SCANNING-TUNNELING-MICROSCOPY OF ORDERED GRAPHITE AND GLASSY-CARBON SURFACES - ELECTRONIC CONTROL OF QUINONE ADSORPTION

Adsorption was examined on STM-characterized graphite and glassy carbon surfaces, in order to relate adsorption behavior to specific surface structures; The adsorption of four electroactive quinones was determined voltammetrically on highly ordered pyrolytic graphite (HOPG) and fractured glassy carbon (GC). The average surface coverage on HOPG was 0.25-0.50, while that on GC was 2.7-4.0, consistent with GC surface roughness. STM of a large number of defects on HOPG yielded an average defect coverage of 0.01 +/- 0.004, much too low to account for the observed adsorption by a simple geometric model. STM and adsorption measurements on identical HOPG surfaces showed that adsorption tracks observed defect area, but with the adsorption about 30 times higher than expected. High-resolution STM of HOPG revealed an electronic perturbation near the step defects which was larger than the defect itself by a factor of about 8. The results are consistent with quinone adsorption to the entire electronically perturbed region rather than to only the physical defect. The results are inconsistent with an adsorption mechanism based on specific chemical sites such as oxides or surface radicals. The results imply that adsorption of quinones on GC and defective HOPG depends on an electronic effect such as an electrostatic attraction between the adsorbate and partial surface charges, rather than a specific chemical effect.