WO2019031564A1

WO2019031564A1 - Method for producing silver nanowires, silver nanowires, silver nanowire ink and transparent conductive film

Info

Publication number: WO2019031564A1
Application number: PCT/JP2018/029841
Authority: WO
Inventors: 王高佐藤
Original assignee: Ｄｏｗａエレクトロニクス株式会社
Priority date: 2017-08-09
Filing date: 2018-08-08
Publication date: 2019-02-14
Also published as: JP6539315B2; JP2019031726A; TWI686484B; TW201917220A; CN111032257A

Abstract

[Problem] To stably produce particularly thin wires in cases where silver nanowires are synthesized by means of an alcohol solvent-based reduction method. [Solution] A method for producing silver nanowires, which comprises a step wherein silver is reduced and precipitated in the form of wires in an alcohol solvent in which a silver compound and an organic protection agent are dissolved. This method for producing silver nanowires is configured such that: a powder which is mainly composed of a polymer having a vinyl pyrrolidone structural unit and contains an acetic acid ester in an amount of from 0.002 to 0.040 mole relative to 1 mole of the polymer is used as a supply source of the organic protection agent; and the silver nanowires have an average diameter DM of 30 nm or less.

Description

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

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

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

Silver nanowires are considered promising as a conductive material for imparting conductivity to transparent substrates. After the liquid containing silver nanowires (silver nanowire ink) is applied to a transparent substrate such as glass, PET (polyethylene terephthalate), PC (polycarbonate), etc., the liquid component is removed by evaporation or the like, the silver nanowires are on the substrate Since a conductive network is formed by contacting each other, a transparent conductive film can be realized.

A transparent conductive film used for a touch panel or the like of an electronic device is required to have clear visibility with little haze, in addition to good conductivity. In order to achieve both conductivity and visibility at a high level in a transparent conductive film using silver nanowires as a conductive material, it is advantageous to apply silver nanowires that are as thin and long as possible.

Conventionally, as a synthesis method of silver nanowires, for example, a silver compound is dissolved in a polyol solvent such as ethylene glycol, and in the presence of a halogen compound and an organic protective agent, a linear shape is obtained using the reducing power of the solvent polyol. A method of precipitating metallic silver (hereinafter referred to as "alcohol solvent reduction method") is known. As the organic protective agent, PVP (polyvinyl pyrrolidone) has generally been widely used. PVP is a suitable organic protective agent for depositing thin and long silver nanowires.

The molecules of the organic protective agent used in the alcohol solvent reduction method adsorb on the surface of the silver nanowire after synthesis, and become a factor governing the dispersibility of the silver nanowire in the liquid medium. Silver nanowires adsorbed with PVP exhibit good dispersibility in water. However, in order to improve the wettability to a substrate such as PET, it is advantageous to apply a silver nanowire ink using a mixed medium of water and an organic solvent (for example, alcohol). In addition, depending on the coating equipment, it may be desirable to apply a silver nanowire ink using a non-aqueous solvent. When considering the dispersibility of silver nanowires in such mixed media and non-aqueous solvents, 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 liquid media other than water. For example, Patent Document 1 is a copolymer having a polymerization composition of vinyl pyrrolidone and Diallyldimethylammonium salt monomer, Patent Document 2 is a copolymer of vinyl pyrrolidone and an acrylate based or methacrylate based monomer, and Patent Document 3 is Vinyl. Copolymers of pyrrolidone and maleimide based monomers are disclosed, respectively. In the alcohol solvent reduction method using these polymers as organic protective agents, it is possible to synthesize thin and long silver nanowires by optimizing the synthesis conditions, as well as or more than using PVP. is there.

JP, 2015-180772, A JP, 2017-78207, A JP, 2016-135919, A

As described above, it is advantageous that the silver nanowires used as the conductive material of the transparent conductive coating film have a thin and long form from the viewpoint of achieving both conductivity and visibility at a high level. The present invention is intended to provide a highly effective technique for producing a particularly thin wire stably when synthesizing silver nanowires by an alcohol solvent reduction method.

The above object is achieved by applying a polymer powder blended with a predetermined amount of acetic acid ester as an organic protective agent source. The following invention is disclosed in the present specification.

[1] A method for producing a silver nanowire comprising the step of reducing and precipitating silver in the form of a wire in an alcohol solvent in which a silver compound and an organic protective agent are dissolved, wherein a vinylpyrrolidone structural unit is used as a source of the organic protective agent. as a main component a polymer with the polymer 1 mole use powder in a proportion of 0.002 to 0.040 moles of acetic acid ester, the preparation of silver nanowires average diameter D _M is 30nm or less.
The use of the powder as a source of [2] an organic protective agent, the average aspect ratio A _M determined by the following equation (1) causes the reduction precipitation of silver nanowires satisfies the following equation (2), [1 ] The manufacturing method of the silver nanowire as described in.
A _M = L _M / D _M (1)
A _M 45 45 D _M- 650 (2)
Here, L _M is the average length (nm) of the silver nanowires, and D _M is the average diameter (nm) of the silver nanowires.
[3] The method for producing a silver nanowire according to the above [1] or [2], wherein the acetate ester is one 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 polymer is vinyl pyrrolidone, diallyl dimethyl ammonium salt, ethyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, N-methyl maleimide, N-ethyl maleimide, N-propyl The method for producing a silver nanowire according to any one of the above [3], which has a polymerization composition with one or more monomers selected from maleimide and N-tert-butyl maleimide.
[6] The method for producing a silver nanowire 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 method according to any one of the above [1] to [6].
[8] A silver nanowires ink, wherein the silver nanowires obtained by the method according to any one of the above [1] to [6] are dispersed in a liquid medium.
[9] A transparent conductive film containing, as a conductive material, the silver nanowire obtained by the 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 silver nanowires follow the definitions below.

[Average length L _M ]
The trace length from one end of one silver nanowire to the other end of a silver nanowire is defined as the length of the wire on an image observed by a field emission scanning electron microscope (FE-SEM). A value obtained by averaging the lengths of the individual silver nanowires present on the microscopic image is defined as an average length L _M. In order to calculate the average length, the total number of wires to be measured is set to 100 or more. Here, in order to evaluate the average length of the stage where the silver nanowires recovered from the solution after the reduction reaction were washed (the stage before being supplied to the purification step such as cross flow filtration) It is necessary to calculate the average length of the wire excluding impurities such as particulate matter and short wire-like products contained in the above. Therefore, wire-like products having a length of less than 3.0 μm are excluded from the measurement target.

[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 one silver nanowire is defined as the diameter of the wire. FIG. 4 exemplifies a TEM bright-field observation image (hereinafter referred to as “TEM image”) of silver nanowires according to the present invention. Each wire can be considered to have a substantially even thickness over its entire length. Therefore, thickness measurement can be performed by selecting a portion that does not overlap other wires. In the TEM image of one field of view, among the silver nanowires observed in the image, the diameters of all the wires except for the wires completely overlapping with other wires and whose diameter is difficult to measure are measured. The operation is performed on a plurality of randomly selected fields of view, the diameters of a total of 100 or more different silver nanowires are determined, the average value of the diameters of the individual silver nanowires is calculated, and the value is defined as the average diameter D _M. Here, as described above, wire-like products having a length of less than 3.0 μm are excluded from the measurement object.

[Average aspect ratio]
The average aspect ratio _AM is calculated by substituting the above average diameter D _M and the average length L _M into the following equation (1). However, it is assumed that D _M and L _M to be substituted into the equation (1) are both values expressed in nm.
A _M = L _M / D _M (1)

According to the present invention, silver nanowires having an average diameter of 30 nm or less, or particularly 28 nm or less, can be stably obtained. When this is used as a conductive material of a transparent conductive film, a transparent conductive film excellent in visibility with little haze can be realized while maintaining high conductivity.

Structural formula of vinyl pyrrolidone structural unit. NMR spectrum of the polymer powder used in Example 1. NMR spectrum of polymer powder used in Comparative Example 1. An example of a TEM image for silver nanowires according to the present invention.

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 classified into a dry powder containing no liquid component and a non-dried powder in which the liquid component is present between solid particles. The latter embodiment includes, for example, solid content in a state of being recovered after completion of solid-liquid separation. The solid particles constituting the polymer powder are considered to be mainly particles in which polymer molecules are aggregated. "Based on a polymer" means that the polymer accounts for at least 50% by mass of the material constituting the powder, but a powder having 90% by mass or more of the polymer is more preferable It is an object of the present invention to be a further preferable object of powder in which the polymer is 95% by mass or more.

[Polymer powder of organic protective agent]
Here, a polymer having a vinylpyrrolidone structural unit is adopted as an organic protective agent covering the metallic silver surface of the silver nanowire. The structural formula of a vinyl pyrrolidone structural unit is shown in FIG. The homopolymer PVP (polyvinyl pyrrolidone) or a copolymer of vinyl pyrrolidone and a monomer other than vinyl pyrrolidone corresponds to the polymer having a vinyl pyrrolidone structural unit. PVP is conventionally used as an organic protective agent suitable for synthesizing practical silver nanowires. However, as described above, in a liquid medium to which an alcohol is added to improve the wettability to a substrate such as PET, there is a disadvantage that the wire dispersibility is lowered. According to the inventor's study, the use of a copolymer of vinyl pyrrolidone and a monomer other than vinyl pyrrolidone can improve the dispersibility in a liquid medium to which an alcohol is added. In addition, it has been confirmed that silver nanowires having a thin and long practical shape can be obtained even with such a copolymer.

However, in recent years, the need for further improvement of the conductivity and low haze of a transparent conductor is increasing. In order to simultaneously realize the improvement of the conductivity and the improvement of the low haze property, it is extremely effective to make the shape of the silver nanowire used as the conductive material thinner to increase the aspect ratio. As a result of the inventor's intensive research, when the polymer having a vinyl pyrrolidone structural unit is used as an organic protective agent in alcohol solvent reduction method to deposit silver nanowires by reductive deposition, the above polymer is used as a source of the organic protective agent. It has been newly found that very thin silver nanowires can be synthesized more stably by using a polymer powder containing as a main component and a predetermined amount of acetate ester. Moreover, the length is sufficiently secured, and silver nanowires having a high average aspect ratio can be recovered.

As polymers having vinylpyrrolidone structural units, PVP (polyvinylpyrrolidone) or copolymers of vinylpyrrolidone with hydrophilic monomers are suitable targets. The latter copolymers include, for example, vinyl pyrrolidone and diallyl dimethyl ammonium salt, ethyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, N-methyl maleimide, N-ethyl maleimide, N- Examples include copolymers having a polymerization composition with one or more monomers selected from propyl maleimide and N-tert-butyl maleimide. The polymerization composition of the copolymer is preferably 0.1 to 10% by weight of monomers other than vinyl pyrrolidone, with the balance being vinyl pyrrolidone.

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

Acetic acid ester may be added as an organic solvent when purifying the produced polymer in the process of synthesizing the above-mentioned polymer. In that case, an acetate ester attached to the polymer molecules is usually present in the powder product. In the case where the amount thereof is less than the predetermined amount required in the present invention (described later) or when using a polymer powder obtained by a synthesis method without adding an acetate ester, for example, by a method such as "polymer purification treatment" An acetic acid ester molecule can be attached to a polymer molecule, and a polymer powder of an organic protective agent whose acetic acid ester content is adjusted to a predetermined range can be prepared. The polymer purification process is a process for removing impurities such as unreacted monomers, polymerization initiators, solvent substances and the like mixed in the product of the synthesized polymer to increase the purity of the polymer. Since the organic protective agent used at the time of reductive deposition of silver nanowires is a hydrophilic polymer with large polarity in general, purification treatment can be performed by the following operations, for example. The organic solvent in which the hydrophilic polymer synthesized by the polymerization reaction is dissolved is composed of a large excess of an ether such as small polar dimethyl ether, diethyl ether, ethyl methyl ether or an acetate such as methyl acetate or ethyl acetate. When dropped in an organic solvent, a highly polar water-soluble polymer precipitates. Since the unreacted monomer, polymerization initiator, solvent substance, etc., which are impurities, are maintained in the above-mentioned organic solvent in a dissolved state, the precipitated solid substance (polymer) is recovered by filtration or the like to reduce impurities. A purified polymer can be obtained. This purification operation is repeated until the desired purity of the polymer is obtained. In the “polymer purification process” performed in this manner, using acetic acid ester as a large excess of small-polarity organic solvent, polymer molecules to which acetic acid molecules are attached can be recovered. The attached amount of acetic acid ester (the content of acetic acid ester in the polymer powder) can be adjusted by controlling the number of operations of the above-mentioned purification.

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

In order to deposit metallic silver in the form of a wire by alcohol solvent reduction method, it is necessary for the polymer molecule of the organic protective agent to be selectively adsorbed on the {100} face of the nucleus crystal that is considered to be multiple twins of silver. It is. Thereby, the growth of the {100} plane is suppressed, the {111} plane preferentially grows, and a linear structure of metallic silver is formed. The selective adsorptivity of polymer molecules is believed to be caused by the interaction between the surface potential of the polymer molecules and the surface potential of the silver crystal face. The surface potential of a polymer molecule changes depending on the attached (adsorbed) state of the organic compound molecule attached to the polymer molecule. That is, the bias of the charge of the polymer molecule changes depending on the kind of the attached organic compound and the amount of adhesion thereof, and the selective adsorption on the {100} plane of silver changes. A polymer powder containing a predetermined amount of acetic acid ester as described later is extremely effective for the synthesis of thin silver nanowires. From this, it is surmised that acetic acid ester is a very effective substance in imparting a surface potential to the polymer having a vinyl pyrrolidone structural unit to improve the selective adsorption to the silver {100} surface. .

Acetic acid ester also functions to clean the {111} crystal face on which silver preferentially precipitates when synthesizing silver nanowires by the alcohol solvent reduction method, ie, an organic protective agent to the {111} crystal face It is thought that it has an action of suppressing the adsorption of molecules and activating the exposed {111} crystal face to promote the deposition of new silver. The action to activate the {111} crystal face is mainly borne by the conventional additives such as halides, but it is speculated that acetic acid ester also acts in a similar manner. If an acetate ester is present in addition to the halogen in the vicinity of the linear structure of metallic silver that has already been deposited, the above-mentioned cleaning action is increased, and the surface in the thickness direction of the linear structure ({100} crystal plane) The relative ease of deposition on the lengthwise exposed surface ({111} crystal plane) relative to the ease of silver deposition is further enhanced, and as a result, the deposition growth of thin nanowires is promoted. It is thought that

There is a problem in increasing the amount of halide added for the purpose of enhancing the activation action of the {111} crystal face. The organic protective agent which coats the surface of the synthesized silver nanowire is attached with a halogen atom such as chlorine added at the time of synthesis, and the halogen atom is entrained into the transparent conductive film in association with the silver nanowire. . According to the inventor's investigation, it is confirmed that when the concentration of chlorine in the transparent conductive film is high, the deterioration with time of the transparent conductive film is promoted, and a problem that the conductivity is deteriorated early tends to occur. In this respect, in the method of strengthening the cleaning action of the {111} crystal face by the addition of acetic acid ester, the problem of the above-mentioned deterioration of the transparent conductive film with time is avoided.

The polymer molecules of the organic protective agent, which are present in the solvent during synthesis of the silver nanowires, preferentially adsorb to the {100} crystal plane of metallic silver, which has a relatively low electron density. Even with the same kind of organic protective agent, the smaller the molecular weight of the polymer, the larger the adsorptivity to metallic silver tends to be, and therefore, using a polymer with a lower molecular weight is advantageous in synthesizing a thin wire. However, when the adsorptive power is increased, adsorption to the {111} crystal plane where precipitation is desired to be promoted preferentially occurs easily. Therefore, when a polymer with an excessively small molecular weight is used as an organic protective agent, a wire with a short length is easily formed, and it becomes difficult to improve the aspect ratio. The above-mentioned selective adsorption of silver {100} crystal faces by acetic acid ester and the cleaning action of {111} crystal faces enhance the ability to select polymers of relatively low molecular weight, and the length is longer and thinner. There is an advantage that the synthesis of the wire becomes easy.

As a result of various investigations, it is assumed that the content of acetic acid ester in the polymer powder is a ratio of 0.002 to 0.040 mol of acetic acid ester to 1 mol of the polymer having a vinylpyrrolidone structural unit which is the main component of the polymer powder. Is desirable. Hereinafter, the molar ratio of acetic acid ester to 1 mole of the polymer may be referred to as "acetic acid ester / polymer molar ratio". The acetate ester / polymer molar ratio is more preferably 0.0025 or more, and may be controlled, for example, in the range of 0.0025 to 0.030. The acetate ester / polymer molar ratio can be determined from the NMR (nuclear magnetic resonance) spectrum of the polymer powder. If the content of acetic acid ester is too low, the above-mentioned action of facilitating the formation of a thin wire can not be sufficiently obtained. On the other hand, if the content is too large, wire aggregation tends to occur depending on the type of liquid medium when producing a silver nanowire dispersion. In addition, when the amount ratio of acetic acid ester to the polymer is seen in mass ratio, for example, when ethyl acetate is applied, 100 parts by mass of polymer having a vinylpyrrolidone structural unit with a weight average molecular weight Mw of about 30,000 to 150,000. Preferably, the content of ethyl acetate is adjusted in the range of 0.2 to 3.5 parts by mass.

The content of “a polymer having a vinylpyrrolidone structural unit” in the polymer powder is desirably 50% by mass or more, and within the range in which silver nanowires can be produced in addition to the above-mentioned predetermined amount of acetic acid ester as the remaining component. You may contain another component. When it is desired to apply a polymer powder in which the purity of the polymer functioning as an organic protective agent is as high as possible, for example, the polymer content is 90% by mass or more, more preferably 95% by mass or more, And what is necessary is just to use what is comprised with the component mixed in the manufacturing process of a polymer. As components to be mixed in the production process of the polymer, there are included VP (vinyl pyrrolidone) as a remaining monomer component, and additives such as TBME (tert-butyl methyl ether) and MIBK (methyl isobutyl ketone). According to the research so far, the lower the content of MIBK, the more advantageous the synthesis of silver nanowires having a large average aspect ratio. The MIBK content is preferably adjusted in the range of 1.0 part by mass or less with respect to 100 parts by mass of the polymer. The amount of polymer powder used at the time of silver nanowire synthesis can be set within the range of the appropriate amount used in the prior art depending on the production conditions.

Examples of acetic acid esters include methyl acetate (C ₃ H ₆ O ₂ ), ethyl acetate (C ₄ H ₈ O ₂ ), propyl acetate (C ₅ H ₁₀ O ₂ ), butyl acetate (C ₆ H ₁₂ O ₂ ) Etc. These acetic acid esters are liquid alone at ordinary temperature as they are, but when attached and attached to the polymer molecule of the organic protective agent in the above-mentioned content range, take the form of a solid substance (powder) as a whole. Acetic acid esters can be used alone or in combination of two or more.

The content of various 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, the ethyl acetate peak appears at around 4.1 ppm or 1.2 to 1.3 ppm, the TBME peak appears at 3.2 to 3.3 ppm or around 1.2 ppm, and the MIBK peak is It appears around 0.9 ppm. The content VP _R of VP is the residual monomer can be obtained by the following equation (3).
VP _R (mol%) = [2 × (I ₁ + I ₂ ) / (3 × I ₃ )] × 100 (3)
Here, I ₁ is the integrated value of the peak (7.0-7.2 ppm) derived from the methine proton related to the C = C double bond of the VP monomer, and I ₂ is related to the C = C double bond of the same monomer integral value of peaks derived from the methylene protons (4.3-4.4ppm), _{I 3} is the integral value of the peak (3.0-3.4ppm) derived from the methylene protons adjacent to the N atom of the polymer.

Acetic acid ester molecules are adsorbed to the molecules of the polymer synthesized using acetic acid ester. A polymer powder product mainly composed of a polymer synthesized by such a method is supplied with an organic protective agent at the time of silver nanowire synthesis when the acetic acid ester content is in the above-mentioned predetermined range. You may use it directly as a source. On the other hand, if the polymer powder product does not contain an acetate ester or if the content is below the above-mentioned predetermined range, the polymer molecules are subjected to a treatment to attach the acetate ester to the polymer molecule. Thus, it is necessary to adjust the acetate content in the polymer powder. In addition, the polymer powder product usually contains impurity components (such as a chain transfer agent component containing sulfur and a residual vinyl pyrrolidone monomer) which are unnecessary for the synthesis of silver nanowires. Hereinafter, a method of performing a process of attaching an acetic acid ester to a polymer molecule using “a purification process of a polymer powder” for reducing the content of impurity components in the polymer powder will be exemplified.

(Example of polymer purification process)
First, a polymer powder to be treated is dissolved in a chloroform solvent to obtain a polymer-containing liquid. In the chloroform solvent, in addition to the polymer concerned, various impurity components are dissolved. When this solution is dropped into a solvent composed of acetic acid ester (for example, ethyl acetate), the polymer is insoluble in the acetic acid ester solvent, and thus, the polymer precipitates in the acetic acid ester solvent. On the other hand, most of the impurity components that are soluble in acetic acid ester solvent remain dissolved in the solution. However, some of them are present in association with the precipitated polymer. The precipitated solid content is filtered and recovered. Ethyl acetate adheres to the recovered solid polymer molecules. The dried solid is again dissolved in fresh chloroform solvent, and the solution is dropped into fresh acetic ester to precipitate a polymer, which is recovered as a solid. The amount of impurities in the polymer powder can be reduced by the purification treatment which repeats the operation of dissolution and precipitation, and the amount of acetic acid ester attached to the polymer molecule can be adjusted.

[Dimensional shape of silver nanowires]
The silver nanowire is preferably as thin and long as possible from the viewpoint of forming a transparent conductive coating film excellent in conductivity and visibility. The present invention is directed to those having an average diameter of 30 nm or less, preferably 28 nm or less. Generally, the larger the average aspect ratio, the better. However, as the average diameter of the wire is smaller, it is advantageous to reduce the haze of the transparent conductive film, and accordingly, the freedom with respect to the average aspect ratio is also expanded. As a result of various investigations, it is preferable that at a later stage the silver nanowire synthesis an average aspect A _M satisfying the following equation (2), those satisfying the following (2) 'expression is more preferred target. However, even when these expressions are satisfied, it is desirable that the average length be 6.5 μm or more.
A _M 45 45 D _M- 650 (2)
A _M 45 45 D _M- 630 (2) '
Here, L _M is the average length (nm) of the silver nanowires, and D _M is the average diameter (nm) of the silver nanowires.

With regard to the average length, it is possible to improve the removal of short wires by performing a purification operation (for example, cross flow purification) of the wires after silver nanowire synthesis. However, the mean diameter is almost determined by whether or not a thin wire is stably synthesized during the reductive deposition reaction. That is, it is very difficult to control the mean diameter thereafter unless thin wires are synthesized. According to the present invention, very thin silver nanowires having an average diameter of 30 nm or less or 28 nm or less can be reductively deposited.

[Synthesis of silver nanowires]
Silver nanowires are synthesized by a method of reducing and precipitating silver in the form of a wire (alcohol solvent reduction method) in an alcohol solvent in which a silver compound and an organic protective agent are dissolved. This method is put to practical use as a method of synthesizing silver nanowires. It is preferable to proceed reduction deposition in an alcohol solvent in which chloride and bromide are dissolved in addition to the silver compound and the organic protective agent. Furthermore, reduction deposition may be allowed to proceed in an alcohol solvent in which an alkali metal hydroxide or aluminum salt is dissolved. For example, the method disclosed in Patent Document 1 can be used. However, in the present invention, as a source of the organic protective agent, a powder comprising a polymer having a vinylpyrrolidone structural unit as a main component and containing an acetic ester at a ratio of 0.002 to 0.040 mol per 1 mol of the polymer Used, the polymer supplied from the powder is dissolved in the alcohol solvent. The acetate ester present in the powder is attached to the polymer molecules and is introduced into the alcohol solvent in association with the polymer.

[Preparation of polymer powder]
(Example of copolymer of vinyl pyrrolidone and diallyl dimethyl ammonium salt)
Synthesized by a method in which 1-vinyl 2-pyrrolidone and diallyldimethylammonium nitrate are dissolved in methyl isobutyl ketone as a solvent as a raw material polymer powder, and a polymerization initiator is added for copolymerization. Several production lots of powder were prepared. In each production lot, the polymerization composition is in common with 1-vinyldipyrrolidone: diallyldimethylammonium nitrate = 99: 1 in molar ratio, but the amount of ethyl acetate used in the synthesis process is different. Some production lots were synthesized without using ethyl acetate. The above-mentioned "polymer purification treatment" is carried out using one or a mixture of two or more kinds of raw material polymer powders, and the number of repetition of dissolution-precipitation operation in the treatment is changed. Thus, polymer powders A to G adjusted to various ethyl acetate contents were produced.

[Analysis of polymer powder]
The content of ethyl acetate, TBME (tert-butyl methyl ether), MIBK (methyl isobutyl ketone) and residual monomer VP (vinyl pyrrolidone) in polymer powder is measured by nuclear magnetic resonance spectroscopy (NMR) 1H It calculated | required from the NMR spectrum. Here, ethyl acetate uses the integral value of the peak around 4.1 ppm, TBME uses the integral value of the peak around 1.2 ppm, and MIBK uses the integral value of the peak around 0.9 ppm, and the mol% of each component is Calculated. The VP content was determined by the equation (3). For measurement of 1 H NMR spectrum, an NMR apparatus manufactured by JEOL Ltd. JNM-LA400 (400 MHz) was used.

Further, the weight average molecular weight Mw of the polymer in the powder was determined by GPC (gel permeation chromatography) under the following conditions.
・ Device: HLC-8320GPC EcoSEC (made by Tosoh Corporation)
・ Column: TSKgel GMPWXL (× 2) + G2500PWXL
Eluent: 100 mM aqueous sodium nitrate solution / acetonitrile = 80/20
・ Flow rate: 1.0 mL / min
Temperature: 40 ° C
Injection volume: 200 μL
-Multi-angle light scattering detector: DAWN HELEOS II (manufactured by Wyatt Technology)
Refractive index (RI) detector: Optilab T-rEX (manufactured by Wyatt Technology)

Example 1
(Synthesis of silver nanowires)
4.84 g of a propylene glycol solution having a lithium chloride content of 10% by mass in 0.1816 g of propylene glycol at normal temperature, 0.1037 g of potassium bromide, 0.426 g of lithium hydroxide, containing aluminum nitrate nonahydrate A solution A was prepared by dissolving 4.994 g of a propylene glycol solution having an amount of 20% by mass and 83.875 g of a polymer powder as a source of the organic protective agent. Here, polymer powder A containing 0.0299 mol of ethyl acetate to 1 mol of a copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt was used as a source of the organic protective agent. In a separate container from this, 67.96 g of silver nitrate is added to a mixed solution of 95.70 g of propylene glycol and 8.00 g of pure water, and the solution B containing silver is dissolved by stirring at 35 ° C. Obtained. The solution A was placed in a reaction vessel and heated from normal temperature to 90 ° C. while stirring at a rotational speed of 175 rpm, and then the whole solution B was added to the solution A from two addition ports over 1 minute . After the addition of solution B was completed, stirring was further maintained and maintained at 90 ° C. for 24 hours. Then, the silver nanowire was synthesize | combined by cooling a reaction liquid to normal temperature.

(Average diameter of silver nanowires, average length measurement)
20 g of the reaction solution cooled to room temperature was taken into a centrifuge tube, 180 g of pure water was added, and centrifugation was performed at 1500 rpm for 15 minutes using a centrifuge. Since the concentrate and the supernatant were observed, the supernatant portion was removed and the concentrate was recovered. This washing operation was repeated several more times to obtain a concentrate. The obtained concentrate was dispersed in pure water. In measuring the length of silver nanowires, the dispersion is placed on an observation table for SEM, water is volatilized on the observation table, and then a field emission scanning electron microscope (manufactured by Hitachi High-Technologies Corporation; S-4700) The observation was performed at an acceleration voltage of 3 kV and a magnification of 1,500 to 2,500 times. For three or more randomly selected fields of view, the average length was measured according to the above definition for all wires whose full length can be confirmed in the field of view. In the diameter measurement, the above dispersion is placed on an observation table for TEM, and bright field image with an acceleration voltage of 100 kV and a magnification of 40,000 to 100,000 by a transmission electron microscope (manufactured by JEOL Ltd .; JEM-1011) The average diameter was measured according to the above definition. The average aspect ratio was determined by substituting the values of the average length and the average diameter into the equation (1). The average diameter of the silver nanowires was 25.1 nm, and the average length was 14.3 μm. The average aspect ratio was 14300 (nm) /25.1 (nm) 570 570. The results are summarized in Table 1 together with other examples and comparative examples. In addition, what converted the composition of polymer powder into the mass ratio is shown in Table 2 with the other Example and the comparative example for reference.

Example 2
When synthesizing silver nanowires, except that a polymer powder B containing 0.0102 mol of ethyl acetate was used as a source of organic protective agent per mol of copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt, The experiment was conducted under the same conditions as in Example 1. As a result, the average diameter of the obtained silver nanowires was 25.3 nm, and the average length was 15.8 μm. The average aspect ratio was 15800 (nm) /25.3 (nm) 625 625.

[Example 3]
When synthesizing silver nanowires, except that polymer powder C containing 0.0031 mol of ethyl acetate to 1 mol of copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt was used as a source of organic protective agent, The experiment was conducted under the same conditions as in Example 1. As a result, the average diameter of the obtained silver nanowires was 26.3 nm, and the average length was 15.8 μm. The average aspect ratio was 15800 (nm) /26.3 (nm) ≒ 601.

Example 4
When synthesizing silver nanowires, except that polymer powder D containing 0.0196 mol of ethyl acetate to 1 mol of copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt was used as a source of organic protective agent, The experiment was conducted under the same conditions as in Example 1. As a result, the average diameter of the obtained silver nanowires was 24.7 nm, and the average length was 16.1 μm. The average aspect ratio was 16100 (nm) /24.7 (nm) ≒ 652.

[Example 5]
When synthesizing silver nanowires, except that a polymer powder E containing 0.0242 moles of ethyl acetate per mole of copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt was used as a source of organic protective agent, The experiment was conducted under the same conditions as in Example 1. As a result, the obtained silver nanowires had an average diameter of 26.8 nm and an average length of 20.4 μm. The average aspect ratio was 20400 (nm) /26.8 (nm) ≒ 761.

Comparative Example 1
Polymer powder F having an ethyl acetate content of 0.000 mol (less than the detection limit) per mol of a copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt as a source of an organic protective agent when synthesizing silver nanowires The experiment was conducted under the same conditions as in Example 1 except that As a result, the obtained silver nanowires had an average diameter of 39.6 nm and an average length of 19.6 μm. The average aspect ratio was 19600 (nm) /39.6 (nm) ≒ 495.

Comparative Example 2
When synthesizing silver nanowires, except that polymer powder G containing 0.0004 mol of ethyl acetate to 1 mol of copolymer of vinyl pyrrolidone and diallyldimethyl ammonium salt was used as a source of organic protective agent, The experiment was conducted under the same conditions as in Example 1. As a result, the average diameter of the obtained silver nanowires was 35.8 nm, and the average length was 14.1 μm. The average aspect ratio was 14100 (nm) /35.8 (nm) ≒ 394.

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 source of organic protective agent, very thin silver nanowires having an average diameter of less than 30 nm Could be synthesized. The average aspect ratio of these silver nanowires was much over 500. On the other hand, in each comparative example using the polymer powder which does not contain the ethyl acetate of the content range prescribed | regulated by this invention, the wire with an average diameter of less than 30 nm was not able to be synthesize | combined.

For reference, the NMR spectrum of the polymer powder A used in Example 1 is shown in FIG. 2 and the NMR spectrum of the polymer powder F used in Comparative Example 1 is shown in FIG. The symbol a in these figures is the peak due to ethyl acetate, the symbol b is the peak due to TBME (tert-butyl methyl ether), the symbol c is the peak due to MIBK (methyl isobutyl ketone), the symbol p is It is a peak attributed to a polymer.

Claims

In a method of producing a silver nanowire having a step of reducing and depositing silver in the form of a wire in an alcohol solvent in which a silver compound and an organic protective agent are dissolved, a polymer having a vinylpyrrolidone structural unit as a source of the organic protective agent A method for producing silver nanowires, comprising: a powder comprising, as a main component, a ratio of 0.002 to 0.040 moles of acetic ester per mole of the polymer, and having an average diameter D M of 30 nm or less.
The use of the powder as a source of organic protective agent, the average aspect ratio A M determined by the following equation (1) causes the reduction precipitation of silver nanowires satisfies the following equation (2), according to claim 1 Silver nanowire manufacturing method.
A M = L M / D M (1)
A M 45 45 D M- 650 (2)
Here, L M is the average length (nm) of the silver nanowires, and D M is the average diameter (nm) of the silver nanowires.
The method for producing silver nanowires according to claim 1 or 2, wherein the acetate ester is one or more of methyl acetate, ethyl acetate, propyl acetate and butyl acetate.
The method for producing silver nanowires according to any one of claims 1 to 3, wherein the polymer is PVP (polyvinyl pyrrolidone) or a copolymer of vinyl pyrrolidone and a hydrophilic monomer.
Said polymers are vinylpyrrolidone, diallyldimethylammonium salt, ethyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, N-methyl maleimide, N-ethyl maleimide, N-propyl maleimide and N The method for producing a silver nanowire according to any one of claims 1 to 3, which has a polymerization composition with one or more monomers selected from -tert-butyl maleimide.
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.
A silver nanowire obtained by the method according to any one of claims 1 to 6.
A silver nanowires ink, wherein the silver nanowires obtained by the method according to any one of claims 1 to 6 are dispersed in a liquid medium.
A transparent conductive film comprising, as a conductive material, silver nanowires obtained by the method according to any one of claims 1 to 6.