WO2016104072A1 - Visible light-responsive photocatalyst - Google Patents

Visible light-responsive photocatalyst Download PDF

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Publication number
WO2016104072A1
WO2016104072A1 PCT/JP2015/083762 JP2015083762W WO2016104072A1 WO 2016104072 A1 WO2016104072 A1 WO 2016104072A1 JP 2015083762 W JP2015083762 W JP 2015083762W WO 2016104072 A1 WO2016104072 A1 WO 2016104072A1
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visible light
core
responsive photocatalyst
band
light responsive
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PCT/JP2015/083762
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French (fr)
Japanese (ja)
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弘文 吉川
真 和泉
恭崇 葛本
吉村　健一
達也 両輪
まみ 森下
朋宏 野澤
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シャープ株式会社
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Priority to JP2016566067A priority Critical patent/JP6446064B2/en
Publication of WO2016104072A1 publication Critical patent/WO2016104072A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/02—Sulfur, selenium or tellurium; Compounds thereof
    • B01J27/04—Sulfides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/02—Sulfur, selenium or tellurium; Compounds thereof
    • B01J27/057—Selenium or tellurium; Compounds thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/24—Nitrogen compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—

Definitions

  • the present invention relates to a visible light responsive photocatalyst.
  • the photocatalyst market and the hydrogen market are expected to continue to grow in the future, and research and development related to high activation of photocatalysts related to water splitting is active.
  • the proportion of ultraviolet light in sunlight is remarkably small at 2%, so there is a limit to high activation, and development of a visible light-responsive photocatalyst is indispensable.
  • the main solutions are the development of new materials for lowering the band gap, the Z-scheme type photocatalyst using a two-step photoexcitation mechanism (two types of photocatalysts for hydrogen generation and oxygen generation, and electron transfer between them) Is developed using reversible redox).
  • Patent Document 1 describes a novel tantalum oxynitride photocatalyst as a new material for reducing the band gap. Further, as a Z scheme type photocatalyst utilizing a two-step photoexcitation mechanism, JP 2005-199187 A (Patent Document 2) discloses a Z scheme type visible light water complete decomposition type catalyst system using Fe 3+ / Fe 2+ redox. Is described.
  • An object of the present invention is to provide a novel visible light responsive photocatalyst that is a photocatalyst of a one-step light absorption process and that can achieve high efficiency.
  • the photocatalyst of the present invention comprises a nanostructure composed of a plurality of semiconductors, the nanostructure has a type II band structure, and each semiconductor has a lower end of the conduction band that is more negative than the reduction potential of water.
  • the upper end of the valence band is more positive than the oxidation potential of water.
  • the nanostructure is, for example, one having a band structure capable of confining electrons or one having a band structure capable of confining holes.
  • the nanostructure has a light absorption characteristic corresponding to the quantum level.
  • the light absorption edge is preferably 440 nm or more.
  • the nanostructure include nanoparticles having a core-shell structure.
  • the core portion and the shell portion are formed of different semiconductors.
  • the shell portion is formed from an oxide semiconductor.
  • the nanostructure is selected from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, cobalt, nickel, tin, chromium, iron, copper, gold and silver. And those carrying fine particles containing at least one element.
  • a highly efficient visible light responsive photocatalyst can be provided.
  • the visible light responsive photocatalyst of the present invention comprises a nanostructure composed of a plurality of semiconductors, the nanostructure has a type II band structure, and each semiconductor has a reduction potential of water at the lower end of the conduction band. It is more negative and the upper end of the valence band is more positive than the oxidation potential of water.
  • the type II band structure is a band structure in which a plurality of different semiconductor materials are in contact with each other, and the band discontinuity amounts have different signs in the valence band and the conduction band. As a result, electrons and holes are confined on different material sides and spatially separated.
  • the light absorption edge (hereinafter also referred to as “band gap”) has the highest valence band energy among a plurality of semiconductors. Due to the transition between the ground quantum level located near the top of the valence band of one semiconductor and the ground quantum level located near the bottom of the conduction band of the second semiconductor having the lowest conduction band energy To do.
  • the energy of the valence band of the first semiconductor is higher than the energy of the valence band of the second semiconductor
  • the energy of the conduction band of the second semiconductor is the conduction band of the first semiconductor. Since the transition between the quantum levels occurs by setting the size and structure to be lower than the energy of the quantum and causing the quantum confinement effect, between the ground quantum levels of the nanostructure band composed of the first semiconductor and the second semiconductor The band gap resulting from this transition is shifted to a longer wavelength side than the bulk band gaps of the first semiconductor and the second semiconductor, and it is possible to realize a longer wavelength of light receiving sensitivity.
  • the visible light response Type photocatalyst can be realized.
  • the band gap resulting from the transition between the ground quantum levels is preferably smaller than the energy having a wavelength of 440 nm or more.
  • the visible light responsive photocatalyst of the present invention is made of a nanostructure, quantum confinement of carriers generated by light absorption becomes possible, and the generated carriers can be efficiently used for catalytic reaction.
  • the nanostructure composed of a plurality of semiconductors is not limited as long as it is a structure capable of quantum confinement of carriers. Core-shell type nanoparticles, nanowires, thin film nanostructures (quantum well superlattices, Quantum dot superlattice) and the like.
  • the nanostructure may be either one capable of electron confinement or one capable of hole confinement.
  • the semiconductor having the conduction band in which the electrons generated by light absorption are located is the second semiconductor, and the semiconductor having the valence band in which the generated holes are located is the first semiconductor.
  • the generated electrons and holes are separated between different semiconductors, and the carrier lifetime is increased. Therefore, according to the nanostructure of the present invention, the photocatalytic activity can be enhanced.
  • the nanostructure of the present invention is a combination of a plurality of semiconductors having a type II band structure.
  • the lower end of the conduction band is more negative than the reduction potential of water
  • the upper end of the valence band is There is no particular limitation as long as it is more positive than the oxidation potential of water.
  • a semiconductor that satisfies such a relationship electrons in the conduction band generated by light absorption are used for water reduction to generate hydrogen, and holes in the valence band generated by light absorption are generated. Used to oxidize water to produce oxygen. That is, a photocatalyst useful for water decomposition can be constituted.
  • the nanostructure of the present invention is a visible light responsive photocatalyst, hydrogen that becomes new energy can be generated using light including visible light, thereby improving the energy conversion efficiency in the photocatalyst.
  • the semiconductor constituting the nanostructure of the present invention include ZnSe, CdS, InGaN, titanium oxide (TiO 2 , SrTiO 3 etc.), tantalum oxide (Ta 2 O 5 , KTaO 3 etc.) and the like.
  • the stability of the nanostructure is improved by forming the outermost surface of the nanostructure (for example, the shell portion of the core-shell structure) from an oxide semiconductor such as titanium oxide or tantalum oxide.
  • an oxide semiconductor such as titanium oxide or tantalum oxide.
  • the surface of the nanostructure carries fine particles containing platinum, palladium, rhodium, ruthenium, iridium, cobalt, nickel, tin, chromium, iron, copper, gold, silver, etc., which promote the oxidation / reduction reaction of water. It may be a configuration. Fine particles such as Pt and NiO promote hydrogen generation, and fine particles such as CoOx and IrO 2 promote oxygen generation.
  • a nanostructure according to an embodiment of the present invention composed of two types of semiconductors will be described by exemplifying the first to sixth embodiments.
  • a semiconductor having a high valence band and conduction band energy level is referred to as a “high energy level semiconductor”, and the valence band and conduction band energy levels.
  • a material with a low level is called a “low energy level semiconductor”.
  • Examples of the “high energy level semiconductor / low energy level semiconductor” combination include “ZnSe / CdS”, “ZnSe / TiO 2 ”, “CdS / TiO 2 ”, “InGaN / TiO 2 ”, and the like.
  • FIG. 1 is a diagram schematically showing a visible light responsive photocatalyst composed of core-shell nanoparticles according to the first embodiment.
  • the nanoparticles 10 include a core 11 and a shell 12 that covers the core 11.
  • the core 11 is made of a low energy level semiconductor
  • the shell 12 is made of a high energy level semiconductor.
  • the diameter of the nanoparticle 10 is 100 nm or less, for example.
  • FIG. 2 is a diagram schematically showing (a) a band structure of a low energy level semiconductor, (b) a band structure of a high energy level semiconductor, and (c) a band structure of the nanoparticles 10.
  • “H 2 / H 2 O” represents the reduction potential of water
  • “O 2 / H 2 O” represents the oxidation potential of water
  • “Eg” represents the band gap.
  • the nanoparticle 10 has a band structure capable of confining electrons.
  • the nanoparticle 10 of the present embodiment has a type II band structure.
  • holes (h + ) and electrons (e ⁇ ) are generated.
  • the holes (h + ) are located in the valence band of the high energy level semiconductor, that is, in the shell 12, and the electrons (e ⁇ ) are in the conduction band of the low energy level semiconductor, that is, in the core 11. Trapped in.
  • Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e ⁇ ) are used for water reduction reaction to generate hydrogen (H 2 ).
  • the generated holes (h + ) and electrons (e ⁇ ) are located in different semiconductors and are spatially separated, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved. Further, since holes (h + ) and electrons (e ⁇ ) are generated in the nanoparticles 10, the generated holes (h + ) and electrons (e ⁇ ) and nanoparticles that serve as the active surface of the photocatalyst. Therefore, the holes (h + ) and electrons (e ⁇ ) can be efficiently used for the oxidation or reduction reaction of water.
  • the band gap resulting from the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor is reduced.
  • a visible light responsive photocatalyst can be configured by appropriately selecting two types of semiconductors.
  • FIG. 3 is a diagram schematically showing a visible light responsive photocatalyst composed of core-shell nanoparticles according to the second embodiment.
  • the nanoparticles 20 include a core 21 and a shell 22 that covers the core 21.
  • the core 21 is made of a high energy level semiconductor
  • the shell 22 is made of a low energy level semiconductor.
  • the diameter of the nanoparticle 20 is, for example, 100 nm or less.
  • FIG. 4 is a diagram schematically showing (a) a band structure of a high energy level semiconductor, (b) a band structure of a low energy level semiconductor, and (c) a band structure of the nanoparticles 20.
  • “H 2 / H 2 O” represents the reduction potential of water
  • “O 2 / H 2 O” represents the oxidation potential of water
  • “Eg” represents the band gap.
  • the nanoparticle 20 has a band structure capable of confining holes.
  • the nanoparticle 20 of the present embodiment has a type II band structure.
  • the nanoparticle 20 absorbs light having energy equal to or higher than the energy corresponding to the band gap, holes (h + ) and electrons (e ⁇ ) are generated.
  • Holes (h + ) are confined in the valence band of the high energy level semiconductor, ie, in the core 21, and electrons (e ⁇ ) are in the conduction band of the low energy level semiconductor, ie, in the shell 22.
  • Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 )
  • electrons (e ⁇ ) are used for water reduction reaction to generate hydrogen (H 2 ).
  • the generated holes (h + ) and electrons (e ⁇ ) are located in different semiconductors and are spatially separated, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved.
  • the generated holes (h + ) and electrons (e ⁇ ) are generated in the nanoparticles 20, the generated holes (h + ) and electrons (e ⁇ ) and nanoparticles that become the active surface of the photocatalyst. Therefore, the holes (h + ) and the electrons (e ⁇ ) can be efficiently used for the water oxidation or reduction reaction.
  • the band gap caused by the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor is reduced.
  • a visible light responsive photocatalyst can be configured by appropriately selecting two types of semiconductors.
  • FIG. 5 is a diagram schematically showing a visible light responsive photocatalyst composed of nanowires according to the third embodiment.
  • the nanowire 30 is provided on a substrate 33 and includes a core 31 and a shell 32 covering the core 31.
  • the diameter of the nanowire 30 is, for example, 100 nm or less.
  • one of the core 31 and the shell 32 is formed of a low energy level semiconductor, and the other is formed of a high energy level semiconductor.
  • the core 31 is formed of a low energy level semiconductor and the shell 32 is formed of a high energy level semiconductor material
  • the relationship between the band structure of each semiconductor and the band structure of the nanowire 30 is shown in FIG. This is the same as the nanoparticle 10 of the embodiment. That is, the nanowire 30 has a band structure that can confine electrons.
  • the core 31 is formed of a high energy level semiconductor and the shell 32 is formed of a low energy level semiconductor
  • the relationship between the band structure of each semiconductor and the band structure of the nanowire 30 is as shown in FIG. This is the same as the nanowire 30 of the embodiment. That is, the nanowire 30 has a band structure that can confine holes.
  • the quantum level near the upper end of the valence band of the high energy level semiconductor and the lower end of the conduction band of the low energy level semiconductor in the nanowire 40 When light having energy equal to or higher than the energy corresponding to the band gap due to the transition of the nearby quantum level is absorbed, holes (h + ) and electrons (e ⁇ ) are generated. Holes (h + ) are confined within the valence band of the high energy level semiconductor, and electrons (e ⁇ ) are confined within the conduction band of the low energy level semiconductor material. Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e ⁇ ) are used for water reduction reaction to generate hydrogen (H 2 ).
  • the generated holes (h + ) and electrons (e ⁇ ) are confined and spatially separated in different semiconductors, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved. Further, since holes (h + ) and electrons (e ⁇ ) are generated in the nanowire 30, the generated holes (h + ) and electrons (e ⁇ ) and the surface of the nanowire 30 that becomes the active surface of the photocatalyst Therefore, holes (h + ) and electrons (e ⁇ ) can be efficiently used for water oxidation reaction or reduction reaction.
  • the band gap caused by the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor is reduced.
  • a visible light responsive photocatalyst can be configured by appropriately selecting the type of semiconductor material.
  • FIG. 6 is a diagram schematically showing a visible light responsive photocatalyst comprising the thin film nanostructure of the fourth embodiment.
  • the thin film nanostructure 40 is provided on a substrate 43 and has a configuration in which a first layer 41 and a second layer 42 are repeatedly laminated.
  • the thicknesses of the first layer 41 and the second layer 42 are each 100 nm or less, for example.
  • one of the first layer 41 and the second layer 42 is formed of a low energy level semiconductor, and the other is formed of a high energy level semiconductor.
  • the thin-film nanostructure 40 has a type II band structure, and a transition between a quantum level near the upper end of the valence band of the high energy level semiconductor and a quantum level near the lower end of the conduction band of the low energy level semiconductor.
  • holes (h + ) and electrons (e ⁇ ) are generated.
  • the hole (h + ) is located in the valence band of the high energy level semiconductor, and the electron (e ⁇ ) is located in the conduction band of the low energy level semiconductor.
  • Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e ⁇ ) are used for water reduction reaction to generate hydrogen (H 2 ).
  • the generated holes (h + ) and electrons (e ⁇ ) are located in different semiconductors and are spatially separated, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved.
  • the band gap due to the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor becomes small.
  • a visible light responsive photocatalyst can be configured by appropriately selecting two types of semiconductors.
  • FIG. 7 is a diagram schematically showing a visible light responsive photocatalyst composed of nanoparticles according to the fifth embodiment.
  • the nanoparticles 50 are composed of a core 51 and a partial shell 52 that partially covers the core 51.
  • the core 51 is formed from a high energy level semiconductor
  • the partial shell 52 is formed from a low energy level semiconductor.
  • the nanoparticle 20 of the second embodiment is such that the partial shell 52 is formed so as to partially cover the core 51 and is not formed so as to cover the entire core 51 like the shell 22.
  • the relationship between the band structure of each semiconductor and the band structure of the nanoparticles 50 is the same as in the second embodiment shown in FIG.
  • the nanoparticle 50 of the present embodiment a part of the core 51 is exposed on the surface of the nanoparticle 50, so that the distance between the hole (h + ) confined in the core 51 and the catalytically active surface is closer.
  • the generated holes (h + ) can be efficiently used by the oxidation reaction of water.
  • the nanoparticle 10 of the first embodiment can also be configured so that the shell covers a part of the core instead of the shell covering the entire core, as in the present embodiment. According to such a configuration, the distance between the electron (e ⁇ ) confined in the core and the catalytically active surface is closer, and the generated electron (e ⁇ ) is efficiently used by the reduction reaction of water. It becomes possible to do.
  • FIG. 8 is a diagram schematically showing a visible light responsive photocatalyst composed of nanowires according to the sixth embodiment.
  • the nanowire 60 is provided on a substrate 63 and includes a core 61 and a shell 62 covering the core 61.
  • One of the core 61 and the shell 62 is formed of a low energy level semiconductor, and the other is formed of a high energy level semiconductor.
  • the nanowire 30 is different from the nanowire 30 of the third embodiment only in that the shell 62 is formed so as to partially cover the core 61 and is not formed so as to cover the entire core 61. Since the relationship between the band structure of each semiconductor and the band structure of the nanowire 60 is the same as in the third embodiment, description thereof is omitted.
  • the distance between the carrier confined in the core 61 and the catalytically active surface is closer, and the generated carrier is It can be efficiently used by the oxidation reaction or reduction reaction of water.
  • Example 1 (Constitution) In Example 1, the nanoparticles of the first embodiment were produced. In the nanoparticles of Example 1, CdS was used as a low energy level semiconductor constituting the core 11, and ZnSe was used as a high energy level semiconductor constituting the shell 12.
  • CdS nanoparticle dispersion solution was obtained by reacting cadmium oxide (0.2 mmol) and sulfur (0.2 mmol) at 250-300 ° C. for 1 hour in a 10 mL mixture of oleylamine / octadecene.
  • a trioctylphosphine solution (precursor solution) containing 0.5 mol / L of diethyl zinc and selenium was prepared at room temperature, and 2 mL of this solution was added to the CdS nanoparticle dispersion solution. After 1 hour, 1 mL of the precursor solution was further added to the reaction solution, and 1.5 mL of the precursor solution was added after 2.5 hours and 4 hours, respectively. After the precursor solution was added for the last time, the mixture was reacted for 1 hour and then cooled to room temperature to obtain a nanoparticle dispersion solution of Example 1.
  • CdS is easily photodegraded in an aqueous solution
  • CdS is used as the material of the core 11 and this is covered with the shell 12, so that the photodecomposition of CdS is suppressed and harmful Cd is eluted. Can be suppressed.
  • a visible light responsive photocatalyst of about 2.2 eV (563 nm) can be obtained based on the band structure of CdS and ZnSe.
  • the maximum theoretical efficiency in the solar energy conversion efficiency is about 14%.
  • the said efficiency was computed using Air Mass0 (AM0) which is a solar spectrum outside the atmosphere as the maximum theoretical efficiency.
  • Example 2 (Constitution) In Example 2, the nanoparticles of the first embodiment were produced. In the nanoparticles of Example 2, TiO 2 was used as the low energy level semiconductor constituting the core 11, and ZnSe was used as the high energy level semiconductor constituting the shell 12.
  • Titanyl sulfate (40 mmol) was dissolved in 250 ml of an alcohol (methanol, ethanol, npropanol) -water mixture at room temperature. The mixture was heated under reflux for 2 hours with stirring to conduct hydrolysis. The product was collected by centrifugation, washed with methanol, and dried under vacuum to obtain nano-sized titanium oxide.
  • an alcohol methanol, ethanol, npropanol
  • the titanium oxide (5-20 mg) was dissolved in a mixture of trioctylphosphine (hereinafter TOP) (2 g) and trioctylphosphine oxide (hereinafter TOPO) (2 g) at 60 ° C. under an argon atmosphere.
  • TOP trioctylphosphine
  • TOPO trioctylphosphine oxide
  • This nanoparticle solution was heated to 260 ° C., a zinc selenide precursor solution (described later) was dropped and reacted for a while, and then a toluene solution was added. Precipitation with methanol gave the nanoparticles of Example 2.
  • the zinc selenide precursor solution was obtained by mixing TOPSe with a concentration of 1.2 mg / mL and an equimolar 2 mol / L dimethylzinc toluene solution to obtain a zinc selenide precursor solution.
  • a visible light responsive photocatalyst of about 1.8 eV (689 nm) can be obtained based on the band structure of TiO 2 and ZnSe.
  • the maximum theoretical efficiency in the solar energy conversion efficiency is about 24% (calculated by AM0).
  • Example 3 (Constitution) In Example 3, the nanoparticles of the second embodiment were produced. In the nanoparticles of Example 3, ZnSe was used as the high energy level semiconductor constituting the core 21, and TiO 2 was used as the low energy level semiconductor constituting the shell 22.
  • Example 3 Methanol was added to the ZnSe nanoparticle solution for precipitation, and re-dispersion in hexane was repeated three times for washing. Titanium tetraisopropoxide (3 mmol) was added thereto and heated at about 80 ° C. for 1 hour to obtain a ZnSe colloid solution surface-protected with titanium tetraisopropoxide. This was hydrolyzed to obtain a nanoparticle dispersion solution of Example 3 in which a core made of ZnSe was coated with a TiO 2 layer.
  • a visible light responsive photocatalyst of about 1.9 eV (652 nm) can be obtained based on the band structure of TiO 2 and ZnSe.
  • the maximum theoretical efficiency in the solar energy conversion efficiency is about 21% (calculated by AM0).
  • Example 4 (Constitution) In Example 4, the nanoparticles of the second embodiment were produced. In the nanoparticles of Example 4, CdS was used as the high energy level semiconductor material constituting the core 21, and TiO 2 was used as the low energy level semiconductor material constituting the shell 22.
  • a visible light responsive photocatalyst of about 2.2 eV (563 nm) can be obtained based on the band structure of TiO 2 and CdS.
  • the maximum theoretical efficiency in the solar energy conversion efficiency is about 14% (calculated by AM0).
  • Example 5 (Constitution) In Example 5, the nanoparticles of the second embodiment were produced. In the nanoparticles of Example 5, InGaN was used as the high energy level semiconductor constituting the core 21, and TiO 2 was used as the low energy level semiconductor constituting the shell 22.
  • a visible light responsive photocatalyst of about 2.7 eV (459 nm) can be obtained based on the band structure of InGaN and TiO 2 .
  • the maximum theoretical efficiency in the solar energy conversion efficiency is about 6% (calculated by AM0).
  • Example 6 the nanowire of the third embodiment was produced.
  • InGaN was used as the high energy level semiconductor constituting the core 31, and TiO 2 was used as the low energy level semiconductor constituting the shell 32.
  • a SiO 2 film having a thickness of about 10 nm was formed on the substrate, and a circular pattern was formed by electron beam drawing and oxide film etching.
  • An InGaN nanowire of the core 31 portion was produced by a selective growth method of MOCVD (metal organic chemical vapor deposition). Thereafter, the shell 32 was grown by a solution method, and a nanowire having a core-shell structure was obtained.
  • MOCVD metal organic chemical vapor deposition
  • Example 7 the thin film nanostructure of the fourth embodiment was produced.
  • ZnSe and CdS were used as the first layer and the second layer.
  • Thin film nanostructures were obtained by repeatedly growing the first and second layers on the substrate by MOCVD. As shown in FIG. 6, the removal of carriers from the first layer can be enhanced by removing other than the initial growth layer of the first layer by etching or the like. Further, a buffer layer or the like may be formed on the substrate.
  • Example 8 the nanoparticles of the fifth embodiment were produced.
  • the nanoparticles are composed of a core 51 and a partial shell 52 that partially covers the core 51.
  • the production method was obtained by adjusting the growth conditions (growth time, growth temperature) during shell growth in the production methods shown in Examples 1 to 5. For example, by shortening the growth time, the growth can be interrupted before the shell material completely covers the core material, and nanoparticles composed of the core 51 and the partial shell 52 partially covering the core 51 are obtained. It was.
  • Example 9 the nanowire of the sixth embodiment was produced.
  • the nanowire 60 is provided on the substrate 63 and includes a core 61 and a shell 62 covering the core 61.
  • the manufacturing method was obtained by adjusting the growth conditions (growth time, growth temperature) during shell growth in the manufacturing method shown in Example 6. For example, by shortening the growth time, the growth can be interrupted before the shell material completely covers the core material, and a nanowire composed of the core 61 and the partial shell 62 that partially covers the core 61 is obtained. .
  • the visible light responsive photocatalyst obtained by the present invention can expand the usable range as compared with a conventional ultraviolet light responsive photocatalyst. Not only outdoors, but preferably indoor use is expected. In room light, ultraviolet light has decreased with the recent trend toward LED, and the characteristics of visible light response are extremely important. In addition, it can be expected to spread to environmental purification and antibacterial home appliances. In addition, the visible light responsive photocatalyst that requires a conventional two-stage light absorption process can be expected to develop into hydrogen production and solar fuel production including artificial photosynthesis, which could not be realized due to low efficiency.

Abstract

This photocatalyst is obtained from a nanostructure configured from multiple semiconductors. The nanostructure has a Type II band structure. For each semiconductor, the lower edge of the conduction band is more negative than the reduction potential of water and the upper edge of the valence band is more positive than the oxidation potential of water.

Description

可視光応答型光触媒Visible light responsive photocatalyst
　本発明は、可視光応答型光触媒に関する。 The present invention relates to a visible light responsive photocatalyst.
　光触媒市場および水素市場は今後も拡大成長が見込まれており、水分解に関わる光触媒の高活性化に関する研究開発が活発である。現行の紫外光応答型光触媒の場合、太陽光に占める紫外光の割合が２％と著しく少ないため高活性化に限界があり、可視光応答型光触媒の開発が不可欠である。主な解決手段としては、低バンドギャップ化に向けた新規材料開発、二段階光励起機構を利用したＺスキーム型光触媒（水素生成用と酸素生成用の２種類の光触媒を用い、両者間の電子伝達を可逆的なレドックスを用いて行う）の開発がある。 The photocatalyst market and the hydrogen market are expected to continue to grow in the future, and research and development related to high activation of photocatalysts related to water splitting is active. In the case of the current ultraviolet light-responsive photocatalyst, the proportion of ultraviolet light in sunlight is remarkably small at 2%, so there is a limit to high activation, and development of a visible light-responsive photocatalyst is indispensable. The main solutions are the development of new materials for lowering the band gap, the Z-scheme type photocatalyst using a two-step photoexcitation mechanism (two types of photocatalysts for hydrogen generation and oxygen generation, and electron transfer between them) Is developed using reversible redox).
　低バンドギャップ化に向けた新規材料として、特開２００７－１７５６５９号公報（特許文献１）には、新規のタンタル系酸窒化物光触媒が記載されている。また、二段階光励起機構を利用したＺスキーム型光触媒として、特開２００５－１９９１８７号公報（特許文献２）には、Ｆｅ３＋／Ｆｅ２＋レドックスを用いたＺスキーム型可視光水完全分解型触媒系が記載されている。 Japanese Unexamined Patent Publication No. 2007-175659 (Patent Document 1) describes a novel tantalum oxynitride photocatalyst as a new material for reducing the band gap. Further, as a Z scheme type photocatalyst utilizing a two-step photoexcitation mechanism, JP 2005-199187 A (Patent Document 2) discloses a Z scheme type visible light water complete decomposition type catalyst system using Fe 3+ / Fe 2+ redox. Is described.
特開２００７－１７５６５９号公報JP 2007-175659 A 特開２００５－１９９１８７号公報JP-A-2005-199187
　しかしながら、特許文献１に記載のタンタル系酸窒化物光触媒は、窒素由来の欠陥が発生しやすく、生成されたキャリアは容易に再結合してしまい高効率化の実現が難しい。また、Ｚスキーム型光触媒においては、原理的に二段階の光吸収プロセスを利用するため、活性に関わるキャリア生成に２光子が必要となり、１光子で活性に関わるキャリアを生成する一段階光吸収プロセスの光触媒に比べて、高効率化に制限が生じてしまう。 However, in the tantalum oxynitride photocatalyst described in Patent Document 1, defects derived from nitrogen tend to occur, and the generated carriers easily recombine, making it difficult to achieve high efficiency. In addition, since the Z-scheme type photocatalyst uses a two-step light absorption process in principle, two photons are required to generate carriers related to activity, and a one-step light absorption process that generates carriers related to activity with one photon. Compared with the photocatalyst, there is a limit to increase in efficiency.
　本発明は、一段階光吸収プロセスの光触媒であって、高効率化の実現が可能な新規の可視光応答型光触媒を提供することを目的とする。 An object of the present invention is to provide a novel visible light responsive photocatalyst that is a photocatalyst of a one-step light absorption process and that can achieve high efficiency.
　本発明の光触媒は、複数の半導体で構成されるナノ構造体からなり、ナノ構造体は、タイプIIのバンド構造を有し、各半導体は、伝導帯の下端が水の還元電位より負であり、価電子帯の上端が水の酸化電位よりも正である。 The photocatalyst of the present invention comprises a nanostructure composed of a plurality of semiconductors, the nanostructure has a type II band structure, and each semiconductor has a lower end of the conduction band that is more negative than the reduction potential of water. The upper end of the valence band is more positive than the oxidation potential of water.
　上記ナノ構造体は、例えば、電子の閉じ込めが可能なバンド構造を有するもの、または正孔の閉じ込めが可能なバンド構造を有するものである。 The nanostructure is, for example, one having a band structure capable of confining electrons or one having a band structure capable of confining holes.
　上記ナノ構造体は、量子準位に対応した光吸収特性を有する。光吸収特性において光吸収端は４４０ｎｍ以上であることが好ましい。 The nanostructure has a light absorption characteristic corresponding to the quantum level. In the light absorption characteristics, the light absorption edge is preferably 440 nm or more.
　上記ナノ構造体の好ましい態様としては、コアシェル構造を有するナノ粒子が挙げられる。このようなコアシェル構造のナノ粒子においては、コア部分とシェル部分とが異なる半導体から形成される。好ましい態様として、シェル部分を酸化物半導体から形成する。 Favorable embodiments of the nanostructure include nanoparticles having a core-shell structure. In such core-shell structured nanoparticles, the core portion and the shell portion are formed of different semiconductors. In a preferred embodiment, the shell portion is formed from an oxide semiconductor.
　また、本発明の可視光応答型光触媒の好ましい態様として、ナノ構造体に、白金、パラジウム、ロジウム、ルテニウム、イリジウム、コバルト、ニッケル、スズ、クロム、鉄、銅、金及び銀からなる群より選択される少なくとも一つの元素を含む微粒子が担持されたものが挙げられる。 Further, as a preferred embodiment of the visible light responsive photocatalyst of the present invention, the nanostructure is selected from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, cobalt, nickel, tin, chromium, iron, copper, gold and silver. And those carrying fine particles containing at least one element.
　本発明によると、高効率な可視光応答型光触媒を提供することができる。 According to the present invention, a highly efficient visible light responsive photocatalyst can be provided.
第１の実施形態のコアシェル型ナノ粒子からなる可視光応答型光触媒を模式的に示す図である。It is a figure which shows typically the visible light responsive photocatalyst which consists of a core-shell type nanoparticle of 1st Embodiment. （ａ）低エネルギー準位半導体のバンド構造と、（ｂ）高エネルギー準位半導体のバンド構造と、（ｃ）ナノ粒子のバンド構造を模式的に示す図である。It is a figure which shows typically the band structure of a (a) low energy level semiconductor, (b) the band structure of a high energy level semiconductor, and (c) the band structure of a nanoparticle. 第２の実施形態のコアシェル型ナノ粒子からなる可視光応答型光触媒を模式的に示す図である。It is a figure which shows typically the visible light responsive photocatalyst which consists of a core-shell type nanoparticle of 2nd Embodiment. （ａ）高エネルギー準位半導体のバンド構造と、（ｂ）低エネルギー準位半導体のバンド構造と、（ｃ）ナノ粒子のバンド構造を模式的に示す図である。It is a figure which shows typically the band structure of (a) a high energy level semiconductor, (b) the band structure of a low energy level semiconductor, and (c) the band structure of a nanoparticle. 第３の実施形態のナノワイヤからなる可視光応答型光触媒を模式的に示す図である。It is a figure which shows typically the visible light responsive photocatalyst which consists of nanowire of 3rd Embodiment. 第４の実施形態の薄膜ナノ構造体からなる可視光応答型光触媒を模式的に示す図である。It is a figure which shows typically the visible light responsive photocatalyst which consists of a thin film nanostructure of 4th Embodiment. 第５の実施形態のナノ粒子からなる可視光応答型光触媒を模式的に示す図である。It is a figure which shows typically the visible light responsive photocatalyst which consists of the nanoparticle of 5th Embodiment. 第６の実施形態のナノワイヤからなる可視光応答型光触媒を模式的に示す図である。It is a figure which shows typically the visible light responsive photocatalyst consisting of the nanowire of 6th Embodiment.
　本発明の可視光応答型光触媒は、複数の半導体で構成されるナノ構造体からなり、ナノ構造体は、タイプIIのバンド構造を有し、各半導体は、伝導帯の下端が水の還元電位より負であり、価電子帯の上端が水の酸化電位よりも正である。 The visible light responsive photocatalyst of the present invention comprises a nanostructure composed of a plurality of semiconductors, the nanostructure has a type II band structure, and each semiconductor has a reduction potential of water at the lower end of the conduction band. It is more negative and the upper end of the valence band is more positive than the oxidation potential of water.
　タイプIIのバンド構造とは、異なる複数の半導体材料が接した構造体において、バンド不連続量が価電子帯と伝導帯でその符号を異にするバンド構造である。その結果、電子と正孔が、それぞれ異なる材料側で閉じ込められ、空間的に分離される。タイプIIのバンド構造であり、量子閉じ込め効果が起きるサイズかつ構造である場合、光吸収端（以下、「バンドギャップ」ともいう）は、複数の半導体の内、価電子帯のエネルギーが最も高い第１の半導体の価電子帯の上端近傍に位置する基底量子準位と、伝導帯のエネルギーが最も低い第２の半導体の伝導帯の下端近傍に位置する基底量子準位との間の遷移に起因する。 The type II band structure is a band structure in which a plurality of different semiconductor materials are in contact with each other, and the band discontinuity amounts have different signs in the valence band and the conduction band. As a result, electrons and holes are confined on different material sides and spatially separated. In the case of a type II band structure that is sized and structured to produce a quantum confinement effect, the light absorption edge (hereinafter also referred to as “band gap”) has the highest valence band energy among a plurality of semiconductors. Due to the transition between the ground quantum level located near the top of the valence band of one semiconductor and the ground quantum level located near the bottom of the conduction band of the second semiconductor having the lowest conduction band energy To do.
　タイプIIのバンド構造によると、第１の半導体の価電子帯のエネルギーが第２の半導体の価電子帯のエネルギーより高く、かつ第２の半導体の伝導帯のエネルギーが第１の半導体の伝導帯のエネルギーより低く、量子閉じ込め効果が起きるサイズかつ構造とすることで、量子準位間の遷移が起きるため、第１の半導体および第２の半導体で構成されるナノ構造帯の基底量子準位間の遷移に起因するバンドギャップが、第１の半導体および第２の半導体のバルクのバンドギャップよりも長波長側にシフトし、受光感度の長波長化を実現することが可能となる。そして、第１の半導体の価電子帯と、第２の半導体の伝導帯との間のバンドギャップが、可視光のエネルギーよりも小さい値となるように半導体材料を選択することにより、可視光応答型光触媒を実現することができる。本発明のナノ構造体においては、基底量子準位間の遷移に起因するバンドギャップが、４４０ｎｍ以上の波長のエネルギーよりも小さいことが好ましい。 According to the type II band structure, the energy of the valence band of the first semiconductor is higher than the energy of the valence band of the second semiconductor, and the energy of the conduction band of the second semiconductor is the conduction band of the first semiconductor. Since the transition between the quantum levels occurs by setting the size and structure to be lower than the energy of the quantum and causing the quantum confinement effect, between the ground quantum levels of the nanostructure band composed of the first semiconductor and the second semiconductor The band gap resulting from this transition is shifted to a longer wavelength side than the bulk band gaps of the first semiconductor and the second semiconductor, and it is possible to realize a longer wavelength of light receiving sensitivity. Then, by selecting a semiconductor material such that the band gap between the valence band of the first semiconductor and the conduction band of the second semiconductor is smaller than the energy of visible light, the visible light response Type photocatalyst can be realized. In the nanostructure of the present invention, the band gap resulting from the transition between the ground quantum levels is preferably smaller than the energy having a wavelength of 440 nm or more.
　本発明の可視光応答型光触媒が、ナノ構造体からなることにより、光吸収で生成されたキャリアの量子閉じ込めが可能となり、生成されたキャリアを触媒反応に効率的に利用することが可能となる。複数の半導体で構成されるナノ構造体としては、キャリアの量子閉じ込めが可能な構造体であれば限定されることはなく、コアシェル型のナノ粒子、ナノワイヤ、薄膜ナノ構造体（量子井戸超格子、量子ドット超格子）等が挙げられる。ナノ構造体は、電子閉じ込めが可能なもの、正孔閉じ込めが可能なもの、いずれであっても構わない。また、ナノ構造体の大きさを適宜調整することにより、量子効果を利用した量子準位を制御することができ、所望の酸化還元反応の酸化還元電位に応じた制御（反応の過電圧制御）が可能となり、所望の酸化還元反応に有用な光触媒を提供することができる。 When the visible light responsive photocatalyst of the present invention is made of a nanostructure, quantum confinement of carriers generated by light absorption becomes possible, and the generated carriers can be efficiently used for catalytic reaction. . The nanostructure composed of a plurality of semiconductors is not limited as long as it is a structure capable of quantum confinement of carriers. Core-shell type nanoparticles, nanowires, thin film nanostructures (quantum well superlattices, Quantum dot superlattice) and the like. The nanostructure may be either one capable of electron confinement or one capable of hole confinement. In addition, by appropriately adjusting the size of the nanostructure, it is possible to control the quantum level using the quantum effect, and the control (reaction overvoltage control) according to the redox potential of the desired redox reaction. It becomes possible to provide a photocatalyst useful for a desired oxidation-reduction reaction.
　タイプIIのバンド構造によると、光吸収により生成された電子が位置する伝導帯を有する半導体は第２の半導体であり、生成された正孔が位置する価電子帯を有する半導体は第１の半導体であり、生成された電子と正孔とが異なる半導体間に分離されキャリア寿命が長くなる。したがって、本発明のナノ構造体によると、光触媒活性を高めることができる。 According to the type II band structure, the semiconductor having the conduction band in which the electrons generated by light absorption are located is the second semiconductor, and the semiconductor having the valence band in which the generated holes are located is the first semiconductor. The generated electrons and holes are separated between different semiconductors, and the carrier lifetime is increased. Therefore, according to the nanostructure of the present invention, the photocatalytic activity can be enhanced.
　本発明のナノ構造体においては、タイプIIのバンド構造となる複数の半導体の組み合わせであって、各半導体については、伝導帯の下端が水の還元電位より負であり、価電子帯の上端が水の酸化電位よりも正であるものであれば特に制限されない。このような関係を満たす半導体を用いることにより、光吸収で生成された伝導帯中の電子が水の還元に利用されて水素が生成され、光吸収で生成された価電子帯中の正孔が水の酸化に利用されて酸素が生成される。すなわち、水の分解に有用な光触媒を構成することができる。また、本発明のナノ構造体は可視光応答型光触媒であることにより、可視光を含む光を用いて新たなエネルギーとなる水素を生成することができるので、光触媒におけるエネルギー変換効率を向上させることができる。本発明のナノ構造体を構成する半導体としては、ＺｎＳｅ、ＣｄＳ、ＩｎＧａＮ、チタン酸化物（ＴｉＯ２、ＳｒＴｉＯ３等）、タンタル酸化物（Ｔａ２Ｏ５、ＫＴａＯ３等）等が挙げられる。なお、ナノ構造体の最表面（例えば、コアシェル構造のシェル部分）が、チタン酸化物またはタンタル酸化物等の酸化物半導体から形成されている構成とすることにより、ナノ構造体の安定性が向上し、水分解用として水溶液中で用いられる場合に半導体材料の水溶液への溶出を抑制することができ、光触媒の耐久性を向上させることができる。 The nanostructure of the present invention is a combination of a plurality of semiconductors having a type II band structure. For each semiconductor, the lower end of the conduction band is more negative than the reduction potential of water, and the upper end of the valence band is There is no particular limitation as long as it is more positive than the oxidation potential of water. By using a semiconductor that satisfies such a relationship, electrons in the conduction band generated by light absorption are used for water reduction to generate hydrogen, and holes in the valence band generated by light absorption are generated. Used to oxidize water to produce oxygen. That is, a photocatalyst useful for water decomposition can be constituted. In addition, since the nanostructure of the present invention is a visible light responsive photocatalyst, hydrogen that becomes new energy can be generated using light including visible light, thereby improving the energy conversion efficiency in the photocatalyst. Can do. Examples of the semiconductor constituting the nanostructure of the present invention include ZnSe, CdS, InGaN, titanium oxide (TiO 2 , SrTiO 3 etc.), tantalum oxide (Ta 2 O 5 , KTaO 3 etc.) and the like. Note that the stability of the nanostructure is improved by forming the outermost surface of the nanostructure (for example, the shell portion of the core-shell structure) from an oxide semiconductor such as titanium oxide or tantalum oxide. However, when used in an aqueous solution for water splitting, elution of the semiconductor material into the aqueous solution can be suppressed, and the durability of the photocatalyst can be improved.
　ナノ構造体の表面には、水の酸化・還元反応を促進する、白金、パラジウム、ロジウム、ルテニウム、イリジウム、コバルト、ニッケル、スズ、クロム、鉄、銅、金、銀等を含む微粒子が担持されている構成であってもよい。Ｐｔ、ＮｉＯ等の微粒子は水素発生を促し、ＣｏＯｘ、ＩｒＯ２等の微粒子は酸素発生を促す。 The surface of the nanostructure carries fine particles containing platinum, palladium, rhodium, ruthenium, iridium, cobalt, nickel, tin, chromium, iron, copper, gold, silver, etc., which promote the oxidation / reduction reaction of water. It may be a configuration. Fine particles such as Pt and NiO promote hydrogen generation, and fine particles such as CoOx and IrO 2 promote oxygen generation.
　以下においては、二種類の半導体で構成される本発明の実施形態に係るナノ構造体について、第１～第６の実施形態を例示して説明する。以下の説明において、二種類の半導体の内、これらの比較において、価電子帯及び伝導帯のエネルギー準位が高い半導体を「高エネルギー準位半導体」と言い、価電子帯及び伝導帯のエネルギー準位が低い材料を「低エネルギー準位半導体」と言う。「高エネルギー準位半導体／低エネルギー準位半導体」組み合わせとしては、「ＺｎＳｅ／ＣｄＳ」、「ＺｎＳｅ／ＴｉＯ２」、「ＣｄＳ／ＴｉＯ２」、「ＩｎＧａＮ／ＴｉＯ２」等が例示される。 In the following, a nanostructure according to an embodiment of the present invention composed of two types of semiconductors will be described by exemplifying the first to sixth embodiments. In the following description, among these two types of semiconductors, in these comparisons, a semiconductor having a high valence band and conduction band energy level is referred to as a “high energy level semiconductor”, and the valence band and conduction band energy levels. A material with a low level is called a “low energy level semiconductor”. Examples of the “high energy level semiconductor / low energy level semiconductor” combination include “ZnSe / CdS”, “ZnSe / TiO 2 ”, “CdS / TiO 2 ”, “InGaN / TiO 2 ”, and the like.
　［第１の実施形態］
　図１は、第１の実施形態のコアシェル型ナノ粒子からなる可視光応答型光触媒を模式的に示す図である。図１に示すように、ナノ粒子１０はコア１１とコア１１を被覆するシェル１２とから構成される。コア１１は低エネルギー準位半導体で形成され、シェル１２は高エネルギー準位半導体から形成されている。ナノ粒子１０の直径は、例えば、１００ｎｍ以下である。 [First Embodiment]
FIG. 1 is a diagram schematically showing a visible light responsive photocatalyst composed of core-shell nanoparticles according to the first embodiment. As shown in FIG. 1, the nanoparticles 10 include a core 11 and a shell 12 that covers the core 11. The core 11 is made of a low energy level semiconductor, and the shell 12 is made of a high energy level semiconductor. The diameter of the nanoparticle 10 is 100 nm or less, for example.
　図２は、（ａ）低エネルギー準位半導体のバンド構造と、（ｂ）高エネルギー準位半導体のバンド構造と、（ｃ）ナノ粒子１０のバンド構造を模式的に示す図である。図２において、「Ｈ２／Ｈ２Ｏ」は水の還元電位を表し、「Ｏ２／Ｈ２Ｏ」は水の酸化電位を表し、「Ｅｇ」はバンドギャップを表す。ナノ粒子１０は、電子の閉じ込めが可能なバンド構造を有する。 FIG. 2 is a diagram schematically showing (a) a band structure of a low energy level semiconductor, (b) a band structure of a high energy level semiconductor, and (c) a band structure of the nanoparticles 10. In FIG. 2, “H 2 / H 2 O” represents the reduction potential of water, “O 2 / H 2 O” represents the oxidation potential of water, and “Eg” represents the band gap. The nanoparticle 10 has a band structure capable of confining electrons.
　図２（ｃ）に示されるように、本実施形態のナノ粒子１０はタイプIIのバンド構造を有する。ナノ粒子１０において、バンドギャップに対応するエネルギー以上のエネルギーを有する光が吸収されると、正孔（ｈ＋）と電子（ｅ－）とが生成される。正孔（ｈ＋）は、高エネルギー準位半導体の価電子帯内に、すなわちシェル１２内に位置し、電子（ｅ－）は、低エネルギー準位半導体の伝導帯内に、すなわちコア１１内に閉じ込められる。そして、正孔（ｈ＋）は水の酸化反応に用いられて酸素（Ｏ２）を発生させ、電子（ｅ－）は水の還元反応に用いられて水素（Ｈ２）を発生させる。 As shown in FIG. 2C, the nanoparticle 10 of the present embodiment has a type II band structure. When light having energy equal to or higher than the energy corresponding to the band gap is absorbed in the nanoparticle 10, holes (h + ) and electrons (e − ) are generated. The holes (h + ) are located in the valence band of the high energy level semiconductor, that is, in the shell 12, and the electrons (e − ) are in the conduction band of the low energy level semiconductor, that is, in the core 11. Trapped in. Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e − ) are used for water reduction reaction to generate hydrogen (H 2 ).
　ナノ粒子１０において、生成した正孔（ｈ＋）と電子（ｅ－）とは、異なる半導体内に位置し空間的に分離されるのでキャリアの再結合が抑制される。したがって、キャリア寿命が増大し、光触媒におけるエネルギー変換効率を向上させることができる。また、ナノ粒子１０内に正孔（ｈ＋）と電子（ｅ－）とが生成されるので、生成した正孔（ｈ＋）及び電子（ｅ－）と、光触媒の活性面となるナノ粒子１０表面との距離が近く、したがって正孔（ｈ＋）及び電子（ｅ－）を効率よく水の酸化反応または還元反応に利用することができる。 In the nanoparticle 10, the generated holes (h + ) and electrons (e − ) are located in different semiconductors and are spatially separated, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved. Further, since holes (h + ) and electrons (e − ) are generated in the nanoparticles 10, the generated holes (h + ) and electrons (e − ) and nanoparticles that serve as the active surface of the photocatalyst. Therefore, the holes (h + ) and electrons (e − ) can be efficiently used for the oxidation or reduction reaction of water.
　ナノ粒子１０においては、高エネルギー準位半導体の価電子帯の上端近傍の量子準位と低エネルギー準位半導体の伝導帯の下端近傍の量子準位の遷移に起因するバンドギャップが小さくなるように二種類の半導体を適宜選択することにより、可視光応答型光触媒を構成することが可能となる。 In the nanoparticle 10, the band gap resulting from the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor is reduced. A visible light responsive photocatalyst can be configured by appropriately selecting two types of semiconductors.
　［第２の実施形態］
　図３は、第２の実施形態のコアシェル型ナノ粒子からなる可視光応答型光触媒を模式的に示す図である。図３に示すように、ナノ粒子２０はコア２１とコア２１を被覆するシェル２２とから構成される。コア２１は高エネルギー準位半導体で形成され、シェル２２は低エネルギー準位半導体から形成されている。ナノ粒子２０の直径は、例えば、１００ｎｍ以下である。 [Second Embodiment]
FIG. 3 is a diagram schematically showing a visible light responsive photocatalyst composed of core-shell nanoparticles according to the second embodiment. As shown in FIG. 3, the nanoparticles 20 include a core 21 and a shell 22 that covers the core 21. The core 21 is made of a high energy level semiconductor, and the shell 22 is made of a low energy level semiconductor. The diameter of the nanoparticle 20 is, for example, 100 nm or less.
　図４は、（ａ）高エネルギー準位半導体のバンド構造と、（ｂ）低エネルギー準位半導体のバンド構造と、（ｃ）ナノ粒子２０のバンド構造を模式的に示す図である。図４において、「Ｈ２／Ｈ２Ｏ」は水の還元電位を表し、「Ｏ２／Ｈ２Ｏ」は水の酸化電位を表し、「Ｅｇ」はバンドギャップを表す。ナノ粒子２０は、正孔の閉じ込めが可能なバンド構造を有する。 FIG. 4 is a diagram schematically showing (a) a band structure of a high energy level semiconductor, (b) a band structure of a low energy level semiconductor, and (c) a band structure of the nanoparticles 20. In FIG. 4, “H 2 / H 2 O” represents the reduction potential of water, “O 2 / H 2 O” represents the oxidation potential of water, and “Eg” represents the band gap. The nanoparticle 20 has a band structure capable of confining holes.
　図４（ｃ）に示されるように、本実施形態のナノ粒子２０はタイプIIのバンド構造を有する。ナノ粒子２０において、バンドギャップに対応するエネルギー以上のエネルギーを有する光が吸収されると、正孔（ｈ＋）と電子（ｅ－）とが生成される。正孔（ｈ＋）は、高エネルギー準位半導体の価電子帯内に、すなわちコア２１内に閉じ込められ、電子（ｅ－）は、低エネルギー準位半導体の伝導帯内に、すなわちシェル２２内に位置する。そして、正孔（ｈ＋）は水の酸化反応に用いられて酸素（Ｏ２）を発生させ、電子（ｅ－）は水の還元反応に用いられて水素（Ｈ２）を発生させる。 As shown in FIG. 4C, the nanoparticle 20 of the present embodiment has a type II band structure. When the nanoparticle 20 absorbs light having energy equal to or higher than the energy corresponding to the band gap, holes (h + ) and electrons (e − ) are generated. Holes (h + ) are confined in the valence band of the high energy level semiconductor, ie, in the core 21, and electrons (e − ) are in the conduction band of the low energy level semiconductor, ie, in the shell 22. Located in. Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e − ) are used for water reduction reaction to generate hydrogen (H 2 ).
　ナノ粒子２０において、生成した正孔（ｈ＋）と電子（ｅ－）とは、異なる半導体内に位置し空間的に分離されるのでキャリアの再結合が抑制される。したがって、キャリア寿命が増大し、光触媒におけるエネルギー変換効率を向上させることができる。また、ナノ粒子２０内に正孔（ｈ＋）と電子（ｅ－）とが生成されるので、生成した正孔（ｈ＋）及び電子（ｅ－）と、光触媒の活性面となるナノ粒子２０表面との距離が近く、したがって正孔（ｈ＋）及び電子（ｅ－）を効率よく水の酸化反応または還元反応に利用することができる。 In the nanoparticle 20, the generated holes (h + ) and electrons (e − ) are located in different semiconductors and are spatially separated, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved. In addition, since holes (h + ) and electrons (e − ) are generated in the nanoparticles 20, the generated holes (h + ) and electrons (e − ) and nanoparticles that become the active surface of the photocatalyst. Therefore, the holes (h + ) and the electrons (e − ) can be efficiently used for the water oxidation or reduction reaction.
　ナノ粒子２０においては、高エネルギー準位半導体の価電子帯の上端近傍の量子準位と低エネルギー準位半導体の伝導帯の下端近傍の量子準位の遷移に起因するバンドギャップが小さくなるように二種類の半導体を適宜選択することにより、可視光応答型光触媒を構成することが可能となる。 In the nanoparticle 20, the band gap caused by the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor is reduced. A visible light responsive photocatalyst can be configured by appropriately selecting two types of semiconductors.
　［第３の実施形態］
　図５は、第３の実施形態のナノワイヤからなる可視光応答型光触媒を模式的に示す図である。図５に示すように、ナノワイヤ３０は、基板３３上に設けられ、コア３１とこれを被覆するシェル３２とから構成されている。ナノワイヤ３０の直径は、例えば、１００ｎｍ以下である。 [Third Embodiment]
FIG. 5 is a diagram schematically showing a visible light responsive photocatalyst composed of nanowires according to the third embodiment. As shown in FIG. 5, the nanowire 30 is provided on a substrate 33 and includes a core 31 and a shell 32 covering the core 31. The diameter of the nanowire 30 is, for example, 100 nm or less.
　ナノワイヤ３０は、コア３１とシェル３２の内、一方を低エネルギー準位半導体で形成し、他方を高エネルギー準位半導体で形成する。コア３１を低エネルギー準位半導体で形成し、シェル３２を高エネルギー準位半導体材料で形成した場合について、各半導体のバンド構造と、ナノワイヤ３０のバンド構造の関係は、図２に示した第１の実施形態のナノ粒子１０と同様となる。すなわち、ナノワイヤ３０は、電子の閉じ込めが可能なバンド構造を有する。また、コア３１を高エネルギー準位半導体で形成し、シェル３２を低エネルギー準位半導体で形成した場合、各半導体のバンド構造と、ナノワイヤ３０のバンド構造の関係は、図４に示した第２の実施形態のナノワイヤ３０と同様となる。すなわち、ナノワイヤ３０は、正孔の閉じ込めが可能なバンド構造を有する。 In the nanowire 30, one of the core 31 and the shell 32 is formed of a low energy level semiconductor, and the other is formed of a high energy level semiconductor. In the case where the core 31 is formed of a low energy level semiconductor and the shell 32 is formed of a high energy level semiconductor material, the relationship between the band structure of each semiconductor and the band structure of the nanowire 30 is shown in FIG. This is the same as the nanoparticle 10 of the embodiment. That is, the nanowire 30 has a band structure that can confine electrons. Further, when the core 31 is formed of a high energy level semiconductor and the shell 32 is formed of a low energy level semiconductor, the relationship between the band structure of each semiconductor and the band structure of the nanowire 30 is as shown in FIG. This is the same as the nanowire 30 of the embodiment. That is, the nanowire 30 has a band structure that can confine holes.
　図２，４いずれのバンド構造の関係をとる場合であっても、ナノワイヤ４０において、高エネルギー準位半導体の価電子帯の上端近傍の量子準位と、低エネルギー準位半導体の伝導帯の下端近傍の量子準位の遷移に起因するバンドギャップに対応するエネルギー以上のエネルギーを有する光が吸収されると、正孔（ｈ＋）と電子（ｅ－）とが生じる。正孔（ｈ＋）は、高エネルギー準位半導体の価電子帯内に閉じ込められ、電子（ｅ－）は、低エネルギー準位半導体材料の伝導帯内に閉じ込められる。そして、正孔（ｈ＋）は水の酸化反応に用いられて酸素（Ｏ２）を発生させ、電子（ｅ－）は水の還元反応に用いられて水素（Ｈ２）を発生させる。 2 and 4, in the nanowire 40, the quantum level near the upper end of the valence band of the high energy level semiconductor and the lower end of the conduction band of the low energy level semiconductor in the nanowire 40. When light having energy equal to or higher than the energy corresponding to the band gap due to the transition of the nearby quantum level is absorbed, holes (h + ) and electrons (e − ) are generated. Holes (h + ) are confined within the valence band of the high energy level semiconductor, and electrons (e − ) are confined within the conduction band of the low energy level semiconductor material. Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e − ) are used for water reduction reaction to generate hydrogen (H 2 ).
　ナノワイヤ３０において、生成した正孔（ｈ＋）と電子（ｅ－）とは、異なる半導体内に閉じ込められ空間的に分離されるのでキャリアの再結合が抑制される。したがって、キャリア寿命が増大し、光触媒におけるエネルギー変換効率を向上させることができる。また、ナノワイヤ３０内に正孔（ｈ＋）と電子（ｅ－）とが生成されるので、生成した正孔（ｈ＋）及び電子（ｅ－）と、光触媒の活性面となるナノワイヤ３０表面との距離が近く、したがって正孔（ｈ＋）及び電子（ｅ－）を効率よく水の酸化反応または還元反応に利用することができる。 In the nanowire 30, the generated holes (h + ) and electrons (e − ) are confined and spatially separated in different semiconductors, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved. Further, since holes (h + ) and electrons (e − ) are generated in the nanowire 30, the generated holes (h + ) and electrons (e − ) and the surface of the nanowire 30 that becomes the active surface of the photocatalyst Therefore, holes (h + ) and electrons (e − ) can be efficiently used for water oxidation reaction or reduction reaction.
　ナノワイヤ３０においては、高エネルギー準位半導体の価電子帯の上端近傍の量子準位と低エネルギー準位半導体の伝導帯の下端近傍の量子準位の遷移に起因するバンドギャップが小さくなるように二種類の半導体材料を適宜選択することにより、可視光応答型光触媒を構成することが可能となる。 In the nanowire 30, the band gap caused by the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor is reduced. A visible light responsive photocatalyst can be configured by appropriately selecting the type of semiconductor material.
　［第４の実施形態］
　図６は、第４の実施形態の薄膜ナノ構造体からなる可視光応答型光触媒を模式的に示す図である。図６に示すように、薄膜ナノ構造体４０は、基板上４３に設けられ、第１層４１と第２層４２とが繰り返し積層されている構成である。第１層４１と第２層４２の厚みは、それぞれ、例えば、１００ｎｍ以下の厚みである。 [Fourth Embodiment]
FIG. 6 is a diagram schematically showing a visible light responsive photocatalyst comprising the thin film nanostructure of the fourth embodiment. As shown in FIG. 6, the thin film nanostructure 40 is provided on a substrate 43 and has a configuration in which a first layer 41 and a second layer 42 are repeatedly laminated. The thicknesses of the first layer 41 and the second layer 42 are each 100 nm or less, for example.
　薄膜ナノ構造体４０は、第１層４１と第２層４２の内、一方を低エネルギー準位半導体で形成し、他方を高エネルギー準位半導体で形成する。薄膜ナノ構造体４０は、タイプIIのバンド構造を有し、高エネルギー準位半導体の価電子帯の上端近傍の量子準位と低エネルギー準位半導体の伝導帯の下端近傍の量子準位の遷移に起因するバンドギャップに対応するエネルギー以上のエネルギーを有する光が吸収されると、正孔（ｈ＋）と電子（ｅ－）とが生成される。正孔（ｈ＋）は、高エネルギー準位半導体の価電子帯内に位置し、電子（ｅ－）は、低エネルギー準位半導体の伝導帯内に位置する。そして、正孔（ｈ＋）は水の酸化反応に用いられて酸素（Ｏ２）を発生させ、電子（ｅ－）は水の還元反応に用いられて水素（Ｈ２）を発生させる。 In the thin film nanostructure 40, one of the first layer 41 and the second layer 42 is formed of a low energy level semiconductor, and the other is formed of a high energy level semiconductor. The thin-film nanostructure 40 has a type II band structure, and a transition between a quantum level near the upper end of the valence band of the high energy level semiconductor and a quantum level near the lower end of the conduction band of the low energy level semiconductor. When light having energy equal to or higher than the energy corresponding to the band gap due to is absorbed, holes (h + ) and electrons (e − ) are generated. The hole (h + ) is located in the valence band of the high energy level semiconductor, and the electron (e − ) is located in the conduction band of the low energy level semiconductor. Holes (h + ) are used for water oxidation reaction to generate oxygen (O 2 ), and electrons (e − ) are used for water reduction reaction to generate hydrogen (H 2 ).
　薄膜ナノ構造体４０において、生成した正孔（ｈ＋）と電子（ｅ－）とは、異なる半導体内に位置し空間的に分離されるのでキャリアの再結合が抑制される。したがって、キャリア寿命が増大し、光触媒におけるエネルギー変換効率を向上させることができる。 In the thin-film nanostructure 40, the generated holes (h + ) and electrons (e − ) are located in different semiconductors and are spatially separated, so that carrier recombination is suppressed. Therefore, the carrier life is increased and the energy conversion efficiency in the photocatalyst can be improved.
　薄膜ナノ構造体４０においては、高エネルギー準位半導体の価電子帯の上端近傍の量子準位と低エネルギー準位半導体の伝導帯の下端近傍の量子準位の遷移に起因するバンドギャップが小さくなるように二種類の半導体を適宜選択することにより、可視光応答型光触媒を構成することが可能となる。 In the thin-film nanostructure 40, the band gap due to the transition between the quantum level near the upper end of the valence band of the high energy level semiconductor and the quantum level near the lower end of the conduction band of the low energy level semiconductor becomes small. Thus, a visible light responsive photocatalyst can be configured by appropriately selecting two types of semiconductors.
　［第５の実施形態］
　図７は、第５の実施形態のナノ粒子からなる可視光応答型光触媒を模式的に示す図である。図７に示すように、ナノ粒子５０はコア５１とコア５１を部分的に被覆する部分シェル５２とから構成される。コア５１は高エネルギー準位半導体から形成され、部分シェル５２は低エネルギー準位半導体から形成されている。第２の実施形態のナノ粒子２０とは、部分シェル５２がコア５１を部分的に被覆するように形成されており、シェル２２のようにコア５１全体を被覆するように形成されていない点のみ異なる。各半導体のバンド構造、ナノ粒子５０のバンド構造の関係は、図４に示した第２の実施形態と同様であるので説明を省略する。 [Fifth Embodiment]
FIG. 7 is a diagram schematically showing a visible light responsive photocatalyst composed of nanoparticles according to the fifth embodiment. As shown in FIG. 7, the nanoparticles 50 are composed of a core 51 and a partial shell 52 that partially covers the core 51. The core 51 is formed from a high energy level semiconductor, and the partial shell 52 is formed from a low energy level semiconductor. The nanoparticle 20 of the second embodiment is such that the partial shell 52 is formed so as to partially cover the core 51 and is not formed so as to cover the entire core 51 like the shell 22. Different. The relationship between the band structure of each semiconductor and the band structure of the nanoparticles 50 is the same as in the second embodiment shown in FIG.
　本実施形態のナノ粒子５０においては、コア５１の一部がナノ粒子５０表面に露出していることにより、コア５１に閉じ込められた正孔（ｈ＋）と触媒活性面との距離がさらに近接し、生成された正孔（ｈ＋）を水の酸化反応により効率的に利用することが可能となる。 In the nanoparticle 50 of the present embodiment, a part of the core 51 is exposed on the surface of the nanoparticle 50, so that the distance between the hole (h + ) confined in the core 51 and the catalytically active surface is closer. Thus, the generated holes (h + ) can be efficiently used by the oxidation reaction of water.
　第１の実施形態のナノ粒子１０についても、本実施形態と同様に、シェルがコア全体を被覆するように構成するのではなく、シェルがコアの一部を被覆するように構成することも可能であり、そのような構成によると、コアに閉じ込められた電子（ｅ－）と触媒活性面との距離がさらに近接し、生成された電子（ｅ－）を水の還元反応により効率的に利用することが可能となる。 The nanoparticle 10 of the first embodiment can also be configured so that the shell covers a part of the core instead of the shell covering the entire core, as in the present embodiment. According to such a configuration, the distance between the electron (e − ) confined in the core and the catalytically active surface is closer, and the generated electron (e − ) is efficiently used by the reduction reaction of water. It becomes possible to do.
　［第６の実施形態］
　図８は、第６の実施形態のナノワイヤからなる可視光応答型光触媒を模式的に示す図である。図８に示すように、ナノワイヤ６０は、基板６３上に設けられ、コア６１とこれを被覆するシェル６２とから構成されている。ナノワイヤ６０は、コア６１とシェル６２の内、一方を低エネルギー準位半導体で形成し、他方を高エネルギー準位半導体で形成する。第３の実施形態のナノワイヤ３０とは、シェル６２がコア６１を部分的に被覆するように形成されており、コア６１全体を被覆するように形成されていない点のみ異なる。各半導体のバンド構造、ナノワイヤ６０のバンド構造の関係は、第３の実施形態と同様であるので説明を省略する。 [Sixth Embodiment]
FIG. 8 is a diagram schematically showing a visible light responsive photocatalyst composed of nanowires according to the sixth embodiment. As shown in FIG. 8, the nanowire 60 is provided on a substrate 63 and includes a core 61 and a shell 62 covering the core 61. One of the core 61 and the shell 62 is formed of a low energy level semiconductor, and the other is formed of a high energy level semiconductor. The nanowire 30 is different from the nanowire 30 of the third embodiment only in that the shell 62 is formed so as to partially cover the core 61 and is not formed so as to cover the entire core 61. Since the relationship between the band structure of each semiconductor and the band structure of the nanowire 60 is the same as in the third embodiment, description thereof is omitted.
　本実施形態のナノワイヤ６０においては、コア６１の一部がナノワイヤ６０表面に露出していることにより、コア６１に閉じ込められたキャリアと触媒活性面との距離がさらに近接し、生成されたキャリアを水の酸化反応または還元反応により効率的に利用することが可能となる。 In the nanowire 60 of the present embodiment, since a part of the core 61 is exposed on the surface of the nanowire 60, the distance between the carrier confined in the core 61 and the catalytically active surface is closer, and the generated carrier is It can be efficiently used by the oxidation reaction or reduction reaction of water.
　以下に実施例を挙げて本発明を具体的に説明するが、本発明はこれらに限定されるものではない。 Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.
　［実施例１］
　（構成）
　実施例１においては、第１の実施形態のナノ粒子を作製した。実施例１のナノ粒子は、コア１１を構成する低エネルギー準位半導体としてＣｄＳを用い、シェル１２を構成する高エネルギー準位半導体としてＺｎＳｅを用いた。 [Example 1]
(Constitution)
In Example 1, the nanoparticles of the first embodiment were produced. In the nanoparticles of Example 1, CdS was used as a low energy level semiconductor constituting the core 11, and ZnSe was used as a high energy level semiconductor constituting the shell 12.
　（製造方法）
　オレイルアミン／オクタデセン１０ｍＬの混合液中で、酸化カドミウム（０．２ｍｍｏｌ）と硫黄（０．２ｍｍｏｌ）を２５０－３００℃で１時間反応させることでＣｄＳナノ粒子分散溶液を得た。０．５ｍｏｌ／Ｌのジエチルジンクとセレンを含むトリオクチルホスフィン溶液（プレカーサ溶液）を室温にて作成し、この溶液２ｍＬを前述のＣｄＳナノ粒子分散溶液に加えた。１時間後、さらにプレカーサ溶液１ｍＬを反応溶液に加え、２．５時間後、４時間後にそれぞれプレカーサ溶液１．５ｍＬを加えた。プレカーサ溶液を最後に加えてから１時間反応させた後、室温まで冷却することで実施例１のナノ粒子の分散溶液を得た。 (Production method)
CdS nanoparticle dispersion solution was obtained by reacting cadmium oxide (0.2 mmol) and sulfur (0.2 mmol) at 250-300 ° C. for 1 hour in a 10 mL mixture of oleylamine / octadecene. A trioctylphosphine solution (precursor solution) containing 0.5 mol / L of diethyl zinc and selenium was prepared at room temperature, and 2 mL of this solution was added to the CdS nanoparticle dispersion solution. After 1 hour, 1 mL of the precursor solution was further added to the reaction solution, and 1.5 mL of the precursor solution was added after 2.5 hours and 4 hours, respectively. After the precursor solution was added for the last time, the mixture was reacted for 1 hour and then cooled to room temperature to obtain a nanoparticle dispersion solution of Example 1.
　ＣｄＳは、水溶液中で光分解しやすいが、本実施例においてはＣｄＳをコア１１の材料として用い、これがシェル１２で覆われる構成であるので、ＣｄＳの光分解が抑制され、有害なＣｄの溶出を抑制することができる。 Although CdS is easily photodegraded in an aqueous solution, in this embodiment, CdS is used as the material of the core 11 and this is covered with the shell 12, so that the photodecomposition of CdS is suppressed and harmful Cd is eluted. Can be suppressed.
　本実施例の可視光応答型光触媒においては、ＣｄＳとＺｎＳｅのバンド構造に基づくと、約２．２ｅＶ（５６３ｎｍ）の可視光応答型光触媒を得ることができる。この場合、太陽光エネルギー変換効率における最大理論効率が約１４％となる。上記効率は、最大理論効率として、大気圏外の太陽光スペクトルであるＡｉｒ　Ｍａｓｓ０（ＡＭ０）を用いて算出した。 In the visible light responsive photocatalyst of this example, a visible light responsive photocatalyst of about 2.2 eV (563 nm) can be obtained based on the band structure of CdS and ZnSe. In this case, the maximum theoretical efficiency in the solar energy conversion efficiency is about 14%. The said efficiency was computed using Air Mass0 (AM0) which is a solar spectrum outside the atmosphere as the maximum theoretical efficiency.
　［実施例２］
　（構成）
　実施例２においては、第１の実施形態のナノ粒子を作製した。実施例２のナノ粒子は、コア１１を構成する低エネルギー準位半導体としてＴｉＯ２を用い、シェル１２を構成する高エネルギー準位半導体としてＺｎＳｅを用いた。 [Example 2]
(Constitution)
In Example 2, the nanoparticles of the first embodiment were produced. In the nanoparticles of Example 2, TiO 2 was used as the low energy level semiconductor constituting the core 11, and ZnSe was used as the high energy level semiconductor constituting the shell 12.
　（製造方法）
　硫酸チタニル（４０ｍｍｏｌ）を２５０ｍｌのアルコール（メタノール、エタノール、ｎプロパノール）-水混合液に室温で溶解した。撹拌下に２時間還流加熱し、加水分解を行なった。生成物は遠心分離して回収、メタノールで洗浄し、真空で乾燥することで、ナノサイズの酸化チタンを得た。 (Production method)
Titanyl sulfate (40 mmol) was dissolved in 250 ml of an alcohol (methanol, ethanol, npropanol) -water mixture at room temperature. The mixture was heated under reflux for 2 hours with stirring to conduct hydrolysis. The product was collected by centrifugation, washed with methanol, and dried under vacuum to obtain nano-sized titanium oxide.
　上記酸化チタン（５－２０ｍｇ）をトリオクチルホスフィン（以下、ＴＯＰ）（２ｇ）とトリオクチルホスフィンオキシド（以下、ＴＯＰＯ）（２ｇ）の混合物にアルゴン雰囲気下、６０℃の条件で溶解した。このナノ粒子溶液を２６０℃に加熱し、セレン化亜鉛の前駆体溶液（後述）を滴下してしばらく反応させた後、トルエン溶液を加えた。メタノールにより沈殿させ、実施例２のナノ粒子を得た。 The titanium oxide (5-20 mg) was dissolved in a mixture of trioctylphosphine (hereinafter TOP) (2 g) and trioctylphosphine oxide (hereinafter TOPO) (2 g) at 60 ° C. under an argon atmosphere. This nanoparticle solution was heated to 260 ° C., a zinc selenide precursor solution (described later) was dropped and reacted for a while, and then a toluene solution was added. Precipitation with methanol gave the nanoparticles of Example 2.
　セレン化亜鉛の前駆体溶液は、１．２ｍｇ／ｍＬの濃度のＴＯＰＳｅと、等モルの２ｍｏｌ／Ｌジメチル亜鉛のトルエン溶液を混合することで、セレン化亜鉛の前駆体溶液を得た。 The zinc selenide precursor solution was obtained by mixing TOPSe with a concentration of 1.2 mg / mL and an equimolar 2 mol / L dimethylzinc toluene solution to obtain a zinc selenide precursor solution.
　本実施例の可視光応答型光触媒においては、コア１１の形成に用いたＴｉＯ２は化合物として非常に安定であるため、化学物質の溶液中への溶出を抑制することができる。 In the visible light responsive photocatalyst of this example, since TiO 2 used for forming the core 11 is very stable as a compound, the elution of the chemical substance into the solution can be suppressed.
　本実施例の可視光応答型光触媒においては、ＴｉＯ２とＺｎＳｅのバンド構造に基づくと、約１．８ｅＶ（６８９ｎｍ）の可視光応答型光触媒を得ることができる。この場合、太陽光エネルギー変換効率における最大理論効率が約２４％となる（ＡＭ０で算出）。 In the visible light responsive photocatalyst of this example, a visible light responsive photocatalyst of about 1.8 eV (689 nm) can be obtained based on the band structure of TiO 2 and ZnSe. In this case, the maximum theoretical efficiency in the solar energy conversion efficiency is about 24% (calculated by AM0).
　［実施例３］
　（構成）
　実施例３においては、第２の実施形態のナノ粒子を作製した。実施例３のナノ粒子は、コア２１を構成する高エネルギー準位半導体としてＺｎＳｅを用い、シェル２２を構成する低エネルギー準位半導体としてＴｉＯ２を用いた。 [Example 3]
(Constitution)
In Example 3, the nanoparticles of the second embodiment were produced. In the nanoparticles of Example 3, ZnSe was used as the high energy level semiconductor constituting the core 21, and TiO 2 was used as the low energy level semiconductor constituting the shell 22.
　（製造方法）
　オレイルアミン７ｍＬを１２５℃で３０分間真空引きした後、窒素ガスフロー中で３２５℃まで加熱する。０．５ｍｏｌ／Ｌのジエチルジンクとセレンを含むトリオクチルホスフィン溶液（プレカーサ溶液）を室温にて作成し、この溶液２ｍＬを前述のオレイルアミンに加えた。１時間後、さらにプレカーサ溶液１ｍＬを反応溶液に加え、２．５時間後、４時間後にそれぞれプレカーサ溶液１．５ｍＬを加える。プレカーサ溶液を最後に加えてから１時間反応させた後、室温まで冷却することでＺｎＳｅナノ粒子を得た。ＺｎＳｅナノ粒子溶液にメタノールを加えて沈殿させ、ヘキサンに再分散させることを３回繰り返して洗浄した。これにチタンテトライソプロポキシド（３ｍｍｏｌ）を加え、約８０℃で１時間加熱することでチタンテトライソプロポキシドにより表面保護されたＺｎＳｅコロイド溶液を得た。これを加水分解することで、ＴｉＯ２層でＺｎＳｅからなるコアがコートされた実施例３のナノ粒子の分散溶液を得た。 (Production method)
After evacuating 7 mL of oleylamine at 125 ° C. for 30 minutes, heat to 325 ° C. in a nitrogen gas flow. A trioctylphosphine solution (precursor solution) containing 0.5 mol / L of diethyl zinc and selenium was prepared at room temperature, and 2 mL of this solution was added to the oleylamine. After 1 hour, 1 mL of the precursor solution is further added to the reaction solution, and 1.5 mL of the precursor solution is added after 2.5 hours and 4 hours, respectively. After the precursor solution was added last, the mixture was reacted for 1 hour, and then cooled to room temperature to obtain ZnSe nanoparticles. Methanol was added to the ZnSe nanoparticle solution for precipitation, and re-dispersion in hexane was repeated three times for washing. Titanium tetraisopropoxide (3 mmol) was added thereto and heated at about 80 ° C. for 1 hour to obtain a ZnSe colloid solution surface-protected with titanium tetraisopropoxide. This was hydrolyzed to obtain a nanoparticle dispersion solution of Example 3 in which a core made of ZnSe was coated with a TiO 2 layer.
　本実施例の可視光応答型光触媒においては、シェル２２の形成に用いたＴｉＯ２は化合物として非常に安定であるため、ナノ粒子全体が分解しにくく、光触媒活性の経時劣化を抑制することができる。また、ＴｉＯ２は超親水性効果を有するので、防汚・セルフクリーニング機能を備えた光触媒を構成することができる。 In the visible light responsive photocatalyst of this example, since TiO 2 used for forming the shell 22 is very stable as a compound, the entire nanoparticle is hardly decomposed and the deterioration of the photocatalytic activity over time can be suppressed. . Moreover, since TiO 2 has a superhydrophilic effect, a photocatalyst having an antifouling / self-cleaning function can be constituted.
　本実施例の可視光応答型光触媒においては、ＴｉＯ２とＺｎＳｅのバンド構造に基づくと、約１．９ｅＶ（６５２ｎｍ）の可視光応答型光触媒を得ることができる。この場合、太陽光エネルギー変換効率における最大理論効率が約２１％となる（ＡＭ０で算出）。 In the visible light responsive photocatalyst of this example, a visible light responsive photocatalyst of about 1.9 eV (652 nm) can be obtained based on the band structure of TiO 2 and ZnSe. In this case, the maximum theoretical efficiency in the solar energy conversion efficiency is about 21% (calculated by AM0).
　［実施例４］
　（構成）
　実施例４においては、第２の実施形態のナノ粒子を作製した。実施例４のナノ粒子は、コア２１を構成する高エネルギー準位半導体材料としてＣｄＳを用い、シェル２２を構成する低エネルギー準位半導体材料としてＴｉＯ２を用いた。 [Example 4]
(Constitution)
In Example 4, the nanoparticles of the second embodiment were produced. In the nanoparticles of Example 4, CdS was used as the high energy level semiconductor material constituting the core 21, and TiO 2 was used as the low energy level semiconductor material constituting the shell 22.
　（製造方法）
　過塩素酸カドミウム（０．２ｍｍｏｌ）とヘキサメタリン酸ナトリウム（０．２ｍｍｏｌ）の水溶液を１０００ｍＬ作製した。その後、溶液中を窒素ガスでバブリングを行い、硫化水素ガス（０．１８ｍｍｏｌ）を激しく攪拌させながら溶液中に注入し、しばらく攪拌を行なった。これにより、ヘキサメタリン酸により安定化された半導体ナノ粒子の溶液を得た。これにオルトチタン酸テトライソプロピル（０．２ｍｍｏｌ）を加え、約８０℃で１時間加熱することでオルトチタン酸テトライソプロピルにより表面保護されたＣｄＳコロイド溶液を得た。これを加水分解することで、ＴｉＯ２層でＣｄＳからなるコアがコートされた実施例４のナノ粒子の分散溶液を得た。 (Production method)
1000 mL of an aqueous solution of cadmium perchlorate (0.2 mmol) and sodium hexametaphosphate (0.2 mmol) was prepared. Thereafter, the solution was bubbled with nitrogen gas, and hydrogen sulfide gas (0.18 mmol) was poured into the solution with vigorous stirring, followed by stirring for a while. Thereby, a solution of semiconductor nanoparticles stabilized with hexametaphosphoric acid was obtained. To this, tetraisopropyl orthotitanate (0.2 mmol) was added and heated at about 80 ° C. for 1 hour to obtain a CdS colloid solution surface-protected with tetraisopropyl orthotitanate. This was hydrolyzed to obtain a nanoparticle dispersion solution of Example 4 in which a core made of CdS was coated with a TiO 2 layer.
　本実施例の可視光応答型光触媒においては、シェル２２の形成に用いたＴｉＯ２は化合物として非常に安定であるため、ナノ粒子全体が分解しにくく、光触媒活性の経時劣化を抑制することができる。また、ＴｉＯ２は超親水性効果を有するので、防汚・セルフクリーニング機能を備えた光触媒を構成することができる。また、コア２１の形成に用いたＣｄＳは、水溶液中で光分解しやすいが、本実施形態においてはＣｄＳをコア１１の材料として用い、これがシェル１２で覆われる構成であるので、ＣｄＳの光分解が抑制され、有害なＣｄの溶出を抑制することができる。 In the visible light responsive photocatalyst of this example, since TiO 2 used for forming the shell 22 is very stable as a compound, the entire nanoparticle is hardly decomposed and the deterioration of the photocatalytic activity over time can be suppressed. . Moreover, since TiO 2 has a superhydrophilic effect, a photocatalyst having an antifouling / self-cleaning function can be constituted. CdS used for forming the core 21 is easily photodegraded in an aqueous solution, but in this embodiment, CdS is used as a material for the core 11 and this is covered with the shell 12, so that CdS is photodegraded. Is suppressed, and the elution of harmful Cd can be suppressed.
　本実施例の可視光応答型光触媒においては、ＴｉＯ２とＣｄＳのバンド構造に基づくと、約２．２ｅＶ（５６３ｎｍ）の可視光応答型光触媒を得ることができる。この場合、太陽光エネルギー変換効率における最大理論効率が約１４％となる（ＡＭ０で算出）。 In the visible light responsive photocatalyst of this example, a visible light responsive photocatalyst of about 2.2 eV (563 nm) can be obtained based on the band structure of TiO 2 and CdS. In this case, the maximum theoretical efficiency in the solar energy conversion efficiency is about 14% (calculated by AM0).
　［実施例５］
　（構成）
　実施例５においては、第２の実施形態のナノ粒子を作製した。実施例５のナノ粒子は、コア２１を構成する高エネルギー準位半導体としてＩｎＧａＮを用い、シェル２２を構成する低エネルギー準位半導体としてＴｉＯ２を用いた。 [Example 5]
(Constitution)
In Example 5, the nanoparticles of the second embodiment were produced. In the nanoparticles of Example 5, InGaN was used as the high energy level semiconductor constituting the core 21, and TiO 2 was used as the low energy level semiconductor constituting the shell 22.
　（製造方法）
　ミリスチン酸インジウム（０．２５ｍｍｏｌ）とミリスチン酸ガリウム（０．２５ｍｍｏｌ）にヘキサデカンチオール（０．５ｍｍｏｌ）とＮａＮＨ２（１．５ｍｍｏｌ）、オクタデセン１０ｍｌを加え、約１８０℃で１時間加熱することで、ヘキサデカンチオールにより表面保護されたＩｎＧａＮコロイド溶液を得た。これにオルトチタン酸テトライソプロピル（０．５ｍｍｏｌ）を加え、約８０℃で１時間加熱することでオルトチタン酸テトライソプロピルにより表面保護されたＩｎＧａＮコロイド溶液を得た。これを加水分解することで、ＴｉＯ２層でＩｎＧａＮからなるコアがコートされた実施例５のナノ粒子の分散溶液を得た。 (Production method)
By adding hexadecanethiol (0.5 mmol), NaNH 2 (1.5 mmol) and octadecene 10 ml to indium myristate (0.25 mmol) and gallium myristate (0.25 mmol), and heating at about 180 ° C. for 1 hour, An InGaN colloid solution surface-protected with hexadecanethiol was obtained. To this, tetraisopropyl orthotitanate (0.5 mmol) was added and heated at about 80 ° C. for 1 hour to obtain an InGaN colloidal solution surface-protected with tetraisopropyl orthotitanate. This was hydrolyzed to obtain a nanoparticle dispersion solution of Example 5 in which a core made of InGaN was coated with a TiO 2 layer.
　本実施例の可視光応答型光触媒においては、シェル２２の形成に用いたＴｉＯ２は化合物として非常に安定であるため、ナノ粒子全体が分解しにくく、光触媒活性の経時劣化を抑制することができる。また、ＴｉＯ２は超親水性効果を有するので、防汚・セルフクリーニング機能を備えた光触媒を構成することができる。 In the visible light responsive photocatalyst of this example, since TiO 2 used for forming the shell 22 is very stable as a compound, the entire nanoparticle is hardly decomposed and the deterioration of the photocatalytic activity over time can be suppressed. . Moreover, since TiO 2 has a superhydrophilic effect, a photocatalyst having an antifouling / self-cleaning function can be constituted.
　本実施例の可視光応答型光触媒においては、ＩｎＧａＮとＴｉＯ２のバンド構造に基づくと、約２．７ｅＶ（４５９ｎｍ）の可視光応答型光触媒を得ることができる。この場合、太陽光エネルギー変換効率における最大理論効率が約６％となる（ＡＭ０で算出）。 In the visible light responsive photocatalyst of this example, a visible light responsive photocatalyst of about 2.7 eV (459 nm) can be obtained based on the band structure of InGaN and TiO 2 . In this case, the maximum theoretical efficiency in the solar energy conversion efficiency is about 6% (calculated by AM0).
　［実施例６］
　実施例６においては、第３の実施形態のナノワイヤを作製した。実施例６のナノワイヤは、コア３１を構成する高エネルギー準位半導体としてＩｎＧａＮを用い、シェル３２を構成する低エネルギー準位半導体としてＴｉＯ２を用いた。基板上に約１０ｎｍ厚のＳｉＯ２を成膜し、電子線描画および酸化膜エッチングにより円状パターンを形成した。ＭＯＣＶＤ（有機金属気相成長法）の選択成長法によりコア３１部分のＩｎＧａＮナノワイヤを作製した。その後、溶液法により、シェル３２を成長し、コアシェル構造のナノワイヤが得られた。 [Example 6]
In Example 6, the nanowire of the third embodiment was produced. In the nanowire of Example 6, InGaN was used as the high energy level semiconductor constituting the core 31, and TiO 2 was used as the low energy level semiconductor constituting the shell 32. A SiO 2 film having a thickness of about 10 nm was formed on the substrate, and a circular pattern was formed by electron beam drawing and oxide film etching. An InGaN nanowire of the core 31 portion was produced by a selective growth method of MOCVD (metal organic chemical vapor deposition). Thereafter, the shell 32 was grown by a solution method, and a nanowire having a core-shell structure was obtained.
　［実施例７］
　実施例７においては、第４の実施形態の薄膜ナノ構造体を作製した。実施例７の薄膜ナノ構造体は、第１層および第２層として、ＺｎＳｅおよびＣｄＳを用いた。ＭＯＣＶＤにより、基板上に第１層および第２層を繰り返し成長することで、薄膜ナノ構造体が得られた。図６のように、第１層の初期成長層以外をエッチング等により取り除くことで、第１層からのキャリア取り出しを高めることができる。また、基板上にバッファー層などを成膜してもよい。 [Example 7]
In Example 7, the thin film nanostructure of the fourth embodiment was produced. In the thin film nanostructure of Example 7, ZnSe and CdS were used as the first layer and the second layer. Thin film nanostructures were obtained by repeatedly growing the first and second layers on the substrate by MOCVD. As shown in FIG. 6, the removal of carriers from the first layer can be enhanced by removing other than the initial growth layer of the first layer by etching or the like. Further, a buffer layer or the like may be formed on the substrate.
　［実施例８］
　実施例８においては、第５の実施形態のナノ粒子を作製した。ナノ粒子はコア５１とコア５１を部分的に被覆する部分シェル５２とから構成される。製造方法は、実施例１～５に示される製造方法において、シェル成長時の成長条件（成長時間、成長温度）を調整することで得た。例えば、成長時間を短くすることで、シェル材料がコア材料を完全に被覆する前に成長中断でき、コア５１とコア５１を部分的に被覆する部分シェル５２とから構成されるナノ粒子が得られた。 [Example 8]
In Example 8, the nanoparticles of the fifth embodiment were produced. The nanoparticles are composed of a core 51 and a partial shell 52 that partially covers the core 51. The production method was obtained by adjusting the growth conditions (growth time, growth temperature) during shell growth in the production methods shown in Examples 1 to 5. For example, by shortening the growth time, the growth can be interrupted before the shell material completely covers the core material, and nanoparticles composed of the core 51 and the partial shell 52 partially covering the core 51 are obtained. It was.
　［実施例９］
　実施例９においては、第６の実施形態のナノワイヤを作製した。ナノワイヤ６０は、基板６３上に設けられ、コア６１とこれを被覆するシェル６２とから構成される。製造方法は、実施例６に示される製造方法において、シェル成長時の成長条件（成長時間、成長温度）を調整することで得た。例えば、成長時間を短くすることで、シェル材料がコア材料を完全に被覆する前に成長中断でき、コア６１とコア６１を部分的に被覆する部分シェル６２とから構成されるナノワイヤが得られた。 [Example 9]
In Example 9, the nanowire of the sixth embodiment was produced. The nanowire 60 is provided on the substrate 63 and includes a core 61 and a shell 62 covering the core 61. The manufacturing method was obtained by adjusting the growth conditions (growth time, growth temperature) during shell growth in the manufacturing method shown in Example 6. For example, by shortening the growth time, the growth can be interrupted before the shell material completely covers the core material, and a nanowire composed of the core 61 and the partial shell 62 that partially covers the core 61 is obtained. .
　本発明により得られた可視光応答型光触媒は、従来の紫外光応答型光触媒に比べて、使用可能範囲を広げることができる。屋外だけでなく、好適には、室内での利用での広がりが期待される。室内光においては、近年のＬＥＤ化に伴い、紫外光が減少しており、可視光応答の特性が極めて重要となる。また、環境浄化、抗菌家電等への広がりが期待できる。また、従来の二段階光吸収プロセスを要する可視光応答型光触媒では低効率のため実現が出来ていなかった水素製造、人工光合成を含めたソーラーフューエル製造への展開が期待できる。 The visible light responsive photocatalyst obtained by the present invention can expand the usable range as compared with a conventional ultraviolet light responsive photocatalyst. Not only outdoors, but preferably indoor use is expected. In room light, ultraviolet light has decreased with the recent trend toward LED, and the characteristics of visible light response are extremely important. In addition, it can be expected to spread to environmental purification and antibacterial home appliances. In addition, the visible light responsive photocatalyst that requires a conventional two-stage light absorption process can be expected to develop into hydrogen production and solar fuel production including artificial photosynthesis, which could not be realized due to low efficiency.
　今回開示された実施形態および実施例はすべての点で例示であって制限的なものではないと考えられるべきである。本発明の範囲は上記した説明ではなくて請求の範囲によって示され、請求の範囲と均等の意味および範囲内でのすべての変更が含まれることが意図される。 The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
　１０，２０，５０　ナノ粒子、１１，２１，３１，５１，６１　コア、１２，２２，３２，６２　シェル、３０，６０　ナノワイヤ、４０　薄膜ナノ構造体、４１　第１層、４２　第２層、５２　部分シェル。 10, 20, 50 nanoparticles, 11, 21, 31, 51, 61 core, 12, 22, 32, 62 shell, 30, 60 nanowire, 40 thin film nanostructure, 41 first layer, 42 second layer, 52 Partial shell.

Claims (8)

  1. 　複数の半導体で構成されるナノ構造体からなり、
    　前記ナノ構造体は、タイプIIのバンド構造を有し、
    　前記各半導体は、伝導帯の下端が水の還元電位より負であり、価電子帯の上端が水の酸化電位よりも正である可視光応答型光触媒。     It consists of nanostructures composed of multiple semiconductors,
    The nanostructure has a type II band structure,
    Each of the semiconductors is a visible light responsive photocatalyst in which the lower end of the conduction band is more negative than the reduction potential of water and the upper end of the valence band is more positive than the oxidation potential of water.
  2. 　前記ナノ構造体は、電子の閉じ込めが可能なバンド構造を有する、請求項１に記載の可視光応答型光触媒。 The visible light responsive photocatalyst according to claim 1, wherein the nanostructure has a band structure capable of confining electrons.
  3. 　前記ナノ構造体は、正孔の閉じ込めが可能なバンド構造を有する、請求項１に記載の可視光応答型光触媒。 The visible light responsive photocatalyst according to claim 1, wherein the nanostructure has a band structure capable of confining holes.
  4. 　量子準位に対応した光吸収特性を有する、請求項１～３のいずれか１項に記載の可視光応答型光触媒。 The visible light responsive photocatalyst according to any one of claims 1 to 3, which has a light absorption characteristic corresponding to a quantum level.
  5. 　前記光吸収特性において光吸収端が４４０ｎｍ以上である、請求項４に記載の可視光応答型光触媒。 The visible light responsive photocatalyst according to claim 4, wherein a light absorption edge in the light absorption property is 440 nm or more.
  6. 　前記ナノ構造体はコアシェル構造を有するナノ粒子であり、
    　前記コアシェル構造において、コア部分とシェル部分とが異なる半導体から形成される、請求項１～５のいずれか１項に記載の可視光応答型光触媒。     The nanostructure is a nanoparticle having a core-shell structure;
    The visible light responsive photocatalyst according to any one of claims 1 to 5, wherein in the core-shell structure, the core portion and the shell portion are formed of different semiconductors.
  7. 　前記コアシェル構造のシェル部分は酸化物半導体からなる、請求項１～６のいずれか１項に記載の可視光応答型光触媒。 The visible light responsive photocatalyst according to any one of claims 1 to 6, wherein a shell portion of the core-shell structure is made of an oxide semiconductor.
  8. 　前記ナノ構造体に、白金、パラジウム、ロジウム、ルテニウム、イリジウム、コバルト、ニッケル、スズ、クロム、鉄、銅、金及び銀からなる群より選択される少なくとも一つの元素を含む微粒子が担持された、請求項１に記載の可視光応答型光触媒。 In the nanostructure, fine particles containing at least one element selected from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, cobalt, nickel, tin, chromium, iron, copper, gold and silver are supported. The visible light responsive photocatalyst according to claim 1.
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