The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the Ln/An oxide particle size from bulk to nano can have a drastic effect on Ln/An oxide properties. We have synthesized a number of Cerium (Ce), Thorium (Th), Uranium (U) and Plutonium (Pu) oxide NPs and performed full characterization of An/Ln oxide NPs at the atomic and molecular levels by a variety of synchrotron-based methods and electronic structure calculations.
The PuO2 NPs were synthesized from precursors with different oxidation states of Pu (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analysed with theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of 2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(IV) oxidation state in all NPs indicate that the structure of PuO2 NPs is very similar to that of bulk PuO2 (Nanoscale (2020) doi:10.1039/D0NR03767B). Moreover, we found evidence that the formation of PuO2 NPs from oxidized Pu(VI) proceeds through the formation of an intermediate Pu(V) solid phase, similar to NH4PuO2CO3, which is stable over a period of several months (VIP in Angew. Chemie Int. Ed. 58 (2019) 17558–17562. doi:10.1002/anie.201911637). Lately, the formation of the PuO2 NPs under acidic pH was studied in detail. It was found the unexpected presence of the Pu(III) oxidation state by applying HERFD methodology and electronic structure calculations (Env.Sci.Nano (2022) 9, 1509-1518, doi: 10.1039/D1EN00666E) which originates from starting solution rather than from the NPs themselves.
Contrary to Pu with its extremely high stability of the Pu(IV) oxidation state, U, upon oxidation, may form various oxides with mixed U oxidation states. Structural and electronic properties of mixed U oxides - U3O7, U4O9, U3O8 - have been studied by valence band RIXS and HERFD methods and a variety of theoretical simulations (Chem. Commun. 54 (2018) 9757–9760. doi:10.1039/C8CC05464A Inorg. Chem. 59 (2020) 4576–4587. doi:10.1021/acs.inorgchem.9b03702). We studied the structure-property relation of small UO2 NPs (<2nm) by HERFD at the U M4 edge (Inorg. Chem. Front., 8, 4, 1102–1110 (2021), doi: 10.1039/D0QI01140A.). As a part of the project, we explored the behaviour of CeO2 NPs of different sizes, which have many industrial and commercial applications. We have demonstrated that in addition to the nanoceria charge stability (Ce(IV)), the formation of hydroxyl groups at the surface profoundly affects the chemical performance of these nanomaterials (Nanoscale 11, 39 18142-18149 (2019), doi:10.1039/C9NR06032D). In addition, the crystal-field splitting of the Ce 5d states into the eg and t2g bands have been investigated (Inorg. Chem. 59 (2020) 5760–5767. doi:10.1021/acs.inorgchem.0c00506). Later, we performed X-ray measurements on a series of oxo-hydroxo polynuclear Ce complexes, which represent the formation of CeO2 small NPs from a few Ce ions (Chem. Mater. 35, 1723-1734, (2023), doi: 10.1021/acs.chemmater.2c03456). The ThO2 NPs in a wide range of particle sizes (from 2.5 to 34 nm) have been made by selecting the chemical precipitation conditions (J. Phys. Chem. C. 123 (2019) 23167–23176, doi:10.1021/acs.jpcc.9b04379). Electronic structure changes have been recorded for small particles with a diameter <2.5 nm and interpreted with the help of ab-initio simulations (Phys. Chem. Chem. Phys. 21 (2019) 10635–10643. doi:10.1039/C9CP01283D.). An innovative approach for accurate structural characterization of actinide nanomaterials has been proposed, based on the case of ThO2 (Chem. – A Eur. J., 27, 1, 252–263, (2021), doi: 10.1002/chem.202003360). We made a first attempt and studied in detail the Oxygen K-edge spectra of ThO2 and CeO2 and interpreted data, based on density functional theory (J. Phys. Chem. C, 127, 3077-3084 (2023), doi: 10.1021/acs.jpcc.2c07771).
A mini-review article (Chem. Comm, 58, 327-342 (2022), doi: 10.1039/D1CC04851A) has been published, where we describe the latest progress in the field of high-energy resolution X-ray spectroscopy at the actinide M4,5 and ligand K edges and we show that the methods are able to a) provide fingerprint information on the actinide oxidation state and ground state character b) probe 5f occupancy, non-stoichiometry, defects, ligand/metal ratio c) investigate the local symmetry and effects of the crystal field.
Overall, more than 40 papers are published in 5 years (Five publications from our group have been selected as front covers and one as a back cover for the corresponding issues in a number of scientific journals). Over these years, a large number of young scientists from many countries have taken part in the progressively developed research work in TOP group.
