the Competition between Planar and Polyhedral ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 7420-7430

The Growth of Copper Clusters over ZnO: the Competition between Planar and Polyhedral Clusters Samuel A. French,*,†,| Alexey A. Sokol,†,‡ C. Richard A. Catlow,†,‡ and Paul Sherwood§ DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W1S 4BS, United Kingdom, Department of Chemistry, UniVersity College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom, and CLRC Daresbury Laboratory, Warrington, WA4 4AD, United Kingdom ReceiVed: October 8, 2007; In Final Form: December 5, 2007

Hybrid quantum and molecular mechanical calculations have been used to investigate the nucleation and growth of copper clusters on the (0001)-Zn polar surface of ZnO. Our method is based on the embedded molecular cluster approach developed to study point defects in polarizable ionic solids, where we make use of the chemically accurate exchange and correlation density functional, B97-1, for the treatment of interactions between the metal cluster and oxide support. Following the initial seeds of cluster growth at the anchor sites on the zinc terminated surface, we identify distinct families of structures for Cu4-Cu7 in the main oxidation states and show when the clusters move from 2- to 3-dimensional entities. Our calculations corroborate and rationalize the experimental evidence of the predominantly neutral nature of larger clusters, with the positive charge concentrating at the anchor site, which is a copper atom occupying a surface vacant zinc site. We unravel the mechanisms for the planar clusters stabilization around the anchor, with the upper copper atoms effectively wetting the ZnO surface, and for the formation of polyhedral configurations prompted by interaction with electron-rich oxide surface ions. From comparison of both binding and nucleation energies, we predict a dynamical equilibrium between planar and polyhedral cluster morphologies for small copper clusters, which can alternate upon cluster growth and chemical interactions. We conclude that the ability of anchor sites to stabilize copper clusters of different morphologies is the key factor that prevents sintering of supported metal clusters and maximizes the surface area of the catalyst.

Introduction The efficacy of the industrial multicomponent Cu/ZnO/Al2O3 catalyst for the reaction of syngas to methanol is not in doubt, but the fundamental processes are still open to debate.1-35 As the worldwide production of methanol plants has increased from 3000 to 5000 tons per day, with planned projects achieving 7000 tons per day, it is of vital importance to provide as much information as possible to aid future catalyst design. Alternative technologies to produce methanol via direct methane oxidation still appear to be some way off, and the conventional precipitated Cu/ZnO/Al2O3 catalyst remains the best currently available. Ongoing challenges include: (i) increasing the catalytic activity by stable high copper surface areas and promoters, (ii) longer catalyst life through resistance to sintering and poisoning processes, and (iii) improved reactor design to maximize heat efficiency and output. While the latter is beyond the scope of the present paper, there is no doubt that by providing the fundamental knowledge of the binding processes of copper to zinc oxide, we will be able to lay the bedrock for future studies that will certainly be able to address the first two issues. In recent years, experimental surface microscopic techniques have progressed towards near atomic resolution, yielding direct confirmation of the formation, structure, and properties of Cu clusters supported on ZnO, in real and model catalytic systems. * To whom correspondence should be addressed. E-mail: [email protected]. † Davy Faraday Research Laboratory. ‡ University College London. § CLRC Daresbury Laboratory. | Current address: JMTC, Sonning Common, Reading, RG4 9NH, UK.

Most recently, the growth of copper on single crystalline ZnO has been investigated by Dulub et al.36 using state-of-the-art scanning tunneling microscopy (STM), complemented by lowenergy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and low-energy He+ ion scattering (LEIS). In particular, they reported that copper grows as 2D clusters at low, 0.001-0.05 equiv monolayers coverage; while at higher (>0.01 ML) coverages 3D clusters start to develop. Remarkably, not only the presence of nanoclusters, but also the change in cluster morphology in situ during exposure to different components of syngas has been investigated using high-resolution transmission electron spectroscopy (HRTEM) by Helveg and Hansen.37 Furthermore, both the STM and HRTEM observation of well-defined and clearly separated clusters on the Znterminated surface of ZnO allowed the work of adhesion of Cu to the support (3.4 ( 0.1 J/m2) to be measured.38,39 Our previous calculations, using a density functional theory (DFT) based embedded cluster approach with a state-of-the-art hybrid exchange and correlation functional have shown that the favored growth mechanism for clusters was for copper atoms to be adsorbed on top of a copper atom anchored in the vacant zinc interstitial surface site (VZISS), shown in Figure 1.40 We predicted that the ground state for copper should be Cu0 but with Cu+ also being possible. However, it was noted that as the cluster size increased the higher-charge species became energetically feasible. The clusters formed for Cu0 and Cu+ were predominantly planar for Cu4. However, on increasing the charge, the position of the copper anchor in the VZISS became lower, leading to a stronger interaction of Cu with the surface ions, which drives the formation of a Cu42+ tetrahedron. We

10.1021/jp709821h CCC: $40.75 © 2008 American Chemical Society Published on Web 04/18/2008

The Growth of Copper Clusters over ZnO

Figure 1. The Cu anchor site on Zn-(0001) surface of ZnO. Atoms in ball-and-stick representation form the QM region embedded in the surface model. Only the topmost surface layer is shown. Red balls are oxygen, gray balls are zinc, and the copper ball is copper; the same color scheme is used throughout the paper.

also published a short description of the structures and nucleation energies for Cu2+ clusters from Cu1-7 41 and used a planar Cu70 prototype cluster for the study of methanol synthesis.42,43 The work of Dulub et al. has confirmed our earlier conclusions for the smallest sizes of clusters at low coverages, showing the cationic nature of Cu. The ZnO substrate is shown to influence strongly the mode of growth, as can be seen by the competition between 2D and 3D clusters. More recently, Meyer and Marx used a periodic slab DFT model to explore the structure and electronic properties of Cu adsorbed on the polar oxygen and zinc-terminated surfaces of ZnO.44 A somewhat simpler and computationally significantly less demanding generalized gradient approximation (GGA) was applied within the DFT formalism along with the plane wave augmented by atomic localized orbitals basis, which allowed investigation of a number of different regimes of adsorption starting with single Cu atoms, moving to monolayers and thin films. In spite of the high quality of these calculations, there remains a doubt as to the reliability of the approach due to a significant underestimation of the electronic band gap for ZnO by the GGA DFT. These problems could significantly affect the predictions of the oxidation state of copper and the binding between the copper and support, which are of primary interest in the present work. Other authors have considered models of the pure (111) surface of copper to examine the reverse reaction of the decomposition of methanol, as in the steam reforming process.45 As the primary active sites are considered to be on copper, and a source of hydrogen is not of consequence, inclusion of ZnO is probably not vital in these simulations. However, it is well known that the Cu/ZnO system is essential for achieving maximum activity in the formation of methanol, which is why we consider it imperative to look at the interface between metal and metal oxide.1-35 It is of interest to compare supported and unsupported Cu clusters, as high-quality experimental46-48 and computational49,50 data become available for smaller gas-phase atomic-size clusters. In particular, the work of Jug et al.49,50 has allowed us to avoid the computationally expensive search of configurational space that would be required to ascertain ground state structures for large gas-phase clusters of copper. We have performed a subset of calculations using the same level of theory and basis set size as for our quantum and molecular mechanical (QM/MM) calculations and found good agreement in final structural properties with those reported by Jug et al. In this paper, we have focused on exploring the potential energy surface for clusters in various oxidation states and exploring the competition between surface and metal. While an exhaustive examination of the potential energy surface is not feasible owing to the computational cost, we have chosen representative starting points to generate specific energy minimum structures, allowing us to compare the relative energies

J. Phys. Chem. C, Vol. 112, No. 19, 2008 7421 of the optimized structures. Using this strategy we can investigate the growth trends for polyhedral and sheetlike structures and the degree of copper interaction with the surface. The electronic structure of supported clusters is then examined focusing on the efficacy of the clusters in trapping charge carriers from the valence and conduction bands of the ZnO support. With the size of cluster that we are able now to present in this work, we can begin to see the initial seeds of the dominant facets of copper metal, namely, the (111) and (110) surfaces. Methodology We have given a detailed discussion elsewhere40,51 of both the technique of hybrid QM/MM modeling and embedding52 as well as the specific details of the solid-state embedding scheme employed in this work. We repeat that in our QM calculations we are using the hybrid exchange and correlation functional B97-153,54 and TZV2P55 basis set for copper, zinc, and oxygen atoms, while our MM model is based on pairwise interatomic potentials along with the shell model and formal ionic charges.56 The interface between QM (GAMESS-UK57) and MM (GULP58) codes is realized by the computational chemistry environment code ChemShell.52 In this work, we have concentrated on the Zn terminated (0001) surface of ZnO. The polar character of this surface necessitates surface reconstruction, which results in approximately 1/4 of all Zn ions in the topmost surface layer being removed. Resultant isolated hollow sites, VZISS, which accommodate the Cu anchor for cluster nucleation, are shown in Figures 1 and 2a. The base structure used for building all the larger clusters was a copper tetrahedron with one ion in the anchor position as this was the most stable structure for the Cu42+ and would be the best starting point for building clusters, approximating the (111) and (110) Cu surfaces (see Figure 2b). The embedded clusters considered in this work include 13 atoms of the support, surrounding the VZISS, and all adsorbate atoms in the QM region; all cations within 3 Å from the QM region are placed in the interface region, bearing all-electron pseudopotentials;59 all other atoms within 13 Å from the VZISS form the active MM region, where all centers are allowed to relax fully, beyond which a further 12 Å-thick hemispherical shell provides the “frozen” MM region. This and similar clusters that represent characteristic ZnO surface sites were previously used to study methanol synthesis,42,43 H2 adsorption,60 and surface-point defects.51 All supported Cu clusters have been considered in three main oxidation states (0, I, and II) in polyhedral and planar configurations. In a number of cases, more than one alternative polyhedral conformation was found, and unless stated otherwise, we present here the lowest energy structures. Results and Discussions Outline of 3D Families. The surfaces of copper metal (a facecentered cubic (fcc) structure) that are of most interest are the (111) and (110); we have therefore built clusters exhibiting facets approximating the initial growth of these surfaces. The Cu42+ tetrahedral unit was used as a seed for building all larger clusters as its structure approximates well both the (111) and (110) surfaces. The (111) surface is smooth exposing a triangular lattice, as shown in Figure 3a. The surface offers the following adsorption sites: on-top, bridging sites between two atoms and hollow sites between three atoms. The corresponding clusters have a planar configuration, in which the atoms remain in contact with the substrate and form a layer parallel with the

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Figure 2. (a) The Cu anchor site on Zn-(0001) surface of ZnO (top view). In this example, the site consists of one Cu0 atom occupying a vacant Zn surface site. Atoms in ball-and-stick representation form the QM region embedded in the surface model. Only the topmost surface layer is shown. The distribution of Cu valence electrons is illustrated with an isoelectron-spin density plot (in cyan at 0.0060, 0.0030, and 0.0015 e/Å3). (b) The Cu42+ tetrahedral cluster used in this work as a seed for construction of larger clusters.

ZnO surface (also known as surface wetting). These clusters were built up by consecutive addition of copper atoms to form triangles with pairs of the upper copper atoms of the base tetrahedron, creating a Cu7 cluster, which has six copper atoms in the upper layer forming a large triangle and one copper ion anchoring the cluster to the surface, as shown in Figure 4c. They approximate the (111) surface structure. In contrast the (110) surface has a corrugated profile, as shown in Figure 3c, with the second layer copper atoms being exposed, due to the substantial gap between surface ions. The surface offers a wide variety of possible adsorption sites, including: on-top and short bridging sites between two atoms in a single row, long bridging sites between two atoms in adjacent rows, and higher coordination sites (in the troughs). For polyhedral clusters, the first copper adatom is placed centrally above the upper triangle leading to a diamond configuration, which is an arrangement of atoms in the hexagonal close-packed (hcp) phase and contrasts to the fcc stacking shown in the bulk. The second copper atom is added to one of the faces of the triangles formed by the first adatom and the upper copper atoms of the base cluster. The third adatom

French et al. is then added to one of the other triangles. After the anchor atom is stabilized in the surface vacant site, these clusters have been built minimizing interaction between additional copper atoms and the ZnO surface. To this end, the polyhedral clusters that have been built have a single exposed ion at the apex of the cluster and triangular faces sloping down to atoms in the lower layer. In our previous publication we introduced gas-phase results for clusters of copper; larger gas-phase clusters of copper have been studied by Jug et al.49,50 We note that these initial positions are only starting configurations, and no constraints are used during geometry optimization. In some cases, there are large movements of the ions away from these initial states, but we still categorize the model by its initial configuration. Neutral Clusters. Planar. The first copper atom added to the base unit is sited directly between two zinc ions and above a surface oxygen ion, thus optimizing the interaction with the support and specifically with surface zinc ions. Figure 4 shows the neutral clusters for Cu50, Cu60, and Cu70, which, although being based on a tetrahedral cluster, expose a planar face in the form of a parallelogram for Cu50, trapezium for Cu60, and triangle for Cu70, which in each case is joined to the surface at the anchor site. The copper bond lengths (2.47 Å) between the upper four copper atoms are on average shorter than those (2.62 Å) between the upper copper atoms and the anchor. We find that the Cu7 planar configuration is triangular in all charge states and presents a facet that closely resembles the (111) surface of the pure metal shown in Figure 3a; the neutral cluster, however, remains the most nearly planar with an exposed surface area (S ) 1/2a2 sin 60°, where a is the side length of the equilateral triangle, which we can take as 3 CuCu distances, taking into account the size of Cu atoms; with 2 Cu-Cu distances, it would be 11 Å2) of ∼24 Å2. In contrast, the outer copper ions lift away from the substrate in the cases of Cu7+ and Cu72+ as described below. Polyhedral. Upon addition of a copper atom in the third layer, directly above the anchor ion, the atom is displaced toward two of the copper atoms in the second layer forming an upper triangle, shown in Figure 5. The most stable structure for Cu50 is polyhedral, which contrasts with Cu60 where the planar geometry is favored, while in the case of Cu70 again the polyhedral cluster is more stable than the planar form. Interestingly, this result would suggest that an open shell configuration of the cluster favors the polyhedral geometry while the planar arrangement has lower energy when all spins are paired. The Cu6 cluster has a parallelogram in the second layer inclined to the surface and still with a single atom in the third layer; however, this atom now interacts with four copper neighbors. The most weakly bound atom moves aside to accommodate a further copper atom in the upper layer, which completes the Cu7 cluster. The upper layer is ∼4.5 Å above the anchor ion. The cluster is built of tetrahedra which are not, however, perfectly symmetrical, either as individual units or as a whole. Insight into the electronic structure and bonding in the clusters is given by the Mulliken spin populations for the Cu70 planar and polyhedral clusters. We find that the charge and spin on the anchor ion change very little between planar and polyhedral clusters and are lower than on any of the copper atoms in upper layers. The planar cluster has a slightly higher average charge of -0.1 e per atom (which is transferred from the surface) compared with the value of close to zero on the copper atoms in the polyhedral cluster. The charge shift shows, unsurprisingly, that the planar clusters interact more with the ZnO surface than


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Figure 3. Copper surfaces: (a) top left (111); (b) bottom left (110); (c) right-side view of (110). Bulk terminations have been used in all cases.

Figure 4. Planar clusters of oxidation state 0: (a) Cu50, (b) Cu60, (c) Cu70.

Figure 5. Polyhedral clusters of oxidation state 0: (a) Cu50, (b) Cu60, (c) Cu70.

do the polyhedral clusters. Copper atoms in the middle layer of both planar and polyhedral clusters pick up most charge from the surface oxygen ions, with the maximum charge on copper being -0.5 e for an atom in the middle layer of the planar cluster. Clusters with Charge +1. Planar. As previously shown, for smaller charged copper clusters,40 the anchor is deeper in the VZISS than for Cu0 clusters and interacts more strongly with surface oxygen ions. As the cluster size increases, the structure of the upper copper layer is also driven to maximize the interactions with surface oxygens tilting the cluster away from a plane parallel to the surface (Figure 6). The most stable

Cu5 cluster has the planar configuration, although there is still a copper atom in an upper layer, that is, 4.3 Å above the anchor copper ion with all ions being in a plane perpendicular to the surface. For the Cu6 cluster, the upper atoms rise forming a distorted trapezium structure in the upper layer. The seventh copper atom completes a distorted triangular facet with one corner pulled down to the surface, by interaction with oxygen, while the other two corners are folded up away from the zinc ions. Comparison of the degree of corrugation of the surface as a function of charge state is described below. For singly charged copper clusters, the charge is largely associated with the anchor ion; the trend in stability does not


French et al.

Figure 6. Planar clusters of oxidation state I: (a) Cu5+, (b) Cu6+, (c) Cu7+.

Figure 7. Polyhedral clusters of oxidations state I: (a) Cu5+, (b) Cu6+, (c) Cu7+.

Figure 8. Planar clusters of oxidation state II: (a) Cu52+, (b) Cu62+, (c) Cu72+.

correspond to the change in the spin state as for the neutral clusters. The planar configuration is more stable for Cu5 and the polyhedral configurations are more stable for Cu6 and Cu7, although we note that the differences in energy between the configurations are an order of magnitude lower for all charge states of Cu5 clusters than those for Cu6 and Cu7 clusters, where the differences between planar and polyhedral configurations range from 0.15 to 0.55 eV. Polyhedral. The addition of a single copper atom, to create a Cu5 cluster, causes one of the copper atoms in the second layer to detach from the initial tetrahedral arrangement and lift up toward the adatom leading to the formation of triangles maximizing interaction of the unpaired spins on the upper copper atoms (Figure 7). In this configuration, one oxygen ion is pulled up out of the surface, interacting strongly with the surface copper ion (1.97 Å) and also with one of the copper atoms in the second layer (1.99 Å). A further copper atom added in the third layer falls over to the center of one of the triangles forming an offset tetrahedron but remains in the upper layer. Interestingly, upon formation of the Cu7 cluster, the copper anchor has become partially detached from the surface, and the copper to oxygen bond lengths are 2.03, 2.04, and 2.78 Å in contrast to Cu6 bond

lengths of 1.90, 1.93, and 2.47 Å. Also there is another Cu-O bond length of 2.0 Å showing that the cluster has not just lifted up but has rolled over to interact at more points with the surface rather than just at the anchor, resulting in a distorted polyhedral structure. Clusters with Charge +2. Planar. For clusters with a +2 charge, there is a tendency for copper to lift away from the surface, particularly in the case of Cu62+ where an atom leaves the second layer and lifts up into the third layer, as shown in Figure 8. (Cu62+ is the only example where two surface oxygen ions are pulled up from the surface layer.) In the case of Cu72+, the two copper atoms at the extremes of the cluster lift away from the surface displaying more of a Cu (110) facet. However, the size of the cluster and the interaction with the surface prevent the cluster from forming the optimum angle of 120° between the copper atoms in the upper layer and those in the second layer, constraining it to 127°, as shown in Figure 9c. The degree of interaction between the cluster and the surface can be correlated with the movement of the outer Cu ions during geometry optimization. In Figure 9, as an example, we compare the three planar Cu7 clusters in 0, I, and II oxidation states.


Figure 9. Planar Cu7 clusters: (a) Cu70, (b) Cu7+, (c) Cu72+.

Clearly from the figure, it can be seen that there is a trend for the outer copper ions to minimize interactions with the surface. The Cu-Cu-Cu angle changes from 171, to 131, to 126° when moving from 0 to II oxidation state. Polyhedral. The Cu52+ cluster is the closest to a trigonal bipyramidal configuration of geometry optimized structures, as shown on the left-hand side of Figure 10. The copper atoms remain very close to where they are placed at the start of the optimization for Cu6 and Cu7, which is not the case for any of the other charge states, as there is a large drive for copper ions to maximize the separation from the surface. In all cases, there is a single oxygen ion pulled out of the surface that interacts strongly with the copper cluster. The relative energy difference between the planar and polyhedral clusters is most marked for Cu2+ clusters, for which it is as large as 0.59 eV, with the polyhedral clusters always being more stable. The two clear advantages of polyhedral clusters over planar clusters are: (i) the polyhedral configuration minimizes the interactions of copper with the surface, and (ii) the polyhedral configuration maximizes the copper to copper interactions allowing the charge to be shared over the maximum number of copper ions. We have shown how the cluster morphology is affected by the change in oxidation state. For charge 0 species, electron pairing dominates and defines the morphology; charge I species are intermediate between 0 and II, and charge II species favor polyhedral species in all cases, as there is maximum overlap between copper ions. In the following section we will compare the stability of the different clusters with respect to electron gain or loss to or from the support. Stability of Charge States. Having defined the minimum energy configurations for each charge state, we now consider which are stable with respect to the potential loss or gain of charge carriers to the valence or conduction bands of the ZnO support. The electronic stability of the system with regard to charge transfer can be ascertained by mapping one-electron energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the copper clusters, as shown in Figure 11. For the charged states we note that oneelectron states are obtained by solving Kohn-Sham equations

J. Phys. Chem. C, Vol. 112, No. 19, 2008 7425 without a contribution to the embedding potential from the polarized surface beyond a 13 Å cutoff radius.51 Furthermore, the conduction states in ZnO are much more strongly delocalised than the valence states with the electron having an effective radius in excess of 15 Å compared to a hole localized within the first coordination sphere of an oxygen ion (see ref 61 and references therein). Therefore, within our embedded cluster approach, the conduction states are reproduced relatively poorly compared to the valence states. To amend this deficiency we introduced a uniform shift of the conduction states downward so that the band gap in our simulations matched the experimental room-temperature value of 3.37 eV. This approximation has been introduced in our earlier work on supported Cu clusters;40 it is further discussed in ref 61, where an alternative procedure for the calculation of electronic levels in ZnO is also given. This consideration shows that the VZISS occupied by a single Cu atom is a special case, where a Cu 4s valence electron will be lost to the conduction band spontaneously. As soon as the next Cu atom is adsorbed on the site the HOMO energy of the Cu cluster drops below the bottom of the surface conduction band, below which it remains as the cluster grows (giving rise to deep-donor states), irrespective of whether the cluster acquires planar or polyhedral morphology. After an initial drop of the HOMO level we observe two overall trends: (i) a gradual reduction in the gradient for both HOMO and LUMO and (ii) the simultaneous narrowing of the gap between cluster HOMO and LUMO levels. More apparent, however, is the oscillation of HOMO and LUMO when moving between clusters with an odd and even number of electrons, showing how electron pairing dominates the stability of Cu(0) clusters, corroborating what we calculated for cluster morphologies. As the LUMO level remains within the conduction band, thermally or chemically activated excitation electrons would be primarily transferred to the conduction band of the support, the energy decreasing with cluster size. With this trend continuing one could expect that larger neutral Cu clusters, beyond a certain size, may again become a source of at least shallow donor states. The observations are in line with experiment where a weakly cationic state was reported for copper on the Zn terminated surface of ZnO at a low coverage limit,36 while with an increase in Cu coverage, neutral copper was found to dominate the system. As we have shown previously,40 during Cu deposition, in the first instance the vacant Zn sites are occupied, creating a positive surface layer with electrons accumulating under the surface. However, as the clusters grow this behavior will change giving rise to the predominantly neutral species, since on adsorption of further Cu atoms the positively charged clusters will form excellent traps for any free charge carriers in the conduction band (see parts a and b of Figure 11). We can also envisage that in the presence of electron scavengers (under oxidizing conditions),

Figure 10. Polyhedral clusters of oxidations state II: (a) Cu52+, (b) Cu62+, (c) Cu72+.


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Figure 11. Positioning of copper cluster (Cun, n ) 1-7) HOMO and LUMO energy levels with respect to Zn-(0001)-ZnO surface bands. (Energy in eV.) (a) Neutral clusters, (b) singly charged clusters, and (c) doubly charged clusters. For Cu1-4 the trends are superimposed as there is no distinction between planar and polyhedral morphologies.


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TABLE 1: Atomization Energies (or Binding Energies per Atom; in kcal mol-1) of Neutral Copper Clusters (See Ref 50 for Further Detail) cluster

this work

LDA/GGA50

experiment

Cu2 Cu3 Cu4 Cu5 Cu6 Cu7

24.1 24.4 32.2 34.6 38.9 40.8

24.2 22.9 32.6 34.8 38.9 40.1

23.5 ( 1.96 (24.0) 24.5 ( 2.86 (23.4) 34.0 ( 3.29 35.8 ( 3.40 39.6 ( 4.14 42.7 ( 5.00

or while a sufficient electrostatic field is maintained between the topmost surface layer and the bulk (at low copper coverage), singly charged copper clusters would coexist with neutral copper clusters. We have previously suggested that Cu2+ species in the ground state form a Cu+ complex with a trapped surface hole polaron, h+. As Figure 11c demonstrates, trapping of a hole is, however, an unfavorable process, which results in an electronically unstable species; for smaller clusters, Cu1-3, the LUMO level of Cu clusters drops below the top of the valence band, which implies that the hole would recombine with any available valence electron of sufficiently high energy, i.e., whose energy is not strongly lowered by interaction with the Cu+. In other words, the hole would hop from the singly charged Cu cluster as could be expected on purely electrostatic grounds. However, in this work we show that starting with Cu4 there appears to be a window in which planar Cu62+ and polyhedral Cu52+ and Cu62+ are stable. In fact, the LUMO level of the polyhedral Cu62+ cluster rises even higher than the corresponding energies of the singly charged and neutral states, which points at its particular stability. For Cu72+, the LUMO levels again dip below the top of the valence band but by only ca. 0.1 eV for the polyhedral cluster. The oscillating character of the HOMO and LUMO energy and its dependence on cluster size, for the polyhedral family, can be expected to carry on to larger cluster sizes. In contrast, the LUMO of the planar clusters after an initial rise, peaking for Cu4, steadily drops. As the planar cluster grows, the next Cu adatoms will be trapped in one of the nearest vacant surface sites, which would change this trend.

Binding and Nucleation Energies. Using our knowledge of the one-electron spectra of the clusters, it is of interest to compare the energetics of the many-electron system. We have not performed an exhaustive study of gas-phase copper clusters, but we have optimized clusters and compared their configurations with those obtained by Jug et al.49,50 We can compare atomization energies obtained for our optimized neutral gasphase clusters with experimental work and the previous calculations of Jug et al., see Table 1. (To make computational data directly comparable, zero-point vibrational energy contributions have been removed from the LDA/GGA values reported in ref 50.) We find good agreement with all previously reported data, which allows us to be confident in the predictive ability of the current approach when extended to the supported clusters, for which no experimental data exist. We have calculated the binding energies of the cluster to the substrate for the neutral and +1 charge states. (The meaningful calculation of binding energies for clusters with higher charge states is questionable, as the reference state clusters in the gas phase are frequently unstable.) We have previously defined binding energies and discussed the trends for smaller clusters;40 therefore, we will focus here on clusters larger than Cu4. From Figure 12 it is clear that the binding energies plateau beyond Cu4 for both charge states confirming our previous conclusions that the interaction with the surface is dominated by the anchoring energy. We, therefore, predict that the plateau will continue until the cluster next encounters a surface vacancy that could act as an additional anchor site. It is of interest to note that there is little dependence on cluster morphology when calculating binding energy as can be seen from the similarity in binding energies for the planar and polyhedral geometries. For the largest planar clusters, we calculated the binding energies per cluster area as 1.7 J/m2 for Cu70 and 3.7 J/m2 for Cu7+, which bracket the experimentally observed value of the adhesion energy of 3.4 J/m2, reported for larger clusters. This finding corroborates the validity of our model, with a predominantly neutral cluster and a charged anchor.

Figure 12. Binding energies for copper clusters to the ZnO substrate. 9, neutral planar copper clusters; 2, neutral polyhedral, (, +1 planar, ×, +1 polyhedral. Cu1-Cu4 as previously described in ref 40 have only been considered in a single morphology.


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Figure 13. Nucleation energy of copper clusters (Cun, n ) 1-7) supported on Zn-(0001)-ZnO surface for (a) polyhedral and (b) planar clusters.

It is however of more interest to compare cluster growth from an anchor atom in different charge states by considering the addition of single atoms. We previously characterized this process by the nucleation energy:

Enuc(n) ) -[Eads(Cun+1(0,+1,+2)/ZnO) - E(Cu) Eads(Cun(0,+1,+2)/ZnO)] which gives the energy of adding an extra single copper atom, E(Cu), from the gas phase, to a pre-adsorbed copper atom or cluster. In our previous study40 (the results of which are also included here) we showed that a neutral copper atom, although relatively weakly bound to ZnO, provides the most energetically favorable adsorption site for further cluster growth. It is clear from the variation of the nucleation energy for planar and polyhedral clusters, shown in parts a and b of Figure

13, that the strength of cluster nucleation is dominated by the electronic structure and is strongly dependent on whether the adatom leads to all spins being paired, which is always the most energetically preferred state. For the larger clusters, of primary interest here, this behavior results in Cu5 and Cu7 clusters following one pattern, with Cu6 being opposite. There is only one case where this statement does not hold good and that is for the Cu22+ cluster, for which the gas-phase cluster is unstable, and we would anticipate that while partially stabilized by the surface this species would quickly acquire electrons from the surface to form a lower charged cluster. We have divided the clusters into two groups, planar and polyhedral, for Cu5 to Cu7 and would need to continue our study to larger clusters to be able to clarify the observed trends, but from our current results, we can see that the change between open- and closed-shell spin states of the cluster leads to an

The Growth of Copper Clusters over ZnO oscillation of the nucleation energy upon introduction of each open shell copper atom. Other features of interest can be described by (i) comparing trends in nucleation energies for the same charge state between the two groups and (ii) comparing all charge states within a group. First, it can be seen that, when comparing Cu0 clusters for both groups, there are large oscillations although they seem to be smaller for the polyhedral group. For Cu+ we can identify a constant oscillation for the planar group with no apparent convergence, which is in contrast to the polyhedral group where there is less oscillation indicating convergence of the nucleation energy irrespective of the spin state of the cluster. For the larger clusters of copper it is clear that the polyhedral group is able to accommodate changes in the spin state resulting from a change in charge state, while the nucleation energy for the planar group remains divergent. The Cu7 cluster exemplifies this behavior with only a 0.5 eV separation in nucleation energy between the Cu70 and Cu7+ polyhedral clusters, while for the planar Cu7 clusters this difference is more than 1 eV. We note that, the polyhedral group is able to accommodate the changes in charge and spin state as all copper atoms interact with the maximum number of other copper atoms meaning that all atoms in the cluster can share charge and spin density. The interaction of ions with the surface is also greatly reduced for polyhedral morphologies, also leading to an enhancement in copper-copper interactions. For the planar configurations, the larger oscillation is due to more ions interacting with the surface, therefore localizing electrons on particular copper atoms, effecting a greater change in stabilization when electrons are paired or unpaired. Interestingly, the nucleation energy for Cu0 adatoms forming polyhedral clusters, upon a Cu2+ anchor, becomes constant before dropping for the Cu7 cluster. Discussion and Conclusions Generally, Cu(0) clusters are electron rich (as any normal metal) and therefore naturally repel oxide anions, while being attracted to Zn cations. As we showed for positively charged Cu1-4 clusters, the charge predominantly localizes on the anchor Cu atom, leaving the second layer of Cu atoms largely neutral. As a result, upper Cu atoms still tend to bind strongly to positive Zn2+ ions, but there also occurs a new trend of interaction with surface oxygen ions closest to the Cu anchor. This binding can be understood as a result of a polarization of oxide anions by the Cu+ ion located in the surface plane. On increasing the charge of the cluster to 2, we observed electron transfer from oxide anions to the anchor, with an electron hole being localized in a hemispherical shell around it. Thus the Cu atoms in the second cluster layer can now bind strongly not only to available Zn ions but also to those oxygen ions that bear the hole. This latter process could be responsible for the tendency towards the formation of polyhedral clusters. Upon interaction with one or two upper Cu atoms, the hole localizes, forming a partial copper oxide. If this were to be the case, other Cu atoms would nucleate around these Cux+ centers effectively destroying the planar structures.40 Our study into the growth of copper clusters has shown the propensity to form polyhedral or planar clusters is controlled at the early stage of nucleation by the charge and spin state. We would therefore expect there to be a dynamical shift upon addition of copper, which would only converge at larger cluster sizes. With regard to catalysis, it is clear that the changes in morphology require little energy, and therefore we would

J. Phys. Chem. C, Vol. 112, No. 19, 2008 7429 anticipate that, during the catalytic cycle, the copper clusters would dynamically adapt to accommodate any changes in charge and spin states of the cluster or the surrounding support. It is clear though why the polar surface of ZnO is the catalytically active face as it is the only one with sufficient anchoring sites for the cluster, allowing it to wet the surface of the support and provide the maximum surface area. The anchor sites act to reduce the sintering of particles to much larger sizes by the tethering of the clusters to the surface. Acknowledgment. We gratefully appreciate discussions with Prof. K. Waugh, Prof. R. Schlo¨gl, Dr. S. Rogers, Prof. F. King, and Dr. M. Watson. The work would not have been possible without the initial studies of Dr. S. Bromley who is thanked for his vital contribution. Computational resources for this work have been provided by the HPCx service via our membership of the Materials Chemistry Consortium and funded by the grant EP/D504872. References and Notes (1) Chinchen, G. C.; Waugh, K. C.; Whan, D. A. Appl. Catal. 1986, 25, 101. (2) Sankar, G.; Vasudevan, S.; Rao, C. N. R. J. Chem. Phys. 1986, 85, 2291. (3) Didziulis, S. V.; Butcher, K. D.; Cohen, S. L.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 7110. (4) Solomon, E. I.; Jones, P. M.; May, J. A. Chem. ReV. 1993, 93, 2623. (5) Yoshihara, J.; Parker, S. C.; Schafer, A.; Campbell, C. T. Catal. Lett. 1995, 31, 313. (6) Yoshihara, J.; Campbell, C. T. J. Catal. 1996, 161, 776. (7) Harikumar, K. R.; Santra, A. K. Solid State Commun. 1996, 99, 403. (8) Harikumar, K. R.; Santra, A. K.; Rao, C. N. R. Appl. Surf. Sci. 1996, 93, 135. (9) Nakamura, J.; Uchijima, T.; Kanai, Y.; Fujitani, T. Catal. Today 1996, 28, 223. (10) Klenov, D. O.; Kryukova, G. N.; Plyasova, L. M. J. Mater. Chem. 1998, 8, 1665. (11) Yoshihara, J.; Campbell, J. M.; Campbell, C. T. Surf. Sci. 1998, 406, 235. (12) Yoshihara, J.; Parker, S. C.; Campbell, C. T. Surf. Sci. 1999, 439, 153. (13) Jansen, W. P. A.; Beckers, J.; Heuvel, J. C. v. d.; Denier v. d. Gon, A. W.; Bliek, A.; Brongersma, H. H. J. Catal. 2002, 210, 229. (14) Fons, P.; Nakahara, K.; Yamada, A.; Iwata, K.; Matsubara, K.; Takasu, H.; Niki, S. Phys. Status Solidi B 2002, 229, 849. (15) Rozovskii, A. Y.; Lin, G. I. Topics Catal. 2003, 22, 137. (16) Kulkarni, G. U.; Rao, C. N. R. Topics Catal. 2003, 22, 183. (17) Nakamura, J.; Choi, Y.; Fujitani, T. Topics Catal. 2003, 22, 277. (18) Caroll, M. C.; Skrotzki, B.; Kurtz, M.; Muhler, M.; Eggeler, G. Scr. Mater. 2003, 49, 527. (19) Raimondi, F.; Wambach, J.; Wokaun, A. Phys. Chem. Chem. Phys. 2003, 5, 4015. (20) Raimondi, F.; Schnyder, B.; Ko¨tz, R.; Schelldorfer, R.; Yung, T.; Wambach, J.; Wokaun, A. Surf. Sci. 2003, 532-535, 383. (21) d’Alnoncourt, R. N.; Kurtz, M.; Wilmer, H.; Lo¨ffler, E.; Hagen, V.; Shen, J.; Muhler, M. J. Catal. 2003, 220, 249. (22) Wagner, J. B.; Hansen, P. L.; Molenbroek, A. M.; Topsøe, H.; Clausen, B. S.; Helveg, S. J. Phys. Chem. B 2003, 107, 7753. (23) Reubroycharoen, P.; Vitidsant, T.; Yoneyama, Y.; Tsubaki, N. Catal. Today 2004, 89, 447. (24) Ressler, T.; Kniep, B. L.; Kasatkin, I.; Schlo¨gl, R. Angew. Chem., Int. Ed. 2005, 44, 4704. (25) Sun, Q. Stud. Surf. Sci. Catal. 2004, 147, 397. (26) Saito, M.; Murata, K. Catal. SurVeys Asia 2004, 8, 285. (27) Waugh, K. C. Solid State Ionics 2004, 168, 327. (28) Girgsdies, F.; Ressler, T.; Wild, U.; Wubben, T.; Balk, T. J.; Dehm, G.; Zhou, L.; Gunther, S.; Artz, E.; Imbihl, R.; Schlo¨gl, R. Catal. Lett. 2005, 102, 91. (29) Batyrev, E. B.; van den Heuvel, J. C.; Beckers, J.; Jansen, W. P. A.; Castricum, H. L. J. Catal. 2005, 229, 136. (30) Yang, R. Q.; Fu, Y. L.; Zhang, Y.; Xu, B. L.; Tsubaki, N. Bull. Chem. Soc. Jpn. 2005, 78, 135. (31) Manzoli, M.; Chiorino, A.; Boccuzzi, F. Appl. Catal. B 2005, 57, 2005. (32) Wang, J.; Burghaus, U. J. Chem. Phys. 2005, 123, 184716.

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