CN111032257A

CN111032257A - Method for manufacturing silver nanowire, silver nanowire ink, and transparent conductive film

Info

Publication number: CN111032257A
Application number: CN201880051097.5A
Authority: CN
Inventors: 佐藤王高
Original assignee: Dowa Electronics Materials Co Ltd
Current assignee: Dowa Electronics Materials Co Ltd
Priority date: 2017-08-09
Filing date: 2018-08-08
Publication date: 2020-04-17
Also published as: WO2019031564A1; JP2019031726A; JP6539315B2; TWI686484B; TW201917220A

Abstract

The subject is as follows: particularly fine wires are stably generated when silver nanowires are synthesized by an alcohol solvent reduction method. The solution is as follows: a method 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, wherein the following powder is used as a supply source of the organic protective agent: the silver nanowire comprises a polymer having a vinylpyrrolidone structural unit as a main component, and an acetate in an amount of 0.002 to 0.040 mol based on 1 mol of the polymer, and has an average diameter D_MIs 30nm or less.

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. Also disclosed are a silver nanowire, a silver nanowire ink and a transparent conductive film, which are obtained by such a 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 (Dia l lyethylenelammonium) 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 monomer. In the alcohol solvent reduction method using these polymers as an organic protective agent, fine and long silver nanowires equal 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. The present invention provides a technique which has a good effect of stably producing a particularly fine silver nanowire when a silver nanowire is synthesized by an alcohol solvent reduction method.

Means for solving the problems

The above object is achieved by using a polymer powder containing a predetermined amount of acetic ester as an organic protective agent supply source. In the present specification, the following inventions are disclosed.

[1]A method 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, wherein the following powder is used as a supply source of the organic protective agent: the silver nanowire comprises a polymer having a vinylpyrrolidone structural unit as a main component, and an acetate in an amount of 0.002 to 0.040 mol based on 1 mol of the polymer, and has an average diameter D_MIs 30nm or less.

[2]According to the above [1]The method for producing a silver nanowire, wherein the powder is used as a supply source of an organic protective agent to reduce and precipitate an average aspect ratio A defined by the following formula (1)_MA silver nanowire satisfying the following expression (2).

A_M＝L_M/D_M…(1)

A_M≧45D_M-650…(2)

Wherein L is_MIs the average length (nm), D, of the silver nanowires_MIs the average diameter (nm) of the silver nanowires.

[3] The method for producing silver nanowires according to the above [1] or [2], wherein the acetate is 1 or 2 or more of methyl acetate, ethyl acetate, propyl acetate, and butyl acetate.

[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 and N-t-butylmaleimide.

[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 ].

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. Among them, since the average length is evaluated at a stage after the silver nanowires recovered from the liquid after the reduction reaction is completed are washed (at a stage before they are supplied to a purification step such as cross-flow filtration), it is necessary to calculate the average length of the wires excluding impurities such as particulate matters and short linear products inevitably contained in the recovered products. Therefore, the linear product having a length of less than 3.0 μm is excluded from the measurement object.

[ 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. In fig. 4 is illustrated a method according to the inventionBright field observation images using TEM of bright silver nanowires (hereinafter referred to as "TEM images"). 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. Wherein the linear product having a length of less than 3.0 μm is excluded from the measurement object as described above.

[ 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, very fine silver nanowires having an average diameter of 30nm or less, or particularly 28nm or less can be stably obtained. 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 NMR spectrum of the polymer powder used in example 1.

FIG. 3 is an NMR spectrum of the polymer powder used in comparative example 1.

Fig. 4 is an example of a TEM image of silver nanowires according to the present invention.

Detailed Description

In the present specification, a powder containing a polymer as a main component is referred to as "polymer powder". The powder is an aggregate of solid particles, and is divided into a dry powder containing no liquid component and an undried powder containing a liquid component between solid particles. The latter form includes, for example, a solid component in a state where solid-liquid separation is completed and recovered. The solid particles constituting the polymer powder are considered to be mainly particles in which polymer molecules are aggregated. The term "mainly composed of a polymer" means that the powder is preferably one in which the polymer accounts for at least 50% by mass, the polymer accounts for 90% by mass or more, and the powder contains the polymer accounts for 95% by mass or more.

[ Polymer powder of organic protecting agent ]

As an organic protective agent for coating the metallic silver surface of the silver nanowire, a polymer having a vinylpyrrolidone structural unit is used herein. The structural formula of the vinylpyrrolidone structural unit is shown in FIG. 1. PVP (polyvinylpyrrolidone), which is a homopolymer, or a copolymer of a monomer other than vinylpyrrolidone and vinylpyrrolidone, is a polymer having a vinylpyrrolidone structural unit. PVP has been used as an organic protectant suitable for use in the synthesis of practical silver nanowires. However, as described above, a liquid medium containing an alcohol to improve wettability to a substrate such as PET has a disadvantage that the linear dispersibility is lowered. According to the studies of the present inventors, by using a copolymer of vinylpyrrolidone and a monomer other than vinylpyrrolidone, dispersibility in a liquid medium to which an alcohol is added can be improved. It was also confirmed that even with such a copolymer, silver nanowires having a long and thin practical shape could be obtained.

Recently, demands for further improvement in the conductivity and low haze of the transparent conductor are increasing. In order to achieve both the improvement of conductivity and the improvement of low haze, it is very effective to use silver nanowires as a conductive material in a finer shape and with an increased aspect ratio. As a result of intensive studies, the present inventors have newly found that: when silver nanowires are reduced and deposited by using a polymer having a vinylpyrrolidone structural unit as an organic protective agent in an alcohol solvent reduction method, a polymer powder containing the polymer as a main component and a predetermined amount of acetate is used as a supply source of the organic protective agent, whereby very fine silver nanowires can be synthesized more stably. Moreover, the length is sufficiently ensured, and silver nanowires having a high average aspect ratio can be recovered.

PVP (polyvinylpyrrolidone) or a copolymer of vinylpyrrolidone and a hydrophilic monomer is a preferable target as a polymer having a vinylpyrrolidone structural unit. 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).

Acetate may be added as an organic solvent when purifying the polymer produced during the synthesis of the above polymer. In this case, usually, acetate attached to the polymer molecule is present in the powder product. When the amount of the organic protective agent to be present does not satisfy the predetermined amount (described later) required in the present invention, or when a polymer powder obtained by a synthesis method in which no acetate is added is used, for example, the polymer powder of the organic protective agent in which the acetate content is adjusted to a predetermined range can be prepared by allowing an acetate molecule to adhere to a polymer molecule by a method such as "polymer purification treatment". The polymer purification treatment is a treatment for removing impurities such as unreacted monomers, polymerization initiators, solvent substances, etc., mixed in the synthesized polymer product, thereby improving the purity of the polymer. Since the organic protective agent used for reductive deposition of silver nanowires is generally a hydrophilic polymer having a large polarity, the purification treatment can be performed by, for example, the following procedure. When an organic solvent in which a hydrophilic polymer synthesized by polymerization is dissolved is dropped into a large excess amount of an organic solvent composed of dimethyl ether having a small polarity, ethers such as diethyl ether and ethyl methyl ether, or acetates such as methyl acetate and ethyl acetate, a water-soluble polymer having a large polarity is precipitated. Since the unreacted monomers, the polymerization initiator, the solvent substances, and the like as impurities are kept dissolved in the organic solvent, the precipitated solid matter (polymer) is collected by filtration or the like, whereby a high-purity polymer with less impurities can be obtained. This purification operation is repeated until a polymer having the desired purity can be obtained. In the "polymer purification treatment" thus performed, when acetate is used as a large excess of organic solvent having a small polarity, polymer molecules having acetate molecules attached thereto can be recovered. The amount of acetate deposited (the acetate content in the polymer powder) can be adjusted by controlling the number of times of the purification process.

In the present invention, a polymer powder having an acetate content within a predetermined range is used as a supply source of the organic protective agent. That is, the polymer powder described above is used as the organic protective agent-containing substance for preparing a liquid in which the organic protective agent is dissolved. In other words, in the solvent when silver nanowires are synthesized by the alcohol solvent reduction method, there is an organic protective agent derived from the polymer powder as described above.

In order to deposit metallic silver in a linear form by the alcohol solvent reduction method, it is necessary to selectively adsorb polymer molecules of the organic protective agent on {100} planes of the nuclear crystals which are thought to be multiple twins of silver. This suppresses the growth of the {100} plane and preferentially grows the {111} plane, thereby forming 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. The surface potential of the polymer molecule changes due to the attachment (adsorption) state of the organic compound molecule attached to the polymer molecule. That is, the deviation of the charge of the polymer molecule (bias り) varies depending on the type of the organic compound to be attached or the amount of the organic compound to be attached, and the selective adsorption property to the {100} plane of silver varies. As described later, the polymer powder containing a predetermined amount of acetate is very effective for synthesis of fine silver nanowires. From this fact, acetate is presumed to be a very effective substance in imparting a surface potential for improving the selective adsorption to the silver {100} plane to a polymer having a vinylpyrrolidone structural unit.

In addition, it is considered that acetate has an effect of purifying the {111} crystal face from which silver is preferentially deposited when silver nanowires are synthesized by an alcohol solvent reduction method, that is, an effect of promoting deposition of new silver by activating the exposed {111} crystal face while suppressing adsorption of organic protective agent molecules to the {111} crystal face. The action of activating the {111} crystal face is mainly responsible for the action of a halide or the like which has been a general additive in the past, but acetate is also presumed to have an action similar to this. It is considered that when acetate is present in addition to halogen in the vicinity of the linear structure of the metal silver that has already been deposited, the above-described purifying action is 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, deposition growth of fine nanowires is promoted.

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 acetate, the problem of the deterioration of the transparent conductive film with time as described above can be avoided.

The polymer molecules of the organic protective agent present in the solvent during the synthesis of the silver nanowires preferentially adsorb to the {100} crystal planes of metallic silver, which has a relatively low electron density. Even if the organic protective agent is the same type, the smaller the molecular weight of the polymer, the greater the adsorption force to metallic silver tends to be, and therefore, it is advantageous to use a polymer having a small molecular weight for synthesizing the fine wire. However, even when the adsorption force is large, adsorption to the {111} crystal plane, which is desired to be preferentially precipitated, is likely to occur. Therefore, when a polymer having an excessively small molecular weight is used as the organic protective agent, short-length threads are easily formed, and it is difficult to improve the aspect ratio. By the above-described reinforcement of the selective adsorption property to the silver {100} crystal plane and the reinforcement of the purification effect of the {111} crystal plane by acetate, there is an advantage that the degree of freedom of selecting a polymer having a relatively low molecular weight can be increased, and a fine and long thread can be easily synthesized.

As a result of various studies, the content of acetate in the polymer powder is preferably set to a ratio of 0.002 to 0.040 mol with respect to 1 mol of a polymer having a vinylpyrrolidone structural unit as a main component of the polymer powder. Hereinafter, the molar ratio of the acetate to 1 mol of the polymer may be referred to as "acetate/polymer molar ratio". The molar ratio of acetate/polymer is more preferably 0.0025 or more, and may be controlled to be, for example, 0.0025 to 0.030. The molar ratio of acetate to polymer can be determined from the NMR (nuclear magnetic resonance) spectrum of the polymer powder. When the content of acetate is too small, the above-mentioned effect of easily forming fine lines cannot be sufficiently obtained. On the other hand, if the content is too large, aggregation of the wires is likely to occur depending on the type of the liquid medium when the silver nanowire dispersion is prepared. When the ratio of the amounts of the acetate and the polymer is observed in terms of mass ratio, for example, when ethyl acetate is used, the content of ethyl acetate is preferably adjusted within a range of 0.2 to 3.5 parts by mass relative to 100 parts by mass of the polymer having a vinylpyrrolidone structural unit with a weight average molecular weight Mw of about 30,000 to 150,000.

The content of the "polymer having a vinylpyrrolidone structural unit" in the polymer powder is preferably 50% by mass or more, and as the remaining component, other components than the above-mentioned predetermined amount of acetate may be contained in the range in which the silver nanowire can be produced. When it is desired to use a polymer powder having a polymer functioning as an organic protective agent and a purity as high as possible, for example, a polymer powder having a polymer content of 90 mass% or more, more preferably 95 mass% or more, and the balance being the predetermined amount of acetate and components mixed in the production process of the polymer can be used. Examples of the component mixed in the production process of the polymer include additives such as TBME (t-butyl methyl ether) and MIBK (methyl isobutyl ketone) in addition to VP (vinyl pyrrolidone) which is a residual monomer component. According to investigations so far, it can be seen that the lower the MIBK content, the more favorable the tendency in synthesizing silver nanowires having a large average aspect ratio. The MIBK content is preferably adjusted within a range of 1.0 part by mass or less with respect to 100 parts by mass of the polymer. The amount of the polymer powder used in the silver nanowire synthesis can be set within a range of an appropriate amount to be used in the conventional art according to the production conditions.

Examples of acetates include: acetic acid methyl ester (C)₃H₆O₂) Ethyl acetate (C)₄H₈O₂) Propyl acetate (C)₅H₁₀O₂) Butyl acetate (C)₆H₁₂O₂) And the like. These acetates are liquid at ordinary temperature when used as monomers, but when present in the above-mentioned content range adhering to the polymer molecules of the organic protective agent, they take the form of a solid substance (powder) as a whole. The acetic acid ester may be used in 1 or 2 or more species.

The contents of the respective components in the polymer powder can be determined from an NMR spectrum measured by nuclear magnetic resonance spectroscopy (NMR). For example, in the NMR spectrum, ethyl acetate peaks appear at around 4.1ppm and around 1.2 to 1.3ppm, TBME peaks appear at around 3.2 to 3.3ppm and around 1.2ppm, and MIBK peaks appear at around 0.9 ppm. Further, the content ratio VP of VP as a residual monomer_RCan be obtained from the following expression (3).

VP_R(mol％)＝[2×(I₁+I₂)/(3×I₃)]×100…(3)

Wherein, I₁Is the integral value of the peak (7.0-7.2ppm) derived from the methyl proton associated with the C ═ C double bond of the VP monomer, I₂Is 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。

Acetate molecules are adsorbed in the molecules of the polymer synthesized using acetate. When the content of acetate is within the above-specified range, the polymer powder product containing the polymer synthesized by the above-described method as the main component can be used as it is as an organic protective agent supply source in the silver nanowire synthesis. On the other hand, when the polymer powder product does not contain acetate or when the content does not satisfy the above-described predetermined range even if it is contained, it is necessary to perform a treatment of attaching acetate to the polymer molecules to adjust the acetate content in the polymer powder. In addition, the polymer powder product generally contains impurity components (a chain transfer agent component containing sulfur, residual vinylpyrrolidone monomer, and the like) which are not necessary for the synthesis of silver nanowires. Hereinafter, a method of performing a treatment of attaching acetate to polymer molecules by "purification treatment of polymer powder" for reducing the content of impurity components in the polymer powder will be described.

(exemplary Polymer purification treatment)

First, a polymer powder as a treatment object was dissolved in a chloroform solvent to obtain a polymer-containing liquid. In the chloroform solvent, various impurity components other than the polymer were dissolved. When the liquid is dropped into a solvent composed of an acetate (for example, ethyl acetate), the polymer is insoluble in the acetate solvent and is precipitated in the acetate solvent. On the other hand, most of the impurity components soluble in the acetate solvent remain dissolved in the liquid and remain. However, some of them are present with accompanying polymer. The solid component precipitated was collected by filtration. Ethyl acetate is attached to the polymer molecules of the recovered solid component. The dried solid was dissolved again in a fresh chloroform solvent, and the solution was added dropwise to fresh acetate to precipitate a polymer, which was recovered as a solid. By repeating the purification treatment of the dissolution and precipitation operations, the amount of impurities in the polymer powder can be reduced, and the amount of acetate adhering to the polymer molecules can be adjusted.

[ 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. The present invention is directed to silver nanowires having an average diameter of 30nm or less, preferably 28nm or less. The larger the average aspect ratio is, the more preferable the average aspect ratio is, in general, the smaller the average diameter of the wire is, the more advantageous the reduction of the haze of the transparent conductive film is, and therefore, the degree of freedom of the average aspect ratio is also expanded along with this. As a result of various studies, the average length and width A satisfying the following formula (2) is preferably selected at the stage after the synthesis of the silver nanowire_MSilver nanowires satisfying the following formula (2)' are more preferable objects. Among them, even when these formulae are satisfied, the average length is preferably 6.5 μm or more.

A_M≧45D_M-650…(2)

A_M≧45D_M-630…(2)’

With respect to average length, this can be increased by removing the stubs by performing a refining operation of the wires (e.g., cross-flow refining) after silver nanowire synthesis. However, the average diameter is determined approximately by whether or not a thin wire is stably synthesized at the time of the reductive precipitation reaction. That is, if a thin line is not synthesized, it is very difficult to control the average diameter thereafter. According to the present invention, very fine silver nanowires having an average diameter of 30nm or less or 28nm or less can be reduced and precipitated.

[ Synthesis of silver nanowires ]

Silver nanowires were synthesized by a method of precipitating silver by reduction in an alcohol solvent in which a silver compound and an organic protective agent are dissolved, to form a linear form (alcohol solvent reduction method). This method has been put into practical use as a method for synthesizing silver nanowires. In addition to the silver compound and the organic protective agent, it is preferable to perform reductive precipitation in an alcohol solvent in which chloride and bromide are dissolved. 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, the method disclosed in patent document 1 can be used. In the present invention, a powder containing a polymer having a vinylpyrrolidone structural unit as a main component and acetate in a proportion of 0.002 to 0.040 mol based on 1 mol of the polymer is used as a supply source of an organic protective agent, and the polymer supplied from the powder is dissolved in the alcohol solvent. The acetate present in the powder adheres to the polymer molecules, with the polymer being introduced into the alcohol solvent.

Examples

[ preparation of Polymer powder ]

(examples of copolymers of vinylpyrrolidone and diallyldimethylammonium salt)

As a raw material polymer powder, several production batches of powder synthesized by this method were prepared by dissolving 1-vinyl 2 pyrrolidone and diallyldimethylammonium nitrate (diisobutyl dimethyl ammonium nitrate) in methyl isobutyl ketone as a solvent and adding a polymerization initiator to copolymerize them. In each manufacturing batch, the polymerization composition was 1-vinyl 2 pyrrolidone in terms of mole ratios: diallyl dimethyl ammonium nitrate ═ 99: 1, but the amount of ethyl acetate used in the synthesis process was different. There were also production lots synthesized without using ethyl acetate. The above-mentioned "polymer purification treatment" was carried out using a powder composed of 1 of these plural kinds of raw polymer powders or a powder obtained by blending 2 or more kinds of raw polymer powders, and polymer powders a to G adjusted to various ethyl acetate contents were prepared by changing the number of repetitions of the dissolution-precipitation operation in the treatment.

[ analysis of Polymer powder ]

The contents of ethyl acetate, TBME (tert-butyl methyl ether), MIBK (methyl isobutyl ketone), and VP (vinyl pyrrolidone) as a residual monomer in the polymer powder were determined from a 1H NMR spectrum measured by nuclear magnetic resonance spectroscopy (NMR). The mole% of ethyl acetate was calculated using the integrated value of the peak near 4.1ppm, the mole% of TBME was calculated using the integrated value of the peak near 1.2ppm, and the mole% of MIBK was calculated using the integrated value of the peak near 0.9 ppm. The VP content is determined by the above formula (3). An NMR apparatus manufactured by JNM-LA400(400MHz) manufactured by Japan Electron society was used for the measurement of the 1H NMR spectrum.

The weight average molecular weight Mw of the polymer in the powder 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.)

[ example 1]

(Synthesis of silver nanowire)

4.84g of a propylene glycol solution having a lithium chloride content of 10 mass%, 0.1037g of potassium bromide, 0.426g of lithium hydroxide, 4.994g of a propylene glycol solution having an aluminum nitrate nonahydrate content of 20 mass%, and 83.875g of polymer powder serving as a supply source of an organic protective agent were dissolved in 8116.3g of propylene glycol at room temperature to prepare a solution A. As a supply source of the organic protective agent, polymer powder a containing 0.0299 mol of ethyl acetate per 1 mol of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used. In a vessel different from this, 67.96g of silver nitrate was added to a mixed solution of 95.70g of propylene glycol and 8.00g of pure water, and the mixture was stirred at 35 ℃ to dissolve the silver nitrate, thereby obtaining a silver-containing solution B. After the solution A was placed in a reaction vessel and heated from room temperature to 90 ℃ while stirring at 175rpm, the total amount of the solution B was added to the solution A from 2 addition ports over 1 minute. After the addition of solution B was completed, the mixture was further kept under stirring at 90 ℃ for 24 hours. Thereafter, the reaction solution was cooled to normal temperature to synthesize silver nanowires.

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

20g of the reaction solution cooled to room temperature was collected in a centrifugal precipitation tube, 180g of pure water was added thereto, and the reaction solution was centrifuged at 1500rpm for 15 minutes in a centrifugal separator. 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 to 2,500. For 3 or more fields of view selected at random, the average length is measured in accordance with the above definition, with respect to all lines in which the entire length can be confirmed within the field of view. In the diameter measurement, the dispersion was collected on an observation stage for TEM, and a bright field image was observed with a transmission electron microscope (JEM-1011, manufactured by JEM corporation) at an acceleration voltage of 100kV and a magnification of 40,000 to 100,000, and the average diameter was measured 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 25.1nm and an average length of 14.3 μm. The average length-diameter ratio of 14300(nm)/25.1(nm) is about equal to 570. The results are summarized in Table 1 together with other examples and comparative examples. For reference, the composition of the polymer powder converted into a mass ratio is shown in table 2 together with other examples and comparative examples.

[ example 2]

An experiment was performed under the same conditions as in example 1 except that polymer powder B containing 0.0102 moles of ethyl acetate per 1 mole of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used as a supply source of the organic protective agent in the synthesis of silver nanowires. As a result, the silver nanowires obtained had an average diameter of 25.3nm and an average length of 15.8 μm. The average length-diameter ratio of 15800(nm)/25.3(nm) is about 625.

[ example 3]

An experiment was performed under the same conditions as in example 1 except that polymer powder C containing 0.0031 mol of ethyl acetate per 1 mol of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used as a source of supplying the organic protective agent when synthesizing the silver nanowires. As a result, the silver nanowires obtained had an average diameter of 26.3nm and an average length of 15.8 μm. The average length-diameter ratio of 15800(nm)/26.3(nm) of about 601.

[ example 4]

An experiment was performed under the same conditions as in example 1 except that polymer powder D containing 0.0196 moles of ethyl acetate per 1 mole of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used as a source of the organic protective agent for synthesizing the silver nanowires. As a result, the silver nanowires obtained had an average diameter of 24.7nm and an average length of 16.1 μm. The average aspect ratio of 16100(nm)/24.7(nm) approximately equal to 652.

[ example 5]

An experiment was performed under the same conditions as in example 1 except that polymer powder E containing 0.0242 mol of ethyl acetate per 1 mol of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used as a supply source of the organic protective agent in the synthesis of silver nanowires. As a result, the silver nanowires obtained had an average diameter of 26.8nm and an average length of 20.4 μm. The average length-diameter ratio of 20400(nm)/26.8(nm) approximately equal to 761.

Comparative example 1

An experiment was performed under the same conditions as in example 1 except that polymer powder F having an ethyl acetate content of 0.000 mol (below the measurement limit) per 1 mol of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used as a supply source of the organic protective agent in the synthesis of silver nanowires. As a result, the silver nanowires obtained had an average diameter of 39.6nm and an average length of 19.6. mu.m. The average aspect ratio of 19600(nm)/39.6(nm) is about equal to 495.

Comparative example 2

An experiment was performed under the same conditions as in example 1 except that polymer powder G containing 0.0004 mol of ethyl acetate per 1 mol of the copolymer of vinylpyrrolidone and diallyldimethylammonium salt was used as a supply source of the organic protective agent in the synthesis of the silver nanowires. As a result, the silver nanowires obtained had an average diameter of 35.8nm and an average length of 14.1 μm. The average length-diameter ratio of 14100(nm)/35.8(nm) is about equal to 394.

[ Table 2]

As can be seen from Table 1: in each example using a polymer powder containing ethyl acetate in the content range specified in the present invention as a supply source of the organic protective agent, very fine silver nanowires having an average diameter of less than 30nm could be synthesized. The average aspect ratio of these silver nanowires is substantially over 500. On the other hand, in each comparative example using a polymer powder containing no ethyl acetate in the content range specified in the present invention, it was not possible to synthesize a yarn having an average diameter of less than 30 nm.

For reference, an NMR spectrum of the polymer powder a used in example 1 is illustrated in fig. 2, and an NMR spectrum of the polymer powder F used in comparative example 1 is illustrated in fig. 3. In these figures, symbol a indicates a peak due to ethyl acetate, symbol b indicates a peak due to TBME (tert-butyl methyl ether), symbol c indicates a peak due to MIBK (methyl isobutyl ketone), and symbol p indicates a peak due to a polymer.

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,

wherein the following powders are used as the supply source of the organic protective agent: comprising a polymer having a vinylpyrrolidone structural unit as a main component, and 0.002 to 0.040 mol of an acetate based on 1 mol of the polymer,

the average diameter D of the silver nanowires_MIs 30nm or less.

2. The method ofThe method for producing a silver nanowire according to claim 1, wherein the powder is used as a source for supplying an organic protective agent to reduce and precipitate an average aspect ratio A defined by the following formula (1)_MA silver nanowire satisfying the following expression (2),

A_M＝L_M/D_M…(1)

A_M≧45D_M-650…(2)

3. The method for producing silver nanowires according to claim 1 or 2, wherein the acetate is 1 or 2 or more of methyl acetate, ethyl acetate, propyl acetate, and butyl acetate.

4. The method for producing silver nanowires according to any one of claims 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 claims 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 and N-t-butylmaleimide.

6. The method for producing silver nanowires according to any one of claims 1 to 5, 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 any one of claims 1 to 6.

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

9. A transparent conductive film comprising the silver nanowires obtained by the production method according to any one of claims 1 to 6 as a conductive material.