Organization of Ionic Liquids with Au

Article pubs.acs.org/Langmuir

Surface Composition/Organization of Ionic Liquids with Au Nanoparticles Revealed by High-Sensitivity Low-Energy Ion Scattering Alan Kauling,† Günter Ebeling,† Jonder Morais,‡ Agílio Pádua,§ Thomas Grehl,∥ Hidde H. Brongersma,⊥,∥ and Jairton Dupont*,† †

Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Post Office Box 15003, Avenida Bento Gonçalves, 9500 Porto Alegre, Rio Grande do Sul, Brazil ‡ Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Post Office Box 15051, Avenida Paulo Gama, 9500 Porto Alegre, Rio Grande do Sul, Brazil § Institut de Chimie de Clermont Ferrand, Université Blaise Pascal and CNRS, 63171 Aubiere, France ∥ ION-TOF GmbH, Heisenbergstraße 15, D-48149 Munster, Germany ⊥ Eindhoven University of Technology, 5612 Eindhoven, Netherlands. S Supporting Information *

ABSTRACT: High-sensitivity low-energy ion scattering (HSLEIS) analysis was used to elucidate the outermost layer of both functionalized and non-functionalized imidazolium ionic liquids (ILs). The IL outermost layer is composed of all atoms of both cations and anions. The HS-LEIS analyses also allow for quantitative measurement of the thickness of IL overlayers on Au nanoparticles prepared by sputter deposition, which was shown to be a monolayer of ions, as predicted by density functional theory calculations.

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INTRODUCTION The possibility of modulating the physicochemical properties of imidazolium-based ionic liquids (ILs) by the appropriate combination of cations and anions allied to their pronounced self-organization1 is among the most important advantages for their use in the preparation of nanomaterials.2 Indeed, stable nanoparticles (NPs) can be easily prepared via sputtering onto ILs, and when the physical parameters are adjusted, it is possible to control the NP size, size distribution, and concentration.3 The IL surface composition, its ion orientation, and the energy of the sputtered atoms also play major roles in the formation of the NPs.4 It is evident that modifying the surface composition of the ILs5 may allow for the design of experiments to produce liquid devices for high-vacuum applications6,7 or the preparation of thin films and NPs by sputter deposition.4 In the case of imidazolium ILs, the surface organization depends mainly upon its composition and ion orientation.4 Indeed, both theoretical and experimental evidence indicate that, in the case of sole non-functionalized imidazolium ILs, both cations and anions are present in the surface region, with the long N-alkyl chains projected into the gas phase.5,8−13 Moreover, perfluorinated anions also tend to populate the vacuum/IL interface within the first few angstroms of the IL surface.8,14 However, experimental details of the surface composition have been obtained from non-functionalized ILs using mainly electron spectroscopic techniques, such © 2013 American Chemical Society

as X-ray photoelectron spectroscopy (XPS), that are not completely surface-specific because most of the signals come from within a few atomic layers of the surface; a small part of the signal comes from much deeper in the sample. Nonetheless, this technique has provided tremendous knowledge not only of the “surface” composition of pure ILs but also of the interaction between ILs and metal complexes and NPs.15,16 Doubtless, much more intimate knowledge of both functionalized and non-functionalized IL surface atom composition and orientation is paramount for the development of tailor-made ILs for “vacuum” applications, such as for the preparation of nanomaterials via chemical vapor deposition methods, in particular sputtering. In this respect, low-energy ion scattering (LEIS) analysis may provide better insights than other available surface technique analytical tools for various reasons.17 In LEIS analysis, the sample is bombarded with noble gas ions at an energy of a few kiloelectronvolts. The energy of the ions that are backscattered by atoms in the outer surface follows from the laws of conservation of energy and momentum. When the energy of these backscattered ions is measured, the masses of the scattering surface atoms can be determined. The measured signal is proportional to the surface coverage of the Received: September 4, 2013 Revised: October 8, 2013 Published: October 23, 2013 14301

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corresponding element and is not influenced by the chemical environment. This thus allows for matrix-independent quantification. Ions that are backscattered by atoms in deeper layers will lose extra energy along the in- and outgoing trajectories. The in-depth distribution of an element can thus be derived from this extra energy loss. At the energies used in LEIS, this non-destructive (static) depth profiling provides information down to a depth of 10 nm. This mode allows for a quantitative measurement of the thickness of organic overlayers, such as Langmuir−Blodgett films or self-assembled monolayers. Thus, LEIS analysis can be used not only to investigate the atomic composition of the surface of ILs but also to measure quantitatively the thickness of IL overlayers on metal NPs prepared by sputter deposition. Recent reviews describe the fundamentals of LEIS and the quantification of the atomic composition of the outer surface, the determination of in-depth profiles, and an overview of many applications of LEIS.17 Indeed, in earlier LEIS experiments on 1-n-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, only the fluorine peak was clearly detectable and no signals attributable to sulfur or oxygen were observed, indicating that the surface outermost atomic layer is strongly enriched in fluorine.9 This result suggests that the IL species prefer to orient themselves with the CF3 groups at the vacuum interface. Thus, it was proposed that the surface of BMI·NTf2 is richer in fluorine than the surfaces of analogous ILs containing fluorinated anions, such as BF4 and PF6, for which previous investigations by direct recoil spectroscopy (DRS) have shown that their composition is the same up to the last atomic layer.18 However, with the development of higher performance instruments, such as the advanced analyzer design of the Qtac100, more accurate measurements can be performed using high-sensitivity low-energy ion scattering (HS-LEIS) analysis.17 We report herein that the use of HS-LEIS allows for the more accurate determination of the surface composition of functionalized and non-functionalized imidazolium ILs and also the IL thickness overlayer on gold NPs prepared by sputtering deposition. Moreover, molecular dynamics simulations have been used to determine the IL composition and orientation.

Table 1. Size of Au NPs Prepared by Sputtering Deposition on ILs for 150 s and 40 mA at 325 V IL

mean size (nm)

minimum size (nm)

maximum size (nm)

standard deviation (nm)

BMI·PF6 BMI·NTf2 (MeOE)MI·NTf2 (BCN)MI·NTf2 (HSE)MI·NTf2

3.7 3.5 4.0 5.0 8.5

1.2 1.2 1.2 2.5 3.7

6.9 6.2 6.8 7.2 22.9

±0.4 ±0.6 ±2.1 ±1.5 ±6.4

These sputtering conditions of relatively low discharge and time were used to avoid the direct diffusion of the sputtered Au species into the IL bulk (see below). The liquid samples were prepared “as is” by pipetting them into stainless-steel sample cups and analyzed using a HS-LEIS Qtac 100 instrument. Two types of measurements were performed: LEIS spectrometry 3 keV 4He+, with and without time-of-flight filtering (ToF filtering) of secondary ions (see Figure 2 and Figure S2 of the Supporting Information, respectively). LEIS spectra of the ILs without gold NPs are presented in Figure 2. Peaks for C, N, O, F, P, and S can be identified in the HS-LEIS spectra of all ILs, indicating that the IL surface is c o m p o s e d o f b o t h i o n s [ im id a z o l i um a n d b is (trifluoromethylsulfonyl)imide or hexafluorophosphate]. Note that this type of composition has been assumed from both theoretical and experimental results using other techniques that are not completely surface-specific.14,18−20 Moreover, the presence of all atoms is in contrast with a previous LEIS study that showed only the presence of fluoride9 and, thus, in the case of BMI·NTf2, shows that HS-LEIS is necessary to obtain more in-depth information about the surface composition of ILs. Note that the impact of the noble gas ions will, in addition to the backscattering of these ions, lead to sputtering of the sample. The contribution of the sputtered (secondary) ions, such as H+, C+, N+, and O+, to the LEIS signal is increasingly important at low energies. This is clearly seen in the background in Figure 2 for energies below 1000 eV. The presence of this background hampers the detection and quantification of light elements, such as C, N, and O. In studies that are focused on functional groups that contain these elements, ToF filtering of the energy-analyzed ions is used to exclude the secondary ions from the LEIS signal.17 The sensitivity for light elements can also be increased significantly using the lighter 3 He + isotope for the analysis. The quantification of C, O, and F with reference samples and the in-depth distribution have been described before.17 In the present study, however, ToF-filtered LEIS and 3He+ ions have not been used to quantify the elemental composition, because the focus is on the Au NPs. Molecular dynamics simulations of the free interface of ILs BMI·NTf2, (MeOE)MI·NTf2, (BCN)MI·NTf2, and (HSE)MI· NTf2 were performed to analyze the surface composition (details of the simulations are given in the Experimental Section). To validate the force field model and the simulation techniques, the surface tension of BMI·NTf2 was calculated from the components of the pressure tensor,21 yielding a value of 27.1 mN/m at 400 K, which compares to the experimental values of 28.4 or 27.7 mN/m taken from the literature.22,23 This kind of agreement is considered good for a totally predictive

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RESULTS AND DISCUSSION Imidazolium ILs chosen for this study (Figure 1) contain different functional groups, alkyl (BMI·PF6), methoxy [(MeO)-

Figure 1. Functionalized and non-functionalized imidazolium ILs used in this study.

EMI·NTf2], nitrile [(BCN)MI·NTf2], and thiol [(HSE)MI· NTf2 and (HSE)MI·PF6] and carboxylate of (HCO2M)MI· NTf2, and thus allow for the formation of different liquid surface compositions and ion orientations. The sputtering of gold foil onto these ILs generates gold NPs of 3−9 nm (Table 1 and Figure S1 of the Supporting Information) under conditions of 40 mA, 325 V, 150 s, and 2 Pa argon work pressure. 14302

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Figure 2. Spectra (3 keV 4He+) of ILs.

Figure 3. Atomic density profiles along z for selected atoms of the cation and anions. N13 denotes the nitrogen atoms of the imidazolium rings (sites of positive partial charge); ONTf2 denotes the oxygen atoms of the anion (sites of negative charge); and terminal atoms denote the functionalized side chains of the cations.

calculation, because the force field parameters were not adjusted to interfacial quantities. In Figure 3, several atomic density profiles are plotted, showing the probability of the presence of selected atoms along the z coordinate. From the z-density profiles, it can be concluded that there is very little charge separation at the interface, because the profiles for the imidazolium headgroup and the O atoms of the anion coincide in terms of interfacial layer extent. Two of the ILs studied, BMI·NTf2 and (MeOE)MI·NTf2, have side chains that protrude into the vacuum, shown by the profiles of their terminal atoms. In the remaining two ILs, (BCN)MI·NTf2 and (HSE)MI·NTf2, which have more polar end groups, there is no evidence of the side chains protruding and the terminal groups are located at the same interfacial layer as the charged groups of the ions. An analogous behavior was reported for ILs with OHfunctionalized side chains.24 The simulation results therefore

agree with the LEIS spectra that show that both cations and anion occupy the interfacial layer. The HS-LEIS spectra for BMI·PF6 and (HSE)MI·PF6 samples also show the presence of P (from the hexafluorophosphate anion) and mainly Si, which is a typical contamination of ILs containing the hexafluorophosphate anion kept in glass vessels, as already observed in TEM samples by energy-dispersive spectrometry (EDS) analysis. Interestingly, the samples with Au NPs do not show Si contamination on the surface of these ILs (see Figure S3 of the Supporting Information), indicating that after sputtering deposition, the silicon contamination is removed from the IL surface probably via a classical sputtering etching.25 The presence of S on the surface of (HSE)MI·PF6 is a strong indication that the SH-containing side chain is not located inside the IL but rather very close to the IL surface. 14303

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Figure 4. Spectra (3 keV 4He+) for the ILs containing Au NPs compared to the spectrum of the Au reference (scaled down by a factor of 0.01). (Inset) Region around the gold peak in the 3 keV 4He+ spectra for the ILs containing gold NPs compared to the spectrum of the gold reference (scaled down by a factor of 0.01).

carboxylate of (HO2CM)MI·NTf2. Thus, modulating the interactions between the incoming gaseous Au atoms and surface composition of the ILs may lead to control of the NP shape and size. Assuming that the strongly coordinating groups, i.e., nitrile and thio groups, are located in the most external position of the surface and pointing out the vacuum, akin to 1alkyl-3-methylimidazolium analogues,5 a relatively strong interaction with the incoming Au atoms and nitrogen and sulfur of the ILs is expected. This coordination of the sputtered Au atoms and/or clusters significantly modifies the way in which the Au NPs grow in comparison to the 1-n-butyl-3methylimidazolium (BMI)-cation-based ILs.32 The coordination of Au atoms diminishes the diffusivity of the atoms and, because of the relatively strong interaction with the surface, propitiates the location of the small Au NPs at the layer immediately below the outermost atomic layer while still covered by a monolayer of ions in the case of functionalized ILs containing thio- and nitrile-coordinating groups. However, in ILs containing very high coordinating carboxylate groups, most of the Au atoms/clusters do not penetrate into the IL bulk (under the sputtering conditions used) and NP growth yields thin films. In contrast, the Au NPs formed on ILs containing alkyl or methoxy groups diffuse deeper into the IL bulk. Indeed, the presence of metal NPs in the most internal layers of ILs containing alkyl groups has been observed by XPS.33 It is known that the increase in the acceleration voltage of the Ar+ ions enhances the average kinetic energy of the sputtered Au atoms. As the molecular arrangement of the surface of the IL gradually changes towards the bulk conformation, the environment in which the Au NPs grow will change dramatically depending upon the depth that the sputtered atoms penetrate into the IL surface. Therefore, a threshold energy for the sputtering atoms that changes the chemical environment where the atoms start the NP growth must exist.4 Moreover, the sputtered atoms are not isoenergetic; rather, their energies follow a Boltzmann distribution.34 Indeed, increasing the discharge voltage of the sputtering process will increase the average translational energy of the sputtered atoms as well the fraction of Au atoms that are above the threshold energy for small Au NP growth. Therefore, the growth mechanism of the Au in the ILs always involves two interconnected parameters: the conformation of

In addition to the C, N, O, F, P, and S peaks observed in the LEIS spectra of the gold-sputtered NP ILs, a feature between 1900 and 2700 eV (Figure 4) is also noticeable. This is caused by gold in layers below the surface, and the shape of this feature represents the in-depth distribution of gold. There is no gold in the outermost atomic layer, but there is gold immediately below it for the samples with ILs containing nitrile [(BCN)MI·NTf2] and thio [(HSE)MI·NTf2] groups. This is an indication that, for this gold depth distribution, the Au NPs are drawn to the liquid surface while still covered by a monolayer of ions. In that case, the depth distribution of gold represents the size of the NPs. The gold depth distribution for Au NPs in (HSE)MI· NTf2 is very narrow, suggesting small particle sizes (∼2 nm), whereas the gold depth distribution for (BCN)MI·NTf2 is broader (see the inset of Figure 4), suggesting larger particle sizes (∼4 nm) at this most external layer. Indeed, these results indicate that some smaller Au NPs are located close to the IL surface, whereas the larger Au NPs migrate deeper into the IL bulk. Indeed, the NP size determined by TEM shows a broad NP size distribution (Table 1), and the location of Au NPs on/ in ILs is probably related to the mechanism of their formation, which depends mainly upon kinetic energy of the sputtered atoms and the coordination ability of the functional groups on the IL. It is plausible to assume that the formation of Au NPs by sputtering follows a similar mechanism of thin-film growth on liquid;6,26−30 that is, the metal film formation proceeds through nucleation, growth of atomic compact clusters, aggregation of branched islands, and finally, the formation of continuous films.31 Indeed, small Au NP growth on the surface of the ILs seems to stop when forming compact clusters followed by the diffusion of the NPs to the liquid bulk phase for ILs containing low coordinating groups [BMI·PF6 and (MeOE)MI·NTf2]. In the case of the ILs containing more coordinating groups, some small NPs are kept on the IL surface but are still covered by a monolayer of ions. Note that it has already been observed that the Au deposition onto ILs may form thin films when low sputtering rates and relatively low discharge voltages are used; otherwise, NPs are produced.32 Moreover, we have observed that Au thin films (discharges from 40 to 400 mA) are formed preferentially when the ionic liquid contains very high coordinating groups, such as 14304

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the molecular species of the IL surface and the energy of the sputtered species. The presence of very high coordinating groups on the IL that are able to maintain the Au atoms/ clusters on the IL surface induces the formation of thin films. In contrast, in ILs with low coordinating functional groups, the NPs diffuse into the liquid bulk, whereas in those containing higher coordinating groups, some small NPs are drawn to the liquid surface while still covered by a monolayer of ions. Note that this type of metallic NP in ILs is solvated preferentially by the charged moieties of the ions, with an interface layer that is one ion thick that has been predicted by density functional theory (DFT) calculations15,35 and estimated by small-angle Xray scattering (SAXS) for NPs in the bulk IL.36

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ASSOCIATED CONTENT

S Supporting Information *

TEM micrographs of the Au nanoparticles in ionic liquids and HS-LEIS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +55-51-3308-7304. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to the INCT-Catal., CAPES, CNPq, and FAPERGS funding agencies.

CONCLUSION

HS-LEIS is revealed to be the technique of choice for the elucidation of the composition and organization of the IL surface and also to measure quantitatively the thickness of ILs overlayers on Au NPs, which was shown to be a monolayer of ions, as predicted by DFT calculations. These findings open exciting perspectives in tuning IL surface effects to design experiments to produce NPs with unusual shapes and sizes. Moreover, the simple change in surface composition of the functionalized ILs might yield new, unexpected NPs for which the synthesis is not straightforward or impossible with classical colloidal methods.

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REFERENCES

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EXPERIMENTAL SECTION

General. The ILs were synthesized as described in the literature37,38 and degassed for approximately 3 h at 298 K under vacuum prior to its introduction in the sputter chamber as liquid substrate for sputter deposition of gold. Deposition for 150 s was performed in a MED 020 (Bal-Tech) in the sputter mode with discharge voltages of 335 V (40 mA) under an argon pressure of 2 Pa at room temperature. In each deposition, a mass of 1.2 g of IL was placed on a Petri plate (3 cm in diameter) and set horizontally in the sputter coater. The liquid surface was located at a distance of 50 mm from the gold target (99.99% purity). The structure, shape, size, and size distribution of the Au NPs (see Figure S3 of the Supporting Information) were investigated using a JEOL JEM1200 transmission electron microscope (TEM) operating at 80 keV. Molecular Simulation Details. The four ILs, BMI·NTf2, (MeOE)MI·NTf2, (BCN)MI·NTf2 and (HSE)MI·NTf2, were represented by all-atom force fields with the same functional form as optimized potentials for liquid simulations−all atom (OPLS−AA),39 with parameters developed specifically for the ions studied here.40 Periodic tetragonal boxes of 50 × 50 × 200 Å were prepared using the Packmol software,41 with a slab of 500 ion pairs centered on the z coordinate. After equilibration, the slab of IL occupied ca. 100 Å in the center of the box, was continuous along the x and y directions through periodic boundary conditions, and had two free interfaces toward vacuum along z. Molecular dynamics simulations were performed using the LAMMPS software.42 Electrostatic and dispersion interactions were calculated at long range using the PPPM method21 with 8 × 8 × 30 grids, a technique suitable for systems with interfaces. Average properties were calculated from 2 ns trajectories at 400 K, maintained by means of a NVT Nosé−Hoover thermostat. HS-LEIS. All samples are analyzed with the Qtac 100 (ION-TOF). Its double toroidal analyzer allows for accurate measurements using HS-LEIS. The 3 keV 4He+ ion fluence for a complete spectrum is 1.4 × 1014 ions/cm2. Assuming a sputter yield of 0.1 atom/He ion, the sputter damage is 1−2% of a monolayer. The change in the surface composition and thickness of the layer covering the NPs is thus negligible (“static” analysis). 14305

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dx.doi.org/10.1021/la403388b | Langmuir 2013, 29, 14301−14306