IMPLEMENTATION OF ELECTROCHEMICALLY SYNTHESIZED SILVER NANOCRYSTALLITES FOR THE PREFERENTIAL SERS ENHANCEMENT OF DEFECT MODES ON THERMALLY ETCHED GRAPHITE SURFACES

Highly oriented pyrolytic graphite (HOPG) surfaces, on which atomicall y well-defined roughness has been introduced via high-temperature gasi fication reactions, are investigated by noncontact mode atomic force m icroscopy (NC-AFM) and Raman spectroscopy both before and after the el ectrochemical deposition of silver nanocrystallites on these surfaces. Exposure of freshly cleaved HOPG surfaces to an O-2-rich ambient at 6 50 degrees C for a few minutes caused the formation of 1-monolayer-dee p, circular etch pits on the HOPG basal plane surface. Silver nanocrys tallites were electrochemically deposited onto these etched surfaces a t two coverages: 0.5 mC cm(-2) (or 5 nmol of Ag-0 cm(-2)) and 2.4 mC c m(-2) (25 nmol of Ag-0 cm(-2)). At the lower coverage, NC-AFM images r evealed that silver decorated only the circumference of the circular e tch pits, forming a uniform annular ring with an apparent diameter of 200-250 Angstrom and a height of similar to 15 Angstrom. At the higher silver coverage, an increase in the height but not the diameter of th is annulus was observed, and additional silver nanostructures-having d imensions of 300-350 Angstrom diameter and 15 Angstrom height-were obs erved on atomically smooth regions of the graphite basal plane. The Ra man spectroscopy of these surfaces was investigated and compared with spectra for nanocrystallite-modified but unetched HOPG basal plane sur faces and thermally etched surfaces on which no silver was deposited. For for thermally etched HOPG surfaces at either silver coverage, SERS -augmented Raman spectra were obtained in which defect modes of the gr aphite surface-derived from ''finite'' graphite domains at the surface -were strongly and preferentially enhanced. In addition, an enhanced b and near 2900 cm(-1) was assigned to v(OH) from carboxylate moieties p resent at step edges based on the basis of the observed pH dependence of the enhancement.