In-situ characterization of metal/electrolyte interfaces: Sulfate adsorption on Cu(111)

As a prototypical example of a metal-electrolyte interface the adsorption o f sulfate on well defined Cu(111) single crystal electrodes from a 5 mM H2S O4 solution has been studied by means of in-situ electrochemical scanning t unneling microscopy (EC-STM) and infrared reflection absorption spectroscop y (IRAS). The EC-STM images show the formation of the well-known Moire-like structure upon adsorption of sulfate on the Cu(111) surface. While IR freq uency data fail to provide information about the anion adsorption site, hig h-resolution STM images taken under specific tunneling conditions also clea rly reveal the C-2v symmetry and the twofold bridging coordination of the s ulfate adsorption complex. Analysis of the IR intensities, however, does sh ow that the sulfate adsorption starts at a potential noticeably lower (less anodic) than the potential of first Moire formation. In this potential reg ion the adsorbed sulfate is very mobile on the surface and difficult to ima ge by STM. Adsorption rate and sulfate coverage are strongly dependent on t he electrode potential. The slow adsorption process leading to a well order ed Moire structure is compared to a fast adsorption process by applying pot ential steps. Under the latter conditions no or only barely ordered sulfate adlayers are observed with STM. As shown by the IR measurements, however, the saturation coverage is hardly affected by the adsorption rate.