The adsorption of acetylene on rhodium-modified colloidal silver, a surface-enhanced Raman study

Surface-enhanced Raman scattering (SERS) from molecules adsorbed on rhodium-modified colloidal silver particles is reported for the first time. Deposition of thin layers of metallic rhodium on the silver surface led to fast aggregation of the sol and to modifications of its SERS spectrum. An intense new band, assigned to the RhO stretching vibration of citrate ions bound to rhodium sites, appeared at 530 cm^-1 in the Raman spectrum after rhodium addition to the suspension. The spectra of acetylene adsorbed on both unmodified silver particles and silver modified by an overlayer of rhodium indicated that acetylene displaced the citrate ions from their adsorption sites. All acetylene spectra were characterized by weak bands at 1990, 2050 and 2150 cm^-1 assigned to σ π-complexes between acetylene and silver, by a silver acetylide peak at 1800 cm^-1 and by an intense band at 1550 cm^-1 due to C=C containing species formed on the surface. However, on the rhodium-modified colloid an additional band, attributed to acetylene σ π-bound to rhodium sites, was observed at 1910-1920 cm^-1. The intensity of the new band was a direct function of the amount of rhodium deposited on the silver. It increased immediately after acetylene adsorption, and later slowly diminished, while simultaneously the 1550 cm^-1 peak became more important. This time evolution was ascribed to a reaction taking place on the surface.