Method for producing photocatalyst

Abstract

The purpose of the present invention is to provide: a method for producing a photocatalyst, with which it is possible to form a photocatalyst layer that has excellent light transmissivity; a method for producing a photocatalyst electrode having a photocatalyst layer that has excellent light transmissivity; a photocatalyst having a photocatalyst layer that has excellent light transmissivity; and a photocatalyst electrode having a photocatalyst layer that has excellent light transmissivity. A method for producing a photocatalyst according to the present invention is characterized in that the method includes a step for subjecting a solution that contains an Fe compound and a Ti compound to a solvothermal reaction, wherein the total concentration of the Fe compound and the Ti compound in the solution is 70 mmol / L or more.

Description

Method for producing photocatalyst

[0001] The present invention relates to a method for producing a photocatalyst capable of forming a photocatalyst layer excellent in light transmittance, a method for producing a photocatalyst electrode having a photocatalyst layer excellent in light transmittance, a photocatalyst having a photocatalyst layer excellent in light transmittance, and a photocatalyst electrode having a photocatalyst layer excellent in light transmittance.

[0002] In recent years, due to the increasing environmental and energy problems, attention has been focused on photocatalyst technology that can produce hydrogen, one of the next-generation energies, using sunlight. When light is irradiated on a photocatalyst, electrons and holes are generated on the catalyst surface, and hydrogen can be obtained by reducing hydrogen ions in water with these electrons. In addition, by applying a voltage between electrodes, it is also possible to produce hydrogen more efficiently at one electrode and oxygen at the other electrode. However, since most of the generated electrons and holes disappear due to recombination, further improvement in light energy conversion efficiency is required for practical use.

[0003] α-Fe2O3 (hematite) is a safe, inexpensive, and stable photocatalyst material, and its application to hydrogen production using sunlight has been expected for a long time. However, hematite has a problem of low light energy efficiency due to recombination of electrons and holes.

[0004] The research group of the present inventors has developed a method for producing a photocatalyst electrode having high catalytic activity in which hematite-based crystal particles are difficult to peel off from a substrate by subjecting a solution of a hematite raw material to a solvothermal reaction (Patent Document 1).

[0005] In general, hematite photocatalysts are excited by short-wavelength light having high energy and cannot utilize long-wavelength light. Therefore, a tandem system has been developed that has a hematite film layer for water electrolysis and a dye-sensitized photoelectrochemical film behind the film layer and can generate electricity using long-wavelength light (Patent Document 2).

[0006] International Publication No. 2019 / 216284 pamphlet, Japanese Patent Application Laid-Open No. 2004-504934

[0007] As mentioned above, a tandem system combining a water electrolysis system and a solar power generation system has been developed (Patent Document 2). However, Patent Document 2 does not consider the light transmittance of the hematite layer that should transmit long-wavelength light. Furthermore, the hematite crystal particles that constitute the photocatalytic electrode developed by the present inventors' research group are mesocrystals, which are aggregates of crystalline nanoparticles that are densely and regularly packed together. However, mesocrystals with relatively large particle sizes scatter long-wavelength light. Therefore, the present invention aims to provide a method for producing a photocatalyst that can form a photocatalytic layer with excellent light transmittance, a method for producing a photocatalytic electrode having a photocatalytic layer with excellent light transmittance, a photocatalyst having a photocatalytic layer with excellent light transmittance, and a photocatalytic electrode having a photocatalytic layer with excellent light transmittance.

[0008] The inventors of the present invention conducted extensive research to solve the above problems. As a result, they discovered that a photocatalyst capable of forming a photocatalytic layer with excellent light transmittance can be produced by subjecting a solution containing at least relatively high concentrations of Fe and Ti compounds to a solvothermal reaction, and thus completed the present invention. The present invention is described below.

[0009] [1] A method for producing a photocatalyst, comprising the step of subjecting a solution containing an Fe compound and a Ti compound to a solvothermal reaction, characterized in that the total concentration of the Fe compound and the Ti compound in the solution is 70 mmol / L or more. [2] The method according to [1], wherein the solution further comprises one or more metal compounds selected from Na compounds, K compounds, Rb compounds, Cs compounds, Mg compounds, Ca compounds, Sr compounds, Ba compounds, Sc compounds, Y compounds, La compounds, Ce compounds, Pr compounds, Nd compounds, Zr compounds, Hf compounds, V compounds, Nb compounds, Ta compounds, Cr compounds, Mo compounds, W compounds, Mn compounds, Re compounds, Ru compounds, Co compounds, Rh compounds, Ir compounds, Ni compounds, Pd compounds, Pt compounds, Cu compounds, Ag compounds, Au compounds, Zn compounds, Cd compounds, Al compounds, Ga compounds, In compounds, Tl compounds, Si compounds, Ge compounds, Sn compounds, Pb compounds, Sb compounds, and Bi compounds. [3] The method according to [1], wherein the solution further comprises one or more metal salts selected from Group 12 metals and Group 14 metals. [4] The method according to any one of the above [1] to [3], further comprising the step of washing the photocatalyst obtained by the solvothermal reaction with water. [5] The method according to the above [4], wherein the precipitate obtained by the solvothermal reaction is washed with water and then further washed with a water-miscible organic solvent.

[0010] [6] A method for manufacturing a photocatalytic electrode, comprising the steps of manufacturing a photocatalyst by the method described in any one of [1] to [5] above, and coating a dispersion of the photocatalyst onto a transparent electrode substrate and then firing it. [7] The method according to [6], further comprising an annealing step.

[0011] [8] A photocatalyst characterized by being composed of Fe2O3 doped with Ti and metals other than Ti, and having an average particle diameter of 200 nm or less. [9] A photocatalytic electrode characterized by having a layer made of the photocatalyst described in [8] on a transparent electrode substrate.

[0012] The photocatalyst produced by the present invention exhibits anisotropy due to its uniform crystal orientation, and likely because of its relatively small crystal size, the catalyst layer formed by the photocatalyst according to the present invention has high light transmittance. Furthermore, the photocatalyst according to the present invention has excellent photohydrolysis activity. Therefore, the present invention is industrially excellent as a technology applicable to the low-cost production of hydrogen from water.

[0013] Figure 1 is a scanning electron microscope image of hematite photocatalytic particles produced by the present invention method and the conventional method. Figure 2 is an XRD chart of hematite photocatalytic particles produced by the present invention method. Figure 3 is a graph showing the transmittance of hematite photocatalytic particles produced by the present invention method and the conventional method. Figure 4 is a photograph showing the transmittance of hematite photocatalytic particles produced by the present invention method and the conventional method. Figure 5 is a graph showing the photohydrolysis activity of Ti-doped hematite photocatalytic particles (Example 1), Ti-Ge-doped hematite photocatalytic particles (Example 3), and Ti-Zn-doped hematite photocatalytic particles (Example 4) produced by the present invention method. Figure 6 is a graph showing the photohydrolysis activity of Ti-doped hematite photocatalytic particles (Example 1), Ti-K-doped hematite photocatalytic particles (Example 5), and Ti-Ba-doped hematite photocatalytic particles (Example 6) produced by the present invention method. Figure 7 is a graph showing the photohydrolysis activity of Ti-doped hematite photocatalytic particles (Example 1), Ti-Sc-doped hematite photocatalytic particles (Example 7), Ti-Zr-doped hematite photocatalytic particles (Example 8), and Ti-In-doped hematite photocatalytic particles (Example 9), which were produced by the method of the present invention. Figure 8 is a graph showing the photohydrolysis activity of Ti-doped hematite photocatalytic particles (Example 1) and Ti-Ge-Zn-doped hematite photocatalytic particles (Example 10), which were produced by the method of the present invention.

[0014] The present invention will be described below, but it is not limited to the following specific examples.

[0015] 1. Sorvothermal reaction step: In this step, a solution containing an Fe compound and a Ti compound, with a total concentration of these Fe and Ti compounds of 70 mmol / L or more, is subjected to a solvothermal reaction.

[0016] In this process, a solution in which the combined concentration of the Fe compound and Ti compound is 70 mmol / L or more is used. By subjecting a relatively high-concentration solution to a solvothermal reaction, it is thought that a large number of fine hematite (α-Fe2O3) seed crystals will be generated, and fine crystals with uniform orientation will be obtained. The concentration of the Fe compound is not particularly limited as long as the Fe compound is soluble under the conditions of the solvothermal reaction, but for example, the concentration of the Fe compound can be 70 mmol / L or more and 500 mmol / L or less. The Fe compound concentration is preferably 80 mmol / L or more or 90 mmol / L or more, more preferably 100 mmol / L or more or 120 mmol / L or more, even more preferably 150 mmol / L or more, and preferably 400 mmol / L or less, and more preferably 300 mmol / L or less or 250 mmol / L or less.

[0017] In this disclosure, a photocatalyst with high photohydrolysis activity can be obtained by doping hematite with Ti at a relatively high concentration. The amount of Ti doped into the hematite is preferably more than 0 at% and 20 at% or less. More preferably 2 at% or more, even more preferably 6 at% or more, even more preferably 15 at% or less, and even more preferably 12 at% or less. The amount of doping can be measured by X-ray fluorescence analysis (XRF).

[0018] The concentration of the Ti compound is not particularly limited as long as the Ti compound is soluble under the conditions of the solvothermal reaction, but for example, the concentration of the Ti compound can be 1 mmol / L or more and 100 mmol / L or less. The Ti compound concentration is preferably 2 mmol / L or more or 5 mmol / L or more, more preferably 10 mmol / L or more or 15 mmol / L or more, preferably 90 mmol / L or less or 70 mmol / L or less, more preferably 50 mmol / L or less or 40 mmol / L or less, and even more preferably 30 mmol / L or less or 25 mmol / L or less. The total concentration of the Fe compound and the Ti compound can be, for example, 70 mmol / L or more and 750 mmol / L or less. The total concentration is preferably 80 mmol / L or more or 90 mmol / L or more, more preferably 100 mmol / L or more or 120 mmol / L or more, even more preferably 150 mmol / L or more or 180 mmol / L or more, and also preferably 500 mmol / L or less or 400 mmol / L or less, and more preferably 300 mmol / L or less or 250 mmol / L or less.

[0019] The catalyst of this invention may be doped with metals other than Ti. The doping metals other than Ti are not particularly limited as long as they can improve photohydrolysis activity, and may be so-called metalloids, but examples include alkali metals such as Na, K, Rb, and Cs; group 2 metals such as Mg, Ca, Sr, and Ba; group 3 metals such as Sc, Y, La, Ce, Pr, and Nd; group 4 metals such as Zr and Hf; group 5 metals such as V, Nb, and Ta; group 6 metals such as Cr, Mo, and W; group 7 metals such as Mn and Re; group 8 metals such as Ru; group 9 metals such as Co, Rh, and Ir; group 10 metals such as Ni, Pd, and Pt; group 11 metals such as Cu, Ag, and Au; group 12 metals such as Zn and Cd; group 13 metals such as Al, Ga, In, and Tl; group 14 metals such as Si, Ge, Sn, and Pb; and group 15 metals such as Sb and Bi. Preferred doping metals other than Ti include alkali metals, group 2 metals, group 3 metals, group 4 metals, group 7 metals, group 11 metals, group 12 metals, group 13 metals, group 14 metals, and group 15 metals, with K, Rb, Cs, Sr, Ba, Sc, Nd, Zr, Hf, Re, Ag, Zn, Cd, In, Tl, Si, Ge, Sn, and Sb being more preferred, and K, Rb, Ba, Re, Zn, Cd, Si, Ge, and Sn being even more preferred. There is no particular upper limit to the number of doping metals containing Ti, but the number can be, for example, 10 or less. The number is preferably 8 or less or 5 or less, more preferably 4 or less, and more preferably 2 or 3. From the viewpoint of photohydrolytic activity, Group 1, Group 2, Group 3, Group 4, Group 12, Group 13, and Group 14 metals are preferred, with Group 12 and Group 14 metals being more preferred.

[0020] The concentrations of metals other than Fe and Ti in the solution subjected to the solvothermal reaction can be adjusted as appropriate from the viewpoint of photohydrolytic activity, for example, between 0.1 mmol / L and 10 mmol / L. Preferably, the concentration is 0.2 mmol / L or higher, more preferably 0.5 mmol / L or higher, preferably 8 mmol / L or lower, and more preferably 5 mmol / L or lower.

[0021] The amount of metals other than Ti doping in hematite can be adjusted as appropriate from the viewpoint of photohydrolysis activity, but for example, it is preferable to have more than 0 at% and 10 at% or less. The doping amount is preferably 0.1 at% or more or 0.5 at% or more, more preferably 1 at% or more or 1.5 at% or more, preferably 8 at% or less, and more preferably 5 at% or less. The doping amount can be measured by X-ray fluorescence analysis (XRF).

[0022] The starting material compounds containing Fe and doped metals are not particularly limited, but examples include halide ion salts such as fluoride ion salts and chloride ion salts; inorganic acid salts such as nitrates and sulfates; organic acid salts such as acetates; metal alkoxides; and complex compounds such as acetylacetone complexes and ethylenediaminetetraacetic acid complexes.

[0023] The solvent in the solution is not particularly limited as long as it has adequate solubility for the starting compound and does not inhibit the reaction. Examples of usable solvents include amide solvents such as dimethylformamide (DMF), diethylformamide (DEF), and dimethylacetamide (DMA); alcohol solvents such as methanol, ethanol, and 2-propanol; acidic solvents such as formic acid and acetic acid; water; and mixtures thereof.

[0024] A solvothermal reaction refers to a reaction carried out under high temperature and high pressure conditions above the boiling point of the solvent used. The reaction temperature can be adjusted as appropriate depending on the solvent, but for example, it can be between 100°C and 300°C. Preferably, the temperature is 120°C or higher, more preferably 150°C or 160°C or higher, preferably 250°C or lower, and more preferably 200°C or lower. The pressure during the reaction is not particularly limited, but for example, the solution can be placed in a pressure-resistant sealed container and heated above the boiling point of the solvent. The reaction pressure can also be adjusted as appropriate, but for example, it can be between 1 MPa and 2 MPa.

[0025] The reaction time can be adjusted as appropriate within the range in which the solvothermal reaction proceeds sufficiently and hematite crystals are obtained, but for example, it can be 10 hours or more and 50 hours or less. Preferably, the reaction time is 12 hours or more, more preferably 15 hours or more, preferably 30 hours or less, and more preferably 20 hours or less.

[0026] After the reaction, general post-treatment is sufficient. For example, first, the reactants are cooled to approximately room temperature. The cooling method is not particularly limited and can be air-cooled, cooled with water, or cooled with air. After cooling to approximately room temperature, the photocatalyst can be separated from the liquid by decantation, centrifugation, or filtration.

[0027] 2. Washing with water step: In this step, the photocatalyst obtained in the solvothermal reaction step 1 is washed with water. Washing the photocatalyst with water suppresses the aggregation of photocatalytic particles and yields a photocatalytic layer with high light transmittance.

[0028] The washing procedure can be carried out using conventional methods, except for the use of water. For example, the photocatalytic particles can be dispersed in an appropriate amount of water, and then separated from the liquid by decantation, centrifugation, filtration, etc.

[0029] 3. Washing step with water-miscible organic solvent In this step, the photocatalyst washed in step 2 with water is washed with a water-miscible organic solvent. Washing the photocatalyst with water can suppress the aggregation of photocatalyst particles, but if a large amount of water remains on the substrate when forming the photocatalyst layer, the arrangement of the photocatalyst particles may be disrupted when the water evaporates. Therefore, it is preferable to remove the water by washing the photocatalyst particles with a water-miscible organic solvent in this step. The washing operation can be carried out in the same manner as the washing operation in step 2 with water, except that a water-miscible organic solvent is used instead of water.

[0030] Water-miscible organic solvents are organic solvents that can be miscible with water without restriction. Examples include alcohol solvents such as methanol, ethanol, and 2-propanol; and ketone solvents such as acetone.

[0031] The photocatalyst obtained by the present invention consists of hematite doped with at least Ti and has a relatively small average particle size. In addition to Ti, it may also be doped with a metal other than Ti. The average particle size is preferably 200 nm or less, and more preferably 150 nm or less. There is no particular lower limit to the average particle size of the photocatalyst; the finer the particle size, the higher the light transmittance of the photocatalyst layer, but for example, 10 nm or more is preferred. In this disclosure, the average particle size of the photocatalyst is defined as the value obtained by, for example, observing the photocatalyst under an electron microscope at a magnification of 10,000x or more and 50,000x or less, selecting at least 30 photocatalyst particles whose longest portions are clear, measuring the length of the longest portions, and dividing the sum of the longest portion lengths by the number of photocatalyst particles. Image analysis software may be used for measuring the longest portion lengths, etc.

[0032] Because the photocatalyst of the present invention has a disc-like shape, it is easy to deposit on a substrate with a uniform orientation, and there is a possibility that a photocatalytic layer with high light transmittance can be formed. Below, a method for forming a photocatalytic layer on a transparent electrode substrate using the photocatalyst of the present invention will be described.

[0033] 4. Photocatalyst Layer Formation Process In this process, a photocatalyst electrode is manufactured by coating a dispersion of the photocatalyst according to the present invention onto a transparent electrode substrate, which is a base material, and then firing it. The solvent for the dispersion is not particularly limited as long as it can disperse the photocatalyst particles well and does not easily remain in the photocatalyst layer, but examples include methanol solvents such as methanol, ethanol, and 2-propanol; water; and mixed solvents thereof.

[0034] The concentration of the photocatalytic dispersion can be adjusted as appropriate within a range that allows for good formation of the photocatalytic layer, for example, between 1 mg / mL and 50 mg / mL. The number of times the photocatalytic dispersion is applied can be adjusted as appropriate, taking into account its concentration and the desired thickness of the photocatalytic layer.

[0035] Examples of transparent electrode substrates include those in which a transparent conductive layer is laminated on a transparent substrate. The transparent substrate is not particularly limited as long as it is light-transmitting, but examples include transparent insulating substrates such as glass substrates. The material of the transparent conductive layer is not particularly limited as long as it is a transparent conductive layer that is both light-transmitting and conductive, but examples include indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO).

[0036] The method for applying the photocatalytic dispersion onto the transparent electrode substrate is not particularly limited, but examples include spin coating, casting, spraying, dipping, and printing.

[0037] After applying a photocatalytic dispersion onto a transparent electrode substrate, the substrate is fired to fix the photocatalytic layer on the transparent electrode substrate and to reduce the number of defects in the photocatalytic particles.

[0038] The firing temperature is preferably 400°C or higher and 1000°C or lower. More preferably 500°C or higher, more preferably 900°C or lower, and even more preferably 800°C or lower. The firing time is preferably 1 minute or more and 48 hours or less after reaching the predetermined firing temperature. More preferably 10 minutes or more, even more preferably 30 minutes or more, even more preferably 20 hours or less or 10 hours or less, and even more preferably 5 hours or less or 2 hours or less.

[0039] Depending on the type of firing apparatus, oxygen deficiency may occur during firing, so an oxygen-containing gas may be supplied during firing. Supplying oxygen may reduce defects in the crystal. The oxygen-containing gas may be oxygen itself, a mixed gas obtained by diluting oxygen with an inert gas such as nitrogen or argon, or air. From a cost standpoint, air is preferred. The oxygen concentration in the oxygen-containing gas can be adjusted as appropriate, but for example, it can be set to 21 ± 3 vol%.

[0040] After firing, general post-treatment may be performed. For example, first, the transparent electrode substrate on which the photocatalyst layer is formed is cooled to approximately room temperature. The cooling method is not particularly limited, and it may be air-cooling, water-cooling, or natural cooling, etc., but rapid cooling is preferred. By rapid cooling, the crystal defects may be reduced. The cooling rate in the case of rapid cooling is not particularly limited and may be adjusted as appropriate. For example, it can be -100°C / h or higher and -1000°C / h or lower. As the cooling rate, -200°C / h or higher is preferred, -300°C / h or higher is more preferred, and -800°C / h or lower or -600°C / h or lower is preferred, -500°C / h or lower is more preferred.

[0041] In order to supply oxygen and perform rapid cooling simultaneously, for example, immediately after firing or when the temperature inside the firing apparatus is sufficiently high after firing, the door of the firing apparatus may be opened to rapidly introduce room-temperature air into the firing apparatus.

[0042] The thickness of the photocatalyst layer is preferably 0.3 μm or more and 2.0 μm or less. Within this range, when water is decomposed, hematite crystal particles are easily exposed to water, and high catalytic activity can be exhibited.

[0043] 5. Annealing step In this step, the fired photocatalyst layer is annealed. By annealing, defects such as internal strain of the hematite crystals forming the photocatalyst layer are reduced, and as a result, it is considered that the recombination of excited electrons and holes generated by light irradiation can be suppressed.

[0044] The annealing conditions may be appropriately adjusted within the range where the defects of the hematite crystals can be reduced and the photocatalytic water decomposition activity is improved. For example, as the annealing temperature, 100°C or higher and 300°C or lower is preferred. As the annealing temperature, 120°C or higher is more preferred, 150°C or higher is even more preferred, and 250°C or lower is more preferred, 200°C or lower is even more preferred. As the annealing time, it is preferably 10 minutes or more and 20 hours or less after reaching the predetermined temperature. As the annealing time, 30 minutes or more or 1 hour or more is more preferred, 2 hours or more is even more preferred, and 10 hours or less is more preferred, 5 hours or less is even more preferred.

[0045] The thickness of the photocatalyst layer is preferably 0.3 μm or more and 2.0 μm or less. Within this range, when water is decomposed, hematite crystal particles are easily exposed to the water, and high catalytic activity can be exhibited.

[0046] The photocatalyst electrode according to the present invention is a photocatalyst electrode for photocatalytic water splitting mainly used for water splitting, and constitutes an electrode of a photocatalytic water splitting cell. The photocatalyst electrode is immersed in water to be decomposed together with the cathode electrode of the counter electrode, and by irradiating light, it constitutes an anode electrode that oxidizes water to form oxygen. That is, the photocatalyst electrode exhibits catalytic activity when irradiated with light. The photocatalyst electrode and the cathode electrode according to the present invention are connected to an auxiliary power source such as a solar cell outside the photocatalytic water splitting cell, and by irradiating light on the photocatalyst electrode and the solar cell, water is oxidized in the electrode of the present invention to generate oxygen, and water is reduced in the cathode electrode to generate hydrogen. In addition, since the photocatalyst electrode according to the present invention has high light transmittance, it is also possible to install a solar cell between the electrode of the present invention and the cathode electrode and perform photocatalytic water splitting and power generation simultaneously by irradiating light from the side of the electrode of the present invention.

[0047] This application claims the benefit of priority based on Japanese Patent Application No. 2025-4509 filed on January 14, 2025. The entire contents of the specification of Japanese Patent Application No. 2025-4509 filed on January 14, 2025 are incorporated herein by reference for reference purposes.

[0048] Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and it is of course possible to appropriately modify and implement within the range that can conform to the gist of the foregoing and following descriptions, and all of them are included in the technical scope of the present invention.

[0049] Example 1: Preparation of mmm-Ti-Fe2O3 crystals and photocatalytic electrodes Iron(III) nitrate nonahydrate (Fe(NO3)3・9H2O, Fujifilm Wako Pure Chemical Industries, Ltd., 9.1 mmol) was mixed with N,N-dimethylformamide (DMF, Fujifilm Wako Pure Chemical Industries, Ltd., 40 mL) and stirred until dissolved. Titanium fluoride (TiF4, Aldrich, 0.9 mmol) was mixed with methanol (CH3OH, Fujifilm Wako Pure Chemical Industries, Ltd., 10 mL) and ultrasonically irradiated until dissolved or dispersed. All solutions were mixed and stirred for 5 minutes to obtain a mixed solution. This mixed solution was sealed in a stainless steel reaction vessel (San-ai Kagaku Co., Ltd.) equipped with a 100 mL polytetrafluoroethylene (PTFE) inner cylinder and heated at 180°C for 24 hours. After that, it was cooled to room temperature to precipitate the hematite photocatalytic particles. After removing the supernatant, the hematite photocatalyst particles were divided into two 50 mL centrifuge tubes using methanol and centrifuged at 9000 rpm for 10 minutes. The precipitate obtained after decantation was dispersed in distilled water (H2O, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 30 mL) and centrifuged at 9000 rpm for 10 minutes. The same washing procedure was repeated twice with distilled water and twice with ethanol (CH3CH2OH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). Ethanol was added to the obtained hematite photocatalyst particles to obtain a dispersion with a concentration of 10 mg / mL. 30 μL of the dispersion was continuously coated 20 times by spin coating onto a substrate with fluorine-doped tin oxide laminated on a glass substrate (hereinafter referred to as "FTO substrate") to form a hematite layer. Furthermore, when a cross-section of the hematite layer formed by continuously coating the same dispersion onto a substrate 10 times was observed under magnification using SEM, the thickness of the hematite layer was found to be approximately 180 nm. Therefore, it is estimated that the thickness of the hematite layer obtained by 20 consecutive coatings is approximately 360 nm. The FTO substrate with the hematite layer was fired in a firing furnace at 700°C for 60 minutes. After the temperature inside the firing furnace fell below 500°C, the door of the firing furnace was opened and rapidly cooled while introducing air. Subsequently, a transparent photocatalytic electrode was fabricated by annealing at 180°C for 3 hours using a natural convection type constant temperature dryer.

[0050] Example 2: Preparation of mm-Ti-Fe2O3 crystals and photocatalytic electrodes Hematite photocatalytic particles were obtained in the same manner as in Example 1, except that the amount of iron(III) nitrate nonahydrate used was changed from 9.1 mmol to 4.55 mmol and the amount of titanium fluoride used was changed from 0.9 mmol to 0.45 mmol. After that, a photocatalytic electrode was obtained in the same manner as in Example 1.

[0051] Example 3: Preparation of (8% Ti, 2% Ge)-Fe2O3 crystals and photocatalytic electrodes 4.5 mmol of iron(III) nitrate nonahydrate was mixed with 20 mL of N,N-dimethylformamide and stirred until dissolved. 0.4 mmol of titanium fluoride and 0.1 mmol of bis(2-carboxyethylgermanium(IV)) sesquioxide (O[Ge(=O)CH2CH2CO2H]2, Aldrich, 0.1 mmol) were each mixed with 2.5 mL of methanol and ultrasonically irradiated until dissolved or dispersed. All solutions were mixed and stirred for 5 minutes to obtain a mixed solution. This mixed solution was sealed in a stainless steel reaction vessel equipped with a 50 mL polytetrafluoroethylene inner cylinder and heated at 180°C for 16 hours. Afterward, it was cooled to room temperature to precipitate the hematite photocatalytic particles. Using the obtained hematite photocatalytic particles, a photocatalytic electrode was prepared in the same manner as in Example 1.

[0052] Example 4: Preparation of (8%Ti, 2%Zn)-Fe2O3 crystals and photocatalytic electrodes. Hematite photocatalytic particles were obtained in the same manner as in Example 3, except that bis(2-carboxyethylgermanium(IV))sesquioxide was replaced with zinc chloride (ZnCl2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then a photocatalytic electrode was prepared in the same manner as in Example 1.

[0053] Example 5: Preparation of (8%Ti, 2%K)-Fe2O3 crystals and photocatalytic electrodes. Hematite photocatalytic particles were obtained in the same manner as in Example 3, except that bis(2-carboxyethylgermanium(IV))sesquioxide was replaced with potassium chloride (KCl, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then a photocatalytic electrode was prepared in the same manner as in Example 1.

[0054] Example 6: Preparation of (8%Ti, 2%Ba)-Fe2O3 crystals and photocatalytic electrodes. Hematite photocatalytic particles were obtained in the same manner as in Example 3, except that bis(2-carboxyethylgermanium(IV))sesquioxide was replaced with barium chloride dihydrate (BaCl2・2H2O, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then a photocatalytic electrode was prepared in the same manner as in Example 1.

[0055] Example 7: Preparation of (8%Ti, 2%Sc)-Fe2O3 crystals and photocatalytic electrodes. Hematite photocatalytic particles were obtained in the same manner as in Example 3, except that bis(2-carboxyethylgermanium(IV))sesquioxide was replaced with scandium(III) acetate hydrate ((CH3CO2)3Sc·xH2O, manufactured by Aldrich). Then, photocatalytic electrodes were prepared in the same manner as in Example 1.

[0056] Example 8: Preparation of (8%Ti, 2%Zr)-Fe2O3 crystals and photocatalytic electrodes. Hematite photocatalytic particles were obtained in the same manner as in Example 3, except that bis(2-carboxyethylgermanium(IV))sesquioxide was replaced with zirconium(IV) chloride (ZrCl4, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then a photocatalytic electrode was prepared in the same manner as in Example 1.

[0057] Example 9: Preparation of (8%Ti, 2%In)-Fe2O3 crystals and photocatalytic electrodes. Hematite photocatalytic particles were obtained in the same manner as in Example 3, except that bis(2-carboxyethylgermanium(IV))sesquioxide was replaced with indium(III) chloride tetrahydrate (InCl3・4H2O, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then a photocatalytic electrode was prepared in the same manner as in Example 1.

[0058] Example 10: Preparation of (8%Ti, 2%Ge, 2%Zn)-Fe2O3 crystals and photocatalytic electrodes 4.4 mmol of iron(III) nitrate nonahydrate was mixed with 20 mL of N,N-dimethylformamide and stirred until dissolved. 0.4 mmol of titanium fluoride, 0.1 mmol of bis(2-carboxyethylgermanium(IV)) sesquioxide, and 0.1 mmol of zinc chloride were each mixed with 1.67 mL of methanol and sonicated until dissolved or dispersed. All solutions were mixed and stirred for 5 minutes to obtain a mixed solution. This mixed solution was sealed in a stainless steel reaction vessel equipped with a 50 mL polytetrafluoroethylene inner cylinder and heated at 180°C for 16 hours. Afterward, it was cooled to room temperature to precipitate the hematite photocatalytic particles. Using the obtained hematite photocatalytic particles, a photocatalytic electrode was prepared in the same manner as in Example 1.

[0059] Comparative Example 1: Preparation of m-Ti-Fe2O3 crystals and photocatalytic electrodes Hematite photocatalytic particles were obtained in the same manner as in Example 1, except that the amount of iron(III) nitrate nonahydrate used was changed from 9.1 mmol to 2.28 mmol and the amount of titanium fluoride used was changed from 0.9 mmol to 0.23 mmol. After that, a photocatalytic electrode was obtained in the same manner as in Example 1.

[0060] Comparative Example 2: Preparation of Ti-Fe2O3 crystals and photocatalytic electrodes Hematite photocatalytic particles were obtained in the same manner as in Example 1, except that the amount of iron(III) nitrate nonahydrate used was changed from 9.1 mmol to 1.14 mmol and the amount of titanium fluoride used was changed from 0.9 mmol to 0.11 mmol. After that, a photocatalytic electrode was obtained in the same manner as in Example 1.

[0061] Test Example 1: SEM Observation The hematite photocatalytic particles of Examples 1 and 2 and Comparative Examples 1 and 2 were observed under magnification using a scanning electron microscope ("JCM-7000 NeoScope," manufactured by JEOL Ltd.), and photographs were taken. Figure 1 shows the electron microscope images of each hematite photocatalytic particle. The obtained scanning electron microscope images were also analyzed using image processing software ("ImageJ," National Institutes of Health, USA), and the longest portion length of 30 particles included in the magnified image of the hematite photocatalytic particles was determined, and the average value was calculated. The results are shown in Table 1.

[0062]

[0063] As shown in Figure 1 and Table 1, it became clear that the size of the resulting hematite photocatalyst particles decreased as the concentration of the metal raw material compound in the solution subjected to the solvothermal reaction increased.

[0064] Test Example 2: Analysis by X-ray Diffraction (XRD) The hematite photocatalyst particles of Example 1 (mmm-Ti-Fe2O3 crystal), the hematite photocatalyst particles of Example 3 ((8%Ti, 2%Ge)-Fe2O3 crystal), and the hematite photocatalyst particles of Example 4 ((8%Ti, 2%Zn)-Fe2O3 crystal) were analyzed using an X-ray diffractometer ("MiniFlex 600-C", Rigaku Corporation). The results are shown in Figure 2. In addition, the isotropic crystal and the peak intensity ratios of the (110) and (006) planes relative to the peak intensity of the (104) plane in each crystal are shown in Table 2.

[0065]

[0066] In an isotropic crystal of Fe2O3, when the peak intensity of the (104) plane is set to 100, the peak intensity ratio of the (110) plane is 70. However, in the hematite photocatalyst particles of Examples 1, 3, and 4, the same peak intensity ratio was 4 or less. Therefore, it is suggested that the crystals of the hematite photocatalyst particles according to the present invention have high light transmittance because their orientation is uniform and they exhibit anisotropy.

[0067] Test Example 3: Light Transmission Test The light transmittance of the photocatalytic electrodes of the FTO substrate, Examples 1 and 2, and Comparative Examples 1 and 2 was measured using an ultraviolet-visible-near-infrared spectrophotometer ("V-770," manufactured by JASCO Corporation). In addition, each photocatalytic electrode was placed on a piece of paper with a mark drawn on it, and a photograph was taken. The light transmittance of each photocatalytic electrode is shown in Figure 3, and the photographs showing the light transmittance are shown in Figure 4.

[0068] As shown in Figure 3, the hematite photocatalyst absorbed and did not transmit light below approximately 450 nm, and electrodes with a hematite photocatalyst layer manufactured by the conventional method (Comparative Examples 1 and 2) did not sufficiently transmit light with wavelengths greater than that. In contrast, electrodes with a hematite photocatalyst layer manufactured by the present invention (Examples 1 and 2) transmitted light with wavelengths greater than that, especially light with wavelengths of approximately 600 to 800 nm, ranging from orange to red, very well.

[0069] Furthermore, as shown in Figure 4, the layers formed with the hematite photocatalysts of Examples 1 and 2, manufactured using the method of the present invention, exhibited excellent transparency, to the point that the marks on the back of the layers were visible. The red color also indicates that they transmit long-wavelength visible light well. This is likely due to the sufficiently small particle size, as shown in the results of Test Example 1, and the uniform orientation of the crystal particles, as shown in the results of Test Example 2.

[0070] Test Example 4: Photohydrolysis Activity Test The working electrode, counter electrode, and reference electrode were immersed in a 1.0 mol / L sodium hydroxide aqueous solution at pH 13.6, and simulated sunlight (AM 1.5 G, 100 mW / cm²) was applied using a solar simulator. 2The current values ​​for each potential were measured by irradiating the working electrode with simulated sunlight. For the direction of irradiation of the working electrode with simulated sunlight, irradiation from the hematite photocatalyst layer side was defined as front irradiation, and irradiation from the FTO substrate side was defined as back irradiation. The photocatalytic electrodes from Examples 1, 3-10 were used as the working electrode, a platinum wire as the counter electrode, and Ag / AgCl as the reference electrode. An electrochemical analyzer ("ALS608E," manufactured by BAS Corporation) and a light source equipped with an AM1.5 filter ("LAX-C100," manufactured by Asahi Spectroscopy Co., Ltd.) were used. The results are shown in Figures 5-8. As shown in Figures 5-8, when using an electrode with a hematite photocatalyst layer according to the present invention, the current value for the applied voltage was high, indicating that water was effectively decomposed into hydrogen and oxygen. Furthermore, when using the hematite photocatalyst according to the present invention, the reason why the water splitting activity does not change significantly whether light is irradiated from the front or the back is thought to be that, due to the high light transmittance of the catalyst layer of the present invention, even when light is irradiated from the front side (hematite photocatalyst layer side), light reaches the hematite photocatalyst close to the FTO substrate and excites the catalyst. In addition, it has been shown that using a hematite photocatalyst doped with one or two additional metals in addition to Ti results in higher activity than using a hematite photocatalyst doped with Ti alone.

Claims

1. A method for producing a photocatalyst, comprising the step of subjecting a solution containing an Fe compound and a Ti compound to a solvothermal reaction, characterized in that the total concentration of the Fe compound and the Ti compound in the solution is 70 mmol / L or more.

2. The method according to claim 1, wherein the solution further comprises one or more metal compounds selected from Na compounds, K compounds, Rb compounds, Cs compounds, Mg compounds, Ca compounds, Sr compounds, Ba compounds, Sc compounds, Y compounds, La compounds, Ce compounds, Pr compounds, Nd compounds, Zr compounds, Hf compounds, V compounds, Nb compounds, Ta compounds, Cr compounds, Mo compounds, W compounds, Mn compounds, Re compounds, Ru compounds, Co compounds, Rh compounds, Ir compounds, Ni compounds, Pd compounds, Pt compounds, Cu compounds, Ag compounds, Au compounds, Zn compounds, Cd compounds, Al compounds, Ga compounds, In compounds, Tl compounds, Si compounds, Ge compounds, Sn compounds, Pb compounds, Sb compounds, and Bi compounds.

3. The method according to claim 1, wherein the solution further comprises a salt of one or more metals selected from Group 12 and Group 14 metals.

4. The method according to claim 1, further comprising the step of washing the photocatalyst obtained by the solvothermal reaction with water.

5. The method according to claim 4, wherein the precipitate obtained by the solvothermal reaction is washed with water and then further washed with a water-miscible organic solvent.

6. A method for manufacturing a photocatalytic electrode, comprising the steps of manufacturing a photocatalyst by the method described in any one of claims 1 to 5, and coating a dispersion of the photocatalyst onto a transparent electrode substrate and then firing it.

7. The method according to claim 6, further comprising an annealing step.

8. A photocatalyst characterized by being composed of Fe2O3 doped with Ti and other metals, and having an average particle diameter of 200 nm or less.

9. A photocatalytic electrode characterized by having a layer made of the photocatalyst described in claim 8 on a transparent electrode substrate.

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