Coupled electrooxidation and electrical conduction in a single gold nanowire

The resistance, R, of single gold nanowires was measured in situ during electrooxidation in aqueous 0.10 M sulfuric acid. Electrooxidation caused the formation of a gold oxide that is approximately 0.8 monolayers (ML) in thickness at +1.1 V vs saturated mercurous sulfate reference electrode (MSE) based upon coulometry and ex situ X-ray photoelectron spectroscopic analysis. As the gold nanowires were electrooxidized, R increased by an amount that depended on the wire thickness, ranging from ΔR/R _0.10V = 14% for a 63 nm (h) × 200 nm (w) wire to 57% for an 18 nm (h) × 95 nm (w) wire at +1.1 V. These nanowires were millimeters in total length, but just 46 μm lengths were exposed to the electrolyte solution. The oxidation process and the accompanying increase in R were reversible: Reduction of the oxide at +0.10 V resulted in recovery of the reduced wire R except for a small resistance offset caused by the dissolution of ≈0.4 ML of gold during each oxidation/reduction cycle. The measured increase in R during oxidation exceeds by a factor of 4 the predicted increases in R associated with the reduction in cross-sectional area of the nanowire and the expected decrease in the specular scattering parameter, p, at the gold-oxide interface at wire surfaces. We propose that this anomalous increase in R is caused by infiltration of the oxide into the nanowire at grain boundaries.