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Photocatalytic water splitting has become a very popular research subject in recent years. Consequently, it is important to report appropriately standardized experimental data, so that each researcher can properly understand the results generated by others. However, experimental methods and measures of photocatalytic performance are not yet sufficiently systematic. In the present manuscript, experimental procedures and standardization of photocatalytic performance are described in relation to the basic theory of photocatalytic water splitting., (© 2024 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

Activating metal ion-doped oxides as visible-light-responsive photocatalysts requires intricate structural and electronic engineering, a task with inherent challenges. In this study, we employed a solid (template)-molten (dopants) reaction to synthesize Bi- and Rh-codoped SrTiO3 (SrTiO3:Bi,Rh) particles. Our investigation reveals that SrTiO3:Bi,Rh manifests as single-crystalline particles in a core (undoped)/shell (doped) structure. Furthermore, it exhibits a well-stabilized Rh3+ energy state for visible-light response without introducing undesirable trapping states. This precisely engineered structure and electronic configuration promoted the generation of high-concentration and long-lived free electrons, as well as facilitated their transfer to cocatalysts for H2 evolution. Impressively, SrTiO3:Bi,Rh achieved an exceptional apparent quantum yield (AQY) of 18.9% at 420 nm, setting a new benchmark among Rh-doped-based SrTiO3 materials. Furthermore, when integrated into an all-solid-state Z-scheme system with Mo-doped BiVO4 and reduced graphene oxide, SrTiO3:Bi,Rh enabled water splitting with an AQY of 7.1% at 420 nm. This work underscores the significance of simultaneous structural and electronic engineering and introduces the solid-molten reaction as a viable approach for this purpose., (© 2024 Wiley‐VCH GmbH.)

Some oxide-based particulate photocatalyst sheets exhibit excellent activity during the water-splitting reaction. The replacement of oxide photocatalysts with narrow-bandgap photocatalysts based on nonoxides could provide the higher solar-to-hydrogen energy conversion efficiencies that are required for practical implementation. Unfortunately, the activity of nonoxide-based photocatalyst sheets is low in many cases, indicating the need for strategies to improve the quality of nonoxide photocatalysts and the charge transfer process. In this work, single-crystalline particulate SrTaO 2 N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La 5 Ti 2 Cu 0.9 Ag 0.1 O 7 S 5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO 2 N with CoO x provided increases activity during photocatalytic water oxidation, giving an apparent quantum yield of 15.7% at 420 nm. A photocatalyst sheet incorporating CoO x -loaded SrTaO 2 N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoO x -loaded SrTaO 2 N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO 2 N particles to the Au layer. This work demonstrates key concepts related to the improvement of nonoxide-based photocatalyst sheets based on facilitating the charge transfer process through appropriate surface modifications., (© 2024 The Authors. Small published by Wiley‐VCH GmbH.)

Green hydrogen production using particulate photocatalyst materials has attracted much attention in recent years because this process could potentially lead to inexpensive and scalable solar-to-chemical energy conversion systems. Although the development of efficient particulate photocatalysts enabling one-step overall water splitting (OWS) with solar-to-hydrogen efficiencies in excess of 10 % remains challenging, promising photocatalyst candidates exhibiting OWS activity have been demonstrated. This review provides a comprehensive introduction to the solar-to-hydrogen energy conversion process of semiconductor photocatalyst materials and highlights recent advances in photocatalytic OWS via both one-step and two-step photoexcitation processes. The review also covers recent developments in the photocatalytic OWS of SrTiO 3 , including the establishment of large-scale photocatalytic systems, interfacial design using cocatalysts to enhance water splitting activity, and its photoelectrochemical (PEC) properties at the electrified solid/liquid interface. In addition, there is a special focus on visible-light-absorbing oxynitride and oxysulfide particulate photocatalysts with absorption edges near 600 nm. Methods for photocatalyst preparation and surface modification, as well as PEC properties, are also discussed. The semiconductor properties of particulate photocatalysts obtained from photoelectroanalytical evaluations using particulate photoelectrodes are evaluated. This review is intended to provide guidelines for the future development of particulate photocatalysts capable of efficient and stable OWS., (© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Overall water splitting (OWS) using semiconductor photocatalysts is a promising method for solar fuel production. Achieving a high quantum efficiency is one of the most important prerequisites for photocatalysts to realize high solar-to-fuel efficiency. In a recent study (Nature 2020, 58, 411-414), a quantum efficiency of almost 100 % has been achieved in an aluminum-doped strontium titanate (SrTiO 3  : Al) photocatalyst. Herein, using the SrTiO 3  : Al as a model photocatalyst, we reveal the criteria for efficient photocatalytic water splitting by investigating the carrier dynamics through a comprehensive photoluminescence study. It is found that the Al doping suppresses the generation of Ti 3+ recombination centers in SrTiO 3 , the surface band bending facilitates charge separation, and the in situ photo-deposited Rh/Cr 2 O 3 and CoOOH co-catalysts render efficient charge extraction. By suppressing photocarrier recombination and establishing a facile charge separation and extraction mechanism, high quantum efficiency can be achieved even on photocatalysts with a very short (sub-ns) intrinsic photocarrier lifetime, challenging the belief that a long carrier lifetime is a fundamental requirement. Our findings could provide guidance on the design of OWS photocatalysts toward more efficient solar-to-fuel conversion., (© 2023 Wiley-VCH GmbH.)

Photocatalytic water splitting is a simple means of converting solar energy into storable hydrogen energy. Narrow-band gap oxysulfide photocatalysts have attracted much attention in this regard owing to the significant visible-light absorption and relatively high stability of these compounds. However, existing materials suffer from low efficiencies due to difficulties in synthesizing these oxysulfides with suitable degrees of crystallinity and particle sizes, and in constructing effective reaction sites. The present work demonstrates the production of a Gd 2 Ti 2 O 5 S 2 (λ<650 nm) photocatalyst capable of efficiently driving photocatalytic reactions. Single-crystalline, plate-like Gd 2 Ti 2 O 5 S 2 particles with atomically ordered surfaces were synthesized by flux and chemical etching methods. Ultrafine Pt-IrO 2 cocatalyst particles that promoted hydrogen (H 2 ) and oxygen (O 2 ) evolution reactions were subsequently loaded on the Gd 2 Ti 2 O 5 S 2 while ensuring an intimate contact by employing a microwave-heating technique. The optimized Gd 2 Ti 2 O 5 S 2 was found to evolve H 2 from an aqueous methanol solution with a remarkable apparent quantum efficiency of 30 % at 420 nm. This material was also stable during O 2 evolution in the presence of a sacrificial reagent. The results presented herein demonstrates a highly efficient narrow-band gap oxysulfide photocatalyst with potential applications in practical solar hydrogen production., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Photocatalytic water splitting is an ideal means of producing hydrogen in a sustainable manner, and developing highly efficient photocatalysts is a vital aspect of realizing this process. The photocatalyst Y 2 Ti 2 O 5 S 2 (YTOS) is capable of absorbing at wavelengths up to 650 nm and exhibits outstanding thermal and chemical durability compared with other oxysulfides. However, the photocatalytic performance of YTOS synthesized using the conventional solid-state reaction (SSR) process is limited owing to the large particle sizes and structural defects associated with this synthetic method. Herein, we report the synthesis of YTOS particles by a flux-assisted technique. The enhanced mass transfer efficiency in the flux significantly reduced the preparation time compared with the SSR method. In addition, the resulting YTOS showed improved photocatalytic H 2 and O 2 evolution activity when loaded with Rh and Co 3 O 4 co-catalysts, respectively. These improvements are attributed to the reduced particle size and enhanced crystallinity of the material as well as the slower decay of photogenerated carriers on a nanosecond to sub-microsecond time range. Further optimization of this flux-assisted method together with suitable surface modification is expected to produce high-quality YTOS crystals with superior photocatalytic activity., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Due to the depletion of fossil fuels and their-related environmental issues, sustainable, clean, and renewable energy is urgently needed to replace fossil fuel as the primary energy resource. Hydrogen is considered as one of the cleanest energies. Among the approaches to hydrogen production, photocatalysis is the most sustainable and renewable solar energy technique. Considering the low cost of fabrication, earth abundance, appropriate bandgap, and high performance, carbon nitride has attracted extensive attention as the catalyst for photocatalytic hydrogen production in the last two decades. In this review, the carbon nitride-based photocatalytic hydrogen production system, including the catalytic mechanism and the strategies for improving the photocatalytic performance is discussed. According to the photocatalytic processes, the strengthened mechanism of carbon nitride-based catalysts is particularly described in terms of boosting the excitation of electrons and holes, suppressing carriers recombination, and enhancing the utilization efficiency of photon-excited electron-hole. Finally, the current trends related to the screening design of superior photocatalytic hydrogen production systems are outlined, and the development direction of carbon nitride for hydrogen production is clarified., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)

The hydrogen evolution reaction (HER) of Rh nanoparticles (RhNP) coated with an ultrathin layer of Cr-oxides (CrO x ) was investigated as a model electrode for the Cr 2 O 3 /Rh-metal core-shell-type cocatalyst system for photocatalytic water splitting. The CrO x layer was electrodeposited over RhNP on a transparent conductive fluorine-doped tin oxide (FTO) substrate. The CrO x layer on RhNP facilitates the electron transfer process at the CrO x /RhNP interface, leading to the increased current density for the HER. Impedance spectroscopic analysis revealed that the CrO x layer transferred protons via the hopping mechanism to the RhNP surface for HER. In addition, CrO x restricted electron transfer from the FTO to the electrolyte and/or RhNP and suppressed the backward reaction by limiting oxygen migration. This study clarifies the crucial role of the ultrathin CrO x layer on nanoparticulate cocatalysts and provides a cocatalyst design strategy for realizing efficient photocatalytic water splitting., (© 2023 Wiley-VCH GmbH.)

Solar-powered one-step-excitation overall water splitting (OWS) using semiconducting materials is a simple means of achieving scalable and sustainable hydrogen production. While tantalum oxynitride (TaON) is one of the few photocatalysts capable of promoting OWS via single-step visible-light excitation, the efficiency of this process remains extremely poor. The present work employed 15 nm amorphous Ta 2 O 5 ⋅3.3 H 2 O nanoparticles as a new precursor together with Zr doping and an optimized nitridation duration to synthesize a TaON-based photocatalyst with reduced particle sizes and low defect densities. Upon loading with Ru/Cr 2 O 3 /IrO 2 cocatalysts, this material exhibited stoichiometric water splitting into hydrogen and oxygen, with an order of magnitude improvement in efficiency. Our findings demonstrate the importance of inventing/selecting the appropriate synthetic precursor and of defect control for fabricating active OWS photocatalysts., (© 2022 Wiley-VCH GmbH.)

Phosphonate groups loaded on the surface of the visible-light-responsive photocatalyst Ru-loaded La,Rh-doped SrTiO 3 (Ru/La,Rh:STO) via a silane-coupling treatment enhance the photocatalytic activity of this material during the hydrogen evolution reaction. Surface modification with an alkylsilane phosphonate accelerates the supply of reactants to active sites and is much more effective at improving the photocatalytic activity than the utilization of a phosphate-buffered electrolyte as a reaction solution. In contrast, the incorporation of amine, sulfonate, and propyl groups does not improve the activity. The effects of these functional groups introduced via silane coupling on the reaction kinetics of hydrogen evolution are evaluated separately from the oxidative reaction using electrochemical methods. It was also demonstrated that the present alkylsilane phosphonate modification increases the photocatalytic activity even under a low photon flux., (© 2020 Wiley-VCH GmbH.)

Sunlight-driven water splitting systems operating under ambient pressure are essential for practical renewable hydrogen production. Printable photocatalyst sheets, composed of a hydrogen evolution photocatalyst (HEP), an oxygen evolution photocatalyst (OEP), and conductive metal nanoparticles, are cost-effective and scalable systems. However, the decrease in water splitting activity under ambient pressure due to reverse reactions hampers their practical application. In this study, coating zirconium oxide (ZrO x ) by facile drop-casting onto a printed photocatalyst sheet, which consists of SrTiO 3  : Rh, BiVO 4  : Mo, and Au nanocolloids as the HEP, OEP, and electron mediator, respectively, effectively maintains the water splitting activity at elevated pressure. The ZrO x -coated photocatalyst sheet retains 90 % and 84 % of the base performance (the pristine sheet at 10 kPa) at 50 and 90 kPa, respectively. Achieving efficient water splitting at the ambient pressure by inexpensive and extensible processes is an important step toward solar hydrogen production., (© 2020 Wiley-VCH GmbH.)

Most CdTe photoanodes and photocathodes show positive and negative photocurrent onset potentials for water oxidation and reduction, respectively, and are thus unable to drive photoelectrochemical (PEC) water splitting without external applied biases. Herein, the activity of a CdTe photoanode having an internal p-n junction during PEC water oxidation was enhanced by applying a CdCl 2 annealing treatment together with surface modifications. The resulting CdTe photoanode generated photocurrents of 1.8 and 5.4 mA cm -2 at 0.6 and 1.2 V RHE , respectively, with a photoanodic current onset potential of 0.22 V RHE under simulated sunlight (AM 1.5G). The CdCl 2 annealing increased the grain sizes and lowered the density of grain boundaries, allowing more efficient charge separation. Consequently, a two-electrode tandem PEC cell comprising a CdTe-based photoanode and photocathode split water without any external bias at a solar-to-hydrogen conversion efficiency of 0.51 % at the beginning of the reaction., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Photoelectrochemical (PEC) water splitting using visible-light-responsive photoelectrodes is the preferred approach to converting solar energy into hydrogen as a renewable energy source. A transparent Ta 3 N 5 photoanode embedded within a PEC cell having a tandem configuration is a promising configuration that may provide a high solar-to-hydrogen energy conversion efficiency. Ta 3 N 5 thin films are typically prepared by heating precursor films in an NH 3 flow at high temperatures, which tends to degrade the transparent conductive layer, such that producing efficient Ta 3 N 5 transparent photoanodes is challenging. Herein, the direct preparation of transparent Ta 3 N 5 photoanodes on insulating quartz substrates was demonstrated without the insertion of a transparent conductive layer. The resulting devices generated a photocurrent of 6.0 mA cm -2 at 1.23 V vs. a reversible hydrogen electrode under simulated sunlight. This study provides a new strategy for the preparation of transparent photoelectrodes that mitigates current challenges., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

LaTaON 2 is a photocatalyst with intense visible light absorption up to 650 nm, but exhibits low H 2 evolution activity owing to uncontrolled facets and high defect densities. In this work, core-shell-structured plate-like LaKNaTaO 5 /LaTaON 2 was synthesized by nitriding a layered perovskite-type LaKNaTaO 5 . The volatilization of K and Na species during the nitridation promoted the rapid transformation of LaKNaTaO 5 into LaTaON 2 along [010] direction with the plate-like shape retained. This yielded high-quality LaTaON 2 shells exposing (010) facets on the lattice-matched LaKNaTaO 5 cores. After loading with a Rh co-catalyst, LaKNaTaO 5 /LaTaON 2 showed photocatalytic H 2 evolution activity four times greater than that obtained from conventional irregular-shaped LaTaON 2 powders and utilized visible light up to 620 nm. This work provides a novel strategy yielding oxynitrides with well-defined facets and low defect densities by selecting lattice-matched oxide precursors containing volatile components., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

La 5 Ti 2 Cu x Ag 1-x S 5 O 7 (x=0-1) is a type of long-wavelength-responsive oxysulfide photocatalysts for hydrogen evolution and has been demonstrated to enable the Z-scheme water-splitting coupling with oxygen evolution photocatalysts (OEPs) in the particulate sheet. Among La 5 Ti 2 Cu x Ag 1-x S 5 O 7 materials, La 5 Ti 2 AgS 5 O 7 was found to have the highest performance on Z-scheme overall water splitting in conjunction with PtO x -WO 3 as an OEP and a triiodide/iodide (I 3 - /I - ) redox couple as a shuttle electron mediator in a powder-suspension system. Loading Pt/NiS on La 5 Ti 2 AgS 5 O 7 benefitted the Z-scheme to achieve an apparent quantum yield of 0.12 % at 420 nm. The results for this powder-suspension system differ from earlier studies on photocatalyst sheet configurations, in which p-type doping and the formation of a solid solution could effectively enhance the water-splitting activity. This work not only demonstrates a La 5 Ti 2 AgS 5 O 7 -based Z-scheme water-splitting photocatalyst but also improves the understanding of the difference between particulate sheets and a powder-suspension system available in an optimal strategy for water splitting., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta 3 N 5 photoanodes were fabricated on n-type GaN/sapphire substrates to promote O 2 evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm -2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta 3 N 5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe 2 (CIS), which absorbs up to 1100 nm. A stand-alone tandem cell with a serially-connected dual-CIS unit terminated with a Pt/Ni electrode was thus constructed for H 2 evolution. This tandem cell exhibited a solar-to-hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The stable and efficient integration of molecular catalysts into p-type semiconductor materials is a contemporary challenge in photoelectrochemical fuel synthesis. Here, we report the combination of a phosphonated molecular Ni catalyst with a TiO 2 -coated La 5 Ti 2 Cu 0.9 Ag 0.1 S 5 O 7 photocathode for visible light driven H 2 production. This hybrid assembly provides a positive onset potential, large photocurrents, and high Faradaic yield for more than three hours. A decisive feature of the hybrid electrode is the TiO 2 interlayer, which stabilizes the oxysulfide semiconductor and allows for robust attachment of the phosphonated molecular catalyst. This demonstration of an oxysulfide-molecular catalyst photocathode provides a novel platform for integrating molecular catalysts into photocathodes and the large photovoltage of the presented system makes it ideal for pairing with photoanodes., (© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Solar energy is a natural and effectively permanent resource and so the conversion of solar radiation into chemical or electrical energy is an attractive, although challenging, prospect. Photo-electrochemical (PEC) water splitting is a key aspect of producing hydrogen from solar power. However, practical water oxidation over photoanodes (in combination with water reduction at a photocathode) in PEC cells is currently difficult to achieve because of the large overpotentials in the reaction kinetics and the inefficient photoactivity of the semiconductors. The development of semiconductors that allow high solar-to-hydrogen conversion efficiencies and the utilization of these materials in photoanodes will be a necessary aspect of achieving efficient, stable water oxidation. This Review discusses advances in water oxidation activity over photoanodes of n-type visible-light-responsive (oxy)nitrides and oxides., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Photoelectrochemical (PEC) water splitting using a combination of a photocathode and photoanode is one of the most promising methods of producing hydrogen from water employing sunlight. Recent reports have shown that surface modification of the photoelectrodes dramatically improves their PEC performance. Bare photoelectrodes often exhibit insufficient depletion regions, undesired surface states and/or degradation due to photocorrosion. It has been demonstrated that surface modifications can tune the flat-band potentials, band-edge potentials, surface states and chemical stabilities of these electrodes and thus improve quantum efficiency, onset potential and durability. This review describes in detail the various surface modification materials that have been developed to date, and the functions of these modifiers. This information is expected to provide guidelines for the future development of photoelectrodes capable of highly efficient and stable PEC water splitting., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

For overall water-splitting systems, it is essential to establish O 2 -insensitive cathodes that allow cogeneration of H 2 and O 2 . An acid-tolerant electrocatalyst is described, which employs a Mo-coating on a metal surface to achieve selective H 2 evolution in the presence of O 2 . In operando X-ray absorption spectroscopy identified reduced Pt covered with an amorphous molybdenum oxyhydroxide hydrate with a local structural order composed of polyanionic trimeric units of molybdenum(IV). The Mo layer likely hinders O 2 gas permeation, impeding contact with active Pt. Photocatalytic overall water splitting proceeded using MoO x /Pt/SrTiO 3 with inhibited water formation from H 2 and O 2 , which is the prevailing back reaction on the bare Pt/SrTiO 3 photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membraneless electrolysis and photocatalysis., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Ta 3 N 5 is a very promising photocatalyst for solar water splitting because of its wide spectrum solar energy utilization up to 600 nm and suitable energy band position straddling the water splitting redox reactions. However, its development has long been impeded by poor compatibility with electrolytes. Herein, we demonstrate a simple sputtering-nitridation process to fabricate high-performance Ta 3 N 5 film photoanodes owing to successful synthesis of the vital TaO δ precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta 3 N 5 by forming a crystalline nitride-on-nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm -2 was obtained with a CoPi/GaN/Ta 3 N 5 photoanode at 1.2 V RHE under simulated sunlight, with O 2 and H 2 generated at a Faraday efficiency of unity over 12 h. Our vapor-phase deposition method can be used to fabricate high-performance (oxy)nitrides for practical photoelectrochemical applications., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A photoelectrochemical (PEC) cell consisting of a Pt-loaded carbon black (Pt/C)-based membrane electrode assembly (MEA) and a particulate SrTiO 3 photoanode effected selective PEC conversion of toluene and water into methylcyclohexane (MCH) at concentrations up to >99 vol %. This cell exhibited 100 % faradaic efficiency (FE) and 18 % incident-photon-to-current conversion efficiency (IPCE) at 320 nm without an external bias voltage in the PEC hydrogenation of pure toluene. It was also found that strong alkaline conditions are beneficial with the present MEA to suppress the competitive side reaction of hydrogen evolution, resulting in a high FE of 94 % even during MCH production from 1 vol % toluene in MCH. This study successfully demonstrated that the present PEC system is capable of producing concentrated MCH as a promising hydrogen carrier and that MCH production from toluene and water represents a means of artificial photosynthesis., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A (ZnSe) 0.85 (CuIn 0.7 Ga 0.3 Se 2 ) 0.15 photocathode with a bilayer structure was fabricated and found to exhibit a photocurrent almost twice that of a photocathode with a monolayer structure during hydrogen evolution from water. The cathodic photocurrent reached maximum values of 12 and 4.9 mA cm -2 at 0 and 0.6 V RHE in a neutral phosphate buffer under simulated sunlight, while the half-cell solar-to-hydrogen conversion efficiency was 3.0 % at 0.6 V RHE , with a maximum value of 3.6 % at 0.45 V RHE . Cross-sectional mapping of the electron-beam-induced current established that the increased photocurrent can be attributed to improved uniformity at the solid-liquid junction in the bilayer sample, which results in enhanced carrier collection., (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An improved variation of highly active/durable O 2 -evolving LaTiO 2 N powder-based photoelectrode has been fabricated by pre-cleaning the powder with mild polysulfonic acid and by homogeneous deposition of CoO x co-catalyst aided by microwave annealing. The treatment in aqueous solution of poly(4-styrene sulfonic acid) results in removal of surface LaTiO 2 N layers, forming fine pores in the crystallites. The CoO x co-catalyst by microwave deposition in Co(NH 3 ) 6 Cl 3 /ethylene glycol homogeneously covers the particle surface. The LaTiO 2 N powder is fabricated into particle-transferred electrodes on Ti thin film supported on solid substrate. The modified LaTiO 2 N grains on the electrode serve as a highly active O 2 -evolving photoanode achieving 8.9 mA cm -2 of the photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 m NaOH (pH 13) under solar-simulator irradiation Airmass 1.5 Global (AM 1.5G). The activity has been much improved, compared with conventional LaTiO 2 N treated in mineral acid or with CoO x deposited by impregnation. The new electrode also exhibits better durability in fixed-potential chronoamperometric tests under AM 1.5G irradiation., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

One of the main targets of studies on water splitting photocatalysts is to develop semiconductor materials with narrower bandgaps capable of overall water splitting for efficient harvesting of solar energy. A series of transition-metal oxynitrides, LaMgx Ta1-xO1+3xN2-3x(x ≥ 1/3), with a complex perovskite structure was reported as the first example of overall water splitting operable at up to 600 nm. The photocatalytic behavior of LaMg1/3Ta2/3O2N was investigated in detail in order to optimize photocatalyst preparation and water-splitting activity. Various attempts exploring photocatalyst preparation steps, that is, cocatalyst selection, coating material and method, and synthesis method for the oxide precursor, revealed photocatalyst structures necessary for achieving overall water splitting. Careful examination of photocatalyst preparation procedures likely enhanced the quality of the produced photocatalyst, leading to a more homogeneous coating quality and semiconductor particles with fewer defects. Thus, the photocatalytic activity for water splitting on LaMg1/3Ta2/3O2N was largely enhanced., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A stand-alone photoelectrochemical (PEC) water-splitting system driven only by sunlight was demonstrated with a tandem-scheme of Pt/CdS/CuGa3 Se5 /(Ag,Cu)GaSe2 photocathode and NiOOH/FeOOH/Mo:BiVO4 photoanode in a neutral phosphate buffer solution as an electrolyte. The as-prepared semi-transparent Mo:BiVO4 layer allows sunlight to pass through the top photoanode and reach the bottom photocathode. Consequently, the tandem cell showed stoichiometric hydrogen and oxygen evolution with a solar-to-hydrogen (STH) conversion efficiency of 0.67 % over 2 h without degradation. The stability and STH efficiency are the highest among similar configuration of PEC tandem cells., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An (oxy)nitride-based heterostructure for powdered Z-scheme overall water splitting is presented. Compared with the single MgTa2O(6-x)N(y) or TaON photocatalyst, a MgTa2O(6-x)N(y)/TaON heterostructure fabricated by a simple one-pot nitridation route was demonstrated to effectively suppress the recombination of carriers by efficient spatial charge separation and decreased defect density. By employing Pt-loaded MgTa2O(6-x)N(y)/TaON as a H2-evolving photocatalyst, a Z-scheme overall water splitting system with an apparent quantum efficiency (AQE) of 6.8% at 420 nm was constructed (PtO(x)-WO3 and IO3(-)/I(-) pairs were used as an O2-evolving photocatalyst and a redox mediator, respectively), the activity of which is circa 7 or 360 times of that using Pt-TaON or Pt-MgTa2O(6-x)N)y) as a H2-evolving photocatalyst, respectively. To the best of our knowledge, this is the highest AQE among the powdered Z-scheme overall water splitting systems ever reported., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A photoelectrochemical (PEC) cell consisting of an n-type CdS single-crystal electrode and a Pt counter electrode with the ruthenium-2,2'-bipyridine complex [Ru(bpy)3](2+/3+) as the redox shuttle in a non-aqueous electrolyte was studied to obtain a higher open-circuit voltage (V(OC)) than the onset voltage for water splitting. A V(OC) of 1.48 V and a short-circuit current (I(SC)) of 3.88 mA cm(-2) were obtained under irradiation by a 300 W Xe lamp with 420-800 nm visible light. This relatively high voltage was presumably due to the difference between the Fermi level of photo-irradiated n-type CdS and the redox potential of the Ru complex at the Pt electrode. The smooth redox reaction of the Ru complex with one-electron transfer was thought to have contributed to the high V(OC) and I(SC). The obtained V(OC) was more than the onset voltage of water electrolysis for hydrogen and oxygen generation, suggesting prospects for application in water electrolysis., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

One of the simplest methods for splitting water into H2 and O2 with solar energy entails the use of a particulate-type semiconductor photocatalyst. To harness solar energy efficiently, a new water-splitting photocatalyst that is active over a wider range of the visible spectrum has been developed. In particular, a complex perovskite-type oxynitride, LaMg(x)Ta(1-x)O(1+3x)N(2-3x)(x≥1/3), can be employed for overall water splitting at wavelengths of up to 600 nm. Two effective strategies for overall water splitting were developed. The first entails the compositional fine-tuning of a photocatalyst to adjust the bandgap energy and position by forming a series of LaMg(x)Ta(1-x)O(1+3x)N(2-3x) solid solutions. The second method is based on the surface coating of the photocatalyst with a layer of amorphous oxyhydroxide to control the surface redox reactions. By combining these two strategies, the degradation of the photocatalyst and the reverse reaction could be prevented, resulting in successful overall water splitting., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The electrostatically derived self-assembly of cationic Zn-Cr-layered double hydroxide (LDH) nanosheets and cationic CdS quantum dots (QDs) with anionic linkers leads to the formation of strongly coupled Zn-Cr-LDH-CdS nanohybrids. The hybridization with Zn-Cr-LDH leads to significant enhancement of the photocatalytic activity of CdS for visible-light-induced H2 generation, a property that is attributed to the depression of electron-hole recombination. In comparison with a direct hybridization method between oppositely charged species, this linker-mediated method provides greater flexibility in controlling the chemical composition and electronic coupling of the nanohybrids. The present hybridization strategy provides a useful method not only to couple two kinds of isocharged nanostructured materials, but also to explore efficient hybrid-type photocatalysts., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Porous films of p-type CuInS2, prepared by sulfurization of electrodeposited metals, are surface-modified with thin layers of CdS and TiO2. This specific porous electrode evolved H2 from photoelectrochemical water reduction under simulated sunlight. Modification with thin n-type CdS and TiO2 layers significantly increased the cathodic photocurrent and onset potential through the formation of a p-n junction on the surface. The modified photocathodes showed a relatively high efficiency and stable H2 production under the present reaction conditions., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A simple and versatile method for the preparation of photocatalyst particulates modified with effective cocatalysts is presented; the method involves the sequential soaking of photocatalyst particulates in solutions containing bifunctional organic linkers and metal ions. The modification of the particulate surfaces is a universal and reproducible method because the molecular linkers utilize strong covalent bonds, which in turn result in modified monolayer with a small but controlled quantity of metals. The photocatalysis results indicated that the CdS with likely photochemically reduced Pd and Ni, which were initially immobilized via ethanedithiol (EDT) as a linker, were highly efficient for photocatalytic hydrogen evolution from Na2S-Na2SO3-containing aqueous solutions. The method developed in this study opens a new synthesis route for the preparation of effective photocatalysts with various combinations of bifunctional linkers, metals, and photocatalyst particulate materials., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Methylcyclohexane (MCH) is regarded as a promising hydrogen carrier that enables hydrogen to be harnessed as an alternate fuel source, which paves the way to a clean-energy future. A photoelectrochemical (PEC) system with a Nb:SrTiO3 photoelectrode for oxygen evolution from an aqueous electrolyte and a Pt/C electrode for toluene (TL) hydrogenation to MCH was investigated under UV irradiation. A Nb:SrTiO3 single-crystal electrode and an ionomer/Pt/C membrane-electrode assembly (MEA) were used as the photoanode and cathode, respectively. A steady-state current density of 0.12 mA cm(-2) was observed for the two-electrode system without any bias voltage for >2 h, and a Faradaic efficiency of 97% was obtained for MCH production from TL. This is the first demonstration of the production of MCH from TL and water using only light energy. This means that light energy was converted directly into MCH from TL and water without any electricity. The PEC properties of the devices are discussed., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

BaNbO2 N is a semiconductor photocatalyst active for water oxidation under visible-light irradiation up to λ=740 nm. It is important to understand the nitridation processes of precursor materials to form BaNbO2 N to tune the physical properties and improve the photocatalytic activity. Comprehensive experiments and analyses of temperatures, durations, ammonia flow rates, and barium/niobium ratios in the precursor during the nitridation process reveals that faster ammonia flow rates and higher barium/niobium ratios in the precursors help to suppress reduction of pentavalent niobium ions in the nitridation products and that the use of a precursor prepared by a soft-chemistry route allows the production of BaNbO2 N at lower temperatures in shorter times than the use of physical mixtures of BaCO3 and Nb2 O5 because the niobium species is dispersed among the barium species. BaNbO2 N prepared by the soft-chemistry route exhibits comparatively higher activity than that prepared from physical mixtures of BaCO3 and Nb2 O5 , probably because of lower nitridation temperatures, which suppress excessive dissociation of ammonia, and thereby reduce pentavalent niobium ions, and intimate interaction of niobium and barium sources, which lowers the densities of mid-gap states associated with defects., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A simple method allows the preparation of core/shell photocatalysts with spatially separated co-catalysts for efficient water splitting. The high activity was attributed to the core/shell structure and separated co-catalysts that assisted separation and collection of the electrons and holes at the respective co-catalysts, owing to active rectification of electron and hole transport (see picture; Eg =2.1 eV)., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Tantalum nitride (Ta3N5) modified with various O2-evolution cocatalysts was employed as a photocatalyst for water oxidation under visible light (λ>420 nm) in an attempt to construct a redox-mediator-free Z-scheme water-splitting system. Ta3N5 was prepared by nitriding Ta2O5 powder under a flow of NH3 at 1023-1223 K. The activity of Ta3N5 for water oxidation from an aqueous AgNO3 solution as an electron acceptor without cocatalyst was dependent on the generation of a well-crystallized Ta3N5 phase with a low density of anionic defects. Modification of Ta3N5 with nanoparticulate metal oxides as cocatalysts for O2 evolution improved water-oxidation activity. Of the cocatalysts examined, cobalt oxide (CoO(x)) was found to be the most effective, improving the water-oxidation efficiency of Ta3N5 by six to seven times. Further modification of CoO(x)/Ta3N5 with metallic Ir as an electron sink allowed one to achieve Z-scheme water splitting under simulated sunlight through interparticle electron transfer without the need for a shuttle redox mediator in combination with Ru-loaded SrTiO3 doped with Rh as a H2-evolution photocatalyst., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A vertically aligned Ta(3)N(5) nanorod photoelectrode is fabricated by through-mask anodization and nitridation for water splitting. The Ta(3)N(5) nanorods, working as photoanodes of a photoelectrochemical cell, yield a high photocurrent density of 3.8 mA cm(-2) at 1.23 V versus a reversible hydrogen electrode under AM 1.5G simulated sunlight and an incident photon-to-current conversion efficiency of 41.3% at 440 nm, one of the highest activities reported for photoanodes so far., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Tungsten carbide exhibits platinum-like behavior, which makes it an interesting potential substitute for noble metals in catalytic applications. Tungsten carbide nanocrystals (≈5 nm) are directly synthesized through the reaction of tungsten precursors with mesoporous graphitic C(3)N(4) (mpg-C(3)N(4)) as the reactive template in a flow of inert gas at high temperatures. Systematic experiments that vary the precursor compositions and temperatures used in the synthesis selectively generate different compositions and structures for the final nanocarbide (W(2)C or WC) products. Electrochemical measurements demonstrate that the WC phase with a high surface area exhibits both high activity and stability in hydrogen evolution over a wide pH range. The WC sample also shows excellent hydrogen oxidation activity, whereas its activity in oxygen reduction is poor. These tungsten carbides are successful cocatalysts for overall water splitting and give H(2) and O(2) in a stoichiometric ratio from H(2)O decomposition when supported on a Na-doped SrTiO(3) photocatalyst. Herein, we present tungsten carbide (on a small scale) as a promising and durable catalyst substitute for platinum and other scarce noble-metal catalysts in catalytic reaction systems used for renewable energy generation., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Poly(triazine imide) was synthesized with incorporation of Li(+) and Cl(-) ions (PTI/Li(+)Cl(-)) to form a carbon nitride derivative. The synthesis of this material by the temperature-induced condensation of dicyandiamide was examined both in a eutectic mixture of LiCl-KCl and without KCl. On the basis of X-ray diffraction measurements of the synthesized materials, we suggest that a stoichiometric amount of LiCl is necessary to obtain the PTI/Li(+)Cl(-) phase without requiring the presence of KCl at 873 K. PTI/Li(+)Cl(-) with modification by either Pt or CoO(x) as cocatalyst photocatalytically produced H(2) or O(2), respectively, from water. The production of H(2) or O(2) from water indicates that the valence and conduction bands of PTI/Li(+)Cl(-) were properly located to achieve overall water splitting. The treatment of PTI/Li(+)Cl(-) with [Pt(NH(3))(4)](2+) cations enabled the deposition of Pt through ion exchange, demonstrating photocatalytic activity for H(2) evolution, while treatment with [PtCl(6)](2-) anions resulted in no Pt deposition. This was most likely because of the preferential exchange between Li(+) ions and [Pt(NH(3))(4)](2+) cations., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Tripping the light fantastic: Despite small band gap energies (1.7-1.8 eV), BaZrO(3)-BaTaO(2)N solid solutions (Zr/Ta≤0.1) are capable of photocatalyzing both water oxidation and reduction even under irradiation above 660 nm. Solar water splitting to form H(2) and O(2) was also demonstrated using a photoelectrochemical cell consisting of a BaZrO(3)-BaTaO(2)N solid solution as an anode and a Pt wire cathode., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A two-step, photocatalytic water splitting system consisting of Pt-loaded TaON (a H(2) evolution photocatalyst), Pt-loaded WO(3) (an O(2) evolution photocatalyst), and an iodate-iodide (IO(3)(-)/I(-)) shuttle redox mediator is investigated under visible light irradiation. Photocatalytic oxidation of water to O(2) and reduction of IO(3)(-) to I(-) proceeded with good selectivity over the Pt-WO(3) photocatalyst, even in the presence of a considerable amount of I(-) anions in the solution. The key difference between the adsorption properties of IO(3)(-) and I(-) anions on WO(3) strongly suggested that the photoexcited electrons could react efficiently with IO(3)(-) adsorbed on WO(3), whereas the photogenerated holes selectively reacted with water molecules owing to the low adsorptivity of I(-) on WO(3). Photocatalytic H(2) evolution on Pt-TaON proceeded efficiently, accompanied by I(-) oxidation to IO(3)(-) due to a substantial amount of adsorption of I(-) anions on the surface, whereas H(2) evolution was significantly inhibited by the competitive adsorption of IO(3)(-), which consumes photoexcited electrons. It was also found that WO(3) photocatalysts loaded with platinum oxide (PtO) showed a much higher activity for O(2) evolution in the presence of the electron acceptor IO(3)(-), compared to those loaded with Pt metal. Overall water splitting at a steady rate was demonstrated using a combination of Pt-TaON and Pt(PtO)-WO(3) in an aqueous NaI solution with neutral or weakly acidic pH values, where the concentration of NaI significantly affected the efficiency., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Photocatalytic activities of perovskite-type niobium oxynitrides (CaNbO₂N, SrNbO₂N, BaNbO₂N, and LaNbON₂) were examined for hydrogen and oxygen evolution from water under visible-light irradiation. These niobium oxynitrides were prepared by heating the corresponding oxide precursors, which were synthesized using the polymerized complex method, for 15 h under a flow of ammonia. They possess visible-light absorption bands between 600-750 nm, depending on the A-site cations in the structures. The oxynitride CaNbO₂N, was found to be active for hydrogen and oxygen evolution from methanol and aqueous AgNO₃, respectively, even under irradiation by light at long wavelengths (λ<560 nm). The nitridation temperature dependence of CaNbO₂N was investigated and 1023 K was found to be the optimal temperature. At lower temperatures, the oxynitride phase is not adequately produced, whereas higher temperatures produce more reduced niobium species (e. g., Nb³(+) and Nb⁴(+)), which can act as electron-hole recombination centers, resulting in a decrease in activity., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Uniform-sized silica nanospheres (SNSs) assembled into close-packed structures were used as a primary template for ordered porous graphitic carbon nitride (g-C(3)N(4)), which was subsequently used as a hard template to generate regularly arranged Ta(3)N(5) nanoparticles of well-controlled size. Inverse opal g-C(3)N(4) structures with the uniform pore size of 20-80 nm were synthesized by polymerization of cyanamide and subsequent dissolution of the SNSs with an aqueous HF solution. Back-filling of the C(3)N(4) pores with tantalum precursors, followed by nitridation in an NH(3) flow gave regularly arranged, crystalline Ta(3)N(5) nanoparticles that are connected with each other. The surface areas of the Ta(3)N(5) samples were as high as 60 m(2) g(-1), and their particle size was tunable from 20 to 80 nm, which reflects the pore size of g-C(3)N(4). Polycrystalline hollow nanoparticles of Ta(3)N(5) were also obtained by infiltration of a reduced amount of the tantalum source into the g-C(3)N(4) template. An improved photocatalytic activity for H(2) evolution on the assembly of the Ta(3)N(5) nanoparticles under visible-light irradiation was attained as compared with that on a conventional Ta(3)N(5) bulk material with low surface area.