Photocatalyst, photoelectrode using same, and methods for producing the same

Abstract

To provide a photocatalyst capable of promoting an H+ supply to a reaction field as an improvement of mass transfer of a reactant, and capable of improving the hydrogen generation activity (activity of water decomposition reaction) as compared with a case where a phosphate buffer solution is used as a reaction solution, and to provide a photoelectrode using the same and a method of manufacturing the photocatalyst and the photoelectrode.SOLUTION: In a photocatalyst 10 according to the present invention, a phosphate group 20 is modified on a surface of a photocatalyst 10a. A method for manufacturing the photocatalyst 10 according to the present invention includes: a phosphate group modification step S1 of modifying the phosphate group 20 on the surface of the photocatalyst 10a; and a hydrogen generation promoter support step S2 of supporting a hydrogen generation promoter 40 on the surfaces of the photocatalysts 10a and 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a photocatalyst that can be used as a photocatalyst for water splitting and that can enhance hydrogen generation activity (water splitting reaction activity), a photoelectrode using the same, and methods for producing the same. [Background technology]

[0002] Since the "Honda-Fujishima effect" was reported, active research has been conducted into the use of electrode-based and powder-based photocatalysts for water splitting, and progress has been made in the development of photocatalytic materials that respond to visible light, which increases the efficiency of solar energy conversion. For water splitting photocatalysts, improving the photocatalytic performance (photocatalytic reaction) is an essential challenge in order to produce more hydrogen.

[0003] Because semiconductor photocatalytic reactions are heterogeneous catalytic reactions that occur only on the catalyst surface, photocatalytic activity is greatly influenced by the efficiency of electron and hole utilization and the adsorption characteristics of the reactant on the semiconductor photocatalyst surface. Therefore, in order to improve performance, it is necessary to devise ways to control factors that affect the photocatalytic reaction (physical properties inside the particle, properties outside the particle) so that they do not lead to a decrease in activity.

[0004] Regarding the characteristics of the exterior (external to the particle) of a semiconductor photocatalyst, the electrons and holes generated by photoexcitation react with reactants on the photocatalyst surface, but this becomes kinetically unfavorable when it comes to multi-electron reactions. In particular, the water splitting reaction, a typical example of a multi-electron reaction, is prone to the uphill type of reverse reaction between the oxygen and hydrogen generated, or the intermediate, so it is necessary to have a reaction field where this is suppressed. A typical example is the support of a promoter such as a precious metal. By contacting an electron acceptor such as a precious metal, it is possible to separate the reaction fields of the charge and redox reaction, and to make kinetic factors favorable, thereby increasing the reaction rate. This method leads to improved photocatalytic reactions. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Hiromu Kumagai, Tsutomu Minegishi, Naotoshi Sato, Taro Yamada, Jun Kubota and Kazunari Domen 「Efficient solar hydrogen production from neutral electrolytes using surface-modified Cu(In,Ga)Se2 photocathodes」 J. Mater. Chem. A 2015,3,8300-8307 https: / / pubs.rsc.org / en / content / articlelanding / 2015 / ta / c5ta01058f#!divAbstract [Non-Patent Document 2] Tatsuya Shinagawa and Kazuhiro Takanabe 「Electrocatalytic Hydrogen Evolution under Densely Buffered Neutral pH Conditions」 J. Phys. Chem. C 2015,119,20453-20458 https: / / pubs.acs.org / doi / pdf / 10.1021 / acs.jpcc.5b05295 [Non-Patent Document 3] Muhammad Qureshi, Angel T.Garcia-Esparza, Tatsuya Shinagawa, Philippe Sautet, Tangui Le Bahers and Kazuhiro Takanabe「Contribution of electrolyte in nanoscale electrolysis of pure and buffered water by particulate photocatalysis」 Sustainable Energy & Fuels 2018,2,2044-2052https: / / pubs.rsc.org / en / content / articlelanding / 2018 / se / c8se00272j#!divAbstract [Summary of the Invention] [Problem to be solved by the invention]

[0006] There have been many previous studies that have promoted photocatalytic water splitting reactions by suppressing the internal recombination of electrons and holes generated by photoexcitation and by increasing the reaction rate by supporting a co-catalyst such as a precious metal. However, there has been little progress in research into promoting photocatalytic reactions by improving the mass transfer (substance diffusion) of reactants in the reaction solution.

[0007] Therefore, the inventors investigated the H in the reaction solution, one of the factors that contribute to photocatalytic performance. + We focused on improving the mass transfer of protons. Excited carriers inside the semiconductor photocatalyst are involved in chemical reactions on the surface, so if a sufficient amount of co-catalyst is supported, the reaction rate between the carriers and the reactants increases, improving activity. However, if the reactants (protons) in the reaction solution do not diffuse to the vicinity of the active sites, the chemical reaction will not proceed, and if the mass transfer of the reactants to the active sites is slower than the reaction rate of the carriers, the overall reaction of the photocatalyst will be dependent on the mass transfer of the reactants.

[0008] For this reason, in (photo)electrochemical and (photo)catalytic water splitting reactions, the hydrogen production activity is influenced by the conditions of the reaction solution. Generally, in strong acids and strong bases, protons and hydroxide ions act as reactants, and unlike other ions in solution, they move in a relay-like fashion via water molecules, resulting in extremely fast mass transfer to the vicinity of the active site. Figure 21 shows an overview of the "method of proton and hydroxide ion transfer."

[0009] However, because strong acids and strong bases are extremely dangerous to handle, the use of neutral (or near-neutral) electrolytes is required for safety reasons. Meanwhile, the reactant in a neutral electrolyte is water molecules, but mass transfer in the solution is very slow, resulting in reduced hydrogen production efficiency. Most water in nature is, of course, a solution near neutral, with a low proton concentration. Therefore, if the photocatalyst surface can be modified to improve proton mass transfer even under neutral conditions, a significant improvement in hydrogen production efficiency can be expected, potentially leading to further advances in hydrogen production using photocatalysts in the future.

[0010] In this regard, previous research in the field of (photo)electrochemistry (Non-Patent Document 1) reported that, as shown in Figure 22(a), the use of phosphate buffer (phosphate buffer solution) improves photoelectrochemical hydrogen production activity compared to the Na2SO4 aqueous solution that had previously been commonly used as a neutral electrolyte. The effect of phosphate buffer on the kinetics of the hydrogen production reaction was also electrochemically evaluated using a Pt disk electrode, as shown in Figure 22(b). It was reported that the use of phosphate buffer in the (photo)electrochemical water splitting reaction reduced the overpotential for hydrogen production even near neutrality, improving the efficiency of hydrogen production. Non-Patent Document 2 also conducted a more detailed electrochemical evaluation of the effect of phosphate buffer on the mass transfer of reactants, and here, as shown in Figure 23, the effect of phosphate ion species (H2PO4 - , HPO4 2- ) itself functions as a reactant, supplying H+ in the reaction solution to the electrode surface, which is thought to contribute to improving mass transfer.

[0011] However, according to Non-Patent Document 3, phosphate buffer can only affect hydrogen production activity at high photon fluxes, and not at low photon fluxes (100 μmol cm -2 h -1It has been shown that the hydrogen production activity is almost the same as when a non-buffered electrolyte is used, and has almost no effect, so phosphate buffers cannot necessarily be said to be the optimal means for stably promoting photocatalytic reactions by improving the mass transfer of reactants. [Means for solving the problem]

[0012] The present invention has been made in view of the above circumstances, and aims to improve the mass transfer of reactants by increasing the amount of H + The present invention aims to provide a photocatalyst that can promote the supply of hydrogen and improve the hydrogen production activity (activity of the water splitting reaction) compared to when a phosphate buffer solution is used as a reaction solution, a photoelectrode using the same, and methods for producing the same.

[0013] In order to achieve the above object, the present invention has the following configuration. According to one example of the photocatalyst of the present invention, the surface of the photocatalyst is modified with a phosphate group. the phosphate group is modified by a silane coupling treatment, and a hydrogen generation promoter is supported on the surface of the photocatalyst. It is characterized by:

[0014] According to this, the phosphate group is converted into the H necessary for the hydrogen production reaction. + By acting as a mediator for the supply of H to the reaction site, the mass transfer of reactants is improved. + The phosphate group is modified on the surface of the photocatalyst, and H + can be supplied directly to the reaction field on the photocatalyst surface, which makes it possible to significantly improve hydrogen production activity.

[0015] Also, By coating the surface of the photocatalyst with a silane coupling agent, the entire surface can be uniformly modified with phosphate groups.

[0016] Also, The hydrogen generation promoter may be ruthenium (Ru).

[0017] The photocatalyst may contain strontium titanate (STO) represented by the chemical formula SrTiO3 or Cu2Snx Ge 1-x A compound (CTGS) represented by S3 (where 0≦x≦1) can be used.

[0018] The strontium titanate is preferably doped with a different metal element, such as lanthanum or rhodium.

[0019] The photocatalyst may be in the form of a powder.

[0020] Furthermore, one example of the photoelectrode according to the present invention is characterized in that a layer of the photocatalyst according to the present invention is formed on a substrate.

[0021] Furthermore, according to one example of the method for producing a photocatalyst according to the present invention, the method includes a phosphate group modification step of modifying the surface of the photocatalyst with a phosphate group, and a hydrogen generation co-catalyst supporting step of supporting a hydrogen generation co-catalyst on the surface of the photocatalyst. and after the phosphoric acid group modification step, the hydrogen generation promoter supporting step is carried out. It is characterized by:

[0023] Also, The method includes a phosphate group modification step of modifying a photocatalyst surface with a phosphate group, and a hydrogen generation co-catalyst supporting step of supporting a hydrogen generation co-catalyst on the photocatalyst surface, In the phosphoric acid group modification step, a photocatalyst is suspended in a solution of a silane coupling agent having a phosphoric acid group. and make a suspension By this, the surface of the photocatalyst can be modified with phosphate groups. Then, the suspension is evaporated to dryness to obtain a photocatalyst powder modified with the phosphate groups.

[0024] Furthermore, one example of the method for producing a photoelectrode according to the present invention is characterized by including a step of forming a film of the photocatalyst produced by the method according to the present invention on a substrate. [Effects of the Invention]

[0025] According to the present invention, H is transferred via the phosphate group modified on the photocatalyst surface. + is supplied directly to the reaction field on the photocatalyst surface, +This has the particularly advantageous effect of facilitating the supply of hydrogen and improving the hydrogen production activity (activity of the water splitting reaction) compared to when a phosphate buffer solution is used as the reaction solution. [Brief explanation of the drawings]

[0026] [Figure 1] 1A and 1B are schematic diagrams showing examples of the surface morphology of a photocatalyst according to an embodiment of the present invention. [Figure 2] Fig. 2(a) is a schematic diagram showing an example of the surface morphology of a photocatalyst according to an embodiment of the present invention, and Fig. 2(b) is the chemical formula of 3-(trihydroxysilyl)propyl methylphosphonate sodium, which is a silane coupling agent. [Figure 3] 1 is a process diagram illustrating an example of a method for producing a photocatalyst according to an embodiment of the present invention. [Figure 4] 1 is a table showing the photocatalytic activity resulting from the surface treatment of the photocatalyst according to Example 1 of the present invention. [Figure 5] 1 is a graph showing the photocatalytic activity depending on the amount of silane coupling agent carried by the photocatalyst according to Example 1 of the present invention. [Figure 6] 1 is a graph showing the photocatalytic activity of the photocatalyst according to Example 1 of the present invention in relation to the concentration of a solution of a silane coupling agent. [Figure 7] 1 is a graph showing the photocatalytic activity of the photocatalyst according to Example 1 of the present invention as a function of the pH of a solution of a silane coupling agent. [Figure 8] 1 is a diagram showing a comparison of the photocatalytic activity between the modification of phosphate groups of the photocatalyst according to Example 1 of the present invention and a phosphate buffer solution. [Figure 9] 1 is a table showing the photocatalytic activity depending on the type (functional group) of the silane coupling agent of the photocatalyst according to Example 1 of the present invention. [Figure 10] 1 is a graph showing the photocatalytic activity of the photocatalyst according to Example 1 of the present invention relative to the amount of light. [Figure 11] Fig. 11(a) is a STEM image of the surface morphology of the photocatalyst according to Example 1 of the present invention. Fig. 11(b) is a diagram showing the results of EDS line analysis of the surface morphology of the photocatalyst according to Example 1 of the present invention. [Figure 12]1 is a table showing the results of XPS analysis of the photocatalyst according to Example 1 of the present invention. [Figure 13] 1 is a diagram showing the diffuse reflectance spectrum according to the Sn / Ge composition of CTGS. [Figure 14] 1 is a graph showing the photocatalytic activity depending on the Sn / Ge composition of the photocatalyst according to Example 2 of the present invention. [Figure 15] 1 is a graph showing the photocatalytic activity of the photocatalyst according to Example 2 of the present invention as a function of the loading of a hydrogen generation promoter. [Figure 16] 1 is a graph showing the photocatalytic activity of the photocatalyst according to Example 2 of the present invention following silane coupling treatment. [Figure 17] 1 is a diagram showing the results of TG-DTA analysis in accordance with a powder recovery method after silane coupling treatment of a photocatalyst according to Example 2 of the present invention. [Figure 18] 10 is a graph showing current-potential curves according to the Sn / Ge composition of the photoelectrode according to Example 3 of the present invention. [Figure 19] 10 is a graph showing current-potential curves according to the Cu composition of the photoelectrode according to Example 3 of the present invention. [Figure 20] 10 is a graph showing current-time curves according to the Sn / Ge composition of the photoelectrode according to Example 3 of the present invention. [Figure 21] FIG. 1 is a schematic diagram illustrating how protons and hydroxide ions move. [Figure 22] FIG. 22(a) is a chart showing comparative data of photoelectrochemical evaluation of photocathodes in previous research, and FIG. 22(b) is a chart showing comparative data of electrochemical evaluation of Pt disk electrodes. [Figure 23] 1 is a conceptual diagram showing the results of other conventional research on electrochemical reactions, and a table showing comparative data. DETAILED DESCRIPTION OF THE INVENTION

[0027] (Photocatalyst) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Figure 1 shows an example of the surface morphology of a photocatalyst (10) according to this embodiment.

[0028] As shown in FIG. 1, the photocatalyst (10) according to this embodiment is characterized in that the surface of the photocatalyst (10a) is modified with a phosphate group (20) (phosphate-based functional group (20)). In this application, the term "phosphate group" refers to a "phosphate-based functional group (phosphate-based functional group)" and broadly includes various types of phosphonic groups. Hereinafter, the photocatalyst in a state modified by modification with a phosphate group (20) will be referred to as the photocatalyst (10), and the modified photocatalyst itself will be referred to as the photocatalyst (10a).

[0029] The photocatalyst (10a) is a semiconductor material that has the function of promoting a water splitting reaction by absorbing light. Electrons and holes generated inside by photoexcitation move to the surface, where the electrons cause a reduction reaction and the holes cause an oxidation reaction, resulting in water splitting and hydrogen being produced by the reduction reaction. Any material having this function can be used as the photocatalyst (10a) according to this embodiment. Examples include "La,Rh:SrTiO3" (La,Rh:STO), "Rh:SrTiO3" (Rh:STO), and "Cu2Sn x Ge 1-x S3" (CTGS), "GaN:ZnO", "La5Ti2CuS5O7", "Sm2Ti2S2O5", "Cu(In,Ga)(S,Se)2" (CIGS system), "CdTe", "Cu2ZnSnS4" (CZTS), etc. Among these, from the viewpoint of using inexpensive, low-toxicity metals and being relatively easy to synthesize, "La,Rh:SrTiO3" (La,Rh:STO), "Cu2Sn x Ge 1-x S3" (CTGS) is suitable.

[0030] In this embodiment, the phosphate groups (20) are modified by a silane coupling treatment (hereinafter, silane coupling may be abbreviated as "SC"). An organosilicon compound having, in its molecule, a reactive group that chemically bonds with inorganic materials and a reactive group that chemically bonds with organic materials is the silane coupling agent (30). In this embodiment, the phosphate groups (20) are modified on the surface of the photocatalyst (10a) via the silane coupling agent (30). More specifically, as shown in FIG. 1 , the surface of the photocatalyst (10a) is coated with a silane coupling agent (30) having the phosphate groups (20), thereby modifying the phosphate groups (20). This results in a structure in which the phosphate groups (20) are modified on the surface of the photocatalyst (10a) via the silane coupling coating (30). However, the "silane coupling coating (30)" referred to here does not only mean a state in which the photocatalytic surface (10a) is completely covered with the silane coupling agent (30), but also a state in which some of the photocatalytic surface (10a) is exposed to the extent that the effect of the silane coupling treatment is not hindered.

[0031] The silane coupling agent (30) is not particularly limited, and an example is "3-(trihydroxysilyl)propyl=sodium methylphosphonate" having a phosphate group. Figure 2(a) shows the surface morphology of photocatalyst (10) in which photocatalyst (10a) is "La,Rh:STO," silane coupling agent (30) is "3-(trihydroxysilyl)propyl=sodium methylphosphonate," and hydrogen generation co-catalyst (40) (described later) is rhodium (Ru). Figure 2(b) shows the chemical formula of "3-(trihydroxysilyl)propyl=sodium methylphosphonate."

[0032] In this way, by modifying the photocatalyst (10a), the phosphate group (20) can be converted into H, which is necessary for the hydrogen production reaction. + (proton) supply mediator. + ) to the reaction field to improve the mass transfer of + This promotes the supply of hydrogen and improves the hydrogen generation activity (photocatalytic activity).+ can be directly supplied to the reaction field on the surface of the photocatalyst (10a, 10) via the phosphate group (20) modified on the surface of the photocatalyst (10a), and thus H + This makes it possible to significantly improve the hydrogen production activity (photocatalytic activity), i.e., the activity of the water splitting reaction, compared to phosphate buffer solution that transports hydrogen (see Figure 8).

[0033] In this embodiment, a hydrogen generation co-catalyst (40) is supported on the surface of the photocatalyst (10a, 10). The hydrogen generation co-catalyst (40) is an electron acceptor such as a precious metal, and when brought into contact with (supported on) the surface of the photocatalyst (10a, 10), it forms a potential gradient in the spatial charge layer near the surface, promoting charge separation of photoexcited carriers (electrons and holes) and improving photocatalytic activity. The hydrogen generation co-catalyst (40) is not particularly limited, and examples include ruthenium (Ru), platinum (Pt), etc.

[0034] By supporting the hydrogen generation promoter (40), as shown in FIG. 2(a), the phosphate group (20) on the surface of the photocatalyst (10a, 10) transfers the captured H to the adjacent hydrogen generation promoter (40) as an active site for the hydrogen generation reaction. + A waste-free H that passes on one after another + As a result, it is possible to form a transport system. + This can further promote the supply of hydrogen, thereby enabling further improvement of the hydrogen generation activity.

[0035] As shown in FIG. 3 , the photocatalyst (10) according to this embodiment can be produced by carrying out a phosphate group modification step (S1) in which the surface of the photocatalyst (10a) is modified with phosphate groups (20) (phosphate-based functional groups (20)), and a hydrogen generation promoter support step (S2) in which the surface of the photocatalyst (10a, 10) is supported with a hydrogen generation promoter (40). The phosphate group modification step (S2) can be carried out by a silane coupling treatment. That is, by suspending photocatalyst (10a) powder in a solution of a silane coupling agent (30) having phosphate groups (20), the surface of the photocatalyst (10a) can be coated with the silane coupling agent (30). The powder can then be recovered from the suspension to easily obtain photocatalyst (10) powder whose surface is modified with phosphate groups (20). According to the silane coupling treatment, the surface of the photocatalyst (10a) can be coated with phosphate groups (20) uniformly and over the entire surface by coating the surface with the silane coupling agent.

[0036] The photocatalyst (10) powder can be recovered by suction filtering and drying the suspension or by evaporating to dryness, but the evaporation to dryness method is preferred (see FIG. 17). This method allows the photocatalyst (10) powder to be recovered with almost no loss of the coated silane coupling agent (30), and makes it possible to more precisely control the amount of modification with the phosphate group (20).

[0037] In this embodiment, the hydrogen generation promoter supporting step (S2) is performed after the phosphate group modifying step (S1) is performed. If the silane coupling treatment (S1) is performed after the hydrogen generation promoter (40) is supported (S2), the hydrogen generation promoter (40) will be covered with the silane coupling agent (30), and the hydrogen will not reach the active site. +In this case, the migration of hydrogen and the reaction at the active sites are hindered, which may result in a decrease in hydrogen generation activity (photocatalytic activity) (see FIG. 4). In contrast, in this embodiment, the entire surface of the photocatalyst (10a) is coated with a silane coupling agent (30) (S1) and then a hydrogen generation co-catalyst (40) is supported (S2). As shown in FIG. 11(a), this allows the hydrogen generation co-catalyst (40) to be scattered on the silane coupling coating (30) on the surface of the photocatalyst (10a). As shown in FIG. 2(a), the hydrogen generation co-catalyst (40) can be exposed on the surface of the photocatalyst (10a, 10) and exposed to the reaction solution. As a result, H + The phosphate group (20) as a mediator and the hydrogen generation promoter (40) as an active site for the hydrogen generation reaction (photocatalytic reaction) are arranged adjacent to each other, and H + It is possible to transport the hydrogen without waste and improve the hydrogen generation activity (photocatalytic activity). The hydrogen generation promoter (40) can be supported on the surface of the photocatalyst (10a, 10) by a known method such as photoprecipitation.

[0038] (photoelectrode) Furthermore, by forming a layer of the photocatalyst (10) powder according to this embodiment on a substrate, it can be used as a photoelectrode for water splitting or hydrogen generation. In the case of the photocatalyst (10), electrons and holes generated internally by photoexcitation cause both a reduction reaction and an oxidation reaction on the surface of the photocatalyst (10). In the case of a photoelectrode, one reaction occurs at the photoelectrode and the other reaction occurs at the counter electrode. The photoelectrode according to this embodiment is constructed using the photocatalyst (10a), which is a p-type semiconductor, and therefore can function as a photocathode, where a reduction reaction occurs on its surface to generate hydrogen (an oxidation reaction occurs on the surface of the counter photoanode to generate oxygen). The photocatalyst (10) powder can be processed into an electrode in which a photocatalyst (10) layer is formed on a substrate by sequentially applying known methods such as drop casting and particle transfer. [Example]

[0039] Next, specific examples of the photocatalyst and the method for producing the same according to the present invention, and an evaluation of the activity of the photocatalyst powder system for hydrogen generation will be described. In this example, we used "La,Rh:SrTiO3" (La,Rh:STO), a photocatalyst powder (10a) doped with La and Rh, to evaluate the photocatalytic activity of the powder after modification with phosphate groups. We evaluated the effects of surface treatment on the photocatalyst under various conditions, such as the order of surface treatment, the amount of silane coupling agent loaded on the photocatalyst under sacrificial reagent conditions, and the concentration and pH of the silane coupling agent solution. We also evaluated the photocatalytic activity of the powder after modification with phosphate groups (silane coupling treatment) using phosphate buffer (KPi buffer) as the reaction solution, or after treatment with a silane coupling agent containing a functional group (amino group) other than phosphate groups at the terminal.

[0040] Strontium titanate (chemical formula: SrTiO3) (STO) can only absorb ultraviolet light of less than 400 nm, but can be made responsive to visible light by doping with different metal elements. Co-doping with multiple elements can also make it responsive to visible light while suppressing a decrease in photocatalytic activity. According to this embodiment, "La,Rh:SrTiO3" (La,Rh:STO), which is co-doped with lanthanum (La) and rhodium (Rh), has a visible light responsiveness of approximately 500 nm. In addition to lanthanum (La) and rhodium (Rh), chromium (Cr), antimony (Sb), tantalum (Ta), etc. may also be doped in appropriate combination as different metal elements.

[0041] The following materials were used in the experiment: The reagents used to synthesize "La,Rh:STO" were SrTiO3 from Fujifilm Wako Pure Chemical Industries, Ltd. (3-1-2 Doshomachi, Chuo-ku, Osaka City, Osaka Prefecture), and La2O3 and Rh2O3 from Kanto Chemical Co., Ltd. (2-2-1 Nihonbashi Muromachi, Chuo-ku, Tokyo). Methanol (CH3OH) (sacrificial reagent) and potassium phosphate (KPi) (KH2PO4 and K2HPO4) (phosphate buffer) used in the sample solution (reaction solution) were obtained from Fujifilm Wako Pure Chemical Industries, Ltd. (3-1-2 Doshomachi, Chuo-ku, Osaka, Osaka Prefecture). Ruthenium chloride trihydrate (RuCl3·3H2O), a promoter source, was purchased from Kanto Chemical Co., Ltd. (2-2-1 Nihonbashi Muromachi, Chuo-ku, Tokyo) and used as is. The silane coupling agents used were "3-(Trihydroxysilyl)propyl methylphosphonate, monosodium salt solution 50 wt% in H2O" with a phosphate group, available from Sigma-Aldrich Japan LLC (5F Arco Tower, 1-8-1 Shimomeguro, Meguro-ku, Tokyo), and "3-Aminopropyltrimethoxysilane" with an amino group, available from Fujifilm Wako Pure Chemical Corporation (3-1-2 Doshomachi, Chuo-ku, Osaka City, Osaka Prefecture).

[0042] The synthesis and preparation of experimental materials were carried out as follows. To synthesize "La,Rh:STO" (doping STO with La and Rh), 1.8349g of SrTiO3, 0.0652g of La2O3, and 0.0508g of Rh2O3 were weighed out and placed in a mortar, followed by wet mixing with ethanol for 1 hour. The powder was then placed in an alumina crucible and fired in a muffle furnace at 1100°C for 6 hours to synthesize "La,Rh:STO."

[0043] For the silane coupling treatment, the reagent was prepared at 50 wt% in H2O, so the preparation was carried out taking into account both the ratio to the photocatalyst and the ratio to the water. After adding the reagent containing the silane coupling agent to the water in a vial under a nitrogen atmosphere, the required photocatalyst powder was added, sonicated for 10 minutes, and stirred for 30 minutes to carry out the silane coupling treatment. The photocatalyst suspension was then suction filtered and dried to recover the "SC / La,Rh:STO" powder.

[0044] Ru was used as a cocatalyst for hydrogen production. 1 mL of water and 1.3 mg of RuCl3·3H2O (cocatalyst source) were dispersed in a 2 cc vial to prepare the Ru precursor. This Ru precursor was then added to the photocatalyst in an amount equivalent to 0.2 wt% of Ru, and photodeposition was carried out under irradiation with a 300 W xenon (Xe) lamp to synthesize "Ru / SC / La,Rh:STO" or "Ru / La,Rh:STO." The wavelength range of the Xe lamp was 420 nm to 800 nm.

[0045] The sample solution (reaction solution) is as follows: The reaction solution was an aqueous methanol solution or a phosphate buffer solution. A 10 vol% methanol solution was prepared using 10 mL of methanol and 90 mL of water. Phosphate buffer solutions were prepared by adding KPi (KH2PO4 and K2HPO4) as free phosphate to a 10 vol% methanol solution, with the following phosphate concentrations (buffer concentrations): 0.93 mM, 5 mM, 25 mM, 50 mM, 0.1 M, 0.5 M, and 1.0 M.

[0046] The measurement procedure is as follows. 0.20 g of photocatalyst powder was placed in a 10 vol% aqueous methanol solution or phosphate buffer solution, and after silane coupling treatment, a Ru precursor was added. In the case of the synthesis of "Ru / SC / La,Rh:STO," the photocatalytic activity was evaluated by irradiating the sample with a 300 W Xe lamp in a closed circulation reactor, measuring the amount of hydrogen produced by gas chromatography, and measuring the photocatalytic activity. The wavelength range of the Xe lamp was 420 nm to 800 nm.

[0047] Using the above experimental setup, the results and considerations of the photocatalytic activity evaluation of "La,Rh:STO" as an activity evaluation for hydrogen production using a photocatalytic powder system are explained below.

[0048] Figure 4 is a chart comparing the photocatalytic activity with and without silane coupling treatment (phosphate group modification) and when the order of surface treatment (silane coupling treatment and hydrogen generation co-catalyst loading) was changed. According to this chart, when the photocatalyst was first subjected to silane coupling treatment (Ru / SC / photocatalyst), it showed better hydrogen generation activity than when Ru was simply loaded onto the photocatalyst without silane coupling treatment (Ru / photocatalyst). Conversely, when Ru was photodeposited and then silane-coupling-treated (SC / Ru / photocatalyst), the hydrogen production activity was lower than when Ru was simply supported (Ru / photocatalyst). This may be because the silane coupling agent is bonded to the Ru, which is the active site. In this state, the silane coupling agent is not bonded to the reactant H + It is thought that the activity decreased rapidly because the activity prevented the enzyme from reaching the active site.

[0049] Figure 5 is a chart comparing the photocatalytic activity when the amount of silane coupling agent loaded on the photocatalyst is changed. The "loading amount of silane coupling agent" here refers to the amount of silane coupling agent added (relative to the photocatalyst). The chart shows that increasing the loading amount of silane coupling agent (from 1:20 to 1:10 SC:photocatalyst) improved activity, with the best activity achieved at a ratio of 1:10 SC:photocatalyst. On the other hand, increasing the loading amount further (from 1:10 to 1:5) resulted in a decrease in activity. In this case, the alkyl chains of the silane coupling agent near the active sites also increased hydrophobicity, presumably resulting in a decrease in activity.

[0050] Figure 6 is a chart comparing the photocatalytic activity when the concentration of the silane coupling agent solution is changed. According to this, the best activity was observed at a concentration of 1 wt%, but a decrease in activity was observed when the concentration was further increased (from 1 wt% to 5 wt%). As with the loading amount condition, when the concentration is increased, the influence of the hydrophobicity of the alkyl chain of the silane coupling agent in the reaction solution becomes stronger, and H + It is thought that the restriction on the movement of the enzymes led to a decrease in activity.

[0051] Figure 7 is a chart comparing the photocatalytic activity when the pH of the silane coupling agent solution was changed. The pH of the solution was adjusted to be acidic by adding hydrochloric acid. This shows that the more acidic the silane coupling agent solution becomes, the more rapidly the photocatalytic activity decreases. The rapid decrease in hydrogen production is thought to be due to factors such as the alteration of the "La,Rh:STO" photocatalyst itself in the acidic solution and changes in the way the silane coupling agent is adsorbed. Therefore, it was shown that the optimal state for the silane coupling agent solution is one in which the pH is not adjusted (i.e., neutral).

[0052] Figure 8 shows the results of evaluating the photocatalytic activity of a photocatalyst without immobilizing phosphate groups on the surface (without modifying it with phosphate-containing alkylsilanol) at various phosphate concentrations (buffer concentrations). The reaction solution was a methanol-water solution to which KPi was added as free phosphate, and the reaction solution was a phosphate buffer solution. The graph compares the activity of the reaction solution with that of a phosphate-immobilized photocatalyst and a solution simply loaded with Ru. The activity of the reaction solution with the same amount of phosphate added (in 0.93 mM KPi) as the amount of silane coupling agent loaded (10 wt%, i.e., SC:photocatalyst = 1:10) was lower than that of the reaction solution with immobilized phosphate groups and that simply loaded with Ru. Furthermore, the activity tended to decrease further as the amount of phosphate (buffer concentration) increased. At 50 mM, the hydrogen peak was almost zero, and no peak was observed above 0.1 mM.

[0053] Figure 9 is a chart showing the results of evaluating the photocatalytic activity of a photocatalyst modified with an alkylsilanol having a phosphate group and a photocatalyst modified with an alkylsilanol having an amino group, which is a functional group other than a phosphate group. This shows that surface modification with a silane coupling agent other than a phosphate group does not achieve the activity of modification with a phosphate group, and that surface modification with a phosphate group is particularly effective in improving photocatalytic activity.

[0054] Figure 10 is a graph showing the photocatalytic activity when the amount of light irradiated onto the photocatalyst is changed. In a methanol aqueous solution or a sodium sulfate aqueous solution without buffer capacity, the photocatalytic activity is H + It is thought that the diffusion of water determines the rate of the hydrogen production reaction, and that at high photon fluxes, water is converted into a reactant, and the rate of the hydrogen production reaction increases depending on the amount of light (light intensity). As shown in Figure 10, in the case of the material not modified with phosphate groups, the rate of hydrogen production increases at low photon fluxes (200 μmol cm -2 h -1 The rate of hydrogen production reaction was almost constant at the + In contrast, in the case of the phosphate group-modified (silane coupling treated) material, the reaction rate increased monotonically depending on the amount of light (light intensity), regardless of the photon flux. This indicates that the phosphate group is H + As mentioned above, according to Non-Patent Document 3, even when a phosphate buffer solution was used as the reaction solution, the concentration of 100 μmol cm -2 h -1 It is said that at such low photon flux levels, the activity is almost the same as when a non-buffered electrolyte is used. This also shows that the phosphate group of the present invention is more effective as a means of improving the mass transfer of reactants than phosphate buffer solutions.

[0055] Next, the evaluation of the properties of the photocatalyst according to the present invention will be described. Figure 11 shows a STEM image (a) and EDS line analysis (b) of the surface morphology of the "Ru / SC / La,Rh:STO" powder. The photocatalyst powder appeared to be small, round particles on the submicron order. Further observation revealed that the very small particles of approximately 10 nm deposited on the "La,Rh:STO" were presumed to be the co-catalyst Ru. However, it was difficult to distinguish the silane coupling agent modification (silane coupling coating) from the STEM image. However, EDS line analysis detected a Ru peak, confirming that the particles deposited on the surface were Ru. Furthermore, Si and P derived from the silane coupling agent were detected throughout the "La,Rh:STO" surface, confirming that the entire photocatalyst surface was coated with the silane coupling agent, regardless of the distribution of Ru.

[0056] Figure 12 shows the chemical states of "La,Rh:STO" and "SC / La,Rh:STO" by XPS analysis. Elemental analysis was performed after correcting the peak positions with carbon. The appearance of the Si peak is due to the silane coupling treatment (derived from the silane coupling agent). On the other hand, the decrease in the Ti peak is due to Ti originating from SrTiO3, and is thought to have been reduced by surface coating by the silane coupling treatment.

[0057] The above experimental results can be summarized as follows: The amount of silane coupling agent carried and the solution conditions contribute to photocatalytic activity, with the highest activity found at a pH of 7, a solution of 1 wt%, and a SC:photocatalyst ratio of 1:10. Surface modification of the reaction solution with a phosphate buffer solution or a silane coupling agent other than a phosphate group did not achieve the best results achieved by modification with a phosphate group, and it was found that surface modification with a phosphate group is particularly effective in improving photocatalytic activity. [Example]

[0058] In this example, the photocatalytic powder (10a) is "Cu2Sn x Ge 1-x S3 (where 0≦x≦1)" (CTGS) was used.

[0059] Chemical formula “Cu2Sn x Ge 1-x The compound (CTGS) represented by "S3 (where 0≦x≦1)" is a p-type semiconductor composed of copper (Cu), tin (Sn), germanium (Ge), and sulfur (S). The Sn / Ge composition (the ratio of Sn to Ge, sometimes referred to as the "Sn / Ge ratio") can be set arbitrarily within the range of 0≦x≦1. The Cu composition may be increased or decreased relative to the stoichiometric composition. Therefore, the term "CTGS" as used herein includes compositions that do not contain Sn or Ge.

[0060] CTGS has recently been attracting attention as a new material for thin-film solar cells. Thin-film solar cells using CTGS have an open-circuit voltage of 300 mV and a short-circuit current density of 1.7 mA / cm under simulated solar irradiation. 2 The external quantum efficiency at each wavelength ranges from 40% to 65% from 300 nm to 1000 nm, demonstrating relatively high performance. Furthermore, the band gap can be controlled in stages from 0.94 eV to 1.30 eV by varying the Sn / Ge ratio. However, CTGS has not been used as a photocatalyst or photoelectrode for water splitting or hydrogen generation, and its activity was unknown. However, as shown in this Example and Example 3, the inventors have discovered that CTGS can be suitably used as a photocatalyst or photoelectrode for water splitting or hydrogen generation.

[0061] As shown in Figure 13, CTGS can absorb infrared light from approximately 800 nm to approximately 1500 nm at any Sn / Ge ratio, and the absorption edge red-shifts to longer wavelengths as the Sn ratio increases. Other p-type semiconductors responsive to near-infrared to infrared light, such as Cu(In,Ga)Se2 (CIGS), CdTe, and Cu2ZnSnS4 (CZTS), use rare or highly toxic metals and require complex synthesis methods. In contrast, the constituent elements of CTGS are relatively inexpensive and safe, and their synthesis is relatively easy. Furthermore, their properties can be controlled by varying the Sn / Ge and Cu compositions. These characteristics make CTGS an ideal material for photocatalysts and photoelectrodes for water splitting and hydrogen generation. In this example, the photocatalytic activity of CTGS was evaluated first without modification with phosphate groups, and then after modification with phosphate groups. In addition, as a method for silane coupling treatment of the photocatalyst surface, the recovery method of silane coupling treated photocatalyst powder was evaluated using CTGS as an example.

[0062] The synthesis and preparation of experimental materials were carried out as follows. CTGS was synthesized with Sn / Ge compositions where the Sn / (Sn+Ge) value, i.e., the x value, was x = 0.38, x = 0.5, x = 0.62, and x = 1, respectively. The specified amounts of Cu2S, SnS2, GeS2, and S were mixed in a mortar for 30 minutes under a nitrogen atmosphere. The synthesized powder was then placed in a quartz ampoule, evacuated, vacuum-sealed, and fired in an electric furnace at 600°C for 15 hours.

[0063] The silane coupling agent used was "3-(trihydroxysilyl)propyl sodium methylphosphonate" with a phosphate group. The silane coupling treatment was performed in the same manner as in Example 1: a reagent containing the silane coupling agent was added to water in a vial under a nitrogen atmosphere, and the required photocatalyst powder was then added. The treatment was then sonicated for 10 minutes and stirred for 30 minutes. Next, in this example, two methods were attempted to recover the "SC / CTGS" powder: suction filtration and drying of the photocatalyst suspension, and evaporation to dryness. Note that suction filtration and evaporation to dryness are both well-known methods for extracting solids from liquids or solutes from solutions.

[0064] Ru was used as a co-catalyst for hydrogen generation. The amount of Ru supported was 1 wt% relative to the photocatalyst, and "Ru / SC / CTGS" was synthesized by the photo-deposition method in the same manner as in Example 1.

[0065] The sample solution (reaction solution) was a 20 mM Na2S / Na2SO3 aqueous solution containing 10 mM Na2S and 10 mM Na2SO3 as sacrificial reagents, or a 10 vol% CH3OH aqueous solution containing CH3OH as a sacrificial reagent.

[0066] The measurement procedure is as follows. 0.10 g of photocatalyst powder was added and suspended in 100 mL of reaction solution. The solution was then irradiated with a 300 W Xe lamp using a closed circulation reactor, and samples were taken. The amount of hydrogen produced was measured by gas chromatography to evaluate the photocatalytic activity. The wavelength range of the Xe lamp was 420 nm to 800 nm.

[0067] Using the above experimental setup, the photocatalytic activity of CTGS was evaluated as an activity evaluation for hydrogen production using a photocatalytic powder system, and the results and considerations of the evaluation of the recovery method for silane coupling-treated "SC / CTGS" are described below.

[0068] Figure 14 is a chart comparing the photocatalytic activity of the photocatalyst "Ru / CTGS" when the Sn / Ge composition is changed. "Ru / CTGS" supports Ru but is not subjected to silane coupling treatment. The reaction solution is a 20 mM Na2S / Na2SO3 aqueous solution. This shows that steady hydrogen generation is confirmed for all compositions, with the maximum hydrogen generation activity (hydrogen generation rate) being observed at Sn / (Sn+Ge) = 0.38 (x = 0.38).

[0069] Figure 15 is a graph comparing the photocatalytic activity of the Ru-loaded photocatalyst "Ru / CTGS" and the Ru-unloaded photocatalyst "CTGS." Both photocatalysts had a Sn / Ge composition x = 0.38 and were not subjected to silane coupling treatment. The reaction solution was a 20 mM Na2S / Na2SO3 aqueous solution. According to this, the Ru-unloaded "CTGS" exhibited lower hydrogen production activity (hydrogen production rate) than the Ru-loaded "Ru / CTGS," but steady hydrogen production was confirmed. This indicates that CTGS is capable of water decomposition, i.e., hydrogen production, to a certain extent, even without the loading of a hydrogen production promoter.

[0070] Figure 16 compares the photocatalytic activity of the silane coupling-treated (phosphate-modified) photocatalyst "Ru / SC / CTGS" with that of the photocatalyst "Ru / CTGS" simply loaded with Ru. Both photocatalysts had a Sn / Ge composition of x = 0.62. The amount of silane coupling agent loaded in the photocatalyst "Ru / SC / CTGS" was 1 wt%, i.e., SC:photocatalyst = 1:100. Furthermore, Ru was loaded after silane coupling treatment. The reaction solution was a 10 vol% CH3OH aqueous solution. The results show that the silane coupling-treated "Ru / SC / CTGS" exhibited improved hydrogen production activity (hydrogen production rate) compared to the "Ru / CTGS" simply loaded with Ru.

[0071] Next, regarding the recovery method of the silane coupling-treated photocatalyst "SC / photocatalyst" powder, Figure 17 is a diagram showing the results of TG-DTA (thermogravimetric-differential thermal analysis) analysis of powder recovered from the "SC / CTGS" suspension by suction filtration and drying, or by evaporation to dryness. SiO2 was used as a model adsorbent. For both photocatalysts, the amount of silane coupling agent supported was "10 wt%, i.e., SC:photocatalyst = 1:10."

[0072] According to this, the powder recovered by evaporation to dryness was confirmed to lose weight with heating. More specifically, the weight loss up to approximately 300°C was due to water evaporation and not SC, while the weight loss from approximately 300°C to approximately 800°C was due to the thermal decomposition of SC. Furthermore, since the weight loss was approximately 10 wt% from approximately 300°C to approximately 800°C, the 10 wt% SC loading was almost completely lost, and the powder was recovered in a silane coupling-treated state. On the other hand, the powder recovered by suction filtration showed almost no weight loss, indicating that most of the SC was lost during suction filtration. This indicates that evaporation to dryness is a suitable method for recovering powder from the "SC / photocatalyst" suspension as a method for silane coupling treatment of the photocatalyst surface. [Example]

[0073] In this example, the photocatalytic powder (10a) is "Cu2Sn x Ge 1-x A photoelectrode using "S3 (where 0≦x≦1)" (CTGS) was fabricated and its photoelectrochemical properties were evaluated.

[0074] The synthesis and preparation of experimental materials were carried out as follows. CTGS was synthesized with Sn / Ge compositions where the Sn / (Sn+Ge) value, i.e., the x value, was x = 0.38, x = 0.5, x = 0.62, and x = 1, respectively. Cu compositions were also synthesized with stoichiometric composition, 1% deficient, 5% deficient, 7% deficient, and 5% excess of the stoichiometric composition. The specified amounts of Cu2S, SnS2, GeS2, and S were mixed in a mortar under a nitrogen atmosphere for 30 minutes. The synthesized powder was then placed in a quartz ampoule, evacuated, vacuum-sealed, and fired in an electric furnace at 800°C for 15 hours.

[0075] 50 mg of each CTGS powder was mixed with 1000 μg of isopropyl alcohol to prepare a drop-cast solution. This solution was sonicated for 10 minutes and then dropped onto a glass substrate. Mo and C were then deposited as contact layers and Ti as a current collecting layer by sputtering on the CTGS layer. The substrate temperature was set to 200°C, and sputtering was performed at 50 W for 1 minute for Mo, 50 W for 5 minutes for C, and 200 W for 3 hours for Ti. Carbon tape was then attached to the Ti current collecting layer, allowing for particle transfer, resulting in a "CTGS / Mo / C / Ti" electrode.

[0076] The electrode was then immersed in a KCN solution for 1 minute for KCN etching and then rinsed with distilled water. CdS was then deposited on the electrode surface by chemical bath deposition (CBD). Finally, Pt was electrodeposited on the electrode surface by photoelectrodeposition. This surface modification resulted in the fabrication of a "Pt / CdS / CTGS / Mo / C / Ti" photoelectrode (sometimes referred to as a "CTGS photoelectrode"). Photoelectrodeposition was performed after OCP and CV during electrochemical measurements.

[0077] The measurement procedure is as follows. The electrodes were connected to conducting wires using indium, and then the indium surface was covered with epoxy resin. Electrochemical measurements were performed using a three-electrode system consisting of a Pt / CdS / CTGS / Mo / C / Ti photoelectrode (photocathode), an Ag / AgCl electrode as the reference electrode, and a Pt electrode as the counter electrode. The electrolyte was a 1M KPi (0.5M KH2PO4 / 0.5M K2HPO4) aqueous solution adjusted to pH 7. OCP and CV were performed, and electrochemical measurements were performed under simulated sunlight (AM1.5G).

[0078] Using the above experimental setup, the results and discussion of the electrochemical characteristics evaluation of the CTGS photoelectrode (photocathode) are described below.

[0079] Figure 18 shows the current-potential curves of CTGS photoelectrodes with different Sn / Ge ratios. Photocurrent due to hydrogen generation was confirmed for all compositions. The photocurrent tended to increase as the Sn ratio increased, reaching a maximum at Sn / (Sn+Ge) = 0.62 (x = 0.62), but then decreased significantly at x = 1. Meanwhile, the photocurrent at high potentials tended to increase with decreasing Sn ratios, reaching a maximum at Sn / (Sn+Ge) = 0.38 (x = 0.38). This suggests that when combining a CTGS photocathode with another anode for two-electrode water splitting, the photocatalyst "Cu2Sn" exhibits a high photocurrent at high potentials around x = 0.38, i.e., 0.35 ≤ x ≤ 0.45. x Ge 1-x S3" (CTGS) powder photoelectrode is suitable.

[0080] Figure 19 shows the current-potential curves of CTGS photoelectrodes with different Cu compositions. The graph shows that the photocurrent tends to increase as the Cu composition decreases, with the maximum current value being reached at 5% deficiency. This is thought to be because holes are generated due to the Cu deficiency, improving the p-type nature of CTGS. On the other hand, the photocurrent decreased when the Cu composition was increased (5% excess). This suggests that when combining a CTGS photocathode with another anode to perform water splitting at two electrodes, the photocatalyst "Cu2Sn" with a Cu composition 4% to 6% below the stoichiometric composition is the best choice.x Ge 1-x S3" (CTGS) powder photoelectrode is suitable.

[0081] Figure 20 shows the current-time curves of CTGS photoelectrodes with different Sn / Ge ratios. Electrochemical measurements were performed with a filter attached to the light source that blocks light below 1000 nm. The results showed that there was no photoresponse when Sn / (Sn + Ge) = 0 (x = 0), but a steady reductive photocurrent was observed when Sn / (Sn + Ge) > 0 (x > 0), demonstrating that infrared light from near-infrared light can be absorbed and used in the reduction reaction.

[0082] The present invention has been described above in various ways using preferred embodiments, but the present invention is not limited to these embodiments, and it goes without saying that many modifications can be made within the scope of the invention without departing from the spirit of the invention. [Explanation of symbols]

[0083] 10, 10a Photocatalyst 20 Phosphate group (phosphate functional group) 30 Silane coupling coating (silane coupling agent) 40 Hydrogen generation promoter

Claims

1. A photocatalyst in which the surface of the photocatalyst is coated with a silane coupling agent having a phosphate group to modify the phosphate group, and a hydrogen generation promoter is supported thereon, the hydrogen generation co-catalyst is formed in a scattered form on a silane coupling coating film made of the silane coupling agent, The photocatalyst is strontium titanate (STO) represented by the chemical formula SrTiO 3 or a compound (CTGS) represented by the chemical formula Cu 2 Sn x Ge 1-x S 3 (where 0≦x≦1), and the strontium titanate is doped with lanthanum and rhodium, and the hydrogen generation promoter is ruthenium. A photocatalyst characterized by:

2. The photocatalyst is a powder. The photocatalyst according to claim 1, characterized in that

3. The photoelectrode comprises the photocatalyst according to claim 1 or 2. A photoelectrode characterized by:

4. a phosphate group modification step of modifying the photocatalyst surface with a phosphate group by silane coupling treatment; and a hydrogen generation co-catalyst supporting step of supporting a hydrogen generation co-catalyst on the surface of the photocatalyst, After the phosphoric acid group modification step is performed, the hydrogen generation promoter supporting step is performed; The photocatalyst is strontium titanate (STO) represented by the chemical formula SrTiO 3 or a compound (CTGS) represented by the chemical formula Cu 2 Sn x Ge 1-x S 3 (where 0≦x≦1), and the strontium titanate is doped with lanthanum and rhodium, and the hydrogen generation promoter is ruthenium. A method for producing a photocatalyst, characterized by:

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