Oxidation of Sn at the Cluster-Support Interface: Sn and Pt-Sn Clusters on TiO2(110)

The growth of Sn and Pt-Sn clusters on TiO2(110) has been studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and density functional theory (DFT). At low Sn coverages (0.02 ML), single-layer high clusters of SnOx are formed with a narrow size distribution and uniform spatial distribution. XPS experiments indicate that these clusters consist of oxidized Sn, and the corresponding reduction in the TiO2 substrate is observed. At higher Sn coverages, the surface is still dominated by two-dimensional clusters of SnOx, but larger three-dimensional clusters of metallic Sn also appear. As the Sn coverage is increased, the number of three-dimensional clusters increases, and the ratio of Sn/SnOx increases, suggesting that SnOx and reduced TiOx form at the cluster-support interface. When Pt is deposited on top of the Sn/SnOx clusters, the relatively mobile Pt atoms diffuse across the TiO2 surface and become incorporated into existing Sn/SnOx clusters. Furthermore, the addition of Pt to the Sn/SnOx clusters causes the reduction of SnOx to metallic Sn and the oxidation of Ti3+ to Ti4+; this behavior is attributed to the formation of Pt-Sn alloy clusters, which results in the diffusion of Sn away from the interface with the TiO2 support. In contrast, when Sn is deposited on an equal coverage of Pt clusters, new Sn/SnOx clusters are formed that coexist with Pt- Sn clusters. However, the surfaces of both Pt on Sn and Sn on Pt clusters are Sn-rich due to the lower surface free energy of Sn compared to Pt. DFT calculations demonstrate that M-TiO2 bonding is favored over M-M bonding for M = Sn, unlike for transition metals such as M = Pt, Au, Ni, and Co. Furthermore, the substantial charge transfer from Sn to TiO2 leads to dipole-dipole repulsion of Sn atoms that prevents agglomeration into the larger clusters that are observed for the mid-late transition metals. DFT studies also confirm that the addition of Pt to a Sn cluster results in strong Pt-Sn bond formation and diminished Sn-O interactions.