Spectroelectrochemistry of colloidal silver

The spectroelectrochemical response of small silver particles was studied in aqueous solution using an optically transparent, thin layer electrode. The position of the surface plasmon band of the colloidal silver was found to depend on the applied electrode potential. It varied from 400 nm in air, corresponding to a redox potential of +0.15 V vs Ag/AgCl, to about 392 nm at -0.6 V vs Ag/AgCl. A value of 80 +/- 10 mu F cm(-2) for the double-layer capacitance of the silver-water interface was obtained on the basis of the spectroelectrochemical shift. The equilibration kinetics of the particles with the electrode obeyed the Cottrell equation. However, the number of electrons transferred at each particle-electrode encounter was found to be potential dependent and reached 1600 +/- 300 at potentials more negative than -0.4 V vs Ag/AgCl. The evidence suggests that this particle charging current occurs via electron tunneling across the particle and electrode double layers and not by contact electrification. Changes in the redox potential of the particles due to added chemical reductants could also be directly monitored by laser doppler electrophoresis.