In situ Video-STM studies of sulfate dynamics on Au(111)

Application of Scanning Tunneling Microscopy in Electrocatalysis and Electrochemistry

Abstract

Scanning tunneling microscopy (STM) has gained increasing attention in the field of electrocatalysis due to its ability to reveal electrocatalyst surface structures down to the atomic level in either ultra-high-vacuum (UHV) or harsh electrochemical conditions. The detailed knowledge of surface structures, surface electronic structures, surface active sites as well as the interaction between surface adsorbates and electrocatalysts is highly beneficial in the study of electrocatalytic mechanisms and for the rational design of electrocatalysts. Based on this, this review will discuss the application of STM in the characterization of electrocatalyst surfaces and the investigation of electrochemical interfaces between electrocatalyst surfaces and reactants. Based on different operating conditions, UHV-STM and STM in electrochemical environments (EC-STM) are discussed separately. This review will also present emerging techniques including high-speed EC-STM, scanning noise microscopy and tip-enhanced Raman spectroscopy.

Graphic Abstract

A Supramolecular Approach to the Preparation of Nanographene Adlayers Using Water-Soluble Molecular Capsules

Polycyclic aromatic hydrocarbons (PAHs) are excellent building blocks for the creation of two-dimensional (2D) nanosheets. However, large PAHs tend to exhibit poor or no solubility in organic solvents and water. To overcome this issue, we employed water-soluble micellar capsules consisting of V-shaped amphiphilic molecules. Characteristic electrochemical behavior was observed in 0.1 m H₂ SO₄ in the presence of the water-soluble capsules containing PAHs, such as ovalene, circobiphenyl, and dicoronylene. Furthermore, under these conditions, PAHs were released from the capsules, resulting in the formation of a 2D adlayer of PAHs at the electrochemical interface. Finally, using electrochemical scanning tunneling microscopy, we demonstrate that our molecular containers based on water-soluble molecular capsules allow the facile preparation of 2D PAH adlayers in addition to structurally controlling nanostructure formation on Au surfaces.

Quantitative aspects of normalized differential reflectance spectroscopy: Pt(111) in aqueous electrolytes

A theoretical model is herein proposed to account for changes in the normalized differential reflectance, ΔR/R, of well-defined single crystal Pt(111) surfaces|aqueous electrolyte interfaces. It assumes that ΔR/R is proportional to the area of the electrode either bare or covered by neutral and/or nominally charged species and, for a specific type of site, is modulated by the applied potential, E. Correlations between the coverage of the various species and E were obtained from data reported in the literature or by coulometric analysis of linear voltammetric scans. Excellent agreement was found for the adsorption/desorption of hydrogen and that of bisulfate from acidic electrolytes both on bare, and cyanide-modified Pt(111). Also discussed are extensions of this technique in the transient mode involving the reduction of adsorbed nitric oxide, NO, on Pt(111).

Quantitative studies of adsorbate dynamics at noble metal electrodes by in situ Video-STM

The surface diffusion of adsorbates at electrochemical interfaces is studied by in situ scanning tunneling microscopy with high temporal resolution, using sulfur and methyl thiolate on c(2 × 2) Cl covered Cu(100), Ag(100), and Au(100) electrode surfaces in 0.01 M HCl solution as an example. While on Au(100) quantitative studies were not possible because of the slow dynamics and high surface defect density, on Cu(100) and Ag(100) a pronounced exponential increase of the jump rates of isolated adsorbates toward more negative potentials was found, indicating a linear decrease of the tracer diffusion barriers with potential. The potential dependence is independent of the adsorbate species, but differs for Cu(100) and Ag(100) substrates. These trends can be explained by electrostatic contributions to the diffusion barrier, caused by the interaction of the adsorbates with the field of the electrochemical double layer, if the presence of the chloride coadsorbate layer is taken into account.

In situ video-STM studies of methyl thiolate surface dynamics and self-assembly on Cu(100) electrodes

The atomic-scale surface dynamic behavior of adsorbed methyl thiolate on Cu(100) electrodes, prepared via the dissociative adsorption of dimethyl disulfide, was studied in 0.01 M HCl solution over a wide regime of coverages. Using video-rate in situ STM, we directly observed the motion of the adsorbates within the c(2 × 2) lattice of the chloride coadsorbates with high spatial and temporal resolution, revealing complex mutual interactions of the organic adsorbates as well as pronounced interactions with Cu adatoms, which significantly affect the thiolate self-assembly. Quantitative measurements of the tracer diffusion of isolated thiolates reveal a 35 meV lower diffusion barrier as compared to that of sulfide adsorbates with a linear potential dependence of 0.5 eV/V. The effective intermolecular interactions between the thiolates resemble those between adsorbed sulfide and are repulsive at the nearest-neighbor distance of a(0) within the c(2 × 2) lattice, attractive at the next-nearest-neighbor distance of √2a(0) and again repulsive at a distance of 2a(0). Thiolates at these small spacings are found to exhibit characteristic collective properties, which are significant for the self-assembly of these species: First, their mobility is greatly enhanced relative to that of isolated thiolates. Second, Cu adatoms can be transiently trapped in between the two thiolates of a metastable dimer with an intermolecular spacing of √2a(0). With increasing coverage, small, highly mobile molecular clusters and subsequently the formation of ordered adlayer domains with a c(2 × 6) structure are observed. Common structural elements of the clusters and c(2 × 6) domains are stripes of thiolate dimers, which are oriented in the [011] direction, spaced at distances of √2a(0) and of which a large fraction is occupied by Cu adatoms. The c(2 × 6) phase can be rationalized as a close-packed arrangement of these dimer stripes. Because of the self-acceleration of the thiolate mobility, the ordering and reorganization of the ordered c(2 × 6) adlayers occur orders of magnitude faster than the surface diffusion of isolated thiolates, illustrating the importance of collective effects in organic self-organization.