LAYER-BY-LAYER ANODIC-DISSOLUTION OF SULFUR-MODIFIED NI(100) ELECTRODES - IN-SITU SCANNING-TUNNELING-MICROSCOPY

In situ scanning tunneling microscopy (STM) was applied to investigate S-modified Ni(100) electrodes in 0.05 M Na2SO4 (pH 3). It was found t hat the monolayer of adsorbed S possessed the c(2 x 2) structure on Ni (100). The S adlayer was found to be very stable even at potentials wh ere hydrogen evolution or anodic dissolution takes place. The passivat ion of the Ni surface due to the formation of oxide layers was complet ely suppressed by the presence of the S adlayer. The anodic dissolutio n of Ni on the S-modified Ni(100) occured only at the step edges, resu lting in the formation of an atomically flat terrace-step structure, T he monoatomic step lines were usually found along the [001] and [010] directions. The etching rate along [010] was faster than that along [0 01]. The anisotropic etching was explained based on a model structure of the S adatoms near the step edges.