Surface Dipoles and Electron Transfer at the Metal Oxide–Metal Interface: A 2PPE Study of Size-Selected Metal Oxide Clusters Supported on Cu(111)

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 coverage-dependent 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.

Influence of Cluster–Support Interactions on Reactivity of Size-Selected NbxOy Clusters

“…39,52 In addition to the secondary electron peak, we also observe a feature associated with the Cu 3d band centered near 6.7 eV, and an asymmetric peak around 8.5 eV which corresponds to a mixture of the surface state (SS) and the unoccupied first image state (IS 1 ) located 0.4 and 0.8 eV below the Fermi level (E F ), respectively. 10,53 The asymmetric peak is quenched as clusters are deposited onto the surface and also when the oxide film is formed. Additionally, a feature attributed to an unoccupied state appears at around 7.4 eV for the Cu 2 O/Cu(111) system.…”

“…56 Here, we set δ equal to θ −0.5 , where θ is the cluster coverage, based on the lattice gas model for randomly distributed adsorbates, appropriate for our experimental conditions. 10,57 The resulting Topping equation written in terms of θ and cluster area (A) is μθ ε…”

“…Both of these trends were observed in our previous studies of W and Mo oxide clusters on Cu(111). 10 The attribution of the observed trends in surface dipole ( Figure 4 and Table 1) to interfacial charge transfer assumes that the contribution from the intrinsic dipole of the cluster is small by comparison. 10 To gain insight into the contribution of charge transfer to the interfacial dipole, we performed Bader charge analyses on the clusters on Cu(111).…”

“…29 In our previous study, we showed that the combination of coverage-dependent 2PPE measurements and DFT calculations is a highly effective tool for understanding electron transfer at the cluster−support interface for a series of metal oxide nanoclusters on Cu(111). 10 Here, we first extend this study to Nb oxide nanoclusters. Specifically, we investigate the effects of cluster size and metal oxidation state on the interfacial dipole by depositing fully oxidized Nb 3 O 7 and Nb 4 O 10 and their "reduced" counterparts, Nb 3 O 5 and Nb 4 O 7 , on Cu(111).…”

Growth modes of thin films of ligand-free metal clusters

“…7,8 So far this technique has been successfully used to investigate the properties of individual deposited clusters. [9][10][11][12][13][14][15][16][17][18][19] In these studies, coalescence is circumvented by dramatically lowering the coverage and/or immobilizing clusters on the surface. Attempts to synthesize cluster thin films by increasing the coverage have failed since deposited clusters merged into larger particles.…”