AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA9
The Influence of Chlorine on the Dispersion of Cu Particles on Cu/ZnO(0001) Model Catalysts

Tuesday, November 3, 1998, 4:40 pm, Room 309

Session: Morton M. Traum Student Award Session
Presenter: A.W. Grant, University of Washington
Authors: A.W. Grant, University of Washington
A. Jamieson, Massachusetts Institute of Technology
T. Evans, University of Manchester, United Kingdom
G. Thornton, University of Manchester, United Kingdom
C.T. Campbell, University of Washington
Correspondent: Click to Email
Adsorbed chlorine is thought to affect the dispersion of metal catalysts on oxide supports. We have studied this by vapor depositing thin films of Cu onto Zn-terminated ZnO(0001), both with and without a pre-exposure of Cl@sub 2@. First, the adsorption of Cl@sub 2@ was studied on ZnO using XPS, LEIS, work function, and band-bending. A close-packed monolayer of Cl adatoms forms at saturation with 0.31 Cl adatoms per Zn site. The work function increase of 2.2 eV indicates anionic Cl@sub ad@. Without Cl@sub ad@, vapor-deposited Cu grows in two-dimensional islands that cover ~33% of the ZnO, after which these islands thicken (i.e., as 3D Cu particles) while the clean ZnO between these Cu islands gets covered with Cu only very slowly. The presence of Cl@sub ad@ decreases the fraction of the surface that is covered by Cu islands by ~3-fold, so Cl@sub ad@ either decreases the number of 2D Cu islands or their critical area before thickening. This is consistent with weaker binding of Cu to the Cl covered surface than to the clean ZnO. In addition, Cl@sub ad@ not only covers the surface of the ZnO, but also covers the surface of the Cu islands, as suggested by the constant Cl-LEIS signal during Cu deposition. This was verified further by following the formate-derived TPD features from HCOOH adsorption, which are distinctly different for Cu and Zn sites on Cl-free Cu/ZnO(0001). Pre-dosed Cl virtually eliminated the 575K Zn-formate peak. When Cu was vapor-deposited onto a Cl-precovered surface, neither the Zn- nor Cu- formate peaks were observed. Apparently, Cu displaces some of the Cl adatoms from Zn sites, and they subsequently cover the Cu islands.