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 Au(111) from acidic aqueous electrolytes has been examined in situ by means of IR reflection-absorption spectroscopy (IRAS) and by atomic-resolution scanning tunneling 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 using STM. Solution conditions that encompassed aqueous sulfuric acid, sulfuric acid/sulfate 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 mentioned type of electrolyte. Essentially the same ordered adlayer structures were deduced by STM to form at suitably high potentials (greater-than-or-equal-to 0.8 V/SCE) in both sulfuric acid and the mixed sulfate/excess perchloric acid media. The adlayer, which exhibits a (square-root3 x square-root7) symmetry, involves a fractional sulfate coverage of 0.2, in accordance with chronocoulometic and radiotracer data. 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-O stretching band v(SO) at 1155-1220 cm-1, the potential-dependent intensity of which correlates with sulfate surface concentrations reported earlier. The appearance of this v(SO) feature is also insensitive to the electrolyte conditions, including pH, consistent with its assignment to adsorbed sulfate rather than bisulfate. The v(SO) frequency exhibits only a slight (ca. 5 cm-1) downshift upon forming the ordered adlayer, indicating that adsorbate ordering incurs no marked changes in sulfate speciation or surface bonding.