Understanding X-ray Photoelectron Spectra of Ionic Liquids: Experiments and Simulations of 1-Butyl-3-methylimidazolium Thiocyanate

We demonstrate a combined experimental and computational approach to probe the electronic structure and atomic environment of an ionic liquid, based on core level binding energies. The 1-butyl-3-methylimidazolium thiocyanate [C₄C₁Im][SCN] ionic liquid was studied using ab initio molecular dynamics, and results were compared against previously published and new experimental X-ray photoelectron spectroscopy (XPS) data. The long-held assumption that initial-state effects in XPS dominate the measured binding energies is proven correct, which validates the established premise that the ground state electronic structure of the ionic liquid can be inferred directly from XPS measurements. A regression model based upon site electrostatic potentials and intramolecular bond lengths is shown to account accurately for variations in core-level binding energies within the ionic liquid, demonstrating the important effect of long-range interactions on the core levels and throwing into question the validity of traditional single ion pair ionic liquid calculations for interpreting XPS data.

Resonant Electron Spectroscopy: Identification of Atomic Contributions to Valence States

Valence electronic structure is crucial for understanding and predicting reactivity. Valence non-resonant X-ray photoelectron spectroscopy (NRXPS) provides a direct method for probing the overall valence electronic structure. However, it is...

Experimental measurement and prediction of ionic liquid ionisation energies

Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs.

Bi(III) halometallate ionic liquids: Interactions and speciation

Bismuth containing compounds are of particular interest for optical or photo-luminescent applications in sensing, bio-imaging, telecommunications, and opto-electronics and as components in non-toxic extremely dense liquids. Bismuth(III) halometallates form highly colored novel ionic liquid based solvents for which experimental characterization and fundamental understanding are limited. In this work, Bismuth(III) halometallates incorporating chloride, bromide, and iodide have been studied via density functional theory employing B3LYP-D3BJ/aug-cc-pVDZ. Lone anions, and anions in clusters with sufficient 1-ethyl-3-methyl-imidazolium [C₂C₁Im]⁺ counter-cations to balance the charge, have been investigated in the gas- phase, and with polarizable continuum solvation. Evaluation of speciation profiles indicates that dimeric or trimeric anions are prevalent. In contrast to analogous Al systems, anions of higher charge (-2, -3) are present. Speciation profiles are similar, but not identical with respect to the halide. The Bi based anions [Bi_mX_n]^x- in the gas phase and generalized solvation environment produce multiple low energy conformers; moreover, key structural interaction patterns emerge from an analysis of ion-pair and neutral-cluster structures (Bi_mX_n)^x-(C₂C₁Im)_x ⁺ for x = 1, 2, and 3. Cation-anion interactions are weak; with Coulombic and dispersion forces predominating, anion-π structures are favored, while significant hydrogen bonding does not occur. Anion to cation charge transfer is minimal, but mutual polarization is significant, leading to local positive regions in the anion electrostatic potential surface. The key features of experimental x-ray photoelectron, UV-Vis spectra, and Raman spectra are reproduced, validating the computational results and facilitating rationalization of key features.

Zinc 1s Valence-to-Core X-ray Emission Spectroscopy of Halozincate Complexes

The Zn 1s valence-to-core (VtC) X-ray emission spectra of seven ionic liquids have been measured experimentally and simulated on the basis of time-dependent density-functional theory (TDDFT) calculations. Six of the ionic liquids were made by mixing [C₈C₁Im]X and Zn(II)X₂ at three different ZnX₂ mole fractions (0.33, 0.50, or 0.67) for X = Cl or Br, and a further ionic liquid was made by mixing [P_6,6,6,14]Cl and a mole fraction of ZnCl₂ of 0.33. Calculations were performed for the [ZnX₄]^2-, [Zn₂X₆]^2-, and [Zn₄X₁₀]^2- ions to capture the expected metal complex speciation. The VtC emission spectra showed three bands arising from single-electron processes that can be assigned to emission from ligand p-type orbitals, zinc d-orbitals, and ligand s-type orbitals. For all seven ionic liquids, the highest occupied molecular orbital arises from the ligand p orbitals, and the spectra for the different size metal complexes for the same X were found to be very similar, in terms of both relative peak intensities and peak energies. For both experiments and TDDFT calculations, there was an energy difference of 0.5 eV between the Cl-based and Br-based metal complexes for the ligand s and p orbitals, while the Zn 3d orbital energies were relatively unaffected by the identity of the ligand. The TDDFT calculations find that for the ions with symmetrically equivalent zinc atoms ([Zn₂X₆]^2- and [Zn₄X₁₀]^2-), the most appropriate core-ionized reference state has a core-hole that is localized on a single zinc atom. In this framework, the spectra for the larger ions can be viewed as a sum of spectra for the tetrahedral complex with a single zinc atom with small variations in the structure of the coordinating ligands. Because the spectra are relatively insensitive to small changes in the geometry of the ligands, this is consistent with the small variation in the spectra measured in the experiment.

Electron spectroscopy of ionic liquids: experimental identification of atomic orbital contributions to valence electronic structure

The atomic contributions to valence electronic structure for 37 ionic liquids (ILs) are identified using a combination of variable photon energy XPS, resonant Auger electron spectroscopy (RAES) and a subtraction method. The ILs studied include a diverse range of cationic and anionic structural moieties. We introduce a new parameter for ILs, the energy difference between the energies of the cationic and anionic highest occupied fragment orbitals (HOFOs), which we use to identify the highest occupied molecular orbital (HOMO). The anion gave rise to the HOMO for 25 of the 37 ILs studied here. For 10 of the ILs, the energies of the cationic and anionic HOFOs were the same (within experimental error); therefore, it could not be determined whether the HOMO was from the cation or the anion. For two of the ILs, the HOMO was from the cation and not from the anion; consequently it is energetically more favourable to remove an electron from the cation than the anion for these two ILs. In addition, we used a combination of area normalisation and subtraction of XP spectra to produce what are effectively XP spectra for individual ions; this was achieved for 10 cations and 14 anions.

Thermal stability of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids: ex situ bulk heating to complement in situ mass spectrometry

Thermal decomposition (TD) products of the ionic liquids (ILs) [CnC1Im][BF4] and [CnC1Im][PF6] ([CnC1Im]+ = 1-alkyl-3-methylimidazolium, [BF4]- = tetrafluoroborate, and [PF6]- = hexafluorophosphate) were prepared, ex situ, by bulk heating experiments in a bespoke setup. The respective products, CnC1(C3N2H2)BF3 and CnC1(C3N2H2)PF5 (1-alkyl-3-methylimidazolium-2-trifluoroborate and 1-alkyl-3-methylimidazolium-2-pentafluorophosphate), were then vaporized and analyzed by direct insertion mass spectrometry (DIMS) in order to identify their characteristic MS signals. During IL DIMS experiments we were subsequently able, in situ, to identify and monitor signals due to both IL vaporization and IL thermal decomposition. These decomposition products have not been observed in situ during previous analytical vaporization studies of similar ILs. The ex situ preparation of TD products is therefore perfectly complimentary to in situ thermal stability measurements. Experimental parameters such as sample surface area to volume ratios are consequently very important for ILs that show competitive vaporization and thermal decomposition. We have explained these experimental factors in terms of Langmuir evaporation and Knudsen effusion-like conditions, allowing us to draw together observations from previous studies to make sense of the literature on IL thermal stability. Hence, the design of experimental setups are crucial and previously overlooked experimental factors.

Atomic charges of sulfur in ionic liquids: experiments and calculations

A wide variety of experimental and computational methods are used to probe sulfur atomic charges in ionic liquids.

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, ced_IP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, ced_C+A, where the ionic vapour constituents are isolated ions. A ced_IP dataset is presented for 64 ILs. For the first time an experimental ced_C+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. ced_C+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between ced_IP and the inverse of the molecular volume. A good linear correlation is found between IL ced_IP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to ced_IP. These findings show that ced_IP is very important for understanding IL intermolecular interactions, in spite of ced_IP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.

A one-pot-one-reactant synthesis of platinum compounds at the nanoscale

The preparation of inorganic nanomaterials with a desired structure and specific properties requires the ability to strictly control their size, shape and composition. A series of chemical reactions with platinum compounds carried out within the 1.5 nm wide channel of single-walled carbon nanotubes (SWNTs) have demonstrated the ability of SWNTs to act as both a very effective reaction vessel and a template for the formation of nanocrystals of platinum di-iodide and platinum di-sulphide, materials that are difficult to synthesise in the form of nanoparticles by traditional synthetic methods. The stepwise synthesis inside nanotubes has enabled the formation of Pt compounds to be monitored at each step of the reaction by aberration-corrected high resolution transmission electron microscopy (AC-HRTEM), verifying the atomic structures of the products, and by an innovative combination of fluorescence-detected X-ray absorption spectroscopy (FD-XAS) and Raman spectroscopy, monitoring the oxidation states of the platinum guest-compounds within the nanotube and the vibrational properties of the host-SWNT, respectively. This coupling of complementary spectroscopies reveals that electron transfer between the guest-compound and the host-SWNT can occur in either direction depending on the composition and structure of the guest. A new approach for nanoscale synthesis in nanotubes developed in this study utilises the versatile coordination chemistry of Pt which has enabled the insertion of the required chemical elements (e.g. metal and halogens or chalcogens) into the nanoreactor in the correct proportions for the controlled formation of PtI₂ and PtS₂ with the correct stoichiometry.

NEXAFS spectroscopy of ionic liquids: experiments versus calculations

Experimental N 1s and S 1s NEXAFS spectra are compared to TD-DFT calculated spectra for 12 ionic liquids.

X-ray spectroscopy for chemistry in the 2-4 keV energy regime at the XMaS beamline: ionic liquids, Rh and Pd catalysts in gas and liquid environments, and Cl contamination in γ-Al2O3

The 2-4 keV energy range provides a rich window into many facets of materials science and chemistry. Within this window, P, S, Cl, K and Ca K-edges may be found along with the L-edges of industrially important elements from Y through to Sn. Yet, compared with those that cater for energies above ca. 4-5 keV, there are relatively few resources available for X-ray spectroscopy below these energies. In addition, in situ or operando studies become to varying degrees more challenging than at higher X-ray energies due to restrictions imposed by the lower energies of the X-rays upon the design and construction of appropriate sample environments. The XMaS beamline at the ESRF has recently made efforts to extend its operational energy range to include this softer end of the X-ray spectrum. In this report the resulting performance of this resource for X-ray spectroscopy is detailed with specific attention drawn to: understanding electrostatic and charge transfer effects at the S K-edge in ionic liquids; quantification of dilution limits at the Cl K- and Rh L3-edges and structural equilibria in solution; in vacuum deposition and reduction of [Rh(I)(CO)2Cl]2 to γ-Al2O3; contamination of γ-Al2O3 by Cl and its potential role in determining the chemical character of supported Rh catalysts; and the development of chlorinated Pd catalysts in `green' solvent systems. Sample environments thus far developed are also presented, characterized and their overall performance evaluated.

X-ray Photoelectron Spectroscopy of Pyridinium-Based Ionic Liquids: Comparison to Imidazolium- and Pyrrolidinium-Based Analogues

We investigate eight 1-alkylpyridinium-based ionic liquids of the form [Cn Py][A] by using X-ray photoelectron spectroscopy (XPS). The electronic environment of each element of the ionic liquids is analyzed. In particular, a reliable fitting model is developed for the C 1s region that applies to each of the ionic liquids. This model allows the accurate charge correction of binding energies and the determination of reliable and reproducible binding energies for each ionic liquid. Shake-up/off phenomena are determinedfor both C 1s and N 1s spectra. The electronic interaction between cations and anions is investigated for both simple ionic liquids and an example of an ionic-liquid mixture; the effect of the anion on the electronic environment of the cation is also explored. Throughout the study, a detailed comparison is made between [C8 Py][A] and analogues including 1-octyl-1-methylpyrrolidinium- ([C8 C1 Pyrr][A]), and 1-octyl-3-methylimidazolium- ([C8 C1 Im][A]) based samples, where X is common to all ionic liquids.

Fine tuning the ionic liquid–vacuum outer atomic surface using ion mixtures

Ionic liquid–vacuum outer atomic surfaces can be created that are remarkably different from the bulk composition.

Directly probing the effect of the solvent on a catalyst electronic environment using X-ray photoelectron spectroscopy

The anion of an ionic liquid can significantly influence the electronic environment of a metal centre, and thus impact upon reaction performance in a model Suzuki cross coupling reaction.

Quantitative adsorbate structure determination for quasicrystals using x-ray standing waves

The quantitative structure determination of adsorbed species on quasicrystal surfaces has so far appeared to present insurmountable problems. The normal incidence standing x-ray wave field technique offers a simple solution, without extensive data sets or large computations. Its application to quasicrystals raises several conceptual difficulties that are related to the phase problem in x-ray diffraction. We demonstrate their solution for the case of Si atoms adsorbed on the decagonal Co-rich modification of the Al-Co-Ni quasicrystal to determine the local structure, comprising 6-atom clusters in particular hollow sites.

Quantized friction across ionic liquid thin films

Ionic liquids - salts in the liquid state under ambient conditions - are of great interest as precision lubricants. Ionic liquids form layered structures at surfaces, yet it is not clear how this nano-structure relates to their lubrication properties. We measured the friction force between atomically smooth solid surfaces across ionic liquid films of controlled thickness in terms of the number of ion layers. Multiple friction-load regimes emerge, each corresponding to a different number of ion layers in the film. In contrast to molecular liquids, the friction coefficients differ for each layer due to their varying composition.

Tuning the electronic environment of cations and anions using ionic liquid mixtures

Ionic liquid mixtures can be used to tune the electronic environment of the ions and hence, their interactions with catalysts.

Preparation and characterisation of high-density ionic liquids incorporating halobismuthate anions

1-Butyl-3-methylimidazolium tetrahalobismuthates. Chlorobismuthates are colourless. Upon substitution of chloride for bromide or iodide the liquids become progressively more yellow or red.

The ionic liquid–vacuum outer atomic surface: a low-energy ion scattering study

We have identified elements present in the ionic liquid–vacuum outer atomic surface of 23 ionic liquids using high sensitivity low-energy ion scattering (LEIS), a very surface sensitive technique.

Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids

Ionic liquids can be intricately nanostructured in the bulk and at interfaces resulting from a delicate interplay between interionic and surface forces. Here we report the structuring of a series of dialkylpyrrolidinium-based ionic liquids induced by confinement. The ionic liquids containing cations with shorter alkyl chain substituents form alternating cation-anion monolayer structures on confinement to a thin film, whereas a cation with a longer alkyl chain substituent leads to bilayer formation. The crossover from monolayer to bilayer structure occurs between chain lengths of n = 8 and 10 for these pyrrolidinium-based ionic liquids. The bilayer structure for n = 10 involves full interdigitation of the alkyl chains; this is in contrast with previous observations for imidazolium-based ionic liquids. The results are pertinent to these liquids' application as electrolytes, where the electrolyte is confined inside the pores of a nanoporous electrode, for example, in devices such as supercapacitors or batteries.

Influence of the ionic liquid/gas surface on ionic liquid chemistry

Applications such as gas storage, gas separation, NP synthesis and supported ionic liquid phase catalysis depend upon the interaction of different species with the ionic liquid/gas surface. Consequently, these applications cannot proceed to the full extent of their potential without a profound understanding of the surface structure and properties. As a whole, this perspective contains more questions than answers, which demonstrates the current state of the field. Throughout this perspective, crucial questions are posed and a roadmap is proposed to answer these questions. A critical analysis is made of the field of ionic liquid/gas surface structure and properties, and a number of design rules are mined. The effects of ionic additives on the ionic liquid/gas surface structure are presented. A possible driving force for surface formation is discussed that has, to the best of my knowledge, not been postulated in the literature to date. This driving force suggests that for systems composed solely of ions, the rules for surface formation of dilute electrolytes do not apply. The interaction of neutral additives with the ionic liquid/gas surface is discussed. Particular attention is focussed upon H(2)O and CO(2), vital additives for many applications of ionic liquids. Correlations between ionic liquid/gas surface structure and properties, ionic liquid surfaces plus additives, and ionic liquid applications are given.

On the diffusion of ferrocenemethanol in room-temperature ionic liquids: an electrochemical study

The electrochemical behaviour of ferrocenemethanol (FcMeOH) has been studied in a range of room-temperature ionic liquids (RTILs) using cyclic voltammetry, chronoamperomery and scanning electrochemical microscopy (SECM). The diffusion coefficient of FcMeOH, measured using chronoamperometry, decreased with increasing RTIL viscosity. Analysis of the mass transport properties of the RTILs revealed that the Stokes-Einstein equation did not apply to our data. The "correlation length" was estimated from diffusion coefficient data and corresponded well to the average size of holes (voids) in the liquid, suggesting that a model in which the diffusing species jumps between holes in the liquid is appropriate in these liquids. Cyclic voltammetry at ultramicroelectrodes demonstrated that the ability to record steady-state voltammograms during ferrocenemethanol oxidation depended on the voltammetric scan rate, the electrode dimensions and the RTIL viscosity. Similarly, the ability to record steady-state SECM feedback approach curves depended on the RTIL viscosity, the SECM tip radius and the tip approach speed. Using 1.3 μm Pt SECM tips, steady-state SECM feedback approach curves were obtained in RTILs, provided that the tip approach speed was low enough to maintain steady-state diffusion at the SECM tip. In the case where tip-induced convection contributed significantly to the SECM tip current, this effect could be accounted for theoretically using mass transport equations that include diffusive and convective terms. Finally, the rate of heterogeneous electron transfer across the electrode/RTIL interface during ferrocenemethanol oxidation was estimated using SECM, and k(0) was at least 0.1 cm s(-1) in one of the least viscous RTILs studied.

Towards a molecular understanding of cation-anion interactions--probing the electronic structure of imidazolium ionic liquids by NMR spectroscopy, X-ray photoelectron spectroscopy and theoretical calculations

Ten [C(8)C(1)Im](+) (1-methyl-3-octylimidazolium)-based ionic liquids with anions Cl(-), Br(-), I(-), [NO(3)](-), [BF(4)](-), [TfO](-), [PF(6)](-), [Tf(2)N](-), [Pf(2)N](-), and [FAP](-) (TfO=trifluoromethylsulfonate, Tf(2)N=bis(trifluoromethylsulfonyl)imide, Pf(2)N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two [C(8)C(1)C(1)Im](+) (1,2-dimethyl-3-octylimidazolium)-based ionic liquids with anions Br(-) and [Tf(2)N](-) were investigated by using X-ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While (1)H NMR spectroscopy is found to probe very specifically the strongest hydrogen-bond interaction between the hydrogen attached to the C(2) position and the anion, a comparative XPS study provides first direct experimental evidence for cation-anion charge-transfer phenomena in ionic liquids as a function of the ionic liquid's anion. These charge-transfer effects are found to be surprisingly similar for [C(8)C(1)Im](+) and [C(8)C(1)C(1)Im](+) salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.

Influence of different anions on the surface composition of ionic liquids studied using ARXPS

Angle-resolved X-ray photoelectron spectroscopy has been used to study the influence of different types of anions on the surface composition of ionic liquids (ILs). We have investigated nine ILs with the same cation, 1-octyl-3-methylimidazolium [C(8)C(1)Im](+), but very different anions. In all cases, an enrichment of the cation alkyl chains is found at the expense of the polar cation head groups and the anions in the first molecular layer. This enhancement effect decreases with increasing size of the anion, which means it is most pronounced for the smallest anions and least pronounced for the largest anions. A simple model is proposed to explain the experimental observations.

Vaporisation of a dicationic ionic liquid

Highest heat of vaporization yet: The dicationic ionic liquid [C(3)(C(1)Im)(2)][Tf(2)N](2) evaporates as a neutral ion triplet. These neutral ion triplets can then be ionised to form singly and doubly charged ions. The mass spectrum exhibits the dication attached to one remaining anion, and the naked dication itself (see picture).

Vapourisation of ionic liquids

Eight common imidazolium based ionic liquids have been successfully evaporated in ultra-high vacuum, their vapours analysed by line of sight mass spectrometry and their heats (enthalpy) of vapourisation determined. They were found to evaporate as ion pairs, with heats of vapourisation which depend primarily on the coulombic interactions within the liquid phase and the gas phase ion pair. An electrostatic model is presented relating the heats of vapourisation to the molar volumes of the ionic liquids.