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Journal of Nuclear Materials 492 (2017) 244e252

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Journal of Nuclear Materials

journal homepage: www.elsevier .com/locate/ jnucmat

First-principles study of water reacting with the (110) surface ofuranium mononitride

Tao Bo a, b, c, Jian-Hui Lan b, Yao-Lin Zhao a, *, Chao-Hui He a, Zhi-Fang Chai b,Wei-Qun Shi b, **

a School of Nuclear Science and Technology, Xi'an Jiaotong University, Xi'an 710049, Chinab Laboratory of Nuclear Energy Chemistry and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics,Chinese Academy of Sciences, Beijing 100049, Chinac China Spallation Neutron Source (CSNS), Institute of High Energy Physics, Chinese Academy of Sciences, Dongguan 523803, China

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 January 2017Received in revised form5 May 2017Accepted 19 May 2017Available online 22 May 2017

Keywords:First principlesUranium mononitrideSurface adsorptionWaterab initio atomistic thermodynamic

* Corresponding author.** Corresponding author.

E-mail addresses: zhaoyaolin@mail.xjtu.edu.cn (Y(W.-Q. Shi).

http://dx.doi.org/10.1016/j.jnucmat.2017.05.0260022-3115/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

The adsorption and dissociation behaviors of water on the UN (110) surface have been investigated byusing DFT þ U method in combination with ab initio atomistic thermodynamic simulations. The moststable adsorption site for H, O, and OH adsorption is the uranium bridge site. For a water monomer, theadsorption energies are �0.90, �3.23, and �4.46 eV for the most stable molecular, partially dissociative,and completely dissociative adsorption, respectively. The dissociation of water from H2O to OH and H hasa very small energy barrier, while from OH to O and H has a high energy barrier of 1.63 eV. The coveragedependence for molecular adsorption is not obvious, while for partially dissociative and completelydissociative adsorption, the coverage dependence is quite obvious. Besides, we have investigated theadsorption of water under different temperature and pressure conditions by using the “ab initio atom-istic thermodynamic” method.

© 2017 Elsevier B.V. All rights reserved.

.-L. Zhao), shiwq@ihep.ac.cn

1. Introduction

With the current global energy crisis, shortage of energy re-sources greatly hampers the development of global economy. Oneimportant way to solve this problem is to develop nuclear power.Uranium mononitride (UN) is considered to be one of the mostpromising fuel materials for the future Generation IV nuclear re-actors [1]. Uranium mononitride fuel has many advantages as

Fig. 1. Side and top views of the surface structure of UN (110) as well as possibleadsorption sites (t for top, b for bridge). The uranium and nitrogen atoms are plotted inblue and yellow, respectively. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252 245

compared to the traditional uranium dioxide fuel, such as greaterthermal conductivity [2,3], higher actinide density, higher meltingpoint (~2850 �C) [4] and easier reprocessing [5]. However, UNmaterials still have some limitations, such as relatively low disso-ciation temperature [6,7] and incompatibility with oxygen andwater [4,8]. Previous experimental studies [9e11] suggested thatinteraction between UN surface and oxygen can result in the oxidegrowth and the decease of thermal conductivity. In addition,experimental studies [4,8,12] also suggested that UN interactionwith water can result in the corrosion of nuclear fuels. Thus, thereactions of UN with oxygen and water affect a lot of processes inthe nuclear fuel cycle such as fabrication and storage of nuclearfuels, and permanent disposal in deep geologic repository for spentfuel [13].

In the past decades, the UN has been widely investigated byusing density function theory (DFT) and DFT þ Umethods [14e20].However, only several studies have been performed on the UNsurfaces. Bocharov et al. [21,22] studied the properties of N and Uvacancies on the UN(001) surface and sub-surface and the incor-poration of O atoms into these vacancies. In another work,Bocharov et al. [23] investigated the mechanism of UN oxidation byincorporation of O into N vacancies on the UN(001) and (110) sur-faces. Zhukovskii et al. [24,25] investigated the properties of oxygenadsorption and dissociation on the UN(001) surface. Li et al. [26]investigated the H, C, and O adsorption on the UN(001) and (111)surfaces by using DFTþUmethod. They also studied the adsorptionof single O2, CO2, and H2O on the UN(001) surface in another work[27]. In our recent studies [28], using the DFT þ Umethod, we havethoroughly investigated the adsorption and dissociation of H2O onthe UN(001) surface.

Based on the above introduction, the investigation of UN(110)surface with H2O by using DFT method has been rarely reported. Inorder to better understand the microscopic material properties, theinitial stage of UN oxidation, and the fuel performance underdifferent operating conditions, we plan to investigate the interac-tion mechanism between UN(110) surface and H2O by using theDFTþUmethod in this work. The research contents include: (1) theadsorption states of H, O, OH, and H2O monomer, (2) the surfacereaction pathways of H, O diffusion and H2O dissociation, (3) theadsorption states of H2O at high coverages, (4) thewater adsorptionstability under different temperature and pressure.

2. Computational methodology

Our DFT calculations were performed using the Vienna ab initioSimulation Package (VASP) [29e31]. The projector augmentedwave (PAW) approach [32,33] was used to represent the electronion interaction. The electron exchange correlation functional wastreated using the generalized gradient approximation (GGA) in theform proposed by Perdew-Burke-Ernzerhof formalism (PBE) [34].The crystal structure of UN is face-centered cubic with sodiumchloride type (space group Fm3m, No. 225). The lattice constant iscalculated to be a ¼ 4.931 Å, which agrees well with the experi-mental value of 4.886 Å [35]. The periodic p(2� 2) slab consisting ofsix atomic monolayers (MLs) was used to simulate the UN(110)surface. Brillouin zone integration was calculated using 2 � 2 � 1Monkhorst-Pack k-point meshes for the surface. A vacuum of 15 Åalong the surface normal directionwas set. The computations wereperformed with the cutoff energy of 550 eV. Spin-polarization wasinduced in all the investigated systems and the Gaussian electronsmearing method with s ¼ 0.10 eV was used. The H 1s1, N 2s2p3, O2s2p4, and U 5f36s2p6d17s2 were treated as valence electrons. Weused antiferromagnetic (AFM) ordering of the magnetic momentsin the overall system. The atomic positions were fully relaxed untilthe maximum force on each atom was smaller than 0.02 eV/Å.

DFT þ Umethod was applied in our calculations in order to dealwith strong on-site Coulomb repulsion of the 5f electrons for ura-nium element [1,36]. Lu et al. [36] concluded that the effectiveHubbard parameter Ueff ¼ 2.0 eV achieves good agreement withexperiments for the phonon density of states and phonon disper-sions. Gryaznov et al. [1] found that the Ueff ¼ 1.85 eV achievescorrectly the UN anti-ferromagnetic ordering, the magnetic mo-ments and the unit cell volume. In our previous work [28], a valuebetween the above two (Ueff ¼ 1.90 eV) was used to investigate theadsorption of water on the UN (001) surface. Thus, in this work, thevalue of Ueff ¼ 1.90 eV was used as the effective Hubbard parameterand the Coulomb U and the exchange energy J for the uraniumelement were set to be 2.40 and 0.50 eV, respectively. In order toevaluate the effects of van der Waals interaction on the adsorptionof water on the UN(110) surface, the empirical dispersion correc-tions have been computed by using the DFT-D3 (BJ) method [37].

The adsorption energy, Ead, is defined as

Ead ¼ E½adsorbate=UN� � E½UN� � E½adsorbate�; (1)

where E[adsorbate/UN] is the total energy of a UN(110) slab withadsorbed adsorbate; E[UN] is the total energy of the UN(110) slab; E[adsorbate] is the total energy of the free adsorbate. For theadsorption of H and O atoms, E[H] and E[O] represent half of thetotal energy of H2 and O2 molecules, respectively. Thus, for theadsorption of single H and O, the adsorption energy represents halfof the adsorption energy for dissociative adsorption of H2 and O2,respectively. The reaction paths were obtained by using theclimbing image nudge elastic band (CI-NEB) method [38e40]. Tostudy the interactions of water molecules with the UN(110) surfacereferring to real reaction conditions, it is significant to take intoaccount water partial pressure and temperature. The “ab initioatomic thermodynamics” approach [41e43] was adopted to studythe water adsorption properties. The detailed description of thismethod is given in the supporting information.

3. Results and discussion

3.1. Adsorption sites

Fig.1 illustrates the schematic side and top views of the UN (110)

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252246

surface as well as its possible adsorption sites. There are fouradsorption sites on the UN (110) surface: nitrogen top (t1), uraniumtop (t2), nitrogen bridge (b1), and uranium bridge (b2) sites. Thenitrogen bridge site (b1) represents the site between two surfacenitrogen atoms, while the uranium bridge site (b2) represents thesite between two surface uranium atoms.

3.2. Adsorption of H, O, OH, and H2O monomer on UN (110) surface

The interaction of water with UN surface is of particular interestfrom nuclear fuel cycle perspectives. In this section, we haveinvestigated the adsorption of H, O, OH, and H2O monomer on theUN (110) surface by considering different initial configurations.

Fig. 2 illustrates top views of the stable adsorption structures ofH, O, OH, and H2O on the UN (110) surface. The correspondingadsorption energies and the structure parameters are shown inTable S1 of the supporting information. The H, O, and OH are alltreated as neutral species. For the adsorption of H atom, the most

Fig. 2. Optimized adsorption structures of (a, b) H, (c) O, (d) OH, and (eeh) H2O on the(2 � 2) UN (110) surface. The hydrogen and oxygen atoms are plotted in green and red,respectively, while the nitrogen and uranium atoms are black and grey, respectively.(For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

stable adsorption configuration is located at the uranium bridgesite (Fig. 2b) with the H atom binding with two surface uraniumatoms. The H-U bonds length are 2.31/2.30 Å and the adsorptionenergy is �0.75 eV. The adsorption configuration at the nitrogentop (Fig. 2a) site is a metastable state. The adsorption energyis þ0.05 eV and the H-N bond length is 1.04 Å. We also calculatedthe adsorption of H atom on the uranium top site. However, theadsorption configuration at the uranium top site is not stablebecause the hydrogen atom will move to an adjacent uraniumbridge site without energy barrier. The diffusion pathway of H fromuranium top site to uranium bridge site is given in Fig. S1 of thesupporting information.

For the adsorption of O atom, the most stable adsorptionconfiguration is located at the uranium bridge site (Fig. 2c) with theO atom coordinating with two surface uranium atoms. The O-Ubonds length are 2.13/2.12 Å and the adsorption energy is�5.92 eV.The low adsorption energy represents strong interaction betweenUN (110) surface and O atom. It also represents that the dissociativeadsorption of O2 on the UN (110) surface is very strong. In ourprevious work [28], we calculated the adsorption energy of O at thehollow site of the UN (001) surface to be �4.67 eV. Bocharov et al.[23] have reported the oxygen adsorption energies of �4.80 eV atthe uranium top site of the UN (001) surface. We also calculated theadsorption of O atom on the nitrogen top site, uranium top site andnitrogen bridge site, respectively. However, these adsorption con-figurations are not stable because the oxygen atomwill move to anadjacent uranium bridge site without energy barrier. The diffusionpathways of O from these three sites to uranium bridge site aregiven in Fig. S2 of the supporting information.

For the adsorption of OH, the most stable adsorption configu-ration (Fig. 2d) is characterized by the OH adsorbed at the uraniumbridge sitewith the OH perpendicular to the surface. The O-U bondslength are 2.40/2.39 Å and the adsorption energy is �5.92 eV. Fromthe above results we conclude that the most stable adsorption sitefor H, O, and OH adsorption is the uranium bridge site.

For H2O adsorption, we considered molecular, partially disso-ciative, and completely dissociative adsorption on the UN (110)surface. Fig. 2eeh illustrates the stable adsorption structures of H2Oon the UN (110) surface. For molecular adsorption, the most stableadsorption configuration (Fig. 2e) is characterized by the oxygenatom of H2O coordinates with a surface uranium atom and ahydrogen atom of H2O binds with a surface nitrogen atom. Theobtained U-O(H2) distance is 2.62 Å. A hydrogen bond with thelength of 1.60 Å is formed between the H atom and the surface Natom. The adsorption energy for this configuration is �0.90 eV.

For partially dissociative adsorption with one OH and one H, weconsidered two stable adsorption structures which are shown inFig. 2f and g. The most stable adsorption structure is shown inFig. 2g. We can see that the OH and H are both adsorbed at uraniumbridge sites. The obtained U-O(H) and U-H distances are 2.37/2.38and 2.24/2.23 Å, respectively, and the adsorption energy for thisconfiguration is �3.23 eV. For the adsorption structure as shown inFig. 2f, the OH adsorbs at the uranium bridge site and the H adsorbsat the nitrogen top site. The obtained U-O(H) and H-N distances are2.42/2.38 and 1.04 Å, respectively. The adsorption energy for thisadsorption configuration is �2.34 eV, which is larger than that ofconfiguration 2 g (�3.23 eV).

For completely dissociative adsorption of H2O, the most stableadsorption configuration is shown in Fig. 2h. The O atom and the Hatoms are all adsorbed at uranium bridge sites with the U-O and U-H distances of 2.13/2.10, 2.28/2.25, and 2.24/2.24 Å, respectively.The adsorption energy for this configuration is �4.46 eV, which ismuch lower than those of molecular and partially dissociativeadsorption. It is concluded that completely dissociative adsorptionis the most stable one for H2O adsorption on the UN (110) surface.

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252 247

In order to understand the effect of slab size on the adsorptionproperties, we calculated the adsorption of H, O, OH, and H2O onthe periodic (3 � 3) UN (110) surface. Fig. 3 illustrates top views ofthe stable adsorption structures of H, O, OH, and H2O on the (3 � 3)UN (110) surface. The adsorption energies for H atom on the ni-trogen top site (Fig. 3a) and uranium bridge site (Fig. 3b) are �0.05and �0.76 eV, respectively, which are a little lower than those onthe (2 � 2) surface. The length of the formed H-N and H-U bondsare 1.03 and 2.28/2.27 Å, respectively, which are comparable tothose on the (2 � 2) surface. The adsorption energies for O and OHon the uranium bridge site (Fig. 3c and d) are �6.34 and �6.43 eV,respectively, which are a little lower than those on the (2 � 2)surface. The O-U bonds length of these two configurations are 2.13/2.13 and 2.37/2.40 Å, respectively, which are comparable to thoseon the (2 � 2) surface. For H2O adsorption, the adsorption energyfor molecular adsorption (Fig. 3e) is�0.97 eV, which is a little lowerthan that on the (2� 2) surface (�0.90 eV). For partially dissociativeadsorption with one OH and one H, the adsorption energiesare �2.44 and �3.37 eV for configuration 3f (Fig. 3f) and configu-ration 3 g (Fig. 3g), respectively, which are a little lower than thoseon the (2 � 2) surface (�2.34 and �3.23 eV). For completely

Fig. 3. Optimized adsorption structures of (a, b) H, (c) O, (d) OH, and (eeh) H2O on the(3 � 3) UN (110) surface. Color code as in Fig. 2. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

dissociative adsorption of H2O (Fig. 3h), the adsorption energyis �4.90 eV, which is a little lower than that on the (2 � 2) surface(�4.46 eV).

From the above results we conclude that the adsorption en-ergies of H, O, OH, and H2O on the periodic (3 � 3) UN (110) surfaceare just a little lower than those on the periodic (2 � 2) UN (110)surface. The small differences in adsorption energies result fromthe effect of adsorbate coverage. On the (3 � 3) UN (110) surface,the repulsive interactions between the adsorbates is small, whichresults in the lower adsorption energies. The calculated bondslength for H, O, OH, and H2O adsorption on the periodic (3 � 3) UN(110) surface are comparable with those on the (2 � 2) UN (110)surface. The calculated surface energies for (2 � 2) and (3 � 3) UN(110) surfaces are 112.11 and 114.06 meV$�2, respectively. We cansee that the two surface energies are very close. From the abovediscussion, we conclude that the (2� 2) slab size is large enough forthe investigation of H2O adsorption on the UN (110) surface.

3.3. H, O diffusion and H2O dissociation on UN (110) surface

On the basis of the stable H, O, and H2O adsorption configura-tions calculated in the last section, we systematically investigatedthe probable reactions pathways of H, O diffusion and H2O disso-ciation on the UN (110) surface. The minimum energy path for H, Odiffusion and H2O dissociation was modeled by the climbing imageNEB method. The adsorption energies of the initial, transition, andfinal states and the corresponding structures are illustrated inFigs. 4e6.

Fig. 4 exhibits two calculated diffusion pathways of H atom onthe UN (110) surface. Table S2 of the supporting information showsthe energies, the reaction barriers, and the structure parameters ofthe adsorption configurations. These two diffusion pathways both

Fig. 4. Potential energy profiles (eV) and structures of intermediates (E) and transitionstates (TS) for H diffusion on the UN (110) surface. Black numbers are energies of all thespecies and red numbers are energy barriers. Color code as in Fig. 2. (For interpretationof the references to colour in this figure legend, the reader is referred to the webversion of this article.)

Fig. 5. Energy profile (eV) and structures of initial state (E1), transition state (TS), andfinal state (E2) for O diffusion on the UN (110) surface. Black numbers are energies ofall the species and red number is energy barrier. Color code as in Fig. 2. (For inter-pretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252248

start at one uranium bridge site (Ea1 and Eb1) and end at anadjacent uranium bridge site (Ea2 and Eb3).

For the first diffusion pathway (Ea1/ Ea2), as shown in Fig. 4a,the H atom is initially adsorbed at a uranium bridge site. Theadsorption energy for this configuration is �0.75 eV. Based on ourcalculation, it needs to overcome an energy barrier of 1.55 eV forthe H atom to diffuse to an adjacent uranium bridge site. Thisdiffusion reaction is exothermic by 0 eV owing to the same struc-ture of the initial and final states.

The second diffusion pathway (Eb1 / Eb2 / Eb3) is investi-gatedwhich is shown in Fig. 4b. The H atom is initially adsorbed at auranium bridge site (Eb1). In the first diffusion process (Eb1 /

Eb2), it needs to overcome an energy barrier of 1.57 eV for the Hatom to diffuse to an adjacent nitrogen top site. The energy barrieris comparable to that of the first diffusion pathway (1.55 eV), for thetwo reactions both need to overcome the U-H bond energy. At thetransition state (TSb1), the breaking U-H distance is 2.33/3.27 Å andthe newly formed N-H bond length is 1.45 Å. The adsorption en-ergies for configuration Eb1 and Eb2 are �0.75 and 0.05 eV,respectively. This reaction is endothermic by 0.80 eV. In the seconddiffusion process (Eb2 / Eb3), the H atom diffuses from the ni-trogen top site to an adjacent uranium bridge site. It needs toovercome an energy barrier of 1.23 eV, which is lower than that of

Fig. 6. Potential energy profiles (eV) for the reactions of H2O dissociation on the UN (110) surColor code as in Fig. 2. (For interpretation of the references to colour in this figure legend,

the first diffusion process (1.57 eV). At the transition state (TSb2),the breaking N-H distance is 1.54 Å and the newly formed U-H bondlength is 2.19 Å. The adsorption energy for configuration Eb3is �0.75 eV. This reaction is exothermic by 0.80 eV. At last, thediffusion of H atom from Eb1 to Eb3 is exothermic by 0 eV and hasan overall energy barrier of 1.57 eV.

Fig. 5 exhibits calculated diffusion pathway of O on the UN (110)surface. The energies, the reaction barriers, and the structure pa-rameters of the adsorption configurations are shown in Table S3 ofthe supporting information. The diffusion pathway (E1/ E2) is thediffusion of O from the uranium bridge site to another uraniumbridge site. At the transition state (TS), the O coordinates with foururanium atoms in the first layer and one uranium atom in thesecond layer. The O-U distances are 2.91, 2.96, 2.89, 2.94, and 2.35 Å,respectively. This diffusion pathway has a high energy barrier of3.65 eV and the reaction heat is 0 eV. The high energy barrier forthis diffusion pathway is owing to the cleavage of the strong O-Ubonds.

Fig. 6 exhibits the reaction pathways of H2O dissociation on theUN (110) surface. Table S4 of the supporting information shows theenergies, the reaction barriers, and the structure parameters of theadsorption configurations. The process begins with the molecularadsorption of H2O on the surface (E1) with the adsorption energyof�0.90 eV. The next step is the H2O dissociation (E1/ E2), whichis exothermic by 1.44 eV and has a small energy barrier of 0.01 eV.The energy barrier (0.01 eV) is really small and maybe within theerror of the calculation. It indicates that the dissociation of H2O onthe UN (110) surface takes place easily. The bond length of N-H1 isshortened from 1.60 Å in E1 via 1.47 Å in TS1 to 1.04 Å in E2, whilethe distance of O-H1 is elongated from 1.06 Å in E1 via 1.11 Å in TS1to 3.09 Å in E2. The next reaction (E2 / E3) is the diffusion of Hatom from the nitrogen top site to the neighboring uranium bridgesite. In the transition state (TS2), the bond length of N-H1 is 1.45 Å,which is 0.41 Å longer than that of E2. This reaction is exothermicby 0.89 eV, which is comparable to that of the diffusion of H atom inthe previous calculation without OH accompanying (0.80 eV).However, the energy barrier of this reaction (0.89 eV) is lower thanthat in the previous calculation without OH accompanying(1.23 eV). The configuration (E3) is the most stable structure ofpartially dissociative adsorption of H2O on the UN (110) surface. Thethird reaction (E3 / E4) is the dissociation of OH into O and H atthe uranium bridge sites. The bond length of O-H2 is elongatedfrom 0.97 Å in E3 via 1.93 Å in TS3 to 5.03 Å in E4. This reaction isexothermic by 1.23 eV and has an energy barrier of 1.63 eV. The

face. Black numbers are energies of all the species and red numbers are energy barriers.the reader is referred to the web version of this article.)

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252 249

configuration (E4) is the most stable structure of completelydissociative adsorption of H2O.

3.4. Adsorption of H2O at higher coverages on UN (110) surface

In this section, we calculated the molecular, partial dissociative,mixed dissociative, and completely dissociative adsorption of two,three, and four water molecules in order to get a better under-standing of water adsorption at different coverages on the UN (110)surface. Fig. 7 illustrates the optimized most stable adsorptionstructures of two, three, and four water molecules on the UN (110)surface. The adsorption energies and the detailed geometrical pa-rameters are listed in Table S5 of the supporting information.

We first discuss the adsorption of two water molecules. Mo-lecular adsorption and partial, mixed, and completely dissociativeadsorption of twowater molecules are investigated. The adsorptionenergy is �2.05 eV for the most stable molecular adsorptionconfiguration as shown in Fig. 7a. The two H2O coordinate with twosurface U atoms with the U-O(H2) distances of 2.66 and 2.60 Å,respectively. A hydrogen bond with the length of 2.10 Å is formedbetween the two water molecules. Fig. 7b shows the most stablepartial dissociative adsorption configuration with two hydroxylions and two hydrogen atoms. The two OH and two H are adsorbedat four uranium bridge sites with the U-O(H) and U-H distances of2.36/2.34, 2.36/2.34, 2.24/2.24, and 2.24/2.24 Å, respectively. Theadsorption energy for this configuration is �6.26 eV. The moststable mixed dissociative adsorption configuration with one OH,one O, and three H is shown in Fig. 7c. In this case, the OH and O areadsorbed at two uranium bridge sites with the U-O and U-O(H)distances of 2.36/2.34 and 2.12/2.12 Å, respectively, and the three Hare adsorbed at two uranium bridge sites and one nitrogen top site,respectively. The adsorption energy is �7.03 eV for this configura-tion. In the completely dissociative adsorption configuration(Fig. 7d), the two O are adsorbed at two uranium bridge sites withthe U-O distances of 2.12/2.12 and 2.12/2.12, respectively, and thefour H are located at two uranium bridge sites and two nitrogen topsites, respectively. The adsorption energy for this configuration

Fig. 7. Optimized (a, e, i) molecular, (b, f, j) partial dissociative, (c, g, k) mixed dissociative,three, and (i, j, k, l) four water molecules on the UN (110) surface. Color code as in Fig. 2. (Forthe web version of this article.)

is �7.48 eV. We can see that completely dissociative adsorptionmode is the most preferable one at the coverages of 2/4 ML on theUN(110) surface.

The molecular and dissociative adsorptions of three watermolecules are also investigated here. Fig. 7e shows the most stablemolecular adsorption configuration, where all the three H2O adsorbon three surface U atoms with the U-O(H2) distances of 2.62, 2.66,and 2.62 Å, respectively. The calculated adsorption energy for thisconfiguration is �2.95 eV. In the most stable partial dissociativeadsorption configuration with three OH and three H (Fig. 7f), thethree OH are adsorbed at three uranium bridge sites with the U-O(H) distances of 2.62, 2.66, and 2.62 Å, respectively, and the threeH are located at one uranium bridge site and two nitrogen top sites,respectively. The calculated adsorption energy for this configura-tion is �7.80 eV. In the most stable mixed dissociative adsorptionconfiguration with one OH, two O, and five H (Fig. 7g), the OH andthe two O are adsorbed at three uranium bridge sites with the U-O(H) and U-O distances of 2.35/2.40, 2.13/2.13, and 2.13/2.13 Å,respectively, and the five H are adsorbed at one uranium bridge siteand four nitrogen top sites, respectively. The calculated adsorptionenergy for this configuration is �9.03 eV. In the most stablecompletely dissociative adsorption configuration with three O andsix H (Fig. 7h), the three O are adsorbed at three uranium bridgesites with the U-O distances of 2.07/2.18, 2.08/2.12, and 2.14/2.10 Å,respectively, and the six H are adsorbed at four nitrogen top sites,one uranium bridge site, and one uranium top site, respectively. AH-H bond of 1.63 Å is formed between the H adsorbed at the ura-nium top site and the H adsorbed at the uranium bridge site. Thecalculated adsorption energy is �8.14 eV for this configuration. Wecan see that mixed dissociative adsorption is the most preferablemode at the water coverage of 3/4 ML.

For four water molecules located on the surface, we also pro-vided the most stable molecular and dissociative adsorption con-figurations. Fig. 7i shows the most stable molecular adsorptionconfiguration, where all the four water molecules adsorb on surfaceU atoms with the U-O(w) distances of 2.65, 2.70, 2.60, and 2.64 Å,respectively. Two hydrogen bonds with the length of 2.04 and

and (d, h, l) completely dissociative adsorption structures of (a, b, c, d) two, (e, f, g, h)interpretation of the references to colour in this figure legend, the reader is referred to

Fig. 8. The relationship between the Gibbs free energy of adsorption (△G) and H2Opartial pressure at (a) 300 K, (b) 600 K, and (c) 900 K, respectively. nH2O represents themost stable adsorption state of n water.

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252250

1.96 Å are formed among the four water molecules. The calculatedadsorption energy for this configuration is �4.13 eV. In the moststable partial dissociative adsorption configuration with four OHand four H (Fig. 7j), all the four OH are adsorbed at uranium bridgesites with the U-O(H) distances of 2.36/2.43, 2.39/2.40, 2.36/2.43,and 2.39/2.40, respectively, and all the four H adsorb at nitrogen topsites. The calculated adsorption energy for this configurationis �8.97 eV. In the most stable mixed dissociative adsorptionconfiguration with three OH, one O, and five H (Fig. 7k), the threeOH and the O are adsorbed at four uraniumbridge sites, and the fiveH are adsorbed at four nitrogen top sites and one uranium bridgesite, respectively. The calculated adsorption energy for thisconfiguration is �9.54 eV. In the most stable completely dissocia-tive adsorption configuration with four O and eight H (Fig. 7l), thefour O are adsorbed at four uranium bridge sites with the U-Odistances of 2.06/2.06, 2.06/2.06, 2.06/2.05, and 2.05/2.06, respec-tively, and the eight H are adsorbed at four nitrogen top sites andfour uranium top sites, respectively. The calculated adsorptionenergy for this configuration is�2.46 eV.We can see that themixeddissociative adsorptionwith three OH, one O, and five H is the mostpreferable adsorption mode at the water coverage of 1 ML.

For molecular adsorption on the UN(110) surface, the averageadsorption energies of one, two, three, and four water moleculesare �0.90 (1/4 ML), �1.03 (2/4 ML), �0.98 (3/4 ML), and �1.03(1ML) eV, respectively. We can see that the coverage dependence isnot obvious for molecular adsorption. The average adsorption en-ergies for partial dissociative adsorption are �3.23 (1/4 ML), �3.13(2/4 ML),�2.60 (3/4 ML), and�2.24 (1ML) eV, respectively. We cansee that the average adsorption energy decreases slightly as thecoverage increases from 1/4 to 2/4 ML but decreases remarkablyfrom 2/4 to 1 ML. The average adsorption energies for completelydissociative adsorption are �4.46 (1/4 ML), �3.74 (2/4 ML), �2.71(3/4 ML), and �0.62 (1 ML) eV, respectively. One can see that theaverage adsorption energy decreases remarkably from 1/4 MLe1 ML. It can be conclude that the coverage dependence isquite obvious for partial dissociative and completely dissociativeadsorption, which results from the lateral electrostatic repulsiveinteractions among the adsorbed species and steric effect.Furthermore, our calculations reveal that the most preferableadsorption modes are completely dissociative adsorption at thecoverages of 1/4 and 2/4 ML, while mixed adsorption at the cov-erages of 3/4 and 1 ML on the UN(110) surface.

3.5. Water adsorption from ab initio atomistic thermodynamics

3.5.1. Water adsorption with pressure and temperatureWe now go on to analyze the water adsorption on the UN(110)

surface under different pressure and temperature conditions byusing the “ab initio atomistic thermodynamics” method. As illus-trated in Eqn (S6) of the supporting information, DG can be used totest the stability of UN(110) surface with adsorption of nH2O underdifferent pressure and temperature conditions, and more negativeDG indicates amore stable structure. On the basis of themost stableadsorption structures at different coverages, we plotted DG as afunction of pH2O (Fig. 8) and T (Fig. 9), respectively. The most stableadsorption states for water at the coverages of 1/4 (1H2O), 2/4(2H2O), 3/4 (3H2O), and 1 (4H2O) ML are considered here.

Firstly, we investigated the effect of H2O partial pressure (pH2O)on the adsorption properties by taking fixed temperatures of 300 K,600 K, and 900 K, respectively. The water partial pressure (pH2O)plays an important role in adsorption behaviors because it reflectsthe H2O equilibrium concentration in the gas phase. Fig. 8aec il-lustrates the relationship between DG and ln(pH2O/p0) for wateradsorption on the UN(110) surface at 300 K, 600 K, and 900 K,respectively. The more negative the DG, the more stable the

adsorbed system. For water adsorption at 300 K (Fig. 8a), theadsorption of three H2O (3/4 ML) is the most stable case withinln(pH2O/p0) < �2. For higher H2O partial pressure within ln(pH2O/p0) > �2, the adsorption of four H2O (1 ML) becomes the moststable case, indicating that the H2O coverage increases. For wateradsorption at 600 K (Fig. 8b), the adsorption of two H2O (2/4 ML) isthe most stable case within ln(pH2O/p0) < �9, and the adsorption ofthree H2O (3/4 ML) is the most stable case within ln(pH2O/p0) > �9,indicating that the H2O coverage increases. We can see that forwater adsorption at 300 K and 600 K, the DG is almost alwaysnegativewithin thewhole range of H2O partial pressure for the fouradsorption structures, indicating that the adsorption of H2O on theUN(110) surface is quite favorable below 600 K. For water adsorp-tion at 900 K (Fig. 8c), the adsorption of one H2O (1/4 ML) is the

Fig. 9. The relationship between the Gibbs free energy of adsorption (△G) and tem-perature (T) at pH2O ¼ 1 � 10�10 atm. nH2O represents the most stable adsorption stateof n water.

Fig. 10. Equilibrium phase diagram of stable H2O coverage on the UN(110) surface as afunction of temperature and H2O partial pressure. UHV represents ultrahigh vacuumcondition.

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252 251

most stable case within ln(pH2O/p0) < �17. As the increasing of H2Opartial pressure, the adsorption configuration with two H2O be-comes themost stable case within�17 < ln(pH2O/p0) < 3. For higherH2O partial pressure within ln(pH2O/p0) > 3, the adsorption of threeH2O is the most stable case. From the above results it can be seenthat increasing H2O partial pressure can strengthen the stability ofall the adsorption structures and increase thewater coverage on theUN(110) surface.

Secondly, we investigated the relationship between DG andtemperature by taking a fixed H2O partial pressure, and the data ofpH2O ¼ 1 � 10�10 atm (ln(pH2O/p0) ¼ -23) was used for discussion.Apart from H2O partial pressure, temperature also plays a signifi-cant role in the water adsorption properties. Fig. 9 illustrates therelationship between DG and temperature for water adsorption onthe UN(110) surface at ln(pH2O/p0) ¼ 23. The adsorption structurewith four H2O molecules (1 ML) is the most stable case withT < 150 K. When the temperature is increased to the range of150 K < T < 420 K, the adsorption configuration with three H2Omolecules (3/4 ML) becomes the most stable case, indicating thatthe H2O coverage decreases. For higher temperatures(420 K< T< 770 K), the H2O coverage continues to decrease and theadsorption structure with two H2O molecules (2/4 ML) becomesthe most stable case. For T > 770 K, the most stable case is thesurface adsorbed with one H2O molecule (1/4 ML). From the aboveresults it can be conclude that increasing temperature can decreasethe stability of all the adsorbed system and decrease the watercoverage on the UN(110) surface.

3.5.2. Phase diagram of stable water coverageIt is meaningful and important experimentally to adjust both

H2O partial pressure and temperature at one time for practicalgoals. Thus, in this section, we investigated the stable wateradsorption states on the UN(110) surface as a function of both H2Opartial pressure and temperature. As shown in Fig. 10, we plottedthe pressure-temperature phase diagram for H2O adsorption atdifferent coverages. The most stable adsorption states for water atthe coverages of 1/4 (1H2O), 2/4 (2H2O), 3/4 (3H2O), and 1 (4H2O)ML are considered here.

The phase diagram has five regions. The blue region representsthe range of H2O partial pressure and temperature, in which theadsorption structure with the water coverage of 1/4 ML is the moststable case. The yellow, red, and green regions correspond to thecoverages of 2/4, 3/4, and 1 ML, respectively. Below these regions,

the water-free surface is possible. We can see that the watercoverage decreases from 1ML to 1/4ML gradually with the increaseof temperature. For example, at H2O partial pressure of ln(pH2O/p0) ¼ -20, the saturated water adsorption (1 ML) has the temper-ature range up to 160 K and the complete desorption of water fromthe surface happens when the temperature rises to 1100 K. Thephase diagram shows clearly that water desorption temperatureincreases with increasing H2O partial pressure. By phase diagramthe water coverage on the UN(110) surface can be determinedunder a certain H2O partial pressure and temperature. Therefore,thewater adsorption phase diagram is informative and it is possibleto adjust the balance between temperature and H2O partial pres-sure for practical uses.

4. Conclusions

In this work, we studied the adsorption and dissociation be-haviors of water on the UN(110) surface by using DFTþUmethod incombination with ab initio atomistic thermodynamic simulations.From our calculations, the most stable adsorption site for H, O, andOH adsorption is the uranium bridge site. For awater monomer, theadsorption energies are �0.90, �3.23, and �4.46 eV for the moststable molecular, partially dissociative, and completely dissociativeadsorption, respectively. The completely adsorption exhibits thelowest adsorption energy, while the molecular adsorption exhibitsthe highest adsorption energy.

According to our calculations, the diffusion pathway of H fromthe uranium bridge site directly to the adjacent uranium bridge sitehas an energy barrier of 1.55 eV, while from the uranium bridge siteto the adjacent nitrogen top site has an energy barrier of 1.57 eV.The diffusion pathway of O from the uranium bridge site to theadjacent uranium bridge site has a high energy barrier of 3.65 eV,indicating the strong stability for O atom adsorption. The dissoci-ation of water from H2O to OH and H has a very small energybarrier, while from OH to O and H has a high energy barrier of1.63 eV. For water adsorption at high coverages, the coveragedependence for molecular adsorption is not obvious, while forpartially dissociative and completely dissociative adsorption, thecoverage dependence is quite obvious. Besides, by using the “abinitio atomistic thermodynamic” method, we found that theadsorption of H2O on the UN(110) surface is quite favorable below600 K, leading to the dissolution and corrosion of the UN fuelmaterials. This work can help to understand the interactionmechanism between UN(110) surface and water in the

T. Bo et al. / Journal of Nuclear Materials 492 (2017) 244e252252

environment.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant Nos. 91426302, 91326202, and21201166), the “Strategic Priority Research Program” of the ChineseAcademy of Sciences (Grant No.XDA030104) and “the FundamentalResearch Funds for the Central Universities”.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jnucmat.2017.05.026.

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