Solid carbon quantum dots

Abstract

The purpose of the present invention is to provide a novel solid carbon quantum dot which exhibits excellent light emission characteristics when irradiated with light. The solid carbon quantum dot, which solves the technical problem, comprises a crystal structure of carbon nitride and a crystal structure of graphite, and has, in an X-ray diffraction spectrum obtained by X-ray diffraction, a peak PCN which has a maximum value in a diffraction angle 2 [theta] range of more than 26 DEG and 29 DEG or less and which belongs to the crystal structure of the carbon nitride, and a peak PCN which has a maximum value in a diffraction angle 2 [theta] range of more than 26 DEG and not more than 29 DEG and which belongs to the crystal structure of the graphite. And a peak PGr1 which has a maximum value within the range of a diffraction angle 2 [theta] of 10-12 degrees inclusive and which belongs to the crystal structure of the graphite. The full width at half maximum of a diffraction peak PGr2 belonging to a crystal structure of graphite at a diffraction angle 2 [theta] = 25 DEG is 2 DEG to 20 DEG, when a line connecting the diffraction angle 2 [theta] to 11 DEG to 15 DEG, a point at which the diffraction (scattering) angle is minimum, and a point at which the diffraction angle 2 [theta] is 34 DEG to 40 DEG, and a point at which the diffraction (scattering) intensity is minimum is defined as a base line.

Description

Technical Field

[0001] This invention relates to a solid carbon quantum dot. Background Technology

[0002] Previously, as white LEDs, there were known devices that combined a blue LED with an inorganic fluorescent material that emitted yellow light (wavelength around 520nm to 580nm) when excited by light emanating from the blue LED. However, inorganic fluorescent materials are expensive, so there is a need to develop alternative materials.

[0003] Carbon quantum dots are known to be stable carbon-based microparticles with particle sizes ranging from a few nm to tens of nm, and they exhibit excellent fluorescence properties. Furthermore, unlike inorganic fluorescent materials, carbon quantum dots do not require the inclusion of rare elements and can be manufactured relatively inexpensively. Therefore, much research has been conducted on carbon quantum dots in recent years. However, the luminescence properties of many carbon quantum dots are still insufficient, requiring further improvement.

[0004] On the other hand, carbon quantum dots are also expected to be developed for applications beyond luminescent materials. For example, Non-Patent Document 1 describes the use of a mixture of carbon nitride and graphene quantum dots as a material for a humidity sensor. Furthermore, Non-Patent Document 2 describes the simultaneous measurement of urea and dopamine using a mixture of carbon nitride and graphene quantum dots. Moreover, Non-Patent Document 3 describes a mixture of sulfur-doped carbon nitride and graphene quantum dots that is useful as an electrode catalyst.

[0005] Existing technical documents

[0006] Non-patent literature

[0007] Non-patent document 1: Morsy et al., Design and implementation of humidity sensor based on carbon nitride modified with graphene quantum dots, NatureportfolioScientific Reports, 2023, No. 23, 2891

[0008] Non-patent document 2: Xia et al., Facile preparation of metal-free graphitic-like carbon nitride / graphene oxide composite for simultaneous determination of uric acid and dopamine, Microchemical Journal, 2023, No. 190, 108726

[0009] Non-patent document 3: Chenyu et al., Sulfur-doped graphitic carbon nitridedecorated with graphene quantum dots for an efficient metal-freeelectrocatalyst, Journal or Materials Chemistry A, 2015, 3, 1841-1846 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] However, the mixture of carbon nitride and graphite (or graphene) quantum dots described in Non-Patent Documents 1, 2 and 3 does not differ significantly from conventional carbon quantum dots in terms of fluorescence properties and cannot be used as a luminescent material.

[0012] This invention was made in view of the above-mentioned technical problems. The object of this invention is to provide a novel solid carbon quantum dot with excellent luminescence properties when illuminated.

[0013] Solution for solving the problem

[0014] This invention provides a solid carbon quantum dot comprising a carbon nitride crystal structure and a graphite crystal structure, exhibiting the following characteristics in its X-ray diffraction spectrum obtained by X-ray diffraction: peak P CN The crystal structure of the carbon nitride has a maximum value in the range of diffraction angle 2θ exceeding 26° and below 29°; and peak P Gr1 The diffraction peak P, belonging to the crystal structure of graphite, is located at a diffraction angle of 2θ = 25°. This peak has a maximum value within the diffraction angle range of 2θ = 10° to 12° and is attributed to the graphite crystal structure. The baseline is defined as the line connecting the point with the minimum diffraction (scattering) angle at 2θ = 11° to 15° and the point with the minimum diffraction (scattering) intensity at 2θ = 34° to 40°.Gr2 The half-width is more than 2° and less than 20°.

[0015] Invention Effects

[0016] This invention provides a novel solid carbon quantum dot that exhibits excellent luminescence properties when illuminated. Attached Figure Description

[0017] Figure 1 The X-ray diffraction spectra of solid carbon quantum dots prepared in Example 1, X-ray diffraction spectra of commercially available carbon nitride, and X-ray diffraction spectra of commercially available graphene carbon quantum dots are shown.

[0018] Figure 2A The X-ray diffraction spectra of carbon nitride, graphitic carbon quantum dots, and mixtures thereof as described in Non-Patent Document 1. Figure 2B The X-ray diffraction spectra of carbon nitride, graphene carbon quantum dots, and mixtures thereof described in Non-Patent Document 2. Figure 2C The X-ray diffraction spectra of carbon sulfide, graphene quantum dots, and mixtures thereof as described in Non-Patent Document 3.

[0019] Figure 3A and Figure 3B All images are taken using a transmission electron microscope to observe the solid carbon quantum dots prepared in Example 1.

[0020] Figure 4 The results are obtained by infrared spectroscopy analysis of the solid carbon quantum dots prepared in Example 1.

[0021] Figure 5A The results are based on X-ray photoelectron spectroscopy analysis of the solid carbon quantum dots prepared in Example 1. Figure 5B The results are based on X-ray photoelectron spectroscopy analysis of commercially available carbon nitride.

[0022] Figure 6 The results are based on X-ray photoelectron spectroscopy analysis of boron in the solid carbon quantum dots prepared in Example 1.

[0023] Figure 7 The X-ray diffraction spectra are those of unpurified and purified solid carbon quantum dots prepared in Example 1, X-ray diffraction spectra of commercially available carbon nitride, and X-ray diffraction spectra of commercially available graphene carbon quantum dots. Detailed Implementation

[0024] In this specification, the range of values ​​indicated by “~” refers to the range of values ​​recorded before and after “~”.

[0025] The solid carbon quantum dots of this invention are novel quantum dots comprising crystal structures of carbon nitride and graphite. In this specification, "crystal structure of carbon nitride" refers to a crystal structure also known as graphitic carbon nitride (g-C3N4), which is a planarly linked six-membered ring structure composed of carbon and nitrogen atoms. Furthermore, "crystal structure of graphite" in this specification includes not only structures formed by stacked graphene atoms but also structures where graphene exists as a single layer. In the solid carbon quantum dots of this invention, a portion of the carbon in the graphite crystal structure can be replaced by other atoms (e.g., nitrogen atoms). Moreover, other atoms (e.g., boron atoms, alkali metals, alkaline earth metals, etc., contained in the raw materials) can be bonded to the aforementioned crystal structures of carbon nitride and graphite, as well as functional groups derived from the raw materials.

[0026] The solid carbon quantum dots of the present invention are solid at 25°C and one atmosphere. Furthermore, the solid carbon quantum dots exhibit peaks at specific positions in the XRD spectrum when analyzed by X-ray diffraction (hereinafter also referred to as "XRD"). Figure 1 The XRD spectrum of the solid carbon quantum dots of the present invention prepared in Example 1 described later is shown in the figure. Figure 1 As shown, the solid carbon quantum dots of the present invention have peak P that has a maximum value in the range of diffraction angle 2θ exceeding 26° and below 29° and belongs to the crystal structure of carbon nitride. CN Furthermore, the solid carbon quantum dots of the present invention also possess a peak P that has a maximum value in the range of diffraction angle 2θ being greater than 10° and less than 12° and belongs to the crystal structure of graphite. Gr1 The peak P Gr1 This peak is attributed to the partially oxidized structure of graphite. During the fabrication of carbon quantum dots, this peak (P) is easily observed when unreacted and byproducts are removed and crystallization occurs. Gr1 The solid carbon quantum dots of this invention also exhibit a diffraction peak P in the XRD spectrum, at a diffraction angle of 2θ = 25°, belonging to the crystal structure of graphite. Gr2 Furthermore, by setting the baseline as the line connecting the point with the minimum diffraction (scattering) intensity at a diffraction angle 2θ of 11° to 15° and the point with the minimum diffraction (scattering) intensity at a diffraction angle 2θ of 34° to 40°, the diffraction peak P located at a diffraction angle 2θ = 25° is determined. Gr2 When the full width at half maximum (FWHM) is 2° or higher and 20° or lower, the FWHM of a peak in an XRD spectrum represents the crystallite size of the crystal structure. The peak P attributable to the crystal structure of graphite... Gr2 The full width at half maximum (FWHM) represents the size of the crystallites in the graphite crystal structure; the larger the FWHM, the smaller the crystallites. At the aforementioned peak P... Gr2With a full width at half maximum (FWHM) of 20°, the crystallite size of graphite is approximately two benzene rings. On the other hand, when peak P... Gr2 When the full width at half maximum (FWHM) of the peak PCr2 is greater than 2°, graphite can be said to exist in the form of microcrystals. Furthermore, when the FWHM of the aforementioned peak PCr2 is greater than 2° and less than 20°, the size of the graphite crystallites becomes extremely small, resulting in more contact points with the crystal structure of carbon nitride. Therefore, solid carbon quantum dots exhibit excellent luminescent properties. Peak P Gr2 The full width at half maximum (FWHM) is more preferably 8° or more and 15° or less. It should be noted that in the XRD spectrum of the solid carbon quantum dots of the present invention, other peaks belonging to the crystal structure of graphite are typically identified in the range of diffraction angle 2θ being 40° or more and 45° or less.

[0027] Here, the XRD spectrum of the solid carbon quantum dots of the present invention is significantly different from the XRD spectrum of a mixture formed by separately synthesizing carbon nitride and graphite (or graphene) quantum dots and mixing them (the mixture described in Non-Patent Document 1 and Non-Patent Document 2 above). Figure 2A The image shows the XRD spectrum described in Non-Patent Document 1 (Non-Patent Document 1's...). Figure 7 ), Figure 2B The image shows the XRD spectrum described in Non-Patent Document 2 (Non-Patent Document 2). Figure 1 D). Figure 2C The XRD spectrum described in Non-Patent Document 3 is shown (Figure 3(d) of Non-Patent Document 2). Figure 2A , Figure 2B as well as Figure 2C As shown, when only carbon nitride and graphite are mixed, no peaks originating from the crystal structure of graphite (graphene) are observed in the range of diffraction angles 2θ above 23° and 26°. Figure 2A and Figure 2B ), or no peaks originating from the crystal structure of carbon nitride were observed in the range of diffraction angles 2θ exceeding 26° and below 29° ( Figure 2C In other words, the carbon quantum dots of the present invention can be described as substances having a crystal structure different from that of a mixture of carbon nitride and graphite quantum dots. Moreover, this crystal structure is believed to contribute to excellent luminescent properties.

[0028] The reasons for the superior luminescence properties of the solid-state carbon quantum dots of the present invention are not yet certain, but it is believed that in these solid-state carbon quantum dots, the crystal structure of carbon nitride and the microcrystalline structure of graphite are not independently mixed, but rather covalently bonded together, forming a complex composite at the molecular level. Furthermore, it is believed that the complex composite of the carbon nitride crystal structure and graphite at the molecular level in these solid-state carbon quantum dots extends the conjugated system. Additionally, the presence of boron atoms and alkali metals and / or alkaline earth metals within the system during the fabrication of the carbon quantum dots may also contribute. In summary, the solid-state carbon quantum dots of the present invention exhibit exceptionally superior luminescence properties compared to conventional quantum dots, for example, displaying a very high internal quantum efficiency when irradiated with excitation light at a wavelength around 400 nm.

[0029] The chemical composition of the solid carbon quantum dots of the present invention is not particularly limited. Relative to the total amount of the solid carbon quantum dots, the amount of carbon atoms is preferably 10% by mass or more and 25% by mass or less, more preferably 12% by mass or more and 18% by mass or less. Furthermore, relative to the total amount of the solid carbon quantum dots, the amount of nitrogen atoms is preferably 15% by mass or more and 35% by mass or less, more preferably 20% by mass or more and 30% by mass or less. In this case, the mass ratio of nitrogen atoms to carbon atoms is preferably 1.0 or more and 2.0 or less, more preferably 1.2 or more and 1.8 or less. Furthermore, the molar ratio of nitrogen atoms to carbon atoms is preferably 0.8 or more and 1.7 or less, more preferably 1.0 or more and 1.5 or less. Moreover, relative to the total amount of the solid carbon quantum dots, the amount of boron atoms is preferably 5% by mass or more and 30% by mass or less, more preferably 10% by mass or more and 15% by mass or less. Furthermore, the mass ratio of boron atoms to carbon atoms is preferably 0.1 or more and 1.5 or less, more preferably 0.5 or more and 1.0 or less. Additionally, the molar ratio of boron atoms to carbon atoms is preferably 0.1 or more and 1.7 or less, more preferably 0.5 or more and 1.2 or less. Moreover, the total amount of alkali metals and alkaline earth metals relative to the total amount of the solid carbon quantum dots is preferably 0.1% by mass or more and 30% by mass or less, more preferably 3.0% by mass or more and 8.0% by mass or less.

[0030] The average particle size of the aforementioned solid carbon quantum dots, as determined by observation using an atomic force microscope (AFM), is preferably 1 nm or more and 100 nm or less, more preferably 1 nm or more and 80 nm or less. When the average particle size of the carbon quantum dots is within this range, the properties characteristic of quantum dots are readily and sufficiently obtained. It should be noted that the aforementioned average particle size of the carbon quantum dots is a value calculated by measuring the diameters of three or more carbon quantum dots using AFM and averaging their values.

[0031] Furthermore, there are no particular limitations on the wavelength of light used to excite the solid carbon quantum dots; these solid carbon quantum dots can typically be excited by light with wavelengths above 200 nm and below 780 nm. In addition, the maximum emission wavelength of the fluorescence emitted by the solid carbon quantum dots under excitation light of this wavelength is typically above 380 nm and below 780 nm, for example, it can be set to above 440 nm and below 580 nm. It should be noted that when irradiated with excitation light, the light emitted by the solid carbon quantum dots is primarily fluorescence. The maximum emission wavelength can be determined by using a spectrophotometer with an integrating sphere unit or similar device to obtain the spectral distribution when irradiated with light of a specified wavelength.

[0032] Furthermore, the internal quantum efficiency of this solid carbon quantum dot when irradiated with light of wavelengths from 200 to 780 nm can be set to 50% or more, and can be further set to 55% or more. Moreover, for example, the internal quantum efficiency when irradiated with light of wavelength 400 nm is preferably 50% or more, more preferably 55% or more, and even more preferably 60% or more. The internal quantum efficiency can also be determined by obtaining the spectral distribution when irradiated with light of wavelength 400 nm using a spectrophotometer or the like equipped with an integrating sphere unit.

[0033] (Methods for manufacturing solid carbon quantum dots)

[0034] There are no particular limitations on the methods for manufacturing the aforementioned solid carbon quantum dots. For example, they can be manufactured using the following two methods. Each method will be described in detail below.

[0035] (1) First manufacturing method

[0036] The first manufacturing method includes the following steps: a step of preparing a homogeneous mixture containing at least carbon atoms, nitrogen atoms, boron atoms, and alkali metals and / or alkaline earth metals (mixture preparation step); a step of heating the mixture in a substantially solvent-free state (heat synthesis step); and a step of purifying the resulting composition (purification step). Each step will be described below.

[0037] • Mixture preparation process

[0038] In the mixture preparation process, a homogeneous mixture comprising at least carbon atoms, nitrogen atoms, boron atoms, and alkali metals and / or alkaline earth metals is prepared. As a specific raw material, it is preferable to use one or more organic compounds having reactive groups (excluding organic compounds containing boron atoms) in combination with one or more boron compounds containing boron atoms. Furthermore, either or both of the aforementioned organic compounds and boron compounds preferably contain alkali metals and / or alkaline earth metals. It should be noted that in this specification, compounds containing boron atoms are referred to as "boron compounds," even if their molecules contain reactive groups; in the case of containing boron atoms, they are also considered boron compounds.

[0039] There are no particular restrictions on the types of alkali or alkaline earth metals that can be contained in organic compounds and boron compounds; examples include lithium, sodium, potassium, magnesium, calcium, and barium. Among these, sodium, potassium, or calcium are preferred from the perspectives of reactivity, safety, and ease of material availability.

[0040] The total amount of alkali metals and alkaline earth metals in the mixture is preferably 0.1 mol% or more and 30 mol% or less relative to the sum of the moles of carbon and nitrogen atoms contained in the organic compound and the boron compound, more preferably 0.5 mol% or more and 20 mol% or less, and even more preferably 1 mol% or more and 10 mol% or less. It should be noted that the alkali metals and alkaline earth metals are preferably mainly derived from the organic compound and the boron compound, but may also be derived from other compounds.

[0041] Organic compounds are any compounds possessing reactive groups. In this specification, a "reactive group" refers to a group bonded to carbon atoms in an organic compound and used in the heating process described later to induce condensation reactions or other reactions between the organic compounds; that is, a group that contributes to the formation of the main framework of solid carbon quantum dots. Specific examples of reactive groups include carboxyl groups, hydroxyl groups, epoxy groups, amide groups, sulfonyl groups, amino groups, and groups formed by their bonding or coordination with alkali metals or alkaline earth metals.

[0042] Organic compounds can consist of only one compound or two or more compounds. However, every organic compound must contain nitrogen atoms. The amount of nitrogen atoms relative to the total amount of the organic compound is preferably 30% by mass or more and 50% by mass or less, more preferably 33% by mass or more and 50% by mass or less, and even more preferably 36% by mass or more and 48% by mass or less. When the amount of nitrogen atoms in the organic compound is 30% by mass or more and 50% by mass or less, the crystal structures of carbon nitride and graphite described above are readily formed.

[0043] Organic compounds can consist only of compounds containing nitrogen atoms and having reactive groups (hereinafter also referred to as "nitrogen-containing organic compounds"), or they can consist of compounds that do not contain nitrogen atoms but have reactive groups (hereinafter also referred to as "nitrogen-free organic compounds") and nitrogen-containing organic compounds.

[0044] Examples of nitrogen-containing organic compounds include amine compounds, nitrogen-containing sugars, imidazoles, triazines, triazoles, triazines, guanidines, oximes, and their salts with alkali metals and / or alkaline earth metals. As such organic compounds, one or more of these substances may be used. Furthermore, these compounds may be solid or liquid at room temperature.

[0045] Examples of amine compounds include: 1,2-phenylenediamine, 1,3-phenylenediamine, 1,4-phenylenediamine, 2,6-diaminopyridine, urea, thiourea, ammonium thiocyanate, ethanolamine, 1-amino-2-propanol, melamine, cyanuric acid, barbituric acid, folic acid, ethylenediamine, diethylenetriamine, triethylenetetramine, polyethyleneimine, dicyandiamine, guanidine, aminoguanidine, formamide, glutamic acid, aspartic acid, cysteine, arginine, histidine, lysine, glutathione, RNA, DNA, cysteine, methionine, homocysteine, taurine, thiamine, N-[3-(trimethoxysilyl)propyl]ethylenediamine, 4,5-difluoro-1,2-phenylenediamine, p-aminobenzenesulfonic acid, o-phosphoserine, 5'-adenosine triphosphate, guanidine phosphate, and guanylurea phosphate. Phosphate), 3-aminopropyltriethoxysilane, etc.

[0046] Examples of nitrogen-containing sugars include glucosamine, chitin, and chitosan. Examples of imidazoles include 1-(trimethylsilyl)imidazolium. Examples of triazines include 1,2,4-triazine; examples of triazoles include 1,3,5-triazine, 1,2,3-triazole, and 1,2,4-triazole. Examples of triazines include 1,3-diphenyltriazine and 1-methyl-3-p-tolyltriazine; examples of guanidines include guanidine and arginine; and examples of oximes include benzamide oxime and p-benzoquinone dioxime. Furthermore, examples of alkali metal salts and alkaline earth metals of the above compounds include monosodium glutamate and sodium imidazole-4-acetate.

[0047] Of the nitrogen-containing organic compounds, amine compounds are preferred from the perspectives of ease of acquisition and reactivity in the heating process described later.

[0048] On the other hand, examples of nitrogen-free organic compounds include carboxylic acids, alcohols, phenols, and sugars, as well as their salts with alkali metals and / or alkaline earth metals. Organic compounds may contain only one of these substances or two or more. Furthermore, these compounds can be solid or liquid at room temperature.

[0049] Carboxylic acids are any compounds that have one or more carboxyl groups in their molecules (excluding compounds that are equivalent to nitrogen-containing organic compounds, phenols, or sugars). Examples of carboxylic acids include: monocarboxylic acids such as formic acid, acetic acid, 3-mercaptopropionic acid, and α-lipoic acid; polycarboxylic acids with two or more groups such as oxalic acid, malonic acid, succinic acid, maleic acid, fumaric acid, itaconic acid, polyacrylic acid, (ethylene dithio)diacetic acid, thiomalic acid, tetrafluoroterephthalic acid, and 2-phosphonobutane-1,2,4-tricarboxylic acid; and hydroxy acids such as citric acid, glycolic acid, lactic acid, tartaric acid, malic acid, and 5-sulfosalicylic acid.

[0050] The alcohol is preferably a monohydric alcohol having one hydroxyl group and six or more carbon atoms; or a polyhydric alcohol having two or more hydroxyl groups (excluding compounds equivalent to nitrogen-containing organic compounds, carboxylic acids, phenols, or sugars). Examples of monohydric alcohols having six or more carbon atoms include higher alcohols such as hexanol and octanol. On the other hand, examples of polyhydric alcohols include ethylene glycol, glycerol, erythritol, pentaerythritol, ascorbic acid, polyethylene glycol, and sorbitol.

[0051] Phenolic compounds are any compounds with a structure in which a hydroxyl group is bonded to a benzene ring (excluding nitrogen-containing organic compounds). Examples of phenols include: phenol, catechol, resorcinol, hydroquinone, phloroglucinol, 1,2,4-phenylpyroglucinol, gallic acid, tannin, lignin, catechin, anthocyanin, rutin, chlorogenic acid, lignan, curcumin, etc.

[0052] Examples of sugars that are nitrogen-free organic compounds include glucose, sucrose, and cellulose. Furthermore, examples of alkali metal salts and alkaline earth metal salts of the above compounds include trisodium citrate dihydrate, tripotassium citrate monohydrate, sodium ascorbate, and calcium acetate.

[0053] The aforementioned nitrogen-free organic compounds preferably have reactive groups that can efficiently undergo condensation reactions with nitrogen-containing organic compounds, and are preferably carboxylic acids, alcohols, phenols, their alkali metal salts, or their alkaline earth metal salts.

[0054] Furthermore, the total amount of the aforementioned organic compounds (nitrogen-containing organic compounds and nitrogen-free organic compounds) in the mixture prepared in the mixture preparation step is preferably 20% by mass or more and 60% by mass or less, more preferably 35% by mass or more and 55% by mass or less, relative to the total amount of organic compounds and boron compounds. When the amount of organic compounds relative to the total amount of organic compounds and boron compounds is within the above range, it is easy to obtain solid carbon quantum dots having the crystal structure of carbon nitride and the crystal structure of graphite described above.

[0055] On the other hand, the boron compound used in the preparation of the mixture can be any compound containing boron atoms, such as elemental boron or a boron-containing compound. The boron compound can consist of one compound or two or more compounds.

[0056] Specific examples of boron compounds include: boron, boric acid, sodium tetraborate, boron oxide, trimethyl borate, triethyl borate, tri(octadecyl) borate, triphenyl borate, 2-ethoxy-4,4,5,5-tetramethyl-1,3,2-dioxoborane, triethanolamine borate, 2,4,6-trimethoxycycloboroxane, tris(trimethylsilyl)boronic acid, tris(2-cyanoethyl)boronic acid, 3-aminophenylboronic acid, 2-anthraboronic acid, 9-anthraboronic acid, phenylboronic acid, 3,5-bis(trifluoromethyl)phenylboronic acid, 4,4'-biphenyldiboronic acid, 2-bromophenylboronic acid, 4-bromo-1-naphthoboronic acid, 3-bromo-2-fluorophenylboronic acid, 4-carboxyphenylboronic acid, 3-cyanophenylboronic acid, 4-cyano 3-fluorophenylboronic acid, 3,5-difluorophenylboronic acid, 4-(diphenylamino)phenylboronic acid, 3-fluorophenylboronic acid, 3-hydroxyphenylboronic acid, 4-mercaptophenylboronic acid, 1-naphthoboronic acid, 9-phenanthroboronic acid, 1,4-phenylene diboronic acid, 1-pyreneboronic acid, 2-aminopyrimidine-5-boronic acid, 2-bromopyridine-3-boronic acid, 2-fluoropyridine-3-boronic acid, 4-pyridineboronic acid, quinoline-8-boronic acid, 4-aminophenylboronic acid pinacol, 3-hydroxyphenylboronic acid pinacol, 4-(4,4,5,5-tetramethyl-1,3,2-dioxoboropentan-2-yl)pyridine, diboronic acid, sodium borohydride, sodium tetrafluoroborate, 1-ethyl-3-methylimidazolium tetrafluoroborate, boron trifluoride, boron tribromide, etc.

[0057] It should be noted that the amount of boron atoms in the boron compound is preferably 3% by mass or more, and more preferably 5% by mass or more, relative to the total amount of the boron compound. When the amount of boron atoms in the boron compound is within this range, boron atoms can be disposed on the surface of the carbon quantum dots.

[0058] The total amount of the aforementioned boron compound relative to the total amount of organic compound and boron compound is preferably 40% by mass or more and 80% by mass or less, more preferably 42% by mass or more and 70% by mass or less, and even more preferably 45% by mass or more and 65% by mass or less. When the amount of boron compound relative to the total amount of organic compound and boron compound is within the above range, it is easy to prepare solid carbon quantum dots having the above-described crystal structures of carbon nitride and graphite.

[0059] It should be noted that, in this process, compounds other than organic compounds and boron compounds may be further mixed without compromising the purpose and effect of the present invention. Preferably, the amount of organic compounds and boron compounds in the mixture is 50% by mass or more, more preferably 70% by mass or more, relative to the total amount of the mixture. When the total amount of the above-mentioned organic compounds and boron compounds is 50% by mass or more, the above-mentioned solid carbon quantum dots can be prepared efficiently. It should also be noted that the compounds other than organic compounds and boron compounds may be compounds containing alkali metals or alkaline earth metals.

[0060] Examples of compounds other than organic and boron compounds include compounds containing phosphorus, sulfur, silicon, or fluorine atoms but lacking the aforementioned reactive groups (hereinafter also referred to as "other compounds"). In this process, when other compounds are mixed, solid carbon quantum dots containing heteroatoms other than nitrogen and boron can be obtained. The mixture may contain only one other compound or may contain two or more other compounds.

[0061] Examples of compounds containing phosphorus atoms include: elemental phosphorus, phosphoric acid, phosphorus oxide, 1-hydroxyethane-1,1-diphosphonic acid, phytic acid, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, o-phosphorylethanolamine, phosphorus chloride, phosphorus bromide, triethyl phosphoroacetate, tetra(hydroxymethyl)phosphonium chloride, methyl phosphate, triethyl phosphite, nitrilotri(methylenephosphonic acid), N,N,N',N'-ethylenediaminetetra(methylenephosphonic acid), etc.

[0062] In addition, examples of compounds containing sulfur atoms include: sulfur, sodium thiosulfate, sodium sulfide, sodium sulfate, sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, sodium hydrosulfide, etc.

[0063] Examples of compounds containing silicon atoms include tetrachlorosilane and tetraethoxysilane.

[0064] Examples of compounds containing fluorine atoms include 2,2,3,3,4,4-hexafluoro-1,5-pentanediol diglycidyl ether, sodium fluoride, etc.

[0065] The amount of other compounds in the mixture is appropriately selected based on the desired amount of heteroatoms, and is generally preferably 0% to 20% by mass relative to the total amount of the mixture, more preferably 3% to 10% by mass. When the amount of other compounds is 3% by mass or more, the effect of adding other compounds is easily obtained. On the other hand, when the amount of other compounds is 10% by mass or less, the amount of organic compounds and boron compounds becomes relatively sufficient, and solid carbon quantum dots can be prepared efficiently.

[0066] Furthermore, layered clay minerals can be added in this process. When layered clay minerals are added, uniformly sized solid carbon quantum dots can be easily prepared using the interlayer spaces of the layered clay minerals as templates. Examples of layered clay minerals include montmorillonite, layered double hydroxides, kaolinite, and mica. Among these, montmorillonite or layered double hydroxides are preferred in terms of having an average interlayer spacing suitable for forming solid carbon quantum dots. It should be noted that layered clay minerals sometimes contain alkali metals or alkaline earth metals, but these alkali metals or alkaline earth metals do not substantially affect the composition of the carbon quantum dots. Therefore, in this specification, the amount of alkali metals or alkaline earth metals contained in the layered clay minerals is not included in the amount of alkali metals or alkaline earth metals in the mixture.

[0067] Montmorillonite is a clay mineral that swells through water, etc. Examples of its composition include: soapstone, montmorillonite, hectorite, beidellite, nontronite, sauconite, stevensite, etc.

[0068] On the other hand, layered double hydroxides are double hydroxides formed by the solid dissolution of trivalent metal ions in divalent metal oxides. Examples of such double hydroxides include hydrotalcite, hydrocalumite, hydromagnesite, and pyroaurite.

[0069] It should be noted that, in order to adjust the average interlayer spacing of layered clay minerals, a solvent can also be used to swell the layered clay minerals. Examples of solvents include: water, methanol, ethanol, hexane, toluene, chloroform, dimethylformamide, and dimethyl sulfoxide. However, in the heating process described later, heating is carried out in a substantially solvent-free manner. Therefore, it is preferable to use a small amount of solvent, and the boiling point of the solvent is preferably lower than the temperature at which organic compounds are carbonized in the heating process (hereinafter also referred to as the "heating temperature"). In other words, in the heating process described later, it is preferable to adjust the amount and type of solvent so that all the solvent evaporates before the temperature of the mixture reaches the heating temperature.

[0070] It should be noted that the amount of layered clay minerals in the mixture is preferably 0% to 20% by mass relative to the total amount of the mixture, more preferably 0% to 10% by mass, and even more preferably substantially free of layered clay minerals. It should be noted that "substantially free" means 5% or less by mass relative to the total amount of the mixture. It should be noted that the amount of layered clay minerals is more preferably 3% or less by mass relative to the total amount of the mixture, more preferably 1% or less by mass, and particularly preferably completely free. When the amount of layered clay minerals is 20% or less by mass, the amount of organic compounds and boron compounds becomes relatively sufficient, enabling efficient preparation of solid carbon quantum dots.

[0071] There are no particular limitations on the mixing method, as long as it can uniformly mix the organic compound, boron compound, and other compounds as needed. For example, it can be mixed while grinding with a mortar and pestle, or while pulverizing with a ball mill or similar device. When the mixture is in solid form, it is preferable to mix in a way that results in a mixture with a substantially uniform particle size.

[0072] When the mixture is in solid form, the particle size can be evaluated by measuring the major diameter of each particle using an optical microscope. The average particle size of the mixture is preferably 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. Furthermore, 99% of the particles contained in the mixture preferably have a particle size of 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. When the particle size of the mixture is within the above range, the mixture is easily and uniformly mixed, the residue of unreacted components can be suppressed, and solid carbon quantum dots can be prepared efficiently.

[0073] Furthermore, when the organic compound, boron compound, or other compound is a liquid, the solid component can be dissolved, miscible, or dispersed in the liquid component for mixing. Alternatively, the materials can be dissolved, miscible, or dispersed in a small amount of solvent for mixing. In this case, the amount and type of solvent are adjusted so that heating can be carried out substantially without solvent in the heating process described later. Specifically, it is preferable to adjust the amount and type of solvent so that all the solvent evaporates before the mixture reaches the heating temperature. It should be noted that the solvent in this specification refers to a compound that is liquid at one atmosphere and 25°C, and is not equivalent to the aforementioned organic compound and boron compound.

[0074] • Heating process

[0075] In the heating process, the mixture, which has been adjusted in the above-described mixture preparation process, is heated in a substantially solvent-free manner. "Substantially solvent-free" in this specification means that the amount of solvent in the mixture at the point in time that the temperature (heating temperature) required to carbonize organic compounds, etc., is 5% by mass or less relative to the total amount of the mixture. More preferably, the amount of solvent in the mixture at the heating temperature is 2% by mass or less, and even more preferably 0% by mass. Therefore, as described above, if it can sufficiently evaporate before the heating temperature, the mixture may contain solvent at the start of heating. It should be noted that compounds that are raw materials for solid carbon quantum dots, i.e., organic compounds, boron compounds, etc., may also be in a liquid state at this heating temperature.

[0076] Heating methods for mixtures can be any methods that can carbonize organic compounds to produce solid carbon quantum dots, including heating with a heater and irradiation with electromagnetic waves.

[0077] When heating the mixture using a heater or similar device, the heating temperature is preferably 70°C or higher and 700°C or lower, more preferably 100°C or higher and 500°C or lower, and even more preferably 100°C or higher and 300°C or lower. Furthermore, the holding time at the heating temperature is preferably 0.01 hours or higher and 45 hours or lower, more preferably 0.1 hours or higher and 30 hours or lower, and even more preferably 0.5 hours or higher and 10 hours or lower. The particle size of the obtained solid carbon quantum dots can be adjusted according to the heating time, thereby adjusting the emission wavelength. Additionally, heating can be performed in a non-oxidizing atmosphere while passing an inert gas such as nitrogen.

[0078] When irradiated with electromagnetic waves (e.g., microwaves), the power is preferably 1W or more and 1500W or less, more preferably 1W or more and 1000W or less. Furthermore, the heating time using electromagnetic waves (e.g., microwaves) is preferably 0.01 hours or more and 10 hours or less, more preferably 0.01 hours or more and 5 hours or less, and even more preferably 0.01 hours or more and 1 hour or less. The particle size of the obtained solid carbon quantum dots can be adjusted according to the irradiation time of the electromagnetic waves, thereby adjusting the emission wavelength.

[0079] The aforementioned electromagnetic wave irradiation can be performed, for example, using a semiconductor-type electromagnetic wave irradiation device. Preferably, the electromagnetic wave irradiation is performed while simultaneously monitoring the temperature of the mixture. More preferably, the electromagnetic waves are irradiated while the heating temperature is adjusted to 70°C or higher and 700°C or lower.

[0080] • Purification process

[0081] In the purification step, the composition obtained in the above heating step is purified to remove unreacted and by-reaction components. There are no particular limitations on the purification method, but washing with an organic solvent is preferred.

[0082] The organic solvent used in the cleaning process may consist of one compound or two or more compounds. The organic solvent may be a polar solvent, a non-polar solvent, or a mixture thereof. More preferably, it contains at least one polar solvent in order to remove unreacted and by-reaction components derived from compounds having reactive groups used in the aforementioned mixture preparation process.

[0083] Specific examples of polar solvents include methanol, ethanol, butanol, 1-propanol, dichloromethane, acetone, ethyl acetate, and acetonitrile. Preferably, a mixed organic solvent consisting of methanol and dichloromethane in a 1:1 ratio is used.

[0084] (2) Second manufacturing method

[0085] The second manufacturing method includes the following steps: a step of preparing a homogeneous mixture containing at least carbon atoms, nitrogen atoms, and boron atoms (mixture preparation step); a step of mixing the mixture with an aqueous solution containing an alkali metal and / or alkaline earth metal (aqueous solution mixing step); a step of heating the aqueous solution and the mixed mixture in a substantially solvent-free manner (heating synthesis step); and a step of purifying the obtained composition (purification step). It should be noted that in the aqueous solution mixing step, the amount of the aqueous solution mixed with the mixture is preferably 200% by mass or less relative to the amount of the mixture. As described above, "substantially solvent-free" in this specification means that the amount of solvent in the mixture reaches the temperature (heating temperature) at which the organic compound, etc., is carbonized, relative to the total amount of the mixture, which is 5% by mass or less. In the second manufacturing method, an aqueous solution containing an alkali metal and / or alkaline earth metal is added in the aqueous solution mixing step, but because the amount of this aqueous solution is sufficiently small, the water in the aqueous solution evaporates before the heating temperature at which the organic compound, etc., is carbonized. Therefore, the heating step can be carried out under substantially solvent-free conditions. Each step will be described below.

[0086] • Mixture preparation process

[0087] In the mixture preparation step, a homogeneous mixture containing at least carbon atoms, nitrogen atoms, and boron atoms is prepared. More specifically, an organic compound having reactive groups but not containing boron atoms is mixed with a boron compound to prepare the mixture. The organic compound and boron compound used in this step may contain alkali metals or alkaline earth metals, or may not contain alkali metals or alkaline earth metals.

[0088] Organic compounds can consist of only one compound or two or more compounds. However, every organic compound must contain nitrogen atoms. The amount of nitrogen atoms relative to the total amount of the organic compound is preferably 20% by mass or more and 50% by mass or less, more preferably 33% by mass or more and 50% by mass or less, and even more preferably 35% by mass or more and 48% by mass or less. When the amount of nitrogen atoms in the organic compound is 20% by mass or more and 50% by mass or less, the crystal structures of carbon nitride and graphite described above are readily formed.

[0089] The organic compound may consist solely of a compound containing only nitrogen atoms and having reactive groups (hereinafter also referred to as "nitrogen-containing organic compounds"), or it may consist of a compound not containing nitrogen atoms but having reactive groups (hereinafter also referred to as "nitrogen-free organic compounds") and a nitrogen-containing organic compound. The nitrogen-containing organic compound and the nitrogen-free organic compound are the same as the nitrogen-containing organic compound and the nitrogen-free organic compound described in the first manufacturing method above.

[0090] On the other hand, any boron compound can be a compound containing boron atoms; for example, it can be elemental boron or a boron-containing compound. A boron compound can consist of one compound or two or more compounds. The boron compound described in the first manufacturing method is the same as that described in the first manufacturing method.

[0091] The total amount of the aforementioned boron compounds is 40% by mass or more and 80% by mass or less relative to the total amount of organic compounds and boron compounds, preferably 42% by mass or more and 70% by mass or less, and more preferably 45% by mass or more and 65% by mass or less. When the amount of boron compounds relative to the total amount of organic compounds and boron compounds is within the above range, it is easy to form solid carbon quantum dots having the crystal structures of carbon nitride and graphite described above.

[0092] It should be noted that, in this process, compounds other than organic compounds and boron compounds may be further mixed without compromising the purpose and effects of the present invention. Preferably, the total amount of organic compounds and boron compounds in the mixture is 50% by mass or more, and more preferably 70% by mass or more, relative to the total amount of the mixture. When the total amount of organic compounds and boron compounds is 50% by mass or more, solid carbon quantum dots can be prepared efficiently. The compounds other than organic compounds and boron compounds are the same as those described in the first manufacturing method.

[0093] There are no particular limitations on the mixing method as long as it can uniformly mix the organic compound, boron compound, and other compounds as needed. For example, it can be mixed while grinding with a mortar and pestle, or while pulverizing with a ball mill or similar device. When the mixture is in solid form, it is preferable to mix in a way that results in a mixture with a substantially uniform particle size. Furthermore, when the organic compound, boron compound, or other compound is in liquid form, the solid component can be dissolved, miscible, or dispersed in the liquid component for mixing. Additionally, the materials can be dissolved, miscible, or dispersed in a small amount of solvent for mixing. In this case, the amount and type of solvent are adjusted so that heating can be carried out substantially solvent-free in the heating process described later. Specifically, it is preferable to adjust the amount and type of solvent so that all the solvent evaporates before the temperature of the mixture reaches the desired heating temperature.

[0094] When the mixture is in solid form, the particle size of the mixture can be evaluated by measuring the major diameter of each particle using an optical microscope. The average particle size of the mixture is preferably 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. Furthermore, 99% of the particles contained in the mixture preferably have a particle size of 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. When the particle size of the mixture is within the above range, the mixture is easily and uniformly mixed, the residue of unreacted components can be suppressed, and solid carbon quantum dots can be prepared efficiently.

[0095] • Aqueous solution mixing process

[0096] The amount of the aqueous solution used in this process should be 200% by mass or less, preferably 150% by mass or less, relative to the mass of the mixture. When the amount of aqueous solution is within this range, all the water evaporates before the temperature of the mixture reaches the desired heating temperature. Therefore, carbon quantum dots can be prepared in a substantially solvent-free manner.

[0097] The types of alkali metals and alkaline earth metals contained in the aqueous solution are not particularly limited, and examples include lithium, sodium, potassium, magnesium, calcium, and barium. The aqueous solution may contain only one alkali metal and alkaline earth metal, or it may contain more than one. It should be noted that, from the viewpoints of reactivity and versatility, sodium, potassium, or calcium are preferred as the alkali metals and alkaline earth metals. The aqueous solution is usually an aqueous solution of an alkali metal salt and / or an alkaline earth metal salt. There are no particular limitations on the counter ions of the alkali metals and alkaline earth metals in the salt; for example, they can be hydroxyl ions, halide ions, etc. Specific examples of the aqueous solution include aqueous solutions of sodium hydroxide, potassium hydroxide, sodium fluoride, sodium chloride, and calcium chloride.

[0098] Furthermore, the concentrations of alkali metal salts and alkaline earth metal salts are acceptable as long as the amounts of alkali metals and alkaline earth metals in the heating process described later are within the desired range.

[0099] • Heating process

[0100] In the heating process, the above-mentioned solid carbon quantum dots are prepared by heating the above-mentioned mixture with an aqueous solution containing alkali metals and / or alkaline earth metals in a substantially solvent-free manner.

[0101] Here, the total amount of alkali metals and alkaline earth metals in the composition (a composition formed by mixing the mixture and the aqueous solution) during the heating process should be 0.1 mol% or more and 30 mol% or less relative to the sum of the molar numbers of carbon and nitrogen atoms contained in the organic compound and the boron compound, preferably 0.5 mol% or more and 20 mol% or less, and more preferably 1 mol% or more and 10 mol% or less. It should be noted that the alkali metals and alkaline earth metals in the composition are mainly derived from the aqueous solution, but as mentioned above, a portion may also be derived from the mixture (e.g., from the organic compound, boron compound, or other compounds).

[0102] In this process, the above mixture is mixed with an aqueous solution and then heated. The heating method can be any method that carbonizes organic compounds to generate solid carbon quantum dots, and examples include heating using a heater or irradiation with electromagnetic waves. The heating temperature, heating time, electromagnetic wave irradiation method, and irradiation time are the same as in the first manufacturing method.

[0103] • Purification process

[0104] In the purification step, the composition obtained in the above heating step is purified to remove unreacted and by-reaction components. There are no particular limitations on the purification method, but washing with an organic solvent is preferred.

[0105] The organic solvent used in the cleaning process may consist of one compound or two or more compounds. The organic solvent may be a polar solvent, a non-polar solvent, or a mixture thereof. More preferably, it contains at least one polar solvent in order to remove unreacted and by-reaction components derived from compounds having reactive groups used in the aforementioned mixture preparation process.

[0106] Specific examples of polar solvents include methanol, ethanol, butanol, 1-propanol, dichloromethane, acetone, ethyl acetate, and acetonitrile. Preferably, a mixed organic solvent consisting of methanol and dichloromethane in a 1:1 ratio is used.

[0107] (Applications of solid carbon quantum dots)

[0108] The aforementioned solid carbon quantum dots exhibit excellent luminescence properties. Therefore, these solid carbon quantum dots can be used for a variety of applications. There are no particular limitations on the applications of solid carbon quantum dots; they can be used in various applications depending on their performance, such as solar cells, displays, anti-counterfeiting inks, quantum dot lasers, biomarkers, lighting materials, thermoelectric materials, photocatalysts, and separation agents for specific substances.

[0109] It should be noted that the aforementioned solid carbon quantum dots are solid at 25°C and one atmosphere, but they can also be used for various purposes in a solution formed by dispersing them in a solvent.

[0110] It should be noted that the carbon quantum dots obtained by the above manufacturing method mainly emit fluorescence with wavelengths above 520 nm and below 580 nm when irradiated with excitation light in the blue wavelength region. Furthermore, the emission wavelength peak is sharp, with high luminescence intensity in the region above 520 nm and below 580 nm, and low luminescence intensity in other wavelength regions. Therefore, these carbon quantum dots are very useful as phosphor materials for white LEDs.

[0111] When the above-mentioned solid carbon quantum dots (phosphor material for white LEDs) are used in white LEDs, white LED elements with excellent luminous efficiency can be produced by combining the carbon quantum dots with blue LEDs with wavelengths above 420 nm and below 480 nm.

[0112] Example

[0113] Hereinafter, specific embodiments and comparative examples of the present invention will be described together, but the present invention is not limited thereto.

[0114] [Example 1]

[0115] (1) Preparation process of raw material mixture

[0116] A mixture was prepared by grinding 0.040 g of trisodium citrate dihydrate, 0.070 g of dicyandiamine, and 0.100 g of boric acid in a mortar and pestle. Then, the granules of each reagent were carefully pulverized into powders of approximately uniform particle size and mixed evenly.

[0117] (2) Heating synthesis process of solid phosphors

[0118] The powdered mixture and stir bar were added to a 15 ml screw-top test tube and sealed with a screw cap with a rubber gasket. Then, while circulating nitrogen gas into the screw-top test tube, a magnetic force from the stir bar was applied to the stir bar to rotate it and stir it. The mixture was then heated at 170 °C for 1.5 hours to synthesize a solid phosphor composition.

[0119] (3) Solvent-based cleaning / purification process

[0120] The total amount of the above-mentioned solid phosphor composition was pulverized into powder using a mortar and pestle. This powder was dispersed in 10 ml of a methanol:dichloromethane (1:1) mixed solvent to obtain a dispersion. The dispersion was then centrifuged (15000 rpm, 5 minutes) to recover the insoluble solid precipitate and remove the solvent and soluble components. This washing process with the solvent was repeated a total of three times. The insoluble solid precipitate was then dried under reduced pressure at 60°C for 6 hours to remove residual solvent, yielding solid carbon quantum dots.

[0121] (4) Confirmation of luminescence state

[0122] The obtained solid carbon quantum dots were clamped onto a KBr plate and pressed to prepare a sample for measurement. Similarly, the sample before the aforementioned cleaning / purification process was also prepared for measurement. Then, the emission wavelength, external quantum efficiency, and internal quantum efficiency when irradiated with excitation light were determined. The wavelength of the excitation light was set to the wavelength of the sample with the highest internal quantum efficiency (maximum excitation wavelength), and the emission wavelength at this point was set as the maximum emission wavelength. The above measurements were performed using a spectrophotometer FP-8500 (manufactured by Nippon Spectrophotometer Co., Ltd.) with an ILF-835 integrating sphere unit. It should be noted that the solid carbon quantum dots emitted yellow fluorescence under irradiation with light at a wavelength of 400 nm, which immediately extinguished when the excitation light was removed. The results are shown in Table 1.

[0123] For comparison, the luminescence states of commercially available carbon nitride (manufactured by Tokyo Chemical Industry Co., Ltd., product code: G0539) and commercially available graphene carbon quantum dots (manufactured by Sigma-Aldrich, catalog number 900726 50MG) were also measured. Furthermore, the luminescence states of a sample prepared by uniformly mixing carbon nitride and graphene carbon quantum dots in a mortar at a mass ratio of 8:2 were also measured. The results are shown in Table 1.

[0124] (5) X-ray diffraction method

[0125] The obtained solid carbon quantum dots were analyzed by X-ray diffraction to obtain their X-ray diffraction spectra. The X-ray diffraction was performed using the following apparatus and conditions.

[0126] Apparatus: PANalytical X'Pert PRO diffractometer, PIXcel detector.

[0127] X-ray source: Cu tube.

[0128] Excitation voltage: 40kV, 40mA.

[0129] Measurement method: θ / 2θ method.

[0130] Slit: DS = 1 / 2°.

[0131] Mask: 10mm.

[0132] Scan range: 5deg~90deg.

[0133] Scanning steps: 0.013deg.

[0134] Scanning speed: 4deg / min.

[0135] The X-ray diffraction spectra obtained for the solid carbon quantum dots of Example 1 are shown below. Figure 1 In this X-ray diffraction spectrum, a peak P, belonging to the crystal structure of carbon nitride (g-C3N4), was confirmed to have a maximum value near a diffraction angle of 28°. CN Furthermore, a peak P, belonging to the crystal structure of graphite, with a maximum value near a diffraction angle of 2θ of 11°, was identified. Gr1 Furthermore, a peak P, belonging to the crystal structure of graphite, was confirmed to be located near a diffraction angle of 25° (2θ). Gr2 It should be noted that the peak P, belonging to the crystal structure of this graphite, is determined by using the line connecting the point with the minimum diffraction (scattering) intensity at a diffraction angle 2θ of 11° to 15° and the point with the minimum diffraction (scattering) intensity at a diffraction angle 2θ of 34° to 40° as the baseline. Gr2 The half-width at half-maximum (HWHM) is 12°. Based on this HWHM, the crystallite size of the graphite crystal structure is calculated to be 0.7 nm. It should be noted that the crystallite size is calculated using the Scherrer formula.

[0136] L=Kλ / (βcosθ)

[0137] (L represents the crystallite size (crystal diameter), K is the Scherrer constant (0.9 in this case), β represents the full width at half maximum (FWHM), and θ represents the diffraction angle).

[0138] For comparison, X-ray diffraction was performed on commercially available carbon nitride (manufactured by Tokyo Chemical Industry Co., Ltd., product code: G0539) and commercially available graphene carbon quantum dots (manufactured by Sigma-Aldrich, catalog number 900726 50MG). Their results are also presented below. Figure 1 .

[0139] Depend on Figure 1 It can be seen that the solid carbon quantum dots prepared above have the crystal structure of carbon nitride and the crystal structure of graphite. In addition, the full width at half maximum (FWHM) of the above peaks also indicates that graphite exists in the form of microcrystals.

[0140] (6) Observation using TEM (transmission electron microscope)

[0141] Images of the above-mentioned solid carbon quantum dots observed using a transmission electron microscope are shown below. Figure 3A and Figure 3B In particular, such as Figure 3B As shown, lattice fringes with d = 0.32 nm were observed in this solid carbon quantum dot. These lattice fringes correspond to the lattice spacing of the (002) plane of carbon nitride, confirming the presence of a carbon nitride crystal structure. Furthermore, lattice fringes with d = 0.21 nm were also observed in this solid carbon quantum dot. These lattice fringes correspond to the lattice spacing of the (100) plane of graphite, providing further evidence of the presence of a graphite crystal structure.

[0142] (7) Analysis using IR (infrared spectroscopy)

[0143] The solid carbon quantum dots were also analyzed using infrared spectroscopy (IR). The IR spectrum at this time is shown in [the figure]. Figure 4 .like Figure 4 As shown, 703cm -1 The peak at 808 cm⁻¹ indicates the presence of aromatic CH₄, suggesting the existence of a graphite crystal structure. Furthermore, the peak at 808 cm⁻¹... -1 The peak indicates the presence of a triazine ring, suggesting the presence of a carbon nitride structure.

[0144] (8) Analysis using XPS (X-ray photoelectron spectroscopy)

[0145] The solid carbon quantum dots were also analyzed using XPS (X-ray photoelectron spectroscopy). The XPS spectrum at this time is shown in... Figure 5A Furthermore, for comparison, commercially available carbon nitride (manufactured by Tokyo Chemical Co., Ltd., product code: G0539) was also analyzed by XPS (X-ray photoelectron spectrometry). The results are presented below. Figure 5B .

[0146] like Figure 5A As shown, the aforementioned solid carbon quantum dots contain numerous graphene-type nitrogen and pyrrole nitrogen atoms. This indicates the presence of nitrogen-doped graphene (graphite) structures. On the other hand, in carbon nitride, such as Figure 5B As shown, the presence of graphene-type nitrogen and pyridine-type nitrogen in a 1:9 ratio is a result that is significantly different from the solid carbon quantum dots of this application.

[0147] Furthermore, analytical results for boron were also shown when analyzing the aforementioned solid carbon quantum dots using XPS (X-ray photoelectron spectroscopy). For example... Figure 6 As shown, the solid carbon quantum dot has BC bonds.

[0148] [Example 2]

[0149] A mixture was prepared by mixing 0.051 g of trisodium citrate dihydrate, 0.088 g of dicyandiamine, and 0.120 g of boric acid using the same method as in Example 1. This mixture was then subjected to a heating synthesis process and a washing / purification process in the same manner as in Example 1 to prepare solid carbon quantum dots. The luminescent properties of the obtained solid carbon quantum dots were evaluated in the same way as in Example 1. Furthermore, XRD analysis was performed. The results are shown in Table 1.

[0150] [Example 3]

[0151] A mixture was prepared by mixing 0.051 g of trisodium citrate dihydrate, 0.096 g of dicyandiamine, and 0.150 g of boric acid using the same method as in Example 1. This mixture was then subjected to a heating synthesis process and a washing / purification process in the same manner as in Example 1 to prepare solid carbon quantum dots. The luminescent properties of the obtained solid carbon quantum dots were evaluated in the same way as in Example 1. Furthermore, XRD analysis was performed. The results are shown in Table 1.

[0152] [Table 1]

[0153]

[0154] As shown in Table 1 above, solid carbon quantum dots with crystal structures of carbon nitride and graphite, where the graphite is microcrystalline (half-width at half maximum of 2° or more), exhibited a very high internal quantum efficiency of over 63% for excitation light at a wavelength of 400 nm (Examples 1-3). Furthermore, the maximum emission wavelength of these solid carbon quantum dots was consistently at 530 nm, and a yellow emission was observed in all cases. It should be noted that the unpurified solid carbon quantum dots from each example were also measured using X-ray diffraction. The diagram at this time is shown in... Figure 7 .like Figure 7 As shown, after purification, a peak P, belonging to the crystal structure of graphite, was identified with a maximum value near a diffraction angle of 2θ = 11°. Gr1 In contrast, without purification, peak P was not identified. Gr1 It was previously believed that unreacted and by-reacted components in unpurified solid carbon quantum dots prevented the layers of graphene from approaching each other, thus hindering the formation of a graphite crystal structure. In contrast, it was argued that the purification process described above removed these components, allowing the graphene structure to approach each other and easily crystallize, confirming the presence of peak P. Gr1 By producing such a graphite crystal structure, both the external and internal quantum efficiencies become good.

[0155] On the other hand, when carbon nitride and graphene quantum dots are excited separately, the internal quantum efficiency is below 4%, which cannot achieve efficient luminescence. Furthermore, when carbon nitride and graphene quantum dots are physically mixed, the internal quantum efficiency is 2%, which cannot achieve the high luminescence efficiency of solid carbon quantum dots mentioned above.

[0156] This application claims priority based on Japanese Patent Application No. 2023-152277, filed on September 20, 2023. The contents of that application specification and drawings are incorporated herein by reference in their entirety.

[0157] Industrial availability

[0158] The solid carbon quantum dots of this invention are novel quantum dots with excellent fluorescence when illuminated. These solid carbon quantum dots can be used, for example, as a substitute material for the yellow phosphor in white LEDs. Furthermore, they can be applied to various lighting materials, thermoelectric materials, and other products.

Claims

1. A solid carbon quantum dot, said solid carbon quantum dot comprising a carbon nitride crystal structure and a graphite crystal structure, In the X-ray diffraction spectrum obtained by X-ray diffraction, the following characteristics are observed: Peak P CN A crystal structure belonging to the carbon nitride with a diffraction angle 2θ that has a maximum value in the range of 26° to 29°; and Peak P Gr1 The crystal structure of the graphite has a maximum value in the range of diffraction angle 2θ between 10° and 12°. When the line connecting the point with the smallest diffraction (scattering) angle (2θ) of 11° to 15° and the point with the smallest diffraction (scattering) intensity (2θ) of 34° to 40° is taken as the baseline, the diffraction peak P, located at diffraction angle 2θ = 25°, belonging to the crystal structure of the graphite, is... Gr2 The half-width is 2° or more and 20° or less.

2. The solid carbon quantum dot according to claim 1, wherein, The peak P Gr2 The half-width is 8° or more and 15° or less.

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