Role of nucleation in the redox reactions of TTF-TCNQ single crystal electrodes in contact with aqueous KBr solutions

Cyclic voltammetry and chronoamperometry experiments have been performed on electrodes prepared from needles of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) single crystals in contact with aqueous 1 M KBr. The data indicate that nucleation and growth mechanisms control the surface redox reactions of the TCNQ acceptor and the TTF donor lattice sites. Experiments with partially immersed electrodes with different faces exposed to the solution suggest that the origin of the nucleation phenomena is inherent in the structure of the TTF-TCNQ crystal. Two transients were seen in the chronoamperometric responses: one with t(max) in the order of 5-10 ms and one with t(max) in the order of 100-200 ms. For the longer transients, a 3-dimensional, instantaneous nucleation model with diffusion-controlled growth of the surface phase gave better fits to the data than two-dimensional models or models based on kinetically controlled growth. The cyclic voltammetric parameters for the TCNQ(0/1-) couple were indicative of a unusual quasi-reversible phenomenon in which cooperative interactions in partially reduced or oxidized surface phases decrease the thermodynamic driving force for the phase transformation. (C) 1998 Elsevier Science Ltd. All rights reserved.