First-principles studies of Au(100)-hex reconstruction in an electrochemical environment

Surface energies of Au(100) p(1x1) and Au(100)-hex as modeled by a p(1x5) unit cell have been calculated as a function of surface charge by the density functional method. When the surface is neutral, the surface energy of Au(100)-hex is lower than that of Au(100), consistent with the experimental observation that a Au(100) surface has a hexagonal, instead of a square top layer. Calculations show that the surface energies of both systems increase when the surfaces are positively charged and there is a crossover with increasing charge so that the Au(100)-square becomes the ground state. This suggests that the surface-to-hexagonal reconstruction observed in this material can be reversed by an external field or surface charging. The required electric field is quite large, but is achievable at metal/electrolyte interfaces. In this paper, we analyze metal/electrolyte interfacial energies and metal surface energies, discuss the possible role of specific adsorption, and compare our results to experiments by converting the calculated surface energies from surface-charge-density dependent to electrode-potential dependent, based on the relationship of the work function and potential of zero charge. Experimental results can be explained to some extent.