IN-SITU SCANNING-TUNNELING-MICROSCOPY OBSERVATIONS OF A DISORDER-ORDER PHASE-TRANSITION IN HYDROGENSULFATE ADLAYERS ON AU(111)

In situ scanning tunnelling microscopy (STM) studies on Au(111) in H2SO4 solutions are presented which resolve an ordered structure in the adsorbate layer with a [GRAPHICS] unit cell and a coverage of 0.4 monolayers at potentials above ca. 0.75 V vs. SCE. From the absence of this structure in Na2SO4 solutions we conclude that this is a hydrogensulfate adsorbate species. The abrupt disorder-order phase transition at ca. 0.75 V vs. SCE coincides with a sharp peak in the voltammogram, underlining the influence of adlayer structural effects on the voltammogram. The phase transition proceeds in a fast and completely reversible way; ordering occurs via an island growth mechanism. The absence of strong Coulombic repulsions points to an effective discharging of the adsorbed anions. The ordered adlayer is stable up to the onset of oxidation at 1.2 V vs. SCE. Comparative studies on Au(110) and Au(100), where no ordered adlayer structures were observed, reveal a strong sensitivity to the substrate geometry. In addition the experimental results provide direct evidence for pronounced electronic effects in anion imaging by STM.