Pattern selection during electropolishing due to double-layer effects

We extend our earlier study of nanoscale pattern formation during electropolishing [Nanotechnology 7, 360 (1996); Phys. Rev. B 56, 12 608 (1997)]. The patterns are attributed to preferential adsorption of organic molecules on the convex portion of the electrode due to its enhanced electric field. This local enhancement occurs because of the effect of surface curvature on the double-layer potential drop. By allowing for transport correction to the double-layer potential drop at thermodynamic equilibrium, we estimate this anodic overpotential to be in the realistic mV range and hence verify the Debye-Huckel approximation used in our model. This small anodic overpotential suggests that pattern formation is a generic electropolishing phenomenon whose only requirement is that the polarizability of the organic additive relative to water must lie within a range specified by our theory. We verify this prediction experimentally with a variety of electrolyte solutions. The voltage ranges for specific hexagonal and ridge patterns are well correlated by our model with only a single parameter. (c) 1999 American Institute of Physics.