Wettability of graphene, water contact angle, and interfacial water structure

Understanding the details of water interacting with graphene is essential for various applications, such as water desalination, energy storage, and catalysis. However, the hydrogen-bonding structure of the water at the graphene-water interface has not been fully understood. Vibrational sum frequency generation (VSFG) spectroscopy is suited to elucidate the water structure at graphene-water interfaces. With varying numbers of graphene layers or tuning the doping level of a single monolayer, the interfacial water structure differs substantially. Specifically, as the number of graphene layers increases, water molecules with non-H-bonded, dangling OH groups become increasingly apparent. The fraction of dangling OH groups inferred from the VSFG spectrum correlates with the water adhesion energy of graphene. This observation suggests that VSFG could be an incisive technique for measuring the water adhesion energy on any spatially confined interface where the water contact angle cannot be measured. We anticipate that VSFG spectroscopy will shed light on the wettability of low-dimensional materials.