INSITU OBSERVATIONS OF ELECTROCHEMICAL INCLUSION OF COPPER AND IRON SPECIES IN A CONDUCTING ORGANIC POLYMER BY USING TIME-RESOLVED X-RAY ABSORPTION-SPECTROSCOPY

A comparative in situ time-resolved x-ray absorption study of the electrochemical inclusion of copper and iron species in poly(3-methylthiophene) (PMeT) is reported. For both treatments, incorporation of metallic ion species and complexation with the sulfur atoms of the polymer backbone lead to significant increase of the ex situ macroscopic conductivity. However, the detailed mechanisms and kinetics of the various complexation processes are specific to the metallic ion. Upon prolonged cathodic polarization of the polymer soaked in a CuCl2 aqueous solution, measurements show that (i) the formation of Cu0 metallic clusters occurs via a series of subsequent reduction steps: Cu+2 -->Cu+1 -->Cu0; (ii) at first, stabilization of the Cu+1 ions in aqueous solution is achieved through the interaction of the copper atom with the oxygen atoms of the SO3CF-3 doping salt and subsequently with the sulfur atoms of the polymer backbone; (iii) the bridging effect resulting from the interaction of the Cu+1 ions with the sulfur atoms located on different polymeric chains is responsible for the increase of ex situ macroscopic conductivity (from 50 to 150 omega-1 cm-1); and (iv) the formation of metallic copper clusters on the polymeric fibers do not affect the conductivity of the system. With the PMeT-FeCl3 system, (i) the interaction of Fe+3 ions with the sulfur atom of the thiophene units immediately follows their incorporation in the polymer; (ii) the first coordination shell of Fe+3 ions is composed of oxygen and sulfur atoms, leading therefore to a picture where the Fe+3 ions bridge two polymeric chains and are responsible for the increase of conductivity; (iii) upon reduction, Fe+2 ions are formed, which are solvated by water molecules and the interaction with the polymer backbone is loose; and (iv) the reduction potential of the Fe+2 + 2e --> Fe0 reaction is too low (compared to the electrochemical reduction potential of the doped PMeT) to allow the formation of conducting polymer-supported metallic iron clusters without dedoping of the conducting polymer.