A. Neckel

Structural and electronic transitions in polyaniline: A Fourier transform infrared spectroscopic study

Fourier transform infrared (FTIR) spectroscopic studies are carried out on different, potentiometric well defined oxidation states of polyaniline in aqueous acidic and organic electrolytes. During the oxidation process ring structures transform from benzenoid into quinoid states. The fully reduced state of polyaniline shows differences in the anion contents in acidic and organic electrolytes. The 400 mV vs saturated calomel electrode (SCE) oxidized state has the maximum number of the intercalated anions in aqueous acidic media in accordance with supporting potentiometric titration experiments. This conducting form of polyaniline shows similar FTIR spectra in organic as well as in acidic media. For the oxidized state at 800 mV vs SCE, a deintercalation of anions in aqueous acidic, or further intercalation in organic electrolyte is observed. Beyond 800 mV vs SCE, polyaniline shows degradation processes in aqueous acidic media which are found to proceed via formation of benzoquinone‐like structures and final...

Protonation and electrochemical redox doping processes ofpolyaniline in aqueous solutions: Investigations using insitu FTIR-ATR spectroscopy and a new dopingsystem

Polyaniline (PANI), one of the most interesting members of the group of conducting polymers, has been the subject of intense experimental and theoretical studies because of its outstanding behaviour during electrochemical redox processes and during changes of pH. In this work the electrochemical doping and protonation processes of PANI are described by using in situ FTIR-ATR and a new doping system NaReO 4 /HReO 4 . Cyclic voltammetry shows that above ca. pH 4 only one wave response can be obtained. With increasing acidity of the electrolyte, a two-wave response develops, the splitting of the waves occurring at pH ca. 3.5–4. By in situ measurements during the base–acid transition process, a distinct picture of protonation of different forms of PANI has been obtained. A scheme for the dopant concentration, as a function of the electrode potential and pH, is presented. The results show that the amount of anions involved in the doping and dedoping processes depends on the pH and the potential. Maximum doping occurs at the intermediate oxidized metallic state. During both processes the electronic structure of the polymer and also the number of protons bound to nitrogen atoms are altered. The mechanisms of the changes under different redox and acid–base conditions are described.

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Abstract Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC 0 75 ) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E γ , thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3 d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E γ , observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.

Analysis of various doping mechanisms in polyaniline by optical, FTIR and Raman spectroscopy

The doping mechanisms of Polyaniline were investigated by various ex situ and in situ spectroscopic techniques. For electrochemical doping two different processes were identified during the oxidation-reduction cycle in an acidic medium. The first occurs via conversion of partly protonated polyeucoemeraldine into the salt form of polyemeraldine resulting in the highly conductive polaron lattice state. It is accompanied by the generation of free carrier absorption indicating a metallic phase in the system. Effective medium dielectric response theories were found to be in good agreement with the observed absorption. The second oxidation process converts the polaron lattice form into polypernigraniline. Simultaneously the free carrier absorption completely vanishes. Both processes are reversible during the electrochemical oxidation and reduction cycle. A bleaching of the free carrier absorption is also observed for the acid-base transition by increasing the pH-value of the electrolyte which supports the interpretation of the free carrier model. The specific IR absorption of the counter ions was found to be much lower in the polaron lattice state as compared to the fully reduced and fully oxidized state. From the analysis of the pH-dependence of the vibrational spectra the transition from the polaron lattice to the unrelaxed benzoid-quinoid form of the ring structure is proved.

The effect of vacancies on the electronic structure of Nbo

Abstract The crystal structure of NbO is considered as a NaCl structure with 25% ordered vacancies in each sublattice. Self-consistent energy band structure calculations using the augmented plane wave (APW) method have been carried out for a hypothetical NbO with the full NaCl structure and for the structure with the vacancies. We find that the introduction of the vacancies leads to (1) a lowering of the Nb- d like bands with respect to the O- p bands, (2) an almost completely filled “vacancy band”, and (3) a splitting and broadening of the O- p like band. These effects are shown to have a significant influence on both the theoretically calculated X-ray photoemission spectra (XPS) and the X-ray emission spectra (XES). All theoretical spectra obtained with the vacancy-structure are found to be in good agreement with experiments in contrast to those spectra calculated for hypothetical NbO in the NaCl structure.

In situ FTIR measurements of polyaniline in the non-metallic phase

Abstract This paper presents results of in situ FTIR measurements during cyclovoltammetric experiments of PANI in pH=4−5 electrolyte. Between −0.2 and +0.8 V vs. SCE only one oxidation-reduction peak is observed. Infrared spectra were recorded simultaneously during the potential sweep. A broad background absorption ⩾ 0.8 eV can be correlated with the existence of acidic centers (bipolarons) in the oxidized form. The absence of a free carrier absorption at 0.5 eV indicates that at this pH-value no metallic state of PANI exists during this redox process. Spectral changes in the region of vibrational absorption show the conversion of benzenoid aromatic rings into quinoid rings during the oxidation and the reverse behaviour during reduction. The salt-base transitions of different oxidation states are also observed with ex situ FTIR spectroscopy and the results are compared with the in situ experiments.

Zur Entmischung von kubischen Mehrstoffcarbiden

ZusammenfassungDie Mischungslücke in den Systemen: TiC−{ZrC, HfC} und VC-{NbC, TaC} wird im Bereich von 1200° C bis zur Temperatur des kritischen Punktes experimentell ermittelt und mit Hilfe der freien Exzeß-Enthalpie von der Form: ΔGE=x(1−x) {(a0+a1T)+(2x−1)(b0+b1T)} berechnet.AbstractThe miscibility gap within the systems: TiC−{ZrC, HfC} and VC—{NbC, TaC} has been experimentally determined in the region from 1200°C up to the temperature of the critical point. A calculation of the binodal curve has been carried out by means of an excess free enthalpy of the form: ΔGE=x(1−x){(a0+a1T)+(2x−1)(b0+b1T)}.

Third generation of conducting polymers: Spectroelectrochemical investigations on viologen functionalized poly (3‐alkylthiophenes)

Results of in situ electron spin resonance, optical absorption, and Fourier transform infrared spectroscopic studies during electrochemical doping and undoping processes of viologen functionalized poly (3‐alkylthiophenes) are reported. Effects of the functional group on the electronic properties of the polythiophene backbone in its conducting state are observed resulting in localization of the charge carriers. Radicals on the viologen side group show hyperfine structured electron spin resonance signals indicating a rapid motional averaging of anisotropic electron‐nuclear couplings. Experimental results are discussed by comparison with other substituted polythiophenes in terms of conformational effects of the acceptor group on the conducting polymer backbone. A structural model with counterion complexation between polythiophene backbone and the viologen functional unit is presented.

Massenspektrometrische Bestimmung thermodynamischer Aktivitäten in Dreistoffsystemen

ZusammenfassungDas für Zweistoffsysteme angegebene Auswerteverfahren zur Ermittlung der freien Zusatzenthalpie ΔGE und der chemischen Zusatzpotentiale μiE aus massenspektrometrischen Daten wird auf Dreistoffsysteme erweitert.AbstractA method for the evaluation of the excessGibbs free energy ΔGE and the excess chemical potentials μiE from mass spectrometric data, previously derived for binary systems, is applied to ternary systems.

Vacancy induced changes in the electronic structure of titanium nitride

Abstract A self-consistent APW band structure calculation has been performed for TiN0.75, assuming long-range ordered vacancies at the nonmetal lattice sites. Two different kinds of titanium atoms occur in this model: Ti[6] atoms that are octahedrally surrounded by six nitrogen atoms and Ti[4] atoms that have only four nitrogen neighbors and are adjacent to two vacancies. The model structure can be described as Ti[4]3Ti[6]N3□N, where □N denotes a nitrogen vacancy. In the densities of states, two sharp vacancy peaks have been found which are not present in stoichiometric TiN. The bonding situation is discussed by means of electron density plots. It is found that the chemical bonding is characteristically influenced by the introduction of vacancies. The calculated XPS and K XES are shown to be in good agreement with the experimental spectra.