JP7743822B2 - Method for producing silver nanoparticles - Google Patents

Description

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

In recent years, metal nanoparticles, which can have properties different from those of bulk materials, have been used and investigated for a variety of applications, including catalysts, ink materials, and electronic component parts.

In the electronics packaging field, metal nanoparticles are also being considered as a lead-free bonding material that can be used for bonding at low temperatures. Lead-free solder is difficult to bond at temperatures below 250°C, but lead-free solder containing metal nanoparticles can make bonding possible at temperatures below 250°C by taking advantage of the properties of metal nanoparticles, namely, that they have a lower melting point than bulk materials, but when used for bonding and sintered, they retain the melting point of bulk materials.

In order to use metal nanoparticles, especially silver nanoparticles, which have a variety of excellent physical and chemical properties in terms of functionality, as a highly heat-resistant bonding material, it is necessary to maintain a constant melting point for the silver nanoparticles. To maintain a constant melting point for silver nanoparticles, it is desirable to reduce the particle size of the silver nanoparticles and narrow the particle size distribution.

For example, Patent Document 1 discloses a method for producing fine silver particles by a wet reduction method, which involves mixing a silver salt-containing solution containing silver salt, a chelating agent, and gelatin with a reducing agent-containing solution containing a reducing agent to induce a reduction reaction and obtain fine silver particles.

Patent Document 2 discloses silver nanoparticles equipped with protective molecules consisting of a primary amine (A) having eight or more carbon atoms and a melting point of 20°C or less, a diamine (B) having a primary amino group and a tertiary amino group, four or more carbon atoms and a melting point of 20°C or less, and a cis-unsaturated primary amine (C) having twelve or more carbon atoms and a melting point of 30°C or less.

Patent Document 3 discloses a method for producing metal nanoparticles, which includes a step of irradiating a raw material solution with microwaves. The raw material solution contains a solvent, one or more chelating agents selected from ethylenediaminetetraacetic acid, ethylenediaminetriacetic acid, ethylenediaminediacetic acid, and salts thereof, and a metal compound, and the molar ratio of the chelating agent to the metal in the metal compound (amount of substance of chelating agent/amount of substance of metal) is 0.067 to 3.00. The raw material solution does not contain a reducing agent selected from formic acid, citric acid, and salts thereof. The method particularly discloses a method using silver as the metal.

Non-Patent Document 1 discloses the properties of reducing agents and protective agents in methods for synthesizing silver particles using sodium borohydride and methods for synthesizing silver particles using organic acids such as citric acid and amino acids.

Japanese Patent Application Laid-Open No. 2007-239077 Japanese Patent Application Laid-Open No. 2016-135916 Japanese Patent Application Laid-Open No. 2019-178355

Restrepo, Cindy Vanessa and Cristian C. Villa, “Synthesis of silver nanoparticles, influence of capping agents, and dependence on size and shape: A review.”, Environmental Nanotechnology, Monitoring & Management, 2021, 100428

While progress is being made in the development of silver nanoparticles that can be used in a variety of applications, the synthesis conditions used in conventional technologies are complex and strict, involving gas generation and fast reaction rates. Furthermore, in conventional methods using citric acid or amino acids, synthesis conditions are limited to those using low concentrations of silver ions due to the formation of salts (insolubilization) between silver ions and citric acid or amino acids. Furthermore, using high concentrations of silver ions raises concerns about accelerated particle growth, i.e., particle coarsening, due to a decrease in the frequency of contact between the silver nanoparticles and the protective agent in the system. Therefore, it is difficult to consistently produce silver nanoparticles with small and uniform particle sizes (here, uniform particle size means a narrow particle size distribution) at low cost.

The present invention aims to provide a method for producing silver nanoparticles that can easily produce small, uniformly sized silver nanoparticles, even when a high concentration of silver ions is present in the solution in order to reduce manufacturing costs through mass production.

After investigating various means for solving the above problems, the inventors discovered that by controlling the molar ratio of chelating agent to silver ions in the method for producing silver nanoparticles, the chelating agent can be used to synthesize silver nanoparticles with a high concentration, and that the chelating agent also quickly acts as a protective agent during the reduction of silver ions, resulting in the production of silver nanoparticles with small and uniform particle sizes, leading to the completion of the present invention.

That is, the gist of the present invention is as follows.
(1) A method for producing silver nanoparticles by reducing silver ions in a reaction solution, comprising:
The reaction solution is
(i) silver ions;
(ii) a reducing agent; and
(iii) a chelating agent; and
(iv) a solvent;
the molar ratio of the chelating agent to the silver ions (chelating agent/silver ions) is 0.12 to 5.0;
method.
(2) The method according to (1), wherein the reducing agent has one or more functional groups selected from an amino group, a hydroxy group, and a carboxy group in the molecule.
(3) The method according to (2), wherein the reducing agent is one or more selected from the group consisting of citric acid, ascorbic acid, L-lysine, and tyrosine.
(4) The method according to any one of (1) to (3), wherein the chelating agent has one or more functional groups selected from an amino group, a hydroxy group, and a carboxy group in the molecule.
(5) The method according to (4), wherein the chelating agent is at least one selected from the group consisting of 1-amino-2-propanol, 1,3-butanediol, 2-amino-1-butanol, ethylenediaminetetraacetic acid, and nitrilotriacetic acid.
(6) The method according to any one of (1) to (5), wherein the concentration of silver ions in the reaction solution is 50 mM or more.
(7) The method according to any one of (1) to (6), wherein the reduction reaction is carried out by irradiating the reaction solution with microwaves, and the output of the microwave irradiation source irradiating the reaction solution is 50 W/mL or more based on the total volume of the reaction solution.
(8) The method according to any one of (1) to (7), wherein the reaction solution does not contain a polymer protecting agent.

The present invention provides a method for producing silver nanoparticles that can easily produce small, uniformly sized silver nanoparticles, even when a high concentration of silver ions is present in the solution in order to reduce manufacturing costs through mass production.

FIG. 1 shows the particle size distribution results of the silver nanoparticles prepared in Example 2. 1 is a TEM photograph of silver nanoparticles prepared in Example 2. 1 is a graph showing the relationship between the molar ratio of EDTA to silver ions and the average particle size of silver nanoparticles based on Comparative Examples 2 to 4 and Examples 1 to 4. 1 is a graph showing the relationship between the molar ratio of EDTA to silver ions and the yield based on Comparative Examples 2 to 4 and Examples 1 to 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. Note that the method for producing silver nanoparticles of the present invention is not limited to the following embodiments, and can be embodied in various forms including modifications and improvements that can be made by those skilled in the art within the scope of the gist of the present invention.

The present invention relates to a method for producing silver nanoparticles by reducing silver ions in a reaction solution, wherein the reaction solution contains (i) silver ions, (ii) a reducing agent, (iii) a chelating agent, and (iv) a solvent, and the molar ratio of the chelating agent to the silver ions (chelating agent/silver ions) is 0.12 to 5.0.

The source of silver ions contained in the reaction solution is not limited as long as it can dissolve in the solvent described below and generate silver ions. Examples include inorganic salts of silver, such as hydrochloride, sulfate, nitrate, and phosphate, and organic salts, such as carboxylate and sulfonate. The source of silver nanoparticles may be prepared, for example, by dissolving a material containing metallic silver or a silver salt in an acid such as nitric acid or aqueous ammonia. It is preferable to use inexpensive silver nitrate as the source of silver ions.

The concentration of silver ions in the reaction solution is not limited and may be, for example, typically 0.1 mmol/L (mM) or higher, preferably 1 mM or higher. Considering the reduction in manufacturing costs through mass production, the concentration of silver ions in the reaction solution is typically 50 mM or higher, preferably 80 mM or higher, and more preferably 100 mM or higher. The upper limit of the silver ion concentration in the reaction solution is not limited as long as the silver ion raw material is present as silver ions in the reaction solution, but is typically 500 mM, preferably 400 mM.

By keeping the silver ion concentration in the reaction solution within this range, the variation in the resulting silver nanoparticles is reduced; in other words, the particle size distribution of the resulting silver nanoparticles is narrowed.

The reducing agent contained in the reaction solution is a compound that can reduce silver ions to silver, whose oxidation number is 0, through a redox reaction. Reducing agents are not limited to, but include, for example, compounds that have one or more functional groups in their molecules selected from amino groups, hydroxy groups, and carboxy groups. Preferred reducing agents are, for example, one or more selected from the group consisting of citric acid, ascorbic acid, L-lysine, and tyrosine.

The amount of reducing agent is not limited as long as it can reduce silver ions to a metal with an oxidation number of 0 through a redox reaction, but it is usually 1.0 to 20.0 equivalents, and preferably 2.0 to 10.0 equivalents, relative to the silver ions.

By using the compounds described above as reducing agents, the carbonyl groups in the amino, hydroxy, and/or carboxy groups contained in the compounds can efficiently reduce silver ions.

The chelating agent contained in the reaction solution is a compound that stabilizes silver ions and silver nanoparticles in the reaction solution. Chelating agents are not limited to, but include, for example, compounds that have one or more functional groups selected from amino groups, hydroxy groups, and carboxy groups within the molecule. Preferred chelating agents are, for example, one or more selected from the group consisting of 1-amino-2-propanol, 1,3-butanediol, 2-amino-1-butanol, ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid.

Chelating agents can control the particle size of the resulting silver nanoparticles by adjusting the number of functional groups contained in the compound; generally, the greater the number of functional groups, the smaller the particle size. Furthermore, amino groups, as a functional group, have a strong adsorption power to the resulting silver nanoparticles and a high level of protection, making them effective in reducing particle size. Carboxy groups, in addition to their protective effect, also have a reducing action, improving the reduction rate of silver ions and effectively reducing particle size. Therefore, by using the compounds described above as chelating agents, the multiple amino and/or carboxyl groups contained in the compound stabilize silver ions, and the particle size of the resulting silver nanoparticles can be reduced based on the number and type of functional groups.

The amount of chelating agent varies depending on the amount of silver ions, and is adjusted so that the molar ratio of chelating agent to silver ions (chelating agent/silver ions) is 0.12 to 5.0, preferably 0.50 to 5.0, and more preferably 0.80 to 3.0.

By determining the amount of chelating agent based on the amount of silver ions, small-sized silver nanoparticles can be produced in high yield, typically 85% or more, and preferably 90% or more.

The solvent contained in the reaction solution is not limited as long as it can dissolve the materials described above. Examples of solvents include low-boiling solvents with a boiling point of 300°C or less. Low-boiling solvents are not limited, but include low-boiling polar solvents such as water, alcohols such as methanol and ethanol, polyhydric alcohol solvents such as ethylene glycol, ketone solvents such as acetone, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), other organic solvents, or mixtures of two or more of these. Water is preferred as the solvent used in the reaction solution.

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

In addition to the materials described above, the reaction solution may also contain additives that are typically used in reaction solutions used in conventional methods for producing silver nanoparticles by reducing silver ions in a reaction solution. However, it is preferable that the reaction solution be composed of silver ions, a reducing agent, a chelating agent, and a solvent.

Therefore, it is preferable that the reaction solution does not contain, as an additive, a polymer protective agent, such as polyvinylpyrrolidone (PVP) or polyacrylamide (PAA), particularly PVP or PAA having a weight-average molecular weight of typically 10,000 g/mol or more, preferably 40,000 g/mol or more.

Because the reaction solution does not contain a polymer protecting agent as an additive, it is possible to prevent the protecting agent from interfering with the coordination of the chelating agent and the silver nanoparticles from becoming coarse in size, thereby enabling the stable production of silver nanoparticles with small and uniform particle sizes.

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 carried out by conventional heating methods, such as heating using a mantle heater, immersion heater, oil bath, or hot water bath, or by using a microwave synthesis apparatus.

In the present invention, in conventional heating methods, the reaction temperature is not limited, but is usually 300°C or less, preferably 50°C to 200°C.

In the present invention, in conventional heating methods, the reaction time is not limited as long as the reaction is complete, but is typically 1 minute to 8 hours, preferably 5 minutes to 1 hour. Completion of the reaction can be confirmed by ICP-AES analysis to confirm that the starting material, silver ions, are no longer present in the solution.

In the method of the present invention using a microwave synthesis apparatus, the reaction solution is irradiated with microwaves to promote the reaction. Therefore, the solvent contained in the reaction solution is a polar solvent that absorbs microwaves when irradiated with microwaves, converts them into thermal energy, and generates heat. Examples of polar solvents include, but are not limited to, low-boiling-point polar solvents such as water and ethanol. Water is preferred as the polar solvent.

In methods using a microwave synthesis apparatus, the material of the container containing the reaction solution is not limited as long as it can uniformly irradiate the raw material solution with microwaves. For example, when microwaves are irradiated onto the raw material solution from outside the reactor through the reactor, materials that transmit microwaves, such as ceramics and glass, can be used. When microwaves are irradiated directly onto the raw material solution from above the reactor, materials that reflect microwaves, such as metals such as aluminum and stainless steel, can be used.

In methods using a microwave synthesis apparatus, microwaves are generated from a microwave irradiation source (microwave oscillator (magnetron)), and the microwave irradiation source can be either a single-mode system or a multi-mode system.

In methods using a microwave synthesis apparatus, the output power of the microwave irradiation source can be varied as appropriate depending on the reaction conditions, for example, the type of reaction, and is not limited to, but is typically 5 W/mL or more, preferably 50 W/mL or more, more preferably 100 W/mL or more, even more preferably 150 W/mL or more, and particularly preferably 200 W/mL or more, based on the total volume of the reaction solution. The upper limit of the output power of the microwave irradiation source can also be varied as appropriate depending on the reaction conditions, for example, the type of reaction, and is not limited to, but is typically 10 kW/mL, preferably 1 kW/mL, based on the total volume of the reaction solution.

In methods using a microwave synthesis apparatus, the frequency of the microwaves generated from the microwave irradiation source can be changed as appropriate and is not limited, but is typically 1 GHz to 10 GHz, preferably 2 GHz to 6 GHz. In the present invention, it is more preferable to use a microwave frequency of 2.45 GHz, which is the frequency of an industrial microwave power source.

In methods using a microwave synthesis apparatus, the temperature of the reaction solution raised by microwave irradiation can be adjusted appropriately depending on the reaction conditions and is not limited. The temperature of the reaction solution should be below the boiling point of the solvent.

When using a microwave synthesis apparatus, the time for irradiating the reaction solution with microwaves can be varied as appropriate depending on the reaction conditions. As long as the reaction is complete, there are no limitations; however, it is typically 10 seconds to 200 minutes, preferably 10 seconds to 60 minutes. Alternatively, the reaction solution can be irradiated with microwaves so as to maintain the desired reaction temperature. As mentioned above, completion of the reaction can be confirmed by ICP-AES analysis, confirming that the starting material, silver ions, are no longer present in the solution.

When using a microwave synthesis apparatus, the total reaction time, including the microwave irradiation time, can be varied appropriately depending on the reaction conditions. It is not limited, but is usually between 10 seconds and 300 minutes, and preferably between 5 and 60 minutes.

The present invention is preferably carried out using a microwave synthesis device, as this increases the rate at which the chelating agent is adsorbed onto the silver nanoparticles. Microwaves activate the functional groups of the chelating agent, increasing the rate at which it is adsorbed onto the silver nanoparticles, making it possible to inhibit the growth of the silver nanoparticles. Furthermore, when microwave irradiation is stopped upon completion of the reaction, the adsorption force of the chelating agent onto the silver nanoparticles decreases, causing the chelating agent to be released from the silver nanoparticles, resulting in a paste of silver nanoparticles with low chelating agent content and low-temperature sinterability.

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 silver 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 either a batch or flow mode.

The solution containing silver 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 silver nanoparticles and/or a dispersion containing silver nanoparticles.

The silver nanoparticles produced by this invention have a small particle size; for example, the average particle size measured by TEM (here, "average particle size" means the average particle size of 300 or more particles, with the particle size being the average of the major and minor axes) is typically 5 nm to 40 nm. Furthermore, the particle size is uniform, i.e., the particle size distribution is narrow; for example, the standard deviation σ of the particle size distribution measured by TEM is 1 nm to 10 nm.

The silver nanoparticles produced by this invention can be used as a highly heat-resistant, lead-free bonding material in the electronics packaging field, in addition to conventional catalysts, electronic component materials, and ink materials.

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 Example 1
(1) Silver nitrate, citric acid, and ethylenediaminetetraacetic acid (EDTA) were dissolved in water so that the concentrations in the reaction solution were 100 mM silver nitrate, 550 mM citric acid, and 50 mM EDTA, respectively, to prepare an aqueous silver nitrate solution, an aqueous EDTA solution, and an aqueous citric acid solution.

(2) First, the silver nitrate aqueous solution prepared in (1) and the EDTA aqueous solution prepared in (1) were mixed at room temperature for 10 minutes to complex the silver ions with EDTA.

(3) Next, the citric acid aqueous solution prepared in (1) was added and mixed to prepare a reaction solution.

(4) The reaction solution was irradiated with microwaves at 200 W/mL based on the total volume of the reaction solution, heating the reaction solution to 100°C, and the temperature was maintained for 15 minutes by irradiating the microwaves in pulses to synthesize silver nanoparticles.

Example 2
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in step (1) of Example 1, the EDTA aqueous solution was prepared so that the concentration of EDTA in the reaction solution was 100 mM.

Example 3
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in step (1) of Example 1, the EDTA aqueous solution was prepared so that the concentration of EDTA in the reaction solution was 200 mM.

Example 4
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in step (1) of Example 1, the EDTA aqueous solution was prepared so that the concentration of EDTA in the reaction solution was 400 mM.

Example 5
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in steps (1) and (2) of Example 1, an aqueous nitrilotriacetic acid solution prepared so that the concentration of nitrilotriacetic acid in the reaction solution was 100 mM was used instead of an aqueous EDTA solution prepared so that the concentration of EDTA in the reaction solution was 50 mM.

Reference example 6
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in step (1), polyvinylpyrrolidone (PVP, weight-average molecular weight: 40,000 g/mol) was further dissolved in water to give a PVP aqueous solution with a concentration of 200 mM in the reaction solution, and in step (3), citric acid was added, and then the PVP aqueous solution prepared in step (1) was further added.

Example 7
In Example 1, in step (1), PVP (weight average molecular weight: 40,000 g/mol) was further dissolved in water to give a PVP aqueous solution with a concentration of 500 mM in the reaction solution, and in step (3), citric acid was added, and then the PVP aqueous solution prepared in step (1) was further added. Except for this, silver nanoparticles were synthesized in the same manner as in Example 1.

Reference example 7
In Example 1, in step (1), PVP (weight average molecular weight: 40,000 g/mol) was further dissolved in water to give a PVP aqueous solution with a concentration of 500 mM in the reaction solution, and in step (3), citric acid was added, and then the PVP aqueous solution prepared in step (1) was further added. Except for this, silver nanoparticles were synthesized in the same manner as in Example 1.

Comparative Example 1
The same procedure as in Example 1 was carried out except that in step (1) of Example 1, the EDTA aqueous solution was prepared so that the concentration of EDTA in the reaction solution was 10 mM. As a result, silver nanoparticles were not produced.

Comparative Example 2
The same procedures were carried out as in Comparative Example 1, except that in steps (1) and (3) of Comparative Example 1, an aqueous 2-propanol solution prepared so that the concentration of 2-propanol in the reaction solution was 550 mM was used instead of an aqueous citric acid solution prepared so that the concentration of citric acid in the reaction solution was 550 mM. As a result, silver nanoparticles were produced.

Comparative Example 3
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in step (1) of Example 1, the EDTA aqueous solution was prepared so that the concentration of EDTA in the reaction solution was 800 mM.

Comparative Example 4
Silver nanoparticles were synthesized in the same manner as in Example 1, except that in step (1) of Example 1, the EDTA aqueous solution was prepared so that the concentration of EDTA in the reaction solution was 1000 mM.

2. Evaluation of Silver Nanoparticles The yield was measured for the silver nanoparticles of Examples 1 to 5 and 8, Comparative Examples 1 to 4 , and Reference Examples 6 and 7 , and further UV-vis spectrum measurement, scanning electron microscope (SEM), and transmission electron microscope (TEM) measurements were performed. In the TEM, the particle size of the particles was defined as the average value of the long and short diameters of the particles, and the average particle size and particle size distribution were calculated from the results of measuring the particle sizes of 300 or more silver nanoparticles.

The results are shown in Table 1.

Furthermore, Figure 1 shows the particle size distribution based on TEM of the silver nanoparticles prepared in Example 2, Figure 2 shows a TEM photograph of the silver nanoparticles prepared in Example 2, Figure 3 shows the relationship between the molar ratio of EDTA to silver ions and the average particle size of the silver nanoparticles based on Comparative Examples 2 to 4 and Examples 1 to 4, and Figure 4 shows the relationship between the molar ratio of EDTA to silver ions and the yield based on Comparative Examples 2 to 4 and Examples 1 to 4.

From Table 1 and Figures 3 and 4, it was found that silver nanoparticles are not produced from reaction solutions in which the molar ratio of chelating agent to silver ions (chelating agent (EDTA)/silver ions) is 0.10 or less, or if produced, the average particle size is large. Therefore, in order to produce silver nanoparticles with a small average particle size, it is preferable that the molar ratio of chelating agent to silver ions (chelating agent (EDTA)/silver ions) is 0.12 or more, preferably 0.50 or more, and particularly 0.80 or more. Furthermore, since the yield decreases as the molar ratio of chelating agent to silver ions (chelating agent (EDTA)/silver ions) increases, it is preferable that the molar ratio of chelating agent to silver ions (chelating agent (EDTA)/silver ions) is 5.0 or less, and more preferably 3.0 or less. Furthermore, it was found that the average particle size of the resulting silver nanoparticles tends to increase when PVP is further added to the reaction solution as a polymer protecting agent. Furthermore, from Example 8, it was found that the reaction can be carried out not only in a microwave synthesis apparatus but also in an oil bath.

Claims (4)

A method for producing silver nanoparticles by reducing silver ions in a reaction solution, comprising:
The reaction solution is
(i) silver ions;
(ii) a reducing agent; and
(iii) a chelating agent; and
(iv) a solvent;
The concentration of silver ions in the reaction solution is 50 mM or more;
the molar ratio of the chelating agent to the silver ions (chelating agent/silver ions) is 0.12 to 5.0;
the reducing agent is one or more selected from the group consisting of citric acid, ascorbic acid, L-lysine, and tyrosine;
The reaction solution does not contain a polymer protecting agent.
method. The method of claim 1, wherein the chelating agent has one or more functional groups selected from an amino group, a hydroxy group, and a carboxy group in the molecule. The method of claim 2, wherein the chelating agent is one or more selected from the group consisting of 1-amino-2-propanol, 1,3-butanediol, 2-amino-1-butanol, ethylenediaminetetraacetic acid, and nitrilotriacetic acid. The method according to any one of claims 1 to 3, wherein the reduction reaction is carried out by irradiating the reaction solution with microwaves, and the output of the microwave irradiation source irradiating the reaction solution is 50 W/mL or more based on the total volume of the reaction solution.