Manufacturing method for ultra-small nanoparticles

Abstract

To provide very small nanoparticles expected to be utilized in the fields, e.g., of catalytic and medical applications, a very small nanoparticle solution and a production method capable of efficiently producing the very small nanoparticles.SOLUTION: A method for producing very small nanoparticles comprises a spark discharge step where repeated spark discharge having a peak current of 80 A or higher, 100 A or higher or 125 A or higher to a space between a pair of electrodes in a liquid to form very small nanoparticles having average particle diameter of 2 nm or lower using the electrodes and / or the liquid as material in the liquid, thus a very small nanoparticle-containing liquid comprising the very small nanoparticles is obtained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing ultra-small nanoparticles formed by repeated spark discharge in a liquid. By law Regarding. [Background technology]

[0002] Nanoparticles of metals, semi-metals, semiconductors, and compounds are being industrialized in a variety of fields, including the environment, IT, printing, and medicine, as catalysts, magnetic materials, battery materials, photoelectric materials, semiconductor materials, medical materials, pharmaceutical materials, and health foods. Nanoparticles have superior surface area, reactivity, catalytic properties, and magnetism compared to bulk materials and powders.

[0003] Conventional techniques for producing nanoparticles include chemical synthesis, sol-gel, laser evaporation, plasma irradiation, atomization, injection, mechanical milling, CVD, PVD, and plasma processes. Plasma processes include arc discharge, corona discharge, glow discharge, streamer discharge, spark discharge, and other discharge-based methods, as well as microwaves. Wire explosion is also a type of plasma generation method. Here, we use repeated spark discharge in a liquid. However, previous methods have the drawback of nanoparticles easily agglomerating. Furthermore, nanoparticles produced by methods other than chemical synthesis are generally large in size. To address this issue, glow discharge in a liquid, as described in Patent Documents 1 and 2, for example, is considered. The present inventors have also disclosed non-patent documents 1 to 5, among others, on the production of nanoparticles using spark discharge in a liquid.

[0004] For example, gold nanoparticles, which are metal nanoparticles, have long been used as staining materials for stained glass and more recently as biological stains for transmission electron microscopy. In the medical field, gold nanoparticles are also used in DNA sensors, in vivo cancer research, nanotoxicology, intracellular probes, optical tweezers, and DNA analysis. Taking advantage of their affinity with living organisms, they are used for biosensing, and coated with highly biocompatible polymers for gene delivery. Medical nanoparticles for cancer treatment and testing are required to be small enough to pass between and inside cells and to incubate with DNA.

[0005] Nanoparticles of platinum, palladium, silver, and copper are widely used as antibacterial materials in medicine, health foods, and cosmetics. Platinum is used as an anticancer agent. Both platinum and palladium have been approved as safe food additives by the Ministry of Health, Labor, and Welfare.

[0006] The development of small nanoparticle catalysts made of platinum (Pt), rhodium (Rh), palladium (Pd), etc. is one of the important issues related to industrial infrastructure, including the environment, resources, and energy. In particular, the development of high-performance precious metal catalysts is an urgent issue due to their scarcity and importance in semiconductor manufacturing, exhaust gas catalysts, and fuel cell catalysts. Platinum is also used as a catalyst for water electrolysis. The creation of small nanoparticles is essential to this task.

[0007] The liquid in which nanoparticles are dispersed is called a dispersoid colloid.

[0008] In general, dispersoid colloid particles universally exhibit interparticle van der Waals forces, which are the sum of intermolecular forces and are synonymous with surface tension. Meanwhile, a potential difference exists on the surface of dispersoid particles due to the difference in polarity between the composition and the solvent, forming a diffuse electric double layer with counterions of opposite sign to the surface potential. Because particles of the same type possess double layers of the same counterions, when particles approach each other, the double layers overlap, creating an osmotic repulsion due to the entropy effect of ion diffusion, preventing particle aggregation and stabilizing the dispersion. Dispersed colloids using water as a dispersion medium are sometimes classified as hydrophobic colloids, those that precipitate easily upon the addition of electrolytes, and hydrophilic colloids, those that do not. Hydrophilic colloids, like hydrophobic colloids, have a surface charge, and are coordinated with numerous water molecules through hydration (solvation), resulting in even stronger steric repulsion and stabilization.

[0009] Gold colloids are colloids in which gold nanoparticles are dispersed in a fluid. The color changes depending on the state of the liquid, but for particles of around 10 nm, they are generally red, and as the particle size decreases, they become pale yellow, and as the particle size increases, they become purple to pale blue, and when the particle size exceeds 100 nm, they become cloudy yellow. Gold colloids have unique optical and electrical properties and are used in electron microscopy, electronics, nanotechnology, materials science, and more. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-167335 [Patent Document 2] Japanese Patent Application Publication No. 2019-094541 [Non-patent literature]

[0011] [Non-Patent Document 1] “Homogeneously alloyed nanoparticles of immiscible Ag-Cu with ultrahigh antibacterial activity”, L. Yang, L. Chen, YC Chen, L. Kang, J. Yu, Y. Wang, C. Lu, T. Mashimo, A. Yoshiasa, CH Lin, Colloids and Surfaces B: Biointerfaces 180, 466-472 (2019).

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[0012] In the field of catalysis, high-performance catalysts such as platinum (Pt), rhodium (Rd), and palladium (Pd) are important in relation to industrial infrastructure, including the environment, resources, and energy. In particular, the development of high-performance precious metal catalysts for exhaust gas catalysts and fuel cell catalysts is an urgent issue due to their scarcity and importance, and the nanoparticles of the present invention are useful in addressing this issue. It is said that the smaller the particle size of a catalyst, the greater the surface area and the better its catalytic properties.

[0013] Nanoparticles of platinum, palladium, silver, and copper are widely used as antibacterial materials in medicine, health foods, and cosmetics. When antibacterial materials are used on humans, nanoparticles with small particle sizes are required.

[0014] In the medical field, gold nanoparticles are used in DNA sensors, in vivo cancer research, nanotoxicology, intracellular probes, optical tweezers, and DNA analysis. Taking advantage of their affinity with living organisms, they are used for biosensing, and coated with highly biocompatible polymers for gene delivery. Medical nanoparticles for cancer treatment and testing must be small enough to pass between and inside cells and to incubate with DNA.

[0015] Nanoparticles are also used as catalysts, and the smaller the size, the larger the surface area, which increases the catalytic effect, so smaller nanoparticles are required. For example, Pd nanoparticles are used in the plating process of semiconductors. In particular, catalysts used in IC and LSI semiconductors must penetrate between the elements that make up the integrated circuit to exert their catalytic properties. The distance between these elements is currently around 10 nm, and in order to improve the performance of IC and LSI semiconductors, smaller palladium nanoparticles of around 2 nm or less are required.

[0016] Ferromagnetic nanoparticles are also used in cancer treatments, utilizing the heating effect of electromagnetic action. Materials such as Fe, Co, and Ni are used for these. If smaller nanoparticles could be obtained, they would be more effective because they could easily penetrate cells.

[0017] As such, there is a demand for the production of smaller metal nanoparticles for use in catalysts, medical applications, and health foods. Chemical reduction methods are used to achieve this, but there are limits to the particle size they can produce. Furthermore, well-dispersed nanoparticles with uniform composition and size are required, but arc discharge, vapor deposition (CVD, PVD), reduction, and other methods produce large aggregates that are difficult to suppress. [Means for solving the problem]

[0018] The present inventors have conducted extensive research to solve the above problems and have found that the following inventions meet the above objectives, thereby completing the present invention.

[0019] <a1>A method for producing extremely small nanoparticles, comprising a spark discharge step in which repeated spark discharges with a peak current of 80 A or more, 100 A or more, or 125 A or more are conducted between a pair of electrodes in a liquid, and nanoparticles having an average particle size of 2 nm or less are formed in the liquid using the electrodes and / or the liquid as materials, thereby obtaining a nanoparticle-containing liquid containing the nanoparticles. <a2>The discharge condition is that a rectangular wave voltage is applied periodically. <a1>A method for producing ultra-small nanoparticles according to claim 1. <a3>The electrode contains any one selected from the group consisting of a metal, a semi-metal, a semiconductor, an element, an alloy, and a compound. <a1>or <a2>A method for producing ultra-small nanoparticles according to claim 1. <a4>The rise and fall times of the current between the electrodes of the spark discharge are 20 μs or less. <a1> ~ <a3>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a5>The discharge conditions are that the voltage rise and fall times are 1 μs or less, and the voltage is applied periodically in a rectangular wave shape with a time width of 10 μs to 200 μs. <a1>A method for producing ultra-small nanoparticles according to claim 1. <a6>The method includes a centrifugation step of centrifuging the nanoparticle-containing liquid and recovering the supernatant. <a1> ~ <a5>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a7>The liquid includes any one selected from the group consisting of water, alcohol, toluene, xylene, an aqueous ammonia solution, sulfur, and carbon tetrachloride. <a1> ~ <a6>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a8>The electrodes are all made of gold, and the average particle size of the nanoparticles is less than 0.6 nm. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a9>The electrodes are all made of palladium, and the average particle size of the nanoparticles is 1.7 nm or less. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a10>The electrodes are all made of platinum, and the average particle size of the nanoparticles is 1.4 nm or less. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a11>The electrode is made of a noble metal element, and the average particle size of the nanoparticles is 2.0 nm or less. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a12>The electrode is made of a base metal, and the average particle size of the nanoparticles is 2.0 nm or less. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a13>The electrode is made of an alloy, and the average particle size of the nanoparticles is 2.0 nm or less. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a14>The electrode is a compound, and the average particle size of the nanoparticles is 2.0 nm or less. <a1> ~ <a7>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims. <a15>The average particle size of the nanoparticles is 1.5 nm or less. <a9> ~ <a14>1. A method for producing ultra-small nanoparticles according to any one of the preceding claims.

[0020] <b1>Ultra-small nanoparticles containing gold, with an average particle size of less than 0.6 nm as measured by dynamic light scattering. <b2>Ultra-small nanoparticles containing palladium, with an average particle size of 1.4 nm or less as measured by dynamic light scattering. <b3>Ultra-small nanoparticles containing palladium, with an average particle size of 1.7 nm or less as measured by dynamic light scattering. <b4>Ultra-small nanoparticles containing platinum, with an average particle size of 1.4 nm or less as measured by dynamic light scattering. <b5>Ultra-small nanoparticles having an average particle size of 2.0 nm or less as measured by dynamic light scattering, and comprising any material selected from the group consisting of metals, semi-metals, and semiconductors. <b6>The average particle size of the ultra-small nanoparticles is 1.5 nm or less <b5>The extremely small nanoparticles described in <b7>The metal is a noble metal, a base metal, an alloy, or a compound. <b5>or <b6>The extremely small nanoparticles described in <b8> <b1> ~ <b7>1. A solution of extremely small nanoparticles in which the nanoparticles according to any one of the above items are dispersed in a solvent.

[0021] [What is spark discharge?] Spark discharge, or spark discharge in English, is a phenomenon in which sparks are observed between electrodes when the voltage exceeds a certain limit, and refers to a discontinuous, transient phenomenon. When the voltage applied between the electrodes is increased, molecules present between the electrodes collide with electrons accelerated by the high voltage and become ionized (called α action). Furthermore, when positive ions generated by ionization collide with the negative electrode, secondary electrons are emitted, causing electrons to be supplied from the negative electrode to the space between the electrodes (called γ action). If the amount of charged particles generated by these two actions is greater than the amount lost to the electrodes or the surrounding space, the amount of charged particles flowing between the electrodes increases like an avalanche, causing a large current to flow between the electrodes.

[0022] Here, the rise or fall time is the time it takes for the current to rise from 10% to 90% of its peak or for the current to fall from 90% to 10% of its peak, respectively. [Effects of the Invention]

[0023] According to the present invention, extremely small nanoparticles that exhibit excellent physical properties for various applications can be efficiently produced. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a diagram illustrating an example of a schematic configuration of a nanoparticle production apparatus used in a method for producing nanoparticles according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram for explaining a circuit diagram of a power supply. [Figure 3] FIG. 10 is a diagram showing an example of a voltage waveform from a power supply (2). [Figure 4A] FIG. 10 is a diagram showing an example of a current waveform when a palladium electrode is used. [Figure 4B] FIG. 10 is a diagram showing an example of a current waveform when a palladium electrode is used. [Figure 4C] FIG. 10 is a diagram showing an example of a current waveform when a palladium electrode is used. [Figure 4D] FIG. 10 is a diagram showing an example of a current waveform when a palladium electrode is used. [Figure 4E] FIG. 10 is a diagram showing an example of a current waveform when a palladium electrode is used. [Figure 5A] 1 is a photograph of a glass container containing a liquid containing palladium nanoparticles. [Figure 5B] 1 is a photograph of a glass container containing a liquid containing palladium nanoparticles. [Figure 5C] 1 is a photograph of a glass container containing a liquid containing palladium nanoparticles. [Figure 5D] 1 is a photograph of a glass container containing a liquid containing palladium nanoparticles. [Figure 6A] This shows the results of observation of palladium nanoparticles using FE-TEM. [Figure 6B] This shows the results of observation of palladium nanoparticles using FE-TEM. [Figure 6C] This shows the results of observation of palladium nanoparticles using FE-TEM. [Figure 6D] This shows the results of observation of palladium nanoparticles using FE-TEM. [Figure 6E] This shows the results of observation of palladium nanoparticles using FE-TEM. [Figure 7A] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7B] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7C] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7D] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7E] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7F] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7G] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 7H] 1 shows the results of measuring the particle size of palladium particles by dynamic light scattering. [Figure 8A] FIG. 10 is a diagram showing an example of a current waveform when a gold electrode is used. [Figure 8B] FIG. 10 is a diagram showing an example of a current waveform when a gold electrode is used. [Figure 8C] FIG. 10 is a diagram showing an example of a current waveform when a gold electrode is used. [Figure 8D] FIG. 10 is a diagram showing an example of a current waveform when a gold electrode is used. [Figure 9A] 1 is a photograph of a glass container containing a liquid containing gold nanoparticles. [Figure 9B] 1 is a photograph of a glass container containing a liquid containing gold nanoparticles. [Figure 9C] 1 is a photograph of a glass container containing a liquid containing gold nanoparticles. [Figure 9D] 1 is a photograph of a glass container containing a liquid containing gold nanoparticles. [Figure 10A] This is the result of observing gold nanoparticles using FE-TEM. [Figure 10B] This is the result of observing gold nanoparticles using FE-TEM. [Figure 10C] This is the result of observing gold nanoparticles using FE-TEM. [Figure 11A] 1 shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11B] 1 shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11C] This shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11D] This shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11E] This shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11F] This shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11G] This shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 11H] This shows the results of measuring the particle size of gold nanoparticles by dynamic light scattering. [Figure 12A] FIG. 10 is a diagram showing an example of a current waveform when a platinum electrode is used. [Figure 12B] FIG. 10 is a diagram showing an example of a current waveform when a platinum electrode is used. [Figure 13A] This is the result of observing platinum nanoparticles using FE-TEM. [Figure 13B] This is the result of observing platinum nanoparticles using FE-TEM. [Figure 14A] 1 shows the results of measuring the particle size of platinum nanoparticles by dynamic light scattering. [Figure 14B] 1 shows the results of measuring the particle size of platinum nanoparticles by dynamic light scattering. DETAILED DESCRIPTION OF THE INVENTION

[0025] The following describes in detail an embodiment of the present invention, but the following description of the constituent elements is one example (typical example) of an embodiment of the present invention, and the present invention is not limited to the following content unless the gist of the present invention is changed. Note that when the expression "~" is used in the drawings, it is used as an expression that includes the number before it but not the number after it. Furthermore, when the expression "~" is used in this specification, it is used as an expression that includes the numbers before and after it.

[0026] [Production method of the present invention] The production method of the present invention includes a spark discharge step in which a spark discharge is repeatedly performed in a liquid between a pair of electrodes selected from the group consisting of metals, semi-metals, semiconductors, simple elements, alloys, and compounds at a discharge condition of a peak current of 80 A or more, preferably 100 A or more or 125 A or more, and the spark discharge forms nanoparticles containing any one selected from the group consisting of metals, semi-metals, semiconductors, simple elements, alloys, and compounds in the liquid, thereby obtaining a nanoparticle-containing liquid containing the nanoparticles.The production method of the present invention makes it possible to efficiently produce nanoparticles smaller than conventional nanoparticles (ultrasmall nanoparticles of 2.0 nm or less).

[0027] [Nanoparticles of the present invention] The nanoparticles of the present invention have an average particle size of 0.1 nm or more and 2.0 nm or less as measured by dynamic light scattering, and comprise any material selected from the group consisting of metals, semi-metals, semiconductors, simple elements, alloys, and compounds.

[0028] The nanoparticles of the present invention have been efficiently obtained with a particle size smaller than that of conventional nanoparticles, and are expected to have dramatically improved surface area, reactivity, catalytic properties, magnetism, etc., and can be used in unprecedented applications such as dyes, catalysts, medical applications, pharmaceutical materials, health foods, magnetic materials, battery materials, photoelectric materials, semiconductor materials, quantum dots, etc.

[0029] The nanoparticles of the present invention can be produced by the production method of the present invention. In this application, the corresponding configurations can be used interchangeably.

[0030] The nanoparticle manufacturing method of the present invention uses a special power source to form extremely small nanoparticles by repeatedly generating spark discharges in a liquid between two electrodes immersed in the liquid. In this manufacturing process, each electrode is filled with the target nanoparticle element or alloy bulk metal, and discharge is performed under conditions that allow the synthesis of small-sized nanoparticles.

[0031] Regarding these discharge conditions, the rise and fall times of the current between the electrodes of the spark discharge are steep, at 10 μs or less or 20 μs or less, respectively. Furthermore, regarding these discharge conditions, it is preferable to use a peak voltage of approximately 160 V to 250 V, with voltage rise and fall times of 1 μs or less, and a peak current of 80 A or more, preferably 100 A or more or 125 A or more. Furthermore, it is preferable to use a voltage that is periodically applied in a rectangular wave shape with a time width of 10 μs to 200 μs. Furthermore, it is preferable to repeatedly generate spark discharge at time intervals of 50 μs to 20 ms. It has been found that this enables the efficient synthesis of nanoparticles smaller than conventional nanoparticles. The present invention is based on these findings.

[0032] I. Nanoparticle Production The manufacturing method of the present invention will be described below with reference to Fig. 1 and other figures. Fig. 1 shows an example of a schematic configuration of a manufacturing apparatus 10 used in the nanoparticle manufacturing method according to the first embodiment of the present invention. The manufacturing apparatus 10 includes, for example, electrodes 101 and 102, a liquid 202 contained in a container 201, and a power supply unit 30. The manufacturing apparatus 10 also includes a vibration unit 40. Note that the vibration unit 40 may be omitted.

[0033] [Pair of electrodes 101, 102] The pair of electrodes 101, 102 serve as electrodes for repeatedly generating spark discharges in (in) the liquid 202. In the manufacturing apparatus 10, for example, one end of each of the electrodes 101, 102 is immersed in the liquid 202, and the other end of each of the electrodes 101, 102 is not immersed in the liquid 202 but is placed in the atmosphere. The ends of the electrodes 101, 102 that are immersed in the liquid 202 are arranged, for example, facing each other with a predetermined gap between them.

[0034] The electrodes 101 and 102 can be, for example, rod-shaped, needle-shaped, or plate-shaped. The electrodes 101 and 102 may be the same size or different sizes. The electrodes 101 and 102 may be made of the same material or different materials. The production volume can be increased simply by increasing the discharge area, for example by increasing the size of the electrodes, which is suitable for mass production of nanoparticles. The discharge area can be increased without particular limitations as long as it is within the range that allows for device structure, current control, etc.

[0035] [Electrode material] The electrodes 101 and 102 are made of an electrically conductive material such as a metal and / or semimetal, or a semiconductor. These electrodes 101 and 102 may be the same or different. For example, when obtaining particles of a single element, it is preferable to use the same electrode of the single element for both electrodes. When obtaining an alloy or compound, different electrodes may be used for both electrodes. When obtaining an alloy or compound, the same electrode of the alloy or compound may be used for both electrodes, or electrodes of the single element may be used for both electrodes to react with the liquid 202.

[0036] [metal] In the present invention, the term "metal" includes elemental metals and alloys, including those essentially consisting of elemental metal elements such as noble metals and base metals, alloys essentially consisting of these metal elements, and alloys containing non-metallic elements that are included when forming an alloy with these metal elements.

[0037] [Precious metals] In this invention, a noble metal is defined as a rare metal that is difficult to form a compound with, or a metal that has a high positive electrode potential compared to a standard hydrogen electrode. Generally, there are eight noble metals: gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os). Furthermore, noble metals occur as simple elements, have low ionization tendency, are resistant to direct reactions with acids, and are resistant to oxidation in air. The noble metal used in this invention may include any one selected from the group consisting of gold (Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), and copper (Cu).

[0038] In particular, when a noble metal is used for the electrodes, it is easy to obtain nanoparticles essentially consisting of the single element of the noble metal used for the electrodes by using the same type of noble metal for both electrodes. For example, gold nanoparticles can be obtained by using a gold electrode, palladium nanoparticles can be obtained by using palladium, and platinum nanoparticles can be obtained by using platinum.

[0039] In particular, nanoparticles of precious metal elements (e.g., Au, Pt, Pd, Ru, Rh or Ag, Cu, etc.) are important as catalysts, medical materials, pharmaceutical materials, and health foods.

[0040] In the field of catalysis, high-performance catalysts such as platinum (Pt), rhodium (Rd), and palladium (Pd) are important in relation to industrial infrastructure such as the environment, resources, and energy. In particular, the development of high-performance noble metal catalysts for exhaust gas catalysts and fuel cell catalysts is an urgent issue due to their scarcity and importance, and the nanoparticles of the present invention are useful in addressing this issue.

[0041] Pt, Pd, and Ru are used as exhaust gas catalysts, Pt and its alloys as fuel cell catalysts, and elemental precious metals are used in hydrogen oxidation reactions. Pd is also used as a catalyst in the manufacturing process of IC and LSI semiconductors.

[0042] In catalysts, as particle size decreases, the surface area increases, significantly improving reactivity and catalytic properties. Therefore, nanoparticles with small particle size are useful.

[0043] [Base metal] In the present invention, the base metal is a metal that has a higher ionization tendency (low stability) than hydrogen and is easily oxidized when heated in air, excluding the above-mentioned noble metals. For example, alkali metals (lithium (Li), sodium (Na), potassium (K), rubidium (Rb), etc.), alkaline earth metals (calcium (Ca), strontium (Sr), barium (Ba), etc.), magnesium group elements (beryllium (Be), magnesium (Mg), zinc (Zn), etc.), aluminum group elements (aluminum (Al), gallium (Ga), indium (In), rare earth elements (yttrium (Y), lanthanum ( These include elements such as titanium (Ti), zirconium (Zr), tin (Sn), hafnium (Hf), and lead (Pb), iron-group elements (iron (Fe), cobalt (Co), and nickel (Ni)), earth elements (vanadium (V), niobium (Nb), and tantalum (Ta)), chromium-group elements (chromium (Cr), molybdenum (Mo), and tungsten (W), and manganese-group elements (manganese (Mn), rhenium (Re)), and so on.

[0044] [alloy] In the present invention, the alloy is a solid solution or intermetallic compound of two or more metal elements or a metal element and a nonmetal element such as carbon, nitrogen, silicon, etc. As these metal elements, the above-mentioned noble metals and base metals can be used.

[0045] [Semimetal] Metalloids exhibit properties intermediate between those of metals and nonmetals. They include carbon, which becomes graphite, and phosphorus, which becomes black phosphorus. Other metalloids include boron, silicon, germanium, arsenic, antimony, tellurium, bismuth, polonium, and astigmatism. The elements contained in these metalloids are the main constituent elements of nanoparticles, and are also constituent elements when forming alloys, compounds, and semiconductors.

[0046] [semiconductor] In the present invention, a semiconductor is a material with electrical conductivity intermediate between that of a conductor and an insulator. Examples include silicon, germanium, selenium, and compound semiconductors. Compound semiconductors include II-VI semiconductors such as CdS, CdSe, CdTe, ZnS, ZnSe, CdS, and ZnO; III-V semiconductors such as GaAs, InP, and GaN; IV-VI semiconductors such as PbS and PbSe; IV compound semiconductors such as SiC and SiGe; and I-III-VI semiconductors such as chalcopyrite semiconductors such as CuInSe2. Semiconductors may also contain semimetallic elements such as carbon.

[0047] [Container 201] The container 201 contains the liquid 202. In order to efficiently cool the liquid 202 and to keep the conditions around the electrodes 101 and 102 in the initial state and to easily control the reaction conditions, the production apparatus 10 may be provided with a mechanism for stirring the liquid 202 or causing a flow in the liquid 202.

[0048] [Liquid 202] The liquid 202 is used to generate in-liquid spark discharge using the pair of electrodes 101, 102, and also to temporarily store the products of the in-liquid spark discharge. Depending on the type of nanoparticles to be produced, the liquid 202 may also serve as a material for nanoparticles. The amount of the liquid 202 is not particularly limited, and may be an amount sufficient to allow at least a portion of the pair of electrodes 101, 102 to be present in the liquid 202. The amount of the liquid 202 may be an amount sufficient to prevent the liquid 202 from scattering due to the in-liquid spark discharge described below, and an amount sufficient to prevent the diffusibility of the liquid 202 from being lost due to the concentration of the products of the in-liquid spark discharge. The temperature of the liquid 202 during discharge is not particularly limited, and may be adjusted appropriately depending on the type of the liquid 202 used.

[0049] Discharge is usually carried out in the range of room temperature to 300°C. However, if the temperature of liquid 202 is too high, the vapor pressure of liquid 202 increases, and if liquid 202 is a flammable liquid, there is a possibility of it catching fire due to discharge. On the other hand, if the temperature of liquid 202 is too low, the viscosity of liquid 202 increases, and there is a possibility that the diffusibility of the product of the spark discharge in the liquid is impaired. Therefore, it is preferable to set the temperature of liquid 202 taking these factors into consideration. The upper limit of the temperature is preferably 200°C or less, and more preferably 150°C or less. The lower limit of the temperature is sufficiently easy to operate at or above room temperature, but it may be 30°C or more, or 40°C or more.

[0050] The liquid 202 is not particularly limited as long as it does not affect the reaction for producing the target product. The liquid 202 may be a mixture of two or more compounds. The liquid 202 can be selected depending on the composition of the target nanoparticles. For example, a liquid 202 can be selected that serves as a reaction site for obtaining nanoparticles made from the material used in the electrode, but whose constituent elements are not incorporated into the nanoparticles. Alternatively, to obtain nanoparticles of an alloy or compound, a liquid containing elements that react appropriately with elements derived from the electrode by spark discharge or the like to form an alloy or compound can be used.

[0051] The liquid 202 can be water, hydrogen peroxide, ammonia water, oils, silicone oil, ionic liquid, carbon tetrachloride, halogen solution, molten sulfur, molten selenium, molten tellurium, saturated hydrocarbons, aromatic hydrocarbons, alcohols, esters, ethers, etc.

[0052] For example, the liquid 202 is preferably selected from the group consisting of water, alcohol, toluene, xylene, aqueous ammonia, sulfur, and carbon tetrachloride. The use of these liquids may facilitate the synthesis of nanoparticles of pure single elements from the material used for the electrodes. In particular, if you do not want to react with the liquid, it is preferable to use carbon tetrachloride or toluene.

[0053] The liquid 202 can be, for example, hexane, octane, decane, cyclohexane, cyclooctane, etc. as saturated hydrocarbons. Benzene, toluene, xylene, naphthalene, etc. as aromatic hydrocarbons. Methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, 1,4-butanediol, etc. as alcohols. Methyl acetate, ethyl acetate, butyl acetate, methyl benzoate, dimethyl phthalate, etc. as esters. Tetrahydrofuran, tetrahydropyran, dipropyl ether, dibutyl ether, diethylene glycol, tetraethylene glycol, etc. as ethers.

[0054] Liquid 202 may be water, saturated hydrocarbons, aromatic hydrocarbons, alcohols, or the like, taking into consideration the dispersion, flammability, and oxidizability of the nanoparticles to be produced. For example, it is more preferable to use methanol or ethanol. Furthermore, when synthesis is performed at high temperatures or when subsequent annealing is performed, oils, silicone oils, ionic liquids, or the like that are stable at high temperatures may be used as liquid 202.

[0055] The liquid 202 may be selected taking into consideration the constituent elements in order to compound or modify the nanoparticles. For example, when the liquid 202 is water, oxide nanoparticles can be efficiently obtained; when the liquid 202 contains molten sulfur or sulfur, sulfide nanoparticles can be efficiently obtained; when the liquid 202 contains molten chalcogenide elements such as Se and Te, chalcogenide compound nanoparticles can be efficiently obtained; and when the liquid 202 is alcohols, carbide or carbon-coated nanoparticles can be efficiently obtained. Furthermore, when the liquid 202 is saturated hydrocarbons, aromatic hydrocarbons, alcohols, halogen solutions such as oxygen chloride, or sulfur, nanoparticles modified with carbon, halogen elements, sulfur, etc. can be efficiently obtained.

[0056] [Nanoparticle composition] In the method of the present invention, nanoparticles of a single metal, semimetal, semiconductor, alloy, compound, or the like can be synthesized using the elements that make up the electrodes 101 and 102 and the single elements that make up the liquid 202.

[0057] When obtaining a single metal, by using a metal element with low reactivity for electrodes 101 and 102 and using a liquid 202 that is also less reactive, and creating conditions in which other elements are less likely to react, it is possible to obtain nanoparticles of the single metal with high purity.

[0058] Furthermore, when obtaining an alloy, an alloy of any composition can be obtained by appropriately combining electrodes 101 and 102 and reacting them with liquid 202. This alloy can also be used to obtain so-called immiscible alloys, which are difficult to mix and produce under other manufacturing conditions.

[0059] In addition, elements contained in the liquid 202 can also be used to form nanoparticles, such as alloy nanoparticles, or nanoparticles of compounds such as oxides, nitrides, sulfides, chalcogenide compounds, carbides, chlorides, and halogen compounds.

[0060] When synthesizing oxide nanoparticles, for example, water or hydrogen peroxide solution is used as the liquid 202. When synthesizing nitride nanoparticles, for example, a nitrogen compound such as ammonia water is used as the liquid 202.

[0061] When nanoparticles of a chalcogen compound such as sulfide are synthesized, a chalcogen substance such as sulfur or selenium that is in a liquid state at high temperatures is used as the liquid 202 .

[0062] When synthesizing carbide nanoparticles or carbon-coated nanoparticles, the liquid 202 may be, for example, alcohols or hydrocarbons.

[0063] When synthesizing nanoparticles of a halogen compound, for example, bromine, oxalic acid, hydrofluoric acid, hydrochloric acid, iodine, a halogen element, or a halogen compound is used as the liquid 202. To obtain an alloy, for example, a salt of an alkali element compound, or an acid or salt containing an alkali element compound may also be used as the liquid 202. A salt of an alkaline earth compound, or an acid containing an alkaline earth compound may also be used as the liquid 202.

[0064] The manufacturing method of the present invention not only produces nanoparticles consisting essentially of the elements used in the electrodes, but also allows the nanoparticles to be doped or surface-modified with elements (e.g., oxygen, nitrogen, sulfur, selenium, carbon, bromine, fluorine, chlorine, or iodine) contained in liquid 202. Furthermore, alkali metals, alkaline earth metals, and other metal elements, such as Na, Mg, K, and Ca, can also be used in liquid 202 as appropriate to modify the surface of the nanoparticles.

[0065] The manufacturing method of the present invention not only produces nanoparticles substantially consisting of the elements used in the electrodes, but also allows the nanoparticles to be doped or surface-modified with a noble metal element (e.g., gold, platinum, palladium, rhodium, ruthenium, silver, copper, etc.) contained in the liquid 202. In this case, a solution of a halogen compound or the like may be used as the liquid.

[0066] Furthermore, in this embodiment, when the liquid 202 is saturated hydrocarbons, aromatic hydrocarbons, alcohols, or the like, a carbon coating can be formed to cover the surface of the nanoparticles. When the surface of the nanoparticles is coated with a carbon coating, the nanoparticles become non-toxic, and nanoparticles with excellent biocompatibility can be formed. Therefore, nanoparticles with the surface coated with a carbon coating are expected to be put to practical use in the medical field (for example, the field of cancer treatment).

[0067] [Power supply device 30] The power supply unit 30 is a device that supplies current and adjusts the voltage and current in the manufacturing equipment 10. The power supply unit 30 is designed using a basic spark generator and resistors. There are two types of basic generators: the conventional old model and the new model, which differ in voltage rise and current value.

[0068] The power supply (1) in Figure 2 is a schematic diagram showing an example of a circuit diagram for an older type of power supply. This pulse generator has a circuit consisting of a thyristor and a capacitor, and an internal resistance of about 10 Ω. It outputs a square wave current with a two-step current rise, current rise and fall times of less than 20 μs, and a peak current of about 5 A to 50 A. This power supply has a long rise time and multiple steps. The current used in this circuit is in the time-averaged range of 0.1 to 5 A.

[0069] Power supply (2) in Figure 2 is a schematic diagram of a new type of power supply suitable for the manufacturing method of the present invention. Figure 3 shows the voltage waveform generated by power supply (2). In power supply (2), the pulse generator that generates the square wave has faster rise and fall times than power supply (1), and can output a voltage of 0.5 μs or less, with a width of 100 μs intervals, a peak voltage of 160 V to 250 V, and a frequency of 5 kHz. With an internal resistance of approximately 2 Ω, it is possible to output a current between the electrodes with rise and fall times of 20 μs or less and a peak current of 80 A to 125 A or more. At 50 A or less, the rise and fall times are 5 μs or less.

[0070] Regarding these discharge conditions, the rise and fall times of the current between the electrodes of the spark discharge are steep, at 10 μs or less or 20 μs or less, respectively. Furthermore, regarding these discharge conditions, it is preferable to use a peak voltage of approximately 160 V to 250 V, with voltage rise and fall times of 1 μs or less, and a peak current of 80 A or more, preferably 100 A or more or 125 A or more. Furthermore, it is preferable to use a voltage that is periodically applied in a rectangular wave shape with a time width of 10 μs to 200 μs. Furthermore, it is preferable to repeatedly generate spark discharge at time intervals of 50 μs to 20 ms. It has been found that this enables the efficient synthesis of nanoparticles smaller than conventional nanoparticles. The present invention is based on these findings.

[0071] [Spark discharge] In the manufacturing method of the present invention, repeated spark discharges are generated in the liquid between two electrodes. This causes the electrode material to instantaneously evaporate and briefly become ionized (plasma). When cooled, the elements aggregate, allowing nanoparticles to be synthesized even under conditions that are easy to manufacture, such as room temperature. When a high potential difference is generated between the electrodes 101 and 102, repeated spark discharges are generated in the liquid between their ends.

[0072] When the frequency of the spark discharge is 5 kHz, the temperature of the area where the discharge is occurring reaches 1000K to 50,000K in part. Therefore, it is possible to melt metals that are difficult to melt, vaporize them, and ionize them (turn them into plasma). Also, the current of the new power supply is larger than that of the old power supply, and the temperature becomes extremely high.

[0073] Furthermore, since the repeated spark discharge in liquid is a spark discharge, the discharge is intermittent and the area where the discharge occurs is sharp and localized. Therefore, the high temperature and high pressure caused by the discharge are concentrated in a narrow area, and the temperature rise of the liquid 202 can be suppressed even if the discharge continues for several hours.

[0074] When the elements from the electrodes are turned into plasma, the element ions tend to bond with each other. As a result, nanoparticles can be synthesized due to the short-term plasma state and the rapid cooling effect of the surrounding liquid. This spark discharge can also synthesize nanoparticles of elemental metals, alloys, compounds, etc., depending on the combination of electrodes 101, 102 and liquid 202.

[0075] [Current and Temperature] In the manufacturing method of the present invention, the peak current of the new power supply (2) is more than twice that of the old power supply (1), at 80 A to 125 A or more. Since electrical energy is proportional to the square of the current if the sample resistance remains unchanged, the new power supply generates more than four times the energy of the old power supply. If we consider that all of this energy is converted into thermal energy, the temperature energy is the specific heat multiplied by the temperature. If the specific heat remains unchanged, the temperature would be more than four times higher. However, in plasma, as the temperature increases, the electronic specific heat increases, so the temperature would be lower. On the other hand, since the resistivity of plasma is generally inversely proportional to the 3 / 2 power of the temperature, the current would be even larger, and the temperature could potentially be higher. In any case, the new power supply is expected to generate at least twice the energy generated by the old power supply.

[0076] This rapid temperature rise, when using the new power supply, generates a plasma with a much higher temperature than the old one, and produces more ions than with the old power supply. The higher the temperature, the more homogeneous nucleation is promoted by the cooling effect of the surrounding liquid, and the more rapid cooling rate suppresses crystal growth than with the old power supply, producing many small clusters. The higher the temperature, the more homogeneous nucleation is promoted by the cooling effect of the surrounding liquid, and the more rapid cooling rate suppresses crystal growth, producing significantly more small clusters. It is believed that these clusters are frozen as they are by rapid cooling, resulting in the production of large quantities of extremely small nanoparticles.

[0077] [Peak current] In the manufacturing method of the present invention, the peak current is preferably 80 A or more, 100 A or more, or 125 A or more. Further increasing the plasma temperature generates a large number of ions, maximizing the quenching rate and further increasing the number of metal nuclei, preventing crystal growth and further reducing particle size. The peak current is 50 A or less with older power supplies. However, by adopting newer power supplies, it can be increased to 80 A to 125 A or more. However, the current may be reduced if the electrodes are in contact or are welded together. Furthermore, the current value depends on the type of electrode and the individual discharge phenomenon, so the current value is not necessarily proportional to the voltage.

[0078] The peak current may be 100 A or more, 125 A or more, or 150 A or more. There is no need to set an upper limit to the peak current, but it may be set to 500 A or less, 250 A or less, or 200 A or less as appropriate, in consideration of factors such as longer rise and fall times, circuit design, and control thereof.

[0079] The width, frequency, rise and fall times of the peak current of the spark discharge may be determined according to the voltage of the power supply device 30 that generates the spark discharge, or may depend on various conditions in the manufacturing apparatus 10.

[0080] The method for producing nanoparticles of the present invention involves forming nanoparticles by repeatedly generating spark discharges in a liquid between two electrodes immersed in the liquid. Each electrode is made of, for example, a single bulk metal of the target nanoparticles. By applying a large current of 80 A or more, preferably 100 A or more or 125 A or more, nanoparticles smaller than those previously produced can be synthesized.

[0081] [Vibration device 40] The manufacturing apparatus 10 may include a vibration device 40. The vibration device 40 applies vibration to the electrodes 101 and 102. By applying vibration to the electrode 20, it is possible to prevent deposits formed on the surface of the electrode 20 from remaining on the surface of the electrode 20, thereby enabling efficient discharge. The vibration device 50 may apply vibration to the electrode 20 periodically or intermittently. It is not necessary to apply vibration.

[0082] In the manufacturing method of the present invention, nanoparticles are synthesized in a liquid. If the synthesized nanoparticles are Brownian particles, the synthesized nanoparticles tend to float in the liquid, preventing aggregation of the nanoparticles. Although the synthesized nanoparticles may settle in the liquid, aggregation of the nanoparticles is unlikely even in this precipitation. Here, aggregation refers to the phenomenon in which nanoparticles are bound to each other by van der Waals forces or the like, making them difficult to separate by stirring. Thus, the manufacturing method of the present invention is less likely to cause aggregation of nanoparticles, allowing the nanoparticles to be used in a dispersed state in the liquid 202. A surfactant may also be used to prevent aggregation.

[0083] The solution in which the nanoparticles formed by the above-described manufacturing method are dispersed in the liquid 202 may be used as the nanoparticle product (nanoparticle solution). Alternatively, the nanoparticles that have settled in the liquid 202 may be separated and used as the nanoparticle product. A centrifugal separation process may be used as a method for separating small nanoparticles from large nanoparticles in the liquid 202. Alternatively, the supernatant liquid (the supernatant liquid of the solution) that is generated in the solution when the nanoparticles floating in the solution settle naturally and form a precipitate layer in the container 30 may be removed from the container 30, and the removed supernatant liquid may be used as the nanoparticle product (nanoparticle solution).

[0084] Here, the nanoparticles do not aggregate and settle, but remain dispersed in the solution, forming dispersoid colloid particles. Generally, dispersoid colloid particles universally exhibit interparticle van der Waals forces, which are the sum of intermolecular forces and are synonymous with surface tension. Meanwhile, a potential difference exists on the surface of dispersoid particles due to the polarity difference between the composition and the solvent, forming a diffuse electric double layer with counterions of opposite sign to the surface potential. Since particles of the same type have counterion double layers of the same type, when particles approach each other, the double layers overlap, and an osmotic repulsion is generated by the entropy effect derived from ion diffusion, preventing particle aggregation and stabilizing the dispersion.

[0085] In addition, when classifying dispersed colloids that use water as a dispersion medium, those that precipitate easily when an electrolyte is added are sometimes called hydrophobic colloids, while those that do not precipitate easily are called hydrophilic colloids. Hydrophilic colloids, like hydrophobic colloids, have a surface charge, and are coordinated with many water molecules through hydration (solvation), which further strengthens the steric repulsion and stabilizes them. Some hydrophilic colloids surround hydrophobic colloids to prevent coagulation, and colloids in this state are called protective colloids. Protective colloids may also have proteins adsorbed to their surface, changing the surface potential and stabilizing them.

[0086] [Surfactants] Nanoparticles in liquid tend to aggregate together, forming larger particles, ceasing to be Brownian particles and causing sedimentation and settling. Surfactants are substances that have parts of their molecules that are compatible with water (hydrophilic groups) and parts that are compatible with oil (lipophilic and hydrophobic groups), and are often called amphiphilic molecules. By forming micelles, vesicles, and lamellar structures, they function to mix polar and non-polar substances uniformly. They also have the effect of weakening surface tension. As a result, nanoparticles can be dispersed in liquid without agglomeration. This is effective for both water and oil. When synthesizing nanoparticles using spark discharge, a surfactant may be added to the solution to prevent aggregation of the nanoparticles as much as possible.

[0087] [Centrifugal separation process] The manufacturing method of the present invention preferably includes a centrifugation step in which a nanoparticle-containing liquid is centrifuged and the supernatant is collected. This involves passing the liquid containing nanoparticles obtained by spark discharge through an ultracentrifuge, allowing large nanoparticles to settle, and then collecting the supernatant, which then selects and collects only the small nanoparticles. This step can also be expected to disperse any aggregates that may have formed due to the aggregation of nanoparticles. For example, an ultracentrifuge CS150NX from Koki Holdings can be used for the centrifugation step. The supernatant liquid in which the nanoparticles are dispersed can be considered a dispersoid colloid.

[0088] When separating by centrifugation, because the nanoparticles obtained by the production method of the present invention are extremely small, it is preferable to use a relative centrifugal acceleration of 100 kG (100,000 G) or more. The relative centrifugal acceleration is more preferably 200 kG or more, more preferably 400 kG or more, and even more preferably 600 kG or more. The centrifugation time depends on the acceleration, but is preferably 1 minute or more, more preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more. The centrifugation conditions are appropriately set taking into account the product of the relative centrifugal acceleration and the centrifugation time. The smaller the particle size of the nanoparticles to be obtained, the higher the acceleration and the longer the time are preferable. The upper limit of the relative centrifugal acceleration does not need to be specified, but it may be 2,000 kG or less, 1,000 kG or less, or 800 kG or less. The centrifugation time may be 120 minutes or less, 90 minutes or less, 60 minutes or less, or 30 minutes or less.

[0089] As described above, the manufacturing method of the present invention can produce various nanoparticles by combining electrodes 101, 102 and liquid 202. Furthermore, by setting the current within a predetermined range, extremely small nanoparticles can be produced more efficiently than conventional methods. Furthermore, this manufacturing method requires low electrical energy and does not require a high vacuum or high-voltage power supply, which reduces capital investment. Furthermore, the equipment is easy to design and manage, and mass production is also easy depending on the size of electrodes 101, 102 and container 201, etc., making the manufacturing method of the present invention suitable for mass production of high-quality nanofunctional materials.

[0090] [Nanoparticles] The nanoparticles of the present invention have an average particle size of 2.0 nm or less as measured by dynamic light scattering, and contain any material selected from the group consisting of metals, metal compounds, and semiconductors.

[0091] In the present invention, nanoparticles are particles ranging in size from nanometer to sub-nanometer. The nanoparticles produced by the production method of the present invention are formed as particles containing elements mainly used as electrodes. The nanoparticle-containing liquid of the present invention is a liquid containing the nanoparticles of the present invention. For example, it is a dispersion (solution) in which the nanoparticles formed by the production method of the present invention are dispersed in a liquid.

[0092] The average particle size of the nanoparticles of the present invention is measured by dynamic light scattering. Dynamic light scattering can be performed using, for example, Otsuka Electronics Co., Ltd.'s "Particle Size / Molecular Weight Measurement System ELSZ-2000S."

[0093] [TEM observation] Transmission electron microscope (TEM) observations were performed using a Philips high-resolution electron microscope (HR-TEM), F20. The resolution of this TEM device is approximately 1 nm, depending on how it is used.

[0094] [Dynamic Light Scattering] In the photon correlation method of dynamic light scattering, the initial slope (decay constant Γ) of the autocorrelation function is determined by analyzing it using the cumulant method. This analysis yields the typical Brownian motion speed (diffusion coefficient D) of particles in a liquid sample. Substituting this into the Einstein-Stokes equation yields the average particle size. The standard deviation (spread width) of each peak determined from the particle size distribution is calculated. The difference between the particle size of each peak calculated above and the particle size of each individual bar is calculated, squared, and then multiplied by the frequency. Dividing this sum by the sum of the frequencies of each peak and taking the square root yields the standard deviation of each peak. In a Gaussian particle size distribution, the minimum particle size is called D0, the peak value is called D50, the maximum value is called D100, and 90% of the minimum and maximum is called D90. The minimum measurement range listed in the ELSZ-2000S particle size and molecular weight measurement system's catalog is 0.6 nm. According to Otsuka Electronics Co., Ltd., this means that they guarantee values ​​up to 0.6 nm, and if a value below that is displayed, it indicates that the average particle size is less than 0.6 nm.

[0095] The average particle size of the nanoparticles of the present invention may be set depending on the type of particle, but can be 2.0 nm or less, preferably 1.7 nm or less, and may be 1.5 nm or less, 1.4 nm or less, or 1.2 nm or less. Furthermore, depending on the type of constituent elements, it may be 1.0 nm or less, 0.8 nm or less, 0.7 nm or less, or 0.6 nm or less, as appropriate. The lower limit of the average particle size is set appropriately depending on the type and size of the elements constituting each nanoparticle, and may be 0.08 nm or more, 0.1 nm or more, 0.2 nm or more, or the like. The average particle size may be the number-average average particle size of particles measured by dynamic light scattering.

[0096] The nanoparticles of the present invention are preferably monodisperse. The term "monodisperse" refers to the fact that, when graphed by dynamic light scattering with particle size and number distribution as axes, there is one peak. This peak is considered to be a peak with a number that is more than half the number of the largest peak in the number distribution.

[0097] The nanoparticles of the present invention preferably have a small deviation in particle size. The nanoparticles of the present invention have an extremely narrow variation from the average particle size, and are easy to handle as nanoparticles containing a large amount of particles with a stable small particle size. The deviation may be 5 or less, 3 or less, 1 or less, 0.7 or less, 0.5 or less, or 0.2 or less, as a standard deviation of particle size (nm).

[0098] The nanoparticles of the present invention preferably have a small 90% particle size (D90) based on the number distribution. The nanoparticles of the present invention are thus obtained as nanoparticles essentially consisting of extremely small particles, and are easy to handle as they have an extremely low probability of containing coarse particles. D90 is preferably 3 times or less the average particle size, more preferably 2.5 times or less, and even more preferably 2.0 times or less. D90 may be set to 5.0 nm or less, 4.0 nm or less, 3.0 nm or less, 2.0 nm or less, 1.5 nm or less, 1.0 nm or less, 0.8 nm or less, or 0.6 nm or less, depending on the type of nanoparticles and the average particle size.

[0099] Dynamic light scattering (DLS) particle size measurements tend to measure the size of aggregates when nanoparticles aggregate and form clumps rather than being dispersed. Therefore, the minimum value is close to the size of a single nanoparticle. In contrast, TEM observation allows for the identification of the shape and size of individual nanoparticles. Therefore, the particle size measured by DLS is consistent with that measured by TEM, and if it is small, it can be interpreted as the size of a single nanoparticle. However, the resolution of TEM observations is limited, and the Philips high-resolution electron microscope (HR-TEM) F20 used in this experiment makes it difficult to accurately measure particle sizes below approximately 2 nm. However, the catalog limit for the particle size measurement range of the dynamic light scattering particle size and molecular weight measurement system ELSZ-2000S used in this experiment is 0.6 nm. The resolution of the Philips high-resolution electron microscope (HR-TEM) F20 is approximately 1 nm, so particles below 2 nm can be detected, but the particle size cannot be accurately determined. Therefore, particle sizes below this size can only be measured by dynamic light scattering.

[0100] [Elements that make up nanoparticles] The nanoparticles of the present invention contain metal and / or semimetal and semiconductor elements. These metals and semiconductors contain at least the elements contained in the materials used for the electrodes described in the manufacturing method of the present invention. In addition to the elements derived from the electrode, they may also contain elements derived from the liquid, as appropriate. The elements constituting the nanoparticles containing elements derived from these electrodes or liquids are the same as those described in the manufacturing method of the present invention.

[0101] The nanoparticles of the present invention may contain, as a constituent element, any one selected from the group consisting of noble metals, base metals, and alloys. Alternatively, the nanoparticles may essentially consist of any one selected from the group consisting of noble metals, base metals, and alloys. These noble metals, base metals, and alloys are as described above in the manufacturing method of the present invention. Similarly, the nanoparticles of the present invention may be semiconductor particles.

[0102] In the field of catalysis, the development of high-performance catalysts such as platinum (Pt), rhodium (Rh), and palladium (Pd) is one of the most important issues related to industrial infrastructure, including the environment, resources, and energy. In particular, the development of high-performance precious metal catalysts for exhaust gas catalysts and fuel cell catalysts is an urgent issue due to their scarcity and importance, and the creation of new nanoparticles is essential to this task. Pt, Pd, and Ru are used as exhaust gas catalysts, while Pt and its alloys are used as fuel cell catalysts, and precious metals are also used as elemental catalysts for hydrogen oxidation reactions. Pd is also used as a catalyst in the manufacturing process of IC and LSI semiconductors.

[0103] In catalysts, as particle size decreases, the surface area increases, significantly improving reactivity and catalytic properties. Therefore, the synthesis of small nanoparticles has become an important issue.

[0104] [Gold (Au) nanoparticles] The nanoparticles of the present invention can be gold nanoparticles consisting essentially of gold. The gold nanoparticles preferably have an average particle size of 0.7 nm or less, as measured by dynamic light scattering. More preferably, the average particle size may be 0.65 nm or less, 0.60 nm or less, or 0.50 nm or less. The lower limit need not be specifically determined and may be set based on the size of the gold element alone. Regarding gold nanoparticles, the particle size of undecagold is approximately 0.82 nm. The nanoparticles of the present invention are provided as nanoparticles with an average particle size smaller than undecagold, and can be more reactive than undecagold. The supernatant liquid in which these nanoparticles are dispersed can be considered a dispersoid colloid.

[0105] [Palladium (Pd) nanoparticles] The palladium nanoparticles of the present invention can be palladium (Pd) nanoparticles consisting essentially of palladium. The Pd nanoparticles preferably have an average particle size of 1.7 nm or less, as measured by dynamic light scattering. More preferably, the particle size may be 1.6 nm or less, 1.5 nm or less, or 1.4 nm or less. The lower limit may not be particularly specified and may be set based on the size of the palladium element alone or its intended use. For example, the lower limit of the average particle size of Pd nanoparticles may be 0.5 nm or more, 0.8 nm or more, or 1.0 nm or more. The nanoparticles of the present invention are provided as nanoparticles with an average particle size smaller than conventional Pd nanoparticles and can have excellent reactivity, etc. The supernatant liquid in which these nanoparticles are dispersed can be considered a dispersoid colloid.

[0106] [Platinum (Pt) nanoparticles] The palladium nanoparticles of the present invention can be platinum (Pt) nanoparticles consisting essentially of platinum. The Pt nanoparticles preferably have an average particle size of 1.7 nm or less, as measured by dynamic light scattering. More preferably, they may be 1.6 nm or less, 1.5 nm or less, or 1.4 nm or less. The lower limit does not need to be specifically determined and may be set based on the size of the platinum element alone or its intended use. For example, the lower limit of the average particle size of Pd nanoparticles may be 0.5 nm or more, 0.8 nm or more, or 1.0 nm or more. The nanoparticles of the present invention are provided as nanoparticles with an average particle size smaller than conventional Pt nanoparticles and can have excellent reactivity, etc. The supernatant liquid in which these nanoparticles are dispersed can be considered a dispersoid colloid.

[0107] Quantum dots, such as CdSe quantum dots, can emit light from blue-green to red (500-650 nm) by varying their particle size from approximately 3 nm to 5 nm. Generally, quantum dot particle size can be controlled by the temperature or reaction time of the thermal decomposition reaction of the organometallic compound used in their synthesis. The band gap of quantum dots also depends on the type of semiconductor. Currently, quantum dots that emit light in the visible to near-infrared range (400-2000 nm) can be synthesized using semiconductors such as ZnSe, CdS, CdSe, CdSeTe, PbS, PbSe, Si, and C. Thus, there is a need for an efficient method for producing smaller nanoparticles. The present invention provides a method for efficiently producing such nanoparticles. Reducing the size of these quantum dots to 3 nm or less would enable them to emit light in the ultraviolet range, dramatically expanding their applications.

[0108] [Nanoparticle morphology] The nanoparticles of the present invention can be used in a state where they are dispersed in a liquid, or can be provided as a mixed composition in which they are combined with materials according to the intended use, such as in the form of a paste.

[0109] [Applications of nanoparticles] Nanoparticles have superior surface area, reactivity, catalytic properties, and magnetism compared to bulk materials and powders. For this reason, they can be used in a variety of fields, including catalysts, magnetic materials, battery materials, semiconductor materials, nanodots, photoelectric materials, medical materials, pharmaceutical materials, health foods, and dyes, as well as in the environment, IT, printing, and medicine.

[0110] [Example] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not changed.

[0111] [Example 1] Example of palladium nanoparticles Production was carried out using a manufacturing apparatus similar to that shown in Figure 1. Two cylindrical electrodes (99.95% purity, 3 mm diameter) of palladium, a simple element, were immersed in a 200 mL beaker containing water as the liquid, with the tip of the electrodes spaced approximately 0.5 mm apart. While the electrodes were vibrated, a spark discharge was induced between the palladium electrodes by repeated current supplied from a power source, repeatedly generating plasma and synthesizing palladium nanoparticles. When testing the effect of the presence or absence of a surfactant, a drop of Kao Corporation's "Cucute" (registered trademark) was added to the liquid in the beaker as the surfactant.

[0112] As shown in Figure 2, power supply 30 is designed using a pulse generator and resistors. There are two types of pulse generators: a conventionally known old model (power supply (1)) and a new, improved version (power supply (2)). As mentioned above, the voltage, current rise, and peak current differ. The pulse generator of the new power supply 30 can output a square-wave voltage with a rise and fall time of 1 μs or less, a width of approximately 100 μs, a peak voltage of approximately 160 V to 250 V, and a frequency of 5 kHz, as shown in Figure 3. Furthermore, the internal resistance of the old model was approximately 10 Ω, while the new model is approximately 2 Ω. The peak current of the old model was 50 A or less, while the new model is 80 A to 125 A or more, with a current width of approximately 10 μs to 50 μs.

[0113] Figure 3 shows an example of the voltage waveform measured by the generator of the new power supply (2). A repeating square wave with a period of 5 kHz and a rise and fall time of less than 0.5 μs, an interval of 100 μs, was obtained.

[0114] In the experiment, the generator voltage setting values ​​for power supply (1) were 60V, 100V, and 200V, and the generator voltage setting values ​​for power supply (2) were 15V and 40V. In this specification, the values ​​shown for the voltages of power supply (1) and power supply (2) are the set voltage values ​​for power supply (1) and power supply (2), and are not the measured values ​​of the output voltages of power supply (1) and power supply (2). Approximately 0.5g of palladium nanoparticles can be obtained by manufacturing them in one hour, on a dry basis. For the shape measurements below, we used nanoparticles manufactured in approximately one hour.

[0115] Figure 4 (Figures 4A to 4E) shows current waveforms when a palladium electrode is used. Figures 4A, 4B, and 4C are waveforms obtained by power supply (1). Figures 4D and 4E are waveforms obtained by power supply (2).

[0116] For power supply (1), the rise time is approximately 15 μs or less for two or one stage at 60 V, 100 V, and 200 V, with a peak current of 50 A or less. For power supply (2), the rise time is 10 μs to 20 μs or less at 15 V and 40 V, with a peak current of 80 A to 125 A or more. In this specification, the values ​​shown for the current of power supply (1) and power supply (2) are measured values ​​of the output current of power supply (1) and power supply (2). The current values ​​are measured using a current transformer (Pearson Electronics Model 4100). The interval is generally 100 μs, but may be slightly faster or more irregular depending on the shaping circuit and discharge characteristics. Here, the current value depends on the type of electrode and the individual discharge phenomenon, so the current value is not necessarily proportional to the voltage.

[0117] [Optical observation and photography] Figure 5 (Figures 5A to 5D) are photographs of glass containers containing palladium nanoparticle-containing liquid. Figure 5A: Power supply (1) 60 V, Figure 5B: Power supply (1) 100 V, Figure 5C: Power supply (1) 200 V. From the left, these are days 0, 1, 2, and 3. Figure 5D: Power supply (2) 5 V to 40 V, more than six months have passed.

[0118] With power supply (1), nanoparticles gradually precipitated each day at all voltages, and by the third day, most of the particles had sunk. However, the settling of Brownian particles slowed as the voltage increased. This indicates that the particle size of the suspended particles decreased as the voltage increased (Figures 5A-5C). With power supply (2) at 15 V, the particles remained dispersed and suspended even after six months (Figure 5D). This suggests that the particle size of the palladium nanoparticles synthesized with power supply (2) was significantly smaller than that of (1). The supernatant liquid in which these nanoparticles were dispersed can be considered a dispersoid colloid.

[0119] In the manufacturing method of the present invention, in order to recover nanoparticles with small particle diameters, the nanoparticle-containing liquid manufactured in Example 1 was subjected to a centrifugation process. The centrifugation process was performed using an ultracentrifuge CS150NX manufactured by Koki Holdings. In this case, eight 4cc centrifuge tubes were used. The centrifugation was performed at an acceleration of 600 kG for 10 minutes.

[0120] [TEM observation] TEM observation was performed to directly observe the shape and size of the nanoparticles. The TEM used was a Tecnai F20 manufactured by FEI. The results of FE-TEM observation of palladium nanoparticles are shown in Figures 6A to 6E. The results are summarized in Table 1.

[0121] As can be seen in Figures 6A-6D, samples obtained using power supply (1) with a peak current of 50 A or less produced nanoparticles with an average particle size of 3 nm or more, with no nanoparticles smaller than 2 nm being obtained. In contrast, samples obtained using power supply (2) with a peak current of 125 A or more produced nanoparticles with a particle size of 2 nm or less, as can be seen in the TEM images in the upper right and lower left of Figure 6E, although their dimensions cannot be accurately determined (arrows in the image). Note that the nanocrystals larger than 2 nm in the 40 V image were used to focus the TEM. Analysis of the diffraction grating confirmed that the nanoparticles were palladium with a face-centered cubic lattice structure.

[0122] The tendency is that the higher the voltage, the smaller the average particle size of the aggregated sample, and the average particle size is also smaller when a surfactant is used. This means that the higher the input energy, the finer the particles become. It can also be said that the surfactant prevents aggregation.

[0123] [Dynamic Light Scattering] The dynamic light scattering method was used to measure particle size using an ELSZ-2000 made by Otsuka Electronics Co., Ltd. This device estimates the particle size of nanoparticles in liquid by measuring the scattered light of a laser beam. The average particle size of palladium nanoparticles in water was measured using dynamic light scattering. The results of measuring the particle size of palladium particles using dynamic light scattering are shown in Figure 7 (Figures 7A to 7H). The results are summarized in Table 1.

[0124] [Scattering intensity distribution] Figures 7A–7F show the results of dynamic light scattering measurements of particle size for samples obtained using power supply (1) with a peak current of 50 A or less. Figures 7G–7I show the results of dynamic light scattering measurements of samples obtained using power supply (2) with a peak current of 125 A or more. The particle size of samples obtained without the centrifugation process using power supply (1) (Figures 7A–7D) is orders of magnitude larger than that obtained using power supply (2) (Figures 7F–7H). TEM observations revealed nanoparticles with average diameters of 3–7 nm, suggesting that the nanoparticles had aggregated to form aggregates. However, no particles were observed in the sample obtained using power supply (1) after the centrifugation process (Figure 7E). This is likely due to the fact that the nanoparticle aggregates obtained using power supply (1) settled during the centrifugation process and were not present in the supernatant, resulting in the absence of the target ultrasmall nanoparticles. Regarding samples obtained using power supply (2) after the centrifugation process, measurements were performed three times on the same sample to ensure reliability. As a result, as can be seen in Figures 7F to 7H, the average particle sizes ultimately obtained were 1.4, 1.6, and 1.7 nm. Therefore, it can be concluded that power supply (1) did not produce nanoparticles with an average size of 3 nm or more but not 2 nm or less, whereas power supply (2) produced extremely small nanoparticles with an average size of 2 nm or less. TEM observation of the particle sizes of the samples obtained through these centrifugation processes revealed that particles with an average size of 2 nm or less were observed, and it can be said that 1.4, 1.6, and 1.7 nm are the particle sizes of individual nanoparticles.

[0125] [Table 1]

[0126] Table 1 summarizes the results of measuring the particle size of Pd. In Table 1, *1 indicates that in FE-TEM observation of power source (2), many nanoparticles with a particle size of 2 nm or less were observed, but the size could not be determined accurately. Using dynamic light scattering, the average particle sizes were measured to be 1, 4, 1.6, and 1.7 nm.

[0127] Table 1 summarizes the results of dynamic light scattering and FE-TEM measurements of the particle size of palladium nanoparticles. Optical observation showed that most of the nanoparticles from power source (1) settled within about three days, while the nanoparticles from power source (2) remained floating even after six months. This indicates that the nanoparticles from power source (2) are significantly smaller than those from (1).

[0128] TEM observations showed that the sample from power source (1) had an average particle size of approximately 3 nm to 7 nm. However, in the sample from power source (2), nanoparticles with an average particle size of 2 nm or less were observed, although their size was too small to be determined. Measurements using dynamic light scattering showed that for samples from power source (1), only the particle size of the aggregates could be measured without the centrifugation process. However, for nanoparticles from the sample from power source (2) that underwent the centrifugation process, the average particle size of the number distribution was 1.4 nm to 1.7 nm after three measurements, which was significantly smaller. Therefore, it was determined that nanoparticles of 2 nm or less were obtained using power source (2).

[0129] This difference in particle size is likely due to the current waveforms flowing through the electrodes. The older power supply (1) had a two-stage or single-stage rise time of about 20 μs at both 60 V and 200 V, with a peak current of less than 50 A. The newer power supply (2) had a current of 80 A to 125 A or more. As mentioned above, the newer power supply (2) had a peak current more than double that of the older power supply (1), resulting in an extremely high temperature rise and the rapid generation of plasma, which produced numerous ions. The higher the temperature, the more homogeneous nuclei were generated by cooling with the surrounding liquid. The intense rapid cooling effect suppressed crystal growth, resulting in the generation of significantly more small clusters. It is believed that these clusters were then frozen in place by the rapid cooling, resulting in the production of a large amount of extremely small nanoparticles.

[0130] The conclusion is that nanoparticles with a particle size of 2.0 nm or less can be efficiently synthesized by using power supply (2) with a voltage rise and fall time of 1 μs or less, and passing a repetitive current to the electrodes with a peak current value of 80 A to 125 A or more and a rise time of 10 μs to 20 μs or less. In this case, the centrifugation process is important for selecting only the smallest nanoparticles and dispersing aggregates.

[0131] [Example 2] Example of gold nanoparticles Gold nanoparticles were produced using the same equipment as used to produce palladium in Example 1. Two cylindrical electrodes (99.99% purity, 3 mm diameter) of gold, a simple element, were immersed in a 200 mL beaker containing water, with the distance between the tips of the electrodes approximately 0.5 mm. While the electrodes were vibrated, a spark discharge was induced between the gold electrodes by repeated current supplied from a power source, repeatedly generating plasma and synthesizing gold nanoparticles. The power supply 30 was similar to that used in Example 1.

[0132] The peak voltage of the generator was set at 60V, 100V, and 200V at power supply (1), and at 200V at power supply (2).

[0133] By completing the production in one hour, approximately 2.1 g of gold nanoparticles can be obtained in dry weight. For the following shape measurements, we used nanoparticles produced in about one hour.

[0134] Figures 8A to 8D show current waveforms when gold electrodes are used. Figures 8A to 8C show waveforms obtained using power supply (1). Figure 8D shows a waveform obtained using power supply (2). With power supply (1), the rise time is approximately 10 to 20 μs or less in two steps or one step at 60 V, 100 V, and 200 V, with a peak current of 50 A or less. With power supply (2), the rise time is 20 μs or less, with a peak current of 125 A or more. The interval is basically 100 μs, but may be slightly earlier or more irregular depending on the shaping circuit and discharge characteristics. Here, the current value depends on the type of electrode and the individual discharge phenomenon, so the current value is not necessarily proportional to the voltage.

[0135] [Observations and Photos] Figures 9A to 9D are photographs of glass containers containing gold nanoparticle-containing liquid. Figure 9A: Power supply (1) 60 V, Figure 9B: Power supply (1) 100 V, Figure 9C: Power supply (1) 200 V. From the left, these are days 0, 1, 2, and 3. Figure 9D: Power supply (2) 200 V, several months have passed.

[0136] The tendency is that the higher the voltage, the longer the nanoparticles remain suspended. In other words, the higher the voltage, the slower the settling rate and the smaller the particle size of the suspended particles.

[0137] With power supply (1), nanoparticles gradually precipitated each day at all voltages, and by the third day, most of the particles had settled. However, the settling of Brownian particles slowed as the voltage increased. This indicates that the particle size of the suspended particles became smaller as the voltage increased (Figures 9A-9C). With power supply (2) at 15 V, the particles remained dispersed and suspended even after several months (Figure 9D). This suggests that the particle size of the gold nanoparticles synthesized with power supply (2) was significantly smaller than that of (1). The supernatant liquid in which these nanoparticles were dispersed can be considered a dispersoid colloid.

[0138] In the manufacturing method of the present invention, the nanoparticle-containing liquid of Example 2 was subjected to a centrifugal separation process in order to recover nanoparticles with small particle sizes. The centrifugal separation process was performed using an ultracentrifuge CS150NX manufactured by Koki Holdings. Eight 4cc centrifuge tubes were used. The process was performed at an acceleration of 600 kG for 10 minutes.

[0139] [TEM] TEM observation was performed to directly observe the shape and size of the nanoparticles. The TEM used was a Tecnai F20 manufactured by FEI. The results of FE-TEM observation of gold nanoparticles are shown in Figures 10A to 10C. The results are summarized in Table 2.

[0140] As shown in Figures 10A and 10B, samples taken with a power supply (1) with a peak current of 50 A or less yielded average particle sizes of 3 nm or more, with no particles smaller than 2 nm. In contrast, samples taken with a power supply (2) with a peak current of 125 A or more yielded particles smaller than 2 nm, as seen in the image at the bottom left of Figure 10C. While operating the TEM, even smaller particles were observed with the naked eye on the monitor. However, these were too small, exceeding the imaging limit of the FE-TEM (approximately 1 nm), making them difficult to record during imaging. Note that the nanocrystals larger than 2 nm in the 200 V image were used for focusing the TEM. Analysis of the diffraction grating confirmed that the nanoparticles were gold with a face-centered cubic lattice structure.

[0141] The tendency is that the higher the voltage, the smaller the average particle size of the aggregated sample, and the average particle size is also smaller when a surfactant is used. This means that the higher the input energy, the finer the particles become. It can also be said that the surfactant prevents aggregation.

[0142] [Dynamic Light Scattering] Dynamic light scattering particle size measurements were performed using the same equipment as used for palladium. The results of dynamic light scattering particle size measurements for gold nanoparticles are shown in Figure 11. The results are summarized in Table 2.

[0143] [Scattering intensity distribution] Figures 11A–11E show the results of dynamic light scattering measurements of particle size for samples obtained using power supply (1) with a peak current of 50 A or less. Figures 6F–6H show the results of dynamic light scattering measurements of samples obtained using power supply (2) with a peak current of 125 A or more. The particle sizes of samples obtained without the centrifugation process using power supply (1) (Figures 11A–11D) are orders of magnitude larger than those obtained using power supply (2) (Figures 11F–11H), except for Figure 11D. TEM observations revealed nanoparticles with an average diameter of approximately 5 nm or larger, suggesting that the nanoparticles have aggregated to form aggregates. However, the particle size of the sample obtained using power supply (1) but then centrifuged (Figure 6E) was 10.6 nm. This is also likely due to some aggregation. To ensure reliability, measurements were performed three times on the same sample obtained using power supply (2) with the centrifugation process. The average particle size was ultimately 0.2 nm for all samples. The standard deviation was 0.1 nm to 0.5 nm, and the 90D was 0.6 nm, demonstrating a significantly smaller size. However, the minimum measurement limit listed in the catalog for this device is 0.6 nm. This means that values ​​up to 0.6 nm are guaranteed, and any value displayed below this indicates an average particle size of less than 0.6 nm. Therefore, the average particle size was determined to be less than 0.6 nm. Therefore, while power supply (1) did not produce nanoparticles with an average diameter of 3 nm or more but less than 2 nm, power supply (2) produced extremely small nanoparticles with an average diameter of less than 0.6 nm. TEM observation of the particles obtained from these centrifugal processing processes confirmed the presence of particles with an average diameter of less than 2 nm, which can be said to represent the size of individual nanoparticles less than 0.6 nm.

[0144] [Table 2]

[0145] Table 2 summarizes the results of gold particle size measurements. In Table 2, *1 indicates that FE-TEM observation of power source (2) confirmed numerous nanoparticles with a particle size of 2 nm or less, but the size could not be determined accurately. Dynamic light scattering measured the average particle size to be less than 0.6 nm.

[0146] Table 2 summarizes the results of the gold nanoparticle particle size measurements using dynamic light scattering and FE-TEM. Optical observation showed that most of the nanoparticles from power source (1) settled within about three days, while those from power source (2) remained floating even after several months. This indicates that the nanoparticles from power source (2) are significantly smaller than those from power source (1).

[0147] TEM observations showed that the sample from power source (1) had an average particle size of approximately 5 nm. However, for the sample from power source (2), nanoparticles of less than 2 nm could be seen with the naked eye on the monitor, but due to the performance of the TEM equipment, they were too small to be imaged. The results of dynamic light scattering measurements showed that for samples from power source (1) without the centrifugation process, only the particle size of the aggregates could be measured. However, for nanoparticles from the sample from power source (2) that had been centrifuged, the average particle size of the number distribution was significantly smaller, less than 0.6 nm, after three measurements. Therefore, it was determined that nanoparticles less than 0.6 nm were obtained using power source (2).

[0148] This difference in particle size is likely due to the current waveforms flowing through the electrodes. The older power supply (1) had a two-stage or single-stage rise time of about 20 μs at both 60 V and 200 V, with a peak current of less than 50 A. The newer power supply (2) also had a current of 80 A to 125 A or more. As mentioned previously, the newer power supply (2) had a peak current more than double that of the older power supply (1), resulting in an extremely high temperature rise and the rapid generation of plasma, which produced numerous ions. The higher the temperature, the more homogeneous nuclei were generated by cooling with the surrounding liquid. The intense rapid cooling effect suppressed crystal growth, resulting in the generation of significantly more small clusters. These clusters were then frozen in place by the rapid cooling, resulting in the production of a large amount of extremely small nanoparticles.

[0149] Furthermore, with palladium, the particle size was approximately 2 nm or less at 40 V of power supply (2), but with gold, it was less than 0.6 nm at 200 V. This is thought to be because with palladium, effects such as welding occurred when the voltage was increased above 40 V, whereas with gold, with power supply (2) at 200 V, the current was increased to over 125 A, reaching nearly 150 A, raising the plasma temperature and maximizing the quenching rate, further raising the plasma temperature, increasing the number of metal nuclei generated, and further reducing the particle size. It is thought that with palladium, the particle size can be made even smaller by using a higher voltage such as 200 V of power supply (2).

[0150] The conclusion is that nanoparticles with a particle size of less than 0.6 nm can be efficiently synthesized by using power supply (2) with a voltage rise and fall time of less than 1 μs, and passing a repetitive current to the electrodes with a peak current value of 80 A to 125 A or more and a current rise time of 10 μs to 20 μs or less. In this case, the centrifugation process is important for selecting only the smallest nanoparticles and dispersing aggregates.

[0151] [Example 3] Example of platinum nanoparticles Production was carried out using a configuration similar to that of the production apparatus shown in Figure 1. Two cylindrical electrodes (99.99% purity, 3 mm diameter) of platinum, a simple element, were immersed in a 200 mL beaker containing water as a liquid, with the distance between the tips of the electrodes being approximately 0.5 mm. While the electrodes were vibrated, a spark discharge was induced between the platinum electrodes by repeated current supplied from a power source, repeatedly generating plasma and synthesizing platinum nanoparticles. The power supply 30 was similar to that used in Example 1.

[0152] In the experiment, the peak voltage of the generator was set at 100V for power supply (1) and 200V for power supply (2).

[0153] By manufacturing for one hour, approximately 0.5 g of platinum nanoparticles can be obtained in dry weight. For the following shape measurements, we used nanoparticles manufactured in approximately one hour.

[0154] 12A and 12B are diagrams showing current waveforms when platinum electrodes are used. Fig. 12A shows the waveform from power supply (1), and Fig. 12B shows the waveform from power supply (2).

[0155] Power source (1) has a two-stage or single-stage rise time of approximately 15 μs or less, with a peak current of 50 A or less. Power source (2) has a rise time of 10 μs to 20 μs or less, with a peak current of 125 A or more. The interval is basically 100 μs, but depending on the shaping circuit and discharge characteristics, it may be slightly earlier or more irregular. Here, the current value depends on the type of electrode and the individual discharge phenomenon, so the current value is not necessarily proportional to the voltage.

[0156] In the manufacturing method of the present invention, in order to recover nanoparticles with small particle sizes, the nanoparticle-containing liquid of Example 3 was subjected to a centrifugation process for both samples (1) and (2). The centrifugation process was performed using an ultracentrifuge CS150NX manufactured by Koki Holdings. In this case, eight 4cc centrifuge tubes were used. The centrifugation was performed at an acceleration of 600 kG for 10 minutes.

[0157] [TEM observation] TEM observation was performed to directly observe the shape and size of the nanoparticles. The TEM used was a Tecnai F20 manufactured by FEI. The results of observation of platinum nanoparticles using FE-TEM are shown in Figures 13A and 13B. The results are summarized in Table 3.

[0158] Samples generated using a power supply with a peak current of 50 A or less (1) had an average particle size of 3 nm or more, whereas samples generated using a power supply with a peak current of 125 A or more (2) were not as clearly observed as Pd or gold. However, while operating the TEM, particles of 2 nm or less were visible on the monitor with the naked eye. However, these were too small, exceeding the imaging limit of the FE-TEM, making them difficult to record during imaging. Note that nanocrystals of 2 nm or more were used for focusing the TEM. Here, the nanoparticles were confirmed to be platinum with a face-centered cubic lattice structure through diffraction grating analysis.

[0159] [Dynamic Light Scattering] Particle size measurement by dynamic light scattering was carried out using the same equipment as that used for palladium. The results of particle size measurement by dynamic light scattering for platinum particles are shown in Figure 14. The results are summarized in Table 3.

[0160] [Scattering intensity distribution] Figure 14A shows the results of dynamic light scattering measurements of particle size for samples obtained using power supply (1) with a peak current of 50 A or less. Figure 14B shows the results of dynamic light scattering measurements of samples obtained using power supply (2) with a peak current of 125 A or more. As can be seen in Figure 14A, the particle size obtained using power supply (1) is orders of magnitude larger than that obtained using power supply (2). TEM observations revealed nanoparticles with an average diameter of 3 nm or more, suggesting that the nanoparticles had aggregated to form aggregates. On the other hand, as can be seen in Figure 14B, the average diameter of platinum nanoparticles obtained using power supply (2) was 1.4 nm, with a standard deviation of 0.1 nm and a 90D of 1.7 nm, confirming their significantly smaller size. Therefore, power supply (1) produced nanoparticles with an average diameter of 3 nm or more but not less than 2 nm, whereas power supply (2) produced extremely small nanoparticles with an average diameter of 2 nm or less. TEM observation confirmed that the average particle size of the samples obtained through these centrifugal processes was 2 nm or less, so the average particle size of 1.4 nm can be said to be the particle size of nanoparticles. The centrifugal process is an important step not only for selecting nanoparticles but also for dispersing aggregates.

[0161] [Table 3]

[0162] Table 3 summarizes the results of measuring the particle size of Pt. In Table 3, *1 indicates that in FE-TEM observation of power source (2), many nanoparticles with a particle size of 2 nm or less were observed, but the size could not be determined accurately. The average particle size was measured to be 1.4 nm using dynamic light scattering.

[0163] TEM observations showed that the sample from power source (1) had an average particle size of approximately 3 to 5 nm. However, for the sample from power source (2), nanoparticles of 2 nm or less could be seen with the naked eye on the monitor, but due to the performance of the TEM equipment, they were too small to be imaged. The results of dynamic light scattering measurements showed that the particle size of the sample from power source (1) was larger than that observed by TEM, but for the nanoparticles obtained by centrifuging the sample from power source (2), the average particle size of the number distribution was 1.4 nm or less, which was significantly smaller. Therefore, it was determined that nanoparticles of 2 nm or less were obtained using power source (2).

[0164] This difference in particle size is likely due to the current waveforms flowing through the electrodes. The older power supply (1) had a two-stage or single-stage rise time of about 20 μs at both 60 V and 200 V, with a peak current of less than 50 A. The newer power supply (2) also had a current of 80 A to 125 A or more. As mentioned previously, the newer power supply (2) had a peak current more than double that of the older power supply (1), resulting in an extremely high temperature rise and the rapid generation of plasma, which produced numerous ions. The higher the temperature, the more homogeneous nuclei were generated by cooling with the surrounding liquid. The intense rapid cooling effect suppressed crystal growth, resulting in the generation of significantly more small clusters. These clusters were then frozen in place by the rapid cooling, resulting in the production of a large amount of extremely small nanoparticles.

[0165] The conclusion is that nanoparticles with a particle size of 2.0 nm or less can be efficiently synthesized by using power supply (2) with a rise and fall time of 1 μs or less, and passing a repetitive current of 80 A to 125 A or more with a rise time of 10 μs to 20 μs or less through the electrodes. In this case, the centrifugation process is important for selecting only the smallest nanoparticles and dispersing aggregates. [Industrial Applicability]

[0166] The nanoparticles of the present invention can be used in various fields such as the environment, IT, printing, and medicine as catalysts, magnetic materials, battery materials, photoelectric materials, medical materials, pharmaceutical materials, and health foods, and are therefore industrially useful. [Explanation of symbols]

[0167] 1...Production equipment, 101,102...Electrode, 201...Container, 202...Liquid, 30...Power supply device, 40...Vibration device. < / b1> < / b8> < / a9> < / a1> < / a1> < / a1> < / a1> < / a1> < / a1> < / a1> < / a1> < / a1> < / a1>

Claims

1. Between a pair of electrodes in a liquid Repeated spark discharges with a peak current of 80A or more are performed. a spark discharge step of forming ultrasmall nanoparticles having an average particle size of 2.0 nm or less in the liquid using the electrode or the electrode and the liquid as materials, thereby obtaining an ultrasmall nanoparticle-containing liquid containing the ultrasmall nanoparticles; the electrode comprises any one selected from the group consisting of a metal, a semi-metal, a semiconductor, an element, an alloy, and a compound; The discharge condition is that a voltage is applied periodically in a rectangular wave form with a time width of 10 μs or more and 200 μs or less. A method for producing extremely small nanoparticles.

2. Between a pair of electrodes in a liquid Repeated spark discharges with a peak current of 100A or more are generated. a spark discharge step of forming ultrasmall nanoparticles having an average particle size of 2.0 nm or less in the liquid using the electrode or the electrode and the liquid as materials, thereby obtaining an ultrasmall nanoparticle-containing liquid containing the ultrasmall nanoparticles; the electrode comprises any one selected from the group consisting of a metal, a semi-metal, a semiconductor, an element, an alloy, and a compound; The discharge condition is that a voltage is applied periodically in a rectangular wave form with a time width of 10 μs or more and 200 μs or less. A method for producing extremely small nanoparticles.

3. Between a pair of electrodes in a liquid Repeated spark discharges with a peak current of 125A or more are performed. a spark discharge step of forming ultrasmall nanoparticles having an average particle size of 2.0 nm or less in the liquid using the electrode or the electrode and the liquid as materials, thereby obtaining an ultrasmall nanoparticle-containing liquid containing the ultrasmall nanoparticles; the electrode comprises any one selected from the group consisting of a metal, a semi-metal, a semiconductor, an element, an alloy, and a compound; The discharge condition is that a voltage is applied periodically in a rectangular wave form with a time width of 10 μs or more and 200 μs or less. A method for producing extremely small nanoparticles.

4. 4. The method for producing ultra-small nanoparticles according to claim 1, wherein the discharge conditions are such that a square-wave voltage is applied periodically.

5. The method for producing ultra-small nanoparticles according to any one of claims 1 to 4, wherein the rise and fall times of the current between the electrodes of the spark discharge are 20 μs or less.

6. 6. The method for producing ultra-small nanoparticles according to claim 1, wherein under discharge conditions, the voltage rise and fall times are 1 μs or less.

7. 7. The method for producing ultrasmall nanoparticles according to claim 1, further comprising a centrifugal separation step of centrifuging the nanoparticle-containing liquid and recovering the supernatant.

8. The method according to any one of claims 1 to 7, wherein the liquid comprises any one selected from the group consisting of water, alcohol, toluene, xylene, an aqueous ammonia solution, sulfur, and carbon tetrachloride.

9. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrodes are both made of gold and the average particle size of the ultra-small nanoparticles is less than 0.6 nm.

10. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrodes are both made of palladium, and the average particle size of the ultra-small nanoparticles is 1.7 nm or less.

11. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrodes are both made of platinum, and the average particle size of the ultra-small nanoparticles is 1.4 nm or less.

12. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrode is made of a noble metal element, and the average particle size of the ultra-small nanoparticles is 2.0 nm or less.

13. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrode is made of a base metal alone, and the average particle size of the ultra-small nanoparticles is 2.0 nm or less.

14. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrode is made of an alloy and the average particle size of the ultra-small nanoparticles is 2.0 nm or less.

15. The method for producing ultra-small nanoparticles according to any one of claims 1 to 8, wherein the electrode is a compound, and the average particle size of the ultra-small nanoparticles is 2.0 nm or less.

16. The method for producing ultra-small nanoparticles according to any one of claims 12 to 15, wherein the average particle size of the ultra-small nanoparticles is 1.5 nm or less.

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