Method for producing silver nanoparticle

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 technology]

[0002] Silver nanoparticles are silver particles with a particle size of 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 that can produce a dense sintered body with low resistance even when sintered at low temperatures is preferable. Silver nanoparticles with a particle size in the range of 5 to 10 nm obtained by liquid phase synthesis have been developed as an ink material suitable for low-temperature sintering.

[0004] For example, US Patent Publication 1 describes a wet milling method for producing monodisperse and stable metallic silver nanoparticles, comprising the steps of a) preparing an aqueous solution of a silver salt, b) preparing an aqueous solution of a reducing agent, c) mixing the aqueous solutions to cause the reaction, and d) separating the mother solution from the silver nanoparticles, characterized in that i) the aqueous solution of the reducing agent contains 0.01% to 20% by weight of a reducing agent of the tannin type, ii) the aqueous solution of the silver salt contains 0.01% to 20% by weight of a soluble silver salt, iii) the reaction occurs by mixing the two aqueous solutions and controlling the pH between 10.5 and 11.5, and iv) 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 reducing solution is adjusted to the alkaline range up to 11.5, the method produces a product with an average particle size of about 10 nm to 20 nm.

[0005] Patent Document 2 discloses a method for producing silver powder particles, comprising the steps of: (a) preparing an acidic aqueous solution of a silver salt, the aqueous solution comprising a water-soluble silver salt dissolved in deionized water; (b) preparing an acidic aqueous solution of a reducing agent and a particle modifier, the aqueous solution of the reducing agent and the particle modifier comprising: (i) a reducing agent selected from the group consisting of ascorbic acid, an ascorbate salt, and mixtures thereof; (ii) nitric acid; (iii) maleic acid; and (iv) deionized water, and adjusting the pH of the aqueous solution of the reducing agent and the particle modifier to 2.5 to 6 by the addition of a base; and (c) stirring the aqueous solution of the silver salt and the aqueous solution of the reducing agent and the surface morphology modifier without stirring each solution during and after preparation. (d) adding, with stirring, the acidic aqueous solution of a silver salt to the acidic aqueous solution of a reducing agent and a particle modifier to obtain a reaction mixture and maintaining the temperature of the reaction mixture at the temperature in (c), wherein the silver powder particles precipitate and are contained in the resulting final aqueous solution; and (e) increasing 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 a silver salt and the acidic aqueous solution of a reducing agent and a particle modifier further comprise gum arabic, and wherein the silver powder particles comprise crystallites with a diameter of 32 nm or less as determined by X-ray diffraction and the Scherrer equation, 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 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, a method for producing silver nanoparticles by liquid phase synthesis is known. However, the conventional method has a problem that the particle size distribution range is wide (for example, in the range of 1 to 100 nm) and the particle size is large. In addition, the conventional method has a problem that the yield of the resulting silver nanoparticles is reduced due to the formation of silver oxide precipitates.

[0008] Therefore, an object of the present invention is 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 present inventors have studied various means for solving the above problems. The present inventors have 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 a reaction temperature in an extremely short time, the formation of silver oxide is substantially suppressed, and the grain growth of silver particles is substantially suppressed, thereby making it possible to obtain the desired silver nanoparticles in a 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 silver salt and an aqueous solution of a reducing agent containing an alkali metal hydroxide or an alkaline earth metal hydroxide; a heating step of mixing an aqueous solution of a silver salt and an aqueous solution of a reducing agent and heating the temperature of the mixture to a reaction temperature of 100° C. or higher within 30 seconds from the start of mixing; a reaction step of maintaining the mixture obtained in the temperature increasing step at a reaction temperature; A method for producing silver nanoparticles, comprising: (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 or higher period of the periodic table. (Embodiment 3) The method of embodiment 2, wherein the alkali metal hydroxide or alkaline earth metal hydroxide is potassium hydroxide. (Embodiment 4) The method of any one of embodiments 1 to 3, wherein the reducing agent is N,N-dimethylformamide, citric acid, a citrate salt, oxalic acid, an oxalate salt, ascorbic acid, an ascorbate salt, formic acid or a formate salt, or a mixture thereof. (Embodiment 5) The method of embodiment 4, wherein the reducing agent is N,N-dimethylformamide. Effect of the Invention

[0011] The present invention makes it possible to provide a means for producing silver nanoparticles having a narrow particle size distribution and a fine particle size in high yield. [Brief description of the drawings]

[0012] [Figure 1] 1 shows transmission electron microscope (TEM) images of silver particles of Example 1 and Comparative Example 1. In the figure, A is the silver particles of Example 1, and B is the silver particles of Comparative Example 1. The scale bars of A and B both indicate 50 nm. [Diagram 2] 1 shows the ultraviolet-visible light absorption spectra of the silver particles of Examples 1, 2, 3, and 4, and Comparative Example 1. In the figure, the horizontal axis represents wavelength (nm), and the vertical axis represents absorbance. [Diagram 3] 1 shows the ultraviolet-visible light absorption spectra of silver particles of Examples 3 and 4 and Comparative Examples 2 and 3. In the figure, the horizontal axis represents wavelength (nm) and the vertical axis represents absorbance. 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 silver nanoparticles.

[0015] In each aspect of the present invention, silver nanoparticles generally mean metal particles containing silver. The silver nanoparticles are preferably metal particles made of silver, and more preferably metal particles made of silver alone. The silver nanoparticles may have a coating of 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, polyacrylates, cyclodextrin, aminopectin, methylcellulose, polyethyleneimine cellulose, aliphatic amines, aliphatic carboxylic acids, and tannic acid. The coating may be formed by the organic compounds listed above alone, or may be formed by a mixture of two or more organic compounds.

[0016] The particle size of the silver nanoparticles obtained by the method of this embodiment is usually 50 nm or less, preferably 45 nm or less, and more preferably 40 nm or less. For example, in the 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 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 embodiment, it is possible to obtain silver nanoparticles having a narrow particle size distribution and a fine particle size compared to silver nanoparticles obtained by the method of the prior art.

[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 a particle size distribution measuring device, for example. 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 by TEM and calculating the average value of the measured values.

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

[0019] [1: Raw material preparation process] The process involves providing an aqueous solution of a silver salt and an aqueous solution of a reducing agent.

[0020] The aqueous solution of silver salt prepared in this step is not limited as long as the silver salt can dissolve in the solvent and generate silver ions. The aqueous solution of silver salt is, for example, an aqueous solution containing silver salt such as inorganic salts of silver, such as hydrochloride, sulfate, nitrate, or phosphate, organic salts 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 with 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 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, even more preferably in the range of 40 to 120 mmol / L, and even more preferably in the range of 50 to 100 mmol / L. The aqueous solution of silver salt may contain a protective agent as desired. The protective agent is preferably selected from the organic compounds exemplified above as the organic compounds that form a coating on the surface of the silver nanoparticles. When the aqueous silver salt solution contains a protective agent, the concentration of the protective agent contained in the aqueous silver salt solution is preferably in the range of 100 to 1,000 mmol / L, more preferably in the range of 100 to 800 mmol / L, more preferably in the range of 200 to 800 mmol / L, and even more preferably in the range of 400 to 700 mmol / L. By using the silver salts exemplified above and, if desired, a protective agent, silver nanoparticles having a narrow particle size distribution and a fine particle size can be obtained in high yield.

[0021] The aqueous solution of the reducing agent prepared in this step is, for example, an aqueous solution containing a reducing agent such as 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. The reducing agent is preferably DMF. The concentration of the reducing agent contained in the aqueous solution of the reducing agent 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, the silver salt can be efficiently reduced to obtain silver nanoparticles having a narrow particle size distribution and a fine particle size in high yield.

[0022] The aqueous solution of the reducing agent prepared in this step contains an alkali metal hydroxide or an alkaline earth metal hydroxide in addition to the reducing agent exemplified above. The alkali metal hydroxide or the alkaline earth metal hydroxide can improve the reaction rate of the reduction reaction of the silver salt and the reducing agent in the reaction step described below. The alkali metal hydroxide or the alkaline earth metal hydroxide contained in the aqueous solution of the reducing agent is preferably a hydroxide of an alkali metal or an alkaline earth metal belonging to the third period or higher of the periodic table, such as potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide or beryllium hydroxide, more preferably potassium hydroxide or sodium hydroxide, and particularly preferably a hydroxide of an alkali metal or an alkaline earth metal belonging to the fourth period or higher of the periodic table, such as potassium hydroxide. The alkali metal hydroxide or the alkaline earth metal hydroxide may be a compound exemplified above used alone or may be used as a mixture of two or more compounds. The concentration of the 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, more preferably in the range of 10 to 150 mmol / L or in the range of 20 to 200 mmol / L, more preferably in the range of 10 to 120 mmol / L or in the range of 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. Since alkali metal or alkaline earth metal ions belonging to the third period or higher, particularly the fourth period or higher, in the period of the periodic table have a large ionic radius, they have a small electrical interaction with hydroxide ions and are easily ionized. For this reason, hydroxides of alkali metals or alkaline earth metals belonging to the third period or higher, particularly the fourth period or higher, in the periodic table are highly basic. Therefore, by carrying out the method of this embodiment using an aqueous solution of the reducing agent containing the hydroxide of the alkali metal or alkaline earth metal exemplified above, the reaction rate of the reduction reaction of the silver salt and the reducing agent can be improved, and the nucleation of silver nanoparticles can be promoted.This makes it possible to suppress aggregation of the silver nanoparticles and to obtain silver nanoparticles having a narrow particle size distribution and a fine particle size in high yield.

[0023] The aqueous solution of the silver salt is substantially free of alkali metal hydroxide or alkaline earth metal hydroxide. In the aqueous solution of the silver salt, the concentration of the alkali metal hydroxide or alkaline earth metal hydroxide 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 case of the conventional method using the liquid phase synthesis method, both the aqueous solution of the silver salt and the aqueous solution of the reducing agent usually contain an alkali or a base. However, when the aqueous solution of the silver salt contains an alkali or a base, by-products such as silver oxide or silver hydroxide may be generated. In contrast, the aqueous solution of the silver salt used in the method of this embodiment is substantially free of alkali metal hydroxide or alkaline earth metal hydroxide. 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 step, the aqueous solution of the silver salt and the aqueous solution of the reducing agent may be prepared by self-preparation of solutions having the characteristics described above, or may be prepared by purchasing pre-prepared aqueous solutions.

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

[0026] In this process, the silver salt aqueous solution and the reducing agent aqueous solution are mixed to cause a reduction reaction between the silver salt and the reducing agent. At this time, the hydrogen ions generated by the reduction reaction are quickly neutralized by the hydroxide of an alkali metal or an alkaline earth metal contained in the reducing agent aqueous solution. Therefore, it is possible to obtain silver nanoparticles having a narrow particle size distribution and a fine particle size in high yield while substantially suppressing the formation of by-products such as silver oxide or silver hydroxide and suppressing the aggregation of silver nanoparticles.

[0027] In this step, the time for increasing 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 increasing the temperature of the mixture exceeds the upper limit, the silver nanoparticles may aggregate, resulting in a broader particle size distribution and / or a larger particle size. Therefore, by increasing the temperature of the mixture within the time range, the aggregation of the silver nanoparticles can be suppressed, and silver nanoparticles having a narrow particle size distribution and a 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 involves 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 lower than the lower limit, the reaction rate of the reduction reaction of the silver salt and the reducing agent will be slow, and the silver nanoparticles may aggregate, resulting in a broader particle size distribution and / or larger particle size. Therefore, by carrying out this process at a reaction temperature in the above range, it is possible to suppress the aggregation of the silver nanoparticles and obtain silver nanoparticles having a narrow particle size distribution and fine particle size in high yield.

[0031] In this process, the reaction time is preferably 30 seconds or more, more preferably 60 seconds or more. If the reaction time is less than the lower limit, the reduction reaction of the silver salt and the reducing agent may not proceed sufficiently. Therefore, by carrying out this process with a reaction time in 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 aspect, silver nanoparticles having a narrow particle size distribution and a fine particle diameter can be produced as compared with silver nanoparticles obtained by the method of the prior art. The silver nanoparticles obtained by the method of this aspect 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 aspect, it is possible to provide an ink material applicable to wiring formation in resin-made printed electronics materials and bonding materials having low heat resistance.

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 a metal source, polyvinylpyrrolidone (PVP) was used as a protective agent, N,N-dimethylformamide (DMF) with a final concentration of 8,900 mmol / L was used as a reducing agent, and water was used as a solvent. An aqueous solution of a 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 the 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 with a transmission electron microscope (TEM). In addition, the particle size distribution of the obtained silver particles was measured using a particle size distribution measuring device. The ultraviolet-visible light absorption spectrum of the obtained silver particles was measured using an ultraviolet-visible light spectrometer. TEM images of the silver particles of Example 1 and Comparative Example 1 are shown in FIG. 1. In the figure, A is the silver particles of Example 1, and B is the silver particles of Comparative Example 1. The scale bars of A and B both indicate 50 nm. The ultraviolet-visible light absorption spectrum of the silver particles of Examples 1, 2, 3, and 4, and Comparative Example 1 is shown in FIG. 2. In the figure, the horizontal axis is the wavelength (nm) and the vertical axis is the absorbance. In addition, the ultraviolet-visible light absorption spectrum of the silver particles of Examples 3 and 4, and Comparative Examples 2 and 3 is shown in FIG. 3. In the figure, the horizontal axis is the wavelength (nm) and the vertical axis is the absorbance.

[0036] Table 1 shows the production conditions for the examples and comparative examples, as well as the particle size distribution of the obtained silver particles 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 about 40 nm (Figure 1A). In contrast, the silver particles of Comparative Example 1 were particles with a particle size of more than 50 nm (Figure 1B). In addition, as shown in Figures 2 and 3, narrow absorption peaks were observed in all of the silver particles of Examples 1 to 4.

[0039] As a result of particle size distribution measurement, it was 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, it was confirmed that the silver particles of Comparative Examples 1 to 3 had D50 values ​​in the range of 40 to 90 nm and D90 values ​​in the range of 60 to 150 nm. These results demonstrate that the silver particles of the present invention have a narrow particle size distribution and a fine particle size.

[0040] 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. A raw material preparation step of preparing an aqueous solution of a silver salt and an aqueous solution of a reducing agent containing an alkali metal hydroxide or an alkaline earth metal hydroxide; a heating step of mixing an aqueous solution of a silver salt and an aqueous solution of a reducing agent and heating the temperature of the mixture to a reaction temperature of 100° C. or higher within 30 seconds from the start of mixing; a reaction step of maintaining the mixture obtained in the temperature increasing step at a reaction temperature; A method for producing silver nanoparticles, comprising:

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

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

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

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

Images