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 (Figure 1a).
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 (AFM, see Figure 1b,c,d), Raman (Figure 1e) and synchrotron radiation spectroscopies (Figure 2), 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).
Figure 1. (a) AFM image of GNBs full of water on TiO2 substrate before annealing. (b) AFM image of the same sample after annealing at 350°C, which induces a clear GNB collapse. (c) AFM height profile and (d) Raman spectra of GNBs before (black curve) and after (red curve) the annealing. Irregularly shaped GNBs with an average height of 6 nm, and lateral size of a few hundreds of nanometers are observed before the annealing, while a nearly flat layer is formed after the annealing.
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 (Figure2a). 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 (Figure 2b). 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 (Figure 2c).
The initial XAS spectrum and the corresponding simulated curve (obtained with the Ligand Field Multiplet, LFM, approach) are consistent with Fe3+ in an octahedral (Oh) configuration typical of FeCl3 compounds. After annealing, the spectrum dramatically changes to a conformation typical of Fe(II) compounds, and the simulation, assuming the Fe2+ in Oh configuration, reproduces quite well the experimental spectrum expected for FeCl2, thus confirming the thermal induced reduction of Fe from 3+ to 2+. The same process, when performed in dry conditions, undergoes a different thermal dehydration and decomposition mechanism which leads to the formation of oxidized products, confirming the need of an aqueous environment to achieve the reduction of Fe ions.
For the easiness of the GNB fabrication and the straightforward extension to a large variety of solutions, we envisage a broad application of the proposed approach to investigate in detail the electronic mechanisms that regulate liquid/solid electron transfer in catalytic and energy conversion related applications.
Figure 2. XPS spectra of (a) O1s, (b) Cl 2p recorded using a photon energy of 596 eV, and (c) XAS spectra of the Fe L3,2-edge together with the corresponding calculated LFM spectra (gray curves). The blue spectra are measured on GNBs filled with FeCl3 aqueous solution; the red spectra are measured on the same sample after the annealing at 250°C. Spectra are measured at normal emission geometry.
S. Nappini1, A. Matruglio1, D. Naumenko1, S. Dal Zilio1, F. Bondino1, M. Lazzarino1, E. Magnano1,3
1 IOM-CNR, Laboratorio TASC, S.S. 14-km 163.5, 34149 Basovizza, Trieste, Italy
2 University of Trieste, Graduate School of Nanotechnology, Piazzale Europa 1, 34127 Trieste, Italy
3 Department of Physics, University of Johannesburg, PO Box 524, Auckland Park, 2006, Johannesburg, South Africa
Silvia Nappini, email: email@example.com
S. Nappini, A. Matruglio; D. Naumenko, S. Dal Zilio, F. Bondino, M. Lazzarino, E. Magnano. "Graphene Nanobubbles on TiO2 for in Operando Electron Spectroscopy of Liquid-Phase Chemistry". Nanoscale 9, 4456 (2017). DOI: 10.1039/C6NR09061C