Waldfried Plieth

Additives in the electrocrystallization process

Abstract Adding organics to a plating bath is the most frequently used method to achieve a special property of a metal layer. Several methods are in use for in situ control of their activity: cyclic voltammetry and laser light scattering are described in this paper. The concept of hard and soft acids and bases is discussed in order to understand competitive adsorption. More molecular information is obtained by spectroscopic methods. As an example, some recent results in monitoring silver deposition by surface enhanced Raman spectroscopy are described.

Raman spectroscopy of titanium dioxide layers

Abstract In situ Raman spectra of electrochemically formed oxide films on Ti electrodes were measured. Below the formation potential of 10V, the spectra showed clear evidence for the formation of Rutile. At higher formation potentials new spectra, neither characteristic for rutile nor for anatase, were observed. Anatase spectra were found after emersion and drying of the oxide film and were also observed when the oxide film was formed in HNO 3 . In all examples, the film appeared to be of a crystalline nature.

Corrosion and Corrosion Protection

Corrosion is responsible for the permanent loss of economic value. Corrosion can affect all materials, but the corrosion of metals and especially iron and steel is the most outstanding example of corrosive degradation and is called electrochemical corrosion. In the case of iron and milled steel, the products of corrosion form an increasingly thicker surface layer of rust. Examples of this process were for many decades the corroding of old cars. Other intensive corrosion damage can be found in the pipe systems of all kinds of cooling devices, where copper is often the material. Water pipes consisted for a long time of lead, but are now usually of zinc coated steel. Corrosion products of lead or zinc deposit in the pipes and cause final degradation. Zinc-coated steel parts in an aggressive atmosphere show first zinc corrosion (white rust) and, finally, the zinc layer is locally destroyed and corrosion of the steel starts (red rust). Different environments cause different types of corrosion. Corrosion is not finished with the formation of solvated metal ions. The metal ions can form secondary corrosion products by reaction with components on the metal surface. One typical reaction is the formation of hydroxides and oxides by reaction with water condensed on the surface in the humid atmosphere of a natural environment. In the presence of oxygen, the metal ions can be oxidized. For example, oxygen can oxidize two-valent iron to three-valent iron. Two- and three-valent iron-oxide-hydroxides form the brown rust on the surface.

Reactive groups on polymer covered electrodes—I. Electrochemical copolymerization of thiophene-3-acetic acid with 3-methylthiophene

Cyclic voltammetry (cv) experiments at Pt electrodes were carried out to produce copoly(3-methyl-2,5-thiophenediyl-2, 5-thiophenediyl-3-acetic acid) (3a) from the monomers 3-methylthiophene (1) and thiophene-3-acetic acid (2a), and to study the mechanism of electrocopolymerization. It was found by means of esteriflcation and amidation and further chemical reactions of the copolymers thus obtained that the COOH groups are covalently bound to the copolymer structure. The charge/discharge reversibility and the overoxidation (irreversible redox process occurring when the covered electrode is scanned beyond the positive potential limit) of 3a and some of its derivatives are demonstrated by representative cyclic voltammograms. In a series of copolymerization experiments the positive potential limit (Epos) was lowered step by step from 2.20 to 1.70V (vs. s AgAgCl) below the potential for the oxidation of 2a (Emox = 2.11 V vs. s AgAgCl) and, yet, the copolymer 3a was obtained. The results of the examination are discussed with the aid of data from cv, MS, FTIR-GIR (Fourier-Transform-Infrared-Grazing Incidence Infrared) spectroscopy, 1H NMR13C NMR spectroscopy, and elementary analyses.

Electrochemical and photoelectrochemical behaviour of passivated Ti and Nb electrodes in nitric acid solutions

Abstract Ti and Nb electrodes were passivated galvanostatically in nitric acid solutions. The electrochemical and photoelectrochemical behaviour of these electrodes was studied and compared with the behaviour of similar electrodes passivated in sulphuric acid solutions under the same conditions. The flatband potential and donor concentration of the passive films were obtained from the corresponding Mott-Schottky plots. The photoelectrochemical investigations were carried out using a pulsed laser which rendered measurements with photon energies smaller than the band gap. The results showed that the behaviour of passivated titanium in nitric acid was differing from its behaviour in sulphuric acid. The behaviour of the passivated niobium was not much affected by the exchange of the two acids.

Photoetching of III/V semiconductors

The dependence of the capacitance—potential plots and of the photocurrent—potential plots of n-GaAs(100) and p-GaAs(100) interfaces in sulfuric acid on the hydrogen peroxide concentration was investigated. Simultaneously, in situ Raman spectroscopy was applied. The mechanism of the photodissolution derived from the investigation suggests an intermediate formation of an As2O3 layer parallel to the dissolution of Ga3+ ions.

Intrinsically Conducting Polymers

Conducting polymers have been synthesized before by mixing polymers with metal or graphite powders. But in the case of a new class of materials, the conductivity was an intrinsic property. Therefore, the name intrinsically conducting polymers (ICPs) was recommended. Conducting properties can be achieved by reduction of the neutral state. An example is the poly(dibutoxyphenylenevinylene). A platinum electrode coated with polymer in acetonitrile showed the transition between neutral and oxidized state at +1V versus a platinum quasi-reference electrode and the transition between neutral and reduced state at –1.8V. The reduced state has received much less attention than the oxidized state. The oxidation process is reversible as it is possible to switch between the oxidized and neutral states. Otherwise, oxidation and reduction of conducting polymers are very complex processes. Therefore, the reversibility is limited and depends on the timescale. In a periodic oxidation–reduction cycle, only quasi-stable oxidized and reduced polymers can be expected. During synthesis or in the process of oxidation the chain of the conjugated polymer is positively charged. The charging can reach a degree of up to 25–30%. Anions are intercalated into the polymer structure to compensate the positive charges of the polymer chain. Conducting polymers can be synthesized by following conventional polymer synthesis routes. One can transfer the Ziegler-Natta synthesis of polyethylene to polyacetylene. The Ziegler–Natta catalyst is solved in large excess in an inert solvent with acetylene streaming over the surface. A black film of polyacetylene is formed on the liquid surface.

Reactive groups on polymer covered electrodes-3. Electrochemical copolymerisation of nitrofunctionalized 3-substituted thiophene derivatives

Abstract Electrochemical copolymerization of 3-(2,4-dinitrostyryl) thiophene (4) and 3-(2,4,6-trinitrostyryl)-thiophene (5) with 3-methylthiophene (6) produced interesting new functionalized polythiophene layers at the surfaces of electrodes suitable for further reactions and which have potential NLO properties.

The influence of the double layer field on the absorption spectrum of adsorbed molecules

Abstract The electrostatic interaction of the double layer field with optically absorbing admolecules can cause modulation reflectance spectra owing to the Stark effect and the dissociation field effect, respectively. The spectra depend on the state of polarization of incident light, the stationary electrode potential, the pH value and other parameters. From the results it could be concluded to the admolecular orientation, the field strength in the double layer and interfacial protonation. This is illustrated for some organic substances.

Preparation of monolayers and multilayers of mixed thallium(III)-lead(IV)-oxides

Anodic depositions of mixed oxides of Tl2O3 and PbO2 from an alkaline thallium(I)-nitrate and lead(II)-nitrate electrolyte were prepared using different current densities. As can be seen from morphological examinations of the oxide surface, it is possible to achieve a homogeneous deposition. With increasing current density the lead/thallium ratio can systematically be increased. The structural analysis shows that at high current density the film exhibits an amorphous structure. Multilayers were produced by pulsed electrolysis. The reports of the literature on superlattice formation could not be confirmed under the experimental conditions applied.