On the water's edge: DIY 2D materials on a liquid-liquid interface

figure: This image depicts the formation of the second layer in a heterolayer coordination nanosheet. The cobalt ions pass through a pre-existing first layer (consisting of the iron coordination center and the pyridine ligand), which has formed at the liquid-liquid interface between water and dichloromethane. They then assemble into a coordinated layer by combining with the pyridine ligand.
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Credit: Hiroshi Nishihara of Tokyo University of Science

The last few decades have witnessed a large amount of research in the field of two-dimensional (2D) materials. True to its name, this thin-film-like material consists of layers only a few atoms thick. Many chemical and physical properties of 2D materials can be adjusted, leading to promising applications in many fields, including optoelectronics, catalysis, renewable energy, and more.

Coordination nanosheets are one of the most interesting types of 2D materials. “Coordination” refers to the effect of metal ions in these molecules, which act as coordination centers. These centers can spontaneously create organized molecular dispositions that span multiple layers in 2D materials. It has attracted the attention of materials scientists because of its beneficial properties. In fact, we’re just starting to scratch the surface of what hetero layer coordination nanosheets – coordination nanosheets whose layers have different atomic compositions – can be offered.

In a recent study first published on June 13, 2022, and featured on the front cover Chemistry—European Journal, a team of scientists from the Tokyo University of Science (TUS) and the University of Tokyo in Japan reported a very simple way to synthesize heterolayer coordination nanosheets. Composed of organic ligands, pyridine, coordination iron and cobalt, these nanosheets self-assemble at the interface between two immiscible liquids in a peculiar way. The study led by Prof. Hiroshi Nishihara of TUS also includes contributions from Mr. Joe Komeda, Dr. Kenji Takada, Dr. Hiroaki Maeda, and Dr. Naoya Fukui from TUS.

To synthesize the heterolayer coordination nanosheets, the team first created a liquid-liquid interface to enable their assembly. They dissolve the tris(terpyridine) ligand in dichloromethane (CH₂Cl₂), an organic liquid that does not mix with water. They then poured a solution of water and ferrous tetrafluoroborate, a chemical that contains iron, over the CH₂Cl₂. After 24 h, the first layer of the coordination nanosheet, bis(terpyridine)iron (or “Fe-tpy”), formed at the interface between the two liquids.

After this, they removed the iron-containing water and replaced it with cobalt-containing water. Over the next few days, a bis(terpyridine)cobalt (or “Co-tpy”) layer forms just below the iron-containing layer at the liquid-liquid interface.

The team made detailed observations of the heterolayer using a variety of advanced techniques, such as scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning transmission electron microscopy. They found that the Co-tpy layer formed neatly beneath the Fe-tpy layer at the liquid-liquid interface. In addition, they can control the thickness of the second layer depending on how long they let the synthesis process run its course.

Interestingly, the team also found that the order of the layers could be swapped simply by changing the order of the synthesis steps. In other words, if they first added a cobalt-containing solution and then replaced it with an iron-containing solution, the synthesized heterolayers would have a cobalt coordination center in the top layer and an iron coordination center in the bottom layer. “Our findings suggest that metal ions can pass through the first layer of the aqueous phase to CH₂Cl₂ phase to react with the pyridine ligand right at the boundary between the nanosheet and the CH₂Cl₂ phase,“explained Prof. Nishihara. “This is the first clarification of the direction of growth of coordination nanosheets at the liquid/liquid interface.”

In addition, the team investigated the oxidation-reduction properties of their coordination nanosheets as well as their electrical rectification characteristics. They found that the heterolayer behaved like a diode in a manner consistent with the Co-tpy and Fe-tpy electronic energy levels. These insights, coupled with the easy synthesis procedures developed by the team, can assist in the design of heterolayer nanosheets made from other materials and adapted for specific electronic applications. “Our synthetic method can be applied to other coordination polymers synthesized at the liquid-liquid interface,” highlight Prof. Nishihara. “Therefore, the results of this study will expand the structural and functional diversity of molecular 2D materials.”

With eyes fixed on the future, the team will continue to investigate chemical phenomena occurring at the liquid-liquid interface, explaining the mechanisms of mass transport and chemical reactions. Their findings could help expand 2D materials design and, hopefully, lead to better performance of optoelectronic devices, such as solar cells.

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Reference

DOI: https://doi.org/10.1002/chem.202201316

About Tokyo University of Science

Tokyo University of Science (TUS) is a well-known and respected university, and the largest private research university specialized in science in Japan, with four campuses in central Tokyo and its suburbs and in Hokkaido. Founded in 1881, the university continues to contribute to the development of Japanese science by instilling a love of science in researchers, technicians, and educators.

With the mission of “Creating science and technology for harmonious development between nature, humans, and society”, TUS has conducted various researches from basic science to applied science. some of the most vital areas today. TUS is a meritocracy where the best in science is recognized and nurtured. It is the only private university in Japan to have produced Nobel Prize winners and the only private university in Asia to have produced Nobel Prize winners in the natural sciences.

Website: https://www.tus.ac.jp/en/mediarelations/

About Professor Hiroshi Nishihara of Tokyo University of Science

Hiroshi Nishihara is Professor of Chemistry at the Tokyo University of Science in Japan. He is a distinguished professor, researcher and pioneer in the field of electrochemistry and the synthesis of conductive metal complex polymers. His research is focused on the manufacture of new electro-functional and photofunctional materials consisting of transition metals and conjugated chains, as well as the discovery of unidirectional electron transfer systems using molecular layer interfaces. He has published over 467 papers with over 14911 citations to his credit.

Funding information

This study was financially supported by JST-CREST JPMJCR15F2, JSPS KAKENHI (grant no. JP19H05460), and the White Rock Foundation.

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Journal

Chemistry – European Journal

Research methods

Experimental study

Research Subject

Cannot be applied

Article title

At the Water’s Edge: Self-assembly of 2D Materials on a Liquid-Liquid Interface

Article Publication Date

13-Jun-2022

COI statement

The authors declare no conflict of interest.

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