Photoelectrode

Abstract

To provide a photoelectrode that can improve an output current density and stability of the output current density.SOLUTION: The photoelectrode of the present invention comprises a substrate and a semiconductor film containing a sulfide semiconductor nanosheet provided on the substrate, and further comprises a metal film containing a metal nanosheet provided between the substrate and the semiconductor film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a photoelectrode including a semiconductor that generates an electromotive force by absorbing light, in which a sulfide semiconductor is used as the semiconductor. [Background technology]

[0002] Various technologies have been developed for photoelectrodes equipped with semiconductors that generate electromotive force by absorbing light, and for photoelectrochemical cells using such electrodes. For example, a conventional technology involves a method for improving the stability of a photoelectrode in a photoelectrochemical reaction using an n-type semiconductor containing a metal element and oxygen as a photoelectrode. This method involves dissolving metal element ions identical to the constituent elements of the semiconductor in the reaction solution, and ensuring that the valence of the metal element ions in the reaction solution is a stable high oxidation state in the steady state of the reaction (Patent Document 1). Another known powder photoelectrode is characterized by a layer of layered oxide nanosheets disposed between a layer of photocatalytic powder (semiconductor) and a conductive layer (Patent Document 2). Meanwhile, sulfide semiconductors are increasingly being used as semiconductors in photoelectrodes due to their excellent response to visible light and high output (output current density). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-17080 [Patent Document 2] Patent Publication No. 2021-127521 Summary of the Invention [Problem to be solved by the invention]

[0004] In the technology described in Patent Document 1, in order to improve the stability of the semiconductor material during the photoreaction and prevent a decrease in efficiency, it is necessary to dissolve metal element ions in the reaction solution of a device using a photoelectrode. As a result, the light transmittance of the reaction solution decreases, resulting in insufficient output and potentially limiting the structure of the device. In addition, dissolving metal element ions in the reaction solution requires more materials, which could increase costs. Furthermore, the effects of using a sulfide semiconductor as the semiconductor are unclear. In addition, in the technology described in Patent Document 2, the activity of the photoelectrode is improved by using a semiconductor in the photocatalyst powder, but because a sulfide semiconductor is not used as the semiconductor, there is a risk of insufficient output.

[0005] The present invention has been made in view of the above-mentioned problems, and has as its main object to provide a photoelectrode that can improve the output current density and the stability of the output current density. [Means for solving the problem]

[0006] In order to solve the above problems, the photoelectrode of the present invention is a photoelectrode comprising a substrate and a semiconductor film provided on the substrate and containing a sulfide semiconductor nanosheet, and further comprising a metal film provided between the substrate and the semiconductor film and containing a metal nanosheet. [Effects of the Invention]

[0007] According to the present invention, it is possible to improve the output current density and the stability of the output current density.

[0008] The objects, configurations, and effects of the present invention other than those described above will become apparent from the following description of the preferred embodiments of the invention. [Brief explanation of the drawings]

[0009] [Figure 1] 1(a) to 1(c) are schematic cross-sectional views showing the photoelectrodes according to the first to third embodiments, respectively, and FIG. 1(d) is a schematic cross-sectional view showing a photoelectrode according to the prior art. [Figure 2] 1 is a flowchart showing an outline of an example of a method for manufacturing a photoelectrode according to an embodiment. [Figure 3] 10 is a flowchart showing an outline of a method for manufacturing a photoelectrode according to a comparative example. [Figure 4] FIG. 1 is a schematic diagram showing a photoelectrochemical cell used to evaluate the photoelectrochemical properties of the photoelectrodes of Examples and Comparative Examples. [Figure 5] Graph (a) shows the output current density (maximum value) and output reduction rate obtained for the photoelectrodes of Example 1 and the comparative example, and graph (b) shows the output current density (initial value) obtained for the photoelectrodes of Examples 1 to 3 and the comparative example. [Figure 6] (a) to (c) are images obtained by observing with an AFM the surface of a sample of a molybdenum sulfide nanosheet film as a semiconductor thin film, the surface of a sample of an iridium oxide nanosheet film as a metal oxide thin film, and the surface of a sample of a metallic iridium nanosheet film as a metal thin film, respectively, formed in Example 1. (d) is an X-ray diffraction pattern of a sample of an iridium oxide nanosheet film as a metal oxide thin film and a sample of a metallic iridium nanosheet film as a metal thin film, both formed in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the photoelectrode of the present invention will be described. First, the photoelectrodes according to the embodiments will be outlined by taking the photoelectrodes according to the first to third embodiments as examples. Figures 1(a) to 1(c) are schematic cross-sectional views showing the photoelectrodes according to the first to third embodiments, respectively, and Figure 1(d) is a schematic cross-sectional view showing a photoelectrode according to the prior art.

[0011] As shown in FIG. 1(a), the photoelectrode 10 according to the first embodiment includes an FTO substrate 2 and a semiconductor thin film (semiconductor film) 4 provided on the FTO substrate 2, and further includes a metal thin film (metal film) 6 provided between the FTO substrate 2 and the semiconductor thin film 4. The FTO substrate 2 is a transparent conductive substrate in which an FTO film (not shown) is provided on the surface of a glass substrate (not shown). The semiconductor thin film 4 is made of a molybdenum sulfide nanosheet film coated with and arranged by molybdenum sulfide nanosheets made of MoS2, and the metal thin film 6 is made of a metal iridium nanosheet film coated with and arranged by metal iridium nanosheets made of Ir. The metal thin film 6 is provided on the surface of the FTO substrate 2 (the surface of the FTO film), and the semiconductor thin film 4 is provided on the surface of the metal thin film 6.

[0012] As shown in FIG. 1(b), a photoelectrode 20 according to the second embodiment includes an FTO substrate 2 and a semiconductor film 14 provided on the FTO substrate 2, and further includes a metal film 16 provided between the FTO substrate 2 and the semiconductor film 14. The semiconductor film 14 includes a first semiconductor thin film 14a and a second semiconductor thin film 14b stacked on the FTO substrate 2. The metal film 16 includes a first metal thin film 16a provided between the FTO substrate 2 and the first semiconductor thin film 14a adjacent to the FTO substrate 2 among the multiple semiconductor thin films of the semiconductor film 14, and a second metal thin film 16b provided between the first semiconductor thin film 14a and the second semiconductor thin film 14b adjacent to each other. The first semiconductor thin film 14a and the second semiconductor thin film 14b are each made of a molybdenum sulfide nanosheet film. The first metal thin film 16a and the second metal thin film 16b are each made of a metal iridium nanosheet film. The first metal thin film 16a is provided on the surface of the FTO substrate 2, and the first semiconductor thin film 14a is provided on the surface of the first metal thin film 16a. The second metal thin film 16b is provided on the surface 1 of the first semiconductor thin film 14a, and the second semiconductor thin film 14b is provided on the surface of the second metal thin film 16b.

[0013] 1(c), a photoelectrode 30 according to the third embodiment is a photoelectrode including an FTO substrate 2 and a semiconductor film 24 provided on the FTO substrate 2, and further including a metal film 26 provided between the FTO substrate 2 and the semiconductor film 24. The semiconductor film 24 includes a first semiconductor thin film 24a to a third semiconductor thin film 24c provided to be stacked on the FTO substrate 2. The metal film 26 includes a first metal thin film 26a provided between the FTO substrate 2 and the first semiconductor thin film 24a adjacent to the FTO substrate 2 among the multiple semiconductor thin films of the semiconductor film 24, a second metal thin film 26b provided between the first semiconductor thin film 24a and the second semiconductor thin film 24b adjacent to each other, and a third metal thin film 26c provided between the second semiconductor thin film 24b and the third semiconductor thin film 24c adjacent to each other. The first semiconductor thin film 24a to the third semiconductor thin film 24c are each made of a molybdenum sulfide nanosheet film, and the first metal thin film 26a to the third metal thin film 26c are each made of a metal iridium nanosheet film. The first metal thin film 26a is provided on the surface of the FTO substrate 2, and the first semiconductor thin film 24a is provided on the surface of the first metal thin film 26a. The second metal thin film 26b is provided on the surface of the first semiconductor thin film 24a, and the second semiconductor thin film 24b is provided on the surface of the second metal thin film 26b. The third metal thin film 26c is provided on the surface of the second semiconductor thin film 24b, and the third semiconductor thin film 24c is provided on the surface of the third metal thin film 26c.

[0014] On the other hand, as shown in FIG. 1(d), a conventional photoelectrode 50 includes an FTO substrate 2 and a semiconductor thin film 54 formed on the FTO substrate 2 and made of a molybdenum sulfide nanosheet film. The semiconductor thin film 54 is disposed directly on the surface of the FTO substrate 2. Therefore, when a conventional photoelectrode is used as an anode in a photoelectrochemical cell, the high concentration of electron and hole trap levels at the interface of the semiconductor thin film on the FTO substrate side and the interfacial resistance may reduce the transport efficiency of electrons excited to the conduction band by light irradiating the semiconductor thin film of the photoelectrode to the counter electrode (cathode). This may result in a decrease in output current density (photocurrent density). Furthermore, in this case, the semiconductor thin film may be decomposed by autoxidation, which is oxidation caused by the trapped holes. This may result in a decrease in the stability of the output current density.

[0015] In contrast, in the photoelectrodes according to the first to third embodiments, a metal thin film (metal film) made of a metallic iridium nanosheet film is provided between an FTO substrate and a semiconductor thin film (semiconductor film) made of a molybdenum sulfide nanosheet film, and the semiconductor thin film is provided on the surface of the metal thin film. This allows the semiconductor thin film to come into surface contact with the metal thin film, and the atoms constituting the molybdenum sulfide nanosheet contained in the semiconductor thin film are closely attached to the metal atoms constituting the metallic iridium nanosheet (metal nanosheet) contained in the metal thin film. Therefore, when the photoelectrodes according to the first to third embodiments are used as an anode in a photoelectrochemical cell, the presence of the metal thin film between the FTO substrate and the semiconductor thin film of the photoelectrode can suppress the influence of high-concentration electron and hole trap levels at the interface of the semiconductor thin film on the FTO substrate side, thereby reducing interface resistance. This improves the transport efficiency of electrons excited to the conduction band by irradiating the semiconductor thin film of the photoelectrode with light to be transported to the counter electrode (cathode). This, in turn, improves the output current density (photocurrent density). Furthermore, in this case, self-oxidation of the semiconductor thin film caused by trapped holes can be suppressed, thereby improving the stability of the output current density. Furthermore, in the photoelectrodes according to the first to third embodiments, the output current density can be further improved by stacking multiple units of semiconductor thin film and metal thin film. Therefore, when the photoelectrodes according to the first to third embodiments are used in photoelectrochemical cells, particularly for generating hydrogen, the amount of hydrogen generated can be increased and the stability of the amount of hydrogen generated can be improved. The photoelectrodes according to the embodiments and the method for manufacturing the same will be described in more detail below.

[0016] 1. Photoelectrode A photoelectrode according to the embodiment is a photoelectrode comprising a substrate and a semiconductor film provided on the substrate and containing a sulfide semiconductor nanosheet, and further comprising a metal film provided between the substrate and the semiconductor film and containing a metal nanosheet.

[0017] The substrate is not particularly limited, but examples thereof include conductive substrates, and among these, transparent conductive substrates such as FTO substrates and ITO substrates are preferred. Note that the FTO substrate is a substrate in which an FTO (fluorine-doped tin oxide) film is provided on the surface of a glass substrate, and the ITO substrate is a substrate in which an ITO (tin-doped indium oxide) film is provided on the surface of a glass substrate.

[0018] The semiconductor film is not particularly limited as long as it contains a sulfide semiconductor nanosheet. The sulfide semiconductor nanosheet is not particularly limited as long as it is a nanosheet composed of a sulfide semiconductor and is obtained by exfoliating an amine intercalation product produced from a precursor inorganic layered substance in a single layer to several layers. Examples of sulfide semiconductor nanosheets include molybdenum sulfide nanosheets composed of MoS2 (molybdenum(IV) sulfide), as well as nanosheets composed of sulfide semiconductors selected from the group consisting of SnS (tin(I) sulfide), WS2 (tungsten(IV) sulfide), HfS2 (hafnium(IV) sulfide), etc. Nanosheets are crystals with an ultrathin film thickness of one to several atoms on the order of nanometers, while possessing two-dimensional anisotropy with in-plane dimensions on the order of micrometers. The average film thickness of the sulfide semiconductor nanosheet is, for example, 0.1 nm to 5 nm, preferably 0.5 nm to 3.5 nm. The average in-plane equivalent circle diameter is, for example, 0.1 μm to 50 μm, preferably 0.5 μm to 10 μm. The average nanosheet film thickness and the average in-plane equivalent circle diameter can be measured, for example, as the average values at five or more locations on the target nanosheet from an AFM (Atomic Force Microscope) image. Examples of semiconductor films include those containing a semiconductor thin film containing a sulfide semiconductor nanosheet. Preferred semiconductor thin films include those made of a sulfide semiconductor nanosheet film. Here, the term "sulfide semiconductor nanosheet film" refers to a film (e.g., a monolayer film) in which sulfide semiconductor nanosheets are arranged, such as a film in which sulfide semiconductor nanosheets are densely arranged. Furthermore, the term "semiconductor thin film made of a sulfide semiconductor nanosheet film" refers to a sulfide semiconductor nanosheet film itself or a multilayer film in which sulfide semiconductor nanosheet films are stacked.

[0019] The metal film is not particularly limited as long as it contains a metal nanosheet. The metal nanosheet is not particularly limited as long as it is a nanosheet made of a metal and is obtained by reducing a precursor metal oxide nanosheet. The metal oxide nanosheet is a nanosheet made of a metal oxide and is obtained by exfoliating a hydrogen ion exchanger produced from a precursor inorganic layered material. Examples of metal nanosheets include metal nanosheets selected from the group consisting of metal iridium nanosheets made from Ir (metallic iridium) (precursor: iridium oxide nanosheets made from IrO (iridium (IV) oxide)), metal ruthenium nanosheets made from Ru (metallic ruthenium) (precursor: ruthenium oxide nanosheets made from RuO (ruthenium (IV) oxide)), metal nickel nanosheets made from Ni (metallic nickel) (precursor: nickel hydroxide nanosheets made from Ni(OH) (nickel (II) hydroxide)), and metal cobalt nanosheets made from Co (metallic cobalt) (precursor: cobalt hydroxide nanosheets made from Co(OH) (cobalt (II) hydroxide)). The average film thickness and average inner circle equivalent diameter of the metal nanosheets are the same as those of sulfide semiconductor nanosheets. Examples of metal films include those containing metal thin films containing metal nanosheets. Preferred metal thin films are metal thin films made from metal nanosheets. Here, the term "metal nanosheet film" refers to a monolayer film in which metal nanosheets are arranged, such as a monolayer film in which metal nanosheets are densely arranged. The term "metal thin film made of a metal nanosheet film" refers to a metal nanosheet film itself or a multilayer film in which metal nanosheet films are stacked.

[0020] The photoelectrode may be a photoelectrode in which the semiconductor film is a semiconductor thin film provided on a substrate and containing a sulfide semiconductor nanosheet, and the metal film is a metal thin film provided between the substrate and the semiconductor thin film (semiconductor film), as in the photoelectrode according to the first embodiment.

[0021] As a photoelectrode, as in the photoelectrodes according to the second and third embodiments, the semiconductor film is preferably stacked on the substrate and includes a plurality of semiconductor thin films containing the sulfide semiconductor nanosheet. The metal film is preferably a photoelectrode including the substrate and one metal thin film containing the metal nanosheet disposed between adjacent ones of the plurality of semiconductor thin films in the semiconductor film, and another metal thin film containing the metal nanosheet disposed between two adjacent ones of the plurality of semiconductor thin films in the semiconductor film. This is because stacking multiple units of semiconductor thin films and metal thin films allows light to be absorbed by the multiple semiconductor thin films, generating a larger electromotive force. Furthermore, the presence of a metal thin film between two adjacent semiconductor thin films improves electron transport efficiency, further improving output current density. Note that as the number of stacked units increases, the total inter-film resistance of the stacked films increases. Therefore, if the number of stacked units exceeds a certain number, the output current density may saturate. In this case, it is preferable to increase the number of stacked units as much as possible within a range in which the output current density does not saturate, for example, to "2." This is because the output current density can be maximized without unnecessarily increasing the number of stacked units.

[0022] 2. Photoelectrode manufacturing method A method for manufacturing a photoelectrode according to the embodiment will now be described. Fig. 2 is a flowchart showing an outline of an example of a method for manufacturing a photoelectrode according to the embodiment.

[0023] In one example of a method for manufacturing a photoelectrode according to an embodiment, first, as shown in FIG. 2, a substrate to be used for manufacturing a photoelectrode, a cationic polymer solution, a metal oxide nanosheet solution, and a sulfide semiconductor nanosheet solution are prepared (S10).

[0024] In this case, the substrate is prepared as explained in the above section "1. Photoelectrode." To prepare the cationic polymer solution, a cationic polymer is dispersed in a dispersion medium (e.g., ultrapure water, etc.), and then, to increase the surface charge of the cationic polymer, NaCl (sodium chloride) or the like is added. Furthermore, to adjust the pH, TBAOHaq. (tetrabutylammonium hydroxide aqueous solution) or the like is added to prepare the cationic polymer solution. Examples of cationic polymers include PDDA (polydiallyldimethylammonium chloride), PEI (polyethyleneimine), and PAH (polyallylamine hydrochloride). The formation of metal oxide thin films and semiconductor thin films can be stabilized by appropriately adjusting the concentration and pH of the cationic polymer solution. From this perspective, it is preferable to adjust the pH of the cationic polymer solution to be the same as that of the nanosheet solution.

[0025] To prepare a metal oxide nanosheet solution, first, an inorganic layered material, which is a precursor of the metal oxide nanosheet, is synthesized. For example, a solid-phase synthesis method is used. In the solid-phase synthesis method, one or more raw material powders are first pulverized and mixed to obtain a raw material mixture of compounds of each element (excluding oxygen) that constitutes the inorganic layered material. Next, the raw material mixture is fired to synthesize the inorganic layered material. Next, a metal oxide nanosheet solution is prepared from the inorganic layered material, and the solution is prepared. First, the inorganic layered material is acid-treated with an aqueous acid solution (e.g., nitric acid solution) to produce a hydrogen ion exchanger having a layered structure. Next, the hydrogen ion exchanger is reacted with a basic substance to exfoliate the metal oxide nanosheets from the hydrogen ion exchanger, thereby preparing a colloidal aqueous solution (raw solution) in which the metal oxide nanosheets are dispersed. The basic substance functions to exfoliate the nanosheets from the hydrogen ion exchanger, and examples of the basic substance include TBAOH (tetrabutylammonium hydroxide). An example of a method for reacting a hydrogen ion exchanger with a basic substance is to mix the hydrogen ion exchanger with an aqueous solution of a basic substance (e.g., TBAOHaq. (tetrabutylammonium hydroxide aqueous solution)). In this method, the mixed aqueous solution may be shaken. Next, the concentration of the colloidal aqueous solution is adjusted using, for example, ultrapure water, and the pH of the colloidal aqueous solution is further adjusted using, for example, hydrochloric acid, to prepare a metal oxide nanosheet solution. By appropriately adjusting the concentration and pH of the metal oxide nanosheet solution, the formation of a metal oxide thin film can be stabilized.

[0026] The sulfide semiconductor nanosheet solution is prepared using the electrochemical exfoliation method. First, MoS2 crystals are dissolved in acetonitrile with THA. + The cathode is immersed in a solution containing Tetraheptylammonium (Tetraheptylammonium) at a predetermined concentration (e.g., 5 g / L), and a negative voltage (e.g., -8 V) is applied for a predetermined time (e.g., 60 minutes), forming THA between the layers of the MoS2 crystal. +are electrochemically intercalated. Next, the intercalated MoS2 crystals are exfoliated by ultrasonic treatment for a predetermined time (e.g., 1 hour) in a solution of PVP (Polyvinylpyrrolidone) dissolved in DMF (N,N-dimethylformamide) at a predetermined concentration (e.g., 0.2 M). Next, the dispersion obtained by exfoliating the MoS2 crystals is redispersed by washing with IPA (Isopropyl alcohol) to remove remaining PVP and other impurities. Next, the dispersion after impurity removal is centrifuged at a predetermined rotation speed (e.g., 5000 rpm) to obtain a sulfide semiconductor nanosheet solution from which unexfoliated material has been removed.

[0027] Next, as shown in Figure 2, the surface of the substrate (the surface on which the film is to be formed) is made hydrophilic (S20). Specifically, first, the surface of the substrate is wiped with a degreaser (e.g., acetone), and then the surface of the substrate is subjected to UV ozone treatment (surface treatment using O3 (ozone) generated by UV (ultraviolet rays)) for a predetermined time (e.g., 20 minutes), and then the surface of the substrate is washed with, for example, ultrapure water to remove organic contaminants from the surface of the substrate. This makes the surface of the substrate hydrophilic.

[0028] Next, as shown in Figure 2, in steps S31 to S62, the hydrophilized substrate is alternately immersed in a cationic polymer solution and a nanosheet solution (metal oxide nanosheet solution or sulfide semiconductor nanosheet solution) using the LBL (Layer By Layer) method, thereby utilizing the electrostatic interaction between the positively charged polymer and the negatively charged nanosheet to obtain a film body including a laminated film in which multiple nanosheet films are adsorbed to each other.

[0029] First, the hydrophilized substrate is immersed in a cationic polymer solution (S31). Next, the immersed substrate is washed with, for example, ultrapure water, and its surface is dried, for example, by a nitrogen gas flow (S32). This results in a polymer-imparted substrate (polymer / substrate) having a cationic polymer attached to its surface.

[0030] Next, the polymer-attached substrate is immersed in the metal oxide nanosheet solution (S41). Next, the polymer-attached substrate after immersion is washed with, for example, ultrapure water, and its surface (the surface on which the polymer is attached) is dried, for example, by a nitrogen gas flow (S42). In this way, a metal oxide thin film (metal oxide film) consisting of a metal oxide nanosheet film is formed on the surface of the polymer-attached substrate, thereby obtaining a film-formed body (metal oxide thin film / polymer / substrate).

[0031] Next, the film-formed body is immersed in a cationic polymer solution (S51). Next, the film-formed body after immersion is washed with, for example, ultrapure water, and its surface (surface of the metal oxide thin film) is dried, for example, by a nitrogen gas flow (S52). This results in a film-formed body (polymer / metal oxide thin film / polymer / substrate) with the polymer applied to the surface of the film-formed body.

[0032] Next, as shown in FIG. 2, the film-formed body is immersed in a sulfide semiconductor nanosheet solution (S61). Next, the immersed film-formed body is washed with, for example, DMF, IPA, or the like, and its surface (the surface on which the polymer is applied) is dried, for example, by a nitrogen gas flow (S62). A semiconductor thin film (semiconductor film) made of a sulfide semiconductor nanosheet film is thus formed on the surface of the film-formed body, thereby obtaining a film-formed body (semiconductor thin film / polymer / metal oxide thin film / polymer / substrate). Note that the immersion times in S31 and S51 are set so that the cationic polymer can be sufficiently applied, the immersion time in S41 is set so that the metal oxide thin film can be sufficiently formed, and the immersion time in S61 is set so that the semiconductor thin film can be sufficiently formed.

[0033] Next, as shown in Fig. 2, it is determined whether the number of stacked units of semiconductor thin film and metal oxide thin film in the film-formed body is a designated number (S70). If the determination is negative, the above steps S31 to S62 are repeated, and the process returns to step S70. As a result, if the designated number of stacked units is n, a stacked body including a substrate and n units (semiconductor thin film / polymer / metal oxide thin film / polymer) stacked on the substrate is obtained as a precursor of a photoelectrode. On the other hand, if the determination is positive, the process proceeds to step S80 described below.

[0034] Next, as shown in Fig. 2, the laminate is heat-treated (S80). Conditions for the heat treatment include, for example, holding the laminate in a reducing atmosphere (e.g., a mixed gas atmosphere of hydrogen gas and argon gas) at a temperature of 300°C to 800°C for 1 hour to 3 hours. By heat-treating the laminate, the metal oxide thin film is reduced to change it into a metal thin film (metal film), and the cationic polymer is eliminated, resulting in adhesion between the semiconductor thin film and the metal thin film. Through the above steps, a photoelectrode (semiconductor thin film / metal thin film / substrate) is manufactured.

[0035] 3. Uses of photoelectrodes The use of the photoelectrode according to the embodiment is not particularly limited, but examples thereof include photoelectrochemical cells (photoelectrochemical batteries), and among these, photoelectrochemical cells for generating hydrogen by electrolyzing water are preferred. In a situation where the production of green hydrogen that can be generated without emitting CO2 is essential to realize a carbon-neutral society, the photoelectrode according to the embodiment can provide a technology for increasing the amount of hydrogen produced and improving the stability of the amount of hydrogen produced. [Example]

[0036] Hereinafter, the photoelectrode according to the embodiment will be described more specifically with reference to examples and comparative examples.

[0037] [Example 1] An example of the photoelectrode according to the first embodiment was manufactured, and the manufacturing method will be described below. First, an FTO substrate, a PDDA solution, an iridium oxide nanosheet solution, and a molybdenum sulfide nanosheet solution to be used for manufacturing a photoelectrode were prepared (S10).

[0038] In this case, to prepare the PDDA solution, PDDA was dispersed in a dispersion medium (ultrapure water) and then TBAOHaq. (tetrabutylammonium hydroxide aqueous solution) was added to prepare a PDDA solution (concentration: 100 g / L, pH = 9.1).

[0039] To prepare the iridium oxide nanosheet solution, we first synthesized an inorganic layered material (layered iridium oxide) composed of IrO2, which is a precursor of iridium oxide nanosheets. This was done using a solid-phase synthesis method. In this solid-phase synthesis method, raw material powders of IrO2 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and K2CO3 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were first crushed and mixed to obtain a raw material mixture. Next, the raw material mixture was fired in an Ar atmosphere at 780°C for 1 hour using an electric furnace to synthesize an inorganic layered material represented by the chemical formula KxIrO2 (0.2≦x≦1). Next, an iridium oxide nanosheet solution was prepared from the inorganic layered material. First, the inorganic layered material was treated with an acid solution (hydrochloric acid solution) for 72 hours to produce a hydrogen ion exchanger with a layered structure represented by the chemical formula HxIrO2 (0.2≦x≦1). Next, the hydrogen ion exchanger was mixed with TBAOHaq (tetrabutylammonium hydroxide aqueous solution) and shaken for 7 days in a shaker to exfoliate the iridium oxide nanosheets into a single layer, preparing a colloidal aqueous solution (stock solution) in which the iridium oxide nanosheets were dispersed. Next, the concentration and pH of the colloidal aqueous solution were adjusted using ultrapure water and hydrochloric acid, respectively, to prepare an iridium oxide nanosheet solution (concentration: 1 g / L, pH = 9.1).

[0040] The sulfide semiconductor nanosheet solution was prepared using an electrochemical exfoliation method. First, MoS2 crystals (natural crystals, sold by Furuuchi Chemical Co., Ltd.) were dissolved in acetonitrile and THA +The cathode was immersed in a solution containing 5 g / L of THA and then a negative voltage of -8 V was applied for 60 minutes, resulting in the formation of THA between the layers of the MoS2 crystals. + The MoS2 crystals were electrochemically intercalated. Next, the intercalated MoS2 crystals were exfoliated by ultrasonic treatment for 1 hour in a solution of 0.2 M PVP dissolved in DMF. Next, the dispersion obtained by exfoliating the MoS2 crystals was redispersed by washing three times with IPA to remove remaining PVP and other impurities. Next, the dispersion after impurity removal was centrifuged at 5000 rpm to obtain a dispersion from which unexfoliated material had been removed. Next, the concentration of the dispersion was adjusted using IPA to prepare a sulfide semiconductor nanosheet solution (concentration: 1 g / L).

[0041] Next, the surface of the FTO substrate (surface of the FTO film) was hydrophilized (S20). Specifically, the surface of the substrate was first wiped with a grease-based cleaning agent (e.g., acetone), then subjected to UV ozone treatment for 20 minutes, and then washed with, for example, ultrapure water to remove organic contaminants from the surface of the substrate. This hydrophilized the surface of the FTO substrate.

[0042] Next, the hydrophilized FTO substrate was immersed in the PDDA solution for 5 minutes (S31). The immersed FTO substrate was then washed with ultrapure water, and its surface was dried under a nitrogen gas flow (S32). This resulted in a PDDA-imparted substrate (PDDA / substrate) with PDDA attached to the surface of the FTO substrate.

[0043] Next, the PDDA-applied substrate was immersed in the iridium oxide nanosheet solution for 5 minutes (S41). Next, the PDDA-applied substrate after immersion was washed with ultrapure water, and its surface (the surface on the PDDA-applied side) was dried using a nitrogen gas flow (S42). As a result, a metal oxide thin film (metal oxide film) consisting of an iridium oxide nanosheet film was formed on the surface of the PDDA-applied substrate, yielding a film-formed body (oxide thin film / PDDA / substrate).

[0044] Next, the film was immersed in a PDDA solution for 5 minutes (S51). After immersion, the film was washed with ultrapure water, and its surface (the surface of the metal oxide thin film) was dried by a nitrogen gas flow (S52). This resulted in a film (PDDA / metal oxide thin film / PDDA / substrate) with PDDA applied to the surface of the film.

[0045] Next, the film-formed body was immersed in a molybdenum sulfide nanosheet solution for 5 minutes (S61). Next, the film-formed body after immersion was washed with IPA, and its surface (the surface on which PDDA was applied) was dried by a nitrogen gas flow (S62). As a result, a semiconductor thin film (semiconductor film) consisting of a molybdenum sulfide nanosheet film was formed on the surface of the film-formed body, thereby obtaining a film-formed body (semiconductor thin film / PDDA / metal oxide thin film / PDDA / substrate). Then, the film-formed body was obtained as a laminate that was a precursor to a photoelectrode.

[0046] Next, the laminate was heat-treated by holding it in a reducing atmosphere (a mixed gas atmosphere of hydrogen gas and argon gas) at 450°C for 2 hours (S80). This reduced the metal oxide thin film, converting it into a metal thin film (metal film) made of metallic iridium nanosheet film composed of Ir, and also eliminated the PDDA, thereby bonding the semiconductor thin film and metal thin film together. Through these steps, a photoelectrode (semiconductor thin film / metal thin film / FTO substrate) was produced.

[0047] [Example 2] An example of a photoelectrode according to the second embodiment was manufactured. In this case, the same manufacturing method as in Example 1 was carried out, except that steps S31 to S62 were repeated twice. As a result, a laminate (semiconductor thin film / PDDA / metal oxide thin film / PDDA / semiconductor thin film / PDDA / metal oxide thin film / PDDA / substrate), which was a precursor of the photoelectrode, was obtained, and then a photoelectrode (semiconductor thin film / metal thin film / semiconductor thin film / metal thin film / FTO substrate) was manufactured via step S80.

[0048] [Example 3] An example of a photoelectrode according to the third embodiment was manufactured. In this case, the same manufacturing method as in Example 1 was carried out, except that steps S31 to S62 were repeated three times. As a result, a laminate (semiconductor thin film / PDDA / metal oxide thin film / PDDA / semiconductor thin film / PDDA / metal oxide thin film / PDDA / semiconductor thin film / PDDA / metal oxide thin film / PDDA / substrate), which was a precursor of the photoelectrode, was obtained, and then a photoelectrode (semiconductor thin film / metal thin film / semiconductor thin film / metal thin film / semiconductor thin film / metal thin film / FTO substrate) was manufactured via step S80.

[0049] [Comparative Example] Unlike Example 1, a photoelectrode according to the prior art was manufactured in which a metal thin film (metal film) was not provided between the FTO substrate and the semiconductor thin film (semiconductor film). FIG. 3 is a flowchart showing an outline of a method for manufacturing a photoelectrode according to a comparative example. As shown in FIG. 3, first, steps S10 to S32 were performed in the same manner as in Example 1 to obtain a PDDA-imparted substrate (PDDA / substrate). Next, the PDDA-imparted substrate was immersed in a molybdenum sulfide nanosheet solution under the same conditions as in S61 of Example 1 (S43). Next, the immersed PDDA-imparted substrate was washed, and its surface (the surface on the PDDA-imparted side) was dried under the same conditions as in S62 of Example 1 (S44). In this way, a semiconductor thin film (semiconductor film) composed of a molybdenum sulfide nanosheet film was formed on the surface of the PDDA-imparted substrate, thereby obtaining a film-formed body (semiconductor thin film / PDDA / substrate). The film-formed body was then obtained as a laminate, which was a precursor of the photoelectrode. Next, the laminate was heat-treated under the same conditions as in step S80 of Example 1 (S82). In this way, a photoelectrode (semiconductor thin film / FTO substrate) was produced.

[0050] [Evaluation of photoelectrochemical properties] The photoelectrochemical properties of the photoelectrodes produced in Examples 1 to 3 and the Comparative Example were evaluated using a photoelectrochemical cell. FIG. 4 is a schematic diagram showing the photoelectrochemical cell used to evaluate the photoelectrochemical properties of the photoelectrodes of the Examples and Comparative Example. The photoelectrochemical cell shown in FIG. 4 includes an electrolytic cell, an electrolyte (aqueous sodium sulfate solution) filled in the electrolytic cell, a photoelectrode (anode), a counter electrode (cathode), and a reference electrode connected to a constant-voltage power supply (potentiostat). In the electrolytic cell, the photoelectrode, the reference electrode, and the counter electrode are arranged so as to be immersed in the electrolyte. In the photoelectrode, the FTO film on the FTO substrate is connected to the constant-voltage power supply. Platinum (Pt) is used as the counter electrode.

[0051] (Evaluation of output current density and output reduction rate) The output current density (maximum value) and output reduction rate were evaluated for the photoelectrodes of Example 1 and Comparative Example. In this case, a photoelectrode of each of Example 1 and Comparative Example was constructed as the photoelectrode, as shown in FIG. 4 . Then, using a constant voltage power supply, the potential of the photoelectrode relative to the RHE (reversible hydrogen electrode) was kept constant at 1.2 V (vs. RHE), and visible light was irradiated onto the surface of the photoelectrode (surface of the semiconductor thin film) from outside the photoelectrochemical cell using a 300 W xenon lamp for 50 minutes or more. In this case, the output current density (photocurrent density) [mA / cm ] was measured using the constant voltage power supply. 2 ] was measured. The maximum value of the output current density during the period from the start of visible light irradiation until 50 minutes had elapsed was determined as the output current density (maximum value). Furthermore, the ratio (percentage) of the output current density measured 50 minutes after the start of visible light irradiation to the output current density (maximum value) was determined as the output reduction rate [%]. Fig. 5(a) is a graph showing the output current density (maximum value) and output reduction rate determined for the photoelectrodes of Example 1 and the comparative example.

[0052] 5(a), the output current density (maximum value) of the photoelectrode of Example 1 was higher than that of the photoelectrode of the Comparative Example, and the output reduction rate of the photoelectrode of Example 1 was lower than that of the photoelectrode of the Comparative Example. It is believed that the presence of the metal thin film in the photoelectrode of Example 1 enabled the output current density to be increased, and the stability of the output current density to be improved.

[0053] (Evaluation of the effect of the number of stacked semiconductor thin film and metal thin film units on output current density) The output current density (initial value) was determined for the photoelectrodes of Examples 1 to 3 and the Comparative Example, and the effect of the number of stacked semiconductor thin film and metal thin film units in the photoelectrode on the output current density (initial value) was evaluated. In this case, a photoelectrode of each of Examples 1 to 3 and the Comparative Example was constructed as the photoelectrode, as shown in FIG. 4 . A constant voltage power supply was used to set the potential of the photoelectrode relative to the RHE (reversible hydrogen electrode) at a constant potential of 1.2 V (vs. RHE), and a 300 W xenon lamp was used to irradiate the surface of the photoelectrode (the surface of the outermost semiconductor thin film) from outside the photoelectrochemical cell for a predetermined time. In this case, the output current density [mA / cm ] was measured using the constant voltage power supply. 2 5(b) is a graph showing the output current densities (initial values) obtained for the photoelectrodes of Examples 1 to 3 and the comparative example.

[0054] As shown in FIG. 5(b), the output current density (initial value) of the photoelectrodes of Examples 1 to 3 all increased compared to the photoelectrode of the comparative example. Comparing the photoelectrodes of Examples 1 to 3, a tendency was observed in which the output current density (initial value) increased as the number of stacked semiconductor thin film and metal thin film units increased. It is believed that as the number of stacked units increased, more semiconductor thin films absorbed light, generating a larger electromotive force. On the other hand, the output current density (initial value) of the photoelectrode of Example 3, in which the number of stacked units was "3," was smaller than that of the photoelectrode of Example 2, in which the number of stacked units was "2." It is believed that as the number of stacked units increased, the total inter-film resistance of the stacked films increased, and therefore, the output current density saturated when the number of stacked units exceeded two.

[0055] [AFM observation] 6(a) to 6(c) are images obtained by AFM observation of the sample surface of a molybdenum sulfide nanosheet film, a semiconductor thin film, a sample surface of an iridium oxide nanosheet film, a metal oxide thin film, and a metal iridium nanosheet film, respectively, formed in Example 1. In FIG. 6(a), it can be seen that molybdenum sulfide nanosheets having a film thickness of about 1 nm and in-plane dimensions on the order of μm form a molybdenum sulfide nanosheet film. In FIGS. 6(b) and 6(c), it can be seen that iridium oxide nanosheets and metal iridium nanosheets having a film thickness of about 1 nm and in-plane dimensions on the order of μm form an iridium oxide nanosheet film and a metal iridium nanosheet film, respectively.

[0056] [Evaluation of XRD patterns] Figure 6(d) shows the in-plane X-ray diffraction patterns of a sample of an iridium oxide nanosheet film, which is a metal oxide thin film, and a sample of a metallic iridium nanosheet film, which is a metal thin film, formed in Example 1. As shown in Figure 6(d), peaks attributed to IrO2 were observed in the in-plane X-ray diffraction pattern of the iridium oxide nanosheet film. On the other hand, peaks attributed to Ir were mainly observed in the in-plane X-ray diffraction pattern of the metallic iridium nanosheet film.

[0057] Although the embodiments of the photoelectrode of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various design modifications can be made within the scope of the spirit of the present invention as set forth in the claims. [Explanation of symbols]

[0058] 2: FTO substrate, 4: semiconductor thin film (semiconductor film), 6: metal thin film (metal film), 10: photoelectrode

Claims

1. A photoelectrode comprising a substrate and a semiconductor film provided on the substrate and containing a sulfide semiconductor nanosheet, The photoelectrode further comprises a metal film provided between the substrate and the semiconductor film and containing a metal nanosheet.

2. the semiconductor film is provided so as to be stacked on the substrate and includes a plurality of semiconductor thin films containing the sulfide semiconductor nanosheets; 2. The photoelectrode according to claim 1, wherein the metal film includes one metal thin film that is provided between the substrate and one of the plurality of semiconductor thin films of the semiconductor film that is adjacent to the substrate and contains the metal nanosheet, and another metal thin film that is provided between two of the plurality of semiconductor thin films of the semiconductor film that are adjacent to each other and contains the metal nanosheet.

Images