Structure of water at ionic liquid/Ag interface probed by surface enhanced Raman spectroscopy

Abstract

The potential-dependent adsorption behavior of water and ionic liquid was probed by surface-enhanced Raman spectroscopy (SERS) at the Ag electrode surface in the ionic liquids containing water with different concentrations. The configuration of water at the ionic liquid/electrode interface and the relationship between the potential of zero charge (pzc) and the molar fraction of water were deduced through the changes in the vibrational frequency of OH stretching mode. A small Stark effect value was determined in the system with lower molar fraction of water. With the increase of the water concentration, the OH stretching vibrational frequency gradually shifted to the high wavenumber region, the pzc was also moved positively, and the Stark effect value of OH stretching vibrational mode increased. It reached about 76 cm−1 V−1 in the 1 mol L−1 [BMIM]Br aqueous solution. These differences were mainly contributed by hydrogen bonding and the configuration of water in the ionic liquid solution. In the solution with lower water content, water molecules existed at the interface layer through hydrogen bonding with cations, while in the higher content solution, the intermolecular hydrogen bonding between water molecules was strengthened and the possibility of the direct interaction between the water molecule and electrode surface increased.