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© 2017 Gold surfaces are widely used in electrochemistry while gold nanoparticles have very many uses, with both the surfaces and the particles often being protected by sulfur-bound organic ligands. The ligands not only provide chemical stability but also directly participate in many desired processes. This review considers the diversity of known atomic structures for gold–sulfur interfaces, how these structures facilitate a diversity of mechanisms in electrochemical applications, and why this is possible based on recent advances in the basic understanding of the electronic structure of gold–sulfur bonds. Believed once to be Au(I)-thiolate in character and hence distinctly different to physisorbed thiols and disulfides, chemisorbed bonds are shown to be Au(0)-thiyls instead. A wide range of in situ STM electrochemical and other data is interpreted from this perspective.

A multiscale density functional theory-quantum mechanics/molecular mechanics (DFT-QM/MM) scheme is presented, based on an efficient electrostatic coupling between the electronic density obtained from a grid-based projector augmented wave (GPAW) implementation of density functional theory and a classical potential energy function. The scheme is implemented in a general fashion and can be used with various choices for the descriptions of the QM or MM regions. Tests on H2O clusters, ranging from dimer to decamer show that no systematic energy errors are introduced by the coupling that exceeds the differences in the QM and MM descriptions. Over 1 ns of liquid water, Born-Oppenheimer QM/MM molecular dynamics (MD) are sampled combining 10 parallel simulations, showing consistent liquid water structure over the QM/MM border. The method is applied in extensive parallel MD simulations of an aqueous solution of the diplatinum [Pt2(P2O5H2)4]4- complex (PtPOP), spanning a total time period of roughly half a nanosecond. An average Pt-Pt distance deviating only 0.01 Å from experimental results, and a ground-state Pt-Pt oscillation frequency deviating by

The synthetic chemistry and spectroscopy of sulfur-protected gold surfaces and nanoparticles is analyzed, indicating that the electronic structure of the interface is Au(0)-thiyl, with Au(I)-thiolates identified as high-energy excited surface states. Density-functional theory indicates that it is the noble character of gold and nanoparticle surfaces that destabilizes Au(I)-thiolates. Bonding results from large van der Waals forces, influenced by covalent bonding induced through s-d hybridization and charge polarization effects that perturbatively mix in some Au(I)-thiolate character. A simple method for quantifying these contributions is presented, revealing that a driving force for nanoparticle growth is nobleization, minimizing Au(I)-thiolate involvement. Predictions that Brust-Schiffrin reactions involve thiolate anion intermediates are verified spectroscopically, establishing a key feature needed to understand nanoparticle growth. Mixing of preprepared Au(I) and thiolate reactants always produces Au(I)-thiolate thin films or compounds rather than monolayers. Smooth links to O, Se, Te, C, and N linker chemistry are established.

Scanning-tunneling microscopy (STM) under electrochemical control (in situ STM) in aqueous solution, combined with a priori density functional theory (DFT) image simulations at room temperature, reveals the atomic nature of the interface between Au(111) and self-assembled monolayers (SAMs) of 1-propanethiol and 1-butanethiol. Use of single-crystal gold substrates allows for both high-resolution images of the surface cell internal structure and the evaluation of pit densities on large surface terraces, while room-temperature STM image simulations facilitate discrimination between possible atomic interface structures. For both adsorbates, the high-coverage c(4 × 2) phase is identified as (3 × 2√3)-4, while the medium-coverage striped phase of 1-propanethiol SAMs is identified as (7 × √3)-4. All of these structures contain two adatom-bound adsorbates of the form RS-Au-SR (R = C nH2n+1S•) per surface unit cell. The observed pit coverages of 2.8-4.0% are much less than those typically found for methanethiol SAMs (ca. 12-20%), indicating that one of the two gold adatoms per cell in 1-propanethiol and 1-butanethiol SAMs is extracted to form a local surface vacancy rather than a coalesced surface pit. The surface vacancy appears free to diffuse within each cell on the STM time scale, with only small STM image changes associated with vacancy localization. Significantly, the c(4 × 2) phases of 1-propanethiol and 1-butanethiol SAMs give quite different STM images. 1-Butanethiol SAMs show characteristics similar to those of longer linear alkanethiols with four bright spots per cell, while the 1-propanethiol SAM displays five bright spots organized in a different pattern. These differences are rationalized by a more uniform vacancy distribution and rigid structure for 1-butanethiol SAMs, compared to the different diffusionally labile vacancy configurations and higher lateral S-C-C-C conformational flexibility found for 1-propanethiol. Also, the differences in interface structure

Self-assembled monolayers (SAMs) formed by chemisorption of a branched-chain alkanethiol, 2-methyl-1-propanethiol, on Au(111) surfaces were studied by in situ scanning tunneling microscopy (STM) under electrochemical potential control and analyzed using extensive density functional theory (DFT) calculations. The SAM forms in the unusual (8 × √3)-4 superlattice, producing a very complex STM image. Seventy possible structures were considered for the SAM, with the calculated lowest-energy configuration in fact predicting the details of the unusual observed STM image. The most stable structure involves two R-S-Au-S-R adatom-mediated motifs per surface cell, with steric-induced variations in the adsorbate alignment inducing the observed STM image contrasts. Observed pits covering 5.6 ± 0.5% of the SAM surface are consistent with this structure. These results provide the missing link from the structural motifs observed on surfaces at low coverage and on gold nanoparticles to the observed spectroscopic properties of high-coverage SAMs formed by methanethiol. However, the significant role attributed to intermolecular steric packing effects suggests a lack of generality for the adatom-mediated motif at high coverage. © 2009 American Chemical Society.

The electrochemical behavior of small metal nanoparticles is governed by Coulomb-like charging and equally spaced charge-transfer transitions. Using electrochemical gating at constant bias voltage, we show, for the first time, that individual nanoparticles can be operated as multistate switches in condensed media at room temperature, displaying distinct peak features in the tunneling current. The tunneling conductance increases with particle charge, suggesting that solvent reorganization and dielectric saturation become increasingly important. © 2007 American Chemical Society.

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