Paper SS+AS+EN+NS-TuM12
Adsorption of Water and Bromine on Gold Nanoclusters Investigated by Neutralization in Low Energy Alkali Ion Scattering

Tuesday, October 20, 2015, 11:40 am, Room 112

Small gold (Au) nanoclusters have been heavily studied because of their intriguingly high catalytic activity, especially when compared to bulk gold. We employ a specialized method of Low Energy Ion Scattering (LEIS) to probe the electronic properties of nanoclusters prepared with a variety of methods. The experiments measure the neutralization probability of singly scattered alkali ions, which is acutely sensitive to the local electrostatic potential a few Å’s above the surface. Because the Au atoms are much more massive than the substrate atoms, this method allows the signal from the nanoclusters to be separated from that of the substrate so that the neutralization reflects the local properties of the cluster surfaces. Earlier work had demonstrated that the neutralization is a function of cluster size, and that it is enhanced for the smallest clusters presumably because they are negatively charged [1]. The work presented here investigates the adsorption of water and Br on Au nanoclusters grown on TiO₂ or SiO₂. There are multiple factors that can contribute to a change in the neutralization of the scattered ions, such as the cluster size, shape, or charge state. When Br attaches to a nanocluster, the neutralization decreases presumably due to charge transfer from the cluster to the electronegative Br atom. Surprisingly, it is found that the neutralization of scattered K⁺ ions increases in the presence of adsorbed water at liquid nitrogen temperatures. Furthermore, the increase of neutralization for adsorbed water is independent of whether the water or the Au is deposited first. Possible explanations for these observations will be discussed.

[1] G.F. Liu, Z. Sroubek and J.A. Yarmoff, Phys. Rev. Lett., 92, 216801 (2004).