Surface X-ray diffraction study of Cu UPD on Au(1 1 1) electrode in 0.5 M H2SO4 solution: the coadsorption structure of UPD copper, hydration water molecule and bisulfate anion on Au(1 1 1)

The structure of underpotential deposition (UPD) copper, a hydration water molecule, and a bisulfate anion on Au(1 1 1) surface in 0.5 M H2SO4 solution has been determined by in situ surface X-ray diffraction study. The bisulfate anion and the UPD copper form a √3×√3R30° structure on an Au(1 1 1) electrode at a potential between 250 and 400 mV. At this potential range, UPD Cu, a bisulfate anion, and a hydration water molecule coadsorb on Au(1 1 1), while the coverages of copper and bisulfate anion are 0.6667 and 0.3333, respectively. The coadsorption structure is represented by Laue symmetry of P31m. On top of each copper atom, a hydration water molecule is accommodated to form a stable and closest-packed water adlayer with a large density. At a more positive potential than 940 mV, the bisulfate anion forms a √3×√7 structure on a bare Au(1 1 1) surface.