JP2005254042A - Photocatalyst having high reaction efficiency - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photocatalyst particle capable of obtaining a high reaction efficiency. <P>SOLUTION: An electronic band energy at only a part area of a surface is varied by crystal surface-selectively etching a surface of the particulate photocatalyst or modifying only the part area of the surface by an organic group. Thereby, since spatial inclination or bending of the energy band is generated at the inside of the photocatalyst and an electron produced by irradiation with a light and a positive hole are immediately moved in a reverse direction, both early re-bonding is prevented. Thereby, the optical reaction efficiency of the catalyst is enhanced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a photocatalyst capable of obtaining high reaction efficiency.

It is known that a photocatalyst represented by titanium dioxide (TiO ₂ ) has an action of causing a water decomposition reaction or decomposing an organic substance or the like by light irradiation. Much research has been conducted on conversion of light energy to chemical energy, decomposition and removal of organic substances, and the like by utilizing such an action. In addition, as an attempt to put it into practical use, for example, a suspension in which photocatalyst particles are dispersed in a solvent is applied to a wall surface or window of a building, and used to decompose dirt adhering to the wall surface of the building. (See Patent Document 1). In addition, it is expected to be applied to medical and organic synthesis fields such as bactericides, cancer therapeutic agents, and catalysts that provide new reaction pathways.

However, at present, the photocatalyst has a very low reaction efficiency, and a large amount of photocatalyst or high-intensity light irradiation is required to sufficiently obtain the above-described effects.
In order to cover this low reaction efficiency, the specific surface area of the catalyst particles is increased by adjusting the calcination temperature or the like when the catalyst is produced (see Patent Document 2).
Further, in recent years, it has been found that the working range of a photocatalyst is expanded from only the conventional ultraviolet light region to the visible light region by doping nitrogen or carbon into a metal oxide (Patent Document 3). reference).

JP 09-300515 A JP 09-000942 A Japanese Unexamined Patent Publication No. 2000-140636

It is difficult to greatly increase the specific surface area by adjusting the firing temperature and the like, and it is difficult to greatly improve the reaction efficiency by the method of Patent Document 2.
According to the method of Patent Document 3 in which the range of action of the photocatalyst is expanded to the visible light region, light in a wider wavelength region can be used as compared with the conventional method, so that the reaction efficiency can be improved. However, this method does not increase the photon yield of the photocatalyst itself, and the extended range of action is in the low energy region, so it is difficult to greatly improve the reaction efficiency.

  The problem to be solved by the present invention is to provide a photocatalyst capable of obtaining a higher reaction efficiency than conventional ones.

  The particulate photocatalyst according to the present invention made to solve the above problems is characterized in that the electron band energy is changed only in a partial region of the particle surface.

  Specific methods for changing the electron band energy in only a partial region of the particle surface include, for example, revealing a specific crystal plane only in a partial region, or modifying the surface of only a partial region with an organic group And so on.

  The reaction efficiency of the photocatalyst particles is low because electrons and holes (photocharge carriers) generated by light irradiation are recombined immediately. For this reason, if it is possible to prevent or delay the recombination of the generated electrons and holes, the reaction efficiency of the photocatalyst can be greatly improved.

The present inventors have found that the electron band energy of the (100) plane of the rutile TiO ₂ photocatalyst is about 0.09 eV higher than the electron band energy of the (110) plane. This indicates that the electronic band energy of the photocatalyst varies depending on the crystal plane. By changing a part of the surface of the photocatalyst particle, the spatial distribution of the energy band in the photocatalyst particle is made asymmetric (into the energy band). It was thought that it would be possible to prevent recombination of electrons and holes by causing inclination and / or bending).

  When the energy band of the photocatalyst particles is spatially asymmetric, that is, when a tilt and / or bend is introduced into the energy band, electrons excited in the conduction band and holes left in the valence band are spatially generated. It is possible to prevent the two from being immediately recombined.

  FIG. 1 is a view showing a state in which light is irradiated to photocatalyst particles having a uniform surface to generate electrons and holes, and FIG. 2 shows a change in electron band energy only in a partial region of the particle surface. It is the figure which expressed the mode at the time of making an electron and a hole generate | occur | produce in the photocatalyst particle which made the energy band asymmetrical.

Photocatalysts can be insulative and semiconductive. First, consider the case where the photocatalyst is insulative.
In photocatalyst particles having a uniform surface as shown in FIG. 1, the valence band (VB) energy and conduction band (CB) energy are uniform across the entire surface and particles, and as shown in FIG. It becomes flat throughout the interior of the particle (this is called “symmetric”). Since electrons and holes generated by light irradiation exist spatially in the vicinity, they recombine within a short time.

  On the other hand, when only a partial region of the particle surface is changed as shown in FIG. 2, the position of the energy band changes (becomes higher / lower) on the surface, and a difference occurs between the other surface or the inside. That is, a spatial tilt or bend occurs in the energy band in the particle (this state is called “asymmetric”). When such photocatalyst particles are irradiated with light, as shown in FIG. 4, the generated electrons and holes immediately move in opposite directions due to the inclination of the band, and are spatially separated. For this reason, recombination of electrons and holes is prevented.

  The case of the semiconductive photocatalyst can be considered in substantially the same manner as the case of the insulating photocatalyst. For example, when the particle is an n-type semiconductor, a Schottky barrier is formed on the particle surface in an equilibrium state in the dark when the particle size is equal to or larger than the width of the space charge layer, and an energy band is formed inside the particle. As shown by a solid line in FIG. In this case, the electrons and holes generated in the light-irradiated particle enter the inside of the particle due to the bending of the energy band in the particle, and the hole moves to the particle surface. Recombination is prevented. However, since electrons entering the inside of the particle cannot react, they accumulate in the conduction band inside the particle, and as a result, the bending of the band is reduced by short-time light irradiation. Such accumulation of electrons proceeds until the electrons can react with the surface species with the band bending reduced. For this reason, as soon as light irradiation is started, the energy band becomes almost flat as shown by the broken line in FIG. Therefore, even in the case of a semiconducting photocatalyst, if the particles have a uniform surface, the effect of separating electrons and holes hardly acts.

On the other hand, in the semiconducting photocatalyst in which only a partial region on the particle surface is changed, as in the case of the insulating photocatalyst, as shown by the broken line in FIG. As a result, electrons and holes generated by light irradiation immediately move in the opposite directions and are spatially separated, thereby preventing recombination of electrons and holes.
In the case of a small semiconducting photocatalyst having a particle size of about 100 nm or less, since there is almost no bending of the energy band as shown by the solid line in FIG. 5, it can be considered in the same manner as in FIGS.

  Common photocatalyst particles have a particle size of several nanometers to several tens of μm, but as a method of changing only a part of the surface area of such small particles, photoetching, surface modification with organic groups, chemical etching, There are crushing and cleavage. In photoetching, a specific crystal plane can be selectively exposed on the surface in a specific direction by using a specific etching method. Such directional / crystal face selective etching occurs only on a part of the entire surface of the particle, so that asymmetry of the entire particle can be realized.

The flat band potential U _fb of the rutile TiO ₂ single crystal (100) and the (110) plane is measured that the electron band energy of the region can be changed by performing the directional / crystal plane selective etching. Was confirmed.

First, a rutile type TiO ₂ single crystal doped with 0.05 wt% Nb and having (100) and (110) cut faces was dipped in a 20% HF aqueous solution for 10 minutes and then pulled up, and then at 600 ° C. in an air atmosphere. Baked for 1 hour. 7 and 8 show atomic force microscope (AFM) images of the (100) and (110) cut surfaces of the TiO ₂ single crystal particles subjected to such treatment. The step-terrace structure is clearly observed on both the (100) plane and the (110) plane. The step heights of these surfaces were measured from the AFM image, and were 0.27 nm and 0.35 nm, respectively, and the step heights estimated from the crystal lattice model (0.25 for the (100) plane and 0.36 nm for the (110) plane) Almost matched. Moreover, when low energy electron diffraction (LEED) images of these surfaces were observed, clear (1 × 1) spots were shown (FIGS. 9 and 10). From these results, it was confirmed that the (100) plane and (110) plane of the TiO ₂ crystal were flattened at the atomic level.

The flat band potential U _fb of the (100) plane and the (110) plane thus flattened was measured as follows. Using the above TiO ₂ crystal as an electrode (reference electrode: silver / silver chloride electrode, saturated potassium chloride aqueous solution), the differential capacitance (C) was measured while changing the electrode potential (U), and this was measured using a Mott-Schottky plot ( U vs. 1 / C ² ). The measurement was performed at a frequency of 10 Hz, 100 Hz, or 1000 Hz.

The plot results are shown in FIGS. As shown in these figures, the plots showed good linearity, and the slopes were almost the same at all frequencies of 10, 100, and 1000 Hz. When U _fb was determined from the x-intercept of the Mott-Schottky plot, it was −0.34 ± 0.01 V for the (100) plane and −0.25 ± 0.01 V for the (110) plane.
Further, when the photocurrent was measured, the rising potential of the photocurrent in the (100) plane was about 0.1 V negative compared with the case of the (110) plane, which coincided with the difference in U _fb (FIG. 13). . Furthermore, when the luminescence intensity was measured, luminescence having a maximum at 840 nm was observed only on the (100) surface and not on the (110) surface (FIG. 14).

When performing photoetching, metal fine particles are supported on photocatalyst particles, and light having a predetermined wavelength is irradiated in an acidic aqueous solution. Electrons generated in the photocatalyst by light irradiation cause a hydrogen generation reaction from the acidic aqueous solution on the metal fine particles, and holes cause an etching reaction of the photocatalyst. As a result, nano-sized pores and grooves in which a predetermined crystal plane is selectively exposed in one direction are formed in the photocatalyst. That is, the surface electron band energy changes only in a partial region.
As the hydrogen generating metal catalyst, Pt, Pd, Ni, Ir, Ru, Rh, Au fine particles and the like can be used. A sulfuric acid aqueous solution or the like can be used as the acidic aqueous solution. The wavelength of the light to irradiate is selected according to the working wavelength of a photocatalyst.

Modification with an organic group in only a part of the surface of the catalyst particle is performed, for example, by adding a photocatalyst into an organic solvent and then adding a small amount of an aqueous solvent to aggregate the catalyst particles to form secondary particles (catalyst particle coagulation). It is possible to carry out the method by adding an organic modifier. In this method, since only the portion appearing on the surface of the secondary particle reacts with the organic modifier, only a partial region of the primary particle (particle simple substance) is modified with the organic group. Further, modification with an organic group can be performed only on a partial region of the surface of the catalyst particle by adjusting the reaction time to control the reaction amount between the catalyst and the organic group.
As the organic modifier, alcohols, organic acids (such as carboxylic acid, benzoic acid, and esters), silane coupling agents, surfactants, complexes, and the like can be used. For example, in the case of titanium dioxide, which is a typical photocatalyst, since it has a Ti-OH group on the surface, it reacts with alcohol to form Ti-OR (R is an alkyl group, etc.), and the surface is ORed. Can be modified with groups.

  In addition, the method of chemically etching the catalyst or crushing and cleaving the catalyst with a ball mill or the like cannot selectively expose a specific surface like photoetching, but the catalyst can be adjusted by adjusting the processing time. It is possible to make it asymmetric.

Changing only a partial region of the surface can be performed for any photocatalyst. For example, TiO _2, N-doped TiO _2, C-doped TiO _2, S-doped TiO _2, Cr; Sb-doped _{TiO 2, LaTiO 2 N, WO} 3, RbNbWO 6, CsNbWO 6, SnO 2, Fe 2 O 3, ZnO, Nb ₂ O ₅ , SrTiO ₃ , La; N-doped SrTiO ₃ , Ta ₂ O ₅ , TaON, Ta ₃ N ₅ , KTaO ₃ , NaTaO ₃ , ZrO ₂ , BiVO ₄ , AgNbO ₃ , Bi ₂ MoO ₆ , Bi ₂ WO ₆ , photocatalysts such as Ag ₃ VO ₄ , In ₂ O ₃ , Cu ₂ O, CdS, ZnS, NaInS ₂ , AgInS ₂ , CuInS ₂ , CuInSe ₂ , AgGaS ₂ , CdSe, GaAs, GaP, and GaN. The crystal structure of each photocatalyst is not particularly limited. For example, TiO ₂ may be any of anatase type, rutile type, or brookite type.

  Changing only a part of the surface of the photocatalyst particle can be applied not only to the single particle but also to other forms such as a thin film photocatalyst in which the particle is fixed on the substrate. .

  A particulate photocatalyst that changes only a partial region of the particle surface and has an asymmetric energy band can prevent the recombination of electrons and holes generated by light irradiation immediately. The rate can be greatly improved. If such a catalyst is used, it is possible to greatly improve the reaction efficiency of a semiconductor light energy conversion device, a light energy environment purification device, or the like.

For example, the up-hill type (energy storage type) reaction typified by water photolysis is generally a multi-electron transfer reaction, so that both the reduction and oxidation reactions caused by electrons and holes react on the surface of the photocatalyst. Form an intermediate. When there is no band inclination or bending, this promotes the recombination of electrons and holes, so that sufficient reaction efficiency cannot be obtained. However, in the photocatalyst according to the present invention, there is a band inclination or bend in the particle, and electrons and holes are spatially separated, so that the above-mentioned recombination process is suppressed and the reaction efficiency is greatly improved. .
As described above, the photocatalyst according to the present invention is particularly effective for an up-hill type reaction, and is combined with a visible light response of a metal oxide photocatalyst by recently developed nitrogen doping or the like, compared with a conventional apparatus. It is possible to manufacture a hydrogen generator by solar water splitting that is very efficient.

By irradiating rutile TiO ₂ fine particles (catalyst society reference catalyst JRC-TIO-5, rutile 94%) with ultraviolet light in 0.1 mM H ₂ PtCl ₆ aqueous solution, Pt is deposited on the surface of TiO ₂ fine particles and supported I let you. The TiO ₂ fine particles were put into a 0.05M sulfuric acid aqueous solution, and the aqueous solution was stirred and irradiated with ultraviolet light for 72 hours to photoetch the catalyst particles.

FIG. 15 is a scanning electron microscope (SEM) photograph of the Pt-supported TiO ₂ fine particles thus obtained. From FIG. 15, it can be seen that nano-sized pores are formed only in part of the surface of the TiO ₂ fine particles by photoetching. In addition, the TiO ₂ catalyst before photoetching did not show 840 nm emission, whereas the TiO ₂ catalyst after photoetching showed 840 nm emission specific to the (100) plane. From this, it was confirmed that the (100) plane appeared on the wall surface of the nano-sized pore.

Using the TiO ₂ fine particles in which only a partial region of the particle surface was changed in this way, a photolysis reaction (photo-Kolbe reaction) of acetic acid was performed as a model reaction. The photo Kolbe reaction is a reaction represented by the following formulas (1) and (2).

FIG. 16 shows the change over time in the amount of methane produced by the photo Kolbe reaction, and FIG. 17 shows the change over time in the amount of ethane. 16 and 17, the solid line represents the generation amount after photoetching, and the broken line represents the generation amount before photoetching.
As shown in FIGS. 16 and 17, the production amounts of methane and ethane as photoproducts were increased by about 2.5 times by photoetching the catalyst.

  From the above, it was shown that the photoreaction efficiency of the catalyst can be greatly improved by applying an etching process or the like to only a part of the catalyst surface to make the energy band of the photocatalyst particles asymmetric. .

The schematic diagram showing a mode when an electron and a hole were produced | generated in the photocatalyst particle | grains which have a uniform surface. The schematic diagram showing a mode when an electron and a hole were produced | generated in the photocatalyst particle which changed only the partial area | region of the surface. Explanatory drawing showing the energy band structure of the insulating photocatalyst which has a uniform surface, the production | generation of an electron and a hole by light irradiation, and the process of these recombination. Explanatory drawing showing the energy band structure made asymmetric by changing only a partial region of the particle surface of the insulating photocatalyst, and electron / hole generation and transfer processes by light irradiation. Explanatory drawing showing the energy band structure of an n-type semiconducting photocatalyst having a uniform surface and electron / hole generation and transfer processes by light irradiation. Explanatory drawing showing the energy band structure made asymmetric by changing only a partial region of the particle surface of the n-type semiconducting photocatalyst, and electron / hole generation and transfer processes by light irradiation. AFM image of (100) cut surface of rutile TiO ₂ single crystal doped with 0.05wt% Nb. AFM image of (110) cut surface of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. LEED image of (100) cut surface of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. LEED image of (110) cut surface of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. The Mott-Schottky plot of the (100) plane of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. The Mott-Schottky plot of the (110) plane of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. The figure showing the relationship between the photocurrent and potential of the (100) plane and (110) plane electrodes of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. The figure showing the relationship between the luminescence intensity and the electrode potential of the (100) and (110) plane electrodes of rutile TiO ₂ single crystal doped with 0.05 wt% Nb. SEM photograph of Pt-supported TiO ₂ fine particles whose surface has been asymmetrical by photoetching. The figure showing the time change of the quantity of methane produced | generated by optical Kolbe reaction. The figure showing the time change of the quantity of the ethane produced | generated by optical Kolbe reaction.

Claims (8)

  A particulate photocatalyst characterized in that the electron band energy changes only in a partial region of the particle surface.   2. The particulate photocatalyst according to claim 1, wherein the electron band energy is changed only in a partial region of the particle surface by crystal plane selective etching.   2. The particulate photocatalyst according to claim 1, wherein the electron band energy is changed only in a partial region of the particle surface due to the modification of the organic group only in the partial region of the particle surface.   Photocatalyst particles carrying a hydrogen-generating metal catalyst on the surface are immersed in an aqueous solution and irradiated with light of a predetermined wavelength to selectively etch the crystal plane, changing the electron band energy only in a part of the particle surface. A method for producing a particulate photocatalyst characterized by comprising:   The method for producing a particulate photocatalyst according to claim 4, wherein the hydrogen generating metal catalyst is any one of Pt, Pd, Ni, Ir, Ru, Rh, Au, or a combination of two or more thereof.   After introducing the photocatalyst into the organic solvent, a small amount of an aqueous solvent is added to agglomerate the catalyst particles, and an organic modifier is added to the photocatalyst suspension in which the catalyst particles are converted into secondary particles. A method for producing a particulate photocatalyst, wherein only a partial region is modified with an organic group, and the electron band energy is changed only in a partial region of the particle surface.   The method for producing a particulate photocatalyst according to claim 6, wherein the organic modifier is any one of an alcohol, an organic acid, a silane coupling agent, a surfactant, or a complex.   A thin-film photocatalyst, wherein the particulate photocatalyst according to any one of claims 1 to 3 is fixed on a substrate.