Wettability of Graphene and Interfacial Water Structure

Understanding the graphene-water interaction, referred to as ‘wettability,’ is important for various applications, such as water desalination, filtration, energy storage, and catalysis. However, most studies on graphene's wettability have been performed with either macroscopic water contact angle measurements or molecular dynamics simulations. The detailed hydrogen-bonding network structure of water molecules at the graphene-water interface has not been fully understood at the molecular level. Here, using vibrational sum frequency generation (VSFG) spectroscopy, we elucidate the interfacial water structure and graphene hydrophobicity at a multilayer graphene-water interface. As the number of graphene layers increases, water molecules with dangling OH group become more populated. We compare the contact angles of water on the multilayer graphene surfaces with VSFG results. An excellent correlation between water adhesion energy of graphene and fraction of dangling OH groups estimated from the water OH stretch VSFG spectrum is established. This observation suggests that the VSFG could be an incisive technique for measuring water's adhesion energy on any spatially confined or blocked surface where the water contact angle cannot be measured. We further anticipate that the VSFG result on the transition from wetting transparency to translucency upon increasing the number of graphene layers will be used to understand the wettability of low-dimensional materials and the role of water structure on electric double layers of graphene-based electrodes.