JP5344731B2 - Visible light responsive semiconductor device and photoelectrode, and light energy conversion system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high performance photo-response semiconductor element using a group of new semiconductors which show response of a cathode photoelectric current with p type property or an anode photoelectric current with n type property in high efficiency and which is stable. <P>SOLUTION: The semiconductor element has a porous thin film of a compound oxide semiconductor containing copper and further at least one element M selected from a group of bismuth, nickel, silicon, titanium, yttrium, alkali earth metal, and lanthanoid formed on a conductive substrate, and has visible light response showing a cathode photoelectric current and/or anode photoelectric current. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a copper-based semiconductor photoelectrode and a light energy conversion system using the same.

Since the report of photoelectrochemical water splitting using n-type titanium dioxide (TiO ₂ ) photoelectrodes, oxide semiconductor photoelectrode systems that produce hydrogen by splitting water using light energy have attracted attention. In order to efficiently use sunlight, development of a visible light responsive semiconductor material is urgently required.
Research using non-oxide semiconductors such as silicon (Si) and gallium phosphide (GaP) is also active, but these are unstable and high in manufacturing cost. On the other hand, a complex oxide semiconductor partially containing metal oxide and oxygen has many stable materials and has an advantage of low manufacturing cost because it can be manufactured relatively easily. In order to use sunlight, it is necessary to increase the area, and the advantage of such an oxide semiconductor is important.
In addition, the electrode structure is also very important for increasing the area and putting it to practical use. The conventional photoelectrode is a pellet type in which the semiconductor powder raw material is baked and hardened, but has a drawback that it is difficult to increase the area. Furthermore, since the pellets are as thick as millimeters, the distance of movement of electrons and holes was large, and charge recombination occurred before the reaction occurred on the electrode surface, resulting in poor efficiency of the electrolysis reaction.

By the way, in recent years, several research examples of creating a porous oxide semiconductor thin film on a conductive glass by a wet method have been reported (for example, see Patent Document 1 and Non-Patent Documents 1 to 4). By using conductive glass, light can be irradiated through the conductive glass, and the distance of charge movement can be shortened. Therefore, it is expected that efficiency can be improved. In addition, since the thin film is in a porous state, the electrolytic solution can penetrate into the thin film, and similarly, the moving distance of the counter charge can be shortened. Therefore, it is expected that the efficiency can be improved.
In order to produce a highly efficient porous oxide semiconductor thin film by a wet method using this structure, the selection of elements is important. Depending on the element, the wet method of thermally decomposing the metal precursor solution is not suitable, and the composite oxide cannot be synthesized, or it is not crushed to form a film, and as a result, the efficiency is often very low. In addition, the absorption wavelength and the semiconductor band structure are determined depending on the types of constituent elements of the composite oxide semiconductor. For solar energy conversion, a semiconductor material that can utilize absorption at a long wavelength as much as possible and has a high quantum yield is desired. However, there are so many kinds of composite semiconductors that they have not been sufficiently searched.
There is a pn characteristic as an important element of semiconductor characteristics. In general, oxides and complex oxide semiconductors are mostly n-type, and there are very few examples of semiconductor photoelectrodes exhibiting p-type characteristics. For example, copper oxide (Cu ₂ O) is unstable and photodecomposes. Moreover, although there are some reports on iron (Fe) -based p-type complex oxide semiconductors (see, for example, Non-Patent Document 5), the photoelectric conversion efficiency is low and the stability is unclear. If there is a stable p-type semiconductor material, the bias is improved by combining a p-type semiconductor electrode as a cathode electrode for generating hydrogen and the like, and an n-type semiconductor electrode as an anode electrode for generating oxygen and the like. Many applications such as the construction of a water splitting system that does not require much can be expected. If the same oxide system is used, the photocatalyst using a combination of p-type and n-type can be easily used by calcination. However, the types of p-type complex oxide semiconductors are very limited and have problems such as potential matching, which has been an obstacle to research in this field. Moreover, although several patent documents regarding a porous thin film electrode are open | released (for example, refer patent document 1), all are related to an n-type semiconductor, Moreover, the semiconductor containing copper is also Cu-In-Zn type | system | group. Only.
Thus, it cannot be said that high-performance p-type oxide semiconductors are sufficiently developed using currently known materials, and new material development is desired.

JP 2005-44758 A C. Santoto, M. Ulmann, J. Augustynski, “J. Phys. Chem. B”, 2001, Vol. 105, p. 936. C. Santoto, M. Odziemkowski, M. Ulmann, J. Augustynski, “J. Am. Chem. Soc.”, 2001, Vol. 123, p.10639. T. Lindgren, H. Wang, N. Beermann, L. Vayssieres, A. Hagfeldt, S. Lindquest, “Sol. Energy Mater. Sol. Cells”, 2002, Vol.71, p.231. G.Zhao, H.Kozuka, H.Lin, M.Takahashi, T.Yoko, “Thin Solid Films”, 1999, Vol.340, p.125. “J. Solid State Chem.”, 1996, Vol. 126, p.227-234.

  The present invention provides a high-performance photoresponsive semiconductor element that uses a group of stable new semiconductors that exhibit a high-efficiency response of cathode photocurrent that is p-type characteristics or anode photocurrent that is n-type characteristics. For the purpose.

As a result of searching for materials and intensive studies, the present inventors have found that the composite oxide semiconductor thin film containing copper and a specific element M has very excellent photocurrent response. The present invention has been made based on such findings.
That is, the present invention is,
(1 ) A porous thin film of a composite oxide semiconductor containing copper and further containing at least one element M selected from the group consisting of bismuth, nickel, silicon, titanium, yttrium, alkaline earth metal, and lanthanoid is conductive. A semiconductor element formed on a substrate, wherein the composite oxide semiconductor is CuBi ₂ O ₄ , CuNi ₂ O ₃ , CuNiO ₂ , CuSiO ₃ , Cu ₃ TiO ₄ , CuYO _2.5 , CuBa ₂ O ₃ , CuCaO _2. , CuLaO _2.5 , or CuMgO ₂ , a visible light-responsive semiconductor element characterized by exhibiting a cathode photocurrent and / or an anode photocurrent,
(2) In the semiconductor device shown both the cathode beam current and anode photocurrent, characterized in that the switches and the anode and the cathode by the potential (1) The semiconductor device according to claim,
( 3 ) The semiconductor element according to (1) or (2), which is produced by a wet method using a solution in which a metal salt of the precursor of the composite oxide semiconductor is dissolved in a solvent,

( 4 ) A photoelectrode comprising the semiconductor element according to any one of (1) to ( 3 ),
( 5 ) The photoelectrode according to item ( 4 ), wherein a p-type semiconductor and an n-type semiconductor are combined.
( 6 ) A photoenergy conversion system that generates light by irradiating the photoelectrode according to the item ( 4 ) or ( 5 ) with light and reducing water on the semiconductor using a cathode photocurrent, and ( 7 ) The present invention provides a light energy utilization system that irradiates light to the photoelectrode described in ( 4 ) or ( 5 ), and oxidizes, reduces, or decomposes harmful substances and organic substances.

The semiconductor element of the present invention is excellent in photocurrent responsiveness with respect to a cathode photocurrent having p-type characteristics or an anode photocurrent having n-type characteristics. Further, the semiconductor element of the present invention can be preferably used as a photoelectric conversion element, can be applied to a photoelectrode, can be switched between an anode and a cathode according to a potential, and is a switching element that controls n-type or p-type. Can also be used.
Moreover, light can be irradiated to the photoelectrode of the present invention, and the generated cathode photocurrent can be used to reduce water and generate hydrogen on the photoelectrode of the present invention, thereby converting light energy into hydrogen. Furthermore, similarly, the photoelectrode of the present invention can be used to oxidize, reduce, and decompose harmful substances and organic substances using light energy.
The semiconductor element of the present invention can be applied not only to wet solar cells but also to solid solar cells and photocatalysts using a combination of p-type and n-type.

Hereinafter, the present invention will be described in detail.
The semiconductor element of the present invention is a visible light responsive semiconductor element in which a porous thin film of a composite oxide semiconductor containing copper (Cu) and a specific element M is formed on a conductive substrate. “Visible light responsiveness” means not only the ability to absorb visible light but also the property that charges generated by irradiation with visible light can be used for the reaction. In the present invention, visible light is preferably 380 to 800 nm, more preferably 400 to 800 nm.

  The specific element M is bismuth (Bi), nickel (Ni), silicon (Si), titanium (Ti), yttrium (Y), alkaline earth metal (Be, Mg, Ca, Sr, Ba, Ra). , At least one element selected from the group consisting of lanthanoids.

  The composition ratio in the composite oxide semiconductor is arbitrary. However, since it is unstable when there is too much Cu with respect to the specific element M, the atomic ratio of Cu / M is preferably 0.1 or more, more preferably 0.1 or more and 10 or less, and still more preferably 0. .2 or more and 3 or less. In addition, it is important that the complex oxide semiconductor is a complex oxide with a fixed structure, and is not a simple oxide mixture.

Specific examples of the composite oxide semiconductor include CuBi ₂ O ₄ , CuNi ₂ O ₃ , CuNiO ₂ , CuSiO ₃ , Cu ₃ TiO ₄ , CuYO _2.5 , CuBa ₂ O ₃ , CuCaO ₂ , CuLaO _2.5 , and CuMgO _2. The present invention is not limited to these examples.

As the conductive substrate, metals such as titanium and stainless steel and conductive glass can be used, and among them, conductive glass is most preferable. It is also possible to use indium tin oxide (ITO) and _{F-SnO 2, Sb-SnO} 2 or the like.

  In the semiconductor element of the present invention, it is important that a porous semiconductor thin film is formed on a conductive substrate. Since the semiconductor film has a porous structure, the electrolytic solution penetrates into the thin film through the pores formed in the semiconductor film, and the semiconductor can contact the electrolytic solution in a large area. As a result, since the movement distance of the electrons generated inside the semiconductor to the electrolytic solution is short, the charges are hardly recombined, and a large photocurrent response can be shown. In the conventional pellet-type electrode, since the pellet is as thick as millimeters, the movement distance of electrons and holes is large, charge recombination occurs before the reaction occurs on the electrode surface, and the efficiency of the electrolysis reaction is poor.

The size of the pores formed in the semiconductor porous film is preferably relatively large from the viewpoint of efficiently moving the electrolyte solution inside the film and diffusing products such as oxygen and hydrogen, If it is too large, the film strength will be weak and it will be difficult for the charge to move. Therefore, a state where pores of various sizes of 5 to 500 nm are combined is preferable, and a combination in the range of 5 to 300 nm is more preferable. . The control of the pore diameter can be adjusted by the molecular weight or the mixture amount of the organic substance to be mixed when the film is fired (see, for example, JP-A-2005-44758). The particle diameter of the semiconductor is preferably a primary particle diameter in the range of 5 to 600 nm, and more preferably in the range of 10 to 100 nm. The specific surface area is preferably in the range of 1~300m ² / g, and more preferably in a range of from 2 to 100 m ² / g.

  The thickness of the thin film is sufficient if it has a film thickness that can absorb light sufficiently, and if it is thicker than that, problems such as cracking and solution transportation and product transportation may occur, leading to performance degradation. Therefore, the semiconductor film thickness is preferably 0.1 to 20 μm, and more preferably 0.2 to 5 μm.

  As the semiconductor thin film preparation method, a wet method is preferable to a physical film formation method such as a vapor deposition method or a sputtering method. As a wet preparation method of a semiconductor film, a metal precursor such as a sol-gel method or a complex polymerization method is dispersed in a solvent and thermally decomposed (baked) after coating, or semiconductor fine particles are prepared in advance by a solid phase method or the like. In addition, there is a method of making a paste and thermally decomposing (baking) after coating. If the melting point is low, a solid phase method may be used. As a coating method, screen printing, a doctor blade method, a spin coating method, a spray method, a dip coating method, or the like can be used.

  The firing temperature must basically be a temperature at which the organic matter mixed above decomposes. However, since the substrate has heat resistance, for example, when a tin oxide conductive glass substrate is used, the heat resistant temperature (about 800 ° C.) or lower is preferable. Moreover, since bismuth (Bi) has a low melting point and is easily sublimated, it is more preferably 600 ° C. or lower, and particularly preferably 350 to 550 ° C. It is also effective to fire in oxygen in order to accelerate the decomposition of organic matter.

  When a semiconductor compound partially containing nitrogen, sulfur, carbon, or the like is manufactured, the oxide film may be treated later with ammonia, hydrogen sulfide, or the like, or the precursor oxide and the nitrogen-containing compound or sulfur-containing compound are treated. A compound may be mixed and fired. In the case of producing a semiconductor compound partially containing nitrogen, sulfur, carbon, etc., the amount of nitrogen or sulfur added is preferably 0.5 mol% or more, more preferably 5 mol% or more, and the upper limit is 20 mol% or less. Is preferable, and 10 mol% or less is more preferable.

  The semiconductor element of the present invention can be preferably used as a photoelectric conversion element and can be applied to a photoelectrode. In addition, the semiconductor element of the present invention may show a response of a cathode photocurrent and / or an anode photocurrent. In this case, it is preferable that the n-type and the p-type can be switched by the potential. By such switching, the semiconductor element of the present invention can be used as a switching element for controlling n-type or p-type.

Next, the light energy conversion system of the present invention will be described.
In the light energy conversion system of the present invention, light energy can be converted to hydrogen by electrolyzing water using the photoelectrode of the present invention. Specifically, the photoelectrode of the present invention is irradiated with light, and the generated cathode photocurrent is used to reduce water on the photoelectrode of the present invention to generate hydrogen. At this time, the photoelectrode of the present invention is used under the condition that the cathode photocurrent flows with a negative bias. Depending on the potential of the semiconductor, a bias may not be necessary.

Similarly, by using the photoelectrode of the present invention, it is possible to oxidize, reduce, and decompose harmful substances and organic substances using light energy.
In order to take advantage of the p-type characteristics of the photoelectrode of the present invention and cause the reduction reaction on the semiconductor and the oxidation reaction to proceed on the counter electrode, the negative electrode is biased with respect to the semiconductor electrode and the cathode photocurrent flows. At this time, a material preferable for oxygen generation such as RuO ₂ / Ti may be used for the counter electrode, or light may be irradiated simultaneously using an n-type semiconductor. In some cases, in order to make use of the n-type characteristics and advance the oxidation reaction on the semiconductor and the reduction reaction on the counter electrode, the semiconductor electrode is biased on the positive side and used under the condition that the anode photocurrent flows.
It is preferable to select an electrolyte solution having a stable electrode composition. In the case of water splitting, it is preferable to avoid using strong acidity or strong alkalinity at a high concentration, and the vicinity of neutrality is more preferable. Examples include Na ₂ SO ₄ and sodium phosphate, low concentrations of NaOH and H ₂ SO ₄ , preferably Na ₂ SO ₄ . Further, sodium acetate or the like may be added.

Next, the photocatalyst of the present invention will be described.
The visible light responsive complex oxide semiconductor described above can also be used as a semiconductor for a photocatalyst. The photocatalyst of the present invention comprises the specific complex oxide semiconductor described above. If a photocurrent can be observed as a porous semiconductor electrode under visible light irradiation, the semiconductor is considered to have charge separation capability inside the particles. The available light absorption range is equivalent to the light region where photocurrent can be observed, and is generally the same as or shorter than the absorption spectrum range.
The particle size of the specific complex oxide semiconductor described above in the photocatalyst is preferably 10 nm to 5000 nm, more preferably 20 nm to 500 nm.
The application of the photocatalyst is limited by the potential of the conduction band and valence band of the semiconductor. In the case of an n-type semiconductor, the observation start potential of the anode photocurrent is close to the conduction charge position. The valence charge potential can also be estimated from the band gap. It can be used for hydrogen generation if the conduction potential is more negative than the H ⁺ / H ₂ potential, and can be used for decomposing harmful substances in the air if it is more negative than the oxygen reduction potential, and more negative than the redox potential in the electrolyte. For example, it can be used to decompose harmful substances using redox (iron, nitric acid, iodine, etc.). The higher the valence charge position, the stronger the oxidizing power and the more toxic substances can be decomposed. If it is more positive than the O ₂ / H ₂ O potential, it can be used for oxygen generation. In the case of a p-type semiconductor, the observation start potential of the cathode photocurrent is close to the valence charge position. The conduction charge potential can also be estimated from the band gap. Combining a p-type semiconductor with a high hydrogen generation capability and an n-type semiconductor with a high oxygen generation capability allows water to be decomposed with almost no bias, and can be used as a photocatalyst for water decomposition.
When the above-described specific complex oxide semiconductor is used as a photocatalyst, the reaction can be advanced not only in the electrolytic solution but also in pure water or a gas phase reaction.
The photocatalyst may be used in a powder suspension state or may be used by being fixed to a stable substrate.
A cocatalyst may be further supported on the photocatalyst for the purpose of enhancing the reaction. Examples of the promoter include noble metals such as platinum (Pt), rhodium (Rh), palladium (Pd), gold (Au), silver (Ag), iridium (Ir), Ni, NiO _x , RuO ₂ , carbon, and the like. is there.

The photoelectrode or photocatalyst of the present invention can be used by combining a semiconductor exhibiting p-type characteristics and a semiconductor exhibiting n-type characteristics among the specific complex oxide semiconductors described above. In addition, the above-described specific complex oxide semiconductor exhibiting p-type characteristics or n-type characteristics and any semiconductor can be used in combination. These photoelectrodes or photocatalysts can be applied to light emitting diodes, solid solar cells, photocatalysts, and the like.
When a p-type semiconductor and an n-type semiconductor are used in combination, the p-type semiconductor and the n-type semiconductor may be in direct contact (pn junction), or a conductive substance such as metal, a semiconductor, or an insulator is interposed therebetween. May be. Examples of the conductive substance include tin oxide, zinc oxide, and indium oxide whose conductivity is increased by doping. Examples of semiconductors that can be interposed include undoped tin oxide, zinc oxide, and indium oxide. A salt containing a redox medium such as NaI or FeSO _{4 is} also used. In the case where an insulator is interposed, since the tunnel current is used, the insulating layer is preferably 100 nm or less.

Examples of the n-type semiconductor include general TiO ₂ , WO ₃ , ZnO, CdS, Ta ₂ O ₅ , Fe ₂ O ₃ , V ₂ O _{5, and the} like, SrTiO ₃ containing these elements, BiVO _{4, and the} like. the composite oxide or oxynitride or _Ta 3 _{N 5,} such as nitride, such as TaON, oxysulfide, M is TiO ₂ -M (wherein, N, S, C, Ni , Cr, Sb, dopant Bi, etc. Or the like.).

The contact method may be simple mixing, but in order to achieve good contact, it is preferable to disperse with a homogenizer or to sufficiently perform mechanical kneading such as a mortar or ball mill. If the particle diameter of the semiconductor particles is small, or if there are many defect levels on the surface, the barrier is small, so that the intervention of another conductive substance is not necessarily required. Further, pressurization, vacuum treatment, or heating may be performed. An organic binder can be added and baked. The heating temperature is generally determined so that the semiconductor bulk is not denatured, but is generally 800 ° C. or lower, preferably 600 ° C. or lower. Also in this case, as described above, a promoter may be further supported on the p-type and / or n-type semiconductor for the purpose of enhancing the reaction on the photocatalyst. Examples of the cocatalyst include noble metals such as Pt, Rh, Pd, Au, Ag, and Ir, Ni, NiO _x , RuO ₂ , and carbon.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited to these.

Example 1
A CuBi ₂ O ₄ electrode was prepared as follows.
Symmetrics Cu coating solution and Bi coating solution were mixed well at a Cu: Bi stoichiometric ratio of 1: 2. The obtained solution was applied to conductive glass (F-SnO ₂ , 10Ω / sq) by a spin coating method, and air-baked at 550 ° C. for 30 minutes. This was repeated 4 times. The film thickness was about 0.5-1 μm. An X-ray diffractometer (XRD; manufactured by MX Labo Mac Science Co., Ltd.) identified CuBi ₂ O ₄ as the main product. The produced film was porous, and the size of the formed particles was about 30 to 40 nm in terms of the particle size observed by SEM.
This electrode was connected to a potentiostat. The counter electrode was Pt wire and the reference electrode was Ag / AgCl. As the electrolytic solution, an aqueous Na ₂ SO ₄ solution was used. UV light of 420 nm or less was cut into a 500 W Xe lamp and irradiated with visible light. The irradiation area was 6 mm in diameter. A cathode photocurrent of −38 μA / cm ² was observed at −0.1 V potential (vs. Ag / AgCl; the same applies hereinafter). The results are shown in Table 1.
In addition, it was found that the natural electrode potential (Vset) shifted positively by light irradiation and operated as a photoelectric conversion element having strong p-type characteristics.
It was also found that when the semiconductor electrode was biased to the positive side, anode photocurrent flowed.

Example 2
A CuBi ₂ O ₄ electrode was prepared by a method different from Example 1 as follows.
Bismuth nitrate pentahydrate (Bi (NO ₃ ) ₃ .5H ₂ O) about 0.7 mol / L acetic acid solution and copper nitrate trihydrate (Cu (NO ₃ ) ₂ .3H ₂ O) about 0. Each of the 08 mol / L acetylacetone solutions was heated to about 100 ° C. to obtain uniform solutions. These two types of solutions are mixed well by reducing the stoichiometric ratio of Cu: Bi by about 10% Bi (ie, molar ratio Cu: Bi = 1: 1.8) to obtain a CuBi ₂ O ₄ precursor. It was. The obtained solution was applied to conductive glass (F-SnO ₂ , 10Ω / sq) by a spin coating method, and air-baked at 550 ° C. for 30 minutes. This was repeated 6 times. The film thickness was about 0.5-1 μm. It was identified by the X-ray diffractometer (XRD; manufactured by MX Labo MacScience) that CuBi ₂ O ₄ was produced almost in a single phase. The produced film was porous, and the size of the formed particles was about 30 to 40 nm in terms of the particle size observed by SEM.
This electrode was connected to a potentiostat. The counter electrode was Pt wire and the reference electrode was Ag / AgCl. As the electrolytic solution, an aqueous Na ₂ SO ₄ solution was used. UV light of 420 nm or less was cut into a 500 W Xe lamp and irradiated with visible light. The irradiation area was 6 mm in diameter. A cathode photocurrent of −92 μA / cm ² was observed at −0.1 V potential (vs. Ag / AgCl; the same applies hereinafter). The results are shown in Table 1.
In addition, it was found that the natural electrode potential (Vset) shifted positively by light irradiation and operated as a photoelectric conversion element having strong p-type characteristics.
In addition, an experiment for confirming stability was performed. A 500 W Xe lamp was directly irradiated. When light irradiation was performed for 10 hours, a substantially constant cathode current continued to be observed. The turnover number of electrons with respect to the semiconductor was 10 or more. In addition, no significant discoloration was observed in the absorption spectrum or XRD pattern after irradiation.
It was also found that when the semiconductor electrode was biased to the positive side, anode photocurrent flowed.

Example 3
In the same manner as in Example 1, CuNi ₂ O ₃ , CuNiO ₂ , CuSiO ₃ , Cu ₃ TiO ₄ , CuYO _2.5 , CuCoO ₂ , CuBa ₂ O ₃ , CuCaO ₂ , CuLaO _2.5 , and CuMgO ₂ are each made of conductive glass as in the first embodiment. A photoelectrode was produced by forming a film thereon.
About the produced photoelectrode, the photocurrent was measured at the potential shown in Table 1 in the same manner as in Example 1. The results are shown in Table 1. As is clear from the results in Table 1, a large cathode photocurrent was observed, and it was found that the device operates as a photoelectric conversion element having a strong p-type characteristic.
It was also found that anode photocurrent flowed when a positive bias was applied to these semiconductor electrodes.

Example 4
Focusing on the fact that the natural electrode potential of CuBi ₂ O ₄ is more positive than the starting voltage of WO ₃ measured under the same conditions, the photocurrent was measured by the bipolar method using WO ₃ as the working electrode and CuBi ₂ O ₄ as the counter electrode. . As a result, a relatively stable photocurrent (60 μA / cm ² ) was observed for 2 hours or more even when the applied bias was 0 V during irradiation with visible light. The number of turnovers in 2 hours was 2 or more, and it was found that the photoelectrochemical decomposition reaction of water proceeded under visible light and no bias conditions.

Comparative Example 1
The semiconductor thin film was formed into a conductive glass in the same manner as in Example 2 except that the bismuth nitrate pentahydrate (Bi (NO ₃ ) ₃ .5H ₂ O) of about 0.7 mol / L acetic acid solution was not used in Example 2. A CuO photoelectrode was produced by forming a film thereon.
When the photocurrent was measured for the produced photoelectrode in the same manner as in Example 2, the electrode changed during the photoreaction, and the photocurrent became zero after 2 hours. Moreover, the number of electron turnovers with respect to the semiconductor was 1 or less. Therefore, it was found that the CuO photoelectrode of Comparative Example 1 was very unstable.

Comparative Example 2
CuMn ₂ O _4, were prepared _{CuB 2 O 6, CuAl 2 O} 4, CuCr 2 O 4 deposited photoelectrode a semiconductor thin film on the conductive glass in the same manner as in each Example 1 About.
About the produced photoelectrode, the photocurrent was measured at the potential shown in Table 2 in the same manner as in Example 1. The results are shown in Table 2. As is clear from the results in Table 2, it was found that the photocurrent was very low as compared with Examples 1 to 3.

Example 5
The photocatalytic activity was measured using CuBi ₂ O ₄ prepared in Example 1. Under the same experimental conditions as in Example 1, the photocurrent before and after the light irradiation was measured for each of the cases where oxygen was introduced into the electrolytic solution after nitrogen purge of the electrolytic solution and the case where oxygen was not introduced into the electrolytic solution.
As a result, the cathode dark current of the CuBi ₂ O ₄ electrode did not change around zero with or without oxygen introduced into the electrolyte, but the cathode photocurrent increased greatly when oxygen was introduced. From this fact, the CuBi ₂ O ₄ semiconductor cannot receive electrons or cannot reduce oxygen in the dark, but on the other hand, it has a feature that electrons excited in the conduction band can reduce oxygen when irradiated with light. all right.

Reference example 1
The photocatalytic activity of the WO ₃ photoelectrode, which is a representative visible light responsive n-type semiconductor, was measured in the same manner as in Example 5. The WO ₃ film was prepared by laminating tungstic acid fine particles and firing at 500 ° C.
As a result, the anode photocurrent of the WO ₃ photoelectrode was greatly increased when organic substances such as aldehydes, alcohols and organic acids were introduced into the electrolyte. From this, it was found that the holes generated in the valence band of the WO ₃ semiconductor by light irradiation can decompose organic substances efficiently. On the other hand, after nitrogen purge of the electrolyte, the cathode photocurrent of the WO ₃ electrode did not change even when oxygen was introduced into the electrolyte. From this, the conduction band electrons of WO ₃ is either difficult to reduce the oxygen in the WO ₃ surface, the speed is found to be slowly be able to reduction.

Example 6
For the purpose of combining the strong oxygen reduction ability of the p-type CuBi ₂ O ₄ semiconductor used in Example 5 with the strong organic matter oxidation ability of the n-type WO ₃ semiconductor used in Reference Example 1, the CuBi ₂ O ₄ electrode The WO ₃ electrode was joined with a conductive wire. Acetaldehyde and oxygen were introduced into the electrolyte, and both electrodes were irradiated with light (300 W Xe lamp, manufactured by Thermax, 1.2 × 1.5 cm).
As a result, a current of 270 μA flowed from the CuBi ₂ O ₄ electrode to the WO ₃ electrode in the conducting wire without bias. This was an order of magnitude higher than the current in the absence of acetaldehyde and oxygen (28 μA). Therefore, without applying a bias from the outside, electrons are organic matter oxidized in WO ₃ electrode with flow to CuBi ₂ O ₄ electrodes, oxygen on CuBi ₂ O ₄ electrode was found to have been reduced.
From the above results, it was found that a hazardous substance decomposition system can be constructed even in an electrochemical cell without bias.

Example 7
The organic matter resolution in a powder photocatalyst combining a p-type semiconductor powder and an n-type semiconductor powder was measured.
First, CuO powders were mixed with Bi ₂ O ₃ powder solid phase synthesis was CuBi ₂ O ₄ by firing at 700 ° C. 24 hours to prepare a CuBi ₂ O ₄ powder was ground in a mortar. On the other hand, a high purity chemical product (99.99%) was used as the WO ₃ powder. Next, CuBi ₂ O ₄ powder and WO ₃ powder were sufficiently kneaded using an agate mortar at a mass ratio of 1 to 2, to prepare a pn junction type powder catalyst. At this time, it is important that both powders are sufficiently in contact from the viewpoint of pn junction, and in the case of a catalyst with insufficient kneading, good performance cannot be obtained.
After 150 mg of the catalyst was charged into the reaction vessel and sealed, acetaldehyde gas (9000 ppm) was introduced using a syringe. A xenon lamp was uniformly irradiated from the bottom of the vial using a rotary sample holder for light irradiation. Thereafter, carbon dioxide and acetaldehyde were quantified. Quantification was performed by gas chromatography (hydrogen flame ionization detector; FID) equipped with a methanizer.
On the other hand, for comparison, only WO ₃ powder not mixed with CuBi ₂ O ₄ powder was used, and light irradiation was performed in the same manner as above to determine carbon dioxide and acetaldehyde.
The time course of the reaction is shown in FIG. FIG. 1 is a graph showing the catalytic activity of the photocatalyst. The vertical axis represents the amount of carbon dioxide generated by oxidation of acetaldehyde, and the horizontal axis represents the light irradiation time. ● is a plot for a pn junction type powder catalyst in which CuBi ₂ O ₄ powder and WO ₃ powder are combined, and ▲ is a plot for WO ₃ powder.
As can be seen from the results of FIG. 1, in the pn junction type powder catalyst, 18000 ppm of CO ₂ was generated after 180 minutes, and it was found that complete oxidation could proceed. Further, the test of reintroducing acetaldehyde using the catalyst as it was was repeated 5 times, but the catalyst activity was not lowered at all and was very stable.
On the other hand, in the case of only WO ₃ powder not mixed with CuBi ₂ O ₄ powder, about 9000 ppm of CO ₂ was generated after 180 minutes, but no further CO ₂ generation was observed and complete oxidation could not be performed.
From these results, the CuBi ₂ O ₄ semiconductor showed excellent catalytic activity, and even with the pn junction type powder catalyst in which the CuBi ₂ O ₄ powder and the WO ₃ powder were mixed and brought into contact, the CuBi ₂ O of Example 6 was used. It was found that toxic substances can be decomposed in the same manner as when the ₄ electrodes and the WO ₃ electrode are joined with a conductive wire.

Example 8
In Example 7, CuBi ₂ O ₄ powder and WO ₃ powder were kneaded at a mass ratio of 1: 2, and then measured in the same manner except that the mixture was heated at 200 ° C. for 1 hour.
As a result, an improvement in activity of about 1.5 times at the initial carbon dioxide gas generation rate was observed as compared with the case of not heating. From this, it was found that the contact between the CuBi ₂ O ₄ powder and the WO ₃ powder was improved by heating, and the catalytic activity was superior to that when not heated.

FIG. 1 is a graph showing the catalytic activity of the photocatalyst.

Claims (7)

A porous thin film of a composite oxide semiconductor containing copper and further containing at least one element M selected from the group consisting of bismuth, nickel, silicon, titanium, yttrium, alkaline earth metal, and lanthanoid is formed on the conductive substrate. In the formed semiconductor device, the composite oxide semiconductor is CuBi ₂ O ₄ , CuNi ₂ O ₃ , CuNiO ₂ , CuSiO ₃ , Cu ₃ TiO ₄ , CuYO _2.5 , CuBa ₂ O ₃ , CuCaO ₂ , CuLaO _2.5. Or a visible light-responsive semiconductor element, which is a CuMgO ₂ and exhibits a cathode photocurrent and / or an anode photocurrent. 2. The semiconductor element according to claim 1, wherein the semiconductor element exhibits both a cathode photocurrent and an anode photocurrent, wherein the anode and the cathode are switched by a potential. 3. The semiconductor element according to claim 1, wherein the semiconductor element is manufactured by a wet method using a solution in which a metal salt of the precursor of the composite oxide semiconductor is dissolved in a solvent. Photoelectrode comprising a semiconductor device according to any one of claims 1-3. 5. The photoelectrode according to claim 4, wherein a p-type semiconductor and an n-type semiconductor are combined. A light energy conversion system for generating hydrogen by irradiating light on the photoelectrode according to claim 4 or 5 and reducing water on a semiconductor using a cathode photocurrent. A light energy utilization system that irradiates light on the photoelectrode according to claim 4 or 5 to oxidize, reduce or decompose harmful substances and organic substances.

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