J. Tamm

AFM studies of polypyrrole film surface morphology. I. The influence of film thickness and dopant nature

In this paper the surface morphology of electrogenerated polypyrrole films of various thicknesses and dopant anions (chloride, sulfate, perchlorate and dodecylsulfate) as studied by atomic force microscopy under ex situ conditions is reported. The roughness characteristics of the film surface are determined and discussed.

AFM studies of polypyrrole film surface morphology II. Roughness characterization by the fractal dimension analysis

The fractal dimension of electrogenerated polypyrrole films with chloride, sulfate, perchlorate and dodecylsulfate (DDS) anions as dopants is investigated by using atomic force microscopy under ex situ conditions. The results for films both as-prepared and after potentiodynamic cycling in various supporting electrolytes is presented and discussed.

Electrochemical properties of polypyrrole/sulphate films

The peculiarity of polypyrrole/sulphate (PPy/SO4) films redox behaviour has been investigated by cyclic voltammetry and electron probe microanalysis. The changes in PPy/SO4 film ionic composition during the redox process, especially the influence of electrolyte cations on its redox properties, were studied in detail. It was established that in the solutions of double charged metal- or bulky tetraalkylammonium cations the redox activity of PPy/SO4 film is suppressed, its mechanism is speculated on the basis of the films morphological peculiarity (compact structure), hydration radii and energy of moving ions, also the aggravations to effective charge balanced intermediate states during redox cycling are taken into account. In the case of tetraalkylammonium ions instead of the charge compensation problem the hydrophilic nature of PPy/SO4 film, and on the other hand the hydrophobicity of cations, causes additional kinetical aggravations or even suppression of film electroactivity. 0 1997 Published by Elsevier Science Ltd

Influence of Anions on Electrochemical Properties of Polypyrrole-Modified Electrodes

The influence of anions ClO4–, NO3–, Cl–, SO42–, and DDS– (dodecyl sulfate) on the cyclic voltammetric response of polypyrrole-modified electrodes is studied. The change in the film composition is examined by electron probe microanalysis. It is established that essential changes in the shape of voltammograms take place during cycling if the anions are not sufficiently freely mobile in the polymer film and insertion of cations from the solution is necessary to guarantee electroneutrality of the system. Some differences between the mobility of Cl– ions and ClO4– or NO3– ions are in good agreement with the results of semi-empirical quantum chemical calculations showing that the interaction of Cl– and Br– ions with pyrrole oligomers is stronger than that of NO3– or ClO4– ions. Nevertheless, it is established that the peak current determined from voltammograms increases linearly with the increase of the scan rate with very high correlation coefficient. It means that it is possible to describe the behavior of ClO4–, NO3– and Cl– ions in the framework of the model of free ions. The redox behavior of the PPy films doped with anions of low mobility such as SO42– and DDS– depends essentially on the nature of cations in the test solution. It is found that the mobility of cations increases in the row Li+ < Na+ < K+ < Cs+. The mobility of DDS– ions in the PPy in ethanolic solution is significantly higher and their electrochemical properties are quite similar to PPy|Cl or NO3– film in aqueous solution.

Nature of ion sensitivity of polypyrrole electrodes

The ion sensitivity of polypyrrole (PPy) films deposited electrochemically onto Pt, glassy carbon and SnO2 substrate was studied by potentiometric measurements. The influence of electrochemical (overoxidation, reduction and potentiodynamic cycling) and chemical (alkaline solution) pre-treatment on the potentiometric behaviour of PPy electrodes was determined. The dependence of the potential of the PPy electrode after long-term conditioning on the log of electrolyte activity of conditioning solution is characterised by low negative (solutions of NaSCN, NaNO3, KCl, or NaF) and low positive (Cs2SO4, Na2SO4 or Li2SO4) slopes. The exceptionally low exchange rate of anions between sulphate-doped PPy films and solution was established by electron probe microanalysis. Short-term (2–5 min) potential responses of PPy electrodes to the change of composition and concentration of electrolyte were studied. PPy electrodes conditioned in NaSCN, NaNO3, KCl, and NaF solutions possess anion sensitivity, and electrodes conditioned in Cs2SO4, Na2SO4 and Li2SO4 solutions, cation sensitivity, independent of the composition of the solution used for the preparation of the PPy film. The short-term response is generally not selective and depends on the arrangement of the ions in the Hofmeister lyophilic series. This potential response is interpreted as the result of asymmetric movement of anions or cations through the PPylsolution boundary.

Redox properties of polypyrrole bilayers

The spatially separated bilayers of polypyrrole (PPy) doped with anions of limited (dodecylsulfate (DDS) and sulfate) and good mobility (chloride) have been prepared by consecutive electrodeposition. The perfect coverage of PPy in the inner layer was obtained already at a very low effective thickness (<0.05 μm) of the outer layer. The peculiarity of the redox behavior of PPy bilayers has been investigated by cyclic voltammetry (CV) and electron probe microanalysis (EPMA). Ion transport through the outer PPy film has been characterized. The cation permeability in the mainly reduced outer component of PPy/DDS/Cl bilayer is limited and increases in the sequence of decreasing hydrated ion radii. The transport of the chloride dopant ions in the course of the reduction process of the inner layer of PPy/Cl/DDS and PPy/Cl/SO4 bilayers through the hydrophobic PPy/DDS and even through the dense but less hydrophobic PPy/SO4 film proceeds relatively easily. The penetration of the anions of the test solution during the reoxidation process of the PPy/Cl inner layer through the PPy/DDS outer layer is, on the contrary, essentially hindered. The reason for the high resistance to ion movement is the hydrophobic character of the PPy/DDS outer layer in which the amount of the electrolyte test solution is too low in order to initiate the anion migration process. Ion transport under conditions of simultaneous redox process in the outer and inner layer is characterized.

Study of the Properties of Electrodeposited Polypyrrole Films

Experimental and theoretical methods have been used for characterization of the properties of polypyrrole films. The AFM studies show that the morphology of polypyrrole (PPy) films on polycrystalline gold electrodes at the first stages of synthesis depends on the structure of the metal surface. It was established that mobility of anions depends remarkably on the rate of electrodeposition of the polymer film. If PPy film was deposited at relatively low current density, mobility of ClO-4 anion was not high enough to guarantee electroneutrality during redox cycling and cations take part in this process especially when Li+ cations were replaced by more mobile К+ cations. Semiempirical (AM1 and PM3) quantum-chemical methods were used for theoretical studies. It was established that different size and charge of the anions together with the variation in doping levels give rise to a different optimal conformation of oligopyrrole cations which, in turn, define the resulting polymer to be either all-anti (common linear chains) or all-syn (formation of helical structures) or a combination of the two.

Voltammetric study of the influence of cations on the redox-switching process of halogenide-doped polypyrrole

Abstract The influence of the nature of cations (K+ and tetraalkylammonium cations TMA+, TEA+, TBA+) on the electrochemical properties of the chloride- and bromide- doped polypyrrole electrodes in aqueous solutions has been studied. The differences in the shape and evolution of the voltammograms during cycling have been explained by the different participation of the ions in the redox-switching process. The dopant ions have been divided into three groups: free moving, blocked and screened. The charge of the blocked ions is compensated during redox-switching by the insertion of the small cations into the polymer film and the screened ions do not participate in redox process. The ratio of the different forms of dopant ions is dependent on both the properties of the film and solution and can be changed during potential cycling. The structural alteration of the polypyrrole frame during the cycling also affects the ratio of the cation-anion participation in the redox-process.

Electrochemical properties of dodecylsulphate doped polypyrrole films

Abstract The electrochemical behavior of dodecylsulphate (DDS) as a surfaceactive dopant of conducting polymer was studied. High selectivity of incorporation of the DDS into the growing polypyrrole film during the anodic polymerization was observed. The DDS ions are firmly fixed and cannot escape from the film and as the oxidation state of the polypyrrole is changed, other ions migrate into or out of the film. Extremely high kinetic resistance of the first reduction process is typical of the DDS doped polypyrrole. The incorporation of cations during the first cycle drastically changes the electrochemical properties of the film.