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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light-to-chemical energetic transduction. This field of research, called “artificial photosynthesis,” is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon-free fuel hydrogen through the light-driven water splitting. In this review, we highlight the recent achievements on light-driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References

The development of a novel energetic scenario based on the use of two renewable resources, water and sunlight, is a challenging solution to the energy crisis the world is currently facing. Conversion of solar energy into chemical energy through the light-driven water splitting indeed generates the environmentally benign oxygen gas and hydrogen, a carbon-free fuel with the highest energy output relative to molecular weight. Thanks to the fuel cell technology, this energetic vector can be converted on request into electricity, with very high energy conversion efficiencies and without exhausting greenhouse gas. Moreover, this approach provides an attractive solution for storing the tremendous amount of sunlight energy falling on earth. In that context, natural photosynthesis is a great source of inspiration for the scientific community. Light-to-chemical energetic transduction is indeed achieved by photosynthetic organisms. Namely, algae and plants use light to extract electrons from water, which is oxidized to O2 (1). Most organisms use these photogenerated electrons to reduce atmospheric carbon dioxide and produce carbohydrates, proteins or lipids as the main constituents of their biomass, but some micro-organisms such as cyanobacteria or microalgae are able, under very specific conditions, to photosynthesize hydrogen as well (2–5). These transformations are realized thanks to a fascinating biologic machinery, presented schematically in Fig. 1, consisting of two large protein complexes, the photosystem I (PS I) and the photosystem II (PS II), assisted by various redox cofactors, and a unique enzyme, the hydrogenase.

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Figure 1.  Schematic representation of the photosynthetic chain in the oxygenic photosynthesis.

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The whole photosynthetic process can be divided into three distinct steps: (1) initial light-harvesting process and local charge separation in PS I and II; (2) proton-coupled electron transfers between redox cofactors along the photosynthetic chain, allowing further spatial charge separation; and (3) multielectronic redox catalysis generating hydrogen and oxygen at remarkable enzymatic sites such as dinuclear metal clusters in hydrogenase (6) and the oxygen-evolving CaMn4 center (OEC) of PS II (1), respectively. As far as the energetic aspect is concerned, the photosynthetic process is a fascinating example of efficiency (7,8). First, a highly ordered array of pigments (the so-called antenna system) absorbing a large range of the visible spectrum converts light into chemical energy at PS II. Second, charge recombination is prevented by the presence of an electron transport chain driving electrons to the PS I. Finally, a second light-harvesting process occurs at PS I, thus providing additional energy to the electrons for their final purpose (either CO2 fixation through the Calvin cycle or hydrogen production at the hydrogenase).

The application of concepts derived from natural photosynthesis is therefore highly attractive for the development of novel hydrogen production technologies. Understanding this biologic process and exploiting this knowledge for designing original synthetic molecular systems achieving light-to-chemical energy conversion is the basis of a large field of research called “artificial photosynthesis” (8–10). Molecular light-harvesting arrays have been developed to mimic the antennae effect, that is collecting the energy of many photons to transfer it directionally to a final acceptor achieving charge separation (Scheme 1A). Dyad and triad models based on the association of a photosensitizer to either an electron donor, an electron acceptor, or both components, have been designed to reproduce light-induced spatial charge separation (Scheme 1B). These first two topics have been for many years the core of artificial photosynthesis, and also the subject of comprehensive reviews (11–21). Significant strategies have been established to reach long-lived charge separation states, which is an important requirement in view of the application of photogenerated electrons in catalysis. These studies have also been a great source of inspiration in the field of organic photovoltaics and dye-sensitized-solar-cells (DSSCs) (22). Such devices are basically constructed on a dyad model (Scheme 1B).

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Figure Scheme 1..  Schematic representation of the different approaches for artificial photosynthesis: (A) light-harvesting array mimicking the collection and transport of energy to a reactive center; (B) dyad and triad models mimicking the charge separation state; (C) homogeneous multielectron photocatalysis for light-driven water splitting.

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Another attractive feature of natural photosynthesis concerns the multielectron redox catalytic processes. Whereas light-driven CO2 reduction for the production of CO or organic products can also be addressed with molecular catalysts (23), we will focus on hydrogen production from sunlight-driven water splitting. Currently, research is focused on the independent development of the two half-reactions, through the combination of a photosensitizer with a suitable catalyst for either the oxidation or the reduction of water, together with a sacrificial electron acceptor or donor, respectively (Scheme 1C). The term “artificial photosynthesis”, formerly devoted to molecular systems, is today also applied to qualify water splitting at a semiconductor–electrolyte interface, which will not be discussed here (24–29). In this review, we will highlight the significant achievements realized in the design of molecular photocatalytic systems for light-driven water oxidation and H2production. Perspectives for the application of these systems to the development of electrodes as part of a photoelectrochemical cell are also discussed.

Molecular catalytic systems for visible light-driven hydrogen evolution

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References

The photocatalytic H2 production is the area of artificial photosynthesis that has enjoyed the biggest development in the last 5 years. In this section, we will focus on the recent advances on noble metal-free molecular catalysts for light-driven hydrogen production. Indeed, although the catalytic performances of platinum group metals for hydrogen production are unrivaled, the limited supply of noble-metal would not sustain a low-cost and wide-scale production of technological devices. The development of new catalysts and photocatalysts for the hydrogen evolution reaction based on earth-abundant first-row transition metals has therefore been a challenge for inorganic chemists. This area of research has been boosted up by the single-crystal structure resolution of hydrogenase (H2ase) enzymes (30–32). These enzymes are the only molecular catalysts capable of catalyzing both proton reduction and hydrogen oxidation with efficiencies comparable to platinum particles (33). Their active site is a binuclear complex based either on iron only, or on a combination of nickel and iron metal ions in a sulfur-rich environment, with Fe ions also coordinating cyanide and carbon monoxide (Fig. 2) (6).

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Figure 2.  Structure of the active sites of [FeFe]-hydrogenases in the reduced active state (X is a hydride ligand, a water molecule or a H2 ligand) and of [NiFe]-hydrogenases in the Ni-C state.

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Consequently, in the past years, multicomponent and supramolecular photocatalytic H2-evolving systems have been developed, based on the assembly of hydrogenases with either synthetic inorganic photosensitizers or PS I (34). In parallel, a bioinspired catalysis approach has been undertaken to generate functional models of hydrogenases, giving a variety of first-row transition metal-based complexes, which have been successfully coupled with photosensitizers to catalyze the visible light-driven hydrogen evolution reaction.

Homogeneous photocatalytic H2-evolving systems and photocatalytic bioconstructs based on hydrogenases

Visible light-induced hydrogen production was reported 20 years ago by Okura et al. with aqueous multicomponent systems comprising the [NiFe] hydrogenase from Desulfovibrio vulgaris, a photosensitizer such as [Ru(bpy)3]2+ or a metallated porphyrin, a sacrificial electron donor, together with an electron relay (methyl viologen or cytochrome c3, the natural redox partner of H2ase) (35–37). Although a combination of the two types of electron carriers (38) or the construction of a cytochrome c3-viologen-ruthenium (II) triad complex (39) were proposed to optimize the electron transfers between the different components, these systems globally suffered from a low efficiency associated with a high oxygen sensitivity.

The first example of a direct light-to-hydrogen conversion system using hydrogenase and PS I was reported in 2006 by the groups of Okura and Friedrich. The authors have designed an artificial fusion protein composed of the membrane-bound [NiFe] hydrogenase from R. eutropha and the peripheral PS I subunit PsaE of T. elongatus. After spontaneous association with PsaE-free PS I, the resulting hydrogenase—PS I complex displayed light-driven hydrogen production at a rate of 0.58 μmol of H2 (mg of Ch1)−1 h−1 in presence of ascorbic acid, DTT and TMPD (electron donor system) (40).

A similar system was constructed by Heberle et al. using the membrane-bound hydrogenase (MBH) from R. eutropha H16 fused with an extrinsic subunit (PsaE) of an histidine-tagged form of PS I (41) (Fig. 3). This system has been immobilized at the surface of a nickel-functionalized gold electrode through a nickel His-Tag interaction. When this electrode is poised at −0.09 V vs NHE, so as to reduce the soluble electron carrier N-methylphenazonium methyl sulfate (PMS), and under visible illumination (λmax = 700 nm), photocurrents are observed (85 nA cm−2), 30% of which being assigned to the hydrogen evolution reaction (41).

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Figure 3.  Structure of the biochemical H2-evolving photoelectrode designed by Heberle et al.

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Recently, Golbeck et al. have developed an original strategy to connect an H2ase to a purified natural photosynthetic component. A dithiol molecule is employed to link via Fe-S coordination bonds a [4Fe-4S] cluster of a C13G mutant of the PS I PsaC subunit to the distal [4Fe-4S] cluster of a C97G mutant of the [FeFe]-hydrogenase from C. acetobutylicum (Fig. 4). Thus, a direct electron transfer is established between the two components, resulting in the production of hydrogen under illumination at a rate of 30.3 μmol of H2 (mg of Ch1)−1 h−1, at pH = 8.3 in the presence of a combination of cytochrome c6, ascorbate and phenazine methosulfate as sacrificial electron donor system. This PS I-H2ase bioconstruct has been found to display a remarkable stability and remained active for more than 2 months, in the absence of oxygen (42,43).

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Figure 4.  Light-driven hydrogen production mediated by a photocatalytic system combining photosystem I and hydrogenase developed by Golbeck et al.

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In a different approach, Armstrong et al. adsorbed the [NiFeSe]-hydrogenase from D. baculatum (a very efficient H2 producer and rather O2-resistant enzyme) onto a thin film of nanocrystalline TiO2, sensitized by a [Ru(bpy)3]2+ complex with phosphonate anchoring groups and deposited on a conducting ITO-coated glass support (Fig. 5) (44,45). Irradiation with visible-light in the presence of a sacrificial electron donor provided an efficient generation of H2, with an initial turnover frequency of 50 s−1 and turnover numbers of 10 000 and 100 based on hydrogenase and photosensitizer respectively, after 8 h of irradiation. However, some photoinstability of the system is observed with time.

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Figure 5.  Structure of the TiO2-based material for H2 photoproduction using surface-immobilized hydrogenases as catalyst.

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Synthetic multicomponent photocatalytic H2-evolving systems

By contrast with the use of biologic material such as hydrogenase or PS I, the synthetic approach benefits from the creativity of chemists that exploit the versatile and modular character of coordination chemistry to develop efficient molecular photosensitizers and catalysts. The last few years have seen tremendous achievements in the area of light-driven hydrogen production, with the emergence of two families of efficient catalysts, the diiron mimics of the active site of [FeFe] hydrogenases (46,47), and the bioinspired cobalt complexes (48–50). As a consequence, sustained efforts have also been invested to optimize the performances of the synthetic photocatalytic systems with the use of a wide range of molecular photosensitizers (PS) highlighted in the next section.

Molecular photosensitizers.  For many years, most of the work in the area of visible-light driven hydrogen production has relied on the use of polypyridyl ruthenium light-harvesting units, such as the prototype complex [Ru(bpy)3]2+ (PS1, Fig. 6).

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Figure 6.  Structure of the ruthenium-based photosensitizers employed in light-driven hydrogen evolution.

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This family of photoactive complexes displays very unique redox and photochemical properties. Ruthenium polypyridine complexes indeed efficiently absorb visible light photons which promote one electron from a metal d orbital to the antibonding π* orbital of a diimine ligand. The singlet excited state (S = 0 spin configuration) formed immediately upon excitation rapidly converts into a triplet state (S = 1) through inter-system crossing (isc in Fig. 7), due to the strong spin-orbit coupling effect of ruthenium. This yields a long-lived emissive metal to ligand charge transfer excited state (3MLCT, written PS* in the following descriptions), that can be described as a RuIII center coordinated to a reduced diimine radical anion ligand. Because of the presence of an electron in the π* antibonding orbital of one ligand, this excited state is a quite powerful reducing agent (Table 1) with a standard potential of −0.86 V vs NHE in the case of PS1. At the same time, due to the presence of an unoccupied d orbital at the metal center, PS* can also act as a powerful oxidizing species (Fig. 7) with E°=0.84 V vs NHE in the case of PS1 (51). The nature and concentration of redox partners then essentially control the reactivity of PS* as either a reducing or oxidizing species. These properties make [Ru(bpy)3]2+ and its derivatives ideal candidates for applications in photocatalytic processes (52).

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Figure 7.  Schematic description of the various states of a photosensitizer relevant for electron transfer processes (PS* indicates the lowest triplet excited state).

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Table 1.   Excitation wavelengths (nm), molecular absorption coefficients (m−1 cm−1), excited states energies (eV) and redox potentials (V vs NHE) of representative molecular photosensitizers.
 λmaxεE(PS+/PS)aE(PS/PS)aE*bE(PS+/PS*)cE(PS*/PS)cReferences
  1. aIf necessary, the original values have been converted according to reference (151). bEnergy of the triplet excited state PS*. cThe excited state redox potentials were calculated using the following equations: E(PS+/PS*) = E(PS+/PS) − E* and E(PS*/PS) = E(PS/PS) + E*.

PS145214600+1.26−1.28+2.04−0.86+0.84(51,52)
PS44166000+1.50−1.17+2.10−0.60+0.93(55,149)
PS6a4609670+0.94−1.08+2.28−1.34+1.20(150)
PS10524+1.03−0.81+1.89−0.86+1.08(65)
PS11550+0.94−0.51+1.83−0.89+1.32(66)

A similar description holds for the excited states of the other metal-diimine photosensitizers (Fig. 8) and also for the organic dyes (Fig. 9) presented below. The photophysical and redox properties of a representative example of each family are given in Table 1.

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Figure 8.  Structure of the iridium-, platinum- and rhenium-based photosensitizers employed in light-driven hydrogen evolution.

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Figure 9.  Structure of the organic dyes employed in light-driven hydrogen evolution.

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The successful application of noble-metal free catalysts in light-driven hydrogen evolution with ruthenium photosensitizers (53,54) has triggered the use of other photoactive complexes, based on iridium (PS4-5), platinum (PS6-7) and rhenium (PS8-9) metal centers (Fig. 8). The iridium photosensitizers are characterized by a higher tunability of their photophysical properties compared with the ruthenium ones (55) and they proved to be more efficient light-harvesting units for hydrogen photoproduction in various photocatalytic systems (55–57).

The platinum chromophores PS6 and PS7 have been extensively studied in combination with the cobaloxime catalysts by Eisenberg et al. (see below), thus allowing to establish some specific trends in the series, such as the lower activity of cyclometallated structures PS7 compared with PS6 (58–60). In a similar study, Castellano et al. have established the influence of phenylacetylide π-conjugation length in PS6e-f on the photocatalytic activity (61).

To date, the most efficient photosensitizers for light-driven hydrogen production are the tricarbonyl-rhenium diimine complexes PS8-9, combined with cobaloximes 3 (57) or 5 (62). Detailed photophysical studies by the group of Hamm et al. have allowed to establish some mechanistic issues related to the cobalt-based photocatalytic systems (62–64). Exchanging the bromide ligand by a thiocyanate (PS9b) in the coordination sphere of Re greatly increased the stability of these systems (62).

As a major breakthrough, the first homogeneous systems exclusively based on earth-abundant elements were reported in 2009 (65). The noble metal-based photosensitizers have been replaced by low cost organic dyes (Fig. 9), such as Eosin Y (PS10) or Rose Bengal (PS11). Other xanthene derivatives such as fluoresceine, lacking bromide or iodide substituents, did not prove active under photocatalytic conditions. Actually, the presence of heavy atoms facilitates inter-system crossing and thus production of a long-lived triplet state, required for an efficient electron transfer to the acceptor. The excited state of these xanthene dyes displays redox properties similar to the noble-metal based PS (Table 1). Photophysical studies have established the feasibility of both reductive quenching in the presence of amine electron donors and oxidative quenching process with a cobaloxime acceptor (66).

The successful implementation of inexpensive organic dyes in light-driven hydrogen production now opens new perspectives for the design and synthesis of novel metal-free photosensitizers with tunable photophysical properties.

Diiron-based catalysts.  Following the first syntheses of biomimetic models of the [FeFe] hydrogenase active site covalently linked to a ruthenium photosensitizer by Åkermark, Sun et al. in 2003 (67,68), various diiron dithiolate [2Fe2S] complexes have been combined to a photosensitizing unit (69–75). However, these early generations were shown to be unsuccessful for light-driven hydrogen evolution. The first catalytically active homogeneous system was reported in 2008 by Sun et al. (76). This three-component system, based on the ruthenium photosensitizers PS2, the diiron dithiolate complexes 1 (Fig. 10) and ascorbic acid as the sacrificial electron donor, generated up to 86 turnover number (TON) based on PS (only 4 TON based on the diiron catalyst) in a CH3CN/H2O mixture, within 3 h under visible-light irradiation. The P(Pyr)3-monosubstituted complex 1b (Fig. 10) proved to be the most efficient catalyst of the series, with an improved photostability. Photostability is a critical issue as many of these organometallic complexes suffer from decomposition by light-induced CO release (77). Recently, replacement of PS2 by the cyclometallated iridium photosensitizer PS4 and of the ascorbic acid by TEA conferred a longer lifetime and a higher activity to the previous photocatalytic system, with 466 TONFe measured over 8 h of irradiation (56). To the best of our knowledge, this result represents the highest TON ever reported with a diiron-based catalyst system.

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Figure 10.  Structure of the diiron complexes catalytically active in light-driven hydrogen evolution.

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In a different approach, Ott et al. have demonstrated that a subtle modification of the catalyst structure could improve its photocatalytic activity. Indeed, under visible-light irradiation, the H2-evolving catalyst 2 (Fig. 10) achieves ca 200 TON of hydrogen in a 2.5 h experiment, in the presence of [Ru(bpy)3]2+ (PS1) and ascorbic acid (sacrificial electron donor) in a 1:1 DMF/H2O solution at pH = 5.5 (78). Replacing the azadithiolate bridge by a 3,6-dichlorobenzene-1,2-dithiolate moiety is proposed to confer a higher stability to the reduced form of the diiron complex, as evidenced by the reversibility of the first reduction featured in its cyclic voltammogram (79).

Cobaloxime and cobalt diimine-dioxime catalysts.  Another class of first-row transition metal complexes that have been successfully employed as H2-evolving catalysts are the cobaloxime series. These cheap nonbiomimetic pseudomacrocyclic bis(dialkylglyoximato)cobalt complexes have been recognized as powerful electrocatalysts for H2 production, notably in terms of working potential and catalytic rate (80–84). The first study using a cobaloxime catalyst in combination with PS1 as a photosensitizer was reported in 1983 by Lehn et al. (85). This family of catalysts for light-driven hydrogen production was revisited by us in 2008, with a series of supramolecular photocatalysts based on a cobaloxime center (54,57). Following this, various photosensitizers have been combined with cobaloxime complexes, either in a supramolecular mode or in multicomponent systems. Nowadays, these cobalt-based entities are among the most employed catalysts for the construction of light-driven systems for H2 evolution.

The structures of various cobaloxime complexes 3–6 evaluated in three-component homogeneous systems are presented in Fig. 11. The substitution pattern has a great influence on their photocatalytic activity. Although displaying a more positive reduction potential, complex 4, bearing four phenyl substituants, is 13 times less active than 3 (66). Comparison of catalysts 5 and 3, in the presence of PS1 and TEA, has established the superiority of the BF2-bridged structures (20 TON in a 1 h experiment) on their H-bridged analog (2 TON in a 4 h experiment, in the presence of an excess of dmgH2 ligand) (54). This has been attributed to the higher stability toward hydrolysis of the BF2-bridged cobaloximes.

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Figure 11.  Representative structures of the cobaloxime catalysts active in light-driven hydrogen evolution.

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Systems based on cobaloxime 3 as the H2-evolving catalyst were further improved in combination with other metal-based photosensitizers such as the cyclometallated iridium diimine PS5 (Fig. 17), or the tricarbonyl-rhenium diimine PS8 (Fig. 8), resulting in H2 production with 165 (15 h) and 273 (15 h) TON respectively (57). The organic dye PS11 has also been successfully employed with up to 327 TON H2 in a 5 h experiment (CH3CN/H2O, 1:2; 10% TEA) (66).

Cobaloximes 6 bearing an axial pyridine ligand have been studied in combination with the platinum-based photosensitizers (PS6-7) (58,59,61). Under the best experimental conditions established by Eisenberg et al. (CH3CN/H2O, 24:1; TEOA 0.27 m; pH = 8.5), 120 TONCo (2150 turnovers with respect to the Pt-sensitizer) are produced during a 10 h experiment. Replacing the Pt-based photosensitizers by commercially available Eosin Y (PS10) resulted in noble-metal free systems able to achieve up to 180 TONCo (CH3CN/H2O, 1:1; 5% TEOA; pH = 7) (65). The stability of photocatalytic system has been further improved by replacing the fluorescein dyes by synthetic rhodamine derivatives containing S or Se atoms in place of O in the xanthene ring (PS12a-c) (86). The performances of a commercially available organic dye, the acriflavine PS13, have also been recently established in combination with catalyst 6a, resulting in H2 production with 66 TONCo (2 h of irradiation; 110 TON with respect to PS13) in the presence of TEOA (2.5%) in a 3:1 mixture of DMF and H2O (87).

Clearly, these studies have demonstrated the potential of cobalt-based catalysts in light-driven hydrogen production. However, the stability of cobaloxime complexes still needs to be improved for the development of long-term stable systems. We specifically addressed this issue with a second generation of catalysts, the cobalt diimine-dioxime [Co(DO)(DOH)pnBr2] complexes bearing a tetradentate ligand instead of two bidentate dioxime ligands and which proved to display excellent electrocatalytic properties for hydrogen evolution and greater stability under acidic conditions as compared with the cobaloximes (88). Recently, complexes 7-8 (Fig. 12) were combined with the cyclometalated iridium photosensitizer PS4 in a 1:1 ratio, to light-drive hydrogen production in a CH3CN/H2O mixture (10% TEA, pH = 10) (89). Catalyst 8 was shown to be the most efficient one with 307 TONCo achieved in a 4 h experiment. However, it has been established that the performance of the photocatalytic system was still limited by the catalyst decomposition. To improve its stability, a fine tuning of the coordination sphere of the cobalt center was successfully undertaken. Addition of two equivalents of triphenylphosphine allowed to reach up to ca 700 TONCo in a 10 h experiment, thanks to the formation of spectroscopically identified Co(I)(DO)(DOHpn)(PPh3) (10) catalytically active species (89).

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Figure 12.  Structure of the cobalt diimine–dioxime catalysts active in light-driven hydrogen evolution.

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Finally, Alberto et al. have recently re-examined the use of cobaloxime 5 in combination with tricarbonyl-rhenium diimine sensitizers (62,63). In the presence of TEOA as the sacrificial electron donor and acetic acid as the proton source in DMF, 75 TON were achieved after 9 h irradiation when PS9a is employed (63). Exchanging the bromide ligand by a thiocyanate one in PS9b conferred a higher stability to the photocatalytic system, thus able to achieve up to 353 TON in a 120 h experiment (62). Under optimized experimental conditions, turnovers up to 1850 per cobalt ion (ca 300 per dimethylglyoxime ligand (dmgH2) added to the solution to restore the integrity of the catalyst) could be achieved, which represents the highest efficiency reported so far for a cobaloxime-based photocatalytic system. Alternatively, tricarbonyl-rhenium diimine complexes have also been successfully used as light-harvesting units in combination with diimine-dioxime cobalt catalysts, resulting in H2 production with up to 550 TONRe (17 TONCo) for complex 9 (64).

Mechanistic issues.  The general mechanisms for light-driven H2-evolution catalyzed by synthetic systems are illustrated in Fig. 13. The process is, in any case, initiated by the absorption of a photon by the photosensitizer, yielding the PS* excited state. From the latter, a first photoinduced electron transfer may take place by an oxidative quenching process involving the catalyst as the electron acceptor and generating the active catalytic reduced species. A second electron transfer process then occurs between the sacrificial electron donor and the resulting oxidized photosensitizer PS+, thus regenerating the PS. An alternative mechanism implies first a reductive quenching of the PS* by the sacrificial electron donor to yield the reduced PS, which subsequently reduces the catalyst to its active state. The following steps, that are metal-centered mechanisms for H2 evolution catalyzed either by a diiron center (47) or by a cobalt catalyst (48,50) have been recently reviewed and are beyond the scope of this article.

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Figure 13.  Schematic representation of the two plausible photosensitizer-based mechanisms ([A] Reductive quenching process; [B] Oxidative quenching process).

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By contrast with multicomponent systems based on platinum colloids or hydrogenase, early studies have revealed that an electron relay between the PS and the molecular synthetic catalyst was not needed to establish an efficient electron transfer (90,91). This is true for all the diiron and cobalt photocatalytic systems described before.

The establishment of one mechanism vs the other will essentially depend on the redox properties of different couples (PS+/PS*, PS*/PS, Cat/Cat, D•+/D), and their relative concentrations. It is well-established that a diiron catalytic center cannot be directly reduced by an excited photosensitizer such as *[Ru(bpy)3]2+ or *[Ir(ppy)2(bpy)]+, because its reduction potential is more negative than the oxidation potential of excited PS*, making the direct electron transfer thermodynamically unfavorable (76). Thus, light-driven hydrogen production with diiron catalysts does occur via pathway (A) (Fig. 13). This reductive quenching mechanism also applies when PS1 and cobaloxime 5 are combined for light-driven hydrogen production (85). However, the mechanistic analysis is more complex for the other cobalt-based photocatalytic systems as both pathways (A) and (B) are in many cases thermodynamically feasible (65,66,89). Morevover, when CoIII catalysts (6–9) are employed, two successive electron transfer steps have to be taken into account. A reductive quenching mechanism has been nevertheless evidenced for the hydrogen-evolving systems comprising catalyst 5 and the rhenium photosensitizers PS9 (62) or for the combination of catalyst 6a with the organic dye PS12c (86). By contrast with the photosensitizers PS6, their cyclometallated platinum analogs PS7 cannot be reductively quenched by TEOA and consequently hydrogen production occurs via the oxidative pathway (B) (59). In the other cases, a reductive quenching cannot be excluded, in particular because of the large excess of sacrificial electron donor employed in the photocatalytic experiments.

The nature of sacrificial electron donor also needs to be considered from a mechanistic point of view. Indeed, in most of the previously described photocatalytic systems, an amine (triethanolamine, TEOA, or triethylamine, TEA) is employed. The radical cation species generated after a first electron transfer to the PS is a strong reducing agent, thus able to furnish a second equivalent of electrons to the system, yielding an aldehyde and a secondary amine by-product (Scheme 2) (62,90,92). For instance, a yield of 70% for this second electron transfer has been determined from kinetic studies on a system comprising PS9b and TEOA (62). This dark process should be systematically taken into account in the determination of quantum yield for the photoproduction of H2.

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Figure Scheme 2..  Decomposition pathways for the sacrificial electron donors TEA (R = H) and TEOA (R = OH) generating one equivalent of electrons (dark process).

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It is clear from the photocatalytic studies described above that optimization of the experimental conditions for a given system is not a trivial process. Indeed, its efficiency strongly relies on a broad range of parameters that cannot be independently optimized. The acidity of the medium is perhaps the most important one to consider. The pH is strongly correlated to the choice of the sacrificial electron donor. Generally, with TEA or TEOA, no H2 production was observed at pH below 6–8 or above 12–13. Indeed, at low pH values, protonation of the amine reduces the amount of redox-active basic form acting as sacrificial electron donor for the regeneration of photosensitizer, whereas at high pH protonation of the catalyst is less favorable. Pronounced solvent effects are also observed for all the photocatalytic systems described in the literature. DMF, CH3CN or acetone generally appear as solvents of choice, in combination with water (up to 66%). Even though such solvent mixtures provide a first step toward using water as a solvent in applications for water splitting, only a limited number of homogeneous photocatalytic systems display activity in pure water (53,64,66). Finally, deactivation of the system is often observed after a few hours of activity, but generally not understood at the molecular level. Bleaching of the dye is the major cause of inactivation in the case of systems based on organic dyes. Metal-polyimine photosensitizers, such as PS1, are also known to photodecompose but rhenium-based photosensitizers are significantly more stable. More generally, it appears crucial to determine whether the deactivation process is an intrinsic feature of the photocatalytic system or is specifically due to the use of a sacrificial electron donor generating potentially damaging radical species. The fact that a mixture of dyes PS12b-c and TEOA rapidly bleaches in the absence of catalyst supports the latter hypothesis.

Supramolecular photocatalysts

Parallel to the development of multicomponent systems described before, efforts have also been devoted toward the synthesis of single-component photocatalysts, either by supramolecular assembly or by covalent linking of the light-harvesting unit to the catalytic one. This design is proposed to optimize electron transfers between these two units and is relevant to the spatially controlled assembly of the various photosynthetic components in the membrane, largely responsible for the efficiency of natural photosynthetic process.

Although the first generations of supramolecular and covalent assemblies of a diiron center with a photosensitizer did not display any activity in light-driven hydrogen production (67–74), the design of biomimetic diiron-based supramolecular photocatalysts is still the subject of intensive studies. Diiron assemblies with Zn(II)-porphyrin (93–95) or rhenium (96) photosensitizing units did evolve traces of H2 under visible light irradiation. However, the efficiency of these dyads remains very low compared with the three-component photocatalytic systems described above.

By contrast, encouraging results have been obtained with cobalt-based supramolecular photocatalysts. Their study was initiated with our reports on a series of supramolecular photocatalysts for H2 production based on cobaloxime centers (Fig. 14). These heterodinuclear complexes were obtained via axial Co coordination of a pyridine-functionalized metal-diimine photosensitizer. Their activity can be tuned through manipulation of (1) the Co coordination sphere; (2) the linker; and (3) the photosensitizing unit. The series of ruthenium-based photocatalytic assemblies 11–13 has for instance established that a conjugated bridge is not required for the activity (54,57,97). Compound 11a with 103 turnovers achieved in the course of a 15 h experiment proves competitive with previously reported Ru-Pt, Ru-Pd and Ru-Rh supramolecular systems containing noble metal-based catalytic centers. As a major drawback and still for unknown reasons, compounds 11a–c required near UV light to drive H2 evolution, whereas 12, bearing substituted phenanthroline ancillary ligands on the Ru center, is active under pure visible light irradiation. Compound 14 with an iridium-based dye achieves 210 TON in the course of a 15 h experiment in good agreement with the observed superiority of Ir photosensitizers over Ru ones. It is important to notice that this supramolecular architecture is more stable than the bi-component catalytic system composed of 3 and PS5 that levels off after 165 TON.

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Figure 14.  Representative structures of the photosensitizer–cobaloxime supramolecular catalysts active in light-driven hydrogen evolution.

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The same design has been applied by the group of Wang and Sun to construct noble-metal free supramolecular devices based on pyridyl-functionalized porphyrin photosensitive units (Fig. 14) (98). Whereas low amounts of H2 are detected with either the Mg-porphyrin 16b or the free-base porphyrin 16c, TONs up to 22 are achieved with the Zn-based device 16a, in a 5 h experiment. On the basis of spectroscopic studies, the authors suggested that an axial weak coordination of the sacrificial electron donor TEA to Zn, allowing an inner-sphere electron transfer process, could account for the highest performance of 16a. Under the same experimental conditions, a three component catalytic system ([Zn(PyTBPP)] + 6a + TEA) does not evolve any detectable amount of H2.

A different strategy, recently developed by Sakai et al., relies on the spontaneous self-assembly of bipy-appended cyclometallated Ir photosensitizers in the presence of CoII ions, thus in situ generating [Co(bpy-L-Ir)n]2+-type species 15 (Fig. 14) (99). In that case, the catalytic center is a [Co(bpy)3]2+ derivative, whose photocatalytic activity has been previously established in the 1980s by Sutin et al. in multicomponent systems with PS1 and PS3 (53,100–103), and further improved with PS4-5 by Bernhard et al. (55). In the presence of TEOA, supramolecular assemblies 15 mediate light-driven H2 production in CH3CN/H2O mixtures with up to 20 TONCo. Multicomponent systems tested under the same experimental conditions were shown to be twice less efficient, again pointing out the importance of supramolecular design.

Finally, a last system has been recently described, with the cobaloxime 14c and the photosensitizer [Ru(bipy)2(2,2′-bipyridine-4,4′-diylbis(phosphonic acid))]2+ electronically connected via a TiO2 nanoparticle (Fig. 15) (104). Both molecules are grafted onto the surface of TiO2via phosphonate anchoring groups, with the aim of taking advantage of the ultrafast electron injection from the excited state of the photosensitizer into the conduction band of TiO2, from which electrons can be further transferred to the catalyst. Hydrogen (53 TON) is evolved when this system is placed at pH 7 in a TEOA aqueous buffer and irradiated with visible light. It still suffers, however, from the photoinstability of ruthenium dye.

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Figure 15.  Structure of the TiO2-based material for H2 photoproduction using a surface-immobilized cobaloxime catalyst (104).

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These studies have pointed out that the supramolecular cobalt-based assemblies are more active than the corresponding multicomponent systems. New photocathodes can thus be developed through the grafting of these original molecular structures onto transparent conducting surfaces.

Light-driven homogeneous water oxidation

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References

Stable and efficient photocatalytic systems for the oxidation of water into molecular oxygen are needed for the development of light-driven water splitting devices. The first molecular complexes active for water oxidation were reported in the early 1980s by Meyer et al. The dinuclear μ-oxo ruthenium complex [(bpy)2(H2O)RuORu(H2O)(bpy)2]4+ (17, Fig. 17), called “blue dimer,” opened the way to the chemistry of high-valent Ru-oxo species for water oxidation (105–107). Research in this field has also been driven by the increased knowledge, at the molecular level, of the structure and mechanism of the oxygen-evolving center (OEC) of PS II (108–110). The most recent evidence indicates that it is constituted of a mixture of Mn and Ca ions in a Mn4Ca stoichiometry, in the arrangement of a Mn3Ca cubane, where ions are bridged by oxo groups, with a fourth manganese ion linked to this structure (111). This unique metal cluster has the potential to accumulate, one by one, four oxidizing equivalents that can be transferred to two metal-bound water molecules. The multielectronic nature of this process is the major difficulty with regard to the development of efficient light-driven synthetic models of PS II. Whereas under electrocatalytic or chemical (CeIV) oxidation the multiple electron transfers are not rate-limiting, the same multielectronic reaction performed under photochemical conditions require stepwise accumulation of redox equivalents, generated by four successive one-photon absorptions. This photocatalytic process will therefore be highly dependent on the photon-flux. Obviously, this kinetic drawback also exists for light-driven hydrogen evolution, however, to a lower extent as it is a two-electron process only. In spite of this limitation, a number of interesting molecular systems have been recently studied.

Photocatalytic bioconstructs based on PS II

By contrast with PS I, photosystem II is a complex multisubunit membrane-associated protein structure difficult to exploit in a bioelectrochemical device. Examples of engineered PS II thus remain scarce. Rögner et al. have succeded in the development of a PS II-modified photoanode. Their strategy relied on the preparation of a His-tagged modified PS II from Thermosynechococcus elongatus, that could be immobilized via metal ion affinity on a gold electrode modified by Ni(II)-nitrilotriacetic (NTA) acid groups (Fig. 16). Photocurrents (up to 14 μA cm−2) associated with oxygen production were observed for a surface coverage corresponding to a monolayer of immobilized enzyme. In this first generation of photoanode, an artificial electron mediator (2,6-dichloro-1,4-benzoquinone) was required to establish communication between the PS II components and the electrode surface (112). The electrode design was further improved by the immobilization of native PS II within the three-dimensional network of an osmium-based redox polymer, also acting as the electron acceptor for the enzyme. Higher amounts of enzyme were grafted and thus photoactive currents as high as 45 μA cm−2 were measured under illumination (2.65 mW cm−2) without the use of any soluble redox mediator (113).

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Figure 16.  Schematic representation of PS II entrapped within an osmium-containing redox polymer, as designed by Rögner et al.

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Synthetic multicomponent photocatalytic oxygen-evolving systems

Studies on light-driven water oxidation have been mainly based on synthetic molecular catalysts (105), associated with ruthenium photosensitizers (PS1 and derivatives).

Mono- and dinuclear ruthenium-based photocatalytic systems.  Following the discovery of the water oxidizing properties of blue dimer 17 (Fig. 17) in the presence of Ce(IV) as a chemical oxidant, a lot of attention has been paid to the development of dinuclear ruthenium complexes as electrocatalysts for water oxidation (106,107). However, examples of functional catalysts for photoinduced water oxidation are few. Grätzel et al. have initially reported that μ-oxo-bridged ruthenium complexes in combination with PS1 generated up to 50 TON O2 under visible light illumination in a 2 h experiment before inactivation of the photosensitizer (114,115).

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Figure 17.  Structures of the ruthenium-based catalysts active in light-driven water oxidation.

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Recently, a breakthrough occurred with the design by Sun et al. of a series of mono- and dinuclear ruthenium complexes 18–20 bearing negatively charged carboxylate ligands (Fig. 17) (116–120). The strong electron-donating ability of the latter greatly lowers the oxidation potentials of these complexes, which consequently successfully match the redox potential of the photosensitizer. Carboxylate-substituted [Ru(bpy)3]2+ derivatives displaying higher oxidizing properties than PS1 have been coupled to catalyst 18 to drive water oxidation under visible light irradiation with up to 370 TON (119). The dinuclear cis analog 19 proved to be more active and stable, with 580 TON O2 evolved (117). The rate and yield of oxygen evolution were found to be strongly correlated with the photosensitizer concentration, employed in large excess (mm range) with regard to the catalyst. However, the stability of these photocatalytic systems, that are active for less than half an hour under irradiation, remains a major issue. Reactivation of such systems is possible, by readjusting the pH to its initial value of 7 and by adding extra photosensitizer and electron acceptor (117,119). An unprecedented TON value of 1270 has been achieved by molecular catalyst 18 after four successive runs (with a derivative of PS1 and S2O82−, pH = 7 buffer solution) (119). The [Ru(terpy)(pic)3]2+ family of catalysts active in CeIV-driven water oxidation previously developed by the group of Thummel (121) has been recently re-examined under photocatalytic conditions by Sun et al. A range of variously substituted structures has been screened and catalyst 21 has proven to be the most efficient one, with 84 TON achieved in a 1 h experiment in the presence of S2O82− (122).

Visible-light driven water oxidation was also demonstrated for the mononuclear catalyst 20 in three-component systems comprising either PS1 or PS3, and [Co(NH3)5Cl]2+ or S2O82− as sacrificial electron acceptors (120). This catalyst was later integrated in a photoelectrochemical device (Fig. 18). Splitting of water from a neutral aqueous electrolyte is observed when a voltage bias is applied (123). However, the formation of catalytically active RuO2 species during the course of photocatalytic reaction cannot be excluded (124).

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Figure 18.  Schematic representation of the voltage-biased photoelectrochemical devices for overall water splitting first reported by Spiccia et al. (catalyst A) then by Sun et al. (catalyst B).

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Recently, Llobet et al. have evaluated the potential of complexes 22 (Fig. 17) in light-driven water oxidation, with PS1 and [Co(NH3)5Cl]2+ as the sacrificial electron acceptor. Low TON (max. 4) is obtained at pH = 7 in a phosphate buffer, in strong contrast with the performances of these catalysts in chemical oxidations, attributed to differences with respect to the pH conditions (pH = 1 in the latter) (125).

These studies have established some trends for the design of efficient water oxidation catalysts: (1) in dinuclear complexes, an organic bridging ligand could provide a higher stability and rigidity than the oxo bridge found in the prototype blue dimer; (2) there is no requirement for polynuclear complexes for catalytic activity; and (3) readjustment of the initial composition of the medium could reactivate the photocatalytic system. The latter issue should be carefully considered in the future for the optimization of light-driven water splitting homogeneous systems.

Ruthenium and cobalt-based polyoxometallates.  Due to the highly oxidizing character of intermediate catalytic species, oxidative degradation of components of the systems, in particular the organic ones, is significant. To circumvent this drawback, inorganic catalysts based on a polyoxometallate (POM) framework, have been independently developed in 2008 by the groups of Hill (126,127) and Bonchio (128). These robust carbon-free polyanionic scaffolds have been used to assemble a tetraruthenium catalytic center, thus conferring a remarkable stability to the system. For example, the tungstosilicate complex [RuIV4(O)4(μ-OH)2(H2O)4(γ-SiW10O36)2]10− 23 (Fig. 19) catalyzes the oxidation of water by CeIV with a TON of 385 and a yield of 90% during a 2 h reaction (128). Successfully, 23 has been coupled with PS1 to catalyze light-driven oxygen evolution with up to 180 TON in a half-an-hour experiment in the presence of S2O82− as the sacrificial electron acceptor (127). Under similar experimental conditions, 120 TON were obtained with the tungstophosphate analog [Cs9][RuIV4(O)5(μ-OH)(H2O)4(γ-PW10O36)2] (129).

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Figure 19.  Light-driven water oxidation system combining the tetrameric ruthenium sensitizer PS14 with the tungstosilicate complex [RuIV4(O)4(μ-OH)2(H2O)4(γ-SiW10O36)2]10– 23.

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It is worth highlighting the use of tetrameric ruthenium sensitizer PS14 (Fig. 19) whose structure is relevant to the antenna systems in the natural light-harvesting process (Scheme 1A). Interestingly, this first generation metallodendrimer displays light-harvesting properties at a wavelength (λmax ≈ 550 nm) at which classical ruthenium photosensitizers are totally ineffective and with a considerably higher extinction coefficient. Its efficiency for photoinduced water oxidation has been previously reported in combination with IrO2 nanoparticles (130). When coupled with POM 23 (Fig. 18), PS14 drives water oxidation with a quantum yield of oxygen production of 30%, significantly larger than the previously reported ones with [Ru(bpy)3]2+ (≈ 5%).

Finally, a novel cobalt-based polyoxometallate recently reported by Hill et al. has proven its superiority compared with the previously reported ruthenium-based homolog systems (131). When assayed in combination with PS1 and S2O82− at pH = 8, this polyanion only consisting of abundant metals catalyzed the light-driven water oxidation with a TON up to 220 and a high photon-to-O2 yield (30%).

Bioinspired manganese-based photocatalytic systems.  Finally, chemical models mimicking the oxygen evolution center have been extensively studied (132–134). The most significant examples are: dinuclear Mn(III) complexes with terpyridine (135) or carboxylate-based ligands (136) and biologically relevant cubane clusters (137). However, their efficiency under homogeneous conditions with a Ce(IV) oxidant remained very low (subcatalytic in most cases). Typically, heterogeneisation of these catalysts provided some benefit in terms of efficiency, thus allowing the development of light-driven catalytic applications.

Following their initial reports that the μ-oxo bridged dimer [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)] 24 (Fig. 20) worked as a water oxidation catalyst when adsorbed on clays (138,139), Yagi et al. have developed a photocatalytic system based on the coadsorption of this cationic manganese complex with PS1 at an interlayer of mica (Fig. 20). Oxygen evolution was detected when the adsorbate was suspended and irradiated in a S2O82−-containing acetate buffer solution. Although up to 88 TONs are measured based on [Ru(bpy)3]2+, it only corresponds to 3.4 TON vs the manganese catalyst, in a 17 h experiment. This low efficiency has been attributed to the instability of ruthenium photosensitizer. A mechanism based on a reductive quenching of PS1* by the manganese catalyst inside the mica layer, followed by regeneration of the photosensitizer by S2O82− is proposed, according to the described photophysical studies (140).

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Figure 20.  Schematic representation of the co-adsorption of the μ-oxo bridged manganese dimer 24 with PS1 in a mica interlayer.

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Recently, an original photoanode (Fig. 18), based on a bioinspired cubane cluster, entrapped in a nafion matrix, and deposited onto a TiO2-supported ruthenium photosensitizer, has been reported by Spiccia et al. (141). This photoanode has been successfully coupled to a platinum counter-electrode and photocurrents up to 25 μA cm−2 were generated under illumination together with evolution of oxygen. To the best of our knowledge, this represents the only active photoelectrode material made from molecular mimics of the OEC. However, there is no evidence that the molecular nature of catalyst is kept intact under the photoelectrocatalytic conditions (142).

This brief survey of light-driven water oxidation raises a number of issues that need to be addressed in the future: (1) the stability of photosensitizer under highly oxidizing conditions has to be significantly improved. In current measurements, PS does not achieve more than a few TON in the presence of S2O82− under irradiation; (2) the stability of catalyst, and especially, the conservation of its molecular nature has to be systematically investigated. In particular, the formation of catalytically active metal oxide nanoparticules has to be ruled out, both in homogeneous studies and heterogeneous systems; and (3) finally, a better understanding of these light-induced processes by undertaking detailed mechanistic studies.

Perspectives

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References

The challenge now is to assemble light-driven hydrogen and oxygen evolution systems together so as to build a fully molecular-based device for overall water splitting. Actually, two very distinct architectures could be considered in that aim:

  • 1
     The first one consists of a fully homogeneous system, where both hydrogen and oxygen are evolved in one compartment (Scheme 3, devices A or B).
  • 2
     In a second variant, a photoelectrochemical cell is constructed by combining a (photo)anode on which the oxidation of water occurs and a (photo)cathode, where the electrons extracted from water react with protons to generate hydrogen, thus converting photon power directly into a chemical fuel (Scheme 3, devices C or D).
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Figure Scheme 3..  Schematic representation of homogeneous (A and B) and PEC (C and D) devices for overall water splitting.

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The obvious drawback of the first solution is that the stoichiometric gas mixture is highly explosive and that hydrogen gas has to be further purified. However, this approach is technically feasible and today appears as the most economically viable (143). To be economically viable, the second architecture requires a photoelectrochemical cell to be coupled with a solar concentrator reflector that will focus solar direct radiation onto the cell (143).

Proofs of concepts for both architectures have been already reported, but using semiconductors and/or composite photovoltaic materials as photosensitizers and noble metals or metal oxide materials as catalysts. State of the art performances for the conversion of the entire visible solar spectrum to chemical energy are 6.3% for a microhomogeneous system reported by Domen et al. (144) (Fig. 21) and 12.4% for the PEC cell from the Turner group at NREL (Fig. 22) (145).

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Figure 21.  Microhomogeneous system for water splitting developed by Domen et al.

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Figure 22.  PEC cell from the Turner group.

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Back to the question whether the molecular photocatalytic systems described above can or not be used for the construction of such devices, and discarding stability considerations, three issues have to be considered. The first requirement is that both catalysts used for H2 and O2 evolution should work under the same operating conditions (medium, pH …) and tolerate the presence of both hydrogen and oxygen. The second requirement derives from thermodynamics. Even if the system is far from being at the equilibrium (this will be discussed below), electron transfers will only occur if the redox potentials of direct partners along the chain are thermodynamically compatible. First, a system with only one [Ru(bpy)3]2+ photosensitizer coupled to two catalysts (Scheme 3, device A) would be active only if the photoexcited PS* is first oxidatively quenched by the H2-evolving catalyst. Indeed, PS* is not oxidizing enough and should be converted into its Ru(III) state to allow electron transfer from the OEC catalyst. In that perspective, one can take advantage of the unique properties of TiO2 particles which oxidatively quench the excited state of metal-polyimine dyes at the femtosecond timescale. Upon excitation, the dye injects an electron into the conduction band of TiO2, which can in turn deliver it to a grafted H2-evolving catalyst (146). Therefore, a design analogous to the one reported by Grätzel et al. in 1981 (146) containing molecular catalysts for H2 and O2 evolution instead of Pt and RuO2 nanoparticules appears promising. Alternatively, the TiO2 electrode could be linked to a separated cathode on which a H2-evolving catalyst is grafted (147,148).

Another solution would consist in using two photosensitizer-catalyst pairs. Basically, the coupling of two light-driven systems can be simply done through a redox mediator (Scheme 3, device B) or by attaching both of them onto a transparent electrode substrate (Scheme 3, device D). The associated drawback could reside in competition between PSs for light harvesting if they absorb in the same region of the visible spectrum.

The third and probably most challenging barrier lies in the fine control of the kinetics of the electron transfer steps. In a way, the half-systems described above are somehow biased as they all use irreversible electron donors (for H2 evolution) or acceptors (for O2 evolution) driving the system. A good alternative resides in the separate immobilization of each system on transparent conducting electrodes. However, whatever the type of architecture (A, B, C, or D) employed for the construction of a light-driven device for overall water splitting, competitive and inefficient recombination pathways need to be considered and avoided. This specific issue can however be addressed thanks to the knowledge acquired in the field of artificial photosynthesis since its pioneering ages. Concepts applied for the mimics of antennae effect, unidirectional electron transfer and formation of long-lived charge separation states (see Scheme 1) could be successfully integrated in these bioinspired photoelectrochemical cells in the future, so as to reproduce the natural photosynthetic process.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References

Acknowledgements— The authors acknowledge CEA for financial support within the “pH2oton” project (program transverse NTE grant).

Author biographies

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References
  • image

[ Eugen S. Andreiadis ]

Eugen S. Andreiadis obtained his Chemical Engineer degree in 2005 as Valedictorian at the Politehnica University of Bucharest, Romania. He received his PhD degree from the University Joseph Fourier of Grenoble in 2009, working as a Marie Curie fellow on luminescent lanthanide complexes for opto-electronics in the group of Marinella Mazzanti. After a first postdoctoral period with Veronique Michelet at Chimie Paris on functionalized gold nanoclusters for catalysis, he joined the group of Marc Fontecave at CEA Grenoble as a postdoctoral fellow working on supramolecular photocatalytic systems for hydrogen production.

  • image

[ Murielle Chavarot-Kerlidou ]

Murielle Chavarot-Kerlidou received her PhD degree in 1998 from the University Joseph Fourier (Grenoble). After a postdoctoral period in the group of Dr. Zoe Pikramenou (University of Birmingham, UK) studying photoinduced supramolecular processes based on luminescent metallocyclodextrins, she spent 2 years in the group of Marc Fontecave to develop chiral-at-metal ruthenium catalysts for enantioselective oxidation. She obtained in 2002 a CNRS position at the University Pierre et Marie Curie (Paris 6), where her research interests dealt with the development of new applications of the arene-tricarbonyl metal complexes. In 2009, she moved to the Laboratory of Chemistry and Biology of Metals to work on hydrogen photoproduction.

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[ Marc Fontecave ]

Marc Fontecave is a member of the French Academy of Science and Professor at the Collège de France in Paris since 2008, where he holds the chair of Chemistry of Biological Processes. He spent 20 years as Professor of the University Joseph Fourier in Grenoble, after a PhD at the Ecole Normale Supérieure in Paris and postdoctoral research at the Karolinska Institute in Stockholm. He is currently the President of the Scientific Council of the Town of Paris. His research group studies the structural and functional properties of complex biologic redox systems mostly implying metal centers and the mechanisms of assembly of these centers, in particular iron-sulfur enzymes, involved in a variety of metabolic and biosynthetic processes. He has developed bioinspired chemical approaches to obtain original molecular catalysts, for example, for hydrogen production.

  • image

[ Vincent Artero ]

Vincent Artero is a graduate of the Ecole Normale Supérieure (Ulm). He received the PhD degree in 2000 from the University Pierre et Marie Curie (Paris 6). His doctoral work under the supervision of Prof. A. Proust dealt with organometallic derivatives of polyoxometalates. After a postdoctoral stay in Aachen with Prof. U. Kölle, he joined in 2001 the Laboratory of Chemistry and Biology of Metals in Grenoble, where he obtained a position in the Life Science Division of the CEA. His current research interests are in the structural and functional modelization of hydrogenases for the design of artificial systems for the photo- and electro-production of hydrogen.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Molecular catalytic systems for visible light-driven hydrogen evolution
  5. Light-driven homogeneous water oxidation
  6. Perspectives
  7. Acknowledgments
  8. Author biographies
  9. References
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