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

The Journal of Chemical Physics

et al. 2015

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Arich menagerie of structures is identified at 5Kfollowing adsorption of lowcoverages (≤0.05 monolayers) of S on Cu(111) at room temperature. This paper emphasizes the reconstructions at the steps. The A-type closepacked step has 1 row of S atoms along its lower edge, where S atoms occupy alternating pseudo-fourfoldhollow (p4fh) sites. Additionally, there are 2 rows of S atoms of equal density on the upper edge, bridging a row of extra Cu atoms, together creating an extended chain. The B-type close-packed step exhibits an even more complex reconstruction, in which triangle-shaped groups of Cu atoms shift out of their original sites and form a base for S adsorption at (mostly) 4fh sites. We propose a mechanism by which these triangles could generate Cu-S complexes and short chains like those observed on the terraces. PHYSICS 142, 194711 (2015) Reconstruction of steps on the Cu(111) surface induced by sulfur A rich menagerie of structures is identified at 5 K following adsorption of low coverages (≤0.05 monolayers) of S on Cu(111) at room temperature. This paper emphasizes the reconstructions at the steps. The A-type close-packed step has 1 row of S atoms along its lower edge, where S atoms occupy alternating pseudo-fourfold-hollow (p4fh) sites. Additionally, there are 2 rows of S atoms of equal density on the upper edge, bridging a row of extra Cu atoms, together creating an extended chain. The B-type close-packed step exhibits an even more complex reconstruction, in which triangle-shaped groups of Cu atoms shift out of their original sites and form a base for S adsorption at (mostly) 4fh sites. We propose a mechanism by which these triangles could generate Cu-S complexes and short chains like those observed on the terraces. C 2015 AIP Publishing LLC.[http://dx

“…30 At very low θ S and at 5 K, S-Ag complexes existed on Ag(111) terraces, both in the form of individual units and concatenated chains. The steps were modified by S adsorption.…”

Section: Discussionmentioning

confidence: 99%

Reconstruction of steps on the Cu(111) surface induced by sulfur

Walen

The Journal of Chemical Physics

et al. 2015

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“…All STM imaging was done at 5 K in vacuum, at a pressure lower than 2.5 x 10 -11 Torr [17]. Assessment of the sulfur coverage, θ S , [the ratio of S atoms to Cu atoms in the (111) plane] was guided by the prior observation that a honeycomb-like reconstruction first appears at θ S ~ 0.05 [18,19].…”

Section: Experimental and Computational Detailsmentioning

confidence: 99%

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

Walen

et al. 2015

Phys. Rev. B

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Sulfur-metal complexes, containing only a few atoms, can open new, highly efficient pathways for transport of metal atoms on surfaces. For example, they can accelerate changes in the shape and size of morphological features, such as nanoparticles, over time. In this study, we perform STM under conditions that are designed to specifically isolate such complexes. We find a new, unexpected S-Cu complex on the Cu(111) surface, which we identify as Cu 2 S 3 . We propose that Cu 2 S 3 enhances mass transport in this system, which contradicts a previous proposal based on Cu 3 S 3 . We analyze bonding within these Cu-S complexes, identifying a new principle for stabilization of sulfur complexes on coinage metal surfaces. Introduction.It has been proposed that metal-adsorbate complexes can greatly accelerate rearrangements of metal nanostructures and surfaces. This issue is of importance for stability of catalysts or nanostructures, and has been the subject of prolonged speculation given that the complexity of such systems typically precludes definitive analysis [1,2]. Nonetheless, evidence continues to accumulate supporting the presence of mobile complexes on surfaces and, by implication, their role in metal transport. 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.The challenge in identifying such complexes is their high mobility, plus their potential condensation into extended ordered structures at moderate to high coverage. Together, these considerations mean that conditions of low temperature and low coverage offer the best chance for isolating and observing such species. The present work is a search for S-Cu complexes under these conditions. Previously, Feibelman [9] proposed that a Cu 3 S 3 complex can enhance metal transport on Cu(111), not because of high mobility (relative to metal adatoms), but rather because of high population (reflecting high stability), combined with moderate mobility (cf. Ref. [1]). The stability of the cluster was attributed to the fact that S atoms could adsorb at pseudo-4-fold-

“…Elsewhere, we have reported that (near-)linear S-M-S units are structural motifs of complexes that form between S and coinage metals M on the Cu(111), Ag(111), and Au(100) surfaces. 16,18,20,51,52 In the case of CuS 2 complexes, we showed that the linear geometry was favored because it maximized the overlap between S porbitals and Cu-d z 2 orbitals. 20 The linearity of X-M-X units may provide an explanation for the different relative stabilities of R-A2 for O and S. In the double-row structures, there is an interaction ∆ between adsorbates, based on the fact that µ O of the double-row is always different than the average of the constituent single-row structures.…”

Section: Discussionmentioning

confidence: 80%

Oxygen and sulfur adsorption on vicinal surfaces of copper and silver: Preferred adsorption sites

The Journal of Chemical Physics

Thiel

2018

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We present an extensive density functional theory (DFT) study of adsorption site energetics for oxygen and sulfur adsorbed on two vicinal surfaces of Cu and Ag, with the goal of identifying the most stable adsorption site(s), identifying trends and common themes, and comparing with experimental work in the literature where possible. We also present benchmark calculations for adsorption on the flat (111) and (100) surfaces. The first vicinal surface is the (211), and results are similar for both metals. We find that the step-doubling reconstruction is favored with both adsorbates and is driven by the creation of a special stable fourfold hollow (4fh) site at the reconstructed step. Zig-zag chain structures consisting of X-M-X units (X = chalcogen, M = metal) at the step edge are considered, in which the special 4fh site is partially occupied. The zig-zag configuration is energetically competitive for oxygen but not sulfur. DFT results for oxygen agree with experiment in terms of the stability of the reconstruction, but contradict the original site assignment. The second vicinal surface is the (410), where again results are similar for both metals. For oxygen, DFT predicts that step sites are filled preferentially even at lowest coverage, followed by terrace sites, consistent with the experiment. For sulfur, in contrast, DFT predicts that terrace sites fill first. Oxygen forms O-M-O rows on the top edge of the step, where it occupies incomplete 4fh sites. This resolves an experimental ambiguity in the site assignment. For both the (211) and (410) surfaces, the interaction energy that stabilizes the X-M-X chain or row correlates with the linearity of the X-M-X unit, which may explain key differences between oxygen and sulfur.