JP2014205899A - Electrochemical reaction apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrochemical reaction apparatus capable of obtaining a stable photocatalytic function.SOLUTION: An electrochemical reaction apparatus 10 according to one embodiment of the present invention includes a photoelectrode 11, a counter electrode 12, and an electrolyte tank 14. The photoelectrode 11 is formed of a gallium oxide single crystal which generates electron-hole pairs by being irradiated with ultraviolet rays. The counter electrode 12 is electrically connected to the photoelectrode 11. The electrolyte tank 14 stores an electrolyte 13 which is in contact with the photoelectrode 11 and the counter electrode 12.

Description

  The present invention relates to an electrochemical reaction apparatus capable of generating hydrogen by electrolysis of water using a photocatalytic reaction.

  As a photocatalyst capable of synthesizing or decomposing a substance by light, a chemically stable semiconductor is usually used. Among these, wide gap semiconductors represented by titanium oxide and gallium nitride have high oxidation or reduction ability, and can be applied to synthesis or decomposition of a wide range of organic and inorganic substances including water decomposition. Furthermore, by using a counter electrode that is electrically connected to the conductive semiconductor single crystal, the oxidation and reduction reaction fields can be spatially separated, and the product can be easily recovered.

  For example, Patent Document 1 describes a photocatalytic reaction device having a photocatalyst body made of gallium nitride (GaN), a metal electrode as a counter electrode thereof, and an electrolytic solution tank for housing them. In the photocatalytic reaction device, when the photocatalyst body is irradiated with excitation light, oxygen is generated by an oxidation reaction of an electrolytic solution (for example, water) on the photocatalyst body side, and hydrogen is generated by a reduction reaction of the electrolytic solution on the metal electrode side. It is configured to generate.

On the other hand, gallium oxide (Ga ₂ O ₃ ) is a semiconductor that is chemically stable and has a large band gap of about 4.7 eV. Compared to titanium oxide and gallium nitride, gallium oxide has the highest conduction band (conduction band) bottom and the lowest valence band top, so it has a higher reducing power and oxidation power than these materials. Is expected to show. For example, Non-Patent Document 1 reports water decomposition using gallium oxide powder.

JP 2007-107043 A

Takashi Hisatomi, et al. Chemical Physics Letters vol.486 (2010) pp.144-146

  However, in general, powder materials cannot exclude the influence of grain boundaries and impurities, and it is difficult to obtain a highly efficient photocatalytic reaction.

  In view of the circumstances as described above, an object of the present invention is to provide an electrochemical reaction device capable of obtaining a highly efficient photocatalytic function.

In order to achieve the above object, an electrochemical reaction device according to one embodiment of the present invention includes a photoelectrode, a counter electrode, and an electrolyte bath.
The photoelectrode is composed of a gallium oxide single crystal that generates electron-hole pairs when irradiated with ultraviolet rays.
The counter electrode is electrically connected to the photoelectrode.
The electrolytic bath contains an electrolytic solution that contacts the photoelectrode and the counter electrode.

It is a diagram illustrating by comparing the band alignment of the β-Ga ₂ O ₃ and other wide-gap semiconductor. It is a schematic block diagram which shows the electrochemical reaction apparatus which concerns on one Embodiment of this invention. It is a schematic diagram which shows the band diagram of the photoelectrode which contacted electrolyte solution. It is a schematic diagram which shows the structure of a 3 electrode photoelectrochemical cell. pH is an experimental result showing the relationship between the capacitance of the Ga ₂ O ₃ single crystal substrate obtained from different electrolyte and potential. Flat band potential of the Ga ₂ O ₃ single crystal substrate, the conduction band bottom, the experimental results showing the dependence on pH of the valence band top of the electrolyte solution. It is an experimental result showing the photoelectrode characteristic of the Ga ₂ O ₃ single crystal substrate. It is an experimental result showing the photoelectrode characteristic of the Ga ₂ O ₃ single crystal substrate. It is a schematic block diagram of the gas collection system used for embodiment of this invention. It is a photograph which shows the mode of the photoelectrolysis of the water in one Embodiment of this invention. It is an experimental result showing the relationship between the total and the amount of generated gas of the Ga ₂ O ₃ single crystal substrate photocurrent. The experimental results showing the correlation between the capacitance of the two different Ga ₂ O ₃ single crystal substrate carrier concentration and potential. Ga ₂ O ₃ which is another experimental result showing a photoelectrode characteristics of the single crystal substrate.

An electrochemical reaction device according to an embodiment of the present invention includes a photoelectrode, a counter electrode, and an electrolyte bath.
The photoelectrode is composed of a gallium oxide single crystal that generates electron-hole pairs when irradiated with ultraviolet rays.
The counter electrode is electrically connected to the photoelectrode.
The electrolytic bath contains an electrolytic solution that contacts the photoelectrode and the counter electrode.

A gallium oxide single crystal used for a photoelectrode is a compound semiconductor having a stoichiometric ratio of gallium (Ga) and oxygen (O) of 2: 3. There are five forms of gallium oxide having different structures of α, β, γ, δ, and ε, and monoclinic β-Ga ₂ O ₃ , which is the most stable structure, is typically used. .

FIG. 1 is a diagram showing the band structure of β-Ga ₂ O ₃ in comparison with the band structure of other wide gap semiconductors. In the figure, the left vertical axis represents the standard hydrogen electrode potential (SHE), and the right vertical axis represents the relative potential with respect to the vacuum level.

The band gap energy of β-Ga ₂ O ₃ is about 4.7 eV, which is wider than other wide gap semiconductors. Further, the edge band of the valence band of β-Ga ₂ O ₃ (the top of the valence band) is lower than that of other semiconductor materials and the redox potential (O ₂ / H ₂ O) of oxygen (largely in the positive direction). ), The edge band of the conduction band (bottom of the conduction band) is higher than that of other semiconductor materials and the redox potential (H ⁺ / H ₂ ) of hydrogen (large in the negative direction). This indicates that β-Ga ₂ O ₃ has a greater overvoltage for water electrolysis at both the cathode and anode compared to other semiconductor materials, while at the same time having sufficient capacity for photoelectrolysis. doing.

  Gallium oxide has a band gap energy of about 4.7 eV as described above, and generates an electron-hole pair that induces a photocurrent when irradiated with light having an energy higher than this energy. Such light is typically ultraviolet light, and more specifically, a photoelectric conversion function can be obtained by irradiation with light in the deep ultraviolet region.

  In this embodiment, since the photoelectrode is composed of a gallium oxide single crystal, it is less susceptible to grain boundaries, impurities, and the like than a photocatalyst composed of gallium oxide powder. Therefore, a highly efficient photocatalytic reaction can be obtained.

  The method for growing the single crystal is not particularly limited, and for example, a FZ (Floating Zone) method, a CZ (Czochralski) method, an EFG (Edge-defined Film-fed Growth) method, or the like is adopted. The crystal growth direction, the pulling direction, etc. are not particularly limited. According to the EFG method, since the single crystal defined by the shape of the upper end portion of the die is grown, the temperature gradient at the crystal growth interface can be extremely reduced. Furthermore, since the gallium oxide melt is supplied through the slit, the evaporation of components in the gallium oxide melt and the composition fluctuation of the gallium oxide melt can be extremely reduced, and a high-quality single crystal can be produced. The growth surface and pulling direction of the single crystal can be set arbitrarily.

  The constituent material of the counter electrode is not particularly limited, and metal materials in general can be widely applied. Typically, platinum (Pt) is used. The counter electrode is electrically connected to the photoelectrode and is immersed in the electrolyte together with the photoelectrode. Typically, the electrons generated by the photocatalytic function of the gallium oxide single crystal move to the counter electrode (cathode) and reduce the electrolyte. When the electrolytic solution is an aqueous solution, hydrogen is generated by the reducing action of water by electrons on the counter electrode surface.

The photoelectrode may be composed of an n-type gallium oxide single crystal substrate having a carrier concentration of 1 × 10 ¹⁶ [cm ⁻³ ] to 1 × 10 ¹⁹ [cm ⁻³ ].
When the carrier concentration exceeds 1 × 10 ¹⁹ [cm ⁻³ ], the light absorption efficiency is significantly reduced. As a result, the generation efficiency of the electron-hole pairs is also reduced, so that a desired photocatalytic function cannot be obtained. . On the other hand, when the carrier concentration is less than 1 × 10 ¹⁶ [cm ⁻³ ], although the light absorption efficiency is improved, recombination of the generated electrons and holes is likely to occur, and the light absorption efficiency reaches its peak. become.

The electrochemical reaction device may further include a bias power source connected between the photoelectrode and the counter electrode.
By applying an external bias between the photoelectrode and the counter electrode, the size of the surface depletion layer of the photoelectrode and its internal electric field can be adjusted. As a result, the catalytic reaction at the photoelectrode can be promoted.

  Hereinafter, an electrochemical device according to an embodiment of the present invention will be described with reference to the drawings. In this embodiment, an example in which the present invention is applied to hydrogen generation by photoelectrolysis of water will be described.

  FIG. 2 is a schematic configuration diagram showing an electrochemical reaction device according to an embodiment of the present invention.

  The electrochemical reaction device 10 of the present embodiment includes a photoelectrode 11, a counter electrode 12 electrically connected to the photoelectrode 11, and an electrolyte solution containing an electrolyte solution 13 that contacts the photoelectrode 11 and the counter electrode 12. And a tank 14.

The photoelectrode 11 is typically composed of an n-type gallium oxide single crystal having an appropriate shape and thickness such as a rectangle, a circle, or an ellipse. In particular, in this embodiment, a β-Ga ₂ O ₃ single crystal substrate is used as the photoelectrode 11. The plane orientation of the photoelectrode 11 is not particularly limited, but in this embodiment, the plane orientation of the main surface is the (010) plane.

  The photoelectrode 11 is connected to the counter electrode 12 via the wiring 15. The wiring 15 is typically made of a metal material, and is preferably a material whose electrical connection with the photoelectrode 11 is an ohmic contact. Alternatively, an appropriate material (for example, metal indium (In)) that can ensure the ohmic contact may be interposed between the photoelectrode 11 and the wiring 15.

  The counter electrode 12 is composed of a platinum (Pt) plate, a wire, or the like. When the counter electrode 12 is composed of a wire, the counter electrode 12 and the wiring 15 may be composed of the same wire material. The counter electrode 12 is arranged to face the photoelectrode 11 in the illustrated example, but it is not limited to this.

The electrolytic solution tank 14 stores an electrolytic solution 13 that contacts the photoelectrode 11 and the counter electrode 12. An appropriate aqueous electrolytic solution is used as the electrolytic solution 13, and for example, an aqueous NaOH solution, an aqueous H ₂ SO ₄ solution, an aqueous Na ₂ SO ₄ solution, or the like is applicable.

  The electrolytic solution tank 14 has an opening 14a on the side surface facing the main surface of the photoelectrode 11, and is configured to be able to irradiate light to the photoelectrode 11 from the outside of the electrolytic solution tank 14 through the opening 14a. The opening 14 a is covered with a light transmissive window member 17 through a seal ring 16.

Β-Ga ₂ O ₃ constituting the photoelectrode 11 has a band gap energy of about 4.7 eV, and generates an electron-hole pair by being irradiated with light having a light energy higher than that energy. . The light having the light energy (excitation light) is typically ultraviolet light, and for example, deep ultraviolet light having a wavelength of 200 nm to 260 nm is suitable.

  The excitation light may be ultraviolet light contained in sunlight, but in the present embodiment, the excitation light further includes an ultraviolet light source 18. As a result, the photoelectrode 11 can be efficiently excited. In the present embodiment, a light source that emits light in the deep ultraviolet region is used as the light source 18.

FIG. 3 is a schematic diagram showing a band structure of the photoelectrode 11 in the electrolytic solution 13. The conduction band edge band (bottom potential of the conduction band) is higher (larger in the negative direction) than the redox potential (H ⁺ / H ₂ ) of hydrogen, and the valence band edge band (top potential of the valence band) is oxygen. Lower than the redox potential (O ₂ / H ₂ O) of (large in the positive direction). The redox potentials of hydrogen and oxygen are 0 V and 1.23 V, respectively, on a standard hydrogen electrode (SHE) as shown in FIG.

  In the vicinity of the surface of the photoelectrode 11 in contact with the electrolytic solution 13, a band bent region, that is, a depletion layer (also referred to as a space charge layer) 11 d is formed. In this state, when the photoelectrode 11 is irradiated with deep ultraviolet rays, electron-hole pairs are generated in the depletion layer. That is, an electron e in the valence band is excited to the conduction band, and a hole h is generated at the original position of the excited electron.

Due to the electric field gradient in the depletion layer 11d, the electrons e diffuse inside the photoelectrode 11, move to the counter electrode 12 through the wiring 15, and one hole h moves to the surface of the photoelectrode 11 (charge separation). ). The electrons e that have moved to the counter electrode 12 reduce the water (H ₂ O) of the electrolyte solution 13 that contacts the counter electrode 12 to generate hydrogen (see the following formula (1)). On the other hand, the holes h that have moved to the surface of the photoelectrode 11 oxidize the hydroxide ions (OH ⁻ ) of the electrolytic solution 13 to generate oxygen (see the following formula (2)).
2H ₂ O + 2e → H ₂ + 2OH ⁻ (1)
4OH ⁻ + 4h ⁺ → 2H ₂ O + O ₂ (2)

  As described above, photoelectrolysis of water is realized. In order to efficiently generate hydrogen at the counter electrode 12, it is necessary to increase the light absorption efficiency of the photoelectrode 11. In order to increase the light absorption efficiency of the photoelectrode 11, it is essential to optimize the thickness (depth) of the depletion layer 11d. The thickness of the depletion layer 11 d is typically adjusted by the carrier concentration of the gallium oxide single crystal constituting the photoelectrode 11.

That is, when the carrier concentration of the Ga ₂ O ₃ single crystal substrate is higher than the optimum value for efficiently absorbing light, the depletion layer becomes thin, and the light absorption efficiency is lowered.

  On the other hand, when the carrier concentration of the substrate is lower than the optimum value, the depletion layer becomes thicker, but light absorption occurs only at a predetermined depth on the surface, so that the light absorption efficiency reaches its peak. Furthermore, if the depletion layer is too thick, the probability that the generated electrons and holes are recombined increases and the efficiency decreases.

For the reasons described above, in this embodiment, the carrier concentration of the photoelectrode 11 (gallium oxide single crystal) is adjusted to, for example, 1 × 10 ¹⁶ [cm ⁻³ ] or more and 1 × 10 ¹⁹ [cm ⁻³ ] or less. The Thereby, the stable photoelectrolysis action of water can be obtained. In terms of the thickness of the depletion layer, when the external bias is 0 [V], the carrier concentration is 1 × 10 ¹⁶ [cm ⁻³ ] and the carrier concentration is 462 nm and the carrier concentration is 1 × 10 ¹⁹ [cm ⁻³ ]. 15 nm. That is, in this embodiment, the thickness of the depletion layer 11d of the photoelectrode 11 can be set in the range of, for example, 15 nm or more and 462 nm or less.

  On the other hand, the depth of the depletion layer 11 d can also be controlled by applying an external bias (potential) between the photoelectrode 11 and the counter electrode 12. In this case, for example, by applying a reverse bias (reverse potential) to the depletion layer 11d, the amounts of oxygen and hydrogen generated in the photoelectrode 11 and the counter electrode 12 can be improved.

  Therefore, the electrochemical reaction device 10 of the present embodiment can replace the wiring 15 between the photoelectrode 11 and the counter electrode 12 with a bias power source 19. The bias power source 19 is a variable power source. The bias power source 19 is configured to apply a reverse bias to the photoelectrode 11 in a predetermined voltage range, but may be configured to apply a bias potential to the photoelectrode 11 also in the forward direction. . In this case, the variable voltage range of the bias power supply 19 is set, for example, to a range of −several V to + several V.

  Hereinafter, the electrochemical reaction device 10 of the present embodiment will be described in detail through the following experimental examples.

[Experimental Example 1]
In this experimental example, the photocatalytic function of an n-type gallium oxide single crystal substrate in which the carrier concentration was controlled to 1 × 10 ¹⁷ cm ⁻³ was verified. The experimental procedure will be described below.

For the verification, a three-electrode photoelectrochemical cell shown in FIG. 4 was used. This cell includes a container 101 having openings H1 and H2 facing each other, a working electrode WE, a counter electrode CE, and a reference electrode RE. An n-type gallium oxide (β-Ga ₂ O ₃ ) single crystal substrate is used for the working electrode WE, a Pt wire is used for the counter electrode CE, and an Ag / AgCl electrode is used for the reference electrode RE. The liquid was placed in contact with the liquid. These three electrodes WE, CE, and RE were connected to a potentiostat 104 and a frequency response analyzer 105.

The electrode potential of the reference electrode RE with respect to the standard hydrogen electrode (SHE) is +2.03, and the electrolytes are 0.05 MH ₂ SO ₄ aqueous solution (pH = 1.91), 0.1 M Na ₂ SO ₄ aqueous solution (pH = 5.92) and 0.1 M aqueous NaOH (pH = 13.2).

The Ga ₂ O ₃ substrate was prepared by slicing a gallium oxide single crystal grown by the FZ method into an elliptical (010) substrate. Ga ₂ O ₃ to one surface of the substrate was fixed through a seal ring 102 to the opening H1, it provided the In terminal 107 to the Ga ₂ O ₃ other surface of the substrate (back). The In bonding to the Ga ₂ O ₃ substrate was ohmic contact by heating at 900 ° C. for 60 seconds in a nitrogen atmosphere.

A crystal window 106 is fixed to the opening H2 of the container 101 through a seal ring 103, and deep ultraviolet light having a wavelength of 254 nm is applied to the working electrode WE (Ga ₂ O ₃ substrate) from the light source (low pressure mercury lamp) through the window 106. The photoelectrode characteristics were evaluated. Monochromatic light from a xenon lamp was used for wavelength sensitivity measurement. The exposed area of the working electrode WE facing the electrolytic solution and the light source was 0.162 cm ² .

First, the flat band potential was determined by the Mott-Schottky correlation shown in the following formula (3).
^{(1 / Cs 2) = 2} (E-Efb- (kT / e)) / (ε · ε r · ε 0 · N D) ... (3)
Here, Cs, e, ε _r , ε ₀ , N _D , E, Efb, and kT are a capacity per unit area, an electronic charge, a relative dielectric constant of β-Ga ₂ O ₃ (ε _r = 10), These are the dielectric constant of vacuum, ionized donor concentration (carrier concentration), electrode potential, flat band potential, and thermal energy.

FIG. 5 shows 1 / Cs ² -E plots obtained from a plurality of electrolytes having different pHs. 1 / Cs ² decreases linearly as E decreases, which is in good agreement with equation (3). From the slope of the plot, the carrier concentration N _D is independent of pH of the electrolyte solution was calculated to be about 4.0 × 10 ¹⁷ cm ^-3.

The thickness (d) of the depletion layer at an externally applied voltage of 0 V on the Ga ₂ O ₃ substrate corresponding to this carrier concentration value is 73 nm. If the light absorption coefficient of the Ga ₂ O ₃ substrate is 2 × 10 ⁵ cm ⁻⁵ , 77% of the incident light is absorbed by the depletion layer.

Efb is obtained from the horizontal intercept of the linear extrapolation of the plot. FIG. 6 shows the pH dependence of Efb. Efb had a first order dependence (linear dependence) as a function of pH and the slope was -57 mV / pH. The value almost coincided with -59 mV / pH which is a theoretical value derived from an equilibrium equation of proton dissolution on the photoelectrode surface represented by the following formula (4).
Ga—OH ⁻ ⇔H ⁺ + Ga—O ²⁻ (4)

The conduction band edge band (Ec) and the valence band edge band (Ev) were calculated from the basic formulas (5) to (7).
Nc = 2Mc (2πme kT / h ² ) ^3/2 (5)
Ec-Efb = kT ln (N D / Nc) ... (6)
Ev−Ec = Eg (7)
Here, Nc, Mc, me and h are the effective state density in the conduction band, the number of equivalent valleys in the conduction band (Mc = 1, the conduction band minimum value for the 4s orbit of Ga is located at the Γ point), The effective electron mass of β-Ga ₂ O ₃ (me = 0.28 m ₀ , m ₀ is the electron's static mass) and the Planck constant.

Ec and Ev were 1.1 V higher than the H ₂ / H ⁺ redox potential and 2.4 V lower than the H ₂ O / O ₂ redox potential, respectively.

Next, the photoelectrochemical reaction was evaluated by measuring the photocurrent. FIG. 7 shows the relationship between the current density (J) and the potential E in a 0.1 M NaOH aqueous solution. In the anodic polarization, a small current value of less than 100 nmA / cm ² was observed in the dark, but a large photocurrent of the order of 10 μA / cm ² was observed when illuminated with 254 nm light at 0.59 mW / cm ² . . The photocurrent increases monotonically as the potential of the working electrode WE increases with respect to the reference electrode RE, which reflects the expansion of the depletion layer and the increase of the electric field in the depletion layer due to the increase in band bending. It is estimated to be.

Wavelength sensitivity characteristics measured in 0.1 M NaOH aqueous solution are shown in black circles in FIG. For comparison, the absorbance spectrum of the β-Ga ₂ O ₃ substrate is also shown by white circles in the figure. In the measurement of wavelength sensitivity characteristics, no external bias (V) was applied between the working electrode WE and the counter electrode CE. The photoelectric conversion efficiency (IPCE: Incident Photon to Current conversion Efficiency) greatly increases under illumination of deep ultraviolet rays (photon energy of 4.7 eV or more), and the rising wavelength corresponds to the fundamental absorption edge. IPCE was 0.14 at a wavelength of 254 nm, and reached a maximum value of 0.36 at 240 nm. Considering that the photocurrent shown in FIG. 7 is not saturated, it is estimated that the efficiency is further improved by controlling the band bending by controlling the carrier concentration of the crystal.

Next, gas generation characteristics were evaluated using the gas collection system shown in FIG. That is, in the quartz beaker 111, the photoelectrode 116 (Ga ₂ O ₃ substrate) used for the working electrode WE and the counter electrode (Pt wire) 117 used for the counter electrode CE are immersed in a 0.1M NaOH aqueous solution, and the source They were connected via a meter 115.

The back surface of the photoelectrode 116, the lead wires, and the lead wires of the counter electrode 117 (regions excluding the tip) were coated with a resin 114 to protect them from electrolysis. The amount of gas generated at the photoelectrode 116 and the counter electrode 117 is measured by the graduated cylinders 112 and 113 respectively disposed immediately above these two electrodes, and the type of gas generated at each electrode is analyzed by gas chromatography. did. In order to estimate the decomposition of Ga ₂ O ₃ in an aqueous solution, ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) was performed on Ga. All the above measurements were performed at room temperature. The source meter 115 is configured to be able to apply an external bias to the photoelectrode 116 and detect a photocurrent.

  FIG. 10 is a photograph showing a gas generation state when an experiment of photoelectrolysis of water in a 0.1 M NaOH aqueous solution was performed using the gas collection system shown in FIG. Bubbles were not observed under room lighting (FIG. 10 (a)). When the photoelectrode 116 was irradiated with light having a wavelength of 254 nm (deep ultraviolet), small bubbles were generated from the counter electrode 117 and the photoelectrode 116 without an external bias (V = 0) (FIG. 10B). The gases generated from the counter electrode 117 and the photoelectrode 118 are identified as hydrogen and oxygen, respectively, which are typical water reduction and n-type photoelectrodes as in the reaction equations (1) and (2) above. Shows oxidation. On the other hand, oxygen gas generation was slight relative to hydrogen. When an external bias of 1V (synonymous with a reverse bias of -1V) was applied, the gas generation amount increased in both electrodes (FIG. 10 (c)).

  FIGS. 11A and 11B show the amounts of gas with respect to the sum of photocurrents when the bias voltages are 0 V and −1 V, respectively. In each figure, the black circle plot indicates the hydrogen gas amount, and the black square plot indicates the oxygen gas amount. The state at this time corresponds to FIG.

  In each bias state, the amount of hydrogen gas was in good agreement with the theoretical value calculated from the sum of photocurrents. This indicates that the electrons from the cathode were spent in the reduction of water as in the above formula (1). With respect to oxidation, the amount of gas generated was considerably smaller than the theoretical value at V = 0 V (FIG. 11 (a)). This means that most of the generated holes were not used for oxidation as in the above formula (2).

  On the other hand, the generation of oxygen gas is greatly improved by an external bias of -1 V (FIG. 11 (b)). Co-production of hydrogen and oxygen in stoichiometric ratio is clearly observed, and these amounts correspond to those estimated from the photocurrent indicating complete water electrolysis. The reason for the improvement of water oxidation is unknown, but it is presumed that an external bias enhances the electric field in the depletion layer and enhances the injection of holes into the electrolyte.

[Experiment 2]
An n-type Ga ₂ O ₃ single crystal substrate having a carrier concentration different from that of Experimental Example 1 was prepared, and this was used as the working electrode WE of the three-electrode photoelectrochemical cell shown in FIG. did. The measurement results are shown in FIG. From the slope of the plot, the carrier concentration of the Ga ₂ O ₃ substrate used in this experimental example was calculated to be 4.7 × 10 ¹⁸ cm ⁻³ .

The thickness (d) of the depletion layer at an externally applied voltage of 0 V on the Ga ₂ O ₃ substrate corresponding to this carrier concentration value is 21 nm. If the light absorption coefficient of the Ga ₂ O ₃ substrate is 2 × 10 ⁵ cm ⁻⁵ , 34% of the incident light is absorbed by the depletion layer.

  Next, the photoelectrochemical reaction was evaluated by measuring the photocurrent. FIG. 13 shows the relationship between the current density (J) and the potential E in the 0.1 M NaOH aqueous solution. Also in this experimental example, as in Experimental example 1, it was confirmed that the photocurrent increased as the potential of the working electrode WE with respect to the reference electrode RE increased.

  The embodiment of the present invention has been described above, but the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  For example, in the above embodiment, an electrochemical reaction apparatus for mainly generating hydrogen by electrolysis of water has been described as an example. However, the present invention is not limited thereto, and for example, for synthesizing or decomposing organic substances and inorganic substances. The present invention can also be applied to an electrochemical reaction apparatus.

DESCRIPTION OF SYMBOLS 10 ... Electrochemical reaction apparatus 11 ... Photoelectrode 12 ... Counter electrode 13 ... Electrolyte solution 14 ... Electrolyte tank 19 ... Bias power supply

Claims (5)

A photoelectrode composed of a gallium oxide single crystal that generates electron-hole pairs upon irradiation with ultraviolet rays; and
A counter electrode electrically connected to the photoelectrode;
An electrochemical reaction device comprising: an electrolytic solution tank containing an electrolytic solution in contact with the photoelectrode and the counter electrode. The electrochemical reaction device according to claim 1,
The said photoelectrode is an electrochemical reaction apparatus comprised with the n-type gallium oxide single crystal substrate which has a carrier concentration of 1 * 10 < ¹⁶ > [cm < ^-3 >] or more and 1 * 10 < ¹⁹ > [cm < ^-3 >] or less. The electrochemical reaction device according to claim 1 or 2,
The electrochemical reaction apparatus, wherein the electrolytic solution is an aqueous electrolytic solution capable of generating hydrogen from the counter electrode by irradiating the photoelectrode with ultraviolet light. The electrochemical reaction device according to any one of claims 1 to 3,
An electrochemical reaction device further comprising a bias power source connected between the photoelectrode and the counter electrode. The electrochemical reaction device according to any one of claims 1 to 4,
An electrochemical reaction device further comprising a light source configured to irradiate the photoelectrode with deep ultraviolet light.