Mixed ion transfer in redox processes of poly(3,4-ethylenedioxythiophene)

A model considering the transfer of counter ions and solvent molecules has been presented to explain the complicated ion transfer in polymer films during electrochemical redox processes. The developed mixed ion transfer model is used to analyze the ion transfer behavior of poly(3,4-ethylenedioxythiophene) during doping and dedoping processes. The results indicate that both anions and cations are involved in n-doping as well as in p-doping processes. The behavior of different electrolyte ions is also studied. The developed model shows that solvent molecules are also transferred in conjunction with the ion transfer.     

Metalloporphyrin electropolymerization: electrochemical quartz crystal microgravimetric studies

The electrochemical quartz crystal microbalance (EQCM) study of several metalloprotoporphyrin (MPP) deposition (Ni(II)PP, Cu(II)PP, Co(II)PP, Zn(II)PP, Fe(III)PP) on gold surfaces from dichloromethane solution is reported. The QCM mass–time evolution during film deposition has shown for all porphyrins studied a monotonic film growth during the positive voltammetric scan which is consistent with the accepted vinyl cation electropolymerization (EP) mechanism [J. Am. Chem. Soc. 105 (1983) 5601]. Quartz crystal impedance at 10 MHz has shown that NiPP, CuPP, ZnPP and FePP films dissipate the acoustic wave much less, rendering acoustically thinner films than a CoPP film that behaves as a loose viscoelastic film. For the former acoustically thin films, gravimetry was possible; unlike CoPP films where the viscoelastic properties are dominant at 10 MHz. Comparison of the redox charge and QCM mass demonstrates that homogeneous chemical reactions coupled to the electroinitiation of vinyl cations should take place, and therefore the application of the Faraday Law to estimate the polymer mass from the oxidation charge is precluded. The optimal EP conditions to deposit single and mixed films are also described.     

Electrochemically induced derivatization of poly(2,2′-bithiophene) and characterization of functionalized polymers by FT-IR microscopy,SEM microanalysis and EQCM

In this paper, we present a study of an innovative way of preparing and characterizing functionalized conducting polymers. Instead of electropolymerizing the pre-functionalized monomer, with a procedure that must be optimized for many experimental aspects, the functionalization of the pre-electropolymerized film on a polarizable electrode is proposed and discussed here. The alkoxylation of pre-electropolymerized poly(2,2′-bithiophene) films, deposited on platinum or gold electrodes, was carried out by electrooxidative treatment of the polymers under several different experimental conditions, in acetonitrile solutions of 2-bromoethanol or 2-chloroethanol. Electrofunctionalization has been carried out potentiostatically, i.e. by keeping the potential at a fixed value, or by cycling it between oxidising and reducing values. The highest yields have been reached potentiostatically. The proposed mechanism for the functionalization reaction is supported by the results of scanning electron microscopy and reflectance-mode FT-IR microscopy, which have been interpreted by means of chemometric techniques.     

Electrochemical dissolution of polythiophene films

Polythiophenes, which are electrically conducting polymers, are generally insoluble except for 3-substituted derivatives with long alkane chains. The electrochemical oxidation of poly(3-methylthiophene) in solution without the monomer caused dissolution of the film at potentials similar to the polymerization potentials until the film was removed completely from the electrode. This is due to oxidative degradation. The kinetics of the dissolution was examined by time-variations of film thickness as well as of the mass by means of EQCM. Dissolved species, which were estimated from UV-, FTIR-spectroscopy and GC–MS, were monomer derivatives with a thiophene ring. When the solution included the monomer of which concentrations were over 6 mM, the oxidation caused not only dissolution but also polymerization.     

Electropreparation and characterization of polyNiTSPc films. An EQCM study

Thin films of polyNi(II)tetrasulfophthalocyanine (polyNiTSPc) electrodeposited on a gold electrode by potential cycling in 0.1 M NaOH solutions containing 1 mM Ni(II) tetrasulfophthalocyanine were studied by cyclic voltammetry, electrochemical quartz crystal microbalance and impedance techniques. Both the mass increase due to the electrodeposited film, and the current of the Ni(II)/Ni(III) process in the film, increase monotonically with the number of electrodeposition cycles, a linear correlation existing between these two parameters. AFM micrographs show that the film is rugose and porous, this porosity being confirmed by the fact that the voltammetric features of gold oxide formation and reduction are unaffected by the film. The cyclic voltammogram of the film in a pH 11 carbonate–hydrogencarbonate buffer shows the characteristic anodic and cathodic peaks of the $\mathrm{Ni}(\mathrm{II})?\mathrm{Ni}(\mathrm{III})$ process, with a large mass decrease upon Ni(II) electrooxidation and an equally large mass increase upon Ni(III) electroreduction. The sign of the mass change is compatible with a β-Ni(OH)₂-character of the film. The large slopes of the plots of mass change vs. charge in the Ni(II)/Ni(III) region clearly show that large fluxes of water and/or ions accompany the electron transfer. The fairly low activation energy of Ni(II) electrooxidation, 38 kJ mol^?1, is in agreement with the moderately non-Nernstian behaviour of the Ni(II)/Ni(III) process. The super-Nernstian dependence on pH of the peak potentials of the Ni(II)/Ni(III) process indicates that the species involved are non-stoichiometric, similarly to the Ni_1 ? xH_x(OH)₂ species in nickel hydroxide.     

Electropolymerization of poly(N-methylaniline) on mild steel: Synthesis,characterization and corrosion protection

Poly(N-methylaniline) (PNMA) coatings on a steel disc electrode have been successfully electrodeposited from N-methylaniline (NMA) in aqueous oxalic acid solution using potentiodynamic, potentiostatic and galvanostatic techniques. For the potentiodynamic synthesis, formation of the PNMA coatings was found to be strongly dependent on the upper potential limits and scan rates. Smooth and strongly adherent PNMA coatings were obtained under potentiodynamic conditions during sequential scanning of the potential region between ?0.5 and 1.0 V, at a scan rate of 10 mV s^?1. The PNMA coatings were characterized by cyclic voltammetry, FT-IR and UV–Vis spectroscopy and SEM techniques. It was shown that NMA converts the iron(III) species to iron(II) chemically, causing a decrease in the current value corresponding to the electrochemical conversion (repassivation). The corrosion behavior of steels covered by polymer was investigated by anodic potentiodynamic polarization technique and the Tafel test. The corrosion resistance of the PNMA coated steel was found to be considerably higher and the corrosion rate was much slower than that of bare steel.     

A polyaminoquinone film for dopamine entrapment and delivery

Dopamine uptake and release was investigated with a quinone-containing conducting film, poly(5-amino-1,4-naphthoquinone) (PANQ), used for its intrinsic cation-exchange properties. The good electroactivity of this film in aqueous medium leads to interesting values of dopamine uptake, up to 100 nmol cm^?2 in a 300 nm thick film. The uptake and release mechanisms indicate a water counterflow during the doping process and competition with protons. The incorporated dopamine can be released by applying an oxidative potential, and such a film has possibilities as a dopamine vector for in vivo applications.     

EQCM study on the oxidation/reduction of bismuth sulfide thin films

The oxidation/reduction of bismuth sulfide thin films deposited on an Au electrode using the successive ionic layer adsorption and reaction method has been investigated by electrochemical quartz crystal microbalance combined with cyclic voltammetry and X-ray photoelectron spectroscopy. It has been shown that the oxidation/reduction of bismuth sulfide film depends on the nature of anion. In the acetate solution the bismuth sulfide film is oxidized at more negative potential values than that in sulfuric acid solution. During the oxidation of bismuth sulfide film the Bi present on the surface of the bismuth sulfide film is oxidized first. This process is followed by subsequent oxidation of bismuth sulfide film alongside the oxidation of Au. In the acetate background solution the oxidation of bismuth sulfide occurs with the whole film stripping and the calculated amount is related to the amount of deposit and corresponds to the thickness of ∼9–10 nm. In sulfuric acid the bismuth sulfide film is not completely stripped out in one anodic cycle. The evaluated amount during the first anodic scan corresponded to the thickness of deposit of ∼3–4 nm. The reduction of bismuth sulfide film took places at more positive potential values in the acetate background solution than that in sulfate background solution at pH 4. After the reduction of bismuth sulfide film to Bi⁰ and S²⁻ in the background solution, depending on the nature of anion, the oxidation of metallic Bi proceeds by two different mechanisms: in the sulfate solution Bi is oxidized to Bi(III) oxygen compounds while in the acetate solution Bi is oxidized by dissolution.     

Spectroelectrochemical and EQCM studies on the oxidation of glycil-peptides in alkaline medium

Electrochemical oxidation of diglycine (G₂), triglycine (G₃) and tetraglycine (G₄) has been studied in an alkaline medium by means of in situ FTIR spectroscopic and electrochemical quartz crystal microbalance (EQCM) methods. The adsorption of the terminal carboxyl group of these peptides occurred around +0.3 V versus Ag ∣ AgCl, and the oxidative decomposition was observed from ca. +1.0 V. A striking difference in the oxidation of amino acid and peptides was that the former reaction continued to proceed with increasing potential but the oxidation of peptides was rather restrained at high positive potentials. The major reason for such constraint was that the glycine or glycil–glycine generated in the anodic oxidation of peptides was in the acid form and its adsorption and following oxidation on the electrode became considerably difficult. Formation of the acid form of glycine or glycil–glycine was attributed to the lowering of pH which occurred because oxygen evolution occurs in further positive polarization. The EQCM data indicated that the adsorption of glycil-peptide occurred from ca. +0.2 V, the adsorbed carboxyl group was decomposed oxidatively at ca. +1.1 V and a part of the remaining adsorbents was detached simultaneously from the electrode with the evolution of oxygen in further positive polarization.     

Mechanism of electropolymerization of phenylacetylene in propylene carbonate

Electrooxidation of phenylacetylene (PAH) in a solution of 0.1 M in LiClO₄ in propylene carbonate (PC) led to the formation of the conducting polymer poly(phenylacetylene) (PPA). A kinetic study has been made by experimental determination of the kinetic parameters. A mechanism is proposed which leads to confirmation of the experimental data. The electropolymerization product was isolated, purified, and characterized. Spectroscopic data showed that the polymer obtained consists in a linear polyene chain with a trans structure. The average molar mass determined was 3095, corresponding to the formation of a polymer with around 30 monomer units and to an electrical efficiency of 3095 g of polymer/mol of electrons.     

Cyclic voltammetric,EQCM and impedance study of the 1-octanol-β-cyclodextrin-electrodeposited platinum system in perchloric medium

The system of 1-octanol-β-cyclodextrin in 0.5 M HClO₄ at Pt electrodeposited on an Au/quartz crystal (Pt/Au/Q) has been studied by cyclic voltammetry (CV), the electrochemical quartz crystal microbalance (EQCM) and impedance measurements. β-Cyclodextrin (β-CD), a cyclic oligomer of seven α-d-glucopyranose units, was chosen for this study because it forms an inclusion complex with primary alcohols and because of its hydrophilic exterior. Previous cycling in 1-octanol profoundly affected (“aged”) the electrodeposited Pt, as is readily seen in subsequent CVs and mass curves in 0.5 M HClO₄. Since adsorbed 1-octanol and β-CD would tend to render the Pt surface hydrophobic and hydrophilic, respectively, these opposite tendencies should be detectable by the EQCM. Effectively, in a positive potential scan, 1-octanol and β-CD added at ?0.22 V produced a mass decrease and increase, respectively, of aged Pt/Au/Q, owing to a decrease and increase of the amount of adsorbed water and/or ions, which was attributed to physisorption of these compounds. With freshly deposited Pt/Au/Q, 1-octanol added at open circuit produced a mass decrease over the whole potential cycle, again evidence of adsorption of a hydrophobic compound.The potential at which the aged Pt/Au/Q electrode immersed in an 1-octanol solution was held while β-CD was added to the electrolyte crucially affected the subsequent voltammogram and mass curve. If β-CD and 1-octanol were added together at ?0.22 V, the current in a subsequent positive scan was the same as in 1-octanol added at ?0.22 V, but the mass was higher, both at ?0.22 V and over the whole positive scan. This mass increase was probably due to physisorption (since the current was unaffected) of the inclusion complex, since β-CD alone did not affect the mass at ?0.22 V. On the contrary, if β-CD was added at open circuit to an electrolyte already containing 1-octanol, also added at open circuit, both the H desorption and Pt oxidation currents were lower than in 1-octanol, indicating a strong interaction of β-CD with the Pt surface, and the mass was also lower over most of the positive scan. Most probably this strong interaction of β-CD involves adsorbed residues formed in the dissociative chemisorption of 1-octanol at open circuit.     

Electrochemical impedance analysis of the redox switching hysteresis of poly(3,4-ethylenedioxythiophene) films

The effect of the redox hysteresis of poly(3,4-ethylenedioxythiophene) films on the electrochemical mechanisms analyzed by impedance has been studied. Impedance spectra were recorded at fixed steady-state potentials under both negative and positive sweeps. The effects of film thickness were also investigated by varying the polymerization charge used. It has been shown that not only do quasi-equilibrium circuit elements (low-frequency capacitance) vary with the potential sweep direction but also kinetics elements (interfacial charge transfer resistance) depend on it, both exhibiting hysteretic effects. The onset of the charge transfer resistance increase is shifted approximately ?300 mV during the negative polarization.     

Anodic oxidation and physicochemical properties of various porphyrin-fluorenes or -spirobifluorenes: Synthesis of new polymers for heterogeneous catalytic reactions

New conducting polymers were obtained by anodic oxidation of various unmetallated or metalloporphyrins bearing fluorene or 9,9′-spirobifluorene groups in meso positions. The anodic behaviour of monomers and polymers is presented. The physicochemical properties (IR, UV–Vis spectroscopy and spectroelectrochemistry, scanning electron microscopy, electronic microanalysis) of these new materials are then discussed.     

A simple way for the electropolymerization of porphyrins

A novel and simple method for the electropolymerization of porphyrins is developed. Polarization of a working electrode at the second ring-oxidation potential of octaethylporphyrins in the presence of appropriate bridging nucleophiles is sufficient to realize the coating of the electrode by a cationic polymeric film. Characterizations of the obtained films are presented.     

EQCM studies on oxidation of metallic nickel electrode in basic solutions

Mixed self-assembled monolayer of thiols were formed on the electrode surface using aromatic and aliphatic thiols and the modified interface were used to demonstrate charge trapping behavior at the modified interface. The electrode surface was modified by two schemes. In Scheme 1, electrode surface modified with methylene blue (MB) shows charge trapping behavior when contacts the electrolyte containing potassium ferricyanide. In Scheme 2, a redox-active bilayer is constructed using hexamineruthenium (II) chloride and methylene blue. This redox-active bilayer assembly exhibits unidirectional flow of current.     

Aniline electropolymerization on platinum and mild steel from neutral aqueous media

Aniline has been electropolymerized on platinum from neutral aqueous electrolytes. The properties of the resulting polyaniline (PANI) films are similar to those obtained in acidic media. PANI films can be generated using multiple cyclic voltammetry or galvanostatically. In the former case, the molecular structure of the films depends on the inversion potential and two types of material can be generated. When potential sweeps are restricted to an inversion potential of 0.8 V, the films do not have a PANI-like structure, whereas an inversion potential of 1 V allows the deposition of short PANI-like chains, as demonstrated by SEC and MALDI-MS techniques. In the galvanostatic mode, the molecular structure of the films depends on the applied current density. There is a current density window which makes it possible to grow PANI films from neutral electrolytes with the same efficiency as in acids and which depends drastically on the experimental conditions. In the case of a 2 M LiClO₄+0.4 M aniline solution, a very high current density can be used for PANI deposition on platinum. These neutral aqueous electrolytes have been used for the deposition of PANI films on mild steel. The films are generated with less metal dissolution than when acid electrolytes are used and exhibit similar anticorrosion properties.     

Interpretation of the ultra-fast electropolymerization of pyrrole in aqueous media on zinc in a one-step process: The specific role of the salicylate salt investigated by X-ray photoelectron spectroscopy (XPS) and by electrochemical quartz crystal microbalance (EQCM)

The use of sodium salicylate as an electrolyte makes it possible to deposit polypyrrole (PPy) films on oxidizable metals such as zinc by the electrochemical oxidation of pyrrole. In spite of the very large difference between the pyrrole and zinc oxidation potentials, which thermodynamically should lead to metal dissolution and not polymer formation, PPy films are formed as easily as on a platinum electrode. X-ray photoelectron spectroscopy (XPS) and in situ electrochemical quartz crystal microbalance (EQCM) experiments reveal that a very thin composite passivating zinc salicylate layer is formed prior to pyrrole electropolymerization and prevents zinc dissolution without inhibiting polymer formation. Ex situ XPS analysis of the surface at different potentials (?1 to 0.7 V) at a grazing and normal incidence of the photoelectrons shows that pyrrole is adsorbed on zinc at an early stage of the polarization and remains on the metal surface up to the beginning of electropolymerization. In situ EQCM measurements indicate that this passivation layer is very thin (about 5 nm), and does not desorb at the potential where pyrrole is oxidized. The rapid oxidation of pyrrole through this passivating layer is explained by the presence of conductive pyrrolic paths inside the salicylate layer which make this composite layer as active as a noble metal.     

On the mechanism of oxidative electropolymerization and film formation for phenanthroline-containing complexes of ruthenium

New experiments involving electrochemistry, Auger spectroscopy and elemental analysis of films and film-forming compounds, as well as X-ray crystallography and electrochemistry of a key model compound, have been used to elucidate the linkage structure and probable mechanism of formation of oxidatively generated polymeric and copolymeric films of phenanthroline-containing complexes of ruthenium. Film formation evidently involves indirect electrochemical activation of coordinated 1,10-phenanthroline to nucleophilic attack by the non-coordinated pyridyl nitrogen of a dipyridyl ligand singly coordinated to a second metal center. The resulting linkage is comprised of a carbon/nitrogen bond with an excess positive charge residing on the nitrogen atom. Electrochemical oxidation of a complexed metal ion (typically Ru(II)) drives the polymerization by: (a) rendering the 4 site of phenanthroline significantly electrophilic, and (b) providing an oxidizing equivalent for eventual H atom elimination and pyridinium ion formation via an internal electron transfer sequence.     

Simultaneous surface plasmon optical and electrochemical investigation of the electropolymerization of bithiophene at elevated pressures

A novel high-pressure set-up is presented that allows for the simultaneous characterization of interfacial reactions and thin films by surface plasmon optical techniques, as well as by electrochemical methods. The current configuration uses a grating coupler for the resonant excitation of surface plasmons at the (gold) metal ∣ dielectric interface with the metallic substrate serving at the same time as the working electrode in a three electrode electrochemical configuration. Using the same electrolyte as the pressure medium experiments up to about 50 MPa are reported. Firstly, some performance characteristics of the cell we discussed on the basis of CV measurements with hexacyanoferrate as a quasi-reversible redox system. Then the electropolymerization of bithiophene in acetonitrile solution at different cell pressures is presented.     

An EQCM investigation of oxidation of formic acid at gold electrode in sulfuric acid solution

The oxidation of HCOOH at Au electrode in sulfuric acid solution was investigated using the electrochemical quartz crystal microbalance (EQCM). The potentiodynamic mass responses in HCOOH-containing solution were compared to that in the background solution. This comparison provided valuable information on the mechanism of HCOOH oxidation. The effect of the potential scan rate on the current peak corresponding to oxide reduction was also discussed.