JP7779087B2 - Coated oxide particles, photocatalyst, and method for producing coated oxide particles - Google Patents

Description

The present invention relates to coated oxide particles, photocatalysts, and methods for producing coated oxide particles.

Research and development of functional composite particles, which are particles that have been combined to impart functionality, has been progressing. Coated particles, in which a coating is formed on the surface of a core particle, have also been proposed as functional composite particles. Coated functional composite particles may have superior electrical and other properties that cannot be achieved with single particles, and are expected to be used in a variety of applications.

For example, Patent Document 1 discloses a powder particle processing method in which powder particles are added and suspended in a benzene solution of the alkoxide of the metal to be coated, water is added to the suspension, the suspension is hydrolyzed, dried, and then calcined (claims of Patent Document 1). Patent Document 1 also describes that the method can be applied to coating oxides that have catalytic properties on the surface of a support, and that a uniform thickness can be coated onto the surface of fine particles using a relatively simple process (page 2, left column, line 17 to right column, line 2 of Patent Document 1).

Patent Document 2 discloses a method for producing oxide-coated microparticles, which comprises mixing microparticles and a metal alkoxide in an aqueous medium and precipitating a metal oxide produced by a hydrolysis reaction of the metal alkoxide on the surface of the microparticles (Claim 1 of Patent Document 2). Patent Document 2 also describes that the oxide-coated microparticles can be used in applications requiring film smoothness and purity (paragraph [0017] of Patent Document 2).

Japanese Unexamined Patent Publication No. 59-025901 Japanese Patent Application Laid-Open No. 2006-188413

As such, functional particles consisting of coated particles in which a coating is formed on the surface of a core particle have been proposed in the past, but there was room for improvement in conventional coated particles.

That is, depending on the application, it is desirable that the functional particles be fine and have high crystallinity. For example, a solid oxide fuel cell comprises an oxygen ion conductor that functions as an electrolyte and an electrode catalyst, and during operation, the reaction is activated at the three-phase interface between the electrolyte, the electrode catalyst, and the reactant gas (H ₂ ). Finely divided electrolyte oxygen ion conductors and electrode catalysts are preferred because they have a large contact area at the interface. Furthermore, oxygen ion conductors and electrode catalysts with poor crystallinity may have poor ion conduction properties and catalytic properties. Therefore, excellent crystallinity is desirable.

Particle size and crystallinity are also important for photocatalytic materials that use solar energy to split water into hydrogen and oxygen. Increasing the crystallinity of photocatalytic particles reduces the number of defect structures that serve as recombination centers for the electrons and holes generated by light irradiation, increasing the splitting efficiency. Furthermore, by miniaturizing photocatalytic particles, the contact area with water can be increased, making it possible to split water more efficiently. Water splitting technology using photocatalytic particles is attracting attention as a technology that can produce hydrogen, a clean energy source that can replace fossil fuels. Therefore, miniaturizing photocatalytic particles and increasing their crystallinity are important in promoting clean energy.

With conventional technology, it has been difficult to achieve both fine and highly crystalline functional particles. For example, heat treatment at high temperatures for a long period of time is effective in increasing the crystallinity of particles. However, when functional particles are heat treated at high temperatures for a long period of time, necking occurs between the particles and the necking grows. As a result, the particles sinter together, causing the particles to become coarse. With conventional technology, it has been difficult to produce fine, highly crystalline coated particles as functional particles.

The present invention was completed based on this knowledge, and aims to provide fine, highly crystalline coated oxide particles and a method for producing them.

The present invention encompasses the following aspects (1) to (18). Note that in this specification, the expression "to" includes both ends of the expression. In other words, "X to Y" is synonymous with "X or greater and Y or less."

(1) Coated oxide particles comprising a core particle and a coating covering the surface of the core particle,
the core particle is composed of a crystalline metal oxide;
the coating contains, as a main component, a crystalline compound consisting of at least one of a metal oxide, an oxynitride, and a nitride,
A coated oxide particle, wherein the crystalline compound is lattice-matched to the crystalline metal oxide at the interface between the coating and the core particle.

(2) The coated oxide particle of (1) above, wherein the crystalline compound has a region that does not have grain boundaries from the interface between the coating and the core particle to the surface of the coating.

(3) Coated oxide particles according to (1) or (2) above, in which the crystalline metal oxide constituting the core particle is in a single crystal state.

(4) The crystalline metal oxide constituting the core particle is aluminum oxide (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), lanthanum aluminate (LaAlO ₃ ), yttrium aluminate (YAlO ₃ ), scandium aluminate (ScAlO ₃ ), lanthanum-strontium aluminate (LaSrAlO ₄ ), lanthanum-strontium gallate (LaSrGaO ₄ ), strontium titanate (SrTiO ₃ ), potassium tantalate (KTaO ₃ ), potassium niobate (KNbO ₃ ), lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO ₃ ), dysprosium scandate (DyScO ₃ ), gadolinium scandate (GdScO ₃ ), neodymium scandate (NdScO ₃ The coated oxide particles according to any one of (1) to ( ₃ ) above, which are at least one selected from the group consisting of magnesium aluminate (MgAl ₂ O ₄ ), magnesium oxide (MgO), titanium oxide (TiO _{2 )} , garnet-type yttrium aluminate (Y ₃ Al ₅ O ₁₂ ; YAG), and aluminum titanate (Al 2 TiO 5 ).

(5) Coated oxide particles according to any one of (1) to (4) above, wherein the average particle size of the core particles is 10 nm or more and 50 μm or less.

(6) Coated oxide particles according to any one of (1) to (5) above, wherein the crystalline compound contained in the coating has a crystallite diameter of 50 Å or more as determined by the Williamson-Hall method.

(7) Coated oxide particles according to any one of (1) to (6) above, wherein the crystalline compound contained in the coating contains at least one metal selected from the group consisting of tantalum (Ta), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), tungsten (W), molybdenum (Mo), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), and zinc (Zn).

(8) Coated oxide particles according to any one of (1) to (7) above, wherein the thickness of the coating is 1 nm or more and 1 μm or less.

(9) Coated oxide particles according to any one of (1) to (8 ₎ above, wherein the core particle is composed of aluminum oxide (Al ₂ O ₃ ) and/or aluminum titanate (Al _{2 TiO 5} ), and the crystalline compound contained in the coating is at least one of tantalum oxide (Ta 2 O 5 ), tantalum oxynitride (TaON), and tantalum nitride (Ta ₃ N ₅ ).

(10) Coated oxide particles according to any one of ( ₁ ) to ( ₈ ) above, wherein the core particle is composed of at least one perovskite compound _selected from the group consisting of lanthanum aluminate (LaAlO ₃ ), yttrium aluminate (YAlO ₃ ), scandium aluminate (ScAlO ₃ ), lanthanum-strontium aluminate (LaSrAlO ₄ ), lanthanum-strontium gallate (LaSrGaO _{4 )} , strontium titanate (SrTiO ₃ ), potassium tantalate (KTaO 3 ), potassium niobate (KNbO ₃ ), dysprosium scandate (DyScO 3 ), gadolinium scandate (GdScO 3 ), and neodymium scandate (NdScO 3 ), and the crystalline compound contained in the coating is at least one of an oxide and an oxynitride having a perovskite structure.

(11) Coated oxide particles according to any one of (1) to (8) above, wherein the core particle is made of magnesium oxide (MgO), and the crystalline compound contained in the coating is at least one of anatase-type titanium oxide (TiO ₂ ) or an oxide and oxynitride having a perovskite structure.

(12) The crystalline compound contained in the coating is iron lanthanate (LaFeO ₃ ), calcium zirconate (CaZrO ₃ ), strontium zirconate (SrZrO ₃ ), barium zirconate (BaZrO ₃ ), calcium hafnate (CaHfO ₃ ), strontium hafnate (SrHfO ₃ ), barium hafnate (BaHfO ₃ ), calcium titanium oxynitride (CaTiON ₂ ), strontium titanium oxynitride (SrTiON ₂ ), barium titanium oxynitride (BaTiON ₂ ), lanthanum titanium oxynitride (LaTiO ₂ N), calcium zirconium oxynitride (CaZrON ₂ ), strontium zirconium oxynitride (SrZrON ₂ ), barium zirconium oxynitride (BaZrON ₂ ), lanthanum zirconium oxynitride (LaZrO ₂ N), lanthanum tantalum oxynitride (LaTaON ₂ ), calcium tantalum oxynitride (CaTaO ₂ N), strontium tantalum oxynitride (SrTaO ₂ N), barium tantalum oxynitride (BaTaO ₂ N), lanthanum niobium oxynitride (LaNbON ₂ ), calcium niobium oxynitride (CaNbO ₂ N), strontium niobium oxynitride (SrNbO ₂ N), barium niobium oxynitride (BaNbO ₂ N), and solid solutions thereof.

(13) The coated oxide particles according to any one of (1) to (12) above, wherein the specific surface area of the coated oxide particles is 0.5 m ² /g or more.

(14) A photocatalyst containing coated oxide particles according to any one of (1) to (13) above.

(15) A method for producing coated oxide particles according to any one of (1) to (13) above, comprising the following steps:
a preparation step of preparing an alcohol-based reaction liquid containing metal oxide particles and an organometallic compound;
a dispersion preparation step of adding water to the alcohol-based reaction solution to hydrolyze the organometallic compound, thereby preparing a dispersion containing coated precursor particles in which coating precursors are precipitated on the surfaces of metal oxide particles;
a separation step of extracting the coated precursor particles from the dispersion; and a firing step of calcining the extracted coated precursor particles to obtain coated oxide particles.
Equipped with
The metal oxide particles have a crystallite diameter of 1000 Å or more as determined by the Williamson-Hall method, and the water content of the alcohol-based reaction liquid prepared in the preparation step is 0.1% by mass or less.

(16) The method according to (15) above, in which the coated precursor particles are calcined in an oxygen-containing atmosphere, thereby forming a coating containing a metal oxide as a main component.

(17) The method according to (15) above, in which the coated precursor particles are calcined in an atmosphere containing an ammonia decomposition gas, thereby forming a coating containing metal oxynitride and/or nitride as a main component.

(18) Any of the methods (15) to (17) above, wherein the coated precursor particles are calcined at a temperature of 600°C or higher and 1100°C or lower.

The present invention provides fine, highly crystalline coated oxide particles and a method for producing the same.

1 shows the X-ray diffraction pattern of the coated oxide particles. 1 shows a SEM backscattered electron image of coated oxide particles. HR-TEM images of coated oxide particles are shown. This shows a high-magnification HR-TEM image of the vicinity of the interface between a metal oxide particle (core particle) and a coating. 1 shows a Cs-TEM image of coated oxide particles. 1 shows a selected area electron diffraction pattern obtained for metal oxide particles (core particles). 1 shows a high-magnification Cs-TEM image of the vicinity of the interface between a metal oxide particle (core particle) and a coating. 1 shows a TEM image of coated oxide particles (Example 1). 1 shows a TEM image of coated oxide particles (Example 2). 1 shows a TEM image of coated oxide particles (Example 3). 1 shows a TEM image of tantalum oxide particles (Comparative Example 1). 1 is a graph showing the results of a catalyst performance test.

A specific embodiment of the present invention (hereinafter referred to as the "present embodiment") will be described. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the gist of the present invention.

<<1. Coated oxide particles >>
The coated oxide particles of this embodiment include a core particle and a coating covering the surface of the core particle. That is, the coated oxide particles include a core particle that functions as a support for the coating and a coating that covers at least a portion of the surface of the core particle. The coating may cover only a portion of the surface of the core particle, or the entire surface. However, to fully utilize the function of the coating, it is preferable that the coating cover a high area (coverage ratio) relative to the total surface area of the core particle. The coverage ratio is preferably 50 area% or more, and more preferably 80 area% or more. In this specification, the term "particle" refers not only to a single particle but also to an aggregate of multiple particles. When referring to an aggregate of multiple particles, "particle" can be replaced with "powder." In this specification, "high crystallinity" refers to a state close to a single crystal. In other words, even in the case of polycrystals, the state in which each crystal is large is referred to as "high crystallinity."

In the coated oxide particles of this embodiment, the core particle is composed of a crystallized metal oxide (crystalline metal oxide). By constructing the core particle from a crystalline metal oxide, it is possible to increase the crystallinity of the coating formed on its surface. Whether the metal oxide constituting the core particle is crystallized can be determined by whether a clear diffraction peak is observed in X-ray diffraction analysis.

In the coated oxide particles of this embodiment, the coating contains a crystalline compound as a primary component. The crystalline compound is at least one of an oxide, oxynitride, and nitride of a metal element. Here, the concept of metal encompasses metalloids. Specifically, metal elements are a collective term for all elements in Groups 1 to 12 of the periodic table except for hydrogen (H), as well as aluminum (Al), gallium (Ga), indium (In), thallium (Tl), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), and polonium (Po) in Groups 13 to 16. The metal is primarily a transition metal, but may be a non-transition metal. Furthermore, oxides are compounds primarily containing a metal and oxygen, oxynitrides are compounds primarily containing a metal, oxygen, and nitrogen, and nitrides are compounds primarily containing a metal and nitrogen.

The coating is a layer formed on the outermost surface of the coated oxide particles, and contributes to the expression of functions based on surface properties. By containing a crystalline compound that is a metal oxide, oxynitride, and/or nitride as the main component, the coated oxide particles can be endowed with a variety of properties, including oxygen ion conductivity, electrocatalytic properties, and photocatalytic properties. The main component is a component whose content is 50% by mass or more. From the perspective of fully expressing the functions based on the metal oxide, oxynitride, and/or nitride, the higher the crystalline compound content, the better. The content is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 99% by mass or more.

If the crystalline compound is an oxide, it may contain only metal and oxygen. If the crystalline compound is an oxynitride, it may contain only metal, oxygen, and nitrogen. If the crystalline compound is a nitride, it may contain only metal and nitrogen. Alternatively, the crystalline compound may contain elements other than metal, oxygen, and nitrogen. If it contains other elements, these elements may be contained in the form of a complex compound or in the form of a solid solution. It may also be doped with an impurity metal.

In the coated oxide particles of this embodiment, the crystalline compound contained in the coating is lattice-matched with the crystalline metal oxide constituting the core particle at the interface between the coating and the core particle. When a single-crystalline core particle is used, a crystalline compound with a lattice constant similar to that of the metal oxide is selected, and a coating is formed under specific conditions, the coating grows epitaxially. As a result, the coating and core particle are bonded together in a lattice-matched state with minimal crystal lattice disorder at their interface. Bonding in a lattice-matched state results in the formation of a coating that inherits the highly crystalline state of the core particle, making it possible to obtain coated oxide particles with excellent properties, including photocatalytic properties. Whether or not lattice matching is achieved can be determined by observing the interface between the coating and core particle with a transmission electron microscope (TEM). Lattice matching can be determined when no clear crystal structure disorder is observed at the interface and the crystal structures of the crystalline compound and the crystalline metal oxide change continuously at the interface. It is most preferable that the coating and core particle are lattice-matched at all interfaces. However, the effects of this embodiment are realized as long as there is lattice matching at at least some of the interfaces. Therefore, this embodiment also includes the case where there is lattice matching at some of the interfaces.

Preferably, the crystalline compound has a region that does not have grain boundaries from the interface between the coating and the core particle to the surface of the coating. In other words, it is preferable that the crystalline structure that makes up the coating is continuous without grain boundaries from the lower surface of the coating, i.e., the interface with the core particle, to the upper surface of the coating, i.e., the surface of the coating. A coating that is formed continuously without grain boundaries has high crystallinity. This makes it possible to further improve the properties of the coated oxide particles.

Preferably, the crystalline metal oxide constituting the core particle is in a single crystal state. By making the core particle in a single crystal state, it is possible to further increase the crystallinity of the coating formed thereon. The concept of a single crystal state includes not only single crystals but also aggregates of multiple single crystals. In other words, the single crystal state also includes a state in which single crystal primary particles are loosely bonded to form aggregated secondary particles, and a state in which several single crystals are bonded together. Whether or not a core particle is in a single crystal state can be determined by examining the electron beam diffraction pattern.

The method for producing single-crystalline core particles (single-crystalline core particles) is not limited as long as it produces single-crystalline particles. However, promising methods include hydrothermal synthesis and flux synthesis, which synthesize core particles from a metal-containing solution. Other methods include wet methods such as coprecipitation and alkoxide methods, and gas-phase methods such as flame spray and CVD. Furthermore, the synthesized particles may be heated and fired in an oxygen-containing atmosphere to promote crystallization. Another effective method is to produce a single-crystalline or polycrystalline ingot using methods such as the Czochralski method, Bridgman method, Verneuil method, or floating zone method, and then pulverize the resulting ingot. Powdering can be achieved by subjecting the ingot to processing such as cutting and/or mechanical crushing.

The material of the metal oxide constituting the core particle is not limited as long as it is crystalline. However, preferably the crystalline metal oxide is aluminum oxide ( _Al2O3 ), zirconium oxide ( _ZrO2 ), lanthanum aluminate ( _LaAlO3 ), yttrium aluminate ( _YAlO3 ), scandium aluminate ( _ScAlO3 ), lanthanum strontium aluminate ( _LaSrAlO4 ), lanthanum strontium gallate ( _LaSrGaO4 ), strontium titanate ( _SrTiO3 ), potassium tantalate ( _KTaO3 ), potassium niobate ( _KNbO3 ), lithium tantalate ( _LiTaO3 ), lithium niobate ( _LiNbO3 ), dysprosium scandate ( _{DyScO3 )} , gadolinium scandate ( _GdScO3 ), neodymium scandate (NdScO3 ₎ ), magnesium aluminate (MgAl ₂ O ₄ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), garnet-type yttrium aluminate (Y ₃ Al ₅ O ₁₂ ; YAG), and aluminum titanate (Al ₂ TiO ₅ ).

The average particle size of the core particles is preferably 10 nm or more and 50 μm or less, more preferably 50 nm or more and 10 μm or less, and even more preferably 100 nm or more and 1 μm or less. When the average particle size is 10 nm or more, the core particles can be uniformly coated without agglomeration. When the average particle size is 50 μm or less, particles with a large specific surface area can be obtained. The average particle size can be determined by observing the core particles with a scanning electron microscope (SEM). The shape of the core particles is not limited. They may be regular shapes such as spherical, plate-like, or rod-like, or they may be irregular. The average particle size of core particles having a shape other than spherical can be determined by measuring the particle size from various directions and calculating the average value.

Preferably, the crystalline compound contained in the coating has a crystallite diameter of 50 Å or more as determined by the Williamson-Hall method. The Williamson-Hall method is a technique for estimating the crystalline state from the peak shape in X-ray diffraction, and allows for the crystallite diameter and lattice distortion to be determined separately. Increasing the crystallite diameter of the crystalline compound increases the crystallinity of the coating, which is the main factor in functional expression, and as a result, it becomes possible to further improve various properties such as photocatalytic properties. A crystallite diameter of 100 Å or more is more preferable, and 200 Å or more is even more preferable. There is no upper limit to the crystallite diameter.

Preferably, the crystalline compound contained in the coating contains at least one metal selected from the group consisting of tantalum (Ta), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), tungsten (W), molybdenum (Mo), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), and zinc (Zn). Crystalline compounds containing these metals have particularly excellent properties, including photocatalytic properties. For example, tantalum oxide and titanium oxide possess photocatalytic activity under ultraviolet light irradiation. Tantalum oxynitride and tantalum nitride possess highly active photocatalytic activity that is responsive to visible light.

The crystalline compound contained in the coating may also be a composite compound of a metal selected from the above group with another metal. Examples of the other metal include at least one metal selected from the group consisting of sodium (Na), potassium (K), lithium (Li), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), lanthanum (La), yttrium (Y), and scandium (Sc). The composite compound is an oxide, oxynitride, and/or nitride, and has a crystalline structure such as a perovskite structure, an ilmenite structure, or a pyrochlore structure. Coated oxide particles having a coating made of such a compound on their surface have particularly excellent properties, including photocatalytic properties.

Preferably, the core particles are composed of at least one perovskite compound _selected from the group consisting of lanthanum aluminate (LaAlO ₃ ), yttrium aluminate (YAlO ₃ ), scandium aluminate (ScAlO ₃ ), lanthanum-strontium aluminate (LaSrAlO ₄ ), lanthanum-strontium gallate (LaSrGaO ₄ ), strontium titanate (SrTiO ₃ ), potassium tantalate (KTaO ₃ ), potassium niobate (KNbO ₃ ), dysprosium scandate (DyScO ₃ ), gadolinium scandate (GdScO ₃ ), and neodymium scandate (NdScO 3 ), and the crystalline compound contained in the coating is at least one of an oxide and an oxynitride having a perovskite structure.

When the core particle is composed of the above-mentioned perovskite-type compound (such as _LaAlO3 ), if the crystalline compound contained in the coating is a composite compound with a perovskite structure, the coating can be epitaxially formed on the core particle, resulting in the growth of a high-quality crystalline coating. This makes it possible to exhibit better photocatalytic properties. The crystalline compound contained in the coating may have the same composition as the perovskite-type compound that constitutes the core particle, or it may have a different composition.

Examples of crystalline compounds contained in the coating include iron lanthanate (LaFeO ₃ ), calcium zirconate (CaZrO ₃ ), strontium zirconate (SrZrO ₃ ), barium zirconate (BaZrO ₃ ), calcium hafnate (CaHfO ₃ ), strontium hafnate (SrHfO ₃ ), barium hafnate (BaHfO ₃ ), calcium titanium oxynitride (CaTiON ₂ ), strontium titanium oxynitride (SrTiON ₂ ), barium titanium oxynitride (BaTiON ₂ ), lanthanum titanium oxynitride (LaTiO ₂ N), calcium zirconium oxynitride (CaZrON ₂ ), strontium zirconium oxynitride (SrZrON ₂ ), barium zirconium oxynitride (BaZrON ₂ ), lanthanum zirconium oxynitride (LaZrO ₂ N), lanthanum tantalum oxynitride (LaTaON ₂ ), calcium tantalum oxynitride (CaTaO ₂ N), strontium tantalum oxynitride (SrTaO ₂ N), barium tantalum oxynitride (BaTaO ₂ N), lanthanum niobium oxynitride (LaNbON ₂ ), calcium niobium oxynitride (CaNbO ₂ N), strontium niobium oxynitride (SrNbO ₂ N), barium niobium oxynitride (BaNbO ₂ N), and solid solutions thereof.

The use of these compounds (such as _LaFeO3 ) improves the crystallinity of the coating formed on the core particles, dramatically improving various properties such as photocatalytic performance. In particular, since these compounds (such as _LaFeO3 ) all have a perovskite structure, they are prone to form solid solutions. Adjusting the composition of the solid solution is advantageous because it allows the lattice constant of the core particles to be controlled to an optimal value so that the coating can easily grow epitaxially. Furthermore, the lattice constant can also be controlled by doping these compounds with different metals.

Preferably, the core particle is made of magnesium oxide (MgO), and the crystalline compound contained in the coating is at least one of anatase titanium oxide (TiO ₂ ) or an oxide and oxynitride having a perovskite structure, such as the above-mentioned crystalline compounds (LaFeO _{3 ,} etc.).

Perovskite-structured composite compounds (oxides, oxynitrides) and perovskite-structure compounds based on these solid solutions are prone to epitaxial growth, dramatically improving the photocatalytic performance of coatings. Furthermore, epitaxial growth of a titanium oxide film on the surface of magnesium oxide (MgO) core particles readily results in an anatase-type crystal structure with excellent photocatalytic performance. Conventional synthesis methods tend to produce titanium oxide with a rutile structure, which has poor photocatalytic performance. Therefore, using magnesium oxide for the core particles is effective in improving photocatalytic performance.

Preferably, the core particles are made of aluminum _titanate ( _Al2TiO5 ), and the crystalline compound contained in the coating is titanium _nitride ( _Ti3N5 ). _Ti3N5 has the same anosovite structure as _Al2TiO5 , and therefore can be easily epitaxially formed on the core particles _, resulting in _a coating with good crystallinity.

Preferably, the core particle is composed of aluminum oxide (Al ₂ O ₃ ), and the crystalline compound contained in the coating is at least one of tantalum oxide (Ta ₂ O ₅ ), tantalum oxynitride (TaON), and tantalum nitride (Ta ₃ N ₅ ). Tantalum oxide is a photocatalytic material that is active under ultraviolet light irradiation, and tantalum oxynitride and tantalum nitride are highly active photocatalytic materials that are responsive to visible light. Therefore, by forming a coating from at least one of tantalum oxide, tantalum oxynitride, and tantalum nitride, it is possible to impart photocatalytic function to ultraviolet light or visible light.

The _composition ratios of tantalum oxide ( _Ta2O5 ), tantalum oxynitride (TaON), and tantalum _nitride ( _Ta3N5 ) contained in the coating may deviate from the stoichiometric composition ratio. That is, in the case of tantalum oxide ( _Ta2O5 ), the O/Ta atomic ratio may deviate from 5/2, or some of the O may be substituted by lattice defects. In the case of tantalum oxynitride (TaON), the O _/ N atomic ratio may deviate from 1, or some of the O or N may be substituted by lattice defects. Furthermore, in the case of tantalum _nitride ( _Ta3N5 ), some of the N may be substituted by O or lattice defects.

Preferably, the coating thickness is 1 nm or more and 10 μm or less. If the coating thickness is too small, it may be difficult to fully utilize the excellent properties of the metal-containing coating. On the other hand, if the coating is too thick, the thick coating may connect the particles together, reducing the specific surface area. The coating thickness is more preferably 1 nm or more and 1 μm or less, even more preferably 5 nm or more and 100 nm or less, and particularly preferably 10 nm or more and 50 nm or less.

Preferably, the amount of coating is 10% by mass or more and 500% by mass or less relative to the amount of core particles. If the amount of coating is too small, it may be difficult to fully utilize the excellent properties of the coating, and if the amount of coating is too large, the specific surface area may decrease. A coating amount of 30% by mass or more and 300% by mass or less is more preferable, and 50% by mass or more and 150% by mass or less is even more preferable.

Preferably, the specific surface area of the coated oxide particles is 0.5 m ² /g or more. By increasing the specific surface area, it becomes possible to fully exert functions based on the surface characteristics. The specific surface area is preferably 1.0 m ² /g or more, more preferably 2.0 m ² /g or more, and even more preferably 3.0 m ² /g or more. There is no upper limit to the specific surface area. However, it is typically 10.0 m ² /g or less.

<<2. Photocatalyst >>
The photocatalyst of this embodiment includes the coated oxide particles described above. This photocatalyst comprises fine coated oxide particles with excellent crystallinity. Therefore, it is expected to have high catalytic performance. For example, coated oxide particles having a coating film containing at least one selected from the group consisting of anatase titanium oxide (TiO ₂ ), tungsten oxide (WO ₃ ), tantalum oxide (Ta ₂ O ₅ ), tantalum oxynitride (TaON), and tantalum nitride (Ta ₃ N ₅ ) exhibit a water splitting reaction. Therefore, a photocatalyst containing this coated oxide particle is useful for water splitting applications. The photocatalyst may contain only the coated oxide particles, or may contain other components. Examples of other components include promoters such as platinum (Pt), rhodium (Rh), and ruthenium oxide (RuO ₂ ).

<<3. Method for producing coated oxide particles>>
The coated oxide particles of this embodiment can be produced by any method as long as they satisfy the above-mentioned requirements. However, a preferred production method includes the following steps: a preparation step of preparing an alcohol-based reaction solution containing metal oxide particles and an organometallic compound; a dispersion preparation step of adding water to the alcohol-based reaction solution to hydrolyze the organometallic compound, thereby preparing a dispersion containing coated precursor particles in which a coating precursor is precipitated on the surfaces of the metal oxide particles; a separation step of extracting the coated precursor particles from the dispersion; and a calcination step of calcining the extracted coated precursor particles to obtain coated oxide particles. The metal oxide particles have a crystallite diameter of 1000 Å or more as determined by the Williamson-Hall method. Furthermore, the water content of the alcohol-based reaction solution prepared in the preparation step is 0.1% by mass or less. Details of each step are described below.

<Reaction solution preparation step>
In the reaction solution preparation step, an alcohol-based reaction solution containing metal oxide particles and an organometallic compound is prepared. The alcohol-based reaction solution is a liquid in which a metal organometallic compound is dissolved in alcohol and metal oxide particles are dispersed. The metal oxide particles are composed of crystalline oxide and are the raw material that will become the core of the coated oxide particles. The organometallic compound is a compound that has a metal-carbon bond and is the raw material for the coating.

The metal oxide particles have a crystallite diameter of 1000 Å or greater, as determined by the Williamson-Hall method. Using metal oxide particles with such a large crystallite diameter can enhance the crystallinity of the coating formed thereon. Specifically, in the subsequent dispersion preparation process, a coating precursor consisting of metal hydroxide and/or oxide precipitates on the surface of the metal oxide particles. Furthermore, in the firing process, the coating precursor transforms into a coating. The oxides, oxynitrides, and/or nitrides that make up the coating tend to have their constituent elements (metal, oxygen, nitrogen) aligned along the lattice planes of the metal oxide particles. Using highly crystalline metal oxide particles as a base can enhance the crystallinity of the coating formed on its surface.

There are no limitations on the material or particle size of the metal oxide particles, so long as the crystallite diameter is 1000 Å or more. However, aluminum oxide (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), lanthanum aluminate (LaAlO ₃ ), yttrium aluminate (YAlO ₃ ), scandium aluminate (ScAlO ₃ ), lanthanum strontium aluminate (LaSrAlO ₄ ), lanthanum strontium gallate (LaSrGaO ₄ ), strontium titanate (SrTiO ₃ ), potassium tantalate (KTaO ₃ ), potassium niobate (KNbO ₃ ), lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO ₃ ), dysprosium scandate (DyScO ₃ ), gadolinium scandate (GdScO ₃ ), neodymium scandate (NdScO ₃ ), magnesium aluminate (MgAl ₂ O 3 ), It is preferable that the metal oxide particles are composed of at least one material selected from the group consisting of magnesium oxide ( _MgO ), titanium oxide (TiO ₂ ), garnet-type yttrium aluminate (Y ₃ Al ₅ O ₁₂ :YAG), and aluminum titanate (Al ₂ TiO ₅ ). The average particle size of the metal oxide particles is preferably 10 nm or more and 50 μm or less, more preferably 50 nm or more and 10 μm or less, and even more preferably 100 nm or more and 1 μm or less. The shape of the metal oxide particles is not limited. They may be regular shapes such as spherical, plate-like, and rod-like, or may be irregular.

The type of metal contained in the organometallic compound is not limited. However, at least one metal selected from the group consisting of tantalum (Ta), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), tungsten (W), molybdenum (Mo), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), and zinc (Zn) is preferred. The type of organometallic compound is also not limited, as long as it is a compound that dissolves in alcohol (alcohol-soluble compound). Typically, it is a metal alkoxide, acetate, and/or acetylacetonate. Alkoxides are preferred, and at least one metal selected from the group consisting of methoxide, ethoxide, propoxide, and butoxide is more preferred.

There are no limitations on the alcohol, as long as it dissolves the organometallic compound. However, at least one alcohol selected from the group consisting of easily available methanol, ethanol, propanol, butanol, and isopropyl alcohol is preferred. A single alcohol may be used alone, or multiple alcohols may be mixed. In addition to the metal oxide and organometallic compound, the reaction solution may also contain additives such as a pH adjuster and a catalyst.

The method for preparing the alcohol-based reaction solution is not limited. For example, it can be prepared by adding and mixing metal oxide particles and an organometallic compound with alcohol. The organometallic compound dissolves in alcohol to form a solution. On the other hand, metal oxide particles do not dissolve in alcohol. Therefore, a slurry-like reaction solution is obtained in which metal oxide particles are dispersed in an alcohol solution in which the organometallic compound is dissolved. In this case, the order of adding the metal oxide particles and the organometallic compound is not limited. An alcohol solution of the organometallic compound may be prepared first, and then the metal oxide particles may be dispersed in this solution. Alternatively, the metal oxide particles may be dispersed in alcohol, and the organometallic compound may be dissolved in the resulting dispersion. The alcohol solution of the organometallic compound may be prepared by dissolving the organometallic compound in alcohol. Alternatively, an alcohol solution containing the organometallic compound can be prepared by reacting a salt such as a metal chloride or hydroxide with alcohol. For example, dissolving tantalum chloride (TaCl ₅ ) in alcohol releases hydrochloric acid, resulting in an alcohol solution in which tantalum alkoxide is dissolved.

In the manufacturing method of this embodiment, it is important to limit the water content of the reaction solution in which the organometallic compound is dissolved to 0.1% by mass or less. Reducing the water content of the reaction solution can improve the crystallinity of the resulting coating. While the details of this mechanism are unclear, it is believed to be as follows: Specifically, metal oxide particles have hydroxyl groups (OH ⁻ ) on their surfaces. When the reaction solution has a low water content, the organometallic compound first adsorbs to the hydroxyl groups on the surface of the metal oxide particles. When water is added in the subsequent dispersion preparation process, a hydrolysis reaction continuously occurs between the organometallic compound adsorbed on the surface and the organic compound in the reaction solution. As a result, a network of metal hydroxides or oxides is formed around the metal oxide particle surfaces, which then becomes the coating precursor. Because metal oxide particles have high crystallinity, the network (coating precursor) formed around the surface has a relatively uniform crystal orientation, and the coating obtained by calcining this coating precursor has high crystallinity. On the other hand, if the reaction solution contains a large amount of water, the dissolved organometallic compound is immediately hydrolyzed, producing metal hydroxides or oxides in the reaction solution. Because these hydroxides and oxides are suspended in the reaction solution, their crystal orientation is random. When water is added in the subsequent dispersion preparation process, crystal growth proceeds around the initial nuclei with random crystal orientation, resulting in a fine polycrystalline coating.

The water content of the reaction solution is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less. The water content can be adjusted by using dehydrated alcohol. For example, while the water content of regular ethanol is high at around 0.2% by mass, the water content of ultra-dehydrated ethanol is 0.001% by mass or less.

<Dispersion liquid preparation process>
In the dispersion preparation step, water is added to the alcohol-based reaction solution to hydrolyze the organometallic compound. This produces a dispersion containing coated precursor particles, in which the coating precursor is precipitated on the surfaces of metal oxide particles. When water is added to the reaction solution, the organometallic compound is hydrolyzed and then undergoes a polycondensation reaction to form a sol containing the metal hydroxide and/or oxide, which precipitates on the surfaces of the oxide particles as the coating precursor.

The amount of water added is preferably between 2 and 50 times the amount (equivalent) required to hydrolyze the organometallic compound. If the amount added is too small, the hydrolysis reaction may not proceed sufficiently, resulting in problems such as poor coating formation. If the amount added is too large, the hydrolysis rate may be too fast, resulting in problems such as an uneven coating. It is also preferable to add water little by little. For example, a burette may be used to intermittently add small amounts of water dropwise, or mist water may be sprayed onto the reaction solution. When adding water dropwise, it is preferable to set the dropwise addition rate to 5 mL/min or less. It is also preferable to stir the reaction solution when adding water. This is because the hydrolysis reaction occurs while the metal oxide particles in the reaction solution are dispersed, allowing for the formation of a uniform coating precursor on the surface of the metal oxide particles.

<Separation process>
In the separation step, the coated precursor particles are removed from the dispersion. Because the coated precursor particles are solid, they can be removed from the dispersion by solid-liquid separation. The separation method is not limited. Known methods such as filtration or centrifugation may be used. If necessary, the coated precursor particles removed from the dispersion may be washed and dried. In the dispersion, the coating precursor on the surface of the metal oxide particles is a sol consisting of a metal hydroxide and/or oxide. When subjected to a drying process, this sol gels, and when further drying is continued to completely remove the liquid, it solidifies. The coating precursor may contain components such as water of crystallization, carbon, chlorine, nitrogen, and/or sulfur. These components are removed by volatilization in the subsequent firing step, so there is no problem even if they are present in the coating precursor.

<Firing process>
In the calcination step, the extracted coated precursor particles are calcined. This causes dehydration and crystallization of the coating precursor, resulting in the formation of a coating composed of oxide, oxynitride, and/or nitride. The calcination conditions can be determined depending on the type of coating desired. For example, the coated precursor particles may be calcined in an oxygen-containing atmosphere. This allows the formation of a coating containing a metal oxide as a main component. The oxygen-containing atmosphere may be air or an oxygen-containing inert atmosphere (e.g., Ar).

The coated precursor particles may be calcined in an atmosphere containing an ammonia decomposition gas. This allows the formation of a coating containing metal oxynitride and/or nitride as a main component. Hydrogen or an inert gas (N ₂ , Ar, He, etc.) may be added to the ammonia decomposition gas as long as the desired coated oxide particles are obtained. The ammonia decomposition gas is a gas produced when ammonia gas (NH ₃ ) is decomposed at a temperature of 500°C or higher, and contains nitrogen (N ₂ ), hydrogen (H ₂ ), and nitrogen hydroxide (NH , NH ₂ , and their radicals).

Incidentally, the coating precursor contains a large amount of oxygen. Therefore, when ammonia decomposition gas oxidizes or nitrides the coating precursor, water is generated as a by-product. The generated water can have a negative effect on obtaining highly crystalline oxynitrides and/or nitrides. When using ammonia decomposition gas, it is desirable to circulate it sufficiently to remove moisture during firing.

Regardless of the atmosphere in which firing is performed, the firing temperature is preferably 600°C or higher and 1100°C or lower. If the firing temperature is lower than 600°C, it becomes difficult to obtain a highly crystalline coating. On the other hand, if the firing temperature is higher than 1100°C, production costs will increase and the resulting coated oxide particles may grow, increasing their specific surface area. The holding time during firing is not limited as long as the desired coated oxide particles are obtained. For example, it may be 0.5 hours or higher and 20 hours or lower. A shorter holding time will result in an oxynitride coating, while a longer holding time will result in a nitride coating.

The coated oxide particles of this embodiment can be produced using the manufacturing method described above. However, the manufacturing method of the coated oxide particles of this embodiment is not limited to using an alcohol solution of an organometallic compound. For example, an aqueous solution of a chloride may be used as the coating raw material instead of an alcohol solution of an organometallic compound. In this case, the pH of the aqueous solution can be adjusted to precipitate a metal hydroxide, thereby forming a coating precursor.

The present invention will be described in more detail using the following examples. However, the present invention is not limited to the following examples.

[Experimental Example A]
In Experimental Example A, aluminum oxide core particles were used, _and their surfaces were coated with tantalum oxide ( _Ta2O5 ), tantalum oxynitride ( _TaON ), or tantalum nitride ( _Ta3N5 ) to produce coated oxide particles, which were then evaluated. For comparison, particles consisting only of tantalum oxide were also produced and evaluated.

(1) Preparation of coated oxide particles [Example 1]
In Example 1, aluminum oxide (Al ₂ O ₃ ) particles coated with tantalum oxide (Ta ₂ O ₅ ) were prepared using aluminum oxide powder and tantalum (V) ethoxide.

<Preparation process>
The raw materials used were aluminum oxide powder (Sumitomo Chemical Co., Ltd.), tantalum (V) ethoxide (Sigma-Aldrich), and ultra-dehydrated ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.). The aluminum oxide powder consisted of polyhedral spherical α-Al ₂ O ₃ single crystal particles with an average particle size of 0.3 μm. The water content of the ultra-dehydrated ethanol was 0.001% by mass or less.

1.0 g of aluminum oxide powder and 1.84 g of tantalum(V) ethoxide were added to 100 mL of ultra-dehydrated ethanol and mixed to obtain a reaction solution. The amount of tantalum in the reaction solution, converted to tantalum oxide, was 100% by mass relative to the aluminum oxide. Next, ultrasonic waves were irradiated onto the reaction solution using an ultrasonic cleaner for 1 minute to disperse the aluminum oxide powder.

<Coating process>
The resulting dispersion was then stirred at 500 rpm using a magnetic stirrer. While stirring, 3.1 mL of ultrapure water was added dropwise to the dispersion at a rate of 1 mL/min. The amount of water added was 15 equivalents. After the addition was complete, stirring was continued for 1 hour to obtain a dispersion. A precipitate formed in the dispersion due to the addition and stirring of the dispersion.

<Solid-liquid separation process>
The resulting precipitate was removed from the dispersion by filtration, washed with water, and then dried in air at 110° C. for 2.5 hours.

<Firing process>
The dried precipitate was fired in air at 1000°C for 1 hour to produce coated oxide particles. The obtained _coated oxide particles were aluminum oxide ( _Al2O3 ) particles coated with _tantalum oxide ( _Ta2O5 ).

[Example 2]
The amount of tantalum in the reaction solution was converted to the amount of tantalum oxide and was 200 mass % relative to the amount of aluminum oxide, and the amount of ultrapure water added was 6.1 mL. Except for this, coated oxide particles were produced under the same conditions as in Example 1. The obtained coated oxide particles were aluminum oxide (Al ₂ O ₃ ) particles coated with tantalum oxide (Ta ₂ O ₅ ).

[Example 3]
The amount of tantalum in the reaction solution was converted to the amount of tantalum oxide and was 300 mass % relative to the amount of aluminum oxide, and the amount of ultrapure water added was 9.2 mL. Except for this, coated oxide particles were produced under the same conditions as in Example 1. The obtained coated oxide particles were aluminum oxide (Al ₂ O ₃ ) particles coated with tantalum oxide (Ta ₂ O ₅ ).

[Example 4]
The particles were fired at 900°C for 1 hour in an atmosphere where ammonia gas was flowing at a flow rate of 0.2 L/min. Except for this, the coated oxide particles were produced in the same manner as in Example 1. The obtained coated oxide particles were aluminum oxide (Al ₂ O ₃ ) particles coated with tantalum oxynitride (TaON).

[Example 5]
The particles were fired at 900°C for 8 hours in an atmosphere where ammonia gas was flowing at a flow rate of 0.2 L _/ min. Except for this, the coated oxide particles were produced in the same manner as in Example 1. The obtained coated oxide particles were aluminum oxide ( _Al2O3 ) particles coated with tantalum nitride _{( Ta3N5} ).

[Comparative Example 1]
Particles consisting solely of tantalum oxide (Ta ₂ O ₅ ), a coating component, were synthesized. Specifically, an ethanol solution containing tantalum (V) ethoxide was prepared under the same conditions as in Example 1, except that aluminum oxide powder was not added. Water was added to this ethanol solution to obtain a precipitate, which was then washed, dried, and calcined to produce tantalum oxide particles.

(2) Evaluation The core particle raw material (aluminum oxide powder) used in Examples 1 to 5 and Comparative Example 1, and the resulting products (coated oxide particles, tantalum oxide particles) were evaluated for various properties as follows.

<Specific surface area>
The BET specific surface areas of the core particle raw material and the product were measured using the _N gas adsorption method as follows: After vacuum degassing the sample at 200°C for 2 hours, nitrogen gas was adsorbed at 77.35K, and the specific surface area was determined by the BET multipoint method.

<XRD>
The core particle raw material and the product were analyzed by X-ray diffraction (XRD) to identify the crystalline phase. The analysis was performed under the following conditions.

- X-ray diffraction equipment: Malvern Panalytical,
X'Pert Pro MRD
-Radiation source: CuKα
-Tube voltage: 45 kV
-Tube current: 40mA
- Scan speed: 5.5°/min - Scan range (2θ): 20 to 60°

The crystallite diameter was determined using the obtained diffraction pattern by the Williamson-Hall method. Specifically, the half-width β [rad] at each diffraction angle θ [rad] was measured, and the relationship between β cos θ and sin θ was plotted. This relationship was then used to calculate the crystallite diameter D using the following equation (1). In equation (1), ε is the lattice strain, and λ is the X-ray wavelength (CuKα1: 1.540598 Å).

<SEM>
The product was observed using a scanning electron microscope (SEM; Carl Zeiss, ULTRA55) at an accelerating voltage of 1 kV.

<HR-TEM>
The product was observed using a high-resolution transmission electron microscope (HR-TEM; Hitachi High-Tech Corporation, HT-TEM H9500) at an accelerating voltage of 300 kV.

<Cs-TEM>
The core particle raw material and the product were observed using a spherical aberration corrected transmission electron microscope (Cs-TEM). First, the thermosetting resin in which the sample was embedded was processed using an ion milling device (Cryo-ion slicer IB-09060CIS manufactured by JEOL Ltd.) to prepare a cross-sectional thin section sample with a thickness of less than 100 nm. Then, this thin section sample was observed using a Cs-TEM (JEOL Ltd., JEM-ARM200F) at an accelerating voltage of 200 kV.

<TEM>
The core particle raw material and the product were observed using a transmission electron microscope (STEM; Hitachi High-Tech Corporation, HD2700) at an accelerating voltage of 200 kV.

<Evaluation of dye decomposition photocatalyst>
The photocatalytic activity of the product was quantitatively evaluated by measuring the decolorization rate of the methyl orange aqueous solution. Specifically, 15 g of a 0.001 w/v% methyl orange aqueous solution was weighed and placed in a petri dish. Next, 15 mg of catalyst powder (coated oxide particles, tantalum oxide particles) was weighed and placed in the methyl orange aqueous solution. The bottom of the dish was placed in an ultrasonic cleaner to disperse the catalyst powder for 5 minutes to prepare a suspension. This suspension was then left in a dark place for 1 hour to allow the powder surface to reach equilibrium with the aqueous solution. Two UV-transmitting filters (Shibuya Optical Co., Ltd., UVF260, synthetic quartz) were placed on the petri dish, and ultraviolet irradiation was performed using an ultraviolet irradiation device (Sen Special Light Sources Co., Ltd., PL17-110, low-pressure mercury lamp SUV110GS-36). UV irradiation was performed for 0, 10, 20, 30, or 40 minutes at an irradiation distance of 7 cm. The low-pressure mercury lamp used generated UV light mainly at wavelengths of 185 nm and 254 nm. On the other hand, the UV transmission filter transmitted over 70% of the 254 nm light and blocked most of the 185 nm light. Therefore, the sample was irradiated mainly with UV light of 254 nm.

After the specified time of UV irradiation, the water evaporated during UV irradiation was weighed, and the amount of ultrapure water equivalent to the weight loss was weighed and added to the reacted suspension to adjust the weight to the initial weight before UV irradiation. Next, the suspension was subjected to solid-liquid separation using a suction filter. The filtrate was transferred to a tube and illuminated with an LED light to confirm complete solid-liquid separation. In other words, if scattered light due to the Dindal effect was observed in the filtrate, this indicated insufficient solid-liquid separation. Therefore, solid-liquid separation using suction filtration was repeated until scattered light from the LED was no longer observed in the filtrate. Next, the wavelength dependence of the absorbance of these solutions was examined using a UV-VIS spectrophotometer (Hitachi High-Tech Fielding Corporation, Spectrophotometer UH4150). Specifically, the solutions were placed in a 10 mm cell (GL Sciences Inc., S10-G-10) and the absorbance at a wavelength of 464 nm was measured. The concentration of methyl orange could be determined from the absorbance at a wavelength of 464 nm. Therefore, the rate at which methyl orange is decomposed due to photocatalytic activity can be evaluated from the absorbance at a wavelength of 464 nm. This process was used to evaluate the photocatalytic activity of the products (coated oxide particles, tantalum oxide particles).

(3) Evaluation Results <Specific Surface Area (Raw Aluminum Oxide Powder and Coated Oxide Particles)>
The specific surface area of the raw aluminum oxide powder used in Examples 1 to 5 was 5.7 m ² /g. The specific surface area of the coated oxide particles obtained in Example 1 was 3.99 m ² /g. It was found that the coated oxide particles had a large specific surface area and were fine.

<XRD (raw aluminum oxide powder and coated oxide particles)>
The X-ray diffraction (XRD) pattern of the coated oxide particles obtained in Example 1 is shown in Figure 1. Figure 1 also shows the diffraction pattern of the raw aluminum oxide powder and the standard diffraction pattern of tantalum oxide (PDF 00-025-0922). In the diffraction pattern of the coated oxide particles, peaks matching the patterns of tantalum oxide and aluminum oxide were detected. This indicated that a tantalum oxide crystalline phase had been formed.

<Crystallite size (raw aluminum oxide powder and coated oxide particles)>
The crystallite diameters of the oxide particles ( _{Al2O3 particles} ) and coating ( _Ta2O5 film) contained in the coated oxide particles obtained in Example 1, together with the crystallite diameter of the raw aluminum oxide powder, are shown in Table 1. The crystallite diameter of the raw aluminum oxide powder was 1000 Å or more, and the crystallite diameter of the coating _{( Ta2O5 film} ) was as large as 280 Å.

<SEM (coated oxide particles)>
Figure 2 shows an SEM backscattered electron image of the coated oxide particles obtained in Example 1. In Figure 2, the bright areas are regions where tantalum (Ta) is present, and the dark areas are regions where aluminum (Al) is present. It was confirmed that the aluminum oxide particles were uniformly coated with tantalum oxide.

<HR-TEM (coated oxide particles)>
An HR-TEM image of the coated oxide particles obtained in Example 1 is shown in Figure 3. In Figure 3, the dark areas are regions where tantalum (Ta) is present, and the light areas are regions where aluminum (Al) is present. It was confirmed that the surfaces of the aluminum oxide particles were almost uniformly coated with tantalum oxide. The tantalum oxide film had a uniform thickness of 14 to 17 nm. Since the tantalum oxide film thickness (14 to 17 nm) was smaller than the crystallite diameter (280 Å), it was thought that the tantalum oxide film was in a single crystal state with no grain boundaries present in the film thickness direction.

A high-magnification HR-TEM image of the vicinity of the interface between the oxide particles (Al ₂ O ₃ particles) and the coating (Ta ₂ O ₅ film) is shown in Figure 4. Lattice fringes of tantalum oxide crystals and aluminum oxide crystals were clearly observed in Figure 4. This indicates that both the tantalum oxide coating and the aluminum oxide particles have excellent crystallinity.

<Cs-TEM (coated oxide particles)>
A Cs-TEM image of the coated oxide particles obtained in Example 1 is shown in Figure 5. In Figure 5, the dark areas are regions where tantalum (Ta) is present, and the light areas are regions where aluminum (Al) is present. It was confirmed that almost the entire surface of the aluminum oxide particles was coated with a tantalum oxide coating.

The selected area electron diffraction pattern obtained for the circled region A in Figure 5 is shown in Figure 6. This region A corresponds to the aluminum oxide particles contained in the coated oxide particles. As shown in Figure 6, periodic diffraction spots were clearly observed. The pattern of these diffraction spots was similar to the electron diffraction simulation pattern of α-type aluminum oxide (α-Al ₂ O ₃ ). This indicated that the aluminum oxide particles contained in the coated oxide particles were single crystals of α-type aluminum oxide.

A high-magnification Cs-TEM image of the vicinity of the interface between the oxide particles (Al ₂ O ₃ particles) and the coating (Ta ₂ O ₅ film) is shown in Figure 7. The crystal structure changed continuously from the aluminum oxide particles to the tantalum oxide film. This indicated that the crystal lattice of the aluminum oxide particles and that of the tantalum oxide film were matched.

The coated oxide particles obtained in Examples 4 and 5 were evaluated, and the same results as in Example 1 were obtained. This shows that even when an oxynitride film (TaON film) or nitride ( _{Ta3N5 film} ) is formed, the oxide particles ( _{Al2O3 particles} ) and coating films (TaON film, _{Ta3N5 film} ) both have excellent crystallinity.

<TEM (Covered oxide particle cross section)>
Cross-sectional TEM images of the coated oxide particles obtained in Examples 1 to 3 are shown in Figures 8 to 10, respectively. When the amount of tantalum oxide was increased to 100 mass% (Figure 8, Example 1), 200 mass% (Figure 9, Example 2), and 300 mass% (Figure 10, Example 3), it was observed that the coating layer became thicker. When the amount of tantalum was increased to 300 mass% (Figure 10, Example 3), the generation of _{Ta2O5 particles} was observed in addition to the coating layer.

Figure 11 shows a TEM image of the tantalum oxide particles obtained in Comparative Example 1. The particle size of the primary particles was approximately 300 nm.

<Evaluation of dye decomposition photocatalyst>
The photocatalytic activities of the coated oxide particles obtained in Examples 1 to 3, the tantalum oxide particles obtained in Comparative Example 1, and the raw aluminum oxide particles are shown in Figure 12. Figure 12 is a graph showing the relationship between ultraviolet (UV) light irradiation time and C/C0, and shows an approximate straight line along with actual measured data. In this graph, a larger C/C0 reduction rate with respect to UV irradiation time indicates higher photocatalytic activity.

With aluminum oxide particles, no decrease in C/C0, i.e., no catalytic activity, was observed even when the UV irradiation time was 40 minutes. On the other hand, in the case of coated oxide particles, catalytic activity was observed even when the amount of tantalum oxide was increased to 100 mass% (Example 1), 200 mass% (Example 2), and 300 mass% (Example 3), and this catalytic activity was greater than the catalytic activity of tantalum oxide (Comparative Example 1). In particular _, when the amount of tantalum oxide was 100 mass% (Example 1), the highest catalytic performance was achieved despite the small amount of _Ta2O5 coating.

[Experimental Example B]
In Experimental Example B, lanthanum aluminate (LaAlO ₃ ) or magnesium oxide (MgO) was used as the core particle. The surface of the core particle was coated with lanthanum-tantalum oxynitride (LaTaON ₂ ) or titanium oxide (TiO ₂ ) to produce coated oxide particles, which were then evaluated. For comparison, particles consisting only of lanthanum-tantalum oxynitride or titanium oxide were also produced and evaluated.

[Example 6]
In Example 6, lanthanum aluminate (LaAlO ₃ ) particles coated with lanthanum-tantalum oxynitride (LaTaON ₂ ) were produced using lanthanum aluminate (LaAlO ₃ ) powder, lanthanum nitrate, and tantalum (V) ethoxide as raw materials.

First, the raw materials used were lanthanum aluminate powder, lanthanum(III) nitrate hexahydrate (Kanto Chemical Co., Inc.), tantalum(V) ethoxide (Sigma-Aldrich), and ultra-dehydrated ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.). The lanthanum aluminate powder was prepared by mechanically grinding a purchased single crystal substrate (manufactured by Crystal Base Co., Ltd.). The purchased single crystal substrate had a double-sided mirror finish, a (100) orientation, and dimensions of 10 x 10 x 0.5 mm. The substrate was manually ground in an agate mortar and then ball milled. The resulting lanthanum aluminate powder was single crystal with an average particle size of 0.5 μm. The water content of the ultra-dehydrated ethanol was less than 0.001% by mass.

Next, 1.0 g of lanthanum aluminate powder, 0.98 g of lanthanum (III) nitrate hexahydrate, and 0.92 g of tantalum (V) ethoxide were added to 100 mL of ultra-dehydrated ethanol and mixed to obtain a reaction solution. The resulting reaction solution was irradiated with ultrasound for 1 minute using an ultrasonic cleaner to disperse the lanthanum aluminate powder.

The resulting dispersion was then stirred at 500 rpm using a magnetic stirrer. While stirring, 3.1 mL of ultrapure water was added dropwise to the dispersion at a rate of 1 mL/min. The amount of water added was 15 equivalents. After the addition was complete, stirring was continued for 1 hour to obtain a dispersion. A precipitate formed in the dispersion as a result of the addition and stirring of the dispersion.

The dried precipitate was calcined at 900°C for 10 hours in an ammonia gas atmosphere flowing at a flow rate of 0.2 L/min to obtain coated oxide particles.

The coated oxide particles obtained in Example 6 were evaluated using X-ray diffraction and a high-resolution transmission electron microscope and a spherical aberration-corrected transmission electron microscope, as in Example 1. The results confirmed that lanthanum aluminate (LaAlO ₃ ) particles coated with lanthanum-tantalum oxynitride (LaTaON ₂ ) were synthesized. The average thickness of the coating was 22 nm, and the specific surface area of the obtained coated oxide particles was 1.3 m ² /g. It was also confirmed that the lanthanum aluminate core particles and the lanthanum-tantalum oxynitride coating were lattice-matched over most of the interface between the core particles and the coating. This indicated that the coating had grown epitaxially on the core particles.

[Comparative Example 2]
In Comparative Example 2, particles consisting only of lanthanum-tantalum oxynitride (LaTaON ₂ ), a coating component, were synthesized. Specifically, the particles were synthesized under the same conditions as in Example 6, except that lanthanum aluminate powder was not added. That is, an ethanol solution containing lanthanum (III) nitrate hexahydrate and tantalum (V) ethoxide was prepared, and water was added to this ethanol solution to obtain a precipitate. The precipitate was then washed and dried, and lanthanum-tantalum oxynitride (LaTaON ₂ ) particles were then produced under an ammonia gas atmosphere. Firing was performed under the same conditions as in Example 6.

The raw materials used in Example 6 and Comparative Example 2 and the resulting products were evaluated for various properties in the same manner as in Example 1. The specific surface area of the lanthanum aluminate core particles was 1.5 m ² /g, while the specific surface area of the particles coated with lanthanum-tantalum oxynitride (Example 6) was 1.3 m ² /g. On the other hand, the specific surface area of the particles consisting only of lanthanum-tantalum oxynitride (Comparative Example 2) was 0.9 m ² /g.

The dye-decomposing photocatalyst was evaluated using the same weight of lanthanum-tantalum oxynitride and an LED lamp (OptiLED, 2.5W) with a wavelength of 455 nm. It was confirmed that the particles of Example 6 were 1.5 times more active than the particles of Comparative Examples 1 and 2.

[Example 7]
In Example 7, coated oxide particles were produced by the same method and under the same conditions as in Example 6, except that magnesium oxide (MgO) powder was used as the core particle raw material instead of lanthanum aluminate (LaAlO ₃ ) powder.

The magnesium oxide (MgO) powder used as the core particle raw material was prepared by mechanically pulverizing a single crystal substrate (manufactured by Crystal Base Co., Ltd.). The purchased single crystal substrate had a double-sided mirror finish, a (100) orientation, and dimensions of 10 x 10 x 0.5 mm. The substrate was manually pulverized in an agate mortar and then ball milled. The magnesium oxide powder prepared was a single crystal powder with an average particle size of 0.4 μm.

The obtained coated oxide particles were magnesium oxide (MgO) particles coated with lanthanum-tantalum oxynitride (LaTaON ₂ ), and the average thickness of the coating was 17 nm and the specific surface area was 2.2 m ² /g.

The dye-decomposing photocatalytic activity of the lanthanum-tantalum oxynitride was evaluated using an LED lamp (OptiLED, 2.5W) with a wavelength of 455 nm, with the same weight of lanthanum-tantalum oxynitride. As a result, it was confirmed that the particles of Example 7 were 1.9 times more active than the particles of Comparative Example 2.

[Example 8]
In Example 8, magnesium oxide (MgO) particles coated with titanium oxide (TiO ₂ ) were produced using magnesium oxide (MgO) powder and titanium tetraisopropoxide as raw materials.

Magnesium oxide powder, titanium tetraisopropoxide (Fujifilm Wako Pure Chemical Industries, Ltd.), and ultra-dehydrated ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.) were prepared as raw materials. The magnesium oxide powder used had an average particle size of 0.5 μm and was prepared using the same procedure as in Example 7. Furthermore, coated oxide particles were prepared using the same method and conditions as in Example 1, except for using the above-mentioned raw materials.

The obtained coated oxide particles had an average coating thickness of 23 nm and a specific surface area of 2.3 m ² /g. The titanium oxide constituting the coating maintained an anatase crystal structure. The coating was also formed epitaxially with respect to the core particles, and had good crystallinity.

[Comparative Example 3]
Particles consisting solely of titanium oxide (TiO ₂ ), a coating component, were synthesized. Specifically, an ethanol solution of titanium tetraisopropoxide was prepared under the same conditions as in Example 8, except that magnesium oxide powder was not added. Water was added to this ethanol solution to obtain a precipitate. This precipitate was washed and dried, and then fired in an air atmosphere under the same conditions as in Example 8 to produce titanium oxide (TiO ₂ ) particles.

The synthesis of titanium oxide particles was confirmed using X-ray diffraction, a high-resolution transmission electron microscope, and a spherical aberration-corrected transmission electron microscope. However, unlike Example 8, the synthesized titanium oxide particles were of the rutile type. The obtained titanium oxide powder (particles) had an average particle size of 700 nm and a specific surface area of 0.9 ^m2 /g.

The dye decomposition photocatalytic properties of Example 8 and Comparative Example 3 by UV irradiation were evaluated using the same procedures and conditions as in Example 1. When the weight of titanium oxide was the same, it was confirmed that the particles of Example 8 had 3.5 times the activity of the particles of Comparative Example 3.

Claims (12)

A coated oxide particle comprising a core particle and a coating covering the surface of the core particle,
the core particle is composed of a crystalline metal oxide;
the coating contains, as a main component, a crystalline compound consisting of at least one of a metal oxide, an oxynitride, and a nitride,
the crystalline compound is lattice-matched with the crystalline metal oxide at the interface between the coating and the core particle;
the crystalline metal oxide constituting the core particle is aluminum oxide (Al ₂ O ₃ ), and the crystalline compound contained in the coating is at least one selected from the group consisting of tantalum oxide (Ta ₂ O ₅ ), tantalum oxynitride (TaON), and tantalum nitride (Ta ₃ N ₅ ); or
the crystalline metal oxide constituting the core particle is lanthanum aluminate (LaAlO ₃ ), and the crystalline compound contained in the coating is lanthanum tantalum oxynitride (LaTaON ₂ ) having a perovskite structure; or
Coated oxide particles, wherein the crystalline metal oxide constituting the core particle is magnesium oxide (MgO), and the crystalline oxide contained in the coating is at least one selected from the group consisting of anatase-type titanium oxide (TiO ₂ ) and lanthanum-tantalum oxynitride (LaTaON ₂ ) having a perovskite structure. The coated oxide particle according to claim 1, wherein the crystalline compound has a region that does not have grain boundaries from the interface between the coating and the core particle to the surface of the coating. The coated oxide particle according to claim 1 or 2, wherein the crystalline metal oxide constituting the core particle is in a single crystal state. The coated oxide particles according to any one of claims 1 to 3 , wherein the average particle size of the core particles is 10 nm or more and 50 µm or less. 5. The coated oxide particle according to claim 1 , wherein the crystalline compound contained in the coating has a crystallite diameter of 50 Å or more as determined by the Williamson-Hall method. The coated oxide particles according to any one of claims 1 to 5 , wherein the coating has a thickness of 1 nm or more and 10 µm or less. The coated oxide particles according to any one of claims 1 to 6 , wherein the coated oxide particles have a specific surface area of 0.5 m ² /g or more. A photocatalyst comprising the coated oxide particles according to any one of claims 1 to 7 . A method for producing the coated oxide particles according to any one of claims 1 to 7 , comprising the following steps:
a preparation step of preparing an alcohol-based reaction liquid containing metal oxide particles and an organometallic compound;
a dispersion preparation step of adding water to the alcohol-based reaction solution to hydrolyze the organometallic compound, thereby preparing a dispersion containing coated precursor particles in which coating precursors are precipitated on the surfaces of metal oxide particles;
a separation step of extracting the coated precursor particles from the dispersion; and a firing step of calcining the extracted coated precursor particles to obtain coated oxide particles.
Equipped with
The metal oxide particles have a crystallite diameter of 1000 Å or more as determined by the Williamson-Hall method, and the water content of the alcohol-based reaction liquid prepared in the preparation step is 0.1% by mass or less. 10. The method of claim 9 , wherein the calcination of the coated precursor particles is carried out in an oxygen-containing atmosphere, thereby forming a coating primarily comprising an oxide of a metal. 10. The method of claim 9 , wherein the calcination of the coated precursor particles is carried out in an atmosphere containing an ammonia decomposition gas, thereby forming a coating containing metal oxynitride and/or nitride as a main component. The method according to any one of claims 9 to 11 , wherein the calcination of the coated precursor particles is carried out at a temperature of 600°C or higher and 1100°C or lower.