A team of scientists from the Tokyo University of Science (TUS) and The University of Tokyo in Japan has found a surprising method for synthesizing heterolayer coordination nanosheets.
Nanosheets are thin, two-dimensional materials that are made up of a single layer of atoms or molecules. They are usually very thin and flexible, which makes them attractive for use in electronics. Nanosheets can be made from a variety of materials, including graphene and transition metal dichalcogenides (TMDCs).
There are various properties that can be fine-tuned in 2D materials, suggesting promising applications in many fields. These include applications in optoelectronics, catalysis, and renewable energy.
Heterolayer Coordination Nanosheets
The formation of the second layer in a heterolayer coordination nanosheet. Credit: Hiroshi Nishihara from Tokyo University of Science
Coordination nanosheets are an especially intriguing class 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. This has attracted the attention of materials scientists due to its favorable properties.
In fact, science has only begun to scratch the surface regarding what heterolayer coordination nanosheets — coordination nanosheets whose layers have different atomic compositions — can offer.
The recently published study features nanosheets made of the organic ligand, terpyridine, coordinating iron, and cobalt. The nanosheets assemble themselves at the interface between two immiscible liquids in a unique way.
Liquid To Liquid
To synthesize the heterolayer coordination nanosheets, the team first constructed the liquid to liquid interface to facilitate assembly. They dissolved tris(terpyridine) ligand in dichloromethane (CH2Cl2), an organic liquid that does not mix with water. This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent.
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”), formed at the interface between both liquids.
They then removed the iron-containing water and replaced it with cobalt-containing water. In the next few days, a bis(terpyridine)cobalt (or “Co-tpy”) layer formed right below the iron-containing one at the liquid–liquid interface.
Ordering The Layers
Detailed observations were conducted of the heterolayer using various advanced techniques, such as scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning transmission electron microscopy.
The team observed the Co-tpy layer formed neatly below the Fe-tpy layer at the liquid–liquid interface. Additionally they could control the thickness of the second layer depending on how long they left the synthesis process run its course.
They also found that the ordering 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 CH2Cl2phase to react with terpyridine ligands right at the boundary between the nanosheet and the CH2Cl2 phase. This is the first ever clarification of the growth direction of coordination nanosheets at a liquid/liquid interface,”
explained Prof. Hiroshi Nishihara from TUS, who led the study.
Diode-like Behaviour
In investigating the electrical rectification characteristics of their coordination nanosheets, the scientists found that the heterolayers behaved much like a diode in a manner consistent with the electronic energy levels of Co-tpy and Fe-tpy.
“Our synthetic method could be applicable to other coordination polymers synthesized at liquid–liquid interfaces. Therefore, the results of this study will expand the structural and functional diversity of molecular 2D materials,”
Prof. Nishihara pointed out. The team plans to continue studying chemical phenomena occurring at liquid–liquid interfaces, clarifying the mechanisms of mass transport and chemical reactions.
Reference: J. Komeda, K. Takada, H. Maeda, N. Fukui, T. Tsuji, H. Nishihara, Chemically Laminated 2D Bis(terpyridine)metal Polymer Films: Formation Mechanism at the Liquid–Liquid Interface and Redox Rectification Chem. Eur. J. 2022, e202201316
