Olaf M. Magnussen

In situ scanning tunnelling microscopy observations of a disorder–order phase transition in hydrogensulfate adlayers on Au(111)

In situ scanning tunnelling microscopy (STM) studies on Au(111) in H2SO4 solutions are presented which resolve an ordered structure in the adsorbate layer with a ([graphic omitted]) unit cell and a coverage of 0.4 monolayers at potentials above ca. 0.75 V vs. SCE. From the absence of this structure in Na2SO4 solutions we conclude that this is a hydrogensulfate adsorbate species. The abrupt disorder-order phase transition at ca. 0.75 V vs. SCE coincides with a sharp peak in the voltammogram, underlining the influence of adlayer structural effects on the voltammogram. The phase transition proceeds in a fast and completely reversible way; ordering occurs via an island growth mechanism. The absence of strong Coulombic repulsions points to an effective discharging of the adsorbed anions. The ordered adlayer is stable up to the onset of oxidation at 1.2 V vs. SCE. Comparative studies on Au(110) and Au(100), where no ordered adlayer structures were observed, reveal a strong sensitivity to the substrate geometry. In addition the experimental results provide direct evidence for pronounced electronic effects in anion imaging by STM.

In-situ STM study of the initial stages of corrosion of Cu(100) electrodes in sulfuric and hydrochloric acid solution

Abstract An in-situ scanning tunneling microscopy (STM) study of Cu(100) electrode surfaces in sulfuric and hydrochloric acid solutions in the potential range −0.6 to −0.1 V versus Ag AgCl (KClsat) is presented, revealing the surface structure and dynamics in the double-layer region and providing detailed structural data on the initial stages of anodic Cu dissolution. After preparation by electropolishing in phosphoric acid, large, atomically flat terraces, separated by frizzy, almost randomly oriented steps are observed in H2SO4 solution in the double-layer region. Atomic-scale observations reveal a (1 × 1) surface lattice in the entire potential range, even during Cu dissolution. In HCl solution this surface morphology and atomic structure are visible only at potentials negative of −0.4 V, whereas above −0.4 V a c(2 × 2) Cl adlattice is observed, together with strong faceting of the steps along the [010] and [001] directions. At low etch rates, the dissolution of clean Cu surfaces proceeds solely by step-flow etching in both electrolytes. In H2SO4 solution the dissolution process is accompanied by strong fluctuations in the step positions and by an increase in step roughness. In HCl solution the [010]- and [001]-oriented steps are stabilized by the c(2 × 2) Cl adlayer and Cu dissolution proceeds by the subsequent removal of complete atomic rows consisting of primitive (√a × √2)R45° units of the (2 × 2) adlattice along these steps. A mechanism is proposed where the dissolution of Cu atoms occurs at two slightly different, structurally well-defined kink sites in the c(2 × 2) lattice, which rapidly travel along the step edge (i.e. along [010] or [001]) during the dissolution process. Adsorbed impurities can locally pin Cu terraces, resulting in the formation of highly anisotropic islands, peninsulas and troughs, or induce the formation of monoatomic etch pits.

An in-situ scanning tunneling microscopy study of electrochemically induced “hex” ↔ (1 × 1) transitions on Au(100) electrodes

Abstract The “hex” ↔ (1 × 1) transitions on Au(100) electrodes immersed into sulfuric acid solutions were investigated from an atomic up to a micrometer scale by in-situ scanning tunneling microscopy. Directly after immersion freshly flame annealed Au(100) crystals are completely covered by a well-ordered “hex” reconstruction, comparable to that observed in UHV. The dependence of this initial surface topography on the sample pretreatment is demonstrated. Domains of parallel running modulation rows, which are the main feature of the reconstructed phase, can have spatial extensions of several micrometers. The potential-induced lifting of the reconstruction starts at the termination of these rows at step edges and domain boundaries. It proceeds by a quasi-one-dimensional growth along the rows, the speed of which is a function of potential. Due to the ~ 25% higher density of the reconstructed surface layer, Au atoms are expelled to the surface during the transition. They form monoatomic islands of isotropic shape, which grow by a ripening process. The reverse (1 × 1)→ “hex” transition proceeds by nucleation and growth of reconstruction domains originating at step edges. A detailed analysis of the mechanisms and interactions of both transitions reveals the importance of surface defects in the kinetic behavior of the phase transition.

Mounting freestanding molecular functions onto surfaces: the platform approach.

A modular system has been developed to mount molecules upright onto metal surfaces in a well controlled geometry. The approach is based on a reactive platform (triazatriangulenium salt) with an electrophilic center. Functional molecules are attached via C-C bond formation. The distance from the surface can be varied by a spacer, and the distance of the functional units from each other by the size of the platform. Self-assembly of the parent triazaangulenium salt as well as the functionalized platforms on Au(111) surfaces results in stable, hexagonally ordered adlayers.

STRUCTURE, DISSOLUTION, AND PASSIVATION OF NI(111) ELECTRODES IN SULFURIC ACID SOLUTION: AN IN SITU STM, X-RAY SCATTERING, AND ELECTROCHEMICAL STUDY

Abstract Results of a detailed study of Ni(111) surfaces in air and in sulfuric acid solution (pH 1.0–2.7) by a combination of STM, surface X-ray scattering using synchrotron radiation, and electrochemical techniques are presented. Ni(111) samples, prepared via annealing in H2 and exposure to air at room temperature, are covered by a smooth three to four layers thick NiO(111) film with parallel (NiO[ 1 1 0 ]∣∣Ni[ 1 1 0 ]) and anti-parallel (NiO[ 1 1 0 ]∣∣Ni[ 1 10 ]) in-plane orientation. Electrochemical reduction at potentials ≤−0.40 VAg/AgCl results in the formation of a well-defined, oxide-free surface with large terraces, a low surface mobility, and a (1×1) lattice on the atomic scale. X-ray reflectivity data indicate vertical lattice expansion for the topmost Ni layer and a strongly bound sulfate or oxygen species. Active Ni dissolution commences at potentials ≥−0.25 VAg/AgCl by a step-flow mechanism, followed by the rapid formation of large three-dimensional etch pits, leading to considerable surface roughening. In situ STM observations of the passive film formation show at potentials ≥−0.10 VAg/AgCl the nucleation and growth of an initial ‘grainy’ phase, which is attributed to a Ni hydroxide, followed by a slower restructuring process. According to our combined STM and SXS data, the resulting steady-state passive film exhibits a duplex structure, with a crystalline, inner NiO(111) layer, consisting of exclusively anti-parallel oriented grains (NiO[ 1 1 0 ]∣∣Ni[ 1 10 ]) which are slightly tilted relative to the substrate lattice, and a porous, probably amorphous hydroxide phase on top. The thickness of the crystalline NiO film increases with potential by 14–17 A V−1. In addition, structural changes of the oxide film during immersion of Ni samples into the sulfuric acid solution at potentials in the passive range and after emersion from the electrolyte were observed, which indicate the slow conversion of the air-formed into the passive oxide and the (partial) reformation of the air-formed oxide, respectively.

In-situ atomic-scale studies of the mechanisms and dynamics of metal dissolution by high-speed STM

Abstract The fundamental atomic-scale processes during active metal dissolution—the local removal/addition of atoms at atomic kinks in the steps on the crystal surface—were directly studied by novel instrumental methods based on in-situ scanning tunneling microscopy (STM), using the dissolution of Cu(100) in 0.01 M HCl solution as an example. Direct observation of these rapid dynamic processes with a novel high-speed electrochemical STM (Video-STM), capable of acquiring up to 25 atomic resolution images per second, reveals that metal dissolution proceeds at a single type of structurally well-defined kinks. The reactivity of these kinks is tentatively explained by the coordination of Cu surface atoms and Cl adsorbates at these sites. The kinks nucleate predominantly at outer terraces corners and can condense into larger facets, resulting in an apparent ‘collective’ local dissolution or growth of terraces. Quantitative data on the kink dynamics were obtained by an alternative approach (TOW-STM) and show pronounced local dissolution/redeposition fluctuations at the individual kinks even at the onset of Cu dissolution with average kink propagation and reaction rates at kinks in the range 103 and 105 atoms s−1, respectively.

Effect of trace amounts of Cl− in Cu underpotential deposition on Au(111) in perchlorate solutions: an in-situ scanning tunneling microscopy study

Abstract The effect of small amounts of chloride, in the concentration range 10 −7 –10 −4 M, on adsorption behavior and adlayer structure in Cu underpotential deposition on Au(111) in HClO 4 solutions was investigated by in-situ scanning tunneling microscopy and cyclic voltammetry. Depending on the Cl − concentration and on the potential two (quasi-) hexagonal adlayer structures — a non-primitive, commensurate (2 × 2) and an incommensurate “(5 × 5)” structure — with average Cu adatom spacings of 3.3 and 3.67A, respectively, were observed. Both structures involve cooperative adsorption of Cu and Cl − , for the latter a CuCl(111)-like bilayer structure is proposed. This is the dominant structure in the major range of potential and Cl − concentration, the (2 × 2) exists only at potentials below 0.14 V versus SCE and if the Cl − concentration does not exceed a critical value. The transition between “(5 × 5)” and (2 × 2) structure proceeds by an island growth mechanism. “(5 × 5)” formation and (2 × 2) → “(5 × 5)” transition are governed by diffusion limited transport of Cl − to the electrode. The data demonstrate that qualitatively different adsorption behavior and adlayer structures may result if the interface concentration of a (co-) adsorbing species drops below a critical value.

The high-resolution diffraction beamline P08 at PETRA III

The new third-generation synchrotron radiation source PETRA III located at the Deutsches Elektronen-Synchrotron DESY in Hamburg, Germany, has been operational since the second half of 2009. PETRA III is designed to deliver hard X-ray beams with very high brilliance. As one of the first beamlines of PETRA III the high-resolution diffraction beamline P08 is fully operational. P08 is specialized in X-ray scattering and diffraction experiments on solids and liquids where extreme high resolution in reciprocal space is required. The resolving power results in the high-quality PETRA III beam and unique optical elements such as a large-offset monochromator and beryllium lens changers. A high-precision six-circle diffractometer for solid samples and a specially designed liquid diffractometer are installed in the experimental hutch. Regular users have been accepted since summer 2010.

Potential-Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular-Scale In Situ Video-STM Study†

Room-temperature ionic liquids are of great current interest for electrochemical applications in material and energy science. Essential for understanding the electrochemical reactivity of these systems are detailed data on the structure and dynamics of the interfaces between these compounds and metal electrodes, which distinctly differ from those in traditional electrolytes. In situ studies are presented of Au(111) electrodes in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSA]) by high-speed scanning tunneling microscopy (video-STM). [BMP][TFSA] is one of the best-understood air and water stable ionic liquids. The measurements provide direct insights into the potential-dependent molecular arrangement and surface dynamics of adsorbed [BMP](+) cations in the innermost layer on the negatively charged Au electrode surface. In particular, two distinct subsequent transitions in the adlayer structure and lateral mobility are observed with decreasing potential.

The structure and phase behavior of electrodeposited halides on single-crystal metal surfaces

Abstract Synchrotron X-ray scattering results of halide monolayers of bromide and iodide on single-crystal electrodes are presented. Both commensurate and incommensurate structures are observed. The incommensurate structures electrocompress with increasing potential. The relative roles of the halide—halide and the substrate—halide interactions are discussed for iodide on Au(111), Ag(111), and Pt(111) and for bromide on Au(111), Ag(111) and Au(100).