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DFT + U Study of the Adsorption and Dissociation of Water on Clean, Defective, and Oxygen-Covered U3Si2{001}, {110}, and {111} Surfaces
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DFT + U Study of the Adsorption and Dissociation of Water on Clean, Defective, and Oxygen-Covered U3Si2{001}, {110}, and {111} Surfaces
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  • Ericmoore Jossou*
    Ericmoore Jossou
    Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada
    *E-mail: [email protected] (E.J.).
    More by Ericmoore Jossou
    Orcidhttp://orcid.org/0000-0002-4792-760X
  • Linu Malakkal
    Linu Malakkal
    Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada
    More by Linu Malakkal
  • Nelson Y. Dzade
    Nelson Y. Dzade
    School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
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    Orcidhttp://orcid.org/0000-0001-7733-9473
  • Antoine Claisse
    Antoine Claisse
    Westinghouse Electric Sweden AB, SE-721 63 Västerås, Sweden
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  • Barbara Szpunar
    Barbara Szpunar
    Department of Physics and Engineering Physics, College of Art and Science, University of Saskatchewan, 116 Science Place, Saskatoon S7N 5E2, Saskatchewan, Canada
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  • Jerzy Szpunar*
    Jerzy Szpunar
    Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada
    *E-mail: [email protected] (J.S.).
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2019, 123, 32, 19453–19467
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https://doi.org/10.1021/acs.jpcc.9b03076
Published July 11, 2019

Copyright © 2019 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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The interfacial interaction of U3Si2 with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U3Si2 surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H2 formation mechanisms. The adsorption strength decreases in the order U3Si2{001} > U3Si2{110} > U3Si2{111} for both molecular and dissociative H2O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H2O on the oxygen-covered U3Si2 surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H2O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces.

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1. Introduction

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There is an increasing interest in the development and use of metallic nuclear fuels such as U3Si2 given the enhanced thermophysical properties compared to traditional uranium dioxide fuel (UO2). (1) However, a key issue to deal with is the ease of oxidation of metallic nuclear fuels in the presence of water, oxygen, or combination of both, which requires the synthesis of U3Si2 in controlled environments. A significant number of earlier experimental works have assessed the behavior of U3Si2 in corrosion-susceptible environments in comparison to other metallic and UO2 fuels. Recently, Nelson et al. investigated the behavior of U3Si2 following exposure to pressurized H2O that is typical in light water reactors (LWRs) at temperatures ranging from 300 to 350 °C. Their results showed that both UN and U3Si5 rapidly pulverize in less than 50 h at 300 °C, while the behavior of U3Si2 was superior but still below the corrosion resistance of UO2 fuel. It is worth mentioning that the mechanism of pulverization of U3Si2 might be due to spallation of UO2 or due to internal hydriding. (2,3) The formation of U3Si2H1.8 with volumetric increase has been shown by experiment and density functional theory (DFT) calculations. (4) In this work, we unraveled the detailed mechanism of the early oxidation of U3Si2 from an atomistic point of view, which is difficult to realize experimentally. Earlier theoretical works have focused on the bulk properties of U3Si2 within the framework of DFT + U formalism and molecular dynamics simulations. (4−10) For instance, Middleburgh et al. (4) investigated the defect evolution in U3Si2 and also proposed a phase diagram capable of predicting fuel behavior during burnup. Furthermore, Wang et al. (7) studied the electronic, structural, and elastic properties showing detailed bonding characteristics of U3Si2 by electron density of states (DOS), charge density, and charge density difference analysis which corroborate the work of Remschnig et al. (11) as regards the metallic nature of U3Si2.
Surface science provides fundamental insight into the chemistry and physics of corrosion in materials, but such experiments are expensive and require dedicated facilities, especially in the study of an actinide containing compound. Hence, theoretical surface science provides an alternative tool for investigating oxidation mechanisms in nuclear fuel materials. Bo et al. (12) modeled the surface properties of the low-index NpO2(111), (110), and (100) surfaces as well as the adsorption and dissociation behaviors of water on these surfaces using DFT + U calculations in combination with ab initio atomistic thermodynamic simulations. Their results showed that water dissociation is enhanced by oxygen vacancy, while coverage of the water molecules plays no significant role during molecular adsorption. (13) More recently, Wellington and co-worker investigated the adsorption and dissociation of water on pristine and reduced UO2 and PuO2 surfaces using the periodic electrostatic-embedded cluster method and Hubbard-corrected periodic conditions implemented in the DFT-based code. Oxygen vacancies were shown to be easily formed on PuO2 compared to UO2, which is due to ease of Pu reduction in comparison to U metal ions. Their results also showed that dissociation is favored over molecular adsorption of water both on defect-free and oxygen deficit surfaces of UO2 and PuO2.
Given the success of first-principles DFT + U calculations in the modeling surfaces, we recently studied the adsorption of molecular and dissociated O2 on stoichiometric U3Si2 lower index surfaces. (14) To further our understanding of the underlying mechanism of oxidation, we have considered the synergetic effect of water and oxygen interaction with U3Si2 perfect surfaces. However, as real surfaces are never perfect but contain defects due to the method of synthesis or service conditions, in the present work, we have explored the effects of U and Si vacancies on the adsorption mechanisms of water and oxygen on the {001}, {110}, and {111} surfaces of U3Si2. Surface vacancies in UN, UO2, PuO2, and CeO2 have been shown in previous studies to affect the oxidation behavior of such surfaces in the presence of oxidizing and hydriding agents. (12,13,15−19) The fundamental aspects of oxygen and water adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are presented. Our results reveal a chemical picture of the initial steps involved in the oxidation process of the U3Si2 surfaces in the presence of oxygen and water (considering both molecular and dissociative adsorption).

2. Computational Methodology

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The first-principles calculations were performed within the plane-wave pseudo-potential DFT technique, (20,21) as implemented in the Quantum ESPRESSO code. (22) The exchange–correlation functional potential was described by the generalized gradient approximation (GGA) in the Wu–Cohen (GGA-WC) formulation. (23) All our calculations deployed the norm-conserving Wu–Cohen (WC) pseudopotentials (where these potentials treat 7p06d15f37s1 as valence electrons for U and 3s23p6 as valence electrons for Si). Due to the onsite Coulomb repulsion among the localized U 5f electrons, we used the Hubbard (DFT + U) correction to account for the strong correlation effect. (24) We have used an effective U value of 1.5 eV, which has been shown to give an accurate description of the structural parameters and the electronic properties of U3Si2. (14)
The Fermi surface effects were treated by the smearing technique of Methfessel–Paxton, (25) using a smearing parameter of 0.02 Ry (0.27 eV). An energy threshold defining self-consistency of the electron density was set to 10–8 eV and a β mixing factor of 0.3. The Brillouin zone integration was performed using 7 × 7 × 10 and 5 × 5 × 1 Monkhorst–Pack (22)k-point grids (centered at the Γ point) for the bulk U3Si2 and the surface models, respectively. Structural relaxation was carried out to minimize the energy using the conjugate gradient method within the Broyden–Fletcher–Goldfarb–Shanno algorithm, (26) until the magnitude of the residual Hell–Feynman force on each relaxed atom reached 0.01 eV Å–1. Visualization and analysis of the structures were performed using the VESTA program. (27,28)
The {001}, {110}, and {111} surfaces were considered for the oxygen and water adsorption calculations as they are the dominant growth facets of U3Si2. The different surface structures were created from the fully optimized bulk tetragonal U3Si2 structure (Figure 1) to eliminate the presence of fictitious forces during surface relaxation using the METADISE code, (29) which ensures the creation of surfaces with a zero dipole moment perpendicular to the surface plane. A vacuum of 15 Å was introduced to the surface models in the z direction, which is large enough to avoid any spurious interactions between periodic slabs. Bader population analysis was carried out on all adsorbate–substrate systems to quantify any charge transfer between the U3Si2 surface and O2/H2O species. (30)

Figure 1

Figure 1. Unit cell of ordered uranium silicide containing two formula units of U3Si2 (10 atoms) (color scheme: U = gray and Si = blue).

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3. Results and Discussion

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3.1. Defective Surface Models

3.1.1. Surface Defect Energies and Stability

Defective surfaces (vacancies) were created by removing one Si/U atom at a time from either the first or second layer (Figure 2) to access their energetics or stability.

Figure 2

Figure 2. Optimized surface geometry of (a) U3Si2{001}, (b) U3Si2{110}, and (c) U3Si2{111} with Si and U1 surface vacancy represented by a rectangular red box in the second and third rows respectively (color scheme: U = gray and Si = blue).

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The silicon vacancies are denoted as VSi, whereas the two considered uranium vacancy sites are denoted as VU1 and VU2, respectively, on the surface. The method of Wellington et al. (19) was employed in the creation of vacancy such that the uranium and silicon atoms removed from the supercell, both from the surface and subsurface layers, are fully coordinated by other quantum mechanically treated atoms and lie close to the center of the supercell to avoid problems that may occur in forming a defect at the edge. (19) The vacancy formation energy was calculated using eq 1
(1)
where Esurface+Xvac is the energy of the relaxed slab with X = Si or U vacancy, Esurface is the energy of the relaxed stoichiometric slab of the same type, and EX is the reference energy for X = Si and U as the energy per atom in its fundamental face-centered cubic and orthorhombic structure, respectively. Presented in Table 1 are the calculated vacancy formation energies on the {001}, {110}, and {111} U3Si2 surfaces. We found higher Si vacancy formation for the {110} surface (2.62 eV) than for the {001} surface (0.16 eV) and {111} surface (1.54 eV), and a similar trend is observed in the subsurface Si vacancies formation. Clearly, Si vacancy on the {110} subsurface (2.62 eV) is close to 2.48 eV, calculated for bulk U3Si2 by Andersson et al. (31) This is not surprising, given that the {110} surface has the lowest surface energy and similar neighboring atom coordination. There are two U sites denoted here as U1 and U2 with vacancy defect formation energies of 1.64 and 2.65 eV, respectively, for the bulk. The first and second layer U vacancy energies were calculated as presented in Table 1. Generally, it is easier to form a vacancy in the surface layer compared to the subsurface, which is due to the difference in the nearest neighbor atoms and the coordination numbers.
Table 1. Uranium and Silicon Vacancy Formation Energies (eV) of the {001}, {110}, and {111} Surfaces of U3Si2
 first layersecond layer
surfaceVSiVU1VU2VSiVU1VU2
{001}0.161.721.871.742.132.88
{110}2.623.182.411.622.032.61
{111}1.543.813.702.441.463.10
The formation of Si and U vacancies resulted in a nonstoichiometric surface, which allows variation in the surface energies as the chemical potential, μ, of Si and U changes. The thermodynamic stability of a given surface, in general, depends on the specific chemical environment. To determine the stability of the surfaces due to vacancies, we calculate the surface energy, γ, as a function of the Si and U chemical potential, respectively. At zero temperature the surface energy of a crystal may be derived from a N-layer slab using eq 2
(2)
In this equation, Esurf is the slab energy with all atomic coordinates relaxed unconstrainedly; NEbulk is the energy of an equal number, N of bulk U3Si2 atoms; A is the area of the slab surface; and the factor of 2 reflects the fact that there are two surfaces for each slab. Due to the surface vacancy defects, the surface energy depends on the specific thermodynamic conditions, i.e., the reservoir with which the atoms of the compound are exchanged in a structural transition. Therefore, the chemical potential of the constituents enters the surface energy. The most stable surface structure is determined by the minimum of the free energy which at zero temperature is given by eq 3
(3)
In the case of U3Si2, eq 3 can be expressed by eq 4 to account for the chemical potential μ of U and Si atoms
(4)
The μU and μSi are bounded by a set of conditions. Assuming U and Si are in thermal equilibrium with the U3Si2 crystal, it would imply that
(5)
Furthermore, since there is no precipitation of U and Si on the U3Si2 surface, the following conditions must also be fulfilled
(6a,b)
Also, the heat of formation of bulk U3Si2, ΔHf (eV f.u.–1), is defined as
(7)
Combining eqs 5, 6a,b, and 7, we obtain a range for possible values of the Si chemical potential
(8)
Combining eqs 4 and 5, gives the surface energy as a function of μSi
(9)
Similar to eq 9, the surface energy can also be expressed as a function μU
(10)
At ambient temperature and pressure, Esurf can be taken as the total energy from DFT calculation neglecting contributions from configurational or vibrational entropies. In Figure 2, we show the results of the calculated surface energies of the {001}, {110}, and {111} surfaces of U3Si2 as functions of the change in silicon chemical potential μSi, ranging from Si4 (−7.46 eV) to that of an isolated Si atom (0 eV), which denotes lower and upper limit of the Si environment, data from NIST database. (32) The lower limit, which is the Si-poor environment, is defined by the decomposition of the silicide into U and Si. Meanwhile, the upper limit is the Si-rich state that corresponds to a situation where the gas phase is so rich in silicon that they condensed on the substrate. In the case of stoichiometric surfaces considered in this work, the surface free energy is independent of μSi as expected, since the coefficient of eq 6a,ba,b containing μSi vanishes. For the nonstoichiometric cases, the surface free energy is a linear function of μSi according to eq 7. It can be seen from Figure 3 that the equilibrium surface stoichiometry of the U3Si2 surfaces can be tuned by changing μSi through the use of different U3Si2 growth (Si vacancy is considered here) and annealing environments. It is clear from the increase in surface energies that the stoichiometric surfaces are found to be more stable than the silicon deficient (silicon vacancy) surfaces. This implies that surface adsorption of an oxidizing adsorbate is more stable on the defective surface in comparison to the stoichiometric surface.

Figure 3

Figure 3. Calculated surface energies of the {001}, {110}, and {111} surfaces of U3Si2 as functions of the change in silicon chemical potential μSi.

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3.2. Adsorption of Water Molecule

The adsorption of the water molecule on the U3Si2 surfaces is an important starting step toward understanding its early oxidation mechanisms. The first interest of this study is therefore to determine the lowest-energy adsorption structures and modes of water on the low-index U3Si2{001}, {110}, and {111} surfaces, and to characterize the strength of their interaction and the extent of O–H bond activation during the dissociation process. Different H2O initial adsorption possibilities, including Hw and Ow head-on configurations, have been subjected to geometry optimization until the residual forces on all atoms were ≤0.03 eV Å–1. Prior to adsorption, we have calculated the reference energies, bond length, bond angle, and vibrational frequencies of one free H2O molecule and compared them with earlier theoretical results and available experimental data. The values of the O–H bond and the H–O–H angle of water are determined to be 0.970 Å and 104.5°, respectively, which are in good agreement with the previous experimental and theoretical values. (33,34) The calculated scissoring bend, asymmetric and symmetric stretching vibrational frequencies are calculated to be 1555, 3523, and 3635 cm–1, which agree well with the experimental results (1596, 3652, and 3756 cm–1), (35) thus making our calculation method reliable.
The adsorption energy (Eads), which characterizes the strength of water adsorption has been defined as the mean adsorption energy per molecule of H2O–U3Si2 interaction using eq 11
(11)
where Esurface+n(H2O) is the total energy of the substrate–adsorbate system in the equilibrium state, Esurface and nEH2O are the total energies for the surface and free molecules, respectively, and n is the number of water molecules in the cell. By this definition, a negative value of Eads indicates an exothermic and stable adsorption, whereas a positive value indicates an endothermic and unstable adsorption.

3.2.1. Water Adsorption and Dissociation on Clean U3Si2{001}

For the adsorption of molecular water on the U3Si2{001} surface, different adsorption sites and configurations were explored to determine the lowest-energy adsorption structures, as presented in Figure 4. The calculated adsorption energies and the optimized interatomic bond distances are summarized in Table 2. The calculated lowest-energy water adsorption structure is presented in Figure 4a, wherein the water molecule adsorbs through the O atom at the U site (U–O = 2.56 Å), releasing an adsorption energy of −3.70 eV. The adsorption of water at the Si site (O–Si = 2.376 Å) released an adsorption energy of −3.11 eV. When the water molecule is adsorbed with the hydrogen atoms pointing toward the surface Si site (Figure 4b), it moved away perpendicularly from the surface during energy minimization until the closest H–Si distance was 3.044 Å. The adsorption energy of this configuration was calculated to be −1.97 eV, while the α(H–O–H) bond angle and O–H bonds are obtained at 102.32° and 0.965/0.967 Å, respectively. Compared to the lowest-energy water adsorption on U3Si2{001}, Bo et al. calculated the adsorption energies of −2.07 and −1.27 eV for the lowest-energy adsorption structures of the water monomer on UN(001) and UO2(110), respectively, which suggest that U3Si2{001} is more reactive toward water adsorption than both UN(001) and UO2(110). (36,37) In all three adsorption modes, the O–H bond lengths were slightly elongated and the α(H–O–H) bonds bond angle was larger compared to the gas-phase free H2O molecule in a vacuum, suggesting that the O–H bonds are activated to some extent when water is adsorbed on U3Si2{001}. In the lowest-energy O–U structure (Figure 4a), the two O–H bonds are calculated at 0.975 and 0.996 Å compared to the gas-phase molecule at 0.970 Å and this is consistent with O–H bond stretching vibrational frequencies presented in Table 1, whereas the α(H–O–H) bonds bond angle is obtained at 107.6° compared to the gas-phase value of 104.5°, which is supported by the red and blue shifts in the bending vibrational frequencies as shown in Table 2. The stretched O–H bond lengths are indicative of weaker O–H bonds, resulting from the π-antibonding occupation. The activated O–H bonds suggest that these molecular adsorption states are likely precursors for H2O dissociation.

Figure 4

Figure 4. Top and side views of the relaxed adsorption structures of molecular H2O adsorbed at: (a) O–U, (b) H–Si, and (c) O–Si (color scheme: U = gray, Si = blue, H = white, and O = red).

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Table 2. Calculated Adsorption Energy (Eads), Relevant Bond Distance (d), Vibrational Frequencies, and Variation of the Total Bader Charge of Molecular (H2O) and Dissociated (OH + H) Adsorbed on the {001}, {110}, and {111} Surfaces of U3Si2
surfaceadsorbateconfig.Eads. (eV)d(O–H1) (Å)d(O–H2) (Å)d(Si–O) (Å)d(U–O) (Å)d(U–H) (Å)d(Si–H) (Å)α(HOH) (deg)νb (cm–1)νs (cm–1)νas (cm–1)Δq (e–)
  H2O 0.9700.970    104.490155535233635 
{001}H2OO–U–3.700.9750.996 2.622  107.6421534339036610.08
O–Si–3.110.9940.9762.3762.562  105.3991543355336750.09
H–Si–1.970.9750.977    102.3231540376738620.05
OH + HOH–Si: H on adj. U–6.810.983  2.598 1.551    2.13
OH–Si: H on Si–5.430.971 1.7412.618 1.501    1.21
OH–Si: H on U–3.610.965 1.644 2.119     1.17
{110}H2OO–Si–2.460.9960.9782.120   107.3431532341836730.15
O–U–0.600.9851.005    104.0411600332635900.16
H–Si0.550.9710.970    103.2321601342535710.05
OH + HOH–Si: H on U–3.240.979 1.676 2.3961.583    0.44
OH–U: H on Si–2.270.974  2.200 1.510    0.17
{111}H2OO–U–1.400.9770.995 2.548  105.4541543343236900.19
H–Si–0.200.9570.977    99.1811575367739480.05
O–Si1.020.9770.982    102.2051622358436910.01
OH + HOH–U: H on Si–3.300.971  2.194 1.697    2.18
OH–Si: H on U–1.210.969 1.702 2.181     0.34
Dissociative adsorption of H2O on the clean U3Si2{001} surface is found to be highly exothermic with adsorption energies of −6.81, −5.43, and −3.61 eV, respectively, for the different configurations presented in Figure 5 and Table 2. Consistent with their stronger adsorption, the collective amount of charge transfer gained by the dissociated H2O species is larger than those of the molecular adsorbed water systems. Due to its metallic nature, the U ions donate more electrons than Si ions during bond formation. The preference for dissociative over molecular adsorption of water can be attributed to the fact that the energy required to break an O–H bond requires less energy compared to the energy released in the formation of the Si–H and U–OH bonds on the U3Si2{001} surface. In like manner, on several metals and oxide surfaces, the dissociative state of H2O is thermodynamically more stable than the molecularly adsorbed state, for example, on Cu and Al2O3, because the breaking of an O–H bond is effectively balanced by the formation of a metal–O and another O–H bond with a surface oxygen. (38,39)

Figure 5

Figure 5. Top and side views of the relaxed adsorption structures of dissociated water configuration with (a) OH–Si: H on adj. U, (b) OH–Si: H on Si, and (c) OH–Si: H on U, on the U3Si2{001}-Si terminated surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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3.2.2. Water Adsorption and Dissociation on Clean U3Si2{110}

As on the U3Si2{001} surface, we have also explored different adsorption configurations of H2O on the {110} surface, including O adsorbed laterally at U or Si sites or H placed head-on Si sites. The optimized adsorption structures are shown in Figure 6, whereas the energies of adsorption, geometric parameters, and Bader charge transfer analysis are presented in Table 2. The lowest-energy water adsorption configuration at U3Si2{110} was calculated to be the O–Si structure (Figure 6a), wherein the water molecule is adsorbed at the Si site through the oxygen atom (O–Si = 2.120 Å), releasing an energy of −2.46 eV. In this structure, we observed elongation of the O–H bond calculated at 0.996 and 0.978 Å, and broadening of the α(H–O–H) bond angle (107.3°). The next stable configuration is the O–U structure (Figure 6b), which released an adsorption energy of −0.60 eV. The interacting O–U distance is calculated at 2.805 Å, and O–H bonds and α(H–O–H) bond angle are calculated at 0.985/1.005 Å and 104.0°, respectively. When the H atoms placed head-on Si (Figure 6c), the adsorption process is found to be endothermic by 0.55 eV and an average O–H bond length is 0.971 Å and there is a preferential Si–O bond formation with an interatomic distance of 1.932 Å after energy minimization. From Bader populations analysis, we found that the adsorption process is characterized by charge transfer from the surface to the water molecule, with the water molecule gaining a charge of 0.15e–, 0.16e–, and 0.05e– in the O–Si, O–U, and H–Si configurations, respectively. We also observed that the change in the vibrational modes and bond strengths is related to the Bader charge transfer between the surface and the adsorbed water molecule (Table 1).

Figure 6

Figure 6. Top and side views of the relaxed adsorption structures of molecular oxygen adsorbed at (a) O–Si, (b) O–U, and (c) H–Si on the U3Si2{110} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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Compared to the molecular adsorption, dissociative water adsorption is found to have average adsorption energies of −3.24 and −2.27 eV for OOH placed head-on Si and head-on U, respectively. For dissociative adsorption through the HOH, we initially placed an HOH above the Si surface. After optimization, a Si–O bond was formed through the OOH atom, which means that dissociative adsorption through the HOH is unstable. The dissociative adsorption structures are characterized by more electrons being transferred from the surface to the dissociating species as evident by the calculated Bader charges of 0.44e– and 0.17e–. The charge transfer from the U3Si2 surface to the OH group is a necessary condition for water to bind to the surface, which is consistent with water dissociation on metal surfaces. (40) Furthermore, the above energetics results indicate that the dissociative adsorption of water is notably stronger than the molecular adsorption on the U3Si2{110} surface. We also observed the formation of U–H with bond lengths of 2.396 and 2.275 Å in the dissociative configurations, which is required for the H+ to achieve stability. In comparison to previous work, the U–H distances are 2.30–2.32 Å in α and β-UH3 configurations. (41) The schematic representations of adsorption structures of dissociated water on the clean U3Si2{110} surface are shown in Figure 7.

Figure 7

Figure 7. Top and side views of the relaxed adsorption structures of dissociated H2O at (a) OH–Si: H on U and (b) OH–U: H on Si on the U3Si2{110} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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3.2.3. Water Adsorption and Dissociation on Clean U3Si2{111}

The geometries of the lowest-energy adsorption structures obtained on the U3Si2{111} surface are shown in Figure 8a–c. Compared to the {001} and {110} surfaces, the water molecules are adsorbed least strongly on the {111} surface. The lowest-energy adsorption structure on the U3Si2{111} surface is calculated to be the O–U configuration (Figure 8a), which released an adsorption energy of −1.40 eV. In this structure, the water molecule binds via the O atom lying laterally above the U atom at an O–U distance of 2.548 Å, with the α(H–O–H) bond angle that slightly increased to 105.5° compared to the gas-phase value of 104.5°. When the H2O molecule is adsorbed with the H atoms pointing toward the surface Si atoms (Figure 8b), an adsorption energy of −0.20 eV was released. The closest distance between H and Si/U sites is calculated to be 2.632/2.942 Å ruling out the formation of silane and uranium hydrides. The O–H bond lengths are elongated within the range of 0.977–0.995 Å while the α(HOH) bond angle increased slightly by 0.964° as presented in Table 2. The adsorption of H2O through Ow is unstable with an endothermic energy of 1.20 eV and tends to move further away from Si after optimization as shown in Figure 8c.

Figure 8

Figure 8. Top and side views of the relaxed adsorption structures of molecular H2O adsorbed at (a) the O–U site, (b) the H–Si site, and (c) the O–Si site on the U3Si2{111} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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For dissociative adsorption of water on U3Si2{111}, the OH radical and one H atom are placed on the surface with two different adsorption modes as shown in Figure 9a,b. In Figure 9a, the OH– forms a bond with a surface U atom while the H atom bonded to a neighboring Si atom. The formed U–O, Si–O, and Si–H bond lengths are 2.655, 1.722, and 1.610 Å, respectively. The calculated adsorption energy for this configuration is −0.76 eV, whereas the adsorption of OOH on U leads to the formation of U–O (2.194 Å) and Si–H (1.697 Å) bonds after optimization requiring an energy of −3.30 eV. The larger adsorption energies suggest a preference for dissociative water adsorption over molecular adsorption on the U3Si2{111} surface, which is consistent with previous studies of uranium-bearing systems. (37,42,43)

Figure 9

Figure 9. Top and side views of the relaxed adsorption structures dissociated water configuration with (a) O–U: H and (b) O–Si: H, on the U3Si2{111}-Si terminated surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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3.3. Effects of Surface Coverage, O-Covered, and Surface Vacancy on Adsorption of Water

3.3.1. Water Adsorption at Higher Coverage

The effect of coverage on the adsorption properties of water on the U3Si2 surfaces was investigated by adsorbing up to four water molecules in a molecular, mixed, and partially dissociative modes. The optimized structures of the most stable adsorption modes are shown in Figure 10, whereas the adsorption energies are reported in Table 3. We found no clear trends in the adsorption energies with an increasing number of water molecules on the U3Si2 surfaces. At the {001} surface, the adsorption energies of one, two, three, and four water molecules representing a coverage of 0.25, 0.50, 0.75, and 1.00 monolayers (ML) are −3.70, −2.83, −3.02, and −3.10 eV, respectively. At the {110} surface, the adsorption energy increased from −2.46 eV for one water molecule to −3.23, −3.06, and −2.58 eV for two, three, and four water molecules, respectively. A decrease in the adsorption energy is observed at the {111} surface with an increasing number of water molecules; −3.30 for one water molecule compared to −1.42, −2.01, and −2.53 eV for the two, three, and four water molecules, respectively.
Table 3. Adsorption Energies (eV) per Water Molecule, Mixed Molecular and Dissociated Water on U3Si2 Surfacesa
adsorbate{001}{110}{111}
1M–3.70–2.46–3.30
2M–2.83–3.23–1.42
3M–3.02–3.06–2.01
4M–3.10–2.58–2.53
1D + 3M–3.25–2.68–2.69
2D + 2M–3.26–2.69–2.65
3D + 1M–4.12–2.72–2.48
4D–3.54–3.68–3.45
a

Where M = H2O and D = OH + H.

Apart from the molecularly adsorbed water molecules, we have also explored mixed adsorption modes, wherein some of the water molecules are dissociated and some remained molecularly adsorbed, and investigated the case of fully dissociated modes in which all of the water molecules are dissociated at full coverage. The optimized structures of the most stable adsorption states are shown in Figure 10, and the adsorption energies are reported in Table 3. The mixed adsorption modes of molecular and dissociative water results in the formation different oxides, hydroxides, and oxygen molecules on the U3Si2 surface depending on the ratio of dissociative to molecularly adsorbed water as shown in Figure 10 for the most stable adsorption energies. The remaining structures are given in the Supporting Information (Figures S1–S3). For instance, on the {001} surface, in the case of 75/25 mix adsorption, the oxygen molecule was formed with a bond length of 0.99 Å directly on the surface Si with a bond distance of 1.70 Å typical of SiO2 formation. The strongest and most stable adsorption modes at the {001} surface are predicted for 3D + 1M, which released an adsorption energy of −4.12 eV, compared to the 4D structures on the {110} and {111} surfaces, which released adsorption energies of −3.68 and −3.45 eV, respectively. Even though U3Si2 is a metallic fuel, it is important to draw a comparison between the silicide and urania, which is the standard fuel for LWRs. Hence, it is interesting to note that studies by Bo et al. (37) on UO2 and recent work by Tegner et al. (44) predicted the mixed 50/50 molecular/dissociative adsorption modes as the most stable adsorption configurations. The differences in the results can be attributed to differences in the crystallographic arrangement of the two systems and their electronic structures, UO2 is a semiconductor with an experimental band gap of 2.1 eV, (45,46) whereas U3Si2 is metallic. Note, however, that the energy difference between the fully dissociative case and the fully molecular case is −0.44, −1.10, and −0.92 eV on the {001}, {110}, and {111} surfaces, respectively. The increase in adsorption energies in the dissociative configuration is due to the formation of stronger intramolecular hydrogen bonds on the crowded surfaces compared to the purely molecular mode.

Figure 10

Figure 10. Top and side view of the relaxed adsorption structures of mixed (dissociative to molecular) 3:1 adsorption of H2O on (a) {001} and fully dissociative adsorption of H2O on (b) the {110} and (c) {111} U3Si2 surface (color scheme: U = gray, Si = blue, H= white, and O = red).

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We also carried out a detailed analysis of the bond lengths for molecular and dissociative adsorption of multiple water molecules on the surfaces as shown in Table 4. We observed a shorter hydrogen bond length for the Si–H compared to U–H on the U3Si2{001}, {110}, and {111} surfaces, which suggest stronger hydrogen bonding in Si–H when it is formed on the surface. There is also strong adsorption of molecular H2O on the surface forming U–H2O and Si–H2O complexes with the exception of the {111} surface where the formation of Si–H2O is not observed.
Table 4. Selected Interatomic Distances (Å) for Molecular and Dissociative Water on the U3Si2{001}, {110}, and {111} Surfaces at Coverages from 0.5 to 1.0 Monolayers (ML)
bond type{001} 0.5–1.0 ML (Å){110} 0.5–1.0 ML (Å){111} 0.5–1.0 ML (Å)
Si–O1.70N/AN/A
Si–H1.50–1.711.57–1.60N/A
Si–OH1.66–1.681.67–1.681.62–1.70
Si–H2O1.89–2.112.02–2.06N/A
U–O2.13N/AN/A
U–H2.17–2.342.34–2.452.16–2.35
U–OHN/A2.31–2.782.48–2.71
U–H2O2.51–2.672.35–2.652.64–2.69

3.3.2. Water Adsorption and Dissociation on O-Covered U3Si2{100}, {110} and {111} Surfaces

Preadsorbed oxygen atoms play an important role in the activation of the O–H bond and further increase the rate of surface oxidation due to rapid water dissociation. (47,48) Hence, it is important to investigate the adsorption behavior of H2O on oxygen-covered U3Si2{001}, {110}, and {111} surfaces. Prior to investigating the adsorption of H2O on oxygen-covered U3Si2{001}, {110}, and {111} surfaces, we have systematically characterized the adsorption of atomic oxygen on different U3Si2 surfaces to elucidate their surface oxide formation. In our previous study, (14) we have provided comprehensive information regarding the mechanism of oxide layer formation on the U3Si2 surface by adsorbed O2 molecule. The adsorption of atomic O is found to be energetically more favorable at U sites than Si sites on the {001}, {110}, and {111} U3Si2 surfaces (Table S4 and Figure S5). The adsorption energies at the U and Si sites were calculated to be −2.44 and −0.67 eV on the {001} surface, −2.71 and −0.77 eV on the {110} surface, and −2.85 and −0.81 eV on the {111} surface. Similar results were predicted for molecular O2 at the different U3Si2 surfaces, which implies that the formation of UO2 on pristine U3Si2 is favored over SiO2. (14)
For the adsorption of water on the O-covered U3Si2 surfaces, various coadsorption structures were explored with the water adsorbed at the neighbor or distant sites from the preadsorbed O atoms. The coadsorption energies between the H2O and O on the U3Si2 surface is calculated as follows
where EH2O, EO2, Esurface, and Esurface+(H2O+O) are the total energy for the free molecule of water, molecular oxygen, the clean U3Si2 surface, and the coadsorbed (H2O + O) + U3Si2 surface systems, respectively. In most cases at the {001} surface, we found that the water molecule coadsorbed with oxygen dissociate to form OH ions due to the attractive force between the hydrogen atoms and preadsorbed O atoms. The most favorable coadsorption mode on the {001} surface is predicted for the configuration in which the water molecule is adsorbed at the U site near the O atom that is preadsorbed at the Si site. Due to the strong attractive force between the two adsorbates, the H2O molecule spontaneously dissociate to form two hydroxyl species, releasing an adsorption energy of −5.47 eV (Table 5), which is consistent with water dissociation on oxygen-covered metal surfaces. (34,49) The U–OH and Si–OH interaction bond lengths are calculated to be 2.339 and 1.641 Å, respectively. Consistent with the strong adsorption, the two OH species draws a combined charge of 1.95e– from the interacting surface species. When the water molecule remained molecularly adsorbed at a Si site near a preadsorbed O atom (Figure 11b), an adsorption energy of −3.54 eV is released.

Figure 11

Figure 11. Top and side views of the relaxed adsorption structures of molecular H2O (a) H–U: O on U, (b) Ow–Si: O on Si, (c) Ow–Si: O on U, and (d) Ow–U: O on Si U3Si2{001} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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Table 5. Calculated Adsorption Energy (Eads) and Relevant Bond Distances for H2O Coadsorbed with Atomic Oxygen on U3Si2{001}, U3Si2{110}, and U3Si2{111} Surfaces
surfaceadsorbateconfig.Eads (eV)d(Ow–H1) (Å)d(Ow–H2) (Å)d(O–H2) (Å)d(Si–O) (Å)d(Si–OH) (Å)d(U–O) (Å)d(U–OH) (Å)α(HOH) (deg)Δq (e–)
{001}H2O + OHw–U: O on U–5.470.977 1.026 1.641 2.339 1.95
Ow–Si: O on Si–3.540.9700.988 1.729 2.525 106.7601.83
Ow–Si: O on U–3.490.975 0.953 1.680 2.525 1.79
Ow–U: O on Si–2.870.973 1.0991.675    1.75
{110}H2O + OOw–U: O on U bridge–7.410.9750.975 2.763 2.268 105.5231.21
Ow–Si: O on U–4.130.982    2.236  0.92
Hw–Si: O on U–3.670.978 1.154   1.80  
{111}H2O + OOw–Si: O on U–1.390.979 1.006 1.710 2.145 1.67
Ow–U: O on Si–0.810.9770.974 1.695 2.567 109.9751.87
Hw–Si: O on U–0.270.974    2.021   
We have also investigated the adsorption and dissociation of water on the U3Si2{110} surface in the presence of preadsorbed atomic oxygen. Three coadsorption modes have been explored with the H2O and O coadsorbed at the neighboring surface U and U bridge sites. The optimized adsorption geometries are shown in Figure 12a–c, while the coadsorption energies and the relevant optimized geometric parameters are summarized in Table 5. When water is adsorbed with the Ow head-on Si and the preadsorbed O atom at the nearest neighbor U–U bridge, the coadsorption energy is −7.41 eV, which is more negative than the sum of the separate adsorption energies (−3.23 eV), suggesting a strong interaction evident by the stretching of the OH bonds and significant transfer of electrons (1.21e–) from Si to the water molecule. It is clear that the oxygen atom has a stronger affinity for the uranium forming a U–O–U complex with a bond angle of 97.65°. Hence, the water molecule preferred to bond with the surface Si atom rather than deprotonate to form hydroxyl ions as was observed on iron sulfide surfaces. (34) When water is coadsorbed with the Ow head-on a neighboring U site, the coadsorption energy is calculated to be −4.13 eV, which is also more negative than the sum of the separate adsorption energies (−3.31 eV), suggesting that the oxygen atom promotes the dissociation of water due to deprotonation resulting in the formation of silane (Si–H), uranium oxide (U–O), and hydroxyl (O–H) species (Figure 12b). In this case, where Hw is placed on the Si atom, we observed a repulsion between H and Si while the nearest O atom forms a bond with one of the Hw leading to the formation of hydroxyl ions as shown in Figure 12c releasing an energy of −2.67 eV.

Figure 12

Figure 12. Top and side views of the relaxed adsorption structures of molecular H2O (a) Ow–Si: O on the U bridge, (b) Ow–Si: O on U, and (c) Hw–Si: O on U on the U3Si2{110} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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The most favorable coadsorbed configuration of the (H2O + O)/U3Si2 system on the {111} surface is found to be the structure with the water molecule adsorbed on Si while one of the hydrogen atoms pointing toward the preadsorbed O atom on the top-U site (Figure 13a) releasing an energy of −1.39 eV, which is more negative than the sum of the separate adsorption energies of 0.39 eV. This is indicative of attraction between the two species on the U3Si2{111} surface for this configuration. This accounts for the dissociation of the water molecule leading to the formation of two Si–OH and U–OH. Although in the adsorption of the O atom on the Si site (Figure 13b) leads to the O atom, preferentially forming a trihedral network with U and Si atoms, this is facilitated by drawing electrons from a metal surface that is easy compared to a Si atom while there is stretching of the OH bonds (0.977 and 0.974 Å) with the Ow forming bond with the surface U atom.

Figure 13

Figure 13. Top and side views of the relaxed adsorption structures of molecular H2O (a) Ow–U: O and (b) Ow–Si: O on the U3Si2{111} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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3.3.3. Water Adsorption on Defective (Nonstoichiometric) Surfaces

Considering that real surfaces are never perfect under reaction conditions and chemical processes often occur at defect sites, we have also investigated water adsorption at the defective surfaces containing one Si or U vacancies at the {001}, {110}, and {111} surfaces. The optimized structure of defective U3Si2 with H2O is shown in Figure 14 and the calculated adsorption energies and optimized geometry parameters are listed in Table 6. At the defective {001}, {110}, and {111} surfaces containing one Si vacancy site (Si-1), the adsorption energies of the water monomer in the molecular state is calculated to be −7.52, −3.18, and −7.0 eV, respectively. In {001} and {110} adsorption structures, there is a complete dissociation of the water molecule which is consistent with the chemisorbed nature of water (16) while the OH bonds on the {111} surface stretch to 1.245 and 0.980 Å, respectively, suggesting the instability of the water molecule on the nonstoichiometric surface. The computed bond distance of U–O (2.163–2.321 Å) and Si–O (1.536–1.892 Å) suggest the formation of UO2 and SiO2.

Figure 14

Figure 14. Top and side views of the relaxed adsorption structures of Si and U vacancy-assisted molecular H2O adsorption (a) H2O–U3Si2–x {001}, (b) H2O–U3Si2–x {110}, (c) H2O–U3Si2–x {111}, (d) H2O–U3–xSi2{001}, (e) H2O–U3–xSi2{110}, and (f) H2O–U3–xSi2{111} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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Table 6. Adsorption Energies (eV) of the Water Molecule on Nonstoichiometric U3Si2{001}, {110}, and {111} Surfaces
adsorption site{001}{110}{111}
Si vacancy–3.43–3.18–2.92
U1 vacancy–4.85–4.35–2.17
U2 vacancy–5.84–4.22–3.54
At the surfaces containing one U vacancy site (U-1), the dissociative adsorption of H2O on the defective U3–xSi2{110} is found to be highly exothermic (Eads = −5.84 eV), which suggests that the defective U3–xSi2{001} favors dissociative H2O adsorption rather than molecular adsorption, similar to the findings on the defective U3–xSi2{001} and {111} surfaces. Bader population analysis reveals that a significant amount of charge (∼1.32e–) was transferred to dissociated water species from the interacting surface species. This is not surprising since vacancies result in the formation of dangling bonds (reactive sites) due to the availability of electrons for transfer to water molecules on the surface.

3.4. Electronic Structure and Bonding Mechanism

Fundamental understanding of the nature of interactions between the H2O molecule and the U3Si2 surfaces and any adsorption-induced changes in the electronic structures of U3Si2 were gained through projected density of states (PDOS) analysis (Figure 15). The empty (filled) electronic states near the Fermi level can accept (donate) more electrons to enhance the reduction (oxidation) reactions. We observed various degrees of shifts in the PDOS, indicative of a change in the electronic structure due to electron transfer from U and Si ions to the water molecules leading to OH bond elongation; a step before deprotonation of the water molecule. The charge gained by the H2O molecule in different adsorption complexes is calculated to be in the range of 0.05–0.11e– at the water–U3Si2{001} surface, 0.04–0.05e– at the water–U3Si2{111} surface, and 0.15–0.25e– at the water–U3Si2{110} surface (Table 3).

Figure 15

Figure 15. (Right) PDOS for the interacting surface U f-states before and after the adsorption of H2O at the (a) water–U3Si2{001} and (b) for interacting surface Si p-states at the water–U3Si2{110} interface, and for the interacting surface U f-states before and after the adsorption of H2O at the (c) water–U3Si2{111} interface. (Left) the corresponding optimized water–U3Si2 interfaces with U–O and Si–O bond lengths.

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There is a possibility of surface oxide formation as shown by the U–O bond lengths of 2.620 and 2.555 Å and Si–O bond length of 2.120 Å on the {001}, {111}, and {110} surfaces, respectively. This suggests that water–U3Si2 oxidation would proceed by any of the following chemical reactions (50,51)
The reaction energy of the different possible reaction pathways is found to be highly exothermic, which suggest that they are thermodynamically favorable and feasible reactions.
We further analyzed the PDOS of the isolated water molecule in the adsorbed state at the various surfaces and compared to the gas-phase molecule (Figure 16). The DOS for the gas-phase H2O molecule is shown in Figure 16a where the molecular orbitals (MOs) are labeled as 2a1, 1b2, 3a1, and 1b1, respectively, while those for the lowest-energy adsorption configurations at the water–U3Si2{001}, {110}, and {111} interfaces are shown in Figure 16b–d, respectively. Due to the strong water–U3Si2 hybridization, electron transfer from the interacting surface U f-states, we observed a shift or disappearance of the 3a1 and 1b1 MOs around the Fermi level of water adsorbed at the {001} and {111} surfaces. At the water–U3Si2{110} interface, we observe a splitting of the 1b1 MO, coupled with a shift toward lower energy levels (Figure 16d), which signifies stabilization of the water molecule via physisorption. The reactivity of the surface is influenced largely by the ease of electron transfer between the surface and the adsorbate. Previous, work by Li et al. showed that the electronic states of the valence band of the surface play a key role during the surface–adsorbate interaction due to the hybridization energy between the bonding and antibonding adsorbate states and the metal valence-bands. (52)

Figure 16

Figure 16. DOS for H2O in the (a) free state and adsorbed in the lowest-energy geometry at the water–U3Si2 interfaces (b–d).

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As the preadsorbed oxygen species enhance the O–H bond activation to dissociate, a necessary step for the formation of surface oxides, we have also characterized the electronic structures of the O-covered U3Si2 surfaces and showed that the U atoms to which atomic oxygen is bound become more positive (1.12e–, 1.20e–, and 1.61e– on the {001}, {110}, and {111} surfaces, respectively) compared to the clean surface U charge of +0.98e– {001}, 0.96e– {110}, and 1.01e– {111}, which, from the qU4+/qU6+ ratio, is enough to suggest that they have been oxidized from U4+ to U6+ (see Table S4 and Figure S5, for full details of O–U3Si2 in terms of bond lengths and charge transfer). We further investigated the nature of the interaction of the bonding between surface U and atomic oxygen by analyzing the projected density of states (PDOS) of the interacting surface U f-states and O p-state, as shown in Figure 17. We observed strong hybridization between the interacting U f-states and O p-state, which is due to the charge transfer from surface U ions into the adsorbed oxygen π orbital, in good agreement with previous studies of oxygen adsorption on metal surfaces (53)

Figure 17

Figure 17. Partial DOS projected on the interacting surface U f-states and O p-states for adsorbed atomic oxygen on the top-U site on (a) {001}, (b) {110}, and (c) {111} surfaces.

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The work function gives a picture of the nature of such electronic interactions; hence, we have calculated the work function of the clean and adsorbate containing U3Si2 surfaces to characterize the level of difficulty for an electron transferring from the surface to the vacuum. The work function is the minimum energy needed to remove an electron from the bulk of a material through a surface to a point outside the material and can be written as
(12)
The potential in the vacuum region (Vvacuum) and the Fermi energy (EFermi) were derived from the same calculation. In practice, this is the energy required at 0 K to remove an electron from the Fermi level of the material to the vacuum potential. We determined the work function of the clean surface and further probed the effect of oxygen on the electronic states of the surface and oxygen-assisted dissociation of H2O.
The work function depends on the crystallographic direction as shown by the results in Table 7. The anisotropic nature of the work function comes from the crystallographic arrangement of the surface planes, which determines the spreading of the electronic charge into the vacuum. (54) Furthermore, the adsorption acts to smoothen the surface electric charge distribution that lowers the work function. The work function is such that Φ(H2O + O) < Φ(H2O) < Φdry due to the partial transfer of electron charge from the substrate to the adsorbate and the resulting adsorption-induced surface dipoles (Table 7). (55)
Table 7. Calculated Work Functions of the Dry (Φdry), Hydrated (ΦH2O), and Coadsorbed H2O + O (ΦH2O+O) U3Si2 Surfaces
surfaceΦdry (eV)ΦH2O (eV)ΦH2O+O (eV)
{001}3.093.052.86
{110}1.891.481.21
{111}2.862.301.96

4. Summary and Conclusions

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We have investigated the adsorption and dissociation reactions of H2O on clean, oxygen-covered and defective U3Si2{001}, {110}, and {100} surfaces using density functional theory calculations. The adsorption energetics and characteristics of molecular water adsorption were compared with full dissociative and mixed (molecular and dissociative) adsorption modes on the U3Si2 surfaces. We showed from our calculations that the major interactions between the adsorbing water molecules and the U3Si2 surfaces occur through oxygen and the surface U or Si site. Compared to the clean surfaces, preadsorbed O atoms are shown to enhance the activation of the O–H bonds of water and their subsequent dissociation reactions to form surface hydroxyl species, which are driven by significant charge transfer from the surface to the adsorbing species. We demonstrated that surface vacancy defects enhance the adsorption and dissociation of H2O compared to a stoichiometric defect-free surfaces. The molecular-level insights derived from this work provide a fundamental understanding of the adsorption processes and mechanisms of the early stage of oxidation of U3Si2 in the presence of oxygen and water and might open new avenues for the rational design of oxidation resistance of metallic fuels for nuclear reactors.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03076.

  • Mixed mode water adsorption configurations and adsorption energies on stoichiometric U3Si2{001}, {110}, and {111} surfaces; calculated adsorption energy (Eads), relevant bond lengths (d) of atomic (O) oxygen on {001}, {110}, and {111} U3Si2 surfaces (PDF)

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Author Information

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  • Corresponding Authors
    • Ericmoore Jossou - Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada;  Orcidhttp://orcid.org/0000-0002-4792-760X;  Email: [email protected]
    • Jerzy Szpunar - Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada;  Email: [email protected]
  • Authors
    • Linu Malakkal - Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon S7N 5A9, Saskatchewan, Canada
    • Nelson Y. Dzade - School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.;  Orcidhttp://orcid.org/0000-0001-7733-9473
    • Antoine Claisse - Westinghouse Electric Sweden AB, SE-721 63 Västerås, Sweden
    • Barbara Szpunar - Department of Physics and Engineering Physics, College of Art and Science, University of Saskatchewan, 116 Science Place, Saskatoon S7N 5E2, Saskatchewan, Canada
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors gratefully acknowledge Compute Canada (WestGrid) and University of Saskatchewan’s Research Cluster (Plato) for a generous amount of CPU time. We also acknowledge the financial support of the Canadian National Science and Engineering Research Council (NSERC) and the University of Saskatchewan’s International Dean’s Scholarship. N.Y.D. acknowledges the U.K. Engineering and Physical Sciences Research Council (EPSRC) for funding (Grant No. EP/S001395/1).

References

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This article references 55 other publications.

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    Recent interest in U3Si2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H2O environments is absent from the literature. The behavior of U3Si2 in H2O contg. atmospheres is investigated and presented in a two-part series of articles aimed to understand the degrdn. mechanism of U3Si2 in H2O. Reported here are thermogravimetric data for U3Si2 exposed to flowing steam at 250-470 °C. Addnl. the response of U3Si2 to flowing Ar-6% H2 from 350 to 400 °C is presented. Microstructural degrdn. is obsd. following hours of exposure at 350 °C in steam. U3Si2 undergoes pulverization on the timescale of minutes when temps. are increased above 400 °C. This mechanism is accelerated in flowing Ar-H2 at the same temps.
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  3. 3
    Nelson, A. T.; Migdisov, A.; Wood, E. S.; Grote, C. J. U3Si2 Behavior in H2O Environments: Part II, Pressurized Water with Controlled Redox Chemistry. J. Nucl. Mater. 2018, 500, 81– 91,  DOI: 10.1016/j.jnucmat.2017.12.026
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    3
    U3Si2 behavior in H2O environments: Part II, pressurized water with controlled redox chemistry
    Nelson, A. T.; Migdisov, A.; Wood, E. Sooby; Grote, C. J.
    Journal of Nuclear Materials (2018), 500 (), 81-91CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    Recent interest in U3Si2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H2O environments is sparse in available literature. The behavior of U3Si2 in H2O contg. atmospheres is investigated and presented in a two-part series of articles. This work examines the behavior of U3Si2 following exposure to pressurized H2O at temps. from 300 to 350 °C. Testing was performed using two autoclave configurations and multiple redox conditions. Use of solid state buffers to attain a controlled water chem. is also presented as a means to test actinide-bearing systems. Buffers were used to vary the hydrogen concn. between 1 and 30 parts per million H2. Testing included UN, U3Si5, and UO2. Both UN and U3Si5 were found to rapidly pulverize in less than 50 h at 300 °C. Uranium dioxide was included as a control for the autoclave system, and was found to be minimally impacted by exposure to pressurized water at the conditions tested for extended time periods. Testing of U3Si2 at 300 °C found reasonable stability through 30 days in 1-5 ppm H2. However, pulverization was obsd. following 35 days. The redox condition of testing strongly affected pulverization. Characterization of the resulting microstructures suggests that the mechanism responsible for pulverization under more strongly reducing conditions differs from that previously identified. Hydride formation is hypothesized to drive this transition. Testing performed at 350 °C resulted in rapid pulverization of U3Si2 in under 50 h.
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  4. 4
    Middleburgh, S. C.; Claisse, A.; Andersson, D. A.; Grimes, R. W.; Olsson, P.; Mašková, S. Solution of Hydrogen in Accident Tolerant Fuel Candidate Material: U3Si2. J. Nucl. Mater. 2018, 501, 234– 237,  DOI: 10.1016/j.jnucmat.2018.01.018
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    Solution of hydrogen in accident tolerant fuel candidate material: U3Si2
    Middleburgh, S. C.; Claisse, A.; Andersson, D. A.; Grimes, R. W.; Olsson, P.; Maskova, S.
    Journal of Nuclear Materials (2018), 501 (), 234-237CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    Hydrogen uptake and accommodation into U3Si2, a candidate accident-tolerant fuel system, has been modelled on the at. scale using the d. functional theory. The soln. energy of multiple H atoms is computed, reaching a stoichiometry of U3Si2H2 which has been exptl. obsd. in previous work (reported as U3Si2H1.8). The absorption of hydrogen is found to be favorable up to U3Si2H2 and the assocd. vol. change is computed, closely matching exptl. data. Entropic effects are considered to assess the dissocn. temp. of H2, estd. to be at ∼800K - again in good agreement with the exptl. obsd. transition temp.
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  5. 5
    Beeler, B.; Baskes, M.; Andersson, D.; Cooper, M. W. D.; Zhang, Y. A modified Embedded-Atom Method interatomic potential for uranium-silicide. J. Nucl. Mater. 2017, 495, 267– 276,  DOI: 10.1016/j.jnucmat.2017.08.025
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    A modified Embedded-Atom Method interatomic potential for uranium-silicide
    Beeler, Benjamin; Baskes, Michael; Andersson, David; Cooper, Michael W. D.; Zhang, Yongfeng
    Journal of Nuclear Materials (2017), 495 (), 267-276CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy fuel benefits from higher thermal cond. and higher fissile d. compared to uranium dioxide (UO2). In order to perform engineering scale nuclear fuel performance simulations, the material properties of the fuel must be known. Currently, the exptl. data available for U-Si fuels is rather limited. Thus, multiscale modeling efforts are underway to address this gap in knowledge. In this study, a semi-empirical modified Embedded-Atom Method (MEAM) potential is presented for the description of the U-Si system. The potential is fitted to the formation energy, defect energies and structural properties of U3Si2. The primary phase of interest (U3Si2) is accurately described over a wide temp. range and displays good behavior under irradn. and with free surfaces. The potential can also describe a variety of U-Si phases across the compn. spectrum.
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  6. 6
    Noordhoek, M. J.; Besmann, T. M.; Andersson, D.; Middleburgh, S. C.; Chernatynskiy, A. Phase Equilibria in the U-Si System from First-Principles Calculations. J. Nucl. Mater. 2016, 479, 216– 223,  DOI: 10.1016/j.jnucmat.2016.07.006
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    Phase equilibria in the U-Si system from first-principles calculations
    Noordhoek, Mark J.; Besmann, Theodore M.; Andersson, David; Middleburgh, Simon C.; Chernatynskiy, Aleksandr
    Journal of Nuclear Materials (2016), 479 (), 216-223CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    D. functional theory calcns. have been used with spin-orbit coupling and on-site Coulomb correction (GGA + U) methods to investigate the U-Si system. Structural prediction methods were employed to identify alternate stable structures. Convex hulls of the U-Si system were constructed for each of the methods to highlight the competing energetics of various phases. For GGA calcns., new structures are predicted to be dynamically stable, but these have not been exptl. obsd. When the GGA + U (Ueff > 1.3 eV) method is considered, the exptl. obsd. structures are predicted to be energetically preferred. Phonon calcns. were used to investigate the energy predictions and showed that the use of the GGA + U method removes the significant imaginary frequencies obsd. for U3Si2 when the correction is not considered. Total and partial electron d. of states calcns. were also performed to understand the role of GGA + U methods and orbitals on the bonding and stability of U-Si compds.
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  7. 7
    Wang, T.; Qiu, N.; Wen, X.; Tian, Y.; He, J.; Luo, K.; Zha, X.; Zhou, Y.; Huang, Q.; Lang, J. First-Principles Investigations on the Electronic Structures of U3Si2. J. Nucl. Mater. 2016, 469, 194– 199,  DOI: 10.1016/j.jnucmat.2015.11.060
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    First-principles investigations on the electronic structures of U3Si2
    Wang, Tong; Qiu, Nianxiang; Wen, Xiaodong; Tian, Yonghui; He, Jian; Luo, Kan; Zha, Xianhu; Zhou, Yuhong; Huang, Qing; Lang, Jiajian; Du, Shiyu
    Journal of Nuclear Materials (2016), 469 (), 194-199CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    U3Si2 has been widely utilized as a high-power uranium fuel for research reactors due to its high d. of uranium. However, theor. investigations on this material are still scarce up to now. For this reason, the computational study via d. functional theory (DFT) is performed on the U3Si2 compd. in this work. The properties of U3Si2, such as stable cryst. structures, d. of states, charge distributions, formation energy of defects, as well as the mech. properties are explored. The calcn. results show that the U3Si2 material is metallic and brittle, which is in good agreement with the previous exptl. observations. The formation energy of uranium vacancy defect is predicted to be the lowest, similar with that of UN. The theor. investigation of this work is expected to provide new insight of uranium silicide fuels.
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  8. 8
    Andersson, D. A.; Liu, X.-Y.; Beeler, B.; Middleburgh, S. C.; Claisse, A.; Stanek, C. R. Density Functional Theory Calculations of Self- and Xe Diffusion in U3Si2. J. Nucl. Mater. 2019, 515, 312– 325,  DOI: 10.1016/j.jnucmat.2018.12.021
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    Density functional theory calculations of self- and Xe diffusion in U3Si2
    Andersson, D. A.; Liu, X.-Y.; Beeler, B.; Middleburgh, S. C.; Claisse, A.; Stanek, C. R.
    Journal of Nuclear Materials (2019), 515 (), 312-325CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    Uranium silicide, U3Si2, has been proposed as an advanced nuclear fuel to be used in light water reactors (LWRs). Development of this alternative to the predominant current fuel, UO2, is motivated by enhanced accident tolerance as a result of higher thermal cond. as well as improved fuel cycle economics through increased uranium d. In order to accurately model the fuel performance of U3Si2, the diffusion rate of point defects, which is related to self-diffusion, and of fission gas atoms must be detd. DFT calcns. are used to predict the U and Si point defect concns., the corresponding self-diffusivities, the preferred Xe trap site and the Xe diffusivity. Effects of irradn. are not considered. A low defect formation energy and a high entropy for Si interstitials give rise to Si-rich non-stoichiometry at elevated temps. Both U and Si self-diffusion and Xe diffusion are anisotropic as a consequence of the tetragonal crystal structure of U3Si2. Si diffusion occurs by interstitial mechanisms in both the a-b plane and along the c axis, while the U c axis diffusion rate is controlled by a vacancy mechanism. Interstitial diffusion of U is very fast in the a-b plane of the U3Si2 crystal structure. Xe atoms prefer to occupy U vacancy trap sites. The highest Xe diffusion rate occurs by a vacancy mechanism in both the a-b plane and along the c axis. The diffusion rate is similar in the a-b plane and along the c axis. U and Si self-diffusion and Xe diffusion are all faster in U3Si2 than intrinsic U and Xe diffusion in conventional UO2 nuclear fuel.
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  9. 9
    Beeler, B.; Baskes, M.; Andersson, D.; Cooper, M. W. D.; Zhang, Y. Molecular Dynamics Investigation of Grain Boundaries and Surfaces in U3Si2. J. Nucl. Mater. 2019, 514, 290– 298,  DOI: 10.1016/j.jnucmat.2018.12.008
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    9
    Molecular dynamics investigation of grain boundaries and surfaces in U3Si2
    Beeler, Benjamin; Baskes, Michael; Andersson, David; Cooper, Michael WD.; Zhang, Yongfeng
    Journal of Nuclear Materials (2019), 514 (), 290-298CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy benefits from higher thermal cond. and higher fissile d. compared to uranium dioxide (UO2). In order to perform engineering scale nuclear fuel performance simulations, the material properties of the fuel must be known. Currently, the exptl. data available for U-Si fuels is rather limited. Thus, multi-scale modeling efforts are underway to address this gap in knowledge. Interfaces play a crit. role in the microstructural evolution of nuclear fuel under irradn., acting both as sinks for point defects and as preferential nucleation sites for fission gas bubbles. In this study, a semi-empirical modified Embedded-Atom Method (MEAM) potential is utilized to investigate grain boundaries and free surfaces in U3Si2. The interfacial energy as a function of temp. is investigated for ten sym. tilt grain boundaries, eight unique free surfaces and voids of radius up to 35 Å. The point defect segregation energy for both U and Si interstitials and vacancies is also detd. for two grain boundary orientations. Finally, the entropy change and free energy change for grain boundaries is calcd. as a function of temp. This is the first study into grain boundary properties of U-Si nuclear fuel.
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  10. 10
    Middleburgh, S. C.; Grimes, R. W.; Lahoda, E. J.; Stanek, C. R.; Andersson, D. A. Non-Stoichiometry in U3Si2. J. Nucl. Mater. 2016, 482, 300– 305,  DOI: 10.1016/j.jnucmat.2016.10.016
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    10
    Non-stoichiometry in U3Si2
    Middleburgh, S. C.; Grimes, R. W.; Lahoda, E. J.; Stanek, C. R.; Andersson, D. A.
    Journal of Nuclear Materials (2016), 482 (), 300-305CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    Uranium silicides, in particular U3Si2, are being explored as an advanced nuclear fuel with increased accident tolerance as well as competitive economics compared to the baseline UO2 fuel. Here we use d. functional theory calcns. and thermochem. anal. to assess the stability of U3Si2 with respect to non-stoichiometry reactions in both the hypo- and hyper-stoichiometric regimes. We find that the degree of non-stoichiometry in U3Si2 is much smaller than in UO2 and at most reaches a few percent at high temp. Non-stoichiometry impacts fuel performance by detg. whether the loss of uranium due to fission leads to a non-stoichiometric U3Si2±x phase or pptn. of a second U-Si phase. We also investigate the U5Si4 phase as a candidate for the equil. phase diagram.
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  11. 11
    Remschnig, K.; Le Bihan, T.; Noël, H.; Rogl, P. Structural Chemistry and Magnetic Behavior of Binary Uranium Silicides. J. Solid State Chem. 1992, 97, 391– 399,  DOI: 10.1016/0022-4596(92)90048-Z
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    Structural chemistry and magnetic behavior of binary uranium silicides
    Remschnig, K.; Le Bihan, T.; Noeel, H.; Rogl, P.
    Journal of Solid State Chemistry (1992), 97 (2), 391-9CODEN: JSSCBI; ISSN:0022-4596.
    Binary uranium silicides have been thoroughly reinvestigated with respect to crystal chem. and magnetic properties; for most compds., magnetic studies have been carried out at low temps. for the first time (2 to 300 K, 0 to 5 T). Formation of all known binary silicides has been confirmed: U3Si, U3Si2 (U3Si2-type), USi (USi-type), U3Si5 (defect AlB2-type), USi1.88 (defect ThSi2-type) and USi3 (Cu3Au-type). At the compn. U3Si∼5 three different phases have been obsd.: the defect AlB2-type as well as a phase sepn. into two orthorhombically distorted AlB2-type related phases. USi2-x with the tetragonal defect ThSi2-type structure as its silicon poor phase boundary was found to be in equil. with USi2-x of the orthorhombic defect GdSi2 type. Precise U-U distances have been derived from x-ray single crystal counter data for U3Si2, for the AlB2-type subcell of U3Si5, and for USi1.84 (defect ThSi2-type). From susceptibility measurements, a band type paramagnetism was obsd. for U3Si2, whereas a temp. independent paramagnetism was confirmed for USi3. Curie-Weiss paramagnetism was encountered within the investigated temp. range for U3Si5 and USi1.88. A modified Curie-Weiss law was revealed for USi with the USi-type, whereas ferromagnetic ordering at Tc = 125 K was obsd. for the oxygen stabilized "USi" with the FeB-type. Magnetism of the uranium silicides is discussed as a function of the distant dependent 5f-5f electron overlap and d-f electron hybridization. No supercond. was obsd. above 2 K.
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  12. 12
    Bo, T.; Lan, J.; Zhao, Y.; Zhang, Y.; He, C.; Chai, Z.; Shi, W. Surface Science Surface Properties of NpO2 and Water Reacting with Stoichiometric and Atomistic Thermodynamics. Surf. Sci. 2016, 644, 153– 164,  DOI: 10.1016/j.susc.2015.09.016
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    12
    Surface properties of NpO2 and water reacting with stoichiometric and reduced NpO2 (111), (110), and (100) surfaces from ab initio atomistic thermodynamics
    Bo, Tao; Lan, Jian-Hui; Zhao, Yao-Lin; Zhang, Yu-Juan; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
    Surface Science (2016), 644 (), 153-164CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    The low-index (111), (110), and (100) surfaces of NpO2, as well as the adsorption and dissocn. of water on these surfaces, have been studied using DFT + U. The calcd. surface energies for the (111), (110), and (100) surfaces are 0.81, 1.14, and 1.67 J m- 2, resp. Based on the calcns., the presence of surface oxygen vacancy enhances the dissocn. adsorption of water mol. There was no significant coverage dependence for mol. adsorption of water on the NpO2 (111), (110), and (100) surfaces. The av. adsorption energy of water in dissociative states increased slightly from 1/4 to 2/4 ML and rose significantly with the coverage increase to 1 ML on these surfaces. Water adsorption on the NpO2 surfaces was analyzed for different temps. and H2O partial pressures, and the authors plotted the pressure-temp. phase diagrams by using the "ab initio atomistic thermodn." approach.
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  13. 13
    Bo, T.; Lan, J.-H.; Zhao, Y.-L.; Zhang, Y.-J.; He, C.-H.; Chai, Z.-F.; Shi, W.-Q. Surface Properties of NpO2 and Water Reacting with Stoichiometric and Reduced NpO2 (111), (110), and (100) Surfaces from Ab Initio Atomistic Thermodynamics. Surf. Sci. 2016, 644, 153– 164,  DOI: 10.1016/j.susc.2015.09.016
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    13
    Surface properties of NpO2 and water reacting with stoichiometric and reduced NpO2 (111), (110), and (100) surfaces from ab initio atomistic thermodynamics
    Bo, Tao; Lan, Jian-Hui; Zhao, Yao-Lin; Zhang, Yu-Juan; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
    Surface Science (2016), 644 (), 153-164CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    The low-index (111), (110), and (100) surfaces of NpO2, as well as the adsorption and dissocn. of water on these surfaces, have been studied using DFT + U. The calcd. surface energies for the (111), (110), and (100) surfaces are 0.81, 1.14, and 1.67 J m- 2, resp. Based on the calcns., the presence of surface oxygen vacancy enhances the dissocn. adsorption of water mol. There was no significant coverage dependence for mol. adsorption of water on the NpO2 (111), (110), and (100) surfaces. The av. adsorption energy of water in dissociative states increased slightly from 1/4 to 2/4 ML and rose significantly with the coverage increase to 1 ML on these surfaces. Water adsorption on the NpO2 surfaces was analyzed for different temps. and H2O partial pressures, and the authors plotted the pressure-temp. phase diagrams by using the "ab initio atomistic thermodn." approach.
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  14. 14
    Jossou, E.; Eduok, U.; Dzade, N. Y.; Szpunar, B.; Szpunar, J. A. Oxidation Behaviour of U3Si2: An Experimental and First Principles Investigation. Phys. Chem. Chem. Phys. 2018, 20, 4708– 4720,  DOI: 10.1039/C7CP07154J
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    Oxidation behaviour of U3Si2: an experimental and first principles investigation
    Jossou, Ericmoore; Eduok, Ubong; Dzade, Nelson Y.; Szpunar, Barbara; Szpunar, Jerzy A.
    Physical Chemistry Chemical Physics (2018), 20 (7), 4708-4720CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    U-contg. metallic systems such as U3Si2 are potential Accident Tolerant Fuels (ATFs) for Light water Reactors (LWRs) and the next generation of nuclear reactors. Their oxidn. behavior, esp. in O and H2O-enriched environments, plays a crit. role in detg. their applicability in com. reactors. The authors have studied the oxidn. behavior of U3Si2 exptl. and by theor. computation. The appearance of oxide signatures was established from XRD and Raman spectroscopic techniques after oxidn. of the solid U3Si2 sample in synthetic air (O and N). The authors have also studied the changes in the electronic structure as well as the energetics of O interactions on the U3Si2 surfaces using 1st principles calcns. in the D. Functional Theory (DFT) formalism. The detailed charge transfer and bond length analyses revealed the preferential formation of mixed oxides of UO2 and SiO2 on the U3Si2{001} surface as well as UO2 alone on the U3Si2{110} and {111} surfaces. The formation of the peroxo (O22-) state confirmed the dissocn. of mol. O before U3Si2 oxidn. Core exptl. analyses of the oxidized U3Si2 samples revealed the formation of higher oxides from Raman spectroscopy and XRD techniques. This work is introduced to further a better understanding of the oxidn. of U-Si metallic fuel compds.
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  15. 15
    Molinari, M.; Parker, S. C.; Sayle, D. C.; Islam, M. S. Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria. J. Phys. Chem. C 2012, 116, 7073– 7083,  DOI: 10.1021/jp300576b
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    15
    Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria
    Molinari, Marco; Parker, Stephen C.; Sayle, Dean C.; Islam, M. Saiful
    Journal of Physical Chemistry C (2012), 116 (12), 7073-7082CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The influence of H2O on the redox properties of ceria is pivotal to its widespread exploitation spanning a variety of applications. Ab initio simulation techniques based on DFT-GGA+U are used to study the H2O-ceria system including associative (H2O) and dissociative (-OH) adsorption/desorption of H2O and the formation of oxygen vacancies in the presence of H2O vapor on the stoichiometric and reduced low index surfaces of ceria at different H2O coverages. Calcns. address the controversy concerning the adsorption of H2O on the CeO2{111}, and new results are reported for the CeO2{110} and {100} surfaces. The simulations reveal strong H2O coverage dependence for dissociatively (-OH) adsorbed H2O on stoichiometric surfaces which becomes progressively destabilized at high coverage, while associative (H2O) adsorption depends weakly on the coverage due to weaker interactions between the adsorbed mols. Anal. of the adsorption geometries suggests that the surface Ce atom coordination controls the strong adhesion of H2O as the av. distance Ce-OW is always 10% greater than the Ce-O distance in the bulk, while the hydrogen bonding network dictates the orientation of the mols. The adsorption energy is predicted to increase on reduced surfaces because oxygen vacancies act as active sites for H2O dissocn. Crucially, by calcg. the heat of redn. of dry and wet surfaces, also H2O promotes further redn. of ceria surfaces and is therefore central to its redox chem. Finally, these simulation approaches can be used to evaluate H2O desorption as a function of temp. and pressure which accords well with exptl. data for CeO2{111}. The authors predict desorption temps. (TD) for CeO2{110} and CeO2{100} surfaces, where exptl. data are not yet available. Such an understanding will help expt. interpret the complex surface/interface redox processes of ceria, which will, inevitably, include H2O.
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  16. 16
    Bo, T.; Lan, J.-H.; Wang, C.-Z.; Zhao, Y.-L.; He, C.-H.; Zhang, Y.-J.; Chai, Z.-F.; Shi, W.-Q. First-Principles Study of Water Reaction and H2 Formation on UO2 (111) and (110) Single Crystal Surfaces. J. Phys. Chem. C 2014, 118, 21935– 21944,  DOI: 10.1021/jp503614f
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    First-Principles Study of Water Reaction and H2 Formation on UO2 (111) and (110) Single Crystal Surfaces
    Bo, Tao; Lan, Jian-Hui; Wang, Cong-Zhi; Zhao, Yao-Lin; He, Chao-Hui; Zhang, Yu-Juan; Chai, Zhi-Fang; Shi, Wei-Qun
    Journal of Physical Chemistry C (2014), 118 (38), 21935-21944CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Mol. and dissociative adsorption behavior of H2O along with the accompanying H2 formation mechanism on the UO2 (111) and (110) surfaces have been investigated by using DFT + U calcns. According to our calcns., the higher stability of the (111) surface leads to higher oxygen vacancy formation energy compared to the (110) surface. On the stoichiometric (111) and (110) surfaces, the first hydrogen atom of water mol. can dissoc. readily with very small or no energy barrier. On the contrary, dissocn. of the second one becomes the rate-detg. step, and water-catalysis leads to the decrease of energy barrier from 0.92 to 0.70 eV and from 2.36 to 1.21 eV on the stoichiometric (111) and (110) surfaces, resp. H2 formation resulting from water dissocn. may undergo two pathways in the presence of surface oxygen vacancy on the reduced UO2 (111) surface. One is characterized by direct combination of two hydrogen atoms of one water mol., and the other is characterized by dissocn. of the first hydrogen atom and its combination with a neighboring surface hydrogen atom. The above two formation pathways possess the energy barriers of 0.56 and 0.53 eV, corresponding to the large reaction energies of -2.62 and -2.64 eV, resp.
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    Fan, J.; Li, C.; Zhao, J.; Shan, Y.; Xu, H. The Enhancement of Surface Reactivity on CeO2 (111) Mediated by Subsurface Oxygen Vacancies. J. Phys. Chem. C 2016, 120, 27917– 27924,  DOI: 10.1021/acs.jpcc.6b07650
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    The Enhancement of Surface Reactivity on CeO2 (111) Mediated by Subsurface Oxygen Vacancies
    Fan, Jing; Li, Chengyang; Zhao, Jinzhu; Shan, Yueyue; Xu, Hu
    Journal of Physical Chemistry C (2016), 120 (49), 27917-27924CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Surface reactivity on metal oxide surfaces and its enhancement play important roles in heterogeneous catalytic reactions. The interactions of O2 and H2O with reduced CeO2 (111) surface are studied by d.-functional theory calcns. The corresponding adsorption geometries, adsorption energies, and reaction barriers are reported. It is found that the diffusion of subsurface oxygen vacancies toward surface can be promoted by the adsorption of O2 on the CeO2 (111) surface. Then those oxygen vacancies diffused onto surface sites will be healed by the adsorbed O2, leaving behind an O adatom on the surface. At moderate temps., the surface O adatom will swap positions with surface lattice O dynamically. The adsorption of H2O may also induce the diffusion of oxygen vacancies from subsurface to surface, leading to the formation of two hydroxyls on the CeO2 (111) surface. The interaction between the paired hydroxyl groups and O2 will result in the formation of water and oxygen adatom on the surface. These results revealed important roles played by the subsurface oxygen vacancies in the enhancement of surface reactivity, esp. when involving the adsorption of water and oxygen.
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    Wellington, J. P. W.; Tegner, B. E.; Collard, J.; Kerridge, A.; Kaltsoyannis, N. Oxygen Vacancy Formation and Water Adsorption on Reduced AnO2{111}, {110}, and {100} Surfaces (An = U, Pu): A Computational Study. J. Phys. Chem. C 2018, 122, 7149– 7165,  DOI: 10.1021/acs.jpcc.7b11512
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    Oxygen Vacancy Formation and Water Adsorption on Reduced AnO2{111}, {110}, and {100} Surfaces (An = U, Pu): A Computational Study
    Wellington, Joseph P. W.; Tegner, Bengt E.; Collard, Jonathan; Kerridge, Andrew; Kaltsoyannis, Nikolas
    Journal of Physical Chemistry C (2018), 122 (13), 7149-7165CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The substoichiometric {111}, {110}, and {100} surfaces of UO2 and PuO2 are studied computationally using two distinct yet related approaches based on d. functional theory (DFT): the periodic electrostatic embedded cluster method and Hubbard-cor. periodic boundary condition DFT. The first and the second layer oxygen vacancy formation energies and geometries are presented and discussed; the energies are found to be substantially larger for UO2 vs. PuO2, a result that traced to the substantially more pos. An(IV)/An(III) redn. potential for Pu and hence relative ease of Pu(III) formation. For {110} and {100} surfaces, the significantly more stable dissociative water adsorption seen previously for stoichiometric surfaces is also found for the defect surfaces. By contrast, the vacancy creation substantially changes the most stable mode of water adsorption on the {111} surface, such that the almost degenerate mol. and dissociative adsorptions on the pristine surface are replaced by a strong preference for dissociative adsorption on the substoichiometric surface. The implications of this result for the formation of H2 are discussed. The generally very good agreement between the data from the embedded cluster and periodic DFT approaches provides addnl. confidence in the reliability of the results and conclusions.
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    QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
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    QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
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    More accurate generalized gradient approximation for solids
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    We present a nonempirical d. functional generalized gradient approxn. (GGA) that gives significant improvements for lattice consts., crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA. The functional is based on a diffuse radial cutoff for the exchange hole in real space, and the analytic gradient expansion of the exchange energy for small gradients. There are no adjustable parameters, the constraining conditions of PBE are maintained, and the functional is easily implemented in existing codes.
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    Evidence is presented that within the d.-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge d. for strongly interacting electron systems. Using a basis-set independent generalization of the LDA + U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator KCuF3.
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    A sampling method is given for Brillouin-zone integration in metals which converges exponentially with the no. of sampling points, without the loss of precision of normal broadening techniques. The scheme is based on smooth approximants to the δ and step functions which are constructed to give the exact result when integrating polynomials of a prescribed degree. In applications to the simple-cubic tight-binding band as well as to band structures of simple and transition metals, significant improvement over existing methods was shown. The method promises general applicability in the fields of total-energy calcns. and many-body physics.
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    Most quantum-chem. calcns. for geometries evaluate first derivs. of the energy with respect to nuclear positions anal. and then use update procedures to build up information on the second derivs. as they step along the potential energy surface toward a min. (stable geometry) or simple saddle point (transition state). The use is described of the Broyden-Fletcher-Goldfarb-Shanno (BFGS) quasi-Newton update used in conjunction with a partial line search. The BFGS is superior to the other update formulas examd. In particular, it is superior to the Murtagh-Sargent (MS) scheme that is commonly used in geometry detns. The advantage of the BFGS update over the MS scheme becomes esp. dramatic for large mol. systems.
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    Momma, K.; Izumi, F. VESTA: A Three-Dimensional Visualization System for Electronic and Structural Analysis. J. Appl. Crystallogr. 2008, 41, 653– 658,  DOI: 10.1107/S0021889808012016
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    Journal of Applied Crystallography (2008), 41 (3), 653-658CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
    A cross-platform program, VESTA, has been developed to visualize both structural and volumetric data in multiple windows with tabs. VESTA represents crystal structures by ball-and-stick, space-filling, polyhedral, wire frame, stick, dot-surface and thermal-ellipsoid models. A variety of crystal-chem. information is extractable from fractional coordinates, occupancies and oxidn. states of sites. Volumetric data such as electron and nuclear densities, Patterson functions, and wavefunctions are displayed as isosurfaces, bird's-eye views and two-dimensional maps. Isosurfaces can be colored according to other phys. quantities. Translucent isosurfaces and/or slices can be overlapped with a structural model. Collaboration with external programs enables the user to locate bonds and bond angles in the 'graphics area', simulate powder diffraction patterns, and calc. site potentials and Madelung energies. Electron densities detd. exptl. are convertible into their Laplacians and electronic energy densities.
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    Momma, K.; Izumi, F. VESTA3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272– 1276,  DOI: 10.1107/S0021889811038970
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    VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data
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    VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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    A new simulation code for modeling extended defects e.g. linear (dislocations) and planar (surfaces and grain boundaries) at the atomistic level is introduced. One of the key components is the ability to calc. the Coulombic potential of a solid with 1-dimensional periodicity. This approach was applied to screw dislocations in MgO and the structure (including core size) and stability of the 〈100〉 and 1/2〈110〉 screw dislocations were evaluated. The 1/2〈110〉 dislocation, which has the shortest Burgers vector, is more stable, as predicted by elasticity theory, although the simulations show that elasticity theory underests. the energy difference. By using this new computer simulation code METADISE, following the approach of Tasker, the structure and energetics of surfaces and interfaces can be calcd. This method was applied to modeling microfaceting, and microfaceted {110} and {111} surfaces of MgO are the most stable forms of these surfaces. The formation energy of tilt grain boundaries in MgO ({h10} and {h20}) as a function of misorientation angle was also studied and for the {h10} series the formation energy is proportional to the interfacial bond d. while no such pattern can be found for the {h20} series.
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    Using first-principles methods, we have investigated the microscopic mechanism for He trapping in two kinds of vacancy defects (monovacancy and 9-atom void) inside vanadium host lattice. In the monovacancy, single He prefers to occupy the octahedral site near vacancy rather than vacancy center. Inside vacancy defects, the He-He equil. distances range in 1.6-2.2 Å. After more He atoms are incorporated, the magnitude of trapping energy decreases and the host lattice expand dramatically. A monovacancy and 9-atom void can host up to 18 and 66 He atoms, resp., with internal pressure up to 7.5 and 19.3 GPa. The at. structures of selected He clusters trapped in vacancies are compared with the gas-phase clusters. The strong tendency of He trapping at vacancies and 9-atom voids provides an explanation for exptl. obsd. He bubble formation at vacancy defects in metals.
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    Dzade, N. Y.; Roldan, A.; de Leeuw, N. H. DFT-D2 Study of the Adsorption and Dissociation of Water on Clean and Oxygen-Covered {001} and {011} Surfaces of Mackinawite (FeS). J. Phys. Chem. C 2016, 120, 21441– 21450,  DOI: 10.1021/acs.jpcc.6b06122
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    DFT-D2 Study of the Adsorption and Dissociation of Water on Clean and Oxygen-Covered {001} and {011} Surfaces of Mackinawite (FeS)
    Dzade, N. Y.; Roldan, A.; de Leeuw, N. H.
    Journal of Physical Chemistry C (2016), 120 (38), 21441-21450CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    A dispersion-cor. d. functional theory study of the adsorption and dissocn. reactions of oxygen and water on the {001} and {011} surfaces of mackinawite (FeS) is presented. A chem. picture of the initial steps of the mackinawite {001} and {011} surfaces oxidn. process in the presence of oxygen and water is presented in the present investigation. Our results show that, while water interacts weakly with the Fe ions on both surfaces and only oxidizes them to some extent, at. and mol. oxygen interact strongly with the FeS{011} surface cations by drawing significant charge from them, thereby oxidizing them from Fe2+ to Fe3+ formal oxidn. state. The authors show from the calcd. adsorption energies and activation energy barriers for the dissocn. of H2O on the clean and oxygen-covered FeS surfaces, that preadsorbed oxygen could easily activate the O-H bond and facilitate the dissocn. of H2O to ferric-hydroxy, Fe3+-OH- on FeS{011}, and to zerovalent sulfur-hydroxyl, S0-OH- on FeS{001}. With the aid of preadsorbed O atom, the activation energy barrier for dissocg. hydrogen atom from H2O decreases from 1.73 to 1.19 eV on the FeS{001}, and from 0.83 to 0.14 eV on the FeS{011}. These findings provide mol.-level insight into the mechanisms of mackinawite oxidn., and are consistent with exptl. results, which have shown that oxygen and water are necessary for the oxidn. process of mackinawite and its possible transformation to pyrite via greigite.
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    Bo, T.; Lan, J.; Zhang, Y.; Zhao, Y.; He, C.; Chai, Z.; Shi, W. Surface of Uranium Mononitride: Energetics And. Phys. Chem. Chem. Phys. 2016, 18, 13255– 13266,  DOI: 10.1039/C6CP01175F
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    Adsorption and dissociation of H2O on the (001) surface of uranium mononitride: energetics and mechanism from first-principles investigation
    Bo, Tao; Lan, Jian-Hui; Zhang, Yu-Juan; Zhao, Yao-Lin; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
    Physical Chemistry Chemical Physics (2016), 18 (19), 13255-13266CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The microscopic adsorption behaviors of water on the UN(001) surface as well as water dissocn. and accompanying H2 formation mechanisms have been investigated on the basis of DFT + U calcns. and ab initio atomistic thermodn. For adsorption of one H2O monomer, the predicted adsorption energies are -0.88, -2.07, and -2.07 eV for the most stable mol., partially dissociative, and completely dissociative adsorption, resp. According to calcns., a water mol. dissocs. into OH and H species via three pathways with small energy barriers of 0.78, 0.72, and 0.85 eV, resp. With the aid of the neighboring H atom, H2 formation through the reaction of H* + OH* can easily occur via two pathways with energy barriers of 0.61 and 0.36 eV, resp. The mol. adsorption of water shows a slight coverage dependence on the surface while this dependence becomes obvious for partially dissociative adsorption as the water coverage increases from 1/4 to 1 ML. In addn., based on the "ab initio atomistic thermodn." simulations, increasing H2O partial pressure will enhance the stability of the adsorbed system and water coverage, while increasing temp. will decrease the H2O coverage. The UN(001) surface reacts easily with H2O at room temp., leading to dissoln. and corrosion of the UN fuel materials.
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    Bo, T.; Lan, J.; Zhao, Y.; Zhang, Y.; He, C.; Chai, Z.; Shi, W. First-Principles Study of Water Adsorption and Dissociation on the UO2. J. Nucl. Mater. 2014, 454, 446– 454,  DOI: 10.1016/j.jnucmat.2014.09.001
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    First-principles study of water adsorption and dissociation on the UO2 (1 1 1), (1 1 0) and (1 0 0) surfaces
    Bo, Tao; Lan, Jian-Hui; Zhao, Yao-Lin; Zhang, Yu-Juan; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
    Journal of Nuclear Materials (2014), 454 (1-3), 446-454CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    The adsorption and dissocn. behaviors of water mol. on the UO2 (1 1 1), (1 1 0) and (1 0 0) surfaces were investigated using first-principles methods within the DFT+U framework. For a single water mol. at 1/4 ML coverage, the mol. adsorption exhibits comparable adsorption energies with the dissociative adsorption on the (1 1 1) surface, while it is far less stable than the dissociative adsorption on the (1 1 0) and (1 0 0) surfaces. We find that the adsorbed mol. and dissociative water tend to cluster on low-index UO2 surfaces by forming hydrogen-bond networks. The adsorption stability of water depends on the synergistic effect of hydrogen bonding interaction and steric effect between adsorbates. The mixed adsorption configuration of mol. and dissociative water in 1:1 mol ratio is found to be thermally more stable on the UO2 (1 1 1) and (1 1 0) surfaces.
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    Shapovalov, V.; Truong, T. N. Ab Initio Study of Water Adsorption on α-Al2O3 (0001) Crystal Surface. J. Phys. Chem. B 2000, 104, 9859– 9863,  DOI: 10.1021/jp001399g
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    Ab initio study of water adsorption on α-Al2O3 (0001) crystal surface
    Shapovalov, Vladimir; Truong, Thanh N.
    Journal of Physical Chemistry B (2000), 104 (42), 9859-9863CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Ab Initio embedded cluster calcns. were performed to study water adsorption on Al-terminated (0001) α-Al2O3 surface. We used the surface charge representation of the embedding electrostatic potential (SCREEP) model to give an accurate representation of the Madelung potential at the adsorption site. The geometry of the cluster was optimized to take into account the surface relaxation. Adsorption energies were obtained using the N-layer integrated MO model (ONIOM). In the case of water adsorption it was found that both dissociative and mol. adsorption min. exist, with adsorption energies of -31.57 and -23.40 kcal/mol, resp., in agreement with expt. Bond orders, covalences and full at. valences were analyzed to investigate the changes in the chem. bonding during adsorption. The results provide some insight into the scrambling of water and crystal oxygen atoms during water desorption in isotopic exchange expts.
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    Pang, Z.; Duerrbeck, S.; Kha, C.; Bertel, E.; Somorjai, G. A.; Salmeron, M. Adsorption and Reactions of Water on Oxygen-Precovered Cu(110). J. Phys. Chem. C 2016, 120, 9218– 9222,  DOI: 10.1021/acs.jpcc.6b00769
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    Adsorption and Reactions of Water on Oxygen-Precovered Cu(110)
    Pang, Zongqiang; Duerrbeck, Stefan; Kha, Calvin; Bertel, Erminald; Somorjai, Gabor A.; Salmeron, Miquel
    Journal of Physical Chemistry C (2016), 120 (17), 9218-9222CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Using scanning tunneling microscopy we studied the adsorption and reactions of water on a Cu(110) surface partially covered with oxygen in the O(2 × 1) phase formed by Cu-O chains. The expts. were carried out from low (77 K) to higher temps. to sequentially open reaction channels that require increased activation energy. At 77 K, water adsorbs at the edges and on top of the Cu-O chains. Heating to 155 K caused water mols. to react with the oxygen atoms in the chains to produce OH. These hydroxyl groups form elongated H-bonded structures mixed with excess unreacted water mols. The Cu atoms freed in the reaction form small clusters at the location of the initial Cu-O chains. Heating to 180 K leads to a second reaction that desorbs all excess water (i.e., water not H-bonded with OH), leaving H2O-OH zigzag chains along [110] directions. At the low oxygen coverage studied here (<0.12 ML) the extent of the partial water dissocn. reaction is stoichiometrically detd. by the amt. of preadsorbed oxygen (H2O + O → 2OH). A third annealing to 280 K resulted in desorption of all water and hydroxyl species and the reappearance of Cu-O chains. After heating to 280 K, the oxygen coverage returns to nearly the same value as that of the initially surface.
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    Chen, L.; Lu, J.; Liu, P.; Gao, L.; Liu, Y.; Xiong, F.; Qiu, S.; Qiu, X.; Guo, Y.; Chen, X. Dissociation and Charge Transfer of H2O on Cu(110) Probed in Real Time Using Ion Scattering Spectroscopy. Langmuir 2016, 32, 12047– 12055,  DOI: 10.1021/acs.langmuir.6b03516
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    Dissociation and Charge Transfer of H2O on Cu(110) Probed in Real Time Using Ion Scattering Spectroscopy
    Chen, Lin; Lu, Jianjie; Liu, Pinyang; Gao, Lei; Liu, Yuefeng; Xiong, Feifei; Qiu, Shunli; Qiu, Xiyu; Guo, Yanling; Chen, Ximeng
    Langmuir (2016), 32 (46), 12047-12055CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Water-Cu(110) interaction is of particular importance during the routine use of graphene-based devices. Water adsorption, dissocn., and desorption at elevated temps. were well studied using the time-of-flight ion scattering technique. Water adsorption meets the first-order Langmuir adsorption model at room temp. The variation of the ratio between residual O and H on the surface with temp. was well detd., which profoundly reveals the dynamical process of surface compn. Also, the change in the surface electronic properties was probed by measuring neg.-ion fractions as a function of the annealing temp. for fast ion scattering. Probably charge transfer is a very sensitive method for studying specific electronic processes in real time.
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    Taylor, C. D.; Lookman, T.; Lillard, R. S. Ab Initio Calculations of the Uranium–Hydrogen System: Thermodynamics, Hydrogen Saturation of a -U and Phase-Transformation to UH3. Acta Mater. 2010, 58, 1045– 1055,  DOI: 10.1016/j.actamat.2009.10.021
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    Ab initio calculations of the uranium-hydrogen system: Thermodynamics, hydrogen saturation of α-U and phase-transformation to UH3
    Taylor, Christopher D.; Lookman, Turab; Lillard, R. Scott
    Acta Materialia (2009), 58 (3), 1045-1055CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
    Total energy calcns. based on d. functional theory (DFT) have been performed for various uranium-hydrogen configurations relevant to the uranium hydriding reaction. Transformation of the supersatd. α-U lattice to the α-UH3 lattice, where α-UH3 is believed to be a precursor to the formation of β-UH3, the stable phase of UH3, was investigated. The total energy DFT calcns. for α- and β-UH3 were validated by comparing the predicted and measured decompn. temps. of the hydride at std. pressure. Calcd. energies also confirm the metastability of α-UH3 vs. β-UH3. Computational group theory and DFT calcns. elucidate this transition, and indicate that the transformation itself is kinetically facile. Therefore it is proposed that the formation of the vol.-expanded H-satd. α-U phase is the primary kinetic barrier to hydride formation.
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    Bo, T.; Lan, J.-H.; Zhang, Y.-J.; Zhao, Y.-L.; He, C.-H.; Chai, Z.-F.; Shi, W.-Q. Adsorption and Dissociation of H2O on the (001) Surface of Uranium Mononitride: Energetics and Mechanism from First-Principles Investigation. Phys. Chem. Chem. Phys. 2016, 18, 13255– 13266,  DOI: 10.1039/C6CP01175F
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    Adsorption and dissociation of H2O on the (001) surface of uranium mononitride: energetics and mechanism from first-principles investigation
    Bo, Tao; Lan, Jian-Hui; Zhang, Yu-Juan; Zhao, Yao-Lin; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
    Physical Chemistry Chemical Physics (2016), 18 (19), 13255-13266CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The microscopic adsorption behaviors of water on the UN(001) surface as well as water dissocn. and accompanying H2 formation mechanisms have been investigated on the basis of DFT + U calcns. and ab initio atomistic thermodn. For adsorption of one H2O monomer, the predicted adsorption energies are -0.88, -2.07, and -2.07 eV for the most stable mol., partially dissociative, and completely dissociative adsorption, resp. According to calcns., a water mol. dissocs. into OH and H species via three pathways with small energy barriers of 0.78, 0.72, and 0.85 eV, resp. With the aid of the neighboring H atom, H2 formation through the reaction of H* + OH* can easily occur via two pathways with energy barriers of 0.61 and 0.36 eV, resp. The mol. adsorption of water shows a slight coverage dependence on the surface while this dependence becomes obvious for partially dissociative adsorption as the water coverage increases from 1/4 to 1 ML. In addn., based on the "ab initio atomistic thermodn." simulations, increasing H2O partial pressure will enhance the stability of the adsorbed system and water coverage, while increasing temp. will decrease the H2O coverage. The UN(001) surface reacts easily with H2O at room temp., leading to dissoln. and corrosion of the UN fuel materials.
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    Tian, X.; Wang, H.; Xiao, H.; Gao, T. Adsorption of Water on UO2 (111) Surface: Density Functional Theory Calculations. Comput. Mater. Sci. 2014, 91, 364– 371,  DOI: 10.1016/j.commatsci.2014.05.009
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    Adsorption of water on UO2 (1 1 1) surface: Density functional theory calculations
    Tian, Xiao-feng; Wang, Hui; Xiao, Hong-xing; Gao, Tao
    Computational Materials Science (2014), 91 (), 364-371CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
    The interaction between the surface of UO2 and mol. water is a serious concern in the range of nuclear waste management. We present a first-principle investigation of the interaction between water and UO2 (1 1 1) surface based on d. functional (DFT) approach. The approxns. of GGA and GGA + U were employed with the projector-augmented-wave method. Both stoichiometric and reduced surfaces were considered in our simulations. We study the at. structures and adsorption energies of various configurations of water adsorption on UO2 (1 1 1) with water coverage of 0.25 ML. The mechanism of the interaction between water (mol. water and dissocd. water) and the surface is discussed in detail. Comparing the adsorption energies for various configurations, both our GGA and GGA + U calcns. suggested that mol. adsorption is more favored than dissociative adsorption of water on defect-free surface, while oxygen vacancy on the surface could make the adsorption of dissocd. water more favorable. Our calcd. results are in good agreement with reported exptl. study and help comprehensive understanding of interactions between water and the stable UO2 (1 1 1) surface.
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    Tegner, B. E.; Molinari, M.; Kerridge, A.; Parker, S. C.; Kaltsoyannis, N. Water Adsorption on AnO2 {111}, {110}, and {100} Surfaces (An = U and Pu): A Density Functional Theory + U Study. J. Phys. Chem. C 2017, 121, 1675– 1682,  DOI: 10.1021/acs.jpcc.6b10986
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    Water Adsorption on AnO2 {111}, {110}, and {100} Surfaces (An = U and Pu): A Density Functional Theory + U Study
    Tegner, Bengt E.; Molinari, Marco; Kerridge, Andrew; Parker, Stephen C.; Kaltsoyannis, Nikolas
    Journal of Physical Chemistry C (2017), 121 (3), 1675-1682CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors report the results of plane-wave d. functional theory calcns. of the interaction of water with the {111}, {110} and {100} surfaces of UO2 and PuO2, using a Hubbard-cor. potential (PBE + U) approach to account for the strongly-correlated 5f electrons. A mix of mol. and dissociative water adsorption is found to be most stable on the {111} surface, whereas the fully dissociative water adsorption is most stable on the {110} and {100} surfaces, leading to a fully hydroxylated monolayer. From these results the authors derive water desorption temps. at various pressures for the different surfaces. These increase in the order {111} < {110} < {100}, and these data are used to propose an alternative interpretation for the two exptl. detd. temp. ranges for water desorption from PuO2.
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    Schoenes, J. Optical Properties and Electronic Structure of UO2. J. Appl. Phys. 1978, 49, 1463– 1465,  DOI: 10.1063/1.324978
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    Optical properties and electronic structure of uranium dioxide
    Schoenes, J.
    Journal of Applied Physics (1978), 49 (3, Pt. 2), 1463-5CODEN: JAPIAU; ISSN:0021-8979.
    The near normal incidence reflectivity of UO2 single crystals was measured from 0.03 to 13 eV. From the reflectivity spectrum, the complex dielec. function ε(ω) = ε1(ω + iε2(ω) was derived by means of the Kramers-Kronig relation. In addn., the absorption coeff. was detd. from a direct transmission measurement on thin single crystal plates in the weakly absorbing spectral region below the absorption edge. An energy level scheme is proposed which allows a self-consistent assignment of the structure in ε2 with optical transitions between max. in the d. of states. The energy gap found at 2.1 ± 0.1 eV is attributed to a 5f2→5f16deg transition. A crystal field splitting 10 Dq = 2.8 eV is derived for the 6d conduction states. Good agreement is obtained within this model with x-ray photoelectron spectroscopy measurements and a recent mol. cluster approxn. It disagrees with a previous interpretation of reflectivity data.
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    Wang, J.; Ewing, R. C.; Becker, U. Electronic Structure and Stability of Hyperstoichiometric UO2+x under Pressure. Phys. Rev. B 2013, 88, 024109  DOI: 10.1103/PhysRevB.88.024109
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    Electronic structure and stability of hyperstoichiometric UO2+x under pressure
    Wang, Jianwei; Ewing, Rodney C.; Becker, Udo
    Physical Review B: Condensed Matter and Materials Physics (2013), 88 (2), 024109/1-024109/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Electronic-Structure and high-pressure phase transitions of stoichiometric uranium dioxide (UO2) and hyperstoichiometric UO2.03 were investigated using first-principles calcns. D. functional theory calcns. using the generalized gradient approxn. and the projector-augmented wave method with the on-site Coulomb repulsive interactions were applied in order to reasonably calc. the equil. vol., total and partial d. of states, band gap of UO2+x, and energetics of the high-pressure phase transitions. Structure optimizations were completed with and without the Hubbard U-ramping method sep. The U-ramping method was intended to remove the metastable states of the 5f electrons of both stoichiometric and hyperstoichiometric UO2+x. Using the Hubbard U parameters (U = 3.8, J = 0.4), whose values are based on the exptl. band gap width of 2.1 eV as a ref., the calcd. cell parameter for the stoichiometric UO2 with cubic fluorite structure is about 1% greater than the exptl. unit cell parameter; in contrast, it is about 1% smaller than the exptl. value without the on-site Coulomb repulsive interactions. For hyperstoichiometric UO2.03 with the cubic fluorite structure, the interstitial oxygen at the octahedral interstitial site induces new bands at the top of the band gap of the stoichiometric UO2, similar to those of the high-pressure phase with orthorhombic cotunnite structure. The orbitals assocd. with the charge transfers to the interstitial oxygen in hyperstoichiometric UO2.03 are partially delocalized and partially localized in both the cubic fluorite structure and orthorhombic cotunnite structure. The energy required for the incorporation of an interstitial O atom is 0.3 eV higher for the orthorhombic phase than for the cubic phase at ambient pressure and increases to 0.5 eV at 10 GPa. The calcd. transition pressures from the cubic to the orthorhombic structure are 18 and 27 GPa for UO2 and UO2.03, resp. The dramatic increase in the calcd. transition pressure for the hyperstoichiometric UO2 is related to structural incompatibility of the interstitial oxygen in the cotunnite structure (high-pressure phase), which is less in the case of the fluorite structure. These results suggest that exptl. detd. pressure values for the phase transition can be significantly affected by small compositional deviations off the ideal stoichiometric UO2. Comparisons of the results using the U-ramping method and without using the U-ramping method suggest that the electronic metastability of UO2 affects the calcd. total energy and electronic structure and could lead to a different local defect configuration for the hyperstoichiometric orthorhombic phase. However, the metastability has a negligible effect on the calcd. phase transition pressure.
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    Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1– 308,  DOI: 10.1016/S0167-5729(01)00020-6
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    The interaction of water with solid surfaces: fundamental aspects revisited
    Henderson, Michael A.
    Surface Science Reports (2002), 46 (1-8), 1-308CODEN: SSREDI; ISSN:0167-5729. (Elsevier Science B.V.)
    A review. Water is perhaps the most important and most pervasive chem. on our planet. The influence of water permeates virtually all areas of biochem., chem. and phys. importance, and is esp. evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled 'The interaction of water with solid surfaces: fundamental aspects' in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the mol.-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodol. has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochem., photoconversion, mineralogy, adhesion, sensors, atm. chem. and tribol. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chem. and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, exptl. studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.
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    Ammon, C.; Bayer, A.; Steinrück, H.-P.; Held, G. Low-Temperature Partial Dissociation of Water on Cu(110). Chem. Phys. Lett. 2003, 377, 163– 169,  DOI: 10.1016/S0009-2614(03)01127-8
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    Low-temperature partial dissociation of water on Cu(1 1 0)
    Ammon, Ch.; Bayer, A.; Steinruck, H.-P.; Held, G.
    Chemical Physics Letters (2003), 377 (1,2), 163-169CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    The low-temp. reactivity of H2O (D2O) adsorbed on clean and oxygen pre-covered Cu(1 1 0) was studied using high resoln. XPS (HRXPS) and LEED. On the clean surface partial dissocn. to hydroxyl was obsd. already at 95 K. Upon annealing to 220 K hydrogen bonded H2O-hydroxyl chains are formed. Upon further annealing H2O desorbs leaving behind a layer of hydroxyl, most of which desorbs recombinatively eventually. With pre-adsorbed oxygen H2O reacts to hydroxyl lifting the added-row reconstruction even <225 K. Upon annealing this adsorbate layer passes through essentially the same stages as without pre-adsorbed oxygen.
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    Jiang, Z.; Fang, T. Dissociation Mechanism of H2O on Clean and Oxygen-Covered Cu(111) Surfaces: A Theoretical Study. Vaccum 2016, 128, 252– 258,  DOI: 10.1016/j.vacuum.2016.03.030
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    Dissociation mechanism of H2O on clean and oxygen-covered Cu (111) surfaces: A theoretical study
    Jiang, Zhao; Fang, Tao
    Vacuum (2016), 128 (), 252-258CODEN: VACUAV; ISSN:0042-207X. (Elsevier Ltd.)
    Using the first-principles calcns. method based on the d. functional theory, the adsorption and dissocn. of water on clean and oxygen-covered Cu (111) surface have been investigated systematically. According to the theor. calcns. results, it was found that H2O prefers to adsorb on the top site, while OH, O and H are favorable on the fcc site. In addn., the structures of the transition states, the corresponding energy barriers and reaction energies were detd. A feasible mechanism on oxygen-covered Cu (111) surface for complete dissocn. of H2O was also put forward. The results showed that the activation barrier for H2O dissocn. decreases obviously with the aid of the oxygen atom. It was proposed that the presence of oxygen atom on the Cu (111) surface can promote the dissocn. of H2O. Compared with our previous reports on Pd and Au surfaces, It was found that a neg. correlation exists between the promoted effect of pre-adsorbed oxygen atom and the adsorption energies on the corresponding surfaces. The results may be useful for computational design, modification and optimization of Cu-based catalysts.
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    Johnson, K.; Ström, V.; Wallenius, J.; Lopes, D. A. Oxidation of Accident Tolerant Fuel Candidates. J. Nucl. Sci. Technol. 2017, 54, 280– 286,  DOI: 10.1080/00223131.2016.1262297
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    Oxidation of accident tolerant fuel candidates
    Johnson, Kyle; Stroem, Valter; Wallenius, Janne; Lopes, Denise Adorno
    Journal of Nuclear Science and Technology (Abingdon, United Kingdom) (2017), 54 (3), 280-286CODEN: JNSTAX; ISSN:0022-3131. (Taylor & Francis Ltd.)
    In this study, the oxidn. of various accident tolerant fuel candidates produced under different conditions have been evaluated and compared relative to the ref. std. - UO2. The candidates considered in this study were UN, U3Si2, U3Si5, and a composite material composed of UN-U3Si2. With the spark plasma sintering (SPS) method, it was possible to fabricate samples of UN with varying porosity, as well as a high-d. composite of UN-U3Si2 (10%). Using thermogravimetry in air, the oxidn. behaviors of each material and the various microstructures of UN were assessed. These results reveal that it is possible to fabricate UN to very high densities using the SPS method, such that its resistance to oxidn. can be improved compared to U3Si5 and UO2, and compete favorably with the principal ATF candidates, U3Si2, which shows a particularly violent reaction under the conditions of this study, and the UN-U3Si2 (10%) composite.
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    Johnson, K. D.; Wallenius, J.; Jolkkonen, M.; Claisse, A. Spark Plasma Sintering and Porosity Studies of Uranium Nitride. J. Nucl. Mater. 2016, 473, 13– 17,  DOI: 10.1016/j.jnucmat.2016.01.037
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    Spark plasma sintering and porosity studies of uranium nitride
    Johnson, Kyle D.; Wallenius, Janne; Jolkkonen, Mikael; Claisse, Antoine
    Journal of Nuclear Materials (2016), 473 (), 13-17CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    In this study, a no. of samples of UN sintered by the SPS method have been fabricated, and highly pure samples ranging in d. from 68% to 99.8%TD - corresponding to an abs. d. of 14.25 g/cm3 out of a theor. d. of 14.28 g/cm3 - have been fabricated. By careful adjustment of the sintering parameters of temp. and applied pressure, the prodn. of pellets of specific porosity may now be achieved between these ranges. The pore closure behavior of the material has also been documented and compared to previous studies of similar materials, which demonstrates that full pore closure using these methods occurs near 97.5% of relative d.
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    Li, J.; Zhu, S.; Li, H.; Oguzie, E. E.; Li, Y.; Wang, F. Bonding Nature of Monomeric H2O on Pd: Orbital Cooperation and Competition. J. Phys. Chem. C 2009, 1931– 1938,  DOI: 10.1021/jp809595y
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    Bonding Nature of Monomeric H2O on Pd: Orbital Cooperation and Competition
    Li, Jibiao; Zhu, Shenglong; Li, Hong; Oguzie, Emeka. E.; Li, Ying; Wang, Fuhui
    Journal of Physical Chemistry C (2009), 113 (5), 1931-1938CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Using d. functional theory (DFT) calcns., the authors provide a unified picture of the bonding nature of a H2O monomer on Pd: varying surface symmetry from hexagonal (Pd{111}) to square (Pd{100}) to rectangular (Pd{110}) lattices. Theor. evidence shows that 3 distinct ranges of whole d-band energy exhibit cooperative roles in the H2O-Pd bonding mechanisms. Bonding states at low-energy resonances (LERs) are assisted by Pauli repulsion-induced electron rearrangement in nonbonding states around d-band centers (intermediate-energy resonances (IERs)) and by depopulation of partially filled antibonding states at high-energy resonances (HERs). Also, the authors have identified the symmetry-tuned lone-pair competitions. As the degree of surface symmetry is reduced, H2O adsorption consistently enhances nonsigma components of lone pair-d overlaps, which originate from distinct balance between electrostatic attraction and Pauli repulsion. Also, the authors found divergency effects of s-d hybridization within the 1st substrate layer.
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Published July 11, 2019

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  • Abstract

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    Figure 1

    Figure 1. Unit cell of ordered uranium silicide containing two formula units of U3Si2 (10 atoms) (color scheme: U = gray and Si = blue).

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    Figure 2

    Figure 2. Optimized surface geometry of (a) U3Si2{001}, (b) U3Si2{110}, and (c) U3Si2{111} with Si and U1 surface vacancy represented by a rectangular red box in the second and third rows respectively (color scheme: U = gray and Si = blue).

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    Figure 3

    Figure 3. Calculated surface energies of the {001}, {110}, and {111} surfaces of U3Si2 as functions of the change in silicon chemical potential μSi.

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    Figure 4

    Figure 4. Top and side views of the relaxed adsorption structures of molecular H2O adsorbed at: (a) O–U, (b) H–Si, and (c) O–Si (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 5

    Figure 5. Top and side views of the relaxed adsorption structures of dissociated water configuration with (a) OH–Si: H on adj. U, (b) OH–Si: H on Si, and (c) OH–Si: H on U, on the U3Si2{001}-Si terminated surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 6

    Figure 6. Top and side views of the relaxed adsorption structures of molecular oxygen adsorbed at (a) O–Si, (b) O–U, and (c) H–Si on the U3Si2{110} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 7

    Figure 7. Top and side views of the relaxed adsorption structures of dissociated H2O at (a) OH–Si: H on U and (b) OH–U: H on Si on the U3Si2{110} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 8

    Figure 8. Top and side views of the relaxed adsorption structures of molecular H2O adsorbed at (a) the O–U site, (b) the H–Si site, and (c) the O–Si site on the U3Si2{111} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 9

    Figure 9. Top and side views of the relaxed adsorption structures dissociated water configuration with (a) O–U: H and (b) O–Si: H, on the U3Si2{111}-Si terminated surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 10

    Figure 10. Top and side view of the relaxed adsorption structures of mixed (dissociative to molecular) 3:1 adsorption of H2O on (a) {001} and fully dissociative adsorption of H2O on (b) the {110} and (c) {111} U3Si2 surface (color scheme: U = gray, Si = blue, H= white, and O = red).

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    Figure 11

    Figure 11. Top and side views of the relaxed adsorption structures of molecular H2O (a) H–U: O on U, (b) Ow–Si: O on Si, (c) Ow–Si: O on U, and (d) Ow–U: O on Si U3Si2{001} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 12

    Figure 12. Top and side views of the relaxed adsorption structures of molecular H2O (a) Ow–Si: O on the U bridge, (b) Ow–Si: O on U, and (c) Hw–Si: O on U on the U3Si2{110} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 13

    Figure 13. Top and side views of the relaxed adsorption structures of molecular H2O (a) Ow–U: O and (b) Ow–Si: O on the U3Si2{111} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 14

    Figure 14. Top and side views of the relaxed adsorption structures of Si and U vacancy-assisted molecular H2O adsorption (a) H2O–U3Si2–x {001}, (b) H2O–U3Si2–x {110}, (c) H2O–U3Si2–x {111}, (d) H2O–U3–xSi2{001}, (e) H2O–U3–xSi2{110}, and (f) H2O–U3–xSi2{111} surface (color scheme: U = gray, Si = blue, H = white, and O = red).

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    Figure 15

    Figure 15. (Right) PDOS for the interacting surface U f-states before and after the adsorption of H2O at the (a) water–U3Si2{001} and (b) for interacting surface Si p-states at the water–U3Si2{110} interface, and for the interacting surface U f-states before and after the adsorption of H2O at the (c) water–U3Si2{111} interface. (Left) the corresponding optimized water–U3Si2 interfaces with U–O and Si–O bond lengths.

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    Figure 16

    Figure 16. DOS for H2O in the (a) free state and adsorbed in the lowest-energy geometry at the water–U3Si2 interfaces (b–d).

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    Figure 17

    Figure 17. Partial DOS projected on the interacting surface U f-states and O p-states for adsorbed atomic oxygen on the top-U site on (a) {001}, (b) {110}, and (c) {111} surfaces.

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  • References

    This article references 55 other publications.

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      White, J. T.; Nelson, A. T.; Dunwoody, J. T.; Byler, D. D.; Safarik, D. J.; McClellan, K. J. Thermophysical Properties of U3Si2 to 1773 K. J. Nucl. Mater. 2015, 464, 275– 280,  DOI: 10.1016/j.jnucmat.2015.04.031
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      Thermophysical properties of U3Si2 to 1773 K
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      Use of U3Si2 in nuclear reactors requires accurate thermophys. property data to capture heat transfer within the core. Compilation of the limited previous research efforts focused on the most crit. property, thermal cond., reveals extensive disagreement. Assessment of this data is challenged by the fact that the crit. structural and chem. details of the material used to provide historic data is either absent or confirms the presence of significant impurity phases. This study was initiated to fabricate high purity U3Si2 to quantify the coeff. of thermal expansion, heat capacity, thermal diffusivity, and thermal cond. from room temp. to 1773 K. Datasets provided in this manuscript will facilitate more detailed fuel performance modeling to assess both current and proposed reactor designs that incorporate U3Si2.
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      Wood, E. S.; White, J. T.; Grote, C. J.; Nelson, A. T. U3Si2 Behavior in H2O: Part I, Fl Owing Steam and the Effect of Hydrogen. J. Nucl. Mater. 2018, 501, 404– 412,  DOI: 10.1016/j.jnucmat.2018.01.002
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      U3Si2 behavior in H2O: Part I, flowing steam and the effect of hydrogen
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      Recent interest in U3Si2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H2O environments is absent from the literature. The behavior of U3Si2 in H2O contg. atmospheres is investigated and presented in a two-part series of articles aimed to understand the degrdn. mechanism of U3Si2 in H2O. Reported here are thermogravimetric data for U3Si2 exposed to flowing steam at 250-470 °C. Addnl. the response of U3Si2 to flowing Ar-6% H2 from 350 to 400 °C is presented. Microstructural degrdn. is obsd. following hours of exposure at 350 °C in steam. U3Si2 undergoes pulverization on the timescale of minutes when temps. are increased above 400 °C. This mechanism is accelerated in flowing Ar-H2 at the same temps.
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      Nelson, A. T.; Migdisov, A.; Wood, E. S.; Grote, C. J. U3Si2 Behavior in H2O Environments: Part II, Pressurized Water with Controlled Redox Chemistry. J. Nucl. Mater. 2018, 500, 81– 91,  DOI: 10.1016/j.jnucmat.2017.12.026
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      U3Si2 behavior in H2O environments: Part II, pressurized water with controlled redox chemistry
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      Recent interest in U3Si2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H2O environments is sparse in available literature. The behavior of U3Si2 in H2O contg. atmospheres is investigated and presented in a two-part series of articles. This work examines the behavior of U3Si2 following exposure to pressurized H2O at temps. from 300 to 350 °C. Testing was performed using two autoclave configurations and multiple redox conditions. Use of solid state buffers to attain a controlled water chem. is also presented as a means to test actinide-bearing systems. Buffers were used to vary the hydrogen concn. between 1 and 30 parts per million H2. Testing included UN, U3Si5, and UO2. Both UN and U3Si5 were found to rapidly pulverize in less than 50 h at 300 °C. Uranium dioxide was included as a control for the autoclave system, and was found to be minimally impacted by exposure to pressurized water at the conditions tested for extended time periods. Testing of U3Si2 at 300 °C found reasonable stability through 30 days in 1-5 ppm H2. However, pulverization was obsd. following 35 days. The redox condition of testing strongly affected pulverization. Characterization of the resulting microstructures suggests that the mechanism responsible for pulverization under more strongly reducing conditions differs from that previously identified. Hydride formation is hypothesized to drive this transition. Testing performed at 350 °C resulted in rapid pulverization of U3Si2 in under 50 h.
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      Middleburgh, S. C.; Claisse, A.; Andersson, D. A.; Grimes, R. W.; Olsson, P.; Mašková, S. Solution of Hydrogen in Accident Tolerant Fuel Candidate Material: U3Si2. J. Nucl. Mater. 2018, 501, 234– 237,  DOI: 10.1016/j.jnucmat.2018.01.018
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      Solution of hydrogen in accident tolerant fuel candidate material: U3Si2
      Middleburgh, S. C.; Claisse, A.; Andersson, D. A.; Grimes, R. W.; Olsson, P.; Maskova, S.
      Journal of Nuclear Materials (2018), 501 (), 234-237CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      Hydrogen uptake and accommodation into U3Si2, a candidate accident-tolerant fuel system, has been modelled on the at. scale using the d. functional theory. The soln. energy of multiple H atoms is computed, reaching a stoichiometry of U3Si2H2 which has been exptl. obsd. in previous work (reported as U3Si2H1.8). The absorption of hydrogen is found to be favorable up to U3Si2H2 and the assocd. vol. change is computed, closely matching exptl. data. Entropic effects are considered to assess the dissocn. temp. of H2, estd. to be at ∼800K - again in good agreement with the exptl. obsd. transition temp.
      >> More from SciFinder ®
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      Beeler, B.; Baskes, M.; Andersson, D.; Cooper, M. W. D.; Zhang, Y. A modified Embedded-Atom Method interatomic potential for uranium-silicide. J. Nucl. Mater. 2017, 495, 267– 276,  DOI: 10.1016/j.jnucmat.2017.08.025
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      A modified Embedded-Atom Method interatomic potential for uranium-silicide
      Beeler, Benjamin; Baskes, Michael; Andersson, David; Cooper, Michael W. D.; Zhang, Yongfeng
      Journal of Nuclear Materials (2017), 495 (), 267-276CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy fuel benefits from higher thermal cond. and higher fissile d. compared to uranium dioxide (UO2). In order to perform engineering scale nuclear fuel performance simulations, the material properties of the fuel must be known. Currently, the exptl. data available for U-Si fuels is rather limited. Thus, multiscale modeling efforts are underway to address this gap in knowledge. In this study, a semi-empirical modified Embedded-Atom Method (MEAM) potential is presented for the description of the U-Si system. The potential is fitted to the formation energy, defect energies and structural properties of U3Si2. The primary phase of interest (U3Si2) is accurately described over a wide temp. range and displays good behavior under irradn. and with free surfaces. The potential can also describe a variety of U-Si phases across the compn. spectrum.
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      Noordhoek, M. J.; Besmann, T. M.; Andersson, D.; Middleburgh, S. C.; Chernatynskiy, A. Phase Equilibria in the U-Si System from First-Principles Calculations. J. Nucl. Mater. 2016, 479, 216– 223,  DOI: 10.1016/j.jnucmat.2016.07.006
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      D. functional theory calcns. have been used with spin-orbit coupling and on-site Coulomb correction (GGA + U) methods to investigate the U-Si system. Structural prediction methods were employed to identify alternate stable structures. Convex hulls of the U-Si system were constructed for each of the methods to highlight the competing energetics of various phases. For GGA calcns., new structures are predicted to be dynamically stable, but these have not been exptl. obsd. When the GGA + U (Ueff > 1.3 eV) method is considered, the exptl. obsd. structures are predicted to be energetically preferred. Phonon calcns. were used to investigate the energy predictions and showed that the use of the GGA + U method removes the significant imaginary frequencies obsd. for U3Si2 when the correction is not considered. Total and partial electron d. of states calcns. were also performed to understand the role of GGA + U methods and orbitals on the bonding and stability of U-Si compds.
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      Wang, T.; Qiu, N.; Wen, X.; Tian, Y.; He, J.; Luo, K.; Zha, X.; Zhou, Y.; Huang, Q.; Lang, J. First-Principles Investigations on the Electronic Structures of U3Si2. J. Nucl. Mater. 2016, 469, 194– 199,  DOI: 10.1016/j.jnucmat.2015.11.060
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      First-principles investigations on the electronic structures of U3Si2
      Wang, Tong; Qiu, Nianxiang; Wen, Xiaodong; Tian, Yonghui; He, Jian; Luo, Kan; Zha, Xianhu; Zhou, Yuhong; Huang, Qing; Lang, Jiajian; Du, Shiyu
      Journal of Nuclear Materials (2016), 469 (), 194-199CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      U3Si2 has been widely utilized as a high-power uranium fuel for research reactors due to its high d. of uranium. However, theor. investigations on this material are still scarce up to now. For this reason, the computational study via d. functional theory (DFT) is performed on the U3Si2 compd. in this work. The properties of U3Si2, such as stable cryst. structures, d. of states, charge distributions, formation energy of defects, as well as the mech. properties are explored. The calcn. results show that the U3Si2 material is metallic and brittle, which is in good agreement with the previous exptl. observations. The formation energy of uranium vacancy defect is predicted to be the lowest, similar with that of UN. The theor. investigation of this work is expected to provide new insight of uranium silicide fuels.
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    8. 8
      Andersson, D. A.; Liu, X.-Y.; Beeler, B.; Middleburgh, S. C.; Claisse, A.; Stanek, C. R. Density Functional Theory Calculations of Self- and Xe Diffusion in U3Si2. J. Nucl. Mater. 2019, 515, 312– 325,  DOI: 10.1016/j.jnucmat.2018.12.021
      8
      Density functional theory calculations of self- and Xe diffusion in U3Si2
      Andersson, D. A.; Liu, X.-Y.; Beeler, B.; Middleburgh, S. C.; Claisse, A.; Stanek, C. R.
      Journal of Nuclear Materials (2019), 515 (), 312-325CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      Uranium silicide, U3Si2, has been proposed as an advanced nuclear fuel to be used in light water reactors (LWRs). Development of this alternative to the predominant current fuel, UO2, is motivated by enhanced accident tolerance as a result of higher thermal cond. as well as improved fuel cycle economics through increased uranium d. In order to accurately model the fuel performance of U3Si2, the diffusion rate of point defects, which is related to self-diffusion, and of fission gas atoms must be detd. DFT calcns. are used to predict the U and Si point defect concns., the corresponding self-diffusivities, the preferred Xe trap site and the Xe diffusivity. Effects of irradn. are not considered. A low defect formation energy and a high entropy for Si interstitials give rise to Si-rich non-stoichiometry at elevated temps. Both U and Si self-diffusion and Xe diffusion are anisotropic as a consequence of the tetragonal crystal structure of U3Si2. Si diffusion occurs by interstitial mechanisms in both the a-b plane and along the c axis, while the U c axis diffusion rate is controlled by a vacancy mechanism. Interstitial diffusion of U is very fast in the a-b plane of the U3Si2 crystal structure. Xe atoms prefer to occupy U vacancy trap sites. The highest Xe diffusion rate occurs by a vacancy mechanism in both the a-b plane and along the c axis. The diffusion rate is similar in the a-b plane and along the c axis. U and Si self-diffusion and Xe diffusion are all faster in U3Si2 than intrinsic U and Xe diffusion in conventional UO2 nuclear fuel.
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    9. 9
      Beeler, B.; Baskes, M.; Andersson, D.; Cooper, M. W. D.; Zhang, Y. Molecular Dynamics Investigation of Grain Boundaries and Surfaces in U3Si2. J. Nucl. Mater. 2019, 514, 290– 298,  DOI: 10.1016/j.jnucmat.2018.12.008
      9
      Molecular dynamics investigation of grain boundaries and surfaces in U3Si2
      Beeler, Benjamin; Baskes, Michael; Andersson, David; Cooper, Michael WD.; Zhang, Yongfeng
      Journal of Nuclear Materials (2019), 514 (), 290-298CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy benefits from higher thermal cond. and higher fissile d. compared to uranium dioxide (UO2). In order to perform engineering scale nuclear fuel performance simulations, the material properties of the fuel must be known. Currently, the exptl. data available for U-Si fuels is rather limited. Thus, multi-scale modeling efforts are underway to address this gap in knowledge. Interfaces play a crit. role in the microstructural evolution of nuclear fuel under irradn., acting both as sinks for point defects and as preferential nucleation sites for fission gas bubbles. In this study, a semi-empirical modified Embedded-Atom Method (MEAM) potential is utilized to investigate grain boundaries and free surfaces in U3Si2. The interfacial energy as a function of temp. is investigated for ten sym. tilt grain boundaries, eight unique free surfaces and voids of radius up to 35 Å. The point defect segregation energy for both U and Si interstitials and vacancies is also detd. for two grain boundary orientations. Finally, the entropy change and free energy change for grain boundaries is calcd. as a function of temp. This is the first study into grain boundary properties of U-Si nuclear fuel.
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    10. 10
      Middleburgh, S. C.; Grimes, R. W.; Lahoda, E. J.; Stanek, C. R.; Andersson, D. A. Non-Stoichiometry in U3Si2. J. Nucl. Mater. 2016, 482, 300– 305,  DOI: 10.1016/j.jnucmat.2016.10.016
      10
      Non-stoichiometry in U3Si2
      Middleburgh, S. C.; Grimes, R. W.; Lahoda, E. J.; Stanek, C. R.; Andersson, D. A.
      Journal of Nuclear Materials (2016), 482 (), 300-305CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      Uranium silicides, in particular U3Si2, are being explored as an advanced nuclear fuel with increased accident tolerance as well as competitive economics compared to the baseline UO2 fuel. Here we use d. functional theory calcns. and thermochem. anal. to assess the stability of U3Si2 with respect to non-stoichiometry reactions in both the hypo- and hyper-stoichiometric regimes. We find that the degree of non-stoichiometry in U3Si2 is much smaller than in UO2 and at most reaches a few percent at high temp. Non-stoichiometry impacts fuel performance by detg. whether the loss of uranium due to fission leads to a non-stoichiometric U3Si2±x phase or pptn. of a second U-Si phase. We also investigate the U5Si4 phase as a candidate for the equil. phase diagram.
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    11. 11
      Remschnig, K.; Le Bihan, T.; Noël, H.; Rogl, P. Structural Chemistry and Magnetic Behavior of Binary Uranium Silicides. J. Solid State Chem. 1992, 97, 391– 399,  DOI: 10.1016/0022-4596(92)90048-Z
      11
      Structural chemistry and magnetic behavior of binary uranium silicides
      Remschnig, K.; Le Bihan, T.; Noeel, H.; Rogl, P.
      Journal of Solid State Chemistry (1992), 97 (2), 391-9CODEN: JSSCBI; ISSN:0022-4596.
      Binary uranium silicides have been thoroughly reinvestigated with respect to crystal chem. and magnetic properties; for most compds., magnetic studies have been carried out at low temps. for the first time (2 to 300 K, 0 to 5 T). Formation of all known binary silicides has been confirmed: U3Si, U3Si2 (U3Si2-type), USi (USi-type), U3Si5 (defect AlB2-type), USi1.88 (defect ThSi2-type) and USi3 (Cu3Au-type). At the compn. U3Si∼5 three different phases have been obsd.: the defect AlB2-type as well as a phase sepn. into two orthorhombically distorted AlB2-type related phases. USi2-x with the tetragonal defect ThSi2-type structure as its silicon poor phase boundary was found to be in equil. with USi2-x of the orthorhombic defect GdSi2 type. Precise U-U distances have been derived from x-ray single crystal counter data for U3Si2, for the AlB2-type subcell of U3Si5, and for USi1.84 (defect ThSi2-type). From susceptibility measurements, a band type paramagnetism was obsd. for U3Si2, whereas a temp. independent paramagnetism was confirmed for USi3. Curie-Weiss paramagnetism was encountered within the investigated temp. range for U3Si5 and USi1.88. A modified Curie-Weiss law was revealed for USi with the USi-type, whereas ferromagnetic ordering at Tc = 125 K was obsd. for the oxygen stabilized "USi" with the FeB-type. Magnetism of the uranium silicides is discussed as a function of the distant dependent 5f-5f electron overlap and d-f electron hybridization. No supercond. was obsd. above 2 K.
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    12. 12
      Bo, T.; Lan, J.; Zhao, Y.; Zhang, Y.; He, C.; Chai, Z.; Shi, W. Surface Science Surface Properties of NpO2 and Water Reacting with Stoichiometric and Atomistic Thermodynamics. Surf. Sci. 2016, 644, 153– 164,  DOI: 10.1016/j.susc.2015.09.016
      12
      Surface properties of NpO2 and water reacting with stoichiometric and reduced NpO2 (111), (110), and (100) surfaces from ab initio atomistic thermodynamics
      Bo, Tao; Lan, Jian-Hui; Zhao, Yao-Lin; Zhang, Yu-Juan; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
      Surface Science (2016), 644 (), 153-164CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      The low-index (111), (110), and (100) surfaces of NpO2, as well as the adsorption and dissocn. of water on these surfaces, have been studied using DFT + U. The calcd. surface energies for the (111), (110), and (100) surfaces are 0.81, 1.14, and 1.67 J m- 2, resp. Based on the calcns., the presence of surface oxygen vacancy enhances the dissocn. adsorption of water mol. There was no significant coverage dependence for mol. adsorption of water on the NpO2 (111), (110), and (100) surfaces. The av. adsorption energy of water in dissociative states increased slightly from 1/4 to 2/4 ML and rose significantly with the coverage increase to 1 ML on these surfaces. Water adsorption on the NpO2 surfaces was analyzed for different temps. and H2O partial pressures, and the authors plotted the pressure-temp. phase diagrams by using the "ab initio atomistic thermodn." approach.
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    13. 13
      Bo, T.; Lan, J.-H.; Zhao, Y.-L.; Zhang, Y.-J.; He, C.-H.; Chai, Z.-F.; Shi, W.-Q. Surface Properties of NpO2 and Water Reacting with Stoichiometric and Reduced NpO2 (111), (110), and (100) Surfaces from Ab Initio Atomistic Thermodynamics. Surf. Sci. 2016, 644, 153– 164,  DOI: 10.1016/j.susc.2015.09.016
      13
      Surface properties of NpO2 and water reacting with stoichiometric and reduced NpO2 (111), (110), and (100) surfaces from ab initio atomistic thermodynamics
      Bo, Tao; Lan, Jian-Hui; Zhao, Yao-Lin; Zhang, Yu-Juan; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
      Surface Science (2016), 644 (), 153-164CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      The low-index (111), (110), and (100) surfaces of NpO2, as well as the adsorption and dissocn. of water on these surfaces, have been studied using DFT + U. The calcd. surface energies for the (111), (110), and (100) surfaces are 0.81, 1.14, and 1.67 J m- 2, resp. Based on the calcns., the presence of surface oxygen vacancy enhances the dissocn. adsorption of water mol. There was no significant coverage dependence for mol. adsorption of water on the NpO2 (111), (110), and (100) surfaces. The av. adsorption energy of water in dissociative states increased slightly from 1/4 to 2/4 ML and rose significantly with the coverage increase to 1 ML on these surfaces. Water adsorption on the NpO2 surfaces was analyzed for different temps. and H2O partial pressures, and the authors plotted the pressure-temp. phase diagrams by using the "ab initio atomistic thermodn." approach.
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    14. 14
      Jossou, E.; Eduok, U.; Dzade, N. Y.; Szpunar, B.; Szpunar, J. A. Oxidation Behaviour of U3Si2: An Experimental and First Principles Investigation. Phys. Chem. Chem. Phys. 2018, 20, 4708– 4720,  DOI: 10.1039/C7CP07154J
      14
      Oxidation behaviour of U3Si2: an experimental and first principles investigation
      Jossou, Ericmoore; Eduok, Ubong; Dzade, Nelson Y.; Szpunar, Barbara; Szpunar, Jerzy A.
      Physical Chemistry Chemical Physics (2018), 20 (7), 4708-4720CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      U-contg. metallic systems such as U3Si2 are potential Accident Tolerant Fuels (ATFs) for Light water Reactors (LWRs) and the next generation of nuclear reactors. Their oxidn. behavior, esp. in O and H2O-enriched environments, plays a crit. role in detg. their applicability in com. reactors. The authors have studied the oxidn. behavior of U3Si2 exptl. and by theor. computation. The appearance of oxide signatures was established from XRD and Raman spectroscopic techniques after oxidn. of the solid U3Si2 sample in synthetic air (O and N). The authors have also studied the changes in the electronic structure as well as the energetics of O interactions on the U3Si2 surfaces using 1st principles calcns. in the D. Functional Theory (DFT) formalism. The detailed charge transfer and bond length analyses revealed the preferential formation of mixed oxides of UO2 and SiO2 on the U3Si2{001} surface as well as UO2 alone on the U3Si2{110} and {111} surfaces. The formation of the peroxo (O22-) state confirmed the dissocn. of mol. O before U3Si2 oxidn. Core exptl. analyses of the oxidized U3Si2 samples revealed the formation of higher oxides from Raman spectroscopy and XRD techniques. This work is introduced to further a better understanding of the oxidn. of U-Si metallic fuel compds.
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    15. 15
      Molinari, M.; Parker, S. C.; Sayle, D. C.; Islam, M. S. Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria. J. Phys. Chem. C 2012, 116, 7073– 7083,  DOI: 10.1021/jp300576b
      15
      Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria
      Molinari, Marco; Parker, Stephen C.; Sayle, Dean C.; Islam, M. Saiful
      Journal of Physical Chemistry C (2012), 116 (12), 7073-7082CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The influence of H2O on the redox properties of ceria is pivotal to its widespread exploitation spanning a variety of applications. Ab initio simulation techniques based on DFT-GGA+U are used to study the H2O-ceria system including associative (H2O) and dissociative (-OH) adsorption/desorption of H2O and the formation of oxygen vacancies in the presence of H2O vapor on the stoichiometric and reduced low index surfaces of ceria at different H2O coverages. Calcns. address the controversy concerning the adsorption of H2O on the CeO2{111}, and new results are reported for the CeO2{110} and {100} surfaces. The simulations reveal strong H2O coverage dependence for dissociatively (-OH) adsorbed H2O on stoichiometric surfaces which becomes progressively destabilized at high coverage, while associative (H2O) adsorption depends weakly on the coverage due to weaker interactions between the adsorbed mols. Anal. of the adsorption geometries suggests that the surface Ce atom coordination controls the strong adhesion of H2O as the av. distance Ce-OW is always 10% greater than the Ce-O distance in the bulk, while the hydrogen bonding network dictates the orientation of the mols. The adsorption energy is predicted to increase on reduced surfaces because oxygen vacancies act as active sites for H2O dissocn. Crucially, by calcg. the heat of redn. of dry and wet surfaces, also H2O promotes further redn. of ceria surfaces and is therefore central to its redox chem. Finally, these simulation approaches can be used to evaluate H2O desorption as a function of temp. and pressure which accords well with exptl. data for CeO2{111}. The authors predict desorption temps. (TD) for CeO2{110} and CeO2{100} surfaces, where exptl. data are not yet available. Such an understanding will help expt. interpret the complex surface/interface redox processes of ceria, which will, inevitably, include H2O.
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    16. 16
      Bo, T.; Lan, J.-H.; Wang, C.-Z.; Zhao, Y.-L.; He, C.-H.; Zhang, Y.-J.; Chai, Z.-F.; Shi, W.-Q. First-Principles Study of Water Reaction and H2 Formation on UO2 (111) and (110) Single Crystal Surfaces. J. Phys. Chem. C 2014, 118, 21935– 21944,  DOI: 10.1021/jp503614f
      16
      First-Principles Study of Water Reaction and H2 Formation on UO2 (111) and (110) Single Crystal Surfaces
      Bo, Tao; Lan, Jian-Hui; Wang, Cong-Zhi; Zhao, Yao-Lin; He, Chao-Hui; Zhang, Yu-Juan; Chai, Zhi-Fang; Shi, Wei-Qun
      Journal of Physical Chemistry C (2014), 118 (38), 21935-21944CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Mol. and dissociative adsorption behavior of H2O along with the accompanying H2 formation mechanism on the UO2 (111) and (110) surfaces have been investigated by using DFT + U calcns. According to our calcns., the higher stability of the (111) surface leads to higher oxygen vacancy formation energy compared to the (110) surface. On the stoichiometric (111) and (110) surfaces, the first hydrogen atom of water mol. can dissoc. readily with very small or no energy barrier. On the contrary, dissocn. of the second one becomes the rate-detg. step, and water-catalysis leads to the decrease of energy barrier from 0.92 to 0.70 eV and from 2.36 to 1.21 eV on the stoichiometric (111) and (110) surfaces, resp. H2 formation resulting from water dissocn. may undergo two pathways in the presence of surface oxygen vacancy on the reduced UO2 (111) surface. One is characterized by direct combination of two hydrogen atoms of one water mol., and the other is characterized by dissocn. of the first hydrogen atom and its combination with a neighboring surface hydrogen atom. The above two formation pathways possess the energy barriers of 0.56 and 0.53 eV, corresponding to the large reaction energies of -2.62 and -2.64 eV, resp.
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    17. 17
      Fan, J.; Li, C.; Zhao, J.; Shan, Y.; Xu, H. The Enhancement of Surface Reactivity on CeO2 (111) Mediated by Subsurface Oxygen Vacancies. J. Phys. Chem. C 2016, 120, 27917– 27924,  DOI: 10.1021/acs.jpcc.6b07650
      17
      The Enhancement of Surface Reactivity on CeO2 (111) Mediated by Subsurface Oxygen Vacancies
      Fan, Jing; Li, Chengyang; Zhao, Jinzhu; Shan, Yueyue; Xu, Hu
      Journal of Physical Chemistry C (2016), 120 (49), 27917-27924CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Surface reactivity on metal oxide surfaces and its enhancement play important roles in heterogeneous catalytic reactions. The interactions of O2 and H2O with reduced CeO2 (111) surface are studied by d.-functional theory calcns. The corresponding adsorption geometries, adsorption energies, and reaction barriers are reported. It is found that the diffusion of subsurface oxygen vacancies toward surface can be promoted by the adsorption of O2 on the CeO2 (111) surface. Then those oxygen vacancies diffused onto surface sites will be healed by the adsorbed O2, leaving behind an O adatom on the surface. At moderate temps., the surface O adatom will swap positions with surface lattice O dynamically. The adsorption of H2O may also induce the diffusion of oxygen vacancies from subsurface to surface, leading to the formation of two hydroxyls on the CeO2 (111) surface. The interaction between the paired hydroxyl groups and O2 will result in the formation of water and oxygen adatom on the surface. These results revealed important roles played by the subsurface oxygen vacancies in the enhancement of surface reactivity, esp. when involving the adsorption of water and oxygen.
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    18. 18
      Zhukovskii, Y.; Bocharov, D.; Gryaznov, D.; Kotomin, E. First Principles Simulations on Surface Properties and Oxidation of Nitride Nuclear Fuels; Advances in Nuclear Fuel; IntechOpen, 2012.
      There is no corresponding record for this reference.
    19. 19
      Wellington, J. P. W.; Tegner, B. E.; Collard, J.; Kerridge, A.; Kaltsoyannis, N. Oxygen Vacancy Formation and Water Adsorption on Reduced AnO2{111}, {110}, and {100} Surfaces (An = U, Pu): A Computational Study. J. Phys. Chem. C 2018, 122, 7149– 7165,  DOI: 10.1021/acs.jpcc.7b11512
      19
      Oxygen Vacancy Formation and Water Adsorption on Reduced AnO2{111}, {110}, and {100} Surfaces (An = U, Pu): A Computational Study
      Wellington, Joseph P. W.; Tegner, Bengt E.; Collard, Jonathan; Kerridge, Andrew; Kaltsoyannis, Nikolas
      Journal of Physical Chemistry C (2018), 122 (13), 7149-7165CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The substoichiometric {111}, {110}, and {100} surfaces of UO2 and PuO2 are studied computationally using two distinct yet related approaches based on d. functional theory (DFT): the periodic electrostatic embedded cluster method and Hubbard-cor. periodic boundary condition DFT. The first and the second layer oxygen vacancy formation energies and geometries are presented and discussed; the energies are found to be substantially larger for UO2 vs. PuO2, a result that traced to the substantially more pos. An(IV)/An(III) redn. potential for Pu and hence relative ease of Pu(III) formation. For {110} and {100} surfaces, the significantly more stable dissociative water adsorption seen previously for stoichiometric surfaces is also found for the defect surfaces. By contrast, the vacancy creation substantially changes the most stable mode of water adsorption on the {111} surface, such that the almost degenerate mol. and dissociative adsorptions on the pristine surface are replaced by a strong preference for dissociative adsorption on the substoichiometric surface. The implications of this result for the formation of H2 are discussed. The generally very good agreement between the data from the embedded cluster and periodic DFT approaches provides addnl. confidence in the reliability of the results and conclusions.
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    20. 20
      Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864– B871,  DOI: 10.1103/PhysRev.136.B864
      There is no corresponding record for this reference.
    21. 21
      Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133– A1138,  DOI: 10.1103/PhysRev.140.A1133
      There is no corresponding record for this reference.
    22. 22
      Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502  DOI: 10.1088/0953-8984/21/39/395502
      22
      QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
      Giannozzi Paolo; Baroni Stefano; Bonini Nicola; Calandra Matteo; Car Roberto; Cavazzoni Carlo; Ceresoli Davide; Chiarotti Guido L; Cococcioni Matteo; Dabo Ismaila; Dal Corso Andrea; de Gironcoli Stefano; Fabris Stefano; Fratesi Guido; Gebauer Ralph; Gerstmann Uwe; Gougoussis Christos; Kokalj Anton; Lazzeri Michele; Martin-Samos Layla; Marzari Nicola; Mauri Francesco; Mazzarello Riccardo; Paolini Stefano; Pasquarello Alfredo; Paulatto Lorenzo; Sbraccia Carlo; Scandolo Sandro; Sclauzero Gabriele; Seitsonen Ari P; Smogunov Alexander; Umari Paolo; Wentzcovitch Renata M
      Journal of physics. Condensed matter : an Institute of Physics journal (2009), 21 (39), 395502 ISSN:.
      QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
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    23. 23
      Wu, Z.; Cohen, R. E. More Accurate Generalized Gradient Approximation for Solids. Phys. Rev. B 2006, 73, 235116  DOI: 10.1103/PhysRevB.73.235116
      23
      More accurate generalized gradient approximation for solids
      Wu, Zhigang; Cohen, R. E.
      Physical Review B: Condensed Matter and Materials Physics (2006), 73 (23), 235116/1-235116/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present a nonempirical d. functional generalized gradient approxn. (GGA) that gives significant improvements for lattice consts., crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA. The functional is based on a diffuse radial cutoff for the exchange hole in real space, and the analytic gradient expansion of the exchange energy for small gradients. There are no adjustable parameters, the constraining conditions of PBE are maintained, and the functional is easily implemented in existing codes.
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    24. 24
      Liechtenstein, A. I.; Anisimov, V. I.; Zaanen, J. Density-Functional Theory and Strong Interactions: Orbital Ordering in Mott-Hubbard Insulators. Phys. Rev. B 1995, 52, R5467– R5470,  DOI: 10.1103/PhysRevB.52.R5467
      24
      Density-functional theory and strong interactions: orbital ordering in Mott-Hubbard insulators
      Liechtenstein, A. I.; Anisimov, V. I.; Zaanen, J.
      Physical Review B: Condensed Matter (1995), 52 (8), R5467-R5470CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Evidence is presented that within the d.-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge d. for strongly interacting electron systems. Using a basis-set independent generalization of the LDA + U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator KCuF3.
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    25. 25
      Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B 1989, 40, 3616– 3621,  DOI: 10.1103/PhysRevB.40.3616
      25
      High-precision sampling for Brillouin-zone integration in metals
      Methfessel, M.; Paxton, A. T.
      Physical Review B: Condensed Matter and Materials Physics (1989), 40 (6), 3616-21CODEN: PRBMDO; ISSN:0163-1829.
      A sampling method is given for Brillouin-zone integration in metals which converges exponentially with the no. of sampling points, without the loss of precision of normal broadening techniques. The scheme is based on smooth approximants to the δ and step functions which are constructed to give the exact result when integrating polynomials of a prescribed degree. In applications to the simple-cubic tight-binding band as well as to band structures of simple and transition metals, significant improvement over existing methods was shown. The method promises general applicability in the fields of total-energy calcns. and many-body physics.
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    26. 26
      Head, J. D.; Zerner, M. C. A Broyden—Fletcher—Goldfarb—Shanno Optimization Procedure for Molecular Geometries. Chem. Phys. Lett. 1985, 122, 264– 270,  DOI: 10.1016/0009-2614(85)80574-1
      26
      A Broyden-Fletcher-Goldfarb-Shanno optimization procedure for molecular geometries
      Head, John D.; Zerner, Michael C.
      Chemical Physics Letters (1985), 122 (3), 264-70CODEN: CHPLBC; ISSN:0009-2614.
      Most quantum-chem. calcns. for geometries evaluate first derivs. of the energy with respect to nuclear positions anal. and then use update procedures to build up information on the second derivs. as they step along the potential energy surface toward a min. (stable geometry) or simple saddle point (transition state). The use is described of the Broyden-Fletcher-Goldfarb-Shanno (BFGS) quasi-Newton update used in conjunction with a partial line search. The BFGS is superior to the other update formulas examd. In particular, it is superior to the Murtagh-Sargent (MS) scheme that is commonly used in geometry detns. The advantage of the BFGS update over the MS scheme becomes esp. dramatic for large mol. systems.
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    27. 27
      Momma, K.; Izumi, F. VESTA: A Three-Dimensional Visualization System for Electronic and Structural Analysis. J. Appl. Crystallogr. 2008, 41, 653– 658,  DOI: 10.1107/S0021889808012016
      27
      VESTA: a three-dimensional visualization system for electronic and structural analysis
      Momma, Koichi; Izumi, Fujio
      Journal of Applied Crystallography (2008), 41 (3), 653-658CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      A cross-platform program, VESTA, has been developed to visualize both structural and volumetric data in multiple windows with tabs. VESTA represents crystal structures by ball-and-stick, space-filling, polyhedral, wire frame, stick, dot-surface and thermal-ellipsoid models. A variety of crystal-chem. information is extractable from fractional coordinates, occupancies and oxidn. states of sites. Volumetric data such as electron and nuclear densities, Patterson functions, and wavefunctions are displayed as isosurfaces, bird's-eye views and two-dimensional maps. Isosurfaces can be colored according to other phys. quantities. Translucent isosurfaces and/or slices can be overlapped with a structural model. Collaboration with external programs enables the user to locate bonds and bond angles in the 'graphics area', simulate powder diffraction patterns, and calc. site potentials and Madelung energies. Electron densities detd. exptl. are convertible into their Laplacians and electronic energy densities.
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    28. 28
      Momma, K.; Izumi, F. VESTA3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272– 1276,  DOI: 10.1107/S0021889811038970
      28
      VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data
      Momma, Koichi; Izumi, Fujio
      Journal of Applied Crystallography (2011), 44 (6), 1272-1276CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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    29. 29
      Watson, G. W.; Kelsey, E. T.; de Leeuw, N. H.; Harris, D. J.; Parker, S. C. Atomistic Simulation of Dislocations, Surfaces and Interfaces in MgO. J. Chem. Soc., Faraday Trans. 1996, 92, 433– 438,  DOI: 10.1039/ft9969200433
      29
      Atomistic simulation of dislocations, surfaces and interfaces in MgO
      Watson, Graeme W.; Kelsey, E. Toby; de Leeuw, Nora H.; Harris, Duncan J.; Parker, Stephen C.
      Journal of the Chemical Society, Faraday Transactions (1996), 92 (3), 433-8CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
      A new simulation code for modeling extended defects e.g. linear (dislocations) and planar (surfaces and grain boundaries) at the atomistic level is introduced. One of the key components is the ability to calc. the Coulombic potential of a solid with 1-dimensional periodicity. This approach was applied to screw dislocations in MgO and the structure (including core size) and stability of the 〈100〉 and 1/2〈110〉 screw dislocations were evaluated. The 1/2〈110〉 dislocation, which has the shortest Burgers vector, is more stable, as predicted by elasticity theory, although the simulations show that elasticity theory underests. the energy difference. By using this new computer simulation code METADISE, following the approach of Tasker, the structure and energetics of surfaces and interfaces can be calcd. This method was applied to modeling microfaceting, and microfaceted {110} and {111} surfaces of MgO are the most stable forms of these surfaces. The formation energy of tilt grain boundaries in MgO ({h10} and {h20}) as a function of misorientation angle was also studied and for the {h10} series the formation energy is proportional to the interfacial bond d. while no such pattern can be found for the {h20} series.
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    30. 30
      Bader, R. F. W. Atoms in Molecules. Acc. Chem. Res. 1985, 18, 9– 15,  DOI: 10.1021/ar00109a003
      30
      Atoms in molecules
      Bader, R. F. W.
      Accounts of Chemical Research (1985), 18 (1), 9-15CODEN: ACHRE4; ISSN:0001-4842.
      A review with 21 refs.
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    31. 31
      Andersson, A. D. Density Functional Theory Calculations of Defect and Fission Gas Properties in U-Si Fuels; Office of Scientific and Technical Information, 2016.
      There is no corresponding record for this reference.
    32. 32
      Li, R.; Zhang, P.; Zhang, C.; Huang, X.; Zhao, J. Vacancy Trapping Mechanism for Multiple Helium in Monovacancy and Small Void of Vanadium Solid. J. Nucl. Mater. 2013, 440, 557– 561,  DOI: 10.1016/j.jnucmat.2013.03.068
      32
      Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid
      Li, Ruihuan; Zhang, Pengbo; Zhang, Chong; Huang, Xiaoming; Zhao, Jijun
      Journal of Nuclear Materials (2013), 440 (1-3), 557-561CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      Using first-principles methods, we have investigated the microscopic mechanism for He trapping in two kinds of vacancy defects (monovacancy and 9-atom void) inside vanadium host lattice. In the monovacancy, single He prefers to occupy the octahedral site near vacancy rather than vacancy center. Inside vacancy defects, the He-He equil. distances range in 1.6-2.2 Å. After more He atoms are incorporated, the magnitude of trapping energy decreases and the host lattice expand dramatically. A monovacancy and 9-atom void can host up to 18 and 66 He atoms, resp., with internal pressure up to 7.5 and 19.3 GPa. The at. structures of selected He clusters trapped in vacancies are compared with the gas-phase clusters. The strong tendency of He trapping at vacancies and 9-atom voids provides an explanation for exptl. obsd. He bubble formation at vacancy defects in metals.
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    33. 33
      Lide, D. R. Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, FL, 2001.
      There is no corresponding record for this reference.
    34. 34
      Dzade, N. Y.; Roldan, A.; de Leeuw, N. H. DFT-D2 Study of the Adsorption and Dissociation of Water on Clean and Oxygen-Covered {001} and {011} Surfaces of Mackinawite (FeS). J. Phys. Chem. C 2016, 120, 21441– 21450,  DOI: 10.1021/acs.jpcc.6b06122
      34
      DFT-D2 Study of the Adsorption and Dissociation of Water on Clean and Oxygen-Covered {001} and {011} Surfaces of Mackinawite (FeS)
      Dzade, N. Y.; Roldan, A.; de Leeuw, N. H.
      Journal of Physical Chemistry C (2016), 120 (38), 21441-21450CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A dispersion-cor. d. functional theory study of the adsorption and dissocn. reactions of oxygen and water on the {001} and {011} surfaces of mackinawite (FeS) is presented. A chem. picture of the initial steps of the mackinawite {001} and {011} surfaces oxidn. process in the presence of oxygen and water is presented in the present investigation. Our results show that, while water interacts weakly with the Fe ions on both surfaces and only oxidizes them to some extent, at. and mol. oxygen interact strongly with the FeS{011} surface cations by drawing significant charge from them, thereby oxidizing them from Fe2+ to Fe3+ formal oxidn. state. The authors show from the calcd. adsorption energies and activation energy barriers for the dissocn. of H2O on the clean and oxygen-covered FeS surfaces, that preadsorbed oxygen could easily activate the O-H bond and facilitate the dissocn. of H2O to ferric-hydroxy, Fe3+-OH- on FeS{011}, and to zerovalent sulfur-hydroxyl, S0-OH- on FeS{001}. With the aid of preadsorbed O atom, the activation energy barrier for dissocg. hydrogen atom from H2O decreases from 1.73 to 1.19 eV on the FeS{001}, and from 0.83 to 0.14 eV on the FeS{011}. These findings provide mol.-level insight into the mechanisms of mackinawite oxidn., and are consistent with exptl. results, which have shown that oxygen and water are necessary for the oxidn. process of mackinawite and its possible transformation to pyrite via greigite.
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    35. 35
      Herzberg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; Lancaster Press: New York, 1946; p 365.
      There is no corresponding record for this reference.
    36. 36
      Bo, T.; Lan, J.; Zhang, Y.; Zhao, Y.; He, C.; Chai, Z.; Shi, W. Surface of Uranium Mononitride: Energetics And. Phys. Chem. Chem. Phys. 2016, 18, 13255– 13266,  DOI: 10.1039/C6CP01175F
      36
      Adsorption and dissociation of H2O on the (001) surface of uranium mononitride: energetics and mechanism from first-principles investigation
      Bo, Tao; Lan, Jian-Hui; Zhang, Yu-Juan; Zhao, Yao-Lin; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
      Physical Chemistry Chemical Physics (2016), 18 (19), 13255-13266CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The microscopic adsorption behaviors of water on the UN(001) surface as well as water dissocn. and accompanying H2 formation mechanisms have been investigated on the basis of DFT + U calcns. and ab initio atomistic thermodn. For adsorption of one H2O monomer, the predicted adsorption energies are -0.88, -2.07, and -2.07 eV for the most stable mol., partially dissociative, and completely dissociative adsorption, resp. According to calcns., a water mol. dissocs. into OH and H species via three pathways with small energy barriers of 0.78, 0.72, and 0.85 eV, resp. With the aid of the neighboring H atom, H2 formation through the reaction of H* + OH* can easily occur via two pathways with energy barriers of 0.61 and 0.36 eV, resp. The mol. adsorption of water shows a slight coverage dependence on the surface while this dependence becomes obvious for partially dissociative adsorption as the water coverage increases from 1/4 to 1 ML. In addn., based on the "ab initio atomistic thermodn." simulations, increasing H2O partial pressure will enhance the stability of the adsorbed system and water coverage, while increasing temp. will decrease the H2O coverage. The UN(001) surface reacts easily with H2O at room temp., leading to dissoln. and corrosion of the UN fuel materials.
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    37. 37
      Bo, T.; Lan, J.; Zhao, Y.; Zhang, Y.; He, C.; Chai, Z.; Shi, W. First-Principles Study of Water Adsorption and Dissociation on the UO2. J. Nucl. Mater. 2014, 454, 446– 454,  DOI: 10.1016/j.jnucmat.2014.09.001
      37
      First-principles study of water adsorption and dissociation on the UO2 (1 1 1), (1 1 0) and (1 0 0) surfaces
      Bo, Tao; Lan, Jian-Hui; Zhao, Yao-Lin; Zhang, Yu-Juan; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
      Journal of Nuclear Materials (2014), 454 (1-3), 446-454CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      The adsorption and dissocn. behaviors of water mol. on the UO2 (1 1 1), (1 1 0) and (1 0 0) surfaces were investigated using first-principles methods within the DFT+U framework. For a single water mol. at 1/4 ML coverage, the mol. adsorption exhibits comparable adsorption energies with the dissociative adsorption on the (1 1 1) surface, while it is far less stable than the dissociative adsorption on the (1 1 0) and (1 0 0) surfaces. We find that the adsorbed mol. and dissociative water tend to cluster on low-index UO2 surfaces by forming hydrogen-bond networks. The adsorption stability of water depends on the synergistic effect of hydrogen bonding interaction and steric effect between adsorbates. The mixed adsorption configuration of mol. and dissociative water in 1:1 mol ratio is found to be thermally more stable on the UO2 (1 1 1) and (1 1 0) surfaces.
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    38. 38
      Shapovalov, V.; Truong, T. N. Ab Initio Study of Water Adsorption on α-Al2O3 (0001) Crystal Surface. J. Phys. Chem. B 2000, 104, 9859– 9863,  DOI: 10.1021/jp001399g
      38
      Ab initio study of water adsorption on α-Al2O3 (0001) crystal surface
      Shapovalov, Vladimir; Truong, Thanh N.
      Journal of Physical Chemistry B (2000), 104 (42), 9859-9863CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      Ab Initio embedded cluster calcns. were performed to study water adsorption on Al-terminated (0001) α-Al2O3 surface. We used the surface charge representation of the embedding electrostatic potential (SCREEP) model to give an accurate representation of the Madelung potential at the adsorption site. The geometry of the cluster was optimized to take into account the surface relaxation. Adsorption energies were obtained using the N-layer integrated MO model (ONIOM). In the case of water adsorption it was found that both dissociative and mol. adsorption min. exist, with adsorption energies of -31.57 and -23.40 kcal/mol, resp., in agreement with expt. Bond orders, covalences and full at. valences were analyzed to investigate the changes in the chem. bonding during adsorption. The results provide some insight into the scrambling of water and crystal oxygen atoms during water desorption in isotopic exchange expts.
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    39. 39
      Pang, Z.; Duerrbeck, S.; Kha, C.; Bertel, E.; Somorjai, G. A.; Salmeron, M. Adsorption and Reactions of Water on Oxygen-Precovered Cu(110). J. Phys. Chem. C 2016, 120, 9218– 9222,  DOI: 10.1021/acs.jpcc.6b00769
      39
      Adsorption and Reactions of Water on Oxygen-Precovered Cu(110)
      Pang, Zongqiang; Duerrbeck, Stefan; Kha, Calvin; Bertel, Erminald; Somorjai, Gabor A.; Salmeron, Miquel
      Journal of Physical Chemistry C (2016), 120 (17), 9218-9222CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Using scanning tunneling microscopy we studied the adsorption and reactions of water on a Cu(110) surface partially covered with oxygen in the O(2 × 1) phase formed by Cu-O chains. The expts. were carried out from low (77 K) to higher temps. to sequentially open reaction channels that require increased activation energy. At 77 K, water adsorbs at the edges and on top of the Cu-O chains. Heating to 155 K caused water mols. to react with the oxygen atoms in the chains to produce OH. These hydroxyl groups form elongated H-bonded structures mixed with excess unreacted water mols. The Cu atoms freed in the reaction form small clusters at the location of the initial Cu-O chains. Heating to 180 K leads to a second reaction that desorbs all excess water (i.e., water not H-bonded with OH), leaving H2O-OH zigzag chains along [110] directions. At the low oxygen coverage studied here (<0.12 ML) the extent of the partial water dissocn. reaction is stoichiometrically detd. by the amt. of preadsorbed oxygen (H2O + O → 2OH). A third annealing to 280 K resulted in desorption of all water and hydroxyl species and the reappearance of Cu-O chains. After heating to 280 K, the oxygen coverage returns to nearly the same value as that of the initially surface.
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    40. 40
      Chen, L.; Lu, J.; Liu, P.; Gao, L.; Liu, Y.; Xiong, F.; Qiu, S.; Qiu, X.; Guo, Y.; Chen, X. Dissociation and Charge Transfer of H2O on Cu(110) Probed in Real Time Using Ion Scattering Spectroscopy. Langmuir 2016, 32, 12047– 12055,  DOI: 10.1021/acs.langmuir.6b03516
      40
      Dissociation and Charge Transfer of H2O on Cu(110) Probed in Real Time Using Ion Scattering Spectroscopy
      Chen, Lin; Lu, Jianjie; Liu, Pinyang; Gao, Lei; Liu, Yuefeng; Xiong, Feifei; Qiu, Shunli; Qiu, Xiyu; Guo, Yanling; Chen, Ximeng
      Langmuir (2016), 32 (46), 12047-12055CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Water-Cu(110) interaction is of particular importance during the routine use of graphene-based devices. Water adsorption, dissocn., and desorption at elevated temps. were well studied using the time-of-flight ion scattering technique. Water adsorption meets the first-order Langmuir adsorption model at room temp. The variation of the ratio between residual O and H on the surface with temp. was well detd., which profoundly reveals the dynamical process of surface compn. Also, the change in the surface electronic properties was probed by measuring neg.-ion fractions as a function of the annealing temp. for fast ion scattering. Probably charge transfer is a very sensitive method for studying specific electronic processes in real time.
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    41. 41
      Taylor, C. D.; Lookman, T.; Lillard, R. S. Ab Initio Calculations of the Uranium–Hydrogen System: Thermodynamics, Hydrogen Saturation of a -U and Phase-Transformation to UH3. Acta Mater. 2010, 58, 1045– 1055,  DOI: 10.1016/j.actamat.2009.10.021
      41
      Ab initio calculations of the uranium-hydrogen system: Thermodynamics, hydrogen saturation of α-U and phase-transformation to UH3
      Taylor, Christopher D.; Lookman, Turab; Lillard, R. Scott
      Acta Materialia (2009), 58 (3), 1045-1055CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
      Total energy calcns. based on d. functional theory (DFT) have been performed for various uranium-hydrogen configurations relevant to the uranium hydriding reaction. Transformation of the supersatd. α-U lattice to the α-UH3 lattice, where α-UH3 is believed to be a precursor to the formation of β-UH3, the stable phase of UH3, was investigated. The total energy DFT calcns. for α- and β-UH3 were validated by comparing the predicted and measured decompn. temps. of the hydride at std. pressure. Calcd. energies also confirm the metastability of α-UH3 vs. β-UH3. Computational group theory and DFT calcns. elucidate this transition, and indicate that the transformation itself is kinetically facile. Therefore it is proposed that the formation of the vol.-expanded H-satd. α-U phase is the primary kinetic barrier to hydride formation.
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    42. 42
      Bo, T.; Lan, J.-H.; Zhang, Y.-J.; Zhao, Y.-L.; He, C.-H.; Chai, Z.-F.; Shi, W.-Q. Adsorption and Dissociation of H2O on the (001) Surface of Uranium Mononitride: Energetics and Mechanism from First-Principles Investigation. Phys. Chem. Chem. Phys. 2016, 18, 13255– 13266,  DOI: 10.1039/C6CP01175F
      42
      Adsorption and dissociation of H2O on the (001) surface of uranium mononitride: energetics and mechanism from first-principles investigation
      Bo, Tao; Lan, Jian-Hui; Zhang, Yu-Juan; Zhao, Yao-Lin; He, Chao-Hui; Chai, Zhi-Fang; Shi, Wei-Qun
      Physical Chemistry Chemical Physics (2016), 18 (19), 13255-13266CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The microscopic adsorption behaviors of water on the UN(001) surface as well as water dissocn. and accompanying H2 formation mechanisms have been investigated on the basis of DFT + U calcns. and ab initio atomistic thermodn. For adsorption of one H2O monomer, the predicted adsorption energies are -0.88, -2.07, and -2.07 eV for the most stable mol., partially dissociative, and completely dissociative adsorption, resp. According to calcns., a water mol. dissocs. into OH and H species via three pathways with small energy barriers of 0.78, 0.72, and 0.85 eV, resp. With the aid of the neighboring H atom, H2 formation through the reaction of H* + OH* can easily occur via two pathways with energy barriers of 0.61 and 0.36 eV, resp. The mol. adsorption of water shows a slight coverage dependence on the surface while this dependence becomes obvious for partially dissociative adsorption as the water coverage increases from 1/4 to 1 ML. In addn., based on the "ab initio atomistic thermodn." simulations, increasing H2O partial pressure will enhance the stability of the adsorbed system and water coverage, while increasing temp. will decrease the H2O coverage. The UN(001) surface reacts easily with H2O at room temp., leading to dissoln. and corrosion of the UN fuel materials.
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    43. 43
      Tian, X.; Wang, H.; Xiao, H.; Gao, T. Adsorption of Water on UO2 (111) Surface: Density Functional Theory Calculations. Comput. Mater. Sci. 2014, 91, 364– 371,  DOI: 10.1016/j.commatsci.2014.05.009
      43
      Adsorption of water on UO2 (1 1 1) surface: Density functional theory calculations
      Tian, Xiao-feng; Wang, Hui; Xiao, Hong-xing; Gao, Tao
      Computational Materials Science (2014), 91 (), 364-371CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
      The interaction between the surface of UO2 and mol. water is a serious concern in the range of nuclear waste management. We present a first-principle investigation of the interaction between water and UO2 (1 1 1) surface based on d. functional (DFT) approach. The approxns. of GGA and GGA + U were employed with the projector-augmented-wave method. Both stoichiometric and reduced surfaces were considered in our simulations. We study the at. structures and adsorption energies of various configurations of water adsorption on UO2 (1 1 1) with water coverage of 0.25 ML. The mechanism of the interaction between water (mol. water and dissocd. water) and the surface is discussed in detail. Comparing the adsorption energies for various configurations, both our GGA and GGA + U calcns. suggested that mol. adsorption is more favored than dissociative adsorption of water on defect-free surface, while oxygen vacancy on the surface could make the adsorption of dissocd. water more favorable. Our calcd. results are in good agreement with reported exptl. study and help comprehensive understanding of interactions between water and the stable UO2 (1 1 1) surface.
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    44. 44
      Tegner, B. E.; Molinari, M.; Kerridge, A.; Parker, S. C.; Kaltsoyannis, N. Water Adsorption on AnO2 {111}, {110}, and {100} Surfaces (An = U and Pu): A Density Functional Theory + U Study. J. Phys. Chem. C 2017, 121, 1675– 1682,  DOI: 10.1021/acs.jpcc.6b10986
      44
      Water Adsorption on AnO2 {111}, {110}, and {100} Surfaces (An = U and Pu): A Density Functional Theory + U Study
      Tegner, Bengt E.; Molinari, Marco; Kerridge, Andrew; Parker, Stephen C.; Kaltsoyannis, Nikolas
      Journal of Physical Chemistry C (2017), 121 (3), 1675-1682CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors report the results of plane-wave d. functional theory calcns. of the interaction of water with the {111}, {110} and {100} surfaces of UO2 and PuO2, using a Hubbard-cor. potential (PBE + U) approach to account for the strongly-correlated 5f electrons. A mix of mol. and dissociative water adsorption is found to be most stable on the {111} surface, whereas the fully dissociative water adsorption is most stable on the {110} and {100} surfaces, leading to a fully hydroxylated monolayer. From these results the authors derive water desorption temps. at various pressures for the different surfaces. These increase in the order {111} < {110} < {100}, and these data are used to propose an alternative interpretation for the two exptl. detd. temp. ranges for water desorption from PuO2.
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    45. 45
      Schoenes, J. Optical Properties and Electronic Structure of UO2. J. Appl. Phys. 1978, 49, 1463– 1465,  DOI: 10.1063/1.324978
      45
      Optical properties and electronic structure of uranium dioxide
      Schoenes, J.
      Journal of Applied Physics (1978), 49 (3, Pt. 2), 1463-5CODEN: JAPIAU; ISSN:0021-8979.
      The near normal incidence reflectivity of UO2 single crystals was measured from 0.03 to 13 eV. From the reflectivity spectrum, the complex dielec. function ε(ω) = ε1(ω + iε2(ω) was derived by means of the Kramers-Kronig relation. In addn., the absorption coeff. was detd. from a direct transmission measurement on thin single crystal plates in the weakly absorbing spectral region below the absorption edge. An energy level scheme is proposed which allows a self-consistent assignment of the structure in ε2 with optical transitions between max. in the d. of states. The energy gap found at 2.1 ± 0.1 eV is attributed to a 5f2→5f16deg transition. A crystal field splitting 10 Dq = 2.8 eV is derived for the 6d conduction states. Good agreement is obtained within this model with x-ray photoelectron spectroscopy measurements and a recent mol. cluster approxn. It disagrees with a previous interpretation of reflectivity data.
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    46. 46
      Wang, J.; Ewing, R. C.; Becker, U. Electronic Structure and Stability of Hyperstoichiometric UO2+x under Pressure. Phys. Rev. B 2013, 88, 024109  DOI: 10.1103/PhysRevB.88.024109
      46
      Electronic structure and stability of hyperstoichiometric UO2+x under pressure
      Wang, Jianwei; Ewing, Rodney C.; Becker, Udo
      Physical Review B: Condensed Matter and Materials Physics (2013), 88 (2), 024109/1-024109/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Electronic-Structure and high-pressure phase transitions of stoichiometric uranium dioxide (UO2) and hyperstoichiometric UO2.03 were investigated using first-principles calcns. D. functional theory calcns. using the generalized gradient approxn. and the projector-augmented wave method with the on-site Coulomb repulsive interactions were applied in order to reasonably calc. the equil. vol., total and partial d. of states, band gap of UO2+x, and energetics of the high-pressure phase transitions. Structure optimizations were completed with and without the Hubbard U-ramping method sep. The U-ramping method was intended to remove the metastable states of the 5f electrons of both stoichiometric and hyperstoichiometric UO2+x. Using the Hubbard U parameters (U = 3.8, J = 0.4), whose values are based on the exptl. band gap width of 2.1 eV as a ref., the calcd. cell parameter for the stoichiometric UO2 with cubic fluorite structure is about 1% greater than the exptl. unit cell parameter; in contrast, it is about 1% smaller than the exptl. value without the on-site Coulomb repulsive interactions. For hyperstoichiometric UO2.03 with the cubic fluorite structure, the interstitial oxygen at the octahedral interstitial site induces new bands at the top of the band gap of the stoichiometric UO2, similar to those of the high-pressure phase with orthorhombic cotunnite structure. The orbitals assocd. with the charge transfers to the interstitial oxygen in hyperstoichiometric UO2.03 are partially delocalized and partially localized in both the cubic fluorite structure and orthorhombic cotunnite structure. The energy required for the incorporation of an interstitial O atom is 0.3 eV higher for the orthorhombic phase than for the cubic phase at ambient pressure and increases to 0.5 eV at 10 GPa. The calcd. transition pressures from the cubic to the orthorhombic structure are 18 and 27 GPa for UO2 and UO2.03, resp. The dramatic increase in the calcd. transition pressure for the hyperstoichiometric UO2 is related to structural incompatibility of the interstitial oxygen in the cotunnite structure (high-pressure phase), which is less in the case of the fluorite structure. These results suggest that exptl. detd. pressure values for the phase transition can be significantly affected by small compositional deviations off the ideal stoichiometric UO2. Comparisons of the results using the U-ramping method and without using the U-ramping method suggest that the electronic metastability of UO2 affects the calcd. total energy and electronic structure and could lead to a different local defect configuration for the hyperstoichiometric orthorhombic phase. However, the metastability has a negligible effect on the calcd. phase transition pressure.
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    47. 47
      Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1– 308,  DOI: 10.1016/S0167-5729(01)00020-6
      47
      The interaction of water with solid surfaces: fundamental aspects revisited
      Henderson, Michael A.
      Surface Science Reports (2002), 46 (1-8), 1-308CODEN: SSREDI; ISSN:0167-5729. (Elsevier Science B.V.)
      A review. Water is perhaps the most important and most pervasive chem. on our planet. The influence of water permeates virtually all areas of biochem., chem. and phys. importance, and is esp. evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled 'The interaction of water with solid surfaces: fundamental aspects' in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the mol.-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodol. has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochem., photoconversion, mineralogy, adhesion, sensors, atm. chem. and tribol. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chem. and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, exptl. studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.
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    48. 48
      Ammon, C.; Bayer, A.; Steinrück, H.-P.; Held, G. Low-Temperature Partial Dissociation of Water on Cu(110). Chem. Phys. Lett. 2003, 377, 163– 169,  DOI: 10.1016/S0009-2614(03)01127-8
      48
      Low-temperature partial dissociation of water on Cu(1 1 0)
      Ammon, Ch.; Bayer, A.; Steinruck, H.-P.; Held, G.
      Chemical Physics Letters (2003), 377 (1,2), 163-169CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      The low-temp. reactivity of H2O (D2O) adsorbed on clean and oxygen pre-covered Cu(1 1 0) was studied using high resoln. XPS (HRXPS) and LEED. On the clean surface partial dissocn. to hydroxyl was obsd. already at 95 K. Upon annealing to 220 K hydrogen bonded H2O-hydroxyl chains are formed. Upon further annealing H2O desorbs leaving behind a layer of hydroxyl, most of which desorbs recombinatively eventually. With pre-adsorbed oxygen H2O reacts to hydroxyl lifting the added-row reconstruction even <225 K. Upon annealing this adsorbate layer passes through essentially the same stages as without pre-adsorbed oxygen.
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    49. 49
      Jiang, Z.; Fang, T. Dissociation Mechanism of H2O on Clean and Oxygen-Covered Cu(111) Surfaces: A Theoretical Study. Vaccum 2016, 128, 252– 258,  DOI: 10.1016/j.vacuum.2016.03.030
      49
      Dissociation mechanism of H2O on clean and oxygen-covered Cu (111) surfaces: A theoretical study
      Jiang, Zhao; Fang, Tao
      Vacuum (2016), 128 (), 252-258CODEN: VACUAV; ISSN:0042-207X. (Elsevier Ltd.)
      Using the first-principles calcns. method based on the d. functional theory, the adsorption and dissocn. of water on clean and oxygen-covered Cu (111) surface have been investigated systematically. According to the theor. calcns. results, it was found that H2O prefers to adsorb on the top site, while OH, O and H are favorable on the fcc site. In addn., the structures of the transition states, the corresponding energy barriers and reaction energies were detd. A feasible mechanism on oxygen-covered Cu (111) surface for complete dissocn. of H2O was also put forward. The results showed that the activation barrier for H2O dissocn. decreases obviously with the aid of the oxygen atom. It was proposed that the presence of oxygen atom on the Cu (111) surface can promote the dissocn. of H2O. Compared with our previous reports on Pd and Au surfaces, It was found that a neg. correlation exists between the promoted effect of pre-adsorbed oxygen atom and the adsorption energies on the corresponding surfaces. The results may be useful for computational design, modification and optimization of Cu-based catalysts.
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    50. 50
      Johnson, K.; Ström, V.; Wallenius, J.; Lopes, D. A. Oxidation of Accident Tolerant Fuel Candidates. J. Nucl. Sci. Technol. 2017, 54, 280– 286,  DOI: 10.1080/00223131.2016.1262297
      50
      Oxidation of accident tolerant fuel candidates
      Johnson, Kyle; Stroem, Valter; Wallenius, Janne; Lopes, Denise Adorno
      Journal of Nuclear Science and Technology (Abingdon, United Kingdom) (2017), 54 (3), 280-286CODEN: JNSTAX; ISSN:0022-3131. (Taylor & Francis Ltd.)
      In this study, the oxidn. of various accident tolerant fuel candidates produced under different conditions have been evaluated and compared relative to the ref. std. - UO2. The candidates considered in this study were UN, U3Si2, U3Si5, and a composite material composed of UN-U3Si2. With the spark plasma sintering (SPS) method, it was possible to fabricate samples of UN with varying porosity, as well as a high-d. composite of UN-U3Si2 (10%). Using thermogravimetry in air, the oxidn. behaviors of each material and the various microstructures of UN were assessed. These results reveal that it is possible to fabricate UN to very high densities using the SPS method, such that its resistance to oxidn. can be improved compared to U3Si5 and UO2, and compete favorably with the principal ATF candidates, U3Si2, which shows a particularly violent reaction under the conditions of this study, and the UN-U3Si2 (10%) composite.
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    51. 51
      Johnson, K. D.; Wallenius, J.; Jolkkonen, M.; Claisse, A. Spark Plasma Sintering and Porosity Studies of Uranium Nitride. J. Nucl. Mater. 2016, 473, 13– 17,  DOI: 10.1016/j.jnucmat.2016.01.037
      51
      Spark plasma sintering and porosity studies of uranium nitride
      Johnson, Kyle D.; Wallenius, Janne; Jolkkonen, Mikael; Claisse, Antoine
      Journal of Nuclear Materials (2016), 473 (), 13-17CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      In this study, a no. of samples of UN sintered by the SPS method have been fabricated, and highly pure samples ranging in d. from 68% to 99.8%TD - corresponding to an abs. d. of 14.25 g/cm3 out of a theor. d. of 14.28 g/cm3 - have been fabricated. By careful adjustment of the sintering parameters of temp. and applied pressure, the prodn. of pellets of specific porosity may now be achieved between these ranges. The pore closure behavior of the material has also been documented and compared to previous studies of similar materials, which demonstrates that full pore closure using these methods occurs near 97.5% of relative d.
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    52. 52
      Li, J.; Zhu, S.; Li, H.; Oguzie, E. E.; Li, Y.; Wang, F. Bonding Nature of Monomeric H2O on Pd: Orbital Cooperation and Competition. J. Phys. Chem. C 2009, 1931– 1938,  DOI: 10.1021/jp809595y
      52
      Bonding Nature of Monomeric H2O on Pd: Orbital Cooperation and Competition
      Li, Jibiao; Zhu, Shenglong; Li, Hong; Oguzie, Emeka. E.; Li, Ying; Wang, Fuhui
      Journal of Physical Chemistry C (2009), 113 (5), 1931-1938CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Using d. functional theory (DFT) calcns., the authors provide a unified picture of the bonding nature of a H2O monomer on Pd: varying surface symmetry from hexagonal (Pd{111}) to square (Pd{100}) to rectangular (Pd{110}) lattices. Theor. evidence shows that 3 distinct ranges of whole d-band energy exhibit cooperative roles in the H2O-Pd bonding mechanisms. Bonding states at low-energy resonances (LERs) are assisted by Pauli repulsion-induced electron rearrangement in nonbonding states around d-band centers (intermediate-energy resonances (IERs)) and by depopulation of partially filled antibonding states at high-energy resonances (HERs). Also, the authors have identified the symmetry-tuned lone-pair competitions. As the degree of surface symmetry is reduced, H2O adsorption consistently enhances nonsigma components of lone pair-d overlaps, which originate from distinct balance between electrostatic attraction and Pauli repulsion. Also, the authors found divergency effects of s-d hybridization within the 1st substrate layer.
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    53. 53
      Akbay, T.; Staykov, A.; Druce, J.; Téllez, H.; Ishihara, T.; Kilner, J. A. The interaction of molecular oxygen on LaO terminated surfaces of La2NiO4. J. Mater. Chem. A 2016, 4, 13113,  DOI: 10.1039/c6ta02715f
      53
      Interaction of molecular oxygen on LaO terminated surfaces of La2NiO4
      Akbay, Taner; Staykov, Aleksandar; Druce, John; Tellez, Helena; Ishihara, Tatsumi; Kilner, John A.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (34), 13113-13124CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temp. electrochem. device applications. The surface chem. of these materials plays a crit. role in detg. the kinetics of oxygen redn. and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden-Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and compn. of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analyzed using first principles calcns. in the D. Functional Theory (DFT) formalism. As for the surface chem., LaO termination rather than NiO2 termination is presumed due to recent exptl. evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighboring La sites to surface species. In addn., pos. charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyze the O-O bond cleavage.
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    54. 54
      Fall, C. Ab Initio Study of the Work Functions of Elemental Metal Crystals; EPFL PP: Lausanne.
      There is no corresponding record for this reference.
    55. 55
      Zhang, W.; Liu, L.; Wan, L.; Liu, L.; Cao, L.; Xu, F.; Zhao, J.; Wu, Z. Electronic Structures of Bare and Terephthalic Acid Adsorbed TiO2(110)-(1 × 2) Reconstructed Surfaces: Origin and Reactivity of the Band Gap States. Phys. Chem. Chem. Phys. 2015, 17, 20144– 20153,  DOI: 10.1039/C5CP01298H
      55
      Electronic structures of bare and terephthalic acid adsorbed TiO2(110)-(1 × 2) reconstructed surfaces: origin and reactivity of the band gap states
      Zhang, Wenhua; Liu, Liming; Wan, Li; Liu, Lingyun; Cao, Liang; Xu, Faqiang; Zhao, Jin; Wu, Ziyu
      Physical Chemistry Chemical Physics (2015), 17 (31), 20144-20153CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Combined core level spectroscopy, valence spectroscopy and d. functional theory studies have probed the terephthalic acid (TPA) adsorption behavior and the electronic structure of the rutile TiO2(110)-(1 × 2) reconstructed surface at room temp. The TiO2(110)-(1 × 2) reconstructed surface exhibits an electron rich nature owing to the unsatd. coordination of the surface terminated Ti2O3 rows. Deprotonation of TPA mols. upon adsorption produces both surface bridging hydroxyl (ObH) and bidentate terephthalate species with a satn. coverage of nearly 0.5 monolayers (ML). In contrast to the TiO2(110)-(1 × 1) surface, the band gap states (BGSs) on the bare (1 × 2) surface exhibit an asym. spectral feature, which is originated from integrated contributions of the Ti2O3 termination and the defects in the near-surface region. The Ti2O3 originated BGSs are found to be highly sensitive to the TPA adsorption, a phenomenon well reproduced by the d. functional theory (DFT) calcns. Theor. simulations of the adsorption process also suggest that the redistribution of the electronic d. on the (1 × 2) reconstructed surface accompanying the hydroxyl formation promotes the disappearance of the Ti2O3-row derived BGS.
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    • Mixed mode water adsorption configurations and adsorption energies on stoichiometric U3Si2{001}, {110}, and {111} surfaces; calculated adsorption energy (Eads), relevant bond lengths (d) of atomic (O) oxygen on {001}, {110}, and {111} U3Si2 surfaces (PDF)


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