Molecular and Electronic Structures at Electrochemical Interfaces from <i>In Situ</i> Resonant X-Ray Diffraction

An original approach to characterize electrochemical interfaces at the atomic level, a challenging topic toward the understanding of electrochemical reactivity, is reported. We employed <i>in situ</i> surface resonant X-ray diffraction experiments combined with their simulation using first-principle density functional theory calculations and were thus able to determine the molecular and electronic structures of the partially ionic layer facing the electrode surface, as well as the charge distribution in the surface metal layers. Pt(111) in an acidic medium at an applied potential excluding specific adsorption was studied. The presence of a positively charged counter layer composed of 1.60 water and 0.15 hydronium molecules per platinum surface unit cell at 2.8 Å from the oppositely charged Pt(111) surface was found. Our results give a unique insight into the water-metal interaction at the electrochemical interfaces.