A (surface-enhanced) Raman spectroscopic study of the adsorption of S2O32-and SCN- on a silver deposit

The study of the adsorption behaviour of SCN- and S2O32- at a silver substr ate contributes substantially to an improved insight into the electrochemic al formation of a silver image in photographic development processes. (Surf ace-enhanced) Raman (SER) spectroscopy and quantum chemical calculations re veal the existence of an S coordination for both species with Ag, forming 1 ,1-AgSCN and 1,1-AgS2O3- surface complexes, respectively, at the Ag electro de surface. At an electrode potential of -0.1 V vs. SCE, the SER peaks of a dsorbed SCN- intensify when the bulk concentration is increased from 1 x 10 (-4) to 6 x 10(-3) M, whereas those of adsorbed S2O32- are already measurab le at a bulk concentration of 1 x 10(-5) M and saturate at 3 x 10(-4) M. In both cases, the Ag-S bond weakens when the potential is lowered from -0.1 to -0.5 V vs. SCE. The correspondence between SER and capacity measurements reveals that a decrease in potential also causes a decrease in the AgSCN s urface concentration. The 1,1-AgS2O3- complex even disappears at potentials below -0.2 V vs. SCE. The formation of a physical adsorption state at lowe r potentials is suggested. (C) 1998 John Wiley & Sons, Ltd.