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

Adsorption was examined on STM-characterized graphite and glassy carbo n surfaces, in order to relate adsorption behavior to specific surface structures; The adsorption of four electroactive quinones was determi ned voltammetrically on highly ordered pyrolytic graphite (HOPG) and f ractured 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 ro ughness. STM of a large number of defects on HOPG yielded an average d efect coverage of 0.01 +/- 0.004, much too low to account for the obse rved adsorption by a simple geometric model. STM and adsorption measur ements on identical HOPG surfaces showed that adsorption tracks observ ed defect area, but with the adsorption about 30 times higher than exp ected. High-resolution STM of HOPG revealed an electronic perturbation near the step defects which was larger than the defect itself by a fa ctor of about 8. The results are consistent with quinone adsorption to the entire electronically perturbed region rather than to only the ph ysical defect. The results are inconsistent with an adsorption mechani sm based on specific chemical sites such as oxides or surface radicals . The results imply that adsorption of quinones on GC and defective HO PG depends on an electronic effect such as an electrostatic attraction between the adsorbate and partial surface charges, rather than a spec ific chemical effect.