Method for reducing metal ions, plate-like metal nanoparticles, composite containing plate-like metal nanoparticles and dispersion thereof, and multibranched metal nanoparticles and method for producing the same

Abstract

To provide a method of reducing a metal ion, for obtaining a metal nano-particle by reducing a metal ion, that has less effect on the environment and is allowed to obtain a metal nano-particle by a simple and energy-saving way and to control the size and shape of the metal nano-particle simply and concretely, a composite containing a sheet-formed metal nano-particle and a dispersion liquid thereof, and a sheet-formed metal nano-particle, and further to provide a novel multi-branched metal nano-particle and a method of producing the same.SOLUTION: A method of reducing a metal ion includes contacting unsaturated fatty acid, preferably alkenyl-carboxylic acid having carbon number 2-23, with a metal ion. A multi-branched metal nano-particle comprises a central portion and a plurality of branch portions extending outward from the central portion, in which the branch portions have a crystal texture including a single crystal and the multi-branched metal nano-particle is entirely of a crystal texture that is polycrystalline.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for reducing metal ions, plate-like metal nanoparticles, a composite containing plate-like metal nanoparticles and a dispersion thereof, and multibranched metal nanoparticles and a method for producing the same. [Background technology]

[0002] Nanoparticles of precious metals such as gold, silver, and platinum, and their dispersions, are typically obtained by adding a reducing agent to a solution containing precious metal ions to reduce the metal ions. For example, Non-Patent Document 1 describes a method for producing gold nanoparticles by reducing gold ions with sodium borohydride (NaBH4). However, the method described in Non-Patent Document 1 often uses harmful reagents such as NaBH4 and alkanethiols (R-SH), and also uses organic solvents such as toluene to dissolve the reducing agent and stabilizer, raising concerns about its impact on the environment. Non-Patent Document 2 describes a method for reducing chloroauric acid at room temperature using cyclohexanone, but this method also requires the use of cyclohexanone, which is harmful, as a reducing agent. Furthermore, the gold nanoparticles produced by this method have an average particle diameter of up to about 400 nm, making it difficult to control their size and shape.

[0003] Most of the methods for producing noble metal nanoparticles that have been reported so far, as described in Non-Patent Document 3, involve multi-step reactions using many different reagents, and the synthesis procedures are complicated. Patent Document 1 describes a method for recovering gold by contacting a solution containing gold ions and chloride ions with dibutyl carbitol to obtain an organic phase containing gold nanoparticles, then adding potassium oxalate to the aqueous phase to transfer the gold ions to the aqueous phase and performing a reduction treatment at around room temperature. However, the method described in Patent Document 1 involves complex steps and there is a risk of contamination with potassium, an alkali metal, which may limit the applications of the recovered gold. Another problem with the method described in Patent Document 1 is that it is difficult to control the size and shape of the resulting gold nanoparticles.

[0004] Among metal nanoparticles, there are those called "multibranched metal nanoparticles" whose surfaces are branched into fine branches. Non-patent documents 4 and 5, for example, report examples of such multibranched metal nanoparticles, such as multibranched metal nanoparticles with sea urchin-like spines and multibranched metal nanoparticles with fine irregularities on the surface. These non-patent documents describe the synthesis of multibranched metal nanoparticles through multistep reactions using a variety of reagents, resulting in a complex synthesis procedure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5351747 [Non-patent literature]

[0006] [Non-Patent Document 1] Brust, M. et al., Synthesis of Thiol-Derivatized Gold Nanoparticles in a Two phase Liquid-Liquid System., J.Chem.Soc., Chem.Commun.1994, 801-802. [Non-patent document 2] MAUppal et al., J. Matter. Chem., A1, 7351 (2013). [Non-patent document 3] H. Liu et al.,CrysEngComm.,2011,13,2281. [Non-patent document 4] Priya Vijayaraghavan et al.,ACS Appl.Mater.Interfaces 2016,8,23909. [Non-patent document 5] Song et al.,J.Mater.Chem.B,2016,4,7112. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, an object of the present invention is to provide a method for reducing metal ions to obtain metal nanoparticles, which has a low environmental impact, is a simple and energy-saving method for obtaining metal nanoparticles, and allows for easy and detailed control of the size and shape of the metal nanoparticles, as well as a composite containing plate-like metal nanoparticles, a dispersion thereof, and plate-like metal nanoparticles. Another object of the present invention is to provide novel multibranched metal nanoparticles and a method for producing the same. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that all of the above problems can be solved by reducing metal ions through contact with unsaturated fatty acids, and have thus completed the present invention. That is, the present invention has the following aspects. [1] A method for reducing metal ions by contacting unsaturated fatty acids with metal ions. [2] The method for reducing metal ions according to [1], wherein the unsaturated fatty acid is an alkenylcarboxylic acid having 2 to 23 carbon atoms. [3] The method for reducing metal ions according to [2], wherein the alkenyl carboxylic acid has a structure represented by the following formula (X1) or (X2): R 1 -CH=CH-COOH (X1) R 2 -CH=CH-R 3 -COOH (X2) (In formula (X1), R 1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. 2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and R 3 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group. [4] A method for reducing metal ions according to any one of [1] to [3], comprising a step of mixing the unsaturated fatty acid and the metal ions in a solution. [5] The method for reducing metal ions according to any one of [1] to [4], wherein the metal ions include precious metal ions. [6] A metal ion reducing agent, including unsaturated fatty acids. [7] The metal ion reducing agent according to [6], wherein the unsaturated fatty acid is an alkenylcarboxylic acid having 2 to 23 carbon atoms. [8] The metal ion reducing agent according to [7], wherein the alkenyl carboxylic acid has a structure represented by the following formula (X1) or the following formula (X2): R 1 -CH=CH-COOH (X1) R 2 -CH=CH-R 3 -COOH (X2) (In formula (X1), R 1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. 2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and R 3 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group. [9] A composite comprising an unsaturated fatty acid and plate-like metal nanoparticles.

[10] The complex according to [9], wherein the unsaturated fatty acid is an alkenylcarboxylic acid having 2 to 23 carbon atoms.

[11] The complex according to

[10] , wherein the alkenyl carboxylic acid has a structure represented by the following formula (X1) or (X2): R 1 -CH=CH-COOH (X1) R 2 -CH=CH-R 3 -COOH (X2) (In formula (X1), R 1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. 2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and R 3represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[12] The complex according to any one of [9] to

[11] , wherein the unsaturated fatty acid is attached to at least one surface of the plate-like metal nanoparticles.

[13] The composite according to any one of [9] to

[12] , wherein the content of the plate-shaped metal nanoparticles relative to the total mass of the composite is 80 mass % or more.

[14] The composite according to any one of [9] to

[13] , wherein the plate-shaped metal nanoparticles include plate-shaped metal nanoparticles of a noble metal.

[15] The composite according to any one of [9] to

[14] , wherein the plate-shaped metal nanoparticles comprise gold plate-shaped metal nanoparticles.

[16] Electrical resistivity is 1×10 -4

[15] The composite according to

[15] , having a resistivity of Ω·m or less.

[17] The composite according to

[15] or

[16] , having a Young's modulus of 1 MPa or less.

[18] A dispersion comprising the complex according to any one of [9] to

[17] .

[19] Plate-shaped metal nanoparticles having a diameter (d) to thickness (t) ratio (d / t) of 100 to 1200, and the thickness (t) being less than 30 nm.

[20] The plate-shaped metal nanoparticles according to

[19] , wherein the plate-shaped metal nanoparticles include plate-shaped metal nanoparticles of a noble metal.

[21] The plate-shaped metal nanoparticles according to

[19] or

[20] , wherein the plate-shaped metal nanoparticles include gold plate-shaped metal nanoparticles.

[22] A multi-branched metal nanoparticle having a core and a plurality of branches extending outward from the core, wherein the crystal structure of the branches comprises a single crystal, and the crystal structure of the entire multi-branched metal nanoparticle is polycrystalline.

[23] The multi-branched metal nanoparticles according to

[22] , wherein the branched portions include a plurality of platelets.

[24] The multi-branched metal nanoparticles according to

[22] or

[23] , wherein the branched portions are composed of a plurality of plate-like pieces extending outward from the central portion in a three-dimensional and irregular manner.

[25] The multi-branched metal nanoparticles according to

[23] or

[24] , wherein the crystal structure of the extending portion of the plate-like piece is a single crystal.

[26] The multibranched metal nanoparticles according to any one of

[23] to

[25] , wherein the average length of the platelets is 20 to 100 nm.

[27] The multi-branched metal nanoparticles according to any one of

[22] to

[26] , wherein the multi-branched metal nanoparticles comprise multi-branched metal nanoparticles of a noble metal.

[28] The multi-branched metal nanoparticles according to any one of

[22] to

[27] , wherein the multi-branched metal nanoparticles comprise gold multi-branched metal nanoparticles.

[29] A dispersion containing the multibranched metal nanoparticles according to any one of

[22] to

[28] .

[30] A complex comprising the multibranched metal nanoparticles according to any one of

[22] to

[28] and an unsaturated fatty acid.

[31] The complex according to

[30] , wherein the unsaturated fatty acid is an alkenyl carboxylic acid having 5 to 23 carbon atoms.

[32] The complex according to

[31] , wherein the alkenyl carboxylic acid has a structure represented by the following formula (X3) or (X4): R 4 -CH=CH-COOH (X3) R 5 -CH=CH-R 6 -COOH (X4) (In formula (X3), R 4 represents an alkyl group having 3 to 16 carbon atoms. 5 represents an alkyl group having 3 to 16 carbon atoms, and R 6 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[33] A method for producing multibranched metal nanoparticles according to any one of

[22] to

[28] , comprising a step of contacting an unsaturated fatty acid with a metal ion.

[34] The method for producing multibranched metal nanoparticles according to

[33] , wherein the unsaturated fatty acid is an alkenylcarboxylic acid having 5 to 23 carbon atoms.

[35] The method for producing multibranched metal nanoparticles according to

[34] , wherein the alkenyl carboxylic acid has a structure represented by the following formula (X3) or (X4): R 4 -CH=CH-COOH (X3) R 5 -CH=CH-R 6 -COOH (X4) (In formula (X3), R 4 represents an alkyl group having 3 to 16 carbon atoms. 5 represents an alkyl group having 3 to 16 carbon atoms, and R 6 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group. [Effects of the Invention]

[0009] According to the present invention, there are provided a method for reducing metal ions to obtain metal nanoparticles, which is a simple, energy-saving method with little environmental impact and enables the production of metal nanoparticles in a simple and precise manner, a composite containing plate-like metal nanoparticles, a dispersion thereof, and plate-like metal nanoparticles. Furthermore, according to the present invention, there are provided novel multibranched metal nanoparticles and a method for producing multibranched metal nanoparticles that is a simple, energy-saving method with little environmental impact. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an electron microscope photograph showing an example of metal nanoparticles obtained by a method according to one embodiment of the present invention. [Figure 2] 1 is an electron microscope photograph showing an example of metal nanoparticles obtained by a method according to another embodiment of the present invention. [Figure 3A] 1 is an example of a conceptual diagram showing a process for producing metal nanoparticles in the first and second embodiments of the present invention. [Figure 3B] 1 is an example of a conceptual diagram showing a process for producing metal nanoparticles in the first and second embodiments of the present invention. [Figure 4] 1 is a transmission electron microscope photograph showing an example of a multibranched metal nanoparticle according to one embodiment of the present invention. [Figure 5] 1 is a transmission electron microscope photograph showing an example of a branched portion of a multi-molecular metallic nanoparticle according to one embodiment of the present invention. [Figure 6] 1 is a photograph of a selected area electron diffraction image showing an example of the overall crystal structure of a multibranched metal nanoparticle according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in detail below, but the present invention is not limited to the following embodiments. [Method for reducing metal ions] A first aspect of the present invention is a method for reducing metal ions by contacting unsaturated fatty acids with metal ions. According to the first aspect, the metal ions are reduced by contacting the unsaturated fatty acids with the metal ions, and metal nanoparticles are produced. The reduction method of this aspect allows for the production of metal nanoparticles in a simple and energy-saving manner. In this specification, the term "metal ion" refers to a cation generated from a metal atom, and the valence thereof is not particularly limited as long as the effect of the present invention is achieved. In this specification, the term "metal nanoparticles" includes both "granular metal nanoparticles" and "plate-like metal nanoparticles." Here, "granular metal nanoparticles" refers to metal nanoparticles whose particle shape is approximately spherical and whose aspect ratio, expressed as the ratio (b / a) of the particle's major axis (b) to its minor axis (a), is 1.0 to 1.5. Furthermore, "plate-like metal nanoparticles" refers to thin particles whose particle shape is plate-like and whose thickness (t) is very small compared to their diameter (d). Here, the diameter (d) of a plate-like metal nanoparticle refers to the diameter of a perfect circle that contains the plate-like metal nanoparticle, when the protruding end of the plate-like metal nanoparticle is circumferentially connected to the other end. The aspect ratio of granular metal nanoparticles can be measured using a scanning electron microscope. In this specification, the aspect ratio refers to a value calculated from the average value of the major axis (b) and minor axis (a) of 10 granular metal nanoparticles. The diameter and thickness of the plate-shaped metal nanoparticles can be measured using a scanning electron microscope and an atomic force microscope. In this specification, the diameter (d) and thickness (t) of the plate-shaped metal nanoparticles are average values ​​of 10 plate-shaped metal nanoparticles.

[0012] <Unsaturated fatty acids> In a first embodiment, metal ions are reduced using an unsaturated fatty acid. An unsaturated fatty acid refers to a fatty acid having at least one unsaturated bond and at least one carboxyl group in its molecular structure. The position of the unsaturated bond is not particularly limited as long as the effects of the present invention are achieved. In this embodiment, the unsaturated fatty acid may be in either a cis or trans form. In one preferred embodiment, the unsaturated fatty acid is preferably an alkenyl carboxylic acid having 2 to 23 carbon atoms. If the alkenyl carboxylic acid has 2 to 23 carbon atoms, metal ions are more likely to be reduced. The number of carbon atoms represents the number of carbon atoms in the alkenyl group. The number of carbon atoms in the alkenyl group is more preferably 2 to 21, and even more preferably 2 to 17. The alkenyl carboxylic acid may have two or more carboxy groups in the molecule. In a more preferred embodiment, the alkenyl carboxylic acid preferably has a structure of the following formula (X1) or (X2). R 1 -CH=CH-COOH (X1) R 2 -CH=CH-R 3 -COOH (X2) (In formula (X1), R 1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. 2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and R 3 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[0013] Alkenyl carboxylic acids having the structure of the above formula (X1) or (X2) are more likely to achieve the effects of reducing metal ions and controlling shape. Among these, alkenyl carboxylic acids having the structure of formula (X2) are more preferred. In addition, R 3 is a CHCH(-COOH)CH group or a CH(-COOH)CH group. Examples of such alkenyl dicarboxylic acids include 2-octenylsuccinic acid, 2-dodecen-1-ylsuccinic acid, octenylsuccinic acid, and dodecenylsuccinic acid, with 2-octenylsuccinic acid and 2-dodecen-1-ylsuccinic acid being particularly preferred.

[0014] <Metal ions> In the first aspect, the metal ions reduced by the unsaturated fatty acid preferably include precious metal ions. Examples of precious metal ions include cations of gold, silver, platinum, palladium, rhodium, iridium, ruthenium, osmium, etc. Among these, the metal ions more preferably include gold, silver, or platinum, and particularly preferably include gold.

[0015] <Rebate conditions> The method for reducing metal ions in the first aspect of the present invention preferably includes a step of contacting unsaturated fatty acids with metal ions in a solution, more preferably in an aqueous solution. Thus, a method for reducing metal ions in an aqueous solution is preferred because it reduces the environmental impact. Furthermore, it is more preferred that the contact between the unsaturated fatty acids and the metal ions is carried out by mixing solutions containing them. The temperature during reduction is preferably from room temperature to 90°C, and more preferably from 20 to 70°C. Here, "room temperature" means room temperature or 20°C. The concentration of the unsaturated fatty acid in the solution is preferably 0.1 to 5 wt %, more preferably 0.8 to 1.5 wt %, based on the total weight of the solution. If the concentration of the unsaturated fatty acid is within this range, metal ions can be easily reduced. The solution may contain other components in addition to the unsaturated fatty acids and metal ions. The other components are not particularly limited as long as they achieve the effects of the present invention, and examples thereof include ethanol. When other components are contained, the amount of the other components is preferably in the range of 0.1 to 50 (V / V%) relative to the solvent, such as water.

[0016] According to the first aspect of the present invention, as described above, metal ions can be easily reduced by contacting them with an unsaturated fatty acid, preferably an alkenyl carboxylic acid having a specific structure. As a result, zero-valent metal nanoparticles are produced. According to the method of the first aspect, metal nanoparticles containing at least one selected from the group consisting of granular metal nanoparticles, plate-like metal nanoparticles, and a composite containing an unsaturated fatty acid and plate-like metal nanoparticles can be obtained. In the first embodiment, the granular metal nanoparticles may be, for example, an alkenyl carboxylic acid having the structure of the formula (X1) in which R 1 They are easily prepared by reducing metal ions using an alkenyl carboxylic acid in which the alkyl group is hydrogen or 1 to 16 carbon atoms. They can also be easily prepared by contacting metal ions with unsaturated fatty acids at room temperature or under heated conditions (e.g., 50 to 60°C). The resulting granular metal nanoparticles have an average particle size of 50 to 120 nm and an aspect ratio in the range of 1 to 1.5. The average particle size of the granular metal nanoparticles refers to the average value of the major axis measurements of 10 granular metal nanoparticles. Furthermore, according to the reduction method of this embodiment, plate-shaped metal nanoparticles having a specific structure can also be obtained. The plate-shaped metal nanoparticles are obtained by using an alkenyl carboxylic acid having the structure of the formula (X2) in which R 2 is an alkyl group having 5 to 16 carbon atoms, R 3 are easily prepared by reducing a metal ion with an alkenyl carboxylic acid that is a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group. The plate-shaped metal nanoparticles obtained by the reduction method of this embodiment have a thickness (t) of several nm, while their diameter (d) grows to several μm. The reduction method of this embodiment makes it possible to obtain thin metal nanoparticles with a very large ratio of thickness (t) to diameter (d) in a simple and energy-saving manner. It is also possible to obtain composites of plate-shaped metal nanoparticles and unsaturated fatty acids. Details of these plate-shaped metal nanoparticles and composites will be described later. The reduction state of the metal ions can also be confirmed visually by a color change. Other specific examples include analysis using absorption spectroscopy, scanning electron microscope, atomic force microscope, transmission electron microscope, and the like.

[0017] As described above, according to the first aspect, metal ions can be reduced to produce metal nanoparticles using a simple and energy-saving method. That is, another aspect of the first aspect of the present invention is the use of an unsaturated fatty acid, preferably an alkenyl carboxylic acid having 2 to 23 carbon atoms, more preferably an alkenyl carboxylic acid having the structure of formula (X1) or (X2), as a reducing agent for metal ions. Furthermore, another aspect of the first aspect of the present invention is a metal ion reducing agent comprising an unsaturated fatty acid, preferably an alkenyl carboxylic acid having 2 to 23 carbon atoms, more preferably an alkenyl carboxylic acid having the structure of formula (X1) or (X2).

[0018] [Metal nanoparticle manufacturing method] A second aspect of the present invention is a method for producing metal nanoparticles, which includes a step of contacting an unsaturated fatty acid with a metal ion. As described above, the inventors have found that metal nanoparticles can be easily produced by reducing metal ions using an unsaturated fatty acid. That is, by contacting an unsaturated fatty acid with a metal ion, the metal ion is reduced, and zero-valent metal nanoparticles can be produced.

[0019] <Unsaturated fatty acids> Examples of the unsaturated fatty acid in the second embodiment include the same as those in the first embodiment. In one preferred embodiment, the unsaturated fatty acid is preferably an alkenyl carboxylic acid having 2 to 23 carbon atoms. If the alkenyl carboxylic acid has 2 to 23 carbon atoms, metal ions are more likely to be reduced. The number of carbon atoms represents the number of carbon atoms in the alkenyl group. The number of carbon atoms in the alkenyl group is more preferably 2 to 21, and even more preferably 2 to 17. The alkenyl carboxylic acid may have two or more carboxy groups in the molecule. In a more preferred embodiment, the alkenyl carboxylic acid preferably has a structure of the following formula (X1) or (X2). R 1 -CH=CH-COOH (X1) R 2 -CH=CH-R 3 -COOH (X2) (In formula (X1), R 1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. 2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and R 3 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[0020] Alkenyl carboxylic acids having the structure of the above formula (X1) or (X2) are more likely to reduce metal ions. Among these, alkenyl carboxylic acids having the structure of formula (X2) are more preferred. In addition, R 3 is a CHCH(-COOH)CH group or a CH(-COOH)CH group. Examples of such alkenyl dicarboxylic acids include 2-octenylsuccinic acid, 2-dodecen-1-ylsuccinic acid, octenylsuccinic acid, and dodecenylsuccinic acid, with 2-octenylsuccinic acid and 2-dodecen-1-ylsuccinic acid being particularly preferred.

[0021] <Metal ions> Examples of metal ions in the second embodiment include the same as those in the first embodiment, and preferred examples are also the same. In the second embodiment, the metal ions are preferably prepared from a metal salt such as chloroauric acid, etc. By dissolving such a metal salt in water, the metal ions can be more easily prepared.

[0022] <Manufacturing process> In the second embodiment, the contact between the unsaturated fatty acid and the metal ion (hereinafter sometimes simply referred to as the "contacting step") is carried out in a solution. The solution is preferably an aqueous solution or a mixed solution containing water. In the second embodiment, the unsaturated fatty acid and the metal ion are preferably contacted by mixing them in a solution. The preferred conditions for the contacting step, i.e., the temperature of the solution, the concentration of the unsaturated fatty acid, the mixing conditions, etc., are the same as those in the first embodiment. By contacting the unsaturated fatty acid with the metal ions under these conditions, the metal ions are easily reduced and metal nanoparticles are produced. Whether the metal ions have been reduced can be confirmed by the same method as in the first embodiment.

[0023] The contacting step reduces the metal ions to produce metal nanoparticles. The metal nanoparticles are obtained in a dispersed state in a solution (a dispersion containing metal nanoparticles). Therefore, the manufacturing method of this embodiment may include a step of recovering the metal nanoparticles after the contacting step. The method for recovering the metal nanoparticles is not particularly limited as long as the effects of the present invention are achieved. For example, the metal nanoparticles may be recovered by a conventionally known method, such as a method in which ethyl acetate or the like is added to the solution to transfer the metal nanoparticles to an organic solvent phase, and then the metal nanoparticles contained in the organic solvent phase are recovered.

[0024] <Metal nanoparticles> According to the second embodiment of the manufacturing method, metal nanoparticles containing at least one selected from the group consisting of granular metal nanoparticles, plate-like metal nanoparticles, and a complex containing an unsaturated fatty acid and plate-like metal nanoparticles can be obtained. In the second embodiment, the granular metal nanoparticles may be, for example, an alkenyl carboxylic acid having the structure of the formula (X1) in which R 1They are more easily prepared by reducing metal ions with an alkenyl carboxylic acid in which the alkyl group is hydrogen or 1 to 16 carbon atoms. They are also more easily prepared by contacting metal ions with unsaturated fatty acids at room temperature or under heated conditions (e.g., 50 to 60°C). The resulting granular nanoparticles have an average particle size of 50 to 120 nm and an aspect ratio of 1 to 1.5. Furthermore, according to the method for producing metal nanoparticles of this embodiment, plate-shaped metal nanoparticles having a specific structure can be obtained. The plate-shaped metal nanoparticles obtained by the production method of this embodiment have a thickness (t) of several nm, while their diameter (d) grows to several μm. The reason why plate-shaped metal nanoparticles with a very large ratio of thickness (t) to diameter (d) can be obtained is thought to be as follows. Unsaturated fatty acids, preferably alkenylcarboxylic acids having 2 to 23 carbon atoms, can form a bilayer structure in solution as shown in Figure 3A or 3B. Such a bilayer structure is likely to form, for example, by heating the fatty acid to 40 to 70°C, dissolving it in a solvent such as water, and then cooling it. When such unsaturated fatty acids are brought into contact with metal ions, a reduction reaction of the metal ions occurs between the bilayer structure, and as a result, metal nanoparticles may be generated between the layers. Since the width of the bilayer structure is controlled in the thickness direction (i.e., in Fig. 3A or Fig. 3B, the height direction of the bilayer structure), the particles tend to grow in the diameter direction (i.e., in Fig. 3A or Fig. 3B, the lateral direction of the bilayer structure). As a result, it is thought that very thin, plate-like metal nanoparticles are generated in which the particles grow large in the diameter (d) direction. That is, according to the method for producing metal nanoparticles of this embodiment, in the alkenyl carboxylic acid having the structure of the formula (X2), R 2 is an alkyl group having 5 to 16 carbon atoms, and R 3 When is a CH2CH(-COOH)CH2 group or a CH(-COOH)CH2 group, it becomes easier to control the shape of the metal nanoparticles to be "very thin plate-like." Furthermore, plate-like metal nanoparticles with such specific shapes can be produced under very mild conditions and by a method that places little strain on the environment. The manufacturing method of this embodiment can also produce a composite of plate-shaped metal nanoparticles and unsaturated fatty acids. Such composites have plasticity and adhesiveness, and therefore tend to have good moldability.

[0025] As described above, the unsaturated fatty acid of the present invention is a single substance that serves three purposes: a reducing agent, a stabilizer (a dispersant and stabilizer for metal nanoparticles), and a shape and size control agent for metal nanoparticles. Therefore, it is not necessary to add other substances that fulfill these roles. Therefore, according to the production method of the second aspect of the present invention, metal nanoparticles can be produced using only three components: an unsaturated fatty acid, metal ions, and a solvent for these components. The composite obtained by this production method can be heated at 100 to 600°C, preferably 210 to 350°C, to remove the unsaturated fatty acids and extract only the plate-shaped metal nanoparticles. Therefore, the second embodiment may include a step of removing the unsaturated fatty acids from the composite. The plate-shaped metal nanoparticles and composite of the present invention will be described in detail below.

[0026] [Plate-shaped metal nanoparticles] A third aspect of the present invention is plate-shaped metal nanoparticles having a diameter (d) to thickness (t) ratio (d / t) of 100 to 1200, with the thickness (t) being less than 30 nm. The plate-shaped metal nanoparticles of this aspect are characterized by a very small thickness (t) relative to their diameter (d). Such plate-shaped metal nanoparticles are advantageous, for example, in terms of film-forming properties. The (d / t) is 100 to 1200, preferably 250 to 800, and more preferably 400 to 800. The thickness (t) is preferably 5 nm or more and less than 30 nm, and more preferably 5 to 12 nm. The diameter (d) of the plate-shaped metal nanoparticles refers to a value measured using a scanning electron microscope. The thickness (t) of the plate-shaped metal nanoparticles refers to a value measured using an atomic force microscope. In the third embodiment, the plate-shaped metal nanoparticles preferably include plate-shaped metal nanoparticles of a noble metal. Examples of noble metals include gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium. Among these, it is more preferable to include plate-shaped metal nanoparticles of gold, silver, or platinum, and it is particularly preferable to include plate-shaped metal nanoparticles of gold.

[0027] The plate-shaped metal nanoparticles in the third aspect can be produced, for example, by the methods described in the first and second aspects of the present invention. As mentioned above, unsaturated fatty acids can form a bilayer structure in a solution such as water. By reducing metal ions in such a bilayer structure, it is possible to produce plate-shaped metal nanoparticles in which the particles have grown large in the diameter direction (i.e., with a large (d / t) ratio). In the first and second aspects, the plate-shaped metal nanoparticles may be obtained as a complex containing unsaturated fatty acids. By heat-treating this complex to remove the unsaturated fatty acids, it is possible to extract only the plate-shaped metal nanoparticles. Such plate-shaped metal nanoparticles can be used for, for example, catalysis, drug delivery, tumor detection, and the like.

[0028] <Aggregation of plate-shaped metal nanoparticles> The plate-shaped metal nanoparticles of the third embodiment can be formed into an aggregate of plate-shaped metal nanoparticles by removing the solvent (water, etc.). The aggregate of plate-shaped metal nanoparticles has plasticity and can be molded by compression. Furthermore, compression significantly improves various physical properties such as electrical conductivity.

[0029] [Complex] A fourth aspect of the present invention is a composite comprising an unsaturated fatty acid and plate-like metal nanoparticles. The methods described in the first and second aspects can reduce metal ions to produce metal nanoparticles. The metal nanoparticles include a composite containing unsaturated fatty acids and plate-like metal nanoparticles. This composite is characterized by growing large in the diameter direction while having a small thickness. In the composite of the fourth aspect of the present invention, the ratio (d / t) of the diameter (d) to the thickness (t) is preferably 100 to 1200, more preferably 250 to 800, and even more preferably 400 to 800. The thickness (t) of the composite is preferably 30 nm or less, more preferably 5 to 15 nm. The diameter (d) of the composite is measured using a scanning electron microscope. The thickness (t) of the composite is measured using an atomic force microscope.

[0030] <Unsaturated fatty acids> Examples of unsaturated fatty acids in the fourth aspect include the same as those in the first and second aspects. In one preferred embodiment, the unsaturated fatty acid is preferably an alkenyl carboxylic acid having 2 to 23 carbon atoms. If the alkenyl carboxylic acid has 2 to 23 carbon atoms, metal ions are more likely to be reduced. The number of carbon atoms represents the number of carbon atoms in the alkenyl group. The number of carbon atoms in the alkenyl group is more preferably 2 to 21, and even more preferably 2 to 17. The alkenyl carboxylic acid may have two or more carboxy groups in the molecule. In a more preferred embodiment, the alkenyl carboxylic acid preferably has a structure of the following formula (X1) or (X2). R 1 -CH=CH-COOH (X1) R 2 -CH=CH-R 3 -COOH (X2) (In formula (X1), R 1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. 2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and R 3represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[0031] Alkenyl carboxylic acids having the structure of the above formula (X1) or (X2) are more likely to reduce metal ions. Among these, alkenyl carboxylic acids having the structure of formula (X2) are more preferred. In addition, R 3 is a CHCH(-COOH)CH group or a CH(-COOH)CH group. Examples of such alkenyl dicarboxylic acids include 2-octenylsuccinic acid, 2-dodecen-1-ylsuccinic acid, octenylsuccinic acid, and dodecenylsuccinic acid, with 2-octenylsuccinic acid and 2-dodecen-1-ylsuccinic acid being particularly preferred.

[0032] In the fourth aspect, the unsaturated fatty acid is preferably attached to at least one surface of the plate-like metal nanoparticles, and more preferably to both surfaces. By attaching the unsaturated fatty acid as a binder to at least one surface of the plate-like metal nanoparticles, adhesiveness and plasticity are more likely to be exhibited. As a result, the formability of the composite is more likely to be improved. Note that the "surface of the plate-like metal nanoparticles" refers to the surface in the diameter (d) direction. The unsaturated fatty acid may be attached to a portion of the surface of the plate-like metal nanoparticles, or to the entire surface. Whether or not unsaturated fatty acids are attached to the surfaces of the plate-shaped metal nanoparticles can be confirmed by elemental analysis using energy dispersive X-ray analysis.

[0033] <Plate-shaped metal nanoparticles> Examples of the plate-shaped metal nanoparticles contained in the composite include the same as those explained in the third embodiment, and preferred examples are also the same. In the fourth aspect, the content of the plate-shaped metal nanoparticles relative to the total mass of the composite is preferably 80 mass% or more, and more preferably 90 to 97 mass%. If the content of the plate-shaped metal nanoparticles relative to the total mass of the composite is within this range, the effect of plasticity is easily exhibited. The content of the plate-shaped metal nanoparticles contained in the composite can be calculated by methods such as energy dispersive X-ray analysis and thermogravimetric differential thermal analysis.

[0034] A preferred example of the fourth aspect is a composite containing gold plate-shaped metal nanoparticles. The composite containing gold plate-shaped metal nanoparticles has an electrical resistivity of 1×10 -4 It is preferable that the resistance is 5×10 Ω·m or less. -7 ~1×10 -4 It is more preferable that the electrical resistivity is Ω·m. The electrical resistivity is a value measured at room temperature using a four-probe device (such as a contact resistance meter) after compressing the composite (60%) at room temperature. The Young's modulus of the composite containing the gold plate-shaped metal nanoparticles is preferably 1 MPa or less, and more preferably 0.1 to 0.8 MPa. The Young's modulus is a value calculated from the relationship between the amount of strain and the direction of unidirectional compressive stress after compressing the composite at room temperature using a small benchtop testing machine (Shimadzu Corporation, product name: EZ-LX).

[0035] <Dispersion containing the complex> Another aspect of the fourth embodiment is a dispersion containing the composite. The dispersion of the composite is not particularly limited as long as it has the effects of the present invention, and examples thereof include water, ethanol, and the like. As described above, the composite of this embodiment is obtained by reducing metal ions in a solution. Therefore, the solution used during production can be used as the dispersion as is. Such a dispersion can be applied to, for example, conductive inks, etc.

[0036] [Multi-branched metal nanoparticles] A fifth aspect of the present invention is a multibranched metal nanoparticle having a central portion and a plurality of branch portions extending outward from the central portion, wherein the crystalline structure of the branch portions comprises a single crystal, and the crystalline structure of the entire multibranched metal nanoparticle is polycrystalline. In this specification, a "multibranched metal nanoparticle" refers to a metal nanoparticle having a central portion and branch portions, and a "branch portion" refers to a branched portion extending outward from the center of the particle. In the multibranched metal nanoparticle of the fifth aspect of the present invention, the crystalline structure of the branch portions comprises a single crystal, and the crystalline structure of the entire multibranched metal nanoparticle is polycrystalline. The crystalline structure of the multibranched metal nanoparticle can be determined by taking electron diffraction images of the branch portions of one multibranched metal nanoparticle and the entire multibranched metal nanoparticle using a transmission electron microscope (TEM). In this specification, the phrase "the crystalline structure of the branch portions comprises a single crystal" means that any of the multiple branch portions present in one multibranched metal nanoparticle contains a single crystal crystalline structure. The branched portion preferably includes a plurality of plate-like pieces. More preferably, the branched portion is composed of a plurality of plate-like pieces extending outward from the center in a three-dimensional and irregular manner. Here, "plate-like piece" refers to a piece that is plate-shaped, and refers to a thin piece whose thickness (t1) is very small compared to its length (d1). The length (d1) of a plate-like piece refers to the diameter of a perfect circle that contains the plate-like piece, when one protruding end of the plate-like piece is in contact with the other end on the circumference. The length (d1) of the piece can be measured using a scanning electron microscope. The average length (d1) of the plate-like pieces av The average length (d1) is preferably 20 to 100 nm, more preferably 40 to 60 nm. av ) means the average length (d1) of 15 platelets contained in one multibranched metal nanoparticle. In one embodiment, the plate-like pieces may be the plate-like metal nanoparticles of the third embodiment.

[0037] FIG. 4 is a transmission electron micrograph showing an example of a multibranched metal nanoparticle according to one embodiment of the present invention. FIG. 5 is a transmission electron micrograph showing an example of a branched portion of a multi-molecular metal nanoparticle according to one embodiment of the present invention. As shown in FIG. 4, the multibranched metal nanoparticle according to the fifth embodiment has branched portions extending outward from the particle. Furthermore, the branched portions extend from the center of the particle. Furthermore, the branched portions have a plate-like shape, as shown in FIGS. 4 and 5. That is, the multibranched metal nanoparticle shown in FIG. 4 is composed of a center and a plurality of plate-like pieces extending three-dimensionally and irregularly outward from the center. In one aspect of the present invention, the crystal structure of the extensions of the platelets is preferably single crystal. Here, the "extensions of the platelets" refers to the portions of the platelets that extend three-dimensionally outward from the center of the multibranched metal nanoparticle and do not overlap with other platelets. The multibranched metal nanoparticle of the fifth aspect is composed of a center and multiple platelets extending outward from the center, and it is preferable that the extensions of the platelets are single crystal and that the crystal structure of the entire multibranched metal nanoparticle is polycrystalline.

[0038] The average particle diameter of the multibranched metal nanoparticles is preferably 100 to 500 nm, more preferably 150 to 350 nm. The particle diameter of the multibranched metal nanoparticles refers to the diameter of a perfect circle that encloses the multibranched metal nanoparticles, with the branched portions of the multibranched metal nanoparticles in contact with the circumference. The particle diameter of the multibranched metal nanoparticles can be measured using a scanning electron microscope. In this specification, the average particle diameter of the multibranched metal nanoparticles refers to the value calculated from the average particle diameter of 15 multibranched metal nanoparticles.

[0039] In the fifth embodiment, the multibranched metal nanoparticles preferably include multibranched metal nanoparticles of a noble metal. Examples of noble metals include gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium. Among these, multibranched metal nanoparticles of gold, silver, or platinum are more preferred, and multibranched metal nanoparticles of gold are particularly preferred.

[0040] <Dispersion containing multibranched metal nanoparticles> Another aspect of the fifth embodiment is a dispersion containing multibranched metal nanoparticles. The dispersion of multibranched metal nanoparticles is not particularly limited as long as it achieves the effects of the present invention, and examples include water, ethanol, and the like. As described later in the method for producing multibranched metal nanoparticles, the multibranched metal nanoparticles of this embodiment are obtained by reducing metal ions in a solution. Therefore, the solution used during production can be used as the dispersion. Such dispersions can be used, for example, in immunostaining and biosensors.

[0041] <Complex containing multibranched metal nanoparticles and unsaturated fatty acids> Another aspect of the fifth embodiment of the present invention is a composite comprising a multibranched metal nanoparticle and an unsaturated fatty acid.

[0042] (unsaturated fatty acids) Examples of unsaturated fatty acids in the fifth aspect include the same as those in the first and second aspects. In one preferred embodiment, the unsaturated fatty acid is an alkenyl carboxylic acid having 5 to 23 carbon atoms. The number of carbon atoms represents the number of carbon atoms in the alkenyl group. The number of carbon atoms in the alkenyl group is more preferably 6 to 8, and even more preferably 6 to 7. The alkenyl carboxylic acid may have two or more carboxy groups in the molecule. In a more preferred embodiment, the alkenyl carboxylic acid preferably has a structure of the following formula (X3) or (X4). R 4 -CH=CH-COOH (X3) R 5 -CH=CH-R 6 -COOH (X4) (In formula (X3), R 4 represents an alkyl group having 3 to 16 carbon atoms. 5 represents an alkyl group having 3 to 16 carbon atoms, and R 6represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[0043] Among these, alkenylcarboxylic acids having the structure of formula (X4) are more preferred. 5 is an alkenyl group having 3 to 4 carbon atoms, R 6 is a CH2CH(-COOH)CH2 group or a CH(-COOH)CH2 group. As such an alkenyl dicarboxylic acid, for example, 2-hexenyl succinic acid is particularly preferred.

[0044] In the fifth aspect, the unsaturated fatty acid is preferably attached to at least a portion of the multi-branched metal nanoparticles. Such a composite is likely to exhibit adhesiveness and plasticity because the unsaturated fatty acid acts as a binder. As a result, the moldability of the composite is likely to be improved. Whether or not unsaturated fatty acids are attached to the surface of the multibranched metal nanoparticles can be confirmed by elemental analysis using energy dispersive X-ray analysis.

[0045] [Method of manufacturing multi-branched metal nanoparticles] A sixth aspect of the present invention is a method for producing the multibranched metal nanoparticles, which includes a step of contacting an unsaturated fatty acid with a metal ion. As described above, the inventors of the present invention have found that metal nanoparticles can be easily produced by reducing metal ions using an unsaturated fatty acid. That is, by contacting an unsaturated fatty acid with a metal ion, the metal ion can be reduced to produce zero-valent metal nanoparticles. They have also found that multibranched metal nanoparticles can be produced by a similar method.

[0046] <Unsaturated fatty acids> Examples of the unsaturated fatty acid in the sixth aspect include the same as those in the first aspect. In one preferred embodiment, the unsaturated fatty acid is preferably an alkenyl carboxylic acid having 5 to 23 carbon atoms. If the alkenyl carboxylic acid has 5 to 23 carbon atoms, multibranched metal nanoparticles are more likely to be produced. The number of carbon atoms represents the number of carbon atoms in the alkenyl group. The number of carbon atoms in the alkenyl group is more preferably 6 to 8, and even more preferably 6 to 7. The alkenyl carboxylic acid may have two or more carboxy groups in the molecule. In a more preferred embodiment, the alkenyl carboxylic acid preferably has a structure of the following formula (X3) or (X4). R 4 -CH=CH-COOH (X3) R 5 -CH=CH-R 6 -COOH (X4) (In formula (X3), R 4 represents an alkyl group having 3 to 16 carbon atoms. 5 represents an alkyl group having 3 to 16 carbon atoms, and R 6 represents an alkylene group having 1 to 4 carbon atoms, a CH2CH(-COOH)CH2 group, or a CH(-COOH)CH2 group.

[0047] Alkenyl carboxylic acids having the structure of the above formula (X3) or (X4) facilitate reduction of metal ions, making it easier to produce multi-branched metal nanoparticles. Among these, alkenyl carboxylic acids having the structure of formula (X4) are more preferred. In addition, R 5 is an alkenyl group having 3 to 4 carbon atoms, R 6 is a CH2CH(-COOH)CH2 group or a CH(-COOH)CH2 group. As such an alkenyl dicarboxylic acid, for example, 2-hexenyl succinic acid is particularly preferred.

[0048] <Metal ions> Examples of metal ions in the sixth embodiment include the same as those in the first embodiment, and preferred examples are also the same. In the sixth embodiment, the metal ions are preferably prepared from a metal salt such as chloroauric acid, etc. By dissolving such a metal salt in water, the metal ions can be more easily prepared.

[0049] <Manufacturing process> In the sixth aspect, the contact between the unsaturated fatty acid and the metal ion (hereinafter sometimes simply referred to as the "contacting step") is carried out in a solution. The solution is preferably an aqueous solution or a mixed solution containing water. In the sixth aspect, the unsaturated fatty acid and the metal ion are preferably contacted by mixing them in a solution. The temperature of the solution in the contacting step is preferably from room temperature to 90° C., more preferably from 20 to 70° C., and particularly preferably from 50 to 70° C. Here, "room temperature" means room temperature or 20° C. The concentration of the unsaturated fatty acid in the solution is preferably 0.1 to 5 wt %, more preferably 0.8 to 1.5 wt %, based on the total weight of the solution. If the concentration of the unsaturated fatty acid is within this range, metal ions can be easily reduced. The solution may contain other components in addition to the unsaturated fatty acids and metal ions. The other components are not particularly limited as long as they achieve the effects of the present invention, and examples thereof include ethanol. When other components are contained, the amount of the other components is preferably in the range of 0.1 to 50 (V / V%) relative to the solvent, such as water.

[0050] According to the method for producing multibranched metal nanoparticles of this embodiment, multibranched metal nanoparticles can be easily prepared under the conditions described above. The time required to prepare multibranched metal nanoparticles, i.e., the time for the contact step, is about 10 seconds to 15 minutes, allowing multibranched metal nanoparticles to be prepared in a short reaction time. Furthermore, by adjusting the time for the contact step and the solution temperature within the aforementioned ranges, it is also possible to control the average particle size of the multibranched metal nanoparticles.

[0051] The contacting step reduces the metal ions to produce multibranched metal nanoparticles. The multibranched metal nanoparticles are obtained in a dispersed state in a solution (a dispersion containing metal nanoparticles). Therefore, the production method of this embodiment may include a step of recovering the metal nanoparticles after the contacting step. The method for recovering the multibranched metal nanoparticles is not particularly limited as long as it has the effect of the present invention, and they may be recovered by a conventionally known method, for example, a method in which ethyl acetate or the like is added to the solution to transfer the metal nanoparticles to an organic solvent phase, and then the metal nanoparticles contained in the organic solvent phase are recovered.

[0052] <Application> The plate-shaped metal nanoparticles according to the third aspect of the present invention can be applied to, for example, catalysts, drug delivery, tumor detection, and the like. Furthermore, the composite according to the fourth aspect of the present invention can be used in, for example, conductive inks, conductive coatings, conductive gap fillers, decorating agents, plastic shaping such as embossing, catalysts, and the like. Furthermore, the multibranched metal nanoparticles according to the fifth aspect of the present invention can be applied to, for example, catalysts, immunostaining, biosensors, and the like. [Example]

[0053] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following description.

[0054] [Example 1] <Synthesis of gold nanoparticles> As the unsaturated fatty acid, in the formula (X2), R 2 is an alkyl group having 16 carbon atoms, and R 3 An alkenyl carboxylic acid with a CH(-COOH)CH group was used to prepare an aqueous solution (1.1 wt%) of unsaturated fatty acid by adding 500 μL of the unsaturated fatty acid to water. This solution was then heated to 69°C, after which a chloroauric acid solution (2 mmol / L) was added and mixed. A color change in the solution was observed, confirming the formation of gold nanoparticles. The solvent was then removed to obtain a complex containing plate-shaped gold nanoparticles and unsaturated fatty acid. The average diameter (d) of the obtained composite was 8 μm, the average thickness (t) was 15 nm, and the (d / t) ratio was 533. The electrical resistivity of the composite was 5×10 -7 The Young's modulus was 0.4 MPa. The metal nanoparticles were evaluated according to the following method.

[0055] <Evaluation of metal nanoparticles> The metal nanoparticles were observed using a scanning electron microscope and an atomic force microscope to identify their types. The average particle size, aspect ratio, diameter (d), and thickness (t) of the obtained metal nanoparticles were measured according to the following conditions. (average particle size, aspect ratio, diameter (d)) Equipment: Scanning electron microscope, model: JSM-6340F (manufactured by JEOL Ltd.) Observation conditions: 10 kV, 10 μA Using the above-mentioned device, the major axis (b), minor axis (a), and particle diameter of 10 granular metal nanoparticles were measured, and the average value was calculated. Similarly, the diameter (d) of 10 plate-shaped metal nanoparticles was measured, and the average value was calculated. (thickness (t)) Equipment: Atomic force microscope (Seiko Instruments Inc.) Unit: SPA-300 Station: SPI 4000 Measurement range: 20 μm x 20 μm Using the above-mentioned device, the thickness (t) of 10 plate-shaped metal nanoparticles was measured, and the average value was calculated.

[0056] The electrical conductivity and Young's modulus of the composite containing metal nanoparticles were measured under the following conditions. (Conductivity evaluation) Equipment: Contact resistance measuring instrument (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name: MCP-T370, four-probe method) The obtained composite was compressed to 80% using a small tabletop testing machine (Shimadzu Corporation, product name: EZ-LX), and then the electrical resistivity was measured using the above measuring device. (Young's modulus evaluation) Equipment: Small tabletop testing machine (Shimadzu Corporation, product name: EZ-LX) The obtained composite was compressed to 60% (strain) at room temperature at a rate of 0.5 mm / min, and then the Young's modulus was calculated from the relationship between the amount of strain and the direction of the compressive stress in one direction.

[0057] [Example 2] As the unsaturated fatty acid, in the formula (X2), R 2 is an alkyl group having 12 carbon atoms, and R 3 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in which the group is CH(-COOH)CH2 was used and the reaction temperature was set at 56°C. As a result, a composite containing plate-like gold nanoparticles and the unsaturated fatty acid was obtained. The gold nanoparticles of the composite were evaluated in the same manner as in Example 1. The average diameter (d) was 6 μm and the average thickness (t) was 8 nm. The (d / t) ratio was 750. The electrical resistivity of the composite was 5 × 10 -7 The Young's modulus was 0.4 MPa.

[0058] [Example 3] As the unsaturated fatty acid, dodecenyl succinic acid (in the formula (X2), R 2 is a nonyl group, and R 3 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid (wherein the unsaturated fatty acid group is CH2CH(-COOH)CH2) was used and the reaction temperature was set at 53°C. As a result, a composite of plate-shaped gold nanoparticles and the unsaturated fatty acid was obtained. The gold nanoparticles of the composite were evaluated in the same manner as in Example 1. The average diameter (d) was 6 μm and the average thickness (t) was 8 nm. The (d / t) ratio was 750. The electrical resistivity of the composite was 5×10 -7 The Young's modulus was 0.4 MPa.

[0059] [Example 4] As the unsaturated fatty acid, in the formula (X2), R 2 is a pentyl group, and R 3Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in which the group is CH2CH(-COOH)CH2 was used and the reaction temperature was set at 53°C. As a result, a composite of plate-shaped gold nanoparticles and the unsaturated fatty acid was obtained. The gold nanoparticles of the composite were evaluated in the same manner as in Example 1. The average diameter (d) was 6 μm and the average thickness (t) was 8 nm. The (d / t) ratio was 750. The electrical resistivity of the composite was 5 × 10 -7 The Young's modulus was 0.4 MPa.

[0060] [Example 5] As the unsaturated fatty acid, in the formula (X2), R 2 is a propyl group, and R 3 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in which the group is CH2CH(-COOH)CH2 was used and the reaction temperature was 53°C. As a result, multibranched gold nanoparticles were obtained. The obtained multibranched gold nanoparticles had platelet-like branched portions, and the branched portions were single crystals. Meanwhile, the crystal structure of the entire multibranched gold nanoparticles was polycrystalline. Furthermore, the average particle diameter of the multibranched gold nanoparticles was 350 nm. The crystal structure and average particle diameter of the multibranched gold nanoparticles were measured by the following methods. (Average particle size) Equipment: Scanning electron microscope, model: JSM-6340F (manufactured by JEOL Ltd.) Observation conditions: 10 kV, 10 μA Using the above-mentioned device, the diameter of the perfect circle containing the multibranched gold nanoparticles was measured to determine the particle size of the multibranched gold nanoparticles. Using the same method, the particle sizes of 15 multibranched gold nanoparticles were measured and the average value was calculated. (crystal structure) Equipment: Transmission electron microscope Model: Tecnai Osiris (manufactured by FEI Company) Using the above-mentioned equipment, the crystal structure was investigated by measuring electron diffraction patterns of the branched portion and one entire multi-branched gold nanoparticle.

[0061] [Example 6] As the unsaturated fatty acid, in the formula (X2), R2 is a methyl group, and R 3 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in which the group is CHCH(-COOH)CH was used and the reaction temperature was set at 53°C. As a result, granular gold nanoparticles were obtained. The average particle size and aspect ratio of the obtained granular gold nanoparticles were calculated in the same manner as in Example 1. The average particle size of the granular gold nanoparticles was found to be 120 nm, and the aspect ratio was 1.5.

[0062] [Example 7] As an unsaturated fatty acid, 5-hexenoic acid (in the formula (X2), R 2 is hydrogen and R 3 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid (wherein the carboxyl group is a propylene group) was used and the reaction temperature was set at 53°C. As a result, granular gold nanoparticles were obtained. The average particle size and aspect ratio of the obtained granular gold nanoparticles were calculated in the same manner as in Example 1. The average particle size of the granular gold nanoparticles was found to be 120 nm, and the aspect ratio was found to be 1.5.

[0063] [Example 8] As an unsaturated fatty acid, trans-2-hexenoic acid (in the formula (X1), R 1 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid (in which the alkyl group is a propyl group) was used and the reaction temperature was set at 53°C. As a result, granular gold nanoparticles were obtained. The average particle size and aspect ratio of the obtained granular gold nanoparticles were calculated in the same manner as in Example 1. The average particle size of the granular gold nanoparticles was found to be 120 nm, and the aspect ratio was found to be 1.5.

[0064] [Example 9] As the unsaturated fatty acid, 4-pentenoic acid (in the formula (X2), R 2 is hydrogen and R 3Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid (in which the alkyl group is an ethylene group) was used and the reaction temperature was set at 53°C. As a result, granular gold nanoparticles were obtained. The average particle size and aspect ratio of the obtained granular gold nanoparticles were calculated in the same manner as in Example 1. The average particle size of the granular gold nanoparticles was found to be 120 nm, and the aspect ratio was found to be 1.5.

[0065] [Example 10] As the unsaturated fatty acid, 3-butenoic acid (in the formula (X2), R 2 is hydrogen and R 3 Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid (in which the carboxyl group is a methylene group) was used and the reaction temperature was set at 53°C. As a result, granular gold nanoparticles were obtained. The average particle size and aspect ratio of the obtained granular gold nanoparticles were calculated in the same manner as in Example 1. The average particle size of the granular gold nanoparticles was found to be 120 nm, and the aspect ratio was 1.5.

[0066] [Comparative Example 1] The reaction was carried out in the same manner as in Example 1 except that dodecyl succinic acid was used instead of the unsaturated fatty acid, but no metal nanoparticles were produced.

[0067] The above results demonstrate that the methods of Examples 1 to 10 facilitate the reduction of metal ions to produce metal nanoparticles. Furthermore, the composites obtained in Examples 1 to 4 had large (d / t) values, indicating that the particles had grown significantly in the diametric direction. Furthermore, in Example 5, in which hexenyl succinic acid was used as the unsaturated fatty acid, multibranched metal nanoparticles were produced. The branched portions of the resulting multibranched metal nanoparticles had a single-crystal crystalline structure, but the overall crystalline structure of each multibranched metal nanoparticle was polycrystalline. On the other hand, as shown in Comparative Example 1, no metal nanoparticles were produced when a fatty acid was used as a reducing agent. These results demonstrate that the metal ion reduction method of the present invention provides metal nanoparticles in a simple, energy-saving manner with minimal environmental impact. It was also demonstrated that the shape of the metal nanoparticles can be controlled within a specific range. Furthermore, the resulting plate-like metal nanoparticles and their composites are expected to be useful in conductive inks, conductive coatings, conductive gap fillers, decorative materials, embossing, and other plastic molding applications.

Claims

1. A reducing agent for metal ions, including precious metal ions, comprising: A metal ion reducing agent comprising an unsaturated fatty acid having a structure represented by the following formula (X2): R ２ -CH=CH-R ３ -COOH ・・・(X2) (In formula (X2), R ２ represents hydrogen or an alkyl group having 1 to 16 carbon atoms; R ３ is CH ２ CH(-COOH)CH ２ group, or CH(—COOH)CH ２ represents a group.)

2. 2. The metal ion reducing agent according to claim 1, wherein the unsaturated fatty acid having the structure of formula (X2) comprises 2-octenylsuccinic acid, 2-dodecen-1-ylsuccinic acid, octenylsuccinic acid, or dodecenylsuccinic acid.

3. A composite comprising the metal ion reducing agent according to claim 1 or 2 and plate-like metal nanoparticles of a noble metal.

4. The composite of claim 3 , wherein the metal ion reducing agent is attached to at least one surface of the plate-like metal nanoparticles.

5. The composite according to claim 3 or 4, wherein the content of the plate-shaped metal nanoparticles relative to the total mass of the composite is 80 mass% or more.

6. 6. The composite of claim 3, wherein the plate-shaped metal nanoparticles comprise gold plate-shaped metal nanoparticles.

7. Electrical resistivity is 1 x 10 －４ The composite according to claim 6, having a resistance of Ω·m or less.

8. 8. The composite according to claim 6 or 7, having a Young's modulus of 1 MPa or less.

9. the composite comprises multi-branched metal nanoparticles containing platelets of the plate-like metal nanoparticles; the multi-branched metal nanoparticles have a central portion and branch portions each consisting of a plurality of the platelets extending outward from the central portion; 9. The composite of claim 3, wherein the crystalline structure of the branches comprises a single crystal and the crystalline structure of the entire multi-branched metal nanoparticle is polycrystalline.

10. 10. The composite according to claim 9, wherein the branched portions are composed of the plurality of plate-like pieces extending outward from the central portion in a three-dimensional and irregular manner.

11. 11. The composite of claim 9 or 10, wherein the average length of the platelets is from 20 to 100 nm.

12. A dispersion comprising the composite of any one of claims 3 to 11.

13. Plate-shaped metal nanoparticles of a precious metal contained in the composite according to any one of claims 3 to 11, The plate-like metal nanoparticles have a diameter (d) to thickness (t) ratio (d / t) of 100 to 1200, and the thickness (t) is less than 30 nm.

Images