Tender X-ray Near-Edge Absorption Fine Structure (TX-NEXAFS, 1.75~6 keV)
Tender X-ray Near Edge X-ray Absorption Fine Structure (TX-NEXAFS) spectroscopy is a cutting-edge analytical method used to delve deep into the electronic and chemical properties of materials. Operating within the tender X-ray energy range (1.75 - 6 keV), this technique allows researchers to investigate atomic arrangements, oxidation states, and electronic configurations with exceptional precision. By irradiating the sample with tender X-rays, core-level electrons undergo photoabsorption, generating an energy spectrum near the absorption edge. Analysis of this spectrum provides crucial insights into the elemental composition, bonding characteristics, and molecular orientation within the material.
The applications of Tender X-ray NEXAFS span across various scientific domains. In materials science, it is instrumental in studying thin films, surfaces, and interfaces, aiding in the understanding of electronic behaviors and structure-property relationships. Catalysis research benefits from this technique by enabling the analysis of surface reactions, catalyst functionality, and active sites. Environmental studies utilize Tender X-ray NEXAFS to investigate pollutants, aerosols, and environmental samples. Additionally, it plays a pivotal role in advancing energy materials research, contributing to the development of innovative materials for energy storage, conversion, and renewable energy applications.
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.