Water Adsorption Control by Surface Nanostructures on Graphene-Related Materials by Grand Canonical Monte Carlo...

The interfaces of carbon materials play an important role in various technological and scientific research fields. Graphene is the fundamental unit of sp² carbon allotropes, and the evaluation of the interfacial properties of graphene-related materials is thus essential to clarify the molecular mechanisms occurring at the interfaces. Ideally, graphene is exclusively composed of sp² carbon atoms; however, some parts of graphene normally contain sp³ carbon atoms with oxygen functional groups, vacancy, and grain boundary defects, and these structural characteristics need to be considered to reveal the interfacial properties. Herein, we demonstrate the interfacial properties of graphene-related materials by analyzing the water adsorption properties of graphene, hydrogenated graphene (graphane), and partially oxidized graphene (named as graphoxide) using grand canonical Monte Carlo simulations. The hydrophobicity evaluated from the simulated water adsorption isotherms followed the order: graphane > graphene > graphoxide with 1% oxygen atomic ratio > graphoxide with 3% oxygen atomic ratio > graphoxide with 5% oxygen atomic ratio. The potential calculations between a single water molecule and graphoxides revealed that the presence of oxygen functional groups enhanced the hydrophilicity of graphoxide. This study also disclosed some differences between the hydrophobic interfaces of graphene and graphane, which have been rarely evaluated. Surprisingly, the hydrophobicity of graphane was higher than that of graphene despite the similar potential well depths between a water molecule and graphene/graphane. This was caused by the restriction of water orientation on graphane; water was preferentially adsorbed on the honeycomb center or concave sites in the initial adsorption, and it was hard to interact with neighboring water molecules. The different structures revealed for the graphene-related materials with nanoscale roughness played important roles in controlling the water vapor adsorption mechanism.