Paper SS-TuP17
The Formation of Iron Oxide Nanoparticles and Thin Films on Au(111)

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Iron-based catalysts, including iron oxides, are an important class of materials with relevance to Fischer-Tropsch catalysis and gas-sensing applications. The controlled growth of nanoparticles and atomically thin films on single crystal surfaces allows for systematic studies of how size, shape, and atomic structure affect the chemical reactivity of these materials. We have studied the formation of monolayer thick iron oxide nanoparticles and thin films on the reconstructed Au(111) surface. STM, XPS, ISS, and LEED were used to evaluate the structure and composition of the iron oxide nanoparticles and films as a function of growth conditions. Iron oxide growth was achieved by first depositing iron nanoparticles on Au(111) at room temperature. At coverages of 0.2-0.5 monolayer (ML), Fe forms triangular islands at the elbows of the Au(111) herringbone reconstruction. At higher coverages, the particles begin to coalesce and the second and third layers of Fe start to form before the first layer is complete. The oxidation of Fe was performed by exposing the nanoparticles to molecular oxygen at 323 K and 423 K, followed by annealing to 500-700 K. XPS results indicate that the Fe is oxidized, with a shift of the Fe 2p peak to a higher binding energy. Atomically-resolved STM images show the formation of iron oxide particles that are hexagonal in shape and one monolayer high with a diameter of 10-20 nm. STM images at higher iron oxide coverages also show evidence of a non-coincidence overlayer lattice with a short periodicity of 0.25-0.3 nm modulated by a larger periodicity of approximately 3.5 nm. The larger periodicity results from a moiré pattern formed between the iron oxide overlayer and the underlying Au(111) surface.