Simulated Photoemission Spectra of Hydroxylated MgO(100) at

Article pubs.acs.org/JPCC

Simulated Photoemission Spectra of Hydroxylated MgO(100) at Elevated Temperatures Lauro Oliver Paz-Borbón,* Anders Hellman, and Henrik Grönbeck Department of Applied Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-41296 Göteborg, Sweden S Supporting Information *

ABSTRACT: Density functional theory has been used to investigate photoemission O1s core-level shifts (CLS) of hydroxylated MgO(100). Rapid proton exchange at elevated temperatures (300 K) yields broad features in the simulated photoemission signal, in good agreement with experimental observations. The results provide further evidence that the stable structure of hydroxylated MgO(100) consists of a partly dissociated water monolayer. Analysis of the CLS for adsorbed hydroxyl groups at different coverage reveals a pronounced effect on hydrogen bonding to neighboring H2O molecules. The inclusion of exact exchange by use of the hybrid PBE0 functional leads to quantitatively similar results as the gradient corrected PBE functional.

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molecule.16,17 The existence of dissociated H2O as well as rapid proton transfer between water molecules and hydroxyl groups for water adsorbed at higher coverage has later been confirmed in several theoretical studies.6,17−19 A partly dissociated water monolayer is, moreover, consistent with experimental work based on metastable low-impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and TPD20,21 as well as HREELS.22,23 X-ray photoelectron spectroscopy (XPS) is a convenient technique to probe the local atomic environment of surface species.24 Early XPS experimental studies showed that H2O readily dissociates at MgO(100) surfaces with defects,25,26 whereas the formation of OH groups could be suppressed by surface annealing.26 The enhanced activity of defect sites has been confirmed by studies of H2O adsorption on submonolayer MgO(100) films supported on Ag(100).27 The presence of a metal support reduces issues associated with surface charging during XPS measurements of oxides. Very recently, Newberg et al. used high-pressure XPS to study H2O adsorption on MgO(100).4 The O1s core-level shifts (CLS) were used to develop an atomistic model for multilayer H2O adsorption in which it was suggested that the first monolayer consists only of hydroxyl groups, whereas the second layer is composed of intact H2O molecules. Extensive hydroxylation of MgO(100) has also been reported at pressures above 10−4 mbar on the basis of XPS and LEED studies by Carrasco et al.3 However, in ref 3 it was suggested that full hydrolysis of the Mg−O surface bonds requires a defective surface that exposes low coordinated sites. It should be noted that MgO(111) represents the stable surface

INTRODUCTION The understanding of how water adsorbs and dissociates on oxide surfaces is important in several applied fields, where heterogeneous catalysis is only one example.1 In this context, the interaction of water with MgO(100) has become a prototype system thanks to experimental and theoretical advantages.2−9 Theoretical work has shown that H2O at low coverage adsorbs molecularly on the pristine MgO(100), whereas dissociation has been observed at defects such as step and corner sites.10,11 The situation is not completely clear at higher coverages. Early experimental studies based on in situ low-energy electron diffraction (LEED)9,12,13 and helium atom scattering (HAS)12,13 measurements reported that molecularly adsorbed water forms a (4 × 2) monolayer at low temperatures (100−180 K), whereas a (3 × 2) overlayer is observed at higher temperatures (above 185 K). Moreover, reflection absorption infrared spectroscopy (RAIRS) experiments in combination with temperature-programmed desorption (TPD) on MgO(100) thin films supported on Mo(100) have been interpreted along the lines of molecularly adsorbed H2O.14 Similar conclusions have more recently been reached on the basis of Fourier transform infrared (FTIR) spectroscopy, although the existence of hydroxyl groups in the monolayer was not ruled out.15 On the theoretical side, there is a general consensus that water is molecularly adsorbed on MgO(100) in the dilute regime, whereas a partially dissociated film is obtained at monolayer coverage. One monolayer is here defined as one water molecule per MgO unit in the surface. The partially dissociated structure was predicted independently by Giordano et al.2 and Odelius16 by use of ab initio molecular dynamics simulations. In particular, it was found that one-third of the water molecules in the monolayer was dissociated. The preference for the partially dissociated monolayer has been rationalized by the formation of two strong H2O−OH bonds for each dissociated H2O © 2012 American Chemical Society

Received: September 27, 2011 Revised: January 7, 2012 Published: January 31, 2012 3545

dx.doi.org/10.1021/jp209336q | J. Phys. Chem. C 2012, 116, 3545−3551

The Journal of Physical Chemistry C

Article

of hydroxylated MgO28−30 and that the transformation of MgO(100) into MgO(111) involves significant mass transfer and may be kinetically controlled. In this work, we have used a combination of static DFT calculations and ab initio molecular dynamics (AIMD) to calculate O1s surface core-level shifts (CLS) of H2O adsorbed on MgO(100). The CLS are investigated as a function of coverage and temperature. The simulated photoemission spectra are in good agreement with the recent measurements,4,31 despite the fact that the calculations clearly prefer a partially dissociated water monolayer. Furthermore, analysis of the calculated CLS for adsorbed hydroxyl groups at different coverages reveals a pronounced effect on the number of coordinated H2O molecules. As the degree of electron localization, in principle, could affect stability and CLS, we have compared the results obtained with a standard gradient corrected functional (PBE) with one including exact-exchange (PBE0). For the investigated systems, we find that PBE and PBE0 yield quantitatively similar results, and the present study provides further evidence that water at monolayer coverage is adsorbed on MgO(100) in a partially dissociated fashion.

The structures are regarded as relaxed when the largest element of the gradient is smaller than 0.025 eV/Å and the change in total energy is less than 3 × 10−6 eV.45 Geometry optimization of surface species is performed only by use of the PBE functional. The total energies within PBE0 are evaluated with the corresponding PBE structures. Adsorption energies (Eads) are calculated according to

Eads = EX/MgO − EX − EMgO

(1)

where X stands for H2O, OH, or atomic H. Negative Eads values indicate exothermic adsorption. The investigated systems are treated in the lowest possible spin state, singlet or doublet. The AIMD simulations are performed by use of the Car− Parrinello approach46 as implemented in the Quantum Espresso code32 with the PBE xc functional. A short time step of 5 au (0.121 fs) is used to integrate the equations of motion, and the fictitious electron mass is set to 500 au. The simulations are carried out in the microcanonical ensemble (NVE) for 10 ps at three different temperatures, namely, 100, 185, and 300 K. The systems were prior to the microcanonical simulations equilibrated by velocity rescaling during 2.5 ps. Calculations of Core-Level Shifts. The O1s core-level shifts are evaluated by the use of a pseudopotential generated with an electron hole in the 1s shell.47 The core-level shift (ΔiCLS) of atom i is in this approach calculated with respect to an oxygen bulk reference represented by an atom in the center of the slab

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COMPUTATIONAL METHOD AND SYSTEMS DFT Calculations and Simulations. Density functional theory (DFT) is employed in an implementation with plane waves and pseudopotentials. In particular, the Quantum Espresso code is used.32 The spin-polarized Perdew−Burke−Ernzerhof (PBE) approximation is applied for the exchange and correlation (xc) functional33 along with the hybrid PBE0 functional.34 PBE0 incorporates a fraction (25%) of Hartree−Fock exact-exchange (EXX), thus contributing to a reduction of the self-interaction error (SIE).34 Norm-conserving pseudopotentials (generated within PBE) are used to describe the interaction between the valence electrons and the ionic cores.35,36 PBE-based pseudopotentials are commonly used also for calculations involving EXX. This is an inconsistency chosen due to the difficulties arising upon construction of pseudopotentials with nonlocal exchange.37,38 The number of electrons treated variationally for each element are: H(1), O(6), and Mg(2). A plane-wave kinetic energy cutoff of 62 Ry is used to expand the Kohn−Sham orbitals. Reciprocal space integration over the Brillouin zone is approximated by finite sampling.39,40 The lattice constant of MgO is calculated to be 4.33 (4.28) Å by use of the PBE (PBE0) functional. This is slightly larger than the experimental value of 4.21 Å41 but in close agreement with previous theoretical work based on PBE (4.2642 and 4.30 Å43), PW91 (4.25 Å),44 and PBE0 (4.21 Å) functionals.42 The O−H bond distance and H−O−H angle in the gas phase water molecule are calculated to be 0.97 Å and 103.9° within PBE. The corresponding values with PBE0 are 0.96 Å and 104.4°, which compare favorably with the experimental values of 0.96 Å and 104.5°. The homolytic gas-phase dissociation of H2O into OH and H is calculated to be 5.20 (5.10) eV within PBE (PBE0). The PBE0 result is close to the experimental value of 5.11 eV. The bulk-like MgO(100) surface is modeled in a slab configuration by use of the 3 × 2 unit cell and five atomic layers. Each surface layer contains 6 O and 6 Mg atoms. Calculations have been performed using a converged 3 × 2 × 1 k-point grid (implying the use of four special k-points of the Monkhorst and Pack grid). Repeated slabs are separated by at least a 10 Å vacuum. Structural optimization is performed with the three bottom layers kept fixed at the corresponding bulk positions.

core − hole ΔCLS = Eicore − hole − Ebulk i

(2)

This approach assumes complete screening of the core hole. The CLS without electronic screening can, in the pseudopotential method, be estimated by calculating the difference in total energy when the pseudopotential without a core hole (VPP) is replaced PP 48 by one with a core hole (VCH ), thus the difference in the expectation values of PP (V PP − VCH )

(3)

with the electronic density that corresponds to the unperturbed ground state. The CLS at elevated temperatures are calculated by extracting atomic configurations every ∼50 fs from the microcanonical trajectories giving 200 configurations for each temperature. A total of 1800 CLS calculations are performed for each temperature. We find that a large number of structural configurations are needed to accurately simulate the photoemission spectra for H2O on MgO.

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RESULTS AND DISCUSSION H2O Adsorption and Dissociation at Low Coverage. Several theoretical studies have considered the adsorption of water monomers on MgO(100),8,10,16,17,19,49 and it is established that H2O adsorbs molecularly atop the Mg cation with the two H atoms facing two surface anions. The adsorption energy (Eads) has been reported to be of the order of −0.4 to −0.5 eV. Two hydroxyl groups are formed upon H2O dissociation: (OH)ads and OsH, where OsH includes an oxygen atom from the surface. The configuration is generally found to be unstable by ∼1.1 eV with respect to the intact water molecule. The present energetic and structural results are shown in Table 1 and Figure 1. The PBE result for the adsorption energy of H2O is in good agreement with the previous theoretical reports as well as the experimental value of −0.52 eV obtained 3546



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Table 1. Adsorption Energies of H2O and Dissociated H2Oa Eads (eV)

H2O

OH + H

OH

H

PBE PBE0

−0.50 −0.44

0.62 0.76

−1.15 −0.76

−0.59 −0.49

underestimated within PBE. In the present work, it is calculated to be 3.47 eV which is 4.3 eV lower than the experimental value of 7.8 eV.50 A substantial improvement is obtained by use of the hybrid PBE0 functional. We obtain a value of 7.38 eV, which is in fair agreement with previous PBE0 reports of 7.24 eV.42 Electronic densities of states (DOS) using PBE and PBE0 for MgO in the bulk phases are given in the Supporting Information. The DOS of H2O adsorbed on MgO(100) within the two functionals are shown in Figure 2. The band gap for MgO(100)

a

The OH and H correspond to results where only one OH group or H atom is considered in the computational cell.

Figure 2. Total electronic DOS for H2O adsorbed on MgO(100) with respect to the highest occupied Kohn−Sham energy. The shaded curve shows the DOS projected onto the water monomer. The one-electron Kohn−Sham energies have been broadened with a 0.1 eV Gaussian.

Figure 1. Structural models of molecular H2O (a), dissociated H2O (b), OH (c), and H (d) on MgO(100). Selected distances are reported in Å. Atomic color codes: green (Mg), red (O), and white (H).

is calculated to be 2.66 and 4.71 eV within PBE and PBE0, respectively. Analysis of the partial density of states for the water molecule reveals that the bonding is mainly of polarization type; the degree of hybridization between H2O and MgO(100) states is minor. The calculated gapbetween the highest occupied and lowest unoccupied statesis 2.68 and 4.83 eV for PBE and PBE0, when H2O is adsorbed on the surface. Adsorption of One and Two Monolayers. As already mentioned, ab initio molecular dynamics (AIMD) simulations have predicted a mixed dissociated/molecular phase to be preferred at monolayer coverage.2,16 The reason for this preference is the strong O−H bonds formed between molecular H2O and (OH)ads.16,17 Although the situation is not completely clear, experimental evidence exists for a partially dissociated monolayer.20,22 Here, we study the energetics of the 1 ML case in PBE as a reference. The relaxed PBE structure in the (3 × 2) surface cell is shown in Figure 3. In the surface cell, two molecules are dissociated

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by LEED adsorption isotherms. The calculated energy difference between the dissociated and molecularly adsorbed water monomer is found to be 1.12 eV, implying endothermic adsorption of (OH)ads + OsH. The (OH)ads group is adsorbed in an atop configuration coordinating to one Mg2+ cation. In Table 1, we also report the adsorption energies for single OH and H with respect to OH and H in the gas phase. These values were obtained by considering only OH (or H) in the computational cell. As these systems have unpaired electrons, they are in some sense hypothetical. However, they are included as references and exemplify the substantial energy gain obtained by electron pairing in the MgO system. A single OH radical is adsorbed in a bridge configuration (between two Mg2+ cations). H forms an OH group with a surface oxygen with a slightly longer O−H distance than in the (OH)ads + OsH structure. This is a structural signature of the incomplete charge separation when only H is considered in the computational cell. The adsorption energy of OH is calculated to be −1.15 eV, whereas it is −0.59 eV for H. By use of the HO−H dissociation energy in the gas phase (5.20 eV), the energy gain owing to electron pairing in the oxide is calculated to be 2.84 eV. The introduction of a fraction of exact exchange by the use of PBE0 does not alter the conclusions. For the adsorbed water monomer, the calculated Eads value is −0.44 eV, close to the PBE value of −0.50 eV. Similarly, the energy difference between the dissociated and molecularly adsorbed water monomer differs by only 0.1 eV. A more pronounced difference is instead found for the adsorbed hydroxyl group. The Eads is calculated to be −0.76 eV within PBE0, which is 0.4 eV lower than the PBE result. The similarity in the PBE and PBE0 results for adsorbed water can be rationalized by the fact that PBE0 mainly affects the separation between occupied and unoccupied states of MgO. The band gap for MgO in the bulk is severely

Figure 3. Side (left) and top (right) views of the structural model of a (3 × 2) partially dissociated H2O ML on MgO(100). The surface cell is indicated. Atomic color codes as in Figure 1.

(forming (OH)ads and OsH), whereas four H2O are intact. Molecular H2O is adsorbed over Mg2+ with one H coordinating toward the (OH)ads. (OH)ads is also adsorbed atop Mg2+. The adsorption energy per H2O molecule is −0.77 eV, thus the stabilization owing to the hydrogen bonding is substantial. 3547



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As the dissociation of H2O in the monolayer is without barriers,17 it is not straightforward to determine the preference of the partly dissociated case to the situation where all the molecules are intact. However, by constrained optimization we estimate the stability of the all-molecular case. In these calculations, the O atoms of the water molecules are kept at a fixed distance normal to the surface, whereas all other degrees of freedom are relaxed. The potential energy minimum within PBE is found to be 2.32 Å above the Mg2+ cations (see Supporting Information). As compared to the partially dissociated case, this structure in PBE is 1.15 eV less stable (the adsorption energy per H2O molecule is −0.57 eV). Within PBE0, the constrained optimization leads to a minimum of the intact H2O molecules at a distance of 2.26 Å above the surface plane (see Supporting Information). The allmolecular configuration is 1.66 eV less stable than the partly dissociated case. Moreover, the adsorption energy per H2O molecule is calculated to be −0.75 eV in the partially dissociated ML, as compared to −0.60 eV in the all-molecular case. Thus, we can conclude that the inclusion of exact exchange does not change the preference for water dissociation in the case of 1 ML. On the basis of high-resolution XPS measurements, it was recently suggested that introduction of a second H2O monolayer would facilitate complete dissociation of the water layer in direct contact with the MgO surface.4 Here, we investigate this situation by relaxing a large set of initial configurations. In particular, several initial configurations were generated with all H2O dissociated in the first layer. The relaxed structure with the lowest energy is reported in Figure 4. As in the case of 1 ML, the

Figure 5. CLS (with respect to an O atom in the bulk) for 1 ML water on the (3 × 2) MgO(100) surface, using PBE (top) and PBE0 (down). The binding energies are obtained by aligning the bulk reference to the corresponding experimental value of 530 eV (dotted line). Colors indicate: blue (H2O), red (OsH), green ((OH)ads), and magenta (Os). The corresponding CLS values are given in ref 51.

toward lower binding energies. The average CLS value is −0.25 eV, thus implying that it is more energetically favorable to make a core hole on the surface O2− sites, as compared to an anion in the bulk. The adsorbed OH groups instead have shifts toward higher binding energies. The CLS for OsH groups are ∼1.8 eV, whereas they are in the range 2.4−2.8 eV for (OH)ads. The largest shifts are calculated for adsorbed water, namely, 3.3−3.7 eV. The CLS are very sensitive to the local environment, and there is an energy spread for each surface species. The introduction of exact exchange in the functional is not found to influence the CLS extensively. The shifts of the Os are slightly less negative, whereas the shifts for OH groups and H2O are larger by ∼0.2 eV. These results indicate that the self-interaction error within standard DFT is not crucial for calculations of corelevel shifts. The core-level shifts discussed above include final state effects. In fact, the electronic relaxation upon the formation of the O1s core hole turns out to be a substantial part of the CLS. The average initial state CLS of the water molecules, (OH)ads, and OsH in the partially dissociated ML are calculated to be 2.11, 1.0, and 1.85 eV, respectively, with the PBE functional. Recently, the O1s binding energy was measured by XPS to investigate water adsorption on MgO(100).4 The binding energy for O1s in the bulk of MgO was determined to be at 530 eV. OH groups and adsorbed water were assigned to peaks shifted 2.1 and 3.4 eV to higher binding energies, respectively. Our results are in good agreement with the measurements. It should be noted that the experiments did not resolve the CLS for OsH and (OH)ads. The calculated mean value for these two types of OH groups is 2.1 eV, thus very close to the experimental value. To elaborate on the O1s CLS in the monolayer, we also considered the dilute regime. As exact exchange was found to be of minor importance for CLS, these calculations were done only within PBE. The final state CLS for adsorbed H2O (see Figure 1a) was calculated to be 3.53 eV, thus very close to the corresponding value in the monolayer. On the contrary, large differences are obtained for the different types of OH groups. For the dissociated molecule (in the structure reported in Figure 1b), the shifts for (OH)ads and OsH are calculated to be −0.67 and 3.07 eV, respectively. Thus, the CLS for O1s in

Figure 4. Side (left) and top (right) views of 2 ML of water relaxed in the (3 × 2) surface cell of MgO(100). The surface cell is indicated. Atomic color codes as in Figure 1.

first water layer consists of two dissociated and four molecular H2O. The second layer is arranged to make hydrogen bonds both within the second layer and to the first layer. In fact, the hydrogen bonds to the second layer make a pair of a H2O molecule and an (OH)ads group in the first layer symmetric in the sense that one proton is optimized to be in between (OH)ads and H2O. This long O−H bond is 1.23 Å. The adsorption energy per water molecule for the 2 ML case is calculated to be −0.62 eV.

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CALCULATION OF THE O1S CORE LEVEL Static Calculations. The calculated O1s core-level shifts of the stable water monolayer are reported in Figure 5 using both the PBE and PBE0 functional.51 The CLS with respect to the bulk are evaluated for all oxygen atoms in the surface and all O atoms originating from the adsorbed water molecules. Thus, in total 13 calculations are performed for each structure. Discussing first the PBE results, oxygen atoms in the surface (Os) without any direct bond to adsorbed species are shifted 3548



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simulated spectrum with three Gaussians clearly shows the signatures of the OsH and (OH)ads groups.52 An increase of the temperature to 185 K results in a slight broadening of the spectrum. This effect is enhanced at 300 K. The broadening is rationalized by the high sensitivity of the CLS to the local environment. As the intact water molecules are further from the surface at higher temperatures, the O1s binding energy is shifted to higher energies by ∼0.3 eV. At high temperatures, the two types of OH groups are difficult to resolve, and some features even appear at negative energies as compared to the bulk component (at zero CLS). The marked temperature dependence is clearly related to the dynamical character of the partially dissociated water monolayer.19 During the simulations, there are frequent events of proton transfers within the monolayer. The barrier for proton transfer in the system is close to zero, as it does not involve rearrangement in electron charge.17,53 The most common process is proton exchange between adsorbed H2O and (OH)ads, which in the photoemission spectrum leads to a smearing of water and OH signatures. In Figure 7, one example of proton transfer between OsH and (OH)ads at 300 K is shown. The structure is initially the

(OH)ads is reversed as compared to the monolayer, and in the OsH group the shift is almost doubled. The sensitivity of the shifts for these species is further exemplified by the results for the adsorption of a single OH (Figure 1c) or an H atom (Figure 1d). The CLS for these cases are calculated to be −0.16 and 2.44 eV, respectively. To investigate the reason for the remarkable observation that the CLS of O1s in (OH)ads in the dilute regime is reversed with respect to the monolayer, we considered the case when the (OH)ads species is coordinated to one and two intact water molecules. (The structures are reported in the Supporting Information.) The latter case has a local geometry that is similar to the monolayer (Figure 3). With one water molecule coordinated to the (OH)ads group, the CLS changes sign and is calculated to be 1.55 eV. The effect of the coordinated H2O is probably somewhat exaggerated as the relaxed geometry should be characterized as two (OH)ads groups with one proton situated between them. The H−(OH)ads distance is in this case 1.23 Å. This configuration, which is similar to the one discussed for two monolayers of water, is a consequence of the symmetry of the system and the fact that transfer of the proton from one (OH)ads group to the other proceeds without substantial rearrangement of the electronic charge. Coordination of a second H2O molecule to (OH)ads increases the O1s binding energy further, and the CLS with respect to the bulk is calculated to be 2.01 eV, thus within ∼0.1 eV of the value in the monolayer. This analysis demonstrates that the CLS are very sensitive to the presence of hydrogen bonds. Elevated Temperatures. To investigate how the photoemission spectrum depends on thermal motion, we study the monolayer in the (3 × 2) cell at three different temperatures, namely, 100, 185, and 300 K. The CLS are calculated from 200 snapshots taken from 10 ps ab initio molecular dynamics trajectories. To facilitate comparisons with experimental data, the CLS are broadened by a 0.1 eV Gaussian. The results are presented in Figure 6.

Figure 7. CLS and bond lengths as a function of time for an OsH group during a simulation at 300 K. Inset (a) shows OsH in the energetically preferred situation when the proton is adsorbed at the surface site. Inset (b) shows a snapshot when the proton has been transferred from the surface site to an (OH)ads group forming water. Red and blue lines indicate the corresponding bond lengths and CLS for the OsH group. The black line shows the calculated CLS of the neighbor (OH)ads group.

one shown in inset (a) with one OsH group and one (OH)ads. The mean CLS for the two OH groups during the first 2.5 ps are 1.98 eV (OsH) and 1.64 eV [(OH)ads]. The results highlight that fairly small fluctuations in the structure may have pronounced effects on the CLS. The standard deviation of the Os−H distance during the first 2.5 ps of the simulation is 0.05 Å. During the same time, the standard deviation of the CLS is 0.45 eV. At 2.8 ps, the CLS of OsH suddenly decreases to negative values, whereas the CLS for (OH)ads increases to ∼3.8 eV. The changes in CLS are related to the transfer of a proton from OsH to (OH)ads, thus forming a surface anion (Os) and a water molecule (see inset (b)). In Figure 7, we also show the O−H bond distance in the OsH group that initially is around 1.0 Å. At the proton transfer, the Os−H distance is abruptly increased to

Figure 6. CLS for 1 ML water on the (3 × 2) MgO(100) surface calculated at three different temperatures (100, 185, and 300 K) using the PBE functional. The binding energies are obtained by aligning the bulk reference to the corresponding experimental value of 530 eV (dotted line). To facilitate a comparison to experimental results, a 0.1 eV Gaussian broadening has been applied.

The spectrum at 100 K is characterized by two main features, which correspond to water molecules (at 3.5 eV) and the two types of hydroxyl groups (2.0 and 2.5 eV). Deconvolution of the 3549



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∼1.6 Å. The distance is reduced to the original value at ∼3.6 ps as the proton is transferred back to the surface anion. This process is responsible for the negative CLS observed in the 300 K spectrum in Figure 6. We have also performed AIMD simulations (10 ps) for 2 ML of H2O adsorbed on MgO(100) at 100, 185, and 300 K. The general temperature dependence is similar to the 1 ML case; the spectrum broadens with increasing temperature (see Supporting Information). The results for the simulation at 185 K are reported in Figure 8 together with the corresponding

The O1s CLS are found to provide sensitive fingerprints of the water monolayer. In particular, clear features of adsorbed H2O and two types of OH groups are observed. The PBE and PBE0 results are similar within ∼0.2 eV. The results for the CLS are in good agreement with recent high-resolution XPS measurements 4,31 and provide means to improve the experimental fitting by separation of features for OH groups adsorbed on the surface and groups including surface anions. However, our simulations do not support the structural model suggested in ref 4 of a fully dissociated water monolayer in contact with the oxide surface. Thermal vibrations and rapid proton transfer at elevated temperatures are found to have pronounced effects on the XPS spectrum. The spectrum is broadened, and the separation between the H2O and OH signatures is smeared. The simulations represent clear examples of how sensitive the XPS spectrum is for minor changes in bond distances.

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ASSOCIATED CONTENT

* Supporting Information S

Electronic density of states for bulk MgO within PBE and PBE0, structural models, and simulated core-level shifts for 2 ML at 100, 185, and 300 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

Figure 8. CLS for 2 ML water on the (3 × 2) MgO(100) surface calculated at 185 K (top) and for the static configuration (bottom) using the PBE functional. The binding energies are obtained by aligning the bulk reference to the corresponding experimental value of 530 eV (dotted line). To facilitate a comparison to experimental results, a 0.1 eV Gaussian broadening has been applied. Colors indicate: blue (H2O), red (OsH), green ((OH)ads), and magenta (Os). The corresponding CLS values are given in ref 54.

*E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Soran Shwan for useful discussions. This work has been funded by FORMAS and the Swedish Research Council. The Competence Centre for Catalysis (KCK) is hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and member companies: AB Volvo, Volvo Car Corporation, Scania CV AB, SAAB Automobile Powertrain AB, Haldor Topsøe A/S, and the Swedish Space Corporation. The calculations have been performed at PDC (Stockholm) and C3SE (Göteborg).

static CLS. The AIMD result for 2 ML differs from the 1 ML spectrum by having a more intense and broader peak for H2O, owing to the extra six water molecules in the second layer. It should be noted that the water layer in direct contact with the surface retains its partially dissociated structure during the simulation. The peaks that correspond to (OH)ads and OsH are just as in the 1 ML case, centered at about 2 and 2.5 eV, respectively. Our results do not reconcile the recently proposed model of a fully hydroxylated MgO(100) surface.4 As the XPS measurements in ref 4 clearly show a large intensity for adsorbed OH groups, we may speculate that surface defects are induced during the experiments that facilitate H2O dissociation and the formation of a fully hydroxylated surface.

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AUTHOR INFORMATION

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REFERENCES

(1) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (2) Giordano, L.; Goniakowski, J.; Suzanne, J. Phys. Rev. Lett. 1998, 81, 1271. (3) Carrasco, E.; Brown, M. A.; Sterrer, M.; Freund, H. J.; Kwapien, K.; Sierka, M.; Sauer, J. J. Phys. Chem. C 2010, 114, 18207. (4) Newberg, J. T.; Starr, D. E.; Yamamoto, S.; Kaya, S.; Kendelewicz, T.; Mysak, E. R.; Porsgaard, S.; Salmeron, M. B.; Brown, G. E.; Nilsson, A.; Bluhm, H. Surf. Sci. 2011, 605, 89. (5) Starr, D. E.; Weis, C.; Yamamoto, S.; Nilsson, A.; Bluhm, H. J. Phys. Chem. C 2009, 113, 7355. (6) Włlodarczyk, R.; Sierka, M.; Kwapien, K.; Sauer, J.; Carrasco, E.; Aumer, A.; Gomes, J. F.; Sterrer, M.; Freund, H. J. J. Phys. Chem. C 2011, 115, 6764. (7) McCarthy, M. I.; Schenter, G. K.; Scamehorn, C. A.; Nicholas, J. B. J. Phys. Chem. 1996, 100, 16989. (8) Carrasco, J.; Illas, F.; Lopez, N. Phys. Rev. Lett. 2008, 100, 016101. (9) Ferry, D.; Picaud, S.; Hoang, P. N. M.; Girardet, C.; Giordano, L.; Demirdjian, B.; Suzanne, J. Surf. Sci. 1998, 409, 101. (10) Langel, W.; Parrinello, M. Phys. Rev. Lett. 1994, 73, 504. (11) Chizellet, C.; Costentin, G.; Che, M.; Delbecq, F.; Sautet, P. J. Phys. Chem. B 2006, 110, 15878. (12) Ferry, D.; Glebov, A.; Senz, V.; Suzanne, J.; Toennies, J. P.; Weiss, H. J. Chem. Phys. 1996, 105, 1697.

CONCLUSIONS

In this work, we have used density functional theory to investigate the stability of hydroxylated MgO(100) and, in particular, to evaluate photoemission O1s core-level shifts (CLS). In agreement with previous studies using standard gradient corrected exchange-correlation functionals, it is found that a partly dissociated water monolayer represents the preferred configuration. We find that inclusion of exactexchange by use of the hybrid PBE0 functional does not alter this conclusion. The partly dissociated monolayer is found to be the preferred configuration also if a second monolayer of water is added. 3550



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3.44, and 3.46 in PBE; 3.48, 3.59, 3.64, and 3.70 in PBE0); (OH)ads (1.81 and 1.85 in PBE; 1.87 and 1.93 in PBE0), and OsH (2.38 and 2.55 in PBE; 2.64 and 2.83 in PBE0). Negative CLS values were calculated for the Os atoms (−0.22, −0.28, −0.31, and −0.34 in PBE; −0.13, −0.19, −0.21, and −0.24 in PBE0). (52) In the experimental setup (see ref 4), the O1s XPS spectra are deconvoluted into three Gaussian−Lorentzian line shapes, i.e., one for the O bulk reference (at 530 eV) and two for the OH and water molecules at 532 and 534 eV, respectively. (53) Grönbeck, H.; Panas, I. Phys. Rev. B 2008, 77, 245419. (54) The calculated (PBE) CLS values (final state) for the 2 ML case, as shown in Figure 8, are: water (2.97, 3.00, 3.19, 3.20, 3.33, 3.52, 3.57, 3.71, and 4.15 eV); (OH)ads (1.68, 1.99, and 2.27 eV); and OsH (2.60 and 2.31 eV). Negative CLS values were calculated for the Os atoms (−0.23, −0.27, −0.28, and −0.33 eV).

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Simulated Photoemission Spectra of Hydroxylated MgO(100) at

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