JP2022151848A - Photocatalytic material production method and photocatalytic material - Google Patents

Abstract

To provide a photocatalytic material with remarkably excellent photocatalytic activity.SOLUTION: A production method according to the present invention includes a nanosizing step of obtaining metal oxide nanoparticles in a rapid temperature rising flow system reaction field of a supercritical or subcritical aqueous solvent, or a nanosizing step of organically modifying metal oxides in a reaction field of a supercritical, subcritical, or gas-phase aqueous solvent to obtain metal oxide nanoparticles. The nanosizing step preferably includes a step that is carried out in the presence of a transition element to obtain metal oxide nanoparticles having organically modified surfaces and doped with the transition element. In addition, the metal oxide nanoparticles are preferably metastable single-crystalline, and preferably are octahedral and/or hexahedral cerium oxide nanoparticles.SELECTED DRAWING: Figure 5

Description

TECHNICAL FIELD The present invention relates to a method for producing a photocatalyst material and a photocatalyst material.

A photocatalytic material exhibits important functions (photocatalytic activity) such as a stain decomposition function and a nitrogen oxide removal function by developing superhydrophilicity and/or generating active oxygen species when exposed to light (ultraviolet light).

Utilizing this function, paints, coating films, etc. containing photocatalyst materials have been proposed and put into practical use as antifouling paints, environment-cleaning paints, coating products, and the like. In recent years, there is an increasing demand for a function to remove harmful substances and microorganisms that are harmful to the health of consumers, especially microorganisms such as bacteria and mold. is expected.

Various materials are known as photocatalyst materials (see Patent Document 1). In addition, when the size of the particles constituting the photocatalyst material is nano-sized, the decrease in porosity due to grain growth is minimized, and the photocatalytic function is higher than that of the photocatalyst material with micro-sized particles. is known (see Patent Document 2). In addition, it is known that particles doped with a photocatalytic material exhibit higher photocatalytic functions than non-doped particles (see Patent Document 3).

Japanese Patent Publication No. 2017-533816 JP 2011-225396 A Japanese Patent Publication No. 2019-531185

By the way, since the photocatalytic reaction occurs on the particle surface, it is desirable to reduce the catalyst particle size and increase the specific surface area in order to improve the catalytic activity per unit volume (weight). The principle of the photocatalytic reaction can be understood as generating an oxidation/reduction reaction at the interface in a solar cell system in which electrons (holes) excited by light are recovered as energy through electrodes. Therefore, the reduction of defects and dangling bonds in the material and at the interface, which has been an issue in the development of solar cell materials, is equally important. Bandgap control of photoresponse by doping is also important. In other words, it is important to synthesize nano-single crystals that can be doped. On the other hand, it is known that metal oxide nanoparticles can be synthesized using supercritical technology, and the nanoparticles obtained are also known to be single crystal particles. It is also known that this can increase the doping amount and that the exposed surface of the nanoparticles can be controlled by allowing organic molecules to coexist. In addition, metal oxide nanoparticles obtained by supercritical technology are known to be excellent as oxygen carrier particles in the chemical loop method (International Publication No. 2015/025941 pamphlet). Therefore, metal oxide nanoparticles obtained by supercritical technology can be utilized as a photocatalyst material.

However, the photocatalyst technology is a technology for changing the electronic state by light energy, as described above. In other words, the photocatalyst technology is a technology that separates electrons and holes by suppressing their recombination and causes chemical reactions between them at the interface. Nanoparticulation of the photocatalyst material using supercritical technology and control of the exposed surface lead to changes in the electronic state of the received light. It is not clear whether the nanoparticle technology is effective, ie whether the metal oxide nanoparticles obtained by supercritical technology have high photocatalytic activity. Moreover, it is not obvious whether a photocatalyst single crystal material with few defects can be synthesized in a non-equilibrium system with a large supercritical reaction field.

The present invention has been made in view of such problems, and experiments have shown that metal oxide nanoparticles obtained using supercritical technology have higher photocatalytic activity than conventionally known photocatalytic materials. It is an object of the present invention to provide a photocatalyst material that is remarkably excellent in photocatalytic activity by clarifying the photocatalytic activity.

The inventors of the present invention have made intensive studies to achieve the above problems, and as a result, obtained metal oxide nanoparticles by processing metal oxide particles in the presence of organic molecules in a reaction field such as supercritical. Thus, the inventors have found that the above problems can be solved, and have completed the present invention. Specifically, the present invention provides the following.

The invention according to a first feature provides a method for producing a photocatalyst material, including a nano-ization step of obtaining metal oxide nanoparticles in a flow system reaction field accompanied by a rapid temperature rise of a supercritical or subcritical aqueous solvent. .

By this method, it is possible to obtain an extremely high degree of supersaturation compared to the conventional liquid-phase method and gas-phase method. can do. As a result, it is possible to provide a photocatalyst material having significantly superior photocatalytic activity compared to conventionally known photocatalyst materials.

The invention according to a second feature comprises a nano-ization step of organically modifying a metal oxide in a reaction field of a supercritical, subcritical, or gas phase aqueous solvent to obtain metal oxide nanoparticles having an organically modified surface. A method for producing a photocatalytic material is provided, comprising:

According to the second aspect of the invention, it is possible to provide a photocatalyst material having significantly superior photocatalytic activity compared to conventionally known photocatalyst materials.

Metal oxides can be further nano-sized by reacting them while capping organic molecules in a supercritical reaction field. In addition, by organically modifying the surface of metal oxide nanoparticles having a hydrophilic surface with a hydrophobic group such as an organic group such as a hydrocarbon, particles that are difficult to recover from an aqueous medium can be easily and reliably collected. It can be separated and recovered by moving to the organic medium side. This organic modification allows the nanoparticles with controlled exposed crystal surfaces to be recovered without changing their shape. In addition, since the photocatalyst device is manufactured by a coating process, it has a high affinity with the solvent, and it is possible to develop low viscosity while dispersing the nanoparticles at a high concentration.

The invention according to a third feature is the invention according to the first or second feature, wherein the nano-forming step is performed in the presence of a transition element, and the surface is organically modified and doped with the transition element. A method is provided comprising the step of obtaining a sintered metal oxide nanoparticle.

According to the invention according to the third feature, it is possible to use metal oxide nanoparticles having a higher doping amount than in the case of doping by other methods, and as a result, a photocatalytic material having even better photocatalytic activity. can provide

The invention according to a fourth aspect provides the method according to any one of the first to third aspects, wherein the metal oxide nanoparticles are metastable and monocrystalline.

A fifth aspect of the invention provides a method according to any one of the first to fourth aspects, wherein the metal oxide nanoparticles include octahedral and/or hexahedral cerium oxide nanoparticles.

According to the inventions according to the fourth and fifth characteristics, the metal oxide nanoparticles are metastable and single-crystalline, and as a result, a photocatalytic material having even better photocatalytic activity can be provided.

A sixth aspect of the invention provides a method according to any one of the first to fifth aspects, wherein the metal oxide nanoparticles have an average particle size of 100 nm or less.

According to the sixth aspect of the invention, since the average particle size is 100 nm or less, it is possible to provide a photocatalyst material having even better photocatalytic activity.

The invention according to a seventh feature is the invention according to any one of the first to sixth features, wherein the metal oxide nanoparticles obtained by the nanoization step are further hydrothermally treated in a supercritical or subcritical reaction field. By doing so, a method is provided which further includes a step of applying high crystallization and low defects.

The invention according to an eighth feature is the invention according to any one of the first to seventh features, wherein the metal oxide nanoparticles obtained by the nanoization step are allowed to coexist with oxygen under hydrothermal conditions, A method is provided further comprising the step of hypoxic depletion.

In non-equilibrium synthesis in a reaction field exemplified by supercriticality, the strain increases due to nanoization of particles, organic modification of the particle surface, and control of the exposed surface of the particles, and this strain affects the electronic state of the crystal. , leading to remarkably large photocatalytic activity.

According to the invention according to the seventh and eighth characteristics, crystal growth occurs by performing hydrothermal treatment in a supercritical or subcritical field after synthesizing the nanocrystalline photocatalyst. This makes it possible to reduce distortion and defects and improve perfect crystallinity.

Above all, in hydrothermal treatment, it forms a homogeneous phase with oxygen gas at any ratio, especially if it is near the critical point. A high oxidation effect can be obtained by using it.

A ninth aspect of the invention provides a photocatalyst material according to any one of the first to eighth aspects, further comprising a calcination step of calcining the organically modified metal oxide nanoparticles. .

Oxygen vacancies are more likely to occur when strain increases due to nano-sizing and exposed surface control. This causes recombination of active species of the photocatalyst. In fact, when measured, trivalent cerium is abundant.

According to the ninth aspect of the invention, after synthesis, the particles are oxidized in an oxygen atmosphere to convert trivalent cerium to tetravalent cerium. In this way, a photocatalyst material having even better photocatalytic activity can be provided by performing the treatment for complete CeO ₂ conversion.

The invention according to a tenth aspect comprises metastable, single-crystalline, octahedral and/or hexahedral cerium oxide nanoparticles, having an average particle size of 100 nm or less, and having a surface organically modified with a hydrophobic group. Provide a photocatalyst material.

The invention according to an eleventh feature comprises metastable single-crystalline octahedral and/or hexahedral cerium oxide nanoparticles having an average particle size of 100 nm or less, and the nanoparticles being an azo dye AO7 solution. A photocatalyst material is provided in which the concentration of the azo dye AO7 contained in the solution after irradiation decreases by 5% or more compared to the concentration before irradiation when the azo dye AO7 is dispersed in the solution and irradiated with ultraviolet light having a wavelength of 365 nm for 2 hours.

According to the tenth and eleventh aspects of the invention, it is possible to provide a photocatalyst material having significantly superior photocatalytic activity compared to conventionally known photocatalyst materials.

The average particle size of 100 nm or less, the organic modification of the particle surface, and the control of the exposed surface of the particles increase the strain, and this strain affects the electronic state of the crystal, leading to a significantly high photocatalytic ability. guessed.

A twelfth aspect of the invention provides a photocatalyst material according to the tenth or eleventh aspect, wherein the lattice strain analyzed by the Hall method is 0.5% or less.

According to the invention according to the twelfth feature, a completely crystalline photocatalyst material with reduced distortion and defects can be obtained, and as a result, a photocatalyst material with superior photocatalytic activity can be provided.

ADVANTAGE OF THE INVENTION According to this invention, the photocatalyst material remarkably excellent in photocatalytic activity can be provided.

FIG. 1 shows a typical reactor utilized for organic modification. FIG. 2 shows the configuration of a representative reaction system apparatus used for the organic modification of the present invention. FIG. 3 is a schematic diagram showing the process of forming a mesoporous material. FIG. 4 is a diagram showing XRD patterns of CeO ₂ nanoparticles produced in Test Example 1-1. FIG. 5 is a graph showing a comparison of photocatalytic activity between test example 1: (plane control 1) with and without organic modification. FIG. 6 is a diagram showing comparison of photocatalytic activity with and without further hydrothermal treatment after nanoization in Test Example 2: (Plane control 2). FIG. 7 is a schematic diagram of a synthesizing apparatus 30 used for synthesizing chromium-doped cerium oxide (Cr—CeO ₂ ). FIG. 8 is a graph showing comparison of photocatalytic activity between test example 3: with and without transition element doping. FIG. 9 is a graph showing comparison of photocatalytic activity with and without calcination after test example 4: freeze-drying. FIG. 10 is a microscopic image of the mesoporous material obtained in Test Example 5 (study focused on surface oxidation state). FIG. 11 is a diagram showing comparison of photocatalytic activity in Test Example 5 above.

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

<Method for producing photocatalyst material>
The method according to the present embodiment includes a nanoization step of obtaining metal oxide nanoparticles in a flow system reaction field accompanied by a rapid temperature rise of a supercritical or subcritical aqueous solvent. Alternatively, it includes a nanoization step of organically modifying a metal oxide in a reaction field of a supercritical, subcritical, or gas phase aqueous solvent to obtain metal oxide nanoparticles having an organically modified surface. By this method, it is possible to obtain an extremely high degree of supersaturation compared to the conventional liquid-phase method and gas-phase method. can do. As a result, it is possible to provide a photocatalyst material having significantly superior photocatalytic activity compared to conventionally known photocatalyst materials.

[Reaction field]
In the case of organically modifying the surface of fine particles, particularly nanoparticles, there is no particular limitation as long as the device can achieve high temperature and high pressure conditions. For example, either a batch system or a flow system can be used. Typical reactors include those shown in FIG. 1, and a system such as that shown in FIG. 2 may be constructed, but if necessary, an appropriate reactor can be constructed.

The water used in the reaction of the present invention may be supercritical water (SCW) or pre-critical water. Pre-critical water includes water in a gaseous phase or a state referred to as water vapor (or steam). In addition, pre-critical water includes water in a state called subcritical water. In the case of water before criticality, it is preferable that water in a liquid state (liquid phase) or a liquid phase is included as a main phase. Under such hydrothermal conditions, it has the ability to form a single phase with relatively heavy hydrocarbons.・Influence on speed) can be greatly controlled.

"Hydrothermal conditions" according to the present invention are defined as water coexistence conditions with the following reaction temperatures: As described above, the "hydrothermal conditions" here include conditions in which gas phase water or water in a state called water vapor (or steam) coexists.

Although not particularly limited, the lower limit of the reaction temperature is preferably 150° C. or higher, more preferably 200° C. or higher, even more preferably 250° C. or higher, and 300° C. or higher. is particularly preferred.

The upper limit of the reaction temperature is preferably 1000° C. or lower, more preferably 600° C. or lower, even more preferably 500° C. or lower, and particularly preferably 450° C. or lower.

The reaction pressure in the present invention is not particularly limited, but may be above atmospheric pressure and below 50 MPa. The reaction pressure is more preferably 5 MPa or higher and may be 40 MPa or lower.

The reaction time of the present invention is not particularly limited, but may be, for example, 1 minute or more and 48 hours or less, more generally 5 minutes or more and 24 hours or less, and more typically may be from 10 minutes to 12 hours.

[Reaction mechanism]
Hydroxyl groups generally exist on the surface of metal oxides in water. This is due to the following reaction equilibrium.
MO+ _H2O =M(OH) (1)

Generally, the reaction is exothermic and the equilibrium shifts to the left at higher temperatures. Further, the reaction by the used surface modifier is as follows, and is due to dehydration reaction.

The leftward reaction (reverse reaction) is a well-known reaction for hydrolysis of alkoxides and the like, and is a reaction that easily occurs by adding water even at around room temperature. Since this reverse reaction is generally an exothermic reaction, it is suppressed on the high temperature side, making the rightward reaction more advantageous. This is the same as the temperature dependence of the dehydration reaction of metal hydroxide in formula (1).

In addition, in the rightward reaction (dehydration), the polarity of the product is lower than that of the reactant system, so the lower the polarity of the solvent, the more advantageous the stability of the product in water. The dielectric constant of water decreases as the temperature rises. At 350° C. or less, the dielectric constant drops to 15 or less. Therefore, the dehydration reaction is accelerated more than the normal temperature effect.

M(OH)+RCOOH=M(OCOR)+ _H2O =MR+ _H2O + _CO2
M(OH)+RCHO=M(OH)CR+ _H2O =MC=R ₊ 2H2O, MCR ₊ 2H2O, MR+H2+ _CO2 (these formulas are defined as ( ₂ ))

Attack of hydroxyl groups by amines is known to proceed in the presence of a strong acid or through substitution with Cl at around room temperature, but exchange with OH occurs in high-temperature, high-pressure water. With regard to organic substances, it has been confirmed that hexanamide and hexanol catalyze carboxylic acid to promote amination of hexanol, and it is presumed that a similar reaction is proceeding.

In general, fine particles produced using a hydrolysis reaction are hydroxides such as Fe(OH) ₃ , and the equilibrium shifts to FeO(OH) and Fe ₂ O ₃ as the temperature rises. The molecular arrangement state shifts from a random amorphous state to an ordered crystalline state at higher temperatures. Using the techniques described in this embodiment, it is possible to obtain highly crystalline nanoparticles that are organically modified.

High crystallinity can be confirmed by electron diffraction, electron micrograph analysis, X-ray diffraction, thermogravimetric analysis, and the like. For example, in electron diffraction, a dot is obtained as a diffraction interference image for a single crystal, a ring is obtained for a polycrystal, and a halo is obtained for an amorphous material. In electron micrographs, if it is a single crystal, the crystal planes are clearly visible, and if it is in a shape in which crystals are visible from above the particles, it is polycrystalline. When polycrystalline primary particles are small and many particles aggregate to form secondary particles, they become spherical. If it is amorphous, it is always spherical. In X-ray diffraction, if it is a single crystal, a sharp peak is obtained. The Sherre equation can be used to estimate the crystallite size from the half-height width of the X-ray peak. If the crystallite size obtained by the evaluation is the same as the particle size evaluated from the electron microscope image, it is evaluated as a single crystal. In thermogravimetric analysis, when heated in a dry inert gas with a thermobalance, the weight decreases due to the evaporation of the adsorbed water around 100°C, and the weight decreases due to dehydration from the inside of the particles up to about 250°C. is seen. An even greater weight loss is observed at 250-400° C. when organic matter is involved. In the case of the particles obtained by the technology of the present invention, even if the temperature is raised to 400° C., the weight loss due to dehydration from the inside of the crystal is 10% or less at maximum, which is larger than that of metal oxide fine particles synthesized at low temperatures. different. Thus, the organically modified metal oxide fine particles obtained according to the present embodiment are characterized by high crystallinity, for example, sharp peaks in X-ray diffraction and dots or rings in electron diffraction. , the dehydration of water of crystallization is 10% or less per dry particle in a thermogravimetric analysis, and/or the primary particles have a crystal face in an electron micrograph.

In fine particles, the surface energy and the external energy such as gravity and electric field compete with each other in relation to the particle size. If the diameter is less than several hundred nanometers, it will not disperse unless a large external force is applied. When the thickness is 50 nm or less, the effect of the surface energy is further increased, and it is extremely difficult to use only the external energy unless the surface properties are controlled and the physical properties of the solvent are not controlled. The technology of this embodiment can solve this problem.

In particular, when the particle size is 10 nm or less, there is no overlapping of quantum states, and the influence of surface electronic states greatly affects bulk physical properties. Therefore, it has been found that physical properties completely different from those of bulk particles can be obtained, that is, the quantum size effect (Kubo effect) appears. Particles with a size of about 10 nm or less can be considered to be a completely different substance, but the technique described in this embodiment can suitably organically modify the fine nanoparticles.

[How to set conditions]
(reaction equilibrium)
Although the reaction conditions for organic modification vary depending on the metal species and the modifier, they are organized as follows.

The reaction proceeds when the equilibrium of equation (1) is on the right side and the equilibrium of equation (2) is on the right side. Since each equilibrium differs depending on the metal species and modifier, the optimum reaction conditions differ. When the temperature is increased, the equilibrium of equation (2) shifts to the right, especially above 350° C., and shifts rapidly to the advancing side, while the equilibrium of equation (1) shifts to the left. Regarding the reaction conditions, refer to the DB of formulas (1) and (2).

If a base or an acid is present, the surface functional group of the metal oxide can be OH, so that the dehydration reaction with the modifier can proceed under these conditions. In that case, since the dehydration reaction is likely to occur in the presence of an acid, the reaction can be advanced by allowing a small amount of acid to coexist at a high temperature.

(phase equilibrium)
Alcohols, aldehydes, carboxylic acids, and amines that are relatively short-chain hydrocarbons are soluble in water, so surface modification of metal oxides with methanol, for example, is possible. However, in the case of longer-chain hydrocarbons, phase separation occurs with the aqueous phase, so even if the above reaction equilibrium is on the advancing side, the metal oxide and the organic modifier in the aqueous phase actually do not react. sometimes not. That is, although introduction of a lipophilic group is relatively easy, phase behavior must be taken into consideration when targeting long-chain hydrocarbons of C3 or higher.

The phase behavior of hydrocarbons and water has already been reported and can be used as a reference. In general, if the gas-liquid critical trajectory is exceeded, a homogeneous phase is formed at an arbitrary ratio. Therefore, by setting such temperature and pressure conditions, favorable reaction conditions can be set.

Further, when the optimum reaction temperature is desired to be lower, it is possible to coexist with a third component for making the water and the organic substance into a homogeneous phase. For example, it is known that the coexistence region of hexanol and water can be formed at a lower temperature by the coexistence of water and ethanol or ethylene glycol, which form a homogeneous phase even at low temperatures. By utilizing it, the reaction between the metal oxide and the organic substance can be carried out. However, in this case, the selection of the third component is important so that the surface modification reaction by the third component does not occur.
As described above, this technique enables long-chain organic modification in water for the first time.

[Organic modification: in-situ surface modification during hydrothermal synthesis]
As described above, the temperature dependence of the formation of hydroxyl groups on the metal oxide surface in formula (1) and the organic modification reaction in formulas (2) and below are in opposite directions. Therefore, especially when the reaction of formula (1) is on the left side, that is, on the dehydration side, setting reaction conditions such as the coexistence of an acid is extremely important and sometimes difficult in order to cause the modification reaction.

In contrast, hydrothermal synthesis in-situ surface modification is an enabling method.

Hydrothermal synthesis proceeds through the following reaction pathway.
Al( _NO3 ) ₃ + _3H2O =Al(OH) ₃ + _3HNO3
nAl(OH) ₃ = nAlO(OH) ₊ nH2O
_nAlO (OH)=n/2Al2O3+ _n / _2H2O

Proceeding along such a reaction route is the same when using other metal species, sulfates, hydrochlorides, and the like. Furthermore, hydrothermal synthesis, for example, can be performed using an apparatus such as that shown in FIG. The technique makes it possible to obtain finer organically modified particles. Also, by adjusting the temperature and pressure, the particle size can be controlled.

As shown here, even if hydroxyl groups are finally eliminated from the surface by the dehydration reaction, many hydroxyl groups are formed on the product or its surface as reaction precursors. If an organic modifier coexists in this reaction field, it is possible to carry out the reaction under conditions where hydroxyl groups are present. In addition, since an acid, which is also a catalyst for advancing the dehydration reaction, coexists in the reaction field, the modification reaction is accelerated. This makes it possible to perform surface modification that could not be performed on oxides.

In the technique described in the present embodiment, the precursor is synthesized once, and then hydrolyzed to synthesize metal oxides and metal hydroxides in a high temperature field to achieve equilibrium to oxides. Furthermore, the surface of fine particles can be organically modified without using radical polymerization substrates, which are sensitive to, for example, oxidizing substances, temperature, light, and the like. Therefore, metal particles and particles with different redox states can be organically modified.

In the present embodiment, a phase state in which water and an organic substance form a uniform phase is used, and the synthesis of an inorganic-organic composite substance is attempted. While synthesizing metal and metal oxide nanoparticles, their surfaces are modified with organic molecules.

Metal oxides can be further nano-sized by reacting them while capping organic molecules in a supercritical reaction field. In addition, by organically modifying the surface of metal oxide nanoparticles having a hydrophilic surface with a hydrophobic group such as an organic group such as a hydrocarbon, particles that are difficult to recover from an aqueous medium can be easily and reliably collected. It can be separated and recovered by moving to the organic medium side. This organic modification allows the nanoparticles with controlled exposed crystal surfaces to be recovered without changing their shape. In addition, since the photocatalyst device is manufactured by a coating process, it has a high affinity with the solvent, and it is possible to develop low viscosity while dispersing the nanoparticles at a high concentration.

The type of metal oxide is not particularly limited as long as it is used as a photocatalyst, but cerium oxide is preferred.

The organic modifier is not particularly limited as long as it is capable of strongly bonding hydrocarbons to the surface of the fine particles, and applications of nanoparticles include the fields of organic chemistry, inorganic materials, and polymer chemistry. can be selected from organic substances that are widely known in fields where The organic modifier includes, for example, an ether bond, an ester bond, a bond via an N atom, a bond via an S atom, a metal-C- bond, a metal-C= bond and a metal-(C=O) Examples include those that allow the formation of a strong bond such as a - bond. The number of carbon atoms of the hydrocarbon is not particularly limited, and those having 1 or 2 carbon atoms can also be used. Hydrogen is preferred, and examples thereof include straight-chain or branched-chain or cyclic hydrocarbons 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. In the case of plural, 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, sugars, and the like. be done.

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

Examples of the hydrocarbon group include optionally substituted linear or branched alkyl groups, optionally substituted cyclic alkyl groups, optionally substituted aryl groups, and optionally substituted aralkyl groups. , 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-, - and S-.

A dye-sensitized solar cell (Grötzel cell) is a system that can utilize light in a wide range of wavelengths by absorbing visible light and transferring its energy to an inorganic solar cell. The organic-inorganic molecules obtained herein can be designed to absorb visible light. In general, a Grätzel cell adsorbs a dye, but this technique enables chemical bonding and an electron conjugated system, enabling more effective electron transfer.

[Further hydrothermal treatment]
The method according to the present embodiment further includes a step of subjecting the metal oxide nanoparticles obtained by the nano-forming step to a hydrothermal treatment in a supercritical or subcritical reaction field to increase crystallization and reduce defects. is preferred. Moreover, it is preferable to further include a step of making the metal oxide nanoparticles obtained by the nano-forming step coexist with oxygen under hydrothermal conditions to reduce oxygen defects.

In non-equilibrium synthesis in a reaction field exemplified by supercriticality, the strain increases due to nanoization of particles, organic modification of the particle surface, and control of the exposed surface of the particles, and this strain affects the electronic state of the crystal. , leading to remarkably large photocatalytic activity.

After synthesizing the nanocrystalline photocatalyst, crystal growth occurs by performing hydrothermal treatment in a supercritical or subcritical field. This makes it possible to reduce distortion and defects and improve perfect crystallinity. Above all, in hydrothermal treatment, it forms a homogeneous phase with oxygen gas at any ratio, especially if it is near the critical point. A high oxidation effect can be obtained by using it.

[Recovery of metal oxide nanoparticles whose surfaces are organically modified]
By organically modifying the surface of metal oxide nanoparticles having a hydrophilic surface with a hydrophobic group such as an organic group such as a hydrocarbon, particles that are difficult to recover from an aqueous medium can be easily and reliably organically treated. It can be separated and recovered by moving to the side of the organic medium. This organic modification allows the nanoparticles with controlled exposed crystal surfaces to be recovered without changing their shape.

In the method according to the present embodiment, it is preferable to organically modify the surface of the metal oxide, and then freeze-dry or supercritically treat the surface, and then recover the organically modified carbon material. When organic modification is performed using subcritical water or supercritical water, the carbon material after the organic modification is hydrophobized, and therefore phase-separates automatically from water. Therefore, it can be recovered well by adding a small amount of organic solvent. However, when an organic solvent is used in the organic modification, the solvent must be removed by drying, and the drying process causes capillary force to agglomerate the carbon material. Therefore, by performing freeze-drying or supercritical treatment, the capillary force can be suppressed to prevent aggregation, and the organically modified carbon material can be recovered with high efficiency. In particular, by performing supercritical carbon dioxide drying, it is possible to completely recover the organic solvent and reuse it. In addition, it becomes possible to separate modifiers that have not been used.

It is more preferable to further calcine the metal oxide nanoparticles after drying. Oxygen vacancies are more likely to occur when strain increases due to nano-sizing and exposed surface control. This causes recombination of active species of the photocatalyst. In fact, when measured, trivalent cerium is abundant. After synthesis, the particles are oxidized in an oxygen atmosphere to convert trivalent cerium to tetravalent cerium. In this way, a photocatalyst material having even better photocatalytic activity can be provided by performing a treatment for complete CeO ₂ conversion.

[Average primary particle size]
The upper limit of the average primary particle size of the metal oxide nanoparticles is not particularly limited, but from the viewpoint of maximizing the exposed surface area (possibility of contact) with the reactant and easy formation of oxygen vacancies due to lattice strain, it is 1 μm or less. is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 30 nm or less, and particularly preferably 20 nm or less.

The lower limit of the average primary particle size of the metal oxide nanoparticles is also not particularly limited. Generally, the nanoparticles are secondaryized by compaction and the secondary particles are used in packed bed reactors for cyclic operation or as circulating fluidized bed particles.

In the present embodiment, the average primary particle size of the metal oxide nanoparticles is determined by taking an image of the oxygen carrier particles with a TEM (transmission electron microscope) and analyzing the TEM image using image analysis/image measurement software. shall be a value. At that time, if the particle size distribution is wide, it is necessary to pay attention to whether the particles within the field of view represent all the particles.

〔Specific surface area〕
From the viewpoint of reaction activity, the specific surface area of the metal oxide nanoparticles may be 5 m ² /g or more and 1000 m ² /g or less, more generally 10 m ² /g or more and 500 m ² /g or less. typically 20 m ² /g or more and 400 m ² /g or less, preferably 30 m ² /g or more and 300 m ² /g or less.

[Crystal form]
From the viewpoint of optimizing the expression of the photocatalytic function, the shape of the metal oxide nanoparticles is required to expose the most active surface, which determines the particle shape. Although the most active surface differs depending on the type of metal oxide nanoparticles, it is the most thermodynamically unstable surface, and information on it can be easily obtained from materials databases and the like. For example, in the case of CeO ₂ , the shape of the oxygen carrier is approximately cubic, and the (100) plane is preferably the main exposed surface.

The particle morphology of the metal oxide is preferably metastable and monocrystalline. Among them, octahedral and/or hexahedral cerium oxide nanoparticles are preferred.

Nanoparticle catalysts of CeO ₂ can take the form of octahedrons or cubes. Also, at this time, the nanoparticle catalyst of CeO ₂ has (111) plane, (110) plane and/or (100) plane as the main exposed plane.

Here, the metal oxide nanoparticles are preferably particles having an active surface exposed to 30% or more of the particle surface. The active plane is the most energetically unstable plane, which is the (100) plane in CeO ₂ . This allows oxygen transfer at low temperatures.

The proportion of the surface of the CeO ₂ nanoparticles where the (100) plane is exposed is preferably 30% or more, more preferably 50% or more, and even more preferably 70% or more. In addition, the ratio of the (100) plane exposed on the surface of the CeO ₂ nanoparticles is measured by TEM.

_The CeO2 nanoparticles are preferably doped with a transition element to enhance activity. Cr, Eu, Co, Y, Ce, Gd, Zr, etc. can be exemplified as the transition elements doped into the _CeO2 nanoparticles. Among them, Cr and Eu are preferable.

The doping amount of the transition element in the _CeO2 nanoparticles is preferably 0.1 mol% or more, more preferably 4 mol% or more, still more preferably 10 mol% or more, and even more preferably 15 mol% or more, relative to the _total mass of CeO2. , 20 mol % or more is more preferable, and 25 mol % or more is particularly preferable. The doping amount of the transition element is preferably as large as possible, but the doping amount is up to 50 mol %.

For CeO2 nanostructures, the _six (100) planes have been shown to have the highest surface energy among the low-index crystallographic planes. This high surface energy is due to the lability of oxygen in the top layer, which acts as a bridging site between the cerium ions. It is believed that this oxygen lability contributes to the high conversion of organics. Oxygen in the top layer of cubic CeO ₂ is released in a temperature and pressure dependent manner. This oxygen species can be transferred to the reactants and decompose them into products. The 4+ valence state of Ce is converted to the 3+ valence state of Ce and becomes unstable. Ceria oxygen vacancies are generated by Ce 3+ , and the vacancies formed on the reduced ceria surface cause a reaction with water molecules to combine with oxygen to form 4+ valence state Ce. Thereby, a photocatalyst function can be exhibited continuously.

[Lattice strain]
Oxygen vacancies are more likely to occur when strain increases due to nano-sizing and exposed surface control. Therefore, the lattice strain is preferably small, and the lattice strain analyzed by the Hall method is preferably 0.5% or less, more preferably 0.2% or less, and 0.1% or less. is more preferred.

[Mesoporous materials]
It can be seen that the metal oxide nanoparticles can exhibit a photocatalytic function even when supported by a mesoporous material, which normally poses a problem of light transmission.

[Method for producing mesoporous material]
For example, when obtaining mesoporous CeO2 as the mesoporous material, mesoporous CeO2 is _nanocast in _three steps, as shown in Fig. 3. First, a precursor solution containing cerium is infiltrated into SBA-15 mesopores with a diameter of about 7 nm (“Ce-precursor-SBA-15” in FIG. 3). Subsequently, the Ce-precursor-SBA-15 is calcined in air at 450° C. to decompose the Ce precursor to produce CeO ₂ (“CeO ₂ -SBA-15” in FIG. 3). Then, the silica in CeO ₂ -SBA-15 is dissolved in an aqueous NaOH solution (pH 12) to leave the mesoporous CeO ₂ structure (“Mesoporous CeO ₂ ” in FIG. 3).

<Example of use of photocatalyst material>
Although not particularly limited, the photocatalyst material described in this embodiment can be used for air purification, deodorization, water purification, antibacterial and antifouling applications.

[Air purification]
Air is polluted by environmental pollutants such as nitrogen oxides (NOx) and sulfur oxides (SOx) emitted from automobile exhaust gas. The atmosphere can be purified by using the photocatalyst material described in this embodiment for building outer walls, road noise barriers, factory exhaust equipment, and the like.

〔deodorize〕
By using the photocatalyst material according to the present embodiment in air cleaners, nursing care products, wallpaper, curtains, etc., ammonia, acetaldehyde (cigarette odor), hydrogen sulfide (rotten egg odor), methyl mercaptan (garlic odor), etc. can be removed.

[Purified water]
By using the photocatalyst material according to the present embodiment for water purifiers, wastewater treatment, and the like, it is possible to decompose and remove organic chlorine compounds such as tetrachlorethylene and trihalomethane contained in water.

〔antibacterial〕
The photocatalyst materials described in this embodiment can be used in tiles, toilets, kitchen utensils and the like to kill bacteria on these surfaces and decompose their dead bodies.

[Anti-fouling]
By using the photocatalyst material according to the present embodiment for the outer walls of buildings, lamp covers, window glass, and the like, it is possible to prevent these from becoming soiled by decomposing the oil on their surfaces.

[Synthesis Example: Synthesis of _CeO2 nanoparticles]
The synthesis of nanoparticles of CeO ₂ , which is a representative metal oxide catalyst, is described below.

Octahedral CeO ₂ nanoparticles can be synthesized by known methods.

Cubic CeO2 nanoparticles _were prepared by (1) preparing a raw material solution in toluene, ( ₂ ) using organic modifiers and, if necessary, transition elements, to produce cubic CeO2 nanoparticles under supercritical water conditions. and ( ₃ ) removing the organic modifier without changing the morphology of the cubic CeO2.

Specifically, the preparation of nanoparticles of cubic CeO ₂ can be carried out as follows. This is a non-limiting example.

A cubic cerium oxide nanoparticle precursor solution is prepared by dissolving an organic modifier, hexanoic acid, a doping agent, a transition element, and Ce(OH) ₄ in toluene. The precursor solution is then mixed with continuous stirring to obtain a clear solution. The precursor solution is mixed with deionized water and rapidly heated to 600-700K using a furnace. The mixture is then cooled. Cubic cerium oxide nanoparticles are obtained as a dispersion in a mixture of water, toluene and unreacted raw materials. The nanoparticles in the toluene phase are added with ethanol and purified by centrifugation and decantation, thereby removing unreacted organic molecules. The particles are dispersed in cyclohexane and freeze-dried under vacuum. The collected nanoparticles are calcined in air at temperatures as high as 300° C. for several hours in order to remove any organic ligands from the surface of the particles. The calcined nanoparticles can be cleaned by centrifugation and decantation, and then dried under reduced pressure, thereby obtaining cubic CeO ₂ nanoparticles.

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

<Test Example 1> Surface Control Part 1: Comparison with and without Organic Modification [Test Example 1-1] Cubic Cerium Oxide with Surface Organically Modified [Synthesis of Cerium Oxide]
Cerium oxide nanoparticles were synthesized by the following method.
This method is briefly described as three steps:
(i) preparing a precursor (raw material) solution in toluene;
( _ii ) synthesizing cubic CeO2 nanoparticles under supercritical water conditions using organic modifiers and optionally transition elements, and ₍ iii) organic modification without changing the morphology of cubic CeO2 The step of removing the pesticide.

(i) Precursor (raw material) solution prepared in toluene In toluene (99.5%, Wako Chemicals), hexanoic acid (99%, Wako Chemicals) as an organic modifier, and Ce(OH) ₄ ( Aldrich Chemicals) was prepared by dissolving the precursor solution. The concentration of hexanoic acid contained in the precursor solution is 0.30 mol/L, and the concentration of Ce(OH) ₄ is 0.050 mol/L. The precursors were mixed with continuous stirring for 40 minutes to obtain a clear solution.

(ii) Synthesis of CeO ₂ nanoparticles The precursor solution was supplied using a high-pressure pump (Nippon Seimitsu Kagaku, NP-KX540) at a flow rate of 7.0 mL/min. At the same time, deionized water was supplied using another pump at a flow rate of 3.0 mL/min. The precursor solution was mixed with deionized water at the junction and rapidly heated to 653 K using a furnace. The residence time in the heating zone was approximately 95 seconds and was estimated from the reactor volume, total flow rate, density of the water and toluene mixture at the mixing point, and reaction temperature and pressure. The mixture was then cooled using a water jacket. The system pressure was maintained at 30 MPa by a back pressure regulator (TESCOM, 26-1700 series). Cerium oxide nanoparticles were obtained as a dispersion in a mixture of water, toluene and unreacted raw materials.

(iii) Removal of Organic Modifiers Samples were left overnight to separate the water and toluene phases. Ethanol was then added into the toluene phase and the mixture was purified by three cycles of centrifugation and decantation to remove unreacted organic molecules. The particles were dispersed in cyclohexane and freeze dried under vacuum for 8 hours.

[Test Example 1-2] CeO ₂ nanoparticles whose surface is not organically modified In the same manner as in Test Example 1-1, except that a precursor (raw material) solution was prepared without adding an organic modifier. Particles were synthesized.

〔evaluation〕
[Nanoparticle form and size]
The morphology and size of the nanoparticles were observed by a transmission electron microscope (TEM, Hitachi H7650) at an accelerating voltage of 100 kV. Fourier transform infrared (FTIR) spectra were obtained using a JASCO FT/IR-680 spectrometer to investigate the chemical bonds and functional groups on the surface of the nanoparticles. Transmission IR spectra were collected from 400 to 4000 cm ⁻¹ . Crystallinity and purity of the particles were identified using X-ray diffraction with Cu Kα radiation (XRD, Rigaku Ultima IV) in a 2θ-θ setup. The 2θ angle was scanned between 20° and 70°.

Then, by comparing the XRD pattern of the nanoparticles with the card of the Joint Committee on Powder Diffraction Standards (JCPDS) obtained from the International Center for Diffraction Data (00-034-0394), the nanoparticles obtained are CeO ₂ It was found to have a crystalline structure. In addition, the size of the crystals evaluated by the Scherrer's formula is about 8 nm for the size obtained by the method of Test Example 1-1, and about 50 nm for the size obtained by the method of Test Example 1-2. Met.

Further, from the results of analysis using TEM, the nanoparticles obtained by the method of Test Example 1-1 are cubic particles surrounded by six {100} planes, and the method of Test Example 1-2 was confirmed to be an octahedral particle surrounded by eight {111} faces. The evolution of the grain shape from octahedral to cubic is due to the preferential interaction of the hexanoic acid ligand molecule with the {001} faces of the truncated octahedron, thereby resulting in { It is believed that the growth rate of crystals in the 001} direction decreased significantly and crystal growth in the {111} direction became dominant, ultimately leading to the formation of nanocubes.

FTIR spectra were obtained to prove the presence of organic molecules chemically bonded to _both sides of the cubic CeO2 nanoparticles. As shown in FIG. 4, a stretching peak appeared in the region of 2900-2970 cm ⁻¹ in the surface-modified nanoparticles. These peaks are assigned to the C—H stretching modes of the methyl and methylene groups in hexanoic acid and are present in the FTIR spectrum of the net modifier and the presence of organic molecules on the surface of the nanoparticles. is shown. In the spectrum of the hexanoic acid-modified nanoparticles, two major peaks at 1531 and 1444 cm ⁻¹ were assigned to the asymmetric and symmetric modes of the carboxylate groups, respectively. This indicates that hexanoic acid is chemically bound to the surface of the cerium oxide nanoparticles through its carboxylate groups.

[Photocatalytic activity]
Azo dye AO7: 0.0002 mol/L, 0.01 g of the nanoparticles of Example 1 or Comparative Example 1 was added to 3 mL, and irradiated with UV at a wavelength of 365 nm for a predetermined time. After a predetermined period of time, the nanoparticles were separated by centrifugation at 10,000 rpm for 15 minutes, and the residual AO7 concentration was measured by UV-vis spectrum intensity at 484 nm. For the irradiation time of 0 h, measurement was performed by immediately centrifuging without UV light irradiation after addition of the catalyst in order to examine the effect of adsorption. The results are shown in FIG.

As shown in FIG. 5, it was confirmed that the photocatalyst nanoparticles whose surfaces were organically modified (Test Example 1-1) had higher photocatalytic activity than those whose surfaces were not organically modified (Test Example 1-2). was done.

<Test Example 2> Surface control 2: Comparison with and without further hydrothermal treatment after _nanoization The photocatalytic activity was measured between the sample (Test Example 2) subjected to "removal" and the sample of Test Example 1-1. The results are shown in FIG.

As shown in FIG. 6, after “( _ii ) Synthesis of CeO nanoparticles”, the photocatalyst nanoparticles that were further hydrothermally treated (Test Example 2) were not subjected to additional hydrothermal treatment (Test Example 1-1). It was confirmed to have a high photocatalytic activity compared to.

In non-equilibrium synthesis in a reaction field exemplified by supercriticality, the strain increases due to nanoization of particles, organic modification of the particle surface, and control of the exposed surface of the particles, and this strain affects the electronic state of the crystal. , leading to remarkably large photocatalytic activity.

After synthesizing the nanocrystalline photocatalyst, crystal growth occurs by performing hydrothermal treatment in a supercritical or subcritical field. This makes it possible to reduce distortion and defects and improve perfect crystallinity. Actually, when the lattice strain by the Hall method was obtained from the XRD data analysis, it was less than 0.1% for the nanoparticles of Test Example 2 and about 0.2% for the nanoparticles of Test Example 1-1. rice field.

Therefore, it is considered that further hydrothermal treatment could reduce distortion and defects, resulting in more remarkable photocatalytic activity.

<Test Example 3> Comparison with and without doping [Test Example 3-1] Chromium-doped cerium oxide (Cr—CeO ₂ )
Chromium-doped cerium oxide (Cr—CeO ₂ ) was synthesized by a reported flow-through supercritical hydrothermal synthesis method. In synthesizing Cr--CeO ₂ , an apparatus 30 shown in FIG. 7 was used.

The apparatus 30 includes a raw material supply unit 31, a supercritical water supply unit 32, a mixing unit 33 that mixes the raw material and the supercritical water, and a cooling unit 34 that cools the mixed liquid of the raw material and the supercritical water and stores it in a container. and

The raw material supply unit 31 has a raw material storage container 31A in which raw materials are stored, and a pump 31B that pumps up the raw materials stored in the raw material storage container 31A toward the mixing unit 33 . Pipes are connected between the raw material container 31A and the pump 31B, and between the pump 31B and the mixing section 33, respectively.

The raw material storage container 31A stores an aqueous solution containing 0.010 mol/l of Ce(OH) ₄ /Cr(OH) ₃ (Ce:Cr=9:1 (molar ratio)). In addition, 0.13 g of decanoic acid is also contained in the raw material container 31A in order to modify the surface of the nanoparticles and induce their anisotropic growth.

The supercritical water supply unit 32 includes a water storage container 32A that stores water, a pump 32B that pumps up the water stored in the water storage container 32A toward the mixing unit 33, and between the pump 32B and the mixing unit 33. and a heating unit 32C that is provided to bring water into a supercritical state. Pipes are connected between the water container 32A and the pump 32B, between the pump 32B and the heating section 32C, and between the heating section 32C and the mixing section 33, respectively.

In this test example, in order to bring water into a supercritical state and then supply the supercritical water to the mixing unit 33, instead of gradually heating the mixture of raw materials and water to finally bring it into a supercritical state, By bringing the supercritical water into direct contact with the raw material, the temperature of the raw material is raised to a supercritical state in an extremely short period of time at a mixing speed. Nanoparticles can be synthesized with high supersaturation.

The temperature and pressure can be controlled in the mixing section 33 .

In this test example, supercritical water preheated in the supercritical water supply unit 32 is supplied to the mixing unit 33 from a pipe separate from the pipe provided in the raw material storage container 31A, and the raw material and supercritical water are mixed. were rapidly mixed to raise the temperature of the raw material to a supercritical state. Although the raw material contains decanoic acid, which is an organic molecule, both the organic molecules and the inorganic aqueous solution form a homogeneous phase in the supercritical state, where particle synthesis occurs. At the reaction tube outlet in the mixing section 33, rapid cooling was performed by an external water cooling device 34A provided in the cooling section 34, and the pressure was then controlled by a back pressure valve (not shown). The reaction temperature in the mixing section 33 was 400° C., the reaction pressure was 30 MPa, and the reaction time was 2 seconds or less. After the hydrothermal reaction, the container 34B containing the post-reaction liquid was cooled in a water bath at room temperature. 5 ml of hexane was used to extract the nanoparticles (hexane phase) from the decanoic acid-modified product. Then, 10 ml of ethanol was added as a poor solvent reagent to the hexane phase to precipitate a precipitate from the hexane phase. Then, centrifugation was performed to obtain cubic Cr _- doped CeO2 nanoparticles.

By this non-equilibrium system synthesis method, it was possible to increase Cr, which can be doped only by several mol % in equilibrium system synthesis using an autoclave at a slow temperature rise, to several tens of mol %.

[Test Example 3-2] Europium-doped cerium oxide (Eu—CeO ₂ )
Europium-doped cerium oxide (Eu—CeO ₂ ) was obtained in the same manner as in Test Example 3-1, except that Eu was used as the transition element constituting the doping agent.

〔evaluation〕
The photocatalytic activity of the nanoparticles according to Test Examples 3-1 and 3-2 was measured in the same manner as in Test Example 1. In addition, as a control, the photocatalytic activity of the CeO ₂ nanoparticles obtained in Test Example 1-2 without using a doping agent was also measured. The results are shown in FIG.

As shown in FIG. 8, the photocatalytic nanoparticles doped with transition elements (Test Examples 3-1 and 3-2) can have higher photocatalytic activity than those without doping (Test Examples 1-2). confirmed.

<Test Example 4> Comparison with and without calcination CeO ₂ nanoparticles were synthesized by the same method as in Test Example 1-2, freeze-dried, and then calcined (Test Example 4-1). The photocatalytic activity was compared with the case without (Test Example 4-2). For calcination, the collected nanoparticles were calcined at 400° C. for 2 hours in air. The calcined nanoparticles were cleaned several times in ethanol and any unreacted molecules were removed by centrifugation and decantation. Finally, the particles were vacuum dried for 6 hours, after which the photocatalytic activity was measured for the calcined nanoparticles. The results are shown in FIG.

As shown in FIG. 9, the photocatalyst nanoparticles that were calcined after freeze-drying (Test Example 4) had higher photocatalytic activity than the photocatalyst nanoparticles that were not calcined (Test Example 1-2). confirmed.

Oxygen vacancies are more likely to occur when strain increases due to nano-sizing and exposed surface control. This causes recombination of active species of the photocatalyst. In fact, when measured, trivalent cerium is abundant. After synthesis, the particles are oxidized in an oxygen atmosphere to convert trivalent cerium to tetravalent cerium. In this way, a photocatalyst material having even better photocatalytic activity can be provided by performing a treatment for complete CeO ₂ conversion.

<Test Example 5> Study focusing on surface oxidation state [Synthesis of SBA-15 mesoporous silica]
Pluronic® P123 (triblock poly(ethylene oxide)-bpoly(propylene oxide)-b-poly(ethylene oxide) copolymer, average molecular weight 5800, Sigma-Aldrich) as structure directing agent5 .14 g was dissolved in 144 mL of water and mixed with 30.96 g of 37% HCl aqueous solution. After slowly adding 11.12 g of tetraethyl orthosilicate (TEOS, manufactured by Sigma-Aldrich) as a silica precursor to this solution, the solution was aged with stirring at 40° C. for 20 hours. This solution was transferred to an autoclave and heated at 100° C. for 24 hours. After cooling, the white solid product was collected by filtration and dried in air at 60°C. The dried samples were calcined in air at 550°C for 5 hours. Pluronic P123 was removed during calcination to yield SBA-15 mesoporous silica.

[Synthesis of CeO2 by _nanocasting ]
Synthesis of the desired mesoporous CeO ₂ was carried out using the SBA-15 mesoporous silica prepared as described above as a solid template. 0.50 g of SBA-15 mesoporous silica was dispersed in 5.0 mL of ethanol (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.). This was stirred at room temperature for 60 minutes. Next, as a cerium source, 0.50 g of ammonium cerium nitrate (Ce(NH ₄ ) ₂ (NO ₃ ) ₆ , manufactured by Kanto Kagaku Co., Ltd.) was dissolved in the dispersion. Ethanol was removed by stirring at 50°C. The dried samples were calcined in air at 450°C for 5 hours. After that, the SBA-15 template was dissolved in a 2.0 M NaOH aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), washed repeatedly with water, and dried at 60° C. to obtain a product (mesoporous CeO ₂ ).

〔evaluation〕
[Microscope image]
Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) images were taken of the nanocast mesoporous CeO ₂ . As the TEM, H-7650 manufactured by Hitachi, Ltd. was used. As the SEM, JSM-7800F Prime manufactured by JEOL Ltd. was used. Results are shown in FIGS. 10A and 10B.

Figures 10A and 10B are TEM and SEM images of _nanocast mesoporous CeO2. _Both images reveal that the mesoporous CeO2 is an assembly of nanowire-like structures. This suggests that it is a replica of the hexagonal pores of SBA-15, but the aggregates were smaller than those of the original SBA-15. All aggregates exhibited mutually isolated high-aspect-ratio morphologies with an average major axis length of about 250 nm. These aggregates are also composed of CeO2 nanoparticles with an _average particle size of about 7 nm.

FIG. 10C is an atomic image of CeO ₂ nanoparticles in mesoporous CeO ₂ observed by HR-TEM. In the HR-TEM image, the {100}, {111}, and {110} lattice fringes clearly appear at interplanar spaces of 0.28, 0.32, and 0.20 nm, _respectively , indicating that the CeO2 nanocrystals have a large area of {100 } face is exposed.

[Photocatalytic activity]
Photocatalytic activity was evaluated for each of the mesoporous CeO ₂ obtained in Test Example 5 and the cubic cerium oxide obtained in Test Example 1-1.

As shown in FIG. 11, it was confirmed that photocatalytic nanoparticles made of a mesoporous material (Test Example 5) have the same photocatalytic activity as cubic photocatalytic nanoparticles (Test Example 1-1).

Claims (12)

A method for producing a photocatalyst material, comprising a nanoization step of obtaining metal oxide nanoparticles in a reaction field of a rapid heating flow system of a supercritical or subcritical aqueous solvent. A method for producing a photocatalyst material, comprising a nanoization step of organically modifying a metal oxide in a reaction field of a supercritical, subcritical, or gas phase aqueous solvent to obtain metal oxide nanoparticles having an organically modified surface. 3. The nano-forming step according to claim 1 or 2, wherein the nano-forming step is carried out in the presence of a transition element and includes a step of obtaining metal oxide nanoparticles whose surfaces are organically modified and doped with the transition element. Production method. 4. The manufacturing method according to any one of claims 1 to 3, wherein the metal oxide nanoparticles are metastable and monocrystalline. 5. The manufacturing method according to any one of claims 1 to 4, wherein the metal oxide nanoparticles comprise octahedral and/or hexahedral cerium oxide nanoparticles. The production method according to any one of claims 1 to 5, wherein the metal oxide nanoparticles have an average particle size of 100 nm or less. Claims 1 to 6, further comprising a step of subjecting the metal oxide nanoparticles obtained by the nano-forming step to hydrothermal treatment in a supercritical or subcritical reaction field to achieve high crystallization and low defects. The manufacturing method according to any one of. 8. The production according to any one of claims 1 to 7, further comprising a step of making the metal oxide nanoparticles obtained by the nanoization step coexist with oxygen under hydrothermal conditions to reduce oxygen defects. Method. 9. The manufacturing method according to any one of claims 1 to 8, further comprising a calcination step of calcining the organically modified metal oxide nanoparticles. comprising metastable, monocrystalline, octahedral and/or hexahedral cerium oxide nanoparticles,
an average particle size of 100 nm or less,
A photocatalytic material whose surface is organically modified with a hydrophobic group. comprising metastable, monocrystalline, octahedral and/or hexahedral cerium oxide nanoparticles,
an average particle size of 100 nm or less,
When the nanoparticles are dispersed in an azo dye AO7 solution and irradiated with ultraviolet light having a wavelength of 365 nm for 2 hours, the concentration of the azo dye AO7 contained in the solution after irradiation decreases by 5% or more compared to the concentration before irradiation. photocatalyst material. 12. The photocatalyst material according to claim 10 or 11, having a lattice strain of 0.5% or less as analyzed by the Hall method.

Images