Following adsorption kinetics at electrolyte/metal interfaces through crystal truncation scattering: sulfur on Au(111)

Adsorption of sulfur on Au(111) surface: An extremely stable configuration

The sulfur adsorption on gold surface is a hot topic in catalysis, electrochemistry and chemical sensors. However, the multiple structures of adsorbed sulfur and sulfur-induced reconstruction in gold substrate topography are still open problems until now. Here we performed an extensively study on sulfur adsorption on Au(111) surface based on First-Principles calculation. Our results show that the sulfur adsorption with additional Au atoms is not favorable. Thus, the well-known lifting of the herringbone reconstruction of Au(111) after sulfur adsorption can't be attributed to the lifting gold atoms. More importantly, we proposed an extremely stable configuration of S-Au(111) surface characterized by (√3 × √3)R30° at 0.33 coverage, in which each S atom is chemisorbed in 3-fold coordinated sites and all the surface-Au atoms are terminated. Finally, the good agreement between our simulated STM and LEED images and experimental observations illuminates the truth that our proposed configuration is also favorable in experiment. This super stable S-adsorbed surface can be used as a starting point for the growth of two dimensional transition metal sulfides.

Molecular Nanowire Bonding to Epitaxial Single-Layer MoS2 by an On-Surface Ullmann Coupling Reaction

Lateral heterostructures consisting of 2D transition metal dichalcogenides (TMDCs) directly interfaced with molecular networks or nanowires can be used to construct new hybrid materials with interesting electronic and spintronic properties. However, chemical methods for selective and controllable bond formation between 2D materials and organic molecular networks need to be developed. As a demonstration of a self-assembled organic nanowire-TMDC system, a method to link and interconnect epitaxial single-layer MoS₂ flakes with organic molecules is demonstrated. Whereas pristine epitaxial single-layer MoS₂ has no affinity for molecular attachment, it is found that single-layer MoS₂ will selectively bind the organic molecule 2,8-dibromodibenzothiophene (DBDBT) in a surface-assisted Ullmann coupling reaction when the MoS₂ has been activated by pre-exposing it to hydrogen. Atom-resolved scanning tunneling microscopy (STM) imaging is used to analyze the bonding of the nanowires, and thereby it is revealed that selective bonding takes place on a specific S atom at the corner site between the two types of zig-zag edges available in a hexagonal single layer MoS₂ sheet. The method reported here successfully combining synthesis of epitaxial TMDCs and Ullmann coupling reactions on surfaces may open up new synthesis routes for 2D organic-TMDC hybrid materials.

Gold adatoms modulate sulfur adsorption on gold

Sulfur adsorption on Au(111) at high coverage has been studied by density functional calculations. In this case S species organize into rectangular structures containing 8 S atoms irrespective of the S source, which have been alternatively assigned to adsorbed monomeric S, adsorbed S₂, adsorbed monomeric plus S₂ species, and gold sulfide. We found that monomeric S at the high coverage organizes into S₂ species that are stabilized into the 8-S structures by Au adatoms, forming gold disulfide complexes (Au-(S₂)₄). The Au atoms could be provided by decomposition of more diluted AuS₃ containing phases, as recently proposed, and direct removal from terraces and step edges, both explaining the surface coverage of vacancy islands coexisting with the 8-S structures. The gold-disulfide complexes capture the disorder shown in the experimental STM images, explain the intrigued features of XPS, and also, give a smooth pathway to gold sulfide formation at higher temperatures. More importantly, the gold-disulfide complexes allow a unified picture of the gold-sulfur surface chemistry at high coverage for thiols and adsorbed sulfur species where the surface chemistry remains under discussion.

Sulfur-driven switching of the Ullmann coupling on Au(111)

We demonstrate a method to selectively switch the Ullmann coupling reaction of 2,8-dibromodibenzothiophene on a Au(111) support. The Ullmann coupling reaction is effective already at low temperature, but the complete inhibition of the same reaction can be achieved on Au(111) pre-exposed to H2S. The marked difference in reactivity of pretreated Au(111) is explained by the S-passivation of free Au atoms emerging from reconstruction sites. The inhibited state can be fully lifted by removing the S via hydrogen gas post-exposure.

A S-Layer Protein of Bacillus anthracis as a Building Block for Functional Protein Arrays by In Vitro Self-Assembly

S-layer proteins create a cell-surface layer architecture in both bacteria and archaea. Because S-layer proteins self-assemble into a native-like S-layer crystalline structure in vitro, they are attractive building blocks in nanotechnology. Here, the potential use of the S-layer protein EA1 from Bacillus anthracis in constructing a functional nanostructure is investigated, and apply this nanostructure in a proof-of-principle study for serological diagnosis of anthrax. EA1 is genetically fused with methyl parathion hydrolase (MPH), to degrade methyl parathion and provide a label for signal amplification. EA1 not only serves as a nanocarrier, but also as a specific antigen to capture anthrax-specific antibodies. As results, purified EA1-MPH forms a single layer of crystalline nanostructure through self-assembly. Our chimeric nanocatalyst greatly improves enzymatic stability of MPH. When applied to the detection of anthrax-specific antibodies in serum samples, the detection of our EA1-MPH nanostructure is nearly 300 times more sensitive than that of the unassembled complex. Together, it is shown that it is possible to build a functional and highly sensitive nanosensor based on S-layer protein. In conclusion, our present study should serve as a model for the development of other multifunctional nanomaterials using S-layer proteins.

Localized dealloying corrosion mediated by self-assembled monolayers used as an inhibitor system

The structure and chemistry of thiol or selenol self-assembled organic monolayers have been frequently addressed due to the unique opportunities in functionalization of materials. Such organic films can also act as effective inhibition layers to mitigate oxidation or corrosion. Cu–Au alloy substrates covered by self-assembled monolayers show a different dealloying mechanism compared to bare surfaces. The organic surface layer inhibits dealloying of noble metal alloys by a suppression of surface diffusion at lower potentials but at higher applied potentials dealloying proceeds in localized regions due to passivity breakdown. We present an in situ atomic force microscopy study of a patterned thiol layer applied on Cu–Au alloy surfaces and further explore approaches to change the local composition of the surface layers by exchange of molecules. The pattern for the in situ experiment has been applied by micro-contact printing. This allows the study of corrosion protection with its dependence on different molecule densities at different sites. Low-density thiol areas surrounding the high-density patterns are completely protected and initiation of dealloying proceeds only along the areas with the lowest inhibitor concentration. Dealloying patterns are highly influenced and controlled by molecular thiol to selenol exchange and are also affected by introducing structural defects such as scratches or polishing defects.

Structure of incommensurate gold sulfide monolayer on Au(111)

We develop an atomic-scale model for an ordered incommensurate gold sulfide (AuS) adlayer which has previously been demonstrated to exist on the Au(111) surface, following sulfur deposition and annealing to 450 K. Our model reproduces experimental scanning tunneling microscopy images. Using state-of-the-art Wannier-function-based techniques, we analyze the nature of bonding in this structure and provide an interpretation of the unusual stoichiometry of the gold sulfide layer. The proposed structure and its chemistry have implications for related S-Au interfaces, as in those involved in self-assembled monolayers of thiols on Au substrates.

Rich Coordination Chemistry of Au Adatoms in Gold Sulfide Monolayer on Au(111)

The rich chemistry of Au nanoparticles and ions has attracted tremendous interest, in part because the surfaces of bulk Au have traditionally been considered to be chemically inert. On the other hand, large-scale mass transport and the formation of vacancy islands have been observed on the Au(111) surface following the deposition of adsorbates, such as sulfur and thiols, that can interact strongly with the Au surface. In this work, we revisit the structure and chemistry of an ordered incommensurate AuS adlayer that forms on Au(111) following the deposition of sulfur and annealing to 450 K. A structural model of this AuS surface phase has not yet been determined experimentally due to the complexity of the system. Here, we use state-of-the-art density functional theory to develop an atomic-scale model that is consistent with the previously reported Au-S stoichiometry. In particular, we introduce theoretical techniques to take into account the charge transfer in an incommensurate system. Our model reproduces convincingly STM images and is remarkably robust. Bonding analysis based on Wannier functions shows that the model exhibits rich coordination chemistry corresponding to different Au oxidation states. The extraordinary stability and rich chemistry of this structure have implications for related S-Au interfaces and previously reported surface features of this system.

Revisiting the S-Au(111) interaction: static or dynamic?

The Au-S interaction is probably the most intensively studied interaction of Au surfaces with nonmetals, as, for example, it plays an important role in Au ore formation(1) and controls the structure and dynamics of thiol-based self-assembled monolayers (SAMs). Various S-induced surface structures on Au(111) were recently reported for different conditions and predominantly interpreted in terms of a static Au surface. Here, we demonstrate that the Au(111) surface exhibits a very dynamic character upon interaction with adsorbed sulfur: large-scale surface restructuring and incorporation of Au atoms into a growing 2D AuS phase were observed in situ. These results provide new insight into the Au-S surface chemistry.