What Vibrational Spectroscopy Tells about Water Structure at the Electrified Palladium–Water Interface

The knowledge of the structure and properties of water adsorbed on metal surfaces is relevant for understanding electrocatalytic reactions. A monolayer of Pd deposited at a gold electrode surface is known to have enhanced electrocatalytic properties. To shed new light on these properties, the present study describes investigations of water structure at monolayer Pd film deposited at a nanostructured gold electrode consisting of ∼50 nm large nanoparticles supported at a Si hemisphere. Attenuated total reflection surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS) was applied to measure vibrational spectra of water molecules at the electrode surface. The surface enhancement of the electric field of the photon allowed one to determine spectra of water molecules interacting with the Pd film. The results show that surface water is a mosaic of "ice-like", "liquid-like", and monomer or small clusters of water molecules. In addition, hydronium ions are present at the Pd surface in an acidic solution. The relative contribution of these structures depends on the pH and electrode potential. The surface water is more ordered at potentials corresponding to the adsorption of hydrogen atoms.