Surface relaxation phenomena at electrified interfaces: Revealing adsorbate, potential, and solvent effects by combined x-ray diffraction, STM and DFT studies

Abstract

Surface relaxation phenomena have been studied in an electrochemical environment using halide modified Cu(100) electrodes as model systems to unravel the impact of the chemical nature of the adsorbed halide, the applied potential, and the presence of solvent species on the surface interlayer spacings. Both, in situ STM and in situ x-ray scattering data point to lateral structures of the adsorbed halides on Cu(100) which are identical for both chloride and bromide. Under saturation conditions both halides form a $p(1\ifmmode\times\else\texttimes\fi{}1)$ adlayer on Cu(100) with reference to a conventional choice of the substrate fcc unit cell. The in situ x-ray scattering data clearly indicate that the copper-halide and the copper-copper interlayer spacings are much more affected by potential changes when bromide is adsorbed on the copper surface and are less affected when chloride is present. This difference in the potential dependence of both halides can be attributed to the larger polarizability of the bromide anion that is almost discharged on the copper surface at the highest applied potentials, while chloride remains largely ionic in the adsorbed state even at the highest applied potential. At the lowest applied potential of ${E}_{\text{work}}=\ensuremath{-}150\text{ }\text{mV}$ [vs reversible hydrogen electrode (RHE)] the Br-Cu and the topmost Cu-Cu layer distances are expanded by 0.150 and $0.058\text{ }\text{\AA{}}$, respectively, with reference to their bulk analogs CuBr and Cu. These spacings continuously contract by up to 0.075 and $0.038\text{ }\text{\AA{}}$ when the electrode potential is increased to ${E}_{\text{work}}=+50\text{ }\text{mV}$ (RHE). Intriguingly, the second Cu layer experiences a potential-dependent buckling due to a different second-shell coordination of Cu by bromide while deeper Cu layers retain the bulk spacing at all potentials. Changes in the halide-copper and the copper-copper interlayer spacings are strongly correlated. An understanding of the in situ x-ray results is achieved by periodic quantum-chemical calculations at density-functional level that allow a modeling of the interfacial structure under consideration of potential and additional solvation effects. The latter originate from interaction of water molecules and counterions in the outer Helmholtz layer with the specifically adsorbed halides in the inner Helmholtz layer.