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

Abstract

The electrodeposition of Ag and Al on flame annealed Au(111) films from an acidic aluminium chloride–1-methyl-3-butylimidazolium chloride room temperature molten salt has been investigated by electrochemical scanning tunneling 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 observed to start near +1.25V vs. Ag/Ag⁺ reference electrode (RE). In the upd range, two dimensional Ag islands form which merge in a coherent Ag monolayer near 0.05 V vs. RE. With further reduction of the potential, a second monolayer grows. The corresponding chronoamperometric measurements show exponential behaviour with time constants of the order 10 s^-1 consistent with a Langmuir adsorption model. In the opd range, a diffusion controlled layer by layer growth of Ag clusters occurs, the bulk Ag⁺ diffusion coefficient being (1.4±0.2)×10^-6 cm² s^-1. Alloying of Ag with codeposited Al from the electrolyte has to be considered. For the Al electrodeposition on Au(111), strong indications for alloying have been observed starting at a potential of +0.95 V vs. Al/Al³⁺ RE. Below +0.55V, 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.