Formation and structural characterization of two-dimensional wetting water layer on graphite (0001).

Understanding the structure and wettability of monolayer water is essential for revealing the mechanisms of nucleation, growth, and chemical reactivity at interfaces. We have investigated the wetting layer formation of water (ice) on the graphite (0001) surface using a combination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). At around monolayer coverages, the LEED pattern showed a (2 × 2) periodicity and STM revealed a hydrogen-bonded hexagonal network. The lattice constant was about 9% larger than that for ice I/I crystals, and the packing density was 0.096 Å. These results indicate that an extended ice network is formed on graphite, different from that on metal surfaces. Graphite is hydrophobic under ambient conditions due to the airborne contaminant but is considered inherently hydrophilic for a clean surface. In this study, the hydrophilic nature of the clean surface has been investigated from a molecular viewpoint. The formation of a well-ordered commensurate monolayer supports that the interaction of water with graphite is not negligible so that a commensurate wetting layer is formed at the weak substrate-molecule interaction limit.