrasonic irradiation for 15 minutes and subsequent freeze-drying to yield 200 mg of graphene precursor. AFM analysis revealed an average thickness of approximately 1.4 nm and a minimum thickness of 0.8 nm. The cathode showed no change in appearance before and after the reaction, confirming its stability under the electrolysis conditions. SEM-EDS analysis revealed an oxygen content of 2.6, confirming the presence of a significant amount of oxygen. Furthermore, electrical conductivity was measured to be 0.142 ± 0.023 S / cm, confirming the decrease in conductivity due to the presence of a large amount of oxygen. [Example]

[0043] The electrolyte was 2M trifluoromethanesulfonic acid, and the current density was 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 2.5, indicating that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 0.083 ± 0.001 S / cm, indicating that the conductivity was reduced due to the presence of a large amount of oxygen. [Example]

[0044] The electrolyte was 2M sodium trifluoromethanesulfonate, and the current density was 100mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 2.4, indicating that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 1.47 ± 0.01 S / cm, indicating that the conductivity was reduced due to the presence of a large amount of oxygen. [Example]

[0045] The electrolyte was 2M sodium trifluoromethanesulfonate, and the current density was 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 2.2, confirming that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 4.31 ± 0.16 S / cm, confirming that the presence of a large amount of oxygen reduced the conductivity. [Example]

[0046] The electrolyte was 2M ammonium trifluoromethanesulfonate, and the current density was 100mA / cm 2An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 2.4, confirming that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 1.90 ± 0.11 S / cm, confirming that the conductivity was reduced due to the presence of a large amount of oxygen. [Example]

[0047] The electrolyte was 2M ammonium trifluoromethanesulfonate, and the current density was 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 2.2, confirming that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 1.59 ± 0.10 S / cm, confirming that the conductivity was reduced due to the presence of a large amount of oxygen. [Example]

[0048] The electrolyte was 2M magnesium trifluoromethanesulfonate, and the current density was 100mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 2.0, indicating that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 0.991 ± 0.059 S / cm, indicating that the conductivity was reduced due to the presence of a large amount of oxygen. [Example]

[0049] The electrolyte used was 2M magnesium trifluoromethanesulfonate, and the current density was 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 1.9, indicating that a considerable amount of oxygen was contained. In addition, when the electrical conductivity was measured, it was found to be 0.415 ± 0.009 S / cm, indicating that the conductivity was reduced due to the presence of a large amount of oxygen. [Example]

[0050] The electrolyte was 2M methanesulfonic acid, and the current density was 100mA / cm 2 The electrochemical method was performed using the same method as in Example 1. The product was brittle due to gas generation from within the graphite, and the graphite sheet was unable to maintain its shape and collapsed. The measurable thickness was 10 mm, indicating a 250-fold expansion. The color also changed from the metallic luster of the graphite sheet to a dark gold color (Figure 1, steps 1-2). This color disappeared when the product was dispersed, confirming that this was a structural color due to light interference and that ions were intercalated within the product. The oxygen content was confirmed by SEM-EDS, revealing that the oxygen content was 4.0, indicating that the amount of oxygen introduced during oxidation was lower than in conventional oxidation and exfoliation methods. Additionally, the electrical conductivity was measured to be 180±4 S / cm, confirming that the low oxygen content contributed to the high electrical conductivity. [Example]

[0051] The electrolyte was 2M methanesulfonic acid, and the current density was 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed using SEM-EDS, and it was found that the oxygen content was C / O = 4.0, which confirmed that the amount of oxygen introduced during oxidation was less than that in the conventional oxidation and stripping method. In addition, the electrical conductivity was measured and found to be 147 ± 6 S / cm, confirming that the electrical conductivity was high due to the low oxygen content. [Example]

[0052] The electrolyte was 2M sodium methanesulfonate, and the current density was 100mA / cm 2An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed using SEM-EDS, and it was found that the oxygen content was C / O = 3.9, which confirmed that the amount of oxygen introduced during oxidation was less than that in the conventional oxidation and stripping method. In addition, when the electrical conductivity was measured, it was found to be 159 ± 19 S / cm, confirming that the electrical conductivity was high due to the low oxygen content. [Example]

[0053] The electrolyte was 2M sodium methanesulfonate, and the current density was 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed using SEM-EDS, and it was found that the oxygen content was C / O = 3.1, which confirmed that the amount of oxygen introduced during oxidation was less than that in the conventional oxidation and stripping method. In addition, when the electrical conductivity was measured, it was found to be 129 ± 10 S / cm, confirming that the electrical conductivity was high due to the low oxygen content. [Example]

[0054] The electrolyte was 2M ammonium methanesulfonate, and the current density was 100mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed using SEM-EDS, and it was found that the oxygen content was C / O = 3.4, which confirmed that the amount of oxygen introduced during oxidation was less than that in the conventional oxidation and stripping method. In addition, when the electrical conductivity was measured, it was found to be 162 ± 9 S / cm, confirming that the electrical conductivity was high due to the low oxygen content. [Example]

[0055] The electrolyte was 2M ammonium methanesulfonate, and the current density was 200mA / cm 2An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed using SEM-EDS, and it was found that the oxygen content was C / O = 2.8, which confirmed that the amount of oxygen introduced during oxidation was less than that in the conventional oxidation and stripping method. In addition, the electrical conductivity was measured and found to be 142 ± 7 S / cm, confirming that the electrical conductivity was high due to the low oxygen content. [Example]

[0056] The electrolyte is saturated magnesium methanesulfonate, and the current density is 100mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 1.3, indicating that a large amount of oxygen was introduced during oxidation. In addition, the electrical conductivity was measured and found to be 29.8 ± 0.3 S / cm, indicating that the conductivity was reduced due to the inclusion of a large amount of oxygen. [Example]

[0057] The electrolyte is saturated magnesium methanesulfonate, and the current density is 200mA / cm 2 An electrochemical method was carried out using the same method as in Example 1. The elements contained were confirmed by SEM-EDS, and it was found that the oxygen content was C / O = 1.1, indicating that a large amount of oxygen was introduced during oxidation. In addition, the electrical conductivity was measured and found to be 48.8 ± 1.7 S / cm, indicating that the conductivity was reduced due to the inclusion of a large amount of oxygen.

[0058] (Comparative Example 1) The graphite was peeled off using a two-electrode system. A polyimide-derived graphite sheet (3 x 7 cm, immersion area 3 x 5 cm) was used as the working electrode, and a platinum wire was used as the counter electrode. 2M sulfuric acid was used as the electrolyte, and the current density was 200 mA / cm. 2The experiment was carried out at 1000 K. When an oxidation current was applied, the reaction proceeded as if the surface was breaking down, and no oxidation was observed, causing expansion. The crude product that floated to the surface also had a black metallic luster, indicating that intercalation had not progressed much. The product was produced and isolated by centrifugation, and then dispersed in water. When the elements contained were confirmed using SEM-EDS, the oxygen content was found to be C / O = 3.5, and graphite was found to remain. In addition, the electrical conductivity was measured to be 115 ± 4 S / cm. [Example]

[0059] The graphite was exfoliated using a two-electrode system. An expanded graphite sheet (1 x 5 cm, immersion area 1 x 3 cm) was used as the working electrode, and a platinum wire was used as the counter electrode. 2 M trifluoromethanesulfonic acid was used as the electrolyte, and the current density was 50 mA / cm. 2 An electrochemical method was carried out using an electrochemical method. By applying an oxidation current, a 0.38 mm thick expanded graphite sheet expanded to a thickness of 8 mm, demonstrating a 20-fold expansion rate (steps 1 to 2). The product was purified and isolated by filtration, and then dispersed in water. When the elements contained were confirmed using SEM-EDS, the oxygen content was found to be C / O = 2.9, confirming that a large amount of oxygen was introduced during oxidation. In addition, when the electrical conductivity was measured, it was found to be 3.53 ± 2.06 S / cm, confirming that the presence of a large amount of oxygen reduced the conductivity. [Example]

[0060] The electrolyte was 2M methanesulfonic acid, and the current density was 50mA / cm 2 An electrochemical method was carried out under the same conditions as in Example 17. Application of an oxidation current caused the 0.38 mm thick expanded graphite sheet to expand to a thickness of 12 mm, demonstrating a 30-fold expansion rate (procedures (1) to (2)). Confirmation of the elements contained in the sheet by SEM-EDS revealed an oxygen content of C / O = 5.3, confirming that the amount of oxygen introduced during oxidation was less than that in conventional oxidation and exfoliation methods. Furthermore, electrical conductivity was measured and found to be 327 ± 18 S / cm, confirming that the low oxygen content resulted in high electrical conductivity.

[0061] (Comparative Example 2) The graphite was exfoliated using a two-electrode system. An expanded graphite sheet (1 x 5 cm, immersion area 1 x 3 cm) was used as the working electrode, and a platinum wire was used as the counter electrode. 2M sulfuric acid was used as the electrolyte, and the current density was 50 mA / cm. 2 An electrochemical method was performed using the ion beam. Application of an oxidizing current caused the material to peel off, crumbling from the surface. The product was purified and isolated by filtration, and then dispersed in water. Confirmation of the elements contained in the material using SEM-EDS revealed an oxygen content of C / O = 3.7 and residual graphite. In addition, electrical conductivity was measured and found to be 228 ± 11 S / cm. [Example]

[0062] Using the products of Examples 1, 9, 17, and 18 and Comparative Examples 1 and 2, thin films of graphene precursors were prepared on glass substrates. The precursor films were heated to 500°C in an electric furnace, and the microstructures before and after heating were measured by Raman spectroscopy. Measurements were performed from 1100 to 3300 cm -1 The Raman spectrum obtained contained the G band (1582 cm ) derived from the planar structure of graphene. -1 ) and the D band (1580 cm ) originating from defects and edges -1 ), as well as the 2D band (2685 cm ) due to the phenon lattice vibration. -1 ) was confirmed. The quality of two-dimensional materials is determined by the ratio of planar structures to defects. D / I G Table 1 shows the I before and after the reaction. D / I G The graphene precursors of Examples 1 and 9, which were made from graphite sheets derived from polyimide films, showed a change in the I ratio. D / I G Although no significant change occurred in I due to heating, in the graphene precursors of Examples 17 and 18, which were made from expanded graphite sheets, I D / I G has decreased significantly, and sp 2 It was confirmed that domain repair had occurred. These graphene precursors were converted to graphene. Similar tests were also performed on the products of Comparative Examples 1 and 2, and ID / I G A slight decrease was confirmed, but the change was not as large as in Examples 17 and 18. These results confirmed that the graphene precursors oxidized and exfoliated with trifluoromethanesulfonic acid and methanesulfonic acid have excellent properties as precursors. [Table 1]

[0063] From the above results, it can be seen that the present invention, which uses a salt of trifluoromethanesulfonic acid or a salt of methanesulfonic acid as an electrolyte when producing a graphene precursor by utilizing an electrochemical reaction, can produce a graphene precursor of high quality with a thinner average particle thickness, which can then be heat-treated to produce graphene.

Claims

1. A method for producing graphene, comprising the steps of: A step of immersing a graphite sheet in an aqueous electrolyte. b) applying a noble potential to the graphite sheet; c) Oxidizing and exfoliating the graphite sheets.

2. In claim 1, the aqueous electrolyte is an electrolyte containing trifluoromethanesulfonic acid or methanesulfonic acid and its salts.

3. In claim 1, the product after oxidation and exfoliation contains graphene precursor exfoliated to a single layer, and graphene is produced by heating to 500°C.

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