Junction photocatalyst

Abstract

To provide a junction photocatalyst having higher catalytic activity than conventional junction photocatalysts.SOLUTION: A junction photocatalyst according to the present invention has a solid mediator between a hydrogen generation photocatalyst and an oxygen generation photocatalyst. The solid mediator and the hydrogen generating photocatalyst are bonded via an ionic polymer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a conjugated photocatalyst having a solid mediator between a hydrogen-evolving photocatalyst and an oxygen-evolving photocatalyst, a method for producing the conjugated photocatalyst, a photocatalytic composite, the use of the conjugated photocatalyst, and a method for producing hydrogen. [Background technology]

[0002] In recent years, technologies that use photocatalysts and sunlight to decompose water and produce hydrogen and oxygen have attracted attention. When decomposing water with photocatalysts, it is preferable to use a photocatalyst that catalyzes both the reduction reaction of water (proton reduction reaction) and the oxidation reaction of water. However, the types of such photocatalysts are limited, and they tend to have low water-decomposing activity.

[0003] Therefore, methods are being investigated to efficiently decompose water by using a combination of photocatalysts that catalyze the reduction reaction of water (hydrogen-generating photocatalysts) and photocatalysts that catalyze the oxidation reaction of water (oxygen-generating photocatalysts).

[0004] For example, a system that uses hydrogen-evolving photocatalysts and oxygen-evolving photocatalysts to decompose water by irradiating it with visible light, thereby generating both hydrogen and oxygen, is called a Z-scheme. In a Z-scheme, for example, a photocatalyst is used that combines an oxygen-evolving photocatalyst that decomposes water with visible light to generate oxygen, a hydrogen-evolving photocatalyst that decomposes water with visible light to generate hydrogen, and a redox medium. In this Z-scheme using photocatalysts, electrons generated by the oxygen-evolving photocatalyst, which do not contribute to the reduction of water, reduce the redox medium, and this reduced redox medium is oxidized by holes generated by the hydrogen-evolving photocatalyst, which do not contribute to the oxidation of water, returning to the redox medium before reduction. This cycle is repeated, enabling the complete decomposition of water (hydrogen:oxygen = 2:1 (stoichiometric ratio)).

[0005] However, in order for the Z scheme using the hydrogen-evolving photocatalyst and oxygen-evolving photocatalyst to function catalytically, an oxidation-reduction medium (e.g., Fe 3+ / Fe 2+ or I - / IO3- Furthermore, this requires a photocatalyst, and there is a problem that the electron transfer efficiency of the redox medium is low, resulting in low water splitting activity. In addition, it is necessary to select an appropriate photocatalyst for the redox potential of the redox medium, which narrows the range of photocatalyst options.

[0006] On the other hand, as a junction-type photocatalyst that does not use an oxidation-reduction medium, for example, Patent Document 1 proposes a photocatalytic layer comprising: a first photocatalytic particle that is visible light responsive for hydrogen generation; a second photocatalytic particle that is visible light responsive for oxygen generation; and a conductive particle that is provided between the first and second photocatalytic particles and has a Fermi level that is negative than the electron energy level at the upper end of the valence band of the first photocatalytic particle and positive than the electron energy level at the lower end of the conduction band of the second photocatalytic particle, wherein the conductive particle is arranged to be connected to the first and second photocatalytic particles.

[0007] Furthermore, Non-Patent Document 1 proposes a Z-scheme photocatalyst with a hierarchical structure consisting of ZnRh2O4 / Au / BiVO4. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2017-124394 [Non-patent literature]

[0009] [Non-Patent Document 1] Toshihiro Takashima et al., J. Mater. Chem. A, 2019, 7, 10372-10378 [Overview of the project] [Problems that the invention aims to solve]

[0010] A bonded photocatalyst in which a hydrogen-generating photocatalyst and an oxygen-generating photocatalyst are bonded via a metal or metal oxide has a very simple structure and exhibits high catalytic activity in the decomposition reaction of water or the like under visible light irradiation. However, there is a need to further enhance the catalytic activity.

[0011] The present invention has been made in view of the above circumstances, and provides a bonded photocatalyst having higher catalytic activity than conventional bonded photocatalysts.

Means for Solving the Problems

[0012] As a result of intensive studies, the present inventor has found that the above problems can be solved by the following bonded photocatalyst.

[0013] That is, the present invention relates to the following 1. to 6. 1. A bonded photocatalyst having a solid mediator between a hydrogen-generating photocatalyst and an oxygen-generating photocatalyst, wherein the solid mediator and the hydrogen-generating photocatalyst are bonded via an ionic polymer. 2. A method for producing the bonded photocatalyst, A method for producing a bonded photocatalyst including the following steps 1 to 4. However, steps 2 and 3 may be performed in any order. Step 1: A step of bonding the solid mediator onto an oxygen-generating photocatalyst by at least one method selected from the group consisting of a photoelectrodeposition method, an impregnation-support method, and a precipitation method using an organic carboxylic acid compound and the solid mediator or a precursor thereof Step 2: A step of introducing an ionic group into the solid mediator to obtain an oxygen-generating photocatalyst bonded with the solid mediator having the ionic group Step 3: A step of introducing an ionic polymer having a charge opposite to that of the ionic group into the hydrogen-generating photocatalyst Step 4: A step of mixing the oxygen-generating photocatalyst bonded with the solid mediator having the ionic group obtained in step 2 and the hydrogen-generating photocatalyst into which the ionic polymer obtained in step 3 has been introduced 3. The method for producing the bonded photocatalyst, A method for manufacturing a bonded photocatalyst including the following steps 1, 2', 3', and 4'. However, steps 2' and 3' can be carried out in any order. Step 1: A step of bonding the solid mediator onto an oxygen-generating photocatalyst by at least one method selected from the group consisting of a photoelectrodeposition method, an impregnation-supported method, and a precipitation method using an organic carboxylic acid compound and the solid mediator or its precursor Step 2': A step of introducing an ionic group into the solid mediator, and further reacting an ionic polymer having a charge opposite to that of the ionic group with the ionic group to obtain an oxygen-generating photocatalyst bonded with the solid mediator having the ionic polymer Step 3': A step of introducing an ionic polymer having a charge opposite to that of the ionic polymer into the hydrogen-generating photocatalyst Step 4': A step of mixing the oxygen-generating photocatalyst bonded with the solid mediator having the ionic polymer obtained in step 2' and the hydrogen-generating photocatalyst into which the ionic polymer obtained in step 3' has been introduced 4. A photocatalyst composite having the bonded photocatalyst on a substrate. 5. Use of the bonded photocatalyst as a photocatalyst for water splitting or for decomposing organic substances. 6. A method for producing hydrogen by irradiating light on the bonded photocatalyst or the photocatalyst composite in the presence of water or alcohol.

Advantages of the Invention

[0014] In the bonded photocatalyst of the present invention, the solid mediator and the hydrogen-generating photocatalyst are bonded via an ionic polymer, so that a strong electrostatic interaction acts, and the hydrogen-generating photocatalyst and the oxygen-generating photocatalyst are bonded via the solid mediator without being separated. Therefore, the catalytic activity is improved, and water can be decomposed into oxygen and hydrogen with high water splitting reaction efficiency.

Brief Description of the Drawings

[0015] [Figure 1] SEM image of Au-BiVO4 of Example 1. [Figure 2] SEM image of the bonded photocatalyst of Example 1. [Figure 3] This shows the volume particle size distribution of the bonded photocatalyst (bonded particles) etc. in Example 1. [Figure 4] This shows the volume particle size distribution of the Ru-supported SrTiO3 and Au-BiVO4 mixed sample of Comparative Example 1. [Modes for carrying out the invention]

[0016] The present invention will be described in detail below.

[0017] <Joined photocatalyst> The bonded photocatalyst of the present invention has a solid mediator between a hydrogen-evolving photocatalyst and an oxygen-evolving photocatalyst, and the solid mediator and the hydrogen-evolving photocatalyst are bonded via an ionic polymer.

[0018] More specifically, the oxygen-evolving photocatalyst is directly or indirectly bonded to a part of the solid mediator, the hydrogen-evolving photocatalyst is directly or indirectly bonded to another part of the solid mediator, and the hydrogen-evolving photocatalyst and the oxygen-evolving photocatalyst are bonded at least via the solid mediator. Preferably, the solid mediator and oxygen-evolving photocatalyst are formed by bonding the solid mediator to the oxygen-evolving photocatalyst using at least one method selected from the group consisting of photoelectrodeposition, impregnation, and precipitation, using the solid mediator or its precursor, as described later. Furthermore, the solid mediator and the hydrogen-generating photocatalyst are joined via an ionic polymer, and it is preferable that the solid mediator has ionic groups, and that the solid mediator and the hydrogen-generating photocatalyst are joined via an ionic polymer having the opposite charge to the ionic groups of the solid mediator (preferably ionic groups introduced into the solid mediator).

[0019] In this invention, "bonding" means a state in which the oxygen-evolving photocatalyst and the hydrogen-evolving photocatalyst are integrated to the extent that they do not separate when performing a photocatalytic reaction. Furthermore, in this invention, "electron-collecting surface" means the exposed crystal surface in the various structures of the oxygen-evolving photocatalyst that is most likely to collect excited electrons. One embodiment of the bonded photocatalyst of the present invention is characterized in which a solid mediator has an ionic group, and a hydrogen-generating photocatalyst has an ionic polymer having a charge opposite to that of the ionic group, and the solid mediator and the hydrogen-generating photocatalyst are bonded together by an ionic bond between the ionic group and the ionic polymer. Furthermore, in one embodiment of the bonded photocatalyst of the present invention, the solid mediator has ionic groups, and the hydrogen-generating photocatalyst has ionic polymer A and, via ionic polymer A, further ionic polymer B having a charge opposite to that of the ionic groups, and the solid mediator and the hydrogen-generating photocatalyst are bonded by ionic bonding between the ionic groups and the ionic polymer B. Furthermore, in one embodiment of the bonded photocatalyst of the present invention, the solid mediator has an ionic group and an ionic polymer B having a charge opposite to that of the ionic group via the ionic group, and the hydrogen-generating photocatalyst has an ionic polymer A, and the solid mediator and the hydrogen-generating photocatalyst are bonded together by an ionic bond between the ionic polymer B of the solid mediator and the ionic polymer A of the hydrogen-generating photocatalyst. When the ionic group of the solid mediator is an anionic group, the ionic polymer A is an anionic polymer and the ionic polymer B is a cationic polymer. When the ionic group of the solid mediator is a cationic group, the ionic polymer A is a cationic polymer and the ionic polymer B is an anionic polymer.

[0020] The hydrogen-generating photocatalyst is not particularly limited and may include, for example, metal oxides such as TiO2, SrTiO3, La2Ti2O7, SnNb2O6, and compounds in which one or more metals such as Cr, Sb, Ta, Ir, and La are doped therein; metal oxynitrides or metal nitrides such as LaTiO2N, BaTaO2N, BaNbO2N, TaON, Ta3N5, and Ge3N4; CuGaS2, CuInS2, Cu(Ga Examples include metal sulfides, metal selenides, or metal sulfose selenides such as In)S2, CuGaSe2, CuInSe2, Cu(Ga,In)Se2, Cu2ZnSnS4(CZTS), and Cu2ZnSn(S,Se)4; and metal acid sulfides or metal acid selenides such as La5Ti2CuS5O7, La5Ti2AgS5O7, La5Ti2CuSe5O7, and La5Ti2AgSe5O7. These may be used individually or in combination of two or more. From the viewpoint of enhancing catalytic activity, preferably one or more are selected from metal oxides, more preferably one or more are selected from transition metal oxides, even more preferably one or more are selected from oxides of elements belonging to Group 4 of the periodic table, and even more preferably SrTiO3.

[0021] The oxygen-evolving photocatalyst is not particularly limited and examples include metal oxides such as BiVO4, TiO2, WO3, SrTiO3, Ag3PO4, SnNb2O6, Bi2WO6, Fe2TiO5, Fe2O3, Bi2MoO6, and compounds obtained by doping these with one or more metals such as Cr, Ni, Sb, Nb, Th, Mo, and W; metal nitrides such as Ta3N5 and Ge3N4; and metal oxynitrides such as LaTiO2N, BaTaO2N, BaNbO2N, and TaON. One of these may be used, or two or more may be used in combination. Of these, from the viewpoint of enhancing catalytic activity, preferably one or more selected from metal oxides, more preferably one or more selected from oxides of transition metals, even more preferably one or more selected from oxides of one or more elements belonging to groups 4 and 5 of the periodic table, and even more preferably BiVO4.

[0022] The hydrogen-generating photocatalyst and the oxygen-generating photocatalyst preferably have a co-catalyst on their surface. This promotes the reduction and oxidation reactions of water and other substances, thereby improving the efficiency of hydrogen and oxygen production.

[0023] Examples of co-catalysts for the hydrogen-generating photocatalyst include transition metals such as Pt, Pd, Ru, Ni, Au, Fe, Ir, and Rh; metal oxides such as NiO, RuO2, IrO2, and Rh2O3; metal sulfides such as NiS and MoS2; and Cr-Rh composite oxides. These may be used individually or in combination of two or more. Of these, from the viewpoint of hydrogen generation efficiency, one or more selected from transition metals are preferred, more preferably one or more selected from metals belonging to Group 10 of the periodic table, and even more preferably Ru.

[0024] Examples of co-catalysts for the oxygen-evolving photocatalyst include transition metals such as Mn, Fe, Co, Ir, Ru, Rh, Ni, Sb, Nb, Th, and Cr; oxides and hydroxides of these transition metals; and so on. One of these may be used, or two or more may be used in combination. Of these, from the viewpoint of oxygen generation efficiency, one or more selected from the oxides of transition metals are preferred.

[0025] Methods for supporting the co-catalyst on the surface of the hydrogen-generating photocatalyst or the oxygen-generating photocatalyst include, for example, photoelectrodeposition, impregnation, adsorption, hydrogen reduction, and electroless plating.

[0026] The impregnation method and the adsorption method described above are methods for dispersing the photocatalyst in a solution in which a co-catalyst precursor is dissolved, and adsorbing the co-catalyst precursor onto the surface of the photocatalyst. Examples of the co-catalyst precursor include chlorides, nitrates, and amine salts of the transition metal.

[0027] It is preferable to support a co-catalyst precursor on the surface of the photocatalyst and then reduce it. Reducing the co-catalyst precursor to a metallic state increases its activity. Examples of methods for reducing the co-catalyst precursor include photoreduction and chemical reduction.

[0028] The photoreduction method is a method of reducing the co-catalyst precursor adsorbed on the photocatalyst by excited electrons generated within the photocatalyst upon irradiation with ultraviolet or visible light. The chemical reduction method is a method of reducing the co-catalyst precursor under a hydrogen gas stream at a temperature of approximately 400°C or lower, preferably 300°C or lower. The co-catalyst supported on the surface of the photocatalyst is in particulate form, and the amount of co-catalyst supported can be adjusted as appropriate.

[0029] The solid mediator is a material that can store excited electrons generated by the oxygen-evolving photocatalyst that do not contribute to the reduction of water, etc., and holes generated by the hydrogen-evolving photocatalyst that do not contribute to the oxidation of water, etc., and can cause a charge recombination reaction between the excited electrons and the holes.

[0030] Examples of the solid mediators include transition metals such as gold, silver, copper, nickel, titanium, manganese, rhodium, palladium, ruthenium, and iridium; transition metal compounds such as oxides, nitrides, and carbides of these transition metals; and doped metal oxides such as tin-doped indium oxide (ITO), metal (B, Al, Ga)-doped zinc oxide, fluorine-doped tin oxide, and antimond-doped tin oxide. These may be used individually or in combination of two or more. From the viewpoint of enhancing catalytic activity, it is preferable to use one or more selected from transition metals or their compounds, more preferably one or more selected from transition metals or their compounds that include elements belonging to Group 11 of the periodic table, even more preferably one or more selected from gold or silver, and even more preferably containing gold.

[0031] The electron-harvesting surface of the oxygen-evolving photocatalyst depends on the material composition and crystal system. One method for identifying the electron-harvesting surface is to contact semiconductor crystal particles, which are the material of the oxygen-evolving photocatalyst, with a precursor solution of a metal such as Pt, Au, or Ag, and then support the metal particles deposited by photodeposition. The crystal plane on which the metal particles have precipitated can then be confirmed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The crystal plane on which the metal particles have precipitated can be identified, for example, by determining the corresponding crystal plane spacing from the lattice fringes observed by TEM.

[0032] The electron-collecting surface of the oxygen-evolving photocatalyst is, for example, a {010} or {040} plane in the case of BiVO4 (monoclinic cerebrolite crystal), a {110} plane in the case of TiO2 (rutile crystal), a {101} plane in the case of TiO2 (anatase crystal), a {002} plane in the case of WO3 (monoclinic crystal), a {110} plane in the case of SrTiO3 (perovskite crystal), and a {110} plane in the case of Ag3PO4 (cubic crystal).

[0033] Furthermore, in the bonded photocatalyst, it is preferable that the hydrogen-generating photocatalyst is selectively bonded to the solid mediator.

[0034] <Method for manufacturing a bonding-type photocatalyst> Examples of methods for producing the bonded photocatalyst of the present invention include bonding the solid mediator to the surface of the oxygen-evolving photocatalyst, and then bonding the hydrogen-evolving photocatalyst to the surface of the solid mediator. Specifically, the following production methods (I) and (II) are examples.

[0035] Examples of the method for producing a conjugated photocatalyst of the present invention include (I) a method for producing a conjugated photocatalyst comprising the following steps 1 to 4. However, steps 2 and 3 may be performed in any order. Step 3 may be performed after step 2, step 2 may be performed after step 3, or they may be performed simultaneously. Step 1: A step of bonding an organic carboxylic acid compound to an oxygen-evolving photocatalyst by at least one method selected from the group consisting of photoelectrodeposition, impregnation and loading, and precipitation, using the solid mediator or its precursor. Step 2: A step to introduce ionic groups into the solid mediator and obtain an oxygen-evolving photocatalyst by bonding the solid mediator having the ionic groups. Step 3: A step of introducing an ionic polymer having a charge opposite to the charge of the ionic group into the hydrogen-generating photocatalyst. Step 4: A step of mixing the oxygen-evolving photocatalyst to which the solid mediator having the ionic group obtained in Step 2 has been bonded with the hydrogen-evolving photocatalyst to which the ionic polymer obtained in Step 3 has been introduced.

[0036] Furthermore, an example of a method for producing a conjugated photocatalyst of the present invention is (II) a method for producing a conjugated photocatalyst comprising the following steps 1, 2', 3', and 4'. However, steps 2' and 3' may be performed in any order. Step 3' may be performed after step 2', or step 2' may be performed after step 3', or they may be performed simultaneously. Step 1: A step of bonding an organic carboxylic acid compound to an oxygen-evolving photocatalyst by at least one method selected from the group consisting of photoelectrodeposition, impregnation and loading, and precipitation, using the solid mediator or its precursor. Step 2': A step to introduce ionic groups into the solid mediator, and further react the ionic groups with an ionic polymer having the opposite charge to the ionic groups, thereby obtaining an oxygen-evolving photocatalyst by bonding the solid mediator having the ionic polymer. Step 3': A step of introducing an ionic polymer having a charge opposite to that of the ionic polymer into the hydrogen-generating photocatalyst. Step 4': A step of mixing the oxygen-evolving photocatalyst to which the solid mediator having the ionic polymer obtained in Step 2' has been bonded with the hydrogen-evolving photocatalyst to which the ionic polymer obtained in Step 3' has been introduced.

[0037] Step 1: A step of bonding an organic carboxylic acid compound to an oxygen-evolving photocatalyst by at least one method selected from the group consisting of photoelectrodeposition, impregnation and loading, and precipitation, using the solid mediator or its precursor.

[0038] In step 1, the method for bonding (supporting) the solid mediator to the surface of the oxygen-evolving photocatalyst is not particularly limited, but it is preferable to use at least one method selected from the group consisting of photoelectrodeposition, impregnation, and precipitation.

[0039] The aforementioned photoelectrodeposition method involves irradiating a dispersion containing a photocatalyst, a metal salt (a precursor of a solid mediator), and an organic carboxylic acid compound with light to reduce the metal salt and deposit metal particles or metal compound particles (solid mediators) onto the surface of the photocatalyst, thereby bonding (supporting) them. Examples of precursors for the solid mediators include ionic salts of mediators such as silver nitrate and gold halides (tetrachloroauric acid).

[0040] The aforementioned impregnation and support method involves mixing a photocatalyst with a solution or dispersion in which a metal salt (precursor of a solid mediator) and an organic carboxylic acid compound are dissolved or dispersed, then removing the solvent by filtration, decantation, or centrifugation, or by heating or reduced pressure to support the metal salt on the surface of the photocatalyst, and then bonding (supporting) metal particles or metal compound particles (solid mediators) to the surface of the photocatalyst by calcining or reducing the metal salt.

[0041] The aforementioned precipitation method involves reducing a metal salt (precursor of the solid mediator) in a solution containing an organic carboxylic acid compound to obtain metal particles or metal compound particles (solid mediator), which are then mixed with the photocatalyst to bond (support) the metal particles or metal compound particles (solid mediator) to the surface of the photocatalyst.

[0042] From the viewpoint of increasing the coverage rate of the solid mediator on the electron collection surface of the oxygen-evolving photocatalyst, the photoelectrodeposition method is preferred. The aforementioned photoelectrodeposition method involves irradiating a dispersion containing an oxygen-evolving photocatalyst, an organic carboxylic acid compound, and a solid mediator or its precursor with light to bond (support) the solid mediator onto the oxygen-evolving photocatalyst. It is believed that the light irradiation generates electrons and holes within the photocatalyst, with the electrons causing the reduction and deposition of the solid mediator, and the holes causing the oxidation of water and other substances. Water is preferred as the solvent for the dispersion, but lower alcohols such as methanol, ethanol, and 2-propanol can also be used as auxiliary agents (sacrificial reagents) to promote the consumption of holes.

[0043] The organic carboxylic acid compound is presumed to contribute to the reduction of the metal salt (precursor of the solid mediator) and is also thought to act as an aid for bonding (supporting) the solid mediator onto the oxygen-evolving photocatalyst. Furthermore, the use of the organic carboxylic acid compound improves the coverage of the solid mediator on the electron-collecting surface of the oxygen-evolving photocatalyst. For example, in the case of BiVO4 (monoclinic celite crystal), the selectivity to the {010} plane, which is the electron-collecting surface, is improved, allowing the solid mediator to be bonded to the oxygen-evolving photocatalyst in a dense state, thereby improving catalytic activity.

[0044] The organic carboxylic acid compound preferably includes at least one selected from the group consisting of ether carboxylates, fatty acids, hydroxy monocarboxylic acids, and polycarboxylic acids, and more preferably includes ether carboxylates, and specifically includes the organic carboxylic acid compounds listed in 1) to 4) below.

[0045] 1) Ether carboxylate Examples of the ether carboxylate mentioned above include those represented by the following general formula (1). R 1 -O-(EO) n -CH2COOH (1) In the above general formula (1), R 1From the viewpoints of water solubility and the function as an auxiliary agent, it is preferably a linear or branched alkyl group or alkenyl group having 4 to 22 carbon atoms, more preferably a linear or branched alkyl group or alkenyl group having 8 to 18 carbon atoms, and still more preferably a linear or branched alkyl group having 8 to 18 carbon atoms (alkyl ether carboxylate). Further, the average number of added moles n of EO (ethyleneoxy group) is preferably 1 to 25, more preferably 2 to 12, and still more preferably 3 to 8 from the viewpoints of water solubility and the function as an auxiliary agent.

[0046] Examples of the ether carboxylate include polyoxyethylene (4.5) lauryl ether acetic acid, etc., and these may be potassium salts, sodium salts, or ammonium salts.

[0047] 2) Fatty acid As the fatty acid, those represented by the following general formula (2) can be preferably exemplified. R 2 -COOH (2) In the general formula (2), R 2 From the viewpoints of water solubility and the function as an auxiliary agent, it is preferably a linear or branched alkyl group or alkenyl group having 4 to 22 carbon atoms, more preferably a linear or branched alkyl group or alkenyl group having 8 to 22 carbon atoms, and still more preferably a linear or branched alkyl group or alkenyl group having 12 to 18 carbon atoms.

[0048] Examples of the fatty acid include oleic acid, etc., and these may be potassium salts, sodium salts, or ammonium salts.

[0049] 3) Hydroxy monocarboxylic acid As the hydroxy monocarboxylic acid, from the viewpoints of water solubility and the function as an auxiliary agent, it preferably has a hydrocarbon group having 2 or more and 12 or less carbon atoms, and more preferably has a hydrocarbon group having 3 or more and 8 or less carbon atoms.

[0050] Examples of the hydroxy monocarboxylic acid include glycolic acid, lactic acid, glyceric acid, gluconic acid, or pantothenic acid.

[0051] 4) Polycarboxylic acids The polycarboxylic acid is preferably a hydrocarbon group having 2 to 12 carbon atoms, and more preferably a hydrocarbon group having 3 to 8 carbon atoms, from the viewpoint of water solubility and the function of an auxiliary agent.

[0052] The polycarboxylic acid is preferably a dicarboxylic acid, such as malic acid, tartaric acid, oxalic acid, malonic acid, maleic acid, succinic acid, or glutaric acid, and the tricarboxylic acid is such as citric acid.

[0053] The organic carboxylic acid compounds described in 1) to 4) above may be used individually or in combination of two or more.

[0054] In step 1, preferably in a dispersion containing a photocatalyst, a metal salt (precursor of the solid mediator), and an organic carboxylic acid compound, the organic carboxylic acid compound is preferably contained (mixed) in an amount of 100 parts by mass or more, more preferably 500 parts by mass or more, even more preferably 1,000 parts by mass or more, and from the viewpoint of increasing the coverage rate of the solid mediator on the oxygen-evolving photocatalyst and improving catalytic activity, per 100 parts by mass of the solid mediator or its precursor (metal salt), the amount is preferably 100,000 parts by mass or less, more preferably 50,000 parts by mass or less, even more preferably 20,000 parts by mass or less, preferably 100 parts by mass or more and 100,000 parts by mass or less, more preferably 500 parts by mass or more and 50,000 parts by mass or less, and even more preferably 1,000 parts by mass or more and 20,000 parts by mass or less.

[0055] In step 1, preferably, in the dispersion, the solid mediator or its precursor (metal salt) is preferably contained (mixed) in an amount of 0.1 parts by mass or more, more preferably 1 part by mass or more, even more preferably 5 parts by mass or more, with respect to 100 parts by mass of the oxygen-evolving photocatalyst and from the viewpoint of increasing the coverage rate of the solid mediator on the oxygen-evolving photocatalyst and improving catalytic activity. Similarly, from the same viewpoint, preferably 1000 parts by mass or less, more preferably 500 parts by mass or less, even more preferably 100 parts by mass or less, preferably 0.1 parts by mass or more and 1000 parts by mass or less, more preferably 1 part by mass or more and 500 parts by mass or less, and even more preferably 5 parts by mass or more and 100 parts by mass or less.

[0056] The irradiated light may be sunlight or artificial light (such as fluorescent lamps, UV lamps, LEDs, mercury lamps, xenon lamps, metal halide lamps, sodium lamps, and halogen lamps). The wavelength of the light is preferably 180 to 1000 nm, more preferably 300 to 800 nm, from the viewpoint of increasing the coverage rate of the solid mediator on the oxygen-evolving photocatalyst.

[0057] The irradiation light intensity is preferably 1 mW / cm², from the viewpoint of increasing the coverage rate of the solid mediator on the electron collection surface of the oxygen-evolving photocatalyst. 2 More preferably 2 mW / cm² 2 More preferably 5 mW / cm² 2 The above is true, and from a similar viewpoint, a preferred 200 mW / cm² 2 The following, more preferably 100 mW / cm² 2 More preferably, 50 mW / cm² 2 The following applies:

[0058] Step 2: This step involves introducing ionic groups into the solid mediator (hereinafter referred to as "Aspect 1") to obtain an oxygen-evolving photocatalyst in which the solid mediator having the ionic groups is bonded.

[0059] To introduce an ionic group into the solid mediator, a compound containing both a group having affinity for the solid mediator and an ionic group can be used. Specifically, when the solid mediator contains gold (atoms), silver, or platinum, from the viewpoint of affinity with the metal, a thiol compound having an ionic group or a selenium compound having an ionic group is preferred, and more preferably, a thiol compound having an ionic group.

[0060] The thiol compound having the ionic group has at least one ionic group (ionic substituent), and among the ionic groups, anionic groups include, for example, sulfonic acid groups, phosphonic acid groups, phosphoric acid groups, and carboxyl groups, and cationic groups include ammonium groups. Among these, thiol compounds having ammonium groups or carboxyl groups are preferred from the viewpoint of enhancing catalytic activity. From a similar viewpoint, the aforementioned ionic group is more preferably an anionic group, and even more preferably a carboxylate anion.

[0061] The thiol compound having the ionic group preferably has an alkylene group with 1 or more carbon atoms, more preferably with 2 or more carbon atoms, and also preferably has an alkylene group with 18 or fewer carbon atoms, more preferably with 14 or fewer carbon atoms, and even more preferably with 8 or fewer carbon atoms.

[0062] From the viewpoint of enhancing catalytic activity, the thiol compound having the ionic group preferably has two or fewer thiol groups, and more preferably has one thiol group.

[0063] If the thiol compounds having the ionic group also have an acidic group, they may be acids, salts, or mixtures thereof.

[0064] Examples of thiol compounds having the aforementioned ionic group include thiomalic acid, 3-mercaptopropionic acid, thioglycolic acid, and (11-mercaptoundecyl)trimethylammonium, which are preferred.

[0065] In step 2, the thiol compound having an ionic group is preferably contained (mixed) in an amount of 0.1 parts by mass or more, more preferably 1 part by mass or more, with respect to 100 parts by mass of oxygen-evolving photocatalyst on which the solid mediator is bonded (supported), from the viewpoint of enhancing catalytic activity. Similarly, from the same viewpoint, it is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, preferably 0.1 parts by mass or more and 100 parts by mass or less, and more preferably 1 part by mass or more and 50 parts by mass or less, and the reaction is carried out.

[0066] Step 2': This step involves introducing ionic groups into the solid mediator ("Aspect 1"), and further reacting the ionic groups with an ionic polymer having a charge opposite to that of the ionic groups (hereinafter referred to as "Aspect 2") to obtain an oxygen-evolving photocatalyst to which the solid mediator having the ionic polymer is bonded.

[0067] Similar to Embodiment 1, an ionic group is introduced into the solid mediator, and an ionic polymer having a charge opposite to that of the ionic group is reacted (ionic bonded) with the ionic group ("Embodiment 2"), thereby obtaining an oxygen-evolving photocatalyst in which the solid mediator into which the ionic group originating from the ionic polymer is bonded.

[0068] As a method for introducing ionic groups into the solid mediator and further reacting (ionic bonding) the ionic groups with an ionic polymer having a charge opposite to that of the ionic groups, for example, (i) a method in which an anionic group is introduced into the solid mediator by mixing and reacting an oxygen-evolving photocatalyst on which the solid mediator is bonded (supported) with a thiol compound having an anionic group, and then mixing and reacting (ionic bonding) a polymer having a cationic group having a charge opposite to that of the anionic group (hereinafter also simply referred to as "cationic polymer"). Furthermore, other methods include (ii) a method in which an oxygen-evolving photocatalyst on which a solid mediator is bonded (supported) is reacted with a thiol compound having a cationic group to introduce a cationic group into the solid mediator, and then a polymer having anionic groups with the opposite charge to the cationic group (hereinafter also simply referred to as "anionic polymer") is mixed and reacted (ionically bonded) thereto. Through these methods, the solid mediator has a structure in which an ionic group and an ionic polymer are ionically bonded to the ionic polymer.

[0069] The term "ionic polymer" refers to either a cationic polymer or anionic polymer. As will be explained later, "ionic polymer A" and "ionic polymer B" are used for convenience to distinguish between the ionic properties of ionic polymers. Specifically, if ionic polymer A is a cationic polymer, then ionic polymer B is an anionic polymer, and if ionic polymer A is an anionic polymer, then ionic polymer B is a cationic polymer.

[0070] Examples of cationic polymers, from the viewpoint of enhancing catalytic activity, include polymers having a quaternary ammonium group, specifically, one or more selected from cationized polysaccharides, polymers or copolymers thereof of diallyl quaternary ammonium salt, polymers or copolymers thereof of (meth)acryloyloxyethyl quaternary ammonium salt, polymers or copolymers thereof of (meth)acrylamidopropyl quaternary ammonium salt, and polymers thereof of dimethylamine epichlorohydrin, among which are (a) cationized cellulose, (b) cationized guar gum, (c) at least one selected from the group consisting of diallyl quaternary ammonium salt polymer and diallyl quaternary ammonium salt / acrylamide copolymer, or (d) (meth)acryloyloxyethyl quaternary ammonium salt polymer and (meth)acryloyloxyethyl quaternary ammonium salt / acrylamide copolymer, (e) dimethylamine epichlorohydrin polymer, etc., with (c) being preferred. The quaternary ammonium group also includes tertiary amines to which protons have been added.

[0071] (a) Cationic cellulose: The degree of cation substitution of the cationized cellulose is preferably 0.01 to 1, and more preferably 0.02 to 0.5, from the viewpoint of enhancing catalytic activity. Furthermore, the weight-average molecular weight of the cationized cellulose is preferably about 100,000 to 8,000,000 from the viewpoint of enhancing catalytic activity. Examples of commercially available products mentioned in (a) above include the product name "Poise C-80H" (manufactured by Kao Corporation) and "Polymer JR-400" (manufactured by Dow Chemical Corporation).

[0072] (b) Cationized guar gum: From the viewpoint of enhancing catalytic activity, the degree of cationic substitution of the cationized guar gum is preferably 0.01 to 1, and more preferably 0.02 to 0.5, of which cationic groups are introduced into the sugar unit. Examples of commercially available products under (b) above include the "Jaguar C-13C" sold by Rhodia Inc. under the trademark name "Jaguar".

[0073] (c) Diallyl quaternary ammonium salt polymers and diallyl quaternary ammonium salt / acrylamide copolymers: The weight-average molecular weight of the diallyl quaternary ammonium salt polymer is preferably about 30,000 to 1,000,000 from the viewpoint of enhancing catalytic activity, and the weight-average molecular weight of the diallyl quaternary ammonium salt / acrylamide copolymer is preferably about 30,000 to 2,000,000, and more preferably about 1,000,000 to 2,000,000 from the viewpoint of enhancing catalytic activity. Examples of commercially available products under (c) above include "Marcoat 100 (weight-average molecular weight: 150,000)" and "Marcoat 550 (weight-average molecular weight: 1,600,000)," which are sold by Noveon under the trademark name "Marcoat."

[0074] (d) (meth)acryloyloxyethyl quaternary ammonium salt polymers and (meth)acryloyloxyethyl quaternary ammonium salt / acrylamide copolymers: The weight-average molecular weight of the methacryloyloxyethyl quaternary ammonium salt / acrylamide copolymer is preferably about 1 million to 10 million, and more preferably about 2 million to 6 million, from the viewpoint of enhancing catalytic activity.

[0075] (e) Dimethylamine epichlorohydrin polymer (poly-2-hydroxypropyldimethylammonium chloride): The weight-average molecular weight of the dimethylamine epichlorohydrin polymer is preferably about 1,000 to 100,000, and more preferably about 3,000 to 10,000, from the viewpoint of enhancing catalytic activity. Examples of commercially available products under (e) above include those sold by Yokkaichi Gosei Co., Ltd. under the trademark name "Catiomaster," such as "Catiomaster PD-7 (weight-average molecular weight: 0.5 million)."

[0076] The anionic polymer is preferably a polymer having a sulfate group, sulfonic acid group, phosphonic acid group, phosphoric acid group, or carboxyl group, and among these, from the viewpoint of enhancing catalytic activity, a polymer having a sulfonic acid group (sulfonic acid-based polymer) or a polymer having a carboxyl group (carboxylic acid-based polymer) is preferred.

[0077] Sodium polystyrene sulfonate is preferred as the polymer having the sulfonic acid group.

[0078] The carboxylic acid polymer is preferably a carboxylic acid polymer having structural units derived from anionic group-containing monomers selected from acrylates, methacrylates, and maleates. While the carboxylic acid polymer is preferably composed of the anionic group-containing monomers, it may also be a copolymer of the anionic group-containing monomers and monomers other than the anionic group-containing monomers.

[0079] The weight-average molecular weight (Mw) of the carboxylic acid polymer is preferably 1,000 or more, more preferably 5,000 or more, and even more preferably 10,000 or more, from the viewpoint of enhancing catalytic activity. Also, from the same viewpoint, it is preferably 500,000 or less, more preferably 100,000 or less, even more preferably 50,000 or less, and even more preferably 35,000 or less.

[0080] The weight-average molecular weight (Mw) of the polymer having the sulfonic acid group is preferably 10,000 or more, more preferably 50,000 or more, and even more preferably 100,000 or more, from the viewpoint of enhancing catalytic activity. Also, from the same viewpoint, it is preferably 5 million or less, more preferably 2 million or less, even more preferably 1 million or less, and even more preferably 600,000 or less.

[0081] Examples of monomers other than the anionic group-containing monomers include alkyl (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate, N-substituted (meth)acrylamides such as N,N-dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide, isopropyl (meth)acrylamide, and tert-butyl (meth)acrylamide, vinyl ethers such as methyl vinyl ether and butyl vinyl ether, styrene, and olefinic hydrocarbons such as ethylene, propylene, and isobutylene. These may be used individually or in combination of two or more. Among these, alkyl (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate are preferred. In this specification, "(meth)acrylic" means "acrylic or methacrylic."

[0082] In this specification, the weight-average molecular weight (Mw) of the cationic polymer and the anionic polymer is the value obtained by gel permeation chromatography (GPC) using polystyrene as the standard substance.

[0083] In step 2', with respect to 100 parts by mass of the oxygen-evolving photocatalyst on which the solid mediator to which the ionic group is introduced is bonded (supported), the ionic polymer is preferably mixed in an amount of 5 parts by mass or more, more preferably 10 parts by mass or more, and also preferably 500 parts by mass or less, and more preferably 100 parts by mass or less, from the viewpoint of enhancing photocatalytic activity.

[0084] Step 3: This step involves introducing an ionic polymer having a charge opposite to that of the ionic group into the hydrogen-generating photocatalyst.

[0085] Step 3': This step involves introducing an ionic polymer having a charge opposite to that of the ionic polymer into the hydrogen-generating photocatalyst.

[0086] Step 3 is a step of introducing an ionic polymer having a charge opposite to that of the ionic groups of the solid mediator in Step 2 into the hydrogen-generating photocatalyst. For example, if the ionic group of the solid mediator in step 2 is an anionic group, the hydrogen-generating photocatalyst may be mixed and dispersed with a polymer having cationic groups to introduce an ionic polymer with the opposite charge, or the hydrogen-generating photocatalyst may be mixed and dispersed with a polymer having anionic groups, and then further mixed and redispersed with a polymer having cationic groups to introduce an ionic polymer with the opposite charge. That is, a bonded photocatalyst having a solid mediator between a hydrogen-evolving photocatalyst and an oxygen-evolving photocatalyst, wherein the solid mediator has an ionic group, and the hydrogen-evolving photocatalyst has an ionic polymer B having a charge opposite to the charge of the ionic group via an ionic polymer A, and the solid mediator and the hydrogen-evolving photocatalyst are bonded together by an ionic bond between the ionic group and the ionic polymer B. This means that the ionic polymer in the hydrogen-generating photocatalyst is ionic polymer A, and via ionic polymer A, there is ionic polymer B having a charge opposite to the charge of the ionic group, and the solid mediator and the hydrogen-generating photocatalyst are joined by ionic bonding between the ionic group and ionic polymer B. Furthermore, if the ionic group is an anionic group, then ionic polymer A is an anionic polymer and ionic polymer B is a cationic polymer. Also, if the ionic group is a cationic group, then ionic polymer A is a cationic polymer and ionic polymer B is an anionic polymer.

[0087] Furthermore, step 3' is a step in which the hydrogen-generating photocatalyst is mixed and dispersed with an ionic polymer having a charge opposite to that of the ionic groups of the solid mediator in step 2', thereby introducing the ionic polymer having a charge opposite to that of the ionic groups into the hydrogen-generating photocatalyst. For example, if the ionic group of the solid mediator in step 2' is an anionic group, further mixing and dispersing with a polymer having cationic groups will result in the solid mediator having a cationic polymer. Alternatively, the hydrogen-generating photocatalyst may be mixed and dispersed with a polymer having anionic groups to introduce an ionic polymer with the opposite charge, or the hydrogen-generating photocatalyst may be mixed and dispersed with a polymer having cationic groups, and then further mixed and redispersed with a polymer having anionic groups to introduce an ionic polymer with the opposite charge. That is, a bonded photocatalyst having a solid mediator between a hydrogen-generating photocatalyst and an oxygen-generating photocatalyst, wherein the solid mediator has an ionic group and an ionic polymer B, the ionic group is bonded to the ionic polymer B, the hydrogen-generating photocatalyst has an ionic polymer A having a charge opposite to that of the ionic polymer B, and the solid mediator and the hydrogen-generating photocatalyst are bonded by an ionic bond between the ionic polymer B of the solid mediator and the ionic polymer A of the hydrogen-generating photocatalyst. This means that the ionic polymer in the hydrogen-generating photocatalyst is ionic polymer A, the solid mediator has an ionic group and an ionic polymer B having a charge opposite to that of ionic polymer A, the ionic group is bonded to ionic polymer B, and the solid mediator and the hydrogen-generating photocatalyst are joined by an ionic bond between the ionic polymer B in the solid mediator and the ionic polymer A in the hydrogen-generating photocatalyst. Furthermore, if the ionic group is an anionic group, then ionic polymer A is an anionic polymer and ionic polymer B is a cationic polymer. Also, if the ionic group is a cationic group, then ionic polymer A is a cationic polymer and ionic polymer B is an anionic polymer.

[0088] The ionic polymer can be the anionic polymer or the cationic polymer described in the above-described embodiment 2.

[0089] In steps 3 and 3', the ionic polymer is preferably included (mixed) in 100 parts by mass of the hydrogen-generating photocatalyst in amounts of 5 parts by mass or more, more preferably 20 parts by mass or more, and similarly, preferably 500 parts by mass or less, more preferably 100 parts by mass or less, preferably 5 parts by mass or more and 500 parts by mass or less, and more preferably 20 parts by mass or more and 100 parts by mass or less.

[0090] Step 4: This step involves mixing the oxygen-evolving photocatalyst, which has the solid mediator having the ionic group obtained in Step 2 joined (supported), with the hydrogen-evolving photocatalyst, which has the ionic polymer obtained in Step 3 introduced into it.

[0091] Step 4': This step involves mixing the oxygen-evolving photocatalyst, which is bonded to the solid mediator having the ionic polymer obtained in Step 2', with the hydrogen-evolving photocatalyst, which is introduced with the ionic polymer obtained in Step 3'.

[0092] In steps 4 and 4', the oxygen-evolving photocatalyst to which the solid mediator having the ionic group is bonded (supported) is mixed with the hydrogen-evolving photocatalyst to which the ionic polymer obtained in steps 3 and 3' has been introduced, thereby bonding the oxygen-evolving photocatalyst and the hydrogen-evolving photocatalyst via the solid mediator. For example, when using an oxygen-evolving photocatalyst with a solid mediator containing anionic groups attached (supported), it is sufficient to use a hydrogen-evolving photocatalyst with cationic groups derived from a cationic polymer. Similarly, when using an oxygen-evolving photocatalyst with a solid mediator containing cationic groups attached (supported), it is sufficient to use a hydrogen-evolving photocatalyst with anionic groups derived from an anionic polymer.

[0093] In step 4, the mixed mass ratio (hydrogen-generating photocatalyst into which the ionic polymer obtained in step 3 has been introduced / oxygen-generating photocatalyst into which the solid mediator having the ionic group obtained in step 2 has been bonded (supported)) is preferably 0.01 or more, more preferably 0.05 or more, even more preferably 0.07 or more, from the viewpoint of increasing catalytic activity, preferably 10 or less, more preferably 5 or less, even more preferably 1 or less, preferably 0.01 to 10, more preferably 0.05 to 5, and even more preferably 0.07 to 1.

[0094] Furthermore, in step 4', the mixed mass ratio (hydrogen-generating photocatalyst with the ionic polymer obtained in step 3' introduced / oxygen-generating photocatalyst with the solid mediator having the ionic polymer obtained in step 2' joined) is preferably 0.01 or higher, more preferably 0.05 or higher, even more preferably 0.07 or higher, and from the viewpoint of enhancing catalytic activity, preferably 10 or lower, more preferably 5 or lower, even more preferably 1 or lower, preferably 0.01 to 10, more preferably 0.05 to 5, and even more preferably 0.07 to 1.

[0095] In steps 4 and 4', by adjusting the mixed mass ratio, the solid mediator and the hydrogen-evolving photocatalyst are bonded via the ionic polymer in both embodiment 1 and embodiment 2. The ionic polymer allows for strong electrostatic interactions, preventing separation of the hydrogen-evolving photocatalyst and the oxygen-evolving photocatalyst. This bonding via the solid mediator improves catalytic activity, enabling the decomposition of water into oxygen and hydrogen with high water-splitting efficiency.

[0096] <Photocatalytic composite> The photocatalytic composite of the present invention has the aforementioned bonded photocatalyst on a substrate.

[0097] The substrate is not particularly limited as long as it can immobilize the bonded photocatalyst on its surface, and examples include: ceramic substrates such as alumina; glass substrates such as soda-lime glass and borosilicate glass; quartz substrates; metal substrates such as titanium, copper, tin, iron, aluminum, and stainless steel; organic substrates such as methacrylic resin, acrylic resin, urethane resin, phenolic resin, melamine resin, urea resin, polyester resin, polycarbonate resin, fluororesin, polyethylene, polypropylene, polystyrene, polyamide, polyimide, polyacetal, polyvinyl chloride, and polyvinylidene chloride; fiber substrates such as glass fibers and carbon fibers; and natural substrates such as paper, bamboo, and wood.

[0098] The substrate preferably has pores, and more preferably has continuous pores. This allows the hydrogen gas generated by the water splitting reaction on the surface of the bonded photocatalyst to reach the outside through the pores, enabling more efficient production of hydrogen gas.

[0099] The method for producing the photocatalytic composite is not particularly limited, and examples include (1) a method of applying a dispersion containing the bonding type photocatalyst onto a substrate, drying it, and firing it if necessary to immobilize the bonding type photocatalyst on the substrate, and (2) a method of applying a dispersion containing the oxygen-evolving photocatalyst onto a substrate, drying it, and firing it if necessary to immobilize the oxygen-evolving photocatalyst on the substrate, then bonding the solid mediator to the surface of the oxygen-evolving photocatalyst in the same manner as above, and then bonding the hydrogen-evolving photocatalyst to the surface of the solid mediator.

[0100] <Applications of bonded photocatalysts and photocatalytic composites> The bonded photocatalyst and photocatalytic composite of the present invention can be used not only as a photocatalyst for catalyzing the decomposition of water and alcohol, but also as a photocatalyst for catalyzing the decomposition of organic matter. For example, it can be used as an environmental purification agent, antibacterial / bactericidal agent, deodorizer, or antifouling agent to detoxify harmful organic compounds, bacteria, and odor-causing substances, as well as inorganic compounds (such as ammonium ions, ammonia, nitrate ions, and nitrite ions).

[0101] <Hydrogen production methods> The hydrogen production method using the bonded photocatalyst or the photocatalytic composite of the present invention is not particularly limited, and includes, for example, a hydrogen production method that includes the step of irradiating the bonded photocatalyst or the photocatalytic composite with light in the presence of water or alcohol to cause a decomposition reaction of water or alcohol and generate at least hydrogen.

[0102] The light used for irradiation may be sunlight or artificial light (such as fluorescent lamps, UV lamps, LEDs, mercury lamps, xenon lamps, metal halide lamps, sodium lamps, and halogen lamps). From the viewpoint of promoting the decomposition reaction, the wavelength of the light is preferably 180 to 1000 nm, more preferably 300 to 800 nm. From the viewpoint of promoting the decomposition reaction, the irradiance of the light irradiation is preferably 1 μW / m². 2 More preferably 1 mW / m 2That concludes the explanation. From the viewpoint of promoting the decomposition reaction, the decomposition reaction temperature is preferably 0°C or higher, and more preferably 15°C or higher.

[0103] The amount of hydrogen generated using the bonded photocatalyst or photocatalytic composite of the present invention (per 55 mg of catalyst) is preferably 0.05 (μmol·h). -1 ) or more, more preferably 0.1 (μmol·h) -1 ) or more, more preferably 0.20 (μmol·h) -1 ) or more, with no particular upper limit, but preferably 1 (mol·h -1 ) The following are listed below.

[0104] With regard to the embodiments described above, the present invention further discloses the following embodiments. [1] A junction-type photocatalyst having a solid mediator between a hydrogen-evolving photocatalyst and an oxygen-evolving photocatalyst, The solid mediator and the hydrogen-generating photocatalyst are bonded together via an ionic polymer, forming a bonded photocatalyst. [2] The bonded photocatalyst according to [1], wherein the solid mediator has an ionic group, and the solid mediator and the hydrogen-generating photocatalyst are bonded via an ionic polymer having a charge opposite to that of the ionic group. [3] The aforementioned solid mediator has an ionic group, The hydrogen-generating photocatalyst comprises the ionic polymer having a charge opposite to that of the ionic group, The bonded photocatalyst according to [1] or [2], wherein the solid mediator and the hydrogen-generating photocatalyst are bonded together by ionic bonding between the ionic group and the ionic polymer. [4] The aforementioned solid mediator has an ionic group, The hydrogen-generating photocatalyst comprises an ionic polymer A, The ionic polymer A further contains an ionic polymer B having a charge opposite to that of the ionic group, The bonded photocatalyst according to [1] or [2], wherein the solid mediator and the hydrogen-generating photocatalyst are bonded together by ionic bonding between the ionic group and the ionic polymer B. [5] The solid mediator has an ionic group and, via the ionic group, an ionic polymer B having a charge opposite to that of the ionic group. The hydrogen-generating photocatalyst has an ionic polymer A, The bonded photocatalyst according to [1] or [2], wherein the solid mediator and the hydrogen-generating photocatalyst are bonded together by an ionic bond between the ionic polymer B of the solid mediator and the ionic polymer A of the hydrogen-generating photocatalyst. [6] The junction-type photocatalyst according to any one of [2] to [5], wherein the ionic group is an anionic group or a cationic group. [7] The bonded photocatalyst according to [6], wherein the anionic group is preferably a sulfonic acid group, a phosphonic acid group, a phosphoric acid group, or a carboxyl group, more preferably a sulfonic acid group or a carboxyl group, and the cationic group is a quaternary ammonium group. [8] The aforementioned ionic group is an anionic group, The ionic polymer A is an anionic polymer, The bonded photocatalyst according to [6] or [7], wherein the ionic polymer B is a cationic polymer. [9] The aforementioned ionic group is a cationic group, The ionic polymer A is a cationic polymer, The bonded photocatalyst according to [6] or [7], wherein the ionic polymer B is an anionic polymer.

[10] The bonded photocatalyst according to [8] or [9], wherein the anionic polymer preferably comprises a polymer having a sulfate group, a sulfonic acid group, a phosphonic acid group, a phosphoric acid group, or a carboxyl group, more preferably a polymer having a sulfonic acid group (sulfonic acid polymer) or a polymer having a carboxyl group (carboxylic acid polymer).

[11] The cationic polymer preferably comprises a polymer having a quaternary ammonium group, more preferably one or more selected from cationized polysaccharides, polymers or copolymers thereof of diallyl quaternary ammonium salts, polymers or copolymers thereof of (meth)acryloyloxyethyl quaternary ammonium salts, polymers or copolymers thereof of (meth)acrylamidopropyl quaternary ammonium salts, and polymers thereof of dimethylamine epichlorohydrin, and more preferably at least one selected from the group consisting of (a) cationized cellulose, (b) cationized guar gum, (c) diallyl quaternary ammonium salt polymers and diallyl quaternary ammonium salt / acrylamide copolymers, (d) (meth)acryloyloxyethyl quaternary ammonium salt polymer, (meth)acryloyloxyethyl quaternary ammonium salt / acrylamide copolymer, or (e) dimethylamine epichlorohydrin polymer, as described in any of [8] to

[10] .

[12] The hydrogen-generating photocatalyst is a metal oxide, and is a junction-type photocatalyst according to any one of [1] to

[11] .

[13] The junction-type photocatalyst according to

[12] , wherein the metal oxide is preferably TiO2, SrTiO3, La2Ti2O7, SnNb2O6, and compounds obtained by doping these with one or more metals of Cr, Sb, Ta, Ir, and La, more preferably SrTiO3.

[14] The oxygen-evolving photocatalyst is a metal oxide, and is a junction-type photocatalyst according to any one of [1] to

[13] .

[15] The aforementioned metal oxide is preferably BiVO4, TiO2, WO3, SrTiO3, Ag3PO4, SnNb2O6, Bi2WO6, Fe2TiO5, Fe2O3, Bi2MoO6, and compounds obtained by doping these with one or more metals such as Cr, Ni, Sb, Nb, Th, Mo, and W, more preferably BiVO4, according to the junction-type photocatalyst described in

[14] .

[16] The solid mediator is a transition metal or a compound thereof, according to any one of [1] to

[15] , a junction-type photocatalyst.

[17] The solid mediator preferably comprises a transition metal such as gold, silver, copper, nickel, titanium, manganese, rhodium, palladium, ruthenium, and iridium, more preferably gold or silver, and even more preferably gold, according to any one of [1] to

[15] .

[18] A bonded photocatalyst according to any one of [1] to

[17] , wherein the solid mediator is bonded to the oxygen-evolving photocatalyst by at least one method selected from the group consisting of photoelectrodeposition, impregnation and loading, and precipitation, using an organic carboxylic acid compound and a solid mediator or its precursor.

[19] A method for producing a junction-type photocatalyst as described in any of [1] to

[18] , A method for producing a bonded photocatalyst, comprising the following steps 1 to 4. However, steps 2 and 3 may be performed in any order. Step 1: A step of bonding an organic carboxylic acid compound to an oxygen-evolving photocatalyst by at least one method selected from the group consisting of photoelectrodeposition, impregnation and loading, and precipitation, using the solid mediator or its precursor. Step 2: A step to introduce ionic groups into the solid mediator and obtain an oxygen-evolving photocatalyst by bonding the solid mediator having the ionic groups. Step 3: A step of introducing an ionic polymer having a charge opposite to the charge of the ionic group into the hydrogen-generating photocatalyst. Step 4: A step of mixing the oxygen-evolving photocatalyst to which the solid mediator having the ionic group obtained in Step 2 has been bonded with the hydrogen-evolving photocatalyst to which the ionic polymer obtained in Step 3 has been introduced.

[20] A method for producing a junction-type photocatalyst as described in any of [1] to

[18] , A method for producing a bonded photocatalyst, comprising the following steps 1, 2', 3', and 4'. However, steps 2' and 3' may be performed in any order. Step 1: A step of bonding an organic carboxylic acid compound to an oxygen-evolving photocatalyst by at least one method selected from the group consisting of photoelectrodeposition, impregnation and loading, and precipitation, using the solid mediator or its precursor. Step 2': A step to introduce ionic groups into the solid mediator, and further react the ionic groups with an ionic polymer having the opposite charge to the ionic groups, thereby obtaining an oxygen-evolving photocatalyst by bonding the solid mediator having the ionic polymer. Step 3': A step of introducing an ionic polymer having a charge opposite to that of the ionic polymer into the hydrogen-generating photocatalyst. Step 4': A step of mixing the oxygen-evolving photocatalyst to which the solid mediator having the ionic polymer obtained in Step 2' has been bonded with the hydrogen-evolving photocatalyst to which the ionic polymer obtained in Step 3' has been introduced. [twenty one] A method for producing a junction-type photocatalyst according to

[19] or

[20] , wherein, in step 1, the photoelectrodeposition method is used, the method further includes irradiating a dispersion containing an oxygen-evolving photocatalyst in addition to the organic carboxylic acid compound and the solid mediator or its precursor with light. [twenty two] A method for producing a conjugated photocatalyst according to any one of

[19] to

[21] , wherein the organic carboxylic acid compound is at least one selected from the group consisting of ether carboxylates, fatty acids, hydroxy monocarboxylic acids, and polycarboxylic acids. [twenty three] The organic carboxylic acid compound includes an ether carboxylate, A method for producing a bonded photocatalyst according to any one of

[19] to

[22] , wherein the solid mediator contains gold. [twenty four] A method for producing a junction-type photocatalyst according to any one of

[19] to

[23] , wherein in step 1, the amount of the organic carboxylic acid compound is preferably 100 parts by mass or more and 100,000 parts by mass or less, more preferably 500 parts by mass or more and 50,000 parts by mass or less, and even more preferably 1,000 parts by mass or more and 20,000 parts by mass or less, with respect to 100 parts by mass of the solid mediator or its precursor (metal salt). [twenty five] A method for producing a junction-type photocatalyst according to any one of

[19] to

[24] , wherein, in step 1, the amount of the solid mediator or its precursor (metal salt) is preferably 0.1 parts by mass or more and 1000 parts by mass or less, more preferably 1 part by mass or more and 500 parts by mass or less, and even more preferably 5 parts by mass or more and 100 parts by mass or less, with respect to 100 parts by mass of the oxygen-evolving photocatalyst.

[26] A method for producing a junction-type photocatalyst according to any one of

[19] to

[25] , wherein in step 2 or step 2', a thiol compound having an ionic group is used in the introduction of an ionic group into the solid mediator.

[27] A method for producing a bonded photocatalyst according to any one of

[19] to

[26] , wherein, in step 2 or step 2', in the introduction of ionic groups to the solid mediator, the thiol compound having ionic groups is reacted with 100 parts by mass of oxygen-evolving photocatalyst on which the solid mediator is bonded (supported) in an amount of preferably 0.1 parts by mass or more and 100 parts by mass or less, more preferably 1 part by mass or more and 50 parts by mass or less.

[28] A method for producing a bonded photocatalyst according to any one of

[19] to

[27] , wherein in step 3 and step 3', the ionic polymer is preferably mixed with 100 parts by mass of the hydrogen generating photocatalyst, preferably in an amount of 5 parts by mass or more and 500 parts by mass or less, and more preferably in an amount of 20 parts by mass or more and 100 parts by mass or less.

[29] In step 4, the mixing mass ratio (hydrogen-generating photocatalyst into which the ionic polymer obtained in step 3 has been introduced / oxygen-generating photocatalyst into which the solid mediator having the ionic group obtained in step 2 has been bonded (supported)) is preferably 0.01 or more and 10 or less, more preferably 0.05 or more and 5 or less, and even more preferably 0.07 or more and 1 or less, according to any one of

[19] ,

[21] to

[28] .

[30] In step 4', the mixing mass ratio (hydrogen-generating photocatalyst obtained in step 3' into which the ionic polymer has been introduced / oxygen-generating photocatalyst obtained in step 2' into which a solid mediator having the ionic polymer has been bonded) is preferably 0.01 or more and 10 or less, more preferably 0.05 or more and 5 or less, and even more preferably 0.07 or more and 1 or less, according to any one of

[20] to

[28] .

[31] A photocatalytic composite having a bonded photocatalyst described in any of [1] to

[18] on a substrate.

[32] Use of a bonded photocatalyst described in any of [1] to

[18] as a photocatalyst for water splitting or a photocatalyst for organic matter decomposition.

[33] A method for producing hydrogen, comprising irradiating light onto a conjugated photocatalyst according to any of [1] to

[18] or a photocatalytic composite according to

[31] in the presence of water or alcohol. [Examples]

[0105] The present invention will be described in detail below based on examples. Unless otherwise specified, reagents used were those manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0106] (Example 1) <Preparation of bismuth vanadate (BiVO4)> 6.06 g of bismuth nitrate pentahydrate and 1.46 g of ammonium vanadate were dissolved in 50 mL of 2 mol / L nitric acid. Then, ammonia water was added dropwise until the pH reached 2.0, and 0.12 g of sodium chloride was added to obtain a yellow-orange dispersion. The dispersion was transferred to a fluororesin container and reacted at 170 °C for 24 hours in an autoclave. After the reaction, the precipitate was separated by suction filtration, and the obtained precipitate was washed with deionized water, then filtered and dried under reduced pressure. The sample obtained after drying was calcined in an electric furnace at 500 °C for 2 hours to obtain yellow powder BiVO4. The prepared BiVO4 particles were confirmed to have a decahedral structure with the {010} plane as the basal plane by SEM (manufactured by JEOL Ltd., JSM-IT500HR) observation. Also, from powder X-ray diffraction measurement (manufactured by Rigaku Corporation, MiniFlex II diffractometer), it was confirmed that the crystal system was monoclinic scheelite crystal.

[0107] <Preparation of Au - BiVO4> 150 mg of the prepared BiVO4 was added to a glass container and dispersed in 20 mL of deionized water. 18.8 mg of tetrachloroauric acid tetrahydrate and 7.76 mL of AKYPO RLM-45NV (manufactured by Kao Corporation, sodium polyoxyethylene (4.5) lauryl ether acetate, active ingredient 24%) were added, and while stirring with a stirrer chip, light irradiation was performed for 5 minutes using a 300 W solar simulator (manufactured by PECCELL Corporation, PEC-L01, current value 7.5 A) to cause a reaction. From a light power meter (manufactured by Hioki Electric Co., Ltd., H3664), the irradiation light intensity at a wavelength of 435 nm set value was 40 mW / cm 2 2. After the reaction, the precipitate was separated by suction filtration, and the obtained precipitate was washed with deionized water, then filtered and dried under reduced pressure to obtain green powder Au - BiVO4. When the prepared Au - BiVO4 particles were observed by SEM, as shown in Fig. 1, an image in which an Au layer was selectively formed on the {010} plane of BiVO4 was obtained.

[0108] <Preparation of MPA-treated Au - BiVO4> 50 mg of the prepared Au-BiVO₄ was added to a glass container, dispersed in 5 mL of deionized water, 12 μL of 3-mercaptopropionic acid (MPA) was added, and the mixture was stirred with a stirrer chip for 2 hours. After the reaction was completed, the precipitate was separated by suction filtration, and the obtained precipitate was washed with deionized water, followed by filtration and drying under reduced pressure to obtain green powder of MPA-treated Au-BiVO₄.

[0109] <Preparation of Cation-Modified Au-BiVO₄> 50 mg of the prepared MPA-treated Au-BiVO₄ was added to a glass container, dispersed in 1 mL of a 0.8 mass% aqueous solution of Merquat 100 (manufactured by Lubrizol Corporation), a cationic polymer (polymer pure content: 8.0 mg), and the mixture was stirred with a stirrer chip for 2 hours. Then, the precipitate was separated by suction filtration, and the obtained precipitate was washed with deionized water, followed by filtration and drying under reduced pressure to obtain green powder of cation-modified Au-BiVO₄. When the volume particle size distribution in water of the obtained cation-modified Au-BiVO₄ was evaluated by laser diffraction scattering measurement (LA960 manufactured by Horiba, Ltd.), it was as shown in Figure 3.

[0110] <Preparation of Ru-Supported SrTiO₃> 120 mg of SrTiO₃ (manufactured by High-Purity Chemical Research Institute) was added to a glass container, dispersed in 160 mL of deionized water, 2.2 mg of ruthenium chloride n-hydrate was added, and ultrasonic dispersion treatment was performed for 15 minutes. 20 mL of methanol was added to the obtained dispersion, and while bubbling argon gas at 20 mL / min for 60 minutes, light irradiation was performed with a 300 W xenon lamp for 120 minutes to cause a reaction and support Ru on SrTiO₃. After the reaction was completed, the precipitate was separated by suction filtration, and the obtained precipitate was washed with deionized water, followed by filtration and drying under reduced pressure to obtain gray powder of Ru-supported SrTiO₃.

[0111] <Preparation of Anion-Modified Ru-Supported SrTiO₃> 100 mg of the prepared Ru-supported SrTiO3 was added to a glass container and dispersed in 1.13 mL of a 4% by mass aqueous solution of the anionic polymer Poiz 520 (weight-average molecular weight: 21,000, a copolymer of sodium acrylate / sodium maleate, manufactured by Kao Corporation) (polymer content: 0.05 g). The mixture was then subjected to ultrasonic irradiation for 30 minutes. The dispersion was then filtered by suction, and the solid material was recovered. After washing the recovered material with deionized water, it was filtered and dried under reduced pressure to obtain an anionically modified Ru-supported SrTiO3 as a gray powder. The volume particle size distribution of the obtained anionically modified Ru-supported SrTiO3 in water was evaluated and is shown in Figure 3.

[0112] <Preparation of junction-type photocatalysts> 50 mg of cation-modified Au-BiVO4 and 5 mg of anion-modified Ru-supported SrTiO3 were added to a glass container and dispersed in 1 mL of deionized water. After stirring for 30 minutes and allowing to stand, all particles in the dispersion were observed to have settled. The volume particle size distribution of the obtained particles was measured and is shown in Figure 3. It was confirmed that no fine particle components derived from Ru-supported SrTiO3 existed alone, and that Au-BiVO4 and Ru-supported SrTiO3 were 100% composited. When these particles (bonded particles) were coated onto a silicon substrate and observed with an SEM after drying, an image was obtained showing Ru-supported SrTiO3 formed on top of Au formed on the {010} plane of BiVO4, as shown in Figure 2.

[0113] <Evaluation of water splitting activity of bonded photocatalysts> The bonded particles obtained above were added to 180 mL of water, and argon gas was bubbled in at 20 mL / min for 30 minutes. The mixture was then irradiated with light using a 300 W xenon lamp. The generated gas was analyzed using a gas chromatograph with a TCD detector. Light irradiation was continued for 5 hours, and the average hydrogen generation rate was calculated. The results are shown in Table 1.

[0114] (Comparative Example 1) When preparing Au-BiVO4, instead of adding AKYPO RLM-45NV, 0.36 mL of methanol was added as a hole sacrificial reagent (auxiliary agent). Furthermore, when preparing the conjugated photocatalyst, 3-mercaptopropionic acid treatment and cation modification of Au-BiVO4 were not performed, and anionic modification of Ru-supported SrTiO3 was not performed. Otherwise, the sample was prepared using the same procedure as in Example 1. Figure 4 shows the volume particle size distribution of the mixed sample of Ru-supported SrTiO3 and Au-BiVO4. The volume particle size distribution of Au-BiVO4 shows a distribution in the 1-5 μm region, which is not observed, indicating that the mixed sample was not uniformly compounded and some Ru-supported SrTiO3 was suspended.

[0115] (Example 2) A conjugated photocatalyst was prepared in the same manner as in Example 1, except that instead of adding AKYPO RLM-45NV when preparing Au-BiVO4, 0.36 mL of methanol was added as a hole sacrificial reagent (auxiliary agent).

[0116] (Example 3) The conjugated photocatalyst was prepared using the same procedure as in Example 1, except that instead of adding AKYPO RLM-45NV when preparing Au-BiVO4, 1.55 mL of potassium oleate FR-14 (manufactured by Kao Corporation) was added.

[0117] (Example 4) <Preparation of cation-modified Ru-supported SrTiO3> 50 mg of anion-modified Ru-supported SrTiO3 powder, prepared in the same manner as in Example 1, was dispersed in 1 mL of a 0.8% by mass aqueous solution of Merquat 100 (manufactured by Lubrizol), a cationic polymer (polymer content 8.0 mg), and the mixture was stirred with a stirrer tip for 10 minutes. After the reaction was complete, the precipitate was separated by suction filtration. The obtained precipitate was washed with deionized water and then dried under reduced pressure to obtain gray powder cationic-modified Ru-supported SrTiO3.

[0118] <Preparation of junction-type photocatalysts> Similar to Example 1, 50 mg of the prepared MPA-treated Au-BiVO4 and 5 mg of cation-modified Ru-supported SrTiO3 were added to a glass container, dispersed in 1 mL of deionized water, and stirred for 30 minutes to prepare a bonded photocatalyst.

[0119] (Example 5) <Preparation of cation-modified Ru-supported SrTiO3> 50 mg of Ru-supported SrTiO3 powder, prepared in the same manner as in Example 1, was dispersed in 1 mL of a 0.8% by mass aqueous solution of Merquat 100 (manufactured by Lubrizol), a cationic polymer (polymer content 8.0 mg), and subjected to ultrasonic irradiation for 30 minutes. The dispersion was then filtered by suction, and the solid material was recovered. After washing the recovered material with deionized water, it was filtered and dried under reduced pressure to obtain gray powder cationic-modified Ru-supported SrTiO3.

[0120] <Preparation of junction-type photocatalysts> Similar to Example 1, 50 mg of the prepared MPA-treated Au-BiVO4 and 5 mg of cation-modified Ru-supported SrTiO3 were added to a glass container, dispersed in 1 mL of deionized water, and stirred for 30 minutes to prepare a bonded photocatalyst.

[0121] (Example 6) In the procedure of Example 1, when preparing cation-modified Au-BiVO4, instead of adding Merquart 100, a cationic polymer, 2-(dimethylamino)ethyl diethyl sulfate polymer of methacrylate (weight-average molecular weight: 160,000, 1 mL of 0.8% by mass aqueous solution (polymer content 8.0 mg)), was added. Furthermore, when preparing the anionically modified Ru-supported SrTiO3, instead of adding Poiz 520, 1.13 mL of a 4% aqueous solution of the anionic polymer sodium polystyrene sulfonate (Na-PSS) (weight-average molecular weight: 500,000 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)) (polymer content 0.05 g) was added. Otherwise, the bonded photocatalyst was prepared using the same procedure as in Example 1.

[0122] Table 1 shows the evaluation results of the water-splitting activity for Examples 1-6 and Comparative Example 1.

[0123] [Table 1]

[0124] From the evaluation results in Table 1 above, it was confirmed that Examples 1 to 6, which used bonded photocatalysts in which a solid mediator and a hydrogen-generating photocatalyst were bonded via an ionic polymer, exhibited superior water-splitting activity compared to Comparative Example 1, which used a bonded photocatalyst in which the solid mediator and hydrogen-generating photocatalyst were not bonded via an ionic polymer. [Industrial applicability]

[0125] The bonded photocatalyst of the present invention is useful as a photocatalyst used for water splitting or organic matter decomposition.

Claims

1. A junction-type photocatalyst having a solid mediator between a hydrogen generating photocatalyst and an oxygen generating photocatalyst, A bonded photocatalyst, in which the solid mediator and the hydrogen generating photocatalyst are bonded via an ionic polymer.

2. The bonded catalyst according to claim 1, wherein the solid mediator has an ionic group, and the solid mediator and the hydrogen generating photocatalyst are bonded via an ionic polymer having an opposite charge to the ionic group.

3. the solid mediator has an ionic group; the hydrogen generating photocatalyst has the ionic polymer having an opposite charge to the charge of the ionic group; 3. The bonded photocatalyst according to claim 1, wherein the solid mediator and the hydrogen generating photocatalyst are bonded together by an ionic bond between the ionic group and the ionic polymer.

4. the solid mediator has an ionic group; The hydrogen generating photocatalyst comprises an ionic polymer A and The ionic polymer B further has an opposite charge to the charge of the ionic group via the ionic polymer A, 3. The bonded photocatalyst according to claim 1, wherein the solid mediator and the hydrogen generating photocatalyst are bonded together by an ionic bond between the ionic group and the ionic polymer B.

5. the solid mediator has an ionic group and, via the ionic group, an ionic polymer B having a charge opposite to that of the ionic group; the hydrogen generating photocatalyst contains an ionic polymer A, 3. The bonded photocatalyst according to claim 1, wherein the solid mediator and the hydrogen generating photocatalyst are bonded by an ionic bond between the ionic polymer B contained in the solid mediator and the ionic polymer A contained in the hydrogen generating photocatalyst.

6. The junction-type photocatalyst according to claim 2 , wherein the ionic group is an anionic group or a cationic group.

7. the ionic group is an anionic group, the ionic polymer A is an anionic polymer, The junction-type photocatalyst according to claim 4 , wherein the ionic polymer B is a cationic polymer.

8. the ionic group is a cationic group, the ionic polymer A is a cationic polymer, The junction-type photocatalyst according to claim 4 , wherein the ionic polymer B is an anionic polymer.

9. The junction-type photocatalyst according to claim 7 , wherein the anionic polymer comprises a sulfonic acid-based polymer or a carboxylic acid-based polymer.

10. 9. The junction-type photocatalyst according to claim 8, wherein the cationic polymer comprises at least one selected from the group consisting of a cationized polysaccharide, a diallyl quaternary ammonium salt polymer or a copolymer thereof, a (meth)acryloyloxyethyl quaternary ammonium salt polymer or a copolymer thereof, a (meth)acrylamidopropyl quaternary ammonium salt polymer or a copolymer thereof, and a dimethylamine epichlorohydrin polymer.

11. The junction-type photocatalyst according to claim 1 or 2, wherein the hydrogen generating photocatalyst is a metal oxide.

12. The junction-type photocatalyst according to claim 1 or 2, wherein the oxygen-generating photocatalyst is a metal oxide.

13. 3. The junction-type photocatalyst according to claim 1, wherein the solid mediator is a transition metal or a compound thereof.

14. 3. The bonded photocatalyst according to claim 1, wherein the solid mediator is bonded onto the oxygen generating photocatalyst by at least one method selected from the group consisting of a photoelectrodeposition method, an impregnation method, and a precipitation method using an organic carboxylic acid compound and a solid mediator or a precursor thereof.

15. A method for producing the junction-type photocatalyst according to claim 1 or 2, A method for producing a junction-type photocatalyst, comprising the following steps 1 to 4, with the exception that steps 2 and 3 may be performed in any order. Step 1: A step of bonding the solid mediator onto the oxygen generating photocatalyst by at least one method selected from the group consisting of a photoelectrodeposition method, an impregnation method, and a precipitation method using an organic carboxylic acid compound and a solid mediator or a precursor thereof. Step 2: A step of introducing an ionic group into the solid mediator to obtain an oxygen generating photocatalyst to which the solid mediator having the ionic group is bonded. Step 3: introducing an ionic polymer having an opposite charge to the charge of the ionic group into the hydrogen generating photocatalyst. Step 4: Mixing the oxygen generating photocatalyst obtained in step 2 to which the solid mediator having an ionic group has been bonded with the hydrogen generating photocatalyst obtained in step 3 to which the ionic polymer has been introduced.

16. A method for producing the junction-type photocatalyst according to claim 1 or 2, A method for producing a junction-type photocatalyst, comprising the following steps 1, 2', 3', and 4', provided that steps 2' and 3' may be performed in any order. Step 1: A step of bonding the solid mediator onto the oxygen generating photocatalyst by at least one method selected from the group consisting of a photoelectrodeposition method, an impregnation method, and a precipitation method using an organic carboxylic acid compound and a solid mediator or a precursor thereof. Step 2': A step of introducing an ionic group into the solid mediator, and further reacting the ionic group with an ionic polymer having an opposite charge to that of the ionic group, to obtain an oxygen generating photocatalyst bonded to a solid mediator having the ionic polymer. Step 3': A step of introducing an ionic polymer having an opposite charge to the charge of the ionic polymer into the hydrogen generating photocatalyst. Step 4': A step of mixing the oxygen generating photocatalyst obtained in step 2' to which the solid mediator having the ionic polymer has been bonded and the hydrogen generating photocatalyst obtained in step 3' to which the ionic polymer has been introduced.

17. 16. The method for producing a junction-type photocatalyst according to claim 15, wherein, when the photoelectrodeposition method is used in step 1, the method further comprises a step of irradiating a dispersion containing an oxygen-generating photocatalyst in addition to the organic carboxylic acid compound and the solid mediator or a precursor thereof with light.

18. 16. The method for producing a junction-type photocatalyst according to claim 15, wherein the organic carboxylic acid compound is at least one selected from the group consisting of ether carboxylates, fatty acids, hydroxymonocarboxylic acids, and polycarboxylic acids.

19. the organic carboxylic acid compound comprises an ether carboxylate; The method for producing a junction-type photocatalyst according to claim 15 , wherein the solid mediator comprises gold.

20. 16. The method for producing a junction-type photocatalyst according to claim 15, wherein a thiol compound having an ionic group is used in the introduction of an ionic group into the solid mediator in the step 2 or the step 2'.

21. A photocatalytic composite comprising the junction-type photocatalyst according to claim 1 or 2 on a substrate.

22. Use of the junction-type photocatalyst according to claim 1 or 2 as a photocatalyst for water splitting or organic matter decomposition.

23. A method for producing hydrogen, comprising irradiating the junction-type photocatalyst according to claim 1 with light in the presence of water or alcohol.

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