The doping of Y2Ti2O5S2 (YTOS) photocatalyst particles with lower-valence Al3+ or Mg2+ cations at Ti4+ sites was investigated using either solid-state reaction (SSR) or flux-mediated crystal growth techniques. Adding Al2O3 or MgO as a dopant source to the precursor mixture slightly decreased the size of the YTOS particles obtained by the SSR method. At relatively low doping levels, Al3+ cations were incorporated into YTOS via the SSR process. However, overly high Al3+ additions caused particulate Al2O3 to deposit on the YTOS surface. Mg2+ cations were only minimally incorporated into the photocatalyst crystals when using the SSR process, resulting in the formation of a separate MgTiOx impurity. The flux method drastically altered the size and morphology of the resulting crystalline photocatalyst particles. Interestingly, either Al3+ or Mg2+ cations could be incorporated into the photocatalyst crystals through this method without generating impurities, meaning that the flux technique promoted doping. The incorporation of Al3+ cations via either the SSR or flux processes improved the photocatalytic activity of YTOS during the hydrogen evolution half-reaction compared with the undoped material. This effect resulted in a maximum hydrogen evolution rate of 113 mu mol h-1 under visible light irradiation.

Solar-driven water splitting offers a direct path to green hydrogen by using sunlight to split water into hydrogen and oxygen, enabling flexible energy storage and hydrogen fuel production. Yet, turning lab-scale demonstrations into widespread deployment requires overcoming intertwined fundamental and engineering challenges. This Voices piece gathers researchers from around the world who work on photocatalytic, photoelectrochemical, and biophotoelectrochemical water splitting to discuss bottlenecks, promising strategies, and the tools most helpful for advancing solar fuels production. They address design principles for materials and devices, emerging configurations and benchmarking concepts, pathways to commercialization, and how related catalytic processes and adjacent fields can guide sustainable hydrogen evolution. What becomes clear is that achieving scalable solar-driven hydrogen production will require integrated, multi-disciplinary efforts that bridge responsible discovery and deployment.

Solar-driven water splitting with semiconductor particulates offers a sustainable pathway for hydrogen production1,2. Two-dimensional (2D) π-conjugated polymers have emerged as promising photocatalysts owing to their cost effectiveness and optoelectronic tunability3,4. However, photoexcited states in polymers are largely confined within π-conjugated 2D planes, making charge carriers vulnerable to recombination. Despite widespread modification of the electronic structure to enhance in-plane charge separation, long-term experimental efforts continue to highlight a persistent bottleneck in quantum efficiency5. To maximize charge utilization efficiency, the main challenge lies in inducing out-of-plane carrier migration, namely, fostering carrier flow through van der Waals-bonded layers. Here, using polymeric carbon nitride crystals as model systems, we demonstrate that out-of-plane carrier transport can be activated over surprisingly long distances (about 200 nm) by applying lateral or vertical internal electric fields by means of encapsulating nanofilms on different polymer facets. The lateral and vertical electric fields boost apparent quantum efficiency for overall water splitting to 53.4% and 82.1%, respectively. Our study introduces a strategy for transitioning from intrinsic 2D-confined excited states into kinetic-driven 3D spatially separated states and paves the way for maximizing energy conversion by polymer photocatalysis.

Perovskite-type oxynitrides offer a promising route for sustainable solar-to-hydrogen energy conversion via one-step-excitation photocatalytic overall water splitting (OWS). However, insufficient charge carrier lifetimes and sluggish surface reaction kinetics, stemming from inadequate control over bulk and surface properties, have thus far limited photocatalytic efficiency. Herein, we demonstrate that the particle size, defect states, and surface properties of BaTaO2N can be effectively tailored by combining precursor engineering with Mg doping, thereby enhancing its OWS activity. Mg-doped BaTaO2N nanocubes with particle sizes of several tens of nanometers were synthesized, and a solar-to-hydrogen energy conversion efficiency an order of magnitude higher than previously reported for BaTaO2N-based photocatalysts was achieved with optimized IrOx and Cr2O3/Ru loading. Mechanistic studies reveal that the dual effects of Mg, namely, passivating bulk defects and tuning surface properties, give rise to long-lived charge carriers and efficient transfer of these carriers to uniformly distributed cocatalyst sites. This work demonstrates that precursor engineering combined with Mg doping enables rational bulk-surface coregulation in oxynitride photocatalysts, providing design principles for developing efficient visible-light-driven OWS systems.

Abstract Cu2(Sn,Ge)S3 (CTGS) is a promising near-infrared-responsive photocathode material whose conduction band minimum can be tuned by varying the Sn/Ge ratio, whereas direct control of the valence band maximum (VBM) remains challenging because of the large Cu 3d contribution to the VBM. This study proposes a strategy for adjusting the VBM position for CTGS photocatalytic particles based on a high degree of substitution of monovalent alkali metal cations at Cu+ sites. Specifically, Li+, Na+, and K+ are introduced during solid-state synthesis under H2S flow while reducing the Cu precursor amount. The results show that Na+ and K+ were mainly enriched near the surfaces of CTGS particles because of their larger ionic radii, whereas Li+ could be incorporated into the bulk of the photocatalyst crystals. Substituting Li+ ions at Cu+ sites reduced the contribution of Cu 3d orbitals to VBM formation, thereby positively shifting the VBM position. Notably, Li+-substituted CTGS was still able to absorb near-infrared light up to 900 nm, despite having a wider bandgap than pristine CTGS. Consequently, a photocathode fabricated from Li+-substituted CTGS particles exhibited an increased cathodic photocurrent of –4.5 mA cm–2 at 0 V vs a reversible hydrogen electrode (RHE) and a positively shifted onset potential of 0.38 VRHE during photoelectrochemical hydrogen evolution under simulated sunlight. Device simulations suggested that the positive VBM shift improves the CdS/CTGS band alignment, increases band bending, and suppresses interfacial recombination. It is evident that doping with Li+ represents a promising approach to significantly adjusting the band structures of Cu-based multielement semiconductors.

Oxynitride photocatalysts drive the hydrogen and oxygen evolution reactions under visible light with sacrificial reagents, but overall water splitting remains challenging, pointing to unresolved interfacial energetics. Here we integrate SrTaO2N and the (Ba x Sr1-x )TaO2N solid solution into particulate photoelectrodes and quantitatively probe their semiconductor properties at the solid/liquid interface. Beyond the rectifying behavior established for single-crystal photoelectrodes, electrochemical measurements using a reversible Fe3+/Fe2+ redox couple identify a defect-derived surface state located at 0.5 V vs the reversible hydrogen electrode in photocatalysts that mediates irreversible electron transfer. While this state enables half-reactions and rationalizes Z-scheme water splitting with suitable mediators, it promotes carrier recombination and thereby suppresses overall water splitting. This study establishes an electrochemical methodology that bridges photocatalyst powders and interfacial energetics and indicates that eliminating or passivating the defect states should enable overall pure water splitting in oxynitrides.

ZnGeN2:ZnO, a solid solution of ZnGeN2 and ZnO, functions as a photocatalyst active for overall water splitting under visible light. The absorption edge wavelength of ZnGeN2:ZnO shifts toward longer wavelength with increasing ZnO content. However, the conventional synthesis by nitridation under an NH3 flow causes the reduction and volatilization of Zn species, which inherently restricts the maximum absorption edge wavelength to 460 nm. In this study, ZnGeN2:ZnO with a ZnO content of 66 mol% and an absorption edge of around 540 nm was synthesized using Zn3N2 as a solid nitrogen source in a sealed evacuated tube. The obtained ZnGeN2:ZnO, when loaded with hydrogen and oxygen evolution cocatalysts, exhibited activity for overall water splitting under visible light irradiation. The photocatalytic activity was enhanced by post-nitridation treatment. This work demonstrates a novel synthetic approach to long-wavelength-responsive ZnGeN2:ZnO and expands its potential for green hydrogen production via sunlight-driven water splitting.

Photocatalytic water splitting with particulate semiconductors offers a scalable route to solar-to-hydrogen (STH) conversion, yet efficiency is limited by optical losses and carrier recombination. We present a predictive framework that integrates first-principles optical spectra, carrier diffusion, multilayer optics, and effective-medium theory. Applied to the visible-light-responsive photocatalyst Gd2Ti2O5S2, the model quantitatively reproduces apparent quantum efficiency (AQE) spectra and enables extraction of carrier diffusion lengths. The calculated absorptance is consistent with Kubelka-Munk theory and its diffuse-reflectance assumptions. Notably, particles larger than the diffusion length can still enhance STH performance via improved light harvesting. However, radiative photon escape, especially near shallow absorption edges, can outweigh recombination losses even in stacked configurations. These results underscore the need for simultaneous optimization of optical and electronic transport properties and challenge the assumption that maximizing photocatalyst loading alone ensures optimal efficiency once particle size is tuned. We also apply the framework to explain the gradual decline in AQE with increasing wavelength observed for particulate SrTiO3:Al photocatalysts that exhibit near-unity AQE.

Perovskite-type tantalum-based oxynitride photocatalysts are promising candidates for water splitting due to their suitable band positions and extended light absorption beyond 600 nm. However, their associated photocatalytic activities and quantum yields remain relatively low. Here, we show that a nano-sized single-crystalline BaxSr1-xTaO2N solid-solution perovskite photocatalyst exhibits state-of-the-art activity in separate oxygen and hydrogen evolution half-reactions. The improved performance is attributed to the nanoscale particle sizes, as well as the reduced defect densities achieved by using a mixed precursor comprising TaS2 and Ta3N5. The half-reaction activities can be modulated by applying a post-synthetic high-temperature treatment. Assessments of charge carrier dynamics, in conjunction with a mechanistic kinetic model, reveal that exponential-tail trap states are formed during this post-treatment. Such trap states, present on the photocatalyst surface, facilitate participation of holes during the oxygen evolution reaction. The development of such solid-solution photocatalysts broadens the range of potential materials for solar-driven hydrogen production. In addition, the present findings are expected to enable the selective tuning of bifunctional photocatalysts for either the hydrogen or oxygen evolution reaction.

The morphology and crystalline structure of semiconductor materials both play important roles in determining the photocatalytic activity of such materials. In this regard, tantalum nitride (Ta3N5) shows promise as a visible-light-responsive photocatalyst for solar-driven water splitting. Even so, the performance of this material is limited by its bulk morphology and by high defect densities and inefficient charge transport. The present work synthesized single-crystalline Ta3N5 nanosheets having reduced defect concentrations and an increased specific surface area via the direct nitridation of two-dimensional TaS2 nanosheets. The Ta3N5 nanosheets had a thickness of approximately 30 nm with well-defined exposed facets and a uniform single-crystalline structure, and so led to a shorter charge-carrier diffusion length along with efficient charge separation and transport. When modified with IrOx as a cocatalyst, these nanosheets provided an apparent quantum yield of 32.4% at 420 nm during photocatalytic oxygen evolution with sacrificial electron acceptors, outperforming Ta3N5 synthesized from Ta2O5. This material was also integrated into Z-scheme photocatalyst sheets together with La5Ti2Cu0.9Ag0.1O7S5 as the hydrogen evolution photocatalyst and carbon nanotubes as the electron mediator. These sheets enabled overall water splitting with stoichiometric H2 and O2 evolution in response to visible light, with a light absorption range extended to approximately 600 nm. This work underscores the critical roles of precursor selection and nanoscale morphological control in the development of photocatalysts with minimal defects and provides new insights expected to advance the field of solar-to-chemical energy conversion.

Photocatalytic water splitting for hydrogen production is a cornerstone of clean energy technology. However, rapidly and accurately evaluating the rate of oxygen evolution remains a bottleneck in material development. This study proposes a combination of microamperometry and light pulse trains to enable high-speed analysis of the oxygen evolution kinetics. A half-day experiment was sufficient to determine the dependence of the evolution rate on a SrTiO3 photocatalyst sheet with seven light power densities ranging from 0.57 to 75 W m- 2. A 1/2order rate law for oxygen evolution was observed at light power densities greater than 14 W m- 2. The random movement and recombination of excited electrons and holes limited the water splitting yield, resulting in the 1/2-order rate law. With smaller power densities, the kinetic order shifted to unity. This shift may affect the water-oxidation yield under natural sunlight, as the power density of UV light on the ground falls in the order of 10 W m- 2. The rate of oxygen evolution was further quantified in solutions with varying oxygen content in a range of 0.02-0.22 mmol l- 1. The presence of oxygen at 0.05 mmol l- 1 negatively affected oxygen evolution. The backward reaction of water splitting likely caused the negative effects observed in the O2-rich solutions. The key concept demonstrated here, reducing the physical distance between the reaction and detection sites in liquid, will be useful to quickly observe the kinetics of many other reactions at liquid-solid interfaces.

Understanding the dynamics of photogenerated hole trapping is essential for the rational design of high-performance photocatalysts. While hole trapping in conventional UV-responsive oxides (Type A) is well described by the small-polaron model involving deeply trapped states, this framework fails to account for the sharp absorption features commonly observed in visible-light-responsive materials. Here, using time-resolved transient absorption spectroscopy on tantalum-based oxynitrides (Type B) and representative hybridized oxides such as BiVO4 (Type C), we show that these sharp spectral features originate from reactive hole states whose energies remain tied to the valence-band edge rather than falling into midgap levels. Our results establish a unified physical picture in which hole trapping is governed by the competition between electronic delocalization and lattice relaxation. We demonstrate that high anion polarizability and/or strong metal-anion d-p hybridization suppresses lattice distortion and kinetically stabilizes shallow, band-edge-proximal hole states. Crucially, this electronic mechanism endows visible-light-responsive photocatalysts with intrinsic defect tolerance as the suppressed lattice relaxation prevents defect sites from acting as deep hole traps. This framework reconciles long-standing discrepancies between the spectroscopic signatures and redox reactivity across different material classes. It further provides design guidelines for developing visible-light-responsive photocatalysts that retain strong oxidative activity.

Photocatalytic overall water splitting using cocatalyst-modified photocatalysts faces a significant challenge to achieve highly efficient performance in pure water, as the back reaction─where the evolved H2 and O2 recombine back into H2O─is also catalyzed by the cocatalysts. Overlayers on modified photocatalysts have been reported to prevent the back reaction and significantly enhance the overall water splitting activity. This review provides a comprehensive overview of the use of overlayers in suppressing the back reaction in photocatalytic overall water splitting and their multifunctional roles in facilitating charge separation and transport, and stabilizing cocatalysts and photocatalysts. The application of overlayers is also explored in suppressing the back reactions in other photocatalytic reactions and systems, including Z-scheme systems, electrochemical and photoelectrochemical water splitting systems, pollutant degradation reactions, and photocatalytic CO2 conversion. Furthermore, this review addresses the challenges facing overlayers, particularly their durability during photoreaction, and suggests future research areas. Overall, the knowledge gained to date emphasizes the potential of overlayers to enhance the efficiency and stability of cocatalyst-modified photocatalysts in the field of photocatalysis.

Photocatalytic overall water splitting remains limited by inefficient charge separation and utilization in reactions. Al-doped SrTiO3 exhibiting near-100% apparent quantum efficiency for overall water splitting indicates nearly complete charge separation and surface catalytic efficiency. Although Al doping has been assumed to enhance charge separation and transfer, the exact role of Al is still unclear. Here, using spatiotemporal surface photovoltage imaging, we show that a gradient Al doping in Al-doped SrTiO3 generates a built-in electric field that drives photogenerated holes from the bulk toward surface trap sites in the form of hydroxylated Al-O-Ti, prolonging their lifetime from ~100 ns to 10 ms. Spectroscopic analyses reveal that these hydroxylated Al sites serve as key centers for water adsorption, facilitating water oxidation. These findings underscore the pivotal role of Al in the spatiotemporal alignment of hole transfer and surface catalytic water oxidation, enabling high-efficiency photocatalysis in overall water splitting.

An Ir nanoparticle (IrNP)/CrO x catalyst on FTO stabilizes native IrO x -coated IrNPs in an electrodeposited CrO x matrix. The CrO x /IrO x interface enhances H 2 evolution, suppressing IrNP aggregation, and enables stable and efficient electrocatalysis.

Z-scheme overall water splitting (ZOWS) systems can provide efficient hydrogen production in response to visible light. However, the integration of narrow-bandgap non-oxide photocatalysts as oxygen evolution photocatalysts (OEPs) in such systems remains underexplored. This lack of research arises because existing OEPs and the associated cocatalysts are typically employed to promote oxygen evolution in aqueous silver nitrate solutions and are less effective in ZOWS systems utilizing reversible redox mediators that are prone to backward reactions. The present work synthesized particulate single-crystalline SrTaO2N with exposed {100} and {110} facets using NaCl as a molten salt. The strategic use of Pt and CoOx as dual cocatalysts to promote reduction and oxidation reactions, respectively, significantly enhanced the reduction of [Co(bpy)3]3+ ions and improved charge carrier separation. The resulting ZOWS system, employing Pt/CoOx/SrTaO2N as the OEP, Cr2O3/Pt/BaTaO2N as the hydrogen evolution photocatalyst and [Co(bpy)3]3+/2+ as the redox mediator, achieved stoichiometric H2 and O2 evolution. This oxynitride-based system extends the visible light range available for ZOWS to nearly 600 nm, providing a new platform for solar hydrogen production. This work provides important insights into the application of narrow-bandgap oxynitride photocatalysts in ZOWS systems.

Cu2Sn0.38Ge0.62S3 (CTGS) crystalline particles grown from a LiCl-RbCl eutectic flux exhibited efficient photoelectrochemical (PEC) hydrogen evolution from water, providing a cathodic photocurrent of -9.6 mA cm-2 at 0 V vs a reversible hydrogen electrode (RHE) under simulated sunlight, and a 45% incident-photon-to-current conversion efficiency (IPCE) at 0 VRHE under 600 nm monochromatic light. The bulk and surface properties of the flux-grown photocatalyst crystals included a low concentration of electronic defects, a relatively narrow particle size distribution with a mean of approximately 10 mu m, the formation of crystal facets and the suppressed formation of oxide-like surface impurities. These factors all contributed to the enhanced PEC performance. CTGS crystals produced in a LiCl-RbCl flux were found to have a greater amount of Cu-deficient secondary phase such as Cu2(Sn,Ge)3S7, resulting in an increased carrier concentration in the bulk material as well as the formation of a preferred interface with an n-type CdS surface modifier. The present synthesis concept based on crystal growth in a flux provides insights into methods of tuning both the bulk and surfaces of photocatalyst crystals, including interfaces formed at heterojunctions.

Z‐scheme systems based on two‐step photoexcitation are thought to be a promising means of converting solar energy into hydrogen using long‐wavelength visible light. Electron transfer between the hydrogen evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP) can greatly affect the performance of such systems, and the use of electron mediators providing efficient electron transfer is thus helpful. Ionic redox couples have been widely employed in particle suspension systems in this regard. In such systems, ions providing electron transfer diffuse through a stirred solution and react on the HEP and OEP. Solid‐state electron mediators can also promote electron transfer while simultaneously holding the HEP and OEP in place. This is a convenient means of immobilizing the particulate photocatalysts and may be applicable to large‐scale production. Accordingly, it would be desirable to increase the efficiency of charge transfer between the HEP and OEP by combining various solid‐state electron conductors. The present review examines the latest advances in Z‐scheme water splitting systems, focusing on solid‐state electron mediators. Both particle suspension and immobilized systems are assessed. Potential approaches to the construction of large‐scale Z‐scheme water splitting operations are discussed in detail together with important future challenges.

LaTiO2N responds to visible light up to a wavelength of 600 nm and so is a promising water splitting photocatalyst. However, structural defects and reduced titanium species generated in this material during the nitridation process promote the recombination of photoexcited electrons and holes, and so reduce its photocatalytic activity. The present work synthesized a Sr-doped La-Ti oxide via flame spray pyrolysis followed by heat treatment, and utilized this oxide as a starting material for the oxynitride. Sr doping promoted the formation of a La-Ti oxide having a perovskite-type structure similar to that of LaTiO2N, whereas heat treatment prior to the nitridation enhanced the crystallinity of the resulting perovskite-type Sr-doped La-Ti oxide. These factors reduced the defect concentration in the LaTiO2N, thereby improving the hydrogen evolution activity of the catalyst threefold. The present study demonstrates a means of suppressing the reduction of metal cations and thereby improving the photocatalytic activity of perovskite-type oxynitrides by controlling the structure and crystallinity of the oxide precursor.