Organic and Inorganic Surfact Chemistry of MXenes

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) show impressive performance in supercapacitors, batteries, electromagnetic interference shielding, and electrocatalysts. These materials combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, and surface-engineered MXenes represent an ideal platform for fundamental and applied studies of interfaces in 2D functional materials.<br/>The comprehensive understanding of MXene surfaces is required for prescriptive engineering of their physical and chemical properties. We discuss general strategies to install and remove surface groups by performing topotactic substitution and reductive elimination reactions. Successful synthesis of MXenes with halido-, oxo-, imido-, thio-, seleno-, or telluro- terminations, as well as bare MXenes (no surface termination) can be synthesized both by traditionally (from MAX phases) and directly synthesized MXenes. We also successfully synthesized a series of hybrid organic-inorganic MXenes by covalently attaching dense carpets of organic surface groups. Since organic and inorganic materials are, in many aspects, complimentary to each other, organic-inorganic MXenes open a pathway to merge the benefits of both worlds into a hybrid matter that combines engineerability of molecules with the electronic, thermal, and mechanical properties of inorganic 2D materials. 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, the strength of electron-phonon coupling, making MXene surfaces not spectators but active contributors to conductivity, superconductivity, and catalytic activity.