Halide adsorption on close-packed metal electrodes

Two mechanisms have been cited as the reason for unexpected work function decrease upon adsorption of electronegative adatoms: electron spillout depletion [Michaelides et al. , Phys. Rev. Lett. , 2003, 90 , 246103] and polarization of the adatom [Roman et al. , Phys. Rev. Lett. , 2013, 110 , 156804]. We attempt to bridge the two pictures in this work. Work function changes due to the adsorption of halides on (111) surfaces of fcc metals (Ca, Sr, Ni, Pd, Pt, Cu, Ag, Au, Al and Pb) were studied using periodic density functional theory. The two mechanisms were found to be clearly independent of each other because of the opposite factors that lead to the work function decrease, and are therefore easy to distinguish. A more general picture of interpreting bond ionicity based on observed work function changes is discussed. In this study, we provide a complete picture of how adatoms can reduce the work function of a metal surface by contrasting two independent mechanisms.