Method for producing carbon quantum dots

Abstract

The purpose of the present invention is to provide a simple production method for carbon quantum dots that mainly emit light having a wavelength of 520-580 nm when irradiated with blue light. The production method for carbon quantum dots that solves said problem has: a step for preparing a mixture including an organic compound that has a reactive group and that does not include boron atoms, and a boron compound; and a step for heating the mixture, substantially without the use of a solvent, in the presence of an alkali metal and / or an alkaline earth metal, and preparing carbon quantum dots. The quantity of nitrogen atoms included by the organic compound is in a specified range, the quantity of the boron compound is in a specified range, and the total quantity of the alkali metal and the alkaline earth metal is in a specified range.

Description

[Technical Field]

[0001] The present invention relates to a method for producing carbon quantum dots. [Background technology]

[0002] Conventionally, a known white LED is a device that combines a blue LED with an inorganic fluorescent material that is excited by the light from the blue LED and emits yellow light (wavelength: approximately 520 nm to 580 nm). However, inorganic fluorescent materials are expensive, and there is a need to develop alternative materials to inorganic fluorescent materials.

[0003] Carbon quantum dots are stable carbon-based particles with particle diameters ranging from several nanometers to several tens of nanometers, and some carbon quantum dots are known to exhibit excellent fluorescence properties. They also have the advantage of being relatively inexpensive to manufacture, without the need to include rare elements, as is the case with inorganic fluorescent materials. Therefore, much research has been conducted on carbon quantum dots in recent years.

[0004] For example, Patent Document 1 describes carbon quantum dots that have a maximum emission wavelength of 549 nm when irradiated with light having a wavelength of 479 nm. This document describes a method for producing the carbon quantum dots, in which a solution obtained by dissolving a carbon source, a nitrogen source, and a boron source in ultrapure water is reacted under high pressure at 200°C for 4 hours. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Chinese Patent Application Publication No. 112028053 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the method of Patent Document 1 requires a large amount of ultrapure water and a high-pressure reaction. Therefore, a large amount of energy is required to produce carbon quantum dots. Furthermore, the method requires various steps such as centrifugation and filtration, making it difficult to efficiently produce carbon quantum dots.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a simple method for producing carbon quantum dots that mainly emit light with a wavelength of 520 nm to 580 nm when irradiated with blue light. [Means for solving the problem]

[0008] The present invention provides a method for producing carbon quantum dots that are solid at 25°C and 1 atmosphere, the method comprising the steps of: preparing a mixture containing an organic compound that has a reactive group but does not contain boron atoms, and a boron compound; and heating the mixture in the presence of an alkali metal and / or alkaline earth metal, substantially without a solvent, to prepare carbon quantum dots, wherein the amount of nitrogen atoms contained in the organic compound is 30% by mass or more and 50% by mass or less, based on the total amount of the organic compound; the amount of the boron compound is 50% by mass or more and 80% by mass or less, based on the total amount of the organic compound and the boron compound; and the total amount of the alkali metal and the alkaline earth metal is 0.1 mol % or more and 30 mol % or less, based on the sum of the moles of carbon atoms and nitrogen atoms contained in the organic compound and the boron compound. [Effects of the Invention]

[0009] According to the method for producing carbon quantum dots of the present invention, it is possible to easily produce carbon quantum dots that mainly emit light with a wavelength of 520 nm to 580 nm when irradiated with blue light. DETAILED DESCRIPTION OF THE INVENTION

[0010] In this specification, a numerical range indicated by "to" means a numerical range including the numerical values ​​written before and after "to".

[0011] The method for producing carbon quantum dots of the present invention is a method for producing solid carbon quantum dots at 25°C and 1 atmosphere. As described above, it has been known to produce carbon quantum dots that have a maximum emission wavelength of 520 nm to 580 nm when irradiated with blue light by hydrothermal synthesis. However, preparing carbon quantum dots using this method has problems such as a complicated production process, a high input of energy, and a long time.

[0012] In contrast, the inventors' intensive research has revealed that a method for producing carbon quantum dots that primarily emit fluorescence with wavelengths of 520 nm to 580 nm when irradiated with blue light, comprising the steps of preparing a mixture containing an organic compound having a reactive group but not containing boron atoms and a boron compound (mixture preparation step), and heating the mixture in the presence of an alkali metal and / or alkaline earth metal substantially without a solvent (heating step), where the amount of nitrogen atoms contained in the organic compound is within a predetermined range, the amount of boron compound relative to the amount of the organic compound and the boron compound is within a predetermined range, and the amounts of the alkali metal and alkaline earth metal are also within predetermined ranges, can easily produce carbon quantum dots. As specific examples of the above method, two production methods are shown below. However, the method for producing carbon quantum dots of the present invention is not limited to these methods.

[0013] (1) First manufacturing method The first method for producing carbon quantum dots is a method that performs the above-mentioned mixture preparation step and the above-mentioned heating step, and includes an alkali metal and / or an alkaline earth metal in either or both of the organic compound and the boron compound used in the mixture preparation step.

[0014] In the first manufacturing method, a mixture containing carbon atoms, nitrogen atoms, boron atoms, and alkali metals and / or alkaline earth metals is prepared in the mixture preparation step, and the mixture is heated substantially without a solvent in the heating step to produce carbon quantum dots. The carbon quantum dots obtained by this manufacturing method have a maximum emission wavelength in the wavelength range of 520 nm to 580 nm, and their emission peak wavelength is very sharp. While the reason for this is unclear, one possible reason is that if a certain amount of alkali metal or alkaline earth metal is present during the production of the carbon quantum dots, this metal is incorporated into the carbon quantum dots, making it easier for the size of the carbon quantum dots to fall within the desired range. It is also thought that the alkali metal or alkaline earth metal ionizes or coordinates with the functional groups of the carbon quantum dots, thereby changing the electronic state of the carbon quantum dots. Furthermore, when the amount of nitrogen atoms and the amount of boron compound are within a predetermined range, the size of the carbon quantum dots is easily controlled and uniform, resulting in a sharp emission wavelength peak.

[0015] Furthermore, as in the prior art documents mentioned above, when carbon quantum dots are prepared in a large amount of solvent, the carbon quantum dots tend to aggregate when the solvent is removed, which can result in quenching and other problems. In contrast, the present production method does not require solvent removal, and the carbon quantum dots are less likely to aggregate and the quenching is less likely to occur. Furthermore, when carbon quantum dots are prepared by heating substantially without a solvent, if the organic compound and the boron compound are solid upon heating, the reaction field of the condensation reaction to produce the carbon quantum dots is limited to the solid-state contact point between the organic compound and the boron compound. In other words, the reaction field of the condensation reaction is locally limited. On the other hand, even if either or both of the organic compound and the boron compound are liquid upon heating, the viscosity of the mixture increases due to the high molecular weight caused by the condensation reaction. Therefore, in this case, the reaction field of the condensation reaction is also localized compared to when carbon quantum dots are prepared in a large amount of solvent. Therefore, the type and particle size of the carbon quantum dots produced are limited, which is thought to result in a higher luminescence quantum yield and a sharper peak in the emission wavelength.

[0016] Furthermore, when carbon quantum dots are produced by heating an organic compound and a boron compound substantially without a solvent, as in this production method, a pressure control device or a solvent removal device is not required, and the input energy amount is very small. Therefore, the desired carbon quantum dots can be produced efficiently and simply. Each step of the first production method will be described below.

[0017] ·Mixture preparation process In the mixture preparation step, a boron compound is mixed with an organic compound having a reactive group but not containing a boron atom to prepare a mixture. In this specification, the term "reactive group" refers to a group that is bonded to a carbon atom in the organic compound and that causes a polycondensation reaction between organic compounds in the heating step described below, contributing to the formation of the main skeleton of the carbon quantum dots. Specific examples of reactive groups include carboxy groups, hydroxy groups, epoxy groups, amide groups, sulfo groups, amino groups, and groups in which alkali metals or alkaline earth metals are bonded or coordinated to these groups. In this specification, a compound containing a boron atom is referred to as a "boron compound." Even if the compound contains a reactive group in its molecule, it is still considered a boron compound if it contains a boron atom.

[0018] Either or both of the organic compound and the boron compound used in this step contain an alkali metal and / or an alkaline earth metal. The amounts of the alkali metal and alkaline earth metal contained in the organic compound and the boron compound may be within the desired ranges as long as the amounts of the alkali metal and alkaline earth metal contained in the heating step described below fall within the desired ranges.

[0019] Here, the type of alkali metal or alkaline earth metal contained in the organic compound or boron compound is not particularly limited, and examples include lithium, sodium, potassium, magnesium, calcium, barium, etc. However, from the viewpoints of reactivity, safety, material availability, etc., sodium, potassium, or calcium is preferred.

[0020] The organic compound used in the mixture preparation step may be any compound that has a reactive group, can produce carbon quantum dots by carbonization (condensation reaction), and has a nitrogen atom content of 30% to 50% by mass relative to the total amount of the organic compound. The nitrogen atom content is preferably 33% to 50% by mass, and more preferably 36% to 48% by mass. When the nitrogen atom content in the organic compound is 30% to 50% by mass, the emission wavelength peak of the carbon quantum dots tends to become sharper and the emission quantum yield tends to increase, as described above. The nitrogen atom content in the organic compound can be determined from the structure of the organic compound.

[0021] The organic compound may contain only one type of compound, or may contain two or more types of compounds. Specifically, it may contain only a compound that contains a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-containing organic compound"), or it may contain a compound that does not contain a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-free organic compound") and a nitrogen-containing organic compound.

[0022] Examples of nitrogen-containing organic compounds include amine compounds, nitrogen-containing sugars, imidazoles, triazines, triazoles, triazenes, guanidines, and oximes, as well as salts of these compounds with alkali metals and / or alkaline earth metals. The organic compounds may contain only one of these compounds or two or more of them. Furthermore, these compounds may be solid or liquid at room temperature.

[0023] 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, dicyandiamide, guanidine, aminoguanidine, formamide, glutamic acid, and the like. These include citric acid, aspartic acid, cysteine, arginine, histidine, lysine, glutathione, RNA, DNA, cysteamine, methionine, homocysteine, taurine, thiamine, N-[3-(trimethoxysilyl)propyl]ethylenediamine, 4,5-difluoro-1,2-phenylenediamine, sulfanilic acid, o-phosphoserine, adenosine 5'-triphosphate, guanidine phosphate, guanylurea phosphate, 3-aminopropyltriethoxysilane, and the like.

[0024] Examples of nitrogen-containing sugars include glucosamine, chitin, chitosan, etc. Examples of imidazoles include 1-(trimethylsilyl)imidazole, etc. Examples of triazines include 1,2,4-triazine, and examples of triazoles include 1,3,5-triazine, 1,2,3-triazole, and 1,2,4-triazole. Examples of triazenes include 1,3-diphenyltriazene and 1-methyl-3-p-tolyltriazene, examples of guanidines include guanidine and arginine, and examples of oximes include benzamide oxime and p-benzoquinone dioxime. Examples of alkali metal and alkaline earth metal salts of the above compounds include sodium glutamate and sodium imidazole-4-acetate, etc.

[0025] Of the above nitrogen-containing organic compounds, amine compounds are preferred from the viewpoints of availability and reactivity in the heating step described below.

[0026] On the other hand, examples of nitrogen-free organic compounds include carboxylic acids, alcohols, phenols, sugars, and salts of these with alkali metals and / or alkaline earth metals. The organic compound may contain only one of these, or two or more of them. Furthermore, these compounds may be solid or liquid at room temperature.

[0027] The carboxylic acid may be any compound having one or more carboxy groups in the molecule (excluding nitrogen-containing organic compounds, phenols, or sugars). Examples of the carboxylic acid include monocarboxylic acids such as formic acid, acetic acid, 3-mercaptopropionic acid, and α-lipoic acid; divalent or higher polyvalent carboxylic acids such as oxalic acid, malonic acid, succinic acid, maleic acid, fumaric acid, itaconic acid, polyacrylic acid, (ethylenedithio)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.

[0028] The alcohol is preferably a monohydric alcohol having one hydroxy group and six or more carbon atoms, or a polyhydric alcohol having two or more hydroxy groups (excluding those 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, sorbitol, etc.

[0029] The phenols may be any compounds having a structure in which a hydroxy group is bonded to a benzene ring (excluding nitrogen-containing organic compounds). Examples of phenols include phenol, catechol, resorcinol, hydroquinone, phloroglucinol, pyrogallol, 1,2,4-trihydroxybenzene, gallic acid, tannin, lignin, catechin, anthocyanin, rutin, chlorogenic acid, lignan, curcumin, etc.

[0030] Examples of sugars, which are non-nitrogen-containing organic compounds, include glucose, sucrose, cellulose, etc. Examples of alkali metal salts and alkaline earth metal salts of the above compounds include trisodium citrate dihydrate, tripotassium citrate monohydrate, sodium ascorbate, calcium acetate, etc.

[0031] The nitrogen-free organic compound preferably has a reactive group that efficiently undergoes a condensation reaction with the nitrogen-containing organic compound, and is preferably a carboxylic acid, an alcohol, a phenol, or an alkali metal salt or alkaline earth metal salt thereof. The amount of the nitrogen-free organic compound in the organic compound is appropriately selected so that the amount of nitrogen atoms in the organic compound is 30% by mass or more and 50% by mass or less.

[0032] The total amount of the organic compounds (nitrogen-containing organic compounds and nitrogen-free organic compounds) may be 20% by mass or more and 50% by mass or less, and preferably 35% by mass or more and 48% by mass or less, based on the total amount of the organic compounds and the boron compounds. When the amount of the organic compounds relative to the total amount of the organic compounds and the boron compounds is within the above range, the emission wavelength peak tends to be sharp, as described above.

[0033] On the other hand, the boron compound used to prepare the mixture may be any compound containing a boron atom, such as boron itself or a compound containing boron. The mixture prepared in this step may contain only one kind of boron compound or two or more kinds of boron compounds.

[0034] Specific examples of boron compounds include boron, boric acid, sodium tetraborate, boron oxide, trimethyl borate, triethyl borate, trioctadecyl borate, triphenyl borate, 2-ethoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, triethanolamine borate, 2,4,6-trimethoxyboroxine, tris(trimethylsilyl)borate, tris(2-cyanoethyl)borate, 3-aminophenyl Boronic acid, 2-anthraceneboronic acid, 9-anthraceneboronic acid, phenylboronic acid, 3,5-bis(trifluoromethyl)phenylboronic acid, 4,4'-biphenyldiboronic acid, 2-bromophenylboronic acid, 4-bromo-1-naphthaleneboronic 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-naphthaleneboronic acid, 9-phenanthreneboronic acid, 1,4-phenylenediboronic acid, 1-pyreneboronic acid, 2-aminopyrimidine-5-boronic acid, 2-bromopyridine-3-boronic acid, 2-fluoropyridine-3-boronic acid, 4 -pyridylboronic acid, quinoline-8-boronic acid, 4-aminophenyl pinacol boronic acid, 3-hydroxyphenyl pinacol boronic acid, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine, diboronic acid, sodium borohydride, sodium tetrafluoroborate, 1-ethyl-3-methylimidazolium tetrafluoroborate, boron trifluoride, boron tribromide, and the like.

[0035] The amount of boron atoms in the boron compound is preferably 3% by mass or more, more preferably 5% by mass or more, based on the total amount of the boron compound. When the amount of boron atoms in the boron compound is within this range, it becomes possible to arrange the boron atoms on the surface of the carbon quantum dots, which tends to increase the luminescence quantum yield.

[0036] The total amount of the boron compound may be 50% by mass or more and 80% by mass or less, preferably 50% by mass or more and 70% by mass or less, and more preferably 52% by mass or more and 65% by mass or less, relative to the total amount of the organic compound and the boron compound. When the amount of the boron compound relative to the total amount of the organic compound and the boron compound is 50% by mass or more, fluorescence is easily emitted, and the emission quantum yield is easily increased and the emission wavelength peak is easily sharpened.

[0037] In this step, compounds other than the organic compound and the boron compound may be further mixed as long as the objectives and effects of the present invention are not impaired. However, the amount of the organic compound and the boron compound in the mixture is preferably 50% by mass or more, more preferably 70% by mass or more, based on the total amount of the mixture. When the total amount of the organic compound and the boron compound is 50% by mass or more, carbon quantum dots can be efficiently prepared. In addition, the compounds other than the organic compound and the boron compound may contain alkali metals or alkaline earth metals, as long as the amount of alkali metals and alkaline earth metals in the heating step described below falls within the desired range.

[0038] Examples of compounds other than organic compounds and boron compounds include compounds containing a phosphorus atom, a sulfur atom, a silicon atom, or a fluorine atom, etc., but not having the above-mentioned reactive groups (hereinafter also referred to as "other compounds"). By mixing other compounds in this process, carbon quantum dots containing heteroatoms other than nitrogen and boron can be obtained. The mixture may contain only one type of other compound, or two or more types.

[0039] Examples of compounds containing a phosphorus atom 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 phosphonoacetate, tetrakis(hydroxymethyl)phosphonium chloride, methyl phosphate, triethyl phosphite, nitrilotris(methylenephosphonic acid), N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid), and the like.

[0040] Examples of compounds containing a sulfur atom include sulfur, sodium thiosulfate, sodium sulfide, sodium sulfate, sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, and sodium hydrosulfide.

[0041] Examples of compounds containing silicon atoms include tetrachlorosilane, tetraethoxysilane, and the like.

[0042] Examples of the compound containing a fluorine atom include 2,2,3,3,4,4-hexafluoro-1,5-pentanediol diglycidyl ether, sodium fluoride, and the like.

[0043] The amount of other compounds in the mixture is appropriately selected according to the desired amount of heteroatoms, but is usually preferably 0% to 20% by mass, and more preferably 3% to 10% by mass, relative to the total amount of the mixture. When the amount of other compounds is 3% by mass or more, the effect of adding the 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 sufficiently large relative to each other, allowing carbon quantum dots to be prepared efficiently.

[0044] Furthermore, a layered clay mineral may be further added in this step. Adding a layered clay mineral makes it easier to prepare carbon quantum dots of uniform size by using the interlayer spaces of the layered clay mineral as a template. Examples of layered clay minerals include smectite, layered double hydroxides, kaolinite, and mica. Among these, smectite or layered double hydroxide is preferred because it has an average interlayer spacing suitable for forming carbon quantum dots. Note that while layered clay minerals may contain alkali metals or alkaline earth metals, these alkali metals and 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 mineral is not included in the amount of alkali metals or alkaline earth metals specified in the processing step.

[0045] Smectite is a clay mineral that swells with water, etc., and examples thereof include saponite, montmorillonite, hectorite, beidellite, nontronite, sauconite, stevensite, and the like.

[0046] On the other hand, layered double hydroxides are double hydroxides in which trivalent metal ions are dissolved in a divalent metal oxide, and examples thereof include hydrotalcite, hydrocalumite, hydromagnesite, and pyroaurite.

[0047] In addition, in order to adjust the average layer spacing of the layered clay mineral, the layered clay mineral may be swollen with a solvent. Examples of solvents include water, methanol, ethanol, hexane, toluene, chloroform, dimethylformamide, and dimethyl sulfoxide. However, in the heating step described below, heating is performed substantially without a solvent. Therefore, it is preferable to use a small amount of solvent, and further, it is preferable that the boiling point of the solvent is lower than the temperature at which the organic compound or the like is carbonized in the heating step (hereinafter also referred to as the "heating temperature"). In other words, it is preferable to adjust the amount and type of solvent so that all of the solvent volatilizes before the temperature of the mixture reaches the heating temperature in the heating step described below.

[0048] The amount of layered clay mineral in the mixture is preferably 0% to 20% by mass, more preferably 0% to 10% by mass, relative to the total amount of the mixture, and even more preferably substantially free of layered clay mineral. "Substantially free" means 5% by mass or less relative to the total amount of the mixture. The amount of layered clay mineral is more preferably 3% by mass or less, even more preferably 1% by mass or less, relative to the total amount of the mixture, and particularly preferably completely free. When the amount of layered clay mineral is 20% by mass or less, the amounts of organic compounds and boron compounds are sufficiently large relative to each other, allowing for efficient preparation of carbon quantum dots.

[0049] The method for mixing the mixture is not particularly limited as long as it allows for uniform mixing of the organic compound, the boron compound, and, if necessary, other compounds or layered clay minerals. For example, they may be mixed by grinding in a mortar or by pulverizing using a ball mill or the like. Furthermore, when the organic compound, boron compound, or other compound is liquid, the solid component may be dissolved, mixed, or dispersed in the liquid component. Alternatively, each material may be dissolved, mixed, or dispersed in a small amount of solvent. In this case, the amount and type of solvent are adjusted so that heating can be performed substantially solvent-free in the heating step described below. Specifically, it is preferable to adjust the amount and type of solvent so that all of the solvent volatilizes before the temperature of the mixture reaches the heating temperature. In this specification, the term "solvent" refers to a compound that is liquid at 25°C under 1 atmosphere and does not correspond to the organic compound or boron compound.

[0050] ·Heating process In the heating step, the mixture prepared in the mixture preparation step is heated substantially without solvent. In this specification, "substantially without solvent" means that the amount of solvent in the mixture is 5% by mass or less relative to the total amount of the mixture when the temperature (heating temperature) at which the organic compound or the like is carbonized is reached. The amount of solvent in the mixture at the heating temperature is more preferably 2% by mass or less, and even more preferably 0% by mass. Therefore, as described above, the mixture may contain a solvent at the start of heating, as long as it is sufficiently volatilizable by the time the heating temperature is reached. Note that the compounds serving as raw materials for carbon quantum dots, i.e., organic compounds, boron compounds, etc., may be liquid at the heating temperature.

[0051] The heating step is carried out in the presence of an alkali metal and / or alkaline earth metal. The total amount of the alkali metal and alkaline earth metal in the mixture during the heating step may be 0.1 mol % to 30 mol %, preferably 0.5 mol % to 20 mol %, and more preferably 1 mol % to 10 mol %, based on the sum of the moles of carbon and nitrogen atoms contained in the organic compound and the boron compound. The alkali metal and alkaline earth metal in the mixture are mainly derived from the organic compound and the boron compound, but a portion may also be derived from other compounds in the mixture.

[0052] The method for heating the mixture may be any method that can carbonize the organic compound or the like to produce carbon quantum dots, and examples thereof include heating with a heater and irradiating with electromagnetic waves.

[0053] When the mixture is heated using a heater or the like, the heating temperature is preferably 70°C to 700°C, more preferably 100°C to 500°C, and even more preferably 100°C to 300°C. The holding time at the heating temperature is preferably 0.01 hours to 45 hours, more preferably 0.1 hours to 30 hours, and even more preferably 0.5 hours to 10 hours. The particle size and, therefore, the emission wavelength of the resulting carbon quantum dots can be adjusted by the heating time. Heating may also be performed in a non-oxidizing atmosphere while circulating an inert gas such as nitrogen.

[0054] When irradiating with electromagnetic waves (e.g., microwaves), the wattage is preferably 1 W or more and 1500 W or less, more preferably 1 W or more and 1000 W 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 resulting carbon quantum dots, and therefore the emission wavelength, can be adjusted by adjusting the irradiation time of the electromagnetic waves.

[0055] The electromagnetic wave irradiation can be carried out using, for example, a semiconductor-type electromagnetic wave irradiation device. The electromagnetic wave irradiation is preferably carried out while checking the temperature of the mixture. The electromagnetic wave irradiation is preferably carried out while adjusting the heating temperature to be 70°C or higher and 700°C or lower.

[0056] Carbon quantum dots are obtained by this heating process. Note that unreacted boron compounds and the like may remain around the carbon quantum dots. Therefore, in order to extract only the carbon quantum dots, the obtained components may be purified by, for example, washing with an organic solvent to remove unreacted materials and by-products.

[0057] (2) Second manufacturing method The second method for producing carbon quantum dots includes the steps of: preparing the mixture; mixing the mixture with an aqueous solution containing an alkali metal and / or an alkaline earth metal (aqueous solution mixing step); and heating the mixture. In the aqueous solution mixing step, the amount of the aqueous solution mixed with the mixture is 200% by mass or less of the amount of the mixture.

[0058] In the second manufacturing method, the mixture preparation step involves preparing a mixture containing at least carbon, nitrogen, and boron atoms; the aqueous solution mixing step involves mixing the mixture with an aqueous solution containing an alkali metal and / or alkaline earth metal; and the heating step involves heating the mixture substantially without a solvent to prepare carbon quantum dots. As described above, "substantially solvent-free" in this specification means that the amount of solvent in the mixture is 5% by mass or less of the total amount of the mixture when the temperature (heating temperature) at which the organic compound or the like is carbonized is reached. 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 the amount of the aqueous solution is sufficiently small that the water in the aqueous solution evaporates by the time the temperature reaches the heating temperature at which the organic compound or the like is carbonized. Therefore, the heating step can be performed substantially without a solvent.

[0059] For the same reasons as in the first manufacturing method, the carbon quantum dots obtained by this manufacturing method have a maximum emission wavelength in the wavelength range of 520 nm to 580 nm, and the emission peak wavelength is very sharp. Furthermore, as described above, when carbon quantum dots are prepared in a large amount of solvent, the carbon quantum dots tend to aggregate when the solvent is removed, which can cause quenching. In contrast, as described above, when carbon quantum dots are prepared substantially without solvent, the obtained carbon quantum dots are less likely to aggregate and the above-mentioned quenching is also less likely to occur.

[0060] Furthermore, when carbon quantum dots are produced by mixing a mixture containing an organic compound and a boron compound with a relatively small amount of aqueous solution, as in the second production method, no pressure control device or solvent removal device is required, and the input energy amount is very small. Therefore, the desired carbon quantum dots can be produced efficiently and easily. Each step of the second production method will be described below.

[0061] ·Mixture preparation process In the mixture preparation step, an organic compound having a reactive group but not containing a boron atom is mixed with a boron compound to prepare a mixture. The organic compound and the boron compound used in this step may or may not contain an alkali metal or an alkaline earth metal.

[0062] The organic compound used in the mixture preparation step may contain only one type of compound, or may contain two or more types of compounds. That is, the organic compound may contain only a nitrogen-containing organic compound, or may be a mixture of a nitrogen-free organic compound and a nitrogen-containing organic compound. The nitrogen-containing organic compound and the nitrogen-free organic compound are the same as those described in the first production method.

[0063] The total amount of the organic compounds (nitrogen-containing organic compounds and nitrogen-free organic compounds) may be 20% by mass or more and 50% by mass or less, and preferably 35% by mass or more and 48% by mass or less, based on the total amount of the organic compounds and the boron compounds. When the amount of the organic compounds relative to the total amount of the organic compounds and the boron compounds is within the above range, the emission wavelength peak tends to be sharp, as described above.

[0064] On the other hand, the boron compound may be any compound containing a boron atom, for example, boron alone or a compound containing boron. The mixture prepared in this step may contain only one type of boron compound, or may contain two or more types. The boron compound is the same as that described in the first production method.

[0065] The total amount of the boron compound may be 50% by mass or more and 80% by mass or less relative to the total amount of the organic compound and the boron compound, preferably 50% by mass or more and 70% by mass or less, and more preferably 52% by mass or more and 65% by mass or less. When the amount of the boron compound relative to the total amount of the organic compound and the boron compound is 50% by mass or more, fluorescence is easily emitted, and the emission quantum yield is easily increased and the emission wavelength peak is easily sharpened.

[0066] In this step, compounds other than the organic compound and the boron compound may be further mixed within a range that does not impair the objectives and effects of the present invention. However, the total amount of the organic compound and the boron compound in the mixture is preferably 50% by mass or more, more preferably 70% by mass or more, based on the total amount of the mixture. When the total amount of the organic compound and the boron compound is 50% by mass or more, carbon quantum dots can be efficiently prepared. The compounds other than the organic compound and the boron compound are the same as those described in the first manufacturing method.

[0067] The method for mixing the mixture is not particularly limited as long as it allows for uniform mixing of the organic compound, the boron compound, and, if necessary, other compounds or layered clay minerals. For example, they may be mixed by grinding in a mortar or by pulverizing using a ball mill or the like. Furthermore, when the organic compound, boron compound, or other compound is liquid, the solid component may be dissolved, mixed, or dispersed in the liquid component and mixed. Alternatively, each material may be dissolved, mixed, or dispersed in a small amount of solvent and mixed. In this case, the amount and type of solvent are adjusted so that heating can be performed substantially solvent-free in the heating step described below. Specifically, it is preferable to adjust the amount and type of solvent so that all of the solvent volatilizes before the temperature of the mixture reaches the desired heating temperature.

[0068] ·Aqueous solution mixing process The amount of the aqueous solution used in this step may be 200% by mass or less, preferably 150% by mass or less, based on the mass of the mixture. When the amount of the aqueous solution is within this range, all of the water evaporates before the temperature of the mixture reaches the desired heating temperature. Therefore, carbon quantum dots can be prepared essentially without using a solvent.

[0069] 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, barium, etc. The aqueous solution may contain only one type of alkali metal or alkaline earth metal, or two or more types. From the viewpoints of reactivity, versatility, etc., sodium, potassium, or calcium is preferred as the alkali metal or alkaline earth metal. The aqueous solution can usually be an aqueous solution of an alkali metal salt and / or an alkaline earth metal salt. The counter ions of the alkali metal and alkaline earth metal in the salt are not particularly limited, and can be, for example, hydroxy ions, halogen ions, etc. Specific examples of aqueous solutions include aqueous sodium hydroxide solutions, aqueous potassium hydroxide solutions, aqueous sodium fluoride solutions, aqueous sodium chloride solutions, and aqueous calcium chloride solutions.

[0070] The concentration of the alkali metal salt or alkaline earth metal salt may be within a range in which the amount of alkali metal or alkaline earth metal in the heating step described below falls within a desired range.

[0071] ·Heating process In the heating step, the composition obtained by mixing the above mixture with an aqueous solution containing an alkali metal and / or alkaline earth metal is heated substantially without a solvent to prepare carbon quantum dots.

[0072] Here, the total amount of alkali metals and alkaline earth metals in the composition (composition obtained by mixing the mixture and the aqueous solution) during the heating step may be 0.1 mol % to 30 mol %, preferably 0.5 mol % to 20 mol %, and more preferably 1 mol % to 10 mol %, of the sum of the moles of carbon atoms and nitrogen atoms contained in the organic compound and the boron compound. Note 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., derived from the organic compound, the boron compound, or other compounds).

[0073] In this process, the mixture is mixed with an aqueous solution and then heated. Any heating method can be used as long as it can carbonize the organic compound or the like to produce carbon quantum dots, and examples of such a method include heating with a heater and irradiating with electromagnetic waves. The heating temperature, heating time, and irradiating method and time of electromagnetic waves are the same as those in the first manufacturing method.

[0074] Carbon quantum dots are obtained by this heating process. Note that unreacted boron compounds and the like may remain around the carbon quantum dots. Therefore, in order to extract only the carbon quantum dots, the obtained components may be purified by, for example, washing with an organic solvent to remove unreacted materials and by-products.

[0075] (3) Physical properties of carbon quantum dots The carbon quantum dots obtained by the first and second manufacturing methods are both solid at 1 atmosphere and 25°C. The average particle size of the carbon quantum dots measured by observing them with 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, it is easy to obtain sufficient quantum dot properties. Note that the average particle size of the carbon quantum dots is preferably measured for three or more carbon quantum dots and the average value thereof is calculated.

[0076] Furthermore, the carbon quantum dots preferably have a maximum emission wavelength of 520 nm to 580 nm, more preferably 530 nm to 550 nm, when irradiated with light at a wavelength of 460 nm. Furthermore, it is preferable that the light emitted when irradiated with light is primarily fluorescence. If the maximum emission wavelength of the fluorescence when irradiated with light at a wavelength of 460 nm falls within the above range, the material can be easily used as a light-emitting material for white LEDs, for example, as a phosphor material for white LEDs that use blue light as excitation light. The maximum emission wavelength can be determined by obtaining the spectral distribution when irradiated with light at a wavelength of 460 nm using a spectrofluorometer or the like equipped with an integrating sphere unit.

[0077] Furthermore, for these carbon quantum dots, the ratio of the emission intensity at a wavelength of 420 nm to the emission intensity at a wavelength of 530 nm when irradiated with light at a wavelength of 340 nm, i.e., (emission intensity at a wavelength of 420 nm) / (emission intensity at a wavelength of 530 nm), is preferably 0.25 or less, more preferably 0.21 or less. This value indicates the extent to which the base of the emission wavelength peak of the carbon quantum dots spreads; the smaller the value, the sharper the emission wavelength peak. Furthermore, when this value is 0.25 or less, the emission intensity around 420 nm is low and the carbon quantum dots emit light efficiently around 530 nm. In other words, this carbon quantum dot can be easily used as a phosphor material for white LEDs. The emission intensity at a wavelength of 420 nm relative to the emission intensity at a wavelength of 530 nm when irradiated with light at a wavelength of 340 nm can be determined by obtaining the spectral distribution when irradiated with light at a wavelength of 340 nm using a spectrofluorophotometer or the like equipped with an integrating sphere unit.

[0078] Furthermore, the internal quantum yield of the carbon quantum dots when irradiated with light having a wavelength of 460 nm is preferably 50% or more, more preferably 55% or more. When the internal quantum yield of the carbon quantum dots is 50% or more, the carbon quantum dots can be easily used for various applications. The internal quantum yield can also be determined by obtaining the spectral distribution when irradiated with light having a wavelength of 460 nm using a spectrofluorometer or the like equipped with an integrating sphere unit.

[0079] The composition of the preferred carbon quantum dots is not particularly limited, but according to the above-mentioned method for producing carbon quantum dots, it is possible to prepare carbon quantum dots having a carbon atom content of 10% by mass or more and 25% by mass or less, a nitrogen atom content of 15% by mass or more and 35% by mass or less, a boron atom content of 5% by mass or more and 30% by mass or less, and an alkali metal or alkaline earth metal content of 0.1% by mass or more and 30% by mass or less.

[0080] (4) Applications of carbon quantum dots The carbon quantum dots obtained by the above-described production method have excellent luminescence properties. Therefore, the carbon quantum dots can be used for various applications. The applications of the carbon quantum dots are not particularly limited, and depending on the performance of the carbon quantum dots, they can be used in, for example, solar cells, displays, security inks, quantum dot lasers, biomarkers, lighting materials, thermoelectric materials, photocatalysts, and separation agents for specific substances.

[0081] The carbon quantum dots described above are solid at 25° C. and 1 atmosphere, but they may be dispersed in a solvent or the like to form a solution for use in various applications.

[0082] The carbon quantum dots obtained by the above-mentioned manufacturing method mainly emit fluorescence with a wavelength of 520 nm to 580 nm when irradiated with excitation light in the blue wavelength range. Furthermore, the emission wavelength peak is sharp, with high emission intensity in the wavelength range of 520 nm to 580 nm and low emission intensity in other wavelength ranges. Therefore, the carbon quantum dots are very useful as a phosphor material for white LEDs.

[0083] When the carbon quantum dots (phosphor material for white LEDs) are used in a white LED, a white LED element with excellent luminous efficiency can be obtained by combining the carbon quantum dots with a blue LED having a wavelength of 420 nm or more and 480 nm or less. [Example]

[0084] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0085] [Example 1] (1) Preparation of carbon quantum dots A mixture was prepared by grinding and mixing organic compounds (0.046 g of trisodium citrate dihydrate and 0.080 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in a mortar. The powder mixture was placed in a 15 ml screw-top test tube and sealed with a rubber gasket. The tube was heated at 170°C for 1.5 hours while circulating nitrogen through it, synthesizing solid carbon quantum dots. The amount of nitrogen atoms in the organic compound and the amount of boron compound relative to the total amount of organic compound and boron compound are shown in Table 1.

[0086] (2) Evaluation of solid-state light-emitting properties The obtained carbon quantum dots were sandwiched between KBr plates and pressed to prepare a measurement sample. The sample was then used to measure the maximum emission wavelength and internal quantum yield when irradiated with light of 460 nm wavelength, as well as the emission intensity at 420 nm relative to the emission intensity at 530 nm when irradiated with light of 340 nm wavelength (emission intensity at 420 nm / emission intensity at 530 nm). The measurements were performed using a spectrofluorometer FP-8500 (manufactured by JASCO Corporation) equipped with an integrating sphere unit ILF-835. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light of 460 nm wavelength, which immediately quenched when the excitation light irradiation was stopped. The results are shown in Table 1.

[0087] [Example 2] A mixture was prepared by mixing organic compounds (0.051 g of tripotassium citrate monohydrate and 0.080 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence under irradiation with light of 460 nm wavelength, which immediately quenched when irradiation with the excitation light was stopped. The results are shown in Table 1.

[0088] [Example 3] A mixture was prepared by mixing organic compounds (0.051 g of tripotassium citrate monohydrate and 0.060 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light having a wavelength of 460 nm, and the fluorescence was immediately extinguished when the irradiation of the excitation light was stopped. The results are shown in Table 1.

[0089] [Example 4] A mixture was prepared by grinding and mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in a mortar. The powder mixture was placed in a 15 mL screw-cap test tube, and 0.3 mL of 10 mass% aqueous potassium hydroxide solution (amount of aqueous solution relative to the mass of the mixture: 129 mass%, amount of potassium relative to the sum of the moles of carbon and nitrogen atoms contained in the mixture: 9 mol%) was added. The tube was then sealed with a rubber gasket-equipped screw-cap cap. The tube was heated at 170°C for 1.5 hours while circulating nitrogen through it, synthesizing solid carbon quantum dots. The solid-state luminescence characteristics of the resulting carbon quantum dots were evaluated in the same manner as in Example 1. The resulting carbon quantum dots emitted yellow fluorescence under irradiation with 460 nm light, which immediately quenched when the excitation light irradiation was stopped. The results are shown in Table 1.

[0090] [Example 5] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 ml of an 8 mass % aqueous solution of sodium hydroxide (amount of aqueous solution relative to the mass of the mixture: 129 mass %, amount of sodium relative to the sum of the moles of carbon and nitrogen atoms contained in the mixture: 9 mol %) in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light at a wavelength of 460 nm, and the fluorescence was immediately extinguished when the irradiation of the excitation light was stopped. The results are shown in Table 1.

[0091] [Example 6] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 ml of a 2% by mass aqueous solution of sodium hydroxide (amount of aqueous solution relative to the mass of the mixture: 120% by mass, amount of sodium relative to the sum of the moles of carbon and nitrogen atoms contained in the mixture: 2 mol%) in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light having a wavelength of 460 nm, and the fluorescence was immediately extinguished when the irradiation with the excitation light was stopped. The results are shown in Table 1.

[0092] [Example 7] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 mL of an 8 mass % aqueous solution of sodium fluoride (amount of aqueous solution relative to the mass of the mixture: 125 mass %, amount of sodium relative to the sum of the moles of carbon and nitrogen atoms contained in the mixture: 9 mol %) in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light at a wavelength of 460 nm, and the fluorescence was immediately extinguished when the irradiation of the excitation light was stopped. The results are shown in Table 1.

[0093] [Example 8] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 mL of an 11% by mass aqueous solution of sodium chloride (amount of aqueous solution relative to the mass of the mixture: 128% by mass, amount of sodium relative to the sum of the moles of carbon and nitrogen atoms contained in the mixture: 9 mol%) in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light at a wavelength of 460 nm, and the fluorescence was immediately extinguished when the irradiation with the excitation light was stopped. The results are shown in Table 1.

[0094] [Example 9] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 ml of a 3 mass % aqueous calcium chloride solution (amount of aqueous solution relative to the mass of the mixture: 120 mass %, amount of calcium relative to the sum of the moles of carbon and nitrogen atoms contained in the mixture: 1 mol %) in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light at a wavelength of 460 nm, and the fluorescence was immediately extinguished when the excitation light irradiation was stopped. The results are shown in Table 1.

[0095] [Comparative Example 1] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted blue fluorescence under irradiation with light of 365 nm wavelength, which immediately quenched when irradiation with the excitation light was stopped. The results are shown in Table 1.

[0096] Comparative Example 2 A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 mL of water in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted blue fluorescence under irradiation with light of 365 nm wavelength, which immediately quenched when the irradiation with the excitation light was stopped. The results are shown in Table 1.

[0097] Comparative Example 3 A mixture was prepared by mixing organic compounds (0.051 g of tripotassium citrate monohydrate and 0.031 g of dicyandiamide) with 0.100 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light having a wavelength of 365 nm, and the fluorescence was immediately extinguished when the irradiation of the excitation light was stopped. The results are shown in Table 1.

[0098] Comparative Example 4 A mixture was prepared by mixing organic compounds (0.051 g of tripotassium citrate monohydrate and 0.120 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow fluorescence when irradiated with light having a wavelength of 365 nm, and the fluorescence was immediately extinguished when the irradiation of the excitation light was stopped. The results are shown in Table 1.

[0099] Comparative Example 5 A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.072 g of a boron compound (boric acid), and 0.3 ml of an 8% by mass aqueous solution of sodium hydroxide in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow-green fluorescence under irradiation with light of 365 nm wavelength, which immediately quenched when the irradiation with the excitation light was stopped. The results are shown in Table 1.

[0100] Comparative Example 6 A mixture was prepared by mixing organic compounds (0.051 g of tripotassium citrate monohydrate and 0.040 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The solid-state luminescence characteristics of the obtained carbon quantum dots were evaluated in the same manner as in Example 1. The obtained carbon quantum dots emitted yellow-green fluorescence under irradiation with light of 365 nm wavelength, which immediately quenched when the irradiation with the excitation light was stopped. The results are shown in Table 1.

[0101] Comparative Example 7 A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide), 0.144 g of a boron compound (boric acid), and 0.3 mL of water in the same manner as in Example 4. The mixture was heated in the same manner as in Example 4 to prepare solid carbon quantum dots. The resulting carbon quantum dots were mixed with 0.3 mL of an 8% by mass aqueous solution of sodium hydroxide and heated at 170°C for 0.5 hours while circulating nitrogen in a screw-cap test tube, yielding solid carbon quantum dots that had undergone alkali post-treatment. The solid-state luminescence characteristics of the resulting carbon quantum dots were evaluated in the same manner as in Example 1. The resulting carbon quantum dots emitted blue-green fluorescence under irradiation with 365 nm light, which immediately quenched when the excitation light irradiation was stopped. The results are shown in Table 1.

[0102] [Comparative Example 8] A mixture was prepared by mixing organic compounds (0.030 g of citric acid and 0.080 g of dicyandiamide) with 0.144 g of a boron compound (boric acid) in the same manner as in Example 1. The mixture was heated in the same manner as in Example 1 to prepare solid carbon quantum dots. The resulting carbon quantum dots were mixed with 0.3 mL of an 8% by mass aqueous solution of sodium hydroxide, and heated at 170°C for 0.5 hours while circulating nitrogen in a screw-cap test tube to obtain solid carbon quantum dots that had undergone alkali post-treatment. The solid-state luminescence characteristics of the resulting carbon quantum dots were evaluated in the same manner as in Example 1. The resulting carbon quantum dots emitted blue-green fluorescence under irradiation with light at a wavelength of 365 nm, which immediately quenched when the excitation light irradiation was stopped. The results are shown in Table 1.

[0103] [Table 1]

[0104] As shown in Table 1 above, when carbon quantum dots were prepared by mixing an organic compound containing nitrogen atoms and an alkali metal and / or alkaline earth metal with a boron compound and heating the mixture substantially without a solvent, the resulting carbon quantum dots had a maximum emission wavelength in the yellow wavelength range (520 nm to 580 nm) and a high emission quantum yield when irradiated with light at a wavelength of 460 nm (Examples 1 to 3, Comparative Examples 3 and 6). However, when the amount of boron compound relative to the total amount of organic compound and boron compound was less than 50% by mass (Comparative Example 4), carbon quantum dots with a low emission quantum yield when irradiated with light at a wavelength of 460 nm were obtained. Furthermore, when the amount of nitrogen atoms in the organic compound was less than 30% by mass relative to the organic compound (Comparative Examples 3 and 6), the emission intensity at a wavelength of 420 nm was higher than the emission intensity at a wavelength of 530 nm when irradiated with light at an excitation wavelength of 340 nm (emission intensity at 420 nm / emission intensity at 530 nm). It can be said that the carbon quantum dots emitted not only yellow fluorescence but also blue light. In contrast, when the amount of nitrogen atoms in the organic compound was within a predetermined range and the amount of the boron compound relative to the total amount of the organic compound and the boron compound was within a predetermined range, the above value was small, and it can be said that yellow light was mainly emitted (Examples 1 to 3).

[0105] On the other hand, when neither the organic compound nor the boron compound contained alkali metals and / or alkaline earth metals, nor was the compound mixed with an alkali metal aqueous solution, the maximum emission wavelength when irradiated with light at a wavelength of 460 nm was lower than the desired range, and the emission intensity at a wavelength of 420 nm (emission intensity at 420 nm / emission intensity at 530 nm) was also higher than the emission intensity at a wavelength of 530 nm when irradiated with light at an excitation wavelength of 340 nm (Comparative Example 1). Furthermore, when carbon quantum dots were prepared by mixing a small amount of water that did not contain alkali metals and / or alkaline earth metals, the emission intensity at a wavelength of 420 nm (emission intensity at 420 nm / emission intensity at 530 nm) was also higher than the emission intensity at a wavelength of 530 nm when irradiated with light at an excitation wavelength of 340 nm (Comparative Example 2).

[0106] In addition, when carbon quantum dots were prepared by preparing a mixture of an organic compound and a boron compound, followed by mixing with an aqueous solution containing an alkali metal and / or alkaline earth metal, and heating the mixture substantially without a solvent, carbon quantum dots having a maximum emission wavelength in the yellow wavelength region (wavelength region of 520 nm to 580 nm) when irradiated with light at a wavelength of 460 nm were obtained (Examples 4 to 9 and Comparative Example 5). However, when the amount of boron compound relative to the total amount of organic compound and boron compound was not within the specified range, the emission intensity at a wavelength of 420 nm was higher than the emission intensity at a wavelength of 530 nm when irradiated with light at an excitation wavelength of 340 nm (emission intensity at 420 nm / emission intensity at 530 nm) (Comparative Example 5). In contrast, when the amount of nitrogen atoms in the organic compound was within the specified range and the amount of boron compound relative to the total amount of organic compound and boron compound was within the specified range, the above value was small, and carbon quantum dots that mainly emitted yellow light were obtained (Examples 4 to 9).

[0107] Furthermore, even when carbon quantum dots were prepared, mixed with an aqueous sodium hydroxide solution, and then heat-treated, the emission intensity at a wavelength of 420 nm (emission intensity at 420 nm / emission intensity at 530 nm) was greater than the emission intensity at a wavelength of 530 nm when irradiated with light having an excitation wavelength of 340 nm (Comparative Examples 7 and 8).

[0108] This application claims priority from Japanese Patent Application No. 2022-044067, filed March 18, 2022, the entire contents of which are incorporated herein by reference. [Industrial Applicability]

[0109] According to the method for producing carbon quantum dots of the present invention, carbon quantum dots that emit light mainly at wavelengths of 520 nm to 580 nm when irradiated with blue light can be easily produced. The carbon quantum dots produced by this method can be used, for example, as a substitute for yellow phosphors in white LEDs. They can also be applied to various other products, such as various lighting materials and thermoelectric materials.

Claims

1. A method for producing carbon quantum dots that are solid at 25°C and 1 atmosphere, A step of preparing a mixture containing an organic compound having a reactive group and not containing a boron atom, and a boron compound; Heating the mixture in the presence of an alkali metal salt and / or an alkaline earth metal salt substantially without a solvent to prepare carbon quantum dots; and the amount of nitrogen atoms contained in the organic compound is 30% by mass or more and 50% by mass or less with respect to the total amount of the organic compound; the amount of the boron compound is 50% by mass or more and 80% by mass or less with respect to the total amount of the organic compound and the boron compound, the total amount of alkali metals and alkaline earth metals (excluding the amount of alkali metals and alkaline earth metals contained in the layered clay mineral) is 0.1 mol % or more and 30 mol % or less based on the sum of the number of moles of carbon atoms and nitrogen atoms contained in the organic compound and the boron compound; Method for producing carbon quantum dots.

2. At least a part of the organic compound and / or the boron compound is an alkali metal salt and / or an alkaline earth metal salt. The method for producing carbon quantum dots according to claim 1 .

3. At least a portion of the organic compound is an alkali metal salt, The organic compound includes either or both of trisodium citrate and tripotassium citrate. The method for producing carbon quantum dots according to claim 2 .

4. After the step of preparing the mixture, and before the step of preparing the carbon quantum dots, further comprising a step of mixing an aqueous solution of an alkali metal salt and / or an alkaline earth metal salt with the mixture; The amount of the aqueous solution is 200% by mass or less relative to the amount of the mixture. The method for producing carbon quantum dots according to claim 1 .

5. The aqueous solution is an aqueous solution of at least one salt selected from the group consisting of potassium hydroxide, sodium hydroxide, sodium fluoride, sodium chloride, and calcium chloride. The method for producing carbon quantum dots according to claim 4 .

6. The organic compound includes a nitrogen-containing organic compound containing a nitrogen atom in the molecule and a nitrogen-free organic compound not containing a nitrogen atom in the molecule. The method for producing carbon quantum dots according to claim 1 .

7. The nitrogen-containing organic compound is an amine compound. The method for producing carbon quantum dots according to claim 6 .

8. The carbon quantum dots are The maximum emission wavelength when irradiated with light having a wavelength of 460 nm is in the wavelength range of 520 nm to 580 nm, When irradiated with light of a wavelength of 340 nm, the emission intensity at a wavelength of 420 nm relative to the emission intensity at a wavelength of 530 nm is 0.25 or less, The internal quantum yield when irradiated with light having a wavelength of 460 nm is 50% or more. The method for producing carbon quantum dots according to any one of claims 1 to 7.

9. The carbon quantum dots are a phosphor material for white LEDs. The method for producing carbon quantum dots according to any one of claims 1 to 7.

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