Solid-state carbon quantum dot and method for producing carbon quantum dot composition

Abstract

The present invention addresses the problem of providing a solid-state carbon quantum dot which has a light emission wavelength in a low-wavelength region and which has high internal quantum efficiency. A solid-state carbon quantum dot that solves the above problem comprises a nitrogen atom and an aluminum atom, has an absorption peak in a range of 1290 cm-1 or more and 1310 cm-1 or less and / or a range of 1560 cm-1 or more and 1615 cm-1 or less in measurement by infrared spectroscopy, and has a maximum light emission wavelength of 300-450 nm.

Description

Method for producing solid carbon quantum dots and carbon quantum dot compositions

[0001] The present invention relates to a method for producing solid carbon quantum dots and carbon quantum dot compositions.

[0002] Carbon quantum dots are stable carbon-based particles with particle diameters ranging from several nanometers to several tens of nanometers. Some carbon quantum dots are known to exhibit excellent fluorescence properties. Such carbon quantum dots are required to have high internal quantum efficiency at any emission wavelength. Furthermore, there is a need for a simple method for producing such carbon quantum dots.

[0003] Here, Non-Patent Document 1 describes a method for preparing carbon quantum dots having an emission wavelength range of 450 nm to 550 nm. In the method described in this document, aluminum chloride, citric acid, and urea are mixed and heated with microwaves to prepare carbon quantum dots. Patent Document 1 also describes a method for preparing carbon quantum dots by mixing citric acid, aluminum acetonate, and aluminum hydroxide and heating the mixture.

[0004] Chinese Patent Application Publication No. 116285973

[0005] Yunyang Zhao, et al., “Aluminum-Based Surface Polymerization on Carbon Dots with Aggregation-Enhanced Luminescence”, J. Phys. Chem. Lett.,2021, 12, 4530-4536

[0006] However, the method described in Non-Patent Document 1 tends to produce carbon quantum dots with a long emission wavelength, making it difficult to prepare carbon quantum dots with a maximum emission wavelength on the short wavelength side. Furthermore, the method described in Patent Document 1 has the problem of low internal quantum efficiency of the carbon quantum dots produced.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a method for producing solid carbon quantum dots that have an emission wavelength in the short wavelength region and a high internal quantum efficiency, and a carbon quantum dot composition containing the same.

[0008] As a result of intensive research, the inventors have found that the above-mentioned problems can be solved by reacting an organic compound containing at least one nitrogen-containing organic compound as a raw material for solid carbon quantum dots in the presence of an aluminum-containing inorganic compound and a basic inorganic compound.

[0009] The present invention is directed to a compound containing nitrogen atoms and aluminum atoms, which has a peak at 1290 cm when measured by infrared spectroscopy. －１ 1310cm or more －１ The range below, and 1560 cm －１ Over 1615cm －１ Provided is a solid carbon quantum dot having an absorption peak in at least one of the following ranges and a maximum emission wavelength of 300 nm or more and 450 nm or less:

[0010] The present invention provides a method for producing a carbon quantum dot composition that contains carbon quantum dots in a solid state at 25°C and 1 atmosphere, the method comprising the steps of preparing a mixture containing an organic compound having a reactive group, an aluminum-containing inorganic compound, and a basic inorganic compound, and heating the mixture to prepare carbon quantum dots, wherein the organic compound contains one or more nitrogen-containing organic compounds that contain nitrogen in the molecule.

[0011] According to the present invention, there are provided solid carbon quantum dots that have an emission wavelength in the short wavelength region and high internal quantum efficiency, and a simple method for producing a carbon quantum dot composition containing the same.

[0012] Fig. 1 is a graph showing the IR spectra of the carbon quantum dots prepared in Examples 1 to 3. Fig. 2 is a graph showing the IR spectra of the carbon quantum dots prepared in Comparative Examples 1 to 4.

[0013] In this specification, a numerical range indicated by "to" means a numerical range including the numerical values ​​written before and after "to".

[0014] 1. Solid Carbon Quantum Dots The solid carbon quantum dots of the present invention (hereinafter also simply referred to as "carbon quantum dots") are solid substances at 25°C and 1 atmosphere, which contain nitrogen atoms and aluminum atoms in addition to the carbon that constitutes the quantum dots. The carbon quantum dots exhibit a peak at 1290 cm in infrared spectroscopy (hereinafter also referred to as "IR measurement"). －１ 1310cm or more －１ The range below, and 1560 cm －１ Over 1615cm －１ It has an absorption peak in at least one of the following ranges. The maximum emission wavelength is 300 nm or more and 450 nm or less. －１ 1310cm or more －１ The absorption peak in the following range indicates the presence of aromatic amines. －１ Over 1615cm －１ The absorption peak in the following range indicates the presence of a primary amine or a secondary amine. The carbon quantum dots of the present invention can also be prepared from raw materials that do not contain aromatic amines, and a purification step may be performed during production. In such cases, the purified carbon quantum dots may have an absorption peak of 1290 cm, which indicates the presence of aromatic amines. －１ 1310cm or more －１ It is important to have an absorption peak in the range below 1560 cm, which indicates the presence of primary or secondary amines. －１ Over 1615cm －１ From the viewpoint of improving quantum efficiency, it is particularly preferable that the compound has both an absorption peak in the following range and an absorption peak in the following range: In IR measurement, having an absorption peak in a specific range means having a peak top in that range.

[0015] Although the structure of the carbon quantum dots of the present invention is unclear, considering the presence of the above-mentioned amines and the emission wavelength, it is presumed that particles (quantum dots) whose main component is carbon exist at the center, and that aluminum ions surround them. More specifically, primary amines, secondary amines, or aromatic amines exist on the surface of the particles (quantum dots) whose main component is carbon, and these amines are in contact with ions containing aluminum (for example, tetrahydroxoaluminate ions ([Al(OH) ４ ] － )) is thought to be coordinated. In carbon quantum dots having such a structure, the presence of aluminum makes the carbon quantum dots less likely to aggregate. Therefore, it is thought that the average particle size can be maintained small, and the maximum emission wavelength falls within the range of 300 nm to 450 nm. Furthermore, by suppressing aggregation, variation in particle size is less likely to occur, and the internal quantum efficiency becomes very high.

[0016] The chemical composition of the carbon quantum dots of the present invention is not particularly limited as long as they contain carbon atoms, nitrogen atoms, and aluminum atoms. The amount of aluminum atoms in the carbon quantum dots may be 0.5% by mass or more, preferably 0.6% by mass or more and 5.5% by mass or less, and more preferably 0.7% by mass or more and 3.5% by mass or less, relative to the total amount of atoms constituting the carbon quantum dots. Furthermore, by adjusting the lower limit to 1.5% by mass or more, it is possible to maintain high internal quantum efficiency and external quantum efficiency. When the amount of aluminum atoms is 0.5% by mass or more, aluminum-containing ions are easily coordinated around the carbon quantum dots, thereby suppressing aggregation of the carbon quantum dots. The amount of aluminum atoms in the carbon quantum dots can be determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy) or the like.

[0017] On the other hand, the amount of nitrogen atoms in the carbon quantum dots is preferably 10% by mass or more and 20% by mass or less, and preferably 11% by mass or more and 18% by mass or less, relative to the total amount of atoms constituting the carbon quantum dots. When the amount of nitrogen atoms in the carbon quantum dots is 10% by mass or more, the amount of nitrogen atoms (amines) generated on the surface of the carbon-based particles (quantum dots) is likely to be sufficient. As a result, aluminum-containing ions are likely to be sufficiently coordinated on the surface side of the carbon quantum dots, which in turn suppresses aggregation of the carbon quantum dots. On the other hand, when the amount of nitrogen atoms is 20% by mass or less, the internal quantum efficiency is likely to be further increased.

[0018] The amount of carbon atoms in the carbon quantum dots is not particularly limited, but is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 30% by mass or less, relative to the total amount of atoms constituting the carbon quantum dots.When the amount of carbon atoms in the carbon quantum dots is within this range, it is easy to obtain desired light emission.The amount of nitrogen atoms and the amount of carbon atoms in the carbon quantum dots are determined by CNH elemental analysis.

[0019] The carbon quantum dots may further contain atoms other than carbon, nitrogen, and aluminum atoms. Examples of other atoms include hydrogen, oxygen, boron, phosphorus, sulfur, silicon, and fluorine atoms. The total amount of other atoms is preferably 0% by mass or more and 10% by mass or less, and more preferably 0% by mass or more and 5% by mass or less, relative to the total amount of carbon quantum dots. The total amount of other atoms is determined by subtracting the amount of aluminum atoms, the amount of nitrogen atoms, and the amount of carbon atoms from the mass of the carbon quantum dots. The type and amount of other atoms can be determined by X-ray photoelectron spectroscopy (XPS measurement) or the like.

[0020] The average particle diameter of the carbon quantum dots measured by observation 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. The average particle diameter of the carbon quantum dots makes it easier to satisfy the maximum emission wavelength, and the internal quantum efficiency is more likely to be increased. The average particle diameter of the carbon quantum dots is a value calculated by measuring the diameters of three or more carbon quantum dots with an AFM and calculating the average value of these diameters.

[0021] The maximum emission wavelength of the carbon quantum dots may be in the range of 300 nm to 450 nm, but is preferably 360 nm to 430 nm. Furthermore, by adjusting the upper limit to a wavelength lower than 400 nm, the carbon quantum dots can be used in applications that use ultraviolet light, such as black lights for identifying counterfeit prevention and UV curing lights for photocurable resins, and by adjusting the lower limit to 400 nm or more, the carbon quantum dots can be used in applications such as fluorescent materials for invisible ink LEDs for counterfeit prevention. In this specification, the maximum emission wavelength refers to the emission wavelength at which the internal quantum efficiency is maximized when irradiated with excitation light at varying wavelengths. Furthermore, light with a wavelength of 200 nm to 400 nm can typically be used as excitation light for exciting the carbon quantum dots. When irradiated with excitation light, the light emitted by the carbon quantum dots is mainly fluorescence. The maximum emission wavelength can be determined by obtaining the spectral distribution when irradiated with light of a predetermined wavelength using a spectrofluorometer or the like equipped with an integrating sphere unit.

[0022] Furthermore, the carbon quantum dots can have an internal quantum efficiency of 20% or more, or even 25% or more, at the maximum emission wavelength. The internal quantum efficiency can also be determined by obtaining the spectral distribution when irradiated with excitation light using a spectrofluorometer or the like equipped with an integrating sphere unit.

[0023] 2. Method for Producing Carbon Quantum Dot Composition The carbon quantum dot composition containing the above-described carbon quantum dots can be produced, for example, by the following method, however, the method for producing the carbon quantum dots is not limited to this method.

[0024] The method includes a step of preparing a mixture containing an organic compound having a reactive group, an aluminum-containing inorganic compound, and a basic inorganic compound (mixture preparation step), and a step of heating the mixture to prepare carbon quantum dots (heating step). If necessary, other steps may be included, and as described below, a purification step may be further included. The method may also include a step of mixing the obtained carbon quantum dots with other components as necessary. The other components to be mixed with the carbon quantum dots are appropriately selected depending on the application, and examples include, but are not limited to, known resins and known solvents.

[0025] Mixture Preparation Step In the mixture preparation step, a mixture containing an organic compound, an aluminum-containing inorganic compound, and a basic inorganic compound is prepared.

[0026] The organic compound is a compound having a reactive group and may be composed of only one type of compound or two or more types of compounds. However, the organic compound includes one or more nitrogen-containing organic compounds that contain nitrogen in the molecule and have a reactive group. That is, the organic compound may be composed of only a nitrogen-containing organic compound, or may be composed of a nitrogen-containing organic compound and a compound that does not contain a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-free organic compound"). In this specification, the term "reactive group" refers to a group that is bonded to a carbon atom in an organic compound and causes a polycondensation reaction between organic compounds in the heating process described below, and contributes 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.

[0027] Here, the nitrogen-containing organic compound may be any compound having a nitrogen atom and a reactive group. The nitrogen-containing organic compound may or may not contain an amine. Even if the nitrogen-containing organic compound itself does not contain an amine, it is believed that the above-mentioned primary amine, secondary amine, or aromatic amine is generated from the nitrogen in the nitrogen-containing organic compound during the heating step described below. Here, examples of the nitrogen-containing organic compound include amine compounds, nitrogen-containing sugars, imidazoles, triazines, triazoles, triazenes, guanidines, and oximes, as well as salts of these with alkali metals and / or alkaline earth metals. As the organic compound, only one of these may be used, or two or more may be used. Furthermore, these compounds may be solid or liquid at room temperature.

[0028] Examples of the amine compound 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. Examples of amino acids 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.

[0029] 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.

[0030] Of the nitrogen-containing organic compounds listed above, amine compounds are preferred from the viewpoints of availability, reactivity in the heating step described below, and ease of arranging amines on the surfaces of particles mainly composed of carbon.

[0031] On the other hand, examples of nitrogen-free organic compounds include carboxylic acids, alcohols, phenols, sugars, and salts thereof with alkali metals and / or alkaline earth metals. The organic compound may contain only one of these, or may contain two or more of these. Furthermore, these compounds may be solid or liquid at room temperature.

[0032] The carboxylic acid may be a compound having one or more carboxy groups in the molecule (excluding those corresponding to 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.

[0033] 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 corresponding 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.

[0034] The phenol may be any compound 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.

[0035] 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.

[0036] 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, an alkali metal salt thereof, or an alkaline earth metal salt thereof.

[0037] The total amount of the organic compounds (nitrogen-containing organic compounds and nitrogen-free organic compounds) in the mixture prepared in the mixture preparation step is preferably 10% by mass or more and 40% by mass or less, more preferably 20% by mass or more and 30% by mass or less, relative to the total amount of the organic compounds, the aluminum-containing inorganic compounds, and the basic inorganic compounds. When the amount of the organic compounds relative to the total amount of the organic compounds, the aluminum-containing inorganic compounds, and the basic inorganic compounds is within this range, the production efficiency of carbon quantum dots is further improved. On the other hand, when the amount is below the above lower limit, the amount of organic compounds in the mixture tends to be relatively small, making it difficult to form carbon quantum dots and reducing the internal quantum efficiency.

[0038] In addition, the amount of the nitrogen-containing organic compound in the organic compound is appropriately selected depending on its type and molecular weight.For example, it is preferable to adjust the amount of the nitrogen-containing organic compound so that the amount of nitrogen atoms is 3% by mass or more and 60% by mass or less, and more preferably 20% by mass or more and 50% by mass or less, relative to the total amount of the organic compound.When the amount of nitrogen atoms in the organic compound is 20% by mass or more and 50% by mass or less, amines are easily generated appropriately around the particles (quantum dots) mainly composed of carbon.

[0039] On the other hand, an aluminum-containing inorganic compound is an inorganic compound containing an aluminum atom. In this specification, an aluminum-containing inorganic compound is a compound that contains an aluminum atom but does not contain a carbon atom. The aluminum atom derived from the aluminum-containing inorganic compound generates an aluminum-containing ion that can coordinate to an amine in a basic aqueous solution. An example of an aluminum-containing ion that can coordinate to an amine is a tetrahydroxoaluminate ion ([Al(OH) ４ ] － In the mixture preparation step, only one aluminum-containing inorganic compound may be used, or two or more aluminum-containing inorganic compounds may be used.

[0040] Examples of the aluminum-containing inorganic compound include aluminum hydroxide, aluminum silicate, aluminum oxide, aluminum nitrate, etc. Among these, aluminum hydroxide and aluminum silicate are preferred in terms of availability, etc.

[0041] The total amount of the aluminum-containing inorganic compound in the mixture prepared in the mixture preparation step is preferably 50% by mass or more and 80% by mass or less, more preferably 60% by mass or more and 70% by mass or less, relative to the total amount of solids in the mixture. If the amount of the aluminum-containing inorganic compound in the mixture is equal to or greater than the lower limit, the outer periphery of the resulting carbon quantum dots is likely to be covered with aluminum (ions containing aluminum). On the other hand, if the amount of the aluminum-containing inorganic compound is equal to or less than the upper limit, the amount of the organic compound becomes relatively large, further improving the production efficiency of carbon quantum dots.

[0042] Here, when the amount (number of moles) of nitrogen atoms contained in the organic compound is taken as 1, the amount (number of moles) of aluminum atoms contained in the aluminum-containing inorganic compound is preferably 0.5 or more and 10.0 or less, and more preferably 1.0 or more and 7.5 or less. When the ratio of the amount of nitrogen atoms to the amount of aluminum atoms is within this range, coordination of the amine and the aluminum-containing ion becomes easier.

[0043] The basic inorganic compound in this specification refers to an inorganic compound that is soluble in water and produces a basic aqueous solution. In the mixture preparation step, only one basic inorganic compound or two or more basic inorganic compounds may be used.

[0044] Examples of basic inorganic compounds include sodium hydroxide, potassium hydroxide, tripotassium phosphate, sodium carbonate, calcium hydroxide, etc. Among these, sodium hydroxide and tripotassium phosphate are preferred as basic inorganic compounds from the viewpoints of availability and ease of handling.

[0045] The amount of the basic inorganic compound in the mixture prepared in the mixture preparation step is preferably 2% by mass or more and 15% by mass or less, more preferably 5% by mass or more and 10% by mass or less, relative to the total amount of solids in the mixture. Furthermore, it is preferably 10 parts by mass or more and 60 parts by mass or less, more preferably 22 parts by mass or more and 40 parts by mass or less, relative to 100 parts by mass of the organic compound. When the amount of the basic inorganic compound relative to the amount of the organic compound is below the upper limit, the organic compound has a sufficient concentration to cause a dehydration condensation reaction, making it possible to efficiently prepare carbon quantum dots. On the other hand, when the amount of the basic inorganic compound relative to the amount of the organic compound is above the lower limit, aluminum-containing ions can be sufficiently generated, even if the organic compound is an acidic substance.

[0046] The amount of the basic inorganic compound in the mixture is preferably 3 to 25 parts by mass, more preferably 6 to 15 parts by mass, relative to 100 parts by mass of the aluminum-containing inorganic compound. When the amount of the basic inorganic compound relative to the amount of the aluminum-containing inorganic compound is within the above range, aluminum-containing ions can be generated more efficiently.

[0047] The mixture may further contain components other than the organic compound, the aluminum-containing inorganic compound, and the basic inorganic compound, provided that the purpose and effect of the present invention are not impaired. Examples of such components include solvents; compounds containing boron, phosphorus, sulfur, silicon, and / or fluorine but not having the above-mentioned reactive groups; layered clay minerals, etc.

[0048] The solvent is not particularly limited as long as it can dissolve the basic inorganic compound and generate aluminum-containing ions, but water is particularly preferred. The solvent may contain a water-miscible organic solvent (e.g., methanol, ethanol, etc.) as long as the purpose and effect of the present invention are not impaired. In the mixture preparation step, the basic inorganic compound and other components may be dissolved in water (solvent) in advance, and then mixed with the other components.

[0049] When water is used as a solvent in the mixture preparation step, the amount is preferably 1 part by weight to 200 parts by weight, more preferably 50 parts by weight to 100 parts by weight, based on the total amount of 100 parts by weight of the organic compound, the aluminum-containing inorganic compound, and the basic inorganic compound. In the present invention, the heating step described below may be performed in the presence of water. However, carbon quantum dots can be prepared more efficiently if there is almost no water in the system at the start of the reaction of the organic compounds (condensation reaction of the organic compounds). Furthermore, by reacting the organic compounds in the absence of water in the system, the resulting carbon quantum dots are less likely to aggregate. Note that even if water (solvent) is not present at the start of the reaction of the organic compounds, water is usually generated by dehydration condensation during the reaction of the organic compounds. Therefore, it is believed that the basic inorganic compound dissolves in the water to generate aluminum-containing ions.

[0050] Examples of compounds containing boron, phosphorus, sulfur, silicon, and / or fluorine but not having the reactive group include boron, phosphorus, sulfur, silicon, or known inorganic and organic compounds containing these. Examples of layered clay minerals include smectite, layered double hydroxide, kaolinite, and mica. The total amount of these compounds is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, even more preferably 5 parts by mass or less, and particularly preferably 3 parts by mass or less, per 100 parts by mass of the total amount of the organic compound, the aluminum-containing inorganic compound, and the basic inorganic compound.

[0051] The method for mixing the mixture is not particularly limited as long as it allows the organic compound, the aluminum-containing inorganic compound, and the basic inorganic compound to be mixed uniformly. For example, the organic compound and the aluminum-containing inorganic compound may be mixed first, and then an aqueous solution containing the basic inorganic compound may be mixed. In this case, the organic compound and the aluminum-containing inorganic compound may be mixed, for example, by grinding them in a mortar or by pulverizing them using a ball mill or the like. Alternatively, the organic compound or the aluminum-containing inorganic compound may be dissolved, mixed, or dispersed in water or a solution containing the basic inorganic compound to prepare a mixed solution.

[0052] In the heating step, the mixture prepared in the mixture preparation step is heated to obtain carbon quantum dots. The heating temperature is not particularly limited as long as it is possible to condense the organic compound and to coordinate aluminum around the particles mainly composed of carbon. The heating temperature is preferably 100°C or higher and 300°C or lower, and more preferably 150°C or higher and 200°C or lower.

[0053] The heating method is not particularly limited as long as it can heat the mixture to the desired temperature. For example, heating with a heater is preferable. Alternatively, heating may be performed in a non-oxidizing atmosphere while circulating an inert gas such as nitrogen. The holding time at the heating temperature is preferably 0.01 to 45 hours, more preferably 0.1 to 30 hours, and even more preferably 0.5 to 10 hours. The particle size of the resulting carbon quantum dots, and therefore the emission wavelength, can be adjusted by the heating time.

[0054] As mentioned above, the mixture may contain a solvent such as water at the heating temperature of the mixture, but it is more preferable to heat the mixture substantially without a solvent. In this specification, "substantially solvent-free" means that the amount of solvent (water) 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. As mentioned above, even if the water added as a solvent evaporates, water is generated by the reaction of the organic compound, which is thought to ionize aluminum and interact with amines derived from the organic compound. Note that ionization of aluminum during the reaction of the organic compound is thought to facilitate the reaction of the organic compound, thereby facilitating the production of quantum dots mainly composed of carbon and the production of amines. Here, the compound serving as the raw material for carbon quantum dots, i.e., the organic compound or the like, may be in a liquid state at the heating temperature.

[0055] Purification step: If necessary, a purification step may be carried out to remove unreacted organic compounds, excess aluminum-containing inorganic compounds, etc. The method for this step is not particularly limited, and can be carried out, for example, by the following method.

[0056] The composition after the heating step is dispersed in water and washed. At this time, the carbon quantum dots are dissolved or dispersed in the water. Meanwhile, the excess aluminum-containing inorganic compound precipitates in the near-neutral water. Therefore, the excess aluminum-containing inorganic compound can be separated by washing with water one or more times.

[0057] Furthermore, the liquid in which the carbon quantum dots are dissolved or dispersed is dried to obtain a solidified product. This may then be washed with an organic solvent. The organic solvent used for washing may be composed of one type of compound or two or more types of compounds. The organic solvent may be a polar solvent, a non-polar solvent, or a mixture thereof. In order to remove unreacted organic compounds and side reaction components, it is more preferable that the solvent contains at least one type of polar solvent.

[0058] Specific examples of polar solvents include methanol, ethanol, butanol, 1-propanol, dichloromethane, acetone, ethyl acetate, and acetonitrile. Among these, it is preferable to use a mixed organic solvent in which methanol and dichloromethane are mixed at a ratio of 1:1. After washing with the organic solvent, the organic solvent may be removed by heating to a temperature of 40°C or higher and 120°C or lower, preferably 40°C or higher and 100°C or lower, and more preferably 50°C or higher and 80°C or lower. The heating temperature can be set appropriately depending on the solvent used in the purification. In terms of preventing structural changes to the carbon quantum dots during the purification process, the upper limit is preferably lower than the temperature used in the heating process. Furthermore, at this time, pressure reduction or the like may be performed as necessary.

[0059] (Uses of Carbon Quantum Dots and Carbon Quantum Dot Compositions) The above-mentioned carbon quantum dots and carbon quantum dot compositions containing them have a maximum emission wavelength in the blue region and high internal quantum efficiency. Therefore, the carbon quantum dots can be used for a variety of 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.

[0060] 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.

[0061] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0062] [Example 1] (1) Preparation of Raw Material Mixture Raw materials were mixed in the composition ratio shown in Table 1 below to obtain a mixture. Specifically, 30 parts by mass of citric acid, 80 parts by mass of dicyandiamide, and 300 parts by mass of aluminum hydroxide were mixed while being ground in a mortar to prepare a mixture. At this time, the granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size, which were then uniformly mixed. Furthermore, 330 parts by mass of an aqueous sodium hydroxide solution (concentration 7.7% by mass) (amount of sodium hydroxide: 25 parts by mass) was mixed to obtain a mixture.

[0063] (2) Heating the mixture The mixture and the stirring bar were placed in a screw-top test tube, which was then sealed with a rubber gasket. While nitrogen was circulating in the test tube, the stirring bar was rotated and stirred by applying magnetic force from a stirrer. The mixture was then heated at 170°C for 1.5 hours to synthesize a solid phosphor composition.

[0064] (3) Solvent Washing and Purification Step The resulting product was dispersed in water and washed with water, and solid precipitates (mainly aluminum hydroxide) were removed by filtration. The resulting filtrate was dried to obtain an intermediate washed product (carbon quantum dots and unreacted materials). This powder was further dispersed in a 1:1 mixed solvent of methanol and dichloromethane to obtain a dispersion. The dispersion was then filtered to recover the insoluble solid precipitate, and the solvent and soluble components were removed. The above solvent washing was repeated three times. The insoluble solid precipitate was then dried under reduced pressure at 60°C for 6 hours to remove the residual solvent, yielding solid carbon quantum dots.

[0065] (4) Confirmation of Light Emission State The obtained solid carbon quantum dots were dispersed, sandwiched between KBr plates, and pressed to prepare measurement samples. The emission wavelength, external quantum efficiency, and internal quantum efficiency were determined when these were irradiated with excitation light. The wavelength of the excitation light was determined as the wavelength at which the internal quantum efficiency of the sample was maximized (maximum excitation wavelength), and the emission wavelength at this time was determined as the maximum emission wavelength. The above measurements were performed using a spectrofluorometer FP-8500 (manufactured by JASCO Corporation) equipped with an integrating sphere unit ILF-835. The solid carbon quantum dots emitted blue fluorescence when irradiated with light at a wavelength of 320 nm, which immediately ceased to emit when the excitation light irradiation was stopped. The maximum emission wavelength and the excitation wavelength at this time are shown in Table 2.

[0066] (5) IR Analysis The solid carbon quantum dots were analyzed by IR to obtain an absorption spectrum. The measurement was carried out using an FT / IR-4100 type A (JASCO Corporation). The measurement conditions were set as follows: Detector: TGS, Number of accumulations: 20, Resolution: 4 cm －１ Zero filling: On Abodization: Cosine Gain: 8 Aperture: 7.1 mm Scan speed: 2 mm / sec Filter: 30000 Hz 1290 cm of the absorption spectrum －１ 1310cm or more －１ The range below, and 1560 cm －１ Over 1615cm －１ It was confirmed whether there were any downward convex peaks in the following ranges. For the carbon quantum dots, peaks were confirmed in both of these ranges. The results are shown in Table 2.

[0067] (6) Nitrogen Atom Content Measurement (CHN Element Analysis) The CHN element content of the solid carbon quantum dots was measured. The measurement was performed using a PE2400 Series II (manufactured by PerkinElmer). The solid carbon quantum dots were placed in a tin sample bottle and then burned with oxygen in a combustion tube for a certain period of time. The generated H ２ O, CO ２、 NO ｘ is sent into the reduction tube, and H ２ O, CO ２ were adsorbed onto the column in this order.ｘ is produced by reducing copper in the reduction tube. ２ After that, N ２ was detected by a TCD (thermal conductivity detector). ２ The column on which CO was adsorbed was heated, and the desorbed CO ２ The gas was detected by TCD. ２ The column on which O was adsorbed was heated, and the desorbed H ２ O was detected using a TCD. The nitrogen atom content was determined from the peak area obtained using a calibration curve prepared in advance from a standard sample (acetanilide). The amount of aluminum atoms in the solid carbon quantum dots was also taken into consideration in the calculation. The lower limit of quantification was calculated to be 0.5 mass%, and if the content was lower than this, it was determined that no nitrogen atoms were present. The results are shown in Table 2.

[0068] (7) Measurement of Aluminum Atom Content (ICP-AES Analysis) 2 mg of the solid carbon quantum was added with 7 ml of concentrated nitric acid and 1 ml of 30% hydrogen peroxide, and microwave decomposition was performed at 210 ° C. for 15 minutes (a total of 45 minutes, including the heating time (30 minutes)). The microwave decomposition was performed using a microwave decomposition system BLADE (manufactured by CEM). The resulting decomposition solution was diluted to 100 mL and subjected to ICP-AES measurement using an ICP optical emission spectrometer SPS3520DD (manufactured by Hitachi High-Tech Science Corporation) to identify the amount of aluminum atoms (internal standard Y (yttrium) 10 ppm added). The lower limit of quantification was calculated to be 0.1% by mass, and if the content was lower than that, it was determined that aluminum atoms were not contained. The results are shown in Table 2.

[0069] Example 2: A mixture was prepared by mixing raw materials in the composition ratios shown in Table 1 below. Specifically, 30 parts by weight of citric acid, 80 parts by weight of dicyandiamide, and 300 parts by weight of aluminum hydroxide were mixed in a mortar while being ground. Granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size and uniformly mixed. Furthermore, 340 parts by weight of an aqueous solution of tripotassium phosphate (concentration 12.8% by weight) (amount of tripotassium phosphate: 44 parts by weight) was added to obtain a mixture. Solid carbon quantum dots were then prepared by carrying out the heating synthesis process and the cleaning and purification process in the same manner as in Example 1. The luminescence characteristics of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. IR analysis, nitrogen atom content measurement, and aluminum atom content measurement were also performed. The results are shown in Table 2.

[0070] Example 3: A mixture was prepared by mixing raw materials in the composition ratios shown in Table 1 below. Specifically, 30 parts by weight of citric acid, 80 parts by weight of dicyandiamide, and 300 parts by weight of aluminum silicate were mixed in a mortar while being ground. Granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size and uniformly mixed. Furthermore, 330 parts by weight of aqueous sodium hydroxide solution (concentration 7.7% by weight) (sodium hydroxide content: 25 parts by weight) was mixed to obtain a mixture. Solid carbon quantum dots were then prepared by carrying out the heating synthesis process and cleaning / purification process in the same manner as in Example 1. The luminescence characteristics of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. IR analysis, nitrogen atom content measurement, and aluminum atom content measurement were also performed. The results are shown in Table 2.

[0071] Comparative Example 1: A mixture was obtained by mixing raw materials in the composition ratios shown in Table 1 below. Specifically, 30 parts by mass of citric acid, 80 parts by mass of dicyandiamide, and 300 parts by mass of aluminum hydroxide were mixed while being ground in a mortar to prepare a mixture. Granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size, which were then uniformly mixed. 300 parts by mass of ion-exchanged water was then added to obtain a mixture. Solid carbon quantum dots were then prepared by carrying out the heating synthesis process and the cleaning and purification process in the same manner as in Example 1. The luminescence characteristics of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. IR analysis, nitrogen atom content measurement, and aluminum atom content measurement were also performed. The results are shown in Table 2.

[0072] Comparative Example 2 A mixture was obtained by mixing raw materials in the composition ratios shown in Table 1 below. Specifically, 30 parts by mass of phloroglucinol dihydrate, 80 parts by mass of L-glutamic acid, and 300 parts by mass of aluminum hydroxide were mixed while being ground in a mortar to prepare a mixture. Granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size, which were then uniformly mixed. Furthermore, 300 parts by mass of ion-exchanged water was added to obtain a mixture. Then, similar to Example 1, a heating synthesis process and a cleaning and purification process were carried out to prepare solid carbon quantum dots. The luminescence properties of the obtained solid carbon quantum dots were evaluated similar to Example 1. IR analysis, nitrogen atom content measurement, and aluminum atom content measurement were also performed. The results are shown in Table 2.

[0073] Comparative Example 3: A mixture was prepared by mixing raw materials according to the composition ratios shown in Table 1 below. Specifically, 30 parts by weight of citric acid and 80 parts by weight of dicyandiamide were mixed in a mortar while being ground. Granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size and then uniformly mixed. Furthermore, 340 parts by weight of an aqueous solution of tripotassium phosphate (concentration 12.8% by weight) (amount of tripotassium phosphate: 44 parts by weight) was added to obtain a mixture. Solid carbon quantum dots were then prepared by carrying out the heating synthesis process and the cleaning and purification process in the same manner as in Example 1. The luminescence characteristics of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. IR analysis, nitrogen atom content measurement, and aluminum atom content measurement were also performed. The results are shown in Table 2.

[0074] Comparative Example 4: A mixture was obtained by mixing raw materials in the composition ratios shown in Table 1 below. Specifically, 20 parts by mass of citric acid, 10 parts by mass of aluminum hydroxide, and 1 part by mass of aluminum acetylacetonate were mixed in a mortar while being ground. Granules of each reagent were thoroughly crushed to obtain powders of approximately uniform particle size, which were then uniformly mixed. 1,000 parts by mass of ion-exchanged water was then added to obtain a mixture. Solid carbon quantum dots were then prepared by carrying out the heating synthesis process and the cleaning and purification process in the same manner as in Example 1. The luminescence characteristics of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. IR analysis, nitrogen atom content measurement, and aluminum atom content measurement were also performed. The results are shown in Table 2.

[0075]

[0076]

[0077] As shown in Tables 1 and 2 above, in Examples 1 to 3 in which carbon quantum dots were prepared by mixing an organic compound including a nitrogen-containing organic compound, an aluminum-containing inorganic compound, and a basic inorganic compound, the obtained carbon quantum dots contained aluminum atoms and nitrogen atoms. Furthermore, in IR measurement, the carbon quantum dots had a peak at 1290 cm －１ 1310cm or more －１ range below 1560 cm (aromatic amines) and－１ Over 1615cm －１ Absorption peaks were observed in the following range (primary amines and / or secondary amines). The IR spectra are shown in Figure 1. Furthermore, the maximum emission wavelengths of all carbon quantum dots were in the range of 300 nm to 450 nm. Furthermore, these carbon quantum dots had internal quantum efficiencies of 27% or higher.

[0078] On the other hand, when carbon quantum dots were prepared by mixing organic compounds and aluminum-containing inorganic compounds without using basic inorganic compounds (Comparative Examples 1 and 2), no absorption peaks derived from aromatic amines, primary amines, and / or secondary amines were observed in IR measurements. One possible reason for this is that the absence of basic inorganic compounds prevented the generation of aluminum-containing ions, and the reaction of the organic compounds did not proceed sufficiently. Furthermore, the maximum emission wavelength of the resulting carbon quantum dots exceeded 520 nm, and their internal quantum efficiency was also very low.

[0079] In Comparative Example 3, which did not use an aluminum-containing inorganic compound, aluminum atoms were not detected in the carbon quantum dots. Furthermore, IR measurements did not reveal any absorption peaks derived from aromatic amines, primary amines, and / or secondary amines. Since aluminum ions were not generated, it is believed that the reaction of the organic compounds did not proceed sufficiently. Furthermore, the maximum emission wavelength was 565 nm, and the internal quantum efficiency was also low.

[0080] In Comparative Example 4, in which the organic compound did not contain a nitrogen-containing organic compound and the carbon quantum dots were prepared without using a basic inorganic compound, the maximum emission wavelength was low, but the internal quantum yield was 0.59, which was very low.

[0081] This application claims priority from Japanese Patent Application No. 2024-097450, filed June 17, 2024. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0082] The carbon quantum dots of the present invention emit light in the blue region and have good internal quantum efficiency, making them suitable for a variety of products, including various lighting materials and thermoelectric materials.

Claims

1. Contains nitrogen and aluminum atoms, and has an IR spectrum of 1290 cm －１ 1310cm or more －１ The following ranges, and 1560 cm －１ Over 1615cm －１ A solid carbon quantum dot having an absorption peak in at least one of the following ranges, and a maximum emission wavelength of 300 nm or more and 450 nm or less:

2. The solid carbon quantum dot according to claim 1, wherein the nitrogen atom content is 10% by mass or more and 20% by mass or less.

3. The solid carbon quantum dots according to claim 1, wherein the content of aluminum atoms is 0.5 mass % or more.

4. A method for producing a carbon quantum dot composition containing solid carbon quantum dots at 25°C and 1 atmosphere, comprising: preparing a mixture containing an organic compound having a reactive group, an aluminum-containing inorganic compound, and a basic inorganic compound; and heating the mixture to prepare carbon quantum dots, wherein the organic compound contains one or more nitrogen-containing organic compounds that contain nitrogen in the molecule.

5. The method for producing a carbon quantum dot composition according to claim 4, wherein the mixture is heated to a temperature of 100°C or higher and 300°C or lower in the step of preparing the carbon quantum dots.

6. The method for producing a carbon quantum dot composition according to claim 4, wherein the mixture further comprises water.

7. The method for producing a carbon quantum dot composition according to claim 4, wherein the amount of the basic inorganic compound in the mixture is 10 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the organic compound.

8. The method for producing a carbon quantum dot composition according to any one of claims 4 to 7, wherein the carbon quantum dots have a maximum emission wavelength of 300 nm or more and 450 nm or less, and the internal quantum efficiency of the carbon quantum dots is 20% or more.

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