WO2020171025A1 - Nanoparticle and method for producing same - Google Patents

Abstract

[Problem] To provide a technique capable of easily and stably providing unstable (including metastable) metal oxide nanoparticles regardless of the type of metal oxide having a tendency to form an unstable phase, and regardless of the average nanoparticle size. [Solution] The present invention provides a nanoparticle having a metastable metal oxide deposited on the surface thereof. The nanoparticle is obtained by using a nano-sized metal oxide particle as a seed particle and performing a hydrothermal treatment on a metal oxide precursor in the presence of the seed particle under subcritical or supercritical water conditions to deposit a metastable metal oxide on the surface of the seed particle. The seed particle is preferably a metastable metal oxide, and the hydrothermal treatment is preferably performed in the present of an organic modifier.

Description

Nanoparticle and method for producing the same

The present invention relates to nanoparticles and a method for producing the same.

In recent years, nanotechnology has attracted attention as a material involved in a wide range of industries such as materials, information, and biotechnology. Nano and is a prefix indicating the amount of 10 ^-9 times the underlying units, 1 10 ⁹ minutes of 1 meter is 1 nanometer. Nanotechnology is science and technology that deals with such a minute world. Particles with a particle size on the order of nanometers, which were born from the fusion of nanotechnology and material science, are called "nanoparticles." Since nanoparticles have a quantum size effect and a large specific surface area, it is known that, unlike bulk and small molecules, they exhibit size-dependent optical characteristics, electromagnetic characteristics, and the like (Non-Patent Documents 1 and 2). Therefore, the application of nanoparticles is advancing in a wide range of fields such as physics, electronic engineering, information engineering, catalytic chemistry, and bioscience. In addition, there are various materials such as metals, metal oxides, and organic substances (Non-Patent Documents 3 and 4).

Nanoparticles can be synthesized by thermal plasma method (gas phase), spray pyrolysis method (gas phase), reverse micelle method (liquid phase), hot soap method (liquid phase), solvothermal method (liquid phase, supercritical). Phase), hydrothermal synthesis method (liquid phase, supercritical phase), and various industrial synthesis methods are known. In either method, the particles to be synthesized can obtain a stable phase under the temperature, pressure and solvent density.

By the way, many metal oxides have different crystal structures and different crystal forms. When the metal oxide is made into nanoparticles, for example, when the target metal oxide is iron(III) oxide, α-iron(III) oxide nanoparticles are obtained at a relatively low temperature.

On the other hand, when the average particle size of the nanoparticles is in the order of several nm or less, the phase state of the metal oxide is unstable due to the surface effect and the nanosize effect as compared with the case where the average particle size is in the order of several tens of nm or more. It is known that there may be a shift to the phase state.

Hiroshi Imahori et al. Nanotechnology (2010) NTS, Nanoparticle Science-From Basic Principle to Application-(2007) J. Prasad Rao et al., Progress in Polymer Science, 36 (2011) 887-913 Stankic et al., JNanobiotechnol, (2016) 14-73

However, even if the average particle size of the nanoparticles is set to the order of several nm or less, the main production phase of the produced nanoparticles will be a stable phase. For example, when the metal oxide is iron (III) oxide, it is extremely difficult to synthesize nanoparticles of γ-phase iron (III) oxide, which is an unstable phase. In addition, it is not possible to synthesize amorphous phase nanoparticles, which are unstable phases when the metal oxide is titanium (IV) oxide.

The present invention has been made in view of such a problem, regardless of the type of metal oxide to form an unstable phase, and regardless of the average particle size of the nanoparticles, unstable An object of the present invention is to provide a technique capable of simply and stably supplying metal oxide nanoparticles of a phase (including a metastable phase).

The present inventors have conducted extensive studies in order to achieve the above-mentioned object. As a result, nano-sized metal particles are used as seed particles, and in the presence of the seed particles, the metal oxide precursor is hydrothermally treated under critical water conditions. By doing so, it is possible to deposit a metastable metal oxide on the surface of the seed particles regardless of the type of metal oxide for which an unstable phase is desired to be formed, and regardless of the average particle size of the seed particles. The present invention has been completed and the present invention has been completed. Specifically, the present invention provides the following.

The invention according to the first feature provides nanoparticles in which a metastable phase metal oxide is deposited on the surface.

The invention according to the second feature provides nanoparticles having a deposition layer in which a metastable phase metal oxide is deposited, the deposition layer including an epitaxial layer, a polycrystalline layer, or an amorphous layer.

According to the inventions of the first and second characteristics, the metastable phase metal oxide nanoparticles, which have hitherto been impossible to realize, are not dependent on the type of the metal oxide for which an unstable phase is formed, It can be used regardless of the average particle size.

Further, since the metastable metal oxide is deposited on the surface of the nanoparticles, it can be applied to a wide range of applications such as semiconductor materials, catalyst materials, biomaterials, magnetic data storage, biosensing and drug delivery. ..

The invention according to the third feature provides the nanoparticles according to the first or second feature, wherein the coefficient of variation obtained by dividing the standard deviation of the average particle diameter by the average particle diameter is 0.15 or less.

Nanoparticles have different physical properties than bulk materials depending on their size. According to the invention of the third aspect, the particle size distribution of the nanoparticles is narrow and the particle size is strictly controlled. Therefore, the physical properties unique to the metastable phase metal oxide nanoparticles can be effectively exhibited. You can

In the invention according to the fourth feature, nano-sized particles of a metal oxide are used as seed particles, and the metal oxide precursor is hydrothermally treated under subcritical water conditions or supercritical water conditions in the presence of the seed particles. The method for producing nanoparticles comprises the step of depositing a metastable phase metal oxide on the surface of the seed particles.

According to the invention of the fourth feature, since the metal oxide precursor is hydrothermally treated in the presence of seed particles, the supersaturation degree/reaction rate can be efficiently controlled depending on the temperature. As a result, the metastable phase metal oxide nanoparticles, which have hitherto been impossible to achieve, are not affected by the type of metal oxide for which the unstable phase is formed, or by the average particle size. Can be supplied without.

By the way, nanoparticles have different physical properties than bulk materials depending on their size. In order to control the physical properties, it is important to control the particle size and shape. However, in principle, it is extremely difficult to obtain a uniform particle size by the synthesis method based on these nucleation and growth mechanisms.

According to the invention of the fourth aspect, since the seed particles are grown by introducing the particle precursor into the seed particles, the range of the particle size distribution of the nanoparticles is narrowed as compared with that before the growth of the seed particles. You can For example, the average particle size of the grown particles becomes 10 to 15 nm because the average particle size of the seed particles having a particle size distribution in the range of 5 nm to 10 nm and double the average particle size grows by 5 nm. The diameter range can be narrowed from 2 times to 1.5 times.

Note that even if the metal oxide precursor is subjected to hydrothermal treatment, the metal oxide precursor changes to a stable phase metal oxide in the absence of seed particles.

The fifth aspect of the invention is the production method according to the fourth aspect, wherein the seed particles are metastable phase metal oxides.

In a sixth aspect of the invention, in the invention according to the fourth or fifth aspect, the metal oxide forming the seed particles has the same chemical formula as the metal oxide forming the seed particles. This is a manufacturing method different from the lattice constant of the metal compound in the stable phase.

The invention according to a seventh feature is the invention according to any one of the fourth to sixth features, wherein the crystal structure of the metal oxide forming the seed particles is the same as that of the metal oxide forming the seed particles. The manufacturing method is different from the crystal structure of the metal compound in the stable phase, which is the chemical formula.

According to the fifth to seventh aspects of the invention, the seed particles are metastable metal oxides, and the lattice constant or crystal structure is different from that of the stable phase. Therefore, as compared with the case where the seed particles are a stable phase metal oxide, the metastable metal oxide of the same quality as the phase constituting the seed particles can be efficiently deposited on the surface of the seed particles.

The invention according to an eighth feature is the invention according to any one of the fourth to seventh features, wherein the content of the nanoparticles contained in the solution under the hydrothermal condition is 0.01 mol/l or more. , The manufacturing method.

According to the invention of the eighth feature, the total surface area of the seed particles supplied to the reactor can be ensured to be a certain amount or more, and when the metal oxide precursor is used for forming a film on the surface of the seed particles, excess metal It is possible to prevent the oxide precursor from remaining and to generate a stable phase metal oxide from the surplus metal oxide precursor.

A ninth aspect of the present invention is the production method according to the third or fourth aspect, wherein the concentration of the metal oxide precursor contained in the solution at the time of performing the hydrothermal treatment is 1 mol/l or less. Is.

▽When synthesizing nanoparticles in the liquid phase, it is known to use a re-dissolution/precipitation method called Ostwald Tryping. When this method is used, relatively small nanoparticles are redissolved and the redissolved nanoparticle components are deposited on relatively large nanoparticles, so that the width of the particle size distribution can be narrowed further.

According to the invention of the ninth feature, since the upper limit is set for the concentration of the metal oxide precursor contained in the solution when the hydrothermal treatment is performed, the principle of the redissolution/precipitation method is applied. Regardless, it is possible to prevent the metal hydroxide from becoming supersaturated in the solution, causing uniform nucleation of the metal oxide precursor, and conversely broadening the particle size distribution.

Also, the crystal growth by the generally known Ostwald lifting requires a long time of several hours to several days, but the invention according to the ninth feature does not require such a long time. In addition, the generally known phase generated by crystal growth by Ostwald lifting is a stable phase, but in the invention according to the ninth feature, a metastable metal oxide is precipitated on the surface of seed particles. It is epoch-making in that it can be done.

The invention according to a tenth feature is the invention according to any one of the fourth to ninth features, wherein the temperature of the hydrothermal treatment is higher than the rate at which the metal oxide precursor is uniformly nucleated. It is a manufacturing method in which the temperature at which the body heterogeneously nucleates is higher.

According to the invention of the tenth feature, since the heterogeneous nucleation of the metal oxide precursor mainly proceeds, the ratio of the metastable phase can be further increased in the metal oxide deposited on the surface.

An invention according to an eleventh feature is the invention according to any one of the fourth to tenth features, wherein the metal oxide precursor is at least one selected from a metal salt, a metal complex, and a metal hydroxide. There is a manufacturing method.

According to the invention of the eleventh feature, since the metal oxide precursor is preferably dissolved in the aqueous solvent, the hydrothermal treatment can be efficiently proceeded.

The twelfth aspect of the invention is the production method according to any one of the fourth to eleventh aspects of the invention, in which the metal oxide precursor is dissolved in a basic solution.

According to the invention of the twelfth feature, the solubility of the metal oxide precursor is higher than that under acidic conditions, and the supersaturation degree of the metal hydroxide in the reaction field of the hydrothermal treatment can be suppressed to be small. The heterogeneous nucleation on the seed particle surface can be dominantly promoted.

The invention according to the thirteenth feature is the invention according to any one of the fourth to twelfth features, wherein the hydrothermal treatment is performed in the presence of an organic modifier.

Normally, the aqueous solvent and organic modifier are phase separated. However, under subcritical or supercritical water conditions, the organic modifier forms a homogeneous phase with water.

According to the invention of the thirteenth feature, the surface of the nanoparticles is capped with the organic modifier, and the surface energy of the nanoparticles can be lowered to form micelles. The presence of the organic modifier forms a complex with the oxide monomer, and because it is more stable than the ion, the solubility of the organic modifier in subcritical water or supercritical water is further increased, thereby, It is possible to proceed with Ostwald Ripening even faster. Then, nanoparticles of metastable phase metal oxide having a diameter smaller than the particle diameter of the seed particles can be synthesized in a larger amount than the total weight of the seed particles. In addition, due to the effect of Ostwald lifting, the width of the particle size distribution can be further narrowed.

A fourteenth aspect of the invention is the invention according to any one of the fourth to thirteenth aspects, wherein a coefficient of variation obtained by dividing the standard deviation of the average particle diameter of the nanoparticles after the step by the average particle diameter. However, it is smaller than the coefficient of variation of the seed particles before undergoing the step.

Nanoparticles have different physical properties than bulk materials depending on their size. According to the invention of the fourteenth feature, since the particle size distribution of the nanoparticles is narrow and the particle size is strictly controlled, it is possible to effectively exhibit the physical properties specific to the metastable phase metal oxide nanoparticles. You can

According to the present invention, a metal oxide of an unstable phase (including a metastable phase) is produced irrespective of the type of metal oxide which is desired to form an unstable phase and the size of the average particle diameter of nanoparticles. It is possible to provide a technique capable of easily and stably supplying nanoparticles.

FIG. 1 is an XRD pattern of particles obtained by hydrothermal treatment of an aqueous iron nitrate solution. FIG. 2(A) is a schematic diagram for explaining uniform nucleation in the reaction field, and FIG. 2(B) is a schematic diagram for explaining nuclei growth of seed particles in the reaction field. FIG. 3 shows the first test condition in Test Example 2. FIG. 4 shows a TEM image of particles synthesized under the first test condition of Test Example 2. FIG. 5 is an XRD pattern of Ni nanoparticles and produced particles under the first test condition of Test Example 2. FIG. 6 shows the second test condition in Test Example 2. FIG. 7 shows TEM images of particles synthesized at each temperature under the second test condition of Test Example 2. FIG. 8 shows Arrhenius plots of homogeneous nucleation, heterogeneous nucleation of TiO _{2 on} the surface of Ni nanoparticles, and growth reaction of TiO ₂ heterogeneously nucleated on the surface of Ni nanoparticles in Test Example 3. FIG. 9 shows a TEM image of particles synthesized at 300° C. for 60 minutes in Test Example 3. FIG. 10 shows a TEM image of particles synthesized at 200° C. for 60 minutes in Test Example 3. FIG. 11 shows HRTEM images of modified ceria nanoparticles when the concentration of the organic modifier was changed in Test Example 4. FIG. 12 shows the particle size distribution of the modified ceria nanoparticles when the concentration of the organic modifier was changed in Test Example 4.

Hereinafter, specific embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments, and is carried out with appropriate modifications within the scope of the object of the present invention. can do.

<Nano particles>
The nanoparticles in the present embodiment are nanoparticles in which a metastable phase metal oxide is deposited on the surface.

The metal oxide is not particularly limited as long as it can form a stable phase and a metastable phase. In the present embodiment, the stable phase means a phase in which the free energy becomes the lowest under a certain temperature and pressure, and corresponds to a true stable state. The metastable phase is not a true stable state and does not exist in a thermal equilibrium state, but it is a phase that can exist tentatively by satisfying a predetermined condition, and is stable unless a large disturbance is given from the outside. Corresponds to the state that can exist. The metastable phase is stable with respect to small turbulence, but becomes unstable when a large turbulence is given from the outside, and changes from the metastable phase to the stable phase.

Examples of metal oxides that can form stable and metastable phases include iron (III) oxide, titanium (IV) oxide, cerium (IV) oxide, and barium titanate.

Iron (III) oxide is represented by the chemical formula Fe ₂ O ₃ , and is also called ferric oxide, hematite, red iron oxide, synthetic maghemite, red iron oxide, and ferric trioxide. Iron (III) oxide can form a stable α phase and metastable β, γ and ε phases.

Titanium (IV) oxide is represented by the chemical formula TiO ₂ , and is also called titanium dioxide, simply titanium oxide, or titania. Titanium oxide (IV) can form a stable phase, rutile type (tetragonal crystal), and metastable phases, anatase type (tetragonal crystal) and brookite type (orthorhombic crystal).

Cerium (IV) oxide is represented by the chemical formula CeO ₂ and is also called ceria. Cerium (IV) oxide can form a crystal form of a fluorite structure that is a stable phase and a cubic crystal form that is a metastable phase.

Barium titanate is an artificial mineral represented by the chemical formula BaTiO ₃ and having a perovskite structure. Barium titanate can form a stable crystal state of cubic crystal and a metastable phase of rhombohedral crystal, orthorhombic crystal, and tetragonal crystal state.

In the nanoparticles according to the present embodiment, an epitaxial layer, a polycrystalline layer, an amorphous layer, etc. are exemplified as examples of the deposition layer in which the metastable phase metal oxide is deposited.

In the present embodiment, the epitaxial layer refers to a layer formed by epitaxial growth, epitaxial growth is one of the thin film crystal growth technology, crystal growth on the surface of the substrate nanoparticles, the substrate nanoparticles of. It refers to the mode of growth that is aligned with the crystal plane.

In the present embodiment, the epitaxial state may be homoepitaxial as the material forming the base material nanoparticles and the material forming the epitaxial layer, or heteroepitaxial as a material in which these materials are different from each other. Good.

In general, for epitaxial growth to occur, it is preferable that the substances constituting the base material nanoparticles and the substances constituting the epitaxial layer have approximately the same lattice constant and that the expansion coefficients of these substances with temperature are close to each other. As an example, the nanoparticles described in the present embodiment naturally include a mode in which an epitaxial layer epitaxially grown with γ-iron(III) oxide crystals is formed on the surface of γ-iron(III) oxide nanoparticles. To do.

In the present embodiment, the polycrystalline layer refers to a layer formed from two or more crystal grains/microcrystals having different crystal orientations in a TEM (transmission electron microscope) microscopic image.

In the present embodiment, the amorphous layer refers to a layer in an amorphous state, that is, a layer that is not in an amorphous state but is in a state close to an amorphous state, and may partially have a crystal structure.  

The lower limit of the average particle size of nanoparticles is not particularly limited. The lower limit of the average particle size of the nanoparticles is preferably 2 nm or more, and more preferably 5 nm or more, from the viewpoint of ensuring the production stability when forming the epitaxial layer on the surface of the seed particles. In addition, the lower limit of the average particle diameter of the nanoparticles is particularly preferably 10 nm or more from the viewpoint that it has been difficult to supply the metal oxide nanoparticles in the metastable phase state.

The upper limit of the average particle size of nanoparticles is not particularly limited, but the maximum exposed surface area (contact possibility) when applied to many fields such as semiconductor materials, catalyst materials, and biomaterials, and the formation of an epitaxial layer on the surface of seed particles When forming the nanoparticles, the upper limit of the average particle diameter of the nanoparticles is preferably 1 μm or less, and more preferably 500 nm or less, from the viewpoint of ensuring a surface area sufficient to form a sufficient epitaxial layer on the surface of the seed particles. It is more preferably 100 nm or less, still more preferably 50 nm or less.

The lower limit of the thickness of the deposited layer is not particularly limited, but when it is applied to many fields such as semiconductor materials, catalyst materials, and biomaterials, the function of forming a metastable phase metal oxide on the surface is sufficient. From the viewpoint of exerting the effect, etc., the lower limit of the thickness of the deposited layer is preferably 0.1 nm or more, more preferably 0.3 nm or more, and particularly preferably 0.5 nm or more.

The upper limit of the thickness of the deposition layer is also not particularly limited, when forming the deposition layer on the surface of the seed particles, the precursor of the metal oxide constituting the deposition layer remains, from the remaining metal oxide precursor stable phase In order to prevent the metal oxide from being produced, the lower limit of the thickness of the deposited layer is preferably 20 nm or less, more preferably 15 nm or less, and particularly preferably 10 nm or less.

Also, a coefficient of variation obtained by dividing the standard deviation of the average particle size by the average particle size can be used as an index showing the narrowness of the particle size distribution. Nanoparticles have different physical properties than bulk materials depending on their size. From the viewpoint of effectively exhibiting the physical properties unique to the metastable phase metal oxide nanoparticles, it is preferable that the coefficient of variation is small. The coefficient of variation is preferably 0.15 or less, more preferably 0.13 or less, and further preferably 0.10 or less.

In the present embodiment, the average particle diameter and layer thickness of various particles are values obtained by capturing an image of a particle with a TEM (transmission electron microscope) and analyzing the TEM image with image analysis/image measurement software. There is.

<Method for producing nanoparticles>
Then, the manufacturing method of the nanoparticle concerning this embodiment is explained. The present production method includes a step of using nano-sized particles of a metal oxide as seed particles and subjecting the metal oxide precursor to hydrothermal treatment under subcritical water conditions or supercritical water conditions in the presence of the seed particles. Through this step, a metastable phase metal oxide can be deposited on the surface of the seed particles.

Hydrothermal treatment of metal oxide precursor in the presence of seed particles enables efficient control of supersaturation and reaction rate depending on temperature. As a result, the metastable phase metal oxide nanoparticles, which have hitherto been impossible to achieve, are not affected by the type of metal oxide for which the unstable phase is formed, or by the average particle size. Can be supplied without.

By the way, nanoparticles have different physical properties than bulk materials depending on their size. In order to control the physical properties, it is important to control the particle size and shape. However, in principle, it is extremely difficult to obtain a uniform particle size by the synthesis method based on these nucleation and growth mechanisms.

According to this manufacturing method, since the particle precursor is introduced into the seed particles to grow the seed particles, the range of the particle size distribution of the nanoparticles can be narrowed compared to before the seed particles are grown. In other words, the coefficient of variation obtained by dividing the standard deviation of the average particle diameter of the nanoparticles after the above step by the average particle diameter can be made smaller than the coefficient of variation of the seed particles before the step. For example, the average particle size of the grown particles becomes 10 to 15 nm because the average particle size of the seed particles having a particle size distribution in the range of 5 nm to 10 nm and double the average particle size grows by 5 nm. The diameter range can be narrowed from 2 times to 1.5 times.

Note that even if the metal oxide precursor is subjected to hydrothermal treatment, the metal oxide precursor changes to a stable phase metal oxide in the absence of seed particles.

[Hydrothermal treatment]
In the present embodiment, the hydrothermal treatment means a treatment for obtaining a metal oxide from the raw material by dissolving and precipitating the raw material in pressurized hot water. Hydrolysis of the raw material produces a hydroxide, and the hydroxide is dehydrated and condensed to produce a metal oxide.
MA _x +xH ₂ O→M(OH) _x +xHA
M(OH) _X → MO _x/2 + (x/2)H ₂ O

ㆍThe hydrothermal treatment is performed under subcritical water conditions or supercritical water conditions. In the present embodiment, the subcritical water condition refers to a hot water state having a temperature and pressure close to the critical point (374° C., 218 atmospheric pressure (22.1 MPa)) in the water state diagram. Further, the supercritical water condition refers to a hot water state having a temperature and a pressure exceeding a critical point. Under subcritical water conditions or supercritical water conditions, the viscosity of the aqueous solution becomes low, whereby excellent penetrating power and vigorous hydrolysis action can be exhibited.

The temperature of the hydrothermal treatment is not particularly limited as long as it is in subcritical water conditions or supercritical water conditions, but the rate at which the metal oxide precursor heterogeneously nucleates is higher than the rate at which the metal oxide precursor uniformly nucleates. Is preferably a higher temperature. Under such temperature conditions, the heterogeneous nucleation of the metal oxide precursor mainly proceeds, so that the ratio of the metastable phase can be further increased in the metal oxide deposited on the surface.

Specifically, the hydrothermal treatment temperature is preferably 450° C. or lower, more preferably 400° C. or lower, and further preferably 300° C. or lower.

[Seed particles]
The seed particle is not particularly limited as long as it is a nano-sized particle made of a metal oxide, but in order to efficiently precipitate the metastable phase metal oxide on the surface of the seed particle, the substance constituting the seed particle is also quasi A stable phase is preferred. Therefore, the seed particle is a metastable phase metal oxide, and the lattice constant and the crystal structure of the metal oxide forming the seed particle have the same chemical formula as that of the metal oxide forming the seed particle. It is preferably different from the lattice constant and crystal structure of the metal compound. For example, when it is desired to deposit γ-phase iron oxide (III) which is a metastable phase on the surface of seed particles, it is preferable to use γ-phase iron oxide (III) as seed particles.

The lower limit of the average particle size of seed particles is not particularly limited. The lower limit of the average particle size of the seed particles is preferably 2 nm or more, and preferably 5 nm or more, from the viewpoint of ensuring the production stability when depositing the metastable phase metal oxide on the surface of the seed particles. Is more preferable. In addition, the lower limit of the average particle diameter of the seed particles is particularly preferably 10 nm or more from the viewpoint that it has been difficult to supply the metal oxide nanoparticles in a metastable phase state.

The upper limit of the average particle size of the seed particles is also not particularly limited, but the maximum exposed surface area (contactability) when applied to many fields such as semiconductor materials, catalyst materials, biomaterials, metastable phase on the surface of seed particles The upper limit of the average particle diameter of the seed particles is preferably 1 μm or less, more preferably 500 nm or less, and more preferably 100 nm or less, from the viewpoint of ensuring a surface area sufficient to precipitate the metal oxide of (1). More preferably, it is particularly preferably 50 nm or less.

The lower limit of the concentration of seed particles contained in subcritical water or supercritical water (hereinafter also referred to as “supercritical water etc.”) when performing hydrothermal treatment is not particularly limited. From the viewpoint of securing a certain amount or more of the total surface area of the seed particles supplied to the reactor, the lower limit of the concentration of the seed particles is preferably 0.01 mol/l or more, and more preferably 0.05 mol/l or more. It is preferably 0.1 mol/l or more, and more preferably 0.1 mol/l or more. By securing the total surface area of the seed particles to a certain level or more, when the metal oxide precursor is subjected to the film formation on the seed particle surface, the excess metal oxide precursor remains and is stable from the excess metal oxide precursor. It is possible to suppress the generation of the metal oxide of the phase.

The upper limit of the concentration of the seed particles is also not particularly limited, but considering the balance of the amounts of the seed particles and the metal oxide precursor, it is preferably 1 mol/l or less, more preferably 0.5 mol/l or less. ..

[Metal oxide precursor]
In the present embodiment, the metal oxide precursor is an intermediate raw material obtained by mixing each raw material component constituting the metal oxide to form a metastable phase in an aqueous solvent, the metal of the metastable phase still It means one that is not an oxide.

The type of the metal oxide precursor is not particularly limited as long as it is a material capable of precipitating a metastable phase metal oxide on the surface of seed particles. Specifically, the metal oxide precursor may be one or more selected from metal salts, metal complexes, and metal hydroxides. With these materials, the metal oxide precursor can be suitably dissolved in the aqueous solvent.

For example, when γ-phase iron (III) oxide is deposited on the surface, the metal oxide precursor is an aqueous iron nitrate solution. When tetragonal barium titanate is deposited on the surface, the metal oxide precursor is a mixed solution of titanate, barium hydroxide, and potassium hydroxide mixed in an aqueous solvent.

Among them, the metal oxide precursor is preferably a metal complex. Since the metal complex is more stable in the aqueous solvent than the metal salt, it is possible to prevent the metal hydroxide from becoming supersaturated in the solution during the hydrothermal treatment of the metal oxide precursor. As a result, it is possible to suppress uniform nucleation of the metal oxide precursor.

Also, the metal oxide precursor is preferably dissolved in a basic solution. In this case, the solubility of the metal oxide precursor is higher than that under acidic conditions, and the supersaturation degree of the metal hydroxide in the reaction field of the hydrothermal treatment can be suppressed to a low level. You can dominate production.

The lower limit of the concentration of the metal oxide precursor contained in supercritical water or the like when performing hydrothermal treatment is not particularly limited, but in consideration of the balance of the amounts of the seed particles and the metal oxide precursor, 0.01 mol/l or more Is preferable, and more preferably 0.05 mol/l or more.

The upper limit of the concentration of the metal oxide precursor is also not particularly limited. Here, in the case of synthesizing nanoparticles in a liquid phase, it is known to use a re-dissolution/precipitation method, which is called Ostwald lifting, in combination. When this method is used, relatively small nanoparticles are redissolved and the redissolved nanoparticle components are deposited on relatively large nanoparticles, so that the width of the particle size distribution can be narrowed further.

When the concentration of the metal oxide precursor contained in the solution when performing the hydrothermal treatment is set to a predetermined threshold value or less, the metal hydroxide is not dissolved in the solution even though the principle of the redissolution/precipitation method is applied. It becomes possible to prevent the supersaturated state, the uniform nucleation of the metal oxide precursor from occurring, and the broadening of the particle size distribution.

From the viewpoint of further narrowing the width of the particle size distribution of nanoparticles after hydrothermal treatment, the upper limit of the concentration of the metal oxide precursor is preferably 1 mol/l or less, and 0.5 mol/l or less. More preferably, it is even more preferably 0.1 mol/l or less.

[Organic modifier]
Although not essential, hydrothermal treatment is preferably performed in the presence of an organic modifier.

Normally, the aqueous solvent and organic modifier are phase separated. However, under subcritical or supercritical water conditions, the organic modifier forms a homogeneous phase with water.

In the present embodiment, the surface of the nanoparticles can be capped with an organic modifier to lower the surface energy of the nanoparticles and form micelles. The presence of the organic modifier forms a complex with the oxide monomer, and because it is more stable than the ion, the solubility of the organic modifier in subcritical water or supercritical water is further increased, thereby, It is possible to proceed with Ostwald Ripening even faster. Then, nanoparticles of metastable phase metal oxide having a diameter smaller than the particle diameter of the seed particles can be synthesized in a larger amount than the total weight of the seed particles. In addition, due to the effect of Ostwald lifting, the width of the particle size distribution can be further narrowed.

The organic modifier is not particularly limited as long as it can strongly bind hydrocarbons to the surface of fine particles, and the application of nanoparticles including the fields of organic chemistry, inorganic materials and polymer chemistry Can be selected from organic substances that are widely known in the field where is expected.

Examples of the organic modifier include ether bond, ester bond, bond via N atom, bond via S atom, metal-C- bond, metal-C= bond and metal-(C=O)-. Those which allow the formation of a strong bond such as a bond of The number of carbon atoms of the hydrocarbon is not particularly limited, and it may be one having 1 or 2 carbon atoms, but is preferably a long-chain hydrocarbon having a chain of 3 or more carbon atoms, for example, Examples thereof include straight-chain or branched-chain or cyclic hydrocarbon having 3 to 20 carbon atoms.

The hydrocarbon may be substituted or unsubstituted. The substituent may be selected from functional groups widely known in the fields of organic chemistry, inorganic materials, polymer chemistry, etc., and one or more substituents may be present. Or a plurality of them may be the same or different.

Examples of the organic modifier include alcohols, aldehydes, ketones, carboxylic acids, esters, amines, thiols, amides, oximes, phosgene, enamines, amino acids, peptides and saccharides. To be

Typical modifiers include, for example, pentanol, pentanal, pentanoic acid, pentanamide, pentanethiol, hexanol, hexanal, hexanoic acid, hexanamide, hexanethiol, heptanol, heptanal, heptanoic acid, heptane amide, heptanethiol, Examples thereof include octanol, octanal, octanoic acid, octanamide, octanethiol, decanol, decanal, decanoic acid, decane amide, and decanethiol.

The hydrocarbon group may be a linear or branched alkyl group which may be substituted, a cyclic alkyl group which may be substituted, an aryl group which may be substituted, or an aralkyl group which may be substituted. , And optionally substituted saturated or unsaturated heterocyclic groups and the like. Examples of the substituent include a carboxy group, a cyano group, a nitro group, a halogen, an ester group, an amide group, a ketone group, a formyl group, an ether group, a hydroxyl group, an amino group, a sulfonyl group, —O—, —NH—, — S- etc. are mentioned.

Hereinafter, the present invention will be specifically described with reference to test examples in the present embodiment, but the present invention is not limited to these.

<Test Example 1> Control of growth crystal phase of iron (III) oxide particles by hydrothermal treatment [Test method]
50 mg of γ-iron oxide nanoparticles (average particle diameter: 3.7 nm) were added as seed particles to 2.2 mL of an aqueous iron nitrate solution (two types of 0.056 mol/l and 0.11 mol/l) (pH 1-2). ). The solution was sealed in a batch reactor (volume: 5 mL) and hydrothermally treated at 200° C. for 10 minutes.

Also, the same test was performed by adjusting the pH of the solution to 11 to 12 using a 1 mol/l NaOH aqueous solution. For comparison, iron nitrate aqueous solution (0.056 mol/l) was hydrothermally treated without adding seed particles at each pH. The obtained particles were analyzed by X-ray diffraction (XRD) and transmission electron microscope (TEM).

〔result〕
First, in consideration of the possibility that the seed particles themselves would dissolve and denature under each condition, a solution containing only seed particles was hydrothermally treated at each pH, but the particles did not dissolve.

Fig. 1 shows an XRD pattern of particles obtained by hydrothermal treatment of an iron nitrate aqueous solution. In the case of hydrothermal treatment of iron nitrate solution without adding seed particles, only α-iron oxide peak was obtained under acidic condition. Further, under basic conditions, a peak of α-iron oxide and a peak of FeO(OH) were obtained. From this result, it is considered that uniform nucleation as shown in FIG. 2(A) occurred in the reaction field and α-iron oxide was generated.

Under the acidic condition, when the iron nitrate concentration was 0.056 mol/l, only the peak of γ-iron oxide was obtained, but under the condition of 0.11 mol/l, the peak of α-iron oxide appeared. .. Under the condition of iron nitrate concentration of 0.056 mol/l, the addition of seed particles causes heterogeneous nucleation, and γ-iron oxide is precipitated on the surface of seed particles, resulting in the nucleus growth as shown in FIG. 2(B). It is believed that However, it is considered that when the concentration of iron nitrate becomes high, the degree of supersaturation in the reaction field becomes large, and the homogeneous nucleation is dominant rather than the heterogeneous nucleation.

On the other hand, under the basic conditions, only the peak of γ-iron oxide was obtained under the conditions where the iron nitrate concentration was 0.056 mol/l and 0.11 mol/l. Since a peak of α-iron oxide was obtained at high concentration under acidic condition, it was suggested that the basic condition is more suitable for heterogeneous nucleation on the particle surface. The solubility of iron hydroxide decreases with increasing temperature under acidic conditions and increases under basic conditions. Under basic conditions, the solubility at reaction temperature is high, so the degree of supersaturation of iron hydroxide in the reaction field becomes small, and it is considered that heterogeneous nucleation on the particle surface predominantly occurred even at high concentrations.

The synthesized Ni / BaTiO ₃ core-shell nanoparticles with impact soluble Ti complex of <Test Example 2> The reaction temperature model, consider the influence of the reaction temperature.

〔Test method〕
[reagent]
The following reagents were used for the test.
・Ni nanoparticles (average particle size: 80 nm)
・Titanium ammonium peroxocitrate aqueous solution (Ti=5wt%)
_{((NH 4) 4 [Ti} 2 (C 6 H 4 O 7) 2 (O 2) 2] · 4H 2 O, Furuuchi Chemical Co., Ltd.)
· Barium hydroxide octahydrate _{(Ba (OH) 2 · 8H} 2 O, Fuji Film Wako Pure Chemical Co., 98.0% purity)
・Potassium hydroxide (KOH, Fujifilm Wako Pure Chemical Industries, Ltd., purity 85.0%)

[Test equipment]
An experiment was conducted using the following device.
・Batch type reactor (5mL), Material: Hastelloy, AKICO
・Shaking type reactor heating and stirring device, AKICO, SAH-R16-500

[Test method]
A transparent water-soluble BaTiO ₃ precursor was obtained by adding an aqueous solution of ammonium titanium peroxocitrate, barium hydroxide octahydrate and citric acid to purified water and stirring the mixture to adjust the pH to make it basic. Ni nanoparticles were added to this precursor solution and dispersed by ultrasonic waves. Then, it was enclosed in a 5 mL batch reactor and the reaction was performed using the reactor. Then, the reactor was cooled with water to complete the reaction. The product was recovered from the reactor, and centrifugally washed and freeze-dried to obtain dry particles. In each case, the Ba/Ti molar ratio was about 1 and the reaction time was 60 minutes.

〔result〕
[Evaluation of generated particles]
First, the generated particles were evaluated. The first test condition is shown in FIG. Further, FIG. 4 shows a TEM image of particles synthesized by changing the concentration of Ni nanoparticles to 4.34 wt% and 0.51 wt %. FIG. 4(A) is a TEM image of the particles produced when the concentration of Ni nanoparticles is 4.34 wt %, and FIG. 4(B) is the case where the concentration of Ni nanoparticles is 0.51 wt %. It is a TEM image of generated particles.

In each case, uniform film formation was confirmed on the surface of Ni nanoparticles. It was also confirmed that the shell thickness increased as the amount of Ni nanoparticles added decreased. When all the surfaces of the Ni nanoparticles are uniformly coated with BaTiO ₃ , the film thickness is expected to be about 4 nm when adding 4.34 wt% of Ni nanoparticles and about 20 nm when adding 0.51 wt%. .. The film thickness was roughly as predicted when 4.8 wt% of Ni nanoparticles was added, but was thinner than the film thickness predicted when 0.51 wt% of Ni nanoparticles was added. It is considered that the reason for this is that when 0.51 wt% of Ni nanoparticles were added, the BaTiO ₃ raw material was not completely consumed for forming the film, and unreacted one remained.

FIG. 5 is an XRD pattern of Ni nanoparticles and produced particles. No peak other than Ni was confirmed in any of the particles, and it was found that the shell structure was amorphous without being crystallized.

[Influence of reaction temperature and pH]
Then, the influence of the reaction temperature on the core-shell structure formation was investigated. The second test condition is shown in FIG. FIG. 7 shows the particles produced at each reaction temperature with and without addition of Ni. It was found that particles were not formed and uniform nucleation did not occur in the case of synthesizing only with the BaTiO ₃ precursor without adding Ni nanoparticles at 100°C. On the other hand, when Ni nanoparticles were added, the surface of Ni nanoparticles was uniformly coated with BaTiO ₃ . From this, it is understood that at 100° C., the condition is that nucleation does not occur and only growth occurs. Further, when synthesized at 200° C., particles were generated only with the BaTiO ₃ precursor, so it can be seen that it is a condition under which uniform nucleation occurs. Even when the Ni nanoparticles were added, the core-shell structure was formed, but the shell structure was rather coarse and uniform nucleation was also occurring.
From the above, it was confirmed that the formation of a uniform core-shell structure is desirable at low temperature.

<Test Example 3> Control of nucleation/nucleus growth Based on Ni/BaTiO ₃ core-shell nanoparticles synthesized using a water-soluble Ti complex as a model, examination on conditions under which nucleation/nucleus growth is dominant from kinetic analysis I went.

〔Test method〕
[reagent]
The following reagents were used for the test.
・Ni nanoparticles (average particle size: 80 nm)
・Titanium ammonium peroxocitrate aqueous solution (Ti=5wt%)
_{((NH 4) 4 [Ti} 2 (C 6 H 4 O 7) 2 (O 2) 2] · 4H 2 O, Furuuchi Chemical Co., Ltd.)
・6 mol/l hydrochloric acid (HCl, Fujifilm Wako Pure Chemical Industries, Ltd.)

[Test equipment]
An experiment was conducted using the following device.
・Batch type reactor (5mL), Material: Hastelloy, AKICO
・Shaking type reactor heating and stirring device, AKICO, SAH-R16-500

[Test method]
A water-soluble Ti precursor was obtained by adding an aqueous solution of ammonium ammonium peroxocitrate to purified water, stirring the mixture, and then acidifying the pH with hydrochloric acid to suppress the hydrolysis rate. Ni nanoparticles were added to this precursor solution and dispersed by ultrasonic waves. After that, the reaction was carried out by enclosing in a 5 mL batch type reactor. The amount of charge to the reactor was adjusted so that the reaction pressure was equal to or higher than the saturated steam pressure. After the reaction, the reactor was cooled with water to complete the reaction. The product was recovered from the reactor, and centrifugally washed and freeze-dried to obtain dry particles.

[Analysis equipment]
The analysis was performed using the following equipment.
・X-ray diffractometer (XRD), RIGAKU, SmartLab 9MTP

The crystallite size was determined using the Halder-Wagner method. The formula is shown below.

In the above formula, β is the integral width, θ is the Bragg angle, K is the Scherrer constant, λ is the wavelength of the X-ray, D is the crystallite size, and ε is the nonuniform lattice strain.

・Transmission electron microscope (TEM), HITACHI, H-7650
・Ultra high resolution aberration correction type analytical electron microscope (STEM/EDS), FEI-Company, Titan ^3TM 60-300
Inductively coupled plasma optical emission spectrometer (ICP-OES), ThermoFisher, iCAP6500

〔result〕
From the kinetic analysis, we examined the conditions under which nucleation and nuclear growth are dominant. FIG. 8 shows Arrhenius plots of uniform nucleation, heterogeneous nucleation of TiO _{2 on} the surface of Ni nanoparticles, and growth reaction of TiO ₂ heterogeneously nucleated on the surface of Ni nanoparticles. Here, the rate constants of the heterogeneous nucleation and growth reaction of TiO ₂ are obtained by taking the difference between the rate constants in the early and late stages of the reaction when Ni nanoparticles are added and the rate constant of the uniform nucleation. When the activation energy is calculated from the slope of this straight line, the activation energy for uniform nucleation is about 73 kJ/mol, the activation energy for the heterogeneous nucleation reaction on the Ni nanoparticle surface is about 46 kJ/mol, and the growth of TiO ₂ The activation energy of the reaction was about 55 kJ/mol. It has been reported that the activation energy for hydrothermal synthesis of TiO ₂ particles when TiO ₂ gel is used as a raw material is 89.1 kJ/mol (Satoshi Uchida et al., Coloring Material, 72 (1999) 680-689). , It was shown that the result of this time is appropriate because it is a value relatively close to the activation energy of uniform nucleation obtained this time. The slight deviation is considered to be due to the difference in the starting materials and the measurement error. In addition, since activation of heterogeneous nucleation occurs more easily than that of homogeneous nucleation, the activation energy is considered to be small. In addition, since the reaction of growing TiO _2, which is a reaction on the same substance, is more likely to occur than the heterogeneous nucleation of TiO _{2 on} the surface of Ni nanoparticles, which is a heterogeneous nucleation with a different substance, Although the activation energy did not change significantly, it is considered that the reaction rate increased as a whole and the straight line was shifted to the direction with a larger rate constant. Further, it can be seen that the straight lines of uniform nucleation and heterogeneous nucleation on the surface of Ni nanoparticles intersect at a temperature of about 272°C. Above this point, the rate of homogeneous nucleation exceeds that of heterogeneous nucleation. Therefore, uniform nucleation is considered to occur preferentially. On the other hand, if the temperature is lower than this point, the rate of heterogeneous nucleation is higher, and therefore it is considered that heterogeneous nucleation occurs preferentially. 9 and 10 show TEM images of particles synthesized under the temperature condition near the intersection. It can be seen that the particles synthesized at 300° C., which is a temperature higher than the intersection, mostly generate uniform nucleation and do not form a core-shell structure. On the other hand, it was confirmed that the particles synthesized at 200° C., which is a temperature lower than the intersection, undergoes uniform nucleation, but heterogeneous nucleation/growth occurs to form a core-shell structure.

From the above, for the formation of the core-shell structure, it is very important to control the reaction rate of the homogeneous nucleation and the heterogeneous nucleation of the shell material on the core particle surface, which is the initial stage of the core-shell structure formation. It was shown that the formation conditions of the core-shell structure can be estimated by analyzing the.

<Test Example 4> Control of crystal growth using an organic modifier A crystal using an organic modifier was modeled on the morphological change of cerium (IV) oxide nanoparticles by supercritical hydrothermal treatment using decanoic acid as an organic modifier. Consider growth control.

〔Test method〕
10.3 mg (0.02 mol/l) of ceria powder modified by decanoic acid (made by ITEC) was transferred to a 5 mL batch reactor. Then 2.5 mL H ₂ O, 0.036 mL (0.06 mol/l), 0.072 mL (0.12 mol/l) or 0.144 mL (0.24 mol/l) decanoic acid without stirring added. The reactor was heated at 400°C for 10 minutes and then quenched in a water bath (20°C). The precipitate was dispersed with hexane, ethanol was added, and the mixture was centrifuged once. The obtained nanocrystal was dissolved in cyclohexane. Samples were analyzed by high resolution transmission electron microscope (HRTEM), Fourier transform infrared spectrometer (FT-IR) and thermogravimetric analysis (TGA).

〔result〕
FIG. 11 shows that the shape and size of the modified nanoparticles changed after supercritical hydrothermal treatment. The morphology of nanoparticles changed from spherical to cubic as the concentration of organic modifier increased. This demonstrates that modified ceria nanoparticles can redissolve and grow in supercritical water. Furthermore, modifiers on the surface of the particles and modifiers in hydrothermal control the growth process.

From FIG. 12, the particle size distribution of the samples treated with the low modifier concentration conditions shows a wide range of particle size distributions, large particle growth and small particle dissolution. It is speculated that the result is due to Ostwald growth. However, the results for the high modifier concentration samples showed a narrow particle size distribution, and the lack of nanoparticles below 4 nm may be due to oriented aggregation.

Claims (14)

Nanoparticles with metastable metal oxides deposited on the surface. Having a deposition layer in which a metastable phase metal oxide is deposited,
The deposition layer comprises nanoparticles, including an epitaxial layer, a polycrystalline layer, or an amorphous layer. 3. The nanoparticles according to claim 1 or 2, wherein the coefficient of variation obtained by dividing the standard deviation of the average particle diameter by the average particle diameter is 0.15 or less. Meta-stable on the surface of the seed particle by hydrothermally treating the nano-sized particles of the metal oxide in the presence of the seed particle in the presence of the seed particle under subcritical water conditions or supercritical water conditions A method for producing nanoparticles, comprising a step of precipitating a metal oxide of a phase. The method according to claim 4, wherein the seed particles are metastable metal oxides. The production according to claim 4 or 5, wherein a lattice constant of the metal oxide forming the seed particles is different from a lattice constant of a metal compound in a stable phase having the same chemical formula as the metal oxide forming the seed particles. Method. 7. The crystal structure of the metal oxide forming the seed particles is different from the crystal structure of a metal compound in a stable phase having the same chemical formula as that of the metal oxide forming the seed particles. The manufacturing method described. The method according to any one of claims 4 to 7, wherein the concentration of the seed particles contained in the solution when the hydrothermal treatment is performed is 0.01 mol/l or more. The production method according to claim 4, wherein the concentration of the metal oxide precursor contained in the solution when the hydrothermal treatment is performed is 1 mol/l or less. 10. The temperature of the hydrothermal treatment is a temperature at which the rate of heterogeneous nucleation of the metal oxide precursor is higher than the rate of uniform nucleation of the metal oxide precursor. The manufacturing method described in. The method according to any one of claims 4 to 10, wherein the metal oxide precursor is one or more selected from metal salts, metal complexes, and metal hydroxides. The manufacturing method according to any one of claims 4 to 11, wherein the metal oxide precursor is dissolved in a basic solution. The method according to any one of claims 4 to 12, wherein the hydrothermal treatment is performed in the presence of an organic modifier. 14. The coefficient of variation obtained by dividing the standard deviation of the average particle diameter of the nanoparticles after the step by the average particle diameter is smaller than the coefficient of variation of the seed particles before the step. The manufacturing method according to any one of 1.

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