1712
J. Phys. Chem. B 1999, 103, 1712-1718
Ab Initio Theory of Metal Deposition on SiO2. 1. Cun (n ) 1-5) Clusters on Nonbridging Oxygen Defects Nuria Lopez and Francesc Illas Departament de Quimica Fisica, UniVersitat de Barcelona, Marti i Franques 1, 08028 Barcelona, Spain
Gianfranco Pacchioni* Istituto Nazionale di Fisica della Materia, Dipartimento di Scienza dei Materiali, UniVersita` di Milano-Bicocca, Via Emanueli, 15 - 20126 Milano, Italy ReceiVed: October 8, 1998; In Final Form: December 18, 1998
The nucleation of small Cu clusters at the surface of SiO2 has been investigated by means of ab initio quantum chemical calculations using cluster models. The interaction of Cun clusters (n )1-5) with a nonbridging oxygen, NBO, a paramagnetic point defect of silica, has been studied by means of DFT calculations using the B3LYP exchange-correlation potential. Different from the regular nondefective surface, characterized by a weak adhesion with metal species, strong metal-oxide interface bonds form at the NBO centers. The Cu clusters are bound with one or two Cu atoms with the NBO forming covalent polar bonds. The partial charge transfer to the oxide favors the formation of electrostatic interactions between the metal cluster and the twocoordinated O atoms of the silica surface. Some observable consequences of the cluster nucleation have been studied, including core level shifts and the appearance of new states in the gap of the oxide.
1. Introduction Metal clusters and thin metal films deposited on oxide surfaces represent an important class of materials. In fact, metal/ oxide interfaces are of great technological importance in catalysis, sensors, coatings, electrochemistry, microelectronics, etc.1,2 Supported metal catalysts are the subject of a large number of fundamental and applied studies.3-5 However, despite this large activity, the level of understanding of the microscopic aspects of the bonding at the interface is still rather poor. The problem is connected to the experimental difficulty to be sensitive to the restricted area of the material where the metal/ oxide contacts are formed and to the fact that oxide surfaces are often reconstructed, highly defective, and sometimes nonstoichiometric. In recent years, the possibility to grow thin oxide films on conducting supports under controlled conditions has opened new perspectives for the study of oxide surfaces and supported metal species.6,7 Application of standard ultrahigh vacuum surface science techniques to the study of metal/oxide interfaces has become possible. This has attracted a considerable theoretical interest, and a few first-principle calculations on the bonding of metals on oxides have been reported in the last 3-4 years.8-13 Most of these studies are dealing with nondefective, unreconstructed surfaces. Few examples of theoretical studies of metal deposition on defective oxide surfaces have been reported recently.14-16 In this work, we consider the interaction of small Cu clusters with the deydroxylated SiO2 surface. We performed high-quality ab initio calculations using cluster models of silica. In a previous study16 of the interaction of Cu, Pd, and Cs atoms with the surface of deydroxilated silica, we have shown that the regular sites of the surface, the four-coordinated Si atoms and the twocoordinated O atoms, do not react strongly with these atomic species, forming rather weak bonds with little covalent character. On the other hand, strong bonds are formed with the point
defects of the silica surface.16 These defects are the paramagnetic nonbridging oxygen, NBO, tSi-O•, and silicon dangling bond, tSi•, and the diamagnetic neutral oxygen vacancy, tSi-Sit (the t symbol indicates three Si-O bonds).17-21 Among these defects, the NBO center is probably the most reactive.22 In this study, we have adsorbed on the NBO center a series of small Cu units, from the monomer to the pentamer, and we have compared the structure and the electronic properties of the resulting supported clusters with those of the corresponding free gas-phase clusters. The choice of studying silica-supported Cu clusters is justified by the existence of a number of experimental studies on the structural and chemical properties of silicasupported copper catalysts.23-28 A direct comparison of experimental and theoretical results is not always possible since very high coverages and samples annealed at high temperatures have been considered in the experiments, leading to considerably different situations (e.g., formation of thick layers or suboxides) from those discussed here (very low coverage at T ) 0 K). Nevertheless, the present investigation is the first theoretical attempt to study metal clusters deposited on silica, and the results provide microscopic information on the very early stages of the nucleation process. 2. Computational Approach To describe the NBO center, we used a cluster model whose structure was derived from that of R-quartz.29 The cluster broken bonds have been saturated by H atoms placed at 0.98 Å from the O atoms along the O-Si bond directions of the perfect crystal. The H atoms have been kept fixed during the geometrical optimization to provide a simple representation of the mechanical embedding of the solid matrix. Recently, we have considered a combined quantum mechanical molecular mechanics, QM+MM, approach for the study of adsorption of atomic copper.15 We found that relatively small SiO2 cluster models
10.1021/jp9840174 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/23/1999
Cun (n ) 1-5) Clusters on Oxygen Defects
J. Phys. Chem. B, Vol. 103, No. 10, 1999 1713
Figure 1. Cluster model (HO)3Si-O-Si(OH)2-O-Si(OH)2-O• of a nonbridging oxygen, NBO, point defect on the silica surface. Small spheres represent hydrogen atoms, dark spheres oxygen atoms, large spheres silicon atoms.
give very similar results as larger models or the more sophisticated QM+MM approach.15 The reason is that the bonding of metal atoms at these defect sites is very local and does depend on the nearest neighbors only. In the present work, the model of the NBO center consists of three SiO4 tetrahedra linked together with an unsaturated O atom, (HO)3Si-O-Si(OH)2-O-Si(OH)2-O•; Figure 1. In the optimal structure, the average Si-O distances are of 1.657 Å and the Si-O-Si and Si-O-H angles are of 145.3° and 128.5°, respectively. The distance of the Si atom from the NBO, d(SiO•), is of 1.673 Å. Cun (n ) 1-5) clusters have been studied in the gas phase and supported on the SiO2 surface model. In both cases, a full geometry optimization without symmetry constraint (C1 symmetry) has been performed. In the optimization of the supported metal clusters also, the silica surface atoms have been let free to relax. Only the peripheral H atoms were kept fixed at their original positions. The changes in d(Si-O•) following the cluster deposition are given in Table 1. In general, the adsorption of the metal cluster has little effect on the other geometrical parameters of the SiO2 substrate model; in particular, the average Si-O-Si angles remain around 145°, i.e., very close to the value of R-quartz. The cluster electronic structure has been computed at the spin polarized DFT level using the Becke’s three parameters hybrid nonlocal exchange functional30 combined with the Lee-YangParr gradient-corrected correlation functional31 (B3LYP). The basis sets used are all electron 6-31G*32 on all Si and O atoms and 3-21G33 on the terminal H atoms. For Cu, we used an effective core potential, ECP, which includes explicitly in the valence the 3s23p63d104s1 electrons of Cu.34 The Cu basis sets is [8s5p4d/3s3p2d].34 The adhesion energies of the cluster to the silica surface as well as the cohesive energies of the metal clusters have not been corrected by the basis set superposition error, BSSE.35 The counterpoise method is particularly suited for systems that do not undergo a strong geometrical relaxation after bond formation. However, in the cases discussed here, the
Figure 2. Model of a supported Cu atom (larger sphere) on a NBO center, tSi-O-Cu. A dotted line indicates the existence of a weak bonding interaction with a bridging oxygen of the silica surface.
geometry relaxation of the metal cluster is so large that the counterpoise procedure does not provide a realistic estimate of the BSSE. Nevertheless, in a previous study on isolated metal atoms interacting with the NBO center, we found BSSE’s of the order of 0.5 eV for the bond of a NBO center with a metal atom.16 Since the bonding of the metal cluster to the NBO center is rather local, we expect a similar BSSE for the binding energy of a cluster with the NBO. All the calculations have been performed with the Gaussian94 program package.36 3. Results and Discussion 3.1. Cluster Geometries, Charge and Spin Distribution. We consider first the geometrical parameters of the Cun/SiO2 interface, i.e., the bond distances of the cluster from the surface. All clusters are directly bound to the NBO with similar O-Cu distances of about 1.9-2.0 Å; see Table 1. Due to bond formation with the Cu clusters, the distance of the Si atom directly bound to the NBO decreases in most cases. While Cu2 is anchored to the NBO center through a single Cu atom, the larger clusters are attached through two Cu atoms; see Figures 2-6. As discussed in our previous paper,16 the partial charge transfer from Cu to SiO2 leads to a depletion of electronic charge from the adsorbed metal atom (or cluster). This leads to the appearance of electrostatic interactions between some atoms of the Cu cluster and the two-coordinated bridging O atoms of the silica surface; see Figures 2-6. All clusters with exception of Cu3 show the formation of this electrostatic interaction. The distances between one of the Cu atoms of the cluster and a bridging oxygen (see r(Cu-O2) in Table 1), 2.0-2.6 Å, are always larger than that of the Cu atoms directly interacting with the NBO (see r(Cu-O1) in Table 1), indicating the different character of the two bonding interactions (mostly electrostatic the first, covalent polar the second). For all cases, the Si-O1Cu angle is close to the typical angles in O covalent compounds (105-120°). We discuss now the geometrical form of free and supported Cun clusters and their electronic structure, starting from the case
TABLE 1: Selected Bond Distances and Angles of a NBO Center Interacting with Cun Clusters at the Cu/SiO2 Interface tSi-O• r(Si-O1), Å r(O1-Cu), Åa r(O2-Cu), Åb R(Si-O1-Cu), deg
1.673
tSi-O-Cu
tSi-O-Cu2
1.614 1.869
1.606 1.865
2.090 106
2.256 130
tSi-O-Cu3
tSi-O-Cu4
tSi-O-Cu5
1.676 2.001 2.038 3.033 121-108
1.622 2.017 2.076 2.116 118-115
1.633 1.994 2.004 2.595 115-112
a Shortest distance(s) of nonbridging oxygen, O , with the Cu atom(s) of the cluster. b Shortest distance of a bridging oxygen, O , from a Cu 1 2 atom of the cluster.
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Figure 3. Model of a supported Cu2 cluster (larger spheres) on a NBO center, tSi-O-Cu2. The optimal Cu-Cu distance is given in Å. A dotted line indicates the existence of a weak bonding interaction with a bridging oxygen of the silica surface.
Figure 5. (a) Model of a supported Cu4 cluster (larger spheres) on a NBO center, tSi-O-Cu4. The optimal Cu-Cu distances are given in Å. A dotted line indicates the existence of a weak bonding interaction with a bridging oxygen of the silica surface. (b) Structure and distances in Å of Cu4 and (in parentheses) Cu4+ gas-phase clusters.
Figure 4. (a) Model of a supported Cu3 cluster (larger spheres) on a NBO center, tSi-O-Cu3. The optimal Cu-Cu distances are given in Å. (b) Structure and distances in Å of Cu3 and (in parentheses) Cu3+ gas-phase clusters.
of a single Cu atom; see Figure 2. Since this has been discussed previously,16 here we recall only the general features of the interaction. Cu forms a strong covalent polar bond with the NBO center; the bond arises from the coupling of the unpaired electrons on the O defect and on Cu. The polarization of the bonding electrons toward the NBO leaves a partial positive charge on the Cu atom which interacts, mainly electrostatically, with the lone pairs of the bridging O atoms of the SiO2 network. This results in the formation of a six-membered cyclic structure similar to that observed recently for Ni2+ ions deposited on amorphous silica.37 The addition of a second atom to the t Si-O-Cu complex results in the formation of a supported Cu dimer. The global ground state of tSi-O-Cu2 is doublet, while gas-phase Cu2 has a 1Σg+ ground state. The Cu-Cu distance in the supported molecule, 2.439 Å, is almost 0.2 Å longer than that in the gas phase, 2.260 Å, but still shorter than the bulk distance, 2.56 Å. The spin is entirely localized on the Cu2 unit,
Figure 6. (a) Model of a supported Cu5 cluster (larger spheres) on a NBO center, tSi-O-Cu5. The optimal Cu-Cu distances are given in Å. A dotted line indicates the existence of a weak bonding interaction with a bridging oxygen of the silica surface. (b) Structure and distances in Å of Cu5. (c) Structure and distances in Å of Cu5+.
in particular on the terminal Cu atom. As for a single Cu atom, we observe an increase of electron density on the NBO shown by the Mulliken population, Table 2, and a modest positive
Cun (n ) 1-5) Clusters on Oxygen Defects
J. Phys. Chem. B, Vol. 103, No. 10, 1999 1715
TABLE 2: Charge and Spin Distribution in Free and SiO2 Supported Cun Clusters tSi-O• a
q(Si) q(O)a q(Cu)b spin density a
+1.16 -0.37 0.93 (O)
tSi-O-Cu
tSi-O-Cu2
tSi-O-Cu3
tSi-O-Cu4
tSi-O-Cu5
+1.27 -0.71 +0.13
+1.19 -0.70 +0.11 0.08 (O) 0.89 (Cu2)
+1.29 -0.71 +0.08
+1.33 -0.71 -0.01 0.03 (O) 0.96 (Cu4)
+1.38 -0.71 -0.05
The data refer to the nonbridging oxygen, NBO, and to the nearest-neighbor Si atom. b Average values.
charge on the Cu atom attached to it. No significant change in the charge of the nearest Si atom is found; see Table 2. The external Cu atom interacts weakly with a bridging oxygen, Figure 3, as shown by the short Cu-O2 distance, Table 1. Gas-phase Cu3 has a doublet ground state and is Jahn-Teller distorted.38 The lowest structure is bent, C2V, with an internal angle of 75.7° and Cu-Cu distances of 2.326 Å. Cu3+, on the other hand, is a closed-shell equilateral triangle with Cu-Cu distances of 2.394 Å. The addition of a Cu atom (doublet) to the tSi-O-Cu2 surface complex (doublet) results in a closedshell tSi-O-Cu3 system; see Figure 4. The supported Cu3 unit interacts with two Cu atoms with the NBO center and assumes an almost perfect equilateral triangle geometry with internal angles of 60 ( 1° and Cu-Cu distances of about 2.4 Å; see Figure 4. Thus, both the structure and the Cu-Cu distances of supported Cu3 are similar to those of gas-phase Cu3+. This is consistent with the idea that the coupling of the unpaired electron on NBO with that of the Cu3 cluster leads to a partial charge transfer which makes the supported Cu3 unit more Cu3+-like. The net charge on the NBO is very similar to what we have found for the Cu and Cu2 adsorbed fragments; see Table 2. The third Cu atom of the supported cluster, however, does not interact with the bridging O atoms of the SiO2 surface (the shorter Cu-O2 distance is 3 Å; Table 1). We cannot exclude that other minima exist on the potential energy surface where the Cu3 unit is closer to the SiO2 cluster. Previous theoretical studies have shown that gas-phase Cu4 has a singlet ground state and a planar rhombic structure,38 a result confirmed by the present DFT-B3LYP calculations; Cu4 is a rhombus with peripheral Cu-Cu distances of 2.455 Å; the short Cu-Cu diagonal is 2.308 Å. Cu4+ is a doublet and the lowest structure is a nearly planar rhombus with Cu-Cu distances of 2.472 Å (2.369 Å is the shortest diagonal). To model a supported Cu tetramer, we started the optimization from the optimal tSi-O-Cu3 complex, Figure 4, and we added a Cu atom above the Cu3 plane, in the direction opposite to the surface; the resulting initial geometry is thus that of a tetrahedron. In this initial structure, the Cu atoms do not form bonds with the surface other than with the NBO. During the geometry optimization, the Cu atom capping the triangular face moves toward the surface resulting in a inverted structure. This can also be described as a distorted tetrahedron, Figure 5, with two Cu atoms interacting with the NBO center and one Cu atom which forms a bond with a SiO2 bridging O (see the short CuO2 distance of 2.116 Å, Table 1). The Cu-Cu distances within the tetramer are in the range 2.38-2.61 Å; the structure can be better described as that of a bent rhombus (butterfly) since the largest Cu-Cu distance is rather long, 2.865 Å. tSi-O-Cu4 has a doublet ground state, with the unpaired electron almost entirely delocalized over the four Cu atoms and little spin density on the NBO; Table 2. The net charges on the Cu atoms are close to zero, Table 2, while the NBO has a more negative charge than in the bare surface. This suggests that even if some charge is transferred from the Cu cluster to the NBO, the resulting small positive charge is delocalized over the entire cluster, a sign of developing metallic character. This is consistent
with the analysis of the core level shifts and of the electronic structure of the metal clusters discussed in the forthcoming sections. Finally, we consider free and supported Cu5 units. Free Cu5 has a planar trapezoidal structure obtained by adding a Cu atom in the plane containing the rhombic Cu4. The internal angles are all close to 60°, while the Cu-Cu distances go from 2.404 to 2.475 Å; see Figure 6. Gas-phase Cu5+ assumes a nonplanar structure with two triangular units linked through a central Cu atom and lying in perpendicular planes; see Figure 6. The structure of supported Cu5 has been obtained starting from that of tSi-O-Cu4 and adding the fifth Cu atom above one triangular face of the nearly tetrahedral Cu4 structure in such a way that the initial geometry is that of a distorted trigonal bipyramid. The geometry optimization, however, leads to an almost flat pentamer which resembles the free Cu5 trapezoidal structure; see Figure 6. The two outermost Cu atoms are bent outside, toward the vacuum. Also Cu5 is bound with two Cu atoms to the NBO; one Cu atom of the cluster has an interaction with a bridging oxygen; see Figure 6. The charge distribution within the cluster and at the interface is very similar to what is found for the smaller clusters; see Table 2. In average, the CuCu distances of supported Cu5, 2.48 Å, are only slightly longer than those in the free cluster, 2.44 Å. Thus, the most noticeable effect of the interaction of the Cu cluster with the SiO2 defective surface is a distortion of the metal structure without a significant alteration of the Cu-Cu distances. The only exception is that of Cu2 where the bond with NBO occurs at the expenses of the localized Cu-Cu σ bond, resulting in a considerable bond elongation. In this respect, Cu2 exhibits a molecular behavior, while slightly larger Cu clusters already show more typical properties of delocalized metal bonds. 3.2. Adhesion, Atomization, and Nucleation Energies. In this section, we describe the energetics of Cun clusters interacting with a NBO center of the SiO2 surface. Isolated Cu atoms interact very weakly with the regular, nondefective, SiO2 surface with Ead ≈ 0.1 eV.16 It is likely that small Cu clusters, which have a larger polarizability, will interact more strongly with the surface, although we do not expect this to be larger than a few tenths of an electron volt. In this respect, the role of defects for the nucleation is crucial. All the clusters considered here, from the monomer to the pentamer, are strongly bound to the NBO, with an adhesion energy which goes from 3.2 to 4.7 eV; see Table 3. These values have been computed for the fully optimized supported clusters with respect to the ground state of the equilibrium gas-phase clusters, Ead ) -[E(Cun/SiO2) E(Cun) - E(SiO2)] (positive values of Ead correspond to bound states). The relatively large oscillations in Ead from cluster to cluster can be attributed to two main effects: the open-shell character of some clusters, Cu, Cu3, Cu5, favors the direct coupling of the unpaired electron on the metal with that of the NBO; closed-shell clusters, Cu2 and Cu4, have to “open” their configuration in order to form a direct covalent bond with the NBO and the adhesion energy is lower. The second contribution to the overall adhesion energy is the presence of “electrostatic” interactions of some of the Cu atoms of the cluster with the
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TABLE 3: Adhesion, Atomization, and Nucleation Energies of Free and Supported Cu Clusters Ead, eVa De/atom, eVb Enuc, eVc
supported free supported free supported
Cu
Cu2
Cu3
Cu4
Cu5
3.96
3.16 1.01 2.59 2.02 1.21
3.46 1.00 2.16 1.00 1.30
3.55 1.31 2.19 2.21 2.30
4.71 1.41 2.35 1.83 3.00
3.96
a Ead ) -[E(Cun/SiO2) - E(Cun) - E(SiO2)]. b De/atom ) -{[E(Cun/SiO2) - nE(Cu) - E(SiO2)]/n}. c Enuc ) -[E(Cun/SiO2) E(Cu) - E(Cun-1/SiO2)].
two-coordinated O atoms of the silica surface. This interaction is present in all systems considered except Cu3. Thus, despite the fact that Cu3 has an open-shell ground state, its adhesion energy is comparable to that of the closed-shell Cu2 and Cu4 clusters; see Table 3. Another quantity of interest is the De/atom defined as De/ atom ) -{[E(Cun/SiO2) - nE(Cu) - E(SiO2)]/n}, which represents the energy required to atomize the cluster. In free Cu clusters, the value of De/atom increases with cluster size and should converge to the cohesive energy of the bulk metal for very large aggregates. For a supported cluster, the atomization energy provides a measure of the additional stability of the cluster due to the bond at the interface. The values reported in Table 3 show that the additional stabilization decreases with the size of the cluster as should be expected but that even for a supported pentamer there is a considerable contribution from the interface bond to the overall stability of the cluster toward atomization. An important quantity determining the cluster growth is the nucleation energy, Enuc, defined as the energy gain due to the addition of an isolated Cu atom to a Cun cluster (Enuc ) -[E(Cun/SiO2) - E(Cu) - E(Cun-1/SiO2) ]). On oxide surfaces nucleation is believed to occur through diffusion of isolated atoms;27 therefore, it is useful to compare the nucleation energy for free and supported clusters. One important aspect, specific to Cu, is that the presence of an unpaired electron on the Cu atom makes the nucleation energy larger for clusters with an open-shell ground state. For the free clusters, this is the case of the dimerization (combination of two 2S Cu atoms) or of the addition of Cu to a Cu3 unit; in these cases, the nucleation energy is more than 2 eV; see Table 3. Addition of a Cu atom to the closed-shell Cu2 and Cu4 gas-phase clusters results in smaller energy gains. On the surface, the situation is reversed since we have considered Cu clusters interacting with an open-shell defect. Therefore, on the surface, the formation of the dimer leads to a gain of 1.2 eV to be compared with Enuc ) 2.0 eV in the gas phase. However, for all clusters considered, the addition of an extra Cu atom leads to a considerable stabilization which is always larger for supported than for free Cu clusters (with the exception of the dimer). This is an important conclusion which shows the role of the substrate in the growth process of
a supported particle. In fact, since isolated Cu atoms are very weakly bound on the regular SiO2 surface, they will diffuse with low activation barriers. The diffusion process will stop only at defects or at sites where the nucleation has already started. It should be noted that the nucleation energy seems to increase with the cluster size for the supported clusters, going from 1.2 eV for the formation of the dimer to 3.0 eV for the case of the pentamer. In other words, the energy gain of the process tSiO-Cun + Cu f tSi-O-Cun+1 seems to increase for larger n. This is in part due to fact that as the cluster becomes larger the cohesive energy increases; another effect, however, is that larger clusters are more “flexible” and can better adapt to the shape of the surface to form “electrostatic” bonds with the O atoms of silica. Of course, by further increasing the cluster size, the nucleation energy will tend first to that of the corresponding isolated Cu particles and then, for larger crystallites, to the cohesive energy of the bulk metal. 3.3. Observable Properties: Core Level Shifts, Gap States, and Metal Work Function. Core level shifts provide an useful measure of the electronic structure of adsorbed species although it is well-known that several, sometimes canceling mechanisms contribute to the final shifts. Core level binding energies have been considered based on the DFT equivalent of the Koopmans’ theorem, IP ) -i, where i is the eigenvalue of the corresponding Kohn-Sham orbital. This procedure is not fully justified in DF theory, but experience shows that it provides an acceptable, and often a good approximation to the final state core level binding energies.39 For O and Si atoms, where an all-electron basis set was used, we have considered the O (1s) and Si (2p) core levels which are usually measured in XPS experiments. For Cu, where an ECP was used, we have considered the semicore 3s levels. In Table 4, we report the computed core level binding energies and the respective shifts of the 1s level of the NBO and of the 2p level of the nearest neighbor Si; we also give the Cu 3s core levels. For the Cu clusters, we report an average value over the atoms of the cluster. The shifts have been determined with respect to the corresponding levels of the (HO)3Si-O-Si(OH)2-O-Si(OH)2-O• model of the surface and free Cun clusters. For all cases, the 1s level of the NBO atom is shifted by 2-3 eV to smaller binding energies; see Table 4. This is consistent with the occurrence of some local charge transfer from the Cun unit to the NBO center of the SiO2 surface, in agreement with the data of Mulliken population shown in Table 2. The change in local potential due to the charge transfer is not restricted to the NBO atom only but has an effect also on the core levels of the nearest Si atom. The 2p levels of Si are shifted to smaller binding energies by about 0.6-1.2 eV for all Cu clusters considered. The shifts computed for the Si and O atoms nearest to the metal cluster are rather large, but the remaining atoms of the SiO2 surface model do not exhibit significant shifts.
TABLE 4: Core Level Shifts at the Cu/SiO2 Interfacea - [Si(2p)], eV ∆ [Si(2p)], eV - [O(1s)], eV ∆ [O(1s)], eV - [Cu(3s)], eV ∆ [Cu(3s)], eV
tSi-O-Cu
tSi-O-Cu2
tSi-O-Cu3
tSi-O-Cu4
tSi-O-Cu5
99.9 -1.2 519.2 -2.8 116.6 +0.8
99.8 -1.3 519.1 -2.9 116.5 +0.6
100.1 -1.0 519.6 -2.4 116.3 -0.1
100.4 -0.7 520.0 -2.0 116.5 0.0
100.5 -0.6 520.3 -1.7 116.5 -0.1
a Reference values for Si(2p) and O(1s) on the NBO cluster model are 101.1 and 522.0 eV, respectively. For Cu, the average on 3s levels for gas-phase clusters is taken as the reference. The corresponding values are: Cu ) 115.8 eV; Cu2 ) 115.9 eV; Cu3 ) 116.4 eV; Cu4 ) 116.5; Cu5 ) 116.6 eV.
Cun (n ) 1-5) Clusters on Oxygen Defects
J. Phys. Chem. B, Vol. 103, No. 10, 1999 1717 TABLE 5: Ionization Potentials, IP ) -E(HOMO), of Free and Supported Cu Clusters
Figure 7. Density of states of a model of the silica surface (---), of a free Cu5 cluster (s), and of a supported Cu5 cluster, tSi-O-Cu5 () ) )). The vertical line indicates the top of the O (2p) valence band (cluster HOMO) in the model of the silica surface. The empty levels of the Cu5 cluster and of the tSi-O-Cu5 surface complex are not shown for clarity.
For the Cu units, we observe a relatively large positive shift of the Cu 3s level in the monomer and in the dimer; see Table 4. The simplest interpretation is that the charge transfer from Cu to the NBO leads to a partially unscreened nuclear potential hence to a stabilization of the core levels. However, some care is necessary since intraunit polarization effects and d-sp hybridization do also contribute to the final shifts.40 The shift to higher binding energies is not seen in the larger clusters, Cu3Cu5, probably because of the metallic screening of the core levels by the delocalized 4sp valence electrons. The consequence is that virtually no shift is found in the Cu 3s and 3p levels of supported clusters compared to the free ones. Experimentally, it has been observed that a ∼40 Å Cu film on SiO2 exhibits a Cu (2p) binding energy of 932.4 eV, identical to that of bulk Cu.26 After annealing, a shift of about 0.6 eV to higher binding energies is observed in Cu (2p) band (BE ) 933.0 eV). Since the binding energy in CuO is 933.6 eV, it has been suggested that the Cu film is partially oxidized, with formation of Cu-O bonds between Cu and SiO2.26 This is the consequence of the annealing process which causes the formation of new interactions at the interface. However, the present study shows that for small clusters deposited at low temperature, the substrate core level shifts should provide a better indication of the interface bond than the adsorbate levels. The fact that these shifts are restricted to the few atoms of the defect may render a detection in XPS difficult because of the low concentration of these defects. Nevertheless, use of synchrotron radiation and accumulation of spectra could allow the detection of these features. Recent metastable impact electron spectroscopy, MIES, experiments on metal atoms and clusters deposited on oxide surfaces have shown the appearance of new states in the gap of the oxide material.41 These states can be attributed to the population of new defects at the surface of the oxide, e.g., F centers in MgO, to the presence of new metal-oxide bonds at the interface, or to features typical of a metal particle (occupied d states, etc.).41 It is therefore of interest to analyze the presence of new states in the gap after metal deposition. As a crude approximation, we use the Kohn-Sham eigenvalues to estimate the gap of silica and the position of new impurity states in the gap. With our cluster model of the SiO2 surface we compute a “HOMO-LUMO” gap of 8.1 eV which is not too far from the experimental value of ∼9 eV for bulk silica.42 In Figure 7, we have reported the density of states, DOS, obtained by Gaussian
IP, eV
Cu
Cu2
Cu3
Cu4
Cu5
free supported
5.18 6.12
5.59 5.15
4.08 5.12
4.75 3.73
4.55 5.32
broadening of the valence one-electron levels of the SiO2 support, of free Cu5, and of the tSi-O-Cu5 surface complex. The DOS of the Cu5 cluster has been determined for exactly the same geometry of supported Cu5; the DOS of fully optimized gas-phase Cu5 is almost identical. In tSi-O-Cu5, we observe the presence of a series of new levels with mainly Cu 3d character which extend from the top of the O (2p) valence band into the gap. These states are located about 1-2 eV above the top of the valence band. The shape and the position of these states is virtually identical to those of the unsupported Cu5 cluster. This shows unambiguously that the gap features are due to the supported metal particle and not to the interface bond. In this respect, it is useful to mention that the presence of the NBO centers on the silica surface results in localized levels which are embodied in the O 2p valence band, not above it. We also found that as the cluster size increases there is a broadening of the feature in the gap which is connected to the presence of more occupied metal states. Therefore, it is expected that by growing even larger clusters these features will appear as broad bands more than as well resolved peaks. An interesting property of free clusters is the first ionization potential, IP, which, for a metallic particle, converges to the metal work function. The study of the IP in supported clusters can provide useful information on the modifications in the electronic structure determined by the interaction with the substrate. Also, in this case the first IP has been measured by the position of the highest occupied level, HOMO, in the Cu cluster; see Table 5. In the free clusters, the IP follows the usual oscillating trend due to the open/closed-shell structure of the clusters with odd/even numbers of valence electrons, respectively. In general, there is a decrease of the IP with increasing cluster size; the IP of Cu5, 4.55 eV, is fortuitously close the metal work function (4.65 eV for the bulk, 4.94 eV for the (111) surface).43 A supported Cu atom exhibits a IP which is almost 1 eV larger than that of free Cu. This is largely due to the fact that the tSi-O-Cu system is closed shell. In fact, supported Cu2, which gives rise to a doublet ground state, has a lower IP than the free dimer. Similar trends are found for larger clusters; see Table 5. From these data, we do not see a net change in the IP as a function of the cluster size for supported clusters compared to the free counterparts. The main conclusion is therefore that the electronic structure of the supported Cu clusters tends to resemble that of the free units as the size becomes larger. Stated differently, the perturbation induced by the bonding with the NBO defect center is rapidly screened by the delocalized metal valence electrons. 4. Conclusions We have performed density functional calculations on the interaction of small Cu clusters with the surface of dehydroxilated silica. The regular, nondefective sites of the silica surface are rather unreactive toward adsorbed metal atoms, as shown by recent first-principle calculations16 and by the low sticking coefficient measured experimentally.24,27 On the other hand, strong bonds form between metal atoms and surface defects.16 One of the dominant point defects at the surface of silica is the nonbridging oxygen, a paramagnetic center which forms in correspondence of broken Si-O bonds. These centers are believed to be among the sites where the nucleation begins.26,27,41
1718 J. Phys. Chem. B, Vol. 103, No. 10, 1999 In this work, we have considered the interaction of small Cu clusters with this specific defect. In general, the Cu clusters form strong bonds with the nonbridging oxygens. The adhesion energy of the entire cluster is of the order of 3-4 eV. This strong interaction arises in part from the formation of a covalent polar bond between the metal cluster and the NBO center and in part from the electrostatic interaction of some of the Cu atoms of the cluster with the twocoordinated oxygens of silica. As a result, the fragmentation energy of the supported cluster increases compared to the free, gas-phase counterparts. The effect of the relatively strong interface bond on the metal-metal distance of the cluster is not large and tends to disappear as the size of the cluster increases. Hence, while the Cu-Cu distances in supported Cu2 and Cu3 are larger than those in the free clusters, in Cu4 and in particular Cu5, the average Cu-Cu distances are close to those of the unsupported clusters. On the other hand, the shape of the supported clusters can differ substantially from that of the gas-phase units. Supported Cu3 has an almost perfect equilateral triangular structure, while free Cu3 is a Jahn-Teller distorted bent molecule. Supported Cu4 assumes a bent rhombic structure, a much more compact structure than the planar rhombic form of free Cu4. We expect that by growing larger Cu particles these will assume nearly spherical three-dimensional structures since the metal-metal bonds will dominate over the weak electrostatic Cu-SiO2 interactions. In this respect, point defects on the SiO2 surface act as strong anchoring sites for the entire cluster limiting the diffusion process and favoring the nucleation. The deposition of Cu clusters on a NBO point defect has some consequences which, in principle, could be observed experimentally. The most important one is that new states appear in the wide gap of the material. These states are located 1-2 eV above the top of the O (2p) valence band and extend into the gap. Depending on the cluster size, however, the feature corresponding to the occupied metal states can give rise to sharp or to broad bands. As the cluster size increases, we expect a considerable broadening of the features. Thus, a strong coverage dependence of the width of the states in the gap is expected. The interaction of a Cu atom or cluster with the NBO center implies the occurrence of a partial charge transfer to the NBO. This is shown in the calculations by the data of Mulliken population analysis which show a moderate increase in the electronic charge associated to this center (about -0.3 e). In correspondence to this change in atomic net charge, there is a significant shift of the (1s) core level of the NBO by 2-3 eV toward smaller binding energies. This shift should result in a new peak or in a shoulder in the O (1s) XPS spectrum, provided that the number of defect centers at the surface is sufficiently high to be detected. In a similar way, the Si atom directly bound to the NBO shows a shift of the 2p level of about 0.6-1 eV to smaller binding energies, consistent with a local increase of the charge density on the oxide surface. On the other hand, only the smaller Cu units considered, the monomer and the dimer, exhibit a significant shift of the core levels, by 0.6-0.8 eV to higher binding energies. The larger clusters show small shifts of the individual atoms,
