CryoEXAFS@KMC-3

EXAFS station with cryogenic or in-beam, operando electrochemistry sample conditions /KMC-3

The instrument CryoEXAFS is dedicated to investigate the short-range environment around selected atomic species in condensed matter by X-ray Absorption Spectroscopy at low temperatures and provides the possibility for EXAFS, XANES and X-ray fluorescence measurements in vacuum. It is installed at the hard x-ray beamline KMC-3 wich is equipped with an x-ray mirror assembly and a double monochromator, which can be removed from the x-ray beam path to allow for experiments with a monochromatic or a white x-ray beam. The energy range extends from 2 keV to 14 keV. In addition, the beamline comprises an ultrafast laser as a pump source for time-resolved experiments.

CryoEXAFS offers XAS experiments in a 5-300 K cryogenic range (liquid Helium cryostat) as well as under in-beam, operando electrochemistry conditions (materials in solution or deposited on surfaces, for example electrodes). Both setups are installed in parallel to enable a rapid change from cryogenic to in-beam, operando electrochemistry mode (Figure 1). Both experiments are performed with a widened beam focus (several mm2 on the sample) thereby avoiding radiation-induced sample modifications (avoidance of radiation damage).

• n-situ characterization of materials under catalysis conditions with low-temperature data collection (freeze-quench approach, Klingan et al., 2018; Pasquini et al, 2019, Bergmann et al., 2020; Smith et al, 2017)
• Operando XAS on films for water oxidation reaction (OER) under electrochemistry conditions at room temperature (Smith et al., 2017; González-Flores et al, 2018)
• XAS on dilute-solution (1 mM) protein samples at cryogenic conditions (Reschke et al., 2019)
• Rapid-scan EXAFS (redox transitions and catalytic processes during cyclic voltammetry)
• EXAFS as a function of chemical composition and/or temperature dependent EXAFS (Kesavan et al., 2020)

Figure 1 Scheme of the experimental setup for XAS at KMC-3 (CryoEXAFS) with cryogenic or in-situ (operando) ambient-temperature sample condition. Two energy-resolving 13-element fluorescence detectors

Methods

EXAFS, NEXAFS, XRF

Remote access

not possible

Figure 2 KMC-3 CryoEXAFS experimental station a) with cryogenic condition utilizing the 13-element Si-drift SSD (Rayspec) that is flanged to the cyrostat, the same SSD is used in b) to acquire data us

The general station setup includes a liquid-helium cryostat (Oxford, ca. 5-300 K), detectors (ion chambers, photodiodes) for transmission-mode XAS, as well as two retractable large-area energy-resolving detectors (liquid-nitrogen-cooled window-less 13-element Ge, Canberra; air/water-cooled Be-window 13-element Si-drift SSD, Rayspec) for fluorescence-mode XAS (up to ca. 400 kcps), which are operated via XIA DXP electronics and can be flanged to the cryostat for in-vacuum operation to yield maximum count rates (Figure 2a). Motorized y/z sample positioning is available for samples in the cryostat as well as in the in-beam, operando electrochemistry sample cells. Fast data acquisition/transfer hard- and software facilitates relatively rapid EXAFS scans, e.g. in ca. 3.0 min to k = 12 Å^-1, as well as fluorescence data acquisition (e.g. at fixed energy) with the energy-resolving detectors with 1 ms per point or less for superior signal contrast in time-resolved applications. Typical applications for the cryogenic set-up are XAS experiments on biological materials (metalloproteins), (diluted) chemical solution samples, and solid-state materials. For the in-situ setup, sample cells and a potentiostat for in-beam electrochemistry are available and typical applications are XAS experiments on electrode-deposited metal catalyst films in contact with electrolyte at room temperature (Figure 2b).