Yvonne Gründer

Initiation and inhibition of dealloying of single crystalline Cu3Au (111) surfaces

Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.

Inhibited and Enhanced Nucleation of Gold Nanoparticles at the Water|1,2-Dichloroethane Interface

The deposition of gold at the interface between immiscible electrolyte solutions has been investigated using reduction of tetrachloroaurate or tetrabromoaurate in 1,2-dichloroethane, with aqueous phase hexacyanoferrate as reducing agent. In a clean environment without defects present at the interface, the Au(III) complex was reduced to the Au(I) complex, but no solid phase formation could be observed. A deposition process could only be observed through the addition of artificial nucleation sites in the form of palladium nanoparticles at the interface. This process could be associated with the reduction of the Au(I) halide complex to metallic gold, by determining the gold reduction potentials in 1,2-dichloroethane. XANES measurements indicate that tetrachloroaurate ion transfers intact into the organic phase, with the central Au atom retaining its oxidation state of +3 and the overall anion remaining charged at -1.

Reconstruction of Cu(100) electrode surfaces during hydrogen evolution.

Electrochemical hydrogen evolution on (100)-oriented copper electrodes is shown to induce a novel surface reconstruction, which substantially influences the rates of this electrochemical reaction. As revealed by in situ video-STM the formation of this phase starts with lateral displacements of Cu surface atoms from lattice positions, resulting in stripe-like structures, followed by expansion of the surface lattice along the stripe direction.

Portable chamber for the study of UHV prepared electrochemical interfaces by hard x-ray diffraction

We report on a new electrochemical cell setup, combined with a portable UHV chamber, for in situ x-ray diffraction using synchrotron radiation. In contrast to more traditional electrochemical sample preparation schemes, atomically clean and well-ordered surfaces are routinely prepared by UHV methods, even in the case of reactive elements or alloys. Samples can be transferred from larger UHV systems into the portable chamber without exposure to ambient air. They can then be studied successively in UHV, in controlled gas atmospheres, and in contact with electrolyte solutions under applied electrochemical potential. The electrochemical setup employs a droplet geometry, which guarantees good electrochemical conditions during in situ x-ray measurements combined with voltammetry. We present first experimental results of Cu deposition on GaAs(001) and on freshly produced nanometric Pd(001) islands on Cu(0.83)Pd(0.17)(001), respectively.

From ultra-high vacuum to the electrochemical interface: x-ray scattering studies of model electrocatalysts.

In-situ surface X-ray scattering (SXS) has become a powerful probe of the atomic structure at the metal-electrolyte interface. In this paper we describe an experiment in which a Pt(111) sample is prepared under ultra-high vacuum (UHV) conditions to have a p(2 x 2) oxygen layer adsorbed on the surface. The surface is then studied using SXS under UHV conditions before successive transfer to a bulk water environment and then to the electrochemical environment (0.1 M KOH solution) under an applied electrode potential. The Pt surface structure is examined in detail using crystal truncation rod (CTR) measurements under these different conditions. Finally, some suggestions for future experiments on alloy materials, using the same methodology, are proposed and discussed in relation to previous results.

High-Speed in situ Surface X-ray Diffraction Studies of the Electrochemical Dissolution of Au(001)

We present in situ X-ray surface diffraction studies of interface processes with data acquisition rates in the millisecond regime, using the electrochemical dissolution of Au(001) in Cl-containing solution as an example. This progress in time resolution permits monitoring of atomic-scale growth and etching processes at solid-liquid interfaces at technologically relevant rates. Au etching was found to proceed via a layer-by-layer mechanism in the entire active dissolution regime up to rates of ∼20 ML/s. Furthermore, we demonstrate that information on the lateral surface morphology and in-plane lattice strain during the electrochemical process can be obtained.

In situ video STM studies of the hydrogen-induced reconstruction of Cu(100): potential and pH dependence

The surface structure of Cu(100) electrodes in perchloric acid solutions of pH 1 to 3 was studied in the potential range of hydrogen evolution by video-rate scanning tunneling microscopy, focusing on the recently reported hydrogen-induced surface reconstruction [H. Matsushima et al., J. Am. Chem. Soc. 2009, 131, 10362]. Potential-dependent measurements reveal a two step formation process: at potentials close to the onset of hydrogen evolution a p(1×8) phase emerges, where Cu surface atoms in stripe-like structures are laterally and vertically displaced; at ≈30 mV more negative potentials a transition to a c(p×8) structure with an expanded Cu surface lattice occurs. Correlation of these observations with electrochemical data and studies on hydrogen interactions with Cu(100) surfaces under vacuum conditions support that these phases are induced by hydrogen in subsurface sites, pointing towards a high hydrogen coverage on this electrode surface under reaction conditions.

Simulation of Surface Resonant X-ray Diffraction

We present an ab initio numerical tool to simulate surface resonant X-ray diffraction experiments. The crystal truncation rods and the spectra around a given X-ray absorption edge are calculated at any position of the reciprocal space. Density functional theory is used to determine the resonant scattering factor of an atom within its local environment and to calculate the diffraction peak intensities for surfaces covered with a thin film or with one or several adsorbed layers. Besides the sample geometry, the collected data also depend on several parameters, such as beam polarization and incidence and exit angles. In order to account for these factors, a numerical diffractometer mimicking the experimental operation modes has been created. Finally two case studies are presented in order to compare our simulations with experimental spectra: (i) a magnetite thin film deposited on a silver substrate and (ii) an electrochemical interface consisting of bromine atoms adsorbed on copper.

Detection and Characterisation of Sub-Critical Nuclei during Reactive Pd Metal Nucleation by X-ray Absorption Spectroscopy

The interfacial reduction of aqueous [PdCl4]2− at the interface with an organic solution of ferrocene has been characterised by X-ray absorption fine structure (XAFS) spectroscopy. Use of a liquid–liquid interface as a model for homogeneous nucleation permits control of the thermodynamic driving force for nucleation, through variation of the [PdCl4]2− and ferrocene concentrations in the bulk of the adjacent phases. We demonstrate that this approach permits characterisation of the system under conditions of (i) no particle nucleation, (ii) fast spontaneous nucleation of stable nanoparticles and (iii) an intermediate state, in which formation of metastable Pd sub-critical nuclei takes place. Analysis of the XAFS spectra in the metastable state revealed a stochastically fluctuating equilibrium in which Pd nuclei are constantly formed and re-dissolved, as evident from oxidation state fluctuations detected by the Pd XAFS. Supersaturation was evidently sufficient to induce nanoparticle formation but insufficient for nuclei to grow beyond the critical cluster size. We were able to maintain a system in this metastable state for several hours. Such sub-critical clusters are predicted by classical nucleation theory, but have not been detected except in liquid-cell TEM imaging and scanning electrochemical microscopy studies.

The Adsorption and Growth of Copper on As-Terminated GaAs(001): Physical Vapour versus Electrochemical Deposition

The adsorption of copper deposited from an effusion cell in ultra high vacuum and electrodeposited from aqueous solution on As-terminated GaAs(001) is investigated by scanning tunneling microscopy, X-ray diffraction, as well as X-ray standing waves in combination with X-ray fluorescence spectroscopy. The analysis of the adsorbate is performed in situ with the samples either in ultra high vacuum or in aqueous electrolyte under potential control. Deposited as Cu ions from sulphuric acid solution, submonolayer coverages of Cu diffuse into the GaAs interface region assumimg predominantly Ga substitutional positions. Different positions with lower symmetry are occupied by Cu if submonolayer are deposited at room temperature in ultra high vacuum. At higher coverage, epitaxial Cu clusters grow by electrodeposition on the As-terminated GaAs(001), but with the Cu lattice of the islands rotated and inclined with respect to the GaAs substrate lattice, leading in total to eight equivalent domains. At comparable coverage, no epitaxial growth was observed for Cu deposited in ultra high vacuum.