The adsorption of sulfate and phosphate on Au(111) and Au(100) electrodes: an in situ STM study

We have studied by in situ STM the adsorption of sulfate and phosphate species on Au(111) and Au(100) single crystal electrodes in acid (0.1 M H2SO4 and 0.1 M H3PO4) and neutral (0.1 M Na2SO4 and 0.1 M KH2PO4 + 0.1 M K2HPO4) solutions. The well-known (bi)sulfate structures on Au(111) and Au(100) in 0.1 M H2SO4 were compared with a new one found for phosphate species on Au(100) in 0.1 M H3PO4. The presence of non-uniform anion-anion distances in all these cases indicates that this is a characteristic feature common to oxoanion adlayers, due to their ability to form hydrogen-bridge bonds through the lone pairs of their oxygen atoms. No ordered adsorption was observed in neutral solutions, which indicates that the coadsorption of hydronium ions is necessary to stabilise the ordered oxoanion adlattices. The absence of current spikes in the cyclic voltammograms of Au(100) in 0.1 M H2SO4 and 0.1 M H3PO4, indicative of the formation of ordered adlayers, was explained by the fact that in these cases the adlayers are composed of many small domains.