Innovative Station for In Situ Spectroscopy

ISISS (Innovative Station for In Situ Spectroscopy) is a project of the Inorganic Chemistry department of the Fritz-Haber-Institut der Max-Planck-Gesellschaft (FHI), Max Planck Institute for Chemical Energy Conversion (MPI-CEC), and HZB/BESSY II in Berlin. The scientific aim at ISISS – an instrument dedicated to near ambient pressure XPS (NAP-XPS) experiments - is to study the electronic surface/near surface structure of functional materials in the presence of a reactive environment. This includes both gas/solid interfaces (e.g. heterogeneous catalysis) and liquid/solid interfaces (e.g. catalytic water splitting).

The working area of the facility is material science in general and catalysis in particular. Our approach includes 3 units that have to complement on another: a state of the art soft X-ray beamline, an endstation for ambient pressure X-ray photoelectron (NAP-XPS) and X-ray absorption spectroscopy (XAS), and an infrastructure on site to perform experiments with a chemical background. In contrast to standard vacuum surface science experiments, in situ experiments require the installation of a complex gas feed and an elaborated gas analytic to follow the conversion of the gas phase during the reaction.

An electrochemical module allows the investigation of solid/liquid interfaces (only aqueous!) using closed flow cells, primarily focused on electrochemical applications. Different approaches are available. 

We highly recommend you to get in contact with us well in advance of the deadline before submitting your proposal.

Electrolyte restrictions for the EC cells/set-up:

Generally,  only chemicals allowed to be used in the preparation lab are allowed to be used in the setup, with additional restrictions described below. Only aqueous solutions are allowed to be used in this setup (also refer to the exclusion list!).

Please note that for  alkaline and neutral electrolytes, the general limit in concentration is 1.0 M, while acidic electrolytes are restricted to 0.1 M. No higher concentrations are allowed to be used inside the setup. Additional restrictions might apply for some particular electrolytes, so please get in contact with us prior to proposal submission.

Selected Applications:

• X-ray photoelectron spectroscopy (XPS) under high vacuum (p=10 &sup-8; mbar) and near ambient pressure conditions (typically 1 mbar)
• X-ray aborption spectroscopy (XAS) at pressure up to 10 mbar with NAP-HE-XPS endstation
• XPS and XPS in confined, specially designed cells for liquid/solid and gas/solid interactions

Fig. 1: picture of NAP-HE-XPS (courtesy of SPECS GmbH, Berlin)

Methods

NEXAFS, XRF, XPD, NAP-XPS, XPS, XPD, Time-resolved PES, Mass Spectrometry

Remote access

not possible

Fig. 2: Scheme of the NAP-HE-XPS endstation installed at the ISISS beamline.
The spectrometer (right site) is displayed retracted from the XPS cell module (left side).

 

Fig. 3: Scheme of the different working electrode approaches in the EC cell

 

Table 1: Gas analytics

 

Table 2: Laboratory facilities at ISISS
 

Electrochemical cell:

We currently offer a 3-electrode flow cell. The cell features exchangeable working electrode (WE) holders that allow different types of experiments, as outlined below and in Fig. 3:

• Working electrode: on SiNx or on ion exchange membrane
• Reference electrode: Ag/AgCl (leak-free)
• Counter electrode: Pt, graphite

Silicon Nitride Membrane Approach:

The catalyst material is deposited onto a SiNx window, and is in direct contact with the electrolyte. The sample is probed through the SiNx membrane, meaning only photon-in/photon-out techniques (fluorescence yield XAS) are possible.

Note that users are fully responsible for procuring and bringing their own SiNx membranes, and for all steps of sample preparation.

Ion Exchange Membrane Approach:

Here, the catalyst is deposited onto a (polymeric) ion exchange membrane, and subsequently covered by a (bi-)layer of graphene, and the sample is mounted such that the catalyst/graphene side faces the analyzer. The graphene is both to provide electrical contact, as well as to retain a pocket of liquid electrolyte in the vacuum. Due to the low thickness of the graphene, electrons of sufficient kinetic energy (> 400–600 eV) can penetrate it to make XPS and electron yield XAS possible. The ion exchange membranes typically are very sensitive to beam damage.

Note that users are fully responsible for purchasing and bringing their own membranes and graphene.

Beamline:

The beamline has to accommodate for a variety of user requirements resulting from the scientific approach as outlined above. The basic design considerations are as follows: 

• A variety of materials with a large diversity in composition should be characterised and diverse scientific problems should be tackled. This requires a beamline that is adaptable to the needs of the users. This comprises a high flexibility concerning the provided photon energy range and spectral resolution.
• Due to the compounds usually studied in catalyst research, the available photon flux at energies between 400 eV and 1200 eV is the main concern covering e.g. the C1s, N1s, O1s and transition metal 2p core levels.
• The X-ray spot size at the sample position is optimised for a best fit to the electrostatic lens system of the in situ apparatus and the electron analyser.
• The beamline should be easy to handle to allow for multi-user operation without elaborated knowledge of technical details.
• An accurate and reliable energy calibration should be feasible as an essential part of high quality XPS studies.

The design considerations resulted in the selection of a plane grating monochromator (PGM). This PGM design is a development based on the Petersen type monochromator at BESSY at which the light is colliminated in the dispersive plane in front of the grating by the mirror M1. In this design, the fix focus constant c = (cos β / cos α) (α: angle of incidence, β: angle of diffraction relative to the grating normal, respectively) is kept constant during the scanning of the photon energy. This allows the free adjustment of the fix-focus constant without movement of the exit slit which can be used to easily optimise the monochromator to the requirements of the users.