Particle morphology often governs the performance of photocatalysts. Despite the growing interest in mixed-anion compounds, robust control of particle morphology and a clear morphology-activity relationship remain to be fully established. Here, we use a flux method to tune the morphology of the layered oxyhalide Bi4NbO8Cl and define how the morphology governs photocatalysis. In situ synchrotron X-ray diffraction analysis during the heating of the precursor/flux mixtures revealed that Bi4NbO8Cl nucleated and grew within the molten flux. The crystallinity, particle size, and surface area were systematically varied by adjusting the calcination temperature and holding time. Under visible light, Bi4NbO8Cl prepared at relatively low temperature (650 degrees C) with a short holding time (5-10 hr) exhibits the highest O2 evolution rates, correlating with smaller particle size and larger surface area. Time-resolved measurements indicate that once the bulk carrier properties are improved by flux synthesis, the morphology, rather than residual differences in bulk kinetics, sets the activity trend within the flux samples. Loading IrOx further enhanced the O2 evolution, and mediator-free interparticle Z-scheme water splitting was demonstrated using the obtained flux-derived Bi4NbO8Cl as an O2-evolving photocatalyst. These results provide practical guidance for controlling the morphology of oxyhalide photocatalysts for efficient solar water splitting.

In Sillén oxyhalides SrBi 3 O 4 X 3 (X = Cl, Br, I) with single/double halogen layers, substitution with larger halogens enables both the conduction band edge lowering and bandgap narrowing, offering a new strategy for band engineering in Sillén oxyhalides.

Metal hexacyanoferrates (MHCFs) are attractive cocatalysts for Z-scheme water splitting owing to their tunable redox properties at C-coordinated FeIII/FeII sites, enabling efficient electron transfer from shuttle redox mediators. MHCFs can incorporate multiple transition metal centers through ambidentate cyanide ligands; however, rational design principles for utilizing redox-active metals at N-coordinated sites remain unclear. Herein, we show that photocatalytic H2 evolution is governed by the relative alignment of redox potentials of MHCF modifiers and aqueous electron donors. A series of MHCFs comprising redox-active metals (i.e., Mn, Fe, Co, and Cu) and redox-inactive metals (i.e., In and Ni) were loaded onto Rh–Cr mixed-oxide-modified TaON, and their electrochemical properties were correlated with photocatalytic H2 evolution using electron donors with different electron-donating abilities. MHCFs containing redox-active metal species with redox potential significantly more negative than those of electron donors (e.g., the N-coordinated FeIII/II in FeHCF and CuII/I in CuHCF) suppressed photocatalytic activity due to backward electron transfer (reduction via photoexcited electrons), whereas MnHCF, with both MnIII/II and FeIII/II potentials more positive than donors, exhibited the highest activity. Furthermore, the incorporation of multiple metals into a Mn-based high-entropy MHCF (K2Mn0.4Fe0.15Co0.15Ni0.15Cu0.15[Fe(CN)6]) improved durability while maintaining appropriate redox potentials, yielding a higher amount of gas evolution compared with MnHCF in Z-scheme water splitting. These findings provide a design strategy for multi-redox-active MHCF cocatalysts, highlighting that the optimal choice and tuning of metal sites can achieve both high efficiency and durability in water splitting systems.

Layered mixed-anion compounds are promising photocatalysts for solar fuel production owing to their visible-light absorption and suitable band-edge positions. However, their two-dimensional crystal structures hinder the spatial separation of reduction and oxidation sites because both photogenerated electrons and holes predominantly migrate in-plane along the layered frameworks. In particular, in plate-like particles reflecting this layered structure, charge carriers must travel relatively long lateral distances. Here, we overcome this limitation for the first time by introducing step structures on the basal planes, which shorten the lateral carrier migration while spatially separating redox sites. Using the layered oxyhalide Bi 4 NbO 8 Cl as a model photocatalyst, nanoscale steps are formed on the basal planes of plate-like crystals via screw-dislocation-driven spiral growth during flux synthesis. Subsequent acid etching converts the basal planes from photogenerated electron-rich to hole-accumulating surfaces, as revealed by surface photovoltage spectroscopy, enabling preferential photodeposition of reduction cocatalysts onto the electron-rich nanosteps. This asymmetric cocatalyst loading promotes anisotropic charge separation, resulting in a 16-fold enhancement in the photocatalytic H 2 evolution rate compared to the untreated sample and achieving one of the highest apparent quantum efficiencies reported for layered oxyhalide photocatalysts. Furthermore, etched samples with nanosteps exhibit enhanced photocatalytic water oxidation under visible-light Z-scheme water splitting, demonstrating the effectiveness of this surface engineering for solar water splitting. This study establishes spiral-growth-induced step formation combined with surface etching as a new design strategy for controlling carrier flow in layered photocatalysts, unlocking their potential for solar energy conversion.

Interlayer-active layered oxysulfides, which exhibit both interlayer hydration and ion-exchange capabilities, have recently emerged as a new class of visible-light-driven photocatalysts for water splitting. However, the utilization of interlayer galleries as reaction fields remains unexplored in oxysulfide photocatalysts. Here we demonstrate the interlayer space of a layered oxysulfide as an active reaction field for H2 evolution. Interlayer-active layered oxysulfides NaMTiO2.2S1.8 (M = Pr, Nd, Sm, Eu, and Gd) were synthesized via a solid-state reaction, yielding submicrometer-sized particles suitable for effective utilization of the interlayer space, markedly smaller than those obtained by the previous H2S-based method. Ni species were introduced into the active interlayer via ion-exchange with interlayer Na+. The interlayer-Ni-loaded NaMTiO2.2S1.8 exhibits significantly enhanced H2 evolution activity compared with surface-Ni-loaded NaMTiO2.2S1.8, owing to more efficient utilization of photogenerated electrons within the particle interior. These results provide a new design concept for layered oxysulfide photocatalysts toward efficient visible-light-driven water splitting.

To realize highly optimized properties and performance of semiconductor photocatalysts, precise control over their composition and the site occupancy of multiple cations and anions is essential. This study demonstrates that Bi2YO4Cl, a multicationic oxyhalide photocatalyst, exhibits markedly higher activity when isovalent Bi-for-Y substitution is suppressed by simply controlling the precursor stoichiometry. The flux synthesis of Bi2YO4Cl under stoichiometric conditions induces the partial substitution of Bi3+ into Y3+ sites, creating localized states near the valence band maximum, which act as hole traps and hinder efficient charge carrier utilization. Using an excess of Y2O3 during the synthesis suppresses the undesired Bi-for-Y substitution, leading to markedly higher H2 and O2 evolution rates under visible-light irradiation. This study highlights the critical importance of precise cation placement for maximizing the photocatalytic performance of multicationic photocatalysts.

Z-scheme water splitting using semiconductor photocatalysts is a promising strategy for achieving sustainable solar hydrogen production. However, in Z-scheme systems, competition for backward electron transfer, which exerts a substantial influence on the overall quantum efficiency, is thermodynamically unavoidable. In this study, a rational strategy is proposed to overcome the backward electron transfer in Z-scheme water-splitting systems by manipulating the electrostatic affinity/repulsion between photocatalyst surfaces and electron mediators. A designed cationic/neutral charge-switchable [Co(bpc)2]+/0 complex selectively suppressed the backward electron transfer caused by the electrostatic repulsion between the oxidised [Co(bpc)2]+ form and positively surface-charged H2-evolving photocatalyst, to which the forward electron transfer from the reduced [Co(bpc)2]0 form should be negligibly influenced by electrostatic interactions. This selective suppression of backward electron transfer enabled by charge-switchable [Co(bpc)2]+/0 is unique and could not be achieved using conventional cationic (e.g. Fe3+/2+) or anionic (e.g. IO3-/I-) redox mediators. As a result, the [Co(bpc)2]+/0 complex mediator provided the best photocatalytic performance for a benchmark H2-evolving SrTiO3:Rh photocatalyst among the conventional redox mediators and yielded a much improved apparent quantum efficiency of 2.7% for overall water splitting using SrTiO3:Rh and Bi4TaO8Cl photocatalysts. This study establishes a molecular design principle for redox mediators to improve Z-scheme water splitting, shifting the focus beyond the conventional emphasis on engineered photocatalyst materials.

Visible-light-driven overall water splitting is a promising route to solar hydrogen production, but achieving it with a single photocatalyst remains difficult. Many visible-light-responsive photocatalysts can drive either H2 or O2 evolution in the presence of suitable electron donors or acceptors, yet fail to drive overall water splitting. Rhodium-doped SrTiO3 (SrTiO3 : Rh) is a representative visible-light-responsive photocatalyst for H2 evolution. However, it does not achieve overall water splitting because of its limited ability to oxidize water to O2, although its conduction-band and Rh-donor levels are thermodynamically suitable for H2 and O2 evolution, respectively. Here, we introduce the concept of a "hole reservoir", defined as an aqueous redox couple that captures photogenerated holes, stores them at potentials more positive than the water oxidation potential, and subsequently delivers them to a water oxidation catalyst to drive O2 evolution. Unlike conventional Z-scheme systems, which couple H2- and O2-evolution photocatalysts through a redox mediator, this design eliminates the need for photoexciting an O2-evolution photocatalyst; instead, water oxidation is catalytically driven by the oxidizing equivalents accumulated within the hole reservoir. We identified [SiW11O39CoIII/II(H2O)]5-/6- (SiCoIII/IIW11) polyoxometalate as an effective hole reservoir that couples H2 evolution on SrTiO3 : Rh with RuO2-catalyzed water oxidation. This strategy enables steady and stoichiometric overall water splitting in a CrO x /Rh/SrTiO3 : Rh-RuO2 suspension as well as simultaneous H2 and O2 evolution over RuO2/Ru/SrTiO3 : Rh, thereby establishing a new framework to convert visible-light-responsive photocatalysts that intrinsically drive only a single half-reaction into overall water-splitting systems.

Redox potential alignment of MHCF cocatalysts enables selective mediator oxidation for Z-scheme water splitting.

Particle morphology often governs the performance of photocatalysts. Despite the growing interest in mixed‐anion compounds, robust control of particle morphology and a clear morphology–activity relationship remain to be fully established. Here, we use a flux method to tune the morphology of the layered oxyhalide Bi 4 NbO 8 Cl and define how the morphology governs photocatalysis. In situ synchrotron X‐ray diffraction analysis during the heating of the precursor/flux mixtures revealed that Bi 4 NbO 8 Cl nucleated and grew within the molten flux. The crystallinity, particle size, and surface area were systematically varied by adjusting the calcination temperature and holding time. Under visible light, Bi 4 NbO 8 Cl prepared at relatively low temperature (650°C) with a short holding time (5–10 hr) exhibits the highest O 2 evolution rates, correlating with smaller particle size and larger surface area. Time‐resolved measurements indicate that once the bulk carrier properties are improved by flux synthesis, the morphology, rather than residual differences in bulk kinetics, sets the activity trend within the flux samples. Loading IrO x further enhanced the O 2 evolution, and mediator‐free interparticle Z‐scheme water splitting was demonstrated using the obtained flux‐derived Bi 4 NbO 8 Cl as an O 2 ‐evolving photocatalyst. These results provide practical guidance for controlling the morphology of oxyhalide photocatalysts for efficient solar water splitting.

The use of biphasic solutions has recently attracted attention as an effective strategy to spatially extend photoinduced charge separation between the photocatalyst and electron donors/acceptors via phase migration, enabling efficient photocatalysis. Herein, key parameters of biphasic photocatalytic systems, namely, diffusion in solution, liquid-liquid phase boundary, and phase transfer equilibrium, were modulated in detail, unveiling design principles that maximise the benefits of biphasic systems.

Chlorophyll (Chl)-a derivatives possessing a carboxylated substituent at the peripheral 20-position were prepared as photosensitizers. The synthetic pigments were adsorbed onto the surface of titanium oxide particles loaded with platinum nanoparticles through the bonding of the carboxy group of the Chls with the oxygen functional groups of TiO2. In an aqueous ascorbic acid solution, the resulting Chl-attaching Pt-TiO2 was illuminated with orange and red light to generate hydrogen gas. The hydrogen evolution reaction was initiated by the electron injection of the Chl photosensitizers excited with the visible light to the TiO2 semiconductor and was completed by the reduction of aqueous protons into H2 on the Pt cocatalyst, while ascorbic acid functioned as an electron donor to the Chl species. The hydrogen evolution activity was dependent on the spacer between the core chlorin π-system and the terminal carboxy group in the Chls. An increase in the spacer length suppressed the undesired back electron transfer from the TiO2 surface to the Chl cation radical to enhance the amount of evolved H2. The facile insertion of a copper atom at the central position of a Chl sensitizer boosted the photocatalytic activity, but its zincation and nickelation diminished the photosensitizing abilities.

Continuous and compositionally controllable band-structure tuning remains a central challenge in visible-light-driven photocatalytic water splitting. Herein, we demonstrate that layered bismuth oxyhalide solid solutions (BiOBr1-xIx) can provide a continuously tunable platform for rational band engineering in visible-light-driven Z-scheme water splitting. By exploiting the intrinsic halide-dependent band tunability of BiOBr1-xIx solid solutions, we achieved a balance between extended visible-light absorption and a sufficient reaction thermodynamic driving force. The optimal composition (x = 0.1) exhibited sustained O2 evolution in the presence of Fe3+ as a reversible electron acceptor, reflecting the interplay between enhanced visible-light absorption and competing thermodynamic and charge-transport limitations. Suppression of iodine depletion via arc plasma deposition of metallic Pt cocatalysts further enhanced the activity by preserving the composition. When integrated as the O2-evolving photocatalyst in a Z-scheme system, stoichiometric and steady overall water splitting was achieved for over 50 h under visible light. These findings establish compositionally tunable bismuth oxyhalide solid solutions as versatile materials for rational valence-band engineering in visible-light-driven water splitting.

Photocatalytic overall water splitting holds great promise for sustainable hydrogen production. For overall water splitting performed by one-step excitation, both the hydrogen evolution and oxygen evolution reactions are hindered kinetically; hence, it is necessary to employ site-selective modification of photocatalysts with hydrogen-evolving and oxygen-evolving cocatalysts. However, critical challenges remain, including the need for cumbersome, multi-step photodeposition processes and durable blocking layers to inhibit the reverse reactions. Here we find a conductive, two-dimensional metal-organic framework to serve as a simple, multifunctional cocatalyst. We loaded this framework on SrTiO3:Al, an overall water-splitting photocatalyst, using a one-step self-assembly method. The framework cocatalyst promoted steady photocatalysis with an apparent quantum efficiency of 31.5% at 350 nm, free from the reverse reaction even without blocking layers. We proposed the operational principles for the cocatalyst activity using spectroscopic, electrochemical and theoretical analyses. This two-dimensional metal-organic framework offers an all-in-one approach for designing efficient and practical one-step excitation overall water-splitting systems.

Z-scheme water splitting with a redox mediator, which transports electrons from an H2- to an O2-evolving photocatalyst, has been widely studied to achieve efficient overall water splitting. However, backward electron transfer with the redox mediator is an intrinsic drawback of Z-scheme water splitting, because it interrupts the desired H2 and O2 evolution. Although a CrOx shell coating on metal cocatalysts on an H2-evolving photocatalyst is widely used to block backward electron transfer, this strategy relies on potentially toxic chromium species of which elution is possible under the photocatalytic operation. Herein, we demonstrate a chromium-free approach that suppresses backward electron transfer by manipulating electrostatic interactions at the photocatalyst-mediator interface. The designed charge-switchable cobalt complex serves as a superior mediator that selectively suppresses backward electron transfer from a positively charged H2-evolving photocatalyst without using a CrOx shell, leading to efficient Z-scheme water splitting with an apparent quantum yield of 7.2%.

Controlling trap depth is crucial to improve photocatalytic activity, but designing such crystal structures has been challenging. In this study, we discovered that in 2D materials like BiOCl and Bi4NbO8Cl, composed of interleaved [Bi2O2]2+ and Cl- slabs, the trap depth can be controlled by manipulating the slab stacking structure. In BiOCl, oxygen vacancies (VO) create deep electron traps, while chlorine vacancies (VCl) produce shallow traps. The depth is determined by the coordination around anion vacancies: VO forms strong σ bonds with Bi-6p dangling bonds below the conduction band minimum (CBM), while those around Cl are parallel, forming weak π-bonding. The strong re-hybridization makes the trap depth deeper. In Bi4NbO8Cl, VCl also creates shallow traps, but VO does not produce deep traps although Bi-6p orbitals are also forming strong σ bonding. This difference is attributed to the difference of the energy level of CBM. In both cases, the CBM consists of Bi-6p orbitals extending into the Cl layers. However, these orbitals are isolated in BiOCl, but those in Bi4NbO8Cl are bonded with each other between neighboring [Bi2O2]2+ layers. This unique bonding-based CBM prevents the formation of deep electron traps, and significantly enhances H2 evolution activity by prolonging the lifetime of highly reactive free electrons.

Layered compounds that utilize interlayer space as a reactive field are known as "interlayer-active" compounds and have been gaining attention, particularly in photocatalysis for water splitting. However, most of the reported "interlayer-active" photocatalysts are oxide semiconductors that possess a wide bandgap. Thus, they cannot utilize visible light essential for efficient water splitting. In this study, we synthesized novel Ruddlesden-Popper (RP) (n = 1) layered oxysulfides, NaMTiO2.2S1.8 (M = Nd, Sm), by heating "interlayer-active" layered oxides, NaMTiO4, under H2S flow. In NaMTiO2.2S1.8, the sulfur atoms occupy the apical oxygen sites and contribute to the elevated valence band maximum (VBM) to enable visible light absorption. Additionally, NaMTiO2.2S1.8 exhibits both proton exchange and interlayer hydration capabilities as well as photocatalytic activity for hydrogen and oxygen evolution under visible light. Hence, NaMTiO2.2S1.8 is the first example of both a n = 1 RP and an "interlayer-active" oxysulfide with the potential for visible-light-driven overall water splitting. The "interlayer-active" RP (n = 1) oxysulfide is expected to find application in various fields beyond photocatalysis by utilizing interlayer reactions such as ion exchange and interlayer hydration.

This study demonstrates the potential of V-based oxyhalides as oxygen-evolving photocatalysts in Z-scheme water-splitting systems.

The rational design of photocatalysts with precise bandgap and band-edge control is crucial for achieving the visible-light-driven conversion of solar to hydrogen. This study demonstrates a novel strategy for band-edge engineering through B-site cation substitution and symmetry evolution in Cu (3d10)-based perovskites. Substitution of Ta5+ (5d0) for Nb5+ (4d0) in CuNbO3 induces sequential phase transitions (Pc-R3c-R3c), accompanied by systematic bandgap modulation from 1.57 to 2.23 eV. The R3c CuTaO3 phase exhibits a much higher conduction band (-0.90 V vs. standard hydrogen electrode) than the H+/H2 potential and a slightly lower valence band than the O2/H2O potential. Visible-light-driven hydrogen evolution occurs efficiently on CuTaO3 with Ru cocatalyst, in the presence of S2-/SO32- sacrificial agents. Our experiments and first-principles calculations reveal that the Ta-for-Nb-substitution widens the bandgap by lowering the valence band maximum via weakened Cu-O hybridization, while simultaneously elevating the conduction band minimum via Ta 5d orbital contributions. The inherent bandgap-narrowing tendency of structural evolution from polar Pc to centrosymmetric R3c symmetry is attenuated by Ta substitution-induced elongation of Cu-O bonds. The interplay between B-site cation engineering and symmetry-induced electronic degeneration establishes a materials design paradigm for visible-light-driven photocatalysis, demonstrating how to enable targeted bandgap optimization by coupling orbital-level modifications and phase transition.

Iodine-based compounds with a Sillen-Aurivillius layered perovskite structure are promising photocatalysts for visible-light-induced water splitting. However, their synthetic method has been limited to solid-state reaction (SSR), which restricts the material tunability and thus photocatalytic performance. Here, we report a liquid-phase synthesis of Sillen-Aurivillius oxyiodide Bi4NbO8I via the flux method. An appropriate choice of reaction conditions, including flux, precursor, and calcination atmosphere/temperature, is required for the single-phase formation due to the complex crystal growth mechanism, as revealed by in situ synchrotron X-ray diffraction measurements. The provided plate-like particles are of excellent crystallinity with size tunability. The superior charge carrier transport property of the flux sample, as shown by time-resolved microwave conductivity measurements, allows its higher photocatalytic water oxidation activity than the sample prepared via conventional SSR. An appropriate surface modification further exploits the superior bulk property of the flux sample, achieving the highest performance reported for oxyiodide photocatalysts with an apparent quantum efficiency for sacrificial O2 evolution of 8.8% at 405 nm. This study provides a solution-based synthetic approach to the complex layered oxyiodides, broadening their potential for solar-to-energy conversion systems.