A Quantum Alloy: The Ligand-Protected Au_25–xAg_x(SR)₁₈ Cluster

Abstract

Recent synthetic advances have produced very small (sub-2 nm), ligand-protected mixed-metal clusters. Realization of such clusters allows the investigation of fundamental questions: (1) Will heteroatoms occupy specific sites within the cluster? (2) How will the inclusion of heteroatoms affect the electronic structure and chemical properties of the cluster? (3) How will these very small mixed-metal systems differ from larger, more traditional alloy materials? In this report we provide experimental and computational characterization of the ligand-protected mixed-metal Au 25−x Ag x (SC 2 H 4 Ph) 18 cluster (abbreviated as Au 25−x Ag x , where x = 0−5 Ag atoms) compared with the unsubstituted Au 25 (SC 2 H 4 Ph) 18 cluster (abbreviated as Au 25 ). Density functional theory analysis has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. X-ray photoelectron spectroscopy revealed Au−Ag state mixing and charge redistribution within the Au 25−x Ag x cluster. Optical spectroscopy and nonaqueous electrochemistry indicate that Ag heteroatoms increased the cluster lowest unoccupied molecular orbital (LUMO) energy, introduced new features in the Au 25−x Ag x absorbance spectrum, and rendered some optical transitions forbidden. In situ spectroelectrochemical experiments revealed charge-dependent Au 25−x Ag x optical properties and oxidative photoluminescence quenching. Finally, O 2 adsorption studies have shown Au 25−x Ag x clusters can participate in photomediated charge-transfer events. These results illustrate that traditional alloy concepts like metal-centered state mixing and internal charge redistribution also occur in very small mixed-metal clusters. However, resolution of specific heteroatom locations and their impact on the cluster's quantized electronic structure will require a combination of computational modeling, optical spectroscopy, and nonaqueous electrochemistry.

Microwave-assisted synthesis of photoluminescent glutathione-capped Au/Ag nanoclusters: A unique sensor-on-a-nanoparticle for metal ions, anions, and small molecules

“…Because the Au/Ag NCs possess a lower 4f 7/2 binding energy than that of bare Au NCs, we believe that this is caused by the Au-Ag charge redistribution that occurred within the clusters, which balances the electron-withdrawing effects of the thiolate ligand. This phenomenon has been previously observed for Au 25-x Ag x clusters [22,47].…”

Interconversion between Superatomic 6-Electron and 8-Electron Configurations of M@Au24(SR)18 Clusters (M = Pd, Pt)

“…In addition, the Pd 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3d 3/2 XPS peak observed at 341.4 eV, which corresponds to Pd 0 , suggests that Pd atom is located at the center of PdAu 24 (SR) 18 ; a positive shift would be expected if it was located at the surface of the core or in the ligand shell. 19,32,38 Similarly, the Pt 4f 5/2 and 4f 7/2 peaks observed at respectively 75.1 and 71.8 eV suggest that the doped Pt atom is located at the center of PtAu 24 (SR) 18 . 32 Transmission electron microscopy (TEM) images of the isolated PdAu 24 (SR) 18 and PtAu 24 (SR) 18 clusters in SI Figure S2 show that the average core sizes are both around 1.…”

Toward the creation of stable, functionalized metal clusters

“…108 18 and reported that, because Ag doping 35 raises the orbital energy of the cluster's LUMO, the HOMO−LUMO gap is increased. 111 All these results indicate that Ag doping is an effective means of creating stable clusters with larger HOMO−LUMO gaps than the original magic Au 25 (SR) 18 . 24 and Au 144-n Ag n (SR) 60 40 Regarding the larger Ag-doped clusters, Dass et al have successfully synthesized larger Ag-doped magic Au n (SR) m clusters, with up to 12 Ag atoms in Au 38 (SC 2 H 4 Ph) 24 and up to 60 in Au 144 (SC 2 H 4 Ph) 60 .…”