H. Siegenthaler

Nanoscale Probes of the Solid — Liquid Interface

In-situ nanoscale probing of the solid-liquid interface, in particular by electrolytic STM and SFM, has gained considerable importance in electrochemistry and has given rise to novel nanoscale applications in electrochemical systems, including first attempts for electrochemical nanostructuring methods. In this report, selected examples are presented to illustrate the present state and future potential of nanoscience in electrochemistry.

Voltammetric investigation of thallium adsorption on silver single crystal electrodes

The adsorption behaviour of Tl+ on Ag(100) and Ag(111) in 0.5 M NaClO4 and Na2SO4 has been studied by voltammetric methods including flow-through thin layer voltammetry (FTTL) and single electrode thin layer voltammetry (STL). The Tl-deposits formed on both electrode substrates exhibit the ideal charge stoichiometry of metal monolayer adsorbates, indicating the absence of anion adsorption. The adsorption on Ag(100) can be characterized by an equilibrium Γ(E)-isotherm compatible with a stepwise formation of three stable sorption layers. The Tl-deposit on Ag(111) features the successive build up of two adsorbate layers. In case of incomplete formation of the first layer, however, a slow structural transformation occurs, involving a strong interaction with the Ag(111) substrate and accompanied by partial Tl-desorption. The complex behaviour of this system is interpreted in terms of internal strain within the Tl-superlattice structures.

In situ scanning tunneling microscopy at potential controlled Ag(100) substrates

Abstract Chemically polished Ag(100) electrodes with and without lead adsorbates and phase deposits were investigated in situ in 0.5 M NaClO 4 by Scanning Tunneling Microscopy (STM), using a potentiostatic STM assembly. During the potential-controlled lead adsorption and phase deposition, the tunneling mode was maintained. On the lead-free electrode surface, parts with flat terraces of about 5–20 nm width, where monoatomic steps are resolved, are interspersed by densely terraced dome-shaped domains. Lead adsorption is accompanied by displacement of steps and smoothing of sub-nm corrugations, as well as by a steeper decay of the tunneling current with tip-substrate separation. Along certain parts of the substrate, repetitive Pb phase deposition/dissolution leads to formation of pronounced nm-scale steps on the Ag substrate.

Electrochemical SPM investigation of the solid electrolyte interphase film formed on HOPG electrodes

Solid electrolyte interphase (SEI) film formation on graphite electrodes was studied on highly oriented pyrolytic graphite (HOPG) in nonaqueous electrolyte by in situ electrochemical atomic force microscopy (AFM). For potentials negative to 0.7 V versus Li|Li+ a SEI film is formed on the HOPG electrode surface. After the first cycle the film is rough and covers the surface of the HOPG electrode only partially. After the second cycle the HOPG surface is fully covered by a compact film. The thickness of the SEI film was measured by increasing the pressure of the AFM tip and thus scraping a part of the electrode surface. In this way a thickness of about 25 nm was found for the SEI film formed after two scan cycles between 3 and 0.01 V versus Li|Li+.

Slow transformation phenomena in underpotential deposits

Abstract Underpotential deposits of metals at foreign electrodes can be metastable systems undergoing slow changes of their structures and electrochemical properties. The behaviour of Tl and Pb deposits at Ag(111) is discussed.

In-situ scanning probe microscopy for the measurement of thickness changes in an electroactive polymer

Using Scanning Probes Methods (SPM), it is possible to monitor dimensional changes of polyaniline (PANI) films in the nm-range. Swelling and shrinking of the polymer during oxidation and reduction could be detected in-situ by this SPM-based technique. For a PANI film with a charge capacity of 8.67 mC/cm2 a thickness change of 26.9 ± 4.3 nm during oxidation was measured by Atomic Force Microsopy (AFM). This result is in qualitative agreement with recent in-situ Spectroscopic Ellipsometry investigations of PANI in our laboratory. The thickness changes determined with Scanning Tunneling Microscopy (STM) are significantly larger than those measured by AFM. Beside first results, intrinsic limits of this methods and the quantitative differences between STM and AFM measurements are discussed.

Electrochemical intercalation of perchlorate ions in HOPG: an SFM/LFM and XPS study

Abstract Highly oriented pyrolitic graphite (HOPG) in perchloric acid was adopted as a model system in order to elucidate the electrochemical anion intercalation process in graphite. The effects of the intercalation process were studied in terms of the changes in surface friction and on the electronic structure of the HOPG. Lateral force microscopy (LFM) combined with cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) indicated that the specific adsorption of perchlorate ions is responsible for changes in friction occurring in proximity of the steps on the HOPG surface. The friction changes reversibly within a narrow potential window preceding intercalation. After an intercalation and deintercalation cycle the change of the friction at a step is irreversible. No change in the friction could be observed on the basal plane. The binding energies in the C 1s, O 1s and Cl 2p XPS spectra of the intercalated compound are shifted relative to those of the non-intercalated host and adsorbed perchlorate ions, which is attributed to a shifted Fermi level.

Anion intercalation into highly oriented pyrolytic graphite studied by electrochemical atomic force microscopy

Abstract Knowledge of the dimensional changes occurring during electrochemical processes is fundamental for understanding of the electrochemical intercalation/insertion mechanism and for evaluation of potential application in electrochemical devices. We studied a highly oriented pyrolitic graphite (HOPG) electrode in perchloric acid, as a model to elucidate the mechanism of electrochemical anion intercalation in graphite. The aim of the work is the local and time dependent investigation of dimensional changes of the host material during electrochemical intercalation processes on the nanometer scale. We used atomic force microscopy (AFM), combined with cyclic voltammetry, as the in situ tool of analysis during intercalation and deintercalation of perchlorate anions. According to the AFM measurements, the HOPG interlayer spacing increases by 32% in agreement with the formation of stage IV of graphite intercalation compounds, when perchlorate anions intercalate. In addition, the local aspect of the process has been demonstrated by revealing coexisting regions with different kinetics for intercalation and deintercalation processes.

Non-equilibrium phenomena at the early stage of formation of lead underpotential adsorbates on electrolytically grown Ag(111) electrode surface

Abstract The phenomena of slow structural transformation in Pb underpotential adsorbate on Ag(111) surface at low coverages are quantitatively investigated.It is shown that these phenomena occur as a result of incorporation of lead atoms mainly in the crystal lattice of the terraces on the adsorbent surface and not in the growth steps as it is the case at high coverages.

Localization of individual biomolecules on sensor surfaces

Abstract Scanning probe microscopy techniques have been used to investigate biosensor surfaces. Complementarily, the surfaces have been characterized using immunoanalytical and radioanalytical methods which average over a large number of biomolecules. This combination of local probe and averaging techniques provides detailed information about the density, the homogeneity and the functionality of biomolecules immobilized on surfaces. Surface analysis at molecular resolution is especially important for the development of sensor surfaces, designed at nanometer scale.