Abstract: The surface-enhanced Raman scattering(SERS) spectrum of the trace amount of water on the silver/acetonitrile interface was investigated as a function of the applied potential and the concentration of water. The results show that several adsorption modes of water exist in the electrochemical double layer of silver. At relatively positive potentials, the water molecule was co-adsorbed with acetonitrile through weak hydrogen bond, which was hardly affected by the increase of the concentration of water. The frequency of ν(O-H) appears at 3 487 cm^-1. When the scanning potentials were negative values, the water molecule turned to interact with the surface complex [Ag(CN)_n]^(n-1)- produced by the dissociation reaction of the solvent acetonitrile. The frequency of ν(O-H) centered at ca. 3 583 cm^-1 which red-shifted with the increase of the concentration of water. At more negative potentials, the hydrogen evolution reaction of water occurs seriously, and the quantity of OH^- produced led to the formation of the crystallite LiOH which had the vibration of ν(O-H) locating at 3 665 cm^-1. The formation of LiOH·H₂O, in which the frequency of ν(O-H) was at 3 563 cm^-1, becomes possible when increasing the quantity of water to 1.0 mol/L in the bulk solution.

Key words: SERS, Nonaqueous system, Acetonitrile, Dissociative adsorption, Water