Photocatalyst and production method for same

Abstract

According to one embodiment of the present disclosure, a photocatalyst includes a polycrystalline ferrite material that has a coercive electric field Ec of 0.1–2.0 MV / m.

Description

Photocatalyst and its manufacturing method

[0001] The present disclosure relates to a photocatalyst and a method for producing the same.

[0002] Ferrite-based materials are inexpensive and readily available because they are mostly composed of iron and oxygen, and because they are oxides, they have excellent chemical stability and are resistant to corrosion. For this reason, they are highly desirable materials for practical use. Ferrite-based materials for photocatalysis include bismuth ferrite (BiFeO) with a perovskite structure. ３ Composite materials combining bismuth ferrite with other materials have been investigated (Patent Documents 1 to 4). Also, the use of polarized bismuth ferrite as a photocatalyst has been investigated (Non-Patent Document 1).

[0003] Chinese Patent Application Publication No. 104646001 Chinese Patent Application Publication No. 104941662 Chinese Patent Application Publication No. 106807400 Chinese Patent Application Publication No. 108114736

[0004] Journal of Alloys and Compounds 783 (2019) 943-951

[0005] Ferrite-based materials are useful because they can be synthesized using readily available industrial materials. However, their photocatalytic activity needs to be further improved.

[0006] An object of the present disclosure is to provide a photocatalyst that uses a ferrite-based material and exhibits good activity, and a method for producing the same.

[0007] The inventors of the present disclosure discovered that a polycrystalline ferrite material having a specific coercive field Ec can be obtained by annealing a ferrite material and then poling it. They then confirmed that the polycrystalline ferrite material having the specific coercive field Ec exhibits good photocatalytic activity, leading to the completion of the present disclosure. Accordingly, the present disclosure has the following aspects.

[0008] [1] A photocatalyst comprising a polycrystalline ferrite material, wherein the polycrystalline ferrite material has a coercive electric field Ec of 0.1 MV / m or more and 2.0 MV / m or less. [2] The photocatalyst according to [1], wherein the polycrystalline ferrite material has a perovskite crystal structure. [3] The photocatalyst according to [1] or [2], wherein the polycrystalline ferrite material is in the form of nanoparticles. [4] The photocatalyst according to [1] or [2], wherein the polycrystalline ferrite material is in the form of fibers.

[0009] [5] A method for producing a photocatalyst, comprising a first step of preparing a polycrystalline ferrite material, a second step of annealing the polycrystalline ferrite material, and a third step of applying an electric field to the polycrystalline ferrite material. [6] A method for producing a photocatalyst according to [5], wherein the method of applying an electric field in the second step is a corona discharge treatment method. [7] A method for producing a photocatalyst according to [5] or [6], wherein the polycrystalline ferrite material is nanoparticles, and the first step is a step of producing nanoparticles of the polycrystalline ferrite material by a solvothermal method. [8] A method for producing a photocatalyst according to [5] or [6], wherein the polycrystalline ferrite material is nanofibers, and the first step is a step of producing nanofibers of the polycrystalline ferrite material by an electrospinning method.

[0010] According to the present disclosure, it is possible to provide a photocatalyst that uses a ferrite-based material and exhibits good activity, and a method for producing the same.

[0011] The nanoparticles obtained in Example 1 (BiFeO ３ 1 is an XRD chart of the nanofiber (BiFeO ３ 1 is an XRD chart of the nanoparticles (BiFeO ３ 1 is an SEM image of the nanofiber (BiFeO ３ 1 is an SEM image of the nanofiber (BiFeO ３1 is a hysteresis loop of the poling treatment. It is a graph showing the results of a photodecomposition experiment of methylene blue using the nanoparticles before and after the poling treatment obtained in Example 1. It is a graph showing the results of a photodecomposition experiment of indigo blue using the nanoparticles before and after the poling treatment obtained in Example 1. It is a graph showing the results of a photodecomposition experiment of indigo blue using the nanofibers before and after the poling treatment obtained in Example 2.

[0012] (Photocatalytic Material) A photocatalyst according to an embodiment of the present disclosure includes a polycrystalline ferrite material. The polycrystalline ferrite material may have any of the following crystal structures: a perovskite structure, a spinel crystal structure, a hexagonal crystal structure, and a garnet crystal structure. Examples of polycrystalline ferrite materials with a perovskite structure include MFeO ３ (wherein M represents at least one rare metal selected from the group consisting of La and Bi) can be used. As the polycrystalline ferrite material of the spinel crystal structure, M I Fe ２ O ４ (However, M I represents at least one magnetic metal selected from the group consisting of Fe, Ni, and Co). As the polycrystalline ferrite material having a hexagonal crystal structure, M II Fe １２ O １９ (However, M II represents at least one metal selected from the group consisting of Ba, Sr, and Pb). As the polycrystalline ferrite material having a garnet crystal structure, RFe ５ O １２ (where R is a rare earth element) can be used.

[0013] The polycrystalline ferrite material may have a perovskite structure, in particular BiFeO ３The photocatalyst may be a polycrystalline bismuth ferrite represented by the formula (1). Polycrystalline bismuth ferrite with a perovskite structure theoretically exhibits diamagnetism, but may exhibit ferromagnetism if the number of crystal defects increases. The polycrystalline bismuth ferrite may be a diamagnetic material or a ferromagnetic material. A photocatalyst containing ferromagnetic polycrystalline bismuth ferrite can be dispersed in contaminated water, for example, and used as a material for promoting the decomposition of pollutants, and then recovered using magnetic force.

[0014] Grain boundaries may exist between crystals of the polycrystalline ferrite material, and the size of the grain boundaries may be such that their presence can be visually determined in an SEM (scanning electron microscope) image.

[0015] The shape of the polycrystalline ferrite material is not particularly limited, and it may be nanoparticles or nanofibers. Nanoparticles refer to particles with a particle diameter of 1000 nm or less, and nanofibers refer to fibrous materials with a diameter in the range of 1 nm to 1000 nm and a length-to-diameter ratio (aspect ratio) of 100 or more.

[0016] (Shape of Nanoparticles) The nanoparticles of polycrystalline ferrite material may be, for example, spherical, rod-shaped, flake-shaped, flower-shaped, amorphous, or may include a combination of any two or more of these shapes. The average aspect ratio (average major axis / average minor axis) of rod-shaped nanoparticles may be less than 100. The ratio of the average major axis to the average thickness (average major axis / average thickness) of flake-shaped nanoparticles may be 100 or more.

[0017] There is no particular limitation on the size of the nanoparticles, and they may have an average particle diameter of, for example, 1 to 1,000 nm. The average particle diameter of the nanoparticles means the arithmetic mean value of the major and minor axes of 10,000 nanoparticles. The major and minor axes of the nanoparticles are values ​​measured using SEM images.

[0018] The nanoparticles may form secondary particles (aggregates). The size of the secondary particles is not particularly limited. For example, the particle diameter of the secondary particles may be 1 μm or more and 10 μm or less. The particle diameter of the secondary particles is a value measured using SEM images.

[0019] (Shape of Nanofiber) Nanofibers of polycrystalline ferrite material may be such that pores and / or cavities are observed on the surface by SEM observation.

[0020] The size of the nanofibers is not particularly limited, and may have, for example, an average diameter of 1 to 1,000 nm and an average length of more than 1,000 nm. The average aspect ratio (average length / average diameter) of the nanofibers may be 100 or more. The average length and average diameter of the nanofibers refer to the arithmetic mean values ​​of the lengths and diameters of 10,000 nanofibers. The length and diameter of the nanofibers are values ​​measured using SEM images.

[0021] (Dielectric Properties of Polycrystalline Ferrite Material) The polycrystalline ferrite material is poled (polarized). The polycrystalline ferrite material is poled so that it has a coercive electric field Ec of 0.1 MV / m or more and 2.0 MV / m or less. The coercive electric field Ec may be in the range of 0.2 MV / m or more and 1.9 MV / m or less, or in the range of 0.4 MV / m or more and 1.7 MV / m or less. Because the coercive electric field Ec is high at 0.1 MV / m or more, it promotes the generation of excited species such as hydroxyl radicals under light irradiation, increasing catalytic activity. Furthermore, because the polycrystalline ferrite material is poled so that the coercive electric field Ec is 2.0 MV / m or less, deterioration during the poling process can be suppressed.

[0022] (Crystallineness of Polycrystalline Ferrite Material) The polycrystalline ferrite material may be an annealed product. The annealed polycrystalline ferrite material has higher crystallinity than before annealing. The higher crystallinity allows the polycrystalline ferrite material to be stably polarized by poling, thereby increasing the catalytic activity of the polycrystalline ferrite material.

[0023] (Supporting of Co-Catalyst) The polycrystalline ferrite material may have co-catalyst particles supported on its surface. For example, metal or metal oxide particles having a Fermi level lower than that of the polycrystalline ferrite material can be used as the co-catalyst particles. Supporting these co-catalyst particles on the polycrystalline ferrite material promotes the generation of hydroxyl radicals upon light irradiation, thereby improving the photocatalytic activity. In the case of polycrystalline bismuth ferrite, the co-catalyst particles may be, for example, particles of any one metal selected from the group consisting of Au, Pd, Ag, and Pt, or an alloy combining two or more of these metals. The surface of the co-catalyst particles may be covered with an oxide such as silica. This structure is called a core-shell structure. Examples of co-catalyst particles with a core-shell structure include Au core-silica shell particles and Ag core-silica shell particles.

[0024] The size of the promoter particles is not particularly limited, and may have an average particle diameter of, for example, 1 to 1,000 nm. When the polycrystalline ferrite material is in the form of nanoparticles, the average particle diameter of the promoter particles may be smaller than the average particle diameter of the nanoparticles of the polycrystalline ferrite material. When the polycrystalline ferrite material is in the form of nanofibers, the average particle diameter of the promoter particles may be smaller than the average diameter of the nanofibers of the polycrystalline ferrite material. The average particle diameter of the promoter particles means the arithmetic mean value of the major axis and minor axis of 10,000 promoter particles. The major axis and minor axis of the promoter particles are values ​​measured using SEM images.

[0025] There are no particular limitations on the amount of promoter particles supported. For example, the amount of promoter particles supported relative to the total mass of the polycrystalline ferrite material and the promoter particles may be in the range of 0.2 mass % to 10 mass %.

[0026] (Band Gap) The polycrystalline ferrite material (nanoparticles or nanofibers) according to this embodiment may have a band gap of 1.0 eV or more and 3.5 eV or less.

[0027] (Mechanism of Action) When the photocatalyst (nanoparticles or nanofibers of polycrystalline ferrite material) of this embodiment is irradiated with light such as ultraviolet light or visible light, electrons are emitted from the surface, generating holes, which react with moisture on the surface to generate excited species such as hydroxyl radicals, which have strong oxidizing power. These excited species can decompose pollutants such as organic compounds that are components of dirt.

[0028] The photocatalyst of this embodiment may be used in the form of nanoparticles or nanofibers. The photocatalyst may also be used in a state where it is fixed to the surface of a substrate. Examples of materials that can be used for the substrate include metal, tile, enamel, cement, concrete, glass, fiber, wood, paper, and plastic. Conventional methods known as catalyst fixing methods, such as sintering the photocatalyst or using a binder, can be used to fix the photocatalyst to the substrate. Furthermore, the photocatalyst may be used in the form of a molded article. Nanoparticles may be molded into a molded product, such as a flat plate, corrugated plate, honeycomb, spherical, or curved shape, by adding a binder as necessary. Nanofibers may also be formed, for example, into a nonwoven fabric in which the nanoparticles are entangled with one another.

[0029] (Method for manufacturing photocatalyst) A method for manufacturing a photocatalyst according to an embodiment of the present disclosure includes a first step of preparing a polycrystalline ferrite material, a second step of annealing the prepared polycrystalline ferrite material, and a third step of applying an electric field to the annealed polycrystalline ferrite material. The polycrystalline ferrite material prepared in the first step may be in the form of nanoparticles or nanofibers.

[0030] (First Step - Nanoparticle Synthesis Method) Methods for synthesizing nanoparticles of polycrystalline ferrite material include a breakdown process and a buildup process. The breakdown process is a process in which coarse particles or lumps of polycrystalline ferrite material are mechanically pulverized using a pulverizer to obtain fine, small nanoparticles. Examples of pulverizers that can be used include a jet mill, a ball mill, a planetary mill, and a bead mill.

[0031] The build-up process is a process for obtaining nanoparticles from polycrystalline ferrite raw materials using physical and chemical reactions. Methods for obtaining nanoparticles include synthesis methods such as gas-phase, liquid-phase, and solid-phase methods. Examples of gas-phase methods include chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD methods include thermal CVD, plasma CVD, and flame methods. Examples of liquid-phase methods include irradiation methods in which raw materials are irradiated with physical energy, sol-gel methods, liquid-phase reduction methods, and solvothermal methods. Irradiation methods include spray pyrolysis, laser decomposition, and ultrasonic methods. Examples of solid-phase methods include solid-phase pyrolysis.

[0032] The nanoparticles are preferably synthesized by the solvothermal method. In the solvothermal method, raw materials for polycrystalline ferrite are reacted in a solvent under high temperature and pressure or in a supercritical state. The solvent can be water, an organic solvent (e.g., ethylene glycol), or a mixture thereof.

[0033] (First Step - Nanofiber Synthesis Method) Electrospinning can be used as a method for synthesizing nanofibers of polycrystalline ferrite material. Synthesis of nanofibers of polycrystalline ferrite material by electrospinning is performed, for example, as follows: A raw material liquid containing a precursor of the polycrystalline ferrite material and a binder resin is placed in a syringe. Next, while applying an electric field to the raw material liquid in the syringe, the raw material liquid is ejected to the outside in the form of fibers, thereby spinning primary fibers. The primary fibers can be obtained by maintaining constant ambient temperature and humidity during spinning. Next, the obtained primary fibers are dried. A constant temperature oven can be used as a drying device. Polyvinylpyrrolidone, for example, can be used as the binder resin in the raw material liquid. The density of the nanofibers and the proportion of pores and / or cavities on the surface can be adjusted by adjusting the amount of binder resin in the raw material liquid.

[0034] (Second Step) In the second step, the polycrystalline ferrite material prepared in the first step is annealed. The annealing temperature is in a range equal to or higher than the temperature at which the crystallinity of the polycrystalline ferrite material improves and lower than the temperature at which the polycrystalline ferrite material sinters or melts. In the case of a polycrystalline bismuth ferrite, the annealing temperature is, for example, in the range of 500°C to 1000°C. The annealing environment may be, for example, an atmospheric environment. The annealing time may be, for example, 30 minutes to 5 hours. A muffle furnace may be used as the heating device.

[0035] (Third Step) In the third step, an electric field is applied to the polycrystalline ferrite material annealed in the second step to perform a poling treatment. The method for applying the electric field is not particularly limited, but examples include a method in which the polycrystalline ferrite material is placed on a metal electrode and then an electric field is applied by corona discharge treatment, and a method in which an acrylic monomer solution containing the polycrystalline ferrite material is cast on a metal electrode to polymerize the acrylic monomer, and then a metal foil is placed on top and an electric field is applied.

[0036] The electric field to be applied is not particularly limited, but a voltage of 100 kV / cm or more is preferably applied. The applied voltage may be in the range of 200 kV / cm to 2000 kV / cm or in the range of 500 kV / cm to 1500 kV / cm.

[0037] In this manner, a polycrystalline ferrite material having the above-mentioned dielectric properties is obtained.

[0038] (Supporting of promoter) When a promoter is supported on the surface of the polycrystalline ferrite material, the step of supporting the promoter may be carried out at any time between the first step and the second step, between the second step and the third step, or after the third step.

[0039] To support metal particles, for example, the polycrystalline ferrite material is dispersed in a solvent containing a metal salt, and then a reducing agent, such as ethanol, is added to the solution to precipitate the metal particles on the surface of the polycrystalline ferrite material.

[0040] In the case of supporting metal oxide particles, for example, a polycrystalline ferrite material is dispersed in a solvent in which an organometallic compound such as TEOS is dissolved, and then the metal oxide particles are precipitated on the surface of the polycrystalline ferrite material by adjusting the pH of the solution or by heating and drying it.

[0041] (Method for Decomposing Pollutants) The photocatalyst of this embodiment can be used as a photocatalyst for decomposing pollutants. Next, a method for decomposing pollutants will be described using contaminated water containing pollutants as an example.

[0042] The method for decomposing pollutants using a photocatalyst of this embodiment includes the steps of contacting contaminated water containing pollutants with the photocatalyst under a light irradiation environment to decompose the pollutants, and separating the contaminated water from the photocatalyst. The photocatalyst may be in the form of nanoparticles or nanofibers, fixed to the surface of a substrate, or in the form of a molded body.

[0043] When the photocatalyst is in the form of nanoparticles or nanofibers, the photocatalyst is dispersed in contaminated water, and after decomposing the contaminants under light irradiation, the photocatalyst and the contaminated water are separated. Various methods used for solid-liquid separation, such as filtration, decantation, and centrifugation, can be used to separate the photocatalyst and the contaminated water. Because nanoparticles or nanofibers have a large surface area, they decompose contaminants more efficiently. When the polycrystalline ferrite material used as the photocatalyst is ferromagnetic, they can be separated using magnetic force, such as by being attracted to a magnet. By using magnetic force, the fine polycrystalline ferrite material can be separated and recovered with high efficiency, reducing the amount of photocatalyst mixed into the contaminated water after treatment. This reduces the burden on the environment.

[0044] When the photocatalyst is fixed to the surface of a substrate or in the form of a molded body, the photocatalyst is brought into contact with contaminated water, the contaminants are decomposed under a light irradiation environment, and then the photocatalyst and the contaminated water are separated. When the photocatalyst is fixed to the surface of a substrate or in the form of a molded body, separation of the photocatalyst and the contaminated water is facilitated. When the photocatalyst is a nonwoven fabric, decomposition of contaminants and separation of the photocatalyst and the contaminated water can be continuously performed by passing the contaminated water through the nonwoven fabric under a light irradiation environment.

[0045] (Information Processing Device) The present disclosure also relates to an information processing device. The information processing device of this embodiment transmits at least photocatalyst information on a photocatalyst, ferroelectricity-related information on the coercive electric field Ec, and photocatalytic activity-related information indicating photocatalytic activity based on the relationship between the photocatalyst-related information and the ferroelectricity-related information. This allows users of photocatalysts to accurately understand the technical significance of photocatalysts that utilize ferroelectricity.

[0046] (Chemicals used) Bismuth nitrate (Bi(NO ３ ) ３ ・5H ２ O, ≥ 98%), iron nitrate (Fe(NO ３ ) ３ ・9H ２ O, ≥ 98%), nickel sulfate (NiSO ４ ・6H ２ O, ≧98%), nitric acid (HNO ３ , 65%) and memalachite green oxalate (MG, ≥90%) were all obtained from Sigma-Aldrich. Potassium hydroxide (KOH, 85%) and urea (99.5%) were purchased from Kanto Chemical Co., Ltd. All reagents were used as received. Deionized water (Milipore System, 18.2 Ω) was used as the solvent.

[0047] Example 1: Preparation of bismuth ferrite nanoparticles 2.43 g of Bi(NO ３ ) ３ ・5H ２ O and 2.02 g of Fe(NO ３ ) ３ ・9H ２ 0, 0.3 g urea, and 2 mL HNO ３The mixture was mixed and then deionized water was added to a final volume of 20 mL. Continuous stirring was performed until all chemicals were completely dissolved. The homogenized solution was then mixed with 60 mL of 14 M KOH aqueous solution. The resulting mixture was transferred to a polytetrafluoroethylene-lined autoclave reactor and heated at 180 °C for 24 hours. After cooling to room temperature, the resulting product was centrifuged, washed with ethanol and deionized water, and finally dried in an oven at 80 °C to obtain nanoparticles. The resulting nanoparticles were thermally annealed in a muffle furnace at 600 °C in an air atmosphere for 2 hours. The annealed nanoparticles were then poled by applying an electric field via corona discharge at an applied voltage of 1000 kV / cm.

[0048] (Example 2) Preparation of bismuth ferrite nanofibers 4.0 g of Bi(NO ３ ) ３ ・5H ２ O and 3.03 g of Fe(NO ３ ) ３ ・9H ２ A bismuth ferrite sol-gel solution was prepared by dissolving bismuth ferrite in 10 mL of 2-methoxyethanol. The pH of the resulting sol-gel solution was adjusted to 3.0-4.0 by adding 0.05 mL of ethanolamine. Five mL of glacial acetic acid was then added to adjust the viscosity of the sol-gel solution. The pH- and viscosity-adjusted sol-gel solution was stirred at room temperature for approximately 2 hours to prepare Solution A. A second solution, Solution B, was prepared by adding 2 g of polyvinylpyrrolidone (PVP) with a mass-average molecular weight (Mw) of 360,000 to 11 g of a dimethylformamide (DMF) / ethanol (1:1 wt / wt) solvent mixture. The resulting Solution B was mechanically stirred for approximately 1 hour. Solution A was then added dropwise to Solution B under constant stirring conditions to obtain a homogeneous feedstock solution for the electrospinning process.

[0049] Bismuth ferrite primary fibers were spun using a nanofiber electrospinning unit (NEU) purchased from Kato Tech Co., Ltd. (Japan) and the resulting feedstock solution. The spinning conditions for the electrospinning unit were an applied voltage of 12 kV, a distance between the needle tip and the collector of approximately 10 cm, and a feedstock solution supply rate of 0.1 mm / min. The primary fibers were dried at 100°C for 1 hour in a thermostatic oven to obtain nanofibers. The resulting nanofibers were thermally annealed in an air atmosphere at 600°C for 2 hours in a muffle furnace. The annealed nanofibers were subjected to a poling treatment using an electric field applied by corona discharge at an applied voltage of 1000 kV / cm.

[0050] [XRD Measurement] The nanoparticles after the poling treatment obtained in Example 1 and the nanofibers after the poling treatment obtained in Example 2 were subjected to XRD measurement under the following conditions. ３ ) XRD chart of the nanoparticles before poling treatment, and XRD chart of BiFeO ３ The standard XRD chart (JCPDS card #71-2494) of the nanofiber (BiFeO) obtained in Example 2 after poling treatment is shown in FIG. ３ ) XRD chart and BiFeO ３ The standard XRD chart (jCPDS card #71-2494) of the above is shown.

[0051] Equipment used: Rigaku MiniFlex Method: 2θ-θ reflection method X-ray used: Cu-Kα ray Scan speed: 1.00° / min Sampling interval: 0.10° Slit width: DS: (variable), SS: 4.2°, RS: 0.3 mm

[0052] From the XRD chart of FIG. 1, it can be seen that the nanoparticles obtained in Example 1 after the poling treatment are BiFeO ３ In addition, the XRD chart of the nanoparticles before poling treatment and the BiFeO ３ Comparison with the standard XRD chart of the nanoparticles after the poling treatment revealed that the perovskite structure was stably maintained.

[0053] From the XRD chart of FIG. 2, the nanofibers obtained in Example 2 after the poling treatment were BiFeO ３ It became clear that...

[0054] [SEM Observation] The nanoparticles obtained in Example 1 after the poling treatment and the nanofibers obtained in Example 2 after the poling treatment were observed using an SEM. ３ ) obtained in Example 2 is shown in Fig. 4. ３ ) shows an SEM image. SEM observation confirmed that the nanoparticles obtained in Example 1 were polycrystalline particles with secondary particle diameters of approximately 1 to 10 μm and primary particle diameters in the range of 1 to 1,000 nm. SEM observation confirmed that the nanofibers obtained in Example 2 had diameters in the range of 1 to 1,000 nm, lengths in the range of 1 to 10 μm, and aspect ratios of 5 or more.

[0055] [Measurement of Optical Absorption Spectra] The optical absorption spectra were measured for the nanoparticles after poling treatment obtained in Example 1 and the nanofibers after poling treatment obtained in Example 2. From the obtained optical absorption spectra, it was confirmed that the band gaps of the nanoparticles obtained in Example 1 and the nanofibers obtained in Example 2 were 2.0 to 2.5 eV.

[0056] [Measurement of Coercive Field Ec] The ferroelectric properties (hysteresis loops) of the nanofibers obtained in Example 2 after thermal annealing and before poling were measured using a piezoresponse force microscope (PFM) equipped with an MFP-3D probe microscope (Asylum Research, Oxford Instruments, UK). The coercive field Ec was calculated from the obtained hysteresis loops. The nanofibers were mounted on the PFM stage using silver paste. The hysteresis loops were measured using a PPP-NCSTPt-20 PFM probe (Nanosensors, Nanoworld AG, Switzerland) in dual AC resonance tracking PFM mode. The hysteresis loops were measured at four locations (Locations 1 to 4) on the nanofiber. The results are shown in Figure 5. The coercive field Ec obtained from each hysteresis loop shown in Figure 5 was 0.45 MV / m at Location 1, 1.7 MV / m at Location 2, and 0.5 MV / m at Location 3. The electrical resistivity was 0.7 MV / m at Loc. 3 and 0.6 MV / m at Loc. 4. From these results, it can be seen that the BiFeO ３ It was confirmed that the coercive field Ec ranged from 0.45 MV / m to 1.7 MV / m.

[0057] [Confirmation of Photocatalytic Activity] (Evaluation of Photocatalytic Performance Under Light Irradiation in the Photodecomposition of Methylene Blue (MB)) Using the nanoparticles obtained in Example 1 as a sample, the photocatalytic performance under light irradiation in the photodecomposition of methylene blue (MB) was evaluated. 20 mL of an MB aqueous solution (concentration: 20 μM) was poured into a quartz tube, and 30 mg of the sample was added to the MB aqueous solution. After the addition, the sample was left to stand in the dark for 120 minutes to achieve adsorption-desorption equilibrium, and then the MB solution was irradiated with 500 mW / cm ２ The photodegradation experiment of MB was carried out by irradiating it with white light using a xenon lamp (LCS-100, 94011A, Newport) operating at 1000 rpm. At designated time intervals, 0.2 mL of the MB aqueous solution was collected and centrifuged to remove the solids dispersed in the solution. The MB concentration (C ｔTo determine the absorbance of MB in aqueous solution at λ = 664 nm, the absorbance was measured using a UV-vis spectrometer (PD-3000UVe, Apel). The results are shown in Figure 6. The photodegradation experiment of MB was carried out using the nanoparticles before and after the poling treatment.

[0058] In FIG. 6, the horizontal axis represents the irradiation time of white light, and the vertical axis (C ｔ / C ０ ) is the MB concentration at the start of the MB photodegradation experiment (C ０ ) is set to 1.0. The results in Figure 6 show that the nanoparticles after poling treatment have a higher effect of promoting the decomposition of MB under light irradiation than the nanoparticles before poling treatment, and are therefore useful as photocatalysts.

[0059] (Evaluation of photocatalytic performance under light irradiation environment in the photodecomposition of indigo blue (IB)) Using the nanoparticles obtained in Example 1 and the nanofibers obtained in Example 2 as samples, the photocatalytic performance under light irradiation environment in the photodecomposition of indigo blue (IB) was evaluated. 20 mL of an IB aqueous solution (concentration: 50 μM) was poured into a quartz tube, and 10 mg of the sample was added to the IB aqueous solution. After the addition, the sample was left to stand in the dark for 120 minutes to achieve adsorption-desorption equilibrium, and then the IB solution was irradiated with 500 mW / cm ２ The photodegradation experiment of IB was carried out by irradiating it with white light using a xenon lamp (LCS-100, 94011A, Newport) operating at 1000 rpm. At designated time intervals, 0.2 mL of the aqueous IB solution was sampled and centrifuged to remove the solids dispersed in the solution. The IB concentration (C ｔ To determine the absorbance of IB, the absorbance of the aqueous IB solution at λ = 611 nm was measured using a UV-vis spectrometer (PD-3000UVe, Apel). The results are shown in Figures 7 and 8. Figure 7 shows the results of a photodegradation experiment of IB using the nanoparticles before and after poling treatment obtained in Example 1, and Figure 8 shows the results of a photodegradation experiment of IB using the nanofibers before and after poling treatment obtained in Example 2.

[0060] 7 and 8, the horizontal axis represents the irradiation time of white light, and the vertical axis (Cｔ / C ０ ) is the IB concentration at the start of the IB photodecomposition experiment (C ０ ) is set to 1.0, which is the relative concentration of IB. From the results of Figures 7 and 8, it can be seen that the nanoparticles and nanofibers after the poling treatment have a higher decomposition promoting effect under the light irradiation environment of IB than before the poling treatment, and the photocatalytic action is improved. In particular, as shown in Figure 7, it can be seen that the nanoparticles after the poling treatment have an 11% improvement in decomposition efficiency under the light irradiation environment of IB compared to the nanoparticles before the poling treatment. This is because the decomposition efficiency of the poled polycrystalline BiFeO ３ This is because the electrons and holes generated by light irradiation in the nanoparticles and nanofibers do not easily recombine, making it easier for hydroxyl radicals to be generated.

[0061] [Additional remarks] Photocatalytic nanoparticles and nanofibers made of the polycrystalline ferrite material disclosed herein can have excellent photocatalytic activity when an electric field is applied, and are also easy to recover, which can contribute to the achievement of Goal 9 of the Sustainable Development Goals (SDGs) led by the United Nations. Goal 9: "Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation."

[0062] The following additionally describes embodiments of the present disclosure.

[0063] [Appendix 1] A photocatalyst containing a polycrystalline ferrite material, the polycrystalline ferrite material having a coercive electric field Ec. The polycrystalline ferrite material contained in the photocatalyst of Appendix 1 has a coercive electric field Ec and is polarized, so electrons and holes generated by irradiation with light are less likely to recombine. This makes it easier for excited species such as hydroxyl radicals to be generated. Therefore, the photocatalyst of Appendix 1 exhibits good activity.

[0064] [Appendix 2] The photocatalyst according to Appendix 1, wherein the polycrystalline ferrite material has a perovskite crystal structure. The polycrystalline ferrite material contained in the photocatalyst of Appendix 2 has a perovskite crystal structure, and therefore has high polarizability and a stable crystal structure. Therefore, the photocatalyst of Appendix 2 exhibits good activity for a longer period of time.

[0065] [Supplementary Note 3] The photocatalyst according to Supplementary Note 1 or 2, wherein the polycrystalline ferrite material is an annealed product. The polycrystalline ferrite material contained in the photocatalyst of Supplementary Note 3 is an annealed product, and has high crystallinity and improved polarizability. Therefore, the photocatalyst of Supplementary Note 3 exhibits good activity for an even longer period of time.

[0066] [Appendix 4] The photocatalyst according to any one of Appendices 1 to 3, wherein the polycrystalline ferrite material is in the form of nanoparticles. Since the polycrystalline ferrite material contained in the photocatalyst of Appendices 4 is in the form of nanoparticles, it is easy to disperse in liquids and can be molded into various shapes. Therefore, the photocatalyst of Appendices 4 can be used for various purposes.

[0067] [Appendix 5] The photocatalyst according to Appendix 4, wherein the nanoparticles of polycrystalline ferrite material have an average particle size of 1 nm to 1000 nm. Since the average particle size of the nanoparticles contained in the photocatalyst of Appendix 5 is within the above range, the surface area is large. Therefore, the photocatalyst of Appendix 5 exhibits further activity.

[0068] [Appendix 6] The photocatalyst according to any one of Appendices 1 to 3, wherein the polycrystalline ferrite material is a nanofiber. Because the polycrystalline ferrite material contained in the photocatalyst of Appendices 6 is a nanofiber, it can be molded into a shape such as a nonwoven fabric. Furthermore, when dispersed in contaminated water, nanofibers have a higher recovery efficiency through a filter than nanoparticles, and filter clogging is suppressed, making them easier to handle. The photocatalytic activity of nanofibers can be improved by forming the nanofibers so that pores or cavities are formed.

[0069] [Appendix 7] The photocatalyst according to Appendix 6, wherein the nanofibers of the polycrystalline ferrite material have an average diameter of 1 nm or more and 1000 nm or less, and a ratio of average length to average diameter (average length / average diameter) of 100 or more. The nanofibers contained in the photocatalyst of Appendix 7 have an average diameter and an average length / average diameter ratio within the above ranges, and therefore can be formed into a nonwoven fabric with high strength.

[0070] [Appendix 8] The photocatalyst according to Appendix 6 or 7, wherein the nanofibers of the polycrystalline ferrite material have pores and / or cavities observed by surface observation using an SEM. The nanofibers contained in the photocatalyst of Appendix 8 have large surface irregularities and a large surface area. Therefore, the photocatalyst of Appendix 8 exhibits further activity.

[0071] [Appendix 9] The photocatalyst according to any one of Appendices 6 to 8, which comprises a plurality of nanofibers of a polycrystalline ferrite material, the plurality of nanofibers being entangled with one another to form a nonwoven fabric. The polycrystalline ferrite material contained in the photocatalyst of Appendices 9 forms a nonwoven fabric, and therefore has high shape stability. Therefore, the photocatalyst of Appendices 9 exhibits good activity for an even longer period of time.

[0072] [Appendix 10] A method for producing a photocatalyst according to any one of Appendices 1 to 9, comprising the steps of: a first step of preparing a polycrystalline ferrite material; a second step of annealing the prepared polycrystalline ferrite material; and a third step of applying an electric field to the annealed polycrystalline ferrite material. According to the method for producing a photocatalyst according to Appendices 10, the polycrystalline ferrite material is annealed to improve the crystallinity of the polycrystalline ferrite material before applying an electric field, so that the resulting polycrystalline ferrite material has high polarity. Therefore, a photocatalyst exhibiting good activity can be produced industrially advantageously.

[0073] [Appendix 11] The method for producing a photocatalyst according to Appendix 10, wherein the method for applying an electric field in the third step is a corona discharge treatment method. According to the method for producing a photocatalyst according to Appendix 11, the electric field is applied by a corona discharge treatment method, so that the photocatalyst can be polarized without being dispersed in the resin, which makes it less likely that localized dielectric breakdown will occur when the electric field is applied, and allows the photocatalyst to be polarized uniformly. Furthermore, corona discharge treatment is a simple method.

[0074] [Appendix 12] The method for producing a photocatalyst according to Appendix 10 or 11, wherein the polycrystalline ferrite material is nanoparticles, and the first step is a step of producing nanoparticles of the polycrystalline ferrite material by a solvothermal method. According to the method for producing a photocatalyst according to Appendix 12, the solvothermal method is used to produce the nanoparticles, and therefore nanoparticles with a stable size can be produced.

[0075] [Appendix 13] The method for producing a photocatalyst according to Appendix 10 or 11, wherein the polycrystalline ferrite material is a nanofiber, and the first step is a step of producing nanofibers of the polycrystalline ferrite material by an electrospinning method. According to the method for producing a photocatalyst according to Appendix 13, since the electrospinning method is used to produce the nanofibers, nanofibers with a stable size can be produced.

[0076] [Appendix 14] The method for producing a photocatalyst according to Appendix 13, wherein the step of producing nanofibers of a polycrystalline ferrite material by electrospinning is a step of obtaining nanofibers of a polycrystalline ferrite material by electrospinning using a raw material liquid containing a precursor of the polycrystalline ferrite material and a binder resin. According to the method for producing a photocatalyst according to Appendix 14, since a raw material liquid containing a precursor of the polycrystalline ferrite material and a binder resin is used, the density of the obtained nanofibers and the proportion of pores and / or cavities on the surface can be adjusted.

[0077] [Supplementary Note 15] A method for decomposing pollutants, comprising the steps of contacting contaminated water containing the pollutants with the photocatalyst of Supplements 1 to 9 in a light irradiation environment to decompose the pollutants, and separating the contaminated water from the photocatalyst. The pollutant decomposition method of Supplementary Note 15 uses the photocatalyst, and therefore has a high ability to decompose pollutants.

[0078] [Supplementary Note 16] The pollutant decomposition method according to Supplementary Note 15, wherein the photocatalyst is a nanoparticle, the step of decomposing the pollutants is carried out by dispersing the photocatalyst in the polluted water, and the step of separating the contaminated water from the photocatalyst is carried out by a solid-liquid separation method. The nanoparticles used in the pollutant decomposition method of Supplementary Note 16 are fine and have a large surface area. Therefore, the pollutant decomposition method of Supplementary Note 16 has a higher pollutant decomposition ability.

[0079] [Appendix 17] The pollutant decomposition method according to Appendix 15, wherein the photocatalyst is a nonwoven fabric in which a plurality of nanofibers are entangled with each other, and the step of decomposing the pollutants and the step of separating the contaminated water from the photocatalyst are carried out by passing the contaminated water through the nonwoven fabric. The photocatalyst used in the pollutant decomposition method according to Appendix 17 is such that the nanofibers are unlikely to be washed away even when the contaminated water is passed through it. Therefore, the pollutant decomposition method according to Appendix 17 imposes a small burden on the environment.

[0080] [Appendix 18] A nonwoven fabric in which a plurality of nanofibers are entangled with each other, the nanofibers containing a polycrystalline ferrite material, and the polycrystalline ferrite material having a coercive electric field Ec. Because the nonwoven fabric of Appendix 18 is polarized, excited species such as hydroxyl radicals are easily generated, and the nanofibers are less likely to flow out even when contaminated water is passed through it. Therefore, the nonwoven fabric of Appendix 18 is useful as a decomposition and removal agent for contaminants contained in contaminated water.

[0081] [Supplementary Note 19] An information processing device that outputs at least photocatalytic information on a photocatalyst, ferroelectricity-related information on a coercive electric field Ec, and photocatalytic activity-related information that indicates photocatalytic activity based on the relationship between the photocatalyst-related information and the ferroelectricity-related information. According to the information processing device of Supplementary Note 19, it is possible to accurately inform users of photocatalysts of the technical significance of photocatalysts that utilize ferroelectricity.

Claims

1. A photocatalyst comprising a polycrystalline ferrite material, wherein the polycrystalline ferrite material has a coercive electric field Ec of 0.1 MV / m or more and 2.0 MV / m or less.

2. The photocatalyst of claim 1, wherein the polycrystalline ferrite material has a perovskite crystal structure.

3. The photocatalyst according to claim 1 or 2, wherein the polycrystalline ferrite material is in the form of nanoparticles.

4. The photocatalyst according to claim 1 or 2, wherein the polycrystalline ferrite material is a nanofiber.

5. A method for producing a photocatalyst, comprising: a first step of preparing a polycrystalline ferrite material; a second step of annealing the polycrystalline ferrite material; and a third step of applying an electric field to the polycrystalline ferrite material.

6. The method for producing a photocatalyst according to claim 5, wherein the method for applying an electric field in the second step is a corona discharge treatment method.

7. The method for producing a photocatalyst according to claim 5 or 6, wherein the polycrystalline ferrite material is nanoparticles, and the first step is a step of producing nanoparticles of the polycrystalline ferrite material by a solvothermal method.

8. The method for producing a photocatalyst according to claim 5 or 6, wherein the polycrystalline ferrite material is a nanofiber, and the first step is a step of producing the nanofiber of the polycrystalline ferrite material by an electrospinning method.

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