Graphene nanobubbles for in operando electron spectroscopy of liquid-phase chemistry

X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chemical properties of materials. Unfortunately this information is scarce when investigating solid/liquid interfaces, chemical or photochemical reactions in ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples operating in atmosphere or in the presence of a solvent. One promising approach to enable the use of conventional electron spectroscopies is the use of thin membrane, such as graphene (Gr), which is transparent to both X-ray photons and photoelectrons. For these purposes, this work proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO₂ (100) rutile single crystal filled with the solution of interest during the fabrication stage .
The formation of irregularly shaped vesicles with an average height of 6 nm and lateral size of a few hundreds of nanometers was proved by using a multi-technique approach involving Atomic Force Microscopy,  Raman, and synchrotron radiation spectroscopies, which have unequivocally demonstrated the presence of water inside the GNBs and the transition to a flat Gr layer after water evaporation by thermal heating up to 350 °C in ultra high vacuum (UHV).
 

GNBs were successfully employed to follow in-operando the thermal-induced reduction of FeCl₃ to FeCl₂ in aqueous solution. In particular, the system was annealed up to 250°C for 1 h with two purposes: on one hand to check the GNB thermal stability and on the other hand to follow the reduction process from Fe³⁺to Fe ²⁺ directly in aqueous environment.
The electronic states of chlorine, iron and oxygen were obtained through a combination of electron spectroscopies (XPS and XAS) in the different phases at the CNR BACH beamline at the Elettra synchrotron facility. The presence of water and its evaporation induced by the annealing was proved by following the evolution of O1s spectra, where the intensity of the typical component of liquid water at 533.5 eV gradually decreases with the temperature . On the contrary Cl 2p and Fe 2p intensities strongly grow as a consequence of the evaporation of water and the increase of Fe and Cl concentration. Interestingly, the initial Cl 2p spectrum is mainly characterized by the presence of a doublet at 198.7 eV and 200.3 eV due to Cl^- ions in FeCl₃ aqueous solution, and a second doublet at 200.1 eV and 201.7 eV can be attributed to Cl-C bond at the Gr interface. After the annealing most of the signal is dominated by a new doublet at 199.4 eV and 201 eV which can be associated to the formation of FeCl₂. Also the analysis of the Fe 2p core level photoemission spectrum confirms the evolution from 3⁺to  2⁺ oxidation state, high spin configuration. In order to clarify this point, the oxidation state of Fe ions in GNBs was investigated by recording the XAS spectra at the Fe L₃₂-edge in total electron yield (TEY), measuring the current on the Gr layer, before and after the annealing. It is important to notice that thanks to the high electrical conductivity of the Gr/liquid interface we can measure a spectrum with a high signal/noise ratio. 
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This work was supported by the Italian MIUR through progetto premiale ABNANOTECH, FIRB FIRB RBAP11ETKA_003 and  the national grant Futuro in ricerca 2012 RBFR128BEC_002 ‘‘Beyond graphene: tailored C-layers for novel catalytic materials and green chemistry’’

  
Graphene nanobubbles on TiO2 for in operando electron spectroscopy of liquid-phase chemistry

DOI: 10.1039/C6NR09061