Surface relaxation and surface stress of Au(1 1 1)

Changes in surface stress and in the top-layer expansion of Au(1 1 1) electrodes in sulfuric acid have been measured as a function of electrode potential by combining surface stress and X-ray diffraction measurements. Both are linear functions of interfacial charge in the electrode potential range of changing anion coverage. Over this range the surface stress changes by −0.5 N m−1 (compressive direction), while the outward top-layer relaxation decreases from +1.5% to +0.2%. The surface stress changes can be rationalized in terms of a jellium model, while ab initio simulations are needed to explain the top-layer expansion. These simulations yield +1.3% relaxation for the uncharged gold surface, in good agreement with the X-ray diffraction measurements. They also demonstrate that the outward relaxation of the surface is curbed in the presence of an electron withdrawing adsorbate (Cl), which mimics the effects of positive surface charging.