Sulfide Surface Dynamics on Cu(100) and Ag(100) Electrodes in the Presence of c(2×2) Halide Adlayers

Abstract

The influence of coadsorbed anions on electrode surface dynamics is studied in situ by direct observation of single adsorbed species via high-speed scanning tunneling microscopy. Using well-defined model systems, sulfide adsorbates on Cu(100) and Ag(100) covered by c(2 2) ordered adlayers of chloride or bromide, the influence of halide coadsorbate and the metal substrate on the dynamic behavior is systematically evaluated and quantitatively analyzed. The sulfide tracer diffusion coefficients exhibit a pronounced exponential potential dependence in all systems, caused by the effect of the double layer's electric field on the diffusion barrier. The sulfide mobility in general decreases with potential, with the surprising exemption of the Br-covered Cu(100) surface where it increases, indicating a different transport mechanism. In contrast to this strong influence of the halide species on the surface diffusion, the sulfide-sulfide interactions seem to be less affected by the nature of the coadsorbate, but depend more strongly on the type of metal substrate. In addition, the dynamic behavior of clusters on the surface, the formation of grooves, caused by adsorbate-induced vacancy generation, and the adlayer structure and dynamics at high sulfide coverage is discussed for bromide-covered surfaces. The data indicate a tendency of bromide to enhance the transfer of sulfide adsorbates to sites within the metal surface layer.

Atomic‐Scale Insights into Electrode Surface Dynamics by High‐Speed Scanning Probe Microscopy

“…At larger local CH 3 S ad coverages, these clusters evolve into islands of a two‐dimensional CH 3 S ad adlayer phase with a c (2×6) structure, which may be viewed as a close‐packed arrangement of CH 3 S ad dimer stripes (Figure b). This structure was also found for alkane thiolates on Cu(100) under UHV conditions, and for adsorbed sulfur on Cu(100) and Ag(100) in an electrochemical environment . As mentioned in Section 3.2, the c (2×6) structure can be rationalised by specific adsorbate–adsorbate interactions in these systems, specifically repulsive interactions at spacings of 2 a 0 .…”

“…[17] Recent Video-STM studies, in which,i nstead of chloride, a c(2 2) adlayero fb romide covered the metal electrode, indeed provided clear evidencefor amajor role of these co-adsorbates. [24,28] In particular, for S ad diffusion on c(2 2)-Br ad -covered Cu(100), an increase in mobility toward more positive potentials was found;t hus indicating ap ositive Dm (Figure 3c). This inverted potential dependence (compared with the Cl ad -coveredC us urface) can lead to orders of magnitude different diffusionr ates and is rather unexpected.…”

“…Recent Video‐STM studies, in which, instead of chloride, a c (2×2) adlayer of bromide covered the metal electrode, indeed provided clear evidence for a major role of these co‐adsorbates . In particular, for S ad diffusion on c (2×2)‐Br ad ‐covered Cu(100), an increase in mobility toward more positive potentials was found; thus indicating a positive Δ μ (Figure c).…”

“…Not all adsorbate systems exhibit the same anion effects, as observed by comparing S ad diffusion on Cl ad ‐ and Br ad ‐covered Ag(100) (see Figure c). Here, a rather similar d E d /d φ dependence was found for the two co‐adsorbed anion species . Still, the corresponding dipole moment changes differed by about 40 %.…”

“…Here, ar ather similar dE d /df dependence was found for the two co-adsorbed anion species. [24] Still, the corresponding dipole moment changes differed by about 40 %. The differencei nd iffusion behaviour on Cu and Ag may originate from the considerably larger Ag lattice spacing, which leads to al ower packing density in the c(2 2) adlayer.…”