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 vio logen were determined on highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC). Provided the HOPG defect density is low, the elect ron-transfer rates of all systems are much slower on the basal plane o f HOPG than on GC. The slow rates on HOPG show a trend with the homoge nous self-exchange rate constants, but in all cases the HOPG rate cons tants are substantially lower than that calculated via Marcus theory f rom 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 presen ce of double-layer or hydrophobic effects. The results are consistent with the semimetal properties of HOPG, which have been invoked to expl ain its low interfacial capacitance. Both the density of electronic st ates (DOS) and carrier density for HOPG are much lower than those for metals. By analogy to theories developed for electron transfer at semi conductor electrodes, the rate depends on an effectively bimolecular r eaction between the redox system and carriers in the electrode. The lo w DOS and carrier density of HOPG leads to low electron-transfer rates compared to those of metals, or to those predicted from exchange rate s. Disorder in the graphite increases electron-transfer rates and the DOS, thus yielding much faster rates on both GC and defective HOPG. Fo r the 14 outer-sphere systems studied here, this electronic factor is much more important than any interaction with specific surface sites p resent 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 interact ions provide inner-sphere routes which have a large effect on the obse rved rate constant.