Method for manufacturing silver nanoparticles

Abstract

To provide means for producing silver nanoparticles having a narrow particle size distribution and a fine particle size in a high yield.SOLUTION: A method for producing silver nanoparticles comprises: a raw material preparation step for preparing an aqueous solution of a silver salt and an aqueous solution of a reducing agent including an alkali metal hydroxide or an alkaline earth metal hydroxide; a temperature-raising step for mixing the aqueous solution of the silver salt and the aqueous solution of the reducing agent and raising the temperature of the mixture to a reaction temperature of 100°C or higher within 30 seconds from the start of mixing; and a reaction step for maintaining the mixture obtained in the temperature-raising step at the reaction temperature.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Silver nanoparticles are silver particles having a particle size of about several nm to several tens of nm. Silver nanoparticles are used as an ink material for patterning in printed electronics.

[0003] As an ink material for patterning in printed electronics, a material capable of obtaining a dense sintered body having a low resistance value even at low-temperature firing is preferable. As an ink material suitable for low-temperature firing, silver nanoparticles having a particle size in the range of 5 to 10 nm obtained by a liquid-phase synthesis method have been developed.

[0004] For example, Patent Document 1 describes a wet pulverization method for producing monodisperse and stable metal silver nanoparticles, which comprises: a) a step of preparing an aqueous solution of a silver salt; b) a step of preparing an aqueous solution of a reducing agent; c) a step of mixing the above aqueous solutions to cause these reactions; and d) a step of separating the mother solution from the above silver nanoparticles. The method is characterized in that: i) the aqueous solution of the reducing agent contains 0.01% by mass to 20% by mass of a tannin-based reducing agent; ii) the aqueous solution of the silver salt contains 0.01% by mass to 20% by mass of a soluble silver salt; iii) the above two aqueous solutions are mixed and the pH is controlled to be between 10.5 and 11.5, whereby the above reaction occurs; and iv) the separation of the mother solution is promoted by changing the zeta potential of the metal particles. The document describes that when the pH of the above reducing solution is adjusted and the pH of the silver salt solution is adjusted to an alkaline range up to 11.5, a product having an average particle size of about 10 nm to 20 nm can be obtained by the above method.

[0005] Patent Document 2 describes a method for producing silver powder particles, comprising: (a) a step of preparing an acidic aqueous solution of a silver salt containing a water-soluble silver salt dissolved in deionized water; (b) a step of preparing an acidic aqueous solution of a reducing agent and a particle modifier, wherein the acidic aqueous solution of the reducing agent and particle modifier comprises (i) a reducing agent selected from the group consisting of ascorbic acid, ascorbate salts, and mixtures thereof; (ii) nitric acid; (iii) maleic acid; and (iv) deionized water, and the pH of the acidic aqueous solution of the reducing agent and particle modifier is adjusted to 2.5 to 6 by adding a base; and (c) stirring the acidic aqueous solution of the silver salt and the acidic solution of the reducing agent and surface morphology modifier during and after preparation. The present invention describes a method comprising: (d) a step of maintaining the same temperature, wherein the temperature is in the range of 10°C to 65°C; (c) a step of adding an acidic aqueous solution of the silver salt to an acidic aqueous solution of the reducing agent and particle modifier while stirring to obtain a reaction mixture, and maintaining the temperature of the reaction mixture at the temperature in (c) above, wherein the silver powder particles precipitate and are included in the final aqueous solution obtained; and (e) a step of raising the temperature of the final aqueous solution to a temperature in the range of 65°C to 80°C while continuing to stir, wherein one or both of the acidic aqueous solution of the silver salt and the acidic aqueous solutions of the reducing agent and particle modifier further contain gum arabic, the silver powder particles contain crystallites with a diameter of 32 nm or less as determined by X-ray diffraction and Scherrer's formula, and the d50 of the silver powder particles is in the range of 0.5 to 3.5 μm. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2010-513718 [Patent Document 2] Special Publication No. 2014-511438 [Overview of the project] [Problems that the invention aims to solve]

[0007] As mentioned above, a method for producing silver nanoparticles by liquid-phase synthesis is known. However, the conventional method has the problem of a wide range of particle size distribution (for example, in the range of 1 to 100 nm) and large particle size. In addition, the conventional method has the problem of a decrease in the yield of silver nanoparticles because a precipitate of silver oxide is formed.

[0008] Therefore, the present invention aims to provide a means for producing silver nanoparticles having a narrow particle size distribution and fine particle size in high yield. [Means for solving the problem]

[0009] The inventors investigated various means to solve the above-mentioned problems. The inventors found that, in a liquid-phase synthesis method, by mixing an aqueous solution of a silver salt that does not contain hydroxide ions with an aqueous solution of a reducing agent that contains hydroxide ions, and raising the temperature of the mixture to the reaction temperature in a very short time, the formation of silver oxides and the grain growth of silver particles can be substantially suppressed, thereby obtaining desired silver nanoparticles in high yield. Based on the above findings, the inventors completed the present invention.

[0010] In other words, the present invention encompasses the following aspects and embodiments. (Embodiment 1) A raw material preparation step is to prepare an aqueous solution of silver salt and an aqueous solution of a reducing agent containing an alkali metal hydroxide or alkaline earth metal hydroxide. A heating step involves mixing an aqueous solution of silver salt and an aqueous solution of a reducing agent, and raising the temperature of the mixture to a reaction temperature of 100°C or higher within 30 seconds from the start of mixing. The reaction step involves maintaining the mixture obtained in the heating step at the reaction temperature. A method for producing silver nanoparticles, including [the specified element]. (Embodiment 2) The method according to Embodiment 1, wherein the alkali metal hydroxide or alkaline earth metal hydroxide is an alkali metal or alkaline earth metal hydroxide belonging to the third period or higher in the periodic table. (Embodiment 3) The method according to Embodiment 2, wherein the alkali metal hydroxide or alkaline earth metal hydroxide is potassium hydroxide. (Embodiment 4) The method according to any one of Embodiments 1 to 3, wherein the reducing agent is N,N-dimethylformamide, citric acid, citrate, oxalic acid, oxalate, ascorbic acid, ascorbate, formic acid or formate, or a mixture thereof. (Embodiment 5) The method according to Embodiment 4, wherein the reducing agent is N,N-dimethylformamide. [Effects of the Invention]

[0011] The present invention provides a means for producing silver nanoparticles having a narrow particle size distribution and fine particle size in high yield. [Brief explanation of the drawing]

[0012] [Figure 1] The images show transmission electron microscope (TEM) images of silver particles from Example 1 and Comparative Example 1. In the figures, A represents the silver particles from Example 1, and B represents the silver particles from Comparative Example 1. The scale bars for both A and B represent 50 nm. [Figure 2] The ultraviolet-visible light absorption spectra of silver particles from Examples 1, 2, 3, and 4, and Comparative Example 1 are shown. In the figure, the horizontal axis represents wavelength (nm), and the vertical axis represents absorbance. [Figure 3] The ultraviolet-visible light absorption spectra of silver particles from Examples 3 and 4, and Comparative Examples 2 and 3 are shown. In the figures, the horizontal axis represents wavelength (nm), and the vertical axis represents absorbance. [Modes for carrying out the invention]

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

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

[0015] In each aspect of the present invention, silver nanoparticles usually mean metal particles containing silver. The silver nanoparticles are preferably metal particles composed of silver, and more preferably metal particles consisting only of silver. The silver nanoparticles may have a coating with an organic compound on the surface of the metal particles. In this case, examples of the organic compound forming the coating include polyvinylpyrrolidone (PVP), dodecylamine (DDA), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylate, cyclodextrin, aminopectin, methylcellulose, polyethyleneimine cellulose, aliphatic amines, aliphatic carboxylic acids, and tannic acid. The coating may be formed of the organic compound exemplified above alone, or may be formed of a mixture of two or more organic compounds.

[0016] The particle size of the silver nanoparticles obtained by the method of this aspect is usually 50 nm or less, preferably 45 nm or less, and more preferably 40 nm or less. For example, in the measurement of the particle size distribution of the silver nanoparticles obtained by the method of this aspect, the D50 value of the silver nanoparticles is usually in the range of 30 to 50 nm, preferably in the range of 30 to 45 nm, and more preferably in the range of 30 to 40 nm. The D90 value of the silver nanoparticles is usually in the range of 50 to 70 nm, preferably in the range of 50 to 65 nm, and more preferably in the range of 50 to 60 nm. By the method of this aspect, silver nanoparticles having a narrow particle size distribution and a fine particle size can be obtained as compared with the silver nanoparticles obtained by the prior art method.

[0017] In each aspect of the present invention, the D10 value, D50 value, and D90 value of the silver nanoparticles can be determined based on the particle size distribution of the silver nanoparticles measured using, for example, a particle size distribution measuring device. Further, the particle size of the silver nanoparticles can be determined, for example, by representing it as the D50 value of the silver nanoparticles, or by measuring the particle sizes of a plurality of silver nanoparticles by TEM and calculating the average value of the measured values.

[0018] The method of this aspect includes a raw material preparation step, a temperature increase step, and a reaction step. Hereinafter, each step will be described in detail.

[0019] [1: Raw material preparation step] This step includes preparing an aqueous solution of a silver salt and an aqueous solution of a reducing agent.

[0020] The aqueous solution of the silver salt prepared in this step is not limited as long as the silver salt can dissolve in a solvent to generate silver ions. The aqueous solution of the silver salt is, for example, an aqueous solution containing a silver salt such as an inorganic salt such as hydrochloride, sulfate, nitrate or phosphate of silver, an organic salt such as carboxylate or sulfonate, or a mixture thereof. The silver salt may be prepared, for example, by dissolving metallic silver or a material containing a silver salt in an acid such as nitric acid or aqueous ammonia. The silver salt is preferably silver nitrate. The concentration of the silver salt contained in the aqueous solution of the silver salt is preferably in the range of 10 to 200 mmol / L, preferably in the range of 10 to 150 mmol / L, more preferably in the range of 10 to 120 mmol / L, still more preferably in the range of 40 to 120 mmol / L, and still more preferably in the range of 50 to 100 mmol / L. The aqueous solution of the silver salt may optionally contain a protective agent. The protective agent is preferably selected from those exemplified above as an organic compound that forms a coating on the surface of the silver nanoparticles. When the aqueous solution of the silver salt contains a protective agent, the concentration of the protective agent contained in the aqueous solution of the silver salt is preferably in the range of 100 to 1,000 mmol / L, preferably in the range of 100 to 800 mmol / L, more preferably in the range of 200 to 800 mmol / L, and still more preferably in the range of 400 to 700 mmol / L. By using the silver salt and optionally the protective agent exemplified above, silver nanoparticles having a narrow particle size distribution and a fine particle size can be obtained in a high yield.

[0021] The aqueous solution of the reducing agent prepared in this process is, for example, an aqueous solution containing a reducing agent such as N,N-dimethylformamide (DMF), citric acid, citrate (e.g., trisodium citrate, disodium citrate, or monosodium citrate), oxalic acid, oxalate (e.g., sodium oxalate), ascorbic acid, ascorbate (e.g., sodium ascorbate), formic acid or formate (e.g., sodium formate), or a mixture thereof. The reducing agent is preferably DMF. The concentration of the reducing agent in the aqueous solution is preferably in the range of 1,000 to 10,000 mmol / L, 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, silver salts can be efficiently reduced to obtain silver nanoparticles having a narrow particle size distribution and fine particle size in high yield.

[0022] The aqueous solution of the reducing agent prepared in this process includes, in addition to the reducing agent exemplified above, an alkali metal hydroxide or an alkaline earth metal hydroxide. The alkali metal hydroxide or alkaline earth metal hydroxide can improve the reaction rate of the reduction reaction between the silver salt and the reducing agent in the reaction process described below. The alkali metal hydroxide or alkaline earth metal hydroxide contained in the aqueous solution of the reducing agent is preferably an alkali metal or alkaline earth metal hydroxide belonging to the third period or higher in the periodic table, for example, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, or beryllium hydroxide, more preferably potassium hydroxide or sodium hydroxide, and particularly preferably an alkali metal or alkaline earth metal hydroxide belonging to the fourth period or higher in the periodic table, for example, potassium hydroxide. The alkali metal hydroxide or alkaline earth metal hydroxide may be used as the compounds exemplified above individually, or as a mixture of two or more compounds. The concentration of alkali metal hydroxide or alkaline earth metal hydroxide contained in the aqueous solution of the reducing agent is preferably in the range of 10 to 200 mmol / L, preferably in the range of 10 to 150 mmol / L or 20 to 200 mmol / L, more preferably in the range of 10 to 120 mmol / L or 40 to 200 mmol / L, even more preferably in the range of 40 to 120 mmol / L, and particularly preferably in the range of 50 to 100 mmol / L. Alkali metal or alkaline earth metal ions belonging to the third period or higher, especially the fourth period or higher, in the periodic table have a large ionic radius, so their electrical interaction with hydroxide ions is small and they ionize easily. For this reason, alkali metal or alkaline earth metal hydroxides belonging to the third period or higher, especially the fourth period or higher, in the periodic table are strongly basic. Therefore, by carrying out the method of this embodiment using an aqueous solution of a reducing agent containing the alkali metal or alkaline earth metal hydroxide exemplified above, the reaction rate of the reduction reaction between the silver salt and the reducing agent can be improved and the nucleation of silver nanoparticles can be promoted.This suppresses the aggregation of silver nanoparticles, making it possible to obtain silver nanoparticles with a narrow particle size distribution and fine particle size in high yield.

[0023] The aqueous solution of the silver salt is substantially free of alkali metal hydroxides or alkaline earth metal hydroxides. In the aqueous solution of the silver salt, the concentration of alkali metal hydroxides or alkaline earth metal hydroxides is usually 1 mmol / L or less, particularly 0.1 mmol / L or less, and preferably below the detection limit (i.e., 0 mmol / L). In the conventional liquid-phase synthesis method, both the aqueous solution of the silver salt and the aqueous solution of the reducing agent usually contain alkalis or bases. However, when the aqueous solution of the silver salt contains alkalis or bases, by-products such as silver oxide or silver hydroxide may be formed. In contrast, the aqueous solution of the silver salt used in the method of this embodiment is substantially free of alkali metal hydroxides or alkaline earth metal hydroxides. Therefore, the formation of by-products such as silver oxide or silver hydroxide can be substantially suppressed, and silver nanoparticles can be obtained in high yield.

[0024] In this process, the aqueous solution of silver salt and the aqueous solution of reducing agent may be prepared by the user themselves, having the characteristics described above, or they may be prepared by purchasing pre-prepared aqueous solutions.

[0025] [2: Heating process] This process involves mixing an aqueous solution of silver salt and an aqueous solution of a reducing agent, and raising the temperature of the mixture to the reaction temperature.

[0026] In this process, the reduction reaction between the silver salt and the reducing agent proceeds by mixing an aqueous solution of silver salt and an aqueous solution of reducing agent. At this time, the hydrogen ions produced by the reduction reaction are rapidly neutralized by alkali metal or alkaline earth metal hydroxides contained in the aqueous solution of the reducing agent. Therefore, while substantially suppressing the formation of by-products such as silver oxide or silver hydroxide, aggregation of silver nanoparticles can be suppressed, and silver nanoparticles with a narrow particle size distribution and fine particle size can be obtained in high yield.

[0027] In this process, the time for raising the temperature of the mixture is preferably within 50 seconds from the start of mixing, more preferably within 30 seconds, even more preferably within 20 seconds, and particularly preferably within 10 seconds. If the time for raising the temperature of the mixture exceeds the above upper limit, the silver nanoparticles may aggregate, resulting in a broader particle size distribution and / or larger particle size. Therefore, by raising the temperature of the mixture within the above time range, aggregation of silver nanoparticles can be suppressed, and silver nanoparticles having a narrow particle size distribution and fine particle size can be obtained in high yield.

[0028] The reaction temperature in this step is the same as the reaction temperature in the reaction step described below. In this step, the reaction in the reaction step can be carried out by raising the temperature of the mixture to the reaction temperature.

[0029] [3: Reaction process] This step includes maintaining the mixture obtained in the heating step at the reaction temperature.

[0030] In this process, the reaction temperature is usually 100°C or higher, and preferably 130°C or higher. If the reaction temperature is below the lower limit, the reaction rate of the reduction reaction between the silver salt and the reducing agent will be slow, and the silver nanoparticles may aggregate, potentially resulting in a broader particle size distribution and / or larger particle size. Therefore, by carrying out this process at a reaction temperature within the above range, aggregation of silver nanoparticles can be suppressed, and silver nanoparticles with a narrow particle size distribution and fine particle size can be obtained in high yield.

[0031] In this process, the reaction time is preferably 30 seconds or longer, and more preferably 60 seconds or longer. If the reaction time is less than the lower limit, the reduction reaction between the silver salt and the reducing agent may not proceed sufficiently. Therefore, by carrying out this process with a reaction time within the above range, silver nanoparticles having a narrow particle size distribution and fine particle size can be obtained in high yield.

[0032] As described in detail above, by the method of this embodiment, silver nanoparticles having a narrow particle size distribution and a fine particle diameter can be produced as compared with the silver nanoparticles obtained by the method of the prior art. The silver nanoparticles obtained by the method of this embodiment can form a dense sintered body that can have a low resistance value even at low-temperature firing. Therefore, by obtaining silver nanoparticles having the above characteristics by the method of this embodiment, an ink material applicable to wiring formation in resin-based printed electronics materials and bonding materials with low heat resistance can be provided.

Example

[0033] 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.

[0034] <I: Production of silver particles> Silver nitrate was used as the metal source, polyvinylpyrrolidone (PVP) was used as the protective agent, N,N-dimethylformamide (DMF) with a final concentration of 8,900 mmol / L was used as the reducing agent, and water was used as the solvent. An aqueous solution of silver salt containing silver nitrate with a final concentration of 100 mmol / L and PVP with a final concentration of 600 mmol / L, as well as an aqueous solution of a reducing agent containing an alkali metal hydroxide or an alkaline earth metal hydroxide at a predetermined concentration and DMF with a final concentration of 8,900 mmol / L were prepared (raw material preparation step). The aqueous solution of silver salt and the aqueous solution of the reducing agent were mixed, and the temperature of the mixture was raised to a reaction temperature of 135°C within a predetermined time from the start of mixing (temperature raising step). The heated mixture was maintained at the reaction temperature for 60 seconds (reaction step).

[0035] <II: Analysis of silver particles> The obtained silver particles were observed using a transmission electron microscope (TEM). The particle size distribution of the obtained silver particles was also measured using a particle size distribution analyzer. The ultraviolet-visible absorption spectra of the obtained silver particles were measured using an ultraviolet-visible light spectrometer. TEM images of the silver particles from Example 1 and Comparative Example 1 are shown in Figure 1. In the figure, A represents the silver particles from Example 1, and B represents the silver particles from Comparative Example 1. The scale bars for both A and B represent 50 nm. The ultraviolet-visible absorption spectra of the silver particles from Examples 1, 2, 3, and 4, and Comparative Example 1 are shown in Figure 2. In the figure, the horizontal axis represents wavelength (nm), and the vertical axis represents absorbance. Furthermore, the ultraviolet-visible absorption spectra of the silver particles from Examples 3 and 4, and Comparative Examples 2 and 3 are shown in Figure 3. In the figure, the horizontal axis represents wavelength (nm), and the vertical axis represents absorbance.

[0036] Table 1 shows the manufacturing conditions for the examples and comparative examples, as well as the particle size distribution of the silver particles obtained for the examples and comparative examples.

[0037] [Table 1]

[0038] As shown in Figure 1, the silver particles of Example 1 were homogeneous particles with a particle size of approximately 40 nm (Figure 1A). In contrast, the silver particles of Comparative Example 1 had a particle size exceeding 50 nm (Figure 1B). Furthermore, as shown in Figures 2 and 3, narrow absorption peaks were observed in all of the silver particles of Examples 1 to 4.

[0039] Particle size distribution measurements confirmed that the silver particles of Examples 1 to 4 had D50 values ​​in the range of 30 to 45 nm and D90 values ​​in the range of 50 to 70 nm. In contrast, the silver particles of Comparative Examples 1 to 3 were found to have D50 values ​​in the range of 40 to 90 nm and D90 values ​​in the range of 60 to 150 nm. These results clearly demonstrate that the silver particles of the present invention have a narrow particle size distribution and fine particle size.

[0040] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are described in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. In addition, it is possible to add, delete, and / or replace some of the configurations in each embodiment with other configurations.

Claims

1. A raw material preparation step involves preparing an aqueous solution of silver salt and an aqueous solution of a reducing agent containing an alkali metal hydroxide or alkaline earth metal hydroxide. A heating step involves mixing an aqueous solution of silver salt and an aqueous solution of a reducing agent, and raising the temperature of the mixture to a reaction temperature of 100°C or higher within 30 seconds from the start of mixing. The reaction step involves maintaining the mixture obtained in the heating step at the reaction temperature. A method for producing silver nanoparticles, including [the specified element].

2. The method according to claim 1, wherein the alkali metal hydroxide or alkaline earth metal hydroxide is an alkali metal or alkaline earth metal hydroxide belonging to the third period or higher in the periodic table.

3. The method according to claim 2, wherein the alkali metal hydroxide or alkaline earth metal hydroxide is potassium hydroxide.

4. The method according to claim 1, wherein the reducing agent is N,N-dimethylformamide, citric acid, citrate, oxalic acid, oxalate, ascorbic acid, ascorbate, formic acid or formate, or a mixture thereof.

5. The method according to claim 4, wherein the reducing agent is N,N-dimethylformamide.

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