▎ 摘　　要

Unlike metals, graphene forms a hydration layer at the electrode/electrolyte interface, which is unusual for a conducting material. Here, the electrostatic properties of the hydration layer are studied by measuring the oscillation of ions at the interface due to an applied AC potential by differential reflectivity during cyclic voltammetry (CV). The amplitude of ion oscillation at picometer range sensitivity on a partially coated Au electrode by a graphene monolayer reveals a remarkable difference between the coated and uncoated part of the electrode. The uncoated Au electrode exhibits the expected behavior, but the oscillation of ions is completely screened over the graphene coated electrode up to 0.2 M of NaCl. The "insulating" behavior is consistent with CV and DPV. The behavior attributed to the hydration layer could be reversibly disrupted to de-screen the electric field to study the electrostatics of graphene/ Au interface. The shift in Fermi level of hydration-layer-free graphene/Au relative to Au is quantitatively consistent with work function transparency of graphene. The cut-off of the emanating field from the underlying electrode in an electrolyte is another example of graphene's unique passivation property that will have profound effect on applications such as, supercapacitors where the charge state of interface is critical.