Maintaining the 2D Structure of MXene via Self-Assembled Monolayers for Efficient Lubrication in High Humidity.

MXene is considered as a promising solid lubricant due to facile shearing ability and tuneable surface chemistry. However, it faces challenges in high-humidity environments where excessive water molecules can significantly impact its 2D structure, thus deteriorating its lubricating properties. In this work, the self-assembled monolayers are formed on MXene by surface chlorination (MXene-Cl) and fluorination (MXene-F), and their friction behaviors in high/low humidity are investigated. The results indicate that MXene-F and MXene-Cl can maintain a relatively constant friction coefficient (CoF) (MXene-F ∼0.76, MXene-Cl ∼0.48) under both high (75%) and low (25%)-relative humidity (RH) environments. Meanwhile, the MXene-F and MXene-Cl display a lower CoF than the pristine MXene (MXene CoF∼1.18) in high humidity. The above phenomena are mainly attributed to the preservation of its 2D layered structure, the increased layer spacing, and superficial partial oxidation for SAMs-functionalized MXene under high humidity during friction. Interestingly, MXene-Cl with moderate water resistance has a lower CoF than that of MXene-F with complete water resistance. The nanostructured water adsorption capacity and larger interlayer spacing of MXene-Cl make it exhibit a lower CoF compared to MXene-F. The findings of this study offer valuable guidance for tailoring MXene by surface chemical functionalization as an efficient solid lubricant in high humidity.