Terminal Groups Enable Tunable, Versatile MXene Materials

MXenes—atomically thin transition metal carbides and nitrides—are drawing fresh attention for one simple reason: their surfaces are never truly bare. During production, the “A” layers of MAX-phase precursors are selectively etched away, leaving behind stacks of Mₙ₊₁Xₙ sheets whose exposed transition-metal sites rapidly react with the surrounding environment. To reduce surface energy, those reactive atoms spontaneously bind with heteroatoms such as oxygen, fluorine, hydroxyl, chlorine, and even chalcogen species, forming terminal groups denoted as Tₓ. These chemical “end-caps” are not decorative; they actively tune the material’s behavior.

A major challenge has been that Tₓ is both crucial and uncertain. “Tailoring the surface terminal groups to make specific MXenes suitable for targeted applications is a critical challenge,” notes Prof. Peng-an Zong of Nanjing Tech University. Prior studies illustrate the payoff: for Nb₂CTₓ MXene, different terminal chemistries can shift the material toward distinct functionalities—for example, Tₓ = −S or −Se has been linked to thermoelectric potential, while Tₓ = −Cl is associated with superconducting behavior. When the terminals are largely −OH, Nb₂CTₓ can even serve as an anode plate in aqueous energy storage, aided by a low work function.

Now, a new review in Nano Research reframes this surface chemistry problem as the central “design knob” for MXenes. The article, titled “Terminal Groups: The Key to Tunable and Versatile MXenes Materials,” compiles structural features and modification strategies for Tₓ, then connects terminal-group chemistry to changes in electrical, optical, magnetic, and mechanical properties. It also surveys how controlling Tₓ can enable applications across sensing, filtration membranes, and catalysis, among others.

The authors emphasize that terminal groups do more than add surface species—they reshape electronic landscapes. While it is generally accepted that terminations alter band structures, the field still lacks universal governing principles. Zong cautions that analyses relying mainly on calculated band structures and density of states (DOS) can become “superficial and intuitive,” because band modulation by Tₓ can be highly complex and counterintuitive.

Beyond prediction, the review highlights an unresolved theoretical bottleneck: establishing robust criteria to identify MXenes that are both high-energy and yet stable while carrying multifunctional surfaces. “Compared to existing research, there are still many unresolved mysteries,” the team comments, pointing to the need for better frameworks that go beyond stability checks and accurately connect chemistry to performance.

The work argues that MXenes’ most distinctive advantage over other 2D families is precisely the tunability of Tₓ. If terminal groups can be engineered with “at will” precision, MXenes could move from laboratory curiosity toward the elusive “universal material” concept—one platform that can be tailored for specific device environments.

The review was published on March 17, 2026 (as reported in the article release). It was supported by Jiangsu PAPD and the Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites.