Effect of applied potential on metal surfaces: Surface energy, Wulff shape and charge distribution

• A new method is proposed to calculate the surface energy under applied potential. • Solvation and applied potential have negligible effect on surface energies and the Wulff shape of the metals considered in this study. • The atom whose charge is most sensitive to applied potential is the most favorable adsorption site and hence, the active site for the electrochemical reaction. • The active sites of a catalyst can be determined by calculating the charge density on the surface atoms under an applied potential, which is computationally less demanding than calculating the adsorption energies of all possible adsorption sites. We use grand canonical density functional theory to predict the surface energies, Wulff shapes, charge distributions and catalytically active sites of different metal surfaces under electrochemical conditions. We propose a method for computing surface energies from grand canonical density functional theory (GC-DFT) calculations of periodic slab models and use it to compute the surface energies of the facets of Pt, Cu, and Ag crystals to predict their Wulff shapes under electrochemical conditions. GC-DFT predicts that, for the pure metals studied, solvation only slightly affects the Wulff shape while applied potentials considerably affect the surface energies and corresponding Wulff shapes. We used Bader charge analysis of GC-DFT computed electron densities to investigate the effect of applied potential on the distribution of electron density over the atoms of the surfaces of Pt, Cu, Ag, and the 75-25 Ag-Pt and Au-Ni alloys. This analysis shows that, under an applied potential, the electron density is unevenly distributed over the surface atoms and that the charges of atoms more exposed to solvent are more sensitive to bias. Our results show that the most sensitive atom to bias can be used to identify the most favorable adsorption site and thus, the active sites of electrochemical reactions.