Effect of decreasing film thickness on the electrochemical responses of cations adsorbed in clay-modified electrodes

Clay-modified electrodes ranging in thickness from 3.4 mu m to 8 nm, as estimated from the clay loadings, were prepared using three different smectites by spin-coating, solvent evaporation or electrophoretic deposition. For all three clays, the voltammetic waves obtained for [Ru(bpy)(3)](2+) or [Os(bpy)(3)](2+) adsorbed in these CMEs were independent of the film thickness for all films thicker than 100 nm. Only in very thin films, <= 40 nm were significant decreases in the peak currents observed. However, when the contributions to the peak currents from the electroactive concentrations, C* and effective diffusion coefficients, D-eff were separated, the values of C* were found to increase with decreasing film thickness, while D-eff decreased by several orders of magnitude. This was attributed to increase contributions to the electrochemical responses from less mobile electrostatically bound cations in the thinner films. Similar variations in C* and D-eff were obtained in films prepared by solvent evaporation. However, C* obtained in 20 nm thick electrodeposited films were significantly lower than in 40 nm spin-coated films. For [Ru(NH3)(6)](3+), the peak currents increased rapidly with the film thickness. However, no significant changes in the values of C* and D-eff with film thickness were found for this ion. This is consistent with the greater mobility of [Ru(NH3)(6)](3+) in clays films that allows a larger fraction of the adsorbed ions to remain electroactive even in thicker films. Results obtained for [Fe(bpy)(3)](2+) were intermediate. While, the peak currents were independent of film thickness, the values of C* or D-eff obtained for this ion were also independent of the clay loadings. (c) 2007 Elsevier Ltd. All rights reserved.