Handout Solar.2

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Acc. Chem. Res. 1995,28, 141-145 141

Artificial Photosynthesis: Solar Splitting of Water toHydrogen and OxygenALLEN .BARD* ND MARYEANNEFox*

Depar tmen t of Chemistry and Biochemistry, Universi ty of Texas a t Aus t in , Aus t in , Texas 78712Rece ived November 16 , 1994

Water SplittingThe maintenance of life on earth, our food, oxygen,and fossil fuels depend upon the conversion of solarenergy into chemical energy by biological photosyn-thesis carried out by green plants and photosyntheticbacteria. In this process sunlight and available abun-dant raw materials (water, carbon dioxide) are con-verted t o oxygen and the reduced organic species thatserve as food and fuel. A long-standing challenge hasbeen the development of a practical artificial photo-synthetic system tha t can roughly mimic the biological

one, not by duplicating the self-organization andreproduction of the biological system nor the aestheticbeauty of trees and plants, but rather by being ableto use sunlight t o drive a thermodynamically uphillreaction of an abundant materials to produce a fuel.In this Account we focus on water splitting, thephotodriven conversion of liquid water to gaseoushydrogen and oxygen:

Beyond the intellectual challenge of designing andfabricating such a system, there are several practicalimplications. H2 could serve directly as a fuel, e.g.,fo r transportation or for the production of electricityin fuel cells, without producing pollutants o r green-house gases upon combustion. For some purposes,however, it might be useful to use the HZas a reactantt o produce a different fuel, such as one that is liquidat the usual temperatures and pressures. Thus, weseek as a Holy Grail a renewable energy sourcedriven by solar energy that produces a clean andstorable fuel.Let us define this Holy Grail more specifically. Wewant an efficient and long-lived system fo r splittingwater t o Ha and 0 2 with light in the terrestrial(AM1.5)olar spectrum at an intensity of one sun. Fora practical system, an energy efficiencyof at least 10%appears to be necessary. This means tha t the HZand0 2 produced in the system have a fuel value of at least10% of the solar energy incident on the system. Inthe southern United States, the instantaneous maxi-mum intensity is of the order of 1 kW/m2 and theaverage 24-h intensity throughout a year is about 250W/m2. Thus, the system should produce H2 at a rateof about 0.7 g/s o r 7.8 L(STP)/s per m2 of collector atmaximum solar intensity. Long-lived implies th at the

Allen J. Bard holds the Hackerman-Welch Regents Chair in Chemistry at TheUniversi ty of Texas and works on the application of electrochemical methods tochemical problems.Marye Anne Fox currently occupies the M. June and J. Virgil Waggoner RegentsChair in Chemistry at the University of Texas and is director of the Center for FastKinetics Research. Her principal research nterests are in physical organic chemistry0001-4842/95/0128-0141$09.00/0

sensitizers and catalysts, as well as the materials ofconstruction, will not be consumed or degraded underirradiation for at least 10 years. The solar spectrumat sea level extends from the near infrared throughthe visible to the near ultraviolet with photon energiesup t o 3.0 eV. This region is not absorbed by wateritself, so photochemical reactions are only possible inthe presence of some recyclable absorbing sensitizer.Finally, fo r practical applications, the cost of H2produced by the system (on an energy equivalentbasis) should be competitive with t ha t of fossil fuels.Although we have defined our Holy Grail in termsof the water-splitting reaction, other chemical solarenergy conversions are possible and have been inves-tigated. For example, there are semiconductor liquidjunction systems that, when irradiated with visiblelight, carry out the reactions 2HBr - H2 + Br2 and2HI - HZ + 1 2 . Indeed, the brine splitting o rphotochloralkali reaction,2H,O + 2C1- hv 20H- + C1, + H, (2)

would probably be more useful than water splitting,but has so far not been achieved without applying anadditional external potential.Many investigations in this field have involvedsacrificial donors, which are reduced materials thatare oxidized more easily than water, fo r example,ethylenediamine tetraacetic acid or triethanolamine.The use of such compounds usually greatly improvesthe efficiency of the solar process, but clearly is not ofinterest in practical systems, especially if the sacrifi-cial donors are more expensive than the H2 produced.It might be possible t o use reduced waste materialsin this role, but is it unlikely that this approach willbe practical in large-scale fuel production.Photoelectrochemical approaches may be useful,however, as a means of water treatment, destroyingorganic wastes and removing metals. There are alsoa number of chemical schemes for converting solarenergy to electrical energy, e.g., in liquid junctionphotovoltaic cells. Indeed, devices with single-crystalsemiconductors have been constructed which showsolar efficienciesof above 10%. It remains to be seenwhether such chemical photovoltaic systems will bepractically competitive with solid state ones based, forexample, on single-crystal or amorphous Si. Althoughmany of these alternative chemical solar energysystems are interesting, we focus here on the water-splitting reaction, because it effectively represents thescientific challenges typical of all such efforts.History and ProgressA. Efficiency. The free energy change fo r reaction1 s AGO = 237.2 kJ/mol o r 2.46 eV/molecule of H2O.

0 995 American Chemical Society


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142 Acc. Chem. Res., Vol.28, No. 3, 1995Since two electrons are involved in the reaction aswritten (n= 2), this corresponds t o 1.23 eV/e, whichis also the standard emf for the reaction. The photonsin the solar spectrum provide sufficient energy to drivethis reaction, but the efficiencyof the reaction dependsupon how the reaction is carried out. It is possible t ocause water splitting thermally with light via concen-trators and a solar furnace by heating water t o 1500-2500 K.I However, the efficiency of this process istypically below 2%, and the cost of the capital equip-ment and material stability problems suggest that thisapproach t o solar water splitting is not a promisingone.Since water itself does not absorb appreciable radia-tion within the solar spectrum, one or more light-absorbing species (photoconverters or sensitizers)must be used to transduce the radiant energy tochemical (or electrical) energy in the form of electronlhole pairs, i.e., t o the oxidizing and reducing potentialneeded t o drive the reaction. The maximum efficiencyfor photochemical solar converters has been consideredin a number of papers2 and depends upon the bandgap (or threshold energy), E,, of the photoconverter.Radiation of energy below E, is not absorbed whilethat above E , is partly lost as heat by internalconversion or intraband thermalization processes.Additional thermodynamic losses occur because theexcited state concentration is only a fraction of thatof the ground sta te and because some excited sta tesare lost through radiative decay.2 When these factorsare taken into account, the threshold photon energyand the maximum efficiency can be calculated. For asingle photoconverter system, wavelengths below 770nm (or energies above 1.6 eV) are required to yield amaximum efficiency of about 30%. Lower photonenergies and higher efficiencies are at tainable if oneemploys two photoconverters. Thus for a system withtwo photoconverters with two different, optimized E,values, one finds a maximum solar efficiency of 41%.2These calculations show that, in principle, the desiredefficiency for water splitt ing is attainable, even witha system involving a single photoconverter.B. Semiconductor Solid State Photovoltaic-Based Systems. A number of different approachesare possible with semiconductors as the photocon-verter. The most direct, brute force, approach employsa solid state photovoltaic solar cell to generate elec-tricity that is then passed into a commercial-typewater electrolyzer (Figure 1A). The maximum theo-retical efficiency for a Si photovoltaic cell is 33%,andthe efficiencies of the best laboratory cells have beenreported t o be about 24%. Commercial single-crystalSi solar cells generally have efficiencies in the 12-16%range. The electrolysis of water a t a reasonablerate in a practical cell requires applied voltagessignificantly larger tha n the theoretical value (1.23Va t 25 "C), and electrolysis energy efficiencies of about60% are typical. Thus, the efficiency of the combinedsolar/electrolyzer system using commercially availablecomponents is close t o the desired 10% defined forsolar hydrogen generation. Moreover, the componentsare rugged and should be long-lived. The problemwith such a system is its cost. Solar photovoltaicscannot currently produce electricity at competitive

(1)E t ievan t , C . Solar Energy Muter . 1991, 4, 413.( Z ) Ar cher , M. D.; Bolton, J. R . J . Phys . Chem. 1990, 4, 8028 a ndreferences therein.

Bard and Fo x

te'

Photovoltaic hv, cell ,/b%-JlectrolysisA n l pB

II 4 2\/n - t w semiconductor semiconductor dye layer

C DFigure 1. Schematic diagrams of different types of semicon-ductor-based systems proposed for solar water splitting: (A )solid sta te photovoltaic cell driving a water electrolyzer; (B ) cellwith immersed semiconductor p/n junction (o r metallsemicon-ductor Schottky junction) as one electrode; (C ) liquid junctionsemiconductor electrode cell; (D ) cell with dye-sensitized semi-conductor electrode.prices, and hydrogen from water electrolyzers issignificantly more expensive than that produced chemi-cally from coal or natu ral gas. Lower cost solar cellsare possible, e.g., through the use of polycrystallineor amorphous Si or other semiconductors (CdS, CdTe,CuInSed, and some improvements in water electro-lyzer efficiency through better configurations andcatalysts and the use of higher temperatures arepossible. However, it probably will be difficult to bringthe overall cost down to levels that make such aconfiguration practical in the foreseeable future.An alternative system involves th e semiconductorphotovoltaic cell immersed directly in the aqueoussystem (Figure 1B). At the least this eliminates thecosts and mechanical difficulties associated with sepa-rate construction and interconnection of solar andelectrochemical cells. In one such system, the elec-trodes are composed of single or multiple semiconduc-tor p/n junctions that are irradiated while they arewithin the cell. This simpler apparatus is attainedat the cost of encapsulating and coating the semicon-ductors t o protect them from th e liquid environmentand probably with a more limited choice of electro-catalyst for 0 2 or H2 evolution. Moreover, the open-circuit photovoltage of a single Si p/n junction is only0.55 V, so at least three of these in series would beneeded t o generate the necessary potential for watersplitting. For example, in a system developed at TexasInstruments3 (TI) p-Si/n-Si junctions were producedon small (0.2 mm diameter) Si spheres embedded inglass and backed by a conductive layer t o form an

(3) Kilby, J. S.; L ath r op , J. W.; Por t e r , W. A. U S . Paten t 4 021 323,1977; U.S. Paten t 4 100 051, 1978; U.S. Paten t 4 136 436, 1979. Se ealso: Johnson, E . L. In Electrochemis try in Ind us try; Landa u, U., Yeager,E . , Kor t an , D ., E ds . ; P l enum: New Yor k , 1982; pp 299-306.


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Artif icial Photosynthesisarray. Each sphere behaved as a photovoltaic cell andproduced about 0.55 V. The use of two arrays,protected with noble metal catalysts (M), i.e., Wp-Si/n-Si and M/n-Sup-Si, connected in series and incontact with HBr, allowed H2 and Bra to be generatedwith about an 8%efficiency. Multiple TI photoarraycells t o carry out water splitting and other reactionsrequiring higher potentials a re possible4 at a consider-able sacrifice in efficiency. Note that, in addition t op/n semiconductor junctions, those between a metaland semiconductor (Schottky barriers) can be used t oproduce a photopotential, e.g., in electrodes such asAdn-Gap, PtSUn-Si, and Pun-GaAs.C. Semiconductor Electrode (Liquid Junc-tion) Systems. Of more interest to chemists aresystems in which the photopotential t o drive thewater-splitting reaction is generated directly at thesemiconductorAiquid interface (Figure IC). In 1839Becquerel noted small photoeffects when metal elec-trodes were irradiated in electrochemical cells.5 Ratherextensive research w as carried out on various metalelectrodes, sometimes covered with oxide or otherfilms, and immersed in a variety of solutions, includingsome containing fluorescent dyes.6 The effects seenwere usually small, and given the state of electro-chemistry and knowledge of the electronic propertiesof solids, the results were generally poorly understood.The discovery of the transistor and interest by chem-ists and physicists in semiconductor materials, notablySi and Ge, led t o more extensive electrochemical andphotoelectrochemical studies, usually with t he goal ofcharacterizing the se mi co nd ~c to r.~ ,~The modern era of semiconductor electrodes andinterest in these in photoelectrochemical devices fo renergy conversion, especially via the water-splittingreaction, can be traced to the work of Honda andFujishima on single-crystal Ti02 electrode^.^ Indeed,water splitting in TiO2-based cells can be accom-plished, but only with an additional electrical bias. Theproblem with Ti02 is that the conduction band is to olow (i.e,, at an insufficiently negative potential) t ogenerate hydrogen at a useful rate. Moreover, becausethe T i 0 2 band gap is large (3.0 eV fo r rutile), only asmall fraction of the solar light is absorbed and theefficiency of TiOz-based cells can never attain thespecified 10% level. Cells with Ti02 electrodes ofvarious types (e.g., single crystal, polycrystalline, hinfilm) have nevertheless been heavily investigated,largely because Ti02 is very stable and is a good modelfo r understanding the semiconductorAiquid interface.The stability of the semiconductor in contact witha liquid and under irradiation is an important factor.To generate oxygen, rather energetic photogeneratedholes are required, and these tend to cause decomposi-tion of the semiconductor. Thus, a key requirementin cells of thi s type, involving a single photojunction,is the discovery of a semiconductor with an appropri-ate band gap (< about 2.5 eV), with the conduction

(4) White , J. R. ; F a n , F.-R. . ; Bard, A. J. J . Electrochem. SOC. 985,(5) Becquerel, E . C. R. Hebd. Seances Acad. Sci. 1839,9, 561.(6 ) Copela nd, A. W .; Black, 0 . D.; Garrett , A. B. C h e m . Reu. 1942,3 1 , 177.(7) Gerischer, H. In Advances in Electrochemistry a nd ElectrochemicalEngineering; Delahay, P., Ed.; Interscience Publishe rs: New York, 1961;Vol. 1, p 139.(8) Myamlin , V. A,; Pleskov, Yu. V. Electrochemistry of Semiconduc-tors; Plen u m Pres s: New York, 1967.(9) Fujish ima, A, ; Honda, K. Nature 1972, 238, 37 .

132,544.

Ace. C hem. Res., Vol . 28, No. 3, 1995 143band sufficiently negative fo r hydrogen evolution andthe valence band sufficiently positive for oxygenevolution, so tha t it remains stable under irradiation.This single-junction semiconductor electrode has notyet been discovered. Indeed, it is only with materialswith band gaps even larger than that of TiO2, likeSrTiOs, that water splitting can be carried out withoutan additional electrical bias. The solar efficiency ofsuch cells is very small.D. Semiconductor Particle Systems. A consid-erable simplification of the apparatus is possible if theelectrochemical cell can be replaced by simple disper-sions of semiconductor particles. In such dispersions,the semiconductor particles can be coated with islandsof metals that behave as catalytic sites, with eachparticle behaving as a microelectrochemical cell.lo i 0 2has been a favorite material, although other com-pounds, such as CdS and ZnO, have also been studied.While a number of interesting photoreactions havebeen carried out, including the use of particles t odestroy organics and t o plate metals from wastewaterll and fo r synthetic purposes,12 reports on theuse of particulate systems fo r water splitting remaincontroversial. A t best the solar efficiencies of pro-cesses reported t o date have been very small (


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144 Ace. Chem. Res. , Vol.28, No. 3, 1995 Bard and Foxlarge fraction of the incident light while still main-taining a high quantum efficiency. A photoelectro-chemical cell based on a ruthenium complex sensitizeradsorbed on Ti02 showed an efficiency above 7% indirect sunlight for conversion of light t o electricity. Inthis example, iodine was generated a t the dye-coatedelectrode and was reduced a t the counter e1ectr0de.l~For water splitting, th e oxidation reaction a t the dye/liquid interface would have t o generate oxygen, aconsiderably more difficult process. Indeed, a funda-mental problem with dye-sensitized systems may bethe photochemical instability of the sensitizer, espe-cially when holes sufficiently energetic t o liberateoxygen are produced during irradiation.F. Homogeneous and MicroheterogeneousSystems. An alternative t o these electrochemicalmethods is t o employ multicomponent solutions ordispersions as complex arrays for multistep watersplitting. Such a system would typically combine anefficient sensitizer, one o r more relays or chargeaccumulation sites, and one or more gas evolutioncatalysts. Much effort ha s been devoted t o under-standing and optimizing the function of each constitu-ent of these arrays , but their combination (even of thebest single components) has s o far not produced thedesired synergism needed t o attain the high solarconversion efficiency that is the goal of this quest.The most thoroughly studied systems employ mi-croheterogeneous arrays, with either the oxidation(H2O- 2 ) or the reduction (HzO-H2)half-reactionsbeing the targeted goal. In such arrays, the excitedsta te of the sensitizer is quenched either reductivelyor oxidatively, and most require a sacrificial electrondonor or acceptor. A major disadvantage of mostsystems studied so far is tha t they do not accomplishtrue water splitting; rather, they focus on eitherhydrogen generation in the presence of a sacrificialelectron donor or oxygen production in the presenceof a sacrificial electron acceptor. Even when opti-mized, a successful melding of these half-reactions intoa simple composite arr ay remains elusive. Further-more, the consumption of a sacrificial reagent inthe optimized half-reactions obviates the economicadvantages of simple water splitting and mars itsenvironmental attractiveness as a means for chemicalproduction of fuel by solar energy conversion. Includ-ing a sacrificial reagent also often completely reversesthe net thermodynamics, converting an inherentlyuphill (photosynthetic) reaction such as water splittinginto a net energetically downhill (photocatalytic)transformation by coupling the endothermic steps withother highly exothermic ones.

In the half-reaction involving the production ofhydrogen, th e following coupled sequence is typicallyfollowed. The key photochemically-driven step is theoxidation of the excited sensitizer S, which is ac-complished by transferring an electron to a moleculeR that acts as a relay t o a proton source, usually inthe presence of a catalyst Cat. The singly oxidizedsensitizer is then reconverted t o its original neutralstate by interaction with a sacrificial electron donorD, which is consumed in a sequence of one or morechemical steps initiated by irreversible electron dona-tion. The radical anion produced upon oxidation ofthe sensitizer acts as a relay reagent, transferring its(15) 'Regan, B. ; Gratzel , M. Nature 1991,353,737

PS *nPS

extra electron, usually in th e presence of a catalyst,to water, thus producing hydrogen gas. The catalystthus functions both to accumulate electrons t o providethe necessary electrochemical potential and the num-ber of electrons for the reduction of water and t o serveas a gas evolution site.For efficient hydrogen evolution, several featuresare necessary: (1) he oxidative trapping must occurfaster than radiative or nonradiative relaxation of thesensitizer excited state; ( 2 ) each of the intermediatecharge relays must be stable in both the oxidized andreduced states so that the sensitizing dye can bemultiply recycled; (3 ) a robust catalyst for efficientevolution of hydrogen must be effective in trappingthe reduced relay; (4) he sacrificial donor must beinexpensive and readily available; and ( 5 )each of thecomponents of the array must be chemically andoptically compatible. In operation, these requirementsare often structurally contradictory, and even when ahigh quantum efficiency (production of fuel per ab-sorbed photon) is observed, low solar efficiency (pro-duction of fuel over the entire range of incidentphotons) is commonly encountered.Lehn and co-workers have provided an early ex-ample of one such ensemble, using a metal complexRu(bpy)~Cls , s sensitizer, triethanolamine as thesacrificial donor, and colloidal platinum generated insitu from KaPtC16 as the gas evolution cata1yst.l6Numerous other variants of these studies were un-dertaken in the subsequent decade,17with sensitizersincluding metal complexes, metalated porphyrins andphthalocyanines, organic dyes, (e.g., xanthines orcyanines), or organic polymers [e.g., poly(viny1py-ridinesll. In such studies, a range of relays (e.g.,viologens, phenanthroline complexes, multivalent cat-ions like V3+or C$+, or redox enzymes like cytochromec3) and catalysts (e.g., metal alloys, enzymes, sup-ported metal oxides, colloidal metals, etc.) were em-ployed.In a typical optimized system [with Ru(bpy)3Cl~ ssensitizer and methyl viologen as relay fo r hydrogenevolution on colloidal platinum or gold in the presenceof sacrificial ethylenediamine tetraacetic acid (EDTA)],hydrogen evolves efficiently, with catalytic turnover(about 100) imited by competing hydrogenation of themethyl viologen relay.'* Turnover efficiency could besubstantially improved by substituting a family ofnewly synthesized viologens that are resistant t ocatalytic hydr~genation'~nd by replacing the hydro-gen evolution catalyst with colloidal platinum. Moststudies with these artificial arrays, however, have

(16, Leh n , J . - M . ; Sau v ag e , J.-P.N O U L ~ .. C h i m . 1977, 1. 449.(17)Gratzel , M . Energy Resources through Photochemistry and Ca-(181 Moradpour, A,:Amouyal, E. : Keller. P.; Kag an , H . N O U L . .. Chim.(19)See, for example: Crutchle y. R. J .: Lever. A. B . P. J . A m . C h e m.

t a l y s i s : Academic Press: New York, 1983.1978,2, 547 .Soc. 1980,102, 7 1 2 8 .


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Artificial Photosynthesisfocused on forward- and back-electron-transfer ratesand on structural improvement of photophysicalproperties: for example, with a complex five-compo-nent antenna system, Sasse and co-workers havereported a quantum efficiency of 93% fo r hydrogenevolution from an a rray containing g-anthracenecar-boxvlate in acetate buffered t o DH in a solution

Ace. Che m. Res. , Vol. 28, No. 3, 1995 145

coniaining EDTA and methyl vioiogen.20Similarly, oxygen evolution could be induced byirradiation with redox catalysts that act as pools of

oxidative equivalent^.^^^^^^^^ In a recent variant, Calza-ferri has employed Ag+-loaded zeolites toward thisobjective, with self-sensitization by 0 2 being demon-strated t o improve02 evolution e ffi ~i en cy .~~n gen-eral, much lower efficiencies are observed fo r theevolution of oxygen than of hydrogen, mainly becauseof the difficulty in accumulating the necessary fouroxidative equivalents required t o convert water t ooxygen.Self-assembling arrays have also been constructedin media in which one or more of the components isconfined within a fured matrix, either a sol-gel or azeolite. In functionalized zeolites, fo r example, Mal-louk and co-workers have shown tha t a substantially

extended charge separation lifetime of 37 ps can leadto a quantum efficiencyfo r charge separation of about17% from flash photolysis me a s ~r e m e nt s, ~ ~lthougha quantum efficiency fo r hydrogen evolution of only0.05%has been reported by Dutta and co-workers fora similarly constructed array.25 Given that very longcharge carrier lifetimes (from excitation of pyrene t oa viologen confined to a silicate matrix in the presenceof a mobile electron relay26)have been reported, onemight expect high efficiencies for hydrogen productionin analogous sol-gels if mass transport limitations canbe overcome.Another option t o a self-assembling, multicompo-nent, microheterogeneous array is using a covalentsupramolecular assembly t o accomplish long-termelectron-hole pair separation. Such arrays containingthree, four, or five absorbers, relays, or traps havebeen described by a number of groups as capable ofrelatively long distance (tens of angstroms) chargeseparation with high quantum efficiency and longlifetimes (83% fo r an electron-transfer pentad with acharge separation lifetime of 55 ps in chl~rofo rm~~) ,

(20) Johanse n, 0.;Mau , A. W. H .; Sass , W. H . F. Chem. Phys. Lett .(21) Lehn, J . -M.; Sauvage, J . - P. ; Ziessel , R. Nouu. J. Chim. 1979,3,(22) Kalya nasunda ram, K. ; Gratzel , M.Angew. Chem.,Int.E d . Engl .(23) Sulzberger , B . ; Calzaferr i , G. J. Photochem. 1982, 19, 321.(24) Krueger , J. S.; Mayer, J. E.; Mallouk, T. E. J . Am. Chem. S O C .(25) Borja, M.; Dut ta , P. K. Nature 1993,362, 43.(26) Slam a-Schw ork, A.; Otto lenghi, M.; Avnir, D. Nature 1992,355,240.(27) Gust, D.; Moore, T. A,; Moore, A. L.; Lee, S. J.;Bi t te r sman n , E.;Luttrull, D. K.; Rehms, A. A,; DeGraziano, J. M.; Ma, X. C.; Gao, F.;Belford, R. E.; Trier , T. T. Science 1990,248, 199.(28) van D amme, H. V. D. ; Nijs, H. ; Fripia t , J. J. J . Mol. Cutal . 1984,27, 123.(29) Katakis, D. F.; Nitsopoulou, C.; Konstantatos, J. ;Vrachnou, E. ;Fa la ra s , P. J . Photochem. Photobiol. A (Chem. )1992,68 , 375.(3 0 ) Smo tk in , E. ; Cervera-March, S.; Bard, A. J . ; Campion, A,; Fox,M. A.; Mallouk, T. E.; Webber, S. E.; White , J. M. J . Phys. Chem. 1987,91, 6.(31)Fan, F.-R. F.; Keil, R. G.; Bard , A. J. J. A m . C h em . S O C . 983,105, 220.(32) Naka to , Y.; U eda, K.; Yano, H.; Tsubomura, H. J. Phy s . Chem.

1983,94, 107, 113.423.1979,18, 701.

1988, 110, 8232.

1988,92,2316.(33)Weller, H. Ber. Bunsen-Ges. Phy s . Chem. 1991,95, 1361.

with the rate of forward electron transfer favored bya factor substantially greater than lo3over the energydissipative back electron transfer.Despite the inherent difficulties encountered inattempts to optimize the oxidative and reductive half-reactions in solution phase systems, it is clear thatimpressive progress has been made. Nonetheless,combinations of such half-reactions t o a more complexarray capable of water splitting (with concomitanthydrogen and oxygen evolution) have proven t o bedifficult. To our knowledge, no truly homogeneousnonsacrificial system fo r water splitting has yet beenreported. Even so , compartmentalized tandem oxida-tion-reduction does take place on several nonelectro-active spatially confined solids, although the catalyticturnover numbers remain small (turnover number ofabout 5 fo r a R~(bpy)3~+/clay/Eu~+rray29, the reac-tions oscillatory, and the net quantum efficiencies low[4% upon excitation of tris[l-(4-methoxyphenyl)-2-phenyl-1,2-ethylenedithioleniclungsten in the vis-ible29 400-500 nm)].

Prospects for the FutureThese scientific difficulties continue to pose tantaliz-ing challenges to scientists bent on devising com-mercially viable means for solar energy conversion,especially as it relates t o chemical fuel production andits contribution to a hydrogen-based economy. Al-though we claim no special insight into the ul timatesolutions t o these problems, we can speculate on thoseresearch directions that may seem most promising.Given the difficulties associated with homogeneousroutes employed so far, it is a fair bet th at heteroge-neous arrays will be needed fo r high-efficiency solarenergy conversion. The problem of accumulatingmultiple redox equivalents fo r the oxidation and

reduction half-reactions also suggests that these sys-tems will require multifunctional components. Onepossibility is the use of multijunction devices, such asthe bipolar CdSe/CoS semiconductor photoelectrodepanel array on which unassisted water photodecom-position can be stoichiometrically attained with anequivalent solar energy efficiencyof about l%.30t isalso likely that new semiconductor materials andmultielectron catalysts will be key componentsof suchheterogeneous arrays, perhaps with small metal clus-ters or films t o protect the semiconductor surface, fo rexample, as noble metal silicides31or microscopicallydiscontinuous metal over layer^.^^ Dramatically newfamilies of photosensitizers might also overcome someof the practical barriers still encountered in attemptst o utilize the whole solar spectrum: the use of nano-particles of one semiconductor on a highly porous filmof Ti0233seems a very interesting possibility. In anycase, it seems clear that a creative breakthrough,probably focused within the boundaries defined byintriguing basic research conducted over the lastdecade, is needed to reach this Grail.

M A F . gratefully acknow ledges useful discussions on theseand related topics with Dr. Edmond Amouya l (Orsay ,France).AR9400812

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Handout Solar.2