Molecular structure at electrode/electrolyte solution interfaces related to electrocatalysis

The potential dependence of the interfacial water structure at Pt and Au thin film electrodes was investigated by sum frequency generation (SFG) spectroscopy in internal reflection mode. In the case of the Pt electrode, two broad peaks were observed in the OH stretching region at ca. 3200 cm(-1) and ca. 3400 cm(-1), which are known to be due to the symmetric OH stretching (v(1)) of tetrahedrally coordinated, i.e., strongly hydrogen bonded "ice-like'' water, and the asymmetric OH stretching (v(3)) of water molecules in a more random arrangement, i.e., weakly hydrogen bonded "liquid-like'' water, respectively. In the case of the Au electrode, however, a 3400 cm(-1) band was dominant in the SFG spectrum, suggesting that the interaction between water molecules and Au and Pt are different, i.e., water molecules are more disordered at the Au surface. The potential dependence of interfacial water during the methanol oxidation reaction on a Pt electrode was also investigated. SFG intensity strongly depended on electrode potential. Several possibilities are suggested for the potential dependence of the SFG intensity.