Hydration Structures of MgO, CaO, and SrO (001) Surfaces - The

Article pubs.acs.org/JPCC

Hydration Structures of MgO, CaO, and SrO (001) Surfaces Milan Ončaḱ ,* Radosław Włodarczyk,† and Joachim Sauer Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany S Supporting Information *

ABSTRACT: Using density functional theory (PBE functional), we show that the degree of surface hydroxylation increases in the MgO, CaO, SrO series, accompanied by an increase in water adsorption energy. Already for water coverage of two monolayers, structures with dissolved M2+ ions are considerably more stable than the intact, nondissolved surface. The dissolved ions above the surface form different patterns including ordered ones (e.g., an infinite stripe) that are preferred for MgO(001) and CaO(001) and disordered ones that are favored for SrO(001). Contrary to previous assignments, an analysis of calculated X-ray photoelectron spectra shows that O(1s) signals arising from OH and H2O groups might coincide in the experimental spectrum.

adsorption of single water molecules.31−36 Gradually increasing water adsorption energies in the MgO−CaO−SrO−BaO series were found, with dissociation of the first adsorbed water molecule being energetically favorable already for the CaO(001) surface.32,34 Several computational studies considered higher water coverages, mostly on the MgO surface,6,7,22,37−45 but also on CaO4,46,47 and BaO.48 Generally, a large fraction of dissociated water molecules was found already for monolayer coverage. A joint experimental-theoretical study suggested formation of a “square pits” pattern in acidic solutions.37 Various adsorption possibilities were considered for the H2O/CaO(001) system,46 including the formation of water chains.4 The H2O/CaO(001) interaction for higher water coverage was explored using Reactive Force Field (ReaxFF),47 this study showed that the CaO(001) surface dissolves extensively at 300 K already for one water monolayer. Finally, the O(1s) XPS signal of water adsorbed on the MgO(001)49 and CaO(001)28 surfaces was recently analyzed using quantum chemical methods.

1. INTRODUCTION Interaction of metal oxides with water represents a complex process:1−5 The metal oxide surface can possess different orientations and surface defects; water molecules may form various surface structures (ice-like nanoclusters, water chains), dissociate, and alter surface properties. In previous publications from this laboratory,6,7 the structures of one to two water monolayers (ML) adsorbed on the MgO(001) surface have been investigated. It was shown that for 1 ML, water partially dissociates on the surface.6 Already for the relatively low coverage of 2 ML, movement of Mg2+ ions out of the MgO(001) surface and formation of unanticipated structures was found to be energetically favorable.7 Here, we explore further such “dissolution” patterns and extend our investigation to the CaO(001) and SrO(001) surfaces. We use “dissolution” in the sense that structures are formed in which M2+ ions have left the surface layer and are located above the MO(001) surface. We focus on trends with increasing hydration in the MgO−CaO−SrO series, and we limit ourselves to a subgroup of possible dissolution patterns. Specifically, we analyze the influence of the surface reconstruction on the formation of OH groups and on the expected XPS signal. The structure of hydrated alkaline earth metals oxide surfaces was subject of various experimental studies.6,8−28 Knowledge on hydration of various MgO facets also comes from experiments on MgO crystals.29,30 For low water coverages of the H2O/MgO(001) system, complex structural patterns have been observed.6,17,23,26 Water forms partially dissociated films on the MgO(001) surface, with different symmetry for different temperature ranges. For both H2O/MgO8−13 and H2O/ CaO10,14−16,28 systems, XPS studies revealed pronounced surface hydroxylation upon hydration. Adsorption of water molecules on alkaline earth metal oxides has also been studied computationally, starting with the © 2016 American Chemical Society

2. METHODS Water on the alkaline earth metal oxide surface was modeled with periodic boundary conditions. We used a four-layer slab with a (4 × 4) MO(001) surface unit cell, M = Mg, Ca, Sr, corresponding to a total composition of M64O64. In the zdirection, a cell vector of 30 Å was used, providing a gap of at least 15 Å between periodic images. Atomic positions of the two lowermost MO layers were kept frozen to mimic the presence of the bulk metal oxide. Only the Γ-point was used in the reciprocal space. Density functional theory (DFT) was Received: July 25, 2016 Revised: October 12, 2016 Published: October 21, 2016 24762

DOI: 10.1021/acs.jpcc.6b07434 J. Phys. Chem. C 2016, 120, 24762−24769

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The Journal of Physical Chemistry C

diagram. Details on these models are given in the Supporting Information (Table S1). Single water molecule in the gas phase was calculated using the energy cutoff of 400 eV in a 12 Å × 12 Å × 12 Å cell.

applied, and the calculations were performed with the Vienna Ab Initio Simulation Package (VASP 5.2).50,51 Plane wave basis sets were applied combined with the projected augmented wave method for the core electrons.52 We employed the Perdew− Burke−Ernzerhof (PBE) density functional53,54 along with the dispersion correction introduced by Grimme (PBE+D2).55 The structures of clean metal oxide surfaces and of surfaces with one water molecule were optimized using standard local optimization algorithms with a cutoff of 400 eV and convergence criteria of 10−6 eV for both electronic and ionic relaxation. To sample structures of hydrated metal oxides surfaces, molecular dynamics was performed at 300 K with a time step of 1 fs. The electronic convergence criterion of 10−4 eV was used. To speed up the molecular dynamics runs, a soft pseudopotential was used for oxygen atoms, along with a lower cutoff of 250 eV. The total running time was 50 ps, the electronic energy was followed along the dynamics. If a new low-lying isomer appeared in last 20 ps (documented by a drop in average energy along the molecules dynamics run of at least 30 kJ/mol), further 20 ps of molecular dynamics were performed. From the last 20 ps of the molecular dynamics trajectory, one structure per 1 ps was taken and optimized to the nearest lying local minimum, first at the energy cutoff of 250 eV and then refined at the cutoff of 400 eV and with standard pseudopotential for the oxygen atom. The most stable one among 20 structures taken from the molecular dynamics was reoptimized using the convergence criteria of 10−6 eV for both electronic and ionic cycles (with the same cutoff energy, 400 eV). Vibrational analysis within the harmonic approximation was performed for each of the optimized structures to validate that the given structure represents a local minimum. XPS spectra were calculated for the most stable structure of a given composition, with the convergence criteria of 10−7 eV. The transition energy was calculated using the initial state approximation to avoid computationally demanding final state approximation.56 The initial state approximation is able to reproduce qualitatively the surface core-level binding energy shifts (SCLS) of the clean MgO surface. The SCLS was calculated to be 0.14 eV for O(1s) and 0.36 eV for Mg(2p); experimentally, it was found to be close to 0 eV for O(1s)57 and about 0.30 and 0.65 eV for MgO(001) films of 4−5.5 and 30 MgO monolayers, respectively.12,57 We note that inclusion of final states effects leads to larger separation between the respective O(1s) peaks.28,49 The width of the XPS spectra was modeled using the Gaussian broadening scheme. The chosen width of 0.3 eV reproduces the experimental peak widths of the water-covered MgO surface.12 The XPS transitions of the lowermost MO layer are not included in the simulated spectra. The lattice constants of MgO, CaO, and SrO structures were obtained using optimization of an MO cell of M 48 O48 composition with an energy cutoff of 600 eV and a (2 × 2 × 2) Monkhorst−Pack k-point mesh. The following lattice constants resulted and were used for structures in the present study: 418.4 pm (MgO), 477.7 pm (CaO), 512.5 pm (SrO), lying within 0.7% with respect to available experimental data (421.2 pm for MgO, 481.1 pm for CaO,58 and 514.0 pm for SrO59). Vibrational zero-point corrections are included in all reported energies. Phase diagrams were constructed from Gibbs free energies obtained within the harmonic approximation. To compare our structures with MgO, CaO, and SrO surfaces with lower water coverage (up to about one monolayer) investigated before,4 we included the respective models into our phase

3. RESULTS AND DISCUSSION 3.1. Structures and Energies. To scan various conformations of water on the (4 × 4) MO(001) surface, we selected 12 different MO·(H2O)n systems, n = 1−32 (see Figure 1 and Figures S1−S3 in the Supporting Information).

Figure 1. Selected structures considered. Red dots represent O2− ions, green and yellow dots represent M2+ ions on the surface and above the surface, respectively. Black dots show surface positions from which M2+ ions were removed. Position of water molecules is omitted for clarity. Specific stable structures for MgO, CaO, and SrO are shown in the second column.

Including 32 water molecules is formally equivalent to including two water monolayers, i.e., two water molecules per one M2+ ion in the surface layer. However, we do not use this nomenclature, as it might be misleading when water monolayers are defined on the basis of the density of water molecules on the surface. Due to the larger lattice constant of, e.g., the SrO(001) surface, the same number of water molecules has a 33% smaller density on the SrO(001) surface than on the MgO(001) surface. To cover various surface dissolution patterns, three groups of structures have been selected. (i) The intact surface with water coverage of 1, 20, and 32 water molecules, denoted as intact-1, intact-20, and intact, respectively. The intact-20 structure was included because it was shown to be particularly stable for the H2O/MgO system.6 (ii) Structures with 32 H2O molecules and 1−8 dissolved M2+ ions with no regular dissolution pattern. These structures are named 1-dis, 2-dis, 4-dis, and 8-dis, representing dissolution of 6%, 13%, 25%, and 50% of M2+ ions, respectively. In the 1-dis and 2-dis structures, the M2+ ions are in direct contact with the surface. Two dissolution patterns are considered for the 4-dis structure, with dissolved ions in contact with the surface (4-dis-A) and separated from the surface by solvent molecules (4-dis-B). Only ions separated from the surface were considered for the 8-dis structure. (iii) Structures with 32 H2O molecules and 4 or 8 dissolved M2+ ions that form an ordered structure, an island or a stripe. One (2 × 2) island structure and three stripe conformations were investigated, single horizontal (stripe-A) and diagonal stripes 24763


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Table 1. Average Adsorption Energy ΔE (in kJ/mol per H2O Molecule) of Water Molecules on the MgO, CaO, and SrO Surface, Number of Water Molecules, nH2O, Dissolution Fraction Used as the Starting Point of Molecular Dynamics, f M, and Hydroxylation Parameter, θOH, As Defined in the Main Text MgO

a

CaO

SrO

structure

nH2O

fM

ΔE

θOH

ΔE

θOH

ΔE

θOH

intact-1 intact-20 intact 1-dis 2-dis 4-dis-A 4-dis-B 8-dis island stripe-A stripe-B stripe-C

1 20 32 32 32 32 32 32 32 32 32 32

0.00 0.00 0.00 0.06 0.13 0.25 0.25 0.50 0.25 0.25 0.25 0.50

−51.8 −82.1a −68.4 −71.0 −71.3 −70.2 −61.6 −63.5 −69.9 −73.7 −72.2 −63.8

0.00 0.63a 0.75 1.00 1.13 1.63 1.88 2.25 1.38 1.75 1.63 1.50

−92.2 −89.0 −76.9 −78.6 −81.1 −81.5 −81.1 −84.7 -

0.13 0.88 0.75 1.25 2.13 2.50 1.50 1.75 -

−123.3 −100.0 −88.0 −93.2 −98.8 −99.3 −92.0 −93.5 -

0.13 1.13 1.13 1.63 2.38 2.75 1.75 1.88 -

For the structure obtained by genetic algorithm search,6 ΔE = −82.9 kJ/mol, θOH = 0.50.

(stripe-B) with respect to orientation of the cell and a horizontal double-stripe structure (stripe-C). Figure 1 shows selected investigated structures. All investigated structures are depicted in Figures S1−S3 in the Supporting Information. Table 1 shows the average adsorption energies per water molecule. To describe the degree of surface hydroxylation, we introduce the hydroxylation parameter θOH, which is also reported in Table 1: n θOH = OH n M2+

The difference between both adsorption energies is less than 1 kJ/mol/H2O, showing that the molecular dynamics approach is able to locate alternative structures that are low in energy. For an intact (4 × 4) MgO(001) surface with 32 water molecules, intact, the adsorption energy amounts to −68 kJ/ mol per H 2 O. As already shown previously, 7 further stabilization might be gained by dissolution of Mg2+ ions from the surface. Dissolution of one Mg2+ ion (i.e., 6% of surface Mg2+ ions, 1-dis structure) leads to stabilization of 2.6 kJ/mol per H2O. Dissolution of a second Mg2+ ion (2-dis) does not stabilize the system significantly, and further dissolution (4dis-A, 4-dis-B, 8-dis) is energetically unfavorable. When more organized structures are formed, i.e., island and stripes, the water adsorption energy increases with respect to the intact structure. In particular, stripe-A and stripe-B structures with infinite stripes are the most stable ones found for the MgO(001)·32 H2O system. The stability of the island structure is comparable to that of the hydrated intact surface, whereas the double-stripe, stripe-C, structure is less stable, similar to the 8-dis isomer. The stripe-A structure was already introduced in our previous publication.7 The increase in the number of dissolved Mg2+ ions is accompanied by an increase in the number of OH groups formed on the surface. Already for the intact structure, a substantial amount of water molecules is dissociated, with 0.75 OH groups per Mg2+ surface ion. The hydroxylation parameter for the 1-dis structure is θOH = 1.00, and it further increases with progressing surface dissolution to 1.13 (2-dis), 1.63 (4disA), and 2.25 (8-dis). This increase is driven by healing of defects created on the surface. As shown in Figure 2a, an Mg2+hole created on the H2O/MgO(001) surface is compensated by formation of up to 4 OH groups on neighboring oxygen positions. For the most stable stripe-A structure, 1.75 OH groups per Mg2+ surface ion are formed, close to 2 for Mg(OH)2, brucite. This is connected with the formation of an OH-layer on the formed stripe of Mg2+ ions, see Figure 2b, which also shows that the coordination of Mg2+ ions in the stripe-A structure closely resembles the one of Mg(OH)2. 3.3. H2O/MgO(001) XPS Spectra. Hydration of the MgO surface is also reflected in the experimental O(1s) and Mg(2p) XPS spectra.8−13 For the intact MgO(001) surface, surface and bulk oxygen ions cannot be experimentally distinguished

where nOH denotes the total number of OH groups and nM2+ the number of M2+ ions in the topmost layer of the respective intact MO surface. For pure MO, θOH = 0, for M(OH)2, θOH = 2. To describe the M2+ dissolution process, we introduce the dissolution fraction f M: 2+ n Mdis fM = n M2+ 2+ where n2+ Mdis is defined as the number of dissolved surface M 2+ ions, i.e., M ions displaced above the MO surface at the beginning of the molecular dynamics run. 3.2. H2O/MgO(001). Adsorption of one water molecule on the MgO(001) surface is not dissociative34 and the adsorption energy of −52 kJ/mol (Table 1) is in reasonable agreement with previous theoretical results of −48 kJ/mol (DFT/ PW91)34 and −46 kJ/mol (DFT/PBE).32 When 20 water molecules are adsorbed on the (4 × 4) MgO(001) surface (intact-20), the average adsorption energy per water molecule increases to −82 kJ/mol. The calculated adsorption enthalpy at 203 K is −87 kJ/mol, close to the measured adsorption enthalpy of −85 ± 2 kJ/mol for one water monolayer60 (we note that the intact-20 structure contains 1.25 water molecules per surface MgO unit, formally equivalent to 1.25 ML). The structure shows already a considerable degree of water dissociation, with 0.63 OH groups produced per Mg2+ surface ion. In Table 1, we also compare the water adsorption energies of the intact-20 structure found within the present study with that found before with the genetic algorithm for the (4 × 2) cell.6

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Figure 2. (a) Healing of an Mg2+-defect by formation of OH groups, top view (left) and isometric view (right) of the upmost surface layer. The black dot shows the position from which the Mg2+ ion was removed. (b) Comparison of the stripe-A structure and Mg(OH)2, showing similar coordination patterns of Mg2+ ions. The dashed line shows the plane along which the stripe-A structure is visualized in the central image, water molecules further from the surface are omitted for clarity, OH groups are shown in darker red color.

Figure 3. Simulated O(1s) and Mg(2p) XPS spectra of various H2O/ MgO(001) structures, along with decomposition into various contributions. Included structures contain 32 water molecules unless stated otherwise. Spectra were shifted by the bulk core-level shift calculated for the waterless MgO(001) surface. Individual contributions from various transitions are given for the intact-20 structure. Color legend: O(1s) − black: total spectrum, red: O2−, green: OH, blue: H2O; Mg(2p) − black: total spectrum, red: second and third layer, green: surface layer. Experimental spectra are included12 for the following relative humidity: O(1s) − 0.03% (light blue), 0.1% (violet), 8% (brown); Mg(2p) − 0.2% (violet).

because of the negligible shift of the O(1s) core-level binding energies.57 After the MgO(001) surface is treated with water, three O(1s) XPS peaks can be distinguished, interpreted as O2− ions of the MgO surface with a shift of −0.5 eV with respect to the intact MgO surface, OH ions with a shift of about +2 eV,6,8,10−13 and, for relative humidity higher than about 2%, an H2O peak appears as a shoulder at about +3 eV.12 According to this interpretation, the OH-peak at +2 eV is formed already for relatively low humidity, assuming an OH-layer on the MgO surface or, in other words, complete dissociation of adsorbed water molecules. However, computational studies showed that a certain minimum loading of intact water molecules on the MgO(001) surface is needed for water dissociation.6,7 In fact, on the intact MgO(001) surface with 1.25 ML of water, the number of OH groups was smaller than the number of nondissociated water molecules6 and the “OH peak” would then include contributions from both OH groups and H2O molecules. Also, the O(1s) peak of Mg(OH)2 measured on mechanically polished magnesium electrodes is shifted by +0.8 eV with respect to the MgO(001) surface.61 This OH peak of Mg(OH)2 is shifted −1 eV away from the peak interpreted as the “OH peak” in experiments on highly ordered MgO(001) films.6,8,10−13 We note, however, that the experiment in ref 61 was performed on polycrystalline samples. A broad Mg(2p) XPS peak has been observed for the hydrated MgO(001) surface,8,12 but shifts to both lower12 and higher binding energies8 in the course of surface hydration were reported. Figure 3 shows simulated O(1s) and Mg(2p) XPS spectra for various structures with 20 and 32 water molecules along with the experimental spectrum for several values of relative humidity.12 The axis origin (0 eV) corresponds to the corelevel shift calculated for the intact MgO(001) surface. The XPS spectrum for the intact-20 structure will be analyzed in more detail as an example. In the O(1s) spectrum, two broad peaks can be distinguished. The first peak near 0 eV has contributions from O2− ions of MgO, most of the signals are located between −0.3 and 0.1 eV. Surface O2− ions contribute to the signal at about −0.5 eV. The second peak at +2 eV is composed of two contributions, originating from OH groups and H2O molecules. The OH contribution spans a region of about 2 eV. Lower transitions at about +1 eV can be attributed to OH groups attached to the surface or above the surface, the transitions at

about +2 eV to surface OH groups. On the contrary, the H2O contribution is relatively sharp, localized within 1 eV. When the final state effects are included, the relative separation of the O(1s) peaks increases, allowing one to distinguish OH and H2O peaks at 100 K, whereas one broad XPS band located between +1 and +4 eV is predicted for simulations at 185 and 300 K.49 For hydrated structures with a larger number of water molecules (32), the position of the O2− component in the XPS O(1s) spectra does not change much, whereas position and width of the OH and H2O peaks is significantly affected by surface dissolution. In the O(1s) spectrum of the intact structure, only two peaks can be distinguished, separated by approximately 2 eV. The OH peak at about +1 eV starts to grow for the 1-dis structure and becomes more pronounced for 2-dis and stripe-A, in accord with the increasing number of OH groups. For the stripe-A structure, the OH peak is well-resolved because of the highly uniform character of formed OH groups (see Figure 2b). Our predicted relative shift is close to the O(1s) Mg(OH)2 peak at +0.8 eV,61 further pointing to the similarity of the stripe-A and brucite Mg(OH)2 structures (see also Figure 2b). The calculated Mg(2p) XPS spectra show a broad peak with two contributions: from inner layers and from the surface layer. With increasing surface dissolution, the peak corresponding to surface Mg2+ ions shifts to higher values, creating a shoulder in the total spectrum. The relative position of the experimental XPS peaks for lower humidity (light blue in Figure 3, top) is well reproduced by our calculations. In the O(1s) spectrum, the O2− peak is shifted to lower energies along with solvation, two peaks at 0 and +2 eV are seen along with hydration of the MgO(001) surface, in full agreement with available experimental 24765


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The Journal of Physical Chemistry C data.8,10−13 Our calculations, however, offer a different assignment, showing that the experimentally assigned “OH peak” may include contributions from both OH groups and H2O molecules. A broad peak at +3−4 eV appears for higher relative humidity and is assigned in the experiment as the H2O peak.12 This peak cannot be distinguished in the simulated spectra. However, XPS spectra were calculated for structure models that contain at most 32 H2O per (4 × 4) MO(001) surface unit cell, which might not be enough to simulate the MgO(001) surface at 8% relative humidity. We also note that, in this study, we limit ourselves to the initial state approximation for calculation of the XPS spectra, while inclusion of the final state effects shifts the relative position of O(1s) peaks to higher values.28,49 If we consider such a shift of calculated peaks for higher water coverages, we might interpret three peaks in, e.g., the stripe O(1s) spectrum (at about 0.0, 1.2, and 2.1 eV), to represent Ox, OH, and H2O peaks in the experimental spectrum for high relative humidity (at about −0.5, 2.0, and 4.0 eV, see brown curve in Figure 3). However, this interpretation is incompatible for lower water coverage where only one peak at about 1.8 eV is seen in the experiment, while our calculations show that both OH groups and intact water molecules are present on the MgO(001) surface under such conditions. In the Mg(2p) spectrum, the calculated shift of 0.4 eV between both components for the intact-20 structure is close to the experimental value of about 0.3 eV for low relative humidity on 4−5.5 ML MgO(001).12 After 32 water molecules are adsorbed, the shift increases to 0.5−0.7 eV, in agreement with experimental results for higher water coverage.12 3.4. H 2O/CaO and H 2O/SrO. The CaO(001) and SrO(001) surfaces are much more reactive toward water than the MgO(001) surface. Dissociation of a single water molecule is thermodynamically favorable for both CaO(001) and SrO(001) surfaces.34 This is reflected in the increased adsorption energies of −92 and −123 kJ/mol, respectively, compared to −52 kJ/mol for MgO(001) (Table 1, PBE+D2). Our calculated values are in good agreement with previous calculations of −87 and −121 kJ/mol for CaO(001) and SrO(001) surfaces, respectively (PBE functional without dispersion).32 For higher water coverages, we considered the intact structures, the nonordered 1-dis, 4-dis-B, and 8-dis dissolved structures, and the ordered dissolved island and stripe-A structures. For the intact structures, there is a constant increase of the adsorption energy of about 10 kJ/mol per water molecule in the MgO−CaO−SrO sequence, and the number of OH groups also increases significantly. For the CaO(001) and SrO(001) surfaces, different trends for structures with high water coverage are observed. The stability of H2O/CaO(001) structures resembles the H2O/ MgO(001) case. The most stable structure is still stripe-A, which is about 8 kJ/mol per H2O more stable than the intact structure. Furthermore, other highly dissolved structures (4dis-B, 8-dis, island) are close in energy, showing a clear tendency of the CaO(001) surface to dissolute in a less ordered manner, in contrast to the trend obtained for the MgO(001) surface. For water on the SrO(001) surface, the tendency to dissolve the surface is even more pronounced. The stripe-A structure is not the most stable structure anymore, but it is still by about 6 kJ/mol per H2O more stable than the intact structure. The most stable structures are 4-dis-B and 8-dis with Sr2+ ions

separated from the surface. They have almost the same stability and are about 11 kJ/mol per H2O more stable than the intact structure. Obviously, nonordered dissolved surface structures are preferred for a given number of water molecules. The calculated XPS spectra of water on the CaO(001) and SrO(001) surfaces resemble the MgO(001) spectra (Figure 4).

Figure 4. O(1s), Ca(2p), and Sr(3p) XPS spectra of various H2O/ CaO (top) and H 2 O/SrO (bottom) structures, along with decomposition into various contributions. Included structures contain 32 water molecules unless stated otherwise. Spectra were shifted by the bulk core-level shift calculated for waterless CaO(001) and SrO(001) surfaces. Individual transitions are analyzed for the intact-20 structure. Color legend: O(1s) − black: total spectrum, red: O2−, green: OH, blue: H2O; Ca(2p)/Sr(3p) − black: total spectrum, red: second and third layer, green: surface layer.

In the O(1s) spectra, three signal contributions can be distinguished: O2− at 0 eV, OH at about +1.0 to +1.5 eV, and H2O at about +2.0 to +2.5 eV. Their shape is affected by the high degree of OH formation on the surface, with a remarkably distinguished OH-peak for the 8-dis and stripe-A structures. For the intact and 1-dis structures, OH and H2O peaks merge into one broad peak. Experimental O(1s) XPS spectra of the hydrated CaO(001) surface are composed of two peaks, interpreted as an O2− peak at 0 eV and an OH peak at about +2.0 to +2.5 eV.10,14−16 For the CaO(001) surface fully immersed in water, the first peak disappears completely.15 The XPS spectra of both O(1s) and Ca(2p) coincide with the spectra of the Ca(OH)2 surface after 24766


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The Journal of Physical Chemistry C full immersion in water.62 Our calculations suggest that the experimental peak at 2.0 to 2.5 eV is composed of both OH and H2O signals. However, it would be necessary to simulate systems with more water molecules to confirm this assignment. The broad peak in the calculated Ca(2p) and Sr(3p) XPS spectra is composed of signals coming from inner layers (at 0 eV) and the surface layer (+0.5 to +1.0 eV). The signal of second and third MO layer is not substantially affected by increasing dissolution of the surface, while the signal originating from the surface layer shifts to higher binding energies, in agreement with experiment,15 which shows only a slight shift of the Ca(2p) peak for low hydration, but a pronounced shift of +1 eV after full immersion in water. The experimental Ca(2p) peak is split into two spin−orbit components,15 but our simulations do not take spin−orbit coupling into account. 3.5. Phase Diagrams. For water on the intact MO(001) surfaces, the water adsorption energy and the degree of surface hydroxylation increase along the M = Mg, Ca, Sr series. However, for each of the studied metal oxide with 32 H2O molecules, we have found a structure with dissolved M2+ ions (i.e., M2+ ions that were moved out of the surface) that is at least 5 kJ/mol/H2O more stable. The relative stability of various surface conformations and the increased reactivity toward water can be summarized in a phase diagram (Figure 5). We included not only the structures

stripe-A is the most stable structure found. The SrO(001) surface interacts with water more strongly. After formation of the intact-1 and intact-2 structures (with one or two water molecules, respectively) for intermediate water pressures and temperatures, formation of the 8-dis structure is predicted. The increased reactivity toward water can be also illustrated by the calculated water pressure needed to adsorb first water molecules at 300 K: 10−2 Pa for MgO, 10−5 Pa for CaO, and 10−10 Pa for SrO. For kinetic reasons, the most stable structures included in Figure 5 might not be observed in experiments. Especially, the formation the highly ordered stripe-A structure might be hindered by a significant barrier, and other dissolved structures can be formed instead. In this context, extensive dissolution of the CaO(001) surface in contact with water was predicted within a reactive force field (ReaxFF) simulation already for one water monolayer (corresponding to 16 H2O in this study).47 The structure presented in the respective publication resembles the 8-dis structure, the second most stable isomer found for the 32H2O/CaO(001) system. Apart from kinetic aspects, our structures are directly relevant for surface science experiments while understanding of dissolution experiments with (nano)crystallites require different models like steps, corners, etc.

■

CONCLUSIONS

Along the MgO−CaO−SrO series, the number of OH groups formed on the water covered (001) surface increases, as well as the tendency to form more disordered “dissolved” structures, i.e., structures with M2+ ions that have moved to positions above the surface. For 32H2O/MgO(001) and 32H2O/ CaO(001) with 16 M2+ ions in the surface layer, we predict that an infinite stripe of M2+ ions formed above the MO surface (stripe-A structure) is the most stable structure at 0 K. For the 32H2O/SrO(001) system, the 4-dis-B and 8-dis structures, which feature many “dissolved” M2+ ions without particular ordering, are the most stable ones. The calculated XPS spectra show deep similarities among all studied metal oxides. Three O(1s) peaks are obtained, for O2− in the MO(001) surface, OH−, and H2O. Within the initial state approximation, the peak interpreted in the experiments as the “OH peak” is predicted to be composed of both OH− and H2O signals.12 For the Mg(2p), Ca(2p), and Sr(3p) spectra, a drift to higher binding energies with increasing dissolution of the surface is predicted. Basic patterns of the experimental XPS signal have been reproduced by the calculations. However, XPS spectra of various isomers are to a large degree similar, hindering deeper structural analysis based solely on the XPS signal. Our results indicate that for all studied metal oxide surfaces, M2+ ion dissolution is thermodynamically favorable already at relatively low water coverage of two water molecules per surface M2+ ions. Further work should investigate the barrier of M2+ ion dissolution to access the kinetics of surface dissolution. Calculated XPS spectra are compatible with the surface dissolution process, they cannot, however, be used to rule out any candidate structure. The predicted structures are compatible with the observed XPS spectra, but we cannot and do not claim that the agreement achieved is proof of their presence. Our most important conclusion is that dissolution of M2+ ions out-of-the-surface and surface roughening are thermody-

Figure 5. Phase diagram for the investigated structures of H2O/ MO(001) (M = Mg, Ca, Sr) systems. MO(001) stands for the clean surface. 1D, 2D, and intact-2 structures were taken from the literature;4 see Supporting Information for details.

investigated in the present paper, but also the most stable ones from ref 4, namely, 1D and 2D structures for the H2O/ CaO(001) system (corresponding to water coverage of about 0.5 and 1 ML, respectively) and the intact-2 structure for the H2O/SrO(001) system (see Supporting Information). Other structures from the respective publication would not be seen in the phase diagram because of their lower stability. The phase diagrams for MgO(001) and CaO(001) are very similar. With increasing water pressure or decreasing temperature, the intact-20 structure is formed, along with 1D and 2D structures for CaO(001). The intact-1 structure is missing from the phase diagram due to its low stability, as already noted elsewhere.4 For higher water pressure or lower temperature, 24767


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(10) Craft, H. S.; Collazo, R.; Losego, M. D.; Sitar, Z.; Maria, J.-P. Surface Water Reactivity of Polycrystalline MgO and CaO Films Investigated Using X-ray Photoelectron Spectroscopy. J. Vac. Sci. Technol., A 2008, 26, 1507−1510. (11) Carrasco, E.; Brown, M. A.; Sterrer, M.; Freund, H.-J.; Kwapien, K.; Sierka, M.; Sauer, J. Thickness-dependent Hydroxylation of MgO(001) Thin Films. J. Phys. Chem. C 2010, 114, 18207−18214. (12) Newberg, J. T.; Starr, D. E.; Yamamoto, S.; Kaya, S.; Kendelewicz, T.; Mysak, E. R.; Porsgaard, S.; Salmeron, M. B.; Brown, G. E., Jr.; Nilsson, A.; et al. Autocatalytic Surface Hydroxylation of MgO(100) Terrace Sites Observed under Ambient Conditions. J. Phys. Chem. C 2011, 115, 12864−12872. (13) Newberg, J. T.; Starr, D. E.; Yamamoto, S.; Kaya, S.; Kendelewicz, T.; Mysak, E. R.; Porsgaard, S.; Salmeron, M. B.; Brown, G. E., Jr.; Nilsson, A.; et al. Formation of Hydroxyl and Water Layers on MgO Films Studied with Ambient Pressure XPS. Surf. Sci. 2011, 605, 89−94. (14) Dahle, S.; Voigts, F.; Maus-Friedrichs, W. In Situ Preparation of Calcium Hydroxide Films. Thin Solid Films 2011, 520, 18−24. (15) Liu, P.; Kendelewicz, T.; Brown, G. E., Jr.; Parks, G. A.; Pianetta, P. Reaction of Water with Vacuum-cleaved CaO(100) Surfaces: An Xray Photoemission Spectroscopy Study. Surf. Sci. 1998, 416, 326−340. (16) Bebensee, F.; Voigts, F.; Maus-Friedrichs, W. The Adsorption of Oxygen and Water on Ca and CaO Films Studied with MIES, UPS and XPS. Surf. Sci. 2008, 602, 1622−1630. (17) Yu, Y. H.; Guo, Q. L.; Liu, S.; Wang, E. G.; Moller, P. J. Partial Dissociation of Water on a MgO(100) Film. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 115414. (18) Coulomb, J. P.; Demirdjian, B.; Ferry, D.; Trabelsi, M. Thermodynamic and Structural Properties of Water Adsorbed Film on MgO(100) Ionic Surface. Adsorption 2013, 19, 861−867. (19) Demirdjian, B.; Suzanne, J.; Ferry, D.; Coulomb, J. P.; Giordano, L. Neutron Diffraction Investigation of Water on MgO(001) Surfaces, from Monolayer to Bulk Condensation. Surf. Sci. 2000, 462, L581− L586. (20) Stirniman, M. J.; Huang, C.; Smith, R. S.; Joyce, S. A.; Kay, B. D. The Adsorption and Desorption of Water on Single Crystal MgO(100): The Role of Surface Defects. J. Chem. Phys. 1996, 105, 1295−1298. (21) Imad-Uddin Ahmed, S.; Perry, S. S.; El-Bjeirami, O. Desorption and Reaction of Water on MgO(100) Studied as a Function of Surface Preparation. J. Phys. Chem. B 2000, 104, 3343−3348. (22) Kim, Y. D.; Lynden-Bell, R. M.; Alavi, A.; Stulz, J.; Goodman, D. W. Evidence for Partial Dissociation of Water on Flat MgO(100) Surfaces. Chem. Phys. Lett. 2002, 352, 318−322. (23) Carrasco, E.; Aumer, A.; Gomes, J. F.; Fujimori, Y.; Sterrer, M. Vibrational Spectroscopic Observation of Ice Dewetting on MgO(001). Chem. Commun. 2013, 49, 4355−4357. (24) Colera, I.; de Segovia, J. L.; Román, E.; Cáceres, D.; Vergara, I.; González, R. Influence of the Surface Structure on the D2O and H218O Adsorption on MgO(100) Thin Films Studied by Electron Stimulated Desorption. Surf. Sci. 2003, 528, 8−14. (25) Ferry, D.; Glebov, A.; Senz, V.; Suzanne, J.; Toennies, J. P.; Weiss, H. Observation of the Second Ordered Phase of Water on the MgO(100) Surface: Low Energy Electron Diffraction and Helium Atom Scattering Studies. J. Chem. Phys. 1996, 105, 1697−1701. (26) Kim, Y. D.; Stultz, J.; Goodman, D. W. Dissociation of Water on MgO(100). J. Phys. Chem. B 2002, 106, 1515−1517. (27) Abriou, D.; Jupille, J. Self-Inhibition of Water Dissociation on Magnesium Oxide Surfaces. Surf. Sci. 1999, 430, L527−L532. (28) Fujimori, Y.; Zhao, X.; Shao, X.; Levchenko, S. V.; Nilius, N.; Sterrer, M.; Freund, H.-J. Interaction of Water with the CaO(001) Surface. J. Phys. Chem. C 2016, 120, 5565−5576. (29) Geysermans, P.; Finocchi, F.; Goniakowski, J.; Hacquart, R.; Jupille, J. Combination of (100), (110) and (111) Facets in MgO Crystals Shapes from Dry to Wet Environment. Phys. Chem. Chem. Phys. 2009, 11, 2228−2233.

namically feasible already for water loadings of about two water molecules per surface M2+ ion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07434. Models recalculated from ref 4.; depiction of structures considered in the present study; Cartesian coordinates and total energies of optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address: Institut für Ionenphysik und Angewandte Physik, LeopoldFranzens-Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria. Telephone: +43 512 507 52696. Present Address †

Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Str. 24−25, 14476 Potsdam-Golm, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the German Research Foundation (DFG) within CRC 1109 “Metal Oxide−Water Interfaces” and the Cluster of Excellence “Unifying Concepts in Catalysis”. M.O. thanks the Alexander von Humboldt Foundation for a fellowship. The authors thank Sergey V. Levchenko and Xunhua Zhao for providing computational details of their study.

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REFERENCES

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Hydration Structures of MgO, CaO, and SrO (001) Surfaces - The

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