Detection of New Features Associated with the Oxidation of Microcrystalline Tetrathiafulvalene Attached to Gold Electrodes by the Simultaneous Application of Electrochemical and Quartz Crystal Microbalance Techniques

Simultaneous voltammetric or double potential step and quartz crystal microbalance experiments combined with the results obtained from optical and surface analysis measurements have allowed new mechanistic aspects of the electrochemical oxidation of microcrystalline particles of tetrathiafulvalene (TTF) attached to a gold substrate, which is placed in aqueous electrolyte media, to be unravelled. The use of the microcrystalline form of the solid rather than thin films makes it possible to obtain short time domain data which enables features for the reduction and the oxidation process consistent with a nucleation process at the solid electrode-aqueous electrolyte interface to be detected. Electrochemical quartz crystal microbalance (EQCM) and electron microprobe analysis experiments confirm that the overall process in the oxidation of microcrystals of TTF in aqueous KBr, KI, KClO₄, NaBr and CsBr electrolyte media involves the uptake of the appropriate nonsolvated anion (X^-) in order to maintain charge neutrality. Reduction of the oxidized TTF leads to the expulsion of the anion. The rates of uptake and expulsion are both very rapid for the bromide ion, but involve slower reaction steps for iodide and perchlorate anions. The oxidation of TTF particles attached to a gold electrode can be described by the equation TTF_(solid)+nX^-_(solution) ⇌ TTFX_n(solid) + ne^- The number of electrons transferred to the solid, n, may theoretically vary over a wide range, but in the case of X^- = Br^- is found to be fixed with only one product formed (n = 0.7). The peak potential and shape of voltammograms have been studied as a function of scan rate, electrolyte concentration and the nature of the anion and cation. A Nernstian change in peak potential is observed when the electrolyte anion concentration is varied. The wave shape, but not the charge, also varies with electrolyte anion and concentration. In contrast, no dependence on the electrolyte cation is found. All the data imply that incorporation of the anion into the solid provides the charge neutralization mechanism. This study confirms that use of microcrystalline forms of a solid enables a wide range of time domains (and techniques) to be applied to electrochemical studies of solids.