Photocatalyst particles, method for producing photocatalyst particles, and method for recovering photocatalyst particles

Abstract

These photocatalyst particles comprise polycrystalline particles of an oxide selected from the group consisting of MFeO3, M2Fe4O9, and M25FeO39 (where M is at least one selected from the group consisting of Bi, Ga, Ni, Co, and Zn). The coefficient of variation in the aspect ratio of the crystal particles constituting the polycrystalline particles is 0.35 to 1.0.

Description

Photocatalyst particles, photocatalyst particle manufacturing method, and photocatalyst particle recovery method

[0001] The present invention relates to photocatalyst particles, a method for producing photocatalyst particles, and a method for recovering photocatalyst particles.

[0002] In recent years, there has been active development in the use of photocatalytic activity. When photocatalyst particles are irradiated with light, the oxidizing power of holes generated by photoexcitation and the strong reducing power of electrons decompose and purify harmful substances, deodorize malodorous gases, and sterilize bacteria, making them useful for environmental purification. Patent Documents 1 to 4 disclose oxides containing Bi as photocatalysts.

[0003] CN104646001 publication CN104941662 publication CN106807400 publication CN108114736 publication

[0004] An object of the present invention is to provide novel photocatalyst particles, a method for producing the same, and a method for recovering the same.

[0005] [1] MFeO ３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９

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[0099] ３ , M ２ Fe ４ O ９, and M ２５ FeO ３９ [6] A method for producing photocatalyst particles, comprising a step of obtaining polycrystalline particles of an oxide selected from the group consisting of (wherein M is at least one selected from the group consisting of Bi, Ga, Ni, Co, and Zn). [6] The method according to [5], further comprising a step of supporting at least one metal particle selected from the group consisting of Au, Pd, Ag, and Pt on the surface of the obtained polycrystalline particles. [7] A method for recovering photocatalyst particles, comprising a step of recovering the photocatalyst particles from a composition containing the photocatalyst particles according to [1] or [2] with a magnet.

[0006] According to the present invention, novel photocatalyst particles, a method for producing the same, and a method for recovering the same are provided.

[0007] The particles obtained in Example 1 (BiFeO ３ ) is an XRD chart of the particles (Bi) obtained in Example 2. ２ Fe ４ O ９ ) obtained in Example 1. ３ ) is a SEM image of the particles (Bi) obtained in Example 2. ２ Fe ４ O ９ ) obtained in Comparative Example 1 (BiFeO ３ ) shows SEM images of the particles obtained in Examples 1 and 2. (a) shows the optical absorption spectrum of the particles obtained in Comparative Example 1. Example 1 (BiFeO ３ ), Example 2 (Bi ２ Fe ４ O ９ ), a 1:1 mixture thereof, and Comparative Example 1 (BiFeO ３ 1 shows first-order reaction plots of the methylene blue decolorization ability of the samples plotted against the sampling time for the Ni particles (a) and the BiFeO particles (b) of Example 1. ３ particles, (c) Bi particles of Example 2 ２ Fe ４ O ９ particles, (d) BiFeO ３1 is a J-H curve of the particles. ３ Photographs of the Bi particle layer of Example 1 after a droplet of water was dropped on the particle layer are shown in (a) and (b), respectively. ２ Fe ４ O ９ Photographs are taken before and after droplets of water are dropped onto a particle layer. (a) is a graph showing a histogram of the aspect ratio of the crystal particles of the polycrystalline particles of Example 1, and (b) is a graph showing a histogram of the particle diameter of the crystal particles of the polycrystalline particles of Example 1. (a) is a graph showing a histogram of the aspect ratio of the crystal particles of the polycrystalline particles of Example 2, and (b) is a graph showing a histogram of the particle diameter of the crystal particles of the polycrystalline particles of Example 2. (a) is a graph showing a histogram of the aspect ratio of the crystal particles of the polycrystalline particles of Comparative Example 1, and (b) is a graph showing a histogram of the particle diameter of the crystal particles of Comparative Example 1. (a), (b), and (c) are, respectively, graphs showing the BiFeO supporting Au particles obtained in Example 3. ３ 1 shows SEM images and the results of elemental analysis for polycrystalline particles with Au loadings of 0.6 wt%, 2.0 wt%, and 5.4 wt%. ３ The MB decomposition activity when the Au loading amount of the polycrystalline particles was 0.6 wt%, 2.0 wt%, and 5.4 wt% was compared with that of BiFeO in Example 1, which had no Au loading amount. ３ 1 is a graph comparing the BiFeO particles of Example 1 with the Au particles in air. ３ (b) shows the state in which polycrystalline particles are attracted to a magnet, and (c) shows the state in which Au particles are supported in water by BiFeO ３ This is a photograph showing polycrystalline particles being attracted to a magnet.

[0008] (Photocatalytic particles) Photocatalytic particles according to one embodiment are MFeO ３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９ The photocatalytic particles are provided with polycrystalline particles of an oxide selected from the group consisting of (wherein M is at least one selected from the group consisting of Bi, Ga, Ni, Co, and Zn).

[0009] In other words, the polycrystalline particles are MFeO３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９ The oxide is formed from a large number of crystal grains of an oxide selected from the group consisting of: The crystal grain boundaries exist between the crystal grains, and the presence of the crystal grain boundaries can be visually determined in the SEM image.

[0010] It is preferable that M contains Bi. There are no particular limitations on the atomic ratio of Bi in M, but it may account for 50% or more, or may account for 70% or more.

[0011] (Composition of crystal particles) MFeO of crystal particles ３ , M ２ Fe ４ O ９ , or M ２５ FeO ３９ may be doped with Ca or Dy.

[0012] (Aspect Ratio of Crystal Particles) The aspect ratio r of a crystal particle is defined as follows for each crystal particle observed in an SEM image of a polycrystalline particle: r = major axis of crystal particle / minor axis of crystal particle.

[0013] The long diameter is the maximum distance between two parallel lines when a crystal particle observed in an SEM image is sandwiched between the two lines. The short diameter is the minimum distance between two parallel lines when a crystal particle observed in an SEM image is sandwiched between the two lines. The long diameter and short diameter directions are not necessarily perpendicular to each other.

[0014] The magnification of the SEM image is preferably set so that about 100 to 200 crystal grains can be observed. The aspect ratio to be measured, i.e., the number of crystal grains, is preferably set to 100 to 500.

[0015] (Arithmetic mean of aspect ratio of crystal grains) Arithmetic mean r of aspect ratio r ａｖ is defined as follows:

[0016] There is no particular limitation on the numerical range of the arithmetic mean aspect ratio of the crystal grains, but it may be 1.90 or more, 2.00 or more, 2.30 or more, 10.0 or less, or 9.00 or less.

[0017] (Standard deviation of aspect ratio of crystal grains) Standard deviation r of aspect ratio r of crystal grains ｓｄ is defined as follows:

[0018] (Variation Coefficient of Aspect Ratio of Crystal Grains) The variation coefficient of aspect ratio of crystal grains is defined as follows.

[0019] In this embodiment, the numerical range of the coefficient of variation of the aspect ratio of the crystal grains is 0.35 to 1.0. A coefficient of variation of the aspect ratio of the crystal grains of 0.35 to 1.0 means that the aspect ratio of the crystal grains varies greatly. The coefficient of variation of the aspect ratio of the crystal grains may be 0.40 or more, 0.45 or more, or 0.50 or more. The coefficient of variation of the aspect ratio of the crystal grains may be 0.95 or less, 0.90 or less, 0.85 or less, or 0.80 or less.

[0020] (Crystal particle size, average particle size, standard deviation of particle size, and coefficient of variation of particle size) The particle size of crystal particles is the arithmetic mean of the major axis and minor axis of crystal particles in an SEM image. The average particle size of crystal particles is the arithmetic mean of the particle sizes of 100 or more crystal particles. The coefficient of variation of the particle size of crystal particles is defined as the standard deviation of the particle size of crystal particles / the average particle size of crystal particles. The average particle size of crystal particles is not particularly limited, but may be, for example, 1 to 1,000 nm. The average particle size of crystal particles may be 2 nm or more, 5 nm or more, 10 nm or more, 20 nm or more, 50 nm or more, 100 nm or more, 150 nm or more, 200 nm or more, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, or 400 nm or less. The coefficient of variation of the grain size of the crystal grains is not particularly limited, but is preferably, for example, 0.35 to 1.0. The coefficient of variation may be 0.30 or more, or 0.40 or more.

[0021] (Shape of Crystal Particles) There are no particular limitations on the shape of the crystal particles, and they may be, for example, spherical, rod-like, flake-like, flower-like, or amorphous, or may include any combination of two or more of these.

[0022] (Average particle size of polycrystalline particles) The polycrystalline particles are MFeO ３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９ The oxide is formed from a large number of crystalline particles of an oxide selected from the group consisting of:

[0023] There is no particular limitation on the particle size of the polycrystalline particles that are secondary particles (aggregates), and it may be, for example, 1 μm or more and 10 μm or less.

[0024] In this specification, the particle size of polycrystalline particles is the arithmetic mean of the major axis and minor axis of polycrystalline particles in an SEM image, and the average particle size of polycrystalline particles is the arithmetic mean of the particle sizes of 100 or more polycrystalline particles.

[0025] (Supporting of co-catalyst) The photocatalyst particles may have metal particles supported on the surface of the polycrystalline particles. The metal in the metal particles may be any one selected from the group consisting of Au, Pd, Ag, and Pt, or an alloy of any combination thereof. The surface of the metal particles may be covered with an oxide such as silica. This structure is called a core-shell structure. Examples of metal particles with a core-shell structure are Au core-silica shell particles and Ag core-silica shell particles.

[0026] The average particle size of the metal particles is not limited, but can be 1 to 10 nm. The particle size of the metal particles is the arithmetic mean of the major and minor axes of the particles in an SEM image. The average particle size of the metal particles is the arithmetic mean of the particle sizes of 100 or more particles.

[0027] There is no particular limitation on the amount of metal particles that serve as promoters, but for example, the amount of metal particles supported can be 0.2 to 10 mass % relative to the total mass of the polycrystalline particles and metal particles.

[0028] (Ferromagnetic) The polycrystalline particles according to this embodiment are ferromagnetic. Ferromagnetic means that a J-H curve (magnetization-external magnetic field curve) obtained by VSM measurement or the like exhibits a loop with hysteresis. The coercive force HcJ of the polycrystalline particles may be 1 kOe or more.

[0029] (Ferroelectricity) The polycrystalline particles according to this embodiment exhibit ferroelectricity in addition to ferromagnetism. Ferroelectricity refers to the electric field-polarization curve exhibiting a loop with hysteresis. The coercive field of the polycrystalline particles may be 1 kV / mm or more.

[0030] (Band Gap) The polycrystalline particles according to this embodiment may have a band gap of 1.0 to 3.5 eV.

[0031] (Mechanism of Action) When ultrasonic waves or ultraviolet or visible light are applied to the photocatalytic particles of this embodiment, excited species such as OH radicals are generated, which can decompose harmful substances such as organic compounds that are components of dirt. In particular, in this embodiment, the polycrystalline particles are made of a specific metal oxide, and the coefficient of variation of the aspect ratio of the crystal particles is appropriately controlled, which leads to a widening of the band gap and is thought to enhance the photocatalytic activity. In addition, since the polycrystalline particles have ferromagnetic and ferroelectric properties, they have the effect of suppressing the recombination of carriers and holes, which enhances the photocatalytic effect.

[0032] The photocatalyst according to this embodiment can be fixed to the surface of a substrate such as metal, tile, enamel, cement, concrete, glass, fiber, wood, paper, or plastic, and then subjected to a photocatalytic reaction. Fixation can be achieved by conventional methods such as sintering the photocatalyst or using a binder. Furthermore, the photocatalyst particles can be mixed with a binder as needed, and molded into a flat, corrugated, honeycomb, spherical, curved, or other shape for use in a photocatalytic reaction.

[0033] (Method for Producing Photocatalyst Particles) A ​​method for producing the photocatalyst particles according to one embodiment will be described.

[0034] The method for producing photocatalyst particles according to this embodiment involves heating a solution of a salt α of at least one metal selected from the group consisting of Bi, Ga, Ni, Co, and Zn and a salt β of Fe to produce MFeO ３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９ (wherein M is at least one selected from the group consisting of Bi, Ga, Ni, Co, and Zn). This process is sometimes referred to as a solvothermal process.

[0035] As the solvent, water, ethylene glycol, etc. can be used, but the so-called hydrothermal synthesis method, which uses water as the solvent, is particularly preferred.

[0036] The metal salt α and the metal salt β are not particularly limited and may each independently be a nitrate, sulfate, hydrogensulfate, acetate, chloride, phosphate, hydrogenphosphate, hydrogencarbonate, carbonate, or the like, but it is preferable that both the metal salt α and the metal β are nitrates.

[0037] MFeO ３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９ When different types of M and Fe are prepared, the atomic ratios of M and Fe may be set to 1:1, 2:4, and 25:1, respectively.

[0038] In this step, first, a solution before heating is prepared.

[0039] Specifically, metal salt α and metal salt β are sufficiently dissolved in a solvent such as water. From the viewpoint of sufficiently dissolving metal salts α and β in the solvent, the pH of the solvent during dissolution is preferably adjusted to 3 to 4 by adding an acid such as nitric acid. Appropriate stirring is preferably performed to ensure sufficient dissolution.

[0040] From the viewpoint of suitably controlling the coefficient of variation of the aspect ratio of the crystal particles, etc., it is preferable to add a base such as KOH to the solution after dissolving the metal salts α and β in the solvent and before heating, which will be described later, to increase the pH of the solution to 12 or more.

[0041] The solvent before or after the addition of the base may further contain a compound that generates an acid or a base when heated in a subsequent step. An example of such a compound is urea, which generates NH ４ and carbon dioxide, which can further increase the pH of the solution.

[0042] The resulting solution is then heated. The heating temperature is not particularly limited, but is preferably 130° C. or higher and 250° C. or lower from the viewpoint of suitably controlling the coefficient of variation of the aspect ratio of the crystal particles. The temperature is preferably lower than the critical temperature of the solvent.

[0043] The pressure during heating is preferably a pressure that can maintain the solvent in a liquid state during heating. Usually, by heating the solvent to the above temperature in a sealed container such as an autoclave, a pressurized state according to the temperature can be obtained by saturated vapor of the solvent.

[0044] The heating time is not particularly limited, but can be from 60 minutes to 48 hours.

[0045] After heating, the solvent containing the solid is cooled to room temperature, and then subjected to solid-liquid separation by centrifugation or the like. If necessary, the solid is washed with ethanol or water and dried to obtain MFeO. ３ , M ２ Fe ４ O ９ , and M ２５ FeO ３９ It is possible to obtain polycrystalline particles of an oxide selected from the group consisting of:

[0046] (Supporting of Promoter) If necessary, metal particles or metal oxide particles that function as a promoter can be supported on the surface of the obtained polycrystalline particles.

[0047] To support metal particles on the surface of polycrystalline particles, for example, the polycrystalline particles may be dispersed in a solvent containing a metal salt, and then a reducing agent, such as ethanol, may be added to the solution to precipitate the metal particles on the surface of the polycrystalline particles.

[0048] When metal oxide particles are supported on the surfaces of polycrystalline particles, for example, the polycrystalline particles are dispersed in a solvent in which an organometallic compound such as TEOS is dissolved, and then the metal oxide particles are precipitated on the surfaces of the polycrystalline particles by adjusting the pH of the solution, heating and drying, or the like.

[0049] (Method for recovering photocatalyst particles) The method for recovering photocatalyst particles according to this embodiment includes a step of recovering photocatalyst particles from the composition containing the above-described photocatalyst particles using a magnet.

[0050] The photocatalyst particles according to this embodiment are ferromagnetic and can be separated by being attracted to a magnet. Compared to photocatalyst particles such as titanium oxide, they are easier to recover from waste photocatalyst products.

[0051] Specifically, the method can be used when separating polycrystalline particles from a composition containing polycrystalline particles and a solvent after the synthesis of the polycrystalline particles, when separating co-catalyst-supported polycrystalline particles from a composition containing co-catalyst-supported crystalline particles and a solvent after the co-catalyst is supported on the polycrystalline particles, and when recovering the photocatalyst particles after putting the photocatalyst particles in wastewater to decompose environmental pollutants, etc.

[0052] (Chemicals used) Bismuth nitrate (Bi(NO ３ ) ３ ・5H ２ O, 98%), iron nitrate (Fe(NO ３ ) ３ ・9H ２ 0, 98%), methanol (98%), tetrachloroaurate(III) trihydrate (HAuCl ４ ・3H ２ O, 99.5%), nitric acid (HNO ３ , 65%) aqueous solution and methylene blue (MB, ≥95%) 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, and deionized water (Milipore System, 18.2 Ω) was used as the solvent.

[0053] (Example 1) BiFeO ３ Preparation of polycrystalline particles BiFeO ３ In the case of 0.005 mol of Bi(NO ３ ) ３ ・5H ２ O, 0.005 mol Fe(NO ３ ) ３ ・9H ２ 0, 0.05 mol urea, 2 mL HNO ３ The mixture was mixed and then deionized water was added to a final volume of 20 ml. Stirring was continued 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, then washed with ethanol and deionized water, and finally dried in an oven at 80 °C to obtain BiFeO of Example 1.３ particles were obtained.

[0054] (Comparative Example 1) BiFeO ３ Preparation of Polycrystalline Particles BiFeO of Comparative Example 1 was prepared in the same manner as in Example 1, except that the reaction in the autoclave was heated at 125°C for 24 hours. ３ particles were obtained.

[0055] (Example 2) Bi ２ Fe ４ O ９ Preparation of polycrystalline particles Bi ２ Fe ４ O ９ In the case of Bi(NO ３ ) ３ ・5H ２ O 0.006 mol, Fe(NO ３ ) ３ ・9H ２ 0.006 mol of O, 0.1 mol of urea, HNO ３ 2 mL of each compound was mixed and then added with deionized water to a total volume of 20 mL. Continuous stirring was performed until all chemicals were completely dissolved. The homogenized solution was then mixed with 60 mL of 10 M KOH aqueous solution. The resulting mixture was transferred to a polytetrafluoroethylene-lined autoclave reactor and heated at 200 °C for 12 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 particles.

[0056] [XRD Measurement] FIGS. 1 and 2 show the particles (BiFeO ３ ) and (Bi obtained in Example 2 ２ Fe ４ O ９ The XRD chart obtained by XRD measurement of the sample was shown below under the following conditions: Instrument used: MiniFlex manufactured by Rigaku Corporation 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

[0057] From the XRD chart of FIG. 1, the particles synthesized in Example 1 are BiFeO ３The particles synthesized in Comparative Example 1 were also found to be BiFeO ３ It became clear that...

[0058] From the XRD chart of FIG. 2, the particles synthesized in Example 2 are Bi ２ Fe ４ O ９ It became clear that...

[0059] [SEM Observation] Figures 3 and 4 show SEM photographs of particles obtained in Examples 1 and 2. Figure 5 shows an SEM photograph of particles obtained in Comparative Example 1. The sample synthesized in Example 1 was confirmed to be polycrystalline particles with a secondary particle diameter of approximately 1 to 10 μm, and the crystal particle diameter was confirmed to be nanoparticles of 1 to 1,000 nm. The sample synthesized in Example 2 was confirmed to be polycrystalline particles with a secondary particle diameter of approximately 1 to 10 μm, and the crystal particle diameter was confirmed to be nanoparticles of 1 to 1,000 nm. The sample synthesized in Comparative Example 1 was confirmed to be polycrystalline particles with a secondary particle diameter of approximately 1 to 10 μm, and the crystal particle diameter was confirmed to be particles of 500 to 2,000 nm.

[0060] [Optical absorption spectrum measurement] Figure 6(a) shows the optical absorption spectrum of the particles obtained in Examples 1 and 2. The band gap of the sample synthesized in Example 1 was confirmed to be 2.0 to 2.5 eV. The band gap of the samples synthesized in Example 2 was confirmed to be 1.53 eV and 2.05 eV. It was confirmed that the photocatalyst particles can absorb visible light. Figure 6(b) shows the optical absorption spectrum of the particles obtained in Comparative Example 1. The band gap of the sample synthesized in Comparative Example 1 was confirmed to be 2.0 eV to 2.2 eV.

[0061] (Confirmation of photocatalytic activity) To evaluate the photocatalytic performance under visible light in the photodegradation of methylene blue (MB), MB was used as a test pollutant. In a typical procedure, 30 mg of a sample was immersed in an MB aqueous solution (20 mL, 20 μM) in a quartz tube. After achieving adsorption-desorption equilibrium in the dark for 120 minutes, the reaction solution was exposed to 500 mW / cm ２Photodegradation experiments of MB were performed using white light irradiation from a xenon lamp (LCS-100, 94011A, Newport) operating at 1000 Hz. At designated time intervals, 0.2 mL of the reaction solution was extracted and centrifuged to remove any solid particles dispersed in the solution. To determine the MB concentration in the reaction solution, the absorbance of the reaction solution at λ = 664 nm was obtained using a UV-vis spectrometer (PD-3000UVe, Apel).

[0062] FIG. 7 shows the results of Example 1 (BiFeO ３ ) and Example 2 (Bi ２ Fe ４ O ９ ) and a 1:1 mixture thereof, and Comparative Example 1 (BiFeO ３ 1 shows a first-order reaction plot of the methylene blue decolorization ability of the sample versus sampling time for BiFeO in Example 1. ３ , Bi in Example 2 ２ Fe ４ O ９ , and BiFeO of Example 1 ３ and Bi in Example 2 ２ Fe ４ O ９ It was found that all of the 1:1 weight ratio mixtures of BiFeO and BiFeO in Comparative Example 1 exhibited photocatalytic activity. ３ The photocatalytic activity of

[0063] (Confirmation of magnetic properties of particles) The magnetic properties of the particles were evaluated using a vibrating sample magnetometer (VSM). Specifically, measurements were performed at 23°C using a TM-VSM1530-HGC-D manufactured by Tamagawa Seisakusho Co., Ltd. The scanning speed was 200 Oe / s and the scanning range was 1.5 T. 0.07 g (BiFeO ３ particles), or 0.15 g (Bi of Example 2 ２ Fe ４ O ９ The particles were placed in a φ5 mm epoxy sample holder for VSM evaluation.

[0064] In FIG. 8, (a) shows Ni particles as a reference, and (b) shows BiFeO particles of Example 1. ３ particles, (c) Bi particles of Example 2 ２ Fe ４ O ９(d) is BiFeO of Comparative Example 1 ３ The JH curves of the particles show that, as in (a), hysteresis loops were observed in (b), (c), and (d), confirming that the particles were ferromagnetic.

[0065] Water contact angle measurement method (confirmation of superhydrophilicity) BiFeO of Example 1 ３ Particles and Bi of Example 2 ２ Fe ４ O ９ The water contact angle of the particles was measured using an automatic contact angle measuring device (SImage AUTO 100, Excimer Corporation). First, approximately 20 mg of particles were placed on a glass substrate. ３ Particles or Bi ２ Fe ４ O ９ The particle layer was flattened to ensure complete coverage. A water droplet with a volume of 5 μL was placed on the BiFeO ３ Particles or Bi ２ Fe ４ O ９ The droplet was dispensed onto an area completely covered with a particle layer. A side-view photograph of the droplet was then captured. The captured image was then analyzed to quantify the contact angle using software built into the contact angle measurement device.

[0066] FIG. 9 shows the BiFeO ３ This is a photograph of the particle layer after a water droplet was dropped on it.

[0067] (a) and (b) of FIG. 10 are, respectively, the Bi ２ Fe ４ O ９ In the case of the particle layer, the photographs are taken before and after droplets were dropped.

[0068] BiFeO of Example 1 ３ The contact angle of the particles is 10° or less, and the Bi of Example 2 ２ Fe ４ O ９ The contact angle of the particles was less than 10°.

[0069] (Measurement of aspect ratio and particle size of crystal particles constituting polycrystalline particles) The aspect ratio and particle size of 100 crystal particles were determined from SEM images of the polycrystalline particles, and the arithmetic mean of the aspect ratio, the coefficient of variation of the aspect ratio, the average particle size, and the coefficient of variation of the particle size were measured according to the above definitions. The results for each of Example 1, Example 2, and Comparative Example 1 are shown in Table 1.

[0070]

[0071] FIG. 11( a ) is a graph showing a histogram of the aspect ratio of the crystal grains of the polycrystalline grains of Example 1, and FIG. 11( b ) is a graph showing a histogram of the grain size of the crystal grains of the polycrystalline grains of Example 1.

[0072] FIG. 12( a ) shows a histogram of the aspect ratio of the crystal grains of the polycrystalline grains of Example 2, and FIG. 12( b ) shows a histogram of the grain size of the crystal grains of the polycrystalline grains of Example 2.

[0073] FIG. 13( a ) shows a histogram of the aspect ratio of the crystal grains of the polycrystalline grains of Comparative Example 1, and FIG. 13( b ) shows a histogram of the grain size of the crystal grains of the polycrystalline grains of Comparative Example 1.

[0074] (Example 3) BiFeO of Example 1 supporting Au particles ３ Preparation of polycrystalline particles 60 mg of BiFeO obtained in the same manner as in Example 1 ３ The particles were dispersed in a solution containing 30 mL of deionized water and 15 mL of ethanol, followed by the addition of 600 μL of 1 M NaOH solution. ３ The slurry was heated to 50°C, and then 150-750 μL of 10 mM HAuCl was added. ４ The solution was added to form BiFeO ３ The amount of Au decorated on the surface was adjusted. The solution was kept at 50°C for 3 hours. Au particle-supported BiFeO ３The polycrystalline particles were centrifuged, washed with deionized water and ethanol, and then dried in an oven at 80°C for later use. The Au content was evaluated using an energy dispersive X-ray analyzer (EDX, EMAX EX-250, HORIBA) equipped with a scanning electron microscope (SEM, S-4300 SE, Hitachi).

[0075] 14(a), (b), and (c) show, in order, the BiFeO of Example 1 carrying Au particles. ３ The graph shows SEM photographs and the results of elemental analysis when the amount of Au carried in the polycrystalline particles is 0.6 wt %, 2.0 wt %, and 5.4 wt %.

[0076] FIG. 15 shows the BiFeO of Example 1 supporting Au particles. ３ The MB decomposition activity when the Au loading amount of the polycrystalline particles was 0.6 wt%, 2.0 wt%, and 5.4 wt% was compared with that of BiFeO in Example 1, which had no Au loading amount. ３ 1 is a graph comparing the photocatalytic activity of the particles with that of the polycrystalline particles, the procedure being the same as described above. The particles carrying Au particles have a higher photocatalytic activity.

[0077] (Recovery of photocatalyst particles by magnetic force) (1) BiFeO of Example 1 carrying approximately 20 mg of Au particles ３ The polycrystalline particles were placed in a 10 mm diameter glass container. When a neodymium magnet (model: TN22-21K-1P, dimensions: 22 mm × 21 mm × 5 mm, adhesive force: 70.56 N, manufactured by Trusco) was placed close to the container, the particles were clearly attracted to the side of the inner wall. A photograph of this situation is shown in Figure 16(a).

[0078] (2) BiFeO of Example 1 supporting Au particles ３ Pure water was added to the glass container containing the polycrystalline particles, and the water level was adjusted to 20 mm. ３ To ensure uniform distribution of the polycrystalline particles, the mixture was placed in an ultrasonic homogenizer. Next, a neodymium magnet was placed near the glass container. The moment the neodymium magnet came into contact with the glass container, the BiFeO 3 of Example 1 carrying several Au particles was released. ３The polycrystalline particles were immediately attracted to the side of the inner wall. After maintaining the magnet in place for 15 minutes, the BiFeO of Example 1 carrying a large amount of Au particles was obtained. ３ The polycrystalline particles were adsorbed to the inner wall near the neodymium magnet, and the solution became transparent. A photograph of this state is shown in Figure 16(b).

[0079] [Additional remarks] The photocatalytic particles disclosed herein can have photocatalytic activity under visible light and are more energy efficient than existing photocatalysts such as titanium oxide. Therefore, they can contribute to 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."

Claims

1. MFeO ３ , M. ２ Fe ４ O ９ , and M ２５ FeO ３９ A photocatalyst particle comprising polycrystalline particles of an oxide selected from the group consisting of (wherein M is at least one selected from the group consisting of Bi, Ga, Ni, Co, and Zn), wherein the coefficient of variation of the aspect ratio of crystal particles constituting the polycrystalline particles is 0.35 to 1.

0.

2. The photocatalyst particles according to claim 1, wherein the coefficient of variation of the particle size of the crystal particles is 0.35 to 1.

0.

3. Photocatalyst particles according to claim 1 or 2, wherein the arithmetic mean of the aspect ratio of the crystal particles is 1.90 to 10.

0.

4. The photocatalyst particle according to claim 1 or 2, further comprising at least one metal particle selected from the group consisting of Au, Pd, Ag, and Pt supported on the surface of the polycrystalline particle.

5. Heat a solution of a salt of at least one metal selected from the group consisting of Bi, Ga, Ni, Co, and Zn and a salt of Fe to obtain MFeO ３ , M. ２ Fe ４ O ９ , and M ２５ FeO ３９ A method for producing photocatalyst particles, comprising a step of obtaining polycrystalline particles of an oxide selected from the group consisting of (wherein M is at least one selected from the group consisting of Bi, Ga, Ni, Co, and Zn).

6. The method of claim 5, further comprising the step of supporting at least one metal particle selected from the group consisting of Au, Pd, Ag, and Pt on the surface of the resulting polycrystalline particles.

7. A method for recovering photocatalyst particles, comprising a step of recovering the photocatalyst particles from a composition containing the photocatalyst particles according to claim 1 or 2 using a magnet.

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