Interaction of a Self-Assembled Ionic Liquid Layer with Graphite(0001): A Combined Experimental and Theoretical Study

Ionic liquids intercalation in titanium carbide MXenes: A first-principles investigation

Herein, we present a density functional theory with dispersion correction (DFT-D) calculations that focus on the intercalation of ionic liquids (ILs) electrolytes into the two-dimensional (2D) Ti3C2Tx MXenes. These ILs include the cation 1-ethyl-3-methylimidazolium (Emim+), accompanied by three distinct anions: bis(trifluoromethylsulfonyl)imide (TFSA-), (fluorosulfonyl)imide (FSA-) and fluorosulfonyl(trifluoromethanesulfonyl)imide (FTFSA-). By altering the surface termination elements, we explore the intricate geometries of IL intercalation in neutral, negative, and positive pore systems. Accurate estimation of charge transfer is achieved through five population analysis models, such as Hirshfeld, Hirshfeld-I, DDEC6 (density derived electrostatic and chemical), Bader, and VDD (voronoi deformation density) charges. In this work, we recommend the DDEC6 and Hirshfeld-I charge models, as they offer moderate values and exhibit reasonable trends. The investigation, aimed at visualizing non-covalent interactions, elucidates the role of cation-MXene and anion-MXene interactions in governing the intercalation phenomenon of ionic liquids within MXenes. The magnitude of this role depends on two factors: the specific arrangement of the cation, and the nature of the anionic species involved in the process.

Molecular Modeling of Water-in-Salt Electrolytes: A Comprehensive Analysis of Polarization Effects and Force Field Parameters in Molecular Dynamics Simulations

Accurate modeling of highly concentrated aqueous solutions, such as water-in-salt (WiS) electrolytes in battery applications, requires proper consideration of polarization contributions to atomic interactions. Within the force field molecular dynamics (MD) simulations, the atomic polarization can be accounted for at various levels. Nonpolarizable force fields implicitly account for polarization effects by incorporating them into their van der Waals interaction parameters. They can additionally mimic electron polarization within a mean-field approximation through ionic charge scaling. Alternatively, explicit polarization description methods, such as the Drude oscillator model, can be selectively applied to either a subset of polarizable atoms or all polarizable atoms to enhance simulation accuracy. The trade-off between simulation accuracy and computational efficiency highlights the importance of determining an optimal level of accounting for atomic polarization. In this study, we analyze different approaches to include polarization effects in MD simulations of WiS electrolytes, with an example of a Na-OTF solution. These approaches range from a nonpolarizable to a fully polarizable force field. After careful examination of computational costs, simulation stability, and feasibility of controlling the electrolyte properties, we identify an efficient combination of force fields: the Drude polarizable force field for salt ions and non-polarizable models for water. This cost-effective combination is sufficiently flexible to reproduce a broad range of electrolyte properties, while ensuring simulation stability over a relatively wide range of force field parameters. Furthermore, we conduct a thorough evaluation of the influence of various force field parameters on both the simulation results and technical requirements, with the aim of establishing a general framework for force field optimization and facilitating parametrization of similar systems.

Structure Formation in an Ionic Liquid Wetting Layer: A Combined STM, IRAS, DFT and MD Study of [C2 C1 Im][OTf] on Au(111)

In a solid catalyst with ionic liquid layer (SCILL), ionic liquid (IL) coatings are used to improve the selectivity of noble metal catalysts. To understand the origins of this selectivity control, we performed model studies by surface science methods in ultrahigh vacuum (UHV). We investigated the growth and thermal stability of ultrathin IL films by infrared reflection absorption spectroscopy (IRAS). We combined these experiments with scanning tunneling microscopy (STM) to obtain information on the orientation of the ions, the interactions with the surface, the intermolecular interactions, and the structure formation. Additionally, we performed DFT calculations and molecular dynamics (MD) simulations to interpret the experimental data. We studied the IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2 C1 Im][OTf] on Au(111) surfaces. We observe a weakly bound multilayer of [C2 C1 Im][OTf], which is stable up to 390 K, while the monolayer desorbs at ∼450 K. [C2 C1 Im][OTf] preferentially adsorbs at the step edges and elbows of the herringbone reconstruction of Au(111). The anion adsorbs via the SO3 group with the molecular axis perpendicular to the surface. At low coverage, the [C2 C1 Im][OTf] crystallizes in a glass-like 2D phase with short-range order. At higher coverage, we observe a phase transition to a 6-membered ring structure with long-range order.

An Overview of Recycling Wastes into Graphene Derivatives Using Microwave Synthesis; Trends and Prospects

It is no secret that graphene, a two-dimensional single-layered carbon atom crystal lattice, has drawn tremendous attention due to its distinct electronic, surface, mechanical, and optoelectronic properties. Graphene also has opened up new possibilities for future systems and devices due to its distinct structure and characteristics which has increased its demand in a variety of applications. However, scaling up graphene production is still a difficult, daunting, and challenging task. Although there is a vast body of literature reported on the synthesis of graphene through conventional and eco-friendly methods, viable processes for mass graphene production are still lacking. This review focuses on the variety of unwanted waste materials, such as biowastes, coal, and industrial wastes, for producing graphene and its potential derivatives. Among the synthetic routes, the main emphasis relies on microwave-assisted production of graphene derivatives. In addition, a detailed analysis of the characterization of graphene-based materials is presented. This paper also highlights the current advances and applications through the recycling of waste-derived graphene materials using microwave-assisted technology. In the end, it would alleviate the current challenges and forecast the specific direction of waste-derived graphene future prospects and developments.

Effects of Ionic Liquids on the Stabilization Process of Gold Nanoparticles

Improving the stability of the gold nanoparticles (AuNPs) is an important challenge in nanoscience, given that the activity and ubiquitous application of the AuNPs in different fields depend largely on their stability in the solution phase. Ionic liquids (ILs) can be used as new alternatives in comparison to water and organic solvents due to their considerable properties to elevate the stability and resistance of the AuNPs against aggregation for a long period of storage. In this study, we employ molecular dynamics simulation and quantum chemistry calculations to investigate the effects of amino acid ILs ([BMIM][Gly], [BMIM][Leu], [BMIM][Pro], [BMIM][Val], and [BMIM][Ala]) on the stability and aggregation process of the AuNPs from the molecular viewpoint. Our results suggest that ILs can prevent AuNP aggregation. These ILs penetrate the solvation shell of the nanoparticles and by increasing the electrostatic repulsions on the surface of the AuNPs improve their stability against aggregation. Moreover, the [BMIM]⁺ cation is more effective on the stability of the AuNPs in comparison with the corresponding anions. The ring of the cation, due to the stronger interaction with the AuNPs compared to the side chain, contributes predominantly to the stability of the nanostructures. Our quantum chemistry calculations confirm that dispersion interactions between the cation and anions of the ILs and the surface of gold play a key role in the stability of the IL-AuNP complexes. [Leu]^- anion has the strongest dispersion interactions with the metal surface and forms the most stable complex with the AuNPs. Overall, the results of this study offer new insights into the properties of amino acid ILs as effective agents to improve the stability of AuNPs for long-term storage.

Atomic force microscopy probing interactions and microstructures of ionic liquids at solid surfaces

Ionic liquids (ILs) are room temperature molten salts that possess preeminent physicochemical properties and have shown great potential in many applications. However, the use of ILs in surface-dependent processes, e.g. energy storage, is hindered by the lack of a systematic understanding of the IL interfacial microstructure. ILs on the solid surface display rich ordering, arising from coulombic, van der Waals, solvophobic interactions, etc., all giving near-surface ILs distinct microstructures. Therefore, it is highly important to clarify the interactions of ILs with solid surfaces at the nanoscale to understand the microstructure and mechanism, providing quantitative structure-property relationships. Atomic force microscopy (AFM) opens a surface-sensitive way to probe the interaction force of ILs with solid surfaces in the layers from sub-nanometers to micrometers. Herein, this review showcases the recent progress of AFM in probing interactions and microstructures of ILs at solid interfaces, and the influence of IL characteristics, surface properties and external stimuli is thereafter discussed. Finally, a summary and perspectives are established, in which, the necessities of the quantification of IL-solid interactions at the molecular level, the development of in situ techniques closely coupled with AFM for probing IL-solid interfaces, and the combination of experiments and simulations are argued.

Water-Controlled Structural Transition and Charge Transfer of Interfacial Ionic Liquids

Clarification of the water-induced structural transitions and electron transfer between ionic liquids (ILs) and a solid surface allows for establishing a unified view of the electrical properties of interfacial ILs via a hitherto unexplored pathway. Here, we propose a simple and effective method to quantitatively identify and extract the transferred electrons between ILs and a solid surface, while demonstrating the critical structural transition of interfacial ILs from ordered stripe structures to disordered aggregation structures. The formation of hydrated anions, rooted in the hydrogen bonds of O-H···O between the anion and water, lies at the tipping point where electron transfer ends and aggregation structure begins. In addition, it is discovered to what extent the hydrophilicity of substrates can affect electron transfer, and a regulation method based on the electric field is explored. These experimental findings may refresh our knowledge of interfacial ILs and provide an effective method for evaluating the intrinsic electrical features of the ILs-solid surface.

The impact of the cation alkyl chain length on the wettability of alkylimidazolium-based ionic liquids at the nanoscale

Ionic liquids (ILs) have been widely used for energy storage and conversion devices due to their negligible vapor pressure, high thermal stability, and outstanding interfacial properties. Notably, the interfacial nanostructure and the wettability of thin ionic liquid films on solid surfaces are of utmost relevance in nanosurface science and technology. Herein, a reproducible physical vapor deposition methodology was used to fabricate thin films of four alkylimidazolium bis(trifluoromethylsulfonyl)imide ILs. The effect of the cation alkyl chain length on the wettability of ILs was explored on different surfaces: gold (Au); silver (Ag); indium-tin oxide (ITO). High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to evaluate the morphology of the produced micro- and nanodroplets and films. SEM and AFM results revealed an island growth for all the ILs deposited on ITO and Ag surfaces, with a lower minimum free area to promote nucleation (MFAN) in Ag and higher wettability for ILs having larger non-polar domains. The low wettability of ITO by the studied ILs was highlighted. For long-chain ILs, nucleation and growth mechanisms were strongly conditioned by coalescence processes. The results also supported the higher affinity of the ILs to the Au surface. The increase in the length of the cation alkyl chain was found to promote a better film adhesion inducing a 2D growth and higher wetting ability.

Investigation of Multilayered Structures of Ionic Liquids on Graphite and Platinum Using Atomic Force Microscopy and Molecular Simulations

The molecular-level orientation and structure of ionic liquids (ILs) at liquid-solid interfaces are significantly different than in the bulk. The interfacial ordering influences both IL properties, such as dielectric constants and viscosity, and their efficacy in devices, such as fuel cells and electrical capacitors. Here, we report the layered structures of four ILs on unbiased, highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy. The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C₄F₉SO₃]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl)imide ([MTBD][Tf₂N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C₄F₉SO₃]). Molecular dynamics simulations provide complementary information on the position and orientation of the ions. These ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]⁺ and [MTBD]⁺ have similar orientations at the surface, but [MTBD]⁺ forms a thinner layer compared to [emim]⁺ on both HOPG and Pt(111). In addition, [Tf₂N]^- shows stronger interactions with Pt(111) surfaces than [C₄F₉SO₃]^-.

Establishing the accuracy of density functional approaches for the description of noncovalent interactions in ionic liquids

We test a number of dispersion corrected versatile Generalized Gradient Approximation (GGA) and meta-GGA functionals for their ability to predict the interactions of ionic liquids, and show that most can achieve energies within 1 kcal mol^-1 of benchmarks. This compares favorably with an accurate dispersion corrected hybrid, ωB97X-V. Our tests also reveal that PBE (Perdew-Burke-Ernzerhof GGA) calculations using the plane-wave projector augmented wave method and Gaussian Type Orbitals (GTOs) differ by less than 0.6 kJ mol^-1 for ionic liquids, despite ions being difficult to evaluate in periodic cells - thus revealing that GTO benchmarks may be used also for plane-wave codes. Finally, the relatively high success of explicit van der Waals density functionals, compared to elemental and ionic dispersion models, suggests that improvements are required for low-cost dispersion correction models of ions.

Interfacial interactions and structures of protic ionic liquids on a graphite surface: A first-principles study and comparison with aprotic ionic liquids

Protic ionic liquids (PILs) have currently been indicated as promising alternative electrolytes in electrical storage devices, such as lithium-ion batteries and supercapacitors. However, compared with the well-studied aprotic ionic liquids (AILs), the knowledge of the interface between PILs and electrode material surfaces is very rare to date. In this work, the adsorption behaviors of three groups of PILs, i.e. pyrrolidinium-based, imidazolium-based, and ammonium-based, on graphite was systematically investigated using first-principles calculations. The corresponding AILs were also taken into account for comparison. The adsorption mechanism of these ILs on the surface is controlled by the interplay of strong electrostatic interactions between adsorbed ions, weak vdW forces between ILs and substrate, and many aromatic interactions including π-π stacking and C-H/N-Hπ contacts. PILs do show quite different preferential interfacial interactions and structures on the graphite surface with respect to AILs, arising mainly from the anion-substrate interactions. Particularly, proton transfer takes place in the PILs consisting of the imidazolium/ammonium cation and the nitrate anion in the gas phase, but it tends to be attenuated or even disappears on graphite caused by interfacial interactions.

The Impact of Water on the Lateral Nanostructure of a Deep Eutectic Solvent–Solid Interface

Deep eutectic solvents (DESs) are tuneable solvents with attractive properties for numerous applications. Their structure–property relationships are still under investigation, especially at the solid–liquid interface. Moreover, the influence of water on interfacial nanostructure must be understood for process optimization. Here, we employ a combination of atomic force microscopy and molecular dynamics simulations to determine the lateral and surface-normal nanostructure of the DES choline chloride:glycerol at the mica interface with different concentrations of water. For the neat DES system, the lateral nanostructure is driven by polar interactions. The surface adsorbed layer forms a distinct rhomboidal symmetry, with a repeat spacing of ~0.9 nm, comprising all DES species. The adsorbed nanostructure remains largely unchanged in 75 mol-% DES compared with pure DES, but at 50 mol-%, the structure is broken and there is a compromise between the native DES and pure water structure. By 25 mol-% DES, the water species dominates the adsorbed liquid layer, leaving very few DES species aggregates at the interface. In contrast, the near-surface surface-normal nanostructure, over a depth of ~3 nm from the surface, remains relatively unchanged down to 25 mol-% DES where the liquid arrangement changed. These results demonstrate not only the significant influence that water has on liquid nanostructure, but also show that there is an asymmetric effect whereby water disrupts the nanostructure to a greater degree closer to the surface. This work provides insight into the complex interactions between DES and water and may enhance their optimization for surface-based applications.

Nanostructure of a deep eutectic solvent at solid interfaces

HYPOTHESIS

Deep eutectic solvents (DESs) are an attractive class of tunable solvents. However, their uptake for relevant applications has been limited due to a lack of detailed information on their structure-property relationships, both in the bulk and at interfaces. The lateral nanostructure of the DES-solid interfaces is likely to be more complex than previously reported and requires detailed, high-resolution investigation.

EXPERIMENTS

We employ a combination of high-resolution amplitude-modulated atomic force microscopy and molecular dynamics simulations to elucidate the lateral nanostructure of a DES at the solid-liquid interface. Specifically, the lateral and near-surface nanostructure of the DES choline chloride:glycerol is probed at the mica and highly-ordered pyrolytic graphite interfaces.

FINDINGS

The lateral nanostructure of the DES-solid interface is heterogeneous and well-ordered in both systems. At the mica interface, the DES is strongly ordered via polar interactions. The adsorbed layer has a distinct rhomboidal symmetry with a repeat spacing of ~0.9 nm comprising all DES species. At the highly ordered pyrolytic graphite interface, the adsorbed layer appears distinctly different, forming an apolor-driven row-like structure with a repeat spacing of ~0.6 nm, which largely excludes the chloride ion. The interfacial nanostructure results from a delicate balance of substrate templating, liquid-liquid interactions, species surface affinity, and packing constraints of cations, anions, and molecular components within the DES. For both systems, distinct near-surface nanostructural layering is observed, which becomes more pronounced close to the substrate. The surface nanostructures elucidated here significantly expand our understanding of DES interfacial behavior and will enhance the optimization of DES systems for surface-based applications.

Growth of Multilayers of Ionic Liquids on Au(111) Investigated by Atomic Force Microscopy in Ultrahigh Vacuum

Understanding the growth of ultrathin films of ionic liquids (ILs) on metal surfaces is of highest relevance for a variety of applications. We present a detailed study of the growth of the wetting layer and successive multilayers of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im][Tf2N]) on Au(111). By atomic force microscopy (AFM) in ultrahigh vacuum, we follow the temperature-dependent behavior between 110 and 300 K at defined coverages. We initially observe the formation of a wetting layer with a thickness of ∼0.37 nm with anions and cations arranged in a checkerboard structure. Stable AFM imaging up to 280 K allows us to follow the IL growing on top of the wetting layer in bilayers with an average thickness of ∼0.71 nm, that is, double the height of the wetting layer, in a bilayer-by-bilayer fashion. This growth behavior is independently confirmed from the surface morphology, as deduced from AFM and angle-resolved X-ray photoelectron spectroscopy. High-resolution AFM images at 110 K allow for identifying the molecular surface structure of the bilayers as a striped phase, which is one of the phases also seen for the wetting layer (Meusel, M.; Lexow, M.; Gezmis, A.; Schotz, S.; Wagner, M.; Bayer, A.; Maier, F.; Steinrück, H. P. Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature. ACS Nano 2020, 14, 9000-9010).

Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature

Ionic liquids (ILs) are used as ultrathin films in many applications. We studied the nanoscale arrangement within the first layer of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C₁C₁Im] [Tf₂N]) on Au(111) between 400 and 110 K in ultrahigh vacuum by scanning tunneling and noncontact atomic force microscopy with molecular resolution. Compared to earlier studies on similar ILs, a different behavior is observed, which we attribute to the small size and symmetrical shape of the cation: (a) In both AFM and STM only the anions are imaged; (b) only long-range-ordered but no amorphous phases are observed; (c) the hexagonal room-temperature phase melts 30-50 K above the IL's bulk melting point; (d) at 110 K, striped and hexagonal superstructures with two and three ion pairs per unit cell, respectively, are found. AFM turned out to be more stable at higher temperature, while STM revealed more details at low temperature.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Controlling the nanoscale friction by layered ionic liquid films

The nanofriction coefficient of ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), on the surfaces of mica and graphite was investigated using atomic force microscopy (AFM). A pronounced layered spatial distribution was found in the IL film formed on the solid substrates and can be divided into 3 well distinguishable regions exhibiting different physical properties with increasing distance from the substrate. We found that the friction coefficient (μ) increases monotonically as the layering thickness decreases, no matter what the thickness of the bulk IL is. This suggests that the layering assembled IL at solid surfaces is more important than the bulk phase in determining the magnitude of the nanoscale friction. The increase in the friction coefficient as the layering thickness decreases is most likely attributed to the assembled ordered IL layers closer to the substrate surfaces having a greater activation barrier for unlocking the surfaces to allow shear.

Chemical potentials of electric double layers at metal-electrolyte interfaces: dependence on electrolyte concentration and electrode materials, and application to field-effect transistors

When a metal is soaked in an electrolyte solution, the metal and solution affect each other through the formation of electric double layers (EDLs) at their interfaces. The EDLs at metal-electrolyte interfaces can realize high-density charge-carrier injections and accumulations, and thus have recently attracted attention for their potential application to energy storage, and electronic and electrochemical devices. In such EDL-based devices, including field-effect transistors (FETs), the potential energy of surface electrons in the metal electrodes (EM) governs the transistor device performance. This is in clear contrast to redox-driven electrochemical devices such as dye-sensitized solar cells and electrochromic devices, whose performance is primarily governed by the potentials of the redox-active species. However, there has been no systematic research to bridge the distance between metal electrons and electrolyte ions. In the present study, we carefully examined the dependence of EM of ITO, Au and Pt electrodes on the concentration of the PEG solutions of LiCl and MgCl2, because it has been well established that the chemical potential of electrolyte solutions is dependent on the solution concentrations. Our results showed that, at the same electrolyte concentration, the values of EM increased in the order of ITO, Au and Pt; moreover, on the same electrode, EM showed linear decreases as a function of the logarithm of the electrolyte concentrations. To understand these behaviors, we developed a theoretical treatment of the EDLs based on the simple Gouy-Chapman model, and obtained the theoretical expressions of EM in terms of the concentration of electrolyte and the work function of the metal electrode (ΦM), which were found to successfully explain the dependences of EM on the electrolyte concentration and the electrode materials. We also examined the EDL-FETs of platinum phthalocyanine (PtPc), with various LiCl-PEG solutions of different concentrations as gate electrolytes. The threshold voltage eVT and EM exhibited a linear relation, which was well explained by the relation between EM and the valence band energy EVB of PtPc. The transfer characteristics at various gate voltage VG were found to be well normalized by a function of eVG + EM.

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li-Containing Ionic Liquids

The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar⁺ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.

Stability and Exchange Processes in Ionic Liquid/Porphyrin Composite Films on Metal Surfaces

In light of increasing interest in the development of organic-organic multicomponent heterostructures on metals, this molecular-scale study investigates prototypical composite systems of ultrathin porphyrin and ionic liquid (IL) films on metallic supports under well-defined ultrahigh vacuum conditions. By means of angle-resolved X-ray photoelectron spectroscopy, we investigated the adsorption, stability, and thermal exchange of the resulting films after sequential physical vapor deposition of the free-base porphyrin 5,10,15,20-tetraphenylporphyrin, 2H-TPP, and the IL 1-methyl-3-octylimidazolium hexafluorophosphate, [C₈C₁Im][PF₆], on Ag(111) and Au(111). 2H-TPP shows two-dimensional growth of up to two closed molecular layers on Ag(111) and Au(111) and three-dimensional island growth for thicker films. IL films on top of a monolayer of 2H-TPP exhibit Stranski-Krastanov-like growth and are stable up to 385 K. The 2H-TPP layer leads to destabilization of the IL films, compared to the IL in direct contact with the bare metals, by inhibiting the specific adsorption of the ions on the metal surfaces. When the porphyrin is deposited on top of [C₈C₁Im][PF₆] at low temperature, the 2H-TPP molecules adsorb on top of the IL film at first but replace the IL at the IL/metal interfaces upon heating above 240 K. This exchange process is most likely driven by the higher adsorption energy of 2H-TPP on Ag(111) and Au(111) surfaces, as compared to the IL. The behavior observed on Ag(111) and Au(111) is identical. The results are highly relevant for the stability of porphyrin/IL-based thin film catalyst systems and molecular devices, and more generally, stacked organic multilayer architectures.

Ionic liquid thin layer-induced memory effects in organic field-effect transistors

We examined the morphologies and structures of pentacene and C60 thin films grown on thin layers of an ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), and found that the characteristics of the films depended significantly on the thickness of DEME-TFSI. In addition, we fabricated organic field-effect transistors (OFETs) of pentacene and C60 in which a thin layer of DEME-TFSI was inserted between the organic semiconductor (pentacene or C60) and the gate insulating layer, and measured their performance in situ. We found that 1.5-2 ML (ML: monolayer) DEME-TFSI produced a large hysteresis loop in the transfer characteristics in these OFETs, but 5 ML DEME-TFSI resulted in the formation of normally-on states with far smaller memory effects. The curvatures of the hysteresis loops were caused by the formation of trap states induced by the DEME-TFSI layers. This novel technique provides a simple tool for creating hysteresis behavior and could potentially be applied to transistor memory devices.

Graphene Meets Ionic Liquids: Fermi Level Engineering via Electrostatic Forces

Graphene-based two-dimensional (2D) materials are promising candidates for a number of different energy applications. A particularly interesting one is in next generation supercapacitors, where graphene is being explored as an electrode material in combination with room temperature ionic liquids (ILs) as electrolytes. Because the amount of energy that can be stored in such supercapacitors critically depends on the electrode-electrolyte interface, there is considerable interest in understanding the structure and properties of the graphene/IL interface. Here, we report the changes in the properties of graphene upon adsorption of a homologous series of alkyl imidazolium tetrafluoroborate ILs using a combination of experimental and theoretical tools. Raman spectroscopy reveals that these ILs cause n-type doping of graphene, and the magnitude of doping increases with increasing cation chain length despite the expected decrease in the density of surface-adsorbed ions. Molecular modeling simulations show that doping originates from the changes in the electrostatic potential at the graphene/IL interface. The findings described here represent an important step in developing a comprehensive understanding of the graphene/IL interface.

Selective Electrochemical Nitrogen Reduction Driven by Hydrogen Bond Interactions at Metal-Ionic Liquid Interfaces

Increasing the activity of the nitrogen reduction reaction while slowing the detrimental hydrogen evolution reaction is a key challenge in current electrocatalysis to provide a sustainable route to ammonia. Recently, nanoparticles in ionic liquid (IL) environments have been found to boost the selectivity of electrochemical synthesis of ammonia from dinitrogen at room temperature. Here, we use for the first time a fully atomistic representation of metal-IL interfaces at the density functional theory level to understand experimental evidence, with particular focus on the rate and selectivity determining formation of N₂H intermediates compared to hydrogen evolution. We find that decorating the metal surface with fluorinated ILs creates specific H-bond interactions between Ru-N₂H and IL anions, stabilizing this intermediate and thus driving the selectivity of the electrochemical process.

Reaction mechanisms at the homogeneous–heterogeneous frontier: insights from first-principles studies on ligand-decorated metal nanoparticles

The frontiers between homogeneous and heterogeneous catalysis are progressively disappearing.

Adsorption of Hydrophobic and Hydrophilic Ionic Liquids at the Au(111) Surface

Electrode-electrolyte microscopic interfacial studies are of great interest for the design and development of functional materials for energy storage and catalysis applications. First-principles-based simulation methods are used here to understand the structure, stability, energetics, and microscopic adsorption mechanism of various hydrophilic and hydrophobic ionic liquids (ILs; 1-butyl 3-methylimidazolium [BMIm]+[X]-, where X = Cl, DCA, HCOO, BF4, PF6, CH3SO3, OTF, and TFSA) interacting with a metallic surface. We have selected the Au(111) surface as a potential electrode model, and our computations show that ILs (anions and cations) adsorb specifically at some selective adsorption sites. Indeed, hydrophilic anions of ILs are strongly adsorbed on the gold surface (via Au-Cl and Au-N bonds at Au(111)), whereas hydrophobic anions are weakly bonded. The [BMIm]+ is always found to be stabilized parallel to the metal surface, irrespective of the nature of the anion, through various kinds of noncovalent interactions. Mulliken, Löwdin, and Hirshfeld charge analyses reveal that there is significant charge transfer between ILs and the surface that may enhance the charge transfer mechanism between the surface and electrolytes for electrochemical applications. Our study shows that the electrostatic and van der Waals interactions are in action at these interfaces. Moreover, we show that there are several covalent and noncovalent interactions between ILs and the metal surface. These interactions play an essential role to maintain the electrostatic behaviors at the solid-liquid interface. The present findings can be helpful to predict specific selectivity and subsequent design of materials for energy harvesting and catalysis applications.

Structure formation and surface chemistry of ionic liquids on model electrode surfaces-Model studies for the electrode | electrolyte interface in Li-ion batteries

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]⁺[TFSI]^-) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO₂(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate-adsorbate and adsorbate-adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li₃N, Li₂S, Li_xSO_y, and Li₂O. In the absence of a [BMP]⁺[TFSI]^- adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO₂) above a critical temperature, forming LiO_x and Ti³⁺ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li⁺ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

The metal–ionic liquid interface as characterized by impedance spectroscopy and in situ scanning tunneling microscopy

Electrochemical measurements including impedance spectroscopy and in situ scanning tunneling microscopy were performed to study the interface between solid electrodes and ionic liquids. We could reveal that the double layer rearrangement processes are not instantaneous, but that the ions can form ordered clusters at the interface.

Intercalation and Deintercalation of Lithium at the Ionic Liquid-Graphite(0001) Interface

The intercalation and deintercalation of lithium (Li) into / out of graphite(0001), which is a highly important process in Li-ion batteries, was investigated under ultrahigh vacuum conditions as a function of temperature, employing X-ray and ultraviolet photoelectron spectroscopy. Both the up-shifts of the core-level binding energy and the lowering of the work function ΔΦ reveal that heating of a monolayer of the battery-relevant ionic liquid (IL) 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]⁺[TFSI]^-) adsorbed on lithiated graphite at 80 K to >230 K facilitates an accumulation of partially charged Li^δ+ atoms at the IL-graphite(0001) interface. This is accompanied by a partial IL decomposition, which is associated with the initial stages of the chemical formation of the solid-electrolyte interphase.

Nanoscale capillary freezing of ionic liquids confined between metallic interfaces and the role of electronic screening

Room-temperature ionic liquids (RTILs) are new materials with fundamental importance for energy storage and active lubrication. They are unusual liquids, which challenge the classical frameworks of electrolytes, whose behaviour at electrified interfaces remains elusive, with exotic responses relevant to their electrochemical activity. Using tuning-fork-based atomic force microscope nanorheological measurements, we explore here the properties of confined RTILs, unveiling a dramatic change of the RTIL towards a solid-like phase below a threshold thickness, pointing to capillary freezing in confinement. This threshold is related to the metallic nature of the confining materials, with more metallic surfaces facilitating freezing. This behaviour is interpreted in terms of the shift of the freezing transition, taking into account the influence of the electronic screening on RTIL wetting of the confining surfaces. Our findings provide fresh views on the properties of confined RTIL with implications for their properties inside nanoporous metallic structures, and suggests applications to tune nanoscale lubrication with phase-changing RTILs, by varying the nature and patterning of the substrate, and application of active polarization.

Ionic interaction-induced assemblies of bimolecular “chessboard” structures

Here we applied ionic interactions as the driving force to fabricate well-ordered bicomponent assemblies by using two porphyrin ions equipped with oppositely-charged groups.

Ionic Liquid Ordering at an Oxide Surface

The interaction of the ionic liquid [C₄ C₁ Im][BF₄ ] with anatase TiO₂ , a model photoanode material, has been studied using a combination of synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The system is of interest as a model for fundamental electrolyte-electrode and dye-sensitized solar cells. The initial interaction involves degradation of the [BF₄ ]^- anion, resulting in incorporation of F into O vacancies in the anatase surface. At low coverages, [C₄ C₁ Im][BF₄ ] is found to order at the anatase(101) surface via electrostatic attraction, with the imidazolium ring oriented 32±4° from the anatase TiO₂ surface. As the coverage of ionic liquid increases, the influence of the oxide surface on the topmost layers is reduced and the ordering is lost.

Fundamental aspects of electric double layer force-distance measurements at liquid-solid interfaces using atomic force microscopy

Atomic force microscopy (AFM) force-distance measurements are used to investigate the layered ion structure of Ionic Liquids (ILs) at the mica surface. The effects of various tip properties on the measured force profiles are examined and reveal that the measured ion position is independent of tip properties, while the tip radius affects the forces required to break through the ion layers as well as the adhesion force. Force data is collected for different ILs and directly compared with interfacial ion density profiles predicted by molecular dynamics. Through this comparison it is concluded that AFM force measurements are sensitive to the position of the ion with the larger volume and mass, suggesting that ion selectivity in force-distance measurements are related to excluded volume effects and not to electrostatic or chemical interactions between ions and AFM tip. The comparison also revealed that at distances greater than 1 nm the system maintains overall electroneutrality between the AFM tip and sample, while at smaller distances other forces (e.g., van der waals interactions) dominate and electroneutrality is no longer maintained.

Computational and Experimental Study of Li-Doped Ionic Liquids at Electrified Interfaces

We evaluate the influence of Li-salt doping on the dynamics, capacitance, and structure of three ionic liquid electrolytes, [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4], using molecular dynamics and polarizable force fields. In this respect, our focus is on the properties of the electric double layer (EDL) formed by the electrolytes at the electrode surface as a function of surface potential (Ψ). The rates of EDL formation are found to be on the order of hundreds of picoseconds and only slightly influenced by the addition of Li-salt. The EDLs of three electrolytes are shown to have different energy storage capacities, which we relate to the EDL formation free energy. The differential capacitance obtained from our computations exhibits asymmetry about the potential of zero charge and is consistent with the camel-like profiles noted from mean field theories and experiments on metallic electrodes. The introduction of Li-salt reduces the noted asymmetry in the differential capacitance profile. Complementary experimental capacitance measurements have been made on our three electrolytes in their neat forms and with Li-salt. The measurements, performed on glassy carbon electrodes, produce U-like profiles, and Li-salt doping is shown to strongly affect capacitance at high magnitudes of Ψ. Differences in the theoretical and experimental shapes and magnitudes of capacitance are rationalized in terms of the electrode surface and pseudocapacitive effects. In both neat and Li-doped liquids, the details of the computational capacitance profile are well described by Ψ-induced changes in the density and molecular orientation of ions in the molecular layer closest to the electrode. Our results suggest that the addition of Li+ induces disorder in the EDL, which originates from the strong binding of anions to Li+. An in-depth analysis of the distribution of Li+ in the EDL reveals that it does not readily enter the molecular layer at the electrode surface, preferring instead to be localized farther away from the surface in the second molecular layer. This behavior is validated through an analysis of the free energy of Li+ solvation as a function of distance from the electrode. Free energy wells are found to coincide with localized concentrations of Li+, the depths of which increase with Ψ and suggest a source of impedance for Li+ to reach the electrode. Finally, we make predictions of the specific energy at ideal graphite utilizing the computed capacitance and previously derived electrochemical windows of the liquids.