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 °C, while the other two freeze
into ice in a stepwise manner above −40 °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.