Center for Functional Nanomaterials Seminar
"In-Situ Imaging of Ag/Ni Oxidation Catalysts Using Low-Energy Electron Microscopy"
Presented by Jens Falta, Institute of Solid State Physics, University of Bremen, Germany
Wednesday, October 9, 2013, 11 am
CFN Conference Room A, Building 735
Hosted by: Sanjaya Senanayake and Jurek Sadowski
LEEM and micro-LEED have been used to investigate growth, atomic structure, and oxidation of ultrathin silver films. For this purpose IV-LEED analysis, based on ab-initio scattering theory, has been expanded to the very low electron energies as commonly used in LEEM. In this talk, I will present results for Ag growth on Ni(111) at growth temperatures between 470 and 850 K. We find that silver grows in a Stranski-Krastanov fashion with a thin wetting layer and a p(7x7) reconstruction at temperatures lower than 700 K. At higher growth temperatures a sqrt(52) x sqrt(52)R13.9° reconstruction is found with profound implications for the growth characteristics of the film. Oxidation of silver films of one- and two-monolayer thicknesses on the Ni(111) surface was investigated at various temperatures. At 500 K and 600 K, we find that exposure to molecular oxygen leads to the destabilization of the Ag film with subsequent relocation of the silver atoms to small few-layer-thick silver patches and concurrent evolution of NiO(111) regions. Subsequent exposure of the oxidized surface to ethylene initiates the transformation of bilayer islands back into monolayer islands, demonstrating at least partial reversibility of the silver relocation process at 600 K. Oxygen exposure of the silver films at 300 K leads to the formation of a continuous NiO(111)-like film consisting of nanosized domains. At 750 K, we observe the formation of a heterogeneous film at the nano-scale, primarily consisting of NiO(111) surface oxide nuclei, separated by oxygen-free Ni(111) terraces. The I-V-analysis suggests an average oxide thickness of two NiO layers.