A New View of Electrochemistry at Highly Oriented Pyrolytic Graphite

Abstract

Major new insights on electrochemical processes at graphite electrodes are reported, following extensive investigations of two of the most studied redox couples, Fe(CN) 6 4−/3− and Ru(NH 3 ) 6 3+/2+ . Experiments have been carried out on five different grades of highly oriented pyrolytic graphite (HOPG) that vary in step-edge height and surface coverage. Significantly, the same electrochemical characteristic is observed on all surfaces, independent of surface quality: initial cyclic voltammetry (CV) is close to reversible on freshly cleaved surfaces (>400 measurements for Fe(CN) 6 4−/3− and >100 for Ru(NH 3 ) 6 3+/2+ ), in marked contrast to previous studies that have found very slow electron transfer (ET) kinetics, with an interpretation that ET only occurs at step edges. Significantly, high spatial resolution electrochemical imaging with scanning electrochemical cell microscopy, on the highest quality mechanically cleaved HOPG, demonstrates definitively that the pristine basal surface supports fast ET, and that ET is not confined to step edges. However, the history of the HOPG surface strongly influences the electrochemical behavior. Thus, Fe(CN) 6 4−/3− shows markedly diminished ET kinetics with either extended exposure of the HOPG surface to the ambient environment or repeated CV measurements. In situ atomic force microscopy (AFM) reveals that the deterioration in apparent ET kinetics is coupled with the deposition of material on the HOPG electrode, while conducting-AFM highlights that, after cleaving, the local surface conductivity of HOPG deteriorates significantly with time. These observations and new insights are not only important for graphite, but have significant implications for electrochemistry at related carbon materials such as graphene and carbon nanotubes.

Impact of Adsorption on Scanning Electrochemical Microscopy Voltammetry and Implications for Nanogap Measurements

“…Fresh HOPG basal surfaces were prepared by gently pressing down Scotch tape on to the sample and pullingoff the top layers, as reported extensively in the literature. 33,[60][61][62][63] Cyclic Voltammetry and SECM Instrumentation cm 2 ) with the counter and reference electrode placed into the droplet. 63 An advantage of this electrochemical cell is that it can be assembled and used within seconds of sample cleavage, 63 minimizing surface contamination.…”

“…We have shown that the behavior of some redox couples change significantly over time, 60,62,63 for a variety of reasons attributed to surface contamination, delamination, surface oxidation and other factors, and it is also known that HOPG is susceptible to atmospheric contamination. 60 Some experiments, such as SECM measurements, where one has to assemble the HOPG sample in a cell before conducting the experiment, may result in unavoidable contamination of the surface, as well as damage to the sample from compression in a cell. 60 As shown in SI, Section S-6, the amount of In the SG/TC mode ( Figure 5 (a (ii))), tip currents are larger than expected and increase as the amount of adsorbed FcTMA + is increased, due to the increased flux of FcTMA 2+ toward the SECM tip from the oxidation of substrate-adsorbed FcTMA + .…”

“…60 Some experiments, such as SECM measurements, where one has to assemble the HOPG sample in a cell before conducting the experiment, may result in unavoidable contamination of the surface, as well as damage to the sample from compression in a cell. 60 As shown in SI, Section S-6, the amount of In the SG/TC mode ( Figure 5 (a (ii))), tip currents are larger than expected and increase as the amount of adsorbed FcTMA + is increased, due to the increased flux of FcTMA 2+ toward the SECM tip from the oxidation of substrate-adsorbed FcTMA + . In this situation, the larger tip current enhancement would lead to an underestimation of tip-substrate distance (if adsorption was neglected by a researcher).…”

Face the Edges: Catalytic Active Sites of Nanomaterials

“…By applying CV for the oxidation of ferrocyanide and KCl solution at highly orient pyrolytic graphite (HOPG) electrode, on behalf of basal plane, and at an edge plane pyrolytic graphite electrode, the calculated electron transfer‐rate constant of edge is 7 orders of magnitude higher than the basal plane 214. In situ AFM215 and high resolution atomic force electrochemical microscopy216 illustrate that the basal plane is not totally inert, the electron can also transfer at pristine graphic basal plane. A recent electrochemical test using region‐specific cover with non‐conducting pinhole‐free thin film on single layer graphene sheets reveals that the edge is over 4 orders of magnitude higher specific capacitance, and has faster electron transfer rate than basal plane 217…”

Molecular Functionalization of Graphite Surfaces: Basal Plane versus Step Edge Electrochemical Activity

“…31,[34][35][36] Studies of basal plane graphite 32,33 are particularly pertinent, in light of the considerable recent revision of, and interest in, the local electrochemical activity of highly oriented pyrolytic graphite (HOPG). [37][38][39][40][41] Previous studies proposed that the step edge density on basal plane HOPG correlated with various electrochemical measurements in aqueous solution, specifically the double layer capacitance (C°), the electron transfer kinetics for the redox couple ferri/ferrocyanide, and the surface coverage (Γ ads ) of adsorbed electroactive anthraquinone-2,6-disulfonate (AQDS). 26,27,42 These studies found that cleaved HOPG surfaces with greater step edge coverage tended to display higher Γ ads for AQDS adsorption (at a particular bulk concentration).…”