Hard X-ray Near Edge Absorption Fine Structure (HX-NEXAFS, 5~27 keV)
Interpretation of XAS
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 Nd 0.1 RuO x . The Ru K-edge of both RuO x shifted to higher energy levels in comparison to Ru foil. This shift is attributed to the presence of Ru-O bonds in RuO 2 and Nd 0.1 RuO x , as illustrated in Figure 2. Additionally, the Ru K- edge energy of Nd 0.1 RuO x surpasses that of RuO 2 , indicating a higher proportion of Ru 4+ in Nd 0.1 RuO x compared to RuO 2 , consistent with the findings from XPS results. The prevalence of Ru 4+ 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.
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 Zn 2+ , 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 ZnSO 4 and 1M Zn(TFSI) 2 . As illustrated in the inset of Figure 3 , the XANES pre-edge of 1M Zn(BBI) 2 shifts to a higher energy compared to 1M ZnSO 4 and 1M Zn(TFSI) 2 . This shift indicates a higher effective valence of Zn 2+ in the former, suggesting that Zn(BBI) 2 exhibits weak solvation, and Zn 2+ possesses a more anion-derived solvation sheath in the 1M Zn(BBI) 2 solution. In contrast, ZnSO 4 and Zn(TFSI) 2 are highly solvated, and the solvation sheaths of Zn 2+ in their 1M aqueous solutions primarily consist of neutral H 2 O molecules.
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.
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 SrVO 3 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 SrVO 3 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 Sr 2+ 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 SrVO 3 thin films.
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.