In-Situ GI-XRD of Electrochemically Formed Thin Gold-Oxide Layers
Authors: Holzer Georg , Kubicek Sabine , Artner Werner , Fafilek Guenter 
Published: December 2020

In-situ XRD analysis is a valuable tool to maximize understanding of complex (electro-)chemical reactions while eliminating errors or changes in the structure when instable compounds or materials that react with air are analysed ex-situ. Surface analysis has been a key aspect in the basic research of metal corrosion. Nevertheless, there is more work to be done to achieve complete understanding which is in part because of the lack of proper in-situ analyses. Unfortunately, previous setups for in-situ XRD analysis come along with their own set of problems. In these constructions a low signal/noise ratio can be seen as the beam has to penetrate the electrolyte. High energy synchrotron radiation and long exposure is needed to get information of surface reactions which makes it very laborous and hard to get high quality data. Various researchers like M. Fleischmann et al [1] introduced new XRD cells in which electrolyte layer thickness was minimized to reduce the energy loss and get more accurate XRD information. However, due to issues with current density distribution and other restrictions like depletion of electroactive species or unfavorable reference electrode position electrochemical processes with high reaction rates cannot be investigated accurately in this setup.

In a new setup S. Reither et al [2] introduced a XRD cell where copper is sputtered on a XRD-transmittable polyimide foil as a working electrode. The XRD beam is directed through the polyimide and the thin metal film where the reaction happens without passing the electrolyte underneath. A normal electrochemical cell can be used at the side of the working electrode with inlets for counter- and reference electrode. Depending on the reaction rate the setup also includes an in- and outlet to circulate electrolyte if required. Gracing incidence XRD geometry allows for improvement of the signal intensity of thin-film surface reactions.

To demonstrate the strength of the setup the oxidation of polycrystalline gold was investigated. Gold is used as the surface reactions are unique compared to other metals due to its stability against oxide formation. L D Burke et al [3] have tried to investigate the reactions taking place at high voltages to create monolayers and thick oxide layers under different conditions. While normal metal oxidation occurs at the surface, gold oxidation occurs beneath a monolayer of adsorbed hydroxy-ions and is catalysed by repulsion triggered ion exchanges at the surface, increasing the influence of conditions and reaction time as well as the current. Our goal is to use the novel in-situ-XRD system to better understand how gold behaves in various electrolytes using both cyclic and constant voltages to create oxide films. It is suspected that gold oxides have porous or amorphous structure. Therefore, a direct XRD-analysis might be difficult. On the other hand, the correlation of electrochemical data with XRD-data allows the indirect categorization between amorphous and crystalline phases at different conditions as pH and temperature.

The details of the in-situ XRD-cell as well as its contribution to analysing gold oxide are presented in this contribution.

Image explanation:

1 .. counter electrode

2 .. polymer window with working electrode

3 .. reference electrode

4, 5 .. solution in- and outlet

[1] M. Fleischmann, A. Oliver, and J. Robinson, ‘In Situ X-ray diffraction studies of

electrode solution interfaces’, Electrochimica Acta, vol. 31, no. 8, pp. 899–906, (1986)

[2] S. Reither, W. Artner, A. Eder, S. Larisegger, M. Nelhiebel, C. Eisenmenger-Sittner, G.

Fafilek, ‘On the In-Situ Grazing Incidence X-Ray Diffraction of Electrochemically Formed

Thin Films’, ECS Transactions, 80 (10) 1231-1238, (2017) 10.1149/08010.1231ecst

[3] L.D. Burke and P.E Nugent, Electrochim. Acta, 42, 399, (1997)

Figure 1