SERS studies on electrode/electrolyte interfacial water .1. Ion effects in the negative potential region

Utilizing a special roughening procedure and a difference spectrum method, we have studied electrolyte ion effect on surface enhanced Raman scattering (SERS) of interfacial water molecules at silver electrodes, for the first time, in solutions free of (pseudo-) halide ions. The SERS obtained from alkali sulfate or perchlorate solutions in the negative potential range (- 2.0 to - 1.3 V) reveals that the cation effect of water on SERS falls into two groups according to the O-H stretching frequencies: Cs+ and Li+ are in one group, while K+ and Na+ are in the other. The formation of electrolyte ion pairs and localized hydrolysis of water molecules around the cation were used to interpret the results. By using a flow cell to replace the electrolyte under potential control, we found that the effect of the anion (e.g. SO42-, ClO4- and OH-) on the SERS of water is not negligible, even at very negative potentials, which may be due to the formation of ion pairs at the electrochemical interface.