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The unprecedented impact of human activity on Earth’s climate and the ongoing increase in global energy demand have made the development of carbon-neutral energy sources ever more important. Hydrogen is an attractive and versatile energy carrier (and important and widely used chemical) obtainable from water through photocatalysis using sunlight, and through electrolysis driven by solar or wind energy1,2. The most efficient solar hydrogen production schemes, which couple solar cells to electrolysis systems, reach solar-to-hydrogen (STH) energy conversion efficiencies of 30% at a laboratory scale3. Photocatalytic water splitting reaches notably lower conversion efficiencies of only around 1%, but the system design is much simpler and cheaper and more amenable to scale-up1,2—provided the moist, stoichiometric hydrogen and oxygen product mixture can be handled safely in a field environment and the hydrogen recovered. Extending our earlier demonstration of a 1-m2 panel reactor system based on a modified, aluminium-doped strontium titanate particulate photocatalyst4, we here report safe operation of a 100-m2 array of panel reactors over several months with autonomous recovery of hydrogen from the moist gas product mixture using a commercial polyimide membrane5. The system, optimized for safety and durability, and remaining undamaged on intentional ignition of recovered hydrogen, reaches a maximum STH of 0.76%. While the hydrogen production is inefficient and energy negative overall, our findings demonstrate that safe, large-scale photocatalytic water splitting, and gas collection and separation are possible. To make the technology economically viable and practically useful, essential next steps are reactor and process optimization to substantially reduce costs and improve STH efficiency, photocatalyst stability and gas separation efficiency. Carbon-neutral hydrogen can be produced through photocatalytic water splitting, as demonstrated here with a 100-m2 array of panel reactors that reaches a maximum conversion efficiency of 0.76%.

Gum arabic (GA) is widely used as an emulsion stabilizer and edible coating, and consists of a complex carbohydrate moiety with a rhamnosyl-glucuronate group capping the non-reducing ends. Enzymes that can specifically cleave the glycosidic chains of GA and modify their properties are valuable tools for structural analysis and industrial application. Cryogenic X-ray crystal structure of GA-specific L-rhamnose-α-1,4-D-glucuronate lyase from Fusarium oxysporum (FoRham1), belonging to the polysaccharide lyase (PL) family 42, has been previously reported. To determine the specific reaction mechanism based on its hydrogen-containing enzyme structure, we performed joint X-ray/neutron crystallography of FoRham1. Large crystals were grown in the presence of L-rhamnose (a reaction product), and neutron and X-ray diffraction datasets were collected at room temperature up to 1.80 and 1.25 Å resolutions, respectively. The active site contained L-rhamnose and acetate, the latter being a partial analog of glucuronate. Incomplete H/D exchange between Arg166 and acetate suggested that a strong salt-bridge interaction was maintained. Doubly deuteronated His105 and deuteronated Tyr150 supported this interaction. The unusually hydrogen-rich environment functions as a charge neutralizer for glucuronate and stabilizes the oxyanion intermediate. The NE2 atom of His85 was deprotonated and formed a hydrogen bond with the deuterated O1 hydroxy of L-rhamnose, indicating the function of His85 as the base/acid catalyst for bond cleavage via β-elimination. Asp83 functions as a pivot between the two catalytic histidine residues by bridging them, and this His–His–Asp structural motif is conserved in the three PL families.Significance StatementAlthough hydrogen transfer plays an important role in enzymatic reactions, hydrogen atoms are generally invisible in macromolecular X-ray crystallography. In the reaction of polysaccharide lyases, substrate activation by negative charge stabilization of uronic acid and base/acid-catalyzed β-elimination reaction have been postulated. Here, we report the neutron crystallography of polysaccharide lyase. Joint X-ray/neutron crystallography of L-rhamnose-α-1,4-D-glucuronate lyase from Fusarium oxysporum (FoRham1) complexed with L-rhamnose was performed, and the hydrogen and deuterium atoms were visualized at a high resolution. FoRham1 catalyzes the specific cleavage of the cap structure of gum arabic, which is useful for various applications in the food, cosmetic, and pharmaceutical industries. A detailed catalytic mechanism for FoRham1 was proposed based on the key structural features of its active site.

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p-n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p-n junction of CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 significantly boosted charge separation. A thin TiO2 coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO2/CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO4:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p-n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.

Tantalum nitride (Ta3N5) is a visible-light-responsive semiconductor that may be capable of achieving the 10% solar-to-hydrogen (STH) efficiency required to allow the commercialization of water splitting systems. However, despite immense research efforts, the highest STH efficiency yet reported for photoanodes based on Ta3N5-nanorods (NRs) is only 2.7%. Therefore, it is imperative to build a theoretical foundation that explains the various loss mechanisms and their correlations with structural and material properties, so as to optimize the performance of this material. The present work devised a detailed numerical model based on an in-depth analysis of the performance characteristics of photoanodes made from either Ba-doped or undoped Ta3N5-NRs. This experimentally calibrated optoelectrical modelling enabled predictions of various factors related to performance loss, including optical effects, charge carrier recombination and resistive loss. Certain physical parameters, such as charge carrier lifetime, diffusion length, hole extraction rate from the NR surfaces to the electrolyte and the series resistance of the photoanode, could also be calculated. The results show that the enhanced performance obtained with Ba doping can be primarily attributed to increases in the carrier lifetime and diffusion length. The present model was recalibrated using experimental data from the literature to examine hidden effects of the NRs’ dimensions on optical and recombination losses. On this basis, various design principles are presented herein that should allow the fabrication of efficient Ta3N5-NRs photoanodes for commercial STH production.

Heterostructure-based photoanodes have been investigated to enhance light absorption and promote the generation and extraction of charge carriers for efficient solar-to-hydrogen energy conversion. ...

Recent advances in neutron crystallographic studies have provided structural bases for quantum behaviors of protons observed in enzymatic reactions. Thus, we resolved the neutron crystal structure of a bacterial copper (Cu) amine oxidase (CAO), which contains a prosthetic Cu ion and a protein-derived redox cofactor, topa quinone (TPQ). We solved hitherto unknown structures of the active site, including a keto/enolate equilibrium of the cofactor with a nonplanar quinone ring, unusual proton sharing between the cofactor and the catalytic base, and metal-induced deprotonation of a histidine residue that coordinates to the Cu. Our findings show a refined active-site structure that gives detailed information on the protonation state of dissociable groups, such as the quinone cofactor, which are critical for catalytic reactions.

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 Ta3 N5 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. Ta3 N5 thin films are typically prepared by heating precursor films in an NH3 flow at high temperatures, which tends to degrade the transparent conductive layer, such that producing efficient Ta3 N5 transparent photoanodes is challenging. Herein, the direct preparation of transparent Ta3 N5 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.

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni3Se4 NCs (sp-Ni3Se4) and phase segregated berzelianite Cu2−xSe (ber-Cu2−xSe)/sp-Ni3Se4 heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni3Se4 phase was formed by cation exchange of ber-Cu2−xSe NCs with Ni2+ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu2−xSe/sp-Ni3Se4 heterostructured NCs. The NCs of sp-Ni3Se4 and ber-Cu2−xSe/sp-Ni3Se4 heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm−2 in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance.

A new route for the preparation of tantalum nitride (Ta3N5) thin films for photoelectrochemical (PEC) applications was demonstrated, based on plasma-enhanced chemical vapor deposition (PCVD). Ta3N5 was produced on a Ta foil substrate using PCVD with a Ta precursor, followed by NH3 nitridation at high temperatures. Various characterization techniques were used to assess the properties of the resulting films, including X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectroscopy, cross-sectional scanning transmission electron microscopy with elemental mapping, cross-sectional high resolution transmission electron spectroscopy, and X-ray photoelectron spectroscopy. This PCVD technique formed an amorphous phase that was converted into a multilayer structure having the composition TaOx/Ta3N5/Ta3N5 + Ta2N/Ta during nitridation. Photoelectrochemical (PEC) trials assessing the progress of the oxygen evolution reaction showed a high photocurrent density of 8.1 mA cm−2 at 1.23 V vs. RHE under simulated solar radiation. This represents one of the highest values ever reported for Ta3N5 without further modification or the addition of a surface layer. The incident photon-to-current efficiency of this specimen reached a maximum of 67% at 500 nm and the device was stable for up to 60 min. The superior PEC performance obtained in this work is attributed to the formation of a highly crystalline, compact and uniform Ta3N5 + Ta2N layer.

CuInS2 photocathodes were prepared and surface modified for photoelectrochemical hydrogen production in a neutral potassium phosphate electrolyte. This work represents the first-ever demonstration that Ni-MoSx provides better catalytic activity than Pt as a hydrogen evolution reaction catalyst on the CuInS2 surface, likely due to the junction formed between the CuInS2 and Ni-MoSx layers.

The IBARAKI biological crystal diffractometer (iBIX) was used in single-crystal time-of-flight neutron diffraction experiments on manganese catalase from Thermus thermophilus. The unit-cell dimensions were 133 × 133 × 133 Å, which is close to the designed maximum limitation of iBIX (135 × 135 × 135 Å). The optimum integration box sizes were set and the degree of integration box overlap was calculated for each Laue spot. Using the overlap ratio as the criterion, the selection of the diffraction intensity data was performed to give a minimum R p.i.m.. Subsequently, diffraction intensity data from Laue spots with overlap ratios ≤0.1 were selected and a complete reflection data set with d min = 2.35 Å was obtained. Joint X-ray and neutron structure refinements were also successfully performed. It was difficult to determine the structures and protonation states of all the oxygen atoms in the manganese cluster owing to the disordered structure. No hydrogen atom was observed on the ordered μ-bridging oxygen atom O1003. Instead, this oxygen atom probably forms a hydrogen bond with Thr39. In addition, the refinements clearly showed the protonation states of the amino acid residues and hydrogen bonds, as observed in Tyr192, Glu167 and Glu280. This first neutron crystal structure of manganese catalase shows that iBIX can provide acceptable diffraction data for neutron single-crystal analyses of at least 2.4 Å resolution within the original targeted unit-cell dimensions of 135 × 135 × 135 Å.

Oxysulfide semiconductors have narrow bandgaps suitable for water splitting under visible-light irradiation, because the electronegative sulfide ions negatively shift the valence band edges of the corresponding oxides1,2. However, the instability of sulfide ions during the water oxidation is a critical obstacle to simultaneous evolution of hydrogen and oxygen3. Here, we demonstrate the activation and stabilization of Y2Ti2O5S2, with a bandgap of 1.9 eV, as a photocatalyst for overall water splitting. On loading of IrO2 and Rh/Cr2O3 as oxygen and hydrogen evolution co-catalysts, respectively, and fine-tuning of the reaction conditions, simultaneous production of stoichiometric amounts of hydrogen and oxygen was achieved on Y2Ti2O5S2 during a 20 h reaction. The discovery of the overall water splitting capabilities of Y2Ti2O5S2 extends the range of promising materials for solar hydrogen production. The instability of sulfide ions during water oxidation prevents simultaneous evolution of hydrogen and oxygen. An oxysulfide semiconductor photocatalyst, Y2Ti2O5S2, is shown to evolve H2 and O2 via a water-splitting reaction under visible-light irradiation.

High-quality needle-like crystals of La5Ti2CuS5O7 (a ternary transition-metallic oxysulfide) as long as 1 mm and a few μm in diameter were grown in CsCl molten-salt flux. This synthesis process was performed in a Bridgman furnace with slow motion of charge across the heating zone. Transmission electron microscopy/diffraction analysis indicated that they are product crystallites without crystalline intergrowth, and that the longitudinal direction of needle crystals is parallel to crystallographic b axis of La5Ti2CuS5O7. Optical polarized ultraviolet–visible reflectance spectroscopy provided the first experimental evidence of the one-dimensional electronic excitation along the b axis of this material. The spectroscopic peaks are assigned to the transitions between the electronic bands aided by numerical calculations on the basis of density functional theory (DFT). The advantages of this synthetic process will be applicable in general for oxysulfides that are useful as optical and photocatalytic materials.

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 Ta3 N5 photoanodes were fabricated on n-type GaN/sapphire substrates to promote O2 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 Ta3 N5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe2 (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 H2 evolution. This tandem cell exhibited a solar-to-hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni3Se4 NCs (sp-Ni3Se4) and phase segregated berzelianite Cu2−xSe (ber-Cu2−xSe)/sp-Ni3Se4 heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni3Se4 phase was formed by cation exchange of ber-Cu2−xSe NCs with Ni2+ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu2−xSe/sp-Ni3Se4 heterostructured NCs. The NCs of sp-Ni3Se4 and ber-Cu2−xSe/sp-Ni3Se4 heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm−2 in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance., Bimetallic selenide nanocrystals formed by cation exchange reaction work as a precursor of efficient water oxidation electrocatalyst.

The unprecedented impact of human activity on Earth's climate and the ongoing increase in global energy demand have made the development of carbon-neutral energy sources ever more important. Hydrogen is an attractive and versatile energy carrier (and important and widely used chemical) obtainable from water through photocatalysis using sunlight, and through electrolysis driven by solar or wind energy

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 (ZrOx ) by facile drop-casting onto a printed photocatalyst sheet, which consists of SrTiO3 : Rh, BiVO4 : Mo, and Au nanocolloids as the HEP, OEP, and electron mediator, respectively, effectively maintains the water splitting activity at elevated pressure. The ZrOx -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.

Photocatalytic water splitting is a viable approach to the large-scale production of renewable solar hydrogen. The apparent quantum yield for this reaction has been improved, but the lifespan of photocatalysts functioning under sunlight at ambient pressure have rarely been examined, despite the critical importance of this factor in practical applications. Herein, we show that Al-doped SrTiO3 (SrTiO3:Al) loaded with a RhCrOx (rhodium chromium oxide) cocatalyst splits water with an apparent quantum yield greater than 50% at 365 nm. Moreover, following the photodeposition of CoOOH and TiO2, this material maintains 80% of its initial activity and a solar-to-hydrogen energy conversion efficiency greater than or equal to 0.3% over a span of 1300 h under constant illumination by simulated sunlight at ambient pressure. This result is attributed to reduced dissolution of Cr in the cocatalyst following the oxidative photodeposition of CoOOH. The photodeposition of TiO2 further improves the durability of this photocatalyst. This work demonstrates a concept that could allow the design of long-term, large-scale photocatalyst systems for practical sunlight-driven water splitting.

A thin film of (CuInS2)x(ZnS)1−x has been developed as a photocathode for solar water splitting for the first time. A superior photoelectrochemical performance has been achieved, mainly attributed to the formation of a solid solution by CuInS2 and ZnS, which proved to be an effective strategy to improve the onset potential and efficiency of CuInS2 photocathodes.

SrTiO3 (STO) has favorable opto-electronic properties for overall water splitting. Nevertheless, realizing a higher efficiency is impeded by its band gap which can only harvest UV light. In order to extend the spectral response towards visible light, STO is (co)doped with lanthanum (La) and rhodium (Rh). However, notwithstanding the amount of visible light absorbed, the H2 evolution rates are remarkably governed by the valence state of Rh, La doping level and ruthenium (Ru) cocatalyst loading. Hence, it is essential to unravel the underlying effect of doping on the photophysical processes to gain insight into material design. To this end, charge carrier dynamics was probed over a wide time (sub-picosecond to microsecond) and spectral (visible to IR) region using transient absorption spectroscopy. Depending on the dopant composition, an interplay between the electron trapping and the kinetics of the electron transfer to the Ru cocatalyst was rationalized. For Rh4+:STO, free electrons probed at 3435 nm decayed virtually completely by 20 ps resulting in a kinetic competition between the electron trapping and the electron transfer to Ru cocatalyst. In the case of Rh3+:STO, free electrons decayed by a factor of three by 100 ps, thus demonstrating the effect of Rh valence state on the electron lifetime. The time constant and quantum yield of electron transfer from Rh3+:STO to the Ru cocatalyst were found to be 1.6 ps and 14.7%, respectively. In addition to a longer electron lifetime, enhanced electron transfer to the Ru cocatalyst makes Rh3+:STO one of the promising photocatalysts for H2 generation. Engineering the energetic position of the dopant within the band gap to avoid undesirable carrier trapping is crucial to enhance the efficiency of photocatalytic reactions.

Transthyretin (TTR) is one of more than 30 amyloidogenic proteins, and the amyloid fibrils found in patients afflicted with ATTR amyloidosis are composed of this protein. Wild-type TTR amyloids accumulate in the heart in senile systemic amyloidosis (SSA). ATTR amyloidosis occurs at a much younger age than SSA, and the affected individuals carry a TTR mutant. The naturally occurring amyloidogenic Y116S TTR variant forms more amyloid fibrils than wild-type TTR. Thus, the Y116S mutation reduces the stability of the TTR structure. A neutron diffraction experiment on Y116S TTR was performed to elucidate the mechanism of the changes in structural stability between Y116S variant and wild-type TTR through structural comparison. Large crystals of the Y116S variant were grown under optimal crystallization conditions, and a single 2.4 mm3crystal was ultimately obtained. This crystal was subjected to time-of-flight (TOF) neutron diffraction using the IBARAKI biological crystal diffractometer (iBIX) at the Japan Proton Accelerator Research Complex, Tokai, Japan (J-PARC). A full data set for neutron structure analysis was obtained in 14 days at an operational accelerator power of 500 kW. A new integration method was developed and showed improved data statistics; the new method was applied to the reduction of the TOF diffraction data from the Y116S variant. Data reduction was completed and the integrated intensities of the Bragg reflections were obtained at 1.9 Å resolution for structure refinement. Moreover, X-ray diffraction data at 1.4 Å resolution were obtained for joint neutron–X-ray refinement.

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

Summary Efficient and scalable sunlight-driven water-splitting systems are essential for practical renewable hydrogen production. Particulate photocatalyst sheets for Z-scheme water splitting can be printed at low cost using inks containing a hydrogen evolution photocatalyst (HEP), an oxygen evolution photocatalyst (OEP), and conductive metal nanoparticles, and can be applied to large areas. However, the metal nanoparticles lead to backward reactions and light absorption. Herein, we demonstrate printable photocatalyst sheets composed of SrTiO3:La,Rh as the HEP, BiVO4:Mo as the OEP, and nanoparticulate indium tin oxide as a transparent and electrochemically inert conductor, for efficient pure-water splitting. The photocatalyst sheets decompose water with a solar-to-hydrogen energy conversion efficiency of 0.4%, which is one of the highest values among Z-scheme overall water-splitting systems prepared without the requirement for a vacuum process. Printable and efficient photocatalyst sheets obtained using cost-effective and readily extensible procedures spread opportunities toward practical solar hydrogen production via the water-splitting reaction.

In this article, the status of the STARGazer data-processing software and its data-processing algorithms are described in detail. The STARGazer data-processing software is used for neutron time-of-flight single-crystal diffraction data collected using the IBARAKI Biological Crystal Diffractometer., The STARGazer data-processing software is used for neutron time-of-flight (TOF) single-crystal diffraction data collected using the IBARAKI Biological Crystal Diffractometer (iBIX) at the Japan Proton Accelerator Research Complex (J-PARC). This software creates hkl intensity data from three-dimensional (x, y, TOF) diffraction data. STARGazer is composed of a data-processing component and a data-visualization component. The former is used to calculate the hkl intensity data. The latter displays the three-dimensional diffraction data with searched or predicted peak positions and is used to determine and confirm integration regions. STARGazer has been developed to make it easier to use and to obtain more accurate intensity data. For example, a profile-fitting method for peak integration was developed and the data statistics were improved. STARGazer and its manual, containing installation and data-processing components, have been prepared and provided to iBIX users. This article describes the status of the STARGazer data-processing software and its data-processing algorithms.

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.

Summary Sunlight-driven photocatalytic water splitting has been investigated as a potentially scalable and economically feasible means of producing renewable hydrogen. However, reactors suitable for efficient water splitting and prompt collection of gaseous products on a large scale have yet to be demonstrated. Here, we demonstrate a sunlight-powered water-splitting reactor using a fixed Al-doped SrTiO 3 photocatalyst and address the key issues in the reactor design associated with the scale-up. A panel reactor filled with only a 1-mm-deep layer of water was capable of rapid release of product gas bubbles without forced convection. A flat panel reactor with 1 m 2 of light-accepting area retained the intrinsic activity of the photocatalyst and achieved a solar-to-hydrogen energy conversion efficiency of 0.4% by water splitting under natural sunlight irradiation. The concept of a readily extensible water-splitting panel is a viable means for large-scale production of low-cost renewable solar hydrogen.

Tantalum nitride (Ta3N5) based photoanodes were overlaid with magnesium-doped gallium nitride (Mg:GaN) thin films by using a plasma-enhanced chemical vapor deposition (PCVD) technique, and subjected to photoelectrochemical activity tests, aiming for a negative shift of onset potential for O2 evolution. A remarkable negative shift of the onset potential was observed after annealing Mg:GaN in N2 gas, reaching 0 V vs. RHE, despite a lower photocurrent than that on bare Ta3N5. Mg:GaN annealed in NH3 exhibited an improvement of the photocurrent. A detailed study of the photoelectrochemical performance for various samples and a thorough characterization have revealed the effects of N2/NH3 post annealing on Mg activation/Ta3N5 damage recovery, controlling the onset potential shift and the current density improvement. N2 post annealing shifted the onset potential to 0 V vs. RHE but decreased the current density. On the other hand, NH3 post annealing slightly shifted the onset potential and increased the current density largely. Despite the current density loss, this onset potential shift unlocks the prospect of unassisted photoelectrochemical water splitting on Ta3N5.

A CuIn1−xGaxSe2 (CIGS) photocathode having a specific composition exhibited remarkable promotion of hydrogen evolution from water, with a half-cell solar-to-hydrogen conversion efficiency of 12.5%. CIGS thin films with various compositions (defined by x = Ga/(In + Ga)) were prepared via a three-stage method employing a vacuum evaporation system. The photocurrent from CIGS photocathode surfaces modified with CdS and Pt (Pt/CdS/CIGS) significantly increased along with x as a result of a reduction in the conduction band offset at the CIGS/CdS interface. Using an optimized aqueous electrolyte resulted in enhanced photocurrents of 28 and 18 mA cm−2 at 0 and 0.6 VRHE, respectively, under simulated AM1.5G sunlight. A tandem-type PEC cell containing the newly developed CIGS photocathode and a BiVO4 semi-transparent photoanode demonstrated stoichiometric hydrogen and oxygen evolution with a solar-to-hydrogen conversion efficiency of 3.7% without the application of an external bias voltage.

Particulate Ta3N5, a material that responds to visible light for photoelectrochemical O2 evolution, was glued to a metallic GaN conducting layer. The electrode was able to oxidize water with 1.8-fold higher efficiency than that without GaN. The GaN layer blocked the hole current from Ta3N5 to the back-contact metal layer and prevented hole–electron recombination.

The high overpotential of the oxygen evolution reaction is a critical issue to be overcome to realize efficient overall water splitting and enable hydrogen generation powered by sunlight. Homogeneous and stable nanoparticles (NPs) dispersed in solvents are useful as both electrocatalysts and cocatalysts of photocatalysts for the electro- and photo-catalytic oxygen evolution reaction, respectively, through their adsorption on various electrode substrates. Here, phase-segregated NiPx@FePyOz core@shell NPs are selectively synthesized by the reaction of Fe(CO)5 with amorphous NiPx seed-NPs. The NiPx@FePyOz NPs on conductive substrates exhibit higher electrocatalytic activity in the oxygen evolution reaction than those of other metal phosphide-based catalysts. The NiPx@FePyOz NPs can also be used as a cocatalyst of an anodic BiVO4 photocatalyst to boost the photocatalytic water oxidation reaction. The excellent catalytic activity and high stability of the NiPx@FePyOz NPs without any post-treatments are derived from in situ activation through both the structural transformation of NiPx@FePyOz into mixed hydroxide species, (Ni, Fe)OxHy, and the spontaneous removal of the insulating organic ligands from NPs to form a smooth and robust (Ni, Fe)OxHy/substrate heterointerface during the oxygen evolution reaction.

Photoelectrochemical water splitting has attracted much attention in recent years as an alternative energy source. However, there have been several significant issues such as low efficiency, high cost and less scalability for its practical application. Here, we show that (CuGa1−yIny)1−xZn2xS2 (CGIZS)-based photocathodes fabricated with a particle transfer method exhibited a photocurrent of 4.5 mA cm−2 at 0.6 V vs. RHE along with a largely positive onset potential of 1.0 V vs. RHE under simulated sunlight (AM 1.5G) and an initial solar-to-hydrogen energy conversion efficiency of 1.1% was obtained with photoelectrochemical water splitting using a two-electrode cell composed of the CGIZS-based photocathode and a BiVO4-based photoanode. CGIZS used for the particle transfer method was obtained in a powder state with high crystallinity by a flux method using a molten-salt of LiCl–KCl, and also formed solid solutions with a chalcopyrite single phase structure in a wide range of Ga/In ratios and Zn contents, in which the absorption edges of photocatalysts were tunable up to 880 nm.

Summary Photoelectrochemical (PEC) water oxidation by semiconductor photoanodes plays a fundamental role for sustainable solar fuel and chemical production. Advanced surface and interface engineering has been demonstrated to be of critical importance for the development of highly efficient and stable water oxidation photoanodes. In this review, we briefly introduce the fundamentals and some general considerations in the development and evaluation of water oxidation photoanodes and the assembly of PEC cells. We summarize several most important roles of surface and interface engineering identified in improving the PEC performance of photoanodes and highlight the most prominent research advancements in these fields. Finally, the outlook for the future development of surface and interface engineering for practical photoelectrochemical water oxidation is concisely discussed.

It remains challenging to develop alternatives, based on widely abundant elements, to highly active Pt catalysts for the hydrogen evolution reaction. Metal sulfide nanoparticles (NPs) are potential candidates for such catalysts because of their metallic features and intrinsic high surface area; however, their poor electrocatalytic activity, owing to the presence of surface ligands, is an obstacle to practical use. Here we report the selective synthesis of oleylamine (OAm)-stabilized Ni₃S₄ (Ni₃S₄/OAm) NPs. The insulating ligands were exchanged with S²¯ and loaded on electrodes by a layer-by-layer (LbL) assembly method with metal cation linkers. The enhanced catalytic properties were confirmed by electrochemical measurements. A LbL assembled film with Ni²+ as a linker (11 cycles) showed an overpotential of 555 mV@-4 mA/cm² at pH 6.8 and a stable current density for 12 h. In addition, when this LbL assembly method was applied to a CdS/Cu(In,Ga)Se₂ photocatalyst electrode, the resulting photoelectrode showed an enhanced photocurrent compared with that of the Ni₃S₄/OAm NPs-spin coated and bare photoelectrodes. These results indicate the versatility of LbL assembly for fabrication of ligand-free NP electrocatalyst films.