Joined body

Abstract

To provide a joined body in which a p-type semiconductor and an n-type semiconductor are combined and which can be used as a photocatalyst efficiently causing an oxidation-reduction reaction.SOLUTION: A joined body in which a Cu-based p-type oxide and an n-type oxide that is M1Fe2O4 or M2WO4 (M1 and M2 are each independently metal elements) are joined.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a conjugate. [Background technology]

[0002] Photocatalysts have attracted attention as a means of cleanly producing hydrogen from air or water. A known example of an article that can be used as a photocatalyst is a bonded structure that combines a p-type semiconductor and an n-type semiconductor. When light is irradiated onto the photocatalyst, electrons in the valence band are excited to the conduction band, generating holes in the valence band. These electrons / holes reduce / oxidize water molecules, allowing clean production of hydrogen and oxygen from water. Such photocatalysts are disclosed, for example, in Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2016-512164 [Patent Document 2] Special Publication No. 2017-528314 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a bonded structure in which a p-type semiconductor and an n-type semiconductor are combined, and which can be used as a photocatalyst that efficiently causes an oxidation-reduction reaction. [Means for solving the problem]

[0005] The present invention includes the following aspects: [Section 1] Cu-based p-type oxide semiconductor, M 1 Fe2O4 or M 2 WO4(M 1 and M 2are independently a metal element) and an n-type oxide semiconductor. [Section 2] Item 1. The conjugate according to item 1, which is a direct Z-scheme type conjugate. [Section 3] Item 3. The junction according to item 1 or 2, wherein a reduction reaction occurs site-selectively on the p-type oxide semiconductor and an oxidation reaction occurs site-selectively on the n-type oxide semiconductor. [Section 4] Item 4. The bonded body according to any one of items 1 to 3, wherein both the p-type oxide semiconductor and the n-type oxide semiconductor have a band gap of 3.0 eV or less. [Section 5] Item 5. The bonded body according to any one of items 1 to 4, wherein at least one of the p-type oxide semiconductor and the n-type oxide semiconductor has a band gap of 2.0 eV or less. [Section 6] Item 6. The bonded body according to any one of items 1 to 5, wherein at least one of the p-type oxide semiconductor and the n-type oxide semiconductor has a band gap of 1.8 eV or less. [Section 7] Item 7. The bonded body according to any one of items 1 to 6, wherein at least one of the p-type oxide semiconductor and the n-type oxide semiconductor has a particle size of 50 nm to 1000 nm. [Section 8] Item 8. The joined body according to any one of items 1 to 7, wherein the Cu-based p-type oxide semiconductor is CuO or Cu2O. [Section 9] M 1 is Zn and M 2 Item 9. The joined body according to any one of items 1 to 8, wherein is Cu. [Section 10] Item 10. The bonded structure according to any one of items 1 to 9, wherein a metal is sandwiched between the n-type oxide semiconductor and the p-type oxide semiconductor, and the metal is in ohmic contact with both semiconductors. [Section 11] Item 11. The bonded structure according to any one of items 1 to 10, which has a protective layer. [Section 12] Item 12. The conjugate according to any one of items 1 to 11, which comprises a co-catalyst. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide a joined body in which a p-type semiconductor and an n-type semiconductor are combined, and which can be used as a photocatalyst that efficiently causes an oxidation-reduction reaction. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing a schematic diagram of the energy gradient of a normal pn junction when the pn junction is irradiated with light. [Figure 2] FIG. 2 is a diagram schematically showing the energy gradient of a direct Z-scheme type bonded body when the bonded body is irradiated with light. [Figure 3] FIG. 3 is a diagram schematically showing the energy gradient of a p-type semiconductor when the semiconductor is irradiated with light in a photoelectrochemical measurement in water. [Figure 4] FIG. 4 is a diagram showing a schematic diagram of the energy gradient of a typical pn junction semiconductor when the semiconductor is irradiated with light in a photoelectrochemical measurement in water. [Figure 5] FIG. 5 is a diagram showing a schematic diagram of the energy gradient of the semiconductor when the direct Z-scheme junction is irradiated with light in a photoelectrochemical measurement in water. [Figure 6] FIG. 6 is a schematic diagram showing one preferred embodiment of the bonded body of the present invention. [Figure 7] FIG. 7 is a schematic diagram showing one preferred embodiment of the bonded structure of the present invention. [Figure 8] FIG. 8 is a schematic diagram showing one preferred embodiment of the bonded body of the present invention. [Figure 9] FIG. 9 shows scanning electron microscope (SEM) images of (A) ZnFe2O4 alone, (B) Cu2O alone, and (C) a ZnFe2O4-Cu2O joint. [Figure 10] FIG. 10 shows the results of analysis of the crystalline phase by X-ray diffraction (XRD) of (A) ZnFe2O4 alone and a ZnFe2O4-Cu2O joint body, and (B) a Cu-based p-type oxide semiconductor. [Figure 11] FIG. 11 is an image showing the results of STEM-EDS analysis of a ZnFe2O4-Cu2O joint. [Figure 12] Figure 12 shows the UV-vis measurement results of (A) ZnFe2O4, CuO, Cu2O, and CuWO4, and the Tauc plots of (B) Cu2O, (C) CuO, ZnFe2O4, and CuWO4. [Figure 13] FIG. 13 shows the results of photoelectrochemical measurements of (A) ZnFe2O4 alone and (B) a ZnFe2O4-Cu2O joint. [Figure 14] FIG. 14 shows the results of IV measurement of a ZnFe2O4-Cu2O joint body. [Figure 15] FIG. 15 shows the results of IV measurements of the Pt—ZnFe2O4—Au sample. [Figure 16] FIG. 16 shows (A) an SEM observation image and (B) the results of XRD measurement of an Fe2O3-Cu2O joint body. [Figure 17] FIG. 17 shows (A) an SEM observation image and (B) the results of XRD measurement of a WO3-Cu2O joint body. [Figure 18] FIG. 18 shows the results of (A) cyclic voltammetry, (B) chronoamperometry, (C) SEM observation images, and (D) XPS measurement of ZnFe2O4 alone. [Figure 19] FIG. 19 shows the results of (A) cyclic voltammetry, (B) chronoamperometry, and (C) SEM observation images of a ZnFe2O4-CuO joint body. [Figure 20] FIG. 20 shows the results of cyclic voltammetry for the TiO2-CuO joint. [Figure 21] FIG. 21 shows the results of IV measurement of a TiO2-CuO joint. DETAILED DESCRIPTION OF THE INVENTION

[0008] The junction of the present invention is a Cu-based p-type oxide semiconductor and M 1 Fe2O4 or M 2 WO4(M 1 and M 2 are junctions in which an n-type oxide semiconductor (which is a metal element) is combined with an n-type oxide semiconductor.

[0009] The Cu-based p-type oxide semiconductor contained in the bonded structure of the present invention is a p-type semiconductor containing a Cu-based oxide. Examples of Cu-based oxides include copper(I) oxide (CuO), copper(II) oxide (CuO), copper peroxide (CuO), mixed-valence compounds of copper oxide, and copper composite oxides.

[0010] The mixed valence compounds of copper oxide are Cu + and Cu 2+ in a given ratio, the formula Cu x The compound represented by the formula: O. x is in the range of 1 to 2, for example, x is 1.01 to 1.99, 1.05 to 1.95, 1.05 to 1.90, 1.05 to 1.80, 1.05 to 1.70, 1.05 to 1.60, 1.05 to 1.50, 1.05 to 1.40, 1.05 to 1.30, 1.05 to 1.20, 1.05 to 1.10, 1.10 ~1.95, 1.10~1.90, 1.10~1.80, 1.10~1.70, 1.10~1.60, 1.10~1.50, 1.10~1.40, 1.10~1.30, 1.10~1.20, 1.20~1.95, 1.20~1.90, 1.20~1.80, 1.20~1.70, 1.20~1. .60, 1.20~1.50, 1.20~1.40, 1.20~1.30, 1.30~1.95, 1.30~1.90, 1.30~1.80, 1.30~1.70, 1.30~1.60, 1.30~1.50, 1.30~1.40, 1.40~1.95, 1.40~1.90, 1.40~1.8 It can be in the range of 0, 1.40-1.70, 1.40-1.60, 1.40-1.50, 1.50-1.95, 1.50-1.90, 1.50-1.80, 1.50-1.70, 1.50-1.60, 1.60-1.95, 1.60-1.90, 1.60-1.80, 1.60-1.70, etc.

[0011] A copper composite oxide is an oxide containing copper and any element other than copper and oxygen, preferably a metal element. Examples of the metal element include Fe, Al, Mn, Cr, W, and Mo. Specific examples of copper composite oxides include, but are not limited to, CuFe2O4, CuAlO2, CuAl2O4, CuMn2O4, CuCr2O4, CuWO4, and CuMoO4.

[0012] The Cu-based p-type oxide semiconductor may contain one or more of the above-mentioned Cu-based oxides, but preferably contains one of them alone.

[0013] In one embodiment, the Cu-based p-type oxide semiconductor is CuFe2O4.

[0014] In a preferred embodiment, the Cu-based p-type oxide semiconductor is CuO or Cu2O.

[0015] The n-type oxide semiconductor constituting the junction of the present invention is M 1 Fe2O4 or M 2 WO4(M 1 and M 2 are each independently a metal element) and are an n-type oxide semiconductor.

[0016] The above M 1 Examples of the elements include Ti, Cr, Mn, Co, Ni, Zn, Mg, Ca, Sr, Ba, and Cd, with Zn being preferred.

[0017] The above M 2 Examples of the elements include Ti, Cr, Mn, Co, Ni, Zn, Mg, Ca, Sr, Ba, and Cd, with Cu being preferred.

[0018] In a preferred embodiment, the n-type oxide semiconductor is ZnFe2O4 or CuWO4.

[0019] The bonded structure of the present invention is a bonded structure in which the above-described p-type oxide semiconductor and n-type oxide semiconductor are bonded. In the present disclosure, "bonding" refers to a state in which at least a portion of two separable members are fixed to each other more firmly than by simple contact. Such "fixation more firmly than by contact" is not particularly limited, but may refer to a state in which the members are adhered, bonded, or bonded to each other by, for example, an anchor effect, intermolecular force, or chemical bond.

[0020] In one embodiment, the bonded structure of the present invention is a laminate formed by directly depositing an n-type semiconductor on a p-type semiconductor. In another embodiment, the bonded structure of the present invention may be a laminate formed by directly depositing a p-type semiconductor on an n-type semiconductor. In another embodiment, the bonded structure of the present invention may be a heteroparticle formed by bonding p-type semiconductor particles and n-type semiconductor particles.

[0021] It is preferable that no reaction phase is formed at the junction between the p-type semiconductor and the n-type semiconductor in the bonded body of the present invention. Here, the reaction phase is a phase formed by a reaction between the p-type semiconductor and the n-type semiconductor. If a reaction phase is formed at the junction between the p-type semiconductor and the n-type semiconductor, the photocatalytic properties of the bonded body may be impaired. For example, if the p-type semiconductor is Cu2O and the n-type semiconductor is Fe2O3 or WO3, a reaction phase will be formed during the process of bonding the two semiconductors, which is undesirable. The presence of a reaction phase at the junction may be detected, for example, by observation with an SEM or X-ray diffraction.

[0022] The bonded body of the present invention may have the property of decomposing water into oxygen and hydrogen upon irradiation with light. The mechanism by which such a property is exhibited is thought to be as follows, without being bound by theory.

[0023] When the bonded body of the present invention is irradiated with light in a certain wavelength range, (i) When the n-type and p-type semiconductors absorb light of a wavelength corresponding to their respective band gaps, electrons in the valence bands of both semiconductors are excited to the conduction band, generating holes in the valence bands; (ii) On the n-type semiconductor, oxygen is produced by an oxidation reaction between the holes generated in the n-type semiconductor and water; (iii) On a p-type semiconductor, hydrogen is produced by the reduction reaction of excited electrons in the p-type semiconductor with water; (iv) Recombination occurs between electrons excited on the n-type semiconductor and holes generated on the p-type semiconductor. The transfer of electrons and holes as in the above (i) to (iv) causes the oxidation-reduction reaction of water to proceed.

[0024] In order for the conjugate to exhibit photocatalytic properties in the water oxidation-reduction reaction, when the conjugate is irradiated with light, the holes generated in the n-type semiconductor must have an oxidizing power sufficient to oxidize water, and the excited electrons in the p-type semiconductor must have a reducing power sufficient to reduce water. To satisfy these conditions, the energy level of the valence band top (VBM) of the n-type semiconductor must be deeper than the potential at which water oxidation occurs (hereinafter also referred to as the "oxygen evolution potential"), and the energy level of the conduction band bottom (CBM) of the p-type semiconductor must be shallower than the potential at which water reduction occurs (hereinafter referred to as the "hydrogen evolution potential").

[0025] Therefore, in a preferred embodiment, the bonded body of the present invention has an n-type oxide semiconductor with a VBM energy level of −5.7 eV (vs. vacuum) or less, and a p-type oxide semiconductor with a CBM energy level of −4.4 eV (vs. vacuum) or more.

[0026] Rectifying pn junctions (hereinafter referred to as "normal pn junctions"), such as those widely used in diodes, do not necessarily exhibit photocatalytic activity toward water even if their energy levels satisfy the above-mentioned conditions. Without being bound by theory, the reason for this is believed to be as follows: When a p-type semiconductor and an n-type semiconductor are joined, the Fermi levels of the two semiconductors match, resulting in an energy gradient between the VBM and CBM of the two semiconductors near the pn junction interface. When light is irradiated, electrons are excited at both the p-type and n-type semiconductor sites, and holes are generated. Figure 1 is a schematic diagram showing the state of the pn junction at this time. As shown in Figure 1, this gradient is inclined in a direction that promotes the migration of electrons excited in the p-type semiconductor to the n-type semiconductor. When electrons excited in the p-type oxide semiconductor migrate to the n-type oxide semiconductor, they lose the reducing power necessary for generating hydrogen from water. Similarly, the gradient is in the direction that promotes the migration of holes generated in the n-type semiconductor to the p-type semiconductor, so that the holes lose the oxidizing power necessary to generate oxygen from water when they move to the p-type oxide semiconductor. This energy gradient at the junction occurs because the Fermi level of the n-type semiconductor is shallower than that of the p-type semiconductor. For these reasons, this pn junction is not suitable as a photocatalyst for water electrolysis.

[0027] On the other hand, when the Fermi level of the n-type semiconductor is equal to or deeper than that of the p-type semiconductor, a different phenomenon occurs. Figure 2 is a schematic diagram showing the state when a bonded structure of both semiconductors is irradiated with light. In this case, the energy gradient near the bonded interface, as shown in Figure 2, is inclined in a direction that prevents electrons excited in the p-type semiconductor from migrating to the n-type semiconductor. Therefore, electrons excited in the p-type oxide semiconductor can maintain high reducing power without migrating to the n-type oxide semiconductor. Similarly, the gradient is inclined in a direction that prevents holes generated in the n-type semiconductor from migrating to the p-type semiconductor, so the holes can maintain high oxidizing power without migrating to the p-type oxide semiconductor. Therefore, when the bonded structure is irradiated with light that can excite electrons in both semiconductors, it exhibits the behaviors (i) to (iv) described above, and therefore the bonded structure can be suitably used as a photocatalyst.

[0028] Therefore, in a preferred embodiment, the junction of the present invention has a Fermi level of the n-type oxide semiconductor that is equal to or deeper than the Fermi level of the p-type oxide semiconductor. In the present disclosure, the movement of electrons and holes as described in (i) to (iv) above is referred to as "Z-scheme excited carrier movement," and a junction that can form a Z-scheme structure by light irradiation due to such a relationship between the Fermi levels of both semiconductors is referred to as a "Z-scheme junction."

[0029] The Fermi levels of p-type and n-type semiconductors may be calculated as approximate values ​​by measuring the ionization potential of the semiconductor using, for example, XPS (X-ray photoelectron spectroscopy) and then calculating the CBM and VBM positions and carrier density estimated from the XPS.

[0030] Even when a normal pn junction is irradiated with light, Z-scheme excited carrier transfer does not occur for the reasons described above. However, by connecting a p-type semiconductor and an n-type semiconductor via a member called a mediator, Z-scheme excited carrier transfer becomes possible. In the present disclosure, a junction that requires a mediator to achieve excited carrier transfer as in the above (i) to (iv) is referred to as an "indirect Z-scheme junction." In contrast, a junction that does not require a mediator to achieve excited carrier transfer as in the above (i) to (iv) is referred to as a "direct Z-scheme junction." The junction of the present invention may preferably be a direct Z-scheme junction, and has the advantage that a mediator is not required when used as a photocatalyst for water.

[0031] Thus, in a preferred embodiment, the conjugate of the present invention is a direct Z-scheme type conjugate.

[0032] The fact that the conjugate of the present invention is a direct Z-scheme conjugate can be confirmed by photoelectrochemical measurement in water using a three-electrode system having a working electrode, a counter electrode, and a reference electrode. In photoelectrochemical measurement using a p-type semiconductor placed alone on the working electrode, the p-type semiconductor is irradiated with light capable of providing energy sufficient to excite electrons, and a negative potential (i.e., a lower potential on the working electrode) is applied. Figure 3 shows a schematic diagram of the state of the p-type semiconductor when the semiconductor is irradiated with light capable of providing energy sufficient to excite electrons and a negative potential is applied (i.e., a lower potential on the working electrode). As shown in Figure 3, the energy gradient at the p-type semiconductor / water interface is inclined in a direction that promotes the migration of holes toward the working electrode. A negative current resulting from the migration of holes toward the working electrode is observed. In this disclosure, the current generated by irradiating a semiconductor with light is referred to as a "photocurrent."

[0033] Conversely, when an n-type semiconductor is placed alone on the working electrode, the energy gradient at the n-type semiconductor / water interface is inclined in a direction that promotes the movement of electrons toward the working electrode, and a positive photocurrent is observed when light is irradiated.

[0034] On the other hand, Figure 4 shows the state of a typical pn junction in which an n-type semiconductor is sandwiched between a p-type semiconductor and a working electrode when a potential is applied under the same conditions as above. As shown in Figure 4, the energy gradient at the pn junction interface tends to move excited carriers generated by light irradiation of the pn junction in the opposite direction to that in the p-type semiconductor alone (i.e., tends to move holes generated in the p-type semiconductor in the direction away from the working electrode). Therefore, in this pn junction, photocurrent is not observed unless a potential larger in the negative direction is applied compared to the case of the p-type semiconductor alone. In other words, the onset potential of the pn junction can be larger than that of the p-type semiconductor alone.

[0035] In contrast, when a direct Z-scheme junction is formed by sandwiching an n-type semiconductor between a p-type semiconductor and a working electrode, the state of the junction when a potential is applied under the same conditions as above is shown in Figure 5. In this case, no depletion layer is formed at the junction interface, and the energy gradient at the junction interface tends to move excited carriers generated by light irradiation in the same direction as in the p-type semiconductor alone. Therefore, the onset potential of the direct Z-scheme junction can be equivalent to the onset potential of the p-type semiconductor alone.

[0036] In this way, by measuring and comparing the onset potential of the p-type semiconductor alone with that of the junction, it is possible to determine whether the junction is a normal pn junction or a direct Z scheme junction. The above-mentioned determination of a direct Z scheme junction by photoelectrochemical measurement may also be performed by measuring and comparing the onset potential of the n-type semiconductor alone with that of the junction. In this case, the junction is arranged so that the p-type semiconductor is sandwiched between the n-type semiconductor and the working electrode, and a potential is applied in the positive direction, allowing photocurrent to be detected using the same procedure as above.

[0037] In the present disclosure, the term "onset potential" refers to the potential at which photocurrent begins to be observed in photoelectrochemical measurement of a semiconductor.

[0038] Therefore, in a preferred embodiment, the junction of the present invention has an onset potential equivalent to that of the n-type semiconductor alone or the p-type semiconductor alone. In another preferred embodiment, the onset potential of the junction of the present invention can be −50 mV to +50 mV relative to the onset potential of the n-type semiconductor alone or the p-type semiconductor alone.

[0039] Another method for confirming that the junction of the present invention is a direct Z-scheme junction is to check whether the junction has rectification properties. Conventional pn junctions are known to exhibit rectification properties because a depletion layer formed at the junction interface prevents carrier movement under reverse bias. On the other hand, in the case of a Z-scheme junction, as mentioned above, no depletion layer is formed at the junction interface, allowing the same amount of current to flow under both forward and reverse bias.

[0040] Therefore, in a preferred embodiment, the junction of the present invention does not exhibit rectification. In another preferred embodiment, the absolute value of the current under forward bias of the junction of the present invention can be 90% to 110%, 92% to 108%, 95% to 105%, 97% to 103%, 99% to 101%, 99.5% to 100.5%, or 99.9% to 100.1% of the absolute value of the current under reverse bias.

[0041] Another method for confirming that the junction of the present invention is a direct Z-scheme junction is to confirm which semiconductors selectively undergo oxidation and reduction reactions when the junction is irradiated with light. As shown in Figure 1, in a typical pn junction, electrons excited on the p-type semiconductor migrate to the n-type semiconductor, accompanied by a decrease in reducing power, due to the energy gradient derived from the depletion layer at the pn junction interface. Therefore, in the pn junction, either a reduction reaction occurs on the n-type semiconductor, or a reduction reaction does not occur due to a decrease in the reducing power of the electrons. For the same reason, in the pn junction, either an oxidation reaction occurs on the p-type semiconductor, or an oxidation reaction does not occur due to a decrease in the oxidizing power of the holes. On the other hand, as shown in Figure 2, in a direct Z-scheme junction, electrons excited on the p-type semiconductor are inhibited from migrating to the n-type semiconductor due to the energy gradient at the junction interface, and are used for the reduction reaction on the p-type semiconductor. Similarly, holes generated on the n-type semiconductor are used for the oxidation reaction on the n-type semiconductor. That is, in a direct Z-scheme junction, a reduction reaction occurs site-selectively on the p-type semiconductor and an oxidation reaction occurs site-selectively on the n-type semiconductor. Therefore, for example, when the junction is irradiated with light in an aqueous solution containing two types of metal ions with different ionization tendencies, if a metal with a lower ionization tendency is deposited site-selectively on the p-type semiconductor and a metal oxide with a higher ionization tendency is deposited site-selectively on the n-type semiconductor, the junction can be determined to be a direct Z-scheme junction.

[0042] In this disclosure, "a reduction reaction occurs site-selectively on a p-type semiconductor" means that 90% or more of the reduction reaction products generated in the entire junction occur on the p-type semiconductor. Similarly, "an oxidation reaction occurs site-selectively on an n-type semiconductor" means that 90% or more of the oxidation reaction products generated in the entire junction occur on the n-type semiconductor.

[0043] The type of metal ion used in the above method is not particularly limited, and metal ions with a higher ionization tendency, such as Mn, Zn, Cr, Fe, Cd, Co, or Ni, may be used, or metal ions with a lower ionization tendency, such as Cu, Ag, or Hg.

[0044] In the above method, the method for detecting the metal or metal oxide deposited on the semiconductor is not particularly limited, and known methods such as energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) may be used.

[0045] Therefore, in a preferred embodiment, in the junction of the present invention, a reduction reaction occurs site-selectively on the p-type semiconductor and an oxidation reaction occurs site-selectively on the n-type semiconductor.

[0046] Another method for confirming that the junction of the present invention is a direct Z-scheme junction may be to observe the potential distribution at the interface between the p-type semiconductor and the n-type semiconductor using various microscopes such as a transmission electron microscope (TEM) or a scanning probe microscope (SPM).

[0047] The band gaps of the p-type semiconductor and n-type semiconductor of the junction of the present invention are not particularly limited, and may be, for example, 3.5 eV or less, 3.2 eV or less, 3.0 eV or less, 2.7 eV or less, 2.5 eV or less, 2.2 eV or less, 2.0 eV or less, or 1.8 eV or less, or may be 1.0 eV or more, or 1.5 eV or more.

[0048] In one embodiment, both the p-type semiconductor and the n-type semiconductor of the junction of the present invention have a band gap of 3.0 eV or less. When both semiconductors have a band gap of 3.0 eV or less, light with a longer wavelength than UV light (e.g., visible light) can be used when the junction is used as a photocatalyst. Since visible light has a higher irradiation intensity than UV light in sunlight, if the junction can utilize visible light, the solar light-to-hydrogen conversion efficiency (hereinafter referred to as STH) can be improved.

[0049] In a preferred embodiment, at least one of the p-type semiconductor and the n-type semiconductor of the junction of the present invention has a band gap of 2.0 eV or less. If the band gap is 2.0 eV or less, light with a wavelength of about 600 nm or more can be used for the redox reaction, thereby further improving the STH. In this case, it is more preferable that the band gaps of both the p-type semiconductor and the n-type semiconductor are 2.0 eV or less.

[0050] In a more preferred embodiment, at least one of the p-type semiconductor and the n-type semiconductor of the junction of the present invention has a band gap of 1.8 eV or less. If the band gap is 1.8 eV or less, light with a wavelength of approximately 700 nm or more can also be used in the redox reaction, thereby further improving the STH. In this case, it is more preferable that the band gaps of both the p-type semiconductor and the n-type semiconductor are 1.8 eV or less.

[0051] The band gap of a semiconductor may be measured by a known measurement method, and in the present invention, the band gap of a semiconductor can be measured by, for example, the method described in the Examples.

[0052] The bonded body of the present invention may be in any form, for example, in the form of a laminate or powder.

[0053] In one embodiment, the bonded body of the present invention is in the form of a laminate.

[0054] When the bonded structure of the present invention is in the form of a laminate, it may be that the entire one surface of an n-type semiconductor is bonded to the entire one surface of a p-type semiconductor, that a portion of one surface of an n-type semiconductor is bonded to the entire one surface of a p-type semiconductor, that the entire one surface of an n-type semiconductor is bonded to a portion of one surface of a p-type semiconductor, or that a portion of one surface of an n-type semiconductor is bonded to a portion of one surface of a p-type semiconductor. Furthermore, the laminate may have a substrate layer, a protective layer, a promoter layer, a surface treatment layer, etc. in addition to the n-type semiconductor layer and the p-type semiconductor layer.

[0055] In the bonded structure of the present invention, it is preferable that either the n-type semiconductor or the p-type semiconductor has a crystal grain size of 50 nm to 1000 nm. With a grain size within this range, when the bonded structure of the present invention is used as a photocatalyst, the number of photons incident on the bonded structure per unit time increases, and catalytic activity can be improved. The grain size is more preferably 50 nm to 500 nm, and even more preferably 50 nm to 200 nm, or 100 nm to 200 nm. It is also more preferable that the grain size of both the n-type semiconductor and the p-type semiconductor is within the above range.

[0056] In one embodiment, the bonded structure of the present invention may be in the form 10 shown in FIG. 6. That is, the bonded structure of the present invention may be in a form in which a first layer 11 containing crystals of either an n-type oxide semiconductor or a p-type oxide semiconductor is bonded to a second layer 12 containing crystals of the other oxide semiconductor. In this case, one or more of the other semiconductors may be bonded to one of the semiconductors. Such a bonded structure can be produced, for example, by coating one oxide semiconductor with the other oxide semiconductor.

[0057] In a preferred embodiment, the bonded structure of the present invention may be in the form 20 shown in FIG. 7 . That is, the bonded structure of the present invention may be in a form in which a specific crystal plane of an n-type oxide semiconductor crystal or a p-type oxide semiconductor crystal 20 is bonded to a specific crystal plane of the other oxide semiconductor crystal 21. In this case, one or more of the other semiconductors may be bonded to one of the semiconductors. The plane orientation of the specific crystal plane is not particularly limited, and may be, for example, any of the (100), (110), and (111) planes. The grain size of the crystal is not particularly limited, but is preferably, for example, 50 nm to 200 nm. Such a bonded structure can be produced, for example, by heteroepitaxially growing one oxide semiconductor on the other oxide semiconductor crystal.

[0058] In one embodiment, the conjugate of the present invention is in powder form.

[0059] When the bonded body of the present invention is in powder form, one particle of an n-type semiconductor or a p-type semiconductor may be bonded to one particle of the other semiconductor. The particle sizes of the two semiconductors may be the same or different. The bonded body of the present invention may also be in the form of, for example, a sheet- or rod-shaped p-type semiconductor or an n-type semiconductor carrying the other powdery semiconductor.

[0060] In one embodiment, the bonded body of the present invention may be in form 30 shown in Fig. 8. That is, the bonded body of the present invention may be in a form in which one or more powder particles of an n-type oxide semiconductor or a p-type oxide semiconductor are bonded onto powder particles of the other semiconductor. Such a bonded body can be produced, for example, by volatilizing the solvent under predetermined conditions in a solution in which powder of one semiconductor is dispersed and a powder raw material of the other semiconductor is dissolved.

[0061] In a preferred embodiment, the conjugate of the present invention is in powder form.

[0062] In a preferred embodiment, the bonded structure of the present invention is a bonded structure in which a conductor that forms ohmic contact with both semiconductors is sandwiched between an n-type oxide semiconductor and a p-type oxide semiconductor. By sandwiching a conductor that forms ohmic contact with both semiconductors in the bonded structure, carrier transfer efficiency can be improved, and catalytic activity can be enhanced.

[0063] The work functions of n-type semiconductors, p-type semiconductors, and conductors are Φ n , Φ m , Φ p Let's say, Φ n ≧Φ m And Φ m ≧Φ p Any conductor that satisfies this relationship will form an ohmic contact with both semiconductors. In the bonded structure of the present invention, examples of conductors that form an ohmic contact with both semiconductors include gold and carbon, which have a work function of about 5.0 eV. Among n-type semiconductors, those with a large work function can form an ohmic contact with gold or carbon.

[0064] When the bonded structure of the present invention is in the form of a laminate or powder, a part of the n-type semiconductor may be in direct bond with the p-type semiconductor, and another part of the n-type semiconductor may be connected to the p-type semiconductor via a conductor that is in ohmic contact with both semiconductors.

[0065] In a preferred embodiment, the conjugate of the present invention has a protective layer.

[0066] The protective layer may be formed on either the p-type semiconductor or the n-type semiconductor, or on both semiconductors. The bonded structure of the present invention is expected to be used in water as a photocatalyst, and providing such a protective layer can suppress deterioration of the bonded structure.

[0067] The protective layer may be any protective layer that can suppress deterioration in water while maintaining the photocatalytic performance of the joined body of the present invention. Examples of such protective layers include, but are not limited to, NiO, Ga2O3, Al2O3, TiO2, CeO2, YO3, and Cr2O3. These may be used alone or in combination of two or more.

[0068] The thickness of the protective layer is not particularly limited, but may be, for example, 5 nm to 50 nm. If the thickness of the protective layer is less than 5 nm, the protective layer will not adequately suppress deterioration of the bonded body, which is undesirable. On the other hand, if the thickness of the protective layer exceeds 50 nm, the photocatalytic performance of the bonded body may be impaired and defects such as cracks may easily occur in the bonded body, which is also undesirable.

[0069] The method for applying the protective layer is not particularly limited, and may be any known method such as spin coating, dip coating, spray coating, roll coating, blade coating, or slit coating.

[0070] In a preferred embodiment, the conjugate of the present invention comprises a promoter.

[0071] The co-catalyst may be supported on either the p-type semiconductor or the n-type semiconductor, or on both semiconductors. The presence of a co-catalyst in the conjugate of the present invention can improve its performance as a photocatalyst. Such a co-catalyst is not particularly limited as long as it has a different composition from the metal particles supported on the semiconductor particles, and various metal catalysts may be used. These co-catalysts may be used alone or in combination of two or more.

[0072] The metal catalyst is not particularly limited, and examples thereof include Cu, Ag, Au, Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, Os, Mn, W, Ta, Ti, Ce, Mg, Cd, Ga, In, and Pb. These metal catalysts may be used alone or in combination. Among these metal catalysts, metals belonging to Groups 8 to 11 of the periodic table (e.g., Cu, Ag, Au, Ni, Pd, Pt, Fe, Ru, Os, Co, Rh, and Ir) are preferred, with Pd, Ag, Pt, Rh, and Ru being more preferred, due to their ability to efficiently transfer electrons. These metals may all exist as elemental metals, or some of them may exist as oxides, hydroxides, or other forms of metals.

[0073] The amount of the co-catalyst supported is not particularly limited, but may be, for example, 0.01 to 50 parts by mass per 100 parts by mass of the semiconductor on which the co-catalyst is supported. If the amount of the co-catalyst supported is less than 0.01 part by mass, the co-catalyst will not be sufficiently effective. If the amount of the co-catalyst supported is more than 50 parts by mass, the co-catalyst may interfere with the light absorption of the assembly, resulting in a decrease in catalytic activity. If the amount of the co-catalyst supported is within the above range, the co-catalyst can suitably improve the photocatalytic performance of the assembly of the present invention.

[0074] <Method of manufacturing the bonded body> The bonded body of the present invention can be produced, for example, by the following method.

[0075] In one embodiment, the method for producing the bonded structure includes the steps of forming a first layer that is a Cu-based p-type oxide semiconductor on a substrate and forming a second layer that is an n-type oxide semiconductor on the first layer. In another embodiment, the method for producing the bonded structure includes the steps of forming a first layer that is an n-type oxide semiconductor on a substrate and forming a second layer that is a Cu-based p-type oxide semiconductor on the first layer.

[0076] The type of the substrate is not particularly limited, but may be an insulating or conductive substrate such as a sapphire substrate, a glass substrate, a Si substrate, or a GaN substrate.

[0077] The method for forming the first layer and the second layer is not particularly limited, but may be, for example, a method of applying a solution containing a predetermined semiconductor component onto a substrate or a semiconductor, followed by a drying step of heating on a hot plate or the like to volatilize the solvent, followed by baking under predetermined conditions. Alternatively, the first layer and the second layer may be formed by chemical vapor deposition (CVD), electrochemical deposition, or the like.

[0078] The predetermined semiconductor component may be the p-type oxide semiconductor or n-type oxide semiconductor of the bonded structure of the present invention itself, or may be a precursor of the p-type oxide semiconductor or n-type oxide semiconductor (hereinafter referred to as a semiconductor precursor). In the present disclosure, the semiconductor precursor refers to a component that can be converted into the p-type oxide semiconductor or n-type oxide semiconductor of the bonded structure of the present invention by a chemical reaction during the manufacturing process of the bonded structure (for example, during baking after applying a solution).

[0079] The solution can be prepared by dissolving the semiconductor component in a predetermined solvent. The type of solvent is not particularly limited, and any organic solvent capable of dissolving the semiconductor component may be used.

[0080] The method for applying the solution is not particularly limited, and the solution may be applied by a known method such as spin coating, dip coating, spray coating, roll coating, blade coating, or slit coating.

[0081] The conditions for the drying step are not particularly limited, but may be, for example, a temperature range near the boiling point of the solvent. By drying at such a temperature, the solvent is removed from the coating film. The drying step may be performed by heating on a hot plate or the like, or by using infrared rays, microwaves, electromagnetic induction, or the like.

[0082] The conditions for the firing are not particularly limited, but may be, for example, heating in an atmosphere of air, oxygen, nitrogen, argon, or the like at a temperature of 400 to 1000°C for several minutes to several hours. Since firing for a long period of time makes it difficult to obtain a dense film due to thermal diffusion, firing may be performed by inserting the sample into an electric furnace heated to a predetermined temperature or in an RTA furnace using infrared heating.

[0083] In another embodiment, the method for producing the junction includes the steps of preparing particles of either a p-type semiconductor or an n-type semiconductor, and supporting the other semiconductor on the p-type semiconductor.

[0084] In the above method, for example, the previously prepared n-type semiconductor ZnFe2O4 is dispersed in an aqueous solution containing copper nitrate, heated at a temperature of 60 to 90°C for a certain period of time to volatilize the solvent, and then heated at a temperature of 400 to 1000°C for several minutes to several hours in a N2 atmosphere, thereby supporting the p-type semiconductor Cu2O on the n-type semiconductor.

[0085] The method for preparing the ZnFe2O4 is not particularly limited, but may be, for example, the following method. Zinc chloride and sodium boron hydroxide are added to water heated to approximately 70°C to 90°C, and the mixture is stirred. The resulting powder is washed by repeated centrifugation and then heated to an elevated temperature, yielding ZnO powder, the precursor to ZnFe2O4. Iron chloride and ascorbic acid are dispersed in an aqueous solution containing this powder, after which hydrazine hydrate is added and the mixture is stirred for a predetermined period of time. The stirred solution is sealed in an autoclave and heated at approximately 150°C to 250°C for 8 to 24 hours to obtain ZnFe2O4 by hydrothermal synthesis.

[0086] The conjugate of the present invention has been described in detail above. Note that the conjugate of the present invention is not limited to those exemplified above.

Example

[0087] Hereinafter, examples of the present invention will be specifically described. However, unless otherwise specified in the specification, the examples do not limit the present invention.

[0088] In the present invention, the measurement of each physical property was performed as follows.

[0089] <X-ray diffraction method (XRD)> Using an X-ray diffractometer SmartLab3 (manufactured by Rigaku Corporation), the crystal phase of each sample was analyzed. Regarding the diffraction peaks of CuKα rays, 2θ-θ scans were performed. For the samples, the films prepared by spin coating were fixed to the sample stage together with the substrates. When fixing, clay was used to adjust the measurement surface. The scan range was 20° to 80°, and the scan speed was 5° / min. The crystal phase was identified by comparing the peak list in the crystal phase database with the measured peaks. It was determined whether crystals other than the target crystal phase were detected.

[0090] <Scanning electron microscope (SEM)> Using a scanning electron microscope S5000 (manufactured by Hitachi High-Technologies Corporation), cross-sectional images of each sample were obtained. A part of the sample was broken, and the fracture surface was observed. The acceleration voltage was 5 kV, and the observation magnification was 10,000 to 100,000 times. The laminated structure, particle size, etc. could be confirmed, and it was determined whether the layer boundaries were clear.

[0091] <Ultraviolet-visible spectroscopy (UV-vis) The absorbance wavelength of each sample was measured using a UV-visible spectrophotometer (Shimadzu Corporation). The wavelength of the incident laser was continuously changed, and the wavelength dependency of the sample's absorbance was evaluated from the intensity of the transmitted light relative to the incident light. The scanning wavelength was 900nm to 190nm, and the scanning speed was 10nm / s. Since the sample can absorb light with energy greater than its band gap (the shorter the wavelength, the greater the energy), it can be seen that the transmitted light intensity decreases (the absorbance increases) when the wavelength is shorter than a certain point. Conversely, the band gap of the sample can be estimated from the rise in absorbance. The Tauc plot is often used to determine the band gap, where the x-axis is hν and the y-axis is (αhν) n The UV-vis results are plotted as follows. h is Planck's constant, and ν is the frequency. ν is calculated by dividing the speed of light c by the wavelength λ. This allows the wavelength λ, which is the x-axis of the UV-vis, to be converted to hν, which is the horizontal axis of the Tauc plot. For n, the band gap of each material is 2 for direct transitions and 1 / 2 for indirect transitions, and is calculated as (αhν) n α is the absorption coefficient, which can be calculated by dividing the absorbance A, which is the vertical axis of UV-vis, by the thickness t and 0.434. 0.434 is log 10 e, which is used to convert from the base of the natural logarithm e to the base of the common logarithm 10. Dividing off this 0.434 has almost no effect on the shape of the Tauc plot. Draw tangent lines before and after the rise of this Tauc plot, and the x-coordinate of the intersection is the band gap.

[0092] <Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (STEM-EDS)> Using STEM-EDS, elemental mapping images of the cross section of each sample were obtained. Samples were obtained by thinly slicing the cross section using a special technique. By performing mapping analysis using EDS, the composition of each layer and the presence or absence of interdiffusion were confirmed.

[0093] <Reference Example 1-1a: Preparation of ZnFe2O4 alone> ZnFe2O4, an n-type oxide semiconductor, was fabricated using the following procedure.

[0094] Zn and Fe acetates were dissolved in an organic solvent at a Zn:Fe ratio of 2:1. A ZnFe2O4 film was formed on quartz glass using a spin coater (Mikasa Corporation), and the solvent was evaporated by heat treatment on a hot plate. The above film formation and solvent evaporation process was repeated to adjust the thickness of the ZnFe2O4 film. This sample was baked in an RTA furnace under an O2 atmosphere at 700°C for 10 minutes.

[0095] <Reference Example 1-1b: Preparation of CuO alone> Cu2O, a p-type oxide semiconductor, was prepared by the following procedure.

[0096] Cu acetate was dissolved in an organic solvent. A CuO film was formed on quartz glass using a spin coater (Mikasa Corporation), and the solvent was evaporated by heat treatment on a hot plate. The above film formation and solvent evaporation procedures were repeated to adjust the thickness of the CuO film. This sample was then baked in an RTA furnace under a N2 atmosphere at 700°C for 10 minutes.

[0097] <Example 1-2: Preparation of ZnFeO-CuO joint body> A ZnFe2O4-Cu2O joint body was fabricated by the following procedure.

[0098] A ZnFe2O4 film was formed on quartz glass using the same procedure as in Reference Example 1-1a, followed by a Cu2O film formed on the ZnFe2O4 film using the same procedure as in Reference Example 1-1b.

[0099] The ZnFe2O4, Cu2O, and ZnFe2O4-Cu2O joints were subjected to SEM observation and XRD measurement. The results are shown in Figures 9 and 10. As shown in Figure 9(C) and Figure 10(A), no reaction phase was formed at the joint interface of the joints, indicating that this combination may be suitable for the joints of the present invention.

[0100] Furthermore, the results of STEM-EDS analysis of the ZnFe2O4-Cu2O joint are shown in Figure 11. As shown in the STEM and EDS images in Figure 11, the layer containing Fe and Zn and the layer containing Cu are clearly separated, and no reaction phase was observed at the joint interface.

[0101] Furthermore, UV-vis measurements were performed on the above ZnFe2O4 and Cu2O alone. The results are shown in Figure 12(A). The Tauc plots are shown in Figure 12(B) (Cu2O, a direct transition semiconductor) and (C) (CuO, ZnFe2O4, and CuWO4, which are indirect transition semiconductors). Tangent lines were drawn before and after the rise of the Tauc plot, and the intersections revealed that the band gaps of ZnFe2O4 and Cu2O were 1.8 eV and 2.5 eV, respectively.

[0102] <Photoelectrochemical measurement> Photoelectrochemical measurements were carried out on each semiconductor in the junction of the present invention. A silver-silver chloride electrode was used as the reference electrode, and the potential of the working electrode (measurement sample) was controlled for comparison. In the case of a p-type electrode, a negative photoresponse current was observed when the potential was controlled to be more negative than the onset potential. On the other hand, in the case of an n-type electrode, a positive photoresponse current was observed when the potential was controlled to be more positive than the onset potential. A 0.1 M aqueous solution of sodium sulfate (pH = 6) was used as the electrolyte. In Figure 13, the potential was calculated based on the silver-silver chloride electrode reference potential (E Ag / AgCl , unit: V vs Ag / AgCl). This is calculated using the following formula: RHE , unit: V vs RHE). In this case, since the pH is 6, 0 V vs Ag / AgCl is approximately 0.55 V vs RHE. TIFF2026020096000002.tif8164

[0103] (Reference Example 1-3a: Preparation of a sample for photoelectrochemical measurement of ZnFe2O4 alone) A sample of ZnFe2O4 alone for photoelectrochemical measurements was prepared using the following procedure.

[0104] In and Sn acetates were dissolved in an organic solvent so that the Sn concentration was 0.5 mol%. A Sn-doped In2O3 (ITO) film was formed on quartz glass using a spin coater (Mikasa Corporation), and the solvent was evaporated by heat treatment on a hot plate. The above film formation and solvent evaporation process was repeated to adjust the thickness of the ITO film. This sample was then baked in an RTA furnace under a N2 atmosphere at 800°C for 10 minutes.

[0105] After the ITO film was formed, a ZnFe2O4 film was formed on the ITO film using the same procedure as in Reference Example 1-1a, with part of the ITO film masked. The ZnFe2O4 film was not formed on the masked area, leaving the ITO exposed after the film formation process. Subsequently, Ag ink was used to connect a conductor to the exposed ITO area. To prevent the exposed ITO area and the metal part of the conductor from coming into contact with water, the conductor was covered with epoxy resin.

[0106] Photoelectrochemical measurements were performed on this measurement sample while irradiating it with ultraviolet light from a UV lamp (manufactured by Hamamatsu Photonics KK) using a potentiostat manufactured by Hokuto Denko.

[0107] (Example 1-3b: Preparation of a sample of ZnFe2O4-Cu2O joint for photoelectrochemical measurement) Using the same procedure as in Reference Example 1-3a, ITO was deposited on quartz glass, and a ZnFe2O4 film was deposited on the ITO film. Then, using the same procedure as in Reference Example 1-1b, a Cu2O film was deposited on the ZnFe2O4 film. Subsequently, a lead wire was connected to the exposed ITO using Ag ink. To prevent the exposed ITO and the metal part of the lead wire from coming into contact with water, they were covered with epoxy resin. Photoelectrochemical measurements were performed on this measurement sample using the same procedure as in Reference Example 1-3a.

[0108] Figure 13 shows the results of photoelectrochemical measurements of the ZnFe2O4 and ZnFe2O4-Cu2O junctions. Photocurrent was observed in both the n-type oxide semiconductor (A) and the junction (B). The n-type oxide semiconductor exhibited a positive photocurrent, indicating its function as a photoanode. The junction exhibited a negative photocurrent, similar to that of Cu2O, indicating its function as a photocathode. The onset potential of the junction was 0.05 V vs. Ag / AgCl, which corresponds to a reversible hydrogen electrode potential of approximately 0.6 V vs. RHE. This is similar to the onset potential of Cu2O alone, as previously reported. An example of this is provided in the supplementary material (Supplementary Figure 7) of the following scientific paper. L.Pan, JHKim, MTMayeretal., NatCatal1, 412-420 (2018).

[0109] <Example 1-4: IV measurement of ZnFe2O4-Cu2O joint body> After preparing a ZnFe2O4-Cu2O joint using the procedure described in Example 1-2, a mask pattern was placed on the Cu2O film and gold was vapor-deposited. The vapor-deposited sample was then measured for current vs. voltage characteristics between the ITO and Au terminals using a source meter (manufactured by Casely) according to the following procedure. The ITO side in contact with the ZnFe2O4 was set as the negative potential side, and the Au side in contact with the Cu2O was set as the positive potential side, and the sample was connected to the source meter.

[0110] The results of the IV measurements are shown in Figure 14. In a typical pn junction, the current increases as the forward bias (application of a positive potential to the Au side) increases, but almost no current flows when the reverse bias (application of a negative potential to the Au side) is small. This difference in current behavior between forward and reverse bias is called rectification. It was found that the ZnFe2O4-Cu2O junction in this example does not exhibit rectification and is an ohmic junction.

[0111] <Reference Example 1-5: Hall effect measurement of CuO> After preparing a p-type oxide semiconductor, CuO, using the same procedure as in Reference Example 1-1b, a mask pattern was placed on the CuO film, and Au was evaporated. The carrier concentration and mobility of the evaporated sample were measured using a ResiTest8400 resistivity / Hall measurement system (manufactured by Toyo Corporation).

[0112] From the above measurements, the carrier concentration of Cu2O was found to be 14, the carrier mobility was 35, and the resistivity was 500 Ω·cm.

[0113] <Reference Example 1-6: Pt rectification of ZnFe2O4> Platinum was sputtered onto quartz glass. Next, a ZnFe2O4 film was formed on the platinum film using the procedure described in Reference Example 1-3a. A mask pattern was then placed over the ZnFe2O4 film, and Au was then vapor-deposited. The current vs. voltage characteristics of the vapor-deposited sample between the Pt-Au terminals were measured using the same procedure as in Example 1-4. The terminals were connected so that Au was at the high potential and Pt was at the low potential.

[0114] The measurement results are shown in Figure 15. From Figure 15, it can be seen that the current increases as the bias voltage is applied to the lower potential side. This means that rectification is exhibited between Pt / ZnFe2O4 on the lower potential side. In other words, it was shown that ZnFe2O4 has a Schottky contact with Pt but an ohmic contact with Au. The fact that ZnFe2O4 has a Schottky contact with Pt, which has a deep work function, indicates that ZnFe2O4 is an n-type semiconductor. Furthermore, the fact that ZnFe2O4 has an ohmic contact with Au suggests that the Fermi level of ZnFe2O4 is deep. From the above, it was found that ZnFe2O4 can be suitably used as an n-type oxide semiconductor in the junction of the present invention.

[0115] <Reference Example 1-7a: Confirmation of the presence or absence of Co photoprecipitation in ZnFe2O4> In order to confirm that a photooxidation reaction occurs on ZnFe2O4, an n-type oxide semiconductor, the following experiment was carried out. First, a sample for photoelectrochemical measurement of ZnFe2O4 alone was prepared using the same procedure as in Reference Example 1-3a.

[0116] The sample was placed in water and subjected to cyclic voltammetry while periodically cycling the light irradiation on and off. The results of the third cycle are shown in Figure 18(A). Next, the sample was placed in a phosphate buffer solution containing a small amount of cobalt(II) acetate and irradiated with light at a constant potential of 0.8 V vs. RHE for 300 seconds. The results are shown in Figure 18(B). As shown in Figures 18(A) and (B), a positive photocurrent was observed when the n-type oxide semiconductor ZnFe2O4 was irradiated with light in water. After the light irradiation, the ZnFe2O4 surface was observed by SEM, revealing white particles less than 10 nm in size. An SEM image is shown in Figure 18(C). Furthermore, the results of XPS analysis of the surface are shown in Figure 18(D). As shown in Figure 18(D), Co with an oxidation state of 2 or higher (e.g., cobalt(III) oxide) was detected on the sample surface. This confirmed that the white particles observed in Figure 18(C) were oxidized and precipitated Co. From the above, it was found that a photooxidation reaction occurs on an n-type oxide semiconductor.

[0117] <Example 1-7b: Confirmation of the Presence or Absence of Co Photoprecipitation on CuO on ZnFeO> To confirm that the redox reaction occurs site-selectively in the junction of the present invention, the following experiment was carried out. First, a sample for photoelectrochemical measurement of a ZnFeO-CuO junction was prepared in the same manner as in Example 1-3b, except that the heat treatment was carried out in an O atmosphere instead of an N atmosphere to convert the p-type oxide semiconductor into CuO instead of CuO.

[0118] The sample was placed in water and subjected to cyclic voltammetry while periodically cycling light on and off. The results of the third cycle are shown in Figure 19(A). Next, the sample was placed in a phosphate buffer solution containing a small amount of cobalt(II) acetate and exposed to light at a constant potential of 0.8 V vs. RHE for 300 seconds. The results are shown in Figure 19(B). As shown in Figures 19(A) and (B), a negative photocurrent was observed when the ZnFe2O4-CuO composite was exposed to light in water. After the exposure, the CuO surface of the sample was observed by SEM. CuO crystals were observed, but no white particles smaller than 10 nm were observed. The SEM image is shown in Figure 19(C). Furthermore, XPS analysis of the surface indicated that Co was at the detection limit, suggesting that little Co had precipitated. Based on these findings, Co with an oxidation state of 2 or higher (e.g., cobalt(III) oxide) was detected on the surface of the Co-CuO sample. This indicates that the white particles observed in Figure 18(C) are oxidized and precipitated Co. From the above, it was found that Co does not oxidize and precipitate in CuO on ZnFe2O4, that is, a reduction reaction occurs selectively on a p-type oxide semiconductor, while a photo-oxidation reaction occurs selectively on an n-type oxide semiconductor.

[0119] As shown in Figure 19(A), the onset potential of the ZnFe2O4-CuO joint was approximately 1.0 V vs. RHE. This onset potential is equivalent to the reported example of CuO alone, shown in Figure 3b (Q. Zhang, B. Zhai, Z. Lin et al., International Journal of Hydrogen Energy 46, 11607-11620 (2021)). This indicates that the ZnFe2O4-CuO joint is a direct Z-scheme joint.

[0120] <Comparative Example 1-1: Example 1 of a failed heterojunction (Fe2O3-Cu2O junction)> Fe 3+A solution containing ions was prepared and applied to a quartz glass substrate using a spin coater (Mikasa), and the solvent was evaporated by heat treatment on a hot plate. The film thickness was adjusted by repeating the process of applying the solution and evaporating the solvent. The Fe2O3 film was then baked using an RTA furnace (ULVAC). The baking conditions were 700°C and 10 minutes (O2 atmosphere). A Cu2O film was formed on the Fe2O3 film using the same procedure as in Reference Example 1-1b.

[0121] The Fe2O3-Cu2O joint was subjected to SEM observation and XRD measurement. The results are shown in Figure 16. (B) As the XRD results show, peaks of CuFe2O3 and CuFeO2 were observed at the joint interface of the joint, indicating the formation of a reaction phase. Therefore, it was shown that the Fe2O3-Cu2O joint is not suitable as a joint of the present invention.

[0122] <Comparative Example 1-2: Example 2 of a failed heterojunction (WO3-Cu2O junction)> W 6+ A solution containing ions was prepared and applied to a quartz glass substrate using a spin coater (Mikasa), and the solvent was evaporated by heat treatment on a hot plate. The process of applying the solution and evaporating the solvent was repeated to adjust the film thickness. Subsequently, a WO - The three films were baked at 700°C for 10 minutes (in an O2 atmosphere). A Cu2O film was formed on the WO3 film in the same manner as in Reference Example 1-1b.

[0123] The WO3-Cu2O joint was subjected to SEM observation and XRD measurement. The results are shown in Figure 17. (B) As the XRD results show, a CuWO4 peak was observed at the joint interface of the joint, indicating the formation of a reaction phase. Therefore, it was shown that the WO3-Cu2O joint is unsuitable for the joint of the present invention.

[0124] Comparative Example 1-3a: Photoelectrochemical Measurement of CuO on TiO An ITO film was formed on quartz glass using the same procedure as in Reference Example 1-3a. A Ti solution was prepared by diluting Ti alkoxide in 2-methoxyethanol. Next, with a portion of the ITO film masked, a TiO film was formed on the ITO film using a spin coater (manufactured by Mikasa Co., Ltd.), and the solvent was evaporated by heat treatment on a hot plate. The above film formation and solvent evaporation procedures were repeated to adjust the thickness of the TiO film. This sample was baked in an RTA furnace under an O atmosphere at 700°C for 10 minutes. Next, Cu acetate was dissolved in an organic solvent, and a CuO film was formed on the TiO film using a spin coater (manufactured by Mikasa Co., Ltd.), and the solvent was evaporated by heat treatment on a hot plate. The above film formation and solvent evaporation procedures were repeated to adjust the thickness of the CuO film. This TiO2-CuO joint sample was baked in an RTA furnace under an O atmosphere at 700°C for 10 minutes. Next, a lead wire was connected to the exposed ITO area using Ag ink. To prevent the exposed ITO area and the metal part of the lead wire from coming into contact with water, the sample was covered with epoxy resin to prepare a sample for photoelectrochemical measurement.

[0125] The sample was placed in water and subjected to cyclic voltammetry while periodically turning on and off light irradiation. The results of the third cycle are shown in Figure 20. As shown in Figure 20, both positive and negative photocurrents were observed upon light irradiation of the sample. Furthermore, the onset potential of CuO (reduction current) was approximately 0.7 V vs. RHE, indicating that a more negative potential was required compared to Example 1-7b. In other words, the onset potential of CuO was worse than that of Example 1-7b. Furthermore, Figure 20 also shows that a positive photocurrent was observed at a potential of 0.8 V vs. RHE, for example. This indicates that irradiating the sample in a Co solution at a constant potential of 0.8 V vs. RHE can result in the oxidative deposition of Co on the CuO. Thus, unlike Example 1-7b, the photoresponse of the surface CuO is inhibited when the n-type oxide semiconductor is TiO2.

[0126] <Comparative Example 1-3b: IV Measurement of TiO2-CuO Joint> A TiO2-CuO composite sample was prepared using the same procedure as in Comparative Example 1-3a, and then an Au electrode was deposited using the same procedure as in Example 1-4, followed by an IV measurement. The results are shown in Figure 21. As shown in Figure 21, rectification was observed in this sample. In composites with such rectification, the photocatalytic reaction was inhibited, which is thought to be the cause of the deterioration of the onset potential. [Industrial Applicability]

[0127] The bonded body of the present invention can be suitably used as a photocatalyst in a wide variety of applications.

Claims

1. a Cu-based p-type oxide semiconductor; M １ Fe ２ O ４ or M ２ WO ４ (M １ and M ２ and each independently represent a metal element) and an n-type oxide semiconductor.

2. The conjugate of claim 1 , which is a direct Z-scheme type conjugate.

3. 3. The junction according to claim 1, wherein a reduction reaction occurs site-selectively on the p-type oxide semiconductor and an oxidation reaction occurs site-selectively on the n-type oxide semiconductor.

4. 4. The bonded structure according to claim 1, wherein both the p-type oxide semiconductor and the n-type oxide semiconductor have a band gap of 3.0 eV or less.

5. 5. The bonded body according to claim 1, wherein at least one of the p-type oxide semiconductor and the n-type oxide semiconductor has a band gap of 2.0 eV or less.

6. 6. The bonded body according to claim 1, wherein at least one of the p-type oxide semiconductor and the n-type oxide semiconductor has a band gap of 1.8 eV or less.

7. 7. The bonded body according to claim 1, wherein at least one of the p-type oxide semiconductor and the n-type oxide semiconductor has a particle size of 50 nm to 1000 nm.

8. The Cu-based p-type oxide semiconductor is CuO or Cu ２ The conjugate according to any one of claims 1 to 7, wherein the aryl group is O.

9. M １ is Zn, and M ２ The joined body according to any one of claims 1 to 8, wherein is Cu.

10. 10. The bonded structure according to claim 1, wherein a metal is sandwiched between the n-type oxide semiconductor and the p-type oxide semiconductor, the metal being in ohmic contact with both semiconductors.

11. The bonded body according to any one of claims 1 to 10, which has a protective layer.

12. The composite according to any one of claims 1 to 11, which comprises a promoter.

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