Effect of thiol self-assembled monolayers and plasma polymer films on dealloying of Cu-Au alloys

We report on the influence of chemical surface modification on the selective dissolution and dealloying of Cu3Au (111) in 0.1 M H2SO4. Cu-Au alloys serve as longstanding model alloys for dealloying and stress corrosion cracking. Employing well-defined hexadecanethiol, mixed-aminobenzenethiol, and plasma-polymerized hexamethyldisiloxane surface layers we obtain detailed atomic-scale insight in the stability of the surfaces. The initial structural evolution of the modified surfaces is tracked by in situ X-ray diffraction using synchrotron radiation. In comparison to the usual sequence of surface states on unmodified Cu3Au (111) the modified surfaces develop a thicker and more stable passive-like Au-rich film below the critical potential. In this regime we observe anodic shifts in the potentials of the surface structural transitions and the suppression of an otherwise observed island morphology. This extreme stability of the modified passive-like surfaces is further confirmed by ICP-MS coupled to a scanning micro-electrochemical flow cell and ex situ SEM. Above the critical potential, the presence of the inhibiting protective layers leads to a localized dealloying mechanism with an altered microstructure of the forming nanoporous film in the form of micro-cracks. Understanding the stability of alloy surfaces is a prerequisite for further applications, e.g. for lithography or sensors.