Method for producing hydrogen boride-containing sheet, hydrogen boride-containing sheet, and use thereof

Abstract

The present disclosure addresses the problem of providing: a hydrogen boride-containing sheet production method with which it is possible to obtain a hydrogen boride-containing sheet larger than conventional products; and a hydrogen boride-containing sheet.　The hydrogen boride-containing sheet production method according to an embodiment of the present disclosure comprises: bringing a metal boride into contact with an ion exchange material to obtain hydrogen boride; and heating a mixed liquid containing the obtained hydrogen boride and a liquid medium on a substrate to form a hydrogen boride-containing sheet on the substrate.　The hydrogen boride-containing sheet according to an embodiment of the present disclosure has a length of at least 100 μm in at least one direction.

Description

Method for manufacturing boron-containing sheet, boron-containing sheet, and use thereof

[0001] The present invention relates to a method for manufacturing a boron-containing sheet, a boron-containing sheet, and uses thereof. This application claims priority based on Japanese Patent Application No. 2023-184054, filed on October 26, 2023, the contents of which are incorporated herein by reference.

[0002] Borohydride (HB) ｎ Borohydride is a material with a structure in which boron atoms form a two-dimensional network via hydrogen atoms. Borohydride has attracted attention because it is lightweight and available in abundant resources (see Patent Document 1).

[0003] International Publication No. 2018 / 074518

[0004] However, the size of the boron hydride sheets synthesized to date has been on the order of 10 μm at most, and larger boron hydride sheets are needed for practical use.

[0005] The problem to be solved by the present invention is to provide a method for producing a boron-containing sheet, which allows for the production of a boron-containing sheet that is larger than conventional boron-containing sheets, and a boron-containing sheet.

[0006] The present invention may include the following aspects. [1] A method for producing a borohydride-containing sheet, comprising: contacting a metal boride with an ion exchange material to obtain borohydride; and heating a mixture containing the obtained borohydride and a liquid medium on a substrate to form a borohydride-containing sheet on the substrate. [2] The method of [1], further comprising evaluating the obtained borohydride by analyzing the borohydride before heating on the substrate. [3] The method of [2], wherein the evaluation comprises determining whether the spectroscopic spectrum of the obtained borohydride contains a peak of sufficient magnitude at a predetermined position. [4] The method of any of [1] to [3], wherein the heating on the substrate is performed at a temperature equal to or higher than the boiling point of the liquid medium (hereinafter, "boiling point" means the boiling point under a pressure different from atmospheric pressure, such as a reduced pressure) and equal to or lower than 200°C. [5] The method according to any one of [1] to [4], further comprising drying the obtained borohydride after obtaining it and mixing the dried borohydride with the liquid medium before heating it on the substrate. [6] The method according to [5], wherein drying the obtained borohydride to form a powder comprises heating the obtained borohydride at a first temperature and heating the obtained borohydride at a second temperature higher than the first temperature. [7] A method for producing a borohydride-containing sheet having an increased size, comprising heating a mixture containing a raw material including the borohydride-containing sheet and a liquid medium on a substrate. [8] The method according to [7], wherein the heating comprises evaporating the liquid medium on the substrate and further heating the borohydride-containing sheet remaining after the liquid medium has evaporated. [9] A borohydride-containing sheet having a length of 100 μm or more in at least one direction.

[10] The borohydride-containing sheet according to [9], having a flat shape.

[11] The borohydride-containing sheet according to [9] or

[10] , having a borohydride content of 10 mass% or more.

[12] A hydrogen storage material, catalyst, electronic material, communication material, electrode material, or fuel material, comprising the borohydride-containing sheet according to any one of [9] to

[11] .

[0007] According to the present invention, the size of the borohydride-containing sheet can be increased.

[0008] Schematic diagram showing a boron-containing sheet 10 according to one embodiment. Flowchart showing a method for manufacturing a boron-containing sheet according to one embodiment. Schematic diagram showing a process for forming a boron-containing sheet according to one embodiment. Schematic diagram showing a process for forming a boron-containing sheet according to one embodiment. Schematic diagram showing a process for forming a boron-containing sheet according to one embodiment. Infrared absorption spectrum of boron-containing sheet produced in Example 1. Photograph of the boron-containing sheet produced in Example 1. Photograph of the boron-containing sheet produced in Example 1. X-ray diffraction (XRD) pattern of the boron-containing sheet produced in Example 1.

[0009] The following describes a method for manufacturing a boron-containing sheet and a boron-containing sheet according to an embodiment. Note that the following embodiment shows one aspect of the present invention, does not limit the present invention, and can be arbitrarily modified within the scope of the technical concept of the present invention. Furthermore, each configuration and each feature of the embodiment can be arbitrarily combined.

[0010] <1. Borohydride-Containing Sheet> A borohydride-containing sheet according to one embodiment is a borohydride-containing sheet having a length in at least one direction of 100 μm or more.

[0011] The present inventors have discovered that the method described below can produce boron-containing sheets larger than conventional ones, as described above. Conventionally known boron-containing sheets have a size on the order of 10 μm at most. No method for producing boron-containing sheets larger than 100 μm has been discovered.

[0012] <1.1 Definition> In this specification, "borohydride" refers to a compound of hydrogen and boron. Specifically, borohydride represented by the chemical formula HB is known. Each boron atom (B) is connected to another via a hydrogen atom (H) to form a two-dimensional network. A borohydride that has formed a two-dimensional network can be represented, for example, by the chemical formula (HB) ｎ(n: natural number) The boron hydride may have a two-dimensional network of continuous honeycomb structures in which boron atoms form six-membered ring structures, or may have a two-dimensional network in which boron atoms form other ring structures such as five-membered ring structures or seven-membered ring structures.

[0013] In this specification, the term "borohydride-containing sheet" refers to a material containing borohydride and having a two-dimensionally extending sheet shape. Note that the borohydride-containing sheet is not limited to one containing only pure borohydride, but may also contain borohydride doped with elements other than boron and hydrogen, or other materials.

[0014] The borohydride-containing sheet contains, for example, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more of borohydride. The borohydride-containing sheet may consist essentially of borohydride only (it may contain inevitable impurities).

[0015] <1.2 Shape of the borohydride sheet> FIG. 1 is a schematic diagram showing an example of a borohydride-containing sheet 10 according to this embodiment. The borohydride-containing sheet 10 has two main surfaces 12a, 12b located opposite each other and a side surface 14 connecting the peripheries of the two main surfaces 12a, 12b. In this case, the main surfaces 12a, 12b are flat surfaces along the xy plane, and the side surface 14 is a surface along the z direction. One or both of the main surfaces 12a, 12b have the largest area on the surface of the borohydride-containing sheet 10. As shown in FIG. 1, the borohydride-containing sheet 10 preferably has a substantially flat shape without a twisted or wavy shape. However, the borohydride-containing sheet does not have to be completely planar; for example, it may have a gently curved surface partially or entirely. Note that the "flat shape" referred to here means that the shape of the borohydride-containing sheet 10 is substantially flat overall, as shown in FIG. 1. Shapes in which the main surface or side of the sheet has slight irregularities, and shapes in which the edge of the sheet has some warping or distortion are also included in the "flat shape."

[0016] The boron-containing sheet 10 has a length measured along at least one direction (for example, the length L1 in the x direction in FIG. 1 ) of 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 300 μm or more, 350 μm or more, 400 μm or more, 450 μm or more, 500 μm or more, 550 μm or more, 600 μm or more, 650 μm or more, 700 μm or more, 750 μm or more, 800 μm or more, 850 μm or more, 900 μm or more, 950 μm or more, or 1 mm or more. Preferably, the lengths of the boron-containing sheet 10 measured along two mutually perpendicular directions (e.g., length L1 in the x direction and length L2 in the y direction in FIG. 1 ) are both 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 300 μm or more, 350 μm or more, 400 μm or more, 450 μm or more, 500 μm or more, 550 μm or more, 600 μm or more, 650 μm or more, 700 μm or more, 750 μm or more, 800 μm or more, 850 μm or more, 900 μm or more, 950 μm or more, or 1 mm or more. Preferably, the boron-containing sheet 10 has a minimum length along the main surfaces 12a, 12b of 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 300 μm or more, 350 μm or more, 400 μm or more, 450 μm or more, 500 μm or more, 550 μm or more, 600 μm or more, 650 μm or more, 700 μm or more, 750 μm or more, 800 μm or more, 850 μm or more, 900 μm or more, 950 μm or more, or 1 mm or more.

[0017] By adjusting conditions such as the borohydride concentration in the solution and the size of the vessel in which the borohydride sheet is prepared, the size of the borohydride-containing sheet 10 can be made very large. There is no particular upper limit to the size of the borohydride-containing sheet 10. For example, the maximum length of the borohydride-containing sheet 10 along the main surfaces 12 a and 12 b may be 10 m or less.

[0018] The thickness D of the boron-containing sheet 10 is not particularly limited as long as it is sufficiently small compared to the two-dimensional extent. For example, the boron-containing sheet 10 may consist of a single layer of boron, or may have a shape in which two or more layers of boron are stacked. The upper limit of the number of layers is not particularly limited. For example, the thickness D of the boron-containing sheet 10 along a direction perpendicular to the main surfaces 12a and 12b is ¼ or less of the shortest length along the main surfaces 12a and 12b. For example, the thickness D of the boron-containing sheet 10 is 1 nm to 100 μm, 10 nm to 50 μm, 100 nm to 20 μm, 500 nm to 10 μm, or 1 μm to 5 μm.

[0019] The size of the boron-containing sheet 10 can be determined by observing a single boron-containing sheet 10 formed as a continuous sheet using an optical microscope or the like and measuring the length of the boron-containing sheet 10 along any direction.

[0020] <1.3 Properties of the borohydride sheet> The borohydride constituting the borohydride-containing sheet 10 according to this embodiment can be amorphous, polycrystalline, or a mixture of these. Therefore, the borohydride-containing sheet 10 has low reflectance in the visible light range and is visible light transmissive. However, the borohydride-containing sheet 10 does not completely transmit visible light. Specifically, the borohydride-containing sheet 10 gradually absorbs light from wavelengths around 800 nm, rapidly increases in absorbance from wavelengths around 500 nm, and almost completely absorbs light with wavelengths of 400 nm or less. As a result, the borohydride-containing sheet 10 appears yellow.

[0021] 1.4 Uses of the borohydride sheet The borohydride-containing sheet 10 according to this embodiment can be used as a hydrogen storage material, catalyst, electronic material, communication material, electrode material, or fuel material, or as a material constituting such functional materials. This is due to the characteristics of borohydride, such as the inclusion of hydrogen atoms in its structure, being lighter than graphene, and having electrical conductivity on the same order as graphene.

[0022] A hydrogen storage material, catalyst, electronic material, communication material, electrode material, or fuel material according to one embodiment includes the borohydride-containing sheet described above.

[0023] According to the borohydride-containing sheet described above, the borohydride sheet can be used in a larger area than before. In general, borohydride has the following characteristics: (a) lightweight, (b) abundant resources, (c) no waste load, and (d) recyclable. Due to these characteristics, borohydride has attracted attention as an energy material. Hydrogen transportation and CO ２ The practical utility of boron hydride in reduction reactions and other applications has been confirmed. It is also theoretically predicted that it will exhibit high performance as a battery material and a communications material. The availability of large-area boron hydride sheets facilitates their social implementation for diverse uses. In particular, boron hydride-containing sheets with a size of 1 mm or more are easily handled by hand. This will greatly expand the industrial applicability of boron hydride sheets. Furthermore, given that millimeter waves are used in the 5G and 6G communications bands, the production of boron hydride-containing sheets with a size of 1 mm or more as described above is expected to lead to their practical application as millimeter wave communications materials.

[0024] 2. Method for Producing a Borohydride-Containing Sheet One embodiment of a method for producing a borohydride-containing sheet includes: (1) contacting a metal boride with an ion exchange material to obtain borohydride; and (2) heating the obtained mixture containing borohydride and a liquid medium on a substrate, thereby forming a borohydride-containing sheet on the substrate.

[0025] <2.1 Definition> In this specification, the term "metal boride" refers to a compound of one or more metal elements and boron. For example, a metal boride has a structure in which boron atoms form a two-dimensional network. For example, a metal boride has a layered structure in which metal cations are inserted between boron layers formed by a two-dimensional network of boron atoms. An example of a metal boride is a compound represented by the composition formula MB ２ An example of a metal boride is magnesium diboride (MgB ２), aluminum diboride (AlB ２ ), tantalum diboride (TaB ２ ), zirconium diboride (ZrB ２ ), rhenium diboride (ReB ２ ), chromium diboride (CrB ２ ), titanium diboride (TiB ２ ), vanadium diboride (VB ２ ), yttrium boride (YB ２ metal diborides such as yttrium chromium boride (YCrB ４ ) and other multi-metal borides; potassium boride (K ２ B ９ , K.B. ９ , K.B. １８ ), lithium boride (Li ２ B ５ Many of the metal diborides have a structure in which the boron atoms form a six-membered ring, and yttrium chromium boride (YCrB ４ ) has a structure in which the boron atom forms a five-membered ring and a seven-membered ring.

[0026] As used herein, "ion exchange material" refers to a material that has ion exchange capacity. For example, the ion exchange material contains ions that can be exchanged with metal cations. Preferably, the ion exchange material exchanges metal cations with protons (H ＋ ) is a material that can exchange ions with the metal boride. Non-limiting examples of ion exchange materials include ion exchange resins. Any ion exchange resin that can exchange ions with the metal boride can be used.

[0027] Non-limiting examples of ion exchange resins include strong acid ion exchange resins and weak acid ion exchange resins. Strong acid ion exchange resins have sulfo groups (-SO ) as ion exchange groups. ３A weakly acidic ion exchange resin is a polymer having a carboxylic acid group (--H). Examples of the base material of such an ion exchange resin include a styrene polymer, a divinylbenzene polymer, and a styrene-divinylbenzene copolymer. A weakly acidic ion exchange resin is a polymer having a carboxylic acid group (--COOH) as an ion exchange group. Examples of the base material of such an ion exchange resin include an acrylic acid (co)polymer and a methacrylic acid (co)polymer.

[0028] <2.2 Details of the manufacturing method> The present inventors have found that the above method can produce a boron-containing sheet having a larger size than conventional methods. Figure 2 is a flowchart showing the manufacturing method of a boron-containing sheet according to Example 1 described below. Figure 2 includes not only essential steps but also optional steps. The above method will be described in detail below with reference to Figure 2.

[0029] <2.2.1 Step (1) of Producing Hydrogen Borate> First, the above step (1) will be described. By bringing the metal boride into contact with the ion exchange material, an ion exchange reaction occurs. The metal cation of the metal boride is converted into a proton (H ＋ ) resulting in borohydride (HB) ｎ is generated (step S200 in FIG. 2).

[0030] The ion exchange reaction preferably proceeds in a liquid. For example, the ion exchange reaction occurs when a metal boride dissolved in a liquid medium comes into contact with an ion exchange material present in a solid state in the liquid medium at a solid-liquid interface. Any liquid medium can be used as long as the ion exchange reaction proceeds. For example, a liquid medium (such as a polar solvent) capable of dissolving the metal boride can be used. Examples of liquid media include inorganic solvents such as water, and organic solvents such as methanol, ethanol, acetonitrile, and N,N-dimethylformamide.

[0031] The concentration of the metal boride in the reaction solution is not particularly limited, but is preferably 0.01 g / L or more and 500 g / L or less. If the concentration of the metal boride is 0.01 g / L or more, a sufficient amount of hydrogen boride can be obtained. If the concentration of the metal boride is 500 g / L or less, excessive aggregation of the product can be suppressed, and a sheet-like product can be obtained.

[0032] Preferably, during or after the ion exchange reaction, impurities such as residues of the ion exchange resin and reaction by-products (e.g., boric acid) are removed (step S202 in FIG. 2). The removal method is not particularly limited, and any method such as filtration, extraction, chromatography, recrystallization, or adsorption can be used. The removal of residues and impurities may be performed at any timing, or may be performed in multiple batches.

[0033] If the produced borohydride is dissolved or dispersed in a liquid medium (hereinafter, a liquid medium in which borohydride is dissolved or dispersed will also be simply referred to as a "sample solution"), the step indicated by the dashed line in Figure 2 can be omitted and the process can proceed to (2). Alternatively, the liquid medium can be removed once to obtain solid borohydride. By removing the liquid medium using any method, such as heating or reducing pressure, a dried powder of borohydride can be obtained (step S204 in Figure 2). If a volatile liquid is used as the liquid medium, the solution can be left at room temperature.

[0034] When the liquid medium is removed by one-stage heating, the heating temperature is preferably equal to or higher than the boiling point of the liquid medium (for example, equal to or higher than 70°C, 80°C, 90°C, or 100°C) and equal to or lower than 200°C. If the heating temperature is equal to or higher than the boiling point of the liquid medium, the liquid medium can be sufficiently vaporized. If the heating temperature is equal to or lower than 200°C, desorption of hydrogen from the borohydride can be suppressed.

[0035] The heating treatment may involve only one heating stage, as described above, or multiple heating stages, as shown in FIG. 2 and in Example 1 described below. The following describes multiple heating stages as in Example 1. For example, as shown in FIG. 2, the heating treatment in step S204 includes step S204a, in which the obtained borohydride is heated at a first temperature, and step S204b, in which the obtained borohydride is heated at a second temperature higher than the first temperature. In step S204a, most of the liquid medium is vaporized, and in step S204b, the liquid medium remaining in the sample is further vaporized to dry the sample. By gradually increasing the temperature in multiple stages in this manner, impurities precipitated during heating can be efficiently separated. The first temperature is not particularly limited as long as it promotes vaporization of the liquid medium, but may be, for example, 0°C to 200°C, 20°C to 150°C, or 70°C to 100°C. The heating time at the first temperature is not particularly limited. The sample can be heated until most of the liquid medium is vaporized and no liquid components are apparent. Note that the first heating step S204b is sufficient to dry the sample. Therefore, natural evaporation without heating or a drying method such as reduced pressure may be used. Heating and drying may be used in combination. The second temperature is preferably equal to or higher than the boiling point of the liquid medium and equal to or lower than 200°C. If the second temperature is equal to or higher than the boiling point of the liquid medium, the liquid medium can be sufficiently evaporated. If the second temperature is equal to or lower than 200°C, desorption of hydrogen from the borohydride can be suppressed. The heating time at the second temperature is not particularly limited, but is, for example, 1 to 2 hours.

[0036] Prior to (2), the obtained boron boride can be evaluated by analyzing the obtained boron boride (step S206 in FIG. 2). The inventors have found that the quality of the boron boride obtained in (1) can affect whether or not a boron boride sheet forms in (2) and the size of the formed boron boride sheet. To obtain a large boron boride sheet, it is important to evaluate the quality of the boron boride prior to (2). Examples of boron boride evaluation targets include the composition, structure, physical properties, purity, and size of the boron boride. For example, the closer the obtained boron boride is to ideal boron boride in terms of composition, chemical structure, crystalline structure, physical properties, purity, etc., the higher the quality of the obtained boron boride sheet is expected to be. As a result, the growth of the boron boride sheet in (2) is less likely to be inhibited, and a larger boron boride sheet is expected to be obtained. Furthermore, the larger the obtained boron boride, the larger the resulting boron boride sheet is expected to be. Samples evaluated as inappropriate can be reused as samples for resynthesis.

[0037] Here, as an example, the evaluation of borohydride by spectroscopic measurement, which will be described in Example 1 below, will be described in detail. For example, the evaluation of borohydride by spectroscopic measurement includes determining whether the obtained spectroscopic spectrum of borohydride contains a peak at a predetermined position. Specifically, in Example 1, the infrared absorption spectrum of the obtained borohydride contains a peak at 1380 cm －１ In addition to the nearby peak, there is a peak at 1280 cm －１ This peak is attributed to the B-H-B bond, and the peak position depends on the boron-boron distance. －１ It was discovered that even if only a small absorption peak appears in the vicinity, a larger boron-containing sheet than conventional ones can be obtained. However, the spectroscopic measurement is not limited to infrared absorption spectroscopy. The peak for confirmation is also the above 1280 cm －１ The borohydride is not limited to nearby absorption peaks, and any spectroscopic feature characteristic of borohydride can be used to evaluate the borohydride.

[0038] One of the reasons for drying the borohydride before step (2) is to facilitate analysis and evaluation of the borohydride. Another reason is to facilitate the removal of impurities. That is, if impurities are still mixed in the dried borohydride, they can be easily separated from the borohydride by separating them during redissolution due to differences in solubility or by classifying them in the dry state.

[0039] If the obtained borohydride is dried as described above, after performing the necessary operations such as the above analysis, the dried borohydride is mixed with a liquid medium to obtain a sample solution in which the borohydride is dissolved or dispersed in the liquid medium, and then proceed to (2) (step S208 in Figure 2). Here, the liquid medium used may be the same liquid medium as used in the previous step, or a different liquid medium. Furthermore, the amount of borohydride in the sample solution after redissolution or redispersion is not particularly limited, but is preferably 1 mg or more and 10 g or less. If the amount of borohydride is 1 mg or more, a borohydride-containing sheet of sufficient size can be obtained. If the amount of borohydride is 10 g or less, excessive aggregation of the borohydride can be suppressed, and a clean borohydride-containing sheet can be obtained.

[0040] 2.2.2 Step (2) of Forming a Borohydride Sheet on a Substrate Next, step (2) will be described. In step (2), to form a borohydride-containing sheet on a substrate, the substrate is filled with a sample solution (step S210 in FIG. 2 ), and then the liquid solvent is evaporated to form a borohydride-containing sheet on the substrate (step S212 in FIG. 2 ).

[0041] The type of substrate is not particularly limited. Any substrate of any shape, material, and size can be used as long as it has a flat surface for forming a borohydride-containing sheet. Non-limiting examples of substrates include glass substrates, silicon substrates, mica substrates, HOPG (highly oriented pyrolytic graphite) substrates, SiC substrates, and SiO ２Examples of suitable substrates include substrates, stainless steel or nickel meshes (preferably with mesh pores smaller than the size of the desired boron-containing sheet, for example, on the order of 1 mm or smaller). It is not important whether or not any of the substrates has a coating such as an oxide film. Furthermore, the term "substrate" as used herein is not limited to an independent plate-like member, but may also refer to a flat portion that constitutes part of a member having a predetermined shape, such as a container.

[0042] The method for filling the substrate with the sample solution is not particularly limited. For example, the substrate may be placed on a horizontal table and the sample solution may be pipetted onto the substrate. Any coating method may be used, such as spin coating, dip coating, bar coating, or spray coating. Here, the method employed in Example 1 (described later) will be described in detail with reference to FIG. 3.

[0043] First, as shown in FIG. 3A , a substrate 32 is placed on the bottom surface of a container 30. Next, the internal space of the container 30 is filled with a sample solution S. As a result, the substrate 32 is immersed in the sample solution S, and the upper surface of the substrate 32 is filled with the sample solution S. Next, as shown in FIG. 3B , the container 30 is heated to evaporate the liquid medium of the sample solution S. Continued heating eventually results in the boron dioxide in the sample solution S remaining on the upper surface of the substrate 32, as shown in FIG. 3C , forming a large-area boron dioxide-containing sheet on the substrate 32. This method can suppress uneven distribution of boron dioxide on the substrate 32 compared to pipetting. Therefore, the uniformity of the boron dioxide-containing sheet formed can be improved. Furthermore, compared to the above-described coating method, a sufficient amount of boron dioxide can fill the upper surface of the substrate 32, allowing the formation of a large boron dioxide-containing sheet.

[0044] The heating temperature is preferably equal to or higher than the boiling point of the liquid medium (for example, 70°C or higher, 80°C or higher, 90°C or higher, or 100°C or higher) and equal to or lower than 200°C. If the heating temperature is equal to or higher than the boiling point of the liquid medium, the liquid medium can be sufficiently evaporated. If the heating temperature is equal to or lower than 200°C, desorption of hydrogen from the borohydride can be suppressed. It is preferable to continue heating after the liquid medium has completely evaporated. The heating time is not particularly limited, but may be, for example, 1 to 2 hours after the liquid medium has completely evaporated.

[0045] Finally, the boron-containing sheet 10 formed on the substrate 32 is recovered. The recovery method is not particularly limited. For example, the boron-containing sheet 10 may be physically peeled off from the substrate 32. A coating of a material to which the boron-containing sheet 10 does not easily adhere may be provided on the substrate 32 in advance. An easily peelable peeling film or the like may be provided on the substrate 32 in advance, and the boron-containing sheet 10 may be recovered from the substrate 32 by peeling off this film. Alternatively, the boron-containing sheet 10 may be used in a state where it is adhered to the substrate 32. Alternatively, the boron-containing sheet 10 can be recovered from the substrate 32 by any other method. Instead of using a substrate 32 separate from the container 30, a flat surface (bottom, side, etc.) of the container 30 may be used as the substrate 32.

[0046] As described above, by heating a mixture containing borohydride and a liquid medium on a substrate, it is possible to form a borohydride-containing sheet on the substrate with a larger area than conventional sheets. The following is a speculation about the mechanism by which such a large area is achieved. However, the following is merely speculation, and the present invention is not limited by theory. The borohydride produced by the ion exchange reaction of the metal boride has a nanosheet shape with a thickness of about several atomic layers. In the sample solution, the borohydride is speculated to exist in a dispersed state, with each individual borohydride nanosheet being solvated. Here, borohydride (HB) is generally referred to as a hydrogen boride nanosheet. ｎIn the case of hydrogen boride nanosheets, all of the outermost electrons of the boron and hydrogen atoms are used in covalent bonds, so there are no intermolecular interactions via unshared electron pairs. Therefore, it is speculated that in the vicinity of the substrate, the interaction between the hydrogen boride nanosheet and the substrate surface is more dominant than the interaction between hydrogen boride nanosheets themselves. Furthermore, considering that the hydrogen boride nanosheet itself has a two-dimensional sheet shape, it is thought that the interaction between the hydrogen boride nanosheet's sheet surface and the substrate surface is greater than the interaction between the edge of the hydrogen boride nanosheet and the substrate surface. Therefore, it is speculated that in the vicinity of the substrate, the hydrogen boride nanosheet tends to adhere to the substrate surface with its sheet surface and the substrate surface approximately parallel. In fact, according to the inventors' investigations, it was confirmed that a hydrogen boride-containing sheet was formed along the substrate surface not only when the substrate was placed horizontally, but also when the substrate was placed vertically. After the substrate surface is covered with hydrogen boride nanosheets, the direct interaction between the hydrogen boride nanosheets in the sample solution and the substrate surface is thought to become weaker. In this case, it is presumed that the next dominant interaction is the interaction between the hydrogen boride nanosheets themselves. Therefore, it is thought that other hydrogen boride nanosheets will pile up on top of the hydrogen boride nanosheets attached to the substrate.

[0047] Because the size of the substrate is larger than the size of each individual hydrogen boride nanosheet, a hydrogen boride sheet larger than the original hydrogen boride nanosheet is formed on the substrate surface as a whole. It is believed that the heating of the sample solution and substrate causes chemical bonds to form between the hydrogen boride nanosheets stacked on the substrate (both along the substrate and in the stacking direction perpendicular to the substrate surface). It is presumed that the bonding of hydrogen boride nanosheets along the substrate surface to form hydrogen boride sheets results in the formation of a large-area hydrogen boride-containing sheet that cannot be achieved by simple growth in solution alone.

[0048] The size of the borohydride-containing sheet can also be adjusted by changing the size of the substrate used.

[0049] The above-described method for producing a boron-containing sheet allows the production of a boron-containing sheet having a larger area than conventional methods. While conventional boron-containing sheets have a size of at most 10 μm × 10 μm, the present method allows the production of boron-containing sheets of 100 μm × 100 μm, 1 mm × 1 mm, or even larger sizes. In other words, the present method increases the area of ​​boron-containing sheets that can be produced by more than 100 times.

[0050] Furthermore, when boron hydride is evaluated by analysis, it is possible to predict whether a large-area boron hydride-containing sheet can be formed based on objective criteria, and therefore large-area boron hydride-containing sheets can be produced with good reproducibility.

[0051] 3. Method for Producing a Borohydride-Containing Sheet with Increased Size> A method for producing a borohydride-containing sheet with increased size according to one embodiment is a method comprising heating a mixture containing a raw material including a borohydride-containing sheet and a liquid medium on a substrate.

[0052] The method of 2. above starts with the synthesis of boron hydride by an ion exchange reaction. On the other hand, the method of producing an increased-size boron hydride-containing sheet according to this embodiment starts with a raw material containing a boron hydride-containing sheet and produces a boron hydride-containing sheet that is larger in size than the raw material boron hydride-containing sheet. For example, the boron hydride-containing sheet obtained by this method has a larger sheet surface area than the raw material boron hydride-containing sheet. In this method, a raw material containing a boron hydride-containing sheet is dissolved or dispersed in a liquid medium, and then steps S210 and S212 of FIG. 2 are performed. As a result, as in the method of 2. above, the raw material boron hydride-containing sheets are stacked on the substrate and bonded to each other. As a result, a boron hydride-containing sheet larger in size than the raw material is formed on the substrate.

[0053] Heating the sample solution includes evaporating the liquid medium on the substrate and further heating the boron-containing sheet remaining after the liquid medium has evaporated. The heat treatment has the significance of both evaporating the liquid medium to dry the boron-containing sheet and bonding the boron-containing nanosheets together to form a large-area boron-containing sheet.

[0054] The present invention will be described below with reference to experimental examples, but the present invention is not limited to the following experimental examples.

[0055] Example 1: 1 g of magnesium boride (MgB) was dissolved in water under an inert gas atmosphere. ２ Magnesium boride crystals (manufactured by Rare Metallic Co., Ltd.) were added to 200 mL of acetonitrile (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and dissolved. Next, 30 mL of ion exchange resin (product name Amberlite IR120B H HG, manufactured by Organo Corporation) was added to the acetonitrile solution. After stirring this magnesium boride solution at room temperature for 72 hours, the residue in the reaction solution was filtered to obtain a yellow filtrate. The filtrate was heated at 70°C for 2 hours to evaporate the acetonitrile, yielding a yellow, treacle-like sample. Impurities precipitated during heating were removed as appropriate. The resulting sample was then further heated at 120°C for 2 hours to obtain a yellow, dry powder sample.

[0056] A part of the obtained powder sample was subjected to infrared spectroscopy. The obtained infrared absorption spectrum is shown in FIG. 4. In the infrared absorption spectrum, －１ An absorption peak was confirmed near 1380 cm. This absorption peak corresponds to the boron-hydrogen bond. －１ Since an absorption peak of sufficient magnitude was confirmed based on the absorption peak of , it was determined that a suitable borohydride sample had been obtained.

[0057] 100 mg of the powder sample was again dissolved in 4 mL of acetonitrile to prepare sample solution S. As shown in FIG. 3A, a flat glass substrate 32 was placed on the bottom of container 30, and sample solution S was poured into container 30. Sample solution S was heated at 70°C to completely evaporate the acetonitrile. It was then further heated at 120°C for 2 hours, resulting in the formation of a borohydride-containing sheet 10 on substrate 32, as shown in FIG. 3C. The borohydride-containing sheet 10 was then peeled off from substrate 32 with a microspatula to obtain the target sample. Note that the target sample was not formed continuously across the entire size of substrate 32, but was divided into small segments by cracks. Each of these segments was a single borohydride-containing sheet 10. FIG. 5 is a photograph of the resulting sample, and FIG. 6 is a photograph of one of the samples shown in FIG. 5.

[0058] Evaluation Example 1: Evaluation of Sample Size The obtained samples were flat sheet-like samples of various sizes. The obtained samples were observed under an optical microscope to evaluate the size of the samples. Most of the obtained samples were 100 μm or more in all directions along the main surface of the sheet. As shown in FIG. 6 , the largest of the obtained samples had a length of 1 mm or more (approximately 2 mm) in multiple directions along the main surface. The thickness in the direction perpendicular to the main surface was approximately 50 μm. When the surfaces of the obtained samples were observed under a scanning electron microscope (SEM), no cracks were observed, and a very clean surface was observed.

[0059] <Evaluation Example 2: Measurement of X-ray Diffraction (XRD) Pattern> Figure 7 shows the XRD pattern of the obtained sample. A pattern resembling that of known borohydride was confirmed. However, the very broad peaks in the XRD pattern suggested that the sample was polycrystalline with very small crystallite sizes, or substantially amorphous (for example, a state in which fine crystallites are randomly scattered within an amorphous structure).

[0060] In Example 1, the sample was completely dried after the reaction and then redissolved in a solvent. However, in Example 2, it was confirmed that the boron-containing sheet 10 could be obtained even if the drying and redissolving steps were omitted. Specifically, magnesium boride (MgB) was used in the same manner as in Example 1. ２ After reacting the ion exchange resin with the boron nitride in acetonitrile, residues and impurities were removed appropriately to obtain a reaction solution containing acetonitrile as a solvent. The reaction solution was then heated at 70°C to completely evaporate the acetonitrile. The reaction solution was then further heated at 120°C for 2 hours to obtain a boron nitride-containing sheet 10 without undergoing drying and redissolution steps.

[0061] Comparative Example 1: 1280 cm in the above infrared spectrum －１ It was confirmed that the absorption peak in the vicinity changes depending on the experimental conditions, the amount of impurities, and other accidental conditions. －１ When the nearby absorption peak was small or the peak was not observed, a boron-containing sheet 10 exceeding 1 mm was not obtained. Therefore, the above-mentioned evaluation by infrared spectrum can be an index for obtaining a large-sized boron-containing sheet 10.

[0062] 10... borohydride-containing sheet, 12... main surface, 14... side surface, 30... container, 32... substrate, S... sample solution

Claims

1. A method for producing a borohydride-containing sheet, comprising: contacting a metal boride with an ion exchange material to obtain borohydride; and forming a borohydride-containing sheet on a substrate by heating a mixture containing the obtained borohydride and a liquid medium on the substrate.

2. The method of claim 1, further comprising evaluating the resulting borohydride by analysis of the borohydride prior to heating on the substrate.

3. The method according to claim 2, wherein said evaluation includes determining whether or not the obtained spectroscopic spectrum of borohydride includes a peak of sufficient magnitude at a predetermined position.

4. The method according to any one of claims 1 to 3, wherein the heating on the substrate is carried out at a temperature equal to or higher than the boiling point of the liquid medium and equal to or lower than 200°C.

5. The method according to any one of claims 1 to 3, further comprising, after obtaining the borohydride and before heating on the substrate, drying the obtained borohydride, and mixing the dried borohydride with the liquid medium.

6. The method of claim 5, wherein drying the resulting borohydride into a powder form comprises: heating the resulting borohydride at a first temperature; and heating the resulting borohydride at a second temperature greater than the first temperature.

7. A method for producing a borohydride-containing sheet of increased size, comprising heating a mixture comprising a feedstock material comprising a borohydride-containing sheet and a liquid medium on a substrate.

8. The method of claim 7, wherein the heating comprises: vaporizing the liquid medium on the substrate; and further heating the borohydride-containing sheet remaining after the liquid medium has vaporized.

9. A borohydride-containing sheet having a length in at least one direction of 100 μm or more.

10. The borohydride-containing sheet of claim 9 having a flat shape.

11. The borohydride-containing sheet according to claim 9 or 10, wherein the borohydride content is 10 mass% or more.

12. A hydrogen storage material, catalyst, electronic material, communication material, electrode material, or fuel material, comprising the borohydride-containing sheet according to claim 9 or 10.

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