A Quantum Alloy: The Ligand-Protected Au25鈥?i>xAgx(SR)18 Cluster

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• 作者：Douglas R. Kauffman ; Dominic Alfonso ; Christopher Matranga ; Huifeng Qian ; Rongchao Jin
• 刊名：The Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：April 18, 2013
• 年：2013
• 卷：117
• 期：15
• 页码：7914-7923
• 全文大小：496K
• 年卷期：v.117,no.15(April 18, 2013)
• ISSN：1932-7455

文摘

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鈥?i>xAg_x(SC₂H₄Ph)₁₈ cluster (abbreviated as Au_25鈥?i>xAg_x, where x = 0鈥? Ag atoms) compared with the unsubstituted Au₂₅(SC₂H₄Ph)₁₈ cluster (abbreviated as Au₂₅). 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鈥揂g state mixing and charge redistribution within the Au_25鈥?i>xAg_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鈥?i>xAg_x absorbance spectrum, and rendered some optical transitions forbidden. In situ spectroelectrochemical experiments revealed charge-dependent Au_25鈥?i>xAg_x optical properties and oxidative photoluminescence quenching. Finally, O₂ adsorption studies have shown Au_25鈥?i>xAg_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鈥檚 quantized electronic structure will require a combination of computational modeling, optical spectroscopy, and nonaqueous electrochemistry.