Electronic Properties of a Monolayer−Electrolyte Interface Obtained from Mechanistic Impedance Analysis
By Gupta, Chaitanya; Shannon, Mark A. & Kenis, Paul J. A.
Published in The Journal of Physical Chemistry C
2009
Abstract
We report here a methodology to measure and characterize the interface properties of thin, insulating films that separate a gold electrode and an electrolyte solution, for cases when the electrolyte solution does not contain electro-active ions in appreciable amounts, by using experimental impedance data. Traditionally, in the absence of redox-active species either in the electrolyte or at the terminal headgroup, monolayer films on polycrystalline gold electrodes have been modeled as ideal dielectric capacitors. However, potential-dependent background currents are usually observed for gold-monolayer-electrolyte systems, even in the absence of redox-active moieties. A qualitative description of the background current density as a barrier-limited flux of charge that applies irrespective of the actual mechanism of charge transport through the monolayer film is proposed in this paper. The potential-dependent, low-frequency impedance data is shown to characterize the properties of this limiting energy barrier. On the basis of this description for the flow of charge through a gold-monolayer-electrolyte system, we modified the equation for charge transport to yield a constitutive expression for the low-frequency admittance of the gold-monolayer-electrolyte system. The derived expression is equally applicable to charge transport via electronic currents through the monolayer phase or by ion migration through pinhole defects in the monolayer film. The mobility and diffusivity of charge carriers within the monolayer film are unknown parameters in the equation for the system admittance. An approach that enables the estimation of these transport parameters is outlined. A comparison of the calculated mobility with published data reveals that electron transport through the monolayer phase, and not ion penetration through pinhole defects, is the dominant mechanism for charge transport. We fit the derived admittance equation containing these estimated transport parameters to the experimentally determined low-frequency admittance to provide a quantitative description of the electronic properties of the monolayer-electrolyte interface in terms of two related physicochemical properties: (a) the equilibrium chemical potential difference between the metal and the monolayer-electrolyte interface and (b) the residual or built-in electric field within the monolayer. These two properties characterize the monolayer-electrolyte interface within a specific potential regime where the current density is shown to be limited by the kinetics of electron transport through the monolayer. We also demonstrate a qualitative analogy between the gold-monolayer-electrolyte system and a Mott-Schottky rectifying barrier, on the basis of our description of the monolayer-electrolyte interface in terms of these two properties, that helps explain the anisotropic current-voltage characteristics of the monolayer-on-gold system. The effects of varying electrolyte pH, monolayer film thickness and terminal headgroup on the two physicochemical properties are also analyzed here.