Anions and the Effect of Supporting Electrolyte at Ag Electrodes

The addition of a supporting electrolyte, such as a potassium halide salt, has the added effect in SERS studies of introducing a species which can adsorb on the metal surface and give SERS bands. For example, at Ag electrodes, in addition to molecular SERS bands, one finds bands for adsorbed halides. 

The major low-frequency bands for the Ag—X stretching vibration on Ag are r(115 cm V90 cm 0, Br (162 cm /l43 cm O, and Cr(240 cm . These surface bands are broader than the SERS bands for molecular vibrations which do not involve atoms directly bonded to the surface, and they also show vibrational frequency shifts with electrode potential (21 cm VV for Ag—CP). Also, the band intensities decrease as the Ag electrode potential is moved in the negative direction. Since the point of zero charge (pzc) is around -0.8 V versus SCE on polycrystalline Ag, there is a sizable potential range on Ag which favors specific anion adsorption. In the absence of a molecular species such as pyridine, the SERS surface band for CP disappears at potentials more negative than ca. -0.6 V versus SCE. In addition to the major surface band, other SERS bands are observed in pure aqueous potassium halide electrolyte. For example, in 1 M KCl at -0.2 V versus SCE at Ag, bands are observed  

In addition to numerous studies of halides, which have been reviewed elsewhere, other anions have been investigated with SERS on Ag electrodes, as indicated in Table VI. There is, in fact, an extensive literature on the SERS of and SCN on Ag. More complicated anions such 

---