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 spec ies on Au(111) and Au(100) single crystal electrodes in acid (0.1 M H2SO4 a nd 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 a ll these cases indicates that this is a characteristic feature common to ox oanion adlayers, due to their ability to form hydrogen-bridge bonds through the lone pairs of their oxygen atoms. No ordered adsorption was observed i n neutral solutions, which indicates that the coadsorption of hydronium ion s 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 dom ains.