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. 2020 Oct 23;11:5368. doi: 10.1038/s41467-020-19212-y

Fig. 4. XPS characterization for W0.2Er0.1Ru0.7O2−δ, Er0.1Ru0.9O2−δ, W0.2Ru0.8O2−δ, and RuO2−δ nanosheets.

Fig. 4

a Ru 3d spectra, b W 4f spectra, c Er 4d spectra, d O 1s spectra for the prepared W0.2Er0.1Ru0.7O2−δ, Er0.1Ru0.9O2−δ, W0.2Ru0.8O2−δ, and RuO2−δ nanosheets. In order to precisely analyze the valance state of Ru, X-ray absorption near-edge spectroscopy (XANES) was applied to characterize RuO2−δ and W0.2Er0.1Ru0.7O2−δ. Ru foil and C-RuO2 were employed as reference materials3,18. Compared with Ru K-edge position of Ru foil, the C-RuO2, RuO2−δ, and W0.2Er0.1Ru0.7O2−δ all shifted to higher energy, resulting from the Ru–O bonds in these materials (Fig. 5a). Additionally, the Ru K-edge spectra in Fig. 5a showed the adsorption energy of the prepared RuO2−δ and W0.2Er0.1Ru0.7O2−δ were different from that for C-RuO2. This result mainly resulted from the fact that Ru valence state in RuO2−δ and W0.2Er0.1Ru0.7O2−δ were mainly dominated Ru4+ accompanied with Rux+ (x < 4)3. Simultaneously, compared with adsorption energy of RuO2−δ, the adsorption energy for W0.2Er0.1Ru0.7O2−δ shifted to lower energy region, indicating that Ru valence state in W0.2Er0.1Ru0.7O2−δ was a little lower than that in RuO2−δ due to introduction of W and Er. Additionally, the adsorption energy (E0) for RuO2−δ (22,119.99 eV) was also a little higher than that for W0.2Er0.1Ru0.7O2−δ (22,118.92 eV). These results were consistent with the valence state analysis in XPS. Furthermore, extended X-ray absorption fine structure (EXAFS) with Fourier transform as well as its counterpart (k3-weighted EXAFS) was applied to analyze the structure of RuO2−δ and W0.2Er0.1Ru0.7O2−δ (Fig. 5b). Compared with the bond of Ru–Ru in Ru foil (2.68 Å), W0.2Er0.1Ru0.7O2−δ exhibited a slightly longer interatomic distance (2.71 Å), which could be related with the strained effect in HRTEM3 (Supplementary Table 7). Additionally, the Ru–Ru and Ru–O bonds in RuO2−δ and W0.2Er0.1Ru0.7O2−δ showed different interatomic distances is due to the existence of lower Rux<4 valence state, compared with that in C-RuO2 (3.12 and 3.56 Å). Besides that, the different Ru–Ru, Ru–O bonds between RuO2−δ and W0.2Er0.1Ru0.7O2−δ should be related with introducing W and Er into RuO2−δ. Furthermore, wavelet transform (WT) for Ru K-edge EXAFS in Fig. 5c–f was applied to exhibit the length changes of Ru–Ru and Ru–O bonds in W0.2Er0.1Ru0.7O2−δ. The intensities at ≈6.5 Å−1 increased gradually, indicating the Rux<4 had strong influence on W0.2Er0.1Ru0.7O2−δ, compared with that for C-RuO2. Besides that, compared with RuO2−δ, the intensities changed slightly at ≈13.5 Å−1 in W0.2Er0.1Ru0.7O2−δ is due to the coordination of Ru–W/Er.