Implementation of Electrochemically Synthesized Silver Nanocrystallites for the Preferential SERS Enhancement of Defect Modes on Thermally Etched Graphite Surfaces

Publisher Website
Google Scholar
Abstract
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 nanostructureshaving dimensions of 300−350 Å diameter and 15 Å heightwere 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 surfacederived from “finite” graphite domains at the surfacewere 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.