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

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 (DFT) 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. To rationalize these findings we extended the Schmickler-Newns-Anderson model Hamiltonian for electron transfer and parametrized it using constant potential DFT. Combining this model with rate theory reveals 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, theory, and simulations in order to understand the nature of heterogeneous electron transfer at complex nanostructured electrode materials.