JP5675224B2 - Visible light water splitting catalyst and photoelectrode manufacturing method - Google Patents

Description

  The present invention relates to a novel visible light water splitting catalyst and a method for producing a photoelectrode, and more specifically, a visible light water splitting catalyst capable of providing a photoelectrode that maintains a photocatalytic function for a long time with respect to visible light irradiation. And a method of manufacturing a photoelectrode.

In recent years, in order to prevent global warming, and to realize a hydrogen society where hydrogen is assumed to be a major energy source as an effort to deplete renewable fuels, fuel cells, hydrogen production technologies, hydrogen storage and transport technologies, etc. Development is active.
Currently, hydrogen can be produced using fossil fuels such as coal, oil and natural gas, but in the future, hydrogen using non-fossil fuels such as water and biomass and clean energy such as sunlight will be used. Manufacturing technology is desired.

By the way, metal oxide semiconductors that receive light such as sunlight and generate a photovoltaic force and cause an electrochemical reaction by the electromotive force are known.
When light having energy higher than the band gap is absorbed by the semiconductor, electrons in the valence band are excited to the conduction band to generate excited electrons, while holes are generated in the valence band. Such excited electrons and holes have a strong reducing power and oxidizing power, respectively, and thus exert a redox action on molecular species in contact with the semiconductor. This redox action is called photocatalysis, and a semiconductor that can show photocatalysis is called a photocatalyst. This photocatalyst immobilized on a conductive substrate is called a photoelectrode, and this is combined with a counter electrode such as platinum, and an appropriate external voltage is applied between the two electrodes, and the photoelectrode is irradiated with light. It is possible to decompose water into hydrogen and oxygen.

As a metal oxide for photocatalyst, a material having a small band gap is required, and titanium dioxide (TiO ₂ ), tungsten oxide (WO ₃ ), diiron trioxide (Fe ₂ O ₃ ), etc. are known. It has been. However, titanium dioxide has a problem that it is almost insensitive to light having a wavelength longer than about 380 nm in the solar spectrum (that is, does not exhibit photocurrent response). Tungsten oxide exhibits photocurrent responsiveness to light with a wavelength of about 460 nm or less, and ferric trioxide to light with a wavelength of about 540 nm or less, but its level is considered to be low and practicality is low. .

For this reason, oxides, oxynitrides or nitrides containing tantalum have been proposed as materials with a small band gap, high photocurrent response to light with a wavelength longer than about 380 nm in the solar spectrum, and a high level of such materials. Has been.
For example, Patent Document 1 discloses a p-type oxide semiconductor made of an X (= II, III, IV, V) group oxide, and at least one of Y (≦ X) group nitrides in the semiconductor, for example, A p-type oxide semiconductor is described which has a metal-nitrogen bond composed of LiN, BeN, MgN, AlN, GaN, etc. and is chemically and electrically stable and capable of realizing a photocatalyst in response to only ultraviolet rays. Yes. As a specific example, a p-type oxide semiconductor of Nb _{2 (1-X)} Ta _X O ₅ (where 0 ≦ x ≦ 1) is described. However, it has not been shown that hydrogen can be generated from water by light irradiation using the p-type oxide semiconductor.

Patent Document 2 discloses that mesoporous tantalum nitride (Ta ₃ N ₅ ) in which a promoter is supported on tantalum nitride having a pore structure has a photocatalytic effect, and the amount of the promoter is 0 of the amount of tantalum nitride. The cocatalyst is selected from the group consisting of Pt, NiO, and RuO ₂ , and, as a specific example, pore tantalum nitride carrying Pt of the cocatalyst is used as the sacrificial reagent. It is described that 13.0 micromol of hydrogen was generated per hour by irradiation with visible light (λ ≧ 420 nm) in an aqueous methanol solution containing 80% by volume of methanol.

Patent Document 3 discloses that a tantalum-based oxynitride represented by the general formula: Ba _5-X La _X Ta ₄ O _15-X N _X (0.25 ≦ x ≦ 1) emits visible light up to about 600 nm. Having a photocatalytic effect of absorbing, Pt, NiO or RuO ₂ supported on the tantalum-based oxynitride as a co-catalyst functions effectively as a photocatalyst for photolyzing water to produce hydrogen; As an example, Ba _4.5 La _0.5 Ta ₄ O _14.5 N _0.5 supporting NiO or RuO ₂ as a cocatalyst produced hydrogen and oxygen by light irradiation with a wavelength longer than 290 nm or 400 nm, Hydrogen is irradiated by light irradiation with a wavelength longer than 290 nm using Ba _5-X La _X Ta ₄ O _15-X N _X (x = 0.25, 0.5, 0.75, 1.0) supporting NiO. And oxygen produced It is described. However, there is no data that quantitatively shows changes with time in the amount of hydrogen generation.

Further, Non-Patent Document 1 discloses a TaON single photoelectrode obtained by laminating tantalum oxynitride: TaON on a conductive substrate and carrying out necking treatment, and IrO ₂ which is a catalyst for oxidizing water on this TaON. An IrO ₂ -TaON photoelectrode is described, and the IrO ₂ -TaON photoelectrode suppresses a decrease in photocurrent and N content after continuous light absorption compared to a TaON single photoelectrode, and the TaON single photoelectrode In this case, the current decreases from about 12 mA to 0 mA due to the continuous light absorption for 5 minutes, whereas the current from the initial about 12 mA is obtained for the IrO ₂ -TaON photoelectrode due to the continuous light absorption for 60 minutes. It is shown that the decrease was reduced to about 17% at about 2 mA after 60 minutes, but the decrease was suppressed. However, even with an IrO ₂ -TaON photoelectrode, the suppression of the photocatalytic function deterioration due to continuous light absorption for 1 hour is insufficient, and since IrO ₂ is expensive, it is difficult to put it into practical use from the viewpoint of industrialization. .

JP 2001-322814 A JP 2007-022858 A JP 2007-175659 A

Journal of American Chemical Society, Volume132, Issue34, 11825-12156 (2010)

Thus, according to the prior art, it was not possible to obtain a visible light water splitting catalyst and a photoelectrode that can maintain a photocatalytic function for a long time with respect to light energy represented by sunlight. .
Accordingly, an object of the present invention is to provide a visible light water splitting catalyst capable of providing a photoelectrode that maintains a photocatalytic function for a long time with respect to visible light irradiation.
Another object of the present invention is to provide a method for producing a photoelectrode capable of maintaining a photocatalytic function for a long time with respect to visible light irradiation.

In the present invention, CoO as a co-catalyst for tantalum oxynitride particles is supported in an amount of 0.1 to 50% by mass with respect to the tantalum oxynitride particles (provided that 0.01% by mass or more in terms of Co is 1 to 1% by mass). The catalyst for visible light water splitting, which is supported by the above method , excluding the supported amount corresponding to the range of mass% or less .
Further, the present invention provides CoO as a co-catalyst for the tantalum oxynitride particles and a supported amount of 0.1 to 50% by mass with respect to the tantalum oxynitride particles (provided that 0.01% by mass in terms of Co). (Excluding the loading amount corresponding to the range of 1% by mass or less), the step of fixing the tantalum oxynitride particles on the conductive substrate, and the electron transfer to the tantalum oxynitride particles. The present invention relates to a photoelectrode manufacturing method including a step of performing a necking process.

ADVANTAGE OF THE INVENTION According to this invention, the visible light water splitting catalyst which can provide the photoelectrode which maintains a photocatalytic function over a long time with respect to visible light irradiation can be obtained.
Furthermore, according to this invention, the photoelectrode which can maintain a photocatalytic function over a long time with respect to visible light irradiation can be obtained easily.

In the visible light water splitting catalyst of the present invention, it is necessary that tantalum oxynitride particles as a photocatalyst carry CoO as a co-catalyst. The photocatalytic function can be maintained for a long time with respect to irradiation.
In the photoelectrode of the present invention, the step of supporting CoO as a co-catalyst on the tantalum oxynitride particles, the step of fixing the tantalum oxynitride particles on the conductive substrate, and the tantalum oxynitride particles It is necessary to include a step of performing a necking process that promotes electron transfer, which can maintain a photocatalytic function for a long time against visible light irradiation typified by sunlight, and a special device. A photoelectrode can be obtained easily and inexpensively without using it.

  In the present specification, the fact that the photocatalytic function can be maintained over a long period of time with respect to visible light irradiation is measured for 1 hour or longer (continuously) as measured by the measurement method described in detail in the Examples section below. This means that the photocurrent density after light absorption is maintained at 50% or more of the initial value of light irradiation.

In the present invention, it is necessary to use a combination of tantalum oxynitride particles and CoO as a promoter.
The tantalum oxynitride particles can be obtained, for example, by bringing commercially available tantalum oxide particles into contact with ammonia under heating at a suitable temperature, for example, 500 to 900 ° C. for about 30 minutes to 24 hours.
In the present invention, a part of the tantalum oxynitride particles, preferably 50% by mass or less, may be used in combination with other metal compound particles having other photocatalytic functions. Examples of the other metal compounds include Ta nitride, oxide, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Tc, and Ru. At least one metal selected from Rh, Pd, Ag, Cd, In, Sn, Sb, Te, La, Hf, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, and Ce And a binary or ternary metal compound of the metal with Ta and oxygen, nitrogen, sulfur, or fluorine. By combining the tantalum oxynitride particles and the other metal compound particles, there is a possibility that a water decomposition reaction can be achieved by light irradiation in a wider wavelength range.
In addition, when the visible light water splitting catalyst of the present invention needs to exhibit p-type semiconductor characteristics or n-type semiconductor characteristics, p-type or n-type can be obtained by doping an impurity element using means well known in the semiconductor industry. It can be.

The CoO is suitable, for example, after adding an aqueous solution of cobalt nitrate, such as cobalt nitrate, cobalt chloride, cobalt acetate, cobalt formate, cobalt acetylacetonate, etc., to tantalum oxynitride particles, followed by drying. It can be produced on tantalum oxynitride particles by decomposing cobalt salt anions by heating at a temperature of, for example, 300 to 500 ° C. for about 15 minutes to 10 hours.
In a preferred embodiment, for example, the cobalt salt is added to a mixture of tantalum oxynitride particles dispersed in water and mixed well, and then dried and heated as described above, to thereby remove the anion of the cobalt salt. It can be decomposed to highly disperse CoO on tantalum oxynitride particles to produce CoO.

  In the visible light water decomposition catalyst of the present invention, the amount of CoO as a cocatalyst is efficiently received from electrons or holes generated in the tantalum oxynitride particles, and the oxidation or reduction reaction proceeds efficiently on the surface thereof. Therefore, the content is preferably 0.1 to 50% by mass, particularly 1 to 25% by mass with respect to the tantalum oxynitride particles, and may be fine particles of several nm to several tens of nm.

In the present invention, the term “co-catalyst” generally promotes hydrogen generation by reduction of water or oxygen generation by oxidation of water by receiving electrons or holes from the photocatalyst while being supported on the surface of the photocatalyst particles. Is.
In the present invention, a part of the CoO, preferably 50% by mass or less, may be used in combination with other metal compound particles having other promoter functions. Examples of the other metal compound include a metal having a specific crystal structure or a compound of metal, oxygen, nitrogen, sulfur, and fluorine. As a promoter material for promoting the reduction of water, a simple metal or a metal oxide such as Ru, Rh, Pd, Ag, Ir, Pt, or Au is used as a promoter material for promoting the oxidation of water. , Mn, Fe, Ni, Cu, Mo, Ru, Rh, Ir, and the like, and metal oxides such as Mn, Mo, Ru, Rh, and Ir are mentioned.

  Although the theoretical elucidation that the photocatalytic function is maintained for a long time with respect to visible light irradiation by the visible light water splitting catalyst of the present invention has not been made, as shown in the Examples section below, Since N / Ta hardly decreased after irradiation with visible light, the reaction in which tantalum oxynitride itself was self-reduced or self-oxidized by electrons or holes generated by light absorption was suppressed by CoO as a cocatalyst. It is thought that this also contributed.

In the photoelectrode according to the present invention, the tantalum oxynitride particles carry CoO as a co-catalyst, the tantalum oxynitride particles are fixed on a conductive substrate, and the tantalum oxynitride particles transfer electrons. It can be obtained by a step of applying a necking treatment to be promoted.
There is no restriction | limiting in particular as said electroconductive board | substrate, For example, various metal plates are mentioned, The titanium plate which has high tolerance with respect to the heat processing in the atmosphere especially applied in the process for obtaining a photoelectrode is suitable. . In addition, by receiving light irradiation from the back side of the fabricated photoelectrode, the charges generated in the photocatalyst near the conductive substrate can be efficiently transferred to the conductive substrate without recombination, and the irradiated light is effectively used. In order to be able to be used, a transparent conductive substrate can be preferably used. Examples of the transparent conductive substrate include an FTO (fluorine-doped tin oxide) substrate and a conductive substrate obtained by coating a metal such as titanium on a glass substrate extremely thinly by sputtering.

In the method for producing a photoelectrode according to the present invention, as a first aspect, a tantalum oxynitride particle is preloaded with CoO as a cocatalyst and fixed on a conductive substrate, and then a necking treatment for promoting electron transfer is performed. .
In the above-described embodiment, first, CoO is previously supported and fixed as a promoter on the tantalum oxynitride particles on the conductive substrate.
As a method for preliminarily supporting CoO as a co-catalyst on the tantalum oxynitride particles on the conductive substrate, CoO as a co-catalyst is supported on the tantalum oxynitride particles as the photocatalyst, for example. A method of suspending the visible light water decomposition catalyst in a solvent and laminating the visible light water decomposition catalyst on a conductive substrate using electrophoresis, or a viscosity containing the visible light water decomposition catalyst Examples thereof include a method of preparing a paste and applying it on a conductive substrate, and a method of preparing a slurry containing the visible light water decomposition catalyst and coating it on the conductive substrate by either a dip coating method or a spray method. In particular, it is preferable to use an electrophoresis method because a uniform porous thin film can be formed with good reproducibility and the film thickness can be easily adjusted.
The electrophoresis method can be carried out, for example, by applying a voltage, for example, a direct current voltage of 5 to 50 V for about 0.1 to 10 minutes in an acetone solution to which iodine is added for visible light water splitting catalyst particles. . After carrying CoO as a co-catalyst on tantalum oxynitride particles on a conductive substrate by electrophoresis, it is usually washed with acetone and then dried at room temperature to form a visible light water decomposition catalyst thin film An electrically conductive substrate can be obtained.

In the first aspect of the method for producing a photoelectrode according to the present invention, a necking treatment is applied to the conductive substrate on which the thin film of the visible light water decomposition catalyst has been formed in the above-described step in order to promote electron transfer.
Examples of the necking treatment include a treatment capable of forming an effective bond between tantalum oxynitride particles. For example, a catalyst particle for visible light water splitting of a photoelectrode is impregnated with a solution containing a tantalum salt, and is optionally added. A heat treatment step of heating at a relatively low temperature, for example, 100 to 750 ° C., particularly 200 to 500 ° C., for about 5 minutes to 5 hours in the atmosphere of, for example, air or an inert atmosphere.
The tantalum salt is not particularly limited, and examples thereof include tantalum pentachloride (TaCl ₅ ), tantalum pentaiodide (TaI ₅ ), tantalum pentafluoride (TaF ₅ ), and tantalum pentabromide (TaBr ₅ ). It is done.
There is no restriction | limiting in particular as a solvent which gives the solution containing the said tantalum salt, For example, alcohol, such as methanol and ethanol, water etc. are mentioned.

  In the first aspect of the method of the present invention, a heat treatment with ammonia that is heated in the presence of ammonia may be further added to the necking treatment. The heat treatment with ammonia can be performed, for example, by heating at a relatively low temperature, for example, 350 to 600 ° C. for about 5 minutes to 5 hours in the presence of ammonia, for example, in a flow of ammonia of 1 to 100 mL / min.

  In the photoelectrode in which a thin film of visible light water splitting catalyst is formed on the conductive substrate, the visible light water splitting catalyst particles are only in physical contact, and electron transfer between the particles hardly occurs. Electrons generated by irradiation are not easily integrated up to the conductive substrate, and it tends to be difficult to obtain a high photocurrent. On the other hand, effective necking between visible light water decomposition catalyst particles is performed by impregnating the catalyst particles for visible light water decomposition of the photoelectrode with a solution containing a tantalum salt and subsequent necking treatment. That is, a junction for electron transfer can be formed.

Alternatively, in the second aspect of the method of the present invention, the tantalum oxynitride particles are fixed on a conductive substrate, and then a necking treatment that promotes electron transfer is performed, and further a heat treatment with ammonia is preferably performed. After that, CoO as a promoter is supported on the tantalum oxynitride particles.
In this second embodiment, the fixing of the tantalum oxynitride particles onto the conductive substrate, the necking treatment for promoting electron transfer, and the loading of CoO as a promoter on the tantalum oxynitride particles are performed as described above. This can be performed in substantially the same manner as in the first aspect.
The heat treatment with additional ammonia may be performed after supporting CoO as a cocatalyst on tantalum oxynitride particles.

The photoelectrode obtained by the method of the present invention can be applied to photoelectrochemical decomposition.
For example, a constant potential is applied to the photoelectrode. In the normal performance evaluation, the so-called “three-pole type” in which the potential is applied to the working electrode is used for the application of the potential, but in the case of industrial application, the operation is performed without using the reference electrode. This can be performed by a so-called “bipolar type” in which only a pole and a counter electrode are used and a potential is applied between both poles. When a visible light water splitting catalyst exhibiting n-type semiconductor characteristics is used as a photoelectrode and used as a working electrode, oxygen is generated by oxidation of water on such a photoelectrode, and hydrogen is generated by reduction of water at the counter electrode. . On the other hand, when a visible light water splitting catalyst exhibiting p-type semiconductor properties is used as a working electrode as a working electrode, hydrogen is generated by reduction of water on such a photoelectrode, and water is oxidized at the counter electrode. Oxygen production proceeds.

The counter electrode that can be used in combination with the photoelectrode in the present invention includes an electrode used in a normal electrolytic reaction, and has a large effective surface area in order to efficiently react charges supplied from the working electrode through the counter electrode. Is preferred. Examples of the counter electrode for hydrogen generation by reduction of water include a mesh electrode made of platinum or nickel, and examples of the counter electrode for oxygen generation by water oxidation include a ruthenium oxide or iridium oxide mesh or a porous electrode.
Alternatively, a photoelectrode using a visible light water splitting catalyst having n-type semiconductor characteristics is used for oxygen generation, and a photoelectrode using a visible light water splitting catalyst having p-type semiconductor characteristics is used for hydrogen generation, It is also possible to decompose water (that is, generate hydrogen) without using the counter electrode by irradiating light while short-circuiting both electrodes or applying an appropriate potential between both electrodes.

  Examples of the supporting electrolyte used when applying the potential include sodium sulfate, sodium hydroxide, sulfuric acid, potassium sulfate, potassium hydroxide, sodium chloride, potassium chloride, sodium phosphate, potassium phosphate, phosphoric acid, nitric acid, Examples include sodium perchlorate, potassium perchlorate, and perchloric acid, and sodium sulfate, sodium phosphate, sodium hydroxide, phosphoric acid, and sulfuric acid are preferable because they do not contribute to the reaction at the above-described constant potential. it can. These electrolytes are desirably mixed and used as appropriate in order to adjust the pH of the aqueous solution between 1 and 13. At this time, it is desirable to use an aqueous solution adjusted to a pH range in which the visible light water splitting catalyst used does not corrode.

Examples of the present invention and comparative examples are shown below. The following examples are illustrative and do not limit the scope of the invention.
In each of the following examples, analysis and evaluation of the obtained visible light water splitting catalyst and photoelectrode were carried out by the following methods. In addition, the following measuring methods are illustrations, Comprising: It can measure using another equivalent apparatus and conditions.

1) Measurement of crystal phase A sample was measured using an X-ray diffractometer ("Rigaku Mini Flex2" manufactured by Rigaku Corporation).
2) Shape observation The shape (film thickness etc.) of the photoelectrode was measured using the scanning electron microscope (Keyence "VE-9800").
3) Chemical composition analysis The composition analysis of the sample was performed using the X-ray photoelectron spectrometer ("JPC-9010MC" by JEOL Ltd.).

4) Measurement of photocurrent value Photocurrent value was measured by the following operation using a cell (internal volume: 100 mL) manufactured by Pyrex Glass.
A platinum mesh electrode was used as the counter electrode, and a silver-silver chloride (Ag / AgCl) electrode was used as the reference electrode. Sodium sulfate equivalent to 0.1 mol / L was dissolved in ultrapure water to obtain a reaction aqueous solution (pH 6.0). 50 mL of this was put into the cell, and bubbling with argon gas was performed for 30 minutes, whereby the air dissolved in the aqueous solution in the cell was replaced with argon. Using a potentiostat (Princeton Applied Research, PARSTAT 2263), visible light was irradiated while scanning the potential applied to the working electrode from negative to positive, and the current value flowing between the counter electrode was measured. . This was evaluated as a current density (mA / cm ² ) per area of the working electrode. Using a xenon lamp (manufactured by Cermax, 300 W) equipped with an ultraviolet cut-off filter (manufactured by HOYA, L-42) as a light source, only visible light having a wavelength of 400 nm or more (irradiation wavelength range: 400 to 700 nm) was irradiated. .

5) Constant potential measurement In order to evaluate the stability of the photoelectrode, visible light was irradiated in a state where a constant potential was applied, and the change with time of the photocurrent density was followed and evaluated.
6) Photoelectrochemical decomposition of water using a photoelectrode A cell made by Pyrex Glass having airtightness, a created photoelectrode as a working electrode, a platinum wire as a counter electrode, and a potentiostat (manufactured by Princeton Applied Research, PARSTAT 2263) ) Was applied with visible light having a wavelength of 400 nm or more as described in 4) above while applying a constant potential to the working electrode. Qualitative and quantitative analysis was performed using a gas chromatograph (manufactured by Agilent Technologies) in which the gas (hydrogen and oxygen) produced at this time was directly connected to the cell. As the supporting electrolyte aqueous solution at this time, an aqueous sodium sulfate solution was used in the same manner as described in 4) above.

Example 1
Synthesis of tantalum oxynitride particles A quartz boat is filled with 10 g of a commercially available tantalum oxide (manufactured by High-Purity Chemical Laboratory) so as to have a width of 2 cm, a length of 3 cm, and a depth of about 2 cm. 3 cm, an inner diameter of 2.6 cm, and a length of 60 cm), and an airtight cap (made of stainless steel) was mounted on both sides and installed in a ceramic annular furnace. Nitrogen was allowed to flow at 100 mL / min for 10 minutes to remove residual air, and then the flow gas was switched to ammonia and allowed to flow at 100 mL / min for 10 minutes. Subsequently, the flow rate of ammonia was adjusted to 60 mL / min, and the temperature of the annular furnace was increased from room temperature to 850 ° C. over about 1 hour, and was maintained for 24 hours after reaching 850 ° C. After that, when the temperature is lowered to 200 ° C. by natural cooling, the gas is switched to nitrogen (100 mL / min) again to replace ammonia with nitrogen, and when the temperature is further lowered to room temperature, the hermetic cap is removed, and the quartz is removed. The boat made was taken out and the powder was collected.
The obtained powder was confirmed to be tantalum oxynitride (TaON) by analysis using an X-ray diffractometer.

Cobalt oxide cocatalyst supported on tantalum oxynitride particles 1 g of the obtained tantalum oxynitride (TaON) particles are placed in an evaporating dish together with a small amount of ultrapure water, and left in an ultrasonic cleaner for 10 minutes to obtain sufficient particles. Dispersed. To this dispersion, a 0.1 mol / L cobalt nitrate aqueous solution is added to the tantalum oxynitride particles so as to be 5 wt% as cobalt oxide (CoO), and the mixture is thoroughly mixed, heated at about 80 ° C. and gradually mixed. After evaporating the water, the nitric acid is decomposed by firing at 400 ° C. for 30 minutes in an electric furnace, and the CoO promoter is supported on the tantalum oxynitride particles in a highly dispersed manner, and the oxynitride is supported with cobalt oxide. Tantalum particles were obtained.

Production of Porous Thin Film Photoelectrode by Electrophoresis Method 40 mg of the obtained cobalt oxide-tantalum oxynitride particles were added to 50 mL of acetone containing 10 mg of iodine and allowed to stand for 10 minutes in an ultrasonic cleaner to disperse the particles. . Two titanium plates (length: 50 mm, width: 15 mm, thickness: 1 mm) were immersed in the obtained dispersion vertically with an interval of about 8 mm, and a potentiostat (Princeton Applied Research, manufactured by Princeton Applied Research, Using PARSTAT 2263), a voltage of 10 V was applied for 3 minutes to laminate the particles on the cathode side titanium plate. Note that the area where the cobalt oxide-tantalum oxynitride particles are laminated is a part of the electrode (length 40 mm, width 15 mm). The titanium plate was removed from the solution, washed with acetone, and then dried at room temperature.
In the obtained laminated titanium plate, 30 to 40 mg of cobalt oxide-tantalum oxynitride particles were laminated on average, and it was confirmed by observation with a scanning electron microscope that the film thickness was about 1.5 to 2.0 μm.

Necking treatment using a tantalum chloride solution The obtained cobalt oxide-tantalum oxynitride particle laminated titanium plate: 10 μL of a tantalum chloride-methanol solution adjusted to 0.1 mol / L was dropped on a thin film electrode and dried. After repeating this operation 10 times, it was baked for 30 minutes at 400 ° C. in air using an electric furnace.

Heat treatment with ammonia The resulting necked photoelectrode is placed in the center of a quartz tube (outer diameter 3 cm, inner diameter 2.6 cm, length 60 cm), and airtight caps (made of stainless steel) are attached to both sides of the ceramic annular furnace. Installed. After nitrogen was circulated at 100 mL / min for 10 minutes to remove residual air, the circulating gas was switched to ammonia and adjusted to 20 mL / min, and the temperature of the annular furnace was increased from room temperature to 450 ° C. over about 30 minutes. The temperature was maintained for 1 hour after reaching 450 ° C. After that, when the temperature is lowered to 200 ° C. by natural cooling, the gas is switched again to nitrogen (100 mL / min) to replace ammonia with nitrogen, and when the temperature is further lowered to room temperature, the hermetic cap is removed. The electrode was removed.

Measurement and Evaluation Photocurrent values were measured using the cobalt oxide-tantalum oxynitride photoelectrode obtained by the series of steps described above. When the working electrode potential is scanned from -1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at -0.5 V, 0 V, +0.5 V, and +1 V are 0.54 mA / cm ² and 0, respectively. It was 0.92 mA / cm ² , 1.95 mA / cm ² , and 4.15 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. The photocurrent densities immediately after the start of light irradiation, 30 minutes later and 1 hour later were 0.96 mA / cm ² , 0.94 mA / cm ² and 0.94 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.37 and 0.31 before and after constant potential measurement for 1 hour.

Moreover, the photoelectrochemical decomposition of water was performed using the cobalt oxide-tantalum oxynitride photoelectrode obtained by the series of steps.
The potential of the working electrode was fixed to +0.6 V with respect to the platinum counter electrode, and the visible light irradiation was performed. By irradiation with light for 4 hours, hydrogen and oxygen became 64.5 μmol and 32.0 μmol, respectively. The current efficiency of hydrogen generation calculated from the number of electrons flowing in the external circuit was about 92%.

Example 2
Cobalt oxide-tantalum oxynitride particles and a photoelectrode were prepared and evaluated in the same manner as in Example 1 except that the amount of cobalt oxide supported on the tantalum oxynitride particles was changed to 3 wt%.
When the working electrode potential is scanned from -1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at -0.5 V, 0 V, +0.5 V, and +1 V are 0.40 mA / cm ² and 0, respectively. It became 0.85 mA / cm ² , 1.55 mA / cm ² , 2.33 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. Immediately after the start of light irradiation, the photocurrent densities after 30 minutes and 1 hour were 0.75 mA / cm ² , 0.66 mA / cm ² , and 0.58 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.39 and 0.32 before and after the constant potential measurement for 1 hour.

Example 3
Cobalt oxide-tantalum oxynitride particles and a photoelectrode were prepared and evaluated in the same manner as in Example 1 except that the amount of cobalt oxide supported on the tantalum oxynitride particles was changed to 7 wt%.
When the working electrode potential is scanned from -1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at -0.5 V, 0 V, +0.5 V, and +1 V are 0.40 mA / cm ² and 0, respectively. It became 0.85 mA / cm ² , 1.55 mA / cm ² , 2.33 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. Immediately after the start of light irradiation, the photocurrent densities after 30 minutes and 1 hour were 0.70 mA / cm ² , 0.63 mA / cm ² , and 0.63 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.35 and 0.33 before and after constant potential measurement for 1 hour.

Example 4
Cobalt oxide-tantalum oxynitride particles and a photoelectrode were prepared and evaluated in the same manner as in Example 1 except that a necking treatment using a tantalum chloride solution was performed but no subsequent heat treatment with ammonia was performed. .
When the working electrode potential is scanned from −1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at −0.5 V, 0 V, +0.5 V, and +1 V are 0.21 mA / cm ² , 0, respectively. .35 mA / cm ² , 0.89 mA / cm ² , and 1.25 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. Immediately after the start of light irradiation, photocurrent densities after 30 minutes and 1 hour were 0.28 mA / cm ² , 0.24 mA / cm ² , and 0.23 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.25 and 0.22 before and after constant potential measurement for 1 hour.

Comparative Example 1
Tantalum oxynitride particles and a photoelectrode were prepared and evaluated in the same manner as in Example 1 except that cobalt oxide was not supported on the tantalum oxynitride particles.
When the working electrode potential is scanned from -1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at -0.5 V, 0 V, +0.5 V, and +1 V are 0.31 mA / cm ² and 0, respectively. It was 0.63 mA / cm ² , 1.22 mA / cm ² , and 2.01 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. Immediately after the start of light irradiation, the photocurrent densities after 30 minutes and 1 hour were 0.70 mA / cm ² , 0.05 mA / cm ² , and 0.00 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.59 and 0.16 before and after constant potential measurement for 1 hour.

Comparative Example 2
Cobalt oxide-tantalum oxynitride particles and a photoelectrode were obtained in the same manner as in Example 1 except that the necking treatment using the tantalum chloride solution of the cobalt oxide-tantalum oxynitride particle laminated titanium plate obtained by electrophoresis was not performed. Were made and evaluated.
When the potential of the working electrode is scanned from -1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at -0.5 V, 0 V, +0.5 V, and +1 V are 0 mA / cm ² and 0 mA / cm, respectively. ² , 0.01 mA / cm ² and 0.05 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. Immediately after the start of light irradiation, the photocurrent densities after 30 minutes and 1 hour were 0.01 mA / cm ² , 0.01 mA / cm ² , and 0.01 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.35 and 0.33 before and after constant potential measurement for 1 hour.

Example 5
Cobalt oxide is used on tantalum oxynitride particles, and after electrophoresis, necking treatment, and heat treatment with ammonia, a cobalt nitrate aqueous solution is dropped, and this is fired at 400 ° C. for 30 minutes in an electric furnace, corresponding to 5 wt% A photoelectrode was prepared and evaluated in the same manner as in Example 1 except that the cobalt oxide was supported.
When the working electrode potential is scanned from -1 V to +1 V with respect to the silver-silver chloride reference electrode, the photocurrent densities at -0.5 V, 0 V, +0.5 V, and +1 V are 0.45 mA / cm ² and 0, respectively. It became 0.87 mA / cm ² , 1.79 mA / cm ² , 2.65 mA / cm ² .
Next, the potential of the working electrode was fixed to +0.4 V with respect to the silver-silver chloride reference electrode, and the change with time in the photocurrent value when the visible light irradiation was performed continuously for 1 hour was followed. Immediately after the start of light irradiation, the photocurrent densities after 30 minutes and 1 hour were 0.80 mA / cm ² , 0.65 mA / cm ² , and 0.57 mA / cm ² , respectively. The ratio of nitrogen to tantalum (N / Ta) in the electrode calculated using XPS was 0.30 and 0.25 before and after constant potential measurement for 1 hour.

From the comparison between the results of Examples 1 to 5 and the results of Comparative Examples 1 and 2, the necking treatment of the tantalum oxynitride particles of the present invention of Examples 1 to 5 was performed after long-time irradiation of continuous irradiation of visible light for 1 hour. In the photoelectrode having the structure of the cobalt oxide-tantalum oxynitride particle-laminated titanium plate subjected to the above, the current density was maintained at a level of 70% or more, whereas the visible light water splitting not supporting CoO of Comparative Example 1 In the photoelectrode using the catalyst for use, the current density is reduced to almost 0%, and the photoelectrode obtained without the necking treatment of Comparative Example 2 has a low absolute value of the current density. Further, in any case of the photoelectrode using the visible light water splitting catalyst supporting CoO, the decrease in N / Ta is small.
Further, from the comparison between the results of Examples 1 to 3 and 5 and the result of Example 4, the effect of increasing the absolute value of the current density by the heat treatment of the cobalt oxide-tantalum oxynitride particle laminated titanium plate: thin film electrode with ammonia. It is understood that it has.
Further, from the comparison between the results of Examples 1 to 3 and the result of Example 5, a photoelectrode obtained by applying a necking treatment to a cobalt oxide-tantalum oxynitride particle previously fixed to a conductive substrate and then a heat treatment with ammonia. The current density is maintained at a higher level of 75% or more after continuous irradiation with visible light for 1 hour as compared with the photoelectrode obtained by finally supporting cobalt oxide.

  According to the present invention, it is possible to obtain a visible light water decomposition catalyst capable of producing hydrogen by decomposing water using sunlight that can maintain a photocatalytic function for a long time with respect to visible light irradiation. A photoelectrode capable of generating hydrogen from sunlight and water using a visible light water splitting catalyst can be obtained simply and inexpensively.

Claims (5)

CoO as a co-catalyst for the tantalum oxynitride particles is supported in an amount of 0.1 to 50% by mass with respect to the tantalum oxynitride particles (however, 0.01% by mass to 1% by mass in terms of Co) The catalyst for visible light water splitting is supported by (1 ) excluding the supported amount corresponding to the range . CoO as a co-catalyst for the tantalum oxynitride particles is supported in an amount of 0.1 to 50% by mass with respect to the tantalum oxynitride particles (however, 0.01% by mass to 1% by mass in terms of Co) A step of supporting the tantalum oxynitride particles on the conductive substrate, and a step of performing a necking treatment to promote electron transfer to the tantalum oxynitride particles, The manufacturing method of the photoelectrode containing this. The manufacturing method according to claim 2 , wherein the necking treatment includes a step of impregnating a catalyst particle for visible light water decomposition of a photoelectrode with a solution containing a tantalum salt, and a subsequent heat treatment step. The manufacturing method according to claim 2 or 3, wherein the heat treatment step is a method of heating at 100 to 750 ° C for 5 minutes to 5 hours. The manufacturing method according to any one of claims 2 to 4 , further comprising a heat treatment step with ammonia after the necking treatment.