Plutonium dioxide nanoparticles puzzle solved through a combination of synchrotron techniques

Nanoscience sometimes produces results that are more mystifying than in any other discipline. Changes in the plutonium dioxide (PuO₂) particle size from bulk to nano could have a dramatic effect on PuO₂ properties. Indeed, plutonium is one of the most complex and fascinating chemical elements in the periodic table. In environmentally-relevant conditions, Pu may exist and even co-exist in four oxidation states, from Pu(III) to Pu(VI). It has been discovered that Pu may be transported by groundwater from contaminated sites on a scale of kilometres when bound to mineral or organic colloids. Recently, it was repeatedly found that PuO_2+x nanoparticles (NPs) are formed during interfacial processes between Pu in different initial oxidation states and various mineral surfaces (hematite, goethite, quartz, and mica) and bacteria. All these results indicate the high importance of PuO₂ NPs in the context of the environmental behaviour of Pu. To improve our understanding of Pu properties, it is necessary to use licensed laboratory facilities to handle radioactive material safely, to combine this with the most powerful experimental methods and to complement the results with theoretical studies. X-rays emitted by a synchrotron can be used to study the atomic structure of such materials because of their penetrative behaviour and their sensitivity to the local environment and the electronic structure of the selected element.

A systematic investigation of PuO₂ NPs was carried out, synthesised using environmentally and waste storage relevant conditions by varying the pH (e.g. pH 8 and pH>10) and the precursor (Pu(III), Pu(IV), Pu(V)). The synthesised PuO₂ NPs were characterised by a variety of experimental methods: extended X-ray absorption fine structure (EXAFS) at BM20, X-ray absorption near-edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L₃ edge at BM20 and at the Pu M₄ edge at ID26, high energy X-ray scattering (HEXS) at ID15A and X-ray diffraction (XRD) at BM20. The theoretical interpretation of the experimental data was based on the Anderson impurity model (AIM) and ab initio calculations. Figure 1 shows a schematic illustration of the research methodology applied to PuO₂ NPs studies.

Figure 1. Schematic illustration of the research methodology applied in the PuO2 NP studies. PuO2 NPs synthesised from different precursors (Pu(III), Pu(IV) and Pu(V)) and at pH 8 and pH >10 were analysed with HRTEM and various synchrotron techniques (EXAFS, XRD, HEXS and HERFD).

The most intriguing question concerning PuO₂ nanoparticles is the potential presence of various oxidation states of plutonium (referred to as PuO_2+x). The formation of PuO₂ NPs has already been investigated by various methods, but none has provided an unambiguous answer regarding the Pu oxidation state. In this work, HERFD was applied at beamlines ID26 and BM20 to obtain straightforward information about the Pu oxidation state for the first time (Figure 2). The HERFD data at the Pu L₃ edge confirm the presence of the Pu(IV) oxidation state as the dominant valence of PuO₂ NPs, as seen from the positions of the Pu white line. However, a minor white line energy shift among the samples is observed (within 0.2-0.4 eV), which prevents exclusion of tiny amounts of other oxidation states being present. Previous experiments showed that HERFD measurements at the actinide M₄ edge, characterised by the actinide 3d-5f electronic transitions, are very powerful for oxidation state identification [1]. The Pu M₄ HERFD spectrum of PuO₂ shows a sharp peak at ∼3970 eV due to the transitions from the 3d_3/2 core level to the unoccupied 5f_5/2 level and a shoulder at higher energy. This splitting of the Pu M₄ main edge transitions is reproduced very well by theoretical calculations and relates to the multiplet splitting of the Pu 5f states. These results confirm the presence of only the Pu(IV) oxidation state in all PuO₂ NPs and settle the controversial debates of the last years about the presence of other oxidation states in nanosystems.

Figure 2. Pu M4 and L3 HERFD experimental data recorded at beamlines ID26 and BM20. Data recorded for six PuO2 NPs samples and compared with a PuO2 reference and theoretical calculations, using the AIM (Anderson impurity model) approximation (left) and the FEFF code (right). The density of states is plotted at the bottom for clarity.

In general, spectroscopic methods with standard resolution have been used extensively in studies of actinide systems in the past years. One of the most popular methods is EXAFS at the Pu L₃ edge, which has been carried out multiple times on PuO₂ NPs by different research groups, but results are heavily debated and are not in agreement with one another. The main discrepancy is related to the different interpretations of the first coordination sphere, which refers to Pu-O interaction and can be classified as:

1. a single Pu-O interaction similar to the one in PuO₂;
2. several Pu-O interactions (with different coordination of O atoms);
3. several Pu-O interactions, including Pu in various oxidation states.

A combination of techniques and theoretical calculations was used in order to find the most consistent model. Pu-O interactions with Pu in various oxidation states can be discarded using the HERFD results. Several complementary methods were used to verify the presence or absence of different Pu(IV)-O contributions. The agreement of the results from the EXAFS shell fit, Monte-Carlo EXAFS simulations, EXAFS Landweber iteration approach and from HEXS indicates unequivocally that only one Pu(IV)-O interaction with a rather symmetric distance distribution (similar to the PuO₂ reference) is present for all six PuO₂ NPs samples.

Moreover, XRD and HEXS experiments, performed at the ID15A beamline (Figure 3) show that PuO₂ NPs are unexpectedly homogeneous and monodisperse. The crystalline structure of the NPs is similar to that of bulk PuO₂. It is surprising and not yet understood why NPs of 2.5 nm are always formed. There are certain difficulties in obtaining bigger PuO₂ NPs (size 5-10 or 15 nm) via the chemical precipitation method.

Figure 3. Size and structure information of PuO2 NPs. a) HRTEM image of NPs from Pu(V) pH 8, b) SAED (selected-area electron diffraction) patterns of particles, the white lines indicate peak positions from a PuO2 standard, c) Intensity profiles extracted from the 2D scattering patterns of the Pu samples, obtained by HEXS at beamline ID15A. d) the corresponding reduced pair distribution functions G(r) obtained by Fourier transformation of the data with Qmax = 26.0 Å-1.

All the collected information plays an important role in explaining plutonium chemistry under real conditions. It will help to advance, to model and to predict long-term Pu release from deep underground repositories of nuclear waste and contaminated sites [2].

Principal publication and authors

E. Gerber (a,b,c), A. Yu. Romanchuk (c), I. Pidchenko (a,b), L. Amidani (a,b), A. Rossberg (a,b), Ch. Hennig (a,b), G.B.M. Vaughan(d), A. Trigub (e), T. Egorova (c), S. Bauters (a,b), T. Plakhova (c), M. O.J.Y. Hunault (f), S. Weiss (b), S.M. Butorin (g), A.C. Scheinost (a,b), S.N. Kalmykov (c,e) and K.O. Kvashnina (a,b,c), The missing pieces of the PuO2 nanoparticles puzzle, Nanoscale (2020); doi: 10.1039/d0nr03767b.
(a) The Rossendorf Beamline at ESRF (BM20), The European Synchrotron, Grenoble (France)
(b) Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, Dresden (Germany)
(c) Lomonosov Moscow State University, Department of Chemistry, Moscow (Russia)
(d) ESRF
(e) National Research Centre “Kurchatov Institute”, Moscow (Russia)
(f) Synchrotron SOLEIL, L’Orme des Merisiers (France)
(g) Molecular and Condensed Matter Physics, Department of Physics and Astronomy, Uppsala University, Uppsala (Sweden)

References
[1] K.O. Kvashnina, A.Y. Romanchuk, I. Pidchenko, L. Amidani, E. Gerber, A. Trigub, A. Rossberg, S. Weiss, K. Popa, O. Walter, R. Caciuffo, A.C. Scheinost, S.M. Butorin, and S.N. Kalmykov, Angew. Chemie Int. Ed. 58, 17558 (2019).
[2] A.Y. Romanchuk, T. V. Plakhova, A.V. Egorov, T.B. Egorova, P.V. Dorovatovskii, Y.V. Zubavichus, A.A. Shiryaev, and S.N. Kalmykov, Dalt. Trans. 47, 11239 (2018).