Structural Evolution of Core–Shell Gold Nanoclusters: Au_n^– (n = 42–50)

Abstract

Gold nanoclusters have attracted great attention in the past decade due to their remarkable size-dependent electronic, optical, and catalytic properties. However, the structures of large gold clusters are still not well-known because of the challenges in global structural searches. Here we report a joint photoelectron spectroscopy (PES) and theoretical study of the structural evolution of negatively charged core-shell gold nanoclusters (Au) for n = 42-50. Photoelectron spectra of size-selected Au clusters are well resolved with distinct spectral features, suggesting a dominating structural type. The combined PES data and density functional calculations allow us to systematically identify the global minimum or candidates of the global minima of these relatively large gold nanoclusters, which are found to possess low-symmetry structures with gradually increasing core sizes. Remarkably, the four-atom tetrahedral core, observed first in Au, continues to be highly robust and is even present in clusters as large as Au. Starting from Au, a five-atom trigonal bipyramidal core appears and persists until Au. Au possesses a six-atom core, while Au and Au feature seven- and eight-atom cores, respectively. Notably, both Au and Au contain a pyramidal Au motif, which is stacked with another truncated pyramid by sharing a common 10-atom triangular face. The present study sheds light on our understanding of the structural evolution of the medium-sized gold nanoclusters, the shells and core as well as how the core-shell structures may start to embrace the golden pyramid (bulk-like) fragment.

Au60–: The Smallest Gold Cluster with the High-Symmetry Icosahedral Core Au13

“…24 Notably, our previous joint experimental and theoretical studies have shown that Au 33 to Au 42 are all core−shell clusters with a highly symmetric four-atom tetrahedral core. [25][26][27] Au 46 and Au 47 are also core−shell clusters with a pyramidal fragment stacked with another truncated pyramid. 27 In this study, our focus is placed on the size range of Au n -(n = 55−60).…”

“…[25][26][27] Au 46 and Au 47 are also core−shell clusters with a pyramidal fragment stacked with another truncated pyramid. 27 In this study, our focus is placed on the size range of Au n -(n = 55−60). We use the unbiased basin-hopping global optimization method combined with density-functional theory (DFT) optimization 28,30 to obtain a much larger population of low-energy isomers.…”

“…Our previous studies have shown that the inclusion of SO effects can give rise to nearly quantitative match between the simulated and measured PE spectra for gold clusters. [25][26][27][31][32][33][34][35][36] The gradient-corrected Perdew−Burke−Ernzerhof (PBE) exchange− correlation functional 37 and the double-numerical polarized (DNP) basis set with effective core potential (ECP), as implemented in the DMol 4.0 program, 38,39 is employed for the geometry optimization of about 1200−2000 isomers obtained from the unbiased basin-hopping search for each size of anionic gold clusters, with convergence of 1 × 10 -5 H. Among the top 100 low-energy isomers, those with the energy gap between the first and second vertical detachment energy (VDE) being close to the measured energy gap between the first and second VDE are selected for reoptimization using a higher-level theory of PBE0/TZP(ECP) (triple-ζ polarization) with inclusion of the relativistic effects under zeroth-order regular approximation (ZORA), as implemented in the ADF2013 software package. [40][41][42] The simulated PE spectra of these ADF-optimized candidate isomers (about 15 for each size of clusters) are then obtained by using the hybrid PBE0 43 functional with inclusion of the spin−orbit (SO) effects (hereafter referred as SO-PBE0 method), as implemented in the NWCHEM 6.6 44 software package.…”

“…Lastly, the simulated PE spectra are used to compare with the measured PE spectra, as done in previous studies. [25][26][27][31][32][33][34][35][36] The isomers that give rise to the best match with the measured PE spectra are identified as the most stable isomers in the experimental cluster beam.…”

Identification of stable adsorption sites and diffusion paths on nanocluster surfaces: an automated scanning algorithm

“…Such clusters are frequently found to be very stable despite their low symmetry. [34][35][36] Their non-trivial geometries however make them challenging targets for traditional MP computational strategies, which depend heavily on symmetry to reduce the complexity of the system and to keep the mental and computational burden tractable. For large clusters we can also relax multiple shells of atoms, thus allowing us to accurately capture most of the relaxation effects.…”