In-situ STM and AFM Studies on Electrochemical Interfaces in imidazolium-based ionic liquids

Manipulation of Contact Angle Hysteresis at Electrified Ionic Liquid-Solid Interfaces

Room-temperature ionic liquids (RTILs) are intriguing fluids that have drawn much attention in applications ranging from tribology and catalysis to energy storage. With strong electrostatic interaction between ions, their interfacial behaviors can be modulated by controlling energetics of the electrified interface. In this work, we report atomic-force-microscope measurements of contact angle hysteresis (CAH) of a circular contact line formed on a micron-sized fiber, which is coated with a thin layer of conductive film and intersects an RTIL-air interface. The measured CAH shows a distinct change by increasing the voltage U applied on the fiber surface. Molecular dynamics simulations were performed to illustrate variations of the solidlike layer in the RTIL adsorbed at the electrified interface. The integrated experiments and computations demonstrate a new mechanism to manipulate the CAH by rearrangement of interfacial layers of RTILs induced by the surface energetics.

Theoretical Insight into the Imidazolium-Based Ionic Liquid Interface Structure and Differential Capacitance on Au(111): Effects of the Cationic Substituent Group

Electric double layers (EDLs) play a key role in the electrochemical and energy storage of supercapacitors. It is important to understand the structure and properties of EDLs. In this work, quantum chemical calculations and molecular dynamics (MD) simulations are used to study the microstructure of EDLs of four different substituents of imidazolium-based bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) on the Au(111) surface. It is shown that the particle interactions influence the different arrangements of the anion and cation. More alkyl substitutions and longer alkyl chains result in a higher ELUMO and thus a stronger interaction energy between cations and electrodes. Strong interactions produce linear patterns of anions/cations on the electrode and a maximum value of differential capacitance near PZC, whereas weak interactions generate worm-like patterns of anions/cations on Au(111) and a minimum value of differential capacitance near the PZC. We hold the opinion that the alkyl substitution has more effects on the EDLs. Our analysis provides a new perspective on EDLs structures at the atomic and molecular level. This study provides a good basis and guidance for further understanding the interface phenomena and characteristics of ionic liquids in electrochemical and energy device applications.

Influence of interfacial water and cations on the oxidation of CO at the platinum/ionic liquid interface

CO oxidation is fundamental to the development of new catalyst materials for fuel cells and key for complete oxidation of small alcohols like methanol or ethanol on Pt catalysts. So far, room-temperature ionic liquids (RTIL) have been used to modify the selectivity and activity in electrocatalysis. In order to understand the mechanism of CO oxidation in RTIL in more detail we have investigated this reaction at the Pt(111)/1-butyl-3-methylimidazolium trifluorosulfonylimide [BMIM][NTf₂] electrode/electrolyte interface as a function of H₂O concentration and electrode potential with in situ sum-frequency generation (SFG) spectroscopy and infrared absorption spectroscopy (IRAS). Using SFG spectroscopy, we address the changes of linearly bonded CO molecules on Pt(111), while we monitor the changes in the bulk electrolyte with IRAS through vibrational bands from H₂O, CO₂ and CO. The presence of water in [BMIM][NTf₂] shifts the onset potential for CO oxidation by more than 200 mV when the water concentration is increased from 0.01 to 1.5 M, which we relate to the incorporation and the availability of water at the electrode/electrolyte interface. The nature of the RTIL cation has also a large effect on the surface excess of H₂O since RTILs like [BMMIM][NTf₂] and [BMPyrr][NTf₂] which are prone to form closed-packed structures, can block the incorporation of water and lead to more sluggish CO oxidation with larger overpotentials and oxidation in a much wider potential range for which we provide evidence by additional SFG measurements. These results clearly show that the choice of the RTIL is important for CO oxidation on Pt(111) electrode surfaces - an observation that is likely highly relevant also to other catalysts and catalytic reactions that require the presence of interfacial water.

Generating intense electric fields in 2D materials by dual ionic gating

The application of an electric field through two-dimensional materials (2DMs) modifies their properties. For example, a bandgap opens in semimetallic bilayer graphene while the bandgap shrinks in few-layer 2D semiconductors. The maximum electric field strength achievable in conventional devices is limited to ≤0.3 V/nm by the dielectric breakdown of gate dielectrics. Here, we overcome this limit by suspending a 2DM between two volumes of ionic liquid (IL) with independently controlled potentials. The potential difference between the ILs falls across an ultrathin layer consisting of the 2DM and the electrical double layers above and below it, producing an intense electric field larger than 4 V/nm. This field is strong enough to close the bandgap of few-layer WSe₂, thereby driving a semiconductor-to-metal transition. The ability to apply fields an order of magnitude higher than what is possible in dielectric-gated devices grants access to previously-inaccessible phenomena occurring in intense electric fields.

The application of electric fields >1 V/nm in solid state devices could provide access to unexplored phenomena, but it is currently difficult to implement. Here, the authors develop a double-sided ionic liquid gating technique to generate electric fields as large as 4 V/nm across few-layer WSe₂, leading to field-induced semiconductor-to-metal transitions.

In Situ Raman Monitoring of Potential-Dependent Adlayer Structures on the Au(111)/Ionic Liquid Interface

Probing the adlayer structures on an electrode/electrolyte interface is one of the most important tasks in modern electrochemistry for clarifying the electrochemical processes. Herein, we have combined cyclic voltammetry and electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy techniques to explore the potential-dependent adlayer structures on Au(111) in a room-temperature ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) without or with pyridine (Py). It is clearly found that the BMI⁺ cations strongly adsorb on the negatively charged surface with a flat-lying orientation, leaving a little space for Py adsorption. Upon increasing the potentials of the electrode, the variations of Raman band intensities and frequencies reveal that the interaction between the BMI⁺ cations and the Au surface becomes weak; meanwhile, the Py adsorption becomes strong, and its geometry turns from flat, tilted to vertical. Finally, BMI⁺ cations desorb and leave plenty of surface sites for Py adsorption in bulk solution, and a N-bonded compact Py adlayer is formed on the very positively charged surface. This causes obvious anodic peaks in cyclic voltammograms, and the peak currents increase with the square root of the scanning rate. The present work provides a fair molecular-level understanding of electrochemical interfaces and molecular adsorption of Py in ionic liquids.

Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces

Charge transfer and energy conversion processes at semiconductor/electrolyte interfaces are controlled by local electric field distributions, which can be especially challenging to measure. Herein we leverage the low vapor pressure and vacuum compatibility of ionic liquid electrolytes to undertake a layer-by-layer, ultra-high vacuum deposition of a prototypical ionic liquid EMIM⁺ (1-ethyl-3-methylimidazolium) and TFSI^- (bis(trifluoromethylsulfonyl)-imide) on the surfaces of different electronic materials. We consider a case-by-case study between a standard metal (Au) and four printed electronic materials, where interfaces are characterized by a combination of X-ray and ultraviolet photoemission spectroscopies (XPS/UPS). For template-stripped gold surfaces, we observe through XPS a preferential orientation of the TFSI anion at the gold surface, enabling large electric fields (∼10⁸ eV m^-1) within the first two monolayers detected by a large surface vacuum level shift (0.7 eV) in UPS. Conversely, we observe a much more random orientation on four printable semiconductor surfaces: methyl ammonium lead triiodide (MAPbI₃), regioregular poly(3-hexylthiophene-2,5-diyl (P3HT)), sol-gel nickel oxide (NiO_x), and PbI_x-capped PbS quantum dots. For the semiconductors considered, the ionization energy (IE) of the ionic liquid at 3 ML coverage is highly substrate dependent, indicating that underlying chemical reactions are dominating interface level alignment (electronic equilibration) prior to reaching bulk electronic structure. This indicates there is no universal rule for energy level alignment, but that relative strengths of Lewis acid/base sites and ion-molecular interactions should be considered. Specifically, for P3HT, interactions are found to be relatively weak and occurring through the π-bonding structure in the thiophene ring. Alternatively, for NiO_x, PbS/PbI_x quantum dots, and MAPbI₃, our XPS data suggest a combination of ionic bonding and Lewis acid/base reactions between the semiconductor and IL, with MAPbI₃ being the most reactive surface. Collectively, our results point towards new directions in interface engineering, where strategically chosen ionic liquid-based anions and cations can be used to preferentially passivate and/or titrate surface defects of heterogeneous surfaces while simultaneously providing highly localized electric fields. These opportunities are expected to be translatable to opto-electronic and electrochemical devices, including energy conversion and storage and biosensing applications.

Characterization of the Interface Structure of 1-Ethyl-2,3-alkylimidazolium Bis(trifluoromethylsulfonyl)imide on a Au(111) Surface with Molecular Dynamics Simulations

As a new type of green electrolyte, ionic liquids have been extensively and successfully used in electrochemical systems. It is extremely important to understand the structure and characteristics of their electric double layers. The microscopic structures of room-temperature ionic liquids 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([Emmim]TFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim]TFSI) were studied on a flat Au(111) surface using molecular dynamics simulations. Since the interactions of [Emmim]TFSI, [Emmim]⁺, and TFSI^- with the Au(111) surface are stronger than those of molecules (or ions) in the [Emim]TFSI system, the linear arrangement of [Emmim]TFSI and the worm-like pattern of the [Emim]TFSI system can be found near the Au(111) surface. Meanwhile, cations are all parallel to the electrode in the [Emmim]TFSI/Au(111) system and tilted toward the surface in the [Emim]TFSI/Au(111) system. TFSI^- presents trans and cis conformations in [Emim]TFSI and [Emmim]TFSI systems adjacent to Au(111), respectively. A Helmholtz-like layer structure with alternating oscillations of anionic and cationic layers can be found in the [Emim]TFSI system, while the molecular layer with cations and anions existing simultaneously can be found in [Emmim]TFSI. Our results confirm that the substitution of hydrogen on C1 by methyl groups in the imidazole ring increases the interaction between the particles. It has also been proved that the change in the anion conformation and cation orientation in the [Emmim]TFSI system can be attributed to the different interaction energies of various particles. The above reasons ultimately make the images on Au(111) different in the two systems. The results provide a new perspective for studying the structure of double layers. They are helpful in deepening the understanding of the interface behavior of ionic liquids and providing a theoretical basis for the design of functional ionic liquids that are suitable for electrochemical equipment.

Batch preparation of gold nanoparticles with highly uniform morphology and tunable plasmonic properties

The plasmonic properties of individual metallic nanostructures are of great importance for application in surface science, materials science, and nanophotonics. Herein, being facilitated with a home-made flow device and pulsed laser irradiation, we proposed a batch preparation protocol towards spherical Au nanoparticles (Au NPs) and cage shell entrapped spherical core nanoparticles (Au@cAu NPs) with highly uniform morphology and a tunable size distribution. The Fano resonance behavior exhibited by the effective interaction between spherical Au NPs and the silicon surface has great potential for the design of ultrasensitive optical sensing devices. In comparison with the spherical Au NP, the individual Au@cAu NP displayed not only a red-shifted and broadened localized surface plasmon resonance (LSPR) scattering peak, but also a higher electromagnetic field enhancement. Therefore, the Au@cAu NPs offer a better choice for plasmonic enhancement-based applications in the red and near-infrared region. In general, the current work provides a new and easy method for the large-scale preparation of gold-based uniform nanostructures, and offers an avenue to understand the interference of different plasmon modes in plasmonic systems, which has potential applications in surface science.

Nanoporous carbon for electrochemical capacitive energy storage

This review summarizes the recent advances of nanoporous carbon materials in the application of EDLCs, including a better understanding of the charge storage mechanisms by combining the advanced techniques and simulations methods.