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Coordination nanosheets are a unique class of two-dimensional (2D) materials that are formed by coordination bonds between planar organic ligands and metal ions. These 2D nanomaterials are increasingly utilized in energy storage, electronic devices, and as electrode-based catalysts due to their excellent electronic, optical, redox properties, and catalytic activity.

Over the last decade, coordination nanosheets composed of various transition metal ions, such as nickel (Ni) ions linked to benzenehexathiol (BHT)—an organic compound—have been successfully synthesized in laboratories. However, their production has relied on a two-phase interfacial reaction that occurs between two immiscible phases of matter.

Furthermore, the selective synthesis of well-organized heterometallic nanosheets, containing two or more metal ions, has proven to be difficult. To address these two major issues limiting the production of novel coordination nanosheets, a team of researchers led by Professor Hiroshi Nishihara, from the Research Institute for Science and Technology (RIST), Tokyo University of Science (TUS), Japan, has conducted a series of innovative experiments.

The research team comprised Miyu Ito, a Master's student at TUS, project researchers Dr. Naoya Fukui and Dr. Kenji Takada, and Dr. Hiroaki Maeda, a lecturer at TUS. Their findings were published online in the journal on May 5, 2025, and selected as the Front Cover of the issue.

"When Ni ions are used in the two-phase interfacial reaction, porous nickelladithiolene (NiDT) and non-porous NiBHT structures are obtained. However, a rational method for selectively synthesizing them has not been fully established until now," says Prof. Nishihara, explaining the motivation behind the present study.

The researchers employed a single-phase reaction of Ni²⁺ ions and BHT to form colloidal solutions of coordination nanosheets. Notably, by controlling the molar ratio of Ni²⁺ ions and BHT, the researchers could selectively synthesize coordination nanosheets of interest in a single-phase reaction. The prepared colloidal coordination nanosheets can be further utilized as inks to coat substrates or electrodes, or as solutions for subsequent chemical reactions.

To validate the utility of coordination nanosheets in colloidal solutions, the researchers coated glassy carbon (GC) with NiDT and NiBHT. During subsequent electrochemical analysis, NiDT showed a broad redox wave, indicating a porous structure, while non-porous NiBHT did not show a redox wave at the corresponding potential. Further testing of the NiDT-coated GC electrode revealed a high potential for the hydrogen evolution reaction as a catalyst.

Drawing inspiration from their research findings, the team successfully synthesized colloidal nanosheet solutions containing copper (Cu) linked to BHT and zinc (Zn) linked to BHT. Furthermore, by adding Cu²⁺ ions to the NiDT colloidal solution containing unreacted BHT, they were able to generate NiCu₂BHT nanosheets, where the Cu²⁺ ions could be introduced into the pores of NiDT. Through a similar strategy, coordination nanosheets of NiZn₂BHT, new compounds were prepared by introducing Zn²⁺ ions into NiDT colloidal solution.

In addition to using colloidal solutions, the researchers turned their attention to a transmetallation reaction—a type of organometallic reaction where metal ions at the coordination center are replaced with other ions to produce heterometallic coordination nanosheets. Remarkably, the first step of the transmetallation reaction of NiBHT with Cu²⁺ metal ions resulted in NiCu₂BHT nanosheets.

"NiCu₂BHT has high crystallinity and electrical conductivity and can be utilized in diverse electronic applications. This shows the superiority of the structure and physical properties of heterometallic coordination nanosheets with a defined structure," states Prof. Nishihara.

Taken together, this study reveals two novel synthesis routes to produce highly conductive coordination nanosheets in an ink-like form, which can be further applied as coatings to various substrates and used as chemical reactants.

Prof. Nishihara concludes by highlighting the future applications of the present research, "The first ink made from coordination nanosheets has made it possible to mass-produce them using printing technology and to apply them directly to devices, marking a major step forward in their practical use as next-generation flexible electronic devices, hydrogen production catalysts, and sensor materials."

Thanks to the researchers of TUS, the promise and potential of low-cost, high-performance nanomaterials can be realized, leading to a sustainable and energy-efficient society.