ANTHRAQUINONEDISULFONATE ADSORPTION, ELECTRON-TRANSFER KINETICS, AND CAPACITANCE ON ORDERED GRAPHITE-ELECTRODES - THE IMPORTANT ROLE OF SURFACE-DEFECTS

The adsorption of anthraquinone-2,6-disulfonate, disodium salt (AQDS), was found to be well behaved at glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) electrodes. On laser-activated GC, the surface excess obeys a Langmuirian isotherm in the concentration range of 5 x 10(-8) to 1 x 10(-5) M with a saturation coverage of 228 pmol/cm2. AQDS at untreated HOPG electrodes was studied more extensively. Adsorption at basal plane HOPG was discovered to occur solely on edge plane defects. Defects on basal plane HOPG can be created adventitiously in the cleaving process or can be controlled by laser irradiation at varying power densities. The amount of AQDS adsorbed correlates with laser power density. Surface excess on HOPG also obeys a Langmuir isotherm with the saturation coverage depending on the defect density. On conventionally cleaved HOPG surfaces, the amount of AQDS adsorption correlates with other electrochemical parameters, the magnitudes of which are known to depend on edge plane density. These parameters are the heterogeneous electron-transfer rate constant of the ferri/ferrocyanide redox couple and differential capacitance. A new low differential capacitance of less than 1.0-mu-F/cm2 has also been observed on near-perfect basal plane HOPG. The unusual ferrocyanide voltammograms observed on low-defect HOPG are consistent with a potential-dependent transfer coefficient.