Morphological instability of crystal growth in nonsteady potentiostatic electrodeposition. I. The mechanism of growth of random crystals in metal deposition

摘要

The growth mechanism of irregular, roundish crystals, which commonly appear in potentiostatic deposition, was studied from the viewpoint of the growth of surface fluctuations which originate at an electrode surface. As a result, it was suggested that such irregular crystals come from the initial surface fluctuations caused by the microscopic dissolution and deposition of the electrode surface in thermodynamical equilibrium. After deriving the relationship between the spatial wave number of the fluctuation and the degree of surface irregularity (dsi), the fluctuation component with a single wave number was examined. As for actual fluctuations, since numerous components with different wave numbers are superimposed, it was necessary for graphic simulations by use of computers to be performed. However, from the analysis of only a single wave component, it was concluded that, at high cathodic overpotential, surface diffusion of adatoms exhibits a remarkable effect, viz., if there is no surface diffusion, it can be predicted that very small crystal peaks are rapidly formed owing to multiple nucleation based on the high supersaturation of adatoms; this prediction, however, is not consistent with actual observations. According to the theoretical equation obtained in the present paper, it was clearly concluded that crystal growth at high cathodic polarizations is moderated by the surface diffusion of adatoms, which are released from the concentrated nucleation sites to active lattice points, such as kinks and steps. Moreover, it was also found that the surface energy of the electrode should become constant in value at high cathodic potentials.