MXenes as Possible Solid Lubricants—Effects of Termination, Vacancies and Intercalated Water on the Resistance to Sliding and Oxidation of Titanium-Based MXenes

Understanding the interlayer interaction in two-dimensional (2D) transition metal carbides and nitrides (MXenes) is important to improve their exfoliation/delamination process and application in (nano)-tribology. The layer-substrate interaction is also essential in (nano)-tribology as effective solid lubricants should be resistant against peeling-off during rubbing. In our study, Ti-based MXenes with both homogeneous and mixed terminations are modeled using density functional theory (DFT). An <i>ad-hoc</i> modified dispersion correction scheme is used, capable of reproducing the results obtained from a higher level of theory. The nature of the interlayer interactions, comprising van der Waals, dipole-dipole, and hydrogen bonding, is discussed along with the effects of MXene sheet’s thickness and C/N ratio. Our results demonstrate that terminations play a major role in regulating MXenes’ interlayer and substrate adhesion to iron and iron oxide and, therefore, lubrication, which is also affected by an external load. Using graphene and MoS₂ as established references, we verify that MXenes’ tribological performance as solid lubricants can be significantly improved by avoiding –OH and –F terminations, which can be done by controlling terminations via post-synthesis processing. Our study on the interaction of Ti-based MXenes with water molecules confirms their extremely high hydrophilicity. The presence of -OH groups and vacancies on the surface inevitably strengthens their interaction with water. The dissociative chemisorption of water is found to be a favorable process when it occurs on both MXene’ edges and surface’ vacancies cluster, leading to material degradation. Our results try to explain the mechanisms of the spontaneous oxidative degradation of MXenes under aqueous environment.<br/>These results are part of the ”Advancing Solid Interface and Lubricants by First Principles Material Design (SLIDE)” project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant agreement No. 865633).