JPH0235037B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method of decomposing water with visible light to obtain oxygen and hydrogen.

[Conventional technology]

It is conventionally known to irradiate an n-type semiconductor electrode with light to decompose water and generate oxygen on the semiconductor surface and hydrogen on the surface of a counter electrode such as platinum. but,
Semiconductors that can be used stably underwater are limited to materials with large band gaps (Eg) such as titanium oxide and zinc oxide, and effective light is limited to the ultraviolet light region of 400 nm or less. All n-type semiconductors that can utilize the visible light range of 400 to 800 nm are inactivated under underwater light irradiation conditions. There are two main reasons for this; one is that holes generated in the so-called depletion layer near the interface, such as in n-CdS and n-GaAs, dissolve the semiconductor in an oxidative manner; -This is the case with Si, where holes form an oxide film and become inactive. How to stabilize and use such a visible n-type semiconductor has been an important issue in this field.

[Problem that the invention seeks to solve]

Therefore, an object of the present invention is to provide a method of decomposing water using visible light using a visible region n-type semiconductor to obtain oxygen and hydrogen.

[Means for solving problems]

The present inventor has proposed that a thin film of a polymer complex having a tris(2,2'-bipyridine)ruthenium complex (hereinafter abbreviated as Ru(bpy) ²⁺ ₃ ) as a pendant group is coated on the surface of an n-type semiconductor in the visible region. found that the semiconductor was stabilized even under underwater light irradiation conditions (References: Rajeshwar, Kaneko, Yamada, Journal of Electrochem. Soc., vol. 130) , 38
Page (1983), Rajeshwar,
Kaneko, Yamada, Noufi, Journal of Physical Chemistry (J.Phys.
Chem.), vol. 89, p. 806 (1985). Furthermore, as a result of intensive research, the inventors discovered that visible light decomposition of water becomes possible when a catalyst for oxidizing water is used in the membrane, leading to the completion of the present invention.

In the present invention, the surface of a visible n-type semiconductor electrode is coated with a film of a polymer complex having a tris(2,2'-bipyridine)ruthenium complex as a pendant group. This modified visible region n-type semiconductor electrode is immersed in an aqueous electrolyte solution, and the electrode surface is irradiated with visible light to decompose water.
This is a method in which oxygen is generated on a type semiconductor electrode and hydrogen is generated on a counter electrode.

The details of the present invention will be explained below.

In order to decompose water with visible light using a semiconductor,
The Eg of the semiconductor must be 3 eV or less, and the positions of its valence band and conduction band must match the redox potential of water. For this reason, there are a limited number of semiconductors that can decompose water with visible light, such as n-type cadmium sulfide (n-CdS,
Eg=2.4eV), n-type cadmium selenide (n-
CdSe, Eg=1.7eV), n-type gallium phosphide (n-
GaP, Eg=2.25eV), n-type indium phosphide (n
−InP), etc. As mentioned above, all of these visible range semiconductors deteriorate in water or under light irradiation conditions, so stabilization is an essential condition.

The polymer complex coated as a film to stabilize such a visible n-type semiconductor has a Ru(bpy) ²⁺ ₃ complex structure as a pendant group, and a typical example thereof is represented by the structural formula .

However, here, M is a monomer such as styrene, acrylic acid or its derivative, N-vinylpyrrolidone, 4-vinylpyridine, etc., x, y, and z are the mole fractions of each repeating unit, and x is 0.01 to 0.98 range, y ranges from 0 to 0.9, z represents a value ranged from 0 to 0.98, and x+y+z=1. This polymer complex consists of a homopolymer of 4-methyl-4'-vinyl-2,2'-bipyridine (when z=0) or a copolymer with other monomer M, cis-Ru (bpy) Synthesized by reacting with ₂ Cl. The molecular weight is preferably 2000 or more.

If M has no affinity for water like styrene,
It can be used as a stable membrane in water and gives good results. If M is a copolymer with strong affinity for water, such as acrylic acid or its derivatives, N-vinylpyrrolidone, or 4-vinylpyridine, the membrane will easily dissolve in water, but if the molecular weight is increased to 10,000 or more, Durable to use. Other polymer complexes with Ru(bpy) ²⁺ ₃ as pendant groups, such as polystyrene with 2,
Compounds having the composition of the following formula, which are obtained by complexation into which a 2'-bipyridyl group is introduced, can also be used as membrane materials.

However, x, y, z, w are the mole fractions of each repeating unit, x is 0.01 to 0.5, y is 0 to 0.5, z
is in the range of 0 to 0.4, w is in the range of 0.5 to 0.9, x+y+z+
w=1.

The above-mentioned polymer complex can be synthesized by the method described in Japanese Patent Publication No. 59-8361.

These polymer complex films can be coated on semiconductors by etching using conventional methods for semiconductors, followed by the so-called casting method in which a solution of the polymer complex is applied and dried, or by conventional methods such as spin coating. In that case, whatever is fine. The film thickness is
A suitable thickness is about 1000 Å to 50 μm. In the present invention, the Ru(bpy) ²⁺ ₃ complex in the film reacts with holes generated near the interface due to photoexcitation of the semiconductor, thereby extracting the holes from the semiconductor and transmitting them to the catalyst for water oxidation. It has a role as a so-called charge relay body.

The metal or metal oxide of Group 1 of the periodic table as a catalyst used in the present invention includes:
Rt, _PtO2 , Ru, _RuO2 , Ir, _IrO2 , Rh, _RhO2 ,
Examples include Ni, NiO ₂ , Fe ₂ O ₃ and the like. This catalyst is used to receive holes generated on the semiconductor surface by light irradiation and oxidize water to generate oxygen. Oxides such as RuO ₂ and PtO ₂ give particularly good results. In order to coexist these catalysts in a polymer complex film, the catalyst powder is placed on the semiconductor surface in advance and then covered with the polymer complex film, or the fine catalyst powder is suspended in a polymer complex solution. A coating film may also be formed. Further, these catalysts can also be made to coexist in the membrane through chemical reactions. For example, after coating a semiconductor with a polymer complex, it is added to a RuO ₄ aqueous solution for 2 seconds to 2 seconds at room temperature.
When immersed for 20 minutes, RuO ₄ is adsorbed into the membrane and simultaneously reduced to RuO ₂ and dispersed in the membrane as fine particles. If the amount of catalyst is too small, the effect will be low, but if it is too large, the amount of light reaching the semiconductor will decrease as a result of absorbing light, which will also reduce efficiency. Therefore, the appropriate amount of catalyst is 0.05 to 5% by weight in the polymer complex membrane.

The electrolyte aqueous solution used in the present invention contains nitrates, sulfates, perchlorates, etc. at 0.01% as an electrolyte.
Examples include aqueous solutions containing ~2M concentrations. In halogenated salts, the halogen ions are oxidized, so
In this case, it is not photolysis of water but photolysis of hydrogen halide.

The modified semiconductor electrode prepared as above,
It is immersed in an aqueous electrolyte solution to serve as a working electrode, the counter electrode is usually platinum, and a reference electrode such as silver chloride or silver-silver chloride is used to control the electrode potential, and photoreactions are carried out in a so-called three-electrode manner. is desirable.

Any visible light source can be used as a light source, such as sunlight,
Examples include fluorescent lamps, incandescent lamps, halogen lamps, projectors, xenon lamps, and mercury lamps.

The cell used is not particularly limited either, but when collecting oxygen and hydrogen separately, a two-chamber cell with separate working electrode and counter electrode sides is used.

〔Effect of the invention〕

According to the present invention, a visible n-type semiconductor can be used stably under underwater light irradiation conditions, and water can be photolyzed by irradiating visible light to produce oxygen and hydrogen.

〔Example〕

The present invention will be explained below with reference to Examples.

Example 1 Ga-coated on the back side of n-CdS single crystal (surface area 0.02 cm ² )
Apply In alloy and bond copper wire here with silver epoxy to make ohmic contact. Surfaces other than the electrode surface are solidified with insulating epoxy resin, and the CdS surface is etched with concentrated hydrochloric acid. In the polymer complex, a compound with a composition in which M is styrene, x = 0.048, y = 0.047, and z = 0.905 is dissolved in DMF (concentration is 1mM in Ru complex units), and this solution is mixed with CdS at a rate of 8μl/ ^cm2 . After spreading and air drying, dry with warm air to form a film. This film thickness is calculated to be approximately 0.28 μm. When this coated CdS is immersed in a RuO ₄ aqueous solution with a concentration of 5 mg/ml, RuO ₄ is adsorbed to the membrane and simultaneously reduced.
RuO ₂ becomes fine powder and is dispersed in the film. The modified CdS prepared in this way was installed in a two-chamber cell, and an aqueous solution containing 0.5M KNO ₃ and 0.1M HNO ₃ was placed in the cell, a platinum wire was used as the counter electrode, and a sweet electrode was used as the reference electrode. Installing. After blowing in argon gas for 1 hour and sealing, the electrode potential was kept at −0.4 V (vs, SCE), and the CdS surface was irradiated with visible light from a 500 W xenon lamp equipped with an ultraviolet and infrared light cut filter. Made it react. Bubbles were observed to be generated on the CdS and platinum surfaces during the photoreaction. Photocurrent is approximately 400μA/cm ²
was maintained at a steady value. After 3 hours of reaction, approximately 7 μl of hydrogen and 3.5 μl of oxygen were evolved. The efficiency of water splitting with respect to the applied photocurrent was 70%.

Example 2 In Example 1, n-CdS was replaced with n-CdS.
A photoreaction was carried out in exactly the same manner as in Example 1, except that CdSe was used and direct sunlight was used instead of a 500W xenon lamp, and after 5 hours of reaction, 10 μl of hydrogen and 5 μl of oxygen were obtained.

Example 3 In Example 1, instead of using the RuO ₄ aqueous solution reduction method, RuO ₄ powder was strongly rubbed onto the surface of the CdS electrode, and an electrode coated with a polymer complex film was used on top of it in the same manner as in Example 1. The photoreaction was carried out in exactly the same manner as in Example 1, except that an aqueous solution containing only 0.5M KNO ₃ was used. After 10 hours of reaction, add 16 μl of hydrogen and
8 μl of oxygen was obtained.

Example 4 In Example 1, n-GaP with a surface area of 0.5 cm ² was used, making ohmic contact with the copper wire using In, etching was performed with a hydrochloric acid-nitric acid mixture, and as a polymer complex, x = 0.04 in the formula, y=0.01,z
= 0.21, w = 0.74, and a chloroform solution (concentration is 1mM in Ru complex units).
A photoreaction was carried out in exactly the same manner as in Example 1, except that a coating film was prepared using the mixture at a rate of 16 μl/cm ² , and after 5 hours, 10 μl of hydrogen and 5 μl of oxygen were obtained.

Example 5 In Example 1, M is methyl methacrylate as a polymer complex, x=0.1, y=0, z=0.9
using 1MK ₂ SO ₄ as an electrolyte aqueous solution.
Using an aqueous solution containing 0.01MH ₂ SO ₄ , the electrode potential was
A photoreaction was carried out in exactly the same manner as in Example 1 except that the voltage was set to 0 V (vs. SCE) and a 150 W halogen lamp was used, and after 10 hours of reaction, 18 μl of hydrogen and 9 μl of oxygen were obtained.

Example 6 A photoreaction was carried out in the same manner as in Example 3 except that n-CdS with a surface area of 0.2 cm ² was used and PtO ₂ powder was used instead of RuO ₂ .
After the reaction time, 48 μl of hydrogen and 24 μl of oxygen were obtained.

Claims (1)

[Claims] 1. The surface of a visible n-type semiconductor electrode is coated with a film of a polymer complex having a tris(2,2'-bipyridine)ruthenium complex as a pendant group,
A metal or metal oxide belonging to group 3 of the periodic table is allowed to coexist in the film as a medium, and the modified visible n-type semiconductor electrode is immersed in an aqueous electrolyte solution, and the electrode surface is irradiated with visible light to remove water. A method in which oxygen is decomposed on a semiconductor electrode, and hydrogen is produced on a counter electrode.