M.A. del Valle

Nucleation and growth mechanisms of poly(thiophene) Part 1. Effect of electrolyte and monomer concentration in dichloromethane

Applying the potential step method, the effects of potential, monomer concentration, nature and concentration of electrolyte on the nucleation and growth processes of polythiophene on a Pt electrode in CH2Cl2 were studied. The current-time transients obtained were fitted using a mathematical equation that considers three contributions corresponding to 2D instantaneous and 3D instantaneous nucleation, both charge-transfer-controlled mechanisms, and 3D progressive nucleation with a diffusion-controlled mechanism. In any condition the principal contribution was always 3D instantaneous nucleation. However, the contribution of 2D instantaneous nucleation is important in the first stages of the nucleation process, indicating that the polymeric deposit initiates by the formation of a two-dimensional film. The contribution of 3D progressive nucleation appears at longer times and it is favoured with increasing monomer concentration. From the general nucleation and growth equation proposed, a relationship for the film thickness variation with time is obtained. Also, a general scheme of the nucleation and growth mechanism for poly(thiophene) is given.

Solvent effect on the nucleation and growth mechanisms of poly(thiophene)

Applying the step potential method, the effect of parameters such as solvent, potential, electrolyte and monomer concentration on the nucleation and growth processes of poly(thiophene) on Pt electrode in tetrabuthylammonium hexafluorophosphate–acetonitrile or dichloromethane has been studied. The j/t transients were generally fitted by means of a mathematical equation that considers different contributions. In acetonitrile the j/t transient (0 30 s, which was fitted by a mathematical equation that included two contributions corresponding to a PN3DCT and PN3Ddif mechanisms. In general, the charge associated to each contribution depended on the solvent, the monomer and electrolyte concentration and the applied potential. However, the PN3DCT (CH3CN) or IN3DCT (CH2Cl2) mechanisms were always the more important contributions. The scanning electron microscopy (SEM) analysis of the deposits morphology are in agreement with the nucleation and growth models that are proposed by this method.

Polythiophene, polyaniline and polypyrrole electrodes modified by electrodeposition of Pt and Pt+Pb for formic acid electrooxidation

The electrosynthesis of polythiophene (PTh), polyaniline (PANI) and polypyrrole (PPy) films modified by dispersion of Pt or Pt+Pb and its employment in the electrocatalytic oxidation of HCOOH are studied and compared. The influence of parameters such as polymer film thickness, the number of dispersed Pt particles, the amount of Pb deposited and the presence of Pb2+ in the electrolyte on the electrooxidation of HCOOH is investigated. Electrode systems including the polymer and a mixture of Pt and Pb particles show a better electrocatalytic activity than electrodes having a polymer–Pt combination or bulk Pt electrodes. Furthermore, during the electrooxidation of HCOOH using polymer–(Pt+Pb) electrodes the presence of fewer poisoning species is observed, indicating that the role of Pb in these electrode systems is in agreement with the Pb adatom effect observed when bulk Pt electrodes are used. However, the presence of Pb(ii) in the electrolyte is not required for the PTh–(Pt+Pb) electrode system and, in addition, a better electrocatalytic effect is obtained in this case. With application of an appropriate E/t program the activity is unchanged over a long time.

Preparation of polythiophene-modified electrodes by electrodeposition of Pt and Pt + Pb. Application to formic acid electro-oxidation

The electrosynthesis of polythiophene (PTh) films containing dispersed Pt or Pt+Pb and its employment in the electrocatalytic oxidation of HCOOH (I) were studied. The influence of PTh film thickness, the amount of dispersed Pt particles, the amount of Pb deposited and the electrode rotation effects on the electro-oxidation of I was investigated. The electrodic system comprising PTh−Pt+Pb shows a better electrocatalytic activity than PTh−Pt and massive Pt electrodes. The current densities observed in the electro-oxidation of I do not depend on the solution stirring, and the presence of Pb2+ in the electrolyte was not required. Furthermore, a greater decrease in poisoning species during the electro-oxidation of I is achieved. By applying an E−t program, which took the oxidation of residual species and the electroadsorption of I into consideration, the activity of the PTh−Pt+Pb electrode was maintained over 36 h without variation. The role assumed by Pb in this electrode system is in accordance with the Pb adatom effect observed in massive Pt electrodes. The performance of the electrode system studied shows the possibility of its employment in fuel cells.

Zinc catechin complexes in aprotic medium. Redox chemistry and interaction with superoxide radical anion

Abstract The redox chemistry of catechin and its zinc(II) complexes has been studied in dimethyl sulfoxide. In the absence of base, catechin undergoes oxidation processes at 0.96 and 1.24 V versus SCE. The first process corresponds to the formation of the quinonic form of the catechol moiety. In the presence of 1 equiv. of base, a stable 1:2 complex is formed with oxidation processes that show up at 0.26 and 0.62 V versus SCE. The voltammetric and spectroscopic characterization of the species produced after the oxidation processes are described. Upon interaction of the complex with superoxide radical anion in dimethyl sulfoxide, its basic character causes the formation of the monoanion of catechin leading to a more stable zinc(II) complex. Protonated superoxide disproportionates to molecular oxygen and peroxide leading to oxidation of the bound ligand. Upon complexation the oxidation potentials decrease, favoring thermodynamically the antioxidant action of this flavonoid.

Influence of the electrochemical conditions on the properties of polymerized carbazole

Polycarbazole (PCZ) was obtained by electrochemical oxidation of carbazole monomers. Two potentials were used: the oxidation potential of the monomer 1700 mV and an over oxidation potential 2500 mV. The carbazole monomers were supplied either in solution in the electrolyte or in the form of pre-deposited thin film evaporated on a working electrode. It is shown that if PCZ is systematically obtained its electrical and morphological properties are strongly dependent on the experimental conditions. The stability of over-oxidized PCZ films corresponds to chain reticulation beyond the first oxidation potential value, with the loss of flexibility of the chains and steric hindrance, which decreases the carrier mobility and, therefore, its conductivity. The polymers obtained at the oxidation of carbazole have conductivity corresponding to higher carrier mobility. These polymers are conjugated. Moreover, in the case of preevaporated carbazole thin film the coverage of the SnO2 is very high and the films are homogenous.

Polycarbazole obtained by electrochemical polymerization of monomers either in solution or in thin film form

Polycarbazole has been synthesized by electrochemistry. The carbazole monomer source consists either in carbazole in solution in the electrolyte or in carbazole deposited in thin film form onto the working electrode. The two families of polymers have been studied by infrared absorption, X-ray diffraction, thermal gravimetric analysis, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, electron spin resonance, and electrical conductivity measurements. It is shown that the polycarbazole films obtained with carbazole deposited in the thin film form exhibit a better polymerization efficiency and an electrical conductivity one order of magnitude higher. This result is in good accordance with a higher spin density and homogeneity. It is proposed that not only are some monomers activated during the evaporation but also that the physical contact between the working electrode and the continuous carbazole films induces the growth of homogenous highly polymerized polycarbazole films. In agreement with this suggestion, the SEM study shows that this type of polycarbazole thin films keep in memory the morphology of the SnO2 under layer.

Synthesis, characterization and electrical properties of poly[bis-(2-aminophenyl)disulfide] and poly[bis-(2-aminophenyl)diselenide]: Part II. XPS, ESR, SEM and conductivity study

Abstract Poly[bis(2-aminophenyl)disulfide] (DSDA) and poly[bis(2-aminophenyl)diselenide] (DSeDA) were synthesized by monomer oxidation with ammonium persulphate at different polymerization times, in 0.5 M H2SO4 and 1 M HCl media. X-ray photoelectron spectroscopy (XPS) analysis of DSDA shows similar percentage of imine and amine groups in the polymer, like emeraldine. DSeDA shows different percentages of imine and amine groups, which corresponds to a state different of emeraldine. When the time of polymerization of DSDA is increased, a small change in the oxidation state is detected, but in DSeDA the number of imine groups decreases. When polymerization is carried out in HCl media, both polymers present a minimum percentage of chlorine unit with covalent linkage to macromolecule. Moreover, electron spin resonance (ESR) study shows the presence of nitrogen and carbon radicals in the structure, and both radicals should be associated to chlorine or sulfate ions. The two polymers present low level of conductivity related to low level of doping. The macromolecules present a granular structure, but DSeDA is more pulverulent.

Synthesis, characterization and electrical properties of poly[bis-(2-aminophenyl)disulfide] and poly[bis(2-aminophenyl)diselenide]

Abstract Polybis(2-aminophenyl)diselenide and polybis(2-aminophenyl)disulfide were chemically synthesized by oxidation of the monomers with ammonium persulphate in 0.5 M H 2 SO 4 and 1 M HCl media, at variable polymerization times. Synthesis of polybis(2-aminophenyl)diselenide for 4.5 h afforded a product with the spectroscopic properties (FT-IR and UV–Vis) similar to those of poly emeraldine. On the other hand, the synthesis of polybis(2-aminophenyl)disulfide with these spectroscopic properties was not so straight forward and, consequently, conditions such as medium, oxidizing agent, temperature, concentration of monomer and oxidizing agent, and polymerization time had to be throughly surveyed. Both emeraldine dichalcogenide polymers proved to be insoluble in common organic solvents, but they dissolve well in concentrated sulphuric acid, yielding blue solutions and PANI-like spectra. They get doped by iodine vapor, but very little doping is obtained in acid aqueous media. Their conductivities attained those of semiconduction.

Synthesis, characterization and electrical properties of poly(dibromoaniline-co-aniline)s

The chemical copolymerization of aniline and 2,5-dibromoaniline or 2,6-dibromoaniline by oxidation with K2Cr2O7 in H2SO4/acetonitrile media has been carried out. Copolymer composition can be effectively controlled varying the monomer feed ratio. When substituted aniline fraction is increased in the copolymer, the electrical conductivity (σ) decreases; this effect is more important when 2,6-dibromoaniline is used. Thus, the conductivity can be controlled in a broad range, from 1.2 to 10−6–10−11 S·cm−1 depending on the substituted aniline and the feed ratio. The relations between copolymer compositions and comonomer feed molar ratios shows that the aniline is slightly more reactive than dibromoanilines during the copolymerization process. All the copolymers were shown to be more processable than polyaniline.