Anthraquinone Monosulfonate Adsorbed on Graphite Shows Two Very Different Rates of Electron Transfer: Surface Heterogeneity Due to Basal and Edge Plane Sites

Abstract

Graphitic electrodes find widespread use throughout electrochemistry; understanding their fundamental electrochemical properties is imperative. It is widely thought that graphite edge plane sites exhibit faster rates of electron transfer as compared to basal plane sites. Hitherto the different rates of electron transfer at the edge and basal sites have been inferred indirectly using diffusional systems. To avoid possible complications we alternatively study a surface-bound system to simplify the interpretation. The voltammetric response of graphitic-surface-bound anthraquinone monosulfonate (AQMS) with varying pH, reveals two distinct voltammetric responses, ascribed as being due to the basal and edge plane sites; where the pK(a) s associated with the reduced anthraquinone are found to differ for the two sites. Through modelling of the system based upon a "scheme of squares" mechanism it is possible to conclude that both the thermodynamics and kinetics of the species differ in the two environments in which the rate of electron transfer at the basal plane site is shown to be 2-3 orders of magnitude slower than that of the edge plane site. This work provides the first example of a voltammetric response which is purely due to electron transfer at a basal plane site. Further, we believe this is the first time a full "scheme of squares" model has been used for the quantitative analysis of a diffusionless 2H(+)2e(-) system.