In Situ Infrared Spectroscopy of Multilayer Copper Thiocyanate Films on Copper Electrodes

We have studied the growth kinetics of multilayer CuSCN films on a Cu(111) surface in aqueous NaSCN solutions using polarization-modulated infrared reflection absorbance spectroscopy (PM-IRRAS) and conventional voltammetric techniques. Formation of the first layer of adsorbed SCN^- species is indicated by the voltammetric results but cannot be resolved spectroscopically. As the potential is increased to -0.430 V (SCE) in 0.1 M NaSCN, a band appears in the PM-IRRAS spectrum at 2173 cm^-1. The intensity of the band reaches a maximum at E = -0.400 V. The frequency of this band is independent of film thickness, electrode potential, and electrolyte composition, which leads us to assign it to the v (C-N) mode of a homogeneous multilayer CuSCN film. At higher potentials, the intensity decreases to about two-thirds of its maximum value and remains stable. We attribute this decrease to a thinning of the CuSCN film brought about by a decrease in the flux of Cu cations into the film, which is caused by the formation of a second, underlying phase of different composition (e.g., a hydrous copper oxide layer).