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

^{For beamtime application at XAS@KMC-3 please read the test below and refer to the instructions in the column on the right side of this webpage.}

The instrument XAS@KMC-3 (previously named CryoEXAFS) is dedicated to advanced X-ray absorption spectroscopy (XAS) techniques to investigate the short-range structure environment (up to ca. 10 Å) around selected atomic species and the redox state in condensed matter (e.g., proteins and synthetic molecules in solution, electrocatalytic films, battery cells, semiconductors, powder materials, etc.). It is installed at the tender/hard X-ray beamline KMC-3, which is equipped with two X-ray mirrors (Pt/Rh coated) for variable beam focus size (~0.35 - 5 mm) and variable spatial position to match the sample position, and a double-crystal monochromator with Si[111] and Si[311] crystal pairs (ca. 2 or 0.2 eV resolution).

XANES and EXAFS spectra can be collected in an energy range of ca. 2,050 eV to 21,000 eV (i.e., P to Mo K-edges and many L- or M-edges of heavier elements). XAS spectra can be collected in fluorescence mode with energy-resolving 7- or 13-element silicon-drift detectors or photodiode/scintillation detectors, or in transmission mode using ion chambers or PD detectors. The beamline has options for incident flux stabilization (MOSTAB) in detuned monochromator mode for harmonics suppression (i.e. at tender energies). 

Five main operation modes of the XAS experiment at KMC-3 are available to users:

1) Cryogenic-temperature CT-XAS in a ca. 5-310 K temperature range in a closed-cycle liquid-helium cryostat with the sample held in a heat-exchange gas (typically 100 mbar He), to probe samples in a static state, as established e.g., using freeze-quench approaches or other treatments.

2) Operando-condition OC-XAS at room temperature, typically on electrochemical or battery (flow-) cells, with samples operated in the X-ray beam using electrochemistry (potentiostats available) or light excitation (pulsed OPO-laser 400-2500 nm, LED-sources), to monitor changes in real time. Diverse sample (flow-) cells for electrochemistry are available and users may also bring their own equipment.

3) Tender-energy TE-XAS at room temperature (ca. 2-4 keV range) in a  sample chamber that can be evacuated or filled with inert gas, facilitating sample change in a few minutes, and using an ion chamber for I0-detection with thin foil windows, facilitating also operando (electrochemical) experiments.

4) Time-resolved TR-XAS using rapid-scan spectra collection (>/= 2 s for a 600 eV EXAFS scan), to track e.g. redox/structure changes in materials during cyclic-voltammetry in minutes or light-induced changes, and collection of timescan traces at fixed excitation energies, for monitoring e.g. edge-energy shifts (>/= 0.1 ms per data point). Both methods use the energy-resolving SD-detectors for superior fluorescence signal contrast.

5) Modulated-excitation ME-XAS with up to 100 kHz excitation pulses (electric potential, light) on samples in the beam during monochromator scans in sec to min and collection of two or more complete XAS spectra, e.g., a spectrum at two potential levels or in dark/light conditions.

Each two of the above experiments (i.e. tender-energy / cryo-XAS or cryo-XAS / operando-XAS) can be carried out in parallel, facilitating for example data collection during electrochemistry followed by low-temperature characterization of the sample. With a medium-thickness (electrochemical film) sample, delivering e.g. 100 kcps fluorescence per detector channel, good EXAFS spectra to k = 12 Å^-1 can be collected within ca. 3 min in a single monochromator scan.

Selected Applications:

• In-situ characterization of materials under catalysis conditions with low-temperature XAS data collection (freeze-quench approach)
• Operando-XAS on electrocatalytic films for water oxidation (OER) or CO2-reduction (CO2RR) under electrochemistry conditions at room temperature
• XAS on dilute-solution (1 mM) protein or synthetic-molecule samples at cryogenic conditions
• Rapid-scan EXAFS (redox transitions and catalytic processes during cyclic voltammetry)
• EXAFS as a function of chemical composition and/or temperature dependent EXAFS

Scheme of the experimental setup for XAS at KMC-3 with cryogenic or operando ambient-temperature sample conditions and two energy-resolving 7/13-element fluorescence SD-detectors.

Methods

EXAFS, NEXAFS, XRF

Remote access

not possible

The XAS endstation at KMC-3. (clockwise) table with setup, lHe-cryostat, electrochemical flow-cell, I0 pressure & cryostat controls & Falcon electronics, tender-energy chamber, 7/13-element SDDs.

The XAS endstation at KMC-3 includes a closed-cycle liquid-helium cryostat (Oxford, ca. 5-310 K), detectors (ion chambers, photodiodes, scintillation/photomultiplier) for I0-detection and transmission-mode XAS, as well as two retractable large-area energy-resolving detectors (7- and 13-element Si-drift detectors, RaySpec) for fluorescence-mode XAS, which are operated via DXP-XMAP (-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 setup are XAS experiments on biological materials (metalloproteins), (diluted) solutions of synthetic molecules, and solid-state materials. For the in-situ setup, sample cells and potentiostats for in-beam electrochemistry are available and typical applications are XAS experiments on electrode-deposited catalyst films in contact with electrolyte at room temperature or monitoring of charge/discharge cycles in battery cells.