JP7779771B2 - Method for producing metal nanoparticles - Google Patents

Description

The present invention relates to a method for producing metal nanoparticles.

Metal nanoparticles, which can have properties different from those of bulk materials, are being used and investigated for a variety of applications, including catalysts, ink materials, and electronic component parts.

For example, Patent Document 1 discloses a conductive wiring material that includes a plurality of first metal nanoparticles and a plurality of second metal nanoparticles having a particle size smaller than that of the first metal nanoparticles, and that can be sintered at a low temperature to melt the second metal nanoparticles and fill the spaces between the first metal nanoparticles.

Patent Document 2 discloses a method for producing silver nanoparticles, characterized by reducing 40 mM or more silver ions in a reaction solution with a silver ion reducing agent in the presence of a particle protecting agent and an element more noble than silver.

Japanese Patent Application Laid-Open No. 2006-279038 Japanese Patent Application Laid-Open No. 2020-183567

In the field of electronics packaging, when a mixture of two types of metal nanoparticles with different particle sizes is used as the metal nanoparticles in the conductive wiring material described in Patent Document 1 or the lead-free bonding material described in Patent Document 2, the sintered body formed by sintering the mixture has a low porosity and, as a result, can have a low volume resistivity.

A mixture of two types of metal nanoparticles with different particle sizes that may be useful in the field of electronics packaging is typically prepared by separately synthesizing two types of metal nanoparticles with different particle sizes, optionally classifying them, and then uniformly mixing them, as described in Patent Document 1. Therefore, the method for producing this mixture has problems such as a large number of steps, low productivity, and high cost. Furthermore, the low-temperature baking temperature described in Patent Document 1 is 180°C, making it difficult to print and bake the mixture on PET, PC, etc.

The present invention aims to provide a method for easily producing a mixture of two types of metal nanoparticles with different particle sizes.

As a result of investigating various means for solving the above-mentioned problems, the inventors discovered that in a method for producing metal nanoparticles by reducing metal ions in a reaction solution, the reaction solution contains a solvent, metal ions, and Compound 1, which acts as a reducing agent by having a standard electrode potential of 0.49 V to 0.80 V, has a molecular weight of 90 g/mol or less, and acts as a first protective agent by being adsorbable to reduced metal nanoparticles, and Compound 2, which has a weight-average molecular weight (Mw) of 10,000 g/mol to 40,000 g/mol, and acts as a second protective agent by being adsorbable to reduced metal nanoparticles. By adjusting the molar ratio of Compound 1 to metal ions (Compound 1/metal ions) to 20 or more and adjusting the metal ion concentration to 50 mmol/L or more, the inventors discovered that two types of metal nanoparticles with different particle sizes can be simultaneously synthesized, thereby completing the present invention.

That is, the gist of the present invention is as follows.
(1) A method for producing metal nanoparticles by reducing metal ions in a reaction solution, comprising:
The reaction solution is
a solvent;
Metal ions,
Compound 1 has a standard electrode potential of 0.49 V to 0.80 V, thereby acting as a reducing agent, and has a molecular weight of 90 g/mol or less and has the ability to adsorb to reduced metal nanoparticles, thereby acting as a first protecting agent;
Compound 2 has a weight average molecular weight (Mw) of 10,000 g/mol to 40,000 g/mol and has the ability to adsorb to reduced metal nanoparticles, thereby acting as a second protective agent;
the molar ratio of compound 1 to metal ion (compound 1/metal ion) is 20 or more;
The method according to any one of claims 1 to 5, wherein the concentration of the metal ions is 50 mmol/L or more.
(2) The method according to (1), wherein the molar ratio of compound 2 to metal ion (compound 2/metal ion) is 4 or more.
(3) The method according to (1) or (2), wherein the metal ion is a silver ion.
(4) The method according to any one of (1) to (3), wherein compound 1 is at least one compound selected from the group consisting of oxalic acid and its salts, and N,N-dimethylformamide.
(5) The method according to any one of (1) to (4), wherein compound 2 is polyvinylpyrrolidone.
(6) The method according to any one of (1) to (5), wherein the reaction is carried out at a temperature of 20°C to 100°C for 5 minutes to 60 minutes.

This invention makes it easy to produce a mixture of two types of metal nanoparticles with different particle sizes.

1 is a diagram illustrating an embodiment of the present invention; FIG. 1 shows TEM images and particle size distributions of Reference Examples 1 and 2. FIG. 1 shows TEM images and particle size distributions of Examples 3 and 4. FIG. 10 is a cross-sectional SEM image of the sintered body of Example 4.

Preferred embodiments of the present invention will now be described in detail.
In this specification, the features of the present invention will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The method for producing metal nanoparticles of the present invention is not limited to the following embodiments, and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art, without departing from the spirit of the present invention.

The present invention relates to a method for producing metal nanoparticles by reducing metal ions in a reaction solution, wherein the reaction solution contains a solvent, metal ions, compound 1, which acts as a reducing agent by having a standard electrode potential of 0.49 V to 0.80 V, has a molecular weight of 90 g/mol or less, and acts as a first protective agent by being adsorbed onto reduced metal nanoparticles, and compound 2, which has a weight-average molecular weight (Mw) of 10,000 g/mol to 40,000 g/mol, and acts as a second protective agent by being adsorbed onto reduced metal nanoparticles; the molar ratio of compound 1 to metal ions (compound 1/metal ions) is 20 or more, and the concentration of the metal ions is 50 mmol/L or more.

The solvent used in the reaction solution of the method of the present invention is not limited. Examples of solvents include low-boiling solvents with a boiling point of 120°C or lower. Low-boiling solvents are not limited, but include low-boiling polar solvents such as water, alcohols such as ethanol, other organic solvents, and mixtures of two or more of these. In the present invention, it is preferable to use water as the solvent.

Using a low-boiling-point solvent as the solvent improves the ease of handling of the solvent and reduces the burden on the environment.

The metal ions in the reaction solution used in the method of the present invention are not limited. Examples of metal ions include ions of metals that make up metal nanoparticles, such as gold, silver, platinum, copper, nickel, iron, and cobalt. In the present invention, it is preferable to use silver ions as the metal ion. Sources of metal ions include, but are not limited to, inorganic salts such as metal hydrochlorides, sulfates, nitrates, and phosphates, and organic salts such as carboxylates, acetates, and sulfonates. In the present invention, it is preferable to use inexpensive nitrates as the source of metal ions.

In the reaction solution used in the method of the present invention, the concentration of metal ions in the reaction solution is 50 mmol/L or more, preferably 100 mmol/L or more. The upper limit of the metal ion concentration in the reaction solution is not limited as long as the metal ion source is present as metal ions in the reaction solution, but is typically 500 mmol/L, preferably 400 mmol/L.

By keeping the concentration of metal ions in the reaction solution within this range, metal nanoparticles can be produced efficiently at high concentrations, significantly increasing the amount of metal nanoparticles that can be produced and recovered at one time, and reducing the time, effort, and cost required to produce metal nanoparticles.

In the reaction solution used in the method of the present invention, Compound 1 has a standard electrode potential of 0.49 V to 0.80 V, thereby acting as a reducing agent for metal ions, and has a molecular weight of 90 g/mol or less. It also has the ability to adsorb to reduced metal nanoparticles, thereby acting as a first protective agent. Here, the "ability to adsorb to metal nanoparticles" of Compound 1 as a protective agent for metal nanoparticles means that Compound 1 binds to part or the entire surface of metal nanoparticles suspended in a solvent, and this characteristic can suppress aggregation of the metal nanoparticles.

Examples of organic functional groups that Compound 1 may have to adsorb to reduced metal nanoparticles include thiol groups, amine groups, carboxy groups, hydroxy groups, and ether groups. Compound 1 may contain two or more of these groups.

Examples of compound 1 include at least one compound selected from the group consisting of oxalic acid and its salts, such as sodium oxalate, N,N-dimethylformamide (DMF), and dimethyl sulfoxide. In the present invention, it is preferable to use at least one compound selected from the group consisting of oxalic acid and its salts and N,N-dimethylformamide as compound 1.

By including Compound 1 in the reaction solution in the method of the present invention, the relatively strong reducing power of Compound 1 allows for simultaneous nucleation of metal nanoparticles, thereby narrowing the particle size distribution of the two types of metal nanoparticles obtained, each with different particle sizes.

In the reaction solution used in the method of the present invention, the molar ratio of compound 1 to metal ions (compound 1/metal ions) is 20 or more, preferably 20 to 80.

By containing compound 1 in the reaction solution in the method of the present invention at a molar ratio to metal ions within the above range, two types of metal nanoparticles with different particle sizes can be easily synthesized.

In the reaction solution used in the method of the present invention, the concentration of compound 1 in the reaction solution is not limited as long as the molar ratio of compound 1 to metal ions is within the above-mentioned range, but is typically 1 mol/L or more, preferably 1 mol/L to 10 mol/L, and more preferably 2 mol/L to 6 mol/L.

In the reaction solution used in the method of the present invention, compound 2 has a weight-average molecular weight (Mw) of 10,000 g/mol to 40,000 g/mol and acts as a second protective agent by being capable of adsorbing to metal nanoparticles produced by reduction. Here, the "adsorbability to metal nanoparticles" of compound 2 as a protective agent for metal nanoparticles means that compound 2 binds to part or the entire surface of metal nanoparticles suspended in a solvent, and this characteristic can suppress aggregation of the metal nanoparticles.

As with compound 1, organic functional groups that compound 2 may have to adsorb to reduced metal nanoparticles include thiol groups, amine groups, carboxy groups, hydroxy groups, and ether groups. Compound 2 may contain two or more of these groups.

Examples of compound 2 include at least one compound selected from the group consisting of polyvinylpyrrolidone (PVP), thiol-based polymers, and polyvinyl alcohol (PVA). In the present invention, it is preferable to use PVP as compound 2.

In the reaction solution of the method of the present invention, the molar ratio of compound 2 to metal ions (compound 2/metal ions) (where the molecular weight of compound 2 is based on the weight average molecular weight) is not limited, but is typically 1 or more, preferably greater than 2, more preferably 4 or more, and even more preferably 4 to 8.

In the reaction solution of the method of the present invention, the molar ratio of compound 1 to compound 2 (compound 1/compound 2) (where the molecular weight of compound 2 is based on the weight average molecular weight) is not limited, but is typically 5 or more, preferably 10 or more, and more preferably 10 to 60.

In the reaction solution used in the method of the present invention, the concentration of compound 2 in the reaction solution (where the molecular weight of compound 2 is based on the weight-average molecular weight) is not limited, but is typically 50 mmol/L or more, preferably 100 mmol/L or more, and more preferably 200 mmol/L to 800 mmol/L.

When the reaction solution in the method of the present invention contains compound 2 in the above-mentioned amount, compound 2 has the effect of reducing the frequency of contact between the nuclei of metal nanoparticles produced in the reaction solution, thereby suppressing excessive coalescence of the nuclei. Compound 2 also exerts a protective effect on the nuclei of coalesced metal nanoparticles and plays a role in aligning the particle size distribution, making it possible to easily synthesize two types of metal nanoparticles with different particle sizes.

Therefore, by including Compound 1 and Compound 2 in the reaction solution in the method of the present invention, the reaction solution contains two types of protective agents that have different adsorption amounts to metal nanoparticles, and each protective agent produces metal nanoparticles with a particle size corresponding to its adsorption amount. Therefore, two types of metal nanoparticles with different particle sizes can be simultaneously synthesized.

In addition to the above-mentioned materials, the reaction solution used in the method of the present invention may also contain sodium hydroxide (NaOH) as a pH adjuster.

In the present invention, the order of addition of each material, addition temperature, mixing method, mixing time, etc. are not limited, and the materials are mixed so as to prepare a homogeneous reaction liquid. In the present invention, the reaction is initiated after a homogeneous reaction liquid has been prepared.

The present invention can be implemented using a heating method.

In the present invention, when the reaction is carried out by heating, the reaction temperature is not limited, but is usually between 20°C and 100°C, preferably between 20°C and 90°C.

In the present invention, the reaction time for the heating method is not limited, but is typically 5 to 60 minutes, preferably 5 to 40 minutes, and more preferably 5 to 30 minutes.

In the present invention, the reaction can be easily carried out by carrying out the reaction using a heating method.

In the present invention, it is preferable to stir the reaction solution using a stirring mechanism, such as a propeller stirrer or a vibration stirrer. By stirring the reaction solution, the two types of metal nanoparticles produced in the reaction solution can be uniformly dispersed, and the reaction solution can be maintained in a homogeneous state.

The present invention may be carried out in a batch system or a flow system. It is preferable to carry out the present invention in a batch system. By carrying out the present invention in a batch system, the synthesis reaction itself can be completed, and the yield of the resulting metal nanoparticles can be improved. Furthermore, the concentration of the reaction solution can be increased, eliminating the problem of metal nanoparticle piping clogging, which can occur in a flow system.

One embodiment of the present invention is schematically shown in Figure 1. Figure 1 shows a scheme for producing a dispersion of silver nanoparticles by reacting a reaction solution containing water (H ₂ O) as a solvent, silver nitrate (AgNO ₃ : 100 mmol/L or more) as a metal ion, DMF (2000 mmol/L or more) as compound 1, and PVP (100 mmol/L or more) as compound 2 at a reaction temperature of 20°C to 100°C for a reaction time of 5 to 60 minutes.

This invention makes it possible to simultaneously prepare two types of metal nanoparticles with different particle sizes, resulting in fewer steps, improved productivity, and lower costs.

The dispersion containing metal nanoparticles obtained by the present invention can be separated and purified (e.g., salting out or centrifugation) using methods known in the art to obtain the desired metal nanoparticles and/or a dispersion containing metal nanoparticles.

The metal nanoparticles produced according to the present invention have two particle sizes, i.e., a bimodal particle size distribution. In this context, a bimodal particle size distribution means that when a histogram of the particle size distribution of the metal nanoparticles is prepared in 2.5 nm increments, the difference between the most frequent value between 0 nm and 10 nm and the most frequent value above 10 nm is 10 nm or more.

The metal nanoparticles obtained by the present invention are a mixture of two types of metal nanoparticles with different particle sizes, and have excellent low-temperature sinterability, for example, at 120°C. Furthermore, sintered bodies formed from these metal nanoparticles have low volume resistivity.

The metal nanoparticles produced by this invention can be used in fields such as catalysts, electronic component materials, and ink materials, as well as conductive wiring materials in the electronics packaging field, and can reduce the number of steps required to produce inks used in wiring boards, for example.

The following describes several examples of the present invention, but it is not intended that the present invention be limited to those examples.

1. Preparation of Silver Nanoparticles A reaction solution was prepared by adding a silver nitrate aqueous solution (concentration: 400 mM) as a metal ion to a PVP aqueous solution (weight average molecular weight: 10,000 g/mol or 40,000 g/mol, concentration: 0.4 mol/L, 0.8 mol/L, 1.6 mol/L, or 2.4 mol/L) as compound 2, followed by N,N-dimethylformamide (DMF) or oxalic acid or ascorbic acid as compound 1, and then adding purified water as a solvent so that the concentrations of each ingredient were as shown in Tables 1 and 2 below. The resulting reaction solution was placed in a 500 mL separable flask and reacted at 90°C (reaction temperature) for 40 minutes (reaction time) while stirring with a magnetic stirrer, thereby preparing a dispersion of silver nanoparticles.

2. TEM observation of silver nanoparticles and particle size distribution measurement
Each of the silver nanoparticles in Reference Examples 1, 2, and 6, Examples 3 to 5, and 7 , and Comparative Examples 1 to 3 was dropped onto a TEM grid and dried to prepare a sample. TEM observation was then performed under the measurement conditions shown in Table 3 below. The projected area circle equivalent diameter of 100 or more silver nanoparticles randomly selected from the TEM image was determined, and the particle size and particle size distribution were measured. The particle size distribution was created using a histogram in 2.5 nm intervals, such as 0 nm to 2.5 nm, 2.5 nm to 5.0 nm, 5.0 nm to 7.5 nm, etc.

The results are shown in Tables 4 and 5.

Comparing the Examples and Comparative Examples from Tables 4 and 5, it was found that a bimodal particle size distribution can be obtained by adding appropriate amounts of Compound 1 and Compound 2 to the reaction solution. Furthermore, a comparison of Examples 4 and 5 confirmed that a bimodal particle size distribution can be obtained when the weight-average molecular weight of Compound 2 is 10,000 g/mol or 40,000 g/mol. Furthermore, a comparison of Examples 4 and 7 and Comparative Example 3 revealed that a bimodal particle size distribution can be obtained by using as Compound 1 a compound capable of adsorbing to reduced metal nanoparticles, i.e., DMF (liquid) or oxalic acid (aqueous solution), which has a standard electrode potential of 0.49 V to 0.80 V and a molecular weight of 90 g/mol or less.

2 and 3 show TEM images and particle size distributions of Reference Examples 1 and 2 and Examples 3 and 4. In Figures 2 and 3, Reference Example 1 showed a bimodal particle size distribution of 6.59±1.60 nm and 19.15±2.09 nm (average particle size: 14.8±6.21 nm), Reference Example 2 showed a bimodal particle size distribution of 6.24±1.96 nm and 21.06±6.00 nm (average particle size: 11.2±8.52 nm), Example 3 showed a bimodal particle size distribution of 4.16±0.92 nm and 18.9±1.59 nm (average particle size: 6.29±5.29 nm), and Example 4 showed a bimodal particle size distribution of 4.02±1.23 nm and 17.2±1.92 nm (average particle size: 6.74±5.53 nm). 2 and 3 show that the addition of compound 2 results in a bimodal particle size distribution, but the bimodality of the particle size distribution depends on the concentration of compound 2; that is, the higher the concentration of compound 2 in the reaction solution, the more pronounced the bimodality. Specifically, it was found that the bimodality can be clearly observed in the particle size distribution of silver nanoparticles, for example, when the molar concentration of silver ions is more than twice, particularly more than four times. This can be inferred from the fact that as the concentration of compound 2 (PVP) increases, PVP becomes more likely to protect the silver nanoparticles, increasing the difference between the mode at 0 nm to 10 nm and the mode at 10 nm or greater, and also tending to result in a smaller average particle size. This result indicates that the number of silver nanoparticles with particle sizes less than 10 nm increases, improving the sinterability of silver nanoparticles.

3. Sinterability Test of Silver Nanoparticles The dispersions of silver nanoparticles from Example 4 and Comparative Example 1 were subjected to ultrafiltration (UF filtration) at room temperature (25°C to 30°C) for 3 hours. Pure water was used as the washing liquid during filtration. The dispersions were then concentrated using an evaporator to obtain dispersions with a solid content of 15% by weight.

The solid content of the dispersion was measured as follows.
First, a small amount of the dispersion was sampled and heated at 150°C for 2 hours to volatilize the solvent, and the residual content of the dispersion was measured. As a result, the residual content of the dispersion was 15.8% by weight of the dispersion. Next, a portion of the residual content was heated to 500°C in a thermogravimetric (TG) measurement device to burn off the organic components, and the solid content of the residual content was measured. As a result, the solid content of the residual content was 95% of the residual content. From the above, the solid content of the dispersion was
0.158 x 0.95 x 100 = 15 (weight%)
It was calculated that:

The resulting dispersion was then dropped into a slide chamber (8-well slide and chamber, manufactured by Watson) and sintered at 120°C for 60 minutes in a Yamato DN63 Constant Temperature Oven (air-blowing low-temperature incubator) to form a film.

The volume resistivity of the resulting film was measured using a resistivity meter (Loresta GX MCP-T700, manufactured by Nitto Seiko Analytech Co., Ltd.). The results are shown in Table 6, and Figure 4 shows a cross-sectional SEM image of the sintered body of Example 4.

Table 6 shows that when fired at the same temperature, the volume resistivity of the sintered body obtained using the silver nanoparticles of Example 4 was lower than that of the sintered body obtained using the silver nanoparticles of Comparative Example 1. This is thought to be because, in the sintered body obtained using the silver nanoparticles of Example 4, the first group of particles, which have a smaller particle size, are melted, filling the spaces between the second group of particles, and also because Compound 2, which exhibits thermal fluidity, is adsorbed as a protective agent.

Furthermore, from Table 6 and Figure 4, it can be seen that the sintered body obtained from the silver nanoparticles of Example 4 had a porosity of 1%, which was smaller than the porosity of Comparative Example 1, confirming that the voids (spaces) between the second group particles in the sintered body obtained from the silver nanoparticles of Example 4 were filled with the first group particles.

Claims (1)

A method for producing silver nanoparticles by reducing silver ions in a reaction solution, comprising:
The reaction solution is
A solvent;
Silver ions and
Compound 1 is at least one compound selected from the group consisting of oxalic acid and its salts, and N,N-dimethylformamide, and acts as a reducing agent and has the ability to adsorb to reduced silver nanoparticles, thereby acting as a first protective agent;
polyvinylpyrrolidone having a weight average molecular weight (Mw) of 10,000 g/mol to 40,000 g/mol and having the ability to adsorb to reduced silver nanoparticles, thereby acting as a second protective agent;
the molar ratio of compound 1 to silver ions (compound 1/ silver ions) is 20 to 80 ;
the molar ratio of polyvinylpyrrolidone to silver ions (polyvinylpyrrolidone/silver ions) is 4 to 8;
The concentration of silver ions is 50 mmol/L to 500 mmol/L ,
The above method , wherein the reaction is carried out at a temperature of 20°C to 100°C for 5 minutes to 60 minutes .