阅读说明：本技术 水分解用光催化剂电极及水分解装置 (Photocatalyst electrode for water splitting and water splitting apparatus ) 是由 小林宏之 折田政宽 于 2019-02-26 设计创作，主要内容包括：本发明的课题在于提供一种起始电位优异的水分解用光催化剂电极及水分解装置。本发明的水分解装置通过对氢产生用光催化剂电极及氧产生用光催化剂电极照射光而从氢产生用光催化剂电极及氧产生用光催化剂电极产生气体,所述水分解装置具有：用于装满电解水溶液的槽；及配置在槽内的氢产生用光催化剂电极及氧产生用光催化剂电极,氢产生用光催化剂电极具有p型半导体层、设置在p型半导体层上的n型半导体层及设置在n型半导体层上的助催化剂,p型半导体层为包含含有Cu、In、Ga及Se的CIGS化合物半导体的半导体层,CIGS化合物半导体中的Ga相对于Ga及In的合计摩尔量的摩尔比为0.4～0.8。(The invention provides a photocatalyst electrode for water splitting and a water splitting device with excellent initial potential. The water splitting apparatus of the present invention generates gas from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, and the water splitting apparatus includes: a tank for filling with an electrolytic aqueous solution; and a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation disposed In the groove, wherein the photocatalyst electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, the p-type semiconductor layer is a semiconductor layer including a CIGS compound semiconductor containing Cu, In, Ga, and Se, and the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4 to 0.8.)

1. A water splitting apparatus that generates a gas from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, the water splitting apparatus comprising:

a tank for filling with an electrolytic aqueous solution; and

the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation disposed in the tank,

the photocatalyst electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer,

the p-type semiconductor layer is a semiconductor layer including a CIGS compound semiconductor containing Cu, In, Ga and Se,

the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4-0.8.

2. The water splitting device of claim 1,

the n-type semiconductor layer contains CdS.

3. The water splitting device of claim 1 or 2, further having a metal layer disposed between the n-type semiconductor layer and the promoter.

4. The water splitting device according to any one of claims 1 to 3,

the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.5-0.7.

5. A water splitting apparatus that generates a gas from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, the water splitting apparatus comprising:

a tank for filling with an electrolytic aqueous solution; and

the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation disposed in the tank,

the photocatalyst electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer,

the difference between the potential p-CBM at the lower end of the conduction band of the p-type semiconductor layer and the potential n-CBM at the lower end of the conduction band of the n-type semiconductor layer, namely the band offset delta E, satisfies the following relation:

ΔE＝(n-CBM)-(p-CBM)≤0.1eV。

6. a photocatalyst electrode for water splitting, which has a semiconductor layer comprising a CIGS compound semiconductor containing Cu, In, Ga and Se,

the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4-0.8.

Technical Field

The present invention relates to a photocatalyst electrode for water splitting and a water splitting apparatus.

Background

From the viewpoint of having excellent solar conversion efficiency, it is known to use a CIGS compound made of an alloy of Cu, In, Ga, and Se as a light absorbing layer of a solar cell. For example, non-patent document 1 discloses a CIGS compound used as a light absorbing layer of a solar cell, which has particularly excellent solar light conversion efficiency when the molar ratio of Ga to the total amount of Ga and In is 0.3.

In recent years, in addition to the photoelectric conversion devices that convert light of the solar cell and the like into electricity, as a method for utilizing solar energy, a technique for producing hydrogen and oxygen by decomposing water using a photocatalyst has been focused.

Prior art documents

Non-patent document

Non-patent document 1: phys. Status Solidi RRL 10, No.8,583-586(2016)/DOI 10.1002/pssr.201600199

Disclosure of Invention

Technical problem to be solved by the invention

In order to increase the amount of gas generated, the photocatalyst electrode for water splitting is required to have a high current value when applied to a water splitting apparatus. In this case, it is also important to extract the potential (initial potential) of the current value.

Here, there is known a water splitting apparatus that uses a system (so-called "Z scheme") in which both hydrogen and oxygen can be generated by splitting water under irradiation of visible light using a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation.

When focusing on a water splitting apparatus having 2 photocatalyst electrodes using such a Z-arrangement, a case where a current is taken out at a potential (initial potential) having a voltage of about half or more of a voltage (about 1.23V) required for water splitting in 1 photocatalyst electrode becomes an indicator of the performance of the water splitting apparatus.

From such a viewpoint, the present inventors have found that a CIGS compound semiconductor used in a solar cell described in the above-mentioned document is applied to a photocatalyst electrode for water splitting, and as a result, the initial potential is insufficient and the performance required for a water splitting device is not satisfied.

Accordingly, an object of the present invention is to provide a photocatalyst electrode for water splitting and a water splitting apparatus having excellent initial potential.

Means for solving the technical problem

As a result of intensive studies on the above-described problems, the present inventors have found that, when a photocatalyst electrode for water splitting having a semiconductor layer including a CIGS compound semiconductor is applied to a water splitting apparatus, the initial potential is excellent as long as the molar ratio of Ga to the total amount of Ga and In the CIGS compound semiconductor is within a predetermined range, and have completed the present invention.

The present inventors have also found that, in a water splitting apparatus including a water splitting photocatalyst electrode having a p-type semiconductor layer and an n-type semiconductor layer, the initial potential of the water splitting photocatalyst electrode is excellent as long as the band offset, which is the difference between the potential at the lower end of the conduction band of the p-type semiconductor layer and the potential at the lower end of the conduction band of the n-type semiconductor layer, is equal to or less than a predetermined value, and have completed the present invention.

That is, the present inventors have found that the above problems can be solved by the following configuration.

[1]

A water splitting apparatus that generates gas from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, the water splitting apparatus comprising:

a tank for filling with an electrolytic aqueous solution; and

the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation disposed in the tank,

the photocatalyst electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer,

the p-type semiconductor layer is a semiconductor layer including a CIGS compound semiconductor containing Cu, In, Ga and Se,

the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4-0.8.

[2]

The water splitting apparatus according to [1], wherein the n-type semiconductor layer contains CdS.

[3]

The water splitting apparatus according to [1] or [2], further comprising a metal layer provided between the n-type semiconductor layer and the co-catalyst.

[4]

The water splitting apparatus according to any one of [1] to [3], wherein a molar ratio of Ga to a total molar amount of Ga and In the CIGS compound semiconductor is 0.5 to 0.7.

[5]

A water splitting apparatus that generates gas from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, the water splitting apparatus comprising:

a tank for filling with an electrolytic aqueous solution; and

the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation disposed in the tank,

the photocatalyst electrode for hydrogen generation has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer,

the band offset Delta E, which is the difference between the potential p-CBM at the lower end of the conduction band of the p-type semiconductor layer and the potential n-CBM at the lower end of the conduction band of the n-type semiconductor layer, satisfies the following relationship.

ΔE＝(n-CBM)-(p-CBM)≤0.1[eV]

[6]

A photocatalyst electrode for water splitting, which has a semiconductor layer comprising a CIGS compound semiconductor containing Cu, In, Ga and Se,

the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4-0.8.

Effects of the invention

As described below, according to the present invention, a photocatalyst electrode for water splitting and a water splitting apparatus having excellent initial potential can be provided.

Drawings

FIG. 1 is a schematic cross-sectional view showing one embodiment of a photocatalyst electrode for water splitting of the present invention.

FIG. 2 is a schematic cross-sectional view showing one embodiment of the photocatalyst electrode for water splitting of the present invention.

Detailed Description

The present invention will be explained below.

In the present invention, the numerical range represented by "to" represents a range in which the numerical values before and after "to" are included as the lower limit value and the upper limit value.

In the present invention, visible light is light of a wavelength visible to the human eye among electromagnetic waves, and specifically, light in a wavelength range of 380 to 780 nm.

In the present specification, excellent initial potential means an initial potential value of 0.6V (vs. rhe) or more. Here, RHE is an abbreviation of reversible hydrogen electrode. When used as a photocatalyst electrode, it is preferable that more current is available at 0.6V (vs. rhe).

Hereinafter, the water splitting apparatus of the present invention will be described in detail for each embodiment.

[ embodiment 1]

In one embodiment of the water splitting apparatus of the present invention, gas is generated from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, the water splitting apparatus including: a tank for filling with an electrolytic aqueous solution; and the hydrogen generating photocatalyst electrode and the oxygen generating photocatalyst electrode disposed In the tank, wherein the hydrogen generating photocatalyst electrode has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a co-catalyst provided on the n-type semiconductor layer, the p-type semiconductor layer is a semiconductor layer including a CIGS compound semiconductor containing Cu, In, Ga, and Se, and the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4 to 0.8. In the present specification, the molar ratio of Ga to the total amount of Ga and In the CIGS compound semiconductor may be simply referred to as "Ga ratio".

The photocatalyst electrode (specifically, the photocatalyst electrode for hydrogen generation) in the water splitting apparatus of embodiment 1 exhibits an excellent initial potential. The reason for this is not clear, but is presumed to be based on the following reason.

That is, when a CIGS compound semiconductor having a Ga ratio of 0.3, which is an optimal composition for a light absorption layer constituting a solar cell, is applied to a photocatalyst electrode, it is considered that a bias potential between a conduction band edge (the bottom of the conduction band) of the CIGS compound semiconductor and a conduction band edge (the bottom of the conduction band) of a layer adjacent thereto (for example, an n-type semiconductor layer described later) is large, and thus the bias potential acts as a barrier to block carrier transport. Therefore, the conduction band edge (the bottommost portion of the conduction band) of the CIGS compound semiconductor is preferably equal to or shallower than the conduction band edge (the bottommost portion of the conduction band) of the layer adjacent thereto (for example, the n-type semiconductor layer described later). Specifically, the difference between the potential p-CBM at the lower end of the conduction band of the p-type semiconductor layer and the potential n-CBM at the lower end of the conduction band of the n-type semiconductor layer, i.e., the band offset Δ E (see the following formula), is preferably 0.1eV or less, and more preferably 0eV or less. The lower limit of Δ E is preferably-0.5 eV or more.

ΔE＝(n-CBM)-(p-CBM)

When the Ga ratio is 0.4 or more, the conduction band edge (the bottom of the conduction band) of the CIGS compound semiconductor becomes shallow, and thus it is estimated that the bias potential of the layer adjacent to the CIGS compound semiconductor becomes small, and as a result, the excellent initial potential is expressed.

In the present invention, the p-CBM and the n-CBM are obtained by adding the potential at the upper end of the valence band obtained by using an atmospheric photoelectron spectrometer (product name "AC-3", RIKEN KEIKI Co., Ltd.) to the value of the band gap obtained by an ultraviolet-visible spectrophotometer (product name "V-770", manufactured by JASCO Corporation).

In the present invention, the potential at the lower end of the conduction band is a value based on the vacuum level (0eV) and the axis is on the negative level side.

Hereinafter, the structure of the water splitting apparatus according to embodiment 1 will be described with reference to the drawings.

Fig. 1 is a side view schematically showing a water splitting apparatus 1 as an example of the water splitting apparatus of embodiment 1. The water splitting apparatus 1 is an apparatus that generates gas from an anode electrode 10 (a photocatalyst electrode for oxygen generation) and a cathode electrode 20 (a photocatalyst electrode for hydrogen generation) by irradiation of light L. Specifically, water is decomposed by the light L to generate oxygen from the anode electrode 10 and hydrogen from the cathode electrode 20.

As shown in fig. 1, the water splitting apparatus 1 includes a tank 40 filled with an electrolytic aqueous solution S, an anode electrode 10 and a cathode electrode 20 disposed in the tank 40, and a separator 30 disposed in the tank 40 between the anode electrode 10 and the cathode electrode 20. The anode electrode 10, the separator 30, and the cathode electrode 20 are arranged in this order in a direction intersecting the traveling direction of the light L.

As the light L to be irradiated, visible light such as sunlight, ultraviolet light, infrared light, or the like can be used, and among them, sunlight with an inexhaustible amount of light is preferable.

< groove >

The inside of the tank 40 is partitioned by the separator 30 into an anode chamber 42 in which the anode electrode 10 is disposed and a cathode chamber 44 in which the cathode electrode 20 is disposed.

The grooves 40 are not limited to this, but are arranged obliquely so that the amount of incident light per unit area with respect to the anode electrode 10 and the cathode electrode 20 is increased. The tank 40 is sealed so that the electrolytic aqueous solution S does not flow out in a state where the tank 40 is inclined.

Specific examples of the material constituting the groove 40 are preferably materials having excellent corrosion resistance (particularly, alkali resistance), and examples thereof include polyacrylate, polymethacrylate, polycarbonate, polypropylene, polyethylene, polystyrene, and glass.

(electrolytic aqueous solution)

As shown in fig. 1, the tank 40 is filled with an electrolytic aqueous solution S, and the anode 10, the cathode 20, and the separator 30 are all immersed in the electrolytic aqueous solution S.

The electrolytic aqueous solution S is a solution obtained by dissolving an electrolyte in water. Specific examples of the electrolyte include sulfuric acid, sodium sulfate, potassium hydroxide, potassium phosphate, and boric acid.

The pH of the electrolytic aqueous solution S is preferably 6 to 11. When the pH of the electrolytic aqueous solution S is within the above range, there is an advantage that safe handling is possible. The pH of the electrolytic aqueous solution S can be measured using a known pH meter, and the measurement temperature is 25 ℃.

The concentration of the electrolyte in the electrolytic aqueous solution S is not particularly limited, but the pH of the electrolytic aqueous solution S is preferably adjusted to be within the above range.

< Anode electrode >

The anode electrode 10 is disposed in the anode electrode chamber 42.

The anode electrode 10 includes a 1 st substrate 12, a 1 st conductive layer 14 disposed on the 1 st substrate 12, and a 1 st photocatalyst layer 16 disposed on the 1 st conductive layer 14. The anode electrode 10 is disposed in the groove 40 so as to form the 1 st photocatalyst layer 16, the 1 st conductive layer 14, and the 1 st substrate 12 in this order from the side of the irradiation light L.

In the example of fig. 1, the anode electrode 10 is a flat plate, but is not limited thereto. The anode electrode 10 may be a punched metal, a mesh, a grid, or a porous body having through pores.

The anode electrode 10 is electrically connected to the cathode electrode 20 through a wire 50. Fig. 1 shows an example in which the anode electrode 10 and the cathode electrode 20 are connected by a lead 50, but the connection method is not particularly limited as long as they are electrically connected.

The thickness of the anode electrode 10 is preferably 0.1 to 5mm, and more preferably 0.5 to 2 mm.

(1 st base plate)

The 1 st substrate 12 is a layer supporting the 1 st conductive layer 14 and the 1 st photocatalyst layer 16.

Specific examples of the material constituting the first substrate 12 include metals, organic compounds (e.g., polyacrylate and polymethacrylate), and inorganic compounds (e.g., SrTiO)₃Etc., metal oxides, glasses, ceramics).

The thickness of the No. 1 substrate 12 is preferably 0.1 to 5mm, more preferably 0.5 to 2 mm.

(conducting layer 1)

The anode electrode 10 has the 1 st conductive layer 14, and thus electrons generated by incidence of light L to the anode electrode 10 move to the 2 nd conductive layer 24 (described later) of the cathode electrode 20 via the lead 50.

Specific examples of the material constituting the first conductive layer 14 include metals (e.g., Sn, Ti, Ta, Au), SrRuO₃ITO (indium tin oxide) and zinc oxide-based transparent conductive materials (Al: ZnO, In: ZnO, Ga: ZnO, etc.). In addition, Al: ZnO, etc. "metal atom: the term "metal oxide" means that a part of a metal (Al: Zn in the case of ZnO) constituting the metal oxide is substituted with a metal atom (Al: Al in the case of ZnO).

The thickness of the 1 st conductive layer 14 is preferably 50nm to 1 μm, and more preferably 100 to 500 nm.

The method for forming the 1 st conductive layer 14 is not particularly limited, and examples thereof include a vapor deposition method (e.g., a chemical vapor deposition method and a sputtering method).

(1 st photocatalyst layer)

When the anode electrode 10 is irradiated with the light L, electrons generated in the 1 st photocatalyst layer 16 move to the 1 st conductive layer 14. On the other hand, the pores (holes) generated in the 1 st photocatalyst layer 16 react with water to generate oxygen from the anode electrode 10.

The thickness of the 1 st photocatalyst layer 16 is preferably 100nm to 10 μm, more preferably 300nm to 2 μm.

As the material constituting the 1 st photocatalyst layer 16, Bi can be mentioned₂WO₆、BiVO₄、BiYWO₆、In₂O₃(ZnO)₃、InTaO₄、InTaO₄: ni (compound: M represents a photo-semiconductor doped with M. the same applies hereinafter), TiO₂：Ni、TiO₂：Ru、TiO₂Rh、TiO₂: Ni/Ta ("Compound: M1/M2" means that M1 and M2 are simultaneously doped in the optical semiconductor, the same applies hereinafter), TiO₂：Ni/Nb、TiO₂：Cr/Sb、TiO₂：Ni/Sb、TiO₂：Sb/Cu、TiO₂：Rh/Sb、TiO₂：Rh/Ta、TiO₂：Rh/Nb、SrTiO₃：Ni/Ta、SrTiO₃：Ni/Nb、SrTiO₃：Cr、SrTiO₃：Cr/Sb、SrTiO₃：Cr/Ta、SrTiO₃：Cr/Nb、SrTiO₃：Cr/W、SrTiO₃：Mn、SrTiO₃：Ru、SrTiO₃：Rh、SrTiO₃：Rh/Sb、SrTiO₃：Ir、CaTiO₃：Rh、La₂Ti₂O₇：Cr、La₂Ti₂O₇：Cr/Sb、La₂Ti₂O₇：Fe、PbMoO₄：Cr、RbPb₂Nb₃O₁₀、HPb₂Nb₃O₁₀、PbBi₂Nb₂O₉、BiVO₄、BiCu₂VO₆、BiSn₂VO₆、SnNb₂O₆、AgNbO₃、AgVO₃、AgLi_1/3Ti_2/3O₂、AgLi_1/3Sn_2/3O₂、WO₃、BaBi_{1- x}InxO₃、BaZr_1-xSn_xO₃、BaZr_1-xGe_xO₃And BaZr_1-xSi_xO₃Isooxide, LaTiO₂N、Ca_0.25La_0.75TiO_2.25N_0.75、TaON、CaNbO₂N、BaNbO₂N、CaTaO₂N、SrTaO₂N、BaTaO₂N、LaTaO₂N、Y₂Ta₂O₅N₂、(Ga_1-xZn_x)(N_1-xO_x)、(Zn_1+xGe)(N₂O_x) (x represents a value of 0 to 1) and TiN_xO_yF_zOxynitride, NbN and Ta of the like₃N₅Iso-nitrides, sulfides such as CdS, selenides such as CdSe, L^x ₂Ti₂S₂O₅(L^x: pr, Nd, Sm, Gd, Tb, Dy, Ho or Er), and oxysulfides containing La and In (Chemistry Letters, 2007,36,854-855), but is not limited to the materials exemplified herein.

The method for forming the 1 st photocatalyst layer 16 is not particularly limited, and examples thereof include a vapor deposition method (e.g., a chemical vapor deposition method, a sputtering method, a pulsed laser deposition method, etc.) and a particle transfer method.

The 1 st photocatalyst layer 16 may carry a co-catalyst on its surface. When the cocatalyst is supported, the initial potential and the gas generation efficiency become good. Specific examples of the cocatalyst are as described above.

The method of supporting the promoter is not particularly limited, and examples thereof include an immersion method (for example, a method of immersing the photocatalyst layer in a suspension containing the promoter) and a vapor deposition method (for example, a sputtering method).

< cathode electrode >

In the cathode electrode 10, a conductive layer 24, a p-type semiconductor layer 26, an n-type semiconductor layer 28, and a co-catalyst 32 are stacked in this order on the surface 22a of the insulating substrate 22. In the example of fig. 1, the semiconductor layer 29 is composed of the p-type semiconductor layer 26 and the n-type semiconductor layer 28.

(insulating substrate)

The insulating substrate 22 is a substrate that supports the conductive layer 24 and the semiconductor layer 29, and is made of a material having electrical insulation properties.

The insulating substrate 22 is not particularly limited, and for example, a soda-lime glass substrate (hereinafter, referred to as an SLG substrate) or a ceramic substrate can be used.

As the insulating substrate 22, a substrate in which an insulating layer is formed on a metal substrate may be used.

In addition, a glass plate such as high strain point glass or alkali-free glass, or a polyimide material can be used for the insulating substrate 22.

The insulating substrate 22 may or may not be a flexible substrate.

The thickness of the insulating substrate 22 is not particularly limited, but may be, for example, about 20 to 20000 μm, preferably 100 to 10000 μm, and more preferably 1000 to 5000 μm.

(conductive layer)

The conductive layer 24 is formed on the surface 22a of the insulating substrate 22, and is used to apply a voltage to the semiconductor layer 29, for example.

The conductive layer 24 is not particularly limited as long as it has conductivity, and is made of, for example, a metal such as Mo, Cr, and W, or a combination thereof. Of these, the conductive layer 24 is also preferably made of Mo.

The conductive layer 24 may have a single-layer structure or a laminated structure such as a 2-layer structure.

The thickness of the conductive layer 24 is generally about 800nm, but the thickness of the conductive layer 24 is preferably 400nm to 1 μm.

(semiconductor layer)

The semiconductor layer 29 generates an electromotive force. The semiconductor layer 29 has a p-type semiconductor layer 26 formed on the surface 24a of the conductive layer 24 and an n-type semiconductor layer 28 formed on the surface 26a of the p-type semiconductor layer 26, and forms a pn junction at the interface between the p-type semiconductor layer 26 and the n-type semiconductor layer 28.

Light incident on the semiconductor layer 29 is absorbed in the semiconductor layer 29, and electrons are excited from the valence band to the conduction band of the semiconductor. In addition, in carriers excited by an internal electric field formed by pn junction, electrons move toward the n-type semiconductor side, and holes move toward the p-type semiconductor side.

Here, in the semiconductor layer 29, the conductivity types of the p-type semiconductor layer 26 and the n-type semiconductor layer 28 can be measured by a measuring device (product name "PN-12 α", manufactured by NAPSON Corporation) using the seebeck effect.

The p-type semiconductor layer 26 is made of a CIGS compound semiconductor containing Cu, In, Ga, and Se. Specifically, the CIGS compound semiconductor containing Cu, In, Ga and Se may be made of Cu (In, Ga) Se₂The compound represented by (I) may be Cu (In, Ga) Se₂Cu (In, Ga) In which a part of Se In (1) is substituted with S (Se, S)₂The compound shown in the specification.

The molar ratio of Ga to the total amount of Ga and In (Ga ratio) In the CIGS compound semiconductor constituting the p-type semiconductor layer 26 is 0.4 to 0.8.

The Ga ratio is 0.4 or more, preferably 0.45 or more, and more preferably 0.5 or more. This makes the conduction band edge of the p-type semiconductor layer 26 shallow, and band discontinuity (band rejection) with the n-type semiconductor layer 28 is eliminated, so that electrons easily flow into the n-type semiconductor layer 28.

There is an advantage that the initial potential is more excellent as the Ga ratio is larger, while there are problems that the band gap becomes wider, the increase in the melting point of the CIGS compound semiconductor inhibits the growth of particles to reduce the particle size, and the crystallinity of the CIGS compound semiconductor is lowered, and the like, when the Ga ratio is too large. In particular, if the Ga ratio is more than 0.7, such a problem becomes remarkable and the current value becomes small. Therefore, the Ga ratio is preferably 0.7 or less, more preferably 0.65 or less, and further preferably 0.6 or less.

In the present invention, the Ga ratio is calculated from elemental analysis of the entire semiconductor layer (CIGS compound semiconductor) by using a high-frequency inductively coupled plasma emission spectroscopy (ICP-AES).

Examples of the method for forming the p-type semiconductor layer 26(CIGS compound semiconductor) include a multi-source Vapor Deposition method (preferably 3-step method), a selenization method, a sputtering method, a hybrid sputtering method, a mechanochemical treatment method, and the like, a screen printing method, a proximity sublimation method, an MOCVD (Metal Organic Chemical Vapor Deposition) method, a spray method (wet film formation method), and the like, and among these, the multi-source Vapor Deposition method is preferable, and the 3-step method is more preferable.

The thickness of the p-type semiconductor layer 26 is preferably 0.5 to 3.0 μm, more preferably 1.0 to 2.0 μm.

As described above, the n-type semiconductor layer 28 forms a pn junction at the interface with the p-type semiconductor layer 26. The n-type semiconductor layer 28 is preferably a layer through which the light L passes so that the incident light L reaches the p-type semiconductor layer 26.

Examples of the material constituting the n-type semiconductor layer 28 include a metal sulfide containing at least one metal element selected from the group consisting of Cd, Zn, Sn, and In. One kind of the metal sulfide may be used alone, or two or more kinds may be used simultaneously.

Examples of the metal sulfide include CdS, ZnS, Zn (S, O), Zn (S, O, OH), and In₂S₃SnS and SnS_xSe_1-x(X represents a value of 0 or more and less than 1), preferably CdS and ZnS, and more preferably CdS. In particular, CdS is preferred from the viewpoint of lattice matching with the CIGS compound semiconductor. In this case, CdS can be formed in a lattice-matched state (epitaxial) on the CIGS compound semiconductor, and defects at the junction interface can be reduced.

The thickness of the n-type semiconductor layer 28 is preferably 10nm to 2 μm, and more preferably 15 to 200 nm.

For the formation of the n-type semiconductor layer 28, for example, a chemical bath deposition method (hereinafter, referred to as CBD method) is used.

In addition, a window layer may be provided on the n-type semiconductor layer 28, for example. The window layer is made of, for example, a ZnO layer having a thickness of about 10 nm.

(Co-catalyst)

The co-catalyst 32 is formed on the semiconductor layer 29, i.e., the surface 28a of the n-type semiconductor layer 28. The cocatalyst 32 enables the initial potential and the photocurrent density of the cathode electrode 20 to be better.

The promoter 32 may be formed over the entire surface of the n-type semiconductor layer 28, or may be formed in an island shape so as to be dispersed.

Examples of the material constituting the Co-catalyst 32 include a monomer composed of Pt, Pd, Ni, Au, Ag, Ru, Cu, Co, Rh, Ir, Mn, or the like, an alloy composed of these monomers and Mn, and an oxide thereof. Among these, Pt, Rh, or Ru is preferable from the viewpoint of further exerting the effect of the present invention.

The size of the cocatalyst 32 is not particularly limited, but is preferably 0.5nm to 1 μm, and the height is preferably about several nm.

The promoter 32 can be formed by, for example, a coating and firing method, a photoelectric deposition method, a vacuum deposition method, a sputtering method, an impregnation method, or the like.

(other Components which may be contained in the cathode electrode)

The cathode electrode 20 may have a metal layer (not shown) between the n-type semiconductor layer 28 and the promoter 32. In this case, the cocatalyst 32 is formed on the surface of the metal layer.

The metal layer can impart conductivity to the surface layer of the n-type semiconductor layer 28. Therefore, carriers (electrons) generated in the semiconductor layer 29 can easily move to the promoter 32 side through the metal layer.

The metal layer is preferably composed of a transition metal of group 4 or more. Examples of the transition metal of group 4 or more include Ti, Zr, Mo, Ta, and W.

The thickness of the metal layer is preferably 8nm or less, and more preferably 6nm or less. The lower limit of the metal layer is not particularly limited as long as the metal layer can satisfactorily exhibit the above-described functions and has a thickness that can be manufactured.

The metal layer can be formed by, for example, sputtering, vacuum evaporation, electron beam evaporation, or the like.

The cathode electrode 20 may have other layers than those described above. Examples of the other layer include a surface protective layer that can be formed on the photocatalyst electrode 32.

In fig. 1, the mode in which the cathode electrode 20 has the insulating substrate 12 is illustrated, but any of these members may be provided as long as the effects of the present invention can be exerted.

< diaphragm >

The separator 30 is disposed between the anode 10 and the cathode 20 so as to prevent the gas generated in the anode 10 and the gas generated in the cathode 20 from being mixed, while allowing the ions contained in the electrolytic aqueous solution S to freely enter and exit the anode chamber 42 and the cathode chamber 44.

The material constituting the separator 30 is not particularly limited, and known ion exchange membranes and the like can be used.

In fig. 1, an example in which the diaphragm 30 is provided is shown, but the present invention is not limited to this, and the diaphragm 30 may not be provided.

< other constitutions >

The gas generated in the anode 10 can be recovered from a pipe, not shown, connected to the anode chamber 42. The gas generated in the cathode electrode 20 can be recovered from a pipe, not shown, connected to the cathode chamber 44.

Although not shown, a supply pipe, a pump, and the like for supplying the electrolytic aqueous solution S may be connected to the tank 40.

Fig. 1 shows an example in which the tank 40 is filled with the electrolytic aqueous solution S, but the present invention is not limited thereto, and the tank 40 may be filled with the electrolytic aqueous solution S when the water splitting apparatus is driven.

In fig. 1, the case where both the anode electrode 10 and the cathode electrode 20 are photocatalyst electrodes is shown, but the present invention is not limited thereto, and only the cathode electrode 20 may be a photocatalyst electrode.

Fig. 1 shows an example in which the anode electrode 10, the separator 30, and the cathode electrode 20 are arranged in this order in a direction intersecting the traveling direction of the light L, but the present invention is not limited to this, and the water splitting apparatus of the present invention may have the configuration shown in fig. 2.

Fig. 2 is a side view schematically showing a water splitting apparatus 100 as an embodiment of the water splitting apparatus of the present invention. The water splitting apparatus 100 is an apparatus that generates gas from the anode electrode 110 and the cathode electrode 120 by irradiation of the light L. Specifically, water is decomposed by the light L to generate oxygen from the anode electrode 110 and hydrogen from the cathode electrode 120.

As shown in fig. 3, the water splitting apparatus 100 includes a tank 40 filled with an electrolytic aqueous solution S, an anode electrode 110 and a cathode electrode 120 disposed in the tank 40, and a separator 30 disposed in the tank 40 between the anode electrode 110 and the cathode electrode 120. The anode electrode 110, the separator 30, and the cathode electrode 120 are arranged in this order along the traveling direction of the light L. The water splitting apparatus 100 is the same as the water splitting apparatus 1 in fig. 1 except that the arrangement of the anode electrode 110, the arrangement of the cathode electrode 120, and the direction of irradiation with the light L are different from those of the water splitting apparatus 1 in fig. 1, and therefore, only different portions will be described.

The anode electrode 110 is disposed in the groove 40 so as to form a 1 st photocatalyst layer 116, a 1 st conductive layer 114, and a 1 st substrate 112 in this order from the side of the irradiation light L.

The cathode electrode 120 is disposed in the groove 40 so as to be the promoter 132, the n-type semiconductor layer 128, the p-type semiconductor layer 126, the conductive layer 124, and the insulating substrate 122 in this order from the side of the light L. The semiconductor layer 129 is composed of the p-type semiconductor layer 126 and the n-type semiconductor layer 128.

The anode electrode 110 and the cathode electrode 120 are disposed in an inclined manner so that the amount of incident light per unit area is increased.

In the water splitting apparatus 100, the 1 st substrate 112 and the 1 st conductive layer 114 are preferably transparent in order to allow the light L to enter the cathode electrode 120. This allows the cathode electrode 120 to use light that cannot be absorbed by the 1 st photocatalyst layer 116, and thus has an advantage of improving the efficiency of light utilization per unit area.

In the present invention, "transparent" means that the light transmittance in a wavelength region of 380nm to 780nm is 60% or more. The light transmittance was measured by a spectrophotometer. As the spectrophotometer, for example, a UV-visible spectrophotometer V-770 (product name) manufactured by JASCOCORATION was used.

The photocatalyst electrode for water splitting has a semiconductor layer of a CIGS compound semiconductor containing Cu, In, Ga and Se, and the molar ratio of Ga to the total molar amount of Ga and In the CIGS compound semiconductor is 0.4 to 0.8. The details of the photocatalyst electrode for water splitting of the present invention are the same as those described for the cathode electrode 20 in the water splitting apparatus 1, and therefore, the description thereof is omitted.

[2 nd embodiment ]

In one embodiment of the water splitting apparatus of the present invention, gas is generated from a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation by irradiating the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation with light, the water splitting apparatus including: a tank for filling with an electrolytic aqueous solution; and the hydrogen generating photocatalyst electrode and the oxygen generating photocatalyst electrode disposed in the groove, wherein the hydrogen generating photocatalyst electrode has a p-type semiconductor layer, an n-type semiconductor layer provided on the p-type semiconductor layer, and a promoter provided on the n-type semiconductor layer, and a band offset Δ E, which is a difference between a potential p-CBM at a lower end of a conduction band of the p-type semiconductor layer and a potential n-CBM at a lower end of a conduction band of the n-type semiconductor layer, satisfies the following relationship.

ΔE＝(n-CBM)-(p-CBM)≤0.1[eV]

The photocatalyst electrode (specifically, the photocatalyst electrode for hydrogen generation) in the water splitting apparatus of embodiment 2 shows an excellent initial potential. The reason for this is not clear, but is presumed to be based on the following reason.

That is, when Δ E is 0.1eV or less, the carrier transport is less likely to be hindered. Therefore, it is assumed that the photocatalyst electrode in the water splitting apparatus according to embodiment 2 shows an excellent initial potential.

In embodiment 2, Δ E is 0.1eV or less, and is more preferably 0eV or less from the viewpoint of further improving the initial potential of the photocatalyst electrode. The lower limit of Δ E is preferably-0.5 eV or more.

The p-type semiconductor layer in embodiment 2 is not particularly limited as long as Δ E can be made 0.1eV or less, and the p-type semiconductor layer described in embodiment 1 is preferably used in view of easy making Δ E0.1 eV or less.

The n-type semiconductor layer in embodiment 2 is not particularly limited as long as Δ E can be made 0.1eV or less, and the n-type semiconductor layer described in embodiment 1 is preferably used in view of easy making Δ E0.1 eV or less.

The water splitting device of embodiment 2 has the same configuration as that of embodiment 1 except for the p-type semiconductor layer and the n-type semiconductor layer, and therefore, the description thereof is omitted.