Solid deposits of osmium bis-bipyridyl triazole chloride: Redox propertiesand electrocrystallisation

Mechanically attached, solid-state films of [Os(bpy)(2) bpt Cl] have been f ormed on platinum microelectrodes and their voltammetric properties investi gated, bpy is 2,2'-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4-triazol e. Scanning electron microscopy reveals that voltammetric cycling in 1.0 M HClO4 converts the amorphous array of microscopically small particles into a plate-like semi-crystalline form. In contrast, crystallisation does not o ccur when the films are cycled in 1.0 M NaClO4. In both electrolytes, the v oltammetric response of these films is reminiscent of that observed for an ideal reversible, solution phase redox couple. Slow and fast scan linear sw eep voltammograms have been used to provide an absolute determination of th e fixed site concentration and apparent diffusion coefficient, D-app. The f ixed site concentration is 1.65 +/-0.05 M for films cycled in either electr olyte and the D-app values increase with increasing electrolyte concentrati on, C-elec. These observations suggest that ion transport rather than the r ate of electron self-exchange limit the overall rate of charge transport th rough these solids. In 1.0 M NaClO4, D-app values for oxidation and reducti on are identical at 8.3 +/-0.5x10(-12) cm(2) s(-1). In 1.0 M HClO4, D-app i s significantly lower and depends on whether the deposit is being oxidised (9.7 +/-0.4x10(-13) cm(2) s(-1)) or reduced (6.3 +/-0.4x10(-13) cm(2) s(-1) ). These data have been used to obtain an insight into the relative importa nce of intra- vs. inter-particle charge transport. When C-elec>0.5 M, the s tandard heterogeneous electron transfer rate constant, k degrees, becomes i ndependent of the electrolyte concentration with a value of 1.7 +/-0.2x10(- 5) cm s(-1) being observed in both 1.0 M NaClO4 and HClO4. Significantly, t he distance normalised heterogeneous electron transfer rate constant for th ese solid state films is almost three orders of magnitude smaller than that found within a spontaneously adsorbed monolayer of the same complex. The i mportance of these results for the rational design of solid-state redox act ive materials for battery, display and sensor applications is considered.