Structure and dynamics of water at water-graphene and water-hexagonal boron-nitride sheet interfaces revealed by ab initio sum-frequency generation spectroscopy.

We simulate sum-frequency generation (SFG) spectra of isotopically diluted water at the water-graphene and water-hexagonal boron-nitride (hBN) sheet interfaces, using ab initio molecular dynamics simulations. A sharp 'dangling' O-D peak around ∼2640 cm-1 appearing in both simulated SFG spectra evidences that both graphene and hBN are hydrophobic. The dangling O-D peak is 10 cm-1 red-shifted at the water-hBN interface relative to the peak at the water-graphene interface. This frequency difference gives a stronger O-DN intermolecular interaction between water and hBN than the O-DC interaction between water and graphene. Accordingly, the anisotropy decay of such a dangling O-D group slows down near hBN compared with near graphene, illustrating that the dynamics of the dangling O-D group are also affected by the stronger O-DN interaction than the O-DC interaction. We discuss molecular-level insights into the structure and dynamics of interfacial water in the context of the friction of hBN and graphene.