Understanding water behaviour on 2D material interfaces through single-molecule motion on h-BN and graphene

Area of Science:

• Surface science
• Materials science
• Physical chemistry

Background:

• Water behavior on 2D materials is vital for sensing, microfluidics, and tribology.
• Graphene-water interactions are well-studied, but water on hexagonal boron nitride (h-BN) is less understood.
• Previous research often overlooked single-molecule dynamics, focusing on multilayer water.

Purpose of the Study:

• To investigate and compare the diffusion dynamics of individual water molecules on h-BN versus graphene.
• To understand the influence of polar B-N bonds in h-BN on water molecule behavior.
• To explore the impact of supporting substrates on water friction on these 2D materials.

Main Methods:

• Helium spin-echo spectroscopy was employed to observe molecular dynamics.
• Ab initio calculations were used to analyze binding energies and activation energies.
• Comparative analysis was performed between water on h-BN/Ni and graphene/Ni.

Main Results:

• Water on h-BN/Ni exhibits coupled rotational-translational motion, unlike the hopping on graphene.
• Water molecules on h-BN demonstrate free rotation around their center of mass.
• The activation energy for water dynamics on h-BN is 2.5 times lower than on graphene.
• Water friction on h-BN/Ni is significantly lower than on graphene/Ni, contrary to free-standing layers.

Conclusions:

• Water transport on polar 2D surfaces like h-BN is fundamentally different from non-polar surfaces like graphene.
• Classical models are insufficient to explain the observed water dynamics on h-BN.
• Substrate interactions play a critical role in determining water friction on 2D materials.
• Findings offer insights for designing microfluidic devices with controlled water mobility.