Crystallization kinetics of water on graphite

Abstract

Graphite is hydrophobic in nature, but the crystallization kinetics and dewetting transition of thin water films deposited onto graphite are distinct from those on typical hydrophobic substrates. To clarify the origin of these behaviors, we investigated the crystallization kinetics of thin water films on graphite in terms of the initial film thickness, deposition temperature, and template effects of adspecies based on reflection high-energy electron diffraction (RHEED) images; the film morphology change was analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The water monolayer nucleates after surface diffusivity occurs at ca. 120 K; the nucleation temperature and time increase with increasing initial film thickness. Crystallites of cubic and hexagonal ices are formed, having preferred orientation [cubic (111) or hexagonal (001)] along the surface normal direction; their relative quantity depends on the initial film thickness and the way of crystallization. Randomly oriented crystallites finally grow via spontaneous nucleation when the film thickness exceeds 7–10 monolayers. The template ordering effects of graphite are quenched when a monolayer of ordered n-octane preexists at the substrate interface. The crystalline ice tends to wet the graphite substrate immediately after nucleation, and the film morphology changes gradually at 130 K because of premelting. The crystallites are ripened via molecular transport through the quasiliquid layer formed at the free surface, grain boundaries, and substrate.