Water at electrode-electrolyte interfaces: combining HOD vibrational spectra with ab initio-molecular dynamics simulations

Abstract We have undertaken a vibrational study of the structure of interfacial water and its potential dependence using H2O:D2O mixtures to explore the O-H and O-D stretching modes of HOD as well as the bending modes of HOD and H2O. Due to the symmetry reduction, some of the complexity characteristic of the vibrational spectrum of water is removed in HOD. Coupled with potential-dependent ab-initio simulations of the gold-water interface, this has enabled a deeper insight into the hydrogen-bond network of interfacial water and into how it is affected by the applied potential. Possibly the most important conclusions of our work are (i) the absence of any ice-like first layer of interfacial water at any potential and (ii) that interfacial water reorients around a stable backbone of hydrogen bonds roughly parallel to the electrode surface. At E > pzc, interfacial water molecules are oriented with the oxygen lone pairs towards the surface and forming exclusively or nearly exclusively hydrogen-donating hydrogen bonds with other water molecules. At E < pzc, the oxygen lone pairs point away from the surface instead, but the population of hydrogen-donating water molecules does not vanish. In fact, the population of hydrogen-accepting water molecules only dominates at considerably negative charge densities, due to the weak interaction of the hydrogen atoms of interfacial water molecules with the Au surface.