Topological and Electron-Transfer Properties of the 2-Thiobarbituric Acid Adlayer on Polycrystalline Gold Electrodes

The voltammetric behavior of [Ru(NH₃)₆]³⁺ on bare gold and that on 2-thiobarbituric acid (TBA)-modified gold surfaces are almost identical, with formal rate constants for the electron-transfer process of 0.25 and 0.21 cm s^-1, respectively. A detailed analysis of the modified surface allowed us to establish that this behavior is due to (i) a high surface coverage of 0.67, (ii) a low adsorption resistance that minimizes the potential drop across the TBA monolayer, (iii) the enhanced hydrophilic character of the modified surface compared with that of bare gold, and (iv) a low decay constant for the electronic coupling of the TBA adlayer that minimizes the tunneling barrier for the electron transfer. The electron-transfer process from Au and Au|TBA electrodes to the soluble [Ru(NH₃)₆]^3+/2+ redox couple can be explained according to the multistate model under the Landau−Zener formalism in the nonadiabatic regime that was recently proposed (Feldberg, S. W.; Sutin, N. Chem. Phys. 2006, 324, 216−225). The behavior of soluble [Ru(NH₃)₆]³⁺ changes from semi-infinite linear diffusion on Au to finite-length bounded on Au|TBA, in agreement with a surface dimension of 2.17 for the TBA adlayer with a bidimensional underlying gold surface. This value for the surface dimension was determined by two essentially different electrochemical techniques with different sensing capabilities:  cyclic voltammetry and electrochemical impedance spectroscopy. The estimated dielectric constant of the adlayer (around 37) and the low potential drop across the monolayer suggest the formation of a “mirror” pattern of water molecules in the diffusion layer, which explains this result.