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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.

Reactions of copper, silver, and gold with the imidazolium-based polyhalide ionic liquid (IL) [C6C1Im][Br2I] are investigated by X-ray photoelectron spectroscopy (XPS), weight loss measurements and gas phase mass spectrometry. All three group 11 metals are strongly corroded by the IL at moderate temperatures, yielding a very high content of dissolved Cu(I), Ag(I), and Au(I) species. The IL-metal solutions are stable against contact with water and air. Replacing imidazolium by inorganic sodium cations decreases metal corrosion rates by orders of magnitude. Our results clearly indicate metal oxidation by iodide from dibromo­iodide anions forming molecular iodine and anionic [Br-M(I)-Br]- (M = Cu, Ag, Au) complexes stabilised by imidazolium counter-cations. From experiments with a trihalide IL with imidazolium methylated in 2-position we rule out imidazole-carbene formation being responsible for the observed corrosion. In contrast to the group 11 metals, molybdenum is inert against the trihalide IL, which is attributed to surface passivation.

The capability for contact formation on indium tin oxide layers of different low-temperature screen printing silver pastes after thermal curing and photonic sintering is evaluated in detail. The 80-nm-thick indium tin oxide layers used in this study are sputtered at three different oxygen gas flows and, consequently, differ by their electrical and optical properties. All pastes are analyzed by means of simultaneous thermogravimetry-differential scanning calorimetry. After printing and contact formation processing, the lateral resistivity of the metal contacts and their contact resistivity to the indium tin oxide layers are determined. Furthermore, the microstructure of the metal electrodes is investigated to gain a deeper understanding of the underlying contact formation mechanisms. Besides low-temperature curing at T curing, low = 180–220 °C, as conventionally applied to silicon heterojunction solar cells, photonic sintering and thermal curing at medium temperatures T curing, mid = 250–350 °C are utilized. The higher curing temperatures in the range of T curing, mid enhance the sintering and densification processes of the different pastes, and thereby, lateral resistivities of 2.4–3.5 μΩ⋅cm and contact resistivities of 1.0–2.5 mΩ⋅cm2 are achieved. The results indicate a promising industry-relevant non-firing-through metallization approach for, e.g., poly-silicon-based structures like tunnel oxide passivating contacts, which allow comparatively high temperatures for contact formation processing. With thermal curing at T curing, low and photonic sintering, both compatible with temperature-sensitive silicon heterojunction solar cells, lateral finger resistivities of 6.5–9.4 μΩ⋅cm and contact resistivities of 1.2–2.5 mΩ⋅cm2 are achieved. Thereby, photonic sintering has the potential of a significantly reduced process time from several minutes down to 1–4 ms.

At 298 K, the surface tension of ionic liquids ( ILs ) of the 1-alkyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide series, [CnC1Im][NTf2], ranges from around 35 mN·m−1 for [C2C1Im][NTf2] to just below 30 mN·m−1 for [C12C1Im][NTf2]. However, the decrease rate along the series is not constant: a large decrease from [C2C1Im][NTf2] to [C8C1Im][NTf2] is followed by almost constant values from [C8C1Im][NTf2] to [C12C1Im][NTf2]. Such behavior is hard to interpret from a molecular point of view without suitable information about the free-surface structure of the different ILs. In this work, we have successfully used the Langmuir principle in combination with structural data obtained from angle-resolved X-ray photoelectron spectroscopy experiments and molecular dynamics simulations, to predict the correct surface tension trend along the IL series. The concepts unveiled for this particular homologous IL family can be easily extended to other systems.

Mixtures of fluorinated and non-fluorinated ionic liquids (ILs) show a distinct structural organization in the bulk and at the surface. To understand how such microscopic effects influence the macroscopic bulk and surface properties of IL mixtures, knowledge of corresponding thermophysical properties including viscosity and surface tension is required yet lacking. With the intention of investigating surface enrichment effects of the fluorinated IL [PFBMIm][PF6] (3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate) in mixtures with the structurally similar, non-fluorinated IL [C4C1Im][PF6] (1-butyl-3-methylimidazolium hexafluorophosphate) observed with angle-resolved X-ray photoelectron spectroscopy (ARXPS), the pendant drop method and surface light scattering (SLS) were applied in the present study to determine surface tension and dynamic viscosity between (293 and 368) K. By adding small amounts of [PFBMIm][PF6] up to 9 mol %, a distinct increase in the viscosity and decrease in the surface tension of the mixtures relative to the properties of pure [C4C1Im][PF6] was found. This behavior reflects the nanosegregated structure in the bulk and at the surface of the binary IL mixtures. Using the results about the pronounced surface enrichment of the fluorinated chain of [PFBMIm][PF6] quantified by ARXPS, a linear mixing rule for the surface tension of the IL mixtures based on the surface tensions of the pure ILs and the surface concentration of their most surface-active groups is suggested.

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 110K in ultra-high vacuum by scanning tunneling and non-contact 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.

The Cover Feature displays a new method to investigate the potential screening at charged electrodes immersed in ionic liquids (ILs) by in situ X‐ray photoelectron spectroscopy. Using binding energy shifts under voltage bias with different ILs combined with Pt and Au electrodes, asymmetric potential screening effects could be attributed to specific attractive ion‐metal interactions and ion size effects. Our approach allows for unraveling interfacial properties of electrodes and ILs under clean ultra‐high vacuum condition. More information can be found in the Communication by F. Greco et al. on page 1365 in Issue 12, 2019 (DOI: https://doi.org/10.1002/open.201900211).[Image: see text]

The pyrolytic treatment of the novel ionic liquid 1-ethyl-3-methyl-imidazolium tetrakis-(1-imidazolyl)borate ([EMIM][BIm4]) allowed the preparation of B/N-doped carbonaceous materials with an exceptional performance in the adsorptive removal of the cationic dye methylene blue (MB) from wastewater. [EMIM][BIm4] was prepared via an easy and scalable synthesis protocol in very good yield. The carbon materials obtained after salt templated pyrolysis of the IL displayed high degrees of heteroatom doping (up to 22 wt% N, 3 wt% B) and high surface areas of up to 1860 m2 g−1. The materials were tested for their performance in the adsorptive removal of MB from contaminated water. Adsorption equilibrium experiments revealed an adsorption capacity exceeding 500 mg g−1, which is, to the best of our knowledge, the highest value reported so far. Time-resolved adsorption experiments showed that the materials not only exhibit high capacity but also very fast adsorption kinetics. Complete decolorization of MB solutions in only 5 min at high MB concentrations of 100 mg L−1 was observed. These results demonstrate that [EMIM][BIm4] is an excellent precursor for the preparation of IL-derived B/N-doped carbon sorbents.

The formation of surface Ga2O3 films on liquid samples of Ga, and Pt–Ga alloys with 0.7 and 1.8 at.% Pt was examined using near-ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS). Thickness, composition and growth of the oxide films were deduced as a function of temperature and Pt content of the alloys, in ultra-high vacuum and at oxygen pressures of 3 × 10− 7, 3 × 10− 3 and 1 mbar. We examined oxide layers up to a thickness of 37 A. Different growth modes were found for oxidation at low and high pressures. The formed Ga2O3 oxide films showed an increased Pt content, while the pristine GaPt alloy showed a surface depletion of Pt at the examined temperatures. Upon growth of Ga2O3 on Pt/Ga alloys a linear increase of Pt content was observed, due to the incorporation of 3.6 at.% Pt in the Ga2O3. The Pt content in Ga2O3, at the examined temperatures and bulk Pt concentrations is found to be independent of pressure, temperature and the nominal Pt content of the metallic alloy.

We report on surface investigations of cesium and tetraphenylphosphonium bis(trifluoromethylsulfonyl)imide (Cs[Tf2N], [PPh4][Tf2N]) molten salts and their mixtures. Depending on composition, these ionic compounds combine low melting points with exceptionally high thermal stability, and thus, exhibit considerably enlarged thermal operation windows compared to conventional ionic liquids. To elucidate their liquid-solid transition behavior in the near-surface region, temperature-dependent angle-resolved X-ray photoelectron spectroscopy (ARXPS) investigations were performed using our unique laboratory electron spectrometer DASSA (“Dual Analyzer System for Surface Analysis”), dedicated for investigations of macroscopic liquid samples. We followed temperature-induced changes in bulk and surface composition within the first nanometers of the neat salt melts and of Cs[Tf2N] : [PPh4][Tf2N] = 3:1 and 1:3 mixtures. In the mixtures, pronounced surface enrichment of Cs+ on expense of the bulky [PPh4]+ ions was found in the liquid state at temperatures close to the liquid-solid transition. In the solid state of Cs[Tf2N]-rich mixtures, [PPh4]+ completely vanishes from the near-surface region probed by XPS.

Group 15 elements in zero oxidation state (P, As, Sb and Bi), also called pnictogens, are rarely used in catalysis due to the difficulties associated in preparing well–structured and stable materials. Here, we report on the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state, suspended in an ionic liquid, with the native atoms ready to interact with external reagents while avoiding aerobic or aqueous decomposition pathways, and on their use as efficient catalysts for the alkylation of nucleophiles with esters. The few layer pnictogen material circumvents the extremely harsh reaction conditions associated to previous superacid–catalyzed alkylations, by enabling an alternative mechanism on surface, protected from the water and air by the ionic liquid. These 2D catalysts allow the alkylation of a variety of acid–sensitive organic molecules and giving synthetic relevancy to the use of simple esters as alkylating agents., We thank the European Research Council (ERC Starting Grant 804110 to G.A., and ERC Advanced Grant 742145 B-PhosphoChem to A.H.) for financial support. The research leading to these results was partially funded by the European Union Seventh Framework Program under grant agreement No. 604391 Graphene Flagship. G.A. has received financial support through the Postdoctoral Junior Leader Fellowship Program from “la Caixa” Banking Foundation (LCF/BQ/PI18/11630018). G.A. thanks support by the Deutsche Forschungsgemeinschaft (DFG; FLAG-ERA AB694/2-1), the Generalitat Valenciana (SEJI/2018/034 grant) and the FAU (Emerging Talents Initiative grant #WS16-17_Nat_04). Financial support by MINECO through the Excellence Unit María de Maeztu (MDM-2015-0538), Severo Ochoa (SEV-2016-0683) and RETOS (CTQ2014-55178-R) program is acknowledged. M.A.R.-C. thanks MINECO for the concession of a FPU fellowship. We also thank the DFG (DFG-SFB 953 “Synthetic Carbon Allotropes”, Project A1), the Interdisciplinary Center for Molecular Materials (ICMM), and the Graduate School Molecular Science (GSMS) for financial support. Research at UCM sponsored by Spanish MINECO/FEDER grant MAT2015-066888-C3-3-R and ERC-PoC-2016 grant POLAR-EM. H.-P.S. thanks the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program for financial support, in the context of an Advanced Investigator Grant granted to him (Grant Agreement No. 693398-ILID). B.S.J.H. and S.S. acknowledge financial support by the DFG within the Cluster of Excellence “Engineering of Advanced Materials” (project EXC 315, Bridge Funding). F.M. acknowledges R. Ransom for very helpful discussions.

In light of the increasing interest in 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 ultra-high 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-tetraphenyl-porphyrin, 2H-TPP, and the IL 1-methyl-3-octylimidazolium hexafluorophosphate, [C8C1Im][PF6], on Ag(111) and Au(111). 2H-TPP shows 2D growth up to two closed molecular layers on Ag(111) and Au(111), and 3D 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 a destabilization of the IL films compared to the IL in direct contact with the bare metals by inhibiting the specific adsorption of the ions to 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 the 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.

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 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 82K, this cation exchange at the IL/solid interface does not occur. Upon heating the film from 82K to RT, we observe the replacement of [C8C1Im]+ by [PFBMIm]+ at the IL/vacuum interface between ~160 and ~220K. No further changes in the surface composition were observed between 220K and RT. Upon further heating of the mixed IL film, we find the complete desorption of [PFBMIm][PF6] from the mixed film below 410K, leaving a WL of pure [C8C1Im][PF6] on Ag(111), which desorbs until 455K.

Group 15 elements in zero oxidation state (P, As, Sb and Bi), also called pnictogens, are rarely used in catalysis due to the difficulties associated in preparing well–structured and stable materials. Here, we report on the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state, suspended in an ionic liquid, with the native atoms ready to interact with external reagents while avoiding aerobic or aqueous decomposition pathways, and on their use as efficient catalysts for the alkylation of nucleophiles with esters. The few layer pnictogen material circumvents the extremely harsh reaction conditions associated to previous superacid–catalyzed alkylations, by enabling an alternative mechanism on surface, protected from the water and air by the ionic liquid. Thus, the Calkyl–O bond of the ester is selectively activated on the catalytic pnictogen surface with respect to the much more reactive neighboring C–C and C=O bonds, allowing the alkylation of a variety of acid–sensitive organic molecules and giving synthetic relevancy to the use of simple esters as alkylating agents.

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.

We show that both tetraphenylphosphonium bis(trifluoromethylsulfonyl)imide ([PPh4][NTf2]) and Cs[NTf2] are low melting salts of exceptionally high and also very similar thermal stability. This similarity indicates that the thermal stability is dominated by the anion. Moreover, eutectic mixtures of [PPh4][NTf2] and Cs[NTf2] with melting points below 100 °C are presented. Surface analysis of the latter in the liquid state reveals a surprising depletion of [PPh4]+ ions from the surface.

Ultrahigh vacuum (UHV) surface science techniques are used to study the heterogeneous catalytic dehydrogenation of a liquid organic hydrogen carrier in its liquid state close to the conditions of real catalysis. For this purpose, perhydrocarbazole (PH), otherwise volatile under UHV, is covalently linked as functional group to an imidazolium cation, forming a non-volatile ionic liquid (IL). The catalysed dehydrogenation of the PH unit as a function of temperature is investigated for a Pt foil covered by a macroscopically thick PH-IL film and for Pd particles suspended in the PH-IL film, and for PH-IL on Au as inert support. X-ray photoelectron spectroscopy and thermal desorption spectroscopy allows us to follow in situ the catalysed transition of perhydrocarbazole to carbazole at technical reaction temperatures. The data demonstrate the crucial role of the Pt and Pd catalysts in order to shift the dehydrogenation temperature below the critical temperature of thermal decomposition.

For equimolar mixtures of ionic liquids with imidazolium-based cations of very different electronic structure, we observe very pronounced surface enrichment effects by angle-resolved X-ray photoelectron spectroscopy (XPS). For a mixture with the same anion, that is, 1-methyl-3-octylimidazolium hexafluorophosphate + 1,3-di(methoxy)imidazolium hexafluorophosphate ([C8C1Im][PF6]+[(MeO)2Im][PF6]), we find a strong enrichment of the octyl chain-containing [C8C1Im]+ cation and a corresponding depletion of the [(MeO)2Im]+ cation in the topmost layer. For a mixture with different cations and anions, that is, [C8C1Im][Tf2N] + [(MeO)2Im][PF6], we find both surface enrichment of the [C8C1Im]+ cation and the [Tf2N]- (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)2Im]+ and [PF6]- are depleted from the surface. We propose that the observed behavior in these mixtures is due to a lowering of the surface tension by the enriched components. Interestingly, we observe pronounced differences in the chemical shifts of the imidazolium ring signals of the [(MeO)2Im]+ cations as compared to the non-functionalized cations. Calculations of the electronic structure and the intramolecular partial charge distribution of the cations contribute to interpreting these shifts for the two different cations.

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 90K, 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/support 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].

An X-ray photoelectron spectroscopy (XPS) study of Pt(II) and Pt(IV) complexes dissolved in ionic liquids (ILs) revealed the occurrence of a rich redox chemistry, which is partly induced by X-ray radiation. Depending on the IL anion either a reduction of Pt(IV) to Pt(II) in the case of ethylsulfate ([EtOSO 3 ] − )-based ILs, or an oxidation of Pt(II) to Pt(IV) in the case of bis[(trifluoromethyl)sulfonyl]imide ([Tf 2 N] − )-based ILs was observed upon X-ray irradiation. Moreover, surface enrichment or depletion effects of the Pt complexes were deduced from angle-resolved XPS (ARXPS) data; while the Pt complexes dissolved in imidazolium-based ILs tended to deplete from the surface, in pyrrolidinium-based solutions a surface enrichment of the Pt complexes was observed.

We have performed a systematic study addressing the surface behavior of a variety of functionalized and non-functionalized ionic liquids (ILs). From angle-resolved X-ray photoelectron spectroscopy, detailed conclusions on the surface enrichment of the functional groups and the molecular orientation of the cations and anions is derived. The systems include imidazolium-based ILs methylated at the C2 position, a phenyl-functionalized IL, an alkoxysilane-functionalized IL, halo-functionalized ILs, thioether-functionalized ILs, and amine-functionalized ILs. The results are compared with the results for corresponding non-functionalized ILs where available. Generally, enrichment of the functional group at the surface is only observed for systems that have very weak interaction between the functional group and the ionic head groups.

We introduce a new method for preparing ultrathin ionic liquid (IL) films on surfaces by means of electrospray ionization deposition (ESID) under ultraclean and well-defined ultra-high-vacuum (UHV) conditions. In contrast to physical vapor deposition (PVD) of ILs under UHV, ESID even allows deposition of ILs, which are prone to thermal decomposition. As proof of concept, we first investigated ultrathin [C8C1Im][Tf2N] (=1-methyl-3-octyl imidazolium bis(trifluoromethyl)imide) films on Au(111) by angle-resolved X-ray photoelectron spectroscopy (ARXPS). Films obtained by ESID are found to be virtually identical to films grown by standard PVD. Thereafter, ESID of [C8C1Im]Cl on Au(111) was studied as a first example of an IL that cannot be prepared as ultrathin film otherwise. [C 8C1Im]Cl forms a wetting layer with a checkerboard arrangement with the cationic imidazolium ring and the chloride anion adsorbed next to each other on the substrate and the alkyl chain pointing toward vacuum. This arrangement within the wetting layer is similar to that observed for [C8C1Im][Tf2N], albeit with a higher degree of order of the alkyl chains. Further deposition of [C8C 1Im]Cl leads to a pronounced island growth on top of the wetting layer, which is independently confirmed by ARXPS and atomic force microscopy. This behavior contrasts the growth behavior found for [C8C 1Im][Tf2N], where layer-by-layer growth on top of the wetting layer is observed. The dramatic difference between both ILs is attributed to differences in the cation-anion interactions and in the degree of order in the wetting layer of the two ILs.

Carbon dioxide (CO2) absorption by the amine-functionalized ionic liquid (IL) dihydroxyethyldimethylammonium taurinate at 310 K was studied using surface- and bulk-sensitive experimental techniques. From near-ambient pressure X-ray photoelectron spectroscopy at 0.9 mbar CO2, the amount of captured CO2 per mole of IL in the near-surface region is quantified to ~0.58 mol, with ~0.15 mol in form of carbamate dianions and ~0.43 mol in form of carbamic acid. From isothermal uptake experiments combined with infrared spectroscopy, CO2 is found to be bound in the bulk as carbamate (with nominally 0.5 mol of CO2 bound per 1 mol of IL) up to ~2.5 bar CO2, and as carbamic acid (with nominally 1 mol CO2 bound per 1 mol IL) at higher pressures. We attribute the fact that at low pressures carbamic acid is the dominating species in the near-surface region, while only carbamate is formed in the bulk, to differences in solvation in the outermost IL layers as compared to the bulk situation.

This perspective analyzes the potential of X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV) conditions to follow chemical reactions in ionic liquids in situ. Traditionally, only reactions occurring on solid surfaces were investigated by X-ray photoelectron spectroscopy (XPS) in situ. This was due to the high vapor pressures of common liquids or solvents, which are not compatible with the required UHV conditions. It was only recently realized that the situation is very different when studying reactions in Ionic Liquids (ILs), which have an inherently low vapor pressure, and first studies have been performed within the last years. Compared to classical spectroscopy techniques used to monitor chemical reactions, the advantage of XPS is that through the analysis of their core levels all relevant elements can be quantified and their chemical state can be analyzed under well-defined (ultraclean) conditions. In this perspective, we cover six very different reactions which occur in the IL, with the IL, or at an IL/support interface, demonstrating the outstanding potential of in situ XPS to gain insights into liquid phase reactions in the near-surface region.

Ultrathin films of two imidazolium-based ionic liquids (ILs), [C 1C1Im][Tf2N] (=1-methyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [C4C1Im][Tf 2N] (=1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) were deposited on mica surfaces by physical vapor deposition in ultrahigh vacuum. Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), the initial wetting behavior, the growth characteristics, and the molecular arrangement of the ions at the interface were investigated. The measurements were performed on freshly air-cleaved mica surfaces with different carbon precoverages. ARXPS clearly reveals that the initial IL adsorption behavior strongly depends on the amount of preadsorbed carbon: On clean mica, 3D growth (complete dewetting) occurs, whereas on a fully carbon covered surface, initially a complete 2D wetting layer forms, followed by 3D growth. © 2013 American Chemical Society.

The investigation of liquid surfaces and interfaces with the powerful toolbox of ultra-high vacuum (UHV)-based surface science techniques generally has to overcome the issue of liquid evaporation within the vacuum system. In the last decade, however, new classes of liquids with negligible vapor pressure at room temperature-in particular, ionic liquids (ILs)-have emerged for surface science studies. It has been demonstrated that particularly angle-resolved X-ray Photoelectron Spectroscopy (ARXPS) allows for investigating phenomena that occur at gas-liquid and liquid-solid interfaces on the molecular level. The results are not only relevant for IL systems but also for liquids in general. In all of these previous ARXPS studies, the sample holder had to be tilted in order to change the polar detection angle of emitted photoelectrons, which restricted the liquid systems to very thin viscous IL films coating a flat solid support. We now report on the concept and realization of a new and unique laboratory "Dual Analyzer System for Surface Analysis (DASSA)" which enables fast ARXPS, UV photoelectron spectroscopy, imaging XPS, and low-energy ion scattering at the horizontal surface plane of macroscopically thick non-volatile liquid samples. It comprises a UHV chamber equipped with two electron analyzers mounted for simultaneous measurements in 0° and 80° emission relative to the surface normal. The performance of DASSA on a first macroscopic liquid system will be demonstrated.

There are strings attached: after linking the reacting groups to head groups of ionic liquids to drastically lower the vapour pressures of the reactants, ordinary liquid-phase organic reactions can be monitored by in situ X-ray photoelectron spectroscopy. This approach is demonstrated for the nucleophilic substitution of an alkyl amine and an alkyl chloride moiety, which are attached to the cation and anion of ionic liquids, respectively.

Incorporating tailored nanostructures into solar cells is a promising way to improve their photovoltaic conversion efficiency. For such solar cells, sputtered indium-doped tin oxide on glass is the standard substrate. As the global resources of indium are very limited, there is the challenge to move to other substrates and thus to synthesize nanostructures on these. We report on the growth and characterization of zinc oxide nanorod arrays on indium-free transparent conducting oxide substrates, in particular on sputtered aluminum-doped ZnO. We present a catalyst-free vapor-transport growth method at 510 °C, which allows for the growth of highly crystalline, well-aligned ZnO nanorods arrays with a tunable length, ranging from hundreds of nanometers to several micrometers. The nanorods exhibit an excellent crystal quality, as shown by photoluminescence measurements and high-resolution transmission electron microscopy. Optical transmission spectra show a sufficient transparency of the substrates, qualifying t...

The Cover Feature shows how coinage metals such as gold, silver, and copper are rapidly dissolved under mild and safe conditions by the ionic liquid 1‐hexyl‐3‐methylimidiazolim dibromoiodide ([C6C1Im][Br2I]), forming metal(I)–dibromo anionic complexes together with molecular iodine, as proven by X‐ray photoelectron spectroscopy, etching experiments, and gas‐phase analysis. Trihalide ionic liquids are shown to be promising candidates for precious‐metal etching and leaching media, and may also open new possibilities for Au(I) catalysis. More information can be found in the Full Paper by F. Maier and co‐workers on page 15 in Issue 1, 2019 (DOI: 10.1002/open.201800149).

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.

The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM][Tf 2N], [Et(EG) 2MIM][Tf 2N], and [Me(EG) 3MIM][Tf 2N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0 degrees and 70 degrees emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surface-active impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf 2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt.

We describe the design of a temperature-controlled flow-through liquid cell dedicated to the study of liquids with soft X-rays. The cell can be operated with internal ambient pressure and is mounted on a standard vacuum manipulator, making it compatible to the ultra-high vacuum environment required in synchrotron beamlines. The liquid is separated from the vacuum by a thin membrane, allowing the use of soft X-ray photon-in–photon-out techniques such as X-ray emission and fluorescence-yield X-ray absorption spectroscopy to study the electronic structure of liquids and liquid–solid interfaces. Special care was taken for a rapid and effective flow of the liquid inside the cell in order to minimize local heating and beam damage effects. To illustrate the capabilities, oxygen K X-ray emission spectra of D 2 O and H 2 O are presented and briefly discussed together with possible problems that may arise from X-ray-induced oxide formation at the membrane–liquid interface.

Photoinduced degradation is a critical obstacle for the real application of novel semiconductors for photovoltaic applications. In this paper, the photoinduced degradation of CH3NH3PbI3 in a vacuum and air (relative humidity 40%) is analyzed by ex situ and advanced in situ technologies. Without light illumination, CH3NH3PbI3 films slowly degrade under vacuum and air within 24 hours. However, we find that CH3NH3PbI3 converts to metallic lead (Pb0) when exposed to vacuum and light illumination. Further, a series of lead salts (e.g. PbO, Pb(OH)2 and PbCO3) are formed when CH3NH3PbI3 is degraded under environmental conditions, i.e. under the combination of light, oxygen and moisture. Photoinduced degradation is found to be determined by the environmental atmosphere as CH3NH3PbI3 films remain very stable under nitrogen conditions. The results from vacuum conditions underpin that the high volatility of the organic component (CH3NH3I) is in conflict with reaching excellent intrinsic stability due to its role in creating ion vacancies. The degradation in air suggests that both oxygen and water contribute to the fast photodecomposition of CH3NH3PbI3 into lead salts rather than water alone. Given these basic yet fundamental understandings, the design of hydrophobic capping layers becomes one prerequisite towards long-term stable perovskite-based devices.

The interaction of cobalt(II) tetraphenylporphyrin (CoTPP) and cobalt(II) tetrakis-(3,5-di-tert-butylphenyl)porphyrin (CoTTBPP) with a Ag(111) surface has been investigated with photoelectron spectroscopy (XPS/UPS). It is demonstrated that these adsorbed metal complexes are excellent model systems for studying the electronic interaction between a coordinated metal ion and a metal surface. The photoelectron spectra and work function data provide evidence that the electronic interaction between the cobalt ion and the silver surface results in a transfer of electron density from the surface to the ion. The presence of an additional electronic state located ∼1 eV above the singly occupied molecular orbital (SOMO) of the metalloporphyrins is consistent with a molecular orbital (MO) model of the Co−Ag interaction as is the fact that the energetic position of this state depends on the distance between the Co ion and the Ag surface. The adsorbate-induced work function changes for the saturated monolayers amount t...

Surface studies of ionic liquids are particularly important for all kinds of multiphasic operations employing ionic liquids, e.g. biphasic homogeneous catalysis or supported ionic liquid phase catalysis. Using X-ray photoelectron spectroscopy (XPS), the surface composition of the model system 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtOSO3] was investigated. By comparing two different samples of this ionic liquid from two different origins, we observed a decisive influence of silicon containing impurities on composition and structure of the surface. For the case of the impurities containing ionic liquid, our angle-dependent XPS data are in agreement with a model of a surface layer consisting of highly oriented ionic liquid molecules. From a fundamental point of view, our study may be of general relevance for the understanding of the chemistry of liquid surfaces in general.

An ordered oxide monolayer with ( 3 × 3 )R30° periodicity was obtained on carbon-terminated 6H–SiC(0001) after preparation with a hydrogen microwave plasma. The stoichiometry of this surface was investigated by X-ray-induced photoelectron spectroscopy. Most components of the Si 2p, C 1s, and O 1s core lines are in agreement with the recently published atomic structure of this ordered monolayer. Beyond this we found hydrocarbon contamination that could be removed by annealing, and small graphitized areas remaining on the surface. We used angle-resolved ultra-violet photoelectron spectroscopy to examine the electronic structure of the valence band. Besides the O 2s and the O 2p lone pair state at 24 eV and 6 eV respectively below the valence band maximum (VBM), we detected a dangling bond state slightly above the VBM and a Si hybrid-O 2p-derived bonding state dispersing between 9.1 and 11.0 eV below the VBM. For the dangling bond state the possibility of a Mott–Hubbard ground state is discussed. In order to clarify the origin of the oxygen-derived states our results are compared with previous investigations of silicon oxides.

We present the first systematic study of the influence of temperature on the degree of surface enrichment of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs). Using angle-resolved X-ray photoelectron spectroscopy, we demonstrate that the degree of surface enrichment strongly decreases with increasing temperature for all the studied ILs. For ILs with the same cation, but different anions, [C8 C1 Im]Br, [C8 C1 Im][TfO] and [C8 C1 Im][Tf2 N], no significant differences of the temperature-induced partial loss of surface enrichment are found. Measurements for [C4 C1 Im][TfO], [C8 C1 Im][TfO] and [C18 C1 Im][TfO] indicate a small effect of the chain length. For [C18 C1 Im][TfO], a continuous decrease of alkyl surface enrichment is found with increasing temperature, with no abrupt changes at the phase-transition temperature from the smectic A to the isotropic phase, indicating that the surface enrichment is not affected by this phase transition.

Ultrathin films of the ionic liquid 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C(1)C(1)Im][Tf(2)N]) were deposited on differently terminated Ni(111) single crystal surfaces. The initial wetting behaviour, the growth characteristics, the molecular arrangement at the interface, and thermal reactivity were investigated using angle-resolved X-ray photoelectron spectroscopy (ARXPS). On clean Ni(111), the initial growth occurs in a layer-by-layer mode. At submonolayer coverages up to at least 0.40 ML, a preferential arrangement of the IL ions in a bilayer structure, with the imidazolium cations in contact with the Ni surface atoms and the anions on top of the cation, is deduced. For higher coverages, a transition to a checkerboard-type arrangement occurs, which is most likely due to repulsive dipole-dipole interactions in the first layer. An overall preference for a checkerboard-type adsorption behaviour, i.e., anions and cations adsorbing next to each other, is found on the oxygen-precovered O(√3×√3)R30° Ni(111) surface. The thermal stability of adsorbed IL layers on Ni(111) and on a fully oxidised Ni(111) surface was studied by heating the layers to elevated temperatures. For clean Ni(111) reversible adsorption takes place. For the oxidised surface, however, only cation-related moieties desorb, starting at ~450 K, while anion-related signals remain on the surface up to much higher temperatures.

Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1–5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of “green” silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts.

We demonstrate the application of in situ X-ray photoelectron spectroscopy (XPS) to monitor organic, liquid-phase reactions. By covalently attaching ionic head groups to the reacting organic molecules, their volatility can be reduced such that they withstand ultra high vacuum conditions. The applied method, which is new for the investigation of organic reactions, allows for following the fate of all elements present in the reaction mixture--except for hydrogen--in a quantitative and oxidation-state-sensitive manner in one experiment. This concept is demonstrated for the alkylation of a tertiary amine attached to an imidazolium or phosphonium moiety by the anion 4-chlorobutylsulfonate ([ClC(4)H(8)SO(3)](-)). In the course of the reaction, the covalently bound chlorine is converted to chloride and the amine to ammonium as reflected by the distinct shifts in the N 1s and Cl 2p binding energies.

Ultrathin films of two imidazolium-based ionic liquids (IL), [C(1)C(1)Im][Tf(2)N] (= 1,3-dimethylimidazolium bis(trifluoromethyl)imide) and [C(8)C(1)Im][Tf(2)N] (= 1-methyl-3-octylimidazolium bis(trifluoromethyl)imide) were prepared on a Au(111) single-crystal surface by physical vapor deposition in ultrahigh vacuum. The adsorption behavior, orientation, and growth were monitored via angle-resolved X-ray photoelectron spectroscopy (ARXPS). Coverage-dependent chemical shifts of the IL-derived core levels indicate that for both ILs the first layer is formed from anions and cations directly in contact with the Au surface in a checkerboard arrangement and that for [C(8)C(1)Im][Tf(2)N] a reorientation of the alkyl chain with increasing coverage is found. For both ILs, geometry models of the first adsorption layer are proposed. For higher coverages, both ILs grow in a layer-by-layer fashion up to thicknesses of at least 9 nm (10 ML). Moreover, beam damage effects are discussed, which are mainly related to the decomposition of [Tf(2)N](-) anions directly adsorbed at the gold surface.

Materials making use of thin ionic liquid (IL) films as support-modifying functional layer open up a variety of new possibilities in heterogeneous catalysis, which range from the tailoring of gas-surface interactions to the immobilization of molecularly defined reactive sites. The present report reviews recent progress towards an understanding of "supported ionic liquid phase (SILP)" and "solid catalysts with ionic liquid layer (SCILL)" materials at the microscopic level, using a surface science and model catalysis type of approach. Thin film IL systems can be prepared not only ex-situ, but also in-situ under ultrahigh vacuum (UHV) conditions using atomically well-defined surfaces as substrates, for example by physical vapor deposition (PVD). Due to their low vapor pressure, these systems can be studied in UHV using the full spectrum of surface science techniques. We discuss general strategies and considerations of this approach and exemplify the information available from complementary methods, specifically photoelectron spectroscopy and surface vibrational spectroscopy.

We measured the density and surface tension of 9 bis[(trifluoromethyl)sulfonyl]imide ([Tf(2)N](-))-based and 12 1-methyl-3-octylimidazolium ([C(8)C(1)Im](+))-based ionic liquids (ILs) with the vibrating tube and the pendant drop method, respectively. This comprehensive set of ILs was chosen to probe the influence of the cations and anions on density and surface tension. When the alkyl chain length in the [C(n)C(1)Im][Tf(2)N] series (n = 1, 2, 4, 6, 8, 10, 12) is increased, a decrease in density is observed. The surface tension initially also decreases but reaches a plateau for alkyl chain lengths greater than n = 8. Functionalizing the alkyl chains with ethylene glycol groups results in a higher density as well as a higher surface tension. For the dependence of density and surface tension on the chemical nature of the anion, relations are only found for subgroups of the studied ILs. Density and surface tension values are discussed with respect to intermolecular interactions and surface composition as determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). The absence of nonvolatile surface-active contaminants was proven by ARXPS.

In hydroformylation, multiphase catalysis is a well-established and industrially realized method for effective catalyst separation and recycling. Aqueous phase liquid–liquid biphasic catalysis was developed through the pioneering discovery of the highly water-soluble ligand tris(3-sodium sulfonatophenyl)phosphine (tppts, 1) by Kuntz in 1976. This ligand concept enabled the industrial realization of aqueous hydroformylation for propene hydroformylation in the Ruhrchemie–Rh ne–Poulenc process. The process went on stream in 1984, and is still operating today (550000 tons per year). However, the limited solubility of higher olefins (>C4) in water has prompted much research activity into alternative polar catalyst media for liquid–liquid multiphase hydroformylation. Among these endeavors, the use of lowmelting salts, so-called ionic liquids (ILs), has attracted particular interest as many ionic liquids show sufficiently high solubilities for higher olefins to allow reasonable reaction rates. First reports on the application of ionic liquids in Rh-catalyzed hydroformylation were published by Chauvin s group in 1995. Already in this first paper, the use of sulfonated triphenylphosphine ligands was highlighted as a crucial precondition to avoid Rh-leaching into the organic product phase. Later, sulfonated triphenylphosphine ligands were also combined with imidazolium counter ions. ColeHamilton and co-workers suggested, for example, [C3mim]2ACHTUNGTRENNUNG[PhP ACHTUNGTRENNUNG(C6H4SO3)2] as a suitable ligand for hydroformylation reactions in the biphasic system ionic liquid/scCO2. [7] The same ligand system was applied recently to an even more efficient catalytic system using the ionic catalyst solution in the form of a supported ionic liquid phase (SILP). Such SILP catalytic systems have also been very successfully applied in continuous gas-phase reactions where the ionic liquid film supported on a highly porous inorganic support is contacted directly with the gas-phase of the reactants to perform continuous hydroformylation using a continuous fixed-bed reactor. In general, hydroformylation in ionic liquids has produced a huge amount of scientific activity over the last 15 years and particular progress was made by the use of regioselective ionic ligand systems and by the application of ionic liquids carrying halide-free, cheap and hydrolytic stable anions. The field has been recently summarized comprehensively in an excellent review by Haumann and Riisager. In multiphase catalysis, interface processes such as substrate diffusion into the catalyst phase, the reaction rate at the phase boundary (in comparison to reaction rate in the bulk), and product diffusion back into the organic phase play a crucial role for the overall performance of the system. Despite this obvious fact, experimental investigations into the chemical nature of the liquid surface of catalytic systems are lacking so far. This is even more surprising in the light of recent theoretical findings by the group of Wipff, which demonstrated that the composition of the catalytic interface may well be very different from the average chemical composition of two adjacent bulk liquid phases. This fact is of high relevance for the detailed understanding of all above-mentioned interface transport and reaction processes. Moreover, the authors demonstrated that the surface-active character of a given type of an IL soluble complex was similar at the IL–vacuum interface and at the IL interface with weakly polar organic solvents. The [a] Dipl.-Chem. C. Kolbeck, Dipl.-Chem. T. Cremer, Dr. F. Maier, Prof. Dr. H.-P. Steinr ck Chair of Physical Chemistry II Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91054 Erlangen (Germany) Fax: (+49)9131-852-8867 E-mail : steinrueck@chemie.uni-erlangen.de [b] Dipl.-Chem. N. Paape, Dr. P. S. Schulz, Prof. Dr. P. Wasserscheid Lehrstuhl f r Chemische Reaktionstechnik Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) [] These authors contributed equally to this work.