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

Abstract

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–D⋯N intermolecular interaction between water and hBN than the O–D⋯C 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–D⋯N interaction than the O–D⋯C interaction. We discuss molecular-level insights into the structure and dynamics of interfacial water in the context of the friction of hBN and graphene.