UC Riverside

Surface science techniques are used to probe Au nanoclusters in an attempt to understand fundamental properties that are related to their unusually high catalytic activity. The main technique used is the neutralization probability of low energy alkali ion scattering. This is performed on Au nanoclusters supported on either TiO2(110) or SiO2.

Au Nanoclusters are deposited onto TiO2(110) via buffer layer assisted growth in ultra-high vacuum, which is a novel process to grow clusters. A thin amorphous solid water buffer layer is condensed onto a TiO2(110) single crystal cooled to 100 K. Au atoms deposited onto this layer arrange themselves into nanoclusters. The sample is then annealed to 320 K to desorb the ASW and enable the clusters to soft-land onto the substrate. Time-of-flight low energy ion scattering, using Li+, Na+ and K+ projectiles, probes the materials during each step of the procedure to measure the surface composition and reveal the details of how the clusters form. The neutralization probability of Na+ ions singly scattered from the Au nanoclusters indicates that they increase in size after annealing and that the magnitude of the increase is a function of the buffer layer thickness. The adsorption of a thin, incomplete water layer prior to Au deposition forms nanoclusters that are possibly even smaller than those produced by direct deposition onto the clean substrate.

The neutralization of low energy Na+ and Li+ ions scattered from Au nanoclusters formed by deposition onto oxide surfaces decreases as the cluster size increases. An explanation for this behavior is provided, which is based on the notion that the atoms in the clusters are not uniformly charged, but that the edge atoms are positively charged while the center atoms are nearly neutral, as reported in the literature. This leads to upward pointing dipoles at the edge atoms that increase the neutralization probability of alkali ions scattered from those atoms. As the clusters increase in size, the number of edge atoms relative to the number of center atoms decreases, so that that the average neutralization also decreases. Calculations employing this model are compared to experimental data and indicate good agreement if the strength of the dipoles at the edge atoms are assumed to decrease with cluster size. This model also explains differences in the neutralization probabilities of scattered Na+ and Li+.

Catalytically active Au nanoclusters on SiO2 are exposed to Br2 and then measured using 1.5 keV Na+ low energy ion scattering. It is found that Br2 adsorbs on the nanoclusters, but not on the substrate nor on bulk Au. These results show that the clusters are able to dissociate the Br2 and then adsorb the individual Br atoms. Results from the literature indicate that the catalytic activity of nanoclusters occurs at the edges and the work from this dissertation confirms that the edge atoms are positively charged. This leads to the conclusion that the outer shells of the electronegative Br atoms become filled so that they ionically bond to the edge atoms of the clusters. Furthermore, Br2 is a known catalytic poison and this work shows how its adsorption blocks sites that would otherwise be involved in a surface chemical reaction.