CORROSION OF ALKANETHIOL-COVERED CU(100) SURFACES IN HYDROCHLORIC-ACID SOLUTION STUDIED BY IN-SITU SCANNING-TUNNELING-MICROSCOPY

The surface structure and corrosion of alkanethiol-covered Cu(100) sur faces in 1 mM HCl was studied by in-situ scanning tunneling microscopy (STM) and complementary electrochemical measurements. The octanethiol (OT) and hexadecanethiol (HDT) monolayers were prepared by spontaneou s adsorption from ethanolic solution onto an electropolished single cr ystal substrate and then immersed into 1 mM HCl solution at potentials between -0.16 and -0.26 V vs Ag/AgCl (KCl sat.). Samples prepared in this way exhibit a well-defined surface morphology, where atomically s mooth Cu terraces, which are found also on thiol-free Cu(100), are cov ered by monoatomically high Cu islands and by pits. Keeping the potent ial in this regime causes slow Cu roughening via the formation of addi tional Cu monolayer islands and pits. This surface restructuring is pr obably caused by exchange of Cu or Cu-thiolates with the electrolyte. The onset of Cu corrosion is shifted anodically to potentials in the r ange -0.12 to -0.10 V, reflecting the inhibition by the thiol layer. C opper corrosion proceeds via the formation of many small etch pits and pronounced surface roughening. At higher potentials and/or after prol onged etch times only few larger pits are observed. Local Cu corrosion within these pits proceeds via a similar mechanism as on bare Cu(100) , suggesting that the inhibiting thiol layer is completely removed at these places.