Controlled Synthesis of Au36(SR)24 (SR = SPh, SC6H4CH3, SCH(CH3)Ph, and SC10H7) Nanoclusters

In the past decade, gold nanoclusters of atomic precision have been demonstrated as novel and promising materials for potential applications in catalysis and biotechnology because of their optical properties and photovoltaics. The Au-36(SR)(24) nanocluster is one of the most well-known clusters, which is directly converted from the Au-38(SR)(24) cluster through a "ligand-exchange" process. It consists of an Au-28 kernel with a truncated face-centered cubic (FCC) framework exposing the (111) and (100) facets. Here we report a simple protocol to prepare Au-36(SR)(24) nanoclusters, ligated by aliphatic and aromatic thiolate ligands (SR = SPh, SC6H4CH3, SCH(CH3)Ph, and SC10H7) via a "size-focusing" process. First, polydisperse Au nanoparticles were synthesized and isolated, and then reacted under harsh conditions in the presence of excess thiol ligands at relatively high etching temperatures (80 degrees C). The as-synthesized Au-36(SR)(24) nanoclusters were characterized and analyzed by UV-Vis absorption spectroscopy, electrospray ionization (ESI), and matrix-assisted laser desorption ionization (MALDI) mass spectrometry, as well as thermogravimetric analysis (TGA). All the Au-36(SR)(24) nanoclusters showed two step-like optical absorption peaks at similar to 370 and 580 nm in the UV-vis spectra. Only one strong set of nanocluster-ion peaks centered at an m/z of 10517.0 was observed in the ESI mass spectrum of the Au-36(SCH(CH3)Ph)(24) nanocluster. This could be assigned to the [Au-36(SCH(CH3)Ph)(24)Cs](+) species, and was a strong indicator of the high purity of the as-obtained Au-36 cluster samples produced on a small scale. The TGA profile showed 31.67% organic weight loss of the nanocluster, matching well with the expected theoretical value of 31.71%. The Au-SR bond in the gold nanoclusters was broken at similar to 180 degrees C in a normal air atmosphere. Fragments of the Au-36(SR)(24) clusters capped with different thiolate ligands, which were mainly caused by the strong laser intensity during the analysis, were detected in the MALDI mass spectra. This interesting phenomenon was also observed in the case of Au-25(SR)(18), and could be due to the inherent properties of the Au-SR bond on the surface of the gold nanoclusters. Finally, the optical properties of the Au-36(SR)(24) nanoclusters were found to be influenced by the capping thiolate ligands. Compared to the UV-Vis spectrum of the Au-36(SCH(CH3)Ph)(24) cluster, the optical spectra of the other three Au-36 clusters were red-shifted (similar to 3 nm for Au-36(SPh)(24), 5 nm for Au-36(SC6H4CH3)(24), and 13 nm for the (Au-36(SNap)(24) clusters). This shift could be explained by the electron transfer occurring from the electron-rich aromatic ligands to the Au kernel. The electron transfer capacity followed the order -SNap > -SC6H4CH3 > -SPh > -SCH(CH3)Ph. Overall, this study demonstrates the effectiveness and promising application of ligand engineering for tailoring the electronic properties of Au nanoclusters.