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 atomically well-defined roughness has been introduced via high-temperature gasification reactions, are investigated by noncontact mode atomic force microscopy (NC-AFM) and Raman spectroscopy both before and after the electrochemical deposition of silver nanocrystallites on these surfaces. Exposure of freshly cleaved HOPG surfaces to an O2-rich ambient at 650 °C for a few minutes caused the formation of 1-monolayer-deep, circular etch pits on the HOPG basal plane surface. Silver nanocrystallites were electrochemically deposited onto these etched surfaces at two coverages: 0.5 mC cm-2 (or 5 nmol of Ag0 cm-2) and 2.4 mC cm-2 (25 nmol of Ag0 cm-2). At the lower coverage, NC-AFM images revealed that silver decorated only the circumference of the circular etch pits, forming a uniform annular ring with an apparent diameter of 200−250 Å and a height of ∼15 Å. At the higher silver coverage, an increase in the height but not the diameter of this annulus was observed, and additional silver nanostructureshaving dimensions of 300−350 Å diameter and 15 Å heightwere observed on atomically smooth regions of the graphite basal plane. The Raman spectroscopy of these surfaces was investigated and compared with spectra for nanocrystallite-modified but unetched HOPG basal plane surfaces 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 graphite surfacederived from “finite” graphite domains at the surfacewere strongly and preferentially enhanced. In addition, an enhanced band near 2900 cm-1 was assigned to νOH from carboxylate moieties present at step edges based on the basis of the observed pH dependence of the enhancement.