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.
Figure 2.14 In situ XAS characterization of CoV-UAH. (a) and (b) Co K-edge XANES data collected on the initial state. (c) and (d) V K-edge XANES data collected on the initial state. Source: Reproduced with permission from Liu et al. [112]. Copyright 2018, The Royal Society of Chemistry.
References
1 1 Kirkland, E.J. (1984). Improved high resolution image processing of bright field electron micrograph: I. Theory. Ultramicroscopy 15: 151–172.
2 2 Lichte, H. (1986). Electron holography approaching atomic resolution. Ultramicroscopy 20: 293–304.
3 3 Scherzer, O. (1947). Spharische und chromatische korrektur von electronen-linsen. Optik 2: 114–132.
4 4 Crewe, A.V., Isaacson, M., and Johnson, D. (1969). A simple scanning electron microscope. Rev Sci. Instrum. 40: 241–246.
5 5 Browning, N.D., Chisholm, M.F., and Pennycook, S.J. (1993). Atomic-resolution chemical analysis using a scanning transmission electron microscope. Nature 366: 143–146.
6 6 Crewe, A.V., Isaacson, M., and Johnson, D. (1971). A high resolution electron spectrometer for use in transmission scanning electron microscopy. Rev. Sci. Instrum. 42: 411–420.
7 7 Muler, D.A., Tzou, Y., Raj, R., and Silcox, J. (1993). Mapping sp2 and sp3 states of carbon at sub-nanometre spatial resolution. Nature 366: 725–727.
8 8 Baston, P.E. (1993). Simultaneous STEM imaging and electron energy-loss spectroscopy with atomic-column sensitivity. Nature 366: 727–728.
9 9 Rose, H. (1990). Outline of a spherically corrected semiaplanatic medium voltage transmission electron microscope. Optik 85: 19–24.
10 10 Meyer, R., Kirkland, A., and Saxton, W. (2004). A new method for the determination of the wave aberration function for high-resolution TEM.: 2. Measurement of the antisymmetric aberrations. Ultramicroscopy 99: 115–123.
11 11 Kuglin, C.D. and Hines, D.C. (1975). The phase correlation image alignment method. Proceedings of the IEEE International Conference on Cybernetics and Society, 163-165.
12 12 Tillmann, K., Thust, A., and Urban, K. (2004). Spherical aberration correction in tandem with exit-plane wave function reconstruction: interlocking tools for the atomic scale imaging of lattice defects in GaAs. Microsc. Microanal. 10: 185–198.
13 13 Nellist, P.D., Behan, G., Kirkland, A.I., and Hetherington, C.J.D. (2006). Confocal operation of a transmission electron microscope with two aberration correctors. Appl. Phys. Lett. 89: 124105.
14 14 Pennycook, S.J., Chisholm, M.F., Varela, M. et al. (2004). Materials applications of aberration-corrected STEM. Microsc. Microanal. 10: 12–13.
15 15 Findlay, S.D., Shibata, N., Sawada, H. et al. (2009). Robust atomic resolution imaging of light elements using scanning transmission electron microscopy. Appl.
