Nature of the Electrical Double Layer on Suspended Graphene Electrodes

The structure of interfacial water near suspended graphene electrodes in contact with aqueous solutions of Na₂SO₄, NH₄Cl, and (NH₄)₂SO₄ has been studied using confocal Raman spectroscopy, sum frequency vibrational spectroscopy, and Kelvin probe force microscopy. SO₄^2- anions were found to preferentially accumulate near the interface at an 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 H-bonded water. No such increase is observed with NH₄Cl at the OCP. The intensity of the dangling O-H bond stretching peak however remains largely unchanged. The degree of orientation of the water molecules as well as the electrical double layer strength increased further when positive voltages are applied. Negative voltages on the other hand produced only small changes in the intensity of the H-bonded water peaks but affected the intensity and frequency of dangling O-H bond peaks. The TOC figure is an oversimplified representation of the system in this work.