Electron transport dynamics in a room-temperature Au nanoparticle molten salt

A room-temperature Au₃₈ nanoparticle polyether melt has been prepared by exchanging poly(ethylene glycol) (PEG) thiolate ligands, HS-C6-PEG₁₆₃, into the organic protecting monolayer of Au ₃₈(PhC2)₂₄ nanoparticles. Spectral and electrochemical properties verify that the Au₃₈ core size is preserved during the exchange. Adding LiClO₄ electrolyte, free PEG plasticizer, and/or partitioned CO₂ leads to an ionically conductive nanoparticle melt, on which voltammetric, chronoamperometric, and impedance measurements have been made, respectively, of the rates of electron and ion transport in the melt. Electron transport occurs by electron self-exchange reactions, or electron hopping, between diffusively relatively immobile Au₃₈⁰ and Au₃₈¹⁺ nanoparticles. The rates of physical diffusion of electrolyte ions (diffusion coefficients DCION) are obtained from ionic conductivities. The measured rates of electron and of electrolyte ion transport are very similar, as are their thermal activation energy barriers, observations that are consistent with a recently introduced ion atmosphere relaxation model describing control of electron transfer in semisolid ion and electron-conductive media. The model has been previously demonstrated using a variety of metal complex polyether melts; the present results extend it to electron transfers between Au nanoparticles. In ion atmosphere relaxation control, measured rates and energy barriers for electron transfer are not intrinsic values but are instead characteristic of competition between back-electron transfer caused by a Coulombic disequilibrium resulting from an electron transfer and relaxation of counterions around donor-acceptor reaction partners so as to reachieve local electroneutrality.