Abstract
AuAg nanoclusters are promising supported co-catalysts for photocatalytic hydrogen reduction. However, beyond the quantum regime (N > 100) little is known about how the electronic properties of these nanoparticles are affected by chemical ordering. We investigate the effects of chemical ordering on the properties of 147-atom cuboctahedral AuAg nanoclusters, using empirical potentials coupled with an atomic-swap basin-hopping search to optimise the elemental distribution, with the lowest energy arrangements then reminimised using Density Functional Theory (DFT). Force-field calculations show Au atoms preferentially occupy sub-surface positions in the bimetallic structures, which results in the formation of a pseudo-onion structure for Ag-rich compositions. At the DFT-level, however, an Ag core surrounded by an Au shell (Ag@Au) is energetically favoured, as electron density can be drawn more readily when Au atoms are positioned on the nanocluster surface, thus resulting in a partial negative charge. Core@shell configurations are analogous to structures that can be chemically synthesised, and further detailed electronic analysis is discussed in the context of nanocluster applications to co-catalysed photocatalysis.
Highlights
Bimetallic nanoclusters (‘‘nanoalloys’’) have garnered much recent interest, as the combination of two metallic species can result in favourable synergistic effects, and lower production costs.[7]
We have investigated occupation of all the possible dopant sites for a single Au atom substituted into Ag147, which results in a composition of Au1Ag146, and repeated the opposite process for Ag included in Au147 to give Au146Ag1, with the results presented in Tables 2 and 3
Positioning of the Au dopant on (c) the (111) surface and (e) edge sites are greater in energy, implying reduced stability when the Au atom is on the surface (EGtoutpta = 0.161 and 0.228 eV, respectively)
Summary
Bimetallic nanoclusters (‘‘nanoalloys’’) have garnered much recent interest, as the combination of two metallic species can result in favourable synergistic effects, and lower production costs.[7]. Larger nanoalloys (10 r x r 100 atoms) favour structures that have an Ag-enriched surface and an Au-enriched core, as calculated using a combined empirical potentials and DFT approach,[47,48,49,50] a recent basin-hopping study by Cerbelaud et al, using an empirical potential that incorporates charge transfer effects, finds that Au atoms on the nanoalloy surface are energetically favourable and in better agreement with equivalent DFT calculations up to N = 201.51 Most recently, the kinetic ordering in 55-atom icosahedral AuAg clusters were investigated by Calvo et al.,[52] using discrete-path sampling with empirical potentials. By determining the most stable and/or the most catalytically suitable arrangements, particular morphologies may be proposed for experimental synthesis in the highlighted photocatalytic applications
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