JP7239297B2 - Method for producing silver nanowires - Google Patents

Description

The present disclosure relates to a method for producing silver nanowires.

Metal nanowires are extremely fine elongated materials with a diameter of usually 20 nm or less and an aspect ratio (that is, length/diameter ratio) of usually 100 or more. It is used for applications such as high-performance ink (ink for electronic circuits, etc.) and high thermal conductivity coolant for automobiles. Among metal nanowires, silver nanowires have the advantage of particularly high conductivity.

Patent Document 1 describes a method for producing silver nanowires, which is characterized by reducing silver ions in a solution containing a reducing compound and a morphology control agent, using silver nanoparticles that have undergone purification treatment as core particles. .

In Patent Document 2, metal nanowires flow along with the flow of a liquid medium in a tubular channel having a porous ceramic filter having an average pore diameter of 1.0 μm or more by the mercury intrusion method on the channel wall surface, and the flow Some of the metal nanowires are discharged out of the tubular channel through the porous ceramic filter together with a part of the liquid medium, and the metal nanowires that have flowed through the channel without being discharged out of the tubular channel. A method for producing metal nanowires is described, in which the is recovered.

Patent Document 3 describes a method for producing silver nanowires, which comprises heating a silver complex in a water solvent at a temperature below the boiling point of the water solvent.

Patent Document 4 discloses a method for producing silver nanowires by heat-treating a precursor containing silver chloride and a reducing agent consisting of a monosaccharide in an aqueous medium at a temperature of 130 to 200° C., wherein the chloride The precursor containing silver is a silver salt aqueous solution with a concentration of 0.01 to 0.10 mol/L and a chlorine-containing substance aqueous solution with a concentration of 0.10 to 0.70 mol/L. It is used so that the molar ratio is 1:1 to 1:7, and is obtained by newly adding an aqueous medium 3 to 10 times the volume of the total amount of the silver salt aqueous solution and the chlorine-containing substance aqueous solution. A method for producing silver nanowires is described, characterized in that

Patent Document 5 discloses 1 selected from poly(N-vinyl-ε-caprolactam), poly(N-vinyl-2-oxazolidone), poly(N-vinylacetamide) and poly(N-methyl-N-vinylacetamide) A method of making silver nanowires is described, comprising heating a silver compound in the presence of one or more polymers.

JP 2012-132082 A JP 2016-55283 A JP 2009-242880 A JP 2010-261090 A JP 2014-189888 A

In silver nanowire products, there are usually impurities such as granular or low aspect ratio silver crystals other than the desired silver nanowires. Since these impurities can cause deterioration of the conductive performance of the silver nanowire product, a method of obtaining a silver nanowire product with few impurities is desired.

For example, the methods described in Patent Documents 1 and 2 relate to providing high-purity metal nanowire products, but the method described in Patent Document 1 requires the use of core particles separately from the silver ion source. , the method described in Patent Document 2 requires an impurity removal step in order to obtain a metal crystal containing the desired metal nanowires with high purity, so any method has a problem that the process is complicated.

In addition, in the method described in Patent Document 2, undesired metal crystals (that is, granular or low aspect ratio metal crystals) are removed by the impurity removal step, so the yield of metal nanowires (that is, the amount of metal used as a raw material) There is also the problem that the ratio of the amount of metal nanowires to the amount) is low.

On the other hand, the methods described in Patent Documents 3 to 5, although the process is relatively simple, suppresses the formation of undesired silver crystals to achieve high purity (that is, the desired silver nanowires in the produced silver crystals). There is room for improvement in terms of obtaining a high percentage) product.

Accordingly, an object of the present disclosure is to provide a method for producing silver nanowires, which is a simple process and capable of producing silver nanowires with high yield and high purity.

Aspects of the present disclosure include the following aspects.

<Aspect 1>
Mixing at least an alkali metal bromide, a first polyvinylpyrrolidone, and a polyol to prepare liquid A;
Mixing at least a silver salt, a second polyvinylpyrrolidone, and a polyol to prepare a liquid B;
mixing liquid A and liquid B to prepare liquid C, and heating liquid C to produce silver nanowires;
including
The number average molecular weight of the second polyvinylpyrrolidone is 1,000 or more and 200,000 or less,
Liquid B contains a complex of silver ions derived from the silver salt and the second polyvinylpyrrolidone,
A method for producing silver nanowires.
<Aspect 2>
Aspect 1 above, wherein the number average molecular weight of the second polyvinylpyrrolidone is less than the number average molecular weight of the first polyvinylpyrrolidone.
<Aspect 3>
The method according to aspect 2 above, wherein the first polyvinylpyrrolidone has a number average molecular weight of 300,000 or more and 1,500,000 or less.
<Aspect 4>
The method according to any one of aspects 1 to 3 above, wherein the second polyvinylpyrrolidone has a number average molecular weight of 5,000 or more and 100,000 or less.
<Aspect 5>
the alkali metal bromide is sodium bromide;
the polyol is ethylene glycol,
the silver salt is silver nitrate;
The method according to any one of aspects 1 to 4 above.

According to the present disclosure, it is possible to provide a method for producing silver nanowires, which is a simple process and capable of producing silver nanowires with high yield and high purity.

1 shows scanning electron microscope (SEM) images of silver crystals obtained in Examples 1 and 2 and Comparative Example 1. FIG. FIG. 4 shows scanning electron microscope (SEM) images of silver crystals obtained in Comparative Examples 2 and 3; FIG. 4 shows scanning electron microscope (SEM) images of silver crystals obtained in Comparative Example 4 and Examples 3-5.

Although exemplary aspects of the present disclosure will be described below, the present disclosure is not limited to the following aspects.

One aspect of the present disclosure is
Mixing at least an alkali metal bromide, a first polyvinylpyrrolidone (also referred to as a first PVP in the present disclosure), and a polyol to prepare liquid A;
Mixing at least a silver salt, a second polyvinylpyrrolidone (also referred to as a second PVP in the present disclosure), and a polyol to prepare a liquid B;
mixing liquid A and liquid B to prepare liquid C, and heating liquid C to produce silver nanowires;
including
The number average molecular weight of the second polyvinylpyrrolidone is 1,000 or more and less than 200,000,
Provided is a method for producing silver nanowires, wherein liquid B contains a complex of silver ions derived from a silver salt and the second polyvinylpyrrolidone.

The generation mechanism of silver nanowires is considered as follows. That is, when silver ions are applied to the silver crystal nucleus in the presence of a reducing agent and a morphology control agent (this is a substance that selectively adsorbs to the [100] face of silver), the silver ions are reduced. When crystal growth occurs by depositing silver on the crystal nucleus, multiple twinned grains can be grown as anisotropic grains due to the contribution of the morphology control agent. This multiply twinned grain grows into a nanowire by further reductive deposition of silver ions. For the production of silver nanowires with high yield and high purity, it is advantageous to prevent excessive generation of crystal nuclei during the reduction reaction, and precomplexing silver is advantageous in preventing excessive generation of crystal nuclei. It is useful for suppression of generation.

The present inventors found that in a method of obtaining silver nanowires by forming a silver complex and then reductively depositing silver ions in the presence of a morphology-controlling agent, a polymer of the same type as the polymer used as the morphology-controlling agent (i.e., the first PVP) ( That is, by using the second PVP) as the ligand of the complex, the affinity between the ligand and the morphology control agent is improved, and the morphology control agent adheres to the [110] face of silver (more specifically, adsorption) is promoted. That is, the formation of a complex of silver with the second PVP not only has the advantage of preventing excessive formation of crystal nuclei due to complex formation, but also the desired nanowires due to better adhesion to the [110] faces of the first PVP. It also provides the advantage of more selective growth to shape.

Therefore, the method of the present disclosure is a simple process that does not require the use of separately produced crystal nuclei, the implementation of a step of separating impurities from the product, etc., while producing the desired silver nanowires with high yield and high efficiency. It has the advantage that it can be produced with purity.

Each step of the method according to one aspect is described below.

<Preparation of liquid A>
Liquid A is obtained by mixing at least an alkali metal bromide, the first PVP, and a polyol. In a typical embodiment, Part A does not contain silver.

In the method of the present disclosure, the alkali metal bromide functions as a shape modifier. The alkali metal bromide is preferably sodium bromide or potassium bromide, more preferably sodium bromide. When an alkali metal bromide and a silver salt coexist in solution C produced by mixing solution A and solution B, they react rapidly to form silver bromide. Silver bromide is sparingly soluble in solution C, but when solution C is heated, the reduction of silver ions progresses and the concentration of silver ions decreases, redissolving silver ions to supply silver ions into the system. Since the progress of redissolution is slower than the production of silver bromide, silver ions are gradually supplied to the crystal growth surface. Thus, the alkali metal bromide contributes to the control of the silver ion concentration in the C solution, and thus to the production of the desired silver nanowires with high purity.

Regarding the amount of alkali metal bromide in solution A, in solution C, the molar ratio of alkali metal bromide to silver (the ratio of the number of moles of alkali metal bromide/the number of moles of silver) is preferably 1×10 ⁻⁷ to 1, It is adjusted to 1×10 ⁻⁶ to 1×10 ⁻¹ , 1×10 ⁻⁴ to 1×10 ⁻² , or 1×10 ⁻³ to 1×10 ⁻² .

The first PVP functions as a morphology control agent. The number average molecular weight of the first PVP is preferably 100,000 or more, 200,000 or more, 300,000 or more, 350,000 or more, or 360,000 or more, preferably 2,000,000 or less from the viewpoint of avoiding excessive interaction between PVP molecules, which are polar molecules, and maintaining good dispersibility of the ingredients in solution C , 1,500,000 or less, or 1,300,000 or less.

In the present disclosure, the number average molecular weight is a value measured in terms of standard polystyrene using gel permeation chromatography.

The amount of the first PVP in solution A is adjusted so that the molar ratio of the first PVP to silver in solution C is within the desired range. In solution C, the molar ratio of the first PVP to silver (the ratio of the number of moles of the first PVP/the number of moles of silver) is preferably 0.5 or more from the viewpoint of growing the desired silver nanowires well. 1.0 or more, or 1.5 or more, preferably 100 or less, 50 or less, 30 or less from the viewpoint of avoiding excessive viscosity increase of C liquid and maintaining good dispersion of the ingredients in C liquid , 10 or less, 5.0 or less, or 3.0 or less.

Polyols function as both reducing agents and solvents. The polyol is preferably a diol from the viewpoint of its good function as a reducing agent and solvent. In one aspect, the diol is an alkylene glycol or a diol having 2 to 5 carbon atoms. The diol is particularly preferably ethylene glycol.

Liquid A can be prepared by mixing at least the above components, for example, at room temperature with stirring. It is not excluded that the liquid A contains optional components other than the above components within a range that does not impair the effects of the present invention.

<Preparation of liquid B>
Liquid B is obtained by mixing at least a silver salt, a second PVP and a polyol.

Examples of the silver salt include silver nitrate, silver carboxylate (silver acetate, silver oxalate, etc.), silver sulfate, silver carbonate, etc. Silver nitrate is preferred from the viewpoint of good progress of the reduction reaction. The silver salt concentration in liquid B is adjusted, for example, so that the silver salt concentration in liquid C is 1 mmol/L to 1000 mmol/L, or 10 mmol/L to 100 mmol/L.

The second PVP functions as a ligand to form a complex with silver ions. The number average molecular weight of the second PVP is preferably smaller than the number average molecular weight of the first PVP. In this case, the first PVP functions well as a morphology control agent due to its relatively high number-average molecular weight, while the second PVP has a relatively low number-average molecular weight, resulting in its ability to complex with silver ions. Excellent for

In one aspect, the number average molecular weight of the second PVP is 1,000 or more, preferably 3,000 or more, 5,000 or more, or 8,000 or more, from the viewpoint of stably existing the complex. , is less than 200,000, preferably 100,000 or less, 80,000 or less, 60,000 or less, or 55,000 or less from the viewpoint of the ability to form a complex with silver ions.

In liquid B, the molar ratio of the second PVP to silver (the ratio of the number of moles of the second PVP/the number of moles of silver) is preferably 0.001 or more from the viewpoint of easily forming a desired amount of complex. 0.005 or more, or 0.008 or more, preferably 0.1 or less, 0.05 or less, 0 0.03 or less, 0.02 or less, or 0.012 or less.

In addition, the molar ratio of N-vinyl-2-pyrrolidone units to silver in solution B (the ratio of the number of moles of N-vinyl-2-pyrrolidone units/the number of moles of silver) allows the desired amount of complex to be easily formed. From the viewpoint, it is preferably 0.1 or more, 0.5 or more, or 0.8, and from the viewpoint of maintaining good adhesion of the first PVP to the [100] surface in liquid C, it is preferably 10 or less. , 5.0 or less, 3.0 or less, 2.0 or less, or 1.2 or less.

As the polyol in liquid B, the same polyol as used in liquid A can be preferably used, and ethylene glycol is particularly preferred.

Liquid B can be prepared by mixing at least the above components at room temperature, for example, under stirring. It is not excluded that the B liquid contains optional components other than the above components within the range that does not impair the effects of the present invention.

<Preparation of solution C>
Next, the prepared A liquid and B liquid are mixed to prepare C liquid. In one embodiment, liquid C is prepared by mixing liquid A and liquid B with stirring. The volume ratio of liquid A: liquid B is preferably 1:1 to 30:1, or 5:1 to 20:1, or 10:1 to 15:1. In one aspect, the mixing temperature may be set between room temperature and the temperature at which silver nanowires are produced, which will be described later. In one aspect, the mixing temperature is set at the temperature at which silver nanowires are produced.

<Generation of silver nanowires>
Then, the liquid C is heated to produce silver nanowires. Heating may be performed, for example, by microwave heating at a heating temperature of preferably 150 to 250° C. or 180 to 200° C. for a heating time of preferably 1 to 10 minutes or 1 to 5 minutes. The resulting silver crystals are cooled and washed with deionized water, hexane, or the like to obtain the desired silver nanowires.

In a typical embodiment, the silver nanowires have a diameter of 20 nm or less and an aspect ratio (length/diameter ratio) of 100 or more. The aspect ratio (length/diameter ratio) of the silver nanowires is preferably 150 or more, 200 or more, 500 or more, or 1000 or more from the viewpoint of obtaining excellent conductive performance. 10000 or less, 5000 or less, or 3000 or less. In one aspect, the ratio of the number of silver crystals having a diameter of 20 nm or less and an aspect ratio of 100 or more to the total number of silver crystals after washing is, for example, 70% or more, 75% or more, 80% or more, without any special impurity removal treatment. % or higher, 85% or higher, or 90% or higher. The number and size of silver crystals can be measured on scanning electron microscope (SEM) images.

EXAMPLES Hereinafter, the present disclosure will be described more specifically with reference to Examples, but the present disclosure is not limited to these Examples.

<Preparation of A liquid, B liquid and C liquid>
Solution A and solution B were mixed so that silver ion molarity [Ag ⁺ ]=42 mM and bromide ion molarity [Br ⁻ ]/silver ion molarity [Ag ⁺ ]=4.2×10 ⁻³ in solution C. prepared.

In 14 mL of ethylene glycol, 2.65 mmol of sodium bromide, and the first PVP in an amount that gives the ratio of PVP molar concentration [PVP]/silver ion molar concentration [Ag ⁺ ] shown in Table 1 in solution C were stirred and mixed at room temperature. Then, liquid A was obtained.

In 1 mL of ethylene glycol, 630 mmol of silver nitrate and the second PVP in an amount that gives the ratio of PVP molar concentration [PVP]/silver ion molar concentration [Ag ⁺ ] shown in Table 1 in solution C are stirred and mixed at room temperature to obtain solution B. got

The above liquid A was placed in a microwave heating device, and the temperature was raised to a set temperature of 175° C. over 30 seconds and then held for 1 minute. Next, the above solution B was added at 175° C. to obtain solution C, and a reduction reaction was allowed to proceed in solution C. After completion of the reaction, silver crystals were obtained through cooling and washing by centrifugation (twice with ethanol and once with pure water).

<Scanning electron microscope (SEM) observation>
The morphology of silver crystals was observed with a scanning electron microscope (SEM) at an accelerating voltage of 15 kV to obtain morphological images shown in FIGS. The aspect ratio of the silver crystal was measured from the morphological image, and those with an aspect ratio of 100 or more were determined to be nanowires. The ratio of the number of silver crystals determined to be nanowires among the total number of silver crystals in the measurement field was defined as the nanowire ratio (%), and the value obtained by subtracting the nanowire ratio (%) from 100% was defined as the impurity ratio (%). Table 1 shows the results.

FIG. 1 is a diagram showing SEM images of silver crystals obtained in Comparative Example 1, Examples 1 and 2, and FIG. 2 is a diagram showing SEM images of silver crystals obtained in Comparative Examples 2 and 3, 3 is a diagram showing SEM images of silver crystals obtained in Comparative Example 4, Examples 3 and 4. FIG. As shown in FIGS. 1-3, each example had a large amount of nanowires and a small amount of impurities, whereas each comparative example had a small amount of wires and a large amount of impurities.

Claims (3)

Mixing at least an alkali metal bromide, a first polyvinylpyrrolidone, and a polyol to prepare liquid A;
Mixing at least a silver salt, a second polyvinylpyrrolidone, and a polyol to prepare a liquid B;
Mixing the A liquid and the B liquid to prepare a C liquid, and heating the C liquid to generate silver nanowires;
including
The first polyvinylpyrrolidone has a number average molecular weight of 300,000 or more and 1,500,000 or less,
The number average molecular weight of the second polyvinylpyrrolidone is smaller than the number average molecular weight of the first polyvinylpyrrolidone, and is 1,000 or more and less than 200,000,
The B liquid contains a complex of silver ions derived from the silver salt and the second polyvinylpyrrolidone,
A method for producing silver nanowires. 2. The method according to claim 1 , wherein the second polyvinylpyrrolidone has a number average molecular weight of 5,000 or more and 100,000 or less. the alkali metal bromide is sodium bromide,
the polyol is ethylene glycol,
wherein the silver salt is silver nitrate;
3. A method according to claim 1 or 2 .

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