Water Nucleation via Transient Bonds to Oxygen Functionalized Graphite

TL;DR

This study investigates how oxygen functionalization on graphite influences initial ice nucleation, growth, and mobility of water molecules, revealing defect sites as key to understanding anti-icing and interstellar ice formation.

Contribution

It combines LT-STM imaging with machine learning and first-principles calculations to elucidate water nucleation mechanisms on oxygen-functionalized graphite surfaces.

Findings

Oxygen atoms serve as nucleation sites for ice growth.

Water molecule mobility is reduced by surface defects.

Larger ice clusters grow via hydrogen bonding to multiple oxygen sites.

Abstract

We present a study the initial stages of ice growth on pristine and oxygen-functionalized highly oriented pyrolytic graphite (O-HOPG), combining low-temperature scanning tunneling microscopy (LT-STM) and machine-learning structural searches. LT-STM images show that oxygen atoms act as nucleation sites for ice growth, and that the size, structure and porosity of the nanometer-sized ice clusters depend strongly on the growth temperature. Machine learning-assisted structural searches and first-principles energy calculations confirm that clusters of water molecules are likely to bind to chemisorbed oxygen atoms through hydrogen bonding. During the early stages of the cluster growth clusters of water molecules are likely to be immobilized by binding to more than one chemisorbed oxygen atom through hydrogen bonding. However, the energy gain by hydrogen bond formation of a molecule, upon incorporation into smaller clusters only bound to a single oxygen atom, is large enough to induce cluster diffusion and favor the growth of larger ice clusters. Our results demonstrate that the mobility of water molecules is significantly lowered in the presence of defects on the surface. The observed lower mobility on defected carbon presented here offers an enhanced understanding of macroscopic anti-icing properties observed for functionalized HOPG under ambient conditions and provides insight into the early stages of ice growth on dust grain surfaces in interstellar space.