Method for producing metal nanoparticles

Abstract

To provide a method for producing an aqueous dispersion of metal nanoparticles having excellent dispersibility and low in an electric conductivity.SOLUTION: To provide a method for producing metal nanoparticles having: a step (i) in which a metal nanoparticle aqueous dispersion comprising a compound having a hydrophilic group as a protective agent for metal nanoparticles, a non-aqueous solvent, a hydrophobing agent, and a salting-out agent are mixed to prepare a mixed liquid; a step (ii) in which the mixed liquid prepared in the step (i) is separated into a first water phase and a metal nanoparticle-comprising non-aqueous solvent phase; a step (iii) in which the non-aqueous solvent phase separated in the step (ii) is taken out, and the non-aqueous solvent phase and a second water phase having an electric conductivity of 200μS / cm or less so as to move the metal nanoparticles to the second water phase; and a step (iv) in which the metal nanoparticle-comprising second water phase moved in the step (iii) is taken out.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for producing metal nanoparticles, particularly aqueous dispersions of metal nanoparticles. [Background technology]

[0002] In recent years, metal nanoparticles, which may have properties different from those of bulk materials, have been used in a variety of applications, such as catalysts and electronic component materials.

[0003] On the other hand, metal nanoparticles are usually obtained as a dispersion, but this dispersion often contains by-products produced during production, and various methods have been devised to remove impurities such as by-products from the dispersion.

[0004] For example, Patent Document 1 describes a method for extracting metal microparticles from an aqueous metal microparticle dispersion containing metal microparticles and a compound (referred to as an aqueous dispersant) that has affinity for water into an organic solvent (referred to as a non-aqueous solvent) that phase-separates from water, the method comprising mixing a non-aqueous solvent, the aqueous metal microparticle dispersion, a release liquid that separates the aqueous dispersant from the metal microparticles, and a compound (referred to as a non-aqueous dispersant) that has affinity for the metal microparticles and the non-aqueous solvent to transfer the metal microparticles to the non-aqueous solvent, and separating the non-aqueous solvent layer in which the metal microparticles are dispersed from the aqueous layer.

[0005] Patent Document 2 describes a method for producing copper nanoparticles, which includes the steps of: preparing a dispersion of copper nanoparticles having an average particle size of 200 nm or less by reducing copper ions in an aqueous solution to which a reducing agent and a dispersant have been added; centrifuging the dispersion of copper nanoparticles after the preparation step to separate it into a liquid phase and a solid phase containing the copper nanoparticles; adding pure water to the solid phase containing the copper nanoparticles after the centrifugation step; and washing the solid phase containing the copper nanoparticles after the pure water addition step using a high-speed rotary homogenizer equipped with a turbine and a stator.

[0006] Patent document 3 describes a method for preparing a non-aqueous dispersion of metal microparticles and / or metal compound microparticles, which is characterized by: (1) contacting an aqueous dispersion of metal microparticles and / or metal compound microparticles with a non-aqueous liquid that undergoes phase separation from water in the presence of a surfactant, then adding a water-soluble inorganic acid salt and / or a water-soluble organic acid salt that has substantially no surfactant activity, thereby transferring the microparticles from the aqueous dispersion into the non-aqueous liquid, forming a two-phase mixture consisting of a non-aqueous dispersion phase in which the microparticles are dispersed and an aqueous phase that is substantially free of the microparticles, and then (2) separating the non-aqueous dispersion from the two-phase mixture. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-270957 [Patent Document 2] Japanese Patent Publication No. 2020-029611 [Patent Document 3] Japanese Patent Application Publication No. 5-271718 Summary of the Invention [Problem to be solved by the invention]

[0008] However, conventional techniques including Patent Documents 1 to 3 have had problems such as the resulting metal nanoparticles agglomerating, or the resulting metal nanoparticle dispersion becoming a non-aqueous dispersion that cannot be used in aqueous solvents such as inks.

[0009] Therefore, an object of the present invention is to provide a method for producing an aqueous dispersion of metal nanoparticles that has excellent dispersibility and low electrical conductivity. [Means for solving the problem]

[0010] The present inventors have investigated various means for solving the above-mentioned problems, and as a result have found that, in a method for producing metal nanoparticles, an aqueous dispersion of metal nanoparticles containing a compound having a hydrophilic group as a protective agent for the metal nanoparticles is mixed with a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent, and the mixture is allowed to stand to separate into a first aqueous phase and a non-aqueous solvent phase (oil phase) containing the metal nanoparticles, and the non-aqueous solvent phase is then removed and brought into contact with a second aqueous phase having an electrical conductivity of 200 μS / cm or less, thereby making it possible to redisperse (re-extract) the metal nanoparticles into the second aqueous phase, thereby completing the present invention.

[0011] That is, the gist of the present invention is as follows. (1) (i) preparing a mixed solution by mixing an aqueous dispersion of metal nanoparticles containing a compound having a hydrophilic group as a protective agent for the metal nanoparticles, a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent; (ii) separating the mixture prepared in step (i) into a first aqueous phase and a non-aqueous solvent phase containing metal nanoparticles; (iii) removing the non-aqueous solvent phase separated in step (ii), contacting the non-aqueous solvent phase with a second aqueous phase having an electrical conductivity of 200 μS / cm or less, and transferring the metal nanoparticles to the second aqueous phase; (iv) removing the second aqueous phase containing the metal nanoparticles transferred in step (iii); A method for producing metal nanoparticles, comprising: [Effects of the Invention]

[0012] The present invention provides a method for producing an aqueous dispersion of metal nanoparticles that has excellent dispersibility and low electrical conductivity. [Brief explanation of the drawings]

[0013] [Figure 1] Photographs showing the states (2), (5), and (7) in Example 3. [Figure 2] 1 is a graph showing the electrical conductivity of the second aqueous phase in which the silver nanoparticles of Examples 1 to 3 were redispersed. [Figure 3]FIG. 2 is a diagram schematically illustrating the transfer of silver nanoparticles from a first aqueous phase to a non-aqueous solvent phase in an example. [Figure 4] FIG. 2 is a diagram schematically illustrating the redispersion of silver nanoparticles from a non-aqueous solvent phase into a second aqueous phase in an example. DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferred embodiments of the present invention will now be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The method for producing metal nanoparticles of the present invention is not limited to the following embodiments, and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art, without departing from the spirit of the present invention.

[0015] The present invention relates to a method for producing metal nanoparticles, comprising the steps of: (i) preparing a mixed solution by mixing an aqueous dispersion of metal nanoparticles containing a compound having a hydrophilic group as a protective agent for the metal nanoparticles with a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent; (ii) separating the mixed solution prepared in step (i) into a first aqueous phase and a non-aqueous solvent phase containing the metal nanoparticles; (iii) removing the non-aqueous solvent phase separated in step (ii) and contacting it with a second aqueous phase having an electrical conductivity equal to or lower than a certain value to transfer the metal nanoparticles to the second aqueous phase; and (iv) removing the second aqueous phase containing the metal nanoparticles transferred in step (iii).

[0016] Each of steps (i) to (iv) will be explained below.

[0017] (i) A step of preparing a mixed solution by mixing an aqueous dispersion of metal nanoparticles containing a compound having a hydrophilic group as a protective agent for the metal nanoparticles, a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent. In step (i) of the present invention, an aqueous dispersion of metal nanoparticles containing a compound having a hydrophilic group as a protective agent for the metal nanoparticles, a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent are mixed, for example, by stirring and / or shaking, to prepare a mixed solution.

[0018] Here, the metal nanoparticle aqueous dispersion is a dispersion containing metal nanoparticles synthesized in an aqueous solvent. It contains metal nanoparticles (e.g., silver nanoparticles, copper nanoparticles, etc.), a protective agent for dispersing the metal nanoparticles in the aqueous solvent (e.g., a compound containing nitrogen or sulfur atoms and having strong adsorption power and hydrophilicity to the metal, such as polyvinylpyrrolidone (PVP)), a counter anion corresponding to the metal ion of the salt used as the raw material for the metal nanoparticles (e.g., nitrate ions when nitrate salts are used), an aqueous solvent (e.g., water, a mixture of water and alcohol (e.g., methanol, ethanol, propanol, etc.)), and a reducing agent (e.g., citric acid, oxalic acid, ascorbic acid, etc.). Examples of metal nanoparticle aqueous dispersions include the dispersions described in JP 2018-135566 A.

[0019] The content of metal nanoparticles in the metal nanoparticle aqueous dispersion is not limited, but from the viewpoint of stable dispersibility, it is usually 0.001 mmol / L to 100 mmol / L, and preferably 0.001 mmol / L to 10 mmol / L, relative to the total volume of the dispersion.

[0020] The non-aqueous solvent is a solvent that undergoes phase separation from the aqueous solvent contained in the aqueous dispersion of metal nanoparticles and is used to extract metal nanoparticles from the aqueous dispersion of metal nanoparticles. Examples of non-aqueous solvents include cyclohexane, n-octane, and 1-octanol.

[0021] The amount of non-aqueous solvent added is not limited, but from the viewpoint of separability from the aqueous solvent contained in the metal nanoparticle aqueous dispersion, it is usually 1 to 3,000 parts by weight, and preferably 10 to 300 parts by weight, per 100 parts by weight of the metal nanoparticle aqueous dispersion.

[0022] A hydrophobizing agent is a compound that hydrophobicizes metal nanoparticles present in an aqueous dispersion of metal nanoparticles and is used to transfer the metal nanoparticles into a non-aqueous solvent. Examples of hydrophobizing agents include, but are not limited to, compounds that have one or more functional groups selected from the group consisting of carbonyl groups, thiol groups, and amine groups as adsorbing sites to metal nanoparticles, and also have a long-chain hydrocarbon group that allows them to exist stably in the non-aqueous solvent (oil phase), such as 1-dodecanethiol and sodium oleate.

[0023] The amount of hydrophobizing agent added is not limited, but from the viewpoint of the efficiency of hydrophobizing the metal nanoparticles present in the metal nanoparticle aqueous dispersion, it is usually 0.01 to 10 parts by weight, and preferably 0.1 to 1 part by weight, per 100 parts by weight of the metal nanoparticle aqueous dispersion.

[0024] The salting-out agent is a compound that salts out a part of the protective agent used to disperse metal nanoparticles in an aqueous solvent, and is used to separate a part of the protective agent adsorbed on the metal nanoparticles from the metal nanoparticles. Examples of salting-out agents include, but are not limited to, compounds that ionize to produce citrate ions, tartrate ions, sulfate ions (SO4 2- ), acetate ion (CH3COO - ), chloride ions (Cl - ), bromide ion (Br - ), nitrate ions (NO3 - ), chlorate ion (ClO3 - ), iodide ion (I - ), or thiocyanate ion (SCN - ) such as sodium chloride and sodium citrate. The salting-out agent is preferably a compound that, upon ionization, generates an ion in the Hofmeister series that has a stronger salting-out effect than acetate ion, and therefore, the salting-out agent is preferably a compound that, upon ionization, generates citrate ions, tartrate ions, or sulfate ions, such as sodium citrate.

[0025] By using as a salting-out agent a compound that generates ions in the Hofmeister series that have a stronger salting-out effect than acetate ions, salting-out can be caused by adding a small amount of the compound. Furthermore, the decrease in the critical micelle concentration of the hydrophobizing agent caused by the salting-out agent is minimized, and the formation of reverse micelles of the hydrophobizing agent in the non-aqueous solvent, i.e., micelles formed by association with lipophilic groups facing outward and hydrophilic groups facing inward, is suppressed. This prevents salts such as the salting-out agent from dissolving in the micelles, prevents the salting-out agent from dissolving in the non-aqueous solvent, and ultimately reduces the electrical conductivity of the second aqueous phase after re-dispersion of the metal nanoparticles in the second aqueous phase.

[0026] The amount of salting-out agent added is not limited, but from the viewpoint of efficient salting-out of the protective agent for dispersing metal nanoparticles in an aqueous solvent, it is usually 0.5 to 10 parts by weight, preferably 0.5 to 5 parts by weight, and more preferably 0.5 to 2 parts by weight per 100 parts by weight of the aqueous dispersion of metal nanoparticles.

[0027] In step (i) of the present invention, the order of addition of the materials, the addition temperature, the mixing method, the mixing time, etc. are not limited, and the materials are mixed so as to be uniformly mixed. For example, in step (i) of the present invention, a non-aqueous solvent is first added to a container containing an aqueous dispersion of metal nanoparticles, typically at 15°C to 30°C, and then a hydrophobizing agent and a salting-out agent are added in that order, and these materials are mixed by stirring typically for 1 minute to 30 minutes to prepare a mixed solution.

[0028] In step (i) of the present invention, by mixing the metal nanoparticle aqueous dispersion, a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent, the metal nanoparticles in the metal nanoparticle aqueous dispersion are hydrophobized by the adsorption of the hydrophobizing agent and extracted into the non-aqueous solvent, and some of the protective agent adsorbed to the metal nanoparticles is aggregated and precipitated by the salting-out agent, and water-soluble compounds, particularly water-soluble compounds other than the metal nanoparticles in the metal nanoparticle aqueous dispersion, remain as they are in the first aqueous phase, which is the medium of the metal nanoparticle aqueous dispersion, and as a result, the metal nanoparticles can be extracted into the non-aqueous solvent.

[0029] (ii) separating the mixture prepared in step (i) into a first aqueous phase and a non-aqueous solvent phase containing metal nanoparticles; In step (ii) of the present invention, the mixed solution prepared in step (i) is separated into a first aqueous phase and a non-aqueous solvent phase containing metal nanoparticles, for example, by leaving it to stand, typically at 15°C to 30°C, typically for 5 minutes to 24 hours.

[0030] In step (ii) of the present invention, the mixture is separated into a first aqueous phase and a non-aqueous solvent phase, resulting in separation of the first aqueous phase in which water-soluble compounds, particularly water-soluble compounds other than the metal nanoparticles in the metal nanoparticle aqueous dispersion, remain dissolved, from the non-aqueous solvent phase into which the metal nanoparticles have been extracted. Note that a portion of the protective agent adsorbed on the salted-out metal nanoparticles precipitates as a solid phase.

[0031] (iii) A step of removing the non-aqueous solvent phase separated in step (ii), contacting the non-aqueous solvent phase with a second aqueous phase having an electrical conductivity equal to or lower than a certain value, and transferring the metal nanoparticles to the second aqueous phase. In step (iii) of the present invention, the non-aqueous solvent phase separated in step (ii) is removed, and the non-aqueous solvent phase is contacted with a second aqueous phase having an electrical conductivity of a certain value or less, for example, typically at 15°C to 30°C, typically for 5 minutes to 24 hours, to transfer the metal nanoparticles to the second aqueous phase.

[0032] Here, the second aqueous phase to be brought into contact with the removed non-aqueous solvent phase is water that undergoes phase separation from the non-aqueous solvent phase and has an electrical conductivity of 200 μS / cm or less, preferably 100 μS / cm or less.

[0033] The electrical conductivity can be measured in accordance with JIS K 0130 "General rules for measuring electrical conductivity."

[0034] The term "contact" refers to contacting the removed non-aqueous solvent phase with the second aqueous phase. Methods for contacting the non-aqueous solvent phase with the second aqueous phase include slowly adding the second aqueous phase to the non-aqueous solvent phase, slowly adding the non-aqueous solvent phase to the second aqueous phase, or mixing the non-aqueous solvent phase with the second aqueous phase, for example by stirring and / or shaking.

[0035] The amount of the second aqueous phase added is not limited, but is usually 10 to 1000 parts by weight, preferably 30 to 300 parts by weight, and more preferably 30 to 200 parts by weight, per 100 parts by weight of the non-aqueous solvent phase.

[0036] By contacting the non-aqueous solvent phase with the second aqueous phase, the metal nanoparticles contained in the non-aqueous solvent phase are transferred from the non-aqueous solvent phase to the second aqueous phase through the interface, i.e., are redispersed and reextracted. In step (iii), the protective agent that was not salted out in step (i) and remained on or attached to the metal nanoparticles contributes to steric stabilization of the redispersed metal nanoparticles in the second aqueous phase.

[0037] (iv) Removing the second aqueous phase containing the metal nanoparticles transferred in step (iii). In step (iv) of the present invention, the second aqueous phase containing the metal nanoparticles transferred in step (iii) is removed.

[0038] The second aqueous phase extracted in step (iv) of the present invention contains almost no components other than the metal nanoparticles, and therefore an aqueous dispersion containing metal nanoparticles with low electrical conductivity can be obtained.

[0039] The metal nanoparticles (aqueous dispersion of metal nanoparticles) produced by the method for producing metal nanoparticles of the present invention have low electrical conductivity equivalent to that of the aqueous dispersion of metal nanoparticles obtained after washing by filtration. Therefore, the present invention makes it possible to easily purify the aqueous dispersion of metal nanoparticles.

[0040] The metal nanoparticles (metal nanoparticle aqueous dispersion) produced by the method for producing metal nanoparticles of the present invention have high purity and can be used as catalysts, electronic component materials, and paints. [Example]

[0041] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0042] 1. Purification of silver nanoparticle aqueous dispersion (1) A silver nanoparticle dispersion prepared by the method described in JP 2018-135566 A was prepared as a metal nanoparticle aqueous dispersion. The silver nanoparticles in the silver nanoparticle dispersion were plate-shaped silver nanoparticles with an average particle size of 40 nm and coated with polyvinylpyrrolidone (PVP, weight-average molecular weight: 10,000 g / mol) as a protective agent. The amounts of various chemicals were adjusted so that the reaction solution contained 3.3 mM silver nitrate (AgNO3), 8 mM PVP, and 12 mM sodium citrate. However, in Comparative Example 3, PVP was not used.

[0043] (2) A non-aqueous solvent phase (cyclohexane) was added to a container containing a silver nanoparticle dispersion in the amounts shown in Table 1 below.

[0044] (3) In the container (2), add sodium oleate as a hydrophobizing agent. [ka] It should be noted that the same results were obtained even when sodium oleate was added to the silver nanoparticle dispersion in advance.

[0045] (4) To eliminate the steric stabilizing effect of the protecting agent, sodium chloride, sodium sulfate, or sodium citrate was added as a salting-out agent to the container of (3).

[0046] (5) The mixture in the container (4) was stirred and allowed to stand for 1 hour, resulting in the migration of the silver nanoparticles from the first aqueous phase to the non-aqueous solvent phase. At this time, the silver nanoparticles were sterically stabilized in the non-aqueous solvent phase by the hydrophobic portion of the hydrophobizing agent. However, in Comparative Example 3, the migration of the silver nanoparticles from the first aqueous phase to the non-aqueous solvent phase did not occur.

[0047] (6) From the vessel (5), only the non-aqueous solvent phase was removed into a separatory funnel and contacted with a second aqueous phase having the electrical conductivity shown in Table 1 below.

[0048] (7) The non-aqueous solvent phase and the second aqueous phase in the separatory funnel (6) were stirred and left to stand for 1 hour, and the movement (re-dispersion) of the silver nanoparticles was confirmed.

[0049] [Table 1]

[0050] Regarding the water with different electrical conductivities in the second aqueous phase brought into contact in Table 1, water with an electrical conductivity of 2 μS / cm was prepared by collecting it from a water purifier, water with an electrical conductivity of 20 μS / cm was prepared by leaving water collected from the water purifier in the air for three days to allow components in the air (such as carbon dioxide) to dissolve therein, and water with an electrical conductivity of 200 μS / cm or higher was prepared by dissolving ions (carbonate ions, bicarbonate ions, and nitrate ions).

[0051] FIG. 1 shows photographs of the states (2), (5) and (7) in Example 3.

[0052] 2.Results The purified aqueous dispersion of silver nanoparticles prepared in 1. Purification of aqueous dispersion of silver nanoparticles was evaluated for migration of the silver nanoparticles into the non-aqueous solvent phase / redispersibility in the second aqueous phase, the electrical conductivity of the second aqueous phase after redispersion, and the dispersion stability of the aqueous dispersion of silver nanoparticles after redispersion. The results are shown in Table 2.

[0053] [Table 2]

[0054] Here, the migration of silver nanoparticles into the non-aqueous solvent phase / redispersion into the second aqueous phase was visually inspected. When the solution from which the silver nanoparticles were migrated became transparent and the concentration gradient of the destination solution was uniform, it was determined that the silver nanoparticles had migrated, and this is indicated by a circle in the table.

[0055] Regarding dispersion stability, when visually confirmed, a state in which no precipitation occurred for 90 days or more or a state in which no concentration gradient was observed was judged to be good dispersion stability, and this is indicated by a circle in the table.

[0056] The electrical conductivity was measured using an electrical conductivity meter (apparatus: HORIBA electrical conductivity meter ES-51, measured at 25°C and atmospheric pressure). Figure 2 shows the electrical conductivities of the redispersed second aqueous phases in Examples 1 to 3.

[0057] Table 2 shows that when silver nanoparticles in an aqueous dispersion of silver nanoparticles do not contain a compound having a hydrophilic group as a protective agent, the silver nanoparticles do not migrate to the non-aqueous solvent phase. Furthermore, Table 2 and Figure 2 show that using sodium sulfate or sodium citrate as a salting-out agent can reduce the electrical conductivity of the second aqueous phase after redispersion into the second aqueous phase compared to when sodium chloride is used as the salting-out agent. It was also found that the electrical conductivity of the second aqueous phase that comes into contact with the non-aqueous solvent phase into which the silver nanoparticles have migrated must be 200 μS / cm or less in order for the silver nanoparticles to be redispersed into the second aqueous phase.

[0058] Although not shown in Tables 1 and 2, in Example 4, an experiment was conducted in which the non-aqueous solvent phase removed from the container (5) was air-dried and then replaced with an aqueous solvent. As a result, the resulting aqueous dispersion of silver nanoparticles had poor dispersion stability.

[0059] FIG. 3 shows a schematic diagram of the transfer of silver nanoparticles from the first aqueous phase to the non-aqueous solvent phase in an example, and FIG. 4 shows a schematic diagram of the redispersion of silver nanoparticles from the non-aqueous solvent phase to the second aqueous phase in an example.

[0060] In Figure 3, first, an aqueous dispersion of silver nanoparticles is mixed with cyclohexane, a non-aqueous solvent phase. The silver nanoparticles (silver nanoplates) in the aqueous dispersion of silver nanoparticles have a hydrophilic surface, a negative charge (ζ potential: -36 mV), and are sterically stabilized by PVP. PVP also affects the shape control (plate formation) of the silver nanoparticles during their formation.

[0061] Sodium oleate is added to the aqueous silver nanoparticle dispersion as a hydrophobizing agent. Sodium oleate may be added to the aqueous silver nanoparticle dispersion before mixing it with cyclohexane. The hydrophobizing agent is a compound that contains a functional group that ligates to silver, such as a carbonyl group, a thiol group, or an amine group, and a long-chain hydrocarbon for stabilization in the non-aqueous solvent phase (compatibility with the non-aqueous solvent phase is important), further hydrophobizing the silver nanoparticles while preventing severe aggregation in the first aqueous phase. When a cationic compound was used as the hydrophobizing agent, aggregation in the first aqueous phase was rapid, and migration of the silver nanoparticles did not occur.

[0062] By adding sodium oleate, sodium oleate coordinates with the surface of silver nanoparticles, and the negative charge of sodium oleate changes the zeta potential of the silver nanoparticles from -36 mV to -49 mV. Although the surface of the silver nanoparticles becomes hydrophobic, they are still dispersed in the first aqueous phase due to the steric stabilization effect of PVP.

[0063] Next, a salting-out agent (electrolyte) such as sodium chloride is added. Compounds with high salting-out ability (according to the Hofmeister series, salting-out ability is chlorine < sulfate < citrate) are advantageous as salting-out agents.

[0064] For example, by adding sodium chloride, a portion of the PVP is salted out, and the steric stabilization effect of the silver nanoparticles is lost. At this time, it is observed that the PVP begins to aggregate in the first aqueous phase. With the steric stabilization effect lost, the silver nanoparticles become hydrophobic as a whole and become unstable in the first aqueous phase, so they migrate to the non-aqueous solvent phase. In the non-aqueous solvent phase, the silver nanoparticles are sterically stabilized by the hydrophobic portion of oleic acid, allowing them to maintain good dispersibility.

[0065] After adding each material, the mixture is stirred and allowed to stand (separate). The silver nanoparticles move into the non-aqueous solvent phase, while unnecessary ionic substances (miscellaneous ions) remain in the first aqueous phase, and the non-aqueous solvent phase and the first aqueous phase separate. Stirring suspends the mixture, increasing the total interfacial area between the water and non-aqueous solvent, and promoting the migration of silver nanoparticles into the non-aqueous solvent phase.

[0066] Next, in Figure 4, only the non-aqueous solvent phase in which silver nanoparticles are dispersed is extracted from the mixture that has separated into two phases, and then the non-aqueous solvent phase is brought into contact with a newly prepared second aqueous phase with an electrical conductivity of 200 μS / cm or less.

[0067] By contacting the non-aqueous solvent phase with the second aqueous phase, the silver nanoparticles in the non-aqueous solvent phase are redispersed in the second aqueous phase.

Claims

1. (i) a step of preparing a mixed solution by mixing an aqueous dispersion of metal nanoparticles containing a compound having a hydrophilic group as a protective agent for the metal nanoparticles, a non-aqueous solvent, a hydrophobizing agent, and a salting-out agent, the hydrophobizing agent is a compound used to transfer the metal nanoparticles into a non-aqueous solvent and to hydrophobize the metal nanoparticles present in the metal nanoparticle aqueous dispersion, the hydrophobizing agent is a compound having one or more functional groups selected from the group consisting of a carbonyl group, a thiol group, and an amine group as a site that adsorbs to the metal nanoparticles, and also having a long-chain hydrocarbon group so that the hydrophobizing agent can exist stably in a non-aqueous solvent; the salting-out agent is a compound that salts out a part of a protective agent used to disperse metal nanoparticles in an aqueous solvent, and is used to separate a part of the protective agent adsorbed on the metal nanoparticles from the metal nanoparticles; the salting-out agent is a compound that, upon ionization, generates citrate ions, tartrate ions, sulfate ions, acetate ions, chloride ions, bromide ions, nitrate ions, chlorate ions, iodide ions, or thiocyanate ions; The process and (ii) separating the mixture prepared in step (i) into a first aqueous phase and a non-aqueous solvent phase containing metal nanoparticles; (iii) removing the non-aqueous solvent phase separated in step (ii), contacting the non-aqueous solvent phase with a second aqueous phase having an electrical conductivity of 200 μS / cm or less, and transferring the metal nanoparticles to the second aqueous phase; (iv) removing the second aqueous phase containing the metal nanoparticles transferred in step (iii).

2. The method according to claim 1, wherein the hydrophobizing agent is 1-dodecanethiol or sodium oleate, and the salting-out agent is sodium chloride, sodium sulfate, or sodium citrate.

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