JP2017082305A - Silver nanowire and method for producing the same, and fluid dispersion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silver nanowire coated with, instead of a polymer protective agent such as PVP (polyvinylpyrrolidone), an organic protective agent having a molecular weight smaller that of the polymer protective agent.SOLUTION: The silver nanowire is provided that has thiol with a molecular weight of 75 to 300 adhered to a surface of a metal silver and has an average diameter of 100 nm or less and an average length of 5 μm. As the thiol, thiol having only one thol group (-S-H) in a molecule is suitable target. For example, it includes 1-dodecanethiol, 1-decanethiol, 1-octanethiol, 3-mercapto-1,2-propanediol, thioglycolic acid monoethanolamine, ammonium thioglycolate or thiomalic acid.SELECTED DRAWING: Figure 13B

Description

  The present invention relates to a silver nanowire useful as a material for forming a transparent conductor, and a method for producing the same. The present invention also relates to a silver nanowire dispersion liquid in which the silver nanowires are dispersed.

  In this specification, a collection of fine metal wires having a thickness of about 200 nm or less is referred to as “nanowires”. In the case of powder, each wire corresponds to “particles” constituting the powder, and nanowires corresponds to “powder” that is a collection of particles.

Silver nanowires are promising as conductive materials for imparting conductivity to transparent substrates. After the liquid in which silver nanowires are dispersed is coated on a transparent substrate such as glass, PET (polyethylene terephthalate), and PC (polycarbonate), the liquid components are removed by evaporation or the like. Since a conductive network is formed by contact, a transparent conductor can be realized. Conventionally, as a transparent conductive material, a metal oxide film typified by ITO is mainly used for applications such as a transparent electrode. However, the metal oxide film has drawbacks such as high film formation cost and weakness against bending, which hinders flexibility of the final product. In addition, high transparency and high conductivity are required for the conductive film of the touch panel sensor, which is one of the main uses of the transparent conductor, but recently, the demand for visibility has become more severe. In the conventional ITO film, it is necessary to increase the thickness of the ITO layer in order to obtain conductivity, but the increase in thickness causes a decrease in transparency and does not improve the visibility.
Silver nanowires are promising in overcoming the above-described drawbacks inherent in metal oxide films represented by ITO.

  As a method for producing silver nanowires, a silver compound is dissolved in a polyol solvent such as ethylene glycol, and a halogen compound and an organic protective agent, PVP (polyvinylpyrrolidone) or a copolymer having a vinylpyrrolidone structural unit, are present. Techniques for depositing linear metallic silver using the reducing power of the solvent polyol are known (Patent Documents 1 to 3, Non-Patent Document 1). A copolymer having PVP or a vinylpyrrolidone structural unit is an extremely effective substance as an organic protective agent for synthesizing silver nanowires with high yield.

US2005 / 0056118 Publication US2008 / 0003130 Publication US2014 / 0178247

J. et al. of Solid State Chem. 1992, 100, 272-280

  Conventional general silver nanowires coated with PVP exhibit good dispersibility in a polar solvent such as water, and thus are usually provided as a silver nanowire dispersion using an aqueous liquid medium. However, depending on the application, it may be desirable to use a silver nanowire dispersion using a nonpolar solvent or a nonpolar non-aqueous solvent. In order to meet such needs, a technique for replacing a polymer protective agent such as PVP, which was necessary for the synthesis of silver nanowires, with a material having good dispersibility with a liquid medium is effective.

  Moreover, it is desirable that the polymer protective agent such as PVP covering the silver nanowires is as small as possible or not present from the viewpoint of improving the conductivity by contact between the wires. However, a polymer protective agent such as PVP is generally difficult to remove effectively by a cleaning operation because one molecule thereof is attached to the metal silver of the wire at many points. It is also difficult to replace the polymer protective agent with another surface protective agent.

  On the other hand, in the formation of a conductive network using silver nanowires, conventionally, conduction is ensured by simple contact between wires. If it becomes possible to join the contact portions of the wires by sintering, it is considered that the conductivity of the conductive network is greatly improved. However, silver nanowires to which a polymer protective agent such as PVP is attached have a high sintering temperature, and it is often difficult to perform a sintering treatment after being applied to a transparent substrate. In particular, PVP also has a problem of being easily fused when heated.

  The present invention is intended to provide a silver nanowire coated with an organic protective agent having a lower molecular weight instead of a polymer protective agent such as PVP.

  As a result of research, the inventors have made it easy to use the silver nanowire obtained by the method of reducing and precipitating silver in the form of a wire in the presence of a polymer protective agent such as PVP in an alcohol solvent as a raw material wire, We found an effective method that can be reliably replaced with a low molecular weight material by operation. Specifically, the above object can be achieved by applying thiol as a protective agent substance to be replaced.

  That is, in the present invention, silver nanowires having an average diameter of 100 nm or less and an average length of 5 μm or more are provided, in which thiols having a molecular weight of 75 to 300 are attached to the surface of metallic silver. What has an average diameter of 50 nm or less and an average length of 10 μm or more is a more preferable target. Those having an average aspect ratio of 250 or more are particularly suitable. Here, the average diameter, average length, and average aspect ratio conform to the following definitions.

[Average diameter]
On a microscopic image (for example, FE-SEM image), in a projection image of a single metal wire, the average value of the diameters when the diameter of the inscribed circle in contact with the contours on both sides in the thickness direction is measured over the entire length of the wire. , Defined as the diameter of the wire. And the value which averaged the diameter of each wire which comprises nanowire (nanowires) is defined as the average diameter of the said nanowire. In order to calculate the average diameter, the total number of wires to be measured is set to 100 or more. However, a wire-like product or a granular product having a length (described later) of less than 0.5 μm is excluded from the measurement target.

[Average length]
On a microscopic image similar to the above, in a projected image of a single metal wire, the length from one end of the wire to the other end of the line passing through the center of the thickness of the wire (that is, the center of the inscribed circle) This is defined as the length of the wire. And the value which averaged the length of each wire which comprises nanowire (nanowires) is defined as the average length of the said nanowire. In order to calculate the average length, the total number of wires to be measured is set to 100 or more. However, wire-like products having a length of less than 0.5 μm and granular products are excluded from the measurement target.
The silver nanowire according to the present invention is composed of a very elongated wire. For this reason, the collected silver nanowires often have a curved string shape rather than a straight rod shape. The inventors have created software for efficiently measuring the above-described wire length on an image for such a curved wire and uses it for data processing.

[Average aspect ratio]
The average aspect ratio is calculated by substituting the above average diameter and average length into the following formula (1).
[Average aspect ratio] = [Average length (nm)] / [Average diameter (nm)] (1)

As the thiol, those having only one thiol group (—S—H) in the molecule are suitable. For example, 1-dodecanethiol (molecular weight 202.4), 1-decanethiol (molecular weight 174.4), 1-octanethiol (molecular weight 146.3), 3-mercapto-1,2-propanediol (molecular weight 108.2) ), Monoethanolamine thioglycolate (molecular weight 153.2), ammonium thioglycolate (molecular weight 109.1), thiomalic acid (molecular weight 150.2), and the like.
The thiol to be attached is not limited to one kind alone, and two or more kinds of thiols may be attached. Further, the position of the thiol group present in the thiol is not limited to the terminal, and may be present at any location in the thiol molecular structure.

The following a to e can be exemplified as specific embodiments of the silver nanowire.
(A) A silver nanowire having an average diameter of 100 nm or less and an average length of 5 μm or more in which a thiol having a molecular weight of 75 to 300 is attached to the surface of metallic silver.
(B) The silver nanowire according to the above (a) having an average diameter of 50 nm or less and an average length of 10 μm or more.
(C) The silver nanowire according to the above (a) or (b), wherein the thiol has only one thiol group in the molecule.
(D) The silver nanowire according to the above a or b, wherein the thiol is one or more selected from the group consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.
(E) In the above a or b, the thiol is at least one selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid. Silver nanowire of description.

  Moreover, as a manufacturing method of the said silver nanowire to which thiol has adhered, the liquid A in which the silver nanowire coat | covered with the polymer is disperse | distributing, and the liquid B in which the thiol of molecular weight 75-300 is melt | dissolving in a container. By mixing, a production method is provided that includes a step of changing the adhesion substance on the surface of metallic silver from the polymer to the thiol.

  Examples of the polymer include PVP (polyvinylpyrrolidone), a copolymer having a vinylpyrrolidone structural unit, and a polymer obtained by polymerizing a polymerizable monomer including acrylamide. The copolymer having a vinylpyrrolidone structural unit is a copolymer having a polymerization composition of vinylpyrrolidone and other monomers. For example, a copolymer of vinylyl pyrrolidone and diallyldimethylammonium nitrate, a copolymer of vinyl pyrrolidone and a maleimide monomer, a copolymer of a copolymer of vinyl pyrrolidone and an acrylate monomer such as ethyl acrylate, hydroxyethyl acrylate, or the like can be exemplified. The weight average molecular weight of the polymer used is, for example, 20,000 to 1,300,000.

  The amount of the polymer adhering to the silver nanowires in the liquid A (that is, the silver nanowires after the silver nanowire synthesis reaction and the washing treatment) is determined by TG-DTA (thermogravimetry-differential thermal analysis) of the dried silver nanowires. Based on the heat loss value, it is, for example, 3 to 15% by mass. When the polymer on the surface of the silver nanowire is substituted with a thiol having a low molecular weight, the thermal loss value of TG-DTA is reduced as compared with the value before the substitution treatment. Examples of the thiol in the liquid B include those described above.

  The liquid A and the liquid B can be mixed in the presence of an amphiphilic substance such as acetone or isopropanol. In particular, when the solvent of the liquid A is a polar solvent and the solvent of the liquid B is a nonpolar solvent, the presence of an amphiphilic substance is extremely effective.

The following f-n can be illustrated as a specific aspect of the silver nanowire manufacturing method.
(F) The liquid A in which silver nanowires coated with a polymer are dispersed and the liquid B in which a thiol having a molecular weight of 75 to 300 is dissolved are mixed, whereby the adhesion substance on the surface of metallic silver is separated from the polymer. The manufacturing method of the silver nanowire which has the process of changing to thiol.
(G) The silver nanowire disperse | distributed to the said liquid A is a manufacturing method of the silver nanowire as described in said f whose average diameter is 100 nm or less and whose average length is 5 micrometers or more.
(H) The silver nanowire dispersed in the liquid A is the method for producing a silver nanowire according to the above f or g, which has an average diameter of 50 nm or less and an average length of 10 μm or more.
(I) The method for producing a silver nanowire according to any one of the above f to h, wherein the thiol has only one thiol group in the molecule.
(J) The method for producing a silver nanowire according to any one of the above f to h, wherein the thiol is one or more selected from the group consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.
(K) The above thiols are one or more selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid. The manufacturing method of the silver nanowire in any one.
(L) The method for producing a silver nanowire according to any one of the above f to k, wherein the polymer is PVP (polyvinylpyrrolidone).
(M) The method for producing a silver nanowire according to any one of the above f to k, wherein the polymer is a copolymer having a vinylpyrrolidone structural unit.
(N) The method for producing a silver nanowire according to any one of the above f to m, wherein the liquid A and the liquid B are mixed in the presence of one or two of acetone and isopropanol.

  Moreover, in this invention, the silver nanowire dispersion liquid in which said silver nanowire to which thiol has adhered is disperse | distributed in a liquid medium is provided. Specific examples thereof include a silver nanowire dispersion liquid in which the silver nanowires according to any one of the above (a) to (e) are dispersed in a liquid medium.

  In the present invention, a silver nanowire having a thiol attached to the surface of metallic silver is disclosed. An organic substance such as a polymer used as a shape control agent at the time of synthesis is attached to the silver nanowire immediately after synthesis by reduction deposition. Since thiol has high adhesion to metallic silver, by adding a surfactant containing thiol to the silver nanowire dispersion after synthesis, organic substances such as polymers attached to the surface of the silver nanowire can be converted into a surfactant containing thiol. Can be replaced. By replacing the organic protective agent attached to the silver nanowire surface, it is possible to disperse silver nanowires that can be dispersed only in a polar solvent into a nonpolar solvent such as toluene or hexane.

  In addition, a monomer having a molecular weight of 300 or less is easily detached from the surface of the silver nanowire at a lower temperature than a polymer such as PVP. Therefore, when forming a transparent conductive film using silver nanowires according to the present invention, the amount of organic matter remaining on the nanowire surface can be reduced by conventional heat treatment after the silver nanowire-containing paint is applied on the substrate. . The smaller the amount of organic matter on the nanowire surface, the higher the probability of contact between the nanowires and the lower the electrical resistance of the conductive network. Since the density of silver nanowires in the film necessary to impart a predetermined conductivity to the conductive film can be reduced, the light transmission is improved accordingly, which is advantageous for the formation of a clear conductive film with less haze. Become.

Structural formula of 1-dodecanethiol. Structural formula of 1-decanethiol. Structural formula of 1-octanethiol. Structural formula of 3-mercapto-1,2-propanediol. Structural formula of thioglycolic acid monoethanolamine. Structural formula of ammonium thioglycolate. Structural formula of thiomalic acid. 2 is an SEM photograph of the polymer (PVP) -coated silver nanowire synthesized in Example 1. FIG. The SEM photograph of the polymer covering silver nanowire before the covering substance replacement process used in Example 1. FIG. The SEM photograph of the thiol covering silver nanowire after the covering substance replacement process obtained in Example 1. The X-ray-diffraction pattern of the silver nanowire before and behind replacement of the coating material in Example 1. The TG curve of the silver nanowire before and after replacement of the coating material in Example 1. 4 is an SEM photograph of a silver nanowire coated with a polymer (a copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate) synthesized in Example 4. FIG. The SEM photograph of the polymer coating silver nanowire before the coating substance replacement process used in Example 4. FIG. The SEM photograph of the thiol covering silver nanowire after the covering substance replacement process obtained in Example 4. The TG curve of the silver nanowire before and behind replacement of the coating material in Example 4.

  Thiol is a substance represented by the general formula R-SH. The thiol group (—S—H) has a property of firmly bonding to a noble metal such as gold or silver. When a thiol is added to a dispersion of silver nanowires to which a surfactant such as PVP is attached, the thiol with higher adhesion to silver tends to preferentially attach to the wire surface. It turns out that the surfactant (organic protective agent) on the surface of the wire can be replaced with thiol by increasing the chance that the thiol is present near the metallic silver surface of the wire by agitating the solution or by inversion mixing. It was. Surfactants such as amines and carboxylic acids do not have the ability to replace polymeric surfactants such as PVP.

The thiol used in the present invention preferably has a molecular weight in the range of 75 to 300. If it is this range, it will be easy to detach | leave at comparatively low temperature in the heat processing at the time of conductive film preparation, and it will be advantageous when obtaining a highly conductive transparent conductive film.
Moreover, it is preferable to employ a thiol having only one thiol group in the molecule. Since the thiol group has a strong binding force to silver, when the molecule contains two or more thiol groups, each thiol group may adhere to another wire. In that case, the individual wires are easily bonded to form an aggregate. In order to disperse each wire independently in the liquid, it is effective to use a thiol having only one thiol group in the molecule.

  Some thiols are soluble in nonpolar solvents and others are soluble in polar solvents. What is necessary is just to select the thiol from which the surface active action according to the objective is obtained. Examples of thiols that dissolve in nonpolar solvents include 1-dodecanethiol, 1-decanethiol, 1-octanethiol, and the like. Examples of thiols that dissolve in polar solvents include 3-mercapto-1,2-propanediol, thioglycolic acid monoethanolamine, ammonium thioglycolate, thiomalic acid, and the like. Here, the nonpolar solvent means a solvent having a solubility parameter (SP value) of 9.5 or less. For example, water, methanol, ethanol, etc. correspond to polar solvents, and hexane, toluene, benzene, etc. correspond to nonpolar solvents.

  When the amount of thiol attached to the silver nanowire is excessive, the amount of thiol remaining without detaching from the wire during the heat treatment during the production of the conductive film increases and the conductivity of the conductive film is insufficiently improved. There is. As a result of various studies, the adhesion amount of thiol is preferably 10.0% by mass or less, based on the total mass of silver nanowires (total of metallic silver and thiol adhering to the surface thereof), and is preferably 5.0% by mass or less. It is more preferable that the amount is not more than mass%. On the other hand, when the attached amount of thiol is too small, there may be a problem of a decrease in dispersibility in liquid or aggregation of silver nanowires. The thiol adhesion amount is preferably 0.5% by mass or more. The attached amount of thiol can be determined from the thermal loss value by TG-DTA (thermogravimetry-differential thermal analysis).

  The shape of the silver nanowire is desirably an average diameter of 100 nm or less and an average length of 5 μm or more, and more preferably an average diameter of 50 nm or less and an average length of 10 μm or more. The smaller the average diameter, the less light is scattered in the transparent conductive film and the higher the light transmission, which is advantageous in producing a clear and highly visible touch panel or display with less haze. The longer the average length, the greater the chance of contact between wires when forming a conductive network, which is advantageous for improving the conductivity of the transparent conductive film, and it is possible to reduce the wire content in the conductive film accordingly. Become. Reduction of the wire content is effective in reducing haze and improving light transmission. However, if the average diameter becomes very thin, the wire is likely to be broken (broken, broken, etc.) during the production process of the conductive film. Therefore, it is necessary to handle it carefully and the productivity tends to be lowered. Usually, the average diameter may be in the range of 10 nm or more. In addition, if the average length is very long, it is also assumed that there is a problem that the dispersion becomes entangled and aggregated in the dispersion liquid or the nozzles are easily clogged when the silver nanowire ink is applied. Usually, the average length may be in the range of 500 μm or less. The above average aspect ratio is more preferably 300 or more.

  Although it is not necessary to pay particular attention to the method for synthesizing the metal nanowire, at present, a synthesis method in a wet process is known. For example, in the case of silver nanowires, the reduction precipitation method (described above) shown in Patent Documents 1 and 2 is known.

  An operation of changing the organic protective agent attached to the synthesized silver nanowire from a polymer such as PVP to a surfactant having a thiol group as a target substance is performed. The silver nanowire of the present invention is characterized in that a surfactant having a thiol group is attached. Since thiol has a property of easily adhering to silver, if a sufficient amount of thiol is present near the surface of the silver nanowire to which a polymer such as PVP is attached, the polymer is detached from the surface of the silver nanowire. The thiol is easily attached, and the replacement reaction can proceed relatively easily.

  Since this reaction proceeds in a solvent, the thiol needs to be dissolved in the solvent. When changing to 1-dodecanethiol, 1-decanethiol, 1-octanethiol, etc. dissolved in a nonpolar solvent, a non-polar solvent such as hexane or toluene is used as Liquid B described later. One kind or two or more kinds of nonpolar solvents may be mixed and used. In addition, when changing to 3-mercapto-1,2-propanediol, thioglycolic acid monoethanolamine, ammonium thioglycolate, thiomalic acid, etc. dissolved in a polar solvent, use a polar solvent such as water, methanol, ethanol, etc. Then, let it be the liquid B described later. One or more polar solvents may be mixed and used.

  If the amount of thiol used is 0.5 mmol or more of thiol with respect to the silver mass of 10 mg of the silver nanowire, the reaction (desorption and adsorption) for replacing the polymer attached to the wire surface with thiol sufficiently proceeds. be able to. When the amount of thiol is small, replacement is insufficient. For example, a polar solvent in which silver nanowires to which PVP is attached is dispersed and a nonpolar solvent (for example, toluene) in which 1-dodecanethiol is dissolved are mixed to change the organic protective agent from PVP to 1-dodecanethiol. When performing the operation, when the amount of 1-dodecanethiol is sufficient, almost all of the PVP on the wire surface is replaced by 1-dodecanethiol, and the silver nanowires all lose their dispersibility in polar solvents and at the same time are nonpolar Dispersibility in the solvent is obtained. As a result, the entire amount of silver nanowires is dispersed in the nonpolar solvent layer among the solvent layers that are separated into two layers. However, when the amount of 1-dodecanethiol added is insufficient, the total amount of PVP cannot be replaced with 1-dodecanethiol, and the wire with PVP attached to part or all of the surface is separated into two layers. It remains in the polar solvent layer.

  When performing the operation of changing the organic protective agent attached to the metallic silver surface of the silver nanowire from the polymer to the thiol, the liquid A in which the silver nanowire coated with the polymer is dispersed and the thiol having a molecular weight of 75 to 300 A liquid B in which is dissolved is prepared. A polymer such as PVP used for reductive synthesis of silver nanowires usually has good dispersibility in water. Silver nanowires coated with a polymer are often stored in a state of being dispersed in an aqueous solvent. The aqueous solvent is a polar solvent.

  In the case of changing to a thiol having a property of being dissolved in a polar solvent, since the liquid B is a polar solvent, by using an aqueous solvent in which silver nanowires coated with a polymer are dispersed as the liquid A, the liquid thiol is coated with the thiol. A silver nanowire dispersion liquid in which silver nanowires are dispersed in a polar solvent (such as an aqueous solvent) can be obtained.

On the other hand, when changing to a thiol having a property of being dissolved in a nonpolar solvent, since the liquid B is a nonpolar solvent, if an aqueous solvent in which silver nanowires coated with a polymer are dispersed is used as it is, the liquid A A and liquid B are separated by forming an interface. In order for thiol adsorption to occur on the surface of the metallic silver wire, the thiol molecule needs to be sufficiently close to the surface of the wire present in the liquid A. If the liquid A and the liquid B that are separated from each other are vigorously stirred, a chance of approaching the wire surface and the thiol molecule can be ensured, but the nanowire is easily damaged by stirring damage. Therefore, it is effective to mix the liquid A and the liquid B in the presence of an amphipathic substance such as acetone or isopropanol in order to make it easier to ensure the close proximity of the wire surface and the thiol molecule even in a mild stirring environment. It is. Mixing in the presence of the amphiphilic substance by adding the amphiphilic substance in liquid A or liquid B in advance, or by placing it in a container for mixing liquid A and liquid B in advance. Matching becomes possible. Since acetone and isopropanol have a solubility parameter (SP value) exceeding 9.5, they are not classified as nonpolar solvents in the present specification, but have affinity for both polar solvents and nonpolar solvents. Using liquid A or liquid B to which such an amphiphilic substance is added, it is easy to secure an opportunity for the wire surface and the thiol molecule to approach when the two solvents are mixed and stirred, and the polymer is replaced with thiol. Can proceed smoothly. In addition, the wire that has been replaced with thiol can easily move into the layer of the nonpolar solvent supplied from the liquid B. In this way, a silver nanowire dispersion liquid in which silver nanowires coated with thiol are dispersed in a nonpolar solvent (hexane, toluene, benzene, etc.) can be obtained.
In the case where acetone or isopropanol is added to an aqueous solvent in which silver nanowires coated with a polymer are dispersed to give liquid A, the amount of acetone or isopropanol added is 0.5 to 2 with respect to 1 volume of the aqueous solvent. .5 volume.
When the dispersion of silver nanowires coated with the polymer is an alcoholic polar solvent such as methanol or ethanol, the interfacial energy between the alcohol and the nonpolar solvent is the interface between water and the nonpolar solvent. Much smaller than energy. Therefore, the alcoholic solvent as described above can be easily replaced with a thiol having dispersibility in a nonpolar solvent even if it is used as it is as the liquid A without adding an amphiphilic substance. be able to.

  The treatment temperature when replacing the organic protective agent is set to a temperature not higher than the boiling point of the solvent used. Since the replacement reaction (desorption of the polymer and adsorption of the thiol) proceeds easily even at room temperature, it is usually performed in the range of 20 to 50 ° C. As for the mixing method of the liquid A and the liquid B, the liquid B is preferably added to the liquid A in which silver nanowires are accommodated. It may be added once, or may be added intermittently or continuously. It is preferable that the liquid A is previously stirred at a speed that does not damage the nanowire as much as possible. Stirring or inversion mixing may be performed after adding all of the liquid B. There is no particular limitation on the atmosphere of the gas phase in which the liquid level of the solvent contacts during the replacement process. An air atmosphere or a nitrogen atmosphere can be applied. The time required for the replacement reaction with thiol is about 10 seconds to 2 minutes from the start of mixing of liquid A and liquid B when replacement is performed at 20 to 50 ° C.

  Silver nanowires that have been replaced with thiol are attached to nonpolar solvents such as hexane and toluene when thiols having carbon chains such as 1-dodecanethiol, 1-decanethiol, and 1-octanethiol are attached. To be distributed. When a thiol having a hydroxy group or a carboxyl group such as 3-mercapto-1,2-propanediol, thioglycolic acid monoethanolamine, ammonium thioglycolate or thiomalic acid is attached, water, methanol, ethanol, etc. In a polar solvent. In this case, the dispersibility with respect to various polar solvents can be controlled by the kind of thiol to be attached.

[Example 1]
<Synthesis of silver nanowires>
PVP (polyvinylpyrrolidone) having a weight average molecular weight of 55,000 was prepared as an organic protective agent.
At room temperature, 2.5 g of PVP and 0.006 g (0.1 mmol) of sodium chloride were added and dissolved in 60 g of ethylene glycol to obtain Solution X. In a separate container, 0.85 g (5.0 mmol) of silver nitrate was added and dissolved in 7.65 g of ethylene glycol to obtain Solution Y.

  After heating the whole amount of the solution X from room temperature to 135 ° C. with stirring at 500 rpm in an air atmosphere, the solution Y was added to the solution X all at once. After the addition of the solution Y was completed, the stirring speed was changed to 100 rpm, and the stirring state was maintained and maintained at 135 ° C. for 3 hours. Thereafter, the reaction solution was cooled to room temperature.

After cooling, washing was performed by the following procedure to obtain a silver nanowire dispersion.
The reaction solution was transferred to a centrifuge tube, 30 mL of distilled water was added, and the mixture was centrifuged under a centrifugal force of 1000 G for 5 minutes. After centrifugation, the supernatant was removed and the solid content was recovered. After 30 mL of methanol was added to the collected solid and dispersed, the mixture was centrifuged under a centrifugal force of 700 G for 5 minutes. After centrifugation, the supernatant was removed and the solid content was recovered. After 30 mL of methanol was added to the collected solid and dispersed, the mixture was centrifuged under a centrifugal force of 250 G for 10 minutes. The supernatant was removed, and the solid content was dispersed again in water to obtain an aqueous dispersion of this silver nanowire.

The dispersion was collected and the solvent water was volatilized on the observation table and then observed with an FE-SEM (field emission scanning electron microscope). As a result, it was confirmed that the solid content was silver nanowires. In this way, silver nanowires coated with PVP were obtained.
FIG. 8 illustrates an SEM photograph of the silver nanowire. In SEM observation, the average diameter and average length were determined according to the above-mentioned definition for five randomly selected visual fields. The total number of wires to be measured is 100 or more. The diameter of the wire was measured from an image taken at a magnification of 150,000 times, and the length of the wire was measured from an image taken at a magnification of 2500 times.
As a result, the average diameter was 65.5 nm, the average length was 11.4 μm, and the average aspect ratio was 11400 nm / 65.5 nm≈174.

<Replacement of silver nanowire coating material with thiol>
1-dodecanethiol having a property of dissolving in a nonpolar solvent was prepared as a thiol to be replaced on the wire surface.
The concentration of the aqueous dispersion of PVP-coated silver nanowires synthesized by the above method was adjusted so that the silver concentration was 0.5% by mass. At room temperature, 5 mL of acetone was added to 10 mL of this silver nanowire dispersion and stirred for 10 seconds. This was designated as Liquid A. Using a container different from this, 1-dodecanethiol was added to 10 mL of hexane, which is a nonpolar solvent, to a concentration of 10 mmol, and sonication was performed for 1 minute. This was designated as Liquid B. After adding the liquid B to the liquid A and confirming that it was separated into two layers, the mixture was stirred for 1 minute. The silver nanowire was dispersed in the hexane solvent layer derived from the liquid B, and the aqueous solvent layer derived from the liquid A became colorless and transparent. The lower aqueous solvent layer was extracted and removed, and the hexane solvent layer in which the silver nanowires were dispersed was collected and transferred to a centrifuge tube, 30 mL of isopropanol was added thereto, and the mixture was centrifuged at a centrifugal force of 250 G for 10 minutes. After centrifugation, the supernatant was removed and the solid content was recovered. After 30 mL of toluene was added to the collected solid and dispersed, the mixture was centrifuged at a centrifugal force of 250 G for 10 minutes. This operation was repeated twice, and the solid content finally obtained was dispersed in toluene to obtain a nonpolar solvent (toluene) dispersion of silver nanowires.

FIG. 9A illustrates an SEM photograph of the polymer-coated silver nanowires before the coating substance changing process (the one in FIG. 8 observed under the same conditions as FIG. 9B described later). FIG. 9B illustrates an SEM photograph of the thiol-coated silver nanowires after the coating material replacement process.
In FIG. 10, the X-ray-diffraction pattern by the Cu-K alpha ray of the silver nanowire before and after replacement | exchange of a coating material is shown. The X-ray diffraction pattern of metallic silver is exhibited both before and after the coating material change.

About the metal nanowire before replacement | exchange of said coating substance and after replacement | exchange, a liquid component was dried in the drying furnace, respectively, and the dry sample was obtained. Using a TG-DTA apparatus (Rigaku Corporation, ThermoPlus TG-8120), 10 mg of the dried sample was heated from room temperature to 600 ° C. at a heating rate of 5 ° C./min. A TG curve was obtained by measuring the weight.
FIG. 11 illustrates a TG curve for the dried silver nanowire sample before and after the coating material change. (Weight of sample at temperature T−weight of sample before temperature rise) / weight loss of sample up to T ° C. (%) expressed by 100 × weight of sample before temperature rise The polymer (PVP) -coated silver nanowires before material change was 3.8%, and the thiol (1-dodecanethiol) -coated silver nanowires after change of the coating material was 1.2%. In this example, a silver nanowire coated with an organic protective agent (surfactant) having good dispersibility in a nonpolar solvent was obtained. Since the surfactant is a thiol having only one thiol group in the molecule, it is adsorbed on the metal silver surface of the wire at one point in the molecule, unlike a polymer with strong multipoint adsorption like PVP. it is conceivable that.

[Example 2]
A nonpolar solvent dispersion of silver nanowires was obtained in the same manner as in Example 1 except that 1-decanethiol was used as a thiol to be replaced on the wire surface in the operation of replacing the silver nanowire coating substance with thiol. The same silver nanowire as in Example 1 (PVP-coated silver nanowire) was used for the replacement operation. From the TG curve obtained under the same conditions as in Example 1, the heat loss up to 600 ° C. of the thiol (1-decanethiol) -coated silver nanowire obtained by changing the coating material obtained in this example was 2.2%. .

Example 3
A nonpolar solvent dispersion of silver nanowires was obtained in the same manner as in Example 1 except that 1-octanethiol was used as the thiol to be replaced on the wire surface in the operation of replacing the silver nanowire coating material with the thiol. The same silver nanowire as in Example 1 (PVP-coated silver nanowire) was used for the replacement operation. From the TG curve obtained under the same conditions as in Example 1, the heat loss up to 600 ° C. of the thiol (1-octanethiol) -coated silver nanowire obtained by changing the coating material obtained in this example was 1.2%. .

Example 4
<Synthesis of silver nanowires>
As an organic protective agent, a copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate (99% by mass of vinylpyrrolidone and 1% by mass of diallyldimethylammonium nitrate was prepared and a weight average molecular weight of 130,000) was prepared.
At room temperature, 5.24 g of vinylpyrrolidone and diallyldimethylammonium nitrate copolymer, 0.041 g of sodium chloride, 0.0068 g of sodium bromide, 0.0506 g of sodium hydroxide, aluminum nitrate nonahydrate in 540 g of ethylene glycol 0.0416 g was added and dissolved to make Solution X. In a separate container, 4.25 g of silver nitrate was added and dissolved in 20 g of ethylene glycol to obtain Solution Y.

  After raising the total amount of the solution X from room temperature to 115 ° C. in an air atmosphere, the whole amount of the solution Y was added to the solution X over 1 minute. After completion of the addition of the solution Y, the stirring state was further maintained and maintained at 115 ° C. for 24 hours. Thereafter, the reaction solution was cooled to room temperature. After cooling, the solid content was washed in the same manner as in Example 1 to obtain an aqueous dispersion of silver nanowires coated with the copolymer.

  FIG. 12 illustrates an SEM photograph of the silver nanowire. As a result of measuring the average diameter and the average length in the same manner as in Example 1, the average diameter was 45 nm, the average length was 15 μm, and the average aspect ratio was 15000 nm / 45 nm≈333.

<Replacement of silver nanowire coating material with thiol>
3-Mercapto-1,2-propanediol having a property of dissolving in a polar solvent was prepared as a thiol to be replaced on the wire surface.
The concentration of the polymer-coated silver nanowire synthesized by the above method was adjusted so that the silver concentration was 0.5% by mass. At room temperature, 5 mL of acetone was added to 10 mL of this silver nanowire dispersion and stirred for 10 seconds. This was designated as Liquid A. Using a container different from this, 3-mercapto-1,2-propanediol was added to 10 mL of pure water so as to have a concentration of 10 mmol, and ultrasonic treatment was applied to dissolve in pure water. This was designated as Liquid B. Liquid B was added to liquid A and stirred for 2 minutes. This liquid was transferred to a centrifuge tube, 30 mL of methanol was added thereto, and the mixture was centrifuged under a centrifugal force of 700 G for 5 minutes. After centrifugation, the supernatant was removed and the solid content was recovered. After 30 mL of methanol was added to the collected solid and dispersed, the mixture was centrifuged under a centrifugal force of 700 G for 5 minutes. This operation was repeated twice, and the solid content finally obtained was dispersed in pure water to obtain a polar solvent (water) dispersion of silver nanowires.

  FIG. 13A illustrates an SEM photograph of the polymer-coated silver nanowires before the coating substance replacement process (observed the above-mentioned FIG. 12 under the same conditions as FIG. 13B described later). FIG. 13B illustrates an SEM photograph of the thiol-coated silver nanowires after the coating material replacement process.

About the metal nanowire before replacement | exchange of said coating substance and after replacement | exchange, a liquid component was dried in the drying furnace, respectively, and the dry sample was obtained. Using a TG-DTA apparatus (Rigaku Corporation, ThermoPlus TG-8120), 10 mg of the dried sample was heated from room temperature to 750 ° C. at a heating rate of 5 ° C./min. A TG curve was obtained by measuring the weight.
FIG. 14 illustrates a TG curve for the dried silver nanowire sample before and after the coating material change. (Weight of sample at temperature T−weight of sample before temperature increase) / weight loss of sample up to T ° C. (%) expressed by 100 × 100% of the heat loss up to T = 750 ° C. The polymer (the copolymer) coated silver nanowires before material change was 15.1%, and the thiol (3-mercapto-1,2-propanediol) coated silver nanowires after change of the coating material was 3.7%. Even with silver nanowires coated with an organic protective agent (surfactant) with good dispersibility in polar solvents such as water, it is not a highly multi-point adsorbing polymer, but coated with thiols that are thought to be adsorbed at one point Was obtained.

Example 5
In the operation of replacing the silver nanowire coating material with thiol, a polar solvent (water) dispersion of silver nanowire was prepared in the same manner as in Example 4 except that monoethanolamine thioglycolate was used as the thiol to be replaced on the wire surface. Obtained. The same silver nanowire as that used in Example 4 (the above-mentioned copolymer-coated silver nanowire) was used for the replacement operation. From the TG curve obtained under the same conditions as in Example 4, the heat loss up to 750 ° C. of the thiol (3-mercapto-1,2-propanediol) -coated silver nanowire obtained by changing the coating material obtained in this example is 2 0.9%.

Example 6
A silver nanowire polar solvent (water) dispersion was obtained in the same manner as in Example 4 except that ammonium thioglycolate was used as the thiol to be replaced on the wire surface in the operation of replacing the silver nanowire coating substance with the thiol. . The same silver nanowire as that used in Example 4 (the above-mentioned copolymer-coated silver nanowire) was used for the replacement operation. From the TG curve obtained under the same conditions as in Example 4, the heat loss up to 750 ° C. of the thiol (ammonium thioglycolate) -coated silver nanowire after changing the coating material obtained in this example was 3.5%. .

Example 7
In the operation of replacing the silver nanowire coating material with thiol, a polar solvent (water) dispersion of silver nanowires was obtained in the same manner as in Example 4 except that thiomalic acid was used as the thiol to be replaced on the wire surface. The same silver nanowire as that used in Example 4 (the above-mentioned copolymer-coated silver nanowire) was used for the replacement operation. From the TG curve obtained under the same conditions as in Example 4, the heat loss to 750 ° C. of the thiol (thiomalic acid) -coated silver nanowires after replacement of the coating material obtained in this example was 5.5%.

[Comparative Example 1]
<Synthesis of silver nanowires>
As an organic protective agent, the same copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate as in Example 4 was prepared.
At room temperature, in 7800 g of propylene glycol (1,2-propanediol), 83.87 g of a copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate, 0.48 g of lithium chloride, 0.13 g of potassium bromide, 0.13 g of lithium hydroxide. Solution X was added by dissolving 48 g and 3.33 g of a 1,2-propanediol solution having an aluminum nitrate nonahydrate content of 20% by mass. In a separate container, 67.96 g of silver nitrate was added and dissolved in 320 g of 1,2-propanediol to obtain Solution Y.

  The total amount of the solution X was raised from room temperature to 115 ° C., and then stirred at 175 rpm for 20 minutes. After the stirring time of 20 minutes, the solution Y was added to the 115 ° C. solution X over 30 seconds with a tube pump, and the stirring state was maintained and maintained at 115 ° C. for 12 hours. A reaction solution in which the precipitation reaction was completed was obtained. Then, it wash | cleaned by the method similar to Example 1, and obtained the aqueous dispersion of the silver nanowire coat | covered with the said copolymer.

<Attempts to replace silver nanowire coating material>
Malonic acid was prepared as an organic substance to be replaced on the wire surface.
The concentration of the polymer-coated silver nanowire synthesized by the above method was adjusted so that the silver concentration was 0.2% by mass. At room temperature, 10 mmol of malonic acid was added to 20 mL of this silver nanowire dispersion and stirred at 40 ° C. for 8 hours. The liquid after stirring was transferred to a centrifuge tube, 150 mL of pure water was added thereto, and the mixture was centrifuged under a centrifugal force of 1860 G for 15 minutes. After centrifugation, the supernatant was removed and the solid content was recovered. 150 mL of pure water was added to the collected solid, and the mixture was centrifuged under a centrifugal force of 1860 G for 15 minutes. This operation was repeated twice, and the solid content finally obtained was dispersed in pure water to obtain a polar solvent (water) dispersion of silver nanowires.

  About the metal nanowire before the replacement | exchange trial of said coating substance, and after the replacement | exchange attempt, the liquid component was dried with the drying furnace, respectively, and the dry sample was obtained. Using a TG-DTA apparatus (TG / DTA6300, manufactured by SII), 10 mg of the dried sample was heated from room temperature to 700 ° C. at a temperature increase rate of 10 ° C./min, and the weight of the sample at each temperature was measured every second. A TG curve was obtained by measurement. As a result, the heat loss up to 700 ° C. of the polymer (copolymer) -coated silver nanowire before the coating material replacement attempt used in this example was 8.1%, and the heat loss of the silver nanowire up to 700 ° C. after the coating material replacement attempt. Was 12.3%. Since the heat loss increased from that of the original polymer, it is considered that the replacement reaction from the polymer to the carboxylic acid did not proceed despite the long stirring of 8 hours. That is, it is considered that a carboxylic acid having no thiol group does not have a strong adsorption ability to silver enough to desorb a polymer adhering to the metal silver surface of the wire.

[Comparative Example 2]
Experiments were performed in the same manner as in Comparative Example 1 except that malic acid was used as the organic material to be replaced on the wire surface in the attempt to replace the silver nanowire coating material. The same silver nanowire as the comparative example 1 (the above-mentioned copolymer-coated silver nanowire) used for the replacement trial operation was used. From the TG curve calculated | required on the conditions similar to the comparative example 1, the heat loss to 700 degreeC of the silver nanowire after an attempt to replace the coating material was 9.7%. Also in this example, the heat loss increased from the original polymer, and it is considered that the replacement reaction from the polymer to the carboxylic acid did not proceed.

[Comparative Example 3]
An experiment was performed in the same manner as in Comparative Example 1 except that ethylenediamine was used as the organic material to be replaced on the wire surface in the attempt to replace the silver nanowire coating material. The same silver nanowire as the comparative example 1 (the above-mentioned copolymer-coated silver nanowire) used for the replacement trial operation was used. From the TG curve calculated | required on the conditions similar to the comparative example 1, the heat loss to 700 degreeC of the silver nanowire after an attempt to replace the coating material was 13.0%. Also in this example, the heat loss increased from that of the original polymer, and it is considered that the replacement reaction from the polymer to the amine did not proceed. In other words, it is considered that the amine having no thiol group does not have a strong adsorption ability to silver enough to desorb the polymer attached to the metal silver surface of the wire.

[Comparative Example 4]
An experiment was performed in the same manner as in Comparative Example 1 except that propanediamine was used as an organic material to be replaced on the wire surface in the attempt to replace the silver nanowire coating material. The same silver nanowire as the comparative example 1 (the above-mentioned copolymer-coated silver nanowire) used for the replacement trial operation was used. From the TG curve calculated | required on the conditions similar to the comparative example 1, the heat loss to 700 degreeC of the silver nanowire after an attempt to replace the coating material was 11.8%. Also in this example, the heat loss increased from that of the original polymer, and it is considered that the replacement reaction from the polymer to the amine did not proceed.

Claims (13)

  Silver nanowires having an average diameter of 100 nm or less and an average length of 5 μm or more, wherein a thiol having a molecular weight of 75 to 300 is attached to the surface of metallic silver.   The silver nanowire according to claim 1, wherein the thiol has only one thiol group in the molecule.   The silver nanowire according to claim 1, wherein the thiol is one or more selected from the group consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.   The silver nanowire according to claim 1, wherein the thiol is at least one selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid. .   By mixing the liquid A in which silver nanowires coated with the polymer are dispersed and the liquid B in which the thiol having a molecular weight of 75 to 300 is dissolved, the adhesion substance on the metal silver surface is changed from the polymer to the thiol. The manufacturing method of the silver nanowire which has a process.   The method for producing silver nanowires according to claim 5, wherein the silver nanowires dispersed in the liquid A have an average diameter of 100 nm or less and an average length of 5 µm or more.   The method for producing a silver nanowire according to claim 5, wherein the thiol has only one thiol group in the molecule.   The method for producing a silver nanowire according to claim 5, wherein the thiol is at least one selected from the group consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.   The silver nanowire according to claim 5, wherein the thiol is at least one selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid. Manufacturing method.   The method for producing a silver nanowire according to claim 5, wherein the polymer is PVP (polyvinylpyrrolidone).   The method for producing a silver nanowire according to claim 5, wherein the polymer is a copolymer having a vinylpyrrolidone structural unit.   The method for producing silver nanowires according to claim 5, wherein the liquid A and the liquid B are mixed in the presence of one or two of acetone and isopropanol.   A silver nanowire dispersion liquid in which the silver nanowires according to claim 1 are dispersed in a liquid medium.