43778-AC5
What is the Upper Limit for the Standard Rate Constant of a Heterogeneous Electron Transfer Reaction?
During the second year of the grant period, we continued our studies of charge transfer reactions at metal/solution nano-interfaces. Electrochemical experiments were carried out in a nanometer-sized cylindrical thin layer cell (TLC) formed by etching the surface of a disk-type platinum nanoelectrode (5- to 150-nm radius).1 Using high frequency ac voltage, the surface of such an electrode was etched to remove a very thin (≥1-nm-thick) layer of Pt. The resulting zeptoliter-scale cavity inside the glass sheath was filled with aqueous solution containing redox species, and the etched electrode was immersed in the dry (no external solution) pool of mercury to produce a TLC. Several approaches based on steady-state voltammetry and scanning electrochemical microscopy (SECM) were developed to independently evaluate the electrode radius and the etched volume. The responses of nano-TLCs could be fitted to the classical electrochemical theory.
The number of redox molecules in the TLC was varied between one and a few hundred by changing its volume and solution concentration. In this way, the transition between random and deterministic number of trapped molecules was observed. High quality steady-state voltammograms of ≥1 molecules were obtained for different neutral and charged redox species and different concentrations of supporting electrolyte. The analysis of such voltammograms yielded information about mass transfer, adsorption, electron transfer kinetics, and double-layer effects at the level of single molecules. Several unusual electrochemical phenomena observed in nano-TLCs are related to their size and unique properties. Among them are the current rectification due to nonpolarizability of the Hg electrode; strong dependence of the response on concentration of supporting electrolyte when the number of ionic species inside TLC becomes too small for the formation of two electrical double layers; and an enhanced voltammetric response to one redox species relative to the other. TLCs can be used as nano-reactors to conduct various electrochemical and chemical processes under controlled experimental conditions including spatial constrains, strong and adjustable electric field, and very fast mass-transport.
We also employed Au nanoelectrodes to investigate the effect of the electrode material on heterogeneous electron transfer (ET) kinetics.2 The extent of adiabaticity can be evaluated by probing the effect of metal on the ET rate. According to Gosavi and Marcus,3 the rate constant is expected to be proportional to the density of electronic states of the metal at the Fermi level (ρF) for nonadiabatic ET and independent of ρF in the adiabatic case. For the former case, they predicted the ratio of standard rate constants of the same ET at Pt and Au electrodes to be ~1.8 (assuming that the reorganization energy is the same for both electrodes) because the ρF of Pt is larger. While this prediction has been corroborated experimentally for ET across self-assembled monolayers,4 no similar results have yet been obtained at bare metal electrodes. We compared the results of kinetic experiments at Au nanoelectrodes to those obtained previously at similarly sized Pt electrodes.5 While for several ET reactions very similar rate constants were determined at Au and Pt surfaces, in one case (Ru(NH3)63+/2+) a significant metal effect on the ET rate was found, indicating that this reaction is not fully adiabatic.
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