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 microba

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.