Stacking of water molecules in hydrophilic graphene oxides characterized by Kelvin probe force microscopy

We directly measured local surface potentials of graphene oxide and poly (sodium 4-styrenesulfonate) intercalated graphene oxide (PSSGO) films, as a function of humidity, using Kelvin probe force microscopy. Our results showed a decrease in the work function of the graphene oxide (GO) film as the humidity increased. This behavior was explained using the first-principle calculation in which water molecules stacked onto the GO film, resulting in a lower local work function and ripples of the surface potential. The ripples directly measured by Kelvin Probe Force microscopy were consistent with the calculation. While the PSSGO film did not show the ripples as the humidity increased. Our results directly showed that absorbed water molecules on the films change the local surface potential and the work function such that its performance in many applications will be affected.