We reveal the ultralow friction or superlubricity of water nanodroplets containing cations and anions on graphene substrates at high ion concentration by molecular dynamics simulations. When the ion concentration is higher than 7 wt.% and the nanodroplet diameter is larger than 10 nm, the friction coefficients of water nanodroplets are lower than 10⁻², and can decrease to the order of 10⁻³ with increasing the ion concentration further. At a certain ion concentration, the optimal nanodroplet diameter of 17–20 nm exists at which the friction coefficient is the lowest. The ultralow friction behaviors of water nanodroplets containing cations and anions are mainly attributed to the opposite variation trends between the interfacial adhesion energy and surface energy of water nanodroplet with ion concentration, and the interfacial hydrophobicity sustained by high ion concentration. These results unveil the essential role of ions in achieving the superlubricity of water nanodroplets.

Lubrication induced by a vertical electric field or bias voltage is typically not applicable to two-dimensional (2D) van der Waals (vdW) crystals. By performing extensive first-principles calculations, we reveal that the interlayer friction and shear resistance of Janus transition metal dichalcogenide (TMD) MoXY (X/Y = S, Se, or Te, and X ≠ Y) bilayers under a constant normal force mode can be reduced by applying vertical electric fields. The maximum interlayer sliding energy barriers between AA and AB stacking of bilayers MoSTe, MoSeTe, and MoSSe decrease as the positive electric field increases because of the more significant counteracting effect from the electric field energy and the more significant enhancement in interlayer charge transfer in AA stacking. Meanwhile, the presence of negative electric fields decreases the interlayer friction of bilayer MoSTe, because the electronegativity difference between Te and S atoms reduces the interfacial atom charge differences between AA and AB stacking. These results reveal an electro-lubrication mechanism for the heterogeneous interfaces of 2D Janus TMDs.

Our extensive first-principles calculations reveal that the chemical activities of monolayer transition metal dichalcogenides (TMDs) MX₂ (M = Mo or W, and X = Te, Se, or S) for water splitting and hydrogen evolution are modified and promoted on their grain boundaries (GBs) when in-plane tensile loadings are applied. Compared with monolayer TMDs without GBs, the flexoelectricity induced by nonuniform deformation and strain gradient significantly enhances the charge polarizations of X and M atoms at the GB sites of monolayer TMDs, which facilitates the dissociation of water molecules on the GB sites and reduces the reaction barrier of hydrogen evolution reaction. The energy barriers of splitting water molecules and hydrogen adsorption free energies on the GB sites decrease with increasing the flexoelectric effect. These results highlight an attractive way of utilizing the flexoelectric effect of GB-containing TMDs to improve their surface catalytic capability for hydrogen generation.