the highly correlated systems such as transition metal oxides and f-electron systems, many-body effects can change the qualitative behavior of the near-edge spectrum while the main effect on the EXAFS region is simply an overall reduction in the amplitude of the fine structure, which is taken into account by the Many-body effects can be traced down to the different energy of photoelectron. The photoelectron with a larger kinetic energy is less affected by the neighboring coordinating atom. Under normal circumstances, it is only scattered by the neighboring coordinating atom. However, if the kinetic energy of the photoelectron is very small, it will be scattered many times by an unknown neighboring coordinating atom scattering. This is the biggest difference between the simplified models of the EXAFS and XANES. Based on single scattering, the EXAFS can generally only give average structural information. The multiple scattering signal that occurs on the high-energy side of the XANES region records the superposition of the scattered waves when scattered by more than one neighbor atom. Therefore, it can reflect the three-dimensional coordination environment of the absorbing atom, combined with the relevant information of the transition, and provide strong evidence to judge the absorption atomic coordination geometry.
The EXAFS has limitations. At high temperature, taking in situ reaction conditions as an example, it is difficult to analyze the EXAFS under such conditions [108]. The XANES is highly sensitive to the local symmetry of the short-range order of absorbing atoms, and the short-range order of matter still exists at high temperatures. Therefore, the XANES is widely applicable. In principle, the XANES can distinguish mixed systems. The reason is that the characteristic of the XANES spectrum is fingerprint authentication, and a mixture of multiple systems can be distinguished.
Although the central atoms are completely different, the lines and shapes of oxides and fluorides with the same short-range order structure in the multiple scattering zone are the same. This has been confirmed by a large number of experimental spectra. This is to identify the coordination geometry of the central atom. At present, the identification of this part of the spectrum is mainly based on experience and comparison with the standard samples.
The EXAFS is also less sensitive to the nonspherical details of the potentials, and a simple overlapped atomic muffin tin potential is adequate for most practical calculations. On the other hand, near-edge spectra can be quite sensitive to the details of charge transfer and changes in Fermi level due to the solid-state effects. Thus, the use of self-consistent potentials and often nonspherical symmetry are essential for accurate calculations of the XANES. Finally, calculations of the single-particle Fermi golden rule must be treated differently in the near-edge region because the path expansion detailed in the equation often fails to converge (or converges very slowly) for low-energy photoelectrons. This slow convergence is caused by two factors. First, the inelastic mean free path becomes large for low energy electrons so that very long paths must be included in the expansion. Second, large angle scattering amplitudes are not small at low energies, so that the XANES signal is not dominated by the nearly linear scattering paths, and all multiple scattering paths must be considered.
2.2.4 Application in Amorphous Nanomaterial Characterization
For the study of the atomic local environment, XAFS is one of the most powerful tools for structural characterization. Because the X-ray absorption spectrum and the coordination structure around the atom have a fingerprint-like correspondence, it can accurately study the structural parameters such as the oxidation state, coordination relationship, bond length, and chaos of the atom to be measured. Of note, the experimental observation is in an atomic short-range scale, which does not reflect whether the sample structure has a long-range order or not. In the following section, we will present some research on amorphous structure characterization by using the XAFS.
Zhang et al. used electrocatalysts operando XAFS to identify the active sites in NiFe PBAs during the OER process [109]. They discovered that the NiFe-PBA decomposed and transferred to amorphous nickel hydroxide with Fe disappearing in the decomposition (as shown in Figure 2.9). By comparing the sample before and after the catalysis process, amorphous nickel hydroxide is considered as the real catalyst in the reaction. It is worth noting that the XAFS result also reveals the reason why the amorphous nickel hydroxide shows a higher catalyst activity. The amorphous structure is of unstable nature and flexible to change; thus, Ni(II) is easier to be oxidized to Ni(III), which is obvious in the absorbing edge (Figure 2.10).
Figure 2.9 (a) SEM image of NiFe Prussian blue analog (NF-PBA). (b) TEM image of NF-PBA; the inset is the electron diffraction pattern. (c) TEM image of NF-PBA-A; inset is the electron diffraction pattern. (d) XRD pattern of NF-PBA-A. Source: Reproduced with permission from Crewe et al. [6]. Copyright 2018, American Chemical Society.
Figure 2.10 Operando Ni K-edge XAS spectra of NF-PBA-A under different potentials. (a) XANES of NF-PBA-A as well as references. Inset shows the shift of the Ni K-edge position. (b) FT-EXAFS of NF-PBA-A. Source: Reproduced with permission from Su et al. [6]. Copyright 2018, American Chemical Society.
After that, Zhang’s group also used the same method to track the phase change in the electrocatalysts LaCo0.8Fe0.2O3−δ (LCF) after in-situ exsolution [110]. In the in-situ XAFS test, they surprisingly found that the Co/Fe metal nanoparticles in the LCF perovskite are transformed into an amorphous (Co/Fe)O(OH) layer with unsaturated coordination of metal ions. They found that cobalt ions treated by high-temperature annealing were reduced to metallic cobalt (Figure 2.11).
Figure 2.11 (a) XRD patterns for LaCo0.8Fe0.2O3−δ (LCF) and the reduced samples at different temperatures. (b) Co K-edge XANES spectra of LCF, LCF-400, and LCF-700 as well as various reference samples. (c) Fourier transform (FT) of the Co K-edge EXAFS. (d) Fe K-edge XANES spectra. (e) FT of the Fe K-edge EXAFS. Source: Reproduced with permission from Song et al. [110]. Copyright 2018, The Royal Society of Chemistry.
Therefore, they performed operando XAS studies to directly monitor the catalytic process
