Potential-Dependent Reorientation of Water Molecules at an Electrode/Electrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy

The structure and orientation of water molecules at a highly ordered Au(111) electrode surface in perchloric acid have been investigated in-situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. This newly developed infrared spectroscopy technique enables the observation of the electrode/electrolyte interface at a very high sensitivity without interference from the bulk solution. The spectrum of the interfacial water significantly differs from that of bulk water and drastically changes in peak frequencies and band widths around the potential of zero charge (pzc) of the electrode and at about 0.3 V positive from the pzc. The interfacial water molecules are weakly hydrogen-bonded at potentials below the pzc and form a strongly hydrogen-bonded ice-like structure at potentials slightly above the pzc. The ice-like structure is broken at more positive potentials due to the specific adsorption of perchlorate ion, where one OH moiety of water is non-hydrogen-bonded and the other OH moiety is hydrogen-bonded to another water molecule. The intensities of the fundamental modes of water are also a strong function of applied potential. They are very weak around the pzc and increase as the potential changes in both positive and negative directions. These results are explained in terms of the potential-dependent reorientation of water molecules from oxygen-up to oxygen-down as the surface charge changes from negative to positive. The adsorption of hydronium and perchlorate ions on gold is also discussed.