JP6656869B2 - Method for producing copper nanoparticles - Google Patents

Description

  The present invention relates to a method for producing copper nanoparticles that can be used for a copper paste or an electrodeposition solution for forming wiring such as a solar cell.

  2. Description of the Related Art In recent years, in solar cells, printed circuit boards, and display devices, wiring using a high conductivity (low resistivity) material that can be sintered at a low temperature has been desired in order to improve power generation efficiency and respond to resin. Among them, wiring materials using silver or copper materials are widely used. Further, at present, silver, which is easy to handle as a wiring material, is often used. On the other hand, silver has a high material unit price, and in recent years, replacement with inexpensive materials such as copper paste has been progressing. However, an oxide film is easily formed on the surface of the copper filler in the copper paste, and high-temperature sintering is required to obtain a low resistance. For example, it is not sufficiently applicable to the formation of wiring on a resin substrate.

  Therefore, nano-sized copper nanoparticles capable of sintering a copper paste at a low temperature have been proposed. When the copper particles are converted to nano-level copper nanoparticles, the surface energy is increased, and the copper nanoparticles are easily bonded to each other without applying large heat energy. Therefore, by obtaining copper nanoparticles that can be sintered at a low temperature, sintering at a low temperature is expected to lower the volume resistivity of the copper wiring.

  For example, (a) preparing a mixed solution containing a copper salt, a dispersant, a reducing agent, and an organic solvent; (b) raising the temperature of the mixed solution to 30 to 50 ° C. and stirring; c) irradiating the mixed solution with microwaves; and (d) obtaining a copper nanoparticle by lowering the temperature of the mixed solution to obtain copper nanoparticles. A method of synthesizing a large amount has been proposed (for example, see Patent Document 1).

  Further, a method is known in which a reducing agent is added to an aqueous solution of a carboxylic acid of a metal salt to obtain metal nanoparticles, particularly silver nanoparticles (for example, see Patent Document 2).

JP 2008-75181 A JP 2010-116626 A

  However, in the configuration described in Patent Literature 1, the solvent is a nonpolar organic solvent such as ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol, and the material itself is costly. In addition, the reaction process of the nanoparticles is irradiated with microwaves to give energy, so that it is easy to catch fire, and there is a problem in terms of environment and safety, such as danger in the manufacturing process.

  In the example described in Patent Document 2, there is a description of a method for producing silver nanoparticles, but there is no description of the example for copper. Therefore, in the production of silver nanoparticles, it is possible to remove the halogen compound, which is the object of Patent Document 2. On the other hand, the method for producing copper nanoparticles cannot be performed based on the example of Patent Document 2. Further, for example, ions or complexes such as silver ions, copper ions, hydroxide ions and carboxy groups have their own equilibrium constants. That is, the concentration of each ion and carboxy group differs depending on each pH, and the reaction process greatly changes for each metal ion. For this reason, even if there is a description about the pH dependence of silver ions and their complexes, it is impossible to predict the pH dependence of copper ions, which are different metal ions, and their complexes.

  Specifically, Patent Document 2 describes silver carboxylate, Ru carboxylate and gold carboxylate, Pt carboxylate, and Pd carboxylate, but does not describe any examples relating to copper. Therefore, it is impossible to obtain copper nanoparticles having a conductivity of the order of μΩcm in the temperature range of 130 ° C. to 400 ° C., which is the firing temperature range described in Patent Document 2.

  In addition, in the case of producing copper nanoparticles, the produced copper is dissolved in an acidic solution having a pH of 7 or less, and thus cannot exist in a state of copper nanoparticles. For this reason, there is a problem that a wiring using copper nanoparticles cannot be formed.

  Furthermore, when the modifying substance is not attached to the surface of the copper nanoparticles even in the alkali range of pH 7 or more, the copper nanoparticles are exposed to the air after drying after the copper nanoparticles are formed, and the copper nanoparticles are exposed to the atmosphere. The particles oxidize quickly. As a result, good conductive properties cannot be obtained in subsequent wiring formation.

  An object of the present invention is to solve the above-mentioned conventional problems and to provide a method of synthesizing copper nanoparticles in an aqueous solution and forming copper nanoparticles for forming a low wiring resistance.

The method for producing copper nanoparticles according to the present invention includes a step of preparing a first aqueous solution containing copper ions and citric acid and adjusted to a range of pH 10 or more and less than pH 12,
A step of preparing a second aqueous solution containing ascorbic acid and adjusted to a range of pH 10 or more and less than pH 12,
Mixing the first aqueous solution and the second aqueous solution, and reducing the copper ions to obtain a dispersion of copper nanoparticles;
including.

  As described above, according to the method for producing copper nanoparticles according to the present invention, by covering the surface of the generated copper nanoparticles with ascorbic acid, copper nanoparticles having high oxidation resistance can be obtained. Thereby, low-temperature sintering and low-resistance copper wiring can be formed.

FIG. 2 is a flowchart of a method for producing copper nanoparticles according to the first embodiment. FIG. 3 is a diagram showing the pH dependence of each concentration of copper ions and copper complexes calculated from equilibrium constants in the first embodiment. FIG. 3 is a diagram showing the pH dependence of the concentration of each ascorbate ion calculated from the equilibrium constant in the first embodiment. FIG. 3 is a diagram showing the pH dependence of the particle size of the copper nanoparticles created in the first embodiment. FIG. 4 is an X-ray diffraction diagram showing pH dependence of crystallinity of copper nanoparticles before sintering.

The method for producing copper nanoparticles according to the first aspect includes a step of preparing a first aqueous solution containing copper ions and citric acid and adjusted to a pH range of pH 10 or more and less than pH 12.
A step of preparing a second aqueous solution containing ascorbic acid and adjusted to a range of pH 10 or more and less than pH 12,
Mixing the first aqueous solution and the second aqueous solution, and reducing the copper ions to obtain a dispersion of copper nanoparticles;
including.

  With the above configuration, ascorbic acid can be firmly attached to the surface of the generated copper nanoparticles, and copper nanoparticles that form low wiring resistance can be manufactured.

The method for producing copper nanoparticles according to the second aspect includes a step of preparing a first aqueous solution containing copper ions and citric acid and adjusted to a pH of 10 or more and less than pH 12;
A step of preparing a second aqueous solution containing a reducing agent and an antioxidant different from the reducing agent and adjusted to a pH range of pH 10 or more and less than pH 12;
Mixing the first aqueous solution and the second aqueous solution, and reducing the copper ions to obtain a dispersion of copper nanoparticles;
including.

  With the above configuration, an antioxidant can be firmly attached to the surface of the generated copper nanoparticles, and copper nanoparticles that form low wiring resistance can be manufactured.

  In the method for producing copper nanoparticles according to a third aspect, in the first or second aspect, the first aqueous solution may be obtained by dissolving powder in the aqueous solution by the copper ion source, or a copper plate. May be dissolved.

  In the method for producing copper nanoparticles according to a fourth aspect, in the third aspect, the copper ion generation source may be selected from the group consisting of copper chloride, copper sulfate, and copper nitrate.

  Hereinafter, a method for producing copper nanoparticles according to an embodiment of the present invention will be described with reference to the accompanying drawings.

(Embodiment 1)
FIG. 1 is a flowchart of a method for producing copper nanoparticles according to Embodiment 1 of the present invention. The production flow of copper nanoparticles will be described with reference to FIG. The method for producing copper nanoparticles has the following three processes.
(1) Prepare a first aqueous solution containing copper ions and citric acid and adjusted to have a pH of 10 or more and less than 12 (treatment 1).
(2) Prepare a second aqueous solution containing ascorbic acid and adjusted to a pH range of 10 or more and less than pH 12 (treatment 2). The pH adjustment of the second aqueous solution may be in a range of pH 10 or more and less than pH 12, and need not be adjusted to the pH of the first aqueous solution.
(3) The first aqueous solution and the second aqueous solution are mixed to reduce copper ions to obtain a dispersion of copper nanoparticles (process 3).
Thus, a dispersion of copper nanoparticles can be obtained.
Here, although the process 1 for preparing the first aqueous solution is described before the process 2 for preparing the second aqueous solution, this is for convenience and does not define the order of the processes. Absent. In other words, the treatment 1 for preparing the first aqueous solution and the treatment 2 for preparing the second aqueous solution may be performed first or simultaneously.

The method for producing copper nanoparticles according to the first embodiment is characterized by containing citric acid and using ascorbic acid which also functions as an antioxidant as a reducing agent. Further, the pH of the first aqueous solution and the second aqueous solution is adjusted to a range of pH 10 or more and less than pH 12. As a result, the complex of copper ion and citric acid, which are precursors of the copper nanoparticles, is stably present regardless of a slight change in pH. Further, the precursor can be reduced by a monovalent ascorbate ion having a stronger reducing property. Furthermore, nanosized copper nanoparticles can be obtained in the above pH range. Furthermore, in the crystallinity of the obtained copper nanoparticles, no peak of copper oxide Cu ₂ O was observed, and it is considered that copper nanoparticles composed mostly of metallic copper Cu were obtained.

Hereinafter, each process of the method for producing copper nanoparticles will be described in more detail.
<Process 1>
In the process 1 for preparing the first aqueous solution, the processes 11, 12, and 13 are performed.
a) First, pure water (deionized water) and granules of dihydrated copper chloride (CuCl ₂ .2H ₂ O) are mixed (treatment 11). The copper chloride is mixed so as not to saturate. Here, copper chloride is used as the copper salt, but the present invention is not limited to this. For example, copper nitrate, copper sulfate, or the like may be used. Alternatively, copper ions may be obtained by dissolving metallic copper in a liquid. The pure water is not limited to deionized water (or deionized water), but may be distilled water or ultrapure water obtained by distilling deionized water.
b) Next, citric acid is added to the aqueous solution of copper chloride obtained in the above treatment 11, and mixed using ultrasonic waves to produce a copper citrate compound which is a complex with copper ions (treatment 12). The mixing method is not limited to the method using ultrasonic waves, and any mixing method that is commonly used can be used.
c) Next, the pH is adjusted to a range of pH 10 or more and less than pH 12 by adding NaOH (treatment 13). Note that the range of pH 10 or more and less than pH 12 in the pH adjustment is derived from FIGS. 2, 3, 4, and 5 described later. Although NaOH was used as the pH adjuster here, the present invention is not limited to this, and a compound containing an alkali metal such as KOH, Ca (OH) ₂ , an alkaline earth metal, and a hydroxide, or an amine compound may be used. .
As described above, a first aqueous solution containing copper ions and citric acid and adjusted to a range of pH 10 or more and less than pH 12 is obtained.

<Process 2>
In the process 2 for preparing the second aqueous solution, the processes 14 and 15 are performed.
d) First, an aqueous solution of ascorbic acid as a reducing agent is prepared (Step 14). For example, a granular ascorbic acid is dissolved in pure water to obtain an aqueous solution of ascorbic acid.
e) Next, NaOH is added to adjust the pH to a range of pH 10 or more and less than pH 12 (Process 15). Here, the reason why the pH of the second aqueous solution is adjusted to a range of pH 10 or more and less than pH 12 similarly to the first aqueous solution is that the first aqueous solution and the second aqueous solution are mixed in the subsequent treatment 3 This is because the change in pH when a suspension of nanoparticles is obtained is kept small. Further, the range of pH 10 or more and less than pH 12 in the pH adjustment is derived from FIGS. 2, 3, 4, and 5 described later. The pH of the second aqueous solution may be adjusted to the same pH as the first aqueous solution. Although NaOH was used as the pH adjuster here, the present invention is not limited to this, and a compound containing an alkali metal such as KOH, Ca (OH) ₂ , an alkaline earth metal, and a hydroxide, or an amine compound may be used. .
As described above, a second aqueous solution containing ascorbic acid and adjusted to a range of pH 10 or more and less than pH 12 is obtained.

<Process 3>
Next, in Process 3 for obtaining a dispersion of copper nanoparticles, Process 16 and Process 17 are performed. Further, treatment 18 may be performed to obtain solid copper nanoparticles.
f) First, the first aqueous solution and the second aqueous solution are mixed (process 16). In this case, the mixing of the first aqueous solution and the second aqueous solution may be performed by any of the following three procedures. For example, the second aqueous solution may be mixed with the first aqueous solution. Further, the first aqueous solution may be mixed with the second aqueous solution. Alternatively, the first aqueous solution and the second aqueous solution may be dropped and mixed in different containers. In order to promote the reduction reaction, it is effective to raise the temperature. For example, the temperature of the first aqueous solution and the second aqueous solution is maintained in advance in a temperature range of 40 ° C. or more and 80 ° C. or less, and preferably in a temperature range of 50 ° C. or more and 70 ° C. or less. You may mix with a 2nd aqueous solution. If the temperature is lower than 40 ° C., the reaction cannot be sufficiently promoted. On the other hand, if it exceeds 80 ° C., evaporation will be intense. As a result, a mixed solution having a temperature range of 40 ° C. to 80 ° C., preferably a temperature range of 50 ° C. to 70 ° C. can be obtained.
g) Subsequently, the temperature range of 40 ° C. or more and 80 ° C. or less, preferably 50 ° C. or more and 70 ° C. or less is maintained, and the mixture is stirred for a time range of 15 minutes or more and 60 minutes or less (processing 17). . If less than 15 minutes, a sufficient reaction is not performed. If it exceeds 60 minutes, it takes too much time, and the cost becomes high. In process 17, a complex ^{_{[(Cu 2+) 2 (C}} 6 H 4 O 7) 4- 2)] and ^{_{[(Cu 2+) (C 6}} H 4 O 7) 4-] is a reducing agent ascorbic Reacts with acids. As a result, the copper ions are reduced from the copper citrate compound, the copper atoms are separated, the copper atoms are attached to the nucleus of a certain copper solid, and the copper nanoparticles are synthesized. As the reaction of the mixed solution proceeds, the color of the mixed solution turns brown, which indicates that copper nanoparticles are generated. The reaction time ranges, for example, from 15 minutes to 60 minutes, and most of the copper citrate compound is copper nanoparticles.
Thus, a dispersion of copper nanoparticles can be obtained. The obtained dispersion liquid of copper nanoparticles may be directly used for production of copper wiring or the like. Alternatively, the dispersion liquid of the copper nanoparticles may be concentrated and used as a paste. Further, the following processing 18 such as filtering may be performed to obtain solid copper nanoparticles.

h) Thereafter, the copper nanoparticles are taken out of the mixture by filtering or concentration to obtain solid copper nanoparticles 4 (processing 18).
As described above, solid copper nanoparticles can be obtained from the dispersion liquid of copper nanoparticles.
Typically, after filtering in process 18, the copper nanoparticles tend to combine with the oxygen in the air and are more likely to oxidize after removal. On the other hand, according to the manufacturing method in the first embodiment, it is considered that ascorbic acid is attached to the entire periphery of the obtained copper nanoparticles. For this reason, it is considered that the oxidation of the copper nanoparticles can be prevented by the antioxidant action of ascorbic acid and can be maintained continuously. Therefore, a low volume resistivity of about 1000 μΩcm is obtained for the conductivity of the copper nanoparticles immediately after filtering.

  In forming the wiring using the copper nanoparticles, since the decomposition temperature of ascorbic acid is 190 ° C., the sintering temperature of the copper wiring must be 190 ° C. or higher. At that time, particularly when ascorbic acid is decomposed, oxygen slightly contained in the surface of the copper nanoparticles reacts with ascorbic acid, thereby providing an effect of further lowering the resistivity of the copper wiring.

<About the range of pH adjustment>
The range in which the pH of the first aqueous solution and the second aqueous solution is adjusted in the processes 13 and 15 will be described below.
FIG. 2 is a diagram showing the concentrations of citric acid molecules and copper ions, each compound and each complex with respect to pH calculated from the equilibrium constant of each ion. In order to obtain copper nanoparticles 4 having good crystallinity and a small particle size from a copper citrate compound which is a precursor of the copper nanoparticles 4, a single ion, complex or compound having the same reaction energy as possible is obtained. Must be a precursor. That is, if the concentration of ions, complexes, or compounds of the reduction target changes due to a slight change in pH during the reduction reaction, the reaction energy of the precursor to be reduced changes. Therefore, there is a possibility that the particle size distribution of the generated copper nanoparticles is widened. From Figure 2, at pH6 or more regions ^{_{[(Cu 2+) 2 (C}} 6 H 4 O 7) 4- 2)] and ^{_{[(Cu 2+) (C 6}} H 4 O 7) 4-] and concentration of certain It turns out that it becomes. That is, in the region of pH 6 or higher, the above condition is satisfied because the concentration of the precursor to be reduced hardly changes even if there is a slight change in pH.
Further, in an acidic region having a pH of less than 7, even if copper nanoparticles are generated by a reduction reaction, the copper nanoparticles are dissolved, so that the copper nanoparticles cannot be stably present. Therefore, an alkaline region having a pH of 7 or more is preferable. As described above, in an aqueous solution having a pH of 7 or more, as an ion, a complex, or a compound containing copper, [(Cu ²⁺ ) ₂ (C ₆ H ₄ O ₇ ^4- ) ₂ ] and [(Cu ²⁺ ) (C ₆ H) ₄ O ₇ ^4- )] exist stably over a wide pH range.

  FIG. 3 is a diagram showing the pH dependence of the concentrations of ascorbic acid and ascorbate ions. From FIG. 3, the divalent ascorbate ion starts increasing from pH 11 and the monovalent ascorbate ion starts decreasing accordingly. At pH 12 or less, monovalent ascorbate ions are greater than divalent ascorbate ions. On the other hand, at pH 12 or higher, divalent ascorbate ions are greater than monovalent ascorbate ions. At a pH of 13 or higher, monovalent ascorbate ions disappear and almost become divalent ascorbate ions. That is, the concentrations of the monovalent ascorbate ion and the divalent ascorbate ion are reversed at about pH12. In addition, monovalent ascorbate ion has a stronger reducing action than divalent ascorbate ion. Therefore, during the reduction reaction with ascorbic acid, the pH at which monovalent ascorbate ions are present is preferably 13 or less, and more preferably 12 or less at which the concentration of monovalent ascorbate ions increases.

FIG. 4 is a diagram showing the pH dependence of the average particle size of the obtained copper nanoparticles when the pH is adjusted with NaOH. From FIGS. 2 and 3, the pH at the time of the reduction of the mixed solution is preferably in the range from pH 7 to pH 12. Therefore, FIG. 4 shows a range of pH 8 to pH 13 as the range of pH adjustment.
As shown in FIG. 4, when the reaction temperature is 50 ° C., copper nanoparticles of about 60 nm, which are minimal at pH 12, are obtained, and copper nanoparticles of about 70 nm are obtained at pH 11. Copper nanoparticles of about 500 nm at pH 10 and about 150 nm at pH 13 are obtained. However, at pH 8 and pH 9, only copper nanoparticles of 1 μm or more were obtained. When the reaction temperature is 70 ° C., about 100 nm of copper nanoparticles, which are minimized at pH 11, are obtained, and at about 12, copper nanoparticles of about 120 nm are obtained. At pH 10 and pH 13, copper nanoparticles of about 300 nm were obtained. However, at pH 8 and pH 9, only copper nanoparticles of 1 μm or more were obtained.
Therefore, from FIG. 4, in order to obtain nano-sized copper particles, the pH range when the first aqueous solution and the second aqueous solution are mixed is preferably in the range of pH 10 or more and pH 13 or less.

FIG. 5 is a powder X-ray diffraction diagram showing the pH dependence of the crystallinity of the copper nanoparticles. A paste was prepared by mixing the obtained copper nanoparticles with ethanol, and the crystallinity of the film applied to the glass substrate in the state of the copper nanoparticles was confirmed by using an X-ray diffraction method.
As shown in FIG. 5, from pH 8 to pH 11, almost only an X-ray diffraction pattern of metallic copper (Cu) is present. At pH 12, along with the X-ray diffraction pattern of copper metal, a slight peak of copper oxide Cu ₂ O was observed, indicating that copper oxide was generated. Further, at pH 13, the peak area of copper oxide is equal to or greater than the peak area of metal copper, and it is considered that the proportion occupied by copper oxide is high. If the copper nanoparticles are sintered in this state, the resulting wiring will contain an oxide, and it is considered that sufficient electrical conductivity cannot be obtained.
From FIG. 5, only the metal copper Cu is obtained without including the peak of copper oxide Cu ₂ O at pH 8 or higher and lower than pH 12.

In consideration of FIGS. 2, 3, 4, and 5, in the processing 13 and the processing 15, the pH range of the first aqueous solution and the second aqueous solution is preferably a pH range of pH 10 or more and less than pH 12. . In the above pH range, the complex serving as the precursor of the copper nanoparticles exists stably regardless of a slight pH change. Further, the precursor can be reduced by a monovalent ascorbate ion having a stronger reducing property. Furthermore, nanosized copper nanoparticles can be obtained in the above pH range. Furthermore, in the crystallinity of the obtained copper nanoparticles, no peak of copper oxide Cu ₂ O was observed, and it is considered that copper nanoparticles composed mostly of metallic copper Cu were obtained.

(Example 1)
A method for producing copper nanoparticles according to Example 1 will be described. In Example 1, copper nanoparticles were manufactured based on the conditions in Embodiment 1.
(1) A first aqueous solution containing 0.1 mol / L of copper ions and 0.6 mol / L of citric acid and adjusted to pH 11 was prepared.
(2) Also, a second aqueous solution having a concentration of ascorbic acid of 0.6 mol / L and adjusted to pH 11 was prepared. Here, since the first aqueous solution was set to pH 11, the second aqueous solution was also set to pH 11.
(3) The first aqueous solution and the second aqueous solution were each heated to 70 ° C. and mixed. Thereafter, the mixture was stirred at 70 ° C. for 60 minutes.
Thus, a dispersion of copper nanoparticles was obtained.
Thereafter, the copper nanoparticles were taken out of the mixed solution by filtering to obtain solid copper nanoparticles. As a conductivity of the nanoparticles immediately after the filtering of the copper nanoparticles, a low volume resistivity of about 1000 μΩcm was obtained.
Further, the filtered copper nanoparticles were dispersed in a solvent (butyl carbitol acetate) to form a 20 μm thick film on glass. The copper nanoparticles were sintered for 1 hour at 200 ° C. in an atmosphere of 4% hydrogen and 96% nitrogen. As a result, a volume resistivity of 10 μΩcm was obtained for the obtained sintered product.

  According to the production method in Example 1, it is considered that ascorbic acid is attached to the entire circumference of the obtained copper nanoparticles. Therefore, it is considered that the oxidation of the copper nanoparticles could be prevented by the antioxidant action of ascorbic acid, and the copper nanoparticles could be maintained continuously. This is because ascorbic acid is known as an antioxidant substance called vitamin C, but ascorbic acid itself is gradually oxidized, so that oxidation of copper nanoparticles can be suppressed continuously. Conceivable.

(Comparative Example 1)
A method for producing copper nanoparticles according to Comparative Example 1 will be described. In Comparative Example 1, copper nanoparticles were produced under substantially the same conditions as in Example 1 except that the reducing agent used in Treatment 14 was changed from ascorbic acid to hydrazine.
The pH dependence of the ascorbic acid ion concentration is as described in FIG. On the other hand, the pH dependence of the hydrazine ion concentration is such that N ₂ H ₅ ⁺ is majority at pH 8 or lower and N ₂ H ₄ is majority at pH 8 or higher. Therefore, at a pH of 8 or more, the reduction potential decreases.
In addition, since the pH dependence of the particle size of the copper nanoparticles when the reducing agent was hydrazine could not be confirmed, copper nanoparticles were synthesized at pH 11 similar to that in Example 1. When the resistivity of the copper wiring obtained by sintering the obtained copper nanoparticles at 200 ° C. was compared, in the case of Example 1 (reducing agent: ascorbic acid), the resistivity was 10 μΩcm. On the other hand, in the case of Comparative Example 1 (reducing agent: hydrazine), the resistivity after sintering was about 10 Ωcm. That is, there was a difference of about 1,000,000 times in resistivity between Example 1 and Comparative Example 1.
Further, also in the case of sintering at 250 ° C., in the case of Example 1, the resistivity was about 1000 μΩcm, and the effect of the antioxidant action by ascorbic acid was large. Also, when the resistivity immediately after the filtering is compared, in Example 1 (reducing agent: ascorbic acid), the copper wiring after sintering of the obtained copper nanoparticles can secure conductivity with a resistivity of about 1000 μΩcm. Was. On the other hand, in Comparative Example 1 (reducing agent: hydrazine), the copper wiring after sintering of the obtained copper nanoparticles was an insulator whose overrange was impossible by the four-terminal method and the resistivity could not be measured.

The reason for the low conductivity (high resistance) after sintering in Comparative Example 1 is considered as follows. That is, hydrazine acts as a reducing agent, but hydrazine does not have antioxidant properties, and it is considered that the surfaces of the reduced copper nanoparticles are not protected by antioxidants. For this reason, after filtering, the copper nanoparticles are exposed to air and oxidation of the surface proceeds, so that it is considered that the copper nanoparticles have low conductivity.
On the other hand, in Example 1, it is considered that ascorbic acid acts not only as a reducing agent but also as having antioxidant properties. Specifically, since the three OH groups of ascorbic acid are strongly linked to the copper atoms constituting the copper nanoparticles, ascorbic acid adheres to the surface of the copper nanoparticles and suppresses the oxidation of copper until sintering. It is thought that there is. In particular, the present inventor has found that ascorbic acid has a high reducing performance when monovalent, and has a strong bond with copper. Therefore, as described above, at pH 10 or more and less than pH 12, the conductivity after sintering is the highest. I found that.

  This time, ascorbic acid was used as a reducing agent and an antioxidant, and it was found that ascorbic acid particularly acts on copper nanoparticles, covers the surface of the copper nanoparticles, and has a remarkable effect of suppressing copper oxidation.

(Embodiment 2)
The method for producing copper nanoparticles according to the second embodiment is different from the method for producing copper nanoparticles according to the first embodiment in that the second aqueous solution contains a reducing agent and an antioxidant different from the reducing agent. In that it contains The other points are substantially the same as those of the first embodiment, and thus the duplicate description will be omitted. Specifically, the method for producing copper nanoparticles has the following three processes.
(1) Prepare a first aqueous solution containing copper ions and citric acid and adjusted to have a pH of 10 or more and less than 12 (treatment 1).
(2) Prepare a second aqueous solution containing a reducing agent and an antioxidant different from the reducing agent and adjusted to have a pH of 10 or more and less than 12 (Process 2-1). In addition to ascorbic acid, for example, hydrazine or uric acid may be used as the reducing agent. When hydrazine or uric acid is used as the reducing agent, an antioxidant to be attached to the surface of the copper nanoparticles may be added separately from the reducing agent. Examples of the antioxidant include a substance having an antioxidant effect and being water-soluble, for example, a vitamin C group such as ascorbic acid, or a water-soluble vitamin B group such as riboflavin and thiamine, or a polyphenol such as flavonoid, isoflavone, and glutathione. May be used.
(3) The first aqueous solution and the second aqueous solution are mixed to reduce copper ions to obtain a suspension of copper nanoparticles (process 3).
Thus, a dispersion of copper nanoparticles can be obtained.
Note that, here, the process 1 for preparing the first aqueous solution is described before the process 2-1 for preparing the second aqueous solution, but this is for convenience and defines the order of the processes. Not something. In other words, the treatment 1 for preparing the first aqueous solution and the treatment 2-1 for preparing the second aqueous solution may be performed first or simultaneously.

  According to the method for producing copper nanoparticles according to Embodiment 2, an antioxidant is attached to the surface of copper nanoparticles obtained by reducing copper ions with a reducing agent by a combination of a reducing agent and an antioxidant. By doing so, the oxidation of the obtained copper nanoparticles can be suppressed.

  ADVANTAGE OF THE INVENTION According to the manufacturing method of the copper nanoparticle which concerns on this invention, the dispersion liquid of the copper nanoparticle which suppressed the oxidation of the copper nanoparticle can be obtained. Therefore, low-resistance copper wiring can be formed using the copper nanoparticles. Further, the copper nanoparticles can also be applied to wiring forming applications requiring a low-resistance and low-temperature process, such as semiconductors, displays, and solar cells.

DESCRIPTION OF SYMBOLS 1 Preparation process of 1st aqueous solution 2 Preparation process of 2nd aqueous solution 3 Mixing process of 1st aqueous solution and 2nd aqueous solution 4 Copper nanoparticle 11 Mixing process of pure water and copper chloride 12 Add citric acid Process 13 Process of adjusting pH to create first aqueous solution 14 Mixing process of pure water and ascorbic acid 15 Process of adjusting pH to create second aqueous solution 16 Mixing of first aqueous solution and second aqueous solution Treatment 17 Stirring while maintaining the temperature of the mixture 18 Treatment for filtering copper nanoparticles from the mixture

Claims (3)

A step of preparing a first aqueous solution containing copper ions and citric acid and adjusted to a range of pH 10 or more and less than pH 12,
A step of preparing a second aqueous solution containing ascorbic acid and adjusted to a range of pH 10 or more and less than pH 12,
Mixing the first aqueous solution and the second aqueous solution, and reducing the copper ions to obtain a dispersion of copper nanoparticles;
A method for producing copper nanoparticles, comprising: Wherein the first aqueous solution, which generates source of the copper ions dissolved in the aqueous solution of the powder, or is obtained by dissolving a copper plate, a manufacturing method of the copper nano-particles according to claim 1. The method for producing copper nanoparticles according to claim 2 , wherein the source of copper ions is selected from the group consisting of copper chloride, copper sulfate, and copper nitrate.

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