Surface diffusion of silver at the silver (111)/liquid-water interface from electrocrystallization measurements

Abstract

The authors observed rhythmic Ag cluster nucleation that was coupled to surface mass transport, and we used this phenomenon to measure D[sub s] for silver adatoms. Low coverages ([theta] [approx] 0.002) of Ag(ad) were generated on a surface that was initially adatom-free by reduction of Ag[sup +](aq) at sparse, redox-active sites. The adatoms diffused from the generation sites across step-free Ag(111) surfaces of known geometry. Diffusing adatoms were trapped at redox-silent sites that were potent catalysts for the nucleation of 2D clusters. The first supercritical cluster to form at a nucleation site swept rapidly over the Ag(111) surface by catalytic reduction of Ag[sup +](aq) at the reactive step edge. The expansion of the monatomic step from the nucleation site yielded a brief charge pulse, a fresh, adatom-free surface, and renewal of the cycle. Regular trains of charge pulses were observed in steady-state experiments. Analysis yielded the locations of the nucleation sites and the following values of D[sub s]: (1.4 [+-] 0.2) [times] 10[sup [minus]6] cm[sup 2]s at 20.0[degrees]C, (3.3 [+-] 1.0) [times] 10[sup [minus]6] cm[sup 2]/s at 35.0[degrees]C, and (4.0 [+-] 0.9) [times] 10[sup [minus]6] cm[sup 2]/s at 50.0[degrees]C ([+-]2[sigma]). These are the same as values predicted by molecular dynamicsmore » simulation for Ag(ad) diffusion on Ag(111) in ultrahigh vacuum, implying that there is little net effect of the interfacial water on Ag(ad) diffusion rates. The mean residence time for Ag(ad) species at the Ag(111)/water interface is > 4 s at these temperatures, corresponding to a standard exchange rate I[degrees] < 40 nA/cm[sup 2] between Ag(ad) and Ag[sup +](aq). In extreme contrast, the authors measure I[degrees] [approx] 1,300 A/cm[sup 2], more than 10 orders of magnitude larger, for the same exchange reaction at the edge of an atomic step on Ag(111). 96 refs., 8 figs.« less