Method for producing graphene oxide

Abstract

To provide a method that enables target graphene oxide to be produced safely, efficiently, and advantageously in comparison with conventional methods.SOLUTION: In a method for producing graphene oxide by oxidizing graphite, target graphene oxide is produced by the following steps: adding a permanganate as an oxidizing agent to a mixed liquid containing graphite and sulfuric acid; and adding a divalent iron salt to the mixed liquid derived from the previous step, to which the permanganate has been added.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present application relates to a method for producing graphene oxide, and in particular to a method that can produce the desired graphene oxide safely, efficiently, and advantageously as compared to conventional methods. [Background technology]

[0002] Graphene oxide (GO) is a sp 2 It has a structure in which functional groups containing oxygen (oxygen functional groups), such as hydroxyl groups and carboxyl groups, have been introduced into graphene, which has a layered structure in which carbon atoms are arranged in a plane by bonds. Focusing on the unique structure and physical properties of graphene oxide, research into its use and application has been progressing in various technical fields in recent years, and it is expected that graphene oxide will be used and utilized, for example, as a catalyst material, an electrode active material for batteries, an electrode material for capacitors, a thermoelectric conversion material, a light-emitting material, a lubricant material, etc.

[0003] As for graphene oxide, which is expected to be used in various technical fields, a method for producing graphene by adding potassium permanganate as an oxidizing agent to a reaction system under ice cooling to oxidize graphene, known as the "Hummers method," has been widely known (see Non-Patent Document 1). In recent years, various production methods improved from the Hummers method and production methods derived from the Hummers method have been proposed in order to improve safety during production and to improve the quality of the graphene oxide obtained (see Patent Documents 1 to 3). In particular, Patent Document 4 proposes a method for efficiently producing graphite oxide (graphene oxide) of stable quality, which includes a step of adding an oxidizing agent to a mixed solution containing graphite and sulfuric acid to oxidize the graphite, and a step of adding the mixed solution obtained in the oxidation step to water or hydrogen peroxide solution in an amount of 200% by mass or more relative to 100% by mass of the mixed solution. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2011-148701 A [Patent Document 2] JP 2002-53313 A [Patent Document 3] JP 2017-88450 A [Patent Document 4] JP 2017-88451 A [Non-patent literature]

[0005] [Non-Patent Document 1] William S. Hummers, et.al, Journal of American Chemical Society, 1958, 80, 1339 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, in a method for producing graphene oxide by adding permanganate as an oxidizing agent to a mixed liquid containing graphite and sulfuric acid to oxidize the graphite, it is widely known that hydrogen peroxide is used as a reaction terminator (quenching agent) for stopping the oxidation reaction, as in the production method described in the above-mentioned Patent Document 4. However, the use of hydrogen peroxide as a quenching agent in producing graphene oxide has the following problems.

[0007] First, hydrogen peroxide is a hazardous material and has a risk of explosion even during storage, so it must be stored in the form of an aqueous solution with a concentration of, for example, about 30 wt%. Furthermore, when hydrogen peroxide acts as a quenching agent, it generates oxygen and heat as decomposition products, which causes rapid expansion and heat generation. Therefore, in order to safely manufacture graphene oxide, hydrogen peroxide with a concentration of about 30 wt% is further diluted and used, and a cooling device is used to cool the reaction tank, etc. Furthermore, hydrogen peroxide also has the risk of decomposition during storage due to light, heat, and contamination (impurities) such as metal ions and metal particles.

[0008] Under such circumstances, the present inventors have conducted various studies on quenching agents used in producing graphene oxide, and have found that the above-mentioned problems associated with the use of hydrogen peroxide water can be advantageously solved when a divalent iron salt such as iron sulfate (II) is used as a quenching agent, leading to the completion of the present invention. That is, the present invention has been made against the background of such circumstances, and the problem to be solved by the present invention is to provide a method that can produce the desired graphene oxide safely, efficiently, and advantageously as compared with conventional methods. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, the present invention provides a method for producing graphene oxide by oxidizing graphite, the method including the steps of adding permanganate as an oxidizing agent to a mixed liquid containing graphite and sulfuric acid, and adding a divalent iron salt to the mixed liquid to which the permanganate has been added.

[0010] Here, in such a method for producing graphene oxide according to the present invention, the divalent iron salt is preferably iron (II) sulfate, and the iron (II) sulfate is added in a ratio of 2.27 to 4.54 parts by mass per 1.00 part by mass of the graphite, calculated as anhydrous.

[0011] In the method for producing graphene oxide according to the present invention, the permanganate is preferably sodium permanganate or potassium permanganate. Effect of the Invention

[0012] As described above, in the graphene oxide production method according to the present invention, a divalent iron salt is used as a reaction terminator (quenching agent) for stopping the oxidation reaction, which advantageously eliminates the problems associated with the use of conventional hydrogen peroxide solution and makes it possible to safely, efficiently, and advantageously produce the desired graphene oxide, more specifically, graphene oxide with an adjusted degree of oxidation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Meanwhile, when producing graphene oxide according to the present invention, first, a step (hereinafter referred to as step I) of adding permanganate as an oxidizing agent to a mixed liquid containing graphite and sulfuric acid is carried out.

[0014] Here, the graphite, sulfuric acid, and permanganate as an oxidizing agent used in the above step I in the present invention can be any of those that have been used in the past for producing graphene oxide, as long as they do not impede the object of the present invention. Specifically, examples of the graphite used in the present invention include natural graphite and various artificial graphites, and as for the sulfuric acid, commercially available concentrated sulfuric acid can be used as it is, or sulfuric acid diluted with water can be used.

[0015] Examples of permanganate salts used as an oxidizing agent include sodium permanganate, potassium permanganate, ammonium permanganate, silver permanganate, zinc permanganate, magnesium permanganate, calcium permanganate, barium permanganate, etc., and one or more of these salts are appropriately selected and used. In the present invention, sodium permanganate or potassium permanganate is advantageously used.

[0016] Such graphite, sulfuric acid, and permanganate are used to add permanganate as an oxidizing agent to a mixed liquid containing graphite and sulfuric acid, and the method and various conditions therefor, as well as additional operations, can be appropriately selected and adopted from methods, etc. adopted in conventional graphene oxide manufacturing methods. For example, in order to improve efficiency in oxidizing graphite, phosphoric acid or the like can be added simultaneously with, before, or after the addition of permanganate to the mixed liquid containing graphite and sulfuric acid.

[0017] The graphene oxide production method according to the present invention has a major technical feature in that a divalent iron salt is used as a reaction terminator. That is, when an oxidizing agent such as a permanganate is added to a mixed solution containing graphite and sulfuric acid, the oxidation of graphite is immediately initiated by permanganate ions, and some of the permanganate ions penetrate between adjacent layers of the graphene oxide produced, chemically bonding the two graphene oxide layers. Such oxidation of graphite and bonding of adjacent graphene oxide layers proceed as long as permanganate ions are present in the reaction system, and therefore various compounds for terminating these reactions have been used as reaction terminators in the past. In conventional graphene oxide production methods, however, hydrogen peroxide solution has generally been used as a reaction terminator. However, as mentioned above, the use of hydrogen peroxide as a reaction stopper has various inherent problems, and a new reaction stopper to replace hydrogen peroxide was needed. As a result of intensive research by the inventors, they found that divalent iron salts function effectively as reaction stoppers even when used instead of hydrogen peroxide, and thus completed the present invention.

[0018] Here, the divalent iron salt used as the reaction stopper in the present invention can be any one as long as it does not inhibit the production of graphene oxide, but iron (II) sulfate is particularly advantageously used. The amount of use is appropriately determined depending on the type of divalent iron salt used and the properties of the target graphene oxide. For example, when iron sulfate (II) is used as the divalent iron salt, if the main purpose is a) to stop the oxidation reaction of graphite only, it is preferable that iron sulfate (II) is used in a ratio of less than 2.27 parts by mass per 1.00 part by mass of graphite, calculated as an anhydride. In addition, if the main purpose is b) to stop the oxidation reaction of graphite and to decompose permanganate ions that exist between adjacent graphene oxide layers and chemically bond the two graphene oxide layers to produce thin-layer graphene oxide having about 3 to 4 layers, it is preferable that iron sulfate (II) is used in a ratio of 2.27 to 4.54 parts by mass per 1.00 part by mass of graphite, calculated as an anhydride. Furthermore, c) when producing graphene oxide having a lower degree of oxidation than general graphene oxide, it is preferable to use iron (II) sulfate in an anhydrous equivalent in a ratio of more than 4.54 parts by mass per 1.00 part by mass of graphite.

[0019] Furthermore, with regard to the method for adding a divalent iron salt as a reaction terminator to the liquid in which graphene oxide production is progressing (reaction liquid: a liquid obtained by adding permanganate to a mixed liquid containing graphite and sulfuric acid), and the various conditions therefor, any method, etc. can be adopted as long as it does not impede the object of the present invention.

[0020] The reaction solution to which the divalent iron salt has been added in this manner is washed according to a conventionally known washing method as necessary, whereby the desired graphene oxide can be obtained as a solid or in the form of a dispersion.

[0021] As described above, in the method for producing graphene oxide according to the present invention, a divalent iron salt is used as a reaction terminator (quenching agent) for terminating the oxidation reaction, and therefore the problems associated with the use of conventional hydrogen peroxide solution are advantageously eliminated. Furthermore, by appropriately adjusting the type of divalent iron salt and the amount used, it is possible to safely, efficiently, and advantageously produce the desired graphene oxide, such as graphene oxide with an adjusted oxidation degree. EXAMPLES

[0022] The present invention will be described in more detail below by showing some examples of the present invention, but it goes without saying that the present invention is not limited in any way by the description of such examples. It should be understood that the present invention can be modified, altered, improved, etc., based on the knowledge of a person skilled in the art, in addition to the following examples and the above specific description, without departing from the spirit of the present invention.

[0023] -Production of graphene oxide (preparation of graphene oxide dispersion)- First, 21.34 g of concentrated sulfuric acid (Wako Grade 1, Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.47 g of natural graphite (product name: MD+595, flake graphite, Fuji Graphite Industries, Ltd.) were added to a 200 mL beaker to prepare a mixed solution a. While stirring the mixed solution a, 2.13 g of concentrated phosphoric acid (Grade 1, Hayashi Pure Chemical Industries, Ltd.) and 1.88 g of potassium permanganate (special reagent grade, Fujifilm Wako Pure Chemical Industries, Ltd.) were further added to the mixed solution a in the beaker to start oxidation of the graphite. After the addition of potassium permanganate was completed, the beaker was heated to raise the liquid temperature in the beaker to 35°C, and after the liquid temperature reached 35°C, stirring in the beaker was continued for 4 hours while maintaining the temperature.

[0024] After stirring was completed, the liquid in the beaker containing the graphite, whose oxidation had either been completed or was still in progress (hereinafter referred to as mixed liquid b), was cooled to room temperature (20°C), and the cooled mixed liquid b was added (dropped) over 15 minutes into a beaker containing 100 g of water (room temperature: 20°C). Note that the water in the beaker was constantly stirred from the start to the end of the addition (dropping) of mixed liquid b, and the temperature of the water in the beaker was maintained at 45°C or lower.

[0025] After the addition (dropping) of the mixed solution was completed, the amount of iron(II) sulfate heptahydrate (special grade, manufactured by Kanto Chemical Co., Ltd.) shown in Table 1 was added to the beaker. Table 1 also shows the amount of iron(II) sulfate heptahydrate added in each example, converted into anhydrous iron(II) sulfate.

[0026] After adding iron (II) sulfate heptahydrate to the beaker, the liquid in the beaker was washed with hydrochloric acid to obtain solid fraction I. This hydrochloric acid washing was carried out by repeating five times (five cycles) the following series of operations: 1) centrifuging the liquid (10,000G, 5 minutes) to separate it into a supernatant and a precipitate, and 2) washing the precipitate with 5% hydrochloric acid (reagent grade 1, Fujifilm Wako Pure Chemical Industries, Ltd.).

[0027] Furthermore, pure water was added to the solid fraction I obtained after the above-mentioned hydrochloric acid washing to prepare a mixture, and the mixture was subjected to a centrifugal separation operation (10,000 G, 5 minutes) and the supernatant liquid was removed. This operation was repeated five times to obtain solid fraction II.

[0028] Pure water was added to the solid content II obtained as described above so that the proportion of solid content II in the prepared liquid was 0.3 mass%, and then centrifugation was performed (6000 G, 5 minutes) to prepare a dispersion containing graphene oxide at a proportion of 0.3 mass%.

[0029] -Evaluation of graphene oxide dispersion- First, the colors (liquid colors) of five types of graphene oxide dispersions obtained according to the above-mentioned method were evaluated. Specifically, five observers directly visually compared each dispersion with a yellow (R: 255, G: 217, B: 0) and black (R: 19, G: 0, B: 18) color sample, and each observer showed the color sample that was judged to be closer. When the observers' judgments differed, the color sample that the majority of observers judged to be closer was recognized as the liquid color of the dispersion. The evaluations of each dispersion according to Examples 1 to 5 are shown in Table 1 below.

[0030] In addition, ultraviolet-visible spectrophotometric measurements (UV measurements) were performed on the five types of graphene oxide dispersions. The dispersion of Example 1 had a peak at 230.2 nm, the dispersions of Examples 2 to 4 had peaks at 230.3 nm, and the dispersion of Example 5 had a peak at 229.2 nm.

[0031] From the above, for the dispersions in which the liquid color was yellow and the peak in UV measurement was 230.3 nm (Examples 2 to 4), the graphene oxide contained therein was designated "GO-a." For the dispersion in which the liquid color was black and the peak in UV measurement was 230.2 nm (Example 1), the graphene oxide contained therein was designated "GO-b." Furthermore, for the dispersion in which the liquid color was black and the peak in UV measurement was 229.2 nm (Example 5), the graphene oxide contained therein was designated "GO-c."

[0032] -Measurement of GO-a yield- Each of the dispersions according to Examples 2 to 4 was heated on a hot plate at 60° C. for 30 minutes to remove moisture. The mass of the resulting residue (GO-a) was measured, and the results are shown in Table 1 below.

[0033] [Table 1]

[0034] First, the peak of UV measurement of graphene oxide is generally confirmed at 230.4±0.2 nm, but the liquid color of the dispersion according to Example 1 was black, whereas the liquid color of each of the dispersions according to Examples 2 to 4 was yellow. The difference between the dispersion according to Example 1 and each of the dispersions according to Examples 2 to 4 is only the amount of iron (II) sulfate heptahydrate used. Therefore, it is judged that the graphene oxide (GO-b) in the dispersion according to Example 1 still has many permanganate ions between adjacent graphene oxide layers, and is in an unpeeled state, compared with the graphene oxide (GO-a) in the dispersion according to Examples 2 to 4. On the other hand, the graphene oxide (GO-c) in the dispersion according to Example 5 obtained using a large amount of iron (II) sulfate heptahydrate has Mn atoms migrated from the permanganate ions that existed between adjacent graphene oxide layers to Mn atoms due to the excess iron (II) sulfate heptahydrate. 7+ It is believed that not only was the compound released in the form of ions, but the graphene oxide layer was also reduced, causing the UV measurement peak to be lower than the UV peak value of general graphene oxide.

[0035] In addition, focusing on the graphene oxide dispersions according to Examples 2 to 4, the yield of graphene oxide (GO-a) increased with an increase in the amount of iron (II) sulfate heptahydrate used.

[0036] As is clear from the above results, by using iron (II) sulfate, which is a divalent iron salt, as a quenching agent (reaction stopper) as in the present invention and by appropriately changing the amount of iron sulfate used, it is possible to form a large amount of permanganate ions (Mn 7+ ions) are intercalated in the unexfoliated graphene oxide, and Mn 7+ It is recognized that the desired graphene oxide, such as a thin-layer graphene oxide from which ions have been desorbed, or even a graphene oxide having a lower degree of oxidation than general graphene oxide, can be advantageously produced.

Claims

1. A method for producing graphene oxide by oxidizing graphite, A step of adding a permanganate as an oxidizing agent to a mixed liquid containing graphite and sulfuric acid; adding a divalent iron salt to the mixture to which the permanganate has been added; A method for producing graphene oxide having the structure.

2. 2. The method for producing graphene oxide according to claim 1, wherein the divalent iron salt is iron (II) sulfate, and the iron (II) sulfate is added in a ratio of 2.27 to 4.54 parts by mass per 1.00 part by mass of the graphite in terms of anhydride.

3. The method for producing graphene oxide according to claim 1 or 2, wherein the permanganate is sodium permanganate or potassium permanganate.

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