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 H₂O 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/HgSO₄, a new 200 cm^-1 mode emerges congruent with hydride formation and surface reconstruction reported in electrochemical scanning tunneling microscopy studies.