SHINERS Study of Chloride Order-Disorder Phase Transition and Solvation of Cu(100).

Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and density functional theory (DFT) are used to probe Cl - adsorption and the order-disorder phase transition associated with the c(2 × 2) Cl - adlayer on Cu(100) in acid media. A two-component ν(Cu-Cl) vibrational band centered near 260 ± 1 cm -1 is used to track the potential dependence of Cl - adsorption. The potential dependence of the dominant 260 cm -1 component tracks the coverage of the fluctional c(2 × 2) Cl - phase on terraces in good agreement with the normalized intensity of the c(2 × 2) superstructure rods in prior surface X-ray diffraction (SXRD) studies. As the c(2 × 2) Cl - coverage approaches saturation, a second ν(Cu-Cl) component mode emerges between 290 and 300 cm -1 that coincides with the onset and stiffening of step faceting where Cl - occupies the threefold hollow sites to stabilize the metal kink saturated Cu <100> step edge. The formation of the c(2 × 2) Cl - adlayer is accompanied by the strengthening of ν(O-H) stretching modes in the adjacent non-hydrogen-bonded water at 3600 cm -1 and an increase in hydronium concentration evident in the flanking H 2 O modes at 3100 cm -1 . The polarization of the water molecules and enrichment of hydronium arise from the combination of Cl - anionic character and lateral templating provided by the c(2 × 2) adlayer, consistent with SXRD studies. At negative potentials, Cl - desorption occurs followed by development of a sulfate ν s (S═O) band. Below -1.1 V vs Hg/HgSO 4 , a new 200 cm -1 mode emerges congruent with hydride formation and surface reconstruction reported in electrochemical scanning tunneling microscopy studies.