Günter Appel

Selective polypyrrole electrodes for quartz microbalances: NO2 and gas flux sensitivities

Abstract Quartz microbalances were produced which have polypyrrole as a conductive polymer electrode. The polymer layers were operated as both electrode and sensitive material to detect the concentration of toxic gases. Tests with NO2 demonstrated detection limits below the MAK values at room temperature. It was also investigated the response to some other noxious compounds and to the total gas flux. The possible mechanisms associated with the sensor properties are pointed out and discussed.

Highly conductive polypyrrole films on non-conductive substrates

Abstract We report on a method to deposit thin films of conductive polypyrrole powder on non-conductive substrates. This is a new method in contrast to the usual way of preparing polypyrrole films by anodic deposition on metal substrates. Polypyrrole tosylate is synthesized by chemical oxidation of pyrrole with ammoniumpersulfate in aqueous solution. We optimized the conductivity by variation of the reaction conditions. Then we produced smooth films of polypyrrole by testing several methods of film deposition. All of these samples were characterized by conductivity and by X-ray photoelectron spectroscopy (XPS) measurements, and compared to electrochemically produced polypyrrole films.

Band bending in doped conducting polypyrrole: interaction with silver

Small amounts of silver were evaporated onto conducting polypyrrole films doped by organic anions like tosylate or copper phthalocyanine trisulfonate. The electronic structure of these systems was studied by photoemission electron microscopy, by photoemission spectroscopy and near edge X-ray absorption spectroscopy. This investigation is an attempt to study the interaction of metallic contacts with organic semiconductors. A shift of the secondary electron onset in the valence band spectra was observed. This shift is caused by the neutralization of positive surface charges by an electron transfer from silver. We conclude that a natural band bending exists in pristine polypyrroles. We determine the flat band situation and describe the electronic structure in a band-model scheme.

The polaron and bipolaron contributions to the electronic structure of polypyrrole films

Abstract UPS and NEXAFS spectroscopy have been used to study the changes in the electronic structure of polypyrrole films by varying the dopant anion. In particular, we find the density of states within the electronic gap to be influenced by the chemical structure of the dopant anion. From the position of the Fermi level in the bandgap, the charge carriers in the polymer can be identified either as polarons or as bipolarons. For the first time, we used copper phthalocyanine trisulfonate as dopant, resulting in a high concentration of bipolarons which have a high contribution from the wavefunctions of the pyrrole–nitrogen atoms. Interaction of the π-systems of the polymer and the dopant causes a change in the chemical nature of the nitrogen from amino-like to imino-like.

The formation of oligomers in the electrolyte upon polymerization of pyrrole

Abstract Pyrrole was polymerized by electrochemical and chemical oxidation of a solution of pyrrole and p-toluene sulfonic acid in acetonitrile. We have investigated the components formed within that electrolyte solution during the polymerization. Both, UV–VIS spectroscopy and size exclusion chromatography give evidence that pyrrole oligomers nP (n≤30) are formed in the solutions during the polymerization process. A model is proposed in which these oligomers act as intermediates of the polymerization reaction. Within that model the deviations from Faradayic behaviour at elevated current densities are explained. It also rationalizes the fact that polymer films with completely different properties can be realized by varying the experimental parameters.

Polypyrrole doped with fluoro-metal complexes: thermal stability and structural properties

Chemically produced polypyrrole powders doped with the fluoro-complexes fluoro-aluminates, fluoro-zirconates, fluoro-titanates and fluorides are synthesized and characterized by both XPS and four-probe conductivity measurements. The results show that the polypyrrole powders produced were doped with fluorides and with fluoro-metal complexes (Zr, Ti, Al) of different coordination numbers. Also, a significant incorporation of sulfates originating from the oxidant was determined. The relative concentration of the sulfates and of the respective fluoro-metal complexes is strongly affected by the size of these dopants and much less from their concentration in the solution. The aging process of these polymers is investigated. We have found that there are two main mechanisms, the oxidation of the polymeric chain by water and oxygen as well as the reaction of fluoride ions with low coordination fluoro-complexes to form stable complexes of higher coordination number. In the case of polypyrrole powder doped with fluorides, the mobility of these counter-ions leads to the formation of gaseous species. Polypyrrole powders doped with fluoro-zirconates and fluoro-titanates exhibit both good stability in contrast to those doped with fluoro-aluminates and fluorides.

Conductivity of silver exposed polypyrrole films

Electrochemically prepared conducting polypyrrole tosylate films were exposed to silver in a wide range of concentrations. Four-probe measurements of the dc conductivity of these films as well as two-probe ac conductivity measurements were performed. Small silver exposures lead to a dramatic change in the surface potentials and electronic structure, whereas the ac- and dc-conductivities appear not to be influenced. In the range of high silver exposures metallic conductivities due to the formation of a massive silver overlayer on top of the polypyrrole films are observed.

Increased sensor sensitivities obtained by polymer-coated quartz microbalances

Abstract Polymer-coated quartz microbalances were produced and studied. They exhibited high sensitivities for heavy noncyclical hydrocarbons and CO 2 concentration measurements. Detection limit of 1 ppm CO 2 and 10 ppm Shellsol (Shell solvent containing: 28.7% C 10 H 22 ; 66.4% C 11 H 24 ; 5% C 12 H 26 ) are obtained and reported. The time constants of the absorption and desorption processes were investigated also. A diffusion-free behaviour was observed and related to the porosity of the layer. Collateral AFM (atomic force microscopy) analysis of the polymer structure was performed. We explain big values of the absorption–desorption time constant for CO 2 and small values for C n H 2 n +2 mixtures in connection with the peculiarities of the chemical reaction involved by the sensing mechanisms.

Doping and defect inhomogeneities of polypyrrole tosylate films as revealed by μ-NEXAFS

The photoemission electron microscope (PEEM) based techniques micro-near-edge X-ray absorption fine structure (μ-NEXAFS) and micro-photoelectron spectroscopy (μ-PES) have been recently installed at the BESSY-I HE-TGM beam lines as novel techniques for material analyses. In the PEEM microscope, photoemitted electrons are imaged by electrostatic lenses having a lateral resolution of 40 nm. Images with a field of view between 800 and 20 μm are obtained. NEXAFS enables the simultaneous recording of the near-edge fine structures of the absorption coefficient while varying the incident photon energy. For μ-PES, at a fixed photon energy, core level and valence band spectra can be recorded by an electrostatic energy analyser to fit in the optical axis of the PEEM microscope. Spectra are recorded by analysing all the electrons of the PEEM image or by selecting segments of the image with a continuous iris aperture. The minimum size of the analysed spot is in the order of a few micrometers. Here, we use these techniques to determine the electronic structure of polypyrrole films within a well defined surface area. We found surface structures of such polypyrrole films which are corrugated in the micron range as well as areas that appear almost smooth. In addition, the PEEM microscope gives patterns due to a non-homogeneous surface potential distribution. In such patterns bright irregular stripes appear between dark patches. The stripes have a typical width in the order of several micrometers.

Nanoparticles and polarons: active centers in thin film sensor devices

Abstract The spectroscopic characterization of localized charges in thin film devices is addressed. We demonstrate their existence and show their particular contribution to the operation of sensor devices. Examples in oxidic and polymeric thin films systems are given in which the localized states dominate the electronic properties.