Andreas Erbe

Mechanism of the potential-triggered surface transformation of germanium in acidic medium studied by ATR-IR spectroscopy

In acidic solution, germanium surfaces undergo a transformation to hydrogen-terminated surfaces at sufficiently negative electrode potentials. Herein, we used in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy coupled to electrochemical experiments to study the details of this surface transformation on Ge(111) and Ge(100) in 0.1 M HClO4. The ATR-IR data gathered during the surface transformation are consistent with an interpretation according to which an intermediate state exists of a surface with mixed termination. In the mixed termination, both H and OH are bound to the surface, which showed a Ge-H stretching mode at ∼2025-2030 cm-1. At sufficiently negative potentials, the surfaces became fully hydrogen terminated. ATR-IR spectra can be understood by assigning the peak at ∼1977-1990 cm-1 to the stretching mode of GeH1 species on Ge(111), and the peak at ∼2000-2015 cm-1 to a stretching mode of GeH2 species on Ge(100). Measurements of the linear dichroism showed the GeH1 species to be oriented predominantly upright. The transition dipole moment of the GeH2 species was oriented parallel to the surface, as expected for an antisymmetric stretching mode.

Waterborne chitosan–epoxysilane hybrid pretreatments for corrosion protection of zinc

Biopolymer-based systems are extensively studied as green alternatives for traditional polymer coatings, e.g., in corrosion protection. Chitosan-epoxysilane hybrid films are presented in this work as a chitosan-based protective system, which could, e.g., be applied in a pretreatment step. For the preparation of the chitosan-epoxysilane hybrid systems, a sol-gel procedure was applied. The function of the silane is to ensure adhesion to the substrate. On zinc substrates, homogeneous thin films with thickness of 50-70u2009nm were obtained after thermal curing. The hybrid-coated zinc substrates were characterized by infrared spectroscopy, ellipsometry, and x-ray photoelectron spectroscopy. As model corrosion experiments, linear polarization resistance was measured, and cathodic delamination of the weak polymer coating poly(vinylbutyral) (PVB) was studied using scanning Kelvin probe. Overall, chitosan-epoxysilane hybrid pretreated samples showed lower delamination rates than unmodified chitosan coatings and pure PVB. Electrochemical impedance spectroscopy confirmed a reduced ion permeability and water uptake by chitosan-epoxysilane films compared to that of a nonmodified chitosan coating. Even though the coatings are hydrophobic and contain water, they slow down cathodic delamination by limiting ion transport.

High Interfacial Charge Storage Capability of Carbonaceous Cathodes for Mg Batteries

A rechargeable Mg battery where the capacity mainly originates from reversible reactions occurring at the electrode/electrolyte interface efficiently avoids the challenge of sluggish Mg intercalation encountered in conventional Mg batteries. The interfacial reactions in a cell based on microwave-exfoliated graphite oxide (MEGO) as the cathode and all phenyl complex (APC) as electrolyte are identified by quantitative kinetics analysis as a combination of diffusion-controlled reactions involving ether solvents ( esols) and capacitive processes. During magnesiation, esols in APC electrolytes can significantly affect the electrochemical reactions and charge transfer resistances at the electrode/electrolyte interface and thus govern the charge storage properties of the MEGO cathode. In APC-tetrahydrofuran (THF) electrolyte, MEGO exhibits a reversible capacity of ∼220 mAh g-1 at 10 mA g-1, while a reversible capacity of ∼750 mAh g-1 at 10 mA g-1 was obtained in APC-1,2-dimethoxyethane (DME) electrolyte. The high capacity improvement not only points to the important role of the esols in the APC electrolytes but also presents a Mg battery with high interfacial charge storage capability as a very promising and viable competitor to the conventional intercalation-based batteries.

Gradient in defect density of ZnO nanorods grown by cathodic delamination, a corrosion process, leads to end-specific luminescence

ZnO nanorods were grown on a zinc substrate via cathodic delamination of a polymer coating, a tailored corrosion process, at room temperature. A comparison between in situ Raman spectra and post mortem cross sectional analysis by Raman spectroscopy, photoluminescence spectroscopy and scanning electron microscopy shows that in the initial stages of the synthesis, preferentially defect rich ZnO grows. At later stages, crystalline wurtzite ZnO growth dominates. The result is nanorod arrays consisting of nanorods with a large density of point defects in the ≈500 nm range near the zinc substrate, and low defect density in the regions further away from the interface. The growth, which proceeds over several hours, can be interrupted at any time. Large salt concentrations in the corrosive medium increase the growth rate, but also the amount of point defects. The resulting rods show strongly position-dependent luminescence and Raman spectra. Different luminescence can thus be selectively excited, based on the position of excitation.

Vibrational spectroscopic study of pH dependent solvation at a Ge(100)-water interface during an electrode potential triggered surface termination transition

The charge-dependent structure of interfacial water at the n-Ge(100)-aqueous perchlorate interface was studied by controlling the electrode potential. Specifically, a joint attenuated total reflection infrared spectroscopy and electrochemical experiment was used in 0.1M NaClO4 at pH ≈ 1-10. The germanium surface transformation to an H-terminated surface followed the thermodynamic Nernstian pH dependence and was observed throughout the entire pH range. A singular value decomposition-based spectra deconvolution technique coupled to a sigmoidal transition model for the potential dependence of the main components in the spectra shows the surface transformation to be a two-stage process. The first stage was observed together with the first appearance of Ge-H stretching modes in the spectra and is attributed to the formation of a mixed surface termination. This transition was reversible. The second stage occurs at potentials ≈0.1-0.3 V negative of the first one, shows a hysteresis in potential, and is attributed to the formation of a surface with maximum Ge-H coverage. During the surface transformation, the surface becomes hydrophobic, and an effective desolvation layer, a hydrophobic gap, developed with a thickness ≈1-3 Å. The largest thickness was observed near neutral pH. Interfacial water IR spectra show a loss of strongly hydrogen-bound water molecules compared to bulk water after the surface transformation, and the appearance of free, non-hydrogen bound OH groups, throughout the entire pH range. Near neutral pH at negative electrode potentials, large changes at wavenumbers below 1000 cm-1 were observed. Librational modes of water contribute to the observed changes, indicating large changes in the water structure.

Cyclodextrin inhibits zinc corrosion by destabilizing point defect formation in the oxide layer

Corrosion inhibitors are added in low concentrations to corrosive solutions for reducing the corrosion rate of a metallic material. Their mechanism of action is typically the blocking of free metal surface by adsorption, thus slowing down dissolution. This work uses electrochemical impedance spectroscopy to show the cyclic oligosaccharide β-cyclodextrin (β-CD) to inhibit corrosion of zinc in 0.1M chloride with an inhibition efficiency of up to 85%. Only a monomolecular adsorption layer of β-CD is present on the surface of the oxide covered metal, with Raman spectra of the interface proving the adsorption of the intact β-CD. Angular dependent X-ray photoelectron spectroscopy (ADXPS) and ultraviolet photoelectron spectroscopy (UPS) were used to extract a band-like diagram of the β-CD/ZnO interface, showing a large energy level shift at the interface, closely resembling the energy level alignment in an n–p junction. The energy level shift is too large to permit further electron transfer through the layer, inhibiting corrosion. Adsorption hence changes the defect density in the protecting ZnO layer. This mechanism of corrosion inhibition shows that affecting the defect chemistry of passivating films by molecular inhibitors maybe a viable strategy to control corrosion of metals.

How to probe structure, kinetics and dynamics at complex interfaces in situ and operando by optical spectroscopy

Techniques using light in a large spectral range are well suited to study interfaces. Photon-based techniques have a special strength when it comes to application in situ and operando. This article introduces some of the most commonly used experimental techniques. Absorption spectroscopy in the far-, mid-, and near-infrared (IR), in the visible (VIS) and ultraviolet (UV) is a simple method to obtain the interface equivalent to the type of information normally obtained by the respective bulk method. Also photoluminescence/fluorescence is used to study interfaces. Polarization, for example, in ellipsometry, or polarization-modulated spectroscopy can increase the interface specificity. The same could be achieved by surface enhancement, for example, in surface-enhanced Raman spectroscopy. IR/VIS sum frequency generation spectroscopy (SFG) has intrinsic surface specificity. Main working principles of these techniques are introduced, with reference to more detailed coverage elsewhere, and examples of application. Overall, the techniques are suitable to yield qualitative and quantitative information on which species are present at interfaces, in which conformation state or which defects develop in solids. Frequently, also information on orientation of species can be obtained.

Sour gas corrosion – corrosion of steels and other metallic materials in aqueous environments containing H2S

The corrosion of metals, mainly steels, in contact with H 2 S-containing solutions is a general problem especially in gas and oil production. The corrosion products are hydrogen and iron sulfides. Penetration of the hydrogen into the material leads to the typical hydrogen-related damage mechanisms. On the other hand, the sulfide corrosion products do not normally form a protective layer, as opposed to oxidic corrosion products in other situations. This contribution summarizes the state-of-the-art in the field and discusses approaches to combat sour corrosion.