The formation and properties of single nuclei

The formation and growth of single metallic nuclei of silver, mercury and copper have been studied at microscopically small electrodes of platinum and carbon. Once formed, metallic nuclei act as point sinks, growing under hemispherical mass-transfer control. The rate of growth at low overpotential is a function of the mean surface concentration as determined by the Nernst relation. The rate of formation of isolated nuclei has been determined as the inverse of the delay time attending their birth as indicated by the onset of the growth current. A distribution of delay times is observed in keeping with the statistical nature of the nucleation process. The nuclei or crystallites formed are spherical droplets in the case of mercury or microscopic single crystals in the case of solid metals, their size being accessible from the current—time integral of their growth. They are stable on open circuit and exhibit the residual overpotential of their excess surface free energy, ie their Gibbs—Kelvin potential. This potential is a linear function of their inverse equivalent spherical radius. The surface tensions calculated from the simplest application of the Gibbs—Kelvin equations appear to be higher than the known or estimated bulk values. The microscopic metallic crystallites have been used as reactive electrodes. The high mass-transfer flux to their surface enabled the exchange current to be determined by a simple, steady state small amplitude dc procedure. These studies confirm the considerable promise of microscopically small electrodes in electrochemistry.