Osamu Tomita

A polyoxometalate hole reservoir bridges a H ₂ -evolving photocatalyst and a water oxidation catalyst, enabling overall water splitting with SrTiO ₃ : Rh.

Metal hexacyanoferrates (MHCFs) are attractive cocatalysts for Z-scheme water splitting owing to their tunable redox properties at the C-coordinated Fe^III/Fe^II 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 the 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, Cu) and redox-inactive metals (i.e., In, Ni) were loaded on Rh-Cr mixed-oxide-modified TaON, and their electrochemical properties were correlated with photocatalytic H₂ evolution using electron donors with different electron-donating ability. MHCFs containing redox-active metal species with redox potential significantly more negative than those of the electron donors (e.g., the N-coordinated Fe^III/Fe^II in FeHCF, and Cu^II/I in CuHCF) suppressed photocatalytic activity due to backward electron transfer (reduction via photoexcited electrons), whereas MnHCF, with both Mn^III/II and Fe^III/Fe^II potentials more positive than the donors, exhibited the highest activity. Furthermore, the incorporation of multiple metals in a Mn-based high-entropy MHCF (K₂Mn_0.4Fe_0.15Co_0.15Ni_0.15Cu_0.15[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.

Key parameters to maximise the benefits of biphasic systems, namely, diffusion in solution, liquid–liquid phase boundary, and phase transfer equilibrium were elucidated.

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. The alternative text for this image may have been generated using AI.

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 CrO_x 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 CrO_x shell, leading to efficient Z-scheme water splitting with an apparent quantum yield of 7.2%.

A designed cationic/neutral charge-switchable Co complex as an electron mediator enabled much improved Z-scheme overall water splitting via manipulation of electrostatic interaction.

Visible-light-driven CO ₂ reduction and H ₂ evolution were demonstrated by a ternary conjugated polymer with the aid of iron and copper species, auxiliary terpyridine ligand, and triethanolamine.

This study demonstrates the first use of interlayer space as photocatalytic reaction sites for hydrogen evolution in an “interlayer-active” layered oxysulfide, NaMTiO _2.2 S _1.8 (M = lanthanoid), by introducing Ni species into the interlayer.

Compositionally tunable BiOBr _{1− x} I _x enables continuous band-structure tuning for visible-light-driven Z-scheme water splitting. BiOBr _0.9 I _0.1 functions as an O ₂ -evolving photocatalyst, enabling stable overall water splitting.