Surface Diffusion of Underpotential-Deposited Lead Adatoms on Gold Nanoelectrodes

A simple and readily applicable voltammetric approach is described to characterize and measure the site-hopping surface diffusion of underpotential-deposited (UPD) metal adatoms at nanoelectrodes. UPD refers to the deposition of atoms on foreign metal supports at potentials lower than those predicted by Nernst law for a bulk deposition. Despite its importance in several fields of catalysis, advanced nanofabrication or even atomic nanoengineering by atomic layer epitaxy, diffusion of UPD adatoms is difficult to observe at micro- and macroelectrodes. In fact, at electrodes of usual dimensions, UPD adatoms surface diffusion is masked by other electrochemical phenomena of greater relative amplitudes. Conversely, at nanowires electrodes sealed in glass, only an extremely small fraction of the surface of the wires is exposed to the electrolyte solution and is rapidly loaded. This allows the spillage of UPD adatoms onto the much larger area of the nanowire rod which is immune to Faradaic reactions due to its isolation from the electrochemical solution by the glass casing. Therefore, surprisingly for a UPD process, voltammetric peaks currents and integral desorption charges primarily reflect these diffusional surface processes so that the integral charges vary linearly with the inverse of the square root of the scan rate. This is easily observable and measurable with usual bench-level electrochemical instrumentation. In this work, by using nanoelectrodes with diameters between 90 and 260 nm, we were able to establish the major involvement of this site-hopping surface diffusion of UPD Pb adatoms on polycrystalline gold (Au) and characterize it quantitatively. The equivalent surface diffusion coefficient of UPD Pb adatoms was determined to be similar to 4.4x10(-11) cm(2) s(-1) at room temperature, corresponding to a Gibbs free energy activation barrier of ca. 18.72 kJ mol(-1) (i. e., 0.19 eV per Pb adatom) for the reaction of inter-sites exchange of Pb adatoms on a polycrystalline Au surface.