Vertical and Lateral Order in Adsorbed Water Layers on Anatase

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Vertical and Lateral Order in Adsorbed Water Layers on Anatase TiO2(101) Antonio Tilocca* and Annabella Selloni Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received April 28, 2004. In Final Form: June 21, 2004 The structure and energetics of thin water overlayers on the (101) surface of TiO2-anatase have been studied through first-principles molecular dynamics simulations at T ) 160 K. At one monolayer coverage, H2O molecules are adsorbed at the 5-fold Ti sites (Ti5c), forming an ordered crystal-like 2D layer with no significant water-water interactions. For an adsorbed bilayer, H2O molecules at both Ti5c and bridging oxygen (O2c) sites form a partially ordered structure, where the water oxygens occupy regular sites but the orientation of the molecules is disordered; in addition, stress-relieving defects are usually present. When a third layer is adsorbed, very limited parallel and perpendicular order is observed above the first bilayer. The calculated energetics of multilayer adsorption is in good agreement with recent temperatureprogrammed desorption data.

1. Introduction Titanium dioxide (TiO2) is a versatile and widely used material, particularly in photocatalysis.1-3 For instance, TiO2 is the semiconductor of choice to promote the photocatalytic degradation of environmentally harmful organic compounds. TiO2 is also biologically and chemically inert, stable to corrosion, and nontoxic, and for these reasons it is used for medical applications such as dental and bone implants.4 As most applications of TiO2 involve an aqueous environment, the interaction of titania surfaces with water is a subject of great interest, not only to surface scientists but also, among others, to geochemists and chemical engineers. More specifically, it is the molecular structure of the first few interfacial water layers which plays a critical role in many processes, for instance affecting the ability of dissolved species to reach the reactive surface sites.5 The structure of adsorbed water overlayers depends on a delicate and complex balance of several factors, such as the symmetry and corrugation of the underlying surface, the spacing between surface adsorption sites, and the relative strength of water-water (w-w) and watersurface (w-s) interactions.6 The general picture is better defined for metal surfaces, where a large wealth of experimental and theoretical studies have in some cases provided a rather good understanding of the structure of adsorbed water layers.6-8 On the other hand, an equally accurate picture is not available yet for nonmetallic substrates such as oxides and salts, due to the greater complexity of their structure as well as to the difficulty in characterizing and obtaining information on welldefined surfaces. For water on TiO2, in particular, despite numerous studies, only fairly recently information concerning well-defined and well-characterized surfaces has become available.3,6 Among these “surface science” studies, * Corresponding author. E-mail: [email protected]. (1) Linsebigler, A.; Lu, G.; Yates, J. T. Chem. Rev. 1995, 95, 735. (2) Gratzel, M. Nature 2001, 414, 338. (3) Diebold, U. Surf. Sci. Rep. 2002, 293, 1. (4) Jones, F. H. Surf. Sci. Rep. 2001, 42, 75. (5) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328. (6) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (7) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 211, 385. (8) Michaelides, A.; Alavi, A.; King, D. A. Phys. Rev. B 2004, 69, 113404.

however, the largest majority so far have focused on the surfaces, especially the (110) surface, of the most stable rutile polymorph,6,9-11 whereas considerably less information is available for the anatase structure, which is more frequently used and more efficient for many photocatalytic applications.12 The most stable surface of TiO2-anatase is the (101) one, which is largely predominant in the equilibrium crystal shape of this polymorph3,13 and is also very frequently exposed by anatase nanocrystals used for solar cell devices. Similarly to rutile (110), also the anatase (101) surface exhibits both undercoordinated 5-fold and fully coordinated 6-fold Ti atoms, as well as 2-fold bridging and 3-fold coordinated oxygen atoms3 (see Figure 1a). However, there are also important structural differences between these two surfaces: for example, the anatase (101) surface is considerably more corrugated than rutile (110), and distances between neighboring Ti5c (∼3.79 Å) are much larger than on rutile (110), for which the corresponding distance (∼2.96 Å) is closer to the O-O distance of ∼2.75 Å in ice. Thus, it seems interesting to investigate how these differences are reflected in the structure of adsorbed water layers. Water on anatase (101) has been recently studied by means of temperature-programmed desorption (TPD) measurements.17 Three desorption states at 160, 190, and 250 K have been observed, which were assigned to multilayer water, H2O coordinated to bridging oxygens (H2O-O2c), and H2O adsorbed on the 5-fold coordinated Ti5c atoms (H2O-Ti5c), respectively. The absence of a hightemperature desorption peak characteristic of hydroxyl groups indicates that no dissociated water is present, which was attributed to the absence of oxygen vacancies (9) Lindan, P. J. D.; Harrison, N. M.; Gillan, M. J. Phys. Rev. Lett. 1998, 80, 762. (10) Schaub, R.; Thorstrup, P.; Laegsgaard, E.; Steensgaard, I.; Norskov, J. K.; Besenbacher, F. Phys. Rev. Lett. 2001, 87, 266104. (11) Langel, W. Surf. Sci. 2002, 496, 141. (12) Kavan, L.; Gra¨tzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716. (13) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. Rev. B 2001, 63, 155409. (14) Henderson, M. A. Langmuir 1996, 12, 5093. (15) Zhang, C.; Lindan, P. J. D. J. Chem. Phys. 2003, 118, 4620. (16) Zhang, C.; Lindan, P. J. D. J. Chem. Phys. 2003, 119, 9183. (17) Herman, G. S.; Dohna`lek, Z.; Ruzycki, N.; Diebold, U. J. Phys. Chem. B 2003, 107, 2788.

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Figure 1. (a) Side view of the anatase (101) slab. Black (gray) spheres represent O (Ti) atoms. (b) Top view of the optimized geometry of an adsorbed water ML.

on the anatase surface. First-principles studies have provided support to the above interpretation by showing that molecular adsorption is indeed favored for water on defect-free anatase (101),18 whereas surface oxygen vacancies should lead to H2O dissociation.19,20 These previous theoretical studies mostly focused on the issue of molecular versus dissociative H2O adsorption. Thus, important questions concerning the intra- and interlayer connectivity and the extent of the surface-induced lateral and perpendicular order in the adsorbed water layers still remain to be investigated. To answer these questions and also make closer contact with the available experimental information,17,21 in the following we report on extensive ab initio molecular dynamics (MD) simulations of one, two, and three adsorbed water monolayers on the stoichiometric anatase (101) surface (an adsorbed monolayer is defined to contain a number of H2O molecules equal to the number of Ti5c sites). 2. Computational Approach The calculations have been performed with the CarParrinello approach,22 using the PBE exchange-correlation functional,23 and ultrasoft pseudopotentials.24 Valence electrons included the O 2s, 2p and Ti 3s, 3p, 3d, 4s shells; the smooth part of the wave functions was expanded in plane waves up to a kinetic energy cutoff of 25 Ry, while the augmented density cutoff was 200 Ry. Due to the large size of the periodic supercell (see below), the k-sampling was restricted to the Γ point. The surface was modeled by four-layer-thick periodically repeated slabs, with the atoms of the bottom layer fixed to their equilibrium bulk positions, and water was adsorbed on the upper surface only. Both (1 × 2) and (1 × 3) supercells were used for our simulations, corresponding to Ti16O32 and Ti24O48 compositions and surface areas of 10.24 × 7.57 and 10.24 × 11.36 Å2, respectively. The perpendicular separation between the slabs was ∼10 Å for the monolayer simulations and was increased to 14.5 Å for simulating adsorption of two and three H2O layers, to minimize the interaction of topmost H2O molecules with the upper periodic image (18) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gra¨tzel, M. Phys. Rev. Lett. 1998, 81, 2954. (19) Tilocca, A.; Selloni, A. J. Chem. Phys. 2003, 119, 7445. (20) Tilocca, A.; Selloni, A. J. Phys. Chem. B 2004, 8, 4743. (21) Nosaka, A. Y.; Fujiwara, T.; Yagi, H.; Akutsu, H.; Nosaka, Y. J. Phys. Chem. B 2004, 108, 9121. (22) Car, R.; Parrinello, M. Phys. Rev. Lett. 1985, 55, 2471. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (24) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892.

of the slab. All approximations were tested in our previous studies of anatase-water systems.13,18,19 The MD trajectories were run at an average temperature of around 160 K, which corresponds to the lowest temperature peak in TPD spectra.17 No water desorption was observed on the time scale of our simulations. Geometry optimizations were carried out by damped dynamics until every component of ionic forces (except on the fixed bottom layer) was below 0.05 eV/Å. 3. Structural Results Monolayer. The minimum energy structure for a water monolayer (ML) on anatase (101) is a 2D commensurate layer, as shown in Figure 1b. The molecules are adsorbed at undercoordinated Ti5c sites, and their molecular dipoles are oriented in such a way to form a zigzag pattern along the [1 h 01] direction. Starting from a slightly disordered variation of this geometry, after a few picoseconds of equilibration, a simulation of several picoseconds at 160 K was carried out. No water dissociation was observed during the whole run, in agreement with earlier predictions.18 The time evolution of the perpendicular distances (z(t)) of water oxygens (Ow) from the surface shows that the H2O molecules remain very close to their adsorption sites throughout the simulation (see Figure 2a), giving rise to a single peak centered around 2 Å in the distance distribution p(z) and confirming the stability of the 2D arrangement. Analysis of the atomic configurations along the MD trajectory shows that no w-w and only weak H2O-O2c hydrogen bonds (Hb’s) are present. The average distance between O atoms belonging to nearest-neighbor adsorbed molecules is 3.8 Å, a value determined by the distance between neighboring Ti5c sites and much larger than the nearest-neighbor O-O distance of ∼2.75 Å in ice. The complete lack of a hydrogen-bond network between H2O molecules distinguishes this ordered, solidlike monolayer from the adsorbed H2O monolayers observed on other oxide surfaces, such as MgO(100), where the distance between surface adsorption sites (2.98 Å) favors the formation of a flat 2D monolayer with Hb’s between neighboring water molecules.25,26 On rutile (110), on the other hand, highresolution electron energy loss spectroscopy (HREELS) (25) Ferry, D.; Picaud, S.; Hoang, P. N. M.; Girardet, C.; Giordano, L.; Demirjian, B.; Suzanne, J. Surf. Sci. 1998, 409, 101. (26) Marmier, A.; Hoang, P. N. M.; Picaud, S.; Girardet, C.; LyndenBell, R. M. J. Chem. Phys. 1998, 109, 3245.

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Figure 2. Distribution p(z) and time evolution z(t) of the perpendicular distance from the surface Ti5c sites of the water oxygen atoms: (a) ML, (b) BL, and (c) TL. In panel b, two distributions are shown, obtained from the MD trajectories of structures BL1 (solid line) and BL2 (dashed line). In panel (c), the arrow points to the z trajectory of the defect H2O-O2c molecule, discussed in the text.

measurements27 found no evidence of w-w Hb’s, whereas calculations suggested a mixed state with H-bonded molecular and dissociated molecules.9 Bilayer. After completion of the first adsorbed monolayer, all Ti5c sites are occupied, so that additional H2O molecules can coordinate either to bridging oxygens, or to other H2O molecules, or to both. To examine these different possibilities, two simulations starting from different initial configurations were carried out. In one case, a second layer of water molecules was arranged with random orientations above the H2O-Ti5c of the first layer, and an equilibration run of a few picoseconds was carried out before the actual simulation. During the equilibration stage, two (out of four) second-layer molecules moved closer to the surface (see Figure 2b), in an intermediate position between the H2O-Ti5c and second H2O layers located at about 2 and 4 Å above the surface, respectively, and formed Hb’s with the surface bridging oxygens (Figure 3a). The formation of H2O-O2c coordination pairs allowed every molecule to become involved in an extended network of hydrogen bonds, thus enhancing the connectivity of the adsorbed H2O overlayer. This structure with two H2O-O2c and two second-layer H2O molecules above the H2O-Ti5c layer (hereafter denoted BL1) remained substantially stable for the rest of the run. The second simulation was started from a local minimum structure with four H2O-Ti5c and four H2O-O2c molecules (Figure 3b). In this ordered arrangement, the molecules are primarily kept in place by direct interactions with the surface Ti5c and O2c atoms; w-w Hb interactions are also present, but their role does not seem to be as important. This initial configuration was largely maintained during the dynamical simulation, except for one H2O-O2c molecule which moved to a “defect” position laterally displaced with respect to the “regular” one dictated by the underlying lattice, without breaking the (27) Henderson, M. A. Surf. Sci. 1996, 355, 151.

Figure 3. Equilibrium structures of the water bilayer and trilayer: (a) BL1 bilayer; (b) BL2 bilayer; (c) BL3 bilayer; (d) trilayer. H2O molecules are represented as ball-and-stick. The oxygen atoms of H2O-Ti5c and H2O-O2c are represented in red and green, respectively, while those of the molecules above are represented in blue.

hydrogen-bond link with O2c. The resulting structure (hereafter denoted BL2) contains two nonequivalent rows of H2O-O2c. The water molecules in one row (denoted H2O-2H in the following) donate both hydrogens to O2c atoms, while H2O-O2c molecules in the other row (denoted H2O-1H in the following) have one of their hydrogens pointing upward. H2O-1H molecules are at higher distance above the surface, so that the peak corresponding to H2OO2c in the height distribution (superimposed in Figure

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Figure 4. Trajectory plots, showing atomic motions of H2O molecules in the TL, projected onto the (x, y) plane: (red) H2O-Ti5c; (green) H2O-O2c; (blue) H2O molecules in the third layer.

2b) is split in two. BL1 and BL2 are found to be substantially degenerate from the energetic point of view. The presence of two nonequivalent H2O-O2c rows in the BL2 structure is somewhat surprising, as on the clean surface the O2c sites are equivalent. We estimated the adsorption energies of H2O-O2c in the two different configurations and found that these are practically identical, 0.64 eV/mol. Additional geometry optimizations showed that a BL structure with all H2O-O2c molecules in the H2O-2H configuration (hereafter denoted BL3; see Figure 3c) is isoenergetic with BL2, while a structure with all H2O-O2c molecules in the H2O-1H configuration is slightly (0.04 eV/mol) less stable. These findings indicate that already at BL coverage a complex balance between H2O-H2O and H2O-surface interactions results in at least three energetically equivalent structures for the BL, characterized by the presence of two different H2O-O2c adsorption modes (BL2 and BL3) or by incomplete occupancy of surface O2c sites (BL1). As a final test, an MD simulation at T ) 160 K was started from BL3. After less than 2 ps, two H2O-2H in the same row switched coordination mode to H2O-1H, leading again to a BL2-type structure. This suggests a significant entropy difference, favoring the less ordered BL1 or BL2 states with respect to BL3. Trilayer. Also in this case, two distinct simulations have been carried out, using different initial conditions as well as different supercells, namely, (1 × 2) and (1 × 3) supercells with 12 and 18 adsorbed H2O molecules, respectively. The results of the two simulations are largely equivalent, so that in the following only the trilayer (TL) structure obtained with the larger supercell is discussed. In this case, the starting configuration was generated by adding a third water layer above the H2O-O2c layer of the

BL2 structure. The positions of the H2O molecules in the third layer were obtained by approximately translating those in the first layer along [101], as suggested in ref 16. From the z(t) plot in Figure 2c, it appears that the first two layers are very well defined and roughly reproduce the peaks observed for BL2, whereas the layering above these two layers is less marked, with two broad peaks centered at 4 and 5.5 Å. During the simulation, one H2OO2c (the same molecule which was in a off-registry position in the BL2 structure) appears to lose its coordination to the O2c site and tends to stay slightly above the second layer. It is likely that the formation of such a “vacancy” is not accidental: indeed a similar event (this time in the first layer of H2O-Ti5c molecules) has been observed also in our other simulation for the trilayer. This suggests that the formation of such defects represents a mechanism to relieve the stress which is present in the first bilayer when all Ti5c and O2c sites are occuped. Further insight into the extent of lateral order is provided by the (x, y)-projected trajectories of the H2O molecules in the various layers (Figure 4). The first two layers clearly reflect the lattice structure of the underlying anatase surface, while the molecules in the third layer (with z > 3.5 Å in Figure 2c) are more mobile and their average positions are only in part correlated with the surface sites. A typical configuration extracted from the TL trajectory is shown in Figure 3c. As in the BL2 structure, two alternate rows with different H2O-O2c coordination are present. There is an extended network of Hb’s involving molecules in the three layers; at variance with the bilayer case, now a stable link between the two rows of H2O-O2c is built through a sequence of Hb bridges along [1 h 01].

Order in Adsorbed Water Layers

4. Discussion and Conclusions A more complete understanding of the water layer structure can be obtained by estimating the relative strength of the w-w and w-s interactions, whose interplay is known to largely determine the growth morphology.6 To this end, we calculated the binding energy of H2O molecules in the various layers. For water at Ti5c sites, we found an adsorption energy ∆Hads) 0.74 eV in the lowcoverage limit and a somewhat smaller value, ∆Hads) 0.69 eV, at ML coverage (configuration in Figure 1b), since in the latter case H2O-O2c interactions are weaker than at low coverage. To evaluate the average binding energy of adsorbed molecules in the second and third layers, we combined the energies of the optimized structures with one, two, and three adsorbed H2O monolayers.28 For H2OO2c (molecules with 2.5 e z e 3.5 Å in Figure 2c), we found a binding energy of 0.65 eV, whereas a binding energy of 0.56 eV was obtained for the third-layer molecules (with z > 3.5 Å). From this, we conclude that water-surface interactions (of the order 0.65-0.70 eV) are somewhat stronger than w-w ones (e0.6 eV). This result together with the considerable mismatch between the anatase and ice lattices accounts well for the observed formation of a (defected) water bilayer in registry with the surface, followed by a less ordered and more mobile overlayer. The above estimates of H2O binding energies allow us to make contact with recent TPD data.17 For H2O-Ti5c, a Redhead analysis29 with a preexponential factor of 1013 yields desorption temperatures in the range of 250-268 K, the highest and lowest temperatures corresponding to low and monolayer coverages, respectively. This agrees well with the TPD experiment,17 where the center of the peak corresponding to H2O-Ti5c shifts by roughly the same amount with increasing coverage up to one ML. For water in the second and third layer, we estimate desorption temperatures of 230 and 200 K, approximately. Although these temperatures are about 40 K larger than the experimental ones, trends are well reproduced by our calculations. It is interesting to compare our results for anatase (101) with recent simulations for rutile (110).9,15 These show that on rutile (110) a water monolayer is characterized by chains of H-bonded H2O-Ti5c molecules, running along the [001] rows of Ti5c sites.9,15 The formation of Hb links between molecules in the same chain is made possible by more favorable Ti5c-Ti5c distances (2.95 Å) compared to anatase (3.8 Å). In the latter case, despite the lack of significant H2O-H2O interactions, we have found that at ML coverage the templating effect of the surface is strong enough to stabilize parallel rows of H2O-Ti5c along the [010] direction. On both the rutile (110) and anatase (101) surfaces, much longer Ti5c-Ti5c distances across the Ti5c rows do not allow the Hb network to extend in that direction, and the parallel water chains do not interact with each other. Half of the water molecules incorporated in a ML are dissociated9 on the perfect rutile surface, while a full molecular ML is stable on anatase.18,20 The larger stability of a mixed ML state on rutile is likely due to the incorporation of (otherwise unstable10) terminal hydroxyls in the 1D chains mentioned above. On both rutile and anatase surfaces, the H2O molecules in the second layer are coordinated to O2c atoms and are H-bonded to the molecules in the first layer.16 On rutile, (28) For 1 and 2 ML, we considered the optimized structures in Figure 1b and Figure 3b. The optimized TL geometry was determined as the most stable structure after local relaxation of five low potential energy configurations along the MD trajectory. (29) Redhead, P. Vacuum 1962, 12, 203.

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the shorter O2c-O2c distances along the [001] rows favor additional Hb’s between second-layer H2O molecules, which are missing on anatase. In the model of ref 16, each H2O layer included only two molecules; thus the formation of nonequivalent parallel H2O-O2c rows, like those we have found for anatase, cannot be observed. Turning to the structure of a third H2O layer, a higher degree of both lateral and perpendicular order is found on rutile (110):16 the third-layer molecules are roughly located above the molecules in the first layer, and their z distribution is more structured than that for anatase, reported in Figure 2c. However, the different behavior of the two H2O in the top layer and their more diffuse p(z) peak are clear indications that the templating effect of the surface is weaker on the top layer. The structure of water on anatase-abundant TiO2 powders has been very recently investigated by solid-state 1 H NMR experiments.21 These have shown the presence of three different layers of H2O molecules, whose mobilities increase considerably from the innermost to the outermost layer. In particular, the innermost layer, which is found to be stable up to 358 K, is formed by very rigid H2O molecules near the surface, while the layers above it are more mobile and are vaporized at that temperature. It is tempting to compare these results with those of our simulations, even though some care should be used because the “pressure-temperature gap” between the NMR experiment (performed in air and at room temperature) and our simulations (perfomed in a vacuum and at low T) may affect the dynamical behavior of the adsorbed molecules.30 In our calculations, the relative mobilities of H2O layers increase going from H2O-Ti5c to H2O-O2c to the H2O molecules above them, as suggested also by the xy-traces in Figure 4. A direct calculation of mean square displacements (MSDs) gives 0.045, 0.06, and 0.31 Å2 for oxygen atoms belonging to H2O-Ti5c, H2O-O2c, and H2O above the first two layers, respectively, in qualitative agreement with the NMR data.31 Another interesting result of ref 21 is that the NMR signal from H2O molecules in the first layer is shifted to larger fields with respect to the layers above, indicating that the molecules in the first layer are less involved in Hb’s. We estimated the number of Hb’s for H2O-Ti5c, H2OO2c, and top-layer H2O by integrating the H2O-H2O radial distribution functions (rdf’s), calculated separately for each water layer, up to R ) 2 Å. At TL coverage, we obtain 2.1, 2.3, and 2.55 Hb’s/molecule for first-, second-, and thirdlayer water, respectively,32 consistent with the interpretation of the NMR chemical shifts in terms of Hb’s. Also, the NMR observation that the water molecules in the outermost layer are not significantly affected by the surface structure21 agrees with the loss of surface-induced order that we have found for the water molecules in the top layer, while the observation of a lower density of H2O molecules in the second layer compared to the third21 may reflect the presence of stress-relieving defects in the second layer, as we have found for structures BL1, BL2, and TL. In conclusion, we have studied the structure of thin water overlayers on anatase (101) by first-principles simulations. Only for one adsorbed monolayer, a perfectly ordered 2D commensurate structure is found to occur. (30) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 1999, 103, 5328. (31) For comparison, note that the MSD for water oxygens in (bulk) hexagonal ice, measured by neutron diffraction, ranges from 0.045 Å2 at 123 K to 0.30 Å2 at 223 K; see: Kuhs, W. F.; Lehmann, M. S. J. Phys. Chem. 1983, 87, 4312. (32) Our calculated numbers of Hb’s do not include H2O-O2c Hb’s, which in any case would further increase the value for second-layer H2O.

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For an adsorbed bilayer, water-surface interactions stronger than water-water ones favor the formation of a regular structure reflecting the order of the underlying anatase surface. However, due to the large mismatch between the anatase and ice lattices, defects are usually present to relieve the stress. Moreover, no clear order is present in the orientation of the second-layer water molecules. Finally, the effect of the mismatch prevails and the surface-induced lateral and perpendicular order

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of the first two layers is only marginally conveyed to the third layer. Acknowledgment. This work was supported by the National Science Foundation under Grant No. CHE0121432. The calculations were performed at the Pittsburgh Supercomputer Center and at the Keck Computational Materials Science Laboratory in Princeton. LA048937R