Method for producing silver nanoparticles

Abstract

To provide means for producing silver nanoparticles having a fine particle diameter with high production efficiency.SOLUTION: An embodiment of the present invention relates to a method for producing silver nanoparticles, the method including: a raw material preparation step of preparing a silver oxide suspension containing a silver oxide and a solvent; and a silver nanoparticle formation step of forming silver nanoparticles by irradiating the silver oxide suspension with a microwave at an absorption rate in a range of 41 to 81 W / mL while allowing the suspension to flow.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing silver nanoparticles. [Background technology]

[0002] Metal nanoparticles are metal particles such as silver, gold, or platinum with a particle size of several nanometers to several tens of nanometers. Metal nanoparticles are used as ink materials for patterning in printed electronics.

[0003] Ink materials for patterning in printed electronics are preferred because they can produce dense sintered bodies with low resistance even when fired at low temperatures. A known method for producing such metal nanoparticles is the liquid-phase synthesis method, in which a solution of metal ions is placed in a reaction vessel and heated and reduced by microwave irradiation.

[0004] For example, Patent Document 1 describes a method for producing silver nanoparticles by irradiating a reaction solution with microwaves. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2024-33912 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, a method for producing silver nanoparticles by a liquid phase synthesis method using microwave irradiation is known. However, in the conventional liquid phase synthesis method, microwave irradiation is performed in a batch process, which has the drawback of long processing time and making it difficult to increase production volume.

[0007] Therefore, an object of the present invention is to provide a means for producing silver nanoparticles having a fine particle size with high production efficiency. [Means for solving the problem]

[0008] The present inventors have investigated various means for solving the above problems. They have found that by irradiating a raw material suspension with microwaves at a predetermined absorption rate while flowing the suspension, silver nanoparticles having a fine particle size can be obtained in high yield within a reaction time of several seconds. Based on this finding, the present inventors have completed the present invention.

[0009] That is, the present invention includes the following aspects and embodiments. (Embodiment 1) A raw material preparation step of preparing a silver oxide suspension containing silver oxide and a solvent; a silver nanoparticle formation step in which the suspension of silver oxide is irradiated with microwaves at an absorption level in the range of 41 to 81 W / mL while flowing the suspension to form silver nanoparticles; A method for producing silver nanoparticles, comprising: [Effects of the Invention]

[0010] The present invention makes it possible to provide a means for producing silver nanoparticles having a fine particle size with high production efficiency. [Brief explanation of the drawings]

[0011] [Figure 1] 1 shows X-ray diffraction (XRD) spectra of silver particles obtained in Examples and Comparative Examples. In the figure, A is the XRD spectrum of the silver particles of Comparative Example 1, and B is the XRD spectrum of the silver particles of Example 3. In the figure, the horizontal axis represents 2θ (°), and the vertical axis represents the spectral intensity (CPS). [Figure 2] Electron microscope images of silver particles obtained in Examples and Comparative Examples. In the figure, A is a scanning electron microscope (SEM) image of silver particles obtained in Comparative Example 2, and B is a transmission electron microscope (TEM) image of silver particles obtained in Example 3. In the figure, the scale bar indicates 50 nm. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the present invention will now be described in detail.

[0013] One aspect of the present invention relates to a method for producing silver nanoparticles.

[0014] The particle size of the silver nanoparticles obtained by the method of this embodiment is usually 100 nm or less, preferably 80 nm or less. For example, in particle size distribution measurement of the silver nanoparticles obtained by the method of this embodiment, the D50 value of the silver nanoparticles is usually in the range of 1 to 100 nm, preferably in the range of 10 to 100 nm, and more preferably in the range of 10 to 80 nm. By the method of this embodiment, silver nanoparticles having a finer particle size can be obtained compared to silver nanoparticles obtained by conventional methods.

[0015] In each aspect of the present invention, the D50 value of the silver nanoparticles can be determined based on the particle size distribution of the silver nanoparticles measured using a particle size distribution analyzer. The particle size of the silver nanoparticles can be expressed as the D50 value of the silver nanoparticles, or can be determined by measuring the particle sizes of multiple silver nanoparticles using an SEM or TEM and calculating the average value of the measured values.

[0016] The method of this embodiment includes a raw material preparation step and a silver nanoparticle formation step. Each step will be described in detail below.

[0017] [1: Raw material preparation process] The process involves providing a silver oxide suspension comprising silver oxide and a solvent.

[0018] The silver oxide prepared in this step is used as a silver source, and the silver oxide is preferably silver oxide (AgO) powder.

[0019] The solvent is not particularly limited, and any solvent may be used as long as it can produce silver nanoparticles by heating with microwave irradiation in the silver nanoparticle formation step described below. The solvent is preferably a polar solvent or ionic liquid that can absorb microwaves. Examples of solvents include low-boiling-point solvents with a boiling point of 300°C or less at one atmosphere. Examples of such low-boiling-point solvents include water; monohydric alcohol solvents such as methanol or ethanol; polyhydric alcohol solvents such as ethylene glycol; ketone solvents such as acetone; and other organic solvents such as dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF), as well as mixtures of two or more of these solvents. By carrying out the method of this embodiment using a silver oxide suspension containing the above-mentioned solvents, silver nanoparticles with a fine particle size can be obtained in high yield.

[0020] The silver oxide concentration in the silver oxide suspension is typically 1 mmol / L or higher, for example, 10 mmol / L or higher. The silver oxide concentration in the silver oxide suspension is typically 600 mmol / L or lower, for example, 300 mmol / L or lower, and particularly 150 mmol / L or lower. The silver oxide concentration in the silver oxide suspension is preferably in the range of 1 to 600 mmol / L, more preferably 10 to 300 mmol / L, and even more preferably 10 to 150 mmol / L. By carrying out the method of this embodiment using a silver oxide suspension having the above-mentioned concentrations, silver nanoparticles with a fine particle size can be obtained in high yield.

[0021] The silver oxide suspension prepared in this step may optionally contain a protective agent. The protective agent serves as a raw material for the organic compound that forms a coating on the surface of the silver nanoparticles. Examples of protective agents include polyvinylpyrrolidone (PVP), dodecylamine (DDA), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylates, cyclodextrin, aminopectin, methylcellulose, polyethyleneimine cellulose, aliphatic amines, aliphatic carboxylic acids, and tannic acid. The organic compounds exemplified above may be used alone or as a mixture of two or more organic compounds. By using the protective agents exemplified above, silver nanoparticles having a fine particle size can be obtained in a higher yield.

[0022] [2: Silver nanoparticle formation process] This process involves irradiating a suspension of silver oxide with microwaves while the suspension is being flowed to form silver nanoparticles.

[0023] This step can be carried out, for example, using a reaction tube for fluidizing the silver oxide suspension and a microwave irradiation device for irradiating microwaves from the outside of the reaction tube to the silver oxide suspension flowing inside the reaction tube. In this embodiment, the inner diameter of the reaction tube is typically in the range of 2 to 10 mm, and the tube length is typically in the range of 10 to 200 mm. Examples of such a reaction tube and microwave irradiation device include a continuous synthesis apparatus having a microwave irradiation device and a reaction tube, as described in PCT / JP2023 / 028440.

[0024] In this step, the flow rate of the silver oxide suspension is preferably in the range of 70 to 300 mL / min, more preferably in the range of 70 to 140 mL / min. The pressure of the silver oxide suspension is preferably in the range of 0 to 1.0 MPa, more preferably in the range of 0 to 0.1 MPa. By flowing the silver oxide suspension under the conditions exemplified above, silver nanoparticles with a fine particle size can be efficiently formed.

[0025] In this step, the frequency of the microwaves irradiated onto the silver oxide suspension is preferably in the range of 0.915 to 2.45 GHz, and the microwave power is preferably in the range of 100 to 2,000 W.

[0026] In this process, a reduction reaction of silver oxide proceeds in the microwave-irradiated silver oxide suspension (e.g., a silver oxide suspension flowing inside a microwave-irradiated reaction tube) to form silver nanoparticles. The microwave absorption level of the silver oxide suspension is typically in the range of 41 to 81 W / mL, particularly in the range of 41 to 60 W / mL. If the microwave absorption level is below the lower limit, the yield of silver nanoparticles may decrease. Furthermore, if the microwave absorption level exceeds the upper limit, the temperature of the reaction system may rise, causing the generated silver particles to sinter and become coarse. Therefore, by performing this process with an absorption level within the above range, silver nanoparticles with a fine particle size can be obtained in high yield.

[0027] In this process, the temperature of the silver oxide suspension (reaction system) in which the reduction reaction of silver oxide proceeds can vary depending on the amount of microwave radiation absorbed, but is typically in the range of 80 to 120°C, particularly 80 to 100°C. If the temperature of the reaction system is below the lower limit, the yield of silver nanoparticles may decrease. Furthermore, if the temperature of the reaction system exceeds the upper limit, the silver particles produced may sinter together and become coarse. Therefore, by performing this process at a temperature within the above range, silver nanoparticles with a fine particle size can be obtained in high yield.

[0028] In this step, the reaction time for the reduction reaction of silver oxide to proceed varies depending on the amount of microwave radiation absorbed, but is usually less than 10 seconds, and particularly less than 1 second. Therefore, by carrying out this step under the conditions described above, silver nanoparticles with a fine particle size can be obtained in a short time.

[0029] As described in detail above, by the method of this aspect, compared with the method of the prior art, silver nanoparticles having a fine particle size can be obtained in a short time and / or with a high yield, so that silver nanoparticles can be produced with high production efficiency. The silver nanoparticles obtained by the method of this aspect can be a dense sintered body that can have a low resistance value even with low-temperature firing. Therefore, by obtaining silver nanoparticles having the above characteristics by the method of this aspect, it is possible to provide an ink material applicable to wiring formation in resin-based printed electronics materials and bonding materials having low heat resistance.

Example

[0030] Hereinafter, the present invention will be described more specifically using examples. However, the technical scope of the present invention is not limited to these examples.

[0031] <I: Production of silver nanoparticles> [I-1: Raw material preparation step] As the silver source, fine powder of silver oxide (Fukuda Metal Foil Powder Industry Co., Ltd.) was used, and as the solvent, a 1:1 mixture of water and N,N-dimethylformamide (DMF) was used. The silver source and the solvent were mixed to prepare a suspension containing 0.15 mol / L of silver oxide and the solvent.

[0032] [I-2: Silver nanoparticle formation step] A continuous synthesis apparatus having a microwave irradiation apparatus (microwave wavelength: 2.45 GHz) and a reaction tube (inner diameter: 4 mm, tube length: 100 mm) described in PCT / JP2023 / 028440 was prepared. The raw material suspensions of the comparative examples and the examples were passed through the reaction tube at a predetermined flow rate. While flowing the raw material suspension inside the reaction tube, the raw material suspension was irradiated with microwaves from the outside of the reaction tube at a predetermined absorption amount. Thereby, silver particles were formed.

[0033] As Comparative Examples 1 and 2, the raw material suspension was put into a beaker, the raw material suspension in the beaker was not irradiated with microwaves, and the raw material suspension was heated to a predetermined temperature (batch process).

[0034] <II: Analysis of silver particles> The production conditions for the examples and comparative examples, as well as the particle sizes of the resulting silver particles, are shown in Table 1. In the table, the silver production rate (%) is the percentage of silver in the product relative to the silver oxide in the raw material input, calculated based on the peak heights of the raw silver oxide and the silver in the product in the X-ray diffraction (XRD) spectrum.

[0035] [Table 1]

[0036] The crystalline structure of the obtained silver particles was analyzed by XRD measurement. The XRD spectrum of the obtained silver particles is shown in Figure 1. In the figure, A is the XRD spectrum of the silver particles of Comparative Example 1, and B is the XRD spectrum of the silver particles of Example 3. In the figure, the horizontal axis is 2θ (°), and the vertical axis is the spectral intensity (CPS). The obtained silver particles were also observed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Figure 2 shows electron microscope images of the obtained silver particles. In the figure, A is an SEM image of the silver particles of Comparative Example 2, and B is a TEM image of the silver particles of Example 3. In the figure, the scale bar indicates 50 nm. Furthermore, the particle size distribution and particle size (D50) of the obtained silver particles were measured using a particle size distribution measuring device.

[0037] As shown in Figure 1A, in Comparative Example 1, in a batch process in which the material was heated to 95°C without microwave irradiation, the raw material silver oxide remained, and the formation of silver particles did not proceed. In contrast, in Comparative Example 2, when the heating temperature was increased to 130°C, the generation of silver was confirmed. However, as shown in Figure 2A, the silver particles formed in Comparative Example 2 sintered together and became coarse.

[0038] When microwaves were irradiated using a continuous synthesis apparatus, the silver particle production rate was low at an absorption dose of 30 W / mL in Comparative Example 3, whereas small silver particles were produced at absorption doses ranging from 41 to 81 W / mL in Examples 1 to 4. The silver particles in Examples 1 to 4 were confirmed to have D50 values ​​of 10 to 80 nm. In contrast, at an absorption dose of 90 W / mL in Comparative Example 4, the temperature rose to 130°C. As a result, the silver particles formed in Comparative Example 4 sintered together and became coarse, as in Comparative Example 2.

[0039] These results demonstrate that the silver particles formed by the method of the present invention are nanoparticles with a fine particle size.

[0040] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to add, delete, and / or replace part of the configuration of each embodiment with other configurations.

Claims

[Claim 1] a raw material preparation step of preparing a silver oxide suspension containing silver oxide and a solvent; a silver nanoparticle formation step in which the suspension of silver oxide is irradiated with microwaves at an absorption level in the range of 41 to 81 W / mL while flowing the suspension to form silver nanoparticles; A method for producing silver nanoparticles, comprising:

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