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 is used to track the potential dependence of Cl adsorption. The potential dependence of the dominant 260 cm 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 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 and an increase in hydronium concentration evident in the flanking HO modes at 3100 cm. 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═O) band. Below -1.1 V vs Hg/HgSO, a new 200 cm mode emerges congruent with hydride formation and surface reconstruction reported in electrochemical scanning tunneling microscopy studies.