Influence of X Elements on the Tribological Properties and Surface Chemistry of MXene Atomic Layers.

MXenes, a class of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides, exhibit promising tribological properties at the nanoscale. However, the influence of X elements on the surface chemistry of MXene atomic layers remains underexplored. Here, we investigate how nitrogen in the TiCN atomic layer modifies its nanotribological behavior compared to TiC. Using friction force microscopy and peak force quantitative nanomechanics , we find that TiCN exhibits a notable increase in friction along with higher adhesion and energy dissipation, which we attribute to enhanced hydroxyl termination, stronger surface dipole interactions, and hydrogen bonding. X-ray photoelectron spectroscopy further reveals that nitrogen incorporation leads to greater electron withdrawal from titanium atoms, resulting in a higher oxidation state and altered surface chemical functionality. These results provide mechanistic insight into how X-element chemistry influences the tribological performance of MXenes, highlighting the importance of surface composition in designing 2D materials for specific applications.