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J. Phys. Chem. C 2007, 111, 8000-8004
Spontaneous Emergence of Cl- Anions from NaCl(100) at Low Relative Humidity Pepa Cabrera-Sanfelix,*,† Daniel Sanchez Portal,‡ Albert Verdaguer,§,∇ George R. Darling,⊥ Miquel Salmeron,∇ and Andres Arnau‡,# Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, San Sebastia´ n 20018, Spain, Unidad de Fı´sica de Materiales, CSIC-UPV, San Sebastia´ n, Spain, Institut Catala de Nanotecnologia, Barcelona, Spain, Lawrence Berkeley Laboratory, UniVersity of California, Berkeley, California 94720, Surface Science Research Centre, Department of Chemistry, UniVersity of LiVerpool, LiVerpool, U.K., and Departamento de Fı´sica de Materiales, UPV/EHU, San Sebastia´ n, Spain ReceiVed: January 22, 2007; In Final Form: April 2, 2007
The emergence of Cl- ions from the surface of alkali halide salts at low relative humidity (RH) is predicted by density functional theory (DFT) calculations and supported by contact potential measurements. We find from DFT that, in the presence of water at the regime of one monolayer coverage on the (100) cleavage plane of NaCl, Cl- ions are displaced at very low energetic cost from their crystal lattice positions toward the plane of water molecules. Starting from plausible low-temperature layered structures, we use total energy DFT to calculate the energy cost to bring Cl- ions to the height of the water layer. We show the importance of the screening of the electrostatic interactions by the water layer in order to explain our findings and to determine the value of the surface dipole as a function of the chloride position. The theoretical surface dipole is used to estimate the concentration of raised Cl- ions at the surface. As dissolved chloride ions are the reactive component of the sea salt aerosols at high RH, we propose that the facile destabilization of Cl- is one of the mechanisms behind the catalytic activity of NaCl at low RH on reactions involving atmospheric gases.
1. Introduction Thin films of water molecules cover most surfaces under ambient conditions. This phenomenon is crucial in biology, material sciences, and chemistry. Consequently, numerous studies are devoted to the subject.1,2 For example, chemical reactions occurring at the wet surface of sea salt aerosols play a fundamental role in tropospheric chemistry.3-5 These reactions, studied in laboratory conditions, show a strong sensitivity to relative humidity (RH) due to the formation of surface adsorbed water.6 It is known that dissolved chloride ions are the reactive component of the salt. However, the role of adsorbed water in the reaction mechanisms at the molecular level is not well understood. Insight into the mechanisms of the catalytic activity of sea salt aerosols above deliquescence has been obtained recently7 using molecular dynamics simulations. A high concentration of halogen ions at the water surface of salt solutions (NaCl, NaBr, and NaI) was predicted. The surface segregation propensity of halogen ions is the basis of models to explain the unexpectedly high Cl2 concentration resulting from the reaction between NaCl particles and O3. The preferential segregation of halogen ions was recently confirmed experimentally by X-ray photoelectron spectroscopy measurements on saturated salt solutions obtained by deliquescence of KBr and KI crystals.8 In the case of NaCl, deliquescence occurs at 75% RH.9 Infrared * To whom correspondence should be addressed. E-mail: swbcasam@ sc.ehu.es. † DIPC. ‡ CSIC-UPV. § Institut Catala de Nanotecnologia. ∇ University of California. ⊥ University of Liverpool. # UPV/EHU.
spectroscopy experiments10 have shown that water adsorption on the NaCl(100) surface under ambient conditions led to surface dissolution at 75% RH, corresponding to a water coverage of 3ML, at which point the salt surface becomes visibly damaged. The coexistence of submonolayer islands and multilayer films at lower coverage has also been demostrated.11 Transmission electron microscopy (TEM) results,12 also at 75% RH, show that microcrystallites of NaCl are formed on NaCl (100) surfaces by recrystallization after dissolution. At lower RH two different regimes, below and above ∼40% RH, are observed in the surface ion mobility of cleaved surfaces of NaCl.12 These two regimes were also observed using atomic force microscopy (AFM).13 Below ∼40% RH only small reversible modifications of the step structure were observed, whereas between 40% and 75% RH large scale modifications of the step structure appeared, followed by deliquescence of the crystal at 75% RH.14 The change of regime at ∼40% RH is related to the completion of the first monolayer of adsorbed water on the NaCl surface.10 Ion mobility on the NaCl surface below 40% RH was recently studied15 using a noncontact AFM electrostatic operation mode, i.e., scanning polarization force microscopy (SPFM), to measure the surface potential of NaCl as a function of RH between 0 and 40%. The results indicated the formation of negative dipoles, first at the step edges and then over the entire terrace, as the surface was progressively covered by water. This observation points toward a preferential detachment of the Cl- ions at 1ML water coverage and could explain the reactivity of NaCl surfaces at low RH. Therefore, as a first step toward understanding the mechanisms at the molecular level, in this work we study the stability of the NaCl(100) surface against 1ML water adsorption. We use several structures in which the molecules are adsorbed
10.1021/jp070548t CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
Emergence of Cl- Anions from NaCl(100) at Low RH
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8001
in their stable quasi-planar configuration on the surface. Further stability is gained by molecular reorientation and formation of hydrogen bonds between the molecules. These configurations can be considered as reasonable candidates for the geometry of one monolayer of water on NaCl(100) at low temperature. We find that the emergence of Cl- ions from their positions in the surface toward the water layer can occur at almost negligible energetic cost. The destabilization occurs when the hydrogen atoms of at least three water molecules point toward a given chloride. Once this initial destabilization takes place, the efficient screening of the electrostatic interactions by the polarizable water layer strongly reduces the energetic cost for the detachment of the chloride. We argue that a similar mechanism can be active at room temperature and thus play a key role to explain some of the experimental data mentioned above. Molecular dynamics simulations7,16,17 will be an important tool to further clarify the effect of temperature on this phenomenon. However, our contact potential measurements support the idea that an appreciable fraction of the chlorine ions have emerged from the surface layer in the presence of a monolayer of water at room temperature. 2. Method 2.1. Theoretical Section. Here, we present theoretical evidence that strongly supports the model of easy displacement of Cl- ions from the NaCl surface lattice into the water monolayer. We use density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP)18-20 and the generalized gradient approximation (GGA).21 Although DFT cannot capture the Van der Waals component of the intermolecular interaction, it has the advantage for the present work of treating all atoms on an equal footing, and treats the electrostatics essentially exactly. Thus it can easily deal with situations where there are substantial rearrangements of atoms, such as found here. We have studied several stable water configurations consistent with the c(4 × 2) symmetry reported by low-energy electron diffraction (LEED) experiments at low temperature (∼100 K).22,23 Therefore, the surface is represented by a periodic supercell/slab formed by four NaCl layers with a c(4 × 2) lateral periodicity. Molecular dynamics simulations of room temperature adsorption on frozen NaCl surfaces indicate that these highly ordered, flat overlayers are an idealization, the true structure in this regime showing some multilayer ordering.16 However local clusters of water molecules resembling our ‘star’ configuration (discussed below) are observable, so our conclusions should not alter on this basis.17 Adsorption of water (four water molecules per cell) has been only considered on one surface of the slab, after confirming identical results using a symmetric configuration with molecules adsorbed on both sides of the slab. Symmetric configurations have been used to calculate the surface dipole variation. For the asymmetric slab the two bottom NaCl layers are fixed during geometry optimization. Relaxations have been performed until all the components of the forces acting on every atom are smaller than 0.03 eV/Å. The adsorption energy per water molecule, Eads, has been calculated from
Eads ) -(EH2O/NaCl - ENaCl - nEHisol2O)/n
(1)
where EH2O/NaCl is the energy of the optimized system, whereas ENaCl and EHisol2O correspond to the energies of the relaxed NaCl(100) surface and isolated water molecule, respectively, and n is the number of molecules adsorbed on the surface (n ) 4 in our cell). Eads calculated in this way, is the result of both
water-water hydrogen bonding and water-surface interaction. The hydrogen-bonding contribution, EH_bonding, has been calculated from
EH_bonding ) -(Ew_layer - nEHisol2O)/n
(2)
where Ew_layer is the energy of the water layer by itself in the adsorbed configuration. The water-surface interaction per molecule has been determined by subtracting EH_bonding from the total adsorption energy of the molecule, Eads:
EH2O-surf ) -(Eads - EH_bonding)
(3)
Notice that we have defined adsorption and other interaction energies above in such a way that their values are positive when the interactions are attractive. 2.2. Experimental Section. These theoretical results are supported by new SPFM data of the variation of the contact potential as a function of RH. Contact potential measurements were carried out at room temperature (21 ( 1 °C) with a homebuilt AFM head24 and a commercial electronic control from RHK Technology Inc., Troy, MI. The microscope head was enclosed in a glove box where humidity control was achieved by circulating either dry or wet N2 (bubbling through Millipore water). The RH was measured using an Omega hygrometer, with an accuracy of (5%. The AFM was operated in the SPFM mode using conductive Pt-coated tips. This provided noncontact topographic images of the NaCl surface either dry or covered with films with simultaneous images of the local contact potential. High-purity (99.999%) NaCl crystals, grown from the melt by the Crystal Growth Laboratory of the University of Utah in Salt Lake City, were cleaved inside the glove box at low humidity ∼5% to obtain a fresh surface that has been never exposed to water before the experiment. Images of both topography and contact potential of the fresh surface were acquired as the RH was increased from ∼5% to 40%. 3. Results and Discussion We have investigated the early stages of anion destabilization by computing the energy associated with a vertical displacement of the Cl- ions into the water monolayer. The most stable configuration of the water monolayer with one water molecule per adsorption site on NaCl(100), according to our DFT calculations, is shown in Figure 1a. We will refer to it as the hexagonal zigzag model in the following. After relaxation, the monolayer is composed of zigzag chains of hydrogen-bonded molecules located between 2.36 Å and 2.40 Å above the NaCl surface. The adsorption energy per molecule is ∼448 meV, with the water-water hydrogen bond accounting for ∼313 meV, and the water-surface interaction contributing ∼135 meV. The NaCl topmost layer only exhibits a small corrugation of ∼0.1 Å. The hydrogen atoms of the water molecules are in general pointing toward the chlorine atoms, with some of them symmetrically surrounded by H atoms (see the Cl- marked with an arrow in Figure 1a). We expect these sites to become less stable and the chlorines to be easily displaced. Notice that there is not an equivalent situation for the Na+, since each water molecule adsorbs with its oxygen atom roughly on top of one cation. Thus, the environment of the Na+ ions is less influenced by the orientation of the neighboring water molecules. To analyze the stability of these peculiar chlorine sites, we increased the height of the Cl- in small steps of 0.2 Å, allowing the structure to relax around the fixed Cl- at every step. Curve A in Figure 2b shows the total energy change (∆E) as a function
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Figure 1. (a) Optimized c(4 × 2) hexagonal zigzag water structure adsorbed on NaCl(100) surface, with adsorption energy per water molecule Eads ) 448 meV. (b) Optimized c(4 × 2) star water structure adsorbed on NaCl(100) surface, with Eads ) 381 meV. In both cases, the black lines delimit the unit cell. Blue and green circles correspond to Na+ and Cl- ions, respectively. The emerging Cl- ion is marked with an arrow. Red and white circles correspond to O and H atoms, respectively.
Figure 2. (a) Side view of the topmost layers of the calculated c(8 × 4) hexagonal zigzag water/NaCl(100) interface structure showing the displacement of Cl- anions into the water layer at ∆zw ) 2.4 Å. It illustrates the structure of the hydrated Cl- anions on the NaCl(100) surface after the chloride emergence. (b) Energy, ∆E (meV), as a function of the Cl- ion displacement distance, ∆z (Å), from the NaCl(100) topmost layer. The three curves correspond to different configurations of the wetting layer: (A) c(4 × 2) hexagonal zigzag; (B) c(8 × 4) hexagonal zigzag, where the concentration of displaced Cl- ions is four times lower than in case A; (C) c(4 × 2) star. The energy zero is always referred to the hexagonal zigzag structures. The inset compares the energy cost of displacing a Cl- anion in dry and wet NaCl(100) surfaces. The wet system corresponds to curve A.
of Cl- height (∆z). The energy cost of this displacement is only 123 meV when the Cl- ion has been displaced by as much as 2.4 Å from the surface NaCl layer. The inset shows a comparison with a similar displacement on the dry NaCl(100) surface, which now requires an energy of ∼1.75 eV. The crucial role of the hydration monolayer is thus evident since the energetic cost of the ionic displacement is lowered by 1 order of magnitude. In fact, the energy cost of such displacement is so low that at room temperature an appreciable concentration of Cl- should spontaneously emerge into the hydration layer at the surface [see Figure 2a]. In order to confirm the preferential rise of the Cl- ion, we have investigated the emergence of one Na+ ion on the same system. The nonequivalent situation of anions and cations, mentioned above, leads to a reorganization of the water layer during this process, due to the displacement and the weakening
of the hydrogen bonds of the water molecule placed on top of the emerging cation. We also found that detachment of one Na+ ion is accompanied by the spontaneous emergence of the closest unstable Cl- ion (at a position equivalent to the one marked with an arrow in Figure 1a). These two events lead to an energy cost for the Na+ ion emergence, ∼596 meV, about five times higher than for the case of the Cl- and are, therefore, in agreement with the large negative surface potential change observed by AFM measurements at RH ∼40% [see Figure 3a]. The very large energy cost of extracting a Na+ ion suggests that even with the inclusion of entropic effects, this should be strongly disfavored compared to Cl- extraction. We have also considered a lower concentration of Cl- ions segregating into the water layer by using a unit cell four times larger. The result is shown in curve B of Figure 2b. The behavior is similar to that in curve A, as expected. However, in this case
Emergence of Cl- Anions from NaCl(100) at Low RH
J. Phys. Chem. C, Vol. 111, No. 22, 2007 8003
Figure 3. (a) Experimental AFM measurements of the surface potential variation, ∆V (volts), as a function of relative humidity (RH). (b) Calculated surface dipole variation, ∆ds (e- Å) as a function of Cl- displacement from the NaCl surface ∆z, for both c(4 × 2) wetting systems: (A) hexagonal zigzag and (B) star.
we find a minimum of energy when the Cl- ions are located at ∆zw ) 2.4 Å above the top NaCl layer, which corresponds to the Cl- ion being coplanar with the water layer. Figure 2a displays a side view of the corresponding NaCl(100) structure, in which displaced Cl- ions are surrounded by four water molecules. It is also important to confirm that the observed phenomenon is not restricted to a particular configuration of the water monolayer. In Figure 1b we show a different structure, that we will refer to as “star” from now on. In this structure the water molecules have been moved closer with hydrogens oriented more exactly to the destabilized Cl- ion in the surface (marked with an arrow in Figure 1b). As a consequence the Cl- ion spontaneously raises ∼1.2 Å toward the water layer during the DFT relaxation.25 The Cl- ion is strongly attracted by the four nearest hydrogen atoms (the H-Cl- distance is ∼2.5 Å) and, in this case, no barrier has to be overcome to reach that height. In spite of this strong structural change the adsorption energy per water molecule is 381 meV, only 15% lower than in the previous “hexagonal zigzag” configuration and comparable to adsorption energies reported in the literature for other structures.26-30 As for the “hexagonal zigzag” we have increased further the height of the raising Cl-ions. The results are shown as curve C in Figure 2b. The energy decreases as a function of the Cl- ion height until it reaches a minimum at ∼2.4 Å. For the “star” configuration the screening of water is more effective, as hydrogen atoms are closer to the Cl- ion and tend to follow the ion along its trajectory. This explains the low-energy cost to bring the Cl- ion even farther above the NaCl (100) surface, as shown by curve C in Figure 2b. We have studied several variants of the “star” structure using a 2 × 2 super-cell and have found that the spontaneous emergence of the Cl- ion during the relaxation process requires that at least three of the surrounding water molecules have one hydrogen atom oriented toward it. However, the destabilization of the chloride seems to be independent of other details of the flat monolayer structures. In order to determine whether the chlorine rises from the lattice as an anion or loses its electron during its displacement, we have calculated the projected density of states for a topmost layer Cl- ion and for a situation where the Cl- is ∼2.4 Å above the surface. In both cases the charge of the Cl- ion is very similar, confirming the ionic character irrespective of the height over the substrate. Since no exchange of electrons takes place during the rising path, the qualitative behavior of the energy as a function of the height of the Cl- ions, and the difference between the dry and wet surfaces, has been reproduced using a simple model where the atoms are treated as point charges located in the structures obtained from the ab initio calculations. This confirms the central role of the electrostatic interactions in governing the physics of the system and the efficient
screening of a single water layer. The large screening comes from the high polarizability of water, which stems from the lowenergy cost necessary to reorient the dipole moment of an individual molecule, as shown by ab initio calculations.31 Thus, the orientation of the molecules toward the chloride is important for the initial stages of the emergence (i.e., destabilize the symmetric position of Cl- in the surface layer). However, the low energetic cost is due to the screening by the surrounding water, which is largely independent of the layer structure. This is particularly important for the applicability of the present results to room-temperature conditions, in which the structure of the layer fluctuates and it is probably different from our ideal low-temperature models. The variation of the surface potential in NaCl terraces as a function of RH, measured by the Kelvin probe method in the AFM experiment, is shown in Figure 3a. From 10% to 40% RH (completion of the water monolayer), the measured surface potential difference was ∆V ≈ -225 mV. According to previous work,15 the appearance of such a negative potential cannot be readily explained without the displacement of the Cl- ions, as found by the present calculations. Assuming that each displaced Cl- ion contributes to the surface dipole an amount ds ) -e∆z, where ∆z is the vertical Cl- ion displacement and e the electron unit charge, we can estimate the surface density of displaced Cl- ions (σ) from the relation ∆V ) σds/, where is the dielectric constant. Using ∆z ) 2.4 Å and ) o, the dielectric constant of vacuum, we estimate σ ∼ 0.5%. However, as we discuss below, once the screening of the water layer is taken into account (increasing the effective ) the concentration of displaced Cl- ions necessary to produce the observed contact potential variation is substantially higher. We performed surface dipole (ds) calculations in the hexagonal zigzag and star wetting systems using a c(4 × 2) unit cell and symmetric slabs containing five NaCl layers and a water layer adsorbed at each side of the slab. In both cases ds was calculated for Cl- at three different ∆z values of 0.0, 1.2, and 2.4 Å. The results for the dipole variation are shown in Figure 3b. The variation is 6-7 times smaller than that estimated from the simple point charge model discussed above. Although this result is slightly influenced by the high density (σ) of displaced Cl- ions in our c(4 × 2) cell calculations, it shows the important role of water in determining the surface potential changes. The very efficient screening associated with the water monolayer is simultaneously responsible for the almost negligible energy cost of the Cl- detachment and for the strong reduction of the corresponding electrostatic dipole (and associated electric field). These results show that the concentration of displaced Cl- ions could be 1 order of magnitude higher than the value obtained from oversimplified estimates. Thus, between 1 and 10% of the surface chlorine ions could be raised in the presence of a monolayer of water.
8004 J. Phys. Chem. C, Vol. 111, No. 22, 2007 4. Conclusions In this work we have reported the facile raising of Cl anions from NaCl(100) at RH lower than 40%, equivalent to the water monolayer regime. Our theoretical research, using DFT calculations, shows that a significant amount of the surface chlorine ions are very likely to emerge in the presence of only one monolayer of water adsorbed on the NaCl(100) surface, already at low temperatures. These findings support the AFM measurements that show a strong negative surface dipole along flat NaCl terraces already at this coverage. The energy associated with the Cl- rising is governed by electrostatic interactions. Thus, the low-energy cost for the anion emergence is a consequence of the efficient screening provided by the highly polarizable water monolayer. We propose that this process may be important to understand the catalytic activity of the NaCl surfaces at low RH, with implications for surface reactions in salt aerosols, where chloride anions play a key role. However, we acknowledge that to claim this to be plausible at ambient conditions, further investigations using MC/MD or ab initio MD are encouraged. Acknowledgment. D.S.-P. and A.A. acknowledge illuminating discussions with J. M. Ugalde. Financial support by UPV/ EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish M.E.C. (Grant No. FIS2004-06490-C3-00), the EU network of excellence FP6-NoE “NANOQUANTA” (Grant No. 5001982), and the Basque Government projects “NANOMATERIALES” and “NANOTRON” within the ETORTEK programme is gratefully acknowledged. A.V. thanks financial support from the Ramon y Cajal program of the Spanish M.E.C. M.S. was supported by the Office of Basic Energy Science, Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 References and Notes (1) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 5. (2) Verdaguer, A.; Sacha, G. M.; Bluhm, H.; Salmeron, M. Chem. ReV. 2006, 106, 1478. (3) Allen, H. C.; Laux, J. M.; Vogt, R.; FinlaysonPitts, B. J.; Hemminger, J. C. J. Phys. Chem. 1996, 100, 6371.
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