Supramolecular Aggregation of Inorganic Molecules at Au(111) Electrodes under a Strong Ionic Atmosphere

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• 作者：Yong-Chun Fu ; Yu-Zhuan Su ; De-Yin Wu ; Jia-Wei Yan ; Zhao-Xiong Xie ; Bing-Wei Mao
• 刊名：Journal of the American Chemical Society
• 出版年：2009
• 出版时间：October 21, 2009
• 年：2009
• 卷：131
• 期：41
• 页码：14728-14737
• 全文大小：537K
• 年卷期：v.131,no.41(October 21, 2009)
• ISSN：1520-5126

文摘

Neutral inorganic molecules are generally weak in surface adsorption and intermolecular interactions. Self-assembly of such types of molecule would provide valuable information about various interactions. At electrochemical interfaces, the relative strength of these interactions may be modified through control of electrode potential and electrolyte, which may lead to the discovery of new structures and new phenomena. However, studies of this nature are as yet lacking. In this work, we consider the covalent-bound semimetal compound molecules, XCl₃ (X = Sb, Bi), as model systems of neutral inorganic molecules to investigate their self-assembly at electrochemical interfaces under a high ionic atmosphere. To fulfill such investigations, in situ STM and cyclic voltammetry are employed, and comparative experiments are performed on Au(111) in ionic liquids as well as aqueous solutions with high ionic strength. In the room temperature ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄), potential-dependent partial charge transfer between the Au surface and XCl₃ molecules creates a molecule−surface interaction and provides the driving force for adsorption of the molecules. Supramolecular aggregations of adsorbed XCl₃ are promoted through chlorine-based short-range intermolecular correlation under crystallographic constraint, while repulsive Coulombic interactions created between the partially charged aggregations facilitate their long-range ordering. For SbCl₃ molecules, hexagonally arranged 6- or 7-member clusters are formed at 0.08 to −0.2 V (vs Pt), which assemble into a secondary (√31 × √31)R8.9° structure. For BiCl₃ molecules, both the 6-membered hexagonal and 3-membered trigonal clusters are formed in the narrow potential range −0.3 to −0.35 V, and are also arranged into an ordered secondary structure. Comparative studies were performed with SbCl₃ in concentrated aqueous solutions containing 2 M HCl to simulate the strong ionic strength of the ionic liquid. Almost identical 6-/7-member clusters and long-range (√31 × √31)R8.9° structure are observed at −0.1 V, demonstrating the crucial role of strong ionic strength in such supramolecular aggregations. However, such supramolecular structures are modified and eventually destroyed as ionic strength is further increased by addition of NaClO₄ up to 6 M. The destructive changes of the supramolecular structures are attributed to the alteration of ion distribution in the double layer from cation-rich to anion-rich at increasing NaClO₄ concentration. This modifies and eventually breaks the balance of intermolecular and molecule−electrolyte interactions. Finally, the dynamic behavior of the SbCl₃ assembly is investigated down to molecular level. It has been demonstrated that the initial stage of assembly follows a two-dimensional nucleation and growth mechanism and has a potential-dependent rate that is closely related to the surface mobility of the SbCl₃ clusters. There is a probability that clusters can escape from an existing assembly domain or insert into a vacancy in such a domain while they can also relax with central or ring members in a dynamic fashion. These phenomena indirectly reflect the dynamic properties of cations from electrolytes at the interface. The rich information contained in the self-assembly behavior of SbCl₃ and BiCl₃ demonstrates that neutral inorganic molecules can be employed for fundamental studies of a variety of interesting issues, especially the interplay of various interfacial interactions.