This image depicts the formation of the second layer in a heterolayer coordination nanosheet. Cobalt ions pass through the pre-existing first layer (composed of iron coordination centers and terpyridine ligands) that has formed at the liquid–liquid interface between water and dichloromethane. They then assemble into coordinated layers by combining with terpyridine ligands. Image: Hiroshi Nishihara from Tokyo University of Science.
This image depicts the formation of the second layer in a heterolayer coordination nanosheet. Cobalt ions pass through the pre-existing first layer (composed of iron coordination centers and terpyridine ligands) that has formed at the liquid–liquid interface between water and dichloromethane. They then assemble into coordinated layers by combining with terpyridine ligands. Image: Hiroshi Nishihara from Tokyo University of Science.

The past few decades have witnessed a great amount of research in the field of two-dimensional (2D) materials. As the name implies, these thin film-like materials are composed of layers that are only a few atoms thick. Many of the chemical and physical properties of 2D materials can be fine-tuned, leading to promising applications in many fields, including optoelectronics, catalysis, renewable energy and more.

Coordination nanosheets are one particularly interesting type of 2D material. The term ‘coordination’ refers to the effect of metallic ions in these molecules, which act as coordination centers. These centers can spontaneously create organized molecular dispositions that span multiple layers in 2D materials, which have attracted the attention of materials scientists due to their favorable properties. In fact, researchers have only begun to scratch the surface regarding what heterolayer coordination nanosheets – which have layers with different atomic compositions – can offer.

In a paper in Chemistry—A European Journal, a team of scientists from Tokyo University of Science (TUS) and the University of Tokyo, both in Japan, report a remarkably simple way to synthesize heterolayer coordination nanosheets. Composed of the organic ligand terpyridine, with iron and cobalt as the coordination centers, these nanosheets assemble themselves at the interface between two immiscible liquids in a peculiar way.

To synthesize the heterolayer coordination nanosheets, the team, led by Hiroshi Nishihara at TUS, first created the liquid–liquid interface. They did this by dissolving a 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, an iron-containing chemical, on top of the CH2Cl2. After 24 hours, the first layer of the coordination nanosheet bis(terpyridine)iron (or ‘Fe-tpy’) began to form at the interface between both liquids.

Following this, they removed the iron-containing water and replaced it with cobalt-containing water. In the next few days, a bis(terpyridine)cobalt (or ‘Co-tp’) layer formed right below the iron-containing one at the liquid–liquid interface.

The team made detailed observations of the heterolayer with various advanced techniques, including scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy and scanning transmission electron microscopy. They found that the Co-tpy layer formed neatly below the Fe-tpy layer at the liquid–liquid interface. Moreover, they were able to control the thickness of the second layer by simply varying how long they let the synthesis process run its course.

Interestingly, the team also found that the order of the layers could be swapped by simply 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 heterolayer would have cobalt coordination centers on the top layer and iron coordination centers on the bottom layer.

“Our findings indicate that metal ions can go through the first layer from the aqueous phase to the CH2Cl2 phase to react with terpyridine ligands right at the boundary between the nanosheet and the CH2Cl2 phase,” explains Nishihara. “This is the first ever clarification of the growth direction of coordination nanosheets at a liquid/liquid interface.”

Additionally, the team investigated the reduction–oxidation properties of their coordination nanosheets, as well as their electrical rectification characteristics. They found that the heterolayers behaved much like a diode in a way that is consistent with the electronic energy levels of Co-tpy and Fe-tpy. These insights, coupled with the easy synthesis procedure developed by the team, could help in the design of heterolayer nanosheets made of other materials and tailored for specific electronics applications.

“Our synthetic method could be applicable to other coordination polymers synthesized at liquid–liquid interfaces,” says Nishihara. “Therefore, the results of this study will expand the structural and functional diversity of molecular 2D materials.”

With eyes set on the future, the team will keep investigating chemical phenomena occurring at liquid–liquid interfaces, elucidating the mechanisms of mass transport and chemical reactions. Their findings can help expand the design of 2D materials and, hopefully, lead to better optoelectronic devices such as solar cells.

This story is adapted from material from Tokyo University of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.