G. Tourillon

Electrochemical polymerization of aromatic amines: IR, XPS and PMT study of thin film formation on a Pt electrode

Films resulting from the anodic oxidation of various monosubstituted aromatic amines have been grafted on Pt electrodes in a CH3CN−NaClO4-pyridine medium. An in situ polaromicrotribometric study of changes occurring on the electrode surface during polarization indicates that these deposits have a stable friction coefficient of 1.3fo. Their analysis by multiple-reflection IR spectroscopy and by X-ray photoelectron spectroscopy shows that these films are organic polymers of emeraldine basic structure and that they probably result from the coupling of electrochemically formed NH. radicals with the monomer. These films are homogeneous, very adherent, thermally stable and free of ionic impurities. In the dry state they display a conductivity of 5×10−13 Ω−1 cm−1 which is characteristic of insulators.

Electrochemical formation of thin polyacetonitrile films on a Pt surface: P.m.t., i.r., x.p.s. and s.i.m.s. analyses and study of formation mechanism

Abstract The modifications occurring on an anodically polarized Pt surface in CH 3 CN-MClO 4 -[M=Li + or N + (R 4 )] or CH 3 CN-LiBF 4 are studied in situ by p.m.t. Under potentiostatic control ( E =2.6 V), formation of a film whose thickness increases over time is observed. For thickness 10–150 nm the friction coefficient is constant ( f =1.5 f 0 ), whereas beyond a thickness of 200 nm, it is very high and fluctuates heavily, thereby expressing not only a major lack of homogeneity in the film surface, but also the weak adherence of the film to the metal substrate. I.r., x.p.s. and s.i.m.s. analysis show the film to be of the polyacetonitrile variety doped with ClO 4 − or BF 4 − ions. When the film is left in a moist atmosphere, partial destruction of the polymer chains occurs and a deposit made up mostly of amides or amide salts is obtained. Polymerization is initiated by radicals ClO 4 . and BF 4 . ; its propagation due to H + ions acting on acetonitrile, leads to the polymer chain Download full-size image . The films are hydrophilic and prove to be heat-resistant when subjected to temperatures up to 200°C for 30 min, but decompose beyond this temperature. Out of the solution these films are non-conductive, but in an electrolytic medium they are highly conductive, as they are easily permeated by the usual ions.

Electronic structure and orientation studies of undoped poly-3-alkyl thiophenes electrochemically deposited on Pt as studied by NEXAFS

Abstract The electronic structure and orientation of films of poly-3-(CH 2 ) n (CH 3 ) thiophenes electrochemically deposited onto Pt have been studied by NEXAFS. The C K edge characteristics reveal that two major effects occur when n is increased from 0 to 10 in the 8 films studied. (1) A continuous decrease in the polymeric 1 s → 2π ∗ intensity was observed due to an overlap between the 2π bonding and 2π ∗ antibonding bands from the polymer and the σ(C 2p-H 1s) and σ ∗ (C 2p-H 1s) bands which develop when the alkyl chain length increases. This leads to a strong modification of the 2π and 2π ∗ densities of states. (2) The orientation of the thiophene ring structure switched from a “lying-down” to an “on-edge” configuration with the long alkyl chain in the latter case being oriented normal to the Pt surface.

Electrical properties of electrochemically prepared thin polyphenylene oxide films on a platinum surface: The role of ionic impurities in electroforming and conduction

Abstract Thin films of polyphenylene oxide with alkyl radical substituents (type A) or with hydroxy- or carboxymethylated functional group substituents (type B) form on the surface of a platinum electrode subjected to anodic polarization in a methanol-NaOH medium containing different types of phenolic monomers such as 2,6- or 3,5-xylenol, ortho- or meta-hydroxylbenzyl alcohol or ortho-hydroxyphenylacetic acid. The current-voltage characteristics of MIM capacitors with type A dielectrics exhibit a strictly capacitive behaviour when they are subject to an electric field; those of capacitors with type B dielectrics indicate that negative resistance and electron emission occur. These different types of conduction are closely related to the presence or absence of impurities in the films. X-ray photoelectron spectroscopy analysis shows that the type A polymers are “pure” and that the type B polymers are doped with sodium ions.

Electrochemical inclusion of metallic clusters in organic conducting polymers: An in situ dispersive X-ray absorption study

Conducting polymers containing metallic clusters are of considerable interest either from a fundamental point of view or by their potential applications. We report here, the first direct kinetic study of the different electrochemical steps occurring during the inclusion of metallic clusters in an organic matrix by using X-ray absorption spectroscopy from a synchrotron radiation source. The system described concerns the growth of copper clusters in poly-3-methyl-thiophene, an organic conducting polymer stable in air and in aqueous medium. The spectra in the vicinity of the Cu K edge (8991 eV) have been recorded continuously from the starting (CuCl2 solution) to the final state (copper cluters in the organic material) during the cathodic polarization of the polymer immersed in the solution. The main points seen through this work can be underlined as follow: (1) The CuCl2 solution is characterized by a strong white line shifted by + 8.5 eV from the edge of the Cu foil, consistent with solvated Cu2+. (2) When the polymer is cathodically polarized, several modifications in the Cu edge spectrum are observed depending on the electrolysis time, (a) A few seconds after the current is applied, Cu + ions are formed and stabilized by the polymeric matrix. (b) After several minutes, the system is evolving to the appearance of Cu0 species on the polymer. (c) For longer polarization time (15–20 min), a new shift toward lower energy is observed and seems to be related with the formation of very small copper clusters. These inclusion processes are completly reversible by changing the potential value. The size and location of the copper aggregates depend on the electrical parameters and on the CuCl2 concentration in the aqueous solution: at high concentration (1–3 M/l), the inclusion along the polymeric fibers is very homogeneous and the cluster sizes lie in the range 1–10 nm; at low concentration (10−2 M/l), copper particles of a few microm are mainly deposited near the metallic electrode.

X-ray absorption spectroscopy of carbonyl basket handle Fe(II) porphyrins: the distortion of the tetrapyrrolic macrocycle

The distortion of the tetrapyrrolic macrocycle, the Fe-C-O bond angle and the Fe local electronic structure of carbonyl basket-handle Fe2+ porphyrins as a function of the basket-handle chain length have been studied by X-ray absorption near edge structure (XANES) spectroscopy, both in the solid state and in toluene solution. The Fe-C-O bond angle has been found to be linear in all compounds while the increasing distortion of the macrocycle with shortening of the chain length is indicated by the multiple scattering resonance in the heme plane appearing at 22 eV in the XANES spectrum.

Time-Resolved X-Ray Absorption Spectroscopy Using an Energy Dispersive Optics: Strengths and Limitations

X-ray Absorption Spectroscopy has undergone a great theoretical and experimental development in the last years. This technique has proved to be a powerful tool in elucidating huge number of questions in materials science. Great interest exists in time-resolved experiments achieved with extreme energy resolution and energy scale stability taking full advantage of the strong correlation between the stereochemical environment of the absorbing atom and the exact shape and position of the absorption edge.

Electrical properties of electrochemically prepared thin polytetrahydrofuran films I: Characterization under A.C. conditions

Abstract The electrical characteristics of platinum-polytetrahydrofuran (poly-THF)- gold capacitors were studied under a.c. conditions at temperatures between 173 and 353 K and at frequencies between 102 and 106 Hz. When the temperature T is 300 K the capacitance C and the loss tangent tan δ are constant in the frequency range 102-105 Hz; when ƒ is greater than 105 Hz the capacitance decreases slightly and the loss tangent increases rapidly from 2 x 10−2 to 0.17. In the range 104−5x 103 Hz the imaginary part e″ of the dielectric constant has two loss peaks which are related to phase changes: one at 243 K corresponding to the glass transition point Tg(U) of the crystalline phase of the poly-THF (itself a semicrystalline polymer) and one at 203 K corresponding to the glass transition point Tg(L) of the amorphous phase. When T is greater than 319 K the loss tangent increases rapidly at very low frequencies (10 2 Hz 3 Hz ) ; 319 K corresponds to the melting point TαC of the crystalline phase. The average dielectric constant is 3.8, a value which is of the same order of magnitude as that of bulk poly-THF. The activation energy of the dielectric is 0.002 eV in the temperature range 173–250 K and is 0.03 eV at higher temperatures. The conduction mechanism for the capacitors studied in this work is of the hopping variety, as shown by the variations of the conductance G with T and ƒ; G is proportional to T − 1 4 at a given frequency, and G is ƒn dependent where n is 0.85 at low frequencies and low temperatures and where n is 2 at high frequencies regardless of the temperature.

XPS study of surface changes in magnesium subjected to attack by an alkyl halide

Abstract Metal-solution interfacial phenomena occuring when magnesium is attacked by a halogen derivative are studied by XPS analysis of the state of the surface formed during this reaction.

Electroforming and conduction in thin electrochemically prepared organic polytetrahydrofuran and polyacetonitrile films

Abstract Pt∣polymer∣metal capacitors (where the polymer is polytetrahydrofuran (poly-THF) or polyacetonitrile) in which the organic dielectric is obtained by electrochemical means evolve very quickly when subjected to an electrical field. Thus, capacitors which undergo electroforming (poly-THF or polyacetonitrile with positive impurities) go from a highly resistive and passive initial state (I) to a final high conduction state (II). In this latter state, negative resistance (NR) I-V characteristics and electron emission are detected. Some of the capacitors obtained with a polyacetonitrile dielectric are preformed and display all the characteristics of state II as soon as they are polarized. In all probability, filaments already existing in the preformed samples account for the electroforming in this dielectric. After polarization, local microscopic flaws are observed on the upper contact surface of Pt∣poly-THF∣Au capacitors. These flaws are not related to electroforming, but rather to electrolytic processes occuring along the filaments, as well as to field-dependent diffusion of the gold from the counterelectrode into the dielectric. These observations indicate highly localized conduction. At high voltages (greater than 20–30 V), a new type of flaw ascribed to microscopic breakdown spots is observed.