ThO2 Nanoparticles Research Articles

Efficient Parameterization of Density Functional Tight-Binding for 5f-Elements: A Th-O Case Study.

Density functional tight binding (DFTB) models for f-element species are challenging to parametrize owing to the large number of adjustable parameters. The explicit optimization of the terms entering the semiempirical DFTB Hamiltonian related to f orbitals is crucial to generating a reliable parametrization for f-block elements, because they play import roles in bonding interactions. However, since the number of parameters grows quadratically with the number of orbitals, the computational cost for parameter optimization is much more expensive for the f-elements than for the main group elements. In this work we present a set of efficient approaches for mitigating the hurdle imposed by the large size of the parameter space. A novel group-by-orbital correction functions for two-center bond integrals was developed. With this approach the number of parameters is reduced, and it grows linearly with the number of elements, maintaining the accuracy and the number of parameters, in the case of f elements, by more than 40%. The parameter optimization step was accelerated by means of the mini-batch BFGS method. This method allows parameter optimizations with much larger training sets than other single batch methods. A stochastic optimizer was employed that helped overcome shallow local minima in the objective function. The proposed algorithm was used to parametrize the DFTB Hamiltonian for the Th-O system, which was subsequently applied to the study of ThO2 nanoparticles. The training set consisted of 6322 unique structures, which is barely feasible with conventional optimization methods. The optimized parameter set, LANL-ThO, displays good agreement with DFT-calculated properties such as energies, forces, and structures for both clusters and bulk ThO2. Benefiting from the fewer number of parameters and lower computational costs for objective function evaluations, this new approach shows its potential applications in DFTB parametrization for elements with high angular momentum, which present a challenge to conventional methods.

Preparation of mesoporous ThO2 nanoparticles: Influence of calcination on morphology and visible-light-driven photocatalytic degradation of indigo carmine and methylene blue

The present article reports the synthesis of thoria nanoparticles (ThO2 NPs) via sol-gel process and examines the effect of calcination temperature of ThO2 on the morphology and photocatalytic degradation of indigo carmine (IC) and methylene blue (MB) under visible-light. As-synthesized white crystals of ThO2 were subjected to calcination at different temperatures, viz. 700 °C (TH-700), 800 °C (TH-800), and 900 °C (TH-900). The effect of calcination temperature on the structural, morphological, thermal, surface area-porosity, and optical properties of ThO2 NPs were investigated by diverse analytical techniques. XRD patterns show the cubic-space group Fm-3m (225) with parameter a = 5.597 Å and reveals crystallite sizes increased with calcination temperature. The bandgap energy was found to be 1.85 eV, 2.33 eV, and 2.71 eV for TH-700, TH-800, and TH-900 NPs, respectively, calculated by Kubelka-Munk (KM) plot. SEM and TEM unveil that the sample TH-700 calcined at a low temperature of 700 °C yields assembled nanosheets, while at higher temperatures, i.e., 800 °C (TH-800) and 900 °C (TH-900), produces agglomerated nanomaterials. Further, TH-700 sample exhibits enhanced photocatalytic degradation within 120 min for both IC and MB dye than TH-800 and TH-900 counterparts. Among the dyes, IC shows improved photocatalytic efficiency than MB for TH-700, owing to the increased optical absorption and improved separation of photogenerated charge carriers. The reusability study of TH-700 reveals that the catalysts were stable up to four successive cycles with no drastic changes in photocatalytic efficiency. Also, systematic photodisintegration of IC was investigated by Liquid chromatography–mass spectrometry (LC–MS).

Facile Preparation of Macro-Microporous Thorium Oxide via a Colloidal Sol-Gel Route toward Safe MOX Fuel Fabrication.

The identification of new colloidal sol-gel routes for the preparation of actinide oxides, which have a homogeneous and accessible porosity that can easily be impregnated by any concentrated actinide solution, opens new perspectives for the preparation of homogeneous nuclear fuel for minor actinide transmutation. This homogeneity allows us to avoid "hot spot" formation due to the local accumulation of more fissile elements. Here, we report the preparation of macro-microporous ThO2 materials by a colloidal sol-gel route. Using a thorium salt with 6-aminocaproic acid as a complexing agent at a controlled pH, we were able to pilot the condensation of thorium hydroxo species forming colloids of tuned nanometric size and thus the sol stability. After a freeze-drying process to concentrate colloids and a thermal treatment allowing complexing agent removal and macroporosity formation by a brutal gas release during combustion, a loose packing of ThO2 nanoparticles with an ordered distribution of interparticular porosity and a fraction of nanometric crystallites, whose size depends on the initial colloidal size, were obtained. The sols, pastes, and final materials were characterized by small- and wide-angle X-ray scattering to determine the colloidal size and the final structure of the materials, which was also confirmed by transmission electron microscopy. The most promising material was finally successfully impregnated by a simulating minor actinide solution and thermally treated to prepare a mixed actinide oxide material. This safe technology, relying on the colloidal sol-gel process and the formulation of complex fluids forming tunable precursors, opens new perspectives for the reuse of nuclear waste solutions as new fuel.

Silver Ion-Induced Formation of Unprecedented Thorium Nonamer Clusters via Lacuna-Construction Strategy

Silver Ion-Induced Formation of Unprecedented Thorium Nonamer Clusters via Lacuna-Construction Strategy

Effective coordination numbers from EXAFS: general approaches for lanthanide and actinide dioxides.

Extended X-ray absorption fine structure (EXAFS) is a comprehensive and usable method for characterizing the structures of various materials, including radioactive and nuclear materials. Unceasing discussions about the interpretation of EXAFS results for actinide nanoparticles (NPs) or colloids were still present during the last decade. In this study, new experimental data for PuO2 and CeO2 NPs with different average sizes were compared with published data on AnO2 NPs that highlight the best fit and interpretation of the structural data. In terms of the structure, PuO2, CeO2, ThO2, and UO2 NPs exhibit similar behaviors. Only ThO2 NPs have a more disordered and even partly amorphous structure, which results in EXAFS characteristics. The proposed new core-shell model for NPs with calculated effective coordination number perfectly fits the results of the variations in a metal-metal shell with a decrease in NP size.

Effect of graphite and graphene oxide on thorium carbide microstructural and thermal properties

Thorium carbide to be tested as target material for the production of 225Ac with the ISOL method, was produced via carbothermal reduction of ThO2 nanoparticles by graphite and graphene oxide, respectively. The use of graphene oxide (GO) as carbon source resulted in a reduced reactivity compared to graphite, confirmed by the presence of unreacted ThO2 mainly in the core of the samples. The reacted ThO2 or ThC2–GO showed a faster reactivity in air, mainly observed as ThC2 amorphization. The specific surface area of the ThC2–GO samples was almost doubled compared to ThC2–graphite samples. The effect of these microstructural features was analysed in terms of thermal diffusivity and calculated thermal conductivity that were both reduced in ThC2–GO samples, however the difference with ThC2–graphite samples decreased at increasing temperature. The present study shows that the use of unreduced GO inhibits the solid-state reaction between ThO2 and C; on the other hand, the high reactivity of the ThC2 so produced is expected to be beneficial for the 225Ac production with the ISOL method, affording a high release efficiency. It is expected that the use of reduced GO could represent a good solution for highly efficient ThC2 targets.

Probing the local structure of nanoscale actinide oxides: a comparison between PuO2 and ThO2 nanoparticles rules out PuO2+x hypothesis.

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO2 and ThO2 nanoparticles displayed as dispersed colloids, nanopowders, or nanostructured oxide powders allow to establish a clear relationship between the size of the nanocrystals constituting these oxides and their corresponding An(iv) local structure investigated by EXAFS spectroscopy. Particularly, the first oxygen shell of the probed An(iv) evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu–O shell on the Fourier transformed EXAFS signal of the PuO2 samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO2+x species.

Size Effects in Nanocrystalline Thoria

The facile chemical precipitation method and subsequent thermal treatment were shown to be suitable for preparation of crystalline ThO2 nanoparticles(NPs) in a wide range of particle sizes (from 2.5 to 34.3 nm). The obtained NPs were investigated with X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption techniques to find out the possible size effects associated with nanocrystalline thoria.

Size-dependent toxicity of ThO2 nanoparticles to green algae Chlorella pyrenoidosa

Thorium (Th) is a natural radioactive element present in the environment and has the potential to be used as a clean nuclear fuel. Relatively little is known about the aquatic toxicity of Th, especially in nanoparticulate form, which may be the main chemical species of Th in the natural waters. In this study, impacts of ThO2 nanoparticles (NPs) with two different sizes (52 ± 5 nm, s-ThO2vs. 141 ± 6 nm, b-ThO2) on a green alga Chlorella pyrenoidosa (C. pyrenoidosa) were evaluated. Results indicated that C. pyrenoidosa was more sensitive to s-ThO2 (96-h EC30 = 64.1 μM) than b-ThO2 (96-h EC30 = 100.2 μM). Exposure to 200 μM of ThO2 NPs reduced the chlorophyll-a and chlorophyll-b contents of the algal cells. At 96 h, SEM and TEM showed that more agglomerates of s-ThO2 than those of b-ThO2 were attached onto the surface of algal cells. Reactive oxygen species (ROS) generation and membrane damage were induced after the attachment of high concentrations of ThO2 NPs. The heteroagglomeration between ThO2 NPs and algal cells and increased oxidative stress might play important roles in the toxicity of ThO2 NPs. To the best of our knowledge, this is the first report on aquatic toxicity of ThO2 NPs.

Probing Site Symmetry Around Eu3+ in Nanocrystalline ThO2 Using Time Resolved Emission Spectroscopy

ThO2:Eu3+ nanoparticles were synthesized at 300°C by combustion route using urea as a fuel and characterized by thermogravimetric/differential thermal analysis, X‐ray diffraction, transmission electron microscopy, and photoluminescence techniques. To investigate the effect of annealing temperature, as synthesized powder were heated further at 500°C, 700°C, and 900°C. It was observed that extent of asymmetry around Eu3+ at 700°C/900°C is very high as compared to as‐prepared or 500°C annealed sample. Based on the time resolved emission spectroscopic investigations, it was inferred that two different types of Eu3+ ions were present in the ThO2 nanoparticles. In ThO2 structure, Eu3+ ions occupy two sites; cubic (Oh) and noncubic (<C2v) as can be confirmed from our emission studies. Short‐lived species T1 (~1.3–3.4 ms) predominates at higher annealing temperature arises because of Eu3+ ions occupying noncubic (<C2v) sites without inversion symmetry, whereas long‐lived species T2 (~4.6–6.6 ms) can be ascribed to Eu3+ ions occupying cubic sites (Oh) with inversion symmetry.

Synthesis and characterization of thorium, uranium and cerium oxide nanoparticles

Abstract We describe the synthesis of cerium, thorium and uranium oxide nanoparticles embedded in a mesoporous matrix as template in a kind of nanocasting technique. The solid matrix is used as a template to obtain and stabilize the actinide oxide nanoparticles. We apply high resolution transmission electron microscopy (HR-TEM) to show evidence of metal oxide incorporation into the matrix pores and analyze their structure. Measured interplanar distances and calculated lattice parameters for synthesized nanosized CeO2−x and ThO2 samples differ from their bulk crystalline counterparts. We obtain with our synthesis CeO2−x particles containing both Ce4+ and larger sized Ce3+. The lattice parameter for these ceria nanoparticles is found to be larger than the bulk value due to the presence of Ce3+ with its larger ionic radius. The presence of Ce3+ was established by means of high resolution X-ray emission spectroscopy (HRXES), applied to the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as one might generally expected for these nanoclusters. However, the lattice distance decrease for our particles is remarkable, up to 5%, indicating that contact with the surrounding silica matrix may exert a bond distance shortening effect such as through significant external pressure on the particle surface.