Coated oxide particles, photocatalyst, and method for producing coated oxide particles

Abstract

To provide a coated oxide particle which is fine and is excellent in crystallinity, and a production method thereof.SOLUTION: A coated oxide particle comprises: a core particle; and a coating with which a surface of the core particle is coated. The core particle is composed of a crystalline metal oxide. The coating comprises a crystalline compound as a main component, the crystalline compound composed of at least one of a metal oxide, an oxynitride, and a nitride. The crystalline compound is lattice-matched to the crystalline metal oxide on an interface between the coating and the core particle.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to coated oxide particles, a photocatalyst, and a method for producing coated oxide particles. [Background technology]

[0002] Research and development of functional composite particles, which are particles that are 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 obtained with single particles, and are expected to be used in a variety of applications.

[0003] For example, Patent Document 1 discloses a method for treating powder particles in which powder particles are added and suspended in a benzene solution of an alkoxide of the metal to be coated, water is added to the suspension to hydrolyze, and the suspension is 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 activity on the surface of a support, and that a uniform thickness can be coated on the surface of fine particles in a relatively simple process (page 2, left column, line 17 to right column, line 2 of Patent Document 1).

[0004] Patent Document 2 discloses a method for producing oxide film-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 where smoothness and purity of the film are required (paragraph

[0017] of Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 59-025901 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-188413 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, functional particles made of coated particles in which a coating is formed on the surface of a core particle have been proposed in the past, but there is still room for improvement in the conventional coated particles.

[0007] That is, depending on the application, it is desirable for the functional particles to 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 (H2). 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.

[0008] 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 has attracted attention as a technology that can produce hydrogen, a clean energy source that can replace fossil fuels. Therefore, it is important to miniaturize and highly crystallize photocatalytic particles in order to promote clean energy.

[0009] With conventional techniques, it has been difficult to achieve both fine and highly crystalline functional particles. For example, heat treatment at high temperature for a long period of time is effective in increasing the crystallinity of particles. However, when functional particles are subjected to heat treatment at high temperature for a long period of time, necking occurs between particles and the necking grows. As a result, the particles sinter together, causing the particles to become coarse. With conventional techniques, it has been difficult to produce fine coated particles with excellent crystallinity as functional particles.

[0010] The present invention was completed based on these findings, and an object of the present invention is to provide fine coated oxide particles with excellent crystallinity, and a method for producing the same. [Means for solving the problem]

[0011] The present invention encompasses the following aspects (1) to (18). In this specification, the expression "to" includes the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0012] (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.

[0013] (2) The coated oxide particle according to (1) above, wherein the crystalline compound has a region having no grain boundary extending from the interface between the coating and the core particle to the surface of the coating.

[0014] (3) Coated oxide particles according to (1) or (2) above, wherein the crystalline metal oxide constituting the core particle is in a single crystal state.

[0015] (4) The crystalline metal oxide constituting the core particle 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), or the like. Titanium (KTaO3), potassium niobate (KNbO3), lithium tantalate (LiTaO3), lithium niobate (LiNbO3), dysprosium scandate (DyScO3), gadolinium scandate (GdScO3), neodymium scandate (NdScO3), magnesium aluminate (MgAl2O4), magnesium oxide (MgO), titanium oxide (TiO2), garnet-type yttrium aluminate (Y3Al5O 12 The coated oxide particles according to any one of (1) to (3) above, which are at least one selected from the group consisting of aluminum oxide (Al 2 TiO 5 ;YAG) and aluminum titanate (Al 2 TiO 5 ).

[0016] (5) The 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.

[0017] (6) The 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.

[0018] (7) The coated oxide particles according to any one of (1) to (6), 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).

[0019] (8) The 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.

[0020] (9) Coated oxide particles according to any one of (1) to (8) above, wherein the core particle is composed of aluminum oxide (Al2O3) and / or aluminum titanate (Al2TiO5), and the crystalline compound contained in the coating is at least one of tantalum oxide (Ta2O5), tantalum oxynitride (TaON), and tantalum nitride (Ta3N5).

[0021] (10) Coated oxide particles according to any one of (1) to (8) above, wherein the core particle is composed of at least one perovskite-type compound selected from the group consisting of 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), dysprosium scandate (DyScO3), gadolinium scandate (GdScO3), and neodymium scandate (NdScO3), and the crystalline compound contained in the coating is at least one of an oxide and an oxynitride having a perovskite-type structure.

[0022] (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 (TiO2) or an oxide and oxynitride having a perovskite structure.

[0023] (12) The crystalline compound contained in the coating is iron lanthanum (LaFeO3), calcium zirconate (CaZrO3), strontium zirconate (SrZrO3), barium zirconate (BaZrO3), calcium hafnate (CaHfO3), strontium hafnate (SrHfO3), barium hafnate (BaHfO3), calcium titanium oxynitride (CaTiON2), strontium titanium oxynitride (SrTiON2), barium titanium oxynitride (BaTiON2), lanthanum titanium oxynitride (LaTiON2N), calcium zirconium oxynitride (CaZrON2), strontium zirconium oxynitride (Sr The coated oxide particles according to (10) or (11) above are at least one selected from the group consisting of ZrON2, barium zirconium oxynitride (BaZrON2), lanthanum zirconium oxynitride (LaZrON2), lanthanum tantalum oxynitride (LaTaON2), calcium tantalum oxynitride (CaTaON2), strontium tantalum oxynitride (SrTaON2), barium tantalum oxynitride (BaTaON2), lanthanum niobium oxynitride (LaNbON2), calcium niobium oxynitride (CaNbON2), strontium niobium oxynitride (SrNbON2), and barium niobium oxynitride (BaNbON2), and solid solutions thereof.

[0024] (13) The specific surface area of ​​the coated oxide particles is 0.5 m 2 The coated oxide particles according to any one of (1) to (12) above, wherein the surface roughness is 0.1 μm or more.

[0025] (14) A photocatalyst comprising the coated oxide particles according to any one of (1) to (13) above.

[0026] (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 removing the coated precursor particles from the dispersion; and a calcination 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 mass % or less.

[0027] (16) The method according to (15) above, wherein the coated precursor particles are calcined in an oxygen-containing atmosphere, thereby forming a coating containing a metal oxide as a main component.

[0028] (17) The method according to (15) above, wherein the coated precursor particles are calcined in an atmosphere containing an ammonia decomposition gas, thereby forming a coating containing a metal oxynitride and / or nitride as a main component.

[0029] (18) The method according to any one of (15) to (17), wherein the coated precursor particles are fired at a temperature of 600°C or higher and 1100°C or lower. [Effects of the Invention]

[0030] According to the present invention, fine coated oxide particles with excellent crystallinity and a method for producing the same are provided. [Brief explanation of the drawings]

[0031] [Figure 1] 1 shows the X-ray diffraction pattern of the coated oxide particles. [Figure 2] 1 shows an SEM backscattered electron image of coated oxide particles. [Figure 3] HR-TEM images of coated oxide particles are shown. [Figure 4] This shows a high-magnification HR-TEM image of the vicinity of the interface between a metal oxide particle (core particle) and a coating. [Figure 5] Cs-TEM image of coated oxide particles. [Figure 6] The selected area electron diffraction pattern obtained for the metal oxide particles (core particles) is shown. [Figure 7] This shows a high-magnification Cs-TEM image of the vicinity of the interface between a metal oxide particle (core particle) and a coating. [Figure 8] 1 shows a TEM image of coated oxide particles (Example 1). [Figure 9] 1 shows a TEM image of coated oxide particles (Example 2). [Figure 10] 1 shows a TEM image of coated oxide particles (Example 3). [Figure 11] 1 shows a TEM image of tantalum oxide particles (Comparative Example 1). [Figure 12] 1 is a graph showing the results of a catalyst performance test. DETAILED DESCRIPTION OF THE INVENTION

[0032] 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 present invention.

[0033] <<1.Coated oxide particles>> The coated oxide particles of this embodiment comprise a core particle and a coating covering the surface of the core particle. That is, the coated oxide particles have a core particle that acts 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 may cover 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, 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."

[0034] In the coated oxide particles of this embodiment, the core particle is composed of a crystallized metal oxide (crystalline metal oxide). By using a crystalline metal oxide to form the core particle, it is possible to enhance the crystallinity of the coating film formed on the surface of the core particle. Whether the metal oxide constituting the core particle is crystallized or not can be determined by whether a clear diffraction peak is observed in X-ray diffraction analysis.

[0035] In the coated oxide particles of this embodiment, the coating contains a crystalline compound as a main component. The crystalline compound is at least one of an oxide, an oxynitride, and a nitride of a metal element. Here, the concept of metal encompasses metalloids. Specifically, the metal element is 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, an oxide is a compound primarily containing a metal and oxygen, an oxynitride is a compound primarily containing a metal, oxygen, and nitrogen, and a nitride is a compound primarily containing a metal and nitrogen.

[0036] The coating is a layer formed on the outermost surface of the coated oxide particles and contributes to the expression of functions based on the surface properties. By including a crystalline compound that is a metal oxide, oxynitride, and / or nitride as the main component, the coated oxide particles can be endowed with various 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 viewpoint of fully expressing the functions based on the metal oxide, oxynitride, and / or nitride, the higher the content of the crystalline compound, 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.

[0037] When the crystalline compound is an oxide, the crystalline compound may contain only metal and oxygen. When the crystalline compound is an oxynitride, the crystalline compound may contain only metal, oxygen, and nitrogen. When the crystalline compound is a nitride, the crystalline compound may contain only metal and nitrogen. Alternatively, the crystalline compound may contain elements other than metal, oxygen, and nitrogen. When the crystalline compound contains other elements, the elements may be contained in the form of a complex compound or a solid solution. Furthermore, the crystalline compound may be doped with an impurity metal.

[0038] 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 core particle in a single crystal state is used, a crystalline compound having 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 the 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, such as photocatalytic properties. Whether or not lattice matching is achieved can be determined by observing the interface between the coating and the 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 the core particle are lattice-matched at all interfaces. However, the effects of this embodiment are achieved as long as lattice matching is achieved at at least a portion of the interface. Therefore, this embodiment also includes a mode in which lattice matching occurs at some interfaces.

[0039] 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. That is, it is preferable that the crystalline structure that constitutes 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.

[0040] 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 film formed thereon. Note that the single crystal state does not only mean a single crystal, but also means an agglomeration of multiple single crystals. BodyThis concept also includes the state in which single crystal primary particles are loosely bound to form agglomerated secondary particles, and the state in which several single crystals are bound together. Whether a core particle is in a single crystal state can be determined by examining the electron diffraction pattern.

[0041] The method for producing core particles in a single crystal state (single crystal core particles) is not limited as long as it can produce particles in a single crystal state. However, methods such as hydrothermal synthesis and flux synthesis, in which core particles are synthesized from a metal-containing solution, are promising. Other methods include wet methods such as coprecipitation and alkoxide methods, and gas-phase methods such as flame spraying 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 crystal or polycrystalline ingot using methods such as the Czochralski method, Bridgman method, Verneuil method, and floating zone method, and then pulverize the resulting ingot. The pulverization can be achieved by subjecting the ingot to processing such as cutting and / or mechanical pulverization.

[0042] The material of the metal oxide constituting the core particle is not limited as long as it is crystalline. However, the crystalline metal oxide is preferably 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), or the like. Titanium (KTaO3), potassium niobate (KNbO3), lithium tantalate (LiTaO3), lithium niobate (LiNbO3), dysprosium scandate (DyScO3), gadolinium scandate (GdScO3), neodymium scandate (NdScO3), magnesium aluminate (MgAl2O4), magnesium oxide (MgO), titanium oxide (TiO2), garnet-type yttrium aluminate (Y3Al5O 12 ;YAG), and aluminum titanate (Al2TiO5).

[0043] The average particle size of the core particles is preferably 10 nm to 50 μm, more preferably 50 nm to 10 μm, and even more preferably 100 nm to 1 μm. 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 shapes other than spherical can be determined by measuring the particle size from various directions and calculating the average value.

[0044] 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 method for estimating the crystalline state from the peak shape in X-ray diffraction, and can determine the crystallite diameter and lattice distortion separately. By increasing the crystallite diameter of the crystalline compound, the crystallinity of the coating, which is the main factor in functional expression, can be improved, and as a result, various properties such as photocatalytic properties can be further improved. The crystallite diameter is more preferably 100 Å or more, and even more preferably 200 Å or more. There is no upper limit to the crystallite diameter.

[0045] 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 are particularly excellent in various properties, such as photocatalytic properties. For example, tantalum oxide and titanium oxide have photocatalytic properties that are active under ultraviolet irradiation. Tantalum oxynitride and tantalum nitride have highly active photocatalytic properties that are responsive to visible light.

[0046] 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 are particularly excellent in various properties, including photocatalytic properties.

[0047] Preferably, the core particles are composed of at least one perovskite-type compound selected from the group consisting of 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), dysprosium scandate (DyScO3), gadolinium scandate (GdScO3), and neodymium scandate (NdScO3), and the crystalline compound contained in the coating is at least one of an oxide and an oxynitride having a perovskite-type structure.

[0048] When the core particle is composed of the 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.

[0049] Examples of crystalline compounds contained in the coating include iron lanthanum (LaFeO3), calcium zirconate (CaZrO3), strontium zirconate (SrZrO3), barium zirconate (BaZrO3), calcium hafnate (CaHfO3), strontium hafnate (SrHfO3), barium hafnate (BaHfO3), calcium titanium oxynitride (CaTiON2), strontium titanium oxynitride (SrTiON2), barium titanium oxynitride (BaTiON2), lanthanum titanium oxynitride (LaTiON2N), calcium zirconium oxynitride (CaZrON2), strontium zirconate (BaTiON2), and lanthanum titanium oxynitride (LaTiON2N). and at least one selected from the group consisting of strontium oxynitride (SrZrON2), barium zirconium oxynitride (BaZrON2), lanthanum zirconium oxynitride (LaZrON2), lanthanum tantalum oxynitride (LaTaON2), calcium tantalum oxynitride (CaTaON2), strontium tantalum oxynitride (SrTaON2), barium tantalum oxynitride (BaTaON2), lanthanum niobium oxynitride (LaNbON2), calcium niobium oxynitride (CaNbON2), strontium niobium oxynitride (SrNbON2), and barium niobium oxynitride (BaNbON2), and solid solutions thereof.

[0050] The use of these compounds (e.g., 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 (e.g., LaFeO3) all have a perovskite structure, they readily form solid solutions. Adjusting the composition of the solid solution advantageously allows the lattice constant of the core particles to be optimally controlled to facilitate epitaxial growth of the coating. Furthermore, the lattice constant can also be controlled by doping these compounds with different metals.

[0051] 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 (TiO2) or an oxide and oxynitride having a perovskite structure. Examples of oxides and oxynitrides having a perovskite structure include the above-mentioned crystalline compounds (LaFeO3, etc.).

[0052] Perovskite-type composite compounds (oxides, oxynitrides) and perovskite-type compounds based on their 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 can easily produce 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.

[0053] 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 grown on the core particles, resulting in a coating with good crystallinity.

[0054] Preferably, the core particle is composed of aluminum oxide (Al2O3), and the crystalline compound contained in the coating is at least one of tantalum oxide (Ta2O5), tantalum oxynitride (TaON), and tantalum nitride (Ta3N5). Tantalum oxide is a photocatalytic material that exhibits activity under ultraviolet light irradiation, while tantalum oxynitride and tantalum nitride are highly active photocatalytic materials that exhibit visible light responsiveness. 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.

[0055] 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.

[0056] Preferably, the thickness of the coating 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, resulting in a decrease in the specific surface area. The thickness of the coating 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.

[0057] Preferably, the amount of the coating is 10% by mass or more and 500% by mass or less relative to the amount of the core particles. If the amount of the coating is too small, it may be difficult to fully utilize the excellent properties of the coating, and if the amount of the coating is too large, the specific surface area may decrease. The amount of the coating is more preferably 30% by mass or more and 300% by mass or less, and even more preferably 50% by mass or more and 150% by mass or less.

[0058] 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 utilize the functions based on the surface characteristics. The specific surface area is 1.0m 2 / g or more is preferable, and 2.0m 2 / g or more is more preferable, and 3.0m 2 / g or more is more preferable. There is no upper limit to the specific surface area. However, it is typically 10.0 m 2 / g or less.

[0059] <<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 made of at least one selected from the group consisting of anatase titanium oxide (TiO2), tungsten oxide (WO3), tantalum oxide (Ta2O5), tantalum oxynitride (TaON), and tantalum nitride (Ta3N5) exhibit water splitting reactions. Therefore, photocatalysts containing these coated oxide particles are 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 (RuO2).

[0060] <<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.

[0061] <Reaction solution preparation process> 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.

[0062] The metal oxide particles have a crystallite diameter of 1000 Å or more as determined by the Williamson-Hall method. Using metal oxide particles with such a large crystallite diameter can enhance the crystallinity of the coating film formed thereon. Specifically, in the subsequent dispersion preparation process, a coating precursor consisting of a metal hydroxide and / or oxide is precipitated on the surface of the metal oxide particles. Furthermore, in the firing process, the coating precursor is transformed into a coating film. The oxides, oxynitrides, and / or nitrides that make up the coating film tend to have their constituent elements (metal, oxygen, and 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 film formed on the surface.

[0063] The metal oxide particles are not limited in terms of material or particle size, as long as their crystallite diameter is 1000 Å or greater. However, examples of suitable metal oxide particles include 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), and potassium tantalate (KTaO3). , potassium niobate (KNbO3), lithium tantalate (LiTaO3), lithium niobate (LiNbO3), dysprosium scandate (DyScO3), gadolinium scandate (GdScO3), neodymium scandate (NdScO3), magnesium aluminate (MgAl2O4), magnesium oxide (MgO), titanium oxide (TiO2), garnet-type yttrium aluminate (Y3Al5O 12It is preferable that the metal oxide particles are composed of at least one material selected from the group consisting of aluminum oxide (YAG), and aluminum titanate (Al2TiO5). The average particle size of the metal oxide particles is preferably 10 nm to 50 μm, more preferably 50 nm to 10 μm, and even more preferably 100 nm to 1 μm. The shape of the metal oxide particles is not limited. They may be of a regular shape such as a sphere, plate, or rod, or may be irregular.

[0064] The type of metal contained in the organometallic compound is not limited. However, at least one 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 (an alcohol-soluble compound). Typically, it is a metal alkoxide, acetate, and / or acetylacetonate. Alkoxides are preferred, and among them, at least one selected from the group consisting of methoxide, ethoxide, propoxide, and butoxide is more preferred.

[0065] The alcohol is not limited 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. Only one type of alcohol may be used alone, or multiple types may be mixed and used. In addition to the metal oxide and organometallic compound, the reaction solution may also contain additives such as a pH adjuster and a catalyst.

[0066] The method for preparing an 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 containing the organometallic compound. 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 then 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 (TaCl5) in alcohol releases hydrochloric acid, resulting in an alcohol solution containing tantalum alkoxide.

[0067] 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 mass % or less. By reducing the water content of the reaction solution, the crystallinity of the coating film that is finally obtained can be improved. Although the details of the mechanism are unclear, it is thought that the metal oxide particles have hydroxyl groups (OH) on their surfaces. -) When the reaction solution contains little water, the organometallic compound first adsorbs to the surface hydroxyl groups of the metal oxide particles. When water is added in the subsequent dispersion preparation process, a hydrolysis reaction occurs continuously between the adsorbed organometallic compound and the organic compound in the reaction solution. As a result, a metal hydroxide or oxide network is formed around the surface of the metal oxide particles, which then becomes the coating precursor. Because metal oxide particles are highly crystalline, the network (coating precursor) formed around the surface has a relatively uniform crystal orientation. Furthermore, the coating obtained by calcining this coating precursor has high crystallinity. In contrast, when 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. These hydroxides and oxides float in the reaction solution, resulting in random crystal orientations. When water is added in the subsequent dispersion preparation process, crystal growth proceeds around the initial nuclei with random crystal orientations, resulting in a fine polycrystalline coating.

[0068] The water content of the reaction solution is preferably 0.01% by mass or less, more preferably 0.005% by mass or less. The water content can be adjusted by using dehydrated alcohol. For example, the water content of normal ethanol is as high as about 0.2% by mass, while the water content of ultra-dehydrated ethanol is 0.001% by mass or less.

[0069] <Dispersion liquid preparation process> In the dispersion preparation process, 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 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.

[0070] The amount of water added is preferably 2 to 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 uneven coating. It is also preferable to add water little by little. For example, a method may be used in which a small amount of water is intermittently dropped using a burette, or a method may be used in which mist-like water is sprayed onto the reaction solution. When adding water dropwise, it is preferable to set the dropping 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.

[0071] <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.

[0072] <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 film made 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 film containing metal oxide as the main component. The oxygen-containing atmosphere may be air or an oxygen-containing inert atmosphere (e.g., Ar).

[0073] The coated precursor particles may be calcined in an atmosphere containing an ammonia decomposition gas. This allows the formation of a coating primarily composed of metal oxynitrides and / or nitrides. Hydrogen or an inert gas (N, Ar, He, etc.) may be added to the ammonia decomposition gas, as long as the desired coated oxide particles are obtained. Ammonia decomposition gas is a gas produced when ammonia gas (NH) is decomposed at temperatures above 500°C, and contains nitrogen (N), hydrogen (H), and nitrogen hydroxide (NH, NH, and their radicals).

[0074] Incidentally, the coating precursor contains a large amount of oxygen. Therefore, when the ammonia decomposition gas oxidizes or nitrides the coating precursor, water is generated as a by-product. The generated water may have a negative effect on obtaining a highly crystalline oxynitride and / or nitride. When using an ammonia decomposition gas, it is desirable to circulate the gas sufficiently to discharge the moisture during firing.

[0075] Regardless of the atmosphere in which the 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 coating with high crystallinity. On the other hand, if the firing temperature exceeds 1100°C, the production cost increases 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. By shortening the holding time, an oxynitride coating can be obtained, and by lengthening the holding time, a nitride coating can be obtained.

[0076] The coated oxide particles of this embodiment can be produced by the above-described production method. However, the production method of the coated oxide particles of this embodiment is not limited to the method 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 the alcohol solution of an organometallic compound. In this case, the pH of the aqueous solution is adjusted to precipitate a metal hydroxide, thereby forming a coating precursor. [Example]

[0077] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0078] [Experimental Example A] In Experiment 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 made only of tantalum oxide were also produced and evaluated.

[0079] (1) Preparation of coated oxide particles [Example 1] In Example 1, aluminum oxide (Al2O3) particles coated with tantalum oxide (Ta2O5) were produced using aluminum oxide powder and tantalum (V) ethoxide.

[0080] <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-AlO single crystal particles with an average particle size of 0.3 μm. The water content of the ultra-dehydrated ethanol was less than 0.001% by mass.

[0081] 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 the amount of tantalum oxide, was 100 mass% relative to the amount of aluminum oxide. Next, the reaction solution was irradiated with ultrasonic waves using an ultrasonic cleaner for 1 minute to disperse the aluminum oxide powder.

[0082] <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 as a result of the addition and stirring.

[0083] <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.

[0084] <Firing process> The dried precipitate was calcined in air at 1000°C for 1 hour to produce coated oxide particles. The resulting coated oxide particles were aluminum oxide (AlO) particles coated with tantalum oxide (TaO).

[0085] [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 (AlO) particles coated with tantalum oxide (TaO).

[0086] [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 (AlO) particles coated with tantalum oxide (TaO).

[0087] [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. Otherwise, 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 oxynitride (TaON).

[0088] [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. Otherwise, the coated oxide particles were produced in the same manner as in Example 1. The obtained coated oxide particles were aluminum oxide (AlO) particles coated with tantalum nitride (TaN).

[0089] [Comparative Example 1] Particles consisting solely of tantalum oxide (Ta2O5) as 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.

[0090] (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.

[0091] <Specific surface area> The BET specific surface areas of the core particle raw material and the product were measured using the N2 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.

[0092] <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.

[0093] -X-ray diffraction equipment: Malvern Panalytical, X'Pert Pro MRD - Source: CuKα -Tube voltage: 45kV -Tube current: 40mA - Scan speed: 5.5° / min - Scan range (2θ): 20~60°

[0094] The crystallite diameter was determined by the Williamson-Hall method using the obtained diffraction pattern. Specifically, the half-width β [rad] at each diffraction angle θ [rad] was measured, and the relationship between β cosθ and sinθ was plotted. Using this relationship, the crystallite diameter D was calculated using the following formula (1). In the following formula (1), ε is the lattice strain, and λ is the X-ray wavelength (CuKα1: 1.540598 Å).

[0095]

number

[0096] <sem> The product was observed using a scanning electron microscope (SEM; Carl ZEISS, ULTRA55) at an accelerating voltage of 1 kV.

[0097] <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.

[0098] <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 less than 100 nm thick. This thin section sample was then observed using a Cs-TEM (JEOL Ltd., JEM-ARM200F) at an accelerating voltage of 200 kV.

[0099] <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.

[0100] <Evaluation of dye decomposition photocatalyst> The photocatalytic activity of the product was quantitatively evaluated by measuring the decolorization rate of the methyl orange solution. Specifically, 15 g of methyl orange solution with a concentration of 0.001 w / v% 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 solution. The bottom of the dish was placed in an ultrasonic cleaner to disperse the catalyst powder for 5 minutes to prepare a suspension. The 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 top of the Petri dish, and UV irradiation was performed using a UV 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 mainly emitted UV light with wavelengths of 185 nm and 254 nm. On the other hand, the UV-transmitting filter transmitted over 70% of the 254 nm light and blocked almost all of the 185 nm light, so the sample was mainly irradiated with 254 nm ultraviolet light.

[0101] After ultraviolet irradiation for a specified time, the evaporated water during ultraviolet irradiation was weighed, and ultrapure water corresponding to the weight loss was weighed and added to the reacted suspension to adjust the weight to the initial weight before ultraviolet irradiation. Next, solid-liquid separation of the suspension was performed using a suction filter, and the filtrate was transferred to a tube and irradiated with an LED light to confirm complete solid-liquid separation. That is, when scattered light due to the Tyndall effect is observed from the filtrate, this means that the solid-liquid separation is insufficient. Therefore, solid-liquid separation by suction filtration was repeated until LED scattered light from the filtrate was no longer observed. Next, the wavelength dependence of the absorbance of these solutions was examined using a UV-VIS spectrophotometer (Hitachi High-Tech Fielding Co., Ltd., Spectrophotometer UH4150). Specifically, the solution was 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 can be determined from the absorbance at a wavelength of 464 nm. Therefore, the decomposition rate of methyl orange due to photocatalytic activity can be evaluated from the absorbance at a wavelength of 464 nm. By this process, the photocatalytic activity of the products (coated oxide particles, tantalum oxide particles) was evaluated.

[0102] (3) Evaluation Results <Specific Surface Area (Raw Material Aluminum Oxide Powder and Coated Oxide Particles)> The specific surface area of the raw material aluminum oxide powder used in Examples 1 to 5 was 5.7 m 2 / g. Also, 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.

[0103] <XRD (Raw Material 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 FIG. 1. In FIG. 1, the diffraction pattern of the raw material aluminum oxide powder and the standard diffraction pattern of tantalum oxide (PDF ********) are shown together. In the diffraction pattern of the coated oxide particles, peaks corresponding to the patterns of tantalum oxide and aluminum oxide were detected. From this, it was found that the crystal phase of tantalum oxide was formed.

[0104] <Crystallite size (starting aluminum oxide powder and coated oxide particles)> The crystallite sizes of the oxide particles (Al2O3 particles) and the coating film (Ta2O5 film) contained in the coated oxide particles obtained in Example 1 are shown in Table 1 together with the crystallite size of the starting aluminum oxide powder. The crystallite size of the starting aluminum oxide powder was 1000 Å or more, and the crystallite size of the coating film (Ta2O5 film) was as large as 280 Å.

[0105]

Table 1

[0109] <Cs-TEM (Coated Oxide Particles)> The Cs-TEM image of the coated oxide particles obtained in Example 1 is shown in Fig. 5. In Fig. 5, the dark part is the region where tantalum (Ta) exists, and the bright part is the region where aluminum (Al) exists. It was confirmed that almost all of the surface of the aluminum oxide particles was coated with a tantalum oxide film.

[0110] The selected area electron diffraction pattern obtained for the region A marked with a circle shown in Fig. 5 is shown in Fig. 6. This region A corresponds to the location of the aluminum oxide particles contained in the coated oxide particles. As shown in Fig. 6, periodic diffraction spots were clearly observed. The pattern of these diffraction spots approximated the electron diffraction simulation pattern of α-type aluminum oxide (α-Al2O3). From this, it was found that the aluminum oxide particles contained in the coated oxide particles are single crystals of α-type aluminum oxide.

[0111] [[ID=eleven]] The high-magnification Cs-TEM image in the vicinity of the interface between the oxide particles (Al2O3 particles) and the film (Ta2O5 film) is shown in Fig. 7. The crystal structure changed continuously from the aluminum oxide particles to the tantalum oxide film. From this, it was found that the crystal lattice of the aluminum oxide particles and the crystal lattice of the tantalum oxide film are in agreement.

[0112] When the coated oxide particles obtained in Examples 4 and 5 were evaluated, the same results as in Example 1 were obtained. From this, it was found that even when an oxynitride film (TaON film) or a nitride (Ta3N5 film) was formed, both the oxide particles (Al2O3 particles) and the films (TaON film, Ta3N5 film) were excellent in crystallinity.

[0113] <TEM (Cross-Section of Coated Oxide Particles)> 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.

[0114] 11 shows a TEM image of the tantalum oxide particles obtained in Comparative Example 1. The particle size of the primary particles was about 300 nm.

[0115] <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 decrease rate with respect to UV irradiation time indicates higher photocatalytic activity.

[0116] 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, with the coated oxide particles, catalytic activity was observed regardless of whether the amount of tantalum oxide was increased to 100 mass% (Example 1), 200 mass% (Example 2), or 300 mass% (Example 3), and this 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.

[0117] [Experimental Example B] In Experiment B, lanthanum aluminate (LaAlO3) or magnesium oxide (MgO) was used as the core particle. The surface of the core particle was coated with lanthanum-tantalum oxynitride (LaTaON2) or titanium oxide (TiO2) to produce coated oxide particles, which were then evaluated. For comparison, particles consisting of only lanthanum-tantalum oxynitride or titanium oxide were also produced and evaluated.

[0118] [Example 6] In Example 6, lanthanum aluminate (LaAlO3) particles coated with lanthanum-tantalum oxynitride (LaTaON2) were prepared using lanthanum aluminate (LaAlO3) powder, lanthanum nitrate, and tantalum (V) ethoxide as raw materials.

[0119] 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 pulverizing a purchased single crystal substrate (Crystal Base Co., Ltd.). The purchased single crystal substrate had a double-sided mirror surface, a (100) orientation, and dimensions of 10 × 10 × 0.5 mm. The lanthanum aluminate powder was manually pulverized in an agate mortar and then ball milled. The lanthanum aluminate powder produced was a single crystal powder with an average particle size of 0.5 μm. The water content of the ultra-dehydrated ethanol was less than 0.001% by mass.

[0120] 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, which was then irradiated with ultrasound for 1 minute using an ultrasonic cleaner to disperse the lanthanum aluminate powder.

[0121] 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.

[0122] The dried precipitate was calcined under the conditions of 900°C x 10 hours in an atmosphere of ammonia gas flowing at a flow rate of 0.2 L / min to obtain coated oxide particles.

[0123] The coated oxide particles obtained in Example 6 were evaluated by X-ray diffraction and by a high-resolution transmission electron microscope and a spherical aberration-corrected transmission electron microscope, as in Example 1. As a result, it was confirmed that lanthanum aluminate (LaAlO3) particles coated with lanthanum-tantalum oxynitride (LaTaON2) had been 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 particle and the lanthanum-tantalum oxynitride coating were lattice-matched in most of the interface between the core particle and the coating. This indicates that the coating grew epitaxially on the core particle.

[0124] Comparative Example 2 In Comparative Example 2, particles consisting solely of the coating component, lanthanum-tantalum oxynitride (LaTaON2), 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 (LaTaON2) particles were synthesized under an ammonia gas atmosphere. Calcination was performed under the same conditions as in Example 6.

[0125] 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, whereas the specific surface area of ​​the particles coated with lanthanum-tantalum oxynitride (Example 6) was 1.3 m 2 On the other hand, the specific surface area of ​​particles consisting only of lanthanum-tantalum oxynitride (Comparative Example 2) was 0.9 m 2 / g.

[0126] The dye-decomposition photocatalytic activity of the lanthanum-tantalum oxynitride was evaluated using an LED lamp (OptiLED, 2.5 W) with a wavelength of 455 nm, with the weights of the lanthanum-tantalum oxynitride being the same. The particles of Example 6 were confirmed to have 1.5 times the activity of the particles of Comparative Examples 1 and 2.

[0127] [Example 7] In Example 7, magnesium oxide (MgO) powder was used as the core particle raw material instead of lanthanum aluminate (LaAlO3) powder, and coated oxide particles were produced using the same method and conditions as in Example 6 except for this.

[0128] 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 pulverization was performed by hand grinding in an agate mortar followed by ball milling. The prepared magnesium oxide powder was a single crystal powder with an average particle size of 0.4 μm.

[0129] The obtained coated oxide particles were magnesium oxide (MgO) particles coated with lanthanum-tantalum oxynitride (LaTaON2). The average thickness of the coating was 17 nm and the specific surface area was 2.2 m 2 / g.

[0130] The lanthanum-tantalum oxynitride was weighed the same and evaluated for its photocatalytic ability to decompose dyes using an LED lamp (OptiLED, 2.5 W) with a wavelength of 455 nm. The results showed that the particles of Example 7 had 1.9 times the activity of the particles of Comparative Example 2.

[0131] [Example 8] In Example 8, magnesium oxide (MgO) particles coated with titanium oxide (TiO2) were produced using magnesium oxide (MgO) powder and titanium tetraisopropoxide as raw materials.

[0132] 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. 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. Coated oxide particles were also prepared using the same method and conditions as in Example 1, except that the above-mentioned raw materials were used.

[0133] 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 that made up the coating retained an anatase crystal structure. It was also formed epitaxially with respect to the core particle, and had good crystallinity.

[0134] Comparative Example 3 Particles consisting solely of titanium oxide (TiO2) as 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 calcined in an air atmosphere under the same conditions as in Example 8 to produce titanium oxide (TiO2) particles.

[0135] 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 average particle size of the obtained titanium oxide powder (particles) was 700 nm, and the specific surface area was 0.9 m. 2 / g.

[0136] 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 compared using the same weight of titanium oxide, it was confirmed that the particles of Example 8 had 3.5 times the activity of the particles of Comparative Example 3.< / tem> < / sem> < / xrd>

Claims

1. 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.

2. The coated oxide particle according to claim 1 , wherein the crystalline compound has a region having no grain boundaries extending from the interface between the coating and the core particle to the surface of the coating.

3. 3. The coated oxide particle according to claim 1, wherein the crystalline metal oxide constituting the core particle is in a single crystal state.

4. 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. 5. The coated oxide particles 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.

6. 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.

7. The specific surface area of ​​the coated oxide particles is 0.5 m ２ The coated oxide particles according to any one of claims 1 to 6, wherein the surface roughness is 1 / g or more.

8. A photocatalyst comprising the coated oxide particles according to any one of claims 1 to 7.

9. 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 removing the coated precursor particles from the dispersion; and a calcination 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. 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.

11. 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.

12. 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.

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