Method for manufacturing noble metal nanoparticle

Abstract

To provide a method for manufacturing noble metal nanoparticles by which it is possible to inexpensively obtain the noble metal nanoparticles from a solution including noble metal ions through a simple procedure by using yeast.SOLUTION: A method for manufacturing noble metal nanoparticles includes: a noble metal adsorption step of adsorbing noble metal on a surface of yeast by adding the yeast to a solution including noble metal ions, followed by stirring; and a nanoparticle formation step of generating noble metal nanoparticles whose mean particle diameter is in a range of 1-100 nm on the surface of the yeast by adding a reductant to the yeast with the noble metal adsorbed thereon. The nanoparticle formation step is the step of bringing reduced vapor including the reductant into contact with the yeast with the noble metal adsorbed thereon under a temperature environment of 40°C or higher.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing precious metal nanoparticles by generating precious metal nanoparticles from a solution containing precious metal ions. [Background technology]

[0002] Noble metal nanoparticles, which are nano-sized particles of precious metals (gold, platinum, silver, palladium, rhodium, iridium, ruthenium, and osmium), are increasingly being used in fields such as catalysts, biosensors, memory, medical materials, and antibacterial materials. Waste materials containing precious metals are attracting attention as a stable source of these precious metal nanoparticles.

[0003] For example, Patent Document 1 discloses a method for reducing and recovering precious metals from microorganisms that have adsorbed precious metal ions, which involves contacting a yeast of the genus Saccharomyces, such as Saccharomyces cerevisiae, with precious metal ions in a liquid having a pH of 4 or less, and separating and baking the yeast within 4 to 5 hours after the start of the contact.

[0004] Furthermore, Patent Document 2 discloses a method for obtaining noble metal nanoparticles by reducing noble metal ions using a chemical reducing agent.

[0005] Furthermore, Patent Document 3 discloses a process for recovering metals from metal oxides or hydroxides without using chemical reducing agents by using iron-reducing bacteria. In this process, known as biomineralization, it is believed that the reducing action of iron-reducing bacteria allows the recovery of reduced metals both inside and outside the bacterial body. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6586690 [Patent Document 2] Japanese Patent Application Publication No. 2023-004079 [Patent Document 3] Patent No. 5090697 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in Patent Document 1, the microorganisms are calcined to recover the precious metals, which makes it difficult to separate and recover the precious metals from the carbonized yeast residue.Furthermore, since a portion of the precious metals evaporates during the calcination, there is also the problem of low yield.

[0008] On the other hand, in Patent Document 2, the chemical reducing agent is added in a liquid state, which increases recovery costs due to excess chemical reducing agent that does not contribute to the reduction reaction, making it difficult to carry out the reduction reaction efficiently.

[0009] Furthermore, the biomineralization described in Patent Document 3 has the problem that the types of metals and microorganisms that can be used are limited. Also, many of the microorganisms used are anaerobic, and equipment to maintain an anaerobic atmosphere is required, resulting in high equipment costs.

[0010] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a method for producing precious metal nanoparticles that uses yeast and enables the production of precious metal nanoparticles from a solution containing precious metal ions through a simple process at low cost. [Means for solving the problem]

[0011] In order to solve the above problems, the method for producing noble metal nanoparticles according to one embodiment of the present invention proposes the following means. (1) A method for producing precious metal nanoparticles according to aspect 1 of the present invention includes a precious metal adsorption step of adding yeast to a solution containing precious metal ions and stirring the mixture to adsorb the precious metal onto the surface of the yeast, and a nanoparticle formation step of adding a reducing agent to the yeast to which the precious metal has been adsorbed to produce precious metal nanoparticles with an average particle size ranging from 1 nm to 100 nm on the surface of the yeast, wherein the nanoparticle formation step is characterized in that it is a step of contacting the yeast to which the precious metal has been adsorbed with reducing vapor containing a reducing agent in a temperature environment of 40°C or higher.

[0012] (2) Aspect 2 of the present invention is characterized in that, in the method for producing precious metal nanoparticles of aspect 1, the yeast includes any one or more species of the genera Saccharomyces, Zygosaccharomyces, Schizosaccharomyces, Debaryomyces, and Candida.

[0013] (3) Aspect 3 of the present invention is characterized in that, in the method for producing precious metal nanoparticles of Aspect 1 or 2, the reducing vapor includes a carrier gas and a vaporized reducing agent, and the reducing agent includes one or more of hydrazine and its salts, borohydride salts, sulfates, thiosulfates, tartrates, phosphinic acid and its salts, formic acid and its salts, acetic acid and its salts, propionic acid and its salts, oxalic acid and its salts, ascorbic acid and its salts, phosphoric acid and its salts, hypophosphorous acid and its salts, citric acid and its salts, transition metal salts, glycine, dimethylamine borane, formaldehyde, and hydrogen.

[0014] (4) A fourth aspect of the present invention is characterized in that, in the method for producing noble metal nanoparticles according to the third aspect, the reducing agent is formic acid and its salts.

[0015] (5) A fifth aspect of the present invention is characterized in that, in the method for producing precious metal nanoparticles according to the third or fourth aspect, the carrier gas is an inert gas that does not chemically react with the reducing agent.

[0016] (6) Aspect 6 of the present invention is characterized in that, in the method for producing precious metal nanoparticles according to any one of aspects 3 to 5, the concentration of the vaporized reducing agent contained in the reducing vapor is in the range of 0.1 mass% or more and 10 mass% or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a method for producing precious metal nanoparticles that uses yeast and enables the production of precious metal nanoparticles from a solution containing precious metal ions in a simple process at low cost. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a flowchart showing a step-by-step method for producing noble metal nanoparticles according to one embodiment of the present invention. [Figure 2] FIG. 1 is a schematic configuration diagram illustrating an example of a reducing vapor supply device. [Figure 3] 1 is a photograph showing the results of Example 1 of the present invention. [Figure 4] 1 is a TEM image showing the results of Example 1 of the present invention. [Figure 5] 1 is a photograph showing the results of Inventive Example 7. [Figure 6] 1 is a TEM image showing the results of Inventive Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, a method for producing precious metal nanoparticles, which is one embodiment of the present invention, will be described with reference to the drawings. Note that the embodiment shown below is specifically described to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0020] FIG. 1 is a flow chart showing the steps of a method for producing noble metal nanoparticles according to one embodiment of the present invention. In the method for producing precious metal nanoparticles of this embodiment, a series of steps will be described, in which precious metals are separated and recovered from, for example, a circuit board (a material containing precious metals) of an electronic device, and the recovered precious metals are made into nanoparticles. The circuit board of an electronic device contains, for example, precious metals such as gold and silver in soldered portions and wiring. Note that the precious metals in this embodiment refer to eight elements: gold, platinum, silver, palladium, rhodium, iridium, ruthenium, and osmium.

[0021] In the method for producing precious metal nanoparticles of this embodiment, first, a solution containing precious metal ions (hereinafter referred to as a precious metal solution) is prepared. Such a precious metal solution can be obtained, for example, by treating a circuit board of an electronic device, which uses precious metals such as gold, silver, and platinum in soldered joints and wiring, with hydrochloric acid and hydrogen peroxide, and then dissolving the circuit board in aqua regia. In particular, by using recycled raw materials such as discarded circuit boards, the precious metal solution can be obtained at low cost.

[0022] Yeast is added to the precious metal solution obtained in this way and stirred, and the precious metal ions contained in the precious metal solution are adsorbed onto the surface of the yeast (precious metal adsorption step S1). In the precious metal adsorption step S1, yeast is added to the precious metal solution and stirred. In this precious metal adsorption step S1, the yeast comes into contact with the precious metal ions contained in the precious metal solution, and the precious metal ions are adsorbed onto the surface of the yeast.

[0023] The amount of yeast added in the precious metal adsorption step S1 may be set to a range of 0.004 to 39 times the mass of the precious metal contained in the precious metal solution. The mass of the precious metal contained in the precious metal solution may be measured in advance using an ICP optical emission spectrometer, an absorptiometer, or the like.

[0024] The yeast used in the noble metal adsorption step S1 may be any yeast that can adsorb noble metal ions. Examples of yeast that can be used in this embodiment include yeasts of the genus Saccharomyces, Candida, Torulopsis, Zygosaccharomyces, Schizosaccharomyces, Pichia, Yarrowia, Hansenula, Kluyveromyces, Debaryomyces, Geotrichum, Wickerhamia, Fellomyces, and Sporobolomyces, and among these, yeasts belonging to the genus Saccharomyces, Zygosaccharomyces, Schizosaccharomyces, and Debaryomyces are particularly preferred.

[0025] Yeasts of the genus Saccharomyces are representative yeasts of the budding yeast family, and may be, for example, S. bayanus, S. boulardii, S. bulderi, S. cariocanus, S. cariocus, S. cerevisiae, S. chevalieri, S. dairenensis, S. ellipsoideus, S. florentinus, S. kluyveri, S. martiniae, S. monacensis, S. norbensis, S. paradoxus, S. pastorianus, S. spencerorum, S. turicensis, S. unisporus, S. uvarum, or S. zonatus.

[0026] Examples of yeasts of the genus Candida include non-pathogenic yeasts of the genus Candida, such as Candida utilis, Candida boidinii, Candida etchellsii, Candida versatilis, and Candida stellata.

[0027] The Zygosaccharomyces genus is a salt-tolerant yeast isolated from miso and soy sauce, such as Z. rouxii. The Schizosaccharomyces genus is a fission yeast, such as S. cryophilus, S. japonicus, S. octosporus, or S. pombe. Another preferred yeast is Debaryomyces hansenii, deposited under accession number NITE BP-01780 (Patent Microorganisms Deposit Center, National Institute of Technology and Evaluation, Room 122, 2-5-8 Kazusa Kamatari, Kisarazu City, Chiba Prefecture, Japan).

[0028] The yeast used in the noble metal adsorption step S1 may be a live cell or a dead cell.

[0029] There are no particular limitations on the pH or temperature of the liquid obtained by adding yeast to the precious metal solution. For example, the pH may be between -3 and 7, which is a strong acidic to neutral range. The temperature is 10°C or higher and 45°C or lower, and preferably 20°C or higher and 35°C or lower.

[0030] The time for carrying out the noble metal adsorption step S1 may be, for example, within a range of 4 hours or less as the time for stirring after adding yeast to the noble metal solution.

[0031] Next, the noble metal solution to which the yeast has been added is subjected to solid-liquid separation by filtration or the like, and the solid phase (yeast with noble metal ions attached to its surface) is washed with ion-exchanged water or pure water. Washing is preferably carried out about three times, for example, by changing the washing water.

[0032] Next, the yeast (solid phase) with the noble metal ions attached to its surface after solid-liquid separation is subjected to a reduction reaction with a reducing agent to produce noble metal nanoparticles (nanoparticle formation step S2). In the nanoparticle formation step S2 of this embodiment, first, the yeast with adsorbed precious metal ions obtained in the precious metal adsorption step S1 is placed in, for example, a chamber (reaction tank) equipped with a temperature control device, and reducing steam is supplied into the chamber from a reducing steam supply device.

[0033] FIG. 2 is a schematic configuration diagram showing an example of a reducing vapor supply device. The reducing vapor supply device 10 supplies an inert gas (nitrogen gas in this embodiment) as a carrier gas from a carrier gas source 11 to each of a first flow path 12 and a second flow path 13. The flow rates of the first flow path 12 and the second flow path 13 can be adjusted by adjustment valves 14 and 15, respectively. The first flow path 12 and the second flow path 13 merge downstream and form a reducing vapor supply path 16, which is supplied into the interior of a chamber 18.

[0034] A bubbling device 17 is disposed midway along the first flow path 12. The bubbling device 17, for example, blows a carrier gas into a liquid reducing agent contained in a vaporization tank to generate reducing vapor (high concentration) containing the vaporized reducing agent and the carrier gas. The generated reducing vapor is mixed and diluted with the carrier gas flowing through the second flow path 13, and then supplied to the chamber 18. That is, the reducing vapor supply device 10 can adjust the concentration of the reducing agent contained in the reducing vapor supplied to the chamber 18 by adjusting the flow rate of the carrier gas flowing through the second flow path 13.

[0035] The reducing agent contained in the reducing vapor may be, for example, one or more of hydrazine and its salts, borohydride salts, sulfates, thiosulfates, tartrates, phosphinic acid and its salts, formic acid and its salts, acetic acid and its salts, propionic acid and its salts, oxalic acid and its salts, ascorbic acid and its salts, phosphoric acid and its salts, hypophosphorous acid and its salts, citric acid and its salts, transition metal salts, glycine, dimethylamine borane, formaldehyde, and hydrogen. Among these, those that are solid at room temperature can be dissolved in an appropriate solvent and then introduced into the bubbling device 17 described above. Those that are gaseous at room temperature can be directly mixed with a carrier gas and supplied to the chamber.

[0036] Of these reducing agents, formic acid (CH2O2) and its salts are used in this embodiment. For formic acid, which is a liquid at room temperature, it can be bubbled as is, while for salts such as sodium formate, it can be dissolved in water and bubbled as a highly concentrated solution. In this embodiment, reducing vapor diluted with nitrogen is used so that the concentration of formic acid vapor is in the range of, for example, 0.1 mass % or more and 10 mass % or less, preferably 1 mass % or more and 5 mass % or less.

[0037] The carrier gas may be an inert gas that does not chemically react with the reducing agent, such as nitrogen, argon, or a mixture thereof. In this embodiment, nitrogen is used as the carrier gas due to its availability and cost.

[0038] The reducing steam generated by the reducing steam supply device described above is introduced into the chamber and contacts the yeast containing the adsorbed precious metal ions. At this time, the temperature of the atmosphere in the chamber is set to 40°C or higher, preferably 50°C or higher. The upper temperature limit may be, for example, 100°C or lower.

[0039] When the reducing steam comes into contact with yeast that has adsorbed precious metal ions in an environment with a temperature of 40°C or higher, the adsorbed precious metals in ionic form are reduced by the reducing agent components and precipitate as precious metal nanoparticles.

[0040] In this way, noble metal nanoparticles having an average particle size in the range of 1 nm or more and 100 nm or less are reduced and generated on the surface of the yeast that has been subjected to the nanoparticle formation step S2.

[0041] Thereafter, the yeast with the noble metal nanoparticles formed on its surface and the reducing agent are subjected to solid-liquid separation by filtration or the like, whereby the yeast and the noble metal nanoparticles can be separated.

[0042] As described above, in the method for producing precious metal nanoparticles of this embodiment, yeast adsorbing precious metal ions is contacted with reducing vapor containing a vaporized reducing agent to reduce the precious metal ions and produce precious metal nanoparticles. As a result, compared to conventional methods of reducing precious metal ions using a liquid reducing agent or a reducing agent solution containing a dissolved reducing agent, laborious and time-consuming operations for evenly applying the reducing agent to the precious metal ions adhering to the yeast, such as controlling the temperature of the entire solution to a uniform temperature or stirring the solution to remove bubbles from the yeast surface, are not required. By using reducing vapor, it is possible to evenly apply the reducing agent to the precious metal ions in a short time without stirring or other operations.

[0043] Furthermore, the precious metal nanoparticles obtained by the method for producing precious metal nanoparticles of this embodiment have significantly less loss than conventional production methods, in which the yeast is baked together to solidify the precious metal, which is then refined and re-melted to produce precious metal nanoparticles, and the yield of precious metal nanoparticles can be improved.

[0044] Although the embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Example]

[0045] To verify the effects of the present invention, samples of Invention Examples 1 to 7 and Comparative Examples 1 to 3 were prepared, and the state of production of noble metal nanoparticles due to plasmon resonance was confirmed from the color change of the baker's yeast. The sample preparation conditions and results are shown in Table 1 below.

[0046] (Example 1 of the present invention) A gold standard stock solution (17595-1B: manufactured by Kanto Chemical Co., Ltd.) was used to prepare a gold concentration of 500 ppm, which was used as a simulated solution of precious metal waste liquid obtained by dissolving waste in acid. 14 cells / m 3 Baker's yeast (Saccharomyces cerevisiae, manufactured by Oriental Yeast Co., Ltd.) was added so that the solution was 100%. After stirring for 5 minutes, the solution was centrifuged to separate the baker's yeast from the solid. As a result, gold ions were adsorbed onto the surface of the baker's yeast, and most of the gold contained in the simulated solution was recovered. Approximately 1 g of this gold ion-loaded baker's yeast was taken and transferred to a screw cap vial. (Although the baker's yeast after centrifugation contains a certain amount of water, it has been confirmed that the results will not change even if the yeast is completely dried and free of water before the subsequent steps.)

[0047] This screw cap was placed in the chamber of a formic acid reflow apparatus (SMT Scope / SK-8000, manufactured by Sanyo Seiko Co., Ltd.), a reducing vapor supply device as shown in Figure 2. The flow rate of nitrogen, the carrier gas, was then adjusted so that reducing vapor with a formic acid concentration of 5% by mass was supplied to the chamber. The overall nitrogen flow rate was fixed at 20 slm. This formic acid concentration was calculated from the vapor pressure of formic acid at 20°C (4.6 kPa). In this state, the atmosphere in the chamber was heated to 90°C and held for 10 minutes. After heating, the screw cap was removed from the chamber, and the color of the baker's yeast was observed.

[0048] As a result, as shown in Figure 3, the baker's yeast, which was white before the reaction, turned reddish-purple after the reaction. This color change reflects plasmon resonance due to the generation of gold nanoparticles, confirming the generation of gold nanoparticles. Figure 4 shows a TEM image (photograph) of the baker's yeast surface after the reaction. The black areas are gold nanoparticles, and it was confirmed that gold nanoparticles with diameters of approximately 5 to 20 nm were actually generated. The average particle diameter was determined by measuring the average diameter of 10 randomly selected gold nanoparticles using the length measurement function of the TEM in a TEM image taken at 360,000x magnification using a transmission electron microscope (JEM-2010F: manufactured by JEOL Ltd.). The resulting average particle diameter of the gold nanoparticles was 10.0 nm.

[0049] (Example 2 of the present invention) The same operation as in Example 1 of the present invention was performed, except that the formic acid concentration of the reducing vapor was set to 3% by mass. As a result, the baker's yeast, which was white before the reaction, changed color to reddish purple after the reaction, confirming the production of gold nanoparticles. Since the color after the reaction was the same as in Example 1 of the present invention, it was confirmed that the average particle size of the gold nanoparticles was approximately 10.0 nm.

[0050] (Example 3 of the present invention) The same operation as in Example 1 of the present invention was performed, except that the formic acid concentration of the reducing vapor was set to 2% by mass. As a result, the baker's yeast, which was white before the reaction, changed color to reddish purple after the reaction, confirming the production of gold nanoparticles. Since the color after the reaction was the same as in Example 1 of the present invention, it was confirmed that the average particle size of the gold nanoparticles was approximately 10.0 nm.

[0051] (Example 4 of the present invention) The same operation as in Example 1 of the present invention was performed, except that the formic acid concentration of the reducing vapor was set to 1% by mass. As a result, the baker's yeast, which was white before the reaction, changed color to reddish purple after the reaction, confirming the production of gold nanoparticles. Since the color after the reaction was the same as in Example 1 of the present invention, it was confirmed that the average particle size of the gold nanoparticles was approximately 10.0 nm.

[0052] (Example 5 of the present invention) The same operation as in Example 1 of the present invention was performed, except that the temperature environment of the chamber was set to 50°C. As a result, the baker's yeast, which was white before the reaction, changed color to reddish purple after the reaction, confirming the production of gold nanoparticles. Since the color after the reaction was the same as in Example 1 of the present invention, it was confirmed that the average particle size of the gold nanoparticles was approximately 10.0 nm.

[0053] (Example 6 of the present invention) Using a gold standard stock solution, prepare a gold concentration of 350 ppm, and add 1 x 10 14 cells / m 3 The same procedure as in Example 1 of the present invention was performed, except that baker's yeast was added so that the adsorption amount per gram of dry baker's yeast was approximately 85 mg, and it is expected that a larger amount of reducing agent would be required than in Example 1 of the present invention. As a result, the baker's yeast, which was white before the reaction, changed color to reddish purple after the reaction, confirming the production of gold nanoparticles. Since the color after the reaction was the same as in Example 1 of the present invention, it was confirmed that the average particle size of the gold nanoparticles was approximately 10.0 nm.

[0054] (Example 7) Palladium standard solution (32788-1B: manufactured by Kanto Chemical Co., Ltd.) was used to prepare a palladium concentration of 200 ppm, and 1 × 10 14 cells / m 3 The same procedure as in Invention Example 1 was carried out, except that baker's yeast was added so that the

[0055] As a result, as shown in Figure 5, the baker's yeast, which was brown before the reaction, turned dark brown after the reaction. This color change reflects the plasmon resonance caused by the production of palladium nanoparticles, confirming the production of palladium nanoparticles. Figure 6 also shows a TEM image (photograph) of the surface of the baker's yeast after the reaction. The black areas are palladium nanoparticles, and it was confirmed that palladium nanoparticles with a particle size of approximately 5 to 20 nm had actually been produced. The average particle size was 8.0 nm.

[0056] (Comparative Example 1) The same operation as in Example 1 was carried out, except that the temperature environment in the chamber was set to 35° C. As a result, it was confirmed that the baker's yeast did not change color after the reaction, and that gold nanoparticles were not produced.

[0057] (Comparative Example 2) The same operation as in Example 1 was carried out, except that only a carrier gas (nitrogen) containing no reducing agent was introduced into the chamber. As a result, it was confirmed that the baker's yeast did not change color after the reaction, and that gold nanoparticles were not produced.

[0058] (Comparative Example 3) The same procedure as in Example 1 was carried out, except that only baker's yeast with no adsorbed precious metal ions was placed in the chamber. As a result, it was confirmed that the baker's yeast did not change color after the reaction, and that gold nanoparticles were not produced.

[0059] [Table 1]

[0060] Based on the above verification results, it was confirmed that by raising the temperature environment to 40°C or higher and applying reducing vapor containing formic acid to yeast that has adsorbed precious metal ions, the precious metal ions are reduced and precious metal nanoparticles are produced on the surface of the yeast. [Industrial Applicability]

[0061] According to the method for producing precious metal nanoparticles of the present invention, when recovering precious metals from a precious metal solution using yeast, reducing vapor containing a reducing agent is applied to yeast that has adsorbed precious metal ions, thereby efficiently producing precious metal nanoparticles in high yield. This makes it possible to produce precious metal nanoparticles, which are increasingly being used in fields such as catalysts, biosensors, memory, medical materials, and antibacterial materials, at low cost using waste circuit boards containing precious metals as raw materials. Therefore, the method has industrial applicability.

Claims

1. a precious metal adsorption step of adding yeast to a solution containing precious metal ions and stirring the mixture to adsorb the precious metals onto the surface of the yeast; a nanoparticle formation step of adding a reducing agent to the yeast to which the precious metal has been adsorbed to generate precious metal nanoparticles having an average particle size in the range of 1 nm or more and 100 nm or less on the surface of the yeast, The method for producing precious metal nanoparticles, wherein the nanoparticle formation step is a step of contacting the yeast having the precious metal adsorbed thereon with reducing vapor containing a reducing agent in an environment at a temperature of 40°C or higher.

2. 2. The method for producing precious metal nanoparticles according to claim 1, wherein the yeast includes one or more of the genera Saccharomyces, Zygosaccharomyces, Schizosaccharomyces, Debaryomyces, and Candida.

3. 3. The method for producing precious metal nanoparticles according to claim 1, wherein the reducing vapor contains a carrier gas and a vaporized reducing agent, and the reducing agent contains one or more of hydrazine and its salts, borohydride salts, sulfates, thiosulfates, tartrates, phosphinic acid and its salts, formic acid and its salts, acetic acid and its salts, propionic acid and its salts, oxalic acid and its salts, ascorbic acid and its salts, phosphoric acid and its salts, hypophosphorous acid and its salts, citric acid and its salts, transition metal salts, glycine, dimethylamine borane, formaldehyde, and hydrogen.

4. 4. The method for producing precious metal nanoparticles according to claim 3, wherein the reducing agent is formic acid and its salts.

5. 4. The method for producing precious metal nanoparticles according to claim 3, wherein the carrier gas is an inert gas that does not chemically react with the reducing agent.

6. The method for producing precious metal nanoparticles according to claim 3 , wherein the concentration of the vaporized reducing agent contained in the reducing vapor is in the range of 0.1 mass % or more and 10 mass % or less.

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