The electrochemistry of supported metal multilayers: In-situ surface x-ray scattering studies

Sisson, Naomi Katie
The electrochemistry of supported metal multilayers: In-situ surface x-ray scattering studies. PhD thesis, University of Liverpool.

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A combination of in-situ surface x-ray scattering (SXS) and cyclic voltammetry (CV) measurements have been performed to develop an increased understanding of supported metal multilayers on single crystal metal substrates. The formation of ordered metal structures and metal oxides are investigated together with the effect of the surface modification on processes such as OH adsorption and hydrogen evolution. A detailed in-situ study of the structure and behaviour of underpotentially deposited (UPD) monolayer and bilayer of Ag on the Au(111) surface is given both before and after transfer of the electrode to alkaline electrolyte. Analysis of the SXS data shows that while the bilayer is stable upon transfer, the Ag monolayer reorders to a partial bilayer. Sulphate anions in the UPD electrolyte stabilise the monolayer as it is formed, however upon transfer to the alkaline electrolyte these are replaced by more specifically adsorbed OH- ions causing the Ag to restructure to the more stable partial bilayer. The Ag modified electrodes show potential response for OH- adsorption similar to that of Ag single crystals. Detailed modelling of the SXS data at the positive potential limit in alkaline electrolyte shows evidence of an ordered OH- layer in the electrochemical double layer. Sub-surface OH indicated in the CV is confirmed in the SXS by a roughening of the topmost Ag layer. The atomic structure and properties of Co thin films electrodeposited onto Au(111) electrode is investigated using STM and SXS. Measurements confirm that the Co initially grows on the surface as a bilayer. Detailed characterisation of the thin film structure was obtained by analysis of the specular CTR. Results show a small outward expansion in the Co film which may be caused by the presence of adsorbed hydrogen on the Co surface. Changes induced by the presence of CO were investigated and it was found that CO has a dramatic effect on the magnetisation but only cause very small changes in the atomic structure of the Co film. The measurements indicate the changes in magnetic behaviour are driven by subtle changes in the electrolyte double layer region adjacent to the Co surface. Thin films of Co were deposited on a Au(111) electrode prior to transfer of the electrode to alkaline electrolyte. Ex-situ AFM imaging revealed that the Co is present on the Au(111) surface in the form of large islands which cover approximately 2.2% of the surface and are located at step edges. Their presence gives rise to a modified reconstruction of the topmost Au layer which was also investigated. Resonant SXS measurements revealed that Co is present both in its metallic state and charged state, most likely in the form Co(OH)2. The Co-modified Au(111) surface shifts the potential for hydrogen evolution positively by ~ 0.1 V compared with that of clean gold.

Item Type: Thesis (PhD)
Additional Information: Date: 2014-04 (completed)
Depositing User: Symplectic Admin
Date Deposited: 04 Aug 2015 10:19
Last Modified: 17 Dec 2022 00:50
DOI: 10.17638/02014339