Independent Geometrical and Electrochemical Characterization of Arrays of Nanometer-Scale Electrodes

Arrays of nanometer-scale electrodes were prepared by cyanide-etching of hexadecanethiol monolayers confined to Cu-underpotential deposition (UPD)-modified Au(111). This process results in fabrication of arrays having average electrode radii ranging from 6 to 80 nm. The arrays were independently characterized by cyclic voltammetry and scanning tunneling microscopy (STM). A recessed disk model was used to calculate an expected limiting current based on the STM-derived geometrical data, and this was correlated with the electrochemically determined value. General agreement was found between these values, but nonidealities in the distribution of electrodes within the arrays led to substantial scatter in the data. We conclude that improved array characterization will result in a reliable means to test for predicted deviation from standard microelectrode theory at nanoscale electrodes. Electrochemical responses were measured for the arrays in the presence of both Ru(NH3)6 and benzoquinone, redox probes having distinctly different heterogeneous electron-transfer rates. Highly irreversible electrochemical kinetics were observed for the kinetically slow redox probe which is a consequence of the small size of these electrodes.