JP6656563B2 - Boron atomic layer sheet and laminated sheet and method for producing the same - Google Patents

Description

  The present invention relates to a boron atomic layer sheet and a laminated sheet and a method for producing the same.

  Nanostructures that precisely control the structure of one-dimensional nanotubes and nanowires, two-dimensional layered materials and nanosheets, three-dimensional porous materials, dendrimers, etc., exhibit various functions and physical properties by utilizing space and shape I do.

  Among them, graphene, which is an atomic layer material of carbon, was discovered in 2004 to have excellent physical properties such as mechanical strength, thermal conductivity, and electrical conductivity, and was obtained by attaching graphite to Scotch tape and peeling it off. Therefore, applied research has been advanced, and, for example, graphene analogs have been studied from the viewpoint of modifying graphene and changing constituent elements.

From the viewpoint of changing constituent elements, boron nitride (BN), silicene (Si), germanene (Ge), borophene (B), and the like are known. Borophene are boron monolayer nanosheets, Wang et al., Performs the synthesis of B ₃₆ clusters of 36 atoms by gas-phase vacuum system, by identifying the structure from a comparison of simulation by photoelectron spectra and theoretical calculations, borophene similar Cluster synthesis was reported (Non-Patent Document 1). After that, borophene as a sheet that spreads not two-dimensionally but as a two-dimensional structure has been proposed by Guissinger et al. (Non-patent document 2) and Wu et al. (Non-patent document 3) to obtain an Ag (111) surface under ultra-high vacuum. Synthesis by vacuum deposition of boron on has been reported. This borophene is a substance that cannot exist in the atmosphere. On the other hand, borophane is a boron monolayer nanosheet protected with hydrogen at the end, and the speed and mechanical strength of Dirac particles are expected to exceed that of graphene, and theoretical calculations predicted that it could exist in the atmosphere, but recently, Its synthesis was reported (Non-Patent Document 4).

Nat Commun 2014, 5, 3113. Science 350, 1513-1516 (2015). Nat. Chem., 8, 563-568 (2016). https://www.tohoku.ac.jp/japanese/2017/09/press20170926-02.html

With the above background, the following inventions are provided based on the knowledge that novel boron atom layer sheets and laminated sheets have been obtained.
[1] An atomic layer sheet having a skeleton element of boron and oxygen and having a molar ratio of oxygen to boron (oxygen / boron) of less than 1.5, which is networked by a non-equilibrium bond having a boron-boron bond.
[2] The atomic layer sheet according to [1], further comprising an alkali metal ion, wherein the molar ratio of the alkali metal ion to boron (alkali metal ion / boron) is less than 1.
[3] The atomic layer sheet of [1] or [2], which is an oxidation product of MBH ₄ (M represents an alkali metal ion).
[4] any atom layer sheet backbone composition is _{B 5 O 3 [1] ~} [3].
[5] The atomic layer sheet according to [4], wherein the skeleton has a three-fold symmetry having a boron-boron bond.
[6] The atomic layer sheet according to [4] or [5], comprising the constituent element X as the skeleton part and the other constituent element Y.
[7] The atomic layer sheet according to [6], wherein the component Y is a terminal site and / or a defective site.
[8] The atomic layer sheet according to [6] or [7], wherein the component Y is a boron oxide site containing B-OH.
[9] In the X-ray photoelectron spectroscopy, any one of [6] to [8] having peaks derived from the B-1s level at 190.5 to 193.0 eV and 192.5 to 194.0 eV, respectively. Atomic layer sheet.
[10] The atomic layer sheet according to [9], wherein a peak at 190.5 to 193.0 eV corresponds to the component X in the X-ray photoelectron spectroscopy measurement.
[11] In the IR measurement, having two peaks derived from BO stretch around 1300～1500Cm ^-1, and a peak derived from BO-H stretching in the vicinity of 3100 cm ^-1 [6] ~ [ 10] The atomic layer sheet according to any one of [1] to [10].
[12] The atomic layer sheet according to [11], wherein, in the IR measurement, a peak on a low wave number side of the two types of peaks derived from BO stretching corresponds to the component X.
[13] A laminated sheet including a plurality of atomic layer sheets according to any one of [1] to [12] and metal ions between the atomic layer sheets.
[14] The laminated sheet according to [13], wherein the metal ion is an alkali metal ion.
[15] The laminated sheet according to [14], wherein the molar ratio of alkali metal ion to boron (alkali metal ion / boron) is less than 1.
[16] A crystal including the laminated sheet according to any one of [13] to [15].
[17] a step of adding MBH ₄ (M represents an alkali metal ion) to a solvent containing an organic solvent under an inert gas atmosphere to prepare a solution, and exposing the solution to an atmosphere containing oxygen A method for producing an atomic layer sheet and / or a laminated sheet containing boron and oxygen, comprising:
[18] The atomic layer sheet has boron and oxygen as skeletal elements and is networked by a non-equilibrium bond having a boron-boron bond, and has a molar ratio of oxygen and boron (oxygen / boron) of less than 1.5. The method according to [17], wherein the laminated sheet includes a plurality of the atomic layer sheets and metal ions between the atomic layer sheets.
[19] The method according to [18], wherein the metal ion is an alkali metal ion, and the molar ratio of the alkali metal ion to boron (alkali metal ion / boron) is less than 1.
[20] A step of adding the laminated sheet of any one of [13] to [15] and at least one selected from crown ethers and cryptands to a solvent containing an organic solvent, and peeling off the laminated sheet. And a method for producing a peeled product of a laminated sheet.
[21] A method for producing a peeled product of a laminated sheet, comprising a step of adding the laminated sheet according to any one of [13] to [15] to an aprotic highly polar solvent and peeling the laminated sheet.
[22] The method according to [20] or [21], wherein the exfoliated material includes a single-layer atomic layer sheet.

  The boron atomic layer sheet and the laminated sheet of the present invention can be expected to be applied to various industries from the structural features disclosed below. Bottom-up synthesis of atomic layer material of boron, liquid phase synthesis under atmospheric pressure, and stability in air are characteristic findings in comparison with the conventional technology, and this boron layered single crystal is physically And a single layer can be formed by a chemical dissolution method. By applying a physical force to the crystal, a sheet material having a thickness corresponding to a single layer can be obtained on the substrate. Further, this layered single crystal is insoluble in an aprotic general organic solvent, but is dissolved by the addition of a cryptand or a crown ether for capturing metal ions between layers. In a state where the metal ions have dissolved, it is assumed that the boron sheet is also dispersed in the solution as a single layer.

It is a photograph of the acicular crystal synthesized in the example. It is the figure which showed the structure of the boron layered crystal by X-ray structure analysis, (a) shows a layered cross section, (b) and (c) show a plane crystal structure. It is the figure which showed the structure of the boron layered crystal by X-ray structure analysis, (a) The estimated structure of the unit cell of a boron atom layer, (b) The estimated structure of the unit cell of a terminal / deletion part, (c) The distance between the BB bond and the BO bond is shown. It is an IR spectrum of boron layered crystal (top) and B (OH) ₃ (bottom). (A) is an XPS spectrum of the boron layered crystal and _{(b) B} 2 _{O 3,} and KBH _4. (A) shows the surface index analysis in the X-ray single crystal structure analysis, and (b) shows the XRD pattern of the boron layered crystal in the capillary. Boron layered crystal, B _{(OH) 3} and _B 2 _{O 3} (a) ultraviolet - visible absorption spectrum, a near infrared absorption spectrum (b). (A) is an SEM image of a boron layered crystal, and (b) is an SEM image of a nanosheet separated from the boron layered crystal by mechanical pressure. It is an AFM image of the nanosheet peeled from the boron layered crystal. It is an AFM image and a height profile of the nanosheet peeled from the boron layered crystal. It is an AFM image of a nanosheet obtained by dissolving a boron layered crystal with crown ether and casting it on a HOPG substrate. It is (a) STEM image of the nanosheet peeled from the boron layered crystal and (b) a high-resolution TEM image. It is a high-resolution TEM image of the lattice pattern of the nanosheet peeled from the boron layered crystal. (A) is a phase transition process from a crystal of a boron layered crystal to a liquid crystal during heating from 50 ° C. to 120 ° C., and (b) is a polarization microscope showing a shape change of the boron layered crystal during cooling from 120 ° C. to 35 ° C. It is a statue.

  Hereinafter, the present invention will be described in detail.

  In the present invention, the “atomic layer sheet” is a monoatomic layer sheet having boron and oxygen as main constituent atoms. In addition to an independent single layer sheet, a single layer existing as a partial component in a laminated sheet It also includes a sheet, a metal ion-containing single-layer sheet in which metal ions for maintaining charge balance are bonded to an independent single-layer sheet, and the like. In this specification, it is also referred to as a boron atomic layer sheet, a nanosheet, or the like. The “laminated sheet” is a layered substance containing this atomic layer sheet and metal ions between the atomic layer sheets, and is also described in this specification as a boron layered crystal or the like.

Borophen is a sheet-like substance composed of boron alone, and its structure and stability are discussed based on the ratio of a triangular lattice formed by boron and hexagonal vacancies composed of sp ² boron. The existence of the triangular lattice is generally considered to be due to the fact that a simple substance and a cluster of boron form a stable structure with the triangular lattice formed by multicenter bonding as a unit unit. In the present invention, the expression "networked by non-equilibrium bonds having boron-boron bonds" expresses a two-dimensional bonding mode in accordance with the discussion of the conventional bonding mode in a boron-containing atomic layer sheet such as borophene. Things.
(Atomic layer sheet)
The atomic layer sheet of the present invention has boron and oxygen as skeleton elements, is networked by a non-equilibrium bond having a boron-boron bond, and has a molar ratio of oxygen to boron (oxygen / boron) of less than 1.5. . In one embodiment, the composition further contains an alkali metal ion, and the molar ratio of the alkali metal ion to boron (alkali metal ion / boron) is less than 1. These specifications are based on the case where the atomic layer sheet is an oxidation product of MBH ₄ (M represents an alkali metal ion), and the conventional boric acid has only a boron-oxygen bond, and is polymerized (polymerized). ) Is considered three-dimensional and does not become an atomic layer sheet. The molar ratio of oxygen to boron (oxygen / boron) may be 1.2 or less, 1.0 or less, 0.8 or less. Further, it may be 0.1 or more and 0.3 or more. The molar ratio of alkali metal ion to boron (alkali metal ion / boron) may be 0.8 or less, 0.6 or less, 0.4 or less. Further, it may be 0.01 or more, 0.05 or more, or 0.1 or more.

As an example of the atomic layer sheet of the present invention as described above, an atomic layer sheet having a skeleton composition of B ₅ O ₃ will be described.
<Atomic layer sheet having a composition of B ₅ O ₃ >
In the above description, the “skeleton” of the atomic layer sheet has a regular structure as shown in FIGS. 2B and 2C and FIGS. 3A and 3C in which the composition is B ₅ O ₃ . And mainly occupy the sheet portion other than the terminal portion and the defective portion.

This atomic layer sheet has a skeletal composition of B ₅ O ₃ . As shown in FIGS. 2 (b) and 2 (c) and FIGS. 3 (a) and 3 (c), it is an atomic layer composed of boron and oxygen, and boron bonded to oxygen is bonded to form a distorted hexagon. While forming a two-dimensionally spread plane.

  Boron atoms are classified into those that occupy the vertices of a hexagon in the unit structure of the crystal and those that occupy each side of the hexagon. Those occupying each side of the hexagon are alternately located inside and outside the side. Thus, the skeleton has a three-fold symmetry of boron-boron bonds.

  Oxygen atoms occupy one of the two locations of the two adjacent boron atoms of the three boron atoms on each side of the hexagon formed by the boron atoms (FIGS. 2 (b) and (c); 3 (a) and (c), oxygen atoms are shown at two locations for convenience, but the occupancy is 0.5 as shown in FIG. 2 (c).)

  The boron-boron bond distance is between 1.6 ° and 1.9 °, and the value obtained by X-ray structural analysis is 1.784 °. This bond distance is a value close to the average value of the distance between the two types of boron-boron bonds present in borophene, and is an intermediate value between the value reported as a single bond and the value reported as an oxygen bridge.

  The value of the boron-oxygen bond distance obtained by X-ray structural analysis is 1.339 ° for boron located on the side of the hexagon and 1.420 ° for boron located at the vertex of the hexagon.

  This atomic layer sheet includes a component X which is a skeleton portion and other components Y. In a typical embodiment, component Y is a terminal site and / or a deletion site.

In an exemplary embodiment, component Y is a boron oxide moiety that includes B-OH. The component Y is a site whose structure is similar to trivalent B ₂ O ₃ or B (OH) ₃ (FIG. 3B), and the bonding state of BO is different from that of the skeleton. According to the identification by measurement of the boron layered crystal including the atomic layer sheet, it is as follows.

In the IR measurement (infrared absorption spectrum), having two peaks derived from BO stretch around 1300～1500Cm ^-1, and a peak derived from BO-H stretching in the vicinity of 3100 cm ^-1. The peak on the low wave number side of the two types of peaks derived from BO expansion and contraction corresponds to the component X. Specifically, among the peaks in the BO region, the peak on the low wavenumber side (around 1350 cm ⁻¹ ) corresponds to the boron sheet of component X, and the BO stretching peak seen in B (OH) ₃ The peak on the high wave number side (around 1420 cm ⁻¹ ) having a similar position corresponds to the component Y. The peak derived from BO-H stretching near 3100 cm ^-1 also corresponds to the component Y.

In X-ray photoelectron spectroscopy, peaks derived from the B-1s level are found at 190.5 to 193.0 eV and 192.5 to 194.0 eV, respectively. The peak at 190.5 to 193.0 eV corresponds to the component X. Specifically, the peak corresponding to the component X is slightly lower in energy than B ₂ O ₃ (193.3 eV) in which boron is in a trivalent state, and therefore, the complete oxidation up to trivalent is possible. Has not progressed. The peak corresponding to the component X is separable into two components, two types of boron in the boron sheet of the component X, that is, one occupying a hexagonal vertex in the unit structure of the crystal, and each side of the hexagon. It corresponds to what occupies. The most oxidized peak at 192.5 to 194.0 eV is consistent with B ₂ O ₃ having trivalent boron, and corresponds to component Y.

  In the ultraviolet-visible absorption spectrum, absorption having an absorption in the ultraviolet region of 250 nm or less, and absorption in the near infrared absorption spectrum including a band derived from the vibration structure of BO or BO-H in the near infrared region of 1000 to 2500 nm. have.

As described above, in the atomic layer sheet, the component X, which is a skeleton portion, has a composition of B ₅ O ₃ , and the component Y, which is a boron oxide portion containing B—OH, has a structure of trivalent B Similar to ₂ O ₃ and B (OH) ₃ . In this atomic layer sheet, the molar ratio of oxygen to boron (oxygen / boron) in the entire sheet containing these constituents X and Y is less than 1.5, 1.2 or less, 1.0 or less. Good. It is 0.6 or more, and may be 0.7 or more.
(Laminated sheet)
The laminated sheet of the present invention includes a plurality of atomic layer sheets as described above and metal ions between the atomic layer sheets. The atomic layer sheet is as described above, has boron and oxygen as skeleton elements, is networked by a non-equilibrium bond having a boron-boron bond, and has a molar ratio of oxygen to boron (oxygen / boron). Is less than 1.5. The crystal of the present invention also includes this laminated sheet.

  In the laminated sheet of the present invention, examples of the metal ion between the atomic layer sheets include an alkali metal ion and an alkaline earth metal ion. Examples of the alkali metal ion include a lithium ion, a sodium ion, a potassium ion, a rubidium ion, a cesium ion and the like. Examples of the alkaline earth metal ion include beryllium ion, magnesium ion, calcium ion, strontium ion, barium ion and the like. Of these, alkali metal ions, particularly potassium ions, are preferred embodiments. The molar ratio of alkali metal ion to boron (alkali metal ion / boron) is less than 1.

In the case of an atomic layer sheet having a composition of B ₅ O ₃ , FIG. 2A is referred to as an example of a laminated sheet. This laminated sheet has a layered structure in which an atomic layer sheet having boron and oxygen as main atoms and metal ions are alternately laminated. In a typical embodiment, the metal ion is located inside the hexagon of boron atoms in the unit structure of the atomic layer sheet in the stacking plane. The crystal is obtained as a rod-shaped single crystal by a manufacturing method described later. In a typical mode including the needle-like single crystal, the crystal growth direction and the c-axis direction, which is the lamination direction, coincide with each other, and the atomic layer sheets are stacked along the growth direction. This laminated sheet (and the crystal) has weak interlayer bonding between the laminated sheets, and can be easily cleaved in a direction perpendicular to the c-axis direction (extending direction) by applying mechanical pressure. For example, the crystal can be cleaved by pressing the HOPG substrate against the crystal from above, and a state in which nanosheets of crystal pieces attached to the surface are stacked can be observed.
(Production method of peeled product of laminated sheet)
In the laminated sheet (and the crystal) of the present invention, the laminated sheet and at least one selected from crown ethers and cryptands are added to a solvent containing an organic solvent, and the laminated sheet can be peeled off. The laminated sheet of the present invention, unlike graphite and the like laminated by van der Waals force, is laminated by an ionic interaction between an anionic boron sheet and a cationic metal ion, so that at least one selected from crown ether and cryptand By capturing metal ions between layers with one type, metal ions can be eluted into the organic solvent, and the laminated sheet can be peeled off while maintaining the sheet structure.

  The exfoliated material includes a single-layer atomic layer sheet. For example, by bringing the solution obtained by the above method into contact with the HOPG substrate and removing the solvent, the crystal fragments attached to the HOPG surface can be observed as a single-layer sheet or a nanosheet close thereto.

  In the above method, the organic solvent is not particularly limited. For example, an aprotic medium-polar solvent (nitrile such as acetonitrile, propionitrile, etc., halogenation such as dichloromethane, dichloroethane, chloroform (trichloromethane), carbon tetrachloride, etc.) Hydrocarbons, ethers such as diethyl ether, tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether; ketones such as acetone, 2-butanone, methyl ethyl ketone, isobutyl methyl ketone, diisobutyl ketone, and cyclohexanone , Ethyl acetate, butyl acetate, propylene glycol monomethyl ether acetate, methyl decanoate, methyl laurate, diiso adipate Preferably contains esters) chill like.

  In addition to these aprotic medium polar solvents, aprotic highly polar solvents (N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide, sulfolane, dihexamethylphosphoric triamide compatible with them) , 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, 1-methyl-2-pyrrolidinone, etc., aprotic low polar solvents (benzene, toluene, xylene and other aromatic hydrocarbons) , Pentane, hexane, cyclohexane, octane and other aliphatic hydrocarbons), protic solvents (methanol, ethanol, 2-propanol, 1-butanol, 1,1-dimethyl-1-ethanol, hexanol, decanol, etc.) Alcohols, carboxylic acids such as formic acid and acetic acid, nitromethane, etc.) It may be a solvent. Further, the solvent containing an organic solvent may be a solvent containing water.

In the above method, the crown ether is a macrocyclic ether represented by (—CH ₂ —CH ₂ —O—) _n , for example, 12-crown-4, 15-crown-5, 18-crown-6. , Dibenzo-18-crown-6, diaza-18-crown-6 and the like. The cryptand is a cage-shaped polydentate ligand composed of two or more rings, and examples thereof include [2.2.2] cryptand.

  The addition amount of at least one selected from crown ethers and cryptands is not particularly limited, but is preferably an excess amount with respect to the laminated sheet.

The laminated sheet (and the crystal) of the present invention can also peel the laminated sheet by dissolving it in an aprotic high-polarity solvent. By bringing the obtained solution into contact with the HOPG substrate and removing the solvent, the crystal fragments adhering to the HOPG surface can be observed as a single-layer sheet or a nanosheet close thereto. Examples of the aprotic highly polar solvent include N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, sulfolane, dihexamethylphosphoric triamide, 1,3-dimethyl-2-imidazolidinone, N , N'-dimethylpropyleneurea, 1-methyl-2-pyrrolidinone and the like.
(Production method of atomic layer sheet and laminated sheet)
An atomic layer sheet and / or a laminated sheet containing boron and oxygen, such as an atomic layer sheet and a laminated sheet of the present invention, can be prepared, for example, by adding MBH ₄ (M is an alkali) in a solvent containing an organic solvent under an inert gas atmosphere. (Indicating metal ions)) to prepare a solution, and exposing the solution to an atmosphere containing oxygen. In the step of exposing to an atmosphere containing oxygen, crystals of an atomic layer sheet or a laminated sheet can be grown.

Examples of the alkali metal ion M of MBH ₄ include an alkali metal ion and an alkaline earth metal ion. Among these, potassium ion is a preferred embodiment.

The concentration of the MBH ₄ is not particularly limited, preferably 0.5 to 10 mm, more preferably 1-2 mM.

The inert gas is not particularly limited as long as it has no reactivity with MBH _4, for example, a rare gas such as argon, nitrogen and the like. For example, in an environment such as a glove box that can block oxygen in the atmosphere, an inert gas having no reactivity with MBH ₄ is substituted, MBH ₄ is added to a solvent containing an organic solvent, and the solution is added. Prepare.

  Examples of the organic solvent include, but are not particularly limited to, for example, aprotic neutral polar solvents (nitrile such as acetonitrile and propionitrile, dichloromethane, dichloroethane, chloroform (trichloromethane), and halogenated hydrocarbons such as carbon tetrachloride; Ethers such as diethyl ether, tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ketones such as acetone, 2-butanone, methyl ethyl ketone, isobutyl methyl ketone, diisobutyl ketone, cyclohexanone, and ethyl acetate Esters such as butyl acetate, propylene glycol monomethyl ether acetate, methyl decanoate, methyl laurate, and diisobutyl adipate It will be preferable to include etc.). In addition to these aprotic medium polar solvents, aprotic highly polar solvents (N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide, sulfolane, dihexamethylphosphoric triamide compatible with them) , 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, 1-methyl-2-pyrrolidinone, etc., aprotic low polar solvents (benzene, toluene, xylene and other aromatic hydrocarbons) , Pentane, hexane, cyclohexane, octane and other aliphatic hydrocarbons), protic solvents (methanol, ethanol, 2-propanol, 1-butanol, 1,1-dimethyl-1-ethanol, hexanol, decanol, etc.) Alcohols, carboxylic acids such as formic acid and acetic acid, nitromethane, etc.) It may be a solvent. Further, the solvent containing an organic solvent may be a solvent containing water.

  The atmosphere containing oxygen is not particularly limited, but releasing in the atmosphere is a preferable embodiment.

  After exposure to an atmosphere containing oxygen, heating may be performed once. The heating temperature is not particularly limited, but is preferably 30 to 40C. The heating time is preferably 30 minutes to 2 hours.

  After exposure to an atmosphere containing oxygen, it is preferable to allow the device to stand still in the atmosphere. The temperature and the time of exposure to the atmosphere containing oxygen are not particularly limited, but from the viewpoint of sufficiently growing the crystal, after heating, the temperature is preferably room temperature (15 to 25 ° C.) and the time is 3 days to 3 days. One month is preferred.

  The atomic layer sheet and the laminated sheet of the present invention can be expected to be applied to a thermotropic liquid crystal which is a heating product and various other industries.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
1. Boron layered single crystal 1-1. Synthesis of Crystal In a glove box under an argon gas atmosphere, a MeOH solution of KBH ₄ (5.0 mg / mL) was added to a solvent of CHCl ₃ : MeCN = 1: 1. Concentration of KBH ₄ was 1.4mM.

  After the resulting solution was released to the atmosphere, it was heated at 40 ° C. for 1 hour. Then, it left still at room temperature for 2 weeks.

After standing, it was confirmed that needle crystals having a length of up to about 2 cm had been formed (FIG. 1).
1-2. Single crystal X-ray structure analysis The obtained needle-shaped crystal was subjected to single crystal X-ray structure analysis.

  As a result of performing single crystal XRD measurement and analyzing the structure, a layered structure in which an atomic layer composed of boron and oxygen and potassium ions are alternately stacked was obtained (FIG. 2A). In the layer of boron and oxygen, it was found that the boron bonded with oxygen formed a two-dimensionally spread atomic layer sheet while bonding to form a distorted hexagon (FIG. 2 (b)). , (C)). It was also found that this boron atomic layer was a perfect plane without distortion.

  As for the occupancy, K is 1, B at the vertex of the hexagon is 1, B is 0.635 on the side of the hexagon, and 0.5 is O. It is considered that O occupies one of two places on each side of the hexagon formed by B (FIG. 2C). The composition was determined in consideration of the fact that the terminal site always exists in the boron sheet (FIGS. 3A and 3B described later).

  The boron-boron bond distance of 1.784 ° was a value close to the average of the distances (1.876 ° and 1.614 °) of the two types of boron-boron bonds present in borophene. BB in the crystal is intermediate between 1.61% of a single bond (Z. Anorg. Allg. Chem. 2017, 643, 517) and 1.824% of an oxygen bridge (Inorg. Chem. 2015, 54, 2910). Value (FIG. 3 (c)).

Since it is expected that the bonding state of BO is different between the boron sheet and its terminal / deleted site, an evaluation of the bonding state in the boron layered crystal by IR measurement was attempted (FIG. 4). As a result, two types of peaks were obtained at around 1300 to 1500 cm ^-1 where BO expansion and contraction were observed (FIG. 4). Among the peaks in the BO region, the broad peak on the high wavenumber side (1420 cm ⁻¹ ) is similar in position to the BO expansion peak seen in B (OH) ₃ , so that the BO region It is considered that of the two types of peaks, the peak on the high energy side is derived from the terminal / deletion site, and the sharp peak on the low wave number side (1350 cm ⁻¹ ) is derived from the skeleton. Further, a peak derived from BO-H stretching was observed around 3100 cm ^-1 , indicating that a B-OH bond was present at the terminal site. From the above, the existence of a B (OH) ₃ similar site as a boron atom layer sheet and its terminal / deletion was suggested in the boron layered crystal.
1-3. Evaluation of oxidation state by XPS measurement and quantification of terminal site XPS measurement was performed to evaluate the oxidation state of boron (FIG. 5). As a result of the measurement, a peak derived from the KBH ₄ B 1s ingredients whereas appearing 185.6EV, the boron layered crystal is shifted to the peak top of about 6eV high energy side, of boron with the formation of crystals Oxidation was suggested (FIGS. 5A and 5B). On the other hand, when compared with B ₂ O ₃ (193.3 eV) in which B is in a trivalent state, it was found that complete oxidation up to trivalent state did not proceed because the energy was slightly lower than that of B ₂ O ₃ (FIG. 5 (a), (b)).

Further, it was found that the broad peak of the obtained boron layered crystal was separable into three components (FIG. 5 (a)). As a result of peak separation, it was found that peak 3 on the most oxidized side coincided with B ₂ O ₃ having trivalent boron, and peaks 1 and 2 were located on the more reduced side. Therefore, it is considered that peak 3 corresponds to a B (OH) ₃ similar terminal site, and peaks 1 and 2 respectively correspond to two types of boron in the boron sheet. As a result of calculating the abundance ratio between the boron sheet and the terminal portion from the area ratio of these peaks, it was found that the ratio of the unit cell was 3.1: 1.0.

  From the single crystal X-ray structure analysis, the plane index of the boron layered crystal was determined, and it was found that the elongation direction of the crystal and the c-axis direction, which is the lamination direction, coincided with each other. It was found that the layers were stacked (FIG. 6A).

The crystal elongation direction can also be confirmed from powder XRD measurement. The powder XRD measurement of the boron layered crystal in the capillary was performed, and the obtained diffraction pattern was compared with a simulation of a diffraction pattern calculated from the crystal structure (FIG. 6B). Since the boron layered crystal has a rod-like shape, it is oriented in the capillary so that the direction of crystal growth is parallel to the tube. Since the X-rays are incident on the rotating capillary in a vertical direction, it was expected that almost no diffraction lines in the crystal elongation direction would be observed. As a result of the measurement, diffraction peaks of planes including only the a and b-axis components such as (100), (110), and (200) were observed at diffraction angles matching the simulation, while peaks including the c-axis component were observed. There was almost no appearance, and only a very weak diffraction peak at (001), which was the layer spacing, was observed. From this, it was confirmed that the lamination direction coincided with the elongation direction of the crystal, and it was found that the rod-shaped crystal was formed by lamination of the boron atom layers.
1-4. Absorption Spectrum of Boron Layered Crystal The absorption spectrum of the boron layered crystal was measured (FIG. 7). A diffuse reflection spectrum was measured in a crystalline state by using a solid diffuse reflection cell, and an absorption spectrum was obtained by performing Kubelka-Munk conversion. As a result of the measurement, absorption was observed in an ultraviolet region of 250 nm or less (FIG. 7A). As a result of calculating the band gap from the absorption edge, it was found that the boron layered crystal was a semiconductor having a band gap of about 5.4 eV.

Further, as a result of spectrum measurement in a long wavelength region, it was found that the boron layered crystal had an absorption in a near infrared region of 1000 to 2500 nm (4000 to 10000 cm ⁻¹ ) (FIG. 7B). In the near-infrared region, B ₂ O ₃ and B (OH) ₃ also exhibit absorption at a wavelength different from that of the boron layered crystal. Therefore, these are absorptions derived from the vibration structure of BO or OH. it is conceivable that.
1-5. Shape Observation by SEM and Mechanical Properties of Boron Layered Crystal As a result of conducting FE-SEM observation to examine the shape of the boron layered single crystal in more detail, it was confirmed that the crystal was a hexagonal prism rod shape (FIG. 8). (A)). When the side portion of the rod was enlarged, it was observed that a layered structure was developed along the crystal elongation direction, and it was found that the stripe pattern of the single crystal was derived from the layered structure.

It has been found that by mechanically applying pressure to the boron layered crystal with a spatula or the like, cleavage can be easily performed in a direction perpendicular to the elongation direction. As a result of observing the cleaved crystals by SEM, it was confirmed that nanosheets were partially generated due to the collapse of the layer structure (FIG. 8B). Also, in some parts, very smooth nanosheet surfaces on the order of microns were observed. Such ease of mechanical peeling suggested that the interlayer bonding of the boron layered crystal was very weak.
1-6. Nanosheet observation by AFM Since it was found that the nanosheet was easily formed by mechanical peeling of the boron layered crystal, the surface of the nanosheet was observed by AFM (FIGS. 9 and 10). The crystal was cleaved by pressing the HOPG substrate against the boron layered crystal from above, and crystal fragments attached to the surface were directly observed by AFM (FIG. 9 (a)). Although there were many parts where the nanosheets were distorted and parts that were not completely horizontal, it was observed that some of the nanosheets were stacked substantially horizontally (FIG. 9B). Since the phase was clearly different between the sheet portion and the base HOPG portion, it was determined that the sheet was a boron sheet. As a result of actually measuring the height of the sheet having the smallest thickness of the shape image, it was found that the sheet had an uneven thickness because the sheet was not completely flat, but the average thickness was about 2.0 nm. (FIG. 10). From the above, it is considered that these sheets are very thin sheets of about one to several layers. As a result of the AFM observation, a plurality of stacked sheets were confirmed, and a single-layer sheet having a height of about 0.9 nm was successfully observed at the thinnest portion. The height of the sheet is about 0.9 nm at the thinnest point, which is correlated with the height of single-layer graphene measured by AFM being 0.8 nm (Science, 2004, 306, 666.). .

Next, we tried to dissolve the boron layered crystal with crown ether and cryptand and make it into a single layer. The crystal was dispersed in a solvent of CHCl ₃ : MeCN = 1: 1, and the boron layered single crystal was dissolved in excess of 18-crown-6 ether or cryptand. This solution was cast on a HOPG substrate and washed with chloroform to remove excess 18-crown-66 ether or cryptand. An attempt was made to observe a single-layer sheet. In the AFM, a crystal fragment attached to the surface was observed by the AFM, and a nanosheet having a height of about 0.9 nm, which was considered to be a single-layer sheet, was observed on the HOPG substrate (FIG. 11: using 18-crown-66 ether). Similarly, the observation of a sheet having a height of about 0.7 nm was successful. From these results, it was suggested that a single layer of the boron layered crystal was achieved by crown ether or the like.
1-7. Nanosheet observation by TEM The shape and surface of the nanosheet were also observed by TEM observation. It is the same as the preparation method of the AFM sample. The crystal is cleaved by pressing a TEM grid with a micromesh from above the boron layered crystal, and the sheet attached to the grid surface is observed by TEM (FIG. 12 (a): STEM). Images, FIG. 12 (b) and FIG. 13: high resolution TEM image). As a result, the laminated structure of the sheet and the nanosheet were directly observed in the STEM (FIG. 12 (a)), and in the high-resolution TEM, a very thin sheet having a lower contrast than the grid mesh was successfully observed (FIG. 12 (b)). . It was confirmed that the thinnest sheet in the observation location had about 15 layers.

Furthermore, by observing these sheets at a high magnification, the lattice was successfully observed (FIG. 13). Hexagonal diffraction points were obtained from some sheet surfaces, and the same hexagonal symmetry as the boron sheet was observed. In some cases, a lattice having an interval of 0.343 nm was also observed. Since this coincides with the layer spacing of the boron layered crystal of 0.347 nm, it was found that the atomic layer stacking was actually measured. From these facts, it was demonstrated that the film could be peeled into a very thin nanosheet by mechanical peeling, and that the interlayer interaction of the boron layered crystal was weak.
2. Transformation of Boron Layered Crystal into Liquid Crystal by Heat The change to liquid crystal can be confirmed by observation with a polarizing microscope. A liquid crystal has a liquid-like fluidity but exhibits an interference color like a crystal under a polarizing microscope. In order to block the influence of oxygen and water, the crystal was vacuum-sealed in a capillary, and changes in the morphology and interference color during the heating process were observed using a polarizing microscope equipped with a heating stage.

  As a result of slowly heating from 50 ° C. to 120 ° C. at a heating rate of 5 ° C./min or less, it was observed that the rod-shaped boron layered crystal began to melt from around 105 ° C. and change its shape to a liquid state (FIG. 14). (A)). Although the shape is liquid, the interference color is observed at the periphery thereof, which indicates that the boron crystal has changed to a liquid crystal instead of a liquid.

  Furthermore, as a result of observing the process of cooling to 35 ° C. at 5 ° C./min after heating to 120 ° C., it was found that the liquid crystal gradually changed into a circular shape (FIG. 14B). Although the interference color is always present at the peripheral edge, the shape has been fluidly changed, which indicates that the liquid crystal is in the liquid crystal state even in the cooling process. From this, it was found that once the temperature of the crystal was changed to liquid crystal by elevating the temperature, the crystal was not transformed again even when cooled to 35 ° C., and the liquid crystal state was maintained. It is considered that the orientation of the liquid crystal is generated by the two-dimensional strong anisotropy of the boron sheet, and the fluidity is expressed by weak interlayer bonding. The reason why the cross-shaped dark portion is visible at the peripheral portion is that the optical axis of the liquid crystal domain is oriented along the direction of the orthogonal polarizing plate, and the polarized light is transmitted without interference. Therefore, it is considered that the boron sheet is oriented concentrically in the liquid crystal.

Claims (20)

A boron and oxygen in the skeleton element, boron - networked by nonequilibrium coupling with boron bond, the molar ratio of oxygen and boron (oxygen / boron) is Ri der less than 1.5, consisting of boron and oxygen An atomic layer sheet in which the atomic layer is anionic . The atomic layer sheet according to claim 1, further comprising an alkali metal ion or an alkaline earth metal ion.   The atomic layer sheet according to claim 1, further comprising an alkali metal ion, wherein a molar ratio of the alkali metal ion to boron (alkali metal ion / boron) is less than 1. Atomic layer sheet according to claim 1 backbone composition is B ₅ O _3.   The atomic layer sheet according to claim 4, wherein the skeleton has a three-fold symmetry having a boron-boron bond. Wherein the components X are skeletal sites, viewed including the other components Y, the component Y is atomic layer sheet according to claim 4 or 5 is a terminal site and / or defect site. The atomic layer sheet according to claim 6, wherein the component Y is a boron oxide site containing B-OH. The atomic layer sheet according to claim 6 or 7, wherein the atomic layer sheet has peaks at 190.5 to 193.0 eV and 192.5 to 194.0 eV respectively derived from the B-1s level in X-ray photoelectron spectroscopy. The atomic layer sheet according to claim 8, wherein a peak of 190.5 to 193.0 eV corresponds to the component X in the X-ray photoelectron spectroscopy. In the IR measurement, any one of claims 6-9 having has two peaks derived from BO stretch around 1300～1500Cm ^-1, and a peak derived from BO-H stretching in the vicinity of 3100 cm ^-1 The atomic layer sheet according to claim 1. 11. The atomic layer sheet according to claim 10, wherein in the IR measurement, a peak on a low wave number side of the two types of peaks derived from BO stretching corresponds to the component X. A laminated sheet comprising a plurality of atomic layer sheets according to any one of claims 1 to 11 and metal ions between the atomic layer sheets, wherein the metal ions are alkali metal ions or alkaline earth metal ions . The laminated sheet according to claim 12, wherein the metal ion is an alkali metal ion, and a molar ratio of the alkali metal ion and boron (alkali metal ion / boron) is less than 1. A crystal comprising the laminated sheet according to claim 12 . Adding MBH ₄ (M represents an alkali metal ion) to a solvent containing an organic solvent under an inert gas atmosphere to prepare a solution;
Exposing the solution to an atmosphere containing oxygen, the method comprising producing an atomic layer sheet and / or a laminated sheet containing boron and oxygen. The atom in which the atomic layer sheet has boron and oxygen as skeletal elements and is networked by non-equilibrium bonds having a boron-boron bond, and has a molar ratio of oxygen to boron (oxygen / boron) of less than 1.5. 16. The method of claim 15, wherein the method is a layer sheet, wherein the laminated sheet includes a plurality of the atomic layer sheets and the alkali metal ions between the atomic layer sheets. 17. The method according to claim 16, wherein the molar ratio of the alkali metal ion to boron (alkali metal ion / boron) is less than 1. A peeled product of a laminated sheet, comprising a step of adding the laminated sheet according to claim 12 or at least one selected from crown ethers and cryptands to a solvent containing an organic solvent, and peeling the laminated sheet. Manufacturing method. A method for producing a peeled product of a laminated sheet, comprising a step of adding the laminated sheet according to claim 12 or 13 to an aprotic highly polar solvent and peeling the laminated sheet. 20. The method according to claim 18 or claim 19, wherein the exfoliate comprises a single layer atomic layer sheet.