In situ atomic force microscopy study of polypyrrole synthesis and the volume changes induced by oxidation and reduction of the polymer

The morphology changes of polypyrrole (PPy) films on polycrystalline platinum, glassy carbon and gold electrodes during synthesis and oxidation–reduction processes have been investigated using in-situ electrochemical AFM. It is found that the morphology of the film at the first stages of synthesis depends on the nature of the electrode and it is different from that of the thick film morphology. Remarkable changes in the morphology of PPy films were observed following synthesis suggesting that PPy films have an extraordinary capacity to alter their morphology over time. Upon reduction (undoping) in aqueous solution of NaClO₄, a perchlorate-doped PPy undergoes initially rapid swelling and subsequent continuous slow shrinking during the first reduction potential step from 0.2 to ?0.8 V, but during subsequent redox potential steps no remarkable changes of volume are observed. In contrast, in an aqueous solution of sodium p-toluenesulfonate, the polymer swells on reduction and shrinks on oxidation during continuous redox potential steps between 0.2 and ?0.8 V.     

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.     

Electrocatalytic oxidation of d-galactose in alkaline medium

The electrocatalytic oxidation of d-galactose was investigated on platinum, gold and nickel electrodes in 0.1 M NaOH medium. The oxidation of galactose on nickel takes place in the NiOOH region and leads to the cleavage of the C–C bonds. This was confirmed by HPLC analyses of electrolyzed solutions which demonstrate relatively high amounts of low molar mass carboxylic acids, i.e., formic and glycolic acids. The oxidation of galactose on platinum is initiated at very low potentials, i.e., ?0.8 V vs. SCE probably without oxygenated species on the electrode surface and gives moderate selectivity towards galactonic acid. The important effect of the lead adatoms on the electrocatalytic properties of platinum was demonstrated by the increase in the yield of galactonic acid from 34% to 67% with addition of 10^?5 M Pb²⁺. On gold, galactose oxidation begins at approximately ?0.5 V vs. SCE and is probably catalysed by the presence of hydrous gold oxides. The best yield of galactonic acid, 86%, was obtained after 6 h of electrolysis using a two potential program: ?0.1 V vs. SCE for 30 s, 1.5 V for 1 s.     

Electrochemical doping of poly(vinyl chloride) obtained by photodehydrochlorination from laminated poly(vinyl chloride) + polypyrrole films

The poly(vinyl chloride) (PVC) in laminated PVC + polypyrrol (PPy) films has been converted to an electrically conducting structure by photodehydrochlorination and subsequent electrochemical doping. The maximum conductivity (6 × 10^?1 S cm^?1) of the PVC film was obtained by potentiostatic oxidation at 0.3 V vs. SCE in HCl solution. The reflection spectra of the PVC film showed a new band around 600 to 800 nm after the electrochemical doping, indicating the formation of a charge-transfer complex. The maximum conductivity of the PVC film was reached with the highest concentration of the charge transfer complex, polyene⁺ … X^?, formed between the photogenerated polyenes and dopant anions.     

Electrochemical properties of natural carotenoids

Carotenoids are widely distributed among plants, certain bacteria and animals, serving for light harvesting and photoprotection in photosynthesis and as antioxidants. Electrochemical data (such as oxidation potentials, reaction rate constants, kinetic equilibrium constants) for 12 naturally occurring carotenoids are summarized in this paper. Electrochemical parameters were estimated by simulating experimental cyclic voltammograms (CVs). Conventional electrochemical techniques (cyclic voltammetry and Osteryoung square-wave voltammetry) were used in these studies. CVs of some natural carotenoids are presented and discussed. The dependence of redox potentials and other kinetic parameters on the structures of carotenoids is summarized. The oxidation potential of neutral carotenoid (E°₁), which corresponds to the formation of cation radicals, varies from 0.50 to 0.72 V versus SCE. The oxidation potential of carotenoid cation radical (E°₂), which corresponds to the formation of a dication, usually lies in the region of 0.52–0.95 V versus SCE. The electrochemical data presented in this article should be helpful in the study of reaction processes in model systems or actual photosynthesis reaction centers.     

Evaluation of the potential dependence of 2D–3D growth rates and structures of polypyrrole films in aqueous solutions of hexafluorates

Polypyrrole hexafluorosilicate, PPYSiF₆, and polypyrrole hexafluoroaluminate, PPYAlF₆, were found to follow the mixed 2DI–3DI kinetic model of electrodeposition in the potentials range (+0.80 V, +0.60 V) and (+0.95 V, +0.55 V) vs. SCE, respectively. The potentiostatic depositions of polypyrrole were performed on polycrystalline gold and on the single layer or bilayer polypyrrole films. Only in the case of the third layer deposition of PPYSiF₆ (on PPYSiF₆) at +0.6 V, the experimental current was found to fit better with the 2DP–3DI model. For electrodeposition of PPYAlF₆ on gold, a slightly better quality fitting with 2DP–3DP or 2DP–3DI kinetic models was observed only for some samples synthesized at 0.55 V. In general, contribution of the 2D structure in the polypyrrole deposits was observed to decrease with an increase of the polarization potential. Conclusions of the mathematical analysis of the current–time functions are in accordance with a texture of the outer surface of the polymer layers visible in SEM micrographs. The apparent rate constants of the lateral and outward growths of polypyrrole increase with potentials in (+0.55, +0.80) V range. Under assumption of the classical electrochemical kinetics law, the anodic transfer coefficients were found lower than 0.5. Values of the outward growth rate constant were of the order of 10⁻⁸–10⁻¹⁰ mol/(s cm²); being approximately one order of magnitude higher for polypyrrole hexafluorosilicate than for polypyrrole hexafluoroaluminate. The lateral growth rates of PPYAlF₆ on gold were found to depend on the electrodeposition potential more significantly than the outward growth rate. An increase in the growth rates with the potential was observed to diminish/vanish at E > +0.80 V, probably due to concurrent degradation processes of polypyrrole chains. The analysis of the deposition currents was done under conditions of the masking double layer currents that were characterized by the relaxation times of the order of seconds or longer.     

The cathodic deprotection of the nitrobenzoyl group from phenyl nitrobenzoates in N,N-dimethylformamide

The reduction of phenyl benzoates with nitro substituents at the 2-, 3- and 4-positions of the benzoates in N, N-dimethylformamide is reported. The phenyl 4- and 3-nitrobenzoate are reduced in two cathodic steps. The first one, at about ?0.9 V vs. SCE, a reversible one-electron process, gives a rather stable anion radical. The second reduction step at potentials between ?1.5 and ?2.0 V vs. SCE leads to formation of the dianion, which decomposes giving free phenol in good yields ( > 80%). On the other hand, the phenyl 2-nitrobenzoate is reduced in one cathodic step. This step occurs at ?0.9 V with formation of an unstable anion radical which decomposes via C-O bond cleavage, giving phenol with a yield of ca. 80%. The mechanisms of the reduction of these compounds are discussed.     

Electrochemical disinfection of marine bacteria attached on a plastic electrode

A plastic electrode, which consisted of graphite and silicone rubber, was employed for the electrochemical disinfection of attached marine bacteria, Vibrio alginolyticus. The viability of the bacteria attached on the electrode depended on the applied potential and time. Marine bacteria attached on a basal plane pyrolytic graphite electrode could be disinfected at potentials above 0.8 V vs. a saturated calomel electrode (SCE) applied for 20 min. The bacteria attached on a graphite—silicone electrode were disinfected at 1.0 V vs. SCE and 1.5 V vs. SCE, and 5.6 × 10³ cells/cm² of attached bacteria were disinfected to less than 5% of the initial number at times above 10 min. The residual chlorine concentration was less than the regulated value (0.02 ppm) and the pH value did not change after a potential of 1.5 V vs. SCE was applied to the graphite—silicone electrode for 30 min.     

Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

An in situ UV-vis and IR spectroelectrochemical study of the deposition of a hydroquinone anion salt on platinum electrodes from dichloromethane solutions

Cyclic voltammetry with a platinum electrode of hydroquinone (BQH₂) solutions in dichloromethane, containing tetrabutylammonium perchlorate supporting electrolyte, shows a sharp asymmetric irreversible oxidation peak at about ?0.3 V (SCE). This feature is seen, in addition to the expected features in this system, when the cycle is extended to potentials more negative than ?0.6 V (SCE). Cyclic voltammetry, in situ UV-vis and infrared spectroelectrochemistry have shown that hydroquinone anion (BQH^?) is formed at negative potentials and this appears to arise via surface decomposition of hydroquinone to p-benzosemiquinone (BQH) followed by reaction of BQH with the p-benzoquinone radical anion (BQ^?). The sharp asymmetric peak in the cyclic voltammograms is due to oxidation of the hydroquinone anion in the insoluble tetrabutylammonium salt on the electrode surface. The oxidation of BQH^? appears to occur via disproportionation of (BQH) and leads to BQH₂ and p-benzoquinone (BQ) as the products.     

Electrochemical polymerization of 3-phenylthiophene

Poly (3-phenylthiophene) (P3PhT) films have been electrochemically synthesized by direct oxidation of 3-phenylthiophene in the electrolyte of pure boron trifluoride diethyl etherate (BFEE). The oxidation potential of the monomer was measured to be 1.29 V (vs. SCE), and about 0.15 V lower than that measured in a neutral medium of acetonitrile. Free-standing P3PhT film with tensile strength of 32–40 MPa has been obtained for the first time. The morphology and the structure of the polymer film have been studied by scanning electron microscopy, infrared and Raman spectroscopies. Raman spectral results demonstrated that the doping level of the P3PhT film increased with film thickness during electrochemical growth process.     

Electrocatalytic oxidations of biological molecules (ascorbic acid and uric acids) at highly oxidized electrodes

Highly oxidized glassy carbon and other metal (gold and platinum) electrode surfaces were generated by applying 2.0 V vs. SCE for 600 s in pH 7 solution. The effects of oxidation on the functional, physical, and chemical characteristics of the electrodes went beyond the usual conditioning of such electrode surfaces through cycling to slightly beyond the medium decomposition potentials. The oxidized electrodes showed enhanced electrochemical activities and lower background currents. Besides cleaning the surface of contaminants introduced in the polishing stage of electrode preparation, application of such high potentials for an extended time interval changed the chemical nature of the glassy carbon surface itself. The chemical changes influenced the electro-oxidation reaction of bio-molecules such as ascorbic acid and uric acid but did not influence the reaction of the ferri- and ferrocyanide system. As a result the biological molecules (uric acid and ascorbic acid) were detected using an electrochemical method individually and simultaneously without the necessity for any electron relay complex.     

Voltammetric studies of electrochemical pretreatment of rotating-disc glassy carbon electrodes in phosphate buffer

Cyclic voltammetry was used to study the formation of graphite oxide film on the surface of rotating-disc glassy carbon electrodes. The film was formed by continuous potential cycling in phosphate buffer electrolyte. It was found that when cycling between the totally oxidized and totally reduced potential ranges (between + 2.3 and ?1.2V vs. SCE), well-shaped redox peaks were obtained. Typical voltammograms comprised one cathodic and two anodic waves, where the anodic waves were directly related to the oxidation of surface-bound products associated with the cathodic process. All three processes were pH dependent. Freshly formed surface graphite oxide films were not stable and would undergo transformation to give the stable state upon potential cycling between potentials ± 1.0V. The ultimate stable voltammograms were characterized by two electrode processes. The quasi-reversible process involved surface quinone-like functional groups, while the other irreversible process might be related to the uptake and release of hydrogen ions at the porous graphite oxide film.     

Electrochemical properties of cation sensitive polypyrrole films

The influence of the electrochemical pretreatment (potentiodynamic cycling) and previous conditioning of thick cation sensitive PPy films (30 μm) doped with sulphate, naphthalene-2-sulfonate (N-2-S) and naphthalene-1,5-disulfonate (N-1,5-DS) on the electrochemical properties were studied. The changes in the chemical composition of the PPy films were determined by electron probe microanalysis (EPMA). The poor mobility of dopant anions in the film and participation of cations in the PPy film redox process is the basis of their cation sensitivity. The influence of potentiodynamic cycling on the properties of these PPy films is correlated with the change of several factors, such as the redox activity in the potential interval of the potentiometric measurements, the redox capacity, the ionic content and the quality of contact with the substrate. The long-term conditioning of the PPy electrode in a certain solution is necessary for the achievement, maintenance or re-establishment of the stable quasi-equilibrium state of PPy film characterized by a quasi-equilibrium potential E_r. The conditioning process of the untreated PPy|N-2-S and PPy|N-1,5-DS electrodes proceeds very slowly. It accelerates after potentiodynamic cycling, enabling the use of the electrodes for potentiometric measurements after only 2 days of conditioning. The potentiodynamic cycling of the PPy electrodes doped with big organic anions improves also their potentiometric response, extending considerably the linear parts of E–log a plots towards low concentrations. The nature of this effect consists in the change of a thick PPy film to become redox active in the potential region of the potentiometric measurements.     

Mass transport behavior of polypyrrole and poly(N-methylpyrrole) films in acetonitrile solutions

The mass transport mechanism of polypyrrole (PPy) and poly(N-methylpyrrole) (PMPy) films in an acetonitrile (AN) solution has been investigated with the cyclic electrochemical quartz crystal microbalance (EQCM) technique. Cations as well as anions take part in ion transport during the redox reaction of PPy films, and the break-in process occurs in the first negative scan. On the other hand, anion transport is dominant during the redox reaction of PMPy films, and no break-in process is observed. Solvent transport takes place in the same direction as cation transport in the case of PPy films, whereas it occurs in the opposite direction to anion transport in the case of PMPy films.     

Electrochemical preparation of polypyrrole-molybdenum trisulfide-tetrathiomolybdate electrode with various amounts of molybdenum species

Polypyrrole films doped with molybdenum trisulfide and tetrathiomolybdate anions have been prepared from an aqueous solution containing pyrrole and ammonium tetrathiomolybdate in the presence of tiron, (4, 5-dihydroxy-1, 3-benzenedisulfonic acid), in the deposition solution. In such a solution the electrodeposition of polypyrrole doped with tetrathiomolybdate anions is in competition with the electrodeposition of molybdenum trisulfide by oxidation of tetrathiomolybdate anions. Our results indicate that the relative amounts of polypyrrole and molybdenum species can be varied over larger ratios when tiron is present in the deposition solution. The variation of the amount of molybdenum species can be explained by considering the number of electrons required to generate one mole of tetrathiomolybdate as dopant for polypyrrole and one mole of molybdenum trisulfide; 14 electrons are required per mole of tetrathiomolybdate incorporated in the polymer in comparison to 2 electrons per mole of molybdenum trisulfide generated directly by electrochemical oxidation of tetrathiomolybdate. Tiron acts as a catalyst for the deposition of polypyrrole and in this case the relative amount of polypyrrole in the composite film electrode is larger. For higher tetrathiomolybdate concentration, the direct formation of molybdenum trisulfide occurs preferentially and the catalytic effect of tiron is less important.     

Analysis of the mechanism for electrodeposition of the ZnSe phase on Cu substrate

Electrodeposition mechanism of selenium, zinc and ZnSe phase was studied with the electrochemical techniques. The chronoamperometry and voltammetry were combined with quartz crystal microbalance technique to analyse the deposition processes. It was found that they strongly depend on the applied potentials. The deposition of pure Se starts at 0 V vs. saturated calomel electrode (SCE) and the efficiency of this process decreases due to H₂Se formation and hydrogen evolution when the more negative potentials are reached. The voltammetric and microgravimetric measurements suggest that the deposition of Zn from the bath containing both ions, follows selenium deposition and synthesis of ZnSe is initiated at −0.6 V vs. SCE. The chronoamperometric transients combined with microgravimetry confirmed the range of potentials in which Se and ZnSe are deposited. Below −0.84 V vs. SCE the formation of H₂Se was observed. Then H₂Se may react with Zn²⁺ ions and another mechanism of ZnSe synthesis is possible.     

Remarkably high activity of electrodeposited IrO2 film for electrocatalytic water oxidation

A homogenous and transparent IrO₂ film was prepared on an ITO electrode by anodic electrodeposition under galvanostatic conditions from an aqueous solution containing 2 mM K₂IrCl₆ and 40 mM oxalic acid that is aged at 37 °C and pH 10 for ca. 10 days. The absorption spectral change of the solution suggested that an IrO₂ colloid is formed in the solution during ca. 10 day-aging. The scanning electron microscopic (SEM) measurement displayed homogeneous deposition of IrO₂ particles with 100–250 nm of a diameter on the surface of the film. The X-ray diffraction (XRD) measurement indicated that IrO₂ in the film is amorphous. The cyclic voltammogram (CV) of the IrO₂-coated ITO electrode dipped in a 0.1 M KNO₃ aqueous solution exhibited a steep rise of an anodic current at 1.0 V vs SCE for catalytic water oxidation, as well as an anodic wave at 0.3 V and a corresponding cathodic wave at ?0.1 V that are assigned as an Ir^IV/Ir^V redox. The anodic current at 1.3 V on the CV was 660 times higher than that for a blank bare ITO electrode. Ir electrodeposited on the ITO electrode was also shown to be electrocatalytically active for water oxidation. However, the anodic current at 1.3 V on the CV for the Ir-coated ITO electrode was 14 times lower than that for an IrO₂-coated electrode in spite of the 34 times higher coverage of Ir. The potential static electrochemical water oxidation using the IrO₂-coated ITO electrode produced a significant amount of O₂ above 1.1 V vs Ag/AgCl, in contrast to no O₂ detected even at 1.3 V using a bare ITO electrode. The maximum turnover frequency (TOF) of the IrO₂ catalyst was provided as 16,400 ± 450 h^?1 at 1.3 V vs Ag/AgCl from the slope of the linear plots of the amount of O₂ vs coverage of IrO₂ in the range of ～1.5 × 10^?9 mol. The TOF was 450 times higher than that (36.4 ± 1.4 h^?1 at 1.3 V) for electrodeposited Ir showing the very high catalytic activity of the IrO₂ film.     

Electrochemical anion doping of poly[(tetraethyldisilanylene) oligot 2,5-thienylene)] derivatives and their p-type semiconducting properties

Chemically synthesized poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] derivatives (DS/mT; m = 3 to 5) have been successfully anion-doped by electrochemical oxidation. Band-gap energies of 2.52, 2.65, 2.82 and 3.27 eV were evaluated for DS5T, DS4T, DS3T and DS2T respectively. The DS5T, DS4T and DS3T films exhibited electrical conductivities of the order of 10^?3 to 10^?4 S cm^?1 when doped with BF₄^?. The work functions of the films changed from 5.1 to ca. 5.5 eV with electrochemical anion doping. In cyclic voltammograms of the polymer films for anion doping and dedoping, an anodic peak potential and a cathodic one (E_pc.) shifted to the positive direction as the number m of thienylene units decreased. E_pcs at a sweep rate of 100 mV s^?1 were about 0.8, 0.9 and 1.0V for DS5T, DS4T and DS3T respectively. Reversible electrochemical doping and dedoping of the DS5T film were feasible when the potential was cycled between 0 and 1.2 V. At potentials more positive than 1.2 V, however, both overoxidation of the oligo(thienylene) unit and Si-Si bond cleavage took place, leading to decreases in conductivity and work function of the film.     

Electrochemical investigations and morphology of poly(4,9-dihydro-o-benzenonaphtho[2,3-c]pyrrole) and poly(acenaphtho[1,2-c]pyrrole)

The electrochemical characteristics and morphology of poly(4,9-dihydro-o-benzenonaphtho[2,3-c]pyrrole) (PDBNP) and poly(acenaphtho[1,2-c]pyrrole) (PANP) films prepared by controlled potential oxidation in acetonitrile containing 0.002 M monomer and 0.1 M tetrabutylammonium perchlorate (TBAP) have been studied. The impedance of PDBNP and PANP films coated onto glassy carbon electrodes measured in 0.1 M TBAP shows that the ionic conductivities of these two films increase with increasing electrode potential (oxidation level) as ClO₄^? ions are incorporated. It is concluded that anion transport is primarily responsible for the ionic conductivities. PANP has a 45° Warburg region at all electrode potentials. However, for PDBNP, the 45° Warburg region is seen only at low electrode potentials. The difference in the mode of charge transport shows that the value of electronic resistance of PDBNP at higher doping levels is similar to the ionic resistance, but for PANP, the electronic resistance is much smaller than the ionic resistance at all doping levels. The apparent electrochemical reversibility was seen to be higher for PDBNP than for PANP from cyclic voltammetry. Evidence for this interpretation is that the ionic conductivities increase dramatically with electrode potentials for PDBNP, indicating that the counterion ClO₄^? moves more easily in PDBNP than PANP. The higher low-frequency capacitance obtained from impedance spectroscopy for PANP is discussed in the light of in situ atomic force microscopy (AFM) observation of the film structure morphology.