Hard X-ray Near Edge Absorption Fine Structure (HX-NEXAFS, 5~27 keV)
Hard X-ray Near Edge Absorption Fine Structure (HX-NEXAFS) is an advanced spectroscopic technique utilized for the in-depth analysis of a material's electronic and chemical composition. Operating in the high-energy range of 5 to 27 keV, HX-NEXAFS utilizes hard X-rays to probe the near-edge absorption of elements in a sample. This method allows researchers to gain detailed insights into the electronic transitions that occur near the absorption edges of specific elements, offering information about bonding states, oxidation states, and coordination environments.
HX-NEXAFS finds applications in diverse scientific fields, including materials science, chemistry, and biology. Its high-energy capabilities enable the investigation of heavier elements and offer a deeper penetration into the sample, making it particularly valuable for studying complex materials, catalysts, and biological specimens. As a non-destructive and highly sensitive technique, HX-NEXAFS provides crucial data for understanding the electronic structure of materials at a molecular level, contributing to advancements in various research areas and technological innovations.
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
To examine the impact of Nd doping, X-ray absorption near-edge spectroscopy (XANES) was conducted in this study to investigate the oxidation state of Ru in Nd0.1RuOx. The Ru K-edge of both RuOx shifted to higher energy levels in comparison to Ru foil. This shift is attributed to the presence of Ru-O bonds in RuO2 and Nd0.1RuOx, as illustrated in Figure 2. Additionally, the Ru K-edge energy of Nd0.1RuOx surpasses that of RuO2, indicating a higher proportion of Ru4+ in Nd0.1RuOx compared to RuO2, consistent with the findings from XPS results. The prevalence of Ru4+ is advantageous for chemical stability. This is crucial, as the low oxidation state of Ru has the potential to transform into high-valence Ru species during intense oxygen evolution reactions, eventually dissolving during the oxygen evolution reaction (OER) process. This phenomenon helps prevent structural collapse and performance degradation.
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.
Energy Storage and Conversion
Interactions among cations, anions, and the solvent within the aqueous electrolyte significantly influence the electrochemical cell's performance. To analyze the local structure of solvated Zn2+, Du et al. conducted synchrotron X-ray absorption near-edge structure (XANES) spectroscopy on 1M Zn(BBI)2 and compared the results with those of 1M ZnSO4 and 1M Zn(TFSI)2. As illustrated in the inset of Figure 3, the XANES pre-edge of 1M Zn(BBI)2 shifts to higher energy compared to 1M ZnSO4 and 1M Zn(TFSI)2. This shift indicates a higher effective valence of Zn2+ in the former, suggesting that Zn(BBI)2 exhibits weak solvation, and Zn2+ possesses a more anion-derived solvation sheath in the 1M Zn(BBI)2 solution. In contrast, ZnSO4 and Zn(TFSI)2 are highly solvated, and the solvation sheaths of Zn2+ in their 1M aqueous solutions primarily consist of neutral H2O molecules.
Reference:
Du H, Dong Y, Li Q, Zhao R, Qi X, Kan WH, et al. A New Zinc Salt Chemistry for Aqueous Zinc‐Metal Batteries. Adv Mater 2023;2210055:2210055.
https://doi.org/10.1002/adma.202210055.
Thin Films
To comprehend the oxidation state of Sr ions, Sharma et al. systematically gathered Sr K-edge XANES spectra from various samples, including the SrO reference powder (representing Sr in a 2+ oxidation state), as well as pristine SrVO3 films and those annealed at 500°C and 700°C. In Figure 4(b), it is evident that the white line peak position of both the pristine and annealed SrVO3 thin films closely aligns with that of the reference SrO sample within the spectral resolution of the utilized beam line. This alignment confirms the presence of Sr2+ ions in the SrVO3 thin films. While there is marginal variation in the white line peak intensity and edge-energy position in the higher temperature annealed samples, this could be attributed to a slightly increased oxidation of surface Sr ions in thinner samples. Notably, higher-temperature annealed samples exhibit lower film thickness and higher surface roughness, as observed in XRR and SEM analyses. The intensities of multiple scattering peaks (between 16,120–16,160 eV) are enhanced in the annealed films, likely due to robust Sr–O/V–O atomic bonding. Nevertheless, no distinct spectral features related to suspicious phases have been observed. Therefore, Sr K-edge XANES validates the absence of notable phase formation during the annealing process for orthorhombic SrVO3 thin films.
Reference:
Sharma A, Varshney M, Cheol Lim W, Shin HJ, Pal Singh J, Ok Won S, et al. Mechanistic insights on the electronic properties and electronic/atomic structure aspects in orthorhombic SrVO3 thin films: XANES-EXAFS study. Phys Chem Chem Phys 2017;19:6397–405.
https://doi.org/10.1039/c6cp08301c.
- 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%