Understanding controls on interfacial wetting at epitaxial graphene: Experiment and theory

Abstract

The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution X-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle θ c = 73°) is substantially smaller than that of multilayer graphene (θ c = 93°). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.2

Epitaxial Graphene and Graphene–Based Devices Studied by Electrical Scanning Probe Microscopy

“…The described experiments indicate that hydrophobicity increases with layer number in both types of graphene, such that SiC → 1 LG → 2 LG → 3 LG on the Si-face (bearing in mind that pristine SiC is hydrophilic) and FLG → MLG on the C-face. These observations obtained from nanoscale SPM measurements are in agreement with our macroscopic measurements of the contact angle performed on different types of graphene [9,78] as well as with results of others [28]. This also agrees well with the results of molecular dynamic modeling of water on epitaxial graphene [27] and is generally consistent with a value of the adsorption (binding) energy for water molecules being significantly larger on pure SiC (−636 meV) [91] than on graphene (18-47 meV) [90], though in the latter case the influence of the substrate was not taken into account.…”

“…Significant effort has been dedicated to both theoretical and experimental investigation of water on graphitic surfaces. In general, the hydrophobic nature of the graphene was commonly observed and revealed on the macroscale as a large contact angle, i.e., ~93°-120°, between a water droplet (~1 μL) and graphene surface, as measured by optical methods and X-ray reflectivity [26][27][28]. However, it has also been shown that few layer graphene on top of different substrates does not significantly change the wettability of such materials as gold, silicon and copper, owing to van der Waals forces dominating the surface-water interactions [29] and, as such, the few layer graphene appears hydrophilic in contradiction to bulk graphite.…”

What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

“…Our results are also consistent with a recent study 53 of the interface between water and epitaxial graphene grown on SiC substrates. In this experiment, the crystal truncation rods of the substrate were measured, which allowed the gap width to be determined with much higher resolution than is possible with the specular reflectivity technique.…”

Insights into electrocatalytic activity of epitaxial graphene on SiC from cyclic voltammetry and ac impedance spectroscopy

“…Parameter σ res is the standard deviation of residuals Here, R is the universal gas constant (8. [20,21,49]. Particularly, this concerns the areas covered only with buffer layer G 0 [21].…”

“…The interaction of the interfacial water with EG grown on SiC has been studied theoretically [18,19] and experimentally by frequency modulation AFM [20], highresolution X-ray analyses [21], vibrational spectroscopy [22], and contact angle goniometry [23]. It has been shown that the G 0 containing high number of sp 3 carbon-bonded inclusions chemically bonds the closest lying water molecules and, probably, blocks electrochemical reactions at the interface.…”