(Invited) Nanogap Voltammetry of Clean Surface of Electron-Beam-Deposited Carbon

Abstract

Clean surfaces of conducting carbon materials are highly demanded for fundamental and applied electrochemistry to mechanistically understand and practically utilize their intrinsic reactivity. In this presentation, we will discuss about the nanoelectrochemical characterization of clean carbon surfaces to gain novel insights into heterogeneous electron-transfer (ET) mechanism with broad implications beyond carbon electrochemistry. Electron-beam deposited carbon (eC) is formed in high vacuum (<6 µtorr) and protected from adventitious contamination by a washable KCl layer deposited without breaking vacuum. Fast ET kinetics of exceedingly flat, clean eC surfaces is reliably measured in ultrapure water using nanogap voltammetry based on scanning electrochemical microscopy and compared quantitatively with Marcus and Frumkin theories. An outer-sphere ET mechanism, which is typically only presumed, is evidenced experimentally for (ferrocenylmethyl)trimethylammonium and tris(1,10-phenanthroline)cobalt(II) (Co(phen)3 2+) by excluding an inner-sphere mechanism. Indeed, the oxidation of Co(phen)3 2+ agrees unprecedentedly well with Marcus theory of adiabatic outer-sphere kinetics in comparison to any redox couple on carbon or metal electrodes reported previously. The coupled reduction of Co(phen)3 3+ deviates from Marcus theory to reveal a unique double-layer effect caused by the adsorption of a redox molecule itself, i.e. Co(phen)3 2+, which contrasts to the Frumkin effect based on the adsorption of inert electrolytes. Remarkably, the Ru(NH3)6 3+/2+ couple exceeds adiabatic limits not only at eC, but also at other carbons and metals, which we attribute to faster inner-sphere ET of adsorbed forms of this couple. In contrast, an inner-sphere pathway is not mediated by the adsorbed Fe(CN)6 3–/4– couple, which dramatically self-decelerates the outer-sphere pathway through a double-layer effect.