Combined Scanning Force Microscopy and Electrochemical Quartz Microbalance in-Situ Investigation of Specific Adsorption and Phase Change Processes at the Silver/Halogenide Interface

Specific adsorption of halogenides and silver halogenide phase formation was investigated by the combination of electrochemical quartz microbalance measurements (EQMB), topographical in-situ scanning force microscopy (SFM), and in-situ lateral force microscopy (LFM). In-situ LFM can be employed to monitor specific adsorption and, more generally, chemical conversion reactions in submonolayers of atomic species which are inaccessible to topological imaging by SFM. Reorganization of the electrochemical double layer during specific adsorption caused nanotribological changes. Hydrated anions in the outer Helmholtz plain are not locally bonded to a specific site and give low LFM friction values. Specifically adsorbed anions and ion pairs, on the other hand, impede the lateral cantilever translation, resulting in increased friction. EQMB measurements yielded data corresponding to the formation of up to one monolayer of specifically adsorbed cation−halogenide ion pairs. Anodic dissolution of silver to AgO- and the formation of Ag2O islands in a halogenide-free alkaline solution contributed to a roughening of the surface. Long range in-situ SFM showed that silver halogenide phases are anodically nonuniformly formed as smooth islands located in no observable correspondence to grain surfaces or boundaries suggesting a dissolution−precipitation growth mechanism. AgI and AgBr phases can almost reversibly be reduced. Irreversible mass gains after an oxidation−reduction cycle can be associated with Ag deposition near surface steps from soluble Ag and AgB species which were dissolved into the electrolyte bulk during previous anodic scans in the dissolution potential range. In the presence of chloride, the silver surface is vigorously electropolished and soluble AgC species evolve, while AgCl islands precipitate on the newly generated surface regardless of the original silver grain topography.