The effect of pH, chloride ion and background electrolyte concentration on the SERS of acidified pyridine solutions

Abstract

Surface enhanced Raman spectra of pyridine are reported as a function of pH, halide concentration and background electrolyte concentration. The assignments of pyridinium bands are given in the range 100–4000 cm −1, and the low frequency band around 235 cm −1 is discussed. It is found that to obtain a pyridinium spectrum, the presence of halide is necessary. Background electrolyte concentration does not affect the intensity of the pyridine spectrum but greatly affects that of pyridinium. On the basis of the dependence of the intensity of pyridinium on chloride and background electrolyte concentrations, pyridinium is considered not be directly adsorbed on the electrode surface but rather located in the diffuse double layer and associated with specifically adsorbed chloride to form an ion pair. The in-situ measurement of pH and the SERS of pyridine and pyridinium during a pH titration reveal a linear relation between surface pH and bulk pH. Specifically adsorbed chloride causes a decrease in the surface pH. This decrease is explained by a shift of the electrostatic potential at the outer Helmholtz plane caused by specific adsorption of chloride.