Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene

TL;DR

This study combines experimental and ab initio methods to reveal how water monomers move on graphene, showing a kinetic barrier to ice nucleation influenced by cooperative repulsive interactions, which could inform control of ice formation.

Contribution

It provides the first detailed experimental and computational insight into water monomer dynamics on graphene, highlighting a kinetic barrier to ice nucleation caused by repulsive forces.

Findings

Water monomers hop via activated processes on graphene.

Repulsive interactions cause cooperative effects in water diffusion.

A kinetic barrier delays ice nucleation on graphene.

Abstract

The interfacial behaviour of water remains a central question to fields as diverse as protein folding, friction and ice formation[1,2]. While the structural and dynamical properties of water at interfaces differ strongly from those in the bulk, major gaps in our knowledge at the molecular level still prevent us from understanding these ubiquitous chemical processes. Information concerning the microscopic motion of water comes mostly from computational simulation[3,4] but the dynamics of molecules, on the atomic scale, is largely unexplored by experiment. Here we present experimental results combined with ab initio calculations to provide a detailed insight into the behaviour of water monomers on a graphene surface. We show that motion occurs by activated hopping on the graphene lattice. The dynamics of water diffusion displays remarkably strong signatures of cooperative behaviour due to repulsive forces between the monomers. The repulsive forces enhance the monomer lifetime ($t_m \approx 3$ s at $T_S = 125$ K) in a $\textit{free-gas}$ phase that precedes the nucleation of ice islands and, in turn, provides the opportunity for our experiments to be performed. Our results give a unique molecular perspective of barriers to ice nucleation on material surfaces, providing new routes to understand and potentially control the more general process of ice formation.