Slippery and Sticky Graphene in Water.

Understanding modulation of water molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials. Here, we examine normal and shear forces on supported single-layer graphene using atomic force microscopy and find that the electrolyte composition affects the molecular slippage of nanometer thick films of aqueous electrolytes along the graphene surface. In light of the shear-assisted thermally activated theory, water molecules along the graphene plane are very mobile when subjected to shear. However, upon addition of an electrolyte, the cations can make water stick to graphene, while ion-specific and concentration effects are present. Recognizing the tribological and tribochemical utility of graphene, we also evaluate the impact of this behavior on its frictional response in the presence of water. It appears that the addition of an electrolyte to pure water causes a reduction of the thermal activation energy and of the shear-activation length at several concentrations, both results conversely affecting the friction force. Further, this work can inspire innovation in research areas where changes of the molecular slippage through the modulation of the doping characteristics of graphene in liquid environment can be of use, including molecular sensing, lubrication, and energy storage.