Paper SS2-MoA9
Deposition of RuSn Nanoparticles onto a Al₂O₃/Ni₃Al(111) Surface from a Gas Phase Organometallic Precursor

Monday, November 9, 2009, 4:40 pm, Room N

Scanning tunneling microscopy (STM) was used to characterize RuSn nanoparticles deposited from a gas phase organometallic precursor (Ru₃(CO)₉(μ-SnPh₂)₃) onto an Al₂O₃ film grown on a Ni₃Al(111) substrate. This novel method allows for the specific preparation of small metal clusters that range between single atoms and larger, bulk-like particles. It also assists in bridging the gap between previous studies based on metal evaporation and industrial catalysis, where the use of precursor compounds for the active species is common. The STM images of the Al₂O₃/Ni₃Al(111) substrate prior to the exposure of the precursor show two distinct areas, with the first one exhibiting the dot structure of the alumina film, as reported in the literature. The second area does not exhibit any regular structure. After exposing the sample to the precursor at room temperature, the precursor was found to adsorb preferentially onto the second area, but also on the first area in significant amounts. In a subsequent series of images acquired for increasingly higher annealing temperatures, the discrepancy between the two areas in the interaction with the adsorbates is strongly increased. While the surface with the dot structure becomes almost fully depleted of the adsorbates, except for a few small (up to 3 nm in diameter) and roundish protrucions, the other area is covered by larger polygonal protrusions. Similarly shaped depressions in the surface are observed along with the protrusions. A control experiment in which the bare Ni₃Al(111) surface was exposed to the precursor indicates that the second area of the Al₂O₃/Ni₃Al(111) surface consists of patches of bare metal. A striking difference between the protrusions on the oxide and the metal is that the size of the latter increases dramatically with increasing annealing temperature, while the former do not grow at all. Nevertheless, both features are observed even up to 925 K, from which it is concluded that the annealing removes the ligands from the metal centers of the precursor molecules, after which the metal nanoparticles can form. These results show that by using an organometallic precursor, small uniform metal nanoparticles with high thermal stability can be successfully deposited onto an oxide thin film. This paves the way for subsequent studies of the surface chemistry associated with model oxide-supported metal catalysts consisting of metal nanoparticles of a uniform size.