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Water adsorption on the stoichiometric and defected Fe(110) surfaces
Accepted Manuscript
Water adsorption on the stoichiometric and defected Fe(110) surfaces Tomasz Ossowski, Juarez L. F. Da Silva, Adam Kiejna PII: DOI: Reference:
S0039-6028(17)30710-0 10.1016/j.susc.2017.10.030 SUSC 21132
To appear in:
Surface Science
Received date: Revised date: Accepted date:
22 September 2017 29 October 2017 29 October 2017
Please cite this article as: Tomasz Ossowski, Juarez L. F. Da Silva, Adam Kiejna, Water adsorption on the stoichiometric and defected Fe(110) surfaces, Surface Science (2017), doi: 10.1016/j.susc.2017.10.030
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Highlights • DFT simultaions of molecular H2O on stoichiometric and defected Fe(110) surfaces • Flat-oriented H2O most preferred on stoichiometric surface, possible H2O rotation. • Stronger H2O bonding to defected surface, possible H2O circulation around vacancies
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• On defected surface H2O prefers sites near vacancy, stoichiometric region unfavorable
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• Van der Waals interactions increase adsorption energy, do not affect the geometry
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Stoichiometric Fe(110): - flat orientation
Defected Fe(110):
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- defects region favored
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- possible rotation
- stoichiometric unfavored
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Abstract.pdf
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Water adsorption on the stoichiometric and defected Fe(110) surfaces Tomasz Ossowskia,∗, Juarez L. F. Da Silvaa,b , Adam Kiejnaa Institute of Experimental Physics, University of Wroclaw, Plac M. Borna 9, 50-204 Wroclaw,Poland b S˜ ao Carlos Institute of Chemistry, University of S˜ ao Paulo, PO Box 780, 13560-970, S˜ ao Carlos, SP, Brazil
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Abstract
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The adsorption of water molecules on defect-free (called here as stoichiometric) and defected Fe(110) surfaces has been investigated using density functional theory (DFT) calculations. It is found that on the stoichiometric surface H2 O molecules do not dissociate spontaneously and adsorbs flat on top of the surface Fe atom. By studying different orientations of the flat lying molecule in different adsorption sites it is found that some of them are degenerated in energy thus suggesting a possibility of molecule rotation around direction normal to the surface. At the vacancy defected surface the water molecule favors undercoordinated adsorption sites at or next to the vacancy edge – not the ones in the stoichiometric region of the surface. Moreover, similarly to the stoichiometric surface, at defected one some different configurations are degenerated in energy, making possible molecules circling around the vacancy. The influence of the van der Waals interactions on the adsorption properties of the system is also considered and discussed.
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Keywords: Iron (110) surface, Vacancies, Defects, Water adsorption, Van der Waals interaction, Molecular water at the surface 1. Introduction
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The interaction of water with metal surfaces is of great importance to a broad range of surface processes and phenomena [1–3]. Water adsorption on iron surfaces has attracted substantial interest due to its relevance to corrosion, electrochemistry, and reduction of iron oxides by hydrogen etc. Among the low-index bcc iron surfaces the most densely packed (110) surface is energetically most stable and dominantly exposed on the iron crystal surfaces [4]. Previous experimental and theoretical studies of water adsorption on iron surfaces have primarily focused on the (100) surface [5–17] which is more reactive than (110). Based on the low energy electron diffraction and Auger electron spectroscopy studies a model of irreversible chemisorption of water on Fe(001) surface via a precursor ∗
Corresponding author Email addresses: [email protected] (Tomasz Ossowski), [email protected] (Juarez L. F. Da Silva), [email protected] (Adam Kiejna)
Preprint submitted to Surface Science
October 30, 2017
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of physisorbed H2 O molecules was proposed [5]. After adsorption molecules are mobile, and can either desorb or dissociate. Adsorbed molecular H2 O was observed[7, 8] by using several experimental techniques, both for low and high water exposure, on clean as well as on oxygen precovered Fe(100) surface. These findings were pretty conflicting with early theoretical studies [6] which considered the mechanism of dissociation of water on a small cluster model of Fe(100), or suggested [9] that for lower coverage water dissociates spontaneously on Fe(100) and (110) surfaces, and a low barrier for H2 O dissociation appears only for higher coverage. Results of several further DFT studies [10–16] allowed to postulate a model of H2 O adsorption at Fe(100) surface according to which water adsorbs molecularly as a mobile precursor state. In agreement with general model for H2 O molecule adsorption on close-packed transition metal surfaces [18, 19] the energetically most favored is its flat orientation on top the substrate atoms, and energy barrier for on-surface H2 O dissociation is observed. Water adsorption on Fe(110) surface was less frequently studied. Dwyer et al. [20], using ultraviolet photoelectron spectroscopy, found water adsorbing molecularly at 160 K, and hydroxyl groups at 225 K on initially clean and oxidized Fe(110) surface. At 360 K, desorption of H2 or H2 O was observed respectively from initially clean or oxidized surface. Bar´o et al. [21], using electron energy loss spectroscopy, found that at 130 K, at low exposure water molecules adsorb on Fe(110) dissociatively while at high exposure molecularly. DFT calculations of Eder et al. [9] showed that at low coverage H2 O molecules adsorb perpendicularly to the iron surface in on-top Fe sites, with barrierless water dissociation to H and OH group [9]. More recent DFT studies [22, 23] reported that energetically most preferred orientation of H2 O is flat lying molecule on the Fe(110). Moreover, the existence of energy barrier of 0.68 eV for H2 O molecule dissociation has been reported.[23] However, a detailed discussion of all possible molecular water precursor geometries on Fe(110) surface and their mobility is lacking. Despite several studies on water/iron system having been reported, there are still open questions to be discussed and answered. The question of water mobility on Fe(110) and Fe(100), and the mechanism of its potential diffusion can be addressed. Moreover, to the best of our knowledge, water adsorption on defected iron surfaces has not been considered yet. To fill this gap, in this work we investigate water adsorption both at stoichiometric and vacancy defected surfaces. We are trying to find basic information being helpful in wider discussion about water mobility on metals surfaces. Furthermore, in water adsorption on metal surfaces the van der Waals (vdW) dispersion interactions play an important role affecting the water/metal bonding [22, 24–26], therefore the effect of the account of vdW interactions on the adsorption properties is also discussed. 2. Theoretical approach and computational details The calculations were performed within the spin-dependent density functional theory (DFT) using the Vienna ab initio simulation package (VASP). [27, 28] The electron exchange-correlation interactions were treated at the generalized gradient approximation (GGA) level using Perdew-Burke-Ernzerhof (PBE) functional form. [29] The empirical van der Waals correction proposed by Grimme et al. [30] was also applied (PBE+D3) to verify the influence of the vdW interactions on the adsorption of water molecules. 4
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It should be noted, that molecular water and molecular water/metal-surface systems are properly described even though for PBE functionals (also including dispersion corrections) liquid water is overstructured [31–33]. The electron ion-core interactions were represented by the projector augmented-wave (PAW) generated potentials [34, 35] supplied with VASP. The Fe 4s1 3d7 and O 2s2 2p4 states were treated explicitly as valence states. The solutions of the Kohn-Sham equations were represented by the plane-wave basis set with the kinetic energy cutoff of 500 eV. For the Brillouin zone integrations a 12 × 12 × 12 Monkhorst-Pack grid [36] of special k-points was used. The second order Methfessel-Paxton method [37] was used to treat fractional occupancies with a smearing width of 0.2 eV. The calculations of bulk α-Fe, were performed using the body centered cubic (bcc) unit cell. The results of PBE calculations are in very good agreement compared with experimental data presented in Supporting Information (SI). The account for the van der Waals interactions results in a smaller lattice constant (2.806 ˚ A). The a0 agrees also very well with other PBE+D3 calculations (2.81 ˚ A [22]). It is seen that using the vdW correction, which is an attractive contribution to the interaction energy, results in a worse agreement of calculated properties of bulk Fe with the experimental data than PBE calculations. However, the use of PBE+D3 does not introduce any substantial changes to character of the electronic structure obtained from PBE (Figure S.1.). The main difference is in a small down-shift with respect to Fermi level observed in the density of electronic states of PBE+D3 in comparison to PBE. The equilibrium lattice constants obtained from PBE and PBE+D3 have been used to calculate the clean, stoichiometric or defected, Fe(110) surfaces and water adsorption thereon. Throughout this paper the term “stoichiometric surface” is used to denote the surface without defects (vacancies). The Fe(110) surface was represented by a (110) oriented slab of nine atomic Fe layers, separated from their periodic images by vacuum region of about 16 ˚ A, using 2 × 2 and 3 × 3 surface unit cell. The bottom three Fe layers were frozen in their bulk positions. A water molecule was adsorbed on one side of the slab. Asymmetry of the slab was compensated by the dipole correction to calculate accurate work function [38]. The special k-points mesh for the reciprocal space sampling was adjusted to obtain the same k-points density for different size of the unit cell. The positions of atoms in all the relaxed layers were optimized until the forces on each atom were smaller than 0.01 eV/˚ A. The adsorption energy of water molecule was calculated from the formula: H O/Fe(110)
Ead = −Etot2
Fe(110)
+ Etot
H2 O + Etot ,
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where E H2 O/Fe(110) is the total energy of the relaxed slab with adsorbed water molecule, E Fe(110) is the total energy of the clean relaxed slab, and E H2 O represents the total energy of a free water molecule. It was calculated in a large rectangular box of dimensions 15×16×16.5 ˚ A3 . The calculated PBE and PBE+D3 geometry of free water molecule compares very well with experimental data (in brackets): the O-H bond length is 0.97 ˚ A (0.96–1.0 ◦ ◦ ˚ A)[1, 39], the HOH angle 104.5 (104.5 [1, 40]).
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Figure 1: Top view of stoichiometric (a), and defected Fe(110) surface with high (b) and lower (c) concentration of surface vacancies. The adsorption sites are marked: on-top (ot, ot1, ot2), pseudothreefold hollow (th), short bridge (sb) and long bridge (lb), at vacancy (av), and defected long bridge (dlb). The dashed yellow circle in figure (c) marks stoichiometric region at defected surface for 3 × 3 unit cell.
3. Results and discussion
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3.1. H2 O on stoichiometric Fe(110) surface Calculated geometric and energetic properties of the stoichiometric Fe(110) surface compare well with experimental and other theoretical results (cf. SI). Account for the vdW correction affects only weakly the properties of the surface. The main difference is observed in the surface energy value which is for PBE+D3 by 0.63 J/m2 higher than that for PBE (2.43 J/m2 ). The work function (4.81 eV) is only little higher than for PBE (4.77 eV). This small increase can be explained by the above mentioned small down-shift of the density of electronic states with respect to Fermi level when the vdW correction is included (cf. Figure S.1). Water molecules have been considered in four different sites: on-top (ot), short bridge (sb), long bridge (lb) and pseudo threefold coordinated hollow (th), with molecule’s oxygen pointing to the adsorption site (Fig.1a). Different initial orientations of vertically or horizontally oriented water at the Fe(110) surface were considered. During the simulation process, all initial configurations relaxed to one of the final geometries with H2 O located in ot, sb or lb site and with perpendicular or flat orientation with respect to the Fe(110). Water molecule placed in th hollow site migrated to ot site. The adsorption in lb site appeared also energetically unfavorable. From the stability point of view, the energetically most preferred geometry is that with H2 O molecule lying almost flat on-top Fe atom, which was also reported as most stable one by other authors for H2 O at Fe(110) [22, 23] as well as at Fe(001) surface [11–15]. Water molecule oriented perpendicularly to the surface in ot or sb sites, as well as in the place between ot and th sites is much less favored. The energetic and geometric parameters of most preferred configurations of the H2 O molecule lying flat on the Fe(110) surface in ot sites are listed in Table 1. It is seen that several final configurations are degenerated in adsorption energy (i.e., their energies differ by less than 5 meV) of about 300 meV for PBE and 500 meV for PBE+D3. Some of them were obtained for water molecules initially placed in th or lb sites. During the relaxation process the H2 O molecule rotated and finally has migrated to the ot site where it adsorbed with almost flat orientation to the Fe(110) surface. The GGA adsorption energy is of similar magnitude as in other theoretical calculations of H2 O on Fe(110): 260 meV [9], 343 6
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Table 1: Adsorption energy, Ead , changes in work function with respect to clean substrate, ∆Φ, Fe–O bond length, and HOH angle, for energetically most preferred configurations of 0.25 ML of water at stoichiometric and 2 × 2 defected Fe(110) surface.
Geometry
Ead (meV) PBE PBE+D3
∆Φ (eV) dFe−O (˚ A) PBE PBE+D3 PBE PBE+D3
]HOH (degree) PBE PBE+D3
H2 O at stoichiometric Fe(110) 296 297 297 271
500 497 500 470
−0.96 −1.13 −0.91 −0.97
−0.83 −1.02 −0.83 −0.90
2.27 2.26 2.27 2.30
H2 O at defected Fe(110) 344 345 348 326 304
586 566 588 494 512
−0.67 −0.44 −0.53 −0.94 −0.88
−0.66 −0.19 −0.39 −1.00 −0.89
2.28 2.25 2.26 2.27 2.29
104.6 105.4 108.1 104.2
104.5 105.2 104.5 104.2
2.25 2.23 2.24 2.25 2.29
104.5 105.0 104.5 105.5 105.0
104.6 104.3 104.5 104.3 105.3
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dsb1 dsb2 dsb3 ot1 dlb
2.25 2.25 2.25 2.29
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0 36.9 75.3 125.2
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meV [22], 380 meV [23]. The existing differences stem presumably from the fact that in ref. [9] perpendicular water was found as most stable configuration, whereas in [22] and [23] lower H2 O concentration (1/6 ML and 1/16 ML, respectively) were considered. The account for dispersion interactions enhances the H2 O adsorption energy by about 200 meV. This enhancement of Ead is of similar magnitude as that reported for water adsorption on Fe(110) (200 meV [22], 240 meV [23]) or other close-packed Cu, Au, and Pt(111) surfaces (200–250 meV [26]). As is seen from Figure 2 the geometries of above configurations are very similar (see also Table 1). However, some differences in the geometry of the systems are visible. Water molecules are not exactly parallel to the surface. The inclination angle of HOH plane to the surface varies in the range 3.2–9.4◦ both for PBE and PBE+D3. For lower H2 O coverage the inclination of about 15.4◦ was reported [22]. For both, PBE as well as for PBE+D3 calculations, depending on the configuration, differences are visible in HOH angle, H2 O inclination to the surface and in its shift from the ideal on-top site. The H2 O oxygen atom, for most energetically stable configurations, is shifted from an ideal on-top position by about 0.1–0.3 ˚ A for PBE as well as for PBE+D3 calculations. The HOH angle varies in the range 104.2– 108.1◦ (PBE) and 104.2–105.2◦ (PBE+D3). For PBE, the OH bond lengths are equal to 0.98 ˚ A. The water molecule binds to the surface through the oxygen atom with the O–Fe bond length of 2.26–2.30 ˚ A. The calculations with vdW correction yield similar properties of bonds, with O–Fe bond shorter only by about 0.01 ˚ A. Similar difference between O–Fe bond length found from PBE (2.21 ˚ A) and PBE+D3 (2.19 ˚ A) have been reported for water molecule adsorbed in 2 × 3 slab [22]. The Bader charges analysis [41] shows a small electron charge transfer (of about 7
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Figure 2: The adsorption energy as a function of the rotation angle of the H2 O molecule around axis perpendicular to the surface, measured with respect to the [001] crystallographic direction, for the most stable, final geometries of H2 O on 2 × 2 Fe(110). Due to the symmetry of the system, the values for 180–360◦ angles are the same as for 0–180◦ .
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0.05–0.1e) from the substrate to the waters oxygen atoms. H2 O gets an electron charge from the nearest Fe atoms. The amount of the charge transfer is similar for PBE and for PBE+D3 calculations. The small electronic charge transfer is connected with H2 O– Fe bonds. However, the presence of H2 O molecule on the surface causes redistribution of the electronic charge in the substrate. It results in a lowering of the surface dipole moment and consequently in a lowering the work function of the system. The degeneration in energy of the above mentioned final configurations means that a water molecule in the ot site, can rotate around direction perpendicular to the Fe(110) surface, with small barrier (30 meV) for this rotation, as observed for H2 O rotated by about 125◦ and 305◦ (cf. Fig. 2.). Moreover, for similar rotation angle, some differences (≤ 3.9◦ ) are visible in the HOH angle, although the geometries have the same total energy. It suggests that, besides rotation in the Fe(110) surface plane, an H2 O molecule can exhibit small oscillations of the HOH angle. Interestingly, for these very similar configurations, small differences in work function can be seen. Depending on H2 O orientation, the work function decreased by 0.91–1.13 eV (PBE) and 0.83–1.04 eV (PBE+D3) in comparison to the value of clean Fe(110) surface. Similar decrease of the work function upon water adsorption (0.71 eV, PBE; 0.63 eV, PBE+D3) was reported previously[22]. A lower H2 O coverage (0.11 ML) gives the same most preferred adsorption site and similar configurations of water molecule at the surface. However, the adsorption energy is higher by about 30 meV, which means that H2 O binding to Fe(110) surface weakens with the coverage. Stronger binding of water at the lower coverage reflects a shorter (by up to 0.06 ˚ A) Fe–O bond length. Its magnitude (2.21–2.25 ˚ A) is similar to the Fe–O bond length reported by other authors [22, 23] for coverages lower than 0.25 ML. Changes in the geometry and work function values are smaller for lower coverage, however, the 8
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character of changes is preserved. Adsorbed water molecule affects only weakly the geometry of the surface. Upon water adsorption interlayer distances in Fe(110) slab increase. However, the changes are small and in most cases limited to the first interlayer distance. For a single H2 O molecule, in 2 × 2 cell, in most stable configuration ∆12 is increased from about 0.24 % to 0.8 %. With vdW correction included, ∆12 is increased from 0.82% for clean surface, to about 1.1 %. The changes of deeper interlayer distances are much lower (≤ 0.1 %) and it may be assumed, that they do not differ from that of the clean Fe(110) surface. Thus finally it can be concluded that upon water adsorption the first interlayer distance is expanded by about 1 % with respect to the bulk distance.
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3.2. H2 O on defected Fe(110) surface To consider a defected Fe(110) surface a single vacancy was created by removing one of the Fe atoms from different layers of the relaxed surface slab, and subsequently the system has been relaxed. It is worth pointing out, that for defected surface, although atoms (except in frozen layers) could optimized their positions in all directions, lateral relaxations have not been observed. The vacancy formation energy calculated as a function of its depth (cf. Fig. S2) increases monotonically that the vacancy is easiest to form in the top surface layer. Therefore water adsorption at defected Fe(110) surface was studied for a substrate geometry with vacancy in the topmost layer of the slab, which is energetically the most stable one. A water molecule has been adsorbed in different adsorption sites presented in Figure 1, applying same procedure as for water adsorption on perfect Fe(110) surface. Similarly to stoichiometric surface, adsorption of water perpendicularly oriented to the defected surface is either unfavorable or unstable, and ends up in flat-lying or tilted molecule’s orientation. Eventually, upon relaxation water molecules were moved either to ot, ot1 sites, or to sites adjacent vacancy (called as defected long (dlb or short (dsb) bridge, or defected hollow sites). The final, most stable geometries of H2 O at 2 × 2 defected Fe(110) are presented in Figure 3 and in Table 1. It can be seen from Table 1 that there are few final configurations for which adsorption energy is within the range of 65 meV (PBE) and 75 meV (PBE+D3). The PBE+D3 gives higher adsorption energy by 200–240 meV. Adsorption close to dlb or to lb sites is weaker than in places close to the defected short bridge. However, the difference in Ead is lower than 70 meV. It may suggest that water molecule is able to change its position and may circle around the vacancy with energy barrier not higher than 70 meV. Interestingly water does not bind in the vacancy site. Water molecule placed initially in vacancy site is moved to one of the final configurations presented in Figure 3. It should be mentioned that flat adsorbed water molecule in ot1 site, which for high defected surface is placed between neighboring vacancies, is only by about 20 meV less stable. It may suggest, that water prefers to jump over ot1 site between neighboring most stable defected positions, than circling around vacancy. On the other hand, it can be understood that the movement of water molecules at highly defected Fe(110) surface may be considered as combination of oscillations around vacancy site and jumps between neighboring defected sites. Final geometries of PBE+D3 configurations are almost the same as for PBE. 9
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Figure 3: Top view of most stable final geometries of H2 O on 2×2 (a-d) and 3×3 (e-h) defected Fe(110) surface. The PBE adsorption energy is listed below pictures. The yellow dashed circles in panels (e) and (h) mark defect and stoichiometric regions, respectively.
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In the case of H2 O adsorption at defected surface, small variation of the HOH angle can be observed, which for the most stable configurations do not exceed 1◦ . The range of O–Fe bond lengths for H2 O at defected surface is similar as for adsorption at stoichiometric surface. For some of the most stable configurations at defected surface, the OH bonds are slightly asymmetric. The bond between O and the H atom located closer to the surface is by 0.01 ˚ A longer than between O and hydrogen atom located further from the surface. However, the OH bond lengths (0.98–0.99 ˚ A) are nearly the same as for adsorption at stoichiometric Fe(110) surface and differ not much from that for a free water molecule (0.97 ˚ A). Similarly to stoichiometric surface, at a defected surface water molecule gains electron charge. The charge transfer of about 0.08–1.1e, mostly from the nearest Fe atoms, is found both for PBE and PBE+D3 calculations. As for a stoichiometric surface, adsorption of H2 O molecule on the defected one results in the charge redistribution leading to reduction of the dipole layer moment and consequently to reduction of the work function. In most stable cases, water adsorption on defected Fe(110) surface reduces work function by 0.4–0.7 eV (PBE) and 0.2–0.7 eV (PBE+D3) compared to stoichiometric system (cf. Table 1). For some less stable cases, especially for vertically oriented H2 O, the changes in work function are larger. The sequence of magnitude of the change in the work function for final geometry is preserved. The largest decrease is observed (cf. Table 1) for the least stable configuration (dth) and the lowest for the most stable one (dsb2). The calculations performed for water adsorption at 3 × 3 defected Fe(110) surface allow to verify if H2 O prefers position on-top of sixfold coordinated substrate Fe atom or one of the positions near to the vacancy A flat lying water molecule was placed in different sites shown in Fig. 1c. An original orientation of the H2 O molecule with H–H 10
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line parallel to surface [001] direction was changed by a rotation of the molecule around the direction perpendicular to the surface by four different angles (22.5, 45, 67.5 and 90◦ ). For the undercoordinated places (ot1 and ot2) rotation in the opposite direction has been also applied. The results show (Fig. 3) that Ead for H2 O adsorbed in the stoichiometric region (ot) is lower by 187–395 meV than in sites near to the vacancy . Thus, it shows that water molecules prefer sites near the vacancy. However, similarly to adsorption at the 2 × 2 defected surface, water does not like to adsorb in the very vacancy site, but in sites at the edge of the vacancy (defected hollow, defected short bridge or defected long bridge). For these sites, Ead varies between 172 and 427 meV. Some configurations are energetically degenerated which suggests possibility of water circling around vacancy. On the other hand, configurations with flat lying H2 O molecule in ot1 sites are only by about 24 meV less favorable. It suggests that there is a quite low energy barrier for H2 O move between neighboring defected sites above ot1 positions, and similarly to the higher defected one, mobility of H2 O molecules at less defected surface is a combination of a circling around vacancy and jumps between defected sites. Similarly to stoichiometric surface, adsorption of water at defected Fe(110) causes extra expansion of the first substrate interlayer distance in comparison with the clean, defected surface. For H2 O at 2 × 2 defected Fe(110), the relaxation of the first interlayer distance (∆12 ) for most energetically stable geometries is equal to −4.6 % (PBE), and −3.9 % (PBE+D3). The ∆23 = 1.4% (PBE) and 1.3% (PBE+D3), and ∆34 = 0.2 % (PBE) and 0.0 % (PBE+D3) are lower than for clean substrate (both for PBE and PBE+D3) by about 0.35 % and 0.20 %, respectively. This means that, compared to the clean, defected substrate, a small contraction of the second and third interlayer distances can be observed upon H2 O adsorption. Simultaneously very small lateral displacements (less than 0.03 ˚ A) of substrate atoms adjacent to H2 O are observed. However, all changes are relatively small which means that adsorbed water does not affect strongly the geometry of the substrate. 4. Summary and conclusions
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We reported results of the theoretical investigation of water adsorption on stoichiometric and defected bcc iron (110) surface using DFT and DFT+D3 calculations taking into account van der Waals correction. It is found that at stoichiometric Fe(110) surface a water molecule adsorbs molecularly on-top of Fe atoms and is oriented horizontally to the surface. There is almost no difference in adsorption energy for system with H2 O rotated around the direction perpendicular to the surface. However, some geometrical differences are visible. Depending on the rotation angle of parallel adsorbed water molecules, the H2 O/Fe(110) system differs in work function changes, and slightly in the geometry. Water molecule adsorbed on a defected surface prefers few different configurations in defected sites nearby the vacancy. A little more preferred is a configuration with one of the hydrogen atoms closer to the vacancy. Flat lying water, adsorbed over on-top Fe atom adjacent to vacancy is only slightly less stable, and H2 O molecule can easy jump between different stable places or simply circle around ot1 site. Adsorption of water at the surface with lower concentration of vacancies shows that water prefers 11
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to adsorb in sites nearby defect than in stoichiometric like region. The account for van der Waals interactions enhances the water adsorption energy and does not substantially affect the geometry of the system. Acknowledgments
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This work was supported by the National Science Centre (NCN), Poland (Grant No. 2012/07/B/ST3/03009). We acknowledge computer time granted by the Interdisciplinary Centre for Mathematical and Computational Modelling (ICM) of the Warsaw University within the project No G44-23. One of us (JLFDS) thanks the University of Wroclaw for a visiting professorship within Human Capital Operational Program – Academy of Development as the Key to Strengthen Human Resources of the Polish Economy.
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References References
[1] P. A. Thiel, T. E. Madey, The Interaction of Water with Solid Surfaces: Fundamental Aspects, Surface Science Reports 7 (1987) 211–385.
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[2] M. A. Henderson, The interaction of water with solid surfaces: fundamental aspects revisited, Surface Science Reports 46 (2002) 1–308. [3] A. Hodgson, S. Haq, Water adsorption and the wetting of metal surfaces, Surface Science Reports 64 (2009) 381–451.
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[4] P. Blo´ nski, A. Kiejna, Structural, Electronic, and Magnetic Properties of bcc Iron Surfaces, Surface Science 601 (2007) 123–133.
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[5] D. J. Dwyer, G. Simmons, R. Wei, A Study of the Initial Reaction of Water Wapor with Fe(001) Surface, Surface Science 64 (1977) 617–632.
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[6] A. B. Anderson, Reactions and Structures of Water on Clean and Oxygen Covered Pt(111) and Fe(100), Surface Science 105 (1981) 159–176.
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[7] W.-H. Hung, J. Schwartz, S. L. Bernasek, Sequential Oxidation of Fe(100) by Water Adsorption: Formation of an Ordered Hydroxylated Surface, Surface Science 248 (1991) 332–342. [8] W.-H. Hung, J. Schwartz, S. Bernasek, Adsorption of H2O on Oxidized Fe(100) Surfaces: Comparison Between the Oxidation of Iron by H2O and O2, Surface Science 294 (1993) 21–32. [9] M. Eder, K. Terakura, J. Hafner, Initial Stages of Oxidation of (100) and (110) Surfaces of Iron Caused by Water, Phys. Rev. B 64 (2001) 115426.
[10] J. M. H. Lo, T. Ziegler, Density Functional Theory and Kinetic Studies of Methanation on Iron Surface, The Journal of Physical Chemistry C 111 (2007) 11012–11025. 12
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[11] S. C. Jung, M. H. Kang, Adsorption of a Water Molecule on Fe(100): DensityFunctional Calculations, Phys. Rev. B 81 (2010) 115460. [12] A. Govender, D. Curulla Ferr´e, J. W. Niemantsverdriet, The Surface Chemistry of Water on Fe(100): A Density Functional Theory Study, ChemPhysChem 13 (2012) 1583–1590.
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[13] R. R. Q. Freitas, R. Rivelino, F. de Brito Mota, C. M. C. de Castilho, Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study, The Journal of Physical Chemistry C 116 (2012) 20306–20314. [14] P. Lazar, M. Otyepka, Dissociation of Water at Iron Surfaces: Generalized Gradient Functional and Range-Separated Hybrid Functional Study, The Journal of Physical Chemistry C 116 (2012) 25470–25477.
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[15] S. Liu, X. Tian, T. Wang, X. Wen, Y.-W. Li, J. Wang, H. Jiao, High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis, The Journal of Physical Chemistry C 118 (2014) 26139–26154. [16] F. Karlick´ y, P. Lazar, M. Dubeck´ y, M. Otyepka, Random Phase Approximation in Surface Chemistry: Water Splitting on Iron, Journal of Chemical Theory and Computation 9 (2013) 3670–3676.
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[17] A. Kiejna, T. Ossowski, Water adsorption on iron surfaces, in: Reference Module in Chemistry. Molecular Sciences and Chemical Engineering, Elsevier, 25-Jul-2017. doi:10.1016/B978-0-12-409547-2.14154-X.
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[18] A. Michaelides, V. A. Ranea, P. L. de Andres, D. A. King, General Model for Water Monomer Adsorption on Close-Packed Transition and Noble Metal Surfaces, Phys. Rev. Lett. 90 (2003) 216102.
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[19] J. Carrasco, A. Hodgson, A. Michaelides, A molecular perspective of water at metal interfaces, Nature Materials 11 (2012) 667–674.
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[20] D. J. Dwyer, S. R. Kelemen, A. Kaldor, The Water Dissociation Reaction on Clean and Oxidized Iron (110), The Journal of Chemical Physics 76 (1982) 1832–1837.
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[21] A. M. Bar´o, W. Erley, The Adsorption of H2O on Fe(100) Studied by EELS (My Comment: This is Fe(110) Surface), Journal of Vacuum Science and Technology 20 (1982) 580–583. [22] P. Tereshchuk, J. L. F. Da Silva, Ethanol and Water Adsorption on Close-Packed 3d, 4d, and 5d Transition-Metal Surfaces: A Density Functional Theory Investigation with van der Waals Correction, The Journal of Physical Chemistry C 116 (2012) 24695–24705. [23] S. Liu, X. Tian, T. Wang, X. Wen, Y.-W. Li, J. Wang, H. Jiao, Coverage Dependent Water Dissociative Adsorption on Fe(110) from DFT Computation, Phys. Chem. Chem. Phys. 17 (2015) 8811–8821. 13
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[24] J. Carrasco, B. Santra, J. c. v. Klimeˇs, A. Michaelides, To Wet or Not to Wet? Dispersion Forces Tip the Balance for Water Ice on Metals, Phys. Rev. Lett. 106 (2011) 026101. [25] K. Tonigold, A. Groß, Dispersive interactions in water bilayers at metallic surfaces: A comparison of the PBE and RPBE functional including semiempirical dispersion corrections, Journal of Computational Chemistry 33 (2012) 695–701.
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[26] R. L. H. Freire, A. Kiejna, J. L. F. Da Silva, Adsorption of water and ethanol on noble and transition-metal substrates: a density functional investigation within van der Waals corrections, Phys. Chem. Chem. Phys. 18 (2016) 29526–29536. [27] G. Kresse, J. Hafner, Ab initio Molecular Dynamics for Liquid Metals, Physical Review B 47 (1993) 558–561.
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[28] G. Kresse, J. Furthm¨ uller, Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set, Physical Review B 54 (1996) 11169– 11186. [29] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation made Simple, Physical Review Letters 77 (1996) 3865–3868.
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[30] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu, The Journal of Chemical Physics 132 (2010) 154104.
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[31] J. VandeVondele, F. Mohamed, M. Krack, J. Hutter, M. Sprik, M. Parrinello, The influence of temperature and density functional models in ab initio molecular dynamics simulation of liquid water, The Journal of Chemical Physics 122 (2005) 014515.
CE
PT
[32] L.-M. Liu, M. Krack, A. Michaelides, Interfacial water: A first principles molecular dynamics study of a nanoscale water film on salt, The Journal of Chemical Physics 130 (2009) 234702.
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[33] K. Forster-Tonigold, A. Groß, Dispersion corrected RPBE studies of liquid water, The Journal of Chemical Physics 141 (2014) 064501. [34] P. E. Bl¨ochl, Projector Augmented-Wave Method, Physical Review B 50 (1994) 17953–17979. [35] G. Kresse, D. Joubert, From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method, Physical Review B 59 (1999) 1758–1775. [36] H. J. Monkhorst, J. D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188–5192. [37] M. Methfessel, A. T. Paxton, High-precision sampling for Brillouin-zone integration in metals, Phys. Rev. B 40 (1989) 3616–3621. 14
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[38] J. Neugebauer, M. Scheffler, Adsorbate-Substrate and Adsorbate-Adsorbate Interactions of Na and K adlayers on Al(111), Phys. Rev. B 46 (1992) 16067–16080. [39] M. D. Harmony, V. W. Laurie, R. L. Kuczkowski, R. H. Schwendeman, D. A. Ramsay, F. J. Lovas, W. J. Lafferty, A. G. Maki, Molecular Structures of GasPhase Polyatomic Molecules Determined by Spectroscopic Methods, Journal of Physical and Chemical Reference Data 8 (1979) 619–722.
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[40] W. S. Benedict, N. Gailar, E. K. Plyler, RotationVibration Spectra of Deuterated Water Vapor, The Journal of Chemical Physics 24 (1956) 1139–1165.
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[41] G. Henkelman, A. Arnaldsson, H. J´onsson, A Fast and Robust Algorithm for Bader Decomposition of Charge Density, Computational Materials Science 36 (2006) 354– 360.
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Water adsorption on the stoichiometric and defected Fe(110) surfaces Tomasz Ossowski, Juarez L. F. Da Silva, Adam Kiejna PII: DOI: Reference:
S0039-6028(17)30710-0 10.1016/j.susc.2017.10.030 SUSC 21132
To appear in:
Surface Science
Received date: Revised date: Accepted date:
22 September 2017 29 October 2017 29 October 2017
Please cite this article as: Tomasz Ossowski, Juarez L. F. Da Silva, Adam Kiejna, Water adsorption on the stoichiometric and defected Fe(110) surfaces, Surface Science (2017), doi: 10.1016/j.susc.2017.10.030
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Highlights • DFT simultaions of molecular H2O on stoichiometric and defected Fe(110) surfaces • Flat-oriented H2O most preferred on stoichiometric surface, possible H2O rotation. • Stronger H2O bonding to defected surface, possible H2O circulation around vacancies
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• On defected surface H2O prefers sites near vacancy, stoichiometric region unfavorable
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• Van der Waals interactions increase adsorption energy, do not affect the geometry
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Stoichiometric Fe(110): - flat orientation
Defected Fe(110):
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- defects region favored
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- possible rotation
- stoichiometric unfavored
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Abstract.pdf
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Water adsorption on the stoichiometric and defected Fe(110) surfaces Tomasz Ossowskia,∗, Juarez L. F. Da Silvaa,b , Adam Kiejnaa Institute of Experimental Physics, University of Wroclaw, Plac M. Borna 9, 50-204 Wroclaw,Poland b S˜ ao Carlos Institute of Chemistry, University of S˜ ao Paulo, PO Box 780, 13560-970, S˜ ao Carlos, SP, Brazil
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a
Abstract
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The adsorption of water molecules on defect-free (called here as stoichiometric) and defected Fe(110) surfaces has been investigated using density functional theory (DFT) calculations. It is found that on the stoichiometric surface H2 O molecules do not dissociate spontaneously and adsorbs flat on top of the surface Fe atom. By studying different orientations of the flat lying molecule in different adsorption sites it is found that some of them are degenerated in energy thus suggesting a possibility of molecule rotation around direction normal to the surface. At the vacancy defected surface the water molecule favors undercoordinated adsorption sites at or next to the vacancy edge – not the ones in the stoichiometric region of the surface. Moreover, similarly to the stoichiometric surface, at defected one some different configurations are degenerated in energy, making possible molecules circling around the vacancy. The influence of the van der Waals interactions on the adsorption properties of the system is also considered and discussed.
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Keywords: Iron (110) surface, Vacancies, Defects, Water adsorption, Van der Waals interaction, Molecular water at the surface 1. Introduction
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The interaction of water with metal surfaces is of great importance to a broad range of surface processes and phenomena [1–3]. Water adsorption on iron surfaces has attracted substantial interest due to its relevance to corrosion, electrochemistry, and reduction of iron oxides by hydrogen etc. Among the low-index bcc iron surfaces the most densely packed (110) surface is energetically most stable and dominantly exposed on the iron crystal surfaces [4]. Previous experimental and theoretical studies of water adsorption on iron surfaces have primarily focused on the (100) surface [5–17] which is more reactive than (110). Based on the low energy electron diffraction and Auger electron spectroscopy studies a model of irreversible chemisorption of water on Fe(001) surface via a precursor ∗
Corresponding author Email addresses: [email protected] (Tomasz Ossowski), [email protected] (Juarez L. F. Da Silva), [email protected] (Adam Kiejna)
Preprint submitted to Surface Science
October 30, 2017
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of physisorbed H2 O molecules was proposed [5]. After adsorption molecules are mobile, and can either desorb or dissociate. Adsorbed molecular H2 O was observed[7, 8] by using several experimental techniques, both for low and high water exposure, on clean as well as on oxygen precovered Fe(100) surface. These findings were pretty conflicting with early theoretical studies [6] which considered the mechanism of dissociation of water on a small cluster model of Fe(100), or suggested [9] that for lower coverage water dissociates spontaneously on Fe(100) and (110) surfaces, and a low barrier for H2 O dissociation appears only for higher coverage. Results of several further DFT studies [10–16] allowed to postulate a model of H2 O adsorption at Fe(100) surface according to which water adsorbs molecularly as a mobile precursor state. In agreement with general model for H2 O molecule adsorption on close-packed transition metal surfaces [18, 19] the energetically most favored is its flat orientation on top the substrate atoms, and energy barrier for on-surface H2 O dissociation is observed. Water adsorption on Fe(110) surface was less frequently studied. Dwyer et al. [20], using ultraviolet photoelectron spectroscopy, found water adsorbing molecularly at 160 K, and hydroxyl groups at 225 K on initially clean and oxidized Fe(110) surface. At 360 K, desorption of H2 or H2 O was observed respectively from initially clean or oxidized surface. Bar´o et al. [21], using electron energy loss spectroscopy, found that at 130 K, at low exposure water molecules adsorb on Fe(110) dissociatively while at high exposure molecularly. DFT calculations of Eder et al. [9] showed that at low coverage H2 O molecules adsorb perpendicularly to the iron surface in on-top Fe sites, with barrierless water dissociation to H and OH group [9]. More recent DFT studies [22, 23] reported that energetically most preferred orientation of H2 O is flat lying molecule on the Fe(110). Moreover, the existence of energy barrier of 0.68 eV for H2 O molecule dissociation has been reported.[23] However, a detailed discussion of all possible molecular water precursor geometries on Fe(110) surface and their mobility is lacking. Despite several studies on water/iron system having been reported, there are still open questions to be discussed and answered. The question of water mobility on Fe(110) and Fe(100), and the mechanism of its potential diffusion can be addressed. Moreover, to the best of our knowledge, water adsorption on defected iron surfaces has not been considered yet. To fill this gap, in this work we investigate water adsorption both at stoichiometric and vacancy defected surfaces. We are trying to find basic information being helpful in wider discussion about water mobility on metals surfaces. Furthermore, in water adsorption on metal surfaces the van der Waals (vdW) dispersion interactions play an important role affecting the water/metal bonding [22, 24–26], therefore the effect of the account of vdW interactions on the adsorption properties is also discussed. 2. Theoretical approach and computational details The calculations were performed within the spin-dependent density functional theory (DFT) using the Vienna ab initio simulation package (VASP). [27, 28] The electron exchange-correlation interactions were treated at the generalized gradient approximation (GGA) level using Perdew-Burke-Ernzerhof (PBE) functional form. [29] The empirical van der Waals correction proposed by Grimme et al. [30] was also applied (PBE+D3) to verify the influence of the vdW interactions on the adsorption of water molecules. 4
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It should be noted, that molecular water and molecular water/metal-surface systems are properly described even though for PBE functionals (also including dispersion corrections) liquid water is overstructured [31–33]. The electron ion-core interactions were represented by the projector augmented-wave (PAW) generated potentials [34, 35] supplied with VASP. The Fe 4s1 3d7 and O 2s2 2p4 states were treated explicitly as valence states. The solutions of the Kohn-Sham equations were represented by the plane-wave basis set with the kinetic energy cutoff of 500 eV. For the Brillouin zone integrations a 12 × 12 × 12 Monkhorst-Pack grid [36] of special k-points was used. The second order Methfessel-Paxton method [37] was used to treat fractional occupancies with a smearing width of 0.2 eV. The calculations of bulk α-Fe, were performed using the body centered cubic (bcc) unit cell. The results of PBE calculations are in very good agreement compared with experimental data presented in Supporting Information (SI). The account for the van der Waals interactions results in a smaller lattice constant (2.806 ˚ A). The a0 agrees also very well with other PBE+D3 calculations (2.81 ˚ A [22]). It is seen that using the vdW correction, which is an attractive contribution to the interaction energy, results in a worse agreement of calculated properties of bulk Fe with the experimental data than PBE calculations. However, the use of PBE+D3 does not introduce any substantial changes to character of the electronic structure obtained from PBE (Figure S.1.). The main difference is in a small down-shift with respect to Fermi level observed in the density of electronic states of PBE+D3 in comparison to PBE. The equilibrium lattice constants obtained from PBE and PBE+D3 have been used to calculate the clean, stoichiometric or defected, Fe(110) surfaces and water adsorption thereon. Throughout this paper the term “stoichiometric surface” is used to denote the surface without defects (vacancies). The Fe(110) surface was represented by a (110) oriented slab of nine atomic Fe layers, separated from their periodic images by vacuum region of about 16 ˚ A, using 2 × 2 and 3 × 3 surface unit cell. The bottom three Fe layers were frozen in their bulk positions. A water molecule was adsorbed on one side of the slab. Asymmetry of the slab was compensated by the dipole correction to calculate accurate work function [38]. The special k-points mesh for the reciprocal space sampling was adjusted to obtain the same k-points density for different size of the unit cell. The positions of atoms in all the relaxed layers were optimized until the forces on each atom were smaller than 0.01 eV/˚ A. The adsorption energy of water molecule was calculated from the formula: H O/Fe(110)
Ead = −Etot2
Fe(110)
+ Etot
H2 O + Etot ,
(1)
where E H2 O/Fe(110) is the total energy of the relaxed slab with adsorbed water molecule, E Fe(110) is the total energy of the clean relaxed slab, and E H2 O represents the total energy of a free water molecule. It was calculated in a large rectangular box of dimensions 15×16×16.5 ˚ A3 . The calculated PBE and PBE+D3 geometry of free water molecule compares very well with experimental data (in brackets): the O-H bond length is 0.97 ˚ A (0.96–1.0 ◦ ◦ ˚ A)[1, 39], the HOH angle 104.5 (104.5 [1, 40]).
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Figure 1: Top view of stoichiometric (a), and defected Fe(110) surface with high (b) and lower (c) concentration of surface vacancies. The adsorption sites are marked: on-top (ot, ot1, ot2), pseudothreefold hollow (th), short bridge (sb) and long bridge (lb), at vacancy (av), and defected long bridge (dlb). The dashed yellow circle in figure (c) marks stoichiometric region at defected surface for 3 × 3 unit cell.
3. Results and discussion
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3.1. H2 O on stoichiometric Fe(110) surface Calculated geometric and energetic properties of the stoichiometric Fe(110) surface compare well with experimental and other theoretical results (cf. SI). Account for the vdW correction affects only weakly the properties of the surface. The main difference is observed in the surface energy value which is for PBE+D3 by 0.63 J/m2 higher than that for PBE (2.43 J/m2 ). The work function (4.81 eV) is only little higher than for PBE (4.77 eV). This small increase can be explained by the above mentioned small down-shift of the density of electronic states with respect to Fermi level when the vdW correction is included (cf. Figure S.1). Water molecules have been considered in four different sites: on-top (ot), short bridge (sb), long bridge (lb) and pseudo threefold coordinated hollow (th), with molecule’s oxygen pointing to the adsorption site (Fig.1a). Different initial orientations of vertically or horizontally oriented water at the Fe(110) surface were considered. During the simulation process, all initial configurations relaxed to one of the final geometries with H2 O located in ot, sb or lb site and with perpendicular or flat orientation with respect to the Fe(110). Water molecule placed in th hollow site migrated to ot site. The adsorption in lb site appeared also energetically unfavorable. From the stability point of view, the energetically most preferred geometry is that with H2 O molecule lying almost flat on-top Fe atom, which was also reported as most stable one by other authors for H2 O at Fe(110) [22, 23] as well as at Fe(001) surface [11–15]. Water molecule oriented perpendicularly to the surface in ot or sb sites, as well as in the place between ot and th sites is much less favored. The energetic and geometric parameters of most preferred configurations of the H2 O molecule lying flat on the Fe(110) surface in ot sites are listed in Table 1. It is seen that several final configurations are degenerated in adsorption energy (i.e., their energies differ by less than 5 meV) of about 300 meV for PBE and 500 meV for PBE+D3. Some of them were obtained for water molecules initially placed in th or lb sites. During the relaxation process the H2 O molecule rotated and finally has migrated to the ot site where it adsorbed with almost flat orientation to the Fe(110) surface. The GGA adsorption energy is of similar magnitude as in other theoretical calculations of H2 O on Fe(110): 260 meV [9], 343 6
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Table 1: Adsorption energy, Ead , changes in work function with respect to clean substrate, ∆Φ, Fe–O bond length, and HOH angle, for energetically most preferred configurations of 0.25 ML of water at stoichiometric and 2 × 2 defected Fe(110) surface.
Geometry
Ead (meV) PBE PBE+D3
∆Φ (eV) dFe−O (˚ A) PBE PBE+D3 PBE PBE+D3
]HOH (degree) PBE PBE+D3
H2 O at stoichiometric Fe(110) 296 297 297 271
500 497 500 470
−0.96 −1.13 −0.91 −0.97
−0.83 −1.02 −0.83 −0.90
2.27 2.26 2.27 2.30
H2 O at defected Fe(110) 344 345 348 326 304
586 566 588 494 512
−0.67 −0.44 −0.53 −0.94 −0.88
−0.66 −0.19 −0.39 −1.00 −0.89
2.28 2.25 2.26 2.27 2.29
104.6 105.4 108.1 104.2
104.5 105.2 104.5 104.2
2.25 2.23 2.24 2.25 2.29
104.5 105.0 104.5 105.5 105.0
104.6 104.3 104.5 104.3 105.3
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dsb1 dsb2 dsb3 ot1 dlb
2.25 2.25 2.25 2.29
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0 36.9 75.3 125.2
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meV [22], 380 meV [23]. The existing differences stem presumably from the fact that in ref. [9] perpendicular water was found as most stable configuration, whereas in [22] and [23] lower H2 O concentration (1/6 ML and 1/16 ML, respectively) were considered. The account for dispersion interactions enhances the H2 O adsorption energy by about 200 meV. This enhancement of Ead is of similar magnitude as that reported for water adsorption on Fe(110) (200 meV [22], 240 meV [23]) or other close-packed Cu, Au, and Pt(111) surfaces (200–250 meV [26]). As is seen from Figure 2 the geometries of above configurations are very similar (see also Table 1). However, some differences in the geometry of the systems are visible. Water molecules are not exactly parallel to the surface. The inclination angle of HOH plane to the surface varies in the range 3.2–9.4◦ both for PBE and PBE+D3. For lower H2 O coverage the inclination of about 15.4◦ was reported [22]. For both, PBE as well as for PBE+D3 calculations, depending on the configuration, differences are visible in HOH angle, H2 O inclination to the surface and in its shift from the ideal on-top site. The H2 O oxygen atom, for most energetically stable configurations, is shifted from an ideal on-top position by about 0.1–0.3 ˚ A for PBE as well as for PBE+D3 calculations. The HOH angle varies in the range 104.2– 108.1◦ (PBE) and 104.2–105.2◦ (PBE+D3). For PBE, the OH bond lengths are equal to 0.98 ˚ A. The water molecule binds to the surface through the oxygen atom with the O–Fe bond length of 2.26–2.30 ˚ A. The calculations with vdW correction yield similar properties of bonds, with O–Fe bond shorter only by about 0.01 ˚ A. Similar difference between O–Fe bond length found from PBE (2.21 ˚ A) and PBE+D3 (2.19 ˚ A) have been reported for water molecule adsorbed in 2 × 3 slab [22]. The Bader charges analysis [41] shows a small electron charge transfer (of about 7
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Figure 2: The adsorption energy as a function of the rotation angle of the H2 O molecule around axis perpendicular to the surface, measured with respect to the [001] crystallographic direction, for the most stable, final geometries of H2 O on 2 × 2 Fe(110). Due to the symmetry of the system, the values for 180–360◦ angles are the same as for 0–180◦ .
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0.05–0.1e) from the substrate to the waters oxygen atoms. H2 O gets an electron charge from the nearest Fe atoms. The amount of the charge transfer is similar for PBE and for PBE+D3 calculations. The small electronic charge transfer is connected with H2 O– Fe bonds. However, the presence of H2 O molecule on the surface causes redistribution of the electronic charge in the substrate. It results in a lowering of the surface dipole moment and consequently in a lowering the work function of the system. The degeneration in energy of the above mentioned final configurations means that a water molecule in the ot site, can rotate around direction perpendicular to the Fe(110) surface, with small barrier (30 meV) for this rotation, as observed for H2 O rotated by about 125◦ and 305◦ (cf. Fig. 2.). Moreover, for similar rotation angle, some differences (≤ 3.9◦ ) are visible in the HOH angle, although the geometries have the same total energy. It suggests that, besides rotation in the Fe(110) surface plane, an H2 O molecule can exhibit small oscillations of the HOH angle. Interestingly, for these very similar configurations, small differences in work function can be seen. Depending on H2 O orientation, the work function decreased by 0.91–1.13 eV (PBE) and 0.83–1.04 eV (PBE+D3) in comparison to the value of clean Fe(110) surface. Similar decrease of the work function upon water adsorption (0.71 eV, PBE; 0.63 eV, PBE+D3) was reported previously[22]. A lower H2 O coverage (0.11 ML) gives the same most preferred adsorption site and similar configurations of water molecule at the surface. However, the adsorption energy is higher by about 30 meV, which means that H2 O binding to Fe(110) surface weakens with the coverage. Stronger binding of water at the lower coverage reflects a shorter (by up to 0.06 ˚ A) Fe–O bond length. Its magnitude (2.21–2.25 ˚ A) is similar to the Fe–O bond length reported by other authors [22, 23] for coverages lower than 0.25 ML. Changes in the geometry and work function values are smaller for lower coverage, however, the 8
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character of changes is preserved. Adsorbed water molecule affects only weakly the geometry of the surface. Upon water adsorption interlayer distances in Fe(110) slab increase. However, the changes are small and in most cases limited to the first interlayer distance. For a single H2 O molecule, in 2 × 2 cell, in most stable configuration ∆12 is increased from about 0.24 % to 0.8 %. With vdW correction included, ∆12 is increased from 0.82% for clean surface, to about 1.1 %. The changes of deeper interlayer distances are much lower (≤ 0.1 %) and it may be assumed, that they do not differ from that of the clean Fe(110) surface. Thus finally it can be concluded that upon water adsorption the first interlayer distance is expanded by about 1 % with respect to the bulk distance.
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3.2. H2 O on defected Fe(110) surface To consider a defected Fe(110) surface a single vacancy was created by removing one of the Fe atoms from different layers of the relaxed surface slab, and subsequently the system has been relaxed. It is worth pointing out, that for defected surface, although atoms (except in frozen layers) could optimized their positions in all directions, lateral relaxations have not been observed. The vacancy formation energy calculated as a function of its depth (cf. Fig. S2) increases monotonically that the vacancy is easiest to form in the top surface layer. Therefore water adsorption at defected Fe(110) surface was studied for a substrate geometry with vacancy in the topmost layer of the slab, which is energetically the most stable one. A water molecule has been adsorbed in different adsorption sites presented in Figure 1, applying same procedure as for water adsorption on perfect Fe(110) surface. Similarly to stoichiometric surface, adsorption of water perpendicularly oriented to the defected surface is either unfavorable or unstable, and ends up in flat-lying or tilted molecule’s orientation. Eventually, upon relaxation water molecules were moved either to ot, ot1 sites, or to sites adjacent vacancy (called as defected long (dlb or short (dsb) bridge, or defected hollow sites). The final, most stable geometries of H2 O at 2 × 2 defected Fe(110) are presented in Figure 3 and in Table 1. It can be seen from Table 1 that there are few final configurations for which adsorption energy is within the range of 65 meV (PBE) and 75 meV (PBE+D3). The PBE+D3 gives higher adsorption energy by 200–240 meV. Adsorption close to dlb or to lb sites is weaker than in places close to the defected short bridge. However, the difference in Ead is lower than 70 meV. It may suggest that water molecule is able to change its position and may circle around the vacancy with energy barrier not higher than 70 meV. Interestingly water does not bind in the vacancy site. Water molecule placed initially in vacancy site is moved to one of the final configurations presented in Figure 3. It should be mentioned that flat adsorbed water molecule in ot1 site, which for high defected surface is placed between neighboring vacancies, is only by about 20 meV less stable. It may suggest, that water prefers to jump over ot1 site between neighboring most stable defected positions, than circling around vacancy. On the other hand, it can be understood that the movement of water molecules at highly defected Fe(110) surface may be considered as combination of oscillations around vacancy site and jumps between neighboring defected sites. Final geometries of PBE+D3 configurations are almost the same as for PBE. 9
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Figure 3: Top view of most stable final geometries of H2 O on 2×2 (a-d) and 3×3 (e-h) defected Fe(110) surface. The PBE adsorption energy is listed below pictures. The yellow dashed circles in panels (e) and (h) mark defect and stoichiometric regions, respectively.
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In the case of H2 O adsorption at defected surface, small variation of the HOH angle can be observed, which for the most stable configurations do not exceed 1◦ . The range of O–Fe bond lengths for H2 O at defected surface is similar as for adsorption at stoichiometric surface. For some of the most stable configurations at defected surface, the OH bonds are slightly asymmetric. The bond between O and the H atom located closer to the surface is by 0.01 ˚ A longer than between O and hydrogen atom located further from the surface. However, the OH bond lengths (0.98–0.99 ˚ A) are nearly the same as for adsorption at stoichiometric Fe(110) surface and differ not much from that for a free water molecule (0.97 ˚ A). Similarly to stoichiometric surface, at a defected surface water molecule gains electron charge. The charge transfer of about 0.08–1.1e, mostly from the nearest Fe atoms, is found both for PBE and PBE+D3 calculations. As for a stoichiometric surface, adsorption of H2 O molecule on the defected one results in the charge redistribution leading to reduction of the dipole layer moment and consequently to reduction of the work function. In most stable cases, water adsorption on defected Fe(110) surface reduces work function by 0.4–0.7 eV (PBE) and 0.2–0.7 eV (PBE+D3) compared to stoichiometric system (cf. Table 1). For some less stable cases, especially for vertically oriented H2 O, the changes in work function are larger. The sequence of magnitude of the change in the work function for final geometry is preserved. The largest decrease is observed (cf. Table 1) for the least stable configuration (dth) and the lowest for the most stable one (dsb2). The calculations performed for water adsorption at 3 × 3 defected Fe(110) surface allow to verify if H2 O prefers position on-top of sixfold coordinated substrate Fe atom or one of the positions near to the vacancy A flat lying water molecule was placed in different sites shown in Fig. 1c. An original orientation of the H2 O molecule with H–H 10
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line parallel to surface [001] direction was changed by a rotation of the molecule around the direction perpendicular to the surface by four different angles (22.5, 45, 67.5 and 90◦ ). For the undercoordinated places (ot1 and ot2) rotation in the opposite direction has been also applied. The results show (Fig. 3) that Ead for H2 O adsorbed in the stoichiometric region (ot) is lower by 187–395 meV than in sites near to the vacancy . Thus, it shows that water molecules prefer sites near the vacancy. However, similarly to adsorption at the 2 × 2 defected surface, water does not like to adsorb in the very vacancy site, but in sites at the edge of the vacancy (defected hollow, defected short bridge or defected long bridge). For these sites, Ead varies between 172 and 427 meV. Some configurations are energetically degenerated which suggests possibility of water circling around vacancy. On the other hand, configurations with flat lying H2 O molecule in ot1 sites are only by about 24 meV less favorable. It suggests that there is a quite low energy barrier for H2 O move between neighboring defected sites above ot1 positions, and similarly to the higher defected one, mobility of H2 O molecules at less defected surface is a combination of a circling around vacancy and jumps between defected sites. Similarly to stoichiometric surface, adsorption of water at defected Fe(110) causes extra expansion of the first substrate interlayer distance in comparison with the clean, defected surface. For H2 O at 2 × 2 defected Fe(110), the relaxation of the first interlayer distance (∆12 ) for most energetically stable geometries is equal to −4.6 % (PBE), and −3.9 % (PBE+D3). The ∆23 = 1.4% (PBE) and 1.3% (PBE+D3), and ∆34 = 0.2 % (PBE) and 0.0 % (PBE+D3) are lower than for clean substrate (both for PBE and PBE+D3) by about 0.35 % and 0.20 %, respectively. This means that, compared to the clean, defected substrate, a small contraction of the second and third interlayer distances can be observed upon H2 O adsorption. Simultaneously very small lateral displacements (less than 0.03 ˚ A) of substrate atoms adjacent to H2 O are observed. However, all changes are relatively small which means that adsorbed water does not affect strongly the geometry of the substrate. 4. Summary and conclusions
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We reported results of the theoretical investigation of water adsorption on stoichiometric and defected bcc iron (110) surface using DFT and DFT+D3 calculations taking into account van der Waals correction. It is found that at stoichiometric Fe(110) surface a water molecule adsorbs molecularly on-top of Fe atoms and is oriented horizontally to the surface. There is almost no difference in adsorption energy for system with H2 O rotated around the direction perpendicular to the surface. However, some geometrical differences are visible. Depending on the rotation angle of parallel adsorbed water molecules, the H2 O/Fe(110) system differs in work function changes, and slightly in the geometry. Water molecule adsorbed on a defected surface prefers few different configurations in defected sites nearby the vacancy. A little more preferred is a configuration with one of the hydrogen atoms closer to the vacancy. Flat lying water, adsorbed over on-top Fe atom adjacent to vacancy is only slightly less stable, and H2 O molecule can easy jump between different stable places or simply circle around ot1 site. Adsorption of water at the surface with lower concentration of vacancies shows that water prefers 11
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to adsorb in sites nearby defect than in stoichiometric like region. The account for van der Waals interactions enhances the water adsorption energy and does not substantially affect the geometry of the system. Acknowledgments
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This work was supported by the National Science Centre (NCN), Poland (Grant No. 2012/07/B/ST3/03009). We acknowledge computer time granted by the Interdisciplinary Centre for Mathematical and Computational Modelling (ICM) of the Warsaw University within the project No G44-23. One of us (JLFDS) thanks the University of Wroclaw for a visiting professorship within Human Capital Operational Program – Academy of Development as the Key to Strengthen Human Resources of the Polish Economy.
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References References
[1] P. A. Thiel, T. E. Madey, The Interaction of Water with Solid Surfaces: Fundamental Aspects, Surface Science Reports 7 (1987) 211–385.
M
[2] M. A. Henderson, The interaction of water with solid surfaces: fundamental aspects revisited, Surface Science Reports 46 (2002) 1–308. [3] A. Hodgson, S. Haq, Water adsorption and the wetting of metal surfaces, Surface Science Reports 64 (2009) 381–451.
ED
[4] P. Blo´ nski, A. Kiejna, Structural, Electronic, and Magnetic Properties of bcc Iron Surfaces, Surface Science 601 (2007) 123–133.
PT
[5] D. J. Dwyer, G. Simmons, R. Wei, A Study of the Initial Reaction of Water Wapor with Fe(001) Surface, Surface Science 64 (1977) 617–632.
CE
[6] A. B. Anderson, Reactions and Structures of Water on Clean and Oxygen Covered Pt(111) and Fe(100), Surface Science 105 (1981) 159–176.
AC
[7] W.-H. Hung, J. Schwartz, S. L. Bernasek, Sequential Oxidation of Fe(100) by Water Adsorption: Formation of an Ordered Hydroxylated Surface, Surface Science 248 (1991) 332–342. [8] W.-H. Hung, J. Schwartz, S. Bernasek, Adsorption of H2O on Oxidized Fe(100) Surfaces: Comparison Between the Oxidation of Iron by H2O and O2, Surface Science 294 (1993) 21–32. [9] M. Eder, K. Terakura, J. Hafner, Initial Stages of Oxidation of (100) and (110) Surfaces of Iron Caused by Water, Phys. Rev. B 64 (2001) 115426.
[10] J. M. H. Lo, T. Ziegler, Density Functional Theory and Kinetic Studies of Methanation on Iron Surface, The Journal of Physical Chemistry C 111 (2007) 11012–11025. 12
ACCEPTED MANUSCRIPT
[11] S. C. Jung, M. H. Kang, Adsorption of a Water Molecule on Fe(100): DensityFunctional Calculations, Phys. Rev. B 81 (2010) 115460. [12] A. Govender, D. Curulla Ferr´e, J. W. Niemantsverdriet, The Surface Chemistry of Water on Fe(100): A Density Functional Theory Study, ChemPhysChem 13 (2012) 1583–1590.
CR IP T
[13] R. R. Q. Freitas, R. Rivelino, F. de Brito Mota, C. M. C. de Castilho, Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study, The Journal of Physical Chemistry C 116 (2012) 20306–20314. [14] P. Lazar, M. Otyepka, Dissociation of Water at Iron Surfaces: Generalized Gradient Functional and Range-Separated Hybrid Functional Study, The Journal of Physical Chemistry C 116 (2012) 25470–25477.
AN US
[15] S. Liu, X. Tian, T. Wang, X. Wen, Y.-W. Li, J. Wang, H. Jiao, High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis, The Journal of Physical Chemistry C 118 (2014) 26139–26154. [16] F. Karlick´ y, P. Lazar, M. Dubeck´ y, M. Otyepka, Random Phase Approximation in Surface Chemistry: Water Splitting on Iron, Journal of Chemical Theory and Computation 9 (2013) 3670–3676.
M
[17] A. Kiejna, T. Ossowski, Water adsorption on iron surfaces, in: Reference Module in Chemistry. Molecular Sciences and Chemical Engineering, Elsevier, 25-Jul-2017. doi:10.1016/B978-0-12-409547-2.14154-X.
ED
[18] A. Michaelides, V. A. Ranea, P. L. de Andres, D. A. King, General Model for Water Monomer Adsorption on Close-Packed Transition and Noble Metal Surfaces, Phys. Rev. Lett. 90 (2003) 216102.
PT
[19] J. Carrasco, A. Hodgson, A. Michaelides, A molecular perspective of water at metal interfaces, Nature Materials 11 (2012) 667–674.
CE
[20] D. J. Dwyer, S. R. Kelemen, A. Kaldor, The Water Dissociation Reaction on Clean and Oxidized Iron (110), The Journal of Chemical Physics 76 (1982) 1832–1837.
AC
[21] A. M. Bar´o, W. Erley, The Adsorption of H2O on Fe(100) Studied by EELS (My Comment: This is Fe(110) Surface), Journal of Vacuum Science and Technology 20 (1982) 580–583. [22] P. Tereshchuk, J. L. F. Da Silva, Ethanol and Water Adsorption on Close-Packed 3d, 4d, and 5d Transition-Metal Surfaces: A Density Functional Theory Investigation with van der Waals Correction, The Journal of Physical Chemistry C 116 (2012) 24695–24705. [23] S. Liu, X. Tian, T. Wang, X. Wen, Y.-W. Li, J. Wang, H. Jiao, Coverage Dependent Water Dissociative Adsorption on Fe(110) from DFT Computation, Phys. Chem. Chem. Phys. 17 (2015) 8811–8821. 13
ACCEPTED MANUSCRIPT
[24] J. Carrasco, B. Santra, J. c. v. Klimeˇs, A. Michaelides, To Wet or Not to Wet? Dispersion Forces Tip the Balance for Water Ice on Metals, Phys. Rev. Lett. 106 (2011) 026101. [25] K. Tonigold, A. Groß, Dispersive interactions in water bilayers at metallic surfaces: A comparison of the PBE and RPBE functional including semiempirical dispersion corrections, Journal of Computational Chemistry 33 (2012) 695–701.
CR IP T
[26] R. L. H. Freire, A. Kiejna, J. L. F. Da Silva, Adsorption of water and ethanol on noble and transition-metal substrates: a density functional investigation within van der Waals corrections, Phys. Chem. Chem. Phys. 18 (2016) 29526–29536. [27] G. Kresse, J. Hafner, Ab initio Molecular Dynamics for Liquid Metals, Physical Review B 47 (1993) 558–561.
AN US
[28] G. Kresse, J. Furthm¨ uller, Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set, Physical Review B 54 (1996) 11169– 11186. [29] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation made Simple, Physical Review Letters 77 (1996) 3865–3868.
M
[30] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu, The Journal of Chemical Physics 132 (2010) 154104.
ED
[31] J. VandeVondele, F. Mohamed, M. Krack, J. Hutter, M. Sprik, M. Parrinello, The influence of temperature and density functional models in ab initio molecular dynamics simulation of liquid water, The Journal of Chemical Physics 122 (2005) 014515.
CE
PT
[32] L.-M. Liu, M. Krack, A. Michaelides, Interfacial water: A first principles molecular dynamics study of a nanoscale water film on salt, The Journal of Chemical Physics 130 (2009) 234702.
AC
[33] K. Forster-Tonigold, A. Groß, Dispersion corrected RPBE studies of liquid water, The Journal of Chemical Physics 141 (2014) 064501. [34] P. E. Bl¨ochl, Projector Augmented-Wave Method, Physical Review B 50 (1994) 17953–17979. [35] G. Kresse, D. Joubert, From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method, Physical Review B 59 (1999) 1758–1775. [36] H. J. Monkhorst, J. D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188–5192. [37] M. Methfessel, A. T. Paxton, High-precision sampling for Brillouin-zone integration in metals, Phys. Rev. B 40 (1989) 3616–3621. 14
ACCEPTED MANUSCRIPT
[38] J. Neugebauer, M. Scheffler, Adsorbate-Substrate and Adsorbate-Adsorbate Interactions of Na and K adlayers on Al(111), Phys. Rev. B 46 (1992) 16067–16080. [39] M. D. Harmony, V. W. Laurie, R. L. Kuczkowski, R. H. Schwendeman, D. A. Ramsay, F. J. Lovas, W. J. Lafferty, A. G. Maki, Molecular Structures of GasPhase Polyatomic Molecules Determined by Spectroscopic Methods, Journal of Physical and Chemical Reference Data 8 (1979) 619–722.
CR IP T
[40] W. S. Benedict, N. Gailar, E. K. Plyler, RotationVibration Spectra of Deuterated Water Vapor, The Journal of Chemical Physics 24 (1956) 1139–1165.
AC
CE
PT
ED
M
AN US
[41] G. Henkelman, A. Arnaldsson, H. J´onsson, A Fast and Robust Algorithm for Bader Decomposition of Charge Density, Computational Materials Science 36 (2006) 354– 360.
15