Self-assembled 2D materials on liquid-liquid interface

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 are just beginning to scratch the surface of what heterolayer coordination nanosheets — coordination nanosheets whose layers have different atomic compositions — can offer.

In a recent study first published on June 13, 2022, and featured on the cover of Chemistry — A 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 show that metal ions can pass through the first layer of the aqueous phase to the 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,” said 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.

Reference:

1. Joe Komeda, Kenji Takada, Hiroaki Maeda, Naoya Fukui, Takuya Tsuji, Hiroshi Nishihara. Chemically Laminated 2D Bis(terpyridine) Metal Polymer Films: Formation Mechanisms at Liquid-Liquid Interfaces and Redox Rectification. Chemistry – European Journal, 2022; DOI: 10.1002/chem.202201316

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