Influence of laser illumination during an oxidation-reduction cycle on the surface structure of silver: Implications for the mechanism of SERS

The structural changes produced by oxidation and reduction of a silver surface in 1 M CKl with and without illumination were examined by scanning electron microscopy. Laser illumination (632.8 nm) during the oxidation-reduction cycle (ORC) produces a surface covered with a high density of sub-μm sized particles. The silver particles are the result of the photolytic reduction of the silver chloride made possible by the presence of the applied electric field within the silver chloride layer which prevents the recombination of the photoelectrons and holes. As the maximum anodization voltage of the ORC is increase to +100 mV vs. SCE, the number of photolytically produced silver particles increases. As the maximum anodization voltage is increased from +100 mV to +200 mV, the individual particle size increases from ≈200 nm to ≈800 nm. The intensity of Raman scattering from water adsorbed on the silver surface was maximized by a laser illuminated ORC with a maximum anodization voltage of +100 mV.Since recent theoretical studies indicate an optimum particle size of r≈50 nm for maximizing the electromagnetic component of the enhancement, the present results indicate that either the individual silver particles contain small scale (≈50 nm) roughness features or the electromagnetic factors are not the sole contributors to the enhancement.