The Effect of Monolayers on Atomic-scale Friction in Aprotic Electrolytes: Underpotential Deposition (UPD) of Silver on I-modified Au(111)

Using lateral force microscopy (LFM), we investigate the frictional behavior on I-modified Au(111) during Ag underpotential deposition (UPD) in aprotic electrolytes. Force-separation (FS) curves clearly show that the solvent forms about 3 ordered layers near the iodine adlayer independent of potential. These ordered layers of solvent can be readily pushed away by the AFM tip. Regardless of solvent, transitions of the lateral adsorbate structure have been observed depending on potential and normal load, respectively. These structural transitions indicate that the structure of adsorbed iodine is very sensitive to the Ag coverage, and the tip penetrates the adsorbed iodine as normal load increases. Friction forces as function of normal load convincingly show the influence of the tip penetration on friction; plowing of the tip in the adsorbate layer leads to an additional friction force depending on potential and solvent. With increasing normal load in Ag+ containing tetraglyme (G4), friction increases steeply as the sliding tip penetrates the iodine adsorbed on Ag adlayers, in agreement with the results in aqueous electrolyte. Furthermore, friction increases more drastically as the concentration of water increases. Considering the high friction on Ag monolayer after the penetration into the iodine, there are two possible causes: 1) the reduced mobility of the iodine on Ag adlayer and 2) the interaction between water adsorbed on the tip and Ag adlayers. However, in Ag+ containing PC, in spite of the penetration of the tip into the iodine adlayer, friction force dependence on load does not show a clear distinction between gold and silver substrates.