Amorphous Nanomaterials. Lin Guo
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 of the electrocatalysts under electrochemical conditions. It is found that homogeneously dispersed Co/Fe metallic nanoparticles socketed in the oxides generate an amorphous (Co/Fe)O(OH) layer with unsaturated coordination of metal ions during the OER electrochemical process in alkaline solution (Figures 2.12 and 2.13).
Guo et al. also used operando XAS to investigate amorphous cobalt hydroxide cages behavior in OER [111]. They synthesized amorphous cobalt hydroxide cages via the hard template method. It is found that the extraordinary OER catalysis performance can be attributed to its amorphous structure. In comparison to the crystal cobalt hydroxide, in-situ XAS revealed that cobalt ions in the amorphous state are easier to be oxidized into +3/+4 valences, which are regarded as the realistic catalyst sites in the reaction. A theory that amorphous structure with structural flexibility can adapt itself during a given catalytic process for enhanced activity was proposed. They also pointed out that the adaption occurs in the first two linear sweep voltammetry (LSV) tests.
Figure 2.12 Operando XAS spectra of (a) Co K-edge XANES of LCF-700 from 1.47 to 1.52 V (vs. RHE) in 0.1 M KOH and (b) Co K-edge FT-EXAFS of LCF-700. Source: Reproduced with permission from Song et al. [110]. Copyright 2018, The Royal Society of Chemistry.
Figure 2.13 Transformation of the catalysts by pretreatment. (a) CV for the AH-Co and the CH-Co catalysts. (b) Co K-edge XANES spectra for of the AH-Co, CH-Co, and COH-Co before pretreatment and their in situ XANES spectra after pretreatment. (c) EPR spectra. (d) XPS spectra. (e) HRTEM images of the AH-Co after the pretreatment (AH-Co-aa). (f) HRTEM images of CH-Co after the pretreatment (CH-Co-aa). (g) Concise schematic diagrams showing the transformation processes of AH-Co and CH-Co in pretreatment. Source: Reproduced with permission from Liu et al. [111]. Copyright 2018, WILEY-VCH.
In addition, Guo’s group addressed a new synergistic effect between cobalt and vanadium in a cobalt–vanadium hydr(oxy)oxide with a high performance in OER catalysis [112]. This system illustrated oxidation or transformation of the Co state with V. The absorption edge of V for ultrathin amorphous cobalt-vanadium bimetal hydr(oxy)oxide (CoV-UAH) shifts to lower energy, implying that the V ion with a lower valence state may appear. While coinciding with the increasing state of Co in the same potential, the decrease of the valence state for the V species may be due to the charge transfer from cobalt to vanadium caused by the strong interaction between them (Figure 2.14).
2.2.5 Summary and Outlook
The XAFS method is the most effective way to explore the structure of amorphous materials. In this chapter, we give a brief introduction of XAFS, including its theory and application. Through these published papers we incited, we can find out that with the development of computer science, some limited conditions to the EXAFS including in-situ test in the past become available. In fact, the XAFS has paradigms of frontier sciences and technologies for catalysts, nanoparticles, and surfaces, such as time-solved analysis of operando chemical states, short-lived dynamics, in-situ imaging of real spaces, and spatially resolved analysis of amorphous materials. With such a powerful tool to reveal the short-range structure and core-level energy, amorphous material research will get a rapid development.