POLYPYRROLE FILM ELECTRODES ELECTROCHEMICALLY DOPED WITH TETRATHIOMOLYBDATE ANIONS - PREPARATION AND CHARACTERIZATION

Polypyrrole films containing tetrathiomolybdate anions and molybdenum trisulfide have been prepared on platinum and tin oxide coated glass electrodes by anodic electrochemical deposition from an aqueous solution containing pyrrole and ammonium tetrathiomolybdate. The scanning electron micrographs of these films show a porous structure. Cyclic voltammetry at a tin oxide electrode in a solution containing pyrrole and tetrathiomolybdate suggests that the formation of the polymer proceeds initially by a nucleation process. The current-time transient for film deposition remains fairly constant for long elapsed times, after the initial decrease and rise, indicating the formation of a fairly conducting layer. A linear relationship is observed between both film thickness, the weight of molybdenum in the polypyrrole-tetrathiomolybdate film and the charge passed during film formation. The cyclic voltammogram of the polypyrrole-tetrathiomolybdate film is characterized by an apparent redox potential, taken as the average between the anodic and cathodic peaks, of approx. -0.3 V vs. SCE. The latter is shifted to a more negative potential in comparison with that of a polypyrrole-chloride film electrode. This negative shift is characteristic of polypyrrole film doped with some "immobile" dopant. In this case the redox processes involve insertion of cations from the supporting electrolyte during film reduction and expulsion during oxidation. This was confirmed by cyclic voltammetry which is strongly dependent on the size of the electrolyte cation and independent of the electrolyte anion. Elemental analysis shows that the polypyrrole-tetrathiomolybdate film contains 70% MoS3 and 30% MoS42- relative to the molybdenum content. The conductivity of a polypyrrole-tetrathiomolybdate film is at least two orders of magnitude larger than that of a molybdenum trisulfide film.