Optimisation of accurate rutile TiO2TiO_2 (110), (100), (101) and (001) surface models from periodic DFT calculations

In this paper, geometric bulk parameters, bulk moduli, energy gaps and relative stabilities of the TiO2 anatase and rutile phases were determined from periodic DFT calculations. Then, for the rutile phase, structures, relaxations and surface energies of the (110), (100), (101) and (001) faces were computed. The calculated surface energies are consistent with the natural rutile powder composition, even if a dependence on the number of layers of the slab used to model the surface was identified. Internal constraints, consisting in freezing some internal layers of the slab to atomic bulk positions, were thus added to mimic the bulk hardness in order to stabilise the computed surface energies for thinner systems. In parallel, the influence of pseudopotentials was studied and it appears that four valence electrons for titanium atoms are sufficient. The aim of this study was to optimise accurate rutile TiO2 surface models that will be used in further calculations to investigate water and uranyl ion sorption mechanisms

Combined investigation of water sorption on TiO2TiO_2 rutile (1 1 0) single crystal face: XPS vs. periodic DFT

XPS and periodic DFT calculations have been used to investigate water sorption on the TiO2 rutile (1 1 0) face. Two sets of XPS spectra were collected on the TiO2 (1 1 0) single crystal clean and previously exposed to water: the first set with photoelectrons collected in a direction parallel to the normal to the surface; and the second set with the sample tilted by 70°, respectively. This tilting procedure promotes the signals from surface species and reveals that the first hydration layer is strongly coordinated to the surface and also that, despite the fact that the spectra were recorded under ultra-high vacuum, water molecules subsist in upper hydration layers. In addition, periodic DFT calculations were performed to investigate the water adsorption process to determine if molecular and/or dissociative adsorption takes place. The first step of the theoretical part was the optimisation of a dry surface model and then the investigation of water adsorption. The calculated molecular water adsorption energies are consistent with previously published experimental data and it appears that even though it is slightly less stable, the dissociative water sorption can also take place. This assumption was considered, in a second step, on a larger surface model where molecular and dissociated water molecules were adsorbed together with different ratio. It was found that, due to hydrogen bonding stabilisation, molecular and dissociated water molecules can coexist on the surface if the ratio of dissociated water molecules is less than ≈33%. These results are consistent with previous experimental works giving a 10–25% range

Molecular approach of the uranyl/mineral interfacial phenomena

Afin de comprendre, à l'échelle moléculaire, les mécanismes impliqués dans les interactions entre radionucléides et minéral, des études thermodynamiques et structurales sont présentées. Les argiles constituant une famille de substrats complexes, il est nécessaire de réaliser ce type d'étude sur des phases unitaires représentatives, afin d'extrapoler ensuite ces résultats aux solides naturels. Sont examinés, dans cette revue, les processus de sorption de l'uranium hexavalent sur la montmorillonite, TiO2 (poudre et monocristal) et également sur l'alumine et la silice. Les constantes de sorption correspondantes sont déterminées en utilisant les résultats structuraux obtenus à l'aide des techniques spectroscopiques suivantes : spectrofluorimétrie laser, spectroscopie de photoélectrons X, génération de seconde harmonique et spectroscopie d'absorption X. De plus, pour TiO2, ces résultats sont comparés à des calculs périodiques utilisant la théorie de la fonctionnelle de la densité, qui permettent d'accéder à la distribution des atomes à l'interface ainsi qu'à la stabilité relative des complexes de surface

Sorption of uranium (VI) species on zircon: structural investigation of the solid/solution interface

RADIOC

X-ray photoreduction of U(VI)-bearing compounds

During XPS analysis, the soft X-ray-induced reduction of metals such as Cr(VI) and Ce(IV) in oxides has been reported in the literature and some mechanisms have been proposed to explain this phenomenon. The reduction of U(VI) by the beam during X-ray Photoelectron Spectroscopy has been already reported in the literature but only for U(VI) sorbed or precipitated onto solids with reducing properties (as micas or pyrites) for whose Fe(II) can also induce the reduction of U(VI), or onto TiO2 whose the photocatalytic properties are well known. The objective of this paper is to investigate the effects of X-ray beam on U(VI) bulk compounds (UO3, UO2(OH)2, (UO2)2SiO4, UO2(CH3COO)2 and UO2C2O4). Successive U4f, U5f, C1s XPS spectra were recorded and compared as a function of the irradiation time. The XPS photoreduction of U(VI) into U(IV) is only observed for uranyl compounds containing organic matter (uranyl acetate and uranyl oxalate). Considering the evolution of the C1s signal during the X-ray irradiation, a significant decrease of the C O component simultaneously to the U(VI) reduction is observed, which suggests a desorption of CO or other volatile organic products from the solid surface. All these results on U(VI) bulk compounds indicate the important role of organic carbon species in the photoreduction process and to explain these observations, a photoreduction mechanism has been suggested

Theoretical first step towards an understanding of the uranyl ion sorption on the rutile TiO2TiO_2(110) face: A DFT periodic and cluster study

First results of a periodic and cluster Density Functional Theory (DFT) study of the uranyl ion (UO22+) sorption onto the rutile TiO2(110) face, based on plane wave and localised basis sets, are presented. A five layers slab with its most internal layer frozen to bulk positions was found to be a good surface model. In a first step and as reference data for the sorption process, the [UO2(H2O)n]2+ systems, with n=4 to 6 were studied. Relative solvation energies confirmed that the uranyl ion adopt a pentacoordinated structure in aqueous solution. From localised approach, an overall 0.91 electron transfer from the first hydration shell to the uranyl ion was calculated. Then, a periodic study of the uranyl sorption on a simplified hydroxylated TiO2(110) surface model was investigated. The resulting optimised structural parameters, for the three possible adsorption sites, show that the sorbed uranyl ion first coordination shell (saturated by three water molecules) plays an important role to model the adsorption process. Both methodologies (plane waves and localised atomic orbitals) were also used with a cluster model and gave similar results in agreement with experimental data. This first step in the understanding of the uranyl ion sorption onto the simplified hydroxylated TiO2(110) surface shows that hydrogen bonds should be included in the model in order to perform a more accurate description of the uranyl ion sorption process. A study with this surface model is currently performed in order to calculate the relative stabilities between the different uranyl adsorption sites and to compare with the experimental data

Periodic Density Functional Theory Investigation of the Uranyl Ion Sorption on the TiO2 Rutile (110) Face

Periodic density functional theory calculations have been performed in order to study the uranyl ion sorption on the TiO2 rutile (110) face. From experimental measurements, two uranyl surface complexes have been observed and the two corresponding sorption sites have been identified. However, from a crystallographic point of view, three different sorption sites can be considered on this face. The corresponding three surface bidentate complexes were modeled and optimized, and their relative energies were calculated. Only 5 kJ/mol separates the two most stable structures, which correspond to the experimental ones. The third surface complex is nearly 10 kJ/mol less stable, in agreement with the fact that it was not observed experimentally

Structural identification of europium(III) adsorption complexes on montmorillonite

RADIOCHA study of trivalent europium retention onto Na-montmorillonite is presented, combining both macroscopic and microscopic points of view. In order to investigate the metal sorption mechanisms at a molecular level and therefore experimentally identify both clay reactive sites and sorption equilibria, laser-induced fluorescence spectroscopy (LIF) and X-ray photoelectron spectroscopy (XPS) on europium ion loaded montmorillonite have been performed. Moreover, since this clay is an alumino-silicated mineral, we have interpreted our experimental results in terms of interactions between a metal ion and a cation exchange site, and distinct aluminol and silanol edge sites. Therefore, identical structural investigations have been carried out on both Eu/alumina and Eu/silica systems. These comparisons have allowed us to determine the nature of the europium surface complexes and thus led to an experimental definition of the sorption equilibria involved in the retention process. The obtained lifetime values and the Eu 3d XPS spectra of europium sorbed on the three solids have shown that this metal is sorbed, on the montmorillonite clay, on exchange sites as an outer-sphere complex and onto both aluminol and silanol edge sites as inner-sphere surface complexes, depending on the pH value and the ionic strength of the suspension