Electrochemical Deposition and Reoxidation of Au at Highly Oriented Pyrolytic Graphite. Stabilization of Au Nanoparticles on the Upper Plane of Step Edges

The electrochemical deposition and reoxidation of Au on the basal plane of highly oriented pyrolytic graphite (HOPG) immersed in a 5 mM AuCl4 -/6 M LiCl solution is reported. Scanning electron microscopy (SEM) and ex-situ atomic force microscopy (AFM) demonstrate that Au nanoparticles, ∼3.3 nm in height and ∼10 nm in diameter, are deposited at times less than ∼1 s. The density of nanoparticles, 6 × 109 cm-2, is of the same order of magnitude as the surface point defect density, suggesting that point defects act as nucleation sites for Au electrodeposition. A small subset of the Au nanoparticles (∼7%) continues to grow between 1 and 50 s, reaching a height of ∼150 nm and a diameter of ∼300 nm. At times greater than 50 s, the larger particles coalesce to yield a surface comprised of a low density (∼2 × 106 cm-2) of micrometer-size Au crystallites surrounded by Au nanoparticles. Double potential step chronocoulometric experiments demonstrate that the electrodeposition of Au is chemically irreversible, a finding supported by SEM and AFM observations of Au nanoparticles and larger crystallites on the surface after long periods of reoxidation (>3600 s). Au nanoparticles are observed to be preferentially deposited on the upper plane of step edges, a consequence of the nonuniform surface electron density that results from relaxation of the graphite lattice near steps.