JP2007117811A - Photocatalyst film, semiconductor photoelectrode for water decomposition, and water decomposition device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photocatalyst film which has sensitivity to both ultraviolet light and visible light, to provide a semiconductor photoelectrode suitable for producing hydrogen and oxygen from water, which has a structure having the photocatalyst and color-increased type solar cell piled up, and to provide a water decomposition device using the same. <P>SOLUTION: The photocatalyst film is provided with a first photocatalyst film 18 formed on a transparent conductive film 19a at the whole face of a transparent substrate 15 and a second photocatalyst film 21 formed on the first photocatalyst film 18, wherein the first photocatalyst film 18 is comprised of a TiO<SB>x</SB>film (x<2.0) containing oxygen and titanium in a stoichiometric rate of oxygen deficiency and the second photocatalyst film 21 is comprised of TiO<SB>2</SB>film. The semiconductor photoelectrode for water decomposition and the water decomposition device using the film are also provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photocatalyst film, a water splitting semiconductor photoelectrode, and a water splitting apparatus using the same, and more particularly, a water splitting apparatus that splits water into hydrogen and oxygen by light and a water splitting semiconductor light used therefor The present invention relates to an electrode and a photocatalytic film.

Recently, in order to prevent global warming, reduction of greenhouse gas emissions is required, and as one of the measures, introduction of clean energy such as wind power and sunlight is promoted. In addition, fuel cells, hydrogen production technologies, hydrogen storage and transport technologies, etc. are being actively researched to realize a hydrogen society where hydrogen is assumed to be a major energy source.
Currently, hydrogen can be produced using fossil fuels such as coal, oil, and natural gas as raw materials, but in the future, hydrogen production technology using non-fossil fuels such as water and biomass and clean energy is desired. ing.

By the way, metal oxides such as titanium dioxide (TiO ₂ ) and strontium titanate (SrTiO ₃ ) are used as semiconductor photocatalysts which receive light such as sunlight and generate photovoltaic power and cause an electrochemical reaction by the photovoltaic power. Physical semiconductors are known. It is known that when a platinum electrode and a titanium dioxide electrode are disposed in water and the titanium dioxide electrode is irradiated with ultraviolet rays, the water can be decomposed into hydrogen and oxygen.

  The conditions of the semiconductor photocatalyst and semiconductor photoelectrode that can electrolyze water and can use sunlight sufficiently are to have a photoelectromotive force higher than the electrolysis voltage of water (theoretical value 1.23V), that is, energy in the conduction band. The level is more negative than the hydrogen generation potential, and the energy level of the valence band is more positive than the oxygen generation potential, and the semiconductor photocatalyst and the semiconductor photoelectrode itself are not chemically dissolved in the electrolyte. It must be stable.

  A typical photocatalyst, titanium dioxide, has an energy band gap of about 3.2 eV and a potential condition necessary for water decomposition, so that water can be decomposed and does not dissolve in the electrolyte. There is a problem that there is almost no sensitivity to light having a wavelength longer than about 380 nm in the sunlight spectrum, and the photoelectric conversion efficiency for sunlight is extremely low.

  In addition, since titanium dioxide satisfies the potential condition necessary for water decomposition as described above, water can be decomposed in principle, but in reality, the energy level of the conduction band of titanium dioxide is higher than the hydrogen generation potential. Since it is only slightly negative, hydrogen cannot be generated sufficiently without bias. Therefore, in general, a chemical bias using a pH difference generated by biasing by using an external power source or by immersing titanium dioxide in an alkaline solution and immersing a platinum electrode in an acidic solution is used.

  When a material having a small energy band gap, for example, about 2.7 eV for tungsten oxide and about 2.3 eV for ferric trioxide is used, light having a wavelength of about 460 nm or less is used for tungsten oxide, and about 540 nm is used for ferric trioxide. The following light can be absorbed. However, the energy level of the conduction band of these materials is more positive than the hydrogen generation potential, and hydrogen cannot be generated without a bias.

Therefore, for example, as described in Patent Document 1 and Patent Document 2, a tandem cell in which a photocatalyst immersed in an electrolyte aqueous solution and a dye-sensitized solar cell are stacked and electrically connected is known. The outline of this tandem cell will be described with reference to FIG. First, the oxide film 3 serving as a photocatalyst absorbs sunlight from ultraviolet to blue or green to generate electrons and holes. The dye-sensitized solar cell superimposed on the back surface of the oxide film 3 absorbs sunlight from green or yellow to red to generate photovoltaic power. The oxide film 3 serving as a photocatalyst and the counter electrode 8 of the dye-sensitized solar cell are electrically connected, and the TiO ₂ film 6 of the dye-sensitized solar cell and the hydrogen generating catalyst cathode 10 are electrically connected 5. Thus, the electromotive force of the dye solar cell functions as a bias, and the energy level of electrons can be pushed more negative than the hydrogen generation potential to generate hydrogen.

In addition, titanium dioxide has almost no sensitivity to light having a wavelength longer than about 380 nm in the solar spectrum, and the photoelectric conversion efficiency with respect to sunlight is extremely low. It is non-patented that the oxygen-deficient titanium oxide film (TiO _x film (x <2.0)) absorbs visible light up to a wavelength of about 450 nm and exhibits photocatalytic activity. It is disclosed in Document 1.

Special table 2003-504799 gazette Special table 2004-504934 gazette Masato Takeuchi, Shoichi Anbo, Takashi Hirao, Nobuhisa Ito, Shinya Iwamoto: Surface Science Vol22, No9 (2001)

  However, when the present inventors examined the oxygen-deficient titanium oxide film described in Non-Patent Document 1, the visible light response of the oxygen-deficient titanium oxide film is manifested by making oxygen less than the stoichiometric ratio. However, it has been found that if there are too many oxygen vacancies, the crystallinity deteriorates and the photoelectric conversion characteristics deteriorate. In particular, when absorption is caused in the visible light region, the photoelectric conversion efficiency in the ultraviolet light region is lowered, and in many cases, only a film having a low photoelectric conversion efficiency can be obtained as a whole.

  Moreover, in the tandem cell structure described in Patent Documents 1 and 2, the photocatalyst film and the dye-sensitized solar cell must be wired with a lead wire or the like, and a problem that requires complicated work when modularizing Is left.

  The present invention has been made in view of the above circumstances, and has a photocatalyst film having sensitivity to both ultraviolet light and visible light, a structure in which this photocatalyst film and a dye-increasing solar cell are overlaid, The present invention provides a water-splitting semiconductor photoelectrode suitable for producing hydrogen and oxygen from water and a water-splitting apparatus using the same.

Thus, according to the present invention, the first photocatalyst film formed on the transparent conductive film on one surface of the transparent substrate and the second photocatalyst film formed on the first photocatalyst film are provided, There is provided a photocatalytic film in which the first photocatalytic film is composed of a TiO _x film (x <2.0) containing oxygen and titanium in a stoichiometric ratio that results in oxygen deficiency, and the second photocatalytic film is composed of a TiO ₂ film. The
According to another aspect of the present invention, the photocatalyst film having the second photocatalyst film / first catalyst film, the first transparent conductive film, the transparent substrate / second transparent conductive film / electrolyte from the light receiving surface side. And at least a dye-sensitized solar cell having a semiconductor layer / metal substrate carrying a dye, and electrically connecting the transparent conductive film on the front and back surfaces of the transparent substrate to the transparent substrate. A semiconductor photoelectrode for water splitting provided with a conductive member is provided.
According to still another aspect of the present invention, the water-splitting semiconductor photoelectrode and the aqueous electrolyte solution are provided in a housing whose light-receiving surface side is transparent, and the housing takes out hydrogen and oxygen. A water splitting device having a mouth is provided.

According to the photocatalyst film of the present invention, good photoelectric conversion characteristics can be obtained not only in the ultraviolet region of light such as sunlight but also in the visible light region.
Further, according to the semiconductor photoelectrode for water splitting of the present invention, it is possible to produce a water splitting apparatus capable of efficiently generating oxygen and hydrogen from water using this semiconductor photoelectrode, and to produce a photocatalyst and dye sensitization. The solar cell can be modularized without using external wiring such as lead wires, and the complexity of modularization can be reduced. In addition, since a conductive member for electrically connecting the first and second transparent conductive films on the front and back surfaces of the transparent substrate is installed, connection by a lead wire is unnecessary, and the photocatalyst film and the dye-sensitized solar The complexity of modularizing the battery can be reduced.
Moreover, according to the water splitting device of the present invention, oxygen and hydrogen can be efficiently generated from water with a simple structure.

The photocatalyst film of the present invention (first invention) has a first photocatalyst film formed on a transparent conductive film on one surface of a transparent substrate and a second photocatalyst film formed on the first photocatalyst film. The first photocatalytic film is composed of a TiO _x film (x <2.0) containing oxygen and titanium at a stoichiometric ratio that results in oxygen deficiency, and the second photocatalytic film is composed of a TiO ₂ film. In addition to the photoelectric conversion function by the TiO ₂ film with respect to the ultraviolet light, the photoelectric conversion function by the TiO _x film with respect to the visible light is added, so that high photoelectric conversion efficiency can be obtained as the entire photocatalyst film.
This photocatalyst film is used for a semiconductor photoelectrode for water splitting which is a second invention described later, and this semiconductor photoelectrode for water splitting is used for a water splitting apparatus which is a third invention described later.
Hereinafter, the present invention will be described in detail with reference to the drawings illustrating embodiments.

<Embodiment 1>
FIG. 1 is a schematic sectional side view showing a water splitting apparatus according to Embodiment 1, and FIG. 2 is a schematic front view of the water splitting apparatus viewed from the light receiving surface side. In addition, in FIG. 1, the arrow shown with the code | symbol 33 represents sunlight.

(Description of photocatalytic film)
Photocatalytic film 22, TiO _x film 18 is an oxygen-deficient titanium oxide film containing oxygen and titanium in a stoichiometric ratio of the oxygen deficiency and (x <2.0), laminated on the TiO _x film 18 It consists of a TiO ₂ film 21. The TiO _x film 18 is formed on the first transparent conductive film 19 a formed on the first surface of the transparent substrate 15, and the TiO ₂ film 21 is formed on the TiO _x film 18. The transparent substrate 15 is a component of the dye-sensitized solar cell S described later.
Here, in the present invention, the oxygen-deficient titanium oxide film means a titanium oxide film having a composition ratio of oxygen atoms to titanium atoms of less than 2.0, particularly 1.99 or less.

FIG. 3 is a graph showing the relationship between absorbance and light wavelength for three types of oxygen-deficient titanium oxide films (TiO _x films: x = 1.9, 1.93, 1.95) and titanium dioxide films having different oxygen atom composition ratios. The data shown in FIG. 3 is obtained by forming each oxygen-deficient titanium oxide film (film thickness: 1 μm) and titanium dioxide film (film thickness: 1 μm) on a transparent substrate and measuring with a visible ultraviolet spectrophotometer. . The composition ratio of oxygen atoms in each TiO _x film (x = 1.9, 1.93, 1.95) was measured by Auger electron spectroscopy.
From FIG. 3, the TiO _x films (x = 1.9, 1.93, 1.95) have a higher absorbance of visible light near 900 to 400 nm than the TiO ₂ films, and the oxygen atom composition ratio exceeds 1.95 and becomes 2. As it approaches, the absorbance of the TiO ₂ film approaches the light absorption in the visible light region. From this, in order that the photocatalytic film in the present invention has good photoelectric conversion characteristics in the visible light region, the composition ratio of oxygen atoms to titanium atoms in the TiO _x film 18 is measured by Auger electron spectroscopy. Is preferably from 1.1.9 to 1.95 (1.9 ≦ x ≦ 1.95). The visible light responsiveness of the TiO _x film 18 is manifested by making it less than the stoichiometric ratio of oxygen atoms to titanium atoms (making it smaller than 2.0), but making x less than 1.9 and oxygen When there are too many defects, crystallinity will deteriorate and photoelectric conversion characteristics will deteriorate.

Also, when as described above is 1.9 ≦ x ≦ 1.95 in the TiO _x layer 18, the thickness of the TiO _x film 18 and the TiO ₂ film 21, good photoelectric to visible light and ultraviolet light It is preferable that the combination provides a conversion characteristic. The combination is an experiment in which the thickness of the TiO _x film 18 is 0.7 to 1 μm and the thickness of the TiO ₂ film 21 is 0.2 to 0.6 μm. Confirmed. Particularly, a combination in which the film thickness of the TiO _x film 18 is 0.7 μm and the film thickness of the TiO ₂ film 21 is 0.4 μm is preferable for obtaining the highest photocurrent value. Therefore, it is not preferable to deviate from the range of the film thickness of 0.7 to 1 μm of the TiO _x film 18 and the film thickness of 0.2 to 0.6 μm of the TiO ₂ film 21 because the photocurrent value decreases. This is because if the film thickness of each photocatalyst film of the present invention is too thin, it is insufficient to absorb light, and if the film thickness is too thick, crystals that act as recombination centers of carriers generated in each photocatalyst film It is thought that this is because the photoelectric conversion efficiency decreases due to the increase in the number of defects. Incidentally, when the lamination order of the TiO _x film 18 and the TiO ₂ film 21 Conversely, the amount of light received ultraviolet light by the TiO ₂ film is ultraviolet light absorption in a portion of the TiO _x film is reduced, ultraviolet light by the TiO ₂ film The photoelectric conversion efficiency of the region is lowered, and as a result, the photoelectric conversion efficiency of the entire photocatalyst layer having two layers is lowered as compared with the present invention.

In the photocatalyst film 22, the TiO _x film 18 is not particularly limited as long as it is a film forming method capable of controlling the composition ratio and the film thickness with high accuracy. For example, a TiO ₂ sintered body having a rutile crystal structure is used as a target. Magnetron sputtering is preferred. In this case, although the film forming conditions depend on the film forming apparatus to be used, for example, the RF power supply is 250 to 350 W, the substrate temperature is 550 to 700 ° C., and the gas composition ratio is Ar: O ₂ = 25: 0 (sccm). The film formation pressure is 1 to 3 Pa, the substrate-target distance is 60 to 80 mm, and the film formation start chamber vacuum is 9 × 10 ⁻⁴ Pa or less. The film thickness of the TiO _x film 18 is adjusted by the film formation time.

The TiO ₂ film 21 is not particularly limited as long as it is a film forming method that can control the film thickness with high accuracy, but the same magnetron sputtering method as that of the TiO _x film 18 can be formed with the same apparatus. In this case as well, although the film forming conditions depend on the film forming apparatus to be used, for example, the RF power is 250 to 350 W, the substrate temperature is 200 to 400 ° C., and the gas composition ratio is Ar: O ₂ = 25: 1 to 5 (Sccm), the film forming pressure is 1 to ³ Pa, the substrate-target distance is 60 to 80 mm, and the vacuum in the film forming start chamber is 2 × 10 ⁻³ Pa or less. The film thickness of the TiO ₂ film 21 is adjusted by the film formation time.

(Description of water splitting device)
The water splitting device includes a water splitting semiconductor photoelectrode 30 and an aqueous electrolyte solution 31 in a housing whose light receiving surface is transparent. The housing includes a frame 32 that fits and holds the periphery of the water-decomposing semiconductor photoelectrode 30, and a cover glass 26 that covers the opening on the light receiving surface side of the frame 32, and is held by the frame 32. Between the water-splitting semiconductor photoelectrode 30 and the cover glass 26, a space for containing the electrolyte aqueous solution 31 is provided.

In the first embodiment, the water-decomposing semiconductor photoelectrode 30 has a quadrangular shape in front view, and the frame 32 is also formed in a quadrangular shape. On the upper side of the frame 32, circulation of the electrolyte aqueous solution 31 and hydrogen (H ₂ ) Two openings 36a and 37a for taking out 35 and oxygen (O ₂ ) 34 are provided, and these openings 36a and 37a are connected to an external electrolyte aqueous solution circulation device and pipes 36 and 37, respectively. It is connected to a known gas separation device provided with a hydrogen separation membrane or a polymer membrane for separating hydrogen 35 and oxygen 34.
As the electrolyte aqueous solution 31, a sodium hydroxide aqueous solution, a potassium sulfate aqueous solution, or the like can be used. The frame 32 is formed of a metal material that is not corroded by the electrolyte aqueous solution 31, such as aluminum, stainless steel, brass, or the like.

  The water-decomposing semiconductor photoelectrode 30 includes a front-view square dye-sensitized solar cell S and a front-view square formed on the light-receiving surface of the dye-sensitized solar cell S via the first transparent conductive film 19a. The photocatalyst film 22 is provided. The photocatalytic film 22 is formed in a size slightly smaller than the dye-sensitized solar cell S.

(Description of dye-sensitized solar cell)
The dye-sensitized solar cell S was formed on a transparent substrate 15 having a first surface on which the first transparent conductive film 19a was formed, and on a second surface opposite to the first surface of the transparent substrate 15. A second transparent conductive film 19b, a metal substrate 25, an electrolyte layer 29 formed between the metal substrate 25 and the second transparent conductive film 19b, a semiconductor layer 27 carrying a dye, and a first transparent conductive film A conductive member 17a for electrically connecting 19a and the second transparent conductive film 19b, one end of which is electrically connected to the second transparent conductive film 19b, and the other end of the photocatalyst along the light receiving surface of the photocatalyst film 22 It has the wire electrode 23 extended without contacting the film 22.

  The transparent substrate 15 is not particularly limited as long as it is usually used for a dye-sensitized solar cell, and examples thereof include a glass substrate, a plastic substrate, and the like. A substrate is preferred. The thickness of the transparent substrate 15 should just be a grade which can support a solar cell structurally, for example, is about 0.1-5 mm. Moreover, the magnitude | size of the transparent substrate 15 should just be a magnitude | size suitable for using as a semiconductor electrode for water splitting, for example, length 100-500 mm and width 100-500 mm are preferable.

The first and second transparent conductive films 19a and 19b are not particularly limited as long as they are usually used for dye-sensitized solar cells. For example, an ITO (In ₂ O ₃ —SnO ₂ ) film, F-doped SnO ₂ film and the like can be mentioned, and as a forming method, known methods such as a vacuum deposition method of a material component, a sputtering method, a thermal CVD method, a gas phase method such as a PVD method, a coating method by a sol-gel method, etc. Can be mentioned. The film thickness of the first and second transparent conductive films 19a and 19b is preferably about 0.1 to 0.5 μm.

  Examples of the metal substrate 25 include stainless steel, Ti, and Pt. A stainless steel substrate is particularly preferable, and a thickness of 0.1 to 1 mm is preferable.

This dye-sensitized solar cell S may be composed of a plurality of solar cells. In the case of FIG. 1, the second transparent conductive film 19b, the semiconductor layer 27, the electrolyte layer 29, and the metal substrate 25 are laminated in this order from the light receiving surface side, and the second transparent conductive film from the light receiving surface side. The dye-sensitized solar cell S is composed of two solar cells of the second solar cell in which the conductive film 19b, the electrolyte layer 29, the semiconductor layer 27, and the metal substrate 25 are laminated in this order. In these two solar cells, the second transparent electrode 19b is divided into two, one semiconductor layer 27 is formed on one second transparent conductive film 19b, and the region facing the other second transparent conductive film 19b is formed. The other semiconductor layer 27 is formed on the metal substrate 25, and the electrolytic solution is poured and sealed between the semiconductor layer 27 and the metal substrate 25 and between the second transparent conductive film 19 b and the semiconductor layer 27. Yes. Examples of the method for separating the second transparent conductive film 19b include a method of removing the second transparent conductive film 19b by scribing with a laser.
Although not shown, the dye-sensitized solar cell S does not divide the second transparent conductive film 19b and forms the semiconductor layer 27 on almost the entire surface of the second transparent conductive film 19b or almost the entire surface of the metal substrate 25. Then, a single solar battery cell may be formed by pouring the electrolyte solution.

  Examples of the semiconductor composing the dye-supported semiconductor layer 27 include known semiconductors such as titanium oxide, zinc oxide, tungsten oxide, barium titanate, strontium titanate, cadmium sulfide, and the like. It can also be used as a mixture. Among these, titanium oxide is particularly preferable from the viewpoint of photoelectric conversion efficiency, stability, and safety. Examples of such titanium oxide include anatase-type titanium oxide, rutile-type titanium oxide, amorphous titanium oxide, various titanium oxides such as metatitanic acid and orthotitanic acid, and titanium oxide-containing composites. It may be.

The semiconductor layer 27 can be formed on the second transparent conductive film 19b and the metal substrate 25 as follows.
First, a semiconductor fine particle as a material is added to a dispersing agent, an organic solvent, water and the like together with an organic compound such as a polymer material, and dispersed to prepare a suspension. The obtained suspension is applied onto the metal substrate 25 by a known method such as a doctor blade method, a squeegee method, a spin coating method, or a screen printing method.

Then, the porous semiconductor layer 27 is obtained by drying and baking the obtained coating film.
In drying / firing, it is necessary to appropriately adjust conditions such as temperature, time, and atmosphere according to the type of metal substrate and semiconductor fine particles to be used. Firing can be performed, for example, in an air atmosphere or an inert gas atmosphere at a temperature of about 50 to 800 ° C. for about 10 seconds to 12 hours. This drying and baking can be performed once at a single temperature or twice or more at different temperatures.
Although the film thickness of the semiconductor layer 27 is not specifically limited, About 0.5-35 micrometers is preferable from viewpoints, such as permeability | transmittance and photoelectric conversion efficiency.

Examples of the semiconductor fine particles include semiconductor particles as described above having a particle diameter of about 1 to 500 nm as observed with a transmission electron microscope. In order to improve the photoelectric conversion efficiency, it is necessary to adsorb more dye to the semiconductor layer 27. For this reason, a semiconductor having a large specific surface area is preferable, and about 1 to 200 m ² / g is preferable.

  The dye that is supported on the semiconductor layer 27 and functions as a photosensitizer is not particularly limited as long as it has absorption in various visible light regions and / or infrared light regions. In order to strongly adsorb the dye to the semiconductor layer 27, the dye molecule has an interlock group such as a carboxyl group, an alkoxy group, a hydroxyl group, a hydroxyalkyl group, a sulfonic acid group, an ester group, a mercapto group, or a phosphonyl group. Among them, a carboxyl group is particularly preferable. The interlock group provides an electrical coupling that facilitates electron transfer between the excited dye and the conduction band edge of the porous semiconductor layer 27.

  Examples of the dye having an interlock group include a ruthenium complex dye, a coumarin dye, an azo dye, a quinone dye, a quinone imine dye, a quinacridone dye, a squarylium dye, a cyanine dye, a merocyanine dye, and triphenylmethane. And dyes, xanthene dyes, porphyrin dyes, phthalocyanine dyes, berylene dyes, indigo dyes, naphthalocyanine dyes, and the like.

Examples of the method for adsorbing the dye on the semiconductor layer 27 include a method in which the semiconductor layer is immersed in a solution in which the dye is dissolved (dye adsorption solution).
The solvent used for dissolving the dye is not particularly limited as long as it can dissolve the dye. Examples thereof include alcohols such as (anhydrous) ethanol, ketones such as acetone, ethers such as diethyl ether and tetrahydrofuran. Nitrogen compounds such as acetonitrile, halogenated aliphatic hydrocarbons such as chloroform, aliphatic hydrocarbons such as hexane, aromatic hydrocarbons such as benzene, esters such as ethyl acetate, water, etc. . Two or more of these solvents can be used in combination.
The dye concentration in the dye adsorption solution can be appropriately adjusted depending on the kind of the dye and solvent to be used, and a higher concentration is preferred to improve the adsorption function. For example, it may be 5 × 10 ⁻⁵ mol / liter or more.
The conditions for immersing the semiconductor layer 27 in the dye adsorption solution, that is, the atmosphere, temperature, pressure, and immersion time are not particularly limited, and are appropriately adjusted according to the dye used, the type of solvent, the concentration of the solution, and the like. be able to. For example, the room temperature is about atmospheric pressure.

  Further, a catalyst layer (for example, platinum 20) may be formed on the second transparent conductive film 19b and the metal substrate 25 in a region where the semiconductor layer 27 is not formed.

The conductive member 17 a is formed by embedding a conductive material in a plurality of through holes formed at predetermined intervals along one side of the rectangular transparent substrate 15. A plurality of other through holes are formed at predetermined intervals along the side of the transparent substrate 15 facing the one side and corresponding to the plurality of conductive members 17a, and conductive members are formed in these through holes. 17b is embedded. The portion having the plurality of conductive members 17b in the transparent substrate 15 is exposed to the electrolyte aqueous solution 31 side. Thus, since the conductive members 17a and 17b for electrically connecting the first and second transparent electrodes 19a and 19b on the front and back surfaces of the transparent substrate 15 are embedded, connection by a lead wire is unnecessary, The complexity of modularizing the photocatalytic film and the dye-sensitized solar cell can be reduced.
One end of each of the wire electrodes 23 is connected to the plurality of conductive members 17b on the transparent substrate 15 by a conductive adhesive, and a plurality of wire electrodes 23 are provided. Further, the wire electrode 23 is bent so as not to come into contact with the photocatalyst film 22, and extends with a gap along the light receiving surface of the photocatalyst. The wire electrode 23 is formed to have an appropriate rigidity with, for example, platinum so that the tip of the wire electrode 23 does not come into contact with the photocatalyst film 22 even when the water splitting device is laid sideways or inclined. Although it is preferable, the tip may be fixed to the cover glass 26. Thus, by arranging the wire electrode close to and facing the photocatalyst film side, it is possible to improve the durability by avoiding the structure in which the dye-sensitized solar cell is immersed in the electrolyte solution, The resistance can be reduced by reducing the distance between the titanium dioxide film to be formed and the wire electrode to be the cathode.

(Production of water splitting device)
Next, an example of a manufacturing procedure of the above-described water splitting device will be described.
First, as shown in FIG. 4A, two rows × 4 through holes having a diameter of 0.5 mm are formed in a glass substrate 15 having a width of 100 mm × a length of 100 mm × a thickness of 1 mm, and Ag paste is dispensed into the through holes. Etc., and fired at about 250 ° C. to form Ag conductive members 17a and 17b. Next, a first transparent conductive film 19a made of ITO (In ₂ O ₃ —SnO ₂ ) is formed on the first surface of the glass substrate 15 so as to cover one row of the Ag conductive members 17a by a sputtering method. The film is formed at 1 μm. Subsequently, the second surface of the glass substrate 15 covers the Ag conductive members 17a and 17b in each row, and the pair of second transparent conductive films 19b separated at the center of the substrate is processed in the same manner as the first transparent conductive film 19a. The film thickness is formed (FIG. 4B).

Next, as shown in FIG. 4C, in the region excluding the Ag conductive member 17b on the first transparent conductive film 19a on the first surface side of the glass substrate 15, a TiO _x film 18 which is an oxygen-deficient titanium oxide film. (X = 1.93) is formed with a size of 95 mm × 90 mm and a film thickness of about 0.7 μm. For film formation, a high-frequency magnetron sputtering apparatus is used, a target is a titanium dioxide (TiO ₂ ) sintered body having a rutile crystal structure with a diameter of 7 inches and a thickness of 5 mm, a glass substrate 15 is placed on a sample holder, and the substrate is heated. A TiO _x film 18 (x <2.0) can be formed by heating to about 600 ° C. with a heater and performing high frequency sputtering. Other film forming conditions can be set within the above range.

Next, as shown in FIG. 4D, using the same apparatus, a TiO ₂ film 21 is formed on the TiO _x film 18 with a film thickness of about 0.4 μm. Thereby, the photocatalytic film 22 in which the TiO _x film 18 and the TiO ₂ film 21 are laminated can be formed. The film forming conditions can be set within the above range.

  Next, the glass substrate 15 is taken out of the chamber, and a titanium oxide layer 27 serving as a power generation layer of the dye-sensitized solar cell is formed on the second transparent conductive film 19b on the second surface side of the glass substrate 15 with a film thickness of 5. -10 μm. The titanium oxide layer 27 can be formed by printing a paste containing titanium oxide particles by screen printing and firing at about 400 ° C. At this time, platinum 20 as a catalyst layer is deposited on the second transparent conductive film 19b after firing by previously thinly applying a methanol solution of chloroplatinic acid on the other second transparent conductive film 19b. (FIG. 4 (e)).

Next, as shown in FIG. 5A, one end of the platinum wire electrode 23 is fixed to each exposed Ag conductive member 17 b with a conductive adhesive 24.
Next, as shown in FIG. 5B, a paste containing titanium oxide particles is printed by a screen printing method on a substantially half region of the stainless steel substrate 25 having the same size as the glass substrate 15 and a thickness of about 0.1 mm. And baking at about 400 ° C. to form a titanium oxide layer 27 to be a power generation layer of the dye-sensitized solar cell with a film thickness of 5 to 10 μm. At this time, platinum 20 as a catalyst layer is deposited on the stainless steel substrate 25 after firing by previously thinly applying a methanol solution of chloroplatinic acid to the remaining almost half of the stainless steel substrate 25.

Thereafter, the glass substrate 15 and the stainless steel substrate 25 that have undergone the above steps are immersed in a dye sensitizer solution for about 12 hours and dried, whereby the dye sensitizer is supported on each titanium oxide layer 27.
Next, as shown in FIG. 5 (c), the glass substrate 15 having the photocatalytic film 22 and the stainless steel substrate 25 are overlapped, and the portion other than the inlet for injecting the electrolyte solution is sealed with an epoxy resin 28. . Then, the iodine electrolyte solution 29 is injected from the injection port, and the injection port is sealed with an epoxy resin to form the water splitting semiconductor photoelectrode 30.
Subsequently, as shown in FIG. 5D, the four sides of the water-decomposing semiconductor photoelectrode 30 and the cover glass 26 are fixed with an aluminum frame 32 to form a module.
Thereafter, an aqueous electrolyte solution 31 made of a 0.1M NaOH aqueous solution is placed between the water splitting semiconductor photoelectrode 30 and the cover glass 26 through one opening 36 of the frame 32 to obtain the water splitting apparatus shown in FIG. Can do.

As shown in FIG. 1, the water splitting device of the present invention irradiates sunlight 33 from the photocatalyst film 22 side of the water splitting semiconductor photoelectrode 30 through the electrolyte aqueous solution 31, thereby Among them, light having a wavelength of about 380 nm or less is absorbed by the TiO ₂ film 21, and light having a wavelength of about 380 to 500 nm is absorbed by the TiO _x film 18 to generate electrons (e ⁻ ) and holes (h ⁺ ). The contact potential difference of the TiO ₂ film 21 and the electrolyte solution 31, the excited electrons in the TiO ₂ film 21 and the TiO _x film 18 is collected in the first transparent conductive film 19a on the glass substrate 15, the Ag conductive member 17a It flows through the second transparent conductive film 19b that becomes the counter electrode of the dye-sensitized solar cell S. On the other hand, the holes move to the surface of the TiO ₂ film 21. Holes on the surface of the TiO ₂ film 21 oxidize water to generate oxygen 34 as shown in the following equation.
4h ⁺ + 2H ₂ O → O ₂ + 4H ⁺

The spectrum of sunlight 33 having a wavelength of about 500 nm or more transmitted through the photocatalyst film 22 is absorbed by the titanium oxide layer 27 of the dye-sensitized solar cell S to generate photovoltaic power. The dye-sensitized solar cell S pushes the energy level of the electrons excited by the photocatalytic film 22 sufficiently more negative than the hydrogen generation potential. Therefore, in the wire electrode 23 that is a hydrogen generation electrode, water molecules are reduced and hydrogen 35 is generated as in the following equation.
4H ^{+ +} 4e ^- → 2H ₂

Thus, water can be decomposed into oxygen and hydrogen by irradiating the water splitting device of the present invention with sunlight. Further, since the photocatalyst film 22 has a titanium dioxide film 21 (TiO ₂ film) laminated on the TiO _x film 18 that is an oxygen-deficient titanium oxide film, the titanium dioxide film 21 is first irradiated with sunlight. When the stacking order of the photocatalytic film 22 is reversed, that is, compared with the case where sunlight is first irradiated to the oxygen-deficient titanium oxide film, the photoelectric conversion efficiency is improved and more hydrogen can be generated. .

<Embodiment 2>
FIG. 6 is a schematic sectional side view showing Embodiment 2 of the water splitting apparatus of the present invention. The second embodiment has the same configuration as that of the first embodiment except that the oxygen 34 and the hydrogen 35 are generated in a separated state in the space containing the electrolyte aqueous solution 31. Hereinafter, points of the second embodiment different from the first embodiment will be mainly described. In FIG. 6, the same elements as those in FIG.

In this water splitting apparatus, an ion conductive film 40 is provided between the photocatalyst film 22 and the wire electrode 23 in the space of the casing, and the aqueous electrolyte solution 31 is applied to both sides of the frame 32 with the ion conductive film 40 interposed therebetween. Openings 38a and 39a are formed to flow out. With this configuration, when sunlight 33 is irradiated on the water splitting device, oxygen 34 is generated from the surface of the TiO ₂ film 21, and hydrogen 35 is generated from the surface of the wire electrode 23. Flows out of the individual openings 38a and 39a together with the electrolyte aqueous solution 31 without passing through the ion conductive film 40. These openings 38a and 39a are connected to an external electrolyte aqueous solution circulation device via a pipe (not shown), and oxygen 34 and hydrogen 35 are respectively recovered at the front portion of the electrolyte aqueous solution circulation device of the pipe. Therefore, the gas separation apparatus required in the first embodiment is not required in the second embodiment. Although not shown, openings for allowing the electrolyte aqueous solution 31 to flow into the space of the housing are formed on both the photocatalyst film 22 side and the wire electrode 23 side of the frame 32, and each opening is connected via a pipe. Connected to the electrolyte aqueous solution circulation device.

<Other embodiments>
In Embodiment 1 and 2, although the case where the dye-sensitized solar cell S was configured by modularizing the first solar cell and the second solar cell, the semiconductor layer 27 of the adjacent solar cell was illustrated. Alternatively, three or more solar cells may be modularized by reversing the stacking order of the electrolyte layers 29. Furthermore, the water-splitting semiconductor photoelectrode described in FIGS. 1 to 6 may be a unit, and a plurality of units of the water-splitting semiconductor photoelectrode may be provided in one housing to be unitized.

A plurality of transparent substrates having a transparent conductive film on one side are prepared, TiO _x films (x = 1.93) having different thicknesses are formed on the transparent conductive film of each transparent substrate, and each TiO _x film is formed on each TiO _x film. Twenty kinds of photocatalytic films were formed by forming TiO ₂ films having different thicknesses.
In forming the TiO _x film, a high-frequency magnetron sputtering apparatus is used, and a target is a titanium dioxide (TiO ₂ ) sintered body having a diameter of 7 inches and a thickness of 5 mm, a substrate temperature of 600 ° C., and a high-frequency power of 350 W. The vacuum degree of the vacuum chamber is 2 Pa, the introduction gas is only argon gas, the substrate-target distance is set to 75 mm, and the film formation time is changed to change the film thickness to 0.5 μm, 0.7 μm, 1 μm, 1.2 μm and Five types of TiO _x films of 1.5 μm were formed.
When forming the TiO ₂ film, the same apparatus was used, the substrate temperature was 400 ° C., the high frequency power was 300 W, the vacuum degree of the vacuum chamber was 2 Pa, the introduced gas was a mixed gas of argon gas and oxygen (mixing ratio 25: 3), the substrate − Four types of TiO ₂ films having thicknesses of 0.2 μm, 0.4 μm, 0.6 μm, and 0.8 μm were formed by changing the film formation time while setting the distance between targets to 75 mm.

(Comparative Example 1)
Comparative Example 1 was obtained by forming only a 1.5 μm-thick TiO _x film (x = 1.93) on the transparent conductive film of the transparent substrate under the same TiO _x film deposition conditions as in the example. .
(Comparative Example 2)
Comparative Example 2 was obtained by forming only a TiO ₂ film having a thickness of 0.8 μm on the transparent conductive film of the transparent substrate under the same TiO ₂ film formation conditions as in the example.

Photoelectrochemical measurements were performed on the photocatalyst films of Examples and Comparative Examples 1 and 2, and the photocurrent was evaluated. The results are shown in Table 1. For the measurement, a 0.1 W NaOH aqueous solution as the electrolytic solution, the counter electrode was Pt, the reference electrode was a silver-silver chloride electrode, and the light source was a 300 W xenon lamp equipped with a cutoff filter having a wavelength of 350 nm or less. As for the photocurrent value, the current value (unit: mA / cm ² ) at 0.5 V of the current-potential curve obtained by electrochemical measurement is shown in Table 1.

From Table 1, than the TiO _x film alone and the TiO ₂ film alone, it found easily laminated film of a TiO _x film and a TiO ₂ film obtained good photoelectric conversion efficiency, further, the thickness of the TiO ₂ film When constant, the TiO _x film has a good photoelectric conversion efficiency of 0.7-1 μm, and when the film thickness of the TiO _x film is constant, the TiO ₂ film has a good 0.2-0.6 μm. It was found that photoelectric conversion efficiency can be obtained. It was also found that high photoelectric conversion efficiency was obtained when the thickness of the TiO _x film was 0.7 to 1 μm and the thickness of the TiO ₂ film was 0.2 to 0.6 μm. Further, a combination in which the film thickness of the TiO _x film 18 is 0.7 to 1 μm and the film thickness of the TiO ₂ film 21 is 0.4 μm can obtain a higher photocurrent value, and particularly the film thickness of the TiO _x film 18 is 0. It was found that the combination of 0.7 μm and TiO ₂ film 21 having a thickness of 0.4 μm can obtain the highest photoelectric conversion efficiency.

Photoelectric conversion at each wavelength of light in a laminated film (Example) of a TiO _x film having a thickness of 0.7 μm and a TiO ₂ film having a thickness of 0.4 μm showing the highest photocurrent value and Comparative Examples 1 and 2 The efficiency is shown in FIG. At this time, light having a specific wavelength was irradiated by combining a 300 W xenon lamp and a wavelength filter. For conversion efficiency, the number of photons is obtained from the energy value of the irradiated light, the number of electrons is obtained from the photocurrent value at 0.5 V of the current-potential curve obtained by electrochemical measurement, and the number of electrons is divided by the number of photons. Asked. The vertical axis of the graph in FIG. 7 is normalized by the maximum value of the obtained photocurrent.

From FIG. 7, it can be seen that the photocatalytic film of the present invention exhibits good photoelectric conversion efficiency even in the ultraviolet light region and has photoelectric conversion characteristics in the visible light region. This result shows that the 0.4 μm-thick titanium dioxide film (TiO ₂ film) in the photocatalyst film of the present invention is sufficient to absorb light of about 380 nm or less, and the 0.7 μm-thick oxygen-deficient titanium oxide film. The film (TiO _x film) is sufficient to absorb light having a wavelength of about 380 to 500 nm, and the titanium dioxide film (TiO ₂ film) and oxygen having few crystal defects that act as recombination centers of generated carriers. This is considered to be due to the combined effect of a decrease in recombination due to the formation of a deficient titanium oxide film (TiO _x film).

  The water splitting apparatus of the present invention is suitable as a hydrogen generating apparatus for producing hydrogen, which is clean energy replacing non-fossil fuels, and generates hydrogen by being installed on the rooftop or roof of a building that is irradiated with sunlight. Can contribute to energy saving.

It is a schematic sectional side view which shows Embodiment 1 of the water splitting device of this invention. It is a front view of the water splitting apparatus of FIG. It is a graph which shows the relationship between the light absorbency and light wavelength about an oxygen deficient titanium dioxide film and a titanium dioxide film. It is a schematic sectional drawing which shows the manufacturing process of the water splitting apparatus of FIG. FIG. 4 is a schematic sectional view showing a continuation of the manufacturing process of FIG. 3. It is a schematic sectional side view which shows Embodiment 2 of a water splitting apparatus. It is a graph which shows the photoelectric conversion efficiency of the photocatalyst film | membrane of an Example. It is a schematic sectional side view which shows the conventional water splitting apparatus.

Explanation of symbols

15 Transparent substrate (glass substrate)
17 Conductive member (Ag conductive member)
18 Oxygen deficient titanium oxide film (TiO _x film)
19a first transparent conductive film 19b second transparent conductive film,
20 Platinum 21 Titanium dioxide film (TiO ₂ film)
22 Photocatalyst film 23 Wire electrode (Pt wire electrode)
24 conductive adhesive 25 metal substrate (stainless steel substrate)
26 Cover glass 27 Semiconductor layer (titanium oxide layer)
28 Epoxy resin 29 Electrolyte layer (iodine electrolyte solution)
DESCRIPTION OF SYMBOLS 30 Semiconductor photoelectrode for water decomposition 31 Electrolyte aqueous solution 32 Frame 33 Sunlight 34 Oxygen 35 Hydrogen 36 Pipe 36a, 37a, 38a, 39a Opening 37 Pipe 40 Ion conductive film S Dye-sensitized solar cell

Claims (7)

A first photocatalyst film formed on a transparent conductive film on one surface of the transparent substrate; and a second photocatalyst film formed on the first photocatalyst film, wherein the first photocatalyst film has an oxygen deficiency. A photocatalytic film comprising a TiO _x film (x <2.0) containing oxygen and titanium in a stoichiometric ratio, wherein the second photocatalytic film comprises a TiO ₂ film. _{2. The} photocatalytic film according to claim 1, wherein the TiO _x film has a thickness of 0.7 to 1 μm, and the TiO ₂ film has a thickness of 0.2 to 0.6 μm. 3. The photocatalytic film according to claim 1, wherein x is 1.9 to 1.95 in the TiO _x film. The photocatalyst film according to any one of claims 1 to 3 having a second photocatalyst film / first catalyst film, a first transparent conductive film, a transparent substrate / second transparent conductive film / And at least a dye-sensitized solar cell having an electrolyte layer and a semiconductor layer / metal substrate carrying a dye,
A water separation semiconductor photoelectrode, wherein a conductive member for electrically connecting the transparent conductive films on the front and back surfaces of the transparent substrate is installed on the transparent substrate.   5. A wire electrode having one end electrically connected to the second transparent conductive film and the other end extending along the light receiving surface of the second photocatalyst film without contacting the second photocatalyst film. A semiconductor photoelectrode for water splitting as described in 1.   The water-decomposing semiconductor photoelectrode according to claim 4 or 5, wherein the dye-sensitized solar cell comprises one or a plurality of solar cells.   A water-resolving semiconductor photoelectrode according to any one of claims 4 to 6 and an aqueous electrolyte solution are provided in a housing having a transparent light-receiving surface side, and the housing has a hydrogen and oxygen outlet. A water splitting device.