Unraveling Compositional Changes of the Double Layer upon Sulfate Adsorption on Au(111) by Surface Plasmon Resonance

The potential-induced sulfate adsorption on (111)-textured Au films in 0.1 M sulfuric acid was studied by surface plasmon resonance (SPR) measurements. Full SPR curves were recorded with millisecond time resolution allowing for simultaneous measurement of SPR and cyclic voltammetry data. We could quantitatively account for the observed SPR shift within a five-phase model, which includes the effects of electron depletion in the topmost electrode layer upon positive polarization. From the refractive index change in the modeled sulfate layer and the sulfate surface excess, as known from the literature, we derived an effective polarizability of αeff = 3.9 Å3 per sulfate species entering the adlayer from the bulk of the solution. Since this value comes close to that of sulfate ions in solution, no significant side processes like exchange of water between the adlayer and the solution are expected. This directly implies compression of the adlayer species upon positive polarization of the electrode and hence a negative reaction volume for the sulfate layer formation process, signaling strong electrostriction of water in the adlayer.