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

Carbon-Induced Changes in the Morphology and Wetting Behavior of Ionic Liquids on the Mesoscale

Thin films of ionic liquids (ILs) have gained significant attention due to their unique properties and broad applications. Extensive research has focused on studying the influence of ILs' chemical composition and substrate characteristics on the structure and morphology of IL films at the nano- and mesoscopic scales. This study explores the impact of carbon-coated surfaces on the morphology and wetting behavior of a series of alkylimidazolium-based ILs. Specifically, this work investigates the effect of carbon coating on the morphology and wetting behavior of short-chain ([C2C1im][NTf2] and [C2C1im][OTf]) and long-chain ([C8C1im][NTf2] and [C8C1im][OTf]) ILs deposited on indium tin oxide (ITO), silver (Ag), and gold (Au) substrates. A reproducible vapor deposition methodology was utilized for the deposition process. High-resolution scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were used to analyze the morphological and structural characteristics of the substrates and obtained IL films. The experimental data revealed that the IL films deposited on carbon-coated Au substrates showed minor changes in their morphology compared to that of the films deposited on clean Au surfaces. However, the presence of carbon coatings on the ITO and Ag surfaces led to significant morphological alterations in the IL films. Specifically, for short-chain ILs, the carbon film surface induced 2D growth of the IL film, followed by subsequent island growth. In contrast, for long-chain ILs deposited on carbon surfaces, layer-by-layer growth occurred without island formation, resulting in highly uniform and coalesced IL films. The extent of morphological changes observed in the IL films was found to be influenced by two crucial factors: the thickness of the carbon film on the substrate surface and the amount of IL deposition.

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.

Thermal Stability and CO Permeability of [C4C1Pyr][NTf2]/Pd(111) Model SCILLs: from UHV to Ambient Pressure

Solid catalysts with ionic liquid layers (SCILLs) are heterogeneous catalysts which benefit significantly in terms of selectivity from a thin coating of an ionic liquid (IL). In the present work, we study the interaction of CO with a Pd model SCILL consisting of a 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-imide ([C4C1Pyr][NTf2]) film deposited on Pd(111). We investigate the CO permeability and stability of the IL film via pressure modulation experiments by infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum (UHV) and at ambient pressure conditions by time-resolved, temperature-programmed, and polarization-modulated (PM) IRAS experiments. In addition, we performed molecular dynamics (MD) simulations to identify adsorption motifs, their abundance, and the influence of CO. We find a strongly bound IL wetting monolayer (ML) and a potentially dewetting multilayer. Molecular reorientation of the IL at the interface and multilayer dewetting allow for the accumulation of CO at the metal/IL interface. Our results confirm that co-adsorption of CO changes the molecular structure of the IL wetting layer which confirms the importance to study model SCILL systems under in situ conditions.

Graphical abstract

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, Wetting, Growth, and Thermal Stability of the Protic Ionic Liquid Diethylmethylammonium Trifluoromethanesulfonate on Ag(111) and Au(111)

We have studied the adsorption, wetting, growth, and thermal evolution of the protic IL diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) on Au(111) and Ag(111). Ultrathin films were deposited at room temperature (RT) and at 90 K, and were characterized in situ by angle-resolved X-ray photoelectron spectroscopy. For both surfaces, we observe that independent of temperature, initially, a closed 2D wetting layer forms. While the film thickness does not increase past this wetting layer at RT, at 200 K and below, "moderate" 3D island growth occurs on top of the wetting layer. Upon heating, on Au(111), the [dema][TfO] multilayers desorb at 292 K, leaving an intact [dema][TfO] wetting layer, which desorbs intact at 348 K. The behavior on Ag(111) is much more complex. Upon heating [dema][TfO] deposited at 90 K, the [dema]⁺ cations deprotonate in two steps at 185 and 305 K, yielding H[TfO] and volatile [dema]⁰. At 355 K, the formed H[TfO] wetting layer partly desorbs (∼50%) and partly decomposes to form an F-containing surface species, which is stable up to 570 K.

Time- and Temperature-Dependent Growth Behavior of Ionic Liquids on Au(111) Studied by Atomic Force Microscopy in Ultrahigh Vacuum

We deposited defined amounts of [C₁C₁Im][Tf₂N] on Au(111) at different temperatures and investigated the morphology and wetting behavior of the deposited films by atomic force microscopy. For multilayer coverages, we observe a drastically different growth behavior when comparing deposition at room temperature (RT) and deposition below 170 K followed by slow annealing to RT. Upon deposition at RT, we find the formation of 2-30 nm high and 50-500 nm wide metastable 3D droplets on top of a checkerboard-type wetting layer. These droplets spread out into stable 2D bilayers, on the time scale of hours and days. The same 2D bilayer structure is obtained after deposition below 170 K and slow annealing to RT. We present a statistical analysis on the time-dependent changes of the shape and volume of the 3D droplets and the 2D bilayers. We attribute the stabilization of the 2D bilayers on the wetting layer and on already formed bilayers to the high degree of order in these layers. Notably, the transformation process from the 3D droplets to 2D bilayer islands is accelerated by tip effects and also X-ray radiation.

Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

The behavior of ionic liquids (ILs) at charged interfaces is pivotal for their application in supercapacitors and electrochemical cells. Recently, we demonstrated for neat ILs that potential screening at polarized electrode interfaces shows a characteristic voltage dependence, as determined in situ by X-ray photoelectron spectroscopy. Herein, we use this fingerprint-type behavior to characterize the nature of the IL/electrode interfaces for IL mixtures of [C₈C₁Im][Tf₂N] and [C₈C₁Im]Cl on Au and Pt electrodes. For Au, the IL/electrode interfaces are dominated by the Cl⁻ anions, even down to a 0.1 mol% [C₈C₁Im]Cl content. In contrast, [Tf₂N]⁻ anions enrich at the IL/Pt electrode interfaces down to 10 mol% [C₈C₁Im][Tf₂N]; only at lower concentrations does a transition to Cl⁻ enrichment occur. These mixture studies demonstrate that even small concentrations of another IL or contamination, e.g. remaining from synthesis, can strongly influence the situation at charged IL interfaces.

The interface of electrodes and IL mixtures has been studied by in situ XPS. We found that the concentration of counterions at the interface can strongly deviate from the bulk composition due to interactions between electrode and IL.

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).

Outstanding Electrode-Dependent Seebeck Coefficients in Ionic Hydrogels for Thermally Chargeable Supercapacitor near Room Temperature

Thermoelectric power generation from waste heat is an important component of future sustainable development. Ion-conducting materials are promising candidates because of their high Seebeck coefficients. This study demonstrates that ionic hydrogels based on imidazolium chloride salts exhibit outstanding Seebeck coefficients of up to 10 mV K^-1. Along with their relatively high ionic conductivities (1.6 mS cm^-1) and extremely low thermal conductivities (∼0.2 W m^-1 K^-1), these hydrogels have good potential for use in heat recovery systems. The voltage behavior in response to temperature difference (stable or transient) differs significantly depending on the metal electrode material. We evaluated the electrode-dependent temperature sensitivity of the double layer capacitance of these hydrogels, which revealed that the thermally induced polarization of ions at the interface is one of the main contributors to the thermovoltage. Our results demonstrate the potential capability for ion and metal interactions to be used as an effective baseline for exploring ionic thermoelectric materials and devices. The developed thermoelectric supercapacitor exhibits reversible charging-discharging behavior under repeated disconnecting-connecting of an external load with a constant temperature difference, which offers a novel strategy for heat-to-electricity energy conversion from steady-temperature heat sources.

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 Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization

The structural dynamics of planar thin films of an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf₂) as a function of surface charge density and thickness were investigated using two-dimensional infrared (2D IR) spectroscopy. The films were made by spin coating a methanol solution of the IL on silica substrates that were functionalized with alkyl chains containing head groups that mimic the IL cation. The thicknesses of the ionic liquid films ranged from ∼50 to ∼250 nm. The dynamics of the films are slower than those in the bulk IL, becoming increasingly slow as the films become thinner. Control of the dynamics of the IL films can be achieved by adjusting the charge density on substrates through multilayer network surface functionalization. The charge density of the surface (number of positively charged groups in the network bound to the surface per unit area) is controlled by the duration of the functionalization reaction. As the charge density is increased, the IL dynamics become slower. For comparison, the surface was functionalized with three different neutral groups. Dynamics of the IL films on the functionalized neutral surfaces are faster than on any of the ionic surfaces but still slower than the bulk IL, even for the thickest films. These results can have implications in applications that employ ILs that have electrodes, such as batteries, as the electrode surface charge density will influence properties like diffusion close to the surface.

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.

Cation Exchange at the Interfaces of Ultrathin Films of Fluorous Ionic Liquids on Ag(111)

In the context of applications with thin ionic liquid (IL) films on solid supports, we studied the ion distribution within mixed thin IL films by angle-resolved X-ray photoelectron spectroscopy. After the deposition of 1-methyl-3-octylimidazolium hexafluorophosphate, [C₈C₁Im][PF₆], on top of a wetting layer (WL) of 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF₆], on Ag(111) at room temperature (RT), we find a preferential enrichment of the [PFBMIm]⁺ cation at the IL/vacuum interface. In a similar deposition experiment at 82 K, this cation exchange at the IL/solid interface does not occur. Upon heating the film from 82 K to RT, we observe the replacement of [C₈C₁Im]⁺ by [PFBMIm]⁺ at the IL/vacuum interface between ∼160 and ∼220 K. No further changes in the surface composition were observed between 220 K and RT. Upon further heating the mixed IL film, we find the complete desorption of [PFBMIm][PF₆] from the mixed film below 410 K, leaving a WL of pure [C₈C₁Im][PF₆] on Ag(111), which desorbs until 455 K.

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].