Hiroaki Maeda

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.

Conventional chemiresistive sensors based on metal oxide semiconductors offer high sensitivity, fast response, and low cost. However, the requirement for high-temperature operation is a major issue. Recently, conductive π-conjugated coordination nanosheets composed of metal ions and π-conjugated planar ligands have attracted attention as active materials for chemiresistive sensors operating at room temperature (RT). However, their tendency toward multilayer formation, driven by strong interlayer interactions, impedes access to metal complexes (active sites), thereby reducing their performance. In this study, we synthesized a composite (NiHATT/CNT) consisting of a coordination nanosheet (composed of nickel ions and the triptycene-based three-branched ligand, 2,3,6,7,14,15-hexaaminotriptycene (HATT)) and a carbon nanotube (CNT). We evaluated the humidity response of the chemiresistive sensors using NiHATT/CNT as the active material. The introduction of the triptycene skeleton allowed the square-planar metal complexes to form perpendicular to the two-dimensional surface, thereby allowing analytes to easily access the active sites. Consequently, the NiHATT/CNT chemiresistive sensor exhibited a significantly greater response than the sensor using an active material consisting of a conventional coordination nanosheet with a planar π-conjugated ligand. This demonstrates that the chemical design that provides high accessibility to the metal complexes in NiHATT by introducing a triptycene skeleton enhances the sensing performance of the NiHATT/CNT composite.

Nickelladithiolene nanosheet, Ni₃BHT, is a two-dimensional material composed of nickel ions and benzenehexathiol (BHT). Ni₃BHT has attracted considerable attention owing to its electrical conductivity. Although conventional Ni₃BHT is obtained as a solid film or powder, recent studies have explored methods for handling Ni₃BHT as a liquid ink, which facilitates industrial applications. One such method involves adding a modulator ligand to control the morphology of Ni₃BHT. In this study, we developed a novel modulator ligand, 4,5-dihexylbenzene-1,2-dithiol (CL1), which afforded a more stable and concentrated Ni₃BHT dispersion than those previously reported. Further investigations suggest that CL1 is incorporated not only at the termini but also within the interior of the Ni₃BHT nanoflakes, based on the consistent interpretation of spectroscopic and morphological data, in the dispersion via the addition of an adequate amount of a modulator. The application of the Ni₃BHT dispersion as a conductive ink was demonstrated. The Ni₃BHT ink exhibited the highest electrical conductivity and colloidal stability at a CL1/BHT ratio of 0.3. These findings pave the way for potential applications of Ni₃BHT in various industries.

Coordination nanosheets are 2D polymers formed by coordination bonds between metal ions and planar organic molecules. They offer high molecular design freedom and unique electronic and chemical properties, making them suitable for various usage. In the two‐phase interfacial reaction of metal ions with benzenehexathiol (BHT), highly conductive coordination nanosheets are generated. For nickel ions, porous NiDT (=Ni ₃ BHT ₂ ) and nonporous NiBHT (=Ni ₃ BHT) structures are obtained. However, a rational selective synthesis method for these structures is not well‐developed. In this study, selective synthesis of NiDT and NiBHT colloidal solutions is achieved by adjusting the BHT to metal ion ratio in a single phase. These colloidal solutions serve as hydrogen evolution catalysts by coating them on an electrode and NiDT is the better catalyst than NiBHT. CuBHT and ZnBHT colloidal solutions are also successfully prepared. Furthermore, nonporous NiCu ₂ BHT and a new material, NiZn ₂ BHT are synthesized by filling the NiDT pores with another metal and BHT. These approaches lead to the production of coordination nanosheet inks and the synthesis of heterometallic coordination nanosheets with controlled molecular structures. The introduction of the second metal significantly changes the electrical conductivity. Furthermore, a method is discovered to convert NiBHT to NiCu ₂ BHT using a transmetallation reaction.

Coordination polymers are attractive materials for various fields of practical application. The high degree of freedom of choice of metal ions and organic ligands plays a critical role in functional diversification. In the present study, we report the liquid–liquid interfacial synthesis of a 2D bis(terpyridine)copper(II) polymer thin film, Cu-tpy. The synthesized Cu-tpy was characterized by various microscopic observations such as TEM, SEM, and AFM, and spectroscopic measurements such as XPS, Raman spectroscopy, SEM/EDS, and UV–Vis spectroscopy. Synchrotron-radiated X-ray scattering confirmed that Cu-tpy was oriented crystalline films. Moreover, Cu-tpy showed electrochemical micro-supercapacitor behavior in the solid-state owing to its ionic nature. This study expands the potential of bis(terpyridine)metal(II) polymers as electro-functional materials.

Development of efficient electrocatalysts for hydrogen evolution reactions (HERs) is necessary to achieve environmentally friendly and sustainable hydrogen production. To reduce cost and to circumvent the scarcity of platinum, the most efficient catalyst for HER, it is essential to develop catalysts using ubiquitous base metals or minimal amounts of precious metals. Bis(diimino)metal (MDI) coordination nanosheets are potential HER catalysts because their electric conductivities, two‐dimensionality, and porous structures provide large surface areas and efficient mass and electron transfer. In addition, with sparse metal arrangements in their chemical structures, nanosheets can reduce the amount of metal needed. We synthesized bis(diimino)palladium coordination nanosheets (PdDI) as a coordination polymer composed of bis(diimino)palladium, with semiconducting characteristics, using gas‐liquid interfacial synthesis and electrochemical oxidation. These electrochemically synthesized PdDIs exhibit remarkable catalytic performance with overpotential reaching 10 mA cm ⁻² of 34 mV, a Tafel slope of 47 mV dec ⁻¹ , and an exchange current density of 2.1 mA cm ⁻² after appropriate activation. This performance is closely comparable to that of metallic platinum. An ex‐situ investigation of the activation process revealed that reduction of the divalent Pd center in bis(diimino)palladium produced a composite of Pd(0) species and PdDI, combining high catalytic activity with smooth electron transfer.

Bis(diimino)metal coordination nanosheets (MHABs) were successfully synthesized in a face-on orientation manner on gold electrodes using electrochemical oxidation methods under optimal reaction conditions.