Abstract
Understanding water behaviour on 2D materials is crucial for applications in sensing, microfluidics, and tribology. While graphene-water interactions are well studied, water on hexagonal boron nitride (h-BN) remains largely unexplored. Despite its structural similarity to graphene, h-BN possesses polar B-N bonds that give rise to distinct electronic and chemical properties. Most previous studies have also focused on multilayer water, leaving single-molecule dynamics poorly understood. Here we show how individual water molecules diffuse on h-BN compared to graphene using helium spin-echo spectroscopy and ab initio calculations. On h-BN/Ni, water exhibits coupled rotational-translational motion, in contrast to the discrete hopping observed on graphene. Water molecules rotate freely around their centre of mass, and although binding energies are similar on both materials, the activation energy for water dynamics on h-BN is 2.5 times lower than on graphene. These dynamics, which classical models fail to capture, highlight the fundamentally different nature of water transport on polar 2D surfaces. We further demonstrate that the supporting substrate strongly influences water friction, with h-BN/Ni showing markedly lower friction than graphene/Ni, opposite to the behaviour of free-standing layers. These findings challenge assumptions and offer insights for designing microfluidic devices requiring precise control of water mobility.