Practical Electron Microscopy and Database

In general, EELS spectra of cerium in materials display two sharp and strong features at 883 and 901 eV corresponding to the cerium M₅ edge and M₄ edge, respectively. Figure 2315a (a) and (b) shows EELS of ceria taken at two different locations of 3D (three-dimensional) ridge-valley structure of a Pt-ceria catalyst. The characteristic peak height inversion and edge splitting present normally oxidation state [1], indicating that a tall M₄ edge is due to +4 valence and a tall M₅ edge signals the presence of significant +3 valence. Therefore, the M₄/M₅ EELS ratio provides a good indication for both reduction potential and valence state. As an example, in Figure 2315a (c) the EELS M₄/M₅ ratio extracted from a line scan across the 3D catalyst indicates the variation of Ce valence across the material and the degree of Ce oxidation.

In some cases, the beam damage of the specimens in TEM measurements can be monitored by observation of energy-loss near-edge structure (ELNES), for instance, in the damage process of CeO₂ specimens. The electron beam damage induces the changes in the spectral shape of a CeO₂ specimen in thickness of 0.17λ within a few seconds. λ is inelastic mean free path. Figure 2315b shows the damage sequence from fresh CeO₂ (A) to damaged CeO₂ (H) with such thickness. After H, the edge showed no further changes with increase of electron dose. The M₅ and M₄ maxima and the peaks Y and Y' are associated with Ce⁴⁺. The electron-beam damaged CeO₂ presents Ce M_4,5 and O K-edge shapes that are consistent with reduction to a Ce³⁺ oxide. During the beam-damage process the spectrum of CeO₂ changes as below:
         i) Decreases in energies of the M₅ and M₄ maxima,
         ii) Changes in shape of the near-edge structure,
         iii) Inversion of the M₅ to M₄ branching ratio,
         iv) Increase in the M₅ to M₄ area ratio.

Figure 2315b. The damage sequence from fresh CeO₂ (A) to damaged CeO₂ (H) in thickness of 0.17λ. The estimated doses in each case are (A) 3 x 10⁵ e/Ų; (B) 6 x 10⁵ e/Ų; (C) 2 x 10⁶ e/Ų; (D) 3 x 10⁶ e/Ų; (E) 7 x 10⁶ e/Ų; (F) 1 x 10⁷ e/Ų; (G) 2 x 10⁷ e/Ų; and (H) 5 x 10⁷ e/Ų. [3]

[1] Wu L, Wiesmann HJ, Moodenbaugh AR, Klie RF, Zhu Y, Welch DO, Suenaga M (2004) Phys Rev B Condens Matter Mater Phys 69(12):125415/1
[2] Uschi M. Graham, Rajesh A. Khatri, Alan Dozier, Gary Jacobs, and Burtron H. Davis, 3D Ridge-Valley Structure of a Pt-Ceria Catalyst: HRTEM and EELS Spectrum Imaging, Catal Lett (2009) 132:335–341.
[3] L.A.J. Garvie and P.R. Buseck, Determination of Ce4+/Ce3+ in electron-beam-damaged CeO2 by electron energy-loss spectroscopy, Journal of Physics and Chemistry of Solids 60 (1999) 1943–1947.