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

As a prototypical example of a metal-electrolyte interface the adsorption of sulfate on well defined Cu(111) single crystal electrodes from a 5 mM H2SO4 solution has been studied by means of in-situ electrochemical scanning tunneling microscopy (EC-STM) and infrared reflection absorption spectroscopy (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 frequency data fail to provide information about the anion adsorption site, high-resolution STM images taken under specific tunneling conditions also clearly reveal the C-2v symmetry and the twofold bridging coordination of the sulfate adsorption complex. Analysis of the IR intensities, however, does show that the sulfate adsorption starts at a potential noticeably lower (less anodic) than the potential of first Moire formation. In this potential region the adsorbed sulfate is very mobile on the surface and difficult to image by STM. Adsorption rate and sulfate coverage are strongly dependent on the electrode potential. The slow adsorption process leading to a well ordered Moire structure is compared to a fast adsorption process by applying potential 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.