Electrochemical in situ STM study of phase formation during Ag and Al electrodeposition on Au(111) from a room temperature molten salt

The electrodeposition of Ag and Al on flame annealed Au(111) films from an acidic aluminium chloride-1-methyl-3-butylimidazolium chloride room tempera ture molten salt has been investigated by electrochemical scanning tunnelin g microscopy, cyclic voltammetry and potential step experiments. The cyclic voltammogram of Ag on Au(111) is characterized by adsorption controlled Ag underpotential deposition (upd) and diffusion controlled Ag overpotential deposition (opd). Starting from the anodic limit, bulk oxidation of Au is o bserved to start near + 1.25 V vs. Ag/Ag+ reference electrode (RE). In the upd range, two dimensional Ag islands form which merge in a coherent Ag mon olayer near 0.05 V vs. RE. With further reduction of the potential, a secon d monolayer grows. The corresponding chronoamperometric measurements show e xponential behaviour with time constants of the order 10 s(-1) consistent w ith a Langmuir adsorption model. In the opd range, a diffusion controlled l ayer by layer growth of Ag clusters occurs, the bulk Ag+ diffusion coeffici ent being (1.4 +/- 0.2) x 10(-6) cm(2) s(-1). Alloying of Ag with codeposit ed Al from the electrolyte has to be considered. For the Al electrodepositi on on Au(111), strong indications for alloying have been observed starting at a potential of + 0.95 V vs. Al/Al3+ RE. Below +0.55 V, the formation of two. dimensional Al islands is seen followed by a three dimensional growth whereby a strong tendency for alloying has to be considered.