Core-Level Spectroscopy to Probe the Oxidation State of Single Europium Atoms

The valence of individual europium atoms confined in carbon nanotubes is successfully measured by using core-level electron energy loss spectroscopy. Changes in the oxidation state at the atomic scale have been observed in Eu atomic chains exposed to oxygen. A transitory behavior has been identified where multiple atoms show a signal consistent with a sum of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. This indicates that single atoms change their valence state multiple times during the reaction, suggesting that oxidation in confined spaces and with extra energy input (from the electron beam) might not be a simple one step electron transfer event.

Figure 1

(a) Example of a typical Eu atomic chain encapsulated in a NT. In this viewing direction the chain is formed by columns of single (periphery) and double atoms (center), as pointed by the blue and red arrows, respectively. The single atomic spectra shown in the manuscript have been measured in the peripheral atoms. The ADF intensity of both types of columns is consistent with 1 and 2 atoms (Fig. S1 in the Supplemental Material [10]). (b) NTs contain pristine Eu chains free of oxygen (O, Supplemental Material [10], Fig. S2). Because of the creation of defects at the nanotube wall by the electron beam, O atoms enter the NT from the local environment. By monitoring the peak position of Eu $M_5$ edge, the oxidation state of all the Eu atoms can be tracked as the reaction proceeds. As the oxidation takes place, ${\mathrm{Eu}}^i$ (${\mathrm{Eu}}^0$ or ${\mathrm{Eu}}^{2+}$) atoms in the initial state sequentially change to the oxidation state ${\mathrm{Eu}}^{3+}$ and the number of the Eu atoms in the mixed state increases (see text).

Figure 2

Three different spectral signatures observed in our oxidation experiments: ${\mathrm{Eu}}^{2+}$ (1128.5 eV), ${\mathrm{Eu}}^{3+}$ (1131.0 eV), and mixed Eu (two peaks). Dots are the experimental data and curves are the smooth data. Each spectrum has been measured in individual atoms.

Figure 3

(a) ADF image showing Eu atomic chains acquired during chemical mapping. The contrast of NT is suppressed and barely seen. (b), (c) Maps for ${\mathrm{Eu}}^{2+}$ and oxide ${\mathrm{Eu}}^{3+}$, respectively. (d) Labeling of Eu valence states (color code as in Fig. 2, orange: initial state, blue: oxidized and green: mixed states) built from the fine structure analysis of Eu $M_5$ peak. (e) Oxygen chemical map. (f) A full chemical map of Eu atoms in different states with the regions where oxygen is present (the red circles mark maxima in the oxygen map, not atomic positions). (g) ADF (black curves), ${\mathrm{Eu}}^{2+}$ (orange curves) and ${\mathrm{Eu}}^{3+}$ (blue curves) profiles across regions marked by arrows and numbers in (a) and (f). In this chemical map the pixel size was 0.6 Å and the exposure time was 100 ms per pixel.

Figure 4

(a)–(d) Temporal sequences of spectra for ${\mathrm{Eu}}^{2+}$ (a), mixed (b), (c) and ${\mathrm{Eu}}^{3+}$ (d) atoms. For each sequence, the spectrum on the top is the temporal average of the four spectra below. From top to bottom the relative starting time of the acquisition are 0, 100, 200, 1900, and 2000 ms. Time delays have been fixed by the exposure time and the sampling of the two dimensional maps. Two dotted vertical lines indicate the references for ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ $M_5$ peak positions.