AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA10
Bimodal Pd Cluster Growth on the Reduced SnO2 (101) Surface

Wednesday, November 2, 2005, 5:00 pm, Room 203

Session: Reactions on Nanoclusters
Presenter: Kh. Katsiev, Tulane University
Authors: Kh. Katsiev, Tulane University
M. Batzill, Tulane University
U. Diebold, Tulane University
Correspondent: Click to Email
Tin oxide is 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 in order to gain new insights into the gas sensing mechanism. The SnO2 substrate exhibits a 1x1 bulk termination, with the surface tin atoms in a reduced Sn2+ charge state. This reduction of the surface Sn atoms results in the formation of a Sn-5s derived surface state. Upon deposition of Pd onto SnO2 (101), bimodal cluster growth was observed. For very low coverages, two-dimensional, round clusters decorate (0-10) -oriented step-edges, a behavior typical for many metal-on-oxide systems. In addition, we observe quasi one-dimensional Pd clusters growing on terraces. In both cases Pd wets the reduced tin oxide surface. The one-dimensional clusters are imaged in scanning tunneling microscopy as straight, parallel nanostructures oriented along the (-101) direction, all with the same characteristic width of 1 nm and a height of 1ML. XPS shows metallic Pd. A 0.45 eV shift in the Pd 3d core level peak position to lower binding energy occurs during the initial stages of the growth. This might be an indication of charge transfer from the Pd clusters to the substrate. Coverage-dependent UPS show that at fractional monolayer coverages, a Pd peak appears at the same position in the band gap as the Sn-5s surface state of reduced SnO2(101), indicating a possible hybridization of Sn-5s with the Pd d-band. This is the first observation of formation of one-dimensional, metallic nanowires on a wide-band gap material.