Molecular Dynamics Study of Water Adsorption Structures on the MgO

Apr 14, 2007 - surface defects was addressed by Barth and Henry.20 They found. 1012-1013 defects/cm2 on flat (100) terraces of UHV-cleaved single MgO ...
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6846

J. Phys. Chem. C 2007, 111, 6846-6851

Molecular Dynamics Study of Water Adsorption Structures on the MgO(100) Surface Karl Jug,* Bettina Heidberg, and Thomas Bredow† Theoretische Chemie, UniVersita¨t HannoVer, Am Kleinen Felde 30, 30167 HannoVer, Germany ReceiVed: NoVember 17, 2006; In Final Form: February 26, 2007

Semiempirical MSINDO calculations were performed for the study of structure and stability of c(4 × 2), p(3 × 2), and (1 × 1) overlayers of water on the MgO(100) surface. Born-Oppenheimer molecular dynamics (MD) techniques were used to simulate the changes in the adsorption at 300 K compared to that at 0 K. It was found that a partially dissociated double layer of the c(4 × 2) structure with coverage θ ) 3/2 attacks the MgO surface at 300 K. The hydroxylated (1 × 1) overlayer structure appears to be equally aggressive toward the MgO surface at this temperature. Nucleation for the process of Mg(OH)2 formation was also observed.

1. Introduction In recent work,1 we studied the stability of water structures on the perfect MgO(100) surface. Overlayers of c(4 × 2), p(3 × 2), and (1 × 1) translational symmetry were considered at 0 K. The role of partial dissociation for the stability of such overlayers was illustrated in the context of the literature on experimental and theoretical work on this subject. Our study was part of an investigation of the periclase (MgO) to brucite (Mg(OH)2) conversion that occurs in cement and is of particular significance in concrete construction.2 The conversion process has also been observed on MgO single crystals.3 We found that partial dissociation enhances the stability of the p(3 × 2) overlayer, in agreement with theoretical studies of other groups.4-10 Beyond this, we could show that partial dissociation enhances also the stability of the c(4 × 2) structure both in the monolayer and double-layer coverage. In the latter, the stability increased with increasing degree of dissociation from 1/6 to 1/3 and finally to 1/2 dissociated water molecules. The calculated stability of partially dissociated water layers is in agreement with recent experimental evidence.11 Also, a fully hydroxylated (1 × 1) structure was studied because such a structure was predicted from experiments.12,13 A figure of such a structure was presented in a review.14 The OH groups were perpendicular to the surface on top of the Mg atoms, and the H atoms were perpendicular to the surface oxygens. A later DFT study15 relaxed these restrictions and allowed the OH groups and the Mg surface atoms to move. The formation of this structure was exothermic, with an adsorption energy Eads ) -6.1 kJ/mol. We showed that these literature structures are too simple and proposed alternative structures. The characterization of the cleaved MgO(100) surface has been the subject of experimental studies. Jupille and coworkers16 found 10-100 nm wide (100) terraces running over distances of microns under ultra-high vacuum (UHV) conditions. It was checked that cleaning of the cleaved MgO surface by heating it to 1200 K under low oxygen pressure did not produce changes in the surface morphology. However, long exposure to ambient air affects the morphology. In a subsequent paper,17 the reactivity of H2O was probed at low-coordinated surface † Present address: Institut fu ¨ r Physikalische und Theoretische Chemie, Universita¨t Bonn, Wegelerstr. 12, 53115 Bonn, Germany.

sites. According to another paper by this group,18 the dissociative adsorption of water also occurs on (100) terraces. Goodman’s group19 studied the sputter-damaged MgO surface and compared it with the vacuum-annealed surface. By the nature of the preparation, such surfaces have a larger number of defects than the vacuum-cleaved surfaces. The question of the density of surface defects was addressed by Barth and Henry.20 They found 1012-1013 defects/cm2 on flat (100) terraces of UHV-cleaved single MgO crystals with dynamic mode scanning force microscopy. First time images with atomic resolution show one square ionic sublattice in its bulklike dimension. Ealet et al.21 studied the interaction of H2O molecules with divacancies at the MgO(100) surface in the limit of very low water coverage by DFT calculations. They concluded that Mg vacancies can play a fundamental role for the hydroxylation of the MgO surface. Such a complex can act as a precursor to the dissociation and dissolution of MgO through the formation of brucite-like intrusions. More recently,22 the interaction of water with extended defects such as mono- and diatomic steps at the MgO(100) surface was studied. It was concluded that characteristics of water adsorption are primarily driven by the coordination number of the surface acid-base pair where the dissociation occurs. The mechanism of acidic dissolution at the MgO(100) surface23 and the water/MgO(100) interface in acidic and basic media24 was studied theoretically. The related effects of such studies were not the goal of our work. On the basis of our previous work,1 we undertook molecular dynamics (MD) calculations in order to understand better whether the clean MgO surface reacts with water when the temperature is increased to room temperature from 0 K. MD studies for the p(3 × 2) structure on the MgO(100) surface were already presented by Marmier et al.6 and Giordano et al.7 up to 600 K. These authors found that partial dissociation of water after adsorption and subsequent molecular desorption are compatible. In the following sections, we sketch the computational details and describe the results of MD calculations on c(4 × 2), p(3 × 2), and (1 × 1) overlayer structures at 300 K. This is a good approximation for real outdoor temperatures relevant for concrete constructions. In the discussion, we shall

10.1021/jp067651n CCC: $37.00 © 2007 American Chemical Society Published on Web 04/14/2007

Water Adsorption Structures on the MgO(100) Surface

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show how these stable structures are deformed by temperature increase and that a reaction with the surface was observed in some cases. 2. Computational Details The calculations were performed with the semiempirical method MSINDO25 in the parametrization for first- and secondrow elements.26 The accuracy of the method for structures and energies has been documented. For the description of hydrogen bonding, which is important in this study, additional 2p functions on H are included.27 There is good agreement of the calculated water dimerization energy with the experimental value. The systems were treated as cyclic clusters. The cyclic cluster model28 is implemented in MSINDO. It simulates periodicity in a finite system, and hence, it is also suitable for the description of translational symmetry in monolayers or multilayers. An embedding procedure takes care of the long-range electrostatic interactions.29 The MSINDO program includes also a BornOppenheimer MD procedure. The description of the details can be found elsewhere.30 We have used a (MgO)96 cyclic cluster of (6 × 4 × 4) shape for the surface simulations. This means that the cluster has a (6 × 4) surface with 4 layers. The choice of a clean surface without defects as a model for the subsequent calculations seems to be justified by the fact that the defect density found in experimental work on surfaces of UHV-cleaved MgO crystals is less than 1%. Much larger surface models would be needed to include realistic defect concentrations in calculations. Since our final goal is the study of the process of water interaction with periclase (MgO) to brucite (Mg(OH)2) formation at ambient temperatures, no calcination process at higher temperatures was considered. 3. Stability of the MgO(100) Surface under the Interaction with Water 3.1. The c(4 × 2) Adsorbate. In our previous work,1 we studied a monolayer and double-layer structure of c(4 × 2) translational symmetry at 0 K because this structure was found at low temperatures from 100 to 180 K by spot profile analysis of low-energy electron diffraction31 and helium atom scattering.32 From molecular beam experiments and temperatureprogrammed desorption, Stirniman et al.33 concluded that a partial second layer was adsorbed on top of the first monolayer. We had, in turn, simulated a coverage of θ ) 3/2 with a double layer, where each layer had an equal number of water molecules so that 3/4 of the MgO surface atoms were covered by water molecules of the first layer. To get a better understanding of atomic processes which occur at the structural phase transition from MgO (periclase) to Mg(OH)2 (brucite), we performed Born-Oppenheimer MD calculations at 300 K for this and the following overlayer structures. In each case, the system was heated up from 0 to 300 K in 200 time steps of 1 fs and then equilibrated using the Nose´-Hoover chain thermostat as implemented in MSINDO.30 In the present case, we considered two starting structures, one without dissociated molecules and a second where 50% of the water molecules were dissociated. The equilibration of the undissociated two-layer adsorbate resulted in the dissociation of 3 out of 18 first-layer water molecules during the simulation time of 3.2 ps. This is in line with the observation from experimental work18 that any exposure of the freshly cleaved MgO surface to water vapor in vacuum dramatically inhibits its reactivity toward water dissociation because no water molecule dissociation was found by us in the second water layer. Figure 1 shows the time development of

Figure 1. Time development of the potential energy of the double layer of the c(4 × 2) adsorbate for a starting structure without dissociated water molecules on the MgO(100) surface; (a) starting structure at 0 ps, (b) with one dissociated water molecule after 0.4 ps shortly after heating up to 300 K, (c) with two dissociated water molecules after 0.6 ps, and (d) with three dissociated water molecules after 3.1 ps.

the potential energy relative to the energy of the optimized structure at 0 K during this time interval. The dissociation of the first, (b), the second, (c), and the third, (d), water molecule is indicated. The corresponding structures are presented in Figure 2. The starting structure (a) was taken as the optimized structure at 0 K.1 Structures c and d were local minima on the energy hypersurface, whereas structure b was unstable and proceeded to structure c. We found a dissociation energy of about -7 kJ/mol per water molecule. After 1 ps, single water molecules of the upper water layer began to fluctuate; after 3 ps, the first molecules were desorbed. The water molecules of the lower water layer remained bound at the MgO surface. No molecules moved from the upper to the lower layer. No reaction of the OH groups with the MgO surface was observed. The MD calculation of the partially dissociated double-layer adsorbate showed a spontaneous attack of the dissociated OH groups on the MgO surface, as shown in Figure 3. The two water layers contained six water molecules in each primitive c(4 × 2) unit cell. The molecules of the lower layer were dissociated. In consequence, 3/4 of the surface Mg atoms were occupied with OH groups, and 1/4 was unoccupied. In the course of the simulation, the OH groups were bridging the unoccupied Mg atoms and finally pulled them out of the surface, at the same time loosening the bonds to the initially occupied Mg atoms. The pulled-out Mg atoms were stabilized in the hydrogen-bonded network structure of OH groups and approximately 1/3 of the undissociated water molecules of the upper layer. In this layer, the remaining water molecules were desorbed step by step. The vacancies in the MgO surface were barely filled by hydrogens bound to the surface oxygens. 3.2. The p(3 × 2) Adsorbate. The p(3 × 2) adsorbate has received much attention in the literature. In our recent work,1 we found an adsorbate structure with 1/3 of the water molecules dissociated as the most stable one, in agreement with experimental4 and other calculated data.8-10 We therefore started with this structure and subjected it to a MD treatment by heating it up to 300 K. The time steps were again 1 fs. The time development of the potential energy relative to the energy of the optimized structure at 0 K is shown in Figure 4. The system was heated up to 300 K after 0.2 ps. The point labeled as (a) shows the potential energy of the system after 0.3 ps. The energy

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Figure 2. Adsorbate structures from the c(4 × 2) double-layer adsorbate without dissociated water molecules on the MgO(100) surface (a) at start, (b) with one dissociated water molecule, (c) with two dissociated water molecules, and (d) with three dissociated water molecules related to Figure 1.

fluctuated up to 2.5 ps, labeled as (b), but the average value remained relatively stable. The snapshots corresponding to structures a and b of these two points are shown in Figure 5. The p(3 × 2) symmetry was lost at 300 K. The partial dissociation of 1/3 of the water molecules was, however, maintained. The loss of symmetry was not connected with a desorption during the time interval of 2.5 ps. The energy was raised by 19 kJ/mol with respect to the initial structure if only the water molecules were moving and by 30 kJ/mol if both the water molecules and the two upper surface layers were allowed to move. No reaction with the surface was observed within the simulation time of 2.8 ps.

Jug et al.

Figure 3. Adsorbate structures from the c(4 × 2) double-layer adsorbate with partial dissociation of water molecules on the MgO(100) surface; (a) starting structure with dissociated water molecules in the lower layer and (b) final structure with energy convergence at 300 K after 1 ps.

3.3. The (1 × 1) Adsorbate. A (1 × 1) adsorbate was found by Liu et al.,12,13 and its structure was also suggested by Daniels et al.14 The structure that we found in our calculation1 was more sophisticated than the one depicted by Liu et al.13 and Daniels et al. The latter contained OH groups on top of surface Mg atoms which were perpendicular to the MgO surface and H atoms on top of the surface oxygens. Our previous study1 and a periodic DFT study14 came to the conclusion that this structure is unstable. Our structure was characterized by OH groups bridging two magnesium surface atoms and dissociated hydrogen atoms bound to a surface oxygen. This stable structure was our starting structure for the MD simulation. The time development of the potential energy relative to the energy of the optimized structure at 0 K is shown in Figure 6. A snapshot (a) shows a maximum at 300 K after the heating-up period of 0.25 ps. After 3.2 ps, a global energy minimum was reached.

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Figure 4. Time development of the potential energy of a monolayer of the p(3 × 2) adsorbate for a starting structure with 1/3 dissociated water molecules on the MgO(100) surface (a) at 0.3 ps shortly after heating up to 300 K and (b) after equilibration at about 2.5 ps. Figure 7. Time development of the temperature of the hydroxylated (1 × 1) adsorbate related to Figure 6.

Figure 5. Adsorbate structures from a p(3 × 2) monolayer adsorbate with 1/3 dissociated water molecules on the MgO(100) surface after (a) 0.3 and (b) 2.5 ps related to Figure 4.

Figure 8. Adsorbate structure from the hydroxylated (1 × 1) adsorbate after (a) 0.25 and (b) 3.2 ps related to Figure 6.

Figure 6. Time development of the potential energy of a monolayer of hydroxylated (1 × 1) adsorbate (a) at 0.25 ps shortly after heating up to 300 K and (b) formation of nuclei after 3.2 ps.

The corresponding temperature development is shown in Figure 7. The structures from snapshots at these simulation times of 0.25 and 3.2 ps are shown in Figure 8. The heating resulted

in a loss of the (1 × 1) symmetry. Most remarkable was the attack of the MgO surface by the bound OH groups. In the structure of snapshot a, we see that, already after the first 0.2 ps, there is a zipper-like distortion of the MgO surface in every second Mg row. In the following 3 ps, approximately one-half of the Mg atoms was pulled out of the MgO surface by the OH groups into a newly forming next layer. Here, the Mg atoms were 5- or 6-fold coordinated, surrounded by OH groups, and they formed a new rough surface structure. The Mg vacancies were filled by hydrogen atoms from the dissociated water molecules. This conversion was accompanied by an energy gain of more than 200 kJ/mol per pulled-out Mg atom. The spontaneous attack of the hydroxylated MgO(100) surface during the MD simulation led to the formation of a first brucite-like “hill” structure where H atoms occupied the Mg vacancies. A continuation of this reaction process should be possible if additional water is available. We started with two

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Jug et al. structure at 0 K and (a) corresponds to the structure, where all six Mg atoms are pulled out after 1.2 ps. The detailed structure corresponding to this snapshot is presented in Figure 10. Here, the pulled-out Mg atoms are labeled I-VI. It should finally be mentioned that almost all of the water molecules, which did not participate in the reaction, were desorbed. 4. Conclusion

Figure 9. Time development of the potential energy of a monolayer of the hydroxylated MgO(100) surface with two layers of physisorbed water molecules on top from the gas phase at 300 K after (a) 1.2 ps.

We could show by Born-Oppenheimer MD simulations that a temperature increase from 0 to 300 K can cause drastic changes in the adsorption behavior of water layers on the perfect MgO(100) surface. From the three kinds of overlayer structures that were stable at 0 K, only one remained stable at 300 K. The most stable p(3 × 2) overlayer with 1/3 of the water molecules dissociated lost its symmetry, but the percentage of dissociated water molecules remained unchanged. The c(4 × 2) doublelayer structure with undissociated water molecules showed partial dissociation in the course of the heating and time development. Our finding that this dissociation occurs only in the first layer confirms the experimental observation of selfinhibition of water dissociation on magnesium oxide surfaces exposed to water vapor after cleavage. The partially dissociated c(4 × 2) double-layer structure showed a spontaneous attack of the OH groups on the MgO surface. The hydroxylated (1 × 1) adsorbate structure underwent a similar change after temperature increase, where a large percentage of Mg atoms was pulled out of the MgO surface. This structure offers an approach to Mg(OH)2 nucleation. This assumption was confirmed by a study where two additional layers of gas-phase water molecules were included in the calculation. Acknowledgment. B. Heidberg thanks Universita¨t Hannover for a graduate fellowship. She also acknowledges support from the Faculty of Architecture. References and Notes

Figure 10. Adsorbate structure from the hydroxylated MgO(100) surface with two layers of water molecules physisorbed on top from the gas phase after (a) 1.2 ps related to Figure 9.

layers of undissociated water molecules. The initial distance between the two layers was chosen as the MgO distance of the cyclic cluster and then optimized. This two-layer system was then placed on top of the hydroxylated surface. To account especially for the brucite nucleation process, which proceeded both from the surface and into the crystal, three and four MgO planes were relaxed. In this way, the results were comparable to the simulation of the monolayer coverage, where three MgO planes were relaxed. At the same time, it could be tested how far a reaction was possible inside the crystal if free motion of the atoms was allowed. If three MgO planes were relaxed, five Mg atoms were pulled out of the surface after 1 ps. For relaxation of four planes, there were six Mg atoms pulled out. This corresponds to 21 and 25%, respectively, of the Mg surface atoms. Figure 9 shows the time development of the potential energy of this process relative to the energy of the optimized

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