Using Light to Probe Surface Electrochemical Processes

Sammanfattning: This thesis shows how various light-based in situ techniques can be used to follow different surface electrochemical processes such as electrochemical deposition into Nanoporous Anodic Aluminium Oxide (NP-AAO) templates and the surface structure of electrodes. Techniques based on both x-rays and visible light were used to follow surface structures and compositions, in situ. The samples were alsoinvestigated with ex situ techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and various other techniques. Templated electrodeposition processes, which can be used for growing various nanostructures with applications in for example gas sensors, catalysts, electrocatalysts and colouring of metal products were studied. Cyclic voltammetry (CV) for investigating electrooxidation and corrosion of model systems as well as real industrial materials was combined with Surface X-ray Diffraction and 2-Dimensional Surface Optical Reflectance.Electrochemical deposition of Sn, Pd and Au into NP-AAO was investigated with both in situ and ex situ methods to gain information about the deposition process, the influence of the NP-AAO template on the growth of the nanostructures and the chemical and crystallographic nature of the grown nanostructures. For the Sn, it was possible to determine that the grown nanostructures were metallic with a beta-Sn crystal structure and a compression of the nanostructures due to the confinement of the pores were measured. Both the Pd and the Au nanostructures were constructedof crystalline grains with grain sizes in the range of the pore diameter. A compressive and tensile strain could be measured across and along thenanostructures, respectively. The combination of 2-Dimensional Surface Optical Reflectance (2D-SOR), an optical microscope, with a modified electrochemical cell, was investigated through cyclic voltammetry measurements on Au, Pd and Super Duplex Stainless Steel and anodization of aluminium. The measured reflectance could be correlated with characteristic features in the CV curve corresponding to oxidation and reduction of the surface. By combining 2D-SOR with High Energy Surface X-ray Diffraction (HESXRD) during CV measurements the reflectance could be correlated with theatomic changes of the Au(111) surface. It was also shown that 2D-SOR could detect monolayer changes of the surface, due to its sensitivity to the increased electron density of the Herringbone reconstruction, a rearrangement of the atoms in the topmost layer of the Au(111) single crystal surface. By applying this method to polycrystalline samples, it was possible to follow how differently oriented grains react differently during the same experimental conditions, showing the possibility for 2D-SOR to image inhomogeneous surfaces.The Au(111) single crystal was further studied using CV and potential stepping going into the oxygen evolution reaction (OER). The combined HESXRD and 2D- SOR measurements detected a roughening of the surface as it was oxidized and during the OER. These measurements also suggest that the oxide on the Au(111) single crystal is amorphous, due to the lack of oxidation peaks in the measured diffraction pattern. With X-ray photoelectron spectroscopy measurements, it could be shown that sulphate species absorb on the surface before any oxide is detected,the oxide is then formed and detected as the sulphate desorbs. The possibility to follow and understand these different surface reactions gives knowledge which in the future could be used for example for the continued development of fuel cells, the development of devices, sensors, catalysts and to develop more corrosion-resistant materials.