Method for producing bismuth vanadate particles, and bismuth vanadate particles

Abstract

An objective of the present disclosure is to provide a method for producing monoclinic bismuth vanadate particles, the method being capable of obtaining bismuth vanadate particles having higher catalytic activity in photoresponsive water decomposition reactions as compared to conventional methods for producing bismuth vanadate particles. The present disclosure provides a method for producing bismuth vanadate particles, said production method including a step α for causing a phase transition in which tetragonal bismuth vanadate is changed to monoclinic bismuth vanadate in the presence of a water-soluble alcohol.

Description

Method for producing bismuth vanadate particles and bismuth vanadate particles

[0001] This disclosure relates to a method for producing monoclinic bismuth vanadate particles.

[0002] As a method for producing hydrogen (green hydrogen) using renewable energy, which is the next-generation energy source for achieving carbon neutrality, research is being conducted on splitting water from solar energy using photoresponsive reactive catalysts (photocatalysts). As a photocatalyst for the above water splitting reaction, bismuth vanadate (BiVO2) ４ ) are known, and in particular, bismuth vanadate with a monoclinic crystal structure is known to have high photocatalytic activity. Regarding bismuth vanadate, Patent Document 1 discloses monoclinic BiVO4 obtained by reacting layered alkali vanadate and bismuth nitrate pentahydrate in water with stirring at room temperature. Patent Document 2 discloses a semiconductor photocatalyst for generating oxygen from water in the presence of a reversible redox medium, characterized in that it has an absorption band in which the maximum light absorption value is in the range of 530 to 590 nm when compared with the diffuse reflectance spectrum of the semiconductor itself after heat treatment at 500°C, and the value obtained by subtracting the absorption rate at the maximum light absorption value (R1) of the base 1000 to 1200 nm (R2) from the absorption rate at the maximum light absorption value (R1) is at least 5%. ４ Semiconductor photocatalysts are disclosed. Non-patent document 1 discloses a method for obtaining BiVO4 by mixing an aqueous solution of bismuth nitrate pentahydrate and an aqueous solution of ammonium vanadate, co-precipitating the mixture, and heating the resulting mixture at 200°C. Patent document 3 discloses a method for producing metal (X)-doped bismuth vanadate, which includes a bismuthization step in which a vanadate salt doped with one or more metals (X) selected from elements in groups 2 to 15 of the periodic table is brought into contact with bismuth ions.

[0003] Furthermore, regarding the improvement of photocatalytic activity, Non-Patent Document 2 states that TiO ２ SrTiO ３ WO ３A method for introducing oxygen vacancies into photocatalytic powders such as the one described is disclosed.

[0004] Japanese Patent Publication No. 2001-2419, Japanese Patent Publication No. 2017-100057, Japanese Patent Publication No. 2016-64976

[0005] Dyes and Pigments 149 (2018), 373-376, "2022 Research Results Report (Research Topic: Development of Highly Active Photocatalysts with Controlled Defect Levels)," Kei Yamagata, [online], accessed October 11, 2024, Internet <URL: https: / / kaken.nii.ac.jp / file / KAKENHI-PROJECT-19H02820 / 19H02820seika.pdf>

[0006] As mentioned above, although various bismuth vanadate compounds and methods for producing them have been disclosed, in order to commercialize the production of green hydrogen, there is a need for photocatalysts with higher catalytic activity in the water splitting reaction.

[0007] This disclosure is made in view of these circumstances. The object of this disclosure is to provide a method for producing monoclinic bismuth vanadate particles that yields bismuth vanadate particles with higher catalytic activity in photoresponsive water splitting reactions than conventional methods for producing bismuth vanadate particles.

[0008] Therefore, the inventors conducted thorough research in consideration of the above problems. As a result, they discovered that by inducing a phase transition in which tetragonal bismuth vanadate is changed to monoclinic bismuth vanadate in the presence of a water-soluble alcohol, bismuth vanadate particles with high photoresponsive water splitting catalytic activity can be obtained.

[0009] In other words, this disclosure provides the inventions described in [1]-

[16] below.

[0010] [1] A method for producing monoclinic bismuth vanadate particles, the method comprising a step α that causes a phase transition, in which tetragonal bismuth vanadate is converted to monoclinic bismuth vanadate in the presence of a water-soluble alcohol, for producing bismuth vanadate particles.

[0011] [2] The above-mentioned method for producing bismuth vanadate particles, comprising a mixing step A of mixing a vanadium compound, a bismuth compound, a solvent or dispersion medium, and a water-soluble alcohol to obtain a mixture a, and a heating step A of heating the mixture a, as described in [1] above.

[0012] [3] The method for producing bismuth vanadate particles according to [2] above, wherein the heating step A includes step α above.

[0013] [4] The method for producing bismuth vanadate particles according to [2] or [3] above, wherein the heating step A is carried out while stirring.

[0014] [5] The above manufacturing method comprises a mixing step B of mixing raw materials containing vanadium atoms and bismuth atoms to obtain a solution b containing vanadium atoms and bismuth atoms, a precipitation step B of obtaining a mixture b having a precipitate containing bismuth vanadate from the solution b, and a heating step B of heating the mixture b under pressure, wherein at least one selected from the group consisting of the solution b and the mixture b contains a water-soluble alcohol, the method for producing bismuth vanadate particles as described in [1] above.

[0015] [6] The method for producing bismuth vanadate particles according to [5], wherein the heating step B includes step α.

[0016] [7] A method for producing bismuth vanadate particles according to [5] or [6], further comprising a stirring step B between the precipitation step B and the heating step B, wherein the mixture b is stirred.

[0017] [8] The method for producing bismuth vanadate particles according to any one of [1] to [7], further comprising a calcination step of calcining the monoclinic bismuth vanadate particles in an inert atmosphere.

[0018] [9] The method for producing bismuth vanadate particles according to [1] above, wherein the water-soluble alcohol is at least one selected from the group consisting of monohydric alcohols having 1 to 6 carbon atoms, glycol ethers having 1 to 10 carbon atoms, and polyhydric alcohols having 2 to 8 carbon atoms.

[0019]

[10] Bismuth vanadate particles in monoclinic form, wherein the particles are doped with nitrogen atoms.

[0020]

[11] The vanadate bismuth particles described in

[10] above, wherein, as measured by XPS (X-ray photoelectron spectroscopy), nitrogen atoms are present in an amount of 0.1 to 10 atomic percent relative to a total of 100 atomic percent of oxygen atoms and nitrogen atoms.

[0021]

[12] The bismuth vanadate particles according to

[10] or

[11] , wherein the bismuth vanadate particles are obtained by the method for producing bismuth vanadate particles described in [5] above.

[0022]

[13] A photocatalytic sheet for water splitting, comprising at least one selected from the group consisting of bismuth vanadate particles obtained by the manufacturing method described in any of [1] to [9] above and monoclinic bismuth vanadate particles doped with nitrogen atoms, photocatalytic particles for hydrogen generation, and a conductive solid.

[0023]

[14] The photocatalytic sheet for water splitting according to

[13] , wherein the conductive solid is at least one selected from the group consisting of carbon nanotubes, silver nanowires, and reduced graphene oxide.

[0024]

[15] The water splitting photocatalyst sheet according to

[13] or

[14] , wherein the water splitting photocatalyst sheet comprises cellulose fiber.

[0025]

[16] A method for producing a photocatalytic sheet for water splitting, comprising the step of forming a sheet using a dispersion containing at least one selected from the group consisting of bismuth vanadate particles obtained by the manufacturing method described in any of [1] to [9] above and monoclinic bismuth vanadate particles doped with nitrogen atoms, and photocatalytic particles for hydrogen generation.

[0026] According to this disclosure, bismuth vanadate particles with high catalytic activity in photoresponsive water splitting reactions can be produced.

[0027] The present disclosure will be described in detail below. While embodiments of the present disclosure will be described below, the present disclosure is not limited to the embodiments described below and may be implemented as appropriate by those skilled in the art, without prejudice to the spirit of the present disclosure.

[0028] <Method for Producing Bismuth Vanadate Particles> The method for producing bismuth vanadate particles according to this disclosure includes step α, which induces a phase transition by changing tetragonal bismuth vanadate to monoclinic bismuth vanadate in the presence of a water-soluble alcohol. It is generally known that the reactivity of bismuth vanadate particles having crystal faces is plane-orient dependent, and it is thought that reduction reactions preferentially occur at the {010} plane and oxidation reactions preferentially occur at the {110} plane. In step α, it is thought that using a water-soluble alcohol will somehow affect the ratio of {010} planes to {110} planes in the crystal of the resulting bismuth vanadate particles, thereby improving the catalytic activity of the photoresponsive water splitting reaction. Step α is not particularly limited as long as a phase transition in which tetragonal bismuth vanadate changes to monoclinic bismuth vanadate proceeds in the presence of a water-soluble alcohol, and can be obtained, for example, by stirring a liquid containing tetragonal bismuth vanadate particles at room temperature (5 to 35°C) for a long time. Furthermore, the execution of step α also includes cases where the above-mentioned phase transition proceeds in the presence of a water-soluble alcohol during the mixing step of mixing the raw materials for bismuth vanadate, the heating step of heating the mixture obtained in the mixing step, or the stirring step of the mixture, as described later. The above-mentioned phase transition of bismuth vanadate from tetragonal to monoclinic can be confirmed by X-ray diffraction (XRD).

[0029] The above-mentioned water-soluble alcohol is not particularly limited as long as it is an organic compound having a hydroxyl group and exhibiting compatibility with water, but one with high solubility in water is preferred. For example, if 0.3 g or more dissolves in 100 ml of water at 25°C, it may be described as "water-soluble". Examples of monohydric alcohols include methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butyl alcohol, 2-butyl alcohol, tert-butyl alcohol, 2-methyl-1-butanol, 3-methyl-2-butanol, 2-ethyl-1-butanol, pentanol, and 1-hexanol. Examples of polyhydric alcohols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol, 2-methyl-2,4-pentanediol, glycerin, and 1,2,6-hexanetriol.Furthermore, glycol ethers may be used as water-soluble alcohols, including 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol, 2-butoxyethanol, 1-methoxy-2-propanol, 1-propoxy-2-propanol, 1-butoxy-2-propanol, 1-methyl-2-(tert-butoxy)ethanol, 2-butoxyethanol, 1-ethoxy-2-propanol, 3-methoxy-1-propanol, 3-ethoxy-1-propanol, 3-methoxybutanol, 4-ethoxybutanol, ethylene glycol monohexyl ether, ethylene glycol dibutyl ether, diethylene glycol monomethyl ether, and diethylene glycol Examples include cellulose monoethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tetraethylene glycol monobutyl ether, tripropylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, 3-methyl-3-methoxy-1-butanol, diethylene glycol monohexyl ether, 1,3-propanediol monomethyl ether, diethylene glycol monoethyl ether, water-soluble polyethylene glycol (PEG), etc. From the viewpoint of high solubility in water, it is preferable to use at least one selected from the group consisting of monohydric alcohols having 1 to 6 carbon atoms, glycol ethers having 1 to 10 carbon atoms, and polyhydric alcohols having 2 to 8 carbon atoms. More preferably, alcohols having 1 to 4 carbon atoms such as methanol, ethanol, 1-propanol, 2-propanol, 1-butyl alcohol, 2-butyl alcohol, and tert-butyl alcohol may be used, and even more preferably methanol, ethanol, 1-propanol, and 2-propanol may be used.

[0030] In step α described above, the phase transition that changes tetragonal bismuth vanadate to monoclinic bismuth vanadate is not particularly limited as long as it is carried out in the presence of a water-soluble alcohol, but it is preferable that it be carried out in a mixture of the water-soluble alcohol and a solvent or dispersion medium other than the water-soluble alcohol. The concentration of the water-soluble alcohol in the mixture is not particularly limited, but is preferably 1 to 50% by mass. More preferably it is 1 to 30% by mass, and even more preferably 1 to 20% by mass. In addition, not only one type of water-soluble alcohol but also two or more types may be used in mixture form. The term "in the presence of" above means that the water-soluble alcohol may be miscible in the liquid phase, attached to, bound to, or adsorbed on the surface of particles, etc., or exist as a complex in the solid or liquid phase. Furthermore, if step α is carried out in the gas phase, it may be in a gaseous state.

[0031] The solvent or dispersion medium described above is preferably an aqueous solvent or dispersion medium from the viewpoint of compatibility with water-soluble alcohols. For example, water is one such example. The water is not particularly limited as long as it does not interfere with the catalytic activity of the bismuth vanadate particles, and for example, pure water, deionized water, distilled water, ultrapure water, etc. can be used.

[0032] The method for producing bismuth vanadate particles according to this disclosure can appropriately utilize known liquid-solid-phase methods, liquid-phase methods, hydrothermal methods, etc., except for the inclusion of step α described above. For example, a production method can include a mixing step of mixing bismuth vanadate raw materials, a heating step of heating the mixture obtained in the mixing step, and a stirring step of the mixture. Among these steps, it is preferable to perform step α during the heating step because it shortens the reaction time. Furthermore, a washing step to wash the particles or precipitate and a drying step to dry the particles or precipitate may be included after or between each of the above steps as needed.

[0033] In one embodiment, the method for producing bismuth vanadate particles of the present disclosure preferably comprises a mixing step A to obtain a mixture a by mixing a vanadium compound, a bismuth compound, a solvent or dispersion medium, and a water-soluble alcohol, and a heating step A to heat the mixture a. In another embodiment, the method for producing bismuth vanadate particles of the present disclosure also comprises a mixing step B to obtain a solution b containing vanadium atoms and bismuth atoms by mixing raw materials containing vanadium atoms and bismuth atoms, a precipitation step B to obtain a mixture b having a precipitate containing bismuth vanadate from the solution b, and a heating step B to heat the mixture b under pressure, wherein at least one selected from the group consisting of the solution b and the mixture b contains a water-soluble alcohol. Hereinafter, the method for producing bismuth vanadate particles of the present disclosure having the above mixing step A and the above heating step A will be referred to as method (A), and the method for producing bismuth vanadate particles of the present disclosure having the above mixing step B, the above precipitation step B and the above heating step B will be referred to as method (B).

[0034] In this specification, "solvent" refers to a liquid that dissolves the vanadium atom raw material or the bismuth atom raw material and makes the respective raw materials invisible to the naked eye. Furthermore, "dispersion medium" refers to a medium in which solids such as particles and precipitates present in each step of the manufacturing process can be dispersed, and the aforementioned raw materials may or may not be dissolved in the medium. In addition, the above "particles" may also be aggregates formed by the aggregation of fine particles, and "dispersion" may include cases in which solids such as particles and precipitates that have settled due to standing are dispersed in the medium by stirring or the like. Furthermore, in this specification, when "bismuth vanadate" is simply referred to, it refers to "bismuth vanadate" as a compound without limiting the crystal system or particle shape, and when "bismuth vanadate particles" is simply referred to, it refers to monoclinic bismuth vanadate particles.

[0035] (Manufacturing Method (A)) The above manufacturing method (A) has the above mixing step A and the above heating step A. The above mixing step A is not particularly limited as long as a vanadium compound, which is a raw material of the mixture a, a bismuth compound, a solvent or a dispersion medium, and a water-soluble alcohol are mixed to obtain the mixture a. For example, the order, means, mixing conditions, etc. for mixing the above raw materials are not particularly limited. Further, in the mixing step, after supplying the above raw materials into a mixing apparatus (hereinafter sometimes referred to as a "reaction vessel"), stirring may be performed. Stirring is preferably performed when there is a liquid (such as a solvent, a dispersion medium, a water-soluble alcohol, etc.) in the apparatus. It may be stirred after all the raw materials of the mixture a are supplied, or the raw materials may be supplied while stirring.

[0036] The above vanadium compound is not particularly limited as long as it contains vanadium element and reacts with a bismuth compound to form bismuth vanadate. Examples thereof include oxides, halides, halogenated oxides, ammonium salts, composite oxides with alkali metals, etc. of bismuth. More specifically, as the above vanadium compound, V ２ O ５ , NH ４ VO ３ , KVO ３ , K ３ V ５ O １４ , KV ３ O ８ , NaVO ３ , Na ３ V ５ O １４ , NaV ３ O ８ , VOCl ３ , VCl ４ , VCl ３ , VBr ３ , VO ２ , V ２ O ３ , V ６ O １３ etc. may be mentioned. Among them, oxides, ammonium salts, and composite oxides with alkali metals of vanadium are preferable, and more preferably V ２ O ５ , NH ４ VO ３ , K ３ V５ O １４ KV ３ O ８ And more preferably V ２ O ５ _K ３ V ５ O １４ NH ４ VO ３ The vanadium compound described above may be in an anhydrous or hydrated form, and may be synthesized by known methods or a commercially available product may be used.

[0037] The above-mentioned bismuth compound is not particularly limited as long as it contains the element bismuth and reacts with a vanadium compound to form bismuth vanadate, but examples include bismuth nitrates, oxides, hydroxides, sulfates, phosphate halides, etc. More specifically as the above-mentioned bismuth compound, Bi(NO) ３ ) ３ , Bi ２ O ３ , BiOH ３ , Bi ２ (SO ４ ) ３ BiCl ３ , BiOCl, BiF ３ BiBr ３ , BiI ３ , BiPO ４ Examples include the following. Among these, bismuth nitrates and oxides are preferred, and more preferably Bi(NO) ３ ) ３ , Bi ２ O ３ The bismuth compound described above may be in the form of an anhydrous or hydrated form, and may be synthesized by known methods or a commercially available product may be used.

[0038] The amounts of vanadium compound and bismuth compound used in the above mixing step A are preferably such that the molar ratio (Bi / V) of bismuth element to vanadium element in the above mixture a is 0.5 / 2.0 to 2.0 / 0.5. More preferably it is 0.7 / 1.3 to 1.3 / 0.7, and even more preferably 0.9 / 1.1 to 1.1 / 0.9.

[0039] Specific examples and preferred forms of the solvent or dispersion medium used in the above mixing step A are as described above.

[0040] As the amount of water-soluble alcohol added in the mixing step A increases, the time required to obtain monoclinic bismuth vanadate particles in the heating step A described later tends to increase. Therefore, the amount of water-soluble alcohol added is preferably 1 to 100 moles per mole of bismuth element in mixture a. More preferably, it is 2 to 50 moles, and even more preferably 5 to 30 moles.

[0041] In the mixing step A described above, an acid may be added further. This can increase the solubility of the vanadium compound and bismuth compound in the solvent or dispersion medium in mixture a. The acid is not particularly limited, but inorganic acids such as nitric acid, hydrochloric acid, and sulfuric acid are preferred. Nitric acid is more preferred.

[0042] When an acid is used in the mixing step A described above, the amount of acid added is not particularly limited, but it is preferably 0.1 to 30 moles per 1 mole of total vanadium and bismuth elements in mixture a. More preferably 0.1 to 20 moles, and even more preferably 0.1 to 15 moles.

[0043] The temperature in mixing step A is not particularly limited, but it may be room temperature (5 to 35°C). Preferably, it may be 10 to 30°C. The mixing time in mixing step A is not particularly limited as long as sufficient mixing is achieved, but from the viewpoint of productivity, it may be, for example, 5 minutes to 24 hours.

[0044] As described above, mixture a refers to a mixture obtained by mixing at least a vanadium compound, a bismuth compound, a solvent or dispersion medium, and a water-soluble alcohol. It may also contain an acid, as described above. Mixture a in mixing step A may have a liquid phase and a solid phase, or it may consist only of a liquid phase, and the ratio of the liquid phase to the solid phase may change over time. The liquid phase in mixing step A contains at least a solvent or dispersion medium, and may also contain dissolved vanadium compounds, bismuth compounds, water-soluble alcohols, etc. The solid phase in mixing step A may contain particles of vanadium compounds and bismuth compounds that did not dissolve in the liquid phase, and may contain bismuth vanadate if the reaction to form bismuth vanadate proceeds simply by mixing, or after starting heating step A described later. Either one of the vanadium compound or the bismuth compound may be dissolved in the liquid phase, both may be dissolved, or neither may be dissolved.

[0045] The heating step A described above is a step of heating the mixture a obtained in the mixing step A described above, and is not particularly limited as long as it is heated within a temperature range in which not all of the water-soluble alcohol contained in mixture a vaporizes. The heating temperature may be, for example, 50°C or higher and below the boiling point of mixture a in the manufacturing environment, and preferably 60°C or higher. The upper limit depends on the boiling point of the water-soluble alcohol, but if the boiling point of the water-soluble alcohol in the manufacturing environment exceeds 100°C, it is preferable that the temperature be 100°C or lower. More preferably, it is 90°C or lower, and even more preferably 80°C or lower.

[0046] The heating time of mixture a in the above heating step A is not particularly limited, but is preferably 5 to 96 hours. More preferably 10 to 72 hours, and even more preferably 24 to 48 hours.

[0047] When the mixture a is heated in the heating step A described above, the formation of tetragonal bismuth vanadate particles is preferentially promoted and precipitates in the reaction vessel. Further heating promotes a phase transition in which tetragonal bismuth vanadate changes to monoclinic bismuth vanadate. After sufficient heating time, the precipitated particles no longer contain tetragonal bismuth vanadate particles, and only monoclinic bismuth vanadate particles remain. At this point, by including a water-soluble alcohol in mixture a, it is possible to improve catalytic activity by influencing the ratio of the reducing surface ({010} plane) to the oxidizing surface ({110} plane), as described above. In other words, the form in which the heating step A includes step α described above is one of the preferred embodiments in this disclosure. As described above, the components and component ratios of mixture a change before, during, and after heating step A, but for convenience, in this specification, it will be referred to as "mixture a" in all cases.

[0048] The heating step A described above is preferably carried out while stirring the mixture a. This makes the phase transition in which tetragonal bismuth vanadate changes to monoclinic bismuth vanadate more likely to occur. The means of stirring are not particularly limited, but a stirring bar or the like can be used. The reaction vessel used in the heating step A is not particularly limited, but from the viewpoint of suppressing the evaporation of water-soluble alcohol, it is preferable to use a sealed container.

[0049] (Manufacturing Method (B)) The above manufacturing method (B) comprises the above mixing step B, the above precipitation step B, and the above heating step B, and it is sufficient that at least one selected from the group consisting of the above solution b and the above mixture b contains a water-soluble alcohol. In the above mixing step B, as long as raw materials containing vanadium atoms and bismuth atoms are mixed to obtain a solution b containing vanadium atoms and bismuth atoms, the type of raw materials, the order in which the raw materials are mixed, the means, the mixing conditions, etc. are not particularly limited, but it is preferable to mix raw materials containing a vanadium compound, a bismuth compound, and a solvent. Furthermore, in the above mixing step B, the solvent, the vanadium compound, and the bismuth compound may be mixed at once (mixing form 1), or one of the vanadium compound and the bismuth compound may be mixed with the solvent, and then the obtained solution may be mixed with the other of the vanadium compound and the bismuth compound (mixing form 2), or the vanadium compound and the bismuth compound may each be mixed with the solvent, and then the vanadium solution and the bismuth solution may be mixed (mixing form 3).

[0050] The vanadium compound and bismuth compound used in the above mixing step B, and specific examples and preferred forms, are the same as those described in the above manufacturing method (A). In particular, it is preferable to use raw materials with high solubility in the solvent described later in mixing step B. For example, as a vanadium compound, NH ４ VO ３ It is preferable to use [this]. Also, as a bismuth compound, Bi(NO) ３ ) ３ It is preferable to use [this method].

[0051] The amounts of vanadium compound and bismuth compound used in the above mixing step B are preferably such that the molar ratio of bismuth element to vanadium element in solution b (Bi / V) is 0.5 / 2.0 to 2.0 / 0.5. More preferably it is 0.7 / 1.3 to 1.3 / 0.7, and even more preferably 0.9 / 1.1 to 1.1 / 0.9.

[0052] Specific examples and preferred forms of the solvent used in the above mixing step B are as described above. Furthermore, in the above mixing step B, it is preferable to use a solvent that dissolves the vanadium and bismuth atom raw materials and is compatible with the following acid in order to obtain solution b. For example, water is preferred.

[0053] In the mixing step B described above, it is preferable to include an acid in order to dissolve each of the above raw materials in the solvent. Specific examples and preferred forms of the acid are as described in the mixing step A described above.

[0054] The acid content in solution b in the above mixing step B is not particularly limited as long as it can dissolve each of the raw materials. For example, it may be 5 to 250 moles per 1 mole total of vanadium and bismuth in solution b. Preferably it is 15 to 75 moles, and more preferably 20 to 60 moles.

[0055] The precipitation step B described above is a step of obtaining a mixture b having a precipitate containing bismuth vanadate from the solution b obtained in the mixing step B described above. To obtain the precipitate, the solubility of the compound containing vanadium atoms and the compound containing bismuth atoms in solution b is reduced, thereby obtaining a precipitate containing bismuth vanadate. For example, if solution b is an aqueous solution containing an acid, the above compounds are dissolved by the acid, so it is possible to add a base such as ammonia to precipitate them, and it is preferable to adjust the pH to 0.1 to 2.0. The pH of the mixture b described above is more preferably 0.2 to 1.8. The precipitation step B described above is preferably carried out while stirring. The means of stirring are not particularly limited, but a stirring bar or the like can be used.

[0056] The precipitation step B described above is not particularly limited as long as it is carried out at a temperature in which the solvent and water-soluble alcohol do not evaporate. For example, it may be carried out at room temperature (5 to 35°C). Preferably, it may be carried out at 10 to 30°C.

[0057] In the above manufacturing method (B), it is sufficient that at least one of the solution b or mixture b contains a water-soluble alcohol. The proportion of water-soluble alcohol in the solution b or mixture b is preferably 1 to 100 moles per mole of bismuth element contained in the solution b or mixture b. More preferably, it is 2 to 50 moles, and even more preferably 5 to 30 moles. In the mixing step B, a water-soluble alcohol may be added to prepare a solution b containing a water-soluble alcohol. In this case, it is preferable to carry out the mixing step B and the precipitation step B under conditions that prevent the water-soluble alcohol from evaporating, so that the mixture b after the precipitation step B also contains a water-soluble alcohol. Furthermore, it is preferable not to add a water-soluble alcohol in the mixing step B, and to add a water-soluble alcohol to the mixture b after the precipitate has formed in the precipitation step B, because this makes it easier to adjust the pH when obtaining the precipitate.

[0058] The heating step B described above is a step of heating the mixture b under pressure, and the heating temperature is preferably 100 to 250°C. More preferably 150 to 240°C, and even more preferably 180 to 230°C. The heating may be carried out by gradually raising the temperature to the heating temperature, or by placing the mixture b in a device that maintains the heating temperature, but in both cases, it is preferable to heat by gradually raising the temperature from the viewpoint of making it easier to apply pressure. The reactor etc. used in the heating step B described above can be any known type and is not particularly limited, but it is preferable to use, for example, a heat-resistant and pressure-resistant vessel (autoclave).

[0059] The heating time of mixture b in the heating step B described above is not particularly limited, but is preferably 5 to 72 hours. More preferably 10 to 48 hours, and even more preferably 15 to 32 hours.

[0060] The pressure applied in the heating step B described above is preferably 0.3 to 5 MPa. More preferably 0.6 to 3.5 MPa, and even more preferably 1.0 to 2.5 MPa.

[0061] Furthermore, "heating while pressurizing" in heating step B above means that the process involves simultaneous pressurization and heating, and the timing of the start of pressurization and the start of heating may be the same or different, and the timing of the end of pressurization and the end of heating may be the same or different. Also, heating and pressurization may be performed once or two or more times.

[0062] By heating mixture b in heating step B, a phase transition is more likely to occur in which tetragonal bismuth vanadate in the precipitate changes to monoclinic bismuth vanadate. Furthermore, by including a water-soluble alcohol in mixture b at this time, it is possible to improve catalytic activity by somehow influencing the ratio of the reducing surface {010} plane to the oxidizing surface {110} plane, as described above. Moreover, when the bismuth vanadate particles obtained after heating step B were measured by XPS, it was found that the particles contained nitrogen atoms. In other words, the form in which heating step B includes step α is one of the preferred embodiments in this disclosure.

[0063] The above manufacturing method (B) preferably includes a stirring step B between the precipitation step B and the heating step B, in which the mixture b is further stirred. Including the stirring step B makes it possible to increase the amount of bismuth vanadate contained in the precipitate. The means of stirring in the stirring step B are not particularly limited, and a stirring bar or the like can be used. The temperature in the precipitation step B is not particularly limited, but is preferably 0 to 80°C. More preferably it is 10 to 60°C. The stirring time is not particularly limited, but from the viewpoint of productivity it may be 10 minutes to 6 hours.

[0064] The above manufacturing methods (A) and (B) may include a purification step to purify the obtained monoclinic bismuth vanadate particles and a drying step to dry the bismuth vanadate particles. Preferably, the method may include the purification step and a drying step to dry the bismuth vanadate particles obtained in the purification step. The purification method in the purification step may be any known method, such as filtration, washing, and recrystallization. The drying method and drying conditions in the drying step are not particularly limited and can be carried out by any known method, such as heating drying using a heating furnace, spray drying, vacuum drying, or drying with a drying agent. Furthermore, heating drying is preferable to shorten the drying time, and the drying temperature is preferably 40 to 200°C. More preferably, it may be 70 to 150°C.

[0065] The method for producing bismuth vanadate particles according to this disclosure preferably further includes a calcination step in which monoclinic bismuth vanadate particles are calcined under an inert atmosphere. This further improves the catalytic activity of the resulting bismuth vanadate particles. Although the detailed mechanism is unknown, it has been found that this effect is more preferably obtained in bismuth vanadate particles that have undergone a phase transition in the presence of a water-soluble alcohol. Non-patent document 2 discloses that in metal oxides exhibiting photocatalytic activity, the activity as a photocatalyst is improved when oxygen vacancies are sparsely present. The aforementioned document states that when oxygen vacancies are present, photoexcited electrons are trapped in the metal cations near the oxygen vacancies, which reduces the collision probability between holes and photoexcited electrons, extends the lifetime of the holes, and consequently improves the activity. Since the photocatalytic activity of the bismuth vanadate particles according to this disclosure is improved when calcined at high temperature under an inert atmosphere, it is presumed that oxygen vacancies or equivalent vacancies are also generated in the calcined bismuth vanadate particles according to this disclosure, although the detailed mechanism is unknown.

[0066] The above calcination step is not particularly limited as long as the monoclinic bismuth vanadate particles of this disclosure are calcined under an inert atmosphere, but it is preferable to calcine the monoclinic bismuth vanadate particles obtained in the above manufacturing methods (A) and (B). Furthermore, in the above manufacturing methods (A) and (B), if the above purification step and drying step are performed, it is preferable to perform the calcination step after these steps.

[0067] In the firing process described above, higher firing temperatures tend to improve catalytic activity, but a tendency for catalytic activity to decrease as the temperature approaches the melting temperature of bismuth vanadate particles was also observed. Therefore, it is preferable that the firing temperature be 300°C or higher and below the melting temperature of bismuth vanadate particles. More preferably, it is 350 to 600°C, and even more preferably 400 to 550°C.

[0068] The firing time in the above firing process is not particularly limited, but is preferably between 5 minutes and 78 hours. More preferably between 10 minutes and 24 hours, and even more preferably between 15 minutes and 6 hours.

[0069] Examples of inert atmospheres used in the above firing process include nitrogen, helium, argon, and other noble gases. Nitrogen and argon are preferred among these.

[0070] The flow rate of the inert gas in the above firing process is not particularly limited, but is preferably 0.1 to 10 L / min. More preferably it is 1.0 to 3.0 L / min.

[0071] The above firing process may be carried out under an inert atmosphere, but the oxygen concentration in the atmosphere is preferably 0.5% or less. More preferably it is 0.0001 to 0.5%.

[0072] <Bismuth Vanadate Particles> One preferred embodiment of the present disclosure is monoclinic bismuth vanadate particles doped with nitrogen atoms. These bismuth vanadate particles can be obtained by the above-described manufacturing method (B). The inventors have also investigated and found that a nitrogen atom peak is detected when measuring bismuth vanadate particles by XPS only when a phase transition is induced in the presence of a water-soluble alcohol. Furthermore, a nitrogen atom peak is also detected when calcination is not performed in a nitrogen atmosphere. Therefore, although the detailed mechanism is unknown, it is presumed that nitrogen atoms contained in the various raw materials mixed in mixing step B are incorporated into the crystal structure of the bismuth vanadate particles due to the presence of a water-soluble alcohol in solution b or mixture b. Note that when XPS measurements were performed on monoclinic bismuth vanadate particles as described later, and a nitrogen atom (N1s) peak was detected, it was considered "doped with nitrogen atoms," and when it was below the detection limit, it was considered "not doped with nitrogen atoms."

[0073] The nitrogen atom content in this embodiment is not particularly limited as long as it does not hinder catalytic activity. However, when the total amount of oxygen and nitrogen atoms is set to 100 atomic%, using the oxygen atom peak (O1s) and nitrogen atom peak (N1s) obtained by measurement using the XPS apparatus described later, the nitrogen atom content may be 0.1 to 20 atomic%, and preferably 0.1 to 10 atomic%.

[0074] In this specification, "catalytic activity" refers to the amount of oxygen produced from an aqueous silver nitrate solution under simulated sunlight irradiation, as measured in the "half-reaction (oxygen production) evaluation" described later. A higher amount of oxygen produced indicates higher catalytic activity of the photoresponsive water splitting reaction.

[0075] <Applications of Bismuth Vanadate Particles> Bismuth vanadate particles obtained by the method for producing bismuth vanadate particles according to this disclosure have high catalytic activity in photoresponsive water splitting reactions and are therefore suitably used as a photocatalyst for water splitting reactions. Furthermore, bismuth vanadate particles according to this disclosure can be used as a catalyst in any chemical reaction in which the oxidation-reduction priority is positively greater than the oxidation-reduction priority of oxygen evolution in the above-mentioned photoresponsive water splitting reaction, and can be used for the decomposition of harmful substances, the purification of pollutants, etc. Examples of the above chemical reactions include aqueous sulfuric acid (HSO4). ４ － ) from persulfuric acid (S ２ O ８ ２－ ), from a sodium chloride (NaCl) solution to hypochlorite (ClO － ), hydrogen peroxide (H) from carbonate aqueous solution ２ O ２ ), iodate (IO ３ － From an aqueous solution containing ) periodate (IO ４ － ), trivalent cerium salt (Ce ３＋ From an aqueous solution containing ) tetravalent cerium salt (Ce ４＋ Examples of reactions with ) and others include, and it can also be applied to the synthesis reaction when synthesizing KA oil from cyclohexane. This disclosure is also a photocatalyst containing bismuth vanadate particles obtained by the manufacturing method of this disclosure.

[0076] <Photocatalytic Sheet for Water Splitting> One embodiment of this disclosure is a photocatalytic sheet for water splitting that uses bismuth vanadate particles obtained by the method for producing bismuth vanadate particles of this disclosure or monoclinic bismuth vanadate particles doped with nitrogen atoms of this disclosure. The bismuth vanadate particles obtained by the method for producing bismuth vanadate particles of this disclosure and the monoclinic bismuth vanadate particles doped with nitrogen atoms of this disclosure can function as photocatalysts for oxygen generation, and are therefore also referred to as oxygen-generating photocatalysts below. The photocatalytic sheet for water splitting has an oxygen-generating photocatalyst, a hydrogen-generating photocatalyst, and a conductive solid such as a solid mediator (hereinafter referred to as a conductive solid) immobilized on a substrate (for example, a substrate or a support other than a substrate). When water is in contact with this photocatalytic sheet for water splitting, the water is split by irradiation with light. In other words, one mode of implementation of the present disclosure is a photocatalytic sheet for water splitting, comprising at least one selected from the group consisting of bismuth vanadate particles obtained by the manufacturing method of the present disclosure and monoclinic bismuth vanadate particles doped with nitrogen atoms, photocatalytic particles for hydrogen generation, and a conductive solid.

[0077] The water-splitting photocatalyst sheet is not particularly limited in its content of oxygen-generating catalyst particles, hydrogen-generating catalyst particles, and conductive solids, as long as it exhibits catalytic activity for a photoresponsive water-splitting reaction. For example, the proportion of oxygen-generating photocatalyst particles is preferably 10 to 90% by mass, when the total amount of oxygen-generating catalyst particles, hydrogen-generating catalyst particles, and conductive solids is taken as 100% by mass. For example, the weight ratio of hydrogen-generating photocatalyst particles to oxygen-generating photocatalyst particles is preferably in the range of 10 / 90 to 90 / 10. It is also preferable to include conductive solids in an amount of 0.1 to 30% by mass relative to the oxygen-generating photocatalyst particles.

[0078] The average primary particle diameter of the photocatalytic particles for oxygen generation and the photocatalytic particles for hydrogen generation may be, for example, 50 μm or less. The lower limit is not particularly limited, but may be, for example, 0.1 μm or more.

[0079] The photocatalytic sheet for water splitting can be a sheet having a flat or substantially flat surface and thickness. It may be a sheet in which the above-mentioned catalyst particles (photocatalytic particles for oxygen generation and photocatalytic particles for hydrogen generation) and conductive solid are immobilized on a substrate, or it may be a sheet in which the above-mentioned catalyst particles and conductive solid are immobilized on any carrier such as a material in which multiple fibers or powders are integrated. Furthermore, the above-mentioned flat or substantially flat surface may have irregularities or pores, and the above-mentioned thickness does not have to be constant. When a substrate is used as the base material, it may be a substrate with a smooth surface such as a glass plate or ceramic plate, a substrate with irregularities on the surface, a substrate with a curved surface, or a substrate with pores such as a filter. It may also be a deformable substrate such as various films. Furthermore, a carrier that forms a sheet when multiple units are integrated (for example, fibers such as cellulose fibers, mesh, particles, etc.) may be used as the base material. Note that such a carrier may be processed into a sheet after the catalyst particles and conductive solid are immobilized on it. Alternatively, a conductive solid may be used as a carrier or substrate, and the above-mentioned catalysts may be immobilized on the surface of the conductive solid.

[0080] (Photocatalytic particles for oxygen generation) Bismuth vanadate particles obtained by the method for producing bismuth vanadate particles of this disclosure can be used. In addition, monoclinic bismuth vanadate particles doped with nitrogen atoms, which is one embodiment of this disclosure, can be used. As each type of bismuth vanadate particle is described above, further explanation is omitted.

[0081] (Hydrogen-generating photocatalytic particles) Hydrogen-generating photocatalytic particles are photocatalytic materials that can generate excited electrons when irradiated with light, for example, and these excited electrons can reduce water to generate hydrogen. As the hydrogen-generating photocatalytic particles, those in which the electron acceptor level existing in the conduction band or the band gap of the hydrogen-generating photocatalytic particles is at a position more negative than the reduction potential of water (0 V vs. NHE (standard hydrogen electrode potential) at pH = 0) can be used. Further, in the present embodiment, in order to be used in the same system as the oxygen-generating photocatalytic particles, it is desirable that the electron donor level existing in the valence band or the band gap of the hydrogen-generating photocatalytic particles is at a position more positive than the conduction band position of the aforementioned oxygen-generating photocatalytic particles. Examples of the hydrogen-generating photocatalytic particles include Rh-doped SrTiO ３ (SrTi<?? １－ｘ Rh ｘ O ３ : x = 0.002 to 0.1), Ir-doped SrTiO ３ (SrTi １－ｘ [[ID=??12]]Ir ｘ O ３ : x = 0.002 to 0.1), Cr-doped SrTiO ３ (SrTi １－ｘ Cr ｘ O ３ : x = 0.002 to 0.1), Cr and Ta-doped SrTiO ３ (SrTi １－ｘ―ｙ Cr ｘ Ta ｙ O ３ : x = 0.002 to 0.1, y = 0.002 to 0.1), La and Rh-doped SrTiO ３ (Sr １－ｘ La ｘ Ti １―ｙ RhyO ３ : x = 0.005 to 0.2, y = 0.005 to 0.2), etc., perovskite-type SrTiO doped with at least one kind of transition metal or noble metal ３ 、Cu ２ O、CuO、CaFe ２ O ４ 、NiO、Bi ２ O ３ 、BiO Ｘ (X = Cl, Br, I), GaN-ZnO solid solution, LaTiO Note: There seems to be some incorrect or incomplete tags in the original text like <?? １－ｘ which are left as is in the translation. If this is a formatting or encoding issue in the original, it might need to be corrected for a more accurate translation.２ N, BaTaO ２ N, BaNbo ２ N, TaON, Ta ３ N ５ , Ge ３ N ４ Oxynitrides or nitrides containing transition metals or typical metals, such as CuGaS ２ CuInS ２ , Cu(Ga,In)S ２ CuGaSe ２ CuInSe ２ , Cu(Ga,In)Se ２ ,Cd ２ ZnSnS ４ (CZTS), Cu ２ Copper-compound selenides containing typical metals such as Ga, In, Al, etc., such as ZnSn(S,Se)4, La ５ Ti ２ CuS ５ O ７ La ５ Ti ２ AgS ５ O ７ La ５ Ti ２ CuSe ５ O ７ La ５ Ti ２ AgSe ５ O ７、 La ５ Ti ２ Cu ０．９ Ag ０．１ O ７ S ５ , Sm ２ Ti ２ O ５ S ２ One or more substances selected from the group consisting of oxysulfoseleminates such as the following are examples.

[0082] (Conductive Solids) The conductive solid (solid mediator) should be one that has the function of transferring electrons to each catalyst particle. Examples of conductive solids include one or more selected from the group consisting of metals such as gold, silver, copper, nickel, rhodium, and palladium, carbon materials, conductive metal oxides such as tin-doped indium oxide (ITO), fluorine-doped tin oxide, antimond-doped tin oxide, ruthenium oxide, iridium oxide, and rhodium oxide, and conductive polymers. Examples of the carbon materials mentioned above include carbon black, Ketjen black, acetylene black, channel black, furnace black, thermal black, lamp black, carbon nanotubes (CNTs), and reduced graphene oxide. Examples of carbon nanotubes include single-walled CNTs and multi-walled CNTs. Among these, gold, carbon nanotubes (CNTs), silver nanowires, reduced graphene oxide, and conductive polymers are preferred. When using materials with fibrous or linear shapes, such as CNTs or silver nanowires, the average diameter (or average fiber diameter in the case of fibers) is preferably 100 nm or less. Furthermore, in the case of solids that have thickness rather than diameter, such as reducing graphene, the thickness is preferably 100 nm or less. The above-mentioned conductive solids may also be used as carriers for supporting the catalyst particles. Examples of such conductive solids include gold, tin-doped indium oxide (ITO), conductive polymers, and various other substrates.

[0083] (Method for manufacturing a photocatalytic sheet for water splitting) The photocatalytic sheet for water splitting can be manufactured by known methods, provided that oxygen-generating photocatalytic particles, hydrogen-generating photocatalytic particles, and a conductive solid can be immobilized. Examples include a dry method in which the above-mentioned various materials are mixed and then processed into pellets or sheets, and a wet method in which the above-mentioned various materials are dispersed in any dispersion, then applied to or supplied to any substrate and dried. In particular, a photocatalytic sheet for water splitting can be easily obtained by using the wet method. That is, one embodiment of the present disclosure is a method for manufacturing a photocatalytic sheet for water splitting, which includes the step of forming a sheet using a dispersion containing bismuth vanadate particles obtained by the method for manufacturing bismuth vanadate particles of the present disclosure, monoclinic bismuth vanadate particles doped with nitrogen atoms of the present disclosure, and hydrogen-generating photocatalytic particles. Furthermore, if a conductive solid is not used as a substrate, it is preferable to include a conductive solid in the dispersion.

[0084] The wet method described above may be a known method, for example, coating methods include screen printing, spray coating, drop casting, and immersion of a predetermined substrate. When removing the dispersion after coating, methods include drying by known methods or separating the dispersion from the catalyst particles and conductive solid by filtration. Alternatively, a dispersion containing the catalyst particles, conductive solid, and carrier may be prepared, and the mixture of catalyst particles, conductive solid, and carrier may be deposited by pouring the dispersion through a mesh such as a sieve to obtain a photocatalytic sheet for water splitting (papermaking method). In this case, cellulose fiber is preferably used as the carrier. In any of these methods, pressurization or heat treatment may be performed as needed.

[0085] The dispersion liquid described above is not particularly limited as long as it can disperse the various materials. Furthermore, it is preferable to use a dispersion liquid that does not excessively dissolve the various materials. Examples include water, water-soluble alcohols, high-boiling point solvents, and high-viscosity solvents.

[0086] Depending on the combination of catalyst particles or particulate conductive solids and the dispersion, aggregation may occur on the catalyst particles alone or on the particulate conductive solids alone. When aggregation occurs on a single component in this way, it can hinder electron transfer. Therefore, when aggregation is likely to occur as described above, it is preferable to aggregate the mixture in advance while the catalyst particles and particulate conductive solids are mixed together. By obtaining a photocatalytic sheet for water splitting using such aggregated mixtures, the decrease in catalytic activity caused by aggregation of a single component can be suppressed. In other words, the method for manufacturing a photocatalytic sheet for water splitting in this embodiment may include the steps of integrating a particle mixture containing oxygen-generating photocatalytic particles, hydrogen-generating photocatalytic particles, and conductive solids, and dispersing the integrated particle mixture in a dispersion, before coating or supplying it to any substrate. In particular, it is preferable to perform the above steps when applying the dispersion by screen printing or spray coating, and it is more preferable to perform the above steps when spray coating.

[0087] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these examples. <Physical property evaluation> The physical properties of the bismuth vanadate particles obtained in the examples and comparative examples were evaluated by following the procedure below.

[0088] (XRD Measurement) For each measurement sample described later, the powder was washed with pure water, dried, and powder X-ray diffraction measurement was performed using an X-ray diffractometer (desktop X-ray diffractometer MiniFlex 600 (manufactured by Rigaku)). From the obtained spectra, tetragonal BiVO2 was identified. ４ Peaks originating from monoclinic BiVO ４ We investigated whether there was a peak originating from [the specified source].

[0089] (Half-reaction (oxygen production) evaluation) To evaluate the catalytic activity of the photoreactive water splitting reaction, the obtained monoclinic BiVO2 ４ The activity of the powder as a photocatalyst was evaluated by measuring the amount of oxygen produced from an aqueous silver nitrate solution under simulated sunlight irradiation. The BiVO of each example and comparative example was used as the photocatalyst in a quartz glass flask with an upward irradiation window. ４ 0.3 g of powder and 0.02 M AgNO as a sacrificial reagent３ 200 ml of aqueous solution was mixed to form the reaction solution. Next, the atmosphere in the reaction system was replaced with Ar gas. Then, while stirring the reaction solution with a stirring bar, simulated sunlight was irradiated for 1 hour from the quartz window side using a 150 W solar simulator (Yamashita Densou; YSS-E40). The gas in the glass flask was sampled with a gas-tight syringe and injected into a gas chromatograph (Agitent, 7890B, TCD detector, CP-Molsieve 5A column) to investigate the amount of oxygen generated over time.

[0090] (XPS Measurement) XPS measurements were performed using an X-ray photoelectron spectroscopy analyzer PHI5000 VerasProbeII (ULVAC-PHI) under the conditions of monochromatic AlKα rays, output power of 25W, and TOA of 45°. Oxygen (Os1) spectra and nitrogen (Ns1) spectra were measured, and the intensity of each spectrum was determined. From the obtained values, a semi-quantitative value was calculated as the nitrogen content (atomic %) when the total value of oxygen and nitrogen atoms was set to 100 atomic %.

[0091] [Example 1] (1-1) Vanadate (K ３ V ５ O １４ KV ３ O ８ For the production of the mixed crystal, K / V = 3.0 / 5.0 (mol ratio), ２ CO ３ Powder 10.45g, V ２ O ５ 23.12 g of powder and 20 mL of ethanol were placed in an agate mortar and mixed for 30 minutes. Next, the resulting mixture was placed in a magnetic crucible and calcined in an electric furnace at 470°C in air for 2 hours. After calcination, the mixture was allowed to cool to room temperature and then crushed. Next, the crushed material was placed in a magnetic crucible and calcined in an electric furnace at 500°C in air for 5 hours. After cooling to room temperature, it was crushed and passed through a 38 μm sieve to remove vanadate (K ３ V ５ O １４ KV ３ O ８ A mixed crystal was obtained. (1-2) BiVO ４Powder Preparation Method To achieve a Bi / V ratio of 1.0 / 1.0 (mol ratio), 2.38 g of vanadate powder obtained in (1-1), 95 g of deionized water, and 9.70 g of bismuth nitrate pentahydrate powder (manufactured by Kanto Chemical; special grade) were placed in a 200 ml glass container, and then 5 g of isopropyl alcohol (hereinafter referred to as "IPA") (Bi:IPA = 1:10 (mol ratio)) was added while stirring (mixing step A). ​​After that, the glass container was sealed and stirred at 70°C for 24 hours (heating step A). ​​When the powder in the glass container was measured by XRD 0.5 hours after the start of heating, tetragonal BiVO was found. ４ A peak originating from was detected, and when the powder in the glass container was measured by XRD 24 hours after the start of heating (after heating), monoclinic BiVO was found. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from were observed. Next, the obtained precipitate was collected by suction filtration, washed with water, and dried at 100°C for 12 hours to obtain monoclinic BiVO2 ４ A powder was obtained.

[0092] [Example 2] Monoclinic BiVO2 was prepared in the same manner as in Example 1, except that 1-butanol was added instead of IPA in (1-2) of Example 1. ４ A powder was obtained. In addition, the powder during heating in the glass container and the powder after heating were subjected to the same BiVO2 process as in Example 1. ４ Upon checking the crystal system, it was found that, similar to Example 1, the powder during heating was tetragonal BiVO2. ４ A monoclinic BiVO2 was detected, and the powder after heating was monoclinic. ４ Only this was detected.

[0093] [Comparative Example 1] Monoclinic BiVO2 was prepared in the same manner as in Example 1, except that IPA was not added in (1-2) of Example 1 and the amount of ion-exchanged water was 100 g. ４ A powder was obtained. In addition, the powder during heating in the glass container and the powder after heating were subjected to the same BiVO2 process as in Example 1. ４ Upon checking the crystal system, it was found that, similar to Example 1, the powder during heating was tetragonal BiVO2. ４ A monoclinic BiVO2 was detected, and the powder after heating was monoclinic. ４Only this was detected.

[0094] [Example 3] Bi / V = 1.0 / 1.0 (mol ratio) ２ O ３ Powder 4.66g, V ２ O ５ 1.819 g of powder and 200 g of 0.5 M nitric acid were placed in a 500 ml glass container, and then 9.6 g of IPA (Bi:IPA = 1:18 (mol ratio)) was added while stirring (mixing step A). ​​The glass container was then sealed and stirred at 70°C for 48 hours (heating step A). ​​One hour after the start of heating, the powder in the glass container was measured by XRD and found to be tetragonal BiVO ４ A peak originating from was detected, and when the powder in the glass container was measured by XRD 48 hours after the start of heating (after heating), monoclinic BiVO was found. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from were observed. Next, the obtained precipitate was collected by suction filtration, washed with water, and dried at 100°C for 12 hours to obtain monoclinic BiVO2 ４ A powder was obtained.

[0095] [Example 4] Monoclinic BiVO2 was prepared in the same manner as in Example 3, except that 2-butanol was added instead of IPA. ４ A powder was obtained. In addition, the powder during heating in the glass container and the powder after heating were subjected to the same BiVO treatment as in Example 3. ４ Upon checking the crystal system, it was found that, similar to Example 3, the powder during heating was tetragonal BiVO2. ４ A monoclinic BiVO2 was detected, and the powder after heating was monoclinic. ４ Only this was detected.

[0096] [Comparative Example 2] Monoclinic BiVO2 was prepared in the same manner as in Example 3, except that IPA was not added. ４ A powder was obtained. In addition, the powder during heating in the glass container and the powder after heating were subjected to the same BiVO treatment as in Example 3. ４ Upon checking the crystal system, it was found that, similar to Example 3, the powder during heating was tetragonal BiVO2. ４ A monoclinic BiVO2 was detected, and the powder after heating was monoclinic. ４Only this was detected.

[0097] [Example 5] (5-1) Preparation of raw material solutions ・Solution A 33 g of 2M nitric acid and 2.426 g of bismuth nitrate pentahydrate powder (manufactured by Kanto Chemical Co., Ltd.; special grade) were placed in a glass container and stirred with a stirring bar at room temperature for 10 minutes. ・Solution B 22 g of 2M nitric acid and 0.585 g of ammonium vanadate powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were placed in a glass container and stirred with a stirring bar at room temperature for 10 minutes. (5-2) BiVO ４ The powder manufacturing solution A and solution B were mixed (Bi:V = 1:1 (mol ratio)) and stirred for 10 minutes to obtain a mixed solution (mixing step B). Next, while stirring the mixed solution, 28% aqueous ammonia (manufactured by Kishida Chemical Co., Ltd.) was gradually added dropwise to adjust the pH to 1 and obtain a precipitate. Next, while stirring the mixture containing the precipitate, 2.2 g of IPA (Kishida Chemical Co., Ltd. Grade 1) was added dropwise (Bi:IPA = 1:17 (mol ratio)) (precipitation step B). After that, it was stirred at room temperature for 3 hours (stirring step B). When the precipitate after stirring was measured by XRD, it was found to be tetragonal BiVO ４ A peak originating from was detected. Next, the mixture containing the precipitate was transferred to a 100 ml Teflon® container, the Teflon® container was sealed in a pressure-resistant container, and treated in a constant temperature bath at 200°C for 24 hours (pressure: 1.6 MPa) (heating step B). Next, the pressure was reduced to atmospheric pressure and simultaneously allowed to cool to room temperature. The obtained precipitate was then collected by suction filtration, washed with water, and dried in a dryer at 100°C for 12 hours to obtain monoclinic BiVO2. ４ A powder was obtained. The powder before drying was found to be monoclinic BiVO2 by XRD measurement. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0098] [Example 6] Monoclinic BiVO2 was prepared in the same manner as in Example 5, except that ethanol was added instead of IPA. ４ A powder was obtained. Furthermore, the precipitate after stirring at room temperature and the powder before drying were subjected to the same BiVO2 analysis as in Example 5. ４ Upon checking the crystal system, it was found that, similar to Example 5, the precipitate was tetragonal BiVO2. ４A monoclinic BiVO2 was detected, and the powder after drying was monoclinic. ４ Only this was detected.

[0099] [Comparative Example 3] Monoclinic BiVO2 was prepared in the same manner as in Example 5, except that IPA was not added. ４ A powder was obtained. Furthermore, the precipitate after stirring at room temperature and the powder before drying were subjected to the same BiVO2 analysis as in Example 5. ４ Upon checking the crystal system, it was found that, similar to Example 5, the precipitate was tetragonal BiVO2. ４ A monoclinic BiVO2 was detected, and the powder after drying was monoclinic. ４ Only this was detected.

[0100] [Example 7] BiVO obtained in Example 5 ４ The powder was transferred to an alumina firing dish and placed inside a tubular furnace. Next, 3.5 L / min of nitrogen was added to the tubular furnace at room temperature. ２ By flowing gas through the tubular furnace, N is introduced from the air. ２ It was replaced with [this]. At this time, the oxygen concentration in the exhaust gas was 0.03%. Next, N ２ The flow rate was changed from 3.5 L / min to 1.6 L / min, N ２ The sample was fired at 500°C for 30 minutes while gas was flowing through it. After firing, the oxygen concentration in the exhaust gas was 0.03%. After firing, the sample was allowed to cool to room temperature, and the firing dish was removed to obtain the fired monoclinic BiVO2. ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0101] [Example 8] First, monoclinic BiVO2 was prepared in the same manner as in Example 5, except that methanol was used instead of IPA in (5-2). ４ A powder was obtained. Next, the obtained BiVO ４ Except for using powder, the same method as in Example 7 was used for calcination, and the calcined monoclinic BiVO ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0102] [Example 9] BiVO obtained in Example 6 ４ Except for using powder, the same method as in Example 7 was used for calcination, and the calcined monoclinic BiVO ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0103] [Example 10] First, monoclinic BiVO2 was prepared in the same manner as in Example 5, except that 1-butanol was used instead of IPA in (5-2). ４ A powder was obtained. Next, the obtained BiVO ４ Except for using powder, the same method as in Example 7 was used for calcination, and the calcined monoclinic BiVO ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0104] [Example 11] BiVO obtained in Example 5 ４ The powder was transferred to an alumina firing dish and placed in a tubular furnace. Next, 3.5 L / min of Ar gas was flowed into the tubular furnace at room temperature to replace the air inside the furnace with Ar. At this time, the oxygen concentration in the exhaust gas was 0.03%. Next, the Ar flow rate was changed from 3.5 L / min to 1.6 L / min, and firing was performed at 500°C for 30 minutes while flowing Ar gas. After firing, the oxygen concentration in the exhaust gas was 0.03%. After firing, the dish was removed and the fired monoclinic BiVO2 was collected. ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0105] [Example 12] BiVO ４ The calcination process was carried out in the same manner as in Example 7, except that the calcination dish containing the powder was placed inside the tubular furnace and the inside of the tubular furnace was not replaced with gas. The calcined monoclinic BiVO ４A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0106] [Example 13] The same method as in Example 7 was used for firing, except that the firing temperature was 400°C, and the fired monoclinic BiVO ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0107] [Example 14] The same method as in Example 7 was used for firing, except that the firing temperature was 600°C, and the fired monoclinic BiVO ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0108] [Comparative Example 4] BiVO obtained in Comparative Example 3 ４ Except for using powder, the same method as in Example 7 was used for calcination, and the calcined monoclinic BiVO ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified source] were observed.

[0109] The bismuth vanadate particles obtained in Examples 1-14 and Comparative Examples 1-4 were evaluated for the above half-reaction (oxygen production). The results are shown in Table 1. The relative values ​​of oxygen production in the water splitting half-reaction in Table 1 are the relative oxygen production amounts when the corresponding comparative examples below are set to 1.0. • Comparative example corresponding to the relative values ​​of Examples 1 and 2: Comparative Example 1 • Comparative example corresponding to the relative values ​​of Examples 3 and 4: Comparative Example 2 • Comparative example corresponding to the relative values ​​of Examples 5-14 and Comparative Example 4: Comparative Example 3

[0110]

[0111] As shown in Table 1, the example in which tetragonal bismuth vanadate was converted to monoclinic bismuth vanadate in the presence of a water-soluble alcohol showed higher catalytic activity of the resulting bismuth vanadate particles than the comparative example in which a water-soluble alcohol was not used during the phase transition from tetragonal to monoclinic. Furthermore, in Comparative Example 4, while the catalytic activity of monoclinic bismuth vanadate particles from Comparative Example 3, which did not use a water-soluble alcohol during the phase transition, did not improve when calcined in an inert atmosphere, the catalytic activity of monoclinic bismuth vanadate particles that underwent the phase transition in the presence of a water-soluble alcohol in Examples 7-11, 13, and 14, which were calcined in an inert atmosphere, showed a significant improvement in catalytic activity. In addition, when comparing the catalytic activity of Examples 1, 3, 5, and 7, in which IPA was added, the order was Example 1 < Example 3 < Example 5 < Example 7, indicating that Example 7 had the highest catalytic activity.

[0112] [Example 15] First, monoclinic BiVO2 was prepared in the same manner as in Example 5, except that the amount of IPA added was 4.5 g in (5-2). ４ A powder was obtained. Furthermore, the precipitate after stirring at room temperature and the powder before drying were subjected to the same BiVO2 analysis as in Example 5. ４ Upon checking the crystal system, it was found that, similar to Example 5, the precipitate was tetragonal BiVO2. ４ A monoclinic BiVO2 was detected, and the powder after drying was monoclinic. ４ Only was detected. Next, the obtained monoclinic BiVO ４ Using the powder, calcination was performed in the same manner as in Example 7 to obtain calcined monoclinic BiVO2. ４ A powder was obtained. The powder after calcination was analyzed using XRD to obtain monoclinic BiVO2. ４ It shows a peak originating from tetragonal BiVO ４ We confirmed that no peaks originating from [the specified factor] were observed. In addition, we measured the relative amount of oxygen produced compared to Comparative Example 3, in accordance with the (half-reaction (oxygen production) evaluation) described above.

[0113] (Measurement of nitrogen atomic content in bismuth vanadate particles) The bismuth vanadate particles obtained in Examples 5, 7, 11, 15 and Comparative Examples 3 and 4 were subjected to the aforementioned XPS measurement, and the semi-quantitative values ​​of nitrogen (N1s) were calculated and are shown in Table 2.

[0114]

[0115] Table 2 shows that in Examples 5, 7, 11, and 15, where monoclinic bismuth vanadate particles were produced using water-soluble alcohol, nitrogen atoms were detected in all of them. Furthermore, in Example 15, where the amount of IPA added was increased compared to the other examples, the semi-quantitative value of nitrogen atoms increased, suggesting that increasing the amount of water-soluble alcohol added increases the amount of doped nitrogen atoms. On the other hand, in Comparative Examples 3 and 4, where water-soluble alcohol was not used, no nitrogen atoms were detected in either of them.

[0116] [Example 16] (6-1) Preparation of photocatalyst for oxygen generation Bismuth vanadate particles obtained by the same method as in Example 7 were prepared.

[0117] (6-2) Preparation of photocatalyst for hydrogen production First, Rh:SrTiO ３ Particles were prepared. 6.67 g (19.6 mmol) of titanium(IV) tetrabutoxide, 200 mL of methanol, 30.74 g (160 mmol) of citric acid, and 3.04 g (20.6 mmol) of strontium carbonate were added to a beaker and stirred at 100°C for 20 hours. After stirring, 0.116 g (20.6 mmol) of rhodium nitrate was added and stirred at 100°C for 30 minutes. Next, 36.52 g (480 mmol) of propylene glycol was added and stirred at 150°C for 16 hours to allow methanol to evaporate from the beaker. The temperature was increased from 200°C to 450°C in 50°C increments, maintaining each temperature for 30 minutes. The mixture was then calcined in an electric furnace in the beaker (500°C, 2 hours). The powder after calcination was ground in an agate mortar, and the powder that passed through a 38 μm sieve was collected. The powder was transferred to an alumina crucible and calcined in an electric furnace at 1100°C for 10 hours to obtain Rh:SrTiO ３ We obtained a particle.

[0118] Next, the obtained Rh:SrTiO ３ Using particles, RuOx-supported Rh:SrTiO ３Particles were prepared by the following method: 0.03 g (0.13 mmol) of ruthenium(III) chloride trihydrate was dissolved in 2.98 g of pure water to prepare an aqueous Ru solution. The Rh:SrTiO solution obtained by the method described above was then prepared. ３ 0.3 g of Rh:SrTiO2 was placed in a cell along with 135 mL of pure water, and ultrasonic waves were applied for 30 minutes. Then, 16 mL of methanol and 0.88 g of the Ru aqueous solution prepared above were added, and light irradiation was performed for 2 hours (150 W solar simulator: Yamashita Densou Co., Ltd.: YSS-E40). The powder after irradiation was collected by suction filtration, dried at 100°C for 2 hours, and RuOx-supported Rh:SrTiO2 was obtained. ３ We obtained particles. We decided to use these particles as a photocatalyst for hydrogen production.

[0119] (6-3) Preparation of photocatalytic sheets for water splitting (screen printing method) 1.25 g of carbon nanotube (CNT) aqueous dispersion (EC-PB; manufactured by Meijo Nanocarbon Co., Ltd., CNT solid content 0.4 wt%), RuOx-supported Rh:SrTiO ３ 0.17g, BiVO ４ 0.33 g of the substance and 0.5 g of water were placed in a container and stirred using a rotational stirring and defoaming apparatus to prepare an evaluation paste. The obtained evaluation paste was printed onto a glass substrate (45 mm x 45 mm, 2.3 mm thick) using an applicator (Gap: 100 μm). Then, it was dried at 50°C for 1 hour to obtain a photocatalytic sheet for water splitting.

[0120] [Example 17] (6-1) to (6-2) were obtained by the same method as in Example 16. (6-3) Preparation of photocatalytic sheet for water splitting (spray method) 1.25 g of carbon nanotube (CNT) aqueous dispersion (EC-PB; manufactured by Meijo Nanocarbon Co., Ltd., CNT solid content 0.4 wt%), RuOx-supported Rh:SrTiO ３ 0.17g, and BiVO ４0.33 g was placed in a container and stirred using a rotational stirring and defoaming apparatus. Then, the container was removed from the apparatus, 20 mL of IPA was added, and ultrasonic waves were irradiated for 5 minutes to disperse the mixture and obtain a spray coating solution. The prepared dispersion was applied to a glass substrate (45 mm x 45 mm, 2.3 mm thick) using a spray gun (PS-9513B-04; manufactured by Anest Iwata Corporation), dried at 50°C for 1 hour, and obtained a photocatalytic sheet for water splitting.

[0121] [Example 18] (6-1) to (6-2) were obtained by the same method as in Example 16. (6-3) Preparation of photocatalytic sheet for water splitting (papermaking method) 10 g of microfibril cellulose fiber (Daicel Mirise: PC110S average fiber diameter: 1.5 μm solid content 35 wt%) was added to 90 g of pure water and stirred with a stirring blade for 3 minutes to obtain a 3.5% KY100G aqueous dispersion. 0.191 g of carbon nanotube (CNT) aqueous dispersion (EC-PB; manufactured by Meijo Nanocarbon Co., Ltd., CNT solid content 0.4 wt%), RuOx supported Rh:SrTiO ３ 0.038g, BiVO ４ 0.076 g and 1.0 g of pure water were placed in a container and mixed, and ultrasonic waves were irradiated for 1 minute to obtain a CNT photocatalyst-containing aqueous dispersion. 0.97 g of 3.5% KY100G aqueous dispersion, the CNT photocatalyst-containing aqueous dispersion, and 4.8 g of water were mixed and stirred with a stirring blade for 3 minutes. Then, the mixture was poured through a sieve (mesh diameter 38 μm, diameter: φ75 mm) and dried at 100°C for 1 hour to obtain a photocatalyst sheet for water splitting. The microfibril cellulose fibers contained in the obtained sheet constitute the total amount of catalyst particles (i.e., BiVO2 ４ Particles and RuOx-supported Rh:SrTiO ３ It was 30 wt% of the total amount of particles.

[0122] (Evaluation of hydrogen and oxygen production by water splitting) The catalytic activity of water splitting by photoreaction was evaluated using a closed-circulation reaction system for photocatalysis (manufactured by Makuhari Chemical Glass Co., Ltd.). The evaluation procedure is as follows. First, the opening was φ41 mm (photocatalyst area: 13.2 cm²). ２The obtained water-splitting photocatalyst sheet was fixed to an open polytetrafluoroethylene jig. Next, the water-splitting photocatalyst sheet fixed above and 118 ml of pure water were placed in a glass cell, the glass cell was sealed with a quartz glass lid, and connected to the above closed-loop reaction system for photocatalysis. Next, the inside of the apparatus (hereinafter referred to as the system) was evacuated, and then Ar replacement was performed. This operation was repeated until the system pressure was 10 kPa. Next, a 300 W xenon light source (Asahi Spectro; MAX-303, mirror module UV-VIS wavelength 300-600 nm, direct-coupled rod lens incident light type) was used as the light source and irradiated for 3 hours. During this time, from the start of irradiation, the amount of hydrogen and oxygen was measured over time using a GC device (GL Sciences Co., Ltd.: GC3210D, TCD detector), and the amount of oxygen and hydrogen produced were investigated, respectively. The obtained results are shown in Table 3. Note that CNT in Table 3 refers to carbon nanotubes, and the content is BiVO2 ４ This represents the ratio when the number of particles is set to 100.

[0123]

[0124] Water splitting tests were conducted using each of the photocatalytic sheets in Examples 16 to 18. In all cases, hydrogen and oxygen were produced upon light irradiation, indicating that these photocatalytic sheets have the function of catalyzing water splitting reactions. Therefore, it can be said that water-splitting photocatalytic sheets were obtained.

Claims

1. A method for producing monoclinic bismuth vanadate particles, the method comprising a step α that induces a phase transition, in which tetragonal bismuth vanadate is converted to monoclinic bismuth vanadate in the presence of a water-soluble alcohol.

2. The method for producing bismuth vanadate particles according to claim 1, comprising a mixing step A of mixing a vanadium compound, a bismuth compound, a solvent or dispersion medium, and a water-soluble alcohol to obtain a mixture a, and a heating step A of heating the mixture a.

3. The method for producing bismuth vanadate particles according to claim 2, wherein the heating step A includes step α.

4. The method for producing bismuth vanadate particles according to claim 2, wherein the heating step A is carried out while stirring.

5. The method for producing bismuth vanadate particles according to claim 1, comprising: a mixing step B of mixing raw materials containing vanadium atoms and bismuth atoms to obtain a solution b containing vanadium atoms and bismuth atoms; a precipitation step B of obtaining a mixture b having a precipitate containing bismuth vanadate from the solution b; and a heating step B of heating the mixture b under pressure, wherein at least one selected from the group consisting of the solution b and the mixture b contains a water-soluble alcohol.

6. The method for producing bismuth vanadate particles according to claim 5, wherein the heating step B includes step α.

7. A method for producing bismuth vanadate particles according to claim 5, further comprising a stirring step B between the precipitation step B and the heating step B, wherein the mixture b is further stirred.

8. The method for producing bismuth vanadate particles according to claim 1, further comprising a calcination step of calcining the monoclinic bismuth vanadate particles in an inert atmosphere.

9. The method for producing bismuth vanadate particles according to claim 1, wherein the water-soluble alcohol is at least one selected from the group consisting of monohydric alcohols having 1 to 6 carbon atoms, glycol ethers having 1 to 10 carbon atoms, and polyhydric alcohols having 2 to 8 carbon atoms.

10. Bismuth vanadate particles in monoclinic form, wherein the particles are doped with nitrogen atoms.

11. The bismuth vanadate particles according to claim 10, wherein, as measured by XPS (X-ray photoelectron spectroscopy), nitrogen atoms are present in an amount of 0.1 to 10 atomic percent relative to a total of 100 atomic percent of oxygen atoms and nitrogen atoms.

12. The bismuth vanadate particles according to claim 10 or 11, wherein the bismuth vanadate particles are obtained by the method for producing bismuth vanadate particles according to claim 5.

13. A photocatalytic sheet for water splitting, comprising at least one selected from the group consisting of bismuth vanadate particles obtained by the manufacturing method described in any one of claims 1 to 9 and monoclinic bismuth vanadate particles doped with nitrogen atoms, hydrogen-generating photocatalytic particles, and a conductive solid.

14. The photocatalytic sheet for water splitting according to claim 13, wherein the conductive solid is at least one selected from the group consisting of carbon nanotubes, silver nanowires, and reduced graphene oxide.

15. The photocatalytic sheet for water splitting according to claim 13 or 14, wherein the photocatalytic sheet for water splitting comprises cellulose fibers.

16. A method for producing a photocatalytic sheet for water splitting, comprising the step of forming a sheet using a dispersion containing at least one selected from the group consisting of bismuth vanadate particles obtained by the manufacturing method described in any one of claims 1 to 9 and monoclinic bismuth vanadate particles doped with nitrogen atoms, and photocatalytic particles for hydrogen generation.

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