JP7828662B2 - Method for reducing metal ions, plate-shaped metal nanoparticles, composites containing plate-shaped metal nanoparticles and their dispersions, and branched metal nanoparticles and methods for producing the same. - Google Patents

Description

This invention relates to a method for reducing metal ions, plate-shaped metal nanoparticles, a composite containing plate-shaped metal nanoparticles and a dispersion thereof, and branched metal nanoparticles and a method for producing them.

Precious metal nanoparticles such as gold, silver, and platinum, and their dispersions, are usually obtained by adding a reducing agent to a solution containing precious metal ions and reducing 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 reducing agents and stabilizers, 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 harmful cyclohexanone as a reducing agent. Furthermore, the gold nanoparticles produced by this method have an average particle size of up to approximately 400 nm, making it difficult to control their size and shape.

Most of the methods for producing precious metal nanoparticles reported to date, as described in Non-Patent Document 3, involve multi-step reactions using various reagents, resulting in complex synthesis procedures.
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 as an aqueous phase to transfer the gold ions to the aqueous phase, and performing a reduction treatment at a temperature near room temperature. However, the method described in Patent Document 1 is complex and there is a risk of contamination with potassium, an alkali metal, which may limit the applications of the recovered gold. Furthermore, even with the method described in Patent Document 1, there is a problem in that it is difficult to control the size and shape of the gold nanoparticles obtained.

Incidentally, some metal nanoparticles have a surface that is finely branched, and are called "branched metal nanoparticles." Examples of such branched metal nanoparticles include those with spines resembling sea urchins and those with fine irregularities on their surface, as reported in Non-Patent Documents 4 and 5. These Non-Patent Documents describe complex synthesis procedures for branched metal nanoparticles, as they involve multi-step reactions using various reagents.

Patent No. 5351747

Brust, M. et al. , Synthesis of Thiol-Derivatized Gold Nanoparticles in a Two phases Liquid-Liquid System. , J. Chem. Soc. , Chem. Commun. 1994, 801-802. M. A. Uppal et al. , J. Matter. Chem. , A1, 7351 (2013). H. Liu et al. , CrysEngComm. , 2011, 13, 2281. Priya Vijayaraghavan et al. , ACS Appl. Mater. Interfaces 2016, 8, 23909. Song et al. , J. Mater. Chem. B, 2016, 4, 7112.

Therefore, the present invention aims to provide a method for reducing metal ions to obtain metal nanoparticles, which is environmentally friendly, simple, and energy-efficient, and allows for easy and precise control of the size and shape of the metal nanoparticles. It also aims to provide a composite containing plate-shaped metal nanoparticles and its dispersion, as well as plate-shaped metal nanoparticles. Furthermore, the present invention aims to provide novel multi-branched metal nanoparticles and a method for producing the same.

As a result of diligent research, the inventors discovered that all of the above problems can be solved by reducing metal ions through contact with unsaturated fatty acids, and thus completed the present invention.
In other words, the present invention has the following aspects.
[1] A method for reducing metal ions by bringing unsaturated fatty acids into contact 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 a metal ion according to [2], wherein the alkenylcarboxylic acid has the structure of 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), ^R1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 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 a metal ion according to any one of [1] to [3], comprising the step of mixing the unsaturated fatty acid and the metal ion in a solution.
[5] A method for reducing a metal ion according to any one of [1] to [4], wherein the metal ion contains a noble metal ion.
[6] Metal ion reducing agent containing 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 alkenylcarboxylic acid has the structure of 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), ^R1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 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-shaped 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 composite according to [10], wherein the alkenylcarboxylic acid has the structure of 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), ^R1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH)CH2 group, or a CH(-COOH) _{CH2 group} .)
[12] The composite according to any one of [9] to [11], wherein the unsaturated fatty acid is attached to at least one surface of the plate-shaped 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% by 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 include gold plate-shaped metal nanoparticles.
[16] The composite material according to [15], wherein the electrical resistivity is 1 × ^10⁻⁴ Ω·m or less.
[17] The composite according to [15] or [16], wherein the Young's modulus is 1 MPa or less.
A dispersion comprising the complex described in any one of items [18], [9], to [17].
[19] Plate-shaped metal nanoparticles having a diameter (d) to thickness (t) ratio (d/t) of 100 to 1200, and a thickness (t) of 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 branched metal nanoparticle having a central part and a plurality of branched parts extending outward from the central part, wherein the crystal structure of the branched parts includes single crystals and the crystal structure of the entire branched metal nanoparticle is polycrystalline.
[23] The branched portion comprises a plurality of plate-like pieces, as described in [22], a multi-branched metal nanoparticle.
[24] The branched portion is composed of a plurality of plate-like pieces that extend outward three-dimensionally and irregularly from the central part, as described in [22] or [23].
[25] The branched metal nanoparticle according to [23] or [24], wherein the crystal structure of the extended portion of the plate-like piece is a single crystal.
[26] The branched metal nanoparticle according to any one of [23] to [25], wherein the average length of the plate-like piece is 20 to 100 nm.
[27] The branched metal nanoparticles according to any one of [22] to [26], wherein the branched metal nanoparticles include branched metal nanoparticles of a noble metal.
[28] The branched metal nanoparticles according to any one of [22] to [27], wherein the branched metal nanoparticles include gold branched metal nanoparticles.
A dispersion containing branched metal nanoparticles as described in any one of items [29], [22], to [28].
A complex comprising branched metal nanoparticles as described in any one of items [30], [22] to [28], and an unsaturated fatty acid.
[31] The complex according to [30], wherein the unsaturated fatty acid is an alkenylcarboxylic acid having 5 to 23 carbon atoms.
[32] The complex according to [31], wherein the alkenylcarboxylic acid has the structure of the following formula (X3) or the following formula (X4).
R ⁴ -CH=CH-COOH...(X3)
R ⁵ -CH=CH-R ⁶ -COOH...(X4)
(In formula (X3), ^R4 represents an alkyl group having 3 to 16 carbon atoms. In formula (X4), ^R5 represents an alkyl group having 3 to 16 carbon atoms, and ^R6 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 branched metal nanoparticles according to any one of [22] to [28], comprising the step of contacting an unsaturated fatty acid with a metal ion.
[34] The method for producing branched 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 branched metal nanoparticles according to [34], wherein the alkenylcarboxylic acid has the structure of the following formula (X3) or the following formula (X4).
R ⁴ -CH=CH-COOH...(X3)
R ⁵ -CH=CH-R ⁶ -COOH...(X4)
(In formula (X3), ^R4 represents an alkyl group having 3 to 16 carbon atoms. In formula (X4), ^R5 represents an alkyl group having 3 to 16 carbon atoms, and ^R6 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH) _CH2 group, or a CH(-COOH) _CH2 group.)

According to the present invention, a method for reducing metal ions to obtain metal nanoparticles is available that is environmentally friendly, simple, and energy-efficient, and allows for easy and precise control of the size and shape of the metal nanoparticles. The invention also provides a composite containing plate-shaped metal nanoparticles and its dispersion, as well as plate-shaped metal nanoparticles. Furthermore, the present invention provides novel multi-branched metal nanoparticles and a method for producing multi-branched metal nanoparticles that is environmentally friendly, simple, and energy-efficient.

This is an electron microscope image showing an example of metal nanoparticles obtained by a method according to one aspect of the present invention. This is an electron microscope image showing an example of metal nanoparticles obtained by a method according to another aspect of the present invention. This is an example of a conceptual diagram illustrating the process of generating metal nanoparticles in the first and second embodiments of the present invention. This is an example of a conceptual diagram illustrating the process of generating metal nanoparticles in the first and second embodiments of the present invention. This is a transmission electron microscope image showing an example of a branched metal nanoparticle according to one embodiment of the present invention. This is a transmission electron microscope image showing an example of a branched portion of a multi-molecule metal nanoparticle according to one embodiment of the present invention. This is a selected-area electron diffraction image showing an example of the overall crystal structure of a multi-branched metal nanoparticle according to one aspect of the present invention.

The present invention will be described in detail below, but the present invention is not limited to the following embodiments.
[Methods for reducing metal ions]
A first aspect of the present invention is a method for reducing metal ions by contacting an unsaturated fatty acid with a metal ion. According to the first aspect, the metal ion is reduced by contacting the unsaturated fatty acid with the metal ion, and metal nanoparticles are produced. According to the reduction method of this aspect, metal nanoparticles can be produced in a simple and energy-saving manner.
In this specification, "metal ion" means a cation generated from a metal atom, and its valency is not particularly limited as long as it has the effect of the present invention.
In this specification, "metal nanoparticles" include both "granular metal nanoparticles" and "plate-like metal nanoparticles." Here, "granular metal nanoparticles" refer to metal nanoparticles whose particle shape approximates a sphere and whose aspect ratio, expressed as the ratio of the major axis (b) to the minor axis (a) (b/a), is between 1.0 and 1.5. "Plate-like metal nanoparticles" refer to thin particles whose particle shape is plate-like and whose thickness (t) is very small relative to the particle diameter (d). Here, the diameter (d) of plate-like metal nanoparticles refers to the diameter of a perfect circle drawn enclosing the plate-like metal nanoparticles, such that one protruding end and the other end of the plate-like metal nanoparticles touch on the circumference. 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 obtained by measuring the long axis (b) and short axis (a) of 10 granular metal nanoparticles.
Furthermore, 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 the average values of 10 plate-shaped metal nanoparticles.

<Unsaturated fatty acids>
In the first embodiment, metal ions are reduced using an unsaturated fatty acid. An unsaturated fatty acid means 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 it provides the effects of the present invention. In this embodiment, the unsaturated fatty acid may be in cis or trans form.
In one preferred embodiment, the unsaturated fatty acid is preferably an alkenylcarboxylic acid having 2 to 23 carbon atoms. If the alkenylcarboxylic acid has 2 to 23 carbon atoms, the metal ions are more easily reduced. The carbon number refers to 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. Furthermore, the alkenylcarboxylic acid may have two or more carboxyl groups in its molecule.
In one more preferred embodiment, the alkenylcarboxylic acid preferably has the structure of 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), ^R1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH)CH2 group, or a CH(-COOH) _{CH2 group} .)

If the alkenyl carboxylic acid has the structure of formula (X1) or (X2) above, the effects of metal ion reduction and shape control are more easily obtained. Of these, the alkenyl carboxylic acid having the structure of formula (X2) is even more preferable. In addition, alkenyl dicarboxylic acids in which ^R3 is a _CH2CH (-COOH) _CH2 group or a CH(-COOH) _CH2 group are particularly preferred. Examples of such alkenyl dicarboxylic acids include 2-octenyl succinic acid, 2-dodecene-1-yl succinic acid, octenyl succinic acid, dodecenyl succinic acid, etc., with 2-octenyl succinic acid and 2-dodecene-1-yl succinic acid being particularly preferred.

<Metal Ions>
In the first embodiment, the metal ions reduced by the unsaturated fatty acid preferably include noble metal ions. Examples of noble metal ions include cations such as gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium. Of these, the metal ions are more preferably gold, silver, or platinum, and particularly preferably gold.

<Conditions for Redemption>
The first aspect of the present invention provides a method for reducing metal ions, which preferably includes a step of contacting an unsaturated fatty acid with a metal ion in a solution, more preferably in an aqueous solution. A method of reducing metal ions in an aqueous solution is preferable because it has a lower environmental impact. Furthermore, it is more preferable that the contact between the unsaturated fatty acid and the metal ion is carried out by mixing a solution containing them.
The reduction temperature is preferably room temperature to 90°C, and more preferably 20°C to 70°C. "Room temperature" refers to room temperature or 20°C.
The concentration of unsaturated fatty acids in the solution is preferably 0.1 to 5 wt%, and more preferably 0.8 to 1.5 wt%, relative to the total weight of the solution. When the concentration of unsaturated fatty acids is within the above range, it becomes easier to reduce metal ions.
The solution may also contain other components besides unsaturated fatty acids and metal ions. These other components are not particularly limited as long as they provide the effects of the present invention, but examples include ethanol. If other components are included, their amount is preferably in the range of 0.1 to 50 (V/V%) relative to a solvent such as water.

According to a 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 can be obtained that include at least one selected from the group consisting of granular metal nanoparticles, plate-like metal nanoparticles, and composites containing an unsaturated fatty acid and plate-like metal nanoparticles.
In the first embodiment, granular metal nanoparticles are easily prepared, for example, by reducing a metal ion using an alkenylcarboxylic acid having the structure of formula (X1), where ^R1 is hydrogen or an alkyl group having 1 to 16 carbon atoms. Alternatively, they can be easily prepared by contacting a metal ion with an unsaturated fatty acid at room temperature or under heated conditions (e.g., 50 to 60°C). The resulting granular metal nanoparticles have an average particle diameter of 50 to 120 nm and an aspect ratio in the range of 1 to 1.5. The average particle diameter of the granular metal nanoparticles refers to the average value obtained by measuring the long axis 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. Plate-shaped metal nanoparticles are easily prepared by reducing metal ions with an alkenylcarboxylic acid having the structure of formula (X2), in which ^R2 is an alkyl group having 5 to 16 carbon atoms, and ^R3 is a _{CH2CH (} -COOH)CH2 group or a CH(-COOH) _CH2 group.
The plate-like metal nanoparticles obtained by the reduction method of this embodiment have a thickness (t) of several nanometers, while their diameter (d) grows to several micrometers. With this reduction method, it is possible to obtain thin metal nanoparticles with a very large ratio of thickness (t) to diameter (d) in a simple and energy-efficient manner. Furthermore, composites of plate-like metal nanoparticles and unsaturated fatty acids can also be obtained. Details of these plate-like metal nanoparticles and composites will be described later. The reduction state of metal ions can also be confirmed by visual color change.
In addition, it can be confirmed using analytical methods such as absorption spectroscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy.

As described above, according to the first embodiment, metal ions can be reduced and metal nanoparticles produced in a simple and energy-saving manner. That is, another aspect of the first embodiment 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 metal ion reducing agent. Furthermore, another aspect of the first embodiment 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).

[Method for manufacturing metal nanoparticles]
A second aspect of the present invention is a method for producing metal nanoparticles, comprising the 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 a metal ion 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.

<Unsaturated fatty acids>
Examples of unsaturated fatty acids in the second embodiment are the same as those in the first embodiment.
In one preferred embodiment, the unsaturated fatty acid is preferably an alkenylcarboxylic acid having 2 to 23 carbon atoms. If the alkenylcarboxylic acid has 2 to 23 carbon atoms, the metal ions are more easily reduced. The carbon number refers to 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. Furthermore, the alkenylcarboxylic acid may have two or more carboxyl groups in its molecule.
In one more preferred embodiment, the alkenylcarboxylic acid preferably has the structure of 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), ^R1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH)CH2 group, or a CH(-COOH) _{CH2 group} .)

If the alkenyl carboxylic acid has the structure of formula (X1) or (X2) above, the metal ion is more easily reduced. Of these, the alkenyl carboxylic acid having the structure of formula (X2) is even more preferred. In addition, alkenyl dicarboxylic acids in which ^R3 is a _CH2CH (-COOH) _CH2 group or a CH(-COOH) _CH2 group are particularly preferred. Examples of such alkenyl dicarboxylic acids include 2-octenyl succinic acid, 2-dodecene-1-yl succinic acid, octenyl succinic acid, dodecenyl succinic acid, etc., with 2-octenyl succinic acid and 2-dodecene-1-yl succinic acid being particularly preferred.

<Metal Ions>
The metal ions in the second embodiment are the same as those in the first embodiment, and the preferred examples are also the same.
In a second embodiment, it is preferable that the metal ions are prepared from a metal salt such as chloroauric acid. By dissolving such a metal salt in water, the metal ions can be prepared more easily.

<Manufacturing process>
In the second embodiment, the contact between the unsaturated fatty acid and the metal ion (hereinafter sometimes simply referred to as the "contact step") is carried out in a solution. Furthermore, the solution is preferably an aqueous solution or a mixed solution containing water. In the second embodiment, it is preferable to bring the unsaturated fatty acid and the metal ion into contact by mixing the unsaturated fatty acid and the metal ion in a solution.
The preferred conditions in the contact process, namely the solution temperature, the concentration of the unsaturated fatty acid, and the mixing conditions, are the same as in the first embodiment. By contacting the unsaturated fatty acid with metal ions under these conditions, the metal ions are easily reduced, and metal nanoparticles are generated. The method for confirming whether the metal ions have been reduced can be the same as in the first embodiment.

The aforementioned contact step reduces metal ions, generating 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 contact step.
The method for recovering the metal nanoparticles is not particularly limited as long as it achieves the effects of the present invention, and may be recovered by conventionally known methods, such as adding ethyl acetate or the like to the solution to transfer the metal nanoparticles to the organic solvent phase, and then recovering the metal nanoparticles contained in the organic solvent phase.

<Metal nanoparticles>
According to the manufacturing method of the second embodiment, metal nanoparticles can be obtained that include at least one selected from the group consisting of granular metal nanoparticles, plate-like metal nanoparticles, and composites containing unsaturated fatty acids and plate-like metal nanoparticles.
In a second embodiment, granular metal nanoparticles are more readily prepared, for example, by reducing a metal ion using an alkenylcarboxylic acid having the structure of formula (X1), where ^R1 is hydrogen or an alkyl group having 1 to 16 carbon atoms. They are also readily prepared by contacting a metal ion with an unsaturated fatty acid 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 in the range of 1 to 1.5.
Furthermore, the method for producing metal nanoparticles according to this embodiment makes it possible to obtain plate-shaped metal nanoparticles having a unique structure. The plate-shaped metal nanoparticles obtained by the production method according to this embodiment have a thickness (t) of several nanometers, while their diameter (d) grows to several micrometers. The reason why plate-shaped metal nanoparticles with such a large ratio of thickness (t) to diameter (d) can be obtained is thought to be due to the following reasons.
Unsaturated fatty acids, preferably alkenylcarboxylic acids having 2 to 23 carbon atoms, may form a bilayer structure in solution, as shown in Figure 3A or Figure 3B. Such a bilayer structure is easily formed, for example, by heating to 40 to 70°C, dissolving in a solvent such as water, and then cooling.
When such unsaturated fatty acids are brought into contact with metal ions, a reduction reaction of the metal ions occurs between the bilayer structures, and as a result, metal nanoparticles may be generated between the layers. Because the width of the bilayer structure is controlled in the thickness direction (i.e., the height direction of the bilayer structure in Figure 3A or Figure 3B), particles tend to grow in the diameter direction (i.e., the lateral direction of the bilayer structure in Figure 3A or Figure 3B). As a result, it is thought that very thin, plate-like metal nanoparticles with large particle growth in the diameter (d) direction are generated. In other words, according to the method for producing metal nanoparticles of this embodiment, when ^R2 is an alkyl group having 5 to 16 carbon atoms and ^R3 is a _CH2CH (-COOH)CH2 group or a CH(-COOH) _CH2 group in an alkenylcarboxylic acid having the structure of formula ( _X2) , it becomes easier to control the shape of the metal nanoparticles to be "very thin and plate-like". Furthermore, plate-like metal nanoparticles having such a specific shape can be produced under very mild conditions and by a method that has little environmental impact.
Furthermore, the manufacturing method of this embodiment can also yield a composite of plate-shaped metal nanoparticles and unsaturated fatty acids. Since such composites have plasticity and adhesiveness, they tend to have good moldability.

As described above, the unsaturated fatty acid of the present invention is a single substance that performs three roles: reducing agent, stabilizer (dispersant and stabilizer for metal nanoparticles), and control of the shape and size of metal nanoparticles. Therefore, there is no need to add other substances that perform these roles. Accordingly, according to the manufacturing method of the second embodiment of the present invention, metal nanoparticles can be produced using only three components: unsaturated fatty acid, metal ions, and their solvents.
Furthermore, the composite obtained by this manufacturing method can be heated at 100 to 600°C, preferably 210 to 350°C, to remove unsaturated fatty acids and extract only plate-shaped metal nanoparticles. Therefore, the second embodiment may include a step of removing unsaturated fatty acids from the composite.
The plate-shaped metal nanoparticles and composites of the present invention will be described in detail below.

[Sheet-shaped metal nanoparticles]
A third aspect of the present invention is a plate-shaped metal nanoparticle having a diameter (d) to thickness (t) ratio (d/t) of 100 to 1200, and a thickness (t) of less than 30 nm. The plate-shaped metal nanoparticle of this aspect is characterized by having a very small thickness (t) relative to its diameter (d). Such plate-shaped metal nanoparticles are advantageous, for example, in terms of film-forming properties.
The (d/t) ratio 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 the value measured using a scanning electron microscope. The thickness (t) of the plate-shaped metal nanoparticles refers to the value measured using an atomic force microscope.
In a third embodiment, the plate-shaped metal nanoparticles preferably include plate-shaped metal nanoparticles of a precious metal. Examples of precious metals include gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium. Of 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.

The plate-shaped metal nanoparticles in the third embodiment can be manufactured, for example, by the methods described in the first and second embodiments of the present invention.
As mentioned above, unsaturated fatty acids can form a bilayer structure in solutions such as water. By reducing metal ions in such a bilayer structure, plate-like metal nanoparticles with a large diameter (i.e., a large (d/t) ratio) can be produced. In the first and second embodiments, the plate-like metal nanoparticles may be obtained as a composite containing unsaturated fatty acids. By heat-treating this composite to remove the unsaturated fatty acids, it is possible to isolate only the plate-like metal nanoparticles.
Such plate-shaped metal nanoparticles can be applied to applications such as catalysts, drug delivery, and tumor detection.

<Aggregate of plate-shaped metal nanoparticles>
In the third embodiment, the plate-shaped metal nanoparticles can form an aggregate of plate-shaped metal nanoparticles by removing the solvent (water, etc.). The aggregate of plate-shaped metal nanoparticles is plastic and can be molded by compression. Furthermore, compression significantly improves various physical properties such as conductivity.

[complex]
A fourth aspect of the present invention is a composite comprising an unsaturated fatty acid and plate-shaped metal nanoparticles.
Metal nanoparticles can be produced by reducing metal ions using the methods described in the first and second embodiments. The metal nanoparticles include a composite comprising an unsaturated fatty acid and plate-shaped metal nanoparticles. This composite is characterized by growing significantly in the diametrical direction while having a small thickness. In the composite of the fourth embodiment of the present invention, the ratio of diameter (d) to thickness (t) (d/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, and more preferably 5 to 15 nm. The diameter (d) of the composite refers to the value measured using a scanning electron microscope. The thickness (t) of the composite refers to the value measured using an atomic force microscope.

<Unsaturated fatty acids>
The unsaturated fatty acids in the fourth embodiment are the same as those in the first and second embodiments.
In one preferred embodiment, the unsaturated fatty acid is preferably an alkenylcarboxylic acid having 2 to 23 carbon atoms. If the alkenylcarboxylic acid has 2 to 23 carbon atoms, the metal ions are more easily reduced. The carbon number refers to 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. Furthermore, the alkenylcarboxylic acid may have two or more carboxyl groups in its molecule.
In one more preferred embodiment, the alkenylcarboxylic acid preferably has the structure of 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), ^R1 represents hydrogen or an alkyl group having 1 to 16 carbon atoms. In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH)CH2 group, or a CH(-COOH) _{CH2 group} .)

If the alkenyl carboxylic acid has the structure of formula (X1) or (X2) above, the metal ion is more easily reduced. Of these, the alkenyl carboxylic acid having the structure of formula (X2) is even more preferred. In addition, alkenyl dicarboxylic acids in which ^R3 is a _CH2CH (-COOH) _CH2 group or a CH(-COOH) _CH2 group are particularly preferred. Examples of such alkenyl dicarboxylic acids include 2-octenyl succinic acid, 2-dodecene-1-yl succinic acid, octenyl succinic acid, dodecenyl succinic acid, etc., with 2-octenyl succinic acid and 2-dodecene-1-yl succinic acid being particularly preferred.

In a fourth embodiment, the unsaturated fatty acid is preferably attached to at least one surface of the plate-shaped metal nanoparticles, and preferably to both surfaces. This attachment of the unsaturated fatty acid as a binder to at least one surface of the plate-shaped metal nanoparticles facilitates the development of tackiness and plasticity. As a result, the moldability of the composite is improved. Note that "the surface of the plate-shaped metal nanoparticles" refers to the surface in the diameter (d) direction. Furthermore, the unsaturated fatty acid may be attached to a portion of the surface of the plate-shaped metal nanoparticles, or to the entire surface.
Whether or not unsaturated fatty acids are attached to the surface of plate-shaped metal nanoparticles can be confirmed by elemental analysis using energy-dispersive X-ray spectroscopy.

<Sheet-shaped metal nanoparticles>
The plate-shaped metal nanoparticles included in the composite are the same as those described in the third embodiment above, and preferred examples are also the same.
In the fourth embodiment, the content of plate-like metal nanoparticles relative to the total mass of the composite is preferably 80% by mass or more, and more preferably 90 to 97% by mass. If the content of plate-like metal nanoparticles relative to the total mass of the composite is within the above range, the plasticity effect is more likely to be exhibited. The content of plate-like metal nanoparticles in the composite can be calculated by methods such as energy-dispersive X-ray analysis or thermogravimetric differential thermal analysis.

One preferred example of the fourth embodiment is a composite containing plate-shaped gold metal nanoparticles. The electrical resistivity of the composite containing plate-shaped gold metal nanoparticles is preferably 1 × ^10⁻⁴ Ω·m or less, and more preferably 5 × ^10⁻⁷ to 1 × ^10⁻⁴ Ω·m. The electrical resistivity refers to the value measured at room temperature using a four-probe method device (such as a contact-type resistance meter) after compressing the composite to 60% under room temperature conditions. Furthermore, the Young's modulus of the composite containing plate-shaped gold metal nanoparticles is preferably 1 MPa or less, and more preferably 0.1 to 0.8 MPa. The Young's modulus refers to the 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 test machine (manufactured by Shimadzu Corporation, product name: EZ-LX).

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

[Multiple branch metal nanoparticles]
A fifth aspect of the present invention is a branched metal nanoparticle having a central part and a plurality of branched parts extending outward from the central part, wherein the crystal structure of the branched parts includes single crystals, and the crystal structure of the entire branched metal nanoparticle is polycrystalline. In this specification, "branched metal nanoparticle" means a metal nanoparticle having a central part and branched parts, and "branched part" means a branched portion that extends outward from the center of the particle. In the branched metal nanoparticle of the fifth aspect of the present invention, the crystal structure of the branched parts includes single crystals, and the crystal structure of the entire branched metal nanoparticle is polycrystalline. The crystal structure of a branched metal nanoparticle can be determined by taking electron diffraction images of the branched parts of a single branched metal nanoparticle and the entire branched metal nanoparticle using a transmission electron microscope (TEM). In this specification, "the crystal structure of the branched parts includes single crystals" means that one of the plurality of branched parts present in a single branched metal nanoparticle contains a single crystal crystal structure.
The branched portion preferably contains a plurality of plate-like fragments. More preferably, the branched portion is composed of a plurality of plate-like fragments that extend three-dimensionally and irregularly outward from the central part. Here, "plate-like fragment" refers to a piece that is plate-shaped, and means a thin piece with a thickness (t1) that is very small relative to the length (d1) of the fragment. The length (d1) of the plate-like fragment refers to the diameter of a perfect circle drawn that encloses the plate-like fragment so that one protruding end of the plate-like fragment and the other end of the plate-like fragment touch on the circumference. The length (d1) of the fragment can be measured using a scanning electron microscope. The average length (d1 _av ) of the plate-like fragments is preferably 20 to 100 nm, and more preferably 40 to 60 nm. The average length (d1 _av ) refers to the average value of the lengths (d1) of 15 plate-like fragments contained in one multi-branched metal nanoparticle.
In one embodiment, the plate-like piece may be a plate-like metal nanoparticle according to the third embodiment.

Figure 4 is a transmission electron microscope image showing an example of a multi-branched metal nanoparticle according to one embodiment of the present invention. Figure 5 is a transmission electron microscope image showing an example of a branched portion of a multi-molecule metal nanoparticle according to one embodiment of the present invention. As shown in Figure 4, the multi-branched metal nanoparticle according to the fifth embodiment has branched portions that extend outward from the particle. Furthermore, the branched portions extend from the center of the particle. Moreover, the branched portions have a plate-like shape as shown in Figures 4 and 5. That is, the multi-branched metal nanoparticle shown in Figure 4 is composed of a central part and a plurality of plate-like pieces that extend outward three-dimensionally and irregularly from the central part.
In one embodiment of the present invention, it is preferable that the crystal structure of the extended portion of the plate-like piece is a single crystal. Here, "extended portion of the plate-like piece" refers to a portion of the plate-like piece that extends three-dimensionally outward from the center of the branched metal nanoparticle and does not overlap with other plate-like pieces. The branched metal nanoparticle of the fifth embodiment is composed of a central part and a plurality of plate-like pieces extending outward from the central part, and it is preferable that the extended portion of the plate-like piece is a single crystal and the crystal structure of the entire branched metal nanoparticle is polycrystalline.

The average particle size of the branched metal nanoparticles is preferably 100 to 500 nm, and more preferably 150 to 350 nm. The particle size of the branched metal nanoparticles refers to the diameter of a perfect circle drawn enclosing the branched metal nanoparticles, such that the branched portion of the nanoparticle touches the circumference. The particle size of the branched metal nanoparticles can be measured using a scanning electron microscope. In this specification, the average particle size of branched metal nanoparticles refers to the value calculated from the average value obtained by measuring the particle size of 15 branched metal nanoparticles.

In a fifth embodiment, the branched metal nanoparticles preferably include branched metal nanoparticles of a noble metal. Examples of noble metals include gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium. Of these, it is more preferable to include branched metal nanoparticles of gold, silver, or platinum, and particularly preferable to include branched metal nanoparticles of gold.

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

<A complex containing multiple branched metal nanoparticles and unsaturated fatty acids>
Another aspect of a fifth aspect of the present invention is a composite comprising branched metal nanoparticles and an unsaturated fatty acid.

(unsaturated fatty acids)
The unsaturated fatty acids in the fifth embodiment are the same as those in the first and second embodiments.
In one preferred embodiment, the unsaturated fatty acid is preferably an alkenylcarboxylic acid having 5 to 23 carbon atoms. The carbon number 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 alkenylcarboxylic acid may also have two or more carboxyl groups in its molecule.
In one more preferred embodiment, the alkenylcarboxylic acid preferably has the structure of the following formula (X3) or the following formula (X4).
R ⁴ -CH=CH-COOH...(X3)
R ⁵ -CH=CH-R ⁶ -COOH...(X4)
(In formula (X3), ^R4 represents an alkyl group having 3 to 16 carbon atoms. In formula (X4), ^R5 represents an alkyl group having 3 to 16 carbon atoms, and ^R6 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH) _CH2 group, or a CH(-COOH) _CH2 group.)

Of these, it is even more preferable that the alkenylcarboxylic acid has the structure of formula (X4). Furthermore, an alkenyl dicarboxylic acid in ^{which R5} is an alkenyl group having 3 to 4 carbon atoms and ^R6 is a _CH2CH (-COOH) _CH2 group or a CH(-COOH) _CH2 group is particularly preferred. As such an alkenyl dicarboxylic acid, for example, 2-hexenyl succinic acid is particularly preferred.

In a fifth embodiment, it is preferable that the unsaturated fatty acid is attached to at least a portion of the branched metal nanoparticles. Such a composite is more likely to exhibit adhesiveness and plasticity because the unsaturated fatty acid acts as a binder. As a result, the moldability of the composite is more easily improved.
Whether or not unsaturated fatty acids are attached to the surface of branched metal nanoparticles can be confirmed by elemental analysis using energy-dispersive X-ray spectroscopy.

[Method for manufacturing branched metal nanoparticles]
A sixth aspect of the present invention is a method for producing branched metal nanoparticles, comprising the 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, metal ions can be reduced to produce zero-valent metal nanoparticles. They have also found that branched metal nanoparticles can be produced by a similar method.

<Unsaturated fatty acids>
Examples of unsaturated fatty acids in the sixth embodiment include those similar to those in the first embodiment.
In one preferred embodiment, the unsaturated fatty acid is preferably an alkenylcarboxylic acid having 5 to 23 carbon atoms. If the alkenylcarboxylic acid has 5 to 23 carbon atoms, branched metal nanoparticles are more easily formed. The carbon number refers to 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. Furthermore, the alkenylcarboxylic acid may have two or more carboxyl groups in its molecule.
In one more preferred embodiment, the alkenylcarboxylic acid preferably has the structure of the following formula (X3) or the following formula (X4).
R ⁴ -CH=CH-COOH...(X3)
R ⁵ -CH=CH-R ⁶ -COOH...(X4)
(In formula (X3), ^R4 represents an alkyl group having 3 to 16 carbon atoms. In formula (X4), ^R5 represents an alkyl group having 3 to 16 carbon atoms, and ^R6 represents an alkylene group having 1 to 4 carbon atoms, a _CH2CH (-COOH) _CH2 group, or a CH(-COOH) _CH2 group.)

If the alkenylcarboxylic acid has the structure of formula (X3) or (X4) above, the metal ions are more easily reduced, and branched metal nanoparticles are more easily formed. Of these, the alkenylcarboxylic acid having the structure of formula (X4) is even more preferred. In addition, alkenyl dicarboxylic acids in which ^R5 is an alkenyl group having 3 to 4 carbon atoms and ^R6 is a _CH2CH (-COOH) _CH2 group or a CH(-COOH) _CH2 group are particularly preferred. As such an alkenyl dicarboxylic acid, for example, 2-hexenyl succinic acid is particularly preferred.

<Metal Ions>
The metal ions in the sixth embodiment are the same as those in the first embodiment, and the preferred examples are also the same.
In the sixth embodiment, it is preferable that the metal ion is prepared from a metal salt such as chloroauric acid. By dissolving such a metal salt in water, the metal ion can be prepared more easily.

<Manufacturing process>
In the sixth aspect, the contact between the unsaturated fatty acid and the metal ion (hereinafter sometimes simply referred to as the "contact step") is carried out in a solution. Furthermore, the solution is preferably an aqueous solution or a mixed solution containing water. In the sixth aspect, it is preferable to bring the unsaturated fatty acid and the metal ion into contact by mixing the unsaturated fatty acid and the metal ion in a solution.
The temperature of the solution during the contact process is preferably room temperature to 90°C, more preferably 20°C to 70°C, and particularly preferably 50°C to 70°C. "Room temperature" refers to room temperature or 20°C.
The concentration of unsaturated fatty acids in the solution is preferably 0.1 to 5 wt%, and more preferably 0.8 to 1.5 wt%, relative to the total weight of the solution. When the concentration of unsaturated fatty acids is within the above range, it becomes easier to reduce metal ions.
The solution may also contain other components besides unsaturated fatty acids and metal ions. These other components are not particularly limited as long as they provide the effects of the present invention, but examples include ethanol. If other components are included, their amount is preferably in the range of 0.1 to 50 (V/V%) relative to a solvent such as water.

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

The aforementioned contact step reduces the metal ions, generating branched metal nanoparticles. The branched 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 contact step.
The method for recovering the branched metal nanoparticles is not particularly limited as long as it achieves the effects of the present invention, and may be recovered by conventionally known methods, such as adding ethyl acetate or the like to the above solution to transfer the metal nanoparticles to the organic solvent phase, and then recovering the metal nanoparticles contained in the organic solvent phase.

<Application>
The plate-shaped metal nanoparticles in the third aspect of the present invention can be applied, for example, to catalysts, drug transport, tumor detection, and the like.
Furthermore, the composite material according to the fourth aspect of the present invention can be applied to, for example, conductive inks, conductive coatings, conductive gap fillers, decorative agents, plastic molding such as embossing, catalysts, and the like.
Furthermore, the branched metal nanoparticles in the fifth aspect of the present invention can be applied to, for example, catalysts, immunostaining, and biosensors.

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

[Example 1]
<Synthesis of gold nanoparticles>
As the unsaturated fatty acid, an alkenyl carboxylic acid in formula (X2) where ^R2 is a C16 alkyl group and ^R3 is a CH(-COOH) _CH2 group was used. 500 μL of the unsaturated fatty acid was added to water to prepare an aqueous solution of the unsaturated fatty acid (1.1 wt%). This aqueous solution was then heated to 69°C, and a chloroauric acid solution (2 mmol/L) was added and mixed. A color change in the aqueous solution was observed, confirming the formation of gold nanoparticles. The solvent was removed to obtain a composite containing plate-like gold nanoparticles and the unsaturated fatty acid.
The average diameter (d) of the obtained composite was 8 μm, and the average thickness (t) was 15 nm. The ratio (d/t) was 533. The electrical resistivity of the composite was 5 × ^10⁻⁷ Ω·m, and the Young's modulus was 0.4 MPa.
The evaluation of metal nanoparticles was performed according to the following method.

<Evaluation of metal nanoparticles>
The types of metal nanoparticles were identified by observing them with a scanning electron microscope and an atomic force microscope. The average particle size, aspect ratio, diameter (d), and thickness (t) of the obtained metal nanoparticles were also 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 apparatus, the major axis (b), minor axis (a), and particle diameter were measured for 10 granular metal nanoparticles, and their average values were calculated. Similarly, the diameter (d) of 10 plate-shaped metal nanoparticles was measured, and its average value was calculated.
(Thickness (t))
Equipment: Atomic force microscope (manufactured by Seiko Instruments Inc.)
Unit: SPA-300 Station: SPI 4000
Measurement range: 20 μm x 20 μm
Using the above apparatus, the thickness (t) of 10 plate-shaped metal nanoparticles was measured, and the average value was calculated.

The conductivity and Young's modulus of composites containing metal nanoparticles were measured under the following conditions.
(Evaluation of electrical conductivity)
Equipment: Contact-type resistance meter (manufactured by Mitsubishi Chemical Analytec Co., Ltd., product name: MCP-T370, four-probe method)
The resulting composite was compressed to 80% using a small benchtop testing machine (manufactured by Shimadzu Corporation, product name: EZ-LX), and then its electrical resistivity was measured using the above measuring instrument.
(Evaluation of Young's modulus)
Equipment: Small benchtop testing machine (manufactured by Shimadzu Corporation, product name: EZ-LX)
The resulting 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 unidirectional compressive stress.

[Example 2]
Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in formula (X2), where ^R2 is a C12 alkyl group and ^R3 is a CH(-COOH) _CH2 group, was used as the unsaturated fatty acid, and the reaction temperature was set to 56°C. As a result, a composite containing plate-like gold nanoparticles and the unsaturated fatty acid was obtained. The gold nanoparticles of the obtained composite were evaluated in the same manner as in Example 1. As a result, 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 Ω·m. The Young's modulus was 0.4 MPa.

[Example 3]
Gold nanoparticles were produced in the same manner as in Example 1, except that dodecenyl succinic acid (an alkenyl carboxylic acid in formula (X2) where ^R2 is a nonyl group and ^R3 is a _CH2CH (-COOH) _CH2 group) was used as the unsaturated fatty acid, and the reaction temperature was set to 53°C. As a result, a composite of plate-like gold nanoparticles and the unsaturated fatty acid was obtained. The gold nanoparticles of the obtained composite were evaluated in the same manner as in Example 1. As a result, 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 Ω·m. The Young's modulus was 0.4 MPa.

[Example 4]
Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in formula (X2), where ^R2 is a pentyl group and ^R3 is a _CH2CH (-COOH) _CH2 group, was used as the unsaturated fatty acid, and the reaction temperature was set to 53°C. As a result, a composite of plate-like gold nanoparticles and the unsaturated fatty acid was obtained. The gold nanoparticles of the obtained composite were evaluated in the same manner as in Example 1. As a result, 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 Ω·m. The Young's modulus was 0.4 MPa.

[Example 5]
Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in formula (X2), where ^R2 is a propyl group and ^R3 is a _CH2CH (-COOH) _CH2 group, was used as the unsaturated fatty acid, and the reaction temperature was set to 53°C. As a result, branched gold nanoparticles were obtained. The obtained branched gold nanoparticles had a branched portion of plate-like fragments, and the branched portion was a single crystal. On the other hand, the overall crystal structure of the branched gold nanoparticles was polycrystalline. Furthermore, the average particle size of the branched gold nanoparticles was 350 nm. The crystal structure and average particle size of the branched gold nanoparticles were measured by the following method.
(Average particle size)
Equipment: Scanning electron microscope, Model: JSM-6340F (manufactured by JEOL Ltd.)
Observation conditions: 10 kV, 10 μA
Using the above apparatus, the diameter of the perfect circle containing the multi-branched gold nanoparticles was measured, and the particle size of the multi-branched gold nanoparticles was determined. In the same manner, the particle size of 15 multi-branched gold nanoparticles was measured, and the average value was calculated.
(Crystal structure)
Equipment: Transmission electron microscope Model: Tecnai Osiris (manufactured by FEI Company)
Using the above apparatus, the crystal structure was investigated by measuring electron diffraction patterns of the branched portion and the entire multi-branched gold nanoparticle.

[Example 6]
Gold nanoparticles were produced in the same manner as in Example 1, except that an alkenyl carboxylic acid in formula (X2), where ^R2 is a methyl group and ^R3 is a _CH2CH (-COOH) _CH2 group, was used as the unsaturated fatty acid, and the reaction temperature was set to 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. As a result, the average particle size of the granular gold nanoparticles was 120 nm, and the aspect ratio was 1.5.

[Example 7]
Gold nanoparticles were produced in the same manner as in Example 1, except that 5-hexenoic acid (an alkenyl carboxylic acid in formula (X2) where ^R2 is hydrogen and ^R3 is a propylene group) was used as the unsaturated fatty acid, and the reaction temperature was set to 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. As a result, the average particle size of the granular gold nanoparticles was 120 nm, and the aspect ratio was 1.5.

[Example 8]
Gold nanoparticles were produced in the same manner as in Example 1, except that trans-2-hexenoic acid (an alkenyl carboxylic acid in formula (X1) where ^R1 is a propyl group) was used as the unsaturated fatty acid and the reaction temperature was set to 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. As a result, the average particle size of the granular gold nanoparticles was 120 nm and the aspect ratio was 1.5.

[Example 9]
Gold nanoparticles were produced in the same manner as in Example 1, except that 4-pentenoic acid (an alkenyl carboxylic acid in formula (X2) where ^R2 is hydrogen and ^R3 is an ethylene group) was used as the unsaturated fatty acid, and the reaction temperature was set to 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. As a result, the average particle size of the granular gold nanoparticles was 120 nm, and the aspect ratio was 1.5.

[Example 10]
Gold nanoparticles were produced in the same manner as in Example 1, except that 3-butenoic acid (an alkenyl carboxylic acid in formula (X2) where ^R2 is hydrogen and ^R3 is a methylene group) was used as the unsaturated fatty acid, and the reaction temperature was set to 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. As a result, the average particle size of the granular gold nanoparticles was 120 nm, and the aspect ratio was 1.5.

[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 an unsaturated fatty acid, but no metal nanoparticles were produced.

From the above results, it was found that in the methods of Examples 1 to 10, metal ions were easily reduced and metal nanoparticles were generated. Furthermore, the composites obtained in Examples 1 to 4 had large (d/t) values, indicating significant particle growth in the diametrical direction. In Example 5, where hexenyl succinic acid was used as the unsaturated fatty acid, branched metal nanoparticles were generated. The obtained branched metal nanoparticles had a single-crystal structure in the branched portions, but the overall crystal structure of a single branched metal nanoparticle was polycrystalline. On the other hand, as shown in Comparative Example 1, when a fatty acid was used as a reducing agent, no metal nanoparticles were generated. From these results, it was found that the metal ion reduction method of the present invention provides a simple and energy-efficient method with minimal environmental impact for obtaining metal nanoparticles. It was also found that the shape of the metal nanoparticles can be controlled within a specific range. Furthermore, the obtained plate-like metal nanoparticles and their composites are expected to have applications in conductive inks, conductive coatings, conductive gap fillers, decorative agents, and plastic molding such as embossing.

Claims (13)

A reducing agent for metal ions, including precious metal ions,
A metal ion reducing agent comprising an unsaturated fatty acid having the structure of the following formula (X2).
R ² -CH=CH-R ³ -COOH...(X2)
(In formula (X2), ^R2 represents hydrogen or an alkyl group having 1 to 16 carbon atoms, and ^R3 represents _two CH2CH(-COOH) _CH groups or _two CH(-COOH)CH groups.) The metal ion reducing agent according to claim 1, wherein the unsaturated fatty acid having the structure of formula (X2) comprises 2-octenyl succinic acid, 2-dodecene-1-yl succinic acid, octenyl succinic acid, or dodecenyl succinic acid. A composite comprising a metal ion reducing agent according to claim 1 or 2 and plate-shaped metal nanoparticles of a noble metal . The composite according to claim 3, wherein the metal ion reducing agent is attached to at least one surface of the plate-shaped metal nanoparticles. 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% by mass or more. The composite according to any one of claims 3 to 5 , wherein the plate-shaped metal nanoparticles include gold plate-shaped metal nanoparticles. The composite according to claim 6 , wherein the electrical resistivity is 1 × ^10⁻⁴ Ω·m or less. The composite according to claim 6 or 7 , wherein the Young's modulus is 1 MPa or less. The composite comprises branched metal nanoparticles including plate-like pieces made of the plate-like metal nanoparticles,
The branched metal nanoparticle has a central part and branched portions consisting of a plurality of plate-like pieces extending outward from the central part.
The composite according to any one of claims 3 to 8 , wherein the crystal structure of the branched portion includes a single crystal, and the crystal structure of the entire multi-branched metal nanoparticle is polycrystalline. The composite according to claim 9 , wherein the branched portion is composed of the plurality of plate-like pieces that extend three-dimensionally and irregularly outward from the central part. The composite according to claim 9 or 10 , wherein the average length of the plate-like pieces is 20 to 100 nm. A dispersion comprising the complex according to any one of claims 3 to 11 . A plate-shaped metal nanoparticle of a noble metal contained in the composite according to any one of claims 3 to 11 ,
Plate-shaped metal nanoparticles having a diameter (d) to thickness (t) ratio (d/t) of 100 to 1200, and a thickness (t) of less than 30 nm.