Patent Application
20220002886
The present invention relates to a method for producing a nitride semiconductor photoelectrode having a photocatalytic function that exhibits a catalytic function by light irradiation to cause a chemical reaction of an oxidation target material or a reduction target material.
As a related art example,
Oxidation Reaction: 2H2O+4h+→O2+4H+ (1)
Reduction reaction: 4H++4e−→2H2 (2)
Specifically, as illustrated in
Non Patent Literature (NPL) 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.
NPL 2: Yoko Ono et al., “Effects of NiO Thin Film Formed on Nitride Semiconductor Electrodes on Photocurrent Characteristics”, 2017 Autumn Meeting of Electrochemical Society of Japan, 1L31.
When the oxidation electrode 112 as illustrated in
The present invention has been made in view of the above-described related-art technique, and is intended to provide a method for producing a nitride semiconductor photoelectrode capable of improving the light energy conversion efficiency.
In order to achieve the above object, a first aspect in accordance with the present invention provides a method for producing a nitride semiconductor photoelectrode, which includes a first step of forming an n-type gallium nitride layer on an insulating or conductive substrate, a second step of forming an indium gallium nitride layer on the n-type gallium nitride layer, a third step of forming a nickel layer on the indium gallium nitride layer, and a fourth step of heat-treating the nickel layer in an oxygen atmosphere.
A second aspect in accordance with the present invention provides the method according to the first aspect, in which metal organic chemical vapor deposition (MOCVD) is used in the first step and the second step.
A third aspect in accordance with the present invention provides the method according to the first or second aspect, in which electron beam (EB) vapor deposition is used in the third step.
A fourth aspect in accordance with the present invention provides the invention according to any one of the first to third aspects, in which the fourth step is performed at a temperature from 250 to 400° C. for a holding time from 30 minutes to 2 hours.
A fifth aspect in accordance with the present invention provides the method according to any one of the first to fourth aspects, in which the nickel layer after the fourth step becomes an oxygen-excessive nickel oxide layer and exhibits characteristics as a p-type semiconductor.
According to the present invention, it is possible to provide a method for producing a nitride semiconductor photoelectrode capable of improving the light energy conversion efficiency.
Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples, and changes may be made without departing from the spirit of the present invention.
In the present invention, in order to reduce the barrier generated in the valence band when the charge generated in the nitride semiconductor thin film is transferred to the oxygen generation co-catalyst, the oxygen generation co-catalyst is heat-treated in an oxygen atmosphere to produce an oxygen-excessive oxide material which is a p-type conductor, and the light energy conversion efficiency is improved by the use of a nitride semiconductor photoelectrode having an interface in which the valence band of the oxygen generation co-catalyst is at a higher level than the valence band of the nitride semiconductor thin film.
A sapphire substrate was used as the substrate. An n-GaN semiconductor thin film doped with silicon was epitaxially grown by MOCVD on a 2-inch sapphire substrate (first step). Ammonia gas and trimethylgallium were used as growth raw materials, silane gas was used as an n-type impurity source, and hydrogen was used as a carrier gas to be fed into a growth furnace. The film thickness of n-GaN was set to 2 μm, which is sufficient to absorb light. The carrier density was 3×1018 cm−3. Thereafter, indium gallium nitride (InGaN) with an indium composition ratio of 5% was grown (second step). Ammonia gas, trimethylgallium, and trimethylindium were used as growth raw materials, and hydrogen was used as a carrier gas to be fed into the growth furnace. The film thickness was 100 nm, which is sufficient to sufficiently absorb light. Then, a 2-inch semiconductor thin film was cleaved into four equal parts, and one of them was used for electrode production. Next, Ni having a film thickness of about 1 nm was EB-deposited on the surface of InGaN (third step). Then, this semiconductor thin film was put into an electric furnace and heat-treated at 200° C. for 15 minutes in an oxygen atmosphere (fourth step).
As a result of elemental analysis of the cross section, Ni and O were detected, indicating that NiO was formed. The TEM observation result of the cross section revealed that the film thickness of NiO was about 2 nm. Further, the titration of the formed NiO revealed that O was excessively contained with respect to Ni.
Nickel oxide, which is a co-catalyst for oxygen generation, exhibits characteristics as a p-type semiconductor when it has an oxygen-excessive composition. As a result, by producing a nitride semiconductor photoelectrode in which oxygen-excessive nickel oxide is formed on the indium gallium nitride semiconductor thin film, holes generated in the indium gallium nitride semiconductor thin film by light irradiation can move to nickel oxide. In this manner, it is possible to improve the light energy conversion efficiency by promoting charge separation (generation and separation of electrons and holes) in the semiconductor electrode.
Next, the redox reaction test in Example 1 will be described. Here again, the configuration diagram of the water decomposition apparatus (
In the redox reaction test, the surface of InGaN was scribed and the surface of n-GaN was exposed. A lead was connected to a part of the exposed n-GaN surface, and soldering was performed using indium (In). Then, it was covered with an epoxy resin so that the indium surface was not exposed. This was installed as the oxidation electrode 112 in
In Example 2, the heat treatment step (fourth step) of Example 1 was performed at 200° C. for 30 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 3, the heat treatment step of Example 1 was performed at 200° C. for 1 hour to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 4, the heat treatment step of Example 1 was performed at 200° C. for 2 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 5, the heat treatment step of Example 1 was performed at 200° C. for 3 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 6, the heat treatment step of Example 1 was performed at 200° C. for 4 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 7, the heat treatment step of Example 1 was performed at 250° C. for 15 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 8, the heat treatment step of Example 1 was performed at 250° C. for 30 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 9, the heat treatment step of Example 1 was performed at 250° C. for 1 hour to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 10, the heat treatment step of Example 1 was performed at 250° C. for 2 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 11, the heat treatment step of Example 1 was performed at 250° C. for 3 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 12, the heat treatment step of Example 1 was performed at 250° C. for 4 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 13, the heat treatment step of Example 1 was performed at 300° C. for 15 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 14, the heat treatment step of Example 1 was performed at 300° C. for 30 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 15, the heat treatment step of Example 1 was performed at 300° C. for 1 hour to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 16, the heat treatment step of Example 1 was performed at 300° C. for 2 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 17, the heat treatment step of Example 1 was performed at 300° C. for 3 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 18, the heat treatment step of Example 1 was performed at 300° C. for 4 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 19, the heat treatment step of Example 1 was performed at 400° C. for 15 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 20, the heat treatment step of Example 1 was performed at 400° C. for 30 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 21, the heat treatment step of Example 1 was performed at 400° C. for 1 hour to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 22, the heat treatment step of Example 1 was performed at 400° C. for 2 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 23, the heat treatment step of Example 1 was performed at 400° C. for 3 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 24, the heat treatment step of Example 1 was performed at 400° C. for 4 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 25, the heat treatment step of Example 1 was performed at 500° C. for 15 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 26, the heat treatment step of Example 1 was performed at 500° C. for 30 minutes to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 27, the heat treatment step of Example 1 was performed at 500° C. for 1 hour to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 28, the heat treatment step of Example 1 was performed at 500° C. for 2 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 29, the heat treatment step of Example 1 was performed at 500° C. for 3 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Example 30, the heat treatment step of Example 1 was performed at 500° C. for 4 hours to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 1, the heat treatment step of Example 8 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 2, the heat treatment step of Example 9 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 3, the heat treatment step of Example 10 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 4, the heat treatment step of Example 14 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 5, the heat treatment step of Example 15 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 6, the heat treatment step of Example 16 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 7, the heat treatment step of Example 20 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 8, the heat treatment step of Example 21 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
In Comparative Example 9, the heat treatment step of Example 22 was performed in an air atmosphere to produce a nitride semiconductor electrode. Other procedures are the same as those in Example 1.
Table 1 shows the amounts of oxygen and hydrogen gases generated 10 hours after the light irradiation in Examples and Comparative Examples. The generated amount of each gas was normalized by the surface area of the semiconductor photoelectrode. It was found that oxygen and hydrogen were generated upon irradiation with light in all cases.
It was found that the generated amounts of hydrogen and oxygen in Examples 8, 9, 10, 14, 15, 16, 20, 21, and 22 were 10 times higher than those in other Examples.
In Examples 1, 2, 3, 4, 5, and 6, the generated amounts of hydrogen and oxygen were low at any heat treatment holding time. The reason for this is likely that Ni was not sufficiently oxidized or NiO was not oxygen-excessive when the heat treatment temperature was 200° C. or lower.
In Examples 25, 26, 27, 28, 29 and 30, the generated amounts of hydrogen and oxygen were low at any heat treatment temperature holding time. The reason for this is likely that Ni rapidly oxidized and thermally expanded to NiO by the high temperature heat treatment when the heat treatment temperature was 500° C. or higher, and voids were generated at the interface with the photocatalytic thin film, whereby deterioration of the electrode performance was started from the voids, and the catalytic activity was almost deactivated after 10 hours.
In Examples 7, 13, and 19, the generated amounts of hydrogen and oxygen were low at any heat treatment temperature. The reason for this is likely that Ni was not sufficiently oxidized or NiO was not oxygen-excessive when the heat treatment holding time was 15 minutes or less.
In Examples 11, 12, 17, 18, 23, and 24, the generated amounts of hydrogen and oxygen were low at any heat treatment temperature. The reason for this is likely that Ni oxidized and thermally expanded to NiO by the heat treatment for a long time when the heat treatment holding time was 3 hours or longer, and voids were generated at the interface with the photocatalytic thin film, whereby deterioration of the electrode performance was started from the voids, and the catalytic activity was almost deactivated after 10 hours.
From these results, the heat treatment conditions for forming NiO are as follows: temperature is from 250 to 400° C., and holding time is from 30 minutes to 2 hours. In addition, the generated amounts of hydrogen and oxygen in Examples 8, 9, 10, 14, 15, 16, 20, 21, 22 were 10 times higher than those in Comparative Examples 1, 3, 4, 5, 6, 7, 8, and 9, indicating that the atmosphere during heat treatment must be an oxygen atmosphere. When the atmosphere is air, NiO does not become oxygen-excessive NiO and holes cannot move through the barrier at the interface between NiO and InGaN, which likely influence the result.
From the above, by performing the heat treatment step in an oxygen atmosphere at a temperature from 250 to 400° C. for a holding time from 30 minutes to 2 hours, the generated amounts of hydrogen and oxygen by the water splitting reaction was increased (the light energy conversion efficiency was increased).
Although some Examples have been described above, it should be understood that the discussion and drawings which form a part of the disclosure are illustrative and are not intended to be limiting. From this disclosure, various alternative embodiments and operation techniques will be apparent to those skilled in the art.
For example, although the fourth step was performed in an electric furnace in Examples, it may be heat treatment on a hot plate installed in a groove box in an oxygen atmosphere. The aqueous solution 111 used for the redox reaction test is not necessarily an aqueous solution of potassium hydroxide, and may be an aqueous solution of potassium hydroxide or hydrochloric acid. The aqueous solution 121 is not necessarily an aqueous solution of potassium hydrogen carbonate, and may be an aqueous solution of sodium hydrogen carbonate, an aqueous solution of potassium chloride, or an aqueous solution of sodium chloride other. Although the target product was hydrogen in Examples, it is possible to generate a carbon compound by a reduction reaction of carbon dioxide or ammonia by a reduction reaction of nitrogen by changing the reduction electrode 122 (for example, Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, or Ru) or the atmosphere in the cell.
As described above, the method for producing a nitride semiconductor photoelectrode according to the embodiment of the present invention includes a first step of forming an n-type gallium nitride layer 2 on an insulating or conductive substrate 1, a second step of forming an indium gallium nitride layer 3 on the n-type gallium nitride layer 2, a third step of forming a nickel layer 4 on the indium gallium nitride layer 3, and a fourth step of heat-treating the nickel layer 4 in an oxygen atmosphere. Thus, by forming nickel on the indium gallium nitride semiconductor thin film and then heat-treating it in an oxygen atmosphere to produce a nitride semiconductor photoelectrode in which oxygen-excessive nickel oxide is formed, an oxygen generation co-catalyst that does not generate a barrier against which holes cannot move is formed on a semiconductor thin film having a high light absorption rate, whereby the light energy conversion efficiency is improved.
In the first step and the second step, it is preferable to use metal organic chemical vapor deposition (MOCVD). As a result, there is less deviation in film thickness and faster growth is possible compared to other methods, which allows efficient production of semiconductor photoelectrodes with reduced charge transfer barriers.
In the third step, it is preferable to use electron beam (EB) vapor deposition. As a result, a thin film with higher purity can be formed as compared with other methods, which allows efficient production of a semiconductor photoelectrode having a reduced charge transfer barrier.
The fourth step is preferably performed at a temperature from 250 to 400° C. for a holding time from 30 minutes to 2 hours. This makes it possible to realize a nitride semiconductor photoelectrode that can maintain high light energy conversion efficiency for a long time.
The nickel layer 4 after the fourth step becomes the oxygen-excessive nickel oxide layer 4 and exhibits characteristics as a p-type semiconductor. This allows holes generated in the indium gallium nitride semiconductor thin film by light irradiation to move to nickel oxide.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-226249 | Dec 2018 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/045259 | 11/19/2019 | WO | 00 |
