Nanostructure and Dynamics of Aprotic Ionic Liquids at Graphite Electrodes as a Function of Potential

Atomic force microscope (AFM) videos reveal the near-surface nanostructure and dynamics of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide (BMIM DCA) and 1-hexyl-3-methylimidazolium dicyanamide (HMIM DCA) above highly oriented pyrolytic graphite (HOPG) electrodes as a function of surface potential. Molecular dynamics (MD) simulations reveal the molecular-level composition of the nanostructures. In combination, AFM and MD show that the near-surface aggregates form via solvophobic association of the cation alkyl chains at the electrode interface. The diffusion coefficients of interfacial nanostructures are ≈0.01 nm2 s-1 and vary with the cation alkyl chain length and the surface potential. For each IL, the nanostructure diffusion coefficients are similar at open-circuit potential (OCP) and OCP + 1V, but BMIM DCA moves about twice as fast as HMIM DCA. At negative potentials, the diffusion coefficient decreases for BMIM DCA and increases for HMIM DCA. When the surface potential is switched from negative to positive, a sudden change in the direction of the nanostructure motion is observed for both BMIM DCA and HMIM DCA. No transient dynamics are noted following other potential jumps. This study provides a new fundamental understanding regarding the dynamics of electrochemically stable ILs at electrodes vital for the rational development of IL-based electrochemical devices.

Surface Chemistry of a [C2C1Im][OTf] (Sub)Wetting Layer on Pt(111): A Combined XPS, IRAS, and STM Study

The concept of a solid catalyst with an ionic liquid layer (SCILL) is a promising approach to improve the selectivity of noble metal catalysts in heterogeneous reactions. In order to understand the origins of this selectivity control, we investigated the growth and thermal stability of ultrathin 1-ethyl-3-methylimidazolium trifluormethanesulfonate [C2C1Im][OTf] films on Pt(111) by infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS) in time-resolved and temperature-programmed experiments. We combined these spectroscopy experiments with scanning tunneling microscopy (STM) to obtain detailed insights into the orientation and adsorption geometry of the ions in the first IL layer. Furthermore, we propose a mechanism for the thermal evolution of [C2C1Im][OTf] on Pt(111). We observe an intact IL layer on the surface at temperatures below 200 K. Adsorbed [C2C1Im][OTf] forms islands, which are evenly distributed over the surface. The [OTf]- anion adsorbs via the SO3 group, with the molecular axis perpendicular to the surface. Anions and cations are arranged next to each other, alternating on the Pt(111) surface. Upon heating to 250 K, we observe changes in geometry and structural distribution. Whereas at low temperature, the ions are arranged alternately for electrostatic reasons, this driving force is no longer decisive at 250 K. Here, a phase separation of two different species is discernible in STM. We propose that this effect is due to a surface reaction, which changes the charge of the adsorbates. We assume that the IL starts to decompose at around 250 K, and thus, pristine IL and decomposition products coexist on the surface. Also, IRAS and XPS show indication of IL decomposition. Further heating leads to increased IL decomposition. The reaction products associated with the anions are volatile and leave the surface. In contrast, the cation fragments remain on the surface up to temperatures above 420 K.

Adsorption and thermal evolution of [C1C1Im][Tf2N] on Pt(111)

In the context of ionic liquid (IL)-assisted catalysis, we have investigated the adsorption and thermal evolution of the IL 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C1Im][Tf2N]) on Pt(111) between 100 and 800 K by angle-resolved X-ray photoelectron spectroscopy and scanning tunneling microscopy. Defined amounts of IL in the coverage range of a complete first wetting layer were deposited at low temperature (100-200 K), and subsequently heated to 300 K, or directly at 300 K. At 100 K, the IL adsorbs as an intact disordered layer. Upon heating to 200 K, the IL stays intact, but forms an ordered and well-oriented structure. Upon heating to 250 K, the surface order increases, but at the same time STM and XPS indicate the onset of decomposition. Upon heating to 300 K, decomposition progresses, such that 50-60% of the IL is decomposed. The anion-related reaction products desorb instantaneously, and the cation-related products remain on the surface. Thereby, the surface is partly passivated, enabling the remaining IL to still be adsorbed intact at 300 K. For IL deposition directly at 300 K, a fraction of the IL instantaneously decomposes, with the anion-related products desorbing, opening free space for further deposition of IL. Hence, cation-related species accumulate at the expense of anions, until one fully closed wetting layer is formed. As a consequence, a higher dose is required to reach this coverage at 300 K, compared to 100-200 K.

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.

Mixing Ionic Liquids Affects the Kinetics and Thermodynamics of the Oxygen/Superoxide Redox Couple in the Context of Oxygen Sensing

The electrochemical oxygen reduction reaction is vital for applications such as fuel cells, metal air batteries and for oxygen gas sensing. Oxygen undergoes a 1-electron reduction process in dry ionic liquids (ILs) to form the electrogenerated superoxide ion that is solvated and stabilized by IL cations. In this work, the oxygen/superoxide (O₂/O₂ ^•-) redox couple has been used to understand the effect of mixing ILs with different cations in the context of developing designer electrolytes for oxygen sensing, by employing cyclic voltammetry at both gold and platinum electrodes. Different cations with a range of sizes, geometries and aromatic/aliphatic character were studied with a common bis(trifluoromethylsulfonyl)imide ([NTf₂]^-) anion. Diethylmethylsulfonium ([S_2,2,1]⁺), N-butyl-N-methylpyrrolidinum ([C₄mpyrr]⁺) and tetradecyltrihexylphosphonium ([P_14,6,6,6]⁺) cations were mixed with a common 1-butyl-3-methylimidazolium ([C₄mim]⁺) cation at mole fractions (x) of [C₄mim]⁺ of 0, 0.2, 0.4, 0.6, 0.8, and 1. Both the redox kinetics and thermodynamics were found to be highly dependent on the cation structure and the electrode material used. Large deviations from "ideal" mixtures were observed for mixtures of [C₄mim][NTf₂] with [C₄mpyrr][NTf₂] on gold electrodes, suggesting a much higher amount of [C₄mim]⁺ ions near the electrode surface despite the large excess of [C₄mpyrr]⁺ in the bulk. The electrical double layer structure was probed for a mixture of [C₄mim]_0.2[C₄mpyrr]_0.8[NTf₂] using atomic force microscopy measurements on Au, revealing that the first layer was more like [C₄mim][NTf₂] than [C₄mpyrr][NTf₂]. Unusually fast kinetics for O₂/O₂ ^•- in mixtures of [C₄mim]⁺ with [P_14,6,6,6]⁺ were also observed in the electrochemistry results, which warrants further follow-up studies to elucidate this promising behavior. Overall, it is important to understand the effect on the kinetic and thermodynamic properties of electrochemical reactions when mixing solvents, to aid in the creation of designer electrolytes with favorable properties for their intended application.

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.

On‐Surface Metathesis of an Ionic Liquid on Ag(111)

We investigated the adsorption, surface enrichment, ion exchange, and on‐surface metathesis of ultrathin mixed IL films on Ag(111). We stepwise deposited 0.5 ML of the protic IL diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) and 1.0 ML of the aprotic IL 1‐methyl‐3‐octylimidazolium hexafluorophosphate ([C₈C₁Im][PF₆]) at around 90 K. Thereafter, the resulting layered frozen film was heated to 550 K, and the thermally induced phenomena were monitored in situ by angle‐resolved X‐ray photoelectron spectroscopy. Between 135 and 200 K, [TfO]⁻ anions at the Ag(111) surface are exchanged by [PF₆]⁻ anions and enriched together with [C₈C₁Im]⁺ cations at the IL/vacuum interface. Upon further heating, [dema][PF₆] and [OMIm][PF₆] desorb selectively at ∼235 and ∼380 K, respectively. Hereby, a wetting layer of pure [C₈C₁Im][TfO] is formed by on‐surface metathesis at the IL/metal interface, which completely desorbs at ∼480 K. For comparison, ion enrichment at the vacuum/IL interface was also studied in macroscopic IL mixtures, where no influence of the solid support is expected.

Adsorption Motifs and Molecular Orientation at the Ionic Liquid/Noble Metal Interface: [C2C1Im][NTf2] on Pt(111)

In solid catalysts with ionic liquid layers (SCILLs), ionic liquid (IL) thin films are used to modify the activity and selectivity of catalytic materials. In this work, we investigated the adsorption behavior of the IL 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁Im][NTf₂] on Pt(111) by combining experimental and theoretical studies. Under ultrahigh vacuum (UHV) conditions, the IL was deposited onto a Pt(111) single crystal surface by physical vapor deposition (PVD) at different surface temperatures (200, 300, and 400 K). The adsorption process was monitored by in situ infrared reflection absorption spectroscopy (IRAS). Complementary to the IRAS studies, we performed density functional theory (DFT) calculations and analyzed the adsorption motifs and orientation of the IL ions. In total, we calculated four different systems: (a) [C₂C₁Im]⁺ and [NTf₂]^- ions in the gas phase; [NTf₂]^- anions in (b) small (4 × 4) and (c) large (6 × 6) Pt(111) supercells; and (d) a complete ion pair of [C₂C₁Im][NTf₂] in a (6 × 6) Pt(111) supercell. Based on DFT, we simulated IR spectra and compared them to the experimental data. Our results suggest that the binding motif and orientation of the IL is strongly dependent on the actual IL coverage. In the monolayer (ML), [NTf₂]^- interacts strongly with the metal surface and adopts a specific orientation in which it interacts with the Pt surface via the SO₂ groups. Also the [C₂C₁Im]⁺ cations adopt a preferential orientation up to coverages of 1 ML. Upon transition to the multilayer region, the specific orientation of the ions is gradually lost.

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.

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.

Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic-Liquid Films and Co3 O4 (111) Thin Films

In this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery‐relevant ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI), Li and a Co₃O₄(111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li‐ion batteries (LIBs). We employed mostly X‐ray photoelectron spectroscopy (XPS) in combination with dispersion‐corrected density functional theory calculations (DFT‐D3). If the surface is pre‐covered by BMP‐TFSI species (model electrolyte), post‐deposition of Li (Li⁺ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li₂NSO₂CF₃, LiF, Li₂S, Li₂O₂, Li₂O, but also kinetic products like Li₂NCH₃C₄H₉ or LiNCH₃C₄H₉ of BMP. Simultaneously, Li adsorption and/or lithiation of Co₃O₄(111) to Li_nCo₃O₄ takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co⁰ could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.

Model Studies on the Ozone-Mediated Synthesis of Cobalt Oxide Nanoparticles from Dicobalt Octacarbonyl in Ionic Liquids

Low-temperature synthesis in ionic liquids (ILs) offers an efficient route for the preparation of metal oxide nanomaterials with tailor-made properties in a water-free environment. In this work, we investigated the role of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4 C1 Pyr][NTf2 ] in the synthesis of cobalt oxide nanoparticles from the molecular precursor Co2 (CO)8 with ozone. We performed a model study in ultra-clean, ultrahigh vacuum (UHV) conditions by infrared reflection absorption spectroscopy (IRAS) using Au(111) as a substrate. Exposure of the pure precursor to ozone at low temperatures results in the oxidation of the first layers, leading to the formation of a disordered Cox Oy passivation layer. Similar protection to ozone is also achieved by deposition of an IL layer onto a precursor film prior to ozone exposure. With increasing temperature, the IL gets permeable for ozone and a cobalt oxide film forms at the IL/precursor interface. We show that the interaction with the IL mediates the oxidation and leads to a more densely packed Cox Oy film compared to a direct oxidation of the precursor.

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.

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.

Potential Screening at Electrode/Ionic Liquid Interfaces from In Situ X-ray Photoelectron Spectroscopy

A new approach to investigate potential screening at the interface of ionic liquids (ILs) and charged electrodes in a two-electrode electrochemical cell by in situ X-ray photoelectron spectroscopy has been introduced. Using identical electrodes, we deduce the potential screening at the working and the counter electrodes as a function of applied voltage from the potential change of the bulk IL, as derived from corresponding core level binding energy shifts for different IL/electrode combinations. For imidazolium-based ILs and Pt electrodes, we find a significantly larger potential screening at the anode than at the cathode, which we attribute to strong attractive interactions between the imidazolium cation and Pt. In the absence of specific ion/electrode interactions, asymmetric potential screening only occurs for ILs with different cation and anion sizes as demonstrated for an imidazolium chloride IL and Au electrodes, which we assign to the different thicknesses of the electrical double layers. Our results imply that potential screening in ILs is mainly established by a single layer of counterions at the electrode.

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.

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.

Anion Exchange at the Liquid/Solid Interface of Ultrathin Ionic Liquid Films on Ag(111)

Thin ionic liquid (IL) films play an important role in many applications. To obtain a better understanding of the ion distribution within IL mixture films, we sequentially deposited ultrathin layers of two ILs with the same cation but different anions onto Ag(111), and monitored their dynamic behaviour by angle‐resolved X‐ray photoelectron spectroscopy. Upon depositing [C₈C₁Im][PF₆] on top of a wetting layer of [C₈C₁Im][Tf₂N] at room temperature (RT), we found a pronounced enrichment of the [Tf₂N]⁻ anions at the IL/vacuum interface, due to a rapid anion exchange at the IL/solid interface. In contrast, at 90 K, the [Tf₂N]⁻ anions remain at the IL/solid interface. Upon heating, we observe a rearrangement of the cations between 140 and 160 K, such that the octyl chains preferentially point towards the vacuum. Above 170 K, the ions start to become mobile, and at 220 K, the anion exchange is completed, with the [Tf₂N]⁻ anions enriched at the IL/vacuum interface in the same way as found for deposition at RT. The temperature range for the anion exchange corresponds well to glass transition temperatures reported in literature. We propose two driving forces to be cooperatively responsible for the replacement/exchange of [Tf₂N]⁻ at the IL/solid interface and its enrichment at the IL/vacuum interface. First, the adsorption energy of [C₈C₁Im][PF₆] is significantly larger than that of [C₈C₁Im][Tf₂N], and second, the surface tension of [C₈C₁Im][Tf₂N] is lower than that of [C₈C₁Im][PF₆].

Anomalous Cooling-Rate-Dependent Charge Transport in Electrolyte-Gated Rubrene Crystals

Although electrolyte gating has been demonstrated to enable control of electronic phase transitions in many materials, long sought-after gate-induced insulator-metal transitions in organic semiconductors remain elusive. To better understand limiting factors in this regard, here we report detailed wide-range resistance-temperature ( R- T) measurements at multiple gate voltages on ionic-liquid-gated rubrene single crystals. Focusing on the previously observed high-bias regime where conductance anomalously decreases with increasing bias magnitude, we uncover two surprising (and related) features. First, distinctly cooling-rate-dependent transport is detected for the first time. Second, power law R- T is observed over a significant T window, which is highly unusual in an insulator. These features are discussed in terms of electronic disorder at the rubrene/ionic liquid interface influenced by (i) cooling-rate-dependent structural order in the ionic liquid and (ii) the intriguing possibility of a gate-induced glassy short-range charge-ordered state in rubrene. These results expose new physics at the gated rubrene surface, pointing to exciting new directions in the field.

Time-dependent changes in the growth of ultrathin ionic liquid films on Ag(111)

Time dependent changes of IL film morphology depend on their molecular structure.

Various amounts of the ionic liquids (ILs) [C₁C₁Im][Tf₂N] and [C₈C₁Im][Tf₂N] were deposited in vacuo by physical vapour deposition (PVD) on single crystalline Ag(111) at room temperature and subsequently monitored by angle-resolved X-ray photoelectron spectroscopy (ARXPS) as a function of time. For very low coverages of up to one closed molecular layer, an initial wetting layer was rapidly formed for both ILs. Deposition of higher amounts of [C₁C₁Im][Tf₂N] revealed an initial three-dimensional film morphology. On the time scale of hours, characteristic changes of the XPS signals were observed. These are interpreted as island spreading and a transformation towards a nearly two dimensional [C₁C₁Im][Tf₂N] film as the final state. In contrast, a film morphology close to 2D was found from the very beginning for [C₈C₁Im][Tf₂N] deposited on Ag(111) demonstrating the influence of the alkyl chain length on the growth kinetics. These studies also highlight the suitability of time-resolved ARXPS for the investigation of IL/solid interfaces, which play a crucial role in IL thin film applications such as in catalysis, sensor, lubrication, and coating technologies.

Modification of the Electrolyte/Electrode Interface for the Template-free Electrochemical Synthesis of Metal Nanowires from Ionic Liquids

In electrochemistry, the electrode/electrolyte interface (EEI) governs the charge/mass-transfer processes and controls the nucleation/growth phenomena. The EEI in ionic liquids (ILs) can be controlled by changing the cation/anion of the IL, salt concentration, electrode potential, and temperature. Here, we show that adding a dopant salt leads to the deposition of nanowires. To illustrate, zinc nanowires were electrodeposited from ZnCl₂/1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate in the presence of GaCl₃ as a dopant salt. The choice of Zn salt and its ratio to GaCl₃ were found to be crucial for Zn nanowires formation. AFM studies revealed that the solvation structure of Au(111)/IL changes significantly in the presence of GaCl₃ and ZnCl₂. Chronoamperometry showed changes in the nucleation/growth process, consequently leading to the formation of nanowires. A similar approach was adopted to synthesize Sn nanowires. Thus, modification of the EEI by adding a dopant to ILs can be a viable method to obtain nanowires.

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.

In Situ Probing of Ion Ordering at an Electrified Ionic Liquid/Au Interface

Charge transport at the interface of electrodes and ionic liquids is critical for the use of the latter as electrolytes. A room-temperature ionic liquid, 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (EMMIM TFSI), is investigated in situ under applied bias voltage with a novel method using low-energy electron and photoemission electron microscopy. Changes in photoelectron yield as a function of bias applied to electrodes provide a direct measure of the dynamics of ion reconfiguration and electrostatic responses of the EMMIM TFSI. Long-range and correlated ionic reconfigurations that occur near the electrodes are found to be a function of temperature and thickness, which, in turn, relate to ionic mobility and different configurations for out-of-plane ordering near the electrode interfaces, with a critical transition in ion mobility for films thicker than three monolayers.

Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes

Atomistic molecular dynamics simulations have been performed to study microscopic the interfacial ionic structures, molecular arrangements, and orientational preferences of trihexyltetradecylphosphonium-bis(mandelato)borate ([P_6,6,6,14][BMB]) ionic liquid confined between neutral and charged gold electrodes. It was found that both [P_6,6,6,14] cations and [BMB] anions are coabsorbed onto neutral electrodes at different temperatures. The hexyl and tetradecyl chains in [P_6,6,6,14] cations lie preferentially flat on neutral electrodes. The oxalato and phenyl rings in [BMB] anions are characterized by alternative parallel-perpendicular orientations in the mixed innermost ionic layer adjacent to neutral electrodes. An increase in temperature has a marginal effect on the interfacial ionic structures and molecular orientations of [P_6,6,6,14][BMB] ionic species in a confined environment. Electrifying gold electrodes leads to peculiar changes in the interfacial ionic structures and molecular orientational arrangements of [P_6,6,6,14] cations and [BMB] anions in negatively and positively charged gold electrodes, respectively. As surface charge density increases (but lower than 20 μC/cm²), the layer thickness of the mixed innermost interfacial layer gradually increases due to a consecutive accumulation of [P_6,6,6,14] cations and [BMB] anions at negatively and positively charged electrodes, respectively, before the formation of distinct cationic and anionic innermost layers. Meanwhile, the molecular orientations of two oxalato rings in the same [BMB] anions change gradually from a parallel-perpendicular feature to being partially characterized by a tilted arrangement at an angle of 45° from the electrodes and finally to a dominant parallel coordination pattern along positively charged electrodes. Distinctive interfacial distribution patterns are also observed accordingly for phenyl rings that are directly connected to neighboring oxalato rings in [BMB] anions.

Switching adsorption and growth behavior of ultrathin [C2C1Im][OTf] films on Au(111) by Pd deposition

Combining in vacuo deposition of ultrathin ionic liquid (UTIL) films with angle-resolved X-ray photoelectron spectroscopy (ARXPS), we demonstrate that by deposition of submonolayer amounts of Pd onto Au(111) the initial growth mode of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C₂C₁Im][OTf]) can be switched from three-dimensional (3D) to two-dimensional (2D) growth, that is, from non-wetting to wetting. On clean Au(111), pronounced 3D growth occurs on top of an initially formed 2D wetting layer with cations and anions next to each other in a checkerboard arrangement. After pre- or postdeposition of only 0.7 ML Pd, two-dimensional layer-by-layer growth is found, which is attributed to strong attractive interactions between [C₂C₁Im][OTf] and surface Pd. For Pd post deposition onto the IL, the ARXPS data revealed particularly strong interactions between the dialkylimidazolium cation and Pd atoms, which considerably reduce the regular surface alloying of Pd with the Au substrate stabilizing Pd at the metal surface. In the context of heterogeneous catalysis using the SCILL (solid catalyst coated with ionic liquid layer) concept, these results directly provide a possible explanation on the molecular level for the beneficial influence of the IL layer in case of heterogeneous metal alloy catalysts.

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.

Ionic-Liquid-Modified Hybrid Materials Prepared by Physical Vapor Codeposition: Cobalt and Cobalt Oxide Nanoparticles in [C1C2Im][OTf] Monitored by In Situ IR Spectroscopy

The synthesis of ionic-liquid-modified nanomaterials has attracted much attention recently. In this study we explore the potential to prepare such systems in an ultraclean fashion by physical vapor codeposition (PVCD). We codeposit metallic cobalt and the room-temperature ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C1C2Im][OTf] simultaneously onto a Pd(111) surface at 100 K. This process is performed under ultrahigh-vacuum (UHV) conditions in the presence of CO, or in the presence of O2 and CO. We use time-resolved (TR) and temperature-programmed (TP) infrared reflection absorption spectroscopy (IRAS) to investigate the formation and stability of the IL-modified Co deposits in situ during the PVD-based synthesis. CO is used as a probe molecule to monitor the growth. After initial growth of flat Co films on Pd(111), multilayers of Co nanoparticles (NPs) are formed. Characteristic shifts and intensity changes are observed in the vibrational bands of both CO and the IL, which originate from the electric field at the IL/Co interface (Stark effect) and from specific adsorption of the [OTf](-) anion. These observations indicate that the Co aggregates are stabilized by mixed adsorbate shells consisting of CO and [OTf](-). The CO coverage on the Co particle decreases with increasing temperature, but some CO is preserved up to the desorption temperature of the IL (370 K). Further, the IL shell suppresses the oxidation of the Co NPs if oxygen is introduced in the PVCD process. Only chemisorbed oxygen is formed at oxygen partial pressures that swiftly lead to formation of Co3O4 in the absence of the IL (5 × 10(-6) mbar O2). This chemisorbed oxygen is found to destabilize the CO ligand shell. The oxidation of Co is not suppressed if IL and Co are deposited sequentially under otherwise identical conditions. In this case we observe the formation of fully oxidized cobalt oxide particles.

Layering of ionic liquids on rough surfaces

Understanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. In this work we have performed an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL-solid interface. Our work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contact.

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

The interaction between (sub)monolayers of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide [BMP](+)[TFSA](-) and graphite(0001), which serves as a model for the anode|electolyte interface in Li-ion batteries, was investigated under ultrahigh vacuum conditions in a combined experimental and theoretical approach. High-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory (DFT-D) calculations were employed. After vapor deposition at 300 K, XPS indicates molecular adsorbates with a 1:1 ratio of cations/anions. Cool down to ∼100 K leads to the formation of an ordered (2D) crystalline phase, which coexists with a mobile (2D) liquid. DFT-D calculations reveal that adsorbed [BMP](+) and [TFSA](-) species are arranged alternately in a row-like adsorption structure (cation-anion-cation-anion) and that adsorption is dominated by dispersion interactions between adlayer and substrate, on the one hand, and electrostatic interactions between the ions in a row, on the other hand. Simulated STM images of that structure closely resemble the experimental molecular resolved STM images and show that the resolved features mostly stem from the cations.

Interaction of the ionic liquid [BMP][TFSA] with rutile TiO2(110) and coadsorbed lithium

The interaction of lithium, [BMP][TFSA] and their mixture with rutile TiO₂(110), the thermal stability of the adlayers and the resulting reaction products are investigated under UHV conditions by STM and XPS.

Pd Nanoparticle Formation in Ionic Liquid Thin Films Monitored by in situ Vibrational Spectroscopy

Ionic liquids (ILs) are flexible reaction media and solvents for the synthesis of metal nanoparticles (NPs). Here, we describe a new preparation method for metallic NPs in nanometer thick films of ultraclean ILs in an ultrahigh vacuum (UHV) environment. CO-covered Pd NPs are formed by simultaneous and by sequential physical vapor deposition (PVD) of the IL and the metal in the presence of low partial pressures of CO. The film thickness and the particle size can be controlled by the deposition parameters. We followed the formation of the NPs and their thermal behavior by time-resolved IR reflection absorption spectroscopy (TP-IRAS) and by temperature-programmed IRAS (TR-IRAS). Codeposition of Pd and [C1C2Im][OTf] in CO at 100 K leads to the growth of homogeneous multilayer films of CO-covered Pd aggregates in an IL matrix. The size of these NPs can be controlled by the metal fraction in the co-deposit. With increasing metal fraction, the size of the Pd NPs also increases. At very low metal content, small Pd carbonyl-like species are formed, which bind CO in on-top geometry only. Upon annealing, the [OTf](-) anion coadsorbs at the NP surface and partially displaces CO. Co-adsorption of CO and IL is indicated by a strong red-shift of the CO stretching bands. While the weakly bound on-top CO is mainly replaced below the melting transition of the IL, coadsorbate shells with bridge-bonded CO and IL are stable well above the melting point. Larger three-dimensional Pd NPs can be prepared by PVD of Pd onto a solid [C1C2Im][OTf] film at 100 K. Upon annealing, on-top CO desorbs from these NPs below 200 K. Upon melting of the IL film, the CO-covered Pd NPs immerse into the IL and again form a stable coadsorbate shell that consists of bridge-bonded CO and the IL.

Adsorption, Desorption, and Reaction of 1-Octyl-3-methylimidazolium Tetrafluoroborate, [C₈C₁Im][BF₄], Ionic Liquid Multilayers on Cu(111)

Multilayers of 1-octyl-3-methylimidazolium tetrafluoroborate [C8C1Im][BF4] have been deposited on a Cu(111) surface by evaporation in UHV. XPS shows that [C8C1Im][BF4] adsorbs without decomposition for substrate temperatures < 300 K. XPS and UPS data indicate that ionic liquid (IL) deposition onto a 120 K Cu(111) surface results in the IL forming multilayers by a simultaneous-multilayer growth process. IL deposition onto a room temperature Cu(111) surface results in a different arrangement where at a coverage of one monolayer the IL forms droplets of about 100 Å height covering only about 1/10th of the surface. Multilayers deposited at 120 K convert to the room temperature arrangement upon heating. Further heating above room temperature causes the IL multilayer droplets to desorb leaving an IL monolayer of ≈6 Å thickness at ≈430 K. At higher temperatures, this monolayer reacts with the surface and BF3 is emitted, leaving products containing C, N, and some F on the surface. We propose a surface reaction where [BF4](-) ions react to form chemisorbed fluorine (Cu-F) and gaseous BF3, with the remaining [C8C1Im](+) decomposing on the Cu(111) in an unidentified manner.

Irreversible structural change of a dry ionic liquid under nanoconfinement

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Nanostructure of the Ionic Liquid-Graphite Stern Layer

Ionic liquids (ILs) are attractive solvents for devices such as lithium ion batteries and capacitors, but their uptake is limited, partially because their Stern layer nanostructure is poorly understood compared to molecular solvents. Here, in situ amplitude-modulated atomic force microscopy has been used to reveal the Stern layer nanostructure of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm TFSI)-HOPG (highly ordered pyrolytic graphite) interface with molecular resolution. The effect of applied surface potential and added 0.1 wt/wt % Li TFSI or EMIm Cl on ion arrangements is probed between ±1 V. For pure EMIm TFSI at open-circuit potential, well-defined rows are present on the surface formed by an anion-cation-cation-anion (A-C-C-A) unit cell adsorbed with like ions adjacent. As the surface potential is changed, the relative concentrations of cations and anions in the Stern layer respond, and markedly different lateral ion arrangements ensue. The changes in Stern layer structure at positive and negative potentials are not symmetrical due to the different surface affinities and packing constraints of cations and anions. For potentials outside ±0.4 V, images are featureless because the compositional variation within the layer is too small for the AFM tip to detect. This suggests that the Stern layer is highly enriched in either cations or anions (depending on the potential) oriented upright to the surface plane. When Li(+) or Cl(-) is present, some Stern layer ionic liquid cations or anions (respectively) are displaced, producing starkly different structures. The Stern layer structures elucidated here significantly enhance our understanding of the ionic liquid electrical double layer.

Potential-Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular-Scale In Situ Video-STM Study

Room-temperature ionic liquids are of great current interest for electrochemical applications in material and energy science. Essential for understanding the electrochemical reactivity of these systems are detailed data on the structure and dynamics of the interfaces between these compounds and metal electrodes, which distinctly differ from those in traditional electrolytes. In situ studies are presented of Au(111) electrodes in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSA]) by high-speed scanning tunneling microscopy (video-STM). [BMP][TFSA] is one of the best-understood air and water stable ionic liquids. The measurements provide direct insights into the potential-dependent molecular arrangement and surface dynamics of adsorbed [BMP](+) cations in the innermost layer on the negatively charged Au electrode surface. In particular, two distinct subsequent transitions in the adlayer structure and lateral mobility are observed with decreasing potential.

Ion structure controls ionic liquid near-surface and interfacial nanostructure

A unique, but unifying, feature of ionic liquids (ILs) is that they are nanostructured on the length scale of the ions; in many ILs well-defined polar and apolar domains exist and may percolate through the liquid. Near a surface the isotropic symmetry of the bulk structure is broken, resulting in different nanostructures which, until now, have only been studied indirectly. In this paper, in situ amplitude modulated atomic force microscopy (AM-AFM) has been used to resolve the 3-dimensional nanostructure of five protic ILs at and near the surface of mica. The surface and near surface structures are distinct and remarkably well-defined, but are very different from previously accepted descriptions. Interfacial nanostructure is strongly influenced by the registry between cations and the mica surface charge sites, whereas near surface nanostructure is sensitive to both cation and anion structure. Together these ILs reveal how interfacial nanostructure can be tuned through ion structure, informing "bottom-up" design and optimisation of ILs for diverse technologies including heterogeneous catalysis, lubrication, electrochemical processes, and nanofluids.