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Strategies for the Development of Visible-light-driven Photocatalysts for Water Splitting

Akihiko Kudo,?Hideki Kato,and Issei Tsuji

(Received August 18,2004;CL-048011)

Abstract

Photocatalysts for water splitting developed by the present au-thors are reviewed.A NiO (0.2wt %)/NaTaO 3:La (2%)photo-catalyst with a 4.1-eV band gap showed high activity for water splitting into H 2and O 2with an apparent quantum yield of 56%at 270nm.Many visible-light-driven photocatalysts have also been developed through band engineering by doping of met-al cations,forming new valence bands with Bi 6s ,Sn 5s ,and Ag 4d orbitals,and by making solid solutions between ZnS with wide band gap and narrow band gap semiconductors.Overall water splitting under visible light irradiation has been achieved by con-struction of a Z-scheme photocatalysis system employing the visible-light-driven photocatalysts for H 2and O 2evolution,and the Fe 3t/Fe 2tredox couple as an electron relay.

Introduction

Photocatalytic reactions have been studied extensively.They are classi?ed into two categories:uphill and downhill reac-tions,as shown in Figure 1.Water splitting into H 2and O 2is ac-companied by a large positive change in the Gibbs free energy (áG o ?237kJ/mol);i.e.,it is an uphill reaction.In this reac-tion,a photon energy is converted into chemical energy,as seen in photosynthesis by green plants.Therefore,this reaction is termed arti?cial photosynthesis.On the other hand,degradation reactions such as the photo-oxidation of organic compounds us-ing oxygen molecules are generally downhill reactions.The re-action proceeds irreversibly.This type of reaction is regarded as a photoinduced reaction and has been extensively studied using titanium dioxide photocatalysts.1

The importance of hydrogen energy has recently been re-recognized because of the interest in clean energy.Hydrogen is mainly produced by steam reforming of hydrocarbons such as methane in industry.Hydrogen must be produced from water using a renewable energy source,if one considers the energy and environmental issues.Therefore,photocatalytic water splitting is a challenging reaction because it is an ultimate solution to these serious problems.

Water splitting has been studied for a long time since the Honda–Fujishima e?ect,which involved a TiO 2semiconductor electrode,was reported.2However,the number of reported pho-tocatalysts that were able to decompose water into H 2and O 2in stoichiometric amounts with reasonable activity has been quite limited.Against such a background,various types of new photo-

catalysts for water splitting have recently been found,3–5and this research ?eld is taking a new turn.In the present paper,new pho-tocatalyst materials,developed mainly by the present authors and aimed at water splitting by means of arti?cial photosynthe-sis,are reviewed.

2.Processes on Photocatalytic Re-actions

Figure 2shows the main processes in a photocatalytic reac-tion.The ?rst step (i)of the photocatalytic reaction is absorption of photons to form electron-hole pairs.The band gap (BG)of a visible-light-driven photocatalyst should be narrower than 3.0eV ( >420nm).Moreover,the conduction and valence band positions should satisfy the energy requirements set by the reduction and oxidation potentials for H 2O,respectively.Therefore,band engineering is necessary for the design of pho-tocatalysts with these properties.

The second step (ii)consists of a charge separation and the migration of photogenerated carriers.The crystal structure and the crystallinity strongly a?ect these processes.The higher the crystalline quality,the smaller is the amount of defects.The de-fects operate as trapping and recombination centers between photogenerated electrons and holes,resulting in a decrease in the photocatalytic activity.Therefore,a high degree of crystal-linity,rather than a high surface area,is required of photocata-lysts,especially for an uphill reaction like water splitting.

The ?nal step (iii)consists of the surface chemical reactions.The important points for this step are surface character (active sites)and quantity (surface area).Cocatalysts such as Pt,NiO,and RuO 2are usually loaded to introduce active sites for H 2evolution because the conduction band levels of many oxide photocatalysts are not high enough to reduce H 2O to produce H 2without catalytic assistance.Active sites for 4-electron oxida-tion of H 2O are required for O 2evolution.Although this reaction is demanding,cocatalysts are unnecessary for oxide photocata-

Highlight Review

P o t e n t i a l

Photon energy conversion reaction

Water splitting

Photoinduced reaction

Decompositionn Figure 1.Types of photocatalytic reactions.

Prof.Akihiko Kudo,?y ;yy Dr.Hideki Kato,y and Mr.Issei Tsuji y y

Department of Applied Chemistry,Faculty of Science,Science University of Tokyo,1-3Kagurazaka,Shinjuku-ku,Tokyo 162-8601yy

Core Research for Evolutional Science and Technology (CREST),Japan Science and Technology Agency (JST)E-mail:a-kudo@rs.kagu.tus.ac.jp

lysts because the valence band is deep enough to oxidize H2O to form O2.

Many photocatalysts are also materials for solar cells,phos-phors,and dielectrics.However,the signi?cant di?erence be-tween the photocatalytic property and the other properties is that chemical reactions are involved in the photocatalytic process, but not in the other properties.

? 3.Highly Active Tantalate Photoca-talysts for Overall Water Splitting un-der UV Irradiation

Before presenting visible-light-driven photocatalysts,we will introduce the highly active photocatalysts for water splitting into H2and O2under UV irradiation.

We have surveyed new photocatalyst materials by paying at-tention to the crystal structure.Table1shows water splitting on various tantalate photocatalysts.6–11These tantalate photocata-lysts were active even without cocatalysts.Moreover,loading

a NiO cocatalyst drastically improved the photocatalytic per-formance.The conduction and valence bands of these tantalate photocatalysts consist of Ta5d and O2p orbitals,respectively. The conduction band level is su?ciently higher than the reduc-tion potential of H2O.The band position changes with the distor-tion as well as the mode of connection of the TaO6units.These tantalate photocatalysts consist of corner-shared TaO6octahedra in their crystal structures.A study on the luminescent properties has concluded that the closer the M–O–M bond angle is to180 , the more the excitation energy is delocalized.12This tendency indicates that photogenerated electron-hole pairs can migrate relatively easily in the corner-shared framework of TaO6units. The present authors have applied this rule to the factors that af-fect photocatalytic performance.8,10The mobility of the elec-tron-hole pair a?ects the photocatalytic activity as well as the conduction band level because it a?ects the probability of elec-trons and holes to reach reaction sites on the surface;this is an important process,as shown in step(ii)in Figure2.

These tantalate photocatalysts were also active for the re-duction of nitrate ions to N2.13NiO/KTaO3:Zr,14NiO/ RbLaTa2O7,15and H2SrTa2O716have been reported as tantalate photocatalysts for water splitting.

NiO/NaTaO3was the most active,as shown in Table1.The photocatalytic activity of NiO/NaTaO3increased remarkably with doping of lanthanoid ions.17An optimized NiO(0.2wt%)/ NaTaO3:La(2%)photocatalyst showed high activity,with an apparent quantum yield of56%for water splitting.Under irradi-ation of the light from a400-W high pressure mercury lamp,H2 and O2evolved at rates of19.8and9.7mmol hà1,respectively, as shown in Figure3.Gas evolution in the form of bubbles was actually observed.The activity was stable for more than400h.

The reaction scheme for the water splitting on the NiO/Na-TaO3:La photocatalyst was clari?ed by nanoscale characteriza-tion,as shown in Figure4.Electron microscope observations re-vealed that the particle size of the NaTaO3:La crystal(0.1–0.7m m)was smaller than that of the nondoped NaTaO3crystal (2–3m m)and that the ordered surface nanosteps were created by lanthanum doping.The small particle size with high crystal-linity was advantageous in terms of increasing the probability of the reactions of photogenerated electrons and holes with water molecules,rather than recombination.Transmission electron mi-croscope observations and extended X-ray absorption?ne struc-ture analyses indicated that the NiO cocatalyst was loaded as ul-tra-?ne NiO particles on the edges of the nanostep structures. The H2evolution site of the edge was e?ectively separated from the O2evolution site of the groove at the surface nanostep struc-ture.This separation is advantageous,especially for water split-ting in order to avoid the back reaction.Doping of Ca,Sr,and Ba Table1.Water splitting on alkali metal and alkaline earth tan-talate photocatalysts

Catalyst BG NiO Activity/m mol h Ref

/eV/mass%H2O2

K3Ta3Si2O13 4.1none53237 K3Ta3Si2O13 4.1 1.33902007 LiTaO3 4.7none4302208 LiTaO3 4.70.198528 NaTaO3 4.0none160868 NaTaO3 4.00.05218011008 KTaO3 3.6none29138 KTaO3 3.60.05738 CaTa2O6 4.0none2188 CaTa2O6 4.00.172328 SrTa2O6 4.4none140669 SrTa2O6 4.40.19604909 BaTa2O6 4.1none33158 BaTa2O6 4.10.36293038 Sr2Ta2O7 4.6none531810 Sr2Ta2O7 4.60.151******** K2PrTa5O15 3.8none10311 K2PrTa5O15 3.80.1155083011 Alkali metal tantalates were prepared in the presence of excess amounts(5%)of alkali metal.

Photocatalyst:1.0g;water:390mL;reaction cell:inner irradi-ation-type reaction cell made of quartz;light source:400-W high pressure mercury lamp.

Reaction condition:

Photocatalyst:1g,

1mmol/L NaOH:390mL,

Inner irradiation reaction cell

made of quartz with 400-W

high pressure mercury lamp

t / h

Figure3.Total gas evolution from water on the NiO/NaTaO3: La photocatalyst under UV irradiation.

surface

reaction sites

(iii) Construction of

for H2 evolution

H2O

Figure2.Processes in photocatalytic reactions.

Published on the web(Advance View)October30,2004;DOI10.1246/cl.2004.1534

also produced the same e?ect as that of La on the formation of the characteristic morphology of NaTaO 3and the improvement of photocatalytic performance.18The dynamics of photogenerat-ed electrons in the NaTaO 3photocatalyst have been studied by time-resolved IR measurements.19

Niobates such as NiO/Sr 2Nb 2O 710,20and NiO/ZnNb 2O 6,21which belong to the same group as the tantalates,were also ac-tive for water splitting,although the loading of the NiO cocata-lyst and pretreatment for its activation were indispensable.The di?erence in the photocatalytic properties between the tantalates and the niobates is mainly due to the conduction band levels.The conduction band energy associated with Ta 5d is higher than that associated with Nb 4d in the same crystal structure.10

4.Visible-light-driven Photocata-lysts for H 2or O 2Evolution Developed by Band Engineering

Suitable band engineering is needed in order to develop new photocatalysts for water splitting under visible light irradiation,as mentioned in Section 2.The strategies are shown in Figure 5.In general,the conduction bands of stable oxide semiconductor photocatalysts are usually based on the metal cations with d 0and d 10con?gurations,and consist of the empty orbitals (LUMOs)of their metal cations.10,22The conduction band (CB)level should be higher than the reduction potential of H 2O,i.e.,that to pro-duce H 2(E o ?0V).On the other hand,the potential of the va-lence band (VB),based on O 2p orbitals (ca.+3eV),is consider-ably more positive than the oxidation potential of H 2O,i.e.,that to produce O 2(E o ?1:23V).Therefore,the band gaps of oxide semiconductor photocatalysts inevitably become wider than the minimum necessary for water splitting.Accordingly,a new va-lence band or an electron donor level (DL)must be formed with orbitals of elements other than O 2p to make the band gap (BG)or the energy gap (EG)narrower.Making a solid solution is also a useful band engineering stratagem.We have developed visible-light-driven photocatalysts according to these strategies,as shown in Table 2.23–36The activities of well-known Pt/CdS and WO 3photocatalysts are also indicated as references.These

reactions were carried out in the presence of sacri?cial reagents.These reactions are regarded as half-reactions for water splitting and are often employed in test reactions of photocatalytic H 2or O 2evolution.

4.1.Transition Metal-doped Photocatalysts

The authors have paid attention to SrTiO 3and ZnS as host photocatalysts,in addition to TiO 2.Pt/SrTiO 3codoped with a combination of either antimony or tantalum and chromium evolved H 2from an aqueous methanol solution under visible light irradiation ( >420nm).23,24The codoping of antimony was also e?ective for a TiO 2photocatalyst doped with chromi-um for O 2evolution from an aqueous silver nitrate solution,whereas TiO 2doped with chromium alone showed no photoca-talytic activity.These photocatalysts showed absorption bands in the visible light region,as shown in Figure 6.The energy gap (EG)transition,not the band gap (BG)transition,from the donor level (DL)formed by Cr 3tto the conduction bands (CBs)of SrTiO 3and TiO 2corresponds to the visible light absorption.The charge balance was maintained by codoping of Sb 5tand Ta 5t,resulting in the suppression of the formation of Cr 6tions and oxygen defects in the lattice.

Rh-doped SrTiO 3loaded with a Pt cocatalyst produced H 2from an aqueous methanol solution with a quantum yield of 5.2%at 420nm.25The visible light response was due to the tran-sition from the electron donor level formed by the Rh ions to the conduction band composed of the Ti 3d orbitals of SrTiO 3.This is a novel oxide photocatalyst that is active for H 2evolution under visible light irradiation.

Cu-and Ni-doped ZnS powders 26,27(Zn 0:957Cu 0:043S and Zn 0:999Ni 0:001S)showed photocatalytic activities for H 2evolu-tion from aqueous potassium sul?te and sodium sul?de solu-tions.ZnS codoped with Pb and Cl was also active for H 2

evo-

Figure 4.Mechanism of water splitting on a surface nanostep of the NiO/NaTaO 3:La photocatalyst.

Table 2.Visible-light-driven photocatalysts for H 2or O 2volution from aqueous solutions in the presence of sacri?cial re-agents

BG Sacri?cial Activity /m mol h à1

Catalyst (EG)reagent

H 2O 2Ref

/eV

Pt/CdS 2.4K 2SO 3850—WO 3

2.8AgNO 3—65Pt/SrTiO 3:Cr,Sb 2.4CH 3OH 78—23Pt/SrTiO 3:Cr,Ta 2.3CH 3OH 70—24TiO 2:Cr,Sb 2.5AgNO 3—4223Pt/SrTiO 3:Rh 2.3CH 3OH 117—25ZnS:Cu 2.5K 2SO 3

450—26ZnS:Ni 2.3Na 2S+K 2SO 3280—27ZnS:Pb,Cl 2.3Na 2S+K 2SO 340—28Pt/SnNb 2O 6

2.3CH 3OH 20—29BiVO 4 2.4AgNO 3—42130AgNbO 3 2.86AgNO 3—3731Ag 3VO 4 2.0AgNO 3—1732Bi 2WO 4 2.8AgNO 3—333Pt/AgInZn 7S 9 2.4Na 2S+K 2SO 3940—34

Pt/NaInS 2 2.3K 2SO 3470—35In 2O 3(ZnO)3 2.6AgNO 3— 1.336Pt/In 2O 3(ZnO)3

2.6CH 3OH

1.1

—

Light source:300-W Xe lamp; >420nm.

Figure 5.Three types of band engineering for the design of

visible-light-driven photocatalysts.

lution.28It is noteworthy that these photocatalysts showed high activities without cocatalysts such as platinum.This means that these photocatalysts possess active catalytic sites and su?ciently high conduction band levels for H 2O reduction to produce H 2,as well as a non-doped zinc sul?de photocatalyst.37The Zn 0:999-Ni 0:001S photocatalyst was also active for the reduction of nitrate and nitrite ions under visible light irradiation in the presence of a sacri?cial reagent.38

These dopants are replacements at metal cation sites.There-fore,doping here means lattice substitution.Doping is regarded as an unsuitable method because,in most cases,the dopant works as a recombination center between photogenerated elec-trons and holes.However,these results indicate that doping is a suitable method,if good combinations of hosts and dopants are chosen.Recently,N-and S-doped TiO 2powders have been studied extensively as visible-light-driven photocatalysts for de-composition reactions.39,40

4.2.Valence Band-controlled Photocatalysts

The formation of a recombination center is not completely suppressed in the doping system mentioned above.The absorp-tion coe?cient should depend on the amount of the dopant.Moreover,the impurity level formed by the doping is usually discrete and is inconvenient for the migration of photogenerated holes in that level.Therefore,a continuous valence band should be formed with some orbitals besides O 2p .Here,Bi 3tand Sn 2t,with ns 2con?gurations,and Ag t,with a d 10con?guration,at-tracted our attention as candidates that can form such valence bands.

SnNb 2O 6is a novel oxide photocatalyst for H 2evolution,as is SrTiO 3:Rh.29BiVO 4,with a scheelite (monoclinic)struc-ture,30AgNbO 3,with a perovskite structure,31and Ag 3VO 432possessed photocatalytic activities for O 2evolution from aque-ous silver nitrate solutions under visible light irradiation ( >420nm).The photocatalytic activity of BiVO 4was much higher than that of commercial WO 3with a 2.8-eV band gap,which is a well-known photocatalyst for O 2evolution reaction under visible light irradiation.41The BiVO 4and AgNbO 3photo-catalysts were also active for the decomposition of the endocrine disrupter 4-nonylphenol under visible light irradiation.42The BiVO 4photocatalyst was synthesized by simply stirring a vana-date powder such as K 3V 5O 14with Bi(NO 3)3.

5H 2O powder as a dispersion in water at room temperature for 3days,as shown in Figure 7.The photocatalytic activity of the scheelite BiVO 4pre-pared by the aqueous process was much higher than that of BiVO 4prepared by a conventional solid-state reaction,even in

the same crystal structure.30

These valence band-controlled photocatalysts possessed steep absorption edges in the visible light region,as shown in Figure 8,being di?erent from the bands consisting of impurity levels,as shown in Figure 6.The steep edges indicate that the visible light absorption is due to a band–band transition.The va-lence bands of BiVO 4,SnNb 2O 6,and AgNbO 3consist of the Bi 6s ,Sn 5s ,and Ag 4d orbitals,respectively,mixed with O 2p ,re-sulting in a raising of the valence band levels and a decrease in the band gaps.It was con?rmed that Ag tand Bi 3twere also e?ective in decreasing the band gaps of AMO 4(M =Mo and W)photocatalysts with scheelite structures,although these photoca-talysts responded to only UV.43Bi 2WO 4,with the Aurivillius structure,was also active for O 2evolution.33

N 2p and S 3p are also suitable orbitals to form valence bands,as seen in nitride,oxynitride,and oxysul?de photocatalysts with visible light response.44

4.3.Solid Solution Photocatalysts

Although metal sul?des always have a problem of photocor-rosion,they are attractive as photocatalysts with visible light re-sponse.A representative photocatalyst is Pt/CdS which is active for H 2evolution under visible light irradiation.The photocorro-sion is considerably suppressed in the presence of sacri?cial re-agents.ZnS is an interesting photocatalyst material from the viewpoint of its preeminent ability to produce H 2,37as con?rmed for Cu-and Ni-doped ZnS.ZnS is able to form (AgIn)x -Zn 2e1àx TS 2solid solutions with a narrow band gap semiconduc-tor,AgInS 2.45The solid solutions include Ag,which is expected to contribute to valence band formation,as observed for an AgNbO 3photocatalyst.31(AgIn)x Zn 2e1àx TS 2solid solutions showed photocatalytic activities for H 2evolution from aqueous solutions containing sacri?cial reagents,SO 32àand S 2à,under visible light irradiation ( >420nm)even without Pt cocata-lysts.34Loading of the Pt cocatalyst improved the photocatalytic activity.Pt (3wt %)-loaded (AgIn)0:22Zn 1:56S 2,with a 2.3-eV

Bi(NO 3)3?5H 2

Vanadates

+H 2O

450 nm

Figure 7.Synthesis of BiVO 4photocatalyst at room tempera-ture and ambient pressure in aqueous media.

350

400

450500550600650

Wavelength / nm

A b s o r b a n c e / a r b .u n i t s

Figure 8.Di?use re?ection spectra of valence band-controlled photocatalysts containing of Bi,Sn,and Ag.

300

400

500600700800

Wavelength / nm

A b s o r b a n c e / a r b . u n i t s

Figure 6.Di?use re?ection spectra of SrTiO 3and TiO 2photo-catalysts codoped with Cr and Sb.

band gap,showed the highest activity for H 2evolution and the apparent quantum yield at 420nm amounted to 20%.H 2gas evolved at a rate of 3.3L m à2h à1under irradiation using a solar simulator (AM 1.5).M I M III S 2(M I :Ag and Cu,M III :Ga and In)and their solid solutions were also active for H 2evolution.NaInS 2was also found to be a unique sul?de photocatalyst with a layered structure.35

It is important to see the action spectrum in order to evaluate a visible-light-driven photocatalyst.The onset of the action spec-trum of the (AgIn)0:22Zn 1:56S 2photocatalyst agreed with the ab-sorption edge,indicating that the reaction proceeded photocata-lytically via the band gap transition from the valence band,con-sisting of S 3p and Ag 4d orbitals,to the conduction band,consist-ing of Zn 4s4p and In 5s5p orbitals,as shown in Figure 9.It was found from SEM and TEM observations that the solid solutions partly had surface nanostep structures on their surfaces.Pt coca-talysts were selectively photodeposited as nano-dots or nano-beads on the edges of the surface nanosteps.The speci?c surface nanostructure was e?ective for the suppression of recombination between photogenerated electrons and holes,and the separation of H 2evolution sites from oxidation reaction sites,as with the NiO/NaTaO 3:La photocatalyst.

These sul?de photocatalysts that show high activities for H 2evolution are expected to turn into a practical application for H 2production using by-products such as hydrogen sul?de emitted from the hydrogenated desulfurization process at petrochemical plants and in the mining industries.

Nb 2O 5–Bi 2O 3,46Ga 2O 3–In 2O 3,47Sr 2Nb 2O 7–Sr 2Ta 2O 7,48SnO 2–TiO 2,49ZnS–CdS,50and CdS–CdSe 51solid solution pho-tocatalysts have been reported,and their photophysical proper-ties and photocatalytic activities are dependent on their compo-sitions.

5.Overall Water Splitting under Visi-ble Light Irradiation by a Two-photon Process

Sayama,Arakawa,and co-workers reported overall water splitting by the Z-scheme for a system that consisted of a Pt/SrTiO 3:Cr,Ta photocatalyst for H 2evolution,24a WO 3photoca-talyst for O 2evolution,41and the IO 3à/I àredox couple.52This system gave an apparent quantum yield of 0.1%and responded to 450nm,which was limited by the band gap of WO 3.

We have evaluated the visible-light-driven photocatalysts listed in Table 2for the construction of Z-scheme photocatalysis systems.Pt/SrTiO 3:Rh functioned as a photocatalyst for H 2pro-

duction using Fe 2tions,while BiVO 4and Bi 2MoO 6were active photocatalysts for O 2production using Fe 3tions.Thus,overall water splitting under visible light irradiation has been achieved by using these photocatalysts,which possess activity for the half-reactions of water splitting and the Fe 3t/Fe 2tredox couple,as shown in Figure 10.53The apparent quantum yields of the (Pt/SrTiO 3:Rh)–(BiVO 4),–(Bi 2MoO 6),and –(WO 3)systems were 0.3,0.2,and 0.2%,respectively,at 440nm.The (Pt/SrTiO 3:Rh)–(BiVO 4)system responded to visible light up to 520nm.

? 6.Summary and Outlook

NiO/NaTaO 3:La was found to be a highly active photocata-lyst for water splitting,a very demanding reaction,under UV light irradiation.This result has proven that highly e?cient wa-ter splitting is actually possible using a particulate photocatalyst system.Moreover,although sacri?cial reagents are needed,var-ious oxide and sul?de photocatalysts for H 2or O 2evolution un-der visible light irradiation have been developed.E?cient H 2evolution was demonstrated for the sul?de solid solution photo-catalysts with a solar simulator (AM 1.5).Overall water splitting has been accomplished by constructing systems involving the two-photon process,combining photocatalysts developed by the present authors.

It is said that the target for photocatalytic water splitting is to develop photocatalysts with a 2-eV band gap and a 30%quan-tum yield.There is still a di?cult barrier to surmount in order to accomplish this target.The present authors believe that it is important to develop a library of photocatalyst materials.This work will clarify the factors dominating photocatalytic proper-ties and give information for the design of highly active photo-catalysts.Continuing this research will lead us to ?nd highly ac-tive photocatalyst systems for H 2production from water using solar light energy to achieve arti?cial photosynthesis.

?Acknowledgements

This work was partly supported by a Grant-in-Aid for Scienti?c Research on Priority Areas (417)from the Ministry of Education,Culture,Sports,Science and Technology of the Japanese Government.Professor Kobayashi of Kurashiki University of Science and the Arts is acknowledged for his help with the DFT calculations.

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Figure 9.Action spectrum for H 2evolution on the (AgIn)0:22-Zn 1:56S 2solid solution photocatalyst.

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