Potential of monolayer charge

Abstract

In this letter, we develop the concept of the potential of monolayer charge (PMC). Its main purpose is to serve as the fundamental reference potential for studying charged interfaces. We estimate PMC values for interfaces between Au(111) surface and frisbee-shaped ions. Density functional theory calculations suggest that increasing ion area shifts the PMC to an experimentally measurable potential range. To guide experimental verification, we have derived an analytical expression, which relates ion area, surface--ion distance, ionic charge, and the corresponding PMC value. Further exploration of the PMC can enrich interfacial electrochemistry and reveal interfacial electrophysics as an independent field.

Figures (4)

• Figure 1: Schematic illustration of the interfacial structure: Potential of zero charge ($\varphi_{0}$); Overscreening potential range (where the surface charge is overcompensated by a monolayer of counterions, which is then balanced by the ions in the following layers); Potential of monolayer charge ($\varphi_{\mathrm{M}}$); and Crowding potential range (where the surface charge can not be compensated by a monolayer of counterions so that counterions also accumulate in the following layer). $\varphi_{\mathrm{M}}$ stands as a milepost between overscreening and crowding. By increasing the ion area, the $\varphi_{\mathrm{M}}$ value can be lowered below the redox potential ($\varphi_\mathrm{redox}$), i. e. fitted into the electrochemical stability window.
• Figure 2: Charge density vs. distance dependence ($\rho(z)$; solid line), showing interfacial charge density fluctuations (perpendicular to the surface plane). Dashed vertical lines denote the electronic distance $l$ between the surface and ionic layer charge planes. Dots mark the positions of the surface and ion nuclei used to define the geometric distance $d$. The side view on the modeled interface is given in the background.
• Figure 3: Potential drop vs. surface dipole dependence ($\varphi(\sigma l)$, data points), indicating a decrease of the absolute $\varphi$ values with an increase of ion area. Ions are illustrated with structural formulae.
• Figure 4: Left: Generated Tersoff--Hamann scanning tunneling microscopy image. Right: The top view on the modeled atomistic structure of the N-CCor$^{+}$ monolayer on the Au(111) surface.