Surface X-ray Diffraction Studies of Electrode/Vacuum and Electrode/Electrolyte Interfaces

Abstract

This thesis presents in situ Surface X-ray Diffraction (SXRD) studies of surfaces and interfaces, in both Ultra High Vacuum (UHV) and an electrochemical environment. Primarily Crystal Truncation Rod (CTR) measurements are utilised to determine a model for the atomic structure at the interface. A SXRD characterisation of the clean Ag(110) and Ag(111) surfaces in UHV were determined as a reference for the rest of the work in this thesis. Following this the growth conditions and structures of a silicene layer on Ag(111) were investigated, by Low Energy Electron Diffraction (LEED) and preliminary SXRD study of the interface structure is presented. A comprehensive study of the Ag(hkl)/alkaline interface is presented. X-ray Voltammetry (XRV) measurements have been performed to determine the potential dependence of the system. CTR measurements have been used to determine the structure at both the electrode and electrolyte sides of the interface. The results reveal large structural changes on the electrolyte side of the interface, with the response of relaxation of the surface layers in the metal. The presence of specifically adsorbed OH on the surface stabilises cations in a compact double layer through non-covalent interactions. The studies were extended to determine the effects of saturating the electrolyte gases, CO and O2 on the double layer structure. The results indicate that double layer structure is subtly perturbed, and hints at a change in the nature of bonding at the interface. Time resolved SXRD measurements are utilised to determine the dynamics of the restructuring of the electrolyte layering at the Ag(111)/Alkaline interface. In order to gain a comprehensive picture of the structural dynamics, two other systems are studied; the Au(111) reconstruction to determine the timescale of the (1 x 1) ↔ (p ×√3) reconstruction, and the underpotential deposition of Ag on Au(111). The results indicate that the mass transport of ions through electrolyte is on a timescale comparable to the charge transfer, whereas the ordering of ions and surface metal atoms occurs on much longer timescales.