SHINERS Study of Chloride Order–Disorder Phase Transition and Solvation of Cu(100)
David Raciti, Eric Cockayne, John Vinson, Kathleen Schwarz, Angela R. Hight Walker, Thomas P. Moffat
Abstract
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.