Structure studies of metal electrodes by in-situ scanning tunneling microscopy

Tracking the surface structure and the influence of cations and anions on the double-layer region of a Au(111) electrode

We examined the electric double-layer (EDL) of a Au(111) electrode in a dilute perchloric acid solution using a combination of capacitance measurements and in situ scanning tunnelling microscopy under electrochemical conditions (ECSTM). The "camel-shaped" capacitance curve of the EDL is studied with different cations and anions, including their impact on the potential of zero charge (PZC). We show that the ECSTM images of thermally reconstructed and of the potential-induced surface reconstruction of Au(111) in perchloric acid electrolyte resemble previous work in sulphuric acid, displaying a herringbone pattern for a thermally reconstructed surface. Once the reconstruction is lifted, the Au(111) forms islands with an average of 1 atomic step height. When the potential is lowered below that of the PZC, the potential-induced surface reconstruction results in a more disoriented pattern than the thermally reconstructed surface. ECSTM images at different potentials are correlated with the voltammogram to understand the time and potential dependence of the surface. This correlation has led to the development of a potential window technique that can be used to reveal the surface structure of Au(111) based on observing the changes in PZC in the voltammogram. This method provides an indirect approach to understanding the surface structure without always relying on ECSTM. From the voltammogram, we also observed that anions (SO42-, CH3SO3-, ClO4-, F-) interact more strongly with the Au(111) surface than the alkali cations. The cation capacitance peak shape does not depend strongly on the identity of the alkali metal cation (Li+, Na+, K+). However, the anion capacitance peak depends strongly on the anion identity. It suggests that some level of specific adsorption cannot be excluded, even for anions that are traditionally not considered to adsorb specifically (perchlorate, fluoride).

Reactivity mapping of nanoscale defect chemistry under electrochemical reaction conditions

Electrocatalysts often show increased conversion at nanoscale chemical or topographic surface inhomogeneities, resulting in spatially heterogeneous reactivity. Identifying reacting species locally with nanometer precision during chemical conversion is one of the biggest quests in electrochemical surface science to advance (electro)catalysis and related fields. Here, we demonstrate that electrochemical tip-enhanced Raman spectroscopy can be used for combined topography and reactivity imaging of electro-active surface sites under reaction conditions. We map the electrochemical oxidation of Au nanodefects, a showcase energy conversion and corrosion reaction, with a chemical spatial sensitivity of about 10 nm. The results indicate the reversible, concurrent formation of spatially separated Au₂O₃ and Au₂O species at defect-terrace and protrusion sites on the defect, respectively. Active-site chemical nano-imaging under realistic working conditions is expected to be pivotal in a broad range of disciplines where quasi-atomistic reactivity understanding could enable strategic engineering of active sites to rationally tune (electro)chemical device properties.

Identifying reacting species locally with nanometer precision is a major challenge in electrochemical surface science. Using operando Raman nanoscopy, authors image the reversible, concurrent formation of nanometer-spatially separated Au₂O₃ and Au₂O species during Au nanodefect oxidation.

A combined UHV-STM-flow cell set-up for electrochemical/electrocatalytic studies of structurally well-defined UHV prepared model electrodes

Atomic scale structural characterization and electrochemical measurements under controlled electrolyte transport conditions are the basis for structure–activity relationships in nanostructured electrodes.

Restructuring of an Ir(210) electrode surface by potential cycling

This study addresses the electrochemical surface faceting and restructuring of Ir(210) single crystal electrodes. Cyclic voltammetry measurements and in situ scanning tunnelling microscopy are used to probe structural changes and variations in the electrochemical behaviour after potential cycling of Ir(210) in 0.1 M H2SO4. Faceted structures are obtained electrochemically as a function of time by cycling at a scanrate of 1 V·s(-1) between -0.28 and 0.70 V vs SCE, i.e., between the onset of hydrogen evolution and the surface oxidation regime. The electrochemical behaviour in sulfuric acid solution is compared with that of thermally faceted Ir(210), which shows a sharp characteristic voltammetric peak for (311) facets. Structures similar to thermally-induced faceted Ir(210) are obtained electrochemically, which typically correspond to polyoriented facets at nano-pyramids. These structures grow anisotropically in a preferred direction and reach a height of about 5 nm after 4 h of cycling. The structural changes are reflected in variations of the electrocatalytic activity towards carbon monoxide adlayer oxidation.

From single crystal surfaces to single atoms: investigating active sites in electrocatalysis

Electrocatalytic processes will undoubtedly be at the heart of energising future transportation and technology with the added importance of being able to create the necessary fuels required to do so in an environmentally friendly and cost effective manner. For this to be successful two almost mutually exclusive surface properties need to be reconciled, namely producing highly active/reactive surface sites that exhibit long term stability. This article reviews the various approaches which have been undertaken to study the elusive nature of these active sites on metal surfaces which are considered as adatoms or clusters of adatoms with low coordination number. This includes the pioneering studies at extended well defined stepped single crystal surfaces using cyclic voltammetry up to the highly sophisticated in situ electrochemical imaging techniques used to study chemically synthesised nanomaterials. By combining the information attained from single crystal surfaces, individual nanoparticles of defined size and shape, density functional theory calculations and new concepts such as mesoporous multimetallic thin films and single atom electrocatalysts new insights into the design and fabrication of materials with highly active but stable active sites can be achieved. The area of electrocatalysis is therefore not only a fascinating and exciting field in terms of realistic technological and economical benefits but also from the fundamental understanding that can be acquired by studying such an array of interesting materials.

Adsorption of Solvent Cations on Au(111) and Au(100) in Alkylimidazolium-Based Ionic Liquids – Worm-Like versus Micelle-Like Structures

By employing high resolution in-situ STM, the adsorption of alkylimidazolium-based cations of EMI + , PMI + , BMI + and OMI + on Au(111) and Au(100) surfaces are investigated systematically. The cation adsorption on both Au(111) and Au(100) are composed of double rows arising from counter-facing imidazolium-based cation pairs. On Au(100), the double rows associated with the four cations show micelle-like appearance along the two √ 2 directions of the Au(100) surface lattice units. The width of the double rows varies depending on the side chain length of the cations, but is constrained by the periodicity along the √ 2 directions. Anions of BF 4 - , PF 6 - , CF3SO 3 - and Tf2N - do not influence the micelle-like adsorption structure. On Au(111), the double rows are formed only when the terraces are etched to several atoms wide. Most likely, the underneath Au surface experiences restructuring to accommodate the double row structure, and the worm-like orientation of the double rows is the consequence of strain release. Both the micelle-like and worm-like adsorption structures would be lifted upon cathodic potential excursions when the surfaces are driven to undergo ordinary Au(100)-hex and Au(111)-(√ 3 × 22) reconstructions. These results reveal that the ordered micelle-like structure on Au(100) and the irregular worm-like structure on Au(111) are of the same nature.

The Electrochemical Development of Pt(111) Stepped Surfaces and Its Influence on Methanol Electrooxidation

The progress in the preparation of new electrode surfaces by electrochemical treatments exhibiting high faradaic efficiencies towards industrial electrocatalytic processes has gained more attention in today's scientific community. Most of the papers report new catalysts dispersed on different substrates, but some fundamental studies required for electrochemical and physical characterizations are sometimes forgotten. In this paper, we make a full staging of two electrochemical treatments that can be conducted to enhance the electrocatalytic activity of platinum surfaces, such as, electrofacetting through square wave potential programs and constant cathodic polarizations in the net hydrogen evolution region. The cathodic treatment applied at −2 V clearly develops (111) stepped planes similarly to the electrofacetting performed after applying the square wave program between 1.40 V and 0.70 V at 2.5 kHz. The X-ray diffraction patterns of both surfaces as well as on other electrofacetted platinum electrodes are obtained for comparison purposes. Moreover, the electrocatalytic activity towards methanol electrooxidation also exhibits equivalent coulombic efficiencies and 200% higher than on polycrystalline platinum as demonstrated by linear sweep voltammetry and potentiostatic current decays.