Manufacturing method of metal nanoparticle

Abstract

To provide means for manufacturing, at a high yield, a metal nanoparticle having a flat shape capable of being a sintered body having high conductivity.SOLUTION: A manufacturing method of a metal nanoparticle having a flat shape includes: a material preparation step of preparing a metallic material aqueous solution containing a metallic material whose concentration is within a range of 5 to 200 mM; a reaction tube preparation step of preparing a reaction tube having a microwave heating part having an inner wall formed with a material having negative electrification characteristics; and a metal nanoparticle formation step of forming a metal nanoparticle through circulating the metallic material aqueous solution at a flow rate within a range of 5 to 300 mL / min to the microwave heating part while irradiating, with microwave, the microwave heating part from an outside of the reaction tube.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing metal 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] As an ink material for patterning in printed electronics, a material that can obtain a dense sintered body that can have a low resistance value even when sintered at a low temperature is preferable. As a material for obtaining a dense sintered body, metal nanoparticles having a flat shape such as a plate shape are preferable. As a method for producing such metal nanoparticles, a liquid phase synthesis method is known in which a solution of metal ions is put into a reaction vessel and heated and reduced by irradiating microwaves.

[0004] For example, Patent Document 1 describes a non-aqueous dispersion of metal nanoplates containing a dispersant and an organic solvent, characterized in that the acid value of the dispersant is 90 or less and the base value of the dispersant is 5 to 100.

[0005] Patent Document 2 describes a method for producing metal microparticles, in which a reaction liquid containing a metal precursor is circulated through a flow tube placed in a microwave irradiation space, and microwaves are irradiated toward the flow tube in the microwave irradiation space from outside the flow tube, heating the reaction liquid in the flow tube by this microwave irradiation and synthesizing metal microparticles, characterized in that a device to which a frequency band of 2.4 to 2.5 GHz is assigned or which is the standard frequency band for the product is used as the microwave generating device, and the average inner diameter of the portion of the flow tube placed in the microwave irradiation field is narrower than 2.84 mm. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2017-119827 A [Patent Document 2] JP 2020-111831 A Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, a method for producing flat metal nanoparticles by liquid phase synthesis is known. Flat metal nanoparticles are expected to produce dense sintered bodies that can have low resistance even when fired at low temperatures. However, in the case of the conventional method, the shape of the metal nanoparticles is controlled using a dispersant, which is an organic compound. Therefore, when the metal nanoparticles are fired to obtain a sintered body of the metal nanoparticles, there is a possibility that the dispersant residue is present in the sintered body. The sintered body containing the dispersant residue may have a decreased electrical conductivity. Thus, in the case of the conventional method, there was a problem that the electrical conductivity of the resulting metal nanoparticles may be decreased.

[0008] Therefore, an object of the present invention is to provide a means for producing metal nanoparticles having a flat shape with a high yield, which can be made into a sintered body having high electrical conductivity. [Means for solving the problem]

[0009] The present inventors have investigated various means for solving the above problems. The present inventors have found that, in a liquid phase synthesis method, by using a reaction tube having an inner wall formed of a material having negative charging properties and adjusting the concentration of the metallic material contained in the aqueous solution of the metallic material and the flow rate of the aqueous solution of the metallic material within a predetermined range, the aggregation of the ions of the metallic material can be optimized, and flat metallic nanoparticles can be obtained in high yield. Based on the above findings, the present inventors have completed the present invention.

[0010] That is, the present invention includes the following aspects and embodiments. (Embodiment 1) A raw material preparation step of preparing an aqueous solution of a metallic material having a concentration in the range of 5 to 200 mM; A reaction tube preparation step of preparing a reaction tube having a microwave heating part with an inner wall formed of a material having a negative charging characteristic; a metal nanoparticle forming step of irradiating microwaves from the outside of the reaction tube toward the microwave heating section while flowing an aqueous solution of a metal material through the microwave heating section at a flow rate in the range of 5 to 300 mL / min to form metal nanoparticles; The present invention relates to a method for producing metal nanoparticles having a flat shape, the method comprising the steps of: (Embodiment 2) The method according to embodiment 1, wherein the material having negative charging properties forming the reaction tube is polytetrafluoroethylene (PTFE). (Embodiment 3) The method of embodiment 1 or 2, wherein the metallic material is a salt of silver, gold, platinum, copper, nickel, iron, cobalt, palladium or zinc, or a mixture thereof. (Embodiment 4) The method of embodiment 3, wherein the metallic material is silver nitrate and the metallic nanoparticles are silver nanoparticles. (Embodiment 5) The concentration c mM of the metallic material contained in the aqueous solution of the metallic material and the flow rate v mL / min of the aqueous solution of the metallic material are as follows: For 0≦c<100, 5≦v<((30 / 19)c+(2700 / 19)); When 100≦c≦200, ((29 / 20)c-140)≦v≦300 5. The method according to any one of claims 1 to 4, wherein the relationship is satisfied: Effect of the Invention

[0011] The present invention makes it possible to provide a means for producing, with a high yield, metal nanoparticles having a flat shape that can be made into a sintered body having high electrical conductivity. [Brief description of the drawings]

[0012] [Figure 1]1 shows a transmission electron microscope (TEM) image of the metal nanoparticles of Example 1. In the figure, the scale bar indicates 50 nm. [Diagram 2] This shows the relationship between the silver salt concentration in the aqueous silver salt solution used in the production of silver particles in Examples 1 to 7 and the flow rate of the aqueous silver salt solution. In the figure, the horizontal axis is the silver salt concentration c (mM), and the vertical axis is the flow rate v (mL / min) of the aqueous silver salt solution. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Preferred embodiments of the present invention will now be described in detail.

[0014] One aspect of the present invention relates to a method for producing metal nanoparticles.

[0015] In each aspect of the present invention, the metal nanoparticles generally refer to fine particles of metals including noble metals or base metals, or alloys thereof. The metal constituting the metal nanoparticles is preferably silver, gold, platinum, copper, nickel, iron, cobalt, palladium, zinc, etc., or alloys thereof, more preferably silver or an alloy containing silver, and even more preferably silver.

[0016] In each embodiment of the present invention, the metal nanoparticles do not usually have a coating of an organic compound on the surface of the metal particles. The amount of carbon contained in the metal nanoparticles obtained by the method of this embodiment is usually 5% by mass or less, and preferably 2% by mass or less. When the metal nanoparticles obtained by the method of this embodiment are fired to obtain a sintered body of the metal nanoparticles, it is possible to substantially prevent organic compounds from remaining in the sintered body. Therefore, a sintered body having high electrical conductivity can be obtained by firing the metal nanoparticles obtained by the method of this embodiment.

[0017] The metal nanoparticles obtained by the method of this embodiment have a flat shape. The probability of flat particles in the metal nanoparticles obtained by the method of this embodiment is usually 20% or more, preferably in the range of 20 to 80%, and more preferably in the range of 20 to 70%. It is expected that the metal nanoparticles having a flat shape will produce a dense sintered body that can have a low resistance value even when sintered at a low temperature. Therefore, by sintering the metal nanoparticles obtained by the method of this embodiment, a sintered body having high conductivity can be obtained.

[0018] In each embodiment of the present invention, the shape of the metal nanoparticles can be determined by, for example, confirming the shape of a plurality of metal nanoparticles by TEM. For example, when the shape of the metal nanoparticles in the TEM image is circular, the crystal growth of the particle is presumed to be highly isotropic, and thus the shape is determined to be spherical. In contrast, when the shape of the metal nanoparticles in the TEM image is polygonal (e.g., triangular or rectangular), the crystal growth of the particle is presumed to be highly anisotropic, and thus the shape is determined to be flat. In addition, the probability of existence of flat particles in the metal nanoparticles can be determined by calculating the percentage of the volume of the flat metal nanoparticles relative to the total volume of the metal nanoparticles in the TEM image.

[0019] The particle size of the metal nanoparticles obtained by the method of this embodiment is usually 100 nm or less, preferably 80 nm or less, and more preferably 50 nm or less. The method of this embodiment can obtain metal nanoparticles having a finer particle size than those obtained by the methods of the prior art.

[0020] In each aspect of the present invention, the particle size of the metal nanoparticles can be determined, for example, based on the particle size distribution of the metal nanoparticles measured using a particle size distribution measuring device, or by measuring the particle sizes of multiple metal nanoparticles by TEM and calculating the average value of the measured values.

[0021] The method of this embodiment includes a raw material preparation step, a reaction tube preparation step, and a metal nanoparticle formation step. Each step will be described in detail below.

[0022] [1: Raw material preparation process] The process includes providing an aqueous solution of a metallic material that includes a metallic material.

[0023] The metal material prepared in this step is not limited as long as the metal salt can dissolve in the solvent and generate metal ions in the aqueous solution. The metal material is, for example, a salt of the metal exemplified above or a mixture thereof, and is preferably a metal salt such as an inorganic salt of the metal exemplified above, such as hydrochloride, sulfate, nitrate, or phosphate, an organic salt of the metal exemplified above, such as carboxylate or sulfonate, or a mixture thereof, and more preferably silver nitrate. The metal salt may be prepared, for example, by dissolving a material containing the metal exemplified above or a salt thereof with an acid such as nitric acid or aqueous ammonia.

[0024] The aqueous solution of the metallic material prepared in this step usually contains the metallic material at a concentration in the range of 5 to 200 mM. The concentration of the metal salt contained in the aqueous solution of the metallic material is preferably in the range of 5 to 100 mM. If the concentration of the metal salt exceeds the upper limit, the resulting metallic particles may become coarse and / or the number of metallic particles having shapes other than flat may increase. If the concentration of the metal salt is less than the lower limit, the yield of the resulting metallic particles may decrease. Therefore, by carrying out the method of this embodiment using an aqueous solution of the metallic material containing the metal salt at a concentration in the above range, metallic nanoparticles having a flat shape can be obtained in high yield.

[0025] The aqueous solution of the metal material prepared in this step usually contains a reducing agent. The reducing agent may be included in the aqueous solution of the metal material, or may be prepared as an aqueous solution separate from the aqueous solution of the metal material. The reducing agent is preferably N,N-dimethylformamide (DMF), citric acid, a citrate salt (e.g., trisodium citrate, disodium citrate, or monosodium citrate), oxalic acid, an oxalate salt (e.g., sodium oxalate), ascorbic acid, an ascorbate salt (e.g., sodium ascorbate), formic acid or a formate salt (e.g., sodium formate), or a mixture thereof, and more preferably DMF. The concentration of the reducing agent is preferably in the range of 1,000 to 10,000 mmol / L, more preferably in the range of 2,000 to 10,000 mmol / L, more preferably in the range of 5,000 to 10,000 mmol / L, and even more preferably in the range of 8,000 to 10,000 mmol / L. By using the reducing agents exemplified above, the metal salt can be efficiently reduced to obtain flat metal nanoparticles in high yield.

[0026] The aqueous solution of the metal material prepared in this step preferably contains a base in addition to the reducing agent exemplified above. The base can improve the reaction rate of the reduction reaction of the metal salt and the reducing agent in the metal nanoparticle formation step described below. The base is preferably an alkali metal hydroxide or an alkaline earth metal hydroxide, or a mixture thereof, more preferably potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, or beryllium hydroxide, or a mixture thereof, even more preferably potassium hydroxide or sodium hydroxide, and particularly preferably potassium hydroxide. By carrying out the method of this embodiment using an aqueous solution of the reducing agent containing the base exemplified above, the reaction rate of the reduction reaction of the metal salt and the reducing agent can be improved, and metal nanoparticles having a flat shape can be obtained in high yield.

[0027] In this step, the aqueous solution of the metal material may be prepared by self-preparation of an aqueous solution having the characteristics described above, or may be prepared by purchasing a pre-prepared aqueous solution.

[0028] [2: Reactor tube preparation process] This step involves preparing a reaction tube for use in the method of this embodiment.

[0029] The reaction tube prepared in this step has at least a microwave heating part. The microwave heating part may be the whole reaction tube or a part of the reaction tube. In the microwave heating part, the aqueous solution of the metal material flowing inside can be heated by irradiating microwaves from the outside.

[0030] The reaction tube prepared in this step, particularly the microwave heating part thereof, is formed of a material having a negative charging characteristic. The material having a negative charge resistance characteristic is preferably a fluororesin, silicon, polyethylene or acrylic resin, or a mixture thereof, more preferably a fluororesin or silicon, even more preferably a fluororesin, and particularly preferably polytetrafluoroethylene (PTFE). All of the materials exemplified above have a negative charge resistance characteristic. In addition, the materials exemplified above have a characteristic of easily transmitting microwaves. Therefore, by carrying out the method of this embodiment using a reaction tube formed of the material exemplified above, it is possible to optimize the aggregation of ions of the metal material and efficiently proceed with the reduction reaction of the ions of the metal material, thereby obtaining metal nanoparticles having a flat shape in a high yield.

[0031] [3: Metal nanoparticle formation process] This process involves irradiating microwaves from outside the reaction tube toward the microwave heating section while flowing an aqueous solution of a metal material through the microwave heating section of the reaction tube to form metal nanoparticles.

[0032] In this step, microwave irradiation may be performed using a microwave irradiation device commonly used in the art. The microwave frequency is preferably in the range of 2 to 3 GHz. The microwave irradiation intensity is preferably in the range of 100 to 2,000 W / mL.

[0033] In this process, a microwave irradiation device is used to irradiate microwaves from the outside of the reaction tube toward the microwave heating section, while an aqueous solution of a metal material is circulated through the microwave heating section of the reaction tube. As a result, the aqueous solution of the metal material circulating through the microwave heating section is heated, and the reduction reaction of the ions of the metal material proceeds to form metal nanoparticles. At this time, the inner wall of the microwave heating section is negatively charged. Therefore, the ions of the metal material are reduced while agglomerating near the inner wall of the microwave heating section, and the metal particles grow into grains. Therefore, by carrying out this process, the aggregation of the ions of the metal material is optimized, and the reduction reaction of the ions of the metal material is efficiently advanced, so that metal nanoparticles having a flat shape can be obtained in a high yield.

[0034] In this process, the flow rate of the aqueous solution of the metallic material is usually in the range of 5 to 300 mL / min, and preferably in the range of 5 to 150 mL / min. If the flow rate of the aqueous solution of the metallic material exceeds the upper limit, the reduction reaction of the ions of the metallic material may not proceed sufficiently. If the flow rate of the aqueous solution of the metallic material is below the lower limit, the ions of the metallic material may aggregate excessively, resulting in coarse metallic particles. Therefore, by carrying out this process under the above conditions, metallic nanoparticles having a flat shape can be obtained in high yield.

[0035] In the method of this embodiment, the concentration of the metallic material contained in the aqueous solution of the metallic material, c mM, and the flow rate of the aqueous solution of the metallic material, v mL / min, are as follows: For 0≦c<100, 5≦v<((30 / 19)c+(2700 / 19)); When 100≦c≦200, ((29 / 20)c-140)≦v≦300 It is preferable to satisfy the relationship. By carrying out the method of this aspect under the conditions that satisfy the relationship, the aggregation of ions of the metal material is optimized, and the reduction reaction of the ions of the metal material proceeds efficiently, so that metal nanoparticles having a flat shape can be obtained in a high yield.

[0036] As described in detail above, by the method of this aspect, metal nanoparticles having a flat shape can be produced as compared with the metal nanoparticles obtained by the method of the prior art. The metal nanoparticles obtained by the method of this aspect can be a dense sintered body that can have a low resistance value even at low-temperature firing. Further, the metal nanoparticles obtained by the method of this aspect do not have a coating with an organic compound on the surface of the metal particles. The sintered body of the metal nanoparticles obtained by the method of this aspect has a volume resistivity of, for example, 40 μΩ·cm or less, preferably 30 μΩ·cm or less, particularly 20 μΩ·cm or less. Therefore, by obtaining the metal nanoparticles having the above characteristics by the method of this aspect, an ink material applicable to wiring formation in a resin-made printed electronics material and a bonding material having low heat resistance can be provided.

Examples

[0037] 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.

[0038] <I: Production of Metal Particles> [I-1: Raw Material Preparation Step] Silver nitrate was used as a metal source, N,N-dimethylformamide (DMF) was used as a reducing agent, potassium hydroxide was used as a base, and water was used as a solvent. In Examples 1 to 7 and Comparative Examples 1 to 3, a protective agent such as polyvinylpyrrolidone (PVP) was not used. In Comparative Example 4, PVP was used as a protective agent. An aqueous solution of a silver salt containing silver nitrate at a predetermined final concentration, and an aqueous solution of a reducing agent containing potassium hydroxide at a final concentration of 120 mmol / L and DMF at a final concentration of 8,000 mmol / L were prepared.

[0039] [I-2: Reaction Tube Preparation Step] A reaction tube having a microwave heating section (outer diameter: 6 mm, inner diameter: 4 mm) with an inner wall formed of a predetermined pipe material having negative charging characteristics was prepared.

[0040] [I-3: Metal Nanoparticle Formation Step] An aqueous solution of a silver salt and an aqueous solution of a reducing agent obtained in the raw material preparation step were mixed and passed through the reaction tube at a predetermined flow rate. At this time, microwaves were irradiated from the outside of the reaction tube toward the microwave heating section (frequency: 2.45 GHz, irradiation intensity: 1,700 W / mL). Thereby, silver particles were formed.

[0041] [II: Analysis of Metal Particles] The obtained silver particles were observed with a transmission electron microscope (TEM). In the TEM image, the shape of the silver particles was confirmed, and flat-shaped particles were discriminated. The percentage of the volume of the flat-shaped silver particles with respect to the total volume of the silver particles in the TEM image was calculated as the flat particle presence probability (%). The TEM image of the silver particles of Example 1 is shown in FIG. 1. In the figure, the scale bar indicates 50 nm.

[0042] The obtained silver particles were sintered under the conditions of 120°C for 1 hour to obtain a sintered body. Based on the mass loss of the obtained sintered body, the carbon content (mass%) contained in the silver particles was calculated. Also, the volume resistivity (μΩ·cm) of the obtained sintered body was measured.

[0043] The production conditions of the examples and comparative examples, and the characteristics of the obtained silver particles of the examples and comparative examples are shown in Table 1. In the table, PTFE represents polytetrafluoroethylene.

[0044]

Table 1

[0045] As shown in FIG. 1, most of the silver particles in Example 1 were fine particles having a flat shape. As a result of the discrimination, it was confirmed that the silver particles in Examples 1 to 7 had a flat particle existence probability in the range of 27 to 64%. In contrast, it was confirmed that the silver particles in Comparative Examples 1 to 4 had a flat particle existence probability of 8% or less. From these results, it was revealed that the silver particles of the present invention are nanoparticles having a flat shape. In addition, it was confirmed that the sintered bodies of the silver particles in Examples 1 to 7 had a volume resistivity in the range of 10.2 to 11.8 μΩ·cm. In contrast, it was confirmed that the sintered bodies of the silver particles in Comparative Examples 1 to 4 had a volume resistivity of 40.3 μΩ·cm or more. From these results, it was confirmed that the silver particles of the present invention can be a dense sintered body that can have a low resistance value even when fired at a low temperature.

[0046] Fig. 2 shows the relationship between the silver salt concentration in the aqueous silver salt solution used in producing silver particles in Examples 1 to 7 and the flow rate of the aqueous silver salt solution. In the figure, the horizontal axis represents the silver salt concentration c (mM), and the vertical axis represents the flow rate v (mL / min) of the aqueous silver salt solution.

[0047] As shown in FIG. 2, the silver salt concentration c mM in the silver salt aqueous solution and the flow rate v mL / min of the silver salt aqueous solution are as follows: For 0≦c<100, 5≦v<((30 / 19)c+(2700 / 19)); When 100≦c≦200, ((29 / 20)c-140)≦v≦300 When the above relationship is satisfied, nanoparticles having a flat shape can be obtained.

[0048] The present invention is not limited to the above-described embodiments, but includes various modified examples. 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 configurations described. In addition, it is possible to add, delete, and / or replace part of the configuration of each embodiment with other configurations.

Claims

1. preparing an aqueous solution of the metallic material having a concentration in the range of 5 to 200 mM; A reaction tube preparation step of preparing a reaction tube having a microwave heating part with an inner wall formed of a material having a negative charging characteristic; a metal nanoparticle forming step of irradiating microwaves from the outside of the reaction tube toward the microwave heating section while flowing an aqueous solution of a metal material through the microwave heating section at a flow rate in the range of 5 to 300 mL / min to form metal nanoparticles; A method for producing metal nanoparticles having a flat shape, comprising:

2. 2. The method according to claim 1, wherein the material having negative charging properties forming the reaction tube is polytetrafluoroethylene (PTFE).

3. 2. The method of claim 1, wherein the metallic material is a salt of silver, gold, platinum, copper, nickel, iron, cobalt, palladium or zinc, or mixtures thereof.

4. The method of claim 3 , wherein the metallic material is silver nitrate and the metallic nanoparticles are silver nanoparticles.

5. The concentration of the metallic material contained in the aqueous solution of the metallic material, c mM, and the flow rate of the aqueous solution of the metallic material, v mL / min, are as follows: When 0≦c<100, 5≦v<((30 / 19)c+(2700 / 19)); When 100≦c≦200, ((29 / 20)c-140)≦v≦300 The method of claim 1 , wherein the relationship is satisfied.

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