Destabilization of Ag nanoislands on Ag(100) by adsorbed sulfur

Abstract

Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS 2 clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at low sulfur coverages, and thus enhancement is minimal. However, higher sulfur coverages force the population of sites adjacent to step edges, so that formation of the cluster is no longer limited by diffusion of sulfur across terraces. Sulfur exerts a much weaker effect on the rate of coarsening on Ag(100) than it does on Ag(111). This is consistent with theory, which shows that the difference between the total energy barrier for coarsening with and without sulfur is also much smaller on Ag(100) than on Ag(111). KeywordsAmes Laboratory, Materials Science and Engineering, adsorption, diffusion barriers, island structure, monolayers, nanostructured materials, silver, sulphur Disciplines Biological and Chemical Physics | Life Sciences | Materials Science and Engineering | Physical Chemistry CommentsReprinted with permission from The Journal of Physical Chemistry C 135, no. 15 (2011) Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS 2 clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at low sulfur coverages, and thus enhancement is minimal. However, higher sulfur coverages force the population of sites adjacent to step edges, so that formation of the cluster is no longer limited by diffusion of sulfur across terraces. Sulfur exerts a much weaker effect on the rate of coarsening on Ag(100) than it does on Ag(111). This is consistent with theory, which shows that the difference between the total energy barrier for coarsening with and without sulfur is also much smaller on Ag(100) than on Ag(111).

Sulfur adsorption on coinage metal(100) surfaces: propensity for metal–sulfur complex formation relative to (111) surfaces

“…[1][2][3] This relates to the broader emerging theme that metal surfaces can be dynamic or fluxional in nature (rather than static or frozen) under operating conditions during catalysis. 4,5 With regard to additiveenhanced transport on coinage metals, previous studies revealed a contrasting dramatic enhancement for Ag(111) 6 versus limited enhancement for Ag(100) 7 upon exposure to S. A similar dramatic enhancement was also observed for Cu(111). 2 While no experiments are currently available for Cu(100), we expect limited enhancement as for Ag(100).…”

“…For higher coverages, island decay was so fast (occurring in less than the acquisition time for a single STM image) that it could not be quantified by STM imaging. In experiments for Ag on Ag(100) at 300 K, 7 the decay of 10 nm 2 islands roughly 5 nm away from an extended step was monitored as a function of y S . The decay time decreases from 70 min at y S E 0.035 ML to 30 min at y S E 0.13 ML, corresponding to only a modest increase in decay rate by a factor of 2.3.…”

Cu2S3complex on Cu(111) as a candidate for mass transport enhancement

“…Thiolates adsorbed on Au(111) [8,28], and thiolates at the periphery of Au nanoclusters, form species that include linear S-M-S units [28,29]. Linear S-M-S complexes (without alkyl ligands) have also been postulated-but not observed directly-on the basis of DFT and experimental data for S/Ag(100) [14], and on the basis of DFT alone for S/Ag(111) [2]. These results suggest that the linear S-M-S unit has generic stability across coinage metals.…”

“…Recently, for instance, Parkinson et al have shown that CO interacts with Pd atoms adsorbed on a Fe 3 O 4 surface, forming a highly-mobile Pd-CO complex [3]. Other adsorbates that form mobile surface complexes with metals include hydrogen [4,5], oxygen [6,7], alkylsulfides [8], and-the subject of this study-sulfur [9][10][11][12][13][14]. The soft metals Cu, Ag, and Au, which are of great interest because of their catalytic and plasmonic properties, are expected to be particularly susceptible to this effect.…”

Communication: Structure, formation, and equilibration of ensembles of Ag-S complexes on an Ag surface

“…These complexes are generally mobile and can even facilitate metal surface dynamics and mass transport across the surface. [8][9][10][11][12][13] More specifically, these complexes can destabilize metal nanostructures and can accelerate coarsening or sintering of arrays of nanoclusters via a reactive version of Ostwald ripening. 11,14 The modern tools of surface science-particularly scanning tunneling microscopy (STM), diffraction techniques, and density functional theory (DFT)-have proven powerful enough to decipher even intricate extended structures, but direct experimental observations of isolated complexes between adsorbates and metal atoms extracted from the surface have been limited, due in part to difficulties in imaging mobile species.…”