光解水综述

ReviewPhotochemical splitting of water for hydrogen productionby photocatalysis:A reviewAdel A.Ismail a,b,c,n,Detlef W.Bahnemann ca Advanced Materials Department,Central Metallurgical R&D Institute,CMRDI,P.O.Box87,Helwan11421,Cairo,Egyptb Advanced Materials and NanoResearch Center,Najran University,P.O.Box:1988,Najran11001,Saudi Arabiac Institute für Technische Chemie,Leibniz Universität Hannover,Callinstrasse3,D-30167Hannover,Germanya r t i c l e i n f oArticle history:Received26January2014Received in revised form21April2014Accepted27April2014Available online2June2014Keywords:Water splittingHydrogen productionPhotocatalystsUV and visible illuminationa b s t r a c tHydrogen production from water using a catalyst and solar energy is an ideal future fuel source.The searchfor suitable semiconductors as photocatalysts for water splitting into molecular hydrogen and oxygen hasbeen considered to be an urgent subject for our daily life.In this review,we aim to focus on the researchefforts that have been made so far for H2generation from water splitting by UV and visible light-drivenphotocatalysis.A number of synthetic modification methods for adapting the electronic structure to enhancethe charge separation in the photocatalyst materials are discussed.Sacrificial reagents and electronmediators for the overall water splitting are also reviewed.The quantum efficiency of photocatalystmaterials upon visible and UV illumination will be reviewed,summarized and discussed.&2014Elsevier B.V.All rights reserved. Contents1.Introduction (86)1.1.Basic principles of photocatalytic hydrogen generation (86)1.2.Main processes of photocatalytic hydrogen generation (87)1.3.Evaluation of photocatalytic water splitting systems (87)2.Photocatalysts for water splitting (87)2.1.UV photocatalysts for water splitting (87)2.1.1.TiO2and titanates as photocatalysts for H2production (87)2.1.2.Nb-and Ta-based oxides (88)2.1.3.W-,Mo-,and V-based oxides (89)2.1.4.d10Metal oxide photocatalysts (89)2.1.5.f0Metal oxide photocatalysts (89)2.1.6.Nonoxide photocatalysts (89)2.2.Visible light active photocatalysts for H2production (89)2.2.1.Efficient separation of photogenerated charge carriers (90)2.2.2.Cocatalyst loading (90)2.2.3.Z-scheme photocatalysts (90)2.2.4.Metal sulfides (92)2.2.5.GaN:ZnO solid solution photocatalysts (92)2.2.6.Overall water splitting employing dye-sensitized solar cells to harvest visible light (95)2.2.7.Electron mediators for overall water splitting (96)3.Main challenges and opportunities (97)4.Conclusions and outlook (97)Contents lists available at ScienceDirectjournal homepage:/locate/solmatSolar Energy Materials&Solar Cells/10.1016/j.solmat.2014.04.0370927-0248/&2014Elsevier B.V.All rightsreserved.n Corresponding author.E-mail address:adelali141@(A.A.Ismail).Solar Energy Materials&Solar Cells128(2014)85–101Acknowledgment (98)References (98)1.IntroductionPhotocatalytic water splitting into H2and O2using semiconduct-ing catalysts has received much attention due to the potential of this technology,as well as the great economic and environmental interest for the production of the clean fuel H2from water using solar energy.Fujishima and Honda demonstrated the potential of TiO2semiconductor materials to split water into H2and O2[1,2]. Their work triggered the development of semiconductor photo-catalysis for a wide range of environmental and energy applications [1,2].The photocatalytic activity of conventional metal-oxide photo-catalysts for the overall water splitting is known to be heavily dependent on the crystallinity and the particle size of the material, as determined by the preparation conditions[3–5].During the past 40years,various photocatalyst materials have been developed to split water into H2and O2under UV and visible light illumination. The direct splitting of water using a particulate photocatalyst would be a good way to produce clean and recyclable H2on a large scale[6–9].A number of photocatalysts have been proposed and achieved high quantum efficiencies under UV illumination.At present,there is a lack of suitable materials with sufficiently band gap positions for overall water splitting,and the stability necessary for practical applications.In general,efficient photocatalytic materi-als contain either transition-metal cations with a d0electronic configuration(e.g.,Ta5þ,Ti4þ,Zr4þ,Nb5þ,Ta5þ,W6þ,and Mo6þ)or typical metal cations with d10electronic configuration (e.g.,In3þand Sn4þ,Ga3þ,Ge4þ,Sb5þ)as principal cation components,the empty d or sp orbitals of which form the bottom of the respective conduction bands[6–9].The tops of the valence bands of metal oxide photocatalysts with d0-or d10-metal cations usually consist of O2p orbitals,which are located at aboutþ3eV or higher versus normal hydrogen electrode(NHE)and,as such, produce a band gap too wide to absorb visible light[9].Also, (oxy)nitrides containing d0transition-metal cations,such as Ta3N5, TaON,and LaTiO2N,are potential photocatalytic materials to achieve water splitting[10].The highest quantum efficiencies(QEs)have been reported for NiO-modified La/KTaO3(QE¼56%)using water under UV light[11],QE¼90%has been obtained for H2generation employing ZnS as photocatalysts in the presence of aqueous Na2S/ Na2SO3as sacrificial reductant under UV illumination[12]and for Cr/Rh-modified GaN/ZnO(QE¼2.5%),in pure water under visible light illumination[13–15].So far,no material capable of catalyzing reaction(Eq.(1))with visible light and a QE larger than10%has been found[15].Therefore,investigating the physical factors that govern the photocatalytic activity for H2production is an important and indispensable issue in the development of highly active photo-catalysts.While a number of visible-light-driven photocatalysts have been proposed as potential candidates for the overall water splitting,a satisfactory material has yet to be devised[16–19].At a power level of1000W/m2the solar energy incident on the earth's surface by far exceeds all human energy needs[20,21].Photovoltaic and electrochemical solar cells that convert solar energy into electricity can reach up to55–77%efficiency[22–25]but remain uneconomical because of high fabrication costs,insufficient light absorption[26],and inefficient charge transfer[22].In a process that mimics photosynthesis;solar energy can also be used to convert water into H2and O2,the fuels of a H2-based energy economy(Eq.(1))H2O-1=2O2ðΔÞþH2ðgÞ;ΔG¼þ237kJ=molð1:3eV=e;λmin¼1100nmÞð1ÞThe development of a photocatalytic system that functions efficiently under visible light,representing almost half of the available solar energy on the surface of the earth,is therefore essential for the practical utilization of solar energy.In this review, we aim to summarize the research efforts having been made so far, for H2generation from water splitting employing UV and visible light-driven photocatalysts in combination with the view on inspiring new ideas to tackle this important challenge.A number of synthetic and modification techniques for adjusting the band structure of the photocatalyst to harvest visible light and to improve the charge separation in photocatalysis are discussed. Sacrificial reagents and electron mediators for the overall water splitting are also reviewed.1.1.Basic principles of photocatalytic hydrogen generationFig.1a shows a schematic diagram of water splitting into H2and O2over photocatalysts.Photocatalysis on semiconductor particles involves three main steps:(i)absorption of photons with energies exceeding the semiconductor bandgap,leading to the generation of electron(eÀ)and hole(hþ)pairs in the semiconductor particles; (ii)charge separation followed by migration of these photogener-ated carriers in the semiconductor particles;(iii)surfacechemicalFig.1.Schematic illustration of water splitting over semiconductor photocatalysts.Source:Ref.[103].A.A.Ismail,D.W.Bahnemann/Solar Energy Materials&Solar Cells128(2014)85–10186reactions between these carriers with various compounds(e.g., H2O);electrons and holes may also recombine with each other without participating in any chemical reactions[3,7].When a photocatalyst is used for water splitting,the energetic position of the bottom of the conduction band must be more negative than the reduction potential of water to produce H2,and that of the top of the valence band must be more positive than the oxidation potential of water to produce O2,as shown in Fig.1b.Furthermore, the photocatalyst must be stable in aqueous solutions under photoirradiation.Scaife noted in1980that it is intrinsically difficult to develop an oxide semiconductor photocatalyst that has both, a sufficiently negative conduction band for H2production and a sufficiently narrow band gap(i.e.,o3.0eV)for visible light absorp-tion because of the highly positive position of the valence band (at ca.þ3.0V vs.NHE)formed by the O2p orbitals[27].To facilitate water oxidation,the potential of the valence band edge must exceed the oxidation potential of water ofþ1.23V vs NHE at pH¼0 (þ0.82V at pH¼7).Based on these parameters,a theoretical semiconductor band-gap energy of$1.23eV is required to drive the water-splitting reaction according to Eq.(1).The smallest band gap achieved so far in a functional catalyst is2.30eV in NiO/RuO2–Ni:InTaO4[28,29].Semiconductors with smaller band gaps or lowerflat-band potentials require a bias voltage or external redox reagents to drive the reaction.Alternatively,two or more small band-gap semicon-ductors can be combined to drive water oxidation/reduction pro-cesses separately via multiphoton processes.It is well known that flat-band potentials strongly depend on ion absorption(protonation of surface hydroxyl groups),crystallographic orientation of the exposed surface,surface defects,and surface oxidation processes [30,31].These factors are rarely considered in the preparation and testing of photochemical water-splitting catalysts[32–34].1.2.Main processes of photocatalytic hydrogen generationThe individual processes involved in the photocatalytic gen-eration of H2are illustrated in Fig.1.They include light absorption of the semiconductor photocatalyst,generation of excited charge carriers(electrons and holes),recombination of these charge carriers,separation of the excited charge carriers,migration of electrons and holes,charge carrier trapping,and transfer of the charge carriers to water or other molecules[3,7].All of these processes affect thefinal generation of H2from the semiconductor photocatalyst system.The total amount of H2generated is deter-mined by the amount of excited electrons at the water/photo-catalyst interface capable of reducing water.After the electron/ hole pairs are created,charge recombination and separation/ migration processes are two important competitive processes inside the semiconductor photocatalyst that largely affect the efficiency of the photocatalytic reaction for water splitting[35]. Charge recombination reduces the number of eÀ/hþpairs by emitting light or generating phonons.This includes both,surface and bulk recombination,and is classified as a deactivation process, and it is ineffective for water splitting.Efficient charge separation and fast charge carrier transport,avoiding any bulk/surface charge recombination,are thus fundamentally important for the photo-catalytic H2generation through water splitting.The reaction of photogenerated H2and O2to form H2O on the photocatalyst surface is normally called“surface backreaction(SBR)”.There are two main approaches to suppress the SBR:one involves the addition of sacrificial reagents into the photocatalytic reaction environment and the second creates a separation of the photo-active sites on the surface of the photocatalysts.In general, sacrificial reagents acting as electron donors or acceptors,respec-tively,drive the reaction into alternative pathways as they are reduced or oxidized,respectively.Taking into consideration the basic mechanism and the individual processes of photocatalytic water splitting,there are two keys for the development of a suitable high-efficiency semiconductor for the visible-light-driven photocatalytic splitting of water into H2and/or O2:(1)a photocatalyst should have a sufficiently narrow band gap (1.23eV o Eg o3.0eV)to both harvest visible light and possess the correct band structure;and(2)photoinduced charges in the photocatalyst should be separated efficiently in order to avoid bulk/surface electron/hole recombination.In addition,both charge carriers must migrate to the photocatalyst surface for H2and/or O2 evolution at the respective photocatalytic active sites[36].It is important that economical,highly efficient photocatalytic systems for light-to-H2energy conversion,in which aqueous solutions containing sacrificial reagents can be used to minimize the back-ward reaction of H2and O2to water on the surface of photo-catalysts,can be constructed.1.3.Evaluation of photocatalytic water splitting systemsThe rate of gas(O2and H2)evolution with units such asμmol hÀ1 is used to enable a measurable comparison between different photocatalysts under similar experimental conditions.The quantum yield,as an extension from the overall quantum yield defined in a homogeneous photochemical system,becomes important and accep-table to evaluate the photocatalytic activity for water splitting. The overall quantum yield is defined for H2and O2formation by Eqs.(2)and(3),respectively[37].The quantum yield is estimated to be smaller than the overall quantum yield because the number of absorbed photons is usually smaller than the number of photons available in the incident light.Activities of photochemical water-splitting catalysts are usually assessed by the rates of evolved gases [mol/h]per catalyst amount[g]under the specified irradiation conditions.From the measured evolution rate[H2],the apparent QE¼2[H2]/I of the catalyst can be calculated using the known photon fluxΙ[mol/s]incident on the reaction mixture.Overall quantum yieldð%Þ¼ð2ÂNumber of evolved H2moleculesÞNumber of incident photonsÂ100ðfor H2evolutionÞð2ÞOverall quantum yieldð%Þ¼ð4ÂNumber of evolvedO2moleculesÞNumber of incident photonsÂ100ðfor O2evolutionÞð3Þ2.Photocatalysts for water splitting2.1.UV photocatalysts for water splittingA wide range of semiconducting materials have been devel-oped as photocatalysts under UV irradiation.On the basis of their electronic configuration properties,these UV-active photocatalysts can be typically classified into four groups:(1)d0metal(Ti4þ, Zr4þ,Nb5þ,Ta5þ,W6þ,and Mo6þ)oxide photocatalysts,(2)d10 metal(In3þ,Ga3þ,Ge4þ,Sn4þ,and Sb5þ)oxide photocatalysts, (3)f0metal(Ce4þ)oxide photocatalysts,and(4)a small group of nonoxide photocatalysts.2.1.1.TiO2and titanates as photocatalysts for H2productionTiO2was thefirst material described as a photochemical water-splitting catalyst(Fig.2).It crystallizes in three structure types: Rutile,Anatase,and Brookite.All modifications contain TiO6 octahedra that are interconnected via two(Rutile),three(Broo-kite),or four(Anatase)common edges and via shared corners,andA.A.Ismail,D.W.Bahnemann/Solar Energy Materials&Solar Cells128(2014)85–10187as a result,the band gaps (3.0eV for Rutile and 3.15eV for Anatase)differ slightly [38].In their 1971/72papers,Fujishima and Honda described an electrochemical cell consisting of a n-type TiO 2(Rutile)anode and a Pt black cathode [1,2].When the cell was irradiated with UV light (o 415nm)from a 500W Xe lamp,O 2evolution was observed at the anode with a current flowing to the Pt counter electrode.Based on the current,a photoelectrochemical ef ficiency of approx.10%was estimated.Under UV irradiation,Pt –TiO 2was found to catalyze the water reduction with iodide as the sacri ficial electron donor,whereas TiO 2was observed to be the water oxidation catalyst itself in the presence of IO 3as the electron acceptor.After both catalysts were combined,H 2and O 2were formed stoichiometrically from a basic (pH ¼11)solution,with iodide serving as a redox shuttle.A similar system was realized with Pt –TiO 2and Pt –WO 3,giving QE ¼4%upon irradiation with UV light [39].Other recent efforts have sought to improve the optical response of TiO 2-based catalysts via doping with C or N,and S [40–43].Pt-modi fied catalysts evolve O 2from aqueous AgNO 3as the sacri ficial electron acceptor and traces of H 2from aqueous methanol as the sacri ficial electron donor under visible light [44].Newly synthesized tailored anatase/brookite mixtures as well as pure brookite TiO 2nanorods were also employed for H 2production.The results achieved so far revealed that anatase/brookite mixtures and brookite nanorods exhibit higher photocatalytic activity than anatase nanoparticles and even higher ef ficiencies than TiO 2P25for the photocatalytic H 2evolution from aqueous methanol solution [45,46].Further-more,mesoporous TiO 2nanostructures have been studied as a route to minimize Pt loading on TiO 2photocatalysts for H 2production.The results indicated that the amount of H 2evolved on 0.2wt%Pt/TiO 2calcined at 4501C is three times higher than that evolved on Pt/TiO 2-P25and twelve times higher than that evolved on Pt/TiO 2calcined at 3501C [47].Also,a series of TiO 2and graphene sheet(GSs)composites were synthesized [48].The photocatalytic activity of these samples was evaluated by H 2evolution from water photo-splitting under UV –vis illumination.On the other hand,when TiO 2is fused with metal oxides (SrO,PbO,etc.)metal titanates with intermediate band gaps can be obtained.Of these,SrTiO 3crystallizes in the Perovskite structure type and has a band gap of 3.2eV,slightly larger than that of TiO 2.It was first employed in 1976as a photocatalyst in a water-splitting electrochemical cell together with p-CdTe or p-GaP photocathodes [49].An optimized version of this system was reported in 1977by Ohashi and co-workers to be the first self-supported photoelec-trochemical cell [50]with a photon to-electron conversion ef fi-ciency of 0.044–0.67%.In 1980,it was shown that NiO-modi fiedSrTiO 3powder splits water vapor stoichiometrically under UV irradiation,while SrTiO 3alone did not show any activity [51].The conditions of the NiO deposition had a strong effect on the catalytic activity [52].Abe et ed Cr/Ta-doped Pt/SrTiO 3together with Pt –WO 3in a two-particle catalyst system for overall water splitting with visible light.Cr/Ta:SrTiO 3–Pt produced H 2and Pt –WO 3produced O 2,with an iodide/iodate redox couple serving as the redox mediator between the two catalysts [53].The effect of metal cocatalysts (Ru,Ir,Pd,Pt,Os,Re,Co)on the water-splitting activity of SrTiO 3was also studied [54,55].From pure water,H 2and O 2were evolved stoichiometrically,with the activity decreas-ing in the order Rh 4Ru 4Re 4Pt 4Ir 4Pd 4Os 4Co.SrTiO 3alone produced H 2but no O 2.A series of layered Perovskites as photochemical water-splitting catalysts was investigated [56,57].The NiO-modi fied catalyst is active for H 2/O 2evolution with a QE up to 12%[56–58].By doping with BaO and upon addition of NaOH to the catalyst suspension,this activity could be further increased to QE ¼50%[56],only slightly below that of the best catalyst,i.e.,La-doped NaTaO 3with QE ¼56%[59,60].2.1.2.Nb-and Ta-based oxidesNb 2O 5has a band gap of 3.4eV and it is not active for pure water splitting under UV irradiation.After modi fication with Pt as a cocatalyst,however,it can ef ficiently produce H 2from aqueous solutions containing methanol as an electron donor [61].Meso-porous Nb 2O 5,demonstrated a photocatalytic activity 20times higher for H 2evolution than a bulk Nb 2O 5without any porosity.Intercalation of In 2O 3into the mesoporous structure further increased the photoactivity of mesoporous Nb 2O 5by 2.7times [62].Besides Nb 2O 5,a large number of niobates can produce H 2and O 2via water splitting upon UV irradiation.K 4Nb 6O 17was developed as the first example of a niobate photocatalyst that showed high and stable activity for H 2evolution from aqueous methanol solution without any assistance from other materials such as the noble metals [63,64].There are various nanocompo-sites of SrNb 2O 6,Sr 2Nb 2O 7,and Sr 5Nb 4O 15that exhibited ef ficient photocatalytic activities for H 2and O 2production under UV irradiation [65–67].In particular,Sr 2Nb 2O 7with a highly donor-doped layered perovskite structure gave quantum yields as high as 23%.The activity of Sr 2Nb 2O 7was further enhanced achieving quantum yields up to 32%[65].Kudo and co-workers reported Ba 5Nb 4O 15with a layered perovskite structure for water splitting loaded with NiO cocatalysts and reported 17%quantum yield upon 270nm illumination [66,67].Partial substitution of Nb 5þwith Zn 2þto obtain BaZn 1/3Nb 2/3O 3with a distorted perovskite struc-ture showed favorable photocatalytic activities under UV irradia-tion [68,69].The photocatalytic activity of ZnNb 2O 6was negligible,whereas NiO-loaded ZnNb 2O 6revealed a high photoactivity [70].Chen and coworkers prepared ABi 2Nb 2O 9(A ¼Ca,Sr,Ba)perovs-kite photocatalysts for both H 2and O 2evolution from aqueous solutions containing sacri ficial reagents [71].The photocatalytic performance decreased in the order of SrBi 2Nb 2O 94BaBi 2N-b 2O 94CaBi 2Nb 2O 9.M 2BiNbO 7(M ¼In 3þ,Ga 3þ)with pyrochlore structures were sensitive to UV irradiation and had the ability to split water [72].Sabio et al.studied the photocatalytic water splitting activities of suspended KCa 2Nb 3O 10nanoscale and bulk particles and compared them using a kinetic model [73].Their model showed that the higher activity of the nanoniobate can be rationalized by shortened transport paths for electrons and holes to the reactive surface,which reduce the lattice recombination.This effect apparently outweighs the increase in surface recombi-nation that is to be expected for the nanomaterial,due to its increased speci fic surfacearea.Fig.2.Schematic representation of a photoelectrochemical cell (PEC).Source :Reprinted with permission from Ref.[1].A.A.Ismail,D.W.Bahnemann /Solar Energy Materials &Solar Cells 128(2014)85–101882.1.3.W-,Mo-,and V-based oxidesPbWO4incorporating a WO4tetrahedron showed high andstable photocatalytic activity for the overall splitting of water[74,75].H2and O2was produced under UV irradiation when RuO2was loaded onto the metal oxide.The photocatalytic performancewas attributed to large dispersions in both the valence andconduction bands.PbMoO4catalyzed the H2evolution fromaqueous methanol solution.It also exhibited photocatalytic O2evolution from aqueous AgNO3solution under UV irradiation withits O2evolution activity being comparable to that observed forTiO2[76].Na2W4O13and Bi2W2O9,with layered structures,wereactive for photocatalytic H2and O2evolution under UV irradiation[77,78].However,Bi2MoO6with a similar structure evolved onlyO2from aqueous AgNO3solution at a low rate[79].A new crystalstructure of VO2,with a body centered-cubic structure(bcc)and alarge optical band gap of2.7eV,surprisingly showed excellentphotocatalytic activity for H2production under UV irradiation[80].The bcc VO2phase exhibited a high quantum efficiency of38.7%when synthesized as nanorods.Also,Bi2GaVO7and Bi2YVO8withtetragonal structures were studied.These two compoundsinitiated both,H2and O2evolution,from water under UV irradia-tion.This is in spite of the fact that both of them showed strongoptical absorption in the visible region(λ4420nm)[81,82].2.1.4.d10Metal oxide photocatalystsVarious typical metal oxides(In3þ,Ga3þ,Ge4þ,Sn4þ,Sb5þ)with d10were shown to be effective photochemical water-splittingcatalysts under UV irradiation.Ni-loaded Ga2O3appears to be oneof the promising photocatalysts for overall water splitting[83].Itsphotocatalytic activity could be effectively improved by the additionof Ca,Cr,Zn,Sr,Ba,and Ta ions[84].Zn ion doping remarkablyimproved the photocatalytic activity,with an apparent quantumyield for Ni/Zn–Ga2O3of QE¼20%.By combining with Lu2O3,theresulting Zn-doped Lu2O3/Ga2O3proved to be a novel compositephotocatalyst for stoichiometric water splitting under UV irradia-tion.NiO was loaded as the cocatalyst,the quantum yield at320nmamounted to6.81%.Ga1.14In0.86O3showed the highest photocataly-tic activity for H2and O2evolution[85].In comparison,Y1.3In0.7O3,showed the highest photocatalytic activity for the overall watersplitting when combined with RuO2as a promoter[86].2.1.5.f0Metal oxide photocatalystsCeO2was reported to show a consistent activity towards O2production in aqueous solutions containing Fe3þand Ce4þaselectron acceptors[87].Sr2þ-doped CeO2was an active photo-catalyst for overall water splitting when RuO2was loaded as apromoter[88].Ce3þsupported zeolites showed higher photoca-talytic activity for water splitting[89].H2and O2evolution wasobserved.Photoirradiation of Ce3þspecies generated electrons(Ce3þþhν-Ce4þþeÀ)that were captured effectively by water molecules resulting the formation of molecular hydrogen.Yuanet al.[90]reported that BaCeO3produced H2and O2from aqueoussolutions containing CH3OH and AgNO3as sacrificial reagents,respectively.2.1.6.Nonoxide photocatalystsZnS in SO32Àsolutions were used for H2production under UVirradiation exhibiting90%quantum yield at313nm[91].CdSenanoribbons in Na2S/Na2SO3solution showed photocatalytic activ-ity for H2evolution under both,UV and visible light illumination.The quantum efficiency for this process has been reported to be0.09%[92].The photocatalytic water-splitting activity of GaN wasfound to be strongly dependent on the crystallinity of the materialand on the cocatalyst employed[93].Modification of well-crystallized GaN with Rh2Ày Cr y O3nanoparticles as a cocatalyst for H2evolution resulted in the stable stoichiometric decomposi-tion of H2O into H2and O2under UV irradiation.RuO2modifica-tion,on the other hand,did not bring about appreciable H2and O2evolution.However,Zn2þ,Mg2þ,and Be2þdoping of GaN con-verted it into a remarkably active and stable photocatalyst.Again,the presence of RuO2as a cocatalyst was required[94].β-Ge3N4 was another effective nitride photocatalyst to show efficientactivity for splitting water into H2and O2when combined withRuO2nanoparticles as reported by Domen's group[95,96].Thephotocatalytic activity of RuO2-loadedβ-Ge3N4was strongly dependent on the reaction conditions employed.The highest activity was obtained when the reaction was carried out in1M aqueous H2SO4solution.Moreover,treatment of as-prepared β-Ge3N4powder under high-pressure ammonia effectively increased the photocatalytic activity by up to4times.This was attributed to a decrease in the density of anion defects in the bulk and at the surface[97].A AgBr/SiO2catalyst showed a stable and high photocatalytic activity for H2generation from CH3OH/H2O solution under UV irradiation.The high activity of this AgBr/SiO2catalyst has been related to photogenerated Ag species acting as sites for H2 formation[98].Very recently,metal-free polymeric photocatalysts have attracted much attention.Wang et al.demonstrated that graphitic carbon nitrides(g-C3N4)can be photocatalytically active for H2evolution from water under visible light irradiation[99]. By introducing a mesoporous structure into polymeric C3N4,the efficiency of H2production from the photochemical reduction of water is increased by one order of magnitude and the quantum efficiency(QE)is1.4%[100,101].2.2.Visible light active photocatalysts for H2productionAlthough,more than100photocatalytic systems based on metal oxides have been reported to be active for the water splitting,most of them require ultraviolet(UV)light(λo400 nm)due to the large bandgap of these semiconductor materials [16–101].Since nearly half of the solar energy incident to the earth's surface lies in the visible region(400nm oλo800nm) (see Fig.3),it is essential to use visible light efficiently to realize H2 production on a large scale by photocatalytic water splitting.A maximum solar conversion efficiency for photocatalytic water splitting with a quantum efficiency of100%can be calculated using the standard solar spectrum.Even if all UV light up to400nm were utilized,the solar conversion efficiency would be only2%, which is similar to the maximum conversion efficiencies of photosynthesis in green plants under normalenvironmentalFig.3.Solar spectrum and maximum solar light conversion efficiencies for water splitting reaction with100%of quantum efficiency.Source:Ref.[103].A.A.Ismail,D.W.Bahnemann/Solar Energy Materials&Solar Cells128(2014)85–10189。