| AVS 65th International Symposium & Exhibition | |
| Surface Science Division | Wednesday Sessions |
| Session SS+HC-WeM |
| Session: | Catalytic Alloys: Understanding Heterogeneity |
| Presenter: | Jun Nakamura, The University of Electro-Communications (UEC-Tokyo), Japan |
| Authors: | A. Akaishi, The University of Electro-Communications (UEC-Tokyo), Japan T. Yonemaru, The University of Electro-Communications (UEC-Tokyo), Japan J.N. Nakamura, The University of Electro-Communications (UEC-Tokyo), Japan |
| Correspondent: | Click to Email |
While graphite is 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 hydrocarbons on the surface are removed [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]. On the other hand, water molecules have been confirmed to form layered structures on a graphene surface [4] and on the surface of carbon nanotubes [5]. But, the wettability of pristine graphene surfaces remains unsettled.
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 [6]. 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.
[1] Z. Li et al., Nat. Mater. 12, 925 (2013)
[2] A. Kozbial et al., Carbon 74, 218 (2014)
[3] T. Werder, J. H. Walther, R. L. Jaffe, T. Halicioglu, and P. Koumoutsakos, J. Chem. Phys. B 107, 1345 (2003)
[4] Y. Maekawa, K. Sasaoka, and T. Yamamoto, Jpn. J. Appl. Phys. 57, 035102 (2018)
[5] Y. Homma et al., Phys. Rev. Lett. 110, 157402 (2013)
[6] A. Akaishi, T. Yonemaru, and J. Nakamura, ACS Omega 2, 2184 (2017)