Environmental cells for vacuum spectroscopies at ambient pressures

The requirement of high vacuum in conventional electron spectroscopy studies such as X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS) imposes a major obstacle for analyzing interfacial physical and chemical processes under ambient pressure conditions. Consequently, there have been large efforts, mainly in the field of heterogeneous catalysis and environmental remediation, to overcome this so-called “pressure gap” in order to observe processes taking place at surfaces and interfaces under in operando conditions, namely elevated or ambient gas pressure or liquid environments. The performance of new materials and devices, in fact, often depends on processes taking place at the interface between an active element and some environment, such as air, water or another fluid. Understanding these processes often requires surface specific spectroscopic data acquired under real operating conditions, which can be challenging because standard approaches such, as said above, generally require high vacuum environments.

Existing approach to this problem relies on unique and expensive apparatuses equipped with electron energy analyzers coupled with sophisticated differentially pumped lenses. Alternatively, liquid micro jets and droplet “trains” methods have also been implemented to probe the analytes inside highly volatile solvents. In spite of the tremendous success, these state-of-the-art instruments are currently accessible only at a few laboratories and synchrotron radiation facilities, thus restricting the span of the research to a small number of users.

Research groups of the Southern Illinois University (USA) lead by Andrei Kolmakov, Northwestern University (USA), Sincrotrone Trieste and Technische Universität München (Germany) have realized a simple environmental cell using graphene oxide windows that are transparent to low-energy electrons (down to 400 eV) produced by soft X-rays. They performed proof-of-principle X-ray Photoelectron Spectroscopy measurements on model samples such as gold nanoparticles placed on the back side of the membrane and aqueous salt solution. Graphene-oxide (GO), graphene and other emerging atomic membrane windows could be used to make low-cost single-use environmental cells that are compatible with commercial X-ray, electron microscopes or optical instruments. In Figure 1 a schematic procedure for the production of the environmental cells is reported. The Graphene Oxide sheets were produced by known synthesis purification procedures; to ensure the mechanical stability of the suspended GO windows, small 3-10 µm holes were milled by a focused ion beam in the primary membranes and the individual ca 100-1000 µm2 GO single sheets were deposited over these orifices by a Langmuir-Blodgett technique. A NaI aqueous solution filled the back side of the membrane and in order to obtain a vacuum compatible E-cell a second Si wafer was placed over the one with the GO window containing the droplet of NaI solution. The entire assembly was sealed with the UV curable adhesive. Due to the few microns size of the GO windows, in situ photoelectron spectroscopy measurements with such a cell require microprobe XPS and a sufficiently intense photon source provided by the scanning photoelectron microscope (SPEM) operated at Elettra. In Figure 2 the O 1s spectra acquired through the GO covered orifice and outside it are shown. The spectrum from the ‘dry’ GO on Au-covered SiN is similar to standard O 1s XPS spectrum of partially reduced GO, with small contribution of H2O presumably trapped at the interface or between GO sheets and the substrate. The O 1s spectrum of the ‘wet’ area filled with the solution is very different, due to pronounced components corresponding to the liquid and vapour phase of water present inside the cell. The presented data are a proof of concept for environmental electron spectroscopy in membrane based E-cells.

Figure 2: Top inset: 40x50 µm2 SPEM image of the GO membrane with 3M NaI aqueous solution on the backside. The bottom inset is enlarged 12x12 µm2 area around the membrane. Point A is at the centre of the GO membrane and point B is ca 30 µm away. Top and bottom spectra were taken at point B and A correspondingly. In addition to common O 1s spectrum from the dry area the spectrum from the membrane contains H2O vapour and liquid contributions from the enclosed cell compartment.

This research was conducted by the following team:

• Andrei. Kolmakov – Southern Illinois University, Carbondale, Illinois, USA.
• Dmitriy A. Dikin, Laura J. Cote, Jiaxing Huang, Northwestern University, Evanston, Illinois 60208, USA.
• Majid Kazemian Abyaneh, Matteo Amati, Luca Gregoratti and Maya Kiskinova, Sincrotrone Trieste S.C.p.A., Trieste, Italy.
• Sebastian Günther, TU München, Chemie Department, Garching, Germany

Research funding: the SIUC part of the research was supported through NSF ECCS-0925837 grant. L.J.C and J.H. were supported by NSF through a CAREER award (DMR 0955612)