Damien Hudry

Non‐aqueous Synthesis of Isotropic and Anisotropic Actinide Oxide Nanocrystals

The huge interest of the scientific community in the controlled synthesis, structural characterization and assembly into 2and 3-dimensional architectures of nano-objects as well as investigations of their corresponding chemical and physical properties cannot be denied anymore. Within the past two decades, it has been shown that size reduction means more than simply making things smaller. Indeed, size decreasing (as well as shape controlling) is a powerful way to tune materials properties (magnetic, electronic, optical, catalytic, etc.). Whereas nanoscience is a very active field when one considers stable elements, it is still in its infancy when dealing with radioactive actinides. Actinide compounds are important in the nuclear industry and actinide-based nano-objects could be used as new building blocks for the preparation of innovative nuclear fuels or as model systems to study the migration of radionuclides in the environment (e.g., in nuclear waste disposal). The actinide series is also characterized by the emergence of 5f electrons in the valence shell. The behaviour of the 5f electrons determines the solid-sate properties of the actinides and their compounds. Compared to the stable elements, questions related to size and shape effects on the physical and chemical properties of actinide compounds are still open and should find their way into the nanoscience. Accordingly, our main goal is dedicated to the controlled synthesis, the structural characterization and the investigation of the properties of actinide-based nano-objects. Here, we report on the controlled synthesis of uranium oxide and thorium oxide nanocrystals (NCs) by a non-aqueous approach. The obtained NCs have been characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The results of this study therefore present an important step for moving to the preparation of transuranium oxide NCs (e.g., NpO2, PuO2). Uranium and thorium oxide NCs were synthesized by the so-called “heating-up” method by using standard air-free techniques. Uranyl acetylacetonate (UO2ACHTUNGTRENNUNG(acac)2) or thorium acetylacetonate (ThACHTUNGTRENNUNG(acac)4) are introduced in a degassed mixture of dibenzyl ether (BnOBn) with different concentrations of stabilizing agents, like oleic acid (OA), oleylACHTUNGTRENNUNGamine (OAm), trioctylamine (N ACHTUNGTRENNUNG(Oct)3) and trioctylphosphine oxide (OP ACHTUNGTRENNUNG(Oct)3). The resulting mixtures are then heated up to 280 8C. After being cooled to room temperature, the NCs are precipitated with ethanol followed by centrifugation and re-dispersion in toluene. Surprisingly, the experimental conditions well-suited for the formation of uranium oxide NCs cannot be applied when considering the formation of thorium oxide NCs. The modification of the reactivity (for a given organic system) as a function of the nature of the actinide precursor and/or the actinide centre seems to be essential when taking into account the synthesis of actinide oxide NCs. Because of these differences in the reactivity of uranium and thorium precursors, different solvent compositions were tested in order to find the best reaction conditions to obtain well defined NCs. A black precipitate can be isolated from the reaction of UO2ACHTUNGTRENNUNG(acac)2 in a mixture of BnOBn/OA/OAm. The PXRD data of the as-prepared compound along with the corresponding Rietveld refinement are presented in Figure 1 a. The PXRD pattern exhibits Bragg reflections characteristic of the fluorite structure (space group Fm-3m). The experimental PXRD pattern was calculated by using the bulk structure of uranium dioxide (UO2). The detailed results of the Rietveld refinement are given in the Supporting Information. The peak broadening is the result of the small size of the coherent domains (the crystallites), which has been estimated with the fundamental approach to be 4.5 nm. Under the same experimental conditions (i.e., BnOBn/ OA/OAm), the reaction of ThACHTUNGTRENNUNG(acac)4 did not give rise to the formation of thorium-based NCs. Indeed, in the presence of [a] Dr. D. Hudry, Dr. C. Apostolidis, Dr. O. Walter, Dr. T. Gouder European Commission: Joint Research Center Institute for Transuranium Elements, P. O. Box 2340 76125 Karlsruhe (Germany) Fax: (+49) 7247-951-599 E-mail : damien.hudry@ec.europa.eu damien.hudry@gmail.com [b] Dr. E. Courtois, Dr. C. K bel Karlsruhe Institute of Technology, Institute of Nanotechnology Hermann-von-Helmholtz-Platz 1 76344 Eggenstein–Leopoldshafen (Germany) [c] Dr. C. K bel Karlsruhe Nano Micro Facility, Hermann-von-Helmholtz-Platz 1 76344 Eggenstein–Leopoldshafen (Germany) [d] Dr. D. Meyer Institut de Chimie S parative de Marcoule, UMR 5257 BP 17171, 30207 Bagnols sur C ze Cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200513.

Controlled Synthesis of Thorium and Uranium Oxide Nanocrystals

Very little is known about the size and shape effects on the properties of actinide compounds. As a consequence, the controlled synthesis of well-defined actinide-based nanocrystals constitutes a fundamental step before studying their corresponding properties. In this paper, we report on the non-aqueous surfactant-assisted synthesis of thorium and uranium oxide nanocrystals. The final characteristics of thorium and uranium oxide nanocrystals can be easily tuned by controlling a few experimental parameters such as the nature of the actinide precursor and the composition of the organic system (e.g., the chemical nature of the surfactants and their relative concentrations). Additionally, the influence of these parameters on the outcome of the synthesis is highly dependent on the nature of the actinide element (thorium versus uranium). By using optimised experimental conditions, monodisperse isotropic uranium oxide nanocrystals with different sizes (4.5 and 10.7 nm) as well as branched nanocrystals (overall size ca. 5 nm), nanodots (ca. 4 nm) and nanorods (with ultra-small diameters of 1 nm) of thorium oxide were synthesised.

Thorium/uranium mixed oxide nanocrystals: Synthesis, structural characterization and magnetic properties

One of the primary aims of the actinide community within nanoscience is to develop a good understanding similar to what is currently the case for stable elements. As a consequence, efficient, reliable and versatile synthesis techniques dedicated to the formation of new actinide-based nano-objects (e.g. nanocrystals) are necessary. Hence, a “library” dedicated to the preparation of various actinidebased nanoscale building blocks is currently being developed. Nanoscale building blocks with tunable sizes, shapes and compositions are of prime importance. So far, the non-aqueous synthesis method in highly coordinating organic media is the only approach which has demonstrated the capability to provide size and shape control of actinide-based nanocrystals (both for thorium and uranium, and recently extended to neptunium and plutonium). In this paper, we demonstrate that the non-aqueous approach is also well adapted to control the chemical composition of the nanocrystals obtained when mixing two different actinides. Indeed, the controlled hot co-injection of thorium acetylacetonate and uranyl acetate (together with additional capping agents) into benzyl ether can be used to synthesize thorium/uranium mixed oxide nanocrystals covering the full compositional spectrum. Additionally, we found that both size and shape are modified as a function of the thorium:uranium ratio. Finally, the magnetic properties of the different thorium/uranium mixed oxide nanocrystals were investigated. Contrary to several reports, we did not observe any ferromagnetic behavior. As a consequence, ferromagnetism cannot be described as a universal feature of nanocrystals of non-magnetic oxides as recently claimed in the literature.

Ultra-Small Plutonium Oxide Nanocrystals: An Innovative Material in Plutonium Science

Apart from its technological importance, plutonium (Pu) is also one of the most intriguing elements because of its non-conventional physical properties and fascinating chemistry. Those fundamental aspects are particularly interesting when dealing with the challenging study of plutonium-based nanomaterials. Here we show that ultra-small (3.2±0.9 nm) and highly crystalline plutonium oxide (PuO2 ) nanocrystals (NCs) can be synthesized by the thermal decomposition of plutonyl nitrate ([PuO2 (NO3 )2 ]⋅3 H2 O) in a highly coordinating organic medium. This is the first example reporting on the preparation of significant quantities (several tens of milligrams) of PuO2 NCs, in a controllable and reproducible manner. The structure and magnetic properties of PuO2 NCs have been characterized by a wide variety of techniques (powder X-ray diffraction (PXRD), X-ray absorption fine structure (XAFS), X-ray absorption near edge structure (XANES), TEM, IR, Raman, UV/Vis spectroscopies, and superconducting quantum interference device (SQUID) magnetometry). The current PuO2 NCs constitute an innovative material for the study of challenging problems as diverse as the transport behavior of plutonium in the environment or size and shape effects on the physics of transuranium elements.

Synthesis of transuranium-based nanocrystals via the thermal decomposition of actinyl nitrates

In this communication, we report on the use of easily accessible actinide precursors to synthesize actinide oxide nanocrystals. Uranyl and neptunyl nitrates have been successfully used as starting materials in the non-aqueous synthesis of AnO2 (An = U, Np) nanocrystals. This communication reports for the first time on the formation of transuranium-based nanocrystals.

High-temperature and melting behaviour of nanocrystalline refractory compounds: an experimental approach applied to thorium dioxide

The behaviour from 1500 K up to melting of nanocrystalline (nc) thorium dioxide, the refractory binary oxide with the highest melting point (3651 K), was explored here for the first time using fast laser heating, multi-wavelength pyrometry and Raman spectroscopy for the analysis of samples quenched to room temperature. Nc-ThO2 was melted at temperatures hundreds of K below the melting temperature assessed for bulk thorium dioxide. A particular behaviour has been observed in the formed liquid and its co-existence with a partially restructured solid, possibly due to the metastable nature of the liquid itself. Raman spectroscopy was used to characterize the thermal-induced structural evolution of nc-ThO2. Assessment of a semi-empirical relation between the Raman active T2g mode peak characteristics (peak width and frequency) and crystallites size provided a powerful, fast and non-destructive tool to determine local crystallites growth within the nc-ThO2 samples before and after melting. This semi-quantitative analysis, partly based on a phonon-confinement model, constitutes an advantageous, more flexible, complementary approach to electron microscopy and powder x-ray diffraction (PXRD) for the crystallite size determination. The adopted experimental approach (laser heating coupled with Raman spectroscopy) is therefore proven to be a promising methodology for the high temperature investigation of nanostructured refractory oxides.