Method for producing metal nanoparticles

Abstract

To provide a method for easily producing a mixture of two types of metal nanoparticles with different particle sizes.SOLUTION: This invention relates to a method for producing metal nanoparticles by reducing metal ions in a reaction solution, wherein the reaction solution comprises: a solvent; metal ions; a compound 1 acting as a reducing agent by having a standard electrode potential of 0.49 V to 0.80 V and a molecular weight of 90 g / mol or less, and further acting as a first protective agent by having an adsorption capacity onto reduced metal nanoparticles; and a compound 2 having a weight average molecular weight (Mw) of 10,000 g / mol to 40,000 g / mol and acting as a second protective agent by having an adsorption capacity onto the reduced metal nanoparticles. A molar ratio of the compound 1 to metal ions (compound 1 / metal ion) is 20 or more, and a concentration of metal ions is 50 mmol / L or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Metal nanoparticles, which may have properties different from those of bulk materials, are being used and investigated for a variety of applications, such as catalysts, ink materials, and electronic component materials.

[0003] For example, Patent Document 1 discloses a conductive wiring material that includes a plurality of first metal nanoparticles and a plurality of second metal nanoparticles having a particle size smaller than that of the first metal nanoparticles, and that can be fired at a low temperature to melt the second metal nanoparticles and fill the spaces between the first metal nanoparticles.

[0004] Patent Document 2 discloses a method for producing silver nanoparticles, characterized in that 40 mM or more silver ions in a reaction solution are reduced with a silver ion reducing agent in the presence of a particle protecting agent and an element more noble than silver. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-279038 [Patent Document 2] Japanese Patent Publication No. 2020-183567 Summary of the Invention [Problem to be solved by the invention]

[0006] In the field of electronics packaging, when a mixture of two types of metal nanoparticles with different particle sizes is used as the metal nanoparticles in the conductive wiring material described in Patent Document 1 or the lead-free bonding material described in Patent Document 2, the sintered body formed by sintering the mixture has a low porosity and, as a result, can have a low volume resistivity.

[0007] A mixture of two types of metal nanoparticles with different particle sizes that may be useful in the field of electronics packaging is typically prepared by separately synthesizing two types of metal nanoparticles with different particle sizes, optionally classifying them, and then uniformly mixing them, as described in Patent Document 1. Therefore, the method for producing this mixture has problems such as a large number of steps, low productivity, and high cost. Furthermore, the low-temperature baking temperature described in Patent Document 1 is 180°C, making it difficult to print and bake on PET, PC, etc.

[0008] Therefore, an object of the present invention is to provide a method for easily producing a mixture of two types of metal nanoparticles with different particle sizes. [Means for solving the problem]

[0009] The present inventors have investigated various means for solving the above-mentioned problems, and as a result, have found that in a method for producing metal nanoparticles by reducing metal ions in a reaction solution, the reaction solution contains a solvent, metal ions, and compound 1, which acts as a reducing agent by having a standard electrode potential of 0.49 V to 0.80 V and has a molecular weight of 90 g / mol or less and is capable of being adsorbed onto reduced metal nanoparticles, thereby acting as a first protective agent, and compound 2, which has a weight-average molecular weight (Mw) of 10,000 g / mol to 40,000 g / mol and is capable of being adsorbed onto reduced metal nanoparticles, thereby acting as a second protective agent, by adjusting the molar ratio of compound 1 to metal ions (compound 1 / metal ions) to 20 or more and adjusting the concentration of metal ions to 50 mmol / L or more, thereby simultaneously synthesizing two types of metal nanoparticles with different particle sizes, thereby completing the present invention.

[0010] That is, the gist of the present invention is as follows. (1) A method for producing metal nanoparticles by reducing metal ions in a reaction solution, comprising: The reaction solution is A solvent; Metal ions, Compound 1 has a standard electrode potential of 0.49 V to 0.80 V, and therefore acts as a reducing agent, and has a molecular weight of 90 g / mol or less, and has the ability to adsorb to reduced metal nanoparticles, and therefore acts as a first protecting agent; Compound 2 has a weight-average molecular weight (Mw) of 10,000 g / mol to 40,000 g / mol and acts as a second protective agent by having the ability to adsorb to reduced metal nanoparticles; Including, the molar ratio of compound 1 to metal ion (compound 1 / metal ion) is 20 or more; Metal ion concentration 50 mmol / L or higher The method. (2) The method according to (1), wherein the molar ratio of compound 2 to metal ion (compound 2 / metal ion) is 4 or more. (3) The method according to (1) or (2), wherein the metal ion is a silver ion. (4) The method according to any one of (1) to (3), wherein compound 1 is at least one compound selected from the group consisting of oxalic acid and its salts, and N,N-dimethylformamide. (5) The method according to any one of (1) to (4), wherein compound 2 is polyvinylpyrrolidone. (6) The method according to any one of (1) to (5), wherein the reaction is carried out at a temperature of 20°C to 100°C for 5 minutes to 60 minutes. [Effects of the Invention]

[0011] According to the present invention, a mixture of two types of metal nanoparticles with different particle sizes can be easily produced. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram illustrating an embodiment of the present invention; [Figure 2] FIG. 1 shows TEM images and particle size distributions of Reference Examples 1 and 2. [Figure 3] FIG. 1 shows TEM images and particle size distributions of Examples 3 and 4. [Figure 4]FIG. 10 is a diagram showing a cross-sectional SEM image of the sintered body of Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0013] Preferred embodiments of the present invention will now be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The method for producing metal nanoparticles of the present invention is not limited to the following embodiments, and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art, without departing from the spirit of the present invention.

[0014] The present invention relates to a method for producing metal nanoparticles by reducing metal ions in a reaction solution, wherein the reaction solution contains a solvent, metal ions, Compound 1, which acts as a reducing agent due to its standard electrode potential being 0.49 V to 0.80 V, has a molecular weight of 90 g / mol or less, and is capable of being adsorbed onto reduced metal nanoparticles, thereby acting as a first protective agent, and Compound 2, which has a weight-average molecular weight (Mw) of 10,000 g / mol to 40,000 g / mol, and is capable of being adsorbed onto reduced metal nanoparticles, thereby acting as a second protective agent, wherein the molar ratio of Compound 1 to the metal ions (Compound 1 / metal ions) is 20 or more, and the concentration of the metal ions is 50 mmol / L or more.

[0015] The solvent used in the reaction solution of the method of the present invention is not limited. Examples of the solvent include low-boiling solvents with a boiling point of 120°C or less. Examples of low-boiling solvents include, but are not limited to, low-boiling polar solvents such as water, alcohols such as ethanol, other organic solvents, and mixtures of two or more of these. In the present invention, it is preferable to use water as the solvent.

[0016] By using a low boiling point solvent as the solvent, the handling of the solvent can be improved and the burden on the environment can be reduced.

[0017] In the reaction solution of the method of the present invention, the metal ions are not limited. Examples of metal ions include ions of metals that constitute metal nanoparticles, such as gold, silver, platinum, copper, nickel, iron, and cobalt. In the present invention, it is preferable to use silver ions as the metal ions. Examples of raw materials for metal ions include, but are not limited to, inorganic salts of metals such as hydrochlorides, sulfates, nitrates, and phosphates, and organic salts such as carboxylates, acetates, and sulfonates. In the present invention, it is preferable to use inexpensive nitrates as the raw material for metal ions.

[0018] In the reaction solution of the method of the present invention, the concentration of the metal ions in the reaction solution is 50 mmol / L or more, preferably 100 mmol / L or more. The upper limit of the concentration of the metal ions in the reaction solution is not limited as long as the raw material of the metal ions is present as metal ions in the reaction solution, but is usually 500 mmol / L, preferably 400 mmol / L.

[0019] By setting the concentration of metal ions in the reaction solution within the above range, metal nanoparticles can be produced efficiently at a high concentration, the amount of metal nanoparticles that can be produced and recovered at one time can be significantly increased, and the time, effort, and cost required for producing metal nanoparticles can be reduced.

[0020] In the reaction solution of the method of the present invention, Compound 1 is a compound that acts as a reducing agent for metal ions by having a standard electrode potential of 0.49 V to 0.80 V, has a molecular weight of 90 g / mol or less, and acts as a first protective agent by having the ability to adsorb to reduced metal nanoparticles. Here, the "ability to adsorb to metal nanoparticles" as a function of Compound 1 as a protective agent for metal nanoparticles means that Compound 1 binds to part or the entire surface of metal nanoparticles suspended in a solvent, and this property can suppress aggregation of the metal nanoparticles.

[0021] Examples of organic functional groups that Compound 1 may have to have the ability to adsorb to reduced metal nanoparticles include thiol groups, amine groups, carboxy groups, hydroxy groups, ether groups, etc. Compound 1 may contain two or more of these groups.

[0022] Examples of Compound 1 include at least one compound selected from the group consisting of oxalic acid and its salts, such as sodium oxalate, N,N-dimethylformamide (DMF), and dimethyl sulfoxide. In the present invention, it is preferable to use at least one compound selected from the group consisting of oxalic acid and its salts, and N,N-dimethylformamide as Compound 1.

[0023] By including compound 1 in the reaction solution in the method of the present invention, the nucleation of metal nanoparticles can be caused simultaneously due to its relatively strong reducing power, and the particle size distribution of each of the two types of metal nanoparticles obtained, each with a different particle size, can be narrowed.

[0024] In the reaction solution in the method of the present invention, the molar ratio of compound 1 to metal ions (compound 1 / metal ions) is 20 or more, preferably 20-80.

[0025] In the method of the present invention, when the reaction solution contains Compound 1 in a molar ratio to metal ions within the above range, two types of metal nanoparticles with different particle sizes can be easily synthesized.

[0026] In the reaction solution in the method of the present invention, the concentration of compound 1 in the reaction solution is not limited as long as the molar ratio of compound 1 to metal ions is within the above range, but is usually 1 mol / L or more, preferably 1 mol / L to 10 mol / L, and more preferably 2 mol / L to 6 mol / L.

[0027] In the reaction solution of the method of the present invention, compound 2 has a weight-average molecular weight (Mw) of 10,000 g / mol to 40,000 g / mol and acts as a second protective agent by having the ability to adsorb to metal nanoparticles produced by reduction. Here, the "ability to adsorb to metal nanoparticles" as a function of compound 2 as a protective agent for metal nanoparticles means that compound 2 binds to part or the entire surface of metal nanoparticles suspended in a solvent, and this property can suppress aggregation of the metal nanoparticles.

[0028] The organic functional groups that compound 2 may have to have the ability to adsorb to reduced metal nanoparticles include a thiol group, an amine group, a carboxy group, a hydroxy group, an ether group, etc., as in compound 1. Compound 2 may contain two or more of the above groups.

[0029] Examples of compound 2 include at least one compound selected from the group consisting of polyvinylpyrrolidone (PVP), thiol-based polymers, and polyvinyl alcohol (PVA). In the present invention, it is preferable to use PVP as compound 2.

[0030] In the reaction solution of the method of the present invention, the molar ratio of compound 2 to metal ions (compound 2 / metal ions) (where the molecular weight of compound 2 is based on the weight-average molecular weight) is not limited, but is usually 1 or more, preferably more than 2, more preferably 4 or more, and even more preferably 4 to 8.

[0031] In the reaction solution in the method of the present invention, the molar ratio of compound 1 to compound 2 (compound 1 / compound 2) (wherein the molecular weight of compound 2 is based on the weight average molecular weight) is not limited, but is usually 5 or more, preferably 10 or more, and more preferably 10 to 60.

[0032] In the reaction solution in the method of the present invention, the concentration of compound 2 in the reaction solution (where the molecular weight of compound 2 is based on the weight average molecular weight) is not limited, but is usually 50 mmol / L or more, preferably 100 mmol / L or more, and more preferably 200 mmol / L to 800 mmol / L.

[0033] When the reaction solution in the method of the present invention contains compound 2 in the above-mentioned amount, compound 2 has the effect of reducing the frequency of contact between the nuclei of metal nanoparticles produced in the reaction solution, thereby suppressing excessive coalescence of the nuclei, and also exerts a protective effect on the nuclei of the coalesced metal nanoparticles, thereby playing a role in aligning the particle size distribution, making it possible to easily synthesize two types of metal nanoparticles with different particle sizes.

[0034] Therefore, since the reaction solution in the method of the present invention contains compound 1 and compound 2, the reaction solution contains two types of protective agents with different adsorption amounts to metal nanoparticles, and each protective agent produces metal nanoparticles with a particle size corresponding to its adsorption amount, thereby allowing two types of metal nanoparticles with different particle sizes to be synthesized simultaneously.

[0035] In addition to the above-mentioned materials, the reaction solution in the method of the present invention may contain sodium hydroxide (NaOH) as a pH adjuster.

[0036] In the present invention, the order of addition of each material, the addition temperature, the mixing method, the mixing time, etc. are not limited, and the materials are mixed so as to prepare a homogeneous reaction liquid. In the present invention, the reaction is started after the homogeneous reaction liquid is prepared.

[0037] The present invention can be carried out by a method involving heating.

[0038] In the present invention, in the method using heating, the reaction temperature is not limited, but is usually 20°C to 100°C, preferably 20°C to 90°C.

[0039] In the present invention, in the method using heating, the reaction time is not limited, but is usually 5 to 60 minutes, preferably 5 to 40 minutes, and more preferably 5 to 30 minutes.

[0040] In the present invention, the reaction can be easily carried out by carrying out the reaction by a heating method.

[0041] In the present invention, the reaction solution is preferably stirred using a stirring mechanism, such as a propeller stirrer, a vibration stirrer, etc. By stirring the reaction solution, the two types of metal nanoparticles produced in the reaction solution can be uniformly dispersed, and the reaction solution can be maintained uniformly.

[0042] The present invention may be carried out in a batch system or a flow system. The present invention is preferably carried out in a batch system. By carrying out the present invention in a batch system, the synthesis reaction itself can be completed, and the yield of the obtained metal nanoparticles can be improved. In addition, the concentration of the reaction solution can be made high, and the problem of clogging of the pipes by metal nanoparticles that can occur in a flow system does not occur.

[0043] One embodiment of the present invention is schematically shown in Figure 1. Figure 1 shows a scheme for producing a dispersion of silver nanoparticles by reacting a reaction solution containing water (HO) as a solvent, silver nitrate (AgNO: 100 mmol / L or more) as a metal ion, DMF (2000 mmol / L or more) as compound 1, and PVP (100 mmol / L or more) as compound 2 at a reaction temperature of 20°C to 100°C for a reaction time of 5 to 60 minutes.

[0044] According to the present invention, two types of metal nanoparticles with different particle sizes can be simultaneously prepared, which results in fewer steps, improved productivity, and lower costs.

[0045] The dispersion liquid containing metal nanoparticles obtained by the present invention can be separated and purified (e.g., salting out or centrifugation) by methods known in the art to obtain the desired metal nanoparticles and / or a dispersion liquid containing metal nanoparticles.

[0046] The metal nanoparticles produced by the present invention have two particle sizes, i.e., a bimodal particle size distribution. Here, in the present invention, a bimodal particle size distribution means that when a histogram of the particle size distribution of the metal nanoparticles is prepared in 2.5 nm intervals, the difference between the mode in the range of 0 nm to 10 nm and the mode above 10 nm is 10 nm or more.

[0047] The metal nanoparticles obtained by the present invention are a mixture of two types of metal nanoparticles with different particle sizes, and have excellent low-temperature sinterability, for example, at 120°C. Furthermore, a sintered body formed from the metal nanoparticles has low volume resistivity.

[0048] The metal nanoparticles produced by the present invention can be used in fields such as catalysts, electronic component materials, and ink materials, as well as conductive wiring materials in the electronics packaging field, and can reduce the number of steps in producing inks used in wiring boards, for example. [Example]

[0049] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0050] 1. Preparation of Silver Nanoparticles A reaction solution was prepared by adding a silver nitrate aqueous solution (concentration: 400 mM) as a metal ion to a PVP aqueous solution (weight average molecular weight: 10,000 g / mol or 40,000 g / mol, concentration: 0.4 mol / L, 0.8 mol / L, 1.6 mol / L, or 2.4 mol / L) as compound 2, followed by N,N-dimethylformamide (DMF) or oxalic acid or ascorbic acid as compound 1, and then adding purified water as a solvent so that the concentrations of each material were as shown in Tables 1 and 2 below. The resulting reaction solution was placed in a 500 mL separable flask and reacted at 90°C (reaction temperature) for 40 minutes (reaction time) while stirring with a magnetic stirrer to prepare a dispersion of silver nanoparticles.

[0051] [Table 1]

[0052] [Table 2]

[0053] 2. TEM observation and particle size distribution measurement of silver nanoparticles Reference Examples 1, 2 and 6, Example 3 ~ 5 and 7 ,and Each of the silver nanoparticles in Comparative Examples 1 to 3 was dropped onto a TEM grid and dried to prepare a sample. TEM observation was then performed under the measurement conditions shown in Table 3 below. The projected area circle equivalent diameter of 100 or more silver nanoparticles randomly selected from the TEM image was determined, and the particle size and particle size distribution were measured. The particle size distribution was created as a histogram in 2.5 nm intervals, such as 0 nm to 2.5 nm, 2.5 nm to 5.0 nm, 5.0 nm to 7.5 nm, etc.

[0054] [Table 3]

[0055] The results are shown in Tables 4 and 5.

[0056] [Table 4]

[0057] [Table 5]

[0058] Comparison of Examples and Comparative Examples from Tables 4 and 5 revealed that a bimodal particle size distribution can be obtained by adding appropriate amounts of Compound 1 and Compound 2 to the reaction solution. Furthermore, comparison of Examples 4 and 5 confirmed that a bimodal particle size distribution can be obtained when the weight-average molecular weight of Compound 2 is 10,000 g / mol or 40,000 g / mol. Furthermore, comparison of Examples 4 and 7 and Comparative Example 3 revealed that a bimodal particle size distribution can be obtained by using, as Compound 1, a compound capable of adsorbing to reduced metal nanoparticles, i.e., DMF (liquid) or oxalic acid (aqueous solution), which has a standard electrode potential of 0.49 V to 0.80 V and a molecular weight of 90 g / mol or less.

[0059] Figures 2 and 3 show Reference Examples 1-2 and Example 3 TEM images and particle size distribution of 4 are shown in Figures 2 and 3. reference In Example 1, the bimodal particle size distribution was 6.59±1.60 nm and 19.15±2.09 nm (mean particle size: 14.8±6.21 nm). referenceIn Example 2, a bimodal particle size distribution of 6.24±1.96 nm and 21.06±6.00 nm (average particle size: 11.2±8.52 nm) was obtained; in Example 3, a bimodal particle size distribution of 4.16±0.92 nm and 18.9±1.59 nm (average particle size: 6.29±5.29 nm) was obtained; and in Example 4, a bimodal particle size distribution of 4.02±1.23 nm and 17.2±1.92 nm (average particle size: 6.74±5.53 nm) was obtained. Figures 2 and 3 show that the addition of compound 2 results in a bimodal particle size distribution, but the bimodality of the particle size distribution depends on the concentration of compound 2; that is, the higher the concentration of compound 2 in the reaction solution, the more pronounced the bimodality. Specifically, in the particle size distribution of silver nanoparticles, a significant bimodality was observed, for example, when the molar concentration of silver ions was more than twice, particularly more than four times, the molar concentration. This suggests that as the concentration of Compound 2 (PVP) increases, PVP becomes more likely to protect the silver nanoparticles, increasing the difference between the most frequent values ​​between 0 and 10 nm and those above 10 nm, and that the average particle size also tends to decrease. This result indicates that the number of silver nanoparticles with a particle size of less than 10 nm increases, improving the sinterability of the silver nanoparticles.

[0060] 3. Sintering test of silver nanoparticles The dispersions of silver nanoparticles from Example 4 and Comparative Example 1 were subjected to ultrafiltration (UF filtration) at room temperature (25°C to 30°C) for 3 hours. Pure water was used as a washing liquid during filtration. The dispersions were then concentrated using an evaporator to obtain dispersions with a solid content of 15% by weight.

[0061] The solid content of the dispersion was measured as follows. First, a small amount of the dispersion was sampled and heated at 150°C for 2 hours to volatilize the solvent, and the residual content of the dispersion was measured. As a result, the residual content of the dispersion was 15.8% by weight of the dispersion. Next, a portion of the residual content was heated to 500°C in a thermogravimetric (TG) measurement device to burn off the organic components, and the solid content of the residual content was measured. As a result, the solid content of the residual content was 95% of the residual content. From the above, the solid content of the dispersion was 0.158×0.95×100=15(weight%) It was calculated that:

[0062] Subsequently, the obtained dispersion was dropped into a slide chamber (slide & chamber 8 well, manufactured by Watson) and sintered at 120°C for 60 minutes in a Yamato DN63 Constant Temperature Oven, a low-temperature incubator with airflow, to form a film.

[0063] The volume resistivity of the obtained film was measured using a resistivity meter (Loresta GX MCP-T700, manufactured by Nitto Seiko Analytech Co., Ltd.) The results are shown in Table 6, and a cross-sectional SEM image of the sintered body of Example 4 is shown in FIG.

[0064] [Table 6]

[0065] Table 6 shows that when fired at the same temperature, the volume resistivity of the sintered body obtained from the silver nanoparticles of Example 4 was lower than that of the sintered body obtained from the silver nanoparticles of Comparative Example 1. This is thought to be because, in the sintered body obtained from the silver nanoparticles of Example 4, the first group particles, which have a small particle size, are melted, thereby filling the spaces between the second group particles, and further because Compound 2, which exhibits thermal fluidity, is adsorbed as a protective agent.

[0066] Furthermore, from Table 6 and FIG. 4, it was confirmed that the sintered body obtained from the silver nanoparticles of Example 4 had a porosity of 1%, which was smaller than the porosity of Comparative Example 1, and that the voids (spaces) between the second group particles in the sintered body obtained from the silver nanoparticles of Example 4 were filled with the first group particles.

Claims

[Claim 1] A method for producing silver nanoparticles by reducing silver ions in a reaction solution, comprising: The reaction solution is A solvent; Silver ions and Compound 1 is at least one compound selected from the group consisting of oxalic acid and its salts, and N,N-dimethylformamide, and acts as a reducing agent and has the ability to adsorb to reduced silver nanoparticles, thereby acting as a first protective agent; Polyvinylpyrrolidone, which has a weight-average molecular weight (Mw) of 10,000 g / mol to 40,000 g / mol and acts as a second protective agent by having the ability to adsorb to reduced silver nanoparticles; Including, the molar ratio of compound 1 to silver ions (compound 1 / silver ions) is 20 to 80; the molar ratio of polyvinylpyrrolidone to silver ions (polyvinylpyrrolidone / silver ions) is 4 to 8; The concentration of silver ions is 50 mmol / L to 500 mmol / L, The reaction is carried out at a temperature of 20°C to 100°C for 5 to 60 minutes. The method.

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