Adiabatic versus Non-Adiabatic Electron Transfer at 2D Electrode Materials

Abstract

<div><div><div><p>Outer-sphere electron transfer (OS-ET) is a cornerstone elementary electrochemical reaction, yet microscopic understanding is largely based on idealized theories, developed in isolation from experiments that themselves are often close to the kinetic (diffusion) limit. Focusing on graphene as-grown on a copper substrate as a model 2D material/metal-supported electrode system, this study resolves the key electronic interactions in OS-ET, and identifies the role of graphene in modulating the electronic properties of the electrode/electrolyte interface. An integrated experimental-theoretical approach combining co-located multi-microscopy, centered on scanning electrochemical cell microscopy (SECCM), with Raman microscopy and field emission-scanning electron microscopy, together with rate theory and density functional theory calculations is used to address OS-ET kinetics of hexaamineruthenium (III/II) chloride, [Ru(NH3)6]3+/2+. The experimental methodology allows spatially-resolved electrochemical measurements to be targeted at distinct regions of monolayer, bilayer and multilayer graphene on copper, with high diffusion rates, to reveal ET kinetics in the order: monolayer > bilayer > multilayer. Theoretical and computational methods combining the Schmickler-Newns-Anderson model, transition state theory, and constant potential DFT reveal that the difference in kinetics at monolayer and bilayer graphene can be rationalized in the context of a dominantly adiabatic mechanism, where the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to electron transfer. This study provides a roadmap for the integration of experiments and theory in order to understand the nature of heterogeneous electron transfer at complex nanostructured electrode materials.</p></div></div></div>

Modulation of the kinetics of outer-sphere electron transfer at graphene by a metal substrate

“…In Liu et al work the authors tend to believe that ET at the copper/graphene electrode is adiabatic or weakly nonadiabatic. 34 In our previous work, 45 under the assumption of the nonadiabatic limit, we qualitatively explained the electrocatalytic effect of the gold substrate, shown in the experiment of Hui et al 24 Here, we continue this study. We investigate how the type of metal changes the electronic properties of the metal/graphene heterostructure, and how these changes affect the kinetics of the nonadiabatic ET at the graphene surface.…”

“…The rate constant values are taken for a distance of 4.5 Å from the graphene surface, which corresponds to the closest approach of the [Ru(NH 3 ) 6 ] 3+/2+ complexes. 34 We see that for platinum, which has the highest r F , such an explanation is quite applicable. Large density of states in Pt is due to the fact that the Fermi level is inside the d-band, while for Au, Cu, Ag the d-band lies below the Fermi level (see Fig.…”

“…The calculated rate constant values are taken for a distance of 4.5 Å, which corresponds to the closest approach of the [Ru(NH 3 ) 6 ] 3+/2+ complexes to the graphene surface. 34 Table 2 shows experimental standard rate constants for [Ru(NH 3 ) 6 ] 3+/2+ at the following electrodes: Au/graphene, Cu/graphene, and quasi free graphene supported on semiconductor surface. It can be seen that when gold and copper substrates are used, the rate constants have similar values (B10 À2 cm s À1 ), which is consistent with the calculations in Fig.…”

“…31,32 The data on the ET adiabaticity at the graphene surface is contradictory. 31,33,34 Nevertheless, there are many confirmations of the nonadiabatic regime of ET at graphene supported on insulating substrates if large organometallic complexes are used for measurements. In ref.…”