JP2010116626A - Method for synthesizing metal nanoparticle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To synthesize metal nanoparticles, which are suitably used as an electrically conductive material, from a metal salt which does not contain a corrosive material and is hardly soluble. <P>SOLUTION: A metal carboxylate aqueous solution 15 is prepared, and thereafter, the metal carboxylate aqueous solution 15 is mixed with carboxylic acid 13 of the kind same as that of the carboxylic acid contained in the aqueous solution so as to prepare a first carboxylate suspension 17. Next, the first carboxylate suspension 17 is mixed with a metal salt aqueous solution 14 containing a metal of the kind same as that of the metal contained in the metal carboxylate aqueous solution 15 so as to prepare a second carboxylate suspension 19, and then, the second carboxylate suspension 19 is mixed with a reducing agent aqueous solution 20, and heating reaction is applied to synthesize metal nanoparticles. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for synthesizing metal nanoparticles from an aqueous metal salt solution.

  Conventionally, metal nanoparticles having a particle size of several nanometers are dramatically different in melting point from those of bulk, and therefore are expected to be applied as conductive pastes that can be used by low-temperature firing. And in order to manufacture such a metal nanoparticle, the method of reducing a metal in a solvent conventionally is known. And, in order to prepare the target metal nanoparticles as a uniform dispersion, it is said that it is necessary to use a raw material metal salt (metal compound) that is soluble in the liquid (solvent). The raw materials have been limited. For example, when silver nanoparticles are obtained, silver nitrate and silver perchlorate have been used exclusively in aqueous solvents, and soluble compounds such as silver complexes have been exclusively used in organic solvents.

  However, as a raw material for silver, silver halide can be easily obtained from the viewpoint that it is already produced in large quantities as a photographic raw material, and it is easy to manage as a solid. It is preferable if the particles can be produced. From this point of view, it is a method for producing silver nanoparticles by reducing a silver salt in a solvent, using silver halide as the silver salt, and as a protective agent for the silver salt as a raw material, There has been proposed a method in which a metal coordination compound that dissolves and coordinates to silver is used as a protective material, and reduction is performed in the presence of a protective agent comprising the compound (see, for example, Patent Document 1). This is because it was found that by using a specific protective agent, a poorly soluble silver salt such as silver halide can be effectively reduced in a solvent to form nanoparticles. According to this method, it is said that silver nanoparticles can be produced from a hardly soluble silver salt.

Japanese Patent Laying-Open No. 2003-253311 (specifications [0002] to [0005])

  However, in the conventional method for producing metal nanoparticles using silver halide as the silver salt, the silver halide used as a raw material contains halogen causing corrosion, and sulfur is used as a part of the raw material. For this reason, when the dispersion liquid in which silver obtained by this production method is dispersed is used as an electronic device wiring as a paste or the like, there still remains a problem of lacking long-term stability to be solved. In particular, since a thiol-based compound is used as a protective agent for silver nanoparticles, it is expected that sulfur will remain in the wiring even after firing. Moreover, in the said invention, since ethanol which is an organic solvent is used for a reaction solvent, it is thought that there exists a subject in an environmental aspect or safety | security.

  An object of the present invention is to synthesize metal nanoparticles from a hardly soluble metal salt without considering corrosive materials and considering environmental and safety aspects, and synthesize metal nanoparticles suitable for use as a conductive material. The object is to provide a method for synthesizing metal nanoparticles.

  As shown in FIG. 1, the first aspect of the present invention is a step of preparing a carboxylic acid metal salt aqueous solution 15 and a carboxylic acid 13 of the same type as the carboxylic acid contained in the aqueous solution 15 in the carboxylic acid metal salt aqueous solution 15. To prepare a first carboxylate suspension 17 and a metal salt aqueous solution 14 containing the same kind of metal as the metal contained in the carboxylic acid metal salt aqueous solution 15 in the first carboxylate suspension 17. To prepare the second carboxylate suspension 19 and to mix the second carboxylate suspension 19 with the reducing agent aqueous solution 20 and heat-react it to synthesize metal nanoparticles. Is a method for synthesizing metal nanoparticles.

In the method for synthesizing metal nanoparticles according to the first aspect, all of the materials other than the raw material metal are composed of C, H, N, and O elements and do not include corrosive substances. For this reason, although metal nanoparticles are synthesized from a hardly soluble metal salt, metal nanoparticles that do not contain corrosive materials suitable for use as a conductive material can be obtained. The metal nanoparticles synthesized by this method can be sintered at a low temperature. For example, in the case of silver nanoparticles, an electrode having a volume resistivity of the order of 10 ⁻⁶ [Ω · cm] It becomes possible to form a conductive reflective film having a certain high reflectance at a low temperature.

  The second aspect of the present invention is an invention based on the first aspect, and is further mixed by stirring the reducing agent aqueous solution 20 in a temperature range of 25 to 95 ° C. The obtained reaction liquid is subjected to solid-liquid separation, and water and a solvent are added to the separated solid content to obtain the metal nanoparticle dispersion liquid 21.

  A third aspect of the present invention is an invention based on the first or second aspect, wherein the metal element contained in the aqueous metal salt solution 14 further contains 75% by mass or more of silver.

  A fourth aspect of the present invention is an invention based on the third aspect, wherein the metal element contained in the aqueous metal salt solution 14 is silver, gold other than silver, platinum, copper, iron, zinc, tin, palladium. And one or more metals selected from the group consisting of ruthenium.

  The fifth aspect of the present invention is the invention based on the first or second aspect, wherein the reducing agent contained in the reducing agent aqueous solution 20 further comprises hydrazine, ascorbic acid, oxalic acid, formic acid and salts thereof. It is characterized by being one or more compounds selected from more.

  A sixth aspect of the present invention is the invention based on the first or second aspect, wherein the reducing agent contained in the reducing agent aqueous solution 20 is selected from the group consisting of sodium borohydride, potassium borohydride and glucose. It is characterized by being one or more compounds.

  A seventh aspect of the present invention is the invention based on the first aspect, wherein the carboxylic acid 13 is selected from glycolic acid, citric acid, acetic acid, malic acid, maleic acid, malonic acid, fumaric acid, succinic acid and tartaric acid. It consists of 1 type, or 2 or more types selected from the group which consists of.

  According to an eighth aspect of the present invention, the metal nanoparticle dispersion liquid 21 obtained by the synthesis method according to the first to seventh aspects is applied as a metal film production composition on a substrate by a wet coating method, followed by firing. And forming a metal film. The method of manufacturing a metal film.

  In the method for synthesizing metal nanoparticles according to the first aspect of the present invention, the carboxylic acid is further mixed with the carboxylic acid metal salt aqueous solution, then the metal salt aqueous solution is mixed, and then the reducing agent aqueous solution is mixed and reacted therewith. Except for these metals, they are all composed of C, H, N, and O elements and do not contain corrosive substances. For this reason, it is possible to obtain metal nanoparticles that do not contain corrosive materials suitable for use as conductive materials, even though they are synthesized from poorly soluble metal salts, taking into consideration environmental and safety aspects. Can do.

It is a figure which shows the flow of the manufacturing method of embodiment of this invention. It is a figure which shows the flow of the manufacturing method of Comparative Examples 1-3.

Next, an embodiment for carrying out the present invention will be described with reference to the drawings.
A. As shown in FIG. 1, the synthesis method of the present invention includes a step of preparing a first carboxylate suspension 17 by mixing a carboxylic acid 13 with a carboxylic acid metal salt aqueous solution 15. In this embodiment, when the carboxylic acid metal salt aqueous solution 15 is prepared by mixing the carboxylic acid metal salt 11 with a basic aqueous solution or an acidic aqueous solution 12 and dissolving in this aqueous solution (hereinafter referred to as “first preparation”). And the carboxylic acid 13 mixed and dissolved in the metal salt aqueous solution 14 and then mixed with the basic aqueous solution or acidic aqueous solution 12 to adjust the pH (hereinafter referred to as “second preparation”). ”).

<Preparation process of carboxylic acid metal salt aqueous solution which is the first mixing process>
(1) First Preparation The carboxylic acid metal salt 11 used for this preparation is a powder. The carboxylic acid contained in this carboxylic acid metal salt 11 is selected from the group consisting of carboxylic acids such as glycolic acid, citric acid, acetic acid, malic acid, maleic acid, malonic acid, fumaric acid, succinic acid, tartaric acid, and salts thereof. Or one or more compounds. The reason why the carboxylic acid is limited to the above is that it functions well as a protective material for modifying the surface of the metal nanoparticles. Moreover, it is because a corrosive substance like thiol etc. is not included. Furthermore, it is because the pH at the time of synthesis | combination can be set to the base side by making these into salts, such as sodium, copper, and ammonia. Examples of the carboxylic acid metal salt 11 include silver citrate, silver acetate, copper acetate, ruthenium acetate, gold acetate, tin maleate, zinc tartrate, and copper tartrate. The metal salt preferably contains silver. When the mass of the whole metal element contained in the metal salt is 100%, the silver preferably occupies 75% by mass or more. Silver may be 100% by weight. When a metal other than silver is included, the element other than silver includes one or more metals selected from gold, platinum, copper, iron, zinc, tin, palladium, and ruthenium. By including silver and a metal other than silver, a mixture of different metal nanoparticles, an alloy, or a so-called core-shell structure in which the other forms an outer shell at the center of one element, in this case, The effect of controlling the reflectance and volume resistivity can be obtained.

The basic aqueous solution or acidic aqueous solution used for this preparation is preferably in the concentration range of 1 to 5M in order to facilitate handling. Examples of the basic aqueous solution include an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, and aqueous ammonia. Examples of the acidic aqueous solution include nitric acid, hydrochloric acid, and acetic acid. The 1st mixing which mixes carboxylic acid metal salt 11 with basic aqueous solution or acidic aqueous solution 12 is performed at 25-50 degreeC in the ratio which a carboxylic acid metal salt melt | dissolves completely.
(2) Second Preparation The carboxylic acid 13 used for this preparation is the same as the carboxylic acid contained in the carboxylic acid metal salt described in the first preparation. The metal salt aqueous solution 14 is prepared by dissolving a metal salt in deionized water at room temperature. Examples of this metal salt include platinum nitrate, platinum chloride, silver nitrate, zinc nitrate, silver acetate, and silver sulfide. The metal contained in this metal salt is the same as the metal described in the first preparation. Further, the basic aqueous solution or acidic aqueous solution used for this preparation is the same as described in the first preparation. When the powdered carboxylic acid 13 is added to and mixed with the aqueous metal salt solution 14 and stirred, metal ions form a carboxylic acid metal salt in the mixed solution. The mixing ratio is the ratio at which the metal element contained in the metal salt aqueous solution is the same as the chemical equivalent constituting the carboxylic acid and chloride, or 10 to 20% of the equivalent to 1 equivalent of carboxylic acid. A proportion is preferred. If it is less than the lower limit, the production of the carboxylic acid metal salt is insufficient and the yield is reduced. If the upper limit is exceeded, the size of the produced nanoparticles will be uneven. Moreover, it is preferable to perform the operation | work which mixes and stirs carboxylic acid in metal salt aqueous solution in the temperature range of 25-95 degreeC under atmospheric pressure, and it is still more preferable to carry out in the temperature range of 30-50 degreeC. After the carboxylic acid 13 is added and mixed in the aqueous metal salt solution 14 and dissolved, the second mixing in which the basic aqueous solution or the acidic aqueous solution 12 is mixed with this solution is performed by adjusting the pH of the solution with the basic aqueous solution or the acidic aqueous solution 12. It adjusts to 7-9 and is performed in order to dissolve the carboxylic acid metal salt precipitated in the solution.

  The first preparation method has the advantage of being simple and having fewer steps than the second preparation method. On the other hand, since the second preparation method does not use an expensive metal carboxylate as a starting material, there is an advantage that it is inexpensive.

<Second mixing step>
In this step, the carboxylic acid metal salt aqueous solution 15 obtained in the first mixing step is mixed with the carboxylic acid of the same type as the carboxylic acid contained in the aqueous solution, that is, the carboxylic acid 13 used in the second preparation. 1 Carboxylate suspension 17 is prepared. When carboxylic acid is mixed in the carboxylic acid metal salt aqueous solution 15, the solubility of the metal salt in the carboxylic acid metal salt aqueous solution 15 is lowered, and most of the carboxylic acid salt is precipitated in the aqueous solution. This mixing is performed by stirring the carboxylic acid metal salt aqueous solution 15 while adding the carboxylic acid 13. The proportion of the metal element contained in the carboxylic acid metal salt aqueous solution is preferably 0.2 to 5 equivalents relative to 1 equivalent of the carboxylic acid contained in the carboxylic acid metal salt aqueous solution 15. If it is less than the lower limit, the production of the carboxylic acid metal salt is insufficient and the yield is reduced. When the upper limit is exceeded, the pH in the reaction system decreases, which causes a problem that a large amount of pH adjusting solution is required when adjusting the pH during synthesis to the base side. Moreover, it is preferable to perform the operation | work which adds and stirs the carboxylic acid 13 to the carboxylic acid metal salt aqueous solution 15 in the temperature range of 25-95 degreeC under atmospheric pressure, and it is still more preferable to carry out in the temperature range of 30-50 degreeC.

<Third mixing step>
In this step, the first carboxylate suspension 17 obtained in the second mixing step is used in the second preparation for the metal salt aqueous solution containing the same type of metal as the metal contained in the carboxylic acid metal salt aqueous solution 15. A second carboxylate suspension 19 is prepared by mixing the existing aqueous metal salt solution 14. This mixing is performed by stirring while dropping the metal salt aqueous solution 14 into the first carboxylate suspension 17. The ratio of dropping is preferably such that the metal element contained in the metal salt aqueous solution 14 is 0.6 to 2 equivalents with respect to 1 equivalent of metal contained in the first carboxylate suspension 17. If the amount is less than the lower limit value, the carboxylic acid metal salt is not sufficiently formed, resulting in a problem that causes a decrease in yield. If the value exceeds the upper limit value, nanoparticles having irregular particle diameters are generated and the specific resistance is increased. Moreover, it is preferable to perform the operation | work which drops and stirs metal salt aqueous solution to the 1st carboxylate suspension 17 in the temperature range of 25-95 degreeC under atmospheric pressure, and is performed in the temperature range of 30-50 degreeC. Is more preferable.

<4th mixing process>
In this process, the reducing agent aqueous solution 20 is added dropwise to the second carboxylate suspension 19 obtained in the third mixing process and stirred, so that the metal carboxylate deposited in the suspension 19 is reduced. As a result, metal nanoparticles are formed. Here, the reducing agent aqueous solution 20 is prepared by dissolving the reducing agent in water as a solvent. Preferably, the reducing agent aqueous solution is prepared at room temperature by dissolving the reducing agent in deionized water. The reducing agent dissolved here consists of one or more compounds selected from the group consisting of hydrazine, ascorbic acid, oxalic acid, formic acid and their salts, or sodium borohydride, potassium borohydride and glucose. It is preferable that it is 1 type, or 2 or more types of compounds chosen from the group. Thereby, even if it does not contain a corrosive substance and remains in the product paste, it can be easily decomposed by firing.

  In the fourth mixing step, the metal carboxylate precipitated in the second carboxylate suspension 19 is reduced by stirring the reducing agent aqueous solution 20 while dropping it into the second carboxylate suspension 19. As a result, metal nanoparticles are formed. The ratio of dropping is preferably such that the reducing agent contained in the reducing agent aqueous solution 20 is 0.4 to 1.5 equivalents with respect to 1 equivalent of metal contained in the second carboxylate suspension 19. If the amount is less than the lower limit, a problem that the yield decreases is caused, and if the value exceeds the upper limit, the particle size of the generated nanoparticles increases and the specific resistance becomes high. Moreover, the operation | work which dripped the reducing agent aqueous solution 20 to the 2nd carboxylate suspension 19 and stirs it is 25-60 degreeC under atmospheric pressure, Preferably it is performed in the temperature range of 30-50 degreeC. When stirred in this temperature range, the average particle size of the generated particles can be reduced to 30 nm or less, and when the film formed using the obtained dispersion using the metal nanoparticles as the electrode-forming composition, the temperature is low. A low volume resistivity can be achieved. Moreover, it is preferable to perform dripping of the reducing agent aqueous solution 20 with respect to the 2nd carboxylate suspension 19 within 10 minutes. When the dropping time exceeds the upper limit value, the particle size of the produced nanoparticles increases, causing a problem that the specific resistance increases.

  Dispersion stability can be increased by removing excess salts from the reaction solution containing the metal nanoparticles after reduction using various separation techniques. Examples of the method for separating excess salts include centrifugation, ultrafiltration, ion exchange membranes, ion exchange resins, and the like. And after apply | coating this metal nanoparticle, the volume resistivity of the electrode obtained by baking generally tends to approach the value of a bulk metal, so that excess salts are removed.

In the method for synthesizing metal nanoparticles of the present invention comprising such steps, metal nanoparticles can be synthesized from a hardly soluble metal salt. In the present invention, the raw material is used to form a salt with a raw material metal and carboxylic acid, or when sodium borohydride or the like is used as a reducing agent, such as sodium or calcium as a basic compound in a basic aqueous solution. Except for boron, it is composed of C, H, N, and O elements and does not contain corrosive substances. For this reason, it becomes possible to obtain metal nanoparticles suitable for use as a conductive material without containing corrosive materials. Moreover, the metal nanoparticles synthesized by the method of the present invention can be sintered at a low temperature. For example, in the case of silver nanoparticles, it is possible to form an electrode having a volume resistivity of the order of 10 ⁻⁶ [Ω · cm] and a conductive reflective film having a high reflectance that is characteristic of silver at a low temperature. .
B. Next, a method for producing a metal film using the metal nanoparticles obtained by the method for synthesizing metal nanoparticles will be described. Specifically, the metal film production method includes a dispersion step of dispersing metal nanoparticles obtained by the synthesis method in a dispersion medium to obtain a metal nanoparticle dispersion liquid, and producing the metal nanoparticle dispersion liquid from a metal film. And a metal film forming step of forming a metal film by applying a wet coating method on a substrate as a composition for use. Details of these steps will be described below.

<Dispersing process>
In this step, a metal nanoparticle dispersion is prepared by adding and mixing the metal nanoparticles obtained by the above synthesis method into a dispersion medium and dispersing the particles in the dispersion medium. The content of the metal nanoparticles including silver nanoparticles in the dispersion is 2.5 to 95.0% by mass, preferably 3.5 to 90% with respect to 100% by mass of the composition comprising the metal nanoparticles and the dispersion medium. It is prepared so that it may contain 0.0 mass%. The dispersion medium may contain 1% by mass or more, preferably 2% by mass or more of water and 2% by mass or more, preferably 3% by mass or more of alcohols with respect to 100% by mass of all the dispersion media. Is preferred. For example, when the dispersion medium is composed of only water and alcohols, it contains 98% by mass of alcohol when it contains 2% by mass of water, and 98% by mass of water when it contains 2% by mass of alcohol.

  Here, the content of the metal nanoparticles including the silver nanoparticles was limited to the range of 2.5 to 95.0% by mass with respect to 100% by mass of the composition composed of the metal nanoparticles and the dispersion medium. If it is less than 5% by mass, the properties of the metal film after firing are not particularly affected, but it is difficult to obtain a metal film having a required thickness. If it exceeds 95.0% by mass, ink or This is because the necessary fluidity as a paste is lost. In addition, the content of water is limited to a range of 1% by mass or more with respect to 100% by mass of all the dispersion media. When the content is less than 1% by mass, a film obtained by applying the composition by a wet coating method is used. It is difficult to sinter at low temperatures, and the conductivity and reflectance of the fired metal film are reduced, so that the content of alcohols is limited to a range of 2% by mass or more with respect to 100% by mass of all dispersion media. If it is less than 2% by mass, it is difficult to sinter the film obtained by applying the composition by the wet coating method at a low temperature in the same manner as described above, and the conductivity and reflectance of the fired metal film are reduced. Because it will do.

  The alcohols are preferably one or more selected from the group consisting of methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, diethylene glycol, glycerol, isobornyl hexanol and erythritol. By passing through the above steps, a dispersion of metal nanoparticles having a uniform shape and particle size can be obtained.

<Metal film formation process>
This step is a step of forming a metal film using the metal nanoparticle dispersion described above. First, the metal nanoparticle dispersion is used as a metal film-forming composition, and this metal film-forming composition is applied onto a substrate by a wet coating method. Coating by this wet coating method is performed so that the thickness after firing is in the range of 0.1 to 2.0 μm, preferably 0.3 to 1.5 μm. The substrate is selected from the group consisting of silicon, glass, ceramics containing a transparent conductive material, a polymer material or a metal substrate, or silicon, glass, ceramics containing a transparent conductive material, a polymer material and a metal. It can be a laminate of two or more types. Moreover, it is preferable that a base material is either a solar cell element or a solar cell element with a transparent metal film. As the transparent metal film, indium tin oxide (ITO), antimony-doped tin oxide (ATO), Nesa (tin oxide SnO ₂ ), IZO (Indium Zic Oxide), AZO (aluminum-doped ZnO) ) And the like. It is preferable that the said metal film formation composition is apply | coated to the surface of the photoelectric conversion semiconductor layer of a solar cell element, or the surface of the transparent metal film of a solar cell element with a transparent metal film.

  The wet coating method is particularly preferably a spray coating method, a dispenser coating method, a spin coating method, a knife coating method, a slit coating method, an inkjet coating method, a screen printing method, an offset printing method or a die coating method. However, the present invention is not limited to this, and any method can be used. The spray coating method is a method in which the metal film forming composition is made into a mist form with compressed air and applied to the substrate, or the metal film forming composition itself is pressurized into a mist form and applied to the substrate. The coating method is, for example, a method in which a metal film forming composition is placed in a syringe and a piston of the syringe is pushed to discharge the metal film forming composition from a fine nozzle at the tip of the syringe and apply it to a substrate. The spin coating method is a method in which a metal film forming composition is dropped on a rotating substrate, and the dropped metal film forming composition is spread on the periphery of the substrate by its centrifugal force. The knife coating method is a knife coating method. A base material with a predetermined gap is provided so as to be movable in the horizontal direction, and the metal film forming composition is supplied onto the base material upstream of this knife, and the base material is moved horizontally toward the downstream side. It is a method to make it.

  The slit coating method is a method in which a metal film forming composition is allowed to flow out of a narrow slit and applied onto a substrate. The ink jet coating method is performed by filling a metal film forming composition into an ink cartridge of a commercially available ink jet printer, This is a method of ink-jet printing on a material. The screen printing method is a method in which a metal film forming composition is transferred to a substrate through a plate image formed thereon using wrinkles as a pattern indicating material. The offset printing method utilizes the water repellency of the ink so that the metal film forming composition attached to the plate is not directly attached to the substrate, but is transferred from the plate to the rubber sheet and then transferred again from the rubber sheet to the substrate. It is a printing method. The die coating method is a method in which a metal film forming composition supplied into a die is distributed by a manifold and extruded from a slit onto a substrate, and the surface of the traveling substrate is applied. The die coating method includes a slot coat method, a slide coat method, and a curtain coat method.

  Next, the base material formed on the upper surface is fired in the air at a temperature of 130 to 400 ° C., preferably 140 to 200 ° C., for 10 minutes to 1 hour, preferably 15 to 40 minutes. Here, the film thickness of the coating film formed on the substrate is limited so that the thickness after firing is in the range of 0.1 to 2.0 μm. If it is a battery, the surface resistance value of the metal film required for it will be insufficient, and if it exceeds 2.0 μm, there will be no problem in characteristics, but the amount of material used will be more than necessary and the material will be wasted. is there. Moreover, the reason for limiting the firing temperature of the coating film formed on the substrate to the range of 130 to 400 ° C. is that the sintering between metal nanoparticles becomes insufficient when the temperature is lower than 130 ° C., and the low temperature process when the temperature exceeds 400 ° C. This is because the production merit cannot be utilized, that is, the manufacturing cost increases and the productivity decreases. Furthermore, the firing time of the coating film formed on the base material was limited to the range of 10 minutes to 1 hour because the sintering between the metal nanoparticles became insufficient when the time was less than 10 minutes, and the property was exceeded when the time exceeded 1 hour. This is because the manufacturing cost increases more than necessary and the productivity decreases.

The metal nanoparticle dispersion liquid that is the metal film forming composition contains a large amount of metal nanoparticles having a primary particle size of 10 to 50 nm and a relatively large size, so that the specific surface area of the metal nanoparticles is reduced. As a result, when a metal film of a solar cell is formed using the metal film forming composition, a metal film containing silver as a main component and containing substantially no organic substance can be obtained. Therefore, even if the solar cell on which the metal film is formed is used for many years, the organic matter is not deteriorated or deteriorated, and the conductivity and reflectance of the metal film are maintained in a high state. An excellent metal film can be obtained. Specifically, an electromagnetic wave having a wavelength of 750 to 1500 nm, that is, a visible light region even after the metal film is accommodated in a constant temperature and humidity chamber having a temperature of 100 ° C. and a humidity of 50%. 80% or more of the electromagnetic wave from the infrared region to the infrared region can be reflected by the metal film, and the conductivity of the metal film, that is, the volume resistivity of the metal film is less than 2 × 10 ⁻⁵ Ω · cm (20 × 10 ⁻⁶ Ω · cm) And can be maintained at a very low value. The solar cell using the metal film formed in this way can maintain high conductivity and high reflectance even when used for many years, and is excellent in aging stability.

  Next, examples of the present invention will be described in detail together with comparative examples.

<Examples 1 to 14>
First, according to the flowchart shown in FIG. 1, the carboxylic acid metal salt aqueous solution 15 was prepared by the second preparation method. Specifically, the carboxylic acid 13 shown in Table 1 is added to the aqueous solution 14 of a metal salt (silver nitrate, gold chloride, platinum chloride, palladium nitrate, ruthenium nitrate) containing Ag, Au, Pt, Pd, and Ru shown in Table 1 below. After mixing and dissolving, the aqueous solution of carboxylic acid metal salt 15 was prepared by adjusting the pH to 8.0 by mixing one or two kinds of aqueous ammonia, aqueous sodium hydroxide and nitric acid as a basic aqueous solution or acidic aqueous solution. (First mixing). The metal salt aqueous solution was prepared as a saturated aqueous solution at room temperature by dissolving the metal salt in deionized water. Subsequently, the carboxylic acid 13 same as the above was mixed into the carboxylic acid metal salt aqueous solution 15 to prepare a first carboxylate suspension 17 (second mixture). Next, the first carboxylate suspension 17 was mixed with the same metal salt aqueous solution 14 as described above to prepare a second carboxylate suspension 19 (third mixture). Furthermore, the aqueous solution 20 of the reducing agent shown in Table 1 was mixed with the second carboxylate suspension 19 and heated to the reaction temperature (55 to 65 ° C.) shown in Table 1 using a water bath. The reducing agent aqueous solution was prepared as a saturated aqueous solution at room temperature by dissolving the reducing agent in deionized water. After reaching the reaction temperature, stirring was continued with a magnetic stirrer at a rate of 400 rpm for a further 4 hours under reflux (fourth mixing). This obtained the metal nanoparticle dispersion liquid.

  As described above, the metal salt aqueous solution and the carboxylic acid were mixed twice in the first mixing, the second mixing, and the third mixing. In Table 1, the ratio of “mixing 1” and “mixing 3” shown in the column of the mixing ratio of carboxylic acids was mixed at the time of the first mixing when the molar amount of all carboxylic acids mixed by this method was 1. The ratio of the carboxylic acid mixed with the carboxylic acid during the third mixing is converted by molar ratio. Similarly, the ratio of “mixing 1” and “mixing 2” shown in the column of the mixing ratio of the metal salts is the same as that of the metal salt mixed at the first mixing when the molar amount of all the metal salts mixed by this method is 1. And the ratio of the metal salt mixed at the time of the second mixing in terms of molar ratio. Furthermore, the molar ratio shown in the column of the reducing agent refers to the molar ratio of the reducing agent to the carboxylate contained in the second carboxylate suspension 19.

<Comparative Examples 1-3>
According to the flow chart shown in FIG. 2, the carboxylic acid aqueous solution shown in Table 1 was mixed and dissolved in the aqueous solution of the metal salt (silver nitrate, gold chloride, platinum chloride) containing Ag, Au, and Pt shown in Table 1 (first mixing). One or two types of aqueous ammonia, aqueous sodium hydroxide, and nitric acid were mixed and dissolved in this mixed solution as a basic aqueous solution or an acidic aqueous solution (second mixing). An aqueous solution of a reducing agent shown in Table 1 was mixed with the obtained carboxylate suspension (third mixing), and a heat reaction was performed in the same manner as in Examples 1 to 14 to obtain a metal nanoparticle dispersion. That is, in Comparative Examples 1 to 3, the metal salt aqueous solution and the carboxylic acid aqueous solution were each mixed once.

  In Examples 1 to 14 and Comparative Examples 1 to 3, after the reaction for 4 hours, the dried product of the reaction solution was evaluated by an X-ray diffraction method. The metal carboxylate added to the dried product of any reaction solution was not detected. In Table 1, the molar ratio of the reducing agent is the molar ratio when the total metal element input amount is 1 mole.

＜比較試験及び評価＞
反応終了後における実施例１〜１４，比較例１〜３の反応液を１０００Ｇで３分間遠心分離した。得られた沈殿物に、沈殿物質量の５０倍の０．１Ｍアンモニア水を加えて１０分間撹拌した後、再度１０００Ｇで３分間遠心分離した。この沈殿物に水を加え、更に限外ろ過法を用いて脱塩した後、エタノールやメタノール等の溶媒を加えて撹拌し、金属ナノ粒子分散液を得た。得られた実施例１〜１４，比較例１〜３における分散液の組成を表２に示す。

<Comparison test and evaluation>
After completion of the reaction, the reaction solutions of Examples 1 to 14 and Comparative Examples 1 to 3 were centrifuged at 1000 G for 3 minutes. To the obtained precipitate, 0.1 M ammonia water 50 times the amount of the precipitated substance was added and stirred for 10 minutes, and then centrifuged again at 1000 G for 3 minutes. Water was added to the precipitate and further desalted using an ultrafiltration method, and then a solvent such as ethanol or methanol was added and stirred to obtain a metal nanoparticle dispersion. Table 2 shows the compositions of the dispersions obtained in Examples 1 to 14 and Comparative Examples 1 to 3.

  The dispersions in Examples 1 to 14 and Comparative Examples 1 to 3 were applied on the base material shown in Table 2 using the coating method shown in Table 2, and in the atmosphere at the firing temperature shown in Table 2 for 30 minutes. Each was fired. Table 2 shows the average particle size of the metal nanoparticles, the composition of the dispersion, the coating method, the type of the substrate, the film thickness after firing, the firing temperature, and the volume resistivity of the obtained coating film. As a method for measuring the average particle diameter of metal nanoparticles, first, the obtained metal nanoparticles are photographed with a TEM (Transmission Electron Microscope) at a magnification of about 500,000 times. Next, a primary particle size is measured for 200 metal nanoparticles from the obtained image, and a particle size distribution is created based on the measurement result. Next, the average value obtained from the created particle size distribution was defined as the average particle size. The volume resistivity was measured and calculated by the four probe method, and the film thickness was measured with a micrometer.

  In the column of alcohols in Table 2, “ME” represents methanol, “ET” represents ethanol, “EG” represents ethylene glycol, “BU” represents butanol, and “PG” represents propylene glycol. “DEG” indicates diethylene glycol, “GL” indicates glycerose, “ER” indicates erythritol, “IH” indicates isobornylhexanol, and “PR” indicates propanol.

Further, in the column of other solvents in Table 2, “A” represents a mixed solution of acetone and isopropyl glycol mixed at a mass ratio of 1: 1, and “B” represents cyclohexane and methyl ethyl ketone at a mass ratio of 1: 1 indicates a mixed liquid, and “C” indicates a mixed liquid in which toluene and hexane are mixed at a mass ratio of 1: 1.

表２から明らかなように、実施例１〜１４では、塗膜の体積抵抗率は、いずれも１０^-6Ω・ｃｍオーダーであり、導電材料として好適な値を示した。また、平均粒径は２０〜４０ｎｍの範囲にあり、いわゆる金属ナノ粒子を形成した。中でもカルボン酸と金属塩を０．４：０．６〜０．５：０．５の割合で２回に分けて混合した場合（実施例３、４、６、７、１０、１１、１４）ナノ粒子の平均粒径は他の混合比の粒径と比べて１０〜２０ｎｍ小さな粒子が得られることを確認した。
一方、原料のカルボン酸と金属塩を２回に分けずに混合した比較例１〜３では、平均粒径が約５０ｎｍで実施例と比較して粒径が大きく、また体積抵抗率は８．７〜１０．４×１０^-6Ωｃｍと実施例１〜１４の３．９〜７．７×１０^-6Ωｃｍより高く、導電材料として好適とはいえなかった。

As is clear from Table 2, in Examples 1 to 14, the volume resistivity of the coating film was on the order of 10 ⁻⁶ Ω · cm, and showed a suitable value as a conductive material. Moreover, the average particle diameter was in the range of 20 to 40 nm, and so-called metal nanoparticles were formed. In particular, when the carboxylic acid and the metal salt are mixed in two portions at a ratio of 0.4: 0.6 to 0.5: 0.5 (Examples 3, 4, 6, 7, 10, 11, 14) It was confirmed that nanoparticles having an average particle size of 10 to 20 nm smaller than the particle size of other mixing ratios were obtained.
On the other hand, in Comparative Examples 1 to 3 in which the raw carboxylic acid and the metal salt were mixed without being divided into two, the average particle size was about 50 nm, the particle size was larger than that of the Example, and the volume resistivity was 8. 7 to 10.4 × 10 ⁻⁶ Ωcm, which is higher than 3.9 to 7.7 × 10 ⁻⁶ Ωcm of Examples 1 to 14, and was not suitable as a conductive material.

11 Carboxylic acid metal salt 12 Basic aqueous solution or acidic aqueous solution 13 Carboxylic acid 14 Metal salt aqueous solution 15 Carboxylic acid metal salt aqueous solution 17 First carboxylate suspension 19 Second carboxylate suspension 20 Reducing agent aqueous solution 21 Metal nano Particle dispersion

Claims (8)

Preparing a carboxylic acid metal salt aqueous solution (15);
Mixing the carboxylic acid metal salt aqueous solution (15) with the carboxylic acid (13) of the same kind as the carboxylic acid contained in the aqueous solution (15) to prepare a first carboxylate suspension (17);
The first carboxylate suspension (17) is mixed with the metal salt aqueous solution (14) containing the same metal as the metal contained in the carboxylic acid metal salt aqueous solution (15) to obtain the second carboxylate suspension. Preparing (19);
Synthesizing metal nanoparticles by mixing the second carboxylate suspension (19) with a reducing agent aqueous solution (20) and reacting with heating.   The reducing agent aqueous solution (20) is mixed by stirring in the temperature range of 25 to 95 ° C. under atmospheric pressure to react with the second carboxylate suspension (19), and the resulting reaction solution is separated into solid and liquid. The method for synthesizing metal nanoparticles according to claim 1, wherein water and a solvent are added to the separated solid to obtain a metal nanoparticle dispersion liquid (21).   The method for synthesizing metal nanoparticles according to claim 1 or 2, wherein the metal element contained in the metal salt aqueous solution (14) contains 75 mass% or more of silver.   The metal element contained in the aqueous metal salt solution (14) contains silver and one or more metals selected from the group consisting of gold, platinum, copper, iron, zinc, tin, palladium and ruthenium other than silver. The method for synthesizing metal nanoparticles according to claim 3.   The metal according to claim 1 or 2, wherein the reducing agent contained in the reducing agent aqueous solution (20) is one or more compounds selected from the group consisting of hydrazine, ascorbic acid, oxalic acid, formic acid and salts thereof. Synthesis method of nanoparticles.   The metal nanoparticles according to claim 1 or 2, wherein the reducing agent contained in the reducing agent aqueous solution (20) is one or more compounds selected from the group consisting of sodium borohydride, potassium borohydride and glucose. Synthesis method.   The carboxylic acid (13) comprises one or more selected from the group consisting of glycolic acid, citric acid, acetic acid, malic acid, maleic acid, malonic acid, fumaric acid, succinic acid and tartaric acid. Synthesis method of metal nanoparticles.   A metal nanoparticle dispersion (21) obtained by the synthesis method according to any one of claims 1 to 7 is coated on a substrate by a wet coating method as a composition for producing a metal film, and fired to form a metal. Forming a film, and a method of manufacturing the metal film.

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