Formation of Water Layers on Graphene Surfaces

Abstract

Although graphitic materials were thought to be hydrophobic, recent experimental results based on contact angle measurements show that the hydrophobicity of graphitic surfaces stems from airborne contamination of hydrocarbons. This leads us to question whether a pristine graphitic surface is indeed hydrophobic. To investigate the water wettability of graphitic surfaces, we use molecular dynamics simulations of water molecules on the surface of a single graphene layer at room temperature. The results indicate that a water droplet spreads over the entire surface and that a double-layer structure of water molecules forms on the surface, which means that wetting of graphitic surfaces is possible, but only by two layers of water molecules. No further water layers can cohere to the double-layer structure, but the formation of three-dimensional clusters of liquid water is confirmed. The surface of the double-layer structure acts as a hydrophobic surface. Such peculiar behavior of water molecules can be reasonably explained by the formation of hydrogen bonds: The hydrogen bonds of the interfacial water molecules form between the first two layers and also within each layer. This hydrogen-bond network is confined within the double layer, which means that no “dangling hydrogen bonds” appear on the surface of the double-layer structure. This formation of hydrogen bonds stabilizes the double-layer structure and makes its surface hydrophobic. Thus, the numerical simulations indicate that a graphene surface is perfectly wettable on the atomic scale and becomes hydrophobic once it is covered by this double layer of water molecules.

Graphene surface structure in aqueous media: Evidence for an air-bubble layer and ion adsorption

“…In addition, the structure of graphene itself can also play an important role in response to the temperaturedependent ion adsorption. A simulation study has reported that water could form an ice-like double layer structure on free standing graphene, preventing the interaction between graphene and the bulk solution [24]. This interfacial water structure was found disrupted at 340 K (67 °C), which is close to where we observed enhanced ion adsorption.…”

“…The ion adsorption was enhanced by heating the solution to 60 ºC, which was retained after cooling. We attributed this change to the higher ion mobility, the higher ion affinity of the formed silanol layer on the substrate, and the disruption of the ordered water structure at high temperatures as reported previously [24].…”

“…Specifically, it has been reported that the hydrophobicity of graphene on a copper foil [13] and a SiC wafer [23] increased with the number of graphene layers. Molecular dynamic simulations showed that free-standing graphene was hydrophobic [23,24], due to the hydrogen bond network within a water double layer formed on the graphene surface.…”

“…affected by the graphene surface structure and related parameters, such as the graphene layer thickness [23,49], the presence of defects [23,48] and adsorbates [24,48], underlying substrates [50], and the doping of graphene [51]. Both the surface energy and the topological features should contribute to the observed wettability of such a heterogeneous material.…”

Gate-controlled photo-oxidation of graphene for electronic structure modification

“…3d. Then, O2 molecules adsorb onto the H2O layer as an over-layer; a study based on molecular dynamics simulation 19 has shown that H2O molecules form a double-layer structure on graphene, but here the second H2O monolayer is omitted for simplicity. When the gate voltage is applied to a graphene FET, electric field lines are formed, as schematically depicted in Fig.…”