Electropolymerization of thiophene on oxidizable metals in organic media

Abstract

The electropolymerization of thiophene on oxidizable metals (Fe, Zn, Al) has been investigated in different organic media (acidic, neutral or basic) with a view to obtaining adherent and homogeneous polythiophene (PT) films. In propylene carbonate (PC), considered as a neutral medium (DN = 15) and a good solvent for the electropolymerization of thiophene on noble metals, it was found that, of several salts, PF6− alone could hinder the anodic dissolution of iron and allow the galvanostatic electropolymerization of thiophene on this metal. In a basic solvent, such as THF (DN = 20), electropolymerization of PT could not be achieved, due to passivation of the metal making the electrode surface highly insulating. The best results were obtained in dichloromethane (DCM), considered as acidic (DN = 4), which allowed the deposition of homogeneous PT films on Fe and Al in potentiodynamic or galvanostatic modes easily, and on Zn in the galvanostatic mode only. Polymer films obtained on these metals in PC and DCM were analysed by IR spectroscopy and XPS; their structure appeared quite similar to those obtained on Pt with, in particular, strictly α, α′ coupling of thiophene units. However, the conductivities of these PT films, related to their doping level, are much lower than on Pt (10 S cm−1), and seem to be correlated with the redox potential of the metal: 10−3 S cm−1 with Fe and Zn, 10−4 S cm−1 with Al. The adherence of PT films synthetized in DCM on Fe and Zn is not very good (maximum adherence about 35% for iron and 0% for Zn). In the case of iron it was found to decrease when the current density increased from 4 to 10 mA cm−2. With Al, the adherence is excellent (100%) and is due to the formation of SAl and CAl bonds, as confirmed by XPS analysis.