Specularity change on a thin gold film surface coated with poly( o-aminophenol) during the polymer redox conversion. The pH effect on the redox sites distribution at the metal ∣ polymer interface

Abstract

The objective of this work was to obtain additional information with respect to that obtained previously by means of cyclic voltammetry and ac impedance measurements, about the redox process of an electroactive polymer, such as poly( o-aminophenol). To this end a non-traditional approach in electrochemistry, the surface resistance technique, was applied. Conversely to other techniques, which are based on complicated models, surface resistance is based on a simple electron dispersion model. The working electrode was a thick poly( o-aminophenol) film (thickness, Q Red ∼2.8 mC cm −2) deposited on a gold film whose thickness ( φ m) was comparable to the mean free path of the conduction electrons on gold. The relative resistance change Δ R/ R of the gold film during the redox conversion of the polymer was monitored. This Δ R/ R change was attributed to the redox conversion from amino to imine groups of this polymer. Three external variables were changed: the anion (perchlorate, sulphate and benzenesulphonate) contacting the polymer film, φ m and the solution pH. The Δ R/ R response is independent of the external anion contacting the polymer when a thick polymer film is used. However, it depend on both φ m and pH. The first dependence allows one to demonstrate that the Δ R/ R change can be interpreted on the basis of an electron scattering process of the conduction electrons from the inside of the gold film at the gold ∣ poly( o-aminophenol) interface. The second one allows one to demonstrate that not only the number of reduced sites of the polymer changes with increasing pH but also their distribution on the metal surface. From the attenuation of Δ R/ R change as a function of the degree of reduction of the polymer, with increasing pH, it was concluded that less compact distributions of redox sites are formed on the metal surface as the pH increases.