High resolution electrochemical STM: New structural results for underpotentially deposited Cu on Au(1 1 1) in acid sulfate solution

Adsorption of sulfate assists Cu monolayer underpotential deposition (upd) on Au(1 1 1) in a unique way, rendering two distinct structural stages: (i) formation of a low-density Cu phase at coverage of 2/3 ML known as the 3 × 3 R 30 ° or honeycomb phase; (ii) formation of a complete monolayer, i.e., Cu-(1 × 1) phase pseudomorphic with respect to underlying Au(1 1 1) substrate. In this paper we present new structural in situ scanning tunneling microscopy (STM) results for this system. We show and discuss the STM imaging of the copper honeycomb superstructure probed underneath the co-adsorbed 3 × 3 R 30 ° sulfate adlayer in the low-density phase. High resolution imaging during the phase transition from the low to high density copper phase unambiguously shows the existence of an ordered sulfate structure p(2 × 2) on the pseudomorphic Cu-(1 × 1) layer. The new structure is seen during the co-existence of two copper phases as well as upon completion of the Cu-(1 × 1) monolayer. While supported by earlier chronocoulometric measurements in the same system, the new structural results raise questions that need to be addressed in a future work.