Probing electrocrystallization of charge transfer salts with the quartz crystal microbalance

Investigation of the electrocrystallization of low-dimensional charge-transfer salts with a piezoelectric quartz crystal microbalance (QCM) is described. Crystal growth of non-conducting [Fe(C₆Me₆)₂ ²⁺] [TCNQF₄ ^-]₂ at gold electrodes of the QCM resulted in the expected frequency decrease during crystallization. The crystallization rate was dependent upon the applied potential, and faradaic efficiencies, determined from comparison of charge and frequency changes, increased with increasing Fe(C₆Me₆)₂ ²⁺ concentration. For electrocrystallization of a low-dimensional conductor, TTFBr_0.7 (TTF = tetrathiafulvalene), the QCM response and current transients suggest 1-D growth at early times; however at long times dendritic or higher dimensionality growth occurs at a significant distance away from the surface of the QCM electrode. Repetitive stepping between potentials at which TTFBr_0.7 is alternately crystallized and reduced (with dissolution) resulted in frequency changes that were consistent with deposition of new material onto persistent TTFBr_0.7 crystal growth centers that were not removed during the reduction step. Mathematical modelling of this behavior suggests that the crystallization rate of TTFBr_0.7 is faster than its subsequent reduction and dissolution, and that growth and dissolution occurs on both the 1-D and long faces of rod-like TTFBr_0.7 deposits.