Heterogeneity of Water Molecules on the Free Surface of Thin Reduced Graphene Oxide Sheets

Reduced graphene oxide (rGO) has numerous potential applications, such as molecular sensor, gas separation membrane, etc. The performance of these devices is often subject to the environment humidity due to the interaction of rGO with water. However, the atomically detailed information on the dynamical and thermodynamic properties of water on the surface of graphene-based materials as well as its underlying molecular mechanism is largely unknown. By performing neutron scattering on hydrated rGO powders, composed by well separated monolayer and few-layer rGO sheets, we found three components of surface water. One remains liquid at -80 degrees C, while the other two freeze into ice in a stepwise manner above -40 degrees C. Although slightly slower than the other two, the nonfreezing water diffuses an order of magnitude faster on rGO than those confined in the hydrophilic bulk phase, such as compact powder or membrane. Complementary molecular dynamics simulation revealed that the heterogeneity of surface water arises from the gradual attenuation of the electrostatic interaction between water and oxide groups on rGO within a few hydration layers. These findings are fundamental for understanding of interfacial hydration and ice formation in many materials, and valuable for various applications using graphene-based materials.