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The initial stage of oxidative CuI film formation on Cu(111) has been studied in an electrochemical environment by means of cyclic voltammetry (CV), in situ scanning tunneling microscopy (STM) and ex situ synchrotron X-ray photoemission spectroscopy (SXPS). Cyclic voltammetric studies indicate a significant acceleration of copper oxidation in the presence of iodide. The reason for that is the iodide-mediated stabilization of cuprous species resulting in a downward shift of the onset potential for copper oxidation. Reactive sites for the copper oxidation followed by iodide complexation are exclusively defects such as substrate step edges. It is the surface-confined supersaturation of mobile CuI species that leads to the two-dimensional (2D) CuI film formation via nucleation and growth of a Cu/I bilayer on top of the preadsorbed iodide phase. In an advanced stage of copper oxidation, however, terraces are directly transformed into the 2D CuI film at the reactive boundary between metallic copper and the grow...

The surface structure of Cu(100) modified by chloride and iodide has been studied in an electrochemical environment by means of in-situ scanning tunneling microscopy in combination with in-situ surface X-ray diffraction with a particular focus on adsorbate and potential dependent surface relaxation phenomena. For positive potentials close to the on-set of the copper dissolution reaction, the X-ray data disclose an extraordinarily large Cu-Cl bond length of 2.61 A for the c(2 x 2)-Cl phase. This finding points to a largely ionic character of the Cu-Cl interaction at the Cu(100) surface, with chloride particles likely to retain their full charge upon adsorption. Together with the positive surface charging at these high potentials, this ionic Cu-Cl bond drives the observed 2.2% outward relaxation between the first two copper layers. These results indicate that the bond between the first and the second copper layer is significantly weakened which appears as the crucial prerequisite for the high surface mobility of copper-chloride species under electrochemical annealing conditions at these high potentials. With 2.51 A the Cu-I bond is 4% shorter than the Cu-Cl bond implying that the nature of the Cu-I bond is mainly covalent. Accordingly, we observe a significant inward relaxation of the top Cu layers upon substituting chloride by iodide at the same electrode potential, which suggests that the iodide adsorption involves charge transfer from the halide to the copper substrate.

Charged organic adsorbates play an important role in a number of electrochemical reactions, e.g. as additives for metal plating relevant for device fabrication in the semiconductor industry. Fundamental investigations are mandatory in order to acquire profound knowledge of the structural and electronic properties of these layers parallel and perpendicular to the surface, and to finally achieve a deeper mechanistic understanding of the kinetics of involved charge transfer reactions taking place at these complex metal/organic/electrolyte interfaces. A key structural motif of these interfaces consists in 'paired' (inorganic)anion/(organic)cation layers that can have an enormous stability even during an ongoing charge transfer reaction. In this contribution we present and discuss a selected case study on the co-adsorption of halide anions and cationic organic molecules that exhibit a pronounced redox activity. It will be demonstrated that their phase behavior at the interface crucially depends on both their particular redox-state and the surface concentration of the halide counter ions. The subtle balance between adsorbate–adsorbate and adsorbate–substrate interaction of the poly-cationic organic layer can be carefully controlled by potential dependent anion adsorption and desorption processes through the organic layer. This process can be followed by in situ high-resolution scanning tunnelling microscopy, while additional information about the structural and chemical state of the respective phase is obtained from in situ X-ray diffraction and ex situ photoelectron spectroscopy.

The adsorption of viologens (1,1'-disubstituted-4,4'-bipyridinium molecules) on a chloride-modified copper electrode has been studied using a combination of cyclic voltammetry (CV), in-situ scanning tunneling microscopy (STM) and ex-situ photoemission spectroscopy (XPS). Two prototypes of viologens could be identified with respect to their redox behavior upon adsorption, namely those which retain (non-reactive adsorption) and those which change their redox state (reactive adsorption) upon interaction with the chloride-modified copper surface at given potential. The first class of viologens represented by 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologens, abbreviated as DBV) can be adsorbed and stabilized on this electrode surface in their di-cationic state at potentials more positive than the reduction potential of the solution species. XPS N1s core level shifts verify that the adsorbed DBV molecules on the electrode are in their oxidized di-cationic state. Electrostatic attraction between the partially solvated viologen di-cations and the anionic chloride layer is discussed as the main driving force for the DBV stabilization on the electrode surface. Analysis of the N1s and O1s core level shifts points to a non-reactive DBV adsorption leaving the DBVads2+ solvation shell partly intact. The laterally ordered DBVads2+ monolayer turns out to be hydrophilic with at least four water molecules per viologen present within this cationic organic film. The analysis of the Cl2p core level reveals that no further chloride species are present at the surface besides those which are specifically adsorbed, i.e. which are in direct contact with the metallic copper surface underneath the organic layer. The reduction of these adsorbed DBVads2+ surface species takes place only in the same potential regime where the solvated DBVaq2+ bulk solution species react and is accompanied by a pronounced structural change from the di-cationic 'cavitand'-structure to a 'stripe'-structure of chains of π-stacked DBV•+ mono-cation radicals as verified by in-situ STM. The second class of viologens represented by 1,1'-diphenyl-4,4'-bipyridinium molecules (diphenyl-viologens, abbreviated as DPV) is much more reactive upon adsorption and cannot be stabilized on the electrode surface in a di-cationic state, at least within the narrow potential window of copper. The N1s core level binding energy indicates only the presence of the corresponding mono-reduced DPVads•+ species on the surface even at potentials more positive than the redox potential of the bulk solution species. This process leads to the formation of a hydrophobic viologen monolayer with stacked polymeric chains as the characteristic structural motif. The wet electrochemical reduction of viologens is further compared with a dry reduction under UHV conditions. The latter reaction inevitably affects the di-cationic viologen species in the course of the photoemission experiment. Slow photoelectrons and secondary electrons are assumed to transform the di-cationic viologens into the corresponding radical mono-cations upon irradiation.

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