WO2019172014A1

WO2019172014A1 - Photocatalytic electrode for electrolysis and electrolysis device

Info

Publication number: WO2019172014A1
Application number: PCT/JP2019/007233
Authority: WO
Inventors: 宏之小林; 政寛折田
Original assignee: 富士フイルム株式会社; 人工光合成化学プロセス技術研究組合
Priority date: 2018-03-06
Filing date: 2019-02-26
Publication date: 2019-09-12
Also published as: JP7026773B2; CN111836779A; JPWO2019172014A1; US20200385873A1

Abstract

The present invention addresses the problem of providing: a photocatalytic electrode for electrolysis that has outstanding onset potential; and an electrolysis device. This electrolysis device causes gases to be generated from a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation. The electrolysis device comprises: a tank for filling with an electrolysis solution; and a photocatalytic electrode for hydrogen generation and a photocatalytic electrode for oxygen generation that are disposed in the tank. The photocatalytic electrode for hydrogen generation comprises a p-type semiconductor layer, an n-type semiconductor layer disposed on the p-type semiconductor layer, and a promoter that is disposed on the n-type semiconductor layer. The p-type semiconductor layer includes a CIGS compound semiconductor that includes copper, indium, gallium, and selenium. The molar ratio of gallium to the total molar quantity of gallium and indium in the CIGS compound semiconductor is 0.4–0.8.

Description

Photocatalytic electrode for water splitting and water splitting device

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

It is known that a CIGS compound made of an alloy of Cu, In, Ga and Se is used as a light absorption layer of a solar cell from the viewpoint of having excellent solar conversion efficiency. For example, in Non-Patent Document 1, as a CIGS compound used for a light absorption layer of a solar cell, when the molar ratio of Ga to the total amount of Ga and In is 0.3, solar conversion efficiency is particularly excellent. It is disclosed.
In recent years, in addition to photoelectric conversion devices that convert light such as the above-described solar cells into electricity, attention has been focused on technologies for producing hydrogen and oxygen by decomposing water using a photocatalyst as a method of utilizing solar energy. Yes.

The photocatalyst electrode for water splitting is required to have a high current value generated when applied to a water splitting device in order to increase the amount of gas generated. In this case, the potential (onset potential) from which the current value can be extracted is also important.
Here, using a photocatalyst electrode for hydrogen generation and a photocatalyst electrode for oxygen generation, a system capable of generating both hydrogen and oxygen by decomposing water under visible light irradiation (so-called “Z scheme”) is used. A water splitting apparatus is known.
When attention is paid to a water splitting apparatus having two photocatalytic electrodes using such a Z scheme, a voltage of about half or more of a voltage (about 1.23 V) required for water splitting is obtained at one photocatalytic electrode. The ability to extract the current at such a potential (onset potential) is a guideline for the performance of the water splitting device.
From such a viewpoint, when the present inventors applied the CIGS compound semiconductor used in the solar cell described in the above document to the photocatalytic electrode for water splitting, the onset potential is insufficient, It was found that the performance required for the device was not satisfied.

Therefore, an object of the present invention is to provide a photocatalyst electrode for water splitting and a water splitting apparatus that are excellent in onset potential.

As a result of earnestly examining the above problems, the present inventors have applied a water-splitting photocatalyst electrode having a semiconductor layer containing a CIGS compound semiconductor to a water-splitting apparatus, with respect to the total amount of Ga and In in the CIGS compound semiconductor. When the molar ratio of Ga is within a predetermined range, the inventors have found that the onset potential is excellent and have reached the present invention.
In addition, in the water splitting apparatus having a water splitting photocatalytic electrode having a p-type semiconductor layer and an n-type semiconductor layer, the inventors of the present invention have the potential at the lower end of the conduction band of the p-type semiconductor layer and the n-type semiconductor layer. When the band offset, which is the difference from the potential at the lower end of the conduction band, is less than or equal to a predetermined value, it was found that the onset potential of the photocatalytic electrode for water splitting is excellent, and the present invention has been achieved.
That is, the present inventors have found that the above problem can be solved by the following configuration.

[1]
A water splitting device that generates gas from the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation by irradiating light to the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation,
A tank for filling the electrolytic solution;
The hydrogen generating photocatalytic electrode and the oxygen generating photocatalytic electrode disposed in the tank,
The photocatalytic electrode for hydrogen generation 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,
The p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga and Se;
The water splitting apparatus, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.
[2]
The water splitting device according to [1], wherein the n-type semiconductor layer contains CdS.
[3]
Furthermore, the water splitting device according to [1] or [2], further including a metal layer provided between the n-type semiconductor layer and the promoter.
[4]
The water splitting device according to any one of [1] to [3], wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.5 to 0.7.
[5]
A water splitting device that generates gas from the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation by irradiating light to the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation,
A tank for filling the electrolytic solution;
The hydrogen generating photocatalytic electrode and the oxygen generating photocatalytic electrode disposed in the tank,
The photocatalytic electrode for hydrogen generation 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,
Water splitting in which the band offset ΔE, 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: apparatus.
ΔE = (n−CBM) − (p−CBM) ≦ 0.1 [eV]
[6]
A semiconductor layer including a CIGS compound semiconductor including Cu, In, Ga, and Se;
The photocatalytic electrode for water splitting, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.

As described below, according to the present invention, a photocatalyst electrode for water splitting and a water splitting device excellent in onset potential can be provided.

It is a typical sectional view showing one embodiment of the photocatalyst electrode for water splitting of the present invention. It is a typical sectional view showing one embodiment of the photocatalyst electrode for water splitting of the present invention.

The present invention will be described below.
In the present invention, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
In the present invention, visible light is light having a wavelength that can be seen by human eyes among electromagnetic waves, and specifically indicates light having a wavelength range of 380 to 780 nm.
In the present specification, being excellent in onset potential means that the value of onset potential is 0.6 V (vs. RHE) or more. Here, RHE is an abbreviation for reversible hydrogen electrode. When used as a photocatalytic electrode, it is preferable that a larger amount of current is obtained at 0.6 V (vs. RHE).

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

[First Embodiment]
One embodiment of the water splitting device of the present invention is configured to irradiate light from the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation by irradiating light to the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation. A water splitting device for filling the electrolytic aqueous solution, the hydrogen generating photocatalyst electrode and the oxygen generating photocatalyst electrode disposed in the tank, and the hydrogen generation The photocatalytic electrode for use 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 the p-type semiconductor layer Is a semiconductor layer containing 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 in the CIGS compound semiconductor is 0.4 to 0.8. is there. In the present specification, the molar ratio of Ga to the total amount of Ga and In in the CIGS compound semiconductor may be simply abbreviated as “Ga ratio”.

The photocatalyst electrode (specifically, the photocatalyst electrode for hydrogen generation) in the water splitting apparatus of the first embodiment exhibits an excellent onset potential. Although the details of this reason are not clear, it is presumed that the reason is as follows.
That is, when a CIGS compound semiconductor with a Ga ratio of 0.3, which is an optimal composition as a light absorption layer constituting the solar cell, is applied to the photocatalytic electrode, the conduction band edge (the bottom of the conduction band) of the CIGS compound semiconductor, This is considered to be because the offset potential with the conduction band edge (the bottom of the conduction band) of the adjacent layer (for example, an n-type semiconductor layer to be described later) is large, which acts as a barrier and inhibits carrier transport. Therefore, the conduction band edge (the bottom part of the conduction band) of the CIGS compound semiconductor should be equal to or shallower than the conduction band edge (the bottom part of the conduction band) of a layer adjacent thereto (for example, an n-type semiconductor layer described later). preferable. Specifically, the band offset ΔE (see the following formula), 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, is 0.1 eV or less is preferable, and 0 eV or less is more preferable. Further, the lower limit of ΔE is preferably −0.5 eV or more.
ΔE = (n-CBM)-(p-CBM)
If 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 as a result, the offset potential between the CIGS compound semiconductor and the adjacent layer is reduced, resulting in excellent onset. It is presumed that it has shown potential.

In the present invention, p-CBM and n-CBM use an atmospheric photoelectron spectrometer (product name “AC-3”, manufactured by Riken Keiki Co., Ltd.) to determine the potential at the upper end of the valence band, and to UV-visible spectroscopy. This is a value obtained by adding a band gap value obtained from a photometer (product name “V-770”, manufactured by JASCO Corporation).
In the present invention, the potential value at the lower end of the conduction band is a value when the axis is on the minus side with the vacuum level as a reference (0 eV).

Below, the structure of the water splitting device of 1st Embodiment is demonstrated, referring drawings.
Drawing 1 is a side view showing typically water splitting device 1 which is an example of a water splitting device of a 1st embodiment. The water splitting device 1 is a device that generates gas from the anode electrode 10 (photocatalytic electrode for oxygen generation) and the cathode electrode 20 (photocatalytic electrode for hydrogen generation) by irradiation with light L. Specifically, water is decomposed by the light L, oxygen is generated from the anode electrode 10, and hydrogen is generated 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 between the anode electrode 10 and the cathode electrode 20. And the diaphragm 30 disposed in the tank 40. The anode electrode 10, the diaphragm 30, and the cathode electrode 20 are arranged in this order along the direction intersecting with the traveling direction of the light L.
As the irradiated light L, visible light such as sunlight, ultraviolet light, infrared light, or the like can be used, and among these, sunlight whose amount is inexhaustible is preferable.

<Tank>
The tank 40 is partitioned by the diaphragm 30 into an anode electrode chamber 42 in which the anode electrode 10 is disposed and a cathode electrode chamber 44 in which the cathode electrode 20 is disposed.
Although not limited to this, the tank 40 is arranged so as to be inclined so that the amount of incident light per unit area with respect to the anode electrode 10 and the cathode electrode 20 becomes large. Moreover, 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.
As a specific example of the material constituting the tank 40, a material excellent in corrosion resistance (particularly alkali resistance) is preferable, and examples thereof include polyacrylate, polymethacrylate, polycarbonate, polypropylene, polyethylene, polystyrene, and glass.

(Electrolytic aqueous solution)
As shown in FIG. 1, the inside of the tank 40 is filled with the electrolytic aqueous solution S, and the whole of the anode electrode 10, the cathode electrode 20 and the diaphragm 30 is immersed in the electrolytic aqueous solution S.
The electrolytic aqueous solution S is a solution in which an electrolyte is dissolved 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 6-11. If the pH of the electrolytic aqueous solution S is within the above range, there is an advantage that it can be handled safely. The pH of the electrolytic aqueous solution S can be measured using a known pH meter, and the measurement temperature is 25 ° C.
The concentration of the electrolyte in the electrolytic aqueous solution S is not particularly limited, but is preferably adjusted so that the pH of the electrolytic aqueous solution S is in the above range.

<Anode electrode>
The anode electrode 10 is disposed in the anode electrode chamber 42.
The anode electrode 10 includes a first substrate 12, a first conductive layer 14 disposed on the first substrate 12, and a first photocatalytic layer 16 disposed on the first conductive layer 14. The anode electrode 10 is disposed in the tank 40 so that the first photocatalyst layer 16, the first conductive layer 14, and the first substrate 12 are arranged in this order from the side irradiated with the light L.
In the example of FIG. 1, the anode electrode 10 has a flat plate shape, but is not limited thereto. The anode electrode 10 may be a punching metal shape, a mesh shape, a lattice shape, or a porous body having penetrating pores.
The anode electrode 10 is electrically connected to the cathode electrode 20 by a conducting wire 50. Although FIG. 1 shows an example in which the anode electrode 10 and the cathode electrode 20 are connected by the conductive wire 50, 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 5 mm, and more preferably 0.5 to 2 mm.

(First substrate)
The first substrate 12 is a layer that supports the first conductive layer 14 and the first photocatalytic layer 16.
Specific examples of the material constituting the first substrate 12 include metals, organic compounds (for example, polyacrylates and polymethacrylates), and inorganic compounds (for example, metal oxides such as SrTiO ₃ , glass, and ceramics).
The thickness of the first substrate 12 is preferably 0.1 to 5 mm, and more preferably 0.5 to 2 mm.

(First conductive layer)
Since the anode electrode 10 has the first conductive layer 14, electrons generated by the incidence of the light L on the anode electrode 10 move to the second conductive layer 24 (described later) of the cathode electrode 20 through the conductive wire 50.
Specific examples of the material constituting the first conductive layer 14 include metals (for example, Sn, Ti, Ta, Au), SrRuO ₃ , ITO (indium tin oxide), and zinc oxide-based transparent conductive materials (Al: ZnO, In: ZnO, Ga: ZnO, etc.). Note that the notation “metal atom: metal oxide” such as Al: ZnO represents a part of metal (Zn in the case of Al: ZnO) constituting the metal oxide with a metal atom (Al: ZnO). In the case, it means one substituted with Al).
The thickness of the first conductive layer 14 is preferably 50 nm to 1 μm, and more preferably 100 to 500 nm.
A method for forming the first conductive layer 14 is not particularly limited, and examples thereof include a vapor deposition method (for example, a chemical vapor deposition method and a sputtering method).

(First photocatalyst layer)
When the light L is applied to the anode electrode 10, electrons generated in the first photocatalytic layer 16 move to the first conductive layer 14. On the other hand, the holes generated in the first photocatalyst layer 16 react with water to generate oxygen from the anode electrode 10.
The thickness of the first photocatalyst layer 16 is preferably 100 nm to 10 μm, and more preferably 300 nm to 2 μm.

Examples of materials that can constitute the first photocatalytic layer 16 include Bi ₂ WO ₆ , BiVO ₄ , BiYWO ₆ , In ₂ O ₃ (ZnO) ₃ , InTaO ₄ , InTaO ₄ : Ni (“Compound: M” is an optical semiconductor. TiO ₂ : Ni, TiO ₂ : Ru, TiO ₂ Rh, TiO ₂ : Ni / Ta (“Compound: M1 / M2” is M1 in the optical semiconductor. And TiO ₂ : Ni / Nb, TiO ₂ : Cr / Sb, TiO ₂ : Ni / Sb, TiO ₂ : Sb / Cu, TiO ₂ : Rh / Sb, TiO ₂ : Rh / Ta, TiO ₂ : Rh / Nb, SrTiO ₃ : Ni / Ta, SrTiO ₃ : Ni / Nb, SrTiO ₃ : Cr, SrTiO ₃ : Cr / Sb, SrTiO ₃ _{_{_{: Cr / Ta, SrTiO 3:}}} Cr / Nb, SrTiO 3: Cr / W, SrTiO 3: Mn, SrTiO 3: Ru, SrTiO 3: Rh, SrTiO 3: Rh / Sb, SrTiO 3: Ir, CaTiO 3: Rh La ₂ Ti ₂ O ₇ : Cr, La ₂ Ti ₂ O ₇ : Cr / Sb, La ₂ Ti ₂ O ₇ : Fe, PbMoO ₄ : Cr, RbPb ₂ Nb ₃ O ₁₀ , HPb ₂ Nb ₃ O ₁₀ , PbBi _{_{_{_{2 Nb 2 O 9, BiVO 4}}}} , BiCu 2 VO 6, BiSn 2 VO 6, SnNb 2 O 6, AgNbO 3, AgVO 3, AgLi 1/3 Ti 2/3 O 2, AgLi 1/3 Sn 2/3 O ₂ , WO ₃ , BaBi _1-x InxO ₃ , BaZr _1-x Sn _x O ₃ , BaZr _1-x Ge _x O ₃ , and Oxides such as BaZr _1-x Si _x O ₃ , LaTiO ₂ N, Ca _0.25 La _0.75 TiO _2.25 N _0.75 , TaON, CaNbO ₂ N, BaNbO ₂ N, CaTaO ₂ N, SrTaO ₂ N, BaTaO ₂ N, LaTaO ₂ N, Y ₂ Ta ₂ O ₅ N ₂ , (Ga _1-x Zn _x ) (N _1-x O _x ), (Zn _{1 + x} Ge) (N ₂ O _x ) (x is , And oxynitrides such as TiN _x O _y F _z , nitrides such as NbN and Ta ₃ N ₅ , sulfides such as CdS, selenides such as CdSe, L ^x ₂ _{_{_{^{Ti 2 S 2 O 5 (L}}}} x: Pr, Nd, Sm, Gd, Tb, Dy, Ho or Er,), as well as La, oxysulfide compound containing In (Chemistry Letters, 2007,36,854-855) can be mentioned But it is not limited to the exemplified materials herein.

The formation method of the first photocatalyst layer 16 is not particularly limited, and examples thereof include a vapor deposition method (for example, a chemical vapor deposition method, a sputtering method, a pulse laser deposition method, etc.) and a particle transfer method.

The first photocatalyst layer 16 may have a promoter supported on its surface. If the promoter is supported, the onset potential and gas generation efficiency will be good. Specific examples of the cocatalyst are as described above.
The method for supporting the cocatalyst 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 cocatalyst) and a vapor phase growth method (for example, a sputtering method). It is done.

<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 promoter 32 are stacked in this order on the surface 22 a of the insulating substrate 22. In the example of FIG. 1, the p-type semiconductor layer 26 and the n-type semiconductor layer 28 constitute a semiconductor layer 29.

(Insulated substrate)
The insulating substrate 22 is a substrate that supports the conductive layer 24 and the semiconductor layer 29, and is made of an electrically insulating material.
The insulating substrate 22 is not particularly limited. For example, a soda lime glass substrate (hereinafter referred to as an SLG substrate) or a ceramic substrate can be used.
The insulating substrate 22 may be a substrate in which an insulating layer is formed on a metal substrate.
The insulating substrate 22 may be made of a glass plate such as high strain point glass and non-alkali glass, or a polyimide material.
The insulating substrate 22 may or may not be flexible.

The thickness of the insulating substrate 22 is not particularly limited, and may be, for example, about 20 to 20000 μm, preferably 100 to 10,000 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 for applying a voltage to the semiconductor layer 29, for example.
Although it will not specifically limit if the conductive layer 24 has electroconductivity, For example, it is comprised with metals, such as Mo, Cr, and W, or what combined these. Among these, the conductive layer 24 is preferably composed of Mo.
The conductive layer 24 may have a single layer structure or a laminated structure such as a two-layer structure.
The thickness of the conductive layer 24 is generally about 800 nm, but the thickness of the conductive layer 24 is preferably 400 nm 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. A pn junction is formed at the interface with the type semiconductor layer 28.
Light incident on the semiconductor layer 29 is absorbed in the semiconductor layer 29 and excites electrons from the valence band of the semiconductor to the conduction band. Of the carriers excited by the internal electric field created by the pn junction, electrons are moved to the n-type semiconductor side and holes are moved to the p-type semiconductor side.
Here, in the semiconductor layer 29, the conduction type of the p-type semiconductor layer 26 and the n-type semiconductor layer 28 can be measured by a measuring device (product name “PN-12α”, manufactured by NAPSON) using the Seebeck effect.

The p-type semiconductor layer 26 is made of a CIGS compound semiconductor containing Cu, In, Ga, and Se. Cu, an In, CIGS compound containing Ga and Se semiconductor, specifically, Cu (In, Ga) may be a compound represented by the Se _2, Cu (In, Ga) in the Se ₂ of Se A compound represented by Cu (In, Ga) (Se, S) ₂ partially substituted with S may be used.
The molar ratio (Ga ratio) of Ga to the total amount of Ga and In 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. As a result, the conduction band edge of the p-type semiconductor layer 26 becomes shallow and the band discontinuity (band barrier) with the n-type semiconductor layer 28 is eliminated, so that electrons easily flow into the n-type semiconductor layer 28.
On the other hand, the larger the Ga ratio is, the better the onset potential is. On the other hand, if the Ga ratio is too large, the band gap is widened, and the grain growth is hindered by the increase in the melting point of the CIGS compound semiconductor, thereby reducing the grain size. In some cases, problems such as a decrease in crystallinity of the CIGS compound semiconductor may occur. In particular, when the Ga ratio exceeds 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 even more preferably 0.6 or less.
In the present invention, the Ga ratio is calculated based on elemental analysis of the entire semiconductor layer (CIGS compound semiconductor) using high frequency inductively coupled plasma emission spectroscopy (ICP-AES).

As a method for forming the p-type semiconductor layer 26 (CIGS compound semiconductor), for example, a multi-source deposition method (preferably a three-step method), a selenization method, a sputtering method, a hybrid sputtering method, a mechanochemical process method, a screen printing method, Examples include a proximity sublimation method, a MOCVD (Metal Organic Chemical Vapor Deposition) method and a spray method (wet film forming method). Among these, a multi-source deposition method is preferable, and a three-stage method is more preferable.

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

The n-type semiconductor layer 28 forms a pn junction at the interface with the p-type semiconductor layer 26 as described above. Further, the n-type semiconductor layer 28 is preferably a layer through which the light L is transmitted 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 metal sulfides containing at least one metal element selected from the group consisting of Cd, Zn, Sn, and In. A metal sulfide may be used individually by 1 type, or may use 2 or more types together.
Examples of the metal sulfide include CdS, ZnS, Zn (S, O), Zn (S, O, OH), In ₂ S ₃ , SnS, and SnS _x Se _1-x (X is 0 or more and less than 1) CdS and ZnS are preferable, and CdS is more preferable. In particular, CdS is preferable from the viewpoint of lattice matching with a CIGS compound semiconductor. In this case, the film can be formed in a state (epitaxial) in which CdS is lattice-matched on the CIGS compound semiconductor, and defects at the junction interface can be reduced.
The film thickness of the n-type semiconductor layer 28 is preferably 10 nm to 2 μm, 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.
For example, a window layer may be provided on the n-type semiconductor layer 28. This window layer is composed of, for example, a ZnO layer having a thickness of about 10 nm.

(Cocatalyst)
The co-catalyst 32 is formed on the semiconductor layer 29, that is, on the surface 28 a of the n-type semiconductor layer 28. The co-catalyst 32 can improve the onset potential and photocurrent density of the cathode electrode 20.
The co-catalyst 32 may be formed on the entire surface of the n-type semiconductor layer 28, or may be formed in an island shape so as to be scattered.
Examples of the material constituting the co-catalyst 32 include simple substances composed of Pt, Pd, Ni, Au, Ag, Ru, Cu, Co, Rh, Ir, Mn, etc., alloys combining them, and oxidation thereof. Things. Among these, Pt, Rh, or Ru is preferable because the effects of the present invention are more exhibited.
The size of the co-catalyst 32 is not particularly limited, and is preferably 0.5 nm to 1 μm, and the height is preferably about several nm.
The co-catalyst 32 can be formed by, for example, a coating baking method, a photo-deposition method, a vacuum deposition method, a sputtering method, an impregnation method, or the like.

(Other members that can be included 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 promoter 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 by the metal layer.
The metal layer is preferably composed of a transition metal of group 4 or higher. Examples of the group 4 or higher transition metal include Ti, Zr, Mo, Ta, and W.
In addition, the thickness of the metal layer is preferably 8 nm or less, and more preferably 6 nm or less. The lower limit of the metal layer is not particularly limited as long as the above-described function can be satisfactorily exhibited and the thickness is manufacturable.
The metal layer can be formed by, for example, a sputtering method, a vacuum evaporation method, an electron beam evaporation method, or the like.

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

In addition, in FIG. 1, although the cathode electrode 20 gave and demonstrated the example which has the insulating substrate 12, as long as the effect of this invention can be exhibited, it has either of these members. It does not have to be.

<Diaphragm>
The diaphragm 30 allows ions contained in the electrolytic aqueous solution S to freely enter and exit the anode electrode chamber 42 and the cathode electrode chamber 44, but the gas generated in the anode electrode 10 and the gas generated in the cathode electrode 20 are not mixed. It is arranged between the anode electrode 10 and the cathode electrode 20.
The material constituting the diaphragm 30 is not particularly limited, and examples thereof include known ion exchange membranes.
In addition, although the example provided with the diaphragm 30 was shown in FIG. 1, it is not limited to this, The diaphragm 30 may not be provided.

<Other configurations>
The gas generated in the anode electrode 10 can be recovered from a pipe (not shown) connected to the anode electrode chamber 42. The gas generated at the cathode electrode 20 can be recovered from a pipe (not shown) connected to the cathode electrode chamber 44.
Although not shown, the tank 40 may be connected to a supply pipe for supplying the electrolytic aqueous solution S, a pump, and the like.

Although FIG. 1 shows an example in which the tank 40 is filled with the electrolytic aqueous solution S, the present invention is not limited to this, and the tank 40 may be filled with the electrolytic aqueous solution S when the water splitting apparatus is driven.
Although FIG. 1 shows a case where both the anode electrode 10 and the cathode electrode 20 are photocatalytic electrodes, the present invention is not limited to this, and only the cathode electrode 20 may be a photocatalytic electrode.

Although FIG. 1 shows an example in which the anode electrode 10, the diaphragm 30 and the cathode electrode 20 are arranged in this order along the direction intersecting the traveling direction of the light L, the present invention is not limited to this, and the water of the present invention is not limited thereto. The decomposition apparatus may have a structure shown in FIG.
FIG. 2 is a side view schematically showing a water splitting device 100 which is an embodiment of the water splitting device of the present invention. The water splitting device 100 is a device that generates gas from the anode electrode 110 and the cathode electrode 120 by irradiation with light L. Specifically, water is decomposed by the light L, oxygen is generated from the anode electrode 110, and hydrogen is generated 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 gap between the anode electrode 110 and the cathode electrode 120. And the diaphragm 30 disposed in the tank 40. The anode electrode 110, the diaphragm 30 and the cathode electrode 120 are arranged in this order along the traveling direction of the light L. The water decomposition apparatus 100 is the same as the water decomposition apparatus 1 except that the arrangement of the anode electrode 110, the arrangement of the cathode electrode 120, and the irradiation direction of the light L are different from the water decomposition apparatus 1 in FIG. Is mainly described.

The anode electrode 110 is disposed in the tank 40 so that the first photocatalyst layer 116, the first conductive layer 114, and the first substrate 112 are arranged in this order from the side irradiated with the light L.
The cathode electrode 120 is arranged so that the promoter 132, the n-type semiconductor layer 128, the p-type semiconductor layer 126, the conductive layer 124, and the insulating substrate 122 are in this order from the side irradiated with the light L. It is arranged in the tank 40. Note that the semiconductor layer 129 includes the p-type semiconductor layer 126 and the n-type semiconductor layer 128.
The anode electrode 110 and the cathode electrode 120 are arranged so as to be inclined so that the amount of incident light per unit area is large.
In the water splitting apparatus 100, the first substrate 112 and the first conductive layer 114 are preferably transparent so that the light L is incident on the cathode electrode 120. Thereby, since the cathode electrode 120 can use the light that the first photocatalyst layer 116 could not absorb, there is an advantage that the light use efficiency per unit area is improved.
In the present invention, “transparent” means that the light transmittance in a wavelength region of 380 nm to 780 nm is 60% or more. The light transmittance is measured with a spectrophotometer. As the spectrophotometer, for example, V-770 (product name) manufactured by JASCO Corporation, which is an ultraviolet-visible spectrophotometer, is used.

The photocatalytic electrode for water splitting of the present invention has a semiconductor layer containing 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 in the CIGS compound semiconductor is 0.4 to 0.8. The details of the photocatalytic electrode for water splitting according to 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.

[Second Embodiment]
One embodiment of the water splitting device of the present invention is configured to irradiate light from the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation by irradiating light to the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation. A water splitting device for filling the electrolytic aqueous solution, the hydrogen generating photocatalyst electrode and the oxygen generating photocatalyst electrode disposed in the tank, and the hydrogen generation The photocatalytic electrode for use 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 the p-type semiconductor layer The band offset ΔE, which is the difference between the potential p-CBM at the lower end of the conduction band of n 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]

The photocatalytic electrode (specifically, the photocatalytic electrode for hydrogen generation) in the water splitting device of the second embodiment exhibits an excellent onset potential. Although the details of this reason are not clear, it is presumed that the reason is as follows.
That is, if ΔE is 0.1 eV or less, carrier transport is hardly inhibited. Therefore, it is estimated that the photocatalytic electrode in the water splitting device of the second embodiment has an excellent onset potential.
In 2nd Embodiment, (DELTA) E is 0.1 eV or less, and 0 eV or less is more preferable from the point which the onset potential of a photocatalyst electrode is more excellent. Further, the lower limit of ΔE is preferably −0.5 eV or more.

The p-type semiconductor layer in the second embodiment is not particularly limited as long as ΔE can be made 0.1 eV or less, but will be described in the first embodiment because ΔE can be easily made 0.1 eV or less. It is preferable to use the p-type semiconductor layer.
The n-type semiconductor layer in the second embodiment is not particularly limited as long as ΔE can be made 0.1 eV or less, but will be described in the first embodiment because ΔE can be easily made 0.1 eV or less. It is preferable to use the n-type semiconductor layer.

In the water decomposing apparatus of the second embodiment, since the configuration other than the p-type semiconductor layer and the n-type semiconductor layer is the same as that of the water decomposing apparatus of the first embodiment, the description thereof is omitted.

Hereinafter, the present invention will be described in more detail based on examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the following examples.

[Example 1]
The photocatalytic electrode of Example 1 was produced as follows.
First, a layer made of Mo having a thickness of 600 nm is formed on the surface of a soda lime glass substrate (insulating substrate) by a DC (direct current) magnetron sputtering method using a magnetron sputtering apparatus (product name “CFS-12P”, manufactured by Shibaura Eletech Corporation). (Conductive layer) was formed.
Next, a layer made of CIGS compound semiconductor (p-type semiconductor layer) having a thickness of 2 μm is formed on the surface of the conductive layer by a three-stage method using a multi-source deposition apparatus (product name “EW-10”, manufactured by Eiko Engineering Co., Ltd.). Formed. Specifically, Cu, In, Ga, and Se were used as the evaporation source, and the first stage substrate temperature was 400 ° C., and the second and third stage substrate temperatures were 520 ° C. The Ga ratio (Ga / (Ga + In)) was adjusted by controlling the deposition rate of Ga and In.
Next, CdS having a thickness of 70 nm is formed on the surface of the p-type semiconductor layer by a chemical bath deposition method (CBD method) using 1.5 mM CdSO ₄ , 1.5 M NH ₄ OH, and 7.5 mM thiourea aqueous solution. A layer made of (n-type semiconductor layer) was prepared. The CBD method was performed at 70 ° C. for 40 minutes.
Next, particles (co-catalyst) having a size of 0.5 nm are formed on the surface of the n-type semiconductor layer by a DC magnetron sputtering method using a magnetron sputtering apparatus (product name “MSP-30T”, manufactured by Vacuum Device Inc.). ) Was supported.
Thus, the photocatalyst electrode of Example 1 (photocatalyst electrode for hydrogen generation) having the same structure as the photocatalyst electrode 10 of FIG. 1 was obtained.

[Examples 2 to 5, Comparative Example 1]
When forming the p-type semiconductor layer, the vapor deposition rate of Ga and In was changed as appropriate so that the Ga ratio became the value shown in Table 1, and the same as the production of the photocatalytic electrode of Example 1. The photocatalyst electrodes of Examples 2 to 5 and Comparative Example 1 (photocatalyst electrodes for hydrogen generation) were obtained.

[Measurement of Ga ratio]
Using an apparatus (product name “ICP-AES 8100”, manufactured by Shimadzu Corporation) whose measurement principle is high frequency inductively coupled plasma optical emission spectrometry (ICP-AES), the entire p-type semiconductor layer (CIGS compound semiconductor) Elemental analysis was performed to calculate the molar amount of Ga and In, and the Ga ratio (Ga / (Ga + In)) was calculated from the molar amount obtained. Specifically, the CIGS compound semiconductor contained in the p-type semiconductor layer was dissolved with an appropriate solution (hydrofluoric acid), and quantitative measurement was performed. The results are shown in Table 1.

[Evaluation test]
<Onset potential measurement>
The onset potential of the produced photocatalytic electrode was evaluated by electrochemical measurement using a three-electrode system using a potentiostat (product name “HZ-7000”, manufactured by Hokuto Denko).
Specifically, a Pyrex glass electrochemical cell was used, the photocatalyst electrodes of the above examples and comparative examples were used as the working electrode, the Ag / AgCl electrode was used as the reference electrode, and the Pt wire was used as the counter electrode. As the electrolytic aqueous solution, an aqueous solution containing 0.5 M sodium sulfate, 0.25 M sodium dihydrogen phosphate and 0.25 M disodium hydrogen phosphate was used.
The inside of the electrochemical cell was filled with argon, and dissolved oxygen and carbon dioxide were removed by sufficiently bubbling before measurement.
As a light source, a solar simulator (AM1.5G) (product name “XES-70S1”, manufactured by Mitsunaga Electric Mfg. Co., Ltd.) was used.
Then, it was swept from 0 V (vs. RHE) to 0.8 V (vs. RHE) at 20 mV / s to obtain a current-potential curve under pseudo-sunlight irradiation by the solar simulator.
Based on the value of the obtained onset potential, evaluation was performed according to the following evaluation criteria. The evaluation results are shown in Table 1.
A: Onset potential value is 0.6 V (vs. RHE) or more B: Onset potential value is less than 0.6 V (vs. RHE)

<Photocurrent density>
The photocurrent density (mA / cm ² ) at 0.6 V (vs. RHE) was read from the current-voltage curve obtained by the onset potential measurement, and evaluated according to the following evaluation criteria. The evaluation results are shown in Table 1.
A: photocurrent density 4mA / cm ² or more B: less than photocurrent density 0 mA / cm ² ultra 4mA / cm ² C: photocurrent density 0 mA / cm ² or less

<Band offset ΔE>
Using the atmospheric photoelectron spectrometer (product name “AC-3”, manufactured by Riken Keiki Co., Ltd.), the potential at the upper end of the valence band of the p-type semiconductor layer of the produced photocatalytic electrode was determined. In addition, the band gap value of the p-type semiconductor layer of the produced photocatalytic electrode was determined using an ultraviolet-visible spectrophotometer (product name “V-770”, manufactured by JASCO Corporation). The band gap value was added to the potential at the upper end of the valence band of the p-type semiconductor layer thus obtained to obtain the p-CBM of the p-type semiconductor layer in the photocatalytic electrode.
In addition, the n-CBM of the n-type semiconductor layer in the photocatalytic electrode was determined in the same manner as the p-CBM measurement except that the n-type semiconductor layer was used.
ΔE was determined from the obtained p-CBM and n-CBM values. The results are shown in Table 1.

As shown in Table 1, when a photocatalytic electrode having a semiconductor layer containing a CIGS compound semiconductor having a Ga ratio of 0.4 to 0.8 was used, it was confirmed that the onset potential was excellent (Examples 1 to 5). ).
In addition, as shown in Table 1, it was confirmed that if a photocatalytic electrode having ΔE of 0.1 eV or less was used, the onset potential was excellent (Examples 1 to 5).
Further, in comparison with Examples 1 to 5, if a photocatalytic electrode having a semiconductor layer containing a CIGS compound semiconductor having a Ga ratio of 0.5 to 0.6 is used (Example 2 and Example 3), the photocurrent density It was confirmed that it was excellent.
On the other hand, as shown in Table 1, when a photocatalytic electrode having a semiconductor layer containing a CIGS compound semiconductor with a Ga ratio of less than 0.4 was used, it was confirmed that the onset potential was inferior (Comparative Example 1).

DESCRIPTION OF SYMBOLS 1,100 Apparatus 10,110 Anode electrode 12,112 1st board | substrate 14,114 1st conductive layer 16,116 1st photocatalyst layer 20,120 Cathode electrode 22,122 Insulation board | substrate 22a, 24a, 26a, 28a Surface 24,124 Conductive layer 26, 126 p-type semiconductor layer 28, 128 n-

type semiconductor layer

29, 129 semiconductor layer 30 diaphragm 32, 132 promoter 40 tank 42 anode electrode chamber 44 cathode electrode chamber 50 conducting wire S electrolyte L light

Claims

A water splitting device that generates gas from the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation by irradiating light to the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation,
A tank for filling the electrolytic solution;
The hydrogen generating photocatalytic electrode and the oxygen generating photocatalytic electrode disposed in the tank,
The photocatalytic electrode for hydrogen generation 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;
The p-type semiconductor layer is a semiconductor layer containing a CIGS compound semiconductor containing Cu, In, Ga and Se;
The water splitting device, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.
The water splitting device according to claim 1, wherein the n-type semiconductor layer contains CdS.
The water splitting device according to claim 1 or 2, further comprising a metal layer provided between the n-type semiconductor layer and the promoter.
The water splitting device according to any one of claims 1 to 3, wherein a molar ratio of Ga to a total molar amount of Ga and In in the CIGS compound semiconductor is 0.5 to 0.7.
A water splitting device that generates gas from the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation by irradiating light to the photocatalyst electrode for hydrogen generation and the photocatalyst electrode for oxygen generation,
A tank for filling the electrolytic solution;
The hydrogen generating photocatalytic electrode and the oxygen generating photocatalytic electrode disposed in the tank,
The photocatalytic electrode for hydrogen generation 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;
Water splitting in which the band offset ΔE, 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: apparatus.
ΔE = (n−CBM) − (p−CBM) ≦ 0.1 [eV]
A semiconductor layer including a CIGS compound semiconductor including Cu, In, Ga, and Se;
The photocatalytic electrode for water splitting, wherein the molar ratio of Ga to the total molar amount of Ga and In in the CIGS compound semiconductor is 0.4 to 0.8.