JP2015063425A - Hydrogen production system using sunlight - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen production system capable of generating hydrogen by using sunlight through irradiating a surface of a photocatalyst with light in a high efficiency.SOLUTION: The hydrogen production system includes: a plurality of lenses 102 for focusing light from the sun 101; a plurality of optical fibers 42 each propagating light focused by each of the plurality of lenses 102; and a hydrogen production apparatus 104 to which each one end of the plurality of optical fibers 42 is connected. The hydrogen production apparatus 104 includes: a water-containing liquid; a photocatalyst; and a plurality of light guide members for emitting light while guiding light made incident into the end part, which are arranged in a vessel. Thereby, it becomes possible to irradiate the photocatalyst with the sunlight, propagated by the optical fibers 42, from the light guide members, and consequently, the water contained in the liquid can be decomposed into hydrogen and oxygen by a photochemical reaction of the photocatalyst, and hydrogen can be produced.

Description

  The present invention relates to a technology for producing hydrogen and oxygen by decomposing water using sunlight.

  An apparatus that decomposes water using a semiconductor or titanium oxide as a photocatalyst to generate hydrogen gas or oxygen gas has been proposed.

  For example, in Patent Document 1, a metal electrode and a nitride semiconductor electrode connected to each other are installed in a solvent, light energy such as sunlight is absorbed by the nitride semiconductor electrode, and the metal electrode or the nitride semiconductor electrode A technique for decomposing a solvent on the surface and generating hydrogen gas or oxygen gas is disclosed.

  In Patent Document 2, an anode and a cathode, one of which is a nitride semiconductor, are electrically connected and immersed in an aqueous solution, a voltage is applied between the anode and the cathode, and light is applied to the nitride semiconductor. Thus, a technique for generating hydrogen gas is disclosed.

  Patent Document 3 is a device that decomposes water and the like with the same configuration as Patent Document 2, and forms a dot-shaped or rod-shaped convex portion on the surface of a nitride semiconductor to improve the reaction efficiency of the surface. Is disclosed.

JP 2003-24764 A International Publication No. 2006/082801 JP 2008-264769 A

  When water is efficiently decomposed by irradiating the photocatalyst with light, it is necessary to increase the surface area of the photocatalyst and irradiate the entire surface of the photocatalyst with light. With the technique proposed in Patent Document 3, it is possible to increase the surface area of a nitride semiconductor, but a technique for efficiently irradiating light on the surface is not disclosed.

  An object of the present invention is to provide a hydrogen production system capable of generating hydrogen by irradiating light on the surface of a photocatalyst with high efficiency using sunlight.

  In order to achieve the above object, in the present invention, a plurality of lenses for condensing sunlight, a plurality of optical fibers for propagating light collected by the plurality of lenses, and one end of the plurality of optical fibers are connected. A hydrogen production system including a hydrogen production apparatus is provided. The hydrogen production apparatus includes a container, a liquid containing water, a photocatalyst, and a plurality of light guide members that emit light while guiding light incident on the end portion. One ends of the plurality of optical fibers are connected to end portions of the plurality of light guide members.

  According to the present invention, sunlight is collected by a plurality of lenses, the collected light is propagated through an optical fiber, is incident on a light guide member, and is emitted while being guided. Light can be irradiated. Therefore, hydrogen gas can be produced by efficiently decomposing water with the photocatalyst.

Explanatory drawing which shows the structure of the hydrogen production system using the sunlight of this invention. Explanatory drawing which shows the example which installed the hydrogen production system of FIG. 1 in the house. The block diagram which shows the outline of the whole structure of the hydrogen production apparatus of 1st Embodiment. (A)-(c) Explanatory drawing which shows the example of a shape of the lens 102 of the system of FIG. Sectional drawing which shows the upper part of the light-guide plate 41 and the photochemical electrode plate (photocatalyst) 3 of the light irradiation part 4 of 1st Embodiment. Sectional drawing which shows the light guide plate 41 of the light irradiation part 4 of 1st Embodiment, the lower part of the photochemical electrode plate 3, and the electricity supply electrode 51. FIG. Sectional drawing which shows the lower part and photoconductive electrode 51 of the photochemical electrode plate 4 of 1st Embodiment. (A) is a front view of the electricity supply electrode 51 carrying the some photochemical electrode plate 4 of 1st Embodiment, (b) is a partial cross section figure of the electricity supply electrode and light irradiation part 4 of figure (a). (A)-(e) Sectional drawing of the processus | protrusion 32 of the photochemical electrode plate 4. FIG. (A)-(c) Sectional drawing of the unevenness | corrugation of the surface of the light-guide plate 41. FIG. (A)-(f) Sectional drawing which shows the manufacturing process of the processus | protrusion 32 of the photochemical electrode plate 4 by MOCVD method. An electron micrograph of the protrusion 32 of the photochemical electrode plate 4. Sectional drawing of the protrusion 32 of the photochemical electrode plate 4. FIG. (A)-(g) Sectional drawing which shows the manufacturing process of the processus | protrusion 32 of the photochemical electrode plate 4 by MBE method. It is a figure which shows the process of forming the electrode pad 40 in the photochemical electrode plate 4, (a-1), (b-1), (c-1) and (e-1) are top views, (a-2) , (B-2), (c-2), (d) and (e-2) are sectional views. Sectional drawing which shows the hydrogen production apparatus of 2nd Embodiment.

  Embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 1 and FIG. 2, the hydrogen production system using sunlight of the present invention propagates a plurality of lenses 102 that collect light from the sun 101 and light collected by the plurality of lenses 101, respectively. And a hydrogen production apparatus 104 to which one ends of the plurality of optical fibers 42 are connected.

  The plurality of lenses 102 is a place where sunlight is irradiated as shown in FIG. 2, and is disposed on a place where an area for arranging the plurality of lenses 102 can be secured, for example, on the roof of a house or on the roof of a building. . Since the hydrogen production apparatus 103 has an area occupied smaller than that of the plurality of lenses 102, it can be arranged at a desired location. The optical fiber 42 propagates the light collected by the lens 102 and guides it to the hydrogen production apparatus 103. Since the optical fiber 42 can be deformed into a desired shape, it can be easily routed from the lens 102 to the hydrogen production apparatus 103. Further, a plurality of optical fibers 42 can be bundled and connected to the hydrogen production apparatus 104.

  As shown in FIG. 3, the hydrogen production apparatus 104 emits a container 2, a liquid 1 containing water, a photocatalyst 3, and a plurality of light that are emitted from the light incident on the end portion while being guided in the container 2. The light guide member 4 is included. One ends of the plurality of optical fibers 42 are connected to end portions of the plurality of light guide members 4. Thereby, since the sunlight which the optical fiber 42 propagated can be irradiated toward the photocatalyst 3 from the light guide member 4, the water contained in the liquid 1 is decomposed into hydrogen and oxygen by the photochemical reaction of the photocatalyst. Hydrogen can be produced.

  Since a large number of lenses 102 can be arranged on the roof of a house or the roof of a building, sunlight can be efficiently collected and irradiated onto the photocatalyst 3 in the hydrogen production apparatus. Therefore, hydrogen can be produced with high efficiency. The plurality of lenses 102 can be arranged with the light incident surfaces facing the upper surface. Thereby, a large number of lenses 102 can be arranged even in a limited area.

  As shown in FIG. 4, the light guide rod 103 can be disposed between the plurality of lenses 102 and the optical fiber 42. By fixing the light guide rod 103 to the plurality of lenses 102 in advance, the optical fiber 42 only needs to be connected to the end of the light guide rod 103, so that the connection between the optical fiber 42 and the lens 102 is facilitated. .

  The shape of the lens 102 may be any shape as long as the top surface is a convex lens and the light from the moving sun can be collected at a predetermined position for a long time. For example, as shown in FIGS. 4A and 4B, a substantially truncated cone shape having a substantially spherical upper surface and a lower surface convex downward can be used. In addition, a spherical lens can be used as the lens 102 as shown in FIG. When the end face of the optical fiber 103 or the end face of the light guide bar 103 is used, the end face of the light guide bar 103 is disposed at the condensing position of the lens 102 so that the light collected by the lens 102 is reflected by the optical fiber 103 or the light guide bar. 103 can be propagated. The light guide rod 103 can be cylindrical or prismatic.

  Hereinafter, the hydrogen production apparatus 104 will be described in detail.

<First Embodiment>
As shown in FIG. 5, the hydrogen production apparatus 104 of the first embodiment includes a plurality of photochemical electrode plates (hereinafter referred to as photochemical electrode plates 3) in which the layer 31 of the photocatalyst 3 is disposed on at least one side, Thus, the structure is arranged in the container 2. The plurality of photochemical electrode plates 3 are arranged with a gap in the container 2 so that the photocatalytic layers 31 face each other. The light guide member 4 has a plate shape, and is disposed in the gap between the photochemical electrode plates 3 arranged in opposition to each other, and irradiates light toward the surface of the photochemical electrode plate 3. Between the light guide member 4 and the photochemical electrode plate 3, there is a space in which the liquid 1 containing water is disposed.

  The container 2 includes a supply port 14 for supplying the liquid 1 containing water, and a gas extraction port 6 for taking out a gas generated by the photochemical electrode plate 3 decomposing water.

  In the present invention, a plurality of photochemical electrode plates 3 can be arranged in a small volume by arranging a plurality of photochemical electrode plates 3 with a gap so that the photocatalytic layers 31 face each other. Further, by arranging the light guide member 4 between the arranged photochemical electrode plates 3, the surface of the photochemical electrode plate 3 does not become a shadow of another photochemical electrode plate with respect to the light guide member 4, and the photochemical electrode plate The entire surface of 3 can be irradiated with light. Since water exists in the space 5 between the photochemical electrode plate 3 and the light guide member 4, the photocatalytic layer 31 of the photochemical electrode plate 3 can decompose water by photochemical reaction and generate gas. Therefore, when installed, the gas generator of the present invention has a small occupation area, and can irradiate light onto the surface of the photochemical electrode with high efficiency to generate gas by a photochemical reaction.

  As the light guide member 4, a light guide plate 41 that sequentially emits light incident on the end as shown in FIG. 5 can be used.

  One end of the optical fiber 42 is connected to the end of the light guide plate 41 as shown in FIG. The other end of the optical fiber 42 is drawn out of the container 2, and the sunlight collected by the lens 102 is guided to the light guide plate 41 and is incident on the light guide plate 41.

  The optical fibers 42 are bundled as shown in FIGS. 1 and 3, drawn into the container 2 from the optical fiber 42 flange 21 of the container 2, and routed to the respective light guide plates 41. By using the optical fiber 42, even if a large number of light guide plates 41 are arranged in the container 2, one or more optical fibers 42 are connected to each light guide plate 41 so that light enters the light guide plate 41. Can do. It is also possible to have a structure in which a plurality of optical fibers 42 are connected to one light guide plate 41 so that light is incident on one light guide plate from the plurality of optical fibers 42. Accordingly, it is possible to emit light having a high intensity from the light guide plate 41. In addition, a large-area light guide plate 41 can be used.

  Moreover, it is desirable that the surface of the light guide plate 41 is formed with a concavo-convex structure 43 that emits light while diffusing light propagating therethrough. Thereby, since the light radiate | emitted from the light-guide plate 41 turns into diffused light, the nonuniformity of the light intensity irradiated to the photocatalyst layer 31 can be reduced.

  Next, the photochemical electrode plate 3 will be described. A nitride semiconductor layer 33 can be used as the photocatalytic layer 31 of the photochemical electrode plate 3. The nitride semiconductor layer 33 is preferably provided with a plurality of protrusions 32 on the surface because the surface area can be increased. The nitride semiconductor layer 33 is preferably a single crystal, and the plurality of protrusions 32 are preferably single crystal protrusions epitaxially grown on the single crystal nitride semiconductor layer 33. This is because the use of the single crystal nitride semiconductor layer 33 and the single crystal protrusions 32 can improve the efficiency of the photochemical reaction.

  The carrier concentration of the single crystal nitride semiconductor layer 33 is preferably higher than the carrier concentration of the protrusions 32. As a result, one of electrons and holes generated when the protrusions 32 and the surface of the nitride semiconductor layer 33 absorb light can flow through the nitride semiconductor layer 33 having a high carrier concentration with low resistance, which will be described later. The metal electrode 7 (see FIG. 3) can be reached with low loss. The decomposition reaction of water is caused in the metal electrode 7 by the electrons or holes that have reached the metal electrode 7.

  As shown in FIGS. 6, 7, and 8, the photochemical electrode plate 3 is mounted on a current-carrying electrode 51 that also serves as a support. An electrode pad 40 is disposed on a part of the nitride semiconductor layer 33 of the photochemical electrode plate 3, and the portion where the electrode pad 40 is disposed is sandwiched between clamp electrodes 141 provided on the energizing electrode 51. The photochemical electrode plate 3 is fixed to the energizing electrode 51 and is electrically connected at the same time. Electrons or holes generated in the protrusions 32 and the nitride semiconductor layer 33 flow through the nitride semiconductor layer 33 as described above, and then flow through the electrode pad 40, the clamp electrode 141, and the conducting electrode 51. Then, the conductor 8 connected to the energizing electrode 51 is cut off and reaches the metal electrode plate 7.

  Also, as shown in FIGS. 8A and 8B, by using a multistage energizing electrode 51, a plurality of photochemical electrode plates 3 having a smaller diameter than the height of the container 2 are arranged side by side, for example, the wafer size. It becomes possible to constitute one photochemical electrode plate 3 having a large area by being arranged in the same plane.

  As shown in FIG. 8B, when the photochemical electrode plate 3 includes the photocatalyst layer 31 on both surfaces, the light guide members 4 can be disposed on both sides of the photochemical electrode plate 3.

  The nitride semiconductor layers 33 and the protrusions 32 of the plurality of photochemical electrode plates 3 may have a surface partly formed of GaN and the other part formed of InGaN. Since GaN absorbs ultraviolet light and InGaN absorbs visible light, the nitride semiconductor layer 33 and the protrusion 32 include regions of GaN and InGaN, so that light having a wide wavelength can be used for the photochemical reaction. It is also possible to add Al to InGaN and use it as AlInGaN. By adding Al, the absorption wavelength can be shifted to the short wavelength side, so that the absorption wavelength of the photocatalyst layer 31 can be controlled in accordance with the wavelength of the light source.

  Next, the structure of the container 2 will be described. As shown in FIG. 3, the container 2 can be divided into a first chamber 11 and a second chamber 12 by a partition wall 9. In both the first chamber 11 and the second chamber 12, the liquid 1 containing water is stored. The photochemical electrode plate 3, the light guide member 4, and the gas outlet 6 are disposed in the first chamber 11. A metal electrode 7 is disposed in the second chamber. The photochemical electrode plate 3 and the metal electrode 7 are connected by a conductive wire 8.

  One of holes and electrons generated by absorption of light by the nitride semiconductor layer 33 and the protrusions 32 of the photochemical electrode plate 3 is used for the decomposition of water on the surfaces of the nitride semiconductor layer 33 and the protrusions 32, and the other One reaches the metal electrode 8 through the conductor 8 and is used for water decomposition on the surface of the metal electrode 8. Therefore, by having a structure in which the container 2 is partitioned into the first chamber 11 and the second chamber 12, different gases are generated in the first chamber 11 and the second chamber 12. The second chamber 12 is provided with a second gas outlet 13 for extracting the gas generated in the second chamber 12. Thereby, since oxygen and hydrogen can be separately taken out from the first chamber 11 and the second chamber 12, respectively, there is an advantage that it is not necessary to separate them after taking out.

  The hydrogen production apparatus according to the first embodiment will be described in more detail.

  Hereinafter, an example in which the n-type nitride semiconductor layer 33 is used as the photocatalytic layer 31 will be described. When the n-type nitride semiconductor layer 33 is used as the photocatalytic layer 31, the photochemical electrode plate 3 serves as an anode to generate oxygen gas, and the metal electrode 7 serves as a cathode to generate hydrogen gas. Therefore, oxygen gas is extracted from the gas outlet 6 of the first chamber 11, and hydrogen gas is extracted from the gas outlet 13 of the second chamber 12 to the cathode chamber.

  In addition, the first chamber 11 and the second chamber 12 are provided with water supply ports 14 and 15 and drain ports 16 and 17 for the liquid 1 containing water, and can supply or drain water as necessary. The container 2 is made of, for example, a vinyl chloride resin. The partition wall 9 may be a wall or an ion permeable membrane.

  For the metal electrode 7, for example, a metal such as platinum, gold, or palladium is used.

As the nitride semiconductor layer 33 of the photochemical electrode plate 3, a GaN single crystal layer or a GaN single crystal layer laminated with an In _1-x Ga _x N (0 ≦ x ≦ 1) single crystal layer is used. The nitride semiconductor layer 33 absorbs light having an energy larger than the band gap among the light irradiated from the light guide member 4, and generates electron-hole pairs. The holes move to the surface of the n-type nitride semiconductor layer 33, and the electrons move to the metal electrode 7 through the conductive wire 8. Water is decomposed by the oxidation-reduction reaction, and oxygen gas is generated from the surface of the nitride semiconductor layer 33 and hydrogen gas is generated from the surface of the metal electrode 7.

The surface of the nitride semiconductor layer 33 is preferably provided with a plurality of protrusions 32 as described above because the surface area can be increased. As the shape of the protrusion 32, a rod-like shape with a sharp tip as shown in FIGS. 9A to 9E and a polygonal pyramid shape can be formed relatively easily. The period of the protrusions can be several hundred nm to several tens of μm, and the diameter of the protrusions can be about several tens of nm to several μm. The material of the protrusion 32 is GaN single crystal, In _1-x Ga _x N (0 ≦ x ≦ 1) single crystal, or Al _x In _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y). ≦ 1, x + y ≦ 1) A single crystal can be formed. Further, as shown in FIG. 9E, an In _1-x Ga _x N (0 ≦ x ≦ 1) layer can be formed only at the tip of the GaN protrusion 32.

  Specifically, in the structure of FIG. 9A, an n-type GaN film (33) is formed on a sapphire substrate 30, and a GaN protrusion 32 is formed thereon. In the structure of FIG. 9B, an n-type GaN film (33) is formed on a sapphire substrate 30, and an InGaN protrusion 32 is formed thereon. In the structure of FIG. 9C, an n-type GaN film (33) and an n-type InGaN film 133 are formed on a sapphire substrate 30, and an InGaN protrusion 32 is formed thereon. 9D, an n-type GaN film (33) is formed on a sapphire substrate 30, a GaN protrusion 32 is formed thereon, and an InGaN layer 132 is formed on the outer periphery of the GaN protrusion 32. ing. In the structure of FIG. 9E, an n-type GaN film (33) is formed on a sapphire substrate 30, a GaN protrusion 32 is formed thereon, and an InGaN layer 132 is formed at the tip of the GaN protrusion 32. ing.

  As the light guide plate 41, an acrylic resin can be used. Other light-transmitting resins, sapphire, quartz, and the like can also be used as the light guide plate 41. The light guide plate 41 includes a light extraction structure such as a fine uneven structure 43 that sequentially emits light from the surface while propagating light.

  A known technique such as connector connection can be used for the end portion of the light guide plate 41 and the connection portion of the optical fiber 41. In order to reduce light loss due to an air gap between the end face of the optical fiber 41 and the end portion of the light guide plate 41, it is preferable to insert a gel-like matching oil (silicone resin) 21 into the gap.

  A fine concavo-convex structure 43 is formed on the surface of the light guide plate 41 for sequentially emitting light propagating through the inside while diffusing. The specific shape of the concavo-convex structure 43 may be any shape, such as a hemispherical shape, a rectangular parallelepiped shape, or a cone shape as shown in FIGS. In order to improve, the shape and density are preferably such that the number of surfaces parallel to the main plane of the light guide plate 41 is as small as possible. Regarding the dimensions, the diameter of the bottom surface of the convex portion of the concavo-convex structure 43 is 1 μm or more and 500 μm or less, preferably 5 μm or more and 50 μm or less.

  In the hydrogen production apparatus of the present embodiment, the interval L between the photochemical electrode plates 3 can be set to 5 mm, for example, and the thickness t of the light guide plate 41 having the concavo-convex structure can be set to 3 mm. As a result, the thickness of the space 5 between the light guide plate 41 and the photochemical electrode plate 3 is less than 1 mm, and the light guide plate 41 and the photochemical electrode plate 3 are close to each other, so that light emitted from the light guide plate 41 is hardly attenuated. The photochemical electrode plate 3 can be irradiated.

  The liquid 1 containing water may be water, but is preferably slightly acidic or alkaline in order to promote the oxidation-reduction reaction and efficiently carry out water decomposition. Moreover, dissolution of the nitride semiconductor layer 33 in the solution can be prevented by using the liquid 1 containing water as the electrolytic solution in the pH range of 5-8. When the liquid 1 is made alkaline, an aqueous solution of sodium hydroxide or potassium hydroxide is used as the liquid 1, and when it is made acidic, an aqueous solution of sulfuric acid or hydrochloric acid is used as the liquid 1. In addition, an aqueous solution of sodium sulfate, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, or the like can be used as the liquid 1. The concentration may be about 1 mol / L or less, for example.

  The operation of the hydrogen production apparatus of FIG. 3 of the present invention will be described. The drain ports 16 and 17 are closed, and the liquid 1 containing water is supplied from the water supply ports 14 and 15 to the first chamber 11 and the second chamber 12 of the container 2. When sunlight enters the light guide plate 41 of the first chamber 11 from the optical fiber 42, the light is diffused and emitted from the surface of the light guide plate 41 while propagating through the light guide plate 41, and on the surface of the opposing photochemical electrode plate 3. The nitride semiconductor layer 33 and the protrusion 32 are irradiated. The nitride semiconductor layer 33 absorbs light having an energy larger than the band gap among the light irradiated from the light guide member 4, and generates electron-hole pairs. The holes move to the surface of the n-type nitride semiconductor layer 33 and decompose water to generate oxygen gas. The electrons flow through the nitride semiconductor layer 33, the electrode pad 40, the clamp electrode 141, and the support / current-carrying electrode 51, reach the conductor 8, and move to the second chamber 12 metal electrode 7 through the conductor 8. Water is decomposed on the surface of the metal electrode 7 to generate hydrogen gas. Oxygen gas is taken out from the gas outlet 6 of the first chamber 11. Hydrogen gas is taken out from the gas outlet 13 of the second chamber 12.

  In this invention, since many photochemical electrode plates 3 can be arrange | positioned in the container 2 and each can be irradiated with sunlight, hydrogen gas and oxygen gas can be manufactured efficiently.

  Here, an example of the manufacturing method of the photochemical electrode plate 3 will be described.

  First, the nanorod-shaped protrusion 32 is grown into the nitride semiconductor layer 33 by a selective growth method using a mask. Of the protrusions 32 in FIGS. 9A to 9E, the protrusions 32 in FIGS. 9A to 9C can be formed by either the MOCVD method or the MBE method. The protrusion 32 in FIG. 9D can be formed by the MOCVD method, and the protrusion 32 in FIG. 9E can be formed by the MBE method.

  A specific example in which the protrusions 32 of FIGS. 9A and 9D are grown on the sapphire substrate 30 by MOCVD will be described below with reference to FIG. However, in FIGS. 9A and 9D, a rod-like protrusion 32 with a sharp tip is shown as an example, but the protrusion 32 to be grown has a hexagonal pyramid shape without a rod portion.

First, a wafer-like sapphire substrate 30 (FIG. 11A) is put into an MOCVD apparatus, and then thermal cleaning is performed at 1000 ° C. for 10 minutes in a hydrogen atmosphere. GaN as a buffer layer was supplied at a temperature of 500 ° C. with trimethylgallium (TMG): 10.4 μmol / min, NH ₃ : 3.3LM (LM is l / min in the standard state) for 3 minutes. The layer is grown at a low temperature. The low-temperature buffer layer is crystallized by heating at 1000 ° C. for 30 seconds. Under a temperature of 1000 ° C., TMG: 45 μmol / min and NH ₃ : 4.4LM are supplied for 60 minutes to grow the underlying GaN layer. The film thickness is about 3 μm. Under a temperature of 1000 ° C., TMG: 45 μmol / min, SiH ₄ : 2.7 × 10 ⁻⁹ μmol / min, NH ₃ : 4.4LM are supplied for 60 minutes, and a Si-doped n-type GaN template layer (33) is provided. Grow (FIG. 11 (b)). The GaN template layer (33) has a thickness of about 3 μm and a carrier concentration of about 5 × 10 ¹⁸ cm ⁻³ .

Next, a SiO ₂ film is formed as a mask layer 81 on the GaN template layer (33) by sputtering (FIG. 11C). The film thickness was 0.5 μm. Thereafter, a resist pattern is formed on the mask layer 81. A circular opening is formed in the resist pattern. The diameter of the opening is, for example, 1 μm. After forming the resist pattern, only the opening of the resist pattern is removed from the mask layer 81 using buffered hydrofluoric acid. As the SiO ₂ removal method, SiO ₂ removal by a dry etching method using a processing gas such as CHF ₃ can also be selected. The resist mask is removed to complete the mask of the mask layer 81 (FIG. 11D).

Next, the wafer on which the mask layer 80 is formed is again put into the MOCVD apparatus. A GaN crystal is selectively grown in the opening of the mask layer 81. An example of growth conditions is shown below. Under a temperature of 870 ° C., TMG: 18 μmol / min, NH ₃ : 4.4LM is supplied for 5 minutes, and a non-doped GaN layer having a height of about 1.4 μm is grown in the opening of the SiO ₂ film. At this time, in the portion of the mask layer 81 covered with the mask, the GaN layer does not grow crystal, and only the opening grows selectively. As a result, the protrusions 32 of the selectively grown GaN layer are formed (FIG. 11E). Thereby, the protrusion of FIG. 9A can be formed. Here, the GaN layer in the selective growth portion is not Si-doped, but may be Si-doped in the same manner as the GaN template layer (33).

Furthermore, an InGaN layer can be further formed on the GaN layer in the selective growth portion. As an example of growth conditions in this case, TMG: 3.6 μmol / min, trimethylindium TMI: 10 μmol / min, NH ₃ : 4.4LM are supplied for 33 seconds at a temperature of 700 ° C., and the film thickness is about 2. A 2 nm InGaN layer is grown (FIG. 11F). Thereby, the protrusion 32 having the structure shown in FIG. 9D is formed.

  Finally, the mask layer 81 can be removed. As a result, the surface of the nitride semiconductor layer 33 can be exposed. Therefore, even on the surface of the nitride semiconductor layer 33, light is absorbed and a pair of electrons and holes is generated, causing water to decompose. Can do. As a method for removing the mask layer 81, wet etching using an aqueous solution such as BHF (buffered hydrofluoric acid) can be used.

  The GaN protrusions 32 obtained by the above growth method have a hexagonal pyramid shape with a height of about 1.4 μm (FIG. 12). FIG. 13 shows the cross-sectional structure.

  Next, a specific example in which the protrusions 32 of FIGS. 9A, 9D, and 9E are grown on the sapphire substrate 30 by MBE will be described with reference to FIG.

A low-temperature buffer layer is formed on the sapphire substrate 30, and an n-type GaN film (33) having a thickness of 2 to 6 μm is formed on the sapphire substrate 30 by using, for example, the MOCVD method (FIGS. 14A and 14B). At this time, Si or the like is added as an n-type dopant, and at least the carrier concentration is 10 ¹⁷ cm ⁻³ or more.

  Next, a Ti thin film or Mo thin film of about 5 nm is deposited as a mask layer 81 on the n-type GaN film (33) by using, for example, EB or sputtering (FIG. 14C). Thereafter, a hole pattern having a period of 400 nm to 4 μm and a diameter of 100 to 550 nm is formed in the mask layer 81 by, for example, FIB or electron drawing and dry etching (FIG. 14D).

After nitriding the surface of the mask layer (Ti or Mo thin film) 81 by RF-MBE, GaN is epitaxially grown on the GaN film (33) exposed from the hole of the mask layer 81 at a growth temperature of 900 ° C. (or a growth temperature of 650). InGaN is epitaxially grown into a nanorod shape at a temperature of 0 ° C.). Thereby, the protrusion 32 of FIGS. 9A and 9B can be formed (FIGS. 14E and 14F). At this time, the high-density dislocations (10 ⁹ to 10 ¹⁰ cm ⁻² ) existing in the underlying n-GaN film (33) bend laterally through the facet surface in the initial stage of nanorod formation, Disappears on the way, and dislocations penetrating to the top decrease drastically. For example, the InGaN layer 132 may be formed on the protrusion 32 of FIG. 14E at a growth temperature of 650 ° C. (FIG. 14G). Thereby, the protrusion 32 of FIG.9 (e) can be formed.

  Next, a procedure for forming the electrode pad 40 on a part of the upper surface of the nitride semiconductor layer 33 will be described with reference to FIGS. In FIG. 15, FIGS. 15 (a-1), (b-1), (c-1) and (e-1) are top views, but in order to make the types of films to be formed easier to understand. Like the cross-sectional view, hatching is applied. 15 (a-2), (b-2), (c-2), and (e-2) are shown in FIGS. 15 (a-1), (b-1), (c-1), ( It is sectional drawing of e-1).

  First, the sapphire substrate 30 on which the nitride semiconductor layer 33 is formed is prepared (FIGS. 15A-1 and 15A-2). The protrusions 32 can be formed in advance on the upper surface of the nitride semiconductor layer 33 by the above-described procedure or the like. However, the protrusion 32 is not formed in the region where the electrode pad 40 is to be formed.

  A resist mask 121 is formed on the upper surface of the nitride semiconductor layer 33 (FIGS. 15B-1 and 15B-2). The resist mask 121 is removed from the region where the electrode pad 40 is to be formed (FIGS. 15C-1 and 15C-2). Thereafter, the resist mask 121 and the nitride semiconductor layer 33 are covered with the electrode film 140 by EB vapor deposition or sputtering (FIG. 15D). Any electrode film 140 may be used as long as it can be electrically bonded to the nitride semiconductor layer 33. For example, the electrode film 140 is formed by sequentially stacking a Ti layer / Al layer / Ti layer / Au layer. Thereafter, the resist mask 121 is removed, whereby the electrode pad 40 having a desired shape is formed (FIGS. 15E-1 and 15E-2).

  As described above, according to the embodiment of the present invention, a plurality of photochemical electrode plates 3 are arranged in a container 2 having a small occupied area and a small volume, and the light guide member 4 is provided on each surface. Since light can be irradiated, the surface area of the photochemical electrode plate 3 that can contribute to the photochemical reaction can be increased. Therefore, the production rates of hydrogen gas and oxygen gas can be increased. In addition, the light use efficiency can be improved by controlling the band gap by controlling the formation of the protrusion 32 and the composition of the nitride semiconductor layer 33.

  In the above-described embodiment, the container 2 is divided into the first chamber 11 and the second chamber 12 to generate oxygen gas and hydrogen gas, respectively, but the present invention is not limited to this configuration. Of course, it is possible to provide only the first chamber 11 and not the second chamber 12. In this case, the metal electrode 7 in the second chamber is not arranged. In the case of a configuration including only the first chamber 11, both hydrogen gas and oxygen gas are generated from the first chamber 11, so that only hydrogen gas is taken out by separating the hydrogen gas and oxygen gas with a hydrogen / oxygen separation filter or the like. be able to.

<Second Embodiment>
A hydrogen production apparatus 104 according to the second embodiment will be described with reference to FIG. In the second embodiment, the photocatalyst 3 is in the form of particles and is dispersed in the liquid 1 containing water and accommodated in the container 2. The plurality of light guide members 4 are inserted into the liquid 1 in which the photocatalyst 3 is dispersed. The ends of the optical fibers 42 are connected to the ends of the light guide members 4. The structure of the light guide member 4 includes a concavo-convex structure 43 on the surface, as in the first embodiment. The light guide members 4 are arranged in the container 2 side by side at intervals.

  Thereby, since the light guide member 4 diffuses and emits light while propagating light inside, the light guide member 4 can efficiently irradiate the photocatalyst dispersed in the liquid 1 with light. The photocatalyst 3 irradiated with light decomposes water contained in the surrounding liquid 1 by a photochemical reaction to generate oxygen and hydrogen. Oxygen and water are taken out from the outlet 6. By separating hydrogen gas and oxygen gas with a hydrogen / oxygen separation filter or the like, only hydrogen gas can be taken out.

  In the second embodiment, the light guide member 4 may be plate-shaped or rod-shaped.

The type of the photocatalyst may be any as long as it can absorb light in the wavelength range of sunlight and decompose water into hydrogen and oxygen. For example, a visible light responsive artificial photosynthesis type photocatalyst having a two-step excitation mechanism is desirable. In the two-stage photoexcitation (Z-scheme) type water splitting system, the water splitting is divided into a hydrogen generating system and an oxygen generating system, and an iodate / iodide (IO ³⁻ / I ⁻ ) is an electron carrier between them. It is connected by reversible ion pairs such as Fe ions. Accordingly, since the energy of light necessary for each system is reduced, it is possible to use long-wavelength visible light with small energy.

Any material may be used for the photocatalyst of the two-stage excitation mechanism, but a typical example of a combination of a photocatalyst for hydrogen generation and a photocatalyst for oxygen generation is shown below. _{Pt / SrTiO 3: Cr-Ta} / WO 3, Pt / TaON-WO 3, Pt / StTiO 3: Rh-BiVO 4, Pt / StTiO 3: Rh-WO 3, Ru / StTiO 3: Rh-BiVO 4, Ru Any of / StTiO ₃ : Rh—WO ₃ , Cr—Rh / GaN: ZnO, and BaTaO ₂ N—WO ₃ , or two or more thereof can be used.

  DESCRIPTION OF SYMBOLS 1 ... Liquid containing water, 2 ... Container, 3 ... Photochemical electrode plate, 4 ... Light guide member, 5 ... Space, 6, 13 ... Gas extraction port, 11 ... 1st chamber, 12 ... 2nd chamber, 30 ... Sapphire Substrate, 31 ... photocatalyst layer, 32 ... protrusion, 33 ... nitride semiconductor layer, 40 ... electrode pad, 41 ... light guide plate, 42 ... optical fiber, 43 ... concave structure, 51 ... current-carrying electrode also serving as a support

Claims (10)

A plurality of lenses for condensing sunlight, a plurality of optical fibers for propagating light collected by the plurality of lenses, and a hydrogen production apparatus to which one end of the plurality of optical fibers is connected,
The hydrogen production apparatus includes a container, a liquid containing water, a photocatalyst, and a plurality of light guide members that emit light while guiding light incident on the end portion, disposed in the container,
One end of the plurality of optical fibers is connected to end portions of the plurality of light guide members. A hydrogen production system using sunlight, wherein:   2. The hydrogen production system according to claim 1, wherein the plurality of lenses are arranged such that a light incident surface faces the upper surface.   3. The hydrogen production system according to claim 1, wherein a light guide bar is disposed between each of the plurality of lenses and the optical fiber. 4. The hydrogen production system according to any one of claims 1 to 3, wherein the hydrogen production apparatus includes a plurality of photochemical electrode plates in which the photocatalyst layer is disposed on at least one side,
The plurality of photochemical electrode plates are arranged with a gap in the container so that the photocatalyst layers face each other,
The light guide member has a plate shape, and is arranged in a gap between the photochemical electrode plates arranged opposite to each other, irradiates light toward the surface of the photochemical electrode plate,
A hydrogen production system using sunlight, wherein there is a space in which the liquid containing water is disposed between the light guide member and the photochemical electrode plate.   5. The hydrogen production system according to claim 1, wherein the container has a supply port for supplying a liquid containing water, and a gas extraction port for taking out a gas generated by decomposing the water by the photocatalyst. And a hydrogen production system using sunlight.   6. The hydrogen production system according to claim 1, wherein unevenness for diffusing emitted light is formed on a surface of the light guide member. Manufacturing system.   7. The hydrogen production system according to claim 4, wherein the photocatalytic layer of the photochemical electrode plate is a nitride semiconductor layer, and the nitride semiconductor layer has a plurality of protrusions on a surface thereof. A hydrogen production system using sunlight.   8. The hydrogen production system according to claim 7, wherein a part of the surface of the photocatalytic layer of the plurality of photochemical electrode plates is formed of GaN, and the other part is formed of InGaN. A hydrogen production system using light. The hydrogen production system according to any one of claims 4 to 8, wherein each of the containers is partitioned into a first chamber and a second chamber that contain the liquid containing water.
The photochemical electrode plate and the light guide member are disposed in the first chamber,
A hydrogen production system using sunlight, wherein a metal electrode is disposed in the second chamber, and the photochemical electrode plate and the metal electrode are connected by a conducting wire.   4. The hydrogen production system according to claim 1, wherein the photocatalyst is dispersed in a liquid containing water and accommodated in the container, and the plurality of light guide members are dispersed with the photocatalyst. A hydrogen production system using sunlight, wherein the system is inserted into the liquid.