September 19, 2003
The Formation and the Spread of MoO3 on Au(111): Study at a Molecular Level
Z. Song, T. Cai, Z. Chang, G. Liu, J.A. Rodriguez, and J. Hrbek
Department of Chemistry, Brookhaven National Laboratory, Upton, NY
The wetting and spreading of metal oxides on catalyst surfaces is one of the important processes for the preparation of highly dispersed monolayer catalytic particles. The formation and the spread of MoO3 on Au(111) has been studied using photoemission and scanning transmission microscopy (STM). Molybdenum particles (~2 nm) on Au(111) were prepared by chemical vapor deposition (CVD) of Mo(CO)6 and oxidized by NO2 at elevated temperatures. The MoO3 spread spontaneously over the surface to form two-dimensional fractal islands of amorphous MoO3. A ramified-cluster-diffusion mechanism is proposed for the spreading of MoO3.
The spontaneous spread of many oxides and salts over catalytic supports has been
used in the chemical industry to prepare highly dispersed
monolayer catalysts, and to manufacture oxide films with specific
electronic properties. Scientists have been discussing the spreading
mechanism of oxides for two decades, trying to explain how the
monolayer-thick film forms and spreads.
We prepared a model system for investigating the formation and the spread of MoO3 on a Au(111) surface. We first deposited Mo on the Au surface and found by x-ray photoemission spectroscopy (XPS) that the Mo is metallic, and free of contaminants such as carbon or oxygen. The Mo growth on the Au surface is self-limited with narrow-sized metal particles about 1.8 nm in diameter. These Mo particles aggregate without coalescence, forming ramified islands with the arms extending preferentially along the fcc troughs or the domain boundaries of the Au(111) herringbone reconstruction (Figure 1).
O2 is not efficient for the oxidation of the Mo on Au, especially for
small Mo coverages. Thus, no oxidized Mo species is observed upon
the reaction with O2 at temperatures up to 850 K (Figure 2). Density
function calculations suggest that the Mo particles may be capped by
a layer of Au and therefore rendered inert. However, the adsorption and
dissociation of NO2 on metal surfaces at elevated temperatures is
known to generate a reactive form of chemisorbed atomic oxygen that
can oxidize the Mo clusters to MoO3 at 500 K, readily as we have
shown in this work (Figure 2). The MoO3 spreads over the Au surface
along the troughs of the Au(111) surface at 500 K, and the spreading
becomes random at 600 K. The important finding is that the MoO3
spreads in a ramified way and forms fractal two-dimensional islands
under ultra-high vacuum (UHV) conditions. Molecule-resolved scanning
transmission microscopy (STM) images show the highly disordered
arrangement of the MoO3 molecules within the islands; the average
distance between two adjacent molecules is ~0.38 nm. The island
covered area increases by a factor of six compared with the original
metallic Mo/Au sample (Figure 1).
Previous studies have suggested three mechanisms for the spontaneous spread of MoO3 over surfaces, namely transportation via gas phase, unrolling-carpet, and free surface diffusion mechanisms. These mechanisms are mutually exclusive, and neither is capable of explaining all of the experimental results alone. We suggest a ramified-cluster-diffusion mechanism for the spreading of MoO3. The spread occurs via the diffusion of MoO3 clusters detached from the bulk MoO3 at temperatures above 500 K. This diffusion is a thermally activated process, and an anisotropic diffusion barrier at the edge of the island leads to the ramified spreading and the formation of the fractal islands. This mechanism explains the process of spreading on the nanoscale for all results in this and previous studies.
BEAMLINE
U7A
FUNDING
U. S. Department of Energy, Division of Chemical Sciences
PUBLICATION
Z. Song, T. Cai, Z. Chang, G. Liu, J.A. Rodriguez, and J. Hrbek,
"The Formation and the Spread of MoO3 on Au(111): Study at a
Molecular Level", J. Am. Chem. Soc. 125 (2003) 8059.