Influences of Surface Substitutional Ti Atom on Hydrogen Adsorption

Oct 27, 2014 - Influence of impurity elements on the corrosion of α-uranium surface: a density functional theory study. Xin Qu , Rusong Li , Bin He ,...
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Influences of Surface Substitutional Ti Atom on Hydrogen Adsorption, Dissociation, and Diffusion Behaviors on the α‑U(001) Surface Peng Shi,† Yu Yang,‡ Bingyun Ao,§ Ping Zhang,‡ and Xiaolin Wang*,† †

China Academy of Engineering Physics, Mianyang 621900, China LCP, Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, China § Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box 718-35, Mianyang 621907, China ‡

S Supporting Information *

ABSTRACT: The hydrogen adsorption, dissociation, and diffusion behaviors on both clean and Ti-doped α-U(001) surfaces are systematically studied with density functional theory method. Through detailed potential energy surface calculations, we find that the dissociation at the bridge sites is energetically more favorable, where the H2 molecule dissociates without any energy barrier and the dissociated hydrogen atoms move into two neighboring 3-fold sites. Once a substitutional Ti atom exists on the α-U(001) surface, the hydrogen molecule similarly dissociates without any energy barriers. However, the diffusion of the dissociated hydrogen atoms is dramatically changed after introduction of a surface substitutional Ti atom. The into-bulk penetration of a hydrogen atom through a defect-free surface is endothermic and needs to overcome an energy barrier of 0.8−0.9 eV. In contrast, the penetration to the subsurface sites near the doped Ti atom is exothermic, and the activation barrier decreases by 0.3−0.4 eV. Our results indicate that surface doped titanium atoms in the outermost layer may behave like hydrogen trapping sites for α-U.

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INTRODUCTION Surface hydrogenation, oxidation, and other corrosion-related reactions of uranium have attracted continuous concern for many years, because these processes are not only of great scientific interest, but also of significant practical importance for nuclear industries.1,2 Many experimental and theoretical studies have been conducted to reveal the detailed mechanisms of surface corrosion in the presence of hydrogen gases, especially the thermochemistry, diffusion, and nucleation sites.1−17 Among the experimental studies, the nucleation sites where uranium hydrides begin to form are the most concerning, since these sites exhibit dramatically greater propensity for further hydrogenation.3,12 It has been revealed that the hydride nuclei are connected with the inclusions,13,18 grain boundaries,8 and other kinds of physical discontinuity9,19 in the metal substrate. However, uranium metal is always ready for reaction with the atmosphere and is easy to cover with an oxide overlayer, which limits our understanding of the interaction of H2 with clean α-U surface from the experimental viewpoint. During the past several years, many theoretical and computational studies have been carried out to understand the interaction of hydrogen and oxygen with metal surface. These works have considered not only ordinary and transition metals, including Be,20,21 Mg,22 Pb,23 and so on, but also rare earth and actinide metals.12−17,24−29 The transformation of uranium atom to UH3 was studied by Balasubramanian et al.12 with an atom-centric approach, which shed light on the interaction between U and H © 2014 American Chemical Society

atoms. However, the important reaction steps of adsorption and dissociation were neglected. A very detailed work focusing on H2 dissociation over the α-U(001) surface was performed by Nie et al.;14 however, as we will discuss later in this paper, the most energetically favorable dissociation path where the two hydrogen atoms dissociated into two neighboring 3-fold sites was missing from their study. Very recently, Taylor et al.15,16 studied the absorption, dissociation, diffusion, trapping, saturation, and phase transformation in detail, and also the influence of the interstitial impurities, which strengthened our current understanding of the U−H interaction. Besides, the H2 dissociation on the γ-U(100) surface was also studied by Yang et al.,17 which revealed that the interaction between H 1s and U 6d electrons plays an important role during the dissociation. Due to the fact that the strength can be enhanced by a quenching method (a precipitant hardening and a later heat treatment process),30,31 the U−Ti alloy has received considerable interest from scientists for technological advances. Previous studies indicate that the hydrogenation behavior of the U−Ti alloy is different from that of pure uranium.9,19 In this paper, to understand the effect of doped titanium on the U−H interaction, we apply density functional theory (DFT) to investigate not only the adsorption and dissociation process on Received: May 26, 2014 Revised: September 22, 2014 Published: October 27, 2014 26634

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clean and Ti-doped α-U(001) surface, but also the diffusion of chemisorbed hydrogen atom in both systems. The results clearly present different hydrogen binding energies and diffusion activation barriers before and after titanium doping. In such a manner, the influence of substantial titanium on H2/Ti-doped αU(001) surface system is discussed in detail.

interlayer distance d12 contracts by an amount of ∼3.7%, while the second outmost interlayer distance d23 expands by an amount ∼1.1%, which agrees very well with previously reported values of −3.5% for d12 and +1.2% for d23.38 After studying the geometry and electronic properties of the αU(001) surface, we build the models to calculate the twodimensional (2D) potential energy surface (PES) cuts for H2 on the relaxed surface. From Figure 1a, one can see that there are five high-symmetry sites on the clean α-U surface, i.e., the top, shortbridge (s-bri), long-bridge (l-bri), hollow1 (e-fcc), and hollow2 (e-hcp) sites. In this study, we construct initial structures by orienting the H2 molecule at the five sites along the x (i.e., [100]), y (i.e., [010]), d (i.e., [(3/2)(1/2)0]), and z (i.e., [001]) directions, separately. Herein, we present the 20 high symmetry channels as x,y,d,z@top, x,y,d,z@s-bri, x,y,d,z@l-bri, x,y,d,z@efcc, and x,y,d,z@e-hcp, respectively. The adsorption energy Ead is calculated according to the following expression:

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COMPUTATION METHOD All the calculations are self-consistently performed within DFT by using the Vienna ab initio simulation package (VASP).32 The generalized gradient approximation (GGA) of the Perdew− Burke−Ernzerhof (PBE)33,34 form is employed for describing the exchange-correlation interactions. A Fermi broadening35 of 0.2 eV is chosen to smear the occupation of the bands around EF by a finite-T Fermi function and extrapolating to T = 0 K. The cutoff energy for the plane-wave expansion is set to be 550 eV. The p(2 × 1) α-U(001) surface is modeled by periodic slabs consisting of five uranium layers separated by a vacuum region of 15 Å. A 5 × 5 × 1 k-point grid within the Monkhorst−Pack36 scheme is employed for the surface system, which has been found to be sufficient for energy convergency. The bottom layer is fixed, while the other four U layers are free to move during the geometry optimizations of the surface. H2 molecule is introduced on one side of the slab, namely, on the top surface. Single-sided adsorption is used in all of the adsorption structure calculations. The calculation of the potential energy surface is interpolated to 210 points with different bond length (dH−H) and height (hH2) of the mass center of H2. The calculated lattice constants of α-U (a = 2.794 Å, b = 5.845 Å, c = 4.897 Å and internal parameter y = 0.097) are in good agreement with the experimental values37 and some other theoretical results.14,15,38 The climbing-image nudged elastic band (CINEB) method39−41 is applied to determine the minimum energy paths for the hydrogen diffusions and the corresponding activation energy barriers.

Ead = −E H2 /U(001) + E U(001) + E H 2

(1)

Here, EH2/U(001), EU(001), and EH2 are the total energies of the optimized slab containing hydrogen, the corresponding clean U(001) slab, and a free H2 molecule, respectively. The calculated 2D PES cuts along the d@top, y@s-bri, d@l-bri, z@l-bri are shown in Figure 2a−d, respectively. From our PES results, it is found the energy profiles are different when the dissociation of H2 molecule starts from different initial states. As shown in Figure 1a, the molecular adsorption state of H2 only occurs when it is parallel to the metal surface and adsorbs along the x,y,d@top channels. Furthermore, the obtained PES results indicate that the most energetically favorable dissociation channel for H2 is along the y@s-bri and d@l-bri channels on the clean α-U(001) surface, which is found to be with no energy barriers. These two channels account for the two hydrogen atoms orienting toward the 3-fold sites on either side of the short- and long-bridge sites, respectively. These two channels seem not to be discussed in the previous theoretical study,14,15 and hence, their conclusion that the dissociation of hydrogen molecules needs to overcome a small energy barrier is not strictly correct. Considering that hydrogen atoms always tend to occupy high-coordinated sites on metal surfaces15,24,42 and also that Dholabhai et al.27 reported a similar dissociation path for H2 dissociation on Am (actinide metal) with very low energy barrier, our result that the two hydrogen atoms dissociate into two 3-fold sites of the α-U(001) surface along the minimum energy path with no energy barrier is physically reasonable. Along the adsorption channels of d@top, x,y@l-bri, the H2 molecule dissociates after overcoming sizable energy barriers. Along all the other adsorption channels including z@top, x,z@s-bri, x,y,d,z@e-fcc, and x,y,d,z@e-hcp, the PES cut results have similar energy distributions with the z@ l-bri channel and thus are not listed. The projected density of states (PDOS) evolutions of the hydrogen atom and the top-most U layer are then analyzed to study the electron interaction along both the d@top and y@s-bri channels. Four points along each minimum energy path are chosen, which are (hH2 = 3.0 Å, dH−H = 0.75 Å), (hH2 = 1.55 Å, dH−H = 1.25 Å), (hH2 = 1.4 Å, dH−H = 1.7 Å), and (hH2 = 1.35 Å, dH−H = 2.22 Å) along the minimum energy path in the d@top channel, and also four other points (hH2 = 3.5 Å, dH−H = 0.75 Å), (hH2 = 2.0 Å, dH−H = 1.2 Å), (hH2 = 1.75 Å, dH−H = 2.2 Å), and (hH2 = 1.4 Å, dH−H = 3.4 Å) along the dissociation path in the channel of y@s-bri, respectively. The obtained PDOS at these points is

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RESULTS AND DISCUSSION Adsorption and Dissociation of Hydrogen Molecule on the α-U(001) Surface. The adsorption of H2 on the α-U(001) surface is studied by using the p(2 × 1) surface cell, as shown in Figure 1. After full surface relaxations, we find that the outmost

Figure 1. (a) p(2 × 1) surface cell of clean α-U(001) and five on-surface adsorption sites: (A) top; (B) short bridge (s-bri); (C) long bridge (lbri); (D) e-fcc; (E) e-hcp. (b) The sketch map showing that the H2 is initially away from the surface with a height of hH2. The blue, gray, and red balls represent for topmost U atoms, subsurface U atoms, and hydrogen atoms, respectively. 26635

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Figure 2. 2D potential energy surface cuts for the adsorption of hydrogen molecular along the (a) d@top, (b) y@s-bri, (c) d@l-bri, (d) z@l-bri on the clean α-U(001) surface. The red circles in parts a and b show the points chosen to calculate the PDOS along the least energy dissociation path.

Figure 3. Projected density of states for H2/α-U(001) system along the minimum energy dissociation paths in the (i) d@top channel with the height of H2 mass center at (a) 3.5, (b) 2.0, (c) 1.75, and (d) 1.4 Å, and also in the (ii) y@s-bri channel with the height of H2 mass center at (e) 1.55 Å, (f) 1.4 Å, (g) 1.35 Å, and (h) 1.35 Å. The Fermi energies are all set to be zero as references.

γ-U (100) surface17 and the electron donor−acceptor model proposed by Balasubramanian et al.12 However, the electron interaction between the adsorbate and the substrate in the d@top channel is different, and where the electron hybridization between hydrogen 1s and uranium 5f could not be neglected. This is because the H−U distance is much shorter at the top sites, which enhances the interaction between H s and U f electronic

listed in Figure 3a−h, respectively. From Figure 3e−h, we find that when the hydrogen molecule gets closer to the α-U(001) surface along the y@s-bri channel, some electrons are donated from the H2 1σg orbital to the U (6d σ) orbital while some electrons are back-donated from the U (6d π) orbital to the H2 1σμ antibonding orbital. The electron interaction behavior agrees well with the adsorption of the H2 along the s@bri channel on the 26636

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Figure 4. 2D PES cuts for the adsorption of hydrogen molecule along the (a) d@top, (b) y@s-bri, (c) d@l-bri, (d) z@l-bri on the UTi surface. The total energy of a free H2 molecule plus that of the Ti-doped α-U(001) surface is set to be zero.

molecular adsorption states of H2 still exist when the hydrogen molecule is parallel to the metal surface, as shown in Figure 4a. Lastly, the H2 dissociation along all other channels is not energetically favorable. These results indicate that the adsorption and dissociation of H2 are just fractionally changed due to the Ti atom doping. The electronic interactions between H2 and the UTi surface, along the most energetically favorable dissociation path y@s-bri, are analyzed by calculating the PDOS of U, Ti, and H atoms at four such points (hH2 = 3.5 Å, dH−H = 0.8 Å), (hH2 = 1.5 Å, dH−H = 1.2 Å), (hH2 = 1.4 Å, dH−H = 1.4 Å), and (hH2 = 1.35 Å, dH−H = 2.1 Å), which are depicted in Figure 5a−d, respectively. In comparison with the PDOS evolution in the same channel between H2 and the clean α-U(001) surface, we find that the d electrons of Ti are also involved in the dissociation process. However, the electron donation and back-donation interaction mechanism is not changed. Hydrogen Diffusion and Penetration on the α-U Surface, and Influences of Ti Doping. To investigate the diffusion behavior of dissociated hydrogen atoms on the clean and Ti doped α-U(001) surfaces, we then perform CNEB calculations to determine the minimum energy paths for diffusion and penetration of the dissociated hydrogen atoms. Four different situations are considered. The first two are the onsurface diffusion between the most stable 3-fold sites e-fcc and ehcp, across the short- and long-bridge sites, respectively, while the other two account for the penetration from on-surface e-fcc (e-hcp) to subsurface site subfcc (subhcp) which are directly below the 3-fold site. The calculated PES are listed in Figure 6a,b. The barriers for the on-surface diffusions are estimated as 0.18 eV (s-bri, very close to 0.175 eV reported by Nie et al.14) and 0.14 eV

states. The latter case agrees well with that discussed by Nie et al.14 Influences of a Surface Substitutional Ti Atom on Hydrogen Adsorption and Dissociation. To investigate influences of surface substitutional Ti atoms on hydrogen adsorption and dissociation, we first study the electronic properties around a surface substitutional Ti atom. After geometry optimization for the α-U(001) surface with a surface substitutional Ti atom, we find that the doped Ti atom does not stay in the same plane with other uranium atoms. This behavior is similar to the surface substitutional doping of a Nb atom on the γU(100) surface.17 The z coordinate of the Ti atoms is found to be ∼0.1 Å lower than other surface U atoms. For simplicity, we will label the α-U(001) surface with a surface substitutional Ti atom as the UTi surface, to differentiate from the clean α-U(001) surface. The relative surface relaxations of the UTi surface are calculated to be Δd12/d0 = −4.4% and Δd23/d0 = +1.3% for the outermost and the second outermost interlayer distances, respectively. The adsorption sites of H2 molecule on the UTi surface are the same as that depicted for the clean α-U(001), as shown in Figure 1. The calculated 2D PES cuts for H2 molecule on the UTi surface along the d@top, y@s-bri, d@l-bri, and z@l-bri channels are shown in Figure 4a−d, respectively. First, the dissociation of H2 molecule along the y@s-bri channel (Figure 4b) is with no energy barriers, which makes this channel the most energetically favorable dissociation path. Second, as shown in Figure 4c, an activation energy of 0.1−0.2 eV is needed for the dissociation of H2 molecule in the d@l-bri channel on Ti-doped α-U(001) surface; on the contrary, in the same channel the dissociation of H2 on the clean α-U(001) surface is with no barrier. Third, the 26637

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Table 1. Binding Energies for Hydrogen Relative to the Hydrogen Atom [E(H)] and the Molecular State [E((1/ 2)H2)] at Various Surface and Subsurface Sites in the α-U Lattice and Ti Doped α-U Latticea α-U(001)

UTi surface

site

[E((1/2)H2)]

[E(H)]

[E((1/2)H2)]

[E(H)]

e-fcc e-hcp Top s-bri l-bri subfcc subhcp

0.49 0.54 −0.27 0.32 0.35 −0.33 −0.23

2.78 2.82 2.01 2.61 2.64 1.96 2.06

0.51 0.61 −0.05 0.36 0.49 −0.08 0.21

2.8 2.90 2.24 2.64 2.78 2.21 2.49

The energies are given in eV H atom−1. A positive (negative) value indicates endothermic (exothermic) reaction.

a

that, for α-U(001)surface, the adsorption energies of a single hydrogen atom at the s-bri, l-bri, hollow sites are exothermic relative to H2; on the contrary, the adsorption energies at the top, subfcc, and subhcp sites on the clean α-U(001) surface are endothermic. These values listed in Table 1 are consistent with those reported by Taylor et al.15 From Table 1, we can also find that the adsorption energies of a single hydrogen atom at the subfcc and subhcp sites for the UTi surface are −0.08 and 0.21 eV, respectively. These values indicate that, for the UTi surface, it is exothermic to trap a single hydrogen atom at the subhcp sites, and it needs to overcome an activation energy barrier of only 0.08 eV to trap a single hydrogen atom at the subfcc sites. Hence, it can be induced that hydrogen atoms are comparatively easy to be trapped at these subsurface sites below the doped titanium atom. Besides, we also investigate the influence of Ti doping on the diffusion of hydrogen atom on the α-U(001) surface. Five different diffusion paths, similar to the cases discussed previously for the clean α-U(001) surface, are considered here. The first three account for the on-surface diffusion from the e-fcc site, to the neighboring e-hcp1 (across the l-bri site), e-hcp2 (across the s-bri site), and e-hcp3 (3-fold sites surrounded by triple uranium atoms) sites, respectively. Since the e-hcp3 site is among uranium atoms, while the e-hcp1 and e-hcp2 sites are near the surface Ti atom, the diffusion path from the e-fcc site to the e-hcp3 site can be seen as escaping from the surface Ti atom, while those to the e-hcp1 and e-hcp2 sites can be seen as moving toward the

Figure 5. PDOS for the H2/Ti-doped α-U(001) surface along the minimum energy path in the y@s-bri channel with the height of H2 mass center as (a) 3.5, (b) 1.5, (c) 1.4, and (d) 1.35 Å. The Fermi energies are all set as zero.

(l-bri), respectively. These energies are very low, and hence, the diffusion of hydrogen on α-U(001) surface is very fast at room temperature.14,15 However, from Figure 6b, we can see that the computed activation energies for into-bulk diffusion of a hydrogen atom are 0.96 eV (e-fcc) and 0.82 eV (e-hcp), respectively. Such high barriers make it difficult for the hydrogen atoms to diffuse into bulk uranium. In order to facilitate the understanding of the dissociated hydrogen atom’s behavior on titanium doped α-U(001)surface, the adsorption energies of hydrogen atom at various sites are summarized in Table 1. The corresponding values for the clean α-U(001)surface are also listed as a reference. It can be found

Figure 6. Potential energy profile for (a) on-surface and (b) penetration-into-subsurface diffusion of hydrogen atom on clean α-U(001) surface. The insets represent the corresponding structures along the diffusion path. 26638

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Figure 7. Potential energy profiles for (a) on-surface, and (b) penetration-into-subsurface diffusion of H atom on UTi surface. The insets represent the corresponding structures along the diffusion path.

contrast, the diffusions to the subsurface sites near the doped Ti atom are exothermic and the activation barriers decrease by 0.3− 0.4 eV. Thus, these regions near doped Ti atoms might behave like hydrogen trapping sites, where hydrogen atoms may accumulate and uranium hydrides may nucleate.

surface-doped Ti atom. The calculated potential energy profiles are shown in Figure 7a. From Figure 7a, the calculated barriers are 0.02, 0.11 and 0.29 eV, respectively, for the on-surface diffusion from the e-fcc site to the neighboring e-hcp(1, 2, 3) sites. This indicates that the on-surface diffusion of hydrogen atom is almost with no barrier. Compared with the diffusion energy barriers on the clean α-U(001) surface, it can be drawn that the energy barrier for a dissociated hydrogen atom running away from a surface-doped Ti atom is larger than that for a dissociated hydrogen atom moving toward a surface-doped Ti atom. Thus, a surface substitutional Ti atom can act as a sink for the dissociated hydrogen atoms. The other two diffusion paths refer to the into-bulk diffusion of hydrogen atom from the e-fcc and e-hcp sites on UTi surface to the corresponding subsurface sites which are directly below the efcc and e-hcp sites (labeled as subfcc and subhcp, respectively). The calculated energy profiles are shown in Figure 7b. From Figure 7b we can see that, the activation energies for hydrogen penetrating into the subsurface are evaluated as 0.63 and 0.45 eV for the initial sites at e-fcc and e-hcp, respectively. These values are 0.33−0.37 eV lower than the corresponding values calculated for the clean α-U(001) surface. These results indicate that faster diffusion rates and energetically more favorable diffusion paths can be provided by the substitutional Ti atoms on the topmost surface.

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ASSOCIATED CONTENT

S Supporting Information *

Structural images in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (No. 9102616, 21371160, and 9043140307) and Foundations for Development of Science and Technology of China Academy of Engineering Physics (No. 2013B0301054, 2014A0301013). Thanks are due to Tao Tang and Gan Li for the fruitful discussions. The computational resources utilized in this research are provided by Shanghai Supercomputer Center.

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CONCLUSION Using density functional theory we have systematically investigated the adsorption and dissociation of hydrogen molecules on the clean and Ti-doped α-U(001) surface. It is found the H−H bond can be broken without conquering any energy barriers in both systems, mainly at the bridge sites when hydrogen atoms point to the 3-fold sites. The electron interaction between U 6d and H 1s plays a dominant role in this process. While at the top site of U atoms, f electrons of U also take part in the electronic interactions. For Ti-doped α-U(001) surface, the Ti 3d electrons also contribute to the bond breaking of the H2 molecule. In addition, the on-surface and into-bulk diffusions of the dissociated hydrogen atom are also studied. It is confirmed that there is small barrier for hydrogen diffusion on the clean and Tidoped α-U(001) surface at room temperature. However, the into-bulk diffusions through a defect-free surface are endothermic and need to overcome activation barriers of 0.8−0.9 eV. In

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dx.doi.org/10.1021/jp505171a | J. Phys. Chem. C 2014, 118, 26634−26640