Gilberto Schiavon

A simple two-band electrode for in situ conductivity measurements of polyconjugated conducting polymers

Abstract A simple and renewable two-band electrode for in situ determination of electronic conductivity in polyconjugated polymers is described. The electrode was tested with electrodeposited polypyrrole, poly- N -methylpyrrole, polyaniline and poly-3-methylthiophene. The dependence of the electrode response on deposition charge and interband gap was investigated. Resistance under semi-infinite volume conditions allows the evaluation of resistivity. Results are in agreement with published data from ex situ measurements.

Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena

Abstract Irreversible changes occurring in the electrochemical reduction of some polythiophenes in acetonitrile have been investigated by cyclic voltammetry (CV), UV-Vis and IR spectroscopy and the electrochemical quartz crystal microbalance (EQCM). Reduction in the presence of alkali metal cations (Li + , Na + , K + and Cs + ) does not result in n-doping but in hydrogenation of the alkene moieties. The promotion of double-bond saturation by alkali metal cations following the order Li + , Na + > K + > Cs + is attributed to their charge-pinning action which decreases as the ion radius is increased. Reversible reduction in tetralkylammonium electrolyte (n-doping) is accompanied by production of hydroxide ions within the polymer which are thereby inserted by subsequent oxidation. The resulting quinone-like groups are responsible for the CV pre-peaks usually observed in conducting polymers after n-doping and previously assigned to trapped charges.

Electrochemical synthesis and characterization of polyconjugated polyfuran

Polyconjugated polyfuran (PF) was produced by electroreduction of 2,5-dibromofuran in acetonitrile using Ni(bipy)32+ as catalyst. Polymerization occurs only in the presence of excess Ni(bipy)32+ at potentials in the range −1.7 to −2.0 V versus Ag/Ag+ via binuclear organonickel complex. IR spectroscopy, elemental analysis and mass spectroscopy confirm that PF is constituted by 2,5-linked furan units and indicate a degree of polymerization > 100. Red PF films (λmax = 420 nm, Eg = 2.2 eV) are reversibly oxidized at 0.45 V and reduced at −2.5 V. Doping with AsF5 gives PF a conductivity of 10−3 S/cm. Polymers produced by anodic coupling of furan were also investigated for comparison. IR spectroscopy and elemental analysis indicate that they display a lower degree of conjugation due to the concomitant acidcatalyzed polymerization.

Anodic oxidation of triphenylphosphine at a platinum electrode in acetonitrile medium

Summary The electrochemical oxidation of Ph 3 P at a platinum electrode in CH 3 CN solvent has been studied using cyclic voltammetry and controlled potential electrolysis. In the anodic reaction, the triphenylphosphinium cation radical initially produced reacts both with Ph 3 P to form a phosphonium cation and with the trace of water present in the solvent to give triphenylphosphine oxide in a ratio depending on water content. When Ph 3 P is oxidized in presence of benzene tetraphenylphosphonium ion is found. The yield of Ph 4 P + in the electrolytic preparation is not very high because a Ph 3 P portion reacts to give Ph 3 PH + and Ph 3 PO.

An electroactive nickel containing polymeric film obtained by electrochemical reduction of an aryl-nickel derivative

Abstract The electrochemical reduction of [Ni(PPh 3 ) 2 ( p -C 6 H 4 Br)Br] in acetonitrile results in a polymeric coating of a nickel-containing poly- p -phenylene, formulated as [-Ni( p -C 6 H 4 ) n -] ( n =6−7) on the basis of elemental analysis, IR, UV, MS and ESCA measurements. This coating is found to undergo a two-electron reversible reduction and a one-electron reversible oxidation with a large potential window (3.6 V), without the polymeric film being destroyed. Moreover, the polymer obtained is shown to be electroactive towards dissolved reactants via permeation or mediated electron transfer, depending on the molecular size of the electroactive species employed.

Electrodeposition of Amorphous Fe2 O 3 Films by Reduction of Iron Perchlorate in Acetonitrile

Electrodeposition of amorphous Fe 2 O 3 films of nanometer size was performed by reduction of Fe(ClO 4 ) 3 or Fe(ClO 4 ) in oxygenate acetonitrile. Cyclic voltammetry has shown that the oxide-film formation occurs via oxidation of electrodeposited metal by dissolved oxygen. The oxide films were characterized by cyclic voltammetry, electrochemical quartz crystal microbalance, X-ray i raction, and UV-visible spectroscopy. The films are reversibly reduced in acetonitrile + LiCIO, in two redox processes to the Fe 3 O 4 and FeO oxides via lithium intercalation. The UV-visible (UV-vis) spectrum displays absorption with an optical gap (1.75 eV) characteristic of amorphous Fe0 3 . Spectroelectrochemistry has shown that reduction causes bleaching of the yellowish films (electrochromic efficiency ≅30 C -1 cm 2 at 400 nm). Heating the films at 500 °C converts them to the crystalline α-Fe 2 O 3 form with loss of electroactivity.

Amperometric monitoring of sulphur dioxide in liquid and air samples of low conductivity by electrodes supported on ion-exchange membranes

An amperometric sensor is described for the determination of sulphur dioxide in both gaseous atmospheres and solutions of low conductivity. It consists of a porous Pt electrode (facing the sample) supported on one face of an ion-exchange membrane (Nafion 417) which serves as a solid polymer electrolyte. The other side of the membrane faces an internal electrolyte solution (1 mol dm–3 aqueous perchloric acid) containing the counter and reference electrodes. This sensor is inserted into a flow cell in which gaseous or electrolyte-free aqueous samples are fed by a peristaltic pump placed in a closed-loop path and SO2 is oxidized at an applied potential of 0.65 V versus Ag–AgCl. The device is found to be characterized by a high current sensitivity and a short response time, 24 A cm–2 mol–1 dm3 and 1 s respectively for gaseous samples; (0.4 A cm–2 mol–1 dm–3 and 4 s, respectively, for water solutions), and by good stability and low background noise. The dynamic range extends up to 2 × 10–4 mol dm–3(gaseous samples) and 1 × 10–3 mol dm–3(water samples) with good linearity, and detection limits of 8 × 10–9 mol dm–3(gaseous samples) and 4 × 10–7 mol dm–3(water samples) are predicted for a signal-to-noise ratio of 3. The advantages offered by this type of sensor over conventional gas-permeation membrane electrodes are discussed.

Electrochemistry of coordination compounds XV. Paramagnetic hydrido complexes of cobalt(II), rhodium(II) and iridium(II)☆

Abstract The electrochemical behaviour of a series of d8-hydrido complexes of cobalt, rhodium and iridium containing phosphorus ligands has been studied. The oxidation proceeds in two one-electron steps involving a rare hydrido complex in the II state, which, in the case of rhodium and iridium, is unstable and decays through a disproportionation pathway. An oxidationreduction mechanism accounting for the chemical and electrochemical results is proposed.

Mixed-valence conduction in redox-substituted polythiophenes. Enhancement of redox electron-exchange rate by polyconjugated chains

Abstract The in situ conductivity of some poly(cyclopentadithiophenes) modified at the 4 position with reducible ( p -nitrophenyl and p-N -methylpyridyl) or oxidizable (ferrocene) redox groups was investigated in acetonitrile. In the nitrophenyl- and pyridyl-functionalized polymers the reduction, which is confined to the pendant redox group, makes the polymer mixed-valence conductive. The conductivity ((3–6) × 10 −3 S cm −1 ), which is the highest reported for a redox polymer, remains within the limits expected for a direct intersite electron hopping. In the ferrocene-functionalized polymers the oxidation involves both the redox group and the backbone. Redox conduction at the ferrocene group is enhanced by hopping through the conductive polymer backbone. The hopping rate is increased by a decrease of the ferrocene-backbone distance and by conjugation of ferrocene with the backbone itself making conductivity increase up to 1 S cm −1 .

Electrodes supported on ion-exchange membranes as sensors in gases and low-conductivity solvents

Abstract An electroanalytical sensor is proposed that is suitable for the detection of electroactive analytes present in gases or low-conductivity solvents where supporting electrolytes cannot be introduced. It consists of a porous working electrode supported on one surface of a cationic ion-exchange membrane (Nafion 417), the other surface of which is in contact with an electrolyte solution containing the counter and reference electrodes. Such an ion-exchange membrane replaces a conventional supporting electrolyte dissolved in the analyte sample and can be regarded as a solid polymer electrolyte (SPE) confined in the close neighbourhood of the working electrode. Alternative procedures followed for coating SPE membranes with various materials (Pt, Au, C or Hg) are described, together with the general properties displayed by the resulting composite electrodes in analyte-free gaseous or liquid media. These assemblies have been used as both voltammetric and amperometric sensors for electroactive analytes present in gases and in aqueous or organic solvents with no supporting electrolyte. The results indicate that their performance is similar to that expected on conventional electrodes, the only difference being a slightly lower degree of reversibility for the electrode processes investigated. Detection limits for some analytes were calculated and the use of SPE electrodes as sensors suitable for the continuous monitoring of electroactive analytes dispersed in gases or non-conductive liquids is reported. Preliminary attempts to use these assemblies for the determination of trace metals in low-conductivity solvents by anodic stripping voltammetry are discussed.