JP2018023940A - Optical semiconductor electrode and optical electrochemical cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical semiconductor electrode capable of effectively using irradiated sunlight for a water splitting reaction.SOLUTION: The optical semiconductor electrode 100 has an optical catalytic effect for splitting water, includes a semiconductor layer 103 formed by a semiconductor with a wavelength of an absorption edge thereof in a longer wavelength side than 450 nm, a reflection factor increasing layer 102 arranged in contact with the semiconductor layer 103 on the semiconductor layer 103 and formed by a metallic material containing at least one kind of element selected from Ag and Al and a conductive substance 101 arranged on the reflection factor increasing layer 102 and formed by a material having larger tensile strength than the metallic material of the reflection factor increasing layer 102.SELECTED DRAWING: Figure 9

Description

  The present disclosure relates to a photosemiconductor electrode and a photoelectrochemical cell.

  In order to solve the serious environmental problems and energy problems and establish a sustainable society, full-scale practical application of renewable energy is required. At present, the spread of a system for storing electric power obtained by a solar cell with a storage battery is spreading. However, the storage battery is heavy and not suitable for movement. Therefore, in the future, use of hydrogen as an energy medium is expected.

The properties of hydrogen are as follows.
-Easy to store and move.
・ Even if burned, the final product is harmless, safe water, and clean.
-It can be converted into electricity or heat by using a fuel cell.
・ It is obtained inexhaustible by water decomposition.

  Photo-semiconductor electrodes that generate hydrogen by decomposing water with sunlight are attracting attention as a technology that can convert solar energy into hydrogen, an energy medium that makes it easy to use solar energy, and research and development aimed at increasing reaction efficiency. It is being advanced. For example, Patent Document 1 discloses an optical semiconductor electrode including a metal substrate and a semiconductor layer formed on the surface of the metal substrate and made of a material having a photocatalytic action.

JP 2006-297300 A

  However, the optical semiconductor electrode of Patent Document 1 has a problem that light incident on the electrode cannot be effectively used for the water splitting reaction. This is because part of the light incident on the electrode reaches the metal substrate after entering the semiconductor layer and is absorbed by the metal substrate. When using sunlight, it is important to use a semiconductor that can absorb light in the visible region that contains a large number of photons. Generally, however, the semiconductor layer is not absorbed by the semiconductor as the wavelength of light increases. Since the ratio of the light which permeate | transmits increases, the ratio of the light absorbed by the metal substrate increases and the utilization efficiency of sunlight falls.

  In order to increase the use efficiency of sunlight for the water splitting reaction by the optical semiconductor electrode, the number of photons absorbed in the semiconductor layer is increased, and the generated photoexcited carriers (electrons and holes) are efficiently used for the water splitting reaction. There is a need to. As a method for increasing the amount of light absorbed by the semiconductor layer, it is conceivable to increase the thickness of the semiconductor layer. However, when the thickness of the semiconductor layer is larger than the distance that the photoexcited carriers can move, the photoexcited carriers recombine before reaching the surface of the semiconductor layer, thereby increasing the efficiency of using light for the water splitting reaction. It is not possible. Therefore, in order to obtain a highly efficient optical semiconductor electrode, the semiconductor layer can sufficiently absorb light in the visible region, and the thickness of the semiconductor layer should be smaller than the distance that photoexcited carriers can move. is necessary.

  Then, this indication aims at providing the photo-semiconductor electrode which can utilize the irradiated sunlight efficiently for water splitting reaction.

This disclosure
A semiconductor layer formed of a semiconductor having a photocatalytic action of decomposing water and having a wavelength at the absorption edge longer than 450 nm;
A reflectance increasing layer disposed on the semiconductor layer in contact with the semiconductor layer and formed of a metal material containing at least one element selected from the group consisting of Ag and Al;
A conductive substrate disposed on the reflectivity increasing layer and formed of a material having a tensile strength greater than the metal material of the reflectivity increasing layer;
An optical semiconductor electrode is provided.

  The photo-semiconductor electrode of this indication can utilize the irradiated sunlight efficiently for a water splitting reaction.

FIG. 1 is a schematic diagram for explaining the reflectance of an interface between a semiconductor layer and a layer that forms an interface with the semiconductor layer. FIG. 2 is a diagram showing the wavelength distribution of the number of photons contained in sunlight. FIG. 3A is a diagram showing how the reflectance changes depending on the values of the refractive index n2 and the extinction coefficient k2 of the material of the reflectance increasing layer when the semiconductor refractive index n1 of the semiconductor layer is 1.5. It is. FIG. 3B is a diagram showing how the reflectance changes depending on the values of the refractive index n2 and the extinction coefficient k2 of the material of the reflectance increasing layer when the semiconductor refractive index n1 of the semiconductor layer is 2.5. It is. FIG. 3C is a diagram showing how the reflectance changes depending on the values of the refractive index n2 and the extinction coefficient k2 of the material of the reflectance increasing layer when the semiconductor refractive index n1 of the semiconductor layer is 3.5. It is. FIG. 4A is a graph showing the value of the refractive index n of Ag and Al for each wavelength. FIG. 4B is a graph showing the extinction coefficient k of Ag and Al for each wavelength. FIG. 5 is a cross-sectional view showing a thin film sample manufactured for measuring the optical constant of niobium oxynitride (NbON). FIG. 6A is a diagram showing actual measurement data in analysis by spectroscopic ellipsometry when measuring the optical constant of niobium oxynitride (NbON). FIG. 6B is a diagram showing optical model data in analysis by spectroscopic ellipsometry when measuring the optical constant of niobium oxynitride (NbON). FIG. 7 is a graph showing the value of the optical constant of niobium oxynitride (NbON) for each wavelength. FIG. 8 is a graph showing the results of calculating the reflectance at the interface between niobium oxynitride (NbON) and a conductor when niobium oxynitride (NbON) is stacked on each conductor material. FIG. 9 is a cross-sectional view illustrating an example of an optical semiconductor electrode according to an embodiment of the present disclosure. FIG. 10 is a schematic diagram illustrating an example of a photoelectrochemical cell according to an embodiment of the present disclosure.

<Background to obtaining one aspect of the present disclosure>
As an optical semiconductor electrode used for the water splitting reaction, an electrode having a semiconductor layer having a photocatalytic action on a conductive substrate such as a metal substrate, such as the electrode disclosed in Patent Document 1, has been proposed. Has been. As the semiconductor layer, a semiconductor having a photocatalytic action for decomposing water attached to the surface is used. Such a conventional optical semiconductor electrode is required to improve efficiency (light utilization efficiency) in which light such as sunlight incident on the electrode is effectively used for the water splitting reaction.

  One means for improving the light utilization efficiency of the optical semiconductor electrode is to increase the amount of light absorbed in the semiconductor layer. When the semiconductor layer is too thin, the ratio of light that cannot be contributed to the water splitting reaction by being transmitted without being absorbed by the semiconductor layer and finally absorbed by the conductive substrate with respect to the entire irradiated light. Get higher. In particular, since the value of the light absorption coefficient of the semiconductor tends to decrease as the wavelength of light increases, the utilization efficiency of light having a wavelength near the absorption edge of the semiconductor decreases. Therefore, if only an increase in the amount of light absorption in the semiconductor layer is taken into consideration, the thickness of the semiconductor layer may be increased. However, when the semiconductor layer is too thick, specifically, when the thickness exceeds the diffusion length of photoexcited carriers generated by photoexcitation, the distance from the electrode surface (semiconductor layer surface) is larger than the diffusion length of photoexcited carriers. The photoexcited carriers generated in (1) disappear due to recombination before moving to the electrode surface and cannot contribute to the water splitting reaction. Therefore, it can be said that the thickness of the semiconductor layer should be as thin as possible from the viewpoint of increasing the utilization efficiency of the photoexcited carriers.

  From the above, in order to increase the light utilization efficiency for the water splitting reaction in the optical semiconductor electrode, the thickness of the semiconductor layer is about the diffusion length of the photoexcited carrier, and how to increase the number of photons that can be absorbed is important.

  As a method for increasing the amount of light absorption in the semiconductor layer without increasing the thickness, a device such as an antireflection film is formed on the surface of the semiconductor layer in a solar cell that is a photoelectric conversion device. However, in the case of an optical semiconductor electrode used for a water splitting reaction, it is necessary to physically contact the semiconductor layer with water, and thus a structure in which the surface of the semiconductor layer is covered with an antireflection film cannot be employed. .

  Therefore, the present inventors have focused on increasing the reflectivity at the interface between the semiconductor layer and the conductive substrate as a method for increasing the number of photons absorbed by the semiconductor layer having a limited thickness through intensive studies. That is, attention was paid to providing a layer (reflectance increasing layer) capable of realizing high reflectance at the interface with the semiconductor layer between the semiconductor layer and the conductive substrate. When the reflectivity at the interface between the semiconductor layer and the reflectivity increasing layer is high, light transmitted through the semiconductor layer is reflected at a high rate at the interface, so that light loss due to light absorption by the conductive substrate is reduced. . Further, since the reflected light again enters the semiconductor layer and is absorbed by the semiconductor layer, as a result, the number of photons absorbed by the semiconductor layer increases. Here, the reflectance (R) at the interface between two different substances (here, the material of the semiconductor layer and the material of the reflectance increasing layer) is the optical constant (refractive index and refractive index) of these substances forming the interface. The extinction coefficient is determined in combination. It is known that the reflectance (R) of the interface between two different substances is obtained by the following formula (1) when light is incident perpendicular to the film surface. In the following formula (1), n1 is the refractive index of the material disposed on the light incident side, that is, here the semiconductor layer material, n2 is the refractive index of the material of the reflectance increasing layer, and k2 is the reflectance. It is the extinction coefficient of the material of the increasing layer. As can be seen from the following equation (1), the reflectance (R) varies depending on the optical constant of the semiconductor layer material and the optical constant of the material of the reflectance increasing layer. The material of the layer cannot be generally determined.

As an example, referring to FIG. 1, titanium dioxide (TiO ₂ ), which is a common semiconductor material, is used as a photocatalytic material for the semiconductor layer (TiO ₂ layer 1), and a Ti layer or a layer that forms an interface with the semiconductor layer is used. Consider the reflectivity of the interface when the Nb layer 2 is used. In the figure, N1 is the complex refractive index of TiO _2, n1 is the refractive index of TiO _2, k1 extinction coefficient of TiO _2, N2 is the complex refractive index of Ti or Nb, n2 is the refractive index of Ti or Nb, k2 is Ti Or the extinction coefficient of Nb.

Since TiO ₂ is a material that can absorb only ultraviolet light (having a wavelength of 400 nm or less), reflection of the interface by the material of the adjacent layer with respect to light having a wavelength of 344 nm (3.6 eV) that can be absorbed. Consider how the rate changes.

Table 1 shows optical constants of Nb, Ti and TiO ₂ with respect to light having a wavelength of 350 nm. When the substance in contact with Nb and Ti is in the atmosphere (refractive index is 1), that is, the reflectivity of the Nb layer and Ti layer in the atmosphere, the results calculated using equations are shown in the upper part of Table 2. From the upper part of Table 2, it can be seen that the reflectance in the atmosphere is substantially the same for both Ti and Nb materials.

Subsequently, the results of calculating the reflectance at the interface between the Nb layer or the Ti layer and the TiO ₂ layer when the TiO ₂ layer is laminated on the Nb layer or the Ti layer are shown in the lower part of Table 2.

According to this result, it can be seen that the reflectance at the interface when the TiO ₂ layer is laminated changes greatly even in the case of a substance (Nb, Ti) having the same degree of reflectance in the atmosphere. Further, from the viewpoint of increasing the light absorption rate in the TiO ₂ layer, a material for forming the TiO ₂ layer and the interface, it can be seen that Ti is more suitable than Nb. Thus, it can be seen that a high reflectivity at the interface can be achieved by selecting a material having an optical constant suitable for the semiconductor to be used and forming a layer in contact with the semiconductor layer with the material.

  Further, the value of the optical constant of the substance varies with the wavelength of light. Therefore, it is necessary to determine the wavelength range of the target light and select a material having an optical constant suitable for the optical constant of the semiconductor in those wavelength ranges as a material for forming the reflectance increasing layer.

  FIG. 2 shows the wavelength distribution of the number of photons contained in sunlight. From this, it can be seen that the number of photons contained in sunlight is large at wavelengths in the visible region (400 nm or more), more in the region of 450 nm or more, and the maximum number of photons in the wavelength region near 500 to 800 nm. Therefore, in order to increase the number of photons that can be absorbed, it is important that the semiconductor used has an absorption edge at a wavelength longer than at least 450 nm, and preferably has a high absorption rate for light of 500 to 800 nm. Furthermore, it is effective to increase the reflectance at the interface between the semiconductor layer and the reflectance increasing layer in this wavelength region.

  On the other hand, in order to perform water splitting by a photocatalytic reaction, it is possible to use photons having energy of 1.23 eV or more theoretically (that is, 1000 nm or less in terms of light wavelength). However, due to the influence of overvoltage in the water splitting reaction, the band gap of the semiconductor actually needs to be larger than 1.23 eV, and the wavelength of light that can be absorbed by the semiconductor is up to the absorption edge of the semiconductor of 1000 nm or less. Become. In addition, since the value of the light absorption coefficient of the semiconductor is smaller as it is closer to the absorption edge, light in the wavelength region near the absorption edge is easily transmitted through the semiconductor layer.

  Only a limited number of semiconductors have been reported as photocatalysts that can be hydrolyzed by a photocatalytic reaction and can absorb light in the visible region. Therefore, it is desirable to select a material suitable for the semiconductor that is promising as a photocatalyst for water splitting as a material for the reflectance increasing layer.

  Here, how the reflectance at the interface between the semiconductor layer and the reflectance increasing layer is determined by the refractive index n1 of the semiconductor of the semiconductor layer and the optical constants (refractive index n2 and extinction coefficient k2) of the material of the reflectance increasing layer. Consider how it will change. When the refractive index n1 of the semiconductor is n1 = 1.5, 2.5, and 3.5, how the reflectance changes depending on the values of the refractive index n2 and the extinction coefficient k2 of the material of the reflectance increasing layer. This is shown in FIGS. 3A to 3C.

  According to the comparison of FIGS. 3A to 3C, it can be seen that the values of n2 and k2 at which the reflectance is minimized change with the change of n1. That is, if a material having n2 and k2 appropriate for the value of the refractive index n1 of the semiconductor is not selected as the material for the reflectance increasing layer, the reflection at the interface between the semiconductor layer and the reflectance increasing layer is almost zero. It is confirmed that the number of photons absorbed in the semiconductor layer cannot be increased. On the other hand, in the case where n1 is any value of 1.5, 2.5, and 3.5, there is a tendency that the reflectivity relatively increases as k2 increases, particularly in the region where k2 is 3 or more. It is confirmed to have a very high reflectivity. In view of this, it is desirable that the value of the extinction coefficient of the material of the reflectance increasing layer in the wavelength region of 500 to 800 nm is 3 or more in order to increase the reflectance in the wavelength region of 500 to 800 nm. Reached the technical idea of Further, from FIGS. 3A to 3C, it can be confirmed that, in the range of the extinction coefficient of 3 or more, it is easy to obtain a high reflectance by setting the refractive index to about 3 or less.

  The reflectance increasing layer needs to have a function as a conductor in order not to prevent electrons formed in the semiconductor layer from moving to the conductive substrate. Therefore, it is desirable that the reflectance increasing layer be formed of a material that can form an ohmic junction with the semiconductor layer. Electron transfer between the semiconductor layer and the reflectivity increasing layer can be hindered by a Schottky barrier formed between them. However, when the semiconductor layer and the reflectance increasing layer form an ohmic junction, no Schottky barrier is generated, and thus electron transfer from the semiconductor layer to the reflectance increasing layer is not hindered. Therefore, when the semiconductor layer and the reflectance increasing layer form an ohmic junction, the photo-semiconductor electrode can efficiently use the generated photoexcited carriers for the water splitting reaction.

  The conductive substrate also supports the semiconductor layer and the reflectance increasing layer, and also plays a role of maintaining the shape of the optical semiconductor electrode. Therefore, the conductive substrate needs to have not only conductivity but also sufficient mechanical strength to support the semiconductor layer and the reflectance increasing layer and to maintain the shape as the optical semiconductor electrode. Moreover, since the photo semiconductor electrode is used in a state immersed in an electrolyte solution, it is desirable that the conductive substrate has corrosion resistance to the electrolyte solution. In addition, assuming that a photohydrogen generation device including a photo semiconductor electrode as a constituent element is installed on the roof of a building or a frame, a conductive substrate that may occupy most of the configuration of the photo semiconductor electrode is: It is also desirable that the weight be light. The material of the conductive substrate is preferably selected in consideration of the above points.

  As a result of intensive studies as described above, the present inventors have reached an optical semiconductor electrode of the following mode that can efficiently use irradiated sunlight for a water splitting reaction.

<Overview of one aspect according to the present disclosure>
An optical semiconductor electrode according to the first aspect of the present disclosure is provided.
A semiconductor layer formed of a semiconductor having a photocatalytic action of decomposing water and having a wavelength at the absorption edge longer than 450 nm;
A reflectance increasing layer disposed on the semiconductor layer in contact with the semiconductor layer and formed of a metal material containing at least one element selected from the group consisting of Ag and Al;
A conductive substrate disposed on the reflectivity increasing layer and formed of a material having a tensile strength greater than the metal material of the reflectivity increasing layer;
including.

  The optical semiconductor electrode according to the first aspect includes a metal material containing at least one element selected from the group consisting of Ag and Al in contact with the semiconductor layer between the semiconductor layer and the conductive substrate The reflectance increasing layer formed in (1) is disposed. The optical constants of Ag and Al are as shown in FIGS. 4A and 4B. As shown in FIG. 4B, Ag and Al have an extinction coefficient k of 3 or more in a wavelength region of 500 to 800 nm. Furthermore, as shown to FIG. 4A, Ag and Al contain the wavelength range whose refractive index n is 3 or less in the wavelength range of 500-800 nm. The reflectance increasing layer formed of a metal material containing at least one element selected from the group consisting of Ag and Al having such an optical constant has a photocatalytic action for decomposing water, and A high reflectance can be realized at the interface with a semiconductor layer formed of a semiconductor whose absorption edge has a wavelength longer than 450 nm. Therefore, in the optical semiconductor electrode according to the first aspect, the light transmitted through the semiconductor layer is reflected at a high rate at the interface between the semiconductor layer and the reflectivity increasing layer with respect to light in the wavelength region containing a large number of photons contained in sunlight. The reflected light again enters the semiconductor layer and is absorbed by the semiconductor layer. As a result, the number of photons absorbed by the semiconductor layer increases. As described above, according to the optical semiconductor electrode according to the first aspect, the number of photons that can be absorbed by the semiconductor layer can be increased without increasing the thickness of the semiconductor layer. Therefore, it can be used for water splitting reaction.

  Furthermore, since the reflectance increasing layer is made of a metal material containing at least one element selected from the group consisting of Ag and Al, the electrons formed in the semiconductor layer move to the conductive substrate. Not disturb. The work function of Ag is 4.26 eV, and the work function of Al is 4.28 eV. Thus, the work functions of Ag and Al are relatively small. Therefore, the reflectance increasing layer also has an advantage that an ohmic junction is easily formed when the semiconductor constituting the semiconductor layer is an n-type semiconductor.

  In addition, there is a concern that the use of Ag as a constituent element of the optical semiconductor electrode is very expensive. However, since the reflectance increasing layer is a layer provided for the purpose of realizing a high reflectance at the interface with the semiconductor layer, the thickness thereof is not particularly limited, and there is no problem even if it is very thin. Therefore, even when a material containing Ag is used for the reflectance increasing layer provided between the semiconductor layer and the conductive substrate, an effect of increasing the reflectance while suppressing an increase in cost can be sufficiently obtained. Further, Ag has a concern that the surface is oxidized by an oxidizing atmosphere and the reflectivity is lowered. However, the corrosion resistance can be improved by alloying Ag with another metal element, and therefore, the process to be used. Depending on the, an alloy may be used. Examples of elements used when alloying Ag include Au and Cu.

  The conductive substrate and the reflectance increasing layer are formed of different materials, and the material of the conductive substrate has a higher tensile strength than the material of the reflectance increasing layer. Therefore, the conductive substrate can have high strength, and as a result, the strength of the optical semiconductor electrode can be improved. Here, the tensile strength of the material of the conductive substrate and the material of the reflectivity increasing layer is the tensile strength obtained in accordance with JIS Z 2241.

  In the second aspect, for example, in the optical semiconductor electrode according to the first aspect, the conductive substrate includes at least one selected from the group consisting of titanium metal (Ti) and stainless steel (SUS). You may go out.

  Ti and SUS have high mechanical strength, are not easily corroded even when immersed in an electrolyte solution, and are relatively light. Therefore, the conductive substrate formed of a material containing Ti and / or SUS has sufficient mechanical strength to support the semiconductor layer and the reflectance increasing layer and maintain the shape as an electrode. Furthermore, since such a conductive substrate also has corrosion resistance to the electrolyte solution, high reliability and stability of the optical semiconductor electrode can be maintained even when the optical semiconductor electrode is used while being immersed in the electrolytic solution. In addition, since the conductive substrate can be reduced in weight, problems are unlikely to occur when a device including an optical semiconductor electrode as a constituent element is installed on a rooftop of a building or a mount.

  In the third aspect, for example, in the optical semiconductor electrode according to the first or second aspect, the semiconductor may be niobium oxynitride (NbON).

  The absorption edge wavelength of NbON is about 600 nm. Therefore, NbON is a semiconductor that can use even light with a wavelength in the visible region, and its band structure has a photocatalytic action suitable for water decomposition. Thus, NbON is a promising semiconductor material as a photocatalyst for water splitting. However, since there has been no report on the optical constant of NbON so far, when NbON is used for the semiconductor layer, what is used for the material of the reflectance increasing layer having an optical constant that realizes a high reflectance at the interface? There was a problem that it was not possible to decide whether it should be done.

  In order to solve the above problem, the present inventors had to try from obtaining the optical constant of NbON. The present inventors made a thin film of NbON by the following method, and attempted to actually measure the chromatic dispersion of the optical constant (refractive index n, extinction coefficient k) of NbON from the measurement by spectroscopic ellipsometry.

(Preparation of NbON thin film)
An NbON thin film was epitaxially formed on a TiO ₂ (101) substrate by a reactive sputtering method using a two-step film forming method. Thus, a thin film sample in which the NbON seed layer 4 and the NbON main growth layer 5 were sequentially arranged on the TiO ₂ substrate 3 as shown in FIG. 5 was produced. Specifically, first, an NbON seed layer 4 having a thickness of about 20 nm was fabricated on a TiO ₂ (101) substrate at 650 ° C. The obtained NbON seed layer 4 was a thin film that was epitaxially grown satisfactorily. Next, the NbON main growth layer 5 was formed on the NbON seed layer 4 as a base at 500 ° C. The NbON thin film obtained by this production method was a high-quality film that achieved both Nb reduction prevention and NbON epitaxial growth.

(Measurement of optical constants by spectroscopic ellipsometry)
Spectroscopic ellipsometry analyzes the optical constant (n: refractive index, k: extinction coefficient) and thin film structure by measuring the change in the polarization state of the light incident on the thin film sample due to light reflection at the sample. It is a technique. FIG. 6A shows the results of Psi (ψ) and Delta (Δ) obtained from the measurements with the incident angles being 65 °, 70 °, and 75 °, and FIG. 6B shows data from the created optical model. When fitting with the actual measurement data shown in FIG. 6A and the optical model data shown in FIG. 6B, it was found that the optical model created this time can accurately fit ψ and Δ at any incident angle (fitting accuracy) The value of MSE, which is an index indicating the value, was 4.167.) Moreover, the film thicknesses of the NbON seed layer 4 and the NbON main growth layer 5 obtained from the created optical model were 12 nm and 60 nm, respectively, and almost coincided with the film thickness estimated from the film formation rate. From the above, it was determined that both the NbON seed layer 4 and the NbON main growth layer 5 were accurately modeled in the optical model created this time. FIG. 7 shows the chromatic dispersion of the NbON optical constant (n: refractive index, k: extinction coefficient) obtained from the optical model of the NbON main growth layer 5. FIG. 7 shows the refractive index of NbON and the optical constants of typical various conductor materials in the wavelength region of 500 to 600 nm, which is important for increasing the number of photons absorbed when NbON is used for the semiconductor layer. 3 shows. Table 4 and FIG. 8 show the results of calculating the reflectance at the interface between NbON and the conductor when NbON is laminated on each conductor material based on Table 3. According to this result, it was found that high reflectance can be obtained by using Ag and / or Al, and it has been found that it is particularly preferable to use Ag.

  As described above, by clarifying the optical constant of NbON for the first time, it is possible to design a structure for maximizing the number of photons that can be absorbed by the semiconductor layer formed of NbON. As a result, the present inventors have combined a semiconductor layer formed of NbON with a reflectance increasing layer formed of a metal material containing at least one element selected from the group consisting of Ag and Al. Moreover, the optical semiconductor electrode according to the second aspect could be reached. The optical semiconductor electrode according to the second aspect is a semiconductor layer and a reflective layer in a wavelength region of 500 to 600 nm, which is an important wavelength region for increasing the number of photons absorbed in the semiconductor layer when NbON is used for the semiconductor layer. It is also possible to increase the reflectance at the interface with the rate increasing layer to 78% or more, for example. Therefore, according to the optical semiconductor electrode which concerns on a 2nd aspect, since the number of photons which a semiconductor layer can absorb can be increased, the irradiated sunlight can be utilized efficiently for a water splitting reaction. Nb-based compounds are suitable for industrial use because of their relatively low cost. Therefore, the optical semiconductor electrode according to the second aspect can reduce the cost.

  In the fourth aspect, for example, in the optical semiconductor electrode according to any one of the first to third aspects, the metal material includes a simple substance of Ag, a simple substance of Al, an alloy containing an Ag element, and an Al element. It may be at least one selected from the group consisting of alloys.

  According to the optical semiconductor electrode according to the fourth aspect, the reflectance at the interface between the semiconductor layer and the reflectance increasing layer is improved to increase the number of photons absorbed in the semiconductor layer, and the corrosion resistance of the reflectance increasing layer is improved. Further demands such as cost reduction can also be realized.

  In the fifth aspect, for example, in the optical semiconductor electrode according to the fourth aspect, the metal material may be at least one selected from the group consisting of a simple substance of Ag and an alloy containing an Ag element. .

  According to the optical semiconductor electrode according to the fifth aspect, the reflectance at the interface between the semiconductor layer and the reflectance increasing layer is improved to increase the number of photons absorbed in the semiconductor layer, and the corrosion resistance of the reflectance increasing layer is improved. Further demands such as cost reduction can also be realized.

  In the sixth aspect, for example, in the optical semiconductor electrode according to any one of the first to fifth aspects, the thickness of the reflectance increasing layer may be smaller than the thickness of the conductive substrate. .

  According to the optical semiconductor electrode according to the sixth aspect, the reflectance at the interface between the semiconductor layer and the reflectance increasing layer is suppressed while suppressing an increase in cost due to the provision of the reflectance increasing layer and an increase in the thickness of the entire electrode. The irradiated sunlight can be efficiently used for the water splitting reaction.

  In the seventh aspect, for example, in the optical semiconductor electrode according to any one of the first to sixth aspects, the thickness of the reflectance increasing layer may be smaller than the thickness of the semiconductor layer.

  According to the optical semiconductor electrode according to the seventh aspect, the reflectance at the interface between the semiconductor layer and the reflectance increasing layer is suppressed while suppressing an increase in cost due to the provision of the reflectance increasing layer and an increase in the thickness of the entire electrode. The irradiated sunlight can be efficiently used for the water splitting reaction.

In the eighth aspect, for example, in the optical semiconductor electrode according to any one of the first to seventh aspects, the ratio of the thickness of the reflectance increasing layer to the thickness of the conductive substrate (the reflectance The thickness of the increasing layer / the thickness of the conductive substrate may be 1 × 10 ⁻⁵ to 1 × 10 ⁻² .

  According to the optical semiconductor electrode according to the eighth aspect, irradiation is performed while increasing the reflectance at the interface between the semiconductor layer and the reflectance increasing layer while suppressing the increase in cost due to the provision of the reflectance increasing layer. Sunlight can be efficiently used for the water splitting reaction.

  In the ninth aspect, for example, in the optical semiconductor electrode according to any one of the first to eighth aspects, the reflectance increasing layer may have a thickness of 10 to 1000 nm.

  According to the optical semiconductor electrode according to the ninth aspect, irradiation is performed while increasing the reflectivity at the interface between the semiconductor layer and the reflectivity increasing layer while suppressing the increase in cost due to the provision of the reflectivity increasing layer. Sunlight can be efficiently used for the water splitting reaction.

  A photoelectrochemical cell according to a tenth aspect of the present disclosure is electrically connected to the optical semiconductor electrode according to any one of the first to ninth aspects and the conductive substrate of the optical semiconductor electrode. A counter electrode, and a container for accommodating the optical semiconductor electrode and the counter electrode.

  Since the photoelectrochemical cell according to the tenth aspect includes the photo-semiconductor electrode according to any one of the first to ninth aspects, the irradiated sunlight is efficiently used for the water splitting reaction. can do.

  In the eleventh aspect, for example, the photoelectrochemical cell according to the tenth aspect further includes an electrolyte solution containing water that is accommodated in the container and is in contact with the surface of the photo-semiconductor electrode and the counter electrode. May be.

  The photoelectrochemical cell which concerns on an 11th aspect can utilize the irradiated sunlight efficiently for a water splitting reaction.

<Embodiment>
Hereinafter, embodiments of the optical semiconductor electrode and the photoelectrochemical cell of the present disclosure will be described in detail. In addition, the following embodiment is an example and this indication is not limited to the following forms.

(Embodiment 1)
One embodiment of the optical semiconductor electrode of the present disclosure will be described.

  FIG. 9 is a cross-sectional view showing an example of the optical semiconductor electrode of the present embodiment. An optical semiconductor electrode 100 shown in FIG. 9 includes a semiconductor layer 103, a reflectance increasing layer 102 disposed on the semiconductor layer 103 in contact with the semiconductor layer 103, and a conductive layer disposed on the reflectance increasing layer 102. And a conductive substrate 101. The upper and lower sides of the optical semiconductor electrode 100 are not limited when installed. Therefore, in other words, the optical semiconductor electrode 100 is disposed on the conductive substrate 101, the reflectance increasing layer 102 disposed on the conductive substrate 101, and on the reflectance increasing layer 102 in contact with the reflectance increasing layer 102. And a semiconductor layer 103 formed. When the optical semiconductor electrode 100 is installed in the device, it is electrically connected to a counter electrode (not shown) via a conductive wire 104 connected to the conductive substrate 101.

  The material forming the conductive substrate 101 has a higher tensile strength than the material forming the reflectivity increasing layer 102. For example, the conductive substrate 101 desirably includes at least one selected from the group consisting of Ti and SUS. Ti and SUS have high mechanical strength, are not easily corroded even when immersed in an electrolyte solution, and are relatively light. Therefore, the conductive substrate 101 formed of a material containing Ti and / or SUS supports the semiconductor layer 103 and the reflectance increasing layer 102 and maintains the shape as an electrode in addition to having conductivity. It also has sufficient mechanical strength to do. Furthermore, since the conductive substrate 101 has corrosion resistance to the electrolyte solution, even when the optical semiconductor electrode 100 is used in a state immersed in the electrolyte solution, high reliability and stability of the optical semiconductor electrode 100 can be maintained. . In addition, since the conductive substrate 101 can be reduced in weight, problems are less likely to occur when a device including the optical semiconductor electrode 100 as a constituent element is installed on a rooftop of a building or a mount.

  The conductive substrate 101 may be formed of only at least one selected from the group consisting of Ti and SUS. However, even in that case, the conductive substrate 101 may contain a component (for example, a component inevitably mixed as an impurity) other than Ti and SUS in a minute amount (for example, about 2% by mass or less). In other words, the conductive substrate 101 may include, for example, 98% by mass or more of at least one selected from the group consisting of Ti and SUS.

  The thickness of the conductive substrate 101 is not particularly limited. The thickness can be appropriately selected according to the material used for the conductive substrate 101, the strength to be realized, and the like. As an example, the thickness of the conductive substrate 101 can be set to 0.1 to 1 mm, for example.

The semiconductor layer 103 is formed of a semiconductor having a photocatalytic action for decomposing water and having a wavelength at the absorption edge longer than 450 nm. Typical examples of such semiconductors include iron oxide (Fe ₂ O ₃ ), tungsten oxide (WO ₃ ), bismuth vanadate (BiVO ₄ ), tantalum nitride (Ta ₃ N ₅ ), and tantalum oxynitride (TaON). And niobium oxynitride (NbON). These materials are photocatalysts having an absorption edge wavelength longer than 450 nm and capable of absorbing light in the visible region, and can utilize light in a wavelength region including many photons in sunlight. is there. Therefore, the material is promising for increasing the utilization efficiency of sunlight as compared with a photocatalyst that can only absorb ultraviolet rays, such as titanium dioxide (TiO ₂ ).

Among the above materials, NbON is preferable as a semiconductor for forming the semiconductor layer 103. NbON has an absorption edge wavelength of about 600 nm. NbON is a semiconductor that can efficiently use light in a wavelength region including a large number of photons in sunlight, and its band structure is suitable for water decomposition. Has a photocatalytic action. In the case of Fe ₂ O ₃ , WO _3, BiVO _4, etc., the lower end of the conduction band and the upper end of the valence band do not sandwich the redox level of water, so no water splitting reaction occurs without an external bias. When the band position sandwiches the redox level of water, theoretically, the water splitting reaction can proceed only by light irradiation. Further, since the Nb element is less expensive than the Ta element, the electrode cost can be reduced as compared with the case where Ta ₃ N ₅ having the same absorption edge wavelength is used.

  It is desirable to set the thickness of the semiconductor layer 103 to about the diffusion length of photoexcited carriers generated by photoexcitation. If the semiconductor layer 103 is too thick, photoexcited carriers generated by photoexcitation may disappear due to recombination before moving to the electrode surface, and it may be difficult to increase the utilization efficiency of the photoexcited carriers. On the other hand, if the semiconductor layer 103 is too thin, it may be difficult to increase the amount of light absorbed by the semiconductor layer 103. Therefore, the thickness of the semiconductor layer 103 is preferably 20 nm or more and 1000 nm or less, for example.

  The reflectivity increasing layer 102 is a layer provided for the purpose of causing light once transmitted through the semiconductor layer 103 to be reflected at the interface between the semiconductor layer 103 and the reflectivity increasing layer 102 and to enter the semiconductor layer 103 again. . Therefore, the reflectance increasing layer 102 achieves a high reflectance at the interface with the semiconductor layer 103, particularly for light in a wavelength region including a large number of photons in sunlight.

  The reflectance increasing layer 102 is formed of a metal material containing at least one element selected from the group consisting of Ag and Al. As described in detail in the section <Summary of One Aspect According to the Present Disclosure>, Ag and Al have a photocatalytic action for decomposing water, and a semiconductor whose absorption edge has a wavelength longer than 450 nm. It has an optical constant that can realize a high reflectance at the interface. Therefore, a layer formed using such a material can achieve high reflectance at the interface with the semiconductor layer 103. As described above, the reflectance (R) at the interface between two different substances (here, the material of the semiconductor layer 103 and the material of the reflectivity increasing layer 102) is expressed by the following equation (1). Is determined by the combination of the optical constants (refractive index and extinction coefficient) of these materials. The reflectance at the interface between the semiconductor layer 103 and the reflectance increasing layer 102 is expressed by the following formula (1), where n1 is the refractive index of the semiconductor forming the semiconductor layer 103, and n2 is the refractive index of the material of the reflectance increasing layer 102. Ratio, k2, can be obtained by using the extinction coefficient of the material of the reflectance increasing layer 102.

  Note that NbON, which is cited as a desirable semiconductor of the semiconductor layer 103, has not been reported since there has been no report on its optical constant. Therefore, conventionally, it has not been possible to select a material that can realize a high reflectance at the interface when in contact with a semiconductor layer formed using NbON. However, it has also been confirmed by the present inventors that the optical constant of NbON is newly obtained by measurement as described above, and that, when combined with Ag and Al, a particularly high reflectance can be realized at the interface. Therefore, the reflectance increasing layer 102 realizes a high reflectance at the interface with the semiconductor layer 103 and greatly increases the amount of light absorption in the semiconductor layer 103 even when the semiconductor layer 103 is formed of NbON. Can do. Note that the method for obtaining the optical constant of NbON and the fact that high reflectivity can be realized at the interface between Ag and Al have been described in detail in the section <Summary of One Aspect According to the Present Disclosure>. Is omitted.

  In addition, since the metal material containing at least one element selected from the group consisting of Ag and Al has high conductivity, the reflectivity increasing layer 102 includes an electron formed in the semiconductor layer 103 as a conductive substrate. It does not prevent moving to 101. In addition, since the work functions of Ag and Al are relatively small, the reflectance increasing layer 102 is likely to form an ohmic junction when the semiconductor constituting the semiconductor layer 103 is an n-type semiconductor such as NbON. Therefore, the electrons from the semiconductor layer 103 to the reflectivity increasing layer 102 easily move, and the generated electrons can be efficiently used for the water splitting reaction.

  As the metal material of the reflectance increasing layer 102, for example, at least one selected from the group consisting of a simple substance of Ag, a simple substance of Al, an alloy containing an Ag element, and an alloy containing an Al element can be used. Further, when at least one selected from the group consisting of a simple substance of Ag and an alloy containing an Ag element is used as the metal material of the reflectance increasing layer 102, a particularly high reflectance can be obtained at the interface with the semiconductor layer 103. Is also possible.

The reflectance increasing layer 102 is higher than the reflectance obtained at the interface between the semiconductor layer 103 and the conductive substrate 101 when the semiconductor layer 103 is disposed in contact with the conductive substrate 101 at the interface with the semiconductor layer 103. Achieve reflectivity. Therefore, the reflectance increasing layer 102 is not required to be formed so thick because it is only necessary to obtain a desired high reflectance at the interface with the semiconductor layer 103. The thickness of the reflectance increasing layer 102 may be smaller than the thickness of the conductive substrate 101 and / or the semiconductor layer 103. The reflectance increasing layer 102 has a ratio of the thickness of the reflectance increasing layer 102 to the thickness of the conductive substrate 101 (the thickness of the reflectance increasing layer / the thickness of the conductive substrate), for example, 1 × 10 ⁻⁵ to It may be 1 × 10 ⁻² . Further, the thickness of the reflectivity increasing layer 102 may be in the range of 10 to 1000 nm, for example.

  Since the optical semiconductor electrode 100 according to the present embodiment has the above-described configuration, the number of photons that can be absorbed by the semiconductor layer 103 can be increased without increasing the thickness of the semiconductor layer 103. Can be efficiently used for water splitting reaction.

(Embodiment 2)
One embodiment of the photoelectrochemical cell of the present disclosure will be described.

  FIG. 10 shows an example of the photoelectrochemical cell of this embodiment. The photoelectrochemical cell 200 shown in FIG. 10 is a container that accommodates the optical semiconductor electrode 100 described in the first embodiment, the counter electrode 210, the electrolyte solution 230 containing water, and the photo semiconductor electrode 100, the counter electrode 210, and the electrolyte solution 220. 220.

  Since the optical semiconductor electrode 100 is as described in the first embodiment, detailed description thereof is omitted here.

  In the container 220, the optical semiconductor electrode 100 and the counter electrode 210 are arranged so that the surfaces thereof are in contact with the electrolyte solution 230. In the photoelectrochemical cell 200 shown in FIG. 10, a portion of the container 220 facing the semiconductor layer 103 of the optical semiconductor electrode 100 disposed in the container 220 (hereinafter, abbreviated as a light incident portion 221) It is made of a material that transmits light such as sunlight. That is, in the photoelectrochemical cell 200, the photo semiconductor electrode 100 is disposed in the container 220 in a direction in which light can enter from the surface of the semiconductor layer 103.

  The conductive substrate 101 and the counter electrode 210 in the optical semiconductor electrode 100 are electrically connected by a conducting wire 104. Here, the counter electrode means an electrode that exchanges electrons with an optical semiconductor electrode without using an electrolyte solution. Therefore, the counter electrode 210 in this embodiment may be electrically connected to the conductive substrate 101 of the optical semiconductor electrode 100, and the positional relationship with the optical semiconductor electrode 100 is not particularly limited. For example, when the semiconductor constituting the semiconductor layer 103 of the optical semiconductor electrode 100 is an n-type semiconductor, the counter electrode 210 is an electrode that receives electrons from the optical semiconductor electrode 100 without passing through the electrolytic solution 230. As the counter electrode 210, it is preferable to use a material having a small overvoltage. For example, it is preferable to use a metal catalyst such as Pt, Au, Ag, Fe, or Ni because the activity of the counter electrode 210 is increased.

  As shown in FIG. 10, the photoelectrochemical cell 200 may further include a separator 240. The interior of the container 220 can be separated by the separator 240 into two regions, a region on the side where the optical semiconductor electrode 100 is disposed and a region where the counter electrode 210 is disposed. The electrolyte solution 230 is accommodated in both areas. The container 220 has an exhaust port 222 for exhausting the gas generated in the region where the optical semiconductor electrode 100 is disposed, and an exhaust port for exhausting the gas generated in the region where the counter electrode 210 is disposed. 223. The container 220 further includes a water supply port 224 for supplying water into the container 220.

  The electrolyte solution 230 is not particularly limited as long as it contains water. The electrolytic solution 230 may be acidic or alkaline. Further, a solid electrolyte can be used instead of the electrolyte solution 230.

  Next, operations of the photo semiconductor electrode 100 and the photoelectrochemical cell 200 will be described. Here, a case where the semiconductor layer 103 of the optical semiconductor electrode 100 is formed of an n-type semiconductor such as NbON will be described as an example.

  When sunlight is incident on the optical semiconductor electrode 100 accommodated in the container 220 and in contact with the electrolyte solution 230 from the light incident part 221 of the container 220 in the photoelectrochemical cell 200, conduction occurs in the semiconductor in the semiconductor layer 103. Electrons are generated in the band and holes are generated in the valence band. The holes generated at this time move to the surface of the semiconductor layer 103 by band bending due to the depletion layer generated by contact with the electrolyte solution 230. On the surface of the semiconductor layer 103, water is decomposed by the following formula (2) to generate oxygen. On the other hand, the electrons move to the conductive substrate 101 by the band bending and reach the counter electrode 210 via the conductive wire 104. At the counter electrode 210, hydrogen is generated according to the following formula (3).

4h ⁺ + 2H ₂ O → O ₂ ↑ + 4H ⁺ (2)
4e ⁻ + 4H ⁺ → 2H ₂ ↑ (3)

  The generated hydrogen and oxygen are separated by the separator 240 in the container, and oxygen is discharged from the exhaust port 222 and hydrogen is discharged from the exhaust port 223. The water to be decomposed is supplied into the container 220 from the water supply port 224.

  As described in the first embodiment, the optical semiconductor electrode 100 can efficiently use the irradiated sunlight for the water splitting reaction. Therefore, the photoelectrochemical cell 200 provided with the photo semiconductor electrode 100 can efficiently use the irradiated sunlight for the water splitting reaction.

  Other configurations other than the photo-semiconductor electrode 100 in the photoelectrochemical cell 200, for example, the counter electrode 210, the container 220, the separator 240, and the like are not particularly limited. Photoelectrics that decompose water and generate a gas such as hydrogen. Known containers, conducting wires, separation membranes and the like used in chemical cells can be used as appropriate.

  Hereinafter, specific examples of the optical semiconductor electrode of the present disclosure will be described.

(First example)
The optical semiconductor electrode of the first example has the same configuration as that shown in FIG. As the substrate 101, a Ti substrate having a thickness of 0.5 mm is used. On the Ti substrate, Ag is deposited with a thickness of about 30 nm as the reflectivity increasing layer 102 by vapor deposition. Subsequently, niobium pentoxide (Nb ₂ O ₅ ) is formed to a thickness of 100 nm as a precursor of NbON as a photocatalyst on the obtained Ag thin film. The Nb ₂ O ₅ thin film is formed by sputtering, and Nb ₂ O ₅ is used as the target material. In this example, the sputtering method is used to form the Nb ₂ O ₅ thin film. However, the method is naturally not limited to this method, and a method capable of forming a film at a low cost under atmospheric pressure such as a spin coating method may be used. Good. From the above, a laminated structure of Nb ₂ O ₅ / Ag / Ti is produced. Subsequently, this laminated structure is set in a Tamman tube furnace (for example, manufactured by Motoyama Co., Ltd.) having a diameter of about 9 cm, and the inside of the furnace is heated at a heating rate of 100 ° C./h under a mixed gas flow of ammonia, oxygen and nitrogen. The temperature is raised from room temperature to 650 ° C. Then, after holding at 650 ° C. for 1 hour, the temperature is lowered at a temperature lowering rate of 100 ° C./h, and the introduced gas is changed from the reaction gas to nitrogen to sufficiently purge. As a result, Nb ₂ O ₅ is nitrided and NbON is synthesized. As a result, an NbON thin film as the semiconductor layer 103 is formed. In this example, NbON is synthesized by nitriding Nb oxide, but it is naturally not limited to this method, and other film forming methods such as reactive sputtering may be used.

  The optical semiconductor electrode which is a laminated structure of NbON / Ag / Ti is produced by the above method. In the produced optical semiconductor electrode, a thin film made of Ag having an optical constant appropriate for NbON is provided as a reflection increasing layer. Therefore, the optical semiconductor electrode of this example has a high reflectance at the NbON / Ag interface, and as a result, the NbON thin film can also absorb the light reflected at the NbON / Ag interface, so the number of photons that can be absorbed by the NbON thin film Is an increase. That is, the photosemiconductor electrode of this example can efficiently use the irradiated sunlight for the water splitting reaction.

(Second example)
In the second example, in the manufacturing method of the first example, the reflectance increasing layer is manufactured using an Ag—Cu alloy instead of Ag. In this case, since the oxidation resistance is improved as compared with the reflectance increasing layer formed of Ag single phase, the oxidation of Ag is suppressed even in an atmosphere where the process of forming NbON oxidizes Ag. However, it is possible to maintain a high reflectance.

(Third example)
In the third example, the reflectance increasing layer is manufactured using an Ag—Au alloy instead of Ag in the manufacturing method of the first example. In this case, since the oxidation resistance is improved as compared with the reflectance increasing layer formed of Ag single phase, the oxidation of Ag is suppressed even in an atmosphere where the process of forming NbON oxidizes Ag. However, it is possible to maintain a high reflectance.

  The optical semiconductor electrode of the present disclosure is useful as an electrode for water splitting using sunlight.

DESCRIPTION OF SYMBOLS 100 Photo-semiconductor electrode 101 Conductive board | substrate 102 Reflectivity increase layer 103 Semiconductor layer 104 Conductive wire 200 Photoelectrochemical cell 210 Counter electrode 220 Container 221 Light incident part 222,223 Exhaust port 224 Water supply port 230 Electrolyte solution 240 Separator

Claims (11)

A semiconductor layer formed of a semiconductor having a photocatalytic action of decomposing water and having a wavelength at the absorption edge longer than 450 nm;
A reflectance increasing layer disposed on the semiconductor layer in contact with the semiconductor layer and formed of a metal material containing at least one element selected from the group consisting of Ag and Al;
A conductive substrate disposed on the reflectivity increasing layer and formed of a material having a tensile strength greater than the metal material of the reflectivity increasing layer;
including,
Photo semiconductor electrode. The conductive substrate includes at least one selected from the group consisting of titanium metal and stainless steel,
The optical semiconductor electrode according to claim 1. The semiconductor is niobium oxynitride (NbON);
The optical semiconductor electrode according to claim 1 or 2. The metal material is at least one selected from the group consisting of a simple substance of Ag, a simple substance of Al, an alloy containing an Ag element, and an alloy containing an Al element.
The optical-semiconductor electrode of any one of Claims 1-3. The metal material is at least one selected from the group consisting of a simple substance of Ag and an alloy containing an Ag element.
The optical semiconductor electrode according to claim 4. The thickness of the reflectance increasing layer is smaller than the thickness of the conductive substrate;
The optical-semiconductor electrode of any one of Claims 1-5. The thickness of the reflectance increasing layer is smaller than the thickness of the semiconductor layer;
The optical semiconductor electrode of any one of Claims 1-6. The ratio of the thickness of the reflectivity increasing layer to the thickness of the conductive substrate (the thickness of the reflectivity increasing layer / the thickness of the conductive substrate) is 1 × 10 ⁻⁵ to 1 × 10 ⁻² . is there,
The photo-semiconductor electrode of any one of Claims 1-7. The thickness of the reflectance increasing layer is 10 to 1000 nm.
The optical semiconductor electrode according to claim 1. An optical semiconductor electrode according to any one of claims 1 to 9,
A counter electrode electrically connected to the conductive substrate of the optical semiconductor electrode;
A container for housing the optical semiconductor electrode and the counter electrode;
Photoelectrochemical cell equipped with. An electrolyte solution containing water, contained in the container, and in contact with the surface of the optical semiconductor electrode and the counter electrode;
The photoelectrochemical cell according to claim 10.

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