JPH1081975A - Coating method of micro-body surface with metal oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a uniform and homogeneous coating film of a metal oxide by dissolving a metallic salt in an organic solvent, adding a micro-body thereinto, dropping an organic solvent containing a hydroxide ion and after dropping, refluxing. SOLUTION: A salt containing the desired metal. oxide is dissolved in the organic solvent having polarity at first and after that, the micro-body having a functional group on the surface is added. And the organic solvent containing the hycdroxide ion is dropped under stirring. At this time, though the metallic ion M becomes a colloid particle of a metallic hydroxide and gradually grows, the colloidal. particle exceeding a certain size becomes unstable because of the reaction in the organic solvent. When the colloidal particle meets the micro- body P having negative charge on the surface, both are rapidly combined with each other as shown by the arrow and the surface of the micro-body P is coated with the metallic hydroxide. After the dropping is finished, the metallic hydroxide is converted to the metal oxide with the heat by refluxing and the desired coating is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a coating film made of a metal oxide on a surface of a minute body, for example, an organic polymer particle or an inorganic minute body, and relates to a conductive material, a magnetic material, and a catalyst. , Pigments, antibacterial agents and the like.

[0002]

2. Description of the Related Art Conventionally, there have been the following methods for forming a coating film made of a metal oxide on the surface of a minute body, but each has its own problems.

Dry blending method: A minute body and a metal oxide are physically and mechanically mixed, and the surface of the minute body is
This is a method in which a metal oxide is attached mainly by heat fusion. Since external energy such as mechanical energy is applied, the minute bodies and the coating film are liable to be deteriorated or deformed. Therefore, the desired product may not be obtained.
Further, it is difficult to obtain a coating film having a uniform thickness or a uniform coating film, and it is difficult to mass-produce the film.

Deposition method: For example, titanium oxide is dispersed in an aqueous medium to form a slurry, a water-soluble zinc compound is added and dissolved, the pH is adjusted, and the mixture is baked and crushed. Is the way. It is difficult to obtain a coating film having a uniform thickness. Also, it is difficult to control the film thickness. Furthermore, since a crushing operation is required, it is troublesome.

[0005] Sol-gel method: A method in which a metal alkoxide solution is adhered to the particle surface, which is hydrolyzed and then fired. It is difficult to obtain a coating film having a uniform thickness. Also, it is difficult to control the film thickness. Furthermore, individual particles are highly likely to aggregate, requiring operations such as pulverization, which is troublesome.

A method of oxidizing a metal coating film: a method of forming a metal coating film by plating or the like and then chemically oxidizing the metal coating film to form a metal oxide coating film. A coating film having a large thickness cannot be obtained.

The present invention has been made to solve the above-mentioned various problems of the conventional method, and it is possible to obtain a uniform and uniform metal oxide coating film, and to easily control the film thickness. It is an object of the present invention to provide a method for coating a surface of a minute body with a metal oxide, which can be performed with a simple facility and with a simple operation.

[0008]

According to the first aspect of the present invention,
In a method of forming a coating film made of a metal oxide on the surface of the microscopic body, a metal salt containing a metal of a target metal oxide is dissolved in an organic solvent having a polar portion in a molecule, and a dissolving step. In the organic solvent, adding a microparticle having a functional group on the surface, the addition step, and in the organic solvent, an organic solvent containing hydroxide ions is dropped under stirring, a dropping step, A reflux step of refluxing or heating and refluxing after the completion of the dropwise addition.

[0009] The invention according to claim 2 is the invention, wherein
In a method of forming a coating film made of a metal oxide, a metal salt containing a metal of a target metal oxide is dissolved in an organic solvent having a polar portion in a molecule, and a dissolving step is performed in the organic solvent. Adding a microparticle having a functional group on the surface, the addition step and, in the organic solvent, an organic solvent containing hydroxide ions is added dropwise with stirring, and the organic solvent is refluxed or heated to reflux, It is characterized by comprising a dropping / refluxing step.

According to a third aspect of the present invention, in the configuration according to the first or second aspect, the fine particles are organic polymer particles.

According to a fourth aspect of the present invention, in the configuration according to the first or second aspect, the minute body is an inorganic minute body.

The fine particles have a particle size of 0.01 to 100 μm.
Fine particles of size m, diameter 0.1-100 μm, length 1
Fibers or whiskers having a size of １０００1000 μm, flakes having a diameter of 0.1 to 600 μm and a thickness of 0.1 to 100 μm, and the like can be given.

Although the metal is not particularly limited, for example, Cu, Mg, Ca, Sr, Ba, Zn, Cd, H
g, Al, Ga, In, Y, B, Si, Ge, Sn, P
b, Ti, Zr, As, Sb, Bi, V, Nb, Ta,
Se, Cr, Mo, W, Mn, Fe, Co, Ni, L
a, Ce, Sm, etc. are used.

Examples of the metal salt include a sulfate, a nitrate, a chloride, a carboxylate, a complex salt, an alkoxide and the like. Examples of the complex salt include an acetylacetone complex salt, an EDTA complex salt, and an ammine complex salt. These metal salts
Used alone or as a mixture.

Examples of the organic solvent include alcohols such as methanol, ethanol, propanol and butanol; polyhydric alcohols such as ethylene glycol; ethers such as ethyl ether, anisole and diphenyl ether;
Ketones such as acetone and methyl ethyl ketone, sulfur compounds such as dimethyl sulfoxide, nitrogen compounds such as N, N-dimethylformamide, and hydrocarbons such as benzene, toluene and xylene are used alone or in combination.

As the organic polymer particles having a functional group on the surface, any of those having a functional group on the surface in advance or those having a functional group added later may be used, but the latter is preferably used. As a method for obtaining the latter, a plasma treatment, a chemical oxidation treatment, an ozone treatment, an ultraviolet irradiation treatment, or the like is employed. As the plasma treatment, low-temperature microwave plasma treatment, high-frequency plasma treatment, or the like using oxygen is used. As the chemical oxidation treatment, a method of immersion in a solution of chromic acid, chloric acid, sulfuric acid-bichromic acid, sulfuric acid-perchloric acid, or the like is used.

The organic solvent containing hydroxide ions includes
NaOH, KOH, NH ₄ OH, is obtained by addition of N ₂ H ₄ · H ₂ O or the like in an organic solvent is used.

The dropping rate is such that a sufficient amount of an organic solvent containing hydroxide ions is dropped over 10 minutes or more, preferably 15 to 60 minutes. However, depending on the type of the metal salt, it may be added all at once.

In the dissolving step of the first or second aspect of the present invention, as shown in FIG. 1, the metal ion M is solvated by its polar part, and the functional group of the microparticle P is also solvated by its polar part. Solvated.

When the organic polymer particles are subjected to, for example, low-temperature oxygen plasma treatment, oxygen-containing functional groups such as a carboxyl group and a phenolic hydroxyl group are imparted to the particle surface, and the particle surface is polarized. On the other hand, inorganic fine particles have a hydroxy group on the surface.

After the dissolving step and the adding step, in the dropping step of the first aspect, as shown in FIG. 2, the metal ions M become colloidal particles of the metal hydroxide and gradually grow larger. The colloid particles are also called polynuclear complex ions or metal oxide ions. These colloidal particles have a positive charge on their surface and are solvated,
The charge decreases as the colloidal particles grow. Incidentally, the stability of the metal ions M and the colloid particles is affected by the strength of the solvation, that is, the amount of charge of the metal and the colloid particles and the strength of the polar portion of the organic solvent. In the present invention, since the reaction is performed not in an aqueous solution but in an organic solvent,
The growing colloidal particles are held in a certain size, but become unstable after the size is exceeded. Then, when the unstable colloidal particles meet the minute body P having a negative charge on the surface, as shown by the arrow in FIG.
Both bind quickly. That is, the coating of the metal hydroxide is performed on the surface of the minute body P. By the way, when the reaction is performed not in the organic solvent but in the aqueous solution, the functional groups of the colloid particles and the microparticles P are both strongly solvated by water molecules. It is held stably until it aggregates with each other and precipitates as a metal hydroxide, and the coating of the metal hydroxide on the surface of the microparticles P is not performed.

In the reflux step of the first aspect, the metal hydroxide is changed to a metal oxide by heat. As a result, the surface of the minute body P is coated with the metal oxide. If it does not completely change to a metal oxide, it may be refluxed or heated under reflux with an organic solvent having a higher boiling point.

In the dropping / refluxing step of the second aspect, the fine particles P
At the same time as the metal hydroxide is precipitated on the surface of the metal oxide. According to this, it is possible to use a metal salt that does not dissolve in an organic solvent unless heated, and further improves the solubility of the metal salt in the organic solvent.

According to the third aspect of the present invention, a metal oxide coating film is formed on the surface of the organic polymer particles.

According to the fourth aspect of the present invention, a coating film of a metal oxide is formed on the surface of the inorganic fine particles.

According to a fifth aspect of the present invention, in the configuration of the first or second aspect, the operation including one step is repeated a plurality of times. Using a metal different from the melting step, in the addition step of the second and subsequent operations, the microparticles coated in the previous operation are added.

According to the fifth aspect of the present invention, a multi-layered metal oxide coating film is formed on the surface of the minute body by the operation repeated a plurality of times.

According to a sixth aspect of the present invention, in the configuration according to the first or second aspect, when refluxing or heating under reflux, all or a part of the organic solvent is removed to concentrate the dissolved components.

The solvent removal rate is 25 to 100%, preferably 50 to 80%.

In the addition step, not all of the metal ions are converted to metal hydroxide, and the amount of metal hydroxide corresponding to the solubility remains in the solvent in a dissolved state. Therefore, when the solubility of the metal hydroxide is large,
The yield of forming the coating film is deteriorated. However, according to the structure of the sixth aspect, the removal of the organic solvent increases the metal ion concentration in the solvent, so that the reaction is promoted and the yield is improved.

In general, the colloidal particles of metal hydroxide have a positive charge, and among the fine particles, the organic polymer particles are formed by the functional groups such as hydroxyl group and carboxyl group on the surface, and the inorganic fine particles are Each is negatively charged by the hydroxy group on its surface. However, when the charge of the colloidal particles is negative, the microparticles may be treated so as to have a functional group having a positive charge, and the coating can be similarly performed. As the processing, for example, microwave plasma processing of a gas such as N ₂ or NH ₃ can be mentioned.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) Organic polymer fine particles were previously
Microwave low-temperature oxygen plasma treatment was performed under the conditions of W, 8 Torr, and 15 min. As a result, oxygen-containing functional groups such as a carboxyl group and a phenolic hydroxyl group were provided on the surface of the organic polymer fine particles. In addition, as this organic polymer fine particle, a brand name "Fine Pearl PB-3000"
(Manufactured by Sumitomo Chemical Co., Ltd.). Its main component was polystyrene, and its average particle size was 15 μm.

First, 0.05 mol of bis (acetylacetonato) zinc was dissolved in 1000 ml of anisole (dissolution step). Next, 6.6 g of the above-mentioned plasma-treated organic polymer fine particles were added to and dispersed in this solution (addition step). Next, 1000 ml of methanol containing 0.2 mol of sodium hydroxide was added dropwise to this solution over 30 minutes with stirring (dropping step). Then, after the completion of the dropwise addition, the mixture was heated under reflux at 150 ° C. for 2 hours, and the solution amount was 500 ml.
The solvent was removed until (25% of the total amount) (reflux step). The fine particles thus obtained were filtered, washed and dried. The obtained fine particles were white. As a result of X-ray diffraction measurement, it was confirmed that the deposit on the surface was zinc oxide.

FIG. 3 is an SEM photograph of the obtained fine particles, and FIG.
Is the S of uncoated polystyrene particles
It is an EM photograph. As can be seen from comparison of both figures, a zinc oxide coating film having a uniform thickness was formed on the surface of the fine particles obtained in the present embodiment. When the amount of zinc oxide carried on the fine particles was measured, it was 4.0.
3 g, and the yield of zinc was 99.0%.

(Embodiment 2) The same treatment as in Embodiment 1 was carried out except that the solvent was not removed in the reflux step. The obtained fine particles are white, and the supported amount of zinc oxide is 2.7.
The yield of zinc was 68.3%.

(Embodiment 3) First, anisole 1000
ml of bis (acetylacetonato) nickel 0.05
mol was dissolved (dissolution step). Next, to this solution:
7.8 g of spherical alumina having an average particle size of 30 μm was added and dispersed (addition step). Next, to this solution was added, with stirring, methanol 1000 containing 0.2 mol of sodium hydroxide.
ml was dropped over 30 minutes (dropping step). After completion of the dropwise addition, the mixture was heated under reflux at 150 ° C. for 2 hours, and the amount of the solution was reduced to 50
The solvent was removed until the volume became 0 ml (25% of the total amount) (reflux step). The fine particles thus obtained were filtered, washed and dried. The obtained fine particles were grayish green. As a result of X-ray diffraction measurement, it was confirmed that the deposit on the surface was nickel monoxide.

FIG. 5 is an SEM photograph of the obtained fine particles, and FIG.
Is SEM of spherical alumina without coating
It is a photograph. As can be seen by comparing the figures, a nickel monoxide coating film having a uniform thickness was formed on the surface of the fine particles obtained in the present embodiment. When the amount of nickel monoxide carried on the fine particles was measured, it was 3.70 g, and the yield of nickel was 99.
1%.

(Comparative Embodiment 1) The dropping step was performed at a stroke without taking any time, the refluxing step was omitted, and the other steps were the same as in Embodiment 1. According to this, a white precipitate was formed, and the coating of the organic polymer fine particles was not performed.

(Embodiment 4) The organic polymer particles were previously subjected to a chemical oxidation treatment by dipping them in a 10% chromic acid solution at 20 ° C. for 1 hour. Thereby, oxygen-containing functional groups such as a carboxyl group and a hydroxyl group were provided on the surface of the organic polymer fine particles. In addition, as the organic polymer fine particles,
Product name "Nylon-6 micro sponge SNP-61
0 "(manufactured by Metal Color Algae Laboratories, Inc.).
Its main component is nylon and its average particle size is 10 μm
Met.

First, cupric sulfate x ₁ mol was dissolved in 1000 ml of methanol (dissolution step). Next, 5.16 g of the above-mentioned chemically oxidized organic polymer fine particles were added to and dispersed in this solution (addition step). next,
To this solution, 1000 ml of isopropyl alcohol containing y ₁ mol of potassium hydroxide was added dropwise over 15 minutes with stirring (dropping step). Note that x ₁ and y ₁ are as shown in Table 1. After completion of the dropwise addition, the mixture was heated under reflux at 75 ° C. for 1 hour, and the solvent was removed until the solution volume reached 600 ml (30% of the total amount) (reflux step). The fine particles thus obtained were filtered, washed and dried. The obtained fine particles were black. As a result of X-ray diffraction measurement, it was confirmed that the deposit on the surface was cupric oxide.

[0041]

[Table 1]

FIG. 7 is a SEM photograph of the obtained fine particles when cupric sulfate is 0.05 mol, FIG. 8 is also a case where the same is 0.10 mol, and FIG. FIG. 10 is an SEM photograph of “Nylon-6 micro sponge SNP-610” without coating. As can be seen from these figures, a cupric oxide coating film having a uniform thickness was formed on the surface of each of the obtained fine particles. Table 2 shows the amount of cupric oxide carried and the yield of copper in the obtained fine particles. As can be seen from Table 2, the supported amount of cupric oxide is proportional to the molarity of the added cupric sulfate.

[0043]

[Table 2]

(Embodiment 5) First, methanol 1000
Then, ₂ ml of zinc acetate was dissolved in each ml (dissolution step). Next, 6.05 g of tungsten carbide having an average particle size of 8.7 μm was added to and dispersed in this solution (addition step). Next, 1000 ml of isopropyl alcohol containing potassium hydroxide y ₂ mol was added to this solution while stirring.
Was added dropwise over 15 minutes (dropping step). The values of x ₂ and y ₂ are as shown in Table 3. Then, after the completion of the dropwise addition, the mixture was heated under reflux at 75 ° C. for 1 hour, and the solution amount was 600 ml.
(30% of the total amount) and the solvent was removed (reflux step). The fine particles thus obtained were filtered, washed and dried. The obtained fine particles were white. As a result of X-ray diffraction measurement, it was confirmed that the deposit on the surface was zinc oxide.

[0045]

[Table 3]

FIG. 11 shows a case where zinc acetate is 0.05 mol, FIG. 12 shows a case where zinc acetate is also 0.10 mol, and FIG.
3 is an SEM photograph of the obtained fine particles in the case of 0.20 mol, and FIG. 14 is an SEM photograph of tungsten carbide without coating. As can be seen from these figures, a zinc oxide coating film having a uniform thickness was formed on the surface of each of the obtained fine particles. Table 4 shows the supported amount of zinc oxide and the yield of zinc in the obtained fine particles. As can be seen from Table 4, the supported amount of zinc oxide is proportional to the molar concentration of the added zinc acetate.

[0047]

[Table 4]

(Embodiment 6) First, tri (acetylacetonato) iron 0.1 was added to 1000 ml of diphenyl ether.
mol was dissolved (dissolution step). Next, to this solution:
The fine particles obtained in the first embodiment, that is, 10.0 g of organic polymer fine particles coated with zinc oxide were added and dispersed (addition step). Next, 1000 ml of dimethyl sulfoxide containing 1.0 mol of hydrazine hydrate was added dropwise to this solution over 60 minutes with stirring (dropping step).
After completion of the dropwise addition, the mixture was heated under reflux at 250 ° C. for 3 hours, and the solvent was removed until the solution amount reached 400 ml (20% of the total amount) (reflux step). The fine particles thus obtained were filtered, washed and dried. The obtained fine particles were reddish brown. As a result of X-ray diffraction measurement, it was confirmed that the deposit on the surface was iron sesquioxide.

FIG. 15 is an SEM photograph of the obtained fine particles. The obtained fine particles, on the surface of the organic polymer fine particles,
A two-layer coating film composed of a coating film of zinc oxide having a uniform thickness and a coating film of iron sesquioxide thereon having a uniform thickness was further formed. The amount of iron sesquioxide supported on the obtained fine particles was measured, and it was 7.93 g, and the yield of iron was 99.4%.

(Embodiment 7) First, tri (acetylacetonato) iron 0.1 was added to 1000 ml of diphenyl ether.
mol was dissolved (dissolution step). Next, to this solution:
The fine particles obtained in Embodiment 3, that is, 10.0 g of spherical alumina coated with nickel oxide were added and dispersed (addition step). Next, 1000 ml of dimethyl sulfoxide containing 1.0 mol of hydrazine hydrate was added dropwise to this solution over 60 minutes with stirring (dropping step). After completion of the dropwise addition, the mixture was heated under reflux at 250 ° C. for 3 hours, and the solvent was removed until the solution amount reached 400 ml (20% of the total amount) (reflux step). The fine particles thus obtained are filtered,
Washed and dried. The obtained fine particles were reddish brown. X
As a result of the line diffraction measurement, it was confirmed that the deposit on the surface was iron sesquioxide.

FIG. 16 is an SEM photograph of the obtained fine particles. The obtained fine particles had a two-layer coating film composed of a coating film of nickel oxide having a uniform thickness and a coating film of iron sesquioxide thereon having a uniform thickness formed on the surface of spherical alumina. Was. The amount of iron sesquioxide carried on the obtained fine particles was measured to be 7.91 g, and the iron yield was 99.1%.

(Another Embodiment) In each of Embodiments 1 to 7, when the dropping step and the refluxing step were performed simultaneously, the same results were obtained.

[0053]

According to the first aspect of the present invention, colloidal particles of metal hydroxide are formed from the metal salt dissolved in the organic solvent in the dissolving step by the dropping step, and the colloidal particles are bonded to the surface of the minute body. Since it can be changed to a metal oxide by the reflux step, a coating film of the metal oxide can be formed on the surface of the minute body.

Further, the film thickness can be easily controlled only by changing the amount of the metal salt.

Further, since each process is simple, all the operations can be performed with simple and simple equipment.

According to the second aspect of the present invention, colloidal particles of metal hydroxide are formed from the metal salt dissolved in the organic solvent in the dissolving step by the dropping / refluxing step, and the colloidal particles are bonded to the surface of the microparticles. At the same time, it can be changed to a metal oxide, so that a coating film of the metal oxide can be formed on the surface of the minute body. The other effects are the same as those of the first aspect.

According to the third aspect of the invention, a coating film of a metal oxide can be formed on the surface of the organic polymer particles.

According to the fourth aspect of the present invention, a coating film of a metal oxide can be formed on the surface of the inorganic fine particles.

According to the fifth aspect of the present invention, it is possible to form a multi-layered metal oxide coating film on the surface of the minute body.

According to the sixth aspect of the invention, the yield of forming a coating film can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a reaction state in a solution in a dissolving step of the present invention.

FIG. 2 is a schematic diagram showing a reaction state in a solution in a dropping step of the present invention.

FIG. 3 is a photograph replacing a drawing showing a particle structure,
3 is an SEM photograph showing the fine particles obtained in Embodiment 1.

FIG. 4 is a photograph replacing a drawing showing a particle structure,
SE showing uncoated polystyrene particles
It is an M photograph.

FIG. 5 is a photograph replacing a drawing showing a particle structure,
9 is an SEM photograph showing the fine particles obtained in Embodiment 3.

FIG. 6 is a photograph replacing a drawing showing a particle structure,
5 is an SEM photograph showing spherical alumina without coating.

FIG. 7 is a photograph replacing a drawing showing a particle structure,
9 is an SEM photograph showing fine particles obtained in a case where cupric sulfate is 0.05 mol in Embodiment 4.

FIG. 8 is a photograph replacing a drawing showing a particle structure,
9 is an SEM photograph showing fine particles obtained when the cupric sulfate is 0.10 mol in Embodiment 4.

FIG. 9 is a photograph replacing a drawing showing a particle structure,
10 is an SEM photograph showing fine particles obtained when the cupric sulfate is 0.20 mol in the fourth embodiment.

FIG. 10 is a SEM photograph showing a non-coated “Nylon-6 micro sponge SNP-610” as a substitute for a drawing showing a particle structure.

FIG. 11 is a photograph instead of a drawing showing a particle structure, and is an SEM photograph showing fine particles obtained in a case where zinc acetate is 0.05 mol in Embodiment 5.

FIG. 12 is a photograph instead of a drawing showing a particle structure, and is an SEM photograph showing fine particles obtained when the amount of zinc acetate is 0.10 mol in Embodiment 5.

FIG. 13 is a photograph replacing a drawing showing a particle structure, and is an SEM photograph showing fine particles obtained when 0.20 mol of zinc acetate is used in Embodiment 5.

FIG. 14 is a SEM photograph showing an uncoated tungsten carbide, which is a photograph replacing a drawing showing a particle structure.

FIG. 15 is a photograph replacing a drawing showing a particle structure, and is an SEM photograph showing the fine particles obtained in Embodiment 6.

FIG. 16 is a SEM photograph showing fine particles obtained in Embodiment 7, which is a photograph replacing a drawing showing a particle structure.

Claims (6)

[Claims] 1. A method for forming a coating film made of a metal oxide on the surface of a minute body, comprising dissolving a metal salt containing a metal of a target metal oxide in an organic solvent having a polar portion in a molecule. Dissolving step, adding a microparticle having a functional group on the surface to the organic solvent, adding the organic solvent containing hydroxide ion in the organic solvent,
A method for coating a surface of a microparticle with a metal oxide, comprising: a dropping step of dropping under stirring; and a refluxing step or heating and refluxing after completion of the dropping. 2. A method for forming a coating film made of a metal oxide on the surface of a minute body, comprising dissolving a metal salt containing a metal of a target metal oxide in an organic solvent having a polar portion in a molecule. Dissolving step, adding a microparticle having a functional group on the surface to the organic solvent, adding the organic solvent containing hydroxide ion in the organic solvent,
A method of coating the surface of the microparticles with a metal oxide, comprising: a step of dropping and refluxing, wherein the organic solvent is refluxed or heated under reflux while dropping under stirring. 3. The method according to claim 1, wherein the fine particles are organic polymer particles. 4. The minute body is an inorganic minute body.
Or the coating method of the metal oxide on the surface of the microparticle as described in 2 above. 5. The method according to claim 1, wherein the operation including one step is repeated a plurality of times. In the second and subsequent operations, a metal different from the previous operation is used, and in the second and subsequent operations, an additional process is performed. 3. The method according to claim 1, wherein the fine particles coated in the previous operation are added. 6. The method of coating a microparticle surface with a metal oxide according to claim 1 or 2, wherein when refluxing or heating under reflux, the organic solvent is removed to remove all or a part of the organic solvent to concentrate the dissolved components.