Graphene manufacturing method

Abstract

To mass-produce graphene at low cost.SOLUTION: The method comprises a recovery step of recovering a gas containing carbon dioxide by removing water from a mixture containing the water and the carbon dioxide, and a generation step of generating graphene by heating the recovered gas containing the carbon dioxide in a quartz vessel at a temperature of not more than 1700°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing graphene. [Background technology]

[0002] There are three conventional methods for producing graphene. The first method is the CVD method, which chemically synthesizes graphene from raw material gases in the gas phase (Patent Document 1). CVD is a method in which a carbon precursor is converted to graphene on a catalyst surface. The second method is the Scotch tape method, which mechanically peels graphene from graphite crystals (Non-Patent Document 1). The Scotch tape method involves attaching tape to a layered substance, peeling it off, and then repeating the same process on the remaining part of the tape to create a single-layer (or a few layers) substance. The third method is the Hummers method, which exfoliates graphene by oxidation in a liquid phase (Patent Document 2). The Hummers method synthesizes graphene by oxidizing graphite. Graphite oxide is prepared from natural flake graphite, and reduced graphene oxide is obtained by chemical reduction of the graphene oxide. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5686418 [Patent Document 2] Japanese Patent Publication No. 2024-022310 [Non-patent literature]

[0004] [Non-Patent Document 1] Novoselov, KS et al. "Electric field effect in atomically thin carbon films" Science 306 (2004) 666-669. Summary of the Invention [Problem to be solved by the invention]

[0005] However, Patent Documents 1 and 2 and Non-Patent Document 1 have the problem that it is difficult to mass-produce graphene at low cost, making them unsuitable for widespread industrial use. For example, the CVD method requires a metal catalyst and a transfer process, making mass production difficult and resulting in high manufacturing costs. The Scotch tape method requires the repeated task of attaching a monolayer film to a substrate, making it unsuitable for commercial graphene production. The Hummers method requires a process to reduce graphene oxide, making mass production difficult and resulting in high manufacturing costs.

[0006] Therefore, an object of the present invention is to mass-produce graphene at low cost. [Means for solving the problem]

[0007] In order to achieve the object of the present invention, the present invention comprises the following features: a recovery step of removing water from a mixture containing water and carbon dioxide to recover the gas containing carbon dioxide, and a production step of heating the recovered gas containing carbon dioxide in a quartz container at a temperature of 1700°C or less to produce graphene. [Effects of the Invention]

[0008] According to the present invention, graphene can be mass-produced at low cost. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows the results of XPS analysis of the graphenes of Examples 1 and 2 and a comparative example (commercially available graphite). DETAILED DESCRIPTION OF THE INVENTION

[0010] The following describes the embodiments in detail. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more features among the multiple features described in the embodiments may be combined in any desired manner.

[0011] <Graphene manufacturing method> A method for producing graphene according to one embodiment includes a recovery step of removing water from a mixture containing water and carbon dioxide to recover a gas containing carbon dioxide, and a production step of producing graphene by heating the recovered gas containing carbon dioxide in a quartz vessel at a temperature of 1700° C. or less. Graphene has electrical conductivity, optical properties, spin transport, magnetic field effects, and the like, and can be used, for example, as a component of electronic devices.

[0012] <Mixture> The mixture contains water and carbon dioxide. Examples of the mixture include natural gas, and gases (hereinafter also referred to as exhaust gas) exhausted from thermal power plants, boilers in manufacturing plants, kilns in cement factories, blast furnaces and converters in steelworks, incinerators, and the like. In a graphene production method according to one embodiment, water may be directly removed from the mixture, and then the carbon dioxide-containing gas may be heated in a quartz container to produce graphene. In a graphene production method according to another embodiment, the mixture may be contacted with calcium hydroxide to recover carbon dioxide as calcium carbonate, and the carbon dioxide may be separated from active components in the natural gas and other component gases in exhaust gases from power plants, etc., and the recovered calcium carbonate may then be calcined and used as the mixture. For convenience of explanation, the graphene production method of the present invention will be described using a graphene production method according to another embodiment as an example.

[0013] (carbon dioxide) The carbon dioxide is brought into contact with an aqueous dispersion of calcium hydroxide, which will be described later, and recovered as calcium carbonate, which will be described later. In this way, the carbon dioxide is treated (consumed) by contacting with the aqueous dispersion of calcium hydroxide. Here, the concentration of carbon dioxide in one embodiment is 5% by volume or more, 10% by volume or more, or 20% by volume or more in the natural gas and exhaust gas. The concentration of carbon dioxide is 50% by volume or less, 40% by volume or less, or 30% by volume or less. The range of the carbon dioxide concentration can be any combination of the above lower limit value and upper limit value.

[0014] <Water dispersion of calcium hydroxide> The aqueous dispersion contains water and calcium hydroxide (slaked lime, Ca(OH)2). The aqueous dispersion may further contain acetonitrile (CH3CN) to adjust the carbon dioxide absorption rate.

[0015] The concentration of calcium hydroxide in an aqueous dispersion containing water and calcium hydroxide is not particularly limited. The concentration of calcium hydroxide, in terms of solid content, is 1% by weight or more, 5% by weight or more, 10% by weight or more, 20% by weight or more, or 30% by weight or more. This improves the carbon dioxide absorption efficiency of the aqueous dispersion. The concentration of calcium hydroxide, in terms of solid content, is 80% by weight or less, 70% by weight or less, 60% by weight or less, 50% by weight or less, or 40% by weight or less. This allows the aqueous dispersion to have an appropriate viscosity and ensures a uniform reaction between calcium hydroxide and carbon dioxide. The range of the calcium hydroxide concentration can be a combination of any of the above lower and upper limits. When the aqueous dispersion contains acetonitrile, the calcium hydroxide concentration can be the concentration relative to the aqueous dispersion containing water, calcium hydroxide, and acetonitrile.

[0016] The concentration of acetonitrile in the aqueous dispersion containing water, calcium hydroxide, and acetonitrile is 1% by weight or more, 5% by weight or more, 10% by weight or more, or 20% by weight or more. The concentration of acetonitrile is 60% by weight or less, 50% by weight or less, 40% by weight or less, or 30% by weight or less. This makes it easy to adjust the carbon dioxide absorption rate in the aqueous dispersion. The range of the acetonitrile concentration can be any combination of the above lower and upper limits.

[0017] In one embodiment, the water dispersion is formed by first preparing a dispersion of water and calcium hydroxide and then adding acetonitrile to the dispersion. The water dispersion of calcium hydroxide can be formed by adding calcium hydroxide or calcium oxide to water. Alternatively, a commercially available dispersion of water and calcium hydroxide can be used as the water dispersion of calcium hydroxide.

[0018] In one embodiment, calcium hydroxide can be obtained by reacting a calcium salt with an alkali metal hydroxide. Examples of calcium salts include calcium chloride and calcium sulfate. Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide.

[0019] The purity of calcium hydroxide is 80 wt % or more, 90 wt % or more, or 95 wt % or more, which improves the carbon dioxide absorption efficiency of the aqueous dispersion.

[0020] In one embodiment, the calcium hydroxide may be a powder. The average particle size (D50) of calcium hydroxide is 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, or 1 μm or less. The average particle size (D50) of calcium hydroxide may be 0.01 μm or more, 0.1 μm or more, 0.5 μm or more, 0.7 μm or more, or 0.9 μm or more. The average particle size (D50) of calcium hydroxide may be a combination of any of the above lower and upper limit values. When calcium hydroxide has such an average particle size, the reaction between calcium hydroxide and carbon dioxide is improved. The average particle size (D50) is a value obtained from a volume-based particle size distribution based on a laser diffraction / scattering method, and the D50 value refers to the particle size (median diameter) at 50% of the cumulative size.

[0021] The water may be any water that functions as a solvent, such as tap water, groundwater, distilled water, and ion-exchanged water.

[0022] (Acetonitrile) The purity of acetonitrile is 90% by weight or more, 95% by weight or more, 98% by weight or more, or 99% by weight or more, which improves the control of the carbon dioxide absorption rate in the aqueous dispersion.

[0023] (Additives) The aqueous dispersion may contain various additives, such as a dispersant. Examples of dispersants include inorganic compound dispersants and polymer surfactants. This improves the dispersibility of calcium hydroxide, even when the solids concentration of calcium hydroxide is high, and ensures a uniform reaction between calcium hydroxide and carbon dioxide. In one embodiment, in preparing a dispersion of water and calcium hydroxide, the dispersant is added to water before adding calcium hydroxide or calcium oxide, and then calcium hydroxide or calcium oxide is added, thereby enabling the calcium hydroxide to be uniformly dispersed.

[0024] <Calcium carbonate> Calcium carbonate is produced by contacting and reacting the carbon dioxide with the aqueous dispersion. The calcium carbonate can be recovered by a conventionally known method such as filtration. As described above, the mixture used in the graphene production method according to another embodiment is obtained by calcining a calcium carbonate precipitate obtained by reacting calcium hydroxide with carbon dioxide.

[0025] (Firing temperature) The temperature at which the calcium carbonate precipitate according to one embodiment is calcined is 600°C or higher, 700°C or higher, 800°C or higher, 850°C or higher, 900°C or higher, or 950°C or higher. The temperature at which the calcium carbonate precipitate is calcined is about 2600°C (the melting point of calcium oxide) or lower, and 1500°C or lower, 1200°C or lower, or 1000°C or lower. The temperature at which the calcium carbonate precipitate is calcined may be any combination of the above-mentioned lower and upper limits. The temperature at which the calcium carbonate precipitate according to one embodiment is calcined is 600 to 1500°C. Calcining within the above temperature range allows sufficient decomposition of calcium carbonate into calcium oxide and carbon dioxide.

[0026] (Baking time) The calcination time for the calcium carbonate precipitate is 1 minute or more, 5 minutes or more, 10 minutes or more, 1 hour or more, 1.5 hours or more, or 2 hours or more. The calcination time for the calcium carbonate precipitate is 7 hours or less, 6 hours or less, 5 hours or less, 4 hours or less, or 3 hours or less. The calcination time for the calcium carbonate precipitate may be any combination of the above lower and upper limits. The calcination time for the calcium carbonate precipitate according to one embodiment is 1 minute to 7 hours. Calcination within the above calcination time range allows sufficient decomposition of calcium carbonate into calcium oxide and carbon dioxide.

[0027] <Recovery process of gas containing carbon dioxide> The recovery step according to one embodiment includes removing water from a mixture containing water and carbon dioxide to recover a gas containing carbon dioxide. The recovered gas containing carbon dioxide is used as a raw material for graphene.

[0028] In the recovery step, for example, a method of cooling the mixture or a method of using an adsorbent can be used. In the recovery step, the above methods may be used alone or in combination.

[0029] (Method of cooling the mixture) The recovery step according to one embodiment includes removing water by cooling the mixture. The temperature to which the mixture is cooled (at 1 atmosphere) is -78°C or higher, -70°C or higher, -60°C or higher, -50°C or higher, -40°C or higher, or -30°C or higher. The temperature to which the mixture is cooled (at 1 atmosphere) is 100°C or lower, 90°C or lower, 80°C or lower, 70°C or lower, 60°C or lower, 50°C or lower, 40°C or lower, 30°C or lower, 20°C or lower, 10°C or lower, 0°C or lower, -10°C or lower, or -20°C or lower. The range of the temperature to which the mixture is cooled may be any combination of the above lower and upper limits.

[0030] The temperature to which the mixture according to one embodiment is cooled (at 1 atmosphere) is -78 to 100°C. When the temperature exceeds 0°C, liquid water can be removed from the mixture, and a gas containing carbon dioxide can be recovered. When the temperature is 0°C or lower, the water in the mixture is frozen, and a gas containing carbon dioxide can be recovered. Note that when the temperature to which the mixture is cooled is -79°C or lower, the carbon dioxide in the mixture freezes, and therefore a gas containing carbon dioxide cannot be recovered from the mixture. On the other hand, when the temperature to which the mixture is cooled exceeds 100°C, the water in the mixture turns into water vapor, and therefore water cannot be removed from the mixture.

[0031] In another embodiment, the mixture is cooled to a temperature (at 1 atmosphere) of -78°C to 0°C. In this temperature range, the water in the mixture can be frozen and a gas containing carbon dioxide can be recovered. Note that the explanation for the case where the mixture is cooled to a temperature of -79°C or lower is the same as above, and therefore will not be repeated.

[0032] The means for cooling the mixture can include, for example, one or more heat exchangers. The heat exchanger includes, for example, a metal tube covered with a coolant. Examples of the metal tube include a stainless steel tube and an aluminum tube. Examples of the coolant include dry ice and liquid nitrogen. By passing the mixture through the heat exchanger, the state of water vapor is changed to water or water to ice, while the state of carbon dioxide is not changed. This allows water to be removed from the mixture and a gas containing carbon dioxide to be recovered.

[0033] (Method using adsorbent) An adsorbent is a substance that absorbs moisture from the air and maintains a dry state. Examples of adsorbents for removing water from a mixture include silica gel, quicklime, calcium chloride, zeolite, and the clay mineral bentonite. The amount of adsorbent used may be any amount that can remove water from the mixture. Furthermore, by leaving the mixture in the presence of the adsorbent, the adsorbent absorbs the water in the mixture but does not absorb carbon dioxide. This allows water to be removed from the mixture and a gas containing carbon dioxide to be recovered. Note that the method using an adsorbent has the advantage that it does not require temperature control of the mixture.

[0034] <Graphene production process> The production process involves heating the recovered carbon dioxide-containing gas in a quartz vessel at a temperature of 1700°C or less to produce graphene.

[0035] (container) The container into which the recovered carbon dioxide-containing gas is introduced is preferably a cylindrical quartz container, due to its heat resistance, light transmittance, and chemical resistance to high heating temperatures. Quartz containers are containers made of quartz glass, which is made from silicon dioxide (SiO2), and contain almost no metal impurities. Examples of quartz glass include fused quartz and synthetic quartz. Fused quartz is produced from quartz powder obtained by melting and refining natural quartz. Synthetic quartz is chemically synthesized using ultra-high-purity silicon tetrachloride and is more pure than fused quartz. For example, the purity of synthetic quartz is 99.99% or higher. Quartz glass has a simple and strong molecular structure and is resistant to thermal deformation. Therefore, the softening point of quartz containers is, for example, 1700°C. Quartz glass, which has excellent physical properties, is used in a wide range of fields, such as optical fibers, optical filters, laboratory equipment, optical lenses, and incinerator viewing windows. Conventional CVD methods generally use metal foil substrates. Therefore, to produce graphene without changing the surface shape of the metal foil substrate and without causing evaporation of the metal foil, plasma treatment must be performed at a temperature sufficiently lower than the melting point of the metal catalyst. For example, when the metal catalyst is a copper foil substrate, which is commonly used in graphene production, treatment must be performed at a temperature sufficiently lower than the melting point of copper (1080°C). On the other hand, since the softening point of the quartz container according to the present invention is approximately 1700°C, the present invention can produce graphene at a higher heating temperature than the CVD method. Thus, the present invention has the advantage of being able to produce graphene over a wider temperature range (e.g., 1000°C or higher) than the CVD method, and therefore can realize mass production of graphene, for example, in commercial plants.

[0036] The shape of the container according to one embodiment is, for example, cylindrical, linear, curved (e.g., U-shaped, V-shaped), or a combination of these shapes. The container according to one embodiment is a sealed reactor that confines introduced carbon dioxide, but is not limited thereto. For example, the container may be a flow reactor. A flow reactor refers to a device that continuously supplies graphene raw materials (i.e., gas containing recovered carbon dioxide) from one end of the reactor and continuously extracts graphene from the other end of the reactor. When the container is a flow reactor, a pressure pump, a temperature sensor, a pressure sensor, a cooling water tank, a pressure control valve, and a graphene recovery container may be optionally provided. When the container is a flow reactor, the graphene production time can be shortened compared to a sealed reactor, enabling mass production of graphene.

[0037] (Heating temperature) According to one embodiment, the temperature to which the carbon dioxide-containing gas is heated in the quartz container is 1700°C or less, 1600°C or less, or 1500°C or less. The temperature to which the carbon dioxide is heated in the quartz container is 1000°C or more, 1100°C or more, or 1200°C or more. The temperature range to which the carbon dioxide-containing gas is heated in the quartz container can be a combination of any of the above lower and upper limit values. This allows graphene to be produced efficiently.

[0038] (Heating time) The time for heating the gas containing carbon dioxide in the quartz container according to one embodiment is 5 minutes or more, 10 minutes or more, or 15 minutes or more. The time for heating the gas containing carbon dioxide in the quartz container is 3 hours or less, 2 hours or less, or 1 hour or less. The time for heating the gas containing carbon dioxide in the quartz container can be any combination of the above lower limit and upper limit values. The time for heating the gas containing carbon dioxide in the quartz container according to one embodiment is 5 minutes to 1 hour. This allows graphene to be produced efficiently.

[0039] (atmosphere) According to one embodiment, the atmosphere inside the quartz container is an inert gas or a vacuum. The atmosphere inside the quartz container during heating can be an inert gas such as nitrogen, argon, or helium, or a vacuum. By generating graphene in an oxygen-free environment, graphene with fewer oxygen functional groups can be obtained, eliminating the graphene reduction process. Note that heat treatment in an atmosphere containing oxygen, such as air, is not preferred because it can cause graphite to burn.

[0040] When the quartz container is evacuated, the pressure is 1 Pa or more, 5 Pa or more, 10 Pa or more, or 20 Pa or more. The pressure is 200 Pa or less, 180 Pa or less, 160 Pa or less, or 140 Pa or less. The pressure range may be any combination of the above lower limit value and upper limit value. This allows graphene to be produced in an oxygen-poor environment, thereby obtaining graphene with fewer oxygen functional groups, and therefore the graphene reduction process can be omitted.

[0041] (catalyst) According to one embodiment, the production process includes heating the recovered carbon dioxide-containing gas in a quartz container without using a catalyst. Examples of the catalyst include metal catalysts made of precious metals such as nickel, copper, cobalt, iridium, and platinum. Single-layer graphene is typically grown on copper foil using CVD. However, CVD processes using metal catalysts require transferring graphene onto an insulating substrate, resulting in increased costs due to the etching and transfer steps of the metal catalyst. Furthermore, the price of metal catalysts can easily rise due to geopolitical risks and resource constraints. Therefore, there is a growing market need for a metal-catalyst-free graphene production method that can reduce the cost of graphene production. The present invention is an excellent invention that can fully address the market need for low-cost graphene production because it can produce graphene in a quartz container commonly used in industry without using a metal catalyst in the production process.

[0042] Hereinafter, embodiments of the present invention will be described with reference to examples. However, the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded. [Example]

[0043] Example 1 7.418 g of calcium hydroxide and a predetermined amount of water were mixed in a container. Natural gas (10.00% CO2 concentration) was then introduced into the container for 5 minutes. The CO2 concentration (volume %) represents the concentration of CO2 in natural gas. Natural gas also contains methane, ethane, nitrogen, butane, and other gases in addition to CO2. After 5 minutes, the introduction of natural gas into the container was stopped. The container was left to stand for 5 minutes, resulting in a calcium carbonate precipitate. Gas samples were taken from the container and analyzed, confirming that 99.99% of the CO2 had been converted (consumed) from the natural gas. The calcium carbonate precipitate was placed in a large electric furnace at 840°C and fired for 10 minutes. A mixture containing carbon dioxide separated from the calcium carbonate and water was collected from the gas supply port at the top of the large electric furnace. The mixture was passed through a heat exchanger (specifically, a stainless steel tube surrounded by dry ice) to remove the water and recover the carbon dioxide-containing gas. The collected gas containing carbon dioxide was trapped in a quartz tube in a large electric furnace, which had been previously evacuated, for 5 minutes, and then heated at 1500°C for 20 minutes. 1.187 g of graphene was produced and deposited inside the quartz tube. The chemical bonding state of the produced graphene was measured using X-ray photoelectron spectroscopy (XPS).

[0044] Example 2 Only the differences from Example 1 will be described. In Example 2, the amount of calcium hydroxide was 3.709 g, the CO2 concentration in the natural gas was 5.00%, the natural gas introduction time was 2.5 minutes, the recovered carbon dioxide-containing gas was heated for 10 minutes, and the amount of graphene produced was 0.579 g. Since graphene was produced using the same production process as Example 1, detailed description will be omitted.

[0045] (Graphene analysis method) X-ray photoelectron spectroscopy (X-ray source: monochromated Al Kα rays) was used to analyze the chemical bonding states of the graphene obtained in Examples 1 and 2 and the comparative example (commercially available graphite).

[0046] FIG. 1 shows the results of XPS analysis of the graphenes of Examples 1 and 2 and a comparative example (commercially available graphite).

[0047] 1, in Example 1 (curve 101) and Example 2 (dashed line 102), a prominent peak in the intensity (vertical axis) of emitted photoelectrons was observed when the binding energy (horizontal axis, binding energy) was 284 to 285, and the intensity of the emitted photoelectrons exceeded 5.0. On the other hand, in the comparative example (curve 103), a slight peak in the intensity of emitted photoelectrons was observed when the binding energy was 283, but the intensity of the emitted photoelectrons decreased at binding energies of 283 and above.

[0048] 1, it was found that the intensity of the emitted photoelectrons in Examples 1 and 2 was about 20 times or more that of the comparative example (commercial graphite). In this way, it was confirmed that graphene was produced by removing water from the mixture and heating the recovered gas containing carbon dioxide at 1500°C in a quartz tube.

[0049] As described above, the present invention has a significant effect of enabling mass production of graphene at lower cost than conventional techniques.

[0050] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0051] 101 curve 102 dashed line 103 Curve

Claims

1. a recovery step of removing the water from a mixture containing water and carbon dioxide to recover the gas containing the carbon dioxide; and a production step of producing graphene by heating the recovered carbon dioxide-containing gas in a quartz container at a temperature of 1700°C or less. How graphene is produced.

2. the recovering step comprises removing the water by cooling the mixture; The method of claim 1.

3. The temperature to which the mixture is cooled (at 1 atmosphere) is −78 to 100° C. The method of claim 2.

4. The generating step includes heating the recovered gas containing carbon dioxide in the quartz vessel without using a catalyst. The method of claim 1.

5. The time for heating the recovered carbon dioxide-containing gas in the quartz container is 5 minutes to 1 hour. The method of claim 1.

6. The atmosphere in the quartz container is an inert gas or a vacuum. The method of claim 1.

7. The mixture is obtained by calcining a calcium carbonate precipitate obtained by reacting calcium hydroxide with carbon dioxide. The method of claim 1.

8. The temperature for firing the calcium carbonate precipitate is 600 to 1500°C. The method of claim 7.

9. The time for calcining the calcium carbonate precipitate is 1 minute to 7 hours. The method of claim 7.

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