Water and electrolyte structure at Ag electrodes in nonaqueous butanol solutions using surface-enhanced Raman scattering

Abstract

Surface-enhanced Raman scattering (SERS) has been used to investigate the interfacial H 2O and electrolyte behavior of LiCl, LiBr, LiI and LiClO 4 at Ag electrodes in several isomers of butanol, including 1-butanol, iso butanol and 2-butanol. The ν(OH) region contains bands originating from trace H 2O that are clearly indicative of the behavior of the electrolyte in the interfacial region. Six ν(OH) bands are observed in the surface spectra depending on the electrode potential. These are assigned on the basis of previous assignments made for similar bands in aqueous solutions and the spectral response of crystalline LiOH. The intensities of these bands are a function of the nature of the anion in the LiX electrolyte, the particular isomer of butanol used as the solvent and the electrode potential. A band at ca. 3500 cm −1 is assigned to H 2O hydrogen-bonded to specifically adsorbed anions at the electrode surface at potentials positive of the potential of zero charge (pzc). A band assigned to H 2O in the primary solvation shell of the Li + is observed at ca. 3570 cm −1 at potentials negative of the pzc. A band thought to be due to “free” H 2O not associated with specific ions, perhaps weakly held in the secondary solvation shell of Li +, is observed between 3600 and 3650 cm −1. Up to three bands are observed from OH − formed at the electrode during the reduction of H 2O at negative potentials. At these potentials, a band observed at 3600 cm −1 is assigned to OH − weakly involved with Li +, perhaps in a solvent-separated ion pair. A second OH − band observed at ca. 3630 cm −1 is assigned to OH − species adsorbed through the oxygen atom to the Ag electrode surface. The sharp band observed at ca. 3660 cm −1 is assigned to crystalline LiOH species which form owing to the poor solubility of this salt in the isomers of butanol. This compound is either precipitated on the electrode surface or found as microcrystallites in the interfacial region.