Structure of water molecules at Pt electrode/electrolyte interface probed by heterodyne-detected second harmonic generation

Heterodyne-detected second harmonic generation (HD-SHG) spectroscopy was used to study the effects of potential and ionic strength on the structure of water molecules at Pt electrode/LiClO4 electrolyte interface. The contribution from Pt electrode and the third-order nonlinear susceptibility, χ(3), contribution from water molecules in the electric double layer (EDL) were separated according to the SHG intensity and phase measurements of Pt/LiClO4 interface at different potentials. The results showed that the contributions from the Pt electrode and water molecules in the EDL both presented a parabolic relationship with the applied potential, and the lowest point of the parabola was close to the potential of zero charge. Compared with the contribution from the Pt electrode, the χ(3) contribution from water molecules in the EDL ranged from 0 to 22%. Moreover, the χ(3) contribution from water molecules in the EDL increased with increasing interfacial potential, which was due to the more ordered arrangement of water molecules in the EDL. In addition, under open circuit potential, the SHG intensity and SHG phase measurements with different concentrations of LiClO4 showed that the χ(3) contribution from water molecules in the EDL increased with the decrease of ionic strength, which was attributed to the increase of the number of ordered water molecules in the EDL. These results provide quantitative information for the χ(3) contribution of water molecules in the EDL to SHG signal at the electrode/dilute electrolyte interfaces.