Main-group-element-based surface functionalization of MXenes toward advanced tribological applications

MXenes are one of the most versatile families of two-dimensional transition-metal carbides and nitrides due to their metallic conductivity, tunable surface chemistry and excellent mechanical properties. Nevertheless, the practical application of MXenes in lubrication, coatings, energy storage and sensing technologies are still limited by surface oxidation, restacking and poor environmental stability. Thus, surface functionalization has become a key strategy for tailoring the interfacial properties of MXenes. Among the various approaches investigated, main-group chemistry provides unique opportunities for rational surface modification via the formation of strong covalent and coordination interactions involving silicon, phosphorus, sulfur, nitrogen and halogen-containing functionalities. These modifications regulate surface energy, interlayer spacing, and oxidation resistance while simultaneously enhancing tribological performance through tribofilm formation and interlayer shear modulation. This review critically summarizes recent advances in main-group chemistry-driven functionalization and doping of MXenes, highlighting structure–property relationships, mechanistic insights, and future opportunities for the development of durable and high-performance MXene-based tribological systems. Besides cataloguing individual functionalization chemistries, this review critically compares silane-, phosphonic acid-, fluorine-, polymer-, intercalation-, and heteroatom doping-based strategies with one another in terms of achievable friction reduction, environmental (humidity/oxidation) tolerance, and mechanistic maturity. Finally, this review explicitly benchmarks main-group-functionalized MXenes against other established two-dimensional (2D) solid lubricants, including graphene/graphene oxide, MoS2/WS2, and hexagonal boron nitride. Based on recent primary literature and review sources in the research areas of MXene synthesis, surface chemistry, and tribology, we also identify specific, outstanding bottlenecks such as termination heterogeneity, limited long-term stability data, and lack of atomic-scale evidence for tribofilm formation, and provide near-, mid-, and long-term research priorities for the translation of main-group-functionalized MXenes into commercially viable lubricant technologies.