First-principles molecular dynamics simulations of uranyl ion interaction at the water/rutile TiO 2 (110) interface K. Sebbari a, b , J. Roques b, ⁎, E. Simoni b , C. Domain a , H. Perron a , H. Catalette c a EDF–R&D, Département Matériaux et Mécanique des Composants, Les Renardières, Ecuelles, F–77818 Moret-sur-Loing Cedex, France b IPN Orsay UMR 8608, Univ. Paris Sud, Bâtiment 100, F–91406 Orsay Cedex, France c EDF–CNPE PENLY, BP 854, F–76370 Neuville les Dieppe, France abstract article info Article history: Received 7 November 2011 Accepted 30 January 2012 Available online 8 February 2012 Keywords: Water Adsorption Rutile TiO 2 Uranyl ion DFT Born–Oppenheimer molecular dynamics The effects of temperature and solvation on uranyl ion adsorption at the water/rutile TiO 2 (110) interface are investigated by Density Functional Theory (DFT) in both static and Born–Oppenheimer molecular dynamics approaches. According to experimental observations, uranyl ion can form two surface complexes in a pH range from 1.5 to 4.5. Based on these observations, the structures of the complexes at 293 K are first calculat- ed in agreement with vacuum static calculations. Then, an increase in temperature (293 to 425 K) induces the reinforcement of uranyl ion adsorption due to the release of water molecules from the solvation shell of uranyl ion. Finally, temperature can modify the nature of the surface species. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Deep geological repository represents a solution for the high level nuclear waste management. Based on the multiple barriers concept, the long-term safety assessment of this storage depends mainly on the ability of the different geological barriers and host matrices to retain radionuclides potentially released [1]. In the case of a water in- filtration for instance, one of the major factors involved in the radio- element retention is their adsorption at the water/mineral interface [2]. Therefore, understanding the interactions between radioelement and minerals, at a molecular level, corresponds to an important point in the evaluation of repository capacities to delay radioelement migration through the geosphere. However, since interactions at water/mineral interfaces are rather complex [3], investigations must be primarily done on a model system. Under environmental conditions, uranium occurs mainly in hexa- valent form UO 2 2+ (the principal species in solution under pH b 4.5 [4]) which can be considered as an oxo-cation model for radionuclide representation [5]. The structure of UO 2 2+ in solution has already been characterised in a number of experimental studies such as X-ray [6–8], Raman [9], NMR [10] and Extended X-Ray Absorption Fine Structure (EXAFS) [11,12] studies, and also by DFT static [13] and DFT-based MD calculations [14,15]. The DFT-based MD studies have described in particular the dynamic features of the UO 2 2+ solvation shells. Similarly, UO 2 2+ adsorption on mineral surfaces has also been the subject of numerous experimental studies, e.g. X-Ray Photoelec- tron Spectroscopy (XPS) and EXAFS studies, that provide local struc- tural information [1,2,4,11,16–22], but also by DFT static calculations [5,23–25]. In particular, due to the stability, the low solubility of titania in a wide pH range and the numerous data on this substrate [26–28], the UO 2 2+ interaction at the water/rutile TiO 2 (110) interface has been widely investigated with both spectroscopic studies [4,11,16,17] and DFT static calculations [5]. According to the experimental conditions (uranium concentration [U] = 10 -4 M, pH range from 1.5 to 4.5), Den Auwer et al. have shown by EXAFS that UO 2 2+ can adsorb on the rutile TiO 2 (110) face by an inner sphere mechanism, with no uranium aggregation phe- nomena, which leads to the formation of a bidentate surface complex [11]. Then, Vandenborre et al. have highlighted two surface com- plexes by combining XPS, Time-Resolved Laser Fluorescence Spec- troscopy (TRLFS), Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Surface Second-Harmonic Generation (SSHG) techniques [4,16,17]. Their studies have revealed that complexes form on two reactive sites with their relative ratio depending on the pH. Using static DFT calculations (vacuum, 0 K) [5], Perron et al. have completed these experimental studies. According to an optimised five-layer slab thickness (with its most internal layer frozen in bulk positions), they have calculated two relative stable complexes on the defect-free rutile TiO 2 (110) face with structural parameters in good agreement with experimental data. This static study has thus shown the capabil- ities of DFT approach to describe radioelement interaction in terms of Surface Science 606 (2012) 1135–1141 ⁎ Corresponding author at: Université de Paris sud 11, Institut de Physique Nucléaire, Bâtiment 100, F-91406 Orsay Cedex, France. Tel.: +33 169156869; fax: +33 169157150. E-mail address: roques@ipno.in2p3.fr (J. Roques). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.01.023 Contents lists available at SciVerse ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc