The investigation of processes in liquid phase is of paramount importance in many fields, from biology to chemistry to energy. Nevertheless the most effective techniques used routinely to obtain information on the electronic structure and chemical properties of materials, such as X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS), operate in vacuum conditions and cannot be directly applied to wet processes.
Recently, graphene has been proposed and demonstrated to work as an X-ray and electron transparent interface between liquid and vacuum condition. Thanks to these characteristics, cells based on thin graphene membranes were used to study the evolution of the electronic properties of liquids by using conventional electron spectroscopies, such as XPS and XAS1–3.
Sealed graphene nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the solution of interest were developed. The formation of GNBs was proved by using a multi-technique approach which involved AFM, Raman and synchrotron radiation spectroscopies, that have unequivocally demonstrated the presence of water inside the GNBs. This system was then successfully employed to follow in-operando two different reactions: the thermal-induced reduction of FeCl3 to FeCl24and the photoinduced reduction of ferromagnetic Prussian Blue (PB) to paramagnetic Prussian White (PW) in aqueous solution. The electronic states of iron and the presence of the solvent were obtained through a combination of core level X-Ray spectroscopies (XPS and XAS) which allowed a comprehensive understanding of the reduction mechanisms and the role of water in both processes.
The fabrication of GNBs is a first step in the direction of a fully functional graphene cell in which the cell itself is, on one hand a “sample holder” and on the other hand a "reactor" that can control the reaction and eventually “take part” to the reaction as a catalyst.
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