Particulate Photocatalysts for Water Splitting: Recent Advances and

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Particulate Photocatalysts for Water Splitting. Recent Advances and Future Prospects Tsuyoshi Takata, and Kazunari Domen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02209 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Energy Letters

Particulate Photocatalysts for Water Splitting. Recent Advances and Future Prospects Tsuyoshi Takata† and Kazunari Domen†,§,*

†Centre

for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553 Japan

§Department

of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Japan.

AUTHOR INFORMATION Corresponding Author * Kazunari Domen, E-mail: [email protected]

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ABSTRACT. Water splitting via photocatalysis has the potential to produce clean, renewable hydrogen. Although various new photocatalysts have been reported, developing semiconductor materials that efficiently convert the energy in sunlight remains the primary challenge. This Perspective focuses on representative examples of particulate photocatalysts for overall water splitting. Both the design of photocatalytic materials and overall systems intended for largescale operation are discussed.

TOC GRAPHICS


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At present, human activities require ever-increasing amounts of energy, and so the development of environmentally-friendly, sustainable energy resources is a pressing concern. The most ideal means of generating clean, sustainable energy would be to convert solar energy into a storable, useable form. This could potentially be accomplished by splitting water into hydrogen and oxygen using solar energy in conjunction with a semiconductor. However, the practical realization of this process remains very challenging. Although the total energy supplied by solar radiation is quite large, the energy density is low. Therefore, both of scalability and conversion efficiency are required to be high for a large quantity of sunlight acceptance and conversion in order to develop viable alternatives to the combustion of fossil fuels. Water splitting into hydrogen and oxygen is an uphill chemical reaction that involves an increase in Gibbs free energy. Therefore, this reaction does not proceed spontaneously and the injection of external energy is necessary. Thermodynamically, the minimum photon energy required to split water based on water oxidation via a four-electron transfer process is 1.23 eV, which corresponds to a wavelength of ca. 1000 nm. Semiconductors or molecular dyes will absorb photons with energy values larger than their bandgap or the gap between their highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively, with the subsequent formation of excited electron-hole pairs. The excited electrons resulting from this process reduce H+ to H2 while the holes oxidize H2O to O2, respectively. The H2 produced via this route can be employed as a clean, renewable and high-density energy carrier, and could also be stored in the form of hydrocarbons, alcohols or NH3 following a subsequent catalytic reaction with CO2 or N2, respectively. Semiconductors are typically used in artificial water photolysis systems rather than dye molecules, even though photosynthesis in plants is driven by an integrated molecular system, because it is difficult to directly mimic natural photosynthesis. Many inorganic semiconductors meant for water



photolysis have been developed, as these compounds tend to resist photodegradation and are able to perform multiple charge transfers. This perspective therefore focuses on recent advances in particulate semiconductor photocatalysts, as well as their future prospects. To date, three main categories of water splitting methods based on semiconductors have been examined.1 These comprise photovoltaic-powered electrolysis (PV-E), photoelectrochemical (PEC) processes and photocatalysis. PV-E techniques have thus far provided the highest solar-tohydrogen energy conversion efficiency (STH) values (up to 20-30%).2-4 However, these methods involve integrated PV device structures that are difficult to fabricate on a large scale. Large PV-E reactors tend to have pH gradients as well as high levels of solution resistance, based on the significant distances between reduction and oxidation sites. Vigorous stirring of the reaction solution and the addition of a large amount of electrolyte are necessary to mitigate these issues, both of which complicate the scaling-up process for the reaction system.5 The same challenges are associated with PEC water splitting devices. In contrast, it is simpler to expand photocatalytic water splitting systems, especially those based on simple powdered photocatalysts, to larger scales. During photocatalysis, the reduction and oxidation steps both occur on individual semiconductor particles, such that mass transfer limitations are negligible. This feature could allow much simpler reactor structures, although there is the associated issue of separating the gaseous H2 and O2 generated during catalysis. Moreover, the STH values exhibited by photocatalysis systems are generally lower than 1%, due to the difficulty inherent in controlling the structure of catalysts with very small particle sizes. As such, each water splitting system presently has merits and demerits, and future research directions should be determined based on careful consideration of these factors. Figure 1A summarizes the basic principles of water splitting on a semiconductor photocatalyst. In this process, photoexcited electrons in the conduction band and holes in the valence band


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migrate to the surface of a semiconductor particle and subsequently reduce H+ to H2 and oxidize H2O to O2, respectively. The primary prerequisites for semiconductors meant to split water are therefore the size and position of the bandgap. Specifically, the conduction-band minimum (CBM) and valence-band maximum (VBM) must straddle the H+/H2 and O2/H2O redox potentials. Because the semiconductor surface is exposed to the reaction solution in photocatalytic and PEC systems, the flat-band potential tends to shift along with the potential of redox species in the solution.6 In contrast, in a typical case of a PV-E system, the band position of the semiconductor does not need to be adjusted relative to the redox potentials of H+/H2 and O2/H2O because only the conductive electrode surface is in contact with the reaction solution. In a photocatalysis system, the band potentials of the semiconductors relative to those of the redox species in the reaction solution is sensitive so as to induce redox reactions. For this reason, various semiconductors have been studied for use as photocatalysts, and tuning the size and position of the bandgap by adjusting the semiconductor composition is currently one of the most


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important aspects of research to improve photocatalysis.7,8

Potential / V vs. NHE (pH 0)

(A) e-

CB

e-

e-

0

H+/H2

hv 1.23 h+

h+ O2/H2O

h+

VB

(B) Potential / V vs. NHE (pH 0)

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e-

CB e-

0 e- CB e-

hv 1.23

O2/H2O h +

h+

VB

e- H+/H 2

hv

e- Ox/Red h +

h+

VB

h+

h+

O2 evolution photocatalyst (OEP)

H2 evolution photocatalyst (HEP)

Figure 1 Basic principles of water splitting on semiconductor photocatalyst. (A) One-step photoexcitation model and (B) Z-scheme water splitting. Another important aspect of photocatalysis is the cocatalyst, which works to adjust the kinetics of the redox reactions. As noted, both reduction and oxidation reactions occur within the same photocatalyst particle, in contrast to the distant reduction and oxidation sites in PV-E or PEC systems. For this reason, cocatalysts are susceptible to crossover reactions, such as oxygen reduction by photoexcited electrons and hydrogen oxidation by holes. In fact, the occurrence of the oxygen reduction reaction (ORR) at the H2 evolution cocatalyst has been a serious issue associated with water splitting systems. This problem has been successfully solved by surface coating of the cocatalyst with a material acting as a molecular sieve that controls the access of reactants and products to the reaction sites.9,10 Thus, band engineering of the semiconductor and


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optimization of the cocatalyst functioning (that is, tuning the thermodynamics and kinetics, respectively) can result in successful water splitting. So-called Z-scheme water splitting, as depicted in Fig. 1B, is another important approach that has been widely researched in recent years.11-13 As the bandgap of the semiconductor becomes smaller, the thermodynamics of the water splitting process become more challenging. Combining two semiconductors and adding their bandgaps, similar to a direct connection between batteries, is expected to overcome this problem. In this model, two different semiconductors are employed to separately evolve H2 and O2 while the remaining holes in one of the materials and the excited electrons in the other are short-circuited via a solution-based redox mediator or a solid-state electron mediator. Compared with the one-step excitation route, this method is advantageous in terms of thermodynamics while more challenging in terms of kinetics. This is because the number of charge transfer steps that must be kinetically controlled increases along with the number of photoexcitation steps. Numerous examples of overall water splitting have been reported, and several recent advances in water splitting photocatalysts are presented herein, along with representative examples. Various oxide photocatalysts, such as TiO2 and SrTiO3, have been applied to overall water splitting.7,14,15 However, these materials are sensitive only to UV light because they tend to have wide bandgaps, primarily as a result of deep VBM levels. For this reason, our group has attempted to design photocatalysts having relatively narrow bandgaps so as to utilize visible light.16-23 In this prior work, certain non-oxides were found to split water into H2 and O2 under visible light irradiation.2023

Nitrides and oxynitrides have shallower VBM levels than those of oxides. This variation in the

VBM is a result of the different energy levels of the main constituents of the N2p and O2p orbitals, which leads to bandgap narrowing.



During a survey of various oxynitrides having similar metallic constituents to those in oxide photocatalysts, Zn-Ga and Zn-Ge quaternary oxynitrides were determined to promote overall water splitting under visible light.20,21 Zn-Ga oxynitrides are solid solutions composed of GaN and ZnO having the same wurtzite crystal structure. Due to their similar lattice parameters, GaN and ZnO form a range of solid solutions, denoted as GaN:ZnO.24-26 The Zn/Ga ratio in such materials can be varied by modifying the nitridation duration because Zn is volatile under the nitridation conditions. Although both of the original compounds are wide bandgap semiconductors responding only to UV light, the formation of their solid solution results in an extension of the absorption edge to the visible region, as shown in Fig. 2A.25,26 Thus, when these materials are modified by the addition of a suitable cocatalyst, overall water splitting proceeds even under visible light. Previous work with GaN:ZnO photocatalysts also identified an effective cocatalyst for overall water splitting.27-29 As noted, one of the main obstacles to efficient overall water splitting is the reverse reaction to form water from H2 and O2, as a result of the crossover reaction (that is, the ORR) on the H2 evolution cocatalyst. The method developed to inhibit the ORR is summarized in Fig. 2B. This technique consists of coating the surface of the cocatalyst (typically Pt or Rh) with a hydrated Cr2O3 nanolayer to prevent the migration of O2 to the surface while simultaneously allowing access to the reactants H+ and H2O. As a result, the cocatalyst selectively promotes the forward reaction while the reverse reaction is inhibited.9 As shown in Fig. 2C, successfully inhibiting the ORR by Cr2O3 deposition on the H2 evolution cocatalyst boosts the


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water splitting activity.30 This is a typical example of rational design of the cocatalyst structure.

Figure 2 (A) Photographic images of GaN, ZnO and series of GaN:ZnO solid solutions. Reprinted from ref (25). (B) Schematic illustration of H2 evolution and ORR inhibition on a core/shell noblemetal/Cr2O3 cocatalyst. Reprinted from ref (9). (C) Data obtained from photocatalytic water splitting on Rh/GaN:ZnO under visible light (λ ≥ 400 nm) (i) with and (ii) without Cr2O3 photodeposition. Reprinted from ref (30). The development of cocatalysts such as this is also indispensable to eventual industrial applications. In full-scale facilities, the water splitting reaction would be conducted at ambient pressure while, in contrast, laboratory-scale studies are commonly conducted using a reduced background pressure. Under these conditions, the O2 partial pressure is lower than the value that would be obtained in a practical scenario, allowing a higher water splitting rate to be maintained. However, when using the cocatalyst system described above, the water splitting rate can be independent of the partial pressure of O2, meaning that this cocatalyst has practical applications.



Although the GaN:ZnO photocatalyst described above was optimized, its usable wavelength region was limited to below 500 nm. Transition metal oxynitrides have been examined as a means of further extending this region, because many of these compounds have optical absorption edges at longer wavelengths. The first reported example of overall water splitting by a transition metal oxynitride employed LaMg1-xTaxO1+3xN2-3x.22 These materials comprise a series of solid solutions between the two perovskite type compounds LaMg2/3Ta1/3O3 and LaTaON2 (Fig. 3A).31-33 The absorption band edge for this series varies from 525 to 640 nm upon decreasing x from 0.6 to 0, as shown in Fig. 3B, and overall water splitting is possible for x ≥ 0.33, following a unique surface modification (Fig. 3C). Specifically, loading a Rh-Cr binary cocatalyst permits photocatalytic H2 and O2 evolution via bandgap excitation. However, this is accompanied by N2 evolution as a result of oxidation of nitrogen species on the surface of the photocatalyst. Moreover, the ORR on the H2 evolution cocatalyst is not completely inhibited for this photocatalyst. Coating the entire surface of the cocatalyst and that of the semiconductor with a thin layer of amorphous hydrous TiO2 (via photodeposition from an aqueous Ti-peroxide solution) inhibits self-oxidation of the photocatalyst as well as the reverse reaction, enabling overall water splitting. In this case, the TiO2 layer functions as a molecular sieve to selectively control the access of reactants and products to the reaction sites.10,22 Thus, a TiO2 layer could serve as an alternative to a Cr2O3 layer. A LaMg1xTaxO1+3xN2-3x

composition with x = 0.33 shows the highest photocatalytic activity, and exhibits


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stable overall water splitting even under visible light (λ ≥ 420 nm) (Fig. 3D).34

Figure 3 (A) Crystal structure and (B) UV-visible diffuse reflectance spectra of LaMgxTa1xO1+3xN2-3x

series of compounds. Reprinted with permission from ref (22) Copyright 2015 Wiley-

VCH. (C) Schematic illustration of water splitting reaction mechanism on surface-modified photocatalyst. (D) Data obtained for overall water splitting on TiO2/SiO2/RhCrOy-modified LaMg0.33Ta0.67O2N under visible light (λ ≥ 420 nm). Reprinted with permission from ref (34) Copyright 2016 Wiley-VCH. Very recently, another example of overall water splitting on the transition metal nitride Ta3N5 was reported.35 A key factor in that work was the design of oxynitride synthesis so as to control the nanostructure and defect state. Ta3N5 is typically obtained via thermal ammonolysis of Ta2O5, but materials prepared using this method are not active for overall water splitting. Ta3N5 has been suggested to have band potentials capable of performing overall water splitting. However, the



synthesis of this material with less defects has been difficult. Defects generate under reductive atmosphere of NH3 during prolonged nitridation. Photoexcited carriers tends to be trapped in such defect levels, which presumably hinders migration of photogenerated carries as well as water splitting. Ta3N5 can also be synthesized by heating an alkali metal tantalate under a dry NH3 flow.36 As an example, heating KTaO3 under a NH3 flow results in the loss of K2O and nitridation of the remaining Ta2O5 to Ta3N5. Thus, the proportion of the Ta3N5 phase increases while that of the KTaO3 phase decreases as the nitridation progresses. Using this method, Ta3N5 crystals can be grown from KTaO3 particles and Ta3N5/KTaO3 composites can be obtained by carefully controlling the nitridation conditions. Fig. 4A,B demonstrates the formation of Ta3N5 nanorods on a crystalline KTaO3 particle. Notably, these nanorods tend to grow on specific crystal faces of the KTaO3 substrate, likely due to similar lattice parameters between the two crystal phases. The composite structure is maintained during short nitridation durations, while prolonged nitridation results in pilling off of Ta3N5 nanorods from the KTaO3 crystals. Samples fabricated using short nitridation durations (i.e., nano-Ta3N5/KTaO3 composites) are capable of overall water splitting under visible light (Fig. 4C,D). The active phase during this process is evidently Ta3N5 rather than nitrogen-doped KTaO3. Limiting the nitridation time maintains the composite structure and results in fewer defects, both of which permit successful overall water splitting.


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Figure 4 (A) Scanning electron microscopic (SEM) and (B) annular dark field-scanning transmission microscopic (ADF-STEM) images of KTaO3 subjected to various nitridation durations. The specimen in the ADF-STEM images was nitrided for 0.25 h and is viewed from the [001] direction of the Ta3N5. (C) Data obtained for overall water splitting on Rh/Cr2O3-modified Ta3N5/KTaO3 synthesized with nitridation time of 0.25 h under visible light (λ ≥ 420 nm) and (D) apparent quantum efficiency as function of incident light wavelength during overall water splitting over this same material. Reprinted with permission from ref (35) Copyright 2018 Springer Nature. The results described in the previous subsection demonstrate that overall water splitting is possible via one-step photoexcitation using non-oxide semiconductors responding up to 600 nm. However, the water splitting efficiency associated with such photocatalysts is presently quite low (apparent quantum efficiency (AQY) = ca. 0.2% at approximately 420 nm), meaning that efficient solar energy conversion is not possible. Another approach to visible light utilization is Z-scheme water splitting combining two different types of semiconductors. One well-developed Z-scheme



water splitting system involves a combination of Rh-doped SrTiO3 and BiVO4 as the H2 evolution photocatalyst (HEP) and O2 evolution photocatalyst (OEP), respectively.37-42 In general, when the HEP and OEP evolve H2 and O2, respectively, holes in the valence band of the HEP and excited electrons in the conduction band of the OEP have to be short-circuited to construct a photocatalytic cycle. The Fe2+/Fe3+, I-/IO3- and Co(bpy)32+/Co(bpy)33+ redox couples are typically employed as aqueous redox mediators for this purpose.11,37,39 However, solid-state electron mediators, including metals, carbon materials and conductive oxides, can also be used for short-circuiting.40-43 In this case, the extent of physical contact between the HEP and OEP via the solid-state electron mediator determines the degree of electrical contact and the resulting water splitting activity. Monoparticle layer of a mixture of La, Rh-doped SrTiO3 and Mo-doped BiVO4 coated on a specific conductive substrate have shown remarkably high activity for water splitting among the ever reported particulate photocatalysts, with an AQY of 33% at approximately 420 nm and an STH of 1.1% (Fig. 5).40 This assembly is fabricated using a particle transfer method,44 depositing a thick coating of Au or C on the photocatalyst particle layer by vacuum evaporation or sputtering, respectively. In this photocatalyst system, the ORR occurs on the conductive substrate and the water splitting rate is significantly affected by the presence of a Au layer but is less affected by a C layer.41 The particle transfer method used to fabricate this system requires the application of a vacuum and a drying process to produce a thick conductive layer. As such, scaling up for the purposes of industrial applications is not realistic. Consequently, only chemical and powder-based synthetic routes are expected to be developed, so as to simplify the fabrication process. The Z-scheme photocatalyst sheet is namely an assembly of miniatures of p-n photoelectrodes with parallel configuration. It is noted that a tandem structure consisting of a transparent conductive film attached with HEP and OEP particles on different sides would enable an optimal sunlight


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absorption. Although fabrication of such a structure is difficult at present, it is an interesting future challenge to optimize an STH value.

Figure 5 (A) Schematic illustration of Z-scheme photocatalyst sheet and (B) data obtained for overall water splitting on SrTiO3:La,Rh/Au/BiVO4:Mo photocatalyst sheet loaded with Ru and Cr2O3 under simulated sunlight at background pressure of 10 kPa and at 331 K. Adopted with permission from ref (40) Copyright 2016 Springer Nature. A former techno-economic analysis has suggested that powder-based photocatalytic systems would be the least expensive route to practical solar H2 production, assuming that a photocatalyst having the required efficiency can be obtained.44 However, this analysis is based on various assumptions regarding costs and lacks complete background information. In fact, scaling up photocatalytic reaction systems and the viability of performing water splitting under actual sunlight have not yet been satisfactorily examined, and future techno-economic analyses should ACS Paragon Plus Environment


incorporate data from bench-scale tests. The lack of such analyses is primarily because there are currently few photocatalysts that function suitably under sunlight at ambient pressure. The aforementioned Rh-Cr bimetallic cocatalyst has potential for such applications, while semiconductor compounds are also under development. It is important to begin examining the scaling up of solar water splitting systems in parallel with the development of such photocatalytic materials. Recently, our group developed the highly active catalyst SrTiO3 using a flux method.46,47 This material is a wide gap semiconductor and exhibits optical absorption only in the UV region (λ ≤ 380 nm), with a small overlap with the sunlight spectrum. However, when functioning under UV radiation, this material does cause water splitting with a high AQY (56% at 365 nm) and an STH of approximately 0.5%. In fact, this material will generate gas bubbles under sunlight, indicating that water splitting proceeds. Although the associated efficiency is low, it was found possible to use this catalyst to fabricate a 1 × 1 m bench-scale prototype of a solar water splitting system operating under natural sunlight. Two key factors were incorporated into the design of this prototype: immobilization of the photocatalyst powder on a substrate and the use of a thin paneltype reactor. In laboratory-scale experiments for photocatalytic reaction, the photocatalyst powder is typically suspended in a stirred reaction solution to ensure sufficient mass transfer. However, this method is not optimal because of the extra energy input required for stirring and the difficult in collecting the powder after use. In addition, tilting the substrate can receive sunlight efficiently. As such, immobilized system is likely to be easier to handle compared with a powder suspension. Immobilizing the photocatalyst powder is typically performed by painting it onto a substrate using a wet process, and the addition of a binder and subsequent heating are commonly employed. However, organic binders can act as contaminants, and heating at temperatures high enough to


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completely remove these contaminants may damage the photocatalyst. SiO2 nanoparticles have been found to function effectively as an inorganic binder while mitigating the above problems.48 In prior work, a RhCrOx/SrTiO3 photocatalyst was deposited on a glass substrate along with a SiO2 binder by drop casting. The SiO2 nanoparticles in the binder also likely promoted the migration of water to the photocatalyst as well as the release of bubbles of the gaseous products from the particulate layer. This occurred because the hydrophilic SiO2 nanoparticles attracted water but repelled the hydrophobic products H2 and O2. Therefore, issues related to mass transfer limitations can be avoided by the addition of SiO2 nanoparticles. Our prior work employed a panel-type reactor made of acrylic material, as illustrated in Fig. 6.47 The reactor had an inner surface area of approximately 1 × 1 m and a 6 mm thickness, respectively. The photocatalyst powder was fixed on glass plates and nine such plates (each 33 × 33 cm in surface area and 2 mm thick) were arrayed on the base of the reactor. The device held approximately 4 kg of water when filled to a depth of 4 mm. The panel reactor was tilted at 1020° to allow it to efficiently accept sunlight. In this case, the lower portion of the reactor experienced a greater load due to the water weight. For this reason, we employed panel-type reactor with a thin water depth to reduce this load. When this panel was exposed to sunlight, bubbles of H2 and O2 were generated from the photocatalyst layers and traveled upward along the inner wall of the window plate. It was necessary to apply a hydrophilic coating to the inner surface of the window to prevent these hydrophobic bubbles from clinging to the inner wall, accumulating in the reactor and creating an explosive mixture. Sunlight-driven photocatalytic water splitting was demonstrated using a SrTiO3 photocatalyst panel on a summer day in Tokyo, providing an STH of approximately 0.4%. This efficiency was essentially the same as that obtained during laboratoryscale experiments. Thus, these trials demonstrate that powder-based photocatalytic water splitting



is readily extensible from the centimeter to the meter size scale. Moreover, such photocatalyst panels could be used as modules in the construction of large-scale solar hydrogen production plants.

Figure 6 (A) Schematic illustration and (B) photographic image of photocatalytic water splitting panel. Adopted with permission from ref (47) Copyright 2018 Elsevier. Over the past two decades, a number of semiconductor photocatalysts have been developed, and significant progress has been achieved in this research field. Increasing concerns regarding energy and environmental issues have also accelerated this research, especially during the last ten years. This work has demonstrated processes for the rational design of water splitting photocatalysts. Tuning the magnitude and position of the bandgaps of semiconductors by composition modification is an important aspect of satisfying the thermodynamic requirements of water splitting. In addition, kinetic control using a cocatalyst has been shown to be indispensable to efficient overall water splitting. In particular, such cocatalysts should promote the forward reaction while inhibiting the reverse reaction. Several examples of overall water splitting via one-step photoexcitation with visible light have been reported using oxynitride photocatalysts, based on thermodynamic and kinetic control. The usable wavelength region has also been extended up to 600 nm by using narrow-gap


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semiconductors. Nevertheless, suitably high STH values have not yet been achieved, primarily because the AQY values for water splitting on these photocatalysts are lower than those associated with wide bandgap oxides. Presently, the STH of approximately 1% exhibited by Z-scheme photocatalyst sheets is the highest reproducible value yet reported, based on oxide photocatalysts sensitive up to approximately 500 nm. Extending the operable wavelength of these sheets up to 600 nm while maintaining the AQY could provide STH values closer to that required for practical solar water splitting. However, upgrading the efficiency to the required level using only simple, scalable methods based on presently available conceptual approaches remains challenging. Thus, more advanced material designs are needed. It is necessary to enhanced the STH above 5 % in photocatalysis system to consider practical application more realistically. Scalability is also an important aspect of designing such systems, because solar water splitting plants will be required to cover large land areas due to the low energy density of sunlight. Powderbased photocatalytic reaction systems have been demonstrated to be readily extensible to large sizes, although a means of separating the H2-O2 mixture must be developed. This would occur primarily in other fields of research. Establishing the technology for the mass production of photocatalysts is also required. The need for bench-scale tests will increase in the near future, as will the necessity for multiple collaborations, and the interplay between various research endeavors is expected to speed the progress of photocatalytic systems toward industrial applications.

AUTHOR INFORMATION E-mail: [email protected] Biographies



Prof. Tsuyoshi Takata received his PhD from Tokyo Institute of Technology in 2000 under Prof. Kazunari Domen’s supervision. He is an Appointed Professor at Shinshu University. His research area is photocatalytic water splitting with a focus on new material design and synthesis. Prof. Kazunari Domen received his PhD(1982) in chemistry from the University of Tokyo. He is a Professor at the University of Tokyo. He has also been a Specially Appointed Professor at Shinshu University since 2017. His research area includes heterogeneous catalysis, materials chemistry, and surface science. His main research interest has been photocatalytic water splitting to generate solar hydrogen.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was primarily supported by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO) and was partly supported by a Grant-in-Aid for Scientific Research (A) (No. 16H02417) and a Grant-in-Aid for Scientific Research (C) (No. 16K06862) from the Japan Society for the Promotion of Science (JSPS)

REFERENCES (1) Walter, G. M.; Warren, L. E.; McKone, J. R.; Bottcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.


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ACS Energy Letters

(2) Nakamura, A.; Ota, Y.; Koike, K.; Hidaka, Y.; Nishioka, K.; Sugiyama, M.; Fujii, K. A 24.4% Solar to Hydrogen Energy Conversion Efficiency by Combining Concentrator Photovoltaic Modules and Electrochemical Cells. Appl. Phys. Express 2015, 8, 107101-107104. (3) Jia, J.; Seitz, L. C.; Bench, J. D..; Huo, Y.; Chen, Y.; Ng, J. W. D.; Biller, T.; Harris, J. S.; Jaramillo, T. F. Solar Water Splitting by Photovoltaic-electrolysis with a Solar-to-Hydrogen Efficiency over 30%. Nat. Commun. 2016, 7, 13237-13241. (4) Turan, B.; Becker, J. P.; Urbain, F.; Finger, F.; Rau, U.; Haas, S. Upscaling of Integrated Photoelectrochemical Water-splitting Devices to Large Areas. Nat. Commun. 2016, 7, 1268112689. (5) Xaing, C.; Weber, A. Z.; Ardo, S.; Berger, A.; Chen, Y.; Coridan, R.; Fountaine, K. T.; Haussener, S.; Hu, S.; Liu, R. et al. Modeling, Simulation, and Implementation of Solar-Driven Water‐Splitting Devices. Angew. Chem. Int. Ed. 2016, 55, 12974-12988. (6) Scaife, D. E. Oxide Semiconductors in Photoelectrochemical Conversion of Solar Energy. Sol. Energy 1980, 25, 41-54. (7) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. (8) Maeda, K.; Domen, K. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. C 2007, 111, 7851-7861. (9) Yoshida, M.; Takanabe, K.; Maeda, K.; Ishikawa, A.; Kubota, J.; Sakata, Y.; Ikezawa, Y.; Domen, K. Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for



Photocatalytic Overall Water Splitting Studied by Model Electrodes. J. Phys. Chem. C 2009, 113, 10151-10157. (10) Takata, T.; Pan, C.; Nakabayashi, M.; Shibata, N.; Domen, K. Fabrication of a Core–ShellType 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. (11) Abe, R. Development of a New System for Photocatalytic Water Splitting into H2 and O2 under Visible Light Irradiation. Bull. Chem. Soc. Jpn. 2011, 84, 1000-1030. (12) Tachibana, Y.; Vayssieres, L.; Durrant, R. J. Artificial Photosynthesis for Solar Water Splitting. Nat. Photonics 2012, 6, 511-518. (13) Cheng, S.; Takata, T.; Domen, K. Recent Developments in Particulate Photocatalysts for Overall Water Splitting. Nat. Rev. Mater. 2017, 2, 17050. (14) Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater., 2008, 20, 35-54. (15) Zhang, G.; Liu, G.; Wang, L.; and Irvine, J. T. S. Inorganic Perovskite Photocatalysts for Solar Energy Utilization. Chem. Soc. Rev. 2016, 45, 5951-5984. (16) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Oxysulfide Sm2Ti2S2O5 as a Stable Photocatalyst for Water Oxidation and Reduction under Visible Light Irradiation (λ ≤ 650 nm). J. Am. Chem. Soc. 2002, 124, 13547-13553. (17) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Ta3N5 as a Novel Visible Light-Driven Photocatalyst (λ

Particulate Photocatalysts for Water Splitting: Recent Advances and

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