ABSTRACT
Large-sized gold Aun- anion clusters exhibit structural characteristics drastically different from other coinage metals. Typically, coinage metal nanoclusters exhibit a 13-atom icosahedral core at the cluster size of 55. Gold clusters, contrarily, do not entail this core until the size reaches 60. Here, we investigated the robustness of the icosahedral core within the large-sized Aun- anion clusters. We found that the icosahedral core persists over the size of range of n = 61-66. To adapt the exceptional robustness of the icosahedral core, the shells of the clusters tend to undergo notable structural deformations with polygonal defects. As the cluster size increases from 61 to 66, the core starts to become distorted at n = 64 and the space between the core and shell becomes enlarged. To our knowledge, this is the first theoretical study that provides the simulated photoelectron spectra of the two largest sized gold clusters: Au65- and Au66-.
ABSTRACT
We report the first joint anion photoelectron spectroscopy and theoretical study on how O2-binding affects the structures of medium even-sized gold clusters, Aun- (n = 20-34), a special size region that entails a variety of distinct structures. Under the temperature conditions in the current photoelectron spectroscopy experiment, O2-bound gold clusters were observed only for n = 22-24 and 34. Nevertheless, O2 binding with the clusters in the size range of n = 20-34 can be still predicted based on the obtained global-minimum structures. Consequently, a series of structural transitions, from the pyramidal to fused-planar to core-shell structures, are either identified or predicted for the AunO2- clusters, where the O2-binding is in either superoxo or peroxo fashion. The identified global-minimum structures of AunO2- (n = 20-34) also allow us to gain improved understanding of why the clusters Aun- (n = 26-32) are less reactive with O2 in comparison to others.
ABSTRACT
We perform a joint photoelectron spectroscopy and theoretical study to investigate CO adsorption sites on midsized gold clusters, Au n- ( n = 21-25), a special size region that bridges the highly symmetric pyramidal cluster Au20- (Li et al. Science 2003, 299, 864) and the prevailing core-shell clusters starting from Au26- (Schaefer et al. ACS Nano 2014, 8, 7413). Particular attention is placed on whether the CO binding can significantly change structures of the host clusters in view of the fact that the size-dependent structural change already occurs for bare gold clusters in this size range. A transition from hollow-tubular to fused-planar structures is identified for the Au nCO- clusters even though the CO molecule mostly binds to an apex gold atom. The computed CO adsorption energy and HOMO-LUMO gap of the gold clusters suggest that among the five gold clusters the Au23- cluster exhibits the strongest CO binding and thereby could be a good catalytic model system.
ABSTRACT
We performed a combined theoretical and experimental photoelectron spectroscopy study of the structural evolution of gold anion clusters Aun- in the size range n = 21-25, a special size range for gold anion clusters where extensive structural changes from the pyramidal structure at Au20- toward the core-shell structure at Au26- were expected to occur. Density functional theory calculations with inclusion of spin-orbit effects were employed to produce the simulated spectra for the selected low-energy isomers obtained from basin-hopping global minimum search. The comparison of these simulated spectra with reasonably well-resolved experimental photoelectron spectra resulted in the identification of the low-lying structures of the gold clusters. The fused-planar and hollow-tubular structures are found dominant in this special size range. The highly stable tetrahedral Au20 unit (viewed as the fragment of face-centered cubic (FCC) bulk gold) was found intact only in the minor isomer at n = 21, whereas hollow-tubular structures were found prevalent in the n = 22-25 range. At n = 25, the dominant structure is a hollow-tubular one with two of gold pyramids fused together, but not a core-shell one as previously believed.
ABSTRACT
Besides the size and structure, compositions can also dramatically affect the properties of alloy nanoclusters. Due to the added degrees of freedom, determination of the global minimum structures for multi-component nanoclusters poses even greater challenges, both experimentally and theoretically. Here we report a systematic and joint experimental/theoretical study of a series of gold-aluminum alloy clusters, AuxAly(-) (x + y = 7,8), with various compositions (x = 1-3; y = 4-7). Well-resolved photoelectron spectra have been obtained for these clusters at different photon energies. Basin-hopping global searches, coupled with density functional theory calculations, are used to identify low-lying structures of the bimetallic clusters. By comparing computed electronic densities of states of the low-lying isomers with the experimental photoelectron spectra, the global minima are determined. It is found that for y ≥ 6 there is a strong tendency to form the magic-number square bi-pyramid motif of Al6(-) in the AuxAly(-) clusters, suggesting that the Al-Al interaction dominates the Au-Au interaction in the mixed clusters. A closely related trend is that for x > 1, the gold atoms tend to be separated by Al atoms unless only the magic-number Al6(-) square bi-pyramid motif is present, suggesting that in the small-sized mixed clusters, Al and Au components do not completely mix with one another. Overall, the Al component appears to play a more dominant role due to the high robustness of the magic-number Al6(-) square bi-pyramid motif, whereas the Au component tends to be either "adsorbed" onto the Al6(-) square bi-pyramid motif if y ≥ 6, or stays away from one another if x < y < 6.
