Ab Initio Simulations of the Interaction between Water and Defects on

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Ab Initio Simulations of the Interaction between Water and Defects on the Calcite (101j 4) Surface Jennifer S. Lardge, Dorothy M. Duffy,* Mike J. Gillan, and Matthew Watkins London Centre for Nanotechnology and Department of Physics and Astronomy, UniVersity College London, Gower Street, London WC1E 6BT, United Kingdom ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: December 17, 2009

The interaction between water and calcite surfaces is relevant to a broad range of technological processes, but a fundamental understanding of the nature of the adsorbed water is still lacking. In an earlier publication we used density functional theory calculations to calculate the interaction between water and perfect (101j 4) calcite surfaces. Water was found to be strongly adsorbed as associated molecules. In this paper water adsorption on (101j 4) calcite surfaces with steps and vacancies is investigated. A water molecule was found to bind more strongly to acute steps than to obtuse steps. The lowest energy position was found to be the base of the step for acute steps and on top of the step for obtuse steps. Water molecules were found to exhibit very strong binding to surface vacancies. Associative adsorption was favored near cation vacancies; however, the water was found to dissociate, to form a bicarbonate ion and a hydroxide ion, near anion vacancies. 1. Introduction The interaction of water with mineral surfaces is fundamental to numerous natural and technological processes. Water interacts strongly with minerals and it has been demonstrated that, in a humid atmosphere, there is generally a layer of water a few angstroms thick (1-3 monolayers) adsorbed on mineral surfaces.1 Calcite appears to have a particularly strong affinity for water, as a 1.7 nm layer (∼5 monolayers) was observed on calcite surfaces down to relatively low humidities.2 This water layer has implications for a range of surface processes, as the water must be displaced before ions or organic molecules can be adsorbed directly onto the surface. In this paper we study the interaction of water with defects on the (101j 4) surface of calcite. Calcite is of particular interest because of its prevalence in geology, biominerals, and in industrial scale. One fundamental question that needs to be addressed is the nature of the adsorbed water. Water is known to dissociate into a proton and a hydroxide ion on some surfaces and this can have strong implications for the surface reactivity. X-ray photoelectron spectroscopy (XPS) experiments3 have indicated the presence of the hydroxide ion on calcite surfaces and these have been interpreted as a surface terminated by bicarbonate and hydroxide ions.4 FTIR experiements5 have detected Ca(OH)2 on the surface of calcite crystals prepared by bubbling CO2 through a Ca(OH)2 slurry but not on pure calcite crystals. Ab initio calculations6,7 have, however, indicated that dissociation is strongly disfavored on the flat defect-free (101j 4) surface; therefore, an alternative interpretation of the experimental results is required. Distinguishing between associated and dissociated water on surfaces experimentally is not straightforward, and the challenges have been summarized in a review by Henderson.8 Water dissociation on some oxide surfaces is quite well established, while on others it is controversial. A comprehensive review of dissociation for oxides, metals, and semiconductors is given in Henderson.8 In some cases dissociation is observed on flat, defect-free surfaces (CaO, for example9), while on others dissociation it mediated by surface defects [the (100) MnO * Corresponding author: e-mail [email protected].

surface, for example10]. Isolated water molecules appear to adsorb associatively on (100) MgO surfaces, but dissociation is favored at higher coverage as it is stabilized by hydrogen bonding.11 Surface steps and kinks also stabilize dissociation on (100) MgO surfaces.12,13 In an earlier publication we carried out a comprehensive ab initio study of water on the flat defect-free (101j 4) calcite surface.7 We found an ordered zigzag arrangement of water molecules parallel to the surface at monolayer coverage and calculated high adsorption energies for coverages of up to 2 monolayers. The ordered monolayer structure was similar to that identified previously by use of classical potentials.14 A metastable configuration for dissociated water was located but it was strongly energetically unfavorable. The energy for dissociated water was found to be 1.7 eV higher than that of associated water. We did not find any evidence that the dissociated molecule could be stabilized by hydrogen bonding to other water molecules. However, real surfaces contain defects such as vacancies, adatoms, and steps, and such defects are known to mediate dissociation on other surfaces. In this paper we investigate water adsorption on defective calcite surfaces. We use density functional theory (DFT) to study the interaction between Ca vacancies, CO3 vacancies, and surface steps with water and to determine whether such defects stabilize dissociated water on the (101j 4) surface of calcite. 2. Calculation Methods We used the Quickstep module15 of the CP2K program suite to perform density functional theory calculations with the PBE functional.16 The Gaussian and plane-wave (GPW) method, which utilizes dual basis sets (Gaussian functions and plane waves) to combine the different efficiencies of local combination of atomic orbital (LCAO) and plane-wave implementations of DFT, was used. Core electrons were represented by GTH pseudopotentials;17 Gaussian basis sets of DZVP quality were used, along with a 300 Ry energy cutoff for the auxiliary planewave basis. The Gaussian basis set for calcium was reoptimized for a singly charged positive ion state, and with a confining parabolic potential starting at a radius of 2.5 bohr added, to

10.1021/jp909593p  2010 American Chemical Society Published on Web 01/25/2010

Interaction between Water and Calcite Surfaces

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2665 Configuration space of the adsorbed water molecules on the surface steps was explored by initiating the simulations with the water molecules at different positions. A static relaxation to the local minimum was carried out until the forces were converged to 0.0045 au/bohr. A 3.5 ps MD (time step 1 fs, 300 K) simulation was then run, with the relaxed configuration as the initial configuration, to explore the local potential energy surface. The final configuration from the MD simulations was relaxed to equilibrium until the forces converged to obtain the adsorption energies and conformations.

Figure 1. Side view of the relaxed configuration of the cell with one Ca vacancy and one CO3 vacancy in the surface layer. Ca ions are shown in green, C in gray, and O in red.

generate basis functions more appropriate for the ionic environment in calcite. In static calculations, all ionic coordinates were optimized by use of the BFGS algorithm18 until the maximum force on any ion was less than 0.0045 au/bohr. Born-Oppenheimer molecular dynamics (BOMD) simulations were carried out within NVE ensembles at nominal temperatures of 270-350 K and the equations of motion were integrated with a time step of 1 fs. The interaction with surface vacancies was investigated by use of a five-layer slab with eight CaCO3 units per layer. One Ca atom and one CO3 group were removed from the upper layer, maintaining overall charge neutrality in the periodic cell. The geometry of the cell was relaxed until the forces converged and the bottom layer of atoms was fixed to represent the constraints imposed by the bulk crystal. The configuration of the relaxed cell is shown in Figure 1. Surface steps are common features of crystal surfaces that are created naturally during both growth and dissolution. In calcite, the low energy steps on the (101j 4) surface are oriented along the 〈4j8j1〉 and 〈4j41〉 directions. The carbonate ions lie at an angle to the (101j 4) surface; therefore, the steps are at an angle (78° or 102°, depending on the direction of the step) to the surface plane. The (318) surface of calcite can be considered to be a (101j 4) surface with a linear array of parallel acute (78°) monatomic surface steps. Similarly, the (31j 2j 16) surface is equivalent to a linear array of obtuse (102°) steps on a (101j 4) surface. Slabs of four layers of both surfaces were created and the structures were relaxed by use of Quickstep until the forces converged. The bottom layer of ions was frozen to represent the constraints of the bulk crystal. The relaxed structures are shown in Figure 2.

3. Results 3.1. Adsorption on a Defect-Free Surface. Earlier DFT calculations7 with VASP (Vienna ab initio simulation package) have investigated the interaction between a water molecule and the defect-free (101j 4) surface. The O atom of the H2O molecule has been found to bind strongly to the surface Ca2+ ion, and two inequivalent orientations were identified. One has the water H atoms oriented toward two different [010] rows of CO3 ions on the surface (Figure 3a) and the other has the H atoms oriented toward one [010] row of CO3 ions (Figure 3b). The former was found to have the lowest energy. The adsorption energy for molecular water in these two configurations was calculated with VASP and Quickstep, in order to compare the results obtained via the two methodologies. The two configurations were investigated for a three-layer slab of calcite, with the bottom layer frozen. γ-Point calculations and the PBE exchange-correlation functional were used for both methods. Results for adsorption energies and interatomic separations for the relaxed molecules are summarized in Table 1 for both methodologies, and the relaxed geometries for the Quickstep calculations are shown in Figure 3. It is clear that the results are in reasonable agreement but the adsorption energies calculated with Quickstep are slightly higher than those calculated with VASP. This is to be expected because of basis set superposition error associated with the local basis set employed by Quickstep. The agreement is sufficiently good to give us confidence in the results obtained by use of Quickstep for the defective surface. The results for a larger unit cell calculated by use of Quickstep are included to test for convergence with cell size. 3.2. Adsorption of Water at Surface Vacancies. The relaxed periodic cell with an anion and a cation surface vacancy (Figure 1) was used to investigate the adsorption on water at vacancies on the (101j 4) calcite surface. The associated water molecule was above the CO3 vacancy and relaxed until the

Figure 2. Relaxed structure of (a) (314j 8) surface (acute steps) and (b) (31j 2j 16) surface (obtuse steps) of calcite. The size of the simulation cell in the visualization has been doubled for clarity.

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Figure 3. Side view along the [010] direction of the relaxed configurations of a water molecule on the (101j 4) calcite surface, calculated by use of Quickstep. Configuration a is 0.2 eV lower in energy than configuration b.

TABLE 1: Adsorption Energies for a Water Molecule on a a Flat (101j 4) Calcite Surface in Two Distinct Orientationsa adsorption energy (eV) VASP (three layers, four CaCO3 units per layer) Quickstep (three layers, four CaCO3 units per layer) Quickstep (four layers, eight CaCO3 units per layer) a

position 1

position 2

-0.63

-0.91

-0.71

-0.93

-0.70

-0.94

See Figure 3. Calculated by use of VASP or Quickstep.

forces converged. The molecule remained associated; the binding energy was found to be -0.61 eV. The relaxed configuration of the associated molecule was used as the initial configuration for a 3.5 ps MD simulation at 300 K. After 0.5 ps the water molecule was observed to dissociate and the proton moved from the water molecule to the carbonate ion, forming a bicarbonate ion. The hydroxide ion moved to a position between two Ca2+ ions. The interatomic distances were monitored during the simulation, and it was found that the OH bond of the bicarbonate ion was very stable (mean bond length of 0.97 Å). The distance between the oxygen of the hydroxide ion and the hydrogen atom of the bicarbonate ion fluctuated quite strongly, as illustrated in the plot of Figure 4. These are the H and O atoms of the original water molecules, and the fluctuations indicate a tendency for reassociation. The final configuration of the 3.5 ps MD simulation was relaxed in a static simulation until the forces converged and the resulting adsorption energy was found to be -1.50 eV. The final conformation is shown in Figure 5a. A similar procedure was carried out for a water molecule over the Ca vacancy. After the initial relaxation, the water molecule relaxed toward the vacancy and the adsorption energy was found to be -0.34 eV. The relaxed configuration was used as an initial configuration for a 3.5 ps MD run (300 K). During the MD simulation, the water moved toward the adjacent Ca2+ ion and the water H atoms pointed toward the carbonate row adjacent to the vacancy. The final configuration of the MD simulation was relaxed until the forces converged and the adsorption energy was found to be -1.60 eV. The final conformation is shown in Figure 5b. 3.3. Adsorption at Surface Steps. The interaction with water molecules with the obtuse calcite surface steps was investigated

Figure 4. Plot of H(water)-O(water) (black line) and H (water)O(carbonate) (gray line) separations during a 3.5 ps MD simulation. The H atom is bonded to the water oxygen at the start of the simulation, but it moves to bond to the carbonate oxygen at around 0.5 ps. The strong oscillations in the Hw-Ow curve shows oscillations of the OH ion toward the bicarbonate ion.

by initiating static relaxation simulations with the water molecule at five different orientations close to the step on a (31j 2j 16) surface. In all cases the final conformation was similar, with the molecule positioned above the Ca ion of the upper level and the H atoms directed toward the adjacent carbonate oxygen atoms on the upper level. The position remained essentially unchanged after a 3.5 ps MD simulations and the final MD configuration was relaxed, resulting in an adsorption energy of -0.97 eV. The binding of a water molecule to the obtuse step is, therefore, slightly stronger than binding to a flat surface. The final conformation is shown in Figure 6a. One calculation was initiated with a dissociated water molecule close to the step; however, this resulted in a positive adsorption energy, and thus there is no evidence of water dissociation at obtuse steps on calcite surfaces. In order to investigate the influence of additional water molecules on dissociation at steps, the density of water molecules at the step edge was increased to one molecule per Ca ion. Each molecule was initially positioned in the low-energy conformation located for the lower water density. A 3.5 ps MD simulation at 300 K revealed no evidence of dissociation and the final adsorption energy per molecule, after static relaxation, was found to be -1.02 eV, slightly lower than the result for the lower water density configuration. A similar procedure was followed for the (314j 8) surface, which is equivalent to a linear array of acute steps. Again five independent initial configurations were used but all relaxed to similar final conformations. The O atom of the water molecule was located in the angle of the step, almost equidistant from two Ca ions on the upper and lower levels, and the H atoms were oriented toward the carbonate O atoms of the upper level. The relaxed configuration with the lowest energy (adsorption energy -1.25 eV) was used as an initial configuration for a 3.5 ps MD simulation and the final configuration was relaxed until the forces converged. The adsorption energy of the relaxed configuration was found to be -1.40 eV and the configuration is shown in Figure 6b. Again a dissociated water molecule on an acute step was found to have a higher energy than the associated molecule. As with the obtuse step, the water density at the step was increased to one water molecule per Ca ion and a 3.5 ps MD simulation did not show any tendency for water dissociation.

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Figure 5. Relaxed configuration of a water molecule adsorbed over (a) an anion vacancy and (b) a cation vacancy on a calcite (101j 4) surface. Note that the water has dissociated over the anion vacancy in panel a and the proton is attached to the carbonate ion to form a bicarbonate ion. The hydroxide ion is positioned between two Ca ions.

Figure 6. Relaxed configuration of a water molecule adsorbed on (a) a (31j 2j 16) surface (obtuse step) and (b) a (314j 8) surface (acute step) of calcite. The size of the simulation cell has been doubled in the visualization for clarity.

4. Summary and Discussion Table 2 summarizes the adsorption energies and interatomic separations calculated, by use of the Quickstep module, for the water molecules adsorbed on defect-free and defective (101j 4) calcite surfaces. It is clear from the table that water binds more strongly to surface vacancies than to steps and that it binds more strongly to acute steps than obtuse steps. The acute step is a particularly favorable binding position because the water oxygen can get close to two Ca ions. The minimum energy position for the water molecule is on top of the step for obtuse steps and on the bottom of the step for the acute step. The strong binding of TABLE 2: Summary of Adsorption Energies and Interatomic Separations position

adsorption energy (eV)

closest Ow-Ca separationa (Å)

closest Hw-OC separationb (Å)

flat surface cation vacancy anion vacancyc obtuse step acute step

-0.94 -1.60 -1.50 -0.97 -1.40

2.47 2.48 2.42 (2.50) 2.53 2.65 (2.74)

1.45 2.05 0.97 2.00 2.88

a Between water oxygen (Ow) and the Ca ion in the relaxed configuration. The numbers in parenthesess indicate that there is a second Ca ion with a comparable separation to the first. b Between water hydrogen (Hw) and carbonate oxygen in the relaxed configuration. c Water is dissociated; therefore, Ow refers to the hydroxide ion and Hw-OC is the bicarbonate bond.

water to the acute step will have implications for crystal growth as the water will have to be displaced before ions can attach to the surface steps. Experiments have shown that calcite precipitation occurs preferentially at obtuse steps on calcite surfaces, which is consistent with the weaker binding of water.19 In all but one case considered in this paper, associative adsorption of water was found to be energetically favored. The exception is adsorption near a carbonate ion vacancy. In this case the water molecule dissociated spontaneously after 0.5 ps of a 300 K MD simulation. The proton attached to the adjacent carbonate ion, to form a bicarbonate ion, and the hydroxide ion moved in between two surface Ca ions. The hydroxide ion oscillated toward the bicarbonate ion during the MD simulation, indicating a tendency toward reassociation. The Ca vacancy did not stabilize dissociated water but it did increase the magnitude of the adsorption energy, with respect to adsorption on the flat surface, and it displayed the strongest binding of any of the defects considered here. 5. Conclusions We have used the efficient CP2Kcode Quickstep to investigate the adsorption of water at defects on the calcite (101j 4) surface. We found adsorption energies for the flat defect-free surface that were comparable with, but slightly higher than, results calculated by use of the VASP DFT program with the same functional. All the defects considered in this paper resulted in enhanced binding over the flat, defect-free surface; however, only the

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anion vacancy induced spontaneous dissociation of the water molecule. The proton attached to a neighboring carbonate ion, forming a bicarbonate, and the hydroxide ion moved between two Ca ions. The magnitude of the adsorption energy was calculated to be 0.56 eV higher than that of the water molecule on the defect-free surface. The binding to the cation vacancy was even stronger, but in this case the water molecule remained associated. Water molecules were also strongly bound to steps on the (101j 4) calcite surface. The binding was particularly strong on the acute step where the water molecule was bonded to the step edge and the water oxygen was almost equidistant between the Ca ions on the upper and lower levels. In contrast, the water molecule was bonded to the Ca ion on the edge of the upper terrace on the obtuse step and the binding energy was only slightly higher (0.03 eV) than on the flat surface. The water molecule did not show any tendency to dissociate on the surface steps, even in the presence of additional water molecules. In summary, water molecules are strongly bound to calcite surfaces and dissociation is generally energetically unfavorable. The only exception we have found is the anion vacancy, where we observed spontaneous dissociation of the water molecule, forming a hydroxide ion and a surface bicarbonate ion. Acknowledgment. We acknowledge funding from EPSRC under Grant GR/S80103/01 and computer time on the Mott2 cluster, funded under Grant GR/S84415/01.

Lardge et al. References and Notes (1) Ewing, G. E. Chem. ReV. 2006, 106, 15111. (2) Stipp et al. Geochim. Cosmochim. Acta, submitted for publication. (3) Stipp, S. L.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1991, 55, 1735. (4) Stipp, S. L. Mol. Simul. 2002, 28, 497. (5) Kuriyavar, S. I.; Vetrivel, R.; Hegde, S. G.; Ramaswamy, A. V.; Chakrabarty, D.; Mahapatra, S. J. Mater. Chem. 2000, 10, 1835. (6) Kerisit, S.; Parker, S. C.; Harding, J. H. J. Phys. Chem. B 2003, 107, 7676. (7) Lardge, J. S.; Duffy, D. M.; Gillan, M. J. J. Phys. Chem. C 2009, 111, 11943. (8) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (9) Iedema, M. J.; Kizhakevariam, N.; Cowin, J. P. J. Phys. Chem B 1998, 102, 693. (10) Kendelewicz, T.; Doyle, C. S.; Carrier, X.; Brown, G. E. Surf. ReV. Lett. 1999, 6, 1255. (11) Giordano, L.; Goniakkowski, J.; Suzanne, J. Phys. ReV. Lett. 1998, 81, 1271. (12) Langel, W.; Parrinello, M. J. Chem. Phys. 1995, 103, 3040. (13) Chizallet, C.; Costentin, G.; Che, M.; Delbecq, F.; Sautet, P. J. Phys. Chem. B 2006, 110, 15878–15886. (14) de Leeuw, N. H.; Parker, S. C. J. Chem. Soc., Faraday Trans. 1997, 93, 467. (15) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Comput. Phys. Commun. 2005, 167, 103. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M.; et al. Phys. ReV. Lett. 1996, 77, 3865. (17) Krack, M. Theor. Chim. Acta 2005, 114, 145. (18) Shanno, D. F. Math. Comput. 1970, 24, 647. (19) Stipp, S. L.; Gutmannsbauer, W.; Lehmann, T. Am. Mineral. 1996, 81, 1.

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