X-ray Structural Studies of Electroactive Films

This thesis describes X-ray studies (XANES, EXAFS and XAS) of the structures of electroactive film modified electrodes and their response to potential. To provide insight into the composition, structure and dynamics of the studied systems EXAFS was combined with other techniques: XPS, electrochemical methods and infrared spectroscopy. The electroactive materials studied were Prussian Blue (PB) and its cobalt and nickel derivatives, iridium oxide and polypyrrole/[P(Mo3O10)4]3- composite films. Selected electroactive materials were diverse, since the study was undertaken with the aim of providing the foundations for future experiments in the area of the dynamic in situ EXAFS research. For cobalt hexacyanoferrate films, cobalt was identified as the active redox site, based on clear evidence of changing bond lengths with potential. This suggests it will be an interesting candidate for future dynamic EXAFS study. The behaviour of PB films was found to be complicated to interpret because both redox sites are Fe-based and there exist additional interstitial iron species. In contrast to cobalt hexacyanoferrate, no significant changes could be observed in the Ni XAS data for the nickel derivative with Fe active redox sites. Polypyrrole/[P(Mo3O10)4]3- composite films showed changes in oxidation state of multiple Mo sites in XPS experiments. Nevertheless, they are not a promising material for a dynamic EXAFS because of the interpretational problems, similar to PB films. Iridium oxide films are another promising candidate. They were successfully characterized in in situ EXAFS configuration and changing bond lengths were observed as the potential (and thus Ir oxidation state) was changed. Additionally the thesis includes nanogravimetric observation of ion exchange characteristics of polypyrrole film p-doping in an ionic liquid. It is shown that polypyrrole film redox switching in the deep eutectic ionic liquid (Ethaline) involves a very different pattern of ion transfers to aqueous media. Ionic liquid in the reduced film provides a reservoir of choline cations whose transfer (in the opposite direction to anion doping) partly satisfies electroneutrality. Overall, the sites of redox activity were identified in a range of multi-site redox systems and structural changes were related to the injected charge. This static study is the first step in identifying systems suitable for X-ray synchrotron-based dynamic studies of electroactive film structure.