TECHNICAL FIELD

The present invention relates to a method for manufacturing silver nanowires which includes causing a silver salt to react in a polyol in the presence of a growth control agent, a halide salt, and an α-hydroxycarbonyl compound having a specific moiety structure.

BACKGROUND ART

In recent years, there has been an increasing use of display devices such as a liquid crystal display, a plasma display, an organic electroluminescence display, and an electronic paper, input sensors such as a touch panel, and solar cells, which takes advantage of sunlight, such as a thin film-type amorphous Si solar cell and a dye-sensitized solar cell. Accordingly, there is an increasing demand for a transparent conductive film which is a member essential to these devices.

As this transparent conductive film, a thin film containing as a main component an ITO (an oxide of In and Sn), which is a transparent oxide, is mainly used. The thin film containing an ITO is generally prepared by a vapor phase method using a sputter apparatus or a vapor deposition apparatus in order to obtain high transparency and high conductivity. However, this preparation method requires a large-scale and complicated apparatus and consumes a large amount of energy. Therefore, there is a need for the development of a technology which can reduce a manufacturing cost and an environmental load. Also, since there is a trend toward increasing the area of a transparent conductive material, the transparent conductive material is increasingly required to be light, flexible, and low in resistance.

Meanwhile, there is research on a transparent conductive film containing nanowires of a metal element, which can be manufactured by a wet method and is high in lightweight properties and flexibility. The nanowires of a metal element, which have a diameter as small as 250 nm or less, have high light transmittance in the visible light region and are expected to be applied as a transparent conductive film to replace an ITO. Especially, a transparent conductive film including silver nanowires has high conductivity and stability and therefore attracts attention.

As a method for manufacturing such silver nanowires, a method of causing a silver compound, which serves as a source of silver nanowires, to react together with a halide ion and a growth control agent with, as a solvent, a polyol such as ethylene glycol to act as a reducing agent is proposed (for example, see Patent Literature 1).

However, the method for manufacturing silver nanowires with a polyol had a problem of reproducibility. That is, since the manufacturing lot of a polyol varies, nanowires having a certain shape (major axis length and diameter) cannot be stably manufactured even when the manufacture is attempted under identical reaction conditions. Therefore, an improvement such as performing a nitrogen purge is made in Patent Literature 2.

Also, the transparent conductive film containing silver nanowires has higher transparency as the diameter of the silver nanowires is smaller. Therefore, finer silver nanowires are preferable for the manufacture of the transparent conductive film. However, silver nanowires having an average diameter of around 75 nm can only be obtained by the method of Patent Literature 2. Accordingly, a method for manufacturing silver nanowires having a small wire diameter is currently under intensive research.

For example, Patent Literature 3 discloses a technology for stably manufacturing thin (average diameter 50 nm or less) and long (average length 10 μm or more) silver nanowires by further using an alkali metal hydroxide and an aluminum salt at a specific ratio in Patent Literature 1. In this method, silver nanowires having a certain shape can be manufactured, but a problem is that time is taken for the manufacture.

Also, Patent Literature 4 presents steps of synthesis and purification for manufacturing silver nanowires having a clearly defined size distribution and discloses that in a coating liquid containing silver nanowires obtained by a specific manufacturing method, more than 80% of silver nanostructures having an aspect ratio of at least 3 has a diameter of less than 25 nm. According to this method, silver nanowires having a certain shape and quality can be manufactured at a high yield, but a problem was that a silver salt is added twice resulting in a complicated synthesis procedure, and time is taken for the manufacture resulting in reduced productivity.

CITATION LIST

Patent Literatures

Patent Literature 1: U.S. Published Patent Application No. 2008/0210052Patent Literature 2: JP-T-2013-503260Patent Literature 3: JP-A-2015-180772Patent Literature 4: JP-T-2017-515983

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In view of the above-described problems in the known technologies in the art, an objective of the present invention is to provide a method for efficiently and stably manufacturing silver nanowires having a certain shape (major axis length and diameter) by a simple method.

Solutions to the Problems

The present inventor intensively conducted research for solving the above-described problems. As a result, it was found that the above-described problems can be solved by a method for manufacturing silver nanowires which includes causing a growth control agent, a halide salt, and a silver salt to react in a polyol in the presence of an α-hydroxycarbonyl compound having a specific moiety structure. Thus, the present invention has been accomplished.

That is, the present invention is as follows.

(1) A method for manufacturing silver nanowires including using a growth control agent and a halide salt in a polyol to obtain silver nanowires from a silver salt, and further using an α-hydroxycarbonyl compound (a) represented by formula (1) below.



(in general formula (1), R indicates any of a hydrogen atom and a hydrocarbon group having 1 to 6 carbon atoms).

(2) The method for manufacturing silver nanowires according to the above-described (1), in which the α-hydroxycarbonyl compound (a) is hydroxyacetone or α-hydroxyacetophenone.

(3) The method for manufacturing silver nanowires according to the above-described (1) or (2), in which the silver salt is silver nitrate.

(4) The method for manufacturing silver nanowires according to any one of the above-described (1) to (3), in which a molar ratio of the α-hydroxycarbonyl compound (a) to silver atoms in the silver salt is 0.20 to 3.0.

(5) The method for manufacturing silver nanowires according to any one of the above-described (1) to (4), including a step of mixing a first solution containing the α-hydroxycarbonyl compound (a) and a second solution containing the silver salt at a reaction temperature over 10 minutes or more.

(6) The method for manufacturing silver nanowires according to any one of the above-described (1) to (5), in which a concentration of the growth control agent to a reaction liquid is 0.40% by mass or more.

(7) The method for manufacturing silver nanowires according to any one of the above-described (1) to (6), in which a molar ratio of halogen atoms in the halide salt to silver atoms in the silver salt is 0.010 to 0.30.

Effects of the Invention

According to the method for manufacturing silver nanowires of the present invention, silver nanowires having a certain shape can be efficiency obtained for a shorter time than the known technologies, even when the manufacturing lot of a polyol varies. Furthermore, the reaction can be performed with higher energy efficiency at a lower temperature than when the α-hydroxycarbonyl compound defined by the present invention is not used.

