Hard X-ray Extended Absorption Fine Structure (HX-EXAFS, 10~30 keV)
Hard X-ray Extended Absorption Fine Structure (HX-EXAFS), operating in the energy range of 14 to 35 keV, is an advanced spectroscopic technique designed to investigate the intricate details of a material's atomic structure. This method is based on Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, which employs high-energy X-rays to probe the local atomic environment of specific elements within a sample. The extension to higher energy levels enhances the penetration depth of the X-rays and expands the applicability of EXAFS to a broader range of materials.
HX-EXAFS is particularly well-suited for studying dense or thick samples, providing detailed insights into the atomic arrangements, coordination, and bonding structures. The higher energy range allows for the analysis of heavier elements and enables the examination of materials with greater depth. This technique finds applications across various scientific disciplines, including materials science, chemistry, and physics, contributing to a deeper understanding of the fundamental properties of materials at the atomic scale.
Interpretation of XAS
Figure 1 displays K-edge XANES of manganese oxides, highlighting a clear link between the oxidation state and edge position. As the oxidation state rises, the absorption edge moves to a higher energy level. Utilizing a more advanced analytical method involves fitting with a linear combination of established references to ascertain the relative contributions of mixed systems. Conversely, determining the oxidation state, or its alteration, usually involves observing a shift in the primary absorption edge.
Reference:
Zimmermann P, Peredkov S, Abdala PM, DeBeer S, Tromp M, Müller C, et al. Modern X-ray spectroscopy: XAS and XES in the laboratory. Coord Chem Rev 2020;423:213466.
https://doi.org /10.1016/j.ccr.2020.213466.
Catalysis Studies
In this study, Li et al. have utilized extended X-ray absorption fine structure (EXAFS) analysis to investigate the coordination structure of Nd0.1RuOx, as illustrated in Figure 2f. The findings reveal that Nd0.1RuOx exhibits a slightly longer Ru-Ru distance (2.87 Å) compared to RuO2 (2.81 Å) and Ru foil (2.67 Å). This elongation is attributed to lattice expansion resulting from the disparity in ionic radii between Ru and Nd. Additionally, the relative intensity of the Ru-Ru bond in Nd0.1RuOx is lower than that in RuO2, a factor expected to mitigate structural changes during catalytic processes.
Reference:
Li L, Zhang G, Xu J, He H, Wang B, Yang Z, et al. Optimizing the Electronic Structure of Ruthenium Oxide by Neodymium Doping for Enhanced Acidic Oxygen Evolution Catalysis. Adv Funct Mater 2023;33:1–9.
https ://doi.org/10.1002/adfm.202213304.
Nanotechnology
To understand how Pb13O8(OH)6(NO3)4 interacts with ZIF-8 in the newly formed nanocomposites, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were utilized. The XANES spectra revealed that the X-ray absorption of Pb in 10% Pb-ZIF-8 is similar to Pb foil, PbO, and PbO2, but it's closest to PbO, indicating that the valence state of Pb in Pb-ZIF-8 is almost +2 (see Figure 3a). The main peak at 1.6 Å in EXAFS is due to the Pb-O/N scattering path. Also, Pb-Pb coordination was observed (see Figure 3b), suggesting there's a bit of Pb clustering in 10% Pb-ZIF-8. To get more detailed information, a technique called wavelet transform (WT) of EXAFS spectra was used, which helped combine information from backscattering atoms, R-space, and k-space in three dimensions (refer to Figure 3c,d).
Reference:
Zeng L, Huang X, Le Y, Zhou X, Zheng W, Brabec CJ, et al. Reversible Growth of Halide Perovskites via Lead Oxide Hydroxide Nitrates Anchored Zeolitic Imidazolate Frameworks for Information Encryption and Decryption. ACS Nano 2023;17:4483–94 .
https://doi.org/10.1021/acsnano.2c10170.
Energy Storage Materials
In-situ X-ray absorption spectroscopy (XAS) demonstrated that when lithium is added, Nb5+ changes to Nb4+. The oxidation state continuously changes during lithiation, as seen in Figure 4b. The extended X-ray absorption fine structure (EXAFS) reveals a two-stage insertion reaction (Figure 4c). Initially, Nb2O5 bond lengths merge at 1.75 Å, indicating increased symmetry due to lithiation. Lithiation is faster at lower Li+ levels, likely due to more available sites. At lower potentials, the new EXAFS peak shifts to longer bond distances (1.85 Å) because of increased Li-O interactions. These findings highlight the importance of an open, layered structure for rapid ion transport in the active material. This study establishes that T-Nb2O5 behaves like a pseudocapacitive material, even though charge storage happens throughout the bulk.
Reference:
Augustyn, V.; Come, J.; Lowe, MA; Kim, JW; Taberna, PL; Tolbert, SH; Abruña, HD; Simon, P.; Dunn, B. High-Rate Electrochemical Energy Storage through Li+ Intercalation Pseudocapacitance. Nat. Mater. 2013, 12 (6), 518–522.
https://doi.org/10.1038/nmat3601.
- Prepare a small zip bag and label it with the sample order number and sample elements
- Put your powder inside the zip bag
- Cover the zip bag with a piece of paper
- Place the zip bag inside a vacuum-sealed bag
- Place it inside another large zip bag and label it with the sample order number, target element wt%, and other elements wt%