CN111032255A

CN111032255A - Method for manufacturing silver nanowire, silver nanowire ink, and transparent conductive film

Info

Publication number: CN111032255A
Application number: CN201880051092.2A
Authority: CN
Inventors: 佐藤王高
Original assignee: Dowa Electronics Materials Co Ltd
Current assignee: Dowa Electronics Materials Co Ltd
Priority date: 2017-09-26
Filing date: 2018-09-20
Publication date: 2020-04-17
Also published as: JP2019059977A; TWI696669B; TW201920518A; WO2019065461A1

Abstract

The subject is as follows: when fine silver nanowires are synthesized by an alcohol solvent reduction method, particularly, wires having a long average length and a large average aspect ratio are stably produced. The solution is as follows: a method for producing silver nanowires, which comprises a step of reducing and precipitating silver into a linear form in an alcohol solvent in which a silver compound and an organic protective agent are dissolved, characterized in that a polymer having a vinylpyrrolidone structural unit is used as the organic protective agent, and the reduction and precipitation are carried out in the liquid in a state where methyl tert-butyl ether is dissolved in the alcohol solvent at a concentration of 0.3 to 25.0 mmol/L.

Description

Method for manufacturing silver nanowire, silver nanowire ink, and transparent conductive film

Technical Field

The present invention relates to a method for producing silver nanowires useful as a conductive material (filler) for a transparent conductive film. Further, the present invention relates to a silver nanowire, a silver nanowire ink, and a transparent conductive film obtained by the production method.

Background

In the present specification, a fine metal wire having a thickness of about 200nm or less is referred to as a "nanowire(s)".

Silver nanowires are gaining attention as conductive materials for imparting conductivity to transparent substrates. When a liquid containing silver nanowires (silver nanowire ink) is applied to a transparent substrate such as glass, PET (polyethylene terephthalate), PC (polycarbonate), or the like, and then a liquid component is removed by evaporation or the like, the silver nanowires are brought into contact with each other on the substrate to form a conductive network, whereby a transparent conductive film can be realized.

Transparent conductive films used for touch panels of electronic devices and the like are required to have good conductivity and also to have clear visibility with little haze. In a transparent conductive film using silver nanowires as a conductive material, it is advantageous to use silver nanowires that are as thin and long as possible in order to achieve both conductivity and visibility at a high level.

Conventionally, as a synthesis method of silver nanowires, for example, the following methods are known: a silver compound is dissolved in a polyol solvent such as ethylene glycol, and metallic silver in a linear shape is precipitated by the reducing power of the polyol as a solvent in the presence of a halogen compound and an organic protective agent (hereinafter, referred to as "alcohol solvent reduction method"). As the organic protective agent, PVP (polyvinylpyrrolidone) has been generally used in many cases. PVP is a suitable organic protectant in precipitating fine and long silver nanowires.

Molecules of the organic protective agent used in the alcohol solvent reduction method are adsorbed on the surface of the synthesized silver nanowires, and become a main cause for dominating the dispersibility of the silver nanowires in the liquid medium. The PVP-adsorbed silver nanowires exhibited good dispersibility with respect to water. However, in order to improve wettability to a substrate such as PET, it is advantageous to apply silver nanowire ink using a mixed medium of water and an organic solvent (e.g., alcohol). In addition, depending on the coating equipment, silver nanowire ink using a nonaqueous solvent may be preferably used. In consideration of the dispersibility of the silver nanowires in such a mixed medium or a nonaqueous solvent, PVP is not necessarily a satisfactory organic protective agent. Recently, various organic protective agents have been developed which can improve the dispersibility of silver nanowires in a liquid medium other than water. For example, patent document 1 discloses a copolymer having a polymerization composition of vinylpyrrolidone and a Diallyldimethylammonium (Diallyldimethylammonium) salt monomer, patent document 2 discloses a copolymer of vinylpyrrolidone and an acrylate or methacrylate monomer, and patent document 3 discloses a copolymer of vinylpyrrolidone and a maleimide (マレイミド) monomer. In the alcohol solvent reduction method using these polymers as an organic protective agent, fine and long silver nanowires equivalent to or better than those using PVP can be synthesized by optimizing the synthesis conditions.

Documents of the prior art

Patent document

Patent document 1 Japanese patent laid-open No. 2015-180772

Patent document 2 Japanese patent laid-open publication No. 2017-78207

Patent document 3, Japanese patent laid-open publication No. 2016-135919

Disclosure of Invention

Problems to be solved by the invention

As described above, the silver nanowires used as the conductive material of the transparent conductive coating film are advantageous in a thin and long form from the viewpoint of achieving both high-level conductivity and visibility. However, even if a fine and long wire can be synthesized, when the wires are likely to form aggregates with each other in the silver nanowire dispersion, not only is the yield of silver reduced and the handling property in the subsequent steps reduced, but the aggregates remaining in the final silver nanowire ink become a cause of reduction in the haze of the transparent conductor and short-circuit in the patterned circuit.

The present invention aims to provide a technique which has a good effect of stably producing particularly long strands and a good effect of suppressing aggregation between the strands to be synthesized, when fine silver nanowires are synthesized by an alcohol solvent reduction method.

Means for solving the problems

The above object is achieved by carrying out a silver precipitation reaction in an alcohol solvent reduction method in an environment where an alkyl ether is present in a solvent at a predetermined concentration. The present specification discloses the following invention.

[1] A process for producing silver nanowires, which comprises a step of precipitating silver into a linear form by reduction in an alcohol solvent in which a silver compound and an organic protective agent are dissolved,

using a polymer having a vinylpyrrolidone structural unit as the organic protective agent,

the reduction precipitation is carried out in the alcohol solvent in a state where the alkyl ether is dissolved in the alcohol solvent at a concentration of 0.3 to 25.0 mol/L.

[2]According to the above [1]The method for producing a silver nanowire comprises reducing and precipitating an average aspect ratio A having an average length of 15 μm or more and an average diameter of 35nm or less and defined by the following formula (1)_MIs a silver nanowire of 800 or more.

A_M＝L_M/D_M…(1)

Wherein L is_MIs a value representing the above average length in nm units, D_MThe average diameter is expressed in nm.

[3] The method for producing silver nanowires according to the above [1] or [2], wherein the alkyl ether is methyl t-butyl ether.

[4] The method for producing silver nanowires according to any one of the above [1] to [3], wherein the polymer is PVP (polyvinylpyrrolidone) or a copolymer of vinylpyrrolidone and a hydrophilic monomer.

[5] The method for producing silver nanowires according to any one of the above [1] to [3], wherein the polymer has a polymerization composition of vinylpyrrolidone and 1 or 2 or more monomers selected from the group consisting of diallyldimethylammonium salt, ethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-t-butylmaleimide, 2-dimethylaminoethyl methacrylate, and 2-diethylaminoethyl methacrylate.

[6] The method for producing silver nanowires according to any one of the above [1] to [5], wherein the polymer has a weight average molecular weight Mw of 30,000 to 300,000.

[7] A silver nanowire obtained by the production method according to any one of the above [1] to [6 ].

[8] A silver nanowire ink, wherein the silver nanowires obtained by the production method according to any one of the above [1] to [6] are dispersed in a liquid medium.

[9] A transparent conductive film comprising, as a conductive material, a silver nanowire obtained by the production method according to any one of the above [1] to [6 ].

The alkyl ether is a compound of the formula R-O-R' having an ether linkage at position 1. Herein, R, R' refers to formula C_nH_2n+1An alkyl group represented by (n is an integer of 1 or more). The alkyl group is a functional group obtained by removing 1 hydrogen from the alkane (アルカン), and is in the case of a straight chain or in the case of a carbon (tertiary carbon) having 3 carbon atoms bonded thereto. Examples of alkyl ethers are the methyl tert-butyl ethers cited above, also known as TBME (tert-butyl methyl ether), according to the structural formula (CH)₃)₃COCH₃And (4) showing.

In the present specification, the average length, average diameter, and average aspect ratio of the silver nanowires are defined as follows.

[ average Length L_M]

On an observation image using a field emission type scanning electron microscope (FE-SEM), a trace length from one end to the other end of a certain 1 silver nanowire is defined as a length of the wire. The average length of each silver nanowire present on the microscope image is defined as the average length L_M. In order to calculate the average length, the total number of lines to be measured is set to 100 or more. The average length was evaluated at a stage after washing the silver nanowires recovered from the solution after completion of the reduction reaction (at a stage before supplying the silver nanowires to a purification step such as cross-flow filtration).

[ average diameter D_M]

On a bright field observation image by a Transmission Electron Microscope (TEM), the distance between contours on both sides in the thickness direction of a certain 1 silver nanowire is defined as the diameter of the wire. A bright field observation image using TEM (hereinafter referred to as "TEM image") of the silver nanowire according to the present invention is illustrated in fig. 4. Each line can be considered to have a substantially uniform thickness over the entire length. Therefore, the thickness measurement can be performed by selecting a portion that does not overlap with other lines. In a TEM image obtained by taking 1 visual field, the diameters of all the silver nanowires observed in the image except for wires which completely overlap with other wires and have difficulty in measuring the diameters are measured, a plurality of visual fields are randomly selected and the operation is performed, the diameters of different silver nanowires of 100 or more in total are obtained, the average value of the diameters of the silver nanowires is calculated, and the average value is defined as an average diameter D_M。

[ average aspect ratio ]

The above average diameter D_MAnd an average length L_MThe average aspect ratio A was calculated by substituting the following formula (1)_M. Wherein D is substituted into formula (1)_M、L_MAre all set to values expressed in nm units.

A_M＝L_M/D_M…(1)

Effects of the invention

According to the present invention, it is possible to stably synthesize fine silver nanowires having an average diameter of, for example, 35nm or less, and particularly long silver nanowires having an average length of 15nm or more and an average aspect ratio of 800 or more. Since the silver nanowires having a long average length as described above can be obtained at the stage of completion of washing after synthesis, silver nanowires having a long average length and a high aspect ratio can be produced with high yield in the subsequent purification operation of adjusting the length distribution of the wires by cross-flow filtration or the like. When the transparent conductive film is used as a conductive material for a transparent conductive film, a transparent conductive film having excellent visibility with less haze can be realized while maintaining high conductivity.

Drawings

FIG. 1 is a structural formula of a vinylpyrrolidone structural unit.

Fig. 2 is an SEM photograph of the silver nanowires obtained in comparative example 1.

Fig. 3 is an SEM photograph of the silver nanowires obtained in example 5.

Detailed Description

As described above, as a method for synthesizing silver nanowires, a method of precipitating silver in a linear form in an alcohol solvent in which a silver compound and an organic protective agent are dissolved, by utilizing the reducing power of the alcohol as the solvent, has been put into practical use. This process is referred to as "alcohol solvent reduction process" in the present specification.

When metallic silver is precipitated in a linear form by the alcohol solvent reduction method, it is necessary to selectively adsorb polymer molecules of the organic protective agent to the {100} plane of the nuclear crystal which is considered to be a multiple twin crystal of silver. This suppresses the growth of the {100} plane, and preferentially grows the {111} plane, which is the closest plane of the silver crystal, to form a linear structure of metallic silver. It is considered that the selective adsorption of the polymer molecules is generated by the interaction of the surface potential of the polymer molecules and the surface potential of the crystal planes of silver. As a polymer having excellent selective adsorption to the {100} plane of silver crystals, a homopolymer (PVP) or a copolymer having a vinylpyrrolidone structural unit is known. The structural formula of the vinylpyrrolidone structural unit is shown in FIG. 1. When silver is reductively deposited in a state where the organic protective agent mainly composed of such a polymer is dissolved in an alcohol solvent, deposition of silver toward the {111} crystal plane occurs preferentially, and a rod-like or wire-like metallic silver structure can be obtained. In order to stably synthesize silver nanowires, a halide or the like having an action of activating a {111} crystal plane is generally present in a solvent.

The present inventors have conducted various studies on a method for synthesizing fine silver nanowires by using a polymer having a vinylpyrrolidone structural unit as an organic protective agent, particularly, a method for increasing the average length of the synthesized wires. As a result, they found that: in addition to the conventionally generally used additives such as halides, the addition of alkyl ethers is very effective. When silver nanowires are synthesized by an alcohol solvent reduction method, it is considered that the alkyl ether has an action of purifying the {111} crystal face from which silver is preferentially deposited, that is, an action of inhibiting adsorption of organic protective agent molecules to the {111} crystal face and activating the exposed {111} crystal face to promote deposition of new silver. The action of activating the {111} crystal face is mainly exerted by a halide or the like which has been a general additive, and it is presumed that an alkyl ether also has an action similar to this action. When an alkyl ether is present in addition to a halogen in the vicinity of the linear structure of the metal silver that has already been deposited, the above-described purifying action is considered to be increased, and the relative ease of deposition of silver on the surface ({100} crystal plane) in the thickness direction of the linear structure and the exposed surface ({111} crystal plane) in the longitudinal direction is further improved, and as a result, a silver nanowire having a large average aspect ratio is easily synthesized.

There is a problem in increasing the amount of halide added for the purpose of enhancing the activation of the {111} crystal plane. Halogen atoms such as chlorine added during synthesis are attached to the organic protective agent coating the surface of the synthesized silver nanowires, and the halogen atoms enter the transparent conductive film with the silver nanowires. According to the investigation by the inventors, it was confirmed that when the chlorine concentration in the transparent conductive film is high, the deterioration of the transparent conductive film with time is promoted, and the problem of early decrease in conductivity is likely to occur. In this regard, in the method of enhancing the purification effect of the {111} crystal plane by adding the alkyl ether, the problem of the deterioration of the transparent conductive film with time as described above can be avoided.

The results of various studies were carried out by adding 0.3mmol/L (═ 0.3X 10) of alkyl ether^-3mol/L) or moreThe silver nanowires are reduced and precipitated in the state of being dissolved in an alcohol solvent, and the effect of increasing the average length of the synthesized silver nanowires can be remarkably exerted. More preferably, the alkyl ether concentration is set to 1.0mmol/L (═ 1.0 × 10)^-3mol/L) above. However, as the alkyl ether concentration increases, the effect of increasing the average length of the silver nanowires becomes saturated. The concentration of alkyl ether in the solvent was 25.0mmol/L (═ 25.0X 10)^-3mol/L) or less, and may be controlled to 20.0mmol/L (═ 20.0 × 10)^-3mol/L) below.

From the viewpoint of the ratio to the amount of the organic protective agent in the alcohol solvent during the reductive precipitation reaction, for example, the amount of the organic protective agent in the liquid alkyl ether is preferably adjusted to 0.003 to 0.30 mol per 1 mol of the polymer having a vinylpyrrolidone structural unit as the organic protective agent. In addition, from the viewpoint of the ratio to the amount of silver, the amount of the alkyl ether present in the liquid at the start of the reaction is preferably adjusted to be in the range of 0.005 to 0.50 mol based on 1 mol of the total amount of silver used in the reaction.

Examples of the alkyl ether include: ethyl methyl ether (CH)₃CH₂OCH₃) Methyl tert-butyl ether ((CH)₃)₃COCH₃) Ethyl isoamyl ether ((CH)₃)₂CHCH₂CH₂OC₂H₅) Ethyl-tert-butyl ether ((CH)₃)₃COC₂H₅) Di-isoamyl ether ((CH)₃)₂CHCH₂CH₂OCH₂CH₂CH(CH₃)₂) Diisopropyl ether ((CH)₃)₂CHOCH(CH₃)₂) Diethyl ether (C)₂H₅OC₂H₅) Dibutyl ether (C)₄H₉OC₄H₉) Dipropyl ether (C)₃H₇OC₃H₇) Dimethyl ether (CH)₃OCH₃) Ethylene glycol monomethyl ether (CH)₃OCH₂CH₂OH), and the like. The alkyl ether may be selected from 1 or 2 or more.

As the polymer having a vinylpyrrolidone structural unit used as the organic protective agent, PVP (polyvinylpyrrolidone), or a copolymer of vinylpyrrolidone and a hydrophilic monomer is a preferable object. Examples of the latter copolymer include copolymers having a polymerization composition of vinylpyrrolidone and 1 or 2 or more kinds of monomers selected from the group consisting of diallyldimethylammonium salt, ethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide and N-t-butylmaleimide. The polymerization composition of the copolymer is preferably 0.1 to 10% by mass of a monomer other than vinylpyrrolidone, and the balance is vinylpyrrolidone.

The weight average molecular weight Mw of the polymer used in the organic protective agent is preferably in the range of 30,000 to 300,000, more preferably in the range of 30,000 to 150,000. Mw can be determined by GPC (gel permeation chromatography).

[ size and shape of silver nanowire ]

From the viewpoint of forming a transparent conductive coating film excellent in conductivity and visibility, the silver nanowires are preferably as thin and long as possible. In the present invention, silver nanowires having an average length of 15 μm or more, an average diameter of 35nm or less, and an average aspect ratio of 800 or more obtained from the above formula (1) are preferable. Silver nanowires having an average length of 15 μm or more and an average diameter of 33nm or less are more preferable. Silver nanowires having an average length of 15 μm or more and an average diameter of 30nm or less are more preferable. In the present invention, since silver nanowires having a long average length and a large average aspect ratio can be obtained in the synthesis stage, the length distribution can be efficiently adjusted in a high yield by, for example, cross-flow purification in the subsequent step.

[ Synthesis of silver nanowires ]

In addition to the presence of the alkyl ether in the alcohol solvent, a conventionally developed technique of the alcohol solvent reduction method can be used. The alcohol solvent is selected from alcohol solvents which have an appropriate reducing power for silver and can precipitate metallic silver in a linear form. For example, ethylene glycol, propylene glycol (1, 2-propanediol), 1, 3-propanediol, and 1, 3-butanediol can be used1, 4-butanediol, and glycerol. These alcohols may be used alone or in combination of 2 or more. As the silver source, a silver compound soluble in an alcohol solvent is used. Examples thereof include: silver nitrate (AgNO) is easily used in consideration of solubility in a solvent and cost, such as silver nitrate, silver acetate, silver oxide, and silver chloride₃). It is preferable to precipitate by reduction in an alcohol solvent in which chloride and bromide are dissolved in addition to the silver compound, the organic protective agent and the alkyl ether. The precipitation may be carried out by reduction in an alcohol solvent in which an alkali metal hydroxide or an aluminum salt is further dissolved. For example, in the method disclosed in patent document 1, an alcohol solvent to which an alkyl ether is added may be used.

Examples

Comparative example 1

(organic protecting agent)

A copolymer powder synthesized by a method of dissolving 1-vinyl 2 pyrrolidone and diallyldimethylammonium nitrate in methylisobutylketone as a solvent and adding a polymerization initiator to copolymerize the resulting solution was prepared. The polymerization composition, in terms of mole ratios, was 1-vinyl 2 pyrrolidone: diallyl dimethyl ammonium nitrate ═ 99: 1.

the copolymer powder was analyzed for its content by measuring its 1H NMR spectrum by nuclear magnetic resonance spectroscopy (NMR) using JNM-LA400(400MHz) manufactured by Japan Electron society. As a result, the molar ratio of polymer 95.53% to residual VP (vinylpyrrolidone): 0.62%, ethyl acetate: 2.79%, methyl tert-butyl ether: 0.00%, methyl isobutyl ketone: 1.06 percent. The integrated value of the peak near 4.1ppm was used to calculate the mole% of ethyl acetate, the integrated value of the peak near 1.2ppm was used to calculate the mole% of methyl t-butyl ether, and the integrated value of the peak near 0.9ppm was used to calculate the mole% of methyl isobutyl ketone. The residual VP content was determined by the following formula (2).

VP_R(mol％)＝[2×(I₁+I₂)/(3×I₃)]×100…(2)

Wherein, I₁Derived from a methyl species associated with the C ═ C double bond of the VP monomerIntegral value, I, of peak (7.0-7.2ppm) of seed (メチンプロトン)₂Is the integral value of the peak (4.3-4.4ppm) derived from the methylene proton associated with the C ═ C double bond of the monomer, I₃Is an integrated value of peaks (3.0 to 3.4ppm) derived from methylene protons adjacent to the N atom of the polymer.

The weight average molecular weight Mw of the copolymer was determined by GPC (gel permeation chromatography) under the following conditions.

An apparatus: HLC-8320GPC EcoSeC (manufactured by DONG ソー Co., Ltd.)

Column: TSKgel GMPWXL (× 2) + G2500PWXL

Eluent: 100mM aqueous sodium nitrate/acetonitrile 80/20

Flow rate: 1.0 mL/min

Temperature: 40 deg.C

Injection amount: 200 μ L

Multi-angle light scatter detectors: DAWN HELEOS II (Wyatt Technology Co., Ltd.)

Refractive Index (RI) detector: optilab T-rEX (Wyatt Technology Co., Ltd.)

As a result, the weight average molecular weight Mw was 89,078.

(Synthesis of silver nanowire)

A solution A was prepared by dissolving 0.302g of a propylene glycol solution having a lithium chloride content of 10 mass%, 0.893g of a propylene glycol solution having a potassium bromide content of 1 mass%, 0.0222g of lithium hydroxide, 0.312g of a propylene glycol solution having an aluminum nitrate nonahydrate content of 20 mass%, and 5.24g of the above-mentioned copolymer powder as a supply source of the organic protective agent in 513.5g of propylene glycol at room temperature. In this example, no alkyl ether was added to solution A. In a vessel different from this, 4.25g of silver nitrate was added to a mixed solution of 5.98g of propylene glycol and 0.5g of pure water, and the mixture was stirred at 35 ℃ to dissolve the silver nitrate, thereby obtaining a silver-containing solution B.

The solution A was put into a reaction vessel and heated from room temperature to 95 ℃ while stirring at 250rpm, and then the total amount of the solution B was added to the solution A for 1 minute from 2 addition ports using a tube pump. After the addition of solution B was completed, 4g of propylene glycol solution was added by using a tube pump to wash the tube to which solution B had adhered, and then the mixture was kept at 95 ℃ for 3.5 hours under stirring, cooled to 85 ℃ over 2.0 hours, and kept at 85 ℃ for 19 hours. Thereafter, the reaction solution was cooled to normal temperature to synthesize silver nanowires.

The reaction solution containing the synthesized silver nanowires at the stage of cooling to room temperature is referred to as a "post-synthesis reaction solution" (the same applies to each example below).

(measurement of average diameter and average Length of silver nanowire)

20g of the post-synthesis reaction solution was collected in a centrifugal precipitation tube, 180g of pure water was added thereto, and the mixture was centrifuged at 1500rpm for 15 minutes in a centrifuge. The concentrate and supernatant were observed, and thus the supernatant fraction was removed and the concentrate was recovered. This washing operation was further repeated several times to obtain a concentrate. The obtained concentrate was dispersed in pure water. In the measurement of the length of the silver nanowires, the dispersion was collected on an observation stage for SEM, water was volatilized on the observation stage, and then the dispersion was observed with a field emission type scanning electron microscope (manufactured by Hitachi ハイテクノロジーズ Co., Ltd.; S-4700) at an acceleration voltage of 3kV and a magnification of 1,500 times. For 3 or more randomly selected fields, the average length is measured as defined above using software (ドクターカンパス) for all lines whose total length can be confirmed within the field. In the diameter measurement, the dispersion was collected on an observation stage for TEM, a bright field image was observed at an acceleration voltage of 100kV and a magnification of 40,000 times with a transmission electron microscope (JEM-1011, manufactured by JE corporation), an observation image was collected, and the collected original image was enlarged to a size of 2 times to accurately measure the diameter, and further, the average diameter was measured by using software (Motic ImagePlus2.1S) in accordance with the above definition. The average aspect ratio is determined by substituting the values of the average length and the average diameter into the above expression (1). The silver nanowires had an average diameter of 26.7nm and an average length of 20.5 μm. The average length-diameter ratio of 20500(nm)/26.7(nm) is about 768. The results are summarized in Table 1 together with other examples and comparative examples.

[ example 1]

An experiment was performed under the same conditions as in comparative example 1 except that 0.0263g of methyl t-butyl ether was further mixed and dissolved in the solution a in addition to the respective substances mixed in comparative example 1 when synthesizing silver nanowires. The concentration of methyl t-butyl ether in the alcohol solvent at the time of starting the precipitation reaction of silver (i.e., at the time of starting the addition of solution B) was 0.598 mmol/L. The silver nanowires obtained under these conditions had an average diameter of 28.4nm and an average length of 23.3 μm. The average aspect ratio of 23300(nm)/28.4(nm) of about 820.

[ example 2]

In the synthesis of silver nanowires, experiments were carried out under the same conditions as in comparative example 1, except that 0.0798g of methyl t-butyl ether was further mixed and dissolved in the solution a in addition to the respective substances mixed in comparative example 1. The concentration of methyl t-butyl ether in the alcohol solvent at the time of starting the precipitation reaction of silver (i.e., at the time of starting the addition of solution B) was 1.811 mmol/L. The silver nanowires obtained under these conditions had an average diameter of 27.8nm and an average length of 24.5 μm. The average length-diameter ratio of 2450 (nm)/27.8(nm) is equal to 881.

[ example 3]

In the synthesis of silver nanowires, experiments were carried out under the same conditions as in comparative example 1, except that 0.1344g of methyl t-butyl ether was further mixed and dissolved as solution a in addition to the respective substances mixed in comparative example 1. The concentration of methyl t-butyl ether in the alcohol solvent at the time of starting the precipitation reaction of silver (i.e., at the time of starting the addition of solution B) was 3.049 mmol/L. The silver nanowires obtained under these conditions had an average diameter of 27.7nm and an average length of 22.7 μm. The average length-diameter ratio of 22700(nm)/27.7(nm) is about 819.

[ example 4]

In the synthesis of silver nanowires, experiments were carried out under the same conditions as in comparative example 1, except that 0.4250g of methyl t-butyl ether was further mixed and dissolved in the solution a in addition to the respective substances mixed in comparative example 1. The concentration of methyl t-butyl ether in the alcohol solvent at the time of starting the precipitation reaction of silver (i.e., at the time of starting the addition of solution B) was 9.643 mmol/L. The silver nanowires obtained under this condition had an average diameter of 27.0nm and an average length of 24.5 μm. The average length-diameter ratio of 2450 (nm)/27.0(nm) is about equal to 907.

[ example 5]

In the synthesis of silver nanowires, experiments were carried out under the same conditions as in comparative example 1, except that 0.5825g of methyl t-butyl ether was further mixed and dissolved in the solution a in addition to the respective substances mixed in comparative example 1. The concentration of methyl t-butyl ether in the alcohol solvent at the time of starting the precipitation reaction of silver (i.e., at the time of starting the addition of solution B) was 13.215 mmol/L. The silver nanowires obtained under these conditions had an average diameter of 27.0nm and an average length of 26.2 μm. The average length-diameter ratio of 26200(nm)/27.0(nm) is about equal to 970.

[ Table 1]

As can be seen from Table 1: when alkyl ether exists in alcohol solvent during silver nanowire synthesis, the average length of the synthesized wires can be obviously increased, and the average length-diameter ratio of the wires accompanied with the average length-diameter ratio is also increased.

For reference, SEM photographs of the silver nanowires obtained in comparative example 1 are illustrated in fig. 2, and SEM photographs of the silver nanowires obtained in example 5 are illustrated in fig. 3.

Measurement of the number of aggregates

Next, the number of aggregated particles was examined as follows for some of the above-described examples (comparative example 1, example 2, and example 5).

500mL of the post-synthesis reaction solution was dispensed and transferred to a 20L PFA bottle, followed by addition of 10kg of acetone and stirring for 15 minutes. Thereafter, the mixture was left to stand for 24 hours. After standing, the concentrate and supernatant were observed, and thus the supernatant part was removed to obtain a concentrate. To the obtained concentrate, an appropriate amount of 1 mass% PVP (polyvinylpyrrolidone) aqueous solution was added, and the mixture was stirred for 3 hours to confirm that the silver nanowires were redispersed. After stirring, 1kg of acetone was added thereto, and the mixture was stirred for 10 minutes and then allowed to stand. After standing, the concentrate and supernatant were re-observed, thus removing the supernatant fraction to give a concentrate. 80g of pure water was added to the obtained concentrate, and stirred for 12 hours to redisperse the silver nanowires. To the re-dispersed silver nanowire dispersion, 1kg of acetone was added, followed by stirring for 30 minutes and then standing. After standing, the concentrate and supernatant were re-observed, thus removing the supernatant fraction to give a concentrate. To the concentrate obtained, an appropriate amount of 0.5 mass% PVP aqueous solution was added, and the mixture was stirred for 12 hours. The stirred liquid containing silver nanowires was diluted with pure water to adjust the content of silver nanowires to 0.05 mass%. Thus, the cleaned liquid containing silver nanowires was obtained.

The number of particles having a particle diameter of 10 μm or more and 300 μm or less present in the silver nanowire-containing liquid after the washing was measured by dark field measurement using a flow particle image analyzer (FPIA-3000S, manufactured by シスメックス Co.). Since the medium of the liquid containing silver nanowires is water, pure water is used as the sheath liquid (シース liquid) in the measurement. The measurement conditions were LPF measurement mode, quantitative counting mode, and 10 times of repeated measurement. The maximum length is limited to 10 to 300 μm, and the average luminance is limited to 0 to 40. The number of aggregates in the silver-nanowire-containing liquid per 55. mu.L was counted as 10 measurement values. The results are shown in Table 2.

[ Table 2]

As can be seen from Table 2: the presence of the alkyl ether in the alcohol solvent at the time of silver nanowire synthesis is effective in suppressing aggregation of the wires with each other, in addition to increasing the average aspect ratio of the synthesized silver nanowires.

Claims

1. A process for producing silver nanowires, which comprises a step of precipitating silver into a linear form by reduction in an alcohol solvent in which a silver compound and an organic protective agent are dissolved,

the reduction precipitation is carried out in the alcohol solvent in a state where the alkyl ether is dissolved in the alcohol solvent at a concentration of 0.3 to 25.0 mmol/L.

2. The method for producing silver nanowires according to claim 1, wherein the average aspect ratio A defined by the following formula (1) and having an average length of 15 μm or more and an average diameter of 35nm or less is reduced and precipitated_MIs a silver nano-wire with the length of more than 800,

A_M＝L_M/D_M…(1)

3. The method for manufacturing silver nanowires of claim 1, wherein the alkyl ether is methyl tert-butyl ether.

4. The method for producing silver nanowires according to claim 1, wherein the polymer is PVP (polyvinylpyrrolidone) or a copolymer of vinylpyrrolidone and a hydrophilic monomer.

5. The method for producing silver nanowires according to claim 1, wherein the polymer has a polymerization composition of vinylpyrrolidone and 1 or 2 or more monomers selected from the group consisting of diallyldimethylammonium salt, ethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-t-butylmaleimide, 2-dimethylaminoethyl methacrylate and 2-diethylaminoethyl methacrylate.

6. The method for producing silver nanowires according to claim 1, wherein the polymer has a weight average molecular weight Mw of 30,000 to 300,000.

7. Silver nanowires obtained by the production method according to claim 1.

8. Silver nanowire ink, wherein the silver nanowires obtained by the production method according to claim 1 are dispersed in a liquid medium.

9. A transparent conductive film comprising the silver nanowire obtained by the production method according to claim 1 as a conductive material.