WO2022209613A1

WO2022209613A1 - Silver nanowire manufacturing method

Info

Publication number: WO2022209613A1
Application number: PCT/JP2022/009791
Authority: WO
Inventors: 葵長谷川; 繁山木; 正彦鳥羽
Original assignee: 昭和電工株式会社
Priority date: 2021-03-31
Filing date: 2022-03-07
Publication date: 2022-10-06
Also published as: TW202245934A

Abstract

[Problem] To provide a silver nanowire manufacturing method by which silver nanoparticles can be efficiently removed from a coarse dispersion liquid containing silver nanowires and the silver nanoparticles. [Solution] A silver nanowire manufacturing method characterized by including a coarse dispersion liquid preparation step for preparing a coarse dispersion liquid containing silver nanowires and silver nanoparticles and a reprecipitation/cleaning step for refining the silver nanowires in the coarse dispersion liquid by means of a reprecipitation method, wherein in the reprecipitation/cleaning step, a series of operations, which comprise (a) a precipitation step for causing silver nanowire-containing precipitates to precipitate by adding a precipitation solvent to the coarse dispersion liquid or to a redispersion liquid described below, (b) a supernatant removal step for removing a supernatant containing at least some of the silver nanoparticles formed by the precipitation of the precipitates, and (c) a redispersion step for obtaining a redispersion liquid by redispersing the precipitates in water by adding water having a specific resistance value of 3.3 MΩ·cm or higher to the remaining precipitates, are repeated several times.

Description

Method for producing silver nanowires

The present invention relates to a method for producing silver nanowires.

　In recent years, silver nanowires have attracted attention as a raw material for highly transparent and highly conductive thin films that can replace the ITO (indium tin oxide) films used in transparent electrodes such as touch panels. Such silver nanowires are generally produced by a so-called polyol reduction method in which a silver compound is heated in the presence of polyvinylpyrrolidone and a polyol such as ethylene glycol (Patent Document 1, Non-Patent Document 1).

When silver nanowires are produced by the polyol reduction method, in addition to the metal nanowires, the synthetic solution contains a polyol solvent, a polymer used as a structure-directing agent, and by-product silver nanoparticles. There is a need. In particular, by-produced silver nanoparticles degrade the transparency of the transparent conductive film, so it is desirable to remove them as much as possible.

A known method for purifying silver nanowires is filtration (dead end filtration) or centrifugal sedimentation of the dispersion of silver nanowires obtained by synthesis. However, in this method, since stress is applied to the silver nanowires during isolation, there is a problem that the silver nanowires tend to agglomerate and re-dispersion becomes difficult as the production scale increases.

The so-called reprecipitation method is suitable for large-scale purification. For example, in Patent Document 2, acetone is used as a poor solvent and water (ion-exchanged water) is used as a good solvent, and the steps of precipitation/supernatant removal/resuspension are repeated several times to obtain a dispersion mainly containing silver nanowires. disclosed.

However, in the reprecipitation method, it is necessary to repeat the steps of precipitation/supernatant removal/resuspension in order to sufficiently remove by-products such as nanoparticles. If it is not below, the silver nanoparticles will not be dispersed in the supernatant, and there is a problem that the number of times of washing increases.

U.S. Pat. No. 7,585,349 Japanese translation of PCT publication No. 2013-502515

Therefore, an object of the present invention is to provide a method for producing silver nanowires that can reduce the number of reprecipitation washings and efficiently remove silver nanoparticles from a coarse dispersion containing silver nanowires and silver nanoparticles. to do.

In order to achieve the above object, as a result of investigation by the present inventors, the number of reprecipitation washings can be reduced by using water as a good solvent for the reprecipitation method and setting the specific resistance value of the water to a certain level or more. I found that it works. That is, the present invention includes the following embodiments.

[1] including a coarse dispersion preparation step of preparing a coarse dispersion containing silver nanowires and silver nanoparticles, and a reprecipitation washing step of purifying the silver nanowires in the coarse dispersion by a reprecipitation method, A method for producing silver nanowires, wherein the reprecipitation washing step repeats a series of operations consisting of the following steps (a), (b), and (c) multiple times: (a) the coarse dispersion or the reprecipitation described later; A sedimentation step of sedimenting a precipitate containing silver nanowires by adding a sedimentation solvent to the dispersion; (b) removing a supernatant liquid containing at least a portion of the silver nanoparticles produced by the sedimentation of the precipitate; (c) a redispersion step of adding water having a resistivity of 3.3 MΩ·cm or more to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid;

[2] The silver according to [1], wherein the series of operations consisting of steps (a), (b), and (c) is repeated until the number of silver nanowires in the redispersion/total number of particles>90%. A method for producing nanowires.

[3] The reprecipitation washing step includes (c′) adding water having a specific resistance of less than 3.3 MΩ·cm to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid. The method for producing silver nanowires according to [1] or [2], which does not include a dispersion step.

[4] Repeating the series of operations consisting of the steps (a), (b) and (c) n times, water having a specific resistance of 18 MΩ cm or more is added at least once in the second and subsequent (c) redispersion steps. The method for producing silver nanowires according to any one of [1] to [3].

[5] When absorption based on silver nanoparticles near 405 nm is observed in the supernatant liquid generated in step (a) in a series of operations from the second time onwards, in step (c) after that, the specific resistance value is 18 MΩ cm or more. The method for producing silver nanowires according to [4], wherein the water of

[6] The method for producing silver nanowires according to any one of [1] to [5], wherein the rough dispersion preparation step includes a step of synthesizing silver nanowires by a polyol reduction method.

[7] The method for producing silver nanowires according to any one of [1] to [6], wherein the precipitation solvent is at least one of a ketone solvent and an organic ester solvent.

[8] The precipitation solvent is at least one selected from the group consisting of acetone, methyl ethyl ketone, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, and propylene glycol monomethyl ether acetate. ] The manufacturing method of the silver nanowire as described in .

According to the present invention, when producing silver nanowires by purifying a silver nanowire coarse dispersion containing silver nanowires and silver nanoparticles using a reprecipitation method, the silver nanowires are washed with a smaller number of reprecipitation washings than before. can be manufactured.

FIG. 10 is a diagram showing absorption spectrum measurement results of supernatant liquids removed in each supernatant removal step (b) in a series of operations of 3rd to 8th reprecipitation washings in the reprecipitation washing step of Example 4;

Hereinafter, modes for carrying out the present invention (hereinafter referred to as embodiments) will be described.

The method for producing silver nanowires according to the present embodiment includes a coarse dispersion preparation step of preparing a coarse dispersion containing silver nanowires and silver nanoparticles, and reprecipitation to refine the silver nanowires in the coarse dispersion. and a reprecipitation washing step, wherein the reprecipitation washing step has a series of operations consisting of the following steps (a), (b), and (c), and the series of operations is repeated multiple times. do.
(a) a sedimentation step of precipitating a precipitate containing silver nanowires by adding a precipitating solvent to the coarse dispersion or the redispersion described later;
(b) a supernatant removing step of removing a supernatant containing at least part of the silver nanoparticles produced by the precipitation of the precipitate;
(c) a redispersion step of redispersing the precipitate in water by adding water having a specific resistance of 3.3 MΩ·cm or more to the remaining precipitate;

(Crude dispersion preparation step)
The method for producing silver nanowires according to the present embodiment first has a coarse dispersion preparation step of preparing a coarse dispersion containing silver nanowires and silver nanoparticles. This coarse dispersion preparation step includes a step of synthesizing the desired silver nanowires. The process for synthesizing silver nanowires is not particularly limited, and known methods can be applied.

For example, it can be synthesized by reducing silver nitrate in the presence of poly-N-vinylpyrrolidone using a poly-ol reduction method (see Chem. Mater., 2002, 14, 4736). Preferably, the production method previously disclosed in WO2017/057326 by the present applicant, that is, a first solution containing an ionic derivative (containing a polyol as a solvent) is kept at 80 to 200 ° C., and the first solution is Then, a second solution (containing a polyol as a solvent) containing a metal salt (silver nitrate) is added to the total number of moles of halogen atoms of the ionic derivative in the first solution and the number of metal atoms of the metal salt supplied for 1 minute. preferably 1 or less, more preferably 0, so that the molar ratio (moles of metal atoms in the metal salt supplied per minute/total number of moles of halogen atoms in the ionic derivative) is less than 10 .22 or less, and the molar ratio between the total number of moles of halogen atoms in the ionic derivative in the first solution and the number of moles of metal atoms in the metal salt (the number of moles of metal atoms in the metal salt/ion (total number of moles of halogen atoms in the organic derivative) is less than 10, and a (co)polymer containing a monomer unit derived from N-vinylpyrrolidone is added as a structure-directing agent to the first solution or the second solution. A method of keeping in at least one of the solutions can be applied. The reaction pressure is usually normal pressure (atmospheric pressure).

The reaction solvent used in the above polyol reduction method includes polyols used as reducing agents, such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, 1 , 2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, glycerin, etc., and at least one selected from the group consisting of these is preferred. Dihydric alcohols are more preferable from the viewpoint of avoiding high viscosity, and among these, ethylene glycol and propylene glycol are more preferable from the viewpoint of economy. The reaction solution after the synthesis reaction contains by-produced silver nanoparticles in addition to the ionic derivative, structure-directing agent, and solvent used in the synthesis together with the desired silver nanowires.

Synthetic silver nanowires are metallic silver with diameters on the order of nanometers, and are conductive materials with linear shapes (including hollow tubular silver nanotubes). In addition, it is preferable that the metal silver of the silver nanowire does not contain a metal oxide in terms of conductive performance, but if air oxidation cannot be avoided, a part (at least a part of the surface) may contain a silver oxide. . The length (diameter) in the minor axis direction of the silver nanowires is preferably an average of 5 nm or more and 90 nm or less, more preferably an average of 10 nm or more and 85 nm or less, still more preferably an average of 10 nm or more and 70 nm or less, particularly preferably an average of 10 nm or more and 50 nm or less. The length in the major axis direction is preferably 1 μm or more and 100 μm or less on average, more preferably 5 μm or more and 95 μm or less on average, still more preferably 5 μm or more and 70 μm or less on average, and particularly preferably 5 μm or more and 50 μm or less on average. As used herein, the term “silver nanowire” means that the aspect ratio represented by a/b is 5 or more, where a is the length in the major axis direction and b is the length (diameter) in the minor axis direction. It is preferably 10 or more, more preferably 50 or more, and even more preferably 100 or more. In addition, the term "silver nanoparticles" as used herein means particles other than the above-mentioned "silver nanowires", has an aspect ratio of less than 5, and is a by-product of the synthesis of silver nanowires. It means a particulate one excluding "silver nanowires".

The ionic derivative is a component that contributes to the growth of metal wires (silver nanowires), and can be applied as long as it is a compound that dissolves in a solvent and can dissociate halogen ions. compounds are preferred. Halogen ions are preferably at least one of chloride ions, bromide ions, and iodine ions, and more preferably contain a compound capable of dissociating chloride ions.

Halides of quaternary ammonium salts include quaternary alkylammonium salts having a total number of carbon atoms of 4 to 20 in the molecule (four alkyl groups are bonded to the nitrogen atom of the quaternary ammonium salt, and each alkyl group is which may be the same or different) are preferred, for example, quaternary ammonium chloride such as tetramethylammonium chloride, tetraethylammonium chloride, tetrapropylammonium chloride, tetrabutylammonium chloride, octyltrimethylammonium chloride, hexadecyltrimethylammonium chloride Chlorides, and quaternary ammonium bromides such as tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide, tetrabutylammonium bromide, octyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide. Any one of these may be used alone, or two or more of them may be used in combination. Ammonium salts obtained by reacting quaternary ammonium hydroxide with hydrogen chloride, hydrogen bromide, and hydrogen iodide can also be used. Since these are in a gaseous state at room temperature, they may be neutralized using an aqueous solution thereof in a polyol solvent, and water and excess hydrogen halide can be distilled off by heating after neutralization.

Among these, halides of quaternary alkylammonium salts having 4 to 16 total carbon atoms in the molecule are more preferable in terms of solubility and efficiency of use, and the longest alkyl chain attached to the nitrogen atom has the highest number of carbon atoms. Halides of quaternary alkylammonium salts having a molecular weight of 12 or less, more preferably 8 or less, are more preferable in terms of efficiency in use because the molecular weight does not become particularly large. From the viewpoint of the wire shape obtained, tetramethylammonium chloride, tetramethylammonium bromide, tetraethylammonium chloride, tetraethylammonium bromide, tetrapropylammonium chloride, tetrapropylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium bromide, Octyltrimethylammonium chloride and octyltrimethylammonium bromide are particularly preferred.

Examples of metal halides include alkali metal halides, alkaline earth metal halides, and metal halides of groups 3 to 12 of the long periodic table.

Alkali metal halides include alkali metal chlorides such as lithium chloride, sodium chloride and potassium chloride, alkali metal bromides such as lithium bromide, sodium bromide and potassium bromide, lithium iodide, sodium iodide and potassium iodide. and alkali metal iodides such as Alkaline earth metal halides include magnesium chloride, magnesium bromide and calcium chloride. Metal halides of groups 3 to 12 of the long periodic table include ferric chloride, cupric chloride, ferric bromide, and cupric bromide. Any one of these may be used alone, or two or more of them may be used in combination.

Among these, it is particularly preferable to contain a compound that dissociates chloride ions for the production of silver nanowires. In order to obtain silver nanowires with a small diameter, it is preferable to use a compound that dissociates chlorine ions and at least one of a compound that dissociates bromide ions and a compound that dissociates iodine ions in combination. When the total number of moles of chlorine atoms in the compound that dissociates chlorine ions is (A), and the total number of moles of iodine atoms in the compound that dissociates bromine atoms and iodine ions is (B), ( When the molar ratio of A)/(B) increases, the wire diameter increases, and when it decreases, the wire diameter decreases. Therefore, the molar ratio of (A)/(B) is preferably 2-8, more preferably 3-6.

The structure-directing agent used in the synthesis is a compound that has the function of one-dimensionally defining the growth direction of the metal particles during synthesis. By using the structure-directing agent, the ratio of silver nanowires formed in the particle formation process can increase In many cases, the structure-directing agent preferentially or selectively adsorbs to specific crystal planes of the target grain and controls the growth orientation by suppressing the growth of the adsorbed planes. This growth orientation can be controlled by adding a structure-directing agent to the polyols and adsorbing it on the surface of the silver nanowires to be produced. The structure-directing agent is preferably a polymer having a weight-average molecular weight of more than 1,000, more preferably 2,000 or more, and even more preferably 10,000 or more. On the other hand, if the weight-average molecular weight of the structure-directing agent is too large, the silver nanowires are more likely to aggregate. Therefore, the weight average molecular weight of the structure-directing agent is preferably 1,500,000 or less, more preferably 1,000,000 or less, and even more preferably 500,000 or less. Examples of the structure-directing agent include poly-N-vinylpyrrolidone (PVP) and a 1:1 copolymer of N-vinylpyrrolidone and vinyl acetate.

As described above, the structure-directing agent controls the wire-like growth of silver nanowires during synthesis of silver nanowires, and also has the effect of preventing aggregation of the generated silver nanowires.

The structure directing agent is preferably contained in the coarse dispersion of silver nanowires in an amount of 0.5% by mass or more, more preferably 0.7 to 7% by mass, and still more preferably 1.0 to 5% by mass. By making it 0.5% by mass or more, aggregation does not occur even when a high-concentration dispersion such as a silver concentration of 1.0% or more is handled. On the other hand, if the concentration of the structure-directing agent is too high, the subsequent purification step will be prolonged and the productivity will be lowered.

If the polyol in the silver nanowire crude dispersion, which is the reaction solution obtained by the synthesis, is too large, the amount of the sedimentation solvent used in the reprecipitation washing step described later will increase, so the polyol is distilled off as necessary. Then, the silver nanowires may be concentrated to some extent (concentration step). However, if the distillation is carried out at an excessively high temperature, there is a risk of aggregation, so it is preferable to carry out the distillation at a pressure of 100 mmHg or less and a temperature of 150° C. or less. Moreover, you may concentrate by adding a poor solvent (for example, ethyl acetate), such as an ester type, as a sedimentation solvent, sedimenting silver nanowires, and removing a polyol and a sedimentation solvent together. In this case, it is preferable to reduce the volume of the coarse dispersion of silver nanowires from 20% by mass to 80% by mass of the original amount. Note that the concentration step is not essential and may be omitted.

(Reprecipitation washing process)
In the method for producing silver nanowires according to the present embodiment, the step of reprecipitating and washing the silver nanowires in the coarse dispersion prepared in the coarse dispersion preparing step is repeated multiple times. The reprecipitation washing step consists of a series of steps (a), (b) and (c) below. That is, a series of steps performed in the order of (a)→(b)→(c) are repeated multiple times. When the (a) sedimentation step is performed for the second time or later, the re-dispersed liquid obtained in the (c) re-dispersion step is used in place of the coarsely dispersed liquid.

[(a) Sedimentation step]
As described above, the coarse dispersion obtained by synthesizing silver nanowires contains, in addition to the target metal nanowires, a synthesis solvent, a polymer used as a structure-directing agent, and silver nanoparticles produced as a by-product during synthesis. Therefore, it is necessary to remove these impurities. Since the silver nanowires, silver nanoparticles, and the like are dispersed in the coarse dispersion, first, a sedimentation solvent is added to the coarse dispersion to sediment the precipitate containing the silver nanowires (precipitation step). Sedimentation of the precipitate can be performed by standing. The standing time is preferably 5 to 20 minutes. If the standing time is too short from this range, the precipitate may not settle completely, and if it is too long, aggregation may occur. The precipitate contains some silver nanoparticles along with the silver nanowires.

The precipitating solvent is a poor solvent in which the structure-directing agent has low solubility, and is preferably at least one of ketone-based solvents and organic ester-based solvents. A ketone-based solvent is a solvent represented by RC(=O)-R' (R and R' each being a hydrocarbon group). Specific examples of ketone solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and benzophenone. The organic ester solvent is represented by R''-C(=O)-OR''' (R'' and R''' are hydrocarbon groups each optionally having a substituent). is a solvent. Specific examples of organic ester solvents include ethyl acetate, n-propyl acetate, isopropyl acetate, allyl acetate, n-butyl acetate, ethyl propionate, and propylene glycol monomethyl ether acetate. Among these solvents, acetone, methyl ethyl ketone, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, and propylene glycol monomethyl ether acetate are preferred from the viewpoint of sedimentation of metal nanowires and solubility in polyols. Since water is used in the re-dispersion step (c) described below, acetone is most preferable from the viewpoint of compatibility with water. The amount of the precipitation solvent to be used is preferably 50 to 2000 parts by mass, more preferably 70 to 600 parts by mass based on 100 parts by mass of the crude silver nanowire dispersion used.

A dispersant (among polymer dispersants, a nonionic dispersant that dissolves in a poor solvent) may be added to the sedimentation solvent (poor solvent). Examples include higher alcohol ethers, alkylphenyl ethers, fatty acid esters, polyhydric alcohol fatty acid ester derivatives, polyoxyethylene polyoxypropylene glycol, and glycerol fatty acid esters. As a result, the dispersant is dissolved not only in the initial coarse dispersion of silver nanowires but also in the poor solvent, so aggregation of silver nanowires can be further suppressed.

[(b) Supernatant removal step]
A supernatant is produced by producing a precipitate containing silver nanowires in the above (a) sedimentation step. This supernatant contains silver nanoparticles by-produced during the synthesis of silver nanowires, a structure-directing agent dissolved in the dispersion medium (synthesis solvent) of the coarse dispersion, a sedimentation solvent, and the like. A supernatant containing at least part of the silver nanoparticles is separated from the precipitate and removed (supernatant removing step). A method for removing the supernatant is not particularly limited. For example, it can be removed by decantation treatment, or it can be removed by suction with a pump.

[(c) Redispersion step]
The sediment in the residue after separating and removing the supernatant contains some silver nanoparticles that have sedimented together with the silver nanowires. In order to separate the silver nanowires and the silver nanoparticles, water having a specific resistance of 3.3 MΩ cm or more is added to the precipitate, so that the silver nanowires and silver nanoparticles contained in the precipitate are separated with a good solvent. A redispersed liquid is obtained by redispersing in a certain amount of water (redispersion step). One of the characteristics of the present invention is that water having a specific resistance of 3.3 MΩ·cm or more is used as the good solvent. As used herein, the term "good solvent" means a dispersion medium capable of uniformly dispersing silver nanowires and silver nanoparticles, and a solvent capable of satisfactorily dissolving a structure-directing agent.

As described above, by using water having a specific specific resistance value as a good solvent in the redispersion step (c), the silver nanoparticles mixed with the silver nanowires in the precipitate can be efficiently redispersed. can. From the results of Examples and Comparative Examples described later, the present inventors found that by controlling the resistivity value of water, the number of times of reprecipitation washing (repeating a series of steps (a), (b), and (c)) was higher than before. It has been found that n can be reduced. The specific resistance value of this water is 3.3 MΩ·cm or more, preferably 5.0 MΩ·cm or more, and more preferably 18 MΩ·cm water, so-called ultrapure water. When the specific resistance value of the water used is less than 3.3 MΩ cm, the silver nanowire ratio (number of silver nanowires/total number of particles) is 90 even if reprecipitation washing is repeated an industrially acceptable number of times, for example, 15 times. % not reached.

As long as the water used as a good solvent has a specific resistance value of 3.3 MΩ·cm or more, the purification method of the water is not particularly limited. For example, distillation of tap water (tap water), purification using an RO (reverse osmosis) membrane, ion exchange resin, or a combination thereof can be mentioned. Since the ionic components in water have the greatest effect on the specific resistance of water, it is most preferable to purify the water to be used by a refining method such as an ion-exchange resin, which has an excellent effect of removing ions. It should be noted that there is no problem even if the above-mentioned water contains trace ion components as long as the specific resistance value is 3.3 MΩ·cm or more.

The above water repeats reprecipitation washing (operation of a series of steps (a), (b), and (c)) n times (n represents an integer of 2 or more), and the same ratio is used in all steps (c) n times. Waters of different resistivity values can be used, but combinations of waters of different resistivity values can also be used. It is preferable to use ultrapure water having a specific resistance of 18 MΩ·cm or more at least once in the second and subsequent (c) redispersion steps. For example, in the initial stage when the structure-directing agent is mainly removed, ion-exchanged water with a specific resistance of 3.3 MΩ cm or more but a relatively small specific resistance is added to the silver nanoparticles in the (a) sedimentation step. It is preferable to use ultrapure water that has a relatively high specific resistance value of 18 MΩ·cm or more after starting to be dispersed in the resulting supernatant. As a result of studies by the present inventors, it was found that the greater the specific resistance of water, the greater the effect of dispersing the silver nanoparticles in the supernatant. At the beginning of the reprecipitation washing process, the concentration of the structure-directing agent contained in the coarse dispersion is first reduced. begins to color, so when combining (a) when coloration is observed in the supernatant liquid generated in the sedimentation process, (c) after the re-dispersion process, use water with a high specific resistance value (high cost) is desirable. That is, when coloring of the supernatant liquid generated in the (a) sedimentation step in the m (m is an integer of 2 or more and n or less) series of operations is observed, the specific resistance in the (c) redispersion step after the m th time It is preferable to use water with a large value (high cost), for example, water with a specific resistance of 18 MΩ·cm or more. The coloration of the dispersion corresponds to the appearance of absorption based on silver nanoparticles around 405 nm by absorption spectrometry of the dispersion. In the absorption spectrum measurement of the dispersion, the supernatant liquid obtained in the (b) supernatant removing step can be used as a sample. By using a combination of water with different specific resistance values, a cost reduction effect can be expected while maintaining the reprecipitation cleaning efficiency.

The amount of water used in the above (c) redispersion step is 25 to 400 parts by weight, preferably 30 to 300 parts by weight, relative to 1 part by weight of silver in the residual liquid containing the precipitated silver nanowires. It is preferably from 50 to 200 parts by mass. If it is less than 25 parts by mass, the concentration of the silver nanowires is too high, making it difficult to redisperse them uniformly. Labor is required.

As described above, in the reprecipitation washing process, a series of operations consisting of steps (a), (b), and (c) are repeated multiple times. By repeating this series of operations, when the concentration of the structure-directing agent in the coarse dispersion falls below the threshold, the silver nanoparticles are well dispersed in the supernatant even after the addition of the precipitating solvent. and silver nanowires can be separated. By setting the specific resistance value of water used in the redispersion step (c) in the series of operations consisting of repeated steps (a), (b), and (c) to 3.3 MΩ cm or more, the number of times n of the series of operations is can be reduced.

The same precipitation solvent can be used in the n-time precipitation step (a), but a different precipitation solvent can be used for each number of times. The amount of the precipitation solvent used in the nth precipitation step (a) is 50 parts by weight with respect to 100 parts by weight of the water used in the (n-1)th redispersion step (c) performed before that. to 500 parts by mass, more preferably 70 to 300 parts by mass. In addition, the amount of the sedimentation solvent used in the first sedimentation step (a) is the same as the amount used in the n-th sedimentation step (a) when water is contained in the coarse dispersion prepared in the coarse dispersion preparation step. Equivalent amounts are preferred.

Even if the coarse dispersion prepared in the coarse dispersion preparation step does not contain water, the precipitate containing the silver nanowires will settle by adding the precipitation solvent. In that case, the amount of the precipitation solvent added is preferably 50 to 500 parts by mass, more preferably 70 to 300 parts by mass, per 100 parts by mass of the crude dispersion. If the coarse dispersion contains no water, it is preferred to use butyl acetate as the precipitating solvent.

The reprecipitation washing step is preferably repeated until the ratio of silver nanowires contained in the dispersion, that is, the number of silver nanowires/(the total number of particles (=the number of silver nanowires + the number of silver nanoparticles))>90%. . The silver nanowire ratio is measured by the method described in Examples below.

The reprecipitation washing step may include steps other than steps (a), (b), and (c) within a range that does not impair the effects of the present invention. Other steps include (c′) a redispersion step of adding water having a specific resistance of less than 3.3 MΩ·cm to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid. be done. In the step (c′), the total number of reprecipitation washing operations until the number of silver nanowires / (the total number of particles (= the number of silver nanowires + the number of silver nanoparticles)) > 90% [(step ( The number of times a) → step (b) → step (c)) and the number of times (step (a) → step (b) → step (c′))] is an industrially acceptable number of times, for example, 15 times Hereinafter, a series of operations [(a) → (b) → (c′)] may be included within a range of preferably 10 times or less, but it is better not to include step (c′). This is preferable because it can reduce the number of times of a series of operations.

After reprecipitation and washing of the silver nanowires, it is possible to carry out a known purification process of the silver nanowires according to the purpose, and prepare a conductive ink containing the silver nanowires.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

Each analysis condition in the examples is as shown below.
<Silver concentration>
Silver concentration is determined using the Volhard method. About 1 g of the sample is weighed into a beaker, and 4 mL of nitric acid (1+1) and 20 mL of pure water are added. Cover the beaker with a watch glass and heat on a hot plate to 150° C. to dissolve the solids. After confirming the dissolution, the heating is stopped and the mixture is allowed to cool. The inner surface of the watch glass and the wall surface of the beaker are washed with pure water to make the liquid volume about 50 mL. To this solution, 5 mL of nitric acid (1+1) and 3 mL of ammonium iron(III) sulfate (3% nitric acid acid) are added and titrated with a 0.01 mol/L ammonium thiocyanate aqueous solution. At this time, the end point is when the solution turns from colorless to light brown.

Based on the titration results, the silver concentration is calculated according to the following formula.
Silver concentration (% by mass) = {(V x c) x 107.9/1000}/m
m: Weight of sample (g)
V: Amount of ammonium thiocyanate aqueous solution consumed for titration up to the endpoint (mL)
c: concentration of ammonium thiocyanate aqueous solution (0.01 mol/L)

Nitric acid (1+1), ammonium iron sulfate (III), and ammonium thiocyanate were all reagents manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd. Ammonium iron (III) sulfate (3% nitric acid) was prepared by mixing 5.17 g of ammonium iron (III) sulfate, 170 g of pure water and 2.00 g of nitric acid. The 0.01 mol/L ammonium thiocyanate aqueous solution was prepared by adding pure water to 38.06 mg of ammonium thiocyanate to make the total amount 50 mL.

<Silver nanowire ratio>
In each of the examples and comparative examples described later, the silver nanowires/aqueous dispersion obtained in step (c) during each series of operations in the reprecipitation washing step was diluted with methanol 300 times by mass to dilute the silver nanowires. A dispersion is made. One drop of the silver nanowire dilute dispersion is dropped on a clean glass plate and dried on a hot plate at 90°C. The glass plate is observed with a laser microscope (Keyence VK-X200) at a magnification of 3000 (measurement field: 260 μm×200 μm) to count the number of silver nanowires and silver nanoparticles. A ratio of silver nanowires in the dispersion (number of silver nanowires/(number of silver nanowires + number of silver nanoparticles)) is calculated.

The methods for purifying each water shown in the examples are as follows.
<Ion-exchanged water>
Tap water was purified with a cartridge type water purifier Demiace DX-15 (manufactured by Kurita Water Industries Ltd.). The specific resistance value was monitored with an attached water quality meter Pure Meter (manufactured by Kurita Water Industries Ltd.) when water was sampled.

<Ultrapure water and distilled water>
Tap water was purified with a distilled water manufacturing apparatus RFD280NC (manufactured by ADVANTEC). As the specific resistance value, the value displayed on the monitor of the device was recorded. Distilled water and ultrapure water obtained by further purifying the distilled water can be produced by this apparatus.

Coarse dispersion liquid preparation step Synthesis example 1
<Production of coarse dispersion of silver nanowires>
667 g of propylene glycol (manufactured by AGC Co., Ltd.) was weighed into a 1 L plastic container, 22.5 g (0.13 mol) of silver nitrate (manufactured by Toyo Kagaku Kogyo Co., Ltd.) was added as a metal salt, and the mixture was stirred for 2 hours at room temperature under light shielding. A silver nitrate solution (second solution) was prepared.

In a 5 L four-necked separable flask equipped with a mechanical stirrer, a metering pump, a reflux tube, a thermometer, and a nitrogen gas introduction tube, 3000 g of propylene glycol and 0.36 g of potassium chloride as an ionic derivative (4. 8 mmol) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.12 g (1.2 mmol) of sodium bromide (manufactured by Manac Co., Ltd.), polyvinylpyrrolidone K-90 (PVP) 72.1 g (manufactured by BASF) as a structure directing agent , Sokalan (registered trademark) K90) was charged and stirred at 150° C. for 1 hour at a rotational speed of 200 rpm using an oil bath as a heat medium, thereby completely dissolving the mixture to obtain a first solution. Silver nanowires were synthesized by connecting the previously prepared silver nitrate solution (second solution) to a metering pump and dropping it into the first solution at a temperature of 150° C. over 2.5 hours. After completion of the dropwise addition, heating and stirring was continued for 30 minutes to complete the reaction.

When the silver concentration of the obtained silver nanowire coarse dispersion was measured using the titration method (Volhardt method), it was 0.4% by mass. In addition, the shape of the contained silver nanowires was observed at arbitrarily 100 points using an SEM (JSM-7000F manufactured by JEOL Ltd.), and when measured, the average diameter was 24 nm and the average length was 13 μm. The resulting coarse dispersion of silver nanowires was directly used as a coarse dispersion in the reprecipitation cleaning step in each of the following examples and comparative examples.

Reprecipitation washing process Example 1
<Reprecipitation cleaning of coarse dispersion of silver nanowires>
101 g of the obtained crude dispersion was placed in a 1 L plastic container, and 98 g of butyl acetate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added over 10 minutes while stirring at 150 rpm using a mechanical stirrer (first time step (a)). After continuing stirring for 10 minutes, the stirring was stopped and the mixture was allowed to stand for 10 minutes to separate the supernatant and the precipitate. After that, 139 g of the supernatant was removed by decantation (first step (b)).

Add 24 g of ion-exchanged water A with an average specific resistance of 10 MΩ cm (varying in the range of 3.3 to 15 MΩ cm during the purification (collection)) to the residual liquid containing the precipitate, and continue stirring for 10 minutes to remove the precipitate. After redispersion (first step (c)), 52 g of acetone were added over 10 minutes (second step (a)). After continuing the stirring for 10 minutes, the stirring was stopped and the mixture was allowed to stand for 10 minutes to separate the supernatant and the precipitate. Thereafter, 70% (95 g) of the total liquid volume of the supernatant was removed by decantation (second step (b)). Add 24 g of ion-exchanged water A with an average specific resistance of 10 MΩ cm (3.3 to 15 MΩ cm) again (second step (c)), and then the second step (a) (b) (c) was repeated, and the number of series of operations (steps (a), (b), and (c)) until the silver nanowire ratio reached >90% was recorded as the number of reprecipitation washings. The results are shown in Table 1.

Example 2
Silver nanowires were purified in the same manner as in Example 1, except that 24 g of ultrapure water with a specific resistance of 18.2 MΩ·cm was used as the good solvent (water) added in step (c). As in Example 1, the number of series of operations (steps (a), (b), and (c)) until the silver nanowire ratio reached >90% was recorded as the number of reprecipitation washings. The results are shown in Table 1.

Example 3
As a good solvent (water) added in step (c), 24 g of ultrapure water with a specific resistance value of 18.2 MΩ cm was added for the first three times, and the remaining times until the silver nanowire ratio reached >90%. Silver nanowires were purified in the same manner as in Example 1, except that 24 g of ion-exchanged water A having an average specific resistance of 10 MΩ·cm (3.3 to 15 MΩ·cm) was used. As in Example 1, the number of series of operations (steps (a), (b), and (c)) until the silver nanowire ratio reached >90% was recorded as the number of reprecipitation washings. The results are shown in Table 1.

Example 4
As a good solvent (water) added in step (c), 24 g of ion-exchanged water A with an average specific resistance value of 10 MΩ cm (3.3 to 15 MΩ cm) is used for the first three times, and the silver nanowire ratio is >90%. Silver nanowires were purified in the same manner as in Example 1 except that 24 g of ultrapure water with a specific resistance of 18.2 MΩ·cm was used for the remaining number of times until reaching the target. As in Example 1, the number of series of operations (steps (a), (b), and (c)) until the silver nanowire ratio reached >90% was recorded as the number of reprecipitation washings. The results are shown in Table 1. The absorption spectrum of a solution obtained by diluting each supernatant liquid obtained by 3 to 8 times of step (a) in the reprecipitation washing step, that is, removed in the immediately following step (b), with methanol to 5 times by mass, was analyzed by UV-visible spectroscopy. When measured with a photometer (manufactured by Shimadzu Corporation, UV-2400PC) (see FIG. 1), absorption based on silver nanoparticles near 405 nm, which was not observed in the supernatant liquid removed in the third step (b), was observed. Since it appeared in the supernatant liquid removed in the fourth step (b), ultrapure water with a specific resistance of 18.2 MΩ·cm was used in the fourth and subsequent steps (c).

Example 5
As a good solvent (water) to be added in step (c), 0.1 mg of sodium chloride (manufactured by Junsei Chemical Co., Ltd.) dissolved in 1500 g of ultrapure water with a specific resistance of 18.2 MΩ cm has a specific resistance of 5.0 MΩ cm. Silver nanowires were purified in the same manner as in Example 1 except that 24 g of water was used. As in Example 1, the number of series of operations (steps (a), (b), and (c)) until the silver nanowire ratio reached >90% was recorded as the number of reprecipitation washings. The results are shown in Table 1.

Comparative example 1
In place of step (c), (c′) a redispersion step of adding water having a specific resistance of less than 3.3 MΩ·cm to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid. As a good solvent (water) to be added, silver nanoparticles were prepared in the same manner as in Example 1 except that 24 g of ion-exchanged water B with an average specific resistance of 2.0 MΩ cm (1.5 to 2.5 MΩ cm) was used. Refined wire. The reprecipitation washing was repeated nine times, but the silver nanowire ratio did not reach >90%. The results are shown in Table 1.

Comparative example 2
In place of step (c), (c′) a redispersion step of adding water having a specific resistance of less than 3.3 MΩ·cm to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid. The silver nanowires were purified in the same manner as in Example 1, except that 24 g of tap water having a specific resistance of 0.005 MΩ·cm was used as the good solvent (water) to be added. The reprecipitation washing was repeated nine times, but the silver nanowire ratio did not reach >90%. The results are shown in Table 1.

Comparative example 3
In place of step (c), (c′) a redispersion step of adding water having a specific resistance of less than 3.3 MΩ·cm to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid. The silver nanowires were purified in the same manner as in Example 1, except that 24 g of distilled water with a specific resistance of 0.1 MΩ·cm was used as the good solvent (water) to be added. The reprecipitation washing was repeated nine times, but the silver nanowire ratio did not reach >90%. The results are shown in Table 1.

Comparative example 4
After the implementation of Comparative Example 1, the good solvent (water) added in step (c) was washed again by reprecipitation using deionized water A with an average specific resistance value of 10 MΩ·cm (3.3 to 15 MΩ·cm) for 10 times. repeated times. The results are shown in Table 1.

Comparative example 5
In place of step (c), (c′) a redispersion step of adding water having a specific resistance of less than 3.3 MΩ·cm to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid. As a good solvent (water) to be added, 24 g of water with a specific resistance value of 0.8 MΩ cm in which 0.9 mg of sodium chloride (manufactured by Junsei Chemical Co., Ltd.) is dissolved in 1500 g of ultrapure water of 18.2 MΩ cm was used. Silver nanowires were purified in the same manner as in Example 1 except for the above. The reprecipitation washing was repeated nine times, but the silver nanowire ratio did not reach >90%. The results are shown in Table 1.

In Examples 1 to 5 using water with a specific resistance value of 3.3 MΩ cm or more in the step (c), the silver nanowire ratio reached >90%, whereas the specific resistance value was less than 3.3 MΩ cm. In Comparative Examples 1 to 3 and 5 using water, the silver nanowire ratio did not reach 90% with the same number of washings, and it was confirmed that by-product nanoparticles could not be removed. Regardless of the presence or absence of purification of water and the purification method, it can be said that the nanoparticle removal efficiency changes depending on the specific resistance value. In Example 2, which uses ultrapure water with particularly high purity, the silver nanowire ratio reaches >90% with fewer washings than in Example 1, which uses only normal ion-exchanged water. It was confirmed that by-product nanoparticles were removed. In addition, when comparing Example 3 and Example 4 in which ion-exchanged water and ultrapure water are combined, it is found that the structure-directing agent is the threshold value rather than using ultrapure water in the first half, in which a large amount of the structure-directing agent remains and the silver nanoparticles are difficult to remove. It was possible to efficiently remove the nanoparticles with a smaller number of washings in the second half when the nanoparticles were more easily dispersed in the supernatant liquid. In Example 2 and Example 4, the cleaning efficiency was substantially the same. In both cases, ultrapure water is used in the second half when the structure-directing agent is below the threshold value and the nanoparticles are easily dispersed in the supernatant liquid, so the cleaning efficiency is good, and ultrapure water is used only in the second half. is more cost effective. In addition, step (c′) was performed instead of step (c) (repeating a series of operations of step (a) → step (b) → step (c′)), and the silver nanowire ratio peaked out. In Comparative Example 4, after the reprecipitation washing step in Comparative Example 1, reprecipitation washing (a series of operations of step (a) → step (b) → step (c)) was repeated with high-purity water having a large specific resistance value. Although the silver nanowire ratio improved, it did not reach 90%. From this, it can be said that it is effective to use high-purity water with a large specific resistance value throughout the re-precipitation washing (in all re-dispersion steps).

Claims

A coarse dispersion preparing step of preparing a coarse dispersion containing silver nanowires and silver nanoparticles, and a reprecipitation washing step of purifying the silver nanowires in the coarse dispersion by a reprecipitation method, wherein A method for producing silver nanowires, wherein the washing step repeats a series of operations consisting of the following steps (a), (b), and (c) multiple times:
(a) a sedimentation step of precipitating a precipitate containing silver nanowires by adding a precipitating solvent to the coarse dispersion or the redispersion described later;
(b) a supernatant removal step of removing a supernatant liquid containing at least a portion of the silver nanoparticles produced by the precipitation of the precipitate;
(c) a redispersion step of adding water having a specific resistance of 3.3 MΩ·cm or more to the remaining precipitate to redisperse the precipitate in water to obtain a redispersion liquid;
The silver nanowires according to claim 1, wherein the series of operations consisting of steps (a), (b), and (c) is repeated until the number of silver nanowires in the redispersion/the total number of particles>90%. Production method.
The re-precipitation washing step includes (c′) a re-dispersion step of adding water having a specific resistance value of less than 3.3 MΩ·cm to the remaining precipitate to re-disperse the precipitate in water to obtain a re-dispersion liquid. The method for producing silver nanowires according to claim 1 or 2, which does not contain.
A series of operations consisting of the steps (a), (b) and (c) are repeated n times, and water having a specific resistance of 18 MΩ cm or more is used at least once in the redispersion step (c) after the second time. Item 4. A method for producing a silver nanowire according to any one of Items 1 to 3.
When absorption based on silver nanoparticles near 405 nm is observed in the supernatant liquid generated in step (a) in a series of operations from the second time onwards, water with a specific resistance value of 18 MΩ cm or more is added in step (c). The method for producing silver nanowires according to claim 4, which is used.
The method for producing silver nanowires according to any one of claims 1 to 5, wherein the coarse dispersion preparation step includes a step of synthesizing silver nanowires by a polyol reduction method.
The method for producing silver nanowires according to any one of claims 1 to 6, wherein the precipitation solvent is at least one of a ketone solvent and an organic ester solvent.
Claim 7, wherein the precipitation solvent is at least one selected from the group consisting of acetone, methyl ethyl ketone, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, and propylene glycol monomethyl ether acetate. production method of silver nanowires.