Semiconductor photocatalysts for water oxidation: current status and challenges.

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Physical ChemistryPhysical Chemistry Chemical Physics Chemical Physics

DOI: 10.1039/C4CP00246F

Semiconductor photocatalysts for water oxidation: current status and challenges Lingling Yang, Han Zhou, Tongxiang Fan* and Di Zhang 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000xc Artificial photosynthesis is a highly-promising strategy to convert solar energy into hydrogen energy for the relief of the global energy crisis. Water oxidation is the bottleneck for its kinetic and energetic complexity in the further enhancement of the overall efficiency of the artificial photosystem. Developing efficient and cost-effective photocatalysts for water oxidation is a growing desire, and semiconductor photocatalysts have recently attracted more attention due to their stability and simplicity. This article reviews the recent advancement of semiconductor photocatalysts with a focus on the relationship between material optimization and water oxidation efficiency. A brief introduction to artificial photosynthesis and water oxidation is firstly given, followed by an explanation of the basic rules and mechanisms of semiconductor particulate photocatalysts for water oxidation as theoretical references for discussions of componential, surface structure, and crystal structure modification. O2-evolving photocatalysts in Zscheme systems are also introduced to demonstrate practical applications of water oxidation photocatalysts in artificial photosystems. The final part proposes some challenges based on the dynamics and energetics of photoholes which are fundamental to the enhancement of water oxidation efficiency, as well as on the simulation of the natural water oxidation that will be a trend in future research. 45

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1. Introduction Sunlight powers all life on the planet through natural photosynthesis and, furthermore, provides the energy for fossil fuels, which are the sediments of ancient biomass over millions of years and a drive force for human technological development. However, this drive is facing a monumental crisis as the rate of its consumption by today’s energy-dependent technology substantially surpasses that of its geological formation. In the light of this consumption rate, the total amount of fossil fuels on the earth will be used up within hundreds of years; thus, changing solar energy into useful forms to replace unsustainable fossil fuels is a growing goal.1 Data2 reveal that over 80% of the total amount of energy used in the world is fossil fuels; solar power, however, accounts for a very small fraction. To address the increasing energy issues, utilization of solar energy on a large scale is of significance, and this depends on its efficient and cost-effective conversion and storage. Actually, nature is intelligent, as it can convert solar energy into stored chemical energy by the biological photosynthesis that is responsible for the maintenance of life on earth and the oxygenated atmosphere. It is the process by which plants, some bacteria, and some protistas use sunlight to transform H2O and CO2 into carbohydrates and release O2. Thehν general formula can be represented as follows3: 6H2O + 6CO2 → C6H12O6 + 6O2. In general, natural photosynthesis is catalyzed by four multi-subunit This journal is © The Royal Society of Chemistry [year]

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membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b6f complex, and F-ATPase. 4 Once light is absorbed by chlorophyll, excited states are triggered, followed by a series of electron-transfer reactions. Water oxidation occurs in PSII composed of a CaMn4 complex which extracts electrons from water; then, electrons go through a series of uphill and downhill redox steps that ultimately lead to the photosystem I (PSI) enabling reduction of hydrogen ions to nicotinamide adenine dinucleotide 2'-phosphate reduced tetrasodium salt (NADPH). The CaMn4 complex goes through a series of S state transitions with increasing oxidizing capacities before oxygen is produced. 4- 6 Since photosynthesis involves many complex steps and each step has energy loss, the overall efficiency of the natural photosynthetic conversion of solar energy into useful forms is quite low. To the best of our knowledge, the maximum conversion efficiency of solar energy to biomass reaches 6%,7 lower than the threshold target of 10% conversion of overall solar energy 8 and about one order of magnitude lower than existing solar-photovoltaic and solarthermal technologies.9 In addition, the area of arable land suitable for plant cultivation is declining globally. In the event of the attainment of sufficient environmental protection measures and the effective utilization of energy, an artificial photosynthetic system should be designed that can roughly mimic the natural photosynthetic system and use sunlight to split water into H2 and O2. In a typical process, antennas harvest light and transfer [journal], [year], [vol], 00–00 | 1

Physical Chemistry Chemical Physics Accepted Manuscript

Published on 24 January 2014. Downloaded by St. Petersburg State University on 27/01/2014 01:17:53.

PERSPECTIVE

Physical Chemistry Chemical Physics

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easily adsorbed on the surface oxygen vacancies13,14 compared to H2 adsorption 15 , 16 and, more undesirably, adsorbs electrons to form O-2 or other species that may inhibit O2 production.8 The above interpretation of the importance and difficulty of water oxidation is illustrated in Fig. 1.

Fig. 1 Schematic representation of the importance and difficulty of water oxidation.

Materials used for water oxidation in the artificial photosynthetic system can be divided into two types: supramolecule and semiconductor. Ru complexes are typical supramolecules for water oxidation, and the turnover frequencies (TOF) reach as high as 1270.17-20 They can also be combined with semiconductors serving as electron acceptors and photosensitizers.20 Despite their high TOF, some supramolecules contain organic ligands that are adequately stable in photovoltaic devices, but unstable in currently prevailing water-splitting devices; this is possibly because likely intermediates with certain oxidizing capacities in water oxidation would degrade all organic ligands, while most semiconductors, especially metal oxides, can resist this degradation. 21 This is a big advantage over supramolecules, so semiconductors are expanding interests in the area of photocatalytic water splitting. There are five categories of semiconductor systems used in water-splitting devices: 22 semiconductor solid state photovoltaic-based systems, semiconductor electrode (liquid junction) systems, semiconductor particle systems, sensitized semiconductor systems(recently popular dye-sensitized solar cells 23 - 25 ), and homogeneous and microheterogeneous systems. Compared to photovoltaic-based and electrode systems, particle systems are dispersions or suspensions of semiconductor photocatalysts and their construction does not require complex devices. Considering stability and simplicity, semiconductor particle photocatalytic systems for water oxidation are quite promising. However, many high quality state-of-the-art reviews are about overall water splitting or H2 production in semiconductor particle photocatalytic systems,10, 26 - 29 focusing on theoretical and experimental discussions about effects of modification methods on light harvesting, charge separation and electron transfer, but few talk about hole dynamics and energetics in determining water oxidation efficiency and correspondent improving strategies. As water oxidation is an important half reaction determining the overall efficiency of artificial photosynthesis, it is significant to review the existing related work and obtain valuable insights for



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excitation energy to reaction centres. Then reaction centres use this excitation energy to transfer photoinduced electrons from donors to acceptors; this generates charge-separated states which remain long enough for the oxidizing and reducing equivalents to migrate to the catalytic sites for water oxidation and fuel production. In the effort to enhance the overall efficiency of artificial photosystems, it has been determined that water oxidation is a bottleneck that presents significant difficulty in the development of effective catalysts for water oxidation.8 As hydrogen energy is sustainable and environmentally-friendly, H2 production on a catalyst is the focus of most investigations. 10 However, O2 evolution should not be ignored, as it provides electrons for the reduction of hydrogen ions to hydrogen gas, i.e., water oxidation is the preliminary reaction required for H2 production. However, it is much more difficult kinetically and energetically. The main reason for this can be summarized as follows. From the formula 2H2O → 4H+ + O2 + 4e-, we can see that generating one O2 molecule requires extraction of four electrons and four protons from two water molecules. Since photons come one-by-one, while the two water molecules must be oxidized in one step, the water oxidation catalyst must somehow be able to accumulate and store four oxidizing electron-holes close together.2 This accumulating and storing capacity refers to kinetic and thermodynamic complexity, meaning multi-electron and multiproton control, and a high capacity (usually high potentials) to store. Nature itself has evolved an advanced photosynthetic system that successfully deals with the four electron transfer process and the difficult formation of the oxygen-oxygen bond in the O2 molecule. The water oxidation catalyst usually changes its charge state (S1 to S4) with increasingly oxidizing capacities to satisfy the needs of sufficiently high potentials.4 More exactly, the charge state of the redox-active metal Mn in the CaMn4 complex varies in each S state (e.g., Mn oxidation states for S1 of 2Mn3+/2Mn4+). While in an artificial photosystem, for example, in a semiconductor photosystem, the top of the valence band of the semiconductor photocatalyst must be deep enough to satisfy the thermodynamic needs, at least more positive than the oxidizing potential of O2/H2O (1.23V vs. NHE, pH=0). Simultaneously, it should not be too deep or it may cause energy waste or wide band gap. Through some modification methods, the valence band maximum will be modified to the more negative position, leading to the narrow band gap; however, it may also cause a lack of thermodynamic drive for photoholes. Therefore, achieving a balance between the efficient use of energy and sufficient reaction drive is difficult. In addition, the O2 formation goes through a much more complicated process compared to H2 in terms of electron transfer and the transformation of a series of intermediates. 11 It is reported that O2 evolution occurs on the seconds time scale (0.27s), much slower than H2 production which occurs on the hundreds of microseconds time scale.8 Furthermore, electron-hole recombination occurs very rapidly with large losses occurring on the submillisecond time scale, leading to low efficiencies for photocatalytic water oxidation. 12 This limits the overall rate of water splitting. Furthermore, O2 evolution is controlled by the interfacial reaction,8 so that the semiconductor should provide a favourable reaction site for the O2 formation process.11 If this were not the case, O2 would be

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future researches to address the complexity of water oxidation and to further enhance its efficiency. Herein, we will mainly review the recent developments and challenges of semiconductor particle photocatalysts for water oxidation that is the half reaction in water splitting sharing the same basic principles with water reduction, meanwhile, differing in many aspects such as the half reaction process and requirements for certain materials. The basic rules and mechanisms of semiconductor particle photocatalysts for water oxidation are firstly introduced, followed by an in-depth discussion of the relationship between material optimization and water oxidation efficiency from the perspectives of composition, surface structure, and crystal structure optimization. In addition, some O2-evolving photocatalysts in a Z-scheme system are also introduced. Finally, conclusions about this review are presented and some challenges are proposed.

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Fig. 2 Schematic representation of the basic processes of photocatalytic water splitting in a semiconductor particle system.

2. Basic rules and mechanisms of semiconductor particle photocatalysts for water oxidation

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The history of semiconductor photocatalysts extends back forty years which was initiated by Fujishima and Honda’s study30 on a TiO2 electrode system. Among the five categories of semiconductor systems mentioned above, particle systems have become more and more popular, as they offer some substantial advantages such as high surface areas beneficial to photocatalysis and simplicity for device construction to avoid harsh outer conditions over electrode systems. There are three important steps in the process of photocatalytic water splitting in a particle system as depicted in Fig.2: electron-hole generation, electronhole separation, and electron-hole transfer. Designing an efficient photocatalyst should consider all of these three elements. For water splitting, the bottom level of the conduction band has to be more negative than the reduction potential of H+/H2 (0 vs. NHE (normal hydrogen electrode)), and the top level of the valence band has to be more positive than the oxidation potential of O2/H2O (1.23V vs. NHE). Fig.3 illustrates the relationship between the band structure of semiconductors and the redox potentials of water splitting. 31 In many situations, sacrificial reagents are needed to evaluate the photocatalytic activity for water splitting as it is a kinetically tough reaction in which excited electrons and holes tend to recombine at the surface, crystal boundaries, and defects of a photocatalyst. For O2 evolution, oxidizing reagents (electron acceptors or electron scavengers), such as Ag+ and Fe3+, are added. They are then consumed by the photoexcited electrons and enrich the holes, resulting in the enhancement of the O2 evolution reaction. Although the reaction mechanisms of natural water oxidation catalysts in terms of electron transfer are introduced in the preceding section, the O2 evolution occurring on a semiconductor photocatalyst is quite different. There are some studies on the water oxidation reaction mechanisms of TiO2. 32 A conventional electron-transfer type mechanism has long established33,34 that the reaction is initiated by electron-transfer-type oxidation of either OH- (or H2O) in solution or Ti-OH at the surface by photogenerated holes. OH- (or H2O) + h+→ ·OH (or ·OH + H+ ) (1) Ti-OHs + h+ → [Ti-OH]+s (2)

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Fig. 3 The relationship between the band structure of semiconductors and the redox potentials of water splitting. Adopted from ref.31.

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However, this conventional mechanism was challenged by Nakamura’s11,35 investigations on the surface state of TiO2 in the process of photocatalysis by in situ IR technology. He proposed a Lewis acid-base mechanism in which the oxygen photoevolution reaction was initiated by a nucleophilic attack of an H2O molecule (Lewis base) to a surface-trapped hole (STH, Lewis acid), accompanied by bond-breaking. Further details of this mechanism are provided in Fig.4a, compared with the conventional Ti-OH oxidation mechanism in Fig.4b. The Lewis acid-base mechanism can be also employed in the O2 evolution on a TaON photocatalyst. 36 Actually, the photocatalytic O2 evolution occurs on a thin Ta-oxide overlayer, and a certain surface peroxo species, which may be tentatively assigned to adsorb HOOH, is formed as an intermediate of the O2 photoevolution reaction. Nakamura’s work provides us with new insights into the photocatalytic O2 evolution mechanism from a molecular level. By using in-situ FTIR absorption and photoluminescence measurements, the primary intermediates are detected as evidence to propose the most plausible dynamic process of O2 photoevolution. This is also the rationale behind the advancement of the Lewis acid-base mechanism compared to the conventional one. The Lewis acid-base mechanism seems more reasonable than the electron-transfer type mechanism, it focuses on the dynamic process of O2 evolution and does not dictate the physical origin of the high overpotential of the oxygen evolution reaction. 37,38 To determine whether this overpotential is due to intrinsic difficulties in the kinetics of the multistep reaction process. 39 Li et al.32


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conducted a detailed investigation of the microscopic mechanisms of the oxygen evolution reaction (OER) on differently-structured anatase surfaces in aqueous surroundings by combining first-principles density functional theory (DFT) calculations and a parallel periodic continuum solvation model. They showed that OER involves the sequential removal of protons from surface oxidative species. The first proton removal, with a high energy cost, leads to the high overpotential, followed by an adsorbed OH state, where the stoichiometric TiO2 surface is oxidized due to the electron flow from the surface to the adsorbed OH group. We demonstrate the process in the form of molecular reaction scheme in Fig.4c instead of the original atomic structure model in their work. They also ruled out the possibility that the presence of d-states helped to stabilize the adsorbed OH via covalent bonding. The covalent bonding identified to be achieved by the p-states of the intermediate oxidative species, and the dstates are critical to facilitate the oxygen evolution. In conclusion, OER is not sensitive to the local surface structure of anatase, but is strongly influenced by the energy position of the maximum of VB. Both the addition of dopants and the change in phase morphology may strongly influence OER activity. By co-doping with Nb + N or Mo + C, the surface OH state can be stabilized due to the extra occupied states above VB; this helps to reduce the overpotential dramatically. In general, apart from experimental studies, first-principles DFT calculations and the parallel periodic continuum solvation model used in their studies are highly powerful tools to deeply explore the complex kinetics and thermodynamics of photocatalytic O2 evolution. With the development of physical chemistry, the trend of future research is to apply cluster study combined with coordination chemistry.38,40 and DFT studies especially on gas adsorption and protonation on the semiconductor surface will keep vigorous.41

3. Semiconductor photocatalysts for water oxidation 35

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Kinetic and thermodynamic requirements must be taken into account when evaluating the efficiency of O2-evolving catalysts. Catalysts need to have turnover frequencies and sizes sufficient for keeping up the solar flux at ground level (1000 Wm2, air mass 1.5) to avoid the waste of incident photons. In addition, they have to operate close to the thermodynamic potential of the redox reaction without the need for excessive driving potentials. 42 Considering the above requirements, the proper selection of materials is crucial for obtaining componential and structural optimization. For a certain material, morphology control is necessary, as it affects both the light harvesting and the gas evolution process.

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Fig. 4 Three different mechanisms of O2 evolution on TiO2. (a)(b)Modified from figure in ref.11. (c)Adapted from ref.32.

3.1 Componential optimization It is well-known that the composition of a material has a major impact on its properties. Through componential optimization, the band structure of semiconductors can be adjusted to our desired degree, and a composite nanostructure can also be obtained to exhibit its synergistic effect. It should be noted that water oxidation photocatalysts containing elements analogous to natural photocatalysis are very promising as natural photosynthesis itself has evolved an advanced system for water oxidation.

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3.1.1 Band structure modification The band structure of a semiconductor plays an important role in determining the absorption band of light and redox potentials of water splitting. By making use of band-structure tailoring, we can develop more visible-light-driven photocatalysts and novel photocatalysts through optimized elemental combination. 3.1.1 .1 Doping Element doping is a general strategy to extend the absorption band of wide band photocatalysts to the visible light region. Some wide band gap photocatalysts, such as TiO2, 43 - 45 and SrTiO3, 46 - 49 are modified by this method to exhibit greater photoactivity under visible light irradiation. Using Ni-doped TiO2 as an example,45 electron donor levels are formed by 3d orbitals of Ni2+ in the forbidden band of TiO2. Upon irradiation, photoexcited electrons transfer from the donor levels to the conduction band of TiO2, thus creating new absorption bands in the visible light region. However, doping one element may produce some undesirable results, as the doped element exists in a high charge state and some deficiencies occur. So, co-doping is a reasonable strategy to suppress some defects. TiO2 co-doped with Sb/Cr43 or Sb/Rh44 shows higher activity than that only doped with Cr and Rh, respectively. Though doping is a route for enhancing the photoactivity of wide band gap photocatalysts under visible light, it sometimes causes low efficiency of water oxidation. The photocatalytic O2 evolution activity of SrTiO3:Ni47 decreases dramatically under both UV light and visible light irradiation compared to SrTiO3; this is due to the difficulty in the hole transportation in the discrete donor level of Ni3+ and a lack of active sites for O2 evolution on the surface of the photocatalyst, as is also observed in SrTiO3 doped with other metals.43 Tang et al.12 conducted a deeper investigation into the lack of water oxidation on N-doped nanocrystalline TiO2 from the perspective



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Table 1 Some common doping methods in TiO2 and SrTiO3 Doping element/wt%

Band gap/eV

Light source/nm

TiO2

None Ni(1.0) Ni(1.0)/Ta(2.0) Ni(1.0)/Nb(2.0) Cr(2.3) Sb(1.25)/Cr(0.5) Sb(2.4)/Cr(1.0) Sb(1.25)/Ni(0.5) Sb(1.25)/Cu(0.5) Rh(1.0)

3.0 2.6 2.6 2.6 2.2 2.2 2.6 2.6

>300 >300 >300 >300 >300 >420 >300 >420 >420

2.0

Rh(1.0)

2.0

Rh(1.0)/Sb(1.0) Rh(1.0)/Sb(2.0)

2.2 2.2

Rh(1.0)/Ta(1.0) Rh(1.0)/Ta(2.0) Rh(1.0)/Nb(1.0) Rh(1.0)/Nb(2.0) None Ni(1.0)

2.2 2.2 2.2 2.2 3.2 1.6

Ni(1.0)/Ta(2.0)

1.7

Ni(1.0)/Ta(2.0)d

2.8

Sb(2.5)/Cr(2)

2.4

Mn(0.5) Ru(0.5) Rh(0.5) Ir(0.5)

2.7 1.9 1.7 2.3

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SrTiO3

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H2 a

O2 b

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>300/>440

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>300 >440 >440 >300 >440 >440 >440 >440 >440 >300 >300 >420 >300 >420 >300 >420 >300 >420 >440 >440 >440 >440

3.8 1.1 2.7 19.5 11.1 0 4.7 0 0.9 35 0.3 0 1.3 0 2.4 0.5 0.9 0.9 2.7 3.9 0 0.4

44 44 44 44 44 44 44 44 44 45 45 45 45 45 45 45 43 43 46 46 46 46

0.06 0.65 0 0.08

74 7.3 0.7 5.3 1.4 3.3 2.4 102 78 0.2 1.7 17.2 8.6

From aqueous methanol solution. b From 0.05 mol/L aqueous silver nitrate solution. c H2 reduced. d Treated with H2 at 470K for 2h.

of the kinetics and thermodynamics of photoholes. They ruled out an electron-hole recombination between the charge carriers trapped at doping-induced states and the reduced oxidative power of the photoholes generated under visible light contributed to the low efficiency of water oxidation. Table 1 lists some common doping methods in TiO2 and SrTiO3. As is seen in the table, doping generally brings about a decrease in O2 evolution. In addition to the above reasons, it is possible that unsuitable doping, e.g., the dopants are not deep in the subsurface, changes the surface geometry, which even influences the hydrophilicity of photocatalysts. 3.1.1.2 Valence band modification For the purpose of enhancing the efficiency of water oxidation, valence band modification is of significance as it can both narrow the band gap and facilitate the mobility of photoholes. Constructing a more negative valence band than O 2p orbitals is a way to narrow the band gap. N and S are the candidate incorporated elements, as their potential energies of p orbitals are higher than that of O 2p orbitals. The valence band of Ta2O5 predominantly consists of O 2p orbitals whose potential energy levels are located at a deep position of about 3 V vs. NHE.;50,51 whereas, Ta3N552 and TaON,53 whose valence bands contain N 2p orbitals, have narrower band gaps than that of Ta2O5. The same

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case is also seen in Sm2Ti2O7, with a wide band gap of 3.6 eV, and Sm2Ti2S2O5 with a much narrower band gap of 2 eV due to the incorporation of S2-.54 In some oxide semiconductor photocatalysts, the valence band is composed of d or s orbitals of the transition metal and O 2p orbitals, resulting in valence band hybridization and making the valence band more dispersed. This favors the mobility of photoexcited holes in the valence band and, thus, promotes the O2 evolution activity. For example, the Zn 3d and O 2p orbitals are hybridized to construct the valence band of Zn3V2O8, showing a higher activity than that of Mg3V2O8 in which no such hybridization effect occurs; this is because the Mg 2p orbitals are not involved in the valence band. Ni3V2O8, however, showed almost no activity as the split Ni 3d orbitals inserted between the O 2p and the V 3d orbitals. 55 Analogous valence band hybridization can be seen in Yi’s report 56 on Ag3PO4 whose valence band was composed of O 2p and Ag 4d orbitals, corresponding to the p-d orbital in Fig.5. The addition of phosphorus into Ag2O creates Ag3PO4 with a band gap of 2.36eV. It has higher photocatalytic O2 evolution activity than that of BiVO4 and WO3 under the same experimental conditions. Surprisingly, it has a considerably high quantum yield (QY) of almost 90% under visible light, far beyond the so far reported best QY of 5.9% for (GaN)x(ZnO)1-x solid solution



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photocatalysts. 57 These elements are similar to those of natural photosynthesis. Apart from p-d hybridization, p-s hybridization also exists. A typical example is BiVO4, where coupling between Bi 6s and O 2p produces antibonding Bi 6s states toward the top of the valence band. The presence of antibonding cation-anion electronic states at lower binding energy is beneficial for hole formation and mobility. 58 The following Table 2 lists typical examples of the valence band hybridization effect in semiconductor photocatalysts. The above studies provide us with the idea that the valance band hybridization effect (e.g., O 2p and Zn 3d) can be utilized by incorporating metal cations (e.g., Zn2+) with the d10 configuration in designing oxide semiconductors with improved photocatalytic activities.

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reveals that it has a wultzite-type structure with the space group of P63mc confirming that oxygen substitutes for nitrogen in the crystal structure, as shown in Fig.6b and c.70 This is responsible for its desirable optical properties. However, the addition of ZnO to GaN causes the zinc- and/or oxygen-related defects that function as recombination centers leading to low quantum yield efficiency (~ 2.5%).69 This can be enhanced to 5.9% by a postcalcination treatment through reduction of the density of defects.57 Following the solid solution (Ga1-xZnx)(N1-xOx), Lee et al. 71 reported another solid solution of zinc oxide and germanium nitride (Zn1+xGe)(N2Ox) (x=0.44), with the band gap of 2.7eV. With RuO2 loading, it exhibits a steady rate of 54.3μmol/h for H2 and 27.5 μmol/h for O2 under UV light and 14.2μ mol/h for H2 and 7.4μmol/h for O2 under visible light irradiation. The large valance band dispersion resulting from the energy difference between O 2p and N 2p orbitals and p-d repulsion between Zn 3d and N 2p + O 2p electrons in the upper valance band contributes to the narrowing of the band gap. It informs us that constructing an oxynitride solid solution photocatalyst that contains d10 configuration elements is a pathway to develop a visible-lightdriven solid solution photocatalyst.

Fig. 5 P-d hybridization effect in the valence band of (a) (b) AgPO4 and (c) (d) Zn3V2O8. Reprinted from ref. 55 and 56. Table 2 Valence band hybridization effect in semiconductor photocatalysts Valence band Photocatalyst hybridization (HOMO coupling) BiVO4 Bi 6s + O 2p Bi2MoO6 Bi 6s + O 2p Bi2Mo2O9 Bi 6s + O 2p Bi2Mo3O12 Bi 6s + O 2p PbWO4 Pb 6s + O 2p Zn3V2O8 Zn 3d + O 2p Ag3VO4 Ag 4d + O 2p Ag3PO4 Ag 4d + O 2p AgTaO3 Ag 4d + O 2p AgNbO3 Ag 4d + O 2p

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Ref. 58,59 60 60 60 61 55 62 56 63 63

3.1.1.3 Solid solution photocatalysts GaN:ZnO (also represented as [Ga1-xZnx][N1-xOx]) has been focused on in much research in recent years due to its potential application in overall water splitting under visible light irradiation. 64-69 Despite the wide band gap of GaN(3.4eV) and ZnO(3.2eV), this solid solution has a narrower band gap lower than both of its constituents, making it a new d10- (oxy)nitride. The visible light absorption of (Ga1-xZnx)(N1-xOx) is due to its band structure and crystal structure. DFT calculations show that the top of the valence band of this solid solution consists of N 2p followed by Zn 3d orbitals, providing p-d repulsion for the valence band maximum without affecting the conduction band minimum,64 which leads to narrowing the band gap, as schematically depicted in Fig. 6a. Crystal structure analysis

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Fig. 6 (a) Schematical illustration of the band structure of (Ga1-xZnx)(N1xOx). (b) Crystal structure of (Ga0.87)(Zn0.13)(N0.83 O0.16) with refined crystallographic parameters and (Ga,Zn)(N,O)4 tetrahedron, and (b) equicontour surface of nuclear density distribution at 0.1 fm/Å3. Reprinted from ref.70.

3.1.2 Composite nanostructure The photocatalytic system, especially the O2 formation process, is a complex one. If we design or fabricate an advanced material based on a composite structure in which every composition contributes its advantage to produce a synergistic effect, it will more or less optimize the process that fulfills the requirements for




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achieving oxygen generation, such as charge separation and intermediates transition. The following section reviews some common composite nanostructures for O2 evolution, such as semiconductor nanostructures loaded with cocatalysts (metal or metal oxide), semiconductor nanojunctions, semiconductorgraphene architectures, and recently-popular biologicallytemplated photocatalytic nanostructures. Metals deposited on semiconductors can promote the charge transfer process, as they act as a reservoir for photogenerated electrons or holes. For example, the photocatalytic H2 generation on Pt, Pd, or Au-deposited TiO2 is tremendously enhanced.10 For water oxidation photocatalysts, loading metal oxides, such as NiO,72 RuO2,65,71 and IrO2,54,73 is an effective method. For semiconductor nanojunction, CdS/TiO2 is a very popular nanostructure, in which photoelectrons in CdS particles can transfer to TiO2 quickly with photoholes left in CdS, causing electron-hole separation and enhancing photocatalytic activity. 7476 Many semiconductor heterojunctions have been constructed to improve H2 production,10 except for Jang’s report 77 on Fe2O3 nanoparticles intercalated into the interlayers of HTiNbO5 and HTiTaO5. This guest-host semiconductor combination system leads to a high photocatalytic O2 evolution instead of H2 production under visible light irradiation. This is because the conduction band bottom of Fe2O3 is located at a less negative position than the reduction potential of H+ to H2, as shown in Fig.7; whereas, the valence band positions are more positive than − the oxidation potential of OH to O2. It should be noted that 78 Wu’s earlier report on the same guest-host structure that had a high photocatalytic H2 activity contained tungsten instead of titanium included in the materials studied here. This indicates that a one-element change may bring about a significant performance difference. So, more investigations, particularly into the energy diagrams of different componential combinations in a multicomponent system, should be conducted so as to predict the possible enhancement of the O2 evolution.

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Fig. 7 An electronic band structure and generation of electrons and holes; (a) HTiNb(Ta)O5 has proper band positions for both water reduction and oxidation, but absorbs only UV light due to large-band gap energy. (b) Fe2O3-HTiNb(Ta)O5 absorbs visible light, but the conduction band position of Fe2O3 is not negative enough to reduce water. Reprinted from ref.77.

Graphene has recently been employed in many fields due to its excellent physicochemical properties. 79 , 80 It is a monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice81 which gives it unusual electronic properties, such as high electron transportation.82,83 Therefore, it is often combined with semiconductor photocatalysts to form a 3D hierarchical architecture in the application of H2 generation, pollutants degradation, and water disinfection.84-86 However, research on its application to O2 evolution is quite scant. Recently, Gunjakar et al.87 synthesized a highly-efficient photocatalyst for visible light

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water oxidation via an electrostatically-derived self-assembly of Zn-Cr-LDH 2D nanoplates with graphene 2D nanosheets to form a highly-porous stacked structure. It reaches a rate of 1.20 mmol h-1g-1 for O2 generation with an unusually high quantum efficiency of 61% at λ=410 nm. This high efficiency is due to the strong electronic coupling between the 2D nanostructured components and the house-of-cards-type stacking of the layered crystallites, respectively, which leads to strong visible light absorption, efficient electron-hole separation, and high surface area. Apart from combination of the 2D structures, Guo et al.88 synthesized WO3 particles on the surface of graphene (GR) sheets by sonochemical reaction of phosphotungstic acid in the presence of grapheme oxide (GO), followed by calcination under nitrogen. Compared to Ng’s report about synthesizing WO3/GR composite through photoassisted in situ reduction of WO3-GO suspension causing aggregation of WO3 particles during the physical mixing,89 more uniform and smaller WO3 particles are obtained on graphene sheets by Guo’s method. The amount of evolved O2 from water for the WO3/GR composite with 40 wt% graphene inside is twice and 1.8 times as much as that for pure WO3 and mixed-WO3/GR, respectively. This higher activity is also attributable to the chemical bonding between WO3 particles and GR sheets. Gunjakar’s and Guo’s work are both about twocomponential semiconductor/graphene composite structure, while Hou et al. 90 developed a Ag3PO4/Ag/AgBr/RGO hybrid composite with high visible-light photocatalytic O2 production activity by a photo-assisted deposition-precipitation reaction, followed by a hydrothermal treatment. The enhancement of O2 evolution is assigned to the conduction band depletion and valence band down shift of Ag3PO4 caused by the addition of Ag/AgBr; RGO also supports this effect in a synergistic manner through delocalization of the transferred charge. Comparing the above three studies, we conclude that the key points in the O2 production enhancement of graphene-based composites are different according to different componential and structural combination. For a two-componential composite, the strong bonding or coupling between the semiconductor and the graphene is dominant; this will be obtained by the more uniform and smaller particles loading on the graphene sheets or the proper staking-type between two layered structures. For a multicomponential composite, the optimized arrangement of these components, e.g., the optimization of RGO loading, is important as it influences the band structure coupling and, thus, the synergistic effect of these components. Hence, future efforts should be made to investigate various complex hybrid structures and relevant modification methods. Using biomass as templates to construct functional photocatalytic architectures is now a research hotpot, 91 as biomass has evolved an advanced structure (e.g., hierarchical porous structure) that may facilitate light harvesting and has some intrinsic functional groups on the surface that are advantageous for assembling components into a functional material. Nam et al. 92 co-assembled iridium oxide hydrosol clusters (IrO2) as catalyst and zinc porphyrins (ZnDPEG) as photosensitizer by using a genetically modified M13 virus as a mediator and porous polymer microgels as an immobilization matrix to improve the structural durability of the assembled nanostructures and to allow the materials to be recycled. This nanostructure has a high


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photocatalytic O2 evolution activity, possibly due to a synergistic result of charge migration between photosensitizers and the close arrangement of the photosensitizers with IrO2. Fig.8 illustrates the assembly method, micrographs of the nanostructure, and the oxygen evolution performance. The use of the M13 virus represents one advantage of biomaterials in facilitating the assembly of multi-component photocatalysts; Yin’s report 93 on C-doped BiVO4 photocatalyst with hierarchical structures templated from butterfly wings also indicates that the synergetic effect of the unique morphology and composition control can lead to high photocatalytic performance. The quasi-honeycomb structure inherited from the Papilio paris butterfly wings promotes light harvesting and in situ carbon doping after calcination improves the visible light absorption, as well as enhances the separation of photogenerated electron–hole pairs. Despite the structural and functional superiority of biological templates, appropriate removal of them afterwards should be subject of concern. There are usually two removal methods: calcination and chemical etching.91 For calcination, carbon will be doped into the host material; this may produce undesirable results discussed in the aforementioned section or have the advantage presented in Yin’s report.93 This is dependent on the calcination temperature, time and atmosphere, and it has to be further investigated.

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attractive ones and have been used as electrocatalysts coated on various types of metal anodes over the past decades. 95 The application of their particle forms can be dated back to the year 1980 when Harriman et al. 96 reported photocatalytic water oxidation using micrometre-sized Co3O4 or Mn2O3 particles driven by a visible light sensitization system; however, the TOF was between 0.035and 0.055s-1. To further enhance the TOF, constructing nanostructured Co and Mn oxides to increase the activity per-site is imperative. Jiao et al.42,97 for the first time, reported efficient oxygen evolution at nanostructured Co3O4 or Mn oxide clusters in mesoporous silica (SBA-15) in aqueous solution under mild temperature and pH conditions using Ru3+(bpy)3/persulfate as the visible light sensitizer system. The TOF can reach as high as 1140s-1 and 3330s-1 per Co3O4 nanocluster and per Mn oxide nanocluster, respectively. The high TOF is attributable to the integrity of the metal oxide nanostructure and the silica support. The nanoclusters possess a very high surface area, and their sharply curved surfaces provide more active reactive sites for the metal centers; whereas, the silica environment affords a stable dispersion for the metal oxide clusters and, more importantly, its high surface area provides an additional benefit of maintaining the rate of the incident solar photons. Fig.9 shows TEM images of Co3O4 clusters on silica support.

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Fig. 8 Regeneration of catalytic materials. (a) Injection part of the glass capillary device for fabrication of the virus-loaded microgels. (b) Monodisperse virus-loaded microgels prepared from the microfluidic device. Scale bar, 100 mm. Scanning electron micrograph (c) and elemental map (d) of the freeze-dried microgel containing IrO2-ZnDPEG nanostructures. Scale bar, 20 mm. Iridium is represented by blue dots. (e) Oxygen evolution profiles from IrO2-ZnDPEG microgels with r=35 (black) and 109 (red). The numbers above the curves indicate TON values. (f) Recycling efficiency of IrO2-ZnDPEG microgels. Reprinted from ref.92.

3.1.3 Earth-abundant transition metal oxide catalysts Considering the turnover frequencies and densities sufficient for keeping up the solar flux, the stability of materials for bearing harsh reaction conditions, and the availability of sources of materials, it is necessary to explore oxides of the earth-abundant first-row transition metals, as inspired by nature’s CaMn4 cluster of PSII.94 Among these metals, Co and Mn oxides are the most

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Fig. 9 TEM images of (a) SBA-15/Co3O4 4% loading. (b) SBA-15/Co3O4 8% loading. (c) Co3O4 nanocluster (8% sample) after removal of the SBA15 silica material using aqueous NaOH as etching reagent. The inset in (b) shows the SAED pattern. (d) Schematic illustration of Co3O4 clusters. Reprinted from ref.95 and 42.

Yusuf98 found that the key factor in determining water oxidation activity of Co3O4-based photocatalysts is not the type of the support but the function it plays as a medium to physically separate Co3O4 nanoclusters from aggregation. In order to obtain highly dispersed cobalt photocatalysts, Ahn 99 prepared small domains of cobalt on silica support. This investigation presents a molecular method to the fabrication of highly dispersed cobalt centers on silica and provides insights into parallel studies on




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other first-transition metals on similar supports.Apart from silicaenhanced Co3O4 clusters, Co3O4 nanocrystals grown on reduced graphene oxide100 or graphitic carbon nitride101 also exhibit highperformance for oxygen evolution reaction (OER). The high activity of these hybrid catalysts is due to the synergetic chemical coupling effects between Co3O4 and graphene or carbon nitride that reveals a new method for developing advanced inorganicorganic photocatalysts for solar energy conversion. 3.1.4 Carbon-based semiconductor-like photocatalysts for water oxidation Carbon materials especially graphite oxide and graphitic carbon nitride are gaining interests because of their 2D configuration and controllable properties. By proper modification, they can behave like semiconductors, thus, have wide applications in photocatalysis. For graphite oxide, a big advantage over conventional semiconductor photocatalysts is its high surface area. In addition, the attached oxygen-containing functional groups on the sheets make graphite oxide more dispersed in water and conduction and valence bands suitable for water splitting. 102 Graphite oxide exhibits p-type conductivity due to oxygen’s higher electronegativity than that of carbon atoms and this, however, hinders hole transfer for water oxidation.103 To enhance its O2 evolution activity, changing its band structure to exhibit ntype conductivity is an effective method. When treated with ammonia gas, graphite oxide was doped with N and its nonirradiative epoxy and carboxyl sites was replaced by nitrogen functionalities, causing N 2p and O 2p orbitals mixing for valence band and enhancing the O2 evolution activity.103 In the light of few reports on photocatalytic O2 evolution of modified graphite oxide, future challenges lie in the modification methods to tune its electronic structure such as doping elements to form electron-donating functional groups for the improvement of hole transfer. For graphitic carbon nitride with polymeric or layered structures, 104 great interests focus on studying the relationship between the specific composition CxNyHz and its structure, thus, developing applications with functional properties in the area of photocatalysis. Among various compositions, graphitic C3N4 (gC3N4) is considered to be a highly-efficient photocatalyst for H2 generation, 105 - 109 whereas it has low O2 evolution activity and produces N2 when irradiated because of its self-decomposition by photoholes. Inspired by the natural water oxidation complex with a metal-componential complicated structure, loading metals or metal oxides like RuO2 on g-C3N4 sheets is feasible as metal components not only stabilize the catalyst but also serve as active sites for water oxidation. 110 To further investigate g-C3N4 in water oxidation or overall water splitting, more investigations have to be conducted about the function of the loaded cocatalysts.

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3.2 Surface structure and morphology control

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As water oxidation occurs on the surface of the semiconductor photocatalyst, the surface nanostructure plays an important role in determining photocatalytic activity and efficiency. In general, different surface structures have different electronic structures and reactive sites that affect hole transfer and surface species adsorption. For example, TiO2 has different photocatalytic reactivities on (110) and (100) surfaces because of the reactive site and the energy level difference,11 as shown in Fig.10. We can obtain the desired exposed crystal facets, i.e., the expected surface structure via crystal facet control. Among the various

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surface structures, nanosteps are gaining attention for their unique properties.

Fig. 10 (a) Schematic crystal models for the (110) and (100) terraces of n-TiO2 (rutile) in contact with an aqueous electrolyte after consideration of surface reconstruction, together with some surface electronic processes. Black sphere = Ti atom, white sphere = O atom, small gray sphere = adsorbed H+ ion or H atom, and large (dark) gray sphere = O atom of adsorbed H2O molecule; h+ = hole. (b) Schematic energy level diagrams for the atomically smooth (100) and (110) surfaces of n-TiO2 (rutile) at pH 0. Reprinted from ref.11.

3.2.1 Crystal facet control In recent years, crystal facet engineering of semiconductor photocatalysts has become a popular field,111 as it is an important strategy for fine-tuning different facets to obtain different physicochemical properties and, thus, optimizing the reactivity of photocatalysts. Accordingly, semiconductors with exposed highindex facets are more reactive due to their high surface energy. However, these highly reactive facets are unstable during the crystal growth process. In order to solve this problem,


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employment of structure-directing agents, such as polymers and inorganic ions, are necessary to control the growth rates of crystal facets. For example, WO3 can be tuned to form a singlecrystalline octahedral bond with {111} facets112 quasi-cubic-like crystal with a nearly equal percentage of {002}, {200} and {020} facets, and a rectangular sheet-like crystal with predominant {002} facets with the addition of acid ingredients or hydrolysis of acid precursors. 113 Wang et al. 114 synthesized monoclinic BiVO4 crystals with preferentially exposed (040) facets via a hydrothermal method by using a trace amount of TiCl3 as the directing agent. They found that the photocatalytic O2 evolution activity of the BiVO4 samples was increased with the increase of the (040)/(110) intensity ratio, indicating that the (040) facet plays a vital role in photocatalytic water oxidation. Despite the higher photoactivity of the (040) plane, the TiCl3 used is poisonous and environmentally-unfriendly. In addition, the removal of directing agents afterwards is a complicated process, and this process has an undesired impact on the adsorbed facet, making it lose many of its active reaction sites. Therefore, achieving crystal facet control without directing agents is of great importance. Xi et al. 115 synthesized m-BiVO4 nanoplates with exposed {001} facets via a facile hydrothermal route without the addition of any template or organic surfactant. Under visible light irradiation, these m-BiVO4 nanoplates showed great photocatalytic activity for O2 evolution. 3.2.2 Surface nanostep Surface steps, specifically step edges, are regarded as the most common intrinsic surface defects and strongly influence the surface chemistry of metal oxides. The surface step structure of metal oxides, especially TiO2, has long been investigated in depth.116 On account of the scarce information on anatase TiO2, Gong et al.117 conducted detailed research on the (101) surface of anatase TiO2. The structure and morphology of anatase TiO2 (101) is shown in Fig.11. It was found that the edge of a monoatomicheight step can be considered as a very narrow slice of a surface. So, it is within our expectation, and first-principles calculations confirm, that the step formation energy of differently-oriented step edges (A-E in Fig.11[c]) scales with the surface energy of the extended facet. Furthermore, the reactivity of a step edge (as tested by the adsorption of water) follows that of the corresponding flat surface. Therefore, the steps on extended surfaces are informative for their surface chemistry, and these are very useful for understanding the reactivity of photocatalysts with certain exposed crystal facets.

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In experimental studies, the scale of steps is often extended to nanometers. This can be seen in La-doped NiO/NaTaO3 photocatalyst with the surface steps of 3-15nm.72 Further investigations showed that the enhanced photocatalytic activity for H2 and O2 evolution was mainly due to the effective separation of reaction sites by these nanosteps in Fig.12a. H2 evolution occurred on ultrafine NiO particles well-dispersed on the edges of the steps; whereas, O2 evolution occurred in the grooves of the steps and, thus, inhibited the back reaction for H2O formation. In addition, based on our general knowledge about light refection and absorption, we propose that this step-like structure enhances light harvesting, especially through multireflection and absorption by the steps, compared to the light path on an atomic smooth surface, as shown in Fig.12b. This may also contribute to the increased oxygen production.

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Fig. 12 (a) Mechanism of highly-efficient photocatalytic water splitting over NiO/NaTaO3:La photocatalysts. (b) Schematic representation of light path through surface steps compared to that on an atomic smooth surface. Reprinted from ref.72. 75

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Fig. 11 (a) Ball-and-stick model of anatase TiO2 (101). Inset: details of the structure. O atoms are red, Ti atoms are grey. This notation is used in all of the figures. (b) STM image (Vsample=+1.5V, Itunnel =1 nÅ, 250×250 Å2) of anatase TiO2(101), showing preferential orientations of monoatomic steps. One isolated trapezoidal island is highlighted by a red circle. (c) Schematic plot of possible island shapes and orientations on anatase TiO2 (101). Five different types of steps are identified and labeled as A–E. Reprinted from ref.117.

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3.3 Crystal structure optimization The crystal structure of a photocatalyst affects its charge separation and mobility and, thus, its photocatalytic activity. The distortion of crystal structure, especially the distortion of the MO6 octahedra, is the important affecting factor. The dipole moment generated by the distortion in crystal structure might improve the separation of electron-hole pairs. 118 For charge mobility, the closer the bond angle of O-M-O is to 180°, the more the excitation energy is delocalized, indicating that the photogenerated electron-hole pairs can move more easily. 119 Furthermore, as the water oxidation photocatalyst mimics the natural one, if the crystal structure contains units that are similar to the natural complex, it will be beneficial to photocatalysis. 3.3.1 Corner-sharing structure of MO6 octahedra WO3, with the corner-sharing structure of WO6 octahedra, is well-known to be a high-performance water oxidation photocatalyst. One reason for this lies in the effective connection of WO6 octahedra that provides a tunnel for carriers to transfer. The similar MO6 octahedra unit is also seen in BiWO6 120 and Bi2MoO6.121 To verify the advantage of this MO6 octahedra unit, Shimodaira et al.60 compared Bi2MoO6 with Bi2Mo2O9 and




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Bi2Mo3O12, in which a certain degree of isolated MoO4 tetrahedra existed. They found that Bi2MoO6 showed a higher O2 evolution activity of 55 μmol/h, compared to Bi2Mo3O12 with O2 evolution activity of 7.6 μmol/h and Bi2Mo2O9 with negligible photocatalytic O2 evolution activity of 1.8μmol/h. The low activity of the latter two Mo-based metal oxides indicates that the larger the degree of isolation of MoO4 tetrahedra, the larger the localization of photogenerated electron-hole pairs. This encourages us to find photocatalysts with the corner-sharing structure of MO6 octahedra to an optimal degree for carriers to transfer. 3.3.2 Substructure analogous to CaMn4 complex In the aforementioned section, that the (040) crystal facet has a positive effect on the photocatalytic O2 evolution of BiVO4 is presented.114 It is worth noting that the (040) facet has the foursquare multi-atomic center BiV4, with Bi located at the center of the square, while the (011) and (110) facets are composed of Bi4V2, as shown in Fig.13. BiV4 is so important as it seems to be structurally analogous to the oxygen-evolving complex CaMn4 in photosynthesis system II,122 which can provide oxo-bridges and multi-coordinate oxygen atoms for the four electron transfer involved in the O2 evolution. As similar examples are few, future challenges lie in investigating the atomic configuration of different crystal facets and identifying this unique substructure. In addition, we must determine whether the substructure actually has a similar O2 evolution process to that of the CaMn4 complex, and not just structural similarity, by using theoretical calculations and chemical tests. If this is confirmed by theoretical and experimental studies, then more important work has to be conducted on exploring synthetic and modification methods to obtain this favorable structure.As similar examples are few, future challenges lie in investigating the atomic configuration of different crystal facets and identifying this unique substructure. In addition, we must determine whether the substructure actually has a similar O2 evolution process to that of the CaMn4 complex, and not just structural similarity, by using theoretical calculations and chemical tests. If this is confirmed by theoretical and experimental studies, then more important work has to be conducted on exploring synthetic and modification methods to obtain this favorable structure.

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Fig. 13 Vertical view of the surface atomic configurations for the facets in monoclinic BiVO4. The slab system of (a) the (040) facet, (c) the (110) facet, and (d) the (011) facet in the monoclinic BiVO4 primitive cell after geometry optimization. (b) is the structure of the CaMn4 clusters. Reprinted from ref.114 and 122.

3.4 O2 -evolving photocatalysts in a Z-scheme system

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A Z-scheme system is a two-photon system that usually consists of a H2-evolving photocatalyst, an O2-evolving photocatalyst, and an electron mediator to accomplish overall water-splitting. Below is a review of some common O2-evolving photocatalysts used for construction of a Z-scheme system. Early in 1997, Sayama et al. 123 found that water could be decomposed into H2 and O2 by a two-step photoexcitation 2+ reaction using a RuO2-WO3 suspension and an Fe3+/Fehν redox 3+ system. The mechanism is formulized as 2H O + 4Fe → O2 + hν 2 4Fe2+ + 4H+, 4Fe2+ → 2H2 + 4Fe3+. This mechanism is similar to that of a Z-scheme reaction in photosynthesis. Although there is not an apparent H2-evolving photocatalyst in the system, it is an embryonic form of the Z-scheme systems studied by scientists. Sayama et al.124, for the first time, used a mixture of Pt-WO3 and Pt-SrTiO3 (Cr–Ta-doped) photocatalysts and an IO3-/I- shuttle redox mediator to accomplish the stoichiometric splitting of water into H2 and O2. The rates of H2 and O2 evolution under visible light irradiation were 0.21 and 0.11 μmol/h, respectively, and the quantum efficiency was estimated to be ca. 0.1% at 420.7 nm. Abe et al.125 found that the Pt-WO3 photocatalysts have a unique property of oxidation which enables preferential oxidation of water to produce O2 in the presence of both IO3- and I-; although water oxidation has the thermodynamic disadvantage compared to the oxidation of I-. Since then, Pt-WO3 has been an effective O2-evolving photocatalyst constructed with H2-evolving photocatalysts, such as Pt/TaON126 and Pt/ZrO2 /TaON.127 Although TiO2 is often used for H2 production, its O2 evolution should not be ignored. Kato43 and Niishiro44 found that TiO2 doped with Sb/Cr and Sb/Rh had a higher O2 evolution activity. This provided inspiration for employing metal-doped TiO2 in an O2 -evolving photocatalyst for construction of a Z-scheme system. Sasaki et al. 128 prepared Sb/Cr- and Sb/Rh-doped TiO2 and constructed it with the H2-evolving photocatalyst Ru/SrTiO3:Rh, respectively. In aqueous H2SO4 solution without an electron mediator, they both can stoichiometrically decompose water. In a conventional Z-scheme system, electron mediators are reduction and oxidation couples. Their mobility in solutions can compete with the reduction of the H2-evolving photocatalyst (PSI) and the oxidation of the O2-evolving photocatalyst (PSII), respectively, to reduce the reaction efficiency. Tada et al. 129 constructed a site selective CdS-Au-TiO2 nanojunction achieving an all-solid-state Z-scheme. CdS plays the role of PSI, TiO2 functions as PSII, and Au is the electron mediator. Although they tested the photocatalytic activity of decomposing organics instead of overall water-splitting, it broadens our horizon for using a three-solid-state system to accomplish overall water-splitting. Sasaki et al. 130 investigated visible-light-driven Z-scheme photocatalysis systems consisting of different metals (Ni, Ru, Rh, Pt, Ag) loaded SrTiO3:Rh for H2 evolution, BiVO4 for O2 evolution, and Fe3+/Fe2+ for an electron mediator. The (Ru/SrTiO3:Rh)-(BiVO4)-(Fe3+/Fe2+) photocatalysis system was found to be the most efficient and stable, giving the rate of H2 and


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O2 evolution of 19 and 9 μmol/h. Sasaki128 later simplified this system by constructing a unique interparticular system without the redox couples, as depicted in Fig.14. Through acidifying the aqueous solution, Ru/SrTiO3:Rh and BiVO4 aggregated to provide effective contact for the electron transfer. A main characteristic of this system is that the photoinduced electrons in the conduction band of BiVO4 transfer to the impurity level of Ru/SrTiO3:Rh via Rh species; only Ru/SrTiO3:Rh containing Rh species with the reversibility of the oxidation state can do this. The same case occurs when Ru/SrTiO3:Rh is combined with WO3, SrTiO3, or TiO2 as the O2-photocatalysts.

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Fig. 14 Mechanism of water splitting using the Z-scheme photocatalysis system driven by electron transfer between H2- and O2- photocatalysts. (a) Suspension of Ru/SrTiO3:Rh and BiVO4 at neutral and acidic conditions. (b) Scheme of photocatalytic water splitting. Reprinted from ref.128.

Similar to CdS–Au–TiO2, Ru/SrTiO3:Rh–BiVO4 can also form an all-solid-state photocatalytic system when reduced graphene oxide is introduced as the electron mediator. 131 It is found that graphene oxide photoreduced by BiVO4 is more miscible in water compared to that photoreduced by Ru/SrTiO3:Rh; this leads to an 8-fold enhancement with regards to H2 and O2 evolution activity. Photoreduced graphene oxide (PRGO) can restore some conductivity by repairing the π-π electron network but becomes more hydrophobic than graphene oxide. 132 - 134 Therefore, achieving a balance between the degree of graphene oxide reduction and the level of hydrophobicity is the key factor for efficient electron transfer in the Ru/SrTiO3:Rh–PRGO-BiVO4 system.

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4. Conclusion and Challenges Although the development of advanced semiconductor photocatalysts for H2 generation has been extensively conducted, the advancement of water oxidation semiconductor photocatalysts has remained comparatively slow, restricting the pace of devising an efficient and cost-effective artificial photosystem. As water oxidation proceeds more difficultly in kinetic and energetic

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aspects than water reduction, there is substantial space, as well as challenges, in designing efficient water oxidation semiconductor photocatalysts. In general, light absorption, electron-hole separation, hole transportation, and surface reaction are the key elements in research and design, requiring materials of optimized composition, favourable structure, and morphology. This review gives an overview of the recent main advancements in the area of semiconductor photocatalysts for water oxidation with a focus on the relationship between materials optimization and water oxidation efficiency with the aim to provide insights into the fabrication of highly-efficient photocatalysts. We now propose some challenges from the perspectives of the dynamics and energetics of photoholes and the simulation of natural water oxidation. Dynamics and energetics of photoholes remain the bottleneck in determining water oxidation efficiency. General modification ways to enhance electron transfer are loading cocatalysts or constructing heterojuctions. However, they are not always effective for hole transfer. Dynamics of holes are much more complicated than that of electrons which make the H2 generation reaction generally obeys a linear law. It is reported that the water oxidation process was controlled by the interfacial reaction obeying a stretched exponential law based on the transient absorption kinetics tests of hole decays using Ag+ as the electron scavenger.8 When the electron scavenger was replaced by Pt, it could not be modelled by a simple exponential law or power law. Thus, one way out to address the relationship between hole dynamics and correspondent improving methods is to investigate hole dynamics using different electron scavengers or cocatalysts to obtain a series of laws as references for studies on effective modification methods. In addition, it is necessary to explore dynamics of holes combined with the kinetical investigation of the electron-hole combination to determine if it is possible to control the recombination before or after some key points on the obtained plots. It is probable that some unusual changes on the plots are indicative of new intermediate reactions. These should be confirmed by further calculations and chemical tests. Talking of energetics of photoholes, it should be kept in mind that O2 production requires sufficient overpotentials (thermodynamic drive) and this seems not that easy as holes will lose part of oxidative capacities leading to lack of thermodynamic drive during the transfer process. A key factor of determining enough overpotentials is the structure of the valence band. Conventional strategies are incorporating elements to produce the valence band hybridization effect facilitating mobility of photoholes and reducing chances of being trapped. It should be noted that the band structure in the bulk photocatalyst is different from that on the surface and the band structure on different crystal facets also varies. It enlightens us to conduct deeper investigations into electronic configurations on different surfaces combined with the crystal facet engineering. As the surface reaction of O2 evolution proceeds more intricately, mechanisms of the surface reaction should be studied with regards to variation of surface species and consequent overpotential changes through integration of coordination chemistry, cluster studies and DFT calculations. Since, in natural photosynthesis, the water oxidation catalyst is composed of an CaMn4 complex that has its own advanced




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process for four electron transfer and O2 formation process, future progress lies in compositional, structural and functional simulation of the natural water oxidation complex. Researches should not be limited to the all-inorganic water oxidation photocatalysts; exploring inorganic-organic ones is promising. Semiconductor-organic composites are quite potential as they not only possess the advantages of semiconductors like band structure tuning and stability but also have the functions of organics such as obtaining various properties through tuning electronic configurations of surface functional groups. Combining graphene oxide with semiconductors or metals has raised some interests as the oxygenic functionalities on the surface of graphene oxide can be optimized for better light harvesting and they can also be modified to have some similarity to the oxygen-network structure of the CaMn4 complex. Developing carbon nitride or boron nitride/ semiconductor composites is another research hotpot. Investigations should be conducted on the optimal band structure alignment through tuning the band structure of carbon nitride or boron nitride and their integration with semiconductors. It should be noted that photoholes are possible to oxidize N instead of O, so it is important to investigate the relationship between oxidizing capacities of photoholes and the reducing capacities of N or O in certain circumstances in order to explore effective methods to suppress N2 production. Nature, our great mother and intelligent teacher, has bestowed advanced components and structures on biomass. Learning from nature to develop bio-inspired materials is a promising tactic. Extensive efforts have been made to mimic the natural photosynthesis for designing highly efficient artificial photocatalytic systems, including exploring reaction mechanisms and fabricating catalysts that have the components of the natural catalysts. However, this is not the state of the art strategy to mimic nature. Structure-function relationship is an important aspect and should be put much value on. With the idea of tailoring and grafting mechanisms of energy absorption, conversion and storage for biomass, artificial synthetic systems with biological architectures as well as analogous functions are prepared and new designing methods for materials structure and composite systems are developed. Nowadays, bio-mimetic photocatalysts replicated from leaves, diatoms, wood, butterflies, etc., have hierarchical structures which can enhance efficiencies of energy conversion and storage.91,135 For example, the synergy of elaborate structures and functional components of green leaves makes them highly complex machineries for photosynthesis and gives them high efficiencies of light harvesting and charge separation. Water oxidation occurs near the grana lamella in the thylakoid in a green leaf. Inspired by this, water oxidation photocatalysts replicated from the structure of the grana lamella in the thylakoid will not only possess unique light harvesting and charge separation processes but also have some similarity to the reaction environment (charge transfer and oxidizing equivalents react) of the natural water oxidation catalyst. This bi-function may make water oxidation photocatalysts highly productive. Another example is diatoms which have 3D regular silica structures with nano- to micrometer dimensions quite efficient in photosynthesis due to their intricate geometries and spectacular patterns of silica-based cell walls. Success has already been

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achieved in understanding the light harvesting mechanisms of diatoms and applying diatom-templated materials in the area of solar cells. Above all, future developments lie in exploring advanced methods to replicate more sophisticated structures of leaves, constructing theoretical calculations and simulation models to better understand the light path in leaves and, for diatom-templated materials, expanding applications in the field of photocatalysts, in particular, water oxidation catalysts.

Acknowledgments

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The authors are grateful for the financial support from the National Natural Science Foundation of China (51172141, 51102163 and 50972090) and the Research Fund for the Doctoral Program of Higher Education (20100073110065 and 20110073120036).

Notes and references Authors Address 75

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Semiconductor water oxidation photocatalysts: a promising strategy to address the bottleneck in an artificial photocatalytic system



DOI: 10.1039/C4CP00246F

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