JP7340224B2 - Nanoparticles and their manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to nanoparticles and methods for producing the same.

In recent years, nanotechnology has attracted attention as it is related to the foundation of a wide range of industries such as materials, information, and biotechnology. Nano is a prefix indicating a quantity 10 ⁻⁹ times the base unit, and 1/10 ⁹ of 1 meter is 1 nanometer. Nanotechnology is a science and technology that deals with this microscopic world. Particles with diameters on the order of nanometers, created through the fusion of nanotechnology and materials science, are called "nanoparticles." Nanoparticles are known to exhibit size-dependent optical properties, electromagnetic properties, etc., unlike bulk or small molecules, because they have a quantum size effect and a large specific surface area (Non-Patent Documents 1 and 2). Therefore, the application of nanoparticles is progressing in a wide range of fields such as physics, electronic engineering, information engineering, catalytic chemistry, and bioscience. In addition, there are a wide variety of materials such as metals, metal oxides, and organic materials (Non-Patent Documents 3 and 4).

Nanoparticle synthesis methods include thermal plasma method (gas phase), spray pyrolysis method (gas phase), reverse micelle method (liquid phase), hot soap method (liquid phase), and solvothermal method (liquid phase, supercritical Various industrial synthesis methods are known, such as hydrothermal synthesis (liquid phase, supercritical phase), and hydrothermal synthesis (liquid phase, supercritical phase). In either method, the particles synthesized have 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 converting a metal oxide into nanoparticles, for example, if the target metal oxide is iron (III) oxide, alpha iron (III) oxide nanoparticles can be obtained at a relatively low temperature.

On the other hand, when the average particle size of nanoparticles is on the order of several nanometers or less, the phase state of the metal oxide becomes unstable due to surface effects and nanosize effects compared to when the average particle size is on the order of several tens of nanometers or more. It is known that there may be a shift to a phase state.

Hiroshi Imahori et al., Nanotechnology (2010) NTS, Nanoparticle Science - From basic principles to applications - (2007) J. Prasad Rao et al., Progress in Polymer Science, 36 (2011) 887-913 Stankic et al., J Nanobiotechnol, (2016) 14-73

However, even if the average particle diameter of the nanoparticles is on the order of several nanometers or less, the main phase of the generated nanoparticles is a stable phase. For example, when the metal oxide is iron (III) oxide, it is said to be extremely difficult to synthesize nanoparticles of iron (III) oxide in the γ phase, which is an unstable phase. In addition, when the metal oxide is titanium (IV) oxide, it is impossible to synthesize nanoparticles in the amorphous phase, which is an unstable phase.

The present invention was made in view of these problems, and is capable of forming an unstable phase regardless of the type of metal oxide desired to form an unstable phase or the average particle size of nanoparticles. The purpose of the present invention is to provide a technology that can easily and stably supply phase (including metastable phase) metal oxide nanoparticles.

As a result of extensive research in order to achieve the above-mentioned problems, the present inventors used nano-sized metal particles as seed particles and hydrothermally treated metal oxide precursors under critical water conditions in the presence of the seed particles. By doing so, a metal oxide in a metastable phase can be precipitated on the surface of the seed particles, regardless of the type of metal oxide for which an unstable phase is to be formed, and regardless of the average particle diameter of the seed particles. They discovered this and completed the present invention. Specifically, the present invention provides the following.

The invention according to the first aspect provides nanoparticles on which a metal oxide in a metastable phase is precipitated.

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

According to the inventions according to the first and second characteristics, metal oxide nanoparticles in a metastable phase, which has been thought to be impossible to realize until now, can be produced regardless of the type of metal oxide for which an unstable phase is to be formed. It can be used regardless of the average particle size.

Since metal oxides in a metastable phase are precipitated on the surface of nanoparticles, they can be applied to a wide range of applications such as semiconductor materials, catalyst materials, biological materials, magnetic data storage, biosensing, and drug delivery. .

The invention according to the third characteristic provides nanoparticles having a coefficient of variation obtained by dividing the standard deviation of the average particle size by the average particle size of 0.15 or less in the invention according to the first or second characteristic.

Nanoparticles have physical properties that differ from bulk materials depending on their size. According to the invention according to the third feature, the particle size distribution of the nanoparticles is narrow and the particle size is strictly controlled, so that physical properties specific to metal oxide nanoparticles in a metastable phase can be effectively exhibited. I can do it.

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

According to the fourth aspect of the invention, since the metal oxide precursor is hydrothermally treated in the presence of seed particles, the degree of supersaturation and reaction rate can be efficiently controlled by temperature. As a result, we have succeeded in producing metal oxide nanoparticles in a metastable phase, which had been thought to be impossible until now, regardless of the type of metal oxide with which we want to form an unstable phase, and regardless of the size of the average particle size. It can be supplied without

By the way, nanoparticles have physical properties that are different from bulk materials depending on their size. In order to control its physical properties, it is important to control the particle size and shape. However, with these synthetic methods based on nucleation and growth mechanisms, it is extremely difficult in principle to obtain uniform particle diameters.

According to the invention according to the fourth feature, since the seed particles are grown by introducing a particle precursor into the seed particles, the range of the particle size distribution of the nanoparticles is narrowed compared to before the seed particles grow. I can do it. For example, if a particle group of seed particles with a particle size distribution in the double average particle size range of 5 nm to 10 nm grows by an average of 5 nm, the average particle size of the grown particles will be 10 to 15 nm, and the average particle size will be 10 to 15 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 invention according to a fifth characteristic is the manufacturing method according to the invention according to the fourth characteristic, wherein the seed particles are metal oxides in a metastable phase.

The invention according to a sixth feature is the invention according to the fourth or fifth feature, wherein the lattice constant of the metal oxide constituting the seed particle is the same chemical formula as the metal oxide constituting the seed particle. This is a manufacturing method that differs from the lattice constant of the stable phase metal compound.

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

According to the invention according to the fifth to seventh characteristics, the seed particles are metal oxides in a metastable phase, and the lattice constant or crystal structure is different from that in the stable phase. Therefore, compared to the case where the seed particles are metal oxides in a stable phase, a metastable metal oxide having the same quality as the phase constituting the seed particles can be efficiently precipitated 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 conditions is 0.01 mol/l or more. , a manufacturing method.

According to the invention according to the eighth feature, the total surface area of the seed particles supplied to the reactor can be secured to a certain level or more, and when the metal oxide precursor is used to form a film on the surface of the seed particles, excess metal The oxide precursor remains, and generation of stable phase metal oxide from the surplus metal oxide precursor can be suppressed.

The invention according to a ninth feature is the manufacturing method according to the third or fourth feature, wherein the concentration of the metal oxide precursor contained in the solution when performing the hydrothermal treatment is 1 mol/l or less. It is.

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

According to the invention according to the ninth feature, since an upper limit is set on the concentration of the metal oxide precursor contained in the solution during hydrothermal treatment, even if the principle of the redissolution/precipitation method is applied. Regardless, the metal hydroxide becomes supersaturated in the solution, uniform nucleation of the metal oxide precursor occurs, and the particle size distribution can be prevented from becoming wider.

Further, although crystal growth by the generally known Ostwald tripening method requires a long time of several hours to several days, the invention according to the ninth feature does not require such a long time. In addition, the phase generated by crystal growth by the generally known Ostwald tripening is a stable phase, but in the invention according to the ninth feature, a metal oxide in a metastable phase is precipitated on the surface of the seed particle. It is revolutionary in that it can be

The invention according to a tenth feature is the invention according to any one of the fourth to ninth features, in which the temperature of the hydrothermal treatment is lower than the uniform nucleation rate of the metal oxide precursor. This is a manufacturing method in which the temperature at which the body undergoes heterogeneous nucleation is greater.

According to the tenth aspect of the invention, since heterogeneous nucleation of the metal oxide precursor mainly proceeds, it is possible to further increase the proportion of metastable phases in the metal oxide deposited on the surface.

The invention according to an eleventh feature is the invention according to any one of the fourth to tenth features, in which the metal oxide precursor is one or more selected from metal salts, metal complexes, and metal hydroxides. There is a manufacturing method.

According to the invention according to the eleventh feature, since the metal oxide precursor is suitably dissolved in the aqueous solvent, the hydrothermal treatment can be carried out efficiently.

The invention according to a twelfth feature is the manufacturing method according to any one of the fourth to eleventh features, wherein the metal oxide precursor is dissolved in a basic solution.

According to the invention according to the twelfth characteristic, the solubility of the metal oxide precursor is higher than that under acidic conditions, and the degree of supersaturation of the metal hydroxide in the reaction field of hydrothermal treatment can be suppressed to a low level, and as a result, Heterogeneous nucleation on the seed particle surface can be dominantly promoted.

The invention according to a 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.

Usually, the aqueous solvent and the organic modifier undergo phase separation. However, in subcritical or supercritical water conditions, the organic modifier forms a homogeneous phase with water.

According to the thirteenth aspect of the invention, the organic modifier caps the surface of the nanoparticles, lowers the surface energy of the nanoparticles, and forms micelles. The presence of the organic modifier forms a complex with the oxide monomer, which is more stable than ions, which further increases the solubility of the organic modifier in subcritical or supercritical water. It is possible to proceed with Ostwald lightning even faster. Then, nanoparticles of metastable phase metal oxide having a diameter smaller than that of the seed particles can be synthesized in a larger amount than the total weight of the seed particles. In addition, the width of the particle size distribution can be further narrowed due to the effect of Ostwald brightening.

The invention according to the fourteenth feature is the invention according to any one of the fourth to thirteenth features, wherein a coefficient of variation is obtained by dividing the standard deviation of the average particle diameter of the nanoparticles after passing through the step by the average particle diameter. is smaller than the coefficient of variation of the seed particles before undergoing the step.

Nanoparticles have physical properties that differ from bulk materials depending on their size. According to the invention according to the fourteenth characteristic, the particle size distribution of the nanoparticles is narrow and the particle size is strictly controlled, so that physical properties specific to metal oxide nanoparticles in a metastable phase are effectively exhibited. I can do it.

According to the present invention, metal oxides in an unstable phase (including a metastable phase) can be formed in an unstable phase (including a metastable phase) regardless of the type of metal oxide desired to form an unstable phase or the average particle diameter of nanoparticles. A technology that can easily and stably supply nanoparticles can be provided.

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 illustrating uniform nucleation in a reaction field, and FIG. 2(B) is a schematic diagram illustrating nucleation of seed particles in the reaction field. FIG. 3 shows the first test conditions in Test Example 2. FIG. 4 shows a TEM image of particles synthesized under the first test conditions of Test Example 2. FIG. 5 is an XRD pattern of Ni nanoparticles and generated particles under the first test conditions of Test Example 2. FIG. 6 shows the second test conditions in Test Example 2. FIG. 7 shows TEM images of particles synthesized at each temperature under the second test conditions of Test Example 2. FIG. 8 shows Arrhenius plots of homogeneous nucleation, heterogeneous nucleation of TiO ₂ on the Ni nanoparticle surface, and growth reaction of TiO ₂ heterogeneously nucleated on the Ni nanoparticle surface 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 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. However, the present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the purpose of the present invention. can do.

<Nanoparticles>
The nanoparticles in this embodiment are nanoparticles on which a metal oxide in a metastable phase is precipitated.

The metal oxide is not particularly limited as long as it can form a stable phase and a metastable phase. In this embodiment, the stable phase refers to a phase in which the free energy is the lowest under a certain temperature and pressure, and corresponds to a true stable state. A metastable phase is not a truly stable state, and refers to a phase that does not exist in thermal equilibrium, but can exist temporarily if certain conditions are met, and is stable unless large external disturbances are applied. It corresponds to a state in which it is possible to exist. The metastable phase is stable against small disturbances, but becomes unstable when large external disturbances are applied, and the metastable phase changes to a stable phase.

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

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

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

Cerium (IV) oxide has the chemical formula _CeO2 and is also called ceria. Cerium (IV) oxide is capable of forming a stable phase, a fluorite crystal structure, and a metastable phase, a cubic crystal structure.

Barium titanate is represented by the chemical formula BaTiO ₃ and is an artificial mineral with a perovskite structure. Barium titanate can form a cubic crystalline state, which is a stable phase, and a rhombohedral, orthorhombic, and tetragonal crystalline state, which are metastable phases.

In the nanoparticles according to the present embodiment, examples of the precipitated layer in which a metastable metal oxide is precipitated include an epitaxial layer, a polycrystalline layer, an amorphous layer, and the like.

In this embodiment, the epitaxial layer refers to a layer formed by epitaxial growth, and epitaxial growth is one of thin film crystal growth techniques, in which crystal growth is performed on the surface of base nanoparticles. A growth pattern that aligns with crystal planes.

In this embodiment, the epitaxial state may be a homoepitaxial state in which the substance forming the base nanoparticle and the substance forming the epitaxial layer are the same, or a heteroepitaxial state in which these substances are different substances. Good too.

Generally, in order for epitaxial growth to occur, it is preferable that the lattice constants of the substance constituting the base nanoparticle and the substance constituting the epitaxial layer are approximately the same, and that the coefficients of expansion due to temperature of these substances are similar. As an example, the nanoparticles described in this embodiment naturally include an embodiment in which an epitaxial layer formed by epitaxial growth of γ-iron(III) oxide crystals is formed on the surface of γ-iron(III) oxide nanoparticles. do.

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

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

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

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

The lower limit of the thickness of the precipitated layer is not particularly limited, but when applied to many fields such as semiconductor materials, catalyst materials, and biological materials, it is necessary to fully utilize the function of the metal oxide in the metastable phase formed on the surface. From the viewpoint of performance, etc., the lower limit of the thickness of the precipitated 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 precipitated layer is not particularly limited, but when forming the precipitated layer on the surface of the seed particles, the precursor of the metal oxide that constitutes the precipitated layer remains, and a stable phase is formed from the remaining metal oxide precursor. In order to prevent the formation of metal oxides, the lower limit of the thickness of the precipitated layer is preferably 20 nm or less, more preferably 15 nm or less, and particularly preferably 10 nm or less.

Further, as an index indicating the narrowness of the particle size distribution, a coefficient of variation obtained by dividing the standard deviation of the average particle size by the average particle size can be used. Nanoparticles have physical properties that differ from bulk materials depending on their size. From the viewpoint of effectively exhibiting the physical properties specific to metal oxide nanoparticles in the metastable phase, the coefficient of variation is preferably smaller. The coefficient of variation is preferably 0.15 or less, more preferably 0.13 or less, and even more preferably 0.10 or less.

In this embodiment, the average particle diameter and layer thickness of various particles are values obtained by capturing images of particles using a TEM (transmission electron microscope) and analyzing the TEM images using image analysis/image measurement software. Assume that there is.

<Method for manufacturing nanoparticles>
Next, a method for manufacturing nanoparticles according to this embodiment will be explained. This production method includes a step of using nano-sized metal oxide particles as seed particles and hydrothermally treating a metal oxide precursor under subcritical water conditions or supercritical water conditions in the presence of the seed particles. Through this step, a metal oxide in a metastable phase can be precipitated on the surface of the seed particles.

By hydrothermally treating a metal oxide precursor in the presence of seed particles, the degree of supersaturation and reaction rate can be efficiently controlled by temperature. As a result, we have succeeded in producing metal oxide nanoparticles in a metastable phase, which had been thought to be impossible until now, regardless of the type of metal oxide with which we want to form an unstable phase, and regardless of the size of the average particle size. It can be supplied without

By the way, nanoparticles have physical properties that are different from bulk materials depending on their size. In order to control its physical properties, it is important to control the particle size and shape. However, with these synthetic methods based on nucleation and growth mechanisms, it is extremely difficult in principle to obtain uniform particle diameters.

According to this manufacturing method, since the seed particles are grown by introducing a particle precursor into 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 going through the above steps by the average particle diameter can be made smaller than the coefficient of variation of the seed particles before going through the steps. For example, if a particle group of seed particles with a particle size distribution in the double average particle size range of 5 nm to 10 nm grows by an average of 5 nm, the average particle size of the grown particles will be 10 to 15 nm, and the average particle size will be 10 to 15 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, hydrothermal treatment refers to a process for obtaining a metal oxide from a raw material by dissolving and precipitating the raw material in pressurized hot water. Hydrolysis of raw materials produces hydroxides, and dehydration condensation of the hydroxides produces metal oxides.
MA _x +xH ₂ O→M(OH) _x +xHA
M(OH) _X → MO _x/2 + (x/2)H ₂ O

Hydrothermal treatment is performed under subcritical water conditions or supercritical water conditions. In this embodiment, the subcritical water condition refers to a hot water state with a temperature and pressure close to the critical point (374° C., 218 atm (22.1 MPa)) in the phase diagram of water. Moreover, supercritical water conditions refer to a hot water state with a temperature and pressure exceeding the critical point. Under subcritical water conditions or supercritical water conditions, the viscosity of the aqueous solution is reduced, so that excellent penetrating power and intense hydrolytic action can be exhibited.

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

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

[Seed particle]
The seed particles are not particularly limited as long as they are nano-sized particles made of metal oxides, but in order to efficiently precipitate the metal oxide in the metastable phase on the surface of the seed particles, the material constituting the seed particles must also be semi-stable. Preferably, it is a stable phase. Therefore, the seed particle is a metal oxide in a metastable phase, and the lattice constant and crystal structure of the metal oxide constituting the seed particle are in the stable phase, which has the same chemical formula as the metal oxide constituting the seed particle. It is preferable that the lattice constant and crystal structure are different from those of the metal compound. For example, when it is desired to precipitate γ-phase iron oxide (III), which is a metastable phase, on the surface of the seed particles, it is preferable to use γ-phase iron oxide (III) as the seed particles.

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

The upper limit of the average particle diameter of the seed particles is not particularly limited, but it is important to maximize the exposed surface area (possibility of contact) when applying to many fields such as semiconductor materials, catalyst materials, and biological materials, and to form a metastable phase on the surface of the seed particles. From the viewpoint of ensuring a surface area sufficient to precipitate metal oxides, 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 100 nm or less. It is more preferable that the particle diameter is 50 nm or less, and 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.") during hydrothermal treatment is not particularly limited. From the viewpoint of ensuring a certain 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, more preferably 0.05 mol/l or more. Preferably, it is more preferably 0.1 mol/l or more. By ensuring the total surface area of the seed particles is above a certain level, when the metal oxide precursor is used to form a film on the surface of the seed particles, an excess of the metal oxide precursor remains, and the metal oxide precursor is stabilized from the excess metal oxide precursor. It is possible to suppress the generation of phase metal oxides.

The upper limit of the concentration of the seed particles is also not particularly limited, but considering the balance between 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 desired to form a metastable phase in an aqueous solvent, and the metal oxide precursor is an intermediate raw material obtained by mixing raw material components constituting the metal oxide desired to form a metastable phase in an aqueous solvent. Refers to substances that have not become oxides.

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

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

Among these, the metal oxide precursor is preferably a metal complex. Metal complexes are more stable in aqueous solvents than metal salts, so when metal oxide precursors are hydrothermally treated, they can prevent metal hydroxides from becoming supersaturated in solution, resulting in As a result, uniform nucleation of metal oxide precursors can be suppressed.

Moreover, it is preferable that the metal oxide precursor is dissolved in a basic solution. In this case, the solubility of the metal oxide precursor is higher than that under acidic conditions, and the degree of supersaturation of the metal hydroxide in the reaction field of hydrothermal treatment can be suppressed to a low level, resulting in heterogeneous nuclei on the surface of the seed particle. Generation can proceed dominantly.

The lower limit of the concentration of metal oxide precursors contained in supercritical water etc. when performing hydrothermal treatment is not particularly limited, but considering the balance between the amounts of seed particles and metal oxide precursors, it should be 0.01 mol/l or more. It is preferable that it is, and it is more preferable that it is 0.05 mol/l or more.

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

If the concentration of metal oxide precursors contained in the solution during hydrothermal treatment is lower than a predetermined threshold, metal hydroxides will be released in the solution despite applying the principle of redissolution/precipitation method. It is possible to prevent the metal oxide precursor from becoming supersaturated, causing uniform nucleation of the metal oxide precursor, and instead widening 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 preferably 0.5 mol/l or less. More preferably, it is 0.1 mol/l or less.

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

Usually, the aqueous solvent and the organic modifier undergo phase separation. However, in subcritical or supercritical water conditions, the organic modifier forms a homogeneous phase with water.

In this embodiment, the surface of the nanoparticles is 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, which is more stable than ions, which further increases the solubility of the organic modifier in subcritical or supercritical water. It is possible to proceed with Ostwald lightning even faster. Then, nanoparticles of metastable phase metal oxide having a diameter smaller than that of the seed particles can be synthesized in a larger amount than the total weight of the seed particles. In addition, the width of the particle size distribution can be further narrowed due to the effect of Ostwald brightening.

The organic modifier is not particularly limited as long as it can strongly bond hydrocarbons to the surface of fine particles, and is suitable for nanoparticle applications including the fields of organic chemistry, inorganic materials, and polymer chemistry. can be selected from organic substances that are widely known in the field.

Examples of organic modifiers include ether bonds, ester bonds, bonds via N atoms, bonds via S atoms, metal-C- bonds, metal-C= bonds, and metal-(C=O)- Examples include those that allow the formation of strong bonds such as bonds of The number of carbon atoms in the hydrocarbon is not particularly limited, and it may be one or two carbon atoms, but long-chain hydrocarbons having three or more carbon atoms are preferred, such as , straight chain, branched chain, or cyclic hydrocarbons having 3 to 20 carbon atoms.

Hydrocarbons 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 of the substituents may be present. In the case of multiple numbers, they may be the same or different.

Examples of organic modifiers include alcohols, aldehydes, ketones, carboxylic acids, esters, amines, thiols, amides, oximes, phosgene, enamines, amino acids, peptides, saccharides, etc. It will be done.

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

The above hydrocarbon group includes an optionally substituted linear or branched alkyl group, an optionally substituted cyclic alkyl group, an optionally substituted aryl group, and an optionally substituted aralkyl group. , an optionally substituted saturated or unsaturated heterocyclic group, and the like. Examples of substituents include carboxy group, cyano group, nitro group, halogen, ester group, amide group, ketone group, formyl group, ether group, hydroxyl group, amino group, sulfonyl group, -O-, -NH-, - Examples include S-.

The present invention will be specifically explained below using test examples in this embodiment, but the present invention is not limited thereto.

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

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

〔result〕
First, considering the possibility that the seed particles themselves would dissolve or denature under various conditions, a solution containing only the seed particles was hydrothermally treated at various pH values, but no dissolution of the particles occurred.

FIG. 1 shows the XRD pattern of particles obtained by hydrothermal treatment of an aqueous iron nitrate solution. In the sample obtained by hydrothermally treating the iron nitrate solution without adding seed particles, only the α-iron oxide peak was obtained under acidic conditions. Moreover, under basic conditions, a peak of α-iron oxide and a peak of FeO(OH) were obtained. This result is considered to be because uniform nucleation as shown in FIG. 2(A) occurred in the reaction field, and α-iron oxide was produced.

Under acidic conditions, only the peak of γ-iron oxide was obtained when the iron nitrate concentration was 0.056 mol/l, but the peak of α-iron oxide appeared under the condition of 0.11 mol/l. . Under the condition of iron nitrate concentration of 0.056 mol/l, the addition of seed particles caused heterogeneous nucleation, and γ-iron oxide precipitated on the surface of the seed particles, resulting in nuclear growth as shown in Figure 2 (B). It is thought that However, when the concentration of iron nitrate became high, the degree of supersaturation in the reaction field increased, and it is thought that homogeneous nucleation occurred more dominantly than heterogeneous nucleation.

On the other hand, under basic conditions, only the peak of γ-iron oxide was obtained under conditions where the iron nitrate concentration was 0.056 mol/l and 0.11 mol/l. Under acidic conditions, a peak of α-iron oxide was obtained at high concentrations, suggesting that basic conditions are more suitable for heterogeneous nucleation on the particle surface. As the temperature increases, the solubility of iron hydroxide decreases under acidic conditions and increases under basic conditions. Under basic conditions, because of the high solubility at the reaction temperature, the degree of supersaturation of iron hydroxide in the reaction field was reduced, and it is thought that heterogeneous nucleation on the particle surface predominantly occurred even at high concentrations.

<Test Example 2> Effect of Reaction Temperature The effect of reaction temperature will be studied using Ni/BaTiO ₃ core-shell nanoparticles synthesized using a water-soluble Ti complex as a model.

〔Test method〕
[reagent]
The following reagents were used in the test.
・Ni nanoparticles (average particle size: 80 nm)
・Ammonium titanium 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 Industries, Ltd., purity 98.0%)
・Potassium hydroxide (KOH, Fuji Film Wako Pure Chemical Industries, Ltd., purity 85.0%)

[Test equipment]
Experiments were conducted using the following equipment.
・Batch reactor (5 mL), material: Hastelloy, AKICO
・Shaking reactor heating 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, stirring, and then adjusting the pH to make it basic. Ni nanoparticles were added to this precursor solution and dispersed using ultrasound. Thereafter, it was sealed in a 5 mL batch reactor, and a reaction was performed using the reactor. Thereafter, the reactor was cooled with water to complete the reaction. The product was collected from the reactor, centrifugally washed and freeze-dried to obtain dry particles. In both cases, 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 conditions are shown in FIG. Further, FIG. 4 shows TEM images of particles synthesized by changing the concentration of Ni nanoparticles to 4.34 wt% and 0.51 wt%. Figure 4(A) is a TEM image of the generated particles when the Ni nanoparticle concentration is 4.34wt%, and Figure 4(B) is a TEM image of the generated particles when the Ni nanoparticle concentration is 0.51wt%. This is a TEM image of generated particles.

In both cases, uniform film formation on the surface of the Ni nanoparticles was confirmed. It was also confirmed that the smaller the amount of Ni nanoparticles added, the thicker the shell became. If BaTiO ₃ is uniformly coated on all surfaces of Ni nanoparticles, it is predicted that the coating thickness will be approximately 4 nm when Ni nanoparticles are added at 4.34 wt%, and approximately 20 nm when 0.51 wt% is added. . When 4.8 wt % of Ni nanoparticles were added, the film thickness was approximately as expected, but when 0.51 wt % of Ni nanoparticles were added, the film thickness was thinner than expected. The reason for this is thought to be that when 0.51 wt% of Ni nanoparticles were added, all of the BaTiO ₃ raw material was not consumed for film formation, and some unreacted material remained.

FIG. 5 is an XRD pattern of Ni nanoparticles and generated particles. No peak other than Ni was observed in any of the particles, indicating that the shell structure was not crystallized and was amorphous.

[Influence of reaction temperature and pH]
Next, we investigated the effect of reaction temperature on core-shell structure formation. The second test conditions are shown in FIG. Further, FIG. 7 shows the particles produced at each reaction temperature when Ni was added and when Ni was not added. It was found that when synthesis was performed using only the BaTiO ₃ precursor without adding Ni nanoparticles at 100° C., no particles were generated, indicating that the conditions were such that uniform nucleation did not occur. On the other hand, when Ni nanoparticles were added, BaTiO ₃ was uniformly coated on the surface of the Ni nanoparticles. From this, it can be seen that 100° C. is a condition where no nucleation occurs and only growth occurs. Further, when the synthesis was performed at 200° C., particles were generated even with only the BaTiO ₃ precursor, which indicates that the conditions are such that uniform nucleation occurs. Even when Ni nanoparticles were added, although a core-shell structure was formed, the shell structure was somewhat rough and uniform nucleation also occurred.
From the above, it was confirmed that low temperatures are more desirable for forming a uniform core-shell structure.

<Test Example 3> Control of nucleation and growth Using a model of Ni/BaTiO ₃ core-shell nanoparticles synthesized using a water-soluble Ti complex, study on the conditions under which nucleation and growth are dominant from kinetic analysis. I did it.

〔Test method〕
[reagent]
The following reagents were used in the test.
・Ni nanoparticles (average particle size: 80 nm)
・Ammonium titanium 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, Fuji Film Wako Pure Chemical Industries, Ltd.)

[Test equipment]
Experiments were conducted using the following equipment.
・Batch reactor (5 mL), material: Hastelloy, AKICO
・Shaking reactor heating stirring device, AKICO, SAH-R16-500

[Test method]
After adding an aqueous solution of ammonium titanium peroxocitrate to purified water and stirring, the pH was made acidic with hydrochloric acid to suppress the rate of hydrolysis, thereby obtaining a water-soluble Ti precursor. Ni nanoparticles were added to this precursor solution and dispersed using ultrasound. Thereafter, it was sealed in a 5 mL batch reactor and a reaction was performed. The amount charged into 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 collected from the reactor, centrifugally washed and freeze-dried to obtain dry particles.

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

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

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

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

〔result〕
From kinetic analysis, we investigated the conditions under which nucleation and growth become dominant. FIG. 8 shows Arrhenius plots of homogeneous nucleation, heterogeneous nucleation of TiO ₂ on the surface of Ni nanoparticles, and growth reactions of TiO ₂ with heterogeneous nucleation on the surface of Ni nanoparticles. Here, the rate constants of the heterogeneous nucleation and growth reactions of TiO ₂ are obtained by taking the difference between the rate constants of the early and late stages of the reaction when Ni nanoparticles are added and the rate constant of homogeneous nucleation. Determining the activation energy from the slope of this straight line, the activation energy for homogeneous nucleation is approximately 73 kJ/mol, the activation energy for the heterogeneous nucleation reaction on the Ni nanoparticle surface is approximately 46 kJ/mol, and the activation energy for TiO ₂ growth is approximately 73 kJ/mol. The activation energy of the reaction was approximately 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 the raw material is 89.1 kJ/mol (Satoshi Uchida et al., Color Materials, 72 (1999) 680-689). , which is relatively close to the activation energy of homogeneous nucleation obtained this time, indicating that the present results are valid. The slight deviation is thought to be due to differences in starting materials and measurement errors. Furthermore, since heterogeneous nucleation occurs more easily than homogeneous nucleation, it is thought that the activation energy is smaller. In addition, the reaction in which TiO ₂ grows on the same material is more likely to occur than the heterogeneous nucleation of TiO ₂ on the Ni nanoparticle surface, which is the heterogeneous nucleation in a different material. Although the activation energy has not changed significantly, the overall reaction rate has increased, and the straight line has shifted in the direction of a larger rate constant. Furthermore, it can be seen that the straight lines of homogeneous nucleation and heterogeneous nucleation on the Ni nanoparticle surface intersect at a temperature of about 272°C. Above this point, the rate of homogeneous nucleation exceeds the rate of heterogeneous nucleation. Therefore, it is thought that homogeneous nucleation occurs preferentially. On the other hand, at temperatures lower than this point, the rate of heterogeneous nucleation is higher, so it is thought that heterogeneous nucleation occurs preferentially. 9 and 10 show TEM images of particles synthesized under temperature conditions near this intersection. It can be seen that most of the particles synthesized at 300° C., which is higher than the intersection point, undergo uniform nucleation and do not form a core-shell structure. On the other hand, in the particles synthesized at 200° C., which is lower than the intersection point, although uniform nucleation occurred, heterogeneous nucleation and growth occurred, and it was confirmed that a core-shell structure was formed.

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

<Test Example 4> Control of crystal growth using an organic modifier Using the morphological change of cerium (IV) oxide nanoparticles due to supercritical hydrothermal treatment using decanoic acid as an organic modifier as a model, crystal growth using an organic modifier was Consider growth control.

〔Test method〕
10.3 mg (0.02 mol/l) of ceria powder modified with decanoic acid (manufactured by ITEC) was transferred to a 5 mL batch reactor. Then 2.5 mL of 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) of decanoic acid were added without stirring. added. The reactor was heated to 400°C for 10 minutes and then quenched in a water bath (20°C). The precipitate was dispersed with hexane, ethanol was added, and centrifuged once. The obtained nanocrystals were dissolved in cyclohexane. Samples were analyzed by high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA).

〔result〕
Figure 11 shows that the shape and size of modified nanoparticles changed after supercritical hydrothermal treatment. As the concentration of organic modifier increased, the morphology of nanoparticles changed from spherical to cubic. This proves that modified ceria nanoparticles can be redissolved and grown in supercritical water. Furthermore, modifiers on the surface of the particles and in the hydrothermal control the growth process.

From FIG. 12, the particle size distribution of the samples processed under low modifier concentration conditions shows a wide range of particle size distribution, growth of large particles and dissolution of small particles. The result is inferred to be due to Ostwald growth. However, the results for the high modifier concentration samples showed a narrow particle size distribution, and the absence of nanoparticles smaller than 4 nm could be attributed to oriented agglomeration.

Claims (14)

Core- shell nanoparticles in which metastable metal oxides are precipitated on the surface of seed particles . It has a precipitated layer of metal oxide in a metastable phase on the surface of the seed particle ,
The deposited layer is a core- shell nanoparticle including an epitaxial layer, a polycrystalline layer, or an amorphous layer. The nanoparticles according to claim 1 or 2, wherein a coefficient of variation obtained by dividing the standard deviation of the average particle diameter by the average particle diameter is 0.15 or less. Nano-sized particles of metal oxide are used as seed particles, and by hydrothermally treating the metal oxide precursor under subcritical or supercritical water conditions in the presence of the seed particles, the surface of the seed particles becomes metastable. A method for producing core- shell nanoparticles, comprising the step of precipitating a metal oxide phase. 5. The manufacturing method according to claim 4, wherein the seed particles are metal oxides in a metastable phase. The production according to claim 4 or 5, wherein the lattice constant of the metal oxide constituting the seed particle is different from the lattice constant of a stable phase metal compound having the same chemical formula as the metal oxide constituting the seed particle. Method. 7. The crystal structure of the metal oxide constituting the seed particle is different from the crystal structure of a stable phase metal compound having the same chemical formula as the metal oxide constituting the seed particle. Manufacturing method described. The manufacturing method according to any one of claims 4 to 7, wherein the concentration of the seed particles contained in the solution when performing the hydrothermal treatment is 0.01 mol/l or more. The manufacturing method according to any one of claims 4 to 8, wherein the concentration of the metal oxide precursor contained in the solution when performing the hydrothermal treatment is 1 mol/l or less. The temperature of the hydrothermal treatment is a temperature at which the rate at which the metal oxide precursor generates heterogeneous nuclei is higher than the rate at which the metal oxide precursor uniformly generates nuclei. The manufacturing method described in. The manufacturing 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 manufacturing method according to any one of claims 4 to 12, wherein the hydrothermal treatment is performed in the presence of an organic modifier. Claims 4 to 13, wherein a coefficient of variation obtained by dividing the standard deviation of the average particle diameter of the nanoparticles after passing through the step by the average particle diameter is smaller than the coefficient of variation of the seed particles before going through the step. The manufacturing method according to any one of.

Images