QUANTITATIVE CORRELATIONS OF HETEROGENEOUS ELECTRON-TRANSFER KINETICS WITH SURFACE-PROPERTIES OF GLASSY-CARBON ELECTRODES

Raman spectra, capacitance (C°), phenanthrenequinone (PQ) adsorption, and heterogeneous electron-transfer rates for ferri/ferrocyanide, dopamine, and ascorbic acid were monitored after fracturing, polishing, and laser activating glassy carbon electrodes (GC-30). Alterations in the Raman spectrum indicate changes in carbon microstructure, while PQ adsorption and C° provide measures of microscopic surface area. It was observed that polishing caused minor changes in carbon disorder and microscopic surface area, but the polished surface had poor electron-transfer kinetics. Laser activation increased k° for Fe(CN)63-/4-by at least a factor of 200 but increased PQ adsorption and C° by less than 50% and had negligible effects on the Raman spectrum. A k° of above 0.5 cm s−1was observed for Fe(CN)6for the first time. A clean, fractured GC surface exhibited a k° of 0.5 cm s−1and was very active toward ascorbic acid and dopamine oxidation. The results are consistent with a surface-cleaning mechanism for laser activation, accompanied by little or no observable surface restructuring or roughening. The results on GC are in contrast to those on laser activation of HOPG, where the mechanism involved formation of active sites. The conclusions reached here permit evaluation of the main variables affecting electron-transfer rate for Fe(CN)6, ascorbic acid, and dopamine on GC. We conclude that the active sites for electron transfer are on graphite edges inherent in the GC structure, and the principal function of the laser is exposure of these sites by removal of chemi- and physisorbed impurities. © 1990, American Chemical Society. All rights reserved.