ANOMALOUSLY SLOW-ELECTRON TRANSFER AT ORDERED GRAPHITE-ELECTRODES - INFLUENCE OF ELECTRONIC FACTORS AND REACTIVE SITES

Electron-transfer rates for 17 inorganic redox systems plus methyl viologen were determined on highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC). Provided the HOPG defect density is low, the electron-transfer rates of all systems are much slower on the basal plane of HOPG than on GC. The slow rates on HOPG show a trend with the homogenous self-exchange rate constants, but in all cases the HOPG rate constants are substantially lower than that calculated via Marcus theory from self-exchange rates. The low HOPG rates do not exhibit any trends with redox system charge or E(1/2), as might be expected in the presence of double-layer or hydrophobic effects. The results are consistent with the semimetal properties of HOPG, which have been invoked to explain its low interfacial capacitance. Both the density of electronic states (DOS) and carrier density for HOPG are much lower than those for metals. By analogy to theories developed for electron transfer at semiconductor electrodes, the rate depends on an effectively bimolecular reaction between the redox system and carriers in the electrode. The low DOS and carrier density of HOPG leads to low electron-transfer rates compared to those of metals, or to those predicted from exchange rates. Disorder in the graphite increases electron-transfer rates and the DOS, thus yielding much faster rates on both GC and defective HOPG. For the 14 outer-sphere systems studied here, this electronic factor is much more important than any interaction with specific surface sites present at defects. The evidence indicates that, for Fe(CN)(6)(-3/-4), Eu-aq(+2/+3), Fe-aq(+2/+3), and V-aq(+2/+3), specific surface interactions provide inner-sphere routes which have a large effect on the observed rate constant.