Theoretical Design of Thorium Nitride MXenes

Actinides with 5f6d7s valence orbitals feature special physicochemical properties different from those of the other elements. Actinide-based two-dimensional (2D) materials combine the distinctive physics of actinides with the quantum size effect of 2D materials, but relevant studies are scarce. Since Th has a valence electron configuration of 6d²7s² like Ti, and Ti-based MXenes show excellent stability and versatile applications, whether Th can form stable MXenes has become an intriguing question. Herein, we designed Th₂N, Th₃N₂, and Th₄N₃ MXenes and investigated their physical properties, functionalization, and potential applications using density functional theory. Their stabilities are validated by global minimum search, phonon spectra, ab initio molecular dynamics, enthalpy of formation, and energy above the hull. All the Th-N MXenes exhibit metallic properties and are stabilized by the electrostatic interaction between Th and N ions, as well as the covalent bonding interaction between the Th 6d/5f and N 2p/2s orbitals. The H-, O-, and F-functionalization₃N₂ MXene improve its stability while preserving its metallicity, and the O-functionalized Th₃N₂ MXene shows promising catalytic activity for hydrogen evolution. The thorium nitride MXenes enrich the family of actinide-based 2D materials and extend our understanding of the structures and properties induced by actinide elements in low-dimensional materials.