Role of Surface Hydroxyl Groups on Zinc Adsorption Characteristics

Article pubs.acs.org/JPCC

Role of Surface Hydroxyl Groups on Zinc Adsorption Characteristics on α‑Al2O3(0001) Surfaces: First-Principles Study Rémi Cavallotti,†,‡,¶ Jacek Goniakowski,*,†,‡ Rémi Lazzari,†,‡ Jacques Jupille,†,‡ Alexey Koltsov,¶ and Didier Loison¶ †

CNRS, UMR 7588, Institut des Nanosciences de Paris, F-75005 Paris, France Sorbonne Universités, UPMC Univ Paris 06, UMR 7588, INSP, F-75005 Paris, France ¶ ArcelorMittal Maizières Research, voie Romaine, F-57280, Maizières lès Metz, France ‡

ABSTRACT: Adhesion at zinc/alumina interface is a key issue in the field of steelmaking industry, where selective Al oxidation, followed by surface segregation of alumina islands, efficiently impedes wetting of anticorrosive Zn coating on the high strength steel grades. Relying on ab initio total energy calculations, we have examined adsorption of Zn adatoms on different terminations of α-Al2O3(0001) surface under both vacuum conditions and in the presence of surface hydroxyls. Surface configurations with strongly bound Zn and thermodynamic conditions necessary for their stability have been identified. We have shown the existence of a wide range of nonextreme oxygen-rich conditions under which Zn tends to spill over the alumina substrate as an array of strongly adsorbed adatoms, rather than to form metallic clusters weakly bound to the substrate. This effect has been assigned to surface non-neutrality, such as due to surface polarity, or to an excess of surface hydroxyls. Moreover, compared to its direct neighbors in the periodic table (Cu, Ag), we have shown that surface structures with strongly bound adatoms can be stabilized already in much more oxygen-poor conditions.

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INTRODUCTION Understanding microscopic mechanisms responsible for adhesion properties at metal/oxide interfaces1,2 is of paramount importance in various domains of applicative research, ranging from microelectronic to corrosion and catalysis3−7 and has motivated many studies dedicated to metal interaction with oxide surfaces. Among them, interface between corundum (or sapphire) Al2O3, which is one of the most common industrial ceramics,8 and transition and noble metals (Ag, Cu, Al, Pd, Pt, Nb, V, etc.) have been the subject of numerous experimental and theoretical studies.9−23 However, while of a strong interest for applications, the interaction of zinc with alumina surfaces remains poorly known.24 The Zn/alumina interface has never been directly investigated despite the widespread use of sapphire in the growth of epitaxial zinc oxide layers.25 Adhesion at zinc/alumina interface is a key issue in the field of steelmaking industry, where galvanization is a common method to protect steel against corrosion. Indeed, steel strips are coated with a zinc layers by either zinc electrodeposition on annealed steel sheets or by hot-dip galvanization, which combines annealing and coating into a single process.26,27 Nowadays, new steel grades known as high strength steels are purposely enriched in strengthening elements, such as Al, Si, Mn, and Cr, so that thinner steel sheets can be used by various manufacturing industries, leading to a reduction of product weight and cost. While the annealing, conducted at 800 °C in a reducing N2−5%H2 gas atmosphere containing only traces of water (approximately −40 °C dew point), is necessary for the recrystallization of steel after cold rolling and for reduction of iron oxides, it results in a selective oxidation of less noble © 2014 American Chemical Society

alloying elements, such as Al and in surface segregation of the oxide particles or films.27,28 In a standard galvanization processing (∼450 °C), these latter are only poorly wetted by zinc alloy, reducing dramatically the quality of the anticorrosive Zn protection. One possible way to improve the wetting of zinc coatings on such Al-rich steel grades passes through a control of adhesion at the zinc/alumina interfaces, aiming to an enhancement of Zn− alumina interaction. The first step, which is the direct motivation of the present study, is the search for configurations in which Zn is strongly bound to alumina surfaces, and the identification of thermodynamic conditions under which such configurations can be stabilized. In this context, Zn/αAl2O3(0001) constitutes a model system. The basal (0001) plane is the most stable α-Al2O3 surface in the vacuum environment.29−31 It terminates a layered structure, in which an oxygen plane and the associated aluminum planes above and below it form a stoichiometric triple layer (−Al/3O/Al−). This structural motif offers three different terminations with different polarity characteristics.32,33 Surface terminated by a single Al plane (stacking sequence Al/3O/Al−Al/3O/Al−etc.) is stoichiometric and nonpolar (type 2 according to Tasker’s classification34). This termination is predicted to be the most stable in vacuum conditions35−39 and is indeed the most observed experimentally.40−43 Conversely, oxygen-rich (3O/ Al−Al/3O/Al−etc.) and aluminum-rich (Al−Al/3O/Al−Al/ Received: February 6, 2014 Revised: April 24, 2014 Published: May 30, 2014 13578

dx.doi.org/10.1021/jp501318p | J. Phys. Chem. C 2014, 118, 13578−13589

The Journal of Physical Chemistry C

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3O/Al−etc.) terminations are characterized by a net dipole moment in the repeat unit in the direction normal to the surface and are polar (type 3 in Tasker’s classification). Polarity impacts the reactivity characteristics of these terminations and makes their stability very sensitive to the environment. The present work deals with the adsorption of zinc on these three nonpolar and polar surfaces. The oxygen-rich termination was shown to interact strongly with metals, which results in its stabilization at metal/alumina interfaces.17,19,21,49 It has also been shown that this termination is stabilized by hydroxylation, so that it becomes thermodynamically the most stable in the presence of water vapor.38,44−47 Surface hydroxylation is of great relevance also for nonpolar alumina termination since, while late transition and noble metals tend to bind weakly to the stoichiometric alumina (0001) termination, it has been shown that the interaction may be sensitive to the presence of water vapor. More specifically, binding of a low coverage of Co, Cu, and Ag adatoms at this surface was found to be substantially enhanced by the coadsorbed surface hydroxyls.48−51 Taking into account the complexity of alumina surfaces under realistic conditions, and the specific properties of different terminations regarding the wetting of metals, the strategy of the present theoretical work is (i) to identify the most stable surface structures in vacuum and in the presence of water vapor and (ii) to deduce the most favorable environmental conditions for a strong Zn binding. To this goal, the nonequivalent alumina terminations and their interaction with zinc adatoms are examined as a function of oxygen and hydrogen chemical potentials. Beyond the evaluation of Zn binding strength, we estimate also the effect of adsorbed metal adatoms on the surfaces stability. This enables an explicit identification of surface configurations favored under different thermodynamic conditions and gives directs guidelines for an experimental improvement of Zn-binding characteristics. Finally, we compare our present results relative to Cu and Ag adsorption characteristics and highlight the Zn-specific features. The paper is organized as follows. After presenting the computational method and settings in Computational Method and Settings, in Results, we report the results on Zn adsorption characteristics on dry and hydroxylated alumina surfaces, including both Zn adsorption energies and the resulting alumina surface energies. We conclude this section with a synthetic stability diagram of bare and Zn-stabilized surfaces. In Discussion, we present a comparative discussion on metalalumina bonding strength for Zn and the selected late transition and noble metals (Cu and Ag).

The essential characteristics of bulk hexagonal Al2O3, bulk ZnO, and bulk Zn obtained with the two approximations are summarized in Table 1. As expected, we find that LDA and Table 1. Comparison of Calculated and Experimental Characteristics of the Three Bulk Materials, as Obtained in LDA and GGA: Lattice Parameters a and c (angstroms), Nearest-Neighbor Al−O, Zn−O, and Zn−Zn Distances d (angstroms), and Atomic Charges (Q)a Al2O3 LDA GGA exp. ZnO LDA GGA exp. Zn LDA GGA exp.

a (Å)

c (Å)

d (Å)

Q

Eform

4.70 4.76 4.7659

12.79 12.99 12.9959

1.83/1.94 1.86/1.97 1.85/1.97

1.62 1.65 −

17.45 15.68 17.43

3.17 3.25 3.2560

5.11 5.24 5.2160

1.94 1.99 1.95

1.20 1.22 −

3.44 3.03 3.64

2.63 2.72 2.6660

4.38 4.53 4.9560

2.67 2.76 2.63

0.00 0.00 −

1.90 1.12 1.36

a

Calculated formation energies (eV/formula unit) of oxides (Al2O3, ZnO) are referred to bulk metal and to free oxygen molecules. This of bulk metal (Zn) is referred to free Zn atoms. Corresponding experimental values of standard formation enthalpies ΔfH° are taken from ref 58.

GGA tend to respectively over- and underestimate the bonding, such that in most cases, the two approximations successfully bracket the experimental estimates. We note, however, that in the particular case of ZnO, the formation energy is better accounted for by the LDA functional. As will be shown in the following section, the bonding overestimation typical for the LDA can also mimic the missing van der Waals interaction, particularly important in the case of weakly interacting adsorbates, such as Zn on stoichiometric alumina surfaces. All Zn/Al2O3(0001) systems were modeled with symmetric alumina slabs composed of six −Al/3O/Al− units, with adspecies adsorbed symmetrically at both slab surfaces. To avoid spurious interactions between periodic images, the slabs were separated by at least 10 Å of vacuum. A (8 × 8 × 1) Monkhorst Pack grid61 was used for k-point sampling of the Brillouin zone of the (1 × 1) surface unit cell. All structures were fully relaxed until residual forces dropped below 0.01 eV/ Å. Ionic charges were estimated with the partition scheme proposed by Bader.62−64 Adsorption energies (per adsorbate) of NX surface ad-species X (X = H, and H2O, Zn, Cu, and Ag) have been evaluated directly from total energy differences:

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COMPUTATIONAL METHOD AND SETTINGS All calculations have been carried out within the density functional theory (DFT) framework and performed with VASP (Vienna ab initio simulation package).52,53 The interaction of valence electrons with ionic cores was described within the projector augmented wave (PAW) method,54,55 and the Kohn− Sham orbitals were developed on a plane-wave basis set with an energy cutoff of 400 eV. As to give more robust grounds to the calculated energetics, the exchange-correlation effects were accounted for at two different levels of approximation. As to partially compensate for the known deficiencies of energetics obtained with standard DFT functionals, we have used in parallel both local density approximation in the form proposed by Ceperley and Alder (LDA-CA)56 and the Perdew−Wang form of generalized gradient approximation (GGA-PW91).57

Eads = −(Eslab + NX − Eslab − NXE X )/NX

Free atom references EX have been used in the case of adsorption of metal adatoms (X = Zn, Cu, and Ag), such that the adsorption energies provide directly the information on the bonding strength between metal adatoms and the oxide surface. Similarly, adsorption energies of H and H2O have been calculated with respect to the corresponding free molecules: H2 H2O (1/2)Emolec and Emolec . In surface energy calculations, we have assumed equilibrium between the surface, the underlying Al2O3 bulk crystal, and the gas atmosphere in contact with the surface, defined by the 13579



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chemical potentials μi(T, pi) of its components (i = O and H). Following the usual procedure of ab initio thermodynamics,65 the difference between slab and bulk alumina Gibbs free energies has been approximated by the corresponding difference of 0 K total energies Eslab − Ebulk: γ(T , p) = (Eslab − E bulk )/2S − [ΔNOμO(T , pO ) + NHμH (T , pH )]/2S

In the above equation ΔNO is the excess/deficiency of oxygen atoms in the slab with respect to stoichiometric alumina, NH is the number of adsorbed H atoms, and the denominator 2S accounts for the two slab surfaces, each of an area S. The chemical potentials μi(T, pi) (i = O and H) are referred to the 0 2,H2 K total energies of the corresponding free molecules EOmolec : i2 μi(T, pi) = (1/2)Emolec + Δμi (T, pi). As a consequence, ΔμO ∼ 0.0 eV corresponds to oxygen-rich conditions (condensation of 2,O3 Al oxygen molecules), whereas ΔμO ∼ (EAl bulk − 2Ebulk)/3 − (1/ O2 2)Emolec ∼ −5.8 eV (LDA) and −5.2 eV (GGA) describes an oxygen-poor environment (decomposition of bulk alumina). Both chemical potentials can be further explicitly linked to the temperature T and to the corresponding partial pressures pi of the gas atmosphere Δμi(T, pi) = Δμi(T, p0) + (1/2)kT ln(pi/ p0). In the case of surfaces with a low surface coverage of metal adatoms X (X = Zn, Cu, and Ag), the above expression of surface energy contains an additional term −NXμX/2S. Contrary to the reference used in calculations of adsorption energies Eads, in case of surface energies γ(T,p), we fix the chemical potential of NX adsorbed metal adatoms to its value in the corresponding bulk metal μX = EXbulk. Such a bulk reference is the best suited for estimation of the overall thermodynamic stability of different surface structures and particularly welladapted for the identification of those with strongly bound metal adatoms. It enables a partial cancellation of the spurious over- and underbinding trends typical for LDA and GGA, respectively. We note that the calculated surface energies may become small and even negative under oxygen-rich conditions, due to energy gain upon formation of X−O bonds. We stress, however, that the formation of surface oxide deposits or of oxide/alumina interface goes beyond the scope of our present study.

Figure 1. Top: schematic representation of the three nonequivalent bare (1 × 1) Al2O3(0001) terminations (red, oxygen; black, aluminum). Bottom: corresponding surface energies (electronVolts/ angstroms squared) as a function of oxygen chemical potential ΔμO (eV). Solid and dashed lines represent LDA and GGA results, respectively.

reproduce correctly the general stability behavior known from earlier studies on alumina surfaces.17,66 The nonpolar stoichiometric (Al) termination is the most stable in the entire range of accessible ΔμO. Energies of both the oxygen-rich (3O) and aluminum-rich (2Al) surfaces are systematically higher due to their polar character. Differences between LDA and GGA results are principally due to the somewhat different bonding strength in the two approximations. This makes LDA surface energies systematically larger and shifts the ΔμO reference toward oxygen-poor conditions. As a consequence, stability inversions between the considered terminations occur systematically at lower ΔμO in LDA (Figure 1), with difference going up to about −0.5 eV. This simple stability diagram requires several comments. On the one hand, the considered surface configuration of the aluminum-rich (2Al) termination is considerably less stable than the stoichiometric (Al) surface with excess Al integrated into bulk aluminum. In the following, we will systematically refer to this more stable configuration as (Al) + AlB. It may be seen as metallic Al deposits at the stoichometric alumina surface, consistent with the aluminum-rich reconstructions observed experimentally in oxygen-poor conditions.67,68 On the other hand, the (1 × 1) configuration of the oxygen-rich termination (3O) (Figure 1) can be additionally stabilized by a removal of a fraction of surface anions. Several such surface structures, denoted (3O)Vx [x represents the number of missing anions per (1 × 1) surface unit cell], are represented in Figure 2. The most stable configuration (3O)V1.5 is obtained by a suppression of 50% of surface anions [e.g., three out of six surface oxygen atoms in a (2 × 1) unit cell], which restores the Al2O3 stoichiometry of the surface layer. We note that, while it is the most stable among the considered O-terminated surfaces,

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RESULTS In the following, we report computational results on the adsorption of low Zn coverage on dry and hydroxylated Al2O3(0001) surfaces. In each case, we start by a synthetic discussion of stability of the corresponding bare terminations (in absence of Zn) as a function of the environment, ΔμO and ΔμH. We systematically consider adsorption of a single Zn adatom per (1 × 1) surface unit cell (1/3 ML). This corresponds to Zn−Zn distances of about 4.7 Å, considerably larger than the interatomic distances in bulk Zn. We also take into account a limited number of configurations with 0.5, 1.0, and 1.5 Zn adatoms per (1 × 1) surface unit cell [obtained by adsorption of 1, 2, and 3 Zn adatoms, respectively, in a (2 × 1) surface cell]. In all such configurations, the coadsorbed Zn adatoms do not form Zn−Zn bonds. Zn Adsorption on Dry Alumina Surfaces. Figure 1 gives energies of the three nonequivalent dry (1 × 1) Al2O3(0001) surfaces as a function of the oxygen chemical potential (ΔμO) calculated within LDA and GGA. Both approximations 13580



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eV/Zn. We note that the standard LDA estimation of adsorption energy (Table 2) differs relatively little from this value, showing that in this case, LDA can efficiently mimic the effect of self-interaction correction. This remains coherent with the estimations of ZnO formation energy in Table 1, where the LDA value approached much better the experimental value. An important issue is the effect of Zn adsorption on the behavior of the otherwise unstable bare oxygen- and aluminumrich terminations. By comparing surface energies of the three Al2O3(0001) terminations either bare (Figure 1) or covered with 1/3 ML of Zn (Figure 3), it is clear that adsorption of Figure 2. Surface energies (LDA) of oxygen-terminated surfaces with a different number of missing surface anions Vx [x = 0···2 per (1 × 1) surface unit cell] as a function of the oxygen chemical potential ΔμO (eV). The energy of the stoichiometric (Al) termination (dashed line) is plotted as a reference.

it nevertheless remains less stable than the stoichiometric (Al) surface, due to the relatively large number of exposed low coordinated atoms. In Table 2, we have summarized the principal characteristics of Zn adsorption at the three (1 × 1) terminations sketched in Table 2. Calculated (LDA) Adsorption Characteristics of Zn Adatoms on the Three Nonequivalent (1 × 1) Terminations of Al2O3(0001): Adsorption Energy (Eads) (eV/Zn), ZnSurface Bond Lengths (Å), and Charge of the Adsorbed Adatoms (QZn)a (Al) (3O) (2Al) a

Eads (eV/Zn)

bond length (Å)

QZn

0.57 (0.20) 7.10 (6.10) 1.60 (0.97)

Zn−O: 2.40, 2.60 Zn−O: 3 × 1.80 Zn−Al: 2.70

+0.06 (+0.03) +1.28 (+1.31) −0.17 (−0.22)

Figure 3. Surface energies of the three nonequivalent (1 × 1) αalumina(0001) terminations with 1/3 ML of adsorbed Zn as a function of oxygen chemical potential ΔμO (eV). Solid and dashed lines represent LDA and GGA results, respectively.

even a low amount of Zn induces substantial changes in the relative surface stability. In line with the existing studies on metal/oxide interfaces,17,49,51 we find that adsorbed Zn considerably extends the range of stability of the two polar terminations. In particular, upon Zn adsorption the (3O) termination becomes the most stable one under oxygen moderate and rich conditions, ΔμO > −2.5 eV. We note that, despite the substantially different Eads (Table 2), differences in LDA and GGA surface energies (Figure 3) are only slightly larger than those reported for the bare surfaces (Figure 1). This is due to a partial cancellation of errors in adsorption and cohesion energies in the Zn adsorption-related contribution to the surface energy −(Eads − EZn bulk)/S. The above data on the Zn interaction with different alumina(0001) surfaces require additional refinements regarding the stability of the Zn deposit and the stoichiometry of the alumina surfaces. First, Zn adatoms weakly bound to the stoichiometric (Al) termination are unstable with respect to Zn atoms in bulk zinc, showing that formation of Zn−Zn bonds is favored over Zn−alumina ones. A more stable configuration, to which we will refer herein as (Al)+ZnB, may be seen as metallic Zn clusters weakly bound to the stoichometric alumina surface, a picture consistent with the existing experimental evidence.24,74 With similar arguments, one expects the formation of metallic (Al,Zn) deposits on the aluminum-rich termination, denoted as (Al)+(Al,Zn)B. On the other hand, taking into account the instability of the oxygen-rich (1 × 1) surface (Figure 1), the considered adsorption configuration is arbitrary. Indeed, as already shown in Figure 2, a considerable stabilization of the bare surface can be gained by removing a fraction of surface anions. In the case of Zn-covered surfaces, a combined stabilizing effect is expected from both the ionized Zn adatoms and from the missing surface anions V. Figure 4 summarizes energies of such (3O)VxZny configurations, in proportion, which assures surface

GGA adsorption energies and adatom charges are given in brackets.

Figure 1: adsorption energies, lengths of Zn-surface bonds, and charges of the adsorbed Zn. In line with the variety of existing data on metal interaction with alumina surfaces, both GGA and LDA results show that Zn adsorption characteristics depend strongly on surface termination. They change from a weak binding with a small charge transfer at the (Al) termination to an extremely strong adsorption, accompanied by ionization of the adsorbed Zn at the (3O) one. We note the particularly large differences between LDA and GGA adsorption energies, which were the reason for the systematic use of the two approximations in the present study. It is worth pointing out that the calculated weak adsorption found for Zn at the (Al) termination is partially due to the neglect of van der Waals interaction in the standard DFT calculations. Indeed, if estimated with a dedicated (e.g., the optB88-vdW)69−71 functional, the Zn adsorption energy is equal to Eads(Al) = 0.46 eV/Zn. This result shows that LDA can efficiently mimic the effect of vdW interactions to the energetics of weakly interacting metal/oxide systems, as it has already been discussed in case of weakly interacting metal/ oxide interfaces.72 On the other hand, adsorption energy of a strongly bound Zn adatom at the (3O) termination may suffer from the know self-interaction error which affects the stability of ZnO systems. Indeed, if estimated within GGA+U scheme (Ud = 12 eV), which have recently been shown to efficiently restore the correct position of the Zn d band and to compensate for the related errors in stability of layered ZnO systems,73 the Zn adsorption energy is equal to Eads(3O) = 7.50 13581



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on the hydrogenated oxygen-rich termination (3O)H3 and on the hydroxylated stoichiometric (Al)H2O surface. As for dry surfaces, we complement the data on Zn adsorption energetics by an estimate of the relative stability of alternative surface structures, either bare or under a low coverage of Zn. Water molecules dissociate readily on the stoichiometric Alterminated alumina(0001) surface,39,45,75−82 with adsorption energy of 1.88 eV/H2O (1.50 eV/H2O in GGA). The OH group issued from water dissociation sits on top of the surface cation, while the hydrogen binds to a surface anion, forming a surface hydroxyl group. Table 3 summarizes the Zn adsorption Figure 4. Surface energies (LDA) of oxygen-terminated Al2O3(0001) surfaces under low coverage of Zn as a function of the oxygen chemical potential ΔμO (eV). Labels (3O)VxZny refer to surface configurations, a different number of adsorbed Zn adatoms y, and missing surface ions (V) x [per (1 × 1) surface unit cell]. The surface energies of (3O)V1.5+ZnB and (Al)+ZnB configurations (dashed lines) are plotted as a reference.

Table 3. Calculated (LDA) Adsorption Characteristics of Zn Adatoms on the Hydroxylated Stoichiometric (Al) Termination: Adsorption Energies Eads (eV/Zn), Zn−O Bond Lengths d(Zn−O) (Å), and Charges of the Adsorbed Adatoms (QZn)a Eads (eV/Zn) config. config. config. config.

neutralization x + y = 1.5 [per (1 × 1) surface unit cell]. Energies of (3O)V1.5+ZnB and (Al)+ZnB configurations, representing Zn clusters weakly bound to (3O)V1.5 and (Al) surfaces, respectively, are plotted as a reference. Figure 4 shows that the relative stability of various oxygenterminated configuration changes as a function of ΔμO, favoring the anion-deficient structures, (3O)V1.5 and (3O)V1Zn0.5 in Opoor conditions, and the (3O)Zn1.5 one, with a complete surface anion layer, in O-rich environments. We note that, since this latter enables a complete compensation of surface polarity, it is indeed more stable than the (3O)Zn structure (Figure 3). It is to be stressed that, despite the fact that Zn strongly binds to all the considered anion-deficient terminations (Eads > EZn bulk), most of the resulting surface structures are not thermodynamically stable. Indeed, Figure 4 shows only two distinct lowest energy surface structures under low coverage of Zn. The weakly bound Zn at the Al-terminated stoichiometric surface [(Al)+ZnB] is by far the most stable in a wide range of Opoor conditions, whereas the oxygen-terminated surface with strongly bound Zn adatoms [(3O)Zn1.5] is favored in O-rich environments. In summary, we have shown that the binding strength of Zn adatoms at dry alumina(0001) depends drastically on the surface termination. It is relatively weak on the stoichiometric (Al) surface, where we expect formation of metallic clusters weakly bound to the oxide. It is particularly strong on the oxygen-rich (3O) surface, where isolated Zn adatoms become ionized upon adsorption, contributing to an efficient stabilization of this polar termination. Comparison of surface energies indicates that the former surface configuration is the most stable in a wide range of oxygen-poor conditions, whereas the latter becomes thermodynamically favored already in oxygenmoderate environments (ΔμO > −2.5 eV). We note however that the actual surface structure observed experimentally in the presence of oxygen may be subject to kinetic hindering. Indeed, although the oxygen-terminated surface stabilized by Zn adatoms is by far the most stable configuration in oxygen-rich conditions, its experimental fabrication would require deposition of zinc on the highly unstable bare oxygen-rich termination. Therefore, less thermodynamically favorable arrangements are likely to appear in the experiments. Zn Adsorption on Hydroxylated Alumina Surfaces. In the following, we report results on Zn adsorption on hydroxylated alumina(0001) terminations, with a special focus

a

A B C D

0.57 1.77 3.32 6.82

(0.09) (0.97) (2.27) (5.48)

d(Zn−O) (Å) − 2.12, 2.19, 2.54 2.18, 2.17, 2.35 1.97, 2.08, 2.11

QZn +0.14 +0.95 +0.72 +1.23

(+0.04) (+0.96) (+0.71) (+1.26)


characteristics on this termination, obtained for hydrogen-rich (Figure 5, configs. A and B) and hydrogen-deficient (Figure 5,

Figure 5. Schematic representation of the four considered configurations of Zn adsorbed at hydroxylated (Al) terminations of alumina (red, oxygen; black, aluminum; blue, zinc; and white, hydrogen).

configs. C and D) conditions. The stability diagram (Figure 6) summarizes the corresponding behavior of surface energies as a function of hydrogen chemical potential ΔμH, for ΔμO = −3.5 eV close to the stability limit of bulk ZnO. Table 3 shows that whereas an incoming Zn atom interacts only weakly with the fully hydroxylated surface, config. A, the interaction is substantially enhanced upon an exchange of Zn with the surface hydrogen, config. B. This exchange, which can be schematized as OH− + Zn → O2− + (ZnH)+, results in an ionization of the adsorbed Zn. In the hydrogen-rich environment, the formal Zn2+ oxidation state of adatoms is enabled by the ionic Zn−H bond and a negative charging of the hydrogen atom. However, since both LDA and GGA predict the Zn− alumina bonding in configs. A and B to be weaker than that of Zn with its own bulk, we expect Zn adatoms to form metallic Zn clusters, weakly bound to the hydroxylated surface [(Al)H2O+ZnB]. This configuration becomes the most stable under extremely H-rich conditions, ΔμH > −0.3 eV (Figure 6). The two hydrogen-deficient configurations (Figure 5) are obtained upon desorption of (1/2)H2 and H2 molecule per surface (1 × 1) unit cell (configs. C and D), respectively. These surface structures are characterized by a much stronger Zn13582



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a wide range of hydrogen-rich environments. Despite similarly large hydrogen adsorption energies [3.13 eV/H and 3.18 eV/H in (3O)H2 and (3O)H1 configurations, respectively], a lower hydrogen coverage is not sufficient for surface neutralization and for its efficient stabilization. Moreover, we note that the most stable (3O)H3 surface configuration hinders the appearance of the less stable hydroxylated stoichiometric termination (Al)H2O, but their stability differs little in the vicinity of the transition point (ΔμH ∼ −2.5 eV). Table 4 summarizes the Zn adsorption characteristics on the hydrogenated (3O) termination, obtained under hydrogen-rich Table 4. Calculated (LDA) Adsorption Characteristics of Zn Adatoms on the Hydrogenated (3O) Surface: Adsorption Energy Eads (eV/Zn), Zn−O Bond Lengths d(Zn−O) (Å), and Charge of the Adsorbed Zn adatom (QZn)a

Figure 6. Surface energies (LDA) of the most stable hydoxylated (Al)H2O termination with adsorbed zinc adatoms as a function of hydrogen chemical potential (ΔμH). As a reference, fully dehydrogenated (Al)ZnO and fully dehydroxylated (Al)+ZnB configurations have also been plotted. Surface energies correspond to ΔμO = −3.5 eV.

Eads (eV/Zn) config. config. config. config.

surface interaction, consistent with the formation of an array of ionized Zn adatoms. The adatoms are the most strongly bound in the dehydrogenated configuration D [(Al)OHZn0.5], obtained upon an exchange of two surface hydrogen atoms by an incoming Zn, which can be schematized as 2H+ + Zn → Zn2+ + H2. Stability diagram (Figure 6) shows that config. D is thermodynamically stable in a relatively wide range of hydrogen-rich environments, −1.2 eV < ΔμH < −0.3 eV. Indeed, since the strong bonding of Zn adsorbates to the nonpolar stoichiometric (Al) termination is due to the excess surface hydroxyl groups, the optimal stability is expected for charge neutral surface configurations (Al)(OH)yZnx, with y = 2x. Full hydrogenation of the oxygen-rich alumina termination (LDA hydrogen adsorption energy Eads = 3.16 eV/H, 2.86 eV/ H in GGA) gives rise to three surface hydroxyl groups per (1 × 1) unit cell. As it has already been reported,37,39,39,44 one of them lies parallel to the surface plane, while the other two sit almost perpendicularly to the surface. Full hydrogenation restores the charge neutrality of this polar termination and results in its substantial stabilization. Indeed, Figure 7 shows that only two surface configurations are thermodynamically stable over the entire range of hydrogen chemical potential: the dry stoichiometric (Al) termination in hydrogen-poor conditions and the fully hydrogenated oxygen-rich (3O)H3 one in

a

A B C D

0.38 0.95 2.74 6.42

(0.07) (0.25) (1.71) (5.67)

d(Zn−O) (Å) 2.60 2.03, 2.05, 1.87 2.01, 2.03, 1.88 1.81, 1.84, 1.79

QZn +0.05 +0.88 +0.52 +1.22

(+0.02) (+0.94) (+0.69) (+1.22)


Figure 8. Schematic representation of the four considered configurations of Zn adatom adsorbed at the hydrogenated (3O) termination of alumina (red, oxygen; black, aluminum; blue, zinc; and white, hydrogen).

(Figure 8, configs. A and B) and hydrogen-deficient (Figure 8, configs. C and D) conditions. We find that an incoming Zn atom interacts very weakly with the fully hydrogenated surface due to the particularly strong passivating effect of surface hydroxyls, config. A. However, a much more stable configuration is obtained upon a 1:1 exchange of Zn with a surface hydrogen, config. B. Similarly to the hydroxylated (Al) surface considered previously, the exchange can be schematized as OH− + Zn → O2− + (ZnH)+ and results in an ionized Zn adatom stabilized by the negatively charged H. As in the case of the stoichiometric termination, Zn adsorption energies in the hydrogen-rich environment are not large enough to favor Znalumina over Zn−Zn bonds, suggesting the formation of metallic Zn clusters weakly bound to the fully hydroxylated surface, (3O)H3+ZnB. Conversely, in hydrogen-deficient conditions, due to the instability of the hydrogen deficient (3O)Hx (x < 3) surface, the Zn−alumina bonds are considerably stronger, consistent with a formation of an array of ionized Zn adatoms. Figure 9 summarizes the results on the relative stability of (3O) surface stabilized by a different number of adsorbed Zn and H atoms, (3O)HxZny [with x and y numbers of H and Zn

Figure 7. Surface energies (LDA) of hydrogenated (3O)H x terminations of alumina as a function of hydrogen chemical potential ΔμH. Dry, hydrogenated, and hydroxylated stoichiometric (Al) surfaces are plotted for comparison. Surface energies correspond to oxygen-rich conditions, ΔμO = 0.0 eV. 13583



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bare and Zn-covered alumina(0001) surface configurations as a function of oxygen and hydrogen chemical potentials. The two diagrams were constructed on the basis of all the surface configurations reported in Figures 1, 2, and 7 (in absence of Zn), and in Figures 3, 4, and 9 (under low Zn coverage). Only a very limited number of surface configurations appears in the two plots. In the absence of Zn adatoms, the diagram is largely dominated by the dry stoichiometric (Al) surface. The range of its stability is particularly large in hydrogen-poor conditions, but it progressively narrows down when available hydrogen atoms efficiently stabilize the fully hydrogenated (3O)H3 (oxygen-moderate and oxygen-rich conditions) and (2Al)H3 (oxygen-poor conditions) terminations. Fully hydroxylated stoichimetric (Al)H2O surface is absent from the diagram because of its somewhat lower stability compared to the (3O)H3 termination (Figure 7). However, the stability difference being relatively small in the vicinity of the transition line, the former is expected to be observed experimentally upon hydroxylation of (Al) surface for kinetic reasons. Presence of a low Zn coverage alters considerably the stability diagram. In hydrogen-rich and in oxygen-poor environments we find principally the bare surface structures with weakly bound metallic Zn deposits. In contrast, several new surface configurations turn out to be stabilized by the metal adatoms under oxygen-rich and hydrogen-moderate and poor conditions. On the one hand, in absence of hydrogen, the polar (3O) termination stabilized by 1.5 ML of Zn [(3O)Zn1.5] is stable in a wide range of oxygen chemical potentials ΔμO > −3.0 eV (−2.4 eV in GGA). On the other hand, in hydrogendeficient environments (ΔμH ∼ −1.2 eV), mixed Zn/OH surface structures are stable at both the stoichiometric [(Al)OHZn0.5] and the oxygen-rich [(3O)HZn] terminations. In all these three cases, the Zn adatoms ionize upon adsorption, providing an efficient neutralization of the net surface charge. Indeed, surface non-neutrality, which makes the corresponding bare terminations unstable, is due either to surface polarity (oxygen-rich termination) or to excess surface hydroxyl groups (stoichiometric termination). Contrary to all the other surface structures present in the diagram, Zn−alumina bonding is particularly strong in these three cases, suggesting that under adequately chosen thermodynamic conditions, a low coverage of Zn adatoms is expected to form an array of strongly bound isolated ions. It is worth pointing out that both LDA and GGA give a qualitatively similar picture of relative surface stability. However, the stability limits of surface configurations are visibly shifted due to substantial differences between adsorption energies obtained within the two approximations. As pointed out before, the largest difference concerns the shift of the ΔμO scale, which systematically pushes all the LDA transition lines toward less oxygen-rich conditions. A similar, but less pronounced effect concerns also the hydrogen reference, shifting the LDA stability limits toward somewhat less hydrogen-rich environments. Beyond these two systematic effects, we notice that GGA predicts a considerably larger stability region for the (Al)OHZn0.5 surface configuration and a visibly reduced one for the (3O)HZn one. In summary, our calculations reveal the existence of a large range of nonextreme oxygen-rich conditions under which the formation of arrays of strongly adsorbed Zn adatoms is favored over the growth of weakly bound metallic clusters. In the case of the oxygen-rich termination, the particularly strong Zn− alumina interaction is due to an efficient polarity compensation

Figure 9. Surface energies (LDA) of (3O) surface configurations (3O)HxZny stabilized by a different number of zinc y and hydrogen x adatoms [x and y per (1 × 1) surface unit cell] as a function of hydrogen chemical potential (ΔμH). Surface energies correspond to ΔμO = −3.5 eV.

adatoms per (1 × 1) surface unit cell]. Considered compositions range from (3O)H0Zn1.5 (entirely Zn-stabilized surface), through (3O)H1Zn1 (config. D), (3O)H2Zn1 (config. C), (3O)H 3Zn1 (config. B), and (3O)H2Zn0.5, up to (3O)H3Zn0 (entirely H-stabilized surface). We have already shown that both Zn adsorption (Figure 4) and a full surface hydroxylation (Figure 7) do efficiently compensate the polarity of this oxygen-rich termination and make it thermodynamically favored in oxygen-rich and hydrogen-rich environments, respectively. Unsurprisingly, the corresponding two surface configurations (3O)Zn1.5 and (3O)H3 + ZnB appear in Figure 9, as the most stable terminations in H-poor and H-rich conditions, respectively. Moreover, polarity compensation can also be obtained by a joint effect of Zn and H adsorbates if x + 2y = 3 [per (1 × 1) surface unit cell]. Indeed, the corresponding configuration D [(3O)HZn] turns out to be thermodynamically favored in a relatively narrow range of −1.2 eV < ΔμH < −0.9 eV. Relative Surface Stability: Effect of Zn Adsorbates. Figure 10 presents the synthetic diagrams of the most stable

Figure 10. Diagrams of the most stable surface configurations of Al2O3(0001) terminations as a function of oxygen and hydrogen chemical potentials ΔμO and ΔμH. Top and bottom panels correspond to bare and Zn-covered surfaces, respectively. Solid and dashed lines represent LDA and GGA results, respectively. 13584



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series is consistent with trends on adatom adsorption reported in the literature.5 At the neutral termination, where the electron transfer toward the oxide substrate is relatively small, two main factors determine the adsorption behavior: the size of the adatom (as estimated, for example, by the atomic radii: Cu, 1.35 Å; Ag, 1.60 Å; Zn, 1.35 Å) and the position of its highest occupied state (estimated, for example, by the first ionization potentials: Cu, 7.7 eV; Ag, 7.6 eV; Zn, 9.4 eV). Copper adatom is relatively small and its occupied states are located higher in energy. This produces shorter Cu−O bonds, a larger electron transfer toward the substrate, and a stronger binding. Despite a similar ionization potential, the much larger size of Ag makes the adatom−surface distance larger compared to that of Cu. This reduces the Ag−alumina electron transfer and weakens the binding strength. The further reduction of the binding strength in the case of Zn is principally driven by its substantially larger ionization potential and the resulting decrease of electron transfer. Due to the closed Zn 4s2 shell; the effect is additionally reinforced by the nearly complete filling of the antibonding O 2p-Zn 3s states. The adsorption behavior at the oxygen-rich (3O) surface is somewhat more complex. Whereas the reduction of the binding strength between Cu and Ag follows the trend discussed for the neutral surface above, the Zn adsorption may seem particular, with the adsorption energy considerably larger than that of Ag. Moreover, while there remains a good correlation between the calculated adsorption energies and adatom−surface bond lengths, this is not the case of the adatom charges. Indeed, since the polarity-driven electron transfer toward the alumina substrate is systematically large at this termination, the metal adatoms are ionized upon adsorption. As a consequence, their final state energetics are influenced by the second ionization potential (Cu, 20.3 eV; Ag, 21.5 eV; Zn, 18.0 eV), which is particularly low in the case of Zn, due to the 3d 104s2 configuration of its valence shell. In this respect, Zn behaves very differently compared to Ag, for which a large second ionization potential and a particularly large ionic radius (Cu, 0.9 Å; Ag, 1.1−1.3 Å; Zn, 0.9 Å) are responsible for the weakest Ag adsorption strength in the considered series. Similar trends are found in metal adsorption energies on both the hydroxylated terminations (Table 6), where the reported data correspond to the most stable configuration for each of the three considered adatoms. In the same way as on

by either the Zn adatoms only (hydrogen-poor conditions), or by a mixed Zn/H surface configuration (hydrogen-moderate conditions). More interestingly, our calculations show that a similar array of strongly adsorbed Zn adatoms may also be stable at the nonpolar stoichiometric alumina termination. In this case, the stabilizing effect is due to an efficient accommodation of the charge excess induced by surface hydroxyl groups by the Zn adsorbates. It is worth to remember that in all these cases the optimal quantity of tightly bound Zn adatoms is entirely determined by the net charge of the corresponding bare surface.

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DISCUSSION Metal adsorption characteristics on alumina surfaces have been extensively studied in the past, with a special focus on late transition and noble metals. In particular, adsorption of Cu and Ag adatoms, the direct neighbors of Zn in the periodic table, has been subject of numerous studies,12,15,16,21,22,48,49,51 motivated by the catalytic properties of these metals. As to highlight the Zn-specific features, in the following we confront the present results on Zn adsorption with the corresponding data on isolated Ag and Cu adatoms. All calculations have been performed with the same computational framework described in Computational Method and Settings. Table 5 summarizes the principal adsorption characteristics on the dry (1 × 1) terminations. We note that both LDA and Table 5. Calculated (LDA) Adsorption Characteristics of Cu, Ag, and Zn Adatoms on the the Dry (1 × 1) Terminations of Al2O3(0001): Adsorption Energy Eads (eV/M), Lengths of Bonds with Surface Anions d(O−M) (Å), and Charge of the Adsorbed Metal Adatom (QM)a M/(Al) Cu Ag Zn M/(3O) Cu Ag Zn a

Eads (eV/M)

d(O−M) (Å)

QM

2.18 (1.10) 1.42 (0.67) 0.57 (0.20)

3 × 2.08 3 × 2.45 2.40, 2.60

+0.30 (+0.27) +0.16 (+0.16) +0.06 (+0.03)

8.21 (6.97) 5.35 (4.34) 7.10 (6.10)

3 × 1.77 3 × 2.00 3 × 1.80

+1.13 (+1.17) +1.15 (+1.00) +1.28 (+1.31)


Table 6. Calculated (LDA) Adsorption Characteristics of Cu, Ag, and Zn Adatoms on the the Hydroxylated Al2O3(0001) Terminations: Adsorption Energy Eads (eV/M), Lengths of Bonds with Surface Anions d(O−M) (Å), and Charge of the Metal Atom (QM)a

GGA give the same trends and that differences induced by a different polar character of the two terminations, discussed in the case of Zn adatoms (Table 2), are present also in case of Cu and Ag. Indeed, the adsorption energies and the electron transfers toward the alumina substrate are systematically large at oxygen-rich surface and much smaller at the neutral termination. Beyond this general characteristic, in the following we will focus on trends in adsorption behavior in the series of the three adatoms (Cu, Ag, and Zn) on each of these two terminations. At the neutral (Al) termination, the binding is the strongest in the case of Cu adatoms and diminishes progressively along the series Cu, Ag, and Zn. This trend is correlated to a decreasing amplitude of electron transfer and to the progressive expansion of adatom−surface bonds. Moreover, the particularly weak binding of Zn is associated with an asymmetric 2-fold bridge adsorption geometry, contrary to the symmetric 3-fold adsorption site found in cases of Cu and Ag. The progressive weakening of the adsorption strength along the considered

M/(Al) H2O Cu Ag Zn M/(3O)H3 Cu Ag Zn

Eads (eV/M)

d(O−M) (Å)

QM

config. D config. C config. D

7.48 (5.85) 4.49 (3.44) 6.82 (5.48)

1.97, 2.05, 2.06 2.33, 2.41, 2.46 1.97, 2.08, 2.11

+0.97 (+1.01) +0.58 (+0.58) +1.23 (+1.26)

config. D config. C config. D

6.98 (6.01) 3.16 (2.19) 6.42 (5.67)

1.79, 1.80, 1.85 2.15, 2.16, 2.21 1.79, 1.81, 1.84

+0.95 (+0.95) +0.55 (+0.57) +1.22 (+1.22)

a

Labels of adsorption configuration refer to Figure 5, [M/(Al)H2O], and Figure 8, [M/(3O)H3]. GGA adsorption energies and adatom charges are given in brackets. 13585



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these (Al)(OH)xMy structures is consistent with the preferential metal oxidation state: Cu2+, Zn2+ (2x = y, config. D, Figure 5), and Ag+ (y = x, config. C, Figure 5). Beyond these systematic similarities, the three diagrams show several differences between the three metals. The most important is the different range of stability of the metalstabilized oxygen-rich surface structures. It extends toward the most oxygen-moderate and poor conditions in the case of Zn (ΔμO > −3.0 eV), but it is substantially reduced in the case of Cu (ΔμO > −1.2 eV) and limited to a narrow stripe of strongly oxygen-rich environments only (ΔμO > −0.3 eV) in the case of Ag. This stability shift is principally driven by the very different formation energies of the corresponding surface oxide species, as estimated from, for example, the experimental values of formation enthalpies of the three bulk oxides (ZnO, 350 kJ/ mol; CuO, 158 kJ/mol; and AgO, 31 kJ/mol). Unsurprisingly, the different stability of the oxide species does also impact the conditions under which excess surface hydroxyls stabilize the ionized metal adatoms at the stoichiometric termination. Indeed, since the corresponding stability regions are located at the intersection of (Al)/(3O)H3 and (Al)/(3O)M 1.5 transition lines, the resulting hydrogen conditions can be deduced from ΔμO of these latter. As a consequence, Ag adatoms require the most hydrogen-poor conditions (ΔμH ∼ −2.5 eV); Cu is efficiently ionized at ΔμH ∼ −2.0 eV, while Zn adspecies are stabilized in relatively hydrogen-rich environment (ΔμH ∼ −1.2 eV). Another important difference between the three diagrams shown in Figure 11 concerns the mixed metal/hydrogen structure at the polar oxygen-rich termination, which appears to be stable only in the case of Zn adatoms. Indeed, the (3O)HZn configuration is thermodynamically favored over a particularly large span of oxygen chemical potentials, whereas this type of surface structure is totally absent from Ag and Cu diagrams. In the absence of direct metal−hydrogen bonds, the existence of this structure is due to the large formation energy of ZnO, which makes the ionic Zn−O bonds fully competitive with the O−H ones. Moreover, compared to (Al)OHZn0.5 termination, the (3O)HZn configuration involves a twice larger number of Zn−O bonds per surface area. In summary, by comparing the adsorption energies of Zn, Cu, and Ag on α-alumina(0001), we conclude that the binding strength of Zn adatoms relative to that of its direct neighbors in the periodic table is strongly influenced by its ionization energetics. Larger first ionization potential of Zn is consistent with the weakest adsorption at the dry neutral termination, where the metal−substrate charge transfer is small and the interaction is weak. In the case of strong-bound ionized adatoms at either oxygen-rich or hydroxylated alumina terminations, the relatively low Zn second ionization potential makes it bind much stronger than Ag and nearly as strong as Cu. Surface stability diagrams of alumina covered with a low metal coverage show that surface structures involving ionized Zn adatoms can be stabilized over a particularly large span of environments. This behavior is driven by the highest ability of Zn to form surface oxide species and is consistent with the largest bulk oxide formation enthalpy. It also determines the good stability of mixed Zn/hydrogen surface structures on the oxygen-rich termination, absent in the cases of Ag and Cu adsorbates.

the dry oxygen-rich surface, the three metal adatoms are ionized upon adsorption. In the case of Cu and Zn, the favored surface configuration (config. D, metal substitutes two surface hydrogens) and large Bader charges point unambiguously toward the 2+ oxidation state of the adsorbed metal. In the case of Ag, the preferential config. C (metal substitutes a single surface hydrogen) and a much smaller QM are consistent with Ag+ oxidation state of adatoms. Since adsorption energies on their own do not constitute the complete guideline to the actual stability of surface configurations, Figure 11 presents schematic surface stability diagrams

Figure 11. Diagrams (LDA) of the most stable surface configurations of Al2O3(0001) terminations with a low coverage of Cu (top), Ag (middle), and Zn (bottom), as a function of oxygen and hydrogen chemical potentials, ΔμO and ΔμH.

of Al2O3(0001) under a low coverage of each of the three considered metals. In all the cases, in oxygen-poor and/or hydrogen-rich conditions, the metal−alumina interaction is weak compared to the metal cohesion energy, which gives preference to a formation of metallic clusters on the oxide substrate. Conversely, in oxygen-rich and hydrogen moderate and poor conditions, surface configurations with strongly bound ionized metal adatoms can be stabilized. On the one hand, in absence of hydrogen, the three metals do efficiently stabilize the oxygen-rich (3O)M1.5 surface structures (M = Cu, Ag, and Zn). Taking into account the 2+ oxidation state of Cu and Zn, this surface composition does indeed provide charge neutrality. In the case of Ag, the expected, charge neutral (3O)Ag3 configuration is destabilized by the Ag−Ag repulsion between first Ag neighbors. On the other hand, at moderate hydrogen chemical potentials, ionized metal atoms can be stabilized on the stoichiometric termination by an excess of surface hydroxyl groups. The precise surface composition of 13586



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(7) Stott, F. H. The Protective Action of Oxide Scales in Gaseous Environments at High Temperature. Rep. Prog. Phys. 1987, 50, 861− 913. (8) Gitzen, W. H. Alumina as a Ceramic Material; American Ceramic Society: Columbus, 1970. (9) Duffy, D. M.; Harding, J. H.; Stoneham, A. M. A Calculation of the Structure and Energy of the NbAl2O3 Interface. Acta Mater. 1996, 44, 3293−3298. (10) Song, G.; Remhof, A.; Theis-Bröhl, K.; Zabel, H. Extraordinary Adhesion of Niobium on Sapphire Substrates. Phys. Rev. Lett. 1997, 79, 5062−5065. (11) Zhukovskii, Y. F.; Alfredsson, M.; Hermansson, K.; Kotomin, E. A.; Nieminen, N. M. Ab initio Simulations of CO Adsorption on Both Clean and Ag-predosed Corundum (0001) Surfaces. In Proc. ECASIA ’97, Wiley: Chichester, West Sussex, England, 1997; pp 197−201. (12) Zhukovskii, Y. F.; Alfredsson, M.; Hermansson, K.; Heifets, E.; Kotomin, E. A. Ab initio Simulations of Silver Film Adhesion on αAl2O3(0001) and MgO(100) Surfaces. Nucl. Instrum. Methods Phys. Res., Sect. B 1998, 141, 73−78. (13) Hojrup Hansen, K.; Worren, T.; Stempel, S.; Laegsgaard, E.; Bäumer, M.; Freund, H.-J.; Besenbacher, F.; Stensgaard, I. Palladium Nanocrystals on Al2O3: Structure and Adhesion Energy. Phys. Rev. Lett. 1999, 83, 4120−4123. (14) Batyrev, I. G.; Alavi, A.; Finnis, M. W.; Deutsch, T. FirstPrinciples Calculations of the Ideal Cleavage Energy of Bulk Niobium(111)/α-Alumina(0001) Interfaces. Phys. Rev. Lett. 1999, 82, 1510−1513. (15) Verdozzi, C.; Jennison, D. R.; Schultz, P. A.; Sears, M. P. Sapphire (0001) Surface, Clean and with d-Metal Overlayers. Phys. Rev. Lett. 1999, 82, 799−802. (16) Zhang, W.; Smith, J. R. Stoichiometry and Adhesion of Nb/ Al2O3. Phys. Rev. B 2000, 61, 16883−16889. (17) Batyrev, I. G.; Alavi, A.; Finnis, M. W. Equilibrium and Adhesion of Nb/sapphire: The Effect of Oxygen Partial Pressure. Phys. Rev. B 2000, 62, 4698−4706. (18) Batyrev, I. G.; Kleinman, L. In-plane Relaxation of Cu(111) and Al(111)/α-Al2O3(0001) Interfaces. Phys. Rev. B 2001, 64, 033410. (19) Gomes, J. R. B.; Illas, F.; Cruz Hernández, N.; Sanz, J. F.; Wander, A.; Harrison, N. M. Surface Model and Exchange-Correlation Functional Effects on the Description of Pd α-Al2O3(0001). J. Chem. Phys. 2002, 116, 1684−1691. (20) Siegel, D. J.; Hector, L. G., Jr.; Adams, J. B. Adhesion, Atomic Structure, and Bonding at the Al(111)/α-Al2O3(0001) Interface: A First Principles Study. Phys. Rev. B 2002, 65, 085415. (21) Todorova, T. K.; Ganduglia-Pirovano, M. V.; Sauer, J. Vanadium Oxides on Aluminum Oxide Supports. 1. Surface Termination and Reducibility of Vanadia Films on α-Al2O3(0001). J. Phys. Chem. B 2005, 109, 23523−23531. (22) Hernandez, N. C.; Graciani, J.; Márquez, A.; Sanz, J. F. Cu, Ag and Au Atoms Deposited on the α-Al2O3(0001) Surface: A Comparative Density Functional Study. Surf. Sci. 2005, 575, 189−196. (23) Briquet, L. G. V.; Catlow, C. R. A.; French, S. A. Comparison of the Adsorption of Ni, Pd, and Pt on the (0001) Surface of α-Alumina. J. Phys. Chem. C 2008, 112, 18948−18954. (24) Rodriguez, J. A.; Kuhn, M.; Hrbek, J. Interaction of Silver, Cesium, and Zinc with Alumina Surfaces: Thermal Desorption and Photoemission Studies. J. Phys. Chem. 1996, 100, 18240−18248. (25) Triboulet, R.; Perriere. Epitaxial Growth of ZnO Films. Prog. Cryst. Growth Charact. Mater. 2003, 47, 65−138. (26) Guttmann, M. Diffusive Phase Transformations in Hot Dip Galvanizing. Mater. Sci. Forum 1994, 155−156, 527−548. (27) Marder, A. R. The Metallurgy of Zinc-Coated Steel. Prog. Mater. Sci. 2000, 45, 191−271. (28) Drillet, P.; Zermout, Z.; Bouleau, D.; Mataigne, J.-M.; Claessens, S. Selective Oxidation of High Si, Mn and Al Steel Grades During Recrystallization Annealing and Steel/Zn Reactivity. Rev. Metall./Cah. d’Inf. Tech. 2004, 10, 831−837.

CONCLUSIONS We have reported results of ab initio calculations on the behavior of Zn adatoms on different terminations of the αAl2O3(0001) surface, with a special focus on the effect of hydroxyls on the Zn bonding to the (nonpolar) stoichiometric and to the (polar) oxygen-rich terminations. For the large variety of surface configurations considered, we have evaluated both the strength of Zn−alumina bonding (adsorption energies) and the relative stability of the corresponding surface structures (surface energies). As to estimate the incertitude inherent to the employed computational approach, we have worked with two different approximations to the exchangecorrelation functional (LDA and GGA). Our results reveal that in a wide range of nonextreme oxygen-rich conditions, Zn favors to spill over the alumina substrate as an array of strongly adsorbed adatoms rather than to form metallic clusters weakly bound to the substrate. The strongly bound Zn adatoms are ionized upon adsorption and contribute to an efficient neutralization of the surface net charge. This occurs at the polar oxygen-rich alumina termination, where either Zn adatoms only (hydrogen-poor conditions), or in a mixed Zn/H surface configuration (hydrogen-moderate conditions) furnish the surface charge necessary for polarity compensation. It also occurs at the stoichiometric alumina termination with a net surface charge induced by an excess of surface hydroxyls. Moreover, we find that the surface structures involving ionized Zn adatoms can be stabilized already in much more oxygen-poor conditions, compared to these involving its direct neighbors in the periodic table (Cu and Ag). This Zn-specific feature, driven by a relatively large ZnO formation enthalpy, is also responsible for the stabilization of the mixed Zn/hydrogen surface structure on the oxygen-rich terminations, which is absent in the cases of Ag and Cu.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Claudine Noguera and Jean-Michel Mataigne for fruitful discussions and acknowledge the financial support from ArcelorMittal Maizières Research (R.C.).

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REFERENCES

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