Chloride-induced morphology transformations of the Cu(110) surface in dilute HCl.

Morphological changes of a bare Cu(110) substrate in 10 mM HCl aqueous solution have been studied using cyclic voltammetry (CV), electrochemical scanning tunneling microscopy (EC-STM), and reflectance anisotropy spectroscopy (RAS). At cathodic potentials more positive than the hydrogen evolution reaction, a bare copper surface (1 × 1) structure is found by EC-STM. At anodic potentials more negative than the copper(II) dissolution reaction, a furrowed structure is found. The governing factor that rules Cu(110)-Cl interface processes is discussed as an interplay among Cl(-) adsorption/desorption, the dynamic rearrangement of the surface atoms on the substrate, and strain in order to reduce the surface energy. The information provided by EC-STM and RAS complements that of CV, supplies detailed information on the surface morphology, and correlates peaking Faraday currents to structural modifications. Furthermore, RAS and EC-STM show changes in the surface appearance in a potential range where no specific charge transfer is observed. CV indicates that the Cu(110) surface chemistry compares much better to that of amorphous Cu than to that of the more stable (100) and (111) surfaces, respectively.

Studying the onset of galvanic steel corrosionin situusing thin films: film preparation, characterization and application to pitting.

This work reports about a novel approach for investigating surface processes during the early stages of galvanic corrosion of stainless steelin situby employing ultra-thin films and synchrotron x-radiation. Characterized by x-ray techniques and voltammetry, such films, sputter deposited from austenitic steel, were found representing useful replicas of the target material. Typical for stainless steel, the surface consists of a passivation layer of Fe- and Cr-oxides, a couple of nm thick, that is depleted of Ni. Films of ≈4 nm thickness were studiedin situin an electrochemical cell under potential control (-0.6 to +0.8 V vs Ag/AgCl) during exposure to 0.1 M KCl. Material transport was recorded with better than 1/10 monolayer sensitivity by x-ray spectroscopy. Leaching of Fe was observed in the cathodic range and the therefor necessary reduction of Fe-oxide appears to be accelerated by atomic hydrogen. Except for minor leaching, reduction of Ni, while expected from Pourbaix diagram, was not observed until at a potential of about +0.8 V Cr-oxide was removed from the steel film. After couple of minutes exposure at +0.8 V, the current in the electrochemical cell revealed a rapid pitting event that was simultaneously monitored by x-ray spectroscopy. Continuous loss of Cr and Ni was observed during the induction time leading to the pitting, suggesting a causal connection with the event. Finally, a spectroscopic image of a pit was recordedex situwith 50 nm lateral and 1 nm depth resolution by soft x-ray scanning absorption microscopy at the Fe L2,3-edges by using a 80 nm film on a SiN membrane, which is further demonstrating the usefulness of thin films for corrosion studies.

Adsorbate Isotherm Analysis by Reflection Anisotropy Spectroscopy on Copper (110) in Hydrochloric Acid.

Reflectance anisotropy spectroscopy (RAS) is a powerful optical probe that works on a polarization contrast basis. It can be operated in any environment, ranging from ultrahigh vacuum to vapor phases and liquids. The measured optical anisotropies are caused by several symmetry breaking effects and are exclusively assigned to the surface for otherwise bulk isotropic materials. In this work, we present a systematic study comprising in situ RAS-transient to assess the surface thermodynamics of the chloride adsorption on Cu(110) upon systematic variations of the applied electrode potentials in comparison to cyclic voltammetry (CV). Numerical time-derivatives of the measured RAS-transients are shown to be exclusively associated with electrical currents of those electrochemical reactions, which change the properties of the electrode surface. The recorded transient line-shapes track the Frumkin type isotherm properties related to chloride coverage. Both connections are theoretically discussed. Owing to the surface and interface specificity, RAS is shown to exhibit a high surface sensitivity. In particular, processes taking place in parallel, namely, the hydrogen evolution reaction (HER) as well as the copper dissolution as Cu+ and Cu2+, do not contribute to the RAS response.

Operando spectroscopy study of the carbon dioxide electro-reduction by iron species on nitrogen-doped carbon.

The carbon-carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (-0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.