2010-JMC-Heterogeneous+photocatalytic+cl...

Heterogeneous photocatalytic cleavage of waterMasaaki Kitano and Michikazu Hara *Received 22nd May 2009,Accepted 27th July 2009First published as an Advance Article on the web 20th August 2009DOI:10.1039/b910180bThe photocatalytic cleavage of water has become of increasing interest in recent years for the manufacture of hydrogen,a clean and renewable energy carrier.The production of H 2usingheterogeneous photocatalysts is reviewed in this feature article.Particulate or electrode heterogeneous catalysts under light irradiation can successively reduce H +into H 2and oxidize water to produce O 2,achieving cleavage of water (H 2O /H 2+1/2O 2).1.IntroductionIn the 21st century,hydrogen is a potential candidate for a clean and high-energy density energy carrier.H 2itself is a clean material,because the combustion of H 2produces only pure water with a large amount of heat release.H 2+1/2O 2/H 2O;D G 0¼À237kJ mol À1(1)However,H 2is currently produced by reforming reactions of hydrocarbon resources such as the steam reforming of methane.1CH 4+2H 2O /4H 2+CO 2;D G 0¼131kJ mol À1(2)In these cases,H 2production is always accompanied by CO 2formation,meaning that such H 2should not be regarded as a clean energy carrier.From this point of view,the only clean hydrogen resource on the earth is water,by the reverse reaction of (1).H 2O /H 2+1/2O 2;D G 0¼237kJ mol À1(3)This reaction is,of course,accompanied by a large amount of energy release;therefore,it is essential that this process utilizes clean and recyclable energies to drive reaction (3).2–5There may be several candidates,such as hydraulic power,wind force and even nuclear power.Among them,solar energy is one of the most favorable energy resources because it is essentially clean and inexhaustible,and it sustains all life on the earth.Table 1provides information regarding solar energy.The sun is steadily emitting a large amount of energy (1.2Â1034J year À1)and only a small portion of it is utilized for energy production on the earth.Approximately one half of the energy (3.0Â1024J year À1)can penetrate the atmosphere to reach the sea and land surfaces.6,7This is the energy available for use on the earth.It should be emphasized that the photosynthesis reaction in nature fixes 0.1%of the solar energy on the earth.The energy consumption in the world by human beings (4.0Â1020J year À1)is about 0.01%of the solar energy.6,7It is also interesting that the total amount of energy accumulated in all fossil resources corresponds to the solar energy received by the earth over only 7days.The amount of solar energy available on the earth far exceeds that required to maintain modern civilization;however,Materials and Structures Laboratory,Tokyo Institute of Technology,4259Nagatsuta,Midori-ku,Yokohama,226-8503,Japan.E-mail:mhara@msl.titech.ac.jp;Fax:+81-45-924-5381;Tel:+81-45-924-5381Masaaki Kitano Masaaki Kitano received his PhD degree at Osaka Prefecture University in 2006.Following postdoctoral research experi-ences in Osaka Prefecture University for a year and Kanagawa Academy of Science and Technology for two years,he joined Prof.Hara’s research groups in Tokyo Institute of Technology as an assistant professor in 2009.He has con-ducted extensive research onphotocatalysts with Prof.Masakazu Anpo at OsakaPrefecture University,and his current research has focused on solid acidcatalysts.Michikazu Hara Michikazu Hara obtained his PhD degree in chemistry at Tokyo Institute of Technology in 1992.In 2006,as a Professor,he moved to the Materials and Structures Laboratory in the same university and concur-rently holds the post of the project leader for the ‘‘Solid acid eco-catalyst’’project at Kana-gawa Academy of Science and Technology (KAST).Dr Hara has conducted aggressiveresearch on the development ofsolid catalysts for the environ-mentally benign production of chemicals and energy.He received a fifth Scientific American 50award (2006)because of the carbon catalyst presented in this feature article.FEATURE ARTICLE /materials |Journal of Materials ChemistryD o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180Bif the surface area used to fix sufficient solar energy to maintain civilization was estimated,it would be very large.Even if a system with a solar energy conversion efficiency of 10%was available,the area required would be in the order of 105km 2.This means that the method of solar energy conversion has to have applicability to such a wide area.The decomposition of water into H 2and O 2using heteroge-neous photocatalysts is one method for the conversion of solar energy into chemicals,2–7including biomass conversion and the electrolysis of water using a solar cell,and was initiated by the work of Honda and Fujishima,who demonstrated that overall water splitting can be achieved using a photoelectrochemical (PEC)cell consisting of a single-crystalline TiO 2(rutile)anode and a Pt cathode under ultraviolet (UV)irradiation and with an external bias.8,92.Bases of photocatalytic water splitting2.1Band structurePhotocatalytic reactions proceed on semiconductor materials,as schematically shown in Fig.1.Semiconductors have a band structure in which the conduction band is separated from the valence band by a band gap of suitable width.When the energy of incident light is larger than that of the band gap,electrons in the valence band are excited to the conduction band and holes are generated in the valence bands.The photogenerated electrons and holes cause redox reactions similar to electrolysis.Water molecules are reduced by electrons to form H 2and oxidized by the holes to form O 2to provide overall water splitting.The important characteristics of the semiconductor photocatalyst materials are the width of the band gap and the levels of the conduction and valence bands.The lower level of the conduction band must be more negative than the redox potential of H +/H 2(0V vs.NHE).Furthermore,the top level of the valence band must be more positive than the redox potential of O 2/H 2O (1.23V).Therefore,the theoretical minimum band gap for water splitting is 1.23eV,which corresponds to light of approximately 1100nm.Not all semiconductors that satisfy the above conditions can decompose water into H 2and O 2.As determined from the band structure,some chalcogenide semiconductors,such as CdS,are regarded as suitable photocatalysts for the cleavage of water under visible light irradiation.However,CdS rather than H 2O isoxidized by photogenerated holes,which is accompanied by the elution of Cd 2+according to Eq.(4).10CdS +2h +/Cd 2++S(4)This reaction is referred to as photocorrosion and is often a disadvantage for metal sulfide photocatalysts.ZnO also undergoes photocorrosion under band gap excitation,even though it is an oxide photocatalyst.ZnO +2h +/Zn 2++1/2O 2(5)As shown in Fig.2,the overall water splitting reaction on a semiconductor photocatalyst occurs in three steps:(1)the photocatalyst absorbs photon energy greater than the band gap energy of the material and generates photoexcited electron-hole pairs in the bulk,(2)the photoexcited carriers separateandFig.1Basic principle of overall water splitting on a heterogeneous photocatalyst.Reprinted with permission from ref.3.Copyright the American Chemical Society2007.Fig.2Processes involved in photocatalytic overall water splitting on a heterogeneous photocatalyst.Reprinted with permission from ref.3.Copyright the American Chemical Society 2007.Table 1Amount of solarenergyD o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180Bmigrate to the surface without recombination,and (3)adsorbed species are reduced and oxidized by the photogenerated electrons and holes to produce H 2and O 2,respectively.The first two steps are strongly dependent on the structural and electronic proper-ties of the photocatalyst.Generally,high crystallinity has a positive effect on catalytic activity,because the density of defects,which act as recombination centers between photo-generated carriers,decreases with increasing crystallinity.However,the third step is promoted by the presence of a solid co-catalyst.The co-catalyst is typically a noble metal (e.g.,Pt,Rh)or a metal oxide (e.g.,NiO,RuO 2),which is loaded onto the photocatalyst surface as a dispersion of nanoparticles to produce active sites and reduce the activation energy for gas evolution.While co-catalysts are essential to bring out the potential pho-tocatalytic performance in photocatalysts,especially visible light-driven photocatalysts,all of the functions of each co-cata-lyst have not yet been clarified;the optimal combination of co-catalyst and photocatalyst is different for each photocatalyst.Therefore,it is important to design both the bulk and surface properties of a photocatalyst material carefully,so as to obtain high photocatalytic activity for the reaction.2.2Water cleavage by particulate photocatalysts and photocatalyst electrodesThe photocatalytic cleavage of water is morphologically divided into water splitting by using particulate photocatalysts and photocatalyst electrodes.The photoelectrochemical cell comprised of TiO 2and Pt electrodes developed by Honda and Fujishima has resulted in a large number of photo-electrochemical cells that are designed specifically for the efficient utilization of solar energy.However,due to the lack of suitable photoelectrode materials with appropriate band gap structures and stability,the systems proposed to date have been developed into complicated integrated multilayer and tandem systems.11–13On the other hand,many solid materials have been developed to date for the cleavage of water using particulate photocatalysts.The concept of water splitting using a particulate photocatalyst is simple.Sunlight shines on photocatalyst particles dispersed in a pool with water,and hydrogen is then readily evolved.More-over,powdered photocatalyst systems are advantageous for large-scale solar water-splitting applications,due to thesimplicity of design.While many serious and fundamental problems remain,such as how to construct such a pool without leaking of evolved H 2and O 2,how to transfer water to a pool in an area under large solar irradiation,how to separate H 2from the explosive H 2and O 2mixture,and total life cycle assessment,studies using particulate photocatalysts have revealed that many solid materials function as stable photocatalysts for water cleavage.These materials will provide new developments and advance water cleavage technology using photoelectrochemical cells that are favorable for the separation of H 2from a H 2and O 2mixture.2.3Materials available for the photocatalytic cleavage of water Until the middle of the 1980s,research on particulate photo-catalysts for overall water splitting was largely devoted to SrTiO 3and TiO 2.14–17Since the discovery of K 4Nb 6O 17,which has a layered structure and high photocatalytic activity,18,19photo-catalysts with unique structures,such as layered,tunnel,and pillared structures,have also been reported.20–25Since the latter half of the 1990s,many tantalates have been reported to be highly active photocatalysts.24,26–30ZrO 2has also been reported to be a unique photocatalyst,exhibiting high activity even without a co-catalyst.31Inoue and co-workers reported typical metal oxides with d 10electronic configuration as a new class of photocatalysts suitable for overall water splitting.32–38Fig.3shows the groups of elements that can form suitable band gap structures of the corresponding oxide materials,and which have been successfully applied as photocatalysts for overall water splitting.In all cases,the oxidation states of the metallic elements in such photocatalysts are the highest,that is,Ti 4+,Zr 4+,Nb 5+,Ta 5+and W 6+in the red group,and Ga 3+,In 3+,Ge 4+,Sn 4+,and Sb 5+in the green group (as indicated in Fig.3).Photocatalysts based on transition-metal cations with empty d orbitals are defined as having a d 0electronic configuration (red group).On the other hand,those based on typical element cations with filled d orbitals are defined as having a d 10electronic configuration (green group).Although other metal oxides with d 0or d 10elec-tronic configurations,such as V 5+,Mo 6+,and Zn 2+,cannot decompose water by themselves,Z-scheme type reaction systems containing these elements function as catalysts for the photo-catalytic cleavage of water (seebelow).Fig.3Principal cation components for photocatalytic overall water splitting.Reprinted with permission from ref.3.Copyright the American Chemical Society 2007.D o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180B3.Wide band gap photocatalysts that decompose water into H 2and O 2under UV-light irradiation3.1Oxides with d 0electronic configuration3.1.1Group 4metal oxides.TiO 2was the most well-known photocatalyst for the cleavage of water under UV-light irradia-tion,since the discovery of the Honda-Fujishima effect,with overall water splitting achieved with a photoelectrochemical cell using a TiO 2electrode with some external bias.8,9However,this abundant oxide particulate itself cannot decompose water without any bias.Photocatalytic cleavage of water does not proceed on TiO 2loaded with Pt,which is usually an excellent co-catalyst for the reduction of H +,because Pt also functions as a catalyst for the rapid formation of H 2O from the H 2and O 2produced.Sato et al.found that NaOH-coated TiO 2decomposes water vapor into H 2and O 2,because the reverse reaction on Pt is suppressed.39The photocatalytic cleavage of water by particulate TiO 2suspended in aqueous solution was realized 25years after the discovery of the Honda-Fujishima effect,although this reaction system requires a large amount of NaCO 3;complex photoreactions with NaCO 3,in addition to the redox reaction on the TiO 2photocatalyst under UV light,result in overall water splitting.40SrTiO 3is also a well-known photocatalyst.The potential of the conduction band edge of SrTiO 3is more negative (ca.À0.3V vs.NHE at pH ¼0)than that of TiO 2(ca.À0.1V vs.NHE at pH ¼0),so that SrTiO 3can be used to split water without an external bias,unlike TiO 2.41,42Domen and co-workers first suc-ceeded in the photocatalytic cleavage of water on a particulate NiO-loaded SrTiO 3photocatalyst suspended in water.17,43–46The NiO co-catalyst for H 2evolution is usually activated by H 2reduction and subsequent O 2oxidation to form a NiO/Ni double layer structure that is convenient for electron migration from the photocatalyst substrate to the co-catalyst.44The pretreated NiO co-catalyst is often denoted as NiOx in the literature.It should be noted that the reverse reaction between H 2and O 2does not proceed on the NiO co-catalyst,which is distinct from the Pt co-catalyst.The excellent NiO co-catalyst has often been employed with many overall water-splitting photocatalysts.Rh is also a suitable co-catalyst for the SrTiO 3photocatalyst.16Many Ti 4+-containing oxide photocatalysts such as Sr 3Ti 2O 7,47Sr 4Ti 3O 10,48La 2Ti 2O 7,25,49–57Ba-doped La 2Ti 2O 7,49KLaZr 0.3Ti 0.7O 458and La 4CaTi 5O 1725that have layered perovskite structures,i.e.perovskite slabs of SrTiO 3,provide high quantum yields for the cleavage of water under UV light.Some Ti 4+-containing oxide photocatalysts exhibit high performance for the cleavage of water,due to their unique structures.A 2La 2Ti 3O 10(A:K or Rb)is a layered Ti 4+-perovskite similar to the Ti 4+-containing oxides previously mentioned,and water molecules are intercalated into the interlayer spaces,as shown in Fig.4.The photocatalytic cleavage of water by NiOx-loaded Rb 2La 2Ti 3O 10is shown in Fig.5.22,59Highly efficient reactions proceed on the photocatalyst,which are attributed to the specific structure of A 2La 2Ti 3O 10with intercalated water molecules in the interlayer spaces,as shown in Fig.6.Holes generated in the layered perovskite oxidize water molecules to produce O 2,and photoelectrons reduce H +to H 2through the NiOx co-catalyst particles loaded only on the external surface ofthe photocatalyst;H 2and O 2evolution at different reaction sites causes efficient charge separation,resulting in the high perfor-mance.Such reaction systems that depend largely on specific structures are also found for Na 2Ti 6O 1320,60–62and BaTi 4O 921,62–67with tunnel structures.In this case,it is expected that distortion in the crystals plays an important role in efficient charge separation.ZrO 2is also one of the group 4oxide photocatalysts able to decompose water into H 2and O 2.31,68–71ZrO 2has a considerably wide gap (ca.5eV),and the potentials of the conductionandFig.4Schematic structures of A 2La 2Ti 3O 10(A:K,Rb).Fig.5Photocatalytic cleavage of water by NiOx-Rb 2La 2Ti 3O 10(A:K,Rb).Catalyst:1.0g;light source:450W high-pressure Hg lamp.The reaction system was evacuated every 5h.Fig.6Schematic mechanism for the cleavage of water by A 2La 2Ti 3O 10.D o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180Brespectively.This means that ZrO 2has excess overpotentials for the reduction of H +and the oxidation of H 2O,resulting in overall water splitting without a co-catalyst.This photocatalyst is active even for CO 2reduction to CO accompanied with O 2evolution by the oxidation of water without any sacrificial reagents.313.1.2Group 5metal oxides.It has been reported that many oxides containing Nb 5+or Ta 5+function as photocatalysts for the cleavage of water.Several Nb 5+oxides such as K 4Nb 6O 17,18,46,72–78Rb 4Nb 6O 17,77Ca 2Nb 2O 7,25Sr 2Nb 2O 7,25,79–81Ba 5Nb 4O 15,82NaCa 2Nb 3O 10,83ZnNb 2O 6,84Cs 2Nb 4O 1185and La 3NbO 755,86are photocatalysts for overall water splitting in UV light.In the case of Ta 5+oxides,Ta 2O 5,68K 2PrTa 5O 15,87K 3Ta 3B 2O 12,88LiTaO 3,89NaTaO 3,26,28KTaO 3,26,28,77AgTaO 3,90Zr-doped KTaO 3,91,92La-doped NaTaO 3,30,93Sr-doped NaTaO 3,94Na 2Ta 2O 6,95K 2Ta 2O 6,95,96CaTa 2O 6,97SrTa 2O 6,97BaTa 2O 6,26,97NiTa 2O 6,26Rb 4Ta 6O 17,77Ca 2Ta 2O 7,95Sr 2Ta 2O 7,79–81,98K 2SrTa 2O 7,99RbNdTa 2O 7,27,100–102H 2La 2/3Ta 2O 7,103K 2Sr 1.5Ta 3O 10,104LiCa 2-Ta 3O 10,105KBa 2Ta 3O 10,25Sr 5Ta 4O 15,98Ba 5Ta 4O 15,106H 1.8Sr 0.81Bi 0.19Ta 2O 7,107Mg-Ta oxide,108LaTaO 4,109and La 3TaO 7,55,86have been reported as active photocatalysts.The band structures of the Ta 5+oxides have excess overpotentials for the reduction of H +and oxidation of H 2O,similar to that for ZrO 2.Therefore,many Ta 5+-based oxides exhibit high photo-catalytic performance for the water splitting reaction,compared with other oxides that have d 0electronic configuration.Among the tantalates,NaTaO 3should be noted as the most active photocatalyst for water splitting.While NiO-loaded NaTaO 3also has highly efficient performance,26,28La(2%)-doped NaTaO 3with a NiO co-catalyst exhibits remarkable perfor-mance for the cleavage of water:the apparent quantum yield reaches 56%,although the photocatalyst functions under UV-light irradiation.30,93The activity is stable for more than 400h under irradiation of light from a 400W high-pressure mercury lamp.The evolution of H 2and O 2as bubbles can be observed when the photocatalyst is irradiated with UV from a 200W Xe-Hg lamp,as shown in Fig.7.5Only light,water and photocatalyst powder are present in the system.It is amazing that the reduction and oxidation of water,completely opposite reactions,simultaneously proceed on the same surface of a nanoparticle.ATaO 3(A:Li,Na,and K)have similar perov-skite-like structures consisting of corner-sharing TaO 6octa-hedra,as shown in Fig.8.5,28However,NaTaO 3has much higher performance than LiTaO 3and KTaO 3,which suggests that the photocatalytic activities of ATaO 3depend largely on the A-site cation in the perovskite-like structure.The bond angles of Ta–O–Ta are 143 (LiTaO 3),163 (NaTaO 3)and 180 (KTaO 3)in octahedral coordination.Wiegel and co-workers have reported the relationship between crystal structures and energy delocal-ization for ATaO 3alkali tantalates.110As the bond angle approaches 180 ,excited energy or electron-hole pairs in the crystal migrate more easily and the band gap becomes narrower.Therefore,the order of the delocalization of excited electrons or electron-hole pairs is LiTaO 3<NaTaO 3<KTaO 3,while the order of the band gap is reversed.The degree of localization affects step (2)in Fig.2,as seen for Na 2Ti 6O 1320,60–62and BaTi 4O 921,62–67with tunnel structures.Thus,it is expected that the high activity of NaTaO 3is due to the suitable conductionband level consisting of Ta 5d and energy delocalization caused by the slight distortion of TaO 6connections.Morphological characteristics also contribute to the highly photocatalytic activity in La(2%)-doped NaTaO 3with NiO -doped NaTaO 3has a smaller particle size (0.1–0.7m m)than non-doped NaTaO 3crystals (2–3m m).It is common that small particle photocatalysts exhibit low crystal-linity,which results in low photocatalytic activity,due to an increase in electron-hole recombination reactions (step (2)in Fig.2).However,the La-doped NaTaO 3particles are not only small,but also have high crystallinity.This is advantageous in terms of increasing the probability of reactions of photo-generated electrons and holes with water molecules,rather than recombination.Ordered nanosteps on the particle surface are also one of the reasons for the high activity of La-doped NaTaO 3.H 2evolution sites at step edges are effectively separated from the O 2evolution sites at the groove of the nanostep struc-ture.This separation of reaction sites is effective to avoid back reactions,especially for the water splitting reaction.Doping ofFig.7Water splitting using a NiO/La-doped NaTaO 3photocatalyst.Reprinted with permission from ref.5.Copyright the Royal Society of Chemistry 2009.Fig.8Crystal and energy structures of alkali tantalate photocatalysts.Reprinted with permission from ref.5.Copyright the Royal Society of Chemistry 2009.D o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180BCa,Sr and Ba also provides the same effect as La doping for the formation of the characteristic morphology of NaTaO 3and improvement of the photocatalytic activity.111Time-resolved infrared (IR)measurements reveal that La doping prolongs the lifetime of photogenerated electrons in the conduction band or the shallow trap level.111The IR absorption is due to electrons photogenerated by band gap excitation at 266nm.The increase in electron lifetime is also one of the factors for improvement of the photocatalytic ability.Thus,the notable high performance of La-doped NaTaO 3is also attributed to the morphological characteristics.3.1.3Other oxides with d 0electronic configuration.Since 2004,oxides containing W 6+or Ce 4+with d 0electronic configu-ration have also been reported to split water.PbWO 4,112,113RbWNbO 6,114RbWTaO 6,114Sr-doped CeO 2,115and BaCeO 3116function as photocatalysts for overall water splitting in UV light.3.2Oxides with d 10electronic configurationThe photocatalytic cleavage of water by oxides with d 0electronic configuration had mainly been studied since the discovery of the Honda-Fujishima effect.However,Inoue et al.reported that AIn 2O 4(A:Ca,Sr),NaSbO 3,and Sr 2SnO 4,oxides with d 10electronic configuration,also function as photocatalysts for overall water splitting in UV light.33This overturned the conventional concepts and direction of study,and had a major impact on photocatalyst development,including visible-light driven photocatalysts mentioned in the following section.Although the lower levels of the conduction bands and top levels of the valence bands of oxide photocatalysts with d 0electronic configuration are usually composed of transition metal d 0and O 2p orbitals,respectively,oxide photocatalysts with d 10electronic configuration such as NaInO 2,36,37CaIn 2O 4,33,36,37,117SrIn 2O 4,33,36,37,117,118LaInO 3,117Y x In 2Àx O 3,119NaSbO 3,33CaSb 2O 6,38Ca 2Sb 2O 7,38Sr 2Sb 2O 7,38Sr 2SnO 4,33ZnGa 2O 4,120Zn 2GeO 4,38LiInGeO 4,121and Ga 2O 3122are distinct from those of oxide photocatalysts with d 0electronic configuration.Density functional theory calculations revealed that the valence and conduction band edges of SrIn 2O 4consist of O 2p orbitals and hybridized orbitals among the In 5s,In 5p and O 2p orbitals,respectively,as shown in Fig.9.117Photoexcited electrons in broad hybridized In 5s5p orbitals with large dispersion can be readily transferred to a RuO 2co-catalyst on the surface without recombination,permitting efficient overall water splitting.3.3Nitrides with d 10electronic configurationNitrides with d 10electronic configuration,such as Ge 3N 4,123–126GaN 127and Mg-doped GaN,128,129are a new series of photo-catalysts available for the cleavage of water under UV-light irradiation.Ge 3N 4(b -Ge 3N 4)exhibits the highest activity for the reaction among these nitrides.The photocatalytic activity of RuO 2-loaded b -Ge 3N 4is considerably decreased with each reaction run of 2h,particularly in the initial period,as shown in Fig.10.The H 2/O 2production ratio throughout these reactions has a stoichiometric ratio of 2,within the experimental error.The total amount of H 2evolved over 24h was 11.2mmol,greater than the amount of catalyst (0.5g,1.8mmol of b -Ge 3N 4),whichclearly demonstrates that RuO 2-loaded b -Ge 3N 4functions as a photocatalyst for overall water splitting.It is estimated,from a comparison with the reported value for NaTaO 3,28that the quantum efficiency of water splitting on b -Ge 3N 4at around 300nm is ca.9%.The electronic structure of b -Ge 3N 4was investigated using plane wave density functional theory (DFT)calculations.Fig.11shows the energy band dispersion and density of states (DOS)for b -Ge 3N 4.The DOSs indicate that the top of the valence bands consist of N 2p orbitals,whereastheFig.9Energy band dispersion diagram and DOS for SrIn 2O 4.Reprinted with permission from ref.117.Copyright the American Chemical Society2003.Fig.10Time course of overall water splitting on RuO 2-loaded b -Ge 3N 4.118The system (0.5g of catalyst in 390cm 3aqueous solution adjusted to pH ¼0using H 2SO 4)was evacuated every 2h,and the reaction was continued to observe the change in activity.The reaction was performed in a quartz reaction vessel with internal irradiation from a high-pressure mercury lamp (450W).Open circles:H 2,closed circles:O 2,triangles:N 2.Reprinted with permission from ref.123.Copyright the American Chemical Society 2005.D o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180Bbottom of the conduction bands is composed of hybridized Ge 4s4p orbitals.The band structure indicates that charge transfer upon photoexcitation occurs from the N 2p orbitals to the hybridized Ge 4s4p orbitals.Oxidation of water to form O 2over existing metal-oxide photocatalysts occurs as a result of contri-butions from photoexcited holes in the valence bands consisting of O 2p orbitals.However,for the present catalyst,the N 2p orbitals in the valence band are also able to generate photoex-cited holes,which can contribute to the photocatalytic oxidation of water.Because hybridized Ge 4s4p orbitals broadly expand,similar to In 5s5p orbitals in the SrIn 2O 4oxide photocatalyst with d 10electronic configuration,photoexcited electrons in such broad hybridized orbitals with large dispersion can be readily transferred to the RuO 2co-catalyst on the surface without recombination,permitting efficient overall water splitting.Transition metal-based (oxy)nitrides with d 0electronic configuration cannot themselves reduce H +and oxidize H 2O simultaneously (see below).The results for Ge 3N 4and GaN indicate that broad hybridized sp orbitals with large dispersion in nitrides with d 10electronic configuration have suitable band structures for overall water splitting,resulting in GaN-ZnO and ZnGeN 2-ZnO solid solution systems that are able to split water in visible light (see below).4.Photocatalytic cleavage of water in visible light4.1StrategyIt is essential to utilize visible light,which represents a large portion of the available solar-energy,for efficient conversion of solar-energy into H 2by the photocatalytic cleavage of water.Photocatalyst that are able to split water under visible light irra-diation must meet all of the following three requirements:(1)the conduction and valence band edges must be more negative than 0V vs.NHE (pH ¼0)and more positive than +1.23V vs.NHE (pH ¼0),as shown in Fig.1,(2)band gap energy smaller than 3eV,and (3)chemical and structural stability during the photo-catalytic reaction.The previously discussed oxide photocatalysts are not active for the water splitting reaction in visible light,due to the O 2p-based valence band.Fig.12shows the relationship between the band structure of a semiconductor and the redox potentials for water splitting.5,130The tops of the valence bands ofoxide photocatalysts are usually composed of O 2p orbitals,which are much more positive than the redox potential of water (+1.23V vs.NHE (pH ¼0)),as shown in Fig.12,and are therefore located around +3V.While this is favorable for the oxidation of water from a thermodynamic point of view,because of the large overpotential,the bottoms of the conduction bands of an oxide photocatalyst must have a more negative potential than 0V to split water into H 2and O 2,inevitably resulting in a wide band gap energy ($3eV (#413nm)),so that most of the visible light ($400nm)cannot be utilized.Although WO 3functions as a stable photocatalyst for O 2evolution under visible light in the presence of an appropriate electron acceptor,the bottom of the conduction band of the material is located at a more positive potential than the potential of water reduction.As a result,WO 3does not have the ability to reduce H +to H 2,some chalcogenide semiconductors,such as CdS,seem to have band structures suitable for the cleavage of water under visible light irradiation,but are not available for the reaction because they are not stable under the photocatalytic reaction conditions.As a result,these conventional materials cannot split water in visible light without the use of an external bias.There are five routes available for the cleavage of water,based on conduction and valence band design for visible light-driven photocatalytic systems:(1)introduction of electron donor levels into the band structures of oxide photocatalysts by doping,(2)formation of a valence band edge composed of orbitals that have more negative potential than O 2p orbitals,(3)formation of a solid solution,(4)construction of Z-scheme photocatalysis system developed by (1)–(3),and (5)construction of a photo-electrochemical cell consisting of visible light-driven thin film photocatalysts.The electron donor level is created above the valence band by doping certain elements into conventional photocatalysts with wide band gaps,such as TiO 2and SrTiO 3,which results in the formation of an energy gap that is smaller than the original band gap.Note that the band gap is distinguished from the energy gap.The energy gap is formed by the impurity level that does not form a complete band.In this case,while conventional photocatalysts with wide band gaps can adsorb visible light,such doped pho-tocatalysts cannot independently decompose water into H 2and O 2in visible light.On the other hand,some metal cations and anions can contribute to valence band formation above the valence band consisting of O 2p orbitals.Making a solid solution is also a useful banddesign.Fig.11DFT calculations for b -Ge 3N 4:(A)energy band dispersion,and (B)density of states.Reprinted with permission from ref.123.Copyright the American Chemical Society2005.Fig.12Relationship between band structure of semiconductor and redox potentials of water splitting.Reprinted with permission from ref.5.Copyright the Royal Society of Chemistry 2009.D o w n l o a d e d b y S u n Y a t -S e n U n i v e r s i t y o n 19 S e p t e m b e r 2010P u b l i s h e d o n 20 A u g u s t 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 910180B。