ABSTRACT
This paper describes microelectrode voltammetry measurements of self-diffusion coefficients and of heterogeneous Co(II/III) electron-transfer rate constants (ko) in undiluted molten salts of three cobalt tris(bipyridine) perchlorate complexes in which the bipyridine ligands are "tailed" with poly(propylene oxide) and poly(ethylene oxide) oligomers. The self-diffusion coefficients are measured with potential step chronoamperometry and range from 10(-12) to 10(-17) cm2/s, while the quasi-reversible reaction rate constants are measured using cyclic voltammetry and small potential steps and range from 10(-7) to 10(-12) cm/s. The ko measurements are unusual in that when rate constants become smaller, the reaction remains quasi-reversible, because of concurrently decreasing self-diffusion rates. The measurements are, furthermore, accomplished in the face of uncompensated resistances that range from mega- to gigaohms, which is made possible by the combination of microelectrode properties and small diffusivities. The melt in which self-diffusion and ko values are smallest is at a temperature below its nominal glassing transition and in the regime of molecule-scale diffusion profiles.
ABSTRACT
A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt.