Functionalization of Oxide Surfaces through Reaction with 1,3

Letter pubs.acs.org/JPCL

Functionalization of Oxide Surfaces through Reaction with 1,3Dialkylimidazolium Ionic Liquids Stefan Schernich,† Mathias Laurin,*,† Yaroslava Lykhach,† Hans-Peter Steinrück,† Nataliya Tsud,‡ Tomás ̌ Skála,‡ Kevin C. Prince,§ Nicola Taccardi,⊥ Vladimír Matolín,‡ Peter Wasserscheid,⊥ and Jörg Libuda† †

Lehrstuhl für Physikalische Chemie II and Erlangen Catalysis Resource Center and ⊥Lehrstuhl für Chemische Reaktionstechnik and Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University, V Holešovičkách 2, 18000 Prague 8, Czech Republic § Sincrotrone Trieste SCpA, Strada Statale 14, km163.5,34149 Basovizza-Trieste, Italy S Supporting Information *

ABSTRACT: Practical applications of ionic liquids (ILs) often involve IL/oxide interfaces, but little is known regarding their interfacial chemistry. The unusual physicochemical properties of ILs, including their exceptionally low vapor pressure, provide access to such interfaces using a surface science approach in ultrahigh vacuum (UHV). We have applied synchrotron radiation photoelectron spectroscopy (SR-PES) to the study of a thin film of the ionic liquid [C6C1Im][Tf2N] prepared in situ in UHV on ordered stoichiometric CeO2(111) and partially reduced CeO2−x. On the partially reduced surface, we mostly observe decomposition of the anion. On the stoichiometric CeO2(111) surface, however, a layer of surface-anchored organic products with high thermal stability is formed upon reaction of the cation. The suggested acid−base reaction pathway may provide well-defined functionalized IL/ solid interfaces on basic oxides. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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tentatively proposed the formation of a carbene species.25 Apart from this explorative study, the chemistry at a reactive interface between ILs and atomically defined surfaces has remained largely unexplored. In this study, we adopt a surface science approach, which allows the preparation of atomically well-defined cerium oxide surfaces.26,27 Ultrapure [C6C1Im][Tf2N] thin films of the order of a single monolayer thickness are deposited in situ by PVD onto atomically clean and ordered cerium oxide surfaces prepared under UHV conditions (Figure 1a); the choice of cerium oxide is driven by the fact that it is an active support exhibiting oxidizing properties. Cerium oxide is also a common support in catalysis,28 sensor technology, and solar energy conversion.29,30 Both stoichiometric CeO2(111)27 and partially reduced CeO2−x26 were grown epitaxially on a Cu(111) single crystal using well-established procedures (details are in the Supporting Information (SI)). The geometric and electronic structures and adsorption and reaction properties of these model supports were characterized in detail previously.26,27,31,32 Our study applies synchrotron radiation photoelectron spectroscopy (SR-PES), a method specific to the elements and their

he exceptional properties of ionic liquids (ILs)1,2 have recently inspired the development of novel materials concepts, with examples in catalysis, solar cells, lubrication, and organic and molecular electronics.3−12 Many of these concepts involve thin IL films supported on solid oxide surfaces. The IL/ oxide interface is therefore at the heart of their functionality, but in most cases, its properties remain poorly understood. A better insight into the related interfacial chemistry should lead to a knowledge-driven design of these interfaces, for example, with the control of reactant interactions in catalysis, the control of adhesion and wetting in lubrication, or the optimization of interfacial electron-transfer processes in solar cells and molecular electronics. Very recently, the IL/solid interface has started to attract the attention of fundamental research (see, e.g., refs 8 and 13−19). The new field of IL surface science20 has emerged as a result of experimental possibilities arising from the low vapor pressure of many ILs.21 IL−support interfaces can be studied under ultrahigh vacuum (UHV) conditions (see, e.g., refs 17, 22, and 23) after in situ physical vapor deposition (PVD) onto atomically defined surfaces.24 However, these studies have so far mostly focused on molecular adsorption or ordering phenomena at the interface, on double or multilayer formation, and on phase transitions. Recently, Cremer et al. reported the reaction of an ionic liquid thin film with an oxide surface using in situ XPS during heating and © 2012 American Chemical Society

Received: November 14, 2012 Accepted: December 10, 2012 Published: December 10, 2012 30

dx.doi.org/10.1021/jz301856a | J. Phys. Chem. Lett. 2013, 4, 30−35

The Journal of Physical Chemistry Letters

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Figure 1. (a) Preparation of the [C6C1Im][Tf2N] films on the stoichiometric and the partially reduced ceria surfaces; (b) synchrotron PES of the C 1s region of the IL films after annealing to successively higher temperatures. The features of the intact IL are shown in blue, and decomposition products are in red.

Figure 1b displays C 1s spectra after PVD of thin films of [C6C1Im][Tf2N] on stoichiometric CeO2(111) and partially reduced CeO2−x. The films were prepared at a substrate temperature of 160 K and annealed stepwise to temperatures above 600 K. We analyzed the data using the procedure applied to synchrotron PES by Lockett33 and validated by density functional calculations,35 where peak 5 at the highest binding energies is attributed to the carbons of the anion and peaks 1−4 are attributed to the four nonequivalent carbons of the cation.

environment. In contrast to laboratory X-ray PES (XPS), synchrotrons provide superior resolution, a higher photon flux, and a tunable photon energy to optimize the photoionization cross sections and information depth.22 However, PES studies of ILs at synchrotrons are extremely rare.33 Due to the high photon flux, beam damage can be a severe problem.34 We carefully checked for it (Figure S2, SI) and only analyze spectra with no or marginal X-ray-induced changes. 31



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Figure 2. Resonant enhancement factor of (a) stoichiometric and (b) partially reduced cerium oxide surfaces as a function of temperature. The resonant enhancement ratio D(Ce3+)/D(Ce4+) was calculated from the resonant enhancements of the respective features in the valence band spectra (see the SI for details). Selected valence band spectra measured off resonance (black), at the Ce3+ resonance (121.4 eV, red) and at the Ce4+ resonance (124.8 eV, blue) on (c) stoichiometric and (d) partially reduced cerium oxide.

Scheme 1. Reaction Mechanisms Suggested for the Interfacial Reactions of [C6C1Im][Tf2N] on (a) Stoichiometric and (b) Reduced Cerium Oxide As a Function of Temperature

SI. The relative contribution of the anion (component 5 in Figure S1, SI) to the total C 1s intensity is slightly lower on the partially reduced surface. The origin of this effect may be a

The alkyl chain is assigned to peak 4, measured at 284.9 and 285.4 eV on stoichiometric and partially reduced ceria, respectively; the other binding energies are provided in the 32



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group and an N-heterocyclic carbene (a similar species was also suggested for [C1C1Im][Tf2N] on NiO25). This carbene could possibly be transformed into a surface-bound imidazole, and we assign the new peak observed above 300 K (Figure 1, im) to this reaction product. The signal of the C1 carbon is expected between the C2 and C3 carbons after carbene formation, which can indeed be deduced from the narrower overall line shape in this temperature region. The next reaction step may be a transalkylation of the carbene resulting from a nucleophilic attack of the surface oxygen, a reaction that was reported in solution.41 This would lead to the formation of a surface hexyloxy species associated with the appearance of the second new feature above 350 K (Figure 1, alkoxy) and the simultaneous decrease of the alkyl signal. The two new features in the C 1s spectra (attributed to surface-bound imidazole and hexyloxy) remain unchanged up to 500 K, demonstrating a very high thermal stability of the anchored species. Above 500 K, these species desorb without any indication of further reaction. We further postulate that the hexyloxy desorbs at least in part as 1-hexene as no reduction of the surface was observed. This leaves a proton at the surface, which may react with the anion, forming an acid that would desorb immediately in vacuum, explaining the decrease of the anion signal already at 400 K. On partially reduced CeO2−x, the situation is entirely different. The aforementioned suggestion that the anion is closer to the surface may explain why the decomposition of the anion is observed here at lower temperatures than on the stoichiometric oxide. The decomposition products of the anion are most likely strongly oxidizing fragments, CF3−, SO32−, and SO42−, and atomic nitrogen, which lead to the reoxidation of the surface. The simultaneous decrease in cationic signal and the associated lack of reaction might indicate that the cations desorb as ion pairs with different counterions than [Tf2N]−, for example, with fluorine atoms released upon anion decomposition. This reaction pathway requires a reduced surface in the first place and would also explain the stability of the anion on stoichiometric CeO2. In conclusion, we have shown that reaction of the IL [C6C1Im][Tf2N] with a stoichiometric CeO2(111) surface leads to the functionalization of the surface by a layer of stable organic reaction products above 350 K. We suggest that the reaction occurs with an acid−base mechanism via a carbene intermediate and a transalkylation. This reaction only occurs on stoichiometric CeO2(111), and the resulting functionalized surface shows exceptional thermal stability, well above the desorption temperature of the pure IL on inert surfaces. On partially reduced CeO2−x, we found no evidence for the reaction of the cation, but instead the anion decomposes. This new example of surface reactions between a clean and atomically well-defined oxide surface and an IL film shows that it is possible to form defined IL/oxide interfaces. This may permit the implementation and design of site- and materialsspecific functionalization pathways. The reaction mechanisms suggested here are triggered by the reducibility and the basicity of the support. We therefore expect them to be transferrable to other imidazolium-based ILs on basic oxides. This would provide means to create well-defined organic IL/oxide interfaces and functionalized oxides of high thermal stability, relevant to applications in catalysis, solar energy conversion, and organic electronics.

damping of the C 1s signal due to the low kinetic energy of the photoelectrons (∼100 eV) resulting in a very high surface sensitivity. This could, for example, suggest that the anion is located closer to the surface22,23 or indicate a different initial wetting behavior on the two surfaces. A reduced distance may be rationalized by Coulombic interactions between the anion and oxygen vacancies at the CeO2−x surface. The nominal intensity ratio of the four nonequivalent carbon atoms in the cation is 1:2:2:5, whereas the experimental ratios are 1:2:2:7.4(±0.3) and 1:2:2:7.2(±0.4) on stoichiometric and partially reduced ceria, respectively (note that the relative intensities of carbons 1−3 were fixed with respect to each other in the corresponding fits). The enhancement of the alkyl signal was attributed to a preferential direction of the alkyl chain toward the vacuum from angle-resolved XPS.22,23,33 We next focus on the changes in the spectral features as a function of the temperature. On both surfaces, the integral C 1s intensity decreases upon annealing up to 300 K (Figures 1 and S1, SI), although desorption of [C4C1Im][Tf2N] was not observed below 373 K on alumina.36 These changes are attributed to the dewetting of the IL film, that is, the formation of small aggregates or droplets. The accompanying relative decrease of the cation signal may be due to temperaturedependent segregation and orientation effects. On reduced CeO2−x, the line shape of the cation peaks stays unaltered over the whole temperature range. The anionic region, however, shows a new peak (at 292.5 eV in Figure 1), indicating decomposition to trifluoromethyl (TFM), starting at around 350 K. On stoichiometric CeO2 , the most important observation is the appearance of two new peaks at 285.8 (im) and 284.3 eV (alkoxy) at 300 K and above, indicating the presence of reaction products of the IL film derived from the cation. Note that the anionic signal does not change until 375 K, indicating that the anion is more stable than on the partially reduced surface. Above this temperature, an additional peak develops at 292.1 eV (Figure 1), which is again assigned to TFM. These observations point toward entirely different reaction mechanisms on the partially reduced and stoichiometric oxides. To better elucidate them, we performed resonant PES (RPES). Measuring the Ce 4d−4f resonance in the valence band region quantifies the relative amounts of Ce3+ and Ce4+. The resonant enhancement ratio, D(Ce3+)/D(Ce4+), thus determines the oxidation state of the cerium oxide surfaces with high sensitivity.37,38 Figure 2 summarizes the RPES data for both surfaces. Remarkably, the oxidation state of the stoichiometric CeO2 surface does not change upon heating, although the cation reacted with the surface above 300 K. The overall reaction mechanism therefore cannot be a redox process, and considering that the oxide is also a base would rather indicate an acid−base reaction mechanism. For CeO2−x, however, D(Ce3+)/D(Ce4+) increases up to 350 K. This behavior originates from the relatively large initial damping of the Ce3+ signal, which is lifted upon dewetting, and not from a surface reduction. The large decrease of D(Ce3+)/D(Ce4+) above 400 K, however, is the result of a reoxidation upon anion decomposition and leads us to postulate a redox reaction mechanism on the reduced surface. From the temperature dependences, we suggest different reaction pathways on stoichiometric CeO2 and partially reduced CeO2−x (Scheme 1). For CeO2, we postulate the reaction of the most acidic proton of the imidazolium ring with the basic, oxygen-terminated oxide39,40 yielding a hydroxyl 33



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EXPERIMENTAL METHODS High-resolution synchrotron radiation PES was performed at the Materials Science Beamline at the Elettra synchrotron facility in Trieste, Italy. Experimental details are provided in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and the precise description of the data fitting procedures. An extra figure with the integrated C 1s features from Figure 1 is also reported. Beam damage was investigated as well, and the interested reader will find the effects discussed there as well. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 9131 8528867. Tel: +49 9131 8527310. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the “Deutsche Forschungsgemeinschaft” (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. S.S. gratefully acknowledges financial support from the “Fonds der Chemischen Industrie”. We acknowledge further support by the Erlangen Catalysis Resource Center and by the COST Action CM1104 “Reducible Oxides Structure and Functions” and the DAAD. P.W. and N.T. acknowledge infrastructural support through an ERC Advanced Investigator Grant. The Materials Science beamline is supported by the Ministry of Education of the Czech Republic under Grants LA08022, LD11047, and LG12003. We further thank Florian Maier for helpful discussion, and we thank our colleagues at Elettra for providing good quality synchrotron light.

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REFERENCES

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(27) Sutara, F.; Cabala, M.; Sedlacek, L.; Skala, T.; Skoda, M.; Matolin, V.; Prince, K. C.; Chab, V. Epitaxial Growth Of Continuous CeO2 (111) Ultra-Thin Films on Cu(111). Thin Solid Films 2008, 516, 6120−6124. (28) Trovarelli, A. Catalysis by Ceria and Related Metals; Imperial College Press: London, 2002. (29) Eranna, G.; Joshi, B. C.; Runthala, D. P.; Gupta, R. P. Oxide Materials for Development of Integrated Gas SensorsA Comprehensive Review. Crit. Rev. Solid State Mater. Sci. 2004, 29, 111−188. (30) Chueh, W. C.; Falter, C.; Abbott, M.; Scipio, D.; Furler, P.; Haile, S. M.; Steinfeld, A. High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria. Science 2010, 330, 1797−1801. (31) Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skála, T.; Bruix, A.; Illas, F.; Prince, K. C.; et al. Support Nanostructure Boosts Oxygen Transfer to Catalytically Active Platinum Nanoparticles. Nat. Mat. 2011, 10, 310−315. (32) Dvorak, F.; Stetsovych, O.; Steger, M.; Cherradi, E.; Matolinova, I.; Tsud, N.; Skoda, M.; Skala, T.; Myslivecek, J.; Matolin, V. Adjusting Morphology and Surface Reduction of CeO2(111) Thin Films on Cu(111). J. Phys. Chem. C 2011, 115, 7496−7503. (33) Lockett, V.; Sedev, R.; Harmer, S.; Ralston, J.; Horne, M.; Rodopoulos, T. Orientation and Mutual Location of Ions at the Surface of Ionic Liquids. Phys. Chem. Chem. Phys. 2010, 12, 13816− 13827. (34) Keppler, A.; Himmerlich, M.; Ikari, T.; Marschewski, M.; Pachomov, E.; Höfft, O.; Maus-Friedrichs, W.; Endres, F.; Krischik, S. Changes of the near-Surface Chemical Composition of the 1-Ethyl-3methylimidazolium Bis(trifluoromethylsulfonyl)imide Room Temperature Ionic Liquid under the Influence of Irradiation. Phys. Chem. Chem. Phys. 2010, 13, 1174−1181. (35) Reinmöller, M.; Ulbrich, A.; Ikari, T.; Preiss, J.; Höfft, O.; Endres, F.; Krischok, S.; Beenken, W. J. D. Theoretical Reconstruction and Elementwise Analysis of Photoelectron Spectra for ImidazoliumBased Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 19526−19533. (36) Sobota, M.; Nikiforidis, I.; Hieringer, W.; Paape, N.; Happel, M.; Steinrück, H.-P.; Görling, A.; Wasserscheid, P.; Laurin, M.; Libuda, J. Toward Ionic-Liquid-Based Model Catalysis: Growth, Orientation, Conformation, and Interaction Mechanism of the [Tf2N]− Anion in [BMIM][Tf2N] Thin Films on a Well-Ordered Alumina Surface. Langmuir 2010, 26, 7199−7207. (37) Matsumoto, M.; Soda, K.; Ichikawa, K.; Tanaka, S.; Taguchi, Y.; Jouda, K.; Aita, O.; Tezuka, Y.; Shin, S. Resonant Photoemission-Study of CeO2. Phys. Rev. B 1994, 50, 11340−11346. (38) Matolín, V.; Cabala, M.; Chab, V.; Matolinova, I.; Prince, K. C.; Skoda, M.; Sutara, F.; Skala, T.; Veltruska, K. A Resonant Photoelectron Spectroscopy Study of Sn(O−X) Doped CeO2 Catalysts. Surf. Interface Anal. 2008, 40, 225−230. (39) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Metal Complexes of N-Heterocyclic Carbenes  A New Structural Principle for Catalysts in Homogeneous Catalysis. Angew. Chem., Int. Ed. 1995, 34, 2371−2374. (40) Holloczki, O.; Gerhard, D.; Massone, K.; Szarvas, L.; Nemeth, B.; Veszpremi, T.; Nyulaszi, L. Carbenes in Ionic Liquids. New J. Chem. 2010, 34, 3004−3009. (41) Wasserscheid, P.; Keim, W. Ionic Liquids  New Solutions for Transition Metal Catalyis. Angew. Chem., Int. Ed. 2000, 39, 3772− 3789.

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Functionalization of Oxide Surfaces through Reaction with 1,3

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