US20200353539A1

US20200353539A1 - Method of producing silver nanoparticles

Info

Publication number: US20200353539A1
Application number: US16/860,496
Authority: US
Inventors: Kazuaki Okamoto; Jyunya Murai; Kensuke Akamatsu
Original assignee: Toyota Motor Corp; Konan University
Current assignee: Toyota Motor Corp; Konan University
Priority date: 2019-05-08
Filing date: 2020-04-28
Publication date: 2020-11-12
Also published as: DE102020110755A1; JP2020183567A; CN111906324A

Abstract

A method of producing silver nanoparticles includes reducing, with a silver ion reducing agent, silver ions of 40 mM or more in a reaction solution in the presence of a particle protective agent and an element more noble than silver.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2019-088310 filed on May 8, 2019, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to a method of producing silver nanoparticles.

2. Description of Related Art

Metal nanoparticles, which may have properties different from those of bulk materials, are used and studied to be used in various applications such as, for example, catalysts, ink materials, and electronic components.
Among the metal nanoparticles, silver nanoparticles have various excellent physical and chemical properties in terms of function, and various research and development have been made on uses and producing methods thereof.
For example, Japanese Unexamined Patent Application Publication No. 2010-285695 (JP 2010-285695 A) discloses silver fine particles obtained by adding, to a polyol solvent, a silver compound having an average particle diameter of 10 μm or less in an amount of 1% by weight to 15% by weight, and a water-soluble polymer, having excellent adsorptivity to silver and serving as a dispersant, in an amount of 5% by weight to 80% by weight with respect to a silver content of the silver compound, and then reducing the solution by heating at 100° C. or lower. The silver fine particles have, each on its surface, a coating layer of the water-soluble polymer, have an average particle diameter of 30 nm or less and a standard deviation a/average particle diameter d of 30% or less, and have monodispersity.
Japanese Unexamined Patent Application Publication No. 2011-225974 (JP 2011-225974 A) discloses a method of producing silver nanoparticles, in which silver nitrate is reduced with citrate in water in the presence of a hydrophilic polymer such as polyvinylpyrrolidone and/or polyvinyl alcohol and an amine compound.

SUMMARY

In the field of electronic packaging, the metal nanoparticles have recently been studied as a lead-free bonding material that can be bonded at low temperatures. It is difficult to bond a lead-free solder at 250° C. or lower. However, it is possible to bond a lead-free solder containing the metal nanoparticles at 250° C. or lower. This is achieved by utilizing the property of the metal nanoparticles having a lower melting point compared to bulk materials, whereas having a melting point equivalent to that of bulk materials after the metal nanoparticles are used for bonding and sintered.
To use the metal nanoparticles, particularly the silver nanoparticles, as a high heat-resistant bonding material, it is necessary to keep the melting point of the silver nanoparticles constant. In order to keep the melting point of the silver nanoparticles constant, it is desirable to narrow a particle size distribution of the silver nanoparticles to improve dispersibility.
However, the method described in JP 2010-285695 A requires, for example, a step of pulverizing a silver compound to prepare a silver compound having a small particle diameter, and the method is complicated.
Further, it is presumed that the dispersibility of the silver nanoparticles obtained by the method described in JP 2011-225974 A is not uniform. The method described in JP 2011-225974 A includes a reaction system of silver nitrate and sodium citrate using water as a solvent. In the reaction system, a nucleation rate of silver varies particularly at low temperatures. This widens the particle size distribution of the silver nanoparticles, and as a result, the dispersibility of the obtained silver nanoparticles is not uniform.
Specifically, in a reaction system in which only the same element as the material of the nanoparticles, namely, silver, is present, after silver nuclei are formed, silver preferentially precipitates and grows on the silver nuclei. Meanwhile, silver nuclei are formed on a free surface locally having high energy. Since particle growth and nucleation occur simultaneously in different reaction fields, the particle diameter of the silver nanoparticles is not uniform.
When a tracing experiment according to JP 2011-225974 A was actually performed, the silver nanoparticles obtained by the method described in JP 2011-225974 A had a standard deviation a/average particle diameter d, which is an index of the dispersibility described in JP 2010-285695 A, of 43%, and did not fall below 30%.
The disclosure provides a method of producing silver nanoparticles having a narrow particle size distribution.
The inventors of the disclosure have studied various means for solving the above-described problems, and have found that, in a method of producing silver nanoparticles, in which silver ions in a reaction solution are reduced with a silver ion reducing agent in the presence of a particle protective agent, it possible to produce silver nanoparticles having a narrow particle size distribution by adjusting the concentration of silver ions to be constant and adding an element more noble than silver, and have thus completed the disclosure.
An aspect of the disclosure relates to a method of producing silver nanoparticles. The method includes reducing, with a silver ion reducing agent, silver ions of 40 mM or more in a reaction solution in the presence of a particle protective agent and an element more noble than silver. In the above aspect, the element more noble than silver may be palladium. In the above aspect, an amount of palladium may be 0.05% by weight to 20% by weight as a palladium metal, based on a weight of silver as a metal. In the above aspect, a solvent used for the reaction solution may be water. In the above aspect, the silver ion reducing agent may be citrate. In the above aspect, the particle protective agent may be polyvinylpyrrolidone. In the above aspect, the method may be carried out using a microwave synthesizer.
According to the disclosure, silver nanoparticles having a narrow particle size distribution can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein:

FIG. 1 is a graph showing a result of a particle size distribution of silver nanoparticles prepared in a first embodiment;

FIG. 2 is a graph showing a result of a particle size distribution of silver nanoparticles prepared in Comparative Example 1; and

FIG. 3 is a graph showing a relationship between silver ion concentration and a particle size variation index in the first embodiment, a second embodiment, and Comparative Examples 1 to 4.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, preferable embodiments of the disclosure will be described in detail. In this specification, features of the disclosure will be described with reference to FIGS. 1 to 3 as appropriate. A method of producing silver nanoparticles according to the disclosure is not limited to the embodiments described below, but may be carried out in various forms in which modifications and improvements may be made by those skilled in the art without departing from the scope of the disclosure.
The disclosure relates to the method of producing the silver nanoparticles, which includes reducing, with a silver ion reducing agent, silver ions having constant concentration in a reaction solution in the presence of a particle protective agent and an element more noble than silver.
Here, a raw material of the silver ions is not limited, but may include, for example, an inorganic salt such as hydrochloride, sulfate, nitrate and phosphate of silver, and an organic salt such as carboxylate and sulfonate of silver. In the disclosure, silver nitrate, which is inexpensive, is preferably used as the raw material for the silver ions.
The concentration of the silver ions in the reaction solution is 40 mmol/L (mM) or more, preferably 50 mM or more. An upper limit of the concentration of the silver ions in the reaction solution is not limited as long as the raw material of the silver ions is present as silver ions in the reaction solution, but is normally 500 mM, preferably 400 mM.
By setting the concentration of the silver ions in the reaction solution in the above range, variations in the obtained silver nanoparticles are reduced, in other words, a particle size distribution of the obtained silver nanoparticles is narrowed.
A solvent used for the reaction solution in the method according to the disclosure is not limited. Examples of the solvent used in the reaction solution in the method according to the disclosure include a low-boiling solvent having a boiling point of 300° C. or lower. Examples of the low-boiling solvent include, but are not limited to, low-boiling polar solvents such as water, alcohol such as ethanol, other organic solvents, and a mixture of two or more thereof. Water is preferably used as the solvent used for the reaction solution.
In the disclosure, by using a low-boiling solvent as the solvent used for the reaction solution, the handleability of the solvent can be improved and a burden on the environment can be reduced.
The particle protective agent is a compound that binds to a part or the entire surface of the silver nanoparticles suspended in the solvent, and is a compound that suppresses aggregation of the silver nanoparticles. Examples of the particle protective agent include, but are not limited to, polyvinylpyrrolidone (PVP), thiol-based agents, and polyvinyl alcohol (PVA). PVP is preferably used as the particle protective agent.
An amount of the particle protective agent is not limited and can be changed depending on a desired particle diameter of the silver nanoparticles.
By using the particle protective agent in the disclosure, the aggregation of the generated silver nanoparticles can be suppressed.
An element more noble than silver is an element that is at a higher potential than silver in standard potential series, in other words, an element that has a lower ionization tendency than silver. Examples of the element more noble than silver include, but are not limited to, palladium (Pd), iridium (Ir), platinum (Pt), and gold (Au). Palladium is preferably used as the element more noble than silver.
The element more noble than silver may be in the form of cations or in the form of nanoparticles. When the element more noble than silver is in the form of cations, the element more noble than silver may be present as an inorganic salt such as hydrochloride, sulfate, nitrate and phosphate, or an organic salt such as carboxylate and sulfonate.
An amount of the element more noble than silver is, as an element, normally 0.01% by weight to 20% by weight, preferably 0.05% by weight to 20% by weight, based on the weight of silver as a metal.
In particular, when the element more noble than silver is palladium, the amount of palladium is, as a palladium metal, normally 0.05% by weight to 20% by weight based on the weight of silver as a metal.
Alternatively, a weight ratio of palladium to silver as a metal is in the range where silver is normally 5 to 20 when palladium is regarded as 1 (1:5 to 1:20).
By adding the element more noble than silver to the reaction system, the element more noble than silver forms nuclei of the nanoparticles before the silver ions do, so that the silver ions can precipitate and grow as silver on the nuclei formed by the element more noble than silver. As a result, the silver nanoparticles having a narrow particle size distribution can be produced even when the silver nanoparticles are produced at low temperatures.
A silver ion reducing agent is a material that can reduce the silver ions to silver having an oxidation number of 0 through an oxidation-reduction reaction.
Examples of the silver ion reducing agent include, but are not limited to, citric acid or a salt thereof, such as trisodium citrate, disodium citrate, monosodium citrate, oxalic acid or a salt thereof, such as sodium oxalate, and ascorbic acid or a salt thereof, such as sodium ascorbate. Citrate is preferably used as the silver ion reducing agent.
The silver ion reducing agent can also act as a reducing agent for the element more noble than silver when the element more noble than silver is present in the form of cations.
The amount of the silver ion reducing agent is not limited as long as the silver ions and, in some cases, the element more noble than silver can be reduced to a metal having the oxidation number of 0 through the oxidation-reduction reaction.
In the disclosure, besides the above materials, ethylenediaminetetraacetic acid (EDTA) and/or a salt thereof can be added.
In the disclosure, the order of adding each material, a temperature during the addition, a mixing method, a mixing time, and the like are not limited, and the materials are mixed to prepare a uniform reaction solution. In the disclosure, the reaction is started after the uniform reaction solution is prepared.
The disclosure can be carried out by a conventional heating method or a method using a microwave synthesizer.
In the disclosure, in the conventional heating method, a reaction temperature is not limited, but is normally 300° C. or lower.
In the disclosure, in the conventional heating method, the reaction time is not limited, but is normally 1 hour to 100 hours, for example, 1 hour to 3 hours, and preferably 24 hours to 100 hours.
In the disclosure, in a method using the microwave synthesizer, the reaction solution is irradiated with microwaves to promote the reaction. Thus, a polar solvent is used as a solvent contained in the reaction solution. The polar solvent absorbs the microwaves when irradiated with the microwaves and converts the microwaves into heat energy, thereby generating heat. Examples of the polar solvent include, but are not limited to, a low-boiling polar solvent such as water and ethanol. Water is preferably used as the polar solvent.
In the method using the microwave synthesizer, a material of a container for accommodating the reaction solution is not limited as long as a raw material solution can be uniformly irradiated with the microwaves. For example, when irradiating the raw material solution with the microwaves from outside a reactor through the reactor, a material that transmits microwaves, for example, ceramics and glass, can be used. When directly irradiating the raw material solution with the microwaves from a position above the raw material solution, a material that reflects the microwaves, for example, a metal such as aluminum and stainless steel can be used.
In the method using the microwave synthesizer, the microwaves are generated by a microwave irradiation source (microwave oscillator (magnetron)), and the microwave irradiation source can use either a single mode system or a multi-mode system.
In the method using the microwave synthesizer, an output of the microwave irradiation source is not limited and can be appropriately changed depending on reaction conditions, for example, a type of the reaction, but based on a total volume of the reaction solution, is normally 100 W/L to 10 kW/L, preferably 100 W/L to 5 kW/L.
In the method using the microwave synthesizer, a frequency of the microwaves generated by the microwave irradiation source is not limited and can be appropriately changed, but is normally 1 GHz to 10 GHz, preferably 2 GHz to 6 GHz. In the disclosure, the frequency of an industrial microwave power supply of 2.45 GHz is preferably used as the microwave frequency.
In the method using the microwave synthesizer, a temperature of the reaction solution heated by the irradiation of the microwaves is not limited and can be appropriately changed depending on the reaction conditions. The temperature of the reaction solution heated by the irradiation of the microwaves only needs to be equal to or lower than the boiling point of the solvent.
In the method using the microwave synthesizer, an irradiation time of the microwaves to the reaction solution is not limited and can be appropriately changed depending on the reaction conditions, but is normally 1 minute to 200 minutes, preferably 1 minute to 80 minutes. Alternatively, the reaction solution can be irradiated with the microwaves so as to maintain a target temperature of the reaction solution.
In the method using the microwave synthesizer, a total reaction time including the irradiation time of the microwaves is not limited and can be appropriately changed depending on the reaction conditions, but is, for example, 1 minute to 300 minutes, preferably 1 minute to 80 minutes.
The disclosure is preferably carried out using the microwave synthesizer from the viewpoint that the entire reaction field can be uniformly heated.
In the disclosure, the reaction solution is preferably stirred by a stirring mechanism such as a propeller stirrer and a vibration stirrer. By stirring the reaction solution, the silver nanoparticles generated in the reaction solution can be uniformly dispersed, and the reaction solution can be kept uniform.
The disclosure may be carried out through a batch system or a flow system. The disclosure is preferably carried out through the batch system. By carrying out the disclosure through the batch system, a synthesis reaction itself can be completed, and a yield of the obtained silver nanoparticles can be improved. Further, the concentration of the reaction solution can be made high, and clogging of a piping of the silver nanoparticles which may occur in the flow system is restrained.
The solution containing the silver nanoparticles obtained according to the disclosure may be subjected to processes such as separation and purification (for example, salting out or centrifugation) by a method known in the technical art to obtain target silver nanoparticles and/or a dispersion containing the target silver nanoparticles.
The silver nanoparticles produced by the method according to the disclosure have uniform particle diameters, that is, a narrow particle size distribution.
The silver nanoparticles produced by the method according to the disclosure can be used as a high heat-resistant lead-free bonding material in the field of electronic packaging, in addition to conventional catalysts, electronic components, ink materials, and the like.
Hereinafter, several embodiments related to the disclosure will be described. The disclosure is not intended to be limited to the embodiments.

1. Preparation of Silver Nanoparticles

First Embodiment

Each of silver nitrate, EDTA, trisodium citrate, and palladium nitrate were dissolved in water. First, an aqueous solution of EDTA was added to an aqueous solution of silver nitrate and the mixture was stirred. Second, an aqueous solution of sodium citrate was added and the mixture was stirred until a silver-EDTA precipitate dissolved. Third, an aqueous solution of palladium nitrate was added and the mixture was stirred. Fourth, an aqueous solution of PVP-polyacrylic acid (PAA) whose pH had been neutralized with sodium hydroxide added thereto was added and the mixture was stirred. Finally, purified water was added and the mixture was stirred to obtain the reaction solution of the concentration shown in Table 1. The concentration of palladium in the reaction solution was 1.3% by weight as a palladium metal based on the weight of silver as a metal.

TABLE 1

Silver nitrate	300	mM
EDTA	300	mM
Sodium citrate	900	mM
PVP
50	mM
Palladium nitrate
4	mM

The reaction solution was heated at 90° C. for 80 minutes in the microwave synthesizer to obtain the silver nanoparticles.

Second Embodiment

The reaction was carried out in the same manner as in the first embodiment except that the concentration of each material was adjusted to the concentration shown in Table 2 to obtain a reaction solution, thereby obtaining the silver nanoparticles.

TABLE 2

Silver nitrate	50	mM
EDTA
50	mM
Sodium citrate	150	mM
PVP	8.3	mM
Palladium nitrate	0.67	mM

Comparative Example 1

The reaction was carried out in the same manner as in the first embodiment except that palladium nitrate was not added, to obtain the silver nanoparticles. Table 3 shows the concentration of each material in the reaction solution of Comparative Example 1.

TABLE 3

Silver nitrate	300	mM
EDTA	300	mM
Sodium citrate	900	mM
PVP
50	mM

Comparative Example 2

The reaction was carried out in the same manner as in Comparative Example 1 except that the concentration of each material was adjusted to the concentration shown in Table 4 to obtain the reaction solution, thereby obtaining the silver nanoparticles.

TABLE 4

Silver nitrate	50	mM
EDTA
50	mM
Sodium citrate	150	mM
PVP	8.3	mM

Comparative Example 3

The reaction was carried out in the same manner as in Comparative Example 1 except that the concentration of each material was adjusted to the concentration shown in Table 5 to obtain the reaction solution, thereby obtaining the silver nanoparticles.

TABLE 5

Silver nitrate	3.3	mM
EDTA	3.3	mM
Sodium citrate	9.9	mM
PVP	0.55	mM

Comparative Example 4

The reaction was carried out in the same manner as in Comparative Example 1 except that the concentration of each material was adjusted to the concentration shown in Table 6 to obtain the reaction solution, thereby obtaining the silver nanoparticles.

TABLE 6

Silver nitrate	5	mM
EDTA
5	mM
Sodium citrate
15	mM
PVP	0.83	mM

2. Evaluation of Silver Nanoparticles

The silver nanoparticles obtained in the first embodiment and Comparative Example 1 were each measured by a particle size distribution analyzer (dynamic light scattering (DLS) method).
FIG. 1 shows the results of the particle size distribution of the silver nanoparticles prepared in the first embodiment, and FIG. 2 shows the results of the particle size distribution of the silver nanoparticles prepared in Comparative Example 1.
As shown in FIGS. 1 and 2, the silver nanoparticles of the first embodiment in which palladium, which is an element more noble than silver, was added to the reaction solution, had a standard deviation a/average particle diameter d, which is an index of dispersibility, of 24%, that is, 30% or less. On the other hand, the silver nanoparticles of Comparative Example 1 in which palladium, which is an element more noble than silver, was not present in the reaction solution, had a standard deviation a/average particle diameter d, which is an index of dispersibility, of 43%.
The reasons of the above results can be considered as follows. By adding palladium, which is more noble than silver, to the reaction system, palladium, which is more noble than silver, formed nuclei of the nanoparticles before the silver ions did. The silver ions precipitated and grew as silver on the nuclei formed by palladium, which is more noble than silver. As a result, silver nanoparticles having a narrower particle size distribution could be produced.

3. Examination Experiment of Silver Ion Concentration

A dispersion liquid in which the silver nanoparticles are dispersed (silver nanoparticle dispersion liquid) shows an absorption spectrum that is dependent on an average particle diameter. For example, when two silver nanoparticle dispersion liquids include different average particle diameters, the absorption spectra of the silver nanoparticle dispersion liquids may also be different.
Considering such properties of the silver nanoparticle dispersion liquid, when silver nanoparticles having different particle diameters are present in a silver nanoparticle dispersion liquid, the absorption spectrum of the silver nanoparticle dispersion liquid correspond to the sum of the absorption spectra of the silver nanoparticles having different particle diameters, and a peak width of an absorption peak in the absorption spectrum of the silver nanoparticle dispersion liquid may become large. In other words, as the variations in the particle size of the silver nanoparticles in the silver nanoparticle dispersion liquid increase, the peak width of the absorption peak in the absorption spectrum of the silver nanoparticle dispersion liquid may also increase.
Utilizing such properties of the silver nanoparticle dispersion liquid, the absorption spectra of the second embodiment and Comparative Examples 1 to 4 were measured, a difference between two absorption wavelengths at half the intensity of the maximum absorption peak wavelength and the maximum absorption peak wavelength were calculated from each of the absorption spectra, and a quotient of the two values was obtained as a “particle size variation index”.
Particle size variation index=Difference between two absorption wavelengths at half the intensity of the maximum absorption peak wavelength/Maximum absorption peak wavelength
FIG. 3 shows the relationship between the silver ion concentration and the particle size variation index. As the particle size variation index of the first embodiment, 0.24 was used as the standard deviation σ/average particle diameter d, which is an index of dispersibility. Considering the fact that the smaller the particle size variation index is, the smaller the variations in silver nanoparticle size is, FIG. 3 shows that when the silver ion concentration is around 40 mM, for example, 50 mM, the obtained silver nanoparticles showed a small variation and the variations in the silver nanoparticles was kept small even when the silver ion concentration was set higher.

Claims

What is claimed is:

1. A method of producing silver nanoparticles, the method comprising reducing, with a silver ion reducing agent, silver ions of 40 mM or more in a reaction solution in a presence of a particle protective agent and an element more noble than silver.

2. The method according to claim 1, wherein the element more noble than silver is palladium.

3. The method according to claim 2, wherein an amount of palladium is 0.05% by weight to 20% by weight as a palladium metal, based on a weight of silver as a metal.

4. The method according to claim 1, wherein a solvent used for the reaction solution is water.

5. The method according to claim 1, wherein the silver ion reducing agent is citrate.

6. The method according to claim 1, wherein the particle protective agent is polyvinylpyrrolidone.

7. The method according to claim 1, wherein the method is carried out using a microwave synthesizer.