A Place in the Sun for Artificial Photosynthesis? - ACS Energy Letters

A Place in the Sun for Artificial Photosynthesis? Jinzhan Su and Lionel Vayssieres* International Research Center for Renewable Energy (IRCRE), School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China ABSTRACT: Artificial photosynthesis enables the possibility of generating clean, sustainable, and large-scale energy resources through water splitting and renewable chemical fuel syntheses. With several decades of research, the scientific community still encounters great academic and technological challenges to efficiently generate hydrogen from water with sunlight at large scale and low cost. Recent developments based on novel system designs have led to significant advances in the fundamental understanding of light-induced charge dynamics and related interfacial chemical reactions and efficiencies. This Review provides a thorough overview of the very recent progress in new materials and photoelectrochemical system designs, surface engineering strategies, and efficient new cocatalysts for hydrogen and oxygen evolution reactions (HER and OER) as well as advanced spectroscopic studies related to photogenerated charge dynamics and interfacial electronic structure. A focus is given on newly developed strategies to improve the rate-determining steps, as well as emerging approaches for surface modifications, surface reaction dynamics, and optimal device structure design from a charge transfer viewpoint. Finally, a promising new system is being proposed and described, derived from the overall understanding, mechanisms, and concepts reported in the literature for efficient and stable watersplitting systems.

I

n our opinion, this decade is witnessing the widest consequences of man-made activities on our planet. The environmental risk for public health is now at a record high and does represent an even greater challenge than global warming and climate change. Indeed, staggering air and water pollution worldwidenow chronic in certain major cities in Asia and in the Western worlddue to toxic gases, chemicals, and ultrafine particles from industry, agriculture, and transportation sectors has become one of the most, if not The most, important problem that humanity is facing. Consequently, it is now crucial to transition to new societies where environmental, energy, and economic policies are no longer based on endless-growth financial models and fossil fuel technologies to substantially decrease our ecological footprint and environmental and health impact. As a matter of fact, everyone is at risk of inflammatory reactions and lung infections to cancer and death. It affects millions every year and not only in emerging countries but in rich nations; furthermore, air and water pollution knows no borders, easily crossing countries and continents, via naturally occurring atmospheric and oceanographic current patterns. The origin of this strong imbalance between human activities and the environment can be found in the endless-growth economic system in place in major countries worldwide. Indeed, this model inherently requires the use of endless cheap energy to be sustained, hence the massive use of coal and fossil fuels as energy sources for a more profitable energy return on energy invested, which might be good for the economy but clearly is not for our environment, health, and sustainable future. Energy conservation has rightfully been pushed forward upon people but turned out not to be as effective as originally thought given the fact that when fuel/energy demand © 2016 American Chemical Society

decreases so does its price, which in turn leads to more consumption (of cheaper energy). Technology, through innovations, has always helped boost the economy and has done it numerous times throughout civilizations, even recently with solid-state lighting technology for instance. However, technological innovation enabling large-scale implementation of a renewable, sustainable, and environmentally friendly substitute energy source into our societies is clearly in itself an entirely different problem and daunting task. Moreover, the transition period time must be contained, ideally within a decade, to avoid devastating environmental pollution and health impacts as well as energy shortages, resources depletion, and world economic crisis. Technological advancements must also involve large-scale, clean, and cost-effective fabrication techniques at moderate to low temperature and be based on highly efficient materials that mostly contain earth-abundant elements, easily extractable and fully recyclable rather than expensive, scarce, and toxic metals, and rare earths. In addition, given that conventional technologies that attempt to improve the efficiency and performance of existing materials and devices by further development along the same incremental approaches are reaching their limits, it is also crucial to develop novel (multi)functional materials where bulk limitations are overcome by changing the fundamental underlying physics and chemistry, by, for example, nanoscale design and quantum confinement effects. Received: April 11, 2016 Accepted: May 17, 2016 Published: May 17, 2016 121

DOI: 10.1021/acsenergylett.6b00059 ACS Energy Lett. 2016, 1, 121−135

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ACS Energy Letters

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Artificial Photosynthesis, which aims to emulate natural processes using man-made devices, is the best way to convert and store solar energy using chemical fuels as feedstock.2 For an artificial system (Figure 1b), a more practical approach is a direct and efficient way to convert abundant materials in nature like (sea)water and CO2 to useful chemicals, for example, H2, CO, and hydrocarbons. Compared to photovoltaic (PV) solar energy conversion, artificial photosynthesis converts sunlight into chemical products. While different from electricity energy storage, chemical fuel storage is more viable for massive energy storage, a requirement for future alternative renewable and sustainable energy sources. The challenges to achieve efficient artificial photosynthesis are now the focus of many worldwide ongoing scientific efforts and will have to be solved in the near future by concomitant multidisciplinary efforts from scientists from all basic sciences as well as engineers from various fields. Over the past few years, significant academic progress has been achieved in many aspects including new materials, more sophisticated structures, systems, and devices, better fundamental understanding of interfacial electronic structure, reaction processes, and physical and chemical phenomena as well as new approaches to enhance each of these aspects.3 Several reviews on novel materials, structure design, and kinetics have been reported.3−7 This Review highlights recent progress and focuses on promising materials and future opportunities in the development of efficient and robust devices as well as new strategies to improve the rate-determining steps, as well as emerging studies on surface modifications, surface reaction dynamics, and optimal design for charge transfer and surface catalytic reactions. Artif icial Photosynthesis with Carbon Compound Production. Carbon compound-based fuel is considered to be a high-density fuel that meets the requirements for an energy carrier in stationary power generation and transportation. It is also regarded as a possible alternative for petrochemicals as fuel feedstock and a carbon source for renewable polymers, plastics, and solvent. Direct CO2 reduction is a promising method for solar energy conversion and storage in the form of chemical fuels. It varies with different reaction pathways/products.8 For these reaction pathways, fewer electrons involved in the reaction required a more negative reduction potential. The one-electron reduction of CO2 to the CO2•− radical anion (E = −1.9 V vs SHE) is a fundamentally challenging process as it requires a high activation energy. Synchronous proton-coupled two-electron reduction of CO2 to CO, formate, or methanol is less restricted in energy cost as it uses H2O as both the electron donor and the proton source. Pioneering work on photocatalytic CO2 reduction was reported by Halmann9 in 1978, who bubbled CO2 over a p-type GaP cathode illuminated with a Hg lamp yielding formic acid, formaldehyde, and methanol. Most of the commonly reported photosynthetic systems mimick the natural photosynthetic process yet incorporate only the most essential steps, that is, CO2 reduction and H2O oxidation. Significant developments in a variety of photocatalytic systems for photochemical CO2 reduction have been achieved and reviewed.10 The reported systems are hybrids of semiconductors or molecular metal complex catalysts or are coupled with cocatalysts and plasmonic photocatalysts working under sunlight irradiation. To achieve efficient CO2 reduction, various new materials are being tested such as g-C3N4, which appeared to be a multifunctional robust photocatalyst. A heterogeneous photocatalyst system that consists of a ruthenium complex and carbon nitride (C3N4) was also developed for the reduction of CO2 into formic acid, reaching a turnover number higher than 1000 and

It is now crucial to transition to new societies where environmental, energy, and economic policies are no longer based on endless-growth financial models and fossil fuel technologies to substantially decrease our ecological footprint and environmental and health impact. Solar Energy Conversion to Chemical Fuels. Solar energy is indeed the largest exploitable renewable energy resource, providing more energy to our blue planet per hour than the total energy consumed by human activities in 1 year. Direct conversion of solar energy into chemical fuels does represent an optimal approach to address the globally growing energy demand in a sustainable way. Photosynthesis is a massive production activity by nature when sunshine is shed on earth. The product is food for life. As those products were converted, stored, and transformed with time, fossil fuel was generated and contained beneath the earth’s crust. Natural photosynthesis, which is conducted by cyanobacteria, algae, and plants, uses solar energy to reduce CO2 to carbohydrates. Cyanobacteria and plants have evolved into highly organized photosynthetic systems. In this system1 (Figure 1a), solar-to-chemical energy

Figure 1. Conversion of solar energy to chemical fuels by natural and artificial photosynthesis approaches. (a) Natural photosynthesis is conducted by complex systems within Photosystems I and II in different organisms. (b) An artificial photosynthesis system is a simplified version with dedicated components for efficient light absorption, charge transport, and surface redox reaction.

conversion was conducted with interconnected light-harvesting systems, highly efficient charge separation functions, and the reaction centers Photosystem I and II (PSI and PSII), followed by a dark reaction of the electron transport chain of the cytochrome with coenzyme NADPH as a redox carrier and the Calvin cycle for CO2 reduction. In PSII, a central pair of chlorophylls, P680, is excited with an electron for CO2 reduction, and then, its oxidized form, P680•+ is be recovered by the electron from a Mn4Ca-oxo cluster, which carries out the biological oxidation of water. Natural photosynthesis is conducted inside of the body of organisms, with the help of numerous proteins, enzymes, and the support of a living environment. This makes the natural photosynthetic machinery a complex system relevant to several different disciplines, such as biochemistry, biophysics, molecular and structural biology, quantum mechanics, as well as physiology and ecology. 122


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an apparent quantum yield of 5.7% at 400 nm.11 Carbon nitride was also coupled with metal oxides as a nature-inspired organic semiconductor-based system.12 Popular systems used nowadays for CO2 reduction consist of a metal complex photocatalyst such as ruthenium complexes13 or a Mn(I) molecular catalyst,14 yet a sacrificial reductant is always required as an electron donor. Direct reduction of CO2 to CO using unstable p-type semiconductors as photocathodes is still under scrutiny because new surface protections are being adopted and shown to be successful, such as TiO2-passivated III−V compounds15 and Cu2O electrodes.16 Metal dots-decorated semiconductors were also used as catalysts17 as noble metals were found to have an enhancing effect over CO2 conversion to hydrocarbons. Heterostructured compounds such as AlGaN/GaN give an efficiency of 0.13%, comparable to that of natural photosynthesis in plants.18 Besides semiconductor catalysis, extensive efforts are currently being invested to re-engineer (genetically) photosynthetic organisms (e.g., green algae and cyanobacteria) for more efficient photobiological biomass and H2 conversion. An autonomous biophotovoltaic cell, which couples two independent processes of light capturing by PS2 and PS1 in a Z-scheme structure, is able to produce both electrical and chemical energy.19 New electrocatalysts are also being developed to reduce the activating energy of CO2 reduction. Partially oxidized atomic cobalt layers show overpotential of only 240 mV and high intrinsic activity toward formate production.20 As the organic chemical production system includes both CO2 reduction and H2O oxidation with quite different reaction conditions, efficient production of organic chemicals remains a very difficult challenge. Artificial organic chemical photosynthesis is still not feasible due to inefficient CO2 reduction process. Artif icial Photosynthesis with Water-Splitting Approach. Hydrogen production is a more favorable and competitive process than CO2 reduction as the overall CO2 reduction yields for most systems are limited. Considering large-scale application using sunlight to convert atmospheric CO2 into organic fuels, CO2 feedstock is an issue compared to hydrogen production as CO2 concentration is relatively low in the atmosphere. Thus, what represents the best feedstock for a synthetic, renewable, and clean fuel? The best nontoxic, most abundant and geographically balanced and freely available resource on earth is (sea)water; hence, hydrogen generation from water using photosynthesis appears as the ideal scenario. The efficient capture of solar energy to power the electrolysis of water, generating hydrogen (H2) as a fuel directly (e.g., in a combustion engine or in fuel cell vehicles) is the key to a clean and sustainable future.

Figure 2. Schematic elementary steps from light energy to chemical bond energy. (a) Light absorption: the oscillating electric field of the incoming light induces an oscillating dipole within the chromophore associated with the photosynthetic system, leading to electron transition to a higher electronic energy state.22 (b) Charge carrier transportation/separation: photoexcited electrons and holes are separated by the space charge regions established at the interfaces, the energy being carried by the electrons and holes. (c) Surface reaction: The electrons and holes reduce and oxidize the water to produce hydrogen and oxygen, respectively, with energy stored in the chemical bonds.

Although numerous new strategies are reported, the fundamental obstacle for efficient solar water splitting remains unchanged.

As depicted in Figure 2, converting light energy to chemical bond energy requires three main steps, which are light absorption, charge carrier transportation/separation, and surface reaction. All three steps should be efficient enough to achieve substantial overall conversion efficiencies. However, to date, there is no single material that meets simultaneously all of the requirements of band gap, band edge position, electronic properties, and long-term stability for efficient water splitting. Back in 2001, a III−V semiconductor junction device consisting of AlGaAs/Si and capable of splitting water at a record efficiency of 18.3% was reported.23 Unfortunately, a stability problem in aqueous solutions prevented its application for practical solar water splitting. With the advent of nanoscience and nanotechnology, material structures and functions can now be engineered and precisely tuned to achieve

Although numerous new strategies are reported, the fundamental obstacle for efficient solar water splitting remains unchanged. The major obstacle in pursuing an efficient and stable solar-to-hydrogen (STH) conversion device is the development of robust and long-lasting water oxidation catalysts (WOCs). The discovery of the photoelectrochemical (PEC) effect of TiO2 by Fujishima and Honda21 in 1972 led to decades of sustained effort to develop a stable and efficient PEC cell and photocatalyst for solar water splitting. Various oxide, sulfide, and nitride semiconductors have been explored. 123


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type II is the ideal band structure for efficient charge separation. An axial heterostructure nanowire can be obtained by conformal deposition of the shell on the nanowire surface, forming a core−shell structure. Since the report of nanostructured WO3/BiVO4 heterojunctions for PEC application,28 this system has become a popular heterostructure. A helical WO3/BiVO4 core−shell heterojunction has been reported reaching a photocurrent density of ∼5.35 ± 0.15 mA/cm2 at 1.23 V versus RHE,29 while for some heterojunctions with straddling gap (type I), such as ZnO/Fe2O3,30 enhanced charge separation was also observed. The energy difference between the conduction band edge and Fermi level can be achieved by adjusting the doping level. Hence, a preferred energy match between the two materials can be achieved with a built-in voltage established by electron flowing from one side to the other until the Fermi levels on both sides equalize. Tree-like hierarchical structures combining the advantages of enhanced light trapping, larger surface area, and better photogenerated electron−hole separation31 do indeed exhibit superior PEC activity. Another efficient way to reduce recombinations consists of depositing an ultrathin layer onto a nanostructure, which not only increases the photon absorption but also decreases the distance for charge carriers to travel before reaching the surface. For example, photocurrents as high as ∼3 mA/cm2 at 1.23 V versus RHE were achieved on an ultrathin hematite film deposited on a 3-D nanophotonic structure.32 Advances over the past few years have demonstrated remarkable improvements in efficiency through nanostructuring and heterojunction design. As a result, the typical photocurrent density for these materials increased29,33 to values of 3−5 mA/cm2. However, nanostructured engineered photoelectrodes seem to have reached their limitations in terms of activity improvement within their absorption range of wavelengths. Exploiting Traditional and New Narrow-Band-Gap Semiconductors. Promising materials with unique properties that could possibly make a breakthrough are under intense scrutiny. For instance, metal-free nitrides such as C3N4 have attracted great attention due to its narrow band gap (2.7 eV) and a conduction band position sufficiently negative to drive proton reduction to hydrogen while showing relatively good stability.34 It was investigated by doping (S, F) or incorporated with other semiconductors, even graphene,35 showing high performance for overall water splitting. Oxynitrides and nitride composites are emerging as popular materials given their narrow band gaps and appropriate band levels for both water oxidation and reduction. For instance, a GaN nanowire with an internal quantum efficiency of ∼51% has been reported.36 Tantalum oxynitride (TaON) and nitride (Ta3N5) were also reported as good overall water-splitting materials with near-ideal band gaps and band edge positions. External quantum efficiency of up to 45% at 400 nm was reported.37 Typical metal oxynitrides with d10 electronic configuration emerged again as they are capable of driving overall water splitting under light with wavelengths below 480 nm. A more complex “perovskite-type” oxynitride, LaMgxTa1−xO1+3xN2−3x (x ≥ 1/3),38 was developed yielding overall water splitting at wavelengths of up to 600 nm. Silicon and III−V semiconductors, which have optimum band gaps, long carrier lifetimes, high charge carrier mobility, and proper band edge positions for unbiased water splitting, are getting substantial interest as new strategies are being developed to protect them against (photo)corrosion in aqueous solutions. This approach appears somehow more promising

enhanced electronic and surface properties as well as overcome bulk limitations. Nanostructural Design. Researchers turned back to earthabundant oxides such as TiO2, WO3, and α-Fe2O3. This return was promoted by the emergence of low-cost nanostructure synthetic procedures such as hydrothermal and templateassisted methods.24 These fabrication techniques are simple and easily accessible to researchers in various fields and draw intense attention to novel nanostructure material development for artificial photosynthesis. In this context, earth-abundant transition-metal oxides became model photoelectrodes intensively investigated for their PEC water-splitting properties.25 For most of them, charge recombination still remains a major barrier for efficient photochemical conversion. The competition between charge transfer (separation) and recombination determines the internal quantum efficiency of the water- splitting reaction. For materials with poor charge mobility, nanostructuring is a successful strategy to overcome the trade-off between light absorption and charge transport to the solid/ electrolyte interface for water oxidation, as depicted in Figure 3a.

Figure 3. Common new strategies to improve overall efficiency. (a) Smart nanostructures: ordered hierarchical structures improve the surface area while keeping grain boundaries at a minimum. (b) Heterojunctions: promote the electron−hole separation by driving them toward opposite directions due to inherent built-in electric field. (c) Surface protection: a passivation layer is conformally deposited to prevent corrosion and eliminate surface state recombination centers. (d) Surface modification: cocatalyst deposition and surface engineering facilitate the water reduction and oxidation reactions.

The natural photosystems have evolved to unique scaffolds that efficiently handle complex photosynthesis processes such as light harvesting and charge collection. These biological structures have been investigated as versatile templates for the assembly of photocatalysta.26 A tree-like structure with “stems” and “branches” was constructed using conductive Al-doped ZnO,27 which is used as conductive scaffolds for efficient electron collection. Heterojunctions. Along with nanostructuring, efforts have also been devoted to the search for a charge-separation-enhancing structure such as heterojunctions, as shown in Figure 3b. In terms of band alignment, three types of semiconductor heterojunctions can be defined, straddling gap (type I), staggered gap (type II), and broken gap (type III), of which 124


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Bisquert and co-workers50 suggested that water oxidation took place predominantly from surface-trapped holes, not directly from valence band holes with the help of a surface-trap-mediated charge transfer process at the hematite/electrolyte interface. Surface modification enables the adjustment of the energetic position of the band edges, demonstrating that it may be possible to engineer the energetics at the semiconductor/electrolyte interface, allowing unbiased water splitting with a single photoelectrode with a band gap of less than 2 eV. Indeed, improving the band edge energetics and the photocurrent onset of GaInP2 by modifying the surface energetics via surface modification by self-assembling with phosphonic acid58 was reported. Titanium dioxide (TiO2) coating on p-type gallium−indium phosphide was also found to be effective at suppressing the reverse process of unproductive recombination by an enhanced charge separation of the excited electrons from the holes.59 Interfaces other than the surface between the active layer and electrolyte were also engineered for enhanced charge separation and transportation. For instance, underlayers working as barrier layers for electron migration from the transparent conducting substrate (typically, fluorine-doped tin oxide, FTO) to the active layer was used to prevent the recombination of electrons in FTO with photoexcited holes in the valence band of the active layer, leading to better charge separation.60 Interface engineering enhancing the contacts between the semiconductor and FTO substrate was achieved by synergistically employing flame reduction and TiCl3 solution reduction.61 It was also reported that sub-10 nm rutile TiO2 nanoparticles in the presence of Ti(III),62 with surface/subsurface defects, not only shifted the top of the valence band upward for band gap narrowing but also promoted better charge carrier separation, reaching state-of-the-art activity for visible-light-driven water splitting. Interface engineering was also used to create homojunctions in Cd0.5Zn0.5S nanorods by twin planes parallel to each other, in which zinc-blende (ZB) and wurtzite (WZ) segments alternatively occurred along the ⟨111⟩ direction. These type-II staggered band alignments between WZ and ZB segments resulted in an intense concentration of homojunctions that are capable of promoting charge separation.63 As a result, the quantum efficiency for the photocatalytic hydrogen production reached a record 62%. Surface Reactions. One of the major obstacles for achieving efficient and stable overall water splitting is directly related to the uncontrolled surface charge properties. The oxygen evolution reaction (OER) and hydrogen evolution reaction

compared to those aiming at overcoming improper intrinsic electronic properties of known stable materials. Surface Passivation. A passivation layer is applied to improve the (photo)chemical stability of the semiconductor when operating in an electrolyte.39 The most effective material, so far, for passivation is TiO2, which has been tested on Si, GaAs, and InP.40 Other materials such as TiO2 or an ultrathin carbon sheath were deposited on Cu2O-based heterojunctions as a surface protection layer against photocorrosion to form buried semiconductor junctions for improved performance in photocurrent density and onset potential.41 Dual-layer thin TiO2/Ni coatings were also utilized to stabilize polycrystalline BiVO4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte.42 SrTiO3 can also act as a transparent protection layer for silicon photocathodes, with the lattice matched, preventing corrosion without compromising photocatalytic redox activity.43 Other narrow-band-gap semiconductors, such as sulfides and nitrides, also face stability issues. The same strategy was used to improve their stability using a protection layer consisting of amorphous TiO2 or noble metals.44,45 Because passivation layers can readily be deposited onto high-surface-area and high-aspect-ratio nanostructures by diverse fabrication techniques, (e.g., atomic layer deposition (ALD), spin coating, electrochemical deposition, sputtering, electron beam evaporation, or dip casting),39 they are being adopted as a simple yet very efficient way to facilitate the use of well-known aqueous-unstable semiconductors. Table 1 summarizes the most common ones along with their performances and stability specifications. Surface Engineering. In addition to protecting the semiconductor from corrosion, surface passivation layers46 are also used to remove the surface states that facilitate surface electron− hole recombination, which is of key importance for the performance of the semiconductors, as depicted in Figure 3c. For instance, improved hematite-based photochemical water splitting has been reported through passivation of surface states using very thin layers, such as FexSn1−xO4, and ALD Al2O3 and TiO2.47 On the other hand, surface states may have positive effects on the charge transport and surface reactions. For example, the so-called “black TiO2” has attracted enormous attention due to its hydrogenation-induced surface disorder48 and unusual photocatalytic activity. It was found that hydrogen doping eliminates the recombination centers at the surface and inhibits the recombination of light-induced electrons and holes.49 Using electrochemical impedance spectroscopy (EIS),

Table 1. Performance of Semiconductor Electrodes with a Passivation Layer for PEC Water Splitting passivation layer semiconductor

material thickness (nm)

Si, GaP, GaAs Si

TiO2 TiO2

4−143 15

1 M KOH, 1.25 Sun 0.5 M H2SO4 100 mW/cm2

condition

performance

Si Si

TiO2 NiO

100 50

InP CH3NH3PbI3

TiO2 Au/Ni

3 80/8

1 M HClO4, AM 1.5G, λ > 635 nm 1 M KOH, 38.6 mW/cm2 AM 1.5G, λ > 635 nm CO2 saturated 0.5 M KCl AM 1.5G, 100 mW/cm2

rutile TiO2 BiVO4 Cu2O/TaON

TiO2 TiO2 C

2 1 3

GaAs/InGaP

TiO2

62.5

1 M KOH, AM 1.5G, 100 mW/cm2 0.1 M KOH pH = 13, 135 mW/cm2 0.5 M NaOH pH = 13.6 AM 1.5G 100 mW/cm2 1.0 M KOH AM 1.5G 125

stability

ref

∼10% decreased after 100 h 2 h @ 27 mA/cm2

40 51

60 h @ 22 mA/cm2 300 h @ 14.5 mA/cm2

52 53 54 55

50% @ 370 nm Faraday eff. 100% 59% @ 400 nm

12 h 14% current remaining for 16 min @ 2 mA/cm2 3 h @ 1.3 mA/cm2 3 h @ 1.4 mA/cm2 1 h @ 2.7 mA/cm2

10.5%

15% decrease after 80 h

57

9.4% efficient Si PV 15.6% @ − 0.32 VRHE >10% Faraday eff. 83.6% 12%

56 42 41


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Table 2. Summary of the Performance of OER and HER Catalysts for Solar Water Splitting electrode

electrolyte

performance (bias vs RHE)

stability

ref

OER 2.71 mA/cm2, 1.23 V 2.73 mA/cm2, 0.6 V 2.5 mA/cm2, 0.6 V 650 μmol/h/g 2.8 mA/cm2, 1.23 V H2: 4030 μmol/h/g, O2: 2030 μmol/h/g 0.7 mA/cm2, 0.6 V 5 mA/cm2, 1.23 V 0.4 mA/cm2, 1.23 V 1.26 mA/cm2, 1.23 V 4.32 mA/cm2, 1.23 V 1.7 mA/cm2, 1.23 V

Co3O4/ BiVO4 FeOOH/NiOOH/BiVO4 CoOx/ NiO/BiVO4 Pt/MnOx/BiVO4 Co−Pi/α-Fe2O3 Ta2O5/Rh2O3/SrTiO3:Sc

0.5 M buffer solution, pH = 7 0.5 M phosphate buffer, pH = 7 0.1 M KPi solution, pH = 7 0.02 M NaIO3 1 M NaOH (pH = 13.6) H2O (250 mL)

Co-Pi/W:BiVO4 Co3O4/Rh/Ta3N5 Ni(OH)2/Fe2O3 Pi-Fe2O3 Co-Pi/Pt:Fe2O3 Ni(OH)2/IrO2/Ti−Fe2O3

0.1 M KPi buffer (pH = 8) 1 M NaOH, pH = 13.6 1 M KOH, pH = 14 0.1 M(K-Pi) buffer, pH = 7 1 M KOH 1 M NaOH, pH = 13.6

MoS2/ZnIn2S4 NiS/g-C3N4−CdS WS2/mpg-CN MoS2/Cu2O MoS2/graphene−CdS

HER 0.43 M Na2S and 0.5 M Na2SO3 153 μmol/h/0.05 g 10 mL triethanolamine; 90 mL distilled water 2563 μmol/g/h lactic acid (10 vol %) 12 μmol/0.05g/h 25% (v/v) methanol mixed with 0.1 M Na2SO4, pH = 7 0.175 mA/cm2, −0.1 V vs SCE 300 mL of lactic acid aqueous solution (20%) 1.8 mmol/0.2 g/h

rGO/TiO2 Ni/RGO

20% v/v methanol aqueous solution 2.0 × 10−4 M eosin Y (EY); 7.7 × 10−2 M trimethylamine (TMA), pH = 10

13996 μmol/g/h 94.3 μmol/0.01g/h

∼15% decay in 600 s no significant decay in 48 h 15% decay in 16 h 20% decay in 75 h no significant decay in 10 h no significant decay 6% decay in 6 h no significant decay no significant decay no significant decay

in 2 h for 170 s in 3 h in 3 h

no significant decay in 15 h 20% decay in the 2nd recycle 70% decay in the 2nd recycle 7% decay after 9 h no significant loss of activity for 5 recycles 70% decay in the 2nd recycle

64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82

only improved its efficiency but also dramatically reduced photocorrosion (maintaining 80% of the photocurrent for 2 h), reaching 6.3 mA at 1.23 V vs RHE. The nanoporous BiVO4 photoanode modified with FeOOH/NiOOH achieved a photocurrent density of 2.73 mA/cm2 at 0.6 V versus RHE.65 CoFeOx is another promising OER electrocatalyst working in alkaline solution with a significant stability94 and can also be used for HER as a bifunctional catalyst. X-ray Spectroscopy Techniques. Interestingly, X-ray absorption spectroscopy (XAS) emerged as a very efficient tool to investigate the activity origin of OERs. The active sites within (Ni,Fe)OOH can be identified by XAS at the Fe edge by using high-energy-resolution fluorescence detection (HERFD).92 The active species in NiFe and NiMn DHs are iron and manganese, while in NiCr DH, nickel is the active species.95 Combining the results of X-ray absorption fine structure (XAFS) measurements with the predictive power of density functional theory (DFT) suggests that in Ni−Fe DH, the Fe−O bond length contraction enables optimal adsorption energies of OER intermediates (i.e., −OH, −O, and −OOH) over Fe sites, which account for the increased OER activity of FexNi1−xOOH. The synergistic effect of Ni and Fe makes the FexNi1−xOOH OER catalysts energetically more favorable than pure NiOOH or FeOOH.96 In situ X-ray absorption near-edge structure (XANES) spectroscopy showed an increase in the oxidation state of Ni from Ni3+ to Ni3.6+ together with a highly covalent Fe(IV)−O bond under the OER process, which confirmed that the charge transfer between Ni and Fe through a “Ni−O−Fe” bond is responsible for the high catalytic activities of NiFe(OH)x.97 In situ O K-edge XAFS was used to directly probe the active species in Ni−Bi electrocatalyst systems during the OER and found that the water oxidation catalysis proceeds at the domain edge of NiO6 octahedra.98 The information gained from XAFS measurements and DFT calculations indicated that the presence of undercoordinated Mn(III)O5 units located at the boundary of the amorphous

(HER) are obviously crucial for the development of efficient solar water-splitting devices.83 Therefore, further improvement must include reducing the bias required to reach high current densities, which is to substantially lower the onset potentials while enhancing the kinetics (Tafel slope). Table 2 summarizes the performance of recently reported OER and HER catalysts for solar water splitting. OER is the process of generating molecular oxygen through electrochemical oxidation of water. This reaction proceeds through multistep proton-coupled electron transfers, which are kinetically sluggish. Furthermore, in most metal oxides, the localized oxygen p nature of the valence band contributes to a low bandwidth (flat band), which leads to large effective masses and low mobility of holes84 and, thus, inefficient interfacial hole transfer. A cocatalyst (or electrocatalyst) is usually needed to reduce the activation energies for the rate-limiting step in water photooxidation, as depicted in Figure 3d. Without such catalysts, photogenerated holes reaching the surface may not accept electrons from water but may instead recombine with electrons in the conduction band of the semiconductor. Traditional effective OER catalysts are typically IrO2 and RuO2, but serious limitations would occur for their large-scale implementation due to their limited availability in nature and very high cost. Earth-abundant cocatalysts have received great attention as they represent the only viable solution to replace scarce and expensive noble metals.85 For instance, Co-based thin films formed by oxidative electrodeposition from Co(II) solutions in the presence of inorganic phosphate, borate, and other buffered electrolytes showing enhanced catalytic reaction have been proposed by Nocera and co-workers.86,87 Indeed, Co−Pi cocatalysts were found to facilitate the water oxidation reaction by suppressing surface recombination on materials such as Fe2O3,88 BiVO4,70 Si,89 TaN,90 FeOOH, and (Ni,Fe)OH.91,92 Ni−Fe layered double hydroxides (LDHs) were also used as efficient cocatalysts for water oxidation on Ta3N5,93 which not 126


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network is essential for catalytic activity. Under external positive bias, the Mn(III)O5 units act as hole traps that trigger the oxidation of neighboring water molecules.99 XAS measurement performed at synchrotron facilities in the soft X-ray range (below 1−2 keV) is also an efficient way to investigate the electronic structure of doped impurity incorporation in a host material. Ti doping is a known correlate to efficiency enhancement of hematite photoanodes due to increased carrier concentrations and carrier diffusion lengths of both electrons and holes. XAS spectra at the O K-edge revealed that Ti incorporation creates new oxygen 2p-hybridized states, overlapping with and distorting the unoccupied Fe 3d−O 2p band of α-Fe2O3. Ti L2,3-edge XAS spectra also showed that titanium is indeed incorporated as Ti(IV), regardless of the fabrication techniques and, therefore, with no long-range titania order detected as well as without the formation of any significant concentrations of Fe(II).100 Polarization-dependent XAS also enabled the study of the orbital anisotropy of hematite nanorod arrays compared to bulk crystals by probing O K- and Fe L2,3-edges.101 The orbitals in the lowest-energy region are found to be strongly hybridized Fe 3d (a1g) orbitals and O2− ligand 2p orbitals, oriented along the c-axis, which is parallel to the substrate surface that is perpendicular to the direction of electron conduction and light propagation in operating electrodes for [110]-oriented α-Fe2O3 nanocrystals. The study also demonstrated that the Fe L3-edge line shape and aspects of polarization dependence can be reproduced by crystal field atomic multiplet calculations of 2p-to-3d transitions for Fe3+ in the D3d point group symmetry of metal ions in the corundum structure. Both the O K-edge and Fe L3-edge spectra possess features that may be related to the high density of surface atoms in this nanoscale system, associated with partial coordination and therefore reduced symmetry compared to that for Fe3+ in bulk crystals. Optimizing OER/HER Performance. With growing numbers of cocatalysts being reported, the next logical step is to further improve their activities. In this context, porous structure engineering has been applied to selectively etching a NiGa LDH nanoplate to topotactically convert them into porous metal chalcogenides, for example, β-Ni(OH)2 and NiSe2, by selective etching.102 The piezotronic effect was also reported to be able to improve the performance of OER catalysts on semiconductor photoelectrodes.103 Combining different cocatalysts as conjugated ones with synergetic effects to further improve their activities has also been proved successful. Indeed, NiOH catalyst is capable of efficiently capturing the photogenerated holes from a host catalyst as a hole-storage layer (HSL) and feeding the efficient IrO2 electrocatalyst with holes for water oxidation.75 For HER, noble metals such as Pt and Rh are excellent promoters for H2 evolution but can also catalyze the (undesirable) back reaction, which limits the STH efficiency in artificial photosynthesis. To avoid such a drawback reaction, transitionmetal oxides such as NiOx and RuO2 that do not exhibit activity for water formation from H2 and O2 are usually used as cocatalysts for overall water splitting. Recently, an oxidized platinum cluster104 was also reported as an efficient HER catalyst while suppressing the undesirable hydrogen backoxidation. NiO/Ni on carbon nanotube sidewalls also acts as a highly effective HER electrocatalyst with activity similar to that of platinum.105 New nonprecious metal electrocatalysts were tested for hydrogen evolution.106 For example, MoS2 has been studied as an electrocatalyst for HER. Layered MoS2/C107

showed high performance. The hydrogen production rate was improved by 28 times after loading Mo3S4 onto NaTaO3.108 Other new compounds such as molybdenum carbides also exhibit remarkable electrocatalytic performance for hydrogen production.109 It has also been found that metal-free cocatalysts such as carbon dots are capable of suppressing back reactions. For instance, the C3N4 photocatalyst does split water into hydrogen and peroxide, a second material, carbon nanodots, breaks the peroxide down before it can damage the generated hydrogen.110 Besides protection, graphene quantum sheets were used as a nonprecious catalyst on Si111 nanowire photocathodes for solar hydrogen generation. Most noble metal-free catalysts are highly desirable for cost effectiveness but still underperform compared to Pt. An alternative strategy is to reduce the use of Pt content while achieving high exchange current density with a small Tafel slope to increase its mass activity. A successful example is the ternary Pt−TiO2−N−rGO nanocomposite, which exhibits superior HER activity with a small Tafel slope.112 Charge Dynamics Studies. Although the phenomenological advantages of material and structural engineering for improving efficiency are now widely confirmed, their atomic scale/microscopic origins are far from understood. Understanding the watersplitting mechanism and electron transport/separation kinetics is essential to develop an efficient and durable solar watersplitting technology in a more rational manner. A complete mechanistic understanding of the rate-determining steps would provide insight into structure−composition−reaction rate relationships to increase the overall kinetics of the reactions. Aiming at the significant improvement of the solar water splitting/artificial photosynthesis overall efficiency, attention should be focused for better fundamental knowledge of the thermodynamics and kinetics of photoelectrodes. Several techniques have been applied to probe the dynamics of photogenerated carriers at the millisecond to second time scales. Indeed, interfacial, surface, and back electron transfer recombination at these time scales have been considered to be at the origin of the transient currents observed under chopped light illumination113 and can be identified by several transient techniques. A simple method is the transient behavior of the photocurrent or photovoltage decay. These behaviors provide insight into surface states, trap charging, and the recombination time scale.

Aiming at the improvement of future solar water splitting/artificial photosynthesis efficiency, attention should be focused on a better fundamental understanding of the thermodynamics and kinetics of the photoelectrodes. Electronic intraband gap surface states promoting charge recombination at the interface can be detected by photoelectrochemical impedance spectroscopy (P-EIS) .114 Photogenerated charge recombination can be probed by intensity modulated photocurrent/photovoltage spectroscopy (IMPS/ IMVS). The electron transport lifetime can be determined by complex plane plots of the IMPS response.115 Transient photocurrent spectroscopy (TPC), such as transient absorption spectroscopy (TAS) and EIS, can also be used to investigate the kinetic competition between electron−hole recombination 127


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and water oxidation.116 Using a first rate law analysis of photoinduced water oxidation on a metal oxide semiconductor surface by photoinduced absorption spectroscopy (PIA), it was found that the accumulation of multiple holes or “oxidizing equivalents” is actually required for efficient water oxidation catalysis.117 TAS is used to measure the electron or hole transfer usually in the presence of a hole or electron scavenger. TAS is essentially laser flash spectroscopy that tracks transient changes in absorption after an excitation pulse. It has allowed researchers to understand the dynamics of charge transfer in many photocatalytic processes by tracking transient absorbance changes at specific wavelengths. For example, it has been found that TiO2 rutile exhibited up to 10× slower recombination kinetics than anatase. However, the photocatalytic activity of rutile was still lower due to the deficiency of its holes to drive efficient and irreversible alcohol oxidation.118 Energy and spatial distribution of intragap trap states in TiO2 photoanodes and their effects on dynamics of electron transport can be investigated using a timeresolved charge extraction experiments (TRCEs).119 Another useful time domain method is photoinduced charge extraction by linearly increasing voltage (photo-CELIV). In this method, (photo)capacitance and (photo)conductance of photoelectrodes are simultaneously measured using a linear increasing voltage. A relaxation time τrel for the photogenerated charge carriers can thus be determined by the recorded capacitance and conductance.100 This technique provides useful information on the number of extractable carriers and their dynamics and helps with the understanding and suppression of loss mechanisms. Time-resolved microwave conductivity (TRMC) measurements120 were also used to reveal the origin of the poor carrier transport properties of undoped BiVO4 due to its low carrier mobility (∼4 × 10−2 cm2 V−1 s−1). Tungsten doping was found to strongly decrease the carrier mobility by introducing intermediate-depth donor defects as carrier traps, but the increased carrier density improves the overall photoresponse. Noncontact atomic force microscopy (AFM) can also be used to identify defect charge states on catalytic surfaces as well as map the charge transfer within individual molecules.121 Time-resolved spectroscopic techniques do offer valuable information about the kinetics of the photoreactions. However, concerted efforts are still needed to better combine these experimental studies with theoretical modeling of the chemical reactions and interfacial electronic structure to gain a more complete understanding of the electron transfer and surface reactions involved in the process. New Systems for Water Splitting. Structural design is an important issue for the fabrication of efficient functional materials as the entire light to chemical production conversion process occurred in several distinct steps. New systems have been designed to address the rate-limiting issues in these steps, as depicted in Figure 4. Novel artificial photosynthesis device configuration could further improve the STH efficiency and may thus provide a highly anticipated breakthrough. Z-Scheme. Z-scheme is a system that consists of two narrowband-gap semiconductors for photo-oxidation and photoreduction and an electron mediator (Figure 4a). Z-schematic water splitting has been reported as a promising approach to overcome the limitation of charge carrier recombination and back reactions of products by spatial isolation of photogenerated electron and holes. Suitable redox couples such as iodate ion (IO3−)/iodide ion (I−), ferric ion (Fe3+)/ferrous ion (Fe2+), or [Co(bpy)3]3+/2+/ [Co(phen)3]3+/2+ are commonly used as electron mediators.122

Figure 4. Schemes of the most common structural designs of artificial photosynthesis systems: (a) Z-scheme, with a redox couple as the electron relay to drive complete water splitting; water reduction and oxidation reactions are occurring at two separated electrodes. (b) Tandem cell, with two stacked semiconductors absorbing light at different wavelength regions to further improve light-harvesting capabilities. (c) Particle system for overall water splitting, with a particle- shaped heterojunction, loaded with HER and OER cocatalysts. (d) Dye-sensitized PEC cells, with WOCs providing an electron for sensitizer regeneration. (e) PV electrolyzer, with a state-of-the-art PV device providing electrical power to drive direct overall water splitting.

Unfortunately, the efficiency is usually limited by the competitive oxidation of the redox couples. Recently, a solid-state electron mediator was used to replace the redox couple.123 In this system, a p-type metal sulfide photocatalyst was used as the H2-evolving photocatalyst and was combined with TiO2 as the O2-evolving photocatalyst and reduced graphene oxide as the solid-state electron mediator to split water into H2 and O2 in stoichiometric amounts. With such a spatial isolation, the metal sulfide in this system was also protected from photocorrosion by its cathodic condition. Systems with other solid electron mediators such as Ag124 or Au125 or even without an electron mediator126 were also reported. As no redox couples were used, the stability as well as the charge transfer were simultaneously improved. Natural−Artificial Hybrid Systems. A natural−artificial hybrid system, also known as a photobiocatalyst, is a system combining a semiconductor as the light harvester and an enzyme activated by the semiconductor. This system generally involves the semiconductor enzyme/bacteria electron transfer process. The most widely studied photobiocatalyst systems so far make use of conduction band electrons of excited semiconductors to promote enzymatic reductions mediated by NAD(+)/NADH and an electron relay.127 A natural PSII based PEC cell was also reported by re-engineering of natural photosynthetic pathways by direct coupling of the water oxidation enzyme, PS II, to the H2-evolving enzyme, hydrogenase. This was achieved by integrating the isolated enzymes into the artificial circuit of a PEC cell.128 128


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Dye-Sensitized PEC Cells. A dye-sensitized PEC cell is the extension of the concept of dye sensitization to the field of photochemical cells (Figure 4d). In this system, WOCs are coupled to the dye molecules to serve as electron donors to regenerate the oxidized dye molecules. Water molecules are oxidized to oxygen and a proton by the WOCs, while the protons are reduced to hydrogen at the counter electrode. For example, a dye-sensitized PEC cell composed of a dye, a WOC, and a mesoporous anatase TiO2 film was synthesized by attaching chromophore−catalyst assemblies onto an electrode surface.140 During the reaction, the photoexcited dye injects an electron into the conduction band of TiO2 and is subsequently reduced by the catalyst, which oxidizes water to generate oxygen and protons. The electron in TiO2 will transport to the counter electrode to reduce protons to hydrogen.141 Another dyesensitized PEC cell with a mesoporous TiO2 photoanode sensitized with an organic dye Lo and loaded with a molecular complex Ru1 as the WOC was used, while for the nanostructured NiO photocathode, an organic dye P1 as the photoabsorber and a molecular complex Co1 as the hydrogen generation catalyst was used.142 However, these cells show low overall solar energy conversion efficiencies as well as poor long-term stability.143 A practical dye-sensitized solar fuel production requires longterm aqueous photocathode stability. New photosensitizers were used to prevent both dye desorption and semiconductor degradation to achieve excellent stability.144 PV Electrolysis. A recent trend for affordable solar water splitting is PV electrolysis, which is an artificial photosynthesis system combining a PV device with a standard water electrolyzer for efficient solar hydrogen production. In this system, water is decomposed by the electrical current produced between the electrodes by the connected PV cell (Figure 4e). Commercial water electrolysis is typically operated at high current densities (1−10 A/cm2). Such high current densities require a higher operating voltage (due to kinetic losses). In such operating conditions, the electrolyte conductivity is an important factor. Because H3O+ and OH− are two of the most conductive ions in aqueous solutions, electrolysis processes need to be carried out in either highly acidic or highly basic conditions. State-of-the-art electrolysis systems give a remarkable electrical-to-chemical energy efficiency of 83.3%.5 Given that organic−inorganic perovskite solar cells (PSCs) have emerged as very promising candidates due to high efficiency, low cost, and low manufacturing energy consumption, earth-abundant-based electrodes were connected to a PSC, which yielded a water-splitting photocurrent density of ∼10 mA/cm2 and an overall STH efficiency of 12.3%.145 However, the stability of the system remains an issue. Another example is the perovskite−hematite tandem cell, in which the two semiconductors act as dual-band-gap absorbers with complementary absorption spectra covering a broad part of the solar spectrum, thus delivering a high efficiency.146 Perovskite/BiVO4/Co−Pi was also found to exhibit a STH of 2.5% at neutral pH.147 This system is a PV device rather than PEC device. A PEC device with a similar structure is the “PV-biased PEC cells”, whose core component is a direct semiconductor/water junction. This structure could be a more straightforward candidate and thus a less expensive approach for solar H2 production. Future Ideal Systems? With so much attention currently devoted to the discovery of novel photocatalysts and optimal architectures for efficient solar fuel applications, these studies keep reminding us that, to the great disappointment of many, there is still no artificial photosynthetic system meeting all of

A natural−artificial hybrid Z-scheme system was reported that integrates plant PSII and inorganic photocatalysts129 (e.g., Ru/SrTiO3:Rh), coupled with [Fe(CN)63−/Fe(CN)64−] as an inorganic electron shuttle. Very recently, a hybrid approach combining inorganic semiconductors with a nonphotosynthetic bacterium was reported. Cadmium sulfide nanoparticles anchored to bacteria served as the light harvester, enabling continuous photosynthesis of acetic acid from carbon dioxide from this “self-augmented” biological system.130 Tandem Cells. Another promising PEC configuration is the tandem structure (Figure 4b). This configuration consists of two stacked semiconductors to address the band edge mismatch or a photoelectrode biased with a solar cell that absorbs longer wavelengths transmitted through the photoelectrode. The Co−Pi/BiVO4:W/Si system was found to reach a STH efficiency of 4.9%.131 Another type of tandem cell consisted of a separated photoanode and photocathode, which utilized different parts of the solar spectrum to achieve overall water splitting without external voltage.132 For instance, a tandem structure allowing sunlight to pass through the top of a NiOOH/FeOOH/Mo:BiVO4 photoanode and reach the bottom of a Pt/CdS/CuGa3Se5/(Ag,Cu)GaSe2 photocathode showed a STH conversion efficiency of 0.67% over 2 h without degradation. III−V tandem devices such as a GaAs/InGaP tandem-junction photoanode with efficient unassisted, solardriven water splitting was also reported.57 This structure exhibited an ηSTH of 10.5% under 1 sun illumination. An in situ surface functionalization procedure was tested on a III−V PV tandem absorber, which achieved a direct solar water-splitting efficiency of 14%,126 breaking the 17 year old record of 12.4%133 for a monolithic water-splitting system. Nanoparticle Systems for Overall Water Splitting. For photocatalysis based on (nano)particle systems, a sacrificial agent is always used as a hole scavenger to protect the photocatalyst from “self-oxidation” by photogenerated holes. Overall water splitting without any sacrificial agents is of particular interest due to its simplicity and potential low cost of operation. One successful example is the Co3O4134 quantum dot photocatalyst capable of splitting pure water into O2 and H2 stoichiometrically under visible-light irradiation. Cobalt(II) oxide (CoO) nanoparticles showed an overall water-splitting efficiency of around 5%.135 This particle system can be also regarded as a wireless version of the PEC cell. The cocatalyst, representing the counter electrode, is placed on the surface of the photocatalyst, representing the working electrode (Figure 4c). These systems showed unexpectedly good stability in aqueous solutions. For example, GaN:ZnO modified with Rh2−yCryO3 showed stable activity for overall water splitting for longer than 3 months.136 Surface modification was also used to prevent back reaction in this compact system. A Cr2O3 shell O2 diffusion barrier was deposited to prevent a water-forming back reaction over Rh in Rh/Cr2O3 incorporated GaN nanowires.137 The cocatalyst Ni metal served as an electron trap (site for water reduction), and NiO as a hole trap (site for water oxidation) was used to improve the overall water-splitting activity of SrTiO3138 under UV light. A metal-based photocatalytic system for harvesting photons for photosynthesis has also been reported. In this system, charge carriers involved in both the oxidation and reduction steps come from the excitation of surface plasmons in a Au nanorod.139 The total efficiency was low (0.1%), but the operational lifetime of the plasmonic device was relatively long and could harvest photons over a wide solar spectrum by nanostructuring design. 129


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the requirements for a large-scale, stable, and efficient solar to fuel conversion. Sustained research studies into material properties engineering and reaction kinetics using sophisticated characterization and smart and original approaches are needed to create the ideal compound and architecture. By in-depth concerted analysis of recent results from the literature and by combining the existing, commonly accepted knowledge, one may derive a possible ideal structure. Except for p−n junctionbased solar cells, two unique solar energy conversion devices, that is, PSC and dye-sensitized solar cells (DSSCs) show good solar-to-electrical power conversion efficiency, both of which rely on remarkable high and broad light absorbance.148 Efficient light absorption is indeed a prerequisite to harvest energy proficiently. In a typical DSSC device, electrons are injected from the dye within hundreds of picoseconds,149 which results in an effective spatial charge separation. This efficient electron injection along with the large interface area from the porous structure ensures a high electron collection and utilization. In PSCs, the low recombination rate of photoexcitation is accompanied by electron and hole separations into distinct regions of the CH3NH3PbI3.150 This was explained by calculations that showed that the conduction band minimum is mainly composed of Pb 6p orbitals while the valence band maximum consists primarily of I 5p orbitals. A photoexcitation with anisotropic charge separation due to a spontaneous photoelectric field enhancement was also reported to not only create highly ordered Co−Pi/BiVO4 nanopyramid arrays151 but also enhance its water oxidation activity in neutral aqueous solutions. However, due to the inherent low hole carrier mobility of BiVO4 and without strong and permanent inner polarization (which exists in perovskite structures), only average performance was obtained. Another example of the effect of the photoinduced electric field has been reported,152 where transient electric fields that form upon photoexcitation within various p-GaInP2 heterostructures were recorded. The results show that the interfacial field drives charge separation and thus significantly reduces charge recombination. Inspired by the aforementioned studies, an efficient lightharvesting and utilization system could be constructed by building an inner field or polarization environment. It has been reported that ferroelectric (FE) layers with strong selfpolarization do significantly improve the charge separation with polarization from an ultrathin BiFeO3 FE layer between the BiVO4/FTO interface.153 FE materials have been investigated for PV applications due to their unique FE−PV effect, which arises from the photogenerated carrier separation by a built-in polarization field. However, most FE materials reported to date are perovskite oxides with large band gaps and relatively poor transport properties, thus not very efficient. However, FE materials can be used for providing a built-in electric field rather than for current generation and transport. Additionally, a hole conductor is typically used in DSSCs and PSCs to extract holes. Commonly used hole conductors are polymeric hole conductor such as P3HT154 and spiro-OMeTAD155 or copper-based p-type semiconductors such as CuSCN156 and CuI.157 Delafossite CuAlO2 has also been reported as an efficient hole conductor158 for p-type TCOs. This unusually high hole mobility arises from a large hybridization of the oxygen orbitals with 3d10 electrons in the Cu+ closed shell, lowering the oxygen character and leading to a dispersive valence band and low effective mass.159 Cu-based materials make good hole conductors and are earth-abundant and nontoxic. We proposed two new concept systems based on a FE material/semiconductor coupled structure for highly efficient

Figure 5. Proposed new systems with a built-in polarization field, (a) Sandwiched structure with electron and hole conductors as “bypass channels” and an electric inner field generated by polarized FE layers. (b) Dispersed FE polarization islands driving charge separation of photogenerated electron−hole pairs.

solar energy conversion devices. As depicted in Figure 5a, a chromophore layer using a narrow-band-gap semiconductor with high light absorption coefficient is attached to electron and hole conductors on each side. Upon excitation of the electron−hole pairs in the chromophore layer, electrons and holes can be efficiently extracted from each side. This structure acts as a bypass channel structure, with photogenerated charges taking two different and dedicated channels and flowing to different surfaces with a minimum possibility of recombination. A local electric field can be built within the chromophore layer using a p−n junction or by a self-polarized FE structure. A local built-in electric field can also be introduced to a semiconductor in a disordered manner. As illustrated in Figure 5b, FE regions are dispersed within the semiconductor, forming polarization islands. The polarization field drives the separation of the photogenerated charges around the islands. The separated electrons and holes are then collected by another type of bypass channel structure, such as graphene for instance, dispersed in the semiconductor. With this purpose-built structure, electron and hole pairs are generated in the semiconductor, separated by the built-in FE polarization field and collected through the bypass channel structures. After the electrons and holes reach their respective surfaces, selected HER and OER cocatalysts (Table 2) drive and boost the water reduction and oxidation reactions highly efficiently, reaching breakthrough record STH efficiency. 130


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scholar at Penn State University. He is now an assistant professor at the School of Energy & Power Engineering and a group leader on Nanophotocatalysts at the International Research Center for Renewable Energy.

Without a doubt, solar water splitting, using the two most abundant, geographically balanced, and free resources available on our blue planet, that is, the Sun and seawater, to make an unlimited amount of clean H2 along with O2 and Cl2 while preventing ocean acidification (due to NaOH generation as a side product) is the way forward.

Lionel Vayssieres got a Ph.D. in Chemistry (Université Pierre & Marie Curie, Paris, 1995) and was a researcher at Uppsala University, LBNL, UT Austin, Stellenbosch University, iThemba LABORATORIES, EPFL, and NIMS. Since 2012, he has been a full-time 1000-talent scholar professor at Xi’an Jiaotong University and cofounder/codirector of the International Research Center for Renewable Energy. http://ircre.xjtu.edu.cn/

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ACKNOWLEDGMENTS Financial support from the Thousand Talents plan and the National Natural Science Foundation of China (No. 51202186, 51236007) is gratefully acknowledged.

Summary and Future Outlook. Despite great academic advances within the past decade, artificial photosynthesis for solar water splitting remains in its infancy and is frankly in critical need of efficiency standardization and certification (similar to the PV industry). This could easily be achieved through intensive industry-driven academic research (or academic-driven industrial research, whichever is the easiest and fastest) to unfold as a viable and striving technology. As important are new global energy and environmental policies, not necessarily in agreement with existing technoeconomic analysis and highest-profit/endless-growthdriven financial systems, from governments worldwide to help implement solar water splitting (along with all other renewable energy sources for that matter) as the cleanest sustainable energy technology for our societies. Moreover, substantial financial support to scientists, engineers, and entrepreneurs should become available to actively spin-off academic research studies and develop businesses and companies for large-scale implementation of solar hydrogen generation. Furthermore, future incentives such as tax return and government subsidies for the public to massively adopt this new technology will also be highly beneficial. It is worth noting that, to date, there are still no solar hydrogen generation companies compared to many for the related PV sector. Despite these facts, we sincerely believe that with worldwide continuous, concerted, and creative efforts from scientists and engineers at academic, national and industrial laboratories, and research institutes as well as with new policies from governmental agencies, the development and implementation of efficient and stable water-splitting technologies for solar production of H2 at low cost and large scale for a clean and environmentally friendly hydrogen-powered societies should ultimately materialize. Without a doubt, using the two most abundant, free, and geographically balanced resources available on our blue planet, that is the sun and seawater, to make an unlimited amount of clean H2 along with O2 and Cl2 while preventing ocean acidification (due to NaOH generation as a side product) is The way forward. Indeed, it will provide a clean and sustainable energy resource while releasing only H2O in the atmosphere as an emission product from combustion and/or fuel cell use. This definitely makes perfect sense and would finally give artificial photosynthesis A Place in the Sun.160

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REFERENCES

(1) Demmig-Adams, B.; Stewart, J. J.; Burch, T. A.; Adams, W. W., III Insights from placing photosynthetic light harvesting into context. J. Phys. Chem. Lett. 2014, 5, 2880−2889. (2) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Artificial photosynthesis for solar water-splitting. Nat. Photonics 2012, 6, 511−518. Nocera, D. G. The artificial leaf. Acc. Chem. Res. 2012, 45, 767−776. (3) Yang, X.; Liu, R.; He, Y.; Thorne, J.; Zheng, Z.; Wang, D. Enabling practical electrocatalyst-assisted photoelectron-chemical water splitting with earth abundant Materials. Nano Res. 2015, 8, 56−81. (4) Zhang, P.; Gao, L.; Song, X.; Sun, J. Micro- and nanostructures of photoelectrodes for solar-driven water splitting. Adv. Mater. 2015, 27, 562−568. (5) Peter, L. M. Photoelectrochemical water splitting. A status assessment. Electroanalysis 2015, 27, 864−871. (6) Dutta, S. K.; Mehetor, S. K.; Pradhan, N. Metal semiconductor heterostructures for photocatalytic conversion of light energy. J. Phys. Chem. Lett. 2015, 6, 936−944. (7) Hisatomi, T.; Takanabe, K.; Domen, K. Photocatalytic watersplitting reaction from catalytic and kinetic perspectives. Catal. Lett. 2015, 145, 95−108. (8) Tu, W.; Zhou, Y.; Zou, Z. Photocatalytic conversion of CO2 into renewable hydrocarbon fuels: State-of-the-art accomplishment, challenges, and prospects. Adv. Mater. 2014, 26, 4607−4626. (9) Halmann, M. Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells. Nature 1978, 275, 115−116. (10) Chen, D.; Zhang, X.; Lee, A. F. Synthetic strategies to nanostructured photocatalysts for CO2 reduction to solar fuels and chemicals. J. Mater. Chem. A 2015, 3, 14487−14516. (11) Kuriki, R.; Sekizawa, K.; Ishitani, O.; Maeda, K. Visible-lightdriven CO2 reduction with carbon nitride: Enhancing the activity of ruthenium catalysts. Angew. Chem., Int. Ed. 2015, 54, 2406−2409. (12) Martin, D. J.; Reardon, P. J. T.; Moniz, S. J. A.; Tang, J. Visible light-driven pure water splitting by a nature-inspired organic semiconductor-based system. J. Am. Chem. Soc. 2014, 136, 12568− 12571. (13) Boston, D. J.; Pachón, Y. M. F.; Lezna, R. O.; de Tacconi, N.; MacDonnell, F. M. Electrocatalytic and photocatalytic conversion of CO2 to methanol using ruthenium complexes with internal pyridyl cocatalysts. Inorg. Chem. 2014, 53, 6544−6553. (14) Fei, H.; Sampson, M. D.; Lee, Y.; Kubiak, C. P.; Cohen, S. M. Photocatalytic CO2 reduction to formate using a Mn (I) molecular catalyst in a robust metal−organic framework. Inorg. Chem. 2015, 54, 6821−6828. (15) Zeng, G.; Qiu, J.; Li, Z.; Pavaskar, P.; Cronin, S. B. CO2 Reduction to methanol on TiO2-passivated GaP photocatalysts. ACS Catal. 2014, 4, 3512−3516. (16) Ba, X.; Yan, L.-L.; Huang, S.; Yu, J.; Xia, X.-J.; Yu, Y. New way for CO2 reduction under visible light by a combination of a Cu

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Jinzhan Su got a B.Sc. in Physics (2005) and a Ph.D. in Thermal Engineering (2011) at Xi’an Jiaotong University and was a visiting 131


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electrode and semiconductor thin film: Cu2O conduction type and morphology effect. J. Phys. Chem. C 2014, 118, 24467−24478. (17) Neaţu, S. t.; Maciá-Agulló, J. A.; Concepción, P.; Garcia, H. Gold−copper nanoalloys supported on TiO2 as photocatalysts for CO2 reduction by water. J. Am. Chem. Soc. 2014, 136, 15969−15976. (18) Deguchi, M.; Yotsuhashi, S.; Hashiba, H.; Yamada, Y.; Ohkawa, K. Enhanced capability of photoelectrochemical CO2 conversion system using an AlGaN/GaN photoelectrode. Jpn. J. Appl. Phys. 2013, 52, 08JF07. (19) Kothe, T.; Plumere, N.; Badura, A.; Nowaczyk, M. M.; Guschin, D. A.; Roegner, M.; Schuhmann, W. Combination of a photosystem 1based photocathode and a photosystem 2-based photoanode to a Zscheme mimic for biophotovoltaic applications. Angew. Chem., Int. Ed. 2013, 52, 14233−14236. (20) Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; et al. Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel. Nature 2016, 529, 68−71. (21) Fujishima, A.; Honda, K. TiO2 photoelectroChem. and photocatalysis. Nature 1972, 238, 37−38. (22) Moore, G. F.; Brudvig, G. W. Energy conversion in photosynthesis: a paradigm for solar fuel production. Annu. Rev. Condens. Matter Phys. 2011, 2, 303−327. (23) Licht, S. Multiple band gap semiconductor/electrolyte solar energy conversion. J. Phys. Chem. B 2001, 105, 6281−6294. (24) Osterloh, F. E. Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting. Chem. Soc. Rev. 2013, 42, 2294− 2320. (25) Du, P.; Eisenberg, R. Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: Recent progress and future challenges. Energy Environ. Sci. 2012, 5, 6012−6021. (26) Nath, K.; Najafpour, M.; Voloshin, R.; Balaghi, S.; Tyystjärvi, E.; Timilsina, R.; Eaton-Rye, J.; Tomo, T.; Nam, H.; Nishihara, H.; et al. Photobiological hydrogen production and artificial photosynthesis for clean energy: from bio to nanotechnologies. Photosynth. Res. 2015, 126, 237−247. (27) Zhang, L.; Reisner, E.; Baumberg, J. J. Al-doped ZnO inverse opal networks as efficient electron collectors in BiVO4 photoanodes for solar water oxidation. Energy Environ. Sci. 2014, 7, 1402−1408. (28) Su, J.; Guo, L.; Bao, N.; Grimes, C. A. Nanostructured WO3/ BiVO4 heterojunction films for efficient photoelectrochemical water splitting. Nano Lett. 2011, 11, 1928−1933. (29) Shi, X.; Choi, I. Y.; Zhang, K.; Kwon, J.; et al. Efficient photoelectrochemical hydrogen production from bismuth vanadatedecorated tungsten trioxide helix nanostructures. Nat. Commun. 2014, 5, 4775. (30) Hsu, Y.-K.; Chen, Y.-C.; Lin, Y.-G. Novel ZnO/Fe2O3 core− shell nanowires for photoelectrochemical water splitting. ACS Appl. Mater. Interfaces 2015, 7, 14157−14162. (31) Yin, Z.; Wang, Z.; Du, Y.; Qi, X.; Huang, Y.; Xue, C.; Zhang, H. Full solution-processed synthesis of all metal oxide-based tree-like heterostructures on fluorine-doped tin oxide for water splitting. Adv. Mater. 2012, 24, 5374−5378. (32) Qiu, Y.; Leung, S.-F.; Zhang, Q.; Hua, B.; Lin, Q.; Wei, Z.; Tsui, K.-H.; Zhang, Y.; Yang, S.; Fan, Z. Efficient photoelectrochemical water splitting with ultrathin films of hematite on three-dimensional nanophotonic structures. Nano Lett. 2014, 14, 2123−2129. (33) Abdi, F. F.; Han, L.; Smets, A. H.; Zeman, M.; Dam, B.; van de Krol, R. Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode. Nat. Commun. 2013, 4, 2195. (34) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76−80. (35) Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z.-X.; Tang, J. Visible-light driven heterojunction photocatalysts for water splitting - a critical review. Energy Environ. Sci. 2015, 8, 731−759. (36) Kibria, M. G.; Zhao, S.; Chowdhury, F. A.; Wang, Q.; Nguyen, H. P. T.; Mi, Z.; Trudeau, M. L.; Guo, H. Tuning the surface Fermi

level on p-type gallium nitride nanowires for efficient overall water splitting. Nat. Commun. 2014, 5, 3825. (37) Dabirian, A.; van de Krol, R. High-temperature ammonolysis of thin film Ta2O5 photoanodes: Evolution of structural, optical, and photoelectrochemical properties. Chem. Mater. 2015, 27, 708−715. (38) Pan, C.; Takata, T.; Nakabayashi, M.; Matsumoto, T.; Shibata, N.; Ikuhara, Y.; Domen, K. A complex perovskite-type oxynitride: The first photocatalyst for water splitting operable at up to 600 nm. Angew. Chem., Int. Ed. 2015, 54, 2955−2959. (39) Liu, R.; Zheng, Z.; Spurgeon, J.; Yang, X. Enhanced photoelectrochemical water-splitting performance of semiconductors by surface passivation layers. Energy Environ. Sci. 2014, 7, 2504−2517. (40) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science 2014, 344, 1005−1009. (41) Hou, J.; Yang, C.; Cheng, H.; Jiao, S.; Takeda, O.; Zhu, H. Highperformance p-Cu2O/n-TaON heterojunction nanorod photoanodes passivated with an ultrathin carbon sheath for photoelectrochemical water splitting. Energy Environ. Sci. 2014, 7, 3758−3768. Dai, P.; Li, W.; Xie, J.; He, Y.; Thorne, J.; McMahon, G.; Zhan, J.; Wang, D. Forming buried junctions to enhance the photovoltage generated by cuprous oxide in aqueous solutions. Angew. Chem., Int. Ed. 2014, 53, 13493−13497. (42) McDowell, M. T.; Lichterman, M. F.; Spurgeon, J. M.; Hu, S.; Sharp, I. D.; Brunschwig, B. S.; Lewis, N. S. Improved stability of polycrystalline bismuth vanadate photoanodes by use of dual-layer thin TiO2/Ni coatings. J. Phys. Chem. C 2014, 118, 19618−19624. (43) Bao, J. Photoelectrochemical water splitting A new use for bandgap engineering. Nat. Nanotechnol. 2015, 10, 19−20. (44) Lichterman, M. F.; Carim, A. I.; McDowell, M. T.; Hu, S.; Gray, H. B.; Brunschwig, B. S.; Lewis, N. S. Stabilization of n-cadmium telluride photoanodes for water oxidation to O2(g) in aqueous alkaline electrolytes using amorphous TiO2 films formed by atomic-layer deposition. Energy Environ. Sci. 2014, 7, 3334−3337. (45) Yu, X.; Shavel, A.; An, X.; Luo, Z.; Ibanez, M.; Cabot, A. Cu2ZnSnS4-Pt and Cu2ZnSnS4-Au heterostructured nanoparticles for photocatalytic water splitting and pollutant degradation. J. Am. Chem. Soc. 2014, 136, 9236−9239. (46) Liu, R.; Zheng, Z.; Spurgeon, J.; Yang, X. Enhanced photoelectrochemical water-splitting performance of semiconductors by surface passivation layers. Energy Environ. Sci. 2014, 7, 2504−2517. (47) Li, X.; Bassi, P. S.; Boix, P. P.; Fang, Y.; Wong, L. H. Revealing the role of TiO2 surface treatment of hematite nanorods photoanodes for solar water splitting. ACS Appl. Mater. Interfaces 2015, 7, 16960− 16966. (48) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 2011, 331, 746−750. (49) Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu, F.; Huang, F.; Lin, J.; Xie, X.; Jiang, M. H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance. Adv. Funct. Mater. 2013, 23, 5444−5450. (50) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. Water oxidation at hematite photoelectrodes: the role of surface states. J. Am. Chem. Soc. 2012, 134, 4294−302. (51) Li, S.; Zhang, P.; Song, X.; Gao, L. Photoelectrochemical hydrogen production of TiO2 passivated Pt/Si-nanowire composite photocathode. ACS Appl. Mater. Interfaces 2015, 7, 18560−18565. (52) Mei, B.; Pedersen, T.; Malacrida, P.; Bae, D.; Frydendal, R.; Hansen, O.; Vesborg, P. C.; Seger, B.; Chorkendorff, I. Crystalline TiO2: A generic and effective electron-conducting protection layer for photoanodes and-cathodes. J. Phys. Chem. C 2015, 119, 15019−15027. (53) Mei, B.; Permyakova, A. A.; Frydendal, R.; Bae, D.; Pedersen, T.; Malacrida, P.; Hansen, O.; et al. Iron-treated NiO as a highly transparent p-type protection layer for efficient Si-based photoanodes. J. Phys. Chem. Lett. 2014, 5, 3456−3461. 132


ACS Energy Letters

Review

(54) Qiu, J.; Zeng, G.; Ha, M.-A.; Ge, M.; Lin, Y.; Hettick, M.; Hou, B.; Alexandrova, A. N.; Javey, A.; Cronin, S. B. Artificial photosynthesis on TiO2-passivated InP nanopillars. Nano Lett. 2015, 15, 6177−6181. (55) Da, P.; Cha, M.; Sun, L.; Wu, Y.; Wang, Z.-S.; Zheng, G. Highperformance perovskite photoanode enabled by Ni passivation and catalysis. Nano Lett. 2015, 15, 3452−3457. (56) Pu, Y.-C.; Ling, Y.; Chang, K.-D.; Liu, C.-M.; Zhang, J. Z.; Hsu, Y.-J.; Li, Y. Surface passivation of TiO2 nanowires using a facile precursor-treatment approach for photoelectrochemical water oxidation. J. Phys. Chem. C 2014, 118 (27), 15086−15094. (57) Verlage, E.; Hu, S.; Liu, R.; Jones, R. J.; Sun, K.; Xiang, C.; Lewis, N. S.; Atwater, H. A. A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III− V light absorbers protected by amorphous TiO 2 films. Energy Environ. Sci. 2015, 8, 3166−3172. (58) MacLeod, B. A.; Steirer, K. X.; Young, J. L.; Koldemir, U.; Sellinger, A.; Turner, J. A.; Deutsch, T. G.; Olson, D. C. Phosphonic acid modification of GaInP2 photocathodes towards unbiased photoelectrochemical water splitting. ACS Appl. Mater. Interfaces 2015, 7, 11346−11350. (59) Yang, Y.; Gu, J.; Young, J. L.; Miller, E. M.; Turner, J. A.; Neale, N. R.; Beard, M. C. Semiconductor interfacial carrier dynamics via photoinduced electric fields. Science 2015, 350, 1061−1065. (60) Hisatomi, T.; Dotan, H.; Stefik, M.; Sivula, K.; Rothschild, A.; Graetzel, M.; Mathews, N. Enhancement in the performance of ultrathin hematite photoanode for water splitting by an oxide underlayer. Adv. Mater. 2012, 24, 2699−2702. (61) Cho, I. S.; Choi, J. M.; Zhang, K.; Kim, S. J.; Jeong, M. J.; Cai, L.; Park, T.; Park, J. H.; Zheng, X. Highly efficient solar water splitting from transferred TiO2 nanotube arrays. Nano Lett. 2015, 15, 5709− 5715. (62) Li, L.; Yan, J.; Guan, N.; Wang, T.; Zhao, Z.-J.; Gong, J.; Zhang, J. Sub-10 nm rutile titanium dioxide nanoparticles for efficient visiblelight-driven photocatalytic hydrogen production. Nat. Commun. 2015, 6, 5881. (63) Liu, M.; Jing, D.; Zhou, Z.; Guo, L. Twin-induced onedimensional homojunctions yield high quantum efficiency for solar hydrogen generation. Nat. Commun. 2013, 4, 2278. (64) Chang, X.; Wang, T.; Zhang, P.; Zhang, J.; Li, A.; Gong, J. Enhanced surface reaction kinetics and charge separation of p−n heterojunction Co3O4/BiVO4 photoanodes. J. Am. Chem. Soc. 2015, 137, 8356−8359. (65) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science 2014, 343, 990−994. (66) Zhong, M.; Hisatomi, T.; Kuang, Y.; Zhao, J.; Liu, M.; Iwase, A.; Jia, Q.; Nishiyama, H.; Minegishi, T.; Nakabayashi, M.; et al. Surface modification of CoOx loaded BiVO4 photoanodes with ultrathin ptype NiO layers for improved solar water oxidation. J. Am. Chem. Soc. 2015, 137, 5053−5060. (67) Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial separation of photogenerated electrons and holes among 010 and 110 crystal facets of BiVO4. Nat. Commun. 2013, 4, 1432. (68) Zhong, D. K.; Cornuz, M.; Sivula, K.; Grätzel, M.; Gamelin, D. R. Photo-assisted electrodeposition of cobalt−phosphate (Co−Pi) catalyst on hematite photoanodes for solar water oxidation. Energy Environ. Sci. 2011, 4, 1759−1764. (69) Takata, T.; Pan, C.; Nakabayashi, M.; Shibata, N.; Domen, K. Fabrication of a core−shell-type photocatalyst via photodeposition of group IV and V transition metal oxyhydroxides: An effective surface modification method for overall water splitting. J. Am. Chem. Soc. 2015, 137, 9627−9634. (70) Zhong, D. K.; Choi, S.; Gamelin, D. R. Near-complete suppression of surface recombination in solar photoelectrolysis by ″Co-Pi″ catalyst-modified W:BiVO4. J. Am. Chem. Soc. 2011, 133, 18370−18377.

(71) Liu, G.; Shi, J.; Zhang, F.; Chen, Z.; Han, J.; Ding, C.; Chen, S.; Wang, Z.; Han, H.; Li, C. A Tantalum nitride photoanode modified with a hole-storage layer for highly stable solar water splitting. Angew. Chem., Int. Ed. 2014, 53, 7295−7299. (72) Young, K. M.; Hamann, T. W. Enhanced photocatalytic water oxidation efficiency with Ni(OH)2 catalysts deposited on α-Fe2O3 via ALD. Chem. Commun. 2014, 50, 8727−8730. (73) Kim, J. Y.; Jang, J. W.; Youn, D. H.; Magesh, G.; Lee, J. S. A stable and efficient hematite photoanode in a neutral electrolyte for solar water splitting: Towards stability engineering. Adv. Energy Mater. 2014, 4, 1400476. (74) Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J.-W.; Kubota, J.; Domen, K.; Lee, J. S. Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting. Sci. Rep. 2013, 3, 2681. (75) Wang, Z.; Liu, G.; Ding, C.; Chen, Z.; Zhang, F.; Shi, J.; Li, C. Synergetic effect of conjugated Ni (OH) 2/IrO2 cocatalyst on Ti doped hematite photoanode for solar water splitting. J. Phys. Chem. C 2015, 119, 19607−19612. (76) Wei, L.; Chen, Y.; Lin, Y.; Wu, H.; Yuan, R.; Li, Z. MoS 2 as non-noble-metal co-catalyst for photocatalytic hydrogen evolution over hexagonal ZnIn 2 S 4 under visible light irradiations. Appl. Catal., B 2014, 144, 521−527. (77) Yuan, J.; Wen, J.; Zhong, Y.; Li, X.; Fang, Y.; Zhang, S.; Liu, W. Enhanced photocatalytic H 2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/gC 3 N 4 heterojunctions. J. Mater. Chem. A 2015, 3, 18244−18255. (78) Hou, Y.; Zhu, Y.; Xu, Y.; Wang, X. Photocatalytic hydrogen production over carbon nitride loaded with WS2 as cocatalyst under visible light. Appl. Catal., B 2014, 156-157, 122−127. (79) Zhao, Y.-F.; Yang, Z.-Y.; Zhang, Y.-X.; Jing, L.; Guo, X.; Ke, Z.; Hu, P.; Wang, G.; Yan, Y.-M.; Sun, K.-N. Cu2O decorated with cocatalyst MoS2 for solar hydrogen production with enhanced efficiency under visible light. J. Phys. Chem. C 2014, 118, 14238− 14245. (80) Chang, K.; Mei, Z.; Wang, T.; Kang, Q.; Ouyang, S.; Ye, J. MoS2/graphene cocatalyst for efficient photocatalytic H2 evolution under visible light irradiation. ACS Nano 2014, 8, 7078−7087. (81) Chen, D.; Zou, L.; Li, S.; Zheng, F. Nanospherical like reduced graphene oxide decorated TiO2 nanoparticles: an advanced catalyst for the hydrogen evolution reaction. Sci. Rep. 2016, 6, 20335. (82) Zhang, W.; Li, Y.; Zeng, X.; Peng, S. Synergetic effect of metal nickel and graphene as a cocatalyst for enhanced photocatalytic hydrogen evolution via dye sensitization. Sci. Rep. 2015, 5, 10589. (83) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137, 4347−4357. (84) Sheng, S.; Fang, G.; Li, C.; Xu, S.; Zhao, X. p-type transparent conducting oxides. Phys. Status Solidi A 2006, 203, 1891−1900. (85) Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z. Earthabundant cocatalysts for semiconductor-based photocatalytic water splitting. Chem. Soc. Rev. 2014, 43, 7787−7812. (86) Kanan, M. W.; Nocera, D. G. In situ formation of an oxygenevolving catalyst in neutral water containing phosphate and Co2+. Science 2008, 321, 1072−1075. (87) Surendranath, Y.; Dinca, M.; Nocera, D. G. Electrolytedependent electrosynthesis and activity of cobalt-based water oxidation catalysts. J. Am. Chem. Soc. 2009, 131, 2615−2620. (88) Carroll, G. M.; Zhong, D. K.; Gamelin, D. R. Mechanistic insights into solar water oxidation by cobalt-phosphate-modified alphaFe2O3 photoanodes. Energy Environ. Sci. 2015, 8, 577−584. (89) Young, E. R.; Costi, R.; Paydavosi, S.; Nocera, D. G.; Bulovic, V. Photo-assisted water oxidation with cobalt-based catalyst formed from thin-film cobalt metal on silicon photoanodes. Energy Environ. Sci. 2011, 4, 2058−2061. (90) Li, Y.; Zhang, L.; Torres-Pardo, A.; Gonzalez-Calbet, J. M.; Ma, Y.; Oleynikov, P.; Terasaki, O.; Asahina, S.; Shima, M.; Cha, D.; Zhao, L.; Takanabe, K.; Kubota, J.; Domen, K. Cobalt phosphate-modified 133


ACS Energy Letters

Review

barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency. Nat. Commun. 2013, 4, 3566. (91) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An advanced Ni−Fe layered double hydroxide electrocatalyst for water oxidation. J. Am. Chem. Soc. 2013, 135, 8452−8455. (92) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Norskov, J. K.; Nilsson, A.; Bell, A. T. Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting. J. Am. Chem. Soc. 2015, 137, 1305−1313. (93) Wang, L.; Dionigi, F.; Nguyen, N. T.; Kirchgeorg, R.; Gliech, M.; Grigorescu, S.; Strasser, P.; Schmuki, P. Tantalum Nitride Nanorod Arrays: Introducing Ni-Fe layered double hydroxides as a cocatalyst strongly stabilizing photoanodes in water splitting. Chem. Mater. 2015, 27, 2360−2366. (94) Kargar, A.; Yavuz, S.; Kim, T. K.; Liu, C. H.; Kuru, C.; Rustomji, C. S.; Jin, S.; Bandaru, P. R. Solution-processed CoFe2O4 nanoparticles on 3D carbon fiber papers for durable oxygen evolution reaction. ACS Appl. Mater. Interfaces 2015, 7, 17851−17856. (95) Diaz-Morales, O.; Ledezma-Yanez, I.; Koper, M. T.; CalleVallejo, F. Guidelines for the rational design of Ni-based double hydroxide electrocatalysts for the oxygen evolution reaction. ACS Catal. 2015, 5, 5380−5387. (96) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel−iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 2014, 136, 6744−6753. (97) Wang, D.; Zhou, J.; Hu, Y.; Yang, J.; Han, N.; Li, Y.; Sham, T.-K. In situ X-ray absorption near edge structure study of advanced NiFe(OH)x electrocatalyst on carbon paper for eater oxidation. J. Phys. Chem. C 2015, 119, 19573−19583. (98) Yoshida, M.; Mitsutomi, Y.; Mineo, T.; Nagasaka, M.; Yuzawa, H.; Kosugi, N.; Kondoh, H. Direct observation of active nickel oxide cluster in nickel− borate Electrocatalyst for Water oxidation by In Situ O K-edge X-ray absorption spectroscopy. J. Phys. Chem. C 2015, 119, 19279−19286. (99) Mattioli, G.; Zaharieva, I.; Dau, H.; Guidoni, L. Atomistic texture of amorphous manganese oxides for electrochemical water splitting revealed by ab initio calculations combined with X-Ray spectroscopy. J. Am. Chem. Soc. 2015, 137, 10254−10267. (100) Kronawitter, C. X.; Zegkinoglou, I.; Shen, S. H.; Liao, P.; Cho, I. S.; Zandi, O.; Liu, Y. S.; Lashgari, K.; Westin, G.; Guo, J. H.; Himpsel, F. J.; Carter, E. A.; Zheng, X. L.; Hamann, T. W.; Koel, B. E.; Mao, S. S.; Vayssieres, L. Titanium incorporation into hematite photoelectrodes: theoretical considerations and experimental observations. Energy Environ. Sci. 2014, 7, 3100−3121. (101) Kronawitter, C. X.; Zegkinoglou, I.; Shen, S.; Guo, J.; Himpsel, F. J.; Mao, S. S.; Vayssieres, L. On the orbital anisotropy in hematite nanorod-based photoanodes. Phys. Chem. Chem. Phys. 2013, 15, 13483−13488. (102) Liang, H.; Li, L.; Meng, F.; Dang, L.; Zhuo, J.; Forticaux, A.; Wang, Z.; Jin, S. Porous two-dimensional nanosheets converted from layered double hydroxides and their applications in electrocatalytic water splitting. Chem. Mater. 2015, 27, 5702−5711. (103) Li, H.; Yu, Y.; Starr, M.; Li, Z.; Wang, X. Piezotronic-enhanced photoelectrochemical reactions in Ni(OH)2 decorated ZnO photoanodes. J. Phys. Chem. Lett. 2015, 6, 3410−3416. (104) Hang Li, Y.; Xing, J.; Jia Chen, Z.; Li, Z.; Tian, F.; Rong Zheng, L.; et al. Unidirectional suppression of hydrogen oxidation on oxidized platinum clusters. Nat. Commun. 2013, 4, 2500. (105) Gong, M.; Zhou, W.; Tsai, M.-C.; Hwang, B.-J.; Zhou, J.; Hu, Y.; Guan, M.; Lin, M.-C.; Zhang, B.; Wang, D.-Y.; Yang, J.; Dai, H.; Pennycook, S. J. Nanoscale nickel oxide/nickel heterostructures for active hydrogen evolution electrocatalysis. Nat. Commun. 2014, 5, 4695. (106) Vesborg, P. C.; Seger, B.; Chorkendorff, I. Recent development in hydrogen rvolution reaction catalysts and their practical implementation. J. Phys. Chem. Lett. 2015, 6, 951−957. Nellist, M.

R.; Laskowski, F. A.; Lin, F.; Mills, T. J.; Boettcher, S. W. Semiconductor−electrocatalyst interfaces: Theory, experiment, and applications in photoelectrochemical water splitting. Acc. Chem. Res. 2016, 49, 733−740. (107) Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575− 6578. (108) Seo, S. W.; Park, S.; Jeong, H.-Y.; Kim, S. H.; Sim, U.; Lee, C. W.; Nam, K. T.; Hong, K. S. Enhanced performance of NaTaO3 using molecular co-catalyst [Mo3S4]4+ for water splitting into H2 and O2. Chem. Commun. 2012, 48, 10452−10454. (109) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X.-Y.; Lou, X. W. Porous molybdenum carbide nano-octahedrons synthesized via confined carburization in metal-organic frameworks for efficient hydrogen production. Nat. Commun. 2015, 6, 6512. (110) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347, 970−974. (111) Sim, U.; Moon, J.; An, J.; Kang, J. H.; Jerng, S. E.; Moon, J.; Cho, S.-P.; Hong, B. H.; Nam, K. T. N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production. Energy Environ. Sci. 2015, 8, 1329−1338. (112) Roy, N.; Leung, K. T.; Pradhan, D. Nitrogen doped reduced graphene oxide based Pt− TiO2 nanocomposites for enhanced hydrogen evolution. J. Phys. Chem. C 2015, 119, 19117−19125. (113) Peter, L. M. Energetics and kinetics of light-driven oxygen evolution at semiconductor electrodes: the example of hematite. J. Solid State Electrochem. 2013, 17, 315−326. (114) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. Water oxidation at hematite photoelectrodes: the role of surface states. J. Am. Chem. Soc. 2012, 134, 4294−4302. (115) Liu, Y.; Li, J.; Li, W.; Yang, Y.; Li, Y.; Chen, Q. Enhancement of the photoelectrochemical performance of WO3 vertical arrays film for solar water splitting by Gadolinium doping. J. Phys. Chem. C 2015, 119, 14834−14842. (116) Le Formal, F.; Pendlebury, S. R.; Cornuz, M.; Tilley, S. D.; Gratzel, M.; Durrant, J. R. Back electron-hole recombination in hematite photoanodes for water splitting. J. Am. Chem. Soc. 2014, 136, 2564−2574. Wu, K.; Chen, Z.; Lv, H.; Zhu, H.; Hill, C. L.; Lian, T. Hole removal rate limits photodriven H2 generation efficiency in CdSPt and CdSe/CdS-Pt semiconductor nanorod−metal tip heterostructures. J. Am. Chem. Soc. 2014, 136, 7708−7716. (117) Le Formal, F.; Pastor, E.; Tilley, S. D.; Mesa, C. A.; Pendlebury, S. R.; Grätzel, M.; Durrant, J. R. Rate law analysis of water oxidation on a hematite surface. J. Am. Chem. Soc. 2015, 137, 6629− 6637. (118) Wang, X.; Kafizas, A.; Li, X.; Moniz, S. J.; Reardon, P. J. T.; Tang, J.; Parkin, I. P.; Durrant, J. R. Transient absorption spectroscopy of anatase and rutile: the impact of morphology and phase on photocatalytic activity. J. Phys. Chem. C 2015, 119, 10439−10447. (119) Wang, Y.; Wu, D.; Fu, L. M.; Ai, X. C.; Xu, D.; Zhang, J. P. Correlation between energy and spatial distribution of intragap trap dtates in the TiO2 photoanode of dye-sensitized solar cells. ChemPhysChem 2015, 16, 2253−2259. (120) Abdi, F. F.; Savenije, T. J.; May, M. M.; Dam, B.; van de Krol, R. The origin of slow carrier transport in BiVO4 thin film photoanodes: A time-resolved microwave conductivity Study. J. Phys. Chem. Lett. 2013, 4, 2752−2757. (121) Altman, E. I.; Baykara, M. Z.; Schwarz, U. D. Noncontact atomic force microscopy: An emerging tool for fundamental catalysis research. Acc. Chem. Res. 2015, 48, 2640−2648. (122) Sasaki, Y.; Kato, H.; Kudo, A. [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ electron mediators for overall water splitting under sunlight irradiation using Z-scheme photocatalyst system. J. Am. Chem. Soc. 2013, 135, 5441−5449. (123) Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A. ZSchematic Water Splitting into H-2 and O-2 Using Metal Sulfide as a 134


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Review

Hydrogen-Evolving Photocatalyst and Reduced Graphene Oxide as a Solid-State Electron Mediator. J. Am. Chem. Soc. 2015, 137, 604−607. (124) Kobayashi, R.; Tanigawa, S.; Takashima, T.; Ohtani, B.; Irie, H. Silver-inserted heterojunction photocatalysts for Z-Scheme overall pure-water splitting under visible-light irradiation. J. Phys. Chem. C 2014, 118, 22450−22456. (125) Li, J.; Cushing, S. K.; Zheng, P.; Senty, T.; Meng, F.; Bristow, A. D.; Manivannan, A.; Wu, N. Solar hydrogen generation by a CdSAu-TiO2 sandwich nanorod array enhanced with Au nanoparticle as electron relay and plasmonic photosensitizer. J. Am. Chem. Soc. 2014, 136, 8438−8449. (126) May, M. M.; Lewerenz, H. J.; Lackner, D.; Dimroth, F.; Hannappel, T. Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure. Nat. Commun. 2015, 6, 8286. (127) Maciá-Agulló, J. A.; Corma, A.; Garcia, H. Photobiocatalysis: The power of combining photocatalysis and enzymes. Chem. - Eur. J. 2015, 21, 10940−10959. (128) Mersch, D.; Lee, C.-Y.; Zhang, J. Z.; Brinkert, K.; FontecillaCamps, J. C.; Rutherford, A. W.; Reisner, E. Wiring of photosystem II to hydrogenase for photoelectrochemical water splitting. J. Am. Chem. Soc. 2015, 137, 8541−8549. (129) Wang, W.; Chen, J.; Li, C.; Tian, W. Achieving solar overall water splitting with hybrid photosystems of photosystem II and artificial photocatalysts. Nat. Commun. 2014, 5, 4647. (130) Sakimoto, K. K.; Wong, A. B.; Yang, P. Self-photosensitization of nonphotosynthetic bacteria for solar-to-chemical production. Science 2016, 351, 74−77. (131) Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R. Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode. Nat. Commun. 2013, 4, 3195. (132) Kim, J. H.; Kaneko, H.; Minegishi, T.; Kubota, J.; Domen, K.; Lee, J. S. Overall Photoelectrochemical Water Splitting using Tandem Cell under Simulated Sunlight. ChemSusChem 2016, 9, 61−66. (133) Khaselev, O.; Turner, J. A. A monolithic photovoltaicphotoelectrochemical device for hydrogen production via water splitting. Science 1998, 280, 425−427. (134) Zhang, N.; Shi, J.; Mao, S. S.; Guo, L. Co3O4 quantum dots: reverse micelle synthesis and visible-light-driven photocatalytic overall water splitting. Chem. Commun. 2014, 50, 2002−2004. (135) Liao, L.; Zhang, Q.; Su, Z.; Zhao, Z.; Wang, Y.; Li, Y.; Lu, X.; Wei, D.; Feng, G.; Yu, Q.; Cai, X.; Zhao, J.; Ren, Z.; Fang, H.; RoblesHernandez, F.; Baldelli, S.; Bao, J. Efficient solar water-splitting using a nanocrystalline CoO photocatalyst. Nat. Nanotechnol. 2013, 9, 69−73. (136) Ohno, T.; Bai, L.; Hisatomi, T.; Maeda, K.; Domen, K. Photocatalytic water splitting using modified GaN:ZnO solid solution under visible light: Long-time operation and regeneration of activity. J. Am. Chem. Soc. 2012, 134, 8254−8259. (137) Wang, D.; Pierre, A.; Kibria, M. G.; Cui, K.; Han, X.; Bevan, K. H.; Guo, H.; Paradis, S.; Hakima, A.-R.; Mi, Z. Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy. Nano Lett. 2011, 11, 2353−2357. (138) Townsend, T. K.; Browning, N. D.; Osterloh, F. E. Overall photocatalytic water splitting with NiOx-SrTiO3 - a revised mechanism. Energy Environ. Sci. 2012, 5, 9543−9550. (139) Mubeen, S.; Lee, J.; Singh, N.; Kraemer, S.; Stucky, G. D.; Moskovits, M. An autonomous photosynthetic device in which all charge carriers derive from surface plasmons. Nat. Nanotechnol. 2013, 8, 247−251. (140) Yu, Z.; Li, F.; Sun, L. Recent advances in dye-sensitized photoelectrochemical cells for solar hydrogen production based on molecular components. Energy Environ. Sci. 2015, 8, 760−775. (141) Swierk, J. R.; McCool, N. S.; Saunders, T. P.; Barber, G. D.; Mallouk, T. E. Effects of electron trapping and protonation on the efficiency of water-splitting dye-sensitized solar cells. J. Am. Chem. Soc. 2014, 136, 10974−10982.

(142) Li, F.; Fan, K.; Xu, B.; Gabrielsson, E.; Daniel, Q.; Li, L.; Sun, L. Organic dye-sensitized tandem photoelectrochemical cell for light driven total water splitting. J. Am. Chem. Soc. 2015, 137, 9153−9159. (143) Sherman, B. D.; Ashford, D. L.; Lapides, A. M.; Sheridan, M. V.; Wee, K.-R.; Meyer, T. J. Light driven water splitting with a molecular electro-aassembly based core/shell photoanode. J. Phys. Chem. Lett. 2015, 6, 3213−3217. (144) Click, K. A.; Beauchamp, D. R.; Huang, Z.; Chen, W.; Wu, Y. Membrane-Inspired acidically stable dye-sensitized photocathode for solar fuel production. J. Am. Chem. Soc. 2016, 138, 1174−1179. (145) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Tilley, S. D.; Gratzel, M.; et al. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014, 345, 1593−1596. (146) Gurudayal; Sabba, D.; Kumar, M. H.; Wong, L. H.; Barber, J.; Grätzel, M.; Mathews, N. Perovskite−hematite tandem cells for efficient overall solar driven water splitting. Nano Lett. 2015, 15, 3833−3839. (147) Chen, Y.-S.; Manser, J. S.; Kamat, P. V. All solution-processed lead halide perovskite-BiVO4 tandem assembly for photolytic solar fuels production. J. Am. Chem. Soc. 2015, 137, 974−981. (148) Boix, P. P.; Agarwala, S.; Koh, T. M.; Mathews, N.; Mhaisalkar, S. G. Perovskite solar cells: Beyond methylammonium lead iodide. J. Phys. Chem. Lett. 2015, 6, 898−907. (149) Koops, S. E.; O’Regan, B. C.; Barnes, P. R.; Durrant, J. R. Parameters influencing the efficiency of electron injection in dyesensitized solar cells. J. Am. Chem. Soc. 2009, 131, 4808−4818. (150) Christians, J. A.; Manser, J. S.; Kamat, P. V. The multifaceted excited state of CH3NH3PbI3. Charge separation, recombination, and trapping. J. Phys. Chem. Lett. 2015, 6, 2086−2095. (151) Wei, Y. K.; Su, J. Z.; Wan, X. K.; Guo, L. J.; Vayssieres, L. Spontaneous photoelectric field-enhancement effect prompts the low cost hierarchical growth of highly ordered heteronanostructures for solar water splitting. Nano Res. 2016, DOI: 10.1007/s12274-016-10509. (152) Yang, Y.; Gu, J.; Young, J. L.; Miller, E. M.; Turner, J. A.; Neale, N. R.; Beard, M. C. Semiconductor interfacial carrier dynamics via photoinduced electric fields. Science 2015, 350, 1061−1065. (153) Dong, W.; Guo, Y.; Guo, B.; Li, H.; Liu, H.; Joel, T. W. Enhanced photovoltaic effect in BiVO4 semiconductor by incorporation with an ultrathin BiFeO3 ferroelectric layer. ACS Appl. Mater. Interfaces 2013, 5, 6925−6929. (154) Zhang, W.; Zhu, R.; Li, F.; et al. High-performance solid-state organic dye sensitized solar cells with P3HT as hole transporter. J. Phys. Chem. C 2011, 115, 7038−7043. (155) Fabregat-Santiago, F.; Bisquert, J.; Cevey, L.; et al. Electron transport and recombination in solid-state dye solar cell with spiroOMeTAD as hole conductor. J. Am. Chem. Soc. 2009, 131, 558−562. (156) Ye, S.; Sun, W.; Li, Y.; et al. CuSCN-based inverted planar perovskite solar cell with an average PCE of 15.6%. Nano Lett. 2015, 15, 3723−3728. (157) Christians, J. A.; Fung, R. C. M.; Kamat, P. V. An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper iodide. J. Am. Chem. Soc. 2014, 136, 758−764. (158) Kawazoe, H.; Yasukawa, M.; et al. p-type electrical conduction in transparent thin films of CuAlO2. Nature 1997, 389, 939−942. (159) Hautier, G.; Miglio, A.; Ceder, G.; et al. Identification and design principles of low hole effective mass p-type transparent conducting oxides. Nat. Commun. 2013, 4, 2292. (160) A Place in the Sun; Paramount Pictures, 1951.

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A Place in the Sun for Artificial Photosynthesis? - ACS Energy Letters

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