Artificial photosynthesis systems for catalytic water oxidation

CHAPTER ONE

Artificial photosynthesis systems for catalytic water oxidation Sheng Ye, Chunmei Ding, Can Li* State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 2. Principles of water oxidation 3. Key parameters for evaluating water oxidation catalysts 3.1 Overpotential 3.2 Activity 3.3 Stability 3.4 Quantum efficiency 4. Analytical approaches for evaluating water oxidation catalysts 4.1 Chemical water oxidation 4.2 Electrocatalytic water oxidation 4.3 Photo(electro)catalytic water oxidation 5. Water oxidation catalysts 5.1 Biological water oxidation catalyst 5.2 Homogeneous water oxidation catalysts 5.3 Heterogeneous water oxidation catalysts 5.4 Hybrid water oxidation catalysts 6. Water oxidation mechanisms 6.1 Water oxidation mechanism for homogeneous catalyst 6.2 Water oxidation mechanism for heterogeneous catalyst 7. Overall water splitting 7.1 Artificial photosynthesis systems 7.2 Nature-artificial hybrid photosynthesis systems 8. Conclusions and perspectives Acknowledgments References

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Abstract The consumption of fossil fuel energy and the resulting environmental pollution have incentivized scientists to attempt to develop renewable, reliable and continuously available energy sources. Solar water splitting to produce hydrogen is one of the Advances in Inorganic Chemistry, Volume 74 ISSN 0898-8838 https://doi.org/10.1016/bs.adioch.2019.03.007

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2019 Elsevier Inc. All rights reserved.

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effective ways to solve the energy and environmental problems. Water oxidation (2H2O ! 4H+ + 4e + O2), as one half of the water-splitting reaction, is the primary reaction of both natural and artificial photosynthesis, thus the development of highly active and robust water oxidation catalysts (WOCs) is extremely important for constructing a sustainable artificial photosynthesis system for solar energy conversion. Molecular catalysts and inorganic nanoparticles, as representatives of homogeneous and heterogeneous catalysts, have their respective advantages and have been widely studied. Moreover, hybrid systems combining the molecular catalysts and inorganic nanoparticles, exhibit unique advantages for water oxidation and bridge the gap between homogeneous catalysis and heterogeneous catalysis. Despite significant efforts made so far, a practically viable catalytic system with sufficient efficiency, stability and low cost is yet to be demonstrated. The present topic mainly focuses on the recent advances on different types of WOCs that are generally screened out by several evaluating approaches, such as chemical water oxidation, electrocatalytic water oxidation and photo(electro)catalytic water oxidation. Furthermore, understanding the water oxidation mechanism, including elucidation of the role of active intermediates during the water oxidation process, is helpful to develop more efficient WOCs.

Abbreviations AQE ALD BD bda BiVO4 bpp BQ CAN CB CeIV CNT CoBi Cp* CPET DLS dpaq EIS EQCN EQE EXAFS Fe-TAML Fh g-C3N4 H2ase H2BQ H2QB

apparent quantum efficiency atomic layer deposition blue dimmer 2,20 -bipyridine-6,60 -dicarboxylate bismuth vanadate bispyridylpyrazolate p-benzoquinone cerium(IV) ammonium nitrate conduction band cerium(IV) carbon nanotubes cobalt borate pentamethylcyclopentadiene concerted proton-electron transfer dynamic light scattering 2-[bis(pyridine-2-ylmethyl]amino-N-quinolin-8-yl-acetamido electrochemical impedance spectroscopy electrochemical quartz crystal nanobalance external quantum efficiency extended X-ray absorption fine structure iron-centered tetra-amido macrocyclic ligand ferrihydrite graphitic carbon nitride hydrogenase hydroquinone hydroquinone B

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HEP HOMOs HSL H2ase IQE IrO2 isoq KIE LDH mcp Mn-NG NaIO4 OAc OEC OEP OER OWS Oxone PCET PEC pGO Phe phen PSII py Py5 QA QB [RuIII(bpy3]3+ RHE salpd SPV SSCE Ta3N5 terpy Ti2Fe2O3 TiO2 TOF TON tpa TyrD TyrZ UV–Vis VB WO3 WOCs XANES

H2-evolution photocatalyst highest occupied molecular orbitals hole storage layer hydrogenase internal quantum efficiency iridium oxide isoquinoline kinetic isotope effect layered double hydroxide N,N0 -dimethyl-N,N0 -bis(2-pyridyl-methyl)-cyclohexane-1,2-diamine manganese embedded in nitrogen-doped graphene sodium periodate acetate oxygen evolving complex O2-evolution photocatalyst oxygen evolution reaction overall water splitting peroxymonosulfate proton-coupled electron transfer photoelectrocatalytic partially oxidized graphene pheophytin 1,10-phenanthroline photosystem II pyridine 2,6-(bis(bis-2-pyridyl)methoxy-methane)pyridine plastoquinone A plastoquinone B ruthenium(III) tris(bipyridine) cation reversible hydrogen electrode propane-l,3-diylbis(salicylideneiminate) surface photovoltage sodium saturated calomel electrode tantalum nitride 2,20 :60 ,200 -terpyridine titanium-doped α-Fe2O3 titanium dioxide turnover frequency turnover number tris(2-pyridylmethyl)amine tyrosine D tyrosine Z ultraviolet–visible valence band tungsten trioxide water oxidation catalysts X-ray absorption near-edge spectroscopy

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1. Introduction Energy and environment issues are the most important scientific challenges in the 21st century. With the rapid growth of the global population, the human demand for energy is dramatically expanding, which not only causes huge consumption of fossil fuels, but also generates greenhouse gases such as carbon dioxide from fossil fuel combustion, thereby affecting the living environment of human beings. The excessive consumption of fossil energy and environmental pollution have led to a determination by people to develop renewable clean energy.1 Hydrogen, featuring a high combustion energy value and pollution-free combustion product, and known as “the oil of the future,” has earned intensive attention nowadays. However, the current main source of hydrogen is still extracted from traditional fossil fuels. Splitting water to produce hydrogen has aroused great interest in academia and industry because of its simple, effective, and environmentally friendly features.2 It has been emphasized that, in the reaction of overall water splitting (OWS) to produce hydrogen, oxidation of water serves as one of the two half reactions, providing electrons and protons for the other half reaction (water reduction).3 However, as a thermodynamically uphill reaction involving multiple electrons transfer, it is considered to be a bottleneck in the process of OWS reaction.4 In recent years, more and more researchers have made significant efforts to investigate water oxidation reactions.5–9 Both homogeneous and heterogeneous catalysts for artificial photosynthesis, have been developed for water oxidation. Molecular catalysts in homogeneous systems are attractive because their structures are readily determined and modified, and their reaction kinetics can be studied in a straightforward manner.5,6 Molecular catalysts exhibit great advantages for the rational design and tunability of water oxidation catalysts (WOCs) to optimize their performances, but they usually possess less stability than heterogeneous catalysts. Relatively, the distinct advantage of heterogeneous catalysts is the excellent chemical stability.7–9 In order to characterize the catalytic performances of WOCs, measurements are generally carried out with a sacrificial oxidant or additional applied bias, such as in chemical water oxidation, electrocatalytic water oxidation and photo(electro)catalytic water oxidation.10 All these approaches can be used, to some degree, to evaluate the catalysts, each with their own advantages and weaknesses, and are all beneficial for the design and construction of highly efficient artificial photosynthesis systems.

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To date, it has been demonstrated, for an industrial application, to be able to split water for H2 evolution by an electrocatalytic method. It should be noted that utilizing solar water splitting successfully to produce H2 is one of the effective ways to solve the energy and environmental problems faced by society; it is known as the “Holy Grail” reaction in the field of chemical science.11,12 In particular, the research and development of water oxidation in artificial photosynthesis are the keys to realizing solar water splitting for future large-scale applications. Natural photosynthesis utilizes light energy to drive the water oxidation, which provides clear guidelines for the design of artificial photosynthetic systems for solar fuel production.13–15 At present, extensive efforts have been made to construct efficient water splitting systems. However, solar water splitting research is still at a fundamental stage, and the energy conversion efficiency is extremely low. Therefore, water splitting is a very attractive, challenging topic and there may be prolonged endeavors before success in development is achieved.

2. Principles of water oxidation The OWS reaction is a thermodynamically uphill process with the change of standard Gibbs free energy, (ΔGθ, 237 kJ mol1).16 The minimum requirement for water splitting is 1.23 eV. In general, OWS contains two half reactions: water oxidation and reduction as follows. Water oxidation reaction : 2H2 O ! 4H + + 4e + O2 

Hydrogen evolution reaction : 4H + 4e ! 2H2 Overall water splitting reaction : 2H2 O ! 2H2 + O2 +

(1) (2) (3)

Water oxidation is the primary reaction of water splitting reaction, involving the multi-electron and multi-proton processes.17 The oxygen evolution reaction (OER) process includes sequential multi-step reactions, each with a single-electron transfer. Notably, energy accumulation at each step results in very sluggish O2-evolution kinetics and thus requires a large overpotential. That is to say, the redox potential/valence band (VB) of the molecular catalyst/ semiconductor is required to be more positive than 1.23 eV, for the consideration of overpotential in the catalytic reactions.11 In this respect, a highly efficient WOC is extremely desirable to overcome the kinetic energy barriers.18 In screening the high active WOCs, several methods are usually used, such as chemical, electrocatalytic and photo(electro)catalytic water oxidation; reactions are kinetically slow without an active catalyst.10 It should be noted that the sluggish kinetics are mainly attributed to many intermediates required to carry out the sequential multi-step reactions.19

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3. Key parameters for evaluating water oxidation catalysts Catalytic performance is the most basic element to determine whether the catalyst can be used for future large-scale applications. There are several parameters to be evaluated to assess the water oxidation performance of a catalyst. Some factors determining the performance are discussed below.

3.1 Overpotential Overpotential is one of the most essential factors for evaluating the performance of WOCs, which is typically the potential difference between the potentials at the current density of 10 mA cm2 and 1.23 VRHE at room temperature.20 In fact, it is difficult to determine the exact value since measured current densities of OER may contribute to the oxidation of the electrocatalyst itself. Therefore, it should be noted that the Faraday efficiency for O2 production must be 100% when measuring the overpotential at 10 mA cm2.7

3.2 Activity The catalytic activity, as a valid comparison, must be referred to the number of exposed active sites on the catalyst. Thus, a simple parameter to describe catalytic activity is turnover frequency (TOF). In 1966, Boudart et al. put forward the concept of the TOF to evaluate the rate of enzyme-catalyzed chemical reactions.21 It is the ratio of the number of the molecules produced per unit time at a single active site. The TOF value is an important parameter to describe the intrinsic rate of a catalytic reaction for a catalyst and for a comparison of the activity of similar catalysts.22,23 The TOF of a catalyst for water oxidation is calculated according to Eq. (4): Number of product molecules Number of active sites  time Number of evolved O2 molecules ¼ Number of active sites  time

TOF ¼

(4)

It should be noted that it is convenient to estimate the TOF value in a homogeneous catalytic system. However, it is difficult to accurately calculate the TOF value for a heterogeneous catalyst as the actual number of active sites on the catalyst surface should be determined.

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3.3 Stability Separate from the TOF, evaluating the catalytic activity, the turnover number (TON) value is an important parameter to evaluate the stability of the catalyst. In homogeneous and heterogeneous catalysis, the TON is a dimensionless number,24,25 which is defined as the number of the molecules produced per catalytic site before deactivation under given reaction conditions. That is to say, the catalyst can achieve the total number of turnovers until it is totally dead, regardless of the reaction time.26 In this respect, an ideal catalyst should have an infinite TON. Thus, the TON represents the maximum yield of products attained from an active catalytic site up to the decay of activity for a specific reaction. The TON of a catalyst for water oxidation is calculated according to Eq. (5): Number of product molecules Number of active sites Number of evolved O2 molecules ¼ Number of active sites

TON ¼

(5)

3.4 Quantum efficiency In addition to the above-mentioned parameters, there are several other factors that also define the performance of a catalyst, such as quantum efficiency, which is usually measured for photocatalysts. It should be noted that different experimental conditions, such as light sources, reaction temperature, and catalyst concentration, are adopted in different research groups. Therefore, it is meaningless to directly compare the rates of gas evolution even if the same photocatalyst is used for water oxidation. Quantum efficiency is an important indicator that requires attention when evaluating the photocatalytic activity for water oxidation, which routinely includes external quantum efficiency (EQE) and internal quantum efficiency (IQE). The EQE is generally referred as the apparent quantum efficiency (AQE). The AQE of photocatalysts for photocatalytic O2 evolution is the ratio of molecular numbers of generated O2 per unit time to the number of incident photons at a given monochromatic wavelength as the following Eq. (6).27,28 EQE ð%Þ ¼ AQE ð%Þ ¼ ¼

Number of reacted electrons  100 Number of incident photons Number of evolved O2 molecules  4  100 Number of incident photons (6)

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The IQE of photocatalysts for photocatalytic O2 evolution is the ratio of molecular numbers of generated O2 per unit time to the number of absorbed photons at a given monochromatic wavelength as the following Eq. (7).27,28 IQE ð%Þ ¼

Number of reacted electrons  100 Number of absorbed photons

Number of evolved O2 molecules  4 ¼  100 Number of absorbed photons

(7)

It can easily be seen that the IQE value is estimated to be higher than the AQE as the extent of incident light is obviously larger than that of absorbed photons. However, it is very difficult to measure exactly the number of photons absorbed by the participating photocatalyst in the solution, due to light scattering and reflection. Accordingly, the AQE is more commonly used to evaluate the performance of a photocatalytic system.

4. Analytical approaches for evaluating water oxidation catalysts When the water oxidation reaction is investigated, the catalytic activity of a potential WOC is examined applying different approaches, for example, chemical water oxidation, electrocatalytic water oxidation, and photo(electro)catalytic water oxidation.10

4.1 Chemical water oxidation In catalyst screening, the reaction of chemical water oxidation is often accompanied by a sacrificial oxidant, which possesses more positive oxidation potential than the water oxidation.29 Cerium(IV) (CeIV) ammonium nitrate (CAN) is the most frequently used oxidant for charactering WOCs; it is a powerful one-electron oxidant. CAN has attracted much attention due to the following reasons: (1) CeIV has a redox potential of approximately 1.75 VNHE.30 (2) Evaluation of WOCs using CAN is relatively convenient as it can only drive the WOC in a one-electron step. Notably, Meyer et al. reported the first example of a homogeneous WOC, a ruthenium “blue dimer (BD),” which was characterized by using CAN.31 Despite extensive evaluation of WOCs using CeIV, there are some obvious disadvantages in the process. When CeIV was used as the oxidant, the solution shows a strongly acidic character (pH <1). If a WOC is unstable in the CeIV solution, the actual activity of a WOC cannot be obtained.

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Therefore, it is important to use an oxidant that enables the WOC to be stable under more neutral conditions. For example, the ruthenium(III) tris(bipyridine) cation ([RuIII(bpy)3]3+) is a mild one-electron oxidant, which shows a redox potential of 1.26 VNHE and can be used at near-neutral conditions.32 Furthermore, it has a significant absorption spectrum that makes it easy to track the consumption of oxidant by measurement of the change of the ultraviolet–visible (UV–Vis) spectrum.33 However, the redox potential of [RuIII(bpy)3]3+ is too negative to drive a number of WOCs.34 Moreover, [RuIII(bpy)3]3+ rapidly decomposes to [RuII(bpy)3]2+ in the solution at a pH >4, even in the solid state. Potassium peroxymonosulfate (Oxone) is a strong two-electron oxidant with a redox potential of 1.82 VNHE.35 It has been demonstrated to be stable up to pH 6. Oxone has been extensively used in characterizing WOCs with the first-row transition metal complexes. However, it fails to drive the water oxidation reaction with a majority of WOCs based on the second and third-row transition metals. This drawback has limited the general use of Oxone in characterization of WOCs. A two-electron oxidant, sodium periodate (NaIO4), possessing a redox potential of 1.60 VNHE (at pH 1) is able to be used under neutral conditions.36 It has been demonstrated to function with both third row and the first row transition metals.37 In addition to the oxidants mentioned above, many other sacrificial oxidants, such as hypochlorite and peroxides have been demonstrated to drive the WOCs well; such oxidants have been used as complementary oxidants.38,39 In brief, various chemical oxidants are used to drive WOCs, each with their own advantages or disadvantages. It is convenient to study WOCs, by characterizing the reaction and screening the assisting chemical oxidants. However, concerns should be given to the origin of O2 evolved from the water for two-electron oxidants, such as Oxone and NaIO4, which could contribute to the origin of O2 by themselves.

4.2 Electrocatalytic water oxidation A water electrolysis cell routinely contains two indispensable parts: the cathode for H2 evolution and the anode for O2 evolution.40 The energy efficiency of water electrolysis is mainly determined by the catalyst, and therefore the development of highly active electrocatalysts has attracted considerable attention. In electrocatalytic OER, an external bias is required to drive the OER. Nevertheless, in order to measure accurately the working

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electrode potential, and thus avoid the pH effect on the applied bias, a reversible hydrogen electrode (RHE) is usually employed as a reference electrode.3 So far, electrocatalytic OER has been applied in various research fields, such as water splitting, fuel cell technology and metal-air batteries. For all these systems, the water oxidation reaction is the primary reaction to carry out their reversible processes together with the reduction reaction.41 Compared with the chemical oxidants, electrochemical methods can also be employed more commonly to evaluate potential WOCs. In general, a WOC is assembled on a conducting material (i.e., SnO2:F and glassy carbon electrode).7 It should be noted that the electrocatalytic performance of water oxidation for the anode is not only related to the ability of the catalyst itself, but also closely related to the techniques, methods and expertise involved in electrode preparation. The exploitation of highly efficient and robust electrocatalysts for OER is crucial for the development of solar water splitting devices. In recent years, more and more researchers have developed hundreds of OER electrocatalysts with different elemental compositions and microstructures prepared by diverse methods.42,43 In addition, many highly sophisticated instruments and advanced characterization methods have helped us to test the intrinsic properties of materials in situ and also monitor the electrochemical efficacy of the catalytic materials during OER.44 This assists in clarifying the reaction mechanism at the atomic level. With these considerations, it is important to tune exactly the size, morphology and elemental composition of the electrocatalysts to increase the active surface area of the materials and improve their electroconductivity. In OER, the overpotential is looked upon the standard to compare the catalytic performances of WOCs,45 which is related to the activation barriers. Therefore, the overpotential can also be considered as a kinetic obstacle for water oxidation. In this regard, the key to realizing highly efficient electrocatalysis is lowering the overpotential required for water oxidation. Thus, it is extremely essential to exploit low-cost electrocatalytic materials with excellent activity and stability at low overpotential for large-scale applications.

4.3 Photo(electro)catalytic water oxidation Photocatalytic technology is a green approach with important application prospects in the energy and environmental fields. A revolutionary breakthrough is considered as the most ideal environmental purification

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Fig. 1 The mechanism of photocatalytic water splitting on semiconductor-based photocatalysts.

technology in the 21st century. Notably, solar water splitting is simple, effective and environmentally friendly, and has stimulated great interest among researchers in order to solve the worldwide energy crisis and associated environmental issues.16 An efficient photocatalytic system usually includes three important components: broad spectral absorption, significant charge separation and rapid surface catalytic reaction, as shown in Fig. 1. Since Fujishima and Honda, in 1972, reported a rutile TiO2 anode coupled with a platinum dark cathode for photoelectrocatalytic (PEC) water splitting,46 considerable effort has been made to construct efficient artificial photosynthesis systems. Although the TiO2 semiconductor has been widely investigated by researchers due to the excellent chemical and photochemical stability, it is limited by working only in the UV light region. In the solar spectrum, the UV region only accounts for 4% of the solar spectrum, whereas the visible light region exceeds 45%. Hence, the development of visible-light-driven photocatalysts is very important. Over the past 40 years, many researchers have reported the development of visible-light-driven photocatalysts by element doping to extend the absorption spectrum, or by exploiting new photocatalytic materials with broad spectral absorption.47 In general, significant progress has been achieved on photo(electro)catalytic water oxidation over the past several decades.8,11 However, there are still enormous challenges, and large-scale applications have yet to be realized.

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5. Water oxidation catalysts This section will highlight and will focus on recent achievements in the design, preparation and performance of a potential WOC that can be the key components of artificial photosynthesis devices for solar fuels production. In the past decades, various kinds of WOCs have been exploited, including biological, homogeneous, heterogeneous and hybrid catalyst systems. To probe the catalytic performances, these WOCs are evaluated by different ways, such as chemical water oxidation, electrocatalytic water oxidation, and photo(electro)catalytic water oxidation.

5.1 Biological water oxidation catalyst 5.1.1 Photosynthetic oxygen-evolving center A photosynthetic oxygen-evolving center, namely, a large protein complex photosystem II (PSII) is found in the thylakoid membranes of photosynthetic organisms (i.e., algae, cyanobacteria, and higher plants); it is the first enzyme for water oxidation to evolve O2.13,14,48,49 PSII utilizes photons to drive the water oxidation reaction at an oxygen evolving complex (OEC), where the light-adsorbing, charge separation and transfer take place efficiently. In general, the electron transfer chain in PSII can be divided into two parts, an acceptor side and a donor side,48,49 as shown in Fig. 2. Firstly, the

Fig. 2 Redox potential scheme of cofactors concomitant with the pathways of charge separation, transport and recombination.

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chlorophyll pigment P680 upon obtaining the light energy is photoexcited into a radical cation, P680%+, with an oxidizing potential of 1.25 VNHE, which is the highest known in biology.50 On the acceptor side of PSII, the electrons are rapidly transferred from the excited state P680%+ to a pheophytin molecule (Phe), and then to the next acceptor, plastoquinone QA, and further to the electron acceptor plastoquinone QB. Thus, the charge separated state is stabilized by the electron transfer chain. Interestingly, after undergoing two-electron and two-proton reductions, the acceptor QB becomes a mobile electron carrier, hydroquinone B (H2QB) and diffuses away from PSII to continue the next electron transport. On the donor side of PSII, the cationic radical P680%+ is reduced by a redox-active tyrosine residue of the D1 subunit, TyrZ (D1Tyr161) to occupy the hole in P680. Subsequently, a neutral tyrosine radical Tyr%Z is generated as an oxidant for oxidizing the CaMn4O5 cluster via proton-coupled electron transfer (PCET) reactions.51 Finally, the OEC couples a series of successive oneelectron reductions to four-electron water oxidation for the liberation of O2 and H+.44 5.1.2 CaMn4O5 cluster and Kok cycle It has been demonstrated that the CaMn4O5 cluster in the PSII reaction center carried out the water oxidation reaction with an extraordinary TOF of 100–400 s1,52 which is much higher than those of most WOCs used in artificial photosynthesis systems. Considering the intimate relationship of structure-activity, the structure of CaMn4O5 cluster may be responsible for the highly efficient water oxidation. A crystallographic structure shows that a CaMn3O4 cubane unit is found in the CaMn4O5 cluster, in which one Ca and three Mn constitute four corners, and four O constitute the alternate corners.53–55 Notably, the fourth Mn is attached to one of the Mn in the cubane via two μ-oxo bridges. The metal sites in the CaMn4O5 cluster are coordinated by one imidazole, six carboxylate residues and four water molecules. The current mechanism of water oxidation was proposed by Kok et al. in the 1970s; it involved five intermediate S-states from S0 to S4 (Fig. 3, the so-called Kok cycle or S-state cycle).56 The progression through the Kok cycle results in the storing of four oxidizing equivalents on the CaMn4O5 cluster.57,58 In reality, the S1-state becomes the dark-stable state because the S0-state is easily oxidized by a second redox-active tyrosine D (YD) during the dark-adaptation.51 The oxidation of each S-state is accompanied by the removal of one electron, while the protons are also released apart from the S1 ! S2 transition in the Kok cycle. Spectroscopic measurements have

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Fig. 3 The Kok cycle showing a consecutive series of five intermediate S-states (S0, S1, S2, S3 and S4) along with the release of electrons and protons.

played a key role in characterizing the S-state transitions. It has been demonstrated that the most possible intermediates, MnIII 2 MnIV 2 for the S1-state and MnIII MnIV 3 for the S2-state can be characterized by X-ray absorption near-edge spectroscopy (EXANES).59 To date, although the S3-state was considered to include a closed cubane, MnIV 3 CaO4, the structure is still not clearly established. Further, because the transient S4-state is not extremely stable, it has not been identified spectroscopically.15 After accumulating four oxidizing equivalents, the oxygen is released, while the CaMn4O5 cluster returned to the ground S0-state. Photosynthetic water oxidation proceeds at the CaMn4O5 cluster, which plays a key role in charge accumulation via the four-electron reaction. It also should be noted that the protein residues around the CaMn4O5 cluster are important for providing the path for electron/proton transport and the release of O2.60,61 Although it is still a great challenge to determine the detailed mechanism of water oxidation, photosynthesis in nature provides the guidelines for the design and construction of artificial photosynthetic systems and considerable efforts are being made to have a better understanding of natural photosynthesis, and apply the knowledge.

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5.2 Homogeneous water oxidation catalysts Molecular catalysts in homogeneous systems are extremely attractive because they can be easily prepared and modified,62 and the reaction kinetics processes can be monitored readily in order to understand the basic catalytic aspect of water oxidation.63 In recent decades, numerous WOCs have been exploited and investigated.5,10,19,64,65 Ligand modification of homogeneous catalysts provides a possible pathway for improving water oxidation performance. In the following sections, we summarize the recent developments in the rapidly growing field of artificial molecular WOCs in homogeneous systems. This section focuses on synthetic molecular catalysts and highlights various approaches that have been adopted in the development of artificial homogeneous WOCs. 5.2.1 Ru-based molecular catalysts A landmark compound, the so-called BD [{RuIII(bpy)2(H2O)}2O]4+ reported by Meyer and co-workers,31 was the first homogeneous complex found to catalyze the oxidation of water to produce O2 in chemical and in electrochemical oxidation. There is an electrochemically catalytic oxidation wave for BD in 0.1 M sulfuric acid at 1.54 VNHE using fast-scan cyclic voltammetry. When CAN as a chemical oxidant is used, the BD is oxidized from RuIII-O-RuIII to RuV-O-RuV by a continuous PCET process. The resultant [(O)RuV(μ-O)RuV(O)]4+ intermediate is considered to be a key species triggering the O2 release. Kinetic studies have shown that the nucleophilic attack of water molecules on the RuV¼O center, yields hydroxo or oxo complexes upon oxidation of the ruthenium center. The water oxidation mechanism for the BD is discussed in detail in the following sections. Following Meyer’s report, a large number of Ru-WOCs have been investigated for chemical, electrochemical and photo(electro)chemical water oxidation catalytic reactions.5,10,19,65,66 Most notably, a mononuclear Ru complex synthesized by Sun’s group demonstrated the highest O2-evolution activity for chemical water oxidation among all homogeneous catalysts.4 In fact, Sun and co-workers have developed a series of Ru(bda)(L2)-type (bda ¼ 2,20 -bipyridine-6,60 -dicarboxylate, L ¼ picoline and derivatives) complexes for water oxidation at relatively low overpotentials.67 Very surprisingly, when the axial picoline ligand was replaced by a π-extended, hydrophobic isoquinoline (isoq) ligand generating Ru(bda)(isoq)2 complex,

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Fig. 4 (A) Molecular structures, (B) O2-evolution performances and (C) CV curves of Ru(bda)(isoq)2 and Ru(bda)(picoline)2 complexes. (D) Proposed mechanism of Ru(bda) (isoq)2 complex with stoichiometric and excess amounts of CeIV at pH 1. Reprinted (adapted) with permission from Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. Nat. Chem. 2012, 4, 418–423. Copyright (2012) Nature Publishing Group.

this change had a dramatic effect on the catalytic activity and resulted in an extraordinary TOF of 300 s1 with CeIV as an oxidant, which is close to that of the natural catalyst CaMn4O5 in PSII (Fig. 4).4 This can be ascribed to noncovalent interactions between isoquinolines, such as ππ stacking, which reduce the energy barrier for the coupling of two RuV¼O species. In addition, theoretical calculations confirmed that isoq ππ stacking plays an important role in stabilizing the transition state of the OO bond formation. Shortly afterwards, Sun et al. improved further the TOF of the Ru(bda)(L2) complex by incorporating the 6-fluoroisoquinoline ligand,68 resulting in the achievement of an extremely high TOF of 1000 s1. The interactions of highly hydrophobic axial ligands can lower the barrier for radical coupling of the two RuV-O units as the key water oxidation intermediates and thus facilitate dimerization as well as O–O bond formation. Although homogeneous Ru(II) complexes have exhibited high catalytic activity, it should be noted here that the light-driven water oxidation performance is still far from that of chemical oxidation due to the

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photodissociation of organic ligands from metal complexes.69 Typically, Ru(bda)(isoq)2 only showed a low TOF of 0.3 s1 using Ru(bpy)32+S2O82, which is three orders of magnitude inferior to the result using CeIV.70 5.2.2 Ir-based molecular catalysts An iridium containing catalyst was known to be the most active and stable WOC for water oxidation for many years, but a report on homogeneous catalysis did not appear until 2000. The origin of molecular Ir catalysts for water oxidation came from the report of Bernhard and co-workers in 2008, showing that single-site cyclometallated Ir complexes demonstrated good water oxidation activity for 7 days with CeIV as a chemical oxidant.71 Electrochemical measurements confirmed that there was a relationship between the onset potential of water oxidation and the IrIV/IrIII redox potential. Computational studies on these Ir complexes indicated that the highest occupied molecular orbitals (HOMOs) revealed the character of mixed metal/ligand, namely, the ligand strongly interacted with Ir metal center via d-π interactions. By introducing substituents, the ligands become either more electron-donating or electron-withdrawing, with respect to the Ir metal center. Such a finding provides the pathway for the design and synthesis of highly active and robust Ir WOCs with a suitable ligand.71 Crabtree and Brudvig et al. developed a novel class of single-site Ir complexes bearing a more electron-donating pentamethylcyclopentadiene (Cp*) ligand to improve further the O2-evolution activity. The catalytic activities of these catalysts were shown to be one order of magnitude higher than those of the previously reported Ir catalysts by Bernhard. Furthermore, such an CpIr*-based WOC was interfaced with a Zn-based porphyrin chromophore co-deposited on a TiO2 photoanode for PEC water oxidation.72 The extensive Ir-based WOCs containing cyclopentadiene-type ligands have shown a tendency to be transformed into catalytically active oxides. This is of special interest for iridium catalysts, because IrOx are excellent catalysts known for O2 evolution.73 Therefore, it is extremely important to determine the molecularity of new synthetic iridium catalysts. To address this challenge, an electrochemical quartz crystal nano-balance (EQCN) was utilized by the group of Crabtree and Brudvig.74 The EQCN is an ideal and powerful tool to probe the homogeneity of the iridium complexes, because it provides accurately a real-time mass change at the surface of the working electrode during electrochemical measurements. With this technology, the electrochemical performances of two water-soluble Cp*Ir-based complexes are compared,74 as shown in Fig. 5. In terms of the tris-aqua complex, an

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A

0.8

0

50

Time (sec) 100

B 150

200

0

50

Time (sec) 100

150

200 0.5

1.5 V

1.5 V

0.2 V

0.2 V

0.4 0.3

Current (mA)

0.6

0.2

0.4

0.1 0.2

Current (mA)

1.0

0.0 0.0 –0.1 –0.2

C

D 800

800

700

700

2–

SO4

Mass (ng)

500 400 300

600

O

2+

Ir H2O OH2 H2O

N

Ir

O O

500 400

CF3

300

200

200

100

100

Mass (ng)

600

0

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Fig. 5 EQCN technology probing the homogeneity of the iridium complexes by comparing current and mass responses of solutions containing the [Cp*Ir] complexes shown as a function of time. Reprinted (adapted) with permission from Schley, N.D.; Blakemore, J.D.; Subbaiyan, N.K.; Incarvito, C.D.; D’Souza, F.; Crabtree, R.H.; Brudvig, G.W. J. Am. Chem. Soc. 2011, 133, 10473–10481. Copyright (2011) American Chemical Society.

excess of oxidizing potential yielded an obvious increase in mass of the electrode and a large pseudo-catalytic current. For the pyridine alkoxide complex, no mass change was detected, indicating such an Ir complex had a homogeneous nature to an extent. It is thus shown that use of the EQCN is a powerful method for differentiating between homogeneous catalysis and heterogeneous catalysis.75 5.2.3 Fe-based molecular catalysts The ubiquitous presence of Fe in redox cofactors and oxygen-containing metalloenzymes in biology, led to Fe complexes receiving increasing attention in this context. In 2010, Bernhard et al. demonstrated for the first time that iron-centered tetra-amido macrocyclic ligand (Fe-TAML) compounds can effectively perform water oxidation to evolve O2 in the presence of CeIV (pH 0.7).37 These Fe-TAML complexes exhibited a first-order water oxidation reaction with an optimal TOF of 1.3 s1. Real-time UV–Vis spectra

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revealed the formation of FeIVOFeIV dimers as the key intermediates during water oxidation processes. Since this report, Fe-TAML complexes have become of widespread interest. As strongly acidic solutions result in the decomposition of Fe-TAML complex, the water oxidation reactions were investigated using NaIO3 or NaClO as a chemical oxidant.37,76 It was significant when a FeV(O) active intermediate was first found to form in pure water when using NaClO as an oxidant. In another similar example, it was found that an Fe-TAML complex showed a homogeneous attribute for photochemical water oxidation with a Ru photosensitizer, and Na2S2O8 as a sacrificial oxidant (Fig. 6).77 It is worth noting that a high valent FeV(O) species is generated as the active intermediate during photocatalytic water oxidation. This is the first example of a molecular Fe complex catalyzing photochemical water oxidation through a FeV(O) intermediate, providing a clear understanding of the water oxidation mechanism. In addition to a typical TAML ligand stabilizing the high valent Fe species, a few other ligands were employed for this purpose, such as 2-[bis (pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido (dpaq) and N,N0 dimethyl-N,N0 -bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine (mcp).78,79 For example, Costas et al. reported highly active Fe WOCs containing mcp ligands.79 These Fe complexes exhibited water oxidation activity when CeIV or NaIO4 was used as the oxidant. An optimal TON reaching up to 1000 was achieved. Although these Fe complexes have demonstrated high TONs for water oxidation, their activities (TOFs) are relatively low. Recently, it was reported that a robust pentanuclear Fe complex can efficiently perform water oxidation

Fig. 6 Probe the FeV(O) active intermediate during homogeneous photochemical water oxidation process for the Fe-TAML complex. Reprinted (adapted) with permission from Panda, C.; Debgupta, J.; Diaz Diaz, D.; Singh, K.K.; Sen Gupta, S.; Dhar, B.B., J. Am. Chem. Soc. 2014, 136, 12273–12282. Copyright (2014) American Chemical Society.

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with a TOF of 1900 s1; this is a value of about three orders of magnitude higher than those of other Fe-based molecular catalysts.80 Electrochemical results revealed the redox flexibility of the system including six different oxidation states; in particular, the FeIII5 state was active for water oxidation. Quantum chemistry calculations indicated that the presence of adjacent active sites facilitated the O–O bond formation with a reaction barrier of less than 10 kcal/mol. 5.2.4 Co-based molecular catalysts Co-based molecular catalysts are most widely studied in non-precious metal WOCs. In 2011, Berlinguette et al. reported a single-site Co-based molecular catalyst [Co(Py5)(OH2)]2+ (Py5 ¼ 2,6-(bis(bis-2-pyridyl)methoxymethane)-pyridine) for electrochemical water oxidation.81 Electrochemical tests showed a concerted proton-electron transfer (CPET) during oxidation of this complex. Control experiments showed that the possibility that the catalytic current stemmed from in situ generated Co-phosphate films could be excluded. Nocera and coworkers found that CoIII corrole complexes can perform electrochemical water oxidation.82 Quantum chemical modeling revealed that the carboxylate units played a key role in activating the attack of H2O molecules on metal-oxo species, as shown in Fig. 7. Interestingly, it was found that the fluorination of the corrole ligand can regulate the electrophilicity of the metal-oxo species and reduce the decomposition of the generated corrole radical cations.83 Cobalt porphyrins have also been studied as WOCs. Sakai et al. reported that Co porphyrins could perform photocatalytic water oxidation effectively.84 Dynamic light scattering (DLS) measurements indicated that there were no nanoparticles generated during the photocatalytic reactions. tBu

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The second-order water oxidation reaction suggested a bimolecular radical coupling process. In a similar manner to the Co corrole complex, the fluorinated Co porphyrin complex was more resistant towards decomposition.85 In addition, Groves et al. synthesized a series of cationic Co porphyrin complexes that were identified as homogeneous electrochemical WOCs.86 In recent years, much effort has been devoted to preparing dinuclear Co-based WOCs; however, there are only a few examples of water oxidation with active binuclear Co catalysts. As an example, the binuclear Co complex with bridged bipyridylpyrazole (bpp) ligands was found to be capable of electrochemical water oxidation under acidic conditions.87 Another example was the binuclear Co complex [(tpa)Co(μ-OH)(μ-O2)Co(tpa)]3+ (tpa ¼ tris(2-pyridylmethyl)amine) used for the light-driven O2 evolution, showing a high TOF of 1.4 s1.88 The homogeneity of the dinuclear Co complex was verified by DLS measurement. The proposed catalytic mechanism for water oxidation involved the production of (O)CoIVCoIV(O) as the key intermediate species for the formation of an O–O bond. Taking inspiration from the core structure of the CaMn4O5 in the PSII, cubane Co complexes, Co4O4(OAc)4(X-py)4 (OAc ¼ acetate, py ¼ pyridine and X ¼ H, Me, t-Bu, OMe, COOMe, CN, Br), have attracted significant interest for both (electro)chemical and photo(electro)chemical water oxidation applications as manifest by numerous reports, Natali, Bonchio, Dismukes, Tilley, Ye and Li et al.89–94 5.2.5 Mn-based molecular catalysts Since the discovery of the oxygen evolving center (PSII) consisting of metal manganese active sites, manganese has also become the focus of design and synthesis of new WOCs. Since the 1980s, more and more mononuclear and multinuclear manganese-based WOCs have been reported for water oxidation.38,95–98 Although a variety of high valence Mn complexes have been prepared, mononuclear Mn complexes are rarely reported to be WOCs. An early example is the MnIII Schiff base complex, [Mn(salpd)(OH2)]2+ (salpd ¼ N,Nbis(salicylidene)propane-1,3-diamine).95 Under visible light irradiation, the [{Mn(salpd)(H2O)}2]2+ complex was found to be photoactive for O2 evolution. In order to mimic PSII, p-benzoquinone (BQ) was employed as a hydrogen acceptor. During the photolysis, the Mn complex is converted to [{Mn(salpd)}2O] and the BQ was reduced to hydroquinone (H2BQ). A pioneering example of a binuclear manganese complex capable of oxidizing water is the Mn porphyrin dimer developed by Naruta et al.96

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The Mn complex showed a high TON of 9.2 for electrocatalytic water oxidation. The formation of O–O bonds occurs through nucleophilic attack of OH– on MnV¼O species, hence producing the MnIV-OO-MnIV species and releasing O2. The next breakthrough on binuclear Mn complex was made in 1999 by the group of Crabtree and Brudvig.38 The dimeric complex, [(H2O)(terpy)Mn(O)2Mn(terpy)(OH2)]3+ (terpy ¼ 2,20 :60 ,200 terpyridine), was the first manganese catalyst capable of driving chemical water oxidation in the presence of HSO5  or ClO. In order to mimic further the structure of the natural OEC (CaMn4O5), tetranuclear manganese-based catalysts with a cubic core structure have been investigated. For example, Bonchio et al. prepared a tetranuclear Mn com 97 IV pound [MnIII 3 Mn O3(CH3COO)3(SiW9O34)]6 . The Mn complex demonstrated effective photocatalytic water oxidation activity with a TOF of 0.71  103 s1 under light illumination, an order of magnitude higher when compared to that of conventional manganese oxide (105 s1). Agapie et al. reported the preparation of a cubane [Mn3CaO4]6+ compound consisting of a [Mn3CaO4] cubane core, which is analogous to the biological CaMn4O5 catalyst.98 However, coordination of a fourth Mn ion onto the [Mn3CaO4] cubane core was never achieved. Recently, an interesting development: Chen and Zhang et al. synthesized a Mn4Ca-cluster (CaMn4O4) closest to the natural CaMn4O5 catalyst, both in terms of the cubane core, and the binding protein groups,99 as shown in Fig. 8. Crystallographic structures revealed that the synthetic complex not only possesses the Mn3CaO4 cubane core, but in addition the fourth Mn was connected to the core. This complex belongs to the S1 state in terms of the Kok cycle. Furthermore, the redox potential of the complex from the S1 state to the S2 state is located at 0.8 VNHE, which is very close to that of the CaMn4O5 catalyst (0.9 V), but is significantly different from the

Fig. 8 Structures of the natural catalyst (CaMn4O5) and the artificial catalyst (CaMn4O4).

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complex reported by Agapie. This indicates that the introduction of the fourth Mn is vital in that now the complex plays a crucial role in regulating the redox potential of the Mn4Ca cluster.

5.3 Heterogeneous water oxidation catalysts Although homogeneous complexes have exhibited superior water oxidation activity, they are difficult to apply in real solar water-splitting systems, mainly due to their poor stability. Comparatively, heterogeneous WOCs are more robust and thus frequently used in artificial photosynthesis. 5.3.1 Commonly used catalysts Among commonly used heterogeneous catalysts, Ru, Ir, Fe, Co, Ni and Mn oxides are all active in performing water oxidation. Some other materials, such as carbides, sulfides, nitrides and phosphides, have also been found to have good water oxidation performances, especially in the field of electrocatalysis. However, these materials usually play a role as a precursor during OER due to the self-oxidation, resulting in the formation of oxides, and thus will not be discussed here. In this section, the subject is a focus on the recent development of typical oxide materials as real active WOCs. 5.3.1.1 IrO2

Noble metal catalysts are widely used for water oxidation due to their high catalytic activity. For example, iridium oxide (IrO2) has been demonstrated as one of the most active and robust WOCs100,101; it possesses advantages of low resistivity and high corrosion-resistance property.102,103 Nevertheless, developing highly efficient and robust WOCs with a small quantity of a noble metal is extremely desirable. There has been a focus on the effect of particle structure and particle dimension upon the activity of Ir-based WOCs. For example, amorphous IrOx is found to be more active than crystalline IrO2, while amorphous IrOx(OH)y nanoparticles exhibit better activity with respect to amorphous IrOx.103 Furthermore, Yagi et al. reported that citrate stabilized IrO2 nanoparticles with a diameter in the range of 50–100 nm showed excellent electrochemical water oxidation performance with a maximum TOF of 6.6 s1 at 581 mV overpotential.104 Murray et al. found that mesoporous IrOx films with a size of 2 nm showed an overpotential of 250 mV at 0.5 mA cm2.105 Mallouk et al. reported in situ formation of 2–5 nm amorphous IrOxxH2O nanoparticles, which showed a low overpotential (200 mV) for O2 evolution at 1.5 mA cm2.106

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Fig. 9 (A) HAADF-STEM image and (B) electrochemical water oxidation performance on the IrO2/CNT catalyst in 1 M KOH or 0.5 M H2SO4 solution. Reprinted (adapted) with permission from Guan, J.; Li, D.; Si, R.; Miao, S.; Zhang, F.; Li, C. ACS Catal. 2017, 7, 5983–5986. Copyright (2017) American Chemical Society.

Sub-nanometric IrO2 clusters supported on multi-walled carbon nanotubes (IrO2/CNT) were synthesized by a chemical vapor deposition method (Fig. 9).107 The resultant IrO2/CNT catalyst with the deposited IrO2 (1.1 nm) demonstrated a high TOF of 11.2 s1 for chemical water oxidation, and the overpotential of electrochemical water oxidation was 249 and 293 mV at 10 mA cm2 in 1.0 M KOH and 0.5 M H2SO4, respectively. Structural characterizations and theoretical calculation indicated that the extraordinary performance of the ultra-small IrO2 species was mainly attributed to the increased number of unsaturated surface Ir atoms and a change of coordination environment.

5.3.1.2 RuO2

In addition to IrO2, RuO2 is also known to be one of the best WOCs in acidic media.7 Typically, IrO2 tends to have higher stability, while RuO2 has higher activity.3 Considering the effect of the crystallinity of RuO2, Imanishi et al. demonstrated that the onset potential of the amorphous RuO2 film for O2 evolution was negatively shifted by 0.06 V from that for the rutile crystalline RuO2.108 This could be attributed to the structural flexibility of the amorphous surface, resulting in the OER activity of amorphous RuO2 being higher than that of rutile crystalline RuO2. Additionally, the particle size has a significant impact on the OER activity. Chorkendorff et al. found that the thermally oxidized Ru nanoparticles with 3–5 nm, exhibited a TOF of 0.65 s1 at 0.25 V overpotential, one order of magnitude higher than those of the current Ru oxides.109

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Although RuO2 has demonstrated excellent OER activity in electrocatalysis, corrosion in the long-term, limits the potential future for application. To resolve this problem, many strategies have been reported, such as fabricating bimetallic oxides (i.e., SrRuO3110 and Y2Ru2O7-δ111) or constructing a core–shell structure (i.e., IrO2@RuO2112). It should be noted that this core–shell structure not only lowers the overpotential (300 mV) but can also enhances the stability, up to 1000 cycles with 96.7% remaining.112

5.3.1.3 CoOx

Cobalt oxides as WOCs have received attention for >70 years. Wang et al. investigated the influence of size of the Co3O4 nanoparticles on their water oxidation activity.113 Photochemical O2 evolution in a Ru(bpy)32+-Na2S2O8 system turns out to depend on the available surface area of Co3O4 nanoparticles; evolution increases with decreasing particle size. Therefore, the surface accessibility of heterogeneous WOCs is a principal factor in influencing the catalytic water oxidation performance. However, the synthesis of ultra-small nanoclusters is extremely difficult. To probe the activity of cobalt oxide with even smaller sizes (<2 nm), CoOx nanoclusters with only 1.5 nm in magnitude were synthesized through mild oxidation of a molecularly dispersed Co4(CO)12 precursor on an alumina support. The TOF value for OER on the CoOx nanoclusters is about six times higher than that of bulk Co3O4 after surface normalization in a photochemical Ru(bpy)32+-Na2S2O8 system.114 This result could be attributed to a different electronic structure or surface structure with comparison to the Co3O4 nanoparticles.115 Beside the available surface areas, the crystallinity is also an important factor that influences the activity of heterogeneous WOCs. Recently, ultra-small amorphous CoOx nanoclusters were synthesized through the mild oxidation of Co2+ in alkaline solution in the presence of a capping agent.116 The TOF value for water oxidation on the amorphous CoOx nanoparticles is up to 8.6 s1 in a photocatalytic Ru(bpy)32+-Na2S2O8 system (Fig. 10A and B), about three orders of magnitude higher than most cobalt-based oxides reported to date. On the basis of extended X-ray absorption fine structure (EXAFS) analysis and first-principles simulations, the atomic structure of the amorphous CoOx nanoparticles was resolved to be composed of a one-dimensional chain of dimeric edge-sharing CoO6 octahedra.

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Very recently, CoOx nanoparticles (<2 nm) as WOCs were anchored on the surface of sulfonated graphite (CoOx@G-Ph-SN).117 The CoOx@ G-Ph-SN nanocomposite exhibited efficient water oxidation activity with a TOF of 1.2 s1 (Fig. 10C and D), which is two orders of magnitude higher than those of conventional Co-based oxides. Moreover, loading the CoOx@G-Ph-SN catalyst onto BiVO4 or a Fe2O3 photoanode can significantly improve the PEC water oxidation performance. 5.3.1.4 NiO

Another first-row transition metal oxide, NiOx, has also attracted much attention in the field of water splitting, since being reported as a WOC by Bode.118 To improve OER activity, many strategies have been adopted, including tuning the particle size, surface energy and microstructure.119–121

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For instance, Bein et al. prepared ultra-small, crystalline, and dispersible NiO nanoparticles (2.5–5 nm) as efficient electrocatalytic WOCs.119 The 3.3 nm nanoparticles demonstrated an extraordinary high TOF of 0.29 s1 at an overpotential of 300 mV for electrochemical water oxidation, higher than expensive IrO2 catalysts. Notably, Ni3+ as an active species is found on the surface of the small NiO particles, and it is commonly thought to be very active for OER. Generally, the surface energy of NiO facets follows the order: {110}  {101} > {113} > {100}.120 As the activity increases with surface energy increase, it has been anticipated that synthesizing ultrafine NiO nanoparticles exposing a high proportion of {110} facets can achieve an efficient OER activity. Based upon this proposition Zhang et al. fabricated ultrafine NiO nanosheets stabilized by TiO2 via calcination of a monolayer layered double hydroxide (LDH) precursor.121 Accordingly, the NiO nanosheets with a high proportion of {110} facets demonstrated outstanding electrocatalytic water oxidation performance. It should be noted that the introduction of Fe dramatically improves the OER activity of NiO due to the enhancement in electrical conductivity by modifying inter-mediate bonds and electronic structures. Bent et al. prepared NiO films by atomic layer deposition (ALD), and its OER activity in a Fe-saturated electrolyte is greatly increased.122 The TOF increases tenfold by moving from an Fe-poor to an Fe-rich KOH electrolyte. Moreover, Boettcher et al. found that even accidental incorporation of iron from KOH electrolyte enhanced catalytic activity.123 5.3.1.5 MnOx

Inspired by natural OEC (CaMn4O5) in PSII, different kinds of manganese oxides have received considerable attention for water oxidation, such as α-, β-, δ- and amorphous MnO2, etc. In 1977, Morita et al. found that MnO2 could perform electrocatalytic water oxidation in alkaline aqueous electrolyte.124 As has been established, the catalytic activity of Mn oxide species depends markedly on their crystallographic structures. As an example, the electrocatalytic activities of MnO2 with various structures follow an order of α-MnO2 > amorphous MnO2 > β-MnO2 > δ-MnO2.125 Notably, α-MnO2 showed an overpotential of 490 mV at 10 mA cm2. The superior OER activity was attributed to several factors including abundant di-μ oxo bridges, mixed valencies and low charge transfer resistances. Furthermore, modulations of the morphologies and basic structures are also very important. Frei et al. fabricated various nanostructured Mn oxides

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on mesoporous silica supports as efficient WOCs, and Mn2O3 exhibited the highest activity.126 Bergmann et al. reported that MnO2 comprised of tunnel structures has a higher activity than that of a layered structure.127 The active sites of both structures are reliant upon the precursors and the oxidation states of Mn. Nine crystalline MnOx samples were evaluated for water oxidation in tandem with different oxidation methods. The results indicated that the identity of the “best” catalyst is subject to the oxidation method used to probe the OER activity.128 In a very important study it was demonstrated that mononuclear manganese embedded in nitrogen-doped graphene (Mn-NG) showed an extraordinary TOF of 214 s1 for chemical water oxidation (Fig. 11),129 which is two orders of magnitude higher than those of reported Mn-based WOCs to date. Structural characterization and DFT calculations revealed that the high activity of Mn-NG could be attributed to the synergistic effect of mononuclear manganese and four coordinated nitrogen atoms embedded in the graphene matrix. 5.3.2 Photo(electro)catalysts Since Fujishima and Honda discovered that a titanium dioxide photoanode can be used for photo(electro)catalytic water splitting under UV irradiation, with an external bias,46 a large number of semiconductor-based materials have been reported to date for this purpose.2,130 The most commonly used semiconductor photo(electro)catalysts for water oxidation include mostly oxides and (oxy)nitrides, as shown in Fig. 12. A

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Fig. 12 Typical examples of semiconductor-based photocatalysts as potential candidates for photo(electro)catalytic water oxidation.

Generally, metal oxides as photocatalytic materials have been widely used for water oxidation. The VB of metal oxides is mainly composed of an O2p orbital, which is located around +3 VRHE.130 Such strong oxidizing potential is enough to oxidize water. To date, a number of metal oxides have been developed for photo(electro)catalytic water oxidation; specifically, TiO2, BiVO4, Fe2O3 and WO3 are representative examples of metal oxide photocatalysts that have been extensively studied. A nitride material, Ta3N5 has recently emerged as a promising photo(electro)catalyst for water oxidation due to its wide range of visible light absorption and suitable band edge position. 5.3.2.1 TiO2

Titanium dioxide (TiO2) is the earliest developed semiconductor photocatalyst for photo(electro)catalytic water splitting. TiO2 has three common polymorphs, including anatase, rutile and brookite, each of which contains TiO6 octahedra.47 Rutile is thermodynamically the most stable phase and has been studied in various photocatalytic applications, but there has not been much focus on the properties of rutile TiO2. Maeda et al. found that the photocatalytic activity for water oxidation on rutile TiO2 depends upon the crystallinity of the material and the density of oxygen vacancies, in the presence of IO3 or Fe3+ as an electron acceptor.131 High-temperature calcination improved crystallinity of the particles and increased the oxygen vacancy density, thus resulting in the enhanced charge transport capability and water oxidation activity.

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Furthermore, loading cocatalysts on the surface of semiconductors is a common method to improve the water oxidation activity. For example, when transition metal oxide clusters (i.e., MnOx, FeOx, CoOx, NiOx, CuOx, IrOx, and RuOx) were finely loaded on rutile TiO2, the charge separation process is significantly promoted, resulting in higher water oxidation activity compared with the pristine rutile TiO2.132 It should be noted that the phase structure of TiO2 has a large influence on its photocatalytic performance. It was found that a mixed-phase structure of TiO2 has much higher H2-evolution activity than that of the pure crystalline phase, and thus the concept of a phase junction is proposed, and is widely recognized.133 The phase junction shows the great potential for efficient charge separation and transfer. Based on the concept, the TiO2 photoanode with the suitable alignment of anatase and rutile phases demonstrates superior activity for PEC water splitting, reaching three and nine times increase in photocurrent density compared with activities for anatase and rutile phase TiO2 photoanode alone, respectively (Fig. 13).134 As can be seen, some effective strategies such as loading cocatalysts and constructing a phase junction have been demonstrated for improving the water oxidation performance on TiO2. As TiO2 can only absorb the sunlight in the UV region (<5%), therefore it is essential that visible-light-responsive photo(electro)catalysts should be developed thus utilizing more visible range sunlight (>40%). 5.3.2.2 BiVO4

The n-type bismuth vanadate (BiVO4) semiconductor has a band gap of 2.4 eV and is an excellent visible light-driven water oxidation photo (electro)catalyst.135 BiVO4 possesses different crystal structures including a tetragonal zircon phase, a monoclinic scheelite phase, and tetragonal scheelite. When silver nitrate is used as an electron acceptor, the monoclinic phase BiVO4 exhibits an excellent photocatalytic oxygen production performance.135 It is noteworthy that a novel scientific phenomenon is spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4,136,137 as shown in Fig. 14. The reduction reaction of photogenerated electrons and the oxidation reaction of photogenerated holes occur in the {010} and {110} crystal facets under light illumination, respectively. Additionally, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} faces via a photodeposition method. The specific sample demonstrated much higher performance than that with randomly distributed cocatalysts in photo(electro)catalytic water oxidation.

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The conduction band (CB) edge position of BiVO4 is very close to the thermodynamic hydrogen production potential, and its onset potential is the most negative among all visible-light-response n-type semiconductor photoanode materials. This results in outstanding performance of BiVO4 in PEC water oxidation. Loading cobalt borate (CoBi) cocatalyst onto BiVO4 not only reduces the onset potential, but also improves the photocurrent density and stability.138 More significantly, Choi et al. prepared a nanoporous BiVO4 electrode with the grain size smaller than the hole diffusion length, thus effectively suppressing the recombination of photogenerated carriers.139 The separation efficiency of electrons and holes is as high as 90% at 1.23 VRHE. Furthermore, after loading FeOOH and NiOOH dual

Fig. 14 SEM images of BiVO4 with and without metal/oxide deposited. (A) BiVO4; (B) Pt/BiVO4; (C) MnOx/BiVO4 and (D) Pt/MnOx/BiVO4. (E) Photocatalytic and (F) photoelectrocatalytic water oxidation performances of BiVO4 with and without cocatalysts. Reprinted (adapted) with permission from Li, R.G.; Zhang, F.X.; Wang, D. G.; Yang, J.X.; Li, M.R.; Zhu, J.; Zhou, X.; Han, H.X.; Li, C. Nat. Commun. 2013, 4, 1432. Copyright (2013) Nature Publishing Group.

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cocatalysts onto BiVO4, the recombination of photogenerated carriers at the interface of WOC/BiVO4 is reduced, and a more favorable Helmholtz layer potential difference is created at the interface of the WOC/electrolyte. The resultant photoanode shows a current density of 2.73 mA cm2 at 0.6 VRHE without obvious decay in 48 h with a STH of 1.75%. Domen et al. prepared a worm-like BiVO4 electrode, which greatly improved the charge separation efficiency and the transmission of incident light by controlling the morphology and nanostructure of the sample.140,141 In addition, in situ growth of NiFeOx cocatalyst onto BiVO4 by photo (electro)oxidation resulted in excellent charge separation efficiency, showing a high STH of >2%. 5.3.2.3 α-Fe2O3

Among various materials used to prepare photoanodes, hematite (α-Fe2O3) has attracted substantial research interest because it is inexpensive, non-toxic, earth-abundant, and has good (photo)chemical stability. α-Fe2O3 possesses a suitable band gap (2.1 eV), particularly, the theoretical STH is as high as 16.5%.142 Due to the increase in one-dimensional anisotropy and charge transport for titanium-doped α-Fe2O3 (TiFe2O3) nanorod arrays,143 the photocurrent density of TiFe2O3 is doubled compared with that of the bulk α-Fe2O3 at 1.50 VRHE. When the Ni(OH)2/IrO2 dual cocatalysts were loaded, the onset potential was negatively shifted by 200 mV (Fig. 15A). It was found that Ni(OH)2 can effectively capture photogenerated holes from

Fig. 15 (A) Current-voltage curves and (B) tested (dot) and simulated (line) results of the photoanodes of Fe2O3-based photoanodes for PEC water oxidation. Reprinted (adapted) with permission from Wang, Z.L.; Liu, G.J.; Ding, C.M.; Chen, Z.; Zhang, F.X.; Shi, J.Y.; Li, C. J. Phys. Chem. C 2015, 119, 19607–19612. Copyright (2015) American Chemical Society.

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TiFe2O3 and promote the interfacial charge transfer between TiFe2O3 and IrO2 (Fig. 15B). The synergistic effect between two cocatalysts and effective surface doping is considered to be the main reason for improving the PEC performance. Interestingly, the crystallinity of cocatalysts also plays a key role in PEC water oxidation. An ultrathin (2 nm) amorphous FeOOH overlayer was deposited on the surface of α-Fe2O3 via a simple precipitation method,144 giving rise to a photocurrent density increase from 0.6 to 1.2 mA cm2 at 1.23 VRHE, and the onset potential was negatively shifted from 0.77 to 0.65 V, with a better stability. These changes are due to the amorphous FeOOH overlayer rendered the surface states passive and improved oxygen evolution kinetics. Hence the crystallinity of a cocatalyst should be considered in the context of PEC water oxidation reactions. Many reports focus on the increase of PEC performance of Fe2O3; however, the reaction mechanism is still not well understood as the photocatalytic water oxidation reaction proceeds through a complicated multistep process. The reaction mechanism of PEC water oxidation on a α-Fe2O3 photoanode was investigated by the group of Zhao.145 The PEC results and H/D kinetic isotope effect (KIE) data in combination with the characterization by electrochemical impedance spectroscopy (EIS), demonstrate the key role of proton transfer in water oxidation. A CPET process was ascertained in the rate-determining step of interfacial hole transport, where electrons are transferred from water molecules to surface-trapped holes, concatenated with proton transfer to water molecules in the solvent. More importantly, the order of proton-electron transfer can be regulated by adding a suitable proton acceptor (alkaline buffer solution) and thus increasing the PEC efficiency by a factor of four. These findings represent a significant bridge between the water oxidation mechanism and performance. 5.3.2.4 WO3

Tungsten trioxide (WO3) contains a perovskite unit structure, which is one of the most attractive n-type semiconductor materials in photo(electro)catalysis.146 The band gap is about 2.7 eV. The VB is composed of filled O2p orbitals, while the CB is formed by empty W5d orbitals. It possesses the following crystal phases: monoclinic II (ε-WO3), triple oblique (δ-WO3), monoclinic I (γ-WO3), orthogonal (β-WO3), tetragonal (α-WO3) and a cubic phase. WO3 has a longer hole diffusion length (150 nm) compared with α-Fe2O3 (2–4 nm), and a higher electron mobility (12 cm2 V1 s1) compared with TiO2 (0.3 cm2 V1 s1).

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Fig. 16 (A) Schematic diagram of photocatalytic water oxidation on Pt/RuOx/WO3 photocatalyst. (B) Time courses of photocatalytic O2-evolution performance on the representative WO3-based photocatalysts under visible-light irradiation (800 > λ > 420 nm, 300 W xenon lamp). Reprinted (adapted) with permission from Ma, S.S.K.; Maeda, K.; Abe, R.; Domen, K. Energy Environ. Sci. 2012, 5, 8390–8397. Copyright (2012) Royal Society of Chemistry.

As shown in Fig. 16, there is an interesting feature; loading a Pt cocatalyst onto WO3 is necessary to drive the oxidation of water with IO3  as an electron acceptor,147 indicating that the Pt cocatalyst is extremely efficient in the activation of IO3  . After loading the second cocatalyst, RuO2 onto Pt/WO3, the performance of the system can be improved further, exhibiting an AQE of 14.4% (λ ¼ 420 nm). This study demonstrates that the photocatalytic activity of bulk WO3 can be greatly improved by loading of reduction and oxidation cocatalysts, further validating the concept of “dual cocatalysts” proposed by the group of Li utilizing CdS.148 Although WO3 is thermodynamically stable enough to resist light corrosion, it has been observed that the photoactivity of WO3 is gradually lost during long-term PEC reaction. Augustynski et al. reported that the loss of photoactivity of WO3 is due to the formation and accumulation of peroxo species on the surface of WO3.149 Thermodynamically, the oxidation of water to O2 (E ¼ 1.23 V) is much easier than the formation of peroxo species (E ¼ 1.78 V). However, due to the slower reaction of the OER, the formation of peroxo species becomes a competitive reaction for O2 production. Therefore, the presence of an efficient cocatalyst is desirable to accelerate the kinetics of O2 production and suppress the formation of peroxide, thereby improving the photostability of WO3. Accordingly, Choi et al. loaded the CoPi cocatalyst onto WO3, and found that CoPi can increase the PEC Faraday efficiency from 61% to 100%, indicating that peroxidic species formation in the CoPi/WO3 system was completely inhibited.150

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5.3.2.5 Ta3N5

In recent years, tantalum nitride (Ta3N5) semiconductor has become a prominent material for water oxidation, because of an ideal band structure (2.1 eV), an attractive theoretical STH efficiency of 15.9% and a theoretical current of 12.9 mA cm2 under AM1.5G irradiation.151 However, when it is used as a photo(electro)catalytic WOC, severe photocorrosion limits its application. So far, various strategies have been explored for improving the photostability of Ta3N5. One of the most common methods is improving the charge separation and transport by loading a cocatalyst. For example, when CoOx cocatalyst was loaded onto Ta3N5, the AQE of water oxidation was up to 5.2% at 500–600 nm.152 The interface between the hydrophobic Ta3N5 and the hydrophilic CoOx is not ideal, and this may reduce the interface charge-transfer efficiency. Interestingly, a MgO nanolayer plays an important role in regulating the interface between Ta3N5 and CoOx (Fig. 17).153 The modified hydrophilic surface will be more favorable for the deposition of hydrophilic CoOx onto Ta3N5, resulting in better interfacial charge transfer and higher water oxidation efficiency. Moreover, the MgO nanolayer acts as a passivation layer, which reduces defects of the Ta3N5 surface and

Fig. 17 (A) Schematic diagram of preparation of CoOx/MgO/Ta3N5 photocatalyst. (B) Rate of oxygen evolution and (C) Decay of transient absorption for the representative Ta3N5-based photocatalysts. Reprinted (adapted) with permission from Chen, S.S.; Shen, S.; Liu, G.J.; Qi, Y.; Zhang, F.X.; Li, C., Angew. Chem. Int. Ed. 2015, 54, 3047–3051. Copyright (2015) Wiley-VCH.

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inhibits recombination of photogenerated carriers. The AQE of this system attains a value as high as 11.3% at 500–600 nm, which is the highest value among the particulate photocatalysts with an absorption edge at 600 nm. Although Ta3N5 exhibits excellent performance for photocatalytic water oxidation, the use of Ag+ ions can result in poor stability due to the deposition of Ag nanoparticles onto Ta3N5. Thus, the construction of a Ta3N5based photoanode for PEC water splitting is a requirement. As stated above, the photocorrosion can be reduced by loading a cocatalyst onto Ta3N5. However, loading IrO2, CoPi, or CoPi/Co(OH)2 cocatalysts only improved the photocurrent density, while the photocorrosion of the Ta3N5 photoanode was not markedly improved.154–156 In order to solve this problem, the first initiative involved the proposal of a concept of a “hole storage layer” (HSL).157 It was found that ferrihydrite (Fh, Fe5HO83H2O) loaded onto Ta3N5 can quickly extract and store photogenerated holes from Ta3N5, thereby preventing the oxidation of Ta3N5 by photogenerated holes. Further, loading Co3O4 as a cocatalyst onto Fh, the resulting composite photoanode showed a current density of 5.2 mA cm2 at 1.23 VRHE for 6 h.157

5.4 Hybrid water oxidation catalysts As stated above, molecular catalysts and semiconductors, as WOCs, have their respective advantages or disadvantages in water oxidation. However, semiconductors generally suffer from poor electron–hole separation without loading a cocatalyst,16 and molecular WOCs are not photoactive without adding a photosensitizer.38 In this context, it has been anticipated that a highly efficient artificial photosynthesis system can be constructed using a combination of robust semiconductor nanoparticles with wide spectral absorption as light harvesters, and molecular catalysts as OECs, thus bridging the gap between homogeneous and heterogeneous catalysis for water oxidation. In early studies, a molecular Ru catalyst loaded on Fe2O3 was reported,158 however, the stability was inadequate and no oxygen was detected. Then, notably, a phosphonate-modified Fe complex was anchored to a WO3 electrode, which dramatically increased the rate and selectivity of water oxidation.159 A series of molecular catalyst/semiconductor hybrid systems were reported for PEC water oxidation.160–164 However, such a hybrid system was rarely applied as the particulate photocatalysts for photocatalytic water oxidation. Interestingly, an artificial photosynthetic system was constructed for photocatalytic water oxidation using BiVO4 as a photosensitizer and a cubic

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Fig. 18 Assembly of artificial photosynthesis systems inspired by nature photosynthesis. (A) Schematic representation and (B) performances of the photocatalytic water oxidation systems. (C) Schematic representation and (D) performances of photoelectrocatalytic water oxidation systems. Reprinted (adapted) with permission from Ye, S.; Chen, R.; Xu, Y.; Fan, F.; Du, P.; Zhang, F.; Zong, X.; Chen, T.; Qi, Y.; Chen, P.; Chen, Z.; Li, C. J. Catal. 2016, 338, 168–173. Copyright (2016) Elsevier and permission from Ye, S.; Ding, C.; Chen, R.; Fan, F.; Fu, P.; Yin, H.; Wang, X.; Wang, Z.; Du, P.; Li, C. J. Am. Chem. Soc. 2018, 140, 3250–3256. Copyright (2018) American Chemical Society.

Co complex as the OEC.93 This system exhibits a high TOF of 2.0 s1 for O2 evolution, with an AQE of 4.5% at 420 nm, which is ninefold higher than that of uncombined BiVO4 (Fig. 18A and B). Spectroscopic results displayed an efficient interfacial hole transfer process from the BiVO4 semiconductor to the molecular Co catalyst. This work is the first case of coupling the semiconductor with a molecular catalyst to achieve improved activity for particulate visible-light-driven O2 evolution. Very recently, the Co cubane/BiVO4 system was further developed in PEC devices by mimicking the functions of PSII (Fig. 18C and D).94 In addition to the light harvester (BiVO4) and the WOC (Co cubane), the introduction of interfacial materials, NiFeLDH and partially oxidized graphene (pGO), is very significant. It should be noted that NiFeLDH as an HSL, collects the holes and protects the BiVO4 semiconductor from

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photocorrosion, which efficiently reduces the electron/hole recombination. More importantly, pGO as biomimetic tyrosine for charge transfer was added between the LDH and Co catalyst, which plays a key role in unidirectionally transporting holes, thus resulting in the long-lived hole accumulation. Accordingly, after loading the LDH and pGO layers, the integrated system exhibited an ultra-low onset potential of 0.17 V, approaching the thermodynamically limiting value. Spatially resolved surface photovoltage (SPV) spectroscopy revealed that the SPV intensity of this overall system is up to an order of magnitude higher than that of a bare BiVO4 photoanode, thus exhibiting a significant improvement in charge separation for a bulk semiconductor. This work highlights the importance of biomimetic strategy and enlightens us on a deeper understanding of the interlayer, which is essential to improve the water oxidation activity in an artificial photosynthesis system, benefiting from mimicking the key functions of PSII. In the same year, Sun et al. prepared a new cubane complex adsorbed on the surface of a porous BiVO4 photoanode via hydrophobic interactions.165 This system demonstrated a high photocurrent density of 5 mA cm2 at 1.23 VRHE under AM1.5G (1 sun) illumination. Various strategies have been applied to the hybrid system, and all of them verify that the molecular catalyst/semiconductor interface plays an important role in promoting charge separation and improving the performance of PEC water oxidation. More significantly, the group of Li constructed an ultra-high performance Ta3N5 photoanode by introducing a passivation layer (TiOx), a [Ni(OH)x/Fh] layer as an HSL, along with dual molecular catalysts.166 The system achieved a current density of 12.1 mA cm2, which is close to the theoretical value. Overall, these results opened up new ways for the rational design and assembly of highly efficient artificial photosynthesis devices based on molecular catalyst/semiconductor hybrid systems for solar energy conversion.

6. Water oxidation mechanisms The mechanism of water oxidation has been the focus of much research over the past 30 years. Proposals regarding the mechanism for water oxidation by WOCs arose from a series of investigations, specifically DFT calculations, analysis of experimental kinetic data, and characterization via employment of highly sophisticated instruments. Two landmark studies are now presented in order to elucidate separately the mechanism of homogeneous and heterogeneous catalysis.31,129

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6.1 Water oxidation mechanism for homogeneous catalyst The first homogeneous catalyst for water oxidation is the [{RuIII(bpy)2 (H2O)}2O]4+ ion, based on an oxo-bridged dimeric system of ruthenium(III).31 The complex can oxidize water to O2 by chemical and electrochemical oxidation. The following section focuses on the catalytic mechanism of water oxidation by chemical oxidation using CeIV as the chemical oxidant (Fig. 19).10 When the Ru complex is added to a solution containing 50 or 100 equivalents of CeIV, oxygen was evolved immediately. Adding 4 equiv. of CeIV results in a rapid disappearance of color followed by a reappearance of color. The production of O2 was consistent with the reappearance of the color. This means that 4 equiv. of CeIII were produced correspondingly with 1 equiv. of O2. Kinetic studies on the stepwise oxidation of RuIIIORuIII to RuVORuV were conducted using stopped-flow spectrometry.167 The RuVORuV oxidation state is accessible by a series of sequential electronproton losses. The high oxidation states are confirmed in the experiments, and the system contains RuIVORuIII, RuVORuIV and RuVORuV. The detailed mechanism of water oxidation by the BD revealed a stepwise oxidation of RuIIIORuIII to RuVORuV.168 The RuIIIORuIII aquo dimer can be first oxidized by CeIV to give a stable RuIVORuIII dimer. The result of a cyclic

Fig. 19 Proposed mechanism of water oxidation by chemical oxidant (CeIV) for the “Blue Dimer,” [{RuIII(bpy)2(H2O)}2O]4+ ion, by Meyer and Co-workers. Reprinted (adapted) with permission from Ka€rka€s, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Chem. Rev. 2014, 114, 11863–12001. Copyright (2014) American Chemical Society.

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voltammogram showed that the RuIIIORuIV/RuIIIORuIII couple appears at 0.77 V vs. a saturated sodium chloride calomel electrode (SSCE). Interestingly, it is unknown whether the further oxidation state of the RuIVORuIII dimer is RuVORuIII or RuIVORuIV, although the full kinetic model predicts the formation of RuIVORuIV, as a one-electron intermediate. Unfortunately, in electrochemical measurements, only the RuVORuIV/RuIVORuIII couple was observed, which indicates that RuIVORuIV is very unstable toward disproportionation into RuIVORuIII and RuVORuIV.169 It is worth noting that the RuVORuV oxidation state with two oxo ligands (Ru¼O) is a prerequisite for accomplishing water oxidation. When the bpy ligand was replaced by the 1,10-phenanthroline (phen) ligand, the RuIIIORuIII dimer can also act as a WOC toward the oxidation of water. A number of WOCs were developed following this report on the BD. Therefore, this set of findings may provide a basic model for the O2-evolution sites in homogeneous WOCs.

6.2 Water oxidation mechanism for heterogeneous catalyst A benchmark WOC in heterogeneous systems, Mn-NG exhibited outstanding activity comparable to that in nature, CaMn4O5 in PSII.129 CV and DFT simulations were performed to understand how the four-electron transfer mechanism takes place during the O2 evolution on the mononuclear Mn of Mn-NG catalyst. Fig. 20A displays the CV of Mn-NG, in which two reversible oxidation waves at E1/2 ¼ 0.74 and 1.36 V can be assigned to two redox reactions A

B MnII/MnIII MnIII/MnIV

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Fig. 20 (A) CV of Mn-NG in Ar-saturated 1.0 M KOH (left) and 1.0 M KOH (right) at a scan rate of 50 mV s1. (B) Me. Reprinted (adapted) with permission from Guan, J.; Duan, Z.; Zhang, F.; Kelly, S.D.; Si, R.; Dupuis, M.; Huang, Q.; Chen, J.Q.; Tang, C.; Li, C. Nat. Catal. 2018, 1, 870. Copyright (2018) Nature Publishing Group.

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involving MnIII/MnII and MnIV/MnIII redox couples, respectively. On the basis of the CV experiment and the DFT simulations on the oxidation states of the mononuclear Mn site at each OER step, a four-step reaction mechanism for water oxidation on the MnN4-G active site could be deduced (Fig. 20B). The first H2O molecule is partly dissociated on the MnII site to form a MnIII-OH intermediate while releasing one proton and one electron. This step is followed by the oxidation of MnIII-OH to form a MnIV¼O site. The second water molecule then acts as a nucleophile and attacks the MnIV¼O species to form a MnIII-OOH intermediate after coupling of the oxygen atoms. Finally, the oxidation of OOH and liberation of O2 from MnIII-OOH with a low activation energy complete the catalytic cycle. Here the highest oxidation state (MnIV) of the Mn ion occurs upon the formation of the O* intermediate. It should be noted that the water oxidation mechanism of such a mononuclear Mn site is different from that for multinuclear complexes, where the rate-determining step has been proposed to be the oxo-oxo coupling step for the O2 release, rather than the electrochemical OH* oxidation step as demonstrated for the mononuclear manganese active site (MnN4-G), in the Mn-NG catalyst.

7. Overall water splitting There have been many investigations upon half reactions of water splitting (H2 or O2 evolution), however, OWS is the final goal. OWS is one of the most exciting and environmentally friendly strategies to develop continuously renewable clean energy. Therefore, the design and ultimately assembly of a complete OWS system for fuel production would be significant achievements. So far, numerous artificial photosynthesis systems have been constructed on semiconductor-based materials for OWS.12,16,170–189

7.1 Artificial photosynthesis systems In general, photocatalytic OWS in artificial photosynthesis can be summarized into two primary approaches: one-step photoexcitation and two-step photoexcitation processes (Fig. 21). 7.1.1 One-step photoexcitation system OWS driven by one-step excited particulate photocatalysts is a magnet for attention because of its cost-effectiveness and manufacture-scalability, as well as high theoretical efficiency of solar energy conversion. A significantly improved AQE of photocatalytic OWS under UV illumination has been seen in the past 40 years. The AQEs have reached 56% at 270 nm on

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Fig. 21 Schematic illustration for two types of photocatalytic overall water-splitting systems: one-step photoexcitation and two-step photoexcitation systems.

NiO:NaTaO3:La,170 71% at 254 nm on Rh0.5Cr1.5O3/Ga2O3:Zn,171 and 69% at 365 nm on MoOy/RhCrOx/SrTiO3:Al.172 However, the theoretical STH efficiency on these UV-responsive photocatalysts is limited to 3.3% even with 100% AQE under UV light, for the reason that UV light only holds 4% of the total solar spectrum. Practical application of this approach to solar energy conversion requires 5–10% STH, requiring a visible-light (λ  400 nm) responsive photocatalyst with high AQE, reportedly there is quite a limited number of them.12 Amongst them, cocatalyst/GaN-ZnO has been demonstrated to be a typical photocatalyst for photocatalytic OWS with relatively high activity and stability, as well as good reproducibility.173,174 In 2005, Maeda and Domen et al. synthesized GaN-ZnO solid solution by nitriding a mixture of Ga2O3 and ZnO powders at high temperature in an NH3 gas flow.175 Although the GaN and ZnO were both UV responsive, the light absorption of a solid solution of GaN and ZnO was extended to the visible region (absorption edge of ca. 500 nm) due to a p-d repulsion effect in the upper VB. RuO2/GaN-ZnO could perform photocatalytic OWS with an average AQE of 0.14% in the range of 300–480 nm. Optimization of a cocatalyst, RhCrOx and passivation of the Zn/N related surface defects, further enhanced the AQE to 2.5% and 5.9% at 420–440 nm.176,177 Actually, GaN-ZnO itself cannot split water photocatalytically. Only when decorated with suitable cocatalysts, could it be active for OWS. It has been confirmed that cocatalysts have crucial effects on the processes of OWS, such as charge separation and transportation, and surface redox reactions.16,178,179 For GaN-ZnO, Rh is an essential component of the efficient cocatalyst (mixed oxide of RhCrOx or a core/shell of Rh@Cr2O3) for photocatalytic OWS, as has been reported.176,177,180,181 Furthermore, other

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Fig. 22 (A) Typical HR-TEM image of Pd@Cr2O3/Ga. (B) The performance of photocatalytic OWS on Pd/GaN-Zn with in situ photodeposition of Cr2O3 under visible light (λ > 420 nm, 300 W Xenon lamp). Reprinted (adapted) with permission from Li, Z.; Zhang, F.; Han, J.; Zhu, J.; Li, M.; Zhang, B.; Fan, W.; Lu, J.; Li, C. Catal. Lett. 2018, 148, 933–939. Copyright (2018) Springer.

metals such as Pt, Pd, Cu and Ni can be effective components of metal-Cr mixed oxides or metal@Cr2O3 core/shell structures for photocatalytic OWS. Nevertheless, their performances are far from those of Rh-based cocatalysts.180,181 Recently, a novel method of ALD was utilized to load uniform ultrafine Pd nanoparticles onto the surface of a GaN-ZnO solid solution (Fig. 22).183 Together with the following in situ photodeposition of Cr2O3 as a blocking layer to suppress the reverse reaction, photocatalytic OWS with comparable activity to that of a typical RhCrOx/GaN-ZnO combination, under the same conditions, was achieved with much less Pd usage (0.13 wt% vs. 1.0 wt%). In this study a Pd@Cr2O3/GaN-ZnO combination could ably split water into stoichiometric H2 and O2 under acidic, neutral and basic conditions. The AQE@420 nm was 1.62%, higher than that of RhCrOx/GaNZnO (1.11%). Most noteworthy, the ALD method applied in this work provides uniform ultrafine nanoparticles and intimate contact between Pd and GaN-ZnO, which is favorable for the interface charge transfer. Therefore, loading noble metals via ALD might stimulate their intrinsically high activity for proton reduction towards a highly effective cocatalyst for photocatalytic OWS. 7.1.2 Two-step photoexcitation (Z-scheme) system In nature, photosynthesis utilizes solar energy to drive water oxidation and fuel production, which resembles a Z-scheme configuration in the matter of

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Fig. 23 Z-scheme diagrams of (A) natural photosynthesis and (B) artificial photosynthesis.

energy distribution (Fig. 23A).184 Inspired by natural photosynthesis, constructing an artificial Z-scheme system, which includes two types of photocatalytic systems that separately produce H2 and O2, is a very effective approach to achieve OWS (Fig. 23B). As an example, Domen et al. reported an effective Z-scheme OWS system using ZrO2-modified TaON as the H2-evolution photocatalyst (HEP) and WO3 as an O2-evolution photocatalyst (OEP), with an IO3  =I redox as the shuttle mediator.185 The AQE can become as high as 6.3% at 420 nm. The high activity is mainly attributed to the efficient reaction of I =IO3  ions on the photocatalysts, which suppresses undesirable reverse reactions. Another interesting example is the use of a (oxy)nitride-based heterostructure for a powdered Z-scheme OWS.186 By employing Pt/ MgTa2O6-xNy/TaON as the HEP and PtOx-WO3 as the OEP, with a IO3  =I redox couple as the electron mediator, a record of AQE of 6.8% was achieved at 420 nm. This was attributed to the hetero-structure of MgTa2O6-xNy/TaON that can inhibit the recombination of carriers by efficient spatial charge separation and decreased defect density.

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This work outlines an effective strategy to improve the H2-evolving rate, especially to promote the separation of photogenerated carriers. However, water oxidation is more challenging than water reduction in the process of OWS reactions. Therefore, the development of OEP is the key to improving further the efficiency of a Z-scheme OWS. Toward this goal, very recently, by employing ZrO2-modified TaON as the HEP and BiVO4 as the OEP, with a [Fe(CN)6]3/[Fe(CN)6]4 redox mediator, Zhang et al. updated the AQE record of a powdered Z-scheme OWS to be 10.3% at 420 nm (Fig. 24).187 Au and CoOx nanoparticles as dual cocatalysts were selectively deposited on the corresponding electronrich {010} and hole-rich {110} facets of BiVO4. The loading of dual cocatalysts significantly accelerated the O2-evolution rate on the OEP, and thus enhanced the efficiency of the OWS reaction. This work highlights the important role of OEP in the construction of a Z-scheme OWS system. In addition to the all-inorganic semiconductor photocatalysts, the robust organic semiconductor graphitic carbon nitride, g-C3N4, can also be integrated with two different metal oxides, BiVO4 and WO3, using a I =IO3  or Fe2+/Fe3+ redox as the shuttle mediator.188 This hybrid system based on inorganic-organic composites has demonstrated a stable and reproducible H2 and O2 evolution rate from water over 14 h under visible light. This finding demonstrates that a robust organic semiconductor, such as g-C3N4, can serve as an HEP to assemble the Z-scheme OWS system,

Fig. 24 (A) Schematic description of Z-scheme OWS process. (B) Time course of photocatalytic OWS performance on the Au/CoOx-BiVO4 photocatalyst (OEP) combined with RhyCr2–yO3-ZrO2/TaON photocatalyst (HEP) under visible-light irradiation (λ > 420 nm, 300 W xenon lamp) at 288 K. Reprinted (adapted) with permission from Qi, Y.; Zhao, Y.; Gao, Y.; Li, D.; Li, Z.; Zhang, F.; Li, C. Joule. 2018, 2, 2393–2402. Copyright (2018) Elsevier.

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which opened up a new method for a low cost and robust artificial photosynthetic system for H2 fuel production from pure water. A solid-state Z-scheme system has been developed by using a metal layer, instead of the shuttle ions. Very recently, Wang et al.189 prepared photocatalyst sheets based on La- and Rh-co-doped SrTiO3 as the HEP, and Mo-doped BiVO4 as the OEP co-embedded into a gold layer. This system demonstrated an unprecedented STH efficiency of 1.1% and an AQE of over 30% at 419 nm; this represents a new record for Z-scheme OWS systems. The novel design of photocatalyst sheets paved a way for efficient and scalable water splitting, using particulate semiconductors. Hence, the artificial Z-scheme OWS system is a typical example that emerges from an insight into natural photosynthesis, and thus may be considered a successful outcome. In comparison with the single one photocatalyst, Z-scheme system, including two types of photocatalysts can utilize visible light more efficiently and has a greater selection of pristine semiconductor materials. Therefore, it may be anticipated that a biomimetic Z-scheme system for OWS can be a rational and effective way to develop artificial photosynthesis systems.

7.2 Nature-artificial hybrid photosynthesis systems The catalytic activity of most artificial photocatalyst systems is lower than that of natural photosynthetic systems, especially the activity of WOCs, which are generally 3–4 orders of magnitude lower than the activity of CaMn4O5 clusters in the native PSII. The construction of natural photosynthesis and artificial photosynthesis hybrid systems can combine the advantages of the two systems.190–196 Wang et al. reported a novel nature-artificial hybrid system using the nature PSII as OEP and artificial Ru/SrTiO3:Rh as HEP, with a [Fe(CN)6]3/[Fe(CN)6]4 redox couple,190 as shown in Fig. 25A. It is worth noting that the electrons from water oxidation by PSII, can transfer to the redox couple and further to the surface of the Ru/SrTiO3:Rh semiconductor (Fig. 25B). This work highlighted the importance of the redox shuttle, which played a key role in electron and proton transfer processes of Z-scheme OWS. In nature, photosynthetic electrons transfer efficiently via the elaborate redox cofactors for generation of the reducing equivalents. Subsequently, Wang et al. developed further a hybrid photosystem based on the previous Z-scheme system by introducing a second redox shuttle (quinone) into the integration of PSII and the inorganic Ru/SrTiO3:Rh photocatalyst.191

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Fig. 25 Schematic representation of a PSII–semiconductor hybrid system for (A, B) photocatalytic and (C) photoelectrocatalytic OWS. Reprinted (adapted) with permission from Wang, W.; Chen, J.; Li, C.; Tian, W. Nat. Commun. 2014, 5, 4647. Copyright (2014) Nature Publishing Group and permission from Li, Z.; Wang, W.Y.; Ding, C.M.; Wang, Z.L.; Liao, S.C.; Li, C. Energy Environ. Sci. 2017, 10, 765–771. Copyright (2017) Royal Society of Chemistry.

It was ascertained that electrons/protons can transfer from PSII to a semiconductor photocatalyst by the quinone molecule. More importantly, the quinoneferricyanide transport relay is found to be much more efficient in comparison with either of them separately for OWS activity.

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Following this concept, a novel CdS-PSII hybrid system was applied in a PEC cell system for OWS,192 where the CdS photocathode and PSII are connected by two redox shuttles (ferricyanide/ferrocyanide and quinone/ hydroquinone), as shown in Fig. 25C. This system achieved OWS with the ratio of 1:2 under AM1.5G irradiation, corresponding to a STH of 0.34%. More importantly, the tandem design greatly improves the light utilization efficiency and the stability of the PEC cell. In a similar approach using the same concept, Wang et al. constructed a new PEC system by combining PSII (at 680 nm) and a silicon-based photochemical battery (λ < 650 nm),193 which took full advantage of complementary solar absorption. Hydrogenases (H2ase) are benchmark electrocatalysts for H2 evolution. Reisner et al. developed a hybrid cell by integrating PSII and H2ase for O2 and H2 evolution, respectively.194 It was shown that quantitative electron transfer from PSII to the hydrogenase occurs with the aid of an applied bias. This system demonstrated a light-to-hydrogen conversion efficiency of 5.4% under low intensity red-light irradiation (10 mW cm2). However, this system cannot achieve unbiased OWS due to the low electrochemical potential. To resolve this difficulty, it is essential to introduce a second lightharvester whose function is to promote the electron energetics. Accordingly, a dye-sensitized TiO2 was connected with PSII, and this provides a sufficient voltage to achieve unbiased OWS.195 Furthermore, a Si-based photocathode connected with an H2ase catalyst in combination with a modified BiVO4 photoanode also demonstrated unbiased OWS in a PEC cell.196 In principle, nature-artificial hybrid systems combine the advantages of natural photosynthesis and artificial photosynthesis, and should provide inspiration for the construction of a novel hybrid system in order to achieve highly efficient Z-scheme water splitting for solar energy conversion.

8. Conclusions and perspectives Water splitting to produce hydrogen, notably solar water splitting, has great potential for addressing and mitigating potential energy crises and thus ameliorating environmental pollution. In the past few decades research momentum has developed with respect to water oxidation, in order to achieve efficient artificial photosynthesis. Significant progress has been made, including the preparation of highly active catalysts, the characterization of surface catalytic reactions, and an increase in the understanding of the mechanism for OER.

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Many WOCs have been developed, including homogeneous, heterogeneous and hybrid systems, each with their own virtues and disadvantages. Various approaches have been adopted to evaluate the performance of the WOCs, such as chemical oxidation, electrocatalysis and photo(electro) catalysis. Subsequently, WOCs that are promising in terms of their properties may lead to their incorporation into solar fuel devices. Indeed, homogeneous catalysts demonstrate high catalytic activity, and possess the advantages of clear active sites and viable reaction kinetics, providing a new window for exploiting more efficient artificial photosynthesis system through a bioinspired method. Nevertheless, the most vital shortcoming of molecular catalysts is the poor stability. In contrast, heterogeneous catalysts exhibit the distinct advantage of excellent stability under the same conditions. However, they have inferior kinetical selectivity, which makes it much more challenging to elucidate the water oxidation mechanisms compared with the mechanisms for molecular catalysts in photo(electro)catalysis. Hybrid systems comprising homogeneous and heterogeneous catalysts, exhibit unique advantages in the combination of these two forms of catalysis, and thus form a “bridge” between them. WOCs have been demonstrated to be an effective modus operandi to lower the energy barrier of the reaction and increase the reaction rate for the assembly of photocatalytic and PEC devices. Nevertheless, two key related issues (i.e. surface engineering and interface engineering) deserve more attention, since each or both may be closely related to the water oxidation efficiency. Notably, trap states of the semiconductor, the lattice or energy level mismatch of semiconductor/catalyst interface, may result in serious recombination during the charge separation and transport process. Therefore, a careful modification of the interface of semiconductor/ catalyst is critical for promoting the charge separation and transfer by loading surface passivation layers (i.e., TiO2, AlOx, SiO2), interfacial transfer layer (biomimetic TyrZ, pGO), and HSL (Fh, Ni(OH)x). The recent advances in water oxidation have paved the way for efficient artificial photosynthesis. However, it is still a great challenge to construct highly efficient and robust artificial photosynthesis system for future scaleup application. Therefore, new catalyst materials and strategies for efficient OER are still imperatively required for the rational design and assembly of highly efficient artificial photosynthesis systems. Additionally, new methodologies and systematic theoretical simulations to provide guidance for the design and synthesis of high active catalysts are also needed in the field of water splitting. Most importantly, much attention should be paid to the engineering of semiconductor/catalyst interface.

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Acknowledgments This work was supported by grants from National Postdoctoral Program for Innovative Talent (Grant No. BX20180296), China Postdoctoral Science Foundation (Grant No. 2018M641720), National Natural Science Foundation of China (Grant No. 21872141, 21633010), Key Research Program of Frontier Sciences, Chinese Academy of Sciences (QYZDYSSW-JSC023), National Key Research and Development Program of China (2017YFA0204800) and Strategic Priority Research Program of Chinese Academy of Sciences (XDB17000000).

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