Adsorption, Desorption, and Reaction of 1-Octyl-3-methylimidazolium

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Adsorption, Desorption, and Reaction of 1‑Octyl-3methylimidazolium Tetrafluoroborate, [C8C1Im][BF4], Ionic Liquid Multilayers on Cu(111) Karen L. Syres† and Robert G. Jones* School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom

Downloaded by FLORIDA ATLANTIC UNIV on September 1, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.langmuir.5b02932

S Supporting Information *

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

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INTRODUCTION Ionic liquids (ILs) are composed of ions, which are held together by a strong Coulomb potential. They have ultralow vapor pressures at room temperature allowing them to be studied using ultrahigh vacuum (UHV) surface science techniques.1 Their low symmetry leads to low glass transition temperatures giving ILs a huge liquid range (>400 K). Studies have shown that ILs self-organize and form layered structures at the IL/vacuum (and IL/air) interface2,3 and at the IL/solid interface.4,5 Such ordering is directly relevant to IL electrochemistry (applications for energy storage in batteries and supercapacitors),6,7 tribology (adhesion and lubrication),8,9 optoelectronic/photovoltaic devices10,11 and supported IL phase (SILP) catalysis.12 In the latter, catalysts are dissolved in a thin film of IL on a high surface area solid support, allowing homogeneous catalytic reactions in a heterogeneous catalyst environment. See ref 13 for a short review of the new field of ionic liquid surface science. Cremer et al. carried out the first single crystal UHV surface science study of IL adsorption, looking at [C1C1Im][Tf2N] on Ni(111) using ARXPS.14 (Note: A range of nonstandardized abbreviations are used for ionic liquids. Those referred to in this paper are given in the Nonstandardized Abbreviations list below.) Submonolayer coverage of the IL formed a bilayer structure with the cations in contact with the metal, but this changed to a checkerboard arrangement of the anions and cations at higher coverages. There is now a reasonable body of work on the adsorption of ILs on coinage metals. Mono- and multilayer adsorption of [C2C1Im][Tf2N] on Au(110) has been studied using LEED, UPS and STM,15 from 128 to 300 K, showing that the IL was ordered by the channelled surface and © XXXX American Chemical Society

that the interaction of the IL with the surface could alter the reconstruction adopted by the substrate. For multilayer adsorption, droplet formation was observed at 300 K. The same IL has also been studied on Au(111) using LEED and helium atom scattering16 which showed formation of successive ordered layers during adsorption and melting from crystalline to liquid phase at 270−280 K. ARXPS has been used to study [C8C1Im][Tf2N] and [C1C1Im][Tf2N] on Au(111) at room temperature,17 where for monolayers the ions were found to adsorb in a coplanar arrangement, with layer by layer growth up to 10 ML. Very recently, video STM of [BMP][TFSA] on Au(111) has been used to image potential dependent molecular arrangements on the negatively charged surface. [BMP][TFSA] adsorption on Au(111) and Ag(111) has been studied using STM, ARXPS, and DFT in the range 100−293 K.18−20 At room temperature, the IL formed a 2D liquid on both surfaces. At 100 K, different adsorbed IL phases coexisted on both surfaces, including long-range ordered phases and a short-range ordered phase (2D glass). The surfaces were deposited at room temperature and cooled rather than formed at low temperatures. Using DFT calculations, identification of cations and anions on the surface was possible and it was possible to deduce that the cations have their pyrrolidinium rings approximately flat to the surface, while the anions have their CF3 groups pointing upward and bind to the surface via their O atoms. STM has also been used to study [Py1,4][FAP] on Au(111)21 where short-range order was observed at 210 K, but it was Received: April 29, 2015 Revised: August 24, 2015

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DOI: 10.1021/acs.langmuir.5b02932 Langmuir XXXX, XXX, XXX−XXX

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these absorbed gases may reside in the ionic underlayer which lies below the octyl chain surface. By depositing ILs by evaporation in vacuum we can study monolayer to multilayer coverages of ILs on solid substrates. Ionic liquids evaporate as ions pairs (anion + cation),1 and hence when adsorbed at a surface they arrive as ion pairs. For monolayer adsorption of [C2C1Im][Tf2N] (Figure 1B) on Au(110),4 the IL was initially found to aggregate together, but then flattened out with time. It was suggested that this behavior was due to the aggregation of the arriving ion pairs into nanodroplets, followed by a breakdown of the ion pairing and flattening out of the droplets. IL nanodroplets have also been studied by Mo et al., where an individual nanodroplet was immobilized in a nanowell on a patterned silicon surface.25 They used AFM to probe the dynamics of the IL nanodroplets at atmosphere. ILs show great promise as lubricants, displaying reduced friction for a number of metals and alloys. However, it has been shown that corrosion can occur during tribological processes and this has been the subject of many studies. Phillips et al. used Mössbauer spectroscopy and X-ray photoelectron spectroscopy to study fluorinated ILs (some containing the BF4 anion) in sliding contact with steel up to temperatures of 300 °C.26 They found that the ILs react with the steel/iron to form FeF2. Espinosa et al. studied the corrosion of copper with various ILs.27 They found that imidazolium based ILs show reactivity toward the copper and that the specific reaction products depend on the anion. The tetrafluoroborate anion decomposes and there is an increase in surface roughness of the Cu surface. Here we investigate the [C8C1Im][BF4]/Cu(111) interface using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) to determine how the IL adsorbs, desorbs, and reacts with the Cu surface. IL is deposited both at 120 K (below the glass transition point of the IL, 192 K28) and at room temperature. Our results show a clear difference between the adsorption behavior at the two temperatures and indicate that the structure created at 120 K transforms into the room temperature structure upon heating. We suggest models for the adsorption behavior and propose that IL droplets are created on the room temperature surface. Our studies of the desorption of IL multilayers from the Cu(111) surface also support droplet formation and indicate that the IL reacts with the Cu surface. A reaction mechanism is discussed.

unclear whether the ions formed coplanar monolayer or a bilayer. Only two studies have been carried out on copper. [BMP][TFSA] adsorbed onto Cu(111) has been studied by STM and XPS.22 At low temperatures (80−200 K), it adsorbs intact forming islands, indicating attractive interactions, but no long-range ordered structures due to low mobility. For deposition at room temperature, the [BMP] remains intact and forms an ordered structure, while the [TFSA] decomposes, probably to adsorbed S, SOx, and CF3. Very recently, DFT and molecular dynamics calculations have been carried out for [mmim][BF4] adsorbed onto Al(111) and Cu (111) surfaces.23 The [BF4]− was found to bond more strongly than the [mmim], and the [mmim] bonded with the imidazolium ring parallel to the surface. Strong lateral interactions were observed such that two adjacent ion pairs bonded to the surface more strongly than double the bonding of one ion pair. Also, both anions and cations exist as fully charged ions on the Cu(111) surface. The barriers to diffusion were found to be rather small. Of particular interest to the present work was that, on both surfaces, the anion−cation interactions are stronger than the ion−metal interactions. Molecular dynamics simulations of multilayers on Al(111) showed that this manifested as formation of tiny droplets of IL. Although droplet simulations were not carried out for the Cu(111) surface, the similarity between the two surfaces indicates that such behavior would probably occur on Cu(111). 1-Octyl-3-methylimidazolium tetrafluoroborate, [C8C1Im][BF4], has a large cation due to its long octyl chain, paired with a small anion (see Figure 1B). On cooling, it forms glasses

Figure 1. (A) 1-Ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C2C1Im][Tf2N], (B) 1-octyl-3-methylimidazolium tetrafluoroborate [C8C1Im][BF4], and (C) schematic of the IL evaporator. The IL is held in a small glass vial and heated from below with a tungsten light bulb.

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EXPERIMENTAL SECTION

Experiments were carried out in a UHV chamber with a base pressure of 5 × 10−10 mbar, previously described in ref 4. XPS measurements were carried out using a PSP TX400 Al Kα X-ray source (1486.6 eV photon energy) at 60° to the surface normal while UPS measurements were carried out using a He lamp (21.2 eV photon energy) also at 60° to the surface normal. Electron energy analysis used a 100 mm mean radius VSW hemispherical electron energy analyzer at normal emission. For UPS measurements, an 18 V negative bias was applied to the sample to accelerate electrons into the analyzer, allowing the secondary electron cutoff to be observed. The average work function for a given surface, φ, was determined using the width of the UP spectrum, A (from the secondary electron cutoff to the copper Fermi level), where φ = 21.2 − A. The Cu(111) crystal (orientated to ±1°) was cleaned by Ar+ ion etching (550 eV) followed by annealing to ∼780 K until XPS and UPS spectra showed only clean surface features with no detectable contamination. The clean surface exhibited a clear, hexagonal, (1 × 1) low energy electron diffraction (LEED) pattern as

(rather than crystallizing) and exhibits strong van der Waals interactions as well as the Coulombic interactions between the ions. By comparison [C2C1Im][Tf2N] (see Figure 1A), which has been used in some of our previous studies, has ions of similar size, which can pack more easily and hence allow crystallization. At the IL/vacuum interface [C8C1Im][BF4] self-assembles so that the octyl chains form an oleaginous layer covering a charged underlayer of the anionic ([BF4]−) and cationic (imidazolium ring) parts of the IL.2 For adsorption of this IL on HOP graphite, the octyl chains are aligned epitaxially along the graphitic lattice24 Some ILs have been found to absorb gases, such as CO2 and SO2, and it has been suggested that B


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expected. A type K thermocouple screwed onto the sample plate next to the crystal provided temperature measurement to ±2 K. The crystal was cooled to 120 K using liquid nitrogen to allow low temperature deposition of the IL. Once cooled, the crystal could be either manually set to a new temperature or heated at a linear rate using a temperature control unit built in-house. The IL [C8C1Im][BF4] was deposited on the Cu(111) crystal by vapor deposition from an evaporator as shown in Figure 1C; see ref 4 for more details. The IL was contained in a glass vial and heated from below via a small halogen bulb (12 V, 20 W). A type K thermocouple in contact with the glass vial was used to measure the temperature of the IL. The IL was degassed prior to use by heating for several days at 150 °C and heating several times to the evaporation temperature ∼250 °C. Once at the evaporation temperature the gate valve to the main chamber and the baffle (see Figure 1C), were opened allowing the IL molecular beam to impinge on the Cu(111) surface. The IL was evaporated onto the sample at an angle of 60° to the surface normal i.e. with the crystal aligned for normal emission to the hemispherical analyzer, allowing UPS or XPS measurements to be taken during deposition.

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dosing rate the F 1s signal grows faster on the 120 K surface than the 303 K surface. After 446 s, the F 1s signal is clearly visible on the 120 K surface, whereas even after 746 s the F 1s signal on the 303 K surface is still relatively small. At both temperatures the same flux of IL impinged on the surface, and at these substrate temperatures the sticking probability of the IL would be expected to be 1, so the same amount of IL was adsorbed on both. See the SI for a rationale for a sticking probability of 1. Figure 3 shows the areas of the F

RESULTS AND DISCUSSION

Adsorption. Before studying the details of the adsorption process, it is important to establish that [C8C1Im][BF4] is deposited intact on the Cu(111) surface (i.e., that the IL does not decompose when heated to ∼250 °C in the evaporator). XPS measurements of multilayers of [C8C1Im][BF4] on Cu(111) show spectra that we would expect for the intact IL and these agree well with XPS measurements carried out on bulk [C8C1Im][BF4];29 see the Supporting Information (SI) for details. As the F 1s signal is the most intense photoelectron peak from the IL, it was used to monitor the adsorption of [C8C1Im][BF4] onto the Cu(111) surface. Figure 2 shows the F 1s signal, taken at ≈30 s intervals as the IL was deposited onto surfaces at 120 K and room temperature (303 K). The spectra have been offset vertically for clarity. Despite the same

Figure 3. F 1s peak areas measured for adsorption at 120 K and at 303 K (shown in Figure 2) as a function of dosing time. The green arrow indicates how the area of the F 1s peak measured at 120 K decreases on heating the surface to 303 K (green data point).

1s spectra measured for adsorption at 120 and 303 K as a function of dosing time (areas were measured following background subtraction). This shows quantitatively how the F signal increases faster at 120 K than 303 K. Following deposition of the IL at 120 K, the surface was then heated to 303 K and the area of the F signal measured again (green data point, Figure 3). The F signal decreased in intensity to match (within error) the corresponding intensity measured when the IL was deposited at 303 K. TPD studies30 show that the IL is not being removed from the surface at these temperatures so the difference in intensity must be due to a structural difference between 120 and 303 K adsorption. For adsorption at 120 K, the IL would be expected to adopt a simultaneous-multilayer growth mode where the neutral ion pairs arrive at the surface and hit and stick where they land, producing a frozen “IL-snow” structure. The term “snow” is used to connote a structure where voids are produced between the randomly adsorbed ion pairs. In this simultaneous multilayer growth mechanism, the average thickness of the adlayer, over a lateral distance of a few molecular diameters, is proportional to the exposure to ionic liquid vapor. Hence, the F 1s signal is the maximum possible for a given exposure. For 303 K adsorption, we suggest that droplets are forming on the surface which reduces the measured F 1s intensity. On heating the 120 K frozen IL-snow structure to 303 K, it transforms to the droplet covered surface with a consequent drop in signal. It should be noted that once the IL-snow has melted to a liquid, subsequent cooling, for this IL, simply causes the liquid to freeze into a glass. For 120 K adsorption we identify the initial linear part of the curve up to ≈150 s, Figure 3, as due to formation of

Figure 2. F 1s signal as [C8C1Im][BF4] is deposited on Cu(111) at (A) 120 K and (B) 303 K. C


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Langmuir approximately the first monolayer. As the IL ion pairs stick where they land, the monolayer here will have some clean surface visible, and there will be some second layer ion pairs on the first layer. This is followed by a decrease in gradient as subsequent multilayers begin to become populated and hence attenuate the signal from beneath them. For 303 K adsorption, the gradient is about 1/4 the size of that for 120 K adsorption over the first 150 s, implying that the droplets of IL are growing for submonolayer coverages and hence there must be clean surface between the droplets. The ratio of the fluorine signal intensity at 120 K, SF,120, to the fluorine signal intensity at 303 K, SF,303, can be calculated using a simple model consisting of a uniform, flat thickness df,120 of IL at 120 K, and uniform cuboid droplets of thickness dd,303 with clean surface between them at 303 K (see Figure 7). Using an inelastic mean free path of 20 Å (see the SI for calculation of mean free path) for the F 1s photoelectron passing through the IL, SF,303/SF,120 = 0.251 for df,120 = 10 Å and dd,303 = 101 Å. This matches, within error, the behavior of the 120 K adsorption curve in Figure 3 which is ≈4× the 303 K curve. So during adsorption at 303 K, the IL clusters as droplets such that for an average thickness of ≈10 Å, the droplets are ≈100 Å high and cover only 1/10th of the clean surface area. Further details of the calculation can be found in the SI. UPS measurements were used to acquire valence band spectra. In the clean UP spectrum (red curve in Figure 4) the

Figure 5. (A) Total intensity at the position of the surface resonance in the Cu valence band (at 2.8 eV from Figure 4) as a function of dosing time for IL deposition on a 120 and 308 K surface. The 120 K data has been scaled up (see text for explanation). The green arrow indicates how the intensity of the surface resonance measured at 120 K increases on heating the surface to 303 K (green data point). (B) Sample work function as a function of dosing time for IL deposition on a 120 and 308 K surface. The green arrow indicates how the sample work function measured at 120 K decreases on heating the surface to 308 K (green data point).

been scaled so that the first data point is the same intensity as the first data point in the 308 K data set. The intensity for 120 K adsorption drops linearly to begin with until a monolayer has been deposited on the surface. The gradient of the graph then decreases. This allows us to identify the approximate monolayer point in the spectra at about 180 s, which is similar to the 150 s found by XPS, above. For deposition on the 308 K surface, the gradient is shallower, in agreement with the model that droplets are forming. However, in this case, a rather clear break in gradient can be seen at 250 s, which implies that although some droplet formation occurs, there is also completion of a monolayer. This contradicts the XPS data above where clean surface between the droplets was postulated. We show below that droplet formation switches on rather abruptly at 300 K. So the difference in adsorption behavior (droplets with clean surface between them or droplets with a monolayer between them) could be due to differences in the exact adsorption temperature and differences in the time taken for adsorption, as a rather large amount of mass transport has to occur when forming droplets. Following 420 s of IL deposition on the 120 K surface, the surface was heated to 308 K. This caused the intensity of the Cu d band in the region of the surface

Figure 4. Cu(111) valence band region as IL was deposited at (A) 120 K and (B) 308 K.

surface state can be seen just below the Fermi level with two intense surface resonances on top of the Cu d band at 2−4 eV binding energy. This is in agreement with previous valence band spectra of clean Cu(111) measured using a He(I) source.31 The feature at about 2.8 eV is the most intense surface resonance. As material is deposited onto the clean surface at 120 and 308 K, Figure 4, the intensities of the Cu surface state and the two surface resonances reduce. Note that the intensity of the spectra for the 120 K surface are lower than the 308 K surface due to a difference in the incident UV intensity for the two experiments. Figure 5A shows the intensity of the surface resonance at 2.8 eV as a function of dosing time on the 120 and 308 K surfaces (intensity at the top of the resonance). The 120 K data set has D


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Langmuir resonance to increase as shown by the green data point in Figure 5A. This is consistent with the low temperature IL-snow multilayer structure transforming into the room temperature droplet structure. As the IL was deposited onto the Cu surface, the work function was determined using the width of the UP spectra. As the work function depends on both the chemical composition of the surface and its morphology, it is useful to detect changes in the surface due to either of these, which can then be correlated to other measurements or data. Figure 5B shows the sample work function as a function of dosing time on the 120 and 308 K surfaces. On the 120 K surface, the work function is initially 4.65 eV; this decreases as the IL is deposited until it reaches a plateau at approximately 4.53 eV at 150 s. This is in reasonable agreement with the change of gradient for the surface resonance in Figure 5A, and for the F 1s peak in Figure 3. It corresponds to completion of approximately a monolayer of IL-snow at 120 K, where the monolayer is molecularly rough with ion pairs mostly in the first layer, but with some in the second and third layers as well. Further adsorption at 120 K leads to no further change of work function. This is consistent with the atomically rough IL-snow surface propagating outward, with the roughness remaining the same even though the thickness increases. The work function from similarly rough surfaces would be expected to be unchanging. On the 308 K surface, the work function is initially 4.59 eV; this decreases and forms a first plateau at 4.32 eV at 250 s. This corresponds to the formation of the first monolayer in agreement with Figure 5A. However, the work function value is 0.2 eV lower than for the same coverage for 120 K adsorption, which indicates that the monolayer at 308 K is indeed significantly different to the monolayer at 120 K. At approximately 380 s, the work function starts decreasing again until it levels off at 4.2 eV at 480 s. This indicates that the surface now resembles a bulklike IL surface with a constant work function of 4.2 eV. When the IL dosed 120 K surface was heated to 308 K, the work function of the sample decreased significantly from 4.53 to 4.34 eV as shown by the green data point, again in agreement with a structural change from ILsnow multilayers to droplets. Figure 6 shows further detail of the deposition of the IL at 120 K and subsequent heating to 308 K. It shows both the valence band region, Figure 6A, and a full spectrum, Figure 6B, including the secondary electron cutoff. The clean Cu valence band flattens out following deposition of the IL at 120 K, and in the wide scan a broad feature at ∼9 eV appears. This feature is from the IL density of states (DOS) of the ion pairs on the surface. The surface was then heated to room temperature. Upon heating, the intensity of the bulk Cu d band (2−4 eV BE) partially returned, and the IL feature at 9 eV in the wide scan decreased and changed shape. The intensity changes further support the idea that the IL coalesces into droplets on heating, causing the Cu d band signal to increase and the IL peak to decrease. The change in shape of the broad feature at ∼9 eV we attribute to the IL changing from frozen ion pairs at 120 K, to ions mixed together as a liquid at 308 K. The 308 K spectrum is in reasonable agreement with UPS spectra of the related IL, [C4C1Im][BF4], measured by Nishi et al.32 The model determined from XPS and UPS data for adsorption of [C8C1Im][BF4] on Cu(111) is shown in Figure 7. For adsorption at room temperature, the IL coalesces on the surface forming droplets with clean surface, or a monolayer, between them. Deposition of [C8C1Im][BF4] at 120 K forms

Figure 6. (A) Valence band region and (B) wide spectrum, for clean Cu(111), IL deposited on Cu(111) at 120 K, and after heating the IL/ Cu surface to 308 K. Arrows indicate particular changes in intensity after heating.

Figure 7. Bottom: Adsorption of IL on Cu(111) at 120 K. Top: Adsorption of IL on Cu(111) at room temperature. The vertical pink arrow shows the conversion to droplets when heated from 120 K to room temperature. df is the thickness of the flat, IL-snow surface; dd is the thickness of the IL droplets; Af is the total area of the sample; Ad is the total area of IL droplets.

an IL-snow multilayer structure by simultaneous-multilayer growth. The IL-snow multilayer consists of ion pairs and has a UP spectrum that is clearly distinguishable from the UP spectrum of the IL liquid at room temperature. The IL-snow multilayer structure converts into the room temperature droplet structure upon heating. Desorption. To study the desorption of [C8C1Im][BF4] from the Cu(111) surface, multilayers of IL-snow were deposited at 120 K. IL was deposited until the Cu 2p3/2 signal was attenuated to 5λ, where λ is the mean free length of the electron. We have calculated a mean free path length of 20 × 10−10 m for an E


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electron traveling at ∼550 eV (the kinetic energy of the Cu 2p3/2 peak) through [C8C1Im][BF4] (see the SI for details). Hence, the IL multilayer was approximately 100 Å (5λ) thick. The surface was then heated linearly from 120 K to ∼510 K while monitoring the F 1s and Cu 2p3/2 signals as shown in Figure 8A and B. The areas of the F 1s and Cu 2p3/2 signals are plotted as a function of temperature in Figure 8C.

Figure 8C, so this transition temperature does not bring about a bulk change to the IL structure. The sharp decrease in F intensity at ∼420 K indicates that this is where the IL droplets, which are multilayers, are desorbed. The sharp increase in Cu intensity at ∼420 K is entirely consistent with this idea. The F intensity does not completely disappear after the multilayers have been removed, suggesting a monolayer of IL is left on the surface at ≈430 K. Further heating to 460 K caused the F 1s intensity to remain fixed, while the Cu intensity increased somewhat. This could be due to some kind of rearrangement of adsorption geometry within the IL monolayer. Another drop in F 1s intensity starts at 480 K, with a concomitant increase in the Cu signal, which suggests that the IL is now decomposing with at least some of the products desorbing from the surface. It can be seen from Figure 8A that there is also a change in binding energy of the F 1s peak as the surface is heated. Figure 8D shows the binding energy of the F 1s peak as the surface is heated from 120 to 510 K. At 120 K, the binding energy is approximately 687.0 eV. As the temperature is increased, the binding energy of the peak begins to decrease. Between 160 and 190 K, the BE appears to experience a plateau at 686.7 eV. A slight increase in the Cu 2p3/2 at 160 K was noted above. We speculate that over this temperature range the IL ion pairs have dissociated into separate ions, but are still locked in a glassy solid structure. Dissociation will move the anion and cation further apart than they were in the ion pair, so the electric field of the cation at the position of the anion will be somewhat reduced, making the anion more negative. This would reduce the BE of the photoelectrons leaving the fluorines in the anion, as observed. At 200 K, the BE begins to decrease again. This is approximately the same as the bulk glass transition temperature (192 K) of the IL.28 So the continuing change in BE is probably a reflection of the glassy IL layer transforming to a liquid, but the liquid is uniform across the surface of the sample. The continuous change in BE would be a consequence of rearrangement of the ions within the multilayer as the temperature rises. Using the same notion that the further apart the anion and cation are, the lower the binding energy of the fluorine photoelectrons, this would be compatible with the ions within the viscous liquid moving to average positions more distant from the positive ions. The BE reaches a flat minimum of 686.0 eV between 300 and 350 K which is consistent with the multilayer transforming to droplets at 300 K. For a mobile liquid, one would not expect the BE to change as there is no further movement of the average positions of the ions relative to each other. The binding energy then begins to increase again at about 400 K which is where the multilayer desorbs to leave a monolayer. This final increase in binding energy is due the observed signal from the IL changing from IL multilayer to IL in contact with the copper surface. There may be a change in ionic position, and there will definitely be a change in the initial state of the IL orbitals due to bonding with the copper, and probably a change in the final state of the outgoing photoelectron wave. As the BE is dependent on all three effects, we cannot ascribe the increase to just one of them. Note that it is difficult to extract the binding energies of the final few spectra around 500 K since there is very little F left on the surface here. By comparing the area of the clean Cu 2p3/2 signal to the area of the Cu 2p3/2 at the monolayer temperature (430 K), we can estimate the thickness of a monolayer. By using λ = 20 Å, the monolayer was found to be ≈6 Å thick. Given that [BF4]−

Figure 8. (A) F 1s and (B) Cu 2p3/2 signals as the IL-snow/Cu(111) surface was heated from 120 K to ∼510 K. (C) Areas under the F 1s and Cu 2p3/2 peaks and (D) binding energy of the F 1s signal during the heating.

The Cu 2p3/2 intensity increases slightly from 120 to 300 K, with a small increase above zero at 160 K. At 300 K, it undergoes an abrupt increase. From 340 to 410 K, the Cu signal remains constant. We interpret this as the IL-snow multilayer of ionic liquid reordering at 300 K to form droplets on a monolayer (or at least a thinner) layer of IL which covers the surface. The behavior of the F 1s peak over this temperature range is a small continuous decrease in intensity, which is consistent with droplet formation at 300 K if the IL is still thick enough to from a strong F peak. This reordering of the IL-snow multilayer occurs at the same temperature, 300 K, at which the adsorption experiments were carried out, which is why the exact behavior of the intensities as measured above at room temperature appear to be very temperature sensitive. As the IL is heated, it passes through the glass transition temperature of 192 K for bulk [C8C1Im][BF4].28 However, no observable change can be detected in either the Cu or the F intensities in F


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Langmuir and the octyl chain are ≈4.5 and ≈4.2 Å in diameter, and that the thickness of a flat lying imidazole ring is ≈4 Å, this thickness is compatible with the IL monolayer consisting of coplanar adsorption of equal numbers of anions and cations with the imidazolium ring of the cation parallel to the surface, and the octyl chain parallel to the surface along its length. We can quantify these changes in photoelectron intensity using a simple model and a mean free path of 20 Å for the Cu 2p3/2 and F 1s photoelectrons through the IL. There are three surfaces as shown in Figure 9: (i) the flat IL-snow multilayer

across the Cu surface. See the SI for further details of the model. Note that the F 1s photoelectron has a kinetic energy of 800 eV, somewhat higher than the Cu 2p3/2 photoelectron, but the inelastic mean free path will only be slightly longer than that for copper, so the same value has been used for both Cu and F calculations. For temperatures above ≈470 K, we suggest that the IL is reacting with the Cu substrate. When the surface is heated to this temperature, the B 1s and F 1s signals decrease more substantially than the C 1s and N 1s signals (see the SI for further details). This suggests that boron and fluorine atoms are removed from the surface and that carbon and nitrogen atoms remain on the surface. A possible reaction is


Cu + [BF4 ]− → Cu − F + BF3 ↑ + e−↓

where [BF4]− ions on the surface react with the copper to form chemisorbed fluorine and BF3, which is desorbed, with the negative charge on the ion being absorbed by the copper surface. This would formally leave the [C8C1Im]+ still on the surface, but the XPS data indicates that decomposition is occurring leaving a residue of unidentified carbon and nitrogen species on the surface. To further characterize the sequence of surface phases formed by desorption of [C8C1Im][BF4] from the Cu(111) surface, UPS measurements were carried out. Approximately five layers of IL were deposited on the Cu(111) surface at 120 K. The surface was then heated in stages from 123 to 523 K. The desorption was monitored by taking a wide UP spectrum at each temperature, shown in Figure 10. These UP spectra are also plotted as a function of temperature as a contour plot shown in Figure 10B, and the work function from the secondary electron cutoff in shown in Figure 11. At 123 K, the Cu valence band cannot be seen, but there is a large broad feature at 5−12 eV binding energy showing the rather well formed 9 eV peak, with a shoulder at 6 eV, due to the ion pairs on the surface. The work function of this surface is 4.21 eV (having dropped from the clean surface value of 4.78 eV when the surface was formed). On heating, the peak at 9 eV becomes noticeably broader by 163 K, which is consistent with the ion pairs dissociating into separate ions, but remaining locked within a glassy solid multilayer. The work function remains unchanged at about 4.2 eV indicating little change in the surface. With continued heating (163−283 K), the peaks change slightly in shape with a general broadening and development of a small peak at 12 eV. However, no particular change is observed at 200 K where the F 1s peak indicated a possible melting transition and the work function does not change either. So although the IL multilayer has melted, there is insufficient change in the surface to affect either the work function or the F 1s peak intensities. At 303−323 K, there is only a very subtle change in the shape of the UP spectra, but a clear change can be seen in the contour plot as the Cu d band begins to emerge (2−4 eV), and also in the intensity of the secondary electron peak at ≈16 eV (blue contours) which

Figure 9. Schematic of three model surfaces: (i) the flat multilayer covered Cu surface at low temperature with an IL thickness df producing signals SCu,f and SF,f, (ii) the droplet covered surface producing signals SCu,d and SF,d, and (iii) the monolayer covered surface with an IL thickness dm producing copper and fluorine signals of intensity SCu,m and SF,m.

covered Cu surface at low temperature with an IL thickness df producing signals SCu,f and SF,f for the copper and fluorine XPS signals, (ii) the droplet covered surface producing signals SCu,d and SF,d, and (iii) the monolayer covered surface with an IL thickness dm producing signals of intensity SCu,m and SF,m. For simplicity, we will assume the droplet surface consists of cuboids of uniform thickness, dd, occupying a fractional area Ad with a monolayer of IL between the drops. For dm = 6 Å, df = 100 Å, and dd = 131 Å, the calculated ratios for the copper 2p3/2 signals, (SCu,m/SCu,d), (SCu,d/SCu,f), and (SCu,m/SCu,f), for the three surfaces are shown in Table 1. These ratios match, within error, those obtained experimentally, Table 1, from Figure 8. A similar calculation for the F 1s photoelectron signal from the IL, using the same mean free path, gives calculated ratios, (SF,m/ SF,d), (SF,d/SF,f), and (SF,m/SF,f), which match the experimental values, Table 1, within error. This supports the model that flat multilayers ≈100 Å thick bunch together at 300 K to form droplets ≈131 Å thick covering ≈3/4 of the surface with a monolayer of IL between the droplets. This is followed by desorption of the droplets at 420 K to leave a monolayer of IL

Table 1. XPS Intensity Ratios for the Monolayer, Droplet, and Flat IL-Snow Covered Surfacesa calcd exptl

(SCu,m/SCu,d)

(SCu,d/SCu,f)

(SCu,m/SCu,f)

(SF,m/SF,d)

(SF,d/SF,f)

(SF,m/SF,f)

4 4 ± 0.7

27 39 ± 22b

107 155 ± 82b

0.32 0.45 ± 0.2

0.82 0.62 ± 0.13

0.26 0.28 ± 0.12

a SCu, Cu 2p3/2 intensity; SF, fluorine 1s intensity. Subscripts: m, monolayer covered surface; d, droplet covered surface; f, flat IL-snow covered surface. bLarge error due small value of SCu,f.

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UP spectrum itself. This suggests that the size and distribution of the IL droplets at the surface are changing in some way, and hence affecting the work function and secondary electron background, but the IL itself is now a liquid and so no particular change occurs in the UP spectra. At 413 K, the contour plot clearly shows desorption of the IL droplets, with a loss of IL peak intensity in the broader 5−15 eV region and the appearance of a clear Cu valence band at 2−4 eV binding energy. The work function here flattens at 3.8 eV and there is a short-range, 413−433 K, where the work function and the UP spectra hardly change, which we identify as approximately monolayer coverage of IL on the surface. In this region, the IL peak is very broad with little structure, reflecting the strong bonding and hence altered band structure of the IL monolayer next to the copper surface. Further heating, 433−463 K, then causes the re-emergence of the Cu d band surface resonance and a flattening of the IL UP peak, both consistent with desorption of part of the ionic liquid to produce some clean surface patches. The work function falls rapidly to a minimum of 3.28 eV at 463 K. This is consistent with the decomposition of the [BF4]− ion to produce BF3, which is desorbed and hence leaves part of the surface clean. We propose that, at 463 K, the ionic liquid cation is still intact on the surface. As it is positively charged, it will have the effect of reducing the work-function to the very low value of 3.28 eV. Further heating, 463−523 K, causes the surface resonance to get larger, the remaining IL peak to flatten to invisibility, and the work function to increase to 3.8 eV at 523 K. Here we propose that the cation is desorbing, and cracking on the surface to leave a small amounts of C, N, and F as observed by XPS. The intensity of the surface resonance at 523 K when compared with the value observed during adsorption (Figure 4) suggests that the surface is probably 80% covered with decomposition product. The low work function of 3.8 eV compared to a clean surface value of 4.78 eV also argues for a distinctly dirty surface.

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Figure 10. (A) Wide UP spectra as the IL/Cu(111) surface was heated in stages from 123 to 523 K. (B) UPS data plotted as a contour plot of intensity versus binding energy and temperature.

CONCLUSIONS [C8C1Im][BF4] can be deposited without decomposition on Cu(111) by vapor deposition. At 120 K, the IL arrives as ion pairs and sticks where it lands forming IL-snow multilayers in a simultaneous multilayer growth mechanism. The IL vacuum interface at this temperature is rough on the atomic scale, but of uniform thickness on larger scales, and the average thickness is proportional to the exposure. UPS of the 120 K IL/Cu(111) surface is different from that of the room temperature IL/ Cu(111) surface, due to the presence of ion pairs. Room temperature (300 K) adsorption leads to the formation of relatively large droplets of IL (≈100 Å thick) covering about 25% of the surface, possibly with clean surface between the droplets. For higher exposures these droplets change in size and distribution in a way that has yet to be determined. IL-snow multilayers of IL deposited at 120 K converted to the room temperature structure upon heating. Flat, uniform multilayers of about 100 Å average thickness at 120 K are thought to consist of ion pairs. On heating to 160 K the ion pairs dissociate to form a glassy solid which is still of uniform thickness. Further heating to 200 K causes the glassy solid to melt but remain of uniform thickness. At 300 K, the uniform layer of ionic liquid coalesce into droplets of about 131 Å thickness covering 75% of the surface, with a monolayer of IL between the droplets. Further heating caused the IL droplets to desorb from the surface, leaving a monolayer of ≈6 Å thickness at ≈430 K. At slightly higher temperatures, the anions

Figure 11. Work function as a surface formed by deposition of ≈5 layers of IL at 120 K is heated from 123 to 523 K.

begins to shift to higher binding energy. The work function also starts to decrease in this region. All three are consistent with the formation of droplets from the previously flat multilayer. Heating from 323 to 403 K causes the work function to continue decreasing, and the secondary electron peak moving to higher binding energies, but with no particular change in the H



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ACKNOWLEDGMENTS Funding from the Engineering and Physical Science Research Council, U.K. (EP/I018093/1) is gratefully acknowledged. We thank P. Licence for providing the ionic liquid.

decompose to produce a surface with a very low work function of 3.28 eV, probably due to remaining adsorbed cations. These then react with the Cu surface to leave submonolayer C, N, and F on the surface. In summary: Adsorption 120 K: Simultaneous multilayer adsorption where ion pairs from the gas phase stick where they land to form IL-snow multilayers. 300 K: Droplet formation occurs immediately for submonolayer coverage with, initially, clean surface between the droplets. Multilayer adsorption leads to thicker droplets with monolayer between the droplets. Lateral size of droplets not known. Desorption 120 K: Multilayers of IL-snow of uniform thickness consisting of ion pairs that stuck where they landed. 120−160 K: Ion pairs dissociating within the IL-snow multilayer to form a uniform thickness glassy solid. 160 K: Uniform thickness glassy solid layer consisting of separate ions. 160−200 K: Annealing of glassy solid layer to form lower energy distribution of ions. 200 K: Glassy solid layer melts to form uniform thickness liquid layer. 200−300 K: Annealing liquid layer, ions becoming sufficiently mobile to achieve bulk liquidlike structure. 300 K: Uniform liquid layer converts to liquid droplets (75% of surface, ≈131 Å thick) with a monolayer of IL between them. 300−400 K: Liquid droplets evolve in size and distribution in a way yet to be determined. 400 K: Liquid droplets desorb leaving a monolayer of IL ≈ 6 Å thick. 430−460 K: Reaction of adsorbed cation with Cu surface occurs, [BF4]− → BF3↑ + Fchem. 460 K: Adsorbed [C8C1Im]+ anion and Fchem left on surface, minimum work function of 3.28 eV observed. 460−520 K: Adsorbed [C8C1Im]+ cation desorbs and/or cracks to leave C, N, and F on the surface.

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NONSTANDARDIZED ABBREVIATIONS [TFSA]≡[Tf2N] bis(trifluoromethylsulfonyl)imide [mmim]≡[C1C1Im] 1,3-dimethylimidazolium [BMP][TFSA] 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C1C1Im][Tf2N] 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C2C1Im][Tf2N] 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C6C1Im][Tf2N] 1-methyl-3-hexylimidazolium bis(trifluoromethylsulfonyl)imide [C8C1Im][BF4] 1-octyl-3-methylimidazolium tetrafluoroborate [C8C1Im][Tf2N] 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [mmim][BF4] 1,3-dimethylimidazolium tetrafluoroborate [Py1,4][FAP] 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate

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REFERENCES

(1) Armstrong, J. P.; Hurst, C.; Jones, R. G.; Licence, P.; Lovelock, K. R. J.; Satterley, C. J.; Villar-Garcia, I. J. Vapourisation of ionic liquids. Phys. Chem. Chem. Phys. 2007, 9 (8), 982−990. (2) Iwahashi, T.; Nishi, T.; Yamane, H.; Miyamae, T.; Kanai, K.; Seki, K.; Kim, D.; Ouchi, Y. Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy. J. Phys. Chem. C 2009, 113 (44), 19237−19243. (3) Carmichael, A. J.; Hardacre, C.; Holbrey, J. D.; Nieuwenhuyzen, M.; Seddon, K. R. Molecular layering and local order in thin films of 1alkyl-3-methylimidazolium ionic liquids using X-ray reflectivity. Mol. Phys. 2001, 99 (10), 795−800. (4) Foulston, R.; Gangopadhyay, S.; Chiutu, C.; Moriarty, P.; Jones, R. G. Mono- and multi-layer adsorption of an ionic liquid on Au(110). Phys. Chem. Chem. Phys. 2012, 14 (17), 6054−6066. (5) Mezger, M.; Schramm, S.; Schroder, H.; Reichert, H.; Deutsch, M.; De Souza, E. J.; Okasinski, J. S.; Ocko, B. M.; Honkimaki, V.; Dosch, H. Layering of [BMIM](+)-based ionic liquids at a charged sapphire interface. J. Chem. Phys. 2009, 131 (9), 9. (6) Galinski, M.; Lewandowski, A.; Stepniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51 (26), 5567−5580. (7) Sato, T.; Masuda, G.; Takagi, K. Electrochemical properties of novel ionic liquids for electric double layer capacitor applications. Electrochim. Acta 2004, 49 (21), 3603−3611. (8) Liu, W. M.; Ye, C. F.; Gong, Q. Y.; Wang, H. Z.; Wang, P. Tribological performance of room-temperature ionic liquids as lubricant. Tribol. Lett. 2002, 13 (2), 81−85. (9) Zhou, F.; Liang, Y. M.; Liu, W. M. Ionic liquid lubricants: designed chemistry for engineering applications. Chem. Soc. Rev. 2009, 38 (9), 2590−2599. (10) Lee, B. R.; Choi, H.; SunPark, J.; Lee, H. J.; Kim, S. O.; Kim, J. Y.; Song, M. H. Surface modification of metal oxide using ionic liquid molecules in hybrid organic-inorganic optoelectronic devices. J. Mater. Chem. 2011, 21 (7), 2051−2053. (11) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D. B.; Zakeeruddin, S. M.; Gratzel, M. Correlation between photovoltaic performance and impedance spectroscopy of dyesensitized solar cells based on ionic liquids. J. Phys. Chem. C 2007, 111 (17), 6550−6560. (12) Mehnert, C. P. Supported ionic liquid phases. Chem. - Eur. J. 2005, 11 (1), 50−56.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02932. Intact deposition of [C8C1Im][BF4] on Cu(111); calculation of mean free path; reaction of [C8C1Im][BF4] with Cu(111)-loss of boron from the surface; XPS intensities for adsorption at low temperatures (flat adsorption) versus droplet formation; constant sticking probability of the ionic liquid (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

K.L.S.: Jeremiah Horrocks Institute, University of Central Lancashire, Preston, Lancashire, PR1 2HE, U.K. Notes

The authors declare no competing financial interest. I


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Langmuir (13) Steinruck, H. P. Surface science goes liquid ! Surf. Sci. 2010, 604 (5−6), 481−484. (14) Cremer, T.; Wibmer, L.; Calderon, S. K.; Deyko, A.; Maier, F.; Steinruck, H. P. Interfaces of ionic liquids and transition metal surfaces-adsorption, growth, and thermal reactions of ultrathin [C(1)C(1)Im][Tf2N] films on metallic and oxidised Ni(111) surfaces. Phys. Chem. Chem. Phys. 2012, 14 (15), 5153−5163. (15) Foulston, R.; Gangopadhyay, S.; Chiutu, C.; Moriarty, P.; Jones, R. G. Mono- and multi-layer adsorption of an ionic liquid on Au(110). Phys. Chem. Chem. Phys. 2012, 14 (17), 6054−6066. (16) McIntosh, E. M.; Ellis, J.; Jardine, P.; Licence, P.; Jones, R. G.; Allison, W. Probing liquid behaviour by helium atom scattering: surface structure and phase transitions of an ionic liquid on Au(111). Chem. Sci. 2014, 5, 667−676. (17) Cremer, T.; Stark, M.; Deyko, A.; Steinruck, H. P.; Maier, F. Liquid/Solid Interface of Ultrathin Ionic Liquid Films: [C(1)C(1)lm][Tf2N] and [C(8)C(1)Im][Tf2N) on Au(111). Langmuir 2011, 27 (7), 3662−3671. (18) Uhl, B.; Cremer, T.; Roos, M.; Maier, F.; Steinruck, H. P.; Behm, J. At the ionic liquid|metal interface: structure formation and temperature dependent behavior of an ionic liquid adlayer on Au(111). Phys. Chem. Chem. Phys. 2013, 15 (40), 17295−17302. (19) Buchner, F.; Forster-Tonigold, K.; Uhl, B.; Alwast, D.; Wagner, N.; Farkhondeh, H.; Gross, A.; Behm, R. J. Toward the Microscopic Identification of Anions and Cations at the Ionic Liquid I Ag(111) Interface: A Combined Experimental and Theoretical Investigation. ACS Nano 2013, 7 (9), 7773−7784. (20) Uhl, B.; Buchner, F.; Alwast, D.; Wagner, N.; Behm, R. J. Adsorption of the ionic liquid BMP TFSA on Au(111) and Ag(111): substrate effects on the structure formation investigated by STM. Beilstein J. Nanotechnol. 2013, 4, 903−918. (21) Waldmann, T.; Huang, H.-H.; Hoster, H. E.; Höfft, O.; Endres, F.; Jürgen Behm, R. Imaging an ionic liquid adlayer by scanning tunneling microscopy at the solid/vacuum interface. ChemPhysChem 2011, 12, 2565−2567. (22) Uhl, B.; Buchner, F.; Gabler, S.; Bozorgchenani, M.; Behm, R. J. Adsorption and reaction of sub-monolayer films of an ionic liquid on Cu(111). Chem. Commun. 2014, 50 (62), 8601−8604. (23) Klaver, T. P. C.; Luppi, M.; Sluiter, M. H. F.; Kroon, M. C.; Thijsse, B. J. DFT Study of 1,3-Dimethylimidazolium Tetrafluoroborate on Al and Cu(111) Surfaces. J. Phys. Chem. C 2011, 115 (30), 14718−14730. (24) Carstens, T.; Gustus, R.; Hofft, O.; Borisenko, N.; Endres, F.; Li, H.; Wood, R. J.; Page, A. J.; Atkin, R. Combined STM, AFM, and DFT Study of the Highly Ordered Pyrolytic Graphite/1-Octyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Interface. J. Phys. Chem. C 2014, 118 (20), 10833−10843. (25) Mo, Y. F.; Pu, J. B.; Huang, F. C. Dynamic forces of ionic liquid nano-droplets measured by atomic force microscope. Colloids Surf., A 2013, 429, 19−23. (26) Phillips, B. S.; John, G.; Zabinski, J. S. Surface chemistry of fluorine containing ionic liquids on steel substrates at elevated temperature using Mossbauer spectroscopy. Tribol. Lett. 2007, 26 (2), 85−91. (27) Espinosa, T.; Sanes, J.; Jimenez, A. E.; Bermudez, M. D. Surface interactions, corrosion processes and lubricating performance of protic and aprotic ionic liquids with OFHC copper. Appl. Surf. Sci. 2013, 273, 578−597. (28) Crosthwaite, J. M.; Muldoon, M. J.; Dixon, J. K.; Anderson, J. L.; Brennecke, J. F. Phase transition and decomposition temperatures, heat capacities and viscosities of pyridinium ionic liquids. J. Chem. Thermodyn. 2005, 37 (6), 559−568. (29) Villar-Garcia, I. J.; Smith, E. F.; Taylor, A. W.; Qiu, F. L.; Lovelock, K. R. J.; Jones, R. G.; Licence, P. Charging of ionic liquid surfaces under X-ray irradiation: the measurement of absolute binding energies by XPS. Phys. Chem. Chem. Phys. 2011, 13 (7), 2797−2808. (30) Buckley, M. An investigation into the interactions of imidazolium-based ionic liquids with small gas molecules and their

structuring at the IL/Au interface. PhD Thesis, The University of Nottingham, 2014. (31) Johnston, S. M.; Mulligan, A.; Dhanak, V.; Kadodwala, M. The bonding of acetone on Cu(111). Surf. Sci. 2004, 548 (1−3), 5−12. (32) Nishi, T.; Iwahashi, T.; Yamane, H.; Ouchi, Y.; Kanai, K.; Seki, K. Electronic structures of ionic liquids [C(n)mim](+) BF4- and [C(n)mim](+) PF6- studied by ultraviolet photoemission, inverse photoemission, and near-edge X-ray absorption fine structure spectroscopies. Chem. Phys. Lett. 2008, 455 (4−6), 213−217.

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Adsorption, Desorption, and Reaction of 1-Octyl-3-methylimidazolium

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