Letter pubs.acs.org/JPCL
Water on the MgO(001) Surface: Surface Reconstruction and Ion Solvation 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: The interaction of water with the MgO(001) surface under ambient conditions is investigated by density functional theory combined with statistical thermodynamics. For water loadings of more than one monolayer, we show that the standard structure model, a fully hydroxylated surface, needs to be revised. Reconstructed surfaces, involving hydrated/hydroxylated Mg2+ ions above the surface, are more stable. These findings provide a consistent picture for surface hydroxylation between low and high water coverage that is in agreement with available XPS data.
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We show that this model with the fully hydroxylated topmost MgO(001) layer is not a stable structure. Instead, we present alternative structural models that feature hydrated/hydroxylated Mg2+ ions above the surface. Using density functional theory (DFT) and statistical thermodynamics, we examine the stability of different surface structures as a function of temperature and pressure. We compare our results with the X-ray photoelectron spectra (XPS) that yield different O 1s signals for O2− ions of the MgO structure, for OH− ions formed on dissociation of H2O, and for molecularly adsorbed H2O.6,13−16 For both ultrahigh vacuum and ambient pressure conditions, the structure models we propose are consistent with the XPS data. Using semiempirical quantum-chemical calculations, Jug et al.24 showed that the hydroxylated monolayer structure4 shown in Figure 1a is not favorable at ambient temperature and suggested another OH monolayer model with OH groups located in bridging sites between two Mg2+ ions; see the Supporting Information (SI).25 We examined both structures by DFT and found that neither of them represents a minimum on the potential energy surface but rather high-order saddle points. Hydroxyl groups recombine easily when the symmetry constraints are lifted and small displacements are made along an imaginary frequency mode, resulting in a structure with only one-quarter of the water monolayer dissociated. (See Table 1.) Hence, a simple monolayer of OH− (Figure 1a) is not a realistic model. For the MgO(001) surfaces with at least one water monolayer, computational studies14,19−26 have found that water tends to partially dissociate on the MgO(001) surface.
nowing the oxide−water interface in atomic detail is prerequisite to understanding diverse and important phenomena such as nucleation, corrosion and dissolution of oxides, mineral weathering, and the surface chemistry of dust particles. Magnesium oxide with its simple rock-salt structure is a popular model system and the interaction of water with various surfaces has been studied,1−3 both experimentally4−18 and computationally,9,13,14,19−26 in a wide range of temperatures and for pressures reaching from high vacuum to ambient ones. For low water coverage on the MgO(001) surface,5,8−12,14,17,18 a clear picture emerges: At low temperatures (100−180 K), water forms a layer with c(4 × 2) symmetry, which transforms to p(3 × 2) symmetry at higher temperatures (185−221 K). In these structures, 1/5 and 1/3, respectively, of the water molecules are dissociated. At higher temperatures, water molecules desorb from the surface. For water coverages beyond a monolayer, the standard structure model, proposed 1965 by Anderson et al.,4 assumes a fully hydroxylated MgO(001) surface (Figure 1a). It is formed by dissociative adsorption of one water molecule per MgO surface ion pair H 2O + [Mg 2 +, O2 −]surf → [OH−Mg 2 +, H+O2 −]surf
(1)
Protons are placed on top of surface oxygen ions, OH− groups are positioned on top of the Mg2+ ions, and nondissociated water molecules are located above the hydroxylated surface layer. Although the multitude of surface OH groups is consistent with numerous experiments performed under ambient water conditions, in his 2006 review Ewing concludes, “What is not settled in the discussion of thin film water on MgO(001) is to what extent the oxide surface has been hydroxylated.”2 © 2015 American Chemical Society
Received: April 29, 2015 Accepted: June 4, 2015 Published: June 5, 2015 2310
DOI: 10.1021/acs.jpclett.5b00885 J. Phys. Chem. Lett. 2015, 6, 2310−2314
Letter
The Journal of Physical Chemistry Letters
found to leave the surface. This was interpreted as a possible first step of brucite (Mg(OH)2) layer formation on the MgO surface. Here we build on these results and investigate two types of surface structures by DFT: (i) Mg(OH)2-like structures and (ii) hydrated MgO(001) surface structures. We apply the PBE functional27,28 augmented with the empirical D2 dispersion term of Grimme (PBE+D).29 Adding the D2 term to PBE is not always an improvement, but this combination has met with success for one water monolayer on MgO(001),14 and it also yields an improved binding energy for an isolated H2O molecule on MgO(001), 60 kJ/mol compared with the observed 63 kJ/mol (SI, Table SI1); see the SI also for further information on the performance of PBE+D compared with PBE. Figure 1 shows the structures obtained and Table 1 shows the water coverage, the number of dissociated water molecules, the energy of formation from the pristine MgO(001) surface and gas-phase water molecules, and the respective surface related Gibbs free energies, whereas the chemical composition and surface area are given in the SI. For the Mg2+(O2−)k(OH−)l (H2O)m surface ions, the coordination numbers k, l, and m, with k + l + m = 6, are given in Table SI5 of the SI. All surface structures include four MgO layers, which correspond to the thickness of thin films studied experimentally.13,15 Mg(OH)2-Like Structures. Because of the exothermicity of the MgO(s) + H2O(g) → Mg(OH)2(s) reaction (−81 kJ/mol, Table SI1 in the SI), the formation of a four-layered Mg(OH)2(0001) surface slab (brucite, Figure 1b) represents the most stable hydration structure of the four-layered MgO surface slab at 0 K (Table 1); however, the experiments performed under ambient conditions show that the presence of the OH groups is limited to the surface.13,15 Moreover, brucite formation would require a deep reconstruction of the MgO(001) surface and is known to be slow requiring hours to days. (See ref 15 and references therein.) To create a single brucite layer on the MgO(001) surface (Figure 1c), one water monolayer is required. To account for the fact that the Mg(OH)2(0001) surface possesses trigonal cell symmetry, while the MgO(001) structure is cubic, we calculated the whole structure in the cubic cell and added the relaxation energy of the brucite layer (see SI). This, however, leads only to a marginal stabilization (Table 1). Hydrated MgO(001) Surface Structures. For low water coverages, 1 ML). In summary, for low OH signal levels, we confirmed previous conclusions14 that a partially dissociated water layer on the MgO(001) surface with p(3 × 2) or c(4 × 2) symmetry is most probably formed.14 For high OH-signal levels, structures with hydrated/hydroxylated Mg2+ ions above the surface explain the experimental data. OH groups are formed both in contact with the Mg2+ ions above the surface and on the surface as a part of the defect healing process (eq 2). Further reconstruction of the hydrated MgO(001) surface can eventually lead to a STRIPE structure. We have presented structure models for hydrated MgO(001) surfaces beyond monolayer coverage that explain observed XPS spectra and, according to our DFT calculations, are stable phases (minima on the potential energy surface). The latter is not true for previous models that assume a full monolayer of hydroxyl groups. In contrast, our models feature reconstructed surfaces with hydrated/hydroxylated Mg2+ ions above the surface layer. The understanding gained for the MgO(001) model surface will be useful when studying the hydration of more complex surfaces, such as LaCoO3 (001);31 however, more work is required to elucidate the role of the proposed structures for corrosion and dissolution processes. Specifically, future work should address the kinetics of the formation of the proposed structures, for example, the fact that the ability of the MgO surfaces to dissociate water is inhibited after several water adsorption−desorption cycles.7
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ASSOCIATED CONTENT
S Supporting Information *
Details of computational methods, benchmarking calculations, interpretation of experiments, and Cartesian coordinates of included structures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00885.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work 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”. MO thanks for a fellowship provided by the Alexander von Humboldt Foundation. The authors gratefully acknowledge the computing time granted by the John von Neumann Institute for Computing (NIC) and provided on the supercomputer JUROPA at Jülich Supercomputing Centre (JSC).
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REFERENCES
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COMPUTATIONAL METHODS The DFT calculations employed the Vienna ab initio simulation package (VASP),32,33 with the projector-augmented wave method (PAW).34 The cutoff energy for plane waves was 600 eV, and the 2 × 2 × 1 Monkhorst−Pack k-point mesh was applied. The slab model was employed for the surface systems with a cell size of 30 Å in the z direction. For the surface cells, we used the lattice constant calculated for the bulk phase for both MgO and Mg(OH)2 at the same level of calculation with the 2 × 2 × 2 Monkhorst−Pack k-point mesh (see SI). For structure optimizations and frequency calculations, an energy convergence criterion of 10−6 eV was applied for both electronic and ionic steps. The two bottom layers were kept fixed to mimic their bulk nature. Molecular dynamics runs were performed with a cutoff energy for plane waves of 300 eV and an energy convergence criterion of 10−4 eV. All reported energies include zero point vibrational energies (ZPE). Gibbs free energies are calculated per surface area for selected 2313
DOI: 10.1021/acs.jpclett.5b00885 J. Phys. Chem. Lett. 2015, 6, 2310−2314
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Letter
NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on June 5, 2015. Eq 1 and main text were updated. The revised paper was reposted on June 10, 2015.
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DOI: 10.1021/acs.jpclett.5b00885 J. Phys. Chem. Lett. 2015, 6, 2310−2314
