Exploring the Role of LiI Additive in Regulating the Morphology of Lithium Deposition by In Situ AFM

The morphology of Li deposition has a profound effect on the cycling performance of a Li-metal battery. However, in situ research studies on the targeted regulation of Li deposition behavior and exploration of the thermodynamic and kinetic factors affecting Li deposition morphology are quite limited. Herein, we employ in situ atomic force microscopy (AFM) to investigate the effects of lithium iodide (LiI) additive on the evolution of Li deposition morphology in propylene carbonate (PC)-based electrolyte. The spherical Li deposits formed via three-dimensional (3D) nucleation and growth partially transform into planar morphology after the addition of 10 mM LiI, leading to a combined 3D nucleation and quasi-two-dimensional (2D) growth behavior. Complemented by X-ray photoelectron spectroscopy (XPS) profiling analysis and AFM nanomechanical characterization, the composition, structure, and properties of solid electrolyte interphases (SEIs) are comprehensively inspected. The SEI formed in the LiI-containing electrolyte bears an inorganic–organic hybrid structure with improved mechanical flexibility, compared with the rigid and thick SEI formed in the basic electrolyte. Meanwhile, the possible interfacial adsorption of Ĩ– contributes to the uniform distribution of Li+ and rapid diffusion of the Li atom. This work reveals the importance of an additive in regulating the Li deposition behavior, which is helpful in improving the Li-metal battery performance

Competitive Adsorption and Surface Alloying: Underpotential Deposition of Sn on Sulfate-Covered Cu(111)

We report an in situ scanning tunneling microscopy study on Sn underpotential deposition (UPD) on sulfate-covered Cu(111) electrode surfaces, which represents a system of anion adsorption in an extreme situation in terms of anion−anion and anion−substrate interactions. Owing to the strong sulfate adsorption, the UPD is initiated with “parasitical” adsorption of Sn adatoms almost exclusively along the periphery of the sulfate adlayer, as manifested in the appearance of brightened Moiré structure modulation at the terrace edge. A Sn-induced local enhancement of tunneling is offered to explain the brightening of the Moiré structure. The initially deposited Sn adatoms displace the sulfate ions from the edge and push themselves toward the interior region of the terrace, maintaining the brightened Moiré pattern at the forefront sites. Consequently, a region free of sulfate is provided for further Sn deposition. In addition to terrace edge reshaping, surface alloying is confirmed by the course-dependent anodic stripping, in which the Sn-covered region of the terrace (excluding the forefront sites) becomes fragmented. The surface alloying is favored, in view of strain relief, by the expanded topmost layer of the reconstructed Cu(111). The overall deposition process may be complete within a time window of several minutes. The present work reveals that when anion−substrate and anion−anion interactions are sufficiently strong and comparable to adatom−substrate interaction, the formation of metal adlayer is severely restricted and novel features of UPD are displayed

Mobility and Reactivity of Oxygen Adspecies on Platinum Surface

The adsorption and mobility of oxygen adspecies on platinum (Pt) surface are crucial for the oxidation of surface-absorbed carbon monoxide (CO), which causes the deactivation of Pt catalyst in fuel cells. By employing nanoelectrode and ultramicroelectrode techniques, we have observed the surface mobility of oxygen adspecies produced by the dissociative adsorption of H₂O and the surface reaction between the oxygen adspecies and the preadsorbed CO on the Pt surface. The desorption charge of oxygen adspecies on a Pt nanoelectrode has been found to be in proportion to the reciprocal of the square root of scan rate. Using this information, the apparent surface diffusion coefficient of oxygen adspecies has been determined to be (5.61 ± 0.84) × 10^–10 cm²/s at 25 °C. During the surface oxidation of CO, two current peaks are observed, which are attributed to CO oxidation at the Pt/electrolyte interface and the surface mobility of the oxygen adspecies on the adjacent Pt surface, respectively. These results demonstrate that the surface mobility of oxygen adspecies plays an important role in the antipoisoning and reactivation of Pt catalyst

Electrochemical Growth of Three-Dimensional Nanostripe Architecture of Antimony on Cu(100)

We report the electrochemical growth of a well-oriented 3D nanostripe architecture of Sb on Cu(100), the system of which has a large crystallographic misfit but still shows thickness-independent growth features. A coincidence between Sb and Cu(100) occurs with 5 × dSb[110] = 6 × dCu[01̄0] and dSb[11̄0] = 2 × dCu[001]. Two observed factors are credited to the formation of periodically separated 2D arrays of ∼1-nm-wide nanostripes in the nanostructure; they are the high number of coincidences in Sb[11̄0] as well as the discontinuity of coincidence lattices in Sb[110] formed to release the residual misfit. The vertical alignment of the nanostripes is achieved following the layer-by-layer epitaxy of nanostripes, leading to the formation of a thick nanostripe film with regular straw-mat-type stacking

Extending the Capability of STM Break Junction for Conductance Measurement of Atomic-Size Nanowires: An Electrochemical Strategy

Extending the Capability of STM Break Junction for Conductance Measurement of Atomic-Size Nanowires: An Electrochemical Strateg

Unraveling the Mechanism of Very Initial Dendritic Growth Under Lithium Ion Transport Control in Lithium Metal Anodes

Lithium metal deposition is strongly affected by the intrinsic properties of the solid-electrolyte interphase (SEI) and working electrolyte, but a relevant understanding is far from complete. Here, by employing multiple electrochemical techniques and the design of SEI and electrolyte, we elucidate the electrochemistry of Li deposition under mass transport control. It is discovered that SEIs with a lower Li ion transference number and/or conductivity induce a distinctive current transition even under moderate potentiostatic polarization, which is associated with the control regime transition of Li ion transport from the SEI to the electrolyte. Furthermore, our findings help reveal the creation of a space-charge layer at the electrode/SEI interface due to the involvement of the diffusion process of Li ions through the SEI, which promotes the formation of dendrite embryos that develop and eventually trigger SEI breakage and the control regime transition of Li ion transport. Our insight into the very initial dendritic growth mechanism offers a bridge toward design and control for superior SEIs

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, 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

Investigation of DNA Orientation on Gold by EC-STM

The immobilization of thiol-derivatized DNA on a Au (111) single crystal surface by self-assembly has been investigated by electrochemical scanning tunneling microscopy (EC-STM). Continuous potential-dependent orientation changes of double-stranded oligodeoxynucleotides (ODN) have been observed in a certain potential range from 200 to 600 mV (versus SCE). It is suggested that the DNA duplexes stand straight on the gold surface at potentials negative of the potential of zero charge (pzc) and then lay down on the surface when the potential shifts positively. These results are in agreement with the expectation based on the Coulombic interaction consideration between negatively charged DNA helices and gold surface. As the applied potential shifts positively, the surface charge changes from negative to positive, that is, the Coulombic force between negatively charged DNA helices and gold surfaces changes from repulsion to attraction. However, for the single-stranded oligodeoxynucleotides, no distinct changes in the surface structure were observed with the applied potential

Self-Assembly of a Rh(I) Complex on Au(111) Surfaces and Its Electrocatalytic Activity toward the Hydrogen Evolution Reaction

The self-assembly of a Wilkinson type of catalyst molecule, trans-RhCl(CO)(PPh3)2, on Au(111) surfaces and its electrocatalytic properties toward the hydrogen evolution reaction (HER) are investigated by employing scanning tunneling microscopy (STM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). The self-assembled monolayers of RhCl(CO)(PPh3)2 are prepared from either dichloromethane or aqueous solutions, but the ordered structures are observed only in atmospheric conditions after solvents evaporate. In the electrolyte solutions, disordered yet uniformly sized spherical clusters of individual molecules are observed as a result of the conformational change of the molecule by the solvation effect of water. The immobilized Rh(I) molecular clusters are electrochemically stable in a wide potential window and exhibit remarkable electrocatalytic activity toward HER in perchloric acid solutions. Several comparative experiments involving similar types of immobilized complexes containing Ru(I) and Ir(I) centers and solution species of RhCl(CO)(PPh3)2 are performed. However, none of them are found to be electroactive to HER. The Tafel slope of HER on the Rh(I) complex modified Au(111) electrode in 0.1 M HClO4 is determined to be −0.061 V, which is almost in the middle of those on bare Au(111) (−0.093 V) and Rh covered (θRh ≈ 0.3) Au(111) (−0.034 V) electrodes. XPS measurements reveal a valence change of Rh(I) to Rh(0), and an oxidative addition and reductive elimination mechanism is suggested for the enhancement of HER

Resolving Fine Structures of the Electric Double Layer of Electrochemical Interfaces in Ionic Liquids with an AFM Tip Modification Strategy

We report enhanced force detection selectivity based on Coulombic interactions through AFM tip modification for probing fine structures of the electric double layer (EDL) in ionic liquids. When AFM tips anchored with alkylthiol molecular layers having end groups with different charge states (e.g., −CH₃, −COO^–, and −NH₃⁺) are employed, Coulombic interactions between the tip and a specified layering structure are intensified or diminished depending on the polarities of the tip and the layering species. Systematic potential-dependent measurements of force curves with careful inspection of layered features and thickness analysis allows the fine structure of the EDL at the Au(111)–OMIPF₆ interface to be resolved at the subionic level. The enhanced force detection selectivity provides a basis for thoroughly understanding the EDL in ionic liquids