Hydration processes on metal surfaces studied by IR and STM: a model for the potential drop across the electric double layers

Abstract

Four different hydration water molecules, a flat monomer, a tilted monomer, a tetramer cluster and an upright monomer, were observed on Ru(0 0 1). In situ scanning tunneling microscopy (STM) images of M(1 1 1)-√3×√7-(HSO − 4+H 5O 2 +) (M=Pt, Ir, Au, Ru(0 0 1)) in H 2SO 4 solution produced a zig-zag chain of hydration water molecules, revealing a large stabilization energy due to the formation of a hydrogen bonding network. Also 2×2-2CO + H 2O structure was observed on both Ru(0 0 1) electrode and Ru(0 0 1) ultra-high vacuum surfaces by STM and low energy electron diffraction. These model double layers including over-layer water molecules form a preferentially ordered structure in terms of hydrogen bonding at a negative electrode potential while also forming a disordered structure with a relatively random orientation in the over layer at a positive electrode potential. The preferential orientation of the large water dipole yields a strong electric field on the surface and lowers the frequencies of the adsorbed bisulfate S–O stretching or the CO stretching absorption band.