SERS studies of methylviologen on silver sol and the effect of halide ions

Abstract

SERS of methylviologen (MV 2+) on silver sol was found to be different from that seen on a roughened surface of a silver electrode. The appearance of SERS signals of MV 2+ radical suggests that the reduction of MV 2+ occurred due to the adsorption of MV 2+ in the inner Helmholtz layer of fresh silver colloid. The active silver clusters located on the surface of silver colloid were regarded as the reducing agent. They can be oxidized or reorganized after the silver sol was aged, leading to a decrease of the Raman intensity. The experimental results concerning the red-shift of the surface plasmon absorption band of fresh silver sol suggested the adsorption of MV 2+ on the active silver cluster. The study on the effect of halide ions, which can greatly decrease the Raman intensities by replacing the ad-MV 2+ molecules from the surface of active silver clusters, supported the above conclusion. Possible orientation of the MV 2+ molecules on silver sol and the peak dependence relation of SERS signals were also discussed.