Layer-dependent frictional properties of Ti3C2Tx MXene nanosheets

MXenes are an emerging class of two-dimensional lamellar materials with exceptional architectural variety, tunable chemical compositions, and chemical inertness. Owing to their unique structure, with a highly tunable interlayer distance and binding properties, MXenes can potentially exhibit excellent nanoscale solid lubrication properties. Herein, we report the layer dependence of the frictional characteristics of atomically thin Ti3C2Tx MXene nanosheets. The frictional properties of the isolated Ti3C2Tx nanosheets deposited on a clean Si surface were investigated using friction force microscopy. The friction decreased with an increase in the number of layers, starting from monolayer Ti3C2Tx. The layer dependence of friction was attributed to the reduced elastic compliance with an increase in the number of Ti3C2Tx layers. A partially suspended Ti3C2Tx monolayer exhibited a higher degree of friction than the substrate-supported Ti3C2Tx monolayer, indicating the role of elastic compliance in the friction mechanism of Ti3C2Tx nanosheets. The reduced elastic compliance of Ti3C2Tx nanosheets decreases the localized out-of-plane puckering-type deformation surrounding the sliding indenter, thereby reducing the resistance to frictional sliding. The results indicate that tuning their out-of-plane mechanical properties by varying the chemical architecture, chemical composition, surface terminations, and interlayer intercalations can render MXenes as potential candidates for versatile nanoscale solid lubrication applications.