Method for manufacturing semiconductor photoelectrodes

Abstract

Provided is a production method for a semiconductor photoelectrode, said method comprising: a first step for forming a semiconductor thin film 12 on an insulative or conductive substrate 11; a second step for forming a metal layer on a surface of the semiconductor thin film 12; and a third step for forming a catalyst layer 13 by oxidizing the metal layer by ozone oxidation.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a semiconductor photoelectrode. [Background technology]

[0002] The device for generating hydrogen through a water splitting reaction using a semiconductor photoelectrode has an oxidation tank and a reduction tank connected via a proton exchange membrane. An aqueous solution and an oxidation electrode are placed in the oxidation tank, and an aqueous solution and a reduction electrode are placed in the reduction tank. The oxidation electrode and reduction electrode are electrically connected by a conductor.

[0003] The water splitting reaction using a photocatalyst consists of a water oxidation reaction and a proton reduction reaction. When light is irradiated onto an n-type photocatalytic material, electrons and holes are generated and separated within the photocatalyst. The holes move to the surface of the photocatalytic material and contribute to the water oxidation reaction. Meanwhile, the electrons move to the reduction electrode and contribute to the proton reduction reaction. Ideally, this oxidation-reduction reaction proceeds, resulting in the water splitting reaction.

[0004] Oxidation reaction: 2H2O + 4h + →O2+4H + Reduction reaction: 4H + +4e - →2H2 [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] S. Yotsuhashi, et al., “CO2 Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics, The Japan Society of Applied Physics, 2012, Volume 51, pp. 02BP07-1-02BP07-3 Summary of the Invention [Problem to be solved by the invention]

[0006] The oxidation electrode is a semiconductor thin film, such as a gallium nitride (GaN) thin film grown on a sapphire substrate, or a heterostructure consisting of gallium nitride, gallium nitride, and aluminum gallium nitride (AlGaN) or gallium nitride and indium gallium nitride (InGaN) layered on a sapphire substrate. When a gallium nitride thin film is irradiated with light in an aqueous solution, oxygen is generated on the surface of the film. However, as shown in the following equation, an etching reaction occurs in which gallium nitride reacts directly with holes, resulting in the loss of the target water oxidation reaction field and a decrease in activity.

[0007] Etching reaction: 2GaN + 6h + +3H2O→N2+6H + +Ga2O3

[0008] To promote the oxygen generation reaction and suppress the etching reaction, a catalytic material such as a NiO layer is formed on the semiconductor surface. To form the NiO layer on the semiconductor surface, a thin metal Ni film is formed on the semiconductor surface, followed by an oxidation process using heat treatment to form the NiO.

[0009] However, when heat-treated, due to the difference in the thermal expansion coefficients between semiconductors and metal oxides (including metals), distortions tend to occur at the interface between the semiconductor and metal oxide, resulting in a loss of adhesion. The thermal expansion coefficient of GaN is 5.6 × 10 -6 K -1 , the thermal expansion coefficients of Ni and NiO are 12 to 14 × 10 -6 K -1 This increases the resistance at the interface between the semiconductor and the metal oxide, which reduces the expected activity of the catalyst layer and the etching reaction suppression effect.

[0010] The present invention has been made in view of the above, and has as its object to improve the light energy conversion efficiency and life span of a semiconductor photoelectrode. [Means for solving the problem]

[0011] A method for manufacturing a semiconductor photoelectrode according to one aspect of the present invention includes the steps of: forming a semiconductor thin film on an insulating or conductive substrate; and forming a metal layer on a surface of the semiconductor thin film. Exposed to ozone at room temperature and oxidizing the metal layer to form a catalyst layer. [Effects of the Invention]

[0012] According to the present invention, the light energy conversion efficiency and life span of a semiconductor photoelectrode can be improved. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode of this embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing an example of the configuration of the semiconductor photoelectrode of this embodiment. [Figure 3] FIG. 3 is a flowchart showing an example of a method for manufacturing the semiconductor photoelectrode of FIG. [Figure 4] FIG. 4 is a flowchart showing an example of a method for manufacturing the semiconductor photoelectrode of FIG. [Figure 5] FIG. 5 is a diagram showing an outline of an apparatus for carrying out an oxidation-reduction reaction test. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments described below, and modifications may be made without departing from the spirit of the present invention.

[0015] [Configuration of semiconductor photoelectrode] Fig. 1 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode of this embodiment. The semiconductor photoelectrode shown in Fig. 1 includes an insulating or conductive substrate 11, a semiconductor thin film 12 disposed on the substrate 11, and a catalyst layer 13 disposed on the semiconductor thin film 12. The catalyst layer 13 is formed by oxidizing a metal layer using ozone oxidation, thereby improving the adhesion of the interface with the semiconductor thin film 12.

[0016] The substrate 11 is an insulating or conductive substrate such as a sapphire substrate, a GaN substrate, a glass substrate, or a Si substrate.

[0017] Gallium nitride, aluminum gallium nitride, or indium gallium nitride is used for the semiconductor thin film 12. Alternatively, metal oxides having photocatalytic functions, such as titanium oxide (TiO2) or tungsten oxide (WO3), or compound semiconductors, such as tantalum nitride (Ta3N5) or cadmium sulfide (CdS), may be used for the semiconductor thin film 12.

[0018] The catalyst layer 13 is made of an oxide of one or more metals selected from the group consisting of Ni, Co, Cu, W, Ta, Pd, Ru, Fe, Zn, and Nb. The thickness of the catalyst layer 13 is preferably 1 to 10 nm, and more preferably 1 to 3 nm, which allows sufficient light transmission. The catalyst layer 13 may cover only a portion of the surface of the semiconductor thin film 12.

[0019] 2, a second semiconductor thin film 14 may be provided between the semiconductor thin film 12 and the catalyst layer 13. That is, the semiconductor photoelectrode in Fig. 2 has the catalyst layer 13 on the second semiconductor thin film 14. The catalyst layer 13 may cover only a portion of the surface of the second semiconductor thin film 14.

[0020] [Method of manufacturing semiconductor photoelectrodes] An example of a method for manufacturing the semiconductor photoelectrode of FIG. 1 will be described with reference to FIG.

[0021] In step 1, a semiconductor thin film 12 is formed on an insulating or conductive substrate 11. The semiconductor thin film 12 may be formed using metal organic chemical vapor deposition (MOCVD).

[0022] In step 2, a metal layer that will become the catalyst layer 13 is formed on the semiconductor thin film 12. The metal layer may be formed by vacuum-depositing a metal on the surface of the semiconductor thin film 12.

[0023] In step 3, the metal layer is oxidized by ozone oxidation to form the catalyst layer 13. For example, the catalyst layer 13 may be formed by subjecting the semiconductor thin film on which the metal layer has been formed to an ozone treatment using a commercially available ozone cleaner.

[0024] An example of a method for manufacturing the semiconductor photoelectrode of Fig. 2 will be described with reference to Fig. 4. The method for manufacturing the semiconductor photoelectrode of Fig. 4 is obtained by adding a step of forming a second semiconductor thin film to the method for manufacturing the semiconductor photoelectrode of Fig. 3.

[0025] In step 1-1, a semiconductor thin film 12 is formed on an insulating or conductive substrate 11.

[0026] In step 1-2, a second semiconductor thin film 14 is formed on the semiconductor thin film 12. The second semiconductor thin film 14 may be formed using an MOCVD method.

[0027] In step 2, a metal layer that will become the catalyst layer 13 is formed on the second semiconductor thin film 14.

[0028] In step 3, the metal layer is oxidized by ozone oxidation to form a catalyst layer 13.

[0029] [Preparation of Examples and Comparative Examples] Examples 1 to 3 in which the semiconductor photoelectrode of this embodiment was fabricated will be described below. Comparative Examples 1 to 3 in which the catalyst layer was formed by heat-treating the metal layer will also be described.

[0030] Example 1 The semiconductor photoelectrode of Example 1 was fabricated using the manufacturing method shown in FIG.

[0031] In step 1, an n-GaN semiconductor thin film was epitaxially grown on a sapphire substrate by MOCVD. Ammonia gas and trimethylgallium were used as growth materials, and hydrogen was used as the carrier gas sent into the growth furnace. Si was used as the dopant element. The n-GaN film thickness was 2 μm. The carrier density was 3 × 10 18 cm -3 It was.

[0032] In step 2, Ni was vacuum-deposited onto the n-GaN semiconductor thin film to a thickness of about 1 nm.

[0033] In step 3, the semiconductor thin film with the Ni layer formed thereon was subjected to ozone oxidation treatment using a Filgen UV ozone cleaner UV253H by exposing it to ozone at a concentration of approximately 200 ppm for 30 minutes at atmospheric pressure and room temperature, forming NiO. TEM observation of the sample cross section revealed that the NiO film thickness was 2 nm.

[0034] Through the above steps, the semiconductor photoelectrode of Example 1 was obtained.

[0035] <Example 2> The semiconductor photoelectrode of Example 2 was fabricated using the manufacturing method shown in FIG.

[0036] In step 1-1, an n-GaN semiconductor thin film was epitaxially grown on a sapphire substrate by MOCVD. Ammonia gas and trimethylgallium were used as growth materials, and hydrogen was used as the carrier gas sent into the growth furnace. Si was used as the dopant element. The n-GaN film thickness was 2 μm. The carrier density was 3 × 10 18 cm -3 It was.

[0037] In step 1-2, Al is deposited on the n-GaN semiconductor thin film. 0.1 Ga 0.9 N semiconductor thin films were epitaxially grown by MOCVD using ammonia gas, trimethylgallium, and trimethylaluminum as growth sources, and hydrogen as the carrier gas sent into the growth furnace.

[0038] In step 2, Al 0.1 Ga 0.9 Ni was vacuum-deposited onto the N semiconductor thin film to a thickness of approximately 1 nm.

[0039] In step 3, the semiconductor thin film on which the Ni layer was formed was subjected to ozone oxidation treatment at atmospheric pressure and room temperature to form NiO, as in Example 1. TEM observation of the sample cross section revealed that the NiO film had a thickness of 2 nm.

[0040] Through the above steps, the semiconductor photoelectrode of Example 2 was obtained.

[0041] Example 3 The semiconductor photoelectrode of Example 3 was fabricated using the manufacturing method shown in Fig. 4. The second semiconductor thin film 14 is different from that of Example 2.

[0042] In step 1-1, an n-GaN semiconductor thin film was epitaxially grown on a sapphire substrate by MOCVD. Ammonia gas and trimethylgallium were used as growth materials, and hydrogen was used as the carrier gas sent into the growth furnace. Si was used as the dopant element. The n-GaN film thickness was 2 μm. The carrier density was 3 × 10 18 cm -3 It was.

[0043] In step 1-2, In was deposited on the n-GaN semiconductor thin film. 0.05 Ga 0.95 N semiconductor thin films were epitaxially grown by MOCVD using ammonia gas, trimethylgallium, and trimethylindium as growth sources, and hydrogen as the carrier gas sent into the growth furnace.

[0044] In step 2, In 0.05 Ga 0.95 Ni was vacuum-deposited onto the N semiconductor thin film to a thickness of approximately 1 nm.

[0045] In step 3, the semiconductor thin film on which the Ni layer was formed was subjected to ozone oxidation treatment at atmospheric pressure and room temperature to form NiO, as in Example 1. TEM observation of the sample cross section revealed that the NiO film had a thickness of 2 nm.

[0046] Through the above steps, the semiconductor photoelectrode of Example 3 was obtained.

[0047] <Comparative example 1> The semiconductor photoelectrode of Comparative Example 1 differs from Example 1 in that after forming a Ni layer, in step 3, a heat treatment was performed in air at 300°C for 1 hour to form NiO. TEM observation of the sample cross section revealed that the NiO film thickness was 2 nm. Other points were the same as in Example 1.

[0048] <Comparative example 2> The semiconductor photoelectrode of Comparative Example 2 differs from Example 2 in that after forming a Ni layer, in step 3, a heat treatment was performed in air at 300°C for 1 hour to form NiO. TEM observation of the sample cross section revealed that the NiO film thickness was 2 nm. Other points were the same as Example 2.

[0049] <Comparative example 3> The semiconductor photoelectrode of Comparative Example 3 differs from Example 3 in that after forming a Ni layer, in step 3, a heat treatment was performed in air at 300°C for 1 hour to form NiO. TEM observation of the sample cross section revealed that the NiO film thickness was 2 nm. Other points were the same as those of Example 3.

[0050] [Oxidation-reduction reaction test] An oxidation-reduction reaction test was carried out for Examples 1 to 3 and Comparative Examples 1 to 3 using the apparatus shown in FIG.

[0051] 5 includes an oxidation tank 110 and a reduction tank 120. An aqueous solution 111 is placed in the oxidation tank 110, and an oxidation electrode 112 is placed in the aqueous solution 111. An aqueous solution 121 is placed in the reduction tank 120, and a reduction electrode 122 is placed in the aqueous solution 121.

[0052] A 1 mol / L aqueous solution of sodium hydroxide was used as the aqueous solution 111 in the oxidation bath 110. A potassium hydroxide aqueous solution or hydrochloric acid may also be used as the aqueous solution 111.

[0053] The semiconductor photoelectrode to be tested was used as the oxidation electrode 112. Specifically, for each of Examples 1 to 3 and Comparative Examples 1 to 3, the n-GaN surface was scribed, a lead wire was connected to part of the surface, soldered using indium, and coated with epoxy resin so that the indium surface was not exposed, and this was installed as the oxidation electrode 112.

[0054] A 0.5 mol / l potassium hydrogen carbonate aqueous solution was used as the aqueous solution 121 in the reduction tank 120. As the aqueous solution 121, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution may also be used.

[0055] Platinum (manufactured by Nilaco) was used for the reduction electrode 122. The reduction electrode 122 may be made of a metal or a metal compound. For example, nickel, iron, gold, silver, copper, indium, or titanium may be used for the reduction electrode 122.

[0056] The oxidation tank 110 and the reduction tank 120 are connected via a proton membrane 130. Protons generated in the oxidation tank 110 diffuse into the reduction tank 120 through the proton membrane 130. Nafion (registered trademark) was used for the proton membrane 130. Nafion is a perfluorocarbon material consisting of a hydrophobic Teflon skeleton made of carbon and fluorine and perfluoro side chains with sulfonic acid groups.

[0057] The oxidation electrode 112 and the reduction electrode 122 are electrically connected by a conductor 132 , and electrons move from the oxidation electrode 112 to the reduction electrode 122 .

[0058] The light source 140 was a 300W high-pressure xenon lamp (illuminance 5mW / cm 2) was used. The light source 140 may be any light source that can emit light of a wavelength that can be absorbed by the material that constitutes the semiconductor photoelectrode installed as the oxidation electrode 112. For example, when the oxidation electrode 112 is made of gallium nitride, the wavelength that can be absorbed by the oxidation electrode 112 is 365 nm or less. The light source 140 may be a light source such as a xenon lamp, a mercury lamp, a halogen lamp, a solar simulant light source, or sunlight, or a combination of these light sources.

[0059] In the oxidation-reduction reaction test, nitrogen gas was flowed at 10 ml / min in each reaction vessel, and the sample area was 1 cm 2 The aqueous solutions 111 and 121 were stirred at the center of the bottom of each reaction vessel at a rotation speed of 250 rpm using a stirring bar and a stirrer.

[0060] After the atmosphere inside the reaction vessel was thoroughly replaced with nitrogen gas, the light source 140 was fixed so as to face the NiO-formed surface of the semiconductor photoelectrode to be tested, which was installed as the oxidation electrode 112, and light was uniformly irradiated onto the semiconductor photoelectrode.

[0061] Immediately after light irradiation, and 50 and 100 hours later, gas samples were collected from each reactor, and the reaction products were analyzed using a gas chromatograph. As a result, it was confirmed that oxygen was produced in the oxidation reactor 110, and hydrogen was produced in the reduction reactor 120.

[0062] In the examples, the target product was hydrogen, but by changing the metal of the reduction electrode (e.g., Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, Ru) or the atmosphere in the cell, it is also possible to produce carbon compounds through the reduction reaction of carbon dioxide, or ammonia through the reduction reaction of nitrogen.

[0063] [Test Results] Table 1 shows the amounts of oxygen and hydrogen gas produced versus light irradiation time in Examples 1 to 3 and Comparative Examples 1 to 3. The amounts of each gas produced were normalized by the surface area of ​​the semiconductor photoelectrode. It was found that oxygen and hydrogen were produced during light irradiation in all examples.

[0064] [Table 1]

[0065] Example 1 had a larger amount of generation immediately after light irradiation than Comparative Example 1. This is believed to be due to a reduction in resistance at the interface between the semiconductor and NiO. Furthermore, when the amounts of generation 50 hours and 100 hours after light irradiation were compared, Example 1 showed a smaller decrease in the amount of generation than Comparative Example 1, demonstrating a longer life. This is believed to be due to the suppression of distortion in each layer near the interface between the semiconductor and NiO, allowing good adhesion to be maintained for a long period of time.

[0066] The results for Examples 2 and 3 and Comparative Examples 2 and 3 were similar.

[0067] As described above, the method for manufacturing a semiconductor photoelectrode of this embodiment includes a first step of forming a semiconductor thin film 12 on an insulating or conductive substrate 11, a second step of forming a metal layer on the surface of the semiconductor thin film 12, and a third step of oxidizing the metal layer by ozone oxidation to form a catalyst layer 13. By using ozone oxidation to oxidize the metal formed on the surface of the semiconductor thin film without a thermal history, the adhesion between the semiconductor thin film and the catalyst layer interface is improved. This can improve the light energy conversion efficiency and lifespan of the semiconductor photoelectrode. [Explanation of symbols]

[0068] 11 Circuit Board 12 Semiconductor thin films 13 Catalyst layer 14 Semiconductor thin films

Claims

1. forming a semiconductor thin film on an insulating or conductive substrate; forming a metal layer on the surface of the semiconductor thin film; and exposing the metal layer to ozone at room temperature to oxidize the metal layer and form a catalyst layer. A method for manufacturing a semiconductor photoelectrode.

2. 2. A method for manufacturing a semiconductor photoelectrode according to claim 1, comprising: forming a second semiconductor thin film on the surface of the semiconductor thin film; In the step of forming the metal layer, the metal layer is formed on the surface of the second semiconductor thin film. A method for manufacturing a semiconductor photoelectrode.

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