Distance-Independent Charge-Transfer Resistance at Gold Electrodes Modified by Thiol Monolayers and Metal Nanoparticles

Abstract

The electrochemical properties of Au electrodes sequentially modified by self-assembled monolayers (SAM) of carboxyl-terminated alkane thiols, ultrathin poly-L-lysine (PLL) film, and diluted monolayers of Au nanoparticles are investigated by electrochemical impedance spectroscopy (EIS). The phenomenological chargetransfer resistance (R ct ) for the hexacyanoferrate redox couple at the equilibrium potential exhibited an exponential increase with increasing methylene units (x) in the SAM. The increase of R ct between x ) 1 and 10 was described by a well-defined decay parameter β ) 1.16 ( 0.04 per methylene unit. This behavior suggests that the kinetics of electron transfer is controlled by coherent electron tunneling across the carboxylterminated SAM. Adsorption of the PLL brings about an average 2.5 times decrease in R ct independent of x. The ultrathin PLL film (thickness less than 1 nm) induces an increase of the surface concentration of the redox couple without affecting the β value observed for the SAM-terminated electrodes. Diluted monolayers of Au nanoparticles with an average 19.2 ( 2.1 nm diameter generate significant changes in the dynamics of electron transfer. In contrast to the behavior in the absence of nanoparticles, a distance-independent R ct was observed for x > 5. Detailed analysis of the electrochemical responses as a function of the particle number density revealed that the rate-determining step is the charging of the nanoparticles by the redox species. It is concluded that the electronic communication between the nanoparticles and the electrode surface over distances as large as 13 Å originates from electron transport through the trapped redox probe. The several orders of magnitude changes of the apparent R ct upon nanoparticle adsorption further suggest that electron transport through the film does not occur via a classical hopping mechanism. A mechanism based on nonthermalized electron transport via the density of the redox probe at the Fermi energy (hot electron transport) is proposed to account for the experimental observations.

Electrocatalysis by crown-type polyoxometalates multi-substituted by transition metal ions; Comparative study

“…3B), that the increase in RCT with layer number for LBL films of both Co6POM and Ni7POM can be described with a single exponential function up to 8 steps of assembly. This effect strongly confirms, that electron transfer between the ferri/ferrocyanide couple and the underlying electrode surface is controlled by coherent electron tunnelling across the LBL film [34], which can be expressed in terms of the tunnelling decay constant  [34,35]:…”

“…The slopes obtained for both POM-based films give almost the same decay constants  approximately of 0.2 per layer. The decay constants for thiol monolayers formed at gold surfaces per methylene unit are up to two times higher than obtained here [34].…”

Tracking Nanoelectrochemistry Using Individual Plasmonic Nanocavities

“…This implies that the resistance of each BPT molecule is far below the quantum conductance, as previously suggested 17 . However this disagrees with [19][20][21][22].…”

Charge Transfer Within Multilayered Films of Gold Nanorods

“…The charge transfer through these types of molecular linkers is characterized by a decay of the tunnelling current with the length of the linker, the tunnelling coefficient being around β≈0.8Å -1 (Adams et al, 2003;Finklea, 1987Finklea, , 1996. However, recent studies demonstrate that the tunnelling decay is not the dominant property of the films when the nanomaterials are tethered at the end of the molecular bridges determining a fast electron transfer process and moreover, this fast electron transfer process is dependent on the size and the shape of the nanomaterials (Bradbury et al, 2008;Chirea et al, 2009Chirea et al, , 2010Kissling et al, 2010). A more detailed research has to be developed in order to fully understand the mechanism behind this fast electron transfer processes taking place at electrodes modified with hydrid nanostructures.…”