Crystalline tantalum oxide particles and method for producing crystalline tantalum oxide particles

Abstract

To provide a nano-sized tantalum oxide particle with high specific surface area and excellent crystallinity, and a manufacturing method thereof.SOLUTION: A crystalline tantalum oxide particle has an average particle diameter (Dm) of 10 to 100 nm and a crystallite diameter (CS) of 10 to 100 nm.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to crystalline tantalum oxide particles and a method for producing crystalline tantalum oxide particles. [Background technology]

[0002] Tantalum oxide (Ta2O5) has high dielectric constant and refractive index, which makes it useful as a material for capacitors and optical coatings. To apply it to these applications, an ink containing tantalum oxide nanoparticles and a binder is prepared, and this ink is applied to a substrate and heat-treated to produce a tantalum oxide film.

[0003] Tantalum oxide is also attracting attention as a photocatalyst material for water splitting, which generates hydrogen from solar energy and water. If hydrogen can be efficiently produced from sunlight, a renewable natural energy source, and water, it will help us transition from a fossil fuel-based society to a hydrogen-based society, where hydrogen is used as an energy source. For this reason, tantalum oxide is a promising water splitting catalyst material.

[0004] For example, Patent Document 1 discloses tantalum oxide particles characterized by a primary particle diameter of 70 nm or less and an optical absorptance of 0.32 or less at a wavelength of 1800 nm measured by diffuse reflectance spectroscopy (Claim 1 of Patent Document 1). Patent Document 1 also describes that the tantalum oxide particles are characterized by having both high crystallinity and a fine primary particle diameter, that they can be used as a photocatalyst for photodecomposition of water, and that they can also be used as a dielectric material for multilayer films and the like (

[0009] ,

[0021] , and

[0022] of Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-004574 Summary of the Invention [Problem to be solved by the invention]

[0006] As such, the use of tantalum oxide particles in a variety of applications, including photocatalysis, has been proposed. Tantalum oxide particles for photocatalysis are desired to have few lattice defects and good crystallinity. If lattice defects exist within the particles, the electrons and holes (positive holes) generated when used as a photocatalyst will recombine at the defects, reducing catalytic activity. Furthermore, it is desirable for tantalum oxide particles for photocatalysis to be minute at the nanometer level. This is because minute particles have a high specific surface area, resulting in many active sites and excellent catalytic performance.

[0007] However, with the techniques proposed so far, it has been difficult to obtain tantalum oxide particles that are nano-level fine and highly crystalline. That is, with the conventional manufacturing methods, high-temperature and long-term firing treatment is required to obtain tantalum oxide particles with high crystallinity. During the firing treatment, grain growth and sintering of the tantalum oxide particles progress, resulting in coarsening of the particles, making it difficult to obtain particles with a high specific surface area.

[0008] The present inventors have conducted extensive research in light of these problems, and as a result have discovered that nano-sized tantalum oxide particles with a large specific surface area and good crystallinity can be obtained by producing them using a chemical vapor reaction method under specific conditions, and that these tantalum oxide particles have excellent photocatalytic performance.

[0009] The present invention was completed based on these findings, and an object of the present invention is to provide nano-sized tantalum oxide particles that have a large specific surface area and good crystallinity, and a method for producing the same. [Means for solving the problem]

[0010] The present invention encompasses the following aspects (1) to (9). In other words, "X~Y" is synonymous with "greater than or equal to X and less than or equal to Y."

[0011] (1) Crystalline tantalum oxide particles having an average particle size (Dm) of 10 nm or more and 100 nm or less and a crystallite size (CS) of 10 nm or more and 100 nm or less.

[0012] (2) The crystalline tantalum oxide particles according to (1) above, wherein the crystal form of the crystalline tantalum oxide particles is at least one of an orthorhombic β type and a hexagonal δ type.

[0013] (3) The BET specific surface area of ​​the crystalline tantalum oxide particles is 5 m 2 / g or more.

[0014] (4) Crystalline tantalum oxide particles according to any one of (1) to (3) above, having a ratio of crystallite size (CS) to average particle size (Dm) of 0.40 or more.

[0015] (5) A method for producing crystalline tantalum oxide particles by a chemical vapor reaction method, comprising the steps of: providing a tantalum source; heating and vaporizing the tantalum source to generate a tantalum-containing gas; and a step of maintaining the tantalum-containing gas at a temperature of 550°C or higher and 1200°C or lower under a pressure of 500 Pa or higher and 50,000 Pa or lower to cause thermal decomposition and oxidation reaction, thereby producing crystalline tantalum oxide particles having an average particle size (Dm) of 10 nm or higher and 100 nm or lower and a crystallite size (CS) of 10 nm or higher and 100 nm or lower; A method comprising:

[0016] (6) The method according to (5) above, wherein the tantalum source is at least one selected from the group consisting of tantalum pentamethoxide, tantalum pentaethoxide, tantalum penta-n-propoxide, tantalum pentaisopropoxide, tantalum penta-n-butoxide, tantalum chloride, and tantalum fluoride.

[0017] (7) The BET specific surface area of ​​the crystalline tantalum oxide particles is 5 m 2 / g or more.

[0018] (8) A photocatalyst comprising crystalline tantalum oxide particles according to any one of (1) to (4) above.

[0019] (9) A method for producing a photocatalyst, comprising the step of producing crystalline tantalum oxide particles by any one of the methods (5) to (7) above. [Effects of the Invention]

[0020] According to the present invention, nano-sized tantalum oxide particles having a large specific surface area and good crystallinity, and a method for producing the same are provided. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram of a powder production apparatus used for synthesis by a chemical vapor reaction method. [Figure 2] 1 shows TEM images of tantalum oxide particles (Examples 1 to 4, Comparative Examples 1 and 2). [Figure 3] 1 shows TEM images of tantalum oxide particles (Comparative Examples 3 to 6). [Figure 4] 1 shows XRD charts of tantalum oxide particles (Examples 1 to 4 and Comparative Examples 1 and 2). [Figure 5] 1 shows XRD charts of tantalum oxide particles (Comparative Examples 2 to 6). [Figure 6] 1 shows the results of TG-MS analysis of tantalum oxide particles (Examples 1 to 4 and Comparative Examples 1 and 2). [Figure 7] 1 shows the results of TG-MS analysis of tantalum oxide particles (Comparative Examples 2 to 6). [Figure 8] 1 shows the diffuse reflectance spectra of tantalum oxide particles (Examples 1 to 4, Comparative Examples 1 and 2). [Figure 9] 1 shows the diffuse reflectance spectra of tantalum oxide particles (Comparative Examples 2 to 6). [Figure 10] The amount of carbon dioxide gas generated by decomposition of acetaldehyde for tantalum oxide particles (Examples 1 to 3 and Comparative Example 2) is shown. [Figure 11] The production rates of hydrogen (H2) and oxygen (O2) gases by water decomposition are shown for tantalum oxide particles (Examples 1 to 4, Comparative Examples 1 to 6). DETAILED DESCRIPTION OF THE INVENTION

[0022] 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.

[0023] 1. Tantalum oxide particles The tantalum oxide particles of this embodiment are crystalline. Whether or not the particles are crystalline can be determined by X-ray diffraction (XRD) analysis. If sharp diffraction peaks based on crystals are clearly observed in an X-ray diffraction chart, the particles can be determined to be crystalline. In this specification, the term "particle" refers to an independent particle or an aggregate of multiple independent particles. When the particle is an aggregate of multiple particles, the term "particle" is synonymous with "powder."

[0024] The tantalum oxide particles of this embodiment have an average particle diameter (Dm) of 10 nm or more and 100 nm or less. Particles with an average particle diameter of less than 10 nm have low crystallinity due to their small crystallite diameter. This may result in insufficient catalytic activity. From the viewpoint of crystallinity, the average particle diameter is preferably 15 nm or more, 20 nm or more, 30 nm or more, 40 nm or more, 50 nm or more, or 60 nm or more. On the other hand, particles with an average particle diameter of more than 100 nm have a too small specific surface area and are therefore unsuitable in terms of catalytic activity. From the viewpoint of specific surface area, the average particle diameter is preferably 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less, 40 nm or less, 30 nm or less, or 20 nm or less. The average particle diameter is determined by observing tantalum oxide particles using a transmission electron microscope (TEM), measuring the particle diameters from the TEM observation image by image processing, and calculating the average particle diameter of 100 particles.

[0025] The tantalum oxide particles of this embodiment have a crystallite diameter (CS) of 10 nm or more and 100 nm or less. The crystallite diameter is an index of particle crystallinity, and it can be said that the larger the crystallite diameter, the fewer lattice defects there are and the higher the crystallinity. If the crystallite diameter is less than 10 nm, the crystallinity may be reduced and the catalytic activity of the particles may be insufficient. This is because particles with low crystallinity have lattice defects within the particles, and electrons and holes generated during use as a catalyst recombine at the defects, adversely affecting the catalytic activity. In particular, amorphous particles have crystallites so small that they cannot be measured, and catalytic activity cannot be expected. From the perspective of crystallinity, the crystallite diameter may be 15 nm or more, 20 nm or more, 30 nm or more, 35 nm or more, 40 nm or more, or 45 nm or more. On the other hand, a larger crystallite diameter inevitably results in a larger particle diameter. If the crystallite diameter is excessively large, the specific surface area decreases, resulting in a decrease in the number of active sites, and even if a co-catalyst is supported, the catalytic performance may not be improved. From the viewpoint of specific surface area, the crystallite diameter may be 50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, or 15 nm or less. The crystallite diameter can be calculated by analyzing the particles using X-ray diffraction (XRD), determining the full width at half maximum (FWHM) of the (001) diffraction peak of tantalum oxide (Ta2O5) in the XRD chart, and then incorporating the obtained FWHM value into Scherrer's formula. Although the crystallite diameter is a measure of the size of the crystallites in the particles, the measurement and calculation methods differ from those for the average particle diameter. Therefore, the crystallite diameter is not necessarily smaller than the average particle diameter.

[0026] The tantalum oxide particles have a crystal form of at least one of orthorhombic β type and hexagonal δ type. Tantalum oxide is known to have α type, β type, and δ type crystal forms. While α type is an unstable phase that forms at relatively high temperatures, β type and δ type are stable phases that form at low temperatures. β type tantalum oxide has an orthorhombic crystal system, and δ type tantalum oxide exhibits a hexagonal crystal system. Both are characterized by being stable at room temperature.

[0027] The tantalum oxide particles preferably have a BET specific surface area of ​​5 m 2 / g or more. Increasing the specific surface area increases the number of active sites, which in turn increases catalytic activity. From the perspective of the number of active sites, the specific surface area is 7m 2 / g or more, 10m 2 / g or more, 20m 2 / g or more, 30m 2 / g or more, or 40m 2 / g or more. On the other hand, if the specific surface area is too high, the particle size becomes too fine, which may result in a decrease in crystallinity. From the viewpoint of crystallinity, the specific surface area is 100 m 2 / g or less, 50m 2 / g or less, 40m 2 / g or less, 30m 2 / g or less, 20m 2 / g or less, or 10m 2 / g or less.

[0028] The tantalum oxide particles preferably have a ratio (CS / Dm) of the crystallite size (CS) to the average particle size (Dm) of 0.40 or more. Increasing CS / Dm makes it possible to further improve the crystallinity of the particles. From the viewpoint of crystallinity, CS / Dm may be 0.60 or more, 0.80 or more, or 1.00 or more. A higher CS / Dm is preferable, but is typically 1.50 or less.

[0029] The tantalum oxide particles preferably have a band gap energy (Eg) of 4.30 eV or more. Due to the quantum size effect, the smaller the particle diameter, the larger the band gap energy. The tantalum oxide particles of this embodiment have a small primary particle diameter and little aggregation. Therefore, the band gap energy is large. The band gap energy may be 4.35 eV or more, or may be 4.40 eV or more.

[0030] The tantalum oxide particles may contain components other than tantalum (Ta) and oxygen (O). However, to fully exhibit properties such as catalytic performance, the content of the other components is preferably low. The content of the other components may be 10.0% by mass or less, 5.0% by mass or less, 2.0% by mass or less, 1.0% by mass or less, 0.5% by mass or less, or 0.1% by mass or less.

[0031] In particular, it is preferable that the tantalum oxide particles have a low carbon content. When tantalum oxide particles are synthesized using a carbon (C)-containing tantalum source, the synthesized tantalum oxide particles may contain carbon components (carbon, organic compounds) derived from the raw materials. If such carbon components are present on the particle surface, the number of active sites may decrease, which may result in a decrease in catalytic performance. From the perspective of catalytic performance, the carbon content may be 5.0 mass% or less, 2.0 mass% or less, 1.5 mass% or less, 1.0 mass% or less, 0.5 mass% or less, or 0.1 mass% or less. The carbon content can be determined by analyzing the particles using thermogravimetry-mass spectrometry or oxygen flow combustion-infrared absorption spectrometry.

[0032] 2. Tantalum oxide particle manufacturing method The tantalum oxide particles of this embodiment are not limited in their manufacturing method as long as they satisfy the above-mentioned requirements. However, it is preferable to synthesize them by a chemical vapor reaction method. The chemical vapor reaction method is a technique in which heated and vaporized raw materials are introduced into a reaction chamber together with a carrier gas, and a reaction is caused to proceed by heating in the reaction chamber. The vaporized raw material particles are thermally decomposed in the reaction chamber to generate tantalum oxide crystals, and the generated tantalum oxide crystals grow to become tantalum oxide particles.

[0033] FIG. 1 shows a schematic diagram of a powder manufacturing apparatus suitable for use in a chemical vapor reaction method. The powder manufacturing apparatus (100) includes a raw material chamber (10), a reaction chamber (20), and a connecting pipe (30) connecting the raw material chamber (10) and the reaction chamber (20). A raw material container (12) capable of storing a compound serving as a raw material for tantalum oxide particles is disposed in the raw material chamber (10). A raw material chamber heater (14) for vaporizing the raw material is disposed around the raw material chamber (10). A carrier gas inlet (16) is disposed in the raw material chamber (10), and a carrier gas is introduced through the carrier gas inlet (16). A reaction chamber connecting port (18) is disposed in the wall of the raw material chamber (10) opposite the carrier gas inlet (16). The reaction chamber connecting port (18) is in communication with a raw material inlet (22) of the reaction chamber (20) via the connecting pipe (30). The connecting pipe (30) is equipped with a valve (32). In the reaction chamber (20), an exhaust port (26) equipped with a recovery filter (24) is provided on the wall opposite to the wall on which the raw material inlet (22) is provided. A reaction heater (28) is also provided around the reaction chamber (20). The reaction in the reaction chamber (20) is carried out under a reduced pressure atmosphere. Therefore, it is preferable that the raw material chamber (10), the reaction chamber (20), and the connecting pipe (30) have a structure that can withstand a reduced pressure atmosphere.

[0034] A particularly suitable method for producing tantalum oxide particles of this embodiment comprises the following steps: a step of preparing a tantalum source (preparation step), a step of heating and vaporizing the prepared tantalum source to generate a tantalum-containing gas (gasification step), and a step of maintaining the tantalum-containing gas at a temperature of 550°C to 1200°C under a pressure of 500Pa to 50,000Pa to cause thermal decomposition and oxidation reaction, thereby producing crystalline tantalum oxide particles with an average particle size (Dm) of 10nm to 100nm and a crystallite size (CS) of 10nm to 100nm (reaction step).Details of each step are described below.

[0035] <Preparation process> In the preparation step, a tantalum (Ta) source is prepared. As the tantalum source, organic tantalum compounds such as tantalum alkoxides (tantalum pentamethoxide, tantalum pentaethoxide, tantalum penta-n-propoxide, tantalum pentaisopropoxide, tantalum penta-n-butoxide, etc.) can be used. Alternatively, inorganic tantalum compounds such as tantalum chloride and tantalum fluoride can be used. However, if the produced tantalum oxide particles are to be used as a photocatalyst, it is desirable to use a tantalum source that does not contain halogen elements. This is because if halogen elements remain in the tantalum oxide particles, the halogen elements may cause a loss of catalytic activity.

[0036] <Gasification process> In the gasification process, a prepared tantalum source is heated and vaporized to generate a tantalum-containing gas. Specifically, the source chamber is heated using a source chamber heater, and the tantalum source charged in the source container is vaporized. In this specification, vaporization includes sublimation. Then, a carrier gas is introduced into the source chamber, and the generated tantalum-containing gas is introduced into the reaction chamber via a connecting pipe. In the reaction chamber, the tantalum-containing gas is thermally decomposed in a subsequent reaction process to generate tantalum oxide crystals, which grow into tantalum oxide particles.

[0037] Gasification parameters during the gasification process, such as the raw material heating temperature, raw material chamber gas pressure, and carrier gas flow rate, are controlled so that the average particle size of the final tantalum oxide particles is 10 nm or more and 100 nm or less. The optimal values ​​of the gasification parameters depend on the size and structure of the equipment, so it is difficult to determine them in general. Therefore, the optimal values ​​can be determined depending on the equipment.

[0038] The raw material heating temperature affects the vaporization or sublimation of the tantalum source. Therefore, by controlling the raw material heating temperature, the amount of tantalum-containing gas generated and introduced into the reaction chamber can be adjusted. A large amount of tantalum-containing gas introduced results in a smaller average particle size of the resulting tantalum oxide particles, while a small amount results in a larger average particle size. In the reaction chamber, tantalum oxide particles grow by adsorption and decomposition of tantalum-containing gas around fine tantalum oxide particles as nuclei. A small amount of tantalum-containing gas introduced results in a smaller number of fine particles serving as nuclei. This results in a larger average particle size. The lower the raw material heating temperature, the smaller the amount of tantalum-containing gas generated, resulting in a larger average particle size of the tantalum oxide particles. On the other hand, the higher the raw material heating temperature, the greater the amount of tantalum-containing gas generated, resulting in a smaller average particle size. The raw material heating temperature is preferably above the vaporization start temperature of the tantalum source and below the vaporization completion temperature. The vaporization start temperature is the temperature at which mass loss begins when the tantalum source is thermally analyzed using a thermogravimetry (TG), and is close to its boiling point if the tantalum source is liquid. On the other hand, the vaporization completion temperature is the temperature at which the tantalum source disappears. The amount of tantalum-containing gas generated varies depending on the configuration of the equipment, but a rate of 1.5 g / hour or more is preferable.

[0039] The gas pressure in the source chamber also affects the vaporization or sublimation of the tantalum source. Therefore, the amount of tantalum-containing gas generated and the average particle size of tantalum oxide particles can be adjusted by controlling the gas pressure in the source chamber. That is, the higher the gas pressure in the source chamber, the longer the residence time of the tantalum-containing gas in the reaction chamber, and therefore the higher the tantalum concentration at the time of nucleation. As a result, the amount of nucleation increases and the average particle size decreases. On the other hand, a source chamber gas pressure of 50,000 Pa or less is preferable. By maintaining the source chamber under a reduced pressure atmosphere, it becomes possible to easily control the vaporization or sublimation of the tantalum source.

[0040] The flow rate of the carrier gas also affects the average particle size of tantalum oxide particles. A high flow rate shortens the residence time of the tantalum-containing gas in the reaction chamber, resulting in a lower tantalum concentration at the time of nucleation. This reduces the amount of nucleation and increases the average particle size. On the other hand, a low flow rate results in a smaller average particle size. The flow rate of the carrier gas is preferably controlled so that the time required for the tantalum source to reach the reaction temperature after reaching the reaction chamber is 0.01 to 20 seconds, and the residence time in the reaction chamber is 0.1 to 30 seconds. A gas containing an inert gas such as argon (Ar) can be used as the carrier gas. The carrier gas is preferably argon (Ar) or a mixed gas of argon (Ar) and oxygen (O), with a mixed gas having an oxygen concentration of 1 to 20% by volume being particularly preferred. Increasing the oxygen concentration to 1% by volume or higher promotes the thermal decomposition of the tantalum source and prevents impurities such as carbon from remaining in the tantalum oxide particles. Furthermore, by keeping the oxygen concentration at 20% by volume or less, a rapid oxidation reaction can be suppressed, and coarsening of tantalum oxide particles can be prevented.

[0041] <Reaction process> In the reaction step, a tantalum-containing gas is maintained at a temperature of 550°C to 1200°C under a pressure of 500 Pa to 50,000 Pa to cause thermal decomposition and oxidation. This produces crystalline tantalum oxide particles with an average particle size of 10 nm to 100 nm and a crystallite size of 10 nm to 100 nm. Specifically, the tantalum-containing gas introduced into the reaction chamber is heated using a reaction heater. As a result, thermal decomposition and oxidation of the tantalum-containing gas occur, producing fine tantalum oxide particles that act as initial nuclei. Continued introduction of the tantalum-containing gas causes the tantalum-containing gas to adsorb onto the surfaces of the produced fine particles, which then undergo further thermal decomposition and oxidation, resulting in the growth of tantalum oxide particles. The resulting tantalum oxide particles have an orthorhombic β-type and / or hexagonal δ-type crystal system.

[0042] The pressure in the reaction step (reaction pressure) affects the shape and production rate of the reaction product. If the reaction pressure is less than 500 Pa, the reaction product will be film-like, making it difficult to obtain tantalum oxide particles. From the perspective of particle properties, the reaction pressure may be 1000 Pa or more, 1500 Pa or more, or 2000 Pa or more. On the other hand, if the reaction pressure exceeds 50,000 Pa, although tantalum oxide particles can be obtained, the production rate will be significantly slower, which may hinder productivity. From the perspective of productivity, the reaction pressure may be 10,000 Pa or less, 7,500 Pa or less, or 5,000 Pa or less.

[0043] The holding temperature (reaction temperature) affects the particle size, crystallinity, and amount of residual impurities of the tantalum oxide particles. If the reaction temperature is below 550°C, it is difficult to obtain tantalum oxide particles with excellent crystallinity. Furthermore, the decomposition reaction of the tantalum-containing gas may not proceed sufficiently, resulting in the risk of unreacted carbon components remaining in the particles as impurities. From the viewpoint of crystallinity and impurities, the reaction temperature may be 600°C or higher, 650°C or higher, 700°C or higher, 750°C or higher, 800°C or higher, 850°C or higher, 900°C or higher, or 950°C or higher. On the other hand, if the reaction temperature exceeds 1200°C, the tantalum oxide particles will become coarse. As a result, the average particle size may exceed 100 nm, or coarse particles with a particle size of 2 μm or more may be generated. From the viewpoint of particle size, the reaction temperature may be 1100°C or less, 1050°C or less, 1000°C or less, 950°C or less, 900°C or less, 850°C or less, 800°C or less, 750°C or less, 700°C or less, or 650°C or less.

[0044] In this manner, the tantalum oxide particles of this embodiment can be produced. The obtained tantalum oxide particles are collected in a collection filter in the reaction chamber. The collected particles are recovered from the filter to obtain tantalum oxide particles.

[0045] Although the production method using a chemical vapor phase reaction has been mainly described, the tantalum oxide particles of this embodiment are not limited to those produced by a chemical vapor phase reaction method. For example, techniques such as aqueous solution thermal decomposition, solid-state reaction, sol-gel method, complex polymerization, and hydrothermal reaction may also be used. Among these, aqueous solution thermal decomposition is a technique in which a metal-containing precursor is used as a raw material, and an aqueous solution containing this metal-containing precursor is heated to evaporate the water solvent, thereby inducing a dehydration polycondensation reaction of the metal-containing precursor. Any technique may be used as long as tantalum oxide particles satisfying the requirements of this embodiment can be obtained.

[0046] 3.Photocatalyst The photocatalyst of this embodiment comprises the above-mentioned crystalline tantalum oxide particles. This photocatalyst can be used to decompose organic substances such as aldehydes and water by ultraviolet irradiation. The photocatalyst may contain tantalum oxide particles alone, or may contain a co-catalyst supported on the surface of the tantalum oxide particles. The co-catalyst may be a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd), gold (Au), silver (Ag) and / or iridium (Ir), a base metal such as nickel (Ni), or nickel oxide (NiO x ) and / or metal oxides such as ruthenium oxide (RuO2).

[0047] 4. Photocatalyst manufacturing method The method for producing a photocatalyst of this embodiment includes the steps of producing crystalline tantalum oxide particles by the above-mentioned method, namely, the steps of preparing a tantalum source (preparation step), heating and vaporizing the prepared tantalum source to generate a tantalum-containing gas (gasification step), and maintaining the tantalum-containing gas at a temperature of 550°C to 1200°C under a pressure of 500 Pa to 50,000 Pa to cause thermal decomposition and oxidation reaction, thereby producing crystalline tantalum oxide particles having an average particle size (Dm) of 10 nm to 100 nm and a crystallite size (CS) of 10 nm to 100 nm (reaction step).

[0048] When the photocatalyst contains tantalum oxide particles alone, the obtained tantalum oxide particles can be used as a photocatalyst as is. When the photocatalyst contains a co-catalyst supported on the surface of the tantalum oxide particles, a step of supporting the co-catalyst on the surface of the obtained tantalum oxide particles (supporting step) may be provided. The supporting step may be performed by a known method. For example, when the co-catalyst is ruthenium oxide (RuO2), a method of impregnating tantalum oxide particles with a RuO2 precursor solution followed by heat treatment can be used. The same applies to catalysts other than RuO2; the supporting method is not limited as long as the co-catalyst is supported. [Example]

[0049] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

[0050] (1) Production of tantalum oxide particles [Example 1] In Example 1, tantalum ethoxide was used as the tantalum source, and tantalum oxide particles were synthesized by a chemical vapor reaction method using the powder production apparatus shown in Figure 1. This powder production apparatus had an internal volume of 400 cm 3 The raw material chamber and the internal volume are 300 cm 3 Specifically, the particles were produced in the following manner.

[0051] First, with the valve closed, the reaction chamber was evacuated and heating of the reaction chamber was then initiated. After the reaction chamber reached the specified temperature, tantalum ethoxide (tantalum source) was charged into the raw material container in the raw material chamber. Next, the valve was opened and heating of the raw material chamber was initiated while flowing a specified flow rate of carrier gas. After the raw material chamber reached the specified temperature, it was maintained at that temperature for 2 hours. Precipitation of tantalum oxide particles began in the reaction chamber due to the heating of the raw material chamber. At this time, the temperatures of the raw material chamber (outside) and the reaction tube inlet (inside) were measured using a thermocouple and were taken as the raw material heating temperature and reaction temperature, respectively. The precipitated tantalum oxide particles were collected in a filter in the reaction chamber. After the precipitation reaction was completed, the tantalum oxide particles collected in the filter were recovered.

[0052] In Example 1, a mixed gas containing 90 vol% argon (Ar) and 10 vol% oxygen (O2) was used as the carrier gas, and the flow rate of the carrier gas was set to 280 SCCM. The raw material heating temperature was 220°C, the gas pressure in the reaction chamber was 5000 Pa, and the reaction temperature was 950°C. The evaporation rate of tantalum ethoxide (tantalum source) was 1.5 g / hour.

[0053] [Example 2] In Example 2, the reaction temperature was set to 835° C. Other than that, tantalum oxide particles were produced in the same manner as in Example 1.

[0054] [Example 3] In Example 3, the reaction temperature was set to 740° C. Other than that, tantalum oxide particles were produced in the same manner as in Example 1.

[0055] [Example 4] In Example 4, the reaction temperature was set to 612° C. Other than that, tantalum oxide particles were produced in the same manner as in Example 1.

[0056] [Comparative Example 1] In Comparative Example 1, the reaction temperature was set to 507° C. Other than that, tantalum oxide particles were produced in the same manner as in Example 1.

[0057] Comparative Example 2 In Comparative Example 2, tantalum oxide particles were synthesized using tantalum ethoxide as a raw material by a conventional hydrolysis method. Specifically, the particles were produced by the following procedure.

[0058] First, 5 g of tantalum ethoxide was added to 10 mL of water and mixed ultrasonically. This caused a hydrolysis reaction to occur, resulting in the formation of a gel. The resulting gel was dried for 3 hours in a dryer set at 80°C, yielding 4.4 g of dried powder. 2.2 g of this powder was placed in an alumina crucible and heated in air at a heating rate of 10°C / min, to a heating temperature of 1000°C, and for a holding time of 1 hour. This yielded a white powder.

[0059] Comparative Example 3 In Comparative Example 3, the heating temperature was set at 900°C. Tantalum oxide particles were produced in the same manner as in Comparative Example 2, except for this difference.

[0060] [Comparative Example 4] In Comparative Example 4, the heating temperature was set at 800°C. Tantalum oxide particles were produced in the same manner as in Comparative Example 2, except for this difference.

[0061] [Comparative Example 5] In Comparative Example 5, the heating temperature was set at 700°C. Tantalum oxide particles were produced in the same manner as in Comparative Example 2, except for this difference.

[0062] [Comparative Example 6] In Comparative Example 6, the heating temperature was set at 600°C. Tantalum oxide particles were produced in the same manner as in Comparative Example 2, except for this difference.

[0063]

Table 1

[0064] (2) Evaluation For the tantalum oxide particles obtained in Examples 1 to 4 and Comparative Examples 1 to 6, evaluations of various properties were conducted as follows. [[ID=3३]]

[0065] <Visual Observation> The tantalum oxide particles were visually observed to examine their color tone.

[0066] <TEM Observation> The tantalum oxide particles were observed using a scanning transmission electron microscope (STEM; Hitachi High-Tech Corporation, HD2700). The particle sizes of 100 particles were measured, and the average value was taken as the average particle size.

[0067] <xrd> X-ray diffraction (XRD) analysis of tantalum oxide particles was performed using a powder X-ray diffractometer (Rigaku, RINT-2200), and based on the obtained results, the generated phase was identified and its crystallinity was evaluated. CuKα rays were used as the radiation source in the analysis.

[0068] Also, based on the XRD analysis results, the crystallite size of the tantalum oxide particles was calculated. Specifically, the XRD chart obtained by the analysis was analyzed, and the full width at half maximum (FWHM) of the (001) diffraction peak of tantalum oxide (Ta2O5) present near 2θ = 22.9° was determined. Then, the obtained FWHM value was introduced into Scherrer's formula to calculate the crystallite size.

[0069] <BET specific surface area> The BET specific surface area of tantalum oxide particles was measured. Specifically, a specific surface area measuring device (Shimadzu, TriStar3000) was used, nitrogen (N2) was used as the adsorption gas, and the specific surface area was measured by the BET multi-point method.

[0070] <Thermal analysis> Thermogravimetry-mass spectrometry (TG-MS; Bruker-AXS, 2020SA / MS9,600) was used to perform thermal analysis on tantalum oxide particles, and thereby the amount of unreacted substances in the particles was examined. The thermal analysis was carried out under the condition of a heating rate of 20 °C / min in an N2-21% O2 gas atmosphere.

[0071] <Diffuse reflectance spectrum> The diffuse reflectance spectrum of tantalum oxide particles in the solid state was examined. The analysis was carried out in the wavelength range of 200 to 800 nm using an ultraviolet-visible spectrophotometer (JASCO, V-570). Then, from the obtained spectrum, using the Tauc Plot, the band gap energy (Eg) was calculated according to the following formula (1). In the following formula (1), α is the absorption coefficient, h is Planck's constant, ν is the frequency. Also, n is a value determined by the type of transition of the sample. Since tantalum oxide (Ta2O,5) shows a direct transition type of transition, n = 1 / 2 was determined.

[0072]

number

[0073] <Photocatalyst evaluation-1> The photocatalytic performance of tantalum oxide particles was evaluated by investigating the decomposition reaction of acetaldehyde (CH3CHO). Specifically, 0.04 g of sample powder was stirred in 100 mL of distilled water to prepare a suspension, which was then filtered through a 3.3 cm diameter filter paper to uniformly immobilize the sample on the filter paper surface. The filter paper with the immobilized sample was placed in a closed circulation system, and the sample was irradiated with ultraviolet light (Hg-Xe lamp, HOYA EXECURE 4000) while a mixed gas of CH3CHO / HO / Air (5 / 10 / 740 Torr) was flowing. The photocatalytic reaction induced by UV irradiation led to the decomposition of acetaldehyde and the production of CO2. The amount of CO2 produced was measured using gas chromatography. A similar measurement was also performed using the filter paper alone without immobilizing the sample, and a reference CO2 production volume was calculated as a blank.

[0074] <Photocatalyst evaluation-2> The photocatalytic performance of tantalum oxide particles was evaluated by investigating the water decomposition reaction. First, RuO2 co-catalyst was supported on tantalum oxide particles by impregnation to prepare RuO2-supported photocatalyst samples for evaluation. Specifically, Ru3(CO) was added to the tantalum oxide particles so that the Ru metal content was 1 mass %. 12 (Aldrich, purity 99%) was weighed. 12 15 mL of tetrahydrofuran (THF) (Nacalai Tesque, purity 99.5%) was added to dissolve the RuO2-supported photocatalyst. Tantalum oxide particles were added to the resulting solution and stirred at 500 rpm for 1 hour using a magnetic stirrer, and then dried under reduced pressure. The powder obtained after drying was oxidized in an electric furnace at 673 K for 5 hours to obtain the RuO2-supported photocatalyst.

[0075] The water decomposition test was carried out using a closed-loop reaction apparatus. This apparatus consisted of a high-vacuum evacuation system, a reaction cell, a piston pump for gas circulation, a pressure gauge, and a gas chromatograph (Shimadzu, GC-390B). First, 0.2 g of the RuO2-supported photocatalyst and 700 mL of distilled water were placed in the reaction cell. Then, the photocatalyst was suspended in water by ultrasonic dispersion and further stirred using a magnetic stirrer. Ar gas was introduced into the degassed reaction system at a pressure of 4 kPa, and the gas was circulated with a piston pump. A 450 W high-pressure mercury lamp (USHIO, UM-452) was used as the light source, and light irradiation was performed by the internal irradiation method. The generated gas was sampled every 15 minutes, and the amount of generated gas was evaluated.

[0076] (3) Evaluation results <Visual observation> The color tones of the tantalum oxide particles of Examples 1 to 4 and Comparative Examples 1 to 6 are summarized in Table 2. White particles were obtained in Examples 1 to 4 and Comparative Examples 2 to 6, while gray particles were obtained in Comparative Example 1. This was considered to be due to the residual organic matter.

[0077] <TEM observation> The TEM photographs of the tantalum oxide particles of Examples 1 to 4 and Comparative Examples 1 to 6 are shown in FIGS. 2(a) to (f) and FIGS. 3(g) to (j), respectively (a: Example 1, b: Example 2, c: Example 3, d: Example 4, e: Comparative Example 1, f: Comparative Example 2, g: Comparative Example 3, h: Comparative Example 4, i: Comparative Example 5, j: Comparative Example 6). The average particle diameters determined from the TEM photographs are summarized in Table 2.

[0078] The samples produced by the chemical vapor reaction method (Examples 1 to 4 and Comparative Example 1) were all composed of single-crystalline primary particles, and their average particle diameters were about 13 to 70 nm. On the other hand, the sample produced by the hydrolysis method (Comparative Example 2) had a relatively large average particle diameter of about 112 nm, and it was confirmed that sintering between particles had progressed. The particle diameters of the primary particles of the samples produced by the hydrolysis method became smaller when synthesized at low temperatures (Comparative Examples 2 to 6). However, severe aggregation was observed in the samples of Comparative Examples 4 to 6 such that it was difficult to distinguish the primary particles.

[0079] <xrd> XRD charts of the tantalum oxide particles of Examples 1 to 4 and Comparative Examples 1 to 6 are shown in FIGS. 4 and 5. In FIGS. 4 and 5, the standard diffraction charts of orthorhombic β-type tantalum oxide (JCPDS card 00-025-0922) and the standard diffraction chart of hexagonal tantalum oxide (JCPDS card 00-019-1299) are shown together. The crystallite diameters determined from the XRD charts are summarized in Table 2.

[0080] As shown in FIG. 4, diffraction peaks of orthorhombic β-type tantalum oxide were observed in Example 1. In Example 2, diffraction peaks that are presumably due to the coexistence of hexagonal δ-type tantalum oxide and orthorhombic β-type tantalum oxide were observed. In Examples 3 and 4, diffraction peaks of hexagonal δ-type tantalum oxide were observed. Also, since the width of each diffraction peak was broad, it was found that the crystallite diameter was small. On the other hand, no diffraction peak was seen in Comparative Example 1, and it was found that this sample was amorphous. Also, in Comparative Example 2, although diffraction peaks of orthorhombic β-type tantalum oxide were observed, the width of the diffraction peak was narrow, and it was confirmed that the crystallite diameter was large. Also, as shown in FIG. 5, in Comparative Examples 3 to 6 produced by the hydrolysis method, structural changes from hexagonal at low temperature to orthorhombic at high temperature could be confirmed, similar to the case of using the chemical vapor reaction method.

[0081] As shown in Table 2, the tantalum oxide particles of Examples 1 to 4 had a crystallite diameter of 14 to 46 nm, which was a value close to the average particle diameter. Therefore, it was confirmed that they had excellent crystallinity. On the other hand, since the sample of Comparative Example 1 was amorphous, the crystallite diameter could not be determined.

[0082] <BET specific surface area> The BET specific surface areas of the tantalum oxide particles of Examples 1 to 4 and Comparative Examples 1 to 6 are summarized in Table 2. It was found that the samples of Examples 1 to 4 and Comparative Example 1 had a large specific surface area and that the tantalum oxide particles were fine. On the other hand, in Comparative Examples 2 to 6 produced by the hydrolysis method, the samples produced at low temperature had a larger specific surface area. However, since there were many aggregated portions, the specific surface area tended to be smaller than that of the chemical vapor method.

[0083] [[ID=ID=18]] <Thermal analysis> The TG-MS analysis results for the tantalum oxide particles of Examples 1 to 4 and Comparative Examples 1 to 6 are shown in Figures 6(a) and 6(b) and Figures 7(a) and 7(b). (a) shows the TG curve, and (b) shows the MS curve. Furthermore, the mass loss at 1200°C determined from the TG curves is summarized in Table 2. The mass loss is believed to be due to unreacted carbon components contained in the tantalum oxide particles. Measurements were performed in an N2 / 21% O2 atmosphere, and CO2 generation was confirmed in all samples within the temperature range of 250 to 500°C. This is believed to be due to the decomposition of organic matter remaining in the samples. Furthermore, in the sample of Comparative Example 1, which was synthesized at a low temperature, rapid CO2 generation was observed around 700°C. It is presumed that crystallization of tantalum oxide (Ta2O5) began around 700°C, leading to the decomposition of the organic matter. The weight loss for all samples in the Examples was 1.3% by mass or less.

[0084] <Diffuse reflectance spectrum> The diffuse reflectance spectra of the tantalum oxide particles of Examples 1 to 4 and Comparative Examples 1 to 6 are shown in Figures 8 and 9. All samples were found to absorb ultraviolet light with wavelengths of 200 to 300 nm. The band gap energies calculated by Turk plots are summarized in Table 2. When the particle size is small, the band gap energy increases due to the quantum size effect. The samples produced by chemical vapor deposition had small primary particle sizes and were not aggregated, which is thought to be why the band gap energy was large.

[0085] <Photocatalyst evaluation-1> The amounts of carbon dioxide gas generated by the decomposition of acetaldehyde for Examples 1 to 3 and Comparative Example 2 are shown in Figure 10. The amount of CO2 generated by the decomposition of acetaldehyde was high in Examples 1 to 3, but low in Comparative Example 2. This indicates that the tantalum oxide crystal particles produced according to this embodiment have higher catalytic performance than tantalum oxide synthesized by conventional technology. This is because the powder of this embodiment has high crystallinity despite its small particle size and high specific surface area.

[0086] <Photocatalyst evaluation-2> Figure 11 shows the rates of production of hydrogen (H2) and oxygen (O2) gas by water decomposition for Examples 1 to 4 and Comparative Examples 1 to 6. The samples of Examples 1 to 4 had high catalytic activity, with Example 4 in particular having the highest catalytic activity. This is thought to be due to the large specific surface area of ​​Example 4. On the other hand, Comparative Example 1 had low catalytic activity due to its amorphous (non-crystalline) structure. Furthermore, Comparative Examples 2 and 4 had high catalytic activity, but Comparative Examples 3, 5, and 6 had low catalytic activity. The catalytic activity results for Comparative Examples 2 to 6 are thought to be due to the fact that although the crystallinity was high, the specific surface area was low, and the aggregation of the samples used for evaluation affected the catalytic activity.

[0087] [Table 2] [Explanation of symbols]

[0088] 10 Raw material room 12 Raw material container 14 Raw material chamber heater 16 Carrier gas inlet 18 Reaction chamber connection 20 Reaction Chamber 22 Raw material input port 24 Collection filter 26 Exhaust port 28 Reaction heater 30 Connecting Pipe 32 valves 100 Powder manufacturing equipment< / xrd> < / xrd>

Claims

1. Crystalline tantalum oxide particles having an average particle size (Dm) of 10 nm or more and 100 nm or less, a crystallite size (CS) of 10 nm or more and 100 nm or less, and a band gap energy (Eg) of 4.30 eV or more.

2. 2. The crystalline tantalum oxide particles according to claim 1, wherein the crystal form of the crystalline tantalum oxide particles is at least one of an orthorhombic β type and a hexagonal δ type.

3. The BET specific surface area of ​​the crystalline tantalum oxide particles is 5 m ２ 3. The crystalline tantalum oxide particles according to claim 1, wherein the crystalline tantalum oxide particles have a molecular weight of 1 / g or more.

4. 4. The crystalline tantalum oxide particles according to claim 1, wherein the ratio of the crystallite size (CS) to the average particle size (Dm) is 0.40 or more.

5. A method for producing crystalline tantalum oxide particles by a chemical vapor reaction method, comprising the steps of: providing a tantalum source; heating and vaporizing the tantalum source to generate a tantalum-containing gas; and a step of maintaining the tantalum-containing gas at a temperature of 550°C or higher and 1200°C or lower under a pressure of 500 Pa or higher and 50,000 Pa or lower to cause thermal decomposition and oxidation reaction, thereby producing crystalline tantalum oxide particles having an average particle size (Dm) of 10 nm or higher and 100 nm or lower and a crystallite size (CS) of 10 nm or higher and 100 nm or lower; A method comprising:

6. 6. The method of claim 5, wherein the tantalum source is at least one selected from the group consisting of tantalum pentamethoxide, tantalum pentaethoxide, tantalum penta-n-propoxide, tantalum pentaisopropoxide, tantalum penta-n-butoxide, tantalum chloride, and tantalum fluoride.

7. The BET specific surface area of ​​the crystalline tantalum oxide particles is 5 m ２ The method according to claim 5 or 6, wherein the saturation coefficient is 1 / g or more.

8. A photocatalyst comprising the crystalline tantalum oxide particles according to any one of claims 1 to 4.

9. A method for producing a photocatalyst, comprising a step of producing crystalline tantalum oxide particles by the method according to any one of claims 5 to 7.

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