Method for producing metal nanoparticles

Abstract

To provide a method for stably producing metal nanoparticles having reduced variation in particle size in relatively high volume by using a liquid phase method.SOLUTION: According to the technique disclosed herein, there is provided a method for producing metal nanoparticles by a liquid phase method. The method includes following steps: a step S1 of preparing a solution to be a reaction system containing: a raw material compound for forming metal nuclei of the metal nanoparticles; and a reducing agent; a step S2 of heating the solution to a predetermined temperature range to form the nuclei by reduction of the raw material compound in the temperature region; a step S3 of growing the nuclei until they become metal nanoparticles of a predetermined particle size by further heating the solution; and a step S4 of quenching the solution to a temperature range where the nuclei growth substantially stops, wherein the average quenching rate is set to be 2°C / min or higher.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing metal nanoparticles, and more particularly to a method for producing metal nanoparticles by a liquid phase method. [Background technology]

[0002] In recent years, research and development of metal fine particles for various applications has been conducted. For example, metal fine particles can be suitably used as conductive materials contained in ceramic electronic components (e.g., circuit boards and multilayer capacitors) mounted on electronic devices, or as exhaust gas purification catalysts contained in the combustion gas exhaust system of vehicles, depending on the metal element. When using metal fine particles for such applications, it is desirable that the shape and particle size of the metal fine particles are almost uniform so that the required characteristics and performance are stably realized. Moreover, in recent years, with the trend toward smaller and more highly integrated electronic devices, there has been a demand for the above-mentioned ceramic electronic components to be smaller and more multi-layered, which has led to an increased demand for technologies to manufacture metal fine particles (i.e., metal nanoparticles) having an average particle size of less than 1 μm.

[0003] Meanwhile, methods for producing metal nanoparticles include various chemical and physical methods. Chemical methods such as liquid phase methods and gas phase methods are often used. For example, as seen in the following Patent Documents 1 to 3, the liquid phase method has been studied as one method capable of mass-producing metal nanoparticles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5706881 [Patent Document 2] JP 2013-7082 A [Patent Document 3] JP 2013-7070 A Summary of the Invention [Problem to be solved by the invention]

[0005] In such a method for producing metal nanoparticles by a liquid phase method, roughly speaking, the desired metal nanoparticles can be obtained by reducing a raw material compound of the metal nanoparticles in a solution using a reducing agent. However, in the conventional liquid-phase method for producing metal nanoparticles, it is important to maintain a uniform temperature distribution in the reaction solution, but for example, when mass-producing metal nanoparticles, such maintenance is difficult. If the temperature distribution in the reaction solution is non-uniform, for example, the growth rate of the metal nanoparticles cannot be adjusted, and a problem of large variation in the particle size of the produced metal nanoparticles may occur. Variation in the particle size of the metal nanoparticles is undesirable because it directly leads to qualitative variation in the final product. Therefore, there has been a demand for the development of a technology that can stably produce metal nanoparticles with reduced variation in particle size in a relatively large amount.

[0006] The present invention has been made to solve these problems, and its object is to provide a method for stably producing metal nanoparticles with reduced particle size variation in a relatively large amount using a liquid phase method. [Means for solving the problem]

[0007] The present inventors have noticed that in the production of metal nanoparticles by a liquid phase method, if there is variation in the timing at which the growth of each nucleus forming the metal nanoparticles substantially stops, the particle size of the metal nanoparticles is likely to vary. Therefore, the present inventors have found that by providing the above production method with a step of rapidly cooling the solution, the variation in particle size of the metal nanoparticles can be suppressed and metal nanoparticles with improved uniformity can be obtained, and have completed the present invention.

[0008] According to the technology disclosed herein, a method for producing metal nanoparticles by a liquid phase method is provided. The method comprises the steps of: A step of preparing a solution that serves as a reaction system containing raw material compounds and a reducing agent for forming metal nuclei of the metal nanoparticles; a step of heating the solution to a predetermined temperature range and forming the nuclei by reducing the raw material compounds in the temperature range; Further heating the solution to grow the nuclei into metal nanoparticles of a desired particle size; and a step of quenching the solution to a temperature range where the nuclei growth substantially stops, wherein the average quenching rate is set to 2° C. / min or more; Includes:

[0009] In this specification and claims, the term "metal nanoparticles" refers to a group of many nanoparticles (i.e., particles), except when referring to a single particle. For example, the metal nanoparticles in a "material containing metal nanoparticles" refer to metal nanoparticles as particles, not individual particles. In Japanese, it is unclear whether to use the singular or plural form, so the above definition is used to clarify the meaning of "metal nanoparticles."

[0010] According to the above-mentioned configuration, it is possible to stably produce a relatively large amount of highly uniform metal nanoparticles in which the occurrence of variation in particle diameter of the metal nanoparticles is highly suppressed. In the conventional technology, a heat treatment is performed to grow the nuclei, and after a predetermined time has elapsed, the heat treatment is stopped to stop the nuclei growth. However, as described above, in the case of producing a relatively large amount of metal nanoparticles, in the liquid phase method, the temperature distribution in the reaction solution is likely to be non-uniform, and even if the heating is stopped, the timing at which the nuclei growth substantially stops may differ for each metal nanoparticle. This may be one of the factors that cause the particle diameter of the metal nanoparticles to vary. According to the technology disclosed herein, the timing at which the nuclei growth substantially stops can be adjusted by including the above-mentioned rapid cooling process. This allows the growth degree of the metal nanoparticles to be adjusted, thereby suppressing the occurrence of variation in particle diameter.

[0011] Preferably, the metal nanoparticles are nanoparticles of any one of an iron group element and a precious metal element, and the raw material compound is composed of a salt or a complex of the metal. According to this configuration, it is possible to suppress the occurrence of variation in particle size of metal nanoparticles in the production of nanoparticles of either an iron group element or a precious metal element, and thus it is possible to stably obtain homogeneous iron group nanoparticles and precious metal nanoparticles. Iron group nanoparticles and noble metal nanoparticles are preferably used as conductive materials contained in ceramic electronic components, for example. Therefore, the above-mentioned configuration can stabilize the quality of ceramic electronic components and, in turn, electronic devices equipped with the ceramic electronic components. In addition, such nanoparticles are preferably used as catalysts for purifying combustion gases (exhaust gases) in vehicles and catalysts contained in electrodes of solar cells, etc. Therefore, the above-mentioned configuration can be said to be preferable for these applications.

[0012] Also preferably, the metal is nickel or silver. According to the manufacturing method having such a configuration, nickel nanoparticles having excellent uniformity or silver particles having excellent uniformity can be obtained.

[0013] In a preferred embodiment of the technology disclosed herein, the reducing agent is composed of an alkylamine. According to the above arrangement, in addition to the above effects, the efficiency of forming metal nanoparticles can be increased. Alkylamines function as reducing agents and can also be solvents in reaction solutions. In addition, alkylamines can also function as dispersants, improving the dispersibility of nuclei and metal nanoparticles in reaction solutions. Even when producing metal nanoparticles that tend to aggregate or settle, alkylamines can impart suitable dispersibility in the solution to prevent such aggregation or settling.

[0014] In a preferred embodiment of the technique disclosed herein, the average quenching rate is set to 5° C. / min or more. According to this configuration, even if the volume of the reaction solution is relatively large, the nucleus growth can be stopped at approximately the same time throughout the reaction solution.

[0015] In addition, in a preferred embodiment of the technology disclosed herein, a cooling pipe capable of continuously supplying cooling water into the inside of the pipe is disposed in the solution, and in the quenching step, cooling water is continuously supplied into the cooling pipe, and the average quenching rate is realized by cooling using the latent heat of the water. According to this configuration, the uniformity of the temperature distribution in the reaction solution can be further improved, and the temperature of the reaction solution can be efficiently lowered uniformly even if the reaction solution has a relatively large volume.

[0016] In a preferred embodiment of the technique disclosed herein, at least the nucleus formation step and the nucleus growth step include blowing an inert gas into the solution. According to this configuration, formation of an oxide film on the surface of the nuclei in the above process can be suppressed, and efficient nuclei growth can be achieved.

[0017] In a preferred embodiment of the technique disclosed herein, the gas species blown into the solution is replaced with an oxidizing gas from the inert gas for a certain period before the end of the nucleus growth step. By injecting an oxidizing gas into the solution, an oxide film is formed on the surface of the nuclei, and the nuclei growth is stopped. According to the above-mentioned configuration, the nuclei growth can be stopped more accurately.

[0018] In a preferred embodiment of the technique disclosed herein, the solution is contained in an induction heating (IH) type heating vessel, and the solution is heated by induction heating the heating vessel. According to this configuration, the effects of the present invention are more effectively exhibited, and metal nanoparticles with superior uniformity can be produced stably in relatively large quantities. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a process diagram illustrating a method for producing metal nanoparticles according to an embodiment. [Diagram 2] 1 is a schematic diagram of a manufacturing apparatus for carrying out a method for manufacturing metal nanoparticles according to an embodiment of the present invention; [Diagram 3] 1 is a graph showing the transition of temperature over time when a method for producing metal nanoparticles (nickel nanoparticles) according to an embodiment is carried out. [Figure 4] 1 is a FE-SEM observation image of nickel nanoparticles according to Sample 2. [Diagram 5] 1 is an FE-SEM observation image of nickel nanoparticles according to Sample 4. [Figure 6] 1 is an FE-SEM observation image of nickel nanoparticles according to Sample 6. [Figure 7] 1 is a FE-SEM observation image of nickel nanoparticles according to Sample 10. [Figure 8] 1 is a FE-SEM observation image of silver nanoparticles according to Sample 13. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Preferred embodiments of the present invention will be described below. Note that matters other than those specifically mentioned in this specification that are necessary for carrying out the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and the technical common sense in the relevant field.

[0021] In this specification, the term "metal nanoparticles" refers to nanoparticles having an average particle diameter of 1 nm or more and less than 1 μm, calculated based on an image observed by a field emission scanning electron microscope (FE-SEM). In a narrower sense, the term refers to nanoparticles having an average particle diameter based on an SEM image of about 20 nm or more and 300 nm or less. For example, fine particles having an average particle diameter based on an SEM image in the range of 20 to 300 nm, 40 to 200 nm, or 50 to 150 nm may be typical examples of the nanoparticles referred to here. In addition, in this specification, the term "average particle diameter" refers to a particle diameter (also referred to as D50 particle diameter) that corresponds to 50% cumulatively from the small diameter side in a particle size distribution based on a number standard based on an image observed by an FE-SEM. In addition, in this specification, the expression "X to Y" indicating a numerical range includes the meaning of X or more and Y or less, as well as "preferably larger than X" and "preferably smaller than Y".

[0022] According to the technology disclosed herein, a method for producing metal nanoparticles by a liquid phase method is provided. Metals constituting metal nanoparticles include various metal elements. Among them, iron group elements (iron (Fe), cobalt (Co), nickel (Ni)), copper (Cu), precious metal elements (gold (Au), silver (Ag), and platinum group elements such as palladium (Pd), platinum (Pt), and rhodium (Rh)), and alloys thereof are suitable as main constituent elements of metal nanoparticles. Among these, the metal nanoparticles are preferably nanoparticles of any metal among iron group elements and precious metal elements. More preferably, the metal nanoparticles are nanoparticles of metal of nickel element or silver element. The main constituent metal element refers to the metal element that constitutes the main component of the metal nanoparticles. Ideally, the metal nanoparticles are composed only of metal elements, but may contain other metal elements or nonmetal elements as impurities. The organic matter content of the total weight (100 wt%) of the metal nanoparticles (referring to the aggregate before calcination) measured based on TG-DTA is preferably approximately 3.5 wt% or less, more preferably 3.0 wt% or less, and particularly preferably 2.5 wt% or less.

[0023] First, materials that are preferably used in the manufacturing method disclosed herein will be described. <Raw material compound> The raw material compound for forming the metal core of the metal nanoparticle is a metal compound containing the metal element that constitutes the metal nanoparticle. As the metal compound, any metal compound used for producing this type of metal nanoparticle can be used without any particular limitation, and can be, for example, a salt, a complex, or an alkoxide. Among them, metal salts and metal complexes are preferred because metal ions are easily dissociated in solution. The salt may be an organic acid salt or an inorganic salt. Examples of organic acid salts include carboxylates such as oxalates, formates, acetates, propionates, malonates, benzoates, and phthalates. Examples of inorganic salts include halide salts such as fluorides, chlorides, bromides, and iodides; sulfates, nitrates, nitrites, and carbonates. Among them, organic acid salts and carbonates, particularly carboxylates, can be preferably used because they are less likely to produce impurities and have a relatively low dissociation temperature (decomposition temperature). The carboxylates may be either anhydrides or hydrates, but typically hydrates can be used from the viewpoint of safety.

[0024] <Reducing agent> As the reducing agent, any reducing agent used for producing this type of metal nanoparticles can be used without any particular limitation, and suitable examples include amine compounds. Any compound having one or more amino groups may be used, and any conventionally known compound that is known to be usable for this type of application can be used without any particular limitation.

[0025] The amine compound that can be used is one that is a solid or liquid at room temperature. It is preferable that the amine compound is liquid at room temperature because it can also function as a solvent in the production of metal nanoparticles. Also, when the amine compound is liquid at room temperature, unreacted amine compound can be more easily removed when washing the produced nanoparticles. When the amine compound is a solid at room temperature, it may be liquid in a step including heating, which is included in the production method disclosed herein, or when the amine compound is a solid at room temperature, it may be dissolved using a solvent.

[0026] Amine compounds are known to have the property of protecting the surfaces of metal nanoparticles when the metal nanoparticles are formed. When an amine compound is attached to the surface of a metal nanoparticle, the dispersibility of the metal nanoparticles in a solution can be improved. In other words, the amine compound can function as a protective agent (dispersant) in a solution. From the viewpoint of the surface protection effect of the metal nanoparticles, the amine compound is preferably a primary amine.

[0027] The amine compound is preferably, for example, one or more alkylamines, and is preferably an amine compound having about 20 or less carbon atoms. As the amine compound, conventionally known ones can be used without any particular limitation.Specific examples include primary aliphatic amines such as n-butylamine, pentylamine, 2-methoxyethylamine, 2-ethoxyethylamine, 3-methoxypropylamine, 3-ethoxypropylamine, octylamine, hexadecylamine, stearylamine, oleylamine, myristylamine, and laurylamine; secondary aliphatic amines such as dimethylamine, diethylamine, methylbutylamine, ethylpropylamine, ethylisopropylamine, and dioctylamine; and tertiary aliphatic amines such as trimethylamine, dimethylethylamine, diethylmethylamine, and trioctylamine; and other amine compounds having a carbon number of 20 or less. Among them, oleylamine is preferred because it is in a liquid state under the temperature conditions of the production method disclosed herein, has excellent reducing action, and has excellent dispersibility, in other words, it can function as at least three agents, namely, a solvent, a reducing agent, and a dispersing agent, in the method.

[0028] <Optional ingredients> The solution may be composed of two components, the raw material compound and the reducing agent, and may contain other optional components as necessary, as long as they do not significantly reduce the effects of the technology disclosed herein. As the optional components, one type may be used alone from among conventionally known components, or two or more types may be used in combination. Examples of the optional components include additives for adjusting the reactivity of metal nanoparticles and improving the dispersion stability. Specific examples include reaction catalysts, reaction regulators, viscosity regulators, dispersants, etc.

[0029] Another example of the optional component is a solvent. Various solvents known to be usable for this type of application can be added to the solution. Such a solvent may contain unavoidable impurities as long as they do not significantly reduce the effects of the technology disclosed herein. Examples of components that may be intentionally or unavoidably contained in the solvent include organic solvents such as alcohol-based solvents, amide-based solvents, ketone-based solvents, ester-based solvents, amine-based solvents, ether-based solvents, nitrile-based solvents, and hydrocarbon-based solvents other than those mentioned above, and water.

[0030] Next, a method for producing metal nanoparticles provided by the technique disclosed herein will be described with reference to FIG. As shown in FIG. 1, the manufacturing method roughly includes a solution preparation step S1, a nucleus formation step S2, a nucleus growth step S3, and a quenching step S4.

[0031] <Solution preparation process> In the solution preparation step S1, first, a solution that serves as a reaction system is prepared, which contains raw material compounds and a reducing agent for forming metal nuclei of metal nanoparticles. The solution is prepared at a temperature at which the raw material compound does not thermally decompose. The solution is prepared at about 40° C. or lower, typically at room temperature (e.g., 25±10° C., preferably 25±5° C.). The solution may be prepared in the air or in an inert atmosphere such as nitrogen, argon, or helium.

[0032] The order of mixing the raw material compound and the reducing agent is not particularly limited. For example, all may be added at the same time and mixed. As described later, when a solvent is used as an optional component, one of the raw material compound and the reducing agent may be dissolved or dispersed in the solvent, and then the other may be added and mixed. It is also preferable to stir the solution when preparing it, since stirring can provide a highly homogeneous solution in a relatively short time.

[0033] <Nucleation process> In the nucleus formation step S2, the solution obtained in the solution preparation step S1 is heated to a first temperature T1. In order to prevent the formation of an oxide film on the surface of the cores, this step is preferably carried out while blowing an inert gas (refers to inert gases such as nitrogen, argon, helium, etc.; the same applies below) into the solution. In addition, from the viewpoint of improving the uniformity of the temperature distribution and concentration distribution in the solution, it is preferable to carry out a stirring operation. The solution contains a raw material compound (metal compound) coordinated with a reducing agent (amine compound). By heating this solution to a first temperature T1, a homogeneous (well-balanced) reaction can be caused to occur on the surface of the metal compound coordinated with the amine compound. This makes it possible to produce clusters that will become the nuclei (precursors) of metal nanoparticles, and metal nanocolloids that are the result of the clusters growing.

[0034] The first temperature T1 is preferably higher than the temperature in the solution preparation step S1 and lower than the thermal decomposition temperature of the raw material compound. The first temperature T1 may vary depending on the composition of the solution, for example, the type and blending ratio of the raw material compound, the type and blending ratio of the reducing agent, the amount and type of the organic solvent added, the amount of water added, etc. From the viewpoint of further increasing the homogeneity of the metal nanoparticles, the first temperature T1 is preferably about 5 to 90°C lower, typically 10 to 80°C lower, and preferably 15 to 70°C lower than the thermal decomposition temperature of the raw material compound to which the reducing agent is coordinated. For example, when a nickel compound coordinated with an amine compound having a thermal decomposition temperature of about 250°C is used, the first temperature T1 can be set to about 160 to 205°C. In addition, when using a silver compound coordinated with an amine compound having a thermal decomposition temperature of about 110° C., the first temperature T1 can be set to about 80 to 90° C. In this manner, T1 may be determined depending on the type of metal element to be produced.

[0035] Up to the first temperature T1, the temperature rise rate ΔT1 can be determined from the viewpoint of improving production efficiency. Although not particularly limited, the temperature rise rate ΔT1 may be approximately 0.1 to 50° C. / min, for example 1 to 20° C. / min.

[0036] In a preferred embodiment, the first temperature T1 is maintained so as not to saturate nucleation. The retention time H1 for maintaining the first temperature T1 is not particularly limited, as it may vary depending on, for example, the first temperature T1 and the composition of the above-mentioned solution (for example, the type of raw material compound, the type of solvent, the amount of water added, etc.). It is preferable that the retention time H1 is set so as not to exceed the saturated concentration of nuclei in the solution. This can prevent fusion of nuclei in the solution. As described in the Examples below, when nickel acetate tetrahydrate is used as a raw material compound and oleylamine is used as a reducing agent, the retention time H1 can be generally 240 minutes or less, for example, 10 to 180 minutes. As described in the examples below, when silver oxalate is used as the raw material compound, n-butylamine and n-octylamine are used as the reducing agents, and n-butanol is used as the solvent, and the first temperature T1 is set to 85°C, the retention time H1 is set to approximately 20 minutes or less, for example, 5 to 15 minutes.

[0037] The nuclei of the metal nanoparticles have a smaller average particle size, for example 10 nm or less, than the metal nanoparticles obtained in the nucleus growth step described below. The amount of nuclei produced in this step can be an important parameter that determines the particle size of the metal nanoparticles. The amount of nuclei produced can be adjusted, for example, by the first temperature T1 or the holding time H1 for holding the first temperature T1. In other words, by varying the above parameters, it is possible to adjust the particle size of the metal nanoparticles to about 20 nm to 300 nm (for example, 50 nm to 150 nm). In general, the higher the first temperature T1 is set, the greater the amount of nuclei produced, and metal nanoparticles with a smaller average particle size are more likely to be obtained.

[0038] The nucleation step S2 may include a heat treatment (i.e., a dehydration treatment) for removing water from the solution. For example, this can be performed as necessary when water is contained in the solution or when a hydrate of a metal salt is used as a raw material compound. From the viewpoint of efficient nucleation, when the dehydration treatment is performed, it is preferable to perform it before increasing the temperature to the first temperature T1. Specifically, the solution can be dehydrated by heating it for about 10 to 180 minutes under temperature conditions of about 120° C. to 150° C.

[0039] <Nucle growth process> In the nucleus growth step S3, the solution at the first temperature T1 is heated to a second temperature T2. This step may be performed consecutively following the nucleus formation step S2, or the solution may be cooled to room temperature (e.g., 25±10° C.) and then heated to the second temperature T2. In order to prevent the formation of an oxide film on the surface of the cores, this step is preferably carried out while blowing an inert gas into the solution. Also, from the viewpoint of improving the uniformity of the temperature distribution and concentration distribution in the solution, it is preferable to carry out a stirring operation. The solution contains nuclei (precursors) of the metal nanoparticles generated in the nucleation step. By heating this solution to a second temperature T2, the zero-valent metal newly generated by the reduction of the raw material compound is stably fused to the nuclei. This allows the nuclei to grow uniformly, and metal nanoparticles are stably generated in the solution.

[0040] The second temperature T2 is a temperature equal to or lower than the thermal decomposition temperature of the raw material compound. The second temperature T2 is preferably a temperature lower than the thermal decomposition temperature of the raw material compound by 0 to 40° C., for example, by 10 to 30° C. The second temperature T2 may vary depending on the amount of the solution, the composition of the solution (for example, the type of raw material compound, the type of solvent, the amount of water added, etc.), etc. When the raw material compound is a nickel compound and the thermal decomposition temperature of the nickel compound coordinated with an amine compound is around 250°C, the second temperature T2 can be set to about 210 to 250°C. When the raw material compound is a silver compound and the thermal decomposition temperature of the silver compound is around 110°C, the second temperature T2 can be set to about 90 to 110°C.

[0041] Although not particularly limited, it is preferable to gradually increase the temperature to the second temperature T2 at a temperature increase rate ΔT2. From the viewpoint of uniformly growing nuclei, the temperature increase rate ΔT2 is preferably smaller than or approximately equal to the temperature increase rate ΔT1, and from the viewpoint of balancing with production efficiency, it is preferable to set the temperature increase rate to about 0.1 to 10° C. / min, for example, 1 to 6° C. / min.

[0042] It is preferable to maintain the second temperature T2 until the decomposition and reduction of the raw material compound are completed. The retention time H2 for maintaining the second temperature T2 is not particularly limited because it may vary depending on, for example, the temperature rise rate ΔT2 and the amount and composition of the above-mentioned solution (e.g., the type of raw material compound, the type of solvent, the amount of water added, etc.). From the viewpoint of suppressing the bonding between grown nuclei and improving production efficiency, the holding time H2 is preferably about 20 minutes or less, for example, 10 to 15 minutes. The sum of the holding times H1 and H2 is preferably about 200 minutes or less, typically 160 minutes or less, for example, 15 to 150 minutes.

[0043] Although not particularly limited, the nucleus growth step S3 may include replacing the gas species to be blown into the solution with an oxidizing gas (e.g., oxygen gas) from an inert gas for a certain period before the end of the step, and blowing in the oxidizing gas. This allows an oxide film to be formed on the surface of the metal nanoparticles generated in the solution. In order to more reliably stop the growth of the metal nanoparticles, it is advisable to carry out this treatment.

[0044] The period for blowing in the oxidizing gas is not particularly limited, but may be the period until an oxide film is formed on at least a part of the surface of the metal nanoparticles. In order to reliably stop the nucleus growth, it is preferable that the period is about 2 to 60 minutes. In order to keep the timing for stopping the nucleus growth constant, it is preferable to replace the gas with the oxidizing gas without changing the temperature from the second temperature T2. After the end of the oxidizing gas blowing period, the gas type blown into the solution may or may not be switched to an inert gas.

[0045] <Quenching process> In the quenching step S4, the solution at the second temperature T2 is cooled to a third temperature T3. This step is performed continuously following the nucleus growth step S3. In this step, it is preferable to perform a stirring operation from the viewpoint of improving the uniformity of the temperature distribution and concentration distribution in the solution. The solution contains metal nanoparticles generated in the nucleus growth step. By quenching the solution at a predetermined average quenching rate, it is possible to synchronize the timing at which the growth of each nucleus contained in the solution stops.

[0046] The third temperature T3 is at least a temperature lower than the first temperature T1 in the nucleation step S2. The third temperature T3 is preferably a temperature lower than the first temperature T1 by 40 to 180° C., for example, by 60 to 160° C. The third temperature T3 may vary depending on the amount of the solution, the composition of the solution (for example, the type of metal compound, the type of solvent, the amount of water added, etc.), etc.

[0047] From the viewpoint of aligning the timing of cessation of nucleus growth, the average quenching rate ΔT3 is preferably set to 2° C. / min or more (e.g., greater than 2° C. / min). The greater the average quenching rate ΔT3, the more likely it is that the effect of aligning the timing of cessation of nucleus growth will improve. From the viewpoint of better exerting the effects of the present invention, it is more preferably set to 4° C. / min or more, and even more preferably to 5° C. / min or more (particularly preferably to 7° C. / min or more). The upper limit of the average quenching rate ΔT3 is preferably set to about 10° C. / min.

[0048] In this manner, metal nanoparticles can be produced in a solution. According to this manufacturing method, metal nanoparticles with little variation from the desired particle size can be obtained with high reproducibility. For example, metal nanoparticles with a coefficient of variation (CV) of the average particle size suppressed to approximately less than 0.16 and 0.15 or less (e.g., 0 or more and less than 0.15) can be obtained.

[0049] The obtained metal nanoparticles can be separated from the solution by, for example, standing or centrifuging. After removing the supernatant, the metal nanoparticles can be obtained by washing with an appropriate solvent and drying. The metal nanoparticles can be used to prepare a conductive paste. Alternatively, after removing the supernatant, the metal nanoparticles may be used in a wet state to prepare a conductive paste. If the metal nanoparticles are used in a wet state to prepare a conductive paste, the aggregation of the metal nanoparticles can be suppressed and the handling can be improved.

[0050] Next, a manufacturing apparatus for carrying out the technique disclosed herein will be described in detail with appropriate reference to Fig. 2. Fig. 2 is a schematic diagram of a manufacturing apparatus for carrying out a method for manufacturing metal nanoparticles according to one embodiment.

[0051] 2, the manufacturing apparatus 100 roughly includes a container 10, an agitator 20, piping 30, a heating device 40, a rapid cooling means 50, a thermocouple 60, and a control device (not shown). A series of operations of the manufacturing apparatus 100 (e.g., opening and closing of valves, operation of the heating device 40 and the rapid cooling means 50, etc.) are controlled by the manufacturing apparatus based on time information and temperature information acquired by the thermocouple 60. The container 10 is a container that contains a solution for producing metal nanoparticles. The container 10 is made of a material that has a thermal conductive property (for example, a metal material such as stainless steel). The container 10 is equipped with an exhaust valve 10a, and the solution contained inside the container 10 can be discharged from the exhaust valve 10a. The size of the container 10 is not particularly limited, but considering that the technology disclosed herein is applied to the production of metal nanoparticles in a relatively large volume (for example, the amount of solution charged is about 50 L to 100 L, or up to about 150 L), it is preferable that the size of the container 10 is such that a solution with a volume of up to about 100 L or more (for example, 150 L to 200 L) can be contained. The stirring device 20 is a device that stirs the material contained in the container 10. From the viewpoint of ease of stirring operation, the stirring device 20 is preferably equipped with a rotating blade as a stirrer. The size and shape of the rotating blade are not particularly limited as long as they can sufficiently stir the material. The piping 30 is configured to be able to introduce gas into the container 10. The piping 30 has a branch point. This allows a plurality of types of gas to be introduced into the container 10. The piping 30 includes a piping 32 for introducing an inert gas into the container 10 through the branch point, and a piping 34 for introducing an oxidizing gas into the container 10 as necessary. The piping 32 includes a valve 321 for adjusting the amount of inert gas introduced. The piping 34 includes a valve 341 for adjusting the amount of oxidizing gas introduced. When the valve 321 is in the "open" state, the inert gas is introduced into the inside of the container 10 (i.e., into the solution) through the piping 32 and the piping 30. When the valve 321 is in the "closed" state, the inert gas is not introduced into the container 10. On the other hand, when the valve 341 is in the "open" state, the oxidizing gas is introduced into the inside of the container 10 (i.e., into the solution) through the piping 34 and the piping 30. When the valve 341 is in the "closed" state, the oxidizing gas is not introduced into the container 10.

[0052] The heating device 40 is configured to be able to heat the solution contained in the container 10. The heating device 40 is not particularly limited as long as the desired temperature control is realized in carrying out the manufacturing method disclosed herein, and a suitable conventional heating device (microwave generator, oil bath, mantle heater, etc.) can be used depending on the amount of solution, the composition of the solution (e.g., the type of metal compound, the type of solvent, the amount of water added, etc.), etc., but it is particularly preferable to adopt an induction heating (IH) type as such a heating device. From the viewpoint of improving the uniformity of the temperature distribution in the solution, it is preferable that the heating device 40 is an induction heating coil. By inductively heating the solution contained in the container 10, the process including heating can be efficiently carried out.

[0053] The quenching means 50 is configured to be able to rapidly cool the solution contained in the container 10. There is no particular limitation on the quenching means 50, but an example of the quenching means 50 is one that utilizes the latent heat of water. As shown in FIG. 2, the quenching means 50 includes an air inlet pipe 52, a cooling water inlet pipe 54, and a cooling pipe 56 capable of continuously supplying air or cooling water to the inside of the pipe. The air inlet pipe 52 includes a valve 521 for adjusting the amount of air introduced into the cooling pipe 56. The cooling water inlet pipe 54 includes a valve 541 for adjusting the amount of cooling water introduced into the cooling pipe 56. The cooling pipe 56 is coiled as shown in the figure, and is disposed in the solution contained in the container 10. The cooling water introduced into the cooling pipe 56 evaporates due to the heat of the container 10. The solution in the container 10 is cooled by the evaporation of the cooling water. When a valve 561 provided on the cooling pipe 56 is opened, water vapor is discharged.

[0054] The thermocouple 60 is a temperature sensor that detects a change in temperature of the container 10. The temperature information acquired by the thermocouple 60 is sent to a control device of the manufacturing apparatus 100, and the operation of the manufacturing apparatus 100 is controlled by the control device.

[0055] Below, a preferred embodiment for producing metal nanoparticles using the above-mentioned materials and the manufacturing apparatus having the above-mentioned configuration will be specifically described using an example in which nickel nanoparticles are produced as the metal nanoparticles; however, this is not intended to limit the metal nanoparticles of the present invention to this.

[0056] <Production of nickel nanoparticles> -Solution preparation process S1- Nickel acetate tetrahydrate as a raw material compound and oleylamine as a reducing agent are charged separately or in a mixed state into a container 10 to obtain a reaction solution for forming nickel nanoparticles. This solution is stirred using a stirring device 20.

[0057] ―Nucleation process S2― Valve 321 is opened to blow inert gas into the solution. Heating device 40 (induction heating coil in this case) is switched on to heat the solution, and when the temperature detected by thermocouple 60 reaches a target temperature within the range of 120°C to 150°C, the temperature increase is stopped and the solution is maintained at about that temperature for about 10 to 180 minutes. This allows nickel acetate tetrahydrate to be dehydrated. After the dehydration treatment is completed, the solution is heated at a temperature increase rate ΔT1. When the temperature of the solution reaches a first temperature T1, the temperature increase is stopped and the solution is maintained at about that temperature for a holding time H1. Here, the temperature T1 can be 160 to 205°C (for example, 180 to 200°C). The holding time H1 can be approximately 240 minutes or less (for example, 10 to 180 minutes). This reduces the nickel compound to form nickel nuclei.

[0058] ―Nucle growth process S3― After the retention time H1 has elapsed, the solution is further heated at a temperature increase rate ΔT2. When the temperature of the solution reaches a second temperature T2, the temperature increase is stopped and the solution is maintained at about that temperature for a retention time H2. Here, the temperature T2 may be about 210 to 250°C. The retention time H2 may be about 10 to 25 minutes. This allows nickel nanoparticles of a predetermined particle size to be obtained.

[0059] When blowing in an oxidizing gas for a certain period before the end of the nucleus growth step S3, close valve 321 to stop blowing inert gas, and open valve 341. Then, the oxidizing gas is blown into the solution through pipe 34. This state is maintained for about 10 to 60 minutes. This forms an oxide film on the surface of the nickel nanoparticles, and further nucleus growth can be stopped.

[0060] -Quick cooling process S4- When the oxidizing gas has been blown in, valve 341 is closed to stop blowing of the oxidizing gas, and valve 321 is opened. Then, the inert gas is blown into the solution through pipe 32. However, the type of gas blown into the solution may be either an oxidizing gas or an inert gas, and replacement with an inert gas is not required. In the quenching step S4, the heater 40 is switched off to stop heating the solution. The valve 541 is opened to supply cooling water from the cooling water inlet pipe 54 to the cooling pipe 56. The supply rate of the cooling water and the temperature of the supplied cooling water are not particularly limited, and may be appropriately adjusted to achieve a desired average quenching rate ΔT3. The cooling water passes through the cooling pipe 56 to cool the solution in the container 10. When the solution is cooled to temperature T3, the discharge valve 10a of the container 10 is opened to remove the solution. The final product, nickel nanoparticles, is obtained from this solution. The temperature T3 can be set to about 20 to 50°C. The cooling water is heated by the heat in the container 10 and turns into steam while flowing through the cooling pipe 56. This evaporation cools the solution. When the valve 561 is opened, the steam (the cooling water flowing through the cooling pipe 56) is discharged.

[0061] According to the technology disclosed herein, it is possible to stably produce a relatively large amount of highly uniform metal nanoparticles in which the occurrence of variation in particle diameter of the metal nanoparticles is highly suppressed by the liquid phase method. Even if the amount of the solution for forming the metal nanoparticles is on the liter scale (for example, 1 L or more, 5 L or more, 10 L or more, 20 L or more, or more), the effect of the technology disclosed herein can be obtained. Specifically, in the process of producing metal nanoparticles, the reaction solution is rapidly cooled at a predetermined cooling rate (i.e., average quenching rate ΔT3), so that nucleus growth can be stopped at approximately the same timing throughout the reaction solution.

[0062] Several embodiments of the present invention will be described below, but it is not intended that the present invention be limited to those embodiments.

[0063] <Example 1. Production example of nickel nanoparticles> Sample 1: A manufacturing example of nickel nanoparticles according to Sample 1 will be described with reference to FIG. 3. The symbols in the following description are the symbols shown in FIG. 2. FIG. 3 is a graph showing the change in temperature over time during the manufacturing of nickel nanoparticles according to one embodiment. The "time" on the horizontal axis indicates the time elapsed during the manufacturing, with the time when the heating of the solution is started being set to zero. The "temperature" on the vertical axis indicates the temperature change of the solution during the manufacturing. In an environment of 25°C, 10 kg of nickel acetate tetrahydrate and 70 L of oleylamine were put into the container 10 of the manufacturing apparatus 100 to obtain a solution for manufacturing nickel nanoparticles. The solution was stirred by the stirring device 20. The rotation speed of the stirring device 20 was 300 rpm. An IH reaction kettle manufactured by Nippon Dennetsu Co., Ltd. was used as the manufacturing apparatus 100. The valve 321 was opened to blow nitrogen gas into the solution at a flow rate of 50 L / min, and the switch of the heating device 40 (here, an induction heating coil) was turned on to raise the temperature to 140°C. The solution was heated at that temperature for 120 minutes. Next, the solution was heated at a heating rate of ΔT1 (4° C. / min). When the temperature of the solution reached a first temperature T1 (200° C.), the heating was stopped and the solution was maintained at about that temperature for 30 minutes. Next, the solution was further heated at a temperature increase rate ΔT2 (3° C. / min). When the temperature of the solution reached a second temperature T2 (230° C.), the temperature increase was stopped and the solution was maintained at about that temperature for 10 minutes. Next, while maintaining this temperature, valve 321 was closed to stop the blowing of nitrogen gas. Valve 341 was opened, and oxygen gas was blown into the solution at a flow rate of 10 L / min through pipe 34. This state was maintained for about 10 minutes. Thereafter, valve 341 was closed to stop the blowing of oxygen gas. Valve 321 was opened, and nitrogen gas was blown into the solution at a flow rate of 2 L / min through pipe 32. Next, the switch of the heating device 40 was turned off to stop heating the solution. The valve 541 was opened, and cooling water was supplied from the cooling water inlet pipe 54 to the cooling pipe 56 at a flow rate of 10 L / min. When the solution was cooled to a temperature T3 (50°C), the discharge valve 10a of the container 10 was opened, and the solution was taken out. The average quenching rate ΔT3 was 7°C / min.

[0064] The solution was then removed from the container 10 via the discharge valve 10a. The solution was left to stand and the supernatant was removed. The precipitate was suspended in hexane and shaken well. The solution was left to stand and the supernatant was removed. The resulting precipitate was dried to obtain nickel nanoparticles of sample 1. It was confirmed by X-ray diffraction that the obtained particles were metallic nickel.

[0065] Samples 2-5: Various conditions for producing nickel nanoparticles are shown in Table 1. Other than that, nickel nanoparticles of Samples 2 to 5 were produced using the same materials and methods as those used for producing Sample 1.

[0066] Samples 6-9: 3.4 kg of nickel acetate tetrahydrate was used as the raw material compound, and 20 L of oleylamine was used as the reducing agent. Various conditions for producing nickel nanoparticles are shown in Table 1. Other than that, nickel nanoparticles of Samples 6 to 9 were produced using the same materials and methods as those used for producing Sample 1.

[0067] Samples 10-12: 100 g of nickel acetate tetrahydrate was used as a raw material compound, and 3 L of oleylamine was used as a reducing agent. A mantle heater was used as the heating device 40. Natural cooling was used as the rapid cooling means. Various conditions for producing nickel nanoparticles were as shown in Table 1. Other than that, nickel nanoparticles according to Samples 10 to 12 were produced using the same materials and methods as those used for producing Sample 1.

[0068] [Table 1]

[0069] [evaluation] (1) FE-SEM Observation The shape of the nickel nanoparticles was observed using a field emission scanning electron microscope (FE-SEM: Hitachi High-Technologies Corporation, S-4700, observation magnification: 50,000 times). For reference, some of the observed images of the nickel nanoparticles of Samples 2, 4, and 6, and the nickel nanoparticles of Sample 10 are shown in Figures 4 to 7, respectively.

[0070] (2) Particle size distribution The particle size distribution of nickel nanoparticles was measured based on the FE-SEM images taken at a magnification of 50,000 times. Specifically, 1000 nickel nanoparticles were randomly extracted from a total of five images taken using image processing software, and their projected areas were calculated. The diameter of a circle having the same area (Heywood diameter) was measured. The cumulative 50% particle size (D50) in the number-based particle size distribution was then calculated as the average particle size (nm). The coefficient of variation (CV) of the average particle size was also calculated. Furthermore, the cumulative 5% particle size (D5) in the number-based particle size distribution and the cumulative 95% particle size (D95) in the number-based particle size distribution were calculated. These results are shown in the corresponding columns in Table 2.

[0071] (3) Measurement of crystallite size For the nickel nanoparticles of each sample, the average crystallite size was calculated based on the diffraction line detected at 2θ = 44.5 ° in the powder X-ray diffraction method using the Scherrer formula. The XRD detector used was SmartLab manufactured by Rigaku Corporation. The results are shown in the corresponding columns in Table 2.

[0072] [Table 2]

[0073] As shown in Figures 4 to 7, Samples 2, 4, and 6 had reduced variation in shape and size. As shown in Table 2, the coefficient of variation (CV) of the average particle size of Samples 1 to 9 was less than 0.16, indicating that the variation in shape and size was suppressed. In addition, the measurement results of D5, D50, and D95 ​​confirmed that Samples 1 to 9 had a sharp particle size distribution.

[0074] Among Samples 1 to 12, the quenching rate ΔT3 of Samples 1 to 5 was 7° C. / min, that of Samples 6 to 9 was 2° C. / min, and that of Samples 10 to 12 was 1° C. / min. From the above results, it was found that in the method for producing metal nanoparticles, when the quenching rate is 2°C / min or more, the variation in shape and size of the metal nanoparticles is suppressed. Furthermore, from the results of Samples 1 to 5, it was confirmed that a higher quenching rate is preferable, and that metal nanoparticles with a narrow particle size distribution are formed when the quenching rate is 5°C / min or more in particular.

[0075] On the other hand, Samples 10 to 12 have a larger coefficient of variation of the average particle size than Samples 1 to 9, and the measurement results of D5, D50, and D95 ​​show that the particle size distribution is larger than that of Samples 1 to 9. In other words, it was found that when the quenching rate is less than 2°C / min, it is difficult to suppress the variation in the shape and size of the metal nanoparticles.

[0076] <Example 2. Production example of silver nanoparticles> Sample 13: In an environment of 25°C, 3.0 L of n-butanol, 3 kg of silver oxalate, 1.6 L of n-butylamine, and 0.5 L of n-octylamine were charged into the container 10 of the manufacturing apparatus 100 to obtain a solution for manufacturing silver nanoparticles. The solution was stirred by the stirring apparatus 20. The rotation speed of the stirring apparatus 20 was 300 rpm. The switch of the heating apparatus 40 (here, an induction heating coil) was turned on, and the solution was heated at a temperature increase rate ΔT1 (4°C / min). When the temperature of the solution reached a first temperature T1 (85°C), the temperature increase was stopped, and the solution was maintained at about that temperature for 10 minutes. Next, the solution was further heated at a heating rate ΔT2 (1° C. / min). When the temperature of the solution reached a second temperature T2 (100° C.), the heating was stopped and the solution was maintained at about that temperature for 10 minutes. Next, the switch of the heating device 40 was turned off to stop heating the solution. The valve 541 was opened to supply cooling water from the cooling water inlet pipe 54 to the cooling pipe 56 at a flow rate of 10 L / min. When the solution was cooled to a temperature T3 (20°C), the discharge valve 10a of the container 10 was opened to remove the solution. The average quenching rate ΔT3 was 4°C / min.

[0077] The solution was then removed from the container 10 via the discharge valve 10a. The solution was left to stand and the supernatant was removed. The precipitate was suspended in ethanol and shaken well. The solution was left to stand and the supernatant was removed. The resulting precipitate was dried to obtain silver nanoparticles of sample 13.

[0078] Samples 14 and 15: Various conditions for producing silver nanoparticles are shown in Table 3. Other than that, silver nanoparticles of Samples 14 and 15 were produced using the same materials and methods as those used for producing Sample 13.

[0079] [Table 3]

[0080] [evaluation] Evaluations (1) to (3) were performed in the same manner as in Example 1. The results are shown in FIG. In the evaluation of (1) and (2) of the above-mentioned Example 1 in Example 2, FE-SEM observation was performed at a magnification of 50,000 times. In addition, in (2), analysis was performed using five images taken. In addition, in the evaluation of (3) of the above-mentioned Example 1 in Example 2, the average crystallite size was calculated based on the diffraction line detected at 2θ=38.2° in the powder X-ray diffraction method using the Scherrer formula.

[0081] [Table 4]

[0082] As shown in FIG. 8, Sample 13 had reduced variation in shape and size. As shown in Table 4, the coefficient of variation (CV) of the average particle size of Samples 13 and 14 was less than 0.16, indicating that the variation in shape and size was suppressed. In addition, the measurement results of D5, D50, and D95 ​​confirmed that Samples 13 and 14 had a sharp particle size distribution. On the other hand, for Sample 15, the coefficient of variation of the average particle size is larger than those of Samples 13 and 14, and the measurement results of D5, D50, and D95 ​​show that the particle size distribution is broader than those of Samples 1 to 9. In other words, it was found that when the quenching rate is less than 2°C / min, it is difficult to suppress the variations in the shape and size of the metal nanoparticles.

[0083] Although the present invention has been described in detail above, these are merely examples, and the present invention can be modified in various ways without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0084] S1 Solution preparation process S2 Nucleation process S3 Nucleus growth process S4 Rapid cooling process 10 containers 10a Exhaust valve 20 Stirring device 30 Piping 32 Piping 321 Valve 34 Piping 341 Valve 40 Heating device 50 Quenching Method 52 Air intake pipe 54 Cooling water introduction pipe 56 Cooling pipe 561 Valve 60 Thermocouple 100 Manufacturing equipment

Claims

1. A method for producing metal nanoparticles by a liquid phase method, comprising the steps of: A step of preparing a solution that serves as a reaction system containing raw material compounds and a reducing agent for forming metal nuclei of the metal nanoparticles; a step of heating the solution at a heating rate ΔT1 to a first temperature T1, the first temperature T1 being higher than the temperature in the step of preparing the solution and lower than the thermal decomposition temperature of the raw material compound, and holding the solution at the first temperature T1 for a holding time H1 to form the nuclei by reduction of the raw material compound; After the step of forming the nuclei, the solution is further heated at a heating rate ΔT2 smaller than the heating rate ΔT1 to a second temperature T2 higher than the first temperature T1 and lower than the thermal decomposition temperature of the raw material compound, and is held at the second temperature T2 for a holding time H2, thereby growing the nuclei into metal nanoparticles having a predetermined particle size; and a step of quenching the solution to a temperature range where the nuclei growth substantially stops, wherein the average quenching rate is set to 2° C. / min or more; Inclusive of The manufacturing method includes: A container for containing the solution; A heating device for heating the solution contained in the container; A rapid cooling means for cooling the solution contained in the container; The manufacturing method is carried out by a manufacturing apparatus comprising: the step of forming the nuclei, the step of growing the nuclei, and the step of quenching the solution are all carried out in the container; the rapid cooling means has a cooling pipe capable of continuously supplying cooling water to an inside of the pipe, the cooling pipe is disposed in the solution contained in the container, A method for producing metal nanoparticles, wherein in the quenching step, cooling water is continuously supplied into the cooling pipe, and the average quenching rate is achieved by cooling using the latent heat of the water.

2. The method according to claim 1 , wherein the metal nanoparticles are nanoparticles of any one of an iron group element and a precious metal element, and the raw material compound is composed of a salt or a complex of the metal.

3. The method of claim 2 , wherein the metal is nickel or silver.

4. The method according to any one of claims 1 to 3, wherein the reducing agent comprises an alkylamine.

5. The method according to any one of claims 1 to 4, wherein the average quenching rate is set to 5°C / min or more.

6. The method according to any one of claims 1 to 5, wherein at least the nucleus formation step and the nucleus growth step include blowing an inert gas into the solution.

7. 7. The method according to claim 6, wherein the inert gas is replaced with an oxidizing gas as the gas species blown into the solution for a certain period before the end of the nucleus growth step.

8. A manufacturing method described in any one of claims 1 to 7, wherein the container is an induction heating (IH) type heating container, and the solution is heated by induction heating the heating container.

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