Electron Transfer Processes on Supported Au Nanoclusters and Nanowires and Substrate Effects

Abstract

The catalytic activity of metal nanoclusters is considered to depend on their size, morphology, and substrate type. Here we address this problem by studying changes in electron transfer processes, that are important in surface chemistry, on the example of the interaction of Li ions with gold nanostructures as a function of their sizes and substrate type. The Au nanoclusters were grown on highly ordered pyrolytic graphite (HOPG) and Al 2 O 3 surfaces. In the case of Al 2 O 3 and sputtered HOPG surface, a wide surface coverage distribution of nanoclusters is formed, whereas on pristine HOPG scanning tunneling microscopy (STM) images show that Au clusters nucleate at step edges and can coalesce into "nanowires". We found that electron transfer is much more probable on small clusters than on bulk Au surfaces. For distributed clusters, electron transfer is most probable for lateral size is of the order of 2−3 nm and height is in the 1 nm range, that is, of the order of a few atomic layers. Interestingly, larger electron transfer rates were found on cluster chains or nanowires nucleated on HOPG step edges in the case of pristine HOPG than on isolated clusters on HOPG planes. Our results suggest that the main effects that are observed are largely related to cluster size and morphology.

Au nanocluster growth on Graphene supported on Ni(111)

“…The data presented here show a reduction in the neutral fraction with cluster size, which is identical to the behavior of Au clusters on TiO2 [44,63,64], SiO2 [49,65], Al2O3 [66] and HOPG [66,67]. This behavior is consistent with the model that ascribes the reduction in NF with Au cluster size to the edge atoms being positively charged while the center atoms are neutral [18].…”

“…Bonding to such π orbitals could remove charge from the edge atoms in a similar manner as for Au bonding to a substrate O atom. The NF for low energy Li + scattered from Au nanoclusters supported on HOPG show a similar decrease of the NF as a function of Au coverage [66,67], which suggests that the bonding of the edge atoms to the substrate is via π orbitals for both single layer Gr and bulk graphite surfaces.…”

Inhomogeneous charge distribution across gold nanoclusters measured by scattered low energy alkali ions

“…Such a behavior has been observed in low energy alkali ion scattering from Au and Ag nanoclusters in Refs. [48,50,51], and a decrease of edge atom charge for clusters that are smaller than those considered here may be the reason. Thus, the extrapolation of the model to a cluster diameter of zero in Figs.…”

“…This is because the alkali LEIS technique is adept for studying surfaces composed of multiple elements and can address questions about the charge state of those elements. Previous results have shown that the ions scattered from Au and Ag nanoclusters on an oxide and other substrates have a much higher neutralization probability than those scattered from the bulk metal, and that neutralization decreases as the cluster size increases [48][49][50][51][52][53][54][55]. Although it is possible that the alkali LEIS neutralization rate and the Au nanocluster catalytic activity are related, such a correlation has not yet been confirmed.…”

“…To explain the unusually high neutralization for alkali ions scattered from small Au nanoclusters, a model is developed here based on the notion that the charge associated with each Au atom in a cluster is different. Previously, the charge on the Au clusters had been considered in aggregate when analyzing ion neutralization [48][49][50][51][52][53][54][55], but the present model is dependent on the differences in charge between the atoms and not on the average charge. Density functional theory (DFT) calculations performed for Au nanoclusters supported on TiO2 find that the charge associated with the edge atoms of the nanocluster are noticeably different from the center atoms [27,31].…”