JP5830527B2 - Semiconductor device, hydrogen production system, and methane or methanol production system - Google Patents

Description

  The present invention relates to a semiconductor element (photoelectrochemical reaction element) for producing a compound as a fuel by reducing carbon dioxide by a chemical reaction induced by light energy, or producing hydrogen by decomposing water. )

  Fossil fuel resources such as oil and coal have a finite amount. On the other hand, the consumption of fossil fuels by humans continues to increase at an accelerating rate, and there is concern about the depletion of fossil fuels. The need for the development of fossil fuel alternative energy resources is increasing year by year.

  Against this background, renewable energies such as sunlight, solar heat, wind power, and geothermal heat have attracted attention, and in recent years, full-scale spread of solar power generation and wind power generation has begun.

  Hydrogen is valuable as clean energy that does not produce carbon dioxide by combustion. Development of hydrogen production technology, hydrogen storage and transport technology is being carried out toward the realization of a society using hydrogen as an energy source. The decomposition of water by the photocatalytic reaction using sunlight or the electrolysis of water using the electromotive force of solar cells can produce hydrogen without consuming fossil fuel. Both the water decomposition reaction by the photocatalytic reaction and the water electrolysis reaction by the solar cell are represented by the following reaction formula (1).

H ₂ O → H ₂ + 1/2 O ₂ (1)
In the photocatalyst, the reaction of the reaction formula (1) occurs on the surface of the catalyst. In the electrolysis of water, as shown in the reaction formula (2) and the reaction formula (3), hydrogen generation and oxygen generation are performed at the cathode and the anode. Happens separately.

The ^{cathode: 2H + + 2e - → H} 2 (2)
_{Anode: H 2 O → 1 / 2O} 2 + 2H + + 2e - (3)
On the other hand, such as methane (reaction formula (4)) and methanol (reaction formula (5)) that can reduce carbon dioxide, which is said to cause global warming, by solar energy, and can be stored for a long time at low cost. Although the practical use of artificial photosynthesis technology for producing fuel compounds is desired, significant improvement in reaction efficiency is a problem.

CO ₂ + 2H ₂ O → CH ₄ + 2O ₂ (4)
CO ₂ + 2H ₂ O → CH ₃ OH + 3 / 2O ₂ (5)
Photocatalysts, electrolysis, electrochemical photovoltaic cells, and the like are known as techniques for decomposing water and reducing carbon dioxide with sunlight.

JP-A-6-126189 JP 2003-288955 A JP 2003-238104 A JP 7-33697 A

Nature 1972, 238, 37-38

  When the photocatalyst absorbs light having energy greater than the band gap, electrons in the valence band are excited to the conduction band, thereby generating electrons having a reducing power and holes having an oxidizing power. Can cause water decomposition and carbon dioxide reduction, respectively. Known photocatalysts include titanium oxide and tungsten oxide. When a platinum promoter is supported on the surface of titanium oxide, the reaction rate is improved. JP-A-6-126189 (Patent Document 1) describes that tantalum oxide and zirconium oxide are suspended in an aqueous solution and water is decomposed under light irradiation to obtain hydrogen and oxygen. The photocatalyst can be said to have a stronger redox reaction force as the band gap is larger, but on the other hand, ultraviolet light with higher energy can be used for the reaction, but there is a problem that visible light and near infrared with low energy cannot be fully utilized. . Another problem is that electron-hole pairs generated by light irradiation are likely to recombine.

  It is possible to decompose water and reduce carbon dioxide by electrolysis, but when electrolysis is performed using system power, the consumption of fossil fuel is promoted and carbon dioxide is discharged as a result. Therefore, it is not desirable as a fuel manufacturing method. Therefore, for example, a method using a solar cell as a power source is described in Japanese Patent Laid-Open No. 2003-288955 (Patent Document 2). This publication describes that an electromotive force necessary for water decomposition reaction is obtained by stacking amorphous silicon solar cells. However, this method cannot fully utilize ultraviolet light. Moreover, since it is necessary to make a multilayered laminated structure, there is a problem that the process is expensive.

  The photoelectrochemical cell performs an oxidation-reduction reaction using a current generated when light is applied to a semiconductor electrode using a circuit composed of a semiconductor electrode and a metal electrode. In Nature 1972, 238, 37-38 (Non-Patent Document 1), there is a description that water was decomposed by a titanium oxide semiconductor electrode and a platinum electrode. The semiconductor electrode is known as the “Honda-Fujishima effect” after the discoverer. Japanese Patent Laid-Open No. 2003-238104 (Patent Document 3) is characterized in that a photocatalyst layer that excites electrons and hole pairs by light irradiation is formed on the surface of a p-type semiconductor layer of a solar cell having a pn junction. There is a description of "optical element for hydrogen generation". In the photoelectrochemical cell, it is necessary to prepare a semiconductor electrode and a metal electrode separately, and to wire them through a resistor, and there is a problem that the system volume increases and the cost increases. Another problem is current loss when the current generated in the semiconductor electrode is taken out to the wiring portion and sent to the metal electrode.

  In response to the problem that the photocatalyst can mainly use only ultraviolet light, Japanese Patent Application Laid-Open No. 7-33697 (Patent Document 4) discloses a methanol synthesis apparatus in which a solution transparent cell and a solar cell are stacked, which performs a photocatalytic reaction using a titanium oxide thin film. Is described. In this device, light in the visible to near-infrared wavelength region, which is not used by titanium oxide, can be used for power generation of solar cells, so light is effectively used, but the power generated by solar cells is used for methanol synthesis. Since it is not used, it is a problem that it is impossible to improve the efficiency of the methanol synthesis reaction.

  The object of the present invention is to make it possible to use light of a wider range of wavelengths in fuel production by carbon dioxide reduction using solar light and hydrogen fuel production by water splitting, and to reduce carbon dioxide with an inexpensive element. It is to improve the efficiency of the reaction and water splitting hydrogen production reaction.

  An electrode layer, a first semiconductor layer, and a first surface and a second surface facing each other. The electrode layer is provided in contact with the first surface, and the first semiconductor layer is in contact with the second surface. The lower end potential of the conduction band of the first semiconductor layer is higher than the hydrogen generation potential, and the upper end potential of the valence band of the second semiconductor layer is higher than the oxygen generation potential. The first semiconductor layer has a lower band gap than that of the second semiconductor layer.

  Since light in a wide wavelength range from ultraviolet to visible and near-infrared can be used, the reaction efficiency of fuel production by carbon dioxide reduction using sunlight and hydrogen fuel production by water splitting is improved.

It is an example of sectional drawing of the semiconductor element of this invention. It is an example of sectional drawing of the semiconductor element of this invention. It is an example of sectional drawing of the semiconductor element of this invention. It is an example of sectional drawing of the semiconductor element of this invention. It is a figure which shows the principle of decomposition | disassembly of water. It is a figure which shows the electric potential of the decomposition reaction of water, and the band gap of a bulk semiconductor. It is an example of sectional drawing of the semiconductor element of this invention. It is an example of reaction implementation using the semiconductor element of this invention. It is an example of reaction implementation using the semiconductor element of this invention. It is an example of reaction implementation using the semiconductor element of this invention. It is an example of reaction implementation using the semiconductor element of this invention. It is an example of reaction implementation using the semiconductor element of this invention. It is a figure which shows the principle of the semiconductor element of this invention. It is a figure which shows the principle of the semiconductor element of this invention.

  The semiconductor device of the present invention is intended to achieve improved utilization efficiency of solar light and improved reaction efficiency in fuel production by carbon dioxide reduction using sunlight and hydrogen fuel production by water splitting.

  First, the principle of fuel production by carbon dioxide reduction using sunlight and hydrogen fuel production by water splitting will be described.

To electrolyze water, an electromotive force of 1.23 V is required. The water electrolysis reaction formula is represented by the following reaction formula (2) and reaction formula (3).
Cathode: 2H ⁺ + 2e ⁻ → H ₂ (2)
_{Anode: H 2 O → 1 / 2O} 2 + 2H + + 2e - (3)
Generally, the potential of the hydrogen generation reaction at the cathode is defined as the standard hydrogen electrode potential and is determined to be 0V. FIG. 5 is an energy diagram showing water electrolysis using a semiconductor. When light (hν) strikes the semiconductor, electrons in the valence band 114 are excited and move to the conduction band 112. As a result, an electron 111-hole 115 pair is generated. The electrons 111 are used in the cathode reaction to generate hydrogen, and the holes 115 are used in the anode reaction to extract electrons from water and generate oxygen. In order for such a reaction to occur, it is necessary that the upper end potential of the valence band 114 is higher than the oxygen generation potential and the lower end potential of the conductor 112 is lower than the hydrogen generation potential.

FIG. 6 shows the relative relationship between the band gap of several semiconductor photocatalyst materials and the electrolysis potential of water at pH7. As can be understood from FIG. 6, it is advantageous that the band gap is larger in order to perform the water decomposition reaction by photoexcitation of the semiconductor. However, high energy is required to generate photoexcitation for a semiconductor having a large band gap, and in the case of titanium oxide (TiO ₂ ), light having a wavelength of 413 nm or more cannot be used. Note that the hydrogen generation potential at pH 0 is 0 V (vs. SHE), and the oxygen generation potential is 1.23 V (vs. SHE).

Next, the reduction reaction of carbon dioxide is also described. Carbon dioxide is decomposed by a multi-electron reduction reaction. Several reduction products are known and the reduction potentials are different. Reaction formula (4 ′) and reaction formula (5 ′) are production reaction formulas of methane and methanol, which are promising reduction products as fuel.
^{_{Methane: CO 2 + 8e - + 8H}} + → CH 4 + 2H 2 O (4 ')
Methanol: CO ₂ + 6e ⁻ + 6H ⁺ → CH ₃ OH + H ₂ O (5 ′)
The reaction potential for methane formation is -0.24 V (vs. SHE), and the reaction potential for methanol formation is -0.38 V (vs. SHE).

  The semiconductor element of the present invention has a wide range by adopting a heterostructure in which a semiconductor layer mainly using ultraviolet light and having a large band gap and a semiconductor layer mainly using visible light and having a small band gap are stacked. It makes light in the wavelength range available. Among the semiconductors constituting the heterostructure, the wide gap semiconductor is arranged on the sunlight incident side, and the narrow gap semiconductor and further the electrodes are arranged in that order in the back. Since an electrode independent from an external power source, wiring, and semiconductor is not necessary and can be used with a simple configuration, a low-cost fuel production system can be constructed.

  By arranging the semiconductor element of the present invention in an aqueous electrolyte solution, it is possible to produce hydrogen from water by sunlight and methane or methanol from carbon dioxide. Moreover, the semiconductor element of this invention can be arrange | positioned in the gas containing a high concentration carbon dioxide like the exhaust gas of a thermal power plant, for example, and methane and methanol can be manufactured from a carbon dioxide.

  The reaction mechanism of this semiconductor element is as follows.

  When the semiconductor element is irradiated with sunlight, an electron-hole pair is generated inside each semiconductor layer. The generated electrons and holes move in opposite directions, and are consumed by the reaction after reaching the electrode and the surface of the first semiconductor layer.

  The first semiconductor layer has a band gap larger than that of the second semiconductor layer, and a semiconductor having a band gap of 1.5 eV or more, preferably 2.0 eV or more is used. Since wide gap semiconductors often have a deep valence band level, it is advantageous to perform an oxidation reaction on the surface of the first semiconductor layer. It is desirable that it does not deteriorate even if it is in contact with an electrolyte aqueous solution or a gas having a mixed composition for a long time. Titanium oxide, which is chemically stable and excellent in photocatalytic activity, is desirable. In addition, an oxide, sulfide, or selenide of at least one element selected from the group consisting of tantalum, tungsten, zinc, tin, iron, copper, vanadium, zirconium, neptunium, molybdenum, cadmium, antimony, and cobalt, silicon carbide , Gallium phosphide, a compound of four elements (copper-indium-gallium-selenium), and a compound of three elements (copper-indium-selenium) can be used. Here, the oxide used for the first semiconductor layer is preferably doped with potassium or strontium in order to improve the photocatalytic activity.

  Since the second semiconductor layer does not undergo oxidation or reduction reaction on its surface, the photocatalyst is a material such as a sulfide-based semiconductor that self-dissolves when used in water, or a solar cell material that degrades in water. Can be used. The material used for the second semiconductor layer is the same material as the first semiconductor (titanium, tantalum, tungsten, zinc, tin, iron, copper, vanadium, zirconium, neptunium, molybdenum, cadmium, antimony, cobalt). An oxide, sulfide, or selenide of at least one element selected from the group, silicon carbide, silicon, gallium phosphide, (copper-indium-gallium-selenium) four-element compound, (copper-indium-selenium) 3 element compound), which is required to have a narrower band gap than the first semiconductor layer. Since an electromotive force for reaction is also required, specifically, it is a semiconductor having a band gap of 0.5 eV or more, preferably 0.8 eV or more.

  There is a demand for the electrode that the overvoltage for a desired chemical reaction is small and that a good contact surface can be formed with the first semiconductor layer without chemically degrading in an aqueous electrolyte solution or mixed composition gas. is there. It is important to improve the reaction efficiency that the reaction substrate (water, carbon dioxide) can be favorably adsorbed on the electrode surface. Examples of materials that can be used for the electrode include platinum, gold, silver, copper, nickel, indium, rhodium, palladium, gallium, zinc, titanium, cobalt, iron, and iridium. Among these, copper is suitable for the production of methane and methanol. Platinum is particularly suitable for hydrogen production because it has a low hydrogen overvoltage and is chemically stable.

  Sparsely adding metal / metal oxide fine particles called cocatalysts to the surface of the first semiconductor layer is effective in improving the reaction efficiency. This is due to an increase in the reaction surface area, an improvement in the adsorptivity of the reaction substrate, and an improvement in the separation of electrons and holes generated inside the semiconductor. Examples of materials that can be used for the promoter include platinum, silver, copper, gold, nickel oxide, ruthenium oxide, and copper oxide. Examples of the method for adding a cocatalyst include fine particle coating, photodeposition, electrodeposition, sputtering, and CVD. The promoter has a diameter of 20 nm or less, desirably 10 nm or less, more desirably 3 nm or less, and the area occupied by the promoter is desirably 5% or less of the surface area of the first semiconductor layer.

  Providing a thin photocatalyst layer on the surface of the first semiconductor layer is effective in improving reaction efficiency. This is particularly effective when a solar cell material having a low photocatalytic activity is used for the first semiconductor layer and the effect of increasing the reaction surface area similar to that of the promoter. The thickness of the photocatalyst layer is 10 nm or less, desirably 5 nm or less, and more desirably 1 nm or less. The effect of reducing the thickness is that the holes can reach the surface of the photocatalyst by the tunnel effect from the first semiconductor layer, and the reaction efficiency is improved. Examples of materials that can be used as the photocatalyst include anatase type titanium oxide, rutile type titanium oxide, brookite type titanium oxide and the like.

  Sealing the side of the element with an insulating material that prevents water, electrolyte, and gas from passing through, as water and electrolyte components permeate from the side of the semiconductor element, and gas intrusion causes deterioration of the element. This is very effective for extending the life of the device. In particular, sealing with a transparent material is effective for improving the reaction efficiency because it does not block sunlight.

  A first embodiment of a semiconductor element is shown in FIG. A second semiconductor layer 102 and a first semiconductor layer 103 are sequentially stacked on the electrode 101 which is a plane. The first and second semiconductor layers are formed by sputtering, CVD, vapor deposition, sol-gel method, or the like. About the formation method of the 1st and 2nd semiconductor layer, hereafter, it is common also in the other form of the photoelectrochemical reaction element of this invention.

  A second embodiment of the semiconductor element is shown in FIG. A second semiconductor layer 102 and a first semiconductor layer 103 are sequentially stacked on the electrode 101 which is a plane. Further, a thin photocatalytic layer 104 is formed on the first semiconductor layer 101. The photocatalyst layer 104 is formed by applying fine particles, applying a metal alkoxide solution and heating crystallization, sputtering, CVD, vapor deposition, or the like.

  A third embodiment of the semiconductor element is shown in FIG. After the second semiconductor layer 102 and the first semiconductor layer 103 are sequentially stacked on the electrode 101 that is a flat surface, the co-catalyst 105 is added discretely. Examples of the method for adding the cocatalyst 105 include coating of fine particles, photodeposition, electrodeposition, sputtering and CVD. The promoter has a diameter of 20 nm or less, desirably 10 nm or less, more desirably 3 nm or less, and the area occupied by the promoter is desirably 5% or less of the surface area of the first semiconductor layer. Examples of materials that can be used for the promoter include platinum, silver, copper, gold, nickel oxide, ruthenium oxide, and copper oxide.

  A fourth embodiment of the semiconductor element is shown in FIG. After the second semiconductor layer 102, the first semiconductor layer 103, and the photocatalyst layer 104 are sequentially stacked on the electrode 101 that is a plane, the promoter 105 is added discretely. The photocatalyst and cocatalyst used here can be the same as those in the second and third embodiments.

  A fifth embodiment of the semiconductor element is shown in FIG. The end surface of the photoelectrochemical reaction element shown as the first to fourth embodiments is covered with a seal 131. FIG. 7 shows an example in which the element of the first embodiment is covered with a seal 131 as an example.

  A method of decomposing water and reducing carbon dioxide using the semiconductor element of the present invention is shown in FIGS. Moreover, although the element of 1st Embodiment is illustrated as an element, it cannot be overemphasized that the element of other embodiment is also the same.

  In FIG. 8, an aqueous electrolyte solution 121 is placed in a container 122, and a photoelectrochemical reaction element is immersed in the aqueous electrolyte solution 121. The container 122 needs to be transparent at least on the surface on which sunlight 124 is incident. That is, it is necessary that ultraviolet light and visible light can pass through the container 122. Desirably, if the side and bottom surfaces are also transparent, the amount of incident sunlight increases, which is convenient.

  When the first semiconductor layer 103 is placed on the incident side of the sunlight 124, the surface of the first semiconductor layer 103 and the electrode are generated by the action of electrons and holes generated inside the element by the incidence of sunlight 124. Gases 120 and 126 are generated on the surface 101. In the decomposition of water, oxygen is generated on the first semiconductor layer 103 side and hydrogen is generated on the electrode 101 side. The generated gas can be taken out from the gas sampling port 123. According to FIG. 8, it is possible to reduce carbon dioxide with sunlight. When carbon dioxide is blown into the electrolyte aqueous solution 121 in advance to obtain a high concentration, the reaction efficiency is improved. The reduction reaction of carbon dioxide is more convenient when the gas sampling port 123 is closed in order to prevent the diffusion of carbon dioxide and collect the reduction product. In the reduction reaction of carbon dioxide, an oxygen generation reaction by oxidation of water occurs on the first semiconductor 103 side, and methane and methanol generation by a reduction reaction of carbon dioxide occurs on the electrode 101 side.

  In FIG. 9, the gas inlet 125 is provided in the container 122, and the reaction efficiency is improved by continuously supplying the source gas containing carbon dioxide during the reaction. When the first semiconductor layer 103 is placed on the incident side of the sunlight 124, the surface of the first semiconductor layer 103 and the electrode are generated by the action of electrons and holes generated inside the element by the incidence of sunlight 124. Gases 120 and 126 are generated on the surface 101. Oxygen generation by water oxidation occurs on the first semiconductor 103 side, and methane and methanol are generated by carbon dioxide reduction reaction on the electrode 101 side.

  In the reaction system shown in FIGS. 8 and 9, the composition of the electrolyte aqueous solution 121 is a composition that can dissolve carbon dioxide at a high concentration in the case of ensuring conductivity or reducing carbon dioxide. The electrolyte includes sodium chloride, potassium chloride, strontium chloride, sodium sulfate, sodium nitrate, sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium sulfate, potassium nitrate, potassium hydroxide, potassium carbonate, potassium bicarbonate, hydrochloric acid, sulfuric acid, Phosphoric acid, etc. are used. In the case of reduction of carbon dioxide, since alkaline can increase the concentration of carbonate ions and bicarbonate ions, it is convenient to use a basic electrolyte such as an alkali metal hydroxide or an alkali metal carbonate.

  FIG. 10 shows an example in which carbon dioxide is reduced by sunlight through a gas phase reaction. A gas blowing port 125 is provided in the container 122, and a source gas containing carbon dioxide can be continuously supplied even during the reaction, which is effective in improving the reaction efficiency. The container 122 is filled with the gas 120 containing carbon dioxide at a high concentration, and the element of the first form installed therein is installed so that the first electrode 103 is on the incident side of the sunlight 124. Irradiation with sunlight 124 causes oxygen generation reaction by oxidation of water vapor on the first semiconductor 103 side, and methane and methanol generation by carbon dioxide reduction reaction on the electrode 101 side.

  Analysis of the composition of the reaction product and measurement of the amount produced can be carried out by gas chromatography and GC-MS. The analysis of the aqueous electrolyte solution can be performed using ion chromatography or high performance liquid chromatography.

  FIG. 11 is a bird's-eye view showing the state and mechanism of the decomposition reaction of water by sunlight in an aqueous solution of a semiconductor element. By irradiation with sunlight 124, electrons 133 and holes 132 are generated inside the first semiconductor layer 103 and the second semiconductor layer 102, and the electrons 133 and holes 132 are separated, and a reduction reaction occurs on the surface of the electrode 101. Production of hydrogen by oxygen and oxygen production by oxidation reaction occur on the surface of the first electrode 103.

  FIG. 12 is an overhead view showing the state and mechanism of carbon dioxide reduction reaction by sunlight in an aqueous solution of a semiconductor element. By irradiation with sunlight 124, electrons 133 and holes 132 are generated inside the first semiconductor layer 103 and the second semiconductor layer 102, and the electrons 133 and holes 132 are separated, and a reduction reaction occurs on the surface of the electrode 101. Production of methane or methanol by oxygen, and oxygen production by water oxidation reaction on the surface of the first electrode 103 occur.

  11 and 12 exemplify the element according to the first embodiment. However, even with elements according to other embodiments, a reaction due to sunlight irradiation occurs by the same reaction mechanism.

  13 and 14 show an operation mechanism of a semiconductor element by a band diagram. In the band diagrams of FIG. 13 and FIG. 14, the upper side of the figure is a negative potential. 13 and 14 show the case of a carbon dioxide reduction reaction, but if the reaction potential of methane and methanol is replaced with a hydrogen generation potential, it becomes an operation mechanism of the water decomposition reaction.

In FIG. 13, the lower end of the conduction band 136 of the first semiconductor layer 103 is more negative than the lower end of the conduction band 136 of the second semiconductor layer 102, and the upper end of the valence band 137 of the first semiconductor layer 103 is , Negative from the upper end of the valence band 137 of the second semiconductor layer 102. In such a structure, since the electrons 111 and the holes 115 are separated and easily flow, the loss of electrons and holes is small and the reaction efficiency can be improved. Combinations of the first semiconductor layer and the second semiconductor layer that can take such a structure include SiC and CdS, TiO ₂ and WO ₃ , ZnS and WO ₃ , TiO ₂ and Si, and the like.

  FIG. 14 shows that the lower end of the conduction band 136 of the first semiconductor layer 103 is more positive than the lower end of the conduction band 136 of the second semiconductor layer 102, and the upper end of the valence band 137 of the first semiconductor layer 103 is , Positive from the upper end of the valence band 137 of the second semiconductor layer 102. In such a structure, since the holes 115 generated in the valence band 137 of the second semiconductor layer 102 do not easily enter the first semiconductor layer 103, the thickness of the first semiconductor layer 103 is very small. The thickness is reduced, and holes generated in the second semiconductor layer 102 by the tunnel effect are extracted.

  Examples will be described below.

  In this example, an example of the semiconductor element of the first embodiment will be described.

  A tungsten oxide layer having a thickness of 100 microns was formed as a second semiconductor layer by sputtering on a platinum electrode having a thickness of 50 microns and 50 mm square. A SiC layer having a thickness of 50 microns was formed as a first semiconductor layer on the tungsten oxide layer.

The obtained photoelectrochemical reaction element was subjected to water decomposition reaction in the configuration of FIG. The size of the transparent reaction vessel was 20 cm square at the bottom, 20 cm in height, and 0.1 M sodium hydroxide aqueous solution was added up to a depth of 10 cm. The photoelectrochemical reaction element was immersed in this aqueous solution with the second semiconductor layer facing upward. When sunlight with an intensity of 1.0 kW / m ² was irradiated from above the container, hydrogen was simultaneously generated from the electrode surface and oxygen was simultaneously generated from the surface of the first semiconductor layer.

  The first semiconductor layer and the second semiconductor layer are replaced with another semiconductor material, and the electrode is made of platinum. A photoelectrochemical reaction element is manufactured by the same method, and the decomposition reaction of water is similarly performed. Hydrogen generation from the surface was confirmed. Table 1 shows the results.

第１の半導体をＳｉＣ、第２の半導体をアナターゼ型酸化チタンとし、電極をプラチナ以外の他の金属に代えた素子も作製し、同様に水の分解反応を行った。表２に結果を示す。表１に示した第１の半導体と第２の半導体の組み合わせに対して、電極をプラチナ以外の金属に変えた素子を用いた場合も水素発生を確認した。

An element in which the first semiconductor was SiC, the second semiconductor was anatase-type titanium oxide, and the electrode was replaced with another metal other than platinum was produced, and the water decomposition reaction was similarly performed. Table 2 shows the results. For the combination of the first semiconductor and the second semiconductor shown in Table 1, hydrogen generation was also confirmed when an element in which the electrode was changed to a metal other than platinum was used.

  In this example, an example of reduction of carbon dioxide by sunlight using the semiconductor element of the first embodiment will be described.

  An amorphous silicon layer having a thickness of 100 microns was formed as a second semiconductor layer on a copper electrode having a thickness of 100 microns and 50 mm square. On the amorphous silicon layer, a titanium oxide layer having a thickness of 50 microns was formed as a first semiconductor layer by a sol-gel method.

The obtained semiconductor element was subjected to carbon dioxide decomposition reaction in the configuration of FIG. The size of the transparent reaction vessel was 20 cm square at the bottom and 20 cm in height, and a 0.1 M sodium hydrogen carbonate aqueous solution was added up to a depth of 10 cm. The semiconductor element was immersed in this aqueous solution with the second semiconductor layer facing upward. When carbon dioxide gas was blown into the aqueous solution and irradiated with sunlight having an intensity of 1.0 kW / m ² from above the container, methane was simultaneously generated from the electrode surface and oxygen was simultaneously generated from the first electrode surface. After the reaction for 1 hour, the components of the aqueous electrolyte solution were analyzed by high performance liquid chromatography. As a result, methanol that was not detected before the reaction was detected after the reaction, and was confirmed to be a reduction product of carbon dioxide. When a similar reaction was performed using a copper electrode and a semiconductor element produced by combining the first semiconductor and the second semiconductor shown in Table 1, generation of methanol and methane, which are reduction products of carbon dioxide, was observed. confirmed. The generation of methanol and methane was also confirmed when the electrodes shown in Table 2 were used instead of the copper electrodes.

The obtained device was also subjected to a carbon dioxide reduction reaction in the gas phase by the method shown in FIG. The reaction vessel used was the same as in the aqueous solution experiment. Before the reaction, carbon dioxide was blown into the container to fully fill it. The carbon dioxide filled inside contained water vapor at 25 degrees and humidity of 80%. When sunlight with an intensity of 1.0 kW / m ² was irradiated from above the container, methane was generated from the electrode surface and oxygen was simultaneously generated from the first electrode surface. After the reaction for 1 hour, the components of the aqueous electrolyte solution were analyzed by high performance liquid chromatography. As a result, methanol that was not detected before the reaction was detected after the reaction, and was confirmed to be a reduction product of carbon dioxide. When a similar reaction was performed using a copper electrode and a semiconductor element produced by combining the first semiconductor and the second semiconductor shown in Table 1, generation of methanol and methane, which are reduction products of carbon dioxide, was observed. confirmed.

  In this example, an example of reduction of carbon dioxide by sunlight using the semiconductor element of the second embodiment will be described.

  An amorphous silicon layer having a thickness of 100 microns was formed as a second semiconductor layer on a copper electrode having a thickness of 100 microns and 50 mm square. A tungsten oxide layer having a thickness of 50 microns was formed as a first semiconductor layer on the amorphous silicon layer by a sol-gel method. Further, a titanium oxide layer having a thickness of 10 nm was formed as a photocatalytic layer on the first semiconductor layer.

The obtained semiconductor element was subjected to carbon dioxide decomposition reaction in the configuration of FIG. The size of the transparent reaction vessel was 20 cm square at the bottom and 20 cm in height, and a 0.1 M sodium hydrogen carbonate aqueous solution was added up to a depth of 10 cm. The photoelectrochemical reaction element was immersed in this aqueous solution with the second semiconductor layer facing upward. When carbon dioxide gas was blown into the aqueous solution and irradiated with sunlight having an intensity of 1.0 kW / m ² from above the container, methane was simultaneously generated from the electrode surface and oxygen was simultaneously generated from the first electrode surface. After the reaction for 1 hour, the components of the aqueous electrolyte solution were analyzed by high performance liquid chromatography. As a result, methanol that was not detected before the reaction was detected after the reaction, and was confirmed to be a reduction product of carbon dioxide. When the amount of methane produced was compared with the device of Example 2, it was confirmed that Example 3 was 1.1 times more.

  In this example, an example of reduction of carbon dioxide by sunlight using the semiconductor elements of the third embodiment and the fourth embodiment will be described.

  An amorphous silicon layer having a thickness of 100 microns was formed as a second semiconductor layer on a copper electrode having a thickness of 100 microns and 50 mm square. A tungsten oxide layer having a thickness of 50 microns was formed as a first semiconductor layer on the amorphous silicon layer by a sol-gel method. On the tungsten oxide layer, platinum ultrafine particles having a diameter of 5 nm were sparsely deposited by vacuum film formation. The area occupied by platinum was 1% with respect to the area of tungsten oxide. This was designated as an element A.

  In the element B, a titanium oxide layer having a thickness of 10 nm was formed as a photocatalytic layer on the first semiconductor layer. Thereafter, platinum ultrafine particles having a diameter of 5 nm were sparsely deposited on the titanium oxide layer by vacuum film formation. The area occupied by platinum was 1% with respect to the area of tungsten oxide.

Each of the obtained semiconductor elements was subjected to carbon dioxide decomposition reaction in the configuration of FIG. The size of the transparent reaction vessel was 20 cm square at the bottom and 20 cm in height, and a 0.1 M sodium hydrogen carbonate aqueous solution was added up to a depth of 10 cm. The photoelectrochemical reaction element was immersed in this aqueous solution with the second semiconductor layer facing upward. When carbon dioxide gas was blown into the aqueous solution and irradiated with sunlight having an intensity of 1.0 kW / m ² from above the container, methane from the electrode surface and oxygen from the first electrode surface both in the device A and device B, respectively. It occurred at the same time. After the reaction for 1 hour, the components of the aqueous electrolyte solution were analyzed by high performance liquid chromatography. As a result, methanol that was not detected before the reaction was detected after the reaction, and was confirmed to be a reduction product of carbon dioxide.

  In this embodiment, an example of side sealing of the photoelectrochemical reaction device of the present invention will be described.

  The side surface of the element of the first embodiment produced in Example 1 was sealed with a transparent photocurable silicone resin. Using this element, water decomposition reaction similar to the method of Example 1 was performed, and the lifetimes of the element of Example 1 and the element subjected to sealing of this example were compared.

The element was continuously irradiated with a pseudo solar light source (intensity 0.7 kW / m ² ) instead of sunlight, and the hydrogen production rate was measured every 2 hours with a gas chromatograph. As a result, in the device of Example 1, the hydrogen generation rate decreased to 10% of the initial value after 100 hours. However, the device of this example showed an initial 90% hydrogen generation rate even after 100 hours. It was confirmed that it was maintained.

101: electrode, 102: second semiconductor layer, 103: first semiconductor layer, 104: photocatalytic layer, 105: promoter, 110: energy, 111: electrons, 112: conduction band, 113: band gap, 114: Valence band, 115: hole, 120: gas, 121: aqueous electrolyte solution, 122: reaction vessel, 123: gas sampling port, 124: sunlight, 125: gas blowing port, 126: gas generated by sunlight irradiation, 131: insulation sealing part, 132: hole, 133: electron, 134: anode, 135: cathode, 136: conduction band, 137: valence band.

Claims (11)

An electrode layer;
A first semiconductor layer;
A second semiconductor having first and second surfaces facing each other, the electrode layer being in contact with the first surface, and the first semiconductor layer being in contact with the second surface And having a layer
The lower end potential of the conduction band of the first semiconductor layer is higher than the hydrogen generation potential from water,
The upper end potential of the valence band of the second semiconductor layer is lower than the oxygen generation potential from water,
The band gap of the first semiconductor layer is larger than the band gap of the second semiconductor layer,
The semiconductor element, wherein the first semiconductor layer is a SiC layer. An electrode layer;
A first semiconductor layer;
A second semiconductor having first and second surfaces facing each other, the electrode layer being in contact with the first surface, and the first semiconductor layer being in contact with the second surface And having a layer
The lower end potential of the conduction band of the first semiconductor layer is higher than the generation potential of methane or methanol from carbon dioxide,
The upper end potential of the valence band of the second semiconductor layer is lower than the oxygen generation potential from water,
The band gap of the first semiconductor layer is larger than the band gap of the second semiconductor layer,
The semiconductor element, wherein the first semiconductor layer is a SiC layer. In claim 1 or 2,
A semiconductor element comprising a photocatalyst layer on a surface of the first semiconductor layer. In claim 1 or 2,
A semiconductor element, wherein a promoter is discretely supported on the surface of the first semiconductor layer. In claim 1 or 2,
A semiconductor device comprising a photocatalyst layer in contact with a surface of the first semiconductor layer facing the second semiconductor layer, and a promoter is discretely supported on the surface of the photocatalyst layer. In claim 1 or 2,
A semiconductor element, wherein a side surface of a stacked body of the first semiconductor layer, the second semiconductor layer, and the electrode layer is sealed with an insulating material. In claim 1 or 2,
The electrode includes at least one metal selected from platinum, gold, silver, copper, nickel, indium, rhodium, palladium, gallium, zinc, titanium, cobalt, iron, and iridium. Semiconductor element. In claim 1 or 2,
The semiconductor element, wherein the second semiconductor layer is a CdS layer. The semiconductor device according to claim 3,
The photocatalyst is a photocatalyst selected from at least one of anatase type titanium oxide, rutile type titanium oxide, and brookite type titanium oxide. The semiconductor device according to claim 4,
The semiconductor device according to claim 1, wherein the promoter is a metal promoter selected from at least one of platinum, silver, copper, gold, nickel oxide, ruthenium oxide, and copper oxide. A transparent container,
A hydrogen production system comprising: the semiconductor element according to claim 1 disposed in the transparent container.

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