Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine large surface-to-volume ratio, mechanical strength, and metallic conductivity with chemically modifiable surfaces. Surface-engineered MXenes represent a powerful platform for fundamental studies of interfaces in low-dimensional functional materials, ultimately leading toward practical applications in energy storage, catalysis, composites, and optoelectronic devices.

Comprehensive understanding of MXene surfaces is required for prescriptive engineering of their physical and chemical properties. We discuss strategies to install and remove surface groups by performing topotactic substitution and reductive elimination reactions. Inorganic MXenes with halido-, oxo-, imido-, thio-, seleno-, or telluro- terminations, as well as bare MXenes (no surface termination) can be synthesized either from MAX phases, or by direct synthesis methods. Hybrid organic-inorganic MXenes can be accessed by covalently attaching dense carpets of different organic surface groups. These hybrid MXenes combine complexity and engineerability of molecules with electronic, thermal, and mechanical properties of inorganic 2D materials. The “organometallic MXenes” with organic surface groups bonded to inorganic 2D sheets through metal-carbon bonds can catalyze novel organic reactions, thus establishing surface-engineered MXenes as an emerging catalytic platform.

The description of MXene surface structure and reactivity requires a mix of concepts from the fields of coordination chemistry, self-assembled monolayers, and surface science. MXene surface groups control biaxial lattice strain, phonon frequencies, electrochemical performance, and strength of electron-phonon coupling, making MXene surfaces not spectators but active contributors to conductivity, superconductivity, and catalytic activity.

Dmitri Talapin

The University of Chicago

Appearances