Tender X-ray Extended Absorption Fine Structure (TX-EXAFS, 1.75~6 keV)
Tender X-ray Extended X-ray Absorption Fine Structure (TX-EXAFS) stands as a sophisticated analytical tool that offers intricate insights into the electronic and chemical properties of materials at the atomic level. By operating within the tender X-ray energy range, typically between 1.75 to 6 keV, TX-EXAFS enables researchers to explore the atomic arrangements, oxidation states, and electronic configurations of materials with remarkable precision. This technique relies on the interaction of tender X-rays with the sample, where core-level electrons undergo photoabsorption near the absorption edge, leading to the generation of an energy spectrum. Through rigorous analysis of this spectrum, researchers can unravel valuable information about the elemental composition, bonding characteristics, and molecular orientation within the material.
The applications of TX-EXAFS span across a wide array of scientific disciplines, making it a versatile and indispensable tool in research and development. In materials science, TX-EXAFS is instrumental in studying thin films, surfaces, and interfaces, shedding light on electronic behaviors and structure-property relationships crucial for material design and optimization. Catalysis research benefits significantly from TX-EXAFS by facilitating the analysis of surface reactions, catalyst functionality, and active sites, aiding in the development of efficient catalytic systems. Furthermore, TX-EXAFS finds extensive use in environmental studies, allowing researchers to investigate pollutants, aerosols, and environmental samples with high sensitivity and specificity. In the realm of energy materials research, TX-EXAFS plays a pivotal role in advancing the understanding of materials used in energy storage, conversion, and renewable energy technologies, contributing to the development of novel materials with enhanced performance and sustainability.
Nano Materials
In this experiment, the authors investigate the chemical bonding of novel platinum-free MoNi4 electrocatalyst nanoparticles. They immobilize these nanoparticles on MoO2, which are formed by the controlled outward diffusion of Ni atoms during the annealing of NiMoO4. The NEXAFS spectra of NiMoO4 and MoO2 with outward-diffused MoNi4 nanoparticles are shown in Figure 1, where Mo-L NEXAFS reveals the transition of Mo(2p) to Mo(4d) electronic states. The NiMoO4 Mo-Lii absorption edge exhibits a main peak at 2630.8 eV and an additional shoulder peak at 2629.4 eV, while the Mo-Liii absorption edge shows a main peak at 2526.0 eV and an additional shoulder peak at 2524.6 eV. The results indicate a 1.4 eV orbital splitting at both the Mo-Lii and Mo-Liii edges, confirming the complete transformation of MoNiO4 to MoO2 and the formation of electro-catalytically active MoNi4 nanoparticles.
References:
S. Werner, P. Guttmann, F. Siewert, A. Sokolov, M. Mast, Q. Huang, Y. Feng, T. Li, F. Senf, R. Follath, Z. Liao, K. Kutukova, J. Zhang, X. Feng, Z.-S. Wang, E. Zschech, G. Schneider, Spectromicroscopy of Nanoscale Materials in the Tender X-Ray Regime Enabled by a High Efficient Multilayer-Based Grating Monochromator. Small Methods 2023, 7, 2201382.
https://doi.org/10.1002/smtd.202201382
Botany
Calcium content in plants is typically measured using spectrophotometry or mass spectrometry, which require large sample sizes and chemical extraction, with limited spatial resolution. Instead, using NEXAFS spectroscopy to measure the calcium-to-carbon mass ratio in plant samples offers spatial resolution without the need for chemical extraction or large sample sizes. Figure 2 shows fluorescence NEXAFS spectra at the calcium K edge of the second, fifth, eighth, and eleventh unextracted onion scales. Measurement results indicate significantly higher calcium mass fractions in the second and fifth layers compared to the eighth and eleventh layers. The numbering of onion scales is as follows: the second scale is the oldest in this study, while the eleventh scale is the youngest onion skin, formed between the tenth and twelfth weeks of onion bulb development, with no significant differences in structure and composition. Although this study only discusses the application of Ca NEXAFS to quantify calcium in dried onion epidermis and sub-embryonic axis, this technique can also be extended to living and complex tissues, such as roots, flowers, grains, and leaves. NEXAFS has been used to characterize complex biological tissues, such as snake scales (Baio et al., 2015), frog tongue mucus (Fowler et al., 2018), and insect cuticles (Baio et al., 2019).
References:
Rongpipi S, Barnes WJ, Siemianowski O, Del Mundo JT, Wang C, Freychet G, Zhernenkov M, Anderson CT, Gomez EW, Gomez ED. Measuring calcium content in plants using NEXAFS spectroscopy. Front Plant Sci. 2023 Aug 16;14:1212126.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10468975/
Chemical
This study investigates the formation of uranium oxidation, a phenomenon occurring at shorter mean free paths (MFP) of 3 keV photons. Therefore, akin to a probe, testing near the Fermi edge confirms the presence or absence of oxidation. As shown in Figure 3, the first peaks at hv=3570 eV and hv=3760 eV provide clear information about uranium oxidation. UM4 and UM5 exhibit similar peaks, albeit with some shifts, indicating they share similar structural characteristics but with slightly different atomic spacings. Due to the stronger sensitivity of X-ray measurements to such oxides, the peaks can be used for in-situ measurement of uranium oxidation. The results show varying degrees of oxidation for both.
References:
J.G. Tobin, S.H. Nowak, S.-W. Yu, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, D. Sokaras, EXAFS as a probe of actinide oxide formation in the tender X-ray regime, Surface Science, 698, 2020, 121607.
https://doi.org/10.1016/j.susc.2020.121607.
Electrochemistry
This study examines lithium-sulfur batteries for automotive applications, utilizing operando sulfur K-edge XANES to analyze the oxidation-reduction chemistry of sulfur and correlate it with chemical mechanisms and local structure. Figure 4 displays XANES data during battery operation, and spectra measured in operando mode confirm variations in cathode composition. Sulfur compounds formed at the cathode can be identified through characteristic energies of edge and pre-edge resonances. For instance, elemental sulfur exhibits a major resonance at 2474 eV, while sulfate absorption shifts to 2479 eV. The primary contribution to this spectrum comes from S6+ compounds in the electrolyte, with the increase in shoulders during discharge indicating the production of polysulfides.
References:
Giuliana Aquilanti et al. Operando characterization of batteries using x-ray absorption spectroscopy: advances at the beamline XAFS at synchrotron Elettra. J. Phys. D: Appl. Phys. 2017, 50, 074001.
https://iopscience.iop.org/article/10.1088/1361-6463/aa519a/meta