SILVER NANOWIRES, AND PRODUCTION METHOD AND DISPERSION OF THE SAME

Abstract

Silver nanowires coated with, instead of a polymer protective agent such as PVP, an organic protective agent having a smaller molecular weight are provided. The silver nanowires have an average diameter of 100 nm or less and an average length of 5 μm or more, and a thiol having a molecular weight of 75 to 300 is adhered on surfaces of the metal silver. The silver nanowires have a thiol containing one thiol group in the structure adhered thereon. A thiol having only one thiol group (—S—H) in a molecule is a suitable target. Examples thereof include 1-dodecanethiol, 1-decanethiol, 1-octanethiol, 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

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

Description of the Related Art

As used herein, “nanowires” refers to an aggregate of fine metal wires having a thickness of approximately 200 nm or less. When likened to powder, an individual wire corresponds to a “particle” constituting powder, and nanowires correspond to “powder” which is an aggregate of particles.

Silver nanowires are expected as a promising conductive material for imparting electric conductivity to a transparent base material. When a transparent base material such as a glass, polyethylene terephthalate (PET), and polycarbonate (PC) is coated with a liquid in which silver nanowires are dispersed, and then the liquid component is removed, for example, by evaporation, silver nanowires come in contact with each other on the base material to thereby form a conductive network, and therefore can realize a transparent conductor. In related arts, as the transparent conductive material, metal oxide films typified by ITO have been mainly used for applications such as a transparent electrode. However, the metal oxide films have drawbacks such as high cost in film formation and low bending resistance which is a factor inhibiting production of flexible end products. In addition, a conductive film for a touch panel sensor which is one of major applications of transparent conductors is required to have high transparency and high electric conductivity, and in addition, a demand for visibility becomes increasingly higher now. In an ITO film in related arts, while the thickness of an ITO layer is required to be increased for enhancing electric conductivity, the increase of the thickness leads to deterioration of transparency, failing to improve the visibility.

Silver nanowires are expected to overcome the above drawbacks inherent in the metal oxide films typified by ITO.

As a method for producing silver nanowires, a technique is known in which a silver compound is dissolved in a polyol solvent such as ethylene glycol and metal silver is deposited in a linear shape by utilizing reducing ability of the solvent polyol in the presence of a halogen compound, and polyvinylpyrrolidone (PVP) or a copolymer having a vinylpyrrolidone constituting unit which is an organic protective agent (Patent Document 1-3, and Non-patent Document 1). PVP and the copolymer having a vinylpyrrolidone constituting unit are highly effective substances as an organic protective agent for synthesizing silver nanowires in a high yield.

RELATED ART DOCUMENT

Patent Document

• Patent Document 1: US 2005/0056118
• Patent Document 2: US 2008/0003130
• Patent Document 3: US 2014/0178247

Non-Patent Document

• Non-patent Document 1: J. of Solid State Chem. 1992, 100, 272-280

SUMMARY OF THE INVENTION

Common silver nanowires coated with PVP in related arts are generally provided as a silver nanowire dispersion using an aqueous liquid medium because of good dispersibility thereof in water and other polar solvents. However, depending on the application, a silver nanowire dispersion using a solvent having small polarity or a nonpolar non-aqueous solvent is in some cases desirably used. For responding such a need, a technique in which the polymer protective agent such as PVP which has been needed for silver nanowires synthesis is substituted with a substance having good dispersibility in a liquid medium is effective.

The polymer protective agent such as PVP with which silver nanowires are coated is desirably adhered in an amount as small as possible or not adhered at all from the viewpoint of improving electric conductivity due to contacts between the wires. However, since one molecule of the polymer protective agent such as PVP adheres to metal silver of the wires at multiple points, it is generally difficult to effectively remove the polymer protective agent through a washing operation. It is also difficult to substitute the polymer protective agent with another surface protective agent.

On the other hand, in formation of a conductive network by silver nanowires, the conduction has been secured by simple contacts between the wires in related arts. If wires can be bonded at the contact points by sintering, the electric conductivity of a conductive network is considered to be greatly enhanced. However, silver nanowires having the polymer protective agent such as PVP adheres thereon have a high sintering temperature, and it is difficult in many cases to sinter the silver nanowires after applying them on a transparent base material. In particular, PVP also has a problem of easy fusing under heat.

The present invention has an object to provide silver nanowires coated with, instead of a polymer protective agent such as PVP, an organic protective agent having a smaller molecular weight.

As a result of studies, the present inventors have found an effective technique for securely substituting a polymer protective agent such as PVP with a substance having a smaller molecular weight with an easy procedure using, as raw material wires, silver nanowires obtained by a method in which silver is reduced and deposited in a wire form in an alcohol solvent in the presence of the polymer protective agent. Specifically, the above object can be achieved by adopting a thiol as the protective agent substance that is the substitute.

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 in which a thiol having a molecular weight of 75 to 300 is adhered on surfaces of metal silver are provided. Silver nanowires having an average diameter of 50 nm or less and an average length of 10 μm or more are a preferred target. Silver nanowires having an average aspect ratio of 250 or more are a particularly suitable target. As used herein, the average diameter, the average length, and the average aspect ratio are defined as follows.

Average Diameter

On a microscopic image (for example, FE-SEM image), diameters of circles inscribed in the opposite outlines in the thickness direction on a projection image of a metal wire are measured along the entire length of the wire, and the average of the diameters is defined as the diameter of the wire. Then, the average of diameters of wires constituting nanowires is defined as the average diameter of the nanowires. For calculating the average diameter, the total number of the wires to be measured is 100 or more. However, wire-like products having a length (described later) less than 0.5 μm and particulate products are excluded from the wires to be measured.

Average Length

On the same microscopic image as above, a length of a metal wire from one end to the other end along a line passing through positions of the thickness centers of the wire (that is, the centers of inscribed circles as described above) on a projection image of the wire is defined as the length of the wire. Then, the average of lengths of wires constituting nanowires is defined as the average length of the nanowires. For calculating the average length, the total number of the wires to be measured is 100 or more. However, wire-like products having a length less than 0.5 μm and particulate products are excluded from the wires to be measured.

The silver nanowires according to the present invention are constituted of wires of a highly elongated shape. For this reason, the silver nanowires aggregated do not have a linear rod shape, but rather have a string-like curved shape in many cases. The present inventors create software for measuring a length of such a wire having a curved shape in an efficient manner on an image and use the software in data processing.

Average Aspect Ratio

The average aspect ratio is calculated by assign the average diameter and the average length described above in the following formula (1).

[Average aspect ratio]=[Average length (nm)]/[Average diameter (nm)]  (1)

As the thiol, a compound having only one thiol group (—S—H) in a molecule is suitable. Examples include 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), and thiomalic acid (molecular weight 150.2).

The thiol to be adhered is not limited to one kind alone, and two or more thiols may be adhered. The position of the thiol group present in the thiol is not limited to the terminal end, and the thiol group may exist at any position in a thiol molecular structure.

As specific aspects of the silver nanowires, the following (a) to (e) may be mentioned.

(a) 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 adhered on surfaces of metal silver.
(b) The silver nanowires according to the above (a), which has an average diameter of 50 nm or less and an average length of 10 μm or more.
(c) The silver nanowires according to the above (a) or (b), wherein the thiol has only one thiol group in a molecule.
(d) The silver nanowires according to the above (a) or (b), wherein the thiol is one or more selected from the group consisting of i-dodecanethiol, 1-decanethiol, and 1-octanethiol.
(e) The silver nanowires according to the above (a) or (b), wherein the thiol is one or more selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.

As a method for producing the silver nanowires having a thiol adhered thereon, there is provided a production method comprising mixing in a container a liquid A in which silver nanowires coated with a polymer is dispersed and a liquid B in which a thiol having a molecular weight of 75 to 300 is dissolved, thereby substituting an adhering substance on metal silver surfaces from the polymer to the thiol.

Examples of the polymer include polyvinylpyrrolidone (PVP), a copolymer having a vinylpyrrolidone constituting unit, and a polymer in which polymerizable monomers including acrylamide are polymerized. The copolymer having a vinylpyrrolidone constituting unit is a copolymer having a polymerized composition of vinylpyrrolidone and another monomer. Examples may include a copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate, a copolymer of vinylpyrrolidone and a maleimide-based monomer, and a copolymer of vinylpyrrolidone and a copolymer of acrylate monomers such as ethyl acrylate and hydroxyethyl acrylate. The weight average molecular weight of the polymer used is, for example, 20,000 to 1,300,000.

The amount of the polymer adhered on silver nanowires in the liquid A (that is, silver nanowires after a synthetic reaction and washing treatment of the silver nanowires) is, for example, 3 to 15% by mass, based on the ignition loss in TG-DTA (thermogravimetry-differential thermal analysis) of dried silver nanowires. When the polymer on surfaces of the silver nanowires is substituted with a thiol having a lower molecular weight, the ignition loss in the TG-DTA is reduced as compared with the value before the substitution treatment. As the thiol in the liquid B, the compounds mentioned above are exemplified.

The liquid A and the liquid B can be mixed in the presence of an amphipathic substance such as acetone and isopropanol. In particular, when a solvent for the liquid A is a polar solvent and a solvent for the liquid B is a nonpolar solvent, the mixing under the presence of an amphipathic substance is highly effective.

As specific aspects of the method for producing silver nanowires, the following (f) to (n) may be mentioned.

(f) A method for producing silver nanowires comprising mixing a liquid A in which silver nanowires coated with a polymer are dispersed and a liquid B in which a thiol having a molecular weight of 75 to 300 is dissolved to thereby substitute an adhering substance on metal silver surfaces from the polymer to the thiol.
(g) The method for producing silver nanowires according to the above (f), 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.
(h) The method for producing silver nanowires according to the above (f) or (g), wherein the silver nanowires dispersed in the liquid A have an average diameter of 50 nm or less and an average length of 10 μm or more.
(i) The method for producing silver nanowires according to any one of the above (f) to (h), wherein the thiol has only one thiol group in a molecule.
(j) The method for producing silver nanowires 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 method for producing silver nanowires according to any one of the above (f) to (h), wherein the thiol is one or more selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.
(l) The method for producing silver nanowires according to any one of the above (f) to (k), wherein the polymer is polyvinylpyrrolidone (PVP).
(m) The method for producing silver nanowires according to any one of the above (f) to (k), wherein the polymer is a copolymer having a vinylpyrrolidone constituting unit.
(n) The method for producing silver nanowires 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.

The present invention also provides a silver nanowire dispersion in which the silver nanowires having a thiol adhered thereon are dispersed in a liquid medium. As a specific aspect, a silver nanowire dispersion in which the silver nanowires as set forth in any one of the above (a) to (e) are dispersed in a liquid medium may be mentioned.

In the present invention, silver nanowires having a thiol adhered on surfaces of metal silver are disclosed. On the silver nanowires immediately after synthesis through reduction and deposition, an organic substance such as a polymer used in the synthesis as a shape controller is adhered. Since thiols have high adhesiveness to metal silver, when a surfactant containing a thiol is added to the silver nanowire dispersion after the synthesis, the organic substance such as a polymer adhered on the surfaces of the silver nanowires can be substituted with the surfactant containing the thiol. By substituting an organic protective agent adhered on the silver nanowire surfaces, it becomes possible to disperse silver nanowires, which can generally be dispersed only in a polar solvent, in a nonpolar solvent such as toluene and hexane.

Monomers having a molecular weight of 300 or less are easily released from surfaces of silver nanowires at a lower temperature compared with a polymer such as PVP. For this reason, when a transparent conductive film is formed using the silver nanowires according to the present invention, through a heat treatment that is performed after a coating material containing the silver nanowires is applied on a base material, the amount of organic substances remaining on the surfaces of the nanowires can be reduced as compared with related arts. A smaller amount of organic substances on surfaces of nanowires leads to increase of probability of contacts between the nanowires and decrease of electric resistance of the conductive network. Since the density of silver nanowires in a film required for imparting a prescribed electric conductivity to a conductive film can be reduced, optical transparency is accordingly enhanced, being advantageous for forming a clear conductive film with less haze.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a structural formula of 1-dodecanethiol.

FIG. 2 shows a structural formula of 1-decanethiol.

FIG. 3 shows a structural formula of 1-octanethiol.

FIG. 4 shows a structural formula of 3-mercapto-1,2-propanediol.

FIG. 5 shows a structural formula of monoethanolamine thioglycolate.

FIG. 6 shows a structural formula of ammonium thioglycolate.

FIG. 7 shows a structural formula of thiomalic acid.

FIG. 8 is an SEM photograph of polymer (PVP)-coated silver nanowires synthesized in Example 1.

FIG. 9A is an SEM photograph of the polymer-coated silver nanowires before coating substance substitution treatment used in Example 1.

FIG. 9E is an SEM photograph of thiol-coated silver nanowires after coating substance substitution treatment obtained in Example 1.

FIG. 10 shows X-ray diffraction patterns of the silver nanowires before and after coating substance substitution in Example 1.

FIG. 11 shows TG curves of the silver nanowires before and after coating substance substitution in Example 1.

FIG. 12 is an SEM photograph of polymer (copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate)-coated silver nanowires synthesized in Example 4.

FIG. 13A is an SEM photograph of the polymer-coated silver nanowires before coating substance substitution treatment used in Example 4.

FIG. 13B is an SEM photograph of thiol-coated silver nanowires after coating substance substitution treatment obtained in Example 4.

FIG. 14 shows TG curves of the silver nanowires before and after coating substance substitution in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

A thiol is a substance represented by the general formula R—SH. A thiol group (—S—H) has a nature of strongly binding to a noble metal such as gold and silver. When a thiol is added to a dispersion of silver nanowires having a surfactant such as PVP adhered thereon, the thiol having a stronger adhesiveness with silver tends to preferentially adhere to wire surfaces. It has been found that by stirring the liquid or mixing the liquid with inversion to increase chances of the thiol getting close to the metal silver surfaces of the wires, it is possible to substitute the surfactant (organic protective agent) on the wire surfaces with the thiol. Some surfactants such as amines and carboxylic acids do not have capability of substituting a high molecular surfactant such as PVP.

The thiol for use in the present invention suitably has a molecular weight in a range of 75 to 300. Within this range, the thiol is easy to be released in a heat treatment in production of a conductive film even at a relatively lower temperature, which is advantageous for obtaining a transparent conductive film with high electric conductivity.

In addition, it is preferred that a thiol having only one thiol group in a molecule is adopted. Since a thiol group has strong binding force to silver, when two or more thiol groups are contained in a molecule, the respective thiol groups may adhere to different wires. In such a case, the individual wires are liable to bond to each other to form agglomerate. In order to disperse the individual wires independently in a liquid, use of a thiol having only one thiol group in a molecule is effective.

Among thiols, there are ones that can dissolve in a nonpolar solvent and ones that can dissolve in a polar solvent. A thiol providing a surfactant action according to the purpose may be selected. Examples of the thiol dissolved in a nonpolar solvent include 1-dodecanethiol, 1-decanethiol, and 1-octanethiol. Examples of the thiol dissolved in a polar solvent include 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid. As used herein, a nonpolar solvent refers to a solvent having a solubility parameter (SP value) of 9.5 or less. For example, water, methanol, ethanol, etc. fall under the polar solvent, and hexane, toluene, benzene, etc. fall under the nonpolar solvent.

When the amount of the thiol adhered to silver nanowires is excessively large, the amount of thiol not leaving the wires but remaining on the wires in the heat treatment in production of a conductive film is increased, and the electric conductivity of the conductive film may not be sufficiently enhanced. According to a result of various studies, the amount of the thiol adhered is preferably 10.0% by mass or less, and more preferably 5.0% by mass or less, relative to the total mass of the silver nanowires (the sum of the metal silver and the thiol adhered to the surfaces thereof). On the other hand, when the amount of the thiol adhered is too small, there may arise problems of reduced dispersibility in the liquid and agglomeration of silver nanowires. The amount of thiol adhered is preferably 0.5% by mass or more. The amount of the thiol adhered can be determined by an ignition loss obtained from TG-DTA (thermogravimetry-differential thermal analysis).

As for the shape of the silver nanowires, the nanowires desirably has an average diameter of 100 nm or less and an average length of 5 μm or more, more preferably has an average diameter of 50 nm or less and an average length of 10 μm or more. A smaller average diameter leads to a smaller scattering of light in the transparent conductive film and higher optical transparency, and therefore is advantageous for producing a clear and highly visible touch panel or display having less haze. A larger average length leads to increased chances of contact between wires in forming a conductive network and therefore is advantageous for enhancing the electric conductivity of the transparent conductive film, accordingly making it possible to reduce the content of wires in the conductive film. The reduction of the wire content is effective for reducing the haze and enhancing the optical transparency. However, when an average diameter is significantly reduced, fracture (break, rupture, etc.) of the wires is liable to occur in the production process of conductive film, and therefore careful handling is required, which may lead to reduction in productivity. In general, an average diameter may be 10 nm or more. In addition, when the average length is significantly increased, there may arise problems that the wires are entangled with each other to agglomerate in a dispersion, or are liable to clog a nozzle in application of a silver nanowire ink. The average length may generally be in a range of 500 μm or less. The average aspect ratio is more preferably 300 or more.

The synthetic method of metal nanowires is not particularly limited, but a synthetic method by a wet process is known at this time. For example, in the case of silver nanowires, the reduction and deposition method disclosed in Patent Documents 1 and 2 (mentioned herein above) is known.

An operation for substitute an organic protective agent adhered on the synthesized silver nanowires from a high molecular compound such as PVP to a surfactant having a thiol group which is a target substance is conducted. The silver nanowires of the present invention are characterized in that a surfactant having a thiol group is adhered. Since thiols has a nature of easily adhering to silver, when a sufficient amount of a thiol exists in the vicinity of surfaces of silver nanowires having a high molecular compound such as PVP adhered thereon, there arises a state where the high molecular compound tends to leave the silver nanowire surfaces and meanwhile the thiol tends to adhere, and the substitution reaction can proceed relatively easily.

Since the reaction proceeds in a solvent, the thiol has to be dissolved in a solvent. When the substitution is made with 1-dodecanethiol, 1-decanethiol, 1-octanethiol, and the like which can be dissolved in a nonpolar solvent, the liquid B described later is prepared using a nonpolar solvent such as hexane and toluene. One kind of the nonpolar solvents may be used alone or two or more kinds thereof may be used in mixture. In the case where the substitution is made with 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, thiomalic acid, and the like which can be dissolved in a polar solvent, the liquid B described later is prepared using a polar solvent such as water, methanol, and ethanol. One kind of the polar solvents may be used alone or two or more kinds thereof may be used in mixture.

When the amount of the thiol used is 0.5 mmol or more relative to 10 mg of the silver mass in the silver nanowires, the reaction (desorption and adsorption) for substituting a polymer adhered to the wire surfaces with the thiol can sufficiently proceed. When the amount of the thiol is lower, the substitution does not proceed sufficiently. For example, in the case where a polar solvent in which silver nanowires having PVP adhered thereon is dispersed and a nonpolar solvent (for example, toluene) in which 1-dodecanethiol is dissolved are mixed to conduct an operation for substituting an organic protective agent from PVP to 1-dodecanethiol, when the amount of 1-dodecanethiol is sufficient, almost all the amount of PVP on the wire surfaces is substituted with 1-dodecanethiol, and all the silver nanowires lose dispersibility in a polar solvent and meanwhile acquire dispersibility in a nonpolar solvent. As a result, all the amount of the silver nanowires becomes dispersed in a nonpolar solvent layer among two solvent layers existing in a separated state. However, when the amount of 1-dodecanethiol added is short, not all the amount of PVP can be substituted with 1-dodecanethiol, wires still having PVP adhered on a part or all of the surface remain in a polar solvent layer among the two layers separated.

In the operation of substituting an organic protective agent adhered on metal silver surfaces of silver nanowires from a polymer to a thiol, a liquid A in which silver nanowires coated with the polymer are dispersed and a liquid B in which a thiol having a molecular weight of 75 to 300 is dissolved are prepared. A polymer such as PVP which is used in reductive synthesis of silver nanowires generally has good dispersibility in water. The silver nanowires coated with the polymer are stored in a state of being dispersed in an aqueous solvent in many cases. The aqueous solvent is a polar solvent.

In the case where the polymer is substituted with a thiol having solubility in a polar solvent, since the liquid B is a polar solvent, by using as the liquid A the aqueous solvent in which silver nanowires coated with the polymer is dispersed, the silver nanowire dispersion in which silver nanowires coated with the thiol is dispersed in a polar solvent (an aqueous solvent, etc.) can be obtained.

On the other hand, in the case where the polymer is substituted with a thiol having solubility in a nonpolar solvent, since the liquid B is a nonpolar solvent, when an aqueous solvent in which silver nanowires coated with the polymer are dispersed is used as it is as the liquid A, the liquid A and the liquid B separate with an interface formed. For causing adsorption of the thiol on wire surfaces of metal silver, molecules of the thiol have to come sufficiently close to the wire surfaces present in the liquid A. When the liquid A and the liquid B which are separated from each other are stirred vigorously, the chances of the wire surfaces and the thiol molecules getting close to each other are secured, but the nanowires are liable to suffer impact of the stirring to be damaged. Thus, in order to easily secure the chances of the wire surfaces and the thiol molecules getting close to each other in a stirring environment as mild as possible, it is effective that the liquid A and the liquid B are mixed in the presence of an amphipathic substance such as acetone and isopropanol. By adding the amphipathic substance into the liquid A or the liquid B in advance or placing the amphipathic substance in a container for mixing the liquid A and the liquid B in advance, the mixing in the presence of the amphipathic substance is possible. Acetone and isopropanol have solubility parameters (SP values) exceeding 9.5, and therefore are not classified into a nonpolar solvent referred to herein, but have affinity to both of a polar solvent and a nonpolar solvent. In the case where the liquid A or the liquid B having such an amphipathic substance added is used, chances of the wire surfaces and the thiol molecules getting close to each other are easily secured when both the solvents are mixed and stirred, the substitution of the polymer with the thiol can proceed smoothly. In addition, the wires after the substitution with the thiol can easily transfer into the nonpolar solvent layer supplied from the liquid B. In this manner, a silver nanowire dispersion in which silver nanowires coated with a thiol are dispersed in a nonpolar solvent (hexane, toluene, benzene, etc.) can be obtained.

When an example where acetone or isopropanol is added to an aqueous solvent in which silver nanowires coated with a polymer are dispersed to prepare the liquid A is mentioned, the amount of acetone or isopropanol added may be 0.5 to 2.5 volume units based on 1 volume unit of the aqueous solvent.

Incidentally, in the case where the dispersion of the silver nanowires coated with a polymer is an alcohol-based polar solvent such as methanol and ethanol, interfacial energy between the alcohol and a nonpolar solvent is significantly smaller compared with interfacial energy between water and the nonpolar solvent. For this reason, also when such an alcohol-based solvent as above is used as it is as the A liquid without addition of any amphipathic substance, the substitution with a thiol having dispersibility into a nonpolar solvent can be achieved relatively easily.

The treatment temperature upon substituting an organic protective agent is a temperature of the boiling point of the solvent used or lower. Since the substitution reaction (desorption of a polymer and adsorption of a thiol) proceeds easily even in normal temperature, the reaction may be performed generally at a temperature in a range of 20 to 50° C. As for the method of mixing the liquid A and the liquid B, it is preferred that the liquid B is added to the liquid A containing silver nanowires. The liquid B may be added at once or may be added intermittently or continuously. The liquid A is preferably brought into a state in advance where the liquid A is stirred at a speed that gives as small as possible damage on the nanowires. The mixture may be stirred or mixed with inversion after the whole volume of the liquid B is added. The atmosphere of the gas phase in contact with the liquid surface of the solvent upon the substitution treatment is not particularly limited. An air atmosphere, a nitrogen atmosphere or the like may be applied. The time required for the substitution reaction with a thiol is, in the case of substitution at 20 to 50° C., from about 10 seconds to about 2 minutes from the start of the mixing of the liquid A and the liquid B.

When a thiol having a carbon chain such as 1-dodecanethiol, 1-decanethiol, and 1-octanethiol is adhered, the silver nanowires after the substitution treatment with the thiol become able to be dispersed in a nonpolar solvent such as hexane and toluene. When a thiol having a hydroxy group or a carboxy group, such as 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate and thiomalic acid is adhered, the silver nanowires can be dispersed in a polar solvent such as water, methanol, and ethanol. In this case, dispersibility to various polar solvents can be controlled depending on the kind of the thiol adhered.

EXAMPLES

Example 1

Synthesis of Silver Nanowires

Polyvinylpyrrolidone (PVP) having a weight average molecular weight of 55,000 was provided as an organic protective agent.

At normal temperature, 2.5 g of PVP and 0.006 g (0.1 mmol) of sodium chloride were added to and dissolved in 60 g of ethylene glycol to prepare a solution X. In a container different from the above, 0.85 g (5.0 mmol) of silver nitrate was added to and dissolved in 7.65 g of ethylene glycol to prepare a solution Y.

Under an air atmosphere, the whole volume of the solution X was heated to 135° C. with stirring at 500 rpm, and then the solution Y was added at once into the solution X. After completing the addition of the solution Y, the stirring speed was changed to 100 rpm, and the solution was maintained at 135° C. for 3 hours while keeping the stirring state. Then, the reaction liquid was cooled to normal temperature.

After cooling, washing was performed according to the following procedure to obtain a dispersion of silver nanowires.

The reaction liquid was transferred into a centrifugal tube, 30 mL of distilled water was added thereto, and the mixture was centrifuged under conditions of a centrifugal force of 1000 G and 5 minutes. After the centrifugation, the supernatant was removed to collect a solid. After 30 mL of methanol was added to the solid collected to disperse the solid therein, the dispersion was centrifuged under conditions of a centrifugal force of 700 G and 5 minutes. After the centrifugation, the supernatant was removed to collect a solid. After 30 mL of methanol was added to the solid collected to disperse the solid therein, the dispersion was centrifuged under conditions of a centrifugal force of 250 G and 10 minutes. The supernatant was removed, and the solid was dispersed again in water, thereby obtaining an aqueous dispersion of the silver nanowires.

A sample was taken from the dispersion, the solvent water was volatilized on an observation stand, and then the sample was observed by FE-SEM (field emission type scanning electron microscope). As a result, the solid was confirmed to be silver nanowires. In this manner, silver nanowires coated with PVP were obtained.

In FIG. 8, an SEM photograph of silver nanowires is illustrated. In the SEM observation, in five fields selected randomly, the average diameter and the average length were determined according to the definition described herein above. The total number of the wires to be measured was 100 or more. The wire diameters were measured on an image taken at a magnification of 150,000, and the wire lengths were measured on an image taken at a magnification of 2,500.

According to the 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.

Substitution of Silver Nanowire Coating Substance with Thiol

As a thiol for substitution on the wire surfaces, 1-dodecanethiol having solubility in a nonpolar solvent was provided.

The aqueous dispersion of the PVP-coated silver nanowires synthesized by the above method was adjusted in concentration so as to give a silver concentration of 0.5% by mass. At a normal temperature, 5 mL of acetone was added to 10 mL of the silver nanowire dispersion, and the mixture was stirred for 10 seconds. The resulting mixture was a liquid A. Using a container different from the above, 1-dodecanethiol was added to 10 mL of hexane which is a nonpolar solvent so as to give a concentration of 10 mmol, and the mixture was centrifuged for 1 minute. The resulting mixture was a liquid B. The liquid B was added to the liquid A, and after confirming separation into two layers, stirring was performed for 1 minute. The silver nanowires were dispersed in a hexane solvent layer originating in the liquid B, and an aqueous solvent layer originating in the liquid A became colorless and transparent. The lower aqueous solvent layer was drawn out and removed, the hexane solvent layer in which the silver nanowires were dispersed was collected and transferred into a centrifugal tube, 30 mL of isopropanol was added thereto, and the mixture was centrifuged under conditions of a centrifugal force of 250 G and 10 minutes. After the centrifugation, the supernatant was removed to collect a solid. After 30 mL of toluene was added to the solid collected to disperse the solid therein, the dispersion was centrifuged under conditions of a centrifugal force of 250 G and 10 minutes. This operation was repeated twice, and the obtained final solid was dispersed in toluene, whereby a nonpolar solvent (toluene) dispersion of silver nanowires was obtained.

In FIG. 9A, an SEM photograph of the polymer-coated silver nanowires before the coating substance substitution treatment (the silver nanowires of FIG. 8 above observed under the same conditions as in FIG. 9B below) is illustrated. In FIG. 9B, an SEM photograph of the thiol-coated silver nanowires after the coating substance substitution treatment is illustrated.

In FIG. 10, X-ray diffraction patterns by Cu-Kα ray of the silver nanowires before and after the coating substance substitution are shown. Both before and after the coating substance substitution, an X-ray diffraction pattern of metal silver is given.

In each of the metal nanowires before the coating substance substitution and those after the substitution, the liquid component was volatilized in a drying oven to obtain a dry sample. Using a TG-DTA apparatus (manufactured by Rigaku Corporation, Thermo Plus TG-8120), 10 mg of the dry sample was heated from a normal temperature to 600° C. at a temperature rising rate of 5° C./min, and the weight of the sample at each temperature was measured every 1 second, thereby obtaining a TG curve.

In FIG. 11, TG curves of the dry samples of the silver nanowires before and after the coating substance substitution are illustrated. When the ignition losses until T=600° C. were compared, with an ignition loss until T° C. being represented by ((sample weight at temperature T)−(sample weight before temperature rising))/(sample weight before temperature rising)×100, the polymer (PVP)-coated silver nanowires before the coating substance substitution had a value of 3.8%, and the thiol (1-dodecanethiol)-coated silver nanowires after the coating substance substitution had a value of 1.2%. In this example, silver nanowires coated with an organic protective agent (surfactant) having good dispersibility in a nonpolar solvent could be obtained. Since the surfactant is a thiol having only one thiol group in a molecule, unlike a polymer having strong multiple-point adsorptivity such as PVP, the surfactant is considered to be adsorbed on a metal silver surface of a wire at one point in the molecule.

Example 2

A nonpolar solvent dispersion of silver nanowires was obtained in the same manner as in Example 1 except that in the substitution operation of silver nanowire coating substance into a thiol, 1-decanethiol was used as the thiol for substitution on the wire surfaces. As the silver nanowires subjected to the substitution operation, the same nanowires as in Example 1 (PVP-coated silver nanowires) were used. According to a TG curve obtained under the same conditions as in Example 1, the ignition loss until 600° C. of the thiol (1-decanethiol)-coated silver nanowires after the coating substance substitution 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 in the substitution operation of silver nanowire coating substance into a thiol, 1-octanethiol was used as the thiol for substitution on the wire surfaces. As the silver nanowires subjected to the substitution operation, the same nanowires as in Example 1 (PVP-coated silver nanowires) were used. According to a TG curve obtained under the same conditions as in Example 1, the ignition loss until 600° C. of the thiol (1-octanethiol)-coated silver nanowires after the coating substance substitution 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 (the copolymer was produced from 99% by mass of vinylpyrrolidone and 1% by mass of diallyldimethylammonium nitrate, weight average molecular weight: 130,000) was provided.

At a normal temperature, 5.24 g of the copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate, 0.041 g of sodium chloride, 0.0072 g of sodium bromide, 0.0506 g of sodium hydroxide, and 0.0416 g of aluminum nitrate nonahydrate were added to and dissolved in 540 g of ethylene glycol to prepare a solution X. In a container different from the above, 4.25 g of silver nitrate was added to and dissolved in 20 g of ethylene glycol to prepare a solution Y.

Under an air atmosphere, the whole volume of the solution X was heated from normal temperature to 115° C. with stirring, and then the whole volume of the solution Y was added to the solution X over 1 minute. After completing the addition of the solution Y, the solution was maintained at 115° C. for 24 hours while further keeping the stirring state. Then, the reaction liquid was cooled to normal temperature. After cooling, the solid was washed in the same manner as in Example 1, whereby an aqueous dispersion of silver nanowires coated with the copolymer was obtained.

In FIG. 12, an SEM photograph of the silver nanowires is illustrated. According to the 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.

Substitution of Silver Nanowire Coating Substance into Thiol

As a thiol for substitution on the wire surfaces, 3-mercapto-1,2-propanediol having solubility in a polar solvent was provided.

The polymer-coated silver nanowires synthesized in the above method were adjusted in concentration so as to give a silver concentration of 0.5% by mass. At normal temperature, 5 mL of acetone was added to 10 mL of the silver nanowire dispersion and the mixture was stirred for 10 seconds. The resulting mixture was a liquid A. Using a container different from the above, 3-mercapto-1,2-propanediol was added to 10 mL of pure water so as to give a concentration of 10 mmol, and dissolved in the pure water with ultrasonication. The resulting mixture was a liquid B. The liquid B was added to the liquid A, and the mixture was stirred for 2 minutes. The liquid was transferred into a centrifugal tube, 30 mL of methanol was added thereto, and the mixture was centrifuged under conditions of a centrifugal force of 700 G and 5 minutes. After the centrifugation, the supernatant was removed to collect a solid. After 30 mL of methanol was added to the solid collected to disperse the solid therein, the dispersion was centrifuged under conditions of a centrifugal force of 700 G and 5 minutes. This operation was repeated twice, and the obtained final solid was dispersed in pure water, whereby a polar solvent (aqueous) dispersion of silver nanowires was obtained.

In FIG. 13A, an SEM photograph of the polymer-coated silver nanowires before the coating substance substitution treatment (the silver nanowires of FIG. 12 above observed under the same conditions as in FIG. 13B below) is illustrated. In FIG. 13B, an SEM photograph of the thiol-coated silver nanowires after the coating substance substitution treatment is illustrated.

In each of the metal nanowires before the coating substance substitution and those after the substitution, the liquid component was volatilized in a drying oven to obtain a dry sample. Using a TG-DTA apparatus (manufactured by Rigaku Corporation, Thermo Plus TG-8120), 10 mg of the dry sample was heated from a normal temperature to 750° C. at a temperature rising rate of 5° C./min, and the weight of the sample at each temperature was measured every 1 second, thereby obtaining a TG curve.

In FIG. 14, TG curves of the dry samples of silver nanowires before and after the coating substance substitution are illustrated. When the ignition losses until T=750° C. were compared, with an ignition loss until T° C. being represented by ((sample weight at temperature T)−(sample weight before temperature rising))/(sample weight before temperature rising)×100, the polymer (the copolymer)-coated silver nanowires before the coating substance substitution had a value of 15.1%, and the thiol (3-mercapto-1,2-propanediol)-coated silver nanowires after the coating substance substitution had a value of 3.7%. Also in silver nanowires coated with an organic protective agent (surfactant) having good dispersibility in a polar solvent such as water, silver nanowires coated not with a polymer having strong multiple-point adsorptivity but with a thiol which is considered to be adsorbed at one point were obtained.

Example 5

A polar solvent (aqueous) dispersion of silver nanowires was obtained in the same manner as in Example 4 except that in the substitution operation of silver nanowire coating substance into a thiol, monoethanolamine thioglycolate was used as the thiol for substitution on the wire surfaces. As the silver nanowires subjected to the substitution operation, the same nanowires as in Example 4 (the copolymer-coated silver nanowires described above) were used. According to a TG curve obtained under the same conditions as in Example 4, the ignition loss until 750° C. of the thiol (3-mercapto-1,2-propanediol)-coated silver nanowires after the coating substance substitution obtained in this example was 2.9%.

Example 6

A polar solvent (aqueous) dispersion of silver nanowires was obtained in the same manner as in Example 4 except that in the substitution operation of silver nanowire coating substance into a thiol, ammonium thioglycolate was used as the thiol for substitution on the wire surfaces. As the silver nanowires subjected to the substitution operation, the same nanowires as in Example 4 (the copolymer-coated silver nanowires described above) were used. According to a TG curve obtained under the same conditions as in Example 4, the ignition loss until 750° C. of the thiol (ammonium thioglycolate)-coated silver nanowires after the coating substance substitution obtained in this example was 3.5%.

Example 7

A polar solvent (aqueous) dispersion of silver nanowires were obtained in the same manner as in Example 4 except that in the substitution operation of silver nanowire coating substance into a thiol, thiomalic acid was used as the thiol for the substitution on wire surfaces. As the silver nanowires subjected to the substitution operation, the same nanowires as in Example 4 (the copolymer-coated silver nanowires described above) were used. According to a TG curve obtained under the same conditions as in Example 4, the ignition loss until 750° C. of the thiol (thiomalic acid)-coated silver nanowires after the coating substance substitution 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 provided.

At a normal temperature, 83.87 g of the copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate, 0.48 g of lithium chloride, 0.13 g of potassium bromide, 0.48 g of lithium hydroxide, 3.33 g of a 1,2-propanediol solution having an aluminum nitrate nonahydrate content of 20% by mass were added to and dissolved in 7800 g of propylene glycol (1,2-propanediol) to prepare a solution X. In a container different from the above, 67.96 g of silver nitrate was added to and dissolved in 320 g of 1,2-propanediol to prepare a solution Y.

The whole volume of the solution X was heated from normal temperature to 115° C., and then stirred at 175 rpm for 20 minutes. After 20 minutes of the stirring, into the solution X at 115° C., the solution Y was added over 30 seconds with a tube pump. The solution was maintained at 115° C. for 12 hours while further keeping the stirring state, thereby obtaining a reaction liquid in which precipitation reaction of silver was completed. Then, washing was performed in the same manner as in Example 1, whereby an aqueous dispersion of silver nanowires coated with the copolymer was obtained.

Attempt of Substitution of Silver Nanowire Coating Substance

Malonic acid was provided as an organic substance for attempting substitution on wire surfaces.

The polymer-coated silver nanowires synthesized by the above method were adjusted in concentration so as to give a silver concentration of 0.2% by mass. At normal temperature, 10 mmol of malonic acid was added to 20 mL of the silver nanowire dispersion, and the mixture was stirred at 40° C. for 8 hours. The liquid after stirring was transferred into a cent fugal tube, 150 mL of pure water was added thereto, and the mixture was centrifuged under conditions of a centrifugal force of 1860 g and 15 minutes. After the centrifugation, the supernatant was removed to collect a solid. To the solid collected, 150 mL of pure water was added, and the mixture was centrifuged under conditions of a centrifugal force of 1860 G and 15 minutes. This operation was repeated twice, and the obtained final solid was dispersed in pure water, whereby a polar solvent (aqueous) dispersion of silver nanowires was obtained.

In each of the metal nanowires before the coating substance substitution attempt and those after the substitution attempt, the liquid component was volatilized in a drying oven to obtain a dry sample. Using a TG-DTA apparatus (manufactured by SII, TG/DTA6300), 10 mg of the dry sample was heated from a normal temperature to 700° C. at a temperature rising rate of 10° C./min, and the weight of the sample at each temperature was measured every 1 second, thereby obtaining a TG curve. According to the result, the ignition loss until 700° C. of a polymer (the copolymer)-coated silver nanowires before the coating substance substitution attempt used in this example was 8.1%, and the ignition loss until 700° C. of the silver nanowires after the coating substance substitution attempt was 12.3%. Since the ignition loss was increased relative to the case of the original polymer, it is considered that, in spite of the stirring for a prolonged time of 8 hours, substitution reaction from the polymer to a carboxylic acid did not proceed. In other words, it is considered that a carboxylic acid having no thiol group does not have adsorptivity to silver that is strong enough to desorb a polymer adhered to metal silver surfaces of wires.

Comparative Example 2

An experiment was performed in the same manner as in Comparative Example 1 except that in the substitution attempt operation of silver nanowire coating substance, malic acid was used as an organic substance for attempting substitution on the wire surfaces. As the silver nanowires subjected to the substitution attempt operation, the same nanowires as in Comparative Example 1 (the copolymer-coated silver nanowires described above) were used. According to a TG curve obtained in the same conditions as in Comparative Example 1, the ignition loss until 700° C. of the silver nanowires after the coating substance substitution attempt was 9.7%. Also in this example, the ignition loss was increased relative to the state with the original polymer, and it is considered that the substitution 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 in the substitution attempt operation of silver nanowire coating substance, ethylene diamine was used as an organic substance for attempting substitution on the wire surfaces. As the silver nanowires subjected to the substitution attempt operation, the same nanowires as in Comparative Example 1 (the copolymer-coated silver nanowires described above) were used. According to a TG curve obtained in the same conditions as in Comparative Example 1, the ignition loss until 700° C. of the silver nanowires after the coating substance substitution attempt was 13.0%. Also in this example, the ignition loss was increased relative to the state with the original polymer, and it is considered that the substitution reaction from the polymer to the amine did not proceed. In other words, it is considered that an amine having no thiol group does not have adsorptivity to silver that is strong enough to desorb a polymer adhered to metal silver surfaces of wires.

Comparative Example 4

An experiment was performed in the same manner as in Comparative Example 1 except that in the substitution attempt operation of silver nanowire coating substance, propane diamine was used as an organic substance for attempting substitution on the wire surfaces. As the silver nanowires subjected to the substitution attempt operation, the same nanowires as in Comparative Example 1 (the copolymer-coated silver nanowires described above) were used. According to a TG curve obtained under the same conditions as in Comparative Example 1, the ignition loss until 700° C. of the silver nanowires after the coating substance substitution attempt was 11.8%. Also in this example, the ignition loss was increased relative to the state with the original polymer, and it is considered that the substitution reaction from the polymer to the amine did not proceed.

Claims

1. 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 adhered on surfaces of metal silver.

2. The silver nanowires according to claim 1, wherein the thiol has only one thiol group in a molecule.

3. The silver nanowires according to claim 1, wherein the thiol is one or more selected from the group consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.

4. The silver nanowires according to claim 1, wherein the thiol is one or more selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.

5. A method for producing silver nanowires comprising mixing a liquid A in which silver nanowires coated with a polymer are dispersed and a liquid B in which a thiol having a molecular weight of 75 to 300 is dissolved to thereby substitute an adhering substance on metal silver surfaces from the polymer to the thiol.

6. 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.

7. The method for producing silver nanowires according to claim 5, wherein the thiol has only one thiol group in a molecule.

8. The method for producing silver nanowires according to claim 5, wherein the thiol is one or more selected from the group consisting of 1-dodecanethiol, l-decanethiol, and 1-octanethiol.

9. The method for producing silver nanowires according to claim 5, wherein the thiol is one or more selected from the group consisting of 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.

10. The method for producing silver nanowires according to claim 5, wherein the polymer is polyvinylpyrrolidone (PVP).

11. The method for producing silver nanowires according to claim 5, wherein the polymer is a copolymer having a vinylpyrrolidone constituting unit.

12. 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.

13. A silver nanowire dispersion, wherein silver nanowires as set forth in claim 1 are dispersed in a liquid medium.