| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Thin Films | Thursday Sessions |
| Session TF-ThM |
| Session: | Graphene |
| Presenter: | Akira Akaishi, The University of Electro-Communications (UEC-Tokyo) and JST-CREST, Japan |
| Authors: | A. Akaishi, The University of Electro-Communications (UEC-Tokyo) and JST-CREST, Japan T. Yonemaru, The University of Electro-Communications (UEC-Tokyo) and JST-CREST J. Nakamura, The University of Electro-Communications (UEC-Tokyo), Tokyo, Japan |
| Correspondent: | Click to Email |
While graphite is well known as hydrophobic material, recent studies have revealed that pristine graphitic surfaces are more likely to be hydrophilic. Hydrophobic/hydrophilic nature is closely related to wettability of surfaces. One of the characteristic measures of wettability is a contact angle that is the angle of the edges of a water droplet placed on target surfaces. It has been reported that the contact angle of water on graphite surfaces decreases as removing hydrocarbons on the surface [1,2]. The contact angle estimated by molecular dynamics (MD) simulations, however, varies depending on a choice of the parameters of interaction potentials between a water molecule and graphitic surfaces [3]. The wettability of pristine graphene surfaces remains unsettled.
We have investigated the behavior of water molecules on graphene surfaces with MD simulations by changing empirical parameters. The simulations are prepared with a graphene sheet and a water droplet which is dropped initially on the graphene surface. With increasing the number of water molecules, the droplet covers the surface and the layered-structure of molecules on the surface is formed. This indicates that, at a level of empirical model, a graphene surface is capable of wetting.
The formation of water layers on the surface can be seen by the density profile of water molecules. The density distribution of oxygen and hydrogen atoms of water along the axis perpendicular to the graphene surface indicates the existence of the layers. Moreover, the distinct peaks of the hydrogen atom distributions indicate that the direction of oxygen-hydrogen (O-H) bonds is not randomly distributed but is certainly oriented. The angle distribution of O-H bonds shows that hydrogen bonds are formed between the water layers on the graphene surface. In other words, the water molecules in the layers tend to form the hydrogen bonding only within the layers. The water molecules above the layer have no unpaired hydrogen bonds that are pointing perpendicular to the layer plane.
References
[1] Z. Li et al., Nat. Mater. 12, 925 (2013)
[2] A. Kozbial et al., Carbon 74, 218 (2014)
[3] T. Werder et al., J.Chem.Phys.B 107, 1345 (2003)