JP5830010B2 - Method for producing nickel-cobalt nanoparticles - Google Patents

Description

  The present invention relates to nanoparticles having a core-shell structure made of different metals.

  Cobalt nanoparticles are cheaper than silver nanoparticles, chemically more stable than copper nanoparticles, and have higher catalytic activity and saturation magnetization than nickel particles, so various catalysts, magnetic materials, fuel cells and multilayer ceramics It is expected to be used for electrodes in capacitors. However, since cobalt is a rare metal and is considerably more expensive than nickel particles, development of cobalt particles that can extract cobalt performance in a smaller amount and a method for producing the same is desired. In order to bring out the performance of cobalt well, its particle size and particle shape must be controlled to be uniform.

  Generally, the manufacturing method of a metal nanoparticle is roughly classified into a physical method and a chemical method. The physical method is a method of pulverizing bulk metal to produce nanoparticles (pulverization method), and the chemical method is a method of generating metal atoms and controlling their aggregation (aggregation method). In the pulverization by the physical method, there is a limit to reducing the particle size, and the shape of the particles varies and it is difficult to make a uniform one. For this reason, the chemical method is more advantageous for controlling the shape and particle size of the particles.

  Moreover, the manufacturing method of the metal nanoparticle by a chemical method is classified into a wet method and a dry method. Known chemical dry methods include CVD (chemical vapor deposition), gas evaporation, laser, sputtering, and metal droplet spraying, and the generated particles are treated at high temperature. It is spherical and has good crystallinity, but has the disadvantages of wide particle size distribution and low productivity. On the other hand, the wet method represented by the chemical reduction method has the advantage of higher productivity and easier particle size control than the dry method. Therefore, many studies have been made with the recent development of nanotechnology. Yes.

  By the way, studies on binary nanoparticles (metal composite nanoparticles) that are expected to exhibit further physical properties as compared with single metal nanoparticles have been widely promoted. Furthermore, focusing on the fact that depending on the application of the nanoparticle, it is required to have a specific property as the surface metal of the particle, and as a binary nanoparticle, a core and shell composed of different metals are also studied. ing.

  As the metal nanoparticles having such a core-shell structure, for example, a metal including a copper core and a thin film layer of metal such as silver, palladium, platinum, or gold surrounding the copper core and having a reduction potential higher than that of copper. Nanoparticles are disclosed (Patent Document 1). The method for producing the metal nanoparticles comprises a copper precursor comprising a copper complex such as copper nitrate, copper chloride, copper formate using tert-butylhydroxytoluene, ascorbic acid, flavonoid, etc. as a reducing agent in a solution containing a primary amine. Forming copper nanoparticles from the body, and forming a metal thin film layer having a reduction potential higher than copper on the surface of the copper nanoparticles. The resulting metal nanoparticles having a core-shell structure have a copper particle as a core and are coated with a noble metal, so that the copper content can be increased while preventing copper oxidation, which is excellent in economic efficiency, In addition, since the thin film layer contains a metal such as silver having an electrical conductivity superior to that of copper, it has an advantage that a wiring having an electrical conductivity superior to that of copper can be formed.

  Further, for example, a metal precursor containing a first metal (for example, cobalt) that is a nanoparticle and a second metal (for example, platinum) having a reduction potential higher than that of the first metal is dissolved in an appropriate organic solvent. In addition, there is disclosed a method for producing core-shell structured metal nanoparticles by mixing each solution and performing a metal substitution reaction between the first metal and the second metal (Patent Document 2).

  In Patent Documents 1 and 2, metal nanoparticles having a core made of a metal having a low reduction potential and a shell made of a metal having a high reduction potential are realized by different methods. However, it is not certain whether the core-shell structured metal nanoparticles made of other dissimilar metals can be appropriately obtained using the manufacturing methods of Patent Documents 1 and 2. In particular, it is difficult to apply the same technique as in Patent Documents 1 and 2 to dissimilar metals having similar oxidation-reduction potentials, such as nickel and cobalt.

On the other hand, the present inventors examined and reported on a Cu—Ni alloy having copper as a core and nickel having a lower reduction potential than copper in the shell (Non-Patent Documents 1 and 2). According to Non-Patent Document 2 in which the investigation of Non-Patent Document 1 was further advanced, the Cu-Ni alloy was prepared by mixing copper formate and nickel formate separately with oleylamine, and the copper formate mixture at room temperature. The nickel formate mixed solution is complexed by heating at 393 K to prepare a precursor, mixed with 1-octal, and then rapidly heated with microwaves to obtain nanoparticles. Copper and nickel redox potential difference from the reducing at different temperatures and particle generation occurs in the ^{(Cu 2+ 433K, Ni 2+ 463K} ), copper nanoparticles generated previously, the copper nano-particles as nuclei as raising the temperature It is considered that a nickel shell is formed on the surface. It has been confirmed that the obtained nanoparticles have a high nickel concentration in the vicinity of the particle surface.

JP 2007-224420 A Japanese Patent Laid-Open No. 2003-055703

Tomohiro Yamauchi, Yasunori Tsukahara, Yuji Wada, “Synthesis and Magnetic Properties of Cu-Ni Alloy Nanoparticles Using Microwave Heating”, Proceedings of the 89th Annual Meeting of the Chemical Society of Japan (2009), 2D2-34 Tomohiro Yamauchi, Yasunori Tsukahara, Yuji Wada, and 1 others, “Synthesis and magnetic properties of Cu core-Ni shell nanoparticles using microwaves”, Proceedings of the 3rd Symposium of Japan Electromagnetic Energy Application, P190, 2009 November 18-20

  An object of the present invention is to provide nickel-cobalt nanoparticles, in which nickel forms a core and cobalt forms a shell layer, and a method for producing the same, which can be suitably used for applications such as catalysts, magnetic materials, and electrodes. .

  The nickel-cobalt nanoparticles according to the present invention include a core made of nickel and a shell made of cobalt substantially covering the entire surface of the core.

  The nickel-cobalt nanoparticles according to the present invention preferably have a nickel content in the range of 30 to 90% by mass, a cobalt content in the range of 10 to 70% by mass, and an average particle size. It is in the range of 10 to 200 nm, and the thickness of the shell is in the range of 1 to 50 nm.

  The method for producing nickel-cobalt nanoparticles according to the present invention includes a step of heating a mixture containing a nickel salt, a cobalt salt and a primary amine to obtain a complexing reaction solution, and heating the complexing reaction solution. Obtaining a nickel-cobalt nanoparticle slurry.

  The method for producing nickel-cobalt nanoparticles according to the present invention includes a step of heating a mixture containing nickel fine particles, a cobalt salt and a primary amine to obtain a complexing reaction solution, and heating the complexing reaction solution. Obtaining a nickel-cobalt nanoparticle slurry.

  The method for producing nickel-cobalt nanoparticles according to the present invention includes a step of heating a mixture containing a cobalt salt and a primary amine to obtain a complexing reaction solution, and adding nickel fine particles to the complexing reaction solution. Heating to obtain a nickel-cobalt nanoparticle slurry.

  In the method for producing nickel-cobalt nanoparticles according to the present invention, preferably, the nickel salt and the cobalt salt are each represented by a linear carboxylic acid group having 1 to 3 carbon atoms or the following structural formula (1). It is characterized by having any one of the following groups.

(Here, the substituents Ra to Rc are groups selected from the group consisting of methyl, ethyl, phenyl and halogen. The substituents Ra to Rc may be the same or different from each other.)

  In the method for producing nickel-cobalt nanoparticles according to the present invention, preferably, the carboxylic acid group is a formic acid group or an acetic acid group, and the group represented by the structural formula (1) is acetylacetonate.

  Moreover, the method for producing nickel-cobalt nanoparticles according to the present invention preferably uses microwaves as a heating means in the step of obtaining the nickel-cobalt nanoparticle slurry.

  Since the nickel-cobalt nanoparticles according to the present invention are composed of a nickel core and a cobalt shell, they are suitable for applications such as catalysts, magnetic materials, and electrodes. Further, by adjusting the cobalt content according to these applications, the cost can be made appropriate, and the performance of cobalt can be maximized in each of the above applications.

  Moreover, according to the manufacturing method of the nickel-cobalt nanoparticle which concerns on this invention, the nickel-cobalt nanoparticle which concerns on this invention can be obtained suitably.

It is a figure which shows the structure of each nickel acetate complex, (a) is a bidentate coordination, (b) is a monodentate coordination, (c) shows the state which carboxylate ion coordinated to the outer sphere, respectively. 2 is a transmission electron microscope (TEM) photograph of the nickel-cobalt nanoparticles obtained in Example 1. FIG. It is a figure which shows the electron diffraction (ED: Electron diffraction) pattern from the TEM photograph area | region of FIG. 2A. It is a figure which shows the mapping image by STEM-EDS of the nickel-cobalt nanoparticle obtained in Example 1. FIG. 2 is a diagram showing the results of a line analysis by STEM-EDS of nickel-cobalt nanoparticles obtained in Example 1. FIG. It is a figure which shows the result of the SQUID magnetic susceptibility measurement of the nickel- cobalt nanoparticle of Example 1, 2 and the nickel particle of the comparative example 8, and a commercially available cobalt particle. It is a figure which shows the mapping image by STEM-EDS of the nickel-cobalt nanoparticle obtained in Example 2. FIG. It is a figure which shows the mapping image by STEM-EDS of the nickel-cobalt nanoparticle obtained in Example 4.

  Embodiments of the present invention will be described below.

[Nickel-cobalt nanoparticles]
The nickel-cobalt nanoparticles according to the present embodiment (hereinafter sometimes simply referred to as “nanoparticles”) are substantially a core made of nickel and substantially cobalt substantially covering the entire surface of the core. Includes a shell consisting of Here, the nanoparticles do not exclude the form in which the core contains a small amount of cobalt and the shell contains a small amount of nickel. In other words, “substantially nickel core” means that a small amount of cobalt inevitably contained in the core is allowed, but most of it consists of nickel, and “substantially cobalt shell”. "Means that a small amount of nickel inevitably contained in the shell is allowed, but most consists of cobalt. Moreover, it is desirable that the nanoparticle shell covers the entire surface of the core. However, the present invention is not limited to this, and does not exclude a form in which nickel is partially exposed. That is, “substantially covering the entire surface of the core” means not only a form in which the shell completely covers the core, but also a discontinuous formation of the shell within a range that does not impair the effect of the present invention (the function of the nanoparticles). This means that the core is partially exposed, but most of the surface of the nickel-cobalt nanoparticles consists of a shell. Moreover, the structure which has an inner shell between the shell of cobalt and nickel may be sufficient as a nanoparticle.

  The nanoparticles according to the present embodiment have, for example, a nickel content of 30 to 90% by mass and a cobalt content of 10 to 70% by mass. The cobalt content of the nanoparticles can be appropriately set in consideration of the characteristics required for the application to be applied, cost, etc., but if it is less than 10% by mass, for example, the core is greatly exposed from the shell, and the catalyst performance and magnetic characteristics are reduced. The effect may not be sufficiently obtained. On the other hand, if it exceeds 70% by mass, the cost increases.

  The average particle diameter of the nanoparticles is, for example, 10 to 200 nm, preferably 10 to 150 nm. If the average particle size of the nanoparticles is less than 10 nm, the agglomeration is severe and the dispersibility is deteriorated, which may make it unsuitable as a paste material such as a catalyst or an electrode. On the other hand, if the average particle diameter of the nanoparticles exceeds 200 nm, the specific surface area becomes small, and for example, high catalyst performance may not be exhibited. In addition, the nanoparticles preferably have a Cv value [coefficient of variation; ratio σ / d of average particle diameter (d) to standard deviation (σ)] of 0.01 ≦ σ / d ≦ 0.5.

  The thickness of the nanoparticle shell is, for example, 1 to 50 nm, preferably 5 to 20 nm. If the thickness of the nanoparticle shell layer is less than 1 nm, the amount of cobalt is so small that the catalytic activity and magnetic properties inherent in cobalt may not be sufficiently exhibited. On the other hand, if it exceeds 50 nm, the cost increases.

  Nanoparticles have various shapes such as spherical, pseudo-spherical, spheroid, cubic, truncated tetrahedral, dihedral pyramidal, octahedral, icosahedral, and icosahedral. However, for example, from the viewpoint of improving the packing density when nickel nanoparticles are used for an electrode of an electronic component, a spherical shape or a pseudospherical shape is preferable, and a spherical shape is more preferable. Here, the shape of the nanoparticles can be confirmed by observing with a scanning electron microscope (SEM).

  The nanoparticles according to the present embodiment described above can adjust the cobalt content according to the use of the catalyst, magnetic material, electrode, etc., to make the cost appropriate, and also draw out the performance of cobalt suitably. be able to.

[Method for producing nickel-cobalt nanoparticles]
Next, a method for producing nickel-cobalt nanoparticles for suitably obtaining nickel-cobalt nanoparticles will be described. A first example to a third example are given as a method for producing nickel-cobalt nanoparticles according to the present embodiment (hereinafter, sometimes simply referred to as “a method for producing nanoparticles”).

<First example>
A first example of a method for producing nanoparticles includes a step of heating a mixture containing a nickel salt, a cobalt salt and a primary amine to obtain a complexing reaction solution, and heating the complexing reaction solution to obtain nickel-cobalt nanoparticles. Obtaining a slurry.

<Second example>
A second example of the method for producing nanoparticles includes a step of heating a mixture containing nickel fine particles, a cobalt salt and a primary amine to obtain a complexing reaction solution, and heating the complexing reaction solution to obtain nickel-cobalt nanoparticles. Obtaining a slurry. That is, nickel fine particles are used in place of the nickel salt of the first example.

<Third example>
A third example of the method for producing nanoparticles includes a step of heating a mixture containing a cobalt salt and a primary amine to obtain a complexing reaction solution, adding nickel fine particles to the complexing reaction solution, and then heating to obtain nickel. A step of obtaining a cobalt nanoparticle slurry; That is, the nickel fine particles of the second example are added not in the complexing reaction solution generation process but in the nickel-cobalt nanoparticle slurry generation process.

  Both the second example and the third example are preferred embodiments. However, when nickel fine particles are added, it is necessary to highly disperse the nickel fine particles in the complexing reaction solution. If sufficient, the cobalt shell may not be formed uniformly. In that sense, the first example that does not require high dispersion processing is the most preferable method.

[Step of obtaining complexing reaction solution]
In this step, a mixture containing nickel salt, cobalt salt and primary amine is heated (first example), or a mixture containing nickel fine particles, cobalt salt and primary amine is heated (second example), Or the complex containing nickel and / or cobalt is produced | generated by heating the mixture containing a cobalt salt and a primary amine (3rd example), and a complexing reaction liquid is obtained. Here, the complexing reaction liquid refers to a reaction product liquid (reaction product) generated by a reaction between a nickel salt and / or a cobalt salt and a primary amine. It is considered that the complexing reaction solution contains, as a complex, a nickel complex and a cobalt complex in the first example, and a cobalt complex in the second example and the third example. By heating the complexing reaction solution, the nickel ions and / or cobalt ions of the nickel complex and / or cobalt complex are reduced, and the carboxylate ions coordinated to the ions are simultaneously decomposed, and finally zero-valent. Ni and / or zero-valent Co are formed, and core-shell structured nanoparticles are formed.

(Nickel salt, cobalt salt)
The nickel salt and the cobalt salt are not particularly limited, but either one or both of a linear carboxylic acid group having 1 to 3 carbon atoms or a group represented by the following structural formula (1) is used. What has is preferable. The group represented by the following structural formula (1) is a 1,3-diketonato group (β-diketonato ligand). The nickel salt and cobalt salt may be the same or different salts. Further, both the nickel salt and the cobalt salt may be anhydrides or hydrates.

(Here, the substituents Ra to Rc are groups selected from the group consisting of methyl, ethyl, phenyl and halogen. The substituents Ra to Rc may be the same or different from each other.)

  Specifically, the nickel salt having a linear carboxylic acid group having 1 to 3 carbon atoms is nickel formate, nickel acetate or nickel propionate, and it is particularly preferable to use nickel formate or nickel acetate. The cobalt salt having a straight-chain carboxylic acid group having 1 to 3 carbon atoms is specifically cobalt formate, cobalt acetate or cobalt propionate, and it is particularly preferable to use cobalt formate or cobalt acetate. These nickel carboxylate and cobalt carboxylate may be anhydrides or hydrates. In place of nickel carboxylate or cobalt carboxylate, nickel chloride (cobalt chloride), nickel nitrate (cobalt nitrate), nickel sulfate (cobalt sulfate), nickel carbonate (cobalt carbonate), nickel hydroxide (cobalt hydroxide), etc. It is also possible to use inorganic salts, but in the case of inorganic salts, dissociation (decomposition) is a high temperature, so that in the process of reducing the dissociated nickel ions (or nickel complexes) and cobalt ions (or cobalt complexes), This is not preferable because heating at a high temperature is required.

  Examples of the group represented by the structural formula (1) include 2,4-pentadionato (also known as acetylacetonato), 2,4-hexadionato, 3,5-heptadionate, 1-phenyl-1,3-butanedionate, 1 -Chlor-1,3-butanedionato and the like are mentioned, among which acetylacetonate is preferably used.

  The blending amount of the nickel salt and the cobalt salt is, for example, 30 to 90 parts by weight of nickel and 10 to 70 parts by weight of cobalt with respect to 100 parts by weight of the total amount of nickel and cobalt in the complexing reaction solution. It is preferable that nickel is 50 to 80 parts by mass and cobalt is 20 to 50 parts by mass. Although the compounding quantity of cobalt can be suitably set in consideration of the characteristics and cost required for the use of the nanoparticles, when it is less than 10 parts by mass with respect to the total amount of nickel and cobalt of 100 parts by mass, for example, There is a possibility that the core is greatly exposed from the shell and the effect of the catalyst performance and magnetic characteristics may not be sufficiently obtained. On the other hand, when it exceeds 70 parts by mass, the cost becomes high.

(Nickel fine particles)
The nickel fine particles used in the second and third examples of the nanoparticle production method are not limited in size, but for example, those having a particle size of 5 to 200 nm are preferably used. When the particle size is less than 5 nm, aggregation is severe and there is a possibility that it is difficult to disperse even in the liquid. On the other hand, when the particle size exceeds 200 nm, the specific surface area of the particles becomes too small, and there is a possibility that sufficient reaction is not performed. The shape of the nickel fine particles is preferably, for example, spherical or pseudospherical, but most preferably spherical.

  The addition amount of the nickel fine particles is preferably 30 to 90 parts by mass, and 50 to 80 parts by mass with respect to 100 parts by mass of the total amount of nickel and cobalt in the complexing reaction solution in terms of metal. Is more preferable.

(Primary amine)
The primary amine can form a complex with a nickel ion or a cobalt ion, and effectively exhibits a reducing ability for the nickel complex (or nickel ion). On the other hand, secondary amines have large steric hindrance, which may hinder good formation of nickel complexes and cobalt complexes, and tertiary amines cannot be used because they do not have the ability to reduce nickel ions and cobalt ions.

  The primary amine is not particularly limited as long as it can form a complex with nickel ions or cobalt ions, and can be solid or liquid at room temperature. Here, room temperature means 20 ° C. ± 15 ° C. The primary amine that is liquid at room temperature also functions as an organic solvent when forming a nickel complex or a cobalt complex. In addition, even if it is a primary amine solid at normal temperature, there is no particular problem as long as it is liquid by heating at 100 ° C. or higher, or can be dissolved using an organic solvent.

  The primary amine also functions as a dispersing agent and can favorably disperse the nickel complex or cobalt complex in the reaction solution. Therefore, after the complex formation, the nickel complex or cobalt complex is heated and decomposed to obtain nanoparticles. Aggregation of particles at the time can be suppressed. The primary amine may be an aromatic primary amine, but an aliphatic primary amine is preferred from the viewpoint of ease of nickel complex formation and cobalt complex formation in the reaction solution. The aliphatic primary amine can control the particle size of the produced nanoparticles, for example, by adjusting the length of its carbon chain, and is particularly advantageous when producing nanoparticles having an average particle size of 10 to 200 nm. It is. From the viewpoint of controlling the particle size of the nanoparticles, the aliphatic primary amine is preferably selected from those having about 6 to 20 carbon atoms. The larger the number of carbons, the smaller the particle size of the resulting nanoparticles. Examples of such amines include octylamine, trioctylamine, dioctylamine, hexadecylamine, dodecylamine, tetradecylamine, stearylamine, oleylamine, myristylamine, and laurylamine. For example, oleylamine exists in a liquid state under the temperature conditions in the nanoparticle production process, and therefore can efficiently proceed with a reaction in a homogeneous solution.

Since the primary amine functions as a surface modifier during the production of the nanoparticles, secondary aggregation can be suppressed even after removal of the primary amine. The primary amine is also preferable from the viewpoint of ease of processing operation in the washing step of separating the solid component of the produced nanoparticles after the reduction reaction and the solvent or the unreacted primary amine and the like. Furthermore, the primary amine is preferably one having a boiling point higher than the reduction temperature from the viewpoint of ease of reaction control when reducing the nickel complex or cobalt complex to obtain nanoparticles. That is, the aliphatic primary amine preferably has a boiling point of 200 ° C. or higher, and preferably has 9 or more carbon atoms. Here, for example, the boiling point of C ₉ H ₂₁ N (nonylamine) of an aliphatic amine having 9 carbon atoms is 201 ° C. The amount of the primary amine is preferably 2 mol or more, more preferably 2.5 mol or more, and more preferably 4 mol or more with respect to 1 mol of the total amount of nickel and cobalt in terms of metal. The upper limit of the amount of primary amine is not particularly limited, but for example, from the viewpoint of productivity, it is preferably about 20 mol or less with respect to 1 mol of the total amount of nickel and cobalt in terms of metal.

A divalent nickel ion is known as a ligand-substituted active species, and the ligand of the complex to be formed may easily change in complex formation by ligand exchange depending on temperature and concentration. For example, in the step of obtaining a reaction liquid by heating a mixture of nickel carboxylate and primary amine, considering steric hindrance such as the carbon chain length of the amine used, for example, carboxylate ions (R ₁ COO as shown in FIG. 1) , R ₂ COO) may be coordinated by either bidentate coordination (a) or monodentate coordination (b), and carboxylate ions are present in the outer sphere when the amine concentration is in large excess. The structure (c) to be taken may be taken. In order to obtain a homogeneous solution at the target reaction temperature (reduction temperature), at least one of the ligands of A, B, C, D, E, and F must be coordinated with a primary amine. . In order to take this state, it is necessary that the primary amine is excessively present in the reaction solution, and it is preferable that at least 2 mol per 1 mol of nickel ions is present, and 2.5 mol or more exist. It is more preferable that 4 mol or more is present.

  Cobalt ions are considered to behave like nickel ions and form complexes. For this reason, the primary amine needs to be present in an excessive amount with respect to the cobalt ion, and is preferably present at least 2 mol, more preferably 2.5 mol or more, relative to 1 mol of nickel ion, It is desirable that 4 mol or more exists.

  Although the complexing reaction can proceed even at room temperature, it is preferable to perform heating at a temperature of 100 ° C. or higher in order to perform the reaction reliably and more efficiently. This heating is particularly advantageous when nickel carboxylate hydrate such as nickel acetate tetrahydrate or cobalt carboxylate hydrate is used as the nickel salt or cobalt salt. The heating temperature is preferably a temperature exceeding 100 ° C., more preferably a temperature of 105 ° C. or more, so that the ligand substitution between the coordinated water coordinated with nickel carboxylate or cobalt carboxylate and primary amine is performed. The reaction is carried out efficiently, the water molecules as the complex ligand can be dissociated, and the water can be discharged out of the system, so that the complex can be formed efficiently. For example, nickel acetate tetrahydrate has a complex structure in which two coordinated water, two acetate ions that are bidentate ligands, and two water molecules exist in the outer sphere at room temperature. In order to efficiently complex the two coordinated water and the primary amine by ligand substitution, it is preferable to dissociate the water molecule as the complex ligand by heating at a temperature higher than 100 ° C. In addition, the heating temperature is 175 ° C. or lower from the viewpoint of surely separating from the subsequent heat reduction process of the nickel complex (or nickel ion) and cobalt complex (or cobalt ion) and completing the preceding complex formation reaction. preferable. Therefore, it is preferable to perform the heating at the time of complex formation at, for example, 105 to 175 ° C. More preferably, heating temperature is 125-160 degreeC.

  The heating time can be appropriately determined according to the heating temperature and the content of each raw material, but is preferably 15 minutes or longer from the viewpoint of reliably completing the complex formation reaction. Although there is no upper limit on the heating time, heating for a long time is useless from the viewpoint of saving energy consumption and process time. The heating method is not particularly limited, and may be heating by a heat medium such as an oil bath or heating by microwave irradiation.

  The complex formation reaction can be confirmed by a change in the color of the solution when the solution obtained by mixing the nickel salt and / or cobalt salt and the primary amine is heated. In addition, this complex formation reaction is carried out by measuring the absorption maximum wavelength of the absorption spectrum observed in the wavelength region of 300 nm to 750 nm using, for example, an ultraviolet / visible absorption spectrum measuring apparatus, and measuring the maximum absorption wavelength of the raw material (for example, nickel acetate). In tetrahydrate, the maximum absorption wavelength is 710 nm.), And this can be confirmed by observing the shift of the reaction solution with respect to.

  After the complex formation between the nickel salt and / or cobalt salt and the primary amine is performed, the resulting reaction solution is heated by a method such as microwave irradiation, as described later, so that nickel of the nickel complex is obtained. Nanoparticles containing nickel and / or cobalt having an oxidation number of zero and finally decomposing the cobalt ions of the ions and / or cobalt complex and simultaneously decomposing nickel ions and carboxylate ions coordinated to cobalt ions. Particles are generated. In general, nickel carboxylate and cobalt carboxylate are hardly soluble under conditions other than using water as a solvent. As a pre-stage of the heat reduction reaction by microwave irradiation, a solution containing nickel carboxylate or cobalt carboxylate is a homogeneous reaction solution. There is a need to. On the other hand, the primary amine used in the present embodiment is liquid at the operating temperature conditions, and is considered to be liquefied by coordination with nickel ions or cobalt ions to form a uniform reaction solution. It is done.

(Organic solvent)
In order to proceed the reaction in a homogeneous solution more efficiently, an organic solvent other than the primary amine may be newly added. When using an organic solvent, the organic solvent may be mixed with the nickel salt or cobalt salt and the primary amine at the same time, but when the nickel salt or cobalt salt and the primary amine are first mixed and complexed, the organic solvent is added. Primary amines are more preferable because they efficiently coordinate to nickel ions and cobalt ions. The organic solvent that can be used is not particularly limited as long as it does not inhibit the complex formation between the primary amine and the nickel ion or cobalt ion. For example, the ether-based organic solvent having 4 to 30 carbon atoms, 7 carbon atoms. -30 saturated or unsaturated hydrocarbon organic solvents, alcohol organic solvents having 8 to 18 carbon atoms, and the like can be used. Further, from the viewpoint of enabling use even under heating conditions such as by microwave irradiation, it is preferable to select an organic solvent having a boiling point of 170 ° C. or higher, more preferably within a range of 200 to 300 ° C. It is better to choose something. Specific examples of such an organic solvent include tetraethylene glycol and n-octyl ether.

[Step of obtaining nickel-cobalt nanoparticle slurry]
In this step, the complexing reaction solution is heated to reduce the nickel complex and / or cobalt complex (nickel ion and / or cobalt ion) to a metal to generate nanoparticles. For example, in the first example, when a mixture of nickel complex and cobalt complex is reduced and thermally decomposed, the nickel complex is thermally decomposed at a lower temperature than the cobalt complex and reduced by the amine, and the nickel particles Is considered to be a core, and a cobalt shell is formed on the surface of the core. Usually, a cobalt complex is dissociated by thermal decomposition, and Co (zero-valent) nanoparticles are formed from the Co ²⁺ complex. On the other hand, in the present invention, in the presence of excess oleylamine, the cobalt complex is reduced from Co ²⁺ to Co (valent 0) at a lower temperature than usual (complex dissociates) using Ni as a catalyst.

  By heating the complexing reaction solution, the complexing reaction solution is reduced and the complex is thermally decomposed. The heating temperature is preferably 200 ° C. or higher, more preferably 220 ° C. or higher, from the viewpoint of efficiently performing the reduction reaction. From the viewpoint of efficiently performing the treatment, it is preferably 270 ° C. or lower, more preferably about 250 ° C. or lower. Taking nickel as an example, in order to produce nanoparticles having a uniform particle size, a nickel complex is uniformly and sufficiently produced in the step of obtaining a complexing reaction solution, and a complexation reaction solution is obtained. It is necessary to simultaneously generate and grow Ni (zero-valent) nuclei generated by reduction of nickel ions in the step of heating nickel to obtain a nickel-cobalt nanoparticle slurry. That is, by adjusting the heating temperature in the step of obtaining the complexing reaction liquid within the above specific range and keeping it lower than the heating temperature in the step of obtaining the nickel-cobalt nanoparticle slurry, the particle size and shape Are easy to produce. For example, if the heating temperature is too high in the step of obtaining a complexing reaction solution, the formation of a nickel complex and the reduction reaction to Ni (zero valence) proceed simultaneously, and the particles having a uniform particle shape are obtained in the step of obtaining a nickel-cobalt nanoparticle slurry. May be difficult to generate. In addition, if the heating temperature in the step of obtaining the nickel-cobalt nanoparticle slurry is too low, the reduction reaction rate to Ni (zero valence) becomes slow and the generation of nuclei is reduced. It is not preferable also in terms of rate.

  In this step, the heat source may be an oil bath or the like, but is preferably a microwave. When the complexing reaction liquid is irradiated with microwaves, the microwave penetrates into the complexing reaction liquid, and rapid heating and uniform heating are performed by internal heating. As a result, the entire complexing reaction solution can be made uniform at a desired temperature, and the reduction, nucleation, and nucleation processes of nickel and cobalt particles occur simultaneously in the entire solution, resulting in a narrow particle size distribution. Monodispersed particles can be easily produced in a short time. In addition, the use wavelength of a microwave is not specifically limited, For example, it is 2.45 GHz.

  The nanoparticle slurry obtained by heating the complexing reaction liquid is, for example, allowed to stand and separate, after removing the supernatant liquid, washed with an appropriate solvent, and dried to obtain nanoparticles.

  In the step of obtaining the nickel-cobalt nanoparticle slurry, the above-mentioned organic solvent may be added to the complexing reaction solution as necessary. As described above, it is a preferred embodiment of the present invention to use the primary amine used in the complex formation reaction as an organic solvent as it is. The solvent added as needed in the step of heating the complexing reaction solution is not particularly limited, and for example, alcohol such as octanol (octyl alcohol), nonpolar solvent, or the like can be used. As described above, when oleylamine is used as the primary amine, the solvent can be omitted.

  The manufacturing method of the nanoparticle of this Embodiment can include arbitrary processes other than the said process. In addition, for example, as described later, an arbitrary treatment such as addition of a surface modifier can be performed. Moreover, since the manufacturing method of the nanoparticle of this Embodiment employ | adopts the reduction | restoration method by the heating using a microwave etc. in the process of obtaining a nickel-cobalt nanoparticle slurry, use of a strong reducing agent is unnecessary. However, it does not prevent the presence of a substance having a reducing action in the complex-forming reaction solution as long as the effects of the invention are not impaired.

(Addition of surface modifier)
In the method for producing nanoparticles according to the present embodiment, as a surface modifier for controlling the particle size of the nanoparticles, for example, polymer resins such as polyvinylpyrrolidone (PVP), polyethyleneimine, polyacrylamide, myristic acid, olein A long-chain carboxylic acid such as an acid or a carboxylate can be added. However, if the surface modification amount of the obtained nanoparticles is large, there is a risk of adverse effects as impurities depending on the use. Therefore, the surface modification amount after washing the obtained nanoparticles is preferably as small as possible. For example, when used as a conductive paste for nickel electrodes, if nickel particles are pasted and fired at a high temperature, the packing density is reduced, and delamination or cracks may occur. Accordingly, the amount of the surface modifier added is preferably in the range of 0.1 to 100 parts by mass with respect to 100 parts by mass of the total amount of nickel elements. The surface modifier may be added at the stage of the mixture of nickel carboxylate and primary amine in the complexing reaction liquid forming step, or may be added to the complexing reaction liquid obtained in the complexing reaction liquid forming step. Preferably, the timing of addition is after the complexing reaction or after the formation of nickel nanoparticles.

  By the method for producing nickel-cobalt nanoparticles according to the present embodiment described above, the average particle diameter is 10 to 200 nm, and the Cv value [coefficient of variation; ratio of average particle diameter (d) to standard deviation (σ) σ / d] has a narrow particle size distribution of 0.01 ≦ σ / d ≦ 0.5, and nickel-cobalt nanoparticles composed of a nickel core and a cobalt shell can be obtained.

  EXAMPLES The present invention will be further described with reference to examples and comparative examples, but the present invention is not limited to the examples described below.

  As for the particle size of the nanoparticles, a photograph of the nanoparticle powder was taken with a transmission electron microscope (TEM), 200 particles were randomly extracted from them, and the average particle size and the standard deviation were obtained. Further, a Cv value (= σ / d) value was obtained from this result. Further, the presence of nickel or cobalt in the obtained particles or their respective concentrations were confirmed by surface analysis and line analysis using a scanning transmission electron microscope (STEM-EDS) equipped with an energy dispersive X-ray analyzer. For the thickness of the cobalt shell layer, as shown in FIG. 4, the nickel and cobalt atom concentrations on a line crossing one particle were calculated by STEM-EDS. This line analysis method cannot calculate the atomic concentration of only the core portion of the generated spherical particles. That is, the analysis in the vicinity of the particle surface shows a simple concentration on the particle surface, but the analysis result in the vicinity of the center of the particle is the sum of the concentration of the metal element on the particle surface and the inside (core). Therefore, in the result of this analysis, the average value of the thickness of the layer in which the strength of cobalt is higher than that of nickel is defined as the thickness of the cobalt shell layer. The composition of the obtained particles was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

Example 1
Oleylamine (275 mmol) was added to cobalt formate dihydrate (12.5 mmol) and nickel acetate tetrahydrate (12.5 mmol), followed by heating at 120 ° C. for 20 minutes under a nitrogen flow to obtain a complexing reaction solution. Next, the complexing reaction solution was heated to 225 ° C. using a microwave, and the temperature was maintained for 30 minutes to obtain a nanoparticle slurry. The nanoparticle slurry was allowed to stand and separated, and the supernatant was removed, followed by washing with hexane three times. Then, it dried with the vacuum dryer maintained at 60 degreeC for 6 hours, and obtained the nanoparticle.

  A TEM (Transmission Electron Microscope) transmission electron microscope (TEM) photograph of the obtained nanoparticles is shown in FIG. 2A, and an ED (Electron diffraction) pattern is shown in FIG. 2B. Spherical uniform particles having an average particle diameter of 82 nm were formed. It can also be seen from the ED pattern that the nanoparticles are fcc-structured metals that do not contain oxides. Moreover, when the metal composition of the particle | grains obtained by ICP-AES was confirmed, since Ni and Co were 47.6 mass% and 47.3 mass%, respectively, mol ratio is Ni / Co = 1.0. Yes, it was in good agreement with the raw material ratio (Ni salt / Co salt charge ratio (mol%)). A photograph of the STEM-EDS mapping image is also shown in FIG. Since nickel is distributed in the center of the nanoparticle and cobalt is distributed on the surface of the nanoparticle, it can be seen that the nanoparticle has a Ni (core) -Co (shell) structure.

  Moreover, from the result of the particle analysis shown in FIG. 4, it can be seen that the thickness of the cobalt shell layer (shell) is approximately 13 nm. Furthermore, the result of the saturation magnetization (unit: emu / g) obtained by SQUID magnetic susceptibility measurement is shown in FIG. Although the saturation magnetization is lower than that of commercially available cobalt nanoparticles (particle size of 50 nm or less, manufactured by Aldrich), it can be seen that the saturation magnetization is significantly increased as compared with the nanoparticles of Comparative Example 8 described later.

(Examples 2-7, Comparative Examples 1-8)
In Examples 2 to 7 and Comparative Examples 1 to 8, according to Example 1 except that the kind of nickel salt and cobalt salt and the kind of heating source and reaction temperature (heating temperature) in the step of obtaining the nanoparticle slurry were changed. Nanoparticles were prepared. The results are shown in Table 1 together with Example 1. In each example and each comparative example, the oleylamine / (Ni salt + Co salt) molar ratio is 10 in all cases. Moreover, the heating method of the complexing reaction liquid was all performed by microwave heating except Example 3 using an oil bath.

  In addition, the result of the SQUID magnetic susceptibility measurement of the nickel-cobalt nanoparticles of Examples 1 and 2, the nickel particles of Comparative Example 8 and the commercially available cobalt particles is shown in FIG. A TEM photograph is shown in FIG. 6, and a TEM photograph of the nickel-cobalt nanoparticles obtained in Example 4 is shown in FIG.

  From Table 1, it is confirmed that nickel-cobalt nanoparticles having an average particle diameter of 10 to 200 nm and a cobalt shell layer of 1 to 50 nm can be obtained by using formic acid, acetate or acetylacetone salt as nickel salt and cobalt salt. It was done. From FIG. 5, it was confirmed that the nickel-cobalt nanoparticles of Examples 1 and 2 were close to the saturation magnetization of cobalt.

(Example 8)
12.5 mmol of cobalt formate dihydrate and 25 mmol of nickel particles having an average particle size of 100 nm and a Cv value of 0.14 (no cobalt salt is used and complexed from nickel acetate tetrahydrate according to the first example above) A complexation reaction solution was obtained by adding 125 mmol of oleylamine to particles prepared by heating and heating it, and then heating it at 120 ° C. for 20 minutes under a nitrogen flow. Next, the complexing reaction solution was heated to 225 ° C. using a microwave, and the temperature was maintained for 30 minutes to obtain a nickel-cobalt nanoparticle slurry.

  The nickel-cobalt nanoparticle slurry was allowed to stand and separated, and the supernatant was removed, followed by washing with hexane three times. Then, it dried for 6 hours with the vacuum dryer maintained at 60 degreeC, and obtained the nickel-cobalt nanoparticle. The average particle diameter of the obtained nickel-cobalt nanoparticles was 120 nm, the Cv value of the particle size distribution was 0.15, and the thickness of the cobalt shell layer was 17 nm.

Example 9
125 mmol of oleylamine was added to 12.5 mmol of cobalt formate dihydrate and heated at 120 ° C. for 20 minutes under a nitrogen flow to obtain a complexing reaction solution. Next, 25 mmol of nickel particles having an average particle diameter of 100 nm and a Cv value of 0.14 were obtained in the complexing reaction solution (cobalt salt was not used, a complexing reaction was obtained from nickel acetate tetrahydrate, The prepared particles; Comparative Example 8) was added and stirred well, then heated to 225 ° C. using microwaves, and the temperature was maintained for 30 minutes to obtain a nickel-cobalt nanoparticle slurry.

  The nickel-cobalt nanoparticle slurry was allowed to stand and separated, and the supernatant was removed, followed by washing with hexane three times. Then, it dried for 6 hours with the vacuum dryer maintained at 60 degreeC, and obtained the nickel-cobalt nanoparticle. The average particle diameter of the obtained nickel-cobalt nanoparticles was 117 nm, the Cv value of the particle size distribution was 0.15, and the thickness of the cobalt shell layer was 15 nm.

As mentioned above, although embodiment of this invention was described in detail for the purpose of illustration, this invention is not restrict | limited to the said embodiment. This international application claims priority based on Japanese Patent Application No. 2010-60773 filed on Mar. 17, 2010, the entire contents of which are incorporated herein by reference.

Claims (7)

The nickel content is in the range of 30 to 90 mass%, the cobalt content is in the range of 10 to 70 mass%, the average particle size is in the range of 10 to 200 nm, and the core made of nickel, A shell made of cobalt covering the entire surface of the core, and a method for producing nickel-cobalt nanoparticles having a thickness of the shell in the range of 1 to 50 nm,
Heating a mixture containing a nickel salt, a cobalt salt and a primary amine to obtain a complexing reaction solution;
Heating the complexing reaction solution to obtain a nickel-cobalt nanoparticle slurry;
A method for producing nickel-cobalt nanoparticles, comprising: The nickel content is in the range of 30 to 90 mass%, the cobalt content is in the range of 10 to 70 mass%, the average particle size is in the range of 10 to 200 nm, and the core made of nickel, A shell made of cobalt covering the entire surface of the core, and a method for producing nickel-cobalt nanoparticles having a thickness of the shell in the range of 1 to 50 nm,
Heating a mixture containing nickel fine particles, cobalt salt and primary amine to obtain a complexing reaction solution;
Heating the complexing reaction solution to obtain a nickel-cobalt nanoparticle slurry;
A method for producing nickel-cobalt nanoparticles, comprising: The nickel content is in the range of 30 to 90 mass%, the cobalt content is in the range of 10 to 70 mass%, the average particle size is in the range of 10 to 200 nm, and the core made of nickel, A shell made of cobalt covering the entire surface of the core, and a method for producing nickel-cobalt nanoparticles having a thickness of the shell in the range of 1 to 50 nm,
Heating a mixture containing a cobalt salt and a primary amine to obtain a complexing reaction solution;
Adding nickel fine particles to the complexing reaction solution and then heating to obtain a nickel-cobalt nanoparticle slurry; and
A method for producing nickel-cobalt nanoparticles, comprising: The nickel-cobalt nanoparticles according to claim 1, wherein the nickel salt has any one of a linear carboxylic acid group having 1 to 3 carbon atoms and a group represented by the following structural formula (1). Manufacturing method.
(Here, the substituents Ra to Rc are groups selected from the group consisting of methyl, ethyl, phenyl and halogen. The substituents Ra to Rc may be the same or different from each other.) The cobalt salt has any one of a straight-chain carboxylic acid group having 1 to 3 carbon atoms and a group represented by the following structural formula (1). The manufacturing method of the nickel-cobalt nanoparticle of description.
(Here, the substituents Ra to Rc are groups selected from the group consisting of methyl, ethyl, phenyl and halogen. The substituents Ra to Rc may be the same or different from each other.)   The nickel-cobalt according to claim 4 or 5, wherein the straight-chain carboxylic acid group having 1 to 3 carbon atoms is a formic acid group or an acetic acid group, and the group represented by the structural formula (1) is acetylacetonato. A method for producing nanoparticles.   The method for producing nickel-cobalt nanoparticles according to any one of claims 1 to 3, wherein a microwave is used as a heating means in the step of obtaining the nickel-cobalt nanoparticle slurry.