In Situ STM Study of the Surface Structure, Dissolution, and Early Stages of Electrochemical Oxidation of the Ag(111) Electrode

The surface structural modifications induced by underpotential anodic oxidation have been studied at the Ag(111)/0.1 M NaOH(aq) interface by in situ electrochemical scanning tunneling microscopy. For E pzc ≤ E ≤ −0.1 V/SHE, no specific surface structure results from the adsorption of hydroxide ions, indicating the formation of a weakly bonded and mobile layer of adsorbed hydroxide species. For −0.05 ≤ E ≤ 0.05 V/SHE, the Ag(111) surface undergoes reconstruction. The terraces become completely covered by irregularly arranged protrusions assigned to nuclei of a 2D layer similar to silver oxide and separated by non-reconstructed regions with adsorbed hydroxide. The 2D oxide islands consist of one reconstructed layer of silver embedded between two layers of subsurface species (oxide) and surface species (oxide and/or hydroxide). Atomic resolution evidences an enlarged Ag−Ag distance consistent with the growth of one monolayer of Ag2O(111). Long-range structural ordering is improved by reducing the rate of reconstruction in the adsorbed layer. Grains assigned to 3D silver oxide nuclei were also observed and possibly result from the aggregation of Ag atoms ejected from the surface lattice during the surface reconstruction. For E = 0.15 V/SHE, dissolution occurs and is strongly influenced by the irregular structure of the reconstructed surface. 2D pits are initiated on the terraces at the less resistant and non-reconstructed sites of the surface. They propagate by preferential dissolution of the topmost metal plane at the newly created step edges. 3D pits initiate and propagate in the second metal plane by the same mechanism. Dissolution is blocked by the ordered regions of the 2D silver oxide monolayer and by 3D oxide nuclei.