Yong-Chun Fu

The Electrode/Ionic Liquid Interface: Electric Double Layer and Metal Electrodeposition

The last decade has witnessed remarkable advances in interfacial electrochemistry in room-temperature ionic liquids. Although the wide electrochemical window of ionic liquids is of primary concern in this new type of solvent for electrochemistry, the unusual bulk and interfacial properties brought about by the intrinsic strong interactions in the ionic liquid system also substantially influence the structure and processes at electrode/ionic liquid interfaces. Theoretical modeling and experimental characterizations have been indispensable in reaching a microscopic understanding of electrode/ionic liquid interfaces and in elucidating the physics behind new phenomena in ionic liquids. This Minireview describes the status of some aspects of interfacial electrochemistry in ionic liquids. Emphasis is placed on high-resolution and molecular-level characterization by scanning tunneling microscopy and vibrational spectroscopies of interfacial structures, and the initial stage of metal electrodeposition with application in surface nanostructuring.

An STM study on nonionic fluorosurfactant zonyl FSN self-assembly on Au(111): large domains, few defects, and good stability.

Nonionic Fluorosurfactant Zonyl FSN self-assembly on Au(111) is investigated with scanning tunneling microscopy under ambient conditions. STM reveals that the FSN forms SAMs on Au(l11) with very large domain size and almost no defects. A (mean square root of 3 x mean square root of 3)R3 degree arrangement of the FSN SAM on Au(111) is observed. The SAMs show excellent chemical stability and last for at least a month in atmospheric conditions. The structure and stability of the FSN SAMs are compared with those of alkanethiols SAMs. It is expected that FSN may serve as a new kind of molecule to form SAMs for surface modification, which would benefit wider applications for various purposes.

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

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, XCl(3) (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 (BMIBF(4)), potential-dependent partial charge transfer between the Au surface and XCl(3) molecules creates a molecule-surface interaction and provides the driving force for adsorption of the molecules. Supramolecular aggregations of adsorbed XCl(3) 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 SbCl(3) molecules, hexagonally arranged 6- or 7-member clusters are formed at 0.08 to -0.2 V (vs Pt), which assemble into a secondary ( radical31 x radical31)R8.9 degrees structure. For BiCl(3) 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 SbCl(3) 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 ( radical31 x radical31)R8.9 degrees 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 NaClO(4) 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 NaClO(4) concentration. This modifies and eventually breaks the balance of intermolecular and molecule-electrolyte interactions. Finally, the dynamic behavior of the SbCl(3) 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 SbCl(3) 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 SbCl(3) and BiCl(3) demonstrates that neutral inorganic molecules can be employed for fundamental studies of a variety of interesting issues, especially the interplay of various interfacial interactions.

An in situ STM study of cobalt electrodeposition on Au(111) in BMIBF4 ionic liquid

The initial stage of Co electrodeposition on Au(111) from CoCl2 and Co(BF4)2 in a room-temperature ionic liquid of BMIBF4 is investigated by in situ STM. Preferential nucleation of Co at structure imperfections of the reconstructed Au(111) surface is observed for deposition from CoCl2 at a surprisingly negative potential (−2.05 V vs. Pt wire). Monoatomic-height Co clusters of 2–3 nm in diameter are formed, which develop into Co islands with size confinement of 3–4 nm after prolonged deposition. The large driving force required for Co deposition from CoCl2 is attributed to the molecular type of the salt in the ionic liquid together with the tip shielding effect. This is proved by the pronounced difference of Co deposition from Co(BF4)2, which takes place at a much less negative potential and proceeds in a 3D progressive nucleation and growth mode without preference in nucleation sites. Surface alloying accompanies the island formation, which is confirmed by pit generation upon stripping of the Co islands. The results are discussed in comparison with features of Co deposition in UHV and in aqueous solutions.

Growth and shape-ordering of iron nanostructures on Au single crystalline electrodes in an ionic liquid: a paradigm of magnetostatic coupling.

Fe electrodeposition on Au(111) and Au(100) in BMIBF(4) ionic liquid is found to form hitherto unreported shape-ordered nanoscale morphologies of pseudorods and pseudosquare rings, respectively, both composed of grains of 4-7 nm. The manner of growth of the square rings is a ring-on-ring structure with enlarging side length and slightly protruding four corners. The generality of the growth mechanism is verified by the formation of almost exactly the same shape-ordered Fe nanostructures on Pt, i.e., pseudorod structure on Pt(111) and pseudosquare rings Pt(100). These structures are explained within the framework of magnetostatic interactions of spontaneously magnetized grains under crystallographic constraint of the substrate surface, which result in an antiparallel arrangement in magnetization of the grains at pseudorods and magnetic flux closure at the pseudosquare rings. The closed magnetic flux further leads to magnetic field-enhanced growth at the four corners and the outer peripheries of the pseudosquare rings. The observed shape-ordering of the Fe thin film serves as a paradigm of magnetostatic coupling, in which the roles of ionic liquid as surfactant and magnetic media may not be underestimated. The present work adds a new dimension to electrodeposition in ionic liquid, by which new magnetic film structures may be expected.

Precursor adsorption of SbBr3 on au(111) and au(100) for antimony underpotential deposition in a BMIBF4 ionic liquid a comparison with SbCl3

By employing in-situ STM, we have studied precursor adsorption and initial stage of Sb UPD on Au(111) and Au(100) in a SbBr3-containing BMIBF4 ionic liquid. It has been observed that the adsorption of SbBr3 selectively takes place on Au(100), in contrast to that of SbCl3, with formation of chains with structure as well as squares with structure. The chains are composed of pairs of SbBr3 molecules, which are bridged with the adjacent SbBr3 molecules in the chain to form the anisotropy along either of the two characteristic directions of Au(100). The square configured structure is composed of periodically arranged monomeric and dimeric SbBr3, which will gradually replace the chain structure. Regardless of the precursor adsorption, however, the initial stage of Sb UPD is characterized by the formation of atomic strips, like in the SbCl3-containing system, which reveals the consistency of discharging behavior of Sb(III) from the SbCl3 and SbBr3 molecules. Together with the findings from the SbCl3-containing system, the results are discussed in view of matching of structures between the pyramidal antimony tri-halides and the crystalline surfaces. The role of the ionic liquid as well as properties of the electrified interface will also be mentioned.