Method for producing layered material nanosheets and layered MoS2 nanosheets

Abstract

This layered material nanosheet production method comprises a detachment step for detaching, in a layer form, layered material particles by irradiating the layered material particles with microwaves while cooling the layered material particles.

Description

[Technical Field]

[0001] This invention relates to a method for producing layered material nanosheets, layered MoS2 nanosheets, and layered graphite nanosheets. [Background technology]

[0002] Layered materials such as molybdenum sulfide (MoS2) and graphite are known to exhibit significant changes in properties such as electrical conductivity and thermal conductivity when they are formed into thin nanosheets with a thickness of nanometers, consisting of one or more layers stacked together. For this reason, nanosheets of layered materials are attracting attention as novel materials.

[0003] As methods for manufacturing layered material nanosheets, bottom-up methods such as CVD (chemical vapor deposition) and PVD (physical vapor deposition) and top-down methods such as peeling methods are known. Peeling methods are a method of peeling off a portion of the layered material particles of the raw material from between layers. Known peeling methods include mechanical peeling, which peels off the layered material using Scotch tape, and liquid phase peeling, which peels off the layered material in a liquid phase. In liquid phase peeling, to facilitate the peeling of the layered material, it has been considered to widen the interlayers by inserting alkali metal ions or ammonium ions between the layers of the layered material (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-73401 [Overview of the project] [Problems that the invention aims to solve]

[0005] Liquid phase exfoliation is an industrially advantageous method compared to CVD and mechanical exfoliation because it is scalable and allows for mass production. However, when alkali metal ions or ammonium ions are inserted between the layers of a layered material, it may be necessary to wash away the ions remaining between the layers of the resulting layered material nanosheet. Furthermore, because the ease of exfoliation of the layered material differs between areas where ions are inserted and areas where they are not, the exfoliation of the layered material proceeds unevenly, and the resulting layered material nanosheet tends to be fine and have low orientation.

[0006] The present invention has been made in view of the above problems, and aims to provide layered material nanosheets such as layered MoS2 nanosheets and layered graphite nanosheets that have little interlayer residue and high orientation, and methods for producing these layered material nanosheets. [Means for solving the problem]

[0007] The inventors have discovered that by cooling the layered material particles and irradiating them with microwaves, it is possible to relatively easily exfoliate the layered material particles in layers without inserting alkali metal ions or ammonium ions between the layers. They then confirmed that the resulting layered material nanosheets exhibit excellent orientation properties, thus completing the present invention. Therefore, the present invention has the following aspects.

[0008] [1] A method for producing a layered material nanosheet, comprising a peeling step of peeling the layered material particles in layers by irradiating them with microwaves while cooling the layered material particles. [2] A method for producing a layered material nanosheet according to [1], comprising a dispersion step of dispersing the layered material particles in a polar solvent, wherein in the peeling step, the layered material particles are peeled off in layers by irradiating the layered material particles with microwaves while cooling the layered material particles by solidifying the polar solvent. [3] A method for producing a layered compound nanosheet according to [1], comprising a dispersion step of dispersing the layered material particles in a nonpolar solvent, wherein in the peeling step, the layered material particles are peeled off in layers by irradiating the layered material particles with microwaves while cooling the nonpolar solvent to 10°C or below. [4] A method for producing a layered material nanosheet according to [1] to [3], wherein microwaves are intermittently irradiated onto the layered material particles. [5] The method for producing a layered material nanosheet according to [1] to [4] above, wherein the layered material particles include at least one particle selected from the group consisting of transition metal chalcogenide particles, molybdenum tungsten particles, and layered carbon particles. [6] The method for producing a layered material nanosheet according to [1] to [5], wherein the layered material particles have a cleavage plane, and the major axis of the cleavage plane is 10 μm or more.

[0009] [7] Contains MoS2, and the X-ray diffraction peak intensity of the (103) plane of the MoS2 is I 103 X-ray diffraction peak intensity I of the (002) plane 002 Ratio I 002 / I 103 A layered MoS2 nanosheet in which the ratio is 5 or higher.

[0010] [8] A layered graphite nanosheet obtained by exfoliating layered carbon particles in layers, wherein the rate of change of the D / G ratio, which is the ratio of the intensity D of the D band to the intensity G of the G band in the Raman spectrum, calculated from the following formula (1), is 5% or more. The rate of change in the D / G ratio = {absolute value of (D / G ratio of layered carbon particles - D / G ratio of layered graphite nanosheets) / D / G ratio of layered carbon particles} × 100 ... (1)

[0011] [9] A layered graphite nanosheet obtained by exfoliating layered carbon particles in layers, wherein the rate of change of the G / 2D ratio, which is the ratio of the intensity G of the G band to the intensity 2D of the 2D band in the Raman spectrum, calculated from the following formula (2), is 2% or more. Change rate of G / 2D ratio = {|(D / G ratio of layered carbon particles - D / G ratio of layered graphite nanosheets)| / D / G ratio of layered carbon particles} × 100 ··· (2)

Advantages of the Invention

[0012] According to the present invention, it becomes possible to provide layer material nanosheets such as layered MoS2 nanosheets and layered graphite nanosheets with less residues between layers and high orientation, and a method for producing these layer material nanosheets.

Brief Description of the Drawings

[0013] [Figure 1] It is a perspective view of a layer material nanosheet obtained by the method for producing a layer material nanosheet according to an embodiment of the present invention. [Figure 2] It is a configuration diagram of an example of a reaction apparatus that can be used in the method for producing a layer material nanosheet according to an embodiment of the present invention. [Figure 3] It is a configuration diagram of another example of a reaction apparatus that can be used in the method for producing a layer material nanosheet according to an embodiment of the present invention. [Figure 4] It is a temperature profile of a frozen MoS2 suspension during microwave irradiation in Experimental Examples A1 to A10. [Figure 5] It is a FE-SEM photograph of the layered MoS2 nanosheets obtained in Experimental Example A2 and Experimental Example A6 and the MoS2 particles before exfoliation treatment. [Figure 6] It is an X-ray diffraction pattern of the MoS2 particles before exfoliation treatment used in Experimental Examples A1 to A10. [Figure 7] It is an X-ray diffraction pattern of the MoS2 nanosheets obtained in Experimental Examples A1 to A10. [Figure 8] It is a FE-SEM photograph of the layered graphite nanosheets obtained in Experimental Example B1 and the graphite particles before exfoliation treatment. [Figure 9] It is a Raman spectrum of the layered graphite nanosheets obtained in Experimental Example B1 and the graphite particles before exfoliation treatment. [Figure 10]This graph shows the temperature change of the frozen liquid obtained in experimental examples C1 to C9, from the time of microwave irradiation until the optical fiber thermometer reads 20°C. [Figure 11] This graph shows the temperature change of the frozen solution obtained in experimental examples C10 to C18, from the time of microwave irradiation until the optical fiber thermometer read 20°C. [Modes for carrying out the invention]

[0014] The following description of this embodiment will be made in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to clearly illustrate the features of the present invention, and the dimensional ratios of each component may differ from those of the actual components. The materials, dimensions, etc., exemplified in the following description are examples only, and the present invention is not limited to them. It can be implemented with appropriate modifications without changing the essence of the invention.

[0015] Figure 1 is a perspective view of a layered material nanosheet obtained by a method for producing a layered material nanosheet according to one embodiment of the present invention. The layered material nanosheet 10 shown in Figure 1 is a laminate formed by stacking unit layers 11. The unit layers 11 are formed by strong bonds such as covalent bonds and ionic bonds between atoms that make up the layered material. The unit layers 11 are stacked by weak bonds such as van der Waals forces. Therefore, the layered material nanosheet 10 has the property (cleavage) in which cracks occur between the unit layers 11 in a direction along the surface of the unit layer 11, causing a portion of the layered material to peel off.

[0016] The layered material nanosheet 10 has upper and lower surfaces 12 and 13. At least one of the upper and lower surfaces 12 and 13 of the layered material nanosheet is a delamination surface. The delamination surface refers to the surface that has been peeled off from the raw material layered material particles (non-sheet-like layered material particles). The delamination surface may have a step formed by the partial peeling of the unit layer 11.

[0017] The shape of the layered material nanosheet 10 is rectangular, but there are no particular restrictions on the shape of the layered material nanosheet 10. The shape of the layered material nanosheet 10 may be, for example, disc-shaped, polygonal, or flaky. The average thickness (T in Figure 1) of the layered material nanosheet 10 is within the range of 2 nm to 10 nm. There are no particular restrictions on the size of the surfaces 12 and 13 of the layered material nanosheet 10. The average maximum diameter (L in Figure 1) of the surfaces 12 and 13 may be, for example, within the range of 100 nm to 100 mm. The average thickness and average maximum diameter of the layered material nanosheet 10 are the average values ​​of the thickness and maximum diameter of 100 layered material nanosheets 10 measured using FE-SEM (Field Emission Scanning Electron Microscope).

[0018] Alkali metal ions and ammonium ions are not present, or are substantially absent, in the interlayers between the unit layers 11 of the layered material nanosheet 10.

[0019] As materials for the layered material nanosheet 10, for example, layered carbon particles, hexagonal boron nitride (h-BN), transition metal chalcogenides (TMDC), metal halides, transition metal oxides, titanium oxide compounds, and MoW (molybdenum tungsten) can be used. Examples of layered carbon particles include graphite and expanded graphite. Examples of transition metal chalcogenides include HfS2, MoS2, NiTe2, PtSe2, and ZrS2. Examples of metal halides include MgBr2, CdI2, AsI3, VI3, SrFCl, PbFI, and Ag2F. The layered material nanosheet 10 is typically a layered MoS2 nanosheet or a layered graphite nanosheet.

[0020] The layered material nanosheet 10 containing MoS2 (layered MoS2 nanosheet) exhibits an X-ray diffraction peak intensity of I of the (103) plane of MoS2. 103 X-ray diffraction peak intensity I of the (002) plane 002 Ratio I 002 / I 103It may be 5 or more. The X-ray diffraction peak of the (002) plane of MoS2 is derived from the unit layer 11 of MoS2. Also, the X-ray diffraction peak of the (103) plane of MoS2 is derived from the stacked structure of the unit layer 11. As the exfoliation of MoS2 progresses and the thickness of the layer-like material nanosheet 10 decreases, the intensity of the X-ray diffraction peak of the (103) plane becomes relatively small, and the X-ray diffraction peak of the (002) plane becomes relatively large, resulting in a ratio I 002 / I 103 becoming larger, and the orientation of the layer-like material nanosheet 10 becoming higher. The ratio I 002 / I 103 may be 1 or more, or may be 10 or more. Also, the ratio I 002 / I 103 may be 300 or less, or may be 200 or less.

[0021] When the layer-like material nanosheet 10 is a layered graphite nanosheet obtained by exfoliating layered carbon particles layer by layer, the change rate of the D / G ratio, which is the ratio of the intensity D of the D band to the intensity G of the G band in the Raman spectrum calculated from the following formula (1), may be 5% or more. Change rate of D / G ratio = {absolute value of (D / G ratio of layered carbon particles - D / G ratio of layered graphite nanosheet) / D / G ratio of layered carbon particles} × 100 ··· (1)

[0022] The G band (1580 - 1600 cm -1 ) of graphite is a signal commonly observed in the carbon skeleton with sp 2 bonding, and the D band (2680 - 2700 cm -1 ) is a band that appears due to structural defects. Therefore, the change rate of the D / G ratio is a measure representing the change in the amount of defects and the change in fragmentation of the unit layer 11 caused by the layered exfoliation of the layered carbon particles. The change rate of the D / G ratio may be 1% or more, or may be 10% or more. Also, the change rate of the D / G ratio may be 300% or less, or may be 200% or less.

[0023] Furthermore, in the case of layered graphite nanosheets, the rate of change of the G / 2D ratio, which is the ratio of the intensity G of the G band to the intensity 2D of the 2D band in the Raman spectrum, calculated from the following equation (2), may be 2% or more. The rate of change in the G / 2D ratio = {absolute value of (D / G ratio of layered carbon particles - D / G ratio of layered graphite nanosheets) / D / G ratio of layered carbon particles} × 100 ... (2)

[0024] Graphite 2D band (2680~2700cm) -1 The ratio (G / 2D) indicates the number of graphite layers. Therefore, the G / 2D change rate is a measure of the stacking state of the graphite layers in unit layer 11. The G / 2D ratio change rate may be 1% or more, or 10% or more. Also, the G / 2D ratio change rate may be 300% or less, or 200% or less.

[0025] The layered graphite nanosheet may have either a change rate of 1% or more between the D / G ratio and the G / 2D ratio, or both may have a change rate of 1% or more.

[0026] The layered material nanosheet 10 of this embodiment, having the above-described structure, has a peelable surface and is obtained by a peeling method, making it industrially advantageous to manufacture. Furthermore, since the layered material nanosheet of this embodiment does not substantially contain alkali metal ions and ammonium ions between the unit layers 11, it is unnecessary to wash away the ions.

[0027] When the layered material nanosheet 10 of this embodiment is a layered MoS2 nanosheet containing MoS2, the X-ray diffraction peak intensity of the (103) plane of MoS2 is I 103 X-ray diffraction peak intensity I of the (002) plane 002 Ratio I 002 / I 103 If the value is 5 or higher, the orientation is high, and properties such as electrical conductivity and thermal conductivity vary significantly compared to non-sheet-like MoS2 particles. For this reason, it can be used as a novel material.

[0028] Furthermore, if the layered material nanosheet 10 of this embodiment is a layered graphite nanosheet obtained by exfoliating layered carbon particles in layers, and the rate of change of the D / G ratio calculated from the above formula (1) is 5% or more, then the change in the amount of defects and fragmentation of the unit layer 11 is large compared to the raw material layered carbon particles, so properties such as electrical conductivity and thermal conductivity vary greatly compared to non-sheet-like layered carbon particles. For this reason, it can be used as a novel material.

[0029] Furthermore, if the layered material nanosheet 10 of this embodiment is a layered graphite nanosheet obtained by exfoliating layered carbon particles in layers, and the rate of change of the G / 2D ratio calculated from equation (2) above is 2% or more, then the stacking state of the graphite layer of the unit layer 11 changes significantly, and properties such as electrical conductivity and thermal conductivity vary greatly compared to non-sheet-like layered carbon particles. For this reason, it can be used as a novel material.

[0030] Next, the method for producing the layered material nanosheet of this embodiment will be described. The method for producing a layered material nanosheet according to this embodiment includes a cooling step and a peeling step.

[0031] In the cooling process, the layered material particles of the raw material are cooled. Cooling of the layered material particles of the raw material may be carried out, for example, by using a nonpolar solvent or a polar solvent as a cooling medium. That is, the layered material particles may be cooled by dispersing them in a nonpolar solvent or a solid polar solvent and then cooling them. The nonpolar solvent may be solid or liquid. The nonpolar solvent and the solid polar solvent that serve as the cooling medium may be cooled to, for example, 10°C or below. The nonpolar solvent may have polarity to the extent that it does not generate heat when irradiated with microwaves. Examples of nonpolar solvents include silicone oil, hexane, and toluene. When a polar solvent is used as a cooling medium, the layered material particles may be dispersed in the polar solvent, and the resulting dispersion may be cooled and solidified (frozen). Examples of polar solvents include water, organic solvents (alcohols, ketones), and mixed solutions of water and organic solvents. Examples of alcohols include methanol, ethanol, and 1-propanol. Examples of ketones include acetaldehyde, acetone, acrolein, benzaldehyde, butyraldehyde, formaldehyde, and propionaldehyde. The concentration of the organic solvent in the mixed solution of water and the organic solvent is, for example, in the range of 0.1 vol% to 20 vol%, preferably in the range of 0.5 vol% to 5.0 vol%.

[0032] The layered material particles used as raw materials are not particularly limited, but examples include layered carbon particles, hexagonal boron nitride (h-BN), transition metal chalcogenides (TMDC), metal halides, transition metal oxides, titanium oxide compounds, and MoW (molybdenum tungsten). Examples of layered carbon particles include graphite and expanded graphite. Examples of transition metal chalcogenides include HfS2, MoS2, NiTe2, PtSe2, and ZrS2. Examples of metal halides include MgBr2, CdI2, AsI3, VI3, SrFCl, PbFI, and Ag2F. The above layered material particles are typically MoS2 or graphite.

[0033] In the peeling process, the layered material particles, cooled in the cooling process, are peeled off in layers by irradiating them with microwaves while continuing to cool them. Microwaves are electromagnetic waves with frequencies in the range of 300 MHz to 300 GHz. Polar materials self-heat by absorbing microwaves. Layered materials have a layered structure in which unit layers 11 are stacked and are polar, so they heat up when irradiated with microwaves. Nonpolar solvents are not polar, so they do not heat up when irradiated with microwaves. Also, polar solvents do not heat up when irradiated with microwaves because the molecules do not rotate or vibrate easily in the solid state. For this reason, layered material particles can be selectively heated by irradiating them with microwaves while they are dispersed in a nonpolar solvent or a solid polar solvent. Furthermore, since the layered material particles self-heat due to microwaves, they can be heated uniformly and rapidly compared to when heat is applied from the outside. On the other hand, since the layered material particles are cooled from the outside via a cooling medium, a temperature distribution is generated inside the layered material particles due to cooling and heating. This temperature distribution causes a sudden thermal stress to act within the layered material particles. This thermal stress causes cracks to form between the layers of the unit layer of the layered material particles, resulting in the layered delamination of the layered material particles.

[0034] Microwave irradiation of layered material particles may be performed continuously or intermittently. If performed intermittently, the microwave irradiation time may be within the range of 1 second to 10 seconds, and the non-irradiation time may be within the range of 1 second to 10 seconds. The duty cycle may be within the range of 30% to 70%.

[0035] Figure 2 is a diagram showing an example of a reaction apparatus that can be used in a method for producing layered material nanosheets according to one embodiment of the present invention. The reaction apparatus 20 shown in Figure 2 comprises a reaction vessel 24 and a cooler 28. The reaction vessel 24 contains a frozen suspension 23 in which layered material particles 21 are dispersed in a solid polar solvent 22. The solid polar solvent 22 is, for example, ice. The frozen suspension 23 can be prepared, for example, by introducing the layered material particles 21 and the polar solvent 22 into the reaction vessel 24, performing dispersion treatment by ultrasonic treatment or the like, and then cooling. The cooler 28 contains a coolant 27. The coolant 27 is, for example, dry ice. Dry ice sublimes at -79°C, and the carbon dioxide used as a raw material is a non-polar substance and therefore does not absorb microwaves. An optical fiber thermometer 26 is also placed on the lid 25 of the reaction vessel 24. The optical fiber thermometer 26 measures the temperature of the frozen suspension 23. When a polar solvent dissolves into a liquid, its temperature rises rapidly upon microwave irradiation. Therefore, it is preferable to irradiate the frozen suspension 23 with microwaves such that the temperature of the frozen suspension 23 is -30°C or lower than the melting point of the polar solvent 22.

[0036] Figure 3 is a diagram showing another example of a reaction apparatus that can be used in a method for producing layered material nanosheets according to one embodiment of the present invention. The reaction apparatus 30 shown in Figure 3 is the same as the reaction apparatus 20 shown in Figure 2, except that it uses a nonpolar solvent as the coolant 37 and a circulating cooler 38 in which a cooling liquid circulates inside as the cooler. Therefore, the same reference numerals are used for common parts and their explanation is omitted.

[0037] The nonpolar solvent of the coolant 37 is, for example, silicone oil. Since silicone oil is a nonpolar solvent, it does not absorb microwaves. The cooling liquid circulating inside the circulating cooler 38 is, for example, silicone oil. In the reaction apparatus 30 shown in Figure 3, instead of the frozen suspension 23, a suspension in which layered material particles 21 are dispersed in a nonpolar solvent may be used. In this case, for example, silicone oil can be used as the nonpolar solvent of the suspension.

[0038] According to the method for manufacturing layered material nanosheets of this embodiment, which has the above configuration, the layered material particles are cooled while being irradiated with microwaves, and the thermal stress generated inside the layered material particles is utilized to peel the layered material particles in layers. Therefore, there is no need to insert alkali metal ions or ammonium ions between the layers of the layered material to widen the interlayer space. Thus, it is unnecessary to wash away any ions remaining between the layers of the obtained layered material nanosheet. Furthermore, since the layered material particles are heated by microwave irradiation, the layered material particles are heated uniformly. As a result, the thermal stress generated inside the layered material particles becomes uniform, making it possible to obtain layered material nanosheets with a large unit layer size and high orientation.

[0039] In the method for producing layered material nanosheets of this embodiment, the layered material particles are dispersed in a non-polar solvent or a solid polar solvent cooled to 10°C or below during the cooling step, allowing for uniform cooling of the layered material particles. This results in more uniform thermal stress being generated within the layered material particles when they are heated. Therefore, it is possible to obtain layered material nanosheets with larger unit layer sizes and higher orientation.

[0040] Furthermore, according to the method for manufacturing layered material nanosheets of this embodiment, in the peeling step, microwaves are intermittently irradiated onto the layered material particles to heat them, thereby increasing the temperature difference between the microwave irradiation and the cessation of irradiation. This increases the thermal stress generated inside the layered material particles. As a result, the peeling of the layered material becomes easier, and the manufacturing speed of layered compound nanosheets can be increased.

[0041] Although one embodiment of the present invention has been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical spirit of the invention. For example, in the method for manufacturing layered material nanosheets of this embodiment, a cooling step is performed before the peeling step, but the method is not limited to this. The layered material particles may be peeled off in layers by irradiating them with microwaves while cooling them, without performing a cooling step. [Examples]

[0042] Next, we will describe the results of experimental examples conducted to confirm the effects of the present invention. The present invention is not limited to the following examples.

[0043] <Experimental Example A: Layered MoS2 Nanosheet> [Experimental Example A1] A MoS2 suspension was obtained by placing 0.5 g of MoS2 particles (average particle size: 50 μm) and 100 mL of pure water into a 100 mL polyethylene bottle and dispersing them by irradiating with ultrasound (50 kHz, 100 W) for 10 minutes. A fiber optic thermometer was inserted into the center of the obtained MoS2 suspension, and then the bottle was placed in liquid nitrogen to rapidly cool the MoS2 suspension and create a frozen MoS2 suspension. The frozen MoS2 suspension was cooled with liquid nitrogen until the temperature of the center was below -100°C. After that, the bottle containing the frozen MoS2 suspension was placed in a 500 mL polyethylene cooling container with a 10 mm thick layer of dry ice at the bottom, and 400 g of dry ice powder was packed between the outer circumference of the bottle and the inner circumference of the polyethylene cooling container.

[0044] A poly bottle was placed on a microwave irradiation sample stage along with a polyethylene cooling container. Microwave irradiation of the frozen MoS2 suspension was started when the temperature of the center of the frozen MoS2 suspension in the poly bottle reached -80°C. Microwave irradiation was stopped when the temperature of the center of the frozen MoS2 suspension reached -30°C. Microwave irradiation was performed using a 2.45 GHz multimode. The microwave output was set to 500 W, and the irradiation condition was continuous. In this way, MoS2 particles were exfoliated to generate MoS2 nanosheets.

[0045] After the delamination process, the poly bottle was removed from the polyethylene cooling container, and the frozen MoS2 suspension was heated and dissolved in a constant temperature bath. The resulting MoS2 suspension was filtered to recover the MoS2 nanosheets.

[0046] [Experimental Examples A2-A10] Except for the microwave output and irradiation conditions used to irradiate the frozen MoS2 suspension as described in Table 1 below, the frozen MoS2 suspension was peeled until the temperature in the center reached -30°C, similar to Experimental Example A1, to generate MoS2 nanosheets. In the irradiation conditions, "continuous" means that the frozen MoS2 suspension is continuously irradiated with microwaves, while "intermittent" means that the MoS2 suspension is irradiated with microwaves for a predetermined irradiation time, and then the microwave irradiation is stopped for a predetermined stop time, and this cycle is repeated.

[0047] [Table 1]

[0048] [Evaluation Results] (Temperature profile of frozen MoS2 suspension during microwave irradiation) Figure 4 shows the temperature profiles of the frozen MoS2 suspensions during microwave irradiation in experimental examples A1 to A10. In the figure, the symbols (1) to (10) represent the temperature profiles of the frozen MoS2 suspensions in experimental examples A1 to A10, respectively. The results in Figure 4 show that the temperature rise of the frozen MoS2 suspension is faster with increasing microwave power and longer irradiation time. With intermittent irradiation, longer irradiation times lead to more cycles of rapid cooling and heating, which is thought to promote more delamination and increase the amount of layered nanosheet material generated.

[0049] (Particle shape observation) The surface morphology of MoS2 nanosheets was observed using a FE-SEM (JSM-7610F, JEOL). For the observation sample, MoS2 nanosheets were dispersed in water, and a few drops of the supernatant were added to a carbon-reinforced microgrid manufactured by Ouken Shoji Co., Ltd., and then dried. Figure 5 shows FE-SEM images of MoS2 nanosheets obtained in Experimental Example A2 and Experimental Example A6, as well as MoS2 particles before the peeling treatment.

[0050] From the FE-SEM images in Figure 5, it was confirmed that the MoS2 nanosheets obtained in Experimental Examples A6 and A2 had stepped surfaces compared to the MoS2 particles before the exfoliation treatment, indicating that partial exfoliation of the unit layer was progressing. In addition, the MoS2 nanosheet obtained in Experimental Example A6 showed a structure sheared at 60 degrees, reflecting the crystal structure of MoS2. Furthermore, the MoS2 nanosheets obtained in Experimental Examples A2 and A6 showed many (001) planes. This suggests that exfoliation was progressing and more of the sheet surface was exposed. From these results, it is considered that microwave irradiation acts on both the exfoliation of the (00l) plane and the shearing of the (hk0) plane.

[0051] (X-ray diffraction pattern) The X-ray diffraction patterns of MoS2 nanosheets were measured using a benchtop powder X-ray diffractometer (D2 PHASER 2nd Gen, Bruker) with CuKα radiation as the X-ray source. Figure 6 shows the X-ray diffraction patterns of MoS2 particles before exfoliation, and Figure 7 shows the X-ray diffraction patterns of MoS2 nanosheets obtained in experimental examples A1 to A10. In the figures, the symbols (1) to (10) represent the temperature profiles of the frozen MoS2 suspensions in experimental examples A1 to A10, respectively. The X-ray diffraction peak intensity I of the (103) plane of MoS2 was also measured. 103 X-ray diffraction peak intensity I of the (002) plane 002 Ratio I 002 / I 103 This is shown in Table 2 below.

[0052] [Table 2]

[0053] From the X-ray diffraction patterns in Figures 6 and 7, no shift in the major X-ray diffraction peaks was observed between the MoS2 particles before exfoliation and the MoS2 nanosheets after exfoliation. This result confirms that the exfoliation treatment does not alter the crystal structure of MoS2. On the other hand, peak intensity ratio I 002 / I 103 Regarding this, a significant difference was observed between MoS2 particles before exfoliation treatment and MoS2 nanosheets after exfoliation treatment. Peak intensity ratio of MoS2 particles before exfoliation treatment I 002 / I 103 While the peak intensity ratio was 2.021, in experimental example A2 (output: 250W, continuous irradiation), it was 24.987, achieving a peak intensity ratio approximately 12 times higher than before the stripping treatment. Furthermore, the peak intensity ratio also increased significantly in experimental example A4 (output: 250W, intermittent irradiation, irradiation time: 7.5 seconds, stop time: 2.5 seconds) and experimental example A9 (output: 500W, intermittent irradiation, irradiation time: 7.5 seconds, stop time: 2.5 seconds). 002 / I 103 In experimental examples A2, A4, and A9, where the peak intensity increased significantly, the low intensity of the X-ray diffraction peak in the (103) plane suggests that the increase in peak intensity was due to a decrease in the number of unit layers forming the stacked structure. In addition, in experimental examples A2, A4, and A9, a slight peak was observed in the (00l) plane, while the (hkl) plane peak was almost negligible. From these results, it can be concluded that in experimental examples A2, A4, and A9, a large number of thin-layer MoS2 nanosheets were generated by the exfoliation treatment.

[0054] <Experimental Example B: Layered graphite nanosheets, layered expanded graphite nanosheets> [Experimental Example B1] 0.05 g of graphite powder (average particle size: 15 μm, D / G ratio: 0.118, G / 2D ratio: 2.289) and 50 mL of pure water were placed in a polypropylene container and dispersed by irradiating with ultrasound (50 kHz, 100 W) for 3 minutes to obtain a graphite suspension with a concentration of 1 g / L. Next, a fiber optic thermometer was inserted into the center of the obtained graphite suspension, and then the poly container was placed in liquid nitrogen for 5 minutes to rapidly cool the graphite suspension and produce a frozen graphite suspension. Subsequently, as in Experimental Example A1, the poly container containing the frozen graphite suspension was placed in a propylene cooling container together with dry ice, and the outer perimeter of the poly container was surrounded by dry ice.

[0055] A poly container was placed on a microwave irradiation sample stage along with a polypropylene cooling container, and microwave irradiation of the frozen graphite suspension was started. The microwave irradiation conditions were: output: 1000W, intermittent irradiation, duty cycle: 50%, pulse period: 10 seconds, irradiation time: 30 minutes. In this way, the graphite particles were exfoliated to generate layered graphite nanosheets. The generated layered graphite nanosheets were collected by filtering.

[0056] The raw material graphite powder and layered graphite nanosheets produced by microwave irradiation were observed using FE-SEM. Figure 8 shows the TE images (transmission electron images) of the graphite powder and layered graphite nanosheets taken with FE-SEM. Figure 8(a) is the FE-SEM image (TE image) of graphite particles, and (b) is the FE-SEM image (TE image) of layered graphite nanosheets. Figure 8 shows that the pores of the filter are more visible through the layered graphite nanosheets compared to the graphite particles. This is because the layered graphite nanosheets have become thinner. Furthermore, the layered graphite nanosheets are smaller in size compared to the graphite particles.

[0057] Furthermore, Raman spectra were measured for layered graphite nanosheets made from graphite powder. The results are shown in Figure 9. The D / G ratio and G / 2D ratio were also measured from the obtained Raman spectra, and the rate of change in the D / G ratio and G / 2D ratio of the layered graphite nanosheets relative to the graphite powder were determined. The results are shown in Table 3 below.

[0058] [Table 3]

[0059] Table 3 shows that the D / G ratio of the layered graphite nanosheets relative to the raw material graphite powder changed by 48.3%, and the G / 2D ratio changed by 20.1%. The high values ​​for the D / G ratio and G / 2D ratio are due to the peeling and fragmentation of graphite particles caused by microwave irradiation, which led to the formation of layered graphite nanosheets.

[0060] [Experimental Examples B2-B3] Layered graphite nanosheets were obtained in the same manner as in Experimental Example B1, except that the microwave pulse period was set to the pulse period shown in Table 4 below. The D / G ratio and G / 2D ratio of the obtained layered graphite nanosheets were measured, and the rate of change of the D / G ratio and the rate of change of the G / 2D ratio were determined. The results are shown in Table 4 below.

[0061] [Table 4]

[0062] The results in Table 4 show that the rate of change in the D / G ratio is high, at over 45% in all cases. This is thought to be because the microwave irradiation caused the delamination and fragmentation of the graphite particles. Furthermore, the rate of change in the G / 2D ratio increases with increasing pulse period. This is thought to be because, as the pulse period increases, the graphite particles are heated locally, and the temperature change when they are cooled by the surrounding ice or dry ice becomes larger, resulting in a stronger thermal shock acting on the graphite particles and accelerating the delamination of the graphite particles.

[0063] [Experimental Examples B4-B6] Layered graphite nanosheets were obtained in the same manner as in Experimental Example B1, except that the graphite concentration in the graphite suspension was set to 3 g / L, and the microwave output and irradiation time were as shown in Table 5 below. The D / G ratio and G / 2D ratio of the obtained layered graphite nanosheets were measured, and the rate of change of the D / G ratio and the rate of change of the G / 2D ratio were determined. The results are shown in Table 5 below.

[0064] [Table 5]

[0065] The results in Table 5 show that the rate of change in the D / G ratio increases with increasing microwave power. This is thought to be because the delamination and fragmentation of graphite particles progressed as the microwave power increased. On the other hand, the rate of change in the G / 2D ratio is less affected by the microwave power.

[0066] [Experimental Example B7] A 50 mL graphite suspension with a graphite concentration of 3 g / L was dispersed by ultrasound irradiation and then frozen in liquid nitrogen. Next, the resulting frozen graphite suspension was irradiated with microwaves until it thawed. The microwave irradiation conditions were: output: 800 W, intermittent irradiation, duty cycle: 50%, pulse period: 10 seconds. This cycle of dispersing the graphite suspension with ultrasound, freezing it in liquid nitrogen, and then irradiating the resulting frozen graphite suspension with microwaves until it thawed was repeated one, two, or three times. The D / G ratio and G / 2D ratio of the resulting layered graphite nanosheets were measured, and the rate of change in the D / G ratio and G / 2D ratio were determined. The results are shown in Table 6 below.

[0067] [Table 6]

[0068] The results in Table 6 show that the rate of change in the D / G ratio decreases as the number of cycles increases. This is thought to be because the effect of structural defects decreases. On the other hand, the rate of change in the G / 2D ratio increases as the number of cycles increases. This is thought to be because the frequency of thermal shock increases as the microwave irradiation cycles increase, leading to the progression of delamination.

[0069] [Experimental Examples B8-B10] Layered expanded graphite nanosheets were obtained in the same manner as in Experimental Example B1, except that expanded graphite powder (D / G ratio: 0.089, G / 2D ratio: 2.2193) was used instead of graphite powder, and the microwave output was set to the output shown in Table 7 below. The D / G ratio and G / 2D ratio of the obtained layered graphite nanosheets were measured, and the rate of change of the D / G ratio and the rate of change of the G / 2D ratio were determined. The results are shown in Table 7 below.

[0070] [Table 7]

[0071] The results in Table 7 show that the rate of change in the D / G ratio increases with increasing microwave power. This is thought to be because the detachment and fragmentation of the expanded graphite particles progressed as the microwave power increased. On the other hand, the rate of change in the G / 2D ratio is less affected by the microwave power.

[0072] <Experimental Example C: Layered Graphite Nanosheets> [Experimental Example C1] A mixture was prepared by adding 0.05 g of graphite powder (average particle size: 15 μm, D / G ratio: 0.037, G / 2D ratio: 2.606) different from that used in Experimental Example B1 to 50 mL of a 0.5 vol% 1-propanol aqueous solution as a dispersion medium. Next, the mixture was stirred at a stirring speed of 400 rpm to disperse the graphite powder. After that, liquid nitrogen was added to the mixture for 2 minutes to cool it. Then, a sack was fixed 1.5 cm from the bottom of the poly container, and liquid nitrogen was added to the mixture through the sack for 10 minutes to freeze the mixture and obtain a frozen solution in which the graphite powder was dispersed. A poly solution (volume: 100 mL) containing the above frozen solution was placed in a 250 mL poly container (polypropylene), and dry ice was filled around it. After inserting a fiber optic thermometer into the frozen solution in the polyethylene solution, the frozen solution was irradiated with microwaves (100W) until the fiber optic thermometer showed 20°C, thereby dissolving the frozen solution and obtaining a dissolved solution. Next, 100 mL of the polyethylene solution (volume: 250 mL) was placed in a constant temperature bath (temperature: 60°C) from a 250 mL polyethylene container. The dissolved solution in the polyethylene container was centrifuged in the constant temperature bath at a rotation speed of 2000 rpm for 2 minutes to recover the graphite powder after microwave irradiation.

[0073] [Experimental Examples C2~C9] Except for using an aqueous solution containing the organic solvents shown in Table 8 at the concentrations shown in Table 8 as the dispersion medium, the frozen solution containing dispersed graphite powder was irradiated with microwaves (100W) until the optical fiber thermometer showed 20°C, in the same manner as in Experimental Example C1.

[0074] (Temperature change of the freezing solution) The temperature change of the frozen solution was measured from the time of microwave irradiation until the optical fiber thermometer showed 20°C. The results are shown in Figure 10. From the results in Figure 10, it can be seen that for the same organic solvent, the time it takes for the temperature to change to 20°C decreases as the concentration of the organic solvent increases. Furthermore, when looking at the cases where the organic solvent concentration is 0.5 vol% and 1.0 vol%, it can be seen that ethanol tends to heat up more easily compared to 1-propanol and acetaldehyde, as the time it takes for the temperature to change to 20°C is shorter.

[0075] (Raman spectrum of graphite powder after microwave irradiation) The Raman spectrum of graphite powder after microwave irradiation was measured. The results are shown in Table 8 below.

[0076] [Table 8]

[0077] The results in Table 8 show that the graphite powder in experimental examples C1-C9 underwent significant changes in the D / G ratio and G / 2D ratio after microwave irradiation, confirming that the graphite powder was delaminated by microwave irradiation. In particular, it was confirmed that at least one of the changes in the D / G ratio and G / 2D ratio increased when using aqueous solutions with 1.0 vol% and 5.0 vol% 1-propanol concentrations, aqueous solution with 5.0 vol% ethanol concentration, and aqueous solutions with 1.0 vol% and 5.0 vol% acetaldehyde concentrations.

[0078] [Experimental Examples C10~C16] Except for using an aqueous solution containing the organic solvents shown in Table 9 at the concentrations shown in Table 9 as the dispersion medium, and setting the microwave output to irradiate the frozen solution to 200W, the procedure was the same as in Experimental Example C1, and the frozen solution was irradiated with microwaves until the optical fiber thermometer showed 20°C.

[0079] (Temperature change of the freezing solution) The temperature change of the frozen solution was measured from the time of microwave irradiation until the optical fiber thermometer showed 20°C. The results are shown in Figure 11. From the results in Figure 11, it can be seen that, for the same organic solvent, the time it takes for the temperature to change to 20°C decreases as the concentration of the organic solvent increases.

[0080] (Raman spectrum of graphite powder after microwave irradiation) The Raman spectrum of graphite powder after microwave irradiation was measured. The results are shown in Table 9 below.

[0081] [Table 9]

[0082] The results in Table 9 show that the graphite powder in experimental examples C10-C16 after microwave irradiation showed a larger rate of change in the D / G ratio compared to experimental examples C1-2, C4-5, and C7-9, which used the same dispersion medium. This is thought to be because the increased microwave output of 200W caused the graphite to heat up significantly, resulting in delamination and simultaneous shearing.

[0083] Furthermore, the organic solvents (1-propanol, ethanol, and acetaldehyde) used in experimental examples C1 to C16 have high reducing properties. Therefore, it is speculated that hydrogen is generated by the oxidation-reduction reaction between water and the organic solvent during microwave irradiation, and that this hydrogen may be causing the interlayers of graphite to widen more easily. [Explanation of symbols]

[0084] 10-layer material nanosheet 11 Unit Layer 12, 13 surface 20 Reactor 21 Layered material particles 22 Polar solvents 23 Frozen suspension 24 Reaction vessel 25 Lid 26 Fiber Optic Thermometer 27 Coolant 28 Cooler 30 Reactor 37 Coolant 38 Circulating cooler

Claims

1. The process includes a peeling step in which layered material particles are peeled off in layers by irradiating them with microwaves while cooling them, A method for producing a layered material nanosheet, comprising a dispersion step of dispersing the layered material particles in a polar solvent, and in the peeling step, the layered material particles are peeled off in layers by irradiating the layered material particles with microwaves while cooling the layered material particles by solidifying the polar solvent.

2. A method for producing a layered material nanosheet according to claim 1, wherein the microwaves are intermittently irradiated onto the layered material particles.

3. The method for producing a layered material nanosheet according to claim 1, wherein the layered material particles include at least one particle selected from the group consisting of transition metal chalcogenide particles, molybdenum tungsten particles, and layered carbon particles.

4. The method for producing a layered material nanosheet according to claim 1, wherein the layered material particles have a cleavage surface, and the major axis of the cleavage surface is 10 μm or more.

5. MoS ２ including the MoS ２ X-ray diffraction peak intensity I of the (103) plane １０３ X-ray diffraction peak intensity I of the (002) plane ００２ Ratio I ００２ / I １０３ Layered MoS ２ Nanosheet.

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