The Structure of the Electrical Double Layer on Suspended Graphene in Contact with Salt Solutions

The structure of interfacial water near suspended graphene in contact with aqueous solutions of Na₂SO₄, NH₄Cl and (NH₄)₂SO₄ was studied using confocal Raman spectroscopy, sum frequency vibrational spectroscopy (SFVS) and Kelvin probe force microscopy (KPFM). SO₄^2- anions were found to preferentially accumulate near the interface at open circuit potential (OCP), creating an electrical field that orients water molecules below the interface, as revealed by the increased intensity of the O-H stretching peak of fully H-bonded water. No such increase is observed with NH₄Cl at OCP. The degree of orientation of the water molecules as well as the electrical double layer strength increased further when positive voltages are applied. We show that preferential anion-accumulation at the interface at OPC is driven by segregation from the solution bulk, and is not driven by electrostatic effects nor by formation of specific chemical bonds, which is impeded by the large energy required to desolvate sulfate anions. The first water layer in contact with graphene has dangling O-H bonds that point to the graphene with an intensity that remains unchanged in both with salt concentration and with increasing positive potentials. However the first water layer undergoes a chemical interaction with graphene at negative values that decreases the intensity of the dangling bond peak and redshifts its frequency, pointing to orbital hybridization between dangling H and graphene.