ANOMALOUS GROWTH BEHAVIOUR OF PD-AU AND AG-AU ALLOY PARTICLES DURING VAPOUR DEPOSITION ON CARBON SUBSTRATES AT ELEVATED TEMPERATURES

摘要

The condensation of Pd-Au and Au-Ag alloy particles from vapour beams of the components on amorphous carbon substrates was investigated by TEM, mostly in-situ. At substrate temperatures in the range from 623 to 723 K, condensation was found to be virtually complete for all components, but nucleation densities of pure Pd or Ag were significantly lower than of Au, under similar conditions, while particle growth speeds were high for Pd and Ag, and low for Au. For the case of Au alloys, rich in Pd or Ag, respectively, two sorts of particles were found to grow simultaneously: relatively few crystallites with high growth speed, and many slowly growing particles in between. After interruptions of the vapour beam, the large crystallites exhibited a significant delay of the order of minutes in resuming growth, depending on composition. Electron diffraction analysis as well as observations of some catalytic activity under oxygen revealed that the large species were rich in Pd, whereas most of the gold was contained in the small particles. It is shown that this complex nucleation and growth behaviour can be explained by differences in the interaction of the metals with each other and in combination with peculiar properties of the a-C substrate. These include the existence of at least two binding states for the metal atoms with energies of around 1 eV for ''regular'' states and in the range from 1.5 to 1.7 eV with abundance of 10(13) to 10(14) cm(-2), probably at porosities in the substrate surface, where metal atoms can temporarily be trapped without acting as nucleation centers. In the case of alloy depositions, these ''active sites'' are initially preferentially occupied by Au atoms, while Pd or Ag adatoms exhibit relatively low binding energies not only to the substrate but also to alloy particles, possibly induced by the presence of the carbon. In particular, the binding energy of Pd seems to be lower on the small particles rich in Au at their surfaces than on the large crystallites rich in Pd. These combined effects would explain selective clustering of Pd adatoms, and the simultaneous growth of small alloy particles. Qualitatively similar results were found for Ag-Au alloys, with Ag behaving like Pd. References: 33