A team led by Prof. Huang Qing at the Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences, has found a new way to “edit” the inner layers of MAX phases, a family of layered compounds. Their study, published in Nature Synthesis, shows how this breakthrough could lead to new two-dimensional (2D) materials with uses in batteries, catalysts, and shielding against electromagnetic interference (EMI).
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MAX phases are compounds made of an early transition metal combined with carbon or nitrogen, arranged in layered structures. This makes them good starting points for 2D materials, which are only a few atoms thick. A well-known group of such materials, MXenes, is usually made by removing metallic A-site atoms from MAX phases. But when non-metallic atoms such as oxygen, sulfur, or phosphorus occupy the A sites, both M–A and M–X sublayers are strongly covalent, blocking the usual etching process.
The researchers discovered that these covalent sublayers behave differently when exposed to high-temperature molten states. By using this difference in reactivity, they were able to structurally modify the sublayers, replacing X-site atoms with non-metals like boron, selenium, sulfur, phosphorus, and carbon. They also showed that Lewis acidic cations can lower the oxidation state of M elements, making it possible to attach more non-metals and turn non-van der Waals MAX phases into van der Waals layered materials. Prof. Huang explained, “By controlling the total formation enthalpy of a reaction, we can achieve targeted replacement of sublayer atoms. This opens a pathway to design layered materials with tailored properties.”
Enthalpy is a scientific term used in chemistry and physics to describe the total heat content of a system. In this research, controlling enthalpy allowed the team to carefully replace certain atoms in the layered structure of MAX phases, ensuring the reaction happened as intended.
Using this approach, the team created a new family of compounds called early transition metal chalcogenide carbides/nitrides (TMXCs). These materials combine features of MXenes and transition metal dichalcogenides (TMDs). The atomic arrangements in monolayer TMXCs are similar to MXenes, but the oxidation states of the M-site atoms can be adjusted by both the substituted X-site atoms and intercalated cations. The researchers also showed that TMXCs can be exfoliated into monolayer nanosheets using electron-donor “chemical scissors,” producing single-atom-thick sheets with unique properties.
Both experiments and calculations confirmed that changing the X-element in the M–X sublayer alters the electronic structure of TMXCs. This ability to control structure and properties could make them useful in electrochemical energy storage, batteries, catalysis, and shielding against EMI.
Source: Chinese Academy of Sciences, Nature
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