Bromide electrosorption on the low-index faces of silver

Abstract

Bromide adsorption on the three low-index faces of silver was investigated from mixed c mol dm^–3 KBr +(0.02 –c) mol dm^–3 KPF₆ solutions over a range of c from 1 × 10^–6 to 5 × 10^–3 mol dm^–3 using the potential-step chronocoulometric technique. The adsorption behaviour on the Ag(110) face shows significant differences with respect to that on the Ag(111) and Ag(100) faces. Thus, the plot of the applied potential E vs. the specifically adsorbed charge density σ_i of the bromide ions, for σ_i equal and opposite to the charge density σ_M on the metal, shows a rather abrupt decrease in slope with a decrease in σ_i. Moreover, the plots of σ_ivs.σ_M at constant c intersect for σ_i≈–35 µC cm^–2. This behaviour is explained by a decrease in the negative value of the surface potential due to electron smoothing with an increase in |σ_i| on Ag(110). The potential difference across the inner layer, once plotted against |σ_i| at constant σ_M, attains a maximum value before shifting linearly towards more negative values with a further increase in |σ_i|; this behaviour, observed on all three low-index faces, is interpreted on the basis of a molecular model of the inner layer which accounts for the reorientation of the water dipoles and for the screening by the inhomogeneous electron gas.