The fluorite-structured actinide dioxides (AnO2, particularly UO2) are the most commonly used fuels in nuclear reactors. Understanding the surface chemistry of the AnO2 series therefore is of both fundamental and technological importance. Many fundamental questions relating to AnO2 surface chemistry need answering for the safe processing, recycling and reuse, long-term storage, and environmental effects of nuclear fuels. Examples of issues relevant to the surface properties of these materials include the atomistic description of AnO2 surface structures, the stability of different surface planes, the effect of defects on the properties of AnO2 surfaces, and the mechanisms of interaction between environmental molecules and AnO2 surfaces. However, our understanding of AnO2 surface chemistry has been so far limited by the difficulties associated with the handling of radioactive actinides in experiments. The computational approaches for the atomic simulation of surfaces have reached a state of maturity such that modeling studies can now supplement experimental efforts.
Limits of actinide dioxide knowledge
Our knowledge of AnO2 surface chemistry is limited primarily due to two factors. First, specialized laboratory equipment is required to handle actinides experimentally because these elements are radioactive. Early actinides such as thorium and uranium are relatively abundant in nature and their most stable isotopes have long radioactive decay times, making them appropriate for laboratory work. The highly radioactive transuranic elements meanwhile have shorter lifetimes and are therefore not found in nature; they are produced artificially and are only available in small quantities. Accordingly, most experimental studies of AnO2 surfaces have focused on UO2, leaving the remainder of the AnO2 series comparatively unexplored. Experimental studies of single-crystallized materials of transuranic oxides are particularly rare in the literature.
The second reason for our limited understanding of AnO2 surface chemistry is that we lack characterization techniques with sufficient precision at the atomic scale. Most of the methods used to probe surface properties, such as X-ray photoelectron spectroscopy (XPS) and Auger Electron Spectroscopy (AES), have penetration depths varying from a few nanometers to micrometers, and the signals from these techniques typically represent an average over these depths. Directly observing the atomic surface structure and chemical properties of AnO2 surfaces using these experimental techniques is challenging without help from atomistic simulations.
Driven by the experimental difficulties studying AnO2 surfaces and the improvement in computational power and methodology, the last few years have seen extensive efforts to simulate AnO2 surfaces using ab-initio electronic structure calculations. However, treatment of actinides using density functional theory (DFT) remains non-trivial due to a combination of factors not present in the lighter elements of the periodic table, e.g.: relativistic effects, strongly-correlated 5f electrons, and noncollinear magnetism. Different approaches have been proposed to circumvent this problem of standard DFT functional. One of the most popular methods is DFT+U, which involves the introduction of an empirical Hubbard U correction term to the Hamiltonian. This method has been widely used because it does not increase the computational cost and it can greatly improve the description of the 5f electrons. Here, we present two examples demonstrating the importance of atomic simulations in understanding AnO2 surface chemistry using DFT studies. The first study focuses on controlling the surface energy of different AnO2 planes to tune the morphology of AnO2 nanoparticles. The second shows the critical effect of surface electronic structure on the reactivity of AnO2 surfaces using water splitting as an example.
Ligand-induced shape transformation of ThO2 nanocrystals
Nanocrystals with size- and shape-dependent properties are of great scientific interest. Remarkable progress has been made in the controlled synthesis of nanocrystals of stable elements in the past two decades, however, our knowledge of actinide nanocrystals has been considerably limited due the difficulties described above. Recently, a non-aqueous surfactant-assisted synthesis has been used for the preparation of actinide oxide nanocrystals (ThO2, UO2, and NpO2), yielding structures with different morphologies such as branched nanocrystals, nanodots, and nanorods. The nanocrystals were synthesized in a mixture of organic molecules including oleic acid, acetylacetone, oleylamine, and trioctylphosphine oxide (Fig. 2). However, both the underlying mechanism that controls the morphologies and the role of different ligands in nanocrystal growth are unclear.
In order to understand the role of ligands in ThO2 growth we performed a DFT study on ThO2 surfaces and their interactions with different surfactant ligands. The low Miller index (111), (110), and (100) lattice planes were considered because their surface energies span a wide range, thus they can represent most surfaces of ThO2 (see the explainer of Miller indices on the opposite page). The thermodynamic stability of ThO2 nanocrystals in different shapes was then evaluated employing a modified Wulff construction as a thermodynamic model. Ligand molecules with long tails were represented using smaller ligands with the same functional group to maintain the same chemistry with reduced computational costs; specifically, we used acetic acid (C2H4O2) to represent oleic acid (C18H34O2). It was found that the (111) surface of ThO2 has the lowest surface energy among the studied models, therefore pristine ThO2 nanocrystals will tend to grow in an octahedral geometry terminated by (111) planes (Fig. 3).
The adsorption of ligands can significantly affect the surface energy for different facets, in turn altering the morphologies of ThO2 nanocrystals due to selective adsorption on different surfaces. In particular, molecules with carboxylate group (–COOH) display stronger affinity for the (110) surface (Fig. 4). The selective adsorption of acetic acid on the (110) surface is attributed to the differences in the surface structures—Th atoms on the (110) surface have a lower coordination number than on the (111) surface, allowing the ligands to bind more strongly. Moreover, the (110) surface has a flat exterior with Th and O atoms on the same plane, while the topmost layer of (111) and (100) surfaces show the O atoms creating stronger repulsion between the ligand/carboxylate O atoms and the surfaces.
The adsorption of ligands can stabilize the growth of different ThO2 surfaces. The more stable surface will grow as the nanocrystal forms, changing the shape, for instance, from octahedral to nanorod in the case of increasing coverage of oleic acid (Fig. 3). Interestingly, other ligands such as acetylacetone, oleylamine, and trioc-tylphosphine oxide do not modify the equilibrium shape of ThO2 nanocrystals. These calculations highlight the critical role of surface-ligand interactions in determining the nanocrystal morphology and may pave the way towards a comprehensive under-standing of the formation and growth of other AnO2 nanocrystals with well-defined sizes and shapes for future applications.
Water splitting on AnO2 surfaces with oxygen vacancies
The interaction of water with AnO2 compounds has attracted a great deal of interest due to its importance in corrosion studies of nuclear fuels. It has been shown that the adsorption of H2O molecules on the perfect UO2 (111) surface is reversible at 300 K, indicating that H2O is weakly adsorbed. In contrast, when oxygen vacancies are present, H2O strongly interacts with the (111) surface and can split into H2. Nonetheless, our understanding of the electronic structure and catalytic properties across the AnO2 surfaces is still lacking, especially with respect to surfaces with oxygen vacancies, and the effects of these vacancies on actinide oxidation states and surface chemical reactions.
Motivated by the aforementioned questions, computational modeling was used to study the electronic structure and water splitting chemistry of ThO2, UO2, and PuO2 (111) surfaces with oxygen vacancies. Because of the formally O2– state of oxygen in these materials, each oxygen vacancy leaves two excess electrons on the surface (we use the term “formal” in chemistry to describe integral electron counting in a molecule). We found that these excess electrons distribute differently depending on the actinide involved (Fig. 5). On the ThO2 surface, the electrons remain at the vacancy site and form a lone pair. At the other extreme, excess electrons localize on two Pu centers on the PuO2 surface, and consequently reduce their oxidation state from Pu4+ to Pu3+ (i.e., each Pu center has formally gained an electron). In contrast, on the UO2 surface, one of the excess electrons remains at the vacancy site and the other is located in a 5f orbital of a neighboring uranium ion.
The differences in the distribution of excess electrons can be explained by the contraction and energy decrease of the 5f orbitals from Th to Pu: the 5f orbitals of Pu are lower in energy than the vacancy states and therefore favor reduction of the metal centers, whereas in Th, the 5f orbitals are much higher in energy, hindering the reduction. The change in electronic structure from ThO2 to PuO2 surfaces with oxygen vacancies leads to different chemical properties and reactivity of the AnO2surfaces. We have demonstrated this in our studies of the splitting of H2O catalyzed by ThO2, UO2, and PuO2 (111) surfaces. On stoichiometric surfaces, the formation of H2 from catalytic splitting of H2O is endothermic (i.e., energy-absorbing) for all three AnO2 (111) surfaces evaluated. In contrast, the H2O splitting reaction and H2 production is spontaneous in the presence of oxygen vacancies. The ThO2 surface shows the most exothermic (energy-releasing) reaction process with small energy barriers. This is due to a lone electron pair at the vacancy site on ThO2 (Fig. 5a) that can readily participate in chemical reactions with H2O. Meanwhile, on the PuO2surface, there is a larger energy barrier for the H2 production, although the H2O adsorption and H2 production is energetically preferred.
Understanding actinide dioxide surface chemistry is relevant to many stages in the nuclear industry and significant work has been performed in the research community to explore the properties and reactivity of AnO2 surfaces. In combination with experimental results, atomic simulations can provide insights into electronic structure and chemical reactions on the AnO2 surfaces. Our recent work has demonstrated the significant role of ligand-surface interactions in the determination of ThO2 nanocrystal morphology, in addition to vacancy formation and its impacts on catalyzing the production of H2 on AnO2 surfaces. Oxygen vacancies are expected to be present on the surfaces of spent nuclear fuels due to radiation damage. If spent fuels are exposed to water in a geological storage facility it will therefore lead to the production of H2. The accumulation of H2 can cause the pressurization of the fuel containers, which is a serious safety issue to consider for long-term storage of spent nuclear fuels.
The authors gratefully acknowledge funding from the Laboratory Directed Research and Development program of Los Alamos National Laboratory (LANL) under project 20160604ECR 20180007DR, and US DOE office of Basic Energy Science under the Heavy Element Chemistry program at LANL. The calculations were performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. G.W. also acknowledges a Director’s Postdoc Fellowship from LANL.
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