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

Abstract

Mechanically attached, solid-state films of [Os(bpy)₂ bpt Cl] have been formed on platinum microelectrodes and their voltammetric properties investigated, bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4-triazole. 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 occur when the films are cycled in 1.0 M NaClO₄. In both electrolytes, the voltammetric response of these films is reminiscent of that observed for an ideal reversible, solution phase redox couple. Slow and fast scan linear sweep voltammograms have been used to provide an absolute determination of the fixed site concentration and apparent diffusion coefficient, D_app. The fixed site concentration is 1.65 ± 0.05 M for films cycled in either electrolyte and the D_app values increase with increasing electrolyte concentration, C_elec. These observations suggest that ion transport rather than the rate of electron self-exchange limit the overall rate of charge transport through these solids. In 1.0 M NaClO₄, D_app values for oxidation and reduction are identical at 8.3 ± 0.5 × 10⁻¹² cm² s⁻¹. In 1.0 M HClO₄, D_app is significantly lower and depends on whether the deposit is being oxidised (9.7 ± 0.4 × 10⁻¹³ cm² s⁻¹) or reduced (6.3 ± 0.4 × 10⁻¹³ cm² s⁻¹). These data have been used to obtain an insight into the relative importance of intra- s. inter-particle charge transport. When C_elec>0.5 M, the standard heterogeneous electron transfer rate constant, k°, becomes independent of the electrolyte concentration with a value of 1.7 ± 0.2 × 10⁻⁵ cm s⁻¹ being observed in both 1.0 M NaClO₄ and HClO₄. Significantly, the distance normalised heterogeneous electron transfer rate constant for these solid state films is almost three orders of magnitude smaller than that found within a spontaneously adsorbed monolayer of the same complex. The importance of these results for the rational design of solid-state redox active materials for battery, display and sensor applications is considered.