Fluorescence-detected quick-scanning X-ray absorption spectroscopy

Abstract

Time-resolved X-ray absorption spectroscopy (XAS) offers the possibility to monitor the state of materials during chemical reactions. While this technique has been established for transmission measurements for a number of years, XAS measurements in fluorescence mode are challenging because of limitations in signal collection as well as detectors. Nevertheless, measurements in fluorescence mode are often the only option to study complex materials containing heavy matrices or in samples where the element of interest is in low concentration. Here, it has been demonstrated that high-quality quick-scanning full extended X-ray absorption fine-structure data can be readily obtained with sub-second time resolution in fluorescence mode, even for highly diluted samples. It has also been demonstrated that in challenging samples, where transmission measurements are not feasible, quick fluorescence can yield significant insight in reaction kinetics. By studying the fast high-temperature oxidation of a reduced LaFe0.8Ni0.8O3 perovskite type, an example where the perovskite matrix elements prevent measurements in fluorescence, it is shown that it is now possible to follow the state of Ni in situ at a 3 s time resolution.

Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film

“…The first calibration curve provides the relation between raw fluorescence signal and absorption K-edge energy position and the second curve provides the relation between K-edge energy position and metal oxidation state. This approach allows us, using standard equipment for fluorescence-detected XAS, to achieve time resolution down to the millisecond (and potentially also microsecond) time range, with a reliability comparable to technically clearly more challenging methods, like energy-dispersive XAS [ 20 , 21 ] or QXAFS spectroscopy [ 19 , 23 , 24 ]. Although promising regarding time-resolved experiments, there are also noteworthy limitations of energy-dispersive XAS and QXAFS.…”

“…The main factor limiting the combined use of operando XAS and electrochemical experiments with typical CV scan rates is the data collection time required for a complete XANES spectrum, which decreases the temporal resolution of the technique. Significant progress has been achieved in the technical development of this approach in recent years [18,19]. Typically, the studies approaching operando XAS measurements aim at quick collection of an entire spectrum, either by using energy-dispersive XAS [20,21] or quick scanning extended X-ray absorption finestructure (QEXAFS) spectroscopy [22][23][24].…”

Redox Dynamics of Active VOx Sites Promoted by TiOx during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

“…A Rh-coated toroidal mirror was used to focus the incident X-ray beam to 500 × 400 μm 2 at the sample position. XAS spectra were recorded in fluorescence mode 52 using a PIPS diode (Mirion Technologies) as a fluorescence detector. The Si(111) channel-cut monochromator was oscillating with a frequency of 1 Hz, which corresponds to a repetition rate of 2 scans/s.…”

Chemical state mapping of simulant Chernobyl lava-like fuel containing material using micro-focused synchrotron X-ray spectroscopy

“…These offer some improved accuracy in the determination of spatial variation in redox gradients; however, accuracy is affected by over-absorption and data normalization. Quick-scanning XAS techniques (Lü tzenkirchen-Hecht et al, 2001;Clark et al, 2020) are nowadays an alternative approach for rapid and accurate quantitative measurement of oxidation state, on a millisecond time scale in fluorescence mode. However, this is currently limited to point measurements for bulk samples (Schroer et al, 2003) and not suitable for heterogeneous sample on the micrometre length scale, since the spot size is relatively large (for example at SuperXAS, 100 mm  100 mm).…”