CN100395895C - Porous thin film semiconductor photoelectrode with visible light response, photoelectrochemical reaction device and preparation - Google Patents

Abstract

A porous thin-film semiconductor photoelectrode with visible light response. This kind of photoelectrochemical cell for energy storage type reaction uses a thin-film semiconductor photoelectrode, which is formed of a composite metal oxide semiconductor with a porous structure; the porous thin-film semiconductor photoelectrode consists of two types Composed of the above elements, at least one element A is one of bismuth, silver, copper, tin, lead, vanadium, indium, praseodymium, chromium and nickel; the other element B is from titanium, niobium, tantalum, zirconium, hafnium , molybdenum, tungsten, zinc, gallium, indium, germanium and tin selected. The diffusion distance of more than 50% of the holes generated in the semiconductor to the surface of the semiconductor electrode is within 500 nm, and the thickness of the semiconductor is less than 50 microns.

Description

Porous thin film semiconductor photoelectrode with visible light response, photoelectrochemical reaction device and preparation

1. Technical field

The present invention uses a thin-film semiconductor photoelectrode with a high-efficiency porous structure in a photoelectrochemical cell (this photoelectrochemical cell mainly converts light energy such as sunlight into chemical energy such as hydrogen), and uses the photoelectrochemical cell to decompose water. .

2. Technical Background

The effective use of renewable energy such as solar energy is very important, especially the technology of using solar energy to split water to produce hydrogen is a very necessary technology for the early practical application of hydrogen fuel cell vehicles and the acceleration of its popularization. However, it is very difficult to realize a cheap and efficient light energy conversion mechanism. The following technologies for splitting water using sunlight are listed below.

First, in the solar cell-electrolysis mechanism, it is impossible to produce hydrogen cheaply even in combination with electrolysis because the cost of generating electricity from solar cells is very expensive.

Secondly, the use of thermochemical decomposition of water to produce hydrogen requires high heat above 700-800°C, and the cycle efficiency is very low.

Although the method of using photocatalyst to photolyze water has been extensively studied in recent years, the photocatalyst with very low performance and high quantum absorption rate can only use ultraviolet rays, and almost no ultraviolet rays below 300nm are contained in sunlight. Therefore, solar energy cannot be utilized at all. Although there have been reports of water splitting using photocatalysts such as TiO ₂ that can use 400nm ultraviolet rays, the highest solar conversion efficiency is less than 0.03%. In addition, there are also some semiconductor photocatalysts with visible light response, but because most of them contain the reaction of methanol and silver nitrate reducing agent and oxidizing agent, they cannot achieve high-efficiency solar energy conversion.

Recently, there has been another report using NiO _x -In _1-x Ni _x TaO ₄ to decompose water into hydrogen and oxygen under visible light, but its quantum absorption rate is 0.66% at 400nm, from which the solar energy conversion efficiency can be deduced About 0.01% or less. In addition, water splitting using non-oxide photocatalysts has not been successful.

From this, it can be said that in terms of photocatalysts, effective light energy conversion using visible light has hardly been realized, and, because hydrogen and oxygen are mixed simultaneously, the cost of separation is high, and the risk of explosion is also high.

On the other hand, the use of semiconductor photoelectrodes for light energy conversion has been widely discussed since a long time ago. After the semiconductor photoelectrode and the Pt counter are irradiated with light at the same time, the holes generated in the valence band of the semiconductor will oxidize water to generate Oxygen, electrons generated in the conduction band reduce water to generate hydrogen on the counter electrode Pt, and the advantage is that hydrogen and oxygen are generated separately. Such semiconductors, such as oxide semiconductors such as TiO ₂ , are stable and inexpensive, but only light with a short wavelength such as ultraviolet rays can be used. If semi-oxide semiconductors such as silicon are used, although most of the visible light can be used, the performance is high, but the cost is high, and it is easy to cause degradation of semiconductor performance. In consideration of cost and stability, it is also desirable to use an oxide-based semiconductor. Photoelectrodes of doped oxide semiconductors with visible light response have been studied from before, but the performance is very low. In the past, visible light responsive photoelectrodes were mainly used to study single crystals and powder high-temperature sintered bodies, which are several millimeters thick and have a small specific surface area. Because the migration distance of charges is very long, it is difficult for charges, especially holes, to migrate on the surface. , the charge has been recombined before the surface reacts, and the performance cannot be improved at all.

In the past, visible light-responsive photoelectrodes mainly used high-temperature sintered semiconductor powders in the granular form (millimeters in thickness), and coated the inside with indium and other ohmic contact metals as connecting wires, and in order to prevent the electrolyte from contacting the metal and wires, Secure it with adhesive. (J.Solid State Electrochem.2(1998)176.Solar Energy Materials 21(1991)335, J.Chimie 4(1980)501, Chem.phys.Lett., 77(1998)6.)

In recent years, regarding the research on semiconductor photoelectrodes, the following reports of porous thin film photoelectrodes with visible light response have appeared. Using porous semiconductor photoelectrodes composed of Fe ₂ O ₃ and WO ₃ can split water very efficiently. In this porous semiconductor photoelectrode, water as a reaction matrix can permeate into the electrode, that is, the diffusion distance of the holes generated inside the semiconductor photoelectrode is smaller than that of the previous electrode, so it is generally believed that the efficiency can be improved. However, these simple oxide semiconductors that respond to visible light have the disadvantage that the energy level of the conduction band is too high. In order to allow the reaction of reducing protons to hydrogen with electrons (the electrons are generated in the conduction band) to proceed, a large amount of electric energy must be input from the outside. This also means that the conversion efficiency of solar energy is reduced. In order to make this kind of porous thin film photoelectrode with visible light response practical, it is urgent to improve the technology of solar energy conversion efficiency. Moreover, WO ₃ is only stable under acidic conditions, and Fe ₂ O ₃ is only stable under alkaline conditions. In this case, Due to the limitations of the conditions of use, new stable materials must also be developed. In addition to these properties, new semiconductor materials must be able to achieve charge separation, and the efficiency of oxygen generation must be improved. The references of the prior art are as follows:

1. J. Phys. Chem. B, 103, (1999) 7184

2. J.Am.Chem.Soc.123, (2001) 10639

3. Contents of the invention

According to the above content, the present invention designs and controls the conductivity and valence band energy level of the semiconductor in the photoelectrochemical reaction that combines a specific semiconductor photoelectrode and its opposite pole so that the reaction can obtain a visible light response, ensuring charge transfer At the same time, the energy level of the conduction band is prevented from becoming large, and because the thin-film semiconductor photoelectrode with a porous structure is used, the diffusion distance of the charge can be reduced to provide a technology that can greatly improve the solar energy conversion efficiency.

According to the present invention, a thin-film semiconductor photoelectrode, a photochemical cell, and a water splitting method shown below can be provided.

(1) The photoelectrochemical cell for the energy storage type reaction uses a thin-film semiconductor photoelectrode, which is formed of a composite metal oxide semiconductor with a visible light response and a porous structure. The porous thin film semiconductor photoelectrode is composed of more than two types of elements, and at least one of these metal elements is required to be selected from elements such as bismuth, silver, copper, tin, lead, vanadium, indium, praseodymium, chromium, and nickel. . Another element B is selected from titanium Ti, niobium Nb, tantalum Ta, zirconium Zr, hafnium Hf, molybdenum Mo, tungsten W, zinc Zn, gallium Ga, indium In, germanium Ge and tin Sn.

The metal element of the metal oxides of the first type is chosen in particular from bismuth, silver, tin or nickel.

(2) This kind of photoelectrochemical cell for energy storage type reaction uses a thin film semiconductor photoelectrode, which contains more than one element in nitrogen and sulfur, and has visible light response at the same time, and has a porous structure. Composed of oxygen-containing compound semiconductors.

(3) The photoelectrode is doped with more than one element in bismuth, iron, silver, copper, lead, vanadium, chromium and nickel by 20-0.5% mol, and mixed with antimony, bismuth, vanadium, niobium and tantalum Formed by co-doping of more than one element.

(4) It is required that more than 50% of the holes generated in the semiconductor have a diffusion distance to the surface of the semiconductor within 500 nm.

(5) It is required to contain bismuth and vanadium, and be composed of a composite metal oxide semiconductor with a visible light response and a porous structure.

(6) The present invention is preferably formed on a light-transmitting substrate.

(7) As described above, the film thickness of the semiconductor is 50 micrometers or less.

(8) A photoelectrochemical reaction cell for energy storage type reaction composed of a semiconductor photoelectrode and its counter-pole. Any thin-film semiconductor photoelectrode mentioned above is required as the semiconductor photoelectrode of the reaction cell.

(9) The photoelectrochemical cell of the present invention is a photoelectrochemical reaction device. Its energy accumulation reaction is required to be the decomposition reaction of water. In a method for splitting water into hydrogen and oxygen using a photoelectrochemical cell, the above-recorded photochemical cell is used.

The further improvement of the present invention is: a simple device that can effectively convert visible light such as sunlight into chemical energy such as hydrogen. The focus of the invention is to provide a porous film semiconductor photoelectrode with visible light response, which makes charge transfer easier, has a relatively high conduction band energy level. In an example, even an electrode with a very low quantum absorption rate, as long as the film-forming method is as perfect as possible, its quantum absorption rate will be close to 100%, so it will make the inexhaustible sunlight and water, effectively It is possible to produce hydrogen at a high level, which is closer to the goal of realizing a hydrogen economy society.

As the semiconductor having a visible light response used in the present invention, it is preferable to use an oxide-based semiconductor containing oxygen atoms in view of stability and economy. Having a visible light response means not only absorbing visible light, but also utilizing the charge generated by visible light irradiation in the reaction.

TiO ₂ , SrTiO ₃ , Ta ₂ O ₅ , WO ₃ and other oxide semiconductors generally have conduction bands on the d orbitals of transition metal atoms and valence bands on the 2p orbitals of oxygen. The charges of these transition metal atoms are Ti ⁴⁺ , Ta ⁵⁺ , w ⁶⁺ , the d orbital at this time is in an electron-free state. After being irradiated by light, the 2p orbital electrons of oxygen will be excited and migrate to the empty d orbitals of transition metals. That is, a hole that loses an electron is formed on the 2p orbital of oxygen. If such a semiconductor is at the same energy level as the 2p orbital of oxygen, the conduction band energy level will dominate the size of the band gap. Therefore, in order to make more extensive use of visible light, the band gap will be smaller, and the conduction band energy level will be A large shift occurs. The larger the shift in the conduction band energy level, the more external bias voltage is required to generate hydrogen at the opposite electrode, resulting in lower energy conversion efficiency. The way to solve this difficulty is to restrict the valence band to atomic orbitals other than oxygen, so that the energy level of the valence band is shifted negatively. The semiconductor photoelectrode with ultraviolet response is doped with transition metals and atomic replacement to control the energy level of the valence band and make it respond to visible light. It has been done on semiconductor electrodes (single crystal and high-temperature sintered body) in the past. But the performance is very low. The reason is that the migration of holes in the valence band is more difficult than that of electrons in the conduction band, and when the doping amount is small, the impurity energy levels will be separated in space, making it very difficult for holes to migrate from the semiconductor body to the surface.

In order to solve this problem, we propose in the present invention to granulate the semiconductor, so that the distance for the charge to migrate to the surface is shortened, so that the oxidation reaction becomes smoother. That is to say, making the semiconductor into a porous film, so that the electrolyte solution has been soaked into the electrode structure inside the film, will make the reaction go smoothly! Similar electrodes have been studied in simple oxides such as Fe ₂ O ₃ and WO ₃ , but they have not been discussed in complex compound oxide semiconductors with limited valence electrons. In the present invention, in order to make the transfer of charge smoother and control the energy band structure of the semiconductor, use (1) compound oxide semiconductors containing two or more metal elements and responding to visible light; (2) containing a part of oxygen Other than negative ionic elements (S or N), oxygen-containing compound semiconductors with visible light response, or (3) compound semiconductors with increased doping amount and co-doping.

Such a composite oxide refers to a substance containing two or more metal elements and having a defined crystal structure. Doping means adding a foreign element to a matrix compound crystal without changing the basic crystal structure of the matrix compound.

The type of semiconductor is, in principle, preferably a composite metal oxide semiconductor that contains element levels other than oxides in the upper part of the valence band energy level and that responds to visible light. Such a composite metal oxide semiconductor is preferably composed of two or more metal elements. Among them, at least one element A is one of bismuth, silver, copper, tin, lead, vanadium, indium, praseodymium, chromium and nickel. Among them, bismuth, silver, tin, and nickel are the best. The combination of two or more metal elements can also be expressed as a combination of B elements opposite to the above-mentioned A elements in addition to the combination of the above-mentioned A elements. At this time, the element B is from Ti, Nb, Ta, Zr, Hf, Select from Mo, W, Zn, Ga, In, Ge and Sn.

In general oxide semiconductors, the valence band is composed of the 2p orbital of oxygen. The energy level at the upper part of the valence band determines the size of the energy gap. In order to reduce the energy gap, it is better to have other atomic orbitals, whose energy level is the same energy level or higher. In addition, even if the energy level of some atomic orbitals is below the top energy level of the valence band, once it forms an orbital hybrid with oxygen, it may reach a new energy level upwards, which can also be used. Such hybrid orbitals can facilitate charge transfer.

With regard to the question of which atomic compound should be used for doping, it has been largely possible to judge recently due to the progress of computational science (density functional method). The best semiconductor used in the present invention is specifically: containing more than one element in the d orbital state, a part of the saturated atomic state (such as chromium, nickel, iron, etc.), a composite oxide semiconductor with visible light response and bismuth, A compound oxide-based semiconductor having visible light response, consisting of one or more elements selected from silver, tin (preferably divalent), lead (preferably divalent), vanadium, indium, and praseodymium.

The semiconductor used in the present invention can also be an oxygen-containing compound semiconductor that contains more than one element in nitrogen and sulfur and has a visible light response, such as nitrogen oxides and oxysulfides. In addition, substances such as carbon oxides also. The above semiconductors, as a result of doping and element replacement, will produce oxygen vacancies and lattice defects, which will also change the energy level of the valence band.

The compound metal oxide semiconductor used in the present invention, the combination of its metal elements is specifically as follows:

Bi/V, Ag/Nb, In/Ni/Ta, Ag/Pr/Ti, Rb/Pb/Nb, In/Zn, Bi/Mo, Bi/W, Ag/V, Pb/Mo/Cr, In/ Zn/Cu, Na/Bi, K/Bi, etc.

The following specific examples of metal oxide semiconductors can be cited: BiVO ₄ , AgNbO ₃ , AgPrTi ₂ O ₆ , RbPb ₂ Nb ₃ O ₁₀ , In ₂ O ₃ -(ZnO) ₃ , Bi ₂ MoO ₆ , Ag ₃ VO ₄ , In ₂ -xZnxCu ₂ O ₅ (x=0~1), ABiO ₂ (A is a monovalent metal such as Na, Ka, Li, Ag), ABiO ₃ (A is a monovalent metal such as Na, K, Li, Ag, etc. Metal).

Oxygen-containing compound semiconductors containing nitrogen or sulfur are composed of (1) metals (2) oxygen (3) N or S and other elements, and the metal elements at this time include Ta, Sm, Ti, Nb, Zr, Hf, Mo , W, Zn, Ga, In, Ge, Sn, Bi, V and Pb and other elements.

Specific examples of the above oxygen-containing compound semiconductors are: TaON, Sm ₂ Ti ₂ S ₂ O ₅ , BaNbO ₂ N, SrTaO ₂ N, LaTaON ₂ , Zr ₂ ON ₂ , Na ₂ TiOS ₂ , ZrOS, Li _7.2 Ti _0.8 O _1.6 N _2.4 , Ta ₅ O _1.81 N _4.79 , Ta _0.48 Zr _0.52 CaO _2.52 N _0.48 and so on.

In the present invention, it is possible to use an oxide-based semiconductor having a doped metal X and a metal Y co-doped structure.

At this time, the metal X is selected from at least chromium (Cr), nickel (Ni), iron (Fe), silver (Ag), lead (Pb), copper (Cu), vanadium (V), and bismuth (Bi). Use more than one element. On the other hand, as metal Y, at least one can be selected from Sb, Bi, V, and Nb. The ratio of metal X to Y: atomic ratio [X]/[Y] is 0.2～ 5. Preferably 0.5-2. As the metal Z containing an oxide semiconductor, Ti, Ta, Zr, Hf, Mo, W, Zn, Ga, In, Te, Sn, Bi, etc. can be used.

Specific examples of oxide-based semiconductors without (X) are:

(1) Simple oxides: TiO ₂ , Ta ₂ O ₅ , ZrO ₂

(2) Composite oxides:

Ti type: SrTiO ₃ , K ₂ La ₂ Ti ₃ O ₁₀ , Rb ₂ La ₂ Ti ₃ O ₁₀ , CsLaTi ₂ NbO ₁₀ , Na ₂ Ti ₆ O ₁₃ , BaTi ₄ O ₉ , etc.

Nb type: K ₄ Nb ₆ O ₁₇ , Rb ₄ Nb ₆ O ₁₇ , Sr ₂ Nb ₂ O ₇ , Na ₄ Nb ₈ P ₄ O ₃₂ , KCa ₂ Nb ₃ O ₁₀ , etc.

Ta type: KTaO ₃ , NaTaO ₃ , BaTaO ₆ , Rb ₄ Ta ₆ O ₁₇ , K ₃ Ta ₃ Si ₂ O ₁₃ , Na ₄ Ta ₈ P ₄ O ₃₂ , K ₂ Ta ₄ O ₁₁ , K ₂ SrTa ₂ O ₇ , Sr ₂ Ta ₂ O ₇ , RbNdTa ₂ O ₇ , LaTaO ₄ and so on.

In type: CaIn ₂ O ₄ , SrIn ₂ O ₄ , etc.

Sn type: Sr ₂ SnO ₄ , Ca ₂ SnO ₄ , etc.

Ga type: CaGa ₂ O ₄ , SrGa ₂ O ₄ , ZnGa ₂ O ₄ , etc.

Ge type: Zn ₂ GeO ₄ etc.

Sb type: NaSbO ₃ , KSbO ₃ , etc.

The combination of oxide-based semiconductor and (X) metal can be expressed as follows;

(1) Semiconductor: tIO ₂ Metal X: Cu, Metal Y: Sb

(2) Semiconductor: TiO ₂ Metal X: Cr, Metal Y: Bi

(3) Semiconductor: TiO ₂ Metal X: Ni, Metal Y: Sb

(4) TiO ₂ /Cr, Sb

(5) Ta ₂ O ₅ /Cr, Sb

(6) NaTaO ₃ /Cr, Sb

In the oxide-based semiconductor doped with metal X and metal Y, the doping amount of metal X is 0.5 to 20% mol, preferably 5 to 10% mol relative to 1 mol of the semiconductor compound. The co-doping amount of co-doping basically requires charge balance, and it is not a big problem if there are some small differences.

In the doping compound, when the energy level of the valence band formed by the doping element is 0.03eV (excitation energy at room temperature) different from the valence band formed by oxygen, if the doping amount is not increased accordingly, holes will be difficult to migrate. As doping compounds, there are mainly elements such as chromium and nickel in a partially saturated electronic state of the d orbital, as well as vanadium, bismuth, silver, tin, and the like. When using base semiconductor metals and doping types with different valence states, in order to neutralize the charge, it is best to co-dope some other valence metals. For example, the types of co-doping are: antimony, bismuth, vanadium, niobium, Tantalum and more.

In order to shorten the migration distance of holes, it is preferable to use a small semiconductor, and the shape may be spherical or rod-like. The holes generated inside the semiconductor are 50%, preferably more than 80%, and the diffusion distance to the semiconductor surface is less than 500nm, preferably less than 10nm, and the porous film is the most excellent. In order to promote charge separation and suppress charge recombination, high crystallinity is required.

4. Specific implementation

Because the semiconductor used in the present invention has a porous structure, the electrolyte mostly passes through the pores of the semiconductor and contacts the substrate. If the surface of the substrate comes into contact with the electrolyte, leakage may occur. At this time, the overall performance can be improved by covering the surface of the substrate with a dense film with almost no pores.

In order to make the migration of the electrolyte inside the membrane and the diffusion of oxygen and other products more effective, the pores of the semiconductor membrane should preferably be larger, but if it is too large, the membrane strength will also be reduced, making charge transfer difficult. Preferably, the state is composed of fine pores with a size of about 5 nm to 500 nm. In order to control the size of the pores, the molecular weight and mixing amount of organic substances mixed during firing of the membrane can be appropriately adjusted.

As for the semiconductor particles constituting the semiconductor film, large particles cause light scattering and can improve light absorption efficiency.

The semiconductor is usually formed on the substrate. At this time, the semiconductor electrode substrate is preferably a transparent conductor such as conductive glass or conductive plastic. Among them, tin oxide-based conductive glass having heat resistance is preferable. The reason for choosing a transparent conductor is that light can pass through from one side of the substrate, shorten the electron migration distance, and thereby suppress the recombination of charges. However, if the semiconductor film is thin, non-transparent substrates such as metals and carbon plates are also acceptable.

It is sufficient for the film thickness of the semiconductor to sufficiently absorb light. If the thickness exceeds this level, cracks will occur, which will hinder the transportation of solutions and products, resulting in a decrease in overall performance. Therefore, the thickness of the semiconductor film should be less than 50 microns, preferably less than 20 microns, more preferably between 1 and 5 microns.

In addition, regarding the semiconductor particles constituting the porous structure semiconductor, if the particles are spherical or powdery, the average particle radius is 3 to 500 nm, preferably 10 to 300 nm. On the other hand, when the particles are columnar, the columnar The average radius is 3-500nm, preferably 10-300nm, and if it is a hollow cylinder, the average thickness of its body wall is 3-1000nm, preferably 10-600nm.

The preparation method of the semiconductor film is as follows: the metal precursor is dispersed in the solvent by the citric acid complex method, the sol-gel method, etc., and the method of thermal decomposition after coating is carried out, and the fine particles of the semiconductor are prepared in advance by the solid phase method. A method of making a paste, applying it, and then thermally decomposing it (firing), etc. If the melting point is low, the solid phase method is also possible. The coating method can be printing, glue-spinning method, thermal spraying method, etc.

In principle, the firing temperature must be the decomposition temperature of the above-mentioned mixed organic matter, but because the substrate has heat resistance, it is desirable to be below the heat-resistant temperature (about 600 degrees) if it is tin oxide-based conductive glass. In order to promote the decomposition of organic matter, firing in oxygen is also effective.

Nitrogen, sulfur, and oxygen-containing compound semiconductors containing carbon can also be synthesized after the oxide film is treated with ammonia and hydrogen sulfide, and it is also possible to mix and fire the precursor compound and nitrogen-containing compound or sulfur-containing compound. The amounts of N and S are at least 0.5 mol % or more, preferably 10 mol % or more, with respect to one oxygen atom in the oxide semiconductor. Its upper limit is generally 80% mol atoms.

The prepared porous semiconductor thin film can improve its overall performance after post-processing. Ti-based semiconductors are immersed in solutions containing Ti such as TiCl ₄ , and Nb and Ta-based semiconductors are immersed in solutions such as alkoxides and chlorides. The Bi-type is immersed in a solution containing bismuth ions, and finally heat-treated to improve crystal defects and necking between grains.

The electrode can not only be mixed with the above-mentioned semiconductors with visible light response, but also can be mixed with other conductive materials. The conductive materials at this time mainly refer to SnO ₂ , TiO ₂ , WO ₃ , In ₂ O ₃ - Inorganic materials such as SnO ₂ where conduction band electrons are easily transferred, and organic materials such as conductive polymers.

The above-mentioned semiconductor film can be deposited as a different semiconductor layer even if it is a single semiconductor, and it is necessary to consider the potential barrier for the transfer of electric charges when depositing.

In this way, when installing wires to the prepared semiconductor electrodes, in order to reduce resistance, it is preferable to use integrated electrodes such as comb electrodes to reduce contact resistance. In addition, indium bonding is also effective.

The photoelectrochemical cell provided by the present invention can effectively carry out energy storage reaction by combining the above-mentioned semiconductor electrode with visible light response and its opposite electrode.

The energy accumulation reaction here means that the free energy change of the reaction is positive, and the basic reaction is the decomposition reaction of water. In addition, this battery can also be used for the decomposition reaction of HI and HBr, and the redox reaction of iodine, etc.

The semiconductor photoelectrode in the present invention can also use a solar cell formed by a combination of redox reactions such as halogens as an electrode.

Regarding the electrolyte composition of the photoelectrochemical cell, when decomposing water, stable electrolytes such as sodium sulfate, sulfuric acid, perchlorate, and sodium hydroxide are used. In principle, it is above 0.1 mol/L, and it can be close to saturation. The pH value is required to be in the range to stabilize the semiconductor electrode. In the decomposition reaction of HI and HBr, HI and HBr can be dissolved in the electrolyte solution. At this time, the solution is not water but an organic solvent.

The electrode of the present invention can also be used to prepare hydrogen from waste liquid containing organic matter and easily oxidizable substances such as organisms. At this time, the organic matter is oxidized on the positive electrode of the semiconductor, which can improve the hydrogen production efficiency. In addition, when the semiconductor electrode in the present invention exhibits p-type characteristics, a reduction reaction such as hydrogen gas generation can proceed on the semiconductor electrode, and an oxidation reaction can proceed on the counter electrode.

Materials suitable for the reaction should be used for the counterpole. If hydrogen gas is generated, Pt, carbon, etc., which have a low overvoltage for generating hydrogen gas, are effective, and inexpensive Co—Mo electrodes can also be used.

Example 1

(1) BiVO ₄ electrode preparation method

Bi(hex) ₃ ((hex) ₃ : the fluorescence group hexachloro-6-carboxyl-fluorescein) was dissolved and mixed into vanadyl acetylacetonate (VO(acac) ₂ ) at a chemical ratio of 1:1. After stirring for one hour, concentrate on the evaporator, add 50vol% polyethylene glycol, and apply the resulting solution on conductive glass (F-SnO ₂ , 10 ohm/cm ² ), burned at a temperature of 500° C. in air for one hour, and so repeated three times, the film thickness was about 0.3 microns, and observed by SEM, it can be seen that a film with large pores of about 100-200 nm was formed. According to XRD, monoclinic BiVO ₄ .

(2) Evaluation of electrodes

Connect this electrode to an electrostatic potentiometer, use Ag/AgCl as the reference electrode, and use Pt as the counter electrode, and carry out the decomposition reaction of water in 0.1mol/L Na ₂ SO ₄ aqueous solution. While changing the bias voltage, while irradiating monochromatic light of various wavelengths, the band gap is about 2.4eV, the open circuit voltage +0.2V (vs.Ag/AgCl), and the band gap of WO ₃ (about 2.7eV) and the open circuit voltage ( +0.25V), the conduction band energy level of BiVO ₄ is smaller than that of WO ₃ , so BiVO ₄ is better. The 6s orbital of Bi constitutes the top of the valence band, which is considered to be the reason for its excellent properties. At 400nm, when the open circuit voltage is 0.6V, the quantum absorption rate is 33%, and the QE is 64% at 0.8V, and the efficiency is very high. high. The solar conversion efficiency is about 0.36%, which is more efficient than photocatalyst devices that cannot split water.

Example 2

(1) Preparation of AgNbO ₃ electrode

Mix _AgNO3 dissolved in methanol and Nb alkoxide dissolved in ethanol according to the stoichiometric ratio of 1:1, after stirring, add 50vol% polyethylene glycol, and apply the obtained solution on the On the conductive glass, it was burned in the air at 550°C for one hour, and this was repeated 5 times. The film thickness was about 0.2 microns. According to XRD, AgNbO ₃ was formed.

(2) Evaluation of electrodes

The evaluation method of the electrode is the same as that of Example 1. The open circuit voltage is submerged in the redox peak of silver, which is difficult to observe and is below 0.2V. In the energy band calculation using density functional function (CASTEP), the conduction band uses the d orbital of Nd, so the conduction band is negative compared with WO ₃ and Fe ₂ O ₃ . As a result of the measurement, at 400nm, when the relative open circuit voltage is 0.5V, the quantum absorption rate (QE) is 1.4%, and the undoped _TiO2 has no absorption at 420nm, and the quantum absorption rate (QE) is 0.4%.

Example 3

(1) Preparation of TiO ₂ electrodes doped with Cr and Sb

Mix titanium isopropoxide, chromium nitrate (Cr: 2.3mol%), and antimony oxide (Sb: 3.5mol%) in an ethanol solvent, and apply the resulting solution on the conductive glass with a doctor blade coating method. Burn at 500°C for one hour, then immerse in TiCl ₄ solution (0.2mol/L) for 18 hours, sinter at 500°C for one hour, repeat this process 3 times, and the film thickness is about 1 micron.

(2) Evaluation of electrodes

The evaluation method of the battery is the same as that of Example 1, and the open circuit voltage is almost the same as that of undoped TiO ₂ . In the energy band calculation using the density functional method (CASTEP), the energy level of the valence band is in the d orbital of Cr, and the conduction band uses the d orbital of Ti. Therefore, the conduction band is negative compared with WO ₃ and Fe ₂ O ₃ .

Undoped _TiO2 has no absorption at 420nm, which corresponds to a quantum absorption rate (QE) of 1% at an open circuit voltage of 0.6V.

Example 4

(1) Modulation of Bi ₂ WO ₆ electrode

Mix the bismuth nitrate and ammonium tungstate dissolved in the nitric acid aqueous solution at a chemical ratio of 1:1, stir and let it stand for 5 days to make it into a gel, disperse the obtained solution with ultrasonic waves, and apply it on the conductive surface with a scraper coating method. On the glass, it was burned in the air at 550°C for one hour, and this was repeated 5 times. According to XRD, it was determined that Bi ₂ WO ₆ was mainly formed.

(2) Evaluation of electrodes

The electrodes were evaluated as in Example 1. The open circuit voltage is -0.12V, which is the most negative in this experiment.

At 400nm, the quantum absorption efficiency (QE) is 1.8% when the open circuit voltage is 0.9V.

Example 5

Indicates the preparation method of N-doped TiO ₂ electrode. Add ammonia to TiCl ₃ to make it precipitate and burn it at 450°C. The color is yellow and the absorption range extends to 470nm. Mix this with acetylacetone to gel it. Coating on conductive glass with doctor blade coating method, and then treating TiCl ₄ , at 440nm, TiO ₂ which does not have visible light response, QE is below 0.2%, but under the same conditions, this N-doped electrode reaches above 0.3%, Based on the relative quantum absorptivity at 400nm, at 440nm, the performance of the N-doped electrode is more than 6 times that of the undoped electrode.

Others such as AgPrTi ₂ O ₆ , RbPb ₂ Nb ₃ O ₁₀ , In ₂ O ₃ -(ZnO) ₃ , Bi ₂ MoO ₆ , Ag ₃ VO ₄ , In ₂ -xZnxCu ₂ O ₅ (x=0～1), NaBiO ₂ , Na, BiO ₃ , etc. are obtained by the method similar to Example 1-5, and have the same performance.

Oxygen-containing compound semiconductors containing nitrogen or sulfur, specific examples are: TaON, Sm ₂ Ti ₂ S ₂ O ₅ , BaNbO ₂ N, SrTaO ₂ N, LaTaON ₂ , Zr ₂ ON ₂ , Na ₂ TiOS ₂ , ZrOS, Li _7.2 Ti _0.8 O _1.6 N _2.4 , Ta ₅ O _1.81 N _4.79 , Ta _0.48 Zr _0.52 CaO _2.52 N _0.48 etc. are obtained by the method similar to Example 1-5, and have the same performance.

Claims (7)

1. The porous thin film semiconductor photoelectrode with visible light response is characterized in that this porous thin film semiconductor photoelectrode is used in the photoelectrochemical cell of the energy storage type reaction, and the porous thin film semiconductor photoelectrode is composed of visible light responsive, porous structure compound Composed of metal oxide semiconductors, composite metal oxide semiconductors are composed of oxygen and two or more metal elements, of which at least one element A is selected from bismuth, silver, copper, tin, lead, vanadium, indium, praseodymium, chromium or nickel ; Another element B is selected from titanium Ti, niobium Nb, tantalum Ta, zirconium Zr, hafnium Hf, molybdenum Mo, tungsten W, zinc Zn, gallium Ga, or germanium Ge, and 50% of the holes generated inside the semiconductor The diffusion distance to the surface of the semiconductor electrode is within 500nm, and the film thickness of the semiconductor electrode is below 50 microns. 2. The porous film semiconductor photoelectrode with visible light response according to claim 1, characterized in that the metal element A is selected from bismuth, silver, tin or nickel. 3. The porous film semiconductor photoelectrode with visible light response according to claim 1 or 2, characterized in that the photoelectrode is an oxide semiconductor co-doped with metal X and metal Y, wherein metal X is selected from Chromium Cr, nickel Ni, iron Fe, silver Ag, lead Pb, copper Cu, vanadium V or bismuth Bi; metal Y is selected from Sb, Bi, V, Nb or Ta tantalum, the atomic ratio of metal X to Y [X] /[Y] ranges from 0.2 to 5. 4. The porous film semiconductor photoelectrode with visible light response according to claim 1 or 2, characterized in that the composite metal oxide semiconductor contains oxides of bismuth and vanadium, and forms a porous structure. 5. by the described porous thin film semiconductor photoelectrode with visible light response of claim 1, it is characterized in that the combination of two or three metal elements in the composite metal oxide semiconductor is specifically as follows: Ag/Nb, In/Ni/Ta, Ag/Pr/Ti, Rb/Pb/Nb, In/Zn, Bi/Mo, Bi/W, Pb/Mo/Cr or In/Zn/Cu,. 6. The porous film semiconductor photoelectrode with visible light response according to claim 1, characterized in that the compound metal oxide semiconductor comprises: AgNbO ₃ , AgPrTi ₂ O ₆ , RbPb ₂ Nb ₃ O ₁₀ , In ₂ O ₃ -( ZnO) ₃ , Bi ₂ MoO ₆ or In _2-x Zn _x Cu ₂ O ₅ (x=0~1). 7. have the photochemical reaction device of the porous thin film semiconductor photoelectrode of visible light response, it is characterized in that use any one porous thin film semiconductor photoelectrode in the claim 1-4 to form on the light-permeable substrate, and as the semiconductor of reaction cell Photoelectrode, the reaction cell is also provided with a counter electrode.