Surface dipoles and electron transfer at the metal oxide-metal interface: A 2PPE study of size-selected metal oxide clusters supported on Cu(111)

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 (Δℙ). 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 Δℙ 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.