Hydrophobic-to-hydrophilic affinity change of sub-monolayer water molecules at water–graphene interfaces

Abstract Investigating the interfacial properties between water and graphene is important, as the water–carbon interface influences various applications. Graphene is the fundamental unit of various carbon allotropes and composed of a single atomic species, although slight oxygen groups are normally attached on it. Therefore, adsorption properties on graphene is inevitable to advance fundamental understating on surface sciences. However, the detection of water adsorption on graphene is considerably hard, because amount of water is tiny and mass production of graphene to a detectable level using a standard adsorption apparatus is extremely difficult. We succeeded to detect water vapor adsorption on several layer graphene using lab-made adsorption apparatus with a quartz crystal microbalance technique. This study derives the properties of water on single-, triple-, and six-layer graphene from the contact angles of water droplets and the water vapor adsorption isotherms. The surface hydrophobicity evaluated from the contact angles and water vapor adsorption isotherms decreased in the following order: single-layer graphene > triple-layer graphene > six-layer graphene. At a relative pressure of 0.4 and water surface densities of 0.5–1.5 mg m−2 (corresponding to 2–5 water layers), the water stability was 1.0 kJ mol−1 smaller on single-layer graphene than on triple- and six-layer graphene. The weak interaction between water molecules and single-layer graphene induced slight water adsorption on graphene. As the water amount increased, the water–graphene interaction energy increased with steps of 0.17–0.18 mg m−2 (corresponding to nearly 50% of a water monolayer), and the water structure correspondingly transformed from cluster to layer. The structure transformation of adsorbed water induced the affinity change from hydrophobic to hydrophilic on graphene interfaces. The mechanism of water–graphene interfaces can be exploited in applications of graphene-based carbon nanomaterials.