AVS 53rd International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM6
Pd Cluster Growth on the SnO2 (101) Surface

Tuesday, November 14, 2006, 9:40 am, Room 2002

Session: Reactivity of Metal Clusters
Presenter: Kh. Katsiev, Tulane University
Authors: Kh. Katsiev, Tulane University
U. Diebold, Tulane University
M. Batzill, Tulane University
Correspondent: Click to Email
Tin oxide is widely used as a solid-state gas sensor for detection of combustible and toxic gases. Its sensitivity and selectivity can be enhanced by catalytically active dopants such as Pd. We have studied the growth of Pd on the SnO2 (101) surface in the sub-monolayer regime using STM, XPS, ARUPS and LEED. The SnO2(101)-(1x1) surface was prepared in stoichiometric (Sn(IV)) and in full- reduced (Sn(II)) form. Palladium was vapor-deposited on both surfaces at room temperature. On the stoichiometric surface round 3-D clusters with an average size of ~1 nm nucleate at surface defects. On the reduced surface, 1-D nanostructures form on the terraces; these are imaged in STM as straight, parallel structures, all with the same characteristic width of one substrate unit cell and height of one ML. The length of the Pd clusters is limited by the size of the substrate's terraces. XPS shows metallic Pd with a 0.45 eV shift in the Pd 3d core level peak position to lower binding energy during the initial stages of the growth. ARUPS measurements show that the position of the Pd-4d valence band "follows" the position of the Sn-5s derived surface state for the reduced SnO2(101) surface. This indicates a strong electronic interaction between the Sn-5s state and the Pd d-band. Clusters nucleated at sputter-created defects are wider and shorter. From LEED it appears that the 1D clusters are commensurate with the substrate in one direction. Formic acid adsorbs on the 1D nanoclusters, but not the SnO2 surface at RT. The clusters are thermally not stable, they coarsen upon annealing to a temperature of 200°. DFT calculations of various test structures suggest that the formation of the 1-dim clusters is dominated by kinetic effects. Pronounced one-dimensional diffusion, combined with strong interaction between Pd and surface Sn appear to be responsible for the formation of the one-dimensional islands.