HREELS and Auger studies of conducting polymers (original) (raw)
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Journal of the Brazilian Chemical Society, 2000
Efeitos de morfologia, espessura, oxidação e iluminação dos filmes de polipirrol dopados com dodecilsulfato podem ser observados qualitativamente por EIS e consistem de variações de valores de resistências e capacitâncias de um circuito equivalente proposto, presentes nas interfaces e no interior do material. O circuito se ajusta bem para filmes sintetizados com densidade de carga de 190 mC cm -2 . Para filmes mais finos os valores calculados por espectroscopia de impedância eletroquímica (EIS) apresentam desvios provavelmente pela ausência de efeitos difusivos. A oxidação dos filmes causa uma diminuição geral da impedância em toda a faixa de freqüências medidas. Os efeitos de morfologia também são observados em todas as freqüências. Os efeitos de iluminação são reversíveis e aparecem, como esperado, apenas quando o polímero está na forma reduzida. A iluminação diminui a resistência interna e a capacitância de carga espacial e aumenta a resistência de transferência de carga e a capacitância de dupla camada.
Impedance spectroscopy of undoped, doped and overoxidized polypyrrole films
Synthetic metals, 1997
The frequency response of polypyrrole thin films was studied in the presence of NO3-, F-, CI-and Br-in the potential region of interest for the study of the insulator-conductor transition. The impedance spectra have been interpreted using the Randles equivalent circuit modified with a constant phase element (CPE), using nonlinear least-squares fitting of the parameters of the circuit to experimental data. The results show that the distribution of relaxation times widens with oxidation of the polymer, indicating the presence of domains with different electrical properties, consistent with hopping of charge carriers in the oxidized state. It is also shown that in F-solution and at moderate potentials, overoxidation leads to irreversible degradation of the electroactive properties, affecting the electronic conductivity and diffusion coefficients of ionic species within the film.
Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole
Australian Journal of Chemistry, 1963
The pyrolysis of tetraiodopyrrole in an inert atmosphere at temperatures from 120-700� yields black, infusible, amorphous polymers insoluble in solvents. Depending on the pyrolysis temperature, iodine may be present in the polymers as iodine of substitution and as chemisorbed molecular iodine, which is very tenaciously held. As a first approximation, the structure may be regarded as a three-dimensional network of pyrrole rings cross-linked in a nonplanar fashion by direct carbon to carbon linkages. The secondary nitrogen atoms form a hydroquine type system which may be oxidized by iodine or molecular oxygen under alkaline conditions. The extent of oxidation depends on the hydroxyl ion concentration. The nonplanarity of the oxidized quinonoid system renders it unstable but stability is enhanced, as in the triphenylmethane dyestuffs, by the formation of a carbinol. Despite their nonplanarity polypyrroles are relatively good conductors of electricity. The resistivity ranges from 1-200 ...
Infrared investigations of pristine, doped and partially doped polypyrrole
Synthetic Metals, 1992
It is well known that pristine (i.e. undoped) polypyrrole is not air-stable. As a result, it is difficult to obtain spectral data for the pristine polymer. Indeed, there are only two reports of infrared (IR) absorption spectra for the pristine material in the literature and, in both cases, the polymer seems to have undergone some air oxidation. It would be highly desirable to have reliable and reproducible IR spectra for the pristine form of this important polymer. We have designed a novel IR spectroelectrochemical cell which allows for electrochemical synthesis and IR and electrochemical analyses of thin polypyrrole films, while rigorously protecting these films from air. We present the IR spectra obtained using this cell for doped, partially doped and pristine polypyrrole.
UV-Visible Spectroelectrochemistry of Conducting Polymers. Energy Linked to Conformational Changes
Langmuir, 1999
Large and reverse hypsochromic and bathochromic shifts on either polaronic (1.29 eV) or bipolaronic (0.68 eV) bands have been determinated from electrochromic polypyrrole films by "in-situ" spectroelectrochemical measurements in the visible region during oxidation/reduction switching. Whatever the rate of the electrochemical reaction, or the electrochemical method used, the energy of the maxima changes linearly with the number of positive charges stored in the chains per monomeric unit. Any possible solvatochromic, ionochromic, or thermochromic effects, related to water or counterion interchange or to thermal heating by the Joule effect, have been experimentally studied and discarded as the origin of this great shift. Only the conformational changes of the polymeric chains during the electrochemically induced swelling/shrinking processes seem to be responsible for this new electrochemical way to store and release molecular energy in a reverse way, observed in polypyrrole.
dc Conduction in electrochemically synthesized polypyrrole films
1998
DC conductivity measurements were performed by modified four-probe rig on electrochemically synthesized polypyrrole films at a temperature range of -30 • C to 120 • C. Conductivity increased with temperature. The temperature dependence of conductivity was very high for lightly doped polymers, decreasing as the doping level increased. The model used to describe the conduction process was the conduction model originally developed for amorphous silicon by Mott and Davis. When applied to conducting polymers, it assumes that electron transport originates from localized or fixed states within the polymer chain. The charge transfer between the chains takes place by hopping, referred to as phonon-assisted hopping, between two localized states. Plots of DC conductivity versus temperature can be parametrized by Mott's Variable Range Hopping conduction model. The DC conductivity of polypyrrole films doped from light to intermediate levels with p-toluene sulphonic acid were measured in the temperature range of 77K to 300K. The localization length of localized electrons was assumed to be 3Å, which is approximately equal to the length of the pyrrole monomer. Mott parameters of polypyrrole films doped with p-TS were evaluated at 300K and 10K. Results were found to be consistent with the Mott's requirement that αR >> 1 . Theoretical values of α and N (EF ) have been determined at approximately 10 8 cm −1 and 10 19 -10 20 cm −3 eV −1 , respectively. Hence according to Mott parameters determined by the experimental data for the p-TS doped polypyrrole samples, Mott parameters are seen to have a better agreement with those expected from disordered systems, particularly for lightly doped samples, indicating the suitability of Mott's model to these samples. The average hopping distance R decreased from 16Åto 4.4Åwith the increase in the doping level from 0.006 M to 0.03 M at 300K, whereas at 10K, R decreased from 37Å to 10Å over the same dopant range.
Advanced Functional Materials, 2014
Coulovoltammetric responses allow a good graphical separation of the electrochemically induced structural changes in conducting polymers. [ 1,2 ] The electrochemically stimulated conformational relaxation (ESCR) model provides a good description and quantification of the chemically induced structural changes. [ 3-13 ] Compacted fi lms have been used to improve metal corrosion protection. [ 14,15 ] The use of different packed conformations as initial states for the study of the fi lm oxidation kinetics drives different kinetic coeffi cients, activation energies, or reaction orders: the kinetic magnitudes include structural information and the chemical kinetics become structural chemical kinetics. [ 16 ] The slow reduction-compaction process was demonstrated to go on until cathodic potential limits higher than-1.5 V were reached. Nevertheless, at more cathodic potential limits than-1.1 V, the reduction-compaction overlaps an irreversible reaction attributed to water electrolysis, with hydrogen release, at the polymer/ metal interface. [ 1 ] Here, the coulovoltammetric experiments will be repeated in different electrolytes by using free-standing polypyrrole fi lms with the metal contact located outside the electrolyte, trying to prove if, as expected from the hypothesis, this irreversible reaction disappears by elimination of the back metal inside the solution. More cathodic potential limits than-1.5 V will be explored in order to follow the reduction-compaction process. Using the theoretical equations from the ESCR model to quantify charges, energies, and the potential ranges for each structural process becomes a quite specialized task. [ 3-13 ] Coulovoltammetric responses from conducting polymers (CPs) coating metal electrodes [ 1 ] become easier graphical tools (useful for non-specialists) to separate and quantify the different structural components, and here we try to extend this methodology to free-standing fi lms as working electrodes. By contrast, with the ESCR model, the conducting-insulator transition model [ 17-38 ] claims that any conducting polymer becomes a fully reduced insulator fi lm after the reduction maximum. There the model states that all the counterions are expelled from the fi lm and the positive charges eliminated from the chains. The subsequent oxidation during the next anodic potential sweep must be initiated at the metal/polymer interface: only from those chains (insulator) in direct contact with the metal the electrons can be extracted by oxidation. In that case, the use of free-standing fi lms with a metal contact
Doping and Dedoping Processes of Polypyrrole: DFT Study with Hybrid Functionals
The Journal of Physical Chemistry C, 2014
Density Functional Theory (DFT) and time depended DFT (TD-DFT) calculations at UB3LYP/6-31G (d) level have been performed to investigate the tunable nature, i.e., doping and de-doping process of polypyrrole (PPy). The calculated theoretical data show strong correlation with the recent experimental reports, which validates our computational protocol. The calculated properties are extrapolated to the polymer (PPy) through a 2 nd order polynomial fit. Changes in band gap, conductivity and resistance of nPy and nPy-X (where n = 1-9 and X = +, NH 3 and Cl)