Potential dependence of the orientation of thiocyanate adsorbed on an iron electrode as probed by surface-enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy has been extended successfully to the study of a bare iron electrode (without any deposition of other noble metals) exposed to the neutral solution containing thiocyanate over a wide potential range from -1.8 to -0.4 V. The applied potential has a significant influence on the Raman frequency shift of SCN- as a result of the electrochemical Stark effect. Both N- and S-coordination modes were observed. Results show that at large negative potentials N-bound SCN- is favored, whereas at relatively positive potentials, S-bound thiocyanate predominates the iron surface. A sudden change in the dnu(CN)/dE value is observed at potentials from ca. -0.75 to -1.1 V and is evidenced for the transition from N-bound thiocyanate to S-bound species.