Surface Dipoles and Electron Transfer at the Metal Oxide–Metal

Article pubs.acs.org/JPCC

Surface Dipoles and Electron Transfer at the Metal Oxide−Metal Interface: A 2PPE Study of Size-Selected Metal Oxide Clusters Supported on Cu(111) Yixiong Yang,†,§ Jia Zhou,‡,∥ Miki Nakayama,‡ Lizhou Nie,† Ping Liu,‡ and Michael G. White*,†,‡ †

Chemistry Department, Stony Brook University, Stony Brook, New York 11794, United States Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

ABSTRACT: Two-photon photoemission spectroscopy (2PPE) was employed to investigate the electronic interactions at the interface of size-selected metal oxide clusters (Mo3O9, W3O9, Ti3O6, Mo3O6, W3O6, and Ti5O10) and a Cu(111) surface. The cluster−Cu interactions were probed by work function shifts measured by 2PPE as a function of local cluster coverage. For all the clusters studied, the work functions shifted to higher energies after cluster deposition, indicating negative interfacial dipole moments pointing toward the surface. The magnitudes of the derived interfacial dipoles are found to be in the order Mo3O9 ≈ W3O9 > W3O6 ≈ Mo3O6 > Ti5O10 > Ti3O6. DFT calculations of the electrostatic potentials at the interface and Bader charge analyses were used to assess the relative contributions of electron transfer and the structure-dependent cluster dipole moment to the observed work function shifts (ΔΦ). For the fully oxidized Mo3O9 and W3O9 clusters (+6 cation oxidation states), DFT calculations indicate that electron transfer from the Cu(111) support to the cluster is the dominant contribution. The smaller interfacial dipole moments for the Mo3O6 and W3O6 clusters are qualitatively consistent with the decreased ability of the reduced cations (+4 oxidation state) to accommodate charge from the Cu surface. The DFT calculations also predict small changes in ΔΦ for the titania clusters on Cu(111) but in the opposite direction of that observed experimentally. In the case of the Ti5O10/Cu(111) surface, this result is due to the net balance of cluster dipole and electron transfer contributions that have opposite signs. Overall, the results presented in this study show that a combination of coveragedependent work function measurements and DFT calculations can be a powerful tool to investigate the electronic interactions, especially electron transfer, at the metal oxide−metal interface.

1. INTRODUCTION Metal oxide supported Cu catalysts have attracted considerable attention because they are active toward several industrially important reactions, e.g., water-gas-shift (WGS),1,2 methanol synthesis,3,4 and methanol steam reforming.5,6 A synergetic effect has been reported between Cu and metal oxide supports where the supports can significantly affect the overall catalytic activity.1,4 The metal oxide support can accomplish this by dispersing the Cu nanoparticles, providing additional sites for reaction and/or modifying the oxidation state of the Cu nanoparticles.4 Additional insight into the Cu−oxide interface has come from recent studies of “inverse” analogues in which oxide nanoparticles are deposited on a Cu(111) surface.7−10 Such inverse catalysts provide convenient model systems for investigating the active phase and chemical role of the oxide in the catalytic process. Specifically, the use of a metal support eliminates surface charging that would otherwise preclude the use of charge particle probes of electronic structure and atomic composition, e.g., photoemission and ion scattering, if a nonreducible metal oxide were used instead. In some cases, the inverse catalyst is actually more active than the conventional catalyst as a result of unique structural and electronic modifications induced by the metal substrate, e.g., higher state of reduction.9 As an example, the CeOx/Cu(111) inverse © XXXX American Chemical Society

system is found to be highly active for promoting the WGS and CO oxidation reactions.7,8 Electronic structure calculations using density functional theory (DFT) attribute the activity for CO oxidation to electron transfer from the Cu support which reduces the ceria nanoparticles and thereby enhances its ability to dissociate oxygen. Recent DFT studies of small oxide nanostructures (MoO3, ZrO2, ZnO, TiO2) on Cu(111) have also correlated interfacial electron transfer with reactivity of the oxide for water dissociation, which is a key step in the WGS reaction.11,12 Experimentally, electron transfer at the metal oxide−metal interface is most often inferred from core level binding energy shifts of the metallic particle or the cations in the oxide support. The former is complicated by the fact that binding energy shifts for metallic particles often include final state effects which can mask initial state shifts due to electron transfer.13−16 Alternatively, measurements of the work function shift between the bare surface and the surface with deposited nanoparticles can be related to the surface dipole moment which includes contributions from electron transfer at the interface. In a recent study, we used a combination of mass-selected cluster Received: April 14, 2014 Revised: May 24, 2014

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impurities could be detected on Cu(111) by Auger electron spectroscopy (AES) using this cleaning procedure. The metal oxide clusters were produced using a DC magnetron sputtering source. The metal targets were sputtered in a gas mixture of ∼2% O2 in Ar. The exact O2 ratio was optimized for the best yield of each cluster of interest. The gasphase cluster cations were mass-selected by a quadrupole filter and deposited onto the clean Cu(111) surface. Cluster deposition and subsequent Auger and 2PPE measurements were performed at room temperature. For all of the clusters studied here, the average cluster kinetic energy was measured to be ∼1 eV, assuring soft-landing conditions, i.e., W3O9 > Ti3O6, Ti5O10. For the reduced Mo3O6 and W3O6

Figure 4. Measured work function shifts with respect to local cluster coverage for metal oxide clusters deposited on the Cu(111) surface: (a) Mo3O9, W3O9, Mo3O6, and W3O3; the inset figure shows a semilog plot of the same work function data at low coverage; (b) Ti3O6 and Ti5O10 (note the smaller range of ΔΦ on the vertical scale). Representative uncertainities in the coverage values are shown for a few data points as horizontal bars; the estimated uncertainty in a work function measurement is comparable to the vertical size of the symbols (±0.01 eV). The solid lines are the least-squares fits of the data to the Topping model (see text for details).

moments are the same for all of the clusters, the magnitude is strongly dependent on the specific cluster. In order to understand such cluster-dependent behavior more quantitatively, the classical Topping model21 was employed to extract interfacial dipole moments from the observed work function shifts. The Topping model relates ΔΦ to the surface dipole and dipole−dipole interactions through the expression40,41 E

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sufficient to predict the magnitude and direction of the surface dipole moment for a specific cluster−support combination. 3.3. DFT Calculations of MxOy/Cu(111) Work Function Shifts. In general, the surface dipole for an adsorbate on a surface has two contributions: the intrinsic dipole of the adsorbate and the dipole associated with electron transfer between the adsorbate and the support.49,50 DFT calculations were employed in this study to identify the origin of the surface dipole for the small oxide clusters deposited on the Cu(111) surface. Specifically, we used DFT to calculate the electrostatic potential along the surface normal from which the surface work function can be extracted.50,51 The overall electrostatic potential of the clusters deposited on Cu(111), Vcluster/Cu(111), can be separated into three components, i.e.,

clusters, Greiner et al. have found a correlation between metal oxide work function and oxidation state of the cation by looking at oxide films that had been reduced by heating and/or sputtering.48 Using their data, the work functions for “bulk reduced” MoO2 and WO2 materials (+4 oxidation states) are 5.9 and 6.1 eV, respectively. The latter are lower than the fully oxidized clusters with +6 oxidation states, and consistent with the lower ΔΦ observed for the “reduced” Mo3O6 and W3O6 clusters. The work functions for the bulk oxides can also be used to calculate the maximum work function shifts (ΔΦmax = Φbulk oxide − ΦCu(111)) that we might expect in the limit of a thick layer of small oxide clusters: +2.1 eV/Mo3O9, +1.9 eV/ W3O9, +1.1 eV/Mo3O6, +1.25 eV/W3O6, and +0.65 eV for Ti3O6 and Ti5O10, respectively. Except for the Ti clusters, these calculated ΔΦmax values are significantly higher than those measured for the Mo and W clusters, even at ∼1 ML coverage (see Figure 4). This comparison suggests that reaching bulk oxide behavior requires significantly thicker cluster films than used in this work. The surface dipole moments, μ, extracted from the data fits in Figure 4 are summarized in Table 1. We note that the derived dipole moments represent the dipole directed along the surface normal, since this is the only component that influences the work function. The magnitude of the surface dipole varies significantly with metal species and oxidation state, and in the case of the Ti3O6 and Ti5O10 clusters, with cluster “size”. If electron transfer were the only contribution, then the negative dipoles would indicate electron transfer from the Cu surface to the oxide clusters in all cases. This is not unexpected from general considerations of metal−semiconductor interfaces, where charge is transferred to the material with the higher work function. The work function of the clean Cu(111) surface measured in this work (4.85 eV) is smaller than the bulk values for all the oxides studied here. The bulk oxide work functions are included for comparison in Table 1. Overall, the magnitudes of the derived surface dipoles follow the same qualitative trend as the work functions for the bulk materials, including the “reduced” bulk oxides MoO2 and WO2. Also included in Table 1 are values of the midgap energies (Emidgap) of the bulk oxides (oxidized and reduced) and the electron affinities (EA) of the gas-phase clusters. The midgap energy represents the solid state equivalent of the Mulliken electronegativity, and is defined as the average of the energies of the conduction band minimum and the valence band maximum. The midgap energy would correspond to the Fermi level for a nondefective metal oxide. The midgap energy values given in Table 1 were determined experimentally for a number of fully oxidized and reduced metal oxides in ref 48. For the bare Cu(111) metal surface, the midgap energy and the work function are the same, i.e., 4.85 eV. Qualitatively, we might expect the bulk oxide midgap energy and the gas-phase electron affinity to be an indicator of the cluster’s ability to induce electron transfer from the Cu surface. As seen in Table 1, the relative magnitudes of the surface dipoles for the MoxOy and WxOy (x/y = 3/6, 3/9) clusters are consistent with the trends in Emidgap and EA, but the latter do not predict the decrease in the surface dipoles observed for the TixOy (x/y = 3/6, 5/10) clusters. As discussed below, the surface dipoles induced by the small oxide clusters include contributions from both electron transfer and the cluster dipole moments, with the latter being strongly dependent on the size and structure of the cluster on the Cu surface. Hence, correlations with electronic properties that can predict the direction of electron transfer may not be

Vcluster/Cu(111) = VCu(111) + Vcluster + Vcharge

(5)

where VCu(111) and Vcluster are the electrostatic potentials of the bare Cu(111) surface and the isolated oxide clusters, respectively. The potential due to the electron transfer between the clusters and the Cu(111) surface, Vcharge, can therefore be calculated as Vcluster/Cu(111) − (VCu(111) + Vcluster). The Vcluster potential is obtained using the optimized structure of the cluster/Cu(111) surface with the Cu(111) substrate removed and the geometry of the cluster frozen. In this way, changes in the cluster structure due to binding on the Cu(111) surface and interactions with neighboring clusters are explicitly included. The change in each electrostatic potential along the surface normal (ΔV) is determined by its asymptotic value below and above the surface plane, which in Figure 5 corresponds to the value as it approaches Z = 0 and Z = 15, respectively. The work function is determined by the electrostatic potential energy in the vacuum near the surface relative to the Fermi energy, i.e., Φ = V − EF; accordingly, the work function shift can be expressed as ΔΦ = ΔVcluster + ΔVcharge

(6)

Figure 5a shows the calculated electrostatic potentials, Vcluster/Cu(111), VCu(111), and Vcluster, along the surface normal for Mo3O9 clusters bonded to the Cu(111) surface. The charge transfer potential, Vcharge, resulting from electron transfer between the Cu(111) surface and the Mo3O9 clusters is also shown. All the potentials are drawn with respect to zero energy in the vacuum. The calculated work function for the bare Cu(111) surface is 4.74 eV, which is in very good agreement with our experimentally measured value of 4.85 eV. The asymptotic shifts in these potentials are also used along with eq 6 to predict an increase in the work function (ΔΦ = +1.46 eV) when Mo3O9 clusters are deposited on the Cu(111) surface, in qualitative agreement with experiment (see Figure 4a). With one Mo3O9 cluster per Cu(111) unit cell, these calculations correspond to a cluster coverage of 0.45 ML. The relative importance of electron transfer to the overall calculated ΔΦ can be inferred from the relative asymptotic shifts of Vcharge and Vcluster shown in Figure 5a. It can be seen that the change in Vcluster from below to above the surface is only −0.02 eV, which indicates that the dipole associated with the Mo3O9 cluster is very small. This is consistent with the highly symmetric structure of Mo3O9 adsorbed on the Cu(111) surface (Figure 2a). Hence, the dominant factor controlling ΔΦ is Vcharge, which exhibits an asymptotic shift of +1.48 eV (Figure 5a). The increase in Vcharge across the surface can be attributed to electron transfer from Cu(111) to the Mo3O9 F

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essentially zero. The larger shift in the cluster dipole potential (ΔVcluster = +0.50 eV) can be traced to the calculated structure of the reduced W3O6 cluster, which is asymmetric along the surface normal with the oxygen atoms bending away from the surface (Figure 2e). By comparison, the structure for the reduced Mo3O6 cluster has a more symmetric charge distribution in the normal direction (Figure 2d) and therefore has a smaller contribution from the cluster dipole potential (ΔVcluster = −0.11 eV). The different cluster structures are also accompanied by differences in Bader electron transfer in both magnitude and sign, i.e., +0.72 e for Mo3O6/Cu(111) and −0.28 e for W3O6/Cu(111). The fact that these Bader charges are smaller than for the fully oxidized clusters (see Table 2), and in the reverse direction for the W3O6 cluster, suggests that the reduced clusters bind to the Cu(111) surface without significant electron transfer. This result may reflect the presence of reduced cations (Mo4+, W4+) which are unable to accommodate more charge from the Cu surface. The overall trend in electron transfer between the Cu(111) surface and the oxidized M3O9 and reduced M3O6 clusters (M = Mo, W) is also consistent with the gas-phase electron affinities which are lower for the M3O6 clusters.52 Unlike the molybdenum and tungsten oxide clusters discussed above, the calculated ΔΦ for the TixOy/Cu(111) surfaces are negative, which is opposite to that observed experimentally (Table 2). For the Ti5O10/Cu(111) surface, the calculated ΔΦ is the result of opposing contributions from the cluster dipole (ΔVcluster = −1.11 eV) and electron transfer (ΔVcharge = +0.74 eV) potentials. The relatively large change in ΔVcluster can be attributed to the asymmetric structure of the supported Ti5O10 cluster along the surface normal (Figure 2f). By comparison, the structure of the Ti3O6 cluster on the Cu(111) surface is more symmetric in the normal direction (Figure 2c), and here, the sign of ΔΦ is determined by the larger change in the charge transfer potential (ΔVcharge = −0.39 eV). Although the calculated electrostatic potentials predict negative work function shifts, Bader charge analyses for both Ti3O6/Cu(111) and Ti5O10/Cu(111) surfaces still indicate that electrons are transferred from the Cu surface to the titania cluster. The latter also show that more electrons are transferred to the larger Ti5O10 cluster (+0.80 e) versus Ti3O6 (+0.33 e). This result is attributed to the Ti5O10 cluster having more Ti4+ cations to which electrons can be transferred from the Cu surface, i.e., a “size” effect. The calculated Bader charges are also consistent with the greater electron affinity of the gas-phase Ti5O10 cluster (4.13 eV) compared to Ti3O6 (3.15 eV) .53 3.4. Comparison with Experimental Results. The DFTcalculated ΔΦ are compared with the experimentally determined values at similar coverage in Table 2. For the Cu(111) surfaces with molybdenum and tungsten oxide clusters, the DFT calculations reproduce the trend of larger ΔΦ for the oxidized versus the reduced clusters. This overall trend is consistent with that seen in the derived interfacial dipole moments (μ) and other electronic properties of the isolated clusters (EA) and the bulk oxides (Φbulk and Emidgap) summarized in Table 1. Quantitatively, the calculated ΔΦ values for the Mo3O9/Cu(111) and W3O9/Cu(111) surfaces are significantly larger than those observed experimentally, whereas the calculated ΔΦ values for the reduced Mo3O6/ Cu(111) and W3O6/Cu(111) surfaces are much closer to the experimental values at the equivalent coverage. The calculated ΔΦ for the Ti3O6/Cu(111) and Ti5O10/Cu(111) surfaces are also relatively small, but as noted earlier, the work functions are

Figure 5. Calculated electrostatic potential energy curves along the surface normal for the (a) Mo3O9/Cu(111) surface and (b) W3O6/ Cu(111) surface. The potentials have been referenced to the vacuum level at zero energy. The distance (Z) denoted on the x-axis corresponds to the distance from the bottom of the four-layer Cu(111) slab. The arrows indicate the changes in cluster (ΔVcluster) and electron transfer (ΔVcharge) potentials from below and above the surface plane. The dotted lines indicate the value of the corresponding potential at Z = 0. The insets show the DFT-optimized structures of the clusters adsorbed on the Cu(111) surface. Atom colors: brown, Cu; green, Mo; blue, W; gray, Ti; red, O.

cluster. This is consistent with the Bader charge analysis, which shows a transfer of 1.40e from the Cu(111) surface to the Mo3O9 cluster. Similar calculations were also performed for the W3O9, Ti3O6, Mo3O6, W3O6, and Ti5O10 clusters on Cu(111), and the results are summarized in Table 2. Given the similarities in cluster structures (Figure 2a and b) and electronic properties (Table 1), it is not surprising that the calculated ΔΦ and ΔV for the W3O9/Cu(111) and Mo3O9/Cu(111) surfaces are very similar. In particular, the Bader electron transfer and ΔVcharge are nearly identical for the two surfaces, with ΔVcharge being the dominant contribution to the calculated work function shift. The slightly larger ΔΦ for the W3O9/Cu(111) surface (+1.57 eV) is due to an increased contribution from the cluster dipole potential (Vcluster = +0.09 eV). This results from a slight deformation of the W3O9 cluster structure when bonded to the Cu(111) surface, and leads to a small cluster dipole that reinforces the electron transfer contribution. For the reduced Mo3O6 and W3O6 clusters (+4 oxidation state) on Cu(111), the calculated ΔΦ are similar but significantly smaller than the fully oxidized Mo3O9 and W3O9 clusters discussed above (see Table 2). For the Mo3O6/ Cu(111) surface, the calculations show that electron transfer (ΔVcharge = +0.31 eV) is the largest contribution to ΔΦ, whereas for the W3O6/Cu(111) surface ΔΦ is largely determined by the change in cluster dipole potential (ΔVcluster = +0.50 eV). The electrostatic potentials for the reduced W3O6 cluster on Cu(111) are shown in Figure 5b, where it can be seen that the change in Vcharge across the surface plane is G

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“reduced” Mo3O6 and W3O6 clusters (+4 cations), and smallest for the Ti3O6 and Ti5O10 systems. Qualitatively, the observed surface dipoles correlate with the work functions of the bulk oxides but less so to the bulk oxide midgap energies or the cluster specific electron affinities. DFT calculations were used to resolve the predicted ΔΦ into contributions from interfacial electron transfer and geometrydependent cluster dipole moments. For the highly symmetric and fully oxidized Mo3O9 and W3O9 clusters, the dominant contributions are electron transfer from the Cu(111) surface to the clusters. These results are consistent with Bader charge analyses which also suggest significant electron transfer to the Mo3O9 and W3O9 clusters. The smaller surface dipole for the “reduced” Mo3O6/Cu(111) surface is attributed to a smaller electron transfer contribution resulting from the presence of Mo4+ cations which can accommodate less charge from the Cu surface. Electron transfer is predicted to be even less for the “reduced” W3O6/Cu(111) surface, and the overall ΔΦ is determined by the cluster dipole contribution resulting from the asymmetric adsorption structure of the W3O6 cluster. The measured surface dipoles on both the Ti3O6/Cu(111) and Ti5O10/Cu(111) surfaces are significantly smaller than the molybdenum or tungsten oxide clusters studied in this work. The DFT calculations also predict small ΔΦ for the titania clusters but in the opposite direction than that observed experimentally. Bader charge analyses suggest a cluster “size” effect in which the larger number of Ti4+ cations in the Ti5O10 cluster are able to accommodate more electron transfer from the Cu(111) surface than the smaller Ti3O6 cluster. In general, the calculated work function shifts at fixed cluster coverage are consistent with the overall trends observed in the surface dipoles, but quantitative agreement is relatively poor. The latter may be due to limitations of DFT when applied to these mixed metal oxide−metal systems, or to differences in the cluster adsorption structures predicted by DFT and those resulting from deposition at room temperature. Nonetheless, the calculations show that electron transfer to metal oxide clusters correlates with the oxidation states of the metal and highlight the fact that the overall surface dipole moment includes contributions from both electron transfer and the dipole associated with the cluster structure. Hence, correlations of reactivity and size/composition of small clusters will strongly depend on the detailed electronic interactions and structure of the specific cluster−surface interface.

predicted to decrease with cluster deposition which is opposite to what is observed experimentally. Differences between the calculated and experimentally measured ΔΦ may be associated with inherent limitations of standard DFT in treating the electronic properties of the small metal oxide clusters. Specifically, DFT under the local or semilocal approximations suffers from the self-interaction error (SIE), which can lead to a qualitatively incorrect description of localized states. To test the effects of SIE, calculations for the TixOy/Cu(111) surfaces were also performed using the hybrid DFT+U method and a U value (4.5 eV) optimized for TiO2 surfaces;29,30 however, the ΔΦ values calculated by DFT+U remain negative as predicted by standard DFT, and even higher values of the U parameter (14−25 eV) had little effect on the results. This suggests that the DFT+U method may still not be sufficiently accurate to describe these supported oxide clusters. In fact, our recent study using the first-principles linear response method shows that the U value is not a constant for a metal oxide but varies with local structure, i.e., metal− oxygen bond length.54 The calculated Ti3O6 and Ti5O10 adsorption structures (Figure 2c and f) exhibit a wide range of Ti−O distances (1.76−2.17 Å) and different local Ti atom environments that may be difficult to describe by a single U parameter. Another factor that may contribute to the observed discrepancies between theory and experiment is associated with cluster structure. The DFT-optimized structures of the metal oxide clusters on the Cu(111) surface correspond to local minima at 0 K and that may not be the same as that formed by deposition at room temperature. The thermal energy of the surface combined with the kinetic energy of the impinging cluster ions could be used to overcome barriers to structural transformations that lead to cluster adsorption structures that are different from the isolated clusters in the gas phase. There are several recent reports of small oxide nanoclusters (e.g., WxOy, V6O12) whose adsorbed structures are different from those in the gas phase as a result of strong support interactions and/or the presence of defects.55−59 Thermal energy may also allow clusters in near proximity to form cluster islands, especially in the higher coverage regions of the deposition area. As seen in this work, the overall surface dipole strongly depends on the adsorbed cluster structure, so significant changes in cluster structure from that used in the DFT calculations will lead to very different predicted work function shifts. Atomic imaging experiments using scanning probes will be needed to address the issue of cluster adsorption structure in the future.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (631) 344-4345. E-mail: [email protected].

4. SUMMARY In this work, experiments and DFT calculations were combined to investigate the electronic interactions of size-selected metal oxide clusters (Mo3O9, W3O9, Mo3O6, W3O6, Ti3O6, and Ti5O10) deposited on a Cu(111) surface. Cluster−support interactions were probed by 2PPE measurements, which allowed us to obtain local work function data over a large range of cluster coverage. For all the clusters included in this study, the work functions shift to higher values with increasing cluster coverage, consistent with surface dipole moments (μ ≤ 0) whose positive ends point toward the surface. Estimates of the interfacial dipole moments were extracted from the measured work function data using the classical Topping model. The derived surface dipoles are largest for the fully oxidized Mo3O9 and W3O9 clusters (+6 cations), less for the

Present Addresses §

Applied Materials, Santa Clara, CA. Department of Chemistry, University of Wisconsin, Madison, WI. ∥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out in the Chemistry Department at Brookhaven National Laboratory under Contract No. DEAC02-98CH10086 with the U.S. Department of Energy (Division of Chemical Sciences). The DFT calculations were performed using computational resources at the Center for Functional Nanomaterials, Brookhaven National Laboratory. H

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