Method for producing niobium oxide nanoparticles and their applications

Abstract

This invention provides a method for producing niobium oxide nanoparticles without using halogen-containing additives, and a technology for utilizing them. [Solution] The following steps (A) to (C): (A) A step of dissolving the niobium complex in a reducing solvent to prepare a precursor solution, (B) A step of mixing the precursor solution with a compound having one or more functional groups selected from ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups to obtain a mixed solution. (C) A method for producing niobium oxide nanoparticles, comprising the step of heating the mixture at 140°C or higher.

Description

[Technical Field]

[0001] This invention relates to a method for producing niobium oxide nanoparticles and to the use of such nanoparticles. [Background technology]

[0002] In recent years, the application of nanoparticles such as quantum dots to light-emitting materials has led to expectations of the creation of novel light-emitting materials (for example, phosphors, electroluminescent (EL) materials, and semiconductor lasers).

[0003] At the nanoscale, where the particle size is 1 to 10 nm and the material consists of approximately 10 to 50 nanoparticles, the band gap can be adjusted by the crystal size of the nanoparticles. Therefore, characteristic luminescence properties that depend not only on the material and composition but also on the particle size can be obtained.

[0004] In other words, the above nanoparticles exhibit an electronic state different from the band structure based on the inherent crystal structure of the normal bulk state, and become quantum dots in which electrons (excitons) are confined in a three-dimensional discretized state.

[0005] Previously developed quantum dot cores have used materials such as compounds containing cadmium metal, which is a toxic substance. Therefore, from the perspective of applying these devices to mass-produced consumer electronics, there has been a demand for the development of non-toxic quantum dots or nanoparticles that do not contain harmful metals.

[0006] Niobium is mainly used as a raw material for the aforementioned low-toxicity and readily available nanoparticles. For example, Patent Document 1 discloses niobium oxide nanoparticles obtained by dissolving a niobium complex in a reducing solvent to obtain a precursor solution, then adding a halogen-containing additive to the solution and heating it. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2022-028630 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the niobium oxide nanoparticles described in Patent Document 1 have the problem of being undesirable in terms of environmental impact because halogen-containing additives are used during manufacturing. In addition, although the aggregation of nanoparticles can be suppressed in the method described in Patent Document 1, it is difficult to control the average particle size of the niobium oxide nanoparticles. [Means for solving the problem]

[0009] To solve the aforementioned problems, the inventors conducted diligent research. As a result, they discovered that by heating a precursor solution containing a niobium complex, a reducing solvent, and a compound containing a specific functional group at 140°C or higher, the average particle size of niobium oxide nanoparticles can be controlled to 100 nm or less without using the aforementioned additives, thus completing the present invention. That is, the present invention includes the following configuration. <1> The following steps (A) to (C): (A) A step of dissolving the niobium complex in a reducing solvent to prepare a precursor solution, (B) A step of mixing the precursor solution with a compound having one or more functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups to obtain a mixed solution. (C) A method for producing niobium oxide nanoparticles, comprising the step of heating the mixture at 140°C or higher. <2> The niobium complex is one or more complexes selected from the group consisting of niobium oxalate, niobium acetate, acetylacetonate niobium, niobium pentachloride, niobium pentabromide, niobium pentaethoxide, and salts thereof. <1> The manufacturing method described above. <3> The reducing solvent is one or more selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetamide, and N-vinylformamide. <1> or <2> The manufacturing method described above. <4> The concentration of the niobium complex in the precursor solution is 0.01 M or higher. <1> ~ <3> A manufacturing method described in any one of the following. <5> The compound to be mixed with the precursor solution in step (B) is one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polyaspartic acid, thioctic acid, 1-dodecanethiol, dodecylamine, and 11-mercaptoundecanoic acid. <1> ~ <4> A manufacturing method described in any one of the following. <6> The viscosity-average molecular weight of the compound mixed with the precursor solution in step (B) is between 10,000 and 500,000. <1> ~ <5> A manufacturing method described in any one of the following. <7> Fourier infrared spectroscopy detected at least one of the following peaks (i) to (iii): (i) 1600–1750 cm⁻¹ -1 (ii) A peak indicating C=O stretching vibration present in (ii) 400~800cm -1 (iii) 400~600 cm² -1 Niobium oxide nanoparticles exhibiting a peak indicating vibrations due to bonding between niobium and sulfur atoms, and in the spectrum measured by X-ray photoelectron spectroscopy (XPS), the peak representing the 3d 5 / 2 orbital, which indicates the electronic state of niobium, is shifted to an energy side of 0.1 eV or more compared to that of the niobium complex. <8> The compound comprises one or more functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups. <7> Niobium oxide nanoparticles as described above. <9> <7> or <8> A light-emitting material containing niobium oxide nanoparticles as described above. <10> <9> A light-emitting element containing the light-emitting material described above. [Effects of the Invention]

[0010] According to one aspect of the present invention, without using an additive containing halogen, the average particle size of niobium oxide nanoparticles can be controlled, and single nano-sized niobium oxide nanoparticles can be easily produced.

Brief Description of Drawings

[0011] [Figure 1] It is an image showing the appearance of a niobium oxide nanoparticle solution according to an example of the present invention. [Figure 2] It is a graph showing the result of measuring the average particle size of niobium oxide nanoparticles according to an example of the present invention by DLS. [Figure 3] It is a graph showing the result of measuring the average particle size of another niobium oxide nanoparticle according to an example of the present invention by DLS. [Figure 4] It is a graph showing the result of measuring the average particle size of yet another niobium oxide nanoparticle according to an example of the present invention by DLS. [Figure 5] It is an observation image and a graph of measuring the average particle size of niobium oxide nanoparticles according to an example of the present invention by TEM. [Figure 6] It is an observation image and a graph of measuring the average particle size of niobium oxide nanoparticles according to an example of the present invention by TEM. [Figure 7] It is an observation image and a graph of measuring the average particle size of niobium oxide nanoparticles according to an example of the present invention by TEM. [Figure 8] It is the fluorescence X-ray analysis result of niobium oxide nanoparticles according to an example of the present invention. [Figure 9] It is the FT-IR analysis result of a sample containing niobium oxide nanoparticles according to an example of the present invention. [Figure 10] It is the TG measurement result of a sample containing niobium oxide nanoparticles according to an example of the present invention. [Figure 11] It is the XPS measurement result of a sample containing niobium oxide nanoparticles according to an example of the present invention.

Modes for Carrying Out the Invention

[0012] [1. Outline of the present invention] A method for producing niobium oxide nanoparticles according to one embodiment of the present invention (hereinafter also referred to as "this production method") comprises the following steps (A) to (C): (A) a step of dissolving a niobium complex in a reducing solvent to prepare a precursor solution; (B) a step of mixing the precursor solution with a compound having one or more functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups; and (C) a step of heating the mixture at 140°C or higher.

[0013] In this specification, niobium oxide nanoparticles may include not only the nanoparticles themselves but also "quantum dots."

[0014] As the particle size of niobium oxide nanoparticles increases, the excitation energy is more likely to transition to the transition state, and the probability of emission decreases; therefore, smaller particle sizes are preferable. Furthermore, niobium oxide nanoparticles can be used in a variety of applications depending on their particle size. In particular, when the particle size is greater than 0 nm and less than 10 nm (single nanoscale), the degrees of freedom of electron motion are extremely restricted, and the kinetic energy of electrons increases. This expands the range of excitation wavelengths and fluorescence wavelengths, resulting in a variety of emission characteristics.

[0015] However, as mentioned above, it was difficult to control the average particle size of niobium oxide nanoparticles using conventional technology. Furthermore, the additives used in conventional technology contain halogens, which are undesirable from an environmental perspective and may corrode electronic components.

[0016] In response to this, the inventors have found that by using a reducing agent as the solvent for the precursor solution, mixing the solution with a compound having a specific functional group, and heating it at a predetermined temperature, the average particle size of niobium oxide nanoparticles can be controlled within a range of 100 nm or less. At this time, it was also possible to control the average particle size to a single nanometer size. Furthermore, the inventors have found that niobium oxide nanoparticles can be produced without using halogen-containing additives.

[0017] Therefore, according to the present invention, it is possible to stably and easily produce niobium oxide nanoparticles with an average particle size of 100 nm or less, and in particular niobium oxide nanoparticles with an average particle size of single nanometer size, without using halogen-containing additives.

[0018] [2. Method for producing niobium oxide nanoparticles] A method for producing niobium oxide nanoparticles according to one embodiment of the present invention is a method comprising the following steps (A) to (C). Step (A): Dissolve the niobium complex in a reducing solvent to prepare a precursor solution. Step (B) is a step in which the precursor solution is mixed with a compound having one or more functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups to obtain a mixed solution. Step (C) A step of heating the mixture to 140°C or higher. <Process (A)> In step (A) of this manufacturing method, a precursor solution is prepared by dissolving the niobium complex in a reducing solvent. Step (A) can be carried out, for example, by mixing the niobium complex with a reducing solvent and dissolving the niobium complex in a reducing agent.

[0019] In one embodiment of the present invention, there may be only one type of niobium complex, or there may be two or more types. In one embodiment of the present invention, the niobium complex may be a commercially available product, or it may be one that has been chemically synthesized by a known method. Because the niobium oxide nanoparticles produced by this manufacturing method contain a reducing agent internally due to step (A), the reducing agent can function as a protective agent for the niobium oxide nanoparticles. The protective ability of the reducing solvent is weak, but as will be described later, the compound used in step (B) functions as a stronger protective agent.

[0020] The niobium oxide nanoparticles preferably contain niobium whose surface valency, as measured by X-ray photoelectron spectroscopy (XPS) described later, is 4-valent and / or 5-valent. From the viewpoint of luminescence characteristics, a 4-valent valency is more preferable.

[0021] Specific examples of niobium complexes include niobium oxalate, niobium acetate, niobium acetylacetonate, niobium pentachloride, niobium pentabromide, niobium pentaethoxide, and salts thereof. Examples of salts include ammonium salts, potassium salts, magnesium salts, sodium salts, and calcium salts. Among these, ammonium niobium oxalate is preferred from the viewpoint of being inexpensive and readily available in large quantities. One or more of the niobium complexes can be used. By using the niobium complex, toxic metals such as cadmium are not used as raw materials for nanoparticles, making it possible to produce non-toxic nanoparticles.

[0022] The reducing agent used as the reducing solvent for the precursor solution is not particularly limited, as long as it is a reducing agent capable of dissolving the niobium complex. One type of reducing agent may be used, or two or more types may be used in mixture form.

[0023] Examples of the reducing agent include N,N-dimethylformamide, N,N-dimethylacetamide, N-vinylformamide, N-methylformamide, and N-methyl-2-pyrrolidone. Among these, N,N-dimethylformamide or N,N-dimethylacetamide is preferred from the viewpoint of efficiently producing niobium oxide nanoparticles.

[0024] The reducing solvent may further contain other solvents in addition to the reducing agent, as long as they do not inhibit the function of the reducing agent. Examples of other solvents include N-methylformamide, N-methyl-2-pyrrolidone, and tetrahydrofuran. The content of other solvents in the solvent is preferably as low as possible, and it is most preferable that the solvent consists of the reducing agent.

[0025] The concentration of the niobium complex in the precursor solution is not particularly limited as long as it is soluble in the reducing agent. Specifically, it is preferably 0.01 M or higher, and more preferably 0.1 M or higher. There is no particular upper limit to the concentration of the niobium complex, but it may be, for example, 0.5 M or lower.

[0026] <Process (B)> Step (B) is a step of mixing the precursor solution with a compound having one or more specific functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups to obtain a mixed solution.

[0027] By mixing the precursor solution with the compound, the compound functions as a protective agent for niobium oxide nanoparticles. This protective ability is stronger than that of the reducing solvent mentioned above. Therefore, variations in the average particle size of the manufactured niobium oxide nanoparticles are reduced, and control of the average particle size becomes possible.

[0028] The method for mixing the precursor solution and the compound is not particularly limited; for example, a method of mixing the two at room temperature using a stirrer with stirring blades can be used.

[0029] These compounds include one or more selected from the group consisting of a C=O structure, an N atom, and an S atom. Among these, the protective agent is preferably a C=O structure from the viewpoint of ease of coordination to niobium. With the above configuration, niobium oxide nanoparticles with an average particle size of 100 nm or less can be stably produced without using halogen-containing additives, and single nano-sized niobium oxide nanoparticles can be easily produced.

[0030] The compound may act as a protective agent for niobium molecules. For this reason, in this specification, the compound mixed with the precursor solution in step (B) is also referred to as the "protective agent" for convenience.

[0031] Based on the results of the examples described later, it is presumed that the mechanism by which the protective agent can control the average particle size of nanoparticles is as follows. However, the following is merely a presumption based on the results of the examples described later and does not limit the action of the protective agent. First, the protective agent has the specific functional group described above. The specific functional group has a structure that can coordinate to at least one surface metal of a nanoparticle. This structure coordinates around the niobium molecule instead of oxygen, causing the protective agent to cover the surface of the niobium molecule and aggregate. It is presumed that this aggregation suppresses the aggregation of niobium molecules among themselves, making it possible to control the average particle size of nanoparticles. Therefore, any compound containing a functional group that has a structure that can coordinate to the surface metal of a nanoparticle can be used as the protective agent.

[0032] The protective agent is not particularly limited as long as it is a compound having the specific functional group. The protective agent may contain only one type of the specific functional group, or it may contain two or more types.

[0033] The protective agent preferably has a viscosity-average molecular weight of 10,000 to 500,000, more preferably 10,000 to 300,000, and even more preferably 10,000 to 100,000. If the viscosity-average molecular weight of the protective agent is within the above range, it becomes easier to control the average particle size of the nanoparticles. In particular, if the molecular weight of the protective agent is in the range of 30,000 to 50,000, it becomes possible to control the average particle size of the nanoparticles within the range of 1 to 100 nm by adjusting the amount of the protective agent mixed, which was difficult with conventional techniques. The viscosity-average molecular weight can be calculated as M (viscosity-average molecular weight) by determining the intrinsic viscosity [η] of the polymer dilution solution using a capillary viscometer and using the viscosity formula [η] = KM (K = constant).

[0034] The specific protective agent is preferably one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polyaspartic acid, thioctic acid, 1-dodecanethiol, dodecylamine, and 11-mercaptoundecanoic acid. Among these, polyvinylpyrrolidone is more preferred.

[0035] The amount of the protective agent mixed in step (B) is preferably 100 or less, more preferably 50 or less, and even more preferably 40 or less, in molar equivalents per mole of niobium in the precursor solution. The lower limit of the amount of the protective agent mixed is not particularly limited, but may be greater than 0. If the amount of the protective agent mixed is within the above range, the average particle size of the niobium oxide nanoparticles can be controlled more precisely.

[0036] In step (B), the precursor solution may contain other components besides the precursor complex, solvent, and protective agent, as long as they do not inhibit the synthesis of niobium oxide nanoparticles. Examples of other components include triphenylphosphine, acetylacetone, and dibenzalacetone.

[0037] <Process (C)> Step (C) is the step of heating the mixture to 140°C or higher.

[0038] The method of heating is not particularly limited, but it is preferable to use conventionally known means such as a heater while stirring. Furthermore, the temperature of the mixture during step (C) is 140°C or higher, more preferably 160°C or higher, and even more preferably 180°C or higher. If the heating temperature is 140°C or higher, one or more elements selected from the group consisting of the C=O structure, N atoms, and S atoms that the protective agent may have will coordinate more easily to the niobium, and the reducing agent will mix efficiently with the niobium oxide nanoparticles. The upper limit of the heating temperature is not particularly limited, but in practice it may be 160°C or lower. It is preferable that the heating temperature during step (C) remain constant.

[0039] The heating time is not particularly limited, but 8 to 24 hours is preferred, 8 to 16 hours is more preferred, and 8 to 10 hours is even more preferred. If the heating time is 8 hours or more, niobium oxide nanoparticles can be produced stably. If the heating time is 24 hours or less, aggregation of nanoparticles is less likely to occur.

[0040] Stirring during step (C) can be carried out using known stirring means such as a stirrer, stirring rod, or stirring blade. Stirring may also be performed by shaking the container containing the precursor solution. The stirring speed should be determined after considering and adopting conditions that are as appropriate.

[0041] Step (C) may be performed after adding the mixture to a solvent that has been preheated, from the viewpoint of efficiently heating the mixture. The temperature, stirring speed, and time during preheating are not particularly limited, but the conditions described above can be selected as appropriate.

[0042] It is preferable to cool the mixture to room temperature after heating. Room temperature means 20-25°C. The cooling method is not particularly limited and may be carried out by known methods. It is preferable to simply leave it undisturbed at room temperature because slow cooling helps stabilize the shape of the niobium oxide nanoparticles.

[0043] [3. Niobium oxide nanoparticles] Niobium oxide nanoparticles according to one embodiment of the present invention were found to have at least one of the following peaks (i) to (iii) detected by Fourier infrared spectroscopy, (i) 1600~1750cm -1 A peak indicating C=O stretching vibration exists. (ii) 400~800cm -1 A peak showing vibration due to the bond (Nb-N bond) between the niobium atom and the nitrogen atom present. (iii) 400~600cm -1 A peak showing vibration due to the bond (Nb-S bond) between the niobium atom and the sulfur atom present. Furthermore, in the spectrum measured by X-ray photoelectron spectroscopy (XPS), the peak representing the 3d 5 / 2 orbital, which indicates the electronic state of niobium, is shifted to an energy side of more than 0.1 eV lower than that of the niobium complex.

[0044] The C=O structure, N atom, or S atom of the protective agent coordinates to the niobium surface, causing the niobium to be reduced. Therefore, in all of the cases described in (i) to (iii), the peak is thought to shift to an energy side 0.1 eV or more lower than that of the niobium complex. The niobium oxide nanoparticles can be produced by the method described in [2. Method for producing niobium oxide nanoparticles].

[0045] The average particle size of the niobium oxide nanoparticles according to one embodiment of the present invention is preferably 1 to 100 nm, more preferably 2 to 90 nm, and even more preferably 5 to 85 nm. In one embodiment, the average particle size of the niobium oxide nanoparticles may be 1 to 10 nm.

[0046] Niobium oxide nanoparticles with an average particle size of, for example, about 10 nm can be used as light-emitting elements. If the average particle size is about 30 to 50 nm, they can be used as film materials. If the average particle size is about 1 to 100 nm, they can be used as catalyst materials. If the average particle size is 10 to 100 nm, they can be used as ink materials.

[0047] The average particle size of the niobium oxide nanoparticles can be measured by dynamic light scattering (DLS). DLS measurement can be performed using the method described later in [Test 1: Measurement of average particle size of niobium oxide nanoparticles by dynamic light scattering (DLS)].

[0048] Preferably, the niobium molecules in the niobium oxide nanoparticles are protected by a molecule used as a reducing agent. Since the niobium oxide nanoparticles can be produced by the method described in [2. Method for Producing Niobium Oxide Nanoparticles], the niobium molecules in the niobium oxide nanoparticles may contain the reducing agent used as a solvent in [2. Method for Producing Niobium Oxide Nanoparticles].

[0049] The niobium oxide nanoparticles contain the protective agent in addition to the reducing agent within the molecule. By containing the protective agent within the molecule, it is presumed that the niobium molecules of the niobium oxide nanoparticles are coordinated by one or more elements selected from the group consisting of the C=O structure, N atoms, and S atoms of the protective agent. This configuration makes aggregation of the niobium oxide nanoparticles less likely to occur, thus eliminating the need to add harmful substances or halogen-containing additives. Furthermore, the average particle size of the niobium oxide nanoparticles can be controlled by adjusting the amount of protective agent added. Moreover, the niobium oxide nanoparticles are stable for long periods in water and air, and are also thermally stable.

[0050] The presence of the protective agent within the niobium oxide nanoparticles can be confirmed, for example, by elemental mapping using X-ray fluorescence analysis. By mapping the position of the Nb atom in the nanoparticle to the positions of the O atom, N atom, or S atom, if the positions of the atoms overlap, it can be said that the protective agent is present.

[0051] When the protective agent coordinates to niobium at C=O, the range of the peak showing C=O stretching vibration observed by Fourier infrared spectroscopy (FT-IR) of the niobium oxide nanoparticles is 1600-1750 cm². -1 And preferably 1650-1750cm-1 、 more preferably 1650 to 1700 m -1 It is.

[0052] Among the above-mentioned protective agents, examples of the protective agent that coordinates with niobium via C=O include polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polyaspartic acid, thioctic acid, and 11-mercaptoundecanoic acid.

[0053] When the protective agent coordinates with niobium via an N atom, the range of the peak showing the vibration due to the Nb-N bond is 400 to 800 cm -1 It is, preferably 400 to 700 cm -1 、 more preferably 500 to 600 cm -1 It is.

[0054] Among the above-mentioned protective agents, an example of the protective agent that coordinates with niobium via an N atom is dodecylamine.

[0055] When the protective agent coordinates with niobium via an S atom, the range of the peak showing the vibration due to the Nb-S bond is 400 to 600 cm -1 It is, preferably 450 to 550 cm -1 It is.

[0056] Among the above-mentioned protective agents, an example of the protective agent that coordinates with niobium via an S atom is 1-dodecanethiol.

[0057] In addition, the peak showing the C=O stretching vibration, the peak showing the vibration due to the Nb-N bond, and the peak showing the vibration due to the Nb-S bond can be confirmed by referring to, for example, the spectral database published by the National Institute of Advanced Industrial Science and Technology. The FT-IR can be performed by the method described in [Test 4: Measurement by Fourier transform infrared spectroscopy (FT-IR)] of the examples described later.

[0058] In the spectrum measured by X-ray photoelectron spectroscopy (XPS), the niobium oxide nanoparticles show a peak representing the 3d 5 / 2 orbital, which indicates the electronic state of niobium, that is shifted to an energy 0.1 eV or more lower than that of the niobium complex. With this configuration, the niobium oxide nanoparticles are in a reduced state compared to the niobium complex, and it can be said that the niobium molecules are protected by the protective agent. The XPS measurement of the nanoparticle spectrum is preferably performed by the method described in the examples below.

[0059] The niobium oxide nanoparticles may contain other components besides those mentioned above, but preferably contain only a reducing agent, niobium, and a protective agent. The niobium oxide nanoparticles may also contain 0 to 10% by weight of other components. Furthermore, from the viewpoint of environmental impact, if the niobium oxide nanoparticles contain halogens, the halogen content is preferably 10% by weight or less, more preferably 1% by weight or less, and even more preferably halogen-free.

[0060] The excitation wavelength of the niobium oxide nanoparticles is not particularly limited, but is preferably 270 to 400 nm, more preferably 300 to 400 nm, and even more preferably 350 to 370 nm. If the excitation wavelength of the niobium oxide nanoparticles is within the above range, the fluorescence intensity is excellent.

[0061] The fluorescence wavelength of the niobium oxide nanoparticles is preferably 400 to 500 nm, more preferably 420 to 480 nm, and even more preferably 440 to 470 nm. If the fluorescence wavelength of the niobium oxide nanoparticles is within the above range, the fluorescence intensity is excellent.

[0062] The fluorescence lifetime of the niobium oxide nanoparticles is preferably 100 nanoseconds or less, more preferably 50 nanoseconds or less, and even more preferably 30 nanoseconds. A fluorescence lifetime of 100 nanoseconds or less is preferable for application to light-emitting devices because the afterglow time of fluorescence emission is sufficiently short.

[0063] [4. Applications of niobium oxide nanoparticles] Niobium oxide nanoparticles according to one embodiment of the present invention can be used, for example, as luminescent materials, fluorescent materials, film materials, ink materials, catalysts, electronic materials, battery materials, and functional ceramics. Niobium oxide nanoparticles with an average particle size of 10 nm or less can be suitably used as luminescent materials or EL luminescent materials.

[0064] The light-emitting material or EL light-emitting material can be used in light-emitting elements, EL light-emitting elements, phosphor application elements, wavelength conversion films, dye lasers, and bioluminescent markers. Examples of the light-emitting element or EL light-emitting element include semiconductor lasers and light-emitting diodes. Examples of the catalyst include Lewis acid catalysts used in organic synthesis. Examples of the battery material include materials for solar cells or lithium-ion batteries.

[0065] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Examples]

[0066] The present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0067] [Manufacturing of niobium oxide nanoparticles] (Manufacturing Example 1) First, 0.1 mmol (33.9 mg) of ammonium niobium oxalate (hereinafter referred to as "ANO") (Companhia Brasileira de Metalurgia e. Mineracao, CBMM) was dissolved in 1 mL of N,N-dimethylformamide (hereinafter referred to as "DMF") (purity: ≥99.7% (by gas chromatography), Fujifilm Wako Pure Chemical Industries) to prepare a 0.1 M precursor solution (step (A)). Hereinafter, ANO is a niobium complex. DMF is the solvent and acts as a reducing agent when heated.

[0068] Polyvinylpyrrolidone K30 (hereinafter referred to as PVPK30) (viscosity-average molecular weight 40,000, manufactured by Tokyo Chemical Industry Co., Ltd.) was added to the precursor solution and mixed (step (B)). The resulting solution was designated as mixture 1. The amount of PVP added was varied within the range of 1 to 20 molar equivalents relative to the amount of niobium.

[0069] Next, 50 mL of DMF was added to a three-necked flask, and the flask was placed on a mantle heater and preheated at 140°C and 1500 rpm for 10 minutes.

[0070] Subsequently, 0.5 mL of the mixture 1 was added to the three-necked flask, and the mixture was heated at 140°C for 8 hours with stirring. After that, it was allowed to stand until it reached room temperature to obtain a solution of niobium oxide nanoparticles (step (C)). This solution was designated as niobium oxide nanoparticle solution K30.

[0071] (Manufacturing example 2) Niobium oxide nanoparticle solution K15 was obtained in the same manner as in Production Example 1, except that polyvinylpyrrolidone K15 (hereinafter referred to as PVPK15) (viscosity-average molecular weight 10,000, manufactured by Tokyo Chemical Industry Co., Ltd.) was added in step (B) instead of PVPK30.

[0072] (Manufacturing Example 3) Niobium oxide nanoparticle solution K90 was obtained in the same manner as in Production Example 1, except that polyvinylpyrrolidone K90 (hereinafter referred to as PVPK15) (viscosity-average molecular weight 360,000, manufactured by Tokyo Chemical Industry Co., Ltd.) was added in step (B) instead of PVPK30.

[0073] Hereinafter, "Nb-PVP-5NPs(K30)" refers to "niobium oxide nanoparticles prepared by adding 5 molar equivalents of PVPK30 to niobium." "Nb NPs-na" refers to "niobium oxide nanoparticles prepared by adding polyvinylpyrrolidone to the precursor solution without adding polyvinylpyrrolidone in step (B)." Furthermore, "ANO" and "PVPK30" refer to the raw materials themselves used in the manufacturing process.

[0074] [Test 1: Measurement of average particle size of niobium oxide nanoparticles by dynamic light scattering (DLS)] Niobium oxide nanoparticle solutions K30 were prepared by mixing PVPK30 with the precursor solution prepared in Production Example 1 in molar equivalents of 1.0, 5.0, 10.0, 20.0, and 40.0 per mole of niobium in the precursor solution. Figure 1 shows the appearance of the prepared solutions. In Figure 1, PVP / Nb represents the molar ratio of PVPK30 to niobium. The upper panel shows the results when irradiated with indoor lighting, and the lower panel shows the results when irradiated with 352 nm wavelength light using a UV light.

[0075] For each of the niobium oxide nanoparticle solutions K30 shown in Figure 1, the average particle size of the niobium oxide nanoparticles was measured by DLS. DLS measurements were performed using a Zetasizer nano ZSP (Malvern Panalytical Ltd., Malvern). Measurement conditions were 25°C, and the measurement was performed in backscattering mode. Each of the niobium oxide nanoparticle solutions K30 was diluted to 0.1 mM with DMF, and measurements were performed using a glass cell. The results are shown in Figure 2.

[0076] Figure 2 is a graph showing the correlation between the molar ratio of PVPK30 used during manufacturing to niobium and the average particle size of the obtained niobium oxide nanoparticles. From Figure 2, it can be seen that when the amount of PVPK30 added was 1 molar equivalent, the average particle size was 83.6 nm, and the average particle size decreased as the amount of PVPK30 added increased. Furthermore, when the amount of PVPK30 added exceeded 10 molar equivalents, the average particle size hardly changed from around 10 nm. When the amount added was small, it is presumed that the amount of PVP coordinated around the niobium was small, causing the niobium to aggregate and the average particle size to increase. On the other hand, when the amount added was large, it is presumed that the aggregation of niobium was suppressed because a large amount of PVP was coordinated around the niobium, resulting in a smaller average particle size.

[0077] The results of measuring the average particle size using niobium oxide nanoparticle solution K15 and niobium oxide nanoparticle solution K90 are shown in Figures 3 and 4, respectively.

[0078] The graph in Figure 3 shows the results when PVPK15 is used instead of PVPK30. From Figure 3, it can be seen that when PVPK15 is used, the average particle size of niobium oxide nanoparticles can be controlled to about 10 nm even with a small amount of additive. This is presumed to be because PVPK15 has a small molecular weight and is easily coordinated around niobium, so even a small amount can reduce the average particle size of niobium oxide nanoparticles to 10 nm or less.

[0079] The graph in Figure 4 shows the case when PVPK90 is used instead of PVPK30. From Figure 4, it can be seen that when PVPK90 is used, the average particle size of the resulting niobium oxide nanoparticles is slightly larger when the amount added is small, and when the amount added exceeds 10 molar equivalents, the average particle size becomes 10 nm or less, similar to the case when PVPK30 is used. Because PVPK90 has a large molecular weight, when one molecule of PVPK90 coordinates to niobium, other molecules have difficulty coordinating around the niobium. As a result, it is presumed that some degree of niobium aggregation occurs, causing the average particle size to be slightly larger.

[0080] From the above, it has been shown that, according to the manufacturing method of one embodiment of the present invention, niobium oxide nanoparticles with a controlled average particle size can be produced without using halogen-containing additives. Furthermore, it has become clear that, according to the manufacturing method, niobium oxide nanoparticles with an average particle size of single nanometers can be easily produced.

[0081] [Test 2: Measurement of average particle size using a transmission electron microscope (TEM)] The average particle size of niobium oxide nanoparticles was measured using a TEM. A field emission transmission electron microscope (JEOL, JEM-ARM200F) was used, and measurements were performed at an acceleration voltage of 200 kV. The solvent in the niobium oxide nanoparticle solution K30 was replaced with 1 mM ethanol, and the solution was dropped directly onto a copper TEM grid for measurement. From the acquired TEM images, the area of ​​each nanoparticle within the TEM image was determined using the image processing software Image J. The particle size was then calculated by inversely determining the frequency of each particle size, and the average particle size was determined. Note that the calculation of particle size was performed assuming that the nanoparticles were perfect circles. The results are shown in Figures 5-7.

[0082] Figure 5 shows an observation image of niobium oxide nanoparticles (Nb-PVP-1NPs(K30)) prepared by adding 1.0 molar equivalent of PVPK30. The observation image shows that there are many nanoparticles with large particle sizes. As shown in the graph, the average particle size was 83.12 ± 17.33 mm, and the proportion of particles with a particle size of 40-80 nm was the largest.

[0083] Figure 6 shows an observation image of niobium oxide nanoparticles (Nb-PVP-5NPs(K30)) prepared by adding 5.0 molar equivalents of PVPK30. From the observation image, it can be seen that there are many nanoparticles with smaller particle sizes than those shown in the observation image of Figure 5. Also, as shown in the graph, the average particle size was 32.89 ± 1.59 mm, and the proportion of particles with a particle size of 20-40 nm was the largest.

[0084] Figure 7 shows an observation image of niobium oxide nanoparticles (Nb-PVP-20NPs(K30)) prepared by adding 20.0 molar equivalents of PVPK30. The observation image reveals the presence of numerous small-particle nanoparticles. Furthermore, as shown in the graph, the average particle size was 2.89 ± 0.19 nm, with most nanoparticles having a particle size of 10 nm or less. Note that the average particle size shown in Figure 7 differs from the DLS measurement result by approximately 3 nm. This is presumed to be because DLS calculates the average particle size of components present in the solution, including the protective agent, while TEM observation measures the average particle size based only on the particle size of the niobium metal.

[0085] From the above, it has been shown that, according to the manufacturing method of one embodiment of the present invention, niobium oxide nanoparticles with an average particle size controlled to 100 nm or less can be produced without using halogen-containing additives.

[0086] [Experiment 3: Elemental mapping by X-ray fluorescence analysis] A niobium oxide nanoparticle solution K30, in which PVPK30 was added to niobium at a molar equivalent of 5.0, was subjected to fluorescence X-ray analysis using a field emission transmission electron microscope (JEOL, JEM-ARM200F) equipped with an energy-dispersive X-ray analyzer. The results are shown in Figure 8.

[0087] In Figure 8, C represents carbon, Nb represents niobium, and O represents oxygen. For reference, a field emission transmission electron microscope (JEOL, JEM-ARM200F) image of niobium oxide nanoparticles is also shown. The overlap of the positions of Nb and O with the positions of the nanoparticles suggests that niobium oxide nanoparticles are formed in the niobium oxide nanoparticle solution.

[0088] [Test 4: Measurement by Fourier Transform Infrared Spectroscopy (FT-IR)] Fourier infrared spectroscopy measurements were performed using a Fourier transform infrared spectrophotometer (Shimadzu Corporation, IRAffinity-1 FT-IR) and IRsolution analysis software. Nb-PVP-5NPs (K30), DMF, and PVPK30 were selected as the target materials. The measurement range was 500-4000 cm². -1 The number of cumulative measurements was set to 16. Liquid DMF was measured using the liquid film method with an NaCl plate. PVPK30 and Nb-PVP-5NPs(K30) were measured using the KBr tablet method. Specifically, nanoparticles and potassium bromide (KBr, manufactured by Fujifilm Wako Pure Chemical Industries) were mixed in a mortar and pestle to form tablets, which were then measured. The results are shown in Figure 9.

[0089] Figure 9 shows the FT-IR measurement results for each sample. From the results shown in Figure 9, the spectrum of Nb-PVP-5NPs(K30) is 1600-1750 cm⁻¹. -1 A peak indicating C=O stretching vibration has been detected, and it can be seen that it is similar to the spectrum of PVPK30. Note that the peak indicating C=O stretching vibration in Nb-PVP-5NPs(K30) is (1656cm²). -1 ) is the peak of PVPK30 (1672cm) -1 The shift to lower wavenumbers compared to ) is thought to be due to the C=O structure contained in PVP coordinating with the niobium nanoparticles. The results shown in Figure 9 suggest that PVP is coordinated to the surface of the niobium nanoparticles as a protective agent.

[0090] [Test 5: Thermogravimetric Analysis (TG)] Thermogravimetric measurements were performed using a differential thermobalance (Rigaku Thermo plus EVO2 TG8121) and Pyris Manager analysis software. The measurement range was 50-500°C, and the heating rate was 5°C / min. The sample was placed in an aluminum sample pan and measured under a nitrogen atmosphere. The results are shown in Figure 10. In Figure 10, TG is the graph of the thermogravimetric measurement results, and DTG is the data obtained by differentiating the TG data using the analysis software (Pyris Manager).

[0091] In the graphs shown in Figure 10, the results for DTG show that the ANO-derived peak present around 130°C to 140°C is almost completely absent in Nb-PVP-5NPs(K30). Therefore, it is presumed that the ANO used as a raw material is converted into another substance within the nanoparticles.

[0092] [Test 6: X-ray photoelectron spectroscopy (XPS)] X-ray photoelectron spectroscopy measurements were performed using an X-ray photoelectron spectroscopy analyzer (Ulvac-PHI 5000 VersaProbe III, manufactured by ULVAC-PHI) and casa xps analysis software. The X-ray source was AlKα. The target samples were ANO only, Nb-PVP-5NPs(K30), and Nb NPs-na.

[0093] For solutions of Nb-PVP-5NPs(K30) and Nb NPs_Na niobium oxide nanoparticles, use an evaporator (80°C, 20hPa), a desiccator (100°C, 1h), and a diaphragm pump (10 2 Pa~10 -1 Pa), as well as a desiccator (100°C, 1h) and a turbomolecular pump (10 -3 ~10 -11 The solvent was removed under reduced pressure using Pa) in this order. The obtained niobium oxide nanoparticles were then mounted on an Ag plate (manufactured by Niraco Co., Ltd., 0.2 mm thick). For ANO, an indium sphere (manufactured by Kojun Chemical Laboratory Co., Ltd.) was mounted on a flattened indium plate. The samples were fixed to the sample stage using carbon tape (manufactured by Kenis Co., Ltd.). All measurement results were corrected for Cls (284.5 eV). The measurement results for each sample are shown in Figure 11.

[0094] As shown in Figure 11, the peak in the spectrum of Nb-PVP-5NPs was shifted to a lower energy side by more than 0.1 eV compared to the peak in the spectrum of the niobium complex ANO. This suggests that the Nb-PVP-5NPs nanoparticles are in a reduced state compared to their precursor ANO, and that the C=O portion of PVP is coordinated to the niobium atoms on the surface of the nanoparticles. On the other hand, Nb NPs_Na is also in a reduced state, which is presumed to be due to reduction by the C=O structure contained in DMF. [Industrial applicability]

[0095] Since the present invention can produce niobium oxide nanoparticles with an average particle size of 100 nm or less, it can be suitably used as a method for providing various light-emitting materials.

Claims

1. The following steps (A) to (C): (A) A step of dissolving the niobium complex in a reducing solvent to prepare a precursor solution, (B) A step of mixing the precursor solution with a compound having one or more functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups to obtain a mixed solution. (C) A method for producing niobium oxide nanoparticles, comprising the step of heating the mixture at 140°C or higher.

2. The production method according to claim 1, wherein the niobium complex is one or more complexes selected from the group consisting of niobium oxalate, niobium acetate, acetylacetonate niobium, niobium pentachloride, niobium pentabromide, niobium pentaethoxide, and salts thereof.

3. The manufacturing method according to claim 1, wherein the reducing solvent is one or more selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetamide, and N-vinylformamide.

4. The manufacturing method according to claim 1, wherein the concentration of the niobium complex in the precursor solution is 0.01 M or higher.

5. The manufacturing method according to claim 1, wherein the compound to be mixed with the precursor solution in step (B) is one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polyaspartic acid, thioctic acid, 1-dodecanethiol, dodecylamine, and 11-mercaptoundecanoic acid.

6. The manufacturing method according to claim 1, wherein the viscosity-average molecular weight of the compound mixed with the precursor solution in step (B) is 10,000 to 500,000.

7. Fourier infrared spectroscopy detected at least one of the following peaks (i) to (iii): (i) 1600-1750cm －１ A peak exhibiting C=O stretching vibration exists. (ii) 400-800cm －１ A peak showing vibration due to the bond between the niobium atom and the nitrogen atom present. (iii) 400-600cm －１ A peak showing vibration due to the bond between the niobium atom and the sulfur atom present. Furthermore, in the spectrum measured by X-ray photoelectron spectroscopy (XPS), the peak representing the 3d 5 / 2 orbital, which indicates the electronic state of niobium, is shifted to an energy side of 0.1 eV or more compared to that of the niobium complex, in the niobium oxide nanoparticles.

8. The niobium oxide nanoparticles according to claim 7, comprising a compound having one or more functional groups selected from the group consisting of ketone groups, aldehyde groups, carboxyl groups, ester groups, amide groups, amine groups, thiol groups, nitrile groups, oxime groups, imine groups, and azo groups.

9. A light-emitting material containing niobium oxide nanoparticles according to claim 7 or 8.

10. A light-emitting element containing the light-emitting material described in claim 9.

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