Structure of Water at the Electrified Platinum−Water Interface:  A Study by Surface-Enhanced Infrared Absorption Spectroscopy

Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode is used to examine the structure of water on a polycrystalline Pt electrode in H<sub>2</sub>SO<sub>4</sub> and HClO<sub>4</sub> as a function of applied potential. The electrode surface covered with CO is used as the reference in recording spectra, which enables us to obtain the <i>absolute</i> infrared spectrum of the interfacial water layer (monolayer or bilayer) in contact with the surface with negligible interference from the bulk water. The spectrum of the interfacial water is largely different from that of bulk water and changes around the potential of zero charge of the electrode. The spectral changes are ascribed to the potential-dependent reorientation of water molecules from a weakly hydrogen-bonded oxygen-up orientation at the negatively charged surface to a strongly hydrogen-bonded nearly flat orientation at the positively charged surface in agreement with theoretical simulations reported in the literature. Clear experimental evidence of the formation of a stable ice-like structured water on the positively charged surface is reported.