Spatially Resolved Raman Spectroscopy of Carbon Electrode Surfaces: Observations of Structural and Chemical Heterogeneity

Raman spectroscopy and Raman imaging were used to examine several types of carbon electrode materials, including glassy carbon (GC) and highly ordered pyrolytic graphite (HOPG). Variations in the intensity ratio of the D and E2g Raman bands across the carbon surface indicated varying carbon microstructure. The D/E2g ratio for polished GC and pyrolytic graphite edge (PG) was relatively constant, while that of basal HOPG and PG varied significantly due to defects. The spatial heterogeneity of Rhodamine 6G Raman intensity following physisorption to carbon surfaces indicated that adsorption occurs at disordered regions, particularly defects on HOPG. This observation provides visual confirmation of previously reported correlations of defect area and physisorption. Chemisorption of dinitrophenylhydrazine was observed only at edge plane regions, confirming the localization of surface carbonyl groups on graphitic edge plane. Finally, chemisorption of nitroazobenzene radical formed from a diazonium precursor occurred at both basal and edge regions, but more rapidly at edge sites. The higher concentrations observed at edges are attributable either to more rapid reduction of the diazonium precursor or to more rapid attack of the radical, compared to basal plane. The results represent the first spatially resolved Raman examination of physiand chemisorption at the monolayer level on carbon surfaces.