The Adsorption of Sulfate on Gold(111) in Acidic Aqueous Media: Adlayer Structural Inferences from Infrared Spectroscopy and Scanning Tunneling Microscopy

The potential-dependent adsorption of sulfate on ordered Au111 from acidic aqueous electrolytes has been examined in-situ by means of infrared reflection-absorption spectroscopy IRAS and by atomic-resolution scanning microscopy STM in order to explore further the nature of the adsorbate bonding and the structural changes attending the formation of the ordered adlayer at high potentials, as observed recently by using STM ref. 8. Solution conditions that encompassed aqueous sulfuric acid, sulfuric acidsulfate electrolytes of varying pH, and dilute sulfate in excess perchloric acid were selected in order to facilitate comparisons with recent adsorbate compositional data extracted from chronocoulometric and radiotracer measurements which utilized the last type of electrolyte ref. 9. Essentially the same ordered adlayer structures were deduced by STM to form at suitably high potentials 0.8 V vs. SCE in both sulfuric acid and the mixed sulfateexcess perchloric acid media. The adlayer, which exhibits a sq. rt.3 x sq. rt.7 symmetry, involves a fractional sulfate coverage of 0.2, in accordance with chronocoulometic and radiotracer data ref. 9. The possibility that the ordered sulfate adlayer incorporates coadsorbed hydronium cations is discussed such coadsorption is suggested by the presence of additional tunneling maxima in the STM images. The IRAS data display a prominent S-0 stretching band ,vso at 1155-1220 cm-l, the potential-dependent intensity of which correlates with the sulfate surface concentrations reported in ref. 9. The appearance of this vso feature is also insensitive to the electrolyte conditions, including pH, consistent with its assignment to adsorbed sulfate, rather than to bisulfate.