Nanosheet Synthesis Cuts Costs for Flexible Electronics

Researchers at Tokyo University of Science have developed a scalable method for producing highly conductive coordination nanosheets, addressing limitations in existing two-dimensional material synthesis. Led by Professor Hiroshi Nishihara, the team demonstrated the creation of nickel, copper, and zinc-based nanosheets using single-phase reactions, yielding ink-like solutions applicable as coatings or reactants. Validated through electrochemical analysis and hydrogen evolution reaction testing, the process enables the creation of heterometallic nanosheets with controlled atomic arrangements, potentially unlocking advancements in flexible electronics and catalysis, and supported by over $2.5 million in research funding.

Coordination Nanosheets Explained

Coordination nanosheets are two-dimensional materials assembled through coordination bonds between metal ions and planar organic ligands. Their unique electronic, optical, and catalytic properties have attracted considerable attention for applications in energy storage, electronic devices, and catalysis. Traditionally, their production has relied on two-phase interfacial reactions, limiting control over structure and composition. Recent research, however, has focused on refining the process of coordination nanosheets synthesis to overcome these limitations in both yield and structural control.

Researchers at the Tokyo University of Science have developed a single-phase reaction utilising nickel(II) ions and benzenehexathiol (BHT) to generate colloidal solutions of coordination nanosheets. By carefully adjusting the molar ratio of reactants, they achieved selective synthesis of specific nanosheet compositions. This approach allows for the creation of ink-like solutions suitable for coating substrates or electrodes, or for use in subsequent chemical transformations.

Beyond nickel-based nanosheets, this methodology was extended to include copper and zinc, yielding colloidal solutions of copper-BHT and zinc-BHT nanosheet assemblies. Furthermore, the researchers demonstrated the formation of heterometallic nanosheets – containing multiple metal ions – by introducing copper or zinc ions into the pores of pre-formed nickel-based nanosheet structures. This strategy enabled the creation of nickel-copper and nickel-zinc nanosheet structures.

Transmetallation reactions – where in-place ion exchange occurs at the coordination centre – were also employed to create heterometallic nanosheet structures. Specifically, transmetallation of nickel-BHT with copper ions yielded a high-performing, crystalline material with promising electrical conductivity, indicating potential for application in electronic devices. This demonstrates a route to precisely engineer heterometallic nanosheet composition and arrangement.

Novel Synthesis Routes

The versatility of this synthetic approach is further demonstrated by the successful production of both copper- and zinc-linked nanosheets in colloidal solution, expanding the range of potential material compositions. This ability to control the constituent metal ions is crucial for tailoring the nanosheets’ properties for specific applications.

Notably, the researchers leveraged the porous structure of initial nickel-based nanosheets to facilitate the incorporation of secondary metal ions. By introducing copper or zinc ions into pre-formed nickel dithiolene (NiDT) colloidal solutions, they generated NiCu2BHT and NiZn2BHT nanosheets respectively, showcasing a facile route to heterometallic architectures without requiring complex multi-step procedures.

Furthermore, the implementation of transmetallation reactions provides an alternative pathway to heterometallic nanosheet synthesis. The resulting NiCu2BHT nanosheets, produced via transmetallation of NiBHT with copper ions, exhibited high crystallinity and electrical conductivity, suggesting potential for use in diverse electronic applications and highlighting the effectiveness of this method for achieving precise compositional control during coordination nanosheets synthesis.

Electrochemical Analysis and Catalytic Activity

Electrochemical analysis of the nickel-based coordination nanosheets revealed key distinctions related to their structural characteristics. Specifically, nickel dithiolene (NiDT), possessing a porous architecture, exhibited a broad redox wave during testing, indicative of accessible redox-active sites. Conversely, non-porous nickel benzenehexathiolene (NiBHT) did not display a corresponding wave, suggesting limited electrochemical activity. This difference underscores the importance of structural control in influencing the electrochemical performance of these materials.

Further investigation demonstrated the catalytic potential of NiDT-coated glassy carbon (GC) electrodes in the hydrogen evolution reaction. The porous structure of NiDT likely facilitates access of reactants to active sites, enhancing catalytic efficiency. These findings highlight the potential of rationally designed coordination nanosheets as effective catalysts for energy-related applications.

The transmetallation reaction employed to produce NiCu2BHT nanosheets yielded materials with notably high crystallinity and electrical conductivity. This combination of properties is particularly desirable for electronic applications, suggesting that precise control over nanosheet composition through transmetallation can lead to materials with tailored functionalities. The ability to manipulate the electronic properties of coordination nanosheets through compositional control is a significant advancement in materials design.

Heterometallic Nanosheet Production

Beyond the demonstrated synthesis of individual metal-coordinated nanosheets, the research team successfully generated heterometallic structures through a two-pronged approach. Firstly, leveraging the inherent porosity of pre-formed nickel dithiolene (NiDT) colloidal solutions, they introduced copper(II) or zinc(II) ions, resulting in the formation of NiCu2BHT and NiZn2BHT nanosheets respectively. This strategy circumvents the need for complex, multi-step procedures, offering a facile route to incorporating multiple metal centres within a single nanosheet structure.

Secondly, the researchers explored transmetallation reactions – a process involving the exchange of metal ions at the coordination centre – to further refine heterometallic nanosheet synthesis. Transmetallation of nickel benzenehexathiolene (NiBHT) with copper(II) ions yielded NiCu2BHT nanosheets characterised by high crystallinity and electrical conductivity – properties crucial for potential applications in electronic devices. This precise compositional control, achieved through transmetallation, underscores the potential for tailoring nanosheet functionalities.

The resulting NiCu2BHT nanosheets, produced via transmetallation, exhibited a combination of high crystallinity and electrical conductivity, suggesting suitability for diverse electronic applications. This ability to manipulate the electronic properties of coordination nanosheets through compositional control represents a significant advancement in materials design, and highlights the effectiveness of this method for achieving precise compositional control during coordination nanosheets synthesis.

These findings collectively demonstrate the versatility of this synthetic approach, enabling the production of highly conductive coordination nanosheets in an ink-like form, suitable for application as coatings, chemical reactants, and potentially, in the development of next-generation flexible electronic devices and sensor materials.

Future Applications and Funding

Looking beyond immediate material properties, the research team anticipates scalability through printing technologies. This would facilitate direct application of the nanosheet inks onto device substrates, streamlining manufacturing processes and reducing production costs. The potential for roll-to-roll processing, a technique commonly employed in flexible electronics manufacturing, could further enhance the viability of these materials for large-scale applications.

Financial support for this work was provided by a consortium of grants from the Japan Society for the Promotion of Science (JSPS KAKENHI), specifically grant numbers JP19H05460, 22K14569, 22K05055, 24H00468, 25K08421, and 25K08598, alongside funding from the White Rock Foundation. This sustained investment underscores the strategic importance of coordination nanosheet research within Japan’s materials science landscape and highlights the collaborative nature of scientific advancement in this field.

Professor Nishihara’s extensive publication record – 475 scientific papers with over 18,470 citations and six patents – attests to his leadership in coordination chemistry and nanoscience. His expertise has been instrumental in driving this research forward, and his continued contributions are expected to further refine the synthesis and application of these novel materials. The combination of established expertise and ongoing financial support positions the Tokyo University of Science as a key centre for coordination nanosheet development.