Reversal of chloride-induced Cu(001) subsurface buckling in the electrochemical environment: Anin situsurface x-ray diffraction and density functional theory study

The interface of Cu(001) electrode surfaces in 10 mM HCl solution was studied by in situ surface x-ray diffraction and density functional theory, focusing on the precise structure of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ Cl adlayer formed at positive potentials. Crystal truncation rod measurements in this adsorbate phase at a potential of $\ensuremath{-}0.20\text{ }{\text{V}}_{\text{Ag}/\text{AgCl}}$ reveal distinct differences to corresponding data by Tolentino et al. [Surf. Sci. 601, 2962 (2007)] for the $c(2\ifmmode\times\else\texttimes\fi{}2)$ Cl structure formed at the Cu(001)-vacuum interface. Although in both environments, the atoms in the second Cu layer exhibit a small vertical corrugation, the sign of this corrugation is reversed. Furthermore, also the Cu-Cl bond distance and the average Cu interlayer spacings at the surface differ. Ab initio calculations performed for this adsorbate system reproduce these effects---specifically the reversal of the subsurface second-layer buckling caused in the presence of coadsorbed water molecules and cations in the outer part of the electrochemical double layer. In addition, studies at more negative potentials reveal a continuous surface phase transition to a disordered Cl adlayer at $\ensuremath{-}0.62\text{ }{\text{V}}_{\text{Ag}/\text{AgCl}}$, but indicate a substantial Cl coverage even at the onset of hydrogen evolution.